Browsing by Author "Gelb, Lev D."
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Item A First Principles Approach to Closing the “10-100 eV GAP” for Electron Thermalization in Wurtzite GaN(May 2023) Nielsen, Dallin O 1993-; Fischetti, Massimo V.; Akin, Bilal; Vandenberghe, William; Cho, Kyeongjae; Gelb, Lev D.Since the 1960s, when radiation-induced disruption of electronic devices in space was first observed, the study of the effects of ionizing radiation on electronics has grown into an extensive field of its own. The present work is concerned with accurately modelling the energy-loss processes that control the thermalization of hot carriers (electrons and/or electron-hole pairs) that are generated by high-energy radiation in wurtzite GaN, using an ab initio approach. Current physical models of the nuclear/particle physics community cover the high-energy range (kinetic energies exceeding ~100 eV), and the electronic-device community has done extensive work in the lower-energy range (below ~10 eV). However, the processes that control the energy losses and thermalization of electrons and holes in the intermediate energy range of about 10-100 eV are poorly known (the “10-100 eV gap”). The aim of this research is to close this gap. To this end, Density Functional Theory (DFT) is utilized to obtain the band structure of GaN for bands reaching energies above 100 eV. Furthermore, charge-carrier scattering rates for the major charge-carrier interactions (phonon scattering, impact ionization, and plasmon emission) are calculated, using the DFT results and first-order perturbation theory (Fermi’s Golden Rule). With this information, the thermalization of electrons starting at 100 eV is simulated in a Monte Carlo code, allowing the electrons to interact stochastically according to the calculated interaction rates and generate electron-hole pairs as they go, which are also tracked in the simulation. Full thermalization of electrons is complete within 1 ps, and that of holes is complete in approximately half the time. Electrons lose 90% of their energy (90 eV) during the first few ~0.1 fs, due to rapid plasmon emission and impact ionization at high energies. The remainder is lost more slowly as phonon emission dominates at lower energies (below ~10 eV). During the thermalization, hot electrons generate electron-hole pairs with an average energy of ~8.9 eV/pair (11-12 pairs per hot electron). Additionally, upon full thermalization, the average electron displacement from its original position is found to be on the order of 100 nm.Item Accounting for the Dead Time in Analysis of Time of Flight Secondary Ion Mass Spectrometry Data(2018-12) Shahrokhesfahani, Negar; Gelb, Lev D.Time of Flight Secondary Ion Mass Spectrometry (ToF SIMS) is a powerful tool for advanced surface analysis. It produces large data sets, which consist of mass spectra at each pixel at an imaged area. Many ToF SIMS instruments use a type of detector that suffers from two problems that lead to non-linearity in the measured sample properties: detector saturation and dead time. Linearity in this type of system is defined as the proportionality of the measured and true ion counts. Non-linearity can influence the interpretation of the data with methods such as multivariate analysis. “Detector saturation” happens when more than one ion arrives at the detector in the time interval related to one specific channel but the detector records only a single count. “Dead time” is when one event happens at a certain channel and the detector become insensitive to subsequent ions arriving within the dead time window. These problems both lead to under-counting of ions. In this thesis, we mainly focus on correcting for the dead time effects. Using extensive simulations, we first characterize the adverse effects of dead time on the output and evaluate quality of existing ways to “correct” for dead time effects. Then, we propose a novel method using the Maximum Likelihood Estimation (MLE) to estimate the true spectrum for the measured data. Specifically, we incorporate the statistical distribution of the dead time affected data in MLE, which leads to a new method for dead time correction.Item Guided Subset Selection Based Unified Active Learning Framework: Formulations, Algorithms and Applications(May 2023) Kothawade, Suraj Nandkishor 1997-; Iyer, Rishabh; Gelb, Lev D.; Lakshman Tamil; Ramakrishnan, Ganesh; Xiang, Yu; Gupta, GopalDeep learning has been successful in a wide variety of domains, ranging from face recognition to self-driving cars. The success of deep learning algorithms is due to large amounts of labeled data, which is easily available in today’s digital world. However, training deep models using large datasets comes with high compute costs and labeling costs. Moreover, large datasets are often naturally plagued with data imperfections like class imbalance, OOD, and redundancy. Training machine learning models using such datasets leads to biased models obtained even after incurring expensive costs. Moreover, these models underperform on rare yet critical scenarios, which can be catastrophic. As one can imagine, one or more of these problems can occur at any point during the development of machine learning models. Hence, there exists a need for a wholistic framework that can serve as a one-stop solution for data-efficiency, model-efficiency and reducing data imperfections. In this dissertation, we focus on