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Klein tunneling, the perfect transmission of a normally incident relativistic particle through an energy barrier, has been tested in various electronic, photonic, and phononic systems. Its potential in guiding and filtering classical waves in the Ultra High Frequency regime, on the other hand, has not been explored. Here, we report the realization of acoustic Klein tunneling in a nanoelectromechanical metamaterial system operating at gigahertz frequencies. The piezoelectric potential profiles are obtained by transmission-mode microwave impedance microscopy, from which reciprocal-space maps can be extracted. The transmission rate of normally incident elastic waves is near unity in the Klein tunneling regime and drops significantly outside this frequency range, consistent with microwave network analysis. Strong angular dependent transmission is also observed by controlling the launching angle of the emitter interdigital transducer. This work broadens the horizon in exploiting high-energy-physics phenomena for practical circuit applications in both classical and quantum regimes., Comment: 15 pages, 5 figures, 14 pages of SI

Quantum valley Hall (QVH) domain wall states are a new class of one-dimensional (1D) one-way conductors that are topologically protected in the absence of valley mixing. Development beyond a single QVH channel raises important new questions as to how QVH channels in close spatial proximity interact with each other, and how that interaction may be controlled. Scalable epitaxial bilayer graphene synthesis produces layer stacking wall (LSW) bundles, where QVH channels are bound, providing an excellent platform to study QVH channel interactions. Here we show that distinct strain sources lead to the formation of both well-separated LSWs and close packed LSW bundles. Comparative studies of electronic transport in these two regimes reveal that close-packed LSW bundles support electrically tunable magnetoconductance. The coexistence of different strain sources offers a potential pathway to realize scalable quantum transport platform based on LSWs where electrically tunability enables programmable functionality.

Topological phononics offers numerous opportunities in manipulating elastic waves that can propagate in solids without being backscattered. Due to the lack of nanoscale imaging tools that aid the system design, however, acoustic topological metamaterials have been mostly demonstrated in macroscale systems operating at low (kilohertz to megahertz) frequencies. Here, we report the realization of gigahertz topological valley Hall effect in nanoelectromechanical AlN membranes. Propagation of elastic wave through phononic crystals is directly visualized by microwave microscopy with unprecedented sensitivity and spatial resolution. The valley Hall edge states, protected by band topology, are vividly seen in both real- and momentum-space. The robust valley-polarized transport is evident from the wave transmission across local disorder and around sharp corners, as well as the power distribution into multiple edge channels. Our work paves the way to exploit topological physics in integrated acousto-electronic systems for classical and quantum information processing in the microwave regime., Comment: 17 pages, 5 figures

Semiconducting monolayer of 2D material are able to concatenate multiple interesting properties into a single component. Here, by combining opto-mechanical and electronic measurements, we demonstrate the presence of a partial 2H-1T phase transition in a suspended 2D monolayer membrane of MoS2. Electronic transport shows unexpected memristive properties in the MoS2 membrane, in the absence of any external dopants. A strong mechanical softening of the membrane is measured concurrently and may only be related to the phase 2H-1T phase transition which imposes a 3percent directional elongation of the topological 1T phase with respect to the semiconducting 2H. We note that only a few percent 2H- 1T phase switching is sufficient to observe measurable memristive effects. Our experimental results combined with First-principles total energy calculations indicate that sulfur vacancy diffusion plays a key role in the initial nucleation of the phase transition. Our study clearly shows that nanomechanics represents an ultrasensitive technique to probe the crystal phase transition in 2D materials or thin membranes. Finally, a better control of the microscopic mechanisms responsible for the observed memristive effect in MoS2 is important for the implementation of future devices., Comment: ACS nano (2020)

