Alexey Razin

Optoelectronic properties of polymer-nanocrystal composites active at near-infrared wavelengths

We report a systematic study of the optoelectronic processes occurring in composites made of near-infrared IR emitting nanocrystals and conjugated polymers. We focus on PbSe and InAs/ZnSe blended with polyphenylenevinylene-type polymers. We find that the process responsible for quenching the visible luminescence of the polymer by the nanocrystal varies depending on the nanocrystal composite. Moreover, the high 66% energy-transfer efficiency from the polymer to the PbSe nanocrystal does result in significant emission at the near IR. Our measurements suggest that the host may be doping the PbSe nanocrystal, thus making the nonradiative Auger process favorable. For InAs we find the energy levels well aligned inside the polymer band gap, making it an efficient charge trap which acts as a luminescence center. Through two-dimensional numerical modeling of the charge transport in such composite films we highlight the importance of morphology nanocrystal distribution control.

Water assisted gate induced temporal surface charge distribution probed by electrostatic force microscopy

In this paper, we present a quantitative method to measure charge density on dielectric layers using electrostatic force microscopy. As opposed to previous reports, our method, which is based on force curve measurements, does not require preliminary knowledge of the tip-sample capacitance and its derivatives. Using this approach, we have been able to quantify lateral and temporal SiO2 surface charge distribution and have unveiled a gate-induced charge redistribution mechanism which takes place in the vicinity of grounded electrodes. We argue that this mechanism constitutes a dominant factor in the hysteresis phenomenon, which is frequently observed in the transfer characteristics of nano-scale devices.

Gate-Induced Modification of Water Adsorption on Dielectrics Probed by EFM and Carbon Nanotube FETs

Humidity plays an important role in molecular electronics. It facilitates charge movement on top of dielectric layers and modifies the device transfer characteristics. Using two different methods to probe temporal charge redistribution on the surface of dielectrics, we were able to extract the surface humidity for the first time. The first method is based on the relaxation time constants of the current through carbon nanotube field-effect transistors (CNTFETs), and the second is based on electric force microscopy (EFM) measurements. Moreover, we found that applying external gate biases modifies the surface humidity. A theoretical model based on dielectrophoretic attraction between the water molecules and the substrate is introduced to explain this observation, and the results support our hypothesis. Furthermore, it is found that upon the adsorption of two to three layers of water the surface conductivity saturates.