Strain In Graphene Research Articles

Laser-Induced Graphene Strain Sensor for Conformable Lip-Reading Recognition and Human–Machine Interaction

Laser-Induced Graphene Strain Sensor for Conformable Lip-Reading Recognition and Human–Machine Interaction

Thermally induced mechanical strain of graphene on copper and other substrates

Raman spectroscopy is employed to investigate the temperature dependence of the phonon frequencies of the G and 2D modes of monolayer graphene grown on copper foil by chemical vapor deposition. Both the G and 2D modes exhibit significant red shift as temperature increases, and the extracted values of the temperature coefficients of the G and 2D modes are about −0.07 cm−1K−1 and −0.15 cm−1K−1, respectively. In addition, we extracted the intrinsic Raman shift of the G mode by determining the thermally induced mechanical strain that is transmitted from the copper substrate to graphene. The extracted intrinsic G mode frequency shift is in very good agreement with earlier reported first principles calculations. Finally, we have calculated the temperature dependence of the substrate-induced strain on graphene when it is deposited or grown to various technologically important substrates, showing that strains as high as 1.1% can be imparted to graphene within a temperature range of 500 K.

Moiré engineering in 2D heterostructures with process-induced strain

We report deterministic control over a moiré superlattice interference pattern in twisted bilayer graphene by implementing designable device-level heterostrain with process-induced strain engineering, a widely used technique in industrial silicon nanofabrication processes. By depositing stressed thin films onto our twisted bilayer graphene samples, heterostrain magnitude and strain directionality can be controlled by stressor film force (film stress × film thickness) and patterned stressor geometry, respectively. We examine strain and moiré interference with Raman spectroscopy through in-plane and moiré-activated phonon mode shifts. Results support systematic C3 rotational symmetry breaking and tunable periodicity in moiré superlattices under the application of uniaxial or biaxial heterostrain. Experimental results are validated by molecular statics simulations and density functional theory based first principles calculations. This provides a method not only to tune moiré interference without additional twisting but also to allow for a systematic pathway to explore different van der Waals based moiré superlattice symmetries by deterministic design.

In-situ thermal Raman spectroscopy of single-layer graphene on different substrates

In this study, we discovered through Raman spectroscopy that single-layered graphene is not destroyed at 600 °C generally but the decomposition temperature depends linearly on the coefficient of thermal expansion (CTE) of the substrate. Moreover, we showed that there is a different phenomenon behind the decomposition of graphene in air and in vacuum. While in both environments, decomposition is initiated by the prolongation of the C-C bonds, in air the increased strain raises the graphene reactivity with oxygen, therefore it is the oxygen which destroys graphene whereas in a vacuum the strain alone causes the decomposition. The maximal strain for the single-layered graphene was measured for both environments. The experiments were carried out on 8 different substrates with very different CTEs. We also calculated the Raman shift thermal coefficient for graphene on Si, Pt, Au, and Cu and discussed the changes in graphene through the very wide temperature changes (−196 to 600 °C).

Tuning the Magnetic Anisotropy of Co–N‐Doped Graphene

Large magnetic anisotropy energy and out‐of‐plane easy magnetization direction are beneficial for the application of 2D ferromagnetism. Although the robust room‐temperature ferromagnetism is reported in Co–N‐doped graphene [Nat. Commun. 2021, 12, 1854], its magnetic anisotropy and easy axis of magnetization are still unclear. Herein, the magnetic anisotropy of CoN4–graphene and CoN3–graphene is systematically studied. The CoN4–graphene has an in‐plane easy magnetization direction, while the easy magnetization direction of CoN3–graphene is out‐of‐plane. The magnetic anisotropy energy can be tuned by injecting charge or applying strain. Especially, for CoN3–graphene, both charge injection and tensile strain can tune the easy magnetization direction from out‐of‐plane to in‐plane. The tunable magnetic anisotropy may promote the application of Co–N‐doped graphene in spintronic devices.