Bilayer two-dimensional (2D) van der Waals (vdW) materials are attracting increasing attention due to their predicted high quality electronic and optical properties. Here we demonstrate dense, selective growth of WSe2 bilayer flakes by chemical vapor deposition with the use of a 1:10 molar mixture of sodium cholate and sodium chloride as the growth promoter to control the local diffusion of W-containing species. A large fraction of the bilayer WSe2 flakes showed a 0 and 60o twist between the two layers, while moire 15 and 30o-twist angles were also observed. Well-defined monolayer-bilayer junctions were formed in the as-grown bilayer WSe2 flakes, and these interfaces exhibited p-n diode rectification and an ambipolar transport characteristic. This work provides an efficient method for the layer-controlled growth of 2D materials, in particular, 2D transition metal dichalcogenides and promotes their applications in next-generation electronic and optoelectronic devices.

Hybrid heterostructures based on bulk GaN and two-dimensional (2D) materials offer novel paths toward nanoelectronic devices with engineered features. Here, we study the electronic properties of a mixed-dimensional heterostructure composed of intrinsic n-doped MoS2 flakes transferred on p-doped GaN(0001) layers. Based on angle-resolved photoemission spectroscopy (ARPES) and high resolution X-ray photoemission spectroscopy (HR-XPS), we investigate the electronic structure modification induced by the interlayer interactions in MoS2/GaN heterostructure. In particular, a shift of the valence band with respect to the Fermi level for MoS2/GaN heterostructure is observed; which is the signature of a charge transfer from the 2D monolayer MoS2 to GaN. ARPES and HR-XPS revealed an interface dipole associated with local charge transfer from the GaN layer to the MoS2 monolayer. Valence and conduction band offsets between MoS2 and GaN are determined to be 0.77 and -0.51 eV, respectively. Based on the measured work functions and band bendings, we establish the formation of an interface dipole between GaN and MoS2 of 0.2 eV., Comment: 13 pages, 5 figures + SI 2 pages, 2 figures

Structural defects in the molybdenum disulfide (MoS2) monolayer are widely known for strongly altering its properties. Therefore, a deep understanding of these structural defects and how they affect MoS2 electronic properties is of fundamental importance. Here, we report on the incorporation of atomic hydrogen in mono-layered MoS2 to tune its structural defects. We demonstrate that the electronic properties of single layer MoS2 can be tuned from the intrinsic electron (n) to hole (p) doping via controlled exposure to atomic hydrogen at room temperature. Moreover, this hydrogenation process represents a viable technique to completely saturate the sulfur vacancies present in the MoS2 flakes. The successful incorporation of hydrogen in MoS2 leads to the modification of the electronic properties as evidenced by high resolution X-ray photoemission spectroscopy and density functional theory calculations. Micro-Raman spectroscopy and angle resolved photoemission spectroscopy measurements show the high quality of the hydrogenated MoS2 confirming the efficiency of our hydrogenation process. These results demonstrate that the MoS2 hydrogenation could be a significant and efficient way to achieve tunable doping of transition metal dichalcogenides (TMD) materials with non-TMD elements., Comment: 15 pages, 6 figures + SI 3 pages 3 figures. Pre-print published with the authorization of ACS Publications

Simpler and more rapid approaches for therapeutic drug-level monitoring are highly desirable to enable use at the point-of-care. We have developed an all-electronic approach for detection of the HIV drug tenofovir based on scalable fabrication of arrays of graphene field-effect transistors (GFETs) functionalized with a commercially available DNA aptamer. The shift in the Dirac voltage of the GFETs varied systematically with the concentration of tenofovir in deionized water, with a detection limit less than 1 ng/mL. Tests against a set of negative controls confirmed the specificity of the sensor response. This approach offers the potential for further development into a rapid and convenient point-of-care tool with clinically relevant performance., Comment: 7 pages, 2 figures

