N-type metal oxide solar cells sensitized by infrared absorbing PbS quantum dots (QDs) represent a promising alternative to traditional photovoltaic devices. However, colloidal PbS QDs capped with pure organic ligand shells suffer from surface oxidation that affects the long term stability of the cells.Application of a passivating CdS shell guarantees the increased long term stability of PbS QDs, but can negatively affect photoinduced charge transfer from the QD to the oxide and the resulting photoconversion efficiency (PCE). For this reason, the characterization of electron injection rates in these systems is very important, yet has never been reported. Here we investigate the photoelectron transfer rate from PbS@CdS core@shell QDs to wide bandgap semiconducting mesoporous films using photoluminescence (PL) lifetime spectroscopy. The different electron affinity of the oxides (SiO 2 , TiO 2 and SnO 2 ), the core size and the shell thickness allow us to fine tune the electron injection rate by determining the width and height of the energy barrier for tunneling from the core to the oxide.

Based on single-nanowire surface photovoltage measurements and finite-element electrostatic simulations, we determine the surface state density, N(s), in individual n-type ZnO nanowires as a function of nanowire diameter. In general, N(s) increases as the diameter decreases. This identifies an important origin of the recently reported diameter dependence of the surface recombination velocity, which has been commonly considered to be independent of the diameter. Furthermore, through the determination of the surface carrier lifetime, we suggest that the diameter dependence of the surface state density accounts for the rather abrupt transition from bulk-limited to surface-limited carrier transport over a narrow nanowire diameter regime (~30-40 nm). These findings are supported by the comparison between bulk-limited and surface-dependent minority carrier diffusion lengths measured at various diameters.

The minority carrier diffusion length, LD, was directly measured in individual ZnO nanowires by a near-field scanning photocurrent microscopy technique. The diameter dependence of LD suggests a diameter-dependent surface electronic structure, particularly an increase in the density of mid-band-gap surface states with the decreasing diameter. This diameter dependence of the surface electronic structure might be a universal phenomenon in wurtzite-type nanostructures, and is critical in interpreting and understanding the effects of surfaces on various material properties.

We show that the amorphization process in phase-change In 2 Se 3 nanowires grown by chemical vapor deposition can be driven by electronic effects and does not require the conventional thermal melt-quench process. In particular, using transmission electron microscopy, in situ single-nanowire Raman spectroscopy, scanning Kelvin probe microscopy, and finite-element simulations, we demonstrate that the electronic amorphization can be achieved under optical excitations at temperatures far below the thermal melting point. The mechanism of this electronic amorphization is likely related to the presence of atomic bonds with different strengths in the crystalline phase In 2 Se 3 and the weakening of the weaker bonds by nonequilibrium electrons. Our findings suggest that In 2 Se 3 is a promising candidate for phase-change memory applications, with potential advantages including energy-efficient memory switching due to the electronic amorphization process and highly stable data storage as a result of a high melting point compared to Ge/Sb−Te alloys. On a more general level, these results indicate the need to take into account the electronic effects in modeling and understanding the phase transition processes in phase-change memories. ■ INTRODUCTIONStructural phase transition in solids is of fundamental interest, as it reflects lattice thermodynamics and kinetics of atomic motion and provides insight into the structure−property relation. From a practical viewpoint, phase-specific material properties can enable phase-change memory applications. In particular, the crystalline−amorphous phase transformation in Ge/Sb−Te alloys and related nanostructures, accompanied by large changes in the optical reflectivity and electrical conductivity, has been extensively studied 1−10 for optical and electronic data storage. It has been the common understanding that amorphization is achieved via thermal melting followed by fast quenching, with the heating provided by intense optical or electric current pulses. However, the amorphization process can also take place in the absence of melting. A recent experimental study 11 based on time-resolved X-ray absorption has shown that nonequilibrium electrons generated by optical excitations can weaken certain types of atomic bonds in Ge/Sb−Te alloys. This electronic effect can lead to lattice instability, which ultimately results in the collapse of the crystalline phase and leads to amorphization at temperatures below the melting point. This is also supported by theoretical studies. 12,13 As the thermal melting is not required, this amorphization process can enable lower energy consumption and faster transformations. 12 Furthermore, the discovery of the electronic amorphization process provides a new perspective in searching for suitable phase-change materials. 13 Particularly, a high melting temperature, which indicates a relatively high crystallization temperature 14 and is thus beneficial for higher amorphous phase stability against self-crystallization, has been so far considered disadvantageous,...

