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Layer-resolved Raman imaging and analysis of parasitic ad-layers in transferred graphene
Abstract
In this report, we demonstrate an applied protocol for layer-resolved Raman imaging and analysis of undesirable ad-layers found in Chemical Vapor Deposition graphene grown on copper foil and transferred onto an oxidized silicon substrate. The method assumes that the intensity of the silicon-related Raman-active mode at 520 cm⁻¹ is attenuated by 2.3 % each time the light passes through a single graphene layer. Upon normalization with respect to a reference graphene-free area, the 520 cm⁻¹ mode relative intensity r-ISi measured in a back-scatter mode follows a univalent function of the number of the graphene layers N. Since N is treated as a continuous argument, it can be ascribed a fractional value and considered statistically. Importantly, the r-ISi offers higher layer differentiation capability and unambiguity than non-functional indicators, including the 2D band width or the 2D-to-G band intensity ratio, thus providing unequivocal evaluation.
... Due to the difficulty of determining the number of graphene layers by conventional microscopy measurements [38], a 3D Raman surface imaging model was used to determine the number of graphene layers [39,40] and generate a three-dimensional image, assuming a constant literature interlayer distance in MLG [41,42]. It relies on the principle of a reduced Raman signal of the substrate underneath the graphene when passing through the material. ...
... The thicknesses of the graphene films were calculated afterward. Fig. 2 summarizes the results obtained by implementing the shadow method analysis [40] to determine the number of graphene layers of each film grown using different catalyst thicknesses. The optical images of the transferred graphene films using 50 nm and 100 nm of Mo catalyst, Fig. 2.a-b have a similar contrast and some brighter areas in the submicrometre scale. ...
... The reference map was created from 1681 measurement points. For more information on the method leading to 3D imaging, the reader can refer to Dobrowolski et al. [39,40]. Supportive standard L.N. Sacco et al. ...
... post-transfer organic residue [19], and uniformity of charge carrier type and sheet density determined by the substrate-related vector of spontaneous polarization [20,21]. ...
... We introduce a practical algorithm in the course of which a QFSgraphene/4H-SiC(0001) structure has its statistical distribution of the number of the QFS graphene layers [19,23] (typically a fractional number between 1.0 and 2.0) extracted from an ellipsometric analysis [22], its hole density outright measured in a Hall effect setup, and its work function numerically calculated via a Density Functional Theory (DFT) model for the previously determined and . With this knowledge, the apex of the measured contact potential difference distribution between the QFS graphene and the silicon tip, or −Si , can be used as a precise reference standard to calculate the work function of any other material under study, or = + ( −Si − −Si ). ...
... Following our experience with the amorphous-Al 2 O 3 /QFS-graphene/4H-SiC(0001) Hall effect sensor [24], the reproducibility of the QFS-graphene/4H-SiC(0001) structure transport properties within a single processing cycle is estimated at ∼10%, measured as a percentile ratio of standard deviation over mean value ( ∕ × 100%). This variation is considered low for graphene technology and possible only because direct epitaxial growth is superior to a polymer-assisted transfer [19]. The vestigial dispersion is a remnant of the fingerprint-like topography of epitaxial graphene, marked with a combination of micrometer-scale terraces and nanometer-high steps [17,18,25]. ...
Kelvin Probe Force Microscopy is a method to assess the contact potential difference between a sample and the probe tip. It remains a relative tool unless a reference standard with a known work function is applied, typically bulk gold or cleaved highly oriented pyrolytic graphite. In this report, we suggest a verifiable, two-dimensional standard in the form of a photolithographically patterned, wire-bonded structure manufactured in the technology of transfer-free p-type hydrogen-intercalated quasi-free-standing epitaxial Chemical Vapor Deposition graphene on semi-insulating high-purity nominally on-axis 4H-SiC(0001). The particular structure has its hole density 𝑝𝑆 = 1.61 × 1013 cm−2 measured through a classical Hall effect, its number of the graphene
layers 𝑁 = 1.74 extracted from the distribution of the ellipsometric angle 𝛹, measured at the angle of incidence AOI = 50◦ and the wavelength 𝜆 = 490 nm, and its work function 𝜙𝐺𝑅 = 4.79 eV postulated by a Density Functional Theory model for the specific 𝑝𝑆 and 𝑁. Following the algorithm, the contact potential difference
between the structure and a silicon tip, verified at 𝛥𝑉𝐺𝑅−Si = 0.64 V, ought to be associated with 𝜙𝐺𝑅 = 4.79 eV and applied as a precise reference value to calculate the work function of an arbitrary material.
