Ph.D. Graduates

2012

2.  Dr. Rudra Kafle, Worcester Polytechnic Institute, MA

Title: Theoretical Study of Bose-Einstein Condensate-Based Atom Michelson Interferometers

1.  Dr. Bill Pandit, University ofUtah

Title: Ultrafast Photophysics of pi-Conjugated Polymers for Organic Photovoltaic Applications

Abstract: In this work we used the pump and probe photomodulation (PM) spectroscopy technique to measure the transient excitation dynamics in various pi- conjugated polymers (PCPs) films and blends with appropriate molecular acceptors. Using two different ultrafast laser systems, we extended the PM spectrum to cover a broad spectral range from 0.25 – 2.5 eV in the time domain from 100 fs to 1 ns with 150 fs time resolution. We also used continuous wave (CW) photomodulation spectroscopy, photoluminescence, electro-absorption, doping-induced absorption, and x-ray diffraction to study the photoexcitations and doping induced states, as well as other optical properties of PCPs and polymer donor-fullerene acceptor blends. In addition to these measurements, we also fabricated organic photovoltaic (OPV) solar cell devices based on poly(3-hexyl-thiophene) [P3HT]/fullerene [PCBM] blend and estimated their power conversion efficiency (PCE) in relation to the optical studies. In (1:1.2) weight ratio of P3HT/PCBM blend that shows maximum donor (D) and acceptor (A) domain separation we found that although the intrachain excitons in the polymer domains decay within ~10 ps, no charge polarons are generated at their expense. Instead there is a built-up of charge-transfer (CT) excitons at the D-A interfaces, which may dissociate into separated ‘free’ polarons in the D and A domains at a later time. Although the CT excitons are photogenerated much faster in D-A blends with a smaller domain size (such as in P3HT with random order), their dissociation is less efficient because of larger binding energy. Our results elucidate the charge photogeneration mechanism in D-A blends, and unravel the important role of the binding energy in generating ‘free’ charge polarons.

We also studied the photophysics of a low band gap polymer, namely poly-thienophene-benzodithiophene (PTB7) film and its blend with acceptor [6,6] phenyl C₇₁ butyric acid methyl ester [PC₇₁BM]. In the CW PM spectrum of PTB7/PC₇₁BM blend, clear signatures of polarons are observed. Whereas PA bands related to triplet excitons and trapped polarons are observed in the PM spectrum of pristine PTB7 film. The transient ultrafast PA of PTB7 is dominated by a singlet exciton band at ~0.95 eV. In the transient ultrafast PA spectrum of PTB7/PC₇₁BM blend we found singlet exciton, charge transfer exciton and a polaron band that are generated simultaneously; this is different from the transient PM spectrum of P3HT/PCBM blend. We also found that the charge transfer exciton in PTB7/PC₇₁BM dissociates faster than in P3HT/PCBM blends. This may be one of the reasons for getting higher PCE of ~7.4 % in the PTB7/PC₇₁BM based solar cells compared to PCE ~4 % in P3HT/PCBM based solar cells.

1.  Dr. Rudra Aryal, University of Miami

2011

12.  Dr. Nabin Kumar Malakar, State University of New York, Albany

Title: Autonomous Entropy-based Intelligent Experimental Design

Abstract: The aim of this thesis is to explore the application of probability and information theory in experimental design, and to do so in a way that combines what we know about inference and inquiry in a comprehensive and consistent manner. Present day scientific frontiers involve data collection at an ever-increasing rate. This requires that we find a way to collect the most relevant data in an automated fashion. By following the logic of the scientific method, we couple an inference engine with an inquiry engine to automate the iterative process of scientific learning. The inference engine involves Bayesian machine learning techniques to estimate model parameters based upon both prior information and   previously collected data, while the inquiry engine implements data-driven exploration. By choosing an experiment whose distribution of expected results has the maximum entropy, the inquiry engine selects the experiment that maximizes the expected information gain. The coupled inference and inquiry engines constitute an autonomous learning method for scientific exploration. We apply it to a robotic arm to demonstrate the efficacy of the method.
Optimizing inquiry involves searching for an experiment that promises, on average, to be maximally informative. If the set of potential experiments is described by many parameters, the search involves a high-dimensional entropy space. In such cases, a brute force search method will be slow and computationally expensive. We develop an entropy-based search algorithm, called nested entropy sampling, to select the most informative experiment. This helps to reduce the number of computations necessary to find the optimal experiment.
We also extended the method of maximizing entropy, and developed a method of maximizing joint entropy so that it could be used as a principle of collaboration between two robots. This is a major achievement of this thesis, as it allows the information-based collaboration between two  robotic units towards a same goal in an automated fashion.

