Our Departmental Reception will be held Sunday, May 20th, from 4:15-6:15pm (drop-in) to celebrate our 2018 graduating seniors!!!
Ahmad Al-Qawasmeh, PhD Candidate
Public Presentation in Olin Physics Building, Room 101
Tuesday, May 8, 2018, at 11:00 AM
Natalie Holzwarth, PhD, Advisor
The defense will follow.
ABSTRACT
The purpose of this work is to develop a detailed understanding of solid state electrolyte materials and to contribute to their development for possible use in Li ion batteries using the framework of first principles computational methods. In particular, we use different computational methods in the framework of density functional theory to perform an in depth study of the structure, Li ion conductivity, and the stability of recently reported promising inorganic solid electrolyte materials. The structure for some materials was reported from experiment and in some cases was predicted from the simulation and validated to be consistent with the experimental data. The Li ion conductivity was studied using the nudged elastic band method and molecular dynamics simulations. The nudged elastic band method was used to analyze the migration barrier of the Li ions. Molecular dynamics simulations were used to analyze the migration of the Li ions by visualizing superposed Li positions over the timescale and temperatures of the simulation and to calculate the ionic conductivity of the material from the mean square displacement of the Li ions. The stability was studied by analyzing the electronic structure of the interface of the material with metallic Li. Four classes of solid electrolytes identified as promising electrolytes in the recent experimental literature were investigated in this work. The first class of materials studied was the alloy system Li3+xAs1-xGexS4 (G. Sahu et al., Journal of Materials Chemistry A, 2, 10396 (2014)) where the simulations were able to model the effects of Ge in enhancing the conductivity of pure Li3AsS4. The second class of materials studied was Li4SnS4 and Li4SnSe4 (T. Kaib et al., Chemistry of Materials, 24, 2211(2012), J. A. MacNeil et al., Journal of Alloys and Compounds, 586, 736 (2013), T. Kaib et al., Chemistry of Materials, 25, 2961 (2013)). The simulations were able to identify the two different crystal structures of the materials and to investigate differences in their conduction properties. The third set of materials studied were two nitrogen rich crystalline lithium oxonitridophosphate materials, Li14P2O3N6 (D. Baumann et al., European Journal of Inorganic Chemistry, 2015, 617 (2015)) and Li7PN4 (W. Schnick et al., Journal of Solid State Chemistry, 37, 101 (1990)). The simulations were able to suggest that these materials are promising solid electrolytes due to their ideal interface properties with metallic Li and their promising ionic conductivity. The fourth project is an ongoing study of the newly synthesized electrolyte Li4PS4I (S. Sedlmaier et al., Chemistry of Materials, 29, 1830 (2017)). The simulations help in the understanding of the structural and ion mobility properties of this material and to study models of interfaces with Li metal.
Zachary Lamport, PhD Candidate
Public Presentation in ZSR Library, Room 404
Thursday, April 19, 2018, at 10:30 AM
Oana Jurchescu, PhD, Advisor
The defense will follow.
ABSTRACT
The electrical properties of devices based on an organic compound result from the structure of the molecules, their solid-state packing, efficiency of charge injection from the electrodes, and the fabrication procedures. The length scales of interest can also vary widely, ranging from a few nanometers in the case of charge transport through single molecules or two-dimensional molecular ensembles, to tens of micrometers in devices focusing on thin films or molecular crystals. The work outlined in this thesis examines the characteristics of electronic devices at both extremes by incorporating organic molecules in molecular rectifiers and organic field-effect transistors (OFETs).
We successfully designed and fabricated molecular rectifiers based on self-assembled monolayers and identified relevant structure-function relationships. We elucidate the dependence of the rectification behavior on molecular length and structure, and found that the degree of rectification is enhanced in shorter molecules and linearly dependent on the strength of the molecular dipole moment. We further developed compounds that, when included into the molecular diodes, rectified current by as much as three orders of magnitude depending on their structure. This performance is on par with that of the best molecular rectifiers obtained on a metallic electrode, but it has the advantage of lower cost and more efficient integration with current silicon technologies, which may yield hybrid systems that can expand the use of silicon towards novel functionalities governed by the molecular species grafted onto its surface.
We then explored charge transport in OFETs using the organic semiconductor 7,14-bis(trimethylsilylethynyl)benzo[k]tetraphene (TMS-BT). We produced thin-film OFETs which exhibited more efficient electronic transport than single crystal devices of the same material, in spite of the inherent presence of grain boundaries. We explained these findings in terms of charge transport anisotropy and electronic trap formation at the interface between the semiconductor and dielectric. We further reduced aggressively the contact resistance in small molecule and polymer OFETs by varying the metal deposition rate, which resulted in over 5 times improved charge carrier mobility compared with the best reported devices with identical composition and structure. The obtained contact resistance normalized over the channel width was 500 Ωcm, and the corresponding devices exhibited charge carrier mobilities of 19.2 cm2/Vs for 2,8-difluoro-5,11-bis(triethylsilylethynyl) anthradithiophene (diF-TES ADT) and 10 cm2/Vs for indacenodithiophene-co-benzothiadiazole copolymer (C16IDTBT), with minimal dependence on the gate voltage.