... graphene layers cast on substrate-related Raman-active modes [11,19]. Specifically, the method assumes that the intensity of the SiC longitudinal optical (LO) A 1 mode at 964 cm −1 is attenuated by 2.3 % [20] each time the light passes through a single graphene layer. ...
... Upon normalization with respect to a reference graphene-free area, the relative intensity at 964 cm −1 measured in a back-scatter geometry is attenuated according to the formula T(N) = (1− ) 2N , where = 1/137 is the fine structure constant [20]. We also postulated that the protocol was universal, had supreme differentiation capability, and lifted the limitation of the author's interpretation of the 2D band Full Width at Half Maximum ( 2D ), the 2D-to-G band intensity ratio (I 2D /I G ) [11,19] or the G and 2D band positions ( G , 2D ). ...
... The shadow method was introduced in Ref. [11] and [19]. It draws from the observation that the transmission-mode opacity of a single graphene layer is dependent solely on the fine structure constant , and defined at = 2.3% of the incident visible light [20], while optical transmittance of a stack of N graphene layers follows a negative ) N , regardless of the stacking sequence [26]. ...
In this report, we present transfer-free p-type hydrogen-intercalated quasi-free-standing epitaxial Chemical
Vapor Deposition graphene on 15-mm × 15-mm semi-insulating vanadium-compensated on-axis 6H–SiC(0001),
characterized in that its room-temperature direct-current Hall-effect-derived hole mobility 𝜇p = 5019 cm2/Vs,
and its statistical number of layers (N), as indicated by the relative intensity of the SiC-related Raman-active
longitudinal optical A1 mode at 964 cm−1, equals N = 1.05. The distribution of the ellipsometric angle 𝛹
measured at an angle of incidence of 50◦ and 𝜆 = 490 nm points out to N = 0.97. The close-to-unity value of
N implies that the material under study is a close-to-perfect quasi-free-standing monolayer, which is further
confirmed by High-Resolution Transmission Electron Microscopy. Therefore, its spectroscopic properties, which
include the Si–H peak at 2131 cm−1, the histograms of 𝛹 and 𝛥, and the Raman G and 2D band positions,
widths, and the 2D-to-G band intensity ratios, constitute a valuable reference for this class of materials.
... The absorbed portion of the light is strictly connected with the fine-structure constant and is equal to πα ≈ 2.3% . In the optical system with a backscatter geometry, light passes through the graphene layers twice each time, losing respectively more of the intensity (Fig. 5a) 12,15 . Taking into account the issue of the fine structure constant and backscattering, it leads Where N is the number of graphene layers and α is a fine-structure constant. ...
... For this PRISM method mode, we have prepared microstructures in a homoepitaxial layer of SiC in the form of an isosceles cross (dimensions of 20 µm × 90 µm ) etched with reactive ions. The nitrogen-doped 5.34-µm-thick homoepitaxial layer was grown on a 15 mm × 15 mm sample cut from a 3-in conducting, n-type (4×10 18 cm -3 ), 364-µm-thick 4H-SiC substrate (SiCrystal GmbH) with a 4 • off-cut from the basal vector [0001] towards the [11][12][13][14][15][16][17][18][19][20] direction. Silane and propane were used as precursors (C : Si = 1.8), and hydrogen as carrier gas 27 . ...
We demonstrate a genuine method for three-dimensional pictorial reconstructions of two-dimensional, three-dimensional, and hybrid specimens based on confocal Raman data collected in a back-scattering geometry of a 532-nm setup. The protocol, or the titular PRISM (Phase-Resolved Imaging Spectroscopic Method), allows for sub-diffractive and material-resolved imaging of the object’s constituent material phases. The spacial component comes through either the signal distal attenuation ratio (direct mode) or subtle light-matter interactions, including interference enhancement and light absorption (indirect mode). The phase component is brought about by scrutinizing only selected Raman-active modes. We illustrate the PRISM approach in common real-life examples, including photolithographically structured amorphous Al2O3, reactive-ion-etched homoepitaxial SiC, and Chemical Vapor Deposition graphene transferred from copper foil onto a Si substrate and AlGaN microcolumns. The method is implementable in widespread Raman apparatus and offers a leap in the quality of materials imaging. The lateral resolution of PRISM is stage-limited by step motors to 100 nm. At the same time, the vertical accuracy is estimated at a nanometer scale due to the sensitivity of one of the applied phenomena (interference enhancement) to the physical property of the material (layer thickness).
