Understanding and Predicting Properties of Low-dimensional Functional Materials from First-principles

A periodic network with uniform single metal active site, in coordination with redox-active organic ligands, is a promising class of materials for next generation single atom catalysts. Towards this quest, in this dissertation I have carried out first-principles density functional theory (DFT) based calculations of the geometrical and electronic structure and magnetic properties of several transition-metal-organic-chains (TM-C) both in gas phase as well as on Au(111) surface. Of particular interest are dipyridyltetrazine (DT), Bis-pyrimidine (BP), and 1,10-phenanthroline5,6-dione, (PDO) ligands used to design the TM-C with several single TM atoms as the coordination center. I have screened several TM atoms to get their coordination geometry (stable structure ) as well as analyzing their chemical activity through adsorption of small molecules on the TM center. Our results suggest that TM atoms with partially occupied d-orbitals exhibit strong affinity, while the TM atoms with fully occupied d-orbitals show weak affinity to the CO and O2 molecule. We also investigate the effect of support (Au(111)) on geometry and charge state in case of V-BP and V-PDO systems, and found that the support not only alters the local coordination of TM-Cs, but also has significant charge transfer from TM-C to Au(111). The tetrazine-based ligand, DT is only able to undergo a two-electron reduction, which limits the complexation to one metal per ligand. We studied the complexation of tetraethyltetraaza- anthraquinone (TAAQ) with elemental Fe, leading to complex metal–organic chains. We utilized the multiple binding pockets of TAAQ and achieve higher metal:ligand (M:L) ratios. Our results of various Fe:TAAQ ratio, suggests that thermodynamically one cannot create FeTAAQ species with higher than 2:1 M:L ratio. The second part of this dissertation deals with electronic structure and excitation spectrum of hydrogenated single layer and clean bilayer MoS2. We calculate the excitation spectrum of single-layer MoS2 at several hydrogen coverages by using Density-Matrix Time-Dependent Density-Functional Theory (TDDFT). Binding energies of the excitons of the hydrogenated MoS2 are relatively large (few tens of meV), making their experimental detection facile and suggesting hydrogenation as a knob for tuning the optical properties of single-layer MoS2. To examine ultrafast charge dynamics in bilayer MoS2, we applied DFT+Liouville equation approach and found that in conjunction with electron-phonon interaction ultrafast charge dynamics has a strong effect on the calculated emission spectrum. Our results reveal the importance of ultrafast charge dynamics in understanding photoemissive properties of a few-layer transition-metal dichalcogenide

Stabilizing intrinsic defects in SnO2_{2}

TThe magnetism and electronic structure of Li-doped SnO$_{2}$ are investigated using first-principles LDA/LDA$+U$ calculations. We find that Li induces magnetism in SnO$_{2}$ when doped at the Sn site but becomes non-magnetic when doped at the O and interstitial sites. The calculated formation energies show that Li prefers the Sn site as compared with the O site, in agreement with previous experimental works. The interaction of Li with native defects (Sn V$_\mathrm{Sn}$ and O V$_\mathrm{O}$ vacancies) is also studied, and we find that Li not only behaves as a spin polarizer, but also a vacancy stabilizer, i.e. Li significantly reduces the defect formation energies of the native defects and helps the stabilization of magnetic oxygen vacancies. The electronic densities of states reveals that these systems, where the Fermi level touches the conduction (valence) band, are non-magnetic (magnetic).cancies. The electronic densities of states reveal that those systems, where the Fermi levels touch the conduction (valence) band, are non-magnetic (magnetic).Comment: Phys. Rev. B (2013), Accepte

Effects on ALT normalization in the first month of treatment by Sofosbuvir/Ribavirin therapy versus Sofosbuvir/Daclatasvir therapy in HCV infected individuals

