2026 Theses Doctoral

Thermodynamic Equilibrium Computation and Synthetic Removal of Atomic Point Defects in Flux-Grown Transition Metal Diselenide Crystals

Holtzman, Luke

Two-dimensional transition metal diselenides (TMDs) are among the most actively studied platforms for correlated-electron physics and post-silicon nanoelectronics. However, their performance is limited by the density of native atomic point defects. Flux synthesis is the premier route to high-quality bulk WSe₂ and MoSe₂. Nevertheless, the chemical identity of the dominant defect, the thermodynamic lower bound on defect density, and the growth mechanism controlling crystal size had not been resolved. This thesis addresses all three within a single flux-synthesis platform.

A generalized temperature- and pressure-dependent framework is used to compute the thermal-equilibrium densities of intrinsic point defects in WSe₂ and MoSe₂ for the first time. The selenium-vacancy equilibrium density is ≤10⁸ cm⁻² under all selenium-rich conditions, three orders of magnitude below the densities measured in flux-grown crystals, establishing that defect density is set by impurity chemistry rather than by thermodynamics. Atomic-resolution scanning-probe microscopy is used to identify the dominant impurity, first establishing that scanning tunneling microscopy images the Wannier electron distribution rather than the atoms themselves by a systematic doping and calibration study.

With the calibrated scanning tunneling microscopy images, the dominant defect is identified as an oxygen substitutional on the selenium site (Oₛₑ). Following identification, a targeted silicon-getter process added to the initial ampule suppresses Oₛₑ density by more than 300-fold. The resulting ultrapure crystals exhibit low-temperature photoluminescence resonances with FWHM as narrow as 3 meV, clear fractional quantum Hall states, and the first defect-free channels in large-area WSe₂ pFET arrays.

In parallel, quench-interrupted syntheses show that dissolution, precipitation, and coarsening all occur during the high-temperature dwell rather than the slow cool, enabling the coarsening activation energies of WSe₂ and MoSe₂ to be measured for the first time and the 1°C/hr slow cool to be replaced by a rapid quench that raises throughput by up to 400% without affecting defect density. Together, these results establish impurity chemistry as the controlling factor in flux-grown TMD defect density and open a new regime of ultrapure two-dimensional semiconductor physics.