... Its exceptional electrical, mechanical, and thermal properties [1][2][3], stemming from its unique honeycomb lattice structure, have attracted attention for a plethora of powerful applications across various fields, including energy storage and conversion [4][5][6], catalysis [7][8][9], sensing [10][11][12], and electronics [13,14]. Moreover, recent advancements in sheet transfer and epitaxial growth have enabled transformative progress in graphene-based device fabrication [15][16][17][18][19]. Paired with refinements in characterization methods [20][21][22], these developments have significantly broadened the potential applications for graphenebased 2D/3D systems, propelling them towards even greater utility and innovation [23][24][25][26][27][28][29][30]. ...
Both intentional and unintentional doping of graphene is a common occurrence, as its carrier concentration can be modulated through various mechanisms. While extensively explored in electronics for achieving desirable conductivity, other aspects of doping remain largely untapped, presenting opportunities for further innovation. This study demonstrates that carrier concentration serves as a powerful and selective tool for modulating the interaction between molecular adsorbates and graphene. The effects are tunable and evident for both n-type and p-type doping, with low-to-medium modulation at doping levels of ±1012e/cm2, and substantial enhancements, with interaction strength increases exceeding 150% and hundreds of meV, at doping levels of ±1013e/cm2. These effects are also molecule specific, with significant enhancements for species such as water (H2O), ammonia (NH3), and aluminum chloride (AlCl3), while having minimal impact on species like hydrogen (H2). This finding not only elucidates the fundamental chemical behavior of graphene but also provides a versatile method to tailor its surface chemistry for applications in sensors, catalysis, and electronic devices. The insights from this research pave the way for advanced material design strategies, leveraging the tunable nature of graphene’s properties to optimize its interaction with various molecular species.
... At 770 K, the dispersion is down to p S (2.6 %), µ p (4.7 %), R S (10.4 %), and S I (2.6 %). This low variation is possible only because direct epitaxial growth does not require a polymer-assisted transfer [26]; it is elementally pure and void of substitutional dopants, and its charge carrier type and density are determined solely by the SiC-related vector of spontaneous polarization. The vestigial dispersion is traced to the developed topography of epitaxial graphene, marked with a fingerprint-like combination of micrometer-scale terraces and nanometer-scale steps [19], [27]. ...
In this letter, we demonstrate a Hall effect sensor in the technology of amorphous-Al2O3-passivated transfer-free
p-type hydrogen-intercalated quasi-free-standing epitaxial Chemical Vapor Deposition graphene on semi-insulating high-purity on-axis 4H-SiC(0001), pre-epitaxially modified with 5-
keV hydrogen (H+) ions. The sensor operates between 305 K and 770 K, with a current-mode sensitivity of ∼75 V/AT and thermal stability below 0.15 %/K (⩽ 0.03 %/K in a narrower range between 305 K and 700 K). It is a promising two-dimensional platform for high-temperature magnetic diagnostics and plasma control systems for modern tokamak fusion reactors.
... Its exceptional electrical, mechanical, and thermal properties [1][2][3], stemming from its unique honeycomb lattice structure, have attracted attention for a plethora of powerful applications across various fields, including energy storage and conversion [4][5][6], catalysis [7][8][9], sensing [10][11][12], and electronics [13,14]. Moreover, recent advancements in sheet transfer and epitaxial growth have enabled transformative progress in graphene-based device fabrication [15][16][17][18][19]. Paired with refinements in characterization methods [20][21][22], these developments have significantly broadened the potential applications for graphenebased 2D/3D systems, propelling them towards even greater utility and innovation [23][24][25][26][27]. ...
Both intentional and unintentional doping of graphene is a common occurrence, as its carrier concentration can be modulated through various mechanisms. While extensively explored in electronics for achieving desirable conductivity, other aspects of doping remain largely untapped, presenting opportunities for further innovation. This study demonstrates that carrier concentration serves as a powerful and selective tool for modulating the interaction between molecular adsorbates and graphene. The effects are tunable and evident for both n-type and p-type doping, with low-to-medium modulation at doping levels of ±1012 e/cm2 , and substantial enhancements, with interaction strength increases exceeding 150% and hundreds of meV, at doping levels of ±1013 e/cm2 . These effects are also molecule-specific, with significant enhancements for species such as water (H2O), ammonia (NH3), and aluminum chloride (AlCl3 ), while having minimal impact on species like hydrogen (H2 ). This finding not only elucidates the fundamental chemical behavior of graphene but also provides a versatile method to tailor its surface chemistry for applications in sensors, catalysis, and electronic devices. The insights from this research pave the way for advanced material design strategies, leveraging the tunable nature of graphene’s properties to optimize its interaction with various molecular species.
