Effect of domain boundaries on the Raman spectra of mechanically strained graphene

Gold-Deposited Graphene Nanosheets for Self-Cleaning Graphene Surface-Enhanced Raman Spectroscopy with Superior Charge-Transfer Contribution

The interaction of graphene with metals initiates charge-transfer interaction-induced chemical enhancements, which critically depend on the doping effect from deposited metallic configurations. In this paper, we have explored the gold nanoparticle-decorated monolayer graphene nanosheets for the large graphene-induced Raman enhancement of adsorbed analytes, indicating the surface-enhanced Raman spectroscopy (SERS) capabilities of metal-doped graphene (G-SERS). Here, the systematically sputtered Au thickness optimization procedure revealed noticeable modifications in the graphene Raman spectra and photoluminescence (PL) background quenching, which indicated favorable charge transfer through n-type doping of chemical vapor deposition-grown graphene nanosheets. The highly consistent, individually distributed morphology of the gold nanoislands over graphene nanosheets depicted a reproducibly uniform G-SERS signal with excellent relative standard deviation values (<5%), resulting in the strongest Raman intensity enhancement factors of ∼108 (MB) (methylene blue) and 107 (DPA) (2,6-pyridinedicarboxylic acid) composed of the weakest PL background. The combined charge-transfer-induced chemical enhancement and electromagnetic enhancement from individual Au nanoislands result in a lowering of detectability down to 10-16 M (MB) and 10-11 M (DPA) concentrations with efficient time-dependent signal stability. Additionally, the GAu demonstrated its effective (∼94.4%) photocatalytic degradation capabilities by decomposing MB dye molecules from a concentration of 1 μM to 2.52 fM within 60 min. Therefore, the prominent charge-transfer contribution through controlled Au decoration over graphene nanosheets provides a potential strategy for fabricating superior SERS sensors and photocatalysts exhibiting adequate signal consistency, stability, and photodegradation efficiency through overcoming the limitations of the traditional sensing platforms.

In-situ atomic level observation of the strain response of graphene lattice

Strain is inevitable in two-dimensional (2D) materials, regardless of whether the film is suspended or supported. However, the direct measurement of strain response at the atomic scale is challenging due to the difficulties of maintaining both flexibility and mechanical stability at low temperature under UHV conditions. In this work, we have implemented a compact nanoindentation system with a size of [Formula: see text] 160 mm[Formula: see text] [Formula: see text] 5.2 mm in a scanning tunneling microscope (STM) sample holder, which enables the reversible control of strain and gate electric field. A combination of gearbox and piezoelectric actuator allowed us to modulate the depth of the indentation continuously with nanometer precision. The 2D materials were transferred onto the polyimide film. Pd clamp was used to enhance the strain transfer from the polyimide from to the 2D layers. Using this unique technique, strain response of graphene lattice were observed at atomic precision. In the relaxed graphene, strain is induced mainly by local curvature. However, in the strained graphene with tented structure, the lattice parameters become more sensitive to the indentor height change and stretching strain is increased additionally. Moreover, the gate controllability is confirmed by measuring the dependence of the STM tip height on gate voltage.

The Graphene Structure’s Effects on the Current-Voltage and Photovoltaic Characteristics of Directly Synthesized Graphene/n-Si(100) Diodes

Graphene was synthesized directly on Si(100) substrates by microwave plasma-enhanced chemical vapor deposition (MW-PECVD). The effects of the graphene structure on the electrical and photovoltaic properties of graphene/n-Si(100) were studied. The samples were investigated using Raman spectroscopy, atomic force microscopy, and by measuring current–voltage (I-V) graphs. The temperature of the hydrogen plasma annealing prior to graphene synthesis was an essential parameter regarding the graphene/Si contact I-V characteristics and photovoltaic parameters. Graphene n-type self-doping was found to occur due to the native SiO₂ interlayer at the graphene/Si junction. It was the prevalent cause of the significant decrease in the reverse current and short-circuit current. No photovoltaic effect dependence on the graphene roughness and work function could be observed.

