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Determining the Thickness of Atomically Thin MoS<sub>2</sub> and WS<sub>2</sub> in the TEM
Abstract
Multislice simulations were used to analyze the reliability of annular dark field scanning transmission electron microscopy (ADF-STEM) imaging and selected-area electron diffraction (SAED) for determining the thickness of MoS2 and WS2 specimens in the aberration-corrected TEM. Samples of 1 to 4 layers in thickness for both 2 H and 1 T polymorphs were studied and tilts up to 500 mrad off of the [0001] zone axis were considered. All thicknesses including the monolayer showed distortions and intensity variations in their ADF-STEM images and SAED patterns as a result of tilt. Both techniques proved to be applicable to distinguish monolayers from multilayers using tilt. Without tilt, neither technique allows unambiguous thickness determination solely by comparing relative intensities of atomic columns in ADF-STEM images or diffraction patterns oriented along at [0001] zone axis, with the exception of monolayer 2 H WS2. However, differentiation is possible using absolute intensities in ADF-STEM images. The analysis of ADF-STEM images and SAED patterns also allows identification of the 2 H and 1 T polymorphs of MoS2 and WS2.
... The SAED pattern (inset given in Fig. 2(D)) shows a continuous ring-like structure suggesting the presence of more number of stacked layers in the bulk MoS 2 . 26 The interlayer spacing of bulk MoS 2 sheets is found to be 0.576 nm from 26 The decrease in lateral dimension with expanded interlayer spacing in the MoS 2 QSs is due to the intercalation of Na + into the layers of bulk MoS 2 sheets during the milling process. Fig. 2(L) depicts the corresponding SAED pattern of MoS 2 QSs which clearly indicated the presence of fewlayered QSs. 26 Fig. 3(A and B) present the AFM 2D and 3D topographs of the MoS 2 QSs which shows the presence of nanometric size MoS 2 sheets in the dimension of 5 to 15 nm (from Fig. 3(A)) with height or thickness up to a maximum of 1.8 nm (from Fig. 3(B)) suggesting the presence of bi-or trilayered MoS 2 QSs. ...
... The SAED pattern (inset given in Fig. 2(D)) shows a continuous ring-like structure suggesting the presence of more number of stacked layers in the bulk MoS 2 . 26 The interlayer spacing of bulk MoS 2 sheets is found to be 0.576 nm from 26 The decrease in lateral dimension with expanded interlayer spacing in the MoS 2 QSs is due to the intercalation of Na + into the layers of bulk MoS 2 sheets during the milling process. Fig. 2(L) depicts the corresponding SAED pattern of MoS 2 QSs which clearly indicated the presence of fewlayered QSs. 26 Fig. 3(A and B) present the AFM 2D and 3D topographs of the MoS 2 QSs which shows the presence of nanometric size MoS 2 sheets in the dimension of 5 to 15 nm (from Fig. 3(A)) with height or thickness up to a maximum of 1.8 nm (from Fig. 3(B)) suggesting the presence of bi-or trilayered MoS 2 QSs. ...
... Fig. 2(L) depicts the corresponding SAED pattern of MoS 2 QSs which clearly indicated the presence of fewlayered QSs. 26 Fig. 3(A and B) present the AFM 2D and 3D topographs of the MoS 2 QSs which shows the presence of nanometric size MoS 2 sheets in the dimension of 5 to 15 nm (from Fig. 3(A)) with height or thickness up to a maximum of 1.8 nm (from Fig. 3(B)) suggesting the presence of bi-or trilayered MoS 2 QSs. Additionally, section analysis was also performed at specic regions to map the thickness of the MoS 2 QSs as shown in Fig. 3(C and D). ...
Molybdenum disulfide (MoS2) is one of the promising electrochemical energy storage material among the recently explored 2D materials beyond the extensively studied graphene sheets. However, the MoS2 in the form of quantum sheets (QSs) has not yet examined for use in energy storage devices (batteries and supercapacitors). Here, we demonstrated the superior electrochemical charge-storage properties of exfoliated MoS2 QSs (with lateral size in the range of 5 to 10 nm) for the first time. A salt-assisted ball milling process was used to prepare MoS2 QSs in gram scale that leads to size confinement in both lateral and vertical orientation. The electrochemical analysis of MoS2 QSs indicated their superior capacitive properties compared to the bulk MoS2 that originates from the combination of quantum capacitance and electrochemical capacitance of MoS2 QSs. The device specific properties of MoS2 QSs were studied by the constructing flexible symmetric supercapacitor (SSC) that demonstrated a high device capacitance (162 F g⁻¹), energy density (14.4 Wh kg⁻¹), good rate capability, and long-cycle life. The energy storage performance metrics of MoS2 QSs based SSC devices were superior compared to the state-of-art of MoS2 based supercapacitors. Furthermore, a solar-driven wireless charging power system comprising the fabricated MoS2 QSs based SSC as energy storage device was illustrated in the view of expanding their utility towards practical applications.
... The plan-view HAADF-STEM images can be used to precisely measure the thickness of atomically-thin 2D materials as the image contrast has a direct correlation with the number of atoms in a given atomic column. 24,25 BAs has an AB stacking of layers where every other layer is half unit-cell shifted in the [100] direction from the layer before, as illustrated in Figure 2a. 14,25 When viewed from the [010] direction, the lateral atomic position of the alternating layers, that are half unit-cell shifted in the [100] direction, can be seen. ...
... 24,25 BAs has an AB stacking of layers where every other layer is half unit-cell shifted in the [100] direction from the layer before, as illustrated in Figure 2a. 14,25 When viewed from the [010] direction, the lateral atomic position of the alternating layers, that are half unit-cell shifted in the [100] direction, can be seen. This configuration results in a distinct contrast in plan-view HAADF-STEM images for the odd and the even numbers of BAs layers.14, 25 For the odd numbered BAs layers, neighboring atomic column-pairs (dumbbells) along the planview direction contain a different number of atoms, resulting in dissimilar intensities in a HAADF-STEM image. ...
... 14,25 When viewed from the [010] direction, the lateral atomic position of the alternating layers, that are half unit-cell shifted in the [100] direction, can be seen. This configuration results in a distinct contrast in plan-view HAADF-STEM images for the odd and the even numbers of BAs layers.14, 25 For the odd numbered BAs layers, neighboring atomic column-pairs (dumbbells) along the planview direction contain a different number of atoms, resulting in dissimilar intensities in a HAADF-STEM image. The intensity difference between adjacent atomic dumbbells is sensitive to the thickness of a BAs nanosheet, when the number of layers is relatively small. ...
Black arsenic (BAs) is a van der Waals layered material with a puckered honeycomb structure and has received increased interest due to its anisotropic properties and promising performance in devices. Here, crystalline structure, thickness-dependent dielectric responses, and ambient stability of BAs nanosheets are investigated using STEM imaging and spectroscopy. Atomic-resolution HAADF-STEM images directly visualize the three-dimensional structure and evaluate the degree of anisotropy. STEM-EELS is used to measure the dielectric response of BAs as a function of the number of layers. Finally, BAs degradation under different ambient environments is studied highlighting high sensitivity to moisture in the air.
... Raman spectra (average of 120 measurements) for (a) 2H-MoS2 (blue) and 1T-MoS2 (red) upon 633 nm excitation, and (b) 2H-WS2 (blue) and 1T-WS2 (red) upon 514 nm excitation, obtained at ambient conditions. very powerful technique for investigating the structure of this kind of nanomaterials.32,[37][38][39] The two different structural phases (2H and 1T) of these two kinds of transition metal dichalcogenide materials (MoS2 and WS2) are depicted inFigure 3. ...
... The two different structural phases (2H and 1T) of these two kinds of transition metal dichalcogenide materials (MoS2 and WS2) are depicted inFigure 3. These high-angle annular dark-field (HAADF) HRS-TEM micrographs show and confirm the clear differences between the 2H and 1T phases of these samples.32,37,38 ...
In the present work, some MoS2 and WS2 nanosheets were prepared and characterized. Depending on the preparation procedures, trigonal prismatic (2H) or octahedral (1T) coordination of the metal atoms were obtained, exhibiting metallic (1T) or semiconducting (2H) character. Both MoS2 and WS2 nanosheets were found exhibiting large nonlinear optical (NLO) response, strongly dependent on their metallic (1T) or semiconducting (2H) character. So, the semiconducting character 2H-MoS2 and 2H-WS2 exhibit positive nonlinear absorption and strong self-focusing, while their metallic character counterparts exhibit strong negative nonlinear absorption and important self-defocusing. In addition, the semiconducting MoS2 and WS2 were found exhibiting important and very broadband optical limiting action extended from 450 to 1750 nm. So, by selecting the crystalline phase of the nanosheets, i.e., their semiconduction/metallic character, their NLO response can be greatly modulated. The results of the present work demonstrate unambiguously that the control of the crystalline phase of MoS2 and WS2 provides an efficient strategy for 2D nanostructures with custom made NLO properties for specific optoelectronic and photonic applications.
... The formation of two dimensional sheets with irregular lateral size via ball milling process in agreement with the recent studies of exfoliation of graphene from graphite via ball milling [33,34]. The HR-TEM micrograph of the bulk MoS 2 shown in Fig. 2(d) represents the presence of thick MoS 2 with nearly lateral size of 700 nm, and the corresponding SAED pattern (shown in inset of Fig. 2(d)) revealed the presence of ring like pattern, thus confirming the existence of more number of layers in the bulk MoS 2 [44]. The HR-TEM micrograph of ball milled MoS 2 (as shown in Fig. 2(e)) revealed the presence of sheets with less thickness and lateral size in the range of 200 nm. ...
... The HR-TEM micrograph of ball milled MoS 2 (as shown in Fig. 2(e)) revealed the presence of sheets with less thickness and lateral size in the range of 200 nm. The SAED pattern (as shown in Fig. 2(f)) revealed the presence of clear diffraction spots suggested that the ball milled MoS 2 sheets are comprised of few layers [44]. These morphological analyses revealed that the ball milling process leads to the formation of few layered MoS 2 sheets via mechanochemical delamination from their bulk counterparts. ...
Two dimensional nanostructures are increasingly used as electrode materials in flexible supercapacitors for portable electronic applications. Herein, we demonstrated a ball milling approach for achieving few layered molybdenum disulfide (MoS2) via exfoliation from their bulk. Physico-chemical characterizations such as X-ray diffraction, field emission scanning electron microscope, and laser Raman analyses confirmed the occurrence of exfoliated MoS2 sheets with few layers from their bulk via ball milling process. MoS2 based wire type solid state supercapacitors (WSCs) are fabricated and examined using cyclic voltammetry (CV), electrochemical impedance spectroscopy, and galvanostatic charge discharge (CD) measurements. The presence of rectangular shaped CV curves and symmetric triangular shaped CD profiles suggested the mechanism of charge storage in MoS2 WSC is due to the formation of electrochemical double layer capacitance. The MoS2 WSC device delivered a specific capacitance of 119 μF cm−1, and energy density of 8.1 nW h cm−1 with better capacitance retention of about 89.36% over 2500 cycles, which ensures the use of the ball milled MoS2 for electrochemical energy storage devices.
... These high-angle annular dark-field (HAADF) HRS-TEM micrographs show and confirm the clear differences between the 2H and 1T phases of these samples. 32,37,38 In Figure 4, some representative UV−vis−NIR absorption spectra of trigonal prismatic and octahedral phase MoS 2 and WS 2 nanosheets are presented, all having a concentration of 1 mg/mL. As shown, the absorption spectrum of 2H-MoS 2 exhibits two distinct peaks located at ∼617 and 679 nm, ascribed to the direct excitonic transitions from the spin−orbit split valence band to the conduction band at the K point of Brillouin zone, namely, the B and A excitons, respectively. ...
... Since WSe 2 becomes a direct band gap semiconductor during the transition from bi-to monolayer, a strong increase in the PL signal and a spectral shift due to the changed band gap can be measured. Later, the spot intensities of the electron diffraction patterns additionally confirm that it is a monolayer (Wu et al., 2014). The mechanical stamping from the flexible gel pack onto the holey, 200 nm thick Si 3 N 4membrane (Plano GmbH) results in some cracks and folds in the monolayer, as shown in Figure 7. ...
