Free-standing Monolayer Graphene Research Articles

Restoring the electronic properties of epitaxial graphene on SiC substrate by Ar intercalation

Thermal decomposition of 6H–SiC(0001) under an argon atmosphere is a promising technique to fabricate large-scale epitaxial graphene monolayers for electronic applications. In this manuscript, we report on the intercalation of argon, hydrogen and oxygen atoms underneath a graphene buffer layer (BL) on SiC(0001), involving bond formation between the intercalated agent and Si atoms. Density Functional Theory calculations were performed to investigate the atomic structure, stability and electronic properties of quasi free-standing monolayer graphene and bilayer graphene, accomplished by intercalation. According to our results, as the Ar intercalated atoms saturate the silicon dangling bonds in the upper SiC surface, the BL is completely detached from the substrate and transformed into free-standing graphene. Ar intercalation would be hindered by the concurrent and thermodynamically more spontaneous adsorption of Ar atop the graphene layers, resulting in a pronounced decrease of the process efficiency. High Ar-coverage regimes and low kinetic barriers for Ar intercalation might shift the intercalation/adsorption balance. These factors as well as strategies such as ion implantation could be explored to identify more favorable conditions to experimentally test Ar intercalation as a simple method to achieve control of the electronic properties of the BL and help preserve the unique characteristics of graphene.

Mechanical response of monolayer graphene via a multi-probe approach

Push-to-pull (PTP) testing is employed to probe the uniaxial tensile response of freestanding monolayer graphene. Various analytical approaches are employed to estimate the elastic modulus of end-clamped graphene samples, combining in-situ Raman spectroscopy and scanning electronic microscope (SEM) measurements. The utilization of spatially resolved Raman-derived strains for assessing the elastic properties of monolayer graphene leads to results consistent with previous experimental and theoretical values of the elastic modulus (approximately 1 TPa). Molecular dynamics (MD) simulations of (pristine and defective) freestanding graphene sheets uniaxially loaded under varying clamping conditions are performed to support the experimental observations. The computational results indicate that the mechanical responses of the sheets are affected by the type, the spatial profile, and the heterogeneity of the clamping. When uniaxial pulling of end-clamped graphene is applied by a substrate adhering to the graphene sheet through van der Waals forces (as in PTP testing), the elastic modulus may be highly underestimated due to often inhomogeneous stress distribution and slippage processes. The MD simulations predict that the elastic modulus of pristine monolayer graphene is approximately 1 TPa, whereas its fracture strength can reach values of up to 110 GPa. Overall, this study underscores the limitations of traditional analyses of PTP experiments (utilizing indentation readouts and SEM imaging) and proposes new potential avenues (involving Raman measurements) for future research on the elastic properties of 2D materials.

Spatial Precision Tailoring the Catalytic Activity of Graphene Monolayers for Designing Janus Swimmers.

Graphene monolayers have interesting applications in many fields due to their intrinsic physicochemical properties, especially when they can be postmodified with high precision. Herein, we describe the highly site-selective functionalization of freestanding graphene monolayers with platinum (Pt) clusters by bipolar electrochemistry. The deposition of such metal spots leads to catalytically active hybrid two-dimensional (2D) nanomaterials. Their catalytic functionality is illustrated by the spatially controlled decomposition of hydrogen peroxide, inducing motion at the water/air interface due to oxygen bubble evolution. A series of such 2D Janus structures with Pt deposition at predefined positions (corners and edges) is studied with respect to the generation of autonomous motion. The type and speed of motion can be fine-tuned by controlling the deposition time and location of the Pt clusters. These proof-of-principle experiments indicate that this type of hybrid 2D object opens up interesting perspectives in terms of applications, such as environmental detection or remediation.