Triaxially strained suspended graphene for large-area pseudo-magnetic fields

Strain-engineered graphene has garnered much attention recently owing to the possibilities of creating substantial energy gaps enabled by pseudo-magnetic fields (PMFs). While theoretical works proposed the possibility of creating large-area PMFs by straining monolayer graphene along three crystallographic directions, clear experimental demonstration of such promising devices remains elusive. Herein, we experimentally demonstrate a triaxially strained suspended graphene structure that has the potential to possess large-scale and quasi-uniform PMFs. Our structure employs uniquely designed metal electrodes that function both as stressors and metal contacts for current injection. Raman characterization and tight-binding simulations suggest the possibility of achieving PMFs over a micrometer-scale area. Current-voltage measurements confirm an efficient current injection into graphene, showing the potential of our devices for a new class of optoelectronic applications. We also theoretically propose a photonic crystal-based laser structure that obtains strongly localized optical fields overlapping with the spatial area under uniform PMFs, thus presenting a practical route toward the realization of graphene lasers.

Real-Time Modulation of Hydrogen Evolution Activity of Graphene Electrodes Using Mechanical Strain

This paper reports the effect of mechanically applied elastic strain on the hydrogen evolution reaction (HER) activity of graphene under acidic conditions. An applied tensile strain of 0.2% on a graphene electrode is shown to lead to a 1-3% increase in the HER current. The tensile strain increases HER activity, whereas compressive strain decreases it. Density functional theory (DFT) calculations using a periodic graphene slab model predict an increase in the adsorption energy of the H atom with growing tensile strain, consistent with an enhancement of the current density in HER, similar to that observed experimentally.

Investigating the Performances of Wide-Field Raman Microscopy with Stochastic Optical Reconstruction Post-Processing

Super-resolution fluorescence microscopy based on localization algorithms has tremendously impacted the field of imaging by improving the spatial resolution of optical measurements with specific blinking fluorophores and concomitant reduction of acquisition time. In vibrational spectroscopy and imaging, various methods have been developed to surpass the diffraction limit including near-field scattering methods, such as in tip-enhanced Raman and infrared spectroscopies. Although these scanning-probe techniques can provide exquisite spatial resolution, they often require long acquisition times and tedious fabrication of nano-scale scanning probes. Herein, stochastic optical reconstruction microscopy (STORM) protocol is applied on Raman measurements acquired using a wide-field home-built microscopy setup. We explore how the fluctuations of the Raman signal acquired over a series of time-lapse images at specific spectral ranges can be exploited with STORM processing, possibly revealing details with improved spatial resolution, under lower irradiance and with faster acquisition speed that cannot be achieved in point scanning mode over the same field of view. Samples studied here include patterned silicon, polystyrene microspheres on a silicon wafer, and graphene on a silicon/silicon dioxide substrate. The outcome presents an effective way to collect Raman images at selected spectral ranges with spatial resolutions of ∼200 nm over a large field of view under 532 nm excitation together with an acquisition speed improved by two orders of magnitude and under a significantly reduced irradiance compared to confocal laser scanning acquisition.

Uniaxially Strained Graphene: Structural Characteristics and G-Mode Splitting

The potential use of graphene in various strain engineering applications requires an accurate characterization of its properties when the material is under different mechanical loads. In this work, we present the strain dependence of the geometrical characteristics at the atomic level and the Raman active G-band evolution in a uniaxially strained graphene monolayer, using density functional theory methods as well as molecular dynamics atomistic simulations for strains that extend up to the structural failure. The bond length and bond angle variations with strain, applied either along the zigzag or along the armchair direction, are discussed and analytical relations describing this dependence are provided. The G-mode splitting with strain, as obtained by first principles' methods, is also presented. While for small strains, up to around 1%, the G-band splitting is symmetrical in the two perpendicular directions of tension considered here, this is no longer the case for larger values of strains where the splitting appears to be larger for strains along the zigzag direction. Further, a crossing is observed between the lower frequency split G-mode component and the out-of-plane optical mode at the Γ point for large uniaxial strains (>20%) along the zigzag direction.

