Ab Initio Study of the Electron-Phonon Interaction in Phosphorene

Apr 9, 2015 10:30am ‐ Apr 9, 2015 10:45am

Identification: O7.08

We perform first-principles calculation of the electron-phonon interactions in phosphorene, a monolayer of black phosphorus, to assess its potential as a thermoelectric material. Electron-phonon matrix elements are extracted from density functional perturbation theory, interpolated to dense meshes using maximally localized Wannier functions, and used in Boltzmann transport equation to calculate electronic transport properties. Simulation results reveal that phosphorene possesses nearly perfect electronic properties for thermoelectric applications: e.g. step-like density of states, anisotropic effective masses and high carrier mobility, which lead to an extraordinary thermoelectric power factor (~1700 ?W/cm-K2 at room temperature, more than 30 times higher than state-of-the-art commercial thermoelectrics). However, the overall thermoelectric performance of phosphorene is largely compromised by its high electronic thermal conductivity. Combined with the calculated lattice thermal conductivity, we predict an optimal zT of ~0.7 in n-type and ~0.8 in p-type up to 800K for a phonon-limited impurity-free phosphorene film.

Planar Sliding Triboelectric Nanogenerator for Active Optical MEMS

Apr 9, 2015 10:30am ‐ Apr 9, 2015 10:45am

Identification: P7.11

With the advantage of a high output voltage, triboelectric nanogenerator is very suitable for actuating capacitive devices. In this paper, a triboelectric nanogenerator (TENG) in planar sliding mode with dual-output voltages is proposed and firstly used for driving and controlling micromechanics. The TENG consists of a freestanding triboelectric-layer and two pairs of orthogonal electrodes. When the triboelectric-layer slides in plane, the dual-output voltages are proportional to the displacements in X and Y directions, respectively and independently. For the capacitive device actuated by the TENG, the driving voltage applied on the device is large if the load capacitance is small. Based on the TENG, an active piezoelectric micro-actuator with two orthogonal positioned piezoelectric bimorphs is developed for two-dimensional direction modulation. The piezoelectric bimorphs are actuated by the TENG and the movements of the light-spot in screen can be controlled and in proportion to the sliding of the triboelectric-layer in plane. Besides, an active electrostatic micro-actuator with two MEMS optical attenuators is also developed for double-channel power modulation. The electrostatic MEMS mirrors are actuated by the TENG and the double-channel attenuations can be controlled by the sliding of the triboelectric-layer in plane, respectively. This work presents the first active micro-actuators driven by the mechanical energy without an external power or mechanical joint, which has demonstrated the TENG�s great capabilities and broad prospects in independent and sustainable self-powered MEMS/NEMS, and opened up a new application of the TENG as the triboelectric-voltage-controlled devices.

N-Type Polymer-Enabled Selective Dispersion of Semiconducting Carbon Nanotubes for Flexible CMOS-Like Logic Circuits

Apr 9, 2015 10:45am ‐ Apr 9, 2015 11:00am

Identification: AA9.05

Single-walled carbon nanotubes (SWNTs) have been widely studied for their applications in transistors and logic circuits due to their excellent electronic properties. However, as-synthesized SWNTs are mixtures of semiconducting SWNTs and metallic SWNTs, hindering their practical deployment in semiconductor-based electronics. Sorting of SWNTs by conjugated polymers (e.g. polythiophene) is of particular interest because of its simplicity, high selectivity and high-yield. Here, we demonstrated the use of high-mobility n-type (electron-transporting) polymer for sorting of high-purity semiconducting SWNTs. The high selectivity of semiconducting SWNTs with n-type polymer is confirmed by Raman spectroscopy, dielectric force spectroscopy and transistor characterizations. In addition, our selected SWNTs have large-diameters than polythiophene-sorted SWNTs and hence are more desirable for electronic applications. Furthermore, by using high-mobility n-type polymer for sorting, we achieved ambipolar SWNT transistors with enhanced electron transport in comparison to polythiophene-sorted SWNTs, even on the flexible substrate. As a result, the ambipolar transistor characteristic allows the fabrication of negated AND (NAND) and negated OR (NOR) logic circuits from the same set of transistors, without the need for doping. The use of n-type polymer for sorting semiconducting SWNTs as well as using the resulting ambipolar SWNT transistors for CMOS-like logic circuits greatly simplify the fabrication of flexible SWNT logic circuits.

