Sreeram Vaddiraju

Near-infrared semiconductor subwavelength-wire lasers

We report near-infrared lasing in the telecommunications band in gallium antimonide semiconductor subwavelength wires. Our results open the possibility of the use of semiconductor subwavelength-wire lasers in future photonic integrated circuits for telecommunications applications.

Hierarchical Multifunctional Composites by Conformally Coating Aligned Carbon Nanotube Arrays with Conducting Polymer

A novel method for the fabrication of carbon nanotube (CNT)-conducting polymer composites is demonstrated by conformally coating extremely high aspect ratio vertically aligned-CNT (A-CNT) arrays with conducting polymer via oxidative chemical vapor deposition (oCVD). A mechanical densification technique is employed that allows the spacing of the A-CNTs to be controlled, yielding a range of inter-CNT distances between 20 and 70 nm. Using this morphology control, oCVD is shown to conformally coat 8-nm-diameter CNTs having array heights up to 1 mm (an aspect ratio of 10(5)) at all inter-CNT spacings. Three phase CNT-conducting polymer nanocomposites are then fabricated by introducing an insulating epoxy via capillary-driven wetting. CNT morphology is maintained during processing, allowing quantification of direction-dependent (nonisotropic) composite properties. Electrical conductivity occurs primarily along the CNT axial direction, such that the conformal conducting polymer has little effect on the activation energy required for charge conduction. In contrast, the conducting polymer coating enhanced the conductivity in the radial direction by lowering the activation energy required for the creation of mobile charge carriers, in agreement with variable-range-hopping models. The fabrication strategy introduced here can be used to create many multifunctional materials and devices (e.g., direction-tailorable hydrophobic and highly conducting materials), including a new four-phase advanced fiber composite architecture.

Selective sensing of volatile organic compounds using novel conducting polymer?metal nanoparticle hybrids

Conducting polymer-metal nanoparticle hybrids, fabricated by assembling metal nanoparticles on top of functionalized conducting polymer film surfaces using conjugated linker molecules, enable the selective sensing of volatile organic compounds (VOCs). In these conducting polymer-metal nanoparticle hybrids, selectivity is achieved by assembling different metals on the same conducting polymer film. This eliminates the need to develop either different polymers chemistries or device configurations for each specific analyte. In the hybrids, chemisorption of the analyte vapor induces charge redistribution in the metal nanoparticles and changes their work function. The conjugated linker molecule causes this change in the work function of the tethered nanoparticles to affect the electronic states in the underlying conducting polymer film. The result is an easily measurable change in the resistance of the hybrid structure. The fabrication of these sensing elements involved the covalent assembly of nickel (Ni) and palladium (Pd) metal nanoparticles on top of poly(3,4-ethylenedioxythiophene-co-thiophene-3-acetic acid), poly(EDOT-co-TAA), films using 4-aminothiophenol linker molecules. The change in resistance of hybrid Pd/poly(EDOT-co-TAA) and Ni/poly(EDOT-co-TAA) hybrid films to acetone and toluene, respectively, is observed to be in proportion to their concentrations. The projected detection limits are 2 and 10 ppm for toluene and acetone, respectively. A negligible response (resistance change) of the Pd/poly(EDOT-co-TAA) films to toluene exposure confirmed its selectivity for detecting acetone. Similarly, lack of response to acetone confirmed the selectivity of the Ni/poly(EDOT-co-TAA) stacks for detecting toluene. It is anticipated that the assembly of other metals such as Ag, Au and Cu on top of poly(EDOT-co-TAA) would provide selectivity for detecting and discriminating other VOCs.

Self-nucleation and growth of group III-antimonide nanowires

In this paper, we show that the growth of III-antimonides can occur via self-catalysis using either group III metal or Sb clusters at their tips. Specific experiments using GaSb and InSb systems show that bulk nucleation and growth of the respective antimonide wires can also occur from mm-sized droplets. The role of equilibrium solubility and the size of the droplet on bulk nucleation versus tip-led growth is discussed.

Engineering Efficient Thermoelectrics from Large-Scale Assemblies of Doped ZnO Nanowires: Nanoscale Effects and Resonant-Level Scattering

Recent studies focusing on enhancing the thermoelectric performance of metal oxides were primarily motivated by their low cost, large availability of the component elements in the earths crust, and their high stability. So far, these studies indicate that n-type materials, such as ZnO, have much lower thermoelectric performance than their p-type counterparts. Overcoming this limitation requires precisely tuning the thermal and electrical transport through n-type metal oxides. One way to accomplish this is through the use of optimally doped bulk assemblies of ZnO nanowires. In this study, the thermoelectric properties of n-type aluminum and gallium dually doped bulk assembles of ZnO nanowires were determined. The results indicated that a high zT of 0.6 at 1000 °C, the highest experimentally observed for any n-type oxide, is possible. The high performance is attributed to the tailoring of the ZnO phase composition, nanostructuring of the material, and Zn-III band hybridization-based resonant scattering.