... The average transmission of N-doped graphene film ranging from 400 nm to 800 nm extracted from Fig. 12 was found to be 95%. This is consistent with the literature related to single layer graphene film [66]. It is well known that the transmittance value of the CdS buffer layer used in CZTS-based thin film solar cell is around 80%. Increasing the transmittance value (above 80%) by employing graphene in the place of the CdS layer may provide more photons arriving at the CZTS absorber layer, thus enhancing solar cell efficiency. ...
... It can be clearly seen that the intensity ratio of 2D and G peaks differs significantly and equals 0.65 ± 0.05, 1.39 ± 0.07, and 2.71 ± 0.10 for three, two, and one layers, respectively. In the literature, there is a very wide spread of these ratios [21][22][23][24][25][26][27][28] which has led to the conclusion that this parameter should not be used to determine the number of layers alone [29]. In this study, however, it becomes evident that the ratio is different for each region and follows the designed pattern. ...
It is of great significance to study new thermal conductivity filler in practical thermal management application. In this study, a multi‐scale assembled thermal conductivity composite Al 2 O 3 ‐ZnO NWs/GO/EP was developed. The test results showed that the composite has good thermal conductivity and mechanical properties with low filler content. When the GO content is 1.5 wt% and the Al 2 O 3 ‐ZnO NWs content is 25 wt%, the thermal conductivity of the composites reaches 2.15 W/mK, which is improved by about 1032% compared with the pure epoxy resin, and the tensile strength of the composites is maintained at a high level. The problems of high filling capacity and poor mechanical properties of alumina filler were effectively solved. It provides a new idea for preparing the filled thermal conductivity composites with low filling capacity and high thermal conductivity.
Epitaxial graphene (EG) grown on the carbon-face of SiC has been shown to exhibit high carrier mobilities, in comparison to other growth techniques amenable to wafer-scale graphene fabrication. The transfer of large area (>mm2) graphene films to substrates amenable for specific applications is desirable. We demonstrate the dry transfer of EG from the C-face of 4H-SiC onto SiO2, GaN and Al2O3 substrates via two approaches using either 1) thermal release tape or 2) a spin-on, chemically-etchable dielectric. We will report on the impact that these transfer processes has upon the electrical properties of the transferred EG films.
We demonstrate a simple method for transferring large areas (up to A4-size sheets) of CVD graphene from copper foils onto a target substrate using a commercially available polyvinyl alcohol polymer foil as a carrier substrate and a commercial hot-roll office laminator. Through the use of terahertz time-domain spectroscopy and Raman spectroscopy, large-area quantitative optical contrast mapping, and the fabrication and electrical characterisation of ~50 individual centimetre-scale van der Pauw field effect devices, we show a non-destructive technique to transfer large-area graphene with low residual doping that is scalable, economical, reproducible, easy to use, and results in less doping and transfer-induced damage than etching or electrochemical delamination transfers. We show that the copper substrate can be used multiple times with minimal loss of material and no observable reduction in graphene quality. We have additionally demonstrated transfer of multilayer hexagonal boron nitride from copper and iron foils. Finally, we note that this approach allows graphene to be supplied on stand-alone polymer supports by CVD graphene manufacturers to end users, with the only equipment and consumables required to transfer graphene onto target substrates being a commercial office laminator and water.
In this report, we demonstrate the preparation method of a multi-layer stack with a pre-defined number of graphene layers, which was obtained using chemical vapor deposition graphene deposited on a copper substrate and subsequently transferred onto a poly(methyl methacrylate) (PMMA) substrate. The prepared multi-layer stack can also be transferred onto an arbitrary substrate and in the end, the polymer can be removed, which in consequence significantly increases the range of possible graphene applications. The multi-layer character was confirmed by optical transmittance measurements and Raman spectroscopy, whereas the microstructure of the multi-layer graphene stack was investigated using Scanning Electron Microscopy. The electrical properties in the function of the number of graphene layers were assessed with standard Hall Effect measurements. Finally, we showed the practical application of the multi-layer graphene stack as a saturable absorber of a mode-locked Er-doped fiber laser.