It is known that 2D materials can exhibit a nonflat topography, which gives rise to an inherent strain. Since local curvature and strain influence mechanical, optical, and electrical properties, but are often difficult to distinguish from each other, a robust measurement technique is needed. In this study, a novel method is introduced, which is capable of obtaining quantitative strain and topography information of 2D materials with nanometer resolution. Relying on scanning nanobeam electron diffraction (NBED), it is possible to disentangle strain from the local sample slope. Using the positions of the diffraction spots of a specimen at two different tilts to reconstruct the locations and orientations of the reciprocal lattice rods, the local strain and slope can be simultaneously retrieved. We demonstrate the differences to strain measurements from a single NBED map in theory, simulation, and experiment. MoS 2 monolayers with different shapes are used as simulation test structures. The slope and height information are recovered, as well as tensile and angular strain which have an absolute difference of less than 0.2% and 0.2° from the theoretical values. An experimental proof of concept using a freely suspended WSe 2 monolayer together with a discussion of the accuracy of the method is provided.
... When applying 2D materials, controlling and identifying their actual layer numbers will be a very important issue to ensure the required performance of 2D material-based devices. As mentioned above, AFM and TEM have been used to locate 2D materials and to count their layer numbers with high spatial resolution; [60,61] these methods are, however, time-consuming and require relatively complex processes for sample preparation. Moreover, irradiation of the electron beam during TEM inspection can result in unexpected sample damage. ...
Optical inspection is a rapid and non-destructive method for characterizing the properties of two-dimensional (2D) materials. With the aid of optical inspection, in situ and scalable monitoring of the properties of 2D materials can be implemented industrially to advance the development and progress of 2D material-based devices toward mass production. This review discusses the optical inspection techniques that are available to characterize various 2D materials, including graphene, transition metal dichalcogenides (TMDCs), hexagonal boron nitride (h-BN), group-III monochalcogenides, black phosphorus (BP), and group-IV monochalcogenides. First, the authors provide an introduction to these 2D materials and the processes commonly used for their fabrication. Then they review several of the important structural properties of 2D materials, and discuss how to characterize them using appropriate optical inspection tools. The authors also describe the challenges and opportunities faced when applying optical inspection to recently developed 2D materials, from mechanically exfoliated to wafer-scale-grown 2D materials. Most importantly, the authors summarize the techniques available for largely and precisely enhancing the optical signals from 2D materials. This comprehensive review of the current status and perspective of future trends for optical inspection of the structural properties of 2D materials will facilitate the development of next-generation 2D material-based devices.
... Krishnamoorthy et al [181] synthesized the delaminated few-layered MoS 2 nanosheets. The HR-TEM and TEM image of the material along with the support of SAED pattern revealed the formation of MoS 2 nanosheets of less thickness and lateral size of about 200 nm [205]. Liu et al [206] synthesized the flower-like MoS 2 on N-doped 3D graphene composite. ...
As globally, the main focus of the researchers is to develop novel electrode materials that exhibit high energy and power density for efficient performance energy storage devices. This review covers the up-to-date progress achieved in transition metal dichalcogenides (TMDs) (e.g. MoS2, WS2, MoSe2, and WSe2) as electrode material for supercapacitors. The TMDs have remarkable properties like large surface area, high electrical conductivity with variable oxidation states. These properties enable the TMDs as the most promising candidates to store electrical energy via hybrid charge storage mechanisms. Consequently, this review article provides a detailed study of TMDs structure, properties, and evolution. The characteristics technique and electrochemical performances of all the efficient TMDs are highlighted meticulously. In brief, the present review article shines a light on the structural and electrochemical properties of transition metal dichalcogenide electrodes. Furthermore, the latest fabricated TMDs based symmetric/asymmetric supercapacitors have also been reported.
... Microscopic studies are mainly focused on the atomic and electronic structures of MoS 2 , using transmission electron microscopy (TEM) and aberration-corrected scanning transmission electron microscopy (Cs-corrected STEM), which can directly observe materials on an atomic level. Crystal and atomic structures with various defects have been discovered using a combination of ab-initio calculations [4,[11][12][13][14][16][17][18][19][20][21][22]. ...
Two-dimensional MoS 2 film can grow on oxide substrates including Al 2 O 3 and SiO 2 . However, it cannot grow usually on non-oxide substrates such as a bare Si wafer using chemical vapor deposition. To address this issue, we prepared as-synthesized and transferred MoS 2 (AS-MoS 2 and TR-MoS 2 ) films on SiO 2 /Si substrates and studied the effect of the SiO 2 layer on the atomic and electronic structure of the MoS 2 films using spherical aberration-corrected scanning transition electron microscopy (STEM) and electron energy loss spectroscopy (EELS). The interlayer distance between MoS 2 layers film showed a change at the AS-MoS 2 /SiO 2 interface, which is attributed to the formation of S–O chemical bonding at the interface, whereas the TR-MoS 2 /SiO 2 interface showed only van der Waals interactions. Through STEM and EELS studies, we confirmed that there exists a bonding state in addition to the van der Waals force, which is the dominant interaction between MoS 2 and SiO 2 . The formation of S–O bonding at the AS-MoS 2 /SiO 2 interface layer suggests that the sulfur atoms at the termination layer in the MoS 2 films are bonded to the oxygen atoms of the SiO 2 layer during chemical vapor deposition. Our results indicate that the S–O bonding feature promotes the growth of MoS 2 thin films on oxide growth templates.
... Krishnamoorthy et al [181] synthesized the delaminated few-layered MoS 2 nanosheets. The HR-TEM and TEM image of the material along with the support of SAED pattern revealed the formation of MoS 2 nanosheets of less thickness and lateral size of about 200 nm [205]. Liu et al [206] synthesized the flower-like MoS 2 on N-doped 3D graphene composite. ...
As globally, the main focus of the researchers is to develop novel electrode materials that exhibit high energy and power density for efficient performance energy storage devices. This review covers the up-to-date progress achieved in transition metal dichalcogenides (TMDs) (e.g. MoS2, WS2, MoSe2, and WSe2) as electrode material for supercapacitors. The TMDs have remarkable properties like: large surface area, high electrical conductivity with variable oxidation states. These properties enable the TMDs as the most promising candidates to store electrical energy via hybrid charge storage mechanisms. Consequently, this review article provides a detailed study of TMDs structure, properties, and evolution. The characteristics technique and electrochemical performances of all the efficient TMDs are highlighted meticulously. In brief, the present review article shines a light on the structural and electrochemical properties of transition metal dichalcogenide electrodes. Furthermore, the latest fabricated TMDs based symmetric/asymmetric supercapacitors have also been reported.
... It may be noted that there is a considerable variation of intensity ratio between the S and Mo atoms obtained along with the Mo-S-Mo directions on both regions ( figure 5(e)). Based on simulated and experimental STEM studies on MoS 2 [34][35][36][37] we can attribute the higher Mo-S intensity ratio to the hexagonal crystal structure or 2H phase, on the other hand, the much lower intensity of the sulfur atoms displayed in the intensity profiles recorded on the left region of the HRSTEM image can be assigned to the octahedrally coordinated 1T-phase MoS 2 , thus confirming the presence of two phases in KCl-MoS 2 samples. ...
Accessing the metastable phases in a controlled fashion can further expand the applications of atomically thin transition metal dichalcogenides (TMDs). Although top-down approaches based on ion intercalation exfoliation have shown to be an effective route to transform 2H phase into 1T and/or 1T′ polytype phases, a bottom-up growth strategy could be more suitable for device integration. Herein, we show that by assisting the atmospheric pressure chemical vapor deposition (APCVD) growth with a specific alkali metal halide (AMH), it possible to induce the direct synthesis of 1T phase domains coexisting with 2H phase structure in micrometer-sized MoS2 monolayer flakes. The photoluminescence emission and structural properties of three different AMH (NaCl, KBr and KCl) MoS2 crystals are compared. Both NaCl and KBr assisted MoS2 monolayers displayed the semiconducting 2H-phase. On the other hand, we demonstrate that KCl promotes the formation of a 1T–2H phase mixture. X-ray photoemission spectroscopy and resonant Raman studies performed on KCl–MoS2 monolayers show the emergence of a second chemical state and 1T Raman bands compared to the rest of the samples. High-resolution scanning transmission electron microscope imaging revealed important changes in the atomic arrangement between 2H and 1T domains, providing clear evidence of the presence of the 1T metastable phase in the lattice. Moreover, the growth 1T domains can also be controlled by modifying the deposition temperature. Our experiments show that the introduction of KCl during the APCVD growth result in stable 1T-MoS2 domains, providing a simple and reproducible route towards the polymorphism phase engineering of layered TMDs using a direct bottom-up approach.
... The atomic displacement parameters of Ti, S, and Fe atoms used for the simulations were 6:5074 Â 10 À4 , 5:041 Â 10 À5 , and 7:0634 Â 10 À3 nm 2 , respectively. 16,17) 3. Results and Discussion ...
... This buckling can create a mistilt along the zdirection and result in differences in the HAADF-STEM intensities of the S atoms in the two grains (Fig. 1c), which is a well-known effect of sample tilt relative to the incident electron beam. 42,43 Although the type A boundary observed here results from the film growth conditions (see Fig. S2), structurally it resembles similar compression lines that have previously been demonstrated to form in TMDs under high-energy electron beams. [44][45][46] However, here they originate from the sub-unit-cell offset present during coalescence, leading to reduced W-W bond distances at the GB, and further leading to compressive strain. ...
Engineering atomic-scale defects is crucial for realizing wafer-scale, single-crystalline transition metal dichalcogenide monolayers for electronic devices. However, connecting atomic-scale defects to larger morphologies poses a significant challenge. Using electron microscopy and atomistic simulations, we provide insights into WS2 crystal growth mechanisms, providing a direct link between synthetic conditions and the microstructure. Dark-field TEM imaging of coalesced monolayer WS2 films illuminates defect arrays that atomic-resolution STEM imaging identifies as translational grain boundaries. Imaging reveals the films to have nearly a single orientation with imperfectly stitched domains. Through atomic-resolution imaging and ReaxFF reactive force field-based molecular dynamics simulations, we observe two types of translational mismatch and discuss their atomic structures and origin. Our results indicate that the mismatch results from relatively fast growth rates. Through statistical analysis of >1300 facets, we demonstrate that the macrostructural features are constructed from nanometer-scale building blocks, describing the system across sub-{\AA}ngstrom to multi-micrometer length scales.
... Many methods allow for the determination of the number of layers in a multilayer sample at a spatial resolution from tens of nanometres to several microns -by an optical contrast [21], atomic force microscopy [22], Raman spectroscopy [23,24] electron energy loss spectroscopy [25,26], or by tilting the sample and measuring it in electron diffraction mode [27]. Simulations have shown that dark-field scanning transmission electron microscopy can be applied to determine the number of layers on a scale of atomic distances [28]. For a bilayer samples, the averaged interlayer distance at sub-Ångstrom precision can be obtained by tilting the sample in selected area electron diffraction measurements and fitting the intensity maxima with the theoretical model [29]. ...
Convergent beam electron diffraction is routinely applied for studying deformation and local strain in thick crystals by matching the crystal structure to the observed intensity distributions. Recently, it has been demonstrated that CBED can be applied for imaging two-dimensional (2D) crystals where a direct reconstruction is possible and three-dimensional crystal deformations at a nanometre resolution can be retrieved. Here, we demonstrate that second-order effects allow for further information to be obtained regarding stacking arrangements between the crystals. Such effects are especially pronounced in samples consisting of multiple layers of 2D crystals. We show, using simulations and experiments, that twisted multilayer samples exhibit extra modulations of interference fringes in CBED patterns, i. e., a CBED moir\'e. A simple and robust method for the evaluation of the composition and the number of layers from a single-shot CBED pattern is demonstrated.
