Zhuangchun Wu

Electronic Junction Control in a Nanotube-Semiconductor Schottky Junction Solar Cell

We exploit the low density of electronic states in single wall carbon nanotubes to demonstrate active, electronic modulation of their Fermi level offset relative to n-type silicon in a nanotube-Si (metal-semiconductor) Schottky junction solar cell. Electronic modulation of the Fermi level offset, the junction interface dipole and a field developed across the depletion layer modifies the built-in potential in the device and its power generation characteristics. As produced (before modulation) devices exhibit ∼8.5% power conversion efficiency (PCE). With active modulation the PCE is continuously and reversibly changed from 4 to 11%.

Carbon nanotube films for room temperature hydrogen sensing.

Thin, uniform, single-walled carbon nanotube films, made by a simple filtration process, subsequently coated with palladium, are shown to be promising detectors of hydrogen. The films detected hydrogen with relative responses of 20% at 100 ppm and 40% at 500 ppm concentrations. Most of the initial film conductance was recovered within 30 s by exposing the samples to air. This quick and easy recoverability make the Pd-coated nanotubes suitable for practical applications in room temperature hydrogen sensing while consuming only approximately 0.25 mW power. The film fabrication process provides highly reproducible control over the film thickness; an important ingredient for commercial production. In the course of this research strong evidence was obtained indicating that sputter deposition of metal onto the nanotubes, even under very low power, short exposure time conditions, does damage to the nanotubes.

Resistivity scaling in single-walled carbon nanotube films patterned to submicron dimensions

The authors describe efficient patterning of transparent, conductive single-walled carbon nanotube thin films by photolithography and e-beam lithography followed by reactive ion etching, and study the transport characteristics of the films patterned down to 200nm lateral dimensions. The resistivity of the films is independent of device length, while increasing over three orders of magnitude compared to the bulk films, as their width and thickness shrink. This behavior is explained by a geometrical argument. Such “top-down” patterning of nanotube films should permit their integration into submicron device structures; however, the strong resistivity scaling will have to be taken into account.

Nanolithographic patterning of transparent, conductive single-walled carbon nanotube films by inductively coupled plasma reactive ion etching

The authors report successful patterning of transparent, conductive single-walled carbon nanotube films down to 100nm lateral dimensions by photolithography or e-beam lithography and subsequent O2 plasma etching using an inductively coupled plasma reactive ion etching (ICP-RIE) system. They systematically study the effect of ICP-RIE etch parameters, such as substrate bias power, chamber pressure, and substrate cooling, on the nanotube film etch rate and etch selectivity. They also characterize the effect of the linewidth etched on the nanotube film etch rate for widths ranging from 50μm down to 100nm. Furthermore, by fabricating standard four point probe structures using the patterning capability developed, the authors investigate the effect of different resist processes on the resistivity of patterned single-walled carbon nanotube films and the effect of ICP reactive ion etching on the resistivity of partially etched nanotube films. In addition, they demonstrate that using an ICP-RIE system provides sign...

Metal-semiconductor-metal photodetectors based on single-walled carbon nanotube film-GaAs Schottky contacts

We demonstrate the Schottky behavior of single-walled carbon nanotube (CNT) film contacts on GaAs by fabricating and characterizing metal-semiconductor-metal (MSM) photodetectors with CNT film electrodes. We extract the Schottky barrier height of CNT film contacts on GaAs by measuring the dark I-V characteristics as a function of temperature. The results show that at temperatures above ∼260 K, thermionic emission of electrons with a barrier height of ∼0.54 eV is the dominant transport mechanism in CNT film–GaAs junctions, whereas at lower temperatures, tunneling begins to dominate suggested by the weak dependence of current on temperature. Assuming an ideal MS diode, this barrier height corresponds to a CNT film workfunction of ∼4.6 eV, which is in excellent agreement with the previously reported values. Furthermore, we characterize the effect of device geometry on the dark current and find that dark currents of the MSM devices scale rationally with device geometry, such as the device active area, finger ...

Electronic properties of metal-semiconductor and metal-oxide-semiconductor structures composed of carbon nanotube film on silicon

We fabricate and experimentally characterize the electrical properties of metal-semiconductor (MS) and metal-oxide-semiconductor (MOS) structures where the metal is single-walled carbon nanotube (CNT) film and the semiconductor is a Si substrate. Our results suggest that for the MS devices thermionic emission is the main high-temperature current transport mechanism, while tunneling becomes the dominant mechanism for MOS devices with thermally grown thin oxide layers between the CNT film and Si. In addition, the CNT film workfunction, a key parameter for the performance of CNT film-based devices, is obtained from the capacitance-voltage measurements on the MOS structures.

Metallic/Semiconducting Nanotube Separation and Ultra‐thin, Transparent Nanotube Films

Perhaps the most wonderful feature of carbon nanotubes is that they are synthesized in both metallic and semiconducting variants, and perhaps the most problematic feature is that they are synthesized in both metallic and semiconducting variants. The intimate mixture hampers numerous envisioned applications and separation of the nanotubes into their respective electronic transport classes has emerged (after high yield synthesis and purification) as the next great materials challenge. Recently, several groups (us among them) have shown progress in attacking the problem. We will elaborate on our bromine exposure/centrifugation based method and the evidence leading to our conclusion that the separation results from an interplay between the nanotubes, bromine and the surfactant. We will also elaborate on our method for production of ultra‐thin (and thereby transparent), optically homogeneous, nanotube films used in the spectroscopic absorbance based assay of the metallic/semiconducting nanotube content. Such f...

Metal-Semiconductor-Metal (MSM) Photodetectors Based on Single-walled Carbon Nanotube Film-Silicon Schottky Contacts

We fabricate and experimentally characterize metal-semiconductor-metal (MSM) photodetectors with CNT film Schottky electrodes on n-type and p-type silicon substrates. We extract a Schottky barrier height of ~0.45 eV and ~0.51 eV for CNT films on n-type and p-type Si respectively. The extracted barrier height corresponds to a CNT film workfunction of 4.5-4.7 eV, which is within the range of the previously reported workfunction values for individual CNTs. Furthermore, we find that while at temperatures above 240°K thermionic emission is the dominant transport mechanism, at lower temperatures tunneling begins to dominate. We also characterize the photoresponse of the CNT film-Si MSM photodetector by illuminating the samples with a 633 nm HeNe laser. We observe that while the photocurrent of the CNT film MSM devices is similar to that of the Ti/Au control samples at high biases, their lower dark current results in a higher photo-to-dark current ratio relative to the control devices. We explain these observations by comparing the two interfaces. This work opens up the possibility of integrating CNT films as transparent and conductive Schottky electrodes in conventional semiconductor electronic and optoelectronic devices.