Nano-Physical Characterization of Chemical Vapor Deposition-Grown Monolayer Graphene for High Performance Electrode: Raman, Surface-Enhanced Raman Spectroscopy, and Electrostatic Force Microscopy Studies

To achieve high-quality chemical vapor deposition of monolayer graphene electrodes (CVD-MG), appropriate characterization at each fabrication step is essential. In this article, (1) Raman spectroscopy/microscopy are employed to unravel the contact effect between the CVD-MG and Cu foil in suspended/supported formation. (2) The Surface-Enhanced Raman spectroscopy (SERS) system is described, unveiling the presence of a z-directional radial breathing-like mode (RBLM) around 150 cm^-1, which matches the Raman shift of the radial breathing mode (RBM) from single-walled carbon nanotubes (SWCNTs) around 150 cm^-1. This result indicates the CVD-MG located between the Au NPs and Au film is not flat but comprises heterogeneous protrusions of some domains along the z-axis. Consequently, the degree of carrier mobility can be influenced, as the protruding domains result in lower carrier mobility due to flexural phonon-electron scattering. A strongly enhanced G-peak domain, ascribed to the presence of scrolled graphene nanoribbons (sGNRs), was observed, and there remains the possibility for the fabrication of sGNRs as sources of open bandgap devices. (3) Electrostatic force microscopy (EFM) is used for the measurement of surface charge distribution of graphene at the nanoscale and is crucial in substantiating the electrical performance of CVD-MG, which was influenced by the surface structure of the Cu foil. The ripple (RP) structures were determined using EFM correlated with Raman spectroscopy, exhibiting a higher tapping amplitude which was observed with structurally stable and hydrophobic RPs with a threading type than surrounding RPs. (4) To reduce the RP density and height, a plausible fabrication could be developed that controls the electrical properties of the CVD-MG by tuning the cooling rate.

Influences of Initial Surface Conditions on Response Characteristics of Amorphous Silicon Films to Nanosecond Laser Irradiation

Although laser-produced micro-/nano-structures have been extensively studied, the effects of the initial surface conditions on the formed micro-/nano-structures have rarely been investigated. In this study, through nanosecond pulsed laser irradiation of unpolished and polished amorphous silicon films, entirely different surface characteristics were observed. The effects of laser irradiation parameters, such as repetition frequency, beam overlap ratio, and scanning velocity, on the surface characteristics were investigated, followed by the characterization of surface roughness, energy-dispersive X-ray spectroscopy, and Raman spectroscopy of the irradiated surfaces. For the unpolished surface, novel micro-protrusions were generated after laser irradiation, whereas no such micro-protrusions were formed on the polished surface. The experimental results indicated that the height of the micro-protrusions could be tuned using laser irradiation parameters and that laser irradiation promoted the crystallization of the amorphous silicon film. Moreover, the formation mechanism of the micro-protrusions was linked to fluctuations of the solid–liquid interface caused by continuous laser pulse shocks at higher repetition frequencies. The findings of this study suggest important correlations between the initial surface conditions and micro-/nano-structure formation, which may enhance our fundamental understanding of the formation of micro-/nano-structures.

3D fuzzy graphene microelectrode array for dopamine sensing at sub-cellular spatial resolution

The development of a high sensitivity real-time sensor for multi-site detection of dopamine (DA) with high spatial and temporal resolution is of fundamental importance to study the complex spatial and temporal pattern of DA dynamics in the brain, thus improving the understanding and treatments of neurological and neuropsychiatric disorders. In response to this need, here we present high surface area out-of-plane grown three-dimensional (3D) fuzzy graphene (3DFG) microelectrode arrays (MEAs) for highly selective, sensitive, and stable DA electrochemical sensing. 3DFG microelectrodes present a remarkable sensitivity to DA (2.12 ± 0.05 nA/nM, with LOD of 364.44 ± 8.65 pM), the highest reported for nanocarbon MEAs using Fast Scan Cyclic Voltammetry (FSCV). The high surface area of 3DFG allows for miniaturization of electrode down to 2 × 2 μm², without compromising the electrochemical performance. Moreover, 3DFG MEAs are electrochemically stable under 7.2 million scans of continuous FSCV cycling, present exceptional selectivity over the most common interferents in vitro with minimum fouling by electrochemical byproducts and can discriminate DA and serotonin (5-HT) in response to the injection of their 50:50 mixture. These results highlight the potential of 3DFG MEAs as a promising platform for FSCV based multi-site detection of DA with high sensitivity, selectivity, and spatial resolution.

