Highly Enhanced Exciton Recombination Rate by Strong Electron− Phonon Coupling in Single ZnTe Nanobelt Qing Zhang, † Xinfeng Liu, † Muhammad Iqbal Bakti Utama, † Jun Zhang, † María de la Mata, ‡ Jordi Arbiol, ‡,§ Yunhao Lu, ∥ Tze Chien Sum,* ,† and Qihua Xiong* ,†,⊥ † Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371 ‡ Institut de Cie ̀ ncia de Materials de Barcelona, ICMAB-CSIC, Campus de la UAB, 08193 Bellaterra, Catalonia, Spain § Institució Catalana de Recerca i Estudis Avanç ats (ICREA), 08010 Barcelona, Catalonia, Spain ∥ Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310058, China ⊥ Division of Microelectronics, School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798 * S Supporting Information ABSTRACT: Electron−phonon coupling plays a key role in a variety of elemental excitations and their interactions in semiconductor nanostructures. Here we demonstrate that the relaxation rate of free excitons in a single ZnTe nanobelt (NB) is considerably enhanced via a nonthermalized hot-exciton emission process as a result of an ultrastrong electron−phonon coupling. Using time-resolved photoluminescence (PL) spec- troscopy and resonant Raman spectroscopy (RRS), we present a comprehensive study on the identification and the dynamics of free/bound exciton recombination and the electron−phonon interactions in crystalline ZnTe NBs. Up to tenth-order longitudinal optical (LO) phonons are observed in Raman spectroscopy, indicating an ultrastrong electron−phonon coupling strength. Temperature-dependent PL and RRS spectra suggest that electron−phonon coupling is mainly contributed from Light hole (LH) free excitons. With the presence of hot-exciton emission, two time constants (∼80 and ∼18 ps) are found in photoluminescence decay curves, which are much faster than those in many typical semiconductor nanostructures. Finally we prove that under high excitation power amplified spontaneous emission (ASE) originating from the electron−hole plasma occurs, thereby opening another radiative decay channel with an ultrashort lifetime of few picoseconds. KEYWORDS: ZnTe nanobelts, exciton dynamics, electron−phonon coupling, photoluminescence, resonant Raman spectroscopy T he understanding and control of the exciton relaxation properties in semiconductor nanostructures is of fundamental scientific interest because of their direct relevance to the practical applications in linear and nonlinear optoelectronic and photovoltaic devices. 1−6 The exciton relaxation process and associated relaxation dynamics are highly dependent on the band structure, the types of phonons involved, and their respective coupling strength to electrons or excitons. 7−9 In polar semiconductors (such as ZnO, CdS, GaAs, etc.), free excitons are strongly coupled to LO phonons via Frö hlich interaction, which can lead to a much faster carrier radiative rate than the nonpolar semiconductors. 10−12 During a typical intravalley relaxation process, the photoexcited carriers lose their excess energy to crystal lattice by emission of either longitudinal optical (LO) or acoustic phonons, relaxing to the k ∼ 0 momentum states. 7 Since the typical relaxation time for LO and acoustic phonons are on the scale of ∼100 fs to ∼100 ps, respectively, which is much shorter than the exciton recombination time scale (∼1 ns), the emission of a nonequilibrium elementary excitation produces “hot excitons” that have been observed in these polar semiconductors. 13−16 The “hot-exciton” emission considerably enhances the radiative rate of the carriers. However, in most of as-reported II−VI semiconductors such as ZnO and CdS, the exciton decay path is still dominated by thermal-equilibrium recombination processes; thus, the decrease of exciton decay rate by electron−phonon coupling is limited, and the exciton lifetime is on the scale of several hundreds of picoseconds or even longer. Recently, through coating a layer of SiO 2 /Ag shell to CdS nanowire, Cho et al. succeeded in tuning the recombination process in CdS nanowires from one at thermal-equilibrium to a hot-exciton recombination process through the plasmon-enhanced exciton−phonon coupling effect, resulting in a decrease of the exciton lifetime by a factor of ∼1000. 3 Herein we demonstrate a very strong electron− phonon coupling in ZnTe NB with which the radiative rate of a single bare ZnTe NBs is significantly enhanced (∼18 ps), involving the so-called “hot-exciton” emission. Received: October 11, 2012 Revised: November 18, 2012 Published: November 21, 2012 Letter pubs.acs.org/NanoLett © 2012 American Chemical Society 6420 dx.doi.org/10.1021/nl3037867 | Nano Lett. 2012, 12, 6420−6427