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Probing the mechanical properties of vertically-stacked ultrathin graphene/Al2O3 heterostructures
Abstract
The superior intrinsic mechanical properties of graphene have been widely studied and utilized to enhance the mechanical properties of various composite materials. However, it is still unclear how heterostructures incorporating graphene behave, and to what extent graphene influences their mechanical response. In this work, a series of graphene/Al2O3 composite films were fabricated via atomic layer deposition of Al2O3 on graphene, and their mechanical behavior was studied using an experimental-computational approach. The inclusion of monolayer chemical vapor deposited graphene between ultrathin Al2O3 films (1.5-4.5 nm thickness) was found to enhance the overall stiffness by as much as 70% compared to a pure Al2O3 film of similar thickness (∼150 GPa to ∼250 GPa). Here, for the first time, the combination of graphene and Al2O3 in vertically-stacked heterostructures results in advanced hybrid films of unprecedented mechanical stiffness that also possess qualities desirable for graphene-based transistors and flexible electronics.
... Only the hexagonal diffraction pattern can be seen and it must be from graphene since at this low-temperature (40−120°C) PE-ALD conditions, the metal-oxide is expected to be amorphous and does not produce diffracted beams. [23][24][25][26]29 Similar results were obtained for Al 2 O 3 /graphene membranes ( Figure S3). ...
... Only a small difference is visible between the loading and unloading curves in both cases, suggesting an elastic deformation process and no sliding between the graphene and TiO 2 layers during indentation, indicative of strong adhesion between graphene and TiO 2 . The effective Young's modulus (E) of the suspended membrane was calculated using eq 1, 15,24,25 which describes the force versus indentation depth of a suspended clamped circular sheet of an isotropic elastic material under a centrally applied load ...
... These values are close to the values reported in the literature. 24,25 The E graphene value, however, is lower than that reported for graphene prepared by mechanical exfoliation of graphite (∼1 TPa), 15 which could be due to defects in the graphene used in this study stemming from the chemical-vapor-deposition process and/or from the transfer processes. Assuming a tip radius between 10 and 50 nm, we estimate that our free-standing metal-oxide membrane can withstand pressures higher than 10 6 Pa, that is, more than 10 bar, which explains its good performance as an environmental-cell window to contain high-pressure gas or liquids. ...
Free-standing ultra-thin (~2 nm) films of several oxides (Al2O3, TiO2, and others) have been developed, which are mechanically robust and transparent to electrons with Ekin ≥ 200 eV, and to photons. We demonstrate their applicability in environmental X-ray photoelectron and infrared spectroscopy for molecular level studies of solid-gas (≥1 bar) and solid-liquid interfaces. These films act both as membranes closing a reaction cell, and as substrates and electrodes for electrochemical reactions. The remarkable properties of such ultra-thin oxides membranes enable atomic/molecular level studies of interfacial phenomena, such as corrosion, catalysis, electrochemical reactions, energy storage, geochemistry, and biology, in a broad range of environmental conditions.
... For the GO nanoparticles, two bands were noticed where the first band (D) was observed at 1341 cm − 1 , while the second band (G) was seen at 1584 cm − 1 (Fig. 2A). As observed in the present report, it is a usual observation to notice two peaks on Raman spectroscopy for graphite-based materials because of the energy shift caused by laser excitation [33]. ...
This study aimed to synthesize and characterize graphene oxide (GO) nanoparticles and study the spectral, microbiological, and mechanical analysis of GO in two types of resin adhesives bonded to two-piece zirconia abutments. GO was synthesized and the nanoparticles were characterized using a scanning electron microscope (SEM). Four experimental groups were formed based on weight percentage (0.25 wt% and 0.5 wt%) of GO added in conventional chemically activated resin cement and the other two groups with dual-cure cement. Two non-modified adhesive groups served as the control groups. Sixty sets of zirconia abutments cemented to screwed titanium bases of implants analogs were divided into 6 groups. Spectral analysis and degree of conversion (DC) were performed using micro-Raman and Fourier-transformed infrared spectroscopy (FTIR). Bacterial viability was studied using the 3-(4,5-dimethylthiazole-2-yl)-2,5-diphenyltetrazoli-umbromide (MTT) assay over the implant. On SEM analysis, GO (thickness of 1-2 nm thick flakes) nanoparticles were uniformly dispersed throughout the adhesive matrix. Raman and FTIR analysis confirmed the presence of D and G bands. Implant specimens treated with 0.5 wt% GO-modified dual-cure adhesive demonstrated the lowest mean score of DC (38.75 ± 6.37). After 1 day and 30 days of incubation with dental adhesives, 0.5 wt% GO-modified chemically activated adhesive specimens showed high antibacterial activity; only 23% and 30% of Streptococcus mutans were able to survive after treating with 0.5 wt% GO-modified chemically activated adhesive, respectively. In conclusion, incorporation of 0.5 wt% of GO in both chemically activated and dual-cure adhesive produced superior antimicrobial and physical characteristics for bonding two-piece zirconia abutments to screwed titanium bases.
