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Semiconductors have long been perceived as a prerequisite for solid-state transistors. Although switching principles for nanometer-scale devices have emerged based on the deployment of two-dimensional (2D) van der Waals heterostructures, tunneling and ballistic currents through short channels are difficult to control, and semiconducting channel materials remain indispensable for practical switching. In this study, we report a semiconductor-less solid-state electronic device that exhibits an industry-applicable switching of the ballistic current. This device modulates the field emission barrier height across the graphene-hexagonal boron nitride interface with ION/IOFF of 106 obtained from the transfer curves and adjustable intrinsic gain up to 4, and exhibits unprecedented current stability in temperature range of 15–400 K. The vertical device operation can be optimized with the capacitive coupling in the device geometry. The semiconductor-less switching resolves the long-standing issue of temperature-dependent device performance, thereby extending the potential of 2D van der Waals devices to applications in extreme environments. In field-effect transistors, a semiconducting channel is indispensable for device switching. Here, the authors demonstrate semiconductor-less switching via modulation of the field emission barrier height across a graphene-hBN interface with ON/OFF ratio of 106.

Temperature-independent magnetoresistance (TIMR) has been studied for applications in magnetic field sensors operating in wide temperature ranges. Graphene is considered as one of the best candidates for achieving nonsaturating and large TIMR through engineering disorders. Nevertheless, large TIMR has not been achieved in disordered graphene with intrinsic defects, such as chemical doping and atomic dislocations. In this work, by introducing extrinsic defects, we realize nonsaturating and large TIMR in monolayer graphene transferred on a BiFeO3 nanoisland array (G/BFO-NIA). Furthermore, the G/BFO-NIA device exhibits a significantly larger MR (∼250% under 9 T) than other materials without gating operation, demonstrating its application feasibility. It is shown that the large MR is a result of the coexistence of electrons and holes with almost the same density, and the observed TIMR originates from the temperature dependence of carrier transport in graphene and of the dielectric property of BFO-NIA.

We have investigated the effect of an Al

Semiconductors have long been perceived as a prerequisite for solid-state transistors. Although switching principles for nanometer-scale devices have emerged based on the deployment of two-dimensional (2D) van der Waals heterostructures, tunneling and ballistic currents through short channels are difficult to control, and semiconducting channel materials remain indispensable for practical switching. In this study, we report a semiconductor-less solid-state electronic device that exhibits an industry-applicable switching of the ballistic current. This device modulates the field emission barrier height across the graphene-hexagonal boron nitride interface with ION/IOFF of 106 obtained from the transfer curves and adjustable intrinsic gain up to 4, and exhibits unprecedented current stability in temperature range of 15–400 K. The vertical device operation can be optimized with the capacitive coupling in the device geometry. The semiconductor-less switching resolves the long-standing issue of temperature-dependent device performance, thereby extending the potential of 2D van der Waals devices to applications in extreme environments., In field-effect transistors, a semiconducting channel is indispensable for device switching. Here, the authors demonstrate semiconductor-less switching via modulation of the field emission barrier height across a graphene-hBN interface with ON/OFF ratio of 106.

The electrical performance of suspended few-layer MoTe2 field-effect-transistors with ionic liquid gating has been investigated. The suspended structure not only enhances the mobility of MoTe2 by removing the influence of the substrate but also allows ions to accumulate on both the top and the bottom surface of MoTe2. The consequent increase in the gate capacitance resulted in an improved subthreshold swing (∼73 mV/dec) and on-off ratio (106) at room temperature for suspended MoTe2 compared to substrate-supported devices. Suspended transistors with ionic liquid gating enable a larger charge density compared to ionic liquid gated supported devices and may provide a useful platform to study screening physics in 2D materials.

We have investigated the effect of an Al2O3passivation layer on the performance of few-layer WS2FETs. While the performance of WS2FETs is often limited by a substantial decrease in carrier mobility owing to charged impurities and a Schottky barrier between the WS2and metal electrodes, the introduction of an Al2O3overlayer by atomic layer deposition (ALD) suppressed the influence of charged impurities by high-κdielectric screening effect and reduced the effective Schottky barrier height. We argue that n-doping of WS2, induced by positive fixed charges formed at Al2O3/WS2interface during the ALD process, is responsible for the reduction of the effective Schottky barrier height in the devices. In addition, the Al2O3passivation layer protected the device from oxidation, and maintained stable electrical performance of the WS2FETs over 57 d. Thus, the ALD of Al2O3overlayer provides a facile method to enhance the performance of WS2FETs and to ensure ambient stability.

