Mesoscale CdTe helices with near-unity enantiomeric excess offer insight into design rules for chiroptical semiconductor materials.

We report a new phase in the binary SnS system, obtained as highly symmetric nanotetrahedra. Due to the nanoscale size and minute amounts of these particles in the synthesis yield, the structure was exclusively solved using electron diffraction methods. The atomic model of the new phase (a = 11.7 Å, P2(1)3) was deduced and found to be associated with the rocksalt-type structure. Kramers-Kronig analysis predicted different optical and electronic properties for the new phase, as compared to α-SnS.

Cryogenic "trapping" was used to obtain the first TEM images of self-assembled monolayers of inorganic anions on a gold nanoparticle. This unique structural information makes it possible to study the formation of a protecting-ligand shell at an unprecedented level of detail. The protecting ligands are polyoxometalates (POMs; alpha-X(n+)W(12)O(40)((8-n)-), X(n+) = Al(3+) and "2H(+)", and alpha-X(n+)W(11)O(39)((12-n)-), X(n+) = P(5+), Si(4+), and Al(3+)) with large negative charges for association with the gold surface and W atoms (Z = 74) for TEM imaging. The POM-anion shells were obtained by ligand exchange from citrate-protected 13.8 nm gold nanoparticles. Replacement of the organic (citrate) by inorganic (tungsten-oxide) ligand shells results in substantial changes in the surface plasmon resonance (SPR). By correlating cryo-TEM images with changes in the SPR, degrees of surface coverage were reliably quantified by UV-visible spectroscopy. Then, the kinetics and thermodynamics of ligand-shell formation were investigated by systematically varying POM structure and charge. Rates of POM association with the gold surface ("nucleation") are inhibited by the electric-potential barrier of the citrate-stabilized particles, while binding affinities increase linearly with the charges (from 5- to 9-) of structurally different POM anions, suggesting that no single orientation ("lattice matching") is required for monolayer self-assembly. Time-dependent cryo-TEM images reveal that monolayer growth occurs via "islands", a mechanism that points to cation-mediated attraction between bound POMs. Complete ligand shells comprised of 330 molecules of alpha-AlW(11)O(39)(9-) (1) possess small net charges (29e from zeta-potential measurements) and short Debye lengths (kappa(-1) = 1.0 nm), which indicate that approximately 99% of the 2970 K(+) counter cations lie within ca. 1.5 nm (approximately 3 hydrated K(+) ion diameters) from the outer surface of the POM shell. Energetic analysis of the 1.57 +/- 0.04 nm center-to-center distance between molecules of 1 further indicates that K(+) ions reside in the ca. 4.5 A spaces between the bound ligands. These findings reveal an important structural role for counter cations within POM ligand shells on gold nanoparticles, analogous to that for cations in the monolayer walls of hollow POM-macroanