Concept study for a high-efficiency nanowire-based thermoelectric

Materials capable of highly efficient, direct thermal-to-electric energy conversion would have substantial economic potential. Theory predicts that thermoelectric efficiencies approaching the Carnot limit can be achieved at low temperatures in one-dimensional conductors that contain an energy filter such as a double-barrier resonant tunneling structure. The recent advances in growth techniques suggest that such devices can now be realized in heterostructured, semiconductor nanowires. Here we propose specific structural parameters for InAs/InP nanowires that may allow the experimental observation of near-Carnot efficient thermoelectric energy conversion in a single nanowire at low temperature

Analysis of surface waves generated on subwavelength-structured silver films

Using transmission electron microscopy (TEM) to analyse the physical-chemical surface properties of subwavlength structured silver films and finite-difference time-domain (FDTD) numerical simulations of the optical response of these structures to plane-wave excitation, we report on the origin and nature of the persistent surface waves generated by a single slit-groove motif and recently measured by far-field optical interferometry. The surface analysis shows that the silver films are free of detectable oxide or sulfide contaminants, and the numerical simulations show very good agreement with the results previously reported.Comment: 9 Figure

Efficiency in nanostructured thermionic and thermoelectric devices

Advances in solid-state device design now allow the spectrum of transmitted electrons in thermionic and thermoelectric devices to be engineered in ways that were not previously possible. Here we show that the shape of the electron energy spectrum in these devices has a significant impact on their performance. We distinguish between traditional thermionic devices where electron momentum is filtered in the direction of transport only and a second type, in which the electron filtering occurs according to total electron momentum. Such 'total momentum filtered' kr thermionic devices could potentially be implemented in, for example, quantum dot superlattices. It is shown that whilst total momentum filtered thermionic devices may achieve efficiency equal to the Carnot value, traditional thermionic devices are limited to efficiency below this. Our second main result is that the electronic efficiency of a device is not only improved by reducing the width of the transmission filter as has previously been shown, but also strongly depends on whether the transmission probability rises sharply from zero to full transmission. The benefit of increasing efficiency through a sharply rising transmission probability is that it can be achieved without sacrificing device power, in contrast to the use of a narrow transmission filter which can greatly reduce power. We show that devices which have a sharply-rising transmission probability significantly outperform those which do not and it is shown such transmission probabilities may be achieved with practical single and multibarrier devices. Finally, we comment on the implications of the effect the shape of the electron energy spectrum on the efficiency of thermoelectric devices.Comment: 11 pages, 15 figure

Charge injection instability in perfect insulators

We show that in a macroscopic perfect insulator, charge injection at a field-enhancing defect is associated with an instability of the insulating state or with bistability of the insulating and the charged state. The effect of a nonlinear carrier mobility is emphasized. The formation of the charged state is governed by two different processes with clearly separated time scales. First, due to a fast growth of a charge-injection mode, a localized charge cloud forms near the injecting defect (or contact). Charge injection stops when the field enhancement is screened below criticality. Secondly, the charge slowly redistributes in the bulk. The linear instability mechanism and the final charged steady state are discussed for a simple model and for cylindrical and spherical geometries. The theory explains an experimentally observed increase of the critical electric field with decreasing size of the injecting contact. Numerical results are presented for dc and ac biased insulators.Comment: Revtex, 7pages, 4 ps figure

Measuring Temperature Gradients over Nanometer Length Scales

When a quantum dot is subjected to a thermal gradient, the temperature of electrons entering the dot can be determined from the dot's thermocurrent if the conductance spectrum and background temperature are known. We demonstrate this technique by measuring the temperature difference across a 15 nm quantum dot embedded in a nanowire. This technique can be used when the dot's energy states are separated by many kT and will enable future quantitative investigations of electron-phonon interaction, nonlinear thermoelectric effects, and the effciency of thermoelectric energy conversion in quantum dots.Comment: 6 pages, 5 figure

Measurement of the scintillation time spectra and pulse-shape discrimination of low-energy beta and nuclear recoils in liquid argon with DEAP-1

The DEAP-1 low-background liquid argon detector was used to measure scintillation pulse shapes of electron and nuclear recoil events and to demonstrate the feasibility of pulse-shape discrimination (PSD) down to an electron-equivalent energy of 20 keV. In the surface dataset using a triple-coincidence tag we found the fraction of beta events that are misidentified as nuclear recoils to be $<1.4\times 10^{-7}$ (90% C.L.) for energies between 43-86 keVee and for a nuclear recoil acceptance of at least 90%, with 4% systematic uncertainty on the absolute energy scale. The discrimination measurement on surface was limited by nuclear recoils induced by cosmic-ray generated neutrons. This was improved by moving the detector to the SNOLAB underground laboratory, where the reduced background rate allowed the same measurement with only a double-coincidence tag. The combined data set contains $1.23\times10^8$ events. One of those, in the underground data set, is in the nuclear-recoil region of interest. Taking into account the expected background of 0.48 events coming from random pileup, the resulting upper limit on the electronic recoil contamination is $<2.7\times10^{-8}$ (90% C.L.) between 44-89 keVee and for a nuclear recoil acceptance of at least 90%, with 6% systematic uncertainty on the absolute energy scale. We developed a general mathematical framework to describe PSD parameter distributions and used it to build an analytical model of the distributions observed in DEAP-1. Using this model, we project a misidentification fraction of approx. $10^{-10}$ for an electron-equivalent energy threshold of 15 keV for a detector with 8 PE/keVee light yield. This reduction enables a search for spin-independent scattering of WIMPs from 1000 kg of liquid argon with a WIMP-nucleon cross-section sensitivity of $10^{-46}$ cm$^2$, assuming negligible contribution from nuclear recoil backgrounds.Comment: Accepted for publication in Astroparticle Physic

