Synchronous Photoluminescence Intermittency (Blinking) along Whole Semiconductor Quantum Wires John J. Glennon, Rui Tang, William E. Buhro,* and Richard A. Loomis* Department of Chemistry and Center for Materials InnoVation, Washington UniVersity in St. Louis, One Brookings DriVe, CB 1134, Saint Louis, Missouri 63130 Received June 19, 2007; Revised Manuscript Received August 15, 2007 ABSTRACT Photoluminescence microscopy studies have detected synchronous-photoluminescence-intensity fluctuations along entire cadmium selenide quantum wires under continuous illumination. While similar photoluminescence blinking has been reported previously for semiconductor quantum dots and rods, the observation of synchronous blinking spanning the entire length of quantum wires, with diameters ≈9 nm and lengths >5 μm, is remarkable. We propose a mechanism to account for the synchronous blinking that is based on a dynamic, photolytic filling of surface-trap sites. The large absorption cross sections, the direct band gaps throughout the visible and near-infrared spectral regions, and the ease of synthesis of semiconductor nanocrystals have led to efforts aimed at increasing the efficiency of photovoltaic devices by incorporating several types of nanostructured semiconductors. Numerous groups have already shown the promise for incorporating semiconductor quantum dots, QDs, and quantum rods, QRs, in photovoltaic devices. 1-3 The inefficient hopping of charge from the QDs to the conductive polymer film in which the QDs are embedded and from the QDs to other QDs is believed to be a limiting factor in the utility of such devices. One-dimensional nanostructures, or nanowires, represent the smallest structures that can in principle efficiently transport charge along prescribed path- ways, and thus information in nanoelectronics. 4 Furthermore, the absorption cross sections of semiconductor nanowires are up to 8× higher per unit volume than those of the widely studied QDs, 5 making them especially attractive for integra- tion into photonics and solar cells. 6 The vast majority of nanowires reported in the literature, however, lack some of the essential properties of chemically prepared semiconductor colloidal QDs that are required for developing more efficient photovoltaics; mainly, most nano- wires do not have the tunability of the band gap absorption and emission energies that arise from controlling the size of the semiconductor near or below the dimensions of twice the exciton Bohr radius where quantum-confinement effects become important. 1,4 Additionally, most syntheses do not produce defect-free nanowires with uniform surface passi- vation that would enable efficient transport of charge along the nanowires. We have previously synthesized semiconductor nanowires with diameters as small as 3.5 nm and with lengths up to tens of micrometers by the solution-liquid-solid, SLS, method. 7-9 The details of the synthesis are provided in Supporting Information. These CdSe nanowires exhibit quantum-confinement effects and are hereafter referred to as quantum wires, QWs. We have shown that the dependence of the change in the band gap energy, ΔE g , which results from quantum confinement, on the diameter of the QWs, d, follows a simple particle-in-a-cylinder quantum-mechanical expression, ΔE g ∝ d -2 , where d is the diameter of the QW. 7,8 Cadmium selenide is a direct band gap semiconductor that has a bulk exciton Bohr radius of 5.6 nm, 10 and the quantum confinement arises when the diameter of the QW < ≈11 nm. 7 Although varying levels of theory can be used to treat the Coulomb and quantum-confinement energies of the electron- hole pairs within the QWs, the total exciton wave function is most often written as a product of wave functions representing the electron, the hole, and the exciton motion along the QW. 11-13 In Cartesian coordinates the total exciton wave function can be written as Ψ(x e ,y e ,x h ,y h ,z,Z) ) ψ e (x e ,y e )ψ h (x h ,y h )φ(z)e (iKZ , with Z and z ) z e - z h represent- ing the position of the center of mass of the exciton and the distance between the positions of the electron and hole along the length of the QW, respectively. 11 Since the Coulomb energy is much weaker than the single particle confinement energy in the radial dimension of the QW, the single-particle states for the electron and hole, ψ e (x e ,y e ) and ψ h (x h ,y h ), can * To whom correspondence should be addressed. E-mail: burho@wustl.edu (W.E.B.); loomis@wustl.edu (R.A.L.). NANO LETTERS 2007 Vol. 7, No. 11 3290-3295 10.1021/nl0714583 CCC: $37.00 © 2007 American Chemical Society Published on Web 10/10/2007