11.  Dr. Naba Raj Paudel, University of Toledo, OH

Title: Stability Issues in Sputtered CdS/CdTe Solar Cells

Abstract: Polycrystalline thin films n-CdS/p-CdTe hetero-junction solar cells have become one of the more promising contenders in thin-film technology. The near optimal band gap energy and high optical absorption of CdTe with a wide variety of fabrication techniques combine to make this technology a cost effective option. One of the most common thin-film deposition techniques, magnetron sputtering, has been using for both CdS and CdTe to achieve efficiency over 13% on commercial FTO coated glass with highly resistive buffer layer. CdS/CdTe cells in superstrate configuration uses elemental Cu and Au as bi-layer back contact in our laboratory; however, the most recent development has been concerned with the stability of device performance due to Cu diffusion. We performed accelerated life testing (ALT) of sputtered cells with large number of parameter variations that showed less than 5% relative loss in efficiency after 900 hrs light soaking under one-sun illumination, without encapsulation, at 85 oC, open-circuit voltage biasing in room ambient when processed with optimal parameters. The stability of these devices was more sensitive to the chloride activation and Cu thickness than CdS and CdTe thickness but independent with the substrate type.

10.  Dr. Pushkal Thapa, Wyane State University

Title:THE STUDY OF WEAK FERROMAGNETISM BY ANDREEV REFLECTION SPIN SPECTROSCOPY

AND DEVELOPMENT OF BIMORPH ELECTROTHERMAL ACTUATORS

Abstract : Point contact Andreev reflection (PCAR) spectroscopy was used to study weakly ferromagnetic materials. We investigated how the spin polarization of itinerant ferromagnet MnSi changes as it undergoes a magnetic phase transition from helical to conical to ferromagnetic phase. Unconventional conductance with periodic conductance oscillations outside the superconducting gap, which may be a signature of triplet superconductivity, has been observed in Nb/MnSi junctions. Another system we have studied was Pd1-xNix. The transport spin polarization of Pd1-xNix with different Ni concentrations was measured and the correlation with the saturation magnetization was studied. A strong ferromagnetic susceptibility enhancement of nearly ferromagnetic Pd by Ni impurities was observed. Surface ferromagnetism has been observed in Pt thin films. This result was supported by first principle density functional calculations. In MnBi, a high Curie temperature, high perpendicular anisotropy, and high spin polarization material, the measured values of the spin polarization were found to be due to the disparity in the Fermi velocities in the electron split bands. Room temperature ferromagnetism and finite low temperature spin polarization have been observed in dilute magnetic semiconductors: undoped and Cr-doped InN.

Bimorph micromechanical actuators fabricated from freestanding metal-coated Si3N4 microstructures have been developed. These actuators have shown a wide range of torsion angles (up to 30o) upon passing a dc current. The quadratic dependence of the torsion angles vs current was measured; this was mostly accounted for by the Joule heating effect and the residual strain in the Si3N4 structures. These electro-thermal actuators could find applications in MEMS technology.

9.  Dr. Rajesh Regmi, Wyane State University

Title:

8.  Dr. Hari Prasad Lamichhane, Georgia State University

Title:

7.  Dr. Som Dahal, Arizona State University

Title:Advanced Nanostructured Concepts in Solar Cells using III-V and Silicon-Based Materials.

