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 67Zn nuclear spins and by the hyperfine interaction of the 7Li, 23Na and 27Al 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 67Zn nuclear spins in the core of ZnO QDs and of the 1H 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. © 2010 The Author(s)

High-frequency EPR, ESE, and ENDOR spectroscopy of Co- and Mn-doped ZnO quantum dots

Co- and Mn-doped ZnO quantum dots (QDs) with ZnO/Zn(OH)2 core-shell structure were studied using high-frequency electron paramagnetic resonance (EPR), electron spin echo, and electron-nuclear double resonance (ENDOR) at low temperature. The shape of the EPR spectrum of cobalt ions was observed to change as a result of Co2+ coupling with optically created shallow donors (SDs). This, along with a shift of SDs line, is a direct demonstration of interaction between the magnetic ion and donor electron in confined system of ZnO QD. ENDOR resonance of the 1H hydrogen nuclei detected by the EPR signal of Co2+ and Mn2+ evidence the hyperfine coupling between these ions, located in the ZnO core, and the protons outside the quantum dot core in the shell. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Dynamic nuclear polarization of Zn 67 and H1 spins by means of shallow donors in ZnO nanoparticles

Dynamic nuclear polarization (DNP) effects are observed of Zn 67 (I=5/2) nuclear spins in ZnO nanoparticles and of H1 (I=1/2) spins of the Zn (OH) 2 capping layer. The almost complete polarization of these nuclear spins is achieved by saturating the electron paramagnetic resonance transition of the shallow interstitial Li donor present in the ZnO nanoparticles. The remarkable aspect is that this DNP is caused by an Overhauser mechanism although the phonons mediating the polarization process do not fit into the nanoparticles. An explanation of this DNP process is presented, and it is shown that this allows for a measurement of the distribution of phonon modes in the nanoparticles. The enhancement of the nuclear polarization also opens the possibility to study semiconductor nanostructures with NMR techniques. © 2009 The American Physical Society

Проблематика переходу до інформаційного суспільства

Аналізуються фундаментальні передумови, що є первинними в процесі творення інформаційного суспільства. Обґрунтовується теза, що електронна готовність та електронне залучення є основоположними факторами переходу суспільства від індустріального до інформаційного устрою. Подано основні характеристики цих понять та наголошено на їх значенні

Direct Observation of Electron-to-Hole Energy Transfer in CdSe Quantum Dots

Euan Hendry, Mattijs Koeberg, F. Wang, H. Zhang, C. de Mello Donegá, D. Vanmaekelbergh, and Mischa Bonn, Physical Review Letters, Vol. 96, article 057408 (2006). "Copyright © 2006 by the American Physical Society."We independently determine the subpicosecond cooling rates for holes and electrons in CdSe quantum dots. Time-resolved luminescence and terahertz spectroscopy reveal that the rate of hole cooling, following photoexcitation of the quantum dots, depends critically on the electron excess energy. This constitutes the first direct, quantitative measurement of electron-to-hole energy transfer, the hypothesis behind the Auger cooling mechanism proposed in quantum dots, which is found to occur on a 1±0.15 ps time scale

Electron paramagnetic resonance based spectroscopic techniques

© 2014 Springer-Verlag Berlin Heidelberg. All rights reserved. This chapter addresses the use of electron paramagnetic resonance based spectroscopic techniques to study nanostructures. Particular attention is given to high frequency electron spin echo, electron-nuclear double resonance and optically detected magnetic resonance spectroscopy

Electronic Structure of ZnO Quantum Dots Studied by High-Frequency EPR, ESE, ENDOR and ODMR Spectroscopy

© 2016 Elsevier Ltd.High-frequency electron paramagnetic resonance (EPR), electron spin echo (ESE), electron-nuclear double resonance (ENDOR) and optically detected magnetic resonance (ODMR) were applied for the investigation of the electronic properties of ZnO colloidal quantum dots (QDs) which consist of a ZnO nanocrystal core and Zn(OH)2 shell. Shallow donors (SDs) in the form of interstitial atoms of lithium and sodium, as well as substituent of the aluminum have been identified and the spatial distribution of their electronic wave functions has been determined in the regime of quantum confinement. Hyperfine interactions as monitored by ENDOR quantitatively reveal the transition from semiconductor to molecular properties upon reduction of the size of the nanoparticles. We studied the process of charge separation in the quantum dot when excited by UV light resulting in the formation of shallow donors and deep acceptors in the paramagnetic state, which recombine when heated, accompanied by thermoluminescence go back again in the non-paramagnetic state. It was shown that in all studied quantum dots deep acceptors are present positioned near the interface and including one sodium atom. ODMR techniques, which is based on EPR detection via photoluminescence or via tunneling afterglow that can be observed after preliminary X-ray or UV irradiation proved to be very useful to study colloidal ZnO NCs. The higher sensitivity of ODMR via afterglow allowed characterization of ZnO QDs dispersed in transparent media, which is an important advantage, since these systems are more relevant for a number of practical applications. A direct evidence of Co (Mn) interaction with SD in the core and hyperfine coupling with 1H in the shell of QDs has been demonstrated in Mn (Co)-doped ZnO QDs, which are promising classes of diluted magnetic semiconductors

Dynamical nuclear polarization and confinement effects in ZnO quantum dots

The spatial distribution of the electronic wave function of a shallow donor (SD) in a ZnO semiconductor quantum dots (QD's) has been determined in the regime of quantum confinement by using the nuclear spins as probes. Hyperfine (HF) interactions as monitored by electron nuclear double resonance spectroscopy quantitatively reveal the transition from semiconductor to molecular properties upon reduction of the size of the nanoparticles. Influence of confinement effect on g-factor value of SD's in ZnO and CdS QD's was displayed. The almost complete dynamic nuclear polarization (DNP) of nuclear spins has been demonstrated can be achieved in ZnO QD's by saturating the EPR transition of the SD present in the QD's with using high-frequency at low temperatures. Polarization of 67Zn nuclear spins in ZnO core and of 1H nuclear spins in the Zn(OH) 2 capping layer have been obtained which manifests itself via the creation of a hole and an antihole in the EPR absorption line of the SD in QD's and a shift of the hole (antihole). The enhancement of the nuclear polarization opens the possibility to study semiconductor nanostructures with NMR techniques. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Nanomaterial-based Sensors for the Study of DNA Interaction with Drugs

The interaction of drugs with DNA has been searched thoroughly giving rise to an endless number of findings of undoubted importance, such as a prompt alert to harmful substances, ability to explain most of the biological mechanisms, or provision of important clues in targeted development of novel chemotherapeutics. The existence of some drugs that induce oxidative damage is an increasing point of concern as they can cause cellular death, aging, and are closely related to the development of many diseases. Because of a direct correlation between the response, strength/ nature of the interaction and the pharmaceutical action of DNA-targeted drugs, the electrochemical analysis is based on the signals of DNA before and after the interaction with the DNA-targeted drug. Nowadays, nanoscale materials are used extensively for offering fascinating characteristics that can be used in designing new strategies for drug-DNA interaction detection. This work presents a review of nanomaterials (NMs) for the study of drug-nucleic acid interaction. We summarize types of drug-DNA interactions, electroanalytical techniques for evidencing these interactions and quantification of drug and/or DNA monitoring