A Universal Low-Temperature One-Step Solution Processing Method for the Deposition of Large-Area Organometallic Halide Perovskite Thin Films for High-Performance Multifunctional Photovoltaics

Apr 9, 2015 10:45am ‐ Apr 9, 2015 11:00am

Identification: C7.05

A new-generation of thin-film photovoltaics have emerged in the past two year with the introduction of solution-processable organometallic halide perovskites (e.g. CH3NH3PbI3) as light absorbers. Owing to the extraordinary crystallization kinetics of such organic-inorganic hybrid perovskites from precursor solutions at elevated temperatures of about 70 to 150 oC, the reproducible deposition of high quality perovskite thin films has been a challenge. To address this issue, we describe here a universal low-temperature one-step solution processing method which enables the deposition of organometallic halide perovskite thin films with superior uniformity over square-centimeter area and smoothness at the nanometer scale. Using this method, CH3NH3PbI3-based photovoltaics can be reproducibly fabricated with high efficiencies up to 15%. It is also demonstrated that ultrathin perovskite films with thickness below 100 nm can form in complete coverage on substrates and their based full devices deliver surprisingly high efficiency, which are promising for building-integrated photovoltaic applications with low lead levels. Furthermore, partial or full substitution of iodine with bromine in the precursor solution allows band-gap tuning of the perovskite thin films, resulting in tunable vivid colors in the solar cells for potential application as decorative or tandem photovoltaics. The simple and mild deposition conditions used here also expand the selection of materials used in electron/hole selective layers and substrates. This makes the resulting solar cells more versatile in terms of multifunctionality and the types of fundamental studies that can be carried out. This new solution-processing method for the deposition of organometallic halide perovskite thin films is potentially low-cost, considering the low temperature, ambient pressure and one-step nature involved. Also, the rapid perovskite conversion from precursor solutions makes this process amenable to roll-to-roll continuous fabrication and automation for high throughput manufacturing.

Modeling of CZT Response to Gamma Photons Using MCNP and Garfield

Apr 9, 2015 10:45am ‐ Apr 9, 2015 11:00am

Identification: CC10.09

CZT is a semiconductor material that promises to be a good candidate for uncooled gamma radiation detectors. However, to date, technological difficulties in production of large size defect-free CZT crystals are yet to be overcome. The most common problem is accumulation of Tellurium precipitates as microscopic inclusions. These inclusions influence the charge collection through charge trapping and electric field distortion. The common work-around solutions are to fabricate pixelated detectors by either grouping together many small volume CZT crystals to act as individual detectors, or to deposit a pixelated grid of electrical contacts on a larger, but defective, crystal, and selectively collect charge. These solutions are satisfactory in an R&D environment, but are unsuitable for mass production and commercial development. Our modeling effort is aimed at quantifying the various contributions of Tellurium (Te) inclusions in CZT crystals to the charge generation, transport, and collection, as a function of inclusions size, position, and concentration. We model the energy deposition of gamma photons in the sensitive volume of the detector using LANL�s MCNP code. The electron-hole pairs produced at the energy deposition sites are then transported through the defective crystal and collected as integral charge at the electrical contact sites using CERN�s Garfield software package. The size and position distribution of Tellurium inclusions is modeled by sampling experimentally measured distributions of such inclusions on a variety of commercially-grown CZT crystals using IR microscopy and image processing software packages.

Textile-Structured Triboelectric Nanogenerators for Wearable Electronics

Apr 9, 2015 10:45am ‐ Apr 9, 2015 11:00am

Identification: P7.12

Wearable and self-powered devices have represented the emerging technology necessary to new life pattern of human. Textile structures are desirable for wearable device that are stretchable and flexible. In this article, a new textile-structured triboelectric nanogenerators are demonstrated for powering to the wearable devices. The tube-type triboelectric nanogenerator consists of PDMS tube, copper wire in center of device, and aluminum wrapped around the outer surface of tube, which are utilized to generate and harvest triboelectricity. We also fabricate the cable-type triboelectric nanogenerator, which is composed of several tube-type nanogenerator with series connection, resulting in 3 times higher output voltage than that of a tube-type nanogenerator. Additionally, textile-type triboelectric nanogenerator is fabricated with weaving from numerous tube-type triboelectric nanogenerator with parallel connection to generate high output current. A maximum output voltages and currents of 50 V and 250 �A for the textile-structured nanogenerator are obtained under the vertical stress of pushing tester. Moreover, the relationship between output performance and the amount of deformation at stretchable loading in the stretchable textile-type triboelectric nanogenerator is also investigated. This work should play an important role in new direction to wearable electronics.