Preferential self-healing at grain boundaries in plasma-treated graphene

Engineering of defects located in grains or at grain boundaries is central to the development of functional materials. Although there is a surge of interest in the formation, migration and annihilation of defects during ion and plasma irradiation of bulk materials, these processes are rarely assessed in low-dimensional materials and remain mostly unexplored spectroscopically at the micrometre scale due to experimental limitations. Here, we use a hyperspectral Raman imaging scheme providing high selectivity and diffraction-limited spatial resolution to examine plasma-induced damage in a polycrystalline graphene film. Measurements conducted before and after very low-energy (11-13 eV) ion bombardment show defect generation in graphene grains following a zero-dimensional defect curve, whereas domain boundaries tend to develop as one-dimensional defects. Damage generation is slower at grain boundaries than within the grains, a behaviour ascribed to preferential self-healing. This evidence of local defect migration and structural recovery in graphene sheds light on the complexity of chemical and physical processes at the grain boundaries of two-dimensional materials.

Probing plasma-treated graphene using hyperspectral Raman

Raman spectroscopy provides rich optical signals that can be used, after data analysis, to assess if a graphene layer is pristine, doped, damaged, functionalized, or stressed. The area being probed by a conventional Raman spectrometer is, however, limited to the size of the laser beam (∼1 µm); hence, detailed mapping of inhomogeneities in a graphene sample requires slow and sequential acquisition of a Raman spectrum at each pixel. Studies of physical and chemical processes on polycrystalline and heterogeneous graphene films require more advanced hyperspectral Raman capable of fast imaging at a high spatial resolution over hundreds of microns. Here, we compare the capacity of two different Raman imaging schemes (scanning and global) to probe graphene films modified by a low-pressure plasma treatment and present an analysis method providing assessments of the surface properties at local defects, grain boundaries, and other heterogeneities. By comparing statistically initial and plasma-treated regions of graphene, we highlight the presence of inhomogeneities after plasma treatment linked to the initial state of the graphene surface. These results provided statistical results on the correlation between the graphene initial state and the corresponding graphene-plasma interaction. This work further demonstrates the potential use of global hyperspectral Raman imaging with advanced Raman spectra analysis to study graphene physics and chemistry on a scale of hundreds of microns.

Transferless Inverted Graphene/Silicon Heterostructures Prepared by Plasma-Enhanced Chemical Vapor Deposition of Amorphous Silicon on CVD Graphene

The heterostructures of two-dimensional (2D) and three-dimensional (3D) materials represent one of the focal points of current nanotechnology research and development. From an application perspective, the possibility of a direct integration of active 2D layers with exceptional optoelectronic and mechanical properties into the existing semiconductor manufacturing processes is extremely appealing. However, for this purpose, 2D materials should ideally be grown directly on 3D substrates to avoid the transferring step, which induces damage and contamination of the 2D layer. Alternatively, when such an approach is difficult—as is the case of graphene on noncatalytic substrates such as Si—inverted structures can be created, where the 3D material is deposited onto the 2D substrate. In the present work, we investigated the possibility of using plasma-enhanced chemical vapor deposition (PECVD) to deposit amorphous hydrogenated Si (a-Si:H) onto graphene resting on a catalytic copper foil. The resulting stacks created at different Si deposition temperatures were investigated by the combination of Raman spectroscopy (to quantify the damage and to estimate the change in resistivity of graphene), temperature-dependent dark conductivity, and constant photocurrent measurements (to monitor the changes in the electronic properties of a-Si:H). The results indicate that the optimum is 100 °C deposition temperature, where the graphene still retains most of its properties and the a-Si:H layer presents high-quality, device-ready characteristics.

Soft Skin Layers Enable Area-Specific, Multiscale Graphene Wrinkles with Switchable Orientations

This paper reports a method to realize crack-free graphene wrinkles with variable spatial wavelengths and switchable orientations. Graphene supported on a thin fluoropolymer and prestrained elastomer substrate can exhibit conformal wrinkling after strain relief. The wrinkle orientation could be switched beyond the intrinsic fracture limit of graphene for hundreds of cycles of stretching and releasing without forming cracks. Mechanical modeling revealed that the fluoropolymer layer mediated the structural evolution of the graphene wrinkles without crack formation or delamination. Patterned fluoropolymer layers with different thicknesses produced wrinkles with controlled wavelengths and orientations while maintaining the mechanical integrity of graphene under high tensile strain.