Abstract: As existing solar cell technologies come closer to their theoretical efficiency, new concepts that overcome the Shockley-Queisser limit and exceed 50% efficiency need to be explored. New materials systems are often investigated to achieve this, but the use of existing solar cell materials in advanced concept approaches is compelling for multiple theoretical and practical reasons. In order to include advanced concept approaches into existing materials, nanostructures are used as they alter the physical properties of these materials. To explore advanced nanostructured concepts with existing materials such as III-V alloys, silicon and/or silicon/germanium and associated alloys, fundamental aspects of using these materials in advanced concept nanostructured solar cells must be understood. Chief among these is the determination and predication of optimum electronic band structures, including effects such as strain on the band structure, and the material’s opto-electronic properties.
Nanostructures have a large impact on band structure and electronic properties through quantum confinement. An additional large effect is the change in band structure due to elastic strain caused by lattice mismatch between the barrier and nanostructured (usually self-assembled QDs) materials. To develop a material model for advanced concept solar cells, the band structure is calculated for single as well as vertical array of quantum dots with the realistic effects such as strain, associated with the epitaxial growth of these materials. The results show significant effect of strain in band structure. More importantly, the band diagram of a vertical array of QDs with different spacer layer thickness show significant change in band offsets, especially for heavy and light hole valence bands when the spacer layer thickness is reduced. These results, ultimately, have significance to develop a material model for advance concept solar cells that use the QD nanostructures as absorbing medium.
The band structure calculations serve as the basis for multiple other calculations. Chief among these is that the model allows the design of a practical QD advanced concept solar cell, which meets key design criteria such as a negligible valence band offset between the QD/barrier materials and close to optimum band gaps, resulting in the predication of optimum material combinations.

6.  Dr. Purushottam Bhandari, University of Miami

Title:The design of a polarimeter and its use for the study of the variation of the down welling polarized radiant distribution in the ocean

Abstract : The spectral polarized radiance distribution provides the most complete description of the light field that can be measured.  However, this is a very difficult parameter to measure near the surface because of its large dynamic range, dependence on incoming sky conditions, and waves at the air-sea interface.  The measurement of the Stokes vector of the downwelling polarized light field requires the combination of at least four images, all of which must be obtained simultaneously. To achieve this, a new polarimeter (which we call DPOL) has been designed, characterized, calibrated and deployed. The description of the DPOL, its calibrations and characterizations are discussed. The uncertainties in the retrieval of Stokes vector and other derived parameters are also discussed. This instrument is equipped with four fish-eye lenses (180° field of view) with polarizers behind each lens in a different orientation, a coherent optical fiber bundle with 4 arms, a spectral filter changer assembly and a charged coupled-device (CCD) imaging camera. With this system, a single image contains 4 separate fisheye images, each a whole hemisphere of the same scene, each with different polarization information. Using these 4 images and applying appropriate calibration parameters allows us to calculate the four-element Stokes vector and then the total degree of polarization and the angle of plane of polarization of the incoming light field in a hemisphere of desired directions. Under the Office of Naval Research RaDyO (Radiance under a Dynamic Ocean) program, DPOL has been used in the Santa Barbara Channel and Hawaii field experiments. In most cases, data on sky polarization were collected with a separate camera (Sky-Cam) simultaneously with the DPOL. The data and results with these two camera systems in these experiments are presented and are compared. Data on the inherent optical properties of water from the same field experiments collected by collaborators will be shown. Our measurements show that very near the surface, for clear sky conditions, the dominant source of polarization is the refracted sky light. As one progresses in the water column, the polarization due to light scattering by the water increases and polarization due to the water becomes dominant. The dependence of the in-water light field polarization on the sky and surface wave conditions, solar zenith and azimuth angles, the depth of the instrument, the viewing angle, the wavelength of light, the inherent optical properties (IOP’s) of water are discussed.

