Search

Copyright © 2019 American Chemical Society. The synthesis of graphene on cubic silicon carbide on silicon pseudosubstrates draws enormous interest due to the potential integration of the 2D material with the well-established silicon technology and processing. However, the control of transport properties over large scales on this platform, essential for integrated electronics and photonics applications, has lagged behind so far, due to limitations such as 3C-SiC/Si interface instability and nonuniform graphene coverage. We address these issues by obtaining an epitaxial graphene (EG) onto 3C-SiC on a highly resistive silicon substrate using an alloy-mediated, solid-source graphene synthesis. We report the transport properties of EG grown over large areas directly on 3C-SiC(100) and 3C-SiC(111) substrates, and we present the corresponding physical models. We observe that the carrier transport of EG/3C-SiC is dominated by the graphene-substrate interaction rather than the EG grain size, sharing the same conductivity and same inverse power law as EG on 4H- or 6H-SiC(0001) substrates - although the grain sizes for the latter are vastly different. In addition, we show that the induced oxidation/silicates at the EG/3C-SiC interface generate a p-type charge in this graphene, particularly high for the EG/3C-SiC(001). When silicates are at the interface, the presence of a buffer layer in the EG/3C-SiC(111) system is found to reduce somewhat the charge transfer. This work also indicates that a renewed focus on the understanding and engineering of the EG interfaces could very well enable the long sought-after graphene-based electronics and photonics integrated on silicon.

Trisodium bismuthide (Na3 Bi) is the first experimentally verified topological Dirac semimetal, and is a 3D analogue of graphene hosting relativistic Dirac fermions. Its unconventional momentum-energy relationship is interesting from a fundamental perspective, yielding exciting physical properties such as chiral charge carriers, the chiral anomaly, and weak anti-localization. It also shows promise for realizing topological electronic devices such as topological transistors. Herein, an overview of the substantial progress achieved in the last few years on Na3 Bi is presented, with a focus on technologically relevant large-area thin films synthesized via molecular beam epitaxy. Key theoretical aspects underpinning the unique electronic properties of Na3 Bi are introduced. Next, the growth process on different substrates is reviewed. Spectroscopic and microscopic features are illustrated, and an analysis of semiclassical and quantum transport phenomena in different doping regimes is provided. The emergent properties arising from confinement in two dimensions, including thickness-dependent and electric-field-driven topological phase transitions, are addressed, with an outlook toward current challenges and expected future progress.

An increase in power consumption necessitates a low-power circuit technology to extend Moore's law. Low-power transistors, such as tunnel field-effect transistors (TFETs), negative-capacitance field-effect transistors (NC-FETs), and Dirac-source field-effect transistors (DS-FETs), have been realised to break the thermionic limit of the subthreshold swing (SS). However, a low-power diode rectifier, which breaks the thermionic limit of an ideality factor (n) of 1 at room temperature, has not been proposed yet. In this study, we have realised a DS Schottky diode, which exhibits a steep-slope characteristic curve, by utilising the linear density of states (DOSs) of graphene. For the developed DS Schottky diode, n, 28 pages, 14 figures, submitted to Nature Communications

We use angle-resolved photoemission spectroscopy to investigate the electronic structure of bilayer graphene at high n-doping and extreme displacement fields, created by intercalating epitaxial monolayer graphene on silicon carbide with magnesium to form quasi-freestanding bilayer graphene on magnesium-terminated silicon carbide. Angle-resolved photoemission spectroscopy reveals that upon magnesium intercalation, the single massless Dirac band of epitaxial monolayer graphene is transformed into the characteristic massive double-band Dirac spectrum of quasi-freestanding bilayer graphene. Analysis of the spectrum using a simple tight binding model indicates that magnesium intercalation results in an n-type doping of 2.1 $\times$ 10$^{14}$ cm$^{-2}$, creates an extremely high displacement field of 2.6 V/nm, opening a considerable gap of 0.36 eV at the Dirac point. This is further confirmed by density-functional theory calculations for quasi-freestanding bilayer graphene on magnesium-terminated silicon carbide, which show a similar doping level, displacement field and bandgap. Finally, magnesium-intercalated samples are surprisingly robust to ambient conditions; no significant changes in the electronic structure are observed after 30 minutes exposure in air., 46 pages including supporting information, 4 figures in the main paper and 9 figures in the supporting information

The electric-field-induced quantum phase transition from topological to conventional insulator has been proposed as the basis of a topological field effect transistor1-4. In this scheme, 'on' is the ballistic flow of charge and spin along dissipationless edges of a two-dimensional quantum spin Hall insulator5-9, and 'off' is produced by applying an electric field that converts the exotic insulator to a conventional insulator with no conductive channels. Such a topological transistor is promising for low-energy logic circuits4, which would necessitate electric-field-switched materials with conventional and topological bandgaps much greater than the thermal energy at room temperature, substantially greater than proposed so far6-8. Topological Dirac semimetals are promising systems in which to look for topological field-effect switching, as they lie at the boundary between conventional and topological phases3,10-16. Here we use scanning tunnelling microscopy and spectroscopy and angle-resolved photoelectron spectroscopy to show that mono- and bilayer films of the topological Dirac semimetal3,17 Na3Bi are two-dimensional topological insulators with bulk bandgaps greater than 300millielectronvolts owing to quantum confinement in the absence of electric field. On application of electric field by doping with potassium or by close approach of the scanning tunnelling microscope tip, the Stark effect completely closes the bandgap and re-opens it as a conventional gap of 90millielectronvolts. The large bandgaps in both the conventional and quantum spin Hall phases, much greater than the thermal energy at room temperature (25millielectronvolts), suggest that ultrathin Na3Bi is suitable for room-temperature topological transistor operation.

