Session Y24: Nanotube Devices and Applications

Self-Assembled, Self-Aligned Carbon Nanotube Thin Film Transistors

Carbon nanotube field effect transistors possess superb device characteristics for electronic applications. However, the non-selective nature of nanotube synthesis, difficulty in accurate nanotube placement, and the high device impedance of single tube devices pose major challenges in the integration of carbon nanotubes in large-scale electronic devices. Here we present a novel approach to address these issues. Carbon nanotubes used in this study have been purified and separated by their electronic structure, where the semiconducting tube percentage is as high as 99{\%}, confirmed by both transport measurements on individual nanotubes and by optical absorption spectra. Through a simple self-assembly technique, we have produced aligned nanotube arrays. Thin film transistors based on these aligned nanotube arrays are fabricated with both back- and top-gate layouts, showing good switching performance and a high drive current. It is found that top-gated and back-gated devices exhibit distinct switching behaviors due to screening effects. Results on device channel length dependence will also be presented.   

Temperature Measurement of Carbon Nanotube FETs by Raman Spectroscopy

Heat dissipation is an important concern for nanoscale electronic devices. Freely suspended carbon nanotubes experience self heating during electron transport due to a lack dissipation channels for acoustic phonons[1]. Nanotubes lying on a SiO2 substrate, however, are often assumed to be in good thermal contact with the underlying substrate [2]. In this work we show that there is substantial self-heating in nanotubes lying on a SiO2 substrate. We use Raman spectroscopy to monitor the temperature of carbon nanotube field effect transistors (FETs) as a function of the applied bias voltage. The temperature is determined from the shift in frequency and the broadening of the high energy Raman modes. Our results suggest that nanotubes FETs on a substrate can reach temperatures upwards of 700K before saturation. [1] Pop et al., PRL 95, 155505 (2005) [2] Lazzeri et al., PRB 73, 165419 (2006)   

Gate-all-around carbon nanotube field-effect transistor

The ultra-thin body of carbon nanotubes allows for aggressive channel length scaling while maintaining excellent gate control. In general, a gate-all-around (GAA) structure is expected to be the ideal geometry that maximizes electrostatic gate control in FETs. Combining the ultra-thin body of a carbon nanotube with a GAA device geometry is a natural choice for ultimate device design. In this talk, we demonstrate a gate-all-around single wall carbon nanotube field-effect transistor. This is the first successful experimental implementation of an off-chip gate and gate-dielectric assembly with subsequent deposition on a suitable substrate.   

MnxGe1-x nanowires field effect transistor for spintronics applications

Group IV Dilute Magnetic Semiconductors (DMS) materials attract much attention not only because of the potential for integration of DMSs with current COMS technology, but also the enhanced spin lifetime and coherent length due to small spin-orbit coupling and lattice inversion symmetry. On the other hand, nanowires are the versatile building blocks for the assembly of functional devices to do fundamental studies in nanoscale. Here we presents Mn$_{x}$Ge$_{1-x}$ (Mn $\sim $ 0.5-1{\%}) nanowires in which there are no detectable secondary phases and the Curie temperature (Tc) is higher than 400 K. Single Mn$_{x}$Ge$_{1-x}$ nanowire back gated field effect transistors (FETs) were fabricated and studied, and $p-$type depletion mode was observed with an on/off ratio of 10$^{4}$, threshold voltage of $\sim $ 0.53 V, maximum transconductance of 0.2 \textit{$\mu $}S, and subthreshold swing (SS) of 210 mV/decade. The mobility was estimated to be around 340 cm$^{2}$/Vs. These results show the high performance of our Mn$_{x}$Ge$_{1-x}$ nanowire FET, which indicates the Mn$_{x}$Ge$_{1-x}$ nanowires could be the promising building blocks for both electrical and spintronics devices.   

Transient Random Telegraph Signal in carbon nanotube field effect transistors

We have studied transient \textit{Random Telegraph Signal} (RTS) induced in carbon nanotube-channel field effect transistors (FETs) by operating them at high bias. RTS arises from the population and depopulation of charge traps at specific energies that are scanned by sweeping the gate in a FET. At high bias, surface adsorbates/dopants interact with the SWNT and produce transient charge traps, which are manifest in the RTS signature. Transient RTS has been seen at temperatures from 200K up to room temperature. We speculate that RTS spectra could provide a characteristic signature of specific adsorbates or adducts on the nanotube channel. This capability is of interest not only for potential sensing technology but also provides a way to introduce controllable quantum interference resonances in the channel transport.   

