Top Graphene Layer Research Articles

Effect of oxygen plasma etching on graphene’s mechanical and electrical properties

This paper reports a study of the effect of oxygen plasma etching on the mechanical and electrical properties of chemical vapor deposited (CVD) multilayer graphene. By means of scan probe microscopy (SPM), it was found that the defects were initially induced to the top graphene layer by oxygen plasma etching, which plays an incentive role in further etching. Oxygen plasma bombarding on surface of graphene enhanced the surface roughness, as well changed the tribological properties. The results of electronic transport measurements show a decrease in mobility with the increase of etching duration. These findings are valuable for studying the effects of plasma etching on graphene, and modifying the physical properties of graphene through artificially generated defects.

Fabrication of graphene–silicon layered heterostructures by carbon penetration of silicon film

A new, easy, in situ technique for fabricating a two-dimensional graphene–silicon layered heterostructure has been developed to meet the demand for integration between graphene and silicon-based microelectronic technology. First, carbon atoms are stored in bulk iridium, and then silicon atoms are deposited onto the Ir(111) surface and annealed. With longer annealing times, the carbon atoms penetrate from the bulk iridium to the top of the silicon and eventually coalesce there into graphene islands. Atomically resolved scanning tunneling microscopy images, high-pass fast Fourier transform treatment and Raman spectroscopy demonstrate that the top graphene layer is intact and continuous, and beneath it is the silicon layer.

Efficient Heterostructures for Combined Interference and Plasmon Resonance Raman Amplification.

The detection, identification, and quantification of different types of molecules and the optical imaging of, for example, cellular processes are important challenges. Here, we present how interference-enhanced Raman scattering (IERS) in adequately designed heterostructures can provide amplification factors relevant for both detection and imaging. Calculations demonstrate that the key factor is maximizing the absolute value of the refractive indices' difference between dielectric and metal layers. Accordingly, Si/Al/Al2O3/graphene heterostructures have been fabricated by optimizing the thickness and roughness and reaching enhancement values up to 700 for 488 nm excitation. The deviation from the calculated enhancement, 1200, is mainly due to reflectivity losses and roughness of the Al layer. The IERS platforms are also demonstrated to improve significantly the quality of white light images of graphene and are foreseen to be adequate to reveal the morphology of 2D and biological materials. A graphene top layer is adequate for most organic molecule deposition and often quenches possible fluorescence, permitting Raman signal detection, which, for a rhodamine 6G (R6G) monolayer, presents a gain of 400. Without graphene, the nonquenched R6G fluorescence is similarly amplified. The wavelength dependence of the involved refractive indices predicts much higher amplification (around 104) for NIR excitation. These interference platforms can therefore be used to gain contrast and intensity in white light, Raman, and fluorescence imaging. We also demonstrate that surface-enhanced Raman scattering and IERS amplifications can be efficiently combined, leading to a gain of >105 (at 488 nm) by depositing a Ag nanostructured transparent film on the IERS platform. When the plasmonic structures deposited on the IERS platforms are optimized, single-molecule detection can be actively envisaged.

Observation of Anomalous Resistance Behavior in Bilayer Graphene

Our measurement results have shown that bilayer graphene exhibits an unexpected sharp transition of the resistance value in the temperature region 200~250 K. We argue that this behavior originates from the interlayer ripple scattering effect between the top and bottom ripple graphene layer. The inter-scattering can mimic the Coulomb scattering but is strongly dependent on temperature. The observed behavior is consistent with the theoretical prediction that charged impurities are the dominant scatters in bilayer graphene. The resistance increase with increasing perpendicular magnetic field strongly supports the postulate that magnetic field induces an excitonic gap in bilayer graphene. Our results reveal that the relative change of resistance induced by magnetic field in the bilayer graphene shows an anomalous thermally activated property.

Design of waveguide-integrated graphene devices for photonic gas sensing

We present waveguide-integrated graphene devices for photonic gas sensing. In a gas environment, graphene’s conductivity is changed by adsorbed gas molecules which serve as charge-carrier donors or acceptors. To accurately probe gas-induced variations in the graphene’s conductivity, we optimize the graphene’s Fermi level and spectral region. Then, we propose graphene-on-silicon and graphene-on-germanium suspended membrane slot waveguides in which propagating light in the waveguide has a strong interaction with the top graphene layer. The gas concentration can be calculated by measuring the spectrum of the optical reflection from the waveguide Bragg grating. The maximum sensitivity of the waveguide-integrated gas sensor can reach one part per million for sensing gaseous nitrogen dioxide. Its sensitivity is about 20 times higher than that of the graphene-covered microfiber sensor and is comparable with that of a graphene plasmonic sensor. The fabrication of the proposed graphene device is CMOS compatible. Our results pave a way for chip-integrated sensitive photonic gas sensors.

