Origins of Unique Optical Properties in Intermediate Band Nanocrystals

Cornell University, Ithaca NY

Investigators

Abstract

PART 1: NON-TECHNICAL SUMMARY When a metal is shrunk down to nanometer sizes, it can obtain new optical absorption features that don’t exist in bigger, bulk materials. These new features, called plasmons, change the color of the metal, so that the commonly expected color becomes something different, like nanoscale gold colloids appearing red. Interestingly, there are some non-metals (semiconductors) that show this same effect at the nanoscale, but the origin and size-dependence of these effects is not well-understood. This project, supported by the Solid State and Materials Chemistry program in the Division of Materials Research, employs state-of-the-art x-ray characterization methods to elucidate unanswered structure-property questions of semiconducting nanocrystals. This project is examining several hypotheses to understand the nature of the semiconductor plasmons, including how excited electronic charges move in the materials and how these are affected by nano-sizing. To accomplish the goals, the PI is synthesizing nanocrystals, and characterizing them with methods including x-ray resonant inelastic x-ray scattering (RIXS) measurements through the quantum confinement size regime. The products of this work will benefit society because these nanocrystals have the potential for photovoltaic applications or they could lead to others such as photothermal therapy of tumor cell annihilation upon laser irradiation. These nanocrystals could also lead to high-frequency, all-optical modulation and switching of light. The accompanying outreach programs invigorate the public excitement about interesting Materials Chemistry through Lending Library demonstration kits on plasmonics. Furthermore, the PI mentors students of color through a program at Cornell. PART 2: TECHNICAL SUMMARY Certain metal chalcogenide nanocrystals have the ability to sustain absorption features that resemble localized surface plasmon resonances (LSPRs) typically seen in metal nanocrystals. The electronic origin of these features in these materials, however, is not completely understood. One theory is that interband electronic transitions lead to the features. This project, supported by the Solid State and Materials Chemistry program in the Division of Materials Research, employs state-of-the-art x-ray characterization methods to determine the influence of quantum confinement on the electronic bands and optical absorption profile of semiconducting nanocrystals. The central hypotheses are that transitions between bands control the absorption features and can be affected by quantum confinement. To test these hypotheses, this project is using synchrotron x-ray spectroscopy, including hard x-ray resonant inelastic x-ray scattering (RIXS) measurements and analysis, of nanocrystal semiconductors through the quantum confinement range. The team is synthesizing a size series of monodisperse nanocrystals, characterizing them with x-ray spectroscopy and optical absorption, and correlating the information with electronic structure models, to elucidate fundamental structure-property relationships. The products of this work will benefit society because these nanocrystals have the potential for photovoltaic applications or they could lead to others such as photothermal therapy of tumor cell annihilation upon laser irradiation. These nanocrystals could lead to high-frequency, all-optical modulation and switching of light. The accompanying outreach programs invigorate the public excitement about interesting Materials Chemistry through Lending Library demonstration kits on plasmonics. Furthermore, the PI mentors students of color through a program at Cornell. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.