We report the observation of unintentionally incorporated nitrogen-related complexes in ZnO and GaN nanowires grown by the catalytic vapor-phase transport method. In particular, our experimental findings from Raman scattering spectroscopy and mass-selected time-of-flight particle emission measurements suggest the presence of interstitial nitrogen molecules that are formed during the nanowire growth. These results may be relevant for many nanowire systems, emphasizing the necessity of more studies on unintentional impurity incorporations in these nanomaterials.

Using an approach combining scanning thermal microscopy (SThM) and spatially revolved Raman spectroscopy, we have investigated quantitatively the heat dissipation characteristics in substrate-supported and suspended (with asymmetric type of contacts) current-carrying GaN nanowires with diameters of ∼40-60 nm, where the phonon confinement is expected to play an important role in thermal transport. In particular, this approach allows direct measurements of nanowire-substrate/electrode interface thermal resistances and the nanowire thermal conductivity. On the basis of these results, the nanowire-substrate thermal transfer was suggested to be the main heat dissipation route, counting for ∼80-93% of the total dissipated heat, whereas the nanowire-electrode interface plays a minor role. The relative significance of nanowire-substrate/electrode interfaces in dissipating heat was further demonstrated in suspended nanowire devices. The measured nanowire thermal conductivity (∼40-60 W/mK) is lower than that in bulk GaN, possibly due to the phonon confinement and boundary scattering effects. Besides providing quantitative insight into heat dissipation characteristics, our results also reveal aspects, particularly the topography-related thermal signals and the relative significance of various tip-sample thermal transfer processes, that are important to advancing the applications of SThM technique in nanoscale thermal characterizations.

We demonstrate a fast and large area-scalable methodology for the fabrication of efficient dye sensitized solar cells (DSSCs) by simple addition of graphene micro-platelets to TiO2 nanoparticulate paste (graphene concentration in the range of 0 to 1.5 wt%). Two dimensional (2D) Raman spectroscopy, scanning electron microscopy (SEM) and atomic force microscopy (AFM) confirm the presence of graphene after 500 C annealing for 30 minutes. Graphene addition increases the photocurrent density from 12.4 mA cm2 in bare TiO2 to 17.1 mA cm2 in an optimized photoanode (0.01 wt% graphene, much lower than those reported in previous studies), boosting the photoconversion efficiency (PCE) from 6.3 up to 8.8%. The investigation of the 2D graphene distribution showed that an optimized concentration is far below the percolation threshold, indicating that the increased PCE does not rely on the formation of an interconnected network, as inferred by prior investigations, but rather, on increased charge injection from TiO2 to the front electrode. These results give insights into the role of graphene in improving the functional properties of DSSCs and identifying a straightforward methodology for the synthesis of new photoanode

Quantum dots (QDs) represent one of the most promising materials for third-generation solar cells due to their potential to boost the photoconversion efficiency beyond the Shockley-Queisser limit. Composite nanocrystals can challenge the current scenario by combining broad spectral response and tailored energy levels to favor charge extraction and reduce energy and charge recombination. We synthesized PbS/CdS QDs with different compositions at the surface of TiO2 nanoparticles assembled in a mesoporous film. The ultrafast photoinduced dynamics and the charge injection processes were investigated by pump-probe spectroscopy. We demonstrated good injection of photogenerated electrons from QDs to TiO2 in the PbS/CdS blend and used the QDs to fabricate solar cells. The fine-tuning of chemical composition and size of lead and cadmium chalcogenide QDs led to highly efficient PV devices (3% maximum photoconversion efficiency). This combined study paves the way to the full exploitation of QDs in next-generation photovoltaic (PV) devices.