We have investigated the performances of barristors with a graphene-tungsten disulfide (WS 2 ) junction by varying the thickness of WS 2 and gate oxide. On-current density ( J ON ) and on- and off-current ratio ( J ON / J OFF ) increases, and sub-threshold swing ( V SS ) decreases with the WS 2 thickness. Also, barristors with thicker WS 2 required less workfunction shift, to switch the barristors. Therefore, unlike the traditional devices, V SS of barristor with gate dielectric 300 nm was smaller than that of 90 nm, when the former is fabricated with thicker WS 2 than the latter. Since materials properties of 2-dimensional semiconductors generally vary with their thickness, the thickness of 2D semiconductors could become a key parameter to engineer the performance of barristors with graphene and the 2D semiconductors.

The evolution of surface cleanliness and the electronic properties-Dirac voltage(V Dirac), hysteresis and mobility (μ) of a graphene field-effect transistor (GFET)-were monitored by measuring lateral force microscopy and drain current (I D) as a function of gate voltage (V G), after mechanically cleaning the surface, scan-by-scan, with contact-mode atomic force microscopy. Both the surface cleanliness and the electronic properties evolved, showing a sudden improvement and then saturation for a mobility of around 2200 cm2 V-1 s-1. We found that the mobility suppression of the as-fabricated GFET deviated from a randomly distributed impurities model, which predicted a greater mobility than obtained from the measured V Dirac. Therefore, the substrate impurities are excluded from the origins of the extraordinary suppression of the mobility, and the possible origin will be discussed.

The temperature ($\it T$) dependence of electrical resistivity in graphene has been experimentally investigated between 10 and 400 K for samples prepared on various substrates; HfO$_2$, SiO$_2$ and h-BN. The resistivity of graphene shows a linear $\it T$-dependence at low $\it T$ and becomes superlinear above a substrate-dependent transition temperature. The results are explained by remote interfacial phonon scattering by surface optical phonons at the substrates. The use of an appropriate substrate can lead to a significant improvement in the charge transport of graphene.

We have investigated the current distribution between individual layers of multilayer MoS2 in a field-effect transistor structure with source and drain electrodes on the top layer. By employing a laser thinning technique, we partially etched down the multilayer in the middle of the device and monitored the electrical current after each etching step. The current was decreased by several orders of magnitude when the upper layers were partially etched away. Our observation suggests that the electrical current of multilayer MoS2 flows mainly through the upper layers close to the source/drain contacts due to relatively large interlayer resistance of MoS2.

We have investigated the effect of laser irradiation on monolayer MoS2 and observed the swellingup of the monolayer from the SiO2 substrate upon laser illumination. The mismatch in the thermal expansion between the substrate and MoS2 can result in the structural deformation. Employing this method, one can induce structural deformation in a desired pattern, and one can demonstrate the patterning of periodic ripples in monolayer MoS2 by using laser irradiation. The controlled fabrication of the ripple structure may be instrumental in understanding the effect of ripples on the interesting physical properties of monolayer MoS2.

Three types of carbon nanoscroll (CNS) structures that are formed when scrolling up graphene sheets are investigated using Raman spectroscopy and atomic force microscopy (AFM). The CNSs were produced from exfoliated monolayer graphene deposited on a Si chip by applying a droplet of isopropyl alcohol (IPA) solution. The three types of CNS are classified as single-elliptical-core, double-elliptical-core (both with large internal volumes) and collapsed ribbon-like, based on AFM surface profile measurements. We discuss the structure and formation of CNS with much larger hollow cores than is commonly assumed and relate this to the large effective 2D bending stiffness of graphene in the IPA solution. The large elliptical core structures show Raman spectra similar to those previously reported for CNS and indicate little interaction between the scrolled layers. The Raman spectra from ribbon-like structures show additional features that are similar to that of folded graphene. These new features can be related to layer breathing modes combined with some resonance enhancement at specific regions of the ribbon-like CNSs that are due to specific twist angles produced when the structure folds/collapses.

