Graphene Buffer Layer Research Articles

Confirmation of the Growth Mechanism of the Buffer Layer in Epitaxial Graphene on SiC

This study substantiates the epigraphene formation theory on SiC, presenting it as freestanding graphene during thermal decomposition epitaxy. It was found that cool down process is responsible for the formation of the graphene buffer layer. Additionally the capping capabilities of the buffer layer have been evaluated using Raman spectroscopy and AFM measurements.

Graphene-Molecule Hybridization at a Ferromagnetic Interface.

The introduction of a graphene (Gr) buffer layer between a ferromagnetic substrate and a metallorganic molecule is known to mediate the magnetic coupling between them, an effect attributed to a weak hybridization between graphene and molecule. In this paper, we present experimental evidence of this effect through a detailed investigation of the frontier electronic properties of iron phthalocyanine deposited on cobalt-supported graphene. Despite being physisorbed, the molecular adsorption on Gr/Co induces a sizeable charge transfer from graphene to the molecular macrocycle leading to the partial occupation of the LUMO and the appearance of an energetically localized hybrid state, which can be attributed to the overlap between the graphene pz state and the molecular macrocycle. Graphene is not inert either; the adsorption of the molecule induces doping and alters the Fermi velocity of both the hybrid minicone state and the Dirac cone. Similar effects are observed when the molecular periphery is decorated with fluorine atoms, known for their electron-withdrawing properties, with minimal changes in the energy alignment.

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.

First-principles analysis of intercalated Pb structures under buffer-layer graphene on SiC(0001): Pb(111)-, plumbene-, and amorphous-like Pb layers

First-principles analysis of intercalated Pb structures under buffer-layer graphene on SiC(0001): Pb(111)-, plumbene-, and amorphous-like Pb layers

Interface and Morphology Engineered Amorphous Si for Ultrafast Electrochemical Lithium Storage.

Ultrafast high-capacity lithium-ion batteries are extremely desirable for portable electronic devices, where Si is the most promising alternative to the conventional graphite anode due to its very high theoretical capacity. However, the low electronic conductivity and poor Li-diffusivity limit its rate capability. Moreover, high volume expansion/contraction upon Li-intake/uptake causes severe pulverization of the electrode, leading to drastic capacity fading. Here, interface and morphology-engineered amorphous Si matrix is being reported utilizing a few-layer vertical graphene (VG) buffer layer to retain high capacity at both slow and fast (dis)charging rates. The flexible mechanical support of VG due to the van-der-Waalsinteraction between the graphene layers, the weak adhesion between Si and graphene, and the highly porous geometry mitigated stress, while the three-dimensional mass loading enhanced specific capacity. Additionally, the high electronic conductivity of VG boosted rate-capability, resulting in a reversible gravimetric capacity of ≈1270mAhg-1 (areal capacity of ≈37µAhcm-2) even after 100 cycles at an ultrafast cycling rate of 20C, which provides a fascinating way for conductivity and stress management to obtain high-performance storage devices.

Graphene coating-modified LiCoO2 films as high-performance cathode material in quasi-solid-state thin-film lithium batteries

LiCoO2 (LCO) films were successfully prepared using a radio frequency magnetron sputtering method, and the interface between the LCO cathode films and Li1.5Al0.5Ge1.5(PO4)3 (LAGP) solid electrolyte was modified by a graphene (G) buffer layer. The composition and morphology of the thin-film samples were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscopy. The LCO thin film annealed at 600 °C had excellent charge transport properties, respectively delivering initial charge and discharge specific capacities of 162.7 mAh·g−1 and 124.5 mAh·g−1 at 0.1C in a half-cell structure. Relaxation time distribution reconstruction impedance analysis and rate characteristic analysis of an LCO/G/LAGP/G/LCO symmetric cell demonstrated that the flexible graphene buffer layer effectively improved the contact impedance of the interface and structure stability of the active materials. After cycling for 1000 h at current densities of 0.02 mA·cm−2 and 0.5 mA·cm−2, the LCO/G/LAGP/G/LCO symmetric cell still maintained stable polarization voltages of 0.05 V and 0.7 V, respectively. In addition, an assembled LCO/G/LAGP/Li quasi-solid-state film lithium battery presented an initial discharge specific capacity of 109.53 mAh·g−1 at 0.1C, with a capacity retention rate of 71.86 % after 100 cycles.