Structural identification of graphene films and nanoislands on 6H-SiC(0001) by direct height measurement

By combining non-contact atomic force microscopy (nc-AFM) and Kelvin probe microscopy (KPFM) in ultra high vacuum environment (UHV), we directly measure the height and work function of graphene monolayer on the Si-face of 6H-SiC(0001) with a precision that allows us to differentiate three different types of graphene structures : zero layer graphene (ZLG), Quasi free-standing monolayer graphene (QFMLG) and bilayer graphene (BLG). The height and work function of ZLG are 2.62 ± 0.22 Å and 4.42 ± 0.05 eV respectively, when they are 4.09 ± 0.11 Å and 4.63 ± 0.05 eV for QFMLG. The work function is 4.83 ± 0.05 eV for the BLG. Unlike any other available technique, the local nc-AFM/KPFM dual probe makes it possible to directly identify the nature of nanometer-sized graphene islands that constitute the early nuclei of graphene monolayer grown on 6H-SiC(0001) by chemical vapor deposition.

Direct milling of 25 nm in diameter gold and graphene nanogears by a focused He+ ion beam

The direct focused helium ion beam milling of solid-state nanogears down to 25 nm in diameter is presented. Sapphire is the supporting surface to release the heat created by this milling process. For this free of resist process, the He+ dosing was first calibrated to limit the lateral fusion in the deposited nanomaterial during the sculpturing of the teeth. 25 nm in diameter solid-state nanogears were fabricated in a 20 nm in thickness Au nanomaterial, in a graphene multilayer and in a monolayer having a van der Waals thickness compatible with a single molecule-gears for transmission of rotation. Optimizing again the He+ dosing for a transmission electron microscope grid, free standing graphene monolayer nanogears were also sculptured down to 40 nm in diameter.

The controllable electronic characteristics and Schottky barrier of graphene/GaP heterostructure via interlayer coupling and in-plane strain

Establishing the desired heterostructures by assembling suitable semiconductor materials has shown significant potential for applications in next-generation micro-nano electronic devices. In this present contribution, we demonstrate the geometric structure and electronic properties of graphene/GaP heterostructure by first-principles calculations. It is found that the heterostructure is characterized by weak interlayer coupling accompanying the stable layer spacing of 3.40 Å and binding energy of −39.35 meV, meaning that the interlayer is dominated by van der Waals (vdW) force. The electronic band structure of free-standing graphene and GaP monolayers are preserved well. Meanwhile, a tiny bandgap of approximatively 20 meV at the Dirac point of graphene is opened, which is attributed to the breakdown of sublattice symmetry. In the ground state, the Schottky contact of p-type is present with the n-type and p-type SBH of 1.71 eV and 0.10 eV, respectively, which can be effectively induced by imposing interlayer coupling as well as in-plane strains. Especially, the transition of p-Schottky contact to p-Ohmic contact occurs when the layer spacing decreases to 3.20 Å or the strain increases to +2 %. These theoretical results may offer potential guiding principle in future electronic devices.

Indirect measurement of the carbon adatom migration barrier on graphene

Although surface diffusion is critical for many physical and chemical processes, including the epitaxial growth of crystals and heterogeneous catalysis, it is particularly challenging to directly study. Here, we estimate the carbon adatom migration barrier on freestanding monolayer graphene by quantifying its temperature-dependent electron knock-on damage. Due to the fast healing of vacancies by diffusing adatoms, the damage rate decreases with increasing temperature. By analyzing the observed damage rates at 300–1073 K using a model describing our finite scanning probe, we find a barrier of (0.33 ± 0.03) eV.

Indirect Measurement of the Carbon Adatom Migration Barrier on Graphene

Although surface diffusion is critical for many physical and chemical processes, including the epitaxial growth of crystals and heterogeneous catalysis, it is particularly challenging to directly study. Here, we estimate the carbon adatom migration barrier on freestanding monolayer graphene by quantifying its temperature-dependent electron knock-on damage. Due to the fast healing of vacancies by diffusing adatoms, the damage rate decreases with increasing temperature. By analyzing the observed damage rates at 300-1073 K using a model describing our finite scanning probe, we find a barrier of (0.33 \pm 0.03) eV.