Recently, attempts to overcome the physical limits of memory devices have led to the development of promising materials and architectures for next-generation memory technology. The selector device is one of the essential ingredients of high-density stacked memory systems. However, complicated constituent deposition conditions and thermal degradation are problematic, even with effective selector device materials. Herein, we demonstrate the highly stable and low-threshold voltages of vanadium pentoxide (V2O5) nanosheets synthesized by facile chemical vapor deposition, which have not been previously reported on the threshold switching (TS) properties. The electrons occupying trap sites in poly-crystalline V2O5 nanosheet contribute to the perfectly symmetric TS feature at the bias polarity and low-threshold voltages in V2O5, confirmed by high-resolution transmission electron microscopy measurements. Furthermore, we find an additional PdO interlayer in V2O5 nanodevices connected with a Pd/Au electrode after th...

The optical constants, bandgaps, and band alignments of mono-, bi-, and trilayer WS2 were experimentally measured, and an extraordinarily high dependency on the number of layers was revealed. The refractive indices and extinction coefficients were extracted from the optical-contrast oscillation for various thicknesses of SiO2 on a Si substrate. The bandgaps of the few-layer WS2 were both optically and electrically measured, indicating high exciton-binding energies. The Schottky-barrier heights (SBHs) with Au/Cr contact were also extracted, depending on the number of layers (1-28). From an engineering viewpoint, the bandgap can be modulated from 3.49 to 2.71 eV with additional layers. The SBH can also be reduced from 0.37 eV for a monolayer to 0.17 eV for 28 layers. The technique of engineering materials' properties by modulating the number of layers opens pathways uniquely adaptable to transition-metal dichalcogenides.

We have investigated the longitudinal magnetoresistance (MR) of thin films of single-walled carbon nanotubes, in pulsed magnetic field up to 32 T. We report the longitudinal MR markedly different from the transverse MR. At T = 38.5 K, the longitudinal MR exhibits a pronounced positive MR and saturates around 20 T, while the transverse MR remained negative for similar carbon nanotube films. With decreasing temperature down to 11 K, the negative contribution to the MR increases at low field and the positive contribution diminishes. The minimum of the MR, thus, appears at a higher field with lowering T , for the temperature range between 11 and 38.5 K. On the other hand, below 10 K, the longitudinal MR behaves similar to the transverse MR. The behaviors of the longitudinal MR are distinguished across T ∼ 11 K, and they might have different origin. We suggest Aharonov–Bohm effect, which tunes the band structure of the CNTs by the field parallel to the tube axis, may play a role.

Ethylenediamine-modified -cyclodextrin (Et-beta-CD) was immobilized on aggregated silver nanoparticle (NP)-embedded silica NPs (SiO2@Ag@Et--CD NPs) for the effective detection of flavonoids. Silica NPs were used as the template for embedding silver NPs to create hot spots and enhance surface-enhanced Raman scattering (SERS) signals. Et-beta-CD was immobilized on Ag NPs to capture flavonoids via host-guest inclusion complex formation, as indicated by enhanced ultraviolet absorption spectra. The resulting SiO2@Ag@Et-beta-CD NPs were used as the SERS substrate for detecting flavonoids, such as hesperetin, naringenin, quercetin, and luteolin. In particular, luteolin was detected more strongly in the linear range 10(-7) to 10(-3) M than various organic molecules, namely ethylene glycol, beta-estradiol, isopropyl alcohol, naphthalene, and toluene. In addition, the SERS signal for luteolin captured by the SiO2@Ag@Et-beta-CD NPs remained even after repeated washing. These results indicated that the SiO2@Ag@Et-beta-CD NPs can be used as a rapid, sensitive, and selective sensor for flavonoids.

We have investigated random telegraph noise (RTN) observed in individual semiconducting carbon nanotubes (CNTs) in Coulomb-blockade regime. RTN characteristics are studied as a function of gate-voltage and drain–source voltage. Our results are explained by the capture and emission of carriers by charge traps in the vicinity of CNTs. Because of the large RTN amplitude, often greater than ∼50% of the total current, RTN measurements can be developed into an effective tool to estimate the trap energy and the location of the trap in CNT quantum dot devices.

We have investigated Hall coefficient and magnetoresistance in thin films of single-walled carbon nanotubes, prepared in four different ways. Hall voltages are linear for all samples in magnetic fields up to 6 T, and the measured carrier density lies in ∼10 21 –10 22 cm −3 . Whereas earlier Hall-effect experiments reported ∼10 18 –10 19 cm −3 for the carrier density, our results are consistent with the theoretically predicted value of ∼10 22 cm −3 , calculated for the aligned metallic CNTs. The signs of the Hall coefficients are positive in general, indicating that majority carriers are holes in these films. In a nanotube film with the lowest conductivity, however, we find the Hall coefficient reverses the sign at low temperature around T = 15 K. The origin of the sign change is not clear. In strongly localized regime, the Hall effect can be anomalous.

