Continuous-Wave and Time-Resolved Optically Detected Magnetic Resonance Studies of Nonetched/Etched InP Nanocrystals

Abstract

The low temperature photoluminescence spectrum of InP nanocrystals (NCs) is composed of a near band-edge exciton and a defect emission band at lower energies. The present study characterized the defect emission utilizing an optically detected magnetic resonance spectroscopy (ODMR) both in continuous-wave and time-resolved modes. The continuous-wave ODMR spectra were measured as a function of the laser power, the microwave power, the microwave modulation frequencies, and of the light polarization. The results showed that the defect emission originated from the trap-to-band luminescence of a weakly coupled electron−hole pair, when the electron is trapped at a phosphorus vacancy, Vp (with angular momention of F = 1/2), while the hole is located at the valence band (with F = 3/2). Spin Hamiltonian simulations of the ODMR lineshape, including hyperfine interactions, revealed that the nonetched samples are dominated by Vp at the surface with two adjacent indium atoms. While treatment with hydrogen floride acid eliminates the surface defects, it leaves behind a small percent of Vp at the core of the NCs, with four indium atoms next neighbors at the vertexes of a tetrahedral site. The time-resolved ODMR measurements further distinguished between radiative and nonradiative processes and followed the spin dynamics at the excited state. Kinetic simulations of the PL intensity response to a microwave square wave modulation indicated that the spin−lattice relaxation time and the radiative lifetime of the weakly coupled electron−hole pair are in the microseconds regime.