Perovskite Chalcogenides are a new class of semiconductors, which have large chemical and structural tunability that translates to tunable band gap in the visible to infrared part of the electromagnetic spectrum. Besides this band gap tunability, they offer a unique opportunity to realize large density of states semiconductors with high carrier mobility. In this talk, I will discuss some of the experimental advances made both in my research group and in the research community on the theory, synthesis of these materials and understanding their optoelectronic properties.
Perovskite structure is composed of an octahedrally coordinated transition metal or main group element with anions such as oxygen, chalcogen or halogens. The octahedra is typically connected in the corners and the voids are filled by alkali, alkaline or rare earth elements. The valence and the size of the cations and anions can lead to different connectivity of these octahedra, which offers a knob to control both the chemical composition and the dimensionality of these materials. Moreover, the large number of elements in the periodic table can be accommodated in these extended perovskite and related structures, which allows us finer knobs to control the physical and chemical properties, in our case, we tailor light-matter interaction precisely over a broad energy range spanning the visible to infrared spectrum. We leverage this effect in early transition metal based perovskite chalcogenides and related phases to achieve properties such as highly anisotropic absorption and refraction (BaTiS3, Sr1+xTiS3), unconventional band gap evolution (BaZrS3 and Ban+1ZrnS3n+1 for n ≥ 1). Finally, I will provide a general outlook for future studies on these exciting new class of materials.
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2. S. Niu et al. Advanced Materials 29, 1604733 (2017).
3. S. Niu et al. Chemistry of Materials, 30 (15), 4897-4901 (2018).
4. S. Niu et al. Chemistry of Materials, 30 (15), 4882-4886 (2018).
Birefringence is a fundamental property of materials that enables optical components such as wave plates and polarizers, and is quantified by the difference between extraordinary and ordinary refractive indices. Solid homogeneous crystals like calcite and rutile are some of the most birefringent materials at visible and near-infrared wavelengths. However, at longer wavelengths (i.e., mid to far infrared) these materials become highly lossy. In the mid infrared, the most birefringent materials that are transparent are significantly less birefringent than their visible counterparts. While structured materials with strong optical anisotropy exist at these wavelengths (i.e., with form birefringence), their utility is limited by fabrication constraints.
In the talk, we will report on a rationally designed and synthesized material, barium titanium sulfide (BaTiS3), which has broadband and giant birefringence surpassing that of any known transparent anisotropic crystal throughout the infrared. We will detail our extensive optical characterization to extract the anisotropic complex refractive index spanning the ultraviolet to the mid infrared by combining generalized spectroscopic ellipsometery and polarized reflection and transmission measurements. We report a difference between the ordinary and extraordinary refractive index of up to 0.76 in a mid-infrared region of transparency, more than twice that of rutile in the visible, and show that the unprecedented optical anisotropy extends to the limit of our detection capabilities (16.7 μm). This material also features highly anisotropic Raman scattering, and we are currently working on measuring polarized infrared photoluminescence measurements to provide further insight into the anisotropy of this unique material.
We present a novel nanotube-on-insulator (NOI) approach to produce high-yield nanotube devices based on aligned
single-walled carbon nanotubes. First, we managed to grow aligned nanotube arrays with controlled density on
crystalline, insulating sapphire substrates, which bear analogy to industry-adopted silicon-on-insulator substrates. Based
on the nanotube arrays, we demonstrated registration-free fabrication of both top-gated and polymer-electrolyte-gated
field-effect transistors with minimized parasitic capacitance. In addition, we have successfully developed a way to
transfer these aligned nanotube arrays to flexible substrates. Our approach has great potential for high-density, largescale
integrated systems based on carbon nanotubes for both micro- and flexible electronics.