Importance of Polaronic Effects for Charge Transport in CdSe Quantum Dot Solids

We developed an accurate model accounting for electron-phonon interaction in colloidal quantum dot supercrystals that allowed us to identify the nature of charge carriers and the electrical transport regime. We find that in experimentally analyzed CdSe nanocrystal solids the electron-phonon interaction is sufficiently strong that small polarons localized to single dots are formed. Charge-carrier transport occurs by small polaron hopping between the dots, with mobility that decreases with increasing temperature. While such a temperature dependence of mobility is usually considered as a proof of band transport, we show that the same type of dependence occurs in the system where transport is dominated by small polaron hopping

Изменчивость биомассы дождевых червей (Lumbricidae) как отклик биоты на различные экологические условия в модельных экспериментах

В експерименті вивчено вплив різних варіантів субстратів, які використовують у лісовій рекультивації, підстилок із листя деревних порід та зволоження на представників грунтових сапрофагів (Lumbricidae). Встановлено достовірний вплив субстратів, підстилок та зволоження на збільшення біомаси дощових черв'яків.Influence of various soil blends used in forest rehabilitation, leaf litters of trees, and humidity levels on soil saprophages (Lumbricidae) is experimentally studied. Significant influence of substrates, litters, and humidity levels on the increase of earthworms' biomass has been determined

Conductive response of a photo-excited sample in a radio-frequent driven resonance cavity

An expression is derived for the perturbative response of a lumped resonance circuit to a sudden change in the circuit parameters. This expression is shown to describe also the photo-induced conductivity of a semiconductor mounted in a single-mode microwave cavity. The power dissipated in the cavity is measured in the two dimensions corresponding to time (after photo-excitation of the sample) and frequency (of the microwave driving source). Analysis of the experimental data for different semiconductor materials demonstrates the general applicability of the presented analytical expression here, by retrieving the time dependence of the sample's photo-induced conductivity.ChemE/Chemical EngineeringApplied Science

From Sphere to Multipod: Thermally Induced Transitions of CdSe Nanocrystals Studied by Molecular Dynamics Simulations

Molecular dynamics (MD) simulations are used to show that a spherical zinc blende (ZB) nanocrystal (NC) can transform into a tetrapod or an octapod as a result of heating, by a local zincblende-to-wurtzite phase transformation taking place in the NC. The partial sphere-to-tetrapod or sphere-to-octapod transition occurs within simulation times of 30 ns and depends on both temperature and NC size. Surprisingly, the wurtzite (WZ) subdomains are not formed through a slip mechanism but are mediated by the formation of highly mobile Cd vacancies on the ZB{111} Cd atomic planes. The total potential energy of a tetrapod is found to be lower than that of a ZB sphere at the same numbers of atoms. The simulation results are in good agreement with experimental highresolution transmission electron microscopy (HR-TEM) data obtained on heated colloidal CdSe NCs

Disorder strongly enhances Auger recombination in conductive quantum-dot solids

Auger recombination (AR) can be an important loss mechanism for optoelectronic devices, but it is typically not very efficient at low excitation densities. Here we show that in conductive quantum-dot solids, AR is the dominant charge carrier decay path even at excitation densities as low as 10?3 per quantum dot, and that AR becomes faster as the charge carrier mobility increases. Monte Carlo simulations reveal that this efficient AR results from charge carrier congregation in ‘Auger hot spots’: lower-energy sites that are present because of energy disorder. Disorder-enhanced AR is a general effect that is expected to be active in all disordered materials. The observed efficient AR is an issue of concern for devices that work at charge carrier densities in excess of ~10?3 charge carriers per quantum dot. At the same time, efficient carrier congregation could be exploited for fast optical switching or to achieve optical gain in the near infrared.ChemE/Chemical EngineeringApplied Science

High charge-carrier mobility enables exploitation of carrier multiplication in quantum-dot films

Carrier multiplication, the generation of multiple electron–hole pairs by a single photon, is of great interest for solar cells as it may enhance their photocurrent. This process has been shown to occur efficiently in colloidal quantum dots, however, harvesting of the generated multiple charges has proved difficult. Here we show that by tuning the charge-carrier mobility in quantum-dot films, carrier multiplication can be optimized and may show an efficiency as high as in colloidal dispersion. Our results are explained quantitatively by the competition between dissociation of multiple electron–hole pairs and Auger recombination. Above a mobility of ~1?cm2?V?1?s?1, all charges escape Auger recombination and are quantitatively converted to free charges, offering the prospect of cheap quantum-dot solar cells with efficiencies in excess of the Shockley–Queisser limit. In addition, we show that the threshold energy for carrier multiplication is reduced to twice the band gap of the quantum dots.ChemE/Chemical EngineeringApplied Science