designing a Guided Active Learning framework that can serve this purpose. In particular, we propose four phases of the Guided Active Learning framework: 1) Seed Set Selection, 2) Discovery, 3) Targeting and, 4) Filtering. This framework is designed to be modular, since different teams can find themselves in requiring to optimize for different phases in this framework. The Seed Set Selection phase focuses on finding an initial set that represents the larger dataset, such that the majority of the information and semantics of the dataset are covered. The Discovery phase focuses on finding unknown instances that do not exist in the current labeled dataset or were potentially missed during the exploration phase. The Targeting phase aims to mitigate data imbalance by finding data points that are semantically similar to rare classes or slices. The Filtering phase focuses on avoiding out-of-distribution and redundant data points from being selected. We provide algorithms and mathematical formulations for executing these phases in several applications. Particularly, we demonstrate the effectiveness of the Guided Active Learning framework on a wide range of real-world domains including autonomous driving, medical imaging and automated speech recognition. We hope that the Guided Active Learning framework will help practitioners navigate data better and improve the performance of their downstream machine learning models.Item Low-Temperature and Photoactivated CVD on Organic Substrates(2021-12-01T06:00:00.000Z) Salazar, Bryan G.; Walker, Amy V.; Kolodrubetz, Michael; Balkus Jr., Kenneth J.; Gelb, Lev D.; Smaldone, Ronald A.Chemical vapor deposition (CVD) is an attractive technique for depositing metallic thin films on organic substrates. However, CVD often uses temperatures > 500 °C to initiate precursor decomposition and generate highly reactive species. This can be problematic when using attempting to deposit on organic thin films as they can degrade at temperatures < 200 °C. Here we offer an alternative to thermal activation by using photolysis to generate reactive species at room temperature. In this work we monitor decomposition pathways of photoactivated precursors by employing TOF SIMS to identify molecular species remaining on the surface as well as test the integrity of the surface post-deposition. XPS is used to identify organic surface- metal interactions, and finally RGA is used to identify gas-phased decomposition products to further identify photolytic pathways. In identifying the decomposition pathway, we aim to use this understanding to further improve the deposition of metal on organic substrates.Item Molecular Dynamics Simulations of Protic Ionic Liquids(2017-12) Taghavi Nasrabadi, Amir; Gelb, Lev D.; Nielsen, Steven O.This dissertation concerns the study of protic ionic liquids (PILs) by means of molecular dynamics (MD) simulations. PILs are a subset of ionic liquids in which cations possess an acidic proton. They have been a focus of intense research in the past decade mainly due to their promising properties. In this dissertation, we first begin with an introduction to PILs, and briefly review their properties and applications in Chapter 1. In Chapter 2, the main features of MD simulations are explained. Empirical force fields used in simulation studies of ILs have often failed to correctly describe their dynamics and transport properties. Chapter 3 describes how to improve the ability of well-known empirical force fields to describe the dynamical properties of tertiary ammonium triflate PILs, by scaling the atomic partial charges of ions using an optimal scaling factor derived from experimental data. Our results show that this method successfully enhances the dynamics of the simulated PILs, and improves the computed transport coefficients without increasing the computational cost. The degree of proton transfer in PILs, which indicates the percentage of ions formed from the reactant acid and base molecules, is a key quantity in this field. However, the effects of this quantity on PIL properties are still not well-understood. In Chapter 4, we try to understand these effects by simulating a family of alkylammonium acetate PILs over the entire range of the degree of proton transfer. Our results show that properties of PILs change dramatically with varying degree of proton transfer. We also use the data obtained to estimate the degree of proton transfer in experimental PILs by comparison with simulation. In Chapter 5, we introduce a rigorous thermodynamic approach by which to calculate the degree of proton transfer and equilibrium constants in PIL media. Our approach is based on a thermodynamic cycle and uses constrained MD simulations to obtain the free energy change associated with the proton transfer reaction. We apply this scheme to trimethylammonium acetate, a tertiary ammonium acetate PIL known for their low degree of proton transfer. Our results show that the proton transfer takes place only partially in this PIL, which is in good agreement with experiment. This approach can effectively be used to predict the degree of proton transfer and equilibrium constant in PILs with variety degrees of proton transfer, which is difficult to assess experimentally. In Chapter 6, we briefly summarize our findings and elaborate on possible future applications of the computational approaches used in this dissertation.Item Nested Sampling of Isobaric Phase Space for