Atomic-scale manipulation of buried graphene–silicon carbide interface by local electric field

Precision of scanning tunneling microscopy (STM) enables control of matter at scales of single atoms. However, transition from atomic-scale manipulation strategies to practical devices encounters fundamental problems in protection of the designer structures formed atop the surface. In this context, STM manipulation of subsurface structures on technologically relevant materials is encouraging. Here, we propose a material platform and protocols for precise manipulation of a buried graphene interface. We show that an electric field from the STM tip reversibly controls breaking and restoring of covalent bonds between the graphene buffer layer and the SiC substrate. The process involves charge redistribution at the atomically sharp interface plane under the epitaxial graphene layer(s). This buried manipulation platform is laterally defined by unit cells from the corresponding (6×6)SiC moiré lattice of the epitaxial graphene. Local and reversible electric-field-induced patterning of graphene heterostructures from the bottom interface creates an alternative architecture concept for their applications.

(Invited) Enabling High Quality Dielectric Passivation on Monolayer WS2 Using a Sacrificial Graphene Oxide Template

Two-dimensional transition metal dichalcogenides (2D TMDC’s) hold a wide variety of applications, among which microelectronic devices. 2D TMDC’s are promising alternatives for today’s silicon-based technology but suffer from various integration challenges. Among others, direct dielectric growth on the 2D surface occurs poorly due to their self-passivated surfaces resulting in island-like growth initiated at defect sites. This work focuses on enabling uniform high-k dielectric nucleation and growth on 2D TMDC’s via a sacrificial, graphene oxide-based buffer layer. Essentially, the role of the sacrificial graphene buffer layer is twofold: 1) serve as a passivation layer, protecting the underlying 2D TMDC (i.e., WS2) during processing and 2) act as a nucleation layer, enabling uniform atomic layer deposition (ALD) high-k dielectric (i.e,. HfO2) growth. A graphene layer is transferred on monolayer WS2, after which polymeric transfer residues are cleaned via a combination of wet treatments and a dry hydrogen downstream plasma exposure. The cleaned graphene is functionalized via a dry UV/O3 oxidative treatment, which enriches its basal plane with carbon-oxygen single bond functionalities by sacrificing its π-network. This way, the graphene carbon lattice itself remains intact and retains its passivation properties. Moreover, a substrate dependency is observed where graphene transferred on WS2 requires longer UV/O3 exposure to ensure functionalization compared to a SiO2 substrate and can be explained by UV-light induced, ultrafast charge transfer between the graphene and WS2 monolayer. The carbon-oxygen groups formed on graphene’s basal plane act as nucleation sites during a subsequent HfO2 ALD process. A constant roughness is noted for the HfO2 film grown on the graphene oxide capped WS2, indicating uniform material deposition even in the early stages of the ALD process. This is in sharp contrast when HfO2 is directly grown on the WS2 monolayer, whose roughness stabilizes only after many ALD cycles (up to 256), demonstrating the poor, island-like growth mechanism. In addition, a similar hafnium areal density is measured for the graphene oxide capped WS2 compared bare SiO2, which was selected to represent a well-functionalized surface and thus confirming the efficiency of the graphene oxide-based nucleation layer. For devices incorporating the graphene oxide buffer layer concept, a crucial question is to what extent the graphene oxide layer gives rise to leakage currents by providing a conductive path between source and drain. Electrical evaluation of a GFET by means of I-V measurements reveal a significant shift in Dirac point towards positive gate voltage for transferred graphene, which is typically associated with p-type doping originating from the polymeric transfer residues still present on the surface. The Dirac point shifts towards 0-point voltage after final hydrogen downstream plasma cleaning, confirming the effectiveness of the established cleaning protocol. Moreover, Graphene’s conductivity is suppressed after UV/O3 to the nA range, as caused by the formation of carbon-oxygen functionalities and associated sacrifice of its π-network. The residual current may be further suppressed by extending the UV/O3 treatment, but nevertheless demonstrates that the graphene oxide buffer layer will not give rise to significant leakage currents.

