Electron Transport and Velocity Oscillations in a Carbon Nanotube

We report Monte Carlo (MC) simulation results showing applied bias dependent spatial velocity oscillations for semiconducting single-walled zig-zag carbon nanotubes (CNTs). MC simulations show velocity oscillations in CNTs at different lengths due to high scattering rates associated with optical phonon emissions in the one-dimensional CNT system. The frequencies of these oscillations are as high as 30 THz. In addition, we investigate average ensemble electron velocities as a function of tube length.     

Probing of contact formation via light emission from organic field-effect transistors

The recent progress on ambipolar organic light-emitting field-effect transistors has opened new possibilities for characterizing the basic physical processes in organic field-effect transistors (OFETs). Here, a way of investigating the contact formation from the light emission of OFETs is presented. In general, in the ambipolar transport regime a spatially controllable recombination zone can be observed in the channel of the transistor by light emission. In the unipolar regimes no light emission is expected due to the fact that only one charge carrier type is accumulated in the channel region and, thus, charge carriers cannot recombine. However, our results indicate that light emission in the unipolar transport regimes is possible due to thermal injection of charge carries at the contacts. It will be demonstrated that emission in the unipolar regime is present in devices providing ohmic contacts and that it can be totally suppressed utilizing non-ohmic contacts which can be achieved by the use of suited metals. Further, employing a double layer of different color emitting acenes in the transistor channel, the vertical movement of the recombination zone can be probed by a color change in the light emission when passing from the unipolar to the ambipolar regime. Both the dependence of the light emission on the utilized contact metal and the color change in the double layer device clearly demonstrate that different conditions apply for the light emission in the unipolar and ambipolar regimes.     

Anomalous kink behavior in the current-voltage characteristics of suspended carbon nanotubes

Electrically-heated suspended, nearly defect-free, carbon nanotubes (CNTs) exhibiting negative differential conductance in the high bias regime experience a sudden drop in current (or “kink”). The bias voltage at the kink (V _kink) is found to depend strongly on gate voltage, substrate temperature, and gas environment. After subtracting the voltage drop across the contacts, however, the kink bias voltages converge around 0.2 V, independent of gate voltage and gas environment. This bias voltage of 0.2 V corresponds to the threshold energy of optical phonon emission. This phenomenon is corroborated by simultaneously monitoring the Raman spectra of these nanotubes as a function of bias voltage. At the kink bias voltage, the G band Raman modes experience a sudden downshift, further indicating threshold optical phonon emission. A Landauer model is used to fit these kinks in various gas environments where the kink is modeled as a change in the optical phonon lifetime, which corresponds to a change in the non-equilibrium factor that describes the existence of hot phonons in the system.      

Enhanced and stable electron emission of carbon nanotube emitter arrays by post-growth hydrofluoric acid treatment

Carbon nanotubes (CNT) have been highlighted as possible candidates for field-emission emitters and vacuum nanoelectronic devices. In this article, we studied the effect of acid treatment of CNTs on field emission from carbon nanotube field emitter arrays (FEAs), grown using the resist-assisted patterning process (RAP). The emission current densities of as grown CNT-FEAs and those which were later immersed in hydrofluoric acid (HF) for 20 s, were 19 μA/cm² and 7.0 mA/cm², respectively, when measured at an anode field of 9.2 V/μm. Hence, the emission current densities after HF treatment are 300 times larger than those of as grown CNT-FEAs. Also, it was observed that a very stable electron emission current was obtained after stressing the CNTs with an electric field of 9.2 V/μm for 800 min in dc-mode, where the emission current non-uniformity was 0.13%. The enhancement in electron emission after HF treatment appears to be due to the effect of fluorine bonding. Also, the electron emission characteristics and structural improvement of CNT-FEAs after HF treatment are discussed.     

Electrically excited, localized infrared emission from single carbon nanotubes

Carbon nanotube field-effect transistors (CNTFETs) produce band gap derived infrared emission under both ambipolar and unipolar transport conditions. We demonstrate here that heterogeneities/defects in the local environment of a CNTFET perturb the local potentials and, as a result, the characteristic bias dependent motion of the ambipolar light emission. Such defects can also introduce localized infrared emission due to impact excitation by carriers accelerated by a voltage drop at the defect. The correlation of the change in the motion of the ambipolarlight emission and of the stationary electroluminescence with the electrical characteristics of the CNTFETs shows that stationaryelectroluminescence can identify "environmental defects" in carbon nanotubes and help evaluate their influence on electrical transport and device operation. A number of different defects are studied involving local dielectric environment changes (partially polymer-covered nanotubes), nanotube-nanotube contacts in looped nanotubes, and nanotube segments close to the electronic contacts. Random defects due to local charging are also observed.     