Supramolecular chemistry protocols applied on surfaces offer compelling avenues for atomic-scale control over organic-inorganic interface structures. In this approach, adsorbate-surface interactions and two-dimensional confinement can lead to morphologies and properties that differ dramatically from those achieved via conventional synthetic approaches. Here, we describe the bottom-up, on-surface synthesis of one-dimensional coordination nanostructures based on an iron (Fe)-terpyridine (tpy) interaction borrowed from functional metal-organic complexes used in photovoltaic and catalytic applications. Thermally activated diffusion of sequentially deposited ligands and metal atoms and intraligand conformational changes lead to Fe-tpy coordination and formation of these nanochains. We used low-temperature scanning tunneling microscopy and density functional theory to elucidate the atomic-scale morphology of the system, suggesting a linear tri-Fe linkage between facing, coplanar tpy groups. Scanning tunneling spectroscopy reveals the highest occupied orbitals, with dominant contributions from states located at the Fe node, and ligand states that mostly contribute to the lowest unoccupied orbitals. This electronic structure yields potential for hosting photoinduced metal-to-ligand charge transfer in the visible/near-infrared. The formation of this unusual tpy/tri-Fe/tpy coordination motif has not been observed for wet chemistry synthetic methods and is mediated by the bottom-up on-surface approach used here, offering pathways to engineer the optoelectronic properties and reactivity of metal-organic nanostructures.

The development of ultrahigh-temperature thermoelectric materials could enable thermoelectric topping of combustion power cycles as well as extending the range of direct thermoelectric power generation in concentrated solar power. However, thermoelectric operation temperatures have been restricted to under 1,500 K due to the lack of suitable materials. Here, we demonstrate a thermoelectric conversion material based on high-temperature reduced graphene oxide nanosheets that can perform reliably up to 3,000 K. After a reduction treatment at 3,300 K, the nanosheet film exhibits an increased conductivity to ~4,000 S cm−1 at 3,000 K and a high power factor S2σ = 54.5 µW cm−1 K−2. We report measurements characterizing the film’s thermoelectric properties up to 3,000 K. The reduced graphene oxide film also exhibits a high broadband radiation absorbance and can act as both a radiative receiver and a thermoelectric generator. The printable, lightweight and flexible film is attractive for system integration and scalable manufacturing. The Carnot efficiency and the power output of thermoelectric power generation increase with temperature but current thermoelectrics are characterized up to 1,500 K. Here, Li et al. develop reduced graphene oxide films that can convert heat up to 3,000 K with high power factors, opening the door for novel applications.

The sub-threshold swing is the fundamental critical parameter determining the operation of a transistor in low-power applications such as switches. It determines the fraction of dissipation due to the gate capacitance used for turning the device on and off, and in a conventional transistor it is limited by Boltzmann's tyranny to kTln(10)/q, or 60 mV per decade. Here, we demonstrate that the sub-threshold swing of a topological transistor, in which conduction is enabled by a topological phase transition via electric field switching, can be sizably reduced in a non-interacting system by modulating the Rashba spin-orbit interaction via a top-gate electric field. We refer to this as the Topological Quantum Field Effect and to the transistor as a Topological Quantum Field Effect transistor (TQFET). By developing a general theoretical framework for quantum spin Hall materials with honeycomb lattices we explicitly show that the Rashba interaction can reduce the sub-threshold swing by more than 25% compared to Boltzmann's limit in currently available materials, but without any fundamental lower bound, a discovery that can guide future materials design and steer the engineering of topological quantum devices., Comment: 14 pages, 4 figures, 1 table

We have studied the ambient air oxidation of chemical vapor deposition (CVD) grown monolayers of the semiconducting transition metal dichalcogenide (S-TMD) WS$_{2}$ using optical microscopy, laser scanning confocal microscopy (LSCM), photoluminescence (PL) spectroscopy, and atomic force microscopy (AFM). Monolayer WS$_{2}$ exposed to ambient conditions in the presence of light (typical laboratory ambient light for weeks, or typical PL spectroscopy map), exhibits damage due to oxidation which can be detected with the LSCM and AFM; though may not be evident in conventional optical microscopy due to poorer contrast and resolution. Additionally, this oxidation was not random, and correlated with 'high-symmetry' and red-shifted areas in the PL spectroscopy map - areas thought to contain a higher concentration of sulfur vacancies. In contrast, samples kept in ambient and darkness showed no signs of oxidation for up to 10 months. Low-irradiance/fluence experiments showed that samples subjected to excitation energies at or above the trion excitation energy (532 nm/2.33 eV and 660 nm/1.88 eV) oxidized in as little as 7 days, even for irradiances and fluences eight and four orders of magnitude lower (respectively) than previously reported. No significant oxidation was observed for 760 nm/1.63 eV light exposure, which lies below the trion excitation energy in WS$_{2}$. The strong wavelength dependence and apparent lack of irradiance dependence suggests that ambient oxidation of WS$_{2}$ is initiated by photon-mediated electronic band transitions, that is, photo-oxidation. These findings have important implications for prior, present and future studies concerning S-TMDs measured, stored or manipulated in ambient conditions., 35 pages, 3 figures, 1 table, submitted to ACS Nano Letters