... The Micro-Raman representative spectra of rGO-5% adhesive demonstrated two vibrational bands; the D band was seen at 1341 cm −1 whereas the G band was perceived at 1584 cm −1 , and this particular finding was similar to a previous study [25]. For the materials having graphite, the observation of two bands is common as they are created due to the shift in energy caused by the laser excitation [34]. Concerning resin tags, rGO-5% demonstrated comparable resin tags ( Figure 4A-C) to EA-rGO-0%. ...
The present study aimed to synthesize and equate the mechanical properties and dentin interaction of two adhesives; experimental adhesive (EA) and 5 wt.% reduced graphene oxide rGO) containing adhesive. Scanning electron microscopy (SEM)-Energy-dispersive X-ray spectroscopy (EDX), Micro-Raman spectroscopy, push-out bond strength test, and Fourier Transform Infrared (FTIR) spectroscopy were employed to study nano-bond strength, degree of conversion (DC), and adhesive-dentin interaction. The EA was prepared, and rGO particles were added to produce two adhesive groups, EA-rGO-0% (control) and rGO-5%. The canals of sixty roots were shaped and prepared, and fiber posts were cemented. The specimens were further alienated into groups based on the root canal disinfection technique, including 2.5% sodium hypochlorite (NaOCl), Photodynamic therapy (PDT), and ER-CR-YSGG laser (ECYL). The rGO nanoparticles were flake-shaped, and EDX confirmed the presence of carbon (C). Micro-Raman spectroscopy revealed distinct peaks for graphene. Push-out bond strength test demonstrated highest values for the EA-rGO-0% group after NaOCl and PDT conditioning whereas, rGO-5% showed higher values after ECYL conditioning. EA-rGO-0% presented greater DC than rGO-5% adhesive. The rGO-5% adhesive demonstrated comparable push-out bond strength and rheological properties to the controls. The rGO-5% demonstrated acceptable DC (although lower than control group), appropriate dentin interaction, and resin tag establishment.
... For the nano-GO particles, two bands were observed where the first band (D) was seen at 1341 cm −1 and the second band (G) was observed at 1584 cm −1 ( Figure 4A). Due to the energy shift caused by laser excitation, observing two peaks on Raman spectroscopy is a common observation for graphite based materials [45], as seen in our study. It is a common finding to observe the characteristic P-O band at 960 cm −1 for HA samples in Raman spectroscopy [46]. ...
The aim was to synthesize and characterize an adhesive incorporating HA and GO nanoparticles. Techniques including scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX), micro-tensile bond strength (µTBS), and micro-Raman spectroscopy were employed to investigate bond durability, presence of nanoparticles inside adhesive, and dentin interaction. Control experimental adhesive (CEA) was synthesized with 5 wt% HA. GO particles were fabricated and added to CEA at 0.5 wt% (HA-GO-0.5%) and 2 wt% GO (HA-GO-2%). Teeth were prepared to produce bonded specimens using the three adhesive bonding agents for assessment of µTBS, with and without thermocycling (TC). The adhesives were applied twice on the dentin with a micro-brush followed by air thinning and photo-polymerization. The HA and GO nanoparticles demonstrated uniform dispersion inside adhesive. Resin tags with varying depths were observed on SEM micrographs. The EDX mapping revealed the presence of carbon (C), calcium (Ca), and phosphorus (P) in the two GO adhesives. For both TC and NTC samples, HA-GO-2% had higher µTBS and durability, followed by HA-GO-0.5%. The representative micro-Raman spectra demonstrated D and G bands for nano-GO particles containing adhesives. HA-GO-2% group demonstrated uniform diffusion in adhesive, higher µTBS, adequate durability, and comparable resin tag development to controls.
Transition metal dichalcogenides (TMDCs) are potential materials for future optoelectronic devices. Grain boundaries (GBs) can significantly influence the optoelectronic properties of TMDC materials. Here, we have investigated the mechanical characteristics of tungsten diselenide (WSe2) monolayers and failure process with symmetric tilt GBs using ReaxFF molecular dynamics simulations. In particular, the effects of topological defects, loading rates, and temperatures are investigated. We considered nine different grain boundary structures of monolayer WSe2, of which six are armchair (AC) tilt structures, and the remaining three are zigzag (ZZ) tilt structures. Our results indicate that both tensile strength and fracture strain of WSe2 with symmetric tilt GBs decrease as the temperature increases. We revealed an interfacial phase transition for high-angle GBs reduces the elastic strain energy within the interface at finite temperatures. Furthermore, brittle cracking is the dominant failure mode in the WSe2 monolayer with tilted GBs. WSe2 GB structures showed more strain rate sensitivity at high temperatures than at low temperatures.
Due to its superior mechanical properties, graphene is widely used as reinforcement materials in nanocomposites. In this work, a series of indentation simulations was performed, using finite element method, to investigate the mechanical properties of graphene/TiO2 and graphene/SnO2 nanocomposite films. The force-displacement curves obtained from simulations were first compared to analytical results, which demonstrates that with increasing the thicknesses of metal oxide layers, the mechanical responses of nanocomposites exhibit a transition from non-linear behaviors to linear behaviors. Furthermore, consistent with literature works, increasing graphene volume fraction can enhance the Young's modulus of the corresponding heterostructure. Interestingly, this enhancement can be modulated by nuances in stacking orders, i.e., layer arrangements, of the nanocomposites. Through analyzing stress and strain distributions, the underlying mechanisms were proposed. Our results reported here provide comprehensive characterizations and understandings on the reinforcement effects of graphene on graphene/metal oxide nanocomposites.
