Optically Detected Spin and Orbit Resonance of Semiconductor Quantum Dots

Abstract

The luminescence spectrum of CdSe/CdS core-shell nanocrystals contains a dominant exciton band located at the CdSe core and an additional weak non-excitonic band, associated with trapped carriers. The present paper describes our efforts to identify the influence of CdSe/CdS interfaces on the localization of photogenerated species, utilizing optically detected magnetic spin and spinorbit resonance spectroscopy (ODMR). The spin resonance (SR) spectrum showed two resonance signals: the first associated with a trapped hole in a symmetric site within the core, and a second, corresponding to trapped electrons in an anisotropic interface defect site. The spin-orbit resonance (SOR) signal data are presented. IntroductionNearly two decades of research have explored the benefits of the optical and opto-electronic properties of semiconductor nanocrystals [1]. These materials exhibit unique chemical and physical properties, differing substantially from those of the corresponding bulk solids. Evidently, these differences are associated with quantumsize effects. Moreover, several recent reports have described the development of coreshell type structures [2] in which an internal semiconductor nanocrystal is coated with an epitaxial layer of a different semiconductor. These structures have shown improvements in photoluminescence (PL) quantum yields and photo-oxidative stability, as compared with the core nanocrystals. For instance, the wide band gap (2.5 eV) of CdS and narrower band gap of CdSe (1.8 eV), cause a localization of electron and hole states within the CdSe core, in a similar manner to that of a quantum well structure. While the excitonic transitions in the CdSe/CdS core-shell samples have been investigated in the last few years [3], the understanding of interface influences on the optical properties has been neglected. Indeed, for crystals in such a small size regime, a large percentage of atoms is on/near the internal interfaces or the external surfaces. These sites may act as electron and hole traps. Thus, the present work describes our attempts to clarify the optical properties associated with trapped carriers.

Size-Dependent Effects in Optically Detected Magnetic Resonance Spectra of CdS Nanocrystals

“…It should also be noted that Cox, by changing the ODMR recording wavelength, registered signals from donor–acceptor pairs with different strengths of the exchange interaction, i.e., located at different distances from each other. In addition, a similar doublet signal was also observed in CdS nanoparticles and was explained by the presence of exchange-coupled donor–acceptor pairs. − Therefore, it is natural to assume that the presence of the doublet ODMR signal in the studied composites is also due to the trapping of photoexcited carriers by donors and acceptors coupled by exchange interactions.…”

“…The unique tunability of inherent physical properties and chemical functionalities of colloidal semiconductor nanocrystals (NCs) has been attracting tremendous fundamental and applied research activities in this area over decades. , Among the variety of NC shapes and dimensions attainable via colloidal synthesis, ultrasmall monodisperse NCs, referred to as magic-size clusters, − ,, have been of interest due to fundamental aspects of very strong spatial confinement of excitations and applied aspects of their inherent blue or broad-band white-light luminescence emission promising applications in light-emitting devices. , The broad-band photoluminescence (PL) emission from ultrasmall colloidal NCs strongly resembles the so-called “defect” or “trap” emission band observed in bulk crystals and NCs of larger sizes. ,− The large width of the “trap” emission bands has been traditionally related to a distribution of surface trap levels or donor–acceptor (D–A) pairs over a broad energy range, with the ensemble broadening being of minor importance. PL studies of individual ultrasmall CdSe NCs , indeed revealed that their spectrum is almost as broad as that of an ensemble.…”

“…The nature of the defect states in II–VI NCs has been studied mainly by the PL spectroscopy. − Electron spin resonance (ESR) and optically detected magnetic resonance (ODMR), which probe microscopic signature of defects directly, − have rarely been applied up to now. − ,, In particular, an earlier ODMR study of large (>2 nm) CdS nanoparticles in phosphate glass revealed unusual spectra explained by signals from electron–hole pairs with a strong exchange interaction. , In our recent ODMR study of ∼5 nm CdS NCs in different polymer matrices, namely, the oxygen-containing polymer PVA (polyvinyl alcohol) and oxygen-free polymer PEI (polyethyleneimine), the differences in nanocomposite properties were correlated to different structures of the NC–polymer interfaces . ODMR studies of other quantum confined semiconductor systems (without magnetic dopants) are still very rare …”