Complex Landau levels and related transport properties in the strained zigzag graphene nanoribbons

The real magnetic fields (MFs) acting on graphene can induce flat real Landau levels (LLs). As an analogy, strains in graphene can produce significant pseudo MFs, triggering the appearance of dispersive pseudo LLs. By analyzing the low-energy effective Hamiltonian, we introduce the concept of the effective orbital MFs to integrate the real MFs and pseudo MFs. Accordingly, we obtain the complex LLs which incorporate the real LLs and pseudo LLs, and calculate the related transport properties. These concepts enable us to uncover the mechanisms driving the fragility of pseudo LLs against disorders and dephasing, proving that tuning the real MFs and Fermi energy can effectively improve the robust performances. Furthermore, the tunability of the valley-polarized currents is also studied, opening up new possibilities for the design of valleytronics devices.

Theoretical study on uniaxial compressive mechanical properties of three-dimensional graphene

Combining molecular dynamics (MD) simulations and continuum modeling, the uniaxial in-plane compressive properties of honeycomb three-dimensional (h3D), triangle-like 3D (t3D) and non-equilateral hexagon 3D (nh3D) graphene are investigated to elucidate configuration-property relationships, and the constitutive relations of three kinds of 3D graphene are derived to evaluate the compressive mechanical properties. Based on theoretical and MD simulation results, it is found that the compressive stress-strain responses of 3D graphene can be divided into five stages. The compressive mechanical properties and deformation mechanisms of each stage are different. When 3D graphene is compressed, graphene sidewalls undergo rotations, bending and buckling deformations, collapses. Because graphene sidewalls of h3D graphene are prone to undergo buckling deformations, there is large locking strain in h3D graphene. For t3D and nh3D graphene, their high plateau stress is attributed to unique configurations. Moreover, we also find that with increasing graphene sidewall width, Young's modulus and plateau stress of 3D graphene gradually decrease. In addition to the above investigations, the energy absorption mechanisms and compression-unloading stress-strain curves of 3D graphene are also studied. It is found that there are extraordinary compression-recovery properties and energy absorption capacities in 3D graphene. Finally, the selection criteria of 3D graphene are proposed.

Strain-Driven Faceting of Graphene-Catalyst Interfaces.

The interfacial interaction of 2D materials with the substrate leads to striking surface faceting affecting its electronic properties. Here, we quantitatively study the orientation-dependent facet topographies observed on the catalyst under graphene using electron backscatter diffraction and atomic force microscopy. The original flat catalyst surface transforms into two facets: a low-energy low-index surface, e.g. (111), and a vicinal (high-index) surface. The critical role of graphene strain, besides anisotropic interfacial energy, in forming the observed topographies is revealed by molecular simulations. These insights are applicable to other 2D/3D heterostructures.

Optics-Free, In Situ Swelling Monitoring of Articular Cartilage with Graphene Strain Sensors.

Articular cartilage derives its load-bearing strength from the mechanical and physiochemical coupling between the collagen network and negatively charged proteoglycans, respectively. Current disease modeling approaches and treatment strategies primarily focus on cartilage stiffness, partly because indentation tests are readily accessible. However, stiffness measurements via indentation alone cannot discriminate between proteoglycan degradation versus collagen degradation, and there is a lack of methods to monitor physiochemical contributors in full-stack tissue. To decouple these contributions, here, we developed a platform that measures tissue swelling in full-depth equine cartilage explants using piezoresistive graphene strain sensors. These piezoresistive strain sensors are embedded within an elastomer bulk and have sufficient sensitivity to resolve minute, real-time changes in swelling. By relying on simple DC resistance measurements over optical techniques, our platform can analyze multiple samples in parallel. Using these devices, we found that cartilage explants under enzymatic digestion showed distinctive swelling responses to a hypotonic challenge and established average equilibrium swelling strains in healthy cartilage (4.6%), cartilage with proteoglycan loss (0.5%), and in cartilage with both collagen and proteoglycan loss (-2.6%). Combined with histology, we decoupled the pathologic swelling responses as originating either from reduced fixed charge density or from loss of intrinsic stiffness of the collagen matrix in the superficial zone. By providing scalable and in situ monitoring of cartilage swelling, our platform could facilitate regenerative medicine approaches aimed at restoring osmotic function in osteoarthritic cartilage or could be used to validate physiologically relevant swelling behavior in synthetic hydrogels.