From Micro-Supercapacitor to Pseudo Capacitor Based on Functionalized Silicon Nanowires Electrodes

Apr 9, 2015 10:45am ‐ Apr 9, 2015 11:00am

Identification: S6.11

Supercapacitor integration in micro-electronic circuits should improve portable devices efficiency1 and work easier with silicon (Si) electrodes. In our previous works we have reported highly doped Si nanowire (SiNW) micro-supercapacitor (�-SC) electrodes showing quasi ideal capacitive behavior2,3. In this work we studied the functionalization of highly doped SiNW to improve the SCs properties by increasing the electrochemical window and/or by changing the charge storage mechanism. Within this context, supercapacitors are classified mainly in electrochemical double layer capacitors (EDLCs) while the pseudo-capacitors require redox reactions or electrochemical faradic reactions (slower than EDLCs). The latter allow an increase in the charge accumulation compared to EDLCs. SiNWs are grown by VLS mechanism via gold catalysis in a CVD reactor, using highly doped Si as a substrate. The performance of the �-SC electrodes was analysed by electrochemical impedance spectroscopy, cyclic voltammetry and galvanostatic charge/discharge cycles using an aprotic ionic liquid PYR13 TFSI ( IOLITEC, Germany) as electrolyte4. Two functionalizations are realized; i) electrodeposition PEDOT films in the presence of PYR13 TFSI, ii) coating with nanocrystalline diamond (NCD) thin films (

Symposium X: Colloidal Nanocrystals: From Individual Quantum Objects to Building Blocks for Honeycomb Semiconductors Hosting Dirac Carriers

Apr 9, 2015 11:00am ‐ Apr 9, 2015 11:45am

Identification: EVT0006

In the last two decades, the wet-chemical synthesis of colloidal semiconductor nanocrystals of II-VI, IV-VI and III-V compounds has taken a high flight. This led to a word wide access to suspensions of nanocrystals with specific size and shape, various surface capping and bright emission. Compared to the typical solid state quantum dots, colloidal quantum dots have several advantages: they are quantum objects that do NOT interact with each other, they are available in high quantities, and they can be used as building blocks for more complex materials.

The optical spectroscopy on ensembles and single colloidal nanocrystals resulted in the demonstration and partial understanding of emission blinking, bi-exciton vs. exciton emission, and type I vs. type II quantum dots. On the other hand, electron-tunnelling spectroscopy on single quantum dots revealed the discrete energy levels of these systems and the degree of quantum coupling for quantum dots in an array.

Accurate Ultrafast and Contact-Free Measurements of Charge Carrier Transport and Dynamics in III�V Nanowires

