Researchers from the University of Illinois at Urbana-Champaign and the University of Washington, Seattle have collaborated to image quantum-dot excited states in three dimensions (3D) for the first time using a single-molecule absorption scanning-tunneling microscope (SMA-STM). A modulated laser beam excites localized electronic transitions (green atoms, as shown in the figure) in a single quantum dot and SMA-STM produces a two-dimensional (2D) projected image of the excited state. The dot is then “rolled” by a single-atom-thick needle to image different orientations in 2D. By comparing different orientations with a full quantum-mechanical calculation of a quantum dot, the 3D electronically excited structure can be reconstructed.
Unlike cryogenic electron tomography methods that create an image using an average of thousands of cryogenically cooled particles that are in different orientations, SMA-STM requires only one particle that is reoriented and does not require cryogenic freezing of the particle, improving understanding of its behavior in the excited state at room temperature. The actual image obtained is a representation of the orbital-density map of the transition excited by the laser and has subnanometer resolution in a field of view of tens of nanometers. Reference: D. Nguyen et al., J. Chem. Phys., 148, 6, 064701 (2018).