Large-area two-dimensional (2D) heterojunctions are promising building blocks of 2D circuits. Understanding their intriguing electrostatics is pivotal but largely hindered by the lack of direct observations. Here graphene–WS2 heterojunctions are prepared over large areas using a seedless ambient-pressure chemical vapor deposition technique. Kelvin probe force microscopy, photoluminescence spectroscopy, and scanning tunneling microscopy characterize the doping in graphene–WS2 heterojunctions as-grown on sapphire and transferred to SiO2 with and without thermal annealing. Both p–n and n–n junctions are observed, and a flat-band condition (zero Schottky barrier height) is found for lightly n-doped WS2, promising low-resistance ohmic contacts. This indicates a more favorable band alignment for graphene–WS2 than has been predicted, likely explaining the low barriers observed in transport experiments on similar heterojunctions. Electrostatic modeling demonstrates that the large depletion width of the graphene–W...

Single- or multi-layer transition metal oxides (TMOs) have a relatively longer history than other atomically thin materials. TMOs comprise many earth-abundant minerals and have been used for millenia as construction materials, pigments, lubricants, for heat management and many other applications. In TMOs, the transition metal s electrons are strongly pulled by oxygen, and consequently the structural, physical and chemical properties are mostly determined by the strongly correlated d electrons. TMOs are also highly tunable owing to the diversity of their chemical composition, crystal structure and relative ease in inducing oxygen defects. Two dimensional (2D) TMOs often show different physical and chemical properties in comparison to their bulk counterparts. These differences give rise to a variety of remarkable electronic properties such as high temperature superconductivity and multiferroicity and also unique optical, mechanical and thermal phenomena. Additionally, reducing the thickness of TMOs can alter their catalytic and chemical characteristics. Despite their unique properties, single- and few-layer TMOs have received relatively little attention compared to other recent families of atomically thin materials such as layered transition metal dichalcogenides. The overarching aim of this review is to bring the unique aspects of TMOs to the attention of the research community to establish a strong future for research on 2D and layered TMOs. In this review, a comprehensive overview regarding 2D and layered TMOs will be presented. The fundamentals and applications of planar TMOs are discussed. The manuscript will also present future prospects and pathways to new developments that are offered by such TMOs.

Topological Dirac semimetals (TDS) are three-dimensional analogues of graphene, with linear electronic dispersions in three dimensions. Nanoscale confinement of TDSs in thin films is a necessary step toward observing the conventional-to-topological quantum phase transition (QPT) with increasing film thickness, gated devices for electric-field control of topological states, and devices with surface-state-dominated transport phenomena. Thin films can also be interfaced with superconductors (realizing a host for Majorana Fermions) or ferromagnets (realizing Weyl Fermions or T-broken topological states). Here we report structural and electrical characterization of large-area epitaxial thin films of TDS Na3Bi on single crystal Al2O3[0001] substrates. Charge carrier mobilities exceeding 6,000 cm(2)/(V s) and carrier densities below 1 × 10(18) cm(-3) are comparable to the best single crystal values. Perpendicular magnetoresistance at low field shows the perfect weak antilocalization behavior expected for Dirac Fermions in the absence of intervalley scattering. At higher fields up to 0.5 T anomalously large quadratic magnetoresistance is observed, indicating that some aspects of the low field magnetotransport (μB1) in this TDS are yet to be explained.

Highly efficient broadband thermal radiation from reduced graphene oxide (RGO) paper mixed with single-walled carbon nanotubes (CNTs) is reported. These RGO-CNT paper ribbons routinely reach 3000 K before failure, with some samples exceeding 3300 K, higher than any other carbon nanomaterial. Excellent performance is achieved, with ≈90% radiation efficiency, 200 000 on/off cycles, and stable operation for more than 50 hours.

