Mechanism of Atomically Resolved Tip-Enhanced Raman Scattering Imaging

Board Of Regents, Nshe, Obo University Of Nevada, Reno, Reno NV

Investigators

Abstract

With support from the Chemical Structure, Dynamics, and Mechanisms A (CSDM-A) program in the Division of Chemistry, Professor Joonhee Lee of the University of Nevada-Reno will visualize the vibrations of single molecules. To image vibrations, incident light should excite each molecule bound to the metal surface and scatter at altered frequencies carrying vibrational information. However, the wavelength of visible light is about 1,000 times greater than the atomic length scale, making most optical spectroscopy unsuitable for probing single molecules. To address this challenge, Professor Lee and his students will precisely engineer and optimize the geometry of a silver needle, which will confine light to its tip. As the tip hovers over an individual molecule, it can excite it with atomic precision and amplify the scattered light, forming vibrational images. Understanding the mechanism of vibrational imaging could lead to the precise determination of photochemical reaction coordinates, which has the potential to aid in the development of photoresists for advanced semiconductor manufacturing. Members of the Lee group supported by this project will reach out to K-12 students in rural areas of Nevada to teach optical spectroscopy labs using materials designed to introduce quantum mechanics. Atomically resolved tip-enhanced Raman spectroscopy (ARTERS) developed in the Lee group will be employed to image the vibrational normal modes of pyridine and its derivatives chemisorbed on Cu single crystal surfaces. Molecules chemisorbed on the metallic surfaces pose significant challenges for understanding their vibrational images for the following reasons: 1) They have a 3-dimensional geometry due to their directional molecule-metal bonds. Consequently, the vibrational images are formed from integrated Raman scattering across multiple focal planes. 2) The molecules are no longer deemed isolated; to accurately model the system, the molecule-substrate complex must be considered. To address these challenges, the Lee group will explore the governing mechanism of vibrational imaging through a joint experimental and theoretical approach. The ongoing feedback between experiment and theory will reveal the governing mechanism in the case of chemisorbed molecules. ARTERS can initiate photochemical reactions and image the products. With a rigorous understanding of the imaging mechanism, precise identification of the products with atomistic details becomes possible. This capability can elucidate the pathways of photochemical reactions and aid in the development of new photoresists. Graduate students supported by this project will disseminate the knowledge gained through the research to K-12 students in rural areas of Nevada. 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.