Graphene-based neuron encapsulation with controlled axonal outgrowth

Neuronal constructs with tuneable 3D geometry can contribute greatly to the construction of brain-like functional tissues for transplantable grafts and robust experimental models. In this study, we propose a self-folding graphene/polymer bilayer film that forms a micro-roll for neuron encapsulation, and highlight the importance of employing pores on the micro-roll to allow neurons to interact with their surroundings. The micro-patterns and varied thicknesses of the bilayer provide control over the 3D geometries of the micro-roll. The pores facilitate the diffusion of reagents, resulting in the adequate loading of probes for imaging and the successful stimulation of the encapsulated neurons. Moreover, the encapsulated neurons inside the micro-roll are functionally integrated into surrounding neuronal networks by extending their axons through the pores. Thus, our method for encapsulating neurons with a porous graphene-laden film allows the construction of precisely shaped neuronal tissues that interact with their surroundings. We believe that the method will open a new avenue for the reconstruction of functional neuronal tissues and is potentially applicable to other self-folding bilayers.

Raman Study of Strain Relaxation from Grain Boundaries in Epitaxial Graphene Grown by Chemical Vapor Deposition on SiC

Strains in graphene play a significant role in graphene-based electronics, but many aspects of the grain boundary effects on strained graphene remain unclear. Here, the relationship between grain boundary and strain property of graphene grown by chemical vapor deposition (CVD) on the C-face of SiC substrate has been investigated by Raman spectroscopy. It is shown that abundant boundary-like defects exist in the graphene film and the blue-shifted 2D-band frequency, which results from compressive strain in graphene film, shifts downward linearly as 1/L_a increases. Strain relaxation caused by grain boundary diffusion is considered to be the reason and the mechanism is analyzed in detail.

Self-Folded Three-Dimensional Graphene with a Tunable Shape and Conductivity

Three-dimensional (3D) graphene architectures are of great interest as applications in flexible electronics and biointerfaces. In this study, we demonstrate the facile formation of predetermined 3D polymeric microstructures simply by transferring monolayer graphene. The graphene adheres to the surface of polymeric films via noncovalent π-π stacking bonding and induces a sloped internal strain, leading to the self-rolling of 3D microscale architectures. Micropatterns and varied thicknesses of the 2D films prior to the self-rolling allows for control over the resulting 3D geometries. The strain then present on the hexagonal unit cell of the graphene produces a nonlinear electrical conductivity across the device. The driving force behind the self-folding process arises from the reconfiguration of the molecules within the crystalline materials. We believe that this effective and versatile way of realizing a 3D graphene structure is potentially applicable to alternative 2D layered materials as well as other flexible polymeric templates.

Polyhydroxybutyrate-co-hydroxyvalerate copolymer modified graphite oxide based 3D scaffold for tissue engineering application

In this study, we have fabricated the PHBV functionalized graphite oxide using freeze drying technique, followed by 'in situ' pay loading of Fe₃O₄ nanoparticles onto the hydrophobic plate of the composite basal plane; thereby, mechanically and thermally stable, bio-imaging Fe₃O₄/GO-g-PHBV composites have been developed. The synthesis of Fe₃O₄/GO-g-PHBV composite was confirmed by field emission SEM and TEM analyses, X-ray diffraction and Fourier transform infrared spectroscopy. The wrapping of PHBV copolymer into the graphene layers was investigated by atomic force microscopy and Raman spectral analyses which provided the shifting of the 2D band with low signal intensity in the range of 2600-3000 cm^-1. The bactericidal activities of the Fe₃O₄/GO-g-PHBV composite films were found to exhibit more efficiency against Gram-negative bacteria strains compared to Gram-positive strains. In vibrating sample magnetometer (VSM) analysis, the zero value of coercivity revealed the super-paramagnetic nature of the Fe₃O₄/GO-g-PHBV composites. The Phantom agar magnetic resonance imaging analysis revealed the efficiency of Fe₃O₄ nanoparticles as a negative contrast (T₂ contrast) along with higher relaxivity value. The significant fibroblast cell (NIH 3T3) adhesion and proliferation (85%) on the Fe₃O₄/GO-g-PHBV composite surface indicated the physiological and biocompatible stability of that composite along with the presence of large π conjugated aromatic domain.