Exploring the Charge Transport in Conjugated Polymers

Conjugated polymers came to an unprecedented epoch that the charge transport is limited only by small disorder within aggregated domains. Accurate evaluation of transport performance is thus vital to optimizing further molecule design. Yet, the routine method by means of the conventional field‐effect transistors may not satisfy such a requirement. Here, it is shown that the extrinsic effects of Schottky barrier, access transport through semiconductor bulk, and concurrent ambipolar conduction seriously influence transport analysis. The planar transistors incorporating ohmic contacts free of access and ambipolar conduction afford an ideal access to charge transport. It is found, however, that only the planar transistors operating in low‐field regime are reliable to explore the inherent transport properties due to the energetic disorder lowering by the lateral field induced by high drain voltage. This work opens up a robust approach to comprehend the delicate charge transport in conjugated polymers so as to develop high‐performance semiconducting polymers for promising plastic electronics.     

Electrical properties of carbon nanotube thin-film transistors fabricated using plasma-enhanced chemical vapor deposition measured by scanning probe microscopy

The electrical properties of carbon nanotube thin-film transistors (CNT-FETs) fabricated using plasma-enhanced chemical vapor deposition (PECVD) were studied by scanning probe microscopy. The measured results suggest the formation of an island structure in the subthreshold regime and the disappearance of the island structure at the ON state. These results were explained by the change in the effective number of CNTs that contributed to the electrical conduction due to the gate-bias-dependent resistance of the semiconducting CNTs. The results obtained by Monte Carlo simulation revealed similar results. The effects of metallic CNTs with defects and the scatter of the drain current in the subthreshold regime were also examined.     

Synthesis and electrochemical application of carbon nanotubes obtained from hexachloroethane

Carbon nanotubes (CNTs) with the average inner (outer) diameter of 10–20 nm (20–40 nm) and length up to 100s of nanometers were synthesized via Wurtz reaction at 400 °C for 12 h, using C₂Cl₆ and Na as reactants. These CNTs, having more defects because of the sp³ bonding raw material of C₂Cl₆, were used as electrode material to detect dopamine (DA) via cyclic voltammetry. The results show that there exists linear relation between peak currents and DA concentration in the range of 2 × 10⁻⁷∼2.8 × 10⁻⁴ mol L⁻¹.The linear regression equation is expressed as Ip (μA) = 0.089 + 0.134c (μmol L⁻¹). This CNTs-modified electrode showed high sensitivity with detection limit of 1 × 10⁻⁷ mol L⁻¹.     

Diode properties of nanotube networks

Single-walled carbon nanotubes (SWCNT) were prepared using iron catalysts deposited by indirect evaporation on silicon substrate covered with 500 nm-thick thermal oxide. Diode SWCNT devices have been fabricated using Au and Al, as the asymmetric metal contacts, and a random network of metallic and semiconducting nanotubes as the device channel. No effort was made to align the SWCNTs or to eliminate metallic nanotubes in our devices. Asymmetric voltage-current behavior was seen. Current rectification was observed in the source-drain bias range of − 3 V to + 3 V. Rectification was somewhat surprising since, although metallic tubes are in the minority (∼ 1/3), they could potentially act as shunts and mask the electric properties of the semiconducting majority. No correlation between electrode spacing and current rectification was observed. The lowest leakage current measured was 1% of the maximum current carrying capacity. Maximum forward-biased current capacities range between 8 μA and 841 μA.     

Intrinsic and extrinsic performance limits of graphene devices on SiO2

The linear dispersion relation in graphene gives rise to a surprising prediction: the resistivity due to isotropic scatterers, such as white-noise disorder or phonons, is independent of carrier density, n. Here we show that electron-acoustic phonon scattering is indeed independent of n, and contributes only 30 Omega to graphene's room-temperature resistivity. At a technologically relevant carrier density of 1 x1012 cm-2, we infer a mean free path for electron-acoustic phonon scattering of >2 microm and an intrinsic mobility limit of 2 x 105 cm2 V-1 s-1. If realized, this mobility would exceed that of InSb, the inorganic semiconductor with the highest known mobility ( approximately 7.7 x 104 cm2 V-1 s-1; ref. 9) and that of semiconducting carbon nanotubes ( approximately 1 x 105 cm2 V-1 s-1; ref. 10). A strongly temperature-dependent resistivity contribution is observed above approximately 200 K (ref. 8); its magnitude, temperature dependence and carrier-density dependence are consistent with extrinsic scattering by surface phonons at the SiO2 substrate and limit the room-temperature mobility to approximately 4 x 104 cm2 V-1 s-1, indicating the importance of substrate choice for graphene devices.     