Advances in techniques for monitoring pH in complex fluids could have significant impact on analytical and biomedical applications ranging from water quality assessment to in vivo diagnostics. We developed flexible graphene microelectrodes (GEs) for rapid (< 5 seconds), very low power (femtowatt) detection of the pH of complex biofluids. The method is based on real-time measurement of Faradaic charge transfer between the GE and a solution at zero electrical bias. For an idealized sample of phosphate buffer solution (PBS), the Faradaic current varied monotonically and systematically with the pH with resolution of ~0.2 pH unit. The current-pH dependence was well described by a hybrid analytical-computational model where the electric double layer derives from an intrinsic, pH-independent (positive) charge associated with the graphene-water interface and ionizable (negative) charged groups described by the Langmuir-Freundlich adsorption isotherm. We also tested the GEs in more complex bio-solutions. In the case of a ferritin solution, the relative Faradaic current, defined as the difference between the measured current response and a baseline response due to PBS, showed a strong signal associated with the disassembly of the ferritin and the release of ferric ions at pH ~ 2.0. For samples of human serum, the Faradaic current showed a reproducible rapid (<20s) response to pH. By combining the Faradaic current and real time current variation, the methodology is potentially suitable for use to detect tumor-induced changes in extracellular pH.

This report describes the development of an aptamer for sensing azole antifungal drugs for therapeutic drug monitoring. Modified Synthetic Evolution of Ligands through Exponential Enrichment (SELEX) was used to discover a DNA aptamer recognizing azole class antifungal drugs. This aptamer undergoes a secondary structural change upon binding to its target molecule as shown through fluorescence anisotropy-based binding measurements. Experiments using circular dichroism spectroscopy, revealed a unique double G-quadruplex structure that was essential and specific for binding to the azole antifungal target. Aptamer-functionalized Graphene Field Effect Transistor (GFET) devices were created and used to measure the binding of strength of azole antifungals to this surface. In total this aptamer and the supporting sensing platform could provide a valuable tool for improving the treatment of patients with invasive fungal infections.

The characterization of protein-nanoparticle assemblies in solution remains a challenge. We demonstrate a technique based on a graphene microelectrode for structural-functional analysis of model systems composed of nanoparticles enclosed in open-pore and closed-pore ferritin molecules. The method readily resolves the difference in accessibility of the enclosed nanoparticle for charge transfer and offers the prospect for quantitative analysis of pore-mediated transport shed light on the spatial orientation of the protein subunits on the nanoparticle surface, faster and with higher sensitivity than conventional catalysis methods.

Scalable production of all-electronic DNA biosensors with high sensitivity and selectivity is a critical enabling step for research and applications associated with detection of DNA hybridization. We have developed a scalable and very reproducible (> 90% yield) fabrication process for label-free DNA biosensors based upon graphene field effect transistors (GFETs) functionalized with single-stranded probe DNA. The shift of the GFET sensor Dirac point voltage varied systematically with the concentration of target DNA. The biosensors demonstrated a broad analytical range and limit of detection of 1 fM for 60-mer DNA oligonucleotide. In control experiments with mismatched DNA oligomers, the impact of the mismatch position on the DNA hybridization strength was confirmed. This class of highly sensitive DNA biosensors offers the prospect of detection of DNA hybridization and sequencing in a rapid, inexpensive, and accurate way.

Liquid-based bio-applications of graphene require a quantitative understanding of the graphene-liquid interface, with the surface charge density of adsorbed ions, the interfacial charge transfer resistance, and the interfacial charge noise being of particular importance. We quantified these properties through measurements of the zero-bias Faradaic charge-transfer between graphene electrodes and aqueous solutions of varying ionic strength using a reproducible, low-noise, minimally perturbative charge measurement technique. The measurements indicated that adsorbed ions had a negative surface charge density of approximately -32.8 mC m-2 and that the specific charge transfer resistance was 6.5pm0.3 M${\Omega}$ cm2. The normalized current noise power spectral density for all ionic concentrations tested collapsed onto a 1/f characteristic with ${\alpha}$=1.1pm0.2. All the results are in excellent agreement with predictions of the theory for the graphene-solution interface. This minimally-perturbative method for monitoring charge-transfer at the sub-pC scale exhibits low noise and ultra-low power consumption (~ fW), making it well-suited for use in low-level bioelectronics in liquid environments.