P. Wilson Cauley, PhD; School of Earth and Space Exploration, Arizona State University
George P. Williams, Jr. Lecture Hall, (Olin 101)
Wednesday, April 11, 2018, at 4:00 PM
There will be a reception with refreshments at 3:30 PM in the lounge. All interested persons are cordially invited to attend.
ABSTRACT
We are well into the Age of Exoplanets with approximately 4000 confirmed to date. Efforts in exoplanet characterization have led to some very precise determinations of planetary densities, compositions, and even accurate maps of active region and spot locations on stellar surfaces. Exoplanet magnetic fields, however, remain elusive. While radio observations continue to push into the low-mass brown dwarf regime, no emission from a planetary-mass object has been confirmed.
I will discuss some of our efforts involving alternate methods for probing exoplanet magnetic fields and how they stack up so far against the prospects for detection via radio emission.
Junwei Xu
Public Presentation in Olin 101
Friday, March 30, 2018, at 10:00 AM
David L. Carroll, PhD, Advisor
The defense will follow.
ABSTRACT
Charge balance in organic light emitting structures is essential to simultaneously achieving high brightness and high efficiency. In DC-driven OLEDs, this is relatively straight forward. However, in the newly emerging, capacitive, field-activated AC-driven organic devices, charge balance can be a challenge. We introduced the concept of gating the compensation charge in AC-driven organic devices and demonstrated that this can result in exceptional increases in device performance. To do this we replaced the insulator layer in a typical field-activated organic light emitting device with a nanostructured, wide band gap semiconductor layer. This layer acts as a gate between the emitter layer and the voltage contact. Time resolved device characterization shows that, at high-frequencies (over 40,000Hz), the semiconductor layer allows for charge accumulation in the forward bias, light generating part of the AC cycle and charge compensation in the negative, quiescent part of the AC cycle.
We also showed that solution-grown films of CsPbBr3 pervoskite nanocrystals imbedded in wide band-gap Cs4PbBr6 can be incorporated as the recombination layer in light-emitting diode structures. More importantly, optical response was studied to shed light on why the very poor light emitter CsPbBr3 becomes a high-efficiency fast emitter when imbedded as nanocrystals in the wider gap host Cs4PbBr6. Kinetics at high carrier density of pure (extended) CsPbBr3 and the nano-inclusion composite were measured and analyzed, indicating second order kinetics in extended and mainly first order kinetics in the confined CsPbBr3, respectively. In these terms, the nano-confinement might be viewed as enforcing mainly geminate recombination of electrons and holes in a material (CsPbBr3) that did not support stable excitons at room temperature. The resulting first-order recombination competes with trapping much more effectively than does bimolecular recombination at the moderate carrier densities typical of LEDs and usual photo-excitation. Analysis of absorption strength of this all-perovskite, all-inorganic imbedded nanocrystal composite relative to pure CsPbBr3 indicated enhanced oscillator strength consistent with earlier published attribution of the subnanosecond exciton radiative lifetime in nano-precipitates of CsPbBr3 in melt-grown CsBr host crystals and CsPbBr3 evaporated films.
Dr. Christopher Tycner, Professor and Chair, Department of Physics, College of Science and Engineering at Central Michigan University
George P. Williams, Jr. Lecture Hall, (Olin 101)
Wednesday, April 4, 2018, at 4:00 PM
There will be a reception with refreshments at 3:30 PM in the lounge. All interested persons are cordially invited to attend.
ABSTRACT
Long-baseline optical and IR interferometers now routinely resolve the disk and disk-like structures around stars. The typical angular scales resolved by current generation of instruments are at the sub-milli-arcsecond level. In this presentation, I will introduce the fundamental principles behind long-baseline optical interferometry, as well as describe the practical implementation based on the Navy Precision Optical Interferometer (NPOI), a joint project between the U.S. Naval Observatory, Navy Research Laboratory, and Lowell Observatory. Specific examples of observations obtained at the NPOI of spatially resolved circumstellar disks will be presented.
I will describe how interferometric observations with complementary spectroscopic data are used to constrain the numerical models of circumstellar disks. Such tests can in turn be used to determine the physical properties of the disks, investigate rotationally enhanced mass-loss processes, and study the effects of a secondary star on a circumprimary disk in binaries.