We study the effect of Cs encapsulation on the transport and noise properties of carbon nanotubes, and report very large two-level current fluctuations observed in the time-traces of the current at low temperatures. When several two-level fluctuations are active at the same time, we find they are interacting each other. Observed noise behaviors are attributed to the movable cluster of Cs ions encapsulated in the carbon nanotube.

We report in-situ measurement of the conductivity during the synthesis of C60-filled single-walled carbon nanotubes (SWNTs), so-called fullerene peapods. The synthesis was performed in a sealed quartz tube at 773 K by using the sublimation of C60 into the hollow space of the SWNTs. The change in the resistance in the SWNT buckypaper was monitored during the C60-filling process, and the temperature dependence of the resistance was compared to that for empty SWNTs and fullerene peapods. The SWNT networks became more metallic due to the encapsulation of C60, and a reduced temperature dependence of the resistivity was observed in the peapods, with an overall decrease in the resistivity. The interaction between the SWNT and C60 molecules, changing the electronic properties of SWNTs, can provide a way to functionalize the SWNTs.

We present results of non-linear magneto resistance (MR) of polyacetylene nanofibers in high magnetic field up to H = 30 T at low temperature T = 1.5 K. The MR was proven to be of the spin origin; it reaches 16% at highest H . Unexpectedly, the MR was suppressed by increasing electric field E , vanishing at E ≳ 5 × 10 4 V/cm. It is understood that the doping induced spinless charged soliton pairs, which are initially confined to a certain distance because of the interchain phase correlations, and are deconfined in high electric fields, resulting in a vanishing magneto resistance (VMR). The role of the specific, degenerate ground state of the polyacetylene is confirmed by parallel studies of the different magneto resistances of polyaniline nanofibers which contrarily is not affected by the electric field.

The switching of resistance between two discrete values, known as random telegraph noise (RTN), was observed in individual single-walled carbon nanotubes (SWNTs) and C60-filled SWNTs (the so-called peapods). The RTN has been studied as a function of bias-voltage and gate-voltage as well as temperature. By analyzing the features of the RTN, we identify three different types of RTN existing in the SWNT related systems. While the RTN can be generated by the various charge traps in the vicinity of the SWNTs, the RTN for metallic SWNTs is mainly due to reversible defect motions between two metastable states, activated by inelastic scattering with ballistic electrons. On the other hand, the noise for peapods can be attributed to the motion of C60 molecules in hollow space of SWNTs.

We investigated the switching of resistance between two discrete values, known as random telegraph noise (RTN), observed in carbon nanotube peapod transistors [single‐walled carbon nanotubes (SWNTs), C60‐peapods, and Cs‐encapsulated SWNTs (so‐called Cs‐peapods)]. By analyzing the features of the RTN, we suggest that this noise for SWNTs is due to the random transition of defects between two metastable states, activated by inelastic scattering with ballistic electrons. The noise for C60‐peapods (Cs‐peapods) is attributed to the motion of C60s (Cs) in the nanotubes.

Three-terminal nanorelay structures were fabricated with multiwall carbon nanotubes (MWNTs). The nanotube relays were deflected by applying a gate voltage until contact (mechanical and/or electrical) was made with a drain electrode, thus closing the circuit. It was possible to achieve multiple switching cycles, showing that carbon nanotubes are suitable and practical systems for developing nanoelectromechanical devices of this kind.

The orientation dependence of magneto-resistance behaviour for a single-walled carbon nanotube (SWCN) rope is reported.A clear delineation of behaviours is observed between applying a magnetic field perpendicular or parallel to the rope axis.For a perpendicular field, monotonic negative magneto-resistance is observed due to two-dimensional weak localisation within the rope. By contrast, for a parallel field, complicated oscillatory behaviour is observed due to the Altshuler–Aronov–Spivak effect around closed electron trajectories on discrete cylinders within the SWCN rope.A dominant oscillatory mode can be identified which corresponds to closed paths around the outer circumference of the SWCN rope.However, due to the composite filamentary nature of the rope, the overall oscillatory behaviour is rather complicated and can be classified as universal conductance fluctuations. With a backgate voltage applied to the sample, Coulomb blockade peaks are observed in the transport current with additional peak structure superimposed due to resonant tunnelling.We find an interesting effect whereby these peaks are suppressed in the presence of a magnetic field. 2002 Elsevier Science B.V. All rights reserved.

