Bottom Graphene Layers Research Articles

Gate‐Dependent Tunnelling Current Modulation of Graphene/hBN Vertical Heterostructures

Combining with layered thin crystalline films, graphene has expanded its application scope beyond the regime where a gapless semimetal cannot serve. Here, we report the modulation of tunneling characteristics in graphene/hexagonal boron nitride (hBN) vertical heterostructure at different interlayer hBN thickness. These results signify an upshift in threshold voltages with hBN layer thickness. Furthermore, the gate‐dependent tunneling characteristics of the device has been demonstrated. The back‐gate voltages are used to adjust the fermi level of bottom graphene layer, which in turns tune the threshold voltages and tunneling current through ultrathin hBN layer. Our findings offer an effective tool to modulate the tunneling characteristics of vertical transistors for their potential applications in high frequency logic and tunnel devices.

Improved field emission performances for graphene/ZnO nanowires/graphene sandwich composites

Graphene/zinc oxide nanowires/graphene (G/ZnO NWs/G) sandwich composites were firstly synthesized on nickel foam by electrostatic spray deposition technique and hydrothermal reaction in sequence. Microstructure and chemical analysis showed the dense and uniform ZnO NWs encapsulated by bottom and upper graphene layers. The G/ZnO NWs/G composites as cold cathodes for field emission (FE) exhibited improved performances with a lower turn-on electric field of 7.45 V/μm, a higher field enhancement factor of 5328 and higher emission current stability over 10 h than G/ZnO NWs. The enhanced FE properties of G/ZnO NWs/G were mainly attributed to its sandwich structure, which introduced more surface defects and numerous emission sites, simultaneously decreased the work function and resistivity of emitters. This investigation paves a new way for developing high-performance cold cathode FE materials.

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.

Interface and interaction of graphene layers on SiC(0001̄) covered with TiC(111) intercalation

It is important to understand the interface and interaction between the graphene layer, titanium carbide [TiC(111)] interlayer, and silicon carbide [SiC(0001[combining macron])] substrates in epitaxial growth of graphene on silicon carbide (SiC) substrates. In this study, the fully relaxed interfaces which consist of up to three layers of TiC(111) coatings on the SiC(0001[combining macron]) as well as the graphene layers interactions with these TiC(111)/SiC(0001[combining macron]) were systematically studied using the density functional theory-D2 (DFT-D2) method. The results showed that the two layers of TiC(111) coating with the C/C-terminated interfaces were thermodynamically more favorable than one or three layers of TiC(111) on the SiC(0001[combining macron]). Furthermore, the bonding of the Ti-hollow-site stacked interfaces would be a stronger link than that of the Ti-Fcc-site stacked interfaces. However, the formation of the C/Ti/C and Ti/C interfaces implied that the first upper carbon layer can be formed on TiC(111)/SiC(0001[combining macron]) using the decomposition of the weaker Ti-C and C-Si interfacial bonds. When growing graphene layers on these TiC(111)/SiC(0001[combining macron]) substrates, the results showed that the interaction energy depended not only on the thickness of the TiC(111) interlayer, but also on the number of graphene layers. Bilayer graphene on the two layer thick TiC(111)/SiC(0001[combining macron]) was thermodynamically more favorable than a monolayer or trilayer graphene on these TiC(111)/SiC(0001[combining macron]) substrates. The adsorption energies of the bottom graphene layers with the TiC(111)/SiC(0001[combining macron]) substrates increased with the decrease of the interface vertical distance. The interaction energies between the bottom, second and third layers of graphene on the TiC(111)/SiC(0001[combining macron]) were significantly higher than that of the freestanding graphene layers. All of these findings provided insight into the growth of epitaxial graphene on TiC(111)/SiC(0001[combining macron]) substrates and the design of graphene/TiC/SiC-based electronic devices.

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.

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.

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.