Liquid-based applications of biomolecule-decorated field-effect transistors (FETs) range from biosensors to in vivo implants. A critical scientific challenge is to develop a quantitative understanding of the gating effect of charged biomolecules in ionic solution and how this influences the readout of the FETs. To address this issue, we fabricated protein-decorated graphene FETs and measured their electrical properties, specifically the shift in Dirac voltage, in solutions of varying ionic strength. We found excellent quantitative agreement with a model that accounts for both the graphene polarization charge and ionic screening of ions adsorbed on the graphene as well as charged amino acids associated with the immobilized protein. The technique and analysis presented here directly couple the charging status of bound biomolecules to readout of liquid-phase FETs fabricated with graphene or other two-dimensional materials.

We study exciton-plasmon coupling in two-dimensional semiconductors coupled with Ag plasmonic lattices via angle-resolved reflectance spectroscopy and by solving the equations of motion (EOMs) in a coupled oscillator model accounting for all the resonances of the system. Five resonances are considered in the EOM model: semiconductor A and B excitons, localized surface plasmon resonances (LSPRs) of plasmonic nanostructures and the lattice diffraction modes of the plasmonic array. We investigated the exciton-plasmon coupling in different 2D semiconductors and plasmonic lattice geometries, including monolayer MoS2 and WS2 coupled with Ag nanodisk and bowtie arrays, and examined the dispersion and lineshape evolution in the coupled systems via the EOM model with different exciton-plasmon coupling parameters. The EOM approach provides a unified description of the exciton-plasmon interaction in the weak, intermediate and strong coupling cases with correctly explaining the dispersion and lineshapes of the complex system. This study provides a much deeper understanding of light-matter interactions in multilevel systems in general and will be useful to instruct the design of novel two-dimensional exciton-plasmonic devices for a variety of optoelectronic applications with precisely tailored responses.

The topologically nontrivial and chemically functional distorted octahedral (T') transition metal dichalcogenides (TMDCs) are a type of layered semimetal that has attracted significant recent attention. However, the properties of monolayer (1L) T'-TMDC, a fundamental unit of the system, is still largely unknown due to rapid sample degradation in air. Here we report that well-protected 1L CVD T'-MoTe2 exhibits sharp and robust intrinsic Raman bands, with intensities about one order of magnitude stronger than those from bulk T'-MoTe2. The high-quality samples enabled us to reveal the set of all nine even-parity zone-center optical phonons, providing reliable fingerprints for the previously elusive crystal. By performing light polarization and crystal orientation resolved scattering analysis, we can effectively distinguish the intrinsic modes from Te-metalloid-like modes A (~122 cm-1) and B (~141 cm-1) which are related to the sample degradation. Our studies offer a powerful non-destructive method for assessing sample quality and for monitoring sample degradation in situ, representing a solid advance in understanding the fundamental properties of 1L-T'-TMDCs.

Active control of light-matter interactions in semiconductors is critical for realizing next generation optoelectronic devices, with tunable control of the systems optical properties via external fields. The ability to manipulate optical interactions in active materials coupled to cavities via geometrical parameters, which are fixed along with dynamic control with applied fields opens up possibilities of controlling exciton lifetimes, oscillator strengths and their relaxation properties. Here, we demonstrate electrical control of exciton-plasmon polariton coupling strength of a two-dimensional semiconductor integrated with plasmonic nanoresonators assembled in a field-effect transistor device between strong and weak coupling limits by electrostatic doping. As a result, the exciton-plasmon polarion dispersion can be altered dynamically with applied electric field by modulating the excitonic properties of monolayer MoS2 arising from many-body effects with carrier concentration. In addition, strong coupling between charged excitons plasmons was also observed upon increased carrier injection. The ability to dynamically control the optical properties of an ultra-thin semiconductor with plasmonic nanoresonators and electric fields demonstrates the versatility of the coupled system and offers a new platform for the design of optoelectronic devices with precisely tailored responses.