Raman fingerprint of the graphene buffer layer grown on the Si‐terminated face of 4H‐SiC(0001): Experiment and theory

AbstractIn this work, we present the results of measurements of the Raman spectrum of the √3x√3R30° reconstruction of graphene grown on 4H‐SiC(0001), the so‐called buffer layer. The extracted Raman spectrum of the buffer layer shows bands, different from those of graphene, which can be attributed to the interaction of the buffer layer with the SiC substrate. In particular, in the high‐wavenumber region, at least three bands are observed in the wavenumber regions 1,350–1,420, 1,470–1,490 and 1,520–1,570 cm−1. The assignment of the buffer layer bands is supported here by tight‐binding simulations of the one‐phonon density of states for structures with a sufficiently large number of Si‐C bilayers for reaching convergence. The converged phonon density of states is found to be in semi‐quantitative agreement with the latter two bands, and therefore, the tight‐binding predictions of the lattice dynamics of the structure can be used for their assignment to buffer layer vibrations. Namely, the Raman band at about 1,550 cm−1 can be assigned to modified in‐plane optical phonon branches of graphene, while the Raman band at about 1,490 cm−1 can be assigned to modified folded parts of these branches inside the Brillouin zone of the buffer layer and can be considered as a Raman fingerprint of the buffer layer.

Preparation of VO2/graphene/SiC film by water vapor oxidation

Abstract Vanadium dioxide (VO2) has attracted extensive attention due to the specific metal-insulator phase transition as well as the wide device applications. The practical performance of VO2-based device strongly depends on the quality of VO2, since the higher quality of VO2 film always shows much more pronounced phase transition behavior. Thus, the preparation of high quality VO2 film is essential and highly desirable. In this work, we have prepared high-quality VO2 film on SiC substrate by water vapor oxidation with graphene (G) buffer layer, which showed excellent phase transformation properties. Compared with the VO2/SiC sample without G buffer layer, the VO2/G/SiC films show the resistance changes up to four-orders of magnitude across the phase transition boundary and superior optoelectronic properties, which indicates the significant role of G layer in the film growth process. The current study not only provides an economical and feasible method for VO2/G/SiC thin film preparation with high quality, but also supply some clues for the application of G-based VO2 devices in the future.

Fermi level depinning via insertion of a graphene buffer layer at the gold–2D tin monoxide contact

Two-dimensional (2D) tin monoxide (SnO) has attracted much attention owing to its distinctive electronic and optical properties, which render itself suitable as a channel material in field effect transistors (FETs). However, upon contact with metals for such applications, the Fermi level pinning effect may occur, where states are induced in its band gap by the metal, hindering its intrinsic semiconducting properties. We propose the insertion of graphene at the contact interface to alleviate the metal-induced gap states. By using gold (Au) as the electrode material and monolayer SnO (mSnO) as the channel material, the geometry, bonding strength, charge transfer and tunnel barriers of charges, and electronic properties including the work function, band structure, density of states, and Schottky barriers are thoroughly investigated using first-principles calculations for the structures with and without graphene to reveal the contact behaviours and Fermi level depinning mechanism. It has been demonstrated that strong covalent bonding is formed between gold and mSnO, while the graphene interlayer forms weak van der Waals interaction with both materials, which minimises the perturbance to the band structure of mSnO. The effects of out-of-plane compression are also analysed to assess the performance of the contact under mechanical deformation, and a feasible fabrication route for the heterostructure with graphene is proposed. This work systematically explores the properties of the Au–mSnO contact for applications in FETs and provides thorough guidance for future exploitation of 2D materials in various electronic applications and for selection of buffer layers to improve metal–semiconductor contact.

Renewable Biomass Flax-Derived Amorphous Carbon@SnO2 with a Graphene Buffer Layer for Enhanced Li Storage Performance

Renewable Biomass Flax-Derived Amorphous Carbon@SnO₂ with a Graphene Buffer Layer for Enhanced Li Storage Performance

Degeneracy in Intercalated Pb Phases under Buffer-Layer Graphene on SiC(0001) and Diffuse Moiré Spots in Surface Diffraction.

First-principles density functional theory (DFT) is used to analyze the stability of Pb intercalated phases under buffer layer graphene on SiC(0001) as a function of the supercell size, Pb coverage, and degree of Pb ordering. By comparing the chemical potentials of such two-dimensional Pb structures, we find that there is a family of structurally distinct thermodynamically preferred Pb subsurface configurations with minute stability differences. These differences are comparable to the thermal energies at about 450 °C, where the Pb intercalated phases are grown. High-resolution surface-diffraction experiments using Spot Profile Analysis Low-Energy Electron Diffraction (SPA-LEED) confirm this high degree of degeneracy of the Pb intercalated phases from broad, low-intensity moiré spots observed exclusively from intercalated Pb. The low intensity of the moiré spots implies the coexistence of structurally different subsurface Pb phases.