Raman spectroscopy and AFM study of 12 C graphene/fullerenes C70 /13 C graphene heterostructure

In our study, we sandwiched fullerenes C70 in between two layers of isotopically labeled graphene and studied this heterostructure by atomic force microscopy and by Raman spectroscopy. Isotope labeling was used to distinguish between the top and the bottom graphene layers in the measured Raman spectra. We analyzed the Raman maps of isotopically labeled graphene–fullerene heterostructures and described the effect of the fullerenes on graphene layers. Apart from the peak parameters obtained directly from fitting the principal Raman modes of graphene, we evaluated strain and doping in both layers employing a correlation analysis of the positions of the G and the 2D bands. Our results show that the top layer is less doped and contains less sub-micron charge inhomogeneities than the bottom layer.

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.

Synchronous growth of AB-stacked bilayer graphene on Cu by simply controlling hydrogen pressure in CVD process

AB-stacked bilayer graphene has attracted considerable attention due to its feasibility of band gap tuning. Although synthesis of bilayer graphene on Cu has been reported using chemical vapor deposition (CVD) through a layer-by-layer growth mechanism, the process is long and complicated due to lack of catalytic assistance of Cu to the second graphene layer growth. Here we show that theoretical modeling demonstrates an alternative synchronous growth of bilayer graphene on Cu is possible by passivating the top graphene nuclei edges with hydrogen to allow carbon diffusion underneath the top graphene nuclei for bottom graphene layer formation. Moreover, such a growth mechanism has been achieved experimentally in a facile CVD method by simply controlling the H2 pressure. Bilayer graphene with high coverage of over ∼95% and a high AB stacking ratio of up to ∼90% has been obtained within a short growth time of 30min. Also, graphene with single, double and multiple layers can be obtained by simply controlling the hydrogen pressure. This result represents the demonstration of the fast synchronous AB-stacked bilayer graphene growth, which is important to scalable manufacture of graphene with controllable layer number and stacking required for practical applications.

Direct transfer and Raman characterization of twisted graphene bilayer

Twisted bilayer graphene (tBLG) is constituted of a two-graphene layer with a mismatch angle θ between the two hexagonal structures. It has recently attracted much attention—thanks to its diverse electronic and optical properties. Here, we study the tBLG fabricated by the direct transfer of graphene monolayer prepared by chemical vapor deposition (CVD) onto another CVD graphene layer remaining attached to the copper foil. We show that high quality and homogeneous tBLG can be obtained by the direct transfer which prevents interface contamination. In this situation, the top graphene layer plays a supporting mechanical role to the bottom graphene layer as confirmed by optical microscopy, scanning electron microscopy, and Raman spectroscopy measurements. The effect of annealing tBLG was also investigated using micro-Raman spectroscopy. The Raman spectra exhibit a splitting of the G peak as well as a change in the 2D band shape indicating a possible decoupling of the two monolayers. We attribute these changes to the different interactions of the top and bottom layers with the substrate.

Self-organized metal-semiconductor epitaxial graphene layer on off-axis 4H-SiC(0001)

The remarkable properties of graphene have shown promise for new perspectives in future electronics, notably for nanometer scale devices. Here we grow graphene epitaxially on an off-axis 4H-SiC(0001) substrate and demonstrate the formation of periodic arrangement of monolayer graphene on planar (0001) terraces and Bernal bilayer graphene on (112 — 0) nanofacets of SiC. We investigate these lateral superlattices using Raman spectroscopy, atomic force microscopy/ electrostatic force microscopy (AFM/EFM) and X-ray and angle resolved photoemission spectroscopy (XPS/ARPES). The correlation of EFM and ARPES reveals the appearance of permanent electronic band gaps in AB-stacked bilayer graphene on (112 — 0) SiC nanofacets of 150 meV. This feature is confirmed by density functional theory (DFT) calculations. The charge transfer between the substrate and graphene bilayer results in an asymmetric charge distribution between the top and the bottom graphene layers opening an energy gap. This surface organization can be thus defined as self-organized metal–semiconductor graphene.