Van der Waals Epitaxy Growth of Bi2Se3 on a Freestanding Monolayer Graphene Membrane: Implications for Layered Materials and Heterostructures

Research on two-dimensional materials has expanded over the past two decades to become a central theme in condensed matter research today. Significant advances have been made in the synthesis and subsequent reassembly of these materials using mechanical methods into a vast array of hybrid structures with novel properties and ever-increasing potential applications. The key hurdles in realizing this potential are the challenges in controlling the atomic structure of these layered hybrid materials and the difficulties in harnessing their unique functionality with existing semiconductor nanofabrication techniques. Here we report on high-quality van der Waals epitaxial growth and characterization of a layered topological insulator on freestanding monolayer graphene transferred to different mechanical supports. This templated synthesis approach enables direct interrogation of interfacial atomic structure of these as-grown hybrid structures and opens a route towards creating device structures with more traditional semiconductor nanofabrication techniques.

Response of free-standing graphene monolayer exposed to ultrashort intense XUV pulse from free-electron laser.

The response of a free-standing graphene monolayer exposed to a few tens of femtoseconds long extreme ultraviolet (XUV) pulse was studied theoretically in order to analyze and compare contributions of various mechanisms to the graphene damage, understood here as a global atomic disintegration. Our simulation results indicate that nonthermal disintegration of the atomic structure is the predominant damage mechanism for a free-standing graphene layer. Only at high absorbed doses, charge-induced disintegration of the graphene structure prevails. We also demonstrate that the progressing damage can be probed by femtosecond optical pulses in the soft UV regime (4eV photon energy). The achieved quantitative understanding of the damage mechanisms may enable a better control of graphene-based devices when they are exposed to x-ray radiation, as well as an efficient processing of graphene layers with ultrashort intense XUV pulses.

Electronic and optical properties of PTCDI adsorbed graphene heterostructure: A first principles study

Density functional theory (DFT) computations were performed to study the change in electronic and optical properties of free-standing graphene monolayer by the adsorption of PTCDI molecule. It is revealed that the K point symmetry of the graphene is broken due to this molecular adsorption. As a consequence, the semi-metallic graphene monolayer turns into a conventional semiconductor at room temperature with conduction and valence bands separated by a gap of ~ 0.21 eV. The LUMO state of PTCDI molecule acts as a localized state in the electronic structure of the graphene/PTCDI system and induces a bandgap in the semi-metallic band structure of graphene. From our calculations, it is also explored that the localized gap state induced conduction gap is created due to the charge transfer mediated molecular sites specific van der waals (vdW) interaction across the graphene/PTCDI interface. The most interesting findings in this system are that the electrons follow the rules of the ordinary two dimensional (2D) materials to participate in the charge conduction process. The effective masses for holes as well as electrons are found to marginally change due to the PTCDI adsorption. Furthermore, the study of their optical properties for both directions of polarization exposes that the optical response of the semiconducting graphene/PTCDI system is highly anisotropic in nature. It is exposed that the sample has birefringence characteristics also. Moreover, the adsorption of a PTCDI molecule leads to notable modifications in the dielectric constant, reflectivity, absorption coefficients, and electron energy loss spectra of pristine graphene. The findings in this study may pave the way towards the development of new graphene-based molecular electronic and optoelectronic devices.

Ultrahigh adhesion between carbon nanotube and free-standing monolayer graphene

Using a self-prepared individual carbon nanotube (CNT) mechanical force sensor, we measured the adhesion between CNT and free-standing monolayer graphene and other bulk substrates. All the measurements were made by using the same CNT force sensor under the same conditions, such as moving speed, observation angle, temperature, and vacuum pressure, confirming the reliability and accuracy of experimental data. The adhesion at contact is proportional to the deformation of the curved CNT, which can be directly measured in a scanning electron microscope. It was found that the deformation of CNT was the largest on the suspended graphene, showing that the suspended graphene has the largest adhesion on CNT. This unusually high adhesion on suspended monolayer graphene is related to the low bending stiffness and extreme flexibility of this atomically thin layer. The main contribution of this work is to demonstrate the unusually high adhesion on suspended graphene experimentally. More advanced modeling needs complicated molecular dynamics simulation and surface energy computation in our future work.

