Graphene is considered a leading candidate to replace conventional transparent conducting electrodes because of its high transparency and exceptional transport properties. The effect of chemical p-type doping on graphene stacks was studied in order to reduce the sheet resistance of graphene films to values approaching those of conventional transparent conducting oxides. In this report, we show that large-area, stacked graphene films are effectively p-doped with nitric acid. The doping decreases the sheet resistance by a factor of 3, yielding films comprising eight stacked layers with a sheet resistance of 90 Omega/(square) at a transmittance of 80%. The films were doped either after all of the layers were stacked (last-layer-doped) or after each layer was added (interlayer-doped). A theoretical model that accurately describes the stacked graphene film system as a resistor network was developed. The model defines a characteristic transfer length where all the channels in the graphene films actively contribute to electrical transport. The experimental data shows a linear increase in conductivity with the number of graphene layers, indicating that each layer provides an additional transport channel, in good agreement with the theoretical model.

The emergence of rapidly expanding infectious diseases such as coronavirus (COVID-19) demands effective biosensors that can promptly detect and recognize the pathogens. Field-effect transistors based on semiconducting two-dimensional (2D) materials (2D-FETs) have been identified as potential candidates for rapid and label-free sensing applications. This is because any perturbation of such atomically thin 2D channels can significantly impact their electronic transport properties. Here, we report the use of FET based on semiconducting transition metal dichalcogenide (TMDC) WSe 2 as a promising biosensor for the rapid and sensitive detection of SARS-CoV-2 in vitro . The sensor is created by functionalizing the WSe 2 monolayers with a monoclonal antibody against the SARS-CoV-2 spike protein and exhibits a detection limit of down to 25 fg/μL in 0.01X phosphate-buffered saline (PBS). Comprehensive theoretical and experimental studies, including density functional theory, atomic force microscopy, Raman and photoluminescence spectroscopies, and electronic transport properties, were performed to characterize and explain the device performance. The results demonstrate that TMDC-based 2D-FETs can potentially serve as sensitive and selective biosensors for the rapid detection of infectious diseases.

We show that the local temperature dependence of thermalized electron and phonon populations along metallic carbon nanotubes is the main reason behind the nonlinear transport characteristics in the high bias regime. Our model is based on the solution of the Boltzmann transport equation considering both optical and zone boundary phonon emission as well as absorption by charge carriers. It also assumes a local temperature along the nanotube, determined self-consistently with the heat transport equation. By using realistic transport parameters, our results not only reproduce experimental data for electronic transport but also provide a coherent interpretation of thermal breakdown under electric stress. In particular, electron and phonon thermalization prohibits ballistic transport in short nanotubes.

There has been recent controversy whether the response seen in carbon nanotube (CNT) chemiresistors is associated with a change in the resistance of the individual nanotubes or changes in the resistance of the junctions. In this study, we carry out a network analysis to understand the relative contributions of the nanotubes and the junctions to the change in resistance of the nanotube network. We find that the dominant mode of detection in nanotube networks changes according to the conductance level (defect level) in the nanotubes. In networks with perfect nanotubes, changes in the junctions between adjacent nanotubes and junctions between the contacts and the CNTs can cause a detectable change in the resistance of the nanotube networks, while adsorption on the nanotubes has a smaller effect. In contrast, in networks with highly defective nanotubes, the changes in the resistance of the individual nanotubes cause a detectable change in the overall resistance of a chemiresistor network, while changes in the junctions have smaller effects. The combinational effect is also observed for the case in between. The results show that the sensing mechanism of a nanotube network can change according to the defect levels of the nanotubes, which may explain the apparently contradictory results in the literature.

Electrostatic screening in multilayer graphene is highly nonlinear due to the vanishing density of states at the Fermi level. Using a discrete model we study the charge screening normal to the layers. Our model shows a strong charge and temperature dependence and has a simple continuum limit at T=0 for undoped systems. Doped systems can exhibit more complex behavior due to minority-carrier screening. Most importantly we find that the screening length can vary more than an order of magnitude depending on the experimental conditions, reconciling the large range of screening lengths reported in previous experiments. This has important consequences for technological applications of multilayer graphene used in electrodes or transistor channels.