Distinctive structures and phase transitions in nanoscale particles and clusters, revealed by the photoelectric effect

Non-technical Abstract: When a metal surface is illuminated by a beam of light, it can emit electrons. This is known as the “photoelectric effect,” and is a foundational milestone of quantum physics. The minimum energy of light quanta required to eject an electron is known as the “work function” of the metal surface. A knowledge of this quantity is needed for the design of a multitude of devices, including light sensors, catalysts, solar energy cells, and even lithium-ion batteries. The same effect takes place at the nanoscale: nanoparticles emit electrons when irradiated by light of sufficiently high frequency. The research team has discovered that a shift in work function value can be used as a highly sensitive and minimally invasive gauge of the underlying changes in phase or structure (e.g., melting or lattice transformations) of both bulk and nanoscale systems. By applying the photoelectric effect to ultrapure individual metal nanoparticles, this project makes it possible to determine work functions with an exceptional accuracy and to detect novel phenomena that are special to the nanoscale regime. For example, some nanoparticles are expected to shrink, rather than expand, upon melting; some may melt at much higher temperatures than the bulk; some display nontrivial structural transitions; and in some nanoparticles three- and two-dimensional phases appear to exist simultaneously. These insights improve the understanding of materials, and have practical implications for areas as diverse as materials science and surface science, energy devices, and the study of liquid metals. Students working on the project will receive training in an interdisciplinary cutting-edge field of science. The activity also promotes undergraduate involvement in research, and serves as a fruitful resource for outreach activities via science fairs, student societies, and exchange programs and meetings. Technical Abstract: Nanoparticles are prepared in the gas phase as a highly pure beam and probed via photoionization as a function of their size and temperature (with the data also extrapolated to the bulk limit). The following systems and materials are investigated: (i) Gallium and bismuth, for which bulk solids compress rather than expand upon melting, but the nanoscale behavior is unknown; (ii) Tin, which exhibits “superheating” (much higher melting points than the bulk); (iii) Sodium and potassium, in which phase transitions broaden due to finite size effects; (iv) Tin alloys, which manifest a steep variation of the melting point with size; (v) Metal-fullerene systems, in which the fullerene core influences the melting transition of the metallic shell; (vi) Lithium, which exhibits an intriguing martensitic transition at a low temperature; and (vii) Zinc, in which two largely independent metallic phases of different dimensionalities appear to coexist within a single nanoparticle. This research provides new insight into fundamental quantum-level condensed matter and nanoscale processes, including the interplay between electronic and structural degrees of freedom, with important implications for the science and application of materials and nanostructures. 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. NSF Award ID: 2619002 | Program: 01002627DB NSF RESEARCH & RELATED ACTIVIT | Principal Investigator: Vitaly Kresin | Institution: University of Southern California, LOS ANGELES, CA | Award Amount: $472,209 View on NSF Award Search: https://www.nsf.gov/awardsearch/show-award/?AWD_ID=2619002 View on Research.gov: https://www.research.gov/awardapi-service/v1/awards/2619002.html