Session C7: Focus Session: Graphene Devices III

Rectification at graphene-semiconductor interfaces

It is now widely recognized that interface between graphene and many semiconductors forms Schottky barriers with rectifying properties. Our work in this area at the University of Florida began in 2009 with our discovery that bulk semimetal graphite when simply pressed against Si, GaAs and 4H-SiC semiconductor substrates readily formed Schottky barriers. Since graphite comprises Bernal-stacked layers of graphene, then the outermost layer, a single sheet of graphene, in contact with the semiconductor plays a major role in the formation of the Schottky barrier. In this talk we follow up on this early work and report on the unusual physics and promising technical applications associated with the formation of Schottky barriers at the interface of graphene and conventional semiconductors. Much of the phenomenology is similar to what is seen with graphite/semiconductor Schottky barriers but with the additional advantages that graphene is flexible, transparent and has a Fermi energy that can be more easily tuned either positively or negatively with respect to the neutrality point by electric fields or chemical doping. Our junctions are fabricated by mechanically transferring chemical vapor deposited graphene onto $n$-type Si, GaAs, 4H-SiC or GaN semiconductor substrates and takes advantage of the strong van der Waals attraction that is accompanied by charge transfer across the interface and the formation of a rectifying (Schottky) barrier. Using current-voltage (I-V), capacitance-voltage (C-V) and Raman measurements we find that thermionic emission theory in conjunction with the Schottky-Mott model within the context of bond-polarization theory provides a surprisingly good description of the electrical properties. We will discuss a number of applications including diode operation to temperatures as high as 550~K, hole doping and associated Fermi level shifts induced by overcoating the graphene with a transparent layer of polymer (TFSA), and demonstration of solar cells with power conversion efficiencies approaching 9{\%}.   

Surface functionalization on graphene through chlorination

Since graphene is an all-surface material, surface functionalization provides effective methods to engineer its electronic properties. Here, we demonstrate that exposure of graphene devices to chlorine plasma in an electron cyclotron resonance (ECR) plasma etcher is an effective way to decrease its sheet resistance, engineer its C/Cl ratio and control the interaction between chlorine and carbon atoms.. First, conductivity of chlorinated graphene increases, due to the hole doping induced by the chlorine plasma. This is further confirmed by the Hall-effect measurements: the hole concentration increased from about 5 $\times$ 10$^{12}$ cm$^{-2}$ to around 1.3 $\times$ 10$^{13}$ cm$^{-2}$. Meanwhile, mobility decreases from about 2500 cm$^{2}$/Vs to 1000 cm$^{\mathrm{2}}$/Vs, which is still very attractive compared to strained silicon films. The sheet resistance of graphene also decreases, which is an overall result of the competition between the decreased mobility and the increased carrier concentration. Raman spectrum analysis on chlorinated graphene samples treated under different RF bias indicated that the interaction between graphene and chlorine underwent three different scenarios under different RF bias conditions: van der Waals bonding, covalent bonding and defects creation. Finally, by tuning the RF bias and treatment time, we can control the C/Cl ratio effectively.   

Theory of nanoscale friction on chemically modified graphene

Recently, it is known from FFM experiments that friction force on graphene is significantly increased by chemical modification such as hydrogenation, oxidization, and fluorination, whereas adhesion properties are altered marginally [1]. A novel nanotribological theory on two-dimensional materials is proposed on the basis of experimental results and first-principles density-functional theory (DFT) calculations. The proposed theory indicates that the total lateral stiffness that is the proportional constant of friction force is mostly associated with the out-of-plane bending stiffness of two-dimensional materials. This contrasts to the case of three-dimensional materials, in which the shear strength of materials determines nanoscale friction. We will discuss details of DFT calculations and how to generalize the current theory to three dimensional materials. [1] S. Kwon, J.-H. Ko, K.-J. Jeon, Y.-H. Kim and J. Y. Park, Nano Lett., dx.doi.org/10.1021/nl204019k (2012).   