Al2O3-graphene nanolayers are widely used within integrated micro/nanoelectronic systems; however, their lifetimes are largely limited by fracture both statically and dynamically. Here, we present a static and fatigue study of thin (1–11 nm) free-standing Al2O3-graphene nanolayers. A remarkable fatigue life of greater than one billion cycles was obtained for films <2.2 nm thick under large mean stress levels, which was up to 3 orders of magnitude longer than that of its thicker (11 nm) counterpart. A similar thickness dependency was also identified for the elastic and static fracture behavior, where the enhancement effect of graphene is prominent only within a thickness of ∼3.3 nm. Moreover, plastic deformation, manifested by viscous creep, was observed and appeared to be more substantial for thicker films. This study provides mechanistic insights on both the static and dynamic reliability of Al2O3-graphene nanolayers and can potentially guide the design of graphene-based devices.
Two-dimensional and quasi-two-dimensional materials are important nanostructures because of their exciting electronic, optical, thermal, chemical and mechanical properties. However, a single-layer nanomaterial may not possess a particular property adequately, or multiple desired properties simultaneously. Recently a new trend has emerged to develop nano-heterostructures by assembling multiple monolayers of different nanostructures to achieve various tunable desired properties simultaneously. For example, transition metal dichalcogenides such as MoS2 show promising electronic and piezoelectric properties, but their low mechanical strength is a constraint for practical applications. This barrier can be mitigated by considering graphene-MoS2 heterostructure, as graphene possesses strong mechanical properties. We have developed efficient closed-form expressions for the equivalent elastic properties of such multi-layer hexagonal nano-hetrostructures. Based on these physics-based analytical formulae, mechanical properties are investigated for different heterostructures such as graphene-MoS2, graphene-hBN, graphene-stanene and stanene-MoS2. The proposed formulae will enable efficient characterization of mechanical properties in developing a wide range of application-specific nano-heterostructures.
Atomically thin boron nitride (BN) nanosheets are important two-dimensional nanomaterials with many unique properties distinct from those of graphene, but investigation into their mechanical properties remains incomplete. Here we report that high-quality single-crystalline mono- and few-layer BN nanosheets are one of the strongest electrically insulating materials. More intriguingly, few-layer BN shows mechanical behaviours quite different from those of few-layer graphene under indentation. In striking contrast to graphene, whose strength decreases by more than 30% when the number of layers increases from 1 to 8, the mechanical strength of BN nanosheets is not sensitive to increasing thickness. We attribute this difference to the distinct interlayer interactions and hence sliding tendencies in these two materials under indentation. The significantly better interlayer integrity of BN nanosheets makes them a more attractive candidate than graphene for several applications, for example, as mechanical reinforcements.
We report on statistical analysis and consistency of electrical performances of devices based on a large scale passivated graphene platform. More than 500 graphene field effect transistors (GFETs) based on graphene grown by chemical vapor deposition and transferred on 4 in. SiO2/Si substrates were fabricated and tested. We characterized the potential of a two-step encapsulation process including an Al2O3 protection layer to avoid graphene contamination during the lithographic process followed by a final Al2O3 passivation layer subsequent to the GFET fabrication. Devices were investigated for occurrence and reproducibility of conductance minimum related to the Dirac point. While no conductance minimum was observed in unpassivated devices, 75% of the passivated transistors exhibited a clear conductance minimum and low hysteresis. The maximum of the device number distribution corresponds to a residual doping below 5 × 10¹¹ cm⁻² (0.023 V/nm). This yield shows that GFETs integrating low-doped graphene and exhibiting small hysteresis in the transfer characteristics can be envisaged for discrete components, with even further potential for low power driven electronics.
The ability to bias chemical reaction pathways is a fundamental goal for chemists and material scientists to produce innovative materials. Recently, two-dimensional materials have emerged as potential platforms for exploring novel mechanically activated chemical reactions. Here we report a mechanochemical phenomenon in graphene oxide membranes, covalent epoxide-to-ether functional group transformations that deviate from epoxide ring-opening reactions, discovered through nanomechanical experiments and density functional-based tight binding calculations. These mechanochemical transformations in a two-dimensional system are directionally dependent, and confer pronounced plasticity and damage tolerance to graphene oxide monolayers. Additional experiments on chemically modified graphene oxide membranes, with ring-opened epoxide groups, verify this unique deformation mechanism. These studies establish graphene oxide as a two-dimensional building block with highly tuneable mechanical properties for the design of high-performance nanocomposites, and stimulate the discovery of new bond-selective chemical transformations in two-dimensional materials.
The sensitivity of graphene based devices to surface adsorbates and charge traps at the
graphene/dielectric interface requires proper device passivation in order to operate them reproducibly under ambient conditions. Here we report on the use of atomic layer deposited aluminum oxide as passivation layer on graphene field effect devices (GFETs). We show that successful passivation produce hysteresis free DC characteristics, low doping level GFETs stable over weeks though operated and stored in ambient atmosphere. This is achieved by selecting proper seed layer prior to deposition of encapsulation layer. The passivated devices are also demonstrated to be robust towards the exposure to chemicals and heat treatments, typically used during device fabrication. Additionally, the passivation of high stability and reproducible characteristics is also shown for functional devices like integrated graphene based inverters.
A nano-scaled coating of titanium oxide (TiO 2) on graphene (G) has been achieved via a novel atomic layer deposition (ALD) method. As a potential supercapacitor material, the TiO 2 -G composites exhibited a capacity of 75 F/g and 84 F/g at a scan rate of 10 mV/s for composites grown using 50 and 100 ALD cycles, respectively. The nearly identical Nyquist plots of the TiO 2 -G composites compared with those of pure graphene demonstrated that the composites possess excellent conductivity for charge transfer and open structures for ion diffusion. In addition, even with 3-4 times additional mass loading (maximum 3.22 mg/cm 2), the composites exhibit no obvious degradation with respect to the electrochemical performance. This ALD approach presents a promising route to synthesize advanced graphene-based nanocomposites for supercapacitor applications.
A room temperature oxygen sensor based on TiO 2 /graphene device was developed with an enhanced sensing performance by UV light. A logarithmic relationship between the sensing response and O 2 concentration (from 134 ppm to 100%) was obtained. The sensor also shows an improved response and recovery with the durations of 193 s and 135 s at RT and atmosphere pressure, respectively. Herein, UV light was employed to excite electron–hole pairs in the TiO 2 film and the photogenerated electrons were scavenged by graphene and percolated to the collecting electrode, while the holes recombined with the electrons within TiO 2 film. This process will deplete the electron further thereby changing the conductivity of the device rapidly resulting in a much better sensing signal.
A method to determine the spring constant of a rectangular atomic force microscope cantilever is proposed that relies solely on the measurement of the resonant frequency and quality factor of the cantilever in fluid (typically air), and knowledge of its plan view dimensions. This method gives very good accuracy and improves upon the previous formulation by Sader &etal; [Rev. Sci. Instrum. 66, 3789 (1995)] which, unlike the present method, requires knowledge of both the cantilever density and thickness.
Graphene Staying Strong
Although exfoliated graphene can be extremely strong, it is produced on too small a scale for materials application. Graphene can be produced on a more practical scale by chemical vapor deposition, but the presence of grain boundaries between crystallites apparently weakens the material. Lee et al. (p. 1073 ) show that postprocessing steps during the removal of the graphene sheets can oxidize the grain boundaries and weaken them. If these steps are avoided, the material is comparable in strength to exfoliated graphene.
The authors reported on investigation of the thermal conductivity of graphene suspended across trenches in Si/SiO2 wafer. The measurements were performed using a noncontact technique based on micro-Raman spectroscopy. The amount of power dissipated in graphene and corresponding temperature rise were determined from the spectral position and integrated intensity of graphene’s G mode. The extremely high thermal conductivity in the range of ∼ 3080–5150 W/m K and phonon mean free path of ∼ 775 nm near room temperature were extracted for a set of graphene flakes. The obtained results suggest graphene’s applications as thermal management material in future nanoelectronic circuits.
The celebrated electronic properties of graphene have opened the way for materials just one atom thick to be used in the post-silicon electronic era. An important milestone was the creation of heterostructures based on graphene and other two-dimensional crystals, which can be assembled into three-dimensional stacks with atomic layer precision. Such layered structures have already demonstrated a range of fascinating physical phenomena, and have also been used in demonstrating a prototype field-effect tunnelling transistor, which is regarded to be a candidate for post-CMOS (complementary metal-oxide semiconductor) technology. The range of possible materials that could be incorporated into such stacks is very large. Indeed, there are many other materials with layers linked by weak van der Waals forces that can be exfoliated and combined together to create novel highly tailored heterostructures. Here, we describe a new generation of field-effect vertical tunnelling transistors where two-dimensional tungsten disulphide serves as an atomically thin barrier between two layers of either mechanically exfoliated or chemical vapour deposition-grown graphene. The combination of tunnelling (under the barrier) and thermionic (over the barrier) transport allows for unprecedented current modulation exceeding 1 × 10(6) at room temperature and very high ON current. These devices can also operate on transparent and flexible substrates.
