Session X14: Nanowires and Nanotubes: Devices and Applications

Conductometric sensor based on Individual CuO Nanowires

CuO nanowires with diameters ranging from 30 to 100 nm are synthesized via a simple thermal oxidation method. High resolution transmission electron microscopy shows that the CuO nanowires have monoclinic crystalline structure. The charge conduction on individual nanowire under transverse electric field exhibits an intrinsic $p$-type semiconducting behavior. The conductivity, charge concentration, and field effect mobility are estimated to be $\sim $1.1 x 10$^{-3}$ S/cm, 8.3x10$^{19}$ cm$^{-3}$, and 2.7x10$^{-3}$ cm$^{2}$/Vs, respectively. Variations in the electrical conductance in different chemical gas environments ($e.g.$ air, NO$_{2}$, and ethanol) are measured on individual CuO nanowire field effect transistors and compared with simulation of surface chemisorption. They show reproducible sensing response and demonstrate excellent sensitivity to the surface adsorbed chemicals. In particular, it is found that the CuO nanowire chemical sensor reveals a reversal response in ethanol vapor at a transition temperature around 300 $^{o}$C. This is attributed to the redox reaction between ethanol and pre-adsorbed oxygen species on the sensor surface.   

Controlling the carrier concentration of Bi2Se3 nanoribbons via doping and gating

Bi2Se3 is one of the few materials identified as a topological insulator; hence, many efforts are underway to measure its electric properties. While recent transport studies of Bi2Se3 nanoribbons have provided strong evidence for the existence of surface states, additional measurements are needed to reveal identifying signatures of the topological states such as dissipationless transport or spin polarized current. The main roadblock to these experiments is the high bulk carrier concentration, which masks the effects of the surface states. Here, we present our efforts to reduce the bulk carrier concentration in Bi2Se3 nanoribbons via doping and gating.   

Memristance in single-component metallic nanowires

In this talk, we will discuss a new type of memristor - one whose state variable is its physical geometry. Using a single metallic material, we employ electromigration to change the resistance of metallic nanowires. The resistive switching is due to the creation/filling-in of voids in the nanowire as atoms are pushed back and forth by the electrical current. Exploiting electromigration in this manner, we repeatedly switch the resistance of single-component metallic nanowires between low and high states over many cycles. This work thus completes the array of fundamental passive circuit elements, now including the memristor, which can be fabricated from a single metallic material.   

Device Properties of Manganite Perovskite Nanowires

The colossal magnetoresistance (CMR) and large spin polarization of La$_{1-x}$Sr$_{x}$MnO$_{3}$ (LSMO) are attractive for fabricating novel spin-based devices. But it has been reported that magnetic fields of several tesla are typically required to observe the CMR effects, limiting the material for potential applications. Few novel approaches to low-field MR effects have been reported in polycrystalline manganite-insulator composite and in trilayer epitaxial thin film systems. However, at higher temperature, the observed low-field CMR significantly decreases and make them less attractive for room temperature spintronic devices. In this work, we present that high quality LSMO nanowire devices exhibit enhanced magnetoresistance behavior under low applied magnetic field (H=500-1000G) at room temperature.   

Electrical rectification in axial \textit{in-situ} doped Ge nanowire \textit{pn} junctions

We demonstrate the vapor-liquid-solid growth of and electrical rectification in axial \textit{in-situ} doped\textit{ pn} junction Ge nanowires (NWs). \textit{In-situ} doping of the NWs was accomplished by introducing dopant gases (diborane and phosphine) during growth, resulting in an axial \textit{pn} junction. Contacts to the wires were defined using e-beam lithography, followed by Ni metallization. Four-point measurements of the fabricated devices at room temperature and at 77 K clearly show rectification with on/off current ratio of more than two orders of magnitude when the bias is applied across the NW \textit{pn} junction. The ideality factor of the junction current points to a significant generation-recombination contribution. The Ohmic characteristics in the $p$ and $n$ regions outside the junction make it possible to estimate the doping levels. We also observed gate control of the NW junction current using the substrate as a back gate. Observed current modulation is in good agreement with the electrostatic depletion of the NWs as a function of diameter and doping.   

