Continuous and Time Resolved Optically Detected Magnetic Resonance Studies of InP Nanoparticles
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Continuous and Time Resolved Optically Detected Magnetic Resonance Studies of InP Nanoparticles L. Langof, E. Ehrenfreund, and E. Lifshitz Solid State Institute, Technion – Israel Institute of Technology, Haifa 32000, Israel O. I. Micic and A. J. Nozik National Renewable Energy Laboratory, Golden, Colorado 80401, USA ABSTRACT Carriers in small colloidal InP nanoparticles are in strong quantum confinement regime. The low temperature photoluminescence spectrum of InP nanoparticles is composed of an excitonic luminescence at high energies and a non-excitonic defect emission band at lower energies. HF etching of the nanoparticles reduces the defect emission and enhances the exciton process. In this work we apply optically detected magnetic resonance spectroscopy (ODMR) both in continuous wave and time resolved mode (TR-ODMR) to study the defect luminescence in InP nanoparticles. The results show that the defect luminescence originates from weakly coupled electron-hole pair, where the electron is trapped at the surface by phosphorous vacancy, Vp, and the hole is located at the valence band. Additionally, the results suggest that the non-etched samples are dominated by Vp at the surface. Those are mainly eliminated upon HF treatment, leaving behind small percent of Vp in the core of the nanoparticle. We also find the electron-hole exchange interaction from circular polarized ODMR measurements. The TR-ODMR measurement further clarifies the spin dynamics and characteristic of the magnetic sites. Fitting these measurements to the simulated response of the PL intensity to the square wave modulated microwave power revealed that the spin relaxation time and radiative lifetime of electron-hole pair in the nanoparticles are in the microseconds regime. INTRODUCTION In recent years, there has been an increase of interest in the scientific and technological aspects of colloidal semiconductor nanoparticles (NPs). These materials exhibit unique chemical and physical properties, differing substantially from those of the corresponding bulk solids. The special properties are associated with the quantum size effect and the control of surface quality. The impact of the quantum size effect in the III-V NPs is of a special interest, due to their large exciton Bohr radius (10-34 nm) and relatively narrow band gap (0.4-1.5 eV). In such a case, the Bohr radius exceeds the NP diameter, which in turn leads to strong confinement of carriers and a relatively large blue shift of the band edge. Although carrier confinement in colloidal III-V NPs is expected to lead to enhanced PL efficiency, this is not frequently observed, presumably due to the trapping of carriers at the surface and non-radiative recombination. Indeed, Micic et al. [1] showed that the growth of InP colloidal NPs under excess indium produces a red luminescence band, while the growth under excess phosphorous eliminates this band. Furthermore, etching of the samples with HF partially quenches the red luminescence from surface traps. This suggests that the red band corresponds to stoic
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