Revealing Strain-Induced Effects in Ultrathin Heterostructures at the Nanoscale

Two-dimensional materials are being increasingly studied, particularly for flexible and wearable technologies because of their inherent thickness and flexibility. Crucially, one aspect where our understanding is still limited is on the effect of mechanical strain, not on individual sheets of materials, but when stacked together as heterostructures in devices. In this paper, we demonstrate the use of Kelvin probe microscopy in capturing the influence of uniaxial tensile strain on the band-structures of graphene and WS₂ (mono- and multilayered) based heterostructures at high resolution. We report a major advance in strain characterization tools through enabling a single-shot capture of strain defined changes in a heterogeneous system at the nanoscale, overcoming the limitations (materials, resolution, and substrate effects) of existing techniques such as optical spectroscopy. Using this technique, we observe that the work-functions of graphene and WS₂ increase as a function of strain, which we attribute to the Fermi level lowering from increased p-doping. We also extract the nature of the interfacial heterojunctions and find that they get strongly modulated from strain. We observe that the strain-enhanced charge transfer with the substrate plays a dominant role, causing the heterostructures to behave differently from two-dimensional materials in their isolated forms.

Carbon Nanotubes versus Graphene as Flexible Transparent Electrodes in Inverted Perovskite Solar Cells

Transparent carbon electrodes, carbon nanotubes, and graphene were used as the bottom electrode in flexible inverted perovskite solar cells. Their photovoltaic performance and mechanical resilience were compared and analyzed using various techniques. Whereas a conventional inverted perovskite solar cells using indium tin oxide showed a power conversion efficiency of 17.8%, the carbon nanotube- and graphene-based cells showed efficiencies of 12.8% and 14.2%, respectively. An established MoO₃ doping was used for  carbon electrode-based devices. The difference in the photovoltaic performance between the carbon nanotube- and graphene-based cells was due to the difference in morphology and transmittance. Raman spectroscopy, and cyclic flexural testing revealed that the graphene-based cells were more susceptible to strain than the carbon nanotube-based cells, though the difference was marginal. Overall, despite higher performance, the transfer step for graphene has lower reproducibility. Thus, the development of better graphene transfer methods would help maximize the current capacity of graphene-based cells.

Multiscale, Hierarchical Patterning of Graphene by Conformal Wrinkling

This paper describes how delamination-free, hierarchical patterning of graphene can be achieved on prestrained thermoplastic sheets by surface wrinkling. Conformal contact between graphene and the substrate during strain relief was maintained by the presence of a soft skin layer, resulting in the uniform patterning of three-dimensional wrinkles over large areas (>cm²). The graphene wrinkle wavelength was tuned from the microscale to the nanoscale by controlling the thickness of the skin layer with 1 nm accuracy to realize a degree of control not possible by crumpling, which relies on delamination. Hierarchical patterning of the skin layers with varying thicknesses enabled multiscale graphene wrinkles with predetermined orientations to be formed. Significantly, hierarchical graphene wrinkles exhibited tunable mechanical stiffness at the nanoscale without compromising the macroscale electrical conductivity.

Deformation of wrinkled graphene

The deformation of monolayer graphene, produced by chemical vapor deposition (CVD), on a polyester film substrate has been investigated through the use of Raman spectroscopy. It has been found that the microstructure of the CVD graphene consists of a hexagonal array of islands of flat monolayer graphene separated by wrinkled material. During deformation, it was found that the rate of shift of the Raman 2D band wavenumber per unit strain was less than 25% of that of flat flakes of mechanically exfoliated graphene, whereas the rate of band broadening per unit strain was about 75% of that of the exfoliated material. This unusual deformation behavior has been modeled in terms of mechanically isolated graphene islands separated by the graphene wrinkles, with the strain distribution in each graphene island determined using shear lag analysis. The effect of the size and position of the Raman laser beam spot has also been incorporated in the model. The predictions fit well with the behavior observed experimentally for the Raman band shifts and broadening of the wrinkled CVD graphene. The effect of wrinkles upon the efficiency of graphene to reinforce nanocomposites is also discussed.

Graphene Mechanics: Current Status and Perspectives

The mechanical properties of 2D materials such as monolayer graphene are of extreme importance for several potential applications. We summarize the experimental and theoretical results to date on mechanical loading of freely suspended or fully supported graphene. We assess the obtained axial properties of the material in tension and compression and comment on the methods used for deriving the various reported values. We also report on past and current efforts to define the elastic constants of graphene in a 3D representation. Current areas of research that are concerned with the effect of production method and/or the presence of defects upon the mechanical integrity of graphene are also covered. Finally, we examine extensively the work related to the effect of graphene deformation upon its electronic properties and the possibility of employing strained graphene in future electronic applications.

Controlled van der Waals epitaxy of monolayer MoS2 triangular domains on graphene

Multilayered heterostructures of two-dimensional materials have recently attracted increased interest because of their unique electronic and optical properties. Here, we present chemical vapor deposition (CVD) growth of triangular crystals of monolayer MoS2 on single-crystalline hexagonal graphene domains which are also grown by CVD. We found that MoS2 grows selectively on the graphene domains rather than on the bare supporting SiO2 surface. Reflecting the heteroepitaxy of the growth process, the MoS2 domains grown on graphene present two preferred equivalent orientations. The interaction between the MoS2 and the graphene induced an upshift of the Raman G and 2D bands of the graphene, while significant photoluminescence quenching was observed for the monolayer MoS2. Furthermore, photoinduced current modulation along with an optical memory effect was demonstrated for the MoS2-graphene heterostructure. Our work highlights that heterostructures synthesized by CVD offer an effective interlayer van der Waals interaction which can be developed for large-area multilayer electronic and photonic devices.

Annealing free, clean graphene transfer using alternative polymer scaffolds

We examine the transfer of graphene grown by chemical vapor deposition (CVD) with polymer scaffolds of poly(methyl methacrylate) (PMMA), poly(lactic acid) (PLA), poly(phthalaldehyde) (PPA), and poly(bisphenol A carbonate) (PC). We find that optimally reactive PC scaffolds provide the cleanest graphene transfers without any annealing, after extensive comparison with optical microscopy, x-ray photoelectron spectroscopy, atomic force microscopy, and scanning tunneling microscopy. Comparatively, films transferred with PLA, PPA, PMMA/PC, and PMMA have a two-fold higher roughness and a five-fold higher chemical doping. Using PC scaffolds, we demonstrate the clean transfer of CVD multilayer graphene, fluorinated graphene, and hexagonal boron nitride. Our annealing free, PC transfers enable the use of atomically-clean nanomaterials in biomolecule encapsulation and flexible electronic applications.

Applications of graphene and related nanomaterials in analytical chemistry

Graphene and its related materials remain a very bright and exciting prospect in analytical chemistry.

Direct assessment of the mechanical modulus of graphene co-doped with low concentrations of boron-nitrogen by a non-contact approach

Boron and nitrogen co-doping has been shown to be an effective way to induce a band gap in graphene for electrical applications but only a few theoretical studies have been done to understand the elastic and mechanical properties of the modified graphene. Until now, no experimental assessment of the mechanical modulus of boron-nitrogen-doped graphene (BNG) has been reported in the literature. Here, we demonstrate a novel non-contact approach to determine the in-plane stiffness of BNG at low BN concentrations. The in-plane stiffness of BNG with 2 at% BN concentration was estimated to be about 309 N m(-1), which is lower than that of pristine graphene, in good agreement with some theoretical studies. Moreover, we correlated the conductivity of BNG with induced strain and found the BNG to be more sensitive than pristine graphene in response to externally applied strain. This result indicates that BNG is a more suitable material than graphene for strain sensor applications.

Structure and transport properties of the interface between CVD-grown graphene domains

During the chemical vapor deposition (CVD) growth of graphene, graphene domains grown on a Cu surface merge together and form a uniform graphene sheet. For high-performance electronics and other applications, it is important to understand the interfacial structure of the merged domains, as well as their influence on the physical properties of graphene. We synthesized large hexagonal graphene domains with controlled orientations on a heteroepitaxial Cu film and studied the structure and properties of the interfaces between the domains mainly merged with the same angle. Although the merged domains have various interfaces with/without wrinkles and/or increased defect-related Raman D-band intensity, the intra-domain transport showed higher carrier mobility reaching 20,000 cm(2) V(-1) s(-1) on SiO2 at 280 K (the mean value was 7200 cm(2) V(-1) s(-1)) than that measured for inter-domain areas, 6400 cm(2) V(-1) s(-1) (mean value 2000 cm(2) V(-1) s(-1)). The temperature dependence of the mobility suggests that impurity scattering dominates at the interface even for the merged domains with the same orientation. This study highlights the importance of domain interfaces, especially on the carrier transport properties, in CVD-grown graphene.