Nanowire-based thermoelectrics

Research on thermoelectrics has seen a huge resurgence since the early 1990s. The ability of tuning a materials electrical and thermal transport behavior upon nanostructuring has led to this revival. Nevertheless, thermoelectric performances of nanowires and related materials lag far behind those achieved with thin-film superlattices and quantum dot-based materials. This is despite the fact that nanowires offer many distinct advantages in enhancing the thermoelectric performances of materials. The simplicity of the strategy is the first and foremost advantage. For example, control of the nanowire diameters and their surface roughnesses will aid in enhancing their thermoelectric performances. Another major advantage is the possibility of obtaining high thermoelectric performances using simpler nanowire chemistries (e.g., elemental and binary compound semiconductors), paving the way for the fabrication of thermoelectric modules inexpensively from non-toxic elements. In this context, the topical review provides an overview of the current state of nanowire-based thermoelectrics. It concludes with a discussion of the future vision of nanowire-based thermoelectrics, including the need for developing strategies aimed at the mass production of nanowires and their interface-engineered assembly into devices. This eliminates the need for trial-and-error strategies and complex chemistries for enhancing the thermoelectric performances of materials.

Thermoelectric properties of large-scale Zn3 P2 nanowire assemblies

Gram quantities of both unfunctionalized and 1,4-benzenedithiol (BDT) functionalized zinc phosphide (Zn3P2) nanowire powders, synthesized using direct reaction of zinc and phosphorus, were hot-pressed into highly dense pellets (≥98% of the theoretical density) for the determination of their thermoelectric performance. It was deduced that mechanical flexibility of the nanowires is essential for consolidating them in randomly oriented fashion into dense pellets, without making any major changes to their morphologies. Electrical and thermal transport measurements indicated that the enhanced thermoelectric performance expected of individual Zn3P2 nanowires is still retained within large-scale nanowire assemblies. A maximum reduction of 28% in the thermal conductivity of Zn3P2 resulted from nanostructuring. Use of nanowire morphology also led to enhanced electrical conductivity in Zn3P2. Interface engineering of the nanowires in the pellets, accomplished by hot-pressing BDT functionalized nanowires, resulted in an increase on both the Seebeck coefficient and the electrical conductivity of the nanowire pellets. It is believed that filtering of low energy carriers resulting from the variation of the chemical compositions at the nanowire interfaces is responsible for this phenomenon. Overall, this study indicated that mechanical properties of the nanowires along with the chemical compositions of their surfaces play a hitherto unknown, but vital, role in realizing highly efficient bulk thermoelectric modules based on nanowires.

Compositional disorder and its effect on the thermoelectric performance of Zn3P2 nanowire?copper nanoparticle composites

Recent studies indicated that nanowire format of materials is ideal for enhancing the thermoelectric performance of materials. Most of these studies were performed using individual nanowires as the test elements. It is not currently clear whether bulk assemblies of nanowires replicate this enhanced thermoelectric performance of individual nanowires. Therefore, it is imperative to understand whether enhanced thermoelectric performance exhibited by individual nanowires can be extended to bulk assemblies of nanowires. It is also imperative to know whether the addition of metal nanoparticle to semiconductor nanowires can be employed for enhancing their thermoelectric performance further. Specifically, it is important to understand the effect of microstructure and composition on the thermoelectric performance on bulk compound semiconductor nanowire-metal nanoparticle composites. In this study, bulk composites composed of mixtures of copper nanoparticles with either unfunctionalized or 1,4-benzenedithiol (BDT) functionalized Zn₃P₂ nanowires were fabricated and analyzed for their thermoelectric performance. The results indicated that use of BDT functionalized nanowires for the fabrication of composites leads to interface-engineered composites that have uniform composition all across their cross-section. The interface engineering allows for increasing their Seebeck coefficients and electrical conductivities, relative to the Zn₃P₂ nanowire pellets. In contrast, the use of unfunctionalized Zn₃P₂ nanowires for the fabrication of composite leads to the formation of composites that are non-uniform in composition across their cross-section. Ultimately, the composites were found to have Zn₃P₂ nanowires interspersed with metal alloy nanoparticles. Such non-uniform composites exhibited very high electrical conductivities, but slightly lower Seebeck coefficients, relative to Zn₃P₂ nanowire pellets. These composites were found to show a very high zT of 0.23 at 770 K, orders of magnitude higher than either interface-engineered composites or Zn₃P₂ nanowire pellets. The results indicate that microstructural composition of semiconductor nanowire-metal nanoparticle composites plays a major role in determining their thermoelectric performance, and such composites exhibit enhanced thermoelectric performance.