the Direct Evaluation of the Isothermal-Isobaric Partition Function of Atomic Systems(American Institute of Physics) Wilson, Blake A.; Gelb, Lev D.; Nielsen, Steven O.; Wilson, Blake A.; Gelb, Lev D.; Nielsen, Steven O.Nested Sampling (NS) is a powerful athermal statistical mechanical sampling technique that directly calculates the partition function, and hence gives access to all thermodynamic quantities in absolute terms, including absolute free energies and absolute entropies. NS has been used predominately to compute the canonical (NVT) partition function. Although NS has recently been used to obtain the isothermal-isobaric (NPT) partition function of the hard sphere model, a general approach to the computation of the NPT partition function has yet to be developed. Here, we describe an isobaric NS (IBNS) method which allows for the computation of the NPT partition function of any atomic system. We demonstrate IBNS on two finite Lennard-Jones systems and confirm the results through comparison to parallel tempering Monte Carlo. Temperature-entropy plots are constructed as well as a simple pressure-temperature phase diagram for each system. We further demonstrate IBNS by computing part of the pressure-temperature phase diagram of a Lennard-Jones system under periodic boundary conditions.Item On the Permeability of Colloidal Gels(Amer Inst Physics, 2019-01-04) Gelb, Lev D.; Graham, Alan L.; Mertz, Alex M.; Koenig, Peter H.; 0000-0003-0291-5098 (Gelb, LD); Gelb, Lev D.We reexamine and refine analytical theories for permeability in colloidal networks, with particular focus on constants and identification of approximations. The new theories are compared against numerical simulations of Stokes flow through the networks and reveal nearly quantitative power-law predictions for both pore size and permeability at low volume fractions, with systematic deviations observed only at high volume fractions. Comparison with two previously published experimental data sets yields mixed results: in one case, very good agreement is found, while in the other, only the scaling is correctly predicted. In fractal gel networks, the permeability is commonly modeled as a power-law function of volume fraction, with the fractal dimension of the network determining the power-law exponent. To quantitatively probe the influence of gel structure on permeability, we investigate this relation in structures generated by diffusion-limited cluster aggregation (DLCA) and reaction-limited cluster aggregation (RLCA) and, for contrast, non-overlapping uniform random dispersions of particles. Geometric analyses are used to determine network pore size distributions, fractal dimensions, and percolation characteristics. High-fidelity simulations of the slow viscous flow of Newtonian fluids are used to obtain first-principles-based velocity and fields and hence network permeabilities. Interestingly, the effective pore size that determines permeability is found to be somewhat larger than that measured by a method based on the insertion of spherical probes. Empirical inclusion of a fractal dimension dependence on volume fraction is found to yield quantitative results for permeabilities over the entire volume fraction range studied, in both DLCA and RLCA materials. Published under license by AIP Publishing.Item Theoretical Study of Electronic Transport in Two-dimensional Materials(2022-12-01T06:00:00.000Z) Gopalan, Sanjay; Nosratinia, Aria; Fischetti, Massimo; Wallace, Robert M.; Vandenberghe, William; Gelb, Lev D.Theoretical methods are critical for evaluating two-dimensional materials as possible channel materials in future field-effect transistors (FETs). The majority of modeling attempts concentrate on electronic transport in ideal free-standing layers, ignoring the dielectric environment’s effect on transport characteristics. We study the carrier transport in two-dimensional (2D) transition metal dichalcogenides (TMDs) by extending our Monte-Carlo model for a free-standing monolayer to include the effect of dielectric environment on the 2D layer. The major effects of the dielectric environment are the dielectric screening and the scattering of electrons with the coupled (hybrid) optical-phonon/plasmon excitations that are present at the interfaces in a field-effect transistors with a channel consisting of a monolayer of a polar semiconductor (TMDs) with top and bottom gates in a double-gate geometry. We perform an extensive study of the electron mobility in supported and/or gated monolayer TMDs in the presence of dielectric environment, to study of the carrier mobility of six TMDs (MoS2, MoSe2, MoTe2, WS2, WSe2, and WTe2) with all TMDs assumed to be supported by SiO2 and with SiO2, HfO2, Al2O3, AlN, and hBN as gate insulators. The carrier mobility improves significantly when considering only the effect of dielectric screening by the free carriers and the surrounding dielectrics. However, these positive effects are countered by the detrimental effects due to the interface plasmon/phonon (IPP) scattering. The carrier mobility decreases significantly below its value in free-standing monolayers with increasing dielectric constant of the insulator(s). The carrier mobility decreases almost monotonically with increasing dielectric constant of the gate insulator, with two exceptions: 1. The beneficial effects of dielectric screening of the ‘out-of-plane’ field lines are seen for hBN, thanks to its relatively