Free-standing PbSe nanocrystals, including quantum wires, multipods, quantum rods, quantum dots, and cubes, were produced in a colloidal solution in the presence of alkyl-diamine solvent at 10−117 °C. The morphology of the nanocrystals was governed by a solvent coordinating molecular template mechanism, which was further adjusted by the temperature and duration of the reaction. Crystalline wires with diameters of approximately 20 nm and lengths of 1−5 μm were formed at the lowest temperatures, while quantum rods (with an aspect ratio of ∼5) and cubes (with 100−500 nm edge) were formed at elevated temperatures.

The optical properties and functionality of air-stable PbSe/PbS core-shell and PbSe/PbSexS1-x core-alloyed shell nanocrystal quantum dots (NQDs) are presented. These NQDs showed chemical robustness over months and years and band-gap tunability in the near infrared spectral regime, with a reliance on the NQD size and composition. Furthermore, these NQDs exhibit high emission quantum efficiencies of up to 65% and an exciton emission band that is narrower than that of the corresponding PbSe NQDs. In addition, the emission bands showed a peculiar energy shift with respect to the relevant absorption band, changing from a Stokes shift to an anti-Stokes shift, with an increase of the NQD diameter. The described core-shell structures and the corresponding PbSe core NQDs were used as passive Q-switches in eye-safe lasers of Er:glass, where they act as saturable absorbers. The absorber saturation investigations revealed a relatively large ground-state cross-section of absorption (sigma gs = 10(-16) - 10(-15) cm2) and a behavior of a "fast" absorber with an effective lifetime of tau eff approximately 4.0 ps is proposed. This lifetime is associated with the formation of multiple excitons at the measured pumping power. The product of sigma gs and tau eff enables sufficient Q-switching performance and tunability in the near infrared spectral regime. The amplified spontaneous emission properties of PbSe NQDs were examined under continuous illumination by a diode laser at room temperature, suitable for standard device conditions. The results revealed a relatively large gain parameter (g = 2.63 - 6.67 cm-1). The conductivity properties of PbSe NQD self-assembled solids, annealed at 200 degrees C, showed an Ohmic behavior at the measured voltages (up to 30 V), which is governed by a variable-range-hopping charge transport mechanism.

For many applications, the presence of oxide on Si nanowires (Si NWs) is undesirable because of the difficulty in controlling the SiO 2 /Si interface properties. Here, we report on the functionalization of 50 nm (in diameter) Si NWs with alkyl chains using a versatile two step chlorination/alkylation process, while preserving the original length and diameter of the NWs. We show that Si NWs terminated with C 1 -C 10 molecules, through Si-C bonds, connect alkyl molecules to 50-100% of the Si atop sites and provide surface stability that depends on the chain length and molecular coverage. These observation were explained by noting that the longer the alkyl chain the higher the concentration of molecule-free pinholes on the Si NW surfaces and, therefore, the easier the oxidation process. Our results provide evidence that alkyl-Si NWs provide stronger Si-C bonds and higher surface stability in ambient conditions than equivalent two-dimensional (2D) Si surfaces having similar or higher initial coverage. The kinetic mechanism of the alkylation process of Si NW surfaces, the oxidation resistance of the modified structures, and the differences from 2D surfaces are discussed in the article.

We report on the functionalization of Si NWs with C 1 -C 6 alkyl chains using a versatile two step chlorination/ alkylation process. We show that Si NWs terminated with C 1 -C 6 molecules, through Si-C bonds, connect alkyl molecules to 50-100% of the Si atop sites and provide surface stability that depends on the chain length and molecular coverage, according to the following order: COur results indicate that the oxidation resistance of (C 1 -C 2 )-Si NWs is significantly higher than equivalent 2D Si(100) surfaces, whereas (C 3 -C 6 )-Si NWs are comparable to 2D (C 3 -C 6 )-Si(100). These discrepancies can be explained as follows: the lower the molecular coverage, the higher the probability for interaction between oxidizing agents (O 2 or H 2 O) and molecule-free sites. Our results are of practical importance when reduced amounts of oxide are required, e.g., for radial epitaxy on NWs to realize vertical P-N junctions for solar cells or for radial Si/Ge superlattices for application in optoelectronics.