Tip-enhanced Raman spectroscopic measurement of stress change in the local domain of epitaxial graphene on the carbon face of 4H-SiC(000–1)

We measured TERS spectra of epitaxial graphene on the carbon face of 4H-SiC(000–1) and compared them with the corresponding normal Raman spectra.

Effects of silicon dioxide surface roughness on Raman characteristics and mechanical properties of graphene

The effects of the surface roughness of a silicon dioxide substrate on the mechanical properties and Raman scattering of graphene prepared by chemical vapor deposition were investigated.

Strain engineering the properties of graphene and other two-dimensional crystals

This perspective discusses recent advances in using strain to engineer the properties of thin-layer materials such as graphene and transition metal dichalcogenides (TMDs).

Enhanced chemical reactivity of graphene induced by mechanical strain

Control over chemical reactivity is essential in the field of nanotechnology. Graphene is a two-dimensional atomic sheet of sp(2) hybridized carbon with exceptional properties that can be altered by chemical functionalization. Here, we transferred single-layer graphene onto a flexible substrate and investigated the functionalization using different aryl diazonium molecules while applying mechanical strain. We found that mechanical strain can alter the structure of graphene, and dramatically increase the reaction rate, by a factor of up to 10, as well as increase the final degree of functionalization. Furthermore, we demonstrate that mechanical strain enables functionalization of graphene for both p- and n-type dopants, where unstrained graphene showed negligible reactivity. Theoretical calculations were also performed to support the experimental findings. Our findings offer a simple approach to control the chemical reactivity of graphene through the application of mechanical strain, allowing for a tuning of the properties of graphene.

On the growth mode of two-lobed curvilinear graphene domains at atmospheric pressure

We demonstrate the chemical vapor deposition (CVD) growth of 2-lobed symmetrical curvilinear graphene domains specifically on Cu{100} surface orientations at atmospheric pressure. We utilize electron backscattered diffraction, scanning electron microscopy and Raman spectroscopy to determine an as-yet unexplored growth mode producing such a shape and demonstrate how its growth and morphology are dependent on the underlying Cu crystal structure especially in the high CH4:H2 regime. We show that both monolayer and bilayer curvilinear domains are grown on Cu{100} surfaces; furthermore, we show that characteristic atmospheric pressure CVD hexagonal domains are grown on all other Cu facets with an isotropic growth rate which is more rapid than that on Cu{100}. These findings indicate that the Cu-graphene complex is predominant mechanistically at atmospheric pressure, which is an important step towards tailoring graphene properties via substrate engineering.

A hydrothermal anvil made of graphene nanobubbles on diamond

The hardness and virtual incompressibility of diamond allow it to be used in high-pressure anvil cell. Here we report a new way to generate static pressure by encapsulating single-crystal diamond with graphene membrane, the latter is well known for its superior nano-indentation strength and in-plane rigidity. Heating the diamond-graphene interface to the reconstruction temperature of diamond (~1,275 K) produces a high density of graphene nanobubbles that can trap water. At high temperature, chemical bonding between graphene and diamond is robust enough to allow the hybrid interface to act as a hydrothermal anvil cell due to the impermeability of graphene. Superheated water trapped within the pressurized graphene nanobubbles is observed to etch the diamond surface to produce a high density of square-shaped voids. The molecular structure of superheated water trapped in the bubble is probed using vibrational spectroscopy and dynamic changes in the hydrogen-bonding environment are observed.

Near-Infrared photoluminescence in the femtosecond time region in monolayer graphene on SiO₂

We investigate the dynamical properties of photoexcited carriers in a single monolayer of graphene at room temperature in air using femtosecond time-resolved luminescence spectroscopy. The luminescence kinetics are observed in the near-infrared region of 0.7-1.4 eV and analyzed based on the two-temperature model describing the cooling of thermalized carriers via the carrier-optical phonon interaction. The observed luminescence in the range 0.7-0.9 eV is well reproduced by the model. In the range 1.0-1.4 eV, however, the luminescence, which decays in ∼300 fs, cannot be reproduced by this model. These results indicate that the carrier system is not completely thermalized in ∼300 fs. We also show the importance of the carrier-doping effect induced by the substrate and surrounding environment in the carrier cooling dynamics and the predominance of optical phonons over acoustic phonons in the carrier-phonon interactions even at a temperature of ∼400 K.