Spectral study of interaction between poly(<Emphasis Type="SmallCaps">l</Emphasis>-lysine)–poly(ethylene glycol)–poly(<Emphasis Type="SmallCaps">l</Emphasis>-lysine) and nucleic acids

Polymer-DNA interactions have attracted considerable interests due to their important application in DNA transfection and cellular drug delivery technologies. In this work, a new detection assay for DNA is proposed with a tri-block copolymer poly(l-lysine)–poly(ethylene glycol)–poly(l-lysine) by resonance light scattering technique with the linear ranges from 0.0656 to 6.56 μg ml⁻¹. The detection limit for DNA is 0.42 ng ml⁻¹. Most coexisting substances do not interfere in the detection. UV-spectra and FTIR-spectra were employed to demonstrate the mechanisms of the interaction that the conformation of the DNA changes because the microenvironment of DNA changes.     

Properties of short channel ballistic carbon nanotube transistors with ohmic contacts

We present self-consistent, non-equilibrium Green's function calculations of the characteristics of short channel carbon nanotube transistors, focusing on the regime of ballistic transport with ohmic contacts. We first establish that the band line-up at the contacts is renormalized by charge transfer, leading to Schottky contacts for small diameter nanotubes and ohmic contacts for large diameter nanotubes, in agreement with recent experiments. For short channel ohmic contact devices, source-drain tunnelling and drain-induced barrier lowering significantly impact the current-voltage characteristics. Furthermore, the ON state conductance shows a temperature dependence, even in the absence of phonon scattering or Schottky barriers. This last result also agrees with recently reported experimental measurements.     

Doping-free fabrication of carbon nanotube thin-film diodes and their photovoltaic characteristics

Random networks of single-walled carbon nanotubes (SWCNTs) were have been grown by chemical vapor deposition on silicon wafers and used for fabricating field-effect transistors (FETs) using symmetric Pd contacts and diodes using asymmetrical Pd and Sc contacts. For a short channel FET or diode with a channel length of about 1 μm or less, the device works in the direct transport regime, while for a longer channel device the transport mechanism changes to percolation. Detailed electronic and photovoltaic (PV) characterizations of these carbon nanotube (CNT) thin-film devices was carried out. While as-fabricated FETs exhibited typical p-type transfer characteristics, with a large current ON/OFF ratio of more than 10⁴ when metallic CNTs were removed via a controlled breakdown, it was found that the threshold voltage for the devices was typically very large, of the order of about 10 V. This situation was greatly improved when the device was coated with a passivation layer of 12 nm HfO₂, which effectively moved the threshold voltages of both FET and diode back to center around zero or turned these device to their OFF states when no bias was applied on the gate. PV measurements were then made on the short channel diodes under infrared laser illumination. It was shown that under an illumination power density of 1.5 kW/cm², the device resulted in an open circuit voltage V _OC = 0.21 V and a short circuit current I _SC = 3.74 nA. Furthermore, we compared PV characteristics of CNT film diodes with different channel lengths, and found that the power transform efficiency decreased significantly when the device changed from the direct transport to the percolation regime.     

Electron structure and activation energy of hydrogen in α-Zr using nonlinear response theory

The electron structure of hydrogen in hcp Zr is calculated by using self-consistent nonlinear screening theory. The host-ion contribution is included through the spherical solid model potential (SSMP). The resulting charge density and scattering phase shifts are used to calculate the activation energy and residual resistivity of hydrogen in α-Zr matrix. The calculated activation energy 0·285 eV is found in reasonably good agreement with experimental value 0·3 eV. The estimated residual resistivity 0·53 μΩ cm/at% for Zr-H system using the scattering phase shifts agrees reasonably well with the observed value 0·27 μΩ cm/at%. The calculated configurational energy shows that hydrogen prefers tetrahedral(T)-sites over octahedral(O)-sites in α-Zr. The strong binding energy of electron-proton suggests that hydrogen forms zirconium hydride.     

