A COMPUTATIONAL STUDY OF PROPERTIES OF CORE-SHELL NANOWIRE HETEROSTRUCTURES USING DENSITY FUNCTIONAL THEORY

Nanoscale systems, especially the one-dimensional semiconducting nanowires, have been the subject of immense research interests due to their potential applications in nanoelectronics and optoelectronics that demand cheaper, smaller, faster, and energy-efficient components. In particular, the core/shell nanostructures, in which the core materials are shielded by materials with larger bandgap called shell, have been shown to enhance the performance of field effect transistors (FETs), solar cells, light emitting diodes (LEDs), and thermoelectric devices due to their outstanding features like valence band offset between the core and shell, higher stability against oxidation, reduction in the surface trap states, diminished nonradiative recombination processes, and enhancement in the carrier multiplication and carrier transport processes. Incorporation of spin functionality via doping of a magnetic impurity into such core/shell (non-magnetic) nanostructures also offers additional advantages for next-generation spin-based electronic devices. Such devices are not only smaller, cost-effective, and non-volatile but also have increased data processing speed, consume less power, and assist reducing heat dissipation compared to the traditional electronic devices. In the first part of my thesis, I have studied the role of Mn and Cr dopants on the electronic structure, magnetic properties, and strain-induced magnetic phase transitions in Ge/Si core/shell nanowire heterostructures using the many-body density functional theory (DFT) approach. Subsequently, I have designed a spin filtering device using Mn-doped Ge/Si core/shell nanowire and a switching device using Cr-doped Ge/Si core/shell nanowire. To understand the spin-transport properties of these devices, I have used a real space orbital based DFT in conjunction with the single-particle non-equilibrium Green’s function approach. In the second part of my thesis, I have studied the effect of size and growth direction on the electronic structure, stability, mechanical, and optical properties for PbTe/PbS core/shell nanowires. To understand the thermodynamic stability of these complex structures, I have performed the ab-initio molecular dynamics simulations that demonstrate the possibilities of core-to-shell diffusion at room temperature in certain growth direction

Comparative Study of Covalent and van der Waals CdS Quantum Dot Assemblies from Many-Body Perturbation Theory

Quantum dot (QD) assemblies are nanostructured networks made from aggregates of QDs and feature improved charge and energy transfer efficiencies compared to discrete QDs. Using first-principles many-body perturbation theory, we systematically compare the electronic and optical properties of two types of CdS QD assemblies that have been experimentally investigated: QD gels, where individual QDs are covalently connected via di- or poly-sulfide bonds, and QD nanocrystals, where individual QDs are bound via van der Waals interactions. Our work illustrates how the electronic, excitonic, and optical properties evolve when discrete QDs are assembled into 1D, 2D, and 3D gels and nanocrystals, as well as how the one-body and many-body interactions in these systems impact the trends as the dimensionality of the assembly increases. Furthermore, our work reveals the crucial role of the covalent di- or poly-sulfide bonds in the localization of the excitons, which highlights the difference between QD gels and QD nanocrystals.Comment: 25 pages, 4 figure

Effect of Different Doses of Sulfur on Growth and Yield of Rapeseed (Brassica campestris var. Lumle Tori)

Sulfur plays an important role in the growth and yield of rapeseed plants. A field experiment was conducted in a randomized complete block design with seven levels of sulfur (60 kg/ha, 50 kg/ha, 40 kg/ha, 30 kg/ha, 20 kg/ha, 10 kg/ha, and 0 kg/ha in three replicates) to evaluate the effect of different doses of sulfur on the growth and yield of rapeseed (Brassica campestris var. Lumle Tori) in Khairahani, Chitwan. Plant height, number of branches per plant, number of pods per plant, pod length, grain per pod, pod weight, fresh weight, dry weight, stover weight, harvest index, and grain yield were recorded.  Significant differences were observed in plant height, yield-related traits, and grain yield. The results showed significant differences between the growth and yield-related traits of the different treatments. Plant height and number of branches increased with increasing sulfur dose, reaching a maximum of 60 kg/ha. The maximum number of pods per plant was observed at 20 kg/ha, and the maximum pod length and grain per pod were observed at 60 kg/ha. Grain yield and harvest index were maximum at 20 kg/ha. The results showed that the maximum grain yield could be obtained by applying 20 kg/ha of sulfur. These findings provide valuable guidance for optimizing agricultural practices to meet the increasing global demand for oilseeds