... Many methods allow for the determination of the number of layers in a multilayer sample at a spatial resolution from tens of nanometres to several microns -by an optical contrast [21], atomic force microscopy [22], Raman spectroscopy [23,24] electron energy loss spectroscopy [25,26], or by tilting the sample and measuring it in electron diffraction mode [27]. Simulations are shown that dark-field scanning transmission electron microscopy can be applied to determine the number of layers on a scale of atomic distances [28]. For a bilayer samples, the averaged interlayer distance at sub-Ångstrom precision can be obtained by tilting the sample in selected area electron diffraction measurements and fitting the intensity maxima with the theoretical model [29]. ...
Convergent beam electron diffraction is routinely applied for studying deformation and local strain in thick crystals by matching the crystal structure to the observed intensity distributions. Recently, it has been demonstrated that CBED can be applied for imaging two-dimensional (2D) crystals where a direct reconstruction is possible and three-dimensional crystal deformations at a nanometre resolution can be retrieved. Here, we demonstrate that second-order effects allow for further information to be obtained regarding stacking arrangements between the crystals. Such effects are especially pronounced in samples consisting of multiple layers of 2D crystals. We show, using simulations and experiments, that twisted multilayer samples exhibit extra modulations of interference fringes in CBED patterns, i. e., a CBED moiré. A simple and robust method for the evaluation of the composition and the number of layers from a single-shot CBED pattern is demonstrated.
... These data reveal a crystalline lattice with few discernible atomic vacancies or line defects ( Fig. 2b and Supplementary Fig. 11). The corresponding Fourier transform of the raw lattice data is consistent with the hexagonal reciprocal lattice pattern of MoS 2 when taken along the 〈0001〉 zone axis (Fig. 2b inset), and can be used to index the longitudinal axis of the crystal to the 〈1210〉 direction 25 . Further analysis of the periphery of the transferred crystal reveals several distinct features ( Fig. 2c and Supplementary Fig. 11). ...
Two-dimensional transition-metal dichalcogenide (TMD) crystals are a versatile platform for optoelectronic, catalytic and quantum device studies. However, the ability to tailor their physical properties through explicit synthetic control of their morphology and dimensionality is a major challenge. Here we demonstrate a gas-phase synthesis method that substantially transforms the structure and dimensionality of TMD crystals without lithography. Synthesis of MoS2 on Si(001) surfaces pre-treated with phosphine yields high-aspect-ratio nanoribbons of uniform width. We systematically control the width of these nanoribbons between 50 and 430 nm by varying the total phosphine dosage during the surface treatment step. Aberration-corrected electron microscopy reveals that the nanoribbons are predominantly 2H phase with zig-zag edges and an edge quality that is comparable to, or better than, that of graphene and TMD nanoribbons prepared through conventional top-down processing. Owing to their restricted dimensionality, the nominally one-dimensional MoS2 nanocrystals exhibit photoluminescence 50 meV higher in energy than that from two-dimensional MoS2 crystals. Moreover, this emission is precisely tunable through synthetic control of crystal width. Directed crystal growth on designer substrates has the potential to enable the preparation of low-dimensional materials with prescribed morphologies and tunable or emergent optoelectronic properties.
... [9] And, using the computer software, Wu et al. estimated the sample thickness versus contrast by ADF-STEM and SAED simulations. [10] Even though TEM is a highly sophisticated microanalysis tool, it is only possible to achieve 2D imaging, [11] and while electron tomography by TEM can be applied to obtain volume distribution of MoS 2 thin films and particles, it is limited to −30°to +30°of probe tilting, as described by Kong et al. [12] Compared to that, laserassisted atom probe tomography (APT) is a characterization technique to determine the chemical and spatial resolution at the 3D nanometric scale. For example, APT was used to determine titanium homogeneous spatial (3D) distribution within a MoS 2 matrix on magnetron-sputtered thin films, as reported by Singh et al. [13] Also, it was used to obtain a spatial distribution on SiGe/Si/SiGe interfaces, in order to determine any migration and volume distribution between species. ...
The molybdenum disulfide (MoS 2 ) and indium tin oxide (ITO) interface were studied by atom probe tomography (APT). Raman spectroscopy, scanning electron microscopy, and grazing-incidence x-ray diffraction measurements were performed as complementary characterization. Results confirm that nanowires plated shape with the 〈110〉-orientation are aligned perpendicular to the ITO film with principal reflections at (002), (100), (101), (201), and Raman spectroscopy vibrational modes at E ¹ 2 g at 378 cm ⁻¹ and A 1g at 407 cm ⁻¹ correspond to 2H-MoS 2 . APT reveals MoS ⁺² , MoS ⁺³ as predominant evaporated molecular ions on the sample, indicating no significant diffusion/segregation of Mo or S species within the ITO layer.
... Thickness identification of 2D materials is essential for their characterization and the implementation of several applications requiring different nanosheet thicknesses at precise sites (accessible by transfer micro-manipulation). Many techniques for counting the number of layers exist, such as atomic force microscopy (AFM) [32] and transmission electron microscopy (TEM) [33]. The physical properties of few-layer 2D crystals are thickness dependent and the number of layers can be identified by photoluminescence (PL) measurements [34,35]. ...
The successful integration of few-layer thick hexagonal boron nitride (hBN) into devices based on two-dimensional materials requires fast and non-destructive techniques to quantify their thickness. Optical contrast methods and Raman spectroscopy have been widely used to estimate the thickness of two-dimensional semiconductors and semi-metals. However, they have so far not been applied to two-dimensional insulators. In this work, we demonstrate the ability of optical contrast techniques to estimate the thickness of few-layer hBN on SiO 2 /Si substrates, which was also measured by atomic force microscopy. Optical contrast of hBN on SiO 2 /Si substrates exhibits a linear trend with the number of hBN monolayers in the few-layer thickness range. We also used bandpass filters (500-650 nm) to improve the effectiveness of the optical contrast methods for thickness estimations. We also investigated the thickness dependence of the high frequency in-plane E 2g phonon mode of atomically thin hBN on SiO 2 /Si substrates by micro-Raman spectroscopy, which exhibits a weak thickness-dependence attributable to the in-plane vibration character of this mode. Ab initio calculations of the Raman active phonon modes of atomically thin free-standing crystals support these results, even if the substrate can reduce the frequency shift of the E2g phonon mode by reducing the hBN thickness. Therefore, the optical contrast method arises as the most suitable and fast technique to estimate the thickness of hBN nanosheets.
... Furthermore, the few-layered structural arrangement of in-situ formed MoS 2 -WS 2 heterostructure was clearly evidenced from the HRTEM image (Fig. 2b). A confined layered crystal lattice structure with interlayer spacing of about 0.67 nm confirmed the correspondence with (002) lattice planes of both hexagonal MoS 2 and WS 2 as both of these TMDS holds similar crystal lattice structure [57,58]. The overall layer number of MoS 2 -WS 2 was observed to be approximately 3e8. ...
A promising electrocatalyst material composed of 2D layered MoS2-WS2 heterostructure hierarchically assembled into a 3D highly interconnected macroporous network of graphene was facilely fabricated. This in-situ synthesis method involves hydrothermal reaction followed by moderate thermal annealing which guarantees the uniform distribution of the MoS2-WS2 heterojunctions within graphene matrix. The presence of 3D conductive and porous graphene network and the combined merits of MoS2 and WS2 endow the resulting 3D MoS2-WS2/graphene nanohybrids with unique conductivity pathways and channels for electrons and with outstanding electrocatalytic performance towards enhanced hydrogen evolution reaction (HER). This 3D nanohybrid delivered the small overpotential of 110 mV, and the small Tafel slope of 41 mV per dec, demonstrating high HER activity. Furthermore, the resulting nanohybrids exhibit excellent stability as very trivial drop in the current density was noticed even after 2000 cycles. The superior electrocatalytic performance of 3D MoS2-WS2/graphene over other non-precious metal electrocatalysts is accredited to the robust synergism of 2D MoS2-WS2 with 3D graphene that offer ample active sites and improved conductivity for HER. The proposed approach can be further extended to modify other layered transition metal dichalcogenides with hierarchical 3D porous structure as a competent electrocatalysts for HER.
... For a few-layer 2H MoS 2 , the position of S atoms in one layer coincides with that of the Mo atoms in the adjacent layer. This gives an appreciable intensity for the peaks corresponding to the S sites 39 , as evident from the intensity profile shown in the bottom panel of Fig. 1(b). We extract a nearest Mo-Mo separation of 3.19 (+/−0.08) ...
Monolithic realization of metallic 1T and semiconducting 2H phases makes MoS2 a potential candidate for future microelectronic circuits. A method for engineering a stable 1T phase from the 2H phase in a scalable manner and an in-depth electrical characterization of the 1T phase is wanting at large. Here we demonstrate a controllable and scalable 2H to 1T phase engineering technique for MoS2 using microwave plasma. Our method allows lithographically defining 1T regions on a 2H sample. The 1T samples show excellent temporal and thermal stability making it suitable for standard device fabrication techniques. We conduct both two-probe and four-probe electrical transport measurements on devices with back-gated field effect transistor geometry in a temperature range of 4 K to 300 K. The 1T samples exhibit Ohmic current-voltage characteristics in all temperature ranges without any dependence to the gate voltage, a signature of a metallic state. The sheet resistance of our 1T MoS2 sample is considerably lower and the carrier concentration is a few orders of magnitude higher than that of the 2H samples. In addition, our samples show negligible temperature dependence of resistance from 4 K to 300 K ruling out any hoping mediated or activated electrical transport.
... The selected area electron diffraction (SAED) (Figure 2b) along the [001] zone axis clearly shows the lack of a diffuse amorphous ring and sharp intensities, confirming the high crystal quality, absence of oxidation, and the monolayer nature of the flake. 44 The combination of AFM, TEM, OM, and DLS has demonstrated the success of lithium borohydride exfoliation for TiS 2 , producing monolayer flakes with a micrometer lateral size. However, it is highly desirable to expedite the exfoliation characterization process. ...
Monolayer TiS2 is the lightest member of the transition metal dichalcogenide family with promising applications in energy storage and conversion systems. The use of TiS2 has been limited by the lack of rapid characterization of layer numbers via Raman spectroscopy and its easy oxidation in wet environment. Here, we demonstrate the layer-number-dependent Raman modes for TiS2. 1T TiS2 presents two characteristics of the Raman active modes, A1g (out-of-plane) and Eg (in-plane). We identified a characteristic peak frequency shift of the Eg mode with the layer number and an unexplored Raman mode at 372 cm–1 whose intensity changes relative to the A1g mode with the thickness of the TiS2 sheets. These two characteristic features of Raman spectra allow the determination of layer numbers between 1 and 5 in exfoliated TiS2. Further, we develop a method to produce oxidation-resistant inks of micron-sized mono- and few-layered TiS2 nanosheets at concentrations up to 1 mg/mL. These TiS2 inks can be deposited to form thin films with controllable thickness and nanosheet density over square centimeter areas. This opens up pathways for a wider utilization of exfoliated TiS2 toward a range of applications.
... Crystal structures with trigonal prismatic coordination are layered in an A-B stacking order; whereas, octahedrally coordinated 2D materials tend to stack in an A-A stacking order. 33 Monolayer h-BN is planar and adopts the honeycomb lattice structure. Multi-layered h-BN preferentially adopts the A-A′ stacking order. ...
Hexagonal boron nitride (h-BN) and semiconducting transition metal dichalcogenides (TMDs) promise greatly improved electrostatic control in future scaled electronic devices. To quantify the prospects of these materials in devices, we calculate the out-of-plane and in-plane dielectric constant from first principles for TMDs in trigonal prismatic and octahedral coordination, as well as for h-BN, with a thickness ranging from monolayer and bilayer to bulk. Both the ionic and electronic contribution to the dielectric response are computed. Our calculations show that the out-of-plane dielectric response for the transition-metal dichalcogenides is dominated by its electronic component and that the dielectric constant increases with increasing chalcogen atomic number. Overall, the out-of-plane dielectric constant of the TMDs and h-BN increases by around 15% as the number of layers is increased from monolayer to bulk, while the in-plane component remains unchanged. Our computations also reveal that for octahedrally coordinated TMDs the ionic (static) contribution to the dielectric response is very high (4.5 times the electronic contribution) in the in-plane direction. This indicates that semiconducting TMDs in the tetragonal phase will suffer from excessive polar-optical scattering thereby deteriorating their electronic transport properties.