Large oscillating non-local voltage in multi-terminal single wall carbon nanotube devices

Spin field-effect transistor has been recently realized in single wall carbon nanotube (SWCNT) devices contacted with NiPd alloy [1]. In order to separate charge related effects from that of pure spin transport we measure a non-local voltage in SWCNTs by using a four-terminal structure. The four contacts divide the tube into three quantum dots (QD) which we control by the back-gate voltage V$_{g}$. We inject the current through the first QD by using excitation voltage of 200 $\mu $V and measure the non-local signal V$_{nl}$ across the third QD. We measure large \textit{oscillating} non-local voltage as a function of V$_{g}$ with amplitude of V$_{nl}\sim $ 2$\mu $V [2]. While the classical resistor model can account for the negative sign of the non-local voltage its large amplitude needs deeper understanding. We discuss the origin of this large non-local signal and its effect on the non-local spin transport measurements in this type of devices. [1] S. Sahoo, et al. Nature Phys. \textbf{1}, 99 (2005). [2] G. Gunnarsson et al., arXiv:0710.0365v1.   

Probing the electrostatics of a metal-carbon nanotube Schottky diode using capacitive measurements

Capacitance-voltage measurement is a technique widely used to characterize metal-semiconductor contacts. We apply this technique to measure the capacitance-voltage across a p-type Schottky contact formed by titanium and a semiconducting carbon nanotube. Ohmic and Schottky contacts are made on the nanotube using palladium and titanium, respectively. The results agree qualitatively with simulations done using a Poisson-Schroedinger solver, considering only the electrostatics. We found additional frequency-dependent effects in the capacitance measurement that indicate the presence of electronic states arising from adsorbates or defects on the length of the nanotube.   

Hypergolic fuel detection using Single Walled Nanotube Networks

Reliable and accurate detection of hypergolic fuels is vital to U. S. Missile Defense Agency. In this research a simple and highly sensitive SWNT network sensor was developed for real time monitoring of hydrazine leaks to ppm level concentrations. Upon exposure to hydrazine vapor, the resistance of n-type (after degassing) nanotubes is observed to decrease rapidly. The response time exhibits a linear dependence on the concentrations of the vapor. It was also found that the resistance of the sample can be recovered by pumping on the sample and exposing to UV light. The experimental results support chemical adsorption of hydrazine on SWNTs. Theoretical results of hydrazine-SWNT interaction [1] are compared with the experimental observations. Results of similar study on ammonia, dimethyl hydrazine, and naphthalene will also be presented. [1] Min Yu, C. S. Jayanthi, Shi-Yu Wu, APS 2008   

Electrically Tunable Spin Polarization in a Carbon-Nanotube Spin Diode

We have studied the current through a carbon nanotube quantum dot with one ferromagnetic and one normal-metal lead. For the values of gate voltage at which the normal lead is resonant with the single available non-degenerate energy level on the dot, we observe a pronounced decrease in the current for one bias direction. We show that this rectification is spin-dependent, and that it stems from the interplay between the spin accumulation and the Coulomb blockade on the quantum dot. Our results imply that the current is spin-polarized for one direction of the bias, and that the degree of spin polarization is fully and precisely tunable using the gate and bias voltages. As the operation of this spin diode does not require high magnetic fields or optics, it could be used as a building block for electrically controlled spintronic devices.   