Current Transport in Graphene/Copper Hybrid Nano Ribbon Interconnect: A First Principle Study

In search of emerging materials replacement of copper interconnect in nanometer CMOS technology nodes, graphene (G) and carbon nanotubes (CNT) have been studied intensively. There is also ongoing research to find hybrid materials, which can have better thermal and electrical conductivity [1]. Recently, reduction in temperature has been observed in graphene encapsulated copper wires [2]. It was argued that this excellent thermal property is inherent to copper not because of graphene. During graphene deposition process, Cu becomes annealed with larger grains. In addition, graphene works as a barrier layer for copper ion to diffuse into dielectric. Motivated by recent experiments we have studied theoretically G/Cu structure in bulk and in one-dimension (1D) as next generation wires for nano-electronic applications. Our purpose is to understand this structure and to study why it outperforms graphene only or Cu only interconnect. In this paper, we report density of states (DOS), current-voltage relation (I-V), and resistance (R) of G/Cu nano ribbon interconnect.We used density functional theory (DFT) with pseudo- potential technique to study electronic band structure within Quantum-Espresso (QE) code. For ballistic transport study, we used Landauer-Buttiker (LB) formalism within Wannier90 Module [3]. We have considered a one-dimensional slice of G/Cu interconnect of width 0.6nm, height 0.8nm and length 10nm. Single layer of graphene is deposited on top of three atomic layers of Cu {111}. Cu {111} is chosen because this plane is closest to graphene lattice within 2% lattice mismatch. Among these three atomic layers, bottom two layers were fixed to their bulk lattice position to represent bulk atoms. Top Cu layer and graphene layer were relaxed to find minimum force position. After obtaining atomic positions from relaxation calculations in QE, we performed self-consistent field calculation and band structure calculation. Bloch states obtained from QE were used as an input for transport calculation in Wannier90 Code. Due to high computational cost, we have limited our study to a 10nm long wire which represent a short local interconnect and a good candidate to study ballistic transport. We believe this geometry will at least serve present purpose to compare performances with graphene only interconnect. I-V characteristics of graphene interconnect and graphene on copper interconnect have been compared. Being one-dimensional wire, we have observed several stairs like and piece wise linear relationships. At zero bias, resistance is found to be the same as quantum resistance for single electronic channel, which is ~h/2e2. Since wire length 10nm is smaller than Cu mean free path (40nm), one should not compare resistivity of G/Cu with Cu only interconnect based on Fuchs-Sondheimer (FS) and Mavadas-Shatzkes (MS) models. For this G/Cu we found resistivity 1.25x10-6 Ω-m, which is one tenth of Cu-bulk resistivity. It is expected by FS & MS theory that Cu resistivity increases with decrease of width. However, in ballistic transport regime FS & MS theories were not applicable. Hence, we have used LB formalism. For comparison, we have studied same geometry for graphene only interconnect using same methodology. From DOS, it is apparent that there is no energy states available near Fermi energy for graphene only interconnect. This is why in I-V characteristics no current is observed near 0V. On other hand, hybrid structure shows better conductivity at low bias than graphene only interconnect. Graphene only nanoribbon might better perform as device not as interconnect. This calculation is useful to understand electrical performances of various nanostructures and to find true contender of next generation interconnect. Acknowledgement: Part of the work is supported by the United States Air Force Research Laboratory under agreement number FA9453-10-1-0002. The U.S. Government is authorized to reproduce and distribute reprints for Government purposes notwithstanding any copyright notation thereon.

Engineering structures and functions of mesenchymal stem cells by suspended large-area graphene nanopatterns

Inspired by the hierarchical nanofibrous and highly oriented structures of natural extracellular matrices, we report a rational design of chemical vapor deposition graphene-anchored scaffolds that provide both physical and chemical cues in a multilayered organization to control the adhesion and functions of cells for regenerative medicine. These hierarchical platforms are fabricated by transferring large graphene film onto nanogroove patterns. The top graphene layer exhibits planar morphology with slight roughness (∼20 nm between peaks) due to the underlying topography, which results in a suspended structure between the nanoridges. We demonstrate that the adhesion and differentiation of human mesenchymal stem cells were sensitively controlled and enhanced by the both the nanotopography and graphene cues in our scaffolds. Our results indicate that the layered physical and chemical cues can affect the apparent cell behaviors, and can synergistically enhance cell functionality. Therefore, these suspended graphene platforms may be used to advance regenerative medicine.

