This tutorial will offer a brief introduction to theory and computations as well as to synthesis and characterization, in particular when applied to sulfides, selenides, tellurides, and multi-anionic systems. Oxide materials will also be included as they relate to chalcogenides more broadly. The focus of the symposium will be on understanding the distinctive nature of this group of materials in the context of materials discovery and design.
Density Functional Theory and Electronic Structure Calculations
Elif Ertekin, University of Illinois at Urbana-Champaign
We will discuss the application of first-principles density functional theory to the predicting bulk and defect properties of chalcogenides. This discussion will include a brief introduction to density functional theory, a practical discussion of how to carry out simulations and an in-depth look at which properties can be reliably predicted, which properties require more scrutiny and why. Our discussion will emphasize and distinguish between ground state and excited state, and equilibrium and nonequilibrium (i.e., response functions) properties of materials. Drawing from the fields of photovoltaics and thermoelectrics, several examples pertaining to chalcogenides will be described, including bulk properties and the prediction of phase stability, and simulations of point defects (formation energies, charge transition levels) and related properties such as dopability and carrier concentrations. This discussion is aimed at providing a basic understanding of the level of accuracy that is achievable or to be expected from density functional theory, as well as to serve as a guide to nonspecialists what to look for when assessing results from electronic structure calculations.
Bulk Crystal Growth and Phase Diagrams for Metal Chalcogenides
Albert Davydov, National Institute of Standards and Technology
The tutorial focuses on single crystal growth of metal chalcogenide electronic materials, including layered van der Waals compounds, such as TMDC semiconductors and semimetals, Bi2(Se,Te)3 based topological materials, InSe and GeSe semiconductors, etc. The experimental methods will cover (1) vapor growth techniques including sublimation/crystallization and chemical vapor transport (CVT), and (2) melt solidification that includes crystallization from near-stoichiometric compositions as well as from "self-flux" or "foreign-element-flux" solutions. Use of phase diagrams will be exemplified to guide choices for the crystal growth approaches; for example, why Bi2Se3, InSe and GeSe can be grown both by CVT and Bridgman (i.e., direct melt solidification) methods, while TMDCs such as MoSe2 and WS2 can only be grown by CVT and not from the melt. The importance of often ignored "pressure variable" when describing phase diagrams for metal-chalcogen systems will also be highlighted.
10:00 am BREAK
Prashun Gorai, Colorado School of Mines and National Renewable Energy Laboratory
This talk will cover the recent developments in high-throughput (HT) computations that have led to a new paradigm in materials discovery and design. First, typical workflows for HT computations will be introduced followed by a discussion of software for HT workflows. Next, several examples of successful chalcogenide materials discovery, enabled by HT computations, will be discussed while also highlighting the challenges and downfalls of HT computations. Finally, materials databases that have resulted from these large-scale computational efforts will be briefly discussed. In closing, we will review the role of data informatics (e.g., data mining, machine learning) in the context of HT computations.
Akshay Singh, Massachusetts Institute of Technology
Now that we have prepared the bulk crystals and thin films of TMDs, how do we characterize the purity of the synthesized materials? Also, how do we qualify these materials for photonics and optoelectronic applications? In this part of the tutorial, we will discuss optical constants measurements using spectroscopic ellipsometry and Fourier transform infrared (FTIR) spectroscopy. We will discuss how performing cross-sectional transmission electron microscopy (TEM, to measure native oxides), atomic force microscopy (AFM, to measure roughness), and x-ray photoelectron spectroscopy (XPS, to measure composition profiles) techniques are important to uncover actual optical constants from thin films and bulk crystals. These techniques are also shown to help differentiate chalcogenides from undesirable oxide phases and improve synthesis procedures.
Tailor-Made Chalcogenide Colloids: Tuning Size, Composition and Structure of Nanomaterials
Maksym Yarema, ETH Zürich
Colloidal chalcogenide nanocrystals are convenient building blocks of various solution-processed devices, such as displays, photovoltaics, thermoelectrics and phase-change memory. Likewise, chalcogenide colloids are handy materials for the fundamental and use-inspired research, featuring size-dependent optical and electronic properties in addition to composition dependences and structure diversity. In this talk, we will summarize the state of the art for the colloidal chalcogenide nanocrystals, outline challenges and future directions in the field. In particular, we will focus on multicomponent nanocrystals, for which composition-dependent effects are superimposed on size dependences. We will also discuss opportunities for chalcogenide colloids with metastable and amorphous structures.