Electron Confinement Effects in the EPR Spectra of Colloidal n-Type ZnO Quantum Dots

Abstract

Additional unpaired electrons have been introduced into colloidal ZnO quantum dots (QDs) photochemically and investigated by experimental and theoretical methods to test various possible descriptions of their wave functions. For n-type ZnO QDs with diameters between 3.0 and 7.0 nm, electron paramagnetic resonance (EPR) spectroscopy reveals size-dependent g* values in the range 1.960 < g* < 1.968 that are temperature independent and that rule out highly localized wave function descriptions. The size dependence of g* is described well using a k · p perturbation expression, indicating similarity between these quantum confined electrons and free carriers in bulk ZnO. Model calculations confirm that significant electron density can reside on the QD surfaces only for surface well depths that can be experimentally excluded. Together, these results allow a firm experimental description of the photogenerated electrons as delocalized within the conduction bands of the colloidal ZnO QDs, making them excellent candidates for investigation of spin effects in semiconductor nanostructures.

A Review of EPR Studies on Magnetization of Nanoparticles of Dilute Magnetic Semiconductors Doped by Transition-Metal Ions

“…Intrinsic defects in dilute magnetic semiconductors determine their electrical and magnetic properties. Investigation of donor and acceptor centers in ZnO nanocrystals by EPR is, therefore, quite important, and many EPR studies have been reported [19,[22][23][24][25][26][27], described briefly as follows. It was found that Li and Na, which are components of commonly used compounds for ZnO nanoparticle synthesis, act as shallow donors, and are located in the core of ZnO nanoparticle [22].…”

“…On the other hand, H has also been found to act as a shallow donor, being located in Zn(OH) 2 capping shell covering the nanocrystal. In addition, Al, Ga, I also act as donors, whereas N acts as a shallow acceptor [19,24]. The effect of electron confinement was investigated in n-type ZnO quantum dots, in which the g value changes from 1.960 to 1.968 for nanoparticles with the sizes 3-7 nm [24].…”

“…In addition, Al, Ga, I also act as donors, whereas N acts as a shallow acceptor [19,24]. The effect of electron confinement was investigated in n-type ZnO quantum dots, in which the g value changes from 1.960 to 1.968 for nanoparticles with the sizes 3-7 nm [24]. Charge-controllable magnetism was observed in ZnO: Mn, in which the injection of electrons activates Mn 2?…”

Heat treatment effects on the surface morphology and optical properties of ZnO nanostructures

“…There is also significant academic research interest in nano-systems as their properties are remarkably different from their bulk materials due to quantum confinement effect [1,2]. In quantum confinement theory, 1-dimensional confinement such as rods and wires increase carrier transport in light harvesting cells [3,4].…”

Modulation of the electron transfer processes in Au–ZnO nanostructures

“…46 By combining NIR and EPR measurements they unequivocally ascribed the signal at g ∼ 1.96 to free conduction band electrons.…”