Spatial Variation of Available Electronic Excitations within Individual Quantum Dots Hee Joon Jung, †,⊥ Neil P. Dasgupta, ‡,⊥ Philip B. Van Stockum, § Ai Leen Koh, ∥ Robert Sinclair, †,∥ and Fritz B. Prinz* ,†,‡ † Department of Materials Science and Engineering, ‡ Department of Mechanical Engineering, § Department of Physics, and ∥ Stanford Nanocharacterization Laboratory, Stanford University, Stanford, California 94305, United States * S Supporting Information ABSTRACT: Quantum dots (QDs) allow for manipulation of the position and energy levels of electrons at sub-10 nm length scales through control of material chemistry, size, and shape. It is known from optical studies that the bandgap of semi- conductor QDs increases as their size decreases due to the narrowing of the quantum confinement potential. The mechanism of quantum confinement also indicates that the localized properties within individual QDs should depend on their shape in addition to their size, but direct observations of this effect have proven challenging due to the limited spatial resolution of measurement techniques at this scale and the ability to remove contributions from the surroundings. Here we present experimental evidence of spatial variations in the lowest available electron transition energy within a series of single electrically isolated QDs due to a dome-shaped geometry, measured using electron energy-loss spectroscopy in a (scanning) transmission electron microscope [(S)TEM-EELS]. We observe a consistent increase in the energy onset of electronic excitations from the lateral center of the dot toward the edges, which we attribute purely to shape. This trend is in qualitative agreement with a simple quantum simulation of the local density of states in a dome- shaped QD. KEYWORDS: Quantum dots, transmission electron microscopy, electron energy-loss spectroscopy, atomic layer deposition W hen matter is confined to sufficiently small regions of space, its electronic properties can change dramatically through the mechanism of quantum confinement. This principle is realized in quantum dots (QDs), which are semiconductor nanocrystals with dimensions below the Bohr exciton radius of the constituent material. 1 Quantum confine- ment entails a significant modification of the electron and hole wave functions inside a material, leading to the possibility of engineering both the spatial and energetic distribution of charges when designing modern solid-state devices such as transistors or solar cells. Further insight into the nature of quantum confinement at the nanometer scale is needed to exploit the degrees of freedom it may provide for the engineering of devices. By probing interband transitions, optical spectroscopy techniques have demonstrated that the QD bandgap increases with decreasing size. 1−4 In addition to these size effects, the bandgap and electronic properties of QDs depend on the nanocrystal shape 5,6 and these properties are expected to exhibit spatial variation within a single nanostructure. However, direct observations of local variations within nanostructures remain challenging due to the limited spatial resolution of measure- ment techniques at this scale. Past studies on localized electronic structure within QDs have primarily been performed using scanning tunneling microscopy (STM) 7−12 and magneto- tunneling spectroscopy (MTS). 13−17 Measurement of the local density of electronic states (LDOS) within single strained epitaxial III−V QDs has revealed states reminiscent of the solutions of the classic particle-in-a-box problem. However, a geometrical interpretation of the effects observed in these studies is obscured by the fact that the QDs are typically in contact with a different semiconductor and tend to be connected to each other by thin wetting layers, both of which can contribute to the signal and lead to nonuniform chemical composition in the dots. 7,11,12 In addition, these techniques only measure individual states rather than directly probing interband transitions. Here we report a study of single, electrically isolated, dome- shaped QDs using electron energy-loss spectroscopy in a (scanning) transmission electron microscope [(S)TEM-EELS]. This technique measures the energy loss spectrum of inelastically scattered electrons that have traversed the sample, revealing a variety of spectroscopic information related to its material properties, such as plasmon energies, elemental composition, and the joint density of electronic states (JDOS) near the band edges. 20 By forming a focused probe in the STEM, localized variations in these properties can be measured with subnm spatial resolution. Using this technique we measure a consistent gradual increase of the lowest available transition energy (LATE) Received: November 29, 2012 Revised: December 23, 2012 Published: December 31, 2012 Letter pubs.acs.org/NanoLett © 2012 American Chemical Society 716 dx.doi.org/10.1021/nl304400c | Nano Lett. 2013, 13, 716−721