Al2O3 atomic layer deposition (ALD) is a model ALD system and Al2O3 ALD films are excellent gas diffusion barrier on polymers. However, little is known about the response of Al2O3 ALD films to strain and the potential film cracking that would restrict the utility of gas diffusion barrier films. To understand the mechanical limitations of Al2O3 ALD films, the critical strains at which the Al2O3 ALD films will crack were determined for both tensile and compressive strains. The tensile strain measurements were obtained using a fluorescent tagging technique to image the cracks. The results showed that the critical tensile strain is higher for thinner thicknesses of the Al2O3 ALD film on heat-stabilized polyethylene naphthalate (HSPEN) substrates. A low critical tensile strain of 0.52% was measured for a film thickness of 80 nm. The critical tensile strain increased to 2.4% at a film thickness of 5 nm. In accordance with fracture mechanics modeling, the critical tensile strains and the saturation crack densities scaled as (1/h)1/2 where h is the Al2O3 ALD film thickness. The fracture toughness for cracking, KIC, of the Al2O3 ALD film was also determined to be KIC = 2.30 MPa m1/2. Thinner Al2O3 ALD film thicknesses also had higher critical strains for cracking from compressive strains. Field-emission scanning electron microscopy (FE-SEM) images revealed that Al2O3 ALD films with thicknesses of 30–50 nm on Teflon fluorinated ethylene propylene (FEP) substrates cracked at a critical compressive strain of ∼1.0%. The critical compressive strain increased to ∼2.0% at a film thickness of ∼20 nm. A comparison of the critical tensile strains on HSPEN substrates and critical compressive strains on Teflon FEP substrates revealed some similarities. The critical strain was ∼1.0% for film thicknesses of 30–50 nm for both tensile and compressive strains. The critical compressive strain then increased more rapidly than the critical tensile strain for thinner films with thicknesses < 30 nm. The high critical tensile and compressive strains for thin Al2O3 ALD films should be very useful for flexible gas diffusion barriers on polymers.
The elastic deformation of few layers ( 5 to 25) of thick, freely suspended MoS2 nanosheets by means of a nanoscopic version of a bending test experiment, carried out with the tip of an atomic force microscope is reported. Young's modulus of these nanosheets is extremely high ( E = 0.33 TPa), comparable to that of graphene oxide, and the deformations are reversible up to tens of nanometers.
We measured the elastic properties and intrinsic breaking strength of free-standing monolayer graphene membranes by nanoindentation in an atomic force microscope. The force-displacement behavior is interpreted within a framework of nonlinear elastic stress-strain response, and yields second- and third-order elastic stiffnesses of 340 newtons per meter (N m(-1)) and -690 Nm(-1), respectively. The breaking strength is 42 N m(-1) and represents the intrinsic strength of a defect-free sheet. These quantities correspond to a Young's modulus of E = 1.0 terapascals, third-order elastic stiffness of D = -2.0 terapascals, and intrinsic strength of sigma(int) = 130 gigapascals for bulk graphite. These experiments establish graphene as the strongest material ever measured, and show that atomically perfect nanoscale materials can be mechanically tested to deformations well beyond the linear regime.
Graphene devices on standard SiO(2) substrates are highly disordered, exhibiting characteristics that are far inferior to the expected intrinsic properties of graphene. Although suspending the graphene above the substrate leads to a substantial improvement in device quality, this geometry imposes severe limitations on device architecture and functionality. There is a growing need, therefore, to identify dielectrics that allow a substrate-supported geometry while retaining the quality achieved with a suspended sample. Hexagonal boron nitride (h-BN) is an appealing substrate, because it has an atomically smooth surface that is relatively free of dangling bonds and charge traps. It also has a lattice constant similar to that of graphite, and has large optical phonon modes and a large electrical bandgap. Here we report the fabrication and characterization of high-quality exfoliated mono- and bilayer graphene devices on single-crystal h-BN substrates, by using a mechanical transfer process. Graphene devices on h-BN substrates have mobilities and carrier inhomogeneities that are almost an order of magnitude better than devices on SiO(2). These devices also show reduced roughness, intrinsic doping and chemical reactivity. The ability to assemble crystalline layered materials in a controlled way permits the fabrication of graphene devices on other promising dielectrics and allows for the realization of more complex graphene heterostructures.
Deposition of high-kappa dielectrics onto graphene is of significant challenge due to the difficulties of nucleating high quality oxide on pristine graphene without introducing defects into the monolayer of carbon lattice. Previous efforts to deposit high-kappa dielectrics on graphene often resulted in significant degradation in carrier mobility. Here we report an entirely new strategy to integrate high quality high-kappa dielectrics with graphene by first synthesizing freestanding high-kappa oxide nanoribbons at high temperature and then transferring them onto graphene at room temperature. We show that single crystalline Al(2)O(3) nanoribbons can be synthesized with excellent dielectric properties. Using such nanoribbons as the gate dielectrics, we have demonstrated top-gated graphene transistors with the highest carrier mobility (up to 23,600 cm(2)/V x s) reported to date, and a more than 10-fold increase in transconductance compared to the back-gated devices. This method opens a new avenue to integrate high-kappa dielectrics on graphene with the preservation of the pristine nature of graphene and high carrier mobility, representing an important step forward to high-performance graphene electronics.
Graphene is the two-dimensional building block for carbon allotropes of every other dimensionality. We show that its electronic structure is captured in its Raman spectrum that clearly evolves with the number of layers. The D peak second order changes in shape, width, and position for an increasing number of layers, reflecting the change in the electron bands via a double resonant Raman process. The G peak slightly down-shifts. This allows unambiguous, high-throughput, nondestructive identification of graphene layers, which is critically lacking in this emerging research area.
We report the measurement of the thermal conductivity of a suspended single-layer graphene. The room temperature values of the thermal conductivity in the range approximately (4.84+/-0.44)x10(3) to (5.30+/-0.48)x10(3) W/mK were extracted for a single-layer graphene from the dependence of the Raman G peak frequency on the excitation laser power and independently measured G peak temperature coefficient. The extremely high value of the thermal conductivity suggests that graphene can outperform carbon nanotubes in heat conduction. The superb thermal conduction property of graphene is beneficial for the proposed electronic applications and establishes graphene as an excellent material for thermal management.
