Single-crystal silicon nanowires with diameters approaching molecular dimensions were synthesized using gold nanocluster-catalyzed 1D growth. High-resolution transmission electron microscopy studies show that silicon nanowires grown with silane reactant in hydrogen are single crystal with little or no visible amorphous oxide down to diameters as small as 3 nm. Structural characterization of a large number of samples shows that the smallest-diameter nanowires grow primarily along the 〈110〉 direction, whereas larger nanowires grow along the 〈111〉 direction. In addition, cross-sectional transmission electron microscopy was used to address the importance of surface energetics in determining the growth direction of the smallest nanowires. The ability to prepare well-defined molecular-scale single-crystal silicon nanowires opens up new opportunities for both fundamental studies and nanodevice applications.

Single-source molecular precursors were used to synthesize II-VI compound semiconductor nanowires for the first time. Cadmium sulfide and zinc sulfide nanowires were prepared using cadmium diethyldithiocarbamate, Cd(S2CNEt2)2, and zinc diethyldithiocarbamate, Zn(S2CNEt2)2, respectively, as precursors in a gold nanocluster-catalyzed vapor-liquid-solid growth process. High-resolution transmission electron microscopy studies show that the CdS and ZnS nanowires are single-crystal wurtzite structures with stoichiometric compositions. In addition, photoluminescence measurements demonstrate that these nanowires exhibit high-quality optical properties. The applicability of our approach to the synthesis of other compound and alloy semiconductors nanowires as well as nanowire heterostructures of these materials is discussed.

We report an approach for guiding and manipulating light on sub-wavelength scales using active nanowire waveguides and devices. Quantitative studies of cadmium sulfide (CdS) nanowire structures show that light propagation takes place with only moderate losses through sharp and even acute angle bends. In addition, measurements demonstrate that efficient injection into and modulation of light through nanowire waveguides are achievable in active devices. The ability to inject, guide, and manipulate light on a sub-wavelength scale using nanowire components that can be assembled into integrated structures represents a promising pathway towards integrated nanoscale photonic systems.

The mechanism of lasing in single cadmium sulfide (CdS) nanowire cavities was elucidated by temperature-dependent and time-resolved photoluminescence (PL) measurements. Temperature-dependent PL studies reveal rich spectral features and show that an exciton-exciton interaction is critical to lasing up to 75 K, while an exciton-phonon process dominates at higher temperatures. These measurements together with temperature and intensity dependent lifetime and threshold studies show that lasing is due to formation of excitons and, moreover, have implications for the design of efficient, low threshold nanowire lasers.

We report the controlled synthesis of axial modulation-doped p-type/intrinsic/n-type (p-i-n) silicon nanowires with uniform diameters and single-crystal structures. The p-i-n nanowires were grown in three sequential steps: in the presence of diborane for the p-type region, in the absence of chemical dopant sources for the middle segment, and in the presence of phosphine for the n-type region. The p-i-n nanowires were structurally characterized by transmission electron microscopy, and the spatially resolved electrical properties of individual nanowires were determined by electrostatic force and scanning gate microscopies. Temperature-dependent current−voltage measurements recorded from individual p-i-n devices show an increase in the breakdown voltage with temperature, characteristic of band-to-band impact ionization, or avalanche breakdown. Spatially resolved photocurrent measurements show that the largest photocurrent is generated at the intrinsic region located between the electrode contacts, with multiplication factors in excess of ca. 30, and demonstrate that single p-i-n nanowires function as avalanche photodiodes. Electron-and hole-initiated avalanche gain measurements performed by localized photoexcitation of the p-type and n-type regions yield multiplication factors of ca. 100 and 20, respectively. These results demonstrate the significant potential of single p-i-n nanowires as nanoscale avalanche photodetectors and open possible opportunities for studying impact ionization of electrons and holes within quasi-one-dimensional semiconductor systems.