Electronic transport in films of colloidal CdSe nanocrystals

We present results for electronic transport measurements on large three-dimensional arrays of CdSe nanocrystals. In response to a step in the applied voltage, we observe a power-law decay of the current over five orders of magnitude in time. Furthermore, we observe no steady-state dark current for fields up to 10^6 V/cm and times as long as 2x10^4 seconds. Although the power-law form of the decay is quite general, there are quantitative variations with temperature, applied field, sample history, and the material parameters of the array. Despite evidence that the charge injected into the film during the measurement causes the decay of current, we find field-scaling of the current at all times. The observation of extremely long-lived current transients suggests the importance of long-range Coulomb interactions between charges on different nanocrystals.Comment: 11 pages, 10 figure

Efficient inverse Auger recombination at threshold in CdSe nanocrystals

We apply the semiempirical nonlocal pseudopotential method to the investigation of prospects for direct carrier multiplication (DCM) in neutral and negatively charged CdSe nanocrystals. In this process, known in the bulk as impact ionization, a highly excited carrier transfers, upon relaxation to the band edge, its excess energy Δ to a valence electron, promoting it across the band gap and thus creating two excitons from one. For excess energies just a few meV above the energy gap Eg (the DCM threshold), we find the following: (i) DCM is much more efficient in quantum dots than in bulk materials, with rates of the order of 1010 s-1. In conventional bulk solids, comparable rates are obtained only for excess energies about 1 eV above Eg. (ii) Unlike the case in the bulk, in both neutral and charged nanocrystals the DCM rate is not an increasing function of the excess energy but oscillates as Δ moves in and out of resonance with the energy of the discrete spectrum of these 0D systems, (iii) The main contribution to the DCM rates is found to come from the dot surface, as in the case of Auger multiexciton recombination rates, (iv) Direct radiative recombination of excited electron-hole pairs and phonon-assisted decay are slower than DCM, but (v) the rate of Auger cooling (where the relaxation energy of an excited electron is used to excite a hole into deeper levels) can be of the same order of magnitude as that of the DCM process. Furthermore, for excess energies well above the DCM threshold, the presence of an energy gap within the hole manifold considerably slows DCM compared to Auger cooling (AC), which is not affected by it. Achieving competitive DCM processes will, therefore, require the suppression of Auger cooling, for example, by removing the hole from the dot or by trapping it at the surface

Off-State Quantum Yields in the Presence of Surface Trap States in CdSe Nanocrystals: The Inadequacy of the Charging Model To Explain Blinking

In the charging model the fluorescence intermittency observed in semiconductor nanocrystals is explained in terms of an Auger-mediated quenching of the photoluminescence in the presence of a single additional charge delocalized in the dot core. The validity of this description has however been challenged by recent experimental results. A realistic scenario that is often proposed to explain the origin of the extra charge, portrays the latter as the result of photogeneration followed by trapping of one of the particles at the surface. The effects of the presence of a surface-localized charge were however not considered in any of the models that led to the challenge. This study addresses this fundamental issue. Using ab-initio-uality wavefunctions within the semiempirical pseudopotential approach, the predictions of the charging model are here thoroughly tested for all possible configurations that could exhibit efficient Auger recombination and, therefore, be responsible for the quenching of the PL in the off state. The results, although confirming the conclusion that the presently accepted charging model is inadequate to explain blinking in NCs, suggest multiple charging as a possible origin of off states with very low quantum ield. A modification to one of the recently proposed phenomenological blinking models is suggested to account for this feature