Graphene has been integrated in many heterogeneous structures in order to take advantage of its superior mechanical properties. However, the complex mechanical response of heterogeneous films incorporating graphene is not well understood. Here, we studied the mechanical behavior of atomic layer deposition (ALD) synthesized TiO2/graphene, as a representative composite material, to understand the mechanical behavior of heterostructures using an experiment-computational approach. The inclusion of graphene was found to significantly enhance the Young’s modulus of TiO2/graphene hetero-films for films below a critical thickness of 3 nm, beyond which the Young’s modulus approaches that of pure TiO2 film. A rule-of-mixtures was found to reasonably estimate the modulus of the TiO2/graphene hetero-film. Experimentally, these hetero-films were observed to fail via brittle fracture. Complimentary finite element modeling demonstrates that graphene serves as a structural support, providing the hetero-film an ability to sustain significantly high stresses at the point of failure initiation. The results and methodology described herein can contribute to the rational design of strong and reliable ultrathin hetero-films for their versatile applications.
2016 Author(s).We report on statistical analysis and consistency of electrical performances of devices based on a large scale passivated graphene platform. More than 500 graphene field effect transistors (GFETs) based on graphene grown by chemical vapor deposition and transferred on 4 in. SiO2/Si substrates were fabricated and tested. We characterized the potential of a two-step encapsulation process including an Al2O3 protection layer to avoid graphene contamination during the lithographic process followed by a final Al2O3 passivation layer subsequent to the GFET fabrication. Devices were investigated for occurrence and reproducibility of conductance minimum related to the Dirac point. While no conductance minimum was observed in unpassivated devices, 75% of the passivated transistors exhibited a clear conductance minimum and low hysteresis. The maximum of the device number distribution corresponds to a residual doping below 5 × 1011 cm−2 (0.023 V/nm). This yield shows that GFETs integrating low-doped graphene and exhibiting small hysteresis in the transfer characteristics can be envisaged for discrete components, with even further potential for low power driven electronics.
In this paper, the mechanical properties of in-plane heterostructure with alternating stripes of graphene grain boundary (GB) and hexagonal boron nitride (h-BN) are investigated using classical Molecular Dynamics method. The graphene GB contains an array of pentagon-heptagon (5–7) defects, and has good interfacial continuity with the lateral BN domain. By studying the dynamic failure process of heterostructure with varying hybridization intervals and GB tilt angles, two different local failure types are noticed. Coupled effects of h-BN hybridization and GB tilt angle on the tensile strength of heterostructure are revealed. For heterostructure with graphene GBs of evenly spaced 5–7 defects, the tensile strength is insensitive to the h-BN hybridization and increases anomalously with the defect density. For heterostructure with graphene GB of unevenly spaced 5–7 defects, lateral h-BN hybridization enhanced the tensile strength of the heterostructure. Such strength enhancement effect is contributed to the bond length mismatch between graphene and h-BN, and deteriorates with the increase of hybridization interval. Our results give helpful insight into the strength characteristics of hybrid two-dimensional nanomaterials based electronic and optical devices.
Crystalline 2D hexagonal boron nitride (2D-hBN) nanosheets are explored as a potential electrocatalyst toward the electroanalytical sensing of dopamine (DA). The 2D-hBN nanosheets are electrically wired via a drop-casting modification process onto a range of commercially available carbon supporting electrodes, including glassy carbon (GC), boron-doped diamond (BDD), and screen-printed graphitic electrodes (SPEs). 2D-hBN has not previously been explored toward the electrochemical detection/electrochemical sensing of DA. We critically evaluate the potential electrocatalytic performance of 2D-hBN modified electrodes, the effect of supporting carbon electrode platforms, and the effect of "mass coverage" (which is commonly neglected in the 2D material literature) toward the detection of DA. The response of 2D-hBN modified electrodes is found to be largely dependent upon the interaction between 2D-hBN and the underlying supporting electrode material. For example, in the case of SPEs, modification with 2D-hBN (324 ng) improves the electrochemical response, decreasing the electrochemical oxidation potential of DA by ?90 mV compared to an unmodified SPE. Conversely, modification of a GC electrode with 2D-hBN (324 ng) resulted in an increased oxidation potential of DA by ?80 mV when compared to the unmodified electrode. We explore the underlying mechanisms of the aforementioned examples and infer that electrode surface interactions and roughness factors are critical considerations. 2D-hBN is utilized toward the sensing of DA in the presence of the common interferents ascorbic acid (AA) and uric acid (UA). 2D-hBN is found to be an effective electrocatalyst in the simultaneous detection of DA and UA at both pH 5.0 and 7.4. The peak separations/resolution between DA and UA increases by ?70 and 50 mV (at pH 5.0 and 7.4, respectively, when utilizing 108 ng of 2D-hBN) compared to unmodified SPEs, with a particularly favorable response evident in pH 5.0, giving rise to a significant increase in the peak current of DA. The limit of detection (3?) is found to correspond to 0.65 ?M for DA in the presence of UA. However, it is not possible to deconvolute the simultaneous detection of DA and AA. The observed electrocatalytic effect at 2D-hBN has not previously been reported in the literature when supported upon carbon or any other electrode. We provide valuable insights into the modifier-substrate interactions of this material, essential for those designing, fabricating, and consequently performing electrochemical experiments utilizing 2D-hBN and related 2D materials.