Q. Ma et al.[1] recently reported a strong photocurrent associated with charge neutrality in graphene devices with non-uniform geometries, which they interpreted as an intrinsic photoresponse enhanced by the momentum non-relaxing nature of electron-electron collisions at charge neutrality. Here we argue that gradients in charge carrier density give rise to a photothermoelectric effect (PTE) which is strongly peaked around charge neutrality, i.e. at p-n junctions, and such p-n junctions naturally arise at the edges of graphene devices due to fringing capacitance. Using known parameters, the PTE effect in the presence of charge density gradients predicts the sign, spatial distribution, gate voltage dependence, and temperature dependence of the photoresponse in non-uniform graphene devices, including predicting the observed sign change of the signal away from charge neutrality, and the non-monotonic temperature dependence, neither of which is explained by the intrinsic photocurrents in graphene. We propose future experiments which may disentangle the contributions of PTE and intrinsic photocurrent in graphene devices., Comment: Nat. Nanotechnol. (2020). Submitted as Matters Arising to Nature Nanotechnology regarding [1] Ma, Q. et al., "Giant intrinsic photoresponse in pristine graphene", Nature Nanotechnology 14, 145 (2019)

2D materials have attracted tremendous attention due to their unique physical and chemical properties since the discovery of graphene. Despite these intrinsic properties, various modification methods have been applied to 2D materials that yield even more exciting results in terms of tunable properties and device performance. Among all modification methods, intercalation of 2D materials has emerged as a particularly powerful tool: it provides the highest possible doping level and is capable of (ir)reversibly changing the phase of the material. Intercalated 2D materials exhibit extraordinary electrical transport as well as optical, thermal, magnetic, and catalytic properties, which are advantageous for optoelectronics, superconductors, thermoelectronics, catalysis and energy storage applications. The recent progress on host 2D materials, various intercalation species, and intercalation methods, as well as tunable properties and potential applications enabled by intercalation, are comprehensively reviewed.

We report here a new type of plasmon resonance that occurs when graphene is connected to a metal. These new plasmon modes offer the potential to incorporate a tunable plasmonic channel into a device with electrical contacts, a critical step toward practical graphene terahertz optoelectronics. Through theory and experiments, we demonstrate, for example, anomalously high resonant absorption or transmission when subwavelength graphene-filled apertures are introduced into an otherwise conductive layer. These tunable plasmon resonances are essential yet missing ingredients needed for terahertz filters, oscillators, detectors, and modulators.

We report a large area terahertz detector utilizing a tunable plasmonic resonance in subwavelength graphene microribbons on SiC(0001) to increase the absorption efficiency. By tailoring the orientation of the graphene ribbons with respect to an array of subwavelength bimetallic electrodes, we achieve a condition in which the plasmonic mode can be efficiently excited by an incident wave polarized perpendicular to the electrode array, while the resulting photothermal voltage can be observed between the outermost electrodes.

Small potential variations in 3D semimetal Na3Bi enable close approach to the Dirac point, allowing exploration of new physics., Topological Dirac semimetals (TDSs) are three-dimensional analogs of graphene, with carriers behaving like massless Dirac fermions in three dimensions. In graphene, substrate disorder drives fluctuations in Fermi energy, necessitating construction of heterostructures of graphene and hexagonal boron nitride (h-BN) to minimize the fluctuations. Three-dimensional TDSs obviate the substrate and should show reduced EF fluctuations due to better metallic screening and higher dielectric constants. We map the potential fluctuations in TDS Na3Bi using a scanning tunneling microscope. The rms potential fluctuations are significantly smaller than the thermal energy room temperature (ΔEF,rms = 4 to 6 meV = 40 to 70 K) and comparable to the highest-quality graphene on h-BN. Surface Na vacancies produce a novel resonance close to the Dirac point with surprisingly large spatial extent and provide a unique way to tune the surface density of states in a TDS thin-film material. Sparse defect clusters show bound states whose occupation may be changed by applying a bias to the scanning tunneling microscope tip, offering an opportunity to study a quantum dot connected to a TDS reservoir.

The stability of the surface of vacuum-cleaved topological insulator Bi2Se3 single crystals is investigated with high-resolution synchrotron-based photoelectron spectroscopy. While the surface is stable at room temperature in vacuum, a Bi2 layer always forms at the surface of Bi2Se3 upon even brief (5 min) exposure to atmosphere. This is accompanied by a depletion of selenium in the near surface region and a 1.4 eV decrease in work function. The Bi2 surface is found to be stable upon return to ultrahigh vacuum conditions but is unstable with prolonged exposure to air, ultimately resulting in two possible different reconstructed surfaces, explaining previous contradictory results on long-term atmosphere exposure of Bi2Se3.

Kelvin probe microscopy in ultrahigh vacuum is used to image the local electrostatic potential fluctuations above hexagonal boron nitride (h-BN) and SiO2, common substrates for graphene. Results are compared to a model of randomly distributed charges in a two-dimensional (2D) plane. For SiO2, the results are well modeled by 2D charge densities ranging from 0.24 to 2.7 × 10(11) cm(-2), while h-BN displays potential fluctuations 1-2 orders of magnitude lower than SiO2, consistent with the improvement in charge carrier mobility for graphene on h-BN compared to SiO2. Electron beam exposure of SiO2 increases the charge density fluctuations, creating long-lived metastable charge populations of ~2 × 10(11) cm(-2) at room temperature, which can be reversed by heating.