Localised strain and doping of 2D materials.

There is a growing interest in 2D materials-based devices as the replacement for established materials, such as silicon and metal oxides in microelectronics and sensing, respectively. However, the atomically thin nature of 2D materials makes them susceptible to slight variations caused by their immediate environment, inducing doping and strain, which can vary between, and even microscopically within, devices. One of the misapprehensions for using 2D materials is the consideration of unanimous intrinsic properties over different support surfaces. The interfacial interaction, intrinsic structural disorder and external strain modulate the properties of 2D materials and govern the device performance. The understanding, measurement and control of these factors are thus one of the significant challenges for the adoption of 2D materials in industrial electronics, sensing, and polymer composites. This topical review provides a comprehensive overview of the effect of strain-induced lattice deformation and its relationship with physical and electronic properties. Using the example of graphene and MoS2 (as the prototypical 2D semiconductor), we rationalise the importance of scanning probe techniques and Raman spectroscopy to elucidate strain and doping in 2D materials. These effects can be directly and accurately characterised through Raman shifts in a non-destructive manner. A generalised model has been presented that deconvolutes the intertwined relationship between strain and doping in graphene and MoS2 that could apply to other members of the 2D materials family. The emerging field of straintronics is presented, where the controlled application of strain over 2D materials induces tuneable physical and electronic properties. These perspectives highlight practical considerations for strain engineering and related microelectromechanical applications.

Impact of Strain in Monolayer Graphene and Related Phenomena

Impact of Strain in Monolayer Graphene and Related Phenomena

In situ measurement of viscoelastic properties of cellular monolayers via graphene strain sensing of elastohydrodynamic phenomena.

Recent advances recognize that the viscoelastic properties of epithelial structures play important roles in biology and disease modeling. However, accessing the viscoelastic properties of multicellular structures in mechanistic or drug-screening applications has challenges in repeatability, accuracy, and practical implementation. Here, we present a microfluidic platform that leverages elastohydrodynamic phenomena, sensed by strain sensors made from graphene decorated with palladium nanoislands, to measure the viscoelasticity of cellular monolayers in situ, without using chemical labels or specialized equipment. We demonstrate platform utility with two systems: cell dissociation following trypsinization, where viscoelastic properties change over minutes, and epithelial-to-mesenchymal transition, where changes occur over days. These cellular events could only be resolved with our platform's higher resolution: viscoelastic relaxation time constants of λ = 14.5 ± 0.4 s-1 for intact epithelial monolayers, compared to λ = 13.4 ± 15.0 s-1 in other platforms, which represents a 30-fold improvement. By rapidly assessing combined contributions from cell stiffness and intercellular interactions, we anticipate that the platform will hasten the translation of new mechanical biomarkers.

Thermal Transport of AlN/Graphene/3C-SiC Typical Heterostructures with Different Crystallinities of Graphene.

