Energy levels of few electron quantum dots imaged and characterized by atomic force microscopy

TL;DR

This paper demonstrates how atomic force microscopy can directly image and characterize the discrete energy levels and electronic structure of individual and coupled quantum dots through electrostatic force detection.

Contribution

It introduces a novel AFM-based method to probe energy levels and degeneracies in quantum dots in situ, including imaging coupled dots and observing temperature-dependent effects.

Findings

Detection of electron addition spectra showing Coulomb blockade.

Observation of degeneracies and temperature-dependent shifts in energy levels.

Imaging of multiple quantum dots and estimation of their coupling strengths.

Abstract

Strong confinement of charges in few electron systems such as in atoms, molecules and quantum dots leads to a spectrum of discrete energy levels that are often shared by several degenerate quantum states. Since the electronic structure is key to understanding their chemical properties, methods that probe these energy levels in situ are important. We show how electrostatic force detection using atomic force microscopy reveals the electronic structure of individual and coupled self-assembled quantum dots. An electron addition spectrum in the Coulomb blockade regime, resulting from a change in cantilever resonance frequency and dissipation during tunneling events, shows one by one electron charging of a dot. The spectra show clear level degeneracies in isolated quantum dots, supported by the first observation of predicted temperature-dependent shifts of Coulomb blockade peaks. Further, by scanning the surface we observe that several quantum dots may reside on what topologically appears to be just one. These images of grouped weakly and strongly coupled dots allow us to estimate their relative coupling strengths.