Domain Boundary Formation Within an Intercalated Pb Monolayer Featuring Charge‐Neutral Epitaxial Graphene

AbstractThe synthesis of new graphene‐based quantum materials by intercalation is an auspicious approach. However, an accompanying proximity coupling depends crucially on the structural details of the new heterostructure. It is studied in detail the Pb monolayer structure after intercalation into the graphene buffer layer on the SiC(0001) interface by means of photoelectron spectroscopy, x‐ray standing waves, and scanning tunneling microscopy. A coherent fraction close to unity proves the formation of a flat Pb monolayer on the SiC surface. An interlayer distance of 3.67 Å to the suspended graphene underlines the formation of a truly van der Waals heterostructure. The 2D Pb layer reveals a quasi ten‐fold periodicity due to the formation of a grain boundary network, ensuring the saturation of the Si surface bonds. Moreover, the densely‐packed Pb layer also efficiently minimizes the doping influence by the SiC substrate, both from the surface dangling bonds and the SiC surface polarization, giving rise to charge‐neutral monolayer graphene. The observation of a long‐ranged () reconstruction on the graphene lattice at tunneling conditions close to Fermi energy is most likely a result of a nesting condition to be perfectly fulfilled.

Polarized Raman scattering spectroscopy of array of embedded graphene ribbons grown on 4H-SiC(0001)

An array of embedded graphene ribbons, whose edges connect to a buffer layer, can be grown on 4H-SiC(0001). The intensity of the Raman D peak of the armchair edge of the ribbon shows the same polarization dependence as that of the non-connected armchair edge of graphene. Considering the Raman scattering process of the D peak at the armchair edge, this polarization dependence indicates that electrons and holes in the embedded graphene by incident photons are scattered back at the boundary of the embedded graphene ribbon and buffer layer. These results show that polarized Raman scattering spectroscopy is useful for investigating the edge structure of embedded graphene.

Improved epitaxy of CsI(Tl) film on Si substrate buffered by graphene for X-ray detection

The growth of the CsI(Tl) scintillators deposited on glass substrates with proper wavelength matching the mainstream semiconductor circuits is crucial for X-ray imaging industry. But the coupling loss between the scintillators and silicon based photo sensors has limited the performance. So fabricating CsI(Tl) films on monocrystalline silicon is greatly important for next-generation high performance X-ray image devices. However, the lattice and thermal mismatch between CsI(Tl) and Si always deteriorates the morphology and structure, and thus degrades the property of the CsI(Tl) scintillation converters. Here, by utilizing crystalline graphene(Gr) as a buffer layer, the van der Waals epitaxial growth of CsI(Tl) film on Si(111) substrate is reported and its advanced applications are demonstrated. The introduced graphene buffer layer extremely frees the stress between CsI(Tl) and Si substrate. Therefore, the CsI(Tl) films show perfect micro-columnar and crystalline structures, and then, exhibit much higher light output and signal to noise ratio, and better modulation transfer functions and a bar pattern X-ray image compared to that on bare Si(111) substrate. This facile method not only lays the foundation for the epitaxy of the alkali halide crystals, including CsI(Tl), onto Si-based detectors and its applications in radiation detection, but also may broaden a new perspective of deposition of CsI(Tl) scintillators into flexible substrates.

Joint Intercalation of Ultrathin Fe and Co Films under a Graphene Buffer Layer on a SiC(0001) Single Crystal

The joint intercalation of Co and Fe atoms under a graphene buffer layer synthesized on a SiC(0001) single crystal has been studied. Intercalation has been performed by means of the alternating deposition of ultrathin Fe and Co metal films on the substrate heated to 450°C with the subsequent heating to 600°C in 15 min. It has been shown that Co and Fe atoms under these conditions are intercalated under graphene, forming compounds with silicon and with each other. The existence of a magnetic order in the system up to room temperature has been demonstrated using a superconducting quantum interferometer. A possible stoichiometry of the formed alloys has been analyzed using data on the shape and magnitude of hysteresis loops. In addition, it has been found that Fe and Co in the system exposed to the atmosphere are not oxidized. Thus, graphene protects the formed system. This study makes contribution to the investigation of graphene in contact with magnetic metals and promotes its application in spintronic and nanoelectronic devices.

