Ultrafast Energy Dynamics of Two Dimensional Semiconducting and Topological Materials

Vasudevan Rajagopal Iyer, Purdue University

Abstract

Invention of the transistor in the 1950s marked a revolution in the history of scientific advancement. Today, our lives are propelled by the tremendous power of electronics which is within everyone’s reach. This has been possible due to the thorough understanding of the transistor and other related electronic components. Silicon, which is abundantly available in nature, has been the material of choice for fabricating most of the electronic components we use today. It is a very robust and well performing semiconductor. However, we are reaching a stage where silicon electronics can no longer be improved, and the scientific community is keenly looking for alternatives to achieve continued progress. The search for faster computing and better electronic devices has led to the opening up of several promising routes, which are being actively researched. These include spintronics, valleytronics, topological computing, quantum computing and exotic two-dimensional (2D) material based systems, to name a few. Each field has seen a tremendous amount of effort in the past decade. Broadly, this thesis explores some of the fundamental aspects of 2D semiconducting and topological materials, via an optical spectroscopy approach. The technique of choice is femtosecond ultrafast pump and probe laser spectroscopy, which has proven to be a powerful tool for investigating fundamental microscopic phenomena. First, we look at emerging 2D materials, black phosphorus and tellurium, which have useful properties such as anisotropic electronic and optical response, good carrier mobility and direct bandgap. Black phosphorus and tellurium are Van der Waals materials, similar to graphene and hence can be cleaved using scotch tape or in liquid phase to obtain thin samples. In this work, a 65 nm black phosphorus flake was obtained and its relaxation following optical excitation was probed from near-infrared (1500 nm) to mid-infrared (4500 nm) wavelengths. The bandgap of the material was clearly resolved and relaxation times in the sub 100 ps range was observed. Wavelength dependent electron-phonon coupling and recombination was observed, which points to the importance of choosing the correct probing energies in ultrafast experiments. An upper estimate of the carrier mobility was obtained from the extracted electron-phonon scattering time. Moreover, the spectral evolution of reflectance was used to obtain hot carrier relaxation times by modeling with a Fermi-Dirac distribution. Lastly, surface oxidation effects were proposed to explain anomalous features in the transient data. Tellurium flakes with thicknesses ranging from 12 nm to 160 nm were obtained by solution growth and a strong dependence of the recombination times on thickness was observed. Thin flakes show a fast decay on the order of 20 ps, whereas thicker flakes have a decay in the 100s of ps range. The recombination mechanism in thin flakes was attributed to fast carrier capture by midgap defect states arising from surface defects and that in the thick flakes to radiative recombination. Recombination coefficients were extracted using a diffusion-recombination model. A surface and bulk scattering model was used to qualitatively explain the observed thickness dependent field effect mobilities in literature. Next, we look at another emerging material, Bi2Te2Se (BTS221). This material is a Van der Waals topological insulator and can be cleaved using scotch tape. Similar to black phosphorus and tellurium, BTS221 is also a direct band gap semiconductor, with a comparable bandgap. However, BTS221 possesses an exotic property due to its topological nature. It has Dirac like energy states on the surface, crossing the bandgap.

Degree

Ph.D.

Advisors

Xu, Purdue University.

Subject Area

Energy|Physics|Analytical chemistry|Atomic physics|Chemistry|Condensed matter physics|Electromagnetics|Optics|Thermodynamics

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