We have experimentally studied the dispersion of optical conductivity in few-layer graphene through reflection spectroscopy at visible wavelengths. A laser scanning microscope (LSM) with a supercontinuum laser source measured the frequency dependence of the reflectance of exfoliated graphene flakes, including monolayer, bilayer and trilayer graphene, loaded on a Si/SiO2 Fabry-Perot resonator in the 545–700 nm range. The complex refractive index of few-layer graphene, n − ik, was extracted from the reflectance contrast to the bare substrate. It was found that each few-layer graphene possesses a unique dispersionless optical index. This feature indicates that the optical conductivity does not simply scale with the number of layers, and that inter-layer electrodynamics are significant at visible energies.

With the continuing miniaturization of semiconductor microelectronics, atomically thin materials are emerging as promising candidate materials for future ultrascale electronics. In particular, the layered transition metal dichalcogenides (TMDs) have attracted a significant amount of attention because of the variety of their electronic properties, depending on the type of transition metal and its coordination within the crystal. Here, we use low-temperature scanning tunneling microscopy (STM) for the structural and electronic phase engineering of the group V TMD niobium diselenide (NbSe2). By applying voltage pulses with an STM tip, we can transform the material crystal phase locally from trigonal prismatic (2H) to octahedral (1T), as confirmed by the concomitant emergence of a characteristic (√13 × √13)R13.9° charge density wave (CDW) order. At 77 K, atomic-resolution STM images of the junction with sublattice detail confirm the successful phase engineering of the material, as we resolve the difference in the Nb coordination evidenced by a slip of the top Se plane. Different 1T-CDW intensities suggest interlayer interactions to be present in 1T-NbSe2. Furthermore, a distinct voltage dependence suggests a complex CDW mechanism that does not just rely on a star-of-David reconstruction as in the case of other 1T-TMDs. Additionally, bias pulses cause surface modifications inducing local lattice strain that favors a one-dimensional charge order over the intrinsic 3 × 3 CDW at 4.5 K for 2H-NbSe2, which can be reversibly manipulated by STM. NRF (Natl Research Foundation, S’pore) Accepted version

Graphene plasmonics has the potential to revolutionize terahertz technology — the last great underdeveloped frequency band of electromagnetic waves. We describe here recent research on large-area tunable graphene plasmonic materials and devices for use in terahertz detectors, modulators, and filters.

Funding for Open Access provided by the UMD Libraries Open Access Publishing Fund., We have experimentally studied the dispersion of optical conductivity in few-layer graphene through reflection spectroscopy at visible wavelengths. A laser scanning microscope (LSM) with a supercontinuum laser source measured the frequency dependence of the reflectance of exfoliated graphene flakes, including monolayer, bilayer and trilayer graphene, loaded on a Si/SiO2 Fabry-Pérot resonator in the 545–700 nm range. The complex refractive index of few-layer graphene, n − ik, was extracted from the reflectance contrast to the bare substrate. It was found that each few-layer graphene possesses a unique dispersionless optical index. This feature indicates that the optical conductivity does not simply scale with the number of layers, and that inter-layer electrodynamics are significant at visible energies.

Hot electron effects in graphene are significant because of graphene's small electronic heat capacity and weak electron-phonon coupling, yet the dynamics and cooling mechanisms of hot electrons in graphene are not completely understood. We describe a novel photocurrent spectroscopy method that uses the mixing of continuous-wave lasers in a graphene photothermal detector to measure the frequency dependence and nonlinearity of hot-electron cooling in graphene as a function of the carrier concentration and temperature. The method offers unparalleled sensitivity to the nonlinearity, and probes the ultrafast cooling of hot carriers with an optical fluence that is orders of magnitude smaller than in conventional time-domain methods, allowing for accurate characterization of electron-phonon cooling near charge neutrality. Our measurements reveal that near the charge neutral-point the nonlinear power dependence of the electron cooling is dominated by disorder-assisted collisions, while at higher carrier concentrations conventional momentum-conserving cooling prevails in the nonlinear dependence. The relative contribution of these competing mechanisms can be electrostatically tuned through the application of a gate voltage -- an effect that is unique to graphene.

It has been shown that graphene exhibits unique electronic, thermal, mechanical, and optical properties. In particular, due to its gapless band structure and linear dispersion relation around the Dirac points, graphene exhibits a strong nonlinear optical response, which has been theoretically predicted to depend on the number of graphene layers. In this Letter, we experimentally validate the theoretical predictions by probing multilayer graphene χ(3) nonlinearities. The intensity of the four-wave mixing signal is observed to grow monotonically as a function of the number of graphene layers, up to a maximum intensity corresponding to ∼32 layers, after which it decreases, well in agreement with theoretical predictions.

A method based on dark field transmission electron microscopy is developed to quantitively investigate the layer number and stacking order of multilayer graphene, demonstrated here on multilayer crystalline graphene synthesized by chemical vapor deposition. Our results show that the relative intensities of first- and second-order diffraction spots and contrast in corresponding dark field images are sufficient to identify the layer number and stacking order of graphene with layer number up to seven (7) or more with few-nanometer spatial resolution.