... The two layers are shifted such that the chalcogens in the top (B) layer align with the metal in the bottom (A) layer. On the other hand octahedrally coordinated 2D materials tend to stack in an A-A' stacking order where the top (A') and bottom (A) layers are not shifted with respect to each other [10]. Monolayer h-BN is a planar 2D material with honeycomb lattice structure and it also follows an A-B stacking order in its bulk form. ...
... 8c shows a high magnification image of the film where the atomic structure of MoS 2 can be observed along the [001] zone axis31 . The lattice parameter of MoS 2 measured in the image is 3.2 A, which is in agreement with previous findings32 .Double gated MoS 2 field effect transistors (FET) were fabricated on a highly doped Si (100) wafer with 285 nm thick SiO 2 dielectric layer using 20 nm and 200 nm Ti and Au contacts. ...
It is of paramount importance to improve the control over large area growth of high quality molybdenum disulfide (MoS2) and other types of 2D dichalcogenides. Such atomically thin materials have great potential for use in electronics, and are thought to make possible the first real applications of spintronics. Here in, a facile and reproducible method of producing wafer scale atomically thin MoS2 layers has been developed using the incorporation of a chelating agent in a common organic solvent, dimethyl sulfoxide (DMSO). Previously, solution processing of a MoS2 precursor, ammonium tetrathiomolybdate ((NH4)2MoS4), and subsequent thermolysis was used to produce large area MoS2 layers. Our work here shows that the use of ethylenediaminetetraacetic acid (EDTA) in DMSO exerts superior control over wafer coverage and film thickness, and the results demonstrate that the chelating action and dispersing effect of EDTA is critical in growing uniform films. Raman spectroscopy, photoluminescence (PL), x-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM) and high-resolution scanning transmission electron microscopy (HR-STEM) indicate the formation of homogenous few layer MoS2 films at the wafer scale, resulting from the novel chelant-in-solution method.
... However, the intensity of diffraction pattern is layer-dependent and periodically changes with the incident beam angle in a cycle of ∼30° [46] The damage can be reduced by using electron energy-loss spectroscopy (EELS) in the characterization of a graphene monolayer [47]. The same techniques have been widely used for other TMDs [48][49][50][51][52] black phosphorene [53] etc. ...
In the great adventure of two-dimensional (2D) materials, the characterization techniques are the lighthouse to guide the investigators across heavy mist and submerged reef. In this review, we highlight the recent achievements in the characterization of the 2D materials. Firstly, the methods to identify the fundamental properties of the 2D materials are introduced. Then, the specific characterization techniques for analyzing electric, optical and chemical properties are summarized with regards to their corresponding fields of applications. It should also be noted that a big challenge remains in the characterizations of the 2D materials in the hybrid or composite and wide acceptance of the characterization standards need to be established to further promote the industrialization of 2D materials in the near future.
... In recent years, a large number of experimental studies of measuring film thickness of 2D materials such as graphene and MoS 2 have been reported by using Raman scattering [11,12], scanning electron microscopy (SEM) [13,14], transmission electron microscopy (TEM) [15,16], atomic force microscopy (AFM) [17,18], optical contrast spectroscopy [19,20] and optical microscopy [21,22]. Most of these methods are applied separately, and significant differences in film thicknesses have been demonstrated when comparing the result of each approach. ...
Thickness measurement plays an important role for characterizing optomechanical behaviors of graphene. From the view of graphene-based Fabry–Perot (F–P) sensors, a simple, nondestructive and in situ method of determining the thickness of nanothick graphene membranes was demonstrated by using optical fiber F–P interference. Few-layer/multilayer graphene sheets were suspendedly adhered onto the endface of a ferrule with a 125 µm inner diameter by van der Waals interactions to construct micro F–P cavities. Along with the Fresnel's law and complex index of refraction of the membrane working as a light reflector of an F–P interferometer, the optical reflectivity of graphene was modeled to investigate the effects of light wavelength and temperature. Then the average thickness of graphene membranes were extracted by F–P interference demodulation, and yielded a very strong cross-correlation coefficient of 99.95% with the experimental results observed by Raman spectrum and atomic force microscope. The method could be further extended for determining the number of layers of other 2D materials.
... When materials are thinned down to atomically thin membranes, thickness begins to play an important role in tailoring their properties, and vice versa, there are a variety of techniques to determine their thicknesses, ranging from image contrast of reflected light microscopy [17][18][19][20][21][22][23], second harmonic microscopy [24][25][26][27], atomic force microscopy, cross-sectional imaging, as well as Raman and photoluminescence spectroscopy [28][29][30][31]. In the field of electron microscopy, there are miscellaneous methods to identify thickness as well, which includes peak shift in plasmon spectroscopy [32,33], direct high-resolution transmission electron microscopy (HRTEM) imaging combined with simulation [34] and linearity in ADF-STEM signals [35,36]. Among the above methods, spectroscopic analysis requires dedicated electron energy loss spectroscopy (EELS) detector and high brightness electron source, and thus limits its use for thickness characterizations. ...
In the current extensive studies of layered two-dimensional (2D) materials, compared to hexagonal structures such as graphene, hBN, and MoS2, low-symmetry 2D materials have shown great potential for applications in anisotropic devices. Rhenium diselenide (ReSe2) possesses the bulk space group xxxx and belongs to the triclinic crystal system with a deformed cadmium-iodide-type structure. Here, we propose an electron diffraction-based method to distinguish the monolayer ReSe2 membrane from multilayer ReSe2 and its two different vertical orientations. Our method is also applicable to other low-symmetry crystal systems, including both triclinic and monoclinic lattices, as long as their third unit-cell basis vectors are not perpendicular to the basal plane. Our experimental results are well explained by kinematical electron diffraction theory and the corresponding simulations. Generalization of our method to other 2D materials, such as graphene, is also discussed.
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... This arrangement is similar to that observed for 1T-MoS 2 . [ 31,32 ] On the other hand, the ADF-STEM image of SnS shows its orthorhombic arrangement, which can be clearly differentiated from the image of SnS 2 . ...
Tin sulfides can exist in a variety of phases and polytypes due to the different oxidation states of Sn. A subset of these phases and polytypes take the form of layered 2D structures that give rise to a wide host of electronic and optical properties. Hence, achieving control over the phase, polytype, and thickness of tin sulfides is necessary to utilize this wide range of properties exhibited by the compound. This study reports on phase-selective growth of both hexagonal tin (IV) sulfide SnS2 and orthorhombic tin (II) sulfide SnS crystals with diameters of over tens of microns on SiO2 substrates through atmospheric pressure vapor-phase method in a conventional horizontal quartz tube furnace with SnO2 and S powders as the source materials. Detailed characterization of each phase of tin sulfide crystals is performed using various microscopy and spectroscopy methods, and the results are corroborated by ab initio density functional theory calculations.
For two-dimensional (2D) materials, the exact thickness of the material often dictates its physical and chemical properties. The 2D quantum material WTe2 possesses properties that vary significantly from a single layer to multiple layers, yet it has a complicated crystal structure that makes it difficult to differentiate thicknesses in atomic-resolution images. Furthermore, its air sensitivity and susceptibility to electron beam-induced damage heighten the need for direct ways to determine the thickness and atomic structure without acquiring multiple measurements or transferring samples in ambient atmosphere. Here, we demonstrate a new method to identify the thickness up to ten van der Waals layers in Td-WTe2 using atomic-resolution high-angle annular dark-field scanning transmission electron microscopy image simulation. Our approach is based on analyzing the intensity line profiles of overlapping atomic columns and building a standard neural network model from the line profile features. We observe that it is possible to clearly distinguish between even and odd thicknesses (up to seven layers), without using machine learning, by comparing the deconvoluted peak intensity ratios or the area ratios. The standard neural network model trained on the line profile features allows thicknesses to be distinguished up to ten layers and exhibits an accuracy of up to 94% in the presence of Gaussian and Poisson noise. This method efficiently quantifies thicknesses in Td-WTe2, can be extended to related 2D materials, and provides a pathway to characterize precise atomic structures, including local thickness variations and atomic defects, for few-layer 2D materials with overlapping atomic column positions.
Black perovskite-phase cesium lead tri-iodide (CsPbI3) has shown great potential in photochemical applications. However, colloidal CsPbI3 still suffers from the existence of ligands attached to the surface of nanocrystals (NCs) and serious photoluminescence (PL) quenching. Here, we develop two different γ-phase CsPbI3 (γ-CsPbI3) NCs/WS2 heterostructures with two different two-dimensional morphologies of WS2 (nanoplates and few-layered WS2 nanosheets) to improve the photocatalytic property of γ-CsPbI3 NCs. The large surface area and the presence of abundant functional groups on the surface of WS2 enable a bonding interaction with precursors in the mixture solution, which can reduce the number of ligands and promote the formation and crystal quality of γ-CsPbI3 NCs. Exhilaratingly, we find that γ-CsPbI3 NCs fabricated with few-layered WS2 nanosheets exhibit a significant enhancement in their photocatalytic property, which may result from the increased amount and improved crystal quality of γ-CsPbI3 NCs and the superior carrier-transport property of few-layered WS2 nanosheets, and these effects are beneficial for producing abundant hydroxyl radicals to completely degrade methylene blue (MB). The fresh γ-CsPbI3 NCs/few-layered WS2 nanosheets show a high photocatalytic degradation efficiency of nearly 100% in 30 min, and completely degrade MB into low-weight and low-toxicity inorganic molecules without any intermediate degradation products.
Black arsenic (BAs) is a van der Waals layered material with a puckered honeycomb structure and has received increased interest due to its anisotropic properties and promising performance in devices. Here, crystalline structure, thickness-dependent dielectric responses, and ambient stability of BAs nanosheets are investigated using scanning transmission electron microscopy (STEM) imaging and spectroscopy. Atomic-resolution high-angle annular dark-field (HAADF)-STEM images directly visualize the three-dimensional structure and evaluates the degree of anisotropy. STEM-electron energy-loss spectroscopy (EELS) is used to measure the dielectric response of BAs as a function of the number of layers. Finally, BAs degradation under different ambient environments is studied highlighting high sensitivity to moisture in the air.
We present density functional studies of the edges of single‐layer 1H‐MoS 2 . This phase presents a rich variability of edges that can influence the morphology and properties of MoS 2 nano‐objects, play an important role in industrial chemical processes, and find future applications in energy storage, electronics and spintronics. For so‐called Mo‐100%S edges we confirm vertical S‐dimers are stable, however we also identify a family of metastable edges combining Mo atoms linked by two‐electron donor symmetrical disulfide ligands and four‐electron donor unsymmetrical disulfide ligands. These may be entropically favoured, potentially stabilizing them at high temperatures as a “liquid edge” phase. For Mo‐50%S edges, S‐bridge structures with 3× periodicity along the edge are the most stable, compatible with a Peierls’ distortion arising from the d‐bands of the edge Mo atoms. We propose an additional explanation for this periodicity via the formation of 3‐centre bonds.
ZrS2, ZrSe2 and mixed alloy ZrSxSe2-x materials were achieved through chemical vapor transport. The incongruent melting system of Zr-S-Se formed crystalline layered flakes as a transport product that grew up to 2 cm in lateral size with cm-scale flakes consistently obtained for the entire compositional range exhibiting visible hexagonal features. Bulk flakes of the series ZrSxSe2-x (x = 0, 0.15, 0.3, 0.6, 1.05, 1.14, 1.51, 1.8 and 2) were analyzed through Raman spectroscopy revealing significant convolution of primary bonding modes and red-shifting of Raman features as a function of increasing sulfur composition. Additionally, activation of new modes not present in the pure compounds are observed as effects which result from disorder introduced into the crystal due to the random mixing of S-Se in the alloying process. Further structural characterization was performed via x-ray diffraction (XRD) on the layered flakes to evaluate the progression of c-plane layer spacing function of alloy composition which was found to range between 6.24 Å for ZrSe2 and 5.85 Å for ZrS2. Estimation of the compositional ratios of the alloy flakes through energy dispersive spectroscopy (EDS) large-area mapping verified the relation of the targeted source stoichiometry represented in the layered flakes. Transmission electron microscopy (TEM) atomic-resolution HAADF-STEM imaging was performed on the representative Zr(S0.5Se0.5)2 alloy to validate the 1T atomic structure and observe the arrangement of the chalcogenide column stacks. Additionally, selected area diffraction pattern generated from the [0001] zone axis reveals the in-plane lattice parameter to be approximately 3.715 Å.