We perform systematic investigations of transport through graphene on hexagonal boron nitride (hBN) substrates, together with confocal Raman measurements and a targeted theoretical analysis, to identify the dominant source of disorder in this system. Low-temperature transport measurements on many devices reveal a clear correlation between the carrier mobility μ and the width n∗ of the resistance peak around charge neutrality, demonstrating that charge scattering and density inhomogeneities originate from the same microscopic mechanism. The study of weak localization unambiguously shows that this mechanism is associated with a long-ranged disorder potential and provides clear indications that random pseudomagnetic fields due to strain are the dominant scattering source. Spatially resolved Raman spectroscopy measurements confirm the role of local strain fluctuations, since the linewidth of the Raman 2D peak—containing information of local strain fluctuations present in graphene—correlates with the value of maximum observed mobility. The importance of strain is corroborated by a theoretical analysis of the relation between μ and n∗ that shows how local strain fluctuations reproduce the experimental data at a quantitative level, with n∗ being determined by the scalar deformation potential and μ by the random pseudomagnetic field (consistently with the conclusion drawn from the analysis of weak localization). Throughout our study, we compare the behavior of devices on hBN substrates to that of devices on SiO2 and SrTiO3, and find that all conclusions drawn for the case of hBN are compatible with the observations made on these other materials. These observations suggest that random strain fluctuations are the dominant source of disorder for high-quality graphene on many different substrates, and not only on hexagonal boron nitride.
Large single-crystal graphene is highly desired and important for the applications of graphene in electronics, as grain boundaries between graphene grains markedly degrade its quality and properties. Here we report the growth of millimetre-sized hexagonal single-crystal graphene and graphene films joined from such grains on Pt by ambient-pressure chemical vapour deposition. We report a bubbling method to transfer these single graphene grains and graphene films to arbitrary substrate, which is nondestructive not only to graphene, but also to the Pt substrates. The Pt substrates can be repeatedly used for graphene growth. The graphene shows high crystal quality with the reported lowest wrinkle height of 0.8 nm and a carrier mobility of greater than 7,100 cm(2) V(-1) s(-1) under ambient conditions. The repeatable growth of graphene with large single-crystal grains on Pt and its nondestructive transfer may enable various applications.
Graphene has been attracting great interest because of its distinctive band structure and physical properties. Today, graphene
is limited to small sizes because it is produced mostly by exfoliating graphite. We grew large-area graphene films of the
order of centimeters on copper substrates by chemical vapor deposition using methane. The films are predominantly single-layer
graphene, with a small percentage (less than 5%) of the area having few layers, and are continuous across copper surface steps
and grain boundaries. The low solubility of carbon in copper appears to help make this growth process self-limiting. We also
developed graphene film transfer processes to arbitrary substrates, and dual-gated field-effect transistors fabricated on
silicon/silicon dioxide substrates showed electron mobilities as high as 4050 square centimeters per volt per second at room
temperature.
We find that the electronic dispersion in graphite gives rise to double resonant Raman scattering for excitation energies up to 5 eV. As we show, the curious excitation-energy dependence of the graphite D mode is due to this double resonant process resolving a long-standing problem in the literature and invalidating recent attempts to explain this phenomenon. Our calculation for the D-mode frequency shift ( 60{\mathrm{cm}}^{\ensuremath{-}1}/\mathrm{eV}) agrees well with the experimental value.
There are few phenomena in condensed matter physics that are defined only by the fundamental constants and do not depend on
material parameters. Examples are the resistivity quantum, h/e2 (h is Planck's constant and e the electron charge), that appears
in a variety of transport experiments and the magnetic flux quantum, h/e, playing an important role in the physics of superconductivity.
By and large, sophisticated facilities and special measurement conditions are required to observe any of these phenomena.
We show that the opacity of suspended graphene is defined solely by the fine structure constant, a = e2/hc � 1/137 (where
c is the speed of light), the parameter that describes coupling between light and relativistic electrons and that is traditionally
associated with quantum electrodynamics rather than materials science. Despite being only one atom thick, graphene is found
to absorb a significant (pa = 2.3%) fraction of incident white light, a consequence of graphene's unique electronic structure.
