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C: Physical Processes in Nanomaterials and Nanostructures
Long-Lived 1D Excitons in Bright CdTe Quantum Wires William M. Sanderson, Fudong Wang, William E. Buhro, and Richard A Loomis J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09588 • Publication Date (Web): 14 Jan 2019 Downloaded from http://pubs.acs.org on January 21, 2019
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Long-lived 1D Excitons in Bright CdTe Quantum Wires William M. Sanderson, Fudong Wang, William E. Buhro and Richard A. Loomis*
Department of Chemistry and Institute of Materials Science and Engineering, Washington University in St. Louis, St. Louis, MO 63130, USA.
ABSTRACT: Time-resolved photoluminescence (PL) intensity decay profiles were recorded for room-temperature ensemble samples of CdTe quantum wires (QWs) with varying PL quantum yields (PL). The PL lifetimes for samples with PL > 4% are nearly an order of magnitude greater than the radiative lifetime of CdTe QDs, 200 ns versus ~25 ns. The photogenerated electron-hole pairs relax to the lowest exciton state, correlating with the 1e and 13/2 quantum-confinement states, and are bound together as one-dimensional (1D) excitons. These 1D excitons have a thermal distribution of translational kinetic energy along the long, unconfined dimension of the QWs. The extended lifetimes are justified via the constraints imposed by the conservation of wave vector (or momentum) and the large mismatch between the wave vector of the moving 1D excitons and of the photons emitted during radiative relaxation. The long charge carrier lifetimes and the dimensionality of these high-quality semiconductor QWs offer distinct advantages for use in photovoltaics.
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INTRODUCTION Semiconductor nanomaterials are the focus of intense study due in large part to their potential for enhancing the efficiencies of photovoltaic (PV) devices.1-5 The ability to tune the band-gap energies and the corresponding absorption and photoluminescence (PL) spectra of these nanomaterials through manipulation of their size, dimensionality, and chemical composition enable a majority of the solar spectrum to be harnessed. The simplest and most widely utilized of these semiconductor nanomaterials are zero-dimensional (0D) quantum dots (QDs) since they can be synthesized with high physical and optical qualities in large quantities using colloidal methods. The optimization of post-synthesis surface passivation schemes6-10 and core-shell syntheses11-14 have enabled non-radiative, charge-carrier relaxation mechanisms to be minimized and near unity PL quantum yields, PL, to be achieved.15 Nevertheless, the efficiencies of PV devices that incorporate these 0D nanostructures are restricted by the imperfect charge hopping that occurs between the QDs packed together in a PV device.16-17 The geometries of one-dimensional (1D) semiconductor nanowires (NWs) intrinsically enable directional charge transport over long distances,18-19 while semiconductor quantum wires (QWs) also offer band-gap tunability through diameter control and quantum-confinement effects.20-22 Strong electron-hole interactions, which are enhanced in QWs over those in the bulk semiconductor, result in large exciton binding energies and the stabilization of photogenerated electron-hole pairs as 1D excitons, even at room temperature.23-25 The form of the particle-in-acylinder quantum-mechanical wavefunctions for electron-hole pairs bound as 1D excitons in ideal QWs with no traps or variations in potential energy along the QWs is 𝛹(𝑟, 𝜃, 𝑍) = 𝛹𝑒 (𝑟, 𝜃)𝛹ℎ (𝑟, 𝜃)𝜙(𝑧𝑒 − 𝑧ℎ )𝑒 ±𝑖𝑘𝑒𝑥𝑐 𝑍 The electron and hole quantum-confinement states are 𝛹𝑒 (𝑟, 𝜃) and 𝛹ℎ (𝑟, 𝜃), and the energies of these states change with the diameter of the QW. The bound electron-hole, hydrogenic states of the 1D excitons are 𝜙(𝑧𝑒 − 𝑧ℎ ), where ze and zh represent the positions of the electrons and holes along the length of the QWs. The free-wave motion of the 1D excitons within the QWs is characterized by the 𝑒 ±𝑖𝑘𝑒𝑥𝑐 𝑍 term, where Z is the center-of-mass position of the exciton along 2 ACS Paragon Plus Environment
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the length of the QWs. kexc is the wave vector of the exciton, which is proportional to the momentum of the exciton, 𝑝𝑒𝑥𝑐 = ℏ𝑘𝑒𝑥𝑐 . It is difficult to synthesize QWs with sufficient quality for electron-hole pairs to behave as ideal particle-in-a-cylinder systems. A good indicator of the quality of a semiconductor QW is PL, the efficiency of spontaneous emission following the photogeneration of an electron-hole pair. In general, QWs with higher irregularities in surface passivation, crystal structure, and diameter will have more non-radiative relaxation pathways and lower PL values than QWs that are nearly ideal. Due to the long lengths of the QWs, the surface areas sampled by photogenerated electron-hole pairs in QWs are significantly larger than in QDs, and the dynamics of 1D excitons within QWs tend to be dominated by non-radiative relaxation pathways that arise from the large number of surface trap sites.26-34 Consequently, the PL values of QWs35-37 are typically