5.  Dr. Narayan Chapagain, Utah State University.

Title: DYNAMICS OF EQUATORIAL SPREAD F USING GROUND-BASED OPTICAL AND RADAR MEASUREMENTS

Abstract: The Earth’s equatorial ionosphere most often shows the occurrence of large plasma density and velocity fluctuations with a broad range of scale sizes and amplitudes. These night time ionospheric irregularities in the F-region are commonly referred to as equatorial spread F (ESF) or plasma bubbles (EPBs). This dissertation focuses on analysis of ground-based optical and radar measurements to investigate the development and dynamics of ESF, which can significantly disrupt radio communication and GPS navigation systems. OI (630.0 nm) airglow image data were obtained by the Utah State University all-sky CCD camera, primarily during the equinox period, from three different longitudinal sectors under similar solar flux conditions: Christmas Island in the Central Pacific Ocean, Ascension Island in South Atlantic, and Brasilia and Cariri in Brazil. Well-defined magnetic field-aligned depletions were observed from each of these sites enabling detailed measurements of their morphology and dynamics. These data have also been used to investigate day-to-day and longitudinal variations in the evolution and distribution of the plasma bubbles, and their nocturnal zonal drift velocities. In particular, comparative optical measurements at different longitudinal sectors illustrated interesting findings. During the post midnight period, the data from Christmas Island consistently showed nearly constant eastward bubble velocity at a much higher value (~80 m/s) than expected, while data from Ascension Island exhibited a most unusual shear motion of the bubble structure, up to 55 m/s, on one occasion with westward drift at low latitude and eastward at higher latitudes, evident within the field of view of the camera.
In addition, long-term radar observations during 1996-2006 from Jicamarca, Peru have been used to study the climatology of post-sunset ESF irregularities. Results showed that the spread F onset times did not change much with solar flux and that their onset heights increased linearly from solar minimum to solar maximum. On average, radar plume onset occurred earlier with increasing solar flux, and plume onset and peak altitudes increased with solar activity. The F-region upward drift velocities that precede spread F onset increased from solar minimum to solar maximum, and were approximately proportional to the maximum prereversal drift peak velocities.

4.  Dr. Trilochan Paudel, Boston College, MA.

Title: NANOSPHERE LITHOGRAPY FOR NANO OPTICAL APPLICATIONS

Abstract: Many different techniques are available to create nanopatterns in nanoscale devices. However, a few are flexible and inexpensive enough to be practical in the nanotechnology. Here, we study the nanosphere lithography (NSL) based on a self-assembly of microspheres. Using this technique, we have developed various patterns in metallic films, ranging from honeycomb arrays of “quasi-triangles” to circular holes. These various patterns have been used subsequently either as nano-optical structures directly, with remarkable optical and plasmonic properties, or as substrates for further nano-processing. In one such nano-processing, the “quasi-triangle” patterns were used as a catalyst for carbon nanotube growth. The resulting aligned arrays of carbon nanotubes were employed in nanocoax solar cells. In another nano-processing, the arrays were used as masks for electrodeposition. In addition to the nano processing and measurements, we have employed the FDTD computer simulations, to develop a full understanding of the nano-optical and plasmonic properties of the developed structures.

3.  Dr. Krishana Parsad Sigdel, Worcester Polytechnic Institute, MA.

Title: Transition Studies of Liquid Crystal Colloids with Solvents and Nano-
solids

Abstract:  Liquid crystals (LCs) are anisotropic fluids that exhibit numerous thermodynamically stable phases in between an isotropic liquid and a three-dimensionally ordered solid. In their simplest ordered phase, the nematic, LCs show orientational order due to molecular self assembly and at the same time maintaining fluid flow properties. In the smectic phase, they show both orientational and partial translational order characterized by a 1-d density wave. Liquid crystalline substances have been extensively studied due to their applications and as important physical models of self-assembly. The effect of the disorder and impurities on LC systems is an important and challenging problem to the fundamental understanding of phases ordering or self-assembly and continually attracts the attention of researchers. The disordered systems often display complex and rich phenomena, being the generalization of the pure (ideal) systems. Disorder can dramatically alter the physical properties of multi-component, composite systems. In particular, the effect of disorder on phase transitions is important as the disorder typically couples to the order parameter, which can be usefully described as a random local field that is conjugate to the order parameter. This is usually realized in systems with random inclusions in a phase ordering media, e.g., a colloidal dispersion of solids in a complex fluid. Another form of disorder is presented by dilution effects, which imposes instead the random breaking or weakening of intermolecular bonds or interactions responsible for the phase ordering. Exploring a good physical system representing random dilution effects in a controlled manner offers a physical probe to unresolved problems in the understanding of mesophasic order.
A series of studies of dilution and different form of disorder effect on liquid crystal phase transitions is reported. We have used high-resolution AC-calorimetry, dielectric spectroscopy as well as polarizing microscopy to characterize the effects of solvent such as hexane, acetone, decane, and nanomaterials such as multiwall carbon nanotubes and ferroelectric nanoparticles on the phase transitions of several liquid crystals. The liquid crystals of interest are: pentylcyanobiphenyl (5CB), octylcyanobiphenyl (8CB), and decylcyanobiphenyl (10CB). Studies have been carried out as a function of solvent, nanotube, and nanoparticles  concentration and temperature spanning the isotropic to nematic (I-N), nematic to smectic-A (N-SmA), and isotropic to smectic-A (I-SmA) phase transitions.