Transfer printing methods are used to pattern and assemble monolithic carbon nanotube (CNT) thin-film transistors on large-area transparent, flexible substrates. Airbrushed CNT thin-films with sheet resistance [email protected]^-^1 at 80% transparency were used as electrodes, and high quality chemical vapor deposition (CVD)-grown CNT networks were used as the semiconductor component. Transfer printing was used to pre-pattern and assemble thin film transistors on polyethylene terephthalate (PET) substrates which incorporated Al"2O"3/poly-methylmethacrylate (PMMA) dielectric bi-layer. CNT-based ambipolar devices exhibit field-effect mobility in range 1-33cm^2/Vs and on/off ratio ~10^3, comparable to the control devices fabricated using Au as the electrode material.

We report the fabrication of single and few-channel carbon nanotube (CNT) Schottky diodes on high-frequency compatible substrates using horizontally aligned CNT arrays and dissimilar metal contacts in a coplanar-waveguide geometry. Rectification of ac signals in the range of 100 MHz-40 GHz is examined and the observed cutoff frequencies are used to compute the Schottky junction capacitance of the devices, which is strongly CNT-length-dependent and appears dominated by stray capacitance between the CNT channel and metal electrode, in agreement with electrostatic simulations.

This abstract is for oral presentation. Carbon nanotube (CNT) thin films are promising electronic materials for a wide range of applications, such as large area transparent and flexible electronics [1], supercapacitors [2], and solar cells [3]. Because the electrical response of a random network of CNTs is determined by percolation of both metallic and semiconducting CNTs, the density of the CNTs determines the electrical behavior of a CNT thin film [4]. Thus, it is possible to realize different CNT-device components by controlling the density. In addition, it is essential to pattern CNT thin films to define individual device components. However, different applications require different patterning approaches. For large area integrated circuits, CNT thin films must be patterned with high resolution and low edge roughness. For flexible electronics on plastic substrates, the patterned CNT thin films must be compatible with additional fabrication steps such as transfer printing. To probe the intrinsic physical properties of CNTs, as-grown CNT thin film patterns are desired without any chemical exposure. Finally, to fully realize the potential of CNT thin film as electronic materials in flexible electronics, patterned CNT thin films have to be placed onto different substrates. Here, we report on controlled growth, patterning, and placement methods that meet these requirements.

For more than two decades, scientists and engineers have focused on impending limitations (from high-power densities and heat distribution to device patterning) that constrain the future miniaturization of conventional silicon technology. Thus far, academic and industrial efforts have risen to the challenge and continue to advance planar silicon processing, pushing traditional microtechnology to the nanometer scale. However, insurmountable limitations, both of physical nature and cost, still loom and motivate the research of new nanomaterials and technologies that have the potential to replace and/or enhance conventional silicon systems. As time has progressed, another Group IV element has emerged as a front-runner, looking beyond silicon, in the form of carbon-based nanotechnology. The focus of this issue is to provide a comprehensive look at the state-of-the-art in carbon-based nanomaterials and nanotechnologies and their potential impact on conventional silicon technologies, which are not limited to electronics but also encompass micro- and nanoelectromechanical systems, optoelectronics, and memory. Recent advances in carbon nanotube growth, sorting, and optoelectronics will be discussed, and the relatively new and surging area of graphene research will be introduced. In addition, progress in controlling the growth and properties of ultrananocrystalline and nanocrystalline diamond thin films will be reviewed. These efforts are multidisciplinary, heavily materials focused, and tend to translate information and ideas to other carbon-based studies (e.g., graphene is the building block of carbon nanotubes).

Graphene, a single atom–thick plane of carbon atoms arranged in a honeycomb lattice, has captivated the attention of physicists, materials scientists, and engineers alike over the five years following its experimental isolation. Graphene is a fundamentally new type of electronic material whose electrons are strictly confined to a two-dimensional plane and exhibit properties akin to those of ultrarelativistic particles. Graphene's two-dimensional form suggests compatibility with conventional wafer processing technology. Extraordinary physical properties, including exceedingly high charge carrier mobility, current-carrying capacity, mechanical strength, and thermal conductivity, make it an enticing candidate for new electronic technologies both within and beyond complementary metal oxide semiconductors (CMOS). Immediate graphene applications include high-speed analog electronics and highly conductive, flexible, transparent thin films for displays and optoelectronics. Currently, much graphene research is focused on generating and tuning a bandgap and on novel device structures that exploit graphene's extraordinary electrical, optical, and mechanical properties.

Airbrushed single walled carbon nanotube (SWCNT) thin films with sheet resistivity 80% are used as electrodes for pentacene and regioregular poly(3-hexylthiophene) (P3HT) thin-film transistors (TFTs). Airbrushed SWCNT electrodes show low contact resistance in a bottom-contact configuration, comparable to (in the case of P3HT) or lower than (in the case of pentacene) Au bottom-contacts. The results show that airbrushed CNT electrodes yield similar performance to CVD-grown CNT electrodes.

Label-free deoxyribonucleic acid (DNA) hybridization detection using carbon nanotube transistor (CNT) arrays is demonstrated. The present scheme is distinguished from other CNT sensing methods as it uses a gate oxide overlayer on top of the carbon nanotubes, which function solely as charge sensors but are not participants in the chemical binding process. Because it involves DNA probe attachment on the gate oxide rather than on the CNT, this approach allows the use of conventional DNA functionalization and bioassay protocols, and is less prone to false positives. The signal sought is a few tens of millivolts in threshold voltage shift due to the increase of surface charges after target hybridization. The hybridization detection is shown to be highly specific and sensitive to a minimum concentration of about 30 nM of 61-mer DNA. Despite differences in the transistor properties due to the spread in the CNT parameters during fabrication, the yields are very high and the sensing characteristics are uniformly consistent in nearly all transistors.

The linear dispersion relation in graphene[1,2] gives rise to a surprising prediction: the resistivity due to isotropic scatterers (e.g. white-noise disorder[3] or phonons[4-8]) is independent of carrier density n. Here we show that acoustic phonon scattering[4-6] is indeed independent of n, and places an intrinsic limit on the resistivity in graphene of only 30 Ohm at room temperature (RT). At a technologically-relevant carrier density of 10^12 cm^-2, the mean free path for electron-acoustic phonon scattering is >2 microns, and the intrinsic mobility limit is 2x10^5 cm^2/Vs, exceeding the highest known inorganic semiconductor (InSb, ~7.7x10^4 cm^2/Vs[9]) and semiconducting carbon nanotubes (~1x10^5 cm^2/Vs[10]). We also show that extrinsic scattering by surface phonons of the SiO2 substrate[11,12] adds a strong temperature dependent resistivity above ~200 K[8], limiting the RT mobility to ~4x10^4 cm^2/Vs, pointing out the importance of substrate choice for graphene devices[13]., 16 pages, 3 figures

we have fabricated transparent electronic devices based on graphene materials with thickness down to one single atomic layer by the transfer printing method. The resulting printed graphene devices retain high field effect mobility and have low contact resistance. The results show that the transfer printing method is capable of high-quality transfer of graphene materials from silicon dioxide substrates, and the method thus will have wide applications in manipulating and delivering graphene materials to desired substrate and device geometries. Since the method is purely additive, it exposes graphene (or other functional materials) to no chemical preparation or lithographic steps, providing greater experimental control over device environment for reproducibility and for studies of fundamental transport mechanisms. Finally, the transport properties of the graphene devices on the PET substrate demonstrate the non-universality of minimum conductivity and the incompleteness of the current transport theory., Comment: 10 pages, 3 figures

A nonlocal Hall bar geometry is used to detect neutral-current Hall effects in graphene on silicon dioxide. Disorder is tuned by the addition of $\mathrm{Au}$ or $\mathrm{Ir}$ adatoms in ultrahigh vacuum. A reproducible neutral-current Hall effect is found in both as-fabricated and adatom-decorated graphene. The Hall angle exhibits a complex but reproducible dependence on gate voltage and disorder, and notably breaks electron-hole symmetry. An exponential dependence on length between Hall and inverse Hall probes indicates a neutral-current relaxation length of approximately $300\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$. The short relaxation length and lack of precession in a parallel magnetic field suggest that the neutral currents are valley currents. No signature of the spin-orbit coupling induced spin Hall effect is observed in the $\mathrm{Au}$- or $\mathrm{Ir}$-decorated graphene. The near lack of temperature dependence from 7 to 300 K is unprecedented among reports of valley Hall effect in graphene, and promising for using controlled disorder for room temperature neutral-current electronics.

We use an ultra-fast near-infrared pulse coincidence technique to study the time, temperature and power dependence of the photoresponse of a bi-metal contacted graphene photodetector. We observe two components of the photovoltage signal. One component is gate-voltage dependent, linear in power at room temperature and sub-linear at low temperature-consistent with the hot-electron photothermoelectric effect due to absorption in the graphene. The power dependence is consistent with supercollision-dominated cooling in graphene. The other component is gate-voltage independent and linear in temperature and power, which we interpret as due to thermoelectricity of the metal electrodes due to differential light absorption.

As the thinnest conductive and elastic material, graphene is expected to play a crucial role in post-Moore era. Besides applications on electronic devices, graphene has shown great potential for nano-electromechanical systems. While interlayer interactions play a key role in modifying the electronic structures of layered materials, no attention has been given to their impact on electromechanical properties. Here we report the positive piezoconductive effect observed in suspended bi- and multi-layer graphene. The effect is highly layer number dependent and shows the most pronounced response for tri-layer graphene. The effect, and its dependence on the layer number, can be understood as resulting from the strain-induced competition between interlayer coupling and intralayer transport, as confirmed by the numerical calculations based on the non-equilibrium Green's function method. Our results enrich the understanding of graphene and point to layer number as a powerful tool for tuning the electromechanical properties of graphene for future applications., Comment: Accepted by Nature Communications; 36 pages, 4 figures, 11 supplementary figures

Graphene decorated with 5d transitional metal atoms is predicted to exhibit many intriguing properties; for example iridium adatoms are proposed to induce a substantial topological gap in graphene. We extensively investigated the conductivity of single-layer graphene decorated with iridium deposited in ultra-high vacuum at low temperature (7 K) as a function of Ir concentration, carrier density, temperature and annealing conditions. Our results are consistent with the formation of Ir clusters of ~100 atoms at low temperature, with each cluster donating a single electronic charge to graphene. Annealing graphene increases the cluster size, reducing the doping and increasing the mobility. We do not observe any sign of an energy gap induced by spin-orbit coupling, possibly due to the clustering of Ir.