Addressing Raman features of individual layers in isotopically labeled Bernal stacked bilayer graphene

In this report important Raman modes for the evaluation of strain in graphene (the 2D and 2D′) are analyzed. The isotope labeling is used to disentangle contribution of individual graphene layers of graphene bilayer to the studied Raman modes. It is shown that for Bernal-stacked bilayers, the 2D and the 2D′ Raman modes have three distinct components that can be assigned to processes originating solely from the top graphene layer, bottom graphene layer, and from a combination of processes originating both from the top and bottom layers. The reported results thus enable addressing the properties of individual graphene layers in graphene bilayer by Raman spectroscopy.

Effects of humidity on the electronic properties of graphene prepared by chemical vapour deposition

The effects of humidity on the electronic properties of mono- and bi-layer graphene prepared by chemical vapour deposition and transferred on SiO2 are investigated via simultaneous global transport and local work function measurements using the van der Pauw method and Kelvin probe force microscopy, respectively. It is found that mono-layer graphene on SiO2 is extremely sensitive to water vapour, with water molecules acting as physisorbed loosely bound p-dopants. In the case of a bi-layer stack, produced by double transfer of two graphene layers, the layers are randomly oriented and decoupled with respect to each other. As a consequence, the bottom layer of the bi-layer stack is mostly affected by substrate charges, while the top graphene layer behaves as a decoupled layer, which is externally doped by the water vapour. Moreover, we provide evidence that ambient humidity is only partly responsible for the p-doping of graphene. These findings will assist in the development of reliable graphene-based electronics such as sensors working in ambient air.

Mixed multilayered vertical heterostructures utilizing strained monolayer WS2.

Creating alternating layers of 2D materials forms vertical heterostructures with diverse electronic and opto-electronic properties. Monolayer WS2 grown by chemical vapour deposition can have inherent strain due to interactions with the substrate. The strain modifies the band structure and properties of monolayer WS2 and can be exploited in a wide range of applications. We demonstrate a non-aqueous transfer method for creating vertical stacks of mixed 2D layers containing a strained monolayer of WS2, with Boron Nitride and Graphene. The 2D materials are all grown by CVD, enabling large area vertical heterostructures to be formed. WS2 monolayers grown by CVD directly on Si substrates with SiO2 surface are easily washed off by water and this makes aqueous based transfer methods challenging for creating vertical stacks on the growth substrate. 2D hexagonal Boron Nitride films are used to provide an insulating layer that limits interactions with a top graphene layer and preserve the strong photoluminescence from the WS2. This transfer method is suitable for layer by layer control of 2D material vertical stacks and is shown to be possible for all CVD grown samples, which opens up pathways for the rapid large scale fabrication of vertical heterostructure systems with atomic thickness depth control and large area coverage.

A Graphene Switching Transistor for Vertical Circuit Design

A graphene switching transistor (GST) is proposed based on graphene-hexagonal boron nitride (hBN)-graphene vertical heterostructure. The carrier transport between source and drain is controlled by the shift in Fermi level in the channel from vertical interlayer tunneling between top and bottom graphene layers through three layers of hBN. Analytical current transport equation is derived and its characteristics are studied. GST operating at 0.1 V supply voltage gives a steep subthreshold slope of 2.97 mV/decade with an on/off current ratio of 2.45 × 104. Drive current of 88 μA/μm2 is estimated for an effective channel area of 0.05 μm2. The voltage transfer characteristic of GST-based vertical inverter is also studied and it is noted that for sharp transition from off to on state. The GST dissipates 15 times less dynamic power and 194 times less off state leakage power in comparison to a projected 2020 nMOSFET.

DFT calculations on atom-specific electronic properties of G/SiC(0001)

We investigate the atom-specific interfacial electronic properties of the epitaxial graphene on Si-terminated SiC substrate using density functional theory (DFT) calculation with van der Waals interaction correction, focusing on the dependency of the local electronic state on the chemical environment. The band structure projected on the respective atomic orbitals of the carbon atoms in the buffer layer and uppermost Si atoms demonstrates that the dangling bonds of these atoms form band structures around the Fermi level. The contribution of each atom to the dangling bond states strongly depends on the chemical environment, i.e., the presence/absence of the interlayer Si-C covalent bond. This difference also affects the atom-specific local density of states of the top-layer graphene through its interaction with the substrate/buffer layer. We demonstrate that the bias voltage dependency of the scanning tunneling spectroscopy (STS) mapping image clearly reflects the presence of the dangling bonds of the buffer layer carbon or uppermost Si atom in the substrate, which would enable the detection of the buried dangling bond with an atomic spatial resolution via STS.