Graphene/hexagonal boron nitride/graphene nanopore for electrical detection of single molecules

Graphene nanopore device, since its proposal, has witnessed tremendous progress toward the goal of single-molecule detection. However, one central challenge of preparing electrodes with nanometer precision on the graphene remains unsolved. Here we show theoretically the feasibility of graphene/hexagonal BN (h-BN)/graphene structure where top graphene layer acts as one electrical contact while the bottom layer as the other. Based on quantum chemistry/nonequilibrium Green’s function investigation, we give clear physical pictures why ABC stacking of the above heterogeneous layers results in excellent insulating of the top and bottom graphene electrodes. On the other hand, when the target molecule is inside the nanopore the background conductance through the h-BN dielectric will not keep decreasing even though more layers of h-BN are inside the nanopore. The mechanism is illustrated as that the presence of the molecule will enhance the vertical transmission through the h-BN dielectric via quantum interference. We employ a single-level molecule model, and show quantitatively that the discussed effect can be utilized as a powerful signal amplifier for the molecule conductance, thus enhancing the measurability of single molecules by 3–4 orders. We report a novel design of graphene as electrical sensor for single-molecule detection, by employing natural self-assembly properties of graphene/multilayered h-BN/graphene for single-molecule detection. The novelty of our design is that we use the top and bottom graphene layers as two separate electrodes while the sandwiched h-BN layers as the insulating dielectric. Our theoretical study indicates that the ordered stacking of h-BN dielectric in the nanopore will result in intriguing quantum interference effects, which can be utilized for enhancing the sensitivity of the proposed device. The astonishing electrical capabilities of graphene films make them ideal for use in ‘nanopore’ sensors that detect minute conductance fluctuations from single molecules, such as DNA, passing through small openings. Masateru Taniguchi and co-workers from Osaka University, Japan, have now calculated a way to improve these devices using hexagonal boron nitride (h-BN), a substance with an atomically flat structure similar to graphene but practically no electric conductivity. Quantum simulations reveal that when h-BN multilayers are sandwiched between two graphene electrodes with a symmetry that repeats every three layers – a so-called ‘ABC’ stacking – the sensor’s background conductance is minimized. The introduction of single molecules into this arrangement causes the amplification of conductance signals by three to four orders of magnitude – a sensitivity boost that may prove invaluable to future biosensing applications.

Attenuation characteristics of the guided THz wave in parallel-plate ferroelectric-graphene waveguides

Guided THz wave characteristics in a parallel-plate waveguide (PPWG) consisting of ferroelectric film (LiNbO3 and LiTaO3) and multilayer graphene (MLG) is studied in this paper, with their low and tunable attenuation valley predicted. The electrical conductivity of MLG is calculated by a set of closed-form equations with the coupling effect between the bottom graphene layer (BGL) and its substrate taken into account carefully, while the dispersive behavior of ferroelectric film itself is described by the Lorentz model over an ultra-wide THz band. It is shown that the guided TM-mode propagation can be adjusted effectively by changing temperature, frequency, optical pumping intensity, MLG layer number, film thickness and its transverse optical-phonon frequency. Moreover, one low attenuation valley of TM-mode in such ferroelectric-graphene waveguide is captured, which can be exploited for developing some THz planar tunable waveguides with ultra-low loss.

Singular Sheet Etching of Graphene with Oxygen Plasma

This paper reports a simple and controllable post-synthesis method for engineering the number of graphene layers based on oxygen plasma etching. Singular sheet etching (SSE) of graphene was achieved with the optimum process duration of 38 seconds. As a demonstration of this SSE process, monolayer graphene films were produced from bilayer graphenes. Experimental investigations verified that the oxygen plasma etching removes a single layer graphene sheet in an anisotropic fashion rather than anisotropic mode. In addition, etching via the oxygen plasma at the ground electrodes introduced fewer defects to the bottom graphene layer compared with the conventional oxygen reactive ion etching using the powered electrodes. Such defects can further be reduced with an effective annealing treatment in an argon environment at 900–1000°C. These results demonstrate that our developed SSE method has enabled a microelectronics manufacturing compatible way for single sheet precision subtraction of graphene layers and a potential technique for producing large size graphenes with high yield from multilayer graphite materials.