Merit, Tenure, and Bureaucratic Behavior: Evidence From a Conjoint Experiment in the Dominican Republic

Bureaucratic behavior in developing countries remains poorly understood. Why do some public servants – yet not others – work hard to deliver public services, misuse state resources, and/or participate in electoral mobilization? A classic answer comes from Weber: bureaucratic structures shift behavior towards integrity, neutrality, and commitment to public service. Our paper conducts the first survey experimental test of the effects of bureaucratic structures. It does so through a conjoint experiment with public servants in the Dominican Republic. Looking at merit examinations and job stability, we find that Weber was right – but only partially. Recruitment by examination curbs corruption and political services by bureaucrats, while enhancing work motivation. Job stability, by contrast, only decreases political services: tenured bureaucrats are less likely to participate in electoral mobilization. Examinations thus enhance the quality of bureaucracy (motivation and lower corruption) and democracy (electoral competition); job stability only enhances the quality of democracy

Epitaxial growth of visible to infra-red transparent conducting In2O3 nanodot dispersions and reversible charge storage as a Li-ion battery anode

peer-reviewedUnique bimodal distributions of single crystal epitaxially grown In2O3 nanodots on silicon are shown to have excellent IR transparency greater than 87% at IR wavelengths up to 4 mu m without sacrificing transparency in the visible region. These broadband antireflective nanodot dispersions are grown using a two-step metal deposition and oxidation by molecular beam epitaxy, and backscattered diffraction confirms a dominant (111) surface orientation. We detail the growth of a bimodal size distribution that facilitates good surface coverage (80%) while allowing a significant reduction in In2O3 refractive index. This unique dispersion offers excellent surface coverage and three-dimensional volumetric expansion compared to a thin film, and a step reduction in refractive index compared to bulk active materials or randomly porous composites, to more closely match the refractive index of an electrolyte, improving transparency. The (111) surface orientation of the nanodots, when fully ripened, allows minimum lattice mismatch strain between the In2O3 and the Si surface. This helps to circumvent potential interfacial weakening caused by volume contraction due to electrochemical reduction to lithium, or expansion during lithiation. Cycling under potentiodynamic conditions shows that the transparent anode of nanodots reversibly alloys lithium with good Coulombic efficiency, buffered by co-insertion into the silicon substrate. These properties could potentially lead to further development of similarly controlled dispersions of a range of other active materials to give transparent battery electrodes or materials capable of non-destructive in situ spectroscopic characterization during charging and discharging.ACCEPTEDpeer-reviewe

Herschel/HIFI observations of O-rich AGB stars : molecular inventory

Spectra, taken with the heterodyne instrument, HIFI, aboard the Herschel Space Observatory, of O-rich asymptotic giant branch (AGB) stars which form part of the guaranteed time key program HIFISTARS are presented. The aim of this program is to study the dynamical structure, mass-loss driving mechanism, and chemistry of the outflows from AGB stars as a function of chemical composition and initial mass. We used the HIFI instrument to observe nine AGB stars, mainly in the H2O and high rotational CO lines We investigate the correlation between line luminosity, line ratio and mass-loss rate, line width and excitation energy. A total of nine different molecules, along with some of their isotopologues have been identified, covering a wide range of excitation temperature. Maser emission is detected in both the ortho- and para-H2O molecules. The line luminosities of ground state lines of ortho- and para-H2O, the high-J CO and NH3 lines show a clear correlation with mass-loss rate. The line ratios of H2O and NH3 relative to CO J=6-5 correlate with the mass-loss rate while ratios of higher CO lines to the 6-5 is independent of it. In most cases, the expansion velocity derived from the observed line width of highly excited transitions formed relatively close to the stellar photosphere is lower than that of lower excitation transitions, formed farther out, pointing to an accelerated outflow. In some objects, the vibrationally excited H2O and SiO which probe the acceleration zone suggests the wind reaches its terminal velocity already in the innermost part of the envelope, i.e., the acceleration is rapid. Interestingly, for R Dor we find indications of a deceleration of the outflow in the region where the material has already escaped from the star.Comment: 6 Figures in the main paper + 12 further figures in the appendix (to be printed in electronic form) Accepted for publication by A&