The Spectroscopy and Microscopy at Atomic Level Laboratory (SMALL), led by Dr. Shaowei Li in the Department of Chemistry and Biochemistry at UC San Diego, explores the quantum properties of single molecules and low-dimensional materials by integrating advanced optical methods with scanning probe microscopy. Our research focuses on uncovering the dynamic reorganization of atoms and molecules during light-induced physical and chemical processes.

A central challenge in modern science is the ability to visualize, manipulate, and track individual molecules in real time. To meet this challenge, we couple photon excitation with electron tunneling at the junction of a scanning tunneling microscope (STM). This unique approach combines the femtosecond temporal resolution of ultrafast lasers with the angstrom-scale spatial precision of tunneling electrons, providing an unprecedented window into molecular-scale quantum phenomena.

At SMALL, we are committed to advancing spectroscopy and microscopy to the atomic limit. By probing the quantum behavior of matter at its most fundamental level, we aim to deepen our understanding of light–matter interactions and drive progress across nanoscience, materials chemistry, and molecular electronics.

Achieving predictive control over molecular assembly at surfaces requires a fundamental understanding of how individual molecules interact, reorganize, and respond to their local environments. Our research explores this challenge through the lens of single-molecule chemistry, where the structure, bonding, and dynamics of individual molecular units can be directly visualized and manipulated. By designing ligands with tunable electronic and steric properties and integrating advanced tools such as scanning tunneling microscopy and inelastic electron tunneling spectroscopy, we reveal how subtle variations in molecular architecture and coordination geometry dictate interfacial reactivity and self-assembly pathways.

Through close coupling of experiment and theory, we uncover how site-specific interactions—governed by orbital hybridization, charge transfer, and local strain—define the stability and function of molecular adsorbates on metal surfaces. This single-molecule perspective provides not only a mechanistic understanding of ligand–metal bonding heterogeneity but also a framework for predictive assembly, where molecular design principles guide the construction of functional nanoscale architectures with atomic precision.

Precise control over molecular reactivity at the single-molecule level represents a cornerstone in the development of molecular electronics and functional nanodevices. Our research investigates how adjustable couplings—between molecules, substrates, and local electromagnetic fields—can be used to tune molecular properties and reaction pathways with atomic precision. By leveraging scanning tunneling microscopy and spectroscopy, we visualize and manipulate the conformational and electronic states of individual molecules in real time.

Through deliberate modulation of intermolecular forces and molecule–surface interactions, we uncover how van der Waals attraction, dipolar coupling, and steric repulsion reshape potential energy landscapes, enabling deterministic control over molecular switching and reactivity. These tunable interactions not only allow selective activation of vibrational modes and transition pathways but also reveal how nanoscale environments can be engineered to enhance or suppress specific chemical outcomes.

By integrating experimental observation with theoretical modeling, this work establishes a framework for nanoscale reaction control, where adjustable couplings serve as a design principle for directing molecular behavior and enabling energy-efficient, multi-state molecular functionalities.

Understanding how light interacts with matter at the level of individual chemical bonds lies at the heart of photochemistry and molecular dynamics. Our research seeks to uncover these interactions with atomic precision by integrating ultrafast optical excitation with scanning tunneling microscopy. This combination enables the direct observation of vibrational and electronic processes that drive molecular transformations on surfaces.

By coupling femtosecond infrared pulses with cryogenic STM, we access broadband vibrational spectra of single molecules with simultaneous ångström-scale spatial resolution—bridging the long-standing gap between ensemble infrared spectroscopy and single-molecule science. Through this approach, we monitor light-induced conformational switching and identify the specific vibrational modes that govern photochemical reactivity at the atomic scale. Our studies reveal how energy is redistributed among vibrational degrees of freedom, how isotopic substitution alters intramolecular coupling, and how delocalized modes mediate structural transitions. These insights uncover the fundamental pathways of energy flow and bond activation that underlie surface photochemistry.

Harnessing quantum degrees of freedom—such as spin, charge, and orbital coupling—within molecular systems offers a promising pathway toward reconfigurable and energy-efficient spintronic devices. Our research focuses on the design and control of quantum materials at interfaces, where molecular orbitals interact strongly with metallic or magnetic substrates to give rise to emergent spin, electronic, and vibrational phenomena. 

By employing low-temperature scanning tunneling microscopy, spin-polarized tunneling spectroscopy, and first-principles modeling, we probe how interfacial interactions modify molecular spin states, exchange coupling, and magnetic anisotropy. Through controlled tuning of molecule–substrate hybridization and intermolecular interactions, we demonstrate reversible switching between distinct spin configurations and magnetic ground states at the single-molecule level. 

These findings reveal how quantum coherence and spin polarization can be engineered through local bonding environments, offering a bottom-up route to reconfigurable molecular spintronics. By integrating experimental precision with theoretical insight, this research establishes design principles for programmable quantum interfaces, paving the way toward spin-based logic, memory, and sensing architectures built from molecular components.