Preparation of large area graphene on SiC(0 0 0 −1) by moderate vacuum technology

Owing to its compatibility with existing Si technology, epitaxial graphene on SiC is expected to replace III-V materials in future. This paper proposes an available approach to overcome the low quality and the high cost properties of traditional epitaxial growth access. The epitaxial graphene grows in low vacuum environment, using Ar as the filling gas. The epitaxial graphene was investigated with optical microscope, atomic force microscopy and Raman spectroscopy. It is found that large area graphene cover on SiC C-face conformally. The Raman results show that the materials obtained in lower vacuum environment are ordered and put free standing monolayer graphene.

OH molecule-involved formation of point defects in monolayer graphene

Point defects in freestanding graphene monolayers such as monovacancies (MVs) and divacancies have been investigated at atomic scale with aberration-corrected transmission electron microscopy and theoretical calculations. In general, these defects can be formed simply by the absence of individual carbon atoms and carbon bond reconstructions in the graphene lattice under electron and ion irradiation. However, in this study, we found that oxygen and hydrogen atoms can be involved in the formation of these point defects caused by the simultaneous detachment of oxygen–carbon atoms. Here we report the effect of the oxygen and hydrogen atoms on the graphene surface forming the point defects under electron beam irradiation, and their role of stabilizing other MVs when composed of 13–5 ring pairs. In addition, theoretical analysis using density functional theory calculations demonstrates that the participating atoms can form the point defects in the intermediate states and stabilize 13–5 ring pairs under electron beam irradiation.

Elastic straining of free-standing monolayer graphene

The sp2 nature of graphene endows the hexagonal lattice with very high theoretical stiffness, strength and resilience, all well-documented. However, the ultimate stretchability of graphene has not yet been demonstrated due to the difficulties in experimental design. Here, directly performing in situ tensile tests in a scanning electron microscope after developing a protocol for sample transfer, shaping and straining, we report the elastic properties and stretchability of free-standing single-crystalline monolayer graphene grown by chemical vapor deposition. The measured Young’s modulus is close to 1 TPa, aligning well with the theoretical value, while the representative engineering tensile strength reaches ~50-60 GPa with sample-wide elastic strain up to ~6%. Our findings demonstrate that single-crystalline monolayer graphene can indeed display near ideal mechanical performance, even in a large area with edge defects, as well as resilience and mechanical robustness that allows for flexible electronics and mechatronics applications.

Position and momentum mapping of vibrations in graphene nanostructures.

Propagating atomic vibrational waves-phonons-determine important thermal, mechanical, optoelectronic and transport characteristics of materials. Thus a knowledge of phonon dispersion (that is, the dependence of vibrational energy on momentum) is a key part of our understanding and optimization of a material's behaviour. However, the phonon dispersion of a free-standing monolayer of a two-dimensional material such as graphene, and its local variations, have remained elusive for the past decade because of the experimental limitations of vibrational spectroscopy. Even though electron energyloss spectroscopy (EELS) in transmission has recently been shown to probe local vibrational charge responses1-4, such studies are still limited by momentum space integration due to the focused beam geometry; they are also restricted to polar materials such as boron nitride or oxides1-4, in which huge signals induced by strong dipole moments are present. On the other hand, measurements on graphene performed by inelastic X-ray (neutron) scattering spectroscopy5-7 or EELS in reflection8,9 do not have any spatial resolution and require large microcrystals. Here we provide a new pathway to determine phonon dispersions down to the scale of an individual free-standing graphene monolayer by mapping the distinct vibrational modes for a large momentum transfer. The measured scattering intensities are accurately reproduced and interpreted with density functional perturbation theory10. Additionally, a nanometre-scale mapping of selected momentum-resolved vibrational modes using graphene nanoribbon structures has enabled us to spatially disentangle bulk, edge and surface vibrations. Our results are a proof-of-principle demonstration of the feasibility of studying local vibrational modes in two-dimensional monolayer materials at the nanometre scale.