We demonstrate strong exciton-plasmon coupling in silver nanodisk arrays integrated with monolayer MoS2 via angle-resolved reflectance microscopy spectra of the coupled system. Strong exciton-plasmon coupling is observed with the exciton-plasmon coupling strength up to 58 meV at 77 K, which also survives at room temperature. The strong coupling involves three types of resonances: MoS2 excitons, localized surface plasmon resonances (LSPRs) of individual silver nanodisks and plasmonic lattice resonances of the nanodisk array. We show that the exciton-plasmon coupling strength, polariton composition and dispersion can be effectively engineered by tuning the geometry of the plasmonic lattice, which makes the system promising for realizing novel two-dimensional plasmonic polaritonic devices.

The manipulation of light-matter interactions in two-dimensional atomically thin crystals is critical for obtaining new optoelectronic functionalities in these strongly confined materials. Here, by integrating chemically grown monolayers of MoS2 with a silver-bowtie nanoantenna array supporting narrow surface-lattice plasmonic resonances, a unique two-dimensional optical system has been achieved. The enhanced exciton-plasmon coupling enables profound changes in the emission and excitation processes leading to spectrally tunable, large photoluminescence enhancement as well as surface-enhanced Raman scattering at room temperature. Furthermore, at low temperatures, due to the decreased damping of MoS2 excitons interacting with the plasmonic resonances of the bowtie array, stronger exciton-plasmon coupling is achieved resulting in a Fano lineshape in the reflection spectrum. The Fano lineshape, which is due to the interference between the pathways involving the excitation of the exciton and plasmon, can be tuned by altering the coupling strengths between the two systems via changing the design of the bowties lattice. The ability to manipulate the optical properties of two-dimensional systems with tunable plasmonic resonators offers a new platform for the design of novel optical devices with precisely tailored responses.

We have developed a novel, all-electronic biosensor for opioids that consists of an engineered mu opioid receptor protein, with high binding affinity for opioids, chemically bonded to a graphene field-effect transistor to read out ligand binding. A variant of the receptor protein that provided chemical recognition was computationally redesigned to enhance its solubility and stability in an aqueous environment. A shadow mask process was developed to fabricate arrays of hundreds of graphene transistors with average mobility of ~1500 cm2 V-1 s-1 and yield exceeding 98%. The biosensor exhibits high sensitivity and selectivity for the target naltrexone, an opioid receptor antagonist, with a detection limit of 10 pg/mL., Comment: Nano Letters 2014

Vapor sensors based on functionalized carbon nanotubes (NTs) have shown great promise, with high sensitivity conferred by the reduced dimensionality and exceptional electronic properties of the NT. Critical challenges in the development of NT-based sensor arrays for chemical detection include the demonstration of reproducible fabrication methods and functionalization schemes that provide high chemical diversity to the resulting sensors. Here, we outline a scalable approach to fabricating arrays of vapor sensors consisting of NT field effect transistors functionalized with single-stranded DNA (DNA-NT). DNA-NT sensors were highly reproducible, with responses that could be described through equilibrium thermodynamics. Target analytes were detected even in large backgrounds of volatile interferents. DNA-NT sensors were able to discriminate between highly similar molecules, including structural isomers and enantiomers. The sensors were also able to detect subtle variations in complex vapors, including mixtures of structural isomers and mixtures of many volatile organic compounds characteristic of humans., Comment: 13 page, 5 figures