Theoretical assessment of feasibility to sequence DNA through interlayer electronic tunneling transport at aligned nanopores in bilayer graphene.

Fast, cost effective, single-shot DNA sequencing could be the prelude of a new era in genetics. As DNA encodes the information for the production of proteins in all known living beings on Earth, determining the nucleobase sequences is the first and necessary step in that direction. Graphene-based nanopore devices hold great promise for next-generation DNA sequencing. In this work, we develop a novel approach for sequencing DNA using bilayer graphene to read the interlayer conductance through the layers in the presence of target nucleobases. Classical molecular dynamics simulations of DNA translocation through the pore were performed to trace the nucleobase trajectories and evaluate the interaction between the nucleobases and the nanopore. This interaction stabilizes the bases in different orientations, resulting in smaller fluctuations of the nucleobases inside the pore. We assessed the performance of a bilayer graphene nanopore setup for the purpose of DNA sequencing by employing density functional theory and non-equilibrium Green’s function method to investigate the interlayer conductance of nucleobases coupling simultaneously to the top and bottom graphene layers. The obtained conductance is significantly affected by the presence of DNA in the bilayer graphene nanopore, allowing us to analyze DNA sequences.

Interlayer interaction and π-band electron occupation on top layers of freshly cleaved bulk HOPG investigated with N(E) C KVV Auger electron spectroscopy

N(E) C KVV Auger spectra (V=σsσpπ) were used for measurement of the π-band electron occupation of five outer layers on freshly cleaved bulk HOPG. The π-band electron occupation of the 1–5 graphene layers was measured relative to the electron concentration in the σp-band. In-depth π-band profiles were obtained by means of variation of the Auger electron takeoff angle within the range of 15–90°. Differences in the π-band electron occupation of the 1–5 graphene layers were determined. The π-band electron occupation varies from 0 at the top graphene layer to that of the π-band electron occupation typical for bulk HOPG at the 5th graphene layer counted from the outer surface. These results are discussed on the basis of the π-band formation under the interlayer interaction of the pz-electrons.

Electrically tunable resonant scattering in fluorinated bilayer graphene

Adatom-decorated graphene offers a promising new path towards spintronics in the ultrathin limit. We combine experiment and theory to investigate the electronic properties of dilutely fluorinated bilayer graphene, where the fluorine adatoms covalently bond to the top graphene layer. We show that fluorine adatoms give rise to resonant impurity states near the charge neutrality point of the bilayer, leading to strong scattering of charge carriers and hopping conduction inside a field-induced band gap. Remarkably, the application of an electric field across the layers is shown to tune the resonant scattering amplitude from fluorine adatoms by nearly twofold. The experimental observations are well explained by a theoretical analysis combining Boltzmann transport equations and fully quantum-mechanical methods. This paradigm can be generalized to many bilayer graphene-adatom materials, and we envision that the realization of electrically tunable resonance may be a key advantage in graphene-based spintronic devices.

Nanoscale stripe arrays templated on Moiré patterns in graphite

Large areas of nanoscale stripe arrays were produced by drop casting silica nanoparticle solutions on highly oriented pyrolytic graphite surfaces at room temperature and imaged with atomic force microscopy. The alignment of the striped areas always reflected the threefold symmetry of the graphite surface. Two different patterns were observed, with different coverages, line separations and mutual orientation, being offset by 30°. Measurement of the relative angles and separations of the line patterns showed a very good match with an underlying Moiré pattern, resulting from the rotation of the top graphene layers. Closer-spaced lines were attributed to the zig-zag direction of the Moiré pattern whereas wider-spaced lines belonged to the armchair direction. The different abundance and apparent difference in long-term stability suggested that stability was governed by the number of reactive vertices per unit area as opposed to the number of vertices per line-length. Whilst sequential images recorded over several days revealed long term stability of all zig-zag arrays, attachment and detachment of single nanoparticles was observed. By contrast, arrays aligned in the armchair direction appeared and vanished collectively, suggesting condensation and evaporation of a fluid of nanoparticles floating on the surface.