UPS of Boron-Sulfur Co-Doped, n-Type Diamond

the exact band structure of the co-doped samples. In this paper, ultraviolet photoelectron spectroscopy ~UPS! is used to characterize the band structure of co-doped diamond samples. The UPS results along with Mott-Schottky analysis are used to determine the position of the Fermi energy relative to the bandedges in the co-doped diamond samples both on the electrochemical and the vacuum scales. Based on the results, a possible model for the observed n-type conductivity in the co-doped diamond samples is proposed and discussed. Experimental Co-doped diamond films were grown homoepitaxially on high pressure, high temperature synthetic ~HPHT !~ 100! diamond samples using an ASTeX microwave plasma reactor. Co-doping was performed using H2S and trimethylboron ~TMB! as sulfur and boron doping additives during diamond growth using methane and hydrogen feed gases. The procedures for co-doping and Mott-Schottky analysis are described in detail elsewhere. 17,18 Mott-Schottky analysis is performed using a platinum wire as counter electrode and a standard calomel electrode ~SCE! as the reference in a 0.5 M H2SO4 electrolyte solution at room temperature. The feed gas compositions used for the growth of the samples in this study are summarized in Table I. All samples were grown using a microwave power of 1000 W. The pressure and substrate temperature were 25 Torr and 750°C, respectively. The first four samples listed in Table I are analyzed using UPS. Mott-Schottky analysis was extensively studied for various co-doped diamond samples, and the data was presented earlier. 17,18 Here, the Mott-Schottky data for the last two samples listed in Table I are presented and analyzed. UPS characterization of the co-doped samples was performed using a multichamber, ultrahigh vacuum ~UHV! surface science facility ~VG Scientific, England/RHK Technology, MI, USA ! comprising of a 150 mm radius CLAM4 hemispherical analyzer and a differentially pumped helium discharge lamp. The ultimate resolution of the UPS data is limited by the natural linewidth of the excitation source at ;0.1 eV. Samples were mounted onto copper holders using copper tape. All spectra were recorded using normal emission and at a constant analyzer energy of 2.5 eV. The surface of the sample was grounded to negate any charging effects during the photoelectron spectra acquisition. The copper holder surface was sputter-cleaned using a 5 keV Ar ion beam and was used as the reference. Following a method described by Diederich et al. 19 spectra were also obtained using a 9.0 V negative bias applied to the sample. As indicated in Ref. 19, the application of a 9.0 V negative bias helped overcome the work function of the spectrometer and increased the secondary emission.

A Scalable Method for the Synthesis of Metal Oxide Nanowires

Large scale synthesis (gram quantities) of nanostructures of transition metal oxides, like tungsten oxide, is essential for their use in catalysis, sensing, photochromic and electrochromic applications. Towards this end, we have developed a vapor phase method for the large scale synthesis of transition metal and metal oxide nanowires, employing no external templates or catalysts. The concept underlying this method was reported previously, and demonstrated specifically with the synthesis of tungsten and tungsten oxide nanowires (1). Briefly, chemical vapor transport of metal oxide onto substrates maintained above the decomposition of the respective metal oxide leads to the formation of metal nanowires. Similar chemical vapor transport of metal oxides onto substrates maintained below the decomposition temperature of the respective metal oxide leads to the formation of the corresponding metal oxide nanowires. Experiments for the demonstration of this concept were performed in a hot-filament chemical vapor deposition (HF-CVD) setup, using metal filaments as the source. One of the key requirements for the process is the supply of low amounts of oxygen, in the range of 0.03-0.1 sccm. Tungsten and tungsten oxide nanowires synthesized using this process are shown in Figures 1(a) and 1(b), respectively. In addition to tungsten and tungsten oxide, tantalum pentoxide, iron, nickel oxide and titania nanowires have also been synthesized using this method(2). A scale-up reactor, capable of producing gram quantities of nanowires is shown in Figure 1(c). The metal oxide nanowires synthesized using the above concept were tested for their performance in sensing, electrochromicity and dispersability. Dispersability of nanomaterials in various solvents is crucial for their use in various applications. It was observed that metal oxides in nanowire format disperse better in various organic solvents compared to those in nanoparticle format (Figure 2). The dispersions of metal oxide nanowires, made without any surfactant addition, were found to be stable over long periods of time. The as-synthesized tungsten oxide nanowire mats (3-D networks) on quartz substrates were tested for gas sensing. In addition, several grams of tungsten oxide nanowires were produced and used in the dispersion and electrochromic application studies. The electrochromicity of tungsten oxide nanowires is found to be highly reversible, with short coloration and bleaching times, compared to tungsten trioxide nanoparticles. Similarly, the dispersion and gas sensing behavior of the synthesized tungsten oxide nanowires in comparison to tungsten oxide nanoparticles will be discussed in detail.