The measured resistance of carbon nanotube networks is often dominated by defects, inter-tube and inter-rope contacts. We show that the peak reported in the frequency-dependent conductivity of single-wall carbon nanotube networks is consistent with metallic conduction interrupted by nonmetallic defects that act as barriers. Such barriers also contribute to the electric field dependence of the conductivity. Using Sheng's model, we calculate the field dependence of fluctuation-assisted tunnelling conduction between metallic regions separated by an insulating barrier, obtaining nonlinearities consistent with our experimental data on carbon nanotube networks.

We have investigated random telegraph noise (RTN) observed in individual metallic carbon nanotubes (CNTs). Mean lifetimes in high- and low-current states, shigh and slow, have been studied as a function of bias-voltage and gate-voltage as well as temperature. By analyzing the statistics and features of the RTN, we suggest that this noise is due to the random transition of defects between two metastable states, activated by inelastic scattering with conduction electrons. Our results indicate an important role of defect motions in the 1=f noise in CNTs., 4 pages

We report on nuclear magnetic resonance on single wall carbon nanotubes. Depending on the chemical preparation the electronic and dynamical properties of carbon nanotubes are presented and discussed. From a room temperature study of the spin lattice relaxation of carbon nanotubes prepared with various catalysts we clearly identified two components. In agreement with previous NMR studies and theoretical predictions, one-third of the intensity of the signal is found with a short relaxation time (about 5 s) attributed to metallic nanobutes while the rest of the signal presents a relaxation time of about 90 s corresponding to semiconducting nanotubes. In the case of oxidized or cut nanotubes only one relaxation time is observed with characteristics similar to the slow component. The disappearance of the fast relaxing component is associated with the absence of metallic nanotubes damaged by the chemical or mechanical treatments. In this case, the T dependence of the spin lattice relaxation reveals the effect of thermally activated small amplitude motions (twistons) of the nanotube in ropes. If diffusion of twistons might induce movement of 13C sites and local magnetic field fluctuations, orientational order could appear below the transition temperature of 170 K. In the last part, we present the theoretical predictions of chemical shift tensor in carbon nanotubes.

International audience

International audience

Carrier mobility in solids is generally limited by electron-impurity or electron-phonon scattering depending on the most frequently occurring event. Three body collisions between carriers and both phonons and impurities are rare; they are denoted supercollisions (SCs). Elusive in electronic transport they should emerge in relaxation processes as they allow for large energy transfers. As pointed out in Ref. \onlinecite{Song2012PRL}, this is the case in undoped graphene where the small Fermi surface drastically restricts the allowed phonon energy in ordinary collisions. Using electrical heating and sensitive noise thermometry we report on SC-cooling in diffusive monolayer graphene. At low carrier density and high phonon temperature the Joule power $P$ obeys a $P\propto T_e^3$ law as a function of electronic temperature $T_e$. It overrules the linear law expected for ordinary collisions which has recently been observed in resistivity measurements. The cubic law is characteristic of SCs and departs from the $T_e^4$ dependence recently reported for metallic graphene below the Bloch-Gr\"{u}neisen temperature. These supercollisions are important for applications of graphene in bolometry and photo-detection.

We have investigated the magnetoresistance of lithographically prepared single-layer graphene nanoribbons in pulsed, perpendicular magnetic fields up to 60 T and performed corresponding transport simulations using a tight-binding model and several types of disorder. In experiment, at high carrier densities we observe Shubnikov-de Haas oscillations and the quantum Hall effect, while at low densities the oscillations disappear and an initially negative magnetoresistance becomes strongly positive at high magnetic fields. The strong resistance increase at very high fields and low carrier densities is tentatively ascribed to a field-induced insulating state in the bulk graphene leads. Comparing numerical results and experiment, we demonstrate that at least edge disorder and bulk short-range impurities are important in our samples., 5 pages, 4 figures, revised version to match published version. First author changed her last name from Schmidmeier to Minke

We report markedly different transport properties of ABA- and ABC-stacked trilayer graphenes. Our experiments in double-gated trilayer devices provide evidence that a perpendicular electric field opens an energy gap in the ABC trilayer, while it causes the increase of a band overlap in the ABA trilayer. In a perpendicular magnetic field, the ABA trilayer develops quantum Hall plateaus at filling factors of \nu = 2, 4, 6... with a step of \Delta \nu = 2, whereas the inversion symmetric ABC trilayer exhibits plateaus at \nu = 6 and 10 with 4-fold spin and valley degeneracy., Comment: 4 pages, 4 figures

