Influence of Supersaturation and Spontaneous Catalyst Formation on the Growth of PbS Wires: Toward a Unified Understanding of Growth Modes

High quality stoichiometric lead sulfide (PbS) wires were synthesized by a simple chemical vapor deposition (CVD) process using pure PbS powder as the material source. Growth mechanisms were systematically investigated under various growth conditions, with three modes of growth identified: direct vapor–liquid–solid (VLS) wire growth nucleating from the substrate surface, bulk PbS crystallites by vapor–solid (VS) deposition, and subsequent VLS growth nucleating on top of the bulk deposition through spontaneously formed catalyst particles. Furthermore, we found that these growth modes can be organized in terms of different levels of supersaturation, with VS bulk deposition dominating at high supersaturation and VLS wire growth on the substrate dominating at low supersaturation. At intermediate supersaturation, the bulk VS deposition can form larger crystallites with domains of similarly oriented wires extending from the flat facets. Both predeposited catalysts and spontaneously formed Pb particles were observed as nucleation catalysts, and their interplay leads to various interesting growth scenarios such as reversely tapered growth with increasing diameter. The VLS growth mechanism was confirmed by the presence of Pb-rich caps revealed in an elaborate cross-sectional transmission electron microscopy (TEM) experiment after focused ion beam milling in a modified lift-out procedure. Temperature-dependent photoluminescence (PL) of PbS wires was performed in the mid-infrared wavelength range for the first time, demonstrating strong light emission from band edge, blue-shifted with increasing temperature. The high optical quality of PbS wires may lead to important applications in mid-infrared photonics. The substrate growth temperature as low as 400 °C allows for silicon-compatible processing for integrated optoelectronics applications

Quaternary Alloy Semiconductor Nanobelts with Bandgap Spanning the Entire Visible Spectrum

Quaternary Alloy Semiconductor Nanobelts with Bandgap Spanning the Entire Visible Spectru

Biomedical RANDO standard male phantom.

<p>Biomedical RANDO standard male phantom.</p

The cross section of the phantom.

<p>It has circular holes (arrows) to place the radiation dose meter. We evaluated the image quality about display of the circular hole (for placing a radiation dose meter) and the boundary of the image.</p

The cross section of the phantom.

<p>It has circular holes (arrows) to place the radiation dose meter. We evaluated the image quality about display of the circular hole (for placing a radiation dose meter) and the boundary of the image.</p

The schematic diagram of the SD decrease and CNR increase rates by 50% ASiR and MBIR (compared with those by FBP).

<p>As the mA value decreased, MBIR brought about gradually augmented variations in the two parameters, whereas 50% ASiR did not give rise to noticeable variations.</p

The schematic diagram of CTDIvol variations according to the mA value.

<p>CTDIvol decreases as the mA value decreases.</p

Image noise analysis based on the different reconstruction models.

<p>The schematic diagram of SD and CNR variations according to the mA values. All three reconstruction algorithms showed increased noise as the tube current decreased: FBP presented the most noticeable variations, followed by 50% ASiR and then by MBIR; the three models showed decreased CNRs as the tube current decreased, but at the same tube current, MBIR presented the highest CNR, which was followed by 50% ASiR and then by FBP.</p

A Top-down Approach to Fabrication of High Quality Vertical Heterostructure Nanowire Arrays

We demonstrate a novel top-down approach for fabricating nanowires with unprecedented complexity and optical quality by taking advantage of a nanoscale self-masking effect. We realized vertical arrays of nanowires of 20−40 nm in diameter with 16 segments of complex longitudinal InGaAsP/InP structures. The unprecedented high quality of etched wires is evidenced by the narrowest photoluminescence linewidth ever produced in similar wavelengths, indistinguishable from that of the corresponding wafer. This top-down, mask-free, large scale approach is compatible with the established device fabrication processes and could serve as an important alternative to the bottom-up approach, significantly expanding ranges and varieties of applications of nanowire technology