It is proven that introduction of graphene into typical heterostructures can effectively reduce the high interfacial thermal resistance in semiconductor chips. The crystallinity of graphene varies greatly; thus, we have investigated the effects of single-crystal and polycrystalline graphene on the thermal transport of AlN/graphene/3C-SiC heterostructures by molecular dynamics. The results show that polycrystalline graphene contributes more to the interfacial thermal conductance (ITC) inside the chip with a maximum increase of 75.09%, which is further confirmed by the energy transport and thermal relaxation time. Multiple analyses indicate that grain boundaries lead to the increase in C-Si covalent bonds, and thus, strong interactions improve the ITC. However, covalent bonding further causes local tensile strain and wrinkles in graphene. The former decreases the ITC, and the latter leads to the fluctuation of the van der Waals interaction at the interface. The combined effect of various influential factors results in the increase in the ITC, which are confirmed by phonon transmission with 0-18 THz. In addition, wrinkles and covalent bonding lead to increased stress concentration in polycrystalline graphene. This leads to a maximum reduction of 19.23% in the in-plane thermal conductivity, which is not conducive to the lateral diffusion of hot spots within the chip. The research results would provide important guidance in designing for high thermal transport performance high-power chips.

Epitaxial growth of ultrathin two-dimensional pentacene film with standing-up molecular geometry on nano-size-curved graphene surface

Molecule packing behavior at organic and graphene interface is essential for graphene-based organic electronics since charge carriers are transported within the very few organic layers nearest to graphene substrate. Although organic molecules especially the ones with planar aromatic rings usually adopt recumbent geometry on graphene substrate owning to the maximized π-π interaction, the alignment of organic molecules may be influenced by modifying the interfacial nature between organic molecules and graphene. Here, epitaxy growth of two-dimensional ultra-thin pentacene film with standing-up molecular geometry was observed for the first time on graphene with periodic nano-sized buckling structure. The curvature and strain of graphene was found collectively responsible for the standing-up geometry of pentacene and its oriented growth on graphene surface. This study provides a feasible way for controlling molecular epitaxy on 2D materials interfaces toward functional heterostructures.

Optical Goos-Hänchen effect in uniaxially strained graphene.

We prove the existence of relatively large Goos-Hänchen (GH) shifts for graphene in the presence of an applied strain in different crystallographic directions for p and s polarized beams. It is shown that GH shifts are smoothly increased by stretching the graphene's lattice. Moreover, we investigate the GH effect for strained graphene as a function of Fermi energy, which can be controlled by external factors such as gate voltage. We show that applied strain along zigzag and armchair orientations gives different results for GH shifts, which could provide a proper tool for the detection of strain in graphene.

Vibrational instabilities in multilayer graphene and graphite: Effects of strain and number of layers

We study the rippling instabilities of multilayer graphene and graphite as a function of strain and number of layers using density-functional theory and density-functional perturbation theory calculations. We find that, for multilayer graphene, a small but finite compressive strain is needed to induce a rippling instability, signaled by the onset of imaginary frequencies near the center of the Brillouin zone. For monolayer graphene, this critical strain is zero within numerical uncertainty. Using insights from continuum elasticity theory, we fit the phonon dispersion of the ZA mode near the zone center to a function ${\ensuremath{\omega}}^{2}(q)={C}_{2}{q}^{2}+{C}_{4}{q}^{4}$ and determine the ${C}_{2}$ and ${C}_{4}$ coefficients as a function of strain and number of layers. In particular, the ${C}_{2}$ coefficient is positive for multilayers in the absence of strain and shows a linear dependence with strain. It assumes negative values for compressive strains beyond a critical value and it is responsible for the appearance of imaginary frequencies in the ZA mode. We discuss these results in light of both continuum and atomistic approaches. Critical parameters vary smoothly with the number of layers from monolayer graphene (two-dimensional) to graphite (three-dimensional).

