High Electron Mobility in Epitaxial Trilayer Graphene on Off-axis SiC(0001)

Effect of solution pH and adsorbent concentration on the sensing parameters of TGN-based electrochemical sensor

The response of trilayer graphene nanoribbon (TGN)-based ion-sensitive field-effect transistor (ISFET) to different pH solutions and adsorption effect on the sensing parameters are analytically studied in this research. The authors propose a TGN-based sensor to electrochemically detect pH. To this end, absorption effect on the sensing area in the form of carrier concentration, carrier velocity, and conductance variations are investigated. Also, the caused electrical response on TGN as a detection element is analytically proposed, in which significant current decrease of the sensor is observed after exposure to high pH values. In order to verify the accuracy of the model, it is compared with recent reports on pH sensors. The TGN-based pH sensor exposes higher current compared to that of carbon nanotube (CNT) counterpart for analogous ambient conditions. While, the comparative results demonstrate that the conductance of proposed model is lower than that of monolayer graphene-counterpart for equivalent pH values. The results confirm that the conductance of the sensor is decreased and V_g-min is obviously right-shifted by increasing value of pH. The authors demonstrate that although there is not the experimental evidence reported in the part of literature for TGN sensor, but the model can assist in comprehending experiments involving nanoscale pH sensors.

Limiting factors for optical switching using nano-structured graphene-based field effect transistors

Thanks to the particular band diagram of graphene, it is recognized as a promising material for developing optoelectronic devices at the nano-scale. In this paper, a functional stack comprised of graphene and other materials is numerically investigated to extract the related capacitance-voltage curve by taking into account practical considerations regarding the nano-structured electronic devices. Polycrystalline silicon gates are used as electrical contacts in this stack, which are considered as semiconductor materials rather than metal contacts owing to the nano-scale dimensions of the constitutive materials. Moreover, graphene is effectively modeled to highlight its presence in the stack. Then, the stack is developed for the construction of a graphene field effect transistor (GFET) in order to examine the speed response of the stack. In this regard, by selecting the carrier mobility of 1500  cm²/(V·s) for graphene and a particular bias condition, the small-signal current gain of the GFET is computed so that according to the simulation results, the intrinsic cutoff frequency of 13.89 GHz is achieved.

Flat Monolayer Graphene Cathodes for Li-Oxygen Microbatteries

Miniature batteries can accelerate the development of mobile electronics by providing sufficient energy to power small devices. Typical microbatteries commonly use thin-film inorganic electrodes based on Li-ion insertion reaction. However, they rely on the complicated thin-film synthesis method of inorganics containing many elements. Graphene, one atomic layer thick carbon sheet, has diverse physical and chemical properties and is compatible with conventional micron-scale device fabrication. Here, we study the use of chemical vapor deposition (CVD) grown monolayer graphene in a two-dimensional configuration, as a future Li-oxygen microbattery cathode. By maximizing the dissolution of discharge intermediates, we obtain 2610 Ah/g_graphene of capacity corresponding to 20% higher areal cathode energy density and 2.7 times higher cathode specific energy than that can be derived from the same volume or mass of conventional Li-ion battery cathode material. Furthermore, a clear observation on the discharge reaction on composite electrodes and their role in the charging reaction was made, thanks to the two-dimensional monolayer graphene Li-oxygen battery cathode. We demonstrate an easy integration of two-dimensional CVD graphene cathode into microscale devices by simply transferring or coating the target device substrate with flexible graphene layers. The ability to integrate and use monolayer graphene on arbitrary device substrates as well as precise control over a chemical derivation of the carbon interface can have a radical impact on future energy-storage devices.

Dirac fermions induced in strained zigzag phosphorus nanotubes and their applications in field effect transistors

In this work, Dirac fermions have been obtained and engineered in one-dimensional (1D) zigzag phosphorus nanotubes (ZPNTs). We have performed a comprehensive first-principles computational study of the electronic properties of ZPNTs with various diameters. The results indicate that as the lattice parameter (L_c) along the axial direction increases, ZPNTs undergo transitions from metal to semimetal and semimetal to semiconductor, whereas Dirac fermions appear at L_c ranging from 3.90 Å to 4.10 Å. In particular, a field effect transistor (FET) based on 12-ZPNT (with 12 unit cells in the transverse direction) exhibits semiconductor behaviors with efficient gate-effect modulation at L_c = 4.60 Å. However, only weak gate modulation is demonstrated when the nanotube becomes a semimetal at L_c = 4.10 Å. This study indicates that ZPNTs are profoundly appealing for applications in strain sensors. Our findings pave the way for the development of high-performance strain-engineered electronics based on Dirac fermions in 1D materials.

Atomic and electronic structure of trilayer graphene/SiC(0001): Evidence of Strong Dependence on Stacking Sequence and charge transfer

The transport properties of few-layer graphene are the directly result of a peculiar band structure near the Dirac point. Here, for epitaxial graphene grown on SiC, we determine the effect of charge transfer from the SiC substrate on the local density of states (LDOS) of trilayer graphene using scaning tunneling microscopy/spectroscopy and angle resolved photoemission spectroscopy (ARPES). Different spectra are observed and are attributed to the existence of two stable polytypes of trilayer: Bernal (ABA) and rhomboedreal (ABC) staking. Their electronic properties strongly depend on the charge transfer from the substrate. We show that the LDOS of ABC stacking shows an additional peak located above the Dirac point in comparison with the LDOS of ABA stacking. The observed LDOS features, reflecting the underlying symmetry of the two polytypes, were reproduced by explicit calculations within density functional theory (DFT) including the charge transfer from the substrate. These findings demonstrate the pronounced effect of stacking order and charge transfer on the electronic structure of trilayer or few layer graphene. Our approach represents a significant step toward understand the electronic properties of graphene layer under electrical field.

Effect of plasma treatment on multilayer graphene: X-ray photoelectron spectroscopy, surface morphology investigations and work function measurements

We report here the effect of plasma treatment on multilayer graphene sheets as determined by X-Ray photoelectron spectroscopy, surface morphology studies using AFM, SEM and TEM along with work function measurements using Kelvin probe technique.