2.  Dr. Prakash Paudel, University of North Texas, TX.

Title:

1.  Dr. Bishwanath Gaire, Kansas State University, USA.

Title: IMAGING OF SLOW DISSOCIATION OF THE LASER INDUCED FRAGMENTATION OF MOLECULAR IONS

Abstract: Lasers are being used widely for the study and manipulation of the dynamics of atomic and molecular targets, and advances in laser technology makes it possible to explore new areas of research — for example attosecond physics. In order to probe the fragmentation dynamics of molecular ions, we have developed a coincidence three-dimensional momentum imaging method that allows the kinematically complete study of all fragments except electrons. Recent upgrades to this method allow the measurement of slow dissociation fragments, down to zero velocity, in intense ultrafast laser fields. Evidences for the low energy breakup are presented using the benchmark molecules diatomic H2+ and polyatomic H3+.

2010

1. Dr. Megh Raj Niraula, Old Dominion University, VA, USA.
2. Dr. Subas Dhakal, Kent State University, OH, USA .
3. Dr. Mukti Aryal, University of Texas, Dallas, USA
4. Dr. Hikmat Bahadur B.C. ,New Mexico State University, NM.
5. Dr. Chandra Thapa, Wayne State University, Michigan, USA
6. Dr. Shyam Raj Badu, State University of NewYork, Albany, USA.

7. Dr. Mahesh Datta Bhatt, University of Tsukaba, Japan

Title: Electronic lectronic structure at organic molecule-metal interface studied by densityfunctional theory

Abstract: The electronic structure of the interface between organic semiconductor and metal was studied by DFT. In the case of BCP on Au, the binding energy and charge transfer were found to be small; however, for BCP on Ca, Mg, Al, and Ag, the binding energy and charge transfer increased with decreasing metal work function. Molecular orbitals of LUMO and the Fermi level in BCP on Ca, Mg, Al, and Ag systems are composed of both BCP and metal derived atomic orbitals, that is, orbital mixing occurred. On the other hand, molecular orbitals of LUMO and the Fermi level in BCP on Au system is composed mainly of BCP and Au, respectively, that is orbital mixing does not occur. Such orbital mixing may play an important role in electron transport at the interface between BCP and metal. In case of CBP and CDBP with metals, the injection barrier for electrons was found sufficient to control the charge transport at the interface. The metal doped bathocuproine with relatively low work function metal certainly improves the electrical properties. In case of organic-organic hetero junctions, the ionic states of HOMO of donors and LUMO of acceptors devices have better performance compared to neutral
states devices.

8. Dr. Pashupati Dhakal, Boston College, MA, USA

Title:Angular Magnetoresistance Oscillations in the Molecular Organic Conductor (DMET)2I3: Experiment and Calculation

Abstract: Quasi-one dimensional (Q1D) molecular organic conductors are among the most exciting materials in condensed matter physics, exhibiting nearly every known ground state. They are highly anisotropic, structurally and electronically, and show large oscillatory phenomena in conductivity for magnetic field rotated in different crystalline planes. Several theoretical works have been published to explain these angular magnetoresistance oscillation (AMRO) effects, but the underlying physics remains illunderstood. Here, we present measurements and calculations of magnetotransport in the molecular organic (super)conductor (DMET)2I3 which detect and simulate all known AMRO phenomena for Q1D systems. Employing, for the first time, the true triclinic crystal structure in the calculations, these results address the mystery of the putative vanishing of the primary AMRO phenomenon, the Lebed magic angle effect, for orientations in which it is expected to be strongest. They also show a common origin for Lebed and so-called “Lee-Naughton” oscillations, and confirm the generalized nature of AMRO in Q1D systems. Furthermore, we report the temperature dependence of the upper critical magnetic field in (DMET)2I3, for magnetic field applied along the intrachain, interchain, and interplane directions. The upper critical field exhibits orbital saturation at low temperature for field in all directions, implying that superconductivity in (DMET)2I3 is conventional spin singlet.