low ionic polarization and the high phonon frequencies resulting from the light weight of the B and N ions. 2. On the contrary, resonance effects among the optical phonons of the substrate, of the TMD layer, and of the top oxide result in a low carrier mobility when AlN and/or Al2O3 are taken as gate insulators. We also evaluate the temperature dependence, carrier density dependence and the dependence of the dielectric constant of the 2D layer on the carrier mobility in the presence of the dielectric environment. We then extend our model to evaluate the high-field characteristics and simulate a 2D material based field effect transistor (FET), considering the TMD as the channel. Despite the TMDs having a low electron mobility, in most cases, under the effect of dielectric environment. Though we see a comparable carrier mobility with hBN as gate- insulator, the comparatively low dielectric constant of hBN negates the scaling benefits of high-κ insulators. Unfortunately, the transport properties of the ideal stack of gate-insulator of HfO2 with SiO2 as substrate and the 2D TMD between them, remain disappointing.Item Toward Understanding Weak Matrix Effects in TOF SIMS(American Vacuum Society) Gelb, Lev D.; Walker, Amy V.; 0000 0001 3758 9240 (Walker, AV); 0000-0003-0291-5098 (Gelb, LD); Gelb, Lev D.; Walker, Amy V.Chemical imaging methods, including imaging mass spectrometry, are increasingly used for the analysis of samples ranging from biological tissues to electronic devices. A barrier to wider adoption of imaging mass spectrometry is the presence of matrix effects which complicate quantitative analysis. Interactions between an analyte molecule and its surroundings (the "matrix") can substantially alter both the yield and type of ions observed. Furthermore, such "intrinsic" effects can be confused with nonlinear response due to detector saturation and other instrument-related complications. As a result, quantitative analyses of time-of-flight secondary ion mass spectrometry (TOF SIMS) data that attempt to account for matrix effects are rare. The authors discuss analysis of such data using maximum a posteriori reconstruction based on physically motivated models, and present progress toward the quantitative extraction of chemical concentration profiles and component spectra in the presence of matrix effects, using mixed self- assembled alkanethiolate monolayers as a test system. The authors demonstrate that the incorporation of matrix effects to lowest order using a series-expansion approach is an effective strategy and that doing so provides improved quantitative performance in measuring surface compositions and can also yield information about interactions between species during the SIMS process. Published by the AVS.Item Towards Understanding the Roles of Organic Bath Additives in Copper Electroless Deposition on Self-assembled Monolayers(December 2022) Vienes, Jevalyne Satparam; Walker, Amy; Makarenkov, Oleg; Gelb, Lev D.; Nielsen, Steven O.; Stefan, Mihaela C.Organic bath additives are commonly used in solution-based deposition techniques to improve the properties of the desired material. However, despite their wide application, studies on understanding the roles of these additives are limited. In this dissertation, we employed multiple surface characterization techniques including X-ray photoelectron spectroscopy (XPS), time-of- flight secondary ion mass spectrometry (TOF SIMS), Raman spectroscopy, scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS), atomic force microscopy (AFM), and optical microscopy to investigate the effect monoethanolamine, diethanolamine, triethanolamine, urea, guanidine, and thiourea as bath additives in the electroless deposition of copper on self-assembled monolayers (SAMs). Using XPS and TOF SIMS, we observed that electrolessly deposited copper is more adherent on the –CH3 terminated SAMs. TOF SIMS spectra also showed that the mono- and triethanolamine interact with the –CH3 terminated SAM via the amine functional group while diethanolamine adsorbs flat on the SAM surface. These interactions allowed the deposition of copper wires via electroless nanowire deposition on micropatterned substrates (ENDOM). Fragment ions in the TOF SIMS spectra showed various interactions of the hydroxyl group of ethanolamines with the –OH terminated SAMs. This indicates that polar ethanolamines interact strongly with the hydroxyl-terminated SAM surface. We also demonstrated a switch from copper electroless deposition at low concentrations of thiourea to copper sulfide deposition at high concentrations of thiourea. We synthesized copper wires using thiourea as bath additive in ENDOM. A method of depositing Cu/CuxS layer via a two-step solution-based technique is also presented. The electron-donating property of the heteroatom in bath additives (urea, guanidine, and thiourea) influences its stabilizing ability. Thiourea inhibited the electroless deposition process which allowed the deposition of small particles resulting in smooth copper film. Also, regardless of the bath additive heteroatom, smoother films were deposited at pH 9. At pH 12, we observed two layers of deposit: (a) ion-by-ion growth of the underlayer and (b) cluster-by-cluster growth of the overlayer. Copper wires deposited using guanidine as bath additive at pH 9 are more uniform. Lastly, we proposed methods to optimize the crossbar wire synthesis using multiple ENDOM processes.