Effect of Different Doses of Calcium and Boron Pre-harvest Spray on Post-harvest Quality of Acid Lime (Citrus aurantifolia var. SunKagati-1)

Acid Lime (Citrus aurantifolia var. SunKagati-1) has been cultivated in 67 districts of Nepal. Its cultivation is becoming increasingly popular among farmers in Nepal. However, adequate post-harvest loss reduction measures are lacking. A field experiment was conducted in lime orchards in Chitwan during August-September 2019 and 2020. Pre-harvest spraying of different concentrations of calcium (0.1, 0.2, and 0.3%) and boron (0.2, 0.4, and 0.6%) in lime fruit trees was done before 45 and 30 days of fruit harvest. Fruits with the same maturity indices were harvested separately from the treated plants. The post-harvest study was conducted for 1 month under laboratory conditions. Different parameters like physiological loss of weight (PLW), decay loss, total soluble solids (TSS), titrable acidity (TA), ascorbic acid content (vitamin C), and freshness were recorded. Minimum PLW (18.2%) and decay loss (14.3%) were observed in boron (@0.6%) treated fruits. At the end of the experiment, minimum TA (1.79 and 2.12%) was recorded from boron (@0.6%) treated fruits in both years. Calcium (@0.3 %) and boron (@0.4 %) treated fruits expressed the maximum values for TSS in 2019 (9.67ºbrix) and 2020 (7.73 ºbrix), respectively. Fruit harvested from fruit trees sprayed with boron (@0.6%) showed the highest ascorbic acid content (55.47 and 49.61 mg/100g) and better lime freshness (2.67 and 3.0) in 2019 and 2020, respectively. This study concluded that the use of boron @0.6% as a pre-harvest spray can prolong the storage life of sour lime and maintain the fruit quality under environmental conditions of a mid-hill situation

PbTe(core)/PbS(shell) Nanowire: Electronic Structure, Thermodynamic Stability, and Mechanical and Optical Properties

Core/shell nanostructures offer exciting opportunities for a wide range of applications from solar cells and light-emitting diodes to field-effect transistors and logic circuits to spin-filtering and switching devices. Here, using first-principles density functional theory, we predict PbTe/PbS core/shell nanowires as semiconductors with direct bandgap along the 111»direction and indirect bandgap in the 200»direction. The inclusion of the spin-orbit coupling shifts the conduction band minimum (dominated by Pb atoms) toward the Fermi energy, thus reducing the energy gap of these nanowires while retaining their semiconducting features. The application of compressive strains (\u3e11.30%) causes semiconductor to metallic phase transitions in these nanowires with transition pressure ranging from ∼3 to ∼6 GPa, which is within the range reported in lead chalcogenides nanowires and nanoparticles. The Young\u27s modulus is ∼20 GPa along the 111»direction and ∼48 GPa in the 200»direction. We also report that the optical absorption in these materials is broad and extends from the infrared to ultraviolet (∼0.39-13 eV) region. Furthermore, our calculations of cohesive energy reveal that wires along the 200»direction are more stable compared to the wires in the 111»direction; ab initio molecular dynamics simulations at room temperature show that the 111»nanowire is more prone to core-to-shell diffusion

