Luminescent Polycrystalline Cadmium Selenide Nanowires

Principle Scientists: Qiguang Li

Q. Li, M.A. Brown, J.C. Hemminger, R.M. Penner*, Luminescent Polycrystalline Cadmium Selenide Nanowires Synthesized by Cyclic Electrodeposition/Stripping Coupled with Step Edge Decoration, Chemistry of Materials 18 (2006) 3432.

We have sought to develop methods for preparing metal chalcogenide nanowires using our Electrochemical Step Edge Decoration (ESED) technique, and to determine whether these polycrystalline nanowires possess technologically useful properties that parallel those of single crystalline semiconductor nanowires. The key properties of interest are: 1) A size-tunable bandgap, 2) band-edge photoluminescence, and 3) band-edge photoconductivity. In this work, we develop a method for synthesizing polycrystalline nanowires composed of CdSe, and we make an initial attempt to characerize the photoluminescence properties of these nanowires.

Figure 1.a) Schematic diagram showing the synthesis of CdSe (red) and CdS/CdS core-shell nanowires (yellow). b) Cyclic voltammograms recorded during the synthesis of CdSe nanowires. Excess cadmium metal is selectively removed from the growing CdSe nanowire during this process. c) Diameter of CdSe nanowires as a function of the number of electrodeposition/stripping (E/S) cycles used for synthesis. d) SEM image of 32 nm CdSe nanowires produced using a single E/S synthesis cycle.

We have developed a method for preparing arrays of polycrystalline CdSe nanowires and CdSe-CdS core-shell nanowires that are 30-300 nm in diameter and more than 100 microns in length. This method, a variant of the electro-chemical step edge decoration (ESED) method, involved the electrodeposition of CdSe selectively at the step edges present on a HOPG surface (Fig 1a). In order to achieve the growth of CdSe nanowires, ESED was coupled with a cyclic electro-deposition/stripping regime in which CdSe and excess elemental selenium and cadmium were first electrodeposited onto HOPG and then elemental Cd and Se were selectively removed by anodic stripping leaving near-stoichiometric CdSe nanowires on the graphite surface (Fig 1b).

Figure 2 a) Photoluminescence (PL) spectra for CdSe nanowires and CdS-capped CdSe nanowires (excitation at 257 nm). No emission for CdS at 2.42 eV is observed in these spectra, but CdSe emission intensity is increased by 15x for the CdS-capped nanowires (b) white light optical micrograph (50 x 50 microns) of CdSe/CdS nanowires (dark lines) on graphite. (c) fluorescence micrograph (excitation at 257 nm) of the same location shown in (b) showing orange photoluminescence from these nanowires.

One conclusion of this study is that polycrystalline CdSe and CdSe/CdS nanowires both produce bandedge photoluminescence emission that is not contaminated with red-shifted trap state emission (Fig 2a). The intensity of the observed PL emission, however, is greater by more than an order of magnitude for the CdSe/CdS core/shell nanowires, suggesting that nonradiative traps at the surface of the CdSe nanowires are eliminated upon the production at the surface of these nanowires of a CdS layer.



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Copyright 2006 R.M. Penner