Watering Graphene for Devices and Electricity

Graphene bring us into a fantastic two-dimensional (2D) age of nanotechnology, which can be fabricated and applied at wafer scale, visible at single layer but showing exceptional properties distinguished from its bulk form graphite, linking the properties of atomic layers with the engineering scale of our mankind. We shown that flow-induced-voltage in graphene can be 20 folds higher than in graphite, not only due to the giant Seebeck coefficient of single layer graphene, but also the exceptional interlayer interaction in few layer graphene. Extremely excitingly, water flow over graphene can generate electricity through unexpected interaction of the ions in the water with the graphene. We also find extraordinary mechanical-electric-magnetic coupling effects in graphene and BN systems. Such extraordinary multifield coupling effects in graphene and functional nanosystems open up new vistas in nanotechnology for efficient energy conversion, self-powering flexible devices and novel functional systems.   

High efficiency graphene solar cell by chemical doping

We demonstrate single layer graphene/n-Si Schottky junction solar cells that under AM1.5 illumination exhibit a power conversion efficiency (PCE) of 8.6{\%}. This performance, achieved by doping the graphene with bis(trifluoromethanesulfonyl)amide (TFSA), exceeds the native (undoped) device performance by a factor of 4.5 and is the highest PCE reported for graphene-based solar cells to date. Current--voltage, capacitance--voltage, and external quantum efficiency measurements show the enhancement to be due to the doping-induced shift in the graphene chemical potential that increases the graphene carrier density (decreasing the cell series resistance) and increases the cell's built-in potential (increasing the open circuit voltage) both of which improve the solar cell fill factor.   

Novel highly conductive graphene-based materials

The development of future flexible and transparent electronics relies on novel materials, which are mechanically flexible, lightweight and low-cost, in addition to being electrically conductive and optically transparent. Currently, tin doped indium oxide (ITO) is the most wide spread transparent conductor in consumer electronics. The mechanical rigidity of this material limits its use for future flexible electronic applications. We report novel graphene-based transparent conductors obtained by intercalating few-layer graphene (FLG) with ferric chloride (FeCl3). Through a combined study of electrical transport and optical transmission measurements we demonstrate that FeCl3 enhances the electrical conductivity of FLG by two orders of magnitude while leaving these materials highly transparent [1]. We find that the optical transmittance in the visible range of FeCl3-FLG is typically between 88{\%} and 84{\%}, whereas the resistivity is as low as 8.8 $\Omega $. These parameters outperform the best values found in ITO (i.e. resistivity of 10 $\Omega $ at an optical transmittance of 85{\%}), making therefore FeCl3-FLG the best candidate for flexible and transparent electronics. \\[4pt] [1] I. Khrapach, F. Withers, T. H. Bointon, D. K. Pplyushkin, W. L. Barnes, S. Russo, M. F. Craciun, Adv. Mater. 24, 2844 (2012).   

Catalytic activity of transition metal-N$_4$ moieties in graphene toward the oxygen reduction reaction: A DFT study

The search for non-precious metal cathode catalysts for the oxygen reduction reaction (ORR) that replace platinum in proton exchange membrane fuel cells is one of the main challenges toward the use of hydrogen as clean energy for transportation. Most current works on ORR catalysts focuses on N-coordinated iron in a carbon matrix. Although the nature of the active site is still a mystery, different carbon-supported Fe-N$_x$ active sites have been proposed. In this work, The O$_2$ dissociation after the interaction with the metal center of M-N$_4$ moieties in graphene (with M = Mn, Fe, and Co) are addressed by density functional theory calculations. Both, saddle points and minimum energy paths for the ORR in the allowed spin channels have been identified. Our results show that the Mn-N$_4$ center in graphene exhibits the lowest activation barrier in all spin channel, less than 1 eV, suggesting improved ORR activity, while for Fe-N$_4$ and Co-N$_4$ they range between 1.2 and 1.6 eV. Our calculations suggest that the O$_2$ dissociation would proceed through different spin channel which would increase the reaction rate, particularly for Mn-O$_2$ and Fe-O$_2$ moieties. We also investigate energetically favorable routes to incorporate the M-N$_4$ centers in graphene.   