Raman spectroscopy of strained monolayer graphene modulated by monodispersed Au nanoparticles

Due to its inherent exceptional mechanical flexibility and strength, graphene provides an ideal platform for strain engineering, enabling versatile modulation and significant enhancement of their optical properties. Both uniaxial and biaxial strain have been achieved by transferring monolayer graphene onto monodispersed Au nanoparticles with negligible charge doping. We present a study on the phonon frequency and crystallographic orientation in graphene monolayer wrinkles under uniaxial and biaxial strain. The G and 2D peaks of graphene exhibit significant red shifts under tensile strain. The doubly degenerate G peak splits in two sub-peaks G− and G+ under larger uniaxial strain because of symmetry breaking, while this phenomenon does not occur in the 2D peak. But the 2D peak is much more sensitive to strain. Further, Raman spectra of the unsplit G and 2D peaks have been proved to show a distinctive polarization dependence. We proposed polarized Raman spectroscopy to determine the crystallographic orientation with respect to the strain of graphene by the unsplit G mode. The polarization dependence of 2D mode offers a unique opportunity to examine the anisotropic modifications of the phonon dispersion and changes in electronic structures. This result is of significant in strain engineering of graphene and other two-dimensional materials.

At Least 10-fold Higher Lubricity of Molecularly Thin D2O vs H2O Films at Single-Layer Graphene-Mica Interfaces.

Interfacial water is a widespread lubricant down to the nanometer scale. We investigate the lubricities of molecularly thin H2O and D2O films confined between mica and graphene, via the relaxation of initially applied strain in graphene employing Raman spectroscopy. Surprisingly, the D2O films are at least 1 order of magnitude more lubricant than H2O films, despite the similar bulk viscosities of the two liquids. We propose a mechanism based on the known selective permeation of protons vs deuterons through graphene. Permeated protons and left behind hydroxides may form ion pairs clamping across the graphene sheet and thereby hindering the graphene from sliding on the water layer. This explains the lower lubricity but also the hindering diffusivity of the water layer, which yields a high effective viscosity in accordance with findings in dewetting experiments. Our work elucidates an unexpected effect and provides clues to the behavior of graphene on hydrous surfaces.

Revisiting Frictional Characteristics of Graphene: Effect of In-Plane Straining

Supreme mechanical performance and tribological properties render graphene a promising candidate as a surface friction modifier. Recently, it has been demonstrated that applying in-plane strain can effectively tune friction of suspended graphene in a reversible manner. However, since graphene is deposited on solid surfaces in most tribological applications, whether such operation will result in a similar modulation effect becomes a critical question to be answered. Herein, by depositing graphene onto a stretchable substrate, the frictional characteristics of supported graphene under a wide range of strain are examined with an in situ tensile loading platform. The experimental results show that friction of supported graphene decreases with increasing graphene strain, similar to the suspended system. However, depending on the adherence state of the graphene/substrate interface, the system exhibits two distinct friction regimes with significantly different strain dependences. Assisted by detailed atomic force microscopy imaging, we attribute the unique behavior to the transition between two friction modulation modes, i.e., contact-quality-dominated friction and puckering-dominated friction. This work provides a more comprehensive view of the influence of strain on surface friction of graphene, which is beneficial for active modulation of graphene friction through strain engineering.

The strain-dependent interfacial thermal resistance at graphene-silicon interface under various deformation conditions

Evaluating the interfacial thermal resistance is a key step in modeling the heat transport in graphene-based thermal interface materials. However, it is always a challenge to accurately estimate such quantity under residual strains which are inevitably induced during fabrication and application of such materials. In this study, the strain effect on the interfacial thermal resistance between graphene and silicon is systematically investigated through molecular dynamics computation. Tensile strain and compressive strain, parallel and perpendicular to the interfaces, are respectively applied to two forms of heterostructures (i.e., supported graphene on Si substrate and embedded graphene between two Si substrates). The results show that the interfacial thermal resistance gradually increases as the tensile strain in graphene increases in the range of 0 to 0.1, mainly caused by the decrease in the overlap of vibrational density of states between graphene and silicon. However, there is no such monotonic increase in the interfacial thermal resistance with the overall tensile strain applied on both graphene and silicon. In addition, as the compressive strain increases, the interfacial thermal resistances under various residual tensile strains gradually decrease and converge. By analysing the contribution of graphene motion in separate directions, we found out that the out-of-plane motion dominates the interfacial heat conduction.