Exposed facets of n-type silicon nanowires (Si NWs) fabricated by a top-down approach are successfully terminated with different organic functionalities, including 1,3-dioxan-2-ethyl, butyl, allyl, and propyl-alcohol, using a two-step chlorination/alkylation method. X-ray photoemission spectroscopy and spectroscopic ellipsometry establish the bonding and the coverage of these molecular layers. Field-effect transistors fabricated from these Si NWs displayed characteristics that depended critically on the type of molecular termination. Without molecules the source-drain conduction is unable to be turned off by negative gate voltages as large as -20 V. Upon adsorption of organic molecules there is an observed increase in the "on" current at large positive gate voltages and also a reduction, by several orders of magnitude, of the "off" current at large negative gate voltages. The zero-gate voltage transconductance of molecule-terminated Si NW correlates with the type of organic molecule. Adsorption of butyl and 1,3-dioxan-2-ethyl molecules improves the channel conductance over that of the original SiO(2)-Si NW, while adsorption of molecules with propyl-alcohol leads to a reduction. It is shown that a simple assumption based on the possible creation of surface states alongside the attachment of molecules may lead to a qualitative explanation of these electrical characteristics. The possibility and potential implications of modifying semiconductor devices by tuning the distribution of surface states via the functionality of attached molecules are discussed.

In this paper, the fundamental advantage of highly conductive transparent polymers as hole transport layers in hybrid solar cells is demonstrated. The substantial efficiency improvement of hybrid n-type silicon (n-Si)/poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS) solar cells by adding organic solvents to the polymer dispersion is investigated, and a model that explains reasons and mechanisms for that improvement is given. Open-circuit voltages of 600 mV were measured, which are comparable to conventional diffused silicon pn-junction wafer cells. It is shown by means of X-ray photoelectron spectroscopy that the PEDOT versus PSS ratio plays an important role for charge carrier transport in the PEDOT:PSS layer as well as for charge carrier separation at the n-Si/PEDOT:PSS interface. A shell of insulating PSS segregates at the surface of PEDOT:PSS grains and represents a considerable barrier for charge carrier transport and charge carrier separation, influencing the conductivity of the polymer film and the open-circuit voltage of a processed solar cell, respectively. It could be demonstrated that a mixing of the PEDOT:PSS polymer blend with the organic solvent dimethylsulfoxide reduces the PSS insulator segregation at the surface of PEDOT:PSS grains and improves the performance of hybrid n-Si/PEDOT:PSS solar cells

Ductile relaxation in cracked metal-organic chemical-vapor-deposition-grownA simple self-catalyzed and mask-free approach will be presented to grow GaN rods and nanorods based on the metal-organic vapor phase epitaxy technique. The growth parameter dependent adjustment of the morphology of the structures will be discussed. Rods and nanorods with diameters reaching from a few lm down to 100 nm, heights up to 48 lm, and densities up to 8Á10 7 cm -2 are all vertically aligned with respect to the sample surface and exhibiting a hexagonal shape with smooth sidewall facets. Optical properties of GaN nanorods were determined using cathodoluminescence. It will be shown that the optical properties can be improved just by reducing the Ga precursor flow. Furthermore, for regular hexagonal shaped rods and nanorods, whispering gallery modes with quality factors up to 500 were observed by cathodoluminescence pointing out high morphological quality of the structures. Structural investigations using transmission electron microscopy show that larger GaN nanorods (diameter > 500 nm) contain threading dislocations in the bottom part and vertical inversion domain boundaries, which separate a Ga-polar core from a N-polar shell. In contrast, small GaN nanorods ($200 nm) are largely free of such extended defects. Finally, evidence for a self-catalyzed, Ga-induced vapor-liquid-solid growth will be discussed. V C 2013 AIP Publishing LLC. [http://dx

Individual PbSe nanocrystals (NCs) were prepared by a chemical reaction between lead-cyclohexanebutirate and tri-butyl-phosphine/selenium precursors in a tri-octyl-phosphine oxide/tri-butyl-phosphine surfactant, at 118 °C. Increasing precursor concentration, accompanied by an additional heating, up to 150 °C, enabled the formation of NCs assemblies:  a reaction time duration of 10−60 min led to the formation of monodispersed spherical polycrystalline assemblies whose average diameters ranged between 50−500 nm. A reaction time duration of >60 min enabled the formation of an ordered wire-like assembly, with a typical width of 60−150 nm and a length of 1−5 μm. The assemblies are presumably stabilized by the NCs−NCs dipole−dipole interactions, with an estimated interaction energy of −28 kJ/mol. The electrical measurement of a single wire-like assembly revealed a conductivity of 7.0 Ω-1cm-1.