Objective: To evaluate the effects on ALT normalization in first month by SOFOS/RIB therapy versus SOFOS/DAC therapy in HCV infected individuals in Pakistan Study Design: Cross sectional comparative study Material and Methods: In a cross sectional analysis in a total of 200 Hepatitis c infected patients, sex, H/O diabetes mellitus, prior interferon therapy, decrease in hemoglobin >2 gm/dl in 1st month and rise in serum bilirubin in 1st month were the qualitative variables and the quantitative variables were age, weight, baseline hemoglobin, baseline bilirubin at week 4 of treatment. The statistical relation of the mentioned variables was checked using SPSS version 15 on the basis of data collected. Results: Out of total 200 patients, 47% (94) were males, 53% (106) were females, 28% (56) patients were diabetic & 44.5% (89) patients had history of prior interferon therapy, 28.5% (57) patients were having low hemoglobin levels before starting above mentioned treatment. Both the groups completed the treatments A & B for 24 &12 weeks respectively & collected data showed the superiority of treatment B to treatment A as no decrease in hemoglobin (p=0.000), & no rise in serum bilirubin(p=0.000) during 1st month of treatment while ,serum bilirubin was 93 % in treatment B and 73 % in treatment A. Conclusion: The results concluded that treatment B (Sofosbuvir / Daclatasvir for 12 weeks) is superior anti hepatitis C therapy as compared to the treatment A (Sofosbuvir / Ribavirin for 24 weeks) in order to achieve ALT normalization in first month of therapy in Pakistani population. Ribavirin should be avoided to prevent hemolytic anemia as well. Keywords: Hepatitis C, Alkaline Phosphatas

Electron Thermalization and Relaxation in Laser-Heated Nickel by Few-Femtosecond Core-Level Transient Absorption Spectroscopy

Direct measurements of photoexcited carrier dynamics in nickel are made using few-femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy at the nickel M$_{2,3}$ edge. It is observed that the core-level absorption lineshape of photoexcited nickel can be described by a Gaussian broadening ($\sigma$) and a red shift ($\omega_{s}$) of the ground state absorption spectrum. Theory predicts, and the experimental results verify that after initial rapid carrier thermalization, the electron temperature increase ($\Delta T$) is linearly proportional to the Gaussian broadening factor $\sigma$, providing quantitative real-time tracking of the relaxation of the electron temperature. Measurements reveal an electron cooling time for 50 nm thick polycrystalline nickel films of 640$\pm$80 fs. With hot thermalized carriers, the spectral red shift exhibits a power-law relationship with the change in electron temperature of $\omega_{s}\propto\Delta T^{1.5}$. Rapid electron thermalization via carrier-carrier scattering accompanies and follows the nominal 4 fs photoexcitation pulse until the carriers reach a quasi-thermal equilibrium. Entwined with a <6 fs instrument response function, carrier thermalization times ranging from 34 fs to 13 fs are estimated from experimental data acquired at different pump fluences and it is observed that the electron thermalization time decreases with increasing pump fluence. The study provides an initial example of measuring electron temperature and thermalization in metals in real time with XUV light, and it lays a foundation for further investigation of photoinduced phase transitions and carrier transport in metals with core-level absorption spectroscopy.Comment: 20 pages, 8 figure

Time-Dependent Density-Functional Theory And Excitons In Bulk And Two-Dimensional Semiconductors

In this work, we summarize the recent progress made in constructing time-dependent density-functional theory (TDDFT) exchange-correlation (XC) kernels capable to describe excitonic effects in semiconductors and apply these kernels in two important cases: a classic bulk semiconductor, GaAs, with weakly-bound excitons and a novel two-dimensional material, MoS2, with very strongly-bound excitonic states. Namely, after a brief review of the standard many-body semiconductor Bloch and Bethe-Salpether equation (SBE and BSE) and a combined TDDFT+BSE approaches, we proceed with details of the proposed pure TDDFT XC kernels for excitons. We analyze the reasons for successes and failures of these kernels in describing the excitons in bulk GaAs and monolayer MoS2, and conclude with a discussion of possible alternative kernels capable of accurately describing the bound electron-hole states in both bulk and two-dimensional materials