© 2015 American Chemical Society. Atomic force microscopy, Kelvin probe force microscopy, and scanning photoluminescence spectroscopy image the progressive postgrowth hydroxylation and hydration of atomically flat Al2O3(0001) under monolayer MoS2, manifested in large work function shifts (100 mV) due to charge transfer (>1013 cm-2) from the substrate and changes in PL intensity, energy, and peak width. In contrast, trapped water between exfoliated graphene and Al2O3(0001) causes surface potential and doping changes one and two orders of magnitude smaller, respectively, and MoS2 grown on hydrophobic hexagonal boron nitride is unaffected by water exposure.

We report on electron transport measurements of single-wall carbon nanotubes in a four-terminal configuration with noninvasive voltage electrodes. The voltage drop is detected using multiwalled carbon nanotubes while the current is injected through nanofabricated Au electrodes. Measurements are carried at high temperature so that the four-terminal resistance directly gives the intrinsic resistance. The resistance is shown to result from weak disorder and from quantum interference effect corrections. In addition, we present Coulomb blockade measurements. The length of the quantum dot that is determined from the level spacing equals the separation between the Au electrodes.

A simple Boltzrnann-equation model for charge transport in semiconducting single-walled carbon nanotubes (s-SWNTs) is developed with two adjustable parameters, the acoustic and optical phonon scattering lengths. The model predicts velocity saturation rather than current saturation in s-SWNTs at high bias, in agreement with a recent experiment. At moderate densities, the model predicts currents in s-SWNTs exceeding 25 μA, the limiting current in long metallic single-walled carbon nanotubes.

We have successfully fabricated devices with isolated single-walled carbon nanotubes (SWNTs) using exclusively standard i-line (365 nm) photolithography. Catalyst islands were patterned with an SU-8-photoresist-based process. This method provides well-defined islands, down to 1 µm in size. The islands are clearly visible with an optical microscope and are used as alignment marks for optical alignment of subsequent layers. SWNTs were grown by chemical vapour deposition (CVD). Contacts to the nanotubes were fabricated by standard photolithography.

Experiments to determine the resistivity and charge-carrier mobility in semiconducting carbon nanotubes are reviewed. Electron transport experiments on long chemical-vapour-deposition-grown semiconducting carbon nanotubes are interpreted in terms of diffusive transport in a field-effect transistor. This allows for extraction of the field-effect and saturation mobilities for hole carriers, as well as an estimate of the intrinsic hole mobility of the nanotubes. The intrinsic mobility can exceed 100 000 cm2 V−1 s−1 at room temperature, which is greater than any other known semiconductor. Scanned-probe experiments show a low degree of disorder in chemical-vapour-deposition-grown semiconducting carbon nanotubes compared with laser-ablation produced nanotubes, and show conductivity and mean-free-path consistent with the high mobility values seen in transport experiments. The application of high-mobility semiconducting nanotubes to charge detection and memory is also reviewed; it is shown that single electronic charges may be detected with a semiconducting nanotube field-effect transistor at operating temperatures up to 200 K.

Semiconducting carbon nanotube transistors with channel lengths exceeding 300 microns have been fabricated. In these long transistors, carrier transport is diffusive and the channel resistance dominates the transport. Transport characteristics are used to extract the field-effect mobility (79 000 cm2/Vs) and estimate the intrinsic mobility (>100 000 cm2/Vs) at room temperature. These values exceed those for all known semiconductors, which bodes well for application of nanotubes in high-speed transistors, single- and few-electron memories, and chemical/biochemical sensors.

We study the response of a graphene photothermoelectric-based detector illuminated by two continuous-wave optical beams. The power and frequency dependence of the photoresponse are used to probe the graphene hot-electron cooling rates and mechanisms.

A high-mobility (9000 cm2/V·s) semiconducting single-walled nanotube transistor is used to construct a nonvolatile charge-storage memory element operating at room temperature. Charges are stored by application of a few volts across the silicon dioxide dielectric between nanotube and silicon substrate, and detected by threshold shift of the nanotube field-effect transistor. The high mobility of the nanotube transistor allows the observation of discrete configurations of charge corresponding to rearrangement of a single or few electrons. These states may be reversibly written, read, and erased at temperatures up to 100 K.

Single-walled carbon nanotubes (SWNTs) have emerged as a very promising new class of electronic materials. The fabrication and electronic properties of devices based on individual SWNTs are reviewed. Both metallic and semiconducting SWNTs are found to possess electrical characteristics that compare favorably to the best electronic materials available. Manufacturability issues, however, remain a major challenge.