Grain Boundaries in Polycrystalline Alloys: Wafer Scale Infrared Detectors (WIRED)

Polycrystalline InAs based materials are potential candidates for low cost wafer-scale detector applications due to their excellent optical properties. However, grain boundaries (GBs) in polycrystalline InAs are very challenging to eliminate and are expected to have a profound effect on the electronic properties of the material and the device efficiency. Thus, an in-depth understanding of the electronic structure of the GBs in InAs and of the role played by the passivation mechanisms is required. We employ first-principles density functional approach to investigate the electronic, structural, and transport properties of GBs in InAs. In particular, we study the energetics and passivation mechanisms of GBs to calculate their relative stability and experimental feasibility. Understanding the exact nature of the GB electronic structure as well as their passivation mechanism is a key step for the further development of wafer-scale InAs based devices.

Hybrid Perovskites

Hybrid organic-inorganic perovskite solar cells have recently emerged as the next-generation photovoltaic technology. Most of the work has been focused on the prototype MAPbI3 perovskite (MA= Methylammonium = CH3NH3+) and its analogues that have lead to power conversion efficiencies in excess of 15%. Despite the huge success, these materials are still non-optimal in terms of optical absorption as the bandgaps are ~1.6 eV and greater.

Thus, investigation and development of perovskites with bandgaps closer to optimal, allowing enhanced spectral absorption, is of great interest. The aim of this project is to perform first-principles calculations to study the structural, optical, and electronic properties of new derivatives of MAPbI3 in which the organic MA cation is replaced by other organic amines of similar size and/or the Pb cation is replaced by similar elements.

Quantum Transport in 2D Heterostructures

Electronic and quantum
 transport properties of 2D heterostructures, both in-plane and out-of-plane configurations, will be investigated using first-principles self-interaction corrected 
density functional theory combined with the non-equilibrium
 Green’s function method primarily for sensing chemical and biological species. In particular, we will simulate the current-voltage (I-V) characteristics of the sensor and characterize the conductance for
 different layers/stacks of 2D heterostructures. Different configurations based on superlattices would provide tailored electronic and optical properties required for variety of sensing applications. Our systematic investigation would provide critical insight in understanding the fundamental working principles of 2D sensors as well as to help in the development of nanoscale sensors with high sensitivity and selectivity.

Carbon Nanotube Gas Sensors

Carbon nanotubes (CNTs) have a wide range of applications in transistors, spintronics, sensors, and solar cells. Due to their “quantum nature” and high surface-to-volume ratio, they are considered as promising candidates for extremely sensitive gas sensors. However, the gas sensing response of pristine CNTs is weak due to the strong sp2 bonds of the carbon network, characterized by a low chemical reactivity. Thus, functionalization of the CNT surface is necessary to obtain a gas sensor with high sensitivity and selectivity. Covalent molecular bonding to nanoscale objects usually is an efficient way to alter the object conducting properties. This is the key concept behind a new generation of nanotube/nanowire based chemical and biological sensors. Most of the complexity in addressing these systems theoretically stems from the size of the simulations. Here we simulate the current-voltage (I-V) characteristics of large sensors

Nanoscale Sensors in Realistic Conditions

Carbon nanotubes (CNTs) have a wide range of applications in transistors, spintronics, sensors, and solar cells. Due to their “quantum nature” and high surface-to-volume ratio, they are considered as promising candidates for extremely sensitive gas sensors. However, the gas sensing response of pristine CNTs is weak due to the strong sp2 bonds of the carbon network, characterized by a low chemical reactivity. Thus, functionalization of the CNT surface is necessary to obtain a gas sensor with high sensitivity and selectivity.

Nanoparticles

The current demand for a large-scale renewable energy source has resulted in an increased interest in photovoltaic technologies. Quantum dot (QD) nanoparticles are receiving significant attention due to their potential use in next-generation photovoltaic devices, as well as LEDs, electronic and optoelectronic devices, field effect transistors (FETs), and biological sensors. Attractive characteristics such as high fluorescence quantum yield, high molar extinction coefficient, ease of solution processing, and size-dependent band gap make quantum dot nanoparticles a desirable candidate for the sensitization of large band gap metal oxides (MO) such as TiO2 (3.2 eV), SnO2 (3.8 eV), and ZnO (3.2 eV) to visible light. One of the remaining challenges in achieving high efficiency devices is a better understanding of the electron transfer dynamics at the QD − MO interface and the subsequent optimization of the method of sensitizer deposition based on this understanding.

• QD Surface Chemistry & Modifications
• Ligand Effects: Linker Molecules
• Electron Transfer Reactions at the QD-Metal Oxide Interface

Complex Transparent Conducting Oxides (TCOs)

Transparent conducting oxides (TCOs) are unique materials that exhibit high optical transparency in the visible region and controllable nearly-metallic electrical conductivity. The range of technological applications and devices which use TCO as a vital component, is remarkable: it includes flat panel displays, electrochromic and smart windows, photovoltaic cells, and invisible transparent electronics.
Until recently, much of the research work and development in the TCO field has been primarily limited to optimizing and improving the performance of conven- tional binary n-type TCOs. However, high worldwide demand for TCO-based technological devices, as well as rapid development of novel TCO-based applications, stimulated the search for alternative TCO materials, those that are more efficient, cost effective, and have broader range of both electrical and optical properties. As a result, a variety of novel TCO materials with optical and electronic properties controllable via chemical composition has been introduced. The novel TCOs include the complex multicomponent TCOs such as InGaZnO4. The main advantage of the multicomponent TCOs is the ability to offer a wide range of tunable electrical and optical properties. These properties include band gaps, band offsets, carrier concentrations, and carrier transport that can be benefitted by a variety of TCO-based applications, such as high performance solar cells and invisible electronics. In addition, multicomponent oxides offer a chance to go beyond the conventional oxides in their compositions, by utilizing the cheap and abundant light-metal oxides.
In this project, first-principles density functional approach is employed to investigate the structural, optical, and electronic properties of several (un)doped (non)stoichiometric multicomponent oxides with layered structure such as InGaZnO4 as candidates for novel transparent conducting oxides. The structural and compositional complexity of these multi-cation materials requires in-depth understanding of the complex defect chemistry in order to determine the carrier source(s). We systematically calculate the formation energies and transition levels of possible acceptor and donor point defects as well as the implied defect complexes  and determine the equilibrium defect and electron densities as a function of growth temperature and oxygen partial pressure. We proposed a carrier generation mechanism explaining the observed intriguing behavior of the conductivity in InGaZnO4.

Research Methods:

• Density Functional Theory (DFT)
• Molecular Dynamics (MD)
• NEGF

DFT Softwares & Packages:

• VASP
• SIESTA
• SMEAGOL
• QUANTUM Espresso
• FLAPW
• LMTO
• Dmol3
• LAMMPS
• NAMD

Scientific Applications:

• MATLAB
• Mathematica
• MathCad
• COMSOL Multiphysics
• Materials Studio
• MedeA

Programming:

• Python
• Git
• Fortran
• Shell/Bash
• LaTeX

Data Visualization & Analysis:

• VESTA
• Xmgrace
• CrystalMaker
• Avogadro
• Xcrysden
• Ymol

• Matplotlib
• Xfig
• OriginPro
• GDIS
• VMD