Digital holography has found applications in many walks of life, from medicine to metrology, due to its ability to measure complex fields. Here, we use the power of digital holography to quantitatively image two-dimensional Transition Metal Dichalcogenides (TMDs) such as MoS2 and WS2 placed on a SiO2/Si substrate and determine their complex refractive indices or layer thicknesses. By considering the different refractive indices of the TMDs as they are thinned down from bulk to monolayers and by holographically capturing both the amplitude and the phase of reflected light, single atomic layers of TMDs, about 0.7 nm thick, can be resolved. Using holography, we also predict the number of layers contained within a thick TMD flake, which shows agreement with results obtained using Atomic Force Microscopy (AFM). A Bland–Altman analysis was performed to compare our experimental results with the standard AFM measurements, yielding a limit of agreement <5 nm for samples with thicknesses ranging from 15 to 60 nm. Our technique is non-contact, non-invasive, does not require scanning, and produces a field of view of a few hundred micrometers by a few hundred micrometers in a single capture. To further our study, we also perform simulations to demonstrate how the thickness of the SiO2 layer and the laser wavelength are critical in optimizing the amplitude and phase response of a two-dimensional material. These simulations can be used as a roadmap to determine the ideal wavelength and SiO2 layer thickness that should be used to accurately determine the refractive index or thickness of any given sample.
We examine the atomic structure of chemical vapour deposition grown multilayer WS2 pyramids using aberration corrected annular dark field scanning transmission electron microscopy coupled with an in situ heating holder. The stacking orders and specific types of defects after partial degradation by S and W atomic loss at high temperature are resolved layer-by-layer. Our study of an individual WS2 pyramid with at least six layers, reveals a mixed 2H and 3R polytype stacking. Etching occurred both top and bottom of the WS2 pyramid, which aids in determining the exact vertical layer stacking configurations in the thicker regions. We provide an extensive catalogue of the contrast profiles associated with defects in WS2 as a function of layer number and stacking type, as imaged using ADF-STEM. These results provide extensive details about the identification of a wide range of defects in S2 layers, and the unique ADF-STEM contrast patterns that arise from complex multilayer stacking.
Engineering phase transitions or finding new polymorphs offers tremendous opportunities for developing functional materials. We reveal that thermally-driven desulfurization of single-crystalline MoS2 samples improves transport properties by reducing the bandgap and further induces metallization. Semi-desulfurization, i.e., removal of the topmost S layer, results in the placement of the exposed Mo layers directly on top of following sub-layers, together with the bottom S layer of the top layer. This homonuclear (AA) stacking derived from AA' stacking of the hexagonal (2H) phase is retained even after further desulfurization of the remained bottom S layer, i.e., full-desulfurization of the top layer. Our findings fundamentally explain why the 2H phase of TMDs is characterized by AA' stacking.
Phase-controlled synthesis of metallic and ambient-stable two-dimensional MX2 (M is Mo or W, and X is S) with 1T octahedral coordination will endow these materials with superior performance as compared to their semiconducting 2H coordination counterparts. Here, we report a clean and facile route to prepare 1T-MoS2 and 1T-WS2 through hydrothermal processing under high magnetic fields. We reveal that the as-synthesized 1T-MoS2 and 1T-WS2 are ambient-stable for more than one year. Electrochemical measurements show that 1T-MoS2 performs much better than 2H-MoS2 as the anode for sodium ion batteries. These results can provide a clean and facile method to prepare ambient-stable 1T-phase MX2.
By applying quantum perturbation theory to two-dimensional excitons in monolayer transition metal dichalcogenides (TMDCs), we develop a theoretical model for two-photon absorption in the near infrared spectral region. By assuming the bandwidth of the final excitonic state to be 0.15 eV, the two-photon absorption coefficients are as high as 50 cm/MW and selenium-based, monolayer TMDCs exhibit greater 2PA coefficients than sulfur-based, monolayer TMDCs. Our model is also compared to the experimental data obtained by Z-scans or nonlinear transmission measurements.
Monolithic realization of metallic 1T and semiconducting 2H polymorphic phases makes MoS2 a potential candidate for future microelectronic circuits. Though co-existence of these phases has been reported, a method for engineering a stable 1T phase in a scalable manner, compatible with the standard device fabrication schemes is yet to emerge. In addition, there are no comprehensive studies on the electrical properties of the 1T phase. In this manuscript, we demonstrate a controllable and scalable 2H to 1T phase engineering technique for MoS2 using Ar + H2 microwave plasma. The technique enables us to realize 1T MoS2 starting from the 2H phase of arbitrary thickness and area. Our method allows lithographically defining continuous 1T regions in a 2H sample. The 1T samples withstand aging in excess of a few weeks in ambience and show a thermal stability up to 300 C, making it suitable for standard device fabrication techniques. We conduct both two-probe and four-probe electrical transport measurements on devices with back-gated field effect transistor geometry in a temperature range of 4 K to 300 K. The 1T samples exhibit Ohmic current-voltage characteristics in all temperature ranges without any dependence to the gate voltage, a signature indicative of metallic state. The sheet resistance of our 1T MoS2 sample is considerably lower than that of 2H samples while the carrier concentration of the 1T sample is few orders of magnitude higher than that of the 2H samples. In addition, our samples show negligible temperature dependence of resistance from 4 K to 300 K ruling out any hoping mediated or activated electrical transport.
The rise of atomically thin materials has the potential to enable a paradigm shift in modern technologies by introducing multi-functional materials in the semiconductor industry. To date the growth of high quality atomically thin semiconductors (e.g. WS2) is one of the most pressing challenges to unleash the potential of these materials and the growth of mono- or bi-layers with high crystal quality is yet to see its full realization. Here, we show that the novel use of molecular precursors in the controlled synthesis of mono- and bi-layer WS2 leads to superior material quality compared to the widely used topotactic transformation of WO3-based precursors. Record high room temperature charge carrier mobility up to 52 cm2/Vs and ultra-sharp photoluminescence linewidth of just 36 meV over submillimeter areas demonstrate that the quality of this material supersedes also that of naturally occurring materials. By exploiting surface diffusion kinetics of W and S species adsorbed onto a substrate, a deterministic layer thickness control has also been achieved promoting the design of scalable synthesis routes.
Sub-angstrom scanning transmission electron microscopy (STEM) allows quantitative column-by-column analysis of crystalline specimens via annular dark-field images. The intensity of electrons scattered from a particular location in an atomic column depends on the intensity of the electron probe at that location. Electron beam channeling causes oscillations in the STEM probe intensity during specimen propagation, which leads to differences in the beam intensity incident at different depths. Understanding the parameters that control this complex behavior is critical for interpreting experimental STEM results. In this work, theoretical analysis of the STEM probe intensity reveals that intensity oscillations during specimen propagation are regulated by changes in the beam’s angular distribution. Three distinct regimes of channeling behavior are observed: the high-atomic-number ( Z ) regime, in which atomic scattering leads to significant angular redistribution of the beam; the low- Z regime, in which the probe’s initial angular distribution controls intensity oscillations; and the intermediate- Z regime, in which the behavior is mixed. These contrasting regimes are shown to exist for a wide range of probe parameters. These results provide a new understanding of the occurrence and consequences of channeling phenomena and conditions under which their influence is strengthened or weakened by characteristics of the electron probe and sample.
MoS2, MoSe2 and WSe2 thin flakes were fabricated by the standard micromechanical cleavage procedures. The thickness and the optical contrast of the atomic thin dichalcogenide flakes on SiO2/Si substrates were measured by atomic force microscopy (AFM) and spectroscopic ellipsometer. A rapid and nondestructive method by using reflection spectra was proposed to identify the layer number of 2D layered transition metal dichalcogenides on SiO2 (275 nm)/Si substrates. The contrast spectra of 2D nanosheets with different layer numbers are in agreement with theoretical calculations based on Fresnel’s law, indicating that this method provides an unambiguous and nondestructive contrast spectra fingerprint for identifying single- and few-layered transition metal dichalcogenides. The results will greatly help in fundamental research and application.
We have investigated the structure of atomic defects within monolayer NbSe2 encapsulated in graphene by combining atomic-resolution transmission electron microscope (TEM) imaging, density functional theory (DFT) calculations and strain mapping using geometric phase analysis (GPA). We demonstrate the presence of stable Nb and Se monovacancies in monolayer material and reveal that Se monovacancies are the most frequently observed defect, consistent with DFT calculations of their formation energy. We reveal that the presence of adventitious impurities in the surroundings stabilises significant quantities of impurity substituted Se divacancies in the lattice. We further observe evidence of Pt substitution into defect sites. The new knowledge we provide concerning the character and relative frequency of different atomic defects provides essential information needed for us to understand and control the electronic, magnetic and superconducting properties of this exciting new two-dimensional material.
Transition metal dichalcogenide MoS2 has attracted significant interest for its unique electronic, optical and catalytic properties. Layered crystalline MoS2 can be mechanically exfoliated into single monolayers. With the advancement of microscopic and analytical techniques, significant insight knowledge has been obtained on the structure and properties of two-dimensional (2D) materials. In this paper, the authors have carried out high resolution transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) to investigate MoS2 layers transferred on silicon substrate. Due to the weak nature of van der Waal forces between the S-S layers, stability of layers poses a challenge at high electron acceleration voltages. Dark field STEM (DF-STEM) and EELS, used on 11 layers of MoS2, reveal an interlayer spacing of similar to 6.25 angstrom, which is consistent with the reported crystal structure of MoS2.
Chlorosulfonic acid assisted exfoliation of MoS2 and WS2 resulted on retaining their semiconducting 2H-phase, while render them soluble. The unparalleled preservation of the semiconducting nature for exfoliated MoS2 and WS2, with chlorosulfonic acid only acting as pure intercalant without modifying or disturbing their electronic properties and structure, sharply contrasts the semiconducting-to-metallic phase-transition observed with the state-of-the-art exfoliation techniques. The assessment of the new findings was accomplished by complementary analytical techniques in solution. Electronic absorption and photoluminescence spectroscopy revealed characteristic bands due to only the semiconducting phase. Most importantly, X-ray photoelectron spectroscopy justified the presence of Mo 3d and S 2s and 2p orbitals as well as those of W 5p and 4fand S 2s and 2p, attributed purely to the 2H-phase. The exfoliated materials were further characterized by Raman spectroscopy and via local (atomic) structural analyses employing HR-STEM imaging.
Synthesis of atomically thin MoS2 layers and its derivatives with large-area uniformity is an essential step to exploit the advanced properties of MoS2 for their possible applications in electronic and optoelectronic devices. In this work, a facile method is reported for the continuous synthesis of atomically thin MoS2 layers at wafer scale through thermolysis of a spin coated-ammonium tetrathiomolybdate film. The thickness and surface morphology of the sheets are characterized by atomic force microscopy. The optical properties are studied by UV–Visible absorption, Raman and photoluminescence spectroscopies. The compositional analysis of the layers is done by X-ray photoemission spectroscopy. The atomic structure and morphology of the grains in the polycrystalline MoS2 atomic layers are examined by high-angle annular dark-field scanning transmission electron microscopy. The electron mobilities of the sheets are evaluated using back-gate field-effect transistor configuration. The results indicate that this facile method is a promising approach to synthesize MoS2 thin films at the wafer scale and can also be applied to synthesis of WS2 and hybrid MoS2-WS2 thin layers.