Detection of adsorbed gas atoms on suspended single-walled carbon nanotube micro-balances

Monolayers of gas atoms or molecules adsorbed on suspended single-walled carbon nanotubes offer the opportunity to study the phases and phase transitions of a unique low dimensional system.~ They are expected to resemble the well studied 2D monolayers on planar graphite, but with tight cylindrical boundary conditions imposed.~ The adsorbed density can be measured by using the nanotube itself as a vibrating microbalance, whose vibration amplitude is detected through the induced modulation of the conductance.~ We are initially studying the noble gases Ne, Kr and Xe, which are attractive for their simplicity and which show discontinuous phase transitions on 2D graphite that from basic considerations should be altered or suppressed as the dimensionality is reduced.~ We will also study oxygen monolayers, because oxygen has more complex 2D ordering on graphite, being magnetic and nonspherical, and because of the surprisingly large doping effect reported for oxygen on nanotubes which remains to be fully understood.~ We will survey the resonant behavior of a number of nanotube microbalances we have made, including examples with quality factor greater than 1000, and the damping effect of a gaseous environment.~ We will then report on our progress in detecting adsorbed layers and phase transitions in them.   

Ab initio design of reallistic nanotube sensors

The understanding of the electronic transport properties of nanoscopic devices present tantalizing possibilities. In particular it has been demonstrated that carbon nanotubes can be used as sensors for hazardous gases. Large scale computer simulations have an important role to play in predicting the transport properties of such systems. In order to do so one must take into account devices which are a few hundred nanometers in length and present defects randomly distributed along the structure. These defects act as binding sites for the molecules one wishes to detect. In this work we initially use density functional theory (DFT) to determine the most likely defects in highly nitrogen-doped carbon nanotubes, and to calculate the dissociation path of ammonia and hydrogen sulphide molecules onto these defects. Finally we use a combination of DFT and recursive Green's functions techniques to first assemble and then calculte the electronic transport properties of nanotubes up to 200 nm in length and with defects randomly distributed along the structure. We demonstrate that these nanotubes present relatively large resistance changes even at low coverages which leads to highly sensitive devices. The result is a new paradigm in computer-aided sensor design, where one can simulate realistic sensors.   

Utilizing Carbon Nanotubes for 1-D Mass Transport

Precision control of the size and placement of materials on the nanoscale creates many opportunities for customizable materials. Recent reports have shown that carbon nanotubes act as efficient one-dimensional mass transport platforms. We have designed nanotube devices on custom fabricated electron transparent substrates compatible with transmission electron microscopy in order to study mass transport mechanisms and applications in situ. Here we report the results of these studies.   

Measurement of metal/carbon nanotube contact resistance by shortening contact length

Estimating the contact resistance to achieve a minimum contact length of a nanotube interconnect to a nanoscale electronic device is a major challenge. In this study, we describe a novel experiment using a focused ion beam to sequentially shorten the contact length between a nanotube and an evaporated metallic film. We develop a theoretical model that relates the measured resistance change as a function of contact length to the intrinsic linear resistivity of the nanotube as well as the specific contact resistivity between the nanotube and the deposited metallic film. In this way, we arrive at an estimate for the optimal contact length of the metal film to the carbon nanotube. The results for Au and Ag contacts to multi-wall carbon nanotubes will be summarized. Our method is quite general and can be used to accurately determine the contact resistance of any metallic film to a wide variety of different nanotubes and nanowires.   

Bolometric Response of a Single-Wall Carbon Nanotube

We report on the low temperature bolometric (thermal) rf response of individual metallic single-wall carbon nanotubes. This response is used to determine the thermal conductance of the nanotube. Previous work has demonstrated heterodyne mixing in individual carbon nanotubes using either an electrical I-V nonlinearity or a gate-modulated conductance. We distinguish between bolometric mixing and the response due to non-thermal electrical nonlinearities. These experiments are a precursor to proposed terahertz measurements of the frequency-dependent bolometric response of an individual single-wall nanotube.   

I-V transport measurements of a single unsupported MWCNT under various bending deformations.

Using a home-made low-temperature high-vacuum probe setup we have obtained more details about the transport characteristics of multiwall carbon nanotubes (MWCNTs). We report our experimental studies on the improvement of the nanowelding between the CNTs and a metallic (W) probe tip in our SEM, which gives a clean and firm contact that satisfies for both electrical and mechanical requirements. We observe hysteresis of the I-V curves between bending and un-bending cycles, effective and efficient fabrication of junctions in the MWCNTs and their respective I-V characteristics, and the deformation-dependent saturation behaviors in the I-V curves of the MWCNTs. All these observations may be qualitatively understood using a simple phenomenological model for localization effects in the deformed hexagonal lattice of graphene.