Silicon and carbon based nanowires

Cataloged from PDF version of article.Nanowires have been an active field of study since last decade. The reduced dimensionality end size allowing electrons can propagate only in one direction has led to quantization which are rather different from the bulk structure. As a result, nanowires having cross section in the range of Broglie wavelength have shown stepwise electrical and thermal conductance, giant Young modulus, stepwise variation of the cross-section etc. Moreover, the atomic structure of nanowires have exhibited interesting regularities which are not known in two or three dimensions. These novel properties of nanowires have been actively explored since last decade in order to find an application in the rapidly developing field of nanotechnology. In the present thesis, we investigated the atomic and electronic structure of a variety of Si and C atom based very thin nanowires starting from linear chain including pentagonal, hexagonal and tubular structures. We found that the C and Si linear chains form double bonds and have high binding energy. Although bulk carbon in diamond structure is an insulator, carbon linear chain is metal and has twice conductance of the gold chain. We carried out an extensive analysis of stability and conductance of the other wires. Our study reveals that Si and C based nanowires generally show metallic properties in spite of the fact that they are insulator or semiconductor when they are in bulk crystal structure. Metallicity occurs due to change in the character and order of bonds.Tongay, SefaattinM.S

Growth and characterization of multiferroic BiMnO3_3 thin films

We have grown epitaxial thin films of multiferroic BiMnO$_3$ using pulsed laser deposition. The films were grown on SrTiO$_3$ (001) substrates by ablating a Bi-rich target. Using x-ray diffraction we confirmed that the films were epitaxial and the stoichiometry of the films was confirmed using Auger electron spectroscopy. The films have a ferromagnetic Curie temperature ($T_C$) of 85$\pm$5 K and a saturation magnetization of 1 $\mu_B$/Mn. The electric polarization as a function of electric field ($P-E$) was measured using an interdigital capacitance geometry. The $P-E$ plot shows a clear hysteresis that confirms the multiferroic nature of the thin films.Comment: 4 pages, 4 figures, submitted to J. Appl. Phy

Interplay of trapped species and absence of electron capture in Moir\'{e} heterobilayers

Moir\'e heterobilayers host interlayer excitons in a natural, periodic array of trapping potentials. Recent work has elucidated the structure of the trapped interlayer excitons and the nature of photoluminescence (PL) from trapped and itinerant charged complexes such as interlayer trions in these structures. In this paper, our results serve to add to the understanding of the nature of PL emission and explain its characteristic blueshift with increasing carrier density, along with demonstrating a significant difference between the interlayer exciton-trion conversion efficiency as compared to both localized and itinerant intra-layer species in conventional monolayers. Our results show the absence of optical generation of trions in these materials, which we suggest arises from the highly localized, near sub-nm confinement of trapped species in these Moir\'e potentials.Comment: 3 figures, Supplementary information available on reques

Ultrafast Charge Transfer in Atomically Thin MoS2/WS2 Heterostructures

Van der Waals heterostructures have recently emerged as a new class of materials, where quantum coupling between stacked atomically thin two-dimensional (2D) layers, including graphene, hexagonal-boron nitride, and transition metal dichalcogenides (MX2), give rise to fascinating new phenomena. MX2 heterostructures are particularly exciting for novel optoelectronic and photovoltaic applications, because 2D MX2 monolayers can have an optical bandgap in the near-infrared to visible spectral range and exhibit extremely strong light-matter interactions. Theory predicts that many stacked MX2 heterostructures form type-II semiconductor heterojunctions that facilitate efficient electron-hole separation for light detection and harvesting. Here we report the first experimental observation of ultrafast charge transfer in photo-excited MoS2/WS2 heterostructures using both photoluminescence mapping and femtosecond (fs) pump-probe spectroscopy. We show that hole transfer from the MoS2 layer to the WS2 layer takes place within 50 fs after optical excitation, a remarkable rate for van der Waals coupled 2D layers. Such ultrafast charge transfer in van der Waals heterostructures can enable novel 2D devices for optoelectronics and light harvesting

Bosonic Mott insulator in WSe2/WS2 moir\'e superlattice

A panoply of unconventional electronic states is recently observed in moir\'e superlattices. On the other hand, similar opportunities to engineer bosonic phases remain largely unexplored. Here we report the observation of a bosonic Mott insulator in WSe2/WS2 moir\'e superlattices composed of excitons, i.e., tightly bound electron-hole pairs. Using a novel pump probe spectroscopy, we find an exciton incompressible state at exciton filling v_ex = 1 and charge neutrality, which we assign to a bosonic Mott insulator. When further varying charge density, the bosonic Mott insulator continuously transitions into an electron Mott insulator at charge filling v_e = 1, suggesting a mixed Mott insulating state in between. Our observations are well captured by a mixed Hubbard model involving both fermionic and bosonic components, from which we extract the on-site Coulomb repulsion to be 15meV and 35meV for exciton-exciton and electron-exciton interactions, respectively. Our studies establish semiconducting moir\'e superlattices as intriguing platforms for engineering novel bosonic phases.Comment: 25 pages, 4+10 figure