Simulating DNA sequencing using graphene nanopores: a QM/MM Nonequilibrium Green's function approach

Graphene is both the thinnest possible membrane and presents exceptional electronic transport properties. This combination could pave the way for applications in devices where high selectivity single molecule detection is required, for example for sequencing DNA. In this work we will present theoretical electronic transport calculations of a possible DNA sequencing device based on graphene nanopores. We consider both single and double layer graphene. The simulations were performed using a QM/MM method which allows us to treat the graphene sheet containing the nanopore and a segment of DNA within the pore via ab initio density functional theory (QM) whereas the effects of the water molecules, the counter-ions and the remainder of the DNA strand are taken into consideration using a classical potential (MM), in such a way that thousands of atoms can be taken into consideration. The arrangement is allowed to evolve in order to sample the configuration space of different basis, and the electronic transport properties along graphene - from a sample of the frames - are subsequently calculated using non-equilibrium Green's functions taking into consideration the solvent effects in the simulation. The effects of the solvent, counter ions and of different stacked basis will be discussed.   

Graphene Nanopores for Single-Molecule DNA Sequencing

We fabricate a nanopore in a suspended single-layer graphene membrane, which serves as a barrier between two aqueous DNA reservoirs. This nanopore device can detect the electrophoretic passage of single or double stranded DNA through transient ionic current blockades caused by DNA obstruction of the pore. Furthermore, a graphene pore, which has atomic thickness, should allow discrimination of different DNA base pairs by ionic current measurements alone. This base discrimination can become the basis of a single-molecule, ultrafast DNA sequencing scheme. We demonstrate the fabrication and evaluate the performance of these graphene nanopore devices.   

Scalable Arrays of DNA-decorated Graphene Chemical Vapor Sensors

Chemical vapor sensors based on biomolecular functionalization of graphene field effect transistor arrays are demonstrated. Novel photolithographic methods were developed to fabricate high quality transistors from CVD-grown graphene. Atomic Force Microscopy was used to verify that the graphene surface remained uncontaminated and was thus suitable for controlled chemical functionalization. Single-stranded DNA was chosen as the functionalizing biomolecule due to its affinity to a wide range of target molecules as well as its $\pi $-$\pi $ stacking interaction with graphene, which allowed functionalization with minimal impact on the transistor mobility. The resulting sensor arrays showed analyte and DNA sequence dependent responses down to parts-per-billion level concentrations. By using large arrays of differently functionalized devices, we distinguished chemically similar analytes and determined electronic signatures indicative of their presence.   

Graphene Nanopres for DNA Fingerprinting

The recent progress in nanopore experiments with transverse current is important for the development of fast, accurate and cheap finger-printing techniques for single nucleotide. Despite its enormous potential for the next generation DNA sequencing technology, the presence of large noise in the temporal spectrum of transverse current remains a big challenge for getting highly accurate interpretation of data. In this paper we present our {\it abinitio} calculations, and propose graphene based device for DNA fingerprinting. We calculate transmission current through graphene for each DNA base (A,C,G,T). As shown in our work, a proper time-series analysis of a signal provides a higher quality information in identifying single bio-molecule is translocating through the nanopores.   

Electrochemistry of Graphene Edge Embedded Nanopores

We demonstrate a stacked graphene- Al$_{2}$O$_{3}$ dielectric nanopore architecture to investigate electrochemical activity at graphene edges. It has proven to be difficult to isolate electrochemical activity at the graphene edges from those at the basal planes [1]. We use 24 nm of Al$_{2}$O$_{3}$ to isolate the graphene basal planes from an ionic fluid environment. Nanopores ranging from 5 to 20 nm are formed by an electron beam sculpting process to expose graphene edges. Electrochemical measurements at isolated graphene edges show current densities as high as 1.2 x 10$^{4}$ A/cm$^{2}$, 300x greater than those reported for carbon nanotubes [2]. Additionally, we modulate nanopore conductance by tuning the graphene edge electrochemical current as a function of the applied bias on the embedded graphene electrode. Our results indicate that electrochemical devices based on graphene nanopores have promising applications as sensitive chemical and biological sensors, energy storage devices, and DNA sequencing.\\[4pt] [1] Ambrosi, \textit{et al.}, Nanoscale \textbf{3}, 2256 (2011);\\[0pt] [2] J. Britto, \textit{et al.}, Adv. Mater. \textbf{11}, 154 (1999)