Spin filtering with Mn-doped Ge-core/Si-shell nanowires

Incorporating spin functionality into a semiconductor core-shell nanowire that offers immunity from the substrate effect is a highly desirable step for its application in next generation spintronics. Here, using first-principles density functional theory that does not make any assumptions of the electronic structure, we predict that a very small amount of Mn dopants in the core region of the wire can transform the Ge-Si core-shell semiconductor nanowire into a half-metallic ferromagnet that is stable at room temperature. The energy band structures reveal a semiconducting behavior for one spin direction while the metallic behavior for the other, indicating 100% spin polarization at the Fermi energy. No measurable shifts in energy levels in the vicinity of Fermi energy are found due to spin-orbit coupling, which suggests that the spin coherence length can be much higher in this material. To further assess the use of this material in a practical device setting, we have used a quantum transport approach to calculate the spin-filtering efficiency for a channel made out of a finite nanowire segment. Our calculations yield an efficiency more than 90%, which further confirms the excellent spin-selective properties of our newly tailored Mn-doped Ge-core/Si-shell nanowires

Cr-Doped Ge-Core/Si-Shell Nanowire: An Antiferromagnetic Semiconductor

An antiferromagnet offers many important functionalities such as opportunities for electrical control of magnetic domains, immunity from magnetic perturbations, and fast spin dynamics. Introducing some of these intriguing features of an antiferromagnet into a low dimensional semiconductor core–shell nanowire offers an exciting pathway for its usage in antiferromagnetic semiconductor spintronics. Here, using a quantum mechanical approach, we predict that the Cr-doped Ge-core/Si-shell nanowire behaves as an antiferromagnetic semiconductor. The origin of antiferromagnetic spin alignments between Cr is attributed to the superexchange interaction mediated by the pz orbitals of the Ge atoms that are bonded to Cr. A weak spin–orbit interaction was found in this material, suggesting a longer spin coherence length. The spin-dependent quantum transport calculations in the Cr-doped nanowire junction reveals a switching feature with a high ON/OFF current ratio (∼41 times higher for the ON state at a relatively small bias of 0.83 V).</p

Cr-Doped Ge-Core/Si-Shell Nanowire: An Antiferromagnetic Semiconductor

An antiferromagnet offers many important functionalities such as opportunities for electrical control of magnetic domains, immunity from magnetic perturbations, and fast spin dynamics. Introducing some of these intriguing features of an antiferromagnet into a low dimensional semiconductor core-shell nanowire offers an exciting pathway for its usage in antiferromagnetic semiconductor spintronics. Here, using a quantum mechanical approach, we predict that the Cr-doped Ge-core/Si-shell nanowire behaves as an antiferromagnetic semiconductor. The origin of antiferromagnetic spin alignments between Cr is attributed to the superexchange interaction mediated by the pz orbitals of the Ge atoms that are bonded to Cr. A weak spin-orbit interaction was found in this material, suggesting a longer spin coherence length. The spin-dependent quantum transport calculations in the Cr-doped nanowire junction reveals a switching feature with a high ON/OFF current ratio (a ;41 times higher for the ON state at a relatively small bias of 0.83 V)

Emergence of Ferromagnetism Due to Spontaneous Symmetry Breaking in a Twisted Bilayer Graphene Nanoflex

Twisted bilayer graphene exhibits many intriguing behaviors ranging from superconductivity to the anomalous Hall effect to ferromagnetism at a magic angle of ∼1°. Here, using a first-principles approach, we reveal ferromagnetism in a twisted bilayer graphene nanoflex. Our results demonstrate that when the energy gap of a twisted nanoflex approaches zero, electronic instability occurs and a ferromagnetic gap state emerges spontaneously to lower the energy. Unlike the observed ferromagnetism at a magic angle in the graphene bilayer, we notice the ferromagnetic phase appearing aperiodically between 0 and 30° in the twisted nanoflex. The origin of electronic instability at various twist angles is ascribed to the several higher-symmetry phases that are broken to lower the energy resulting from an aperiodic modulation of the interlayer interaction in the nanoflex. Besides unraveling a spin-pairing mechanism for the reappearance of the nonmagnetic phase, we have found orbitals at the boundary of nanoflex contributing to ferromagnetism