We developed a scalable, label-free all-electronic sensor for D-glucose based on a carbon nanotube transistor functionalized with pyrene-1-boronic acid. This sensor responds to glucose in the range 1 uM - 100 mM, which includes typical glucose concentrations in human blood and saliva. Control experiments establish that functionalization with the boronic acid provides high sensitivity and selectivity for glucose. The devices show better sensitivity than commercial blood glucose meters and could represent a general strategy to bloodless glucose monitoring by detecting low concentrations of glucose in saliva., Comment: 8 pages, 4 figures, to appear in Applied Physics Letters

Despite the great promise of carbon nanotube field effect transistors (CNT FETs) for applications in chemical and biochemical detection, a quantitative understanding of sensor responses is lacking. To explore the role of electrostatics in sensor transduction, experiments were conducted with a set of highly similar compounds designed to adsorb onto the CNT FET via a pyrene linker group and take on a set of known charge states under ambient conditions. Acidic and basic species were observed to induce threshold voltage shifts of opposite sign, consistent with gating of the CNT FET by local charges due to protonation or deprotonation of pyrene compounds by interfacial water. The magnitude of the gate voltage shift was controlled by the distance between the charged group and the CNT. Additionally, functionalization with an un-charged pyrene compound showed a threshold shift ascribed to its molecular dipole moment. This work illustrates a method to produce CNT FETs with controlled values of the turnoff gate voltage, and more generally, these results will inform the development of quantitative models for the response of CNT FET chemical and biochemical sensors., Comment: 6 pages, 5 figures, appears in JACS

We examined the potential of antibody-functionalized single-walled carbon nanotube (SWNT) field-effect transistors (FETs) for use as a fast and accurate sensor for a Lyme disease antigen. Biosensors were fabricated on oxidized silicon wafers using chemical vapor deposition grown carbon nanotubes that were functionalized using diazonium salts. Attachment of Borrelia burgdorferi (Lyme) flagellar antibodies to the nanotubes was verified by Atomic Force Microscopy and electronic measurements. A reproducible shift in the turn-off voltage of the semiconducting SWNT FETs was seen upon incubation with Borrelia burgdorferi flagellar antigen, indicative of the nanotube FET being locally gated by the residues of flagellar protein bound to the antibody. This sensor effectively detected antigen in buffer at concentrations as low as 1 ng/ml, and the response varied strongly over a concentration range coinciding with levels of clinical interest. Generalizable binding chemistry gives this biosensing platform the potential to be expanded to monitor other relevant antigens, enabling a multiple vector sensor for Lyme disease. The speed and sensitivity of this biosensor make it an ideal candidate for development as a medical diagnostic test., Comment: 10 pages, 3 figures, appears in Biosensors and Bioelectronics

We developed a novel detection method for osteopontin (OPN), a new biomarker for prostate cancer, by attaching a genetically engineered single chain variable fragment (scFv) protein with high binding affinity for OPN to a carbon nanotube field-effect transistor (NTFET). Chemical functionalization using diazonium salts is used to covalently attach scFv to NT-FETs, as confirmed by atomic force microscopy, while preserving the activity of the biological binding site for OPN. Electron transport measurements indicate that functionalized NT-FET may be used to detect the binding of OPN to the complementary scFv protein. A concentration-dependent increase in the source-drain current is observed in the regime of clinical significance, with a detection limit of approximately 30 fM. The scFv-NT hybrid devices exhibit selectivity for OPN over other control proteins. These devices respond to the presence of OPN in a background of concentrated bovine serum albumin, without loss of signal. Based on these observations, the detection mechanism is attributed to changes in scattering at scFv protein-occupied defect sites on the carbon nanotube sidewall. The functionalization procedure described here is expected to be generalizable to any antibody containing an accessible amine group, and to result in biosensors appropriate for detection of corresponding complementary proteins at fM concentrations., Comment: 11 pages, 5 figures, appears in ACS Nano