We report an optoelectronic mixer based on chemical vapour-deposited
graphene. Our device consists in a coplanar waveguide that integrates a
graphene channel, passivated with an atomic layer-deposited Al2O3 film. With
this new structure, 30 GHz optoelectronic mixing in commercially-available
graphene is demonstrated for the first time. In particular, using a 30 GHz
intensity-modulated optical signal and a 29.9 GHz electrical signal, we show
frequency downconversion to 100 MHz. These results open promising perspectives
in the domain of optoelectronics for radar and radio-communication systems.
In this work, atomic layer deposition is applied to coat graphene nanosheets with TiO2. The produced TiO2-Graphene (TiO2-Gr) composites are characterized by X-ray photoelectron spectroscopy, fourier transform infrared spectroscopy, and transmission electron microscopy. It is revealed that TiO2 are effectively deposited on the surface of graphene. The coatings have a highly controlled thickness. The electrochemical properties of the obtained TiO2-Gr composites are investigated for detection of lead (Pb2+) and cadmium (Cd2+) ions by differential pulse anodic stripping voltammetry (DPASV). It is found that the TiO2-Gr composite exhibits improved sensitivity for detection of these metal ions. The linear dynamic ranges are from 1.0×10-8 M to 3.2×10-5 M for Pb2+ and 6.0×10-7 M to 3.2×10-5 M for Cd2+, respectively. The detection limits (S/N = 3) are estimated to be 1.0×10-10 M for Pb2+ and 2.0×10-9 M for Cd2+, respectively.
The production of high-quality graphene-oxide interfaces is normally achieved by graphene growth via chemical vapour deposition on a metallic surface, followed by transfer of the C layer onto the oxide, by atomic layer and physical vapour deposition of the oxide on graphene or by carbon deposition on top of oxide surfaces. These methods, however, come with a series of issues: they are complex, costly and can easily result in damage to the carbon network, with detrimental effects on the carrier mobility. Here we show that the growth of a graphene layer on a bimetallic Ni3Al alloy and its subsequent exposure to oxygen at 520 K result in the formation of a 1.5 nm thick alumina nanosheet underneath graphene. This new, simple and low-cost strategy based on the use of alloys opens a promising route to the direct synthesis of a wide range of interfaces formed by graphene and high-κ dielectrics.
Layered transition-metal dichalcogenides comprise a category of two-dimensional materials that offer exciting properties, including metallic and semi-conducting electrical capabilities, fluorescence and fast heterogeneous electron transfer. To date, these materials have mostly been employed in energy-storage and generation devices. However, in very recent times, there was a significant emerging trend in their utilization in analytical chemistry. Hence, this review aims to provide an introduction to this new trend for the analytical community.
Elastic properties of materials are an important factor in their integration
in applications. Chemical vapor deposited (CVD) monolayer semiconductors are
proposed as key components in industrial-scale flexible devices and building
blocks of 2D van der Waals heterostructures. However, their mechanical and
elastic properties have not been fully characterized. Here we report high 2D
elastic moduli of CVD monolayer MoS2 and WS2 (~ 170 N/m), which is very close
to the value of exfoliated MoS2 monolayers and almost half the value of the
strongest material, graphene. The 2D moduli of their bilayer heterostructures
are lower than the sum of 2D modulus of each layer, but comparable to the
corresponding bilayer homostructure, implying similar interactions between the
hetero monolayers as between homo monolayers. These results not only provide
deep insight to understanding interlayer interactions in 2D van der Waals
structures, but also potentially allow engineering of their elastic properties
as desired.
In the development of lithium ion batteries for electric vehicle applications, much effort has been made in fabrication and optimization of active materials. Mechanical properties of the electrode laminate, which are also very critical determining the durability of the cell, have long been ignored. In this study, graphite laminates of the same chemical composition are prepared by using different mixing technologies. With similar breaking strength, Young’s modulus of the laminate is found to be very different according to the mixing order. Electrochemical performances of the graphite laminates are investigated against Li anode and Li[Ni1/3Co1/3Mn1/3]O2 cathode, respectively. Long-term cycling performance is closely related with the Young’s modulus of the graphite laminate. Whereas the laminates of low modulus cycles well during the investigated 600 deep charge–discharge cycles, the electrodes of high modulus fail at around 350 cycles. Electrochemical degradation mechanism for the laminate of high modulus is associated with the significant impedance rise aroused from mechanical failure. After the long-term cycling test, lots of cracks are observed on the high modulus electrode along the particle boundaries while the mechanical integrity of the low modulus laminate is well maintained.
