Spin-dependent electronic processes and long-lived spin coherence of deep-level trap sites in CdS nanocrystals

Abstract

Carrier trapping in colloidal nanocrystals represents a major energy loss mechanism for excitonic states crucial to devices. Surprisingly little is known about the influence of the spin degree of freedom on the nature of these intrinsic trap centers or the types of coupling that these states experience. Here, a pulsed microwave optically detected magnetic resonance study is presented that aims to probe the interaction pathways existing between shallow band-edge trap states and the deep-level emissive chemical defect states responsible for the broad, low-energy emission common to CdS nanocrystals. Due to long spin coherence times (${T}_{2}$) of these states, Rabi flopping detected in the luminescence under magnetic resonance provides access to information regarding the modes of coupling of shallow-trapped electron-hole pairs, both of isolated species and of those in proximity to the emissive defect. Corresponding optically detected spin-echo experiments expose an extraordinarily long intrinsic spin coherence time (${T}_{2}\ensuremath{\approx}1.6$ \ensuremath{\mu}s) for colloidal nanocrystals, and an electron spin-echo envelope modulation indicative of local spin interactions. This effect provides opportunities for gaining the detailed chemical and structural information needed in order to eliminate energy loss mechanisms during the synthetic process.