Antibody-functionalized carbon nanotube devices have been suggested for use as bacterial detectors for monitoring of food purity in transit from the farm to the kitchen. Here we report progress towards that goal by demonstrating specific detection of Salmonella in complex nutrient broth solutions using nanotube transistors functionalized with covalently-bound anti-Salmonella antibodies. The small size of the active device region makes them compatible with integration in large-scale arrays. We find that the on-state current of the transistor is sensitive specifically to the Salmonella concentration and saturates at low concentration (<1000 cfu/ml). In contrast, the carrier mobility is affected comparably by Salmonella and other bacteria types, with no sign of saturation even at much larger concentrations (10^8 cfu/ml)., Comment: 8 pages, 3 figures, 1 table, appears in AIP Advances

We report electronic measurements on high-quality graphene nanoconstrictions (GNCs) fabricated in a transmission electron microscope (TEM), and the first measurements on GNC conductance with an accurate measurement of constriction width down to 1 nm. To create the GNCs, freely-suspended graphene ribbons were fabricated using few-layer graphene grown by chemical vapor deposition. The ribbons were loaded into the TEM, and a current-annealing procedure was used to clean the material and improve its electronic characteristics. The TEM beam was then used to sculpt GNCs to a series of desired widths in the range 1 - 700 nm; after each sculpting step, the sample was imaged by TEM and its electronic properties measured in-situ. GNC conductance was found to be remarkably high, comparable to that of exfoliated graphene samples of similar size. The GNC conductance varied with width approximately as, where w is the constriction width in nanometers. GNCs support current densities greater than 120 \muA/nm2, two orders of magnitude higher than has been previously reported for graphene nanoribbons and 2000 times higher than copper., Comment: 17 pages, 4 figures. Accepted by Nano Letters

We report a method to pattern monolayer graphene nanoconstriction field effect transistors (NCFETs) with critical dimensions below 10 nm. NCFET fabrication is enabled by the use of feedback controlled electromigration (FCE) to form a constriction in a gold etch mask that is first patterned using conventional lithographic techniques. The use of FCE allows the etch mask to be patterned on size scales below the limit of conventional nanolithography. We observe the opening of a confinement-induced energy gap as the NCFET width is reduced, as evidenced by a sharp increase in the NCFET on/off ratio. The on/off ratios we obtain with this procedure can be larger than 1000 at room temperature for the narrowest devices; this is the first report of such large room temperature on/off ratios for patterned graphene FETs., Comment: 18 pages, 6 figures, to appear in Small

Graphene is a true two dimensional material with exceptional electronic properties and enormous potential for practical applications. Graphene's promise as a chemical sensor material has been noted but there has been relatively little work on practical chemical sensing using graphene, and in particular how chemical functionalization may be used to sensitize graphene to chemical vapors. Here we show one route towards improving the ability of graphene to work as a chemical sensor by using single stranded DNA as a sensitizing agent. The resulting broad response devices show fast response times, complete and rapid recovery to baseline at room temperature, and discrimination between several similar vapor analytes., Comment: 7 pages, To appear in Applied Physics Letters

We observe that gold atoms deposited by physical vapor deposition onto few layer graphenes condense upon annealing to form nanoparticles with an average diameter that is determined by the graphene film thickness. The data are well described by a theoretical model in which the electrostatic interactions arising from charge transfer between the graphene and the gold particle limit the size of the growing nanoparticles. The model predicts a nanoparticle size distribution characterized by a mean diameter D that follows a scaling law D proportional to m^(1/3), where m is the number of carbon layers in the few layer graphene film., Comment: 15 pages, 4 figures