As one of the most promising negative electrode materials in lithium-ion batteries (LIBs), SnO2 experiences intense investigation due to its high specific capacity and energy density, relative to conventional graphite anodes. In this study, for the first time, atomic layer deposition (ALD) is used to deposit SnO2, containing both amorphous and crystalline phases, onto graphene nanosheets (GNS) as anodes for LIBs. The resultant SnO2-graphene nanocomposites exhibit a sandwich structure, and, when cycled against a lithium counter electrode, demonstrate a promising electrochemical performance. It is demonstrated that the introduction of GNS into the nanocomposites is beneficial for the anodes by increasing their electrical conductivity and releasing strain energy: thus, the nanocomposite electrode materials maintain a high electrical conductivity and flexibility. It is found that the amorphous SnO2-GNS is more effective than the crystalline SnO2-GNS in overcoming electrochemical and mechanical degradation; this observation is consistent with the intrinsically isotropic nature of the amorphous SnO2, which can mitigate the large volume changes associated with charge/discharge processes. It is observed that after 150 charge/discharge cycles, 793 mA h g-1 is achieved. Moreover, a higher coulombic efficiency is obtained for the amorphous SnO2-GNS composite anode. This study provides an approach to fabricate novel anode materials and clarifies the influence of SnO2 phases on the electrochemical performance of LIBs.
Atomically-thin forms of layered materials, such as conducting graphene, insulating hexagonal boron nitride (hBN), and semiconducting molybdenum disulfide (MoS2), have generated great interests recently due to the possibility of combining diverse atomic layers by mechanical 'stacking' to create novel materials and devices. In this work, we demonstrate field-effect transistors (FETs) with MoS2 channels, hBN dielectric, and graphene gate electrodes. These devices show field effect mobilities of up to 45 cm2/Vs and operating gate voltage below 10V, with greatly reduced hysteresis. Taking advantage of the mechanical strength and flexibility of these materials, we demonstrate integration onto a polymer substrate to create flexible and transparent FETs that show unchanged performance up to strain of 1.5 %. These heterostructure devices consisting of ultrathin two-dimensional (2D) materials open up a new route toward high performance flexible and transparent electronics.
In this paper, a method is presented to create and characterize mechanically robust, free-standing, ultrathin, oxide films with controlled, nanometer-scale thickness using atomic layer deposition (ALD) on graphene. Aluminum oxide films were deposited onto suspended graphene membranes using ALD. Subsequent etching of the graphene left pure aluminum oxide films only a few atoms in thickness. A pressurized blister test was used to determine that these ultrathin films have a Young's modulus of 154 ± 13 GPa. This Young's modulus is comparable to much thicker alumina ALD films. This behavior indicates that these ultrathin two-dimensional films have excellent mechanical integrity. The films are also impermeable to standard gases suggesting they are pinhole-free. These continuous ultrathin films are expected to enable new applications in fields such as thin film coatings, membranes, and flexible electronics.
We report the development of a multilayered graphene-Al(2)O(3) nanopore platform for the sensitive detection of DNA and DNA-protein complexes. Graphene-Al(2)O(3) nanolaminate membranes are formed by sequentially depositing layers of graphene and Al(2)O(3), with nanopores being formed in these membranes using an electron-beam sculpting process. The resulting nanopores are highly robust, exhibit low electrical noise (significantly lower than nanopores in pure graphene), are highly sensitive to electrolyte pH at low KCl concentrations (attributed to the high buffer capacity of Al(2)O(3)), and permit the electrical biasing of the embedded graphene electrode, thereby allowing for three terminal nanopore measurements. In proof-of-principle biomolecule sensing experiments, the folded and unfolded transport of single DNA molecules and RecA-coated DNA complexes could be discerned with high temporal resolution. The process described here also enables nanopore integration with new graphene-based structures, including nanoribbons and nanogaps, for single-molecule DNA sequencing and medical diagnostic applications.
Research in electronic nanomaterials, historically dominated by studies of nanocrystals/fullerenes and nanowires/nanotubes, now incorporates a growing focus on sheets with nanoscale thicknesses, referred to as nanomembranes. Such materials have practical appeal because their two-dimensional geometries facilitate integration into devices, with realistic pathways to manufacturing. Recent advances in synthesis provide access to nanomembranes with extraordinary properties in a variety of configurations, some of which exploit quantum and other size-dependent effects. This progress, together with emerging methods for deterministic assembly, leads to compelling opportunities for research, from basic studies of two-dimensional physics to the development of applications of heterogeneous electronics.
Sensitive platform: The use of graphene oxide (GO) as a platform for the sensitive and selective detection of DNA and proteins is presented. The interaction of GO and dye-labeled single-stranded DNA leads to quenching of the dye fluorescence. Conversely, the presence of a target DNA or protein leads to the binding of the dye-labeled DNA and target, releasing the DNA from GO, thereby restoring the dye fluorescence (see picture).
We investigate atomic layer deposition (ALD) of metal oxide on pristine and functionalized graphene. On pristine graphene, ALD coating can only actively grow on edges and defect sites, where dangling bonds or surface groups react with ALD precursors. This affords a simple method to decorate and probe single defect sites in graphene planes. We used perylene tetracarboxylic acid (PTCA) to functionalize the graphene surface and selectively introduced densely packed surface groups on graphene. Uniform ultrathin ALD coating on PTCA graphene was achieved over a large area. The functionalization method could be used to integrate ultrathin high-kappa dielectrics in future graphene electronics.