Tuning the n-type contact of graphene on Janus MoSSe monolayer by strain and electric field

The interfacial contact between graphene and Janus MoSSe monolayer was investigated using first-principles calculations. Results show that the intrinsic electronic properties of the freestanding graphene and Janus MoSSe monolayer are well preserved in the van der Waals (vdW) graphene/MoSSe heterostructures. An n-type Schottky contact is formed in the interface of graphene and Janus MoSSe monolayer with a Schottky barrier height (SBH) of 0.07 eV. The SBH can be tuned by the external electric field and in-plane mechanical strain. SBH decreases with the increase of positive electric field and tensile strain. The n-type Schottky contact transfers to the Ohmic contact when the applied electric field is larger than 0.4 V/Å or the tensile strain exceeds 8%. These results can provide helpful guide in the design of nanoscale devices based on the vdW heterostructures.

Broadband light-trapping enhancement of graphene absorptivity

Low optical absorptivity of graphene limits the quantum efficiency of graphene-based optoelectronic devices. We demonstrate a broadband enhancement of absorptivity in an architecture of graphene intercalation inside a simple-cubic woodpile photonic crystal. Significant light-trapping effect is achieved due to spectrally broad and dense resonances in the architecture, especially modes propagating nearly parallel to the interface between the photonic crystal and the background material. We demonstrate an overall absorptivity $\ensuremath{\sim}10$ to 17 times larger than the free-standing bare graphene monolayer with bandwidth to central frequency ratio 0.3, and $\ensuremath{\sim}5$ to 11 times larger with bandwidth to central frequency ratio 1.33. A distributed intercalation of three graphene monolayers proves a $\ensuremath{\sim}70%$ overall absorptivity with bandwidth to central frequency ratio 0.3.

Nanographene growing on free-standing monolayer graphene

Contamination on graphene is a critical problem that has limited the applications of graphene despite its innate versatility. However, the structures of such contamination remain unclear. In this study, we investigated the structure of a contaminated region growing on free-standing monolayer graphene in situ at room temperature using an aberration-corrected scanning transmission electron microscope. Energy-dispersive X-ray spectroscopy showed that carbon atoms were the dominant element of the growing contamination. We quantitatively confirmed that the growing contamination consisted of several layers of nanoscale graphene flakes on pristine single-layer graphene. Electron energy-loss spectroscopy indicated that the hexagonal carbon lattice deforms considerably from the pristine one at the higher layers. Analysis of the three-dimensional atomic structure revealed that the atoms in the third layer were the suppliers of the second layer at the step edge. In addition, we found that the lattice constant of pristine graphene in the region of the first layer was larger than that of the bulk graphite crystals, and that it gradually decreased as the second layer grew on the first layer. The findings of this paper will be helpful not only for the control of nanographene but also for a basic understanding of the intrinsic features of graphene.

Critical assessment of charge transfer estimates in non-covalent graphene doping

Non-covalent doping by pure charge transfer complexes is one possible solution to tune at low-cost electronic properties of carbon-based nanostructures, more specifically to enhance their conductivity. Here, we present a thorough density functional theory-based study of charge transfer estimates, by comparing available integration/partitioning scheme of the electronic density in periodic boundary conditions, as well as the influence of the exchange-correlation term, the cornerstone of DFT by testing various exchange-correlation functionals. Our test case is made of a freestanding graphene monolayer in interaction with two prototypical donor/acceptor molecules: TTF and TCNE. These results illustrate the role played by the exact exchange in the description of charge transfer processes, as well as the difference between the density-based and wavefunction-based partitioning schemes used in this study. When using hybrid functionals, charge transfer are usually smaller than when using standard generalized gradient approximations, especially for the donor molecule. In terms of electronic density partitioning schemes, both strategies provide quite similar charge transfers; however, each intra-molecular decomposition presents very distinct features, making the discussion of atomic charge reorganization on the electron/donor molecule highly dependent on the selected partitioning scheme.