Publications

1999
Yun-Wei Cao, Julia Aksenton, Victor Soloviev, and Uri Banin. 1999. “Colloidal synthesis and properties of InAs/InP and InAs/CdSe core/shell nanocrystals.” MRS Online Proceedings Library Archive, 571. Publisher's Version Abstract

High-temperature colloidal synthesis of InAs/InP and InAs/CdSe core/shell nanocrystal quantum dots is reported. InP and CdSe shells with several thicknesses were grown on InAs cores ranging in diameter between 20 to 50 Å. Optical spectra, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and X-ray diffraction (XRD) were used to analyze the core/shell quantum dots and determine their chemical composition, average size, size distributions, and structures. The experimental results indicate that shell growth is uniform, expitaxial, and controllable. For both InP and CdSe shells, growth is accompanied by a red shift of the band gap energy as a result of the extension of the electron wavefunction into the shell region. An increase of the room temperature photoluminescence quantum yield by a factor of∼4 is observed with CdSe shell growth on InAs Cores. The growth of InP shells, however, quenches the photoluminescence quantum yield. The difference is assigned to outer surface effects in core/shell nanocrystals.

Alvin T Yeh, Giulio Cerullo, Uri Banin, Alf Mews, Paul A Alivisatos, and Charles V Shank. 1999. “Dynamics of exciton localization in CdS/HgS quantum-dot quantum wells.” Physical Review B, 59, 7, Pp. 4973. Publisher's Version Abstract

Localization of carrier wave functions to the quantum-well portion of the CdS/HgS quantum-dot quantum well (QDQW) is investigated. Nanosecond hole-burning (HB) spectra measure the photoinduced exciton coupling to a 250−cm−1 HgS phonon mode indicative of localization. Femtosecond pump-probe spectroscopy of these QDQW, however, show the photoinduced exciton couples to coherent 300−cm−1 CdS longitudinal optical-phonon modes, which is indicative of delocalization throughout the QDQW.  Femtosecond HB and three pulse pump-dump experiments reveal these results are dependent on the time scale of the experiment. These experiments indicate that the initially photoexcited electron and hole wave functions are weakly confined to the HgS monolayer. Only after long times (∼400 fs) will the exciton localize to the HgS well. These results indicate that the primary optical interaction excites electrons from a delocalized QDQW ground state and not from a localized HgS well state.

Uri Banin, Yunwei Cao, David Katz, and Oded Millo. 1999. “Identification of atomic-like electronic states in indium arsenide nanocrystal quantum dots.” Nature, 400, 6744, Pp. 542. Publisher's Version Abstract

Semiconductor quantum dots, due to their small size, mark the transition between molecular and solid-state regimes, and are often described as ‘artificial atoms’ (13). This analogy originates from the early work on quantum confinement effects in semiconductor nanocrystals, where the electronic wavefunctions are predicted4 to exhibit atomic-like symmetries, for example ‘s ’ and ‘p ’. Spectroscopic studies of quantum dots have demonstrated discrete energy level structures and narrow transition linewidths5,6,7,8,9, but the symmetry of the discrete states could be inferred only indirectly. Here we use cryogenic scanning tunnelling spectroscopy to identify directly atomic-like electronic states with s and p character in a series of indium arsenide nanocrystals. These states are manifest in tunnelling current–voltage measurements as two- and six-fold single-electron-charging multiplets respectively, and they follow an atom-like Aufbau principle of sequential energy level occupation10.

Yun‐Wei Cao and Uri Banin. 1999. “Synthesis and characterization of InAs/InP and InAs/CdSe core/shell nanocrystals.” Angewandte Chemie International Edition, 38, 24, Pp. 3692-3694. Publisher's Version Abstract

The effect of the outer surface of core/shell nanocrystals on the fluorescence quantum yield was observed for InAs/InP and InAs/CdSe core/shells (see picture). For InAs/CdSe we observed substantial enhancement of the fluorescence quantum yield compared to the InAs core, and up to two times larger than the laser dye IR‐140. Such core/shell nanocrystals have potential use as biological fluorescent markers in the near IR spectral range.