Atomistic approach to study charge and current distribution in ultra-scaled SiGe/Si core/shell nanowire FETs

Recent development in the fabrication processes have enabled the manufacturing of ultra-scaled, high mobility SiGe/Si core/shell nanowire FETs (NWFETs). These NWFETs are meant to break the speed bottleneck of CMOS devices by enhancing the performance of pMOS devices as well as making nMOS devices faster. These devices operate under strong geometrical and potential confinements where quantum effects are dominant. In this work we study these ultra-scaled NWFETs using an atomistic Tight-Binding based modified Virtual Crystal Approximation method, for electronic structure calculation, coupled to a top-of-the-barrier quantum transport model. Variation in the bandstructure with Si shell thickness and Ge concentration in the core are studied. This allows us to capture the effect of bandstructure on the charge and current distribution in these NWFETs. Higher Ge concentration and optimal Si shell thickness are necessary to provide good performance in both n and p type devices. Further improvement in the performance of p-type devices can be achieved by using high-k gate dielectric material.   

Full 3D Quantum Transport Simulation of Interface Roughness in Nanowire FETs

Silicon-silicon dioxide interface roughness has been identified as a critical device characteristic in MOS devices a long time ago. Down-scaling the diameter of silicon nanowires increases the relative importance of the details on their surfaces. In this work, we numerically investigate the effects of surface roughness in ultra-scaled silicon nanowire field-effect transistors (NWFETs) using a full 3D quantum transport simulator based on the atomistic sp$^{3}$d$^{5}$s* tight-binding model. The interface roughness between the silicon and the silicon dioxide atoms is generated in a real-space atomistic representation using an experimentally derived autocovariance function. Through statistical evaluations of different atomistic nanowire configurations, it is found that interface roughness alters the conduction band edge throughout the nanowire structure leading to resonance peaks in the transmission probability and localization of the density-of-states . These effects cause a reduction of the drain current in the ON-state of the NWFETs. A threshold voltage fluctuation is also observed in rough nanowire samples. The reduction and the fluctuation of the ON-current and of the threshold voltage caused by interface roughness increase as the diameter of the NWFETs decreases. This behavior is illustrated for NWFETs with a diameter of 2 and 3nm.   

A Feedback-Stabilized Platform for the Study of the Electrical and Mechanical Properties of Atomic Point Contacts

The ability to create and maintain contacts of atomic dimensions between a probe tip and a surface enables the study of a wide range of electronic and mechanical properties of nanowires, single-atom chains and single molecules. Such experiments require that the relative tip-surface position be held stable to better than atomic dimensions for extended periods of time. In this work, we describe recent experimental results from a feedback-stabilized break junction (FSBJ) instrument that uses high-resolution Fabry-Perot interferometry to measure and control tip-surface position with better than 5 pm long-term stability in vacuum at 4 K. Experiments with gold contacts reveal complex electron transport behavior through gold nanowires and single-atom chains, including the observation of stable integer and non-integer quantum conduction states and variations in non-ballistic transport as a function of atomic chain length. Correlations are observed between measured experimental conduction states and calculated density-functional-theory-based results for one- and two-dimensional structures. Extension of the FSBJ to the measurement of atomic bond strength and stiffness will also be discussed.   

Large Area Applications and Current Transport Considerations in Carbon Nanotube Composites

Carbon nanotube (CNT) electronics can be broadly split into to single or few CNT-based architectures, such as that found in a transistor, or multiple nanotube based architectures such as in films or composites. Potential large area applications of the later include transparent electronics, nanocomposites for displays or lighting or in sensor technology. Crucial to the development of these applications is an understanding of the factors that control the current transport. In particular a key question to be asked is `what is the rate limiting step for electron transport?' We have studied the field induced electron emission with low volume fractions of CNTs in PVA. We find that excellent emission can be found with as low as 1 vol. {\%} CNTs - one of the lowest values reported for a nanocomposite cathode [1]. Below this concentration we find the rate limiting step for emission to be controlled by transport through the composite. We generalize our conclusions for other large area CNT based applications such as transparent electronics and next generation technologies. [1] Thomas Connolly, Richard C. Smith, Yenny Hernandez, Yurii Gun'ko, Jonathan N. Coleman and J. David Carey, Small \textbf{5}, 826 (2009).   