Subwavelength Graphene-Based Plasmonic THz Switches and Logic Gates

In this paper, we report on the design procedure for developing subwavelength graphene-based plasmonic waveguide, performing as a THz switch or an AND/OR logic gate. The propagation length of the surface plasmons (SPs), stimulated by a 6 THz TM polarized incident wave along this waveguide with a top graphene layer whose chemical potential is held at $\mu_{\rm C}=300~{\hbox{meV}}$ (ON state) is more than 35 times larger than that in the waveguide with $\mu_{\rm C}= 0~{\hbox{eV}}$ (OFF state). Numerical results, obtained from full wave simulations using a finite element method, also show that the modulation depth density obtained for the straight plasmonic switching waveguide, whose length is just about 20% of the incident wavelength, is larger than those reported to date. Moreover, we also designed a logic AND gate composed of a straight waveguide, a Y-branch switch, and a logic OR gate composed of two face to face Y-branches, whose total lengths are ${\sim} {\hbox{37\%}}$ , ${\sim} {\hbox{45\%}}$ , and ${\sim} {\hbox{53\%}}$ of the incident wavelength, respectively. Simulations show that the maximum ON/OFF ratios for these subwavelength plasmonic waveguides that occur between their ‘1 1’ and ‘0 0’ logical states are ${\sim} {\hbox{41.37}}$ , ${\sim} {\hbox{39.87}}$ , and ${\sim} {\hbox{40.76}}~{\hbox{dB}}$ , respectively. These numerical data also show that the modulation depth densities obtained for these devices are also greater than those reported to date. The proposed graphene-based plasmonic switches and gates offer potential building blocks for the future digital plasmonic circuits operating around 6 THz.

Evolution of temperature-induced strain and doping of double-layer graphene: An in situ Raman spectral mapping study

Using Raman spectral-mapping, we investigated in situ temperature dependence (100–300 K) of strain and doping of isotopically labelled double-layer graphene, which allows us to simultaneously and independently follow the strain and doping in both layers of the double layer. We find that the strain in both layers of the double-layer graphene is almost the same and becomes larger when the temperature is decreased. The bottom graphene layer, in contact with the Si/SiO2 substrate has a stronger doping than the top graphene layer which is in contact with the graphene bottom layer. We also observed that the spatial distribution of the temperature-induced strain and doping is not homogeneously distributed within the substrate.

First-principles simulations of a graphene-based field-effect transistor

We improvise an approach to carry out first-principles simulations of graphene-based vertical field-effect tunneling transistors that consist of a graphene$|h\ensuremath{-}\mathrm{BN}|$graphene multilayer structure. Within the density functional theory framework, we exploit the effective screening medium (ESM) method to properly treat boundary conditions for electrostatic potentials and investigate the effect of gate voltage. The distribution of free carriers and the band structure of both top and bottom graphene layers are calculated self-consistently. The dielectric properties of $h\text{-BN}$ thin films sandwiched between graphene layers are computed layer-by-layer following the theory of microscopic permittivity. We find that the permittivities of BN layers are very close to that of crystalline $h\text{-BN}$. The effect of interface with graphene on the dielectric properties of $h\text{-BN}$ is weak according to an analysis on the interface charge redistribution.

Efficient photoinduced charge accumulation in reduced graphene oxide coupled with titania nanosheets to show highly enhanced and persistent conductance.

Tuning of the electrical properties of graphene via photoexcitation of a heteroassembled material has started to attract attention for electronic and optoelectronic applications. Actually photoinduced carrier doping from the hexagonal boron nitride (h-BN) substrate greatly modulated the transport property of the top layer graphene, showing promising potential for this approach. However, for practical applications, the large scale production of this two-dimensional heterostructure is needed. Here, a superlattice film constructed from reduced graphene oxide (rGO) and photoactive titania nanosheets (Ti0.87O2(0.52-)) was employed as a channel to construct a field effect transistor (FET) device, and its UV light response on the electrical transport property was examined. The UV light illumination induced significant improvement of the electrical conductance by ∼7 times on the basis of simultaneous enhancements of the electron carrier concentration and its mobility in rGO. Furthermore, the polarity of the FET response changed from ambipolar to n-type unipolar. Such modulated properties persisted in vacuum even after the UV light was turned off. These interesting behaviors may be explained in terms of photomodulation effects from Ti0.87O2(0.52-) nanosheets. The photoexcited electrons in Ti0.87O2(0.52-) are injected into rGO to increase the electron carrier concentration as high as 7.6×10(13) cm(-2). On the other hand, the holes are likely trapped in the Ti0.87O2(0.52-) nanosheets. These photocarriers undergo reduction and oxidation of oxygen and water molecules adsorbed in the film, respectively, which act as carrier scattering centers, contributing to the enhancement of the carrier mobility. Since the film likely contains more water molecules than oxygen, upon extinction of UV light, a major portion of electrons (∼80% of the concentration at the UV off) survives in rGO, showing the highly enhanced conductance for days. This surpassing photomodulated FET response and its persistency observed in the present superlattice system of rGO/Ti0.87O2(0.52-) are noteworthy compared with previous studies such as the device with a heteroassembly of graphene/h-BN.