All-Transfer Electrode Interface Engineering Toward Harsh-Environment-Resistant MoS2 Field-Effect Transistors.

Nanoscale electronic devices that can work in harsh environments are in high demand for wearable, automotive, and aerospace electronics. Clean and defect-free interfaces are of vital importance for building nanoscale harsh-environment-resistant devices. However, current nanoscale devices are subject to failure in these environments, especially at defective electrode-channel interfaces. Here, harsh-environment-resistant MoS2 transistors are developed by engineering electrode-channel interfaces with an all-transfer of van der Waals electrodes. The delivered defect-free, graphene-buffered electrodes keep the electrode-channel interfaces intact and robust. As a result, the as-fabricated MoS2 devices have reduced Schottky barrier heights, leading to a very large on-state current and high carrier mobility. More importantly, the defect-free, hydrophobic graphene buffer layer prevents metal diffusion from the electrodes to MoS2 and the intercalation of water molecules at the electrode-MoS2 interfaces. This enables high resistances of MoS2 devices with all-transfer electrodes to various harsh environments, including humid, oxidizing, and high-temperature environments, surpassing the devices with other kinds of electrodes. The work deepens the understanding of the roles of electrode-channel interfaces in nanoscale devices and provides a promising interface engineering strategy to build nanoscale harsh-environment-resistant devices.

Thermodynamics and kinetics of Pb intercalation under graphene on SiC(0001)

SiC-supported graphene intercalated by a two-dimensional Pb monolayer can provide an appealing platform for spintronic applications. Such a monolayer structure is thermodynamically ultrastable, as observed in recent experiments. However, important fundamentals such as the structure of intercalated phases, locations of intercalated atoms, thermodynamic preference for intercalation, and intercalation pathways for this system have not yet been understood conclusively. In this work, extensive density functional theory calculations are performed to assess Pb intercalation thermodynamics and kinetics under graphene on SiC(0001). We find that intercalation of isolated Pb atoms is strongly disfavored over adsorption on top of graphene. However, intercalation of complete Pb layers in the gallery between SiC and graphene buffer layer is strongly favored over supported Pb monolayers and moderately favored over formation of supported large three-dimensional Pb islands. We also find that initiation of intercalation either by individual Pb atoms directly penetrating graphene or by hopping under a static graphene step edge is energetically prohibitive at experimental temperatures. Consequently, more complex intercalation pathways are operative and further analyzed. We demonstrated that once an intercalated Pb monolayer forms around a graphene step edge, a facile Pb mass transport by Pb vacancy-mediated diffusion can be triggered for continued growth of the intercalated monolayer.

Toward a Mechanistic Understanding of the Formation of 2D-GaNx in Epitaxial Graphene

Ultrathin 2D-GaNx can be formed by Ga intercalation into epitaxial graphene (EG) on SiC followed by nitridation in ammonia. Defects in the graphene provide routes for intercalation, but the nature and role of the defects have remained elusive. Here we examine the influence of graphene layer thickness and chemical functionalization on Ga intercalation and 2D-GaNx formation using a combination of experimental and theoretical studies. Thin buffer layer regions of graphene near steps on SiC readily undergo oxygen functionalization when exposed to air or a He/O2 plasma in contrast to thicker regions which are not chemically modified. Oxygen functionalization is found to inhibit Ga intercalation leading to accumulation of Ga droplets on the surface. In contrast, Ga readily intercalates between EG and SiC in the thicker graphene regions that do not contain oxygen. When NH3 annealing is carried out immediately after Ga exposure, 2D-GaNx formation is observed only in the oxygen-functionalized regions, and Ga intercalated under thicker nonfunctionalized graphene does not convert to GaNx. Density functional theory calculations demonstrate that oxygen functionalization of graphene alters the binding energy of Ga and NH3 species to the graphene surface. The presence of hydroxyl groups on graphene inhibits binding of Ga to the surface; however, it enhances the chemical reactivity of the graphene surface to NH3 which, in turn, enhances Ga binding and facilitates the formation of 2D-GaNx. By modifying the EG process to produce oxygen-functionalized buffer layer graphene, uniformly intercalated 2D-GaNx is obtained across the entire substrate surface.