We have investigated the magnetoconductance of semiconducting carbon nanotubes (CNTs) in pulsed, parallel magnetic fields up to 60 T, and report the direct observation of the predicted band-gap closure and the reopening of the gap under variation of the applied magnetic field. We also highlight the important influence of mechanical strain on the magnetoconductance of the CNTs., Comment: 4 pages, 4 figures

The electronic spectra of long carbon nanotubes (CNTs) can, to a very good approximation, be obtained using the dispersion relation of graphene with both angular and axial periodic boundary conditions. In short CNTs one must account for the presence of open ends, which may give rise to states localized at the edges. When a magnetic field is applied parallel to the tube axis, it modifies both momentum quantization conditions, causing hitherto extended states to localize near the ends. We study analytically and numerically the appearance and evolution of this peculiar localization phenomenon in CNTs of any nonarmchair chirality, including the electron spin. Conductance calculations show different evolution of spin up and down states in increasing magnetic field.

The magneto-conductance of an open carbon nanotube (CNT)-quantum wire was measured in pulsed magnetic fields. At low temperatures we find a peculiar split magneto-conductance peak close to the charge neutrality point. Our analysis of the data reveals that this splitting is intimately connected to the spin-orbit interaction and the tube chirality. Band structure calculations suggest that the current in the peak regions is highly spin-polarized, which calls for application in future CNT-based spintronic devices., Comment: 4 pages, 3 figures

We report the low temperature current-voltage (I-V) characteristics studies in quasi-one dimensional conducting polymer nanofibers. We find a threshold voltage Vt below which little current flows at temperatures below 30-40 K. For V > Vt current scales as (V/Vt - 1)^zeta, where zeta ~ 1.8-2.1 at high biases. Differential conductance oscillations are found whose magnitude increases as temperature decreases below 10 K. We attribute the observed low temperature I-V behavior to Coulomb blockade effects with a crossover to Luttinger liquid-like behavior at high temperature. We demonstrate that at low temperatures such a doped conjugated polymer fiber can be considered as an array of small conducting regions separated by nanoscale barriers, where the Coulomb blockade tunneling is the dominant transport mechanism., 5 pages, 3 figures; accepted for publication in Phys. Rev. B, Brief Reports (2005)

Chemical modification by SOCl2 of an entangled network of purified single-wall carbon nanotubes, also known as 'bucky paper', is reported to profoundly change the electrical and mechanical properties of this system. Four-probe measurements indicate a conductivity increase by up to a factor of 5 at room temperature and an even more pronounced increase at lower temperatures. This chemical modification also improves the mechanical properties of SWNT networks. Whereas the pristine sample shows an overall semiconducting character, the modified material behaves as a metal. The effect of SOCl2 is studied in terms of chemical doping of the nanotube network. We identified the microscopic origin of these changes using SEM, XPS, NEXAFS, EDX, and Raman spectroscopy measurements and ab initio calculations. We interpret the SOCl2-induced conductivity increase by p-type doping of the pristine material. This conclusion is reached by electronic structure calculations, which indicate a Fermi level shift into the valence band, and is consistent with the temperature dependence of the thermopower.

We report experimental results of the electric transport properties of single-walled carbon nanotubes (SWNTs) functionalized by plasma ion irradiation method, where purified SWNTs and Cs-encapsulating SWNTs are used. SWNTs bundles are well dropped between source and drain electrodes of the field effect transistor (FET) configuration. Voltage-current characteristics, gate bias dependence, and measuring temperature dependence are investigated. It is found that purified SWNTs exhibit p-type semiconducting behavior. Transport measurements for Cs encapsulating individual SWNTs have also been performed, the result of which is discussed.

The current-voltage (I-V) characteristic of single-wall carbon nanotube (SWNT) network was investigated at low temperatures. At T∼ 1.6K, S-shaped negative differential resistance (NDR) was observed, which was systematically examined under the variation of temperature, magnetic field in transverse direction and distances between probes. S-shaped NDR was explained by the nondestructive impact ionization of carriers and the resulting rmation of inhomogeneous spatial structures such as high-current filaments. Critical electric field for impact ionization was estimated to be about IV/ cm.