9. Dr. Madhab Neupane, Boston College, MA, USA

Title: Angle-Resolved Photoemission Studies on Ruthenates and Iron-Based Superconductors

Abstract: Angle-resloved photoemission spectroscopy (ARPES) is a powerful technique to study the electronic structure in solids. Its unique ability of resolving the energy and momentum information of electrons inside a solid provides an essential tool in measuring the electronic structure of solids. ARPES has made great contributions in the understanding of correlated system such as high-Tc superconductors and ruthenates.
The Metal-insulator transition is a fundamental problem in condensed matter physics. The calcium substituted strontium ruthenate, Ca(2-x)SrxRuO4, provides a good platform to study the metal-insulator transition in multi-orbital systems. This system has a complex phase diagram that evolves from a p-wave superconductor to a Mott insulator. One of important projects of this thesis focuses on Ca(2-x)SrxRuO4.
The growing evidence for coexistence of itinerant electrons and local moments in transition metals with nearly degenerate d orbitals suggests that one or more electron orbitals undergo a Mott transition while the others remain itinerant. We have observed a novel orbital selective Mott transition (OSMT) in Ca1.8Sr0.2RuO4  by ARPES. While we observed two sets of dispersing bands and Fermi surfaces (FSs) associated with the doubly-degenerate dyz and dzx orbitals, the Fermi surface associated with the dxy orbital which has a wider bandwidth is missing as a consequence of selective Mott localization. Our theoretical calculations have demonstrated that this unusual OSMT is mainly driven by the combined effects of inter-orbital carrier transfer, superlattice potentials and orbital degeneracy, whereas the bandwidth difference plays a less important role.
Another important project of this thesis focuses on the recently discovered iron-pnictides superconductors. The idea of inter-FS scattering associated with the near-nesting condition has been proposed to explain the superconductivity in the pnictides.  The near-nesting condition varies upon the carrier doping which shifts the chemical potential. We have performed a systematic photoemission study of the chemical potential shift as a function of doping in a pnictide system based on BaFe2As2. The experimentally determined chemical potential shift is consistent with the prediction of a rigid band shift picture by the renormalized first-principle band calculations. This leads to an electron-hole asymmetry (EHA) due to different Fermi velocities for different FS sheets, which can be calculated from the Lindhard function of susceptibility. This built-in EHA from the band structure, which is fully consistent with the experimental phase diagram, strongly supports that inter-FS scattering over the near-nesting Fermi surfaces plays a vital role in the superconductivity of the iron pnictides.

10. Dr. Samir Guragain, Florida Institute of Technology, FL

Title: Muon endcap alignment for the CMS experiment and its effect on the search
for Z’ bosons in the dimuon channel at LHC.

Abstract: The first 7 TeV proton-proton collisions produced by the LHC have been recorded by the CMS experiment in 2010. The CMS muon endcap alignment system succeeded in tracking muon detector movements of up to 18 mm and rotations of a few milliradians under magnetic forces during the system com-missioning at full magnetic field in 2008. This dissertation describes in detail the reconstruction of chamber positions from alignment data. The system achieved chamber alignment precisions of 220 – 340 μm and 200 μrad. Systematic errors on displacements are estimated to be less than 500 μm. This dissertation describes the expected effect of muon misalignments on the search for Z’ → μ+ μ− using fully reconstructed sets of simulated events of proton-proton collisions at   √s = 7 TeV with the CMS experiment. The simulation results show that the expected pT resolution for muons in the end-cap is about 14.4% (4.8%) with the startup (ideal) alignment scenario using a 1.2 TeV/c2 Z’ sample. The impact of systematic biases in the muon end-cap positions and rotations on the pT resolution is also studied and quantified Using the MC samples, the discovery potential for Z’ with different muon misalignments and integrated luminosities is evaluated. A CMS detector better aligned than with the current startup alignment requires significantly less data.Preliminary results for dimuon events using ∼ 300 nb−1 of first collision data at  √s = 7 TeV with the CMS experiment are presented.