The 1+1 layer folded graphene sheets that deviate from AB stacking are successfully fabricated and their electronic structures are investigated by Raman spectroscopy. Significant blue shift of the 2D band of folded graphene compared to that of single layer graphene (SLG) is observed. This is attributed to SLG-like electronic structure of folded graphene but with slowing down of Fermi velocity (as much as ~5.6%). Different amount of blue shift of 2D band is observed for different folded graphenes, which may correspond to the different twist angle and/or separation between the two layers, resulting in different Fermi velocity of folded graphenes. Electronic structure of 1+1 folded graphene samples with different stacking order (twist and separation between the two layers, and in-plane shift of the two layers) can be investigated by Raman spectroscopy. Comment: 18 pages, 4 figures, 2 tables, accepted by Phys. Rev. B
A simple, non-invasive method using Raman spectroscopy for the estimation of
the thickness of graphene layers grown epitaxially on silicon carbide (SiC) is
presented, enabling simultaneous determination of thickness, grain size and
disorder using the spectra. The attenuation of the substrate Raman signal due
to the graphene overlayer is found to be dependent on the graphene film
thickness deduced from X-ray photoelectron spectroscopy and transmission
electron microscopy of the surfaces. We explain this dependence using an
absorbing overlayer model. This method can be used for mapping graphene
thickness over a region and is capable of estimating thickness of multilayer
graphene films beyond that possible by XPS and Auger electron spectroscopy
(AES).
In this report, we demonstrate a method for the enhancement of Raman active modes of hydrogen-intercalated quasi-free-standing epitaxial chemical vapor deposition graphene and the underlying semi-insulating 6H–SiC(0001) substrate through constructive signal interference within atomic-layer-deposited amorphous Al2O3 passivation. We find that an optimum Al2O3 thickness of 85 nm for the graphene 2D mode and one of 82 nm for the SiC longitudinal optical A1 mode at 964 cm–1 enable a 60% increase in their spectra intensities. We demonstrate the method’s efficiency in Raman-based determination of the dielectric thickness and high-resolution topographic imaging of a graphene surface.
A metrological framework for statistical analysis of number of layers and stacking order in mass-produced graphene using Raman spectroscopy is presented. The method is based on two complementary protocols, denominated by 2D and G. The 2D–protocol is based on the parameterized principal component analysis of the two-phonon 2D band, and it measures interlayer coupling. A neural-network algorithm for spectral denoising was also developed to improve the outcome. The G–protocol explores the intensity of the bond-stretching G band, and provides information about the number of layers. The method is suitable for automated statistical analysis of heterogeneous graphene-based systems with relatively low computational cost, as shown here for graphene flakes prepared by the liquid-phase exfoliation of graphite.
The interest in graphene has surged over the past few years due to its exceptional mechanical, thermal, electrical, and optical properties, as well as its potential for flexible electronic applications. The traditional batch process for graphene synthesis and transfer need to be replaced by a high throughput, low-cost manufacturing process in order to enable mass production of graphene-based devices. Roll-to-roll (R2R) manufacturing and its related enabling technologies have been employed for both graphene growth and transfer, the two essential steps in graphene fabrication. It has been demonstrated that large-scale graphene production is feasible and has the potential to achieve economic success in the near future. This review provides an analysis of the state-of-the-art R2R manufacturing techniques for large-scale graphene fabrication. Different growth and transfer methods are compared. The benefit, limitation, and future outlook of R2R graphene fabrication are discussed. It is expected that through this review a benchmark is established for future development in R2R large-scale graphene production.
Two-dimensional (2D) materials are characterised by their strong intraplanar bonding but weak interplanar interaction. Interfaces between neighboring 2D layers or between 2D overlayers and substrate surfaces provide intriguing confined spaces for chemical processes, which have stimulated a new area of "chemistry under 2D cover". In particular, well-defined 2D material overlayers such as graphene, hexagonal boron nitride, and transition metal dichalcogenides have been deposited on solid surfaces, which can be used as model systems to understand the new chemistry. In the present review, we first show that many atoms and molecules can intercalate ultrathin 2D materials supported on solid surfaces and the space under the 2D overlayers has been regarded as a 2D nanocontainer. Moreover, chemical reactions such as catalytic reactions, surface adlayer growth, chemical vapor deposition, and electrochemical reactions occur in the 2D confined spaces, which further act as 2D nanoreactors. It has been demonstrated that surface chemistry and catalysis are strongly modulated by the 2D covers, resulting in weakened molecule adsorption and enhanced surface reactions. Finally, we conclude that the confinement effect of the 2D cover leads to new chemistry in a small space, such as "catalysis under cover" and "electrochemistry under cover". These new concepts enable us to design advanced nanocatalysts encapsulated with 2D material shells which may present improved performance in many important processes of heterogeneous catalysis, electrochemistry, and energy conversion.