Single-layered molybdenum disulphide with a direct bandgap is a promising two-dimensional material that goes beyond graphene for the next generation of nanoelectronics. Here, we report the controlled vapour phase synthesis of molybdenum disulphide atomic layers and elucidate a fundamental mechanism for the nucleation, growth, and grain boundary formation in its crystalline monolayers. Furthermore, a nucleation-controlled strategy is established to systematically promote the formation of large-area, single- and few-layered films. Using high-resolution electron microscopy imaging, the atomic structure and morphology of the grains and their boundaries in the polycrystalline molybdenum disulphide atomic layers are examined, and the primary mechanisms for grain boundary formation are evaluated. Grain boundaries consisting of 5- and 7- member rings are directly observed with atomic resolution, and their energy landscape is investigated via first-principles calculations. The uniformity in thickness, large grain sizes, and excellent electrical performance signify the high quality and scalable synthesis of the molybdenum disulphide atomic layers.
Two dimensional (2D) materials with a monolayer of atoms represent an ultimate control of material dimension in the vertical direction. Molybdenum sulfide (MoS2) monolayers, with a direct bandgap of 1.8 eV, offer an unprecedented prospect of miniaturizing semiconductor science and technology down to a truly atomic scale. Recent studies have indeed demonstrated the promise of 2D MoS2 in fields including field effect transistors, low power switches, optoelectronics, and spintronics. However, device development with 2D MoS2 has been delayed by the lack of capabilities to produce large-area, uniform, and high-quality MoS2 monolayers. Here we present a self-limiting approach that can grow high quality monolayer and few-layer MoS2 films over an area of centimeters with unprecedented uniformity and controllability. This approach is compatible with the standard fabrication process in semiconductor industry. It paves the way for the development of practical devices with 2D MoS2 and opens up new avenues for fundamental research.
We report a controllable wet method for effective decoration of 2-dimensional (2D) molybdenum disulfide (MoS2) layers with Au nanoparticles (NPs). Au NPs can be selectively formed on the edge sites or defective sites of MoS2 layers. The Au-MoS2 nano-composites are formed by non-covalent bond. The size distribution, morphology and density of the metal nanoparticles can be tuned by changing the defect density in MoS2 layers. Field effect transistors were directly fabricated by placing ion gel gate dielectrics on Au-decorated MoS2 layers without the need to transfer these MoS2 layers to SiO2/Si substrates for bottom gate devices. The ion gel method allows probing the intrinsic electrical properties of the as-grown and Au-decorated MoS2 layers. This study shows that Au NPs impose remarkable p-doping effects to the MoS2 transistors without degrading their electrical characteristics.
Recent progress in large-area synthesis of monolayer molybdenum disulphide, a new two-dimensional direct-bandgap semiconductor, is paving the way for applications in atomically thin electronics. Little is known, however, about the microstructure of this material. Here we have refined chemical vapour deposition synthesis to grow highly crystalline islands of monolayer molybdenum disulphide up to 120 μm in size with optical and electrical properties comparable or superior to exfoliated samples. Using transmission electron microscopy, we correlate lattice orientation, edge morphology and crystallinity with island shape to demonstrate that triangular islands are single crystals. The crystals merge to form faceted tilt and mirror twin boundaries that are stitched together by lines of 8- and 4-membered rings. Density functional theory reveals localized mid-gap states arising from these 8-4 defects. We find that mirror twin boundaries cause strong photoluminescence quenching whereas tilt boundaries cause strong enhancement. Meanwhile, mirror twin boundaries slightly increase the measured in-plane electrical conductivity, whereas tilt boundaries slightly decrease the conductivity.
We report ab-initio calculations of the phonon dispersion relations of the
single-layer and bulk dichalcogenides MoS2 and WS2. We explore in detail the
behavior of the Raman active modes, A1g and E2g as a function of the number of
layers. In agreement with recent Raman spectroscopy measurements [C. Lee et.
al., ACS Nano Vol. 4, 2695 (2010)] we find that the A1g mode increases in
frequency with increasing layer number while the E2g mode decreases. We explain
this decrease by an enhancement of the dielectric screening of the long-range
Coulomb interaction between the effective charges with growing number of
layers. This decrease in the long-range part over-compensates the increase of
the short-range interaction due to the weak inter-layer interaction.
Boron nitride (BN) sheets were exfoliated, and proof of the presence of single and double layers was obtained via electron diffraction and plasmon electron energy loss spectroscopy. A plasmon structure unique to mono- and bi-layer BN was established, and was accompanied by WIEN2K DFT calculations. The latter reproduced plasmon energies and general plasmon structure well; however, the detailed shape of the π-plasmon of the calculated pure BN spectra shows discrepancies with the experimental data. The theoretical models were then modified to include impurity atoms. Both oxygen and carbon impurities were considered, as well as different structures, including singular oxygen atoms and oxygen next to carbon atoms. These various configurations were obtained by analyzing atomic-resolution high-angle annular dark field (HAADF) images. Using these modified models, a π-plasmon structure close to the experimentally observed structure could be simulated.
The remarkable properties of graphene have renewed interest in inorganic, two-dimensional materials with unique electronic and optical attributes. Transition metal dichalcogenides (TMDCs) are layered materials with strong in-plane bonding and weak out-of-plane interactions enabling exfoliation into two-dimensional layers of single unit cell thickness. Although TMDCs have been studied for decades, recent advances in nanoscale materials characterization and device fabrication have opened up new opportunities for two-dimensional layers of thin TMDCs in nanoelectronics and optoelectronics. TMDCs such as MoS(2), MoSe(2), WS(2) and WSe(2) have sizable bandgaps that change from indirect to direct in single layers, allowing applications such as transistors, photodetectors and electroluminescent devices. We review the historical development of TMDCs, methods for preparing atomically thin layers, their electronic and optical properties, and prospects for future advances in electronics and optoelectronics.
We describe a two-step synthesis of pure multiwall MoS2 nanotubes with a high degree of homogeneity in size. The Mo6S4I6 nanowires grown directly from elements under temperature gradient conditions in hedgehog-like assemblies were used as precursor
material. Transformation in argon-H2S/H2 mixture leads to the MoS2 nanotubes still grouped in hedgehog-like morphology. The described method enables a large-scale production of MoS2 nanotubes and their size control. X-ray diffraction, optical absorption and Raman spectroscopy, scanning electron microscopy
with wave dispersive analysis, and transmission electron microscopy were used to characterize the starting Mo6S4I6 nanowires and the MoS2 nanotubes. The unit cell parameters of the Mo6S4I6 phase are proposed. Blue shift in optical absorbance and metallic behavior of MoS2 nanotubes in two-probe measurement are explained by a high defect concentration.
Inorganic Nanotubes–MoS2–Conductivity–Defects–Mo6S4I6
If they could be easily exfoliated, layered materials would become a diverse source of two-dimensional crystals whose properties
would be useful in applications ranging from electronics to energy storage. We show that layered compounds such as MoS2, WS2, MoSe2, MoTe2, TaSe2, NbSe2, NiTe2, BN, and Bi2Te3 can be efficiently dispersed in common solvents and can be deposited as individual flakes or formed into films. Electron
microscopy strongly suggests that the material is exfoliated into individual layers. By blending this material with suspensions
of other nanomaterials or polymer solutions, we can prepare hybrid dispersions or composites, which can be cast into films.
We show that WS2 and MoS2 effectively reinforce polymers, whereas WS2/carbon nanotube hybrid films have high conductivity, leading to promising thermoelectric properties.
The electronic properties of ultrathin crystals of molybdenum disulfide
consisting of N = 1, 2, ... 6 S-Mo-S monolayers have been investigated by
optical spectroscopy. Through characterization by absorption,
photoluminescence, and photoconductivity spectroscopy, we trace the effect of
quantum confinement on the material's electronic structure. With decreasing
thickness, the indirect band gap, which lies below the direct gap in the bulk
material, shifts upwards in energy by more than 0.6 eV. This leads to a
crossover to a direct-gap material in the limit of the single monolayer. Unlike
the bulk material, the MoS2 monolayer emits light strongly. The freestanding
monolayer exhibits an increase in luminescence quantum efficiency by more than
a factor of 1000 compared with the bulk material.
The properties of many nanoscale devices are sensitive to local atomic configurations, and so elemental identification and electronic state analysis at the scale of individual atoms is becoming increasingly important. For example, graphene is regarded as a promising candidate for future devices, and the electronic properties of nanodevices constructed from this material are in large part governed by the edge structures. The atomic configurations at graphene boundaries have been investigated by transmission electron microscopy and scanning tunnelling microscopy, but the electronic properties of these edge states have not yet been determined with atomic resolution. Whereas simple elemental analysis at the level of single atoms can now be achieved by means of annular dark field imaging or electron energy-loss spectroscopy, obtaining fine-structure spectroscopic information about individual light atoms such as those of carbon has been hampered by a combination of extremely weak signals and specimen damage by the electron beam. Here we overcome these difficulties to demonstrate site-specific single-atom spectroscopy at a graphene boundary, enabling direct investigation of the electronic and bonding structures of the edge atoms-in particular, discrimination of single-, double- and triple-coordinated carbon atoms is achieved with atomic resolution. By demonstrating how rich chemical information can be obtained from single atoms through energy-loss near-edge fine-structure analysis, our results should open the way to exploring the local electronic structures of various nanodevices and individual molecules.
Direct imaging and chemical identification of all the atoms in a material with unknown three-dimensional structure would constitute a very powerful general analysis tool. Transmission electron microscopy should in principle be able to fulfil this role, as many scientists including Feynman realized early on. It images matter with electrons that scatter strongly from individual atoms and whose wavelengths are about 50 times smaller than an atom. Recently the technique has advanced greatly owing to the introduction of aberration-corrected optics. However, neither electron microscopy nor any other experimental technique has yet been able to resolve and identify all the atoms in a non-periodic material consisting of several atomic species. Here we show that annular dark-field imaging in an aberration-corrected scanning transmission electron microscope optimized for low voltage operation can resolve and identify the chemical type of every atom in monolayer hexagonal boron nitride that contains substitutional defects. Three types of atomic substitutions were found and identified: carbon substituting for boron, carbon substituting for nitrogen, and oxygen substituting for nitrogen. The substitutions caused in-plane distortions in the boron nitride monolayer of about 0.1 A magnitude, which were directly resolved, and verified by density functional theory calculations. The results demonstrate that atom-by-atom structural and chemical analysis of all radiation-damage-resistant atoms present in, and on top of, ultra-thin sheets has now become possible.
Following the invention of electron optics during the 1930s, lens aberrations have limited the achievable spatial resolution to about 50 times the wavelength of the imaging electrons. This situation is similar to that faced by Leeuwenhoek in the seventeenth century, whose work to improve the quality of glass lenses led directly to his discovery of the ubiquitous "animalcules" in canal water, the first hints of the cellular basis of life. The electron optical aberration problem was well understood from the start, but more than 60 years elapsed before a practical correction scheme for electron microscopy was demonstrated, and even then the remaining chromatic aberrations still limited the resolution. We report here the implementation of a computer-controlled aberration correction system in a scanning transmission electron microscope, which is less sensitive to chromatic aberration. Using this approach, we achieve an electron probe smaller than 1 A. This performance, about 20 times the electron wavelength at 120 keV energy, allows dynamic imaging of single atoms, clusters of a few atoms, and single atomic layer 'rafts' of atoms coexisting with Au islands on a carbon substrate. This technique should also allow atomic column imaging of semiconductors, for detection of single dopant atoms, using an electron beam with energy below the damage threshold for silicon.