The realization of single-molecule electronic devices, in which a nanometer-scale molecule is connected to macroscopic leads, requires the reproducible production of highly ordered nanoscale gaps in which a molecule of interest is electrostatically coupled to nearby gate electrodes. Understanding how the molecule-gate coupling depends on key parameters is crucial for the development of high-performance devices. Here we directly address this, presenting two- and three-dimensional finite-element electrostatic simulations of the electrode geometries formed using emerging fabrication techniques. We quantify the gate coupling intrinsic to these devices, exploring the roles of parameters believed to be relevant to such devices. These include the thickness and nature of the dielectric used, and the gate screening due to different device geometries. On the single-molecule (~1nm) scale, we find that device geometry plays a greater role in the gate coupling than the dielectric constant or the thickness of the insulator. Compared to the typical uniform nanogap electrode geometry envisioned, we find that non-uniform tapered electrodes yield a significant three orders of magnitude improvement in gate coupling. We also find that in the tapered geometry the polarizability of a molecular channel works to enhance the gate coupling.

Graphene is a purely two-dimensional material that has extremely favorable chemical sensor properties. It is known, however, that conventional nanolithographic processing typically leaves a resist residue on the graphene surface, whose impact on the sensor characteristics of the system has not yet been determined. Here we show that the contamination layer both degrades the electronic properties of the graphene and masks graphene s intrinsic sensor responses. The contamination layer chemically dopes the graphene, enhances carrier scattering, and acts as an absorbent layer that concentrates analyte molecules at the graphene surface, thereby enhancing the sensor response. We demonstrate a cleaning process that verifiably removes the contamination on the device structure and allows the intrinsic chemical responses of graphene to be measured., Comment: 9 pages and 2 figures

We measured the electronic properties and gas sensing responses of template-grown poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS)-based nanowires. The nanowires have a "striped" structure (gold-PEDOT/PSS-gold), typically 8um long (1um-6um-1um for each section, respectively) and 220 nm in diameter. Single-nanowire devices were contacted by pre-fabricated gold electrodes using dielectrophoretic assembly. A polymer conductivity of 11.5 +/- 0.7 S/cm and a contact resistance of 27.6 +/- 4 kOhm were inferred from measurements of nanowires of varying length and diameter. The nanowire sensors detect a variety of odors, with rapid response and recovery (seconds). The response (R-R0)/R0 varies as a power law with analyte concentration., Comment: 4 figures 8 pages, add 2 references

Graphene-derived nanomaterials are emerging as ideal candidates for postsilicon electronics. Elucidating the electronic interaction between an insulating substrate and few-layer graphene (FLG) films is crucial for device applications. Here, we report electrostatic force microscopy (EFM) measurements revealing that the FLG surface potential increases with film thickness, approaching a "bulk" value for samples with five or more graphene layers. This behavior is in sharp contrast with that expected for conventional conducting or semiconducting films, and derives from unique aspects of charge screening by graphene's relativistic low energy carriers. EFM measurements resolve previously unseen electronic perturbations extended along crystallographic directions of structurally disordered FLGs, likely resulting from long-range atomic defects. These results have important implications for graphene nanoelectronics and provide a powerful framework by which key properties can be further investigated.

We demonstrate a method by which few-layer graphene samples can be etched along crystallographic axes by thermally activated metallic nanoparticles. The technique results in long (>1 micron) crystallographic edges etched through to the insulating substrate, making the process potentially useful for atomically precise graphene device fabrication. This advance could enable atomically precise construction of integrated circuits from single graphene sheets with a wide range of technological applications.

Chemical gas sensors based on nanowires can find a wide range of applications in clinical assaying, environmental emission control, explosive detection, agricultural storage and shipping, and workplace hazard monitoring. Sensors in the forms of nanowires are expected to have significantly enhanced performance due to high surface-volume ratio and quasi-one-dimensional confinement in nanowires. Indeed, chemical gas sensors based on nanowires with a ppb level sensitivity have been demonstrated. In this review, the fundamental aspects on (i) methods of nanowire synthesis (ii) performance of nanowire sensors, (iii) chemiresistors, transistor sensors, and their sensing mechanism, and (iv) assembly technologies will be summarized and discussed. The prospects of the future research on chemical gas sensors based on nanowires will be also addressed., Comment: 33 pages 17 figures