The growth of graphene on single crystal Cu(111) has been achieved by thermal decomposition of ethylene in an ultrahigh vacuum chamber for the first time. The structural and electronic properties of graphene on Cu(111) have been investigated by scanning tunneling microscopy and spectroscopy. The nucleation of monolayer islands and two predominant domain orientations have been observed, which lead to the formation of numerous domain boundaries with increasing coverage. These results reveal that reducing the density of domain boundaries is one challenge of growing high-quality graphene on copper.
C. S. Han and co-workers report on page 6394 the clean, fast, and inexpensive direct transfer of large-area monolayer CVD graphene from Cu foil to various substrates such as PET, PDMS, and glass using a mechano-electro-thermal (MET) process. This process leads to ultraconformal contact between the graphene and the target substrate, which increases the higher adhesion energy of the graphene to the substrate over Cu foil. This transferred graphene presents both excellent quality (with no residues or defects) and remarkable mechanical and electrical stability.
We report on the electrical characterization and Raman spectroscopy of chemical vapor deposition copper-grown graphene transferred onto a Si/SiO2 substrate by high-speed (1 mm/s) electrochemical delamination. We determine graphene’s sheet resistance, carrier mobility, and concentration as well as its physical quality as a function of the electolyte concentration. Graphene’s electrical properties are investigated with standard Hall measurements in van der Pauw geometry and a contactless method that employs a single-post dielectric resonator operating at microwave frequencies. These properties are related to the widely used copper etching technique. The results prove that the high-speed electrochemical delamination provides good-quality graphene within a short time scale.
X-ray diffraction, Raman spectroscopy and Optical absorption estimates of the thickness of graphene multi layer stacks (number of graphene layers) are presented for three different growth techniques. The objective of this work was focused on comparison and reconciliation of the two already widely used methods for thickness estimates (Raman and Absorption) with the calibration of the X-ray method as far as Scherer constant K is concerned and X-ray based Wagner-Aqua extrapolation method.
The growth of graphene on Cu substrates by plasma enhanced chemical vapor deposition (PE-CVD) was investigated and its growth mechanism was discussed. At a substrate temperature of 500°C, formation of graphene was found to precede the growth of carbon nanowalls (CNWs), which are often fabricated by PE-CVD. The growth of graphene was investigated in various conditions, changing the plasma power, gas pressures, and the substrate temperature. The catalytic nature of Cu also affects the growth of monolayer graphene at high substrate temperatures, while the growth at low temperatures and growth of multilayer graphene are dominated mostly by radicals generated in the plasma.
A Raman spectrum of a solid contains information about its vibrational and electronic properties. Collecting spectral data with spatial resolution and encoding it in a 2D plot generates images with information complementary to optical and scanning force imaging. In the case of few-layer graphene the frequency of the G line and especially the width of the D′ line turn out to be sensitive to single layers. The thickness of the few-layer graphene flake is reflected in the intensity of the G line and in the reduced intensity of the dominant peak of the underlying silicon oxide.
Raman spectroscopy is an integral part of graphene research. It is used to determine the number and orientation of layers, the quality and types of edge, and the effects of perturbations, such as electric and magnetic fields, strain, doping, disorder and functional groups. This, in turn, provides insight into all sp(2)-bonded carbon allotropes, because graphene is their fundamental building block. Here we review the state of the art, future directions and open questions in Raman spectroscopy of graphene. We describe essential physical processes whose importance has only recently been recognized, such as the various types of resonance at play, and the role of quantum interference. We update all basic concepts and notations, and propose a terminology that is able to describe any result in literature. We finally highlight the potential of Raman spectroscopy for layered materials other than graphene.
The growth of bilayer and multilayer graphene on copper foils was studied by isotopic labeling of the methane precursor. Isotope-labeled graphene films were characterized with micro-Raman mapping and Time-of-Flight Secondary Ion Mass Spectrometry (TOF SIMS). During growth at high temperature the adlayers formed simultaneously and beneath the top, continuous layer of graphene and the Cu substrate; the adlayers share the same nucleation center and all adlayers nucleating in one place have the same edge termination. These results suggest that adlayer growth proceeds by catalytic decomposition of methane (or CHx, x<4) trapped in a "nano-CVD" chamber between the first layer and the substrate. Based on these results, submillimeter bilayer graphene was synthesized by applying a much lower growth rate.