Apr 9, 2015 12:30pm ‐ Apr 9, 2015 1:00pm

Identification: S7.01

III�V nanowires exhibit outstanding potential as nanocomponents for future electronic and optoelectronic devices. In order to develop these nanowire-based technologies further, it is essential to control the electronic properties of the nanowires, and to understand the limitations on charge carrier transport and dynamics. Unfortunately, measuring nanowire electronic properties using traditional contact-based techniques has proved challenging, because forming electrical contacts to nanoscale structures is technically difficult and introduces artefacts. To avoid these problems, non-contact probes of nanowire conductivity are highly desirable. Optical pump�terahertz probe (OPTP) spectroscopy is one such non-contact probe, capable of measuring carrier transport and dynamics with sub-picosecond temporal resolution. It is therefore ideally suited to studies of nanowires at room temperature. OPTP spectroscopy provides two principal measurements: photoconductivity decays and spectra. Photoconductivity decays provide information on the charge carrier lifetime and surface recombination velocity. Photoconductivity spectra of nanowires exhibit a pronounced surface plasmon mode, and from these spectra, electron mobilities can be readily extracted. This study spans a variety of technologically important III�V nanowires and heterostructure nanowires, including GaAs, InAs, InP, and GaAs/AlGaAs core�shell nanowires. Of all nanowires studied, InAs nanowires exhibited the highest electron mobilities of over 6000 cm2V-1s-1. InP nanowires exhibited the lowest surface recombination velocity of just 170 cm/s. This makes InP nanowires promising for applications which require long charge carrier lifetimes, such as photovoltaics. However, the electron mobility, measured as below 600 cm2V-1s-1, was strongly limited by the high density of stacking faults in these predominantly wurtzite InP nanowires. This points to the importance of controlling the crystal phase of InP nanowires for future device applications. Bare GaAs nanowires featured the highest surface recombination velocities (> 105 cm/s), and photoconductivity lifetimes below 5 ps. Bare GaAs nanowires also exhibited significantly lower charge carrier mobilities than their bulk GaAs counterparts, due to carrier scattering at the nanowire surface. To improve the lifetimes and mobilities, we investigated engineering the GaAs nanowire surface by overcoating with AlGaAs shells. This increased the photoconductivity lifetime in the GaAs core by almost 3 orders of magnitude to up to 1.6 ns. These GaAs/AlGaAs core�shell nanowires achieved room temperature electron mobilities above 2500 cm2V-1s-1 in the GaAs core. This electron mobility is significantly higher than in bare GaAs nanowires, and is approaching values typical of high quality bulk GaAs. The long photoconductivity lifetime and high electron mobility suggest the immediate suitability of these nanowires for optoelectronic devices.

Enantioselective Control of Lattice and Shape Chirality in Inorganic Nanocrystals Using Chiral Biomolecules

Apr 9, 2015 12:45pm ‐ Apr 9, 2015 1:00pm

Identification: MM10.02

In the last decade there has been a lot of interest in inorganic nanosystems that exhibit chirality and chiroptical activity.1 Three different approaches have been demonstrated for that purpose. First, induction of weak chiroptical effects in the plasmon or exciton resonances of nanocrystals of achiral materials with achiral shapes (such as gold, silver and cadmium/zinc chalcogenides), when these are capped with chiral surfactant molecules. Second, formation of chiral shaped nanostructures, mostly by lithographic or vapor deposition methods. Third, assembly of achiral nanocrystals into chiral superstructures. In all these systems the lattice symmetry is high and the crystal structure itself is achiral. More recently we have demonstrated an alternative approach for realization of chirality and stronger chiroptical effects in inorganic nanostructures. In our work we study inorganic materials that crystallize in chiral space groups, such as mercury sulfide,2 tellurium and selenium.3 The handedness of the crystal can be controlled when nanocrystals of these materials are grown in the presence of strongly binding chiral biomolecules. Furthermore, in the case of tellurium, the lattice chirality can be translated to the overall shape, on a 100 nm scale. Hence, control over chirality at two different size hierarchies is achieved. This is a unique demonstration of formation of chiral shapes in colloidal synthesis of inorganic nanocrystals. The chiral inorganic systems evolve by initial formation of very small clusters followed by a unique self-assembly process. As a result, interesting analogies to molecular crystals could be looked at. Different stages of nucleation and growth can be conveniently monitored by spectroscopic measurements in solution (as opposed to bulk crystals where this is more challenging). The shape and crystal structure are also easily imaged on the nanometer scale by electron microscopy (as opposed to organic crystals where this is more challenging). These new materials should be useful for a range of applications such as metamaterials fabrication, optics and asymmetric catalysis. On a more fundamental level, we believe that these are excellent model systems for studies of chiral crystallization and separation, and the interaction of chiral biomolecules with chiral crystals. 1.Ben-Moshe, A.; Maoz, B. M.; Govorov, A. O.; Markovich, G. "Chirality and Chiroptical Effects in Inorganic Nanocrystal Systems with Plasmon and Exciton Resonances" Chem. Soc. Rev. 42, 7028-7041 (2013). 2.Ben-Moshe, A.; Govorov, A. O.; Markovich, G. "Enantioselective Synthesis of Intrinsically Chiral Mercury Sulfide Nanocrystals" Angew. Chem. Int. Ed. 52, 1275-1279 (2013) 3.Ben-Moshe, A.; Wolf, S. G.; Bar-Sadan, M.; Houben, L.; Fan, Z.; Govorov, A. O.; Markovich, G. "Enantioselective control of lattice and shape chirality in inorganic nanocrystals using chiral biomolecules" Nat. Comm. 5, 4302 (2014)