High-Frequency EPR and ENDOR Spectroscopy of Mn2+ Ions in CdSe/CdMnS Nanoplatelets.

Semiconductor colloidal nanoplatelets based of CdSe have excellent optical properties. Their magneto-optical and spin-dependent properties can be greatly modified by implementing magnetic Mn2+ ions, using concepts well established for diluted magnetic semiconductors. A variety of magnetic resonance techniques based on high-frequency (94 GHz) electron paramagnetic resonance in continuous wave and pulsed mode were used to get detailed information on the spin structure and spin dynamics of Mn2+ ions in core/shell CdSe/(Cd,Mn)S nanoplatelets. We observed two sets of resonances assigned to the Mn2+ ions inside the shell and at the nanoplatelet surface. The surface Mn demonstrates a considerably longer spin dynamics than the inner Mn due to lower amount of surrounding Mn2+ ions. The interaction between surface Mn2+ ions and 1H nuclei belonging to oleic acid ligands is measured by means of electron nuclear double resonance. This allowed us to estimate the distances between the Mn2+ ions and 1H nuclei, which equal to 0.31 ± 0.04, 0.44 ± 0.09, and more than 0.53 nm. This study shows that the Mn2+ ions can serve as atomic-size probes for studying the ligand attachment to the nanoplatelet surface.

Creation of Negatively Charged Boron Vacancies in Hexagonal Boron Nitride Crystal by Electron Irradiation and Mechanism of Inhomogeneous Broadening of Boron Vacancy-Related Spin Resonance Lines.

Optically addressable high-spin states (S ≥ 1) of defects in semiconductors are the basis for the development of solid-state quantum technologies. Recently, one such defect has been found in hexagonal boron nitride (hBN) and identified as a negatively charged boron vacancy (VB-). To explore and utilize the properties of this defect, one needs to design a robust way for its creation in an hBN crystal. We investigate the possibility of creating VB- centers in an hBN single crystal by means of irradiation with a high-energy (E = 2 MeV) electron flux. Optical excitation of the irradiated sample induces fluorescence in the near-infrared range together with the electron spin resonance (ESR) spectrum of the triplet centers with a zero-field splitting value of D = 3.6 GHz, manifesting an optically induced population inversion of the ground state spin sublevels. These observations are the signatures of the VB- centers and demonstrate that electron irradiation can be reliably used to create these centers in hBN. Exploration of the VB- spin resonance line shape allowed us to establish the source of the line broadening, which occurs due to the slight deviation in orientation of the two-dimensional B-N atomic plains being exactly parallel relative to each other. The results of the analysis of the broadening mechanism can be used for the crystalline quality control of the 2D materials, using the VB- spin embedded in the hBN as a probe.

High-Frequency EPR and ENDOR Spectroscopy on Semiconductor Quantum Dots.

It is shown that high-frequency electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) spectroscopy are excellent tools for the investigation of the electronic properties of semiconductor quantum dots (QDs). The great attractions of these techniques are that, in contrast to optical methods, they allow the identification of the dopants and provide information about the spatial distribution of the electronic wave function. This latter aspect is particularly attractive because it allows for a quantitative measurement of the effect of confinement on the shape and properties of the wave function. In this contribution EPR and ENDOR results are presented on doped ZnO QDs. Shallow donors (SDs), related to interstitial Li and Na and substitutional Al atoms, have been identified in this material by pulsed high-frequency EPR and ENDOR spectroscopy. The shallow character of the wave function of the donors is evidenced by the multitude of ENDOR transitions of the (67)Zn nuclear spins and by the hyperfine interaction of the (7)Li, (23)Na and (27)Al nuclear spins that are much smaller than for atomic lithium, sodium and aluminium. The EPR signal of an exchange-coupled pair consisting of a shallow donor and a deep Na-related acceptor has been identified in ZnO nanocrystals with radii smaller than 1.5 nm. From ENDOR experiments it is concluded that the deep Na-related acceptor is located at the interface of the ZnO core and the Zn(OH)(2) capping layer, while the shallow donor is in the ZnO core. The spatial distribution of the electronic wave function of a shallow donor in ZnO semiconductor QDs has been determined in the regime of quantum confinement by using the nuclear spins as probes. Hyperfine interactions as monitored by ENDOR spectroscopy quantitatively reveal the transition from semiconductor to molecular properties upon reduction of the size of the nanoparticles. In addition, the effect of confinement on the g-factor of SDs in ZnO as well as in CdS QDs is observed. Finally, it is shown that an almost complete dynamic nuclear polarization (DNP) of the (67)Zn nuclear spins in the core of ZnO QDs and of the (1)H nuclear spins in the Zn(OH)(2) capping layer can be obtained. This DNP is achieved by saturating the EPR transition of SDs present in the QDs with resonant high-frequency microwaves at low temperatures. This nuclear polarization manifests itself as a hole and an antihole in the EPR absorption line of the SD in the QDs and a shift of the hole (antihole). The enhancement of the nuclear polarization opens the possibility to study semiconductor nanostructures with nuclear magnetic resonance techniques.

Observation of the triplet metastable state of shallow donor pairs in AlN crystals with a negative-U behavior: a high-frequency EPR and ENDOR Study.

Theoretical predictions about the n-type conductivity in nitride semiconductors are discussed in the light of results of a high-frequency EPR an ENDOR study. It is shown that two types of effective-mass-like, shallow donors with a delocalized wave function exist in unintentionally doped AlN. The experiments demonstrate how the transformation from a shallow donor to a deep (DX) center takes place and how the deep DX center can be reconverted into a shallow donor forming a spin triplet and singlet states.

Hydrogen: a relevant shallow donor in zinc oxide.

Electron paramagnetic resonance and Hall measurements show consistently the presence of two donors ( D1 and D2) in state-of-the-art, nominally undoped ZnO single crystals. Using electron nuclear double resonance it is found that D1 shows hyperfine interaction with more than 50 shells of surrounding 67Zn nuclei, proving that it is a shallow, effective-mass-like donor. In addition D1 exhibits a single interaction with a H nucleus ( a(H) = 1.4 MHz), thus H is the defining element. It is in agreement with the prediction of Van de Walle [Phys. Rev. Lett. 85, 1012 (2000)] that H acts as a donor in ZnO. The concentration of D1 is 6x10(16) cm(-3) emphasizing its relevance for carrier statistics and applications.