Investigation of charge transport in organic polymer donor/acceptor photovoltaic materials

π-conjugated organic semiconductors have long been used as either holes or electrons transport materials. Recently, ambipolar charge carrier transport in these materials have been reported in many investigations. In this paper, we report on the basis of experimental results that the organic semiconductor (donor/acceptor) materials can be as good electrons transporters as these materials are holes transporters. In our study, the solution-processed unipolar diodes based on organic materials P3HT, VOPCPhO, and their blends with PCBM have been fabricated. The I–V characteristics of these diodes have been analyzed in the space-charge-limited current regime. The values of the electron and hole mobilities for the materials were found in the range of 10^?4–10^?5?cm²/Vs.     

Effect of carbon nanotube reinforcement on fracture strength of composite adhesive joints

This study investigated the use of carbon nanotubes (CNTs) as an epoxy adhesive additive for adhesive joints between steel–composite interfaces and composite–composite interfaces. The study also examined the effect of CNT functionalization to improve CNT dispersion and thus improve joint strength. Specimens were constructed by adhesively bonding two parallel coupons, with a starting crack at one end. The specimens were loaded to final failure in three-point bending for Mode II fracture. Critical strain energy release rate was used to compare fracture properties of each set of specimens. It was shown that additions of multi-walled CNTs on the order of 1 wt% with diameters on the order of 30 nm and lengths 5–20 μm enhanced fracture toughness for both steel–composite and composite–composite adhesive joints tested. However, other combinations of CNTs could significantly decrease fracture properties, likely due to agglomeration issues. Functionalization of nanotubes showed some limited promise. Scanning electron microscopy validated the improved dispersion of CNTs using functionalization, but also highlighted the shortening effects due to the harsh chemical treatment. In summary, the study illustrates the importance of various CNT parameters on fracture properties, and encourages further investigation and optimization of these parameters for applications of interest.     

Lattice dynamics of solid Ne

Previously derived interatomic potentials for Ne are used to examined selected solid state properties. The self-consistent phonon harmonic approximation with suitable corrections for cubic anharmonic terms is used to calculate phonon dispersion curves at 5 K anda=4.454 Å. The isochoric temperature shifts between 5 and 22 K of phonons propagating in the [100] direction are evaluated using two approximations for the phonon self-energy. ThepV isotherm at 4.2 K and the volume dependence of the elastic constants and the zero-temperature Debye theta from the heat capacity are also calculated. There is reasonable agreement with experiment at low temperatures but near the melting point there is a real discrepancy with recent Brillouin scattering data and reasons for this are discussed.     

Temperature-Dependent Thermal Boundary Conductance at Al/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and Pt/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> interfaces

With the ever-decreasing size of microelectronic devices, growing applications of superlattices, and development of nanotechnology, thermal resistances of interfaces are becoming increasingly central to thermal management. Although there has been much success in understanding thermal boundary conductance at low temperatures, the current models applied at temperatures more common in device operation are not adequate due to our current limited understanding of phonon transport channels. In this study, the scattering processes in Al and Pt films on Al₂O₃ substrates are examined by transient thermoreflectance testing at high temperatures. At high temperatures, traditional models predict the thermal boundary conductance to be relatively constant in these systems due to assumptions about phonon elastic scattering. Experiments, however, show an increase in the conductance indicating potential inelastic phonon processes.     

Asymptotic estimation of the threshold power and anti-Stokes combination frequencies of stimulated temperature scattering in solid-state microcavities for the optical range

Asymptotic estimates of the threshold power and anti-Stokes combination frequencies of stimulated temperature scattering (STS) have been obtained for spherical fused-quartz microcavities for the optical range. A three-mode regime of interaction of the temperature and electromagnetic modes of the whispering gallery type is considered. It is shown that a threshold STS power of is on the order of 50 μ W for quartz spheres with a radius of 40 μ m at a pumping radiation wavelength of 840 nm. Microcavities featuring the whispering gallery modes are promising elements for the construction of resonance bolometers and microlasers.     

Computational study of exciton generation in suspended carbon nanotube transistors

Optical emission from carbon nanotube transistors (CNTFETs) has recently attracted significant attention due to its potential applications. In this paper, we use a self-consistent numerical solution of the Boltzmann transport equation in the presence of both phonon and exciton scattering to present a detailed study of the operation of a partially suspended CNTFET light emitter, which has been discussed in a recent experiment. We determine the energy distribution of hot carriers in the CNTFET and, as reported in the experiment, observe localized generation of excitons near the trench-substrate junction and an exponential increase in emission intensity with a linear increase in current versus gate voltage. We further provide detailed insight into device operation and propose optimization schemes for efficient exciton generation; a deeper trench increases the generation efficiency, and use of high-k substrate oxides could lead to even larger enhancements.