Optical determination of crystal phase in semiconductor nanocrystals

Abstract

Optical, electronic and structural properties of nanocrystals fundamentally derive from crystal phase. This is especially important for polymorphic II–VI, III–V and I-III-VI2 semiconductor materials such as cadmium selenide, which exist as two stable phases, cubic and hexagonal, each with distinct properties. However, standard crystallographic characterization through diffraction yields ambiguous phase signatures when nanocrystals are small or polytypic. Moreover, diffraction methods are low-throughput, incompatible with solution samples and require large sample quantities. Here we report the identification of unambiguous optical signatures of cubic and hexagonal phases in II–VI nanocrystals using absorption spectroscopy and first-principles electronic-structure theory. High-energy spectral features allow rapid identification of phase, even in small nanocrystals (∼2 nm), and may help predict polytypic nanocrystals from differential phase contributions. These theoretical and experimental insights provide simple and accurate optical crystallographic analysis for liquid-dispersed nanomaterials, to improve the precision of nanocrystal engineering and improve our understanding of nanocrystal reactions.

Thermally-induced reversible structural isomerization in colloidal semiconductor CdS magic-size clusters

“…For the II–VI semiconductor bulk and RQDs, thermally-induced phase transformations from zinc-blende to wurtzite structures have been documented. The two structures, however, have nearly identical bandgap energies, and optical absorption spectroscopy has not been shown to be a practical tool to meticulously monitor the transformation kinetics of the bulk materials and RQDs 51 – 55 . On the other hand, for the two types of MSCs discussed in the present work, there is a significant difference in bandgap energies.…”

Coherent exciton-vibrational dynamics and energy transfer in conjugated organics

“…Interestingly, for materials in the “intermediate coupling regime”, there exists an ample amount of spectroscopic evidence for robust coherent electron-vibrational dynamics, which persists over long (frequently picosecond) timescales at ambient conditions, in spite of structural disorder, noise, and environmental fluctuations that may be present. Subsequently, several recent reviews 1 – 3 , 7 suggested that coherence is a highly non-trivial and very important factor, which can be used to achieve specific functionalities in chemical and biological systems provided that underpinning design principles 25 , 26 are well understood.…”

Nonequilibrium BN-ZnO: Optical properties and excitonic effects from first principles

“…Fortunately, highly accurate theoretical-spectroscopy techniques can now directly provide experiment with guidance needed to reliably distinguish different phases optically, even in nanostructures. 21 Here we combine first-principles simulations with Maxwell modeling to derive an optical signature that can be used to unambiguously identify BN contributions in ZnO samples using optical-absorption experiments. To this end, we study structural, electronic, and optical properties of bulk BN-ZnO and WZ-ZnO using density functional and manybody perturbation theory.…”