Graphene has attracted a lot of interest for fundamental studies as well as for potential applications. Till now, micromechanical cleavage (MC) of graphite has been used to produce high-quality graphene sheets on different substrates. Clear understanding of the substrate effect is important for the potential device fabrication of graphene. Here we report the results of the Raman studies of micromechanically cleaved monolayer graphene on standard SiO2 (300 nm)/Si, single crystal quartz, Si, glass, polydimethylsiloxane (PDMS), and NiFe. Our data suggests that the Raman features of monolayer graphene are independent of the substrate used; in other words, the effect of substrate on the atomic/electronic structures of graphene is negligible for graphene made by MC. On the other hand, epitaxial monolayer graphene (EMG) on SiC substrate is also investigated. Significant blueshift of Raman bands is observed, which is attributed to the interaction of the graphene sheet with the substrate, resulting in the change of lattice constant and also the electronic structure.
Because of its superior stretchability, graphene exhibits rich structural deformation behaviours and its strain engineering has proven useful in modifying its electronic and magnetic properties. Despite the strain-sensitivity of the Raman G and 2D modes, the optical characterization of the native strain in graphene on silica substrates has been hampered by excess charges interfering with both modes. Here we show that the effects of strain and charges can be optically separated from each other by correlation analysis of the two modes, enabling simple quantification of both. Graphene with in-plane strain randomly occurring between -0.2% and 0.4% undergoes modest compression (-0.3%) and significant hole doping on thermal treatments. This study suggests that substrate-mediated mechanical strain is a ubiquitous phenomenon in two-dimensional materials. The proposed analysis will be of great use in characterizing graphene-based materials and devices.
We have measured the Raman spectra of one to three layer graphene as a function of laser excitation energy. The observed G′ band Raman peak (∼2650 cm−1) intensity decreases with increasing numbers of graphene layers. The electronic energy band structure calculated by the extended tight binding model shows that there are four and nine possible optical processes in double resonance theory for the Raman G′ band of double and triple layers graphene, respectively. Raman intensity calculations show that each peak position depends on its wavevector, and then the G′ band of double and triple layer graphene has three and five components, respectively.
Layer number and stacking order of few-layer graphene (FLG) are of particular interest since they directly determine the performance of graphenebased electronic devices. By analyzing Raman spectra and Raman images, quantitative indices are extracted to discriminate the thickness of ABstacked FLG from single- to five-layer graphene; a few key spectral characteristics are also identified for FLG with misoriented stacking electronic structure graphene, Raman spectroscopy, stacking.
Results of room-temperature Raman scattering studies of ultrathin graphitic films supported on Si (100)/SiO2 substrates are reported. The results are significantly different from those known for graphite. Spectra were collected using 514.5 nm radiation on films containing from n = 1 to 20 graphene layers, as determined by atomic force microscopy. Both the first- and second-order Raman spectra show unique signatures of the number of layers in the film. The nGL film analogue of the Raman G-band in graphite exhibits a Lorentzian line shape whose center frequency shifts linearly relative to graphite as approximately 1/n (for n = 1 omegaG approximately 1587 cm-1). Three weak bands, identified with disorder-induced first-order scattering, are observed at approximately 1350, 1450, and 1500 cm-1. The approximately 1500 cm-1 band is weak but relatively sharp and exhibits an interesting n-dependence. In general, the intensity of these D-bands decreases dramatically with increasing n. Three second-order bands are also observed (approximately 2450, approximately 2700, and 3248 cm-1). They are analogues to those observed in graphite. However, the approximately 2700 cm-1 band exhibits an interesting and dramatic change of shape with n. Interestingly, for n < 5 this second-order band is more intense than the G-band.
We present Raman spectroscopy measurements on single- and few-layer graphene
flakes. Using a scanning confocal approach we collect spectral data with
spatial resolution, which allows us to directly compare Raman images with
scanning force micrographs. Single-layer graphene can be distinguished from
double- and few-layer by the width of the D' line: the single peak for
single-layer graphene splits into different peaks for the double-layer. These
findings are explained using the double-resonant Raman model based on ab-initio
calculations of the electronic structure and of the phonon dispersion. We
investigate the D line intensity and find no defects within the flake. A finite
D line response originating from the edges can be attributed either to defects
or to the breakdown of translational symmetry.
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