Phase transformation of PbSe/CdSe nanocrystals from core-shell to Janus structure studied by photoemission spectroscopy

Photoelectron spectroscopic measurements have been performed, with synchrotron radiation on PbSe/CdSe heteronanocrystals that initially consist of core-shell structures. The study of the chemical states of the main elements in the nanocrystals shows a reproducible and progressive change in the valence-band and core-level spectra under photon irradiation, whatever the core and shell sizes are. Such chemical modifications are explained in light of transmission electron microscopy observations and reveal a phase transformation of the nanocrystals: The core-shell nanocrystals undergo a morphological change toward a Janus structure with the formation of semidetached PbSe and CdSe clusters. Photoelectron spectroscopy gives new insight into the reorganization of the ligands anchored at the surface of the nanocrystals and the modification of the electronic structure of these heteronanocrystals

HRTEM, HAADF and HR-EELS investigations of colloidal PbSe nanoparticles.

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MICROWAVE AND INFRARED SPECTRA OF URETHANE

Author Institution: PhLAM, UMR 8523 CNRS - Universite Lille 1, F-59655, Villeneuve d'Ascq Cedex, France; Institute of Radio Astronomy of NASU, Chervonopraporna 4, 61002 Kharkov, Ukraine; LADIR, UMR 7075 CNRS - Universite Pierre et Marie Curie, Bat. F74 - Case 49, 4 place Jussieu, F-75252, Paris Cedex 05, FranceThe rotational spectrum of urethane (H$_2$NCO$_2$CH$_2$CH$_3$) was investigated in the frequency range 5--20 GHz using a MB-MWFT spectrometer. The accurate values of rotational, centrifugal distortion and quadrupole coupling constants were determined for the two most stable conformers. Previous MW results from the literature [1] have been revised for conformer I. Additional measurements in the frequency range 50--150 GHz are in progress, using a conventional absorption spectrometer. Also the nuclear spin-spin interactions are studied from additional hyperfine splittings observed in the MWFT spectra. The Fourier transform infrared spectra of urethane have been recorded in the 1000--1900 cm$^{-1}$ region at a 0.1 cm$^{-1}$ resolution in a supersonic jet seeded with argon. Maximum absorption signals are obtained by controlling the temperature of the compound and the argon flow. Preliminary bands assignments are proposed on the grounds of FTIR spectra analysis, conformational equilibrium and {\em ab initio} harmonic frequencies. In order to confirm these assignments, higher level {\em ab initio} calculations of the total electronic energy and anharmonic frequencies within the adiabatic approximation [2], for each conformer, are presently under way. \vspace{1em} 1. K. M. Marstokk and H. M{\o}llendal, Acta Chem. Scandinavia 53 (1999) 329. 2. M. Goubet, B. Madebene and M. Lewerenz, Chimia 58 (2004) 291. \vspace{1em

Anomalous quantum confinement of the longitudinal optical phonon mode in PbSe quantum dots

We have investigated the diameter dependence of the Raman spectra of lead selenide nanocrystals. The first-order Raman peak at about 136 cm−1 and its second-order overtone at twice this wavenumber move up in energy with decreasing nanocrystal diameter. This anomalous behavior is interpreted in terms of quantum confinement of the longitudinal optical (LO) phonon whose frequency displays a minimum at in the dispersion of bulk PbSe. We perform ab initio calculations of the phonons of PbSe slabs with up to 15 layers. The LO mode perpendicular to the slab shifts indeed upwards with decreasing layer thickness, thus validating the interpretation of the anomalous radius dependence of the Raman spectra in terms of quantum confinement

Charge carrier identification in tunneling spectroscopy of core-shell nanocrystals

Semiconductor PbSe/CdSe core-shell nanocrystals (NCs) in a double barrier tunnel junction have been investigated by means of scanning tunneling spectroscopy at low temperature. From the analysis of the differential conductance peak position as a function of the potential distribution in both potential barriers, we demonstrate a unipolar transport regime for a large amount of NCs. The same charge carriers are injected on both sides of the zero-conductance gap, and the peaks observed at higher energy arise from the charging of the NCs. Similar results are obtained for CdSe/CdS dot-in-rod NCs, indicating that the addition of a shell favors transitions between different charge states rather than single particle excited states. Further characterization of the PbSe/CdSe core-shell NCs by x-ray photoemission spectroscopy reveals that the variations in the transport properties from NC to NC are explained by the occurrence of unprotected PbSe facets that have different orientations in the junction