The recent discovery of graphene has sparked much interest, thus far focused on the peculiar electronic structure of this material, in which charge carriers mimic massless relativistic particles. However, the physical structure of graphene--a single layer of carbon atoms densely packed in a honeycomb crystal lattice--is also puzzling. On the one hand, graphene appears to be a strictly two-dimensional material, exhibiting such a high crystal quality that electrons can travel submicrometre distances without scattering. On the other hand, perfect two-dimensional crystals cannot exist in the free state, according to both theory and experiment. This incompatibility can be avoided by arguing that all the graphene structures studied so far were an integral part of larger three-dimensional structures, either supported by a bulk substrate or embedded in a three-dimensional matrix. Here we report on individual graphene sheets freely suspended on a microfabricated scaffold in vacuum or air. These membranes are only one atom thick, yet they still display long-range crystalline order. However, our studies by transmission electron microscopy also reveal that these suspended graphene sheets are not perfectly flat: they exhibit intrinsic microscopic roughening such that the surface normal varies by several degrees and out-of-plane deformations reach 1 nm. The atomically thin single-crystal membranes offer ample scope for fundamental research and new technologies, whereas the observed corrugations in the third dimension may provide subtle reasons for the stability of two-dimensional crystals.
A quadrupole-octupole spherical aberration corrector has been designed and built for a modified VG HB5 STEM. The corrector fits between the second condenser and the objective lens, and consists of 6 separate stages. Each stage contains one strong quadrupole, one strong octupole and 12 auxiliary trim coils. The corrector is provided with autotuning software that measures all the relevant aberrations on-line and tunes the corrector automatically. Preliminary tests of the corrector have verified the correction principle, and shown that it can readily compensate for parasitic aberrations arising from misalignment.
The image contrast and specimen resolution of point scatterers embedded in thick crystalline material are calculated for the high-resolution scanning transmission electron microscope. A static periodic potential is assumed for the crystal, thereby neglecting inelastic scattering. Owing to the reciprocity theorem, the results are also valid for the fixed-beam electron microscope. To account for thick crystalline layers the propagation of a convergent incident electron wave within the crystal is calculated, employing the dynamic theory of electron diffraction. The analytical formulas are evaluated numerically for different thicknesses of crystalline aluminum layers. The results indicate that the crystalline foils can be considered as fiber-optic plates for electrons. When the atom rows are parallel to the axis of the incident beam, the electrons channel along the atom rows, so beam broadening is largely avoided for thin crystals. At the defocus of the objective less is chosen optimally, the beam broadens only in proportion to the square root of the crystal thickness.
The scattering of electrons by three-dimensional potential fields, and, in particular, the potential fields associated with a crystal lattice, is considered in terms of the new approach to physical optics proposed by Cowley & Moodie.
By combining high-resolution transmission electron microscopy experiments and first-principles calculations, we study production, diffusion, and agglomeration of sulfur vacancies in monolayer MoS2 under electron irradiation. Single vacancies are found to be mobile under the electron beam and tend to agglomerate into lines. Different kinds of such extended defects are identified in the experiments, and their atomic structures and electronic properties are determined with the help of calculations. The orientation of line defects is found to be sensitive to mechanical strain. Our calculations also indicate that the electronic properties of the extended defects can be tuned by filling vacancy lines with other atomic species, thereby suggesting a way for strain and electron-beam-assisted engineering of MoS2-based nanostructures.
The band offsets and heterostructures of monolayer and few-layer transition-metal dichalcogenides MX2 (M = Mo, W; X = S, Se, Te) are investigated from first principles calculations. The band alignments between different MX2 monolayers are calculated using the vacuum level as reference, and a simple model is proposed to explain the observed chemical trends. Some of the monolayers and their heterostructures show band alignments suitable for potential applications in spontaneous water splitting, photovoltaics, and optoelectronics. The strong dependence of the band offset on the number of layers also implicates a possible way of patterning quantum structures with thickness engineering.
Recent dramatic progress in studying various two-dimensional (2D) atomic crystals and their heterostructures calls for better and more detailed understanding of their crystallography, reconstruction, stacking order, etc. For this, direct imaging and identification of each and every atom is essential. Transmission Electron Microscopy (TEM) and Scanning Transmission Electron Microscopy (STEM) are ideal, and perhaps the only tools for such studies. However, the electron beam can in some cases induce dramatic structure changes and radiation damage becomes an obstacle in obtaining the desired information in imaging and chemical analysis in the (S)TEM. This is the case of 2D materials such as molybdenum disulfide MoS2, but also of many biological specimens, molecules and proteins. Thus, minimizing damage to the specimen is essential for optimum microscopic analysis. In this letter we demonstrate, on the example of MoS2, that encapsulation of such crystals between two layers of graphene allows for a dramatic improvement in stability of the studied 2D crystal, and permits careful control over the defect nature and formation in it. We present STEM data collected from single layer MoS2 samples prepared for observation in the microscope through three distinct procedures. The fabricated single layer MoS2 samples were either left bare (pristine), placed atop a single-layer of graphene or finally encapsulated between single graphene layers. Their behaviour under the electron beam is carefully compared and we show that the MoS2 sample 'sandwiched' between the graphene layers has the highest durability and lowest defect formation rate compared to the other two samples, for very similar experimental conditions.
We have calculated the temperature dependence of the atomic mean-square displacements (MSD's) for a reasonable harmonic lattice dynamical model considered to be appropriate to 2HMoS2. Thermal expansion effects were estimated in a crude way using previous model calculations and thermal expansion data. Our results are shown to obey the Lindeman melting criterion.
Advanced Computing in Electron Microscopy, 2nd Edition, brings together diverse information on image simulation. An invaluable resource, this book provides information on various methods for numerical computation of high resolution conventional and scanning transmission electron microscope images. This text will serve as a great tool for students at the advanced undergraduate or graduate level, as well as experienced researchers in the field. This enhanced second edition includes: descriptions of new developments in the field updated references additional material on aberration corrected instruments and confocal electron microscopy expanded and improved examples and sections to provide stronger clarity. © Springer Science+Business Media, LLC 2010. All rights reserved.
The multislice method, pioneered by Cowley and Moodie, has recently been adapted to simulate annular dark-field scanning transmission electronmicroscope (ADF STEM) images. This paper presents a series of calculations using this new approach with experimental parameters appropriate for a VGHB501 STEM to investigate the visibility of single heavy adatoms on thin crystalline silicon membranes. The tendency for electrons to channel along columns of atoms in crystals can greatly increase the intensity incident on an adatom on the exit surface, thereby increasing the adatom visibility. The simulations indicate that an adatom on the exit surface on a column of crystal atoms is up to three times as visible as an adatom on the entrance surface, and that the adatom remains highly visible as the crystal thickness is increased. Tilting the specimen or displacing the adatom from the column appears to lower the visibility of the adatom dramatically. These calculations suggest that, with the appropriate imaging conditions, a single gold adatom may be visible on at least 235 of (111) silicon.
The photocurrent spectra of single crystals of the semiconducting group VI transition metal dichalcogenides (MoSâ, WSâ, WSeâ, and MoSeâ) were measured as a function of crystal orientation and surface morphology as well as the polarization and the angle of incidence of the incident radiation. The spectra were analyzed with a band edge analysis revised to include the diffusion of carriers, and estimates of the transition energies were obtained. Structure corresponding to the excitonic transitions was also observed. The results were discussed with respect to the applicability of these materials to solar energy conversion.
One of the biggest obstacles in improving the resolution of the electron microscope has always been the blurring of the image caused by lens aberrations. Here we report a solution to this problem for a medium-voltage electron microscope which gives a stunning enhancement of image quality.
Not all crystals form atomic bonds in three dimensions. Layered crystals, for instance, are those that form strong chemical
bonds in-plane but display weak out-of-plane bonding. This allows them to be exfoliated into so-called nanosheets, which can
be micrometers wide but less than a nanometer thick. Such exfoliation leads to materials with extraordinary values of crystal
surface area, in excess of 1000 square meters per gram. This can result in dramatically enhanced surface activity, leading
to important applications, such as electrodes in supercapacitors or batteries. Another result of exfoliation is quantum confinement
of electrons in two dimensions, transforming the electron band structure to yield new types of electronic and magnetic materials.
Exfoliated materials also have a range of applications in composites as molecularly thin barriers or as reinforcing or conductive
fillers. Here, we review exfoliation—especially in the liquid phase—as a transformative process in material science, yielding
new and exotic materials, which are radically different from their bulk, layered counterparts.
The frozen phonon technique is introduced as a means of including the effects of thermal vibrations in multislice calculations of CBED patterns. This technique produces a thermal diffuse background, Kikuchi bands and a Debye-Waller factor, all of which are neglected in the standard multislice calculation. The frozen phonon calculations match experimental silicon (100) CBED patterns for specimen thicknesses of up to at least 550 angstrom. The best-fit silicon r.m.s. vibration amplitude at near room temperature was determined to be 0.085 (5) angstrom. As an independent check of validity, a comparison of calculated CBED, experimental CBED and electron energy loss spectroscopy (EELS) data was also performed. The frozen phonon technique provides an improved theoretical basis for the simulation of CBED and therefore annular dark field scanning transmission electron microscope imaging.
Molybdenum disulfide (MoS2) is systematically studied using Raman spectroscopy with ultraviolet and visible laser lines. It is shown that only the Raman frequencies of and peaks vary monotonously with the layer number of ultrathin MoS2 flakes, while intensities or widths of the peaks vary arbitrarily. The coupling between electronic transitions and phonons are found to become weaker when the layer number of MoS2 decreases, attributed to the increased electronic transition energies or elongated intralayer atomic bonds in ultrathin MoS2. The asymmetric Raman peak at 454 cm−1, which has been regarded as the overtone of longitudinal optical M phonons in bulk MoS2, is actually a combinational band involving a longitudinal acoustic mode (LA(M)) and an optical mode (). Our findings suggest a clear evolution of the coupling between electronic transition and phonon when MoS2 is scaled down from three- to two-dimensional geometry.
Monolayer molybdenum disulfide (MoS2) is a two-dimensional direct band gap semiconductor with unique mechanical, electronic, optical, and chemical properties that can be utilized for novel nanoelectronics and optoelectronics devices. The performance of these devices strongly depends on the quality and defect morphology of the MoS2 layers. Here we provide a systematic study of intrinsic structural defects in chemical vapor phase grown monolayer MoS2, including point defects, dislocations, grain boundaries, and edges, via direct atomic resolution imaging, and explore their energy landscape and electronic properties using first-principles calculations. A rich variety of point defects and dislocation cores, distinct from those present in graphene, were observed in MoS2. We discover that one-dimensional metallic wires can be created via two different types of 60° grain boundaries consisting of distinct 4-fold ring chains. A new type of edge reconstruction, representing a transition state during growth, was also identified, providing insights into the material growth mechanism. The atomic scale study of structural defects presented here brings new opportunities to tailor the properties of MoS2 via controlled synthesis and defect engineering.
Ultrathin two-dimensional nanosheets of layered transition metal dichalcogenides (TMDs) are fundamentally and technologically intriguing. In contrast to the graphene sheet, they are chemically versatile. Mono- or few-layered TMDs - obtained either through exfoliation of bulk materials or bottom-up syntheses - are direct-gap semiconductors whose bandgap energy, as well as carrier type (n- or p-type), varies between compounds depending on their composition, structure and dimensionality. In this Review, we describe how the tunable electronic structure of TMDs makes them attractive for a variety of applications. They have been investigated as chemically active electrocatalysts for hydrogen evolution and hydrosulfurization, as well as electrically active materials in opto-electronics. Their morphologies and properties are also useful for energy storage applications such as electrodes for Li-ion batteries and supercapacitors.
Objective molecular dynamics simulations coupled with a density functional-based tight-binding model indicated that a stress-free single-walled (14,6) MoS2 nanotube exhibits a torsional deformation of 0.87 deg/nm. Simulated electron diffraction patterns and atomic-resolution annular dark field scanning transmission electron microscopy (ADF-STEM) images of the computed nanotube structures show promise that this peculiar feature can be identified experimentally. The small intrinsic twist removes the translational periodicity prescribed by the rolled-up construction and defines a nanotube for which the atomic order is most fundamentally described by the objective structures concept.