11. Dr. Naresh Shakya, Kent State University, OH

Title: Studies of Electronic Transport in Novel Smectic and Discotic Liquid Crystalline Organic Semiconductors

Abstract: Organic semiconductors (OSs) have stirred huge commercial interest due to their potential applications in electronic and optoelectronic devices such as field effect transistors, photovoltaic cells, and organic light-emitting diodes. Major benefits of OSs over conventional semiconductors include mechanical flexibility, low temperature processing, very low cost, and ease of fabrication in large area electronic devices on plastic and paper substrates. Liquid crystals (LCs) are particularly interesting classes of OSs, both from the standpoints of fundamental physics and practical applications. Systems we studied include a thiophene-benzene-thiophene-based smectic (1,4-di-(5-n-tridecylthien-2-yl)-benzene). This material exhibited polaron band behavior with very impressive hole transport (> 0.1 cm2/Vs with the smectic-F phase templating large domains of more ordered phases with very large mobilities. The mobilities are high enough to be of practical interest.
Another project involved calamitic LCs with pyridine-thiophene-thiophene-pyridine cores (5, 5’-Di-(alkyl-pyridin-yl)-2, 2’ bithiophenes). We found both electron and hole mobilities to be strongly electric field dependent but very weakly dependent on temperature. Pyridine-based LCs often exhibit very high order smectic phases and are therefore of interest as OSs. However, the mobilities of these materials were found quite low, even in high-order phases. We were able to describe our data using Basseler’s theory of hopping conduction in disordered systems.
We also studied charge transport in a triphenylene-based discotic LC (1-nitro-2, 3, 6, 7, 10, 11-hexakis (pentyloxy) triphenylene). This material showed strong temperature and field dependent hole mobilities described by disorder dominated one-dimensional hopping. Since the columnar phase exists over a wide range of temperatures, such photo-conducting materials may be very useful for applications in electronics.
Finally, we developed a technique to measure charge carrier mobility in freely suspended films of LCs in high vacuum. Here, the external field can be coupled easily to the molecular order, no electrodes contact the sample, and extremely high voltages can be applied. Also, both hole and electron mobility (which depends on high purity and absence of oxygen), and samples with a very wide range of thickness may be studied.

12. Dr. Ishwor Kumar Shrestha, University of Nevada, Reno

Title:

13. Dr. Giri Raj Joshi, Boston College, MA

Title: Study of thermoelectric properties of reconstructed p-type SiGe, BiTe, BiSb, and hal-Heusler bulk material

Abstract:Silicon germanium alloys (SiGe) have long been used in thermoelectric modules for deep-space missions to convert radio-isotope heat into electricity. They also hold promise in terrestrial applications such as waste heat recovery. The performance of these materials depends on the dimensionless figure-of-merit ZT (= S2σ T/k), where S is the Seebeck coefficient, σ the electrical conductivity, k the thermal conductivity, and T is the absolute temperature. Since 1960 efforts have been made to improve the ZT of SiGe alloys, however, the ZT of p-type SiGe has remained low.  In recent years, many studies have shown a significant enhancement of ZT in other material systems by utilizing a nanostructuring approach to reduce the thermal conductivity by scattering phonons more effectively than electrons. Here we observed a peak ZT of 0.95 at 850 oC by using a low-cost and mass-production nanocomposite process, which is about 50% improvement in comparison to the previously reported peak ZT value. The ZT enhancement mainly comes from a large reduction in the thermal conductivity due to the increased phonon scattering at the grain boundaries and crystal defects formed by lattice distortion, with some contribution from the increased electron power factor at high temperatures. Moreover, nanocomposite approach has also been used to study the thermoelectric properties of other material systems such as bismuth telluride (BiTe), bismuth antimony (BiSb), and half-Heusler phases and we observed a significant improvement in peak ZT of nanostructured n-type half-Heusler compounds from 0.8 to 1.0. The nanocomposite approach could also be applicable to many other thermoelectric materials that are useful for automotive, industrial waste heat recovery, space power generation, or solar power conversion applications.

14. Dr. Krishana Lamichhane, University of Nevada, Reno

Title:

Abstract:

2009

1. Dr. Indra Dev Sahu, State University of NewYork, Albany, USA.
2. Dr. Laxman Mainali, State University of NewYork, Albany, USA.
3. Dr. Himal Khatri, University of Toledo, OH, USA.
4. Dr. Rajendra Dahal, Kansas State University, USA.
5. Dr. Bed Pantha, Kansas State University, USA.
6. Dr. Durga Kafle, Utah State University.
7. Dr. Milan Poudel, Texas A&M University, TX.