Single-wall carbon nanotube diode AC-DC power converter

Increasing demand for low power consumption in electronics combined with continued miniaturization of electronic device components in today's analogue/digital circuits have reduced the supply voltage to sub-one volt levels. The supply voltage to integrated circuit devices are provided by AC-DC converters, which converts high frequency ac voltage to a conditioned dc output voltage at a given power level The low-voltage AC-DC converters generally consist of a diode rectifier, which consists of several diodes, an inductor and capacitor. The critical feature of the AC-DC diode converter is its high rectification, which allows large current to flow only in one direction or for one phase of the AC input. In this paper we report the observation of AC-DC half-wave conversion in the range of 1-1000 Hz by single-wall carbon nanotube diode rectifiers, which show a high degree of rectification ($\sim $10$^{5})$. The conversion factor remains constant over the tested frequency range of 1-200 Hz, but decreases slightly between 200-1000 Hz.   

Solution-processable SWNT transistors: Toward high performance and full semiconducting device yield

The major hurdle to scale up SWNT network field-effect transistors (FETs) is the difficulty in obtaining high mobility and full semiconductor device yield due to the co-existence of metallic and semiconducting tubes in fabricated networks. We demonstrate that the radical initiator 1,1'-azobis(cyanocyclohexane) (ACN) allows for release of radicals which preferentially react with small and metallic SWNTs. By applying this reaction to CoMoCat SWNTs, metallic tubes are electrically suppressed and semiconducting thin-film transistors can be readily made with almost full semiconductor device yield. The effective mobility can be raised to $\sim $10 cm2/V$\cdot $s with increasing the network thickness to $\sim $20 nm while keeping the on-off ratio higher than 10000. Moreover, top-gated devices with polymer-ionic liquid mixture as the dielectrics are demonstrated to be almost hysteresis-free and with low threshold voltage, promising applications in low-cost printable electronics.   

The possibility of high-performance n-type carbon nanotube devices by fullerene functionalization: A first-principles study

The successive appearance of low-dimensional carbon nanomaterials, such as zero-dimensional [60]fullerene (C60), one-dimensional carbon nanotube (CNT), and two-dimensional graphene, has been an inexhaustible source for the study of novel scientific principles and the search of advanced technological applications. Recently, about a decade after the discovery of the first C60-CNT hybrid form, carbon nanopeapod, a new hybrid C60-CNT nanostructure termed as ``carbon nanobud'' has been synthesized [1]. Extending our earlier study of polymerized C60 nanowires [2], we here apply a first-principles computational approach [3] to consider the C60 functionalization of CNTs as a scheme to engineer the CNT-metal contacts to produce reliable high-performance n-type CNT devices. References: [1] A.G. Nasibulin, et al., Nat. Nanotechnol. 2, 156 (2007). [2] G. I. Lee, J. K. Kang, and Y.-H. Kim, J. Phys. Chem. C 112, 7029 (2008). [3] Y.-H. Kim et al., Phys. Rev. Lett. 94, 156801 (2005).   

Fabrication of Sub-Micron, Multi-Channel, Single Walled Carbon Nanotube Devices

Single-Walled Carbon Nanotubes (SWCNTs) possess exceptional electronic properties making them leading candidates for future device technologies. The current bottleneck for integration of SWCNTs is the limitations of the processing techniques available to address challenges such as selective placement, doping and differing electronic types. This talk will focus on using chemical methods to address these challenges. SWCNTs are first purified using a step-gradient and then chemically modified to reduce the contribution of the metallic species. The SWCNTs are then assembled into trenches to induce alignment of the SWCNTs. Devices containing multiple SWCNTs are then fabricated with sub-micron channel lengths. The device properties and effects of the chemical processes are evaluated.   

Spectroscopic Study of Chromophore/Single-Walled Carbon Nanotubes Hybrid Structures

Chromophores have been used to functionalize individual single-walled carbon nanotubes (SWNTs) for potential applications in optoelectronic devices. In such a system, the molecular photoabsorption can be synthetically tuned independently of the the nanotube structure. Recently, we have developed a nanotube-based transistor in which azobenzene-based chromophores were attached to SWNTs by means of an anthracene tether. UV light induces isomerization of the chromophore, changing the molecular dipole and photogating the SWNTs, shifting the threshold voltage and increasing the conductivity of the nanotube transistor. Here we report the results of spectroscopic analysis of chromophore functionalized SWNTs in orthodichlorobenzene. We examine the effect of changing the tether to pyrene on both the absorption and fluorescence emission spectrum. We will outline our studies on the cis-trans isomerization and creation of charge separated state in the attached chromophore in both solution and FET architecture. Research supported by the U.S. Department of Energy, Office of Basic Energy Science, Division of Materials Sciences and Engineering under Award {\#}ER46590.