Self-focusing of an atomic-scale high-energy electron wave packet by channeling along a zone axis in crystalline silicon is directly measured by scanning transmission electron microscopy using thin epitaxial SrTiO3(100) islands grown on Si(100) as test objects. As the electron probe propagates down a silicon atom column, it is progressively focused onto the column, resulting in a fourfold increase in the scattered signal at the channeling maximum. This results in an enhancement of the visibility of the SrTiO3 islands, which is lost if the sample is flipped upside down and the channeling occurs only after the probe scatters off the SrTiO3 layer. The evolution of the probe wave function calculated by the multislice method accurately predicts the trends in the channeling signal on an absolute thickness scale. We find that while electron channeling enhances the visibility of on-column atoms, it suppresses the contribution from off-column atoms. It can therefore be used as a filter to selectively image the atoms that are most aligned with the atomic columns of the substrate. By using this technique, coherent islands can be distinguished from relaxed islands. For SrTiO3 films formed in a topotactic reaction on Si(100), we show that only ∼55% of the SrTiO3 is aligned with the Si atom columns. The fraction of coherent SrTiO3 islands on Si(100) can be increased by choosing growth conditions away from equilibrium.
Plasmon spectroscopy of the thinnest possible membrane, a single layer of carbon atoms: graphene, has been carried out in conjunction with ab initio calculations of the low loss function. We observe π and π+σ-surface plasmon modes in free-standing single sheets at 4.7 and 14.6 eV, which are substantially redshifted from their values in graphite. These modes are in very good agreement with the theoretical spectra, which find the π- and π+σ in-plane modes of graphene at 4.8 and 14.5 eV. We also find that there is little loss caused by out-of-plane modes for energies less than about 10 eV.
A combination of scanning transmission electron microscopy, electron energy loss spectroscopy and ab initio calculations reveal striking electronic structure differences between two distinct single substitutional Si defect geometries in graphene. Optimised acquisition conditions allow for exceptional signal-to-noise levels in the spectroscopic data. The near-edge fine structure can be compared with great accuracy to simulations and reveal either an sp2-like configuration for a trivalent Si substitution or a more complicated hybridised structure for a tetravalent Si impurity.
We evaluate the probe forming capability of a JEOL 2200FS transmission electron microscope equipped with a spherical aberration (Cs) probe corrector. The achievement of a real space sub-Angstrom (0.1 nm) probe for scanning transmission electron microscopy (STEM) imaging is demonstrated by acquisition and modeling of high-angle annular dark-field STEM images. We show that by optimizing the illumination system, large probe currents and large collection angles for electron energy loss spectroscopy (EELS) can be combined to yield EELS fine structure data spatially resolved to the atomic scale. We demonstrate the probe forming flexibility provided by the additional lenses in the probe corrector in several ways, including the formation of nanometer-sized parallel beams for nanoarea electron diffraction, and the formation of focused probes for convergent beam electron diffraction with a range of convergence angles. The different probes that can be formed using the probe corrected STEM opens up new applications for electron microscopy and diffraction.
Reflectivity spectra from the basal planes of crystals of 2H-NbSe2, 2H-TaSe2, 2H-TaS2 and 1T-TaS2 have been measured in the energy range 25 meV to 14 eV at room temperature and liquid nitrogen temperature. Using extrapolations of the experimental data to 0 eV and 30 eV, a Kramers-Kronig analysis of the data has been performed. The optical data so obtained has been discussed and interpreted in terms of the proposed band model for these materials.
Nanoscale heterostructures with quantum dots, nanowires, and nanosheets have opened up new routes toward advanced functionalities and implementation of novel electronic and photonic devices in reduced dimensions. Coherent and passivated heterointerfaces between electronically dissimilar materials can be typically achieved through composition or doping modulation as in GaAs/AlGaAs and Si/NiSi or heteroepitaxy of lattice matched but chemically distinct compounds. Here we report that single layers of chemically exfoliated MoS(2) consist of electronically dissimilar polymorphs that are lattice matched such that they form chemically homogeneous atomic and electronic heterostructures. High resolution scanning transmission electron microscope (STEM) imaging reveals the coexistence of metallic and semiconducting phases within the chemically homogeneous two-dimensional (2D) MoS(2) nanosheets. These results suggest potential for exploiting molecular scale electronic device designs in atomically thin 2D layers.
Multislice simulations in the transmission electron microscope (TEM) were used to examine changes in annular-dark-field scanning TEM (ADF-STEM) images, conventional bright-field TEM (BF-CTEM) images, and selected-area electron diffraction (SAED) patterns as atomically thin hexagonal boron nitride (h-BN) samples are tilted up to 500 mrad off of the [0001] zone axis. For monolayer h-BN the contrast of ADF-STEM images and SAED patterns does not change with tilt in this range, while the contrast of BF-CTEM images does change; h-BN multilayer contrast varies strongly with tilt for ADF-STEM imaging, BF-CTEM imaging, and SAED. These results indicate that tilt series analysis in ADF-STEM image mode or SAED mode should permit identification of h-BN monolayers from raw TEM data as well as from quantitative post-processing.
Single-layer molybdenum disulfide (MoS2) is a newly emerging two-dimensional semiconductor with a potentially wide range of applications in the fields of nanoelectronics and energy harvesting. The fact that it can be exfoliated down to single-layer thickness makes MoS2 interesting both for practical applications and for fundamental research, where the structure and crystalline order of ultrathin MoS2 will have a strong influence on electronic, mechanical, and other properties. Here, we report on the transmission electron microscopy study of suspended single- and few-layer MoS2 membranes with thicknesses previously determined using both optical identification and atomic force microscopy. Electron microscopy shows that monolayer MoS2 displays long-range crystalline order, although surface roughening has been observed with ripples which can reach 1 nm in height, just as in the case of graphene, implying that similar mechanisms are responsible for the stability of both two-dimensional materials. The observed ripples could explain the degradation of mobility in MoS2 due to exfoliation. We also find that symmetry breaking due to the reduction of the number of layers results in distinctive features in electron-beam diffraction patterns of single- and multilayer MoS2, which could be used as a method for identifying single layers using only electron microscopy. The isolation of suspended single-layer MoS2 membranes will improve our understanding of two-dimensional systems, their stability, and the interplay between their structures, morphologies, and electrical and mechanical properties.
Annular dark field scanning transmission electron microscope (ADF-STEM) images allow detection of individual dopant atoms located on the surface of or inside a crystal. Contrast between intensities of an atomic column containing a dopant atom and a pure atomic column in ADF-STEM image depends strongly on specimen parameters and microscope conditions. Analysis of multislice-based simulations of ADF-STEM images of crystals doped with one substitutional dopant atom for a wide range of crystal thicknesses, types and locations of dopant atom inside the crystal, and crystals with different atoms reveal some interesting trends and non-intuitive behaviours in visibility of the dopant atom. The results provide practical guidelines to determine the optimal microscope and specimen conditions to detect a dopant atom in experiment, obtain information about the 3-d location of a dopant atom, and recognize cases where detecting a single dopant atom is not possible.
A finely focused angstrom-sized coherent electron probe produces a convergent beam electron diffraction pattern composed of overlapping orders of diffracted disks that sensitively depends on the probe position within the unit cell. By incoherently averaging these convergent beam electron diffraction patterns over many probe positions, a pattern develops that ceases to depend on lens aberrations and effective source size, but remains highly sensitive to specimen thickness, tilt, and polarity. Through a combination of experiment and simulation for a wide variety of materials, we demonstrate that these position averaged convergent beam electron diffraction patterns can be used to determine sample thicknesses (to better than 10%), specimen tilts (to better than 1mrad) and sample polarity for the same electron optical conditions and sample thicknesses as used in atomic resolution scanning transmission electron microscopy imaging. These measurements can be carried out by visual comparison without the need to apply pattern-matching algorithms. The influence of thermal diffuse scattering on patterns is investigated by comparing the frozen phonon and absorptive model calculations. We demonstrate that the absorptive model is appropriate for measuring thickness and other specimen parameters even for relatively thick samples (>50nm).
This paper reports on a method to obtain atomic resolution Z-contrast (high-angle annular dark-field) images with intensities normalized to the incident beam. The procedure bypasses the built-in signal processing hardware of the microscope to obtain the large dynamic range necessary for consecutive measurements of the incident beam and the intensities in the Z-contrast image. The method is also used to characterize the response of the annular dark-field detector output, including conditions that avoid saturation and result in a linear relationship between the electron flux reaching the detector and its output. We also characterize the uniformity of the detector response across its entire area and determine its size and shape, which are needed as input for image simulations. We present normalized intensity images of a SrTiO(3) single crystal as a function of thickness. Averaged, normalized atom column intensities and the background intensity are extracted from these images. The results from the approach developed here can be used for direct, quantitative comparisons with image simulations without any need for scaling.
As silicon-based transistors in integrated circuits grow smaller, the concentration of charge carriers generated by the introduction of impurity dopant atoms must steadily increase. Current technology, however, is rapidly approaching the limit at which introducing additional dopant atoms ceases to generate additional charge carriers because the dopants form electrically inactive clusters. Using annular dark-field scanning transmission electron microscopy, we report the direct, atomic-resolution observation of individual antimony (Sb) dopant atoms in crystalline Si, and identify the Sb clusters responsible for the saturation of charge carriers. The size, structure, and distribution of these clusters are determined with a Sb-atom detection efficiency of almost 100%. Although single heavy atoms on surfaces or supporting films have been visualized previously, our technique permits the imaging of individual dopants and clusters as they exist within actual devices.
We have achieved atomic-resolution imaging of single dopant atoms buried inside a crystal, a key goal for microelectronic device characterization, in Sb-doped Si using annular dark-field scanning transmission electron microscopy. In an amorphous material, the dopant signal is largely independent of depth, but in a crystal, channeling of the electron probe causes the image intensity of the atomic columns to vary with the depths of the dopants in each column. We can determine the average dopant concentration in small volumes, and, at low concentrations, the depth in a column of a single dopant. Dopant atoms can also serve as tags for experimental measurements of probe spreading and channeling. Both effects remain crucial even with spherical aberration correction of the probe. Parameters are given for a corrected Bloch-wave model that qualitatively describes the channeling at thicknesses 20 nm, but does not account for probe spreading at larger thicknesses. In thick samples, column-to-column coupling of the probe can make a dopant atom appear in the image in a different atom column than its physical position.
We report free-standing atomic crystals that are strictly 2D and can be viewed as individual atomic planes pulled out of bulk crystals or as unrolled single-wall nanotubes. By using micromechanical cleavage, we have prepared and studied a variety of 2D crystals including single layers of boron nitride, graphite, several dichalcogenides, and complex oxides. These atomically thin sheets (essentially gigantic 2D molecules unprotected from the immediate environment) are stable under ambient conditions, exhibit high crystal quality, and are continuous on a macroscopic scale.
Graphene is a rapidly rising star on the horizon of materials science and condensed-matter physics. This strictly two-dimensional material exhibits exceptionally high crystal and electronic quality, and, despite its short history, has already revealed a cornucopia of new physics and potential applications, which are briefly discussed here. Whereas one can be certain of the realness of applications only when commercial products appear, graphene no longer requires any further proof of its importance in terms of fundamental physics. Owing to its unusual electronic spectrum, graphene has led to the emergence of a new paradigm of 'relativistic' condensed-matter physics, where quantum relativistic phenomena, some of which are unobservable in high-energy physics, can now be mimicked and tested in table-top experiments. More generally, graphene represents a conceptually new class of materials that are only one atom thick, and, on this basis, offers new inroads into low-dimensional physics that has never ceased to surprise and continues to provide a fertile ground for applications.
The formation and utility of sub-angstrom to nanometer-sized electron probes
- J Wen
- J Mabon
- C Lei
- S Burdin
- E Sammann
- I Petrov
- A B Shah
- V Chobpattana
- J Zhang
- K Ran
- J Zuo
- S Mishina
- T Aoki
J. Wen, J. Mabon, C. Lei, S. Burdin, E. Sammann, I. Petrov, A.B. Shah, V. Chobpattana, J. Zhang, K. Ran, J. Zuo, S. Mishina, T. Aoki, The formation and utility of sub-angstrom to nanometer-sized electron probes, Microsc. Microanal. 16 (2010) 183–193.