Singular Sheet Etching of Graphene with Oxygen Plasma

Abstract This paper reports a simple and controllable post-synthesis method for engineering the number of graphene layers based on oxygen plasma etching. Singular sheet etching (SSE) of graphene was achieved with the optimum process duration of 38 seconds. As a demonstration of this SSE process, monolayer graphene films were produced from bilayer graphenes. Experimental investigations verified that the oxygen plasma etching removes a single layer graphene sheet in an anisotropic fashion rather than anisotropic mode. In addition, etching via the oxygen plasma at the ground electrodes introduced fewer defects to the bottom graphene layer compared with the conventional oxygen reactive ion etching using the powered electrodes. Such defects can further be reduced with an effective annealing treatment in an argon environment at 900–1000°C. These results demonstrate that our developed SSE method has enabled a microelectronics manufacturing compatible way for single sheet precision subtraction of graphene layers and a potential technique for producing large size graphenes with high yield from multilayer graphite materials.

Terahertz wave responses of dispersive few-layer graphene (FLG) and ferroelectric film composite

Terahertz wave responses of few-layer graphene (FLG) and ferroelectric film (LiNbO3) composite are studied using our developed algorithm based on the spectral-domain exponential matrix (SDEM) technique. The electrical conductivity of such FLG is calculated by a set of closed-form equations with coupling effect between the bottom graphene layer (BGL) and the substrate treated in an appropriate way, while the dispersive properties of the LiNbO3 film are described by the Lorentz model over an ultra-wide THz band. It is shown that both transmittance and reflectance of the composite can be adjusted effectively by changing the number of layers in FLG, the optical pumping intensity and the transverse optical-phonon frequency of LiNbO3. Moreover, producing such FLG and ferroelectric film composite is technically feasible, which can be exploited for further developing some novel planar THz structures.

Raman identification of edge alignment of bilayer graphene down to the nanometer scale.

The ideal edges of bilayer graphene (BLG) are that the edges of the top and bottom graphene layers (GLs) of BLG are well-aligned. Actually, the alignment distance between the edges of the top and bottom GLs of a real BLG can be as large as the submicrometer scale or as small as zero, which cannot be distinguished using an optical microscope. Here, we present a detailed Raman study on BLG at its edges. If the alignment distance of the top and bottom GLs of BLG is larger than the laser spot, the measured D mode at the edge of the top GL of BLG shows a similar spectral profile to that of disordered BLG. If the alignment distance is smaller than the laser spot, the D mode at a real BLG edge shows three typical spectral profiles similar to that at the edge of SLG, that of the well-aligned edge of BLG, or a combination of both. We show the sensitivity and ability of Raman spectroscopy to acquire the alignment distance between two edges of top and bottom GLs of BLG as small as several nanometers, which is far beyond the diffraction limit of a laser spot. This work opens the possibility to probe the edge alignment of multi-layer graphene.

Study on the gain characteristics of terahertz surface plasma in optically pumped graphene multi-layer structures

Based on the developed optically pumped graphene multilayer terahertz surface plasma structures, this paper calculates the real part of propagation index and amplification coefficient in optically pumped graphene multilayer structures, discusses the inluences of momentum relaxation time, temperature, numbers of grapheme layers, and the quasi-Fermi energy in the topmost grapheme layer on the real part of propagation index and amplification coefficient. It is shown that when the real part of dynamic conductivity becomes negative in the terahertz range of frequencies in the optically pumped graphene multilayer structures, the surface plasma of graphene layers can achieve gain. By comparing the peeling-graphene-structure with the graphene structure that has a high conducting bottom graphene layer in optically pumped scheme, it can be said that the surface plasma of the peeling-graphene-structure can get a high efficient amplification. Meanwhile, the structure having properly numbers of graphene layers can get a larger amplification than the simple graphene structure in an optically pumped scheme at low temperatures.