Jason Howard, PhD Candidate
Public Presentation in Olin 107
Tuesday, April 17, 2018, at 2:00 PM
Natalie Holzwarth, PhD Advisor
The defense will follow.
ABSTRACT
In this work materials with possible applications in all solid state Li-ion batteries are explored using computational methods within the framework of density functional theory and kinetic Monte-Carlo. The density functional theory simulations use fundamental quantum mechanics along with some approximations to produce accurate models of real materials. A smaller portion of the work uses kinetic Monte Carlo to provide qualitative information about the convergence properties of transport coefficients. The materials Li2+xSnO3 and Li2+xSnS3 are studied in the context of electrodes for Li-ion batteries. Their structures are calculated, conduction pathways for the Li-ions predicted, open cell voltages calculated, and reactivity with lithium at the surface studied. The results for these materials provided insight into existing experimental data from the literature and made predictions for open cell voltages that had not yet been measured. The materials Li4SnS4, Li2OHCl and Li2OHBr are studied in the context of solid state electrolytes for Li-ion batteries. The structural properties are explored for some materials by calculating Helmholtz free energies to help understand temperature dependent phases. First-principles molecular dynamics are performed on some of these materials to gain insight into the mechanisms for Li-ion diffusion, which is related to the Li-ion conductivity. The molecular dynamics simulations of these materials are also used to calculate order parameters, such as time averaged site occupancy, which provide insight into temperature dependent aspects of their structure. The computations using kinetic Monte-Carlo are limited to the study of the convergence properties of transport coefficients on a lattice equivalent to the Li lattice of Li2OHCl. These Monte-Carlo simulations provide critical insight on the level of statistics needed to converge the transport coefficients related to ionic conductivity. As a whole the simulations in this research provide atomistic level knowledge of real world energy storage materials.
David Montgomery, PhD Candidate
Public Presentation in Olin 107
Monday, April 9, 2018, at 12:00 PM
David L. Carroll, PhD, Advisor
The defense will follow.
ABSTRACT
This work focuses on the combination of thermoelectric and piezoelectric materials into a new hybrid generator. It was discovered that a hybrid thermoelectric piezoelectric generator results in a meta-structure that creates a coupling field effect at the interface between the thermoelectric and piezoelectric films that produces more power than the sum of the individual generators. This coupling field effect causes a modification of the thermoelectric properties causing an observed 468% increase in total power output. In addition to this coupling effect, the first functional thermoelectric and piezoelectric generator design is presented. This is achieved by integrating a flexible continuous alternating p- and n-type semiconductor thermoelectric generator into the electrode of the piezoelectric film. This design overcomes major issues previously preventing the two materials to be combined in a single generator architecture. This functional thermoelectric piezoelectric generator can achieve 89% of the theoretical thermoelectric power and 540% increase in the piezoelectric power due to the geometry of the structure. A spray doping synthesis method is presented that was used to create the continuous alternating p- and n-type semiconductor film. Spray doping achieves that same thermoelectric properties of solution doping but greatly simplifies the fabrication of a thermoelectric generator. Finally an optimized thermoelectric generator is presented that overcomes many of the current issues plaguing other thin film designs. The optimized structure is robust and is compatible with numerous synthesis methods and materials used in thin film thermoelectrics.
Ryan Melvin.
PhD Candidate, Department of Physics, Wake Forest University.
George P. Williams, Jr. Lecture Hall, (Olin 101)
Wed. Mar. 14, 2018, at 4:00 PM
There will be a reception with refreshments at 3:30 PM in the lounge. All interested persons are cordially invited to attend.
ABSTRACT
Applying statistical and machine learning, I have addressed key issues in the field of computational biophysics. The guiding principle in this work has been removing bias and conveying uncertainty. To that end, I have contributed numerous methods for interpreting biopolymer ensemble data without the need for prior knowledge or setting of biasing parameters. Additionally, in all of these works, I have provided a careful discussion of the limits of these methods and how researchers might visually convey the inherent uncertainty, including displaying what are effectively error bars on biopolymer structures. I have worked to remove bias even in estimating the amount of sampling needed for any time-dependent multi-dimensional process. These contributions may move the field forward in its ability to remove bias and convey uncertainty in statistically rigorous ways.
After introducing these methods, I proceed with applications of them to the study of a chemotherapeutic nucleic acid called F10 – a 10mer of 5-fluoro-2′-deoxyuridine-5′-O-monophosphate. Here I uncover the mechanism for a previously observed interaction with zinc and magnesium, leading to a general investigation of F10’s interactions with metal ions. I conclude by proposing a stabilizing chemical perturbation to the polymer and discussing implications for drug delivery.
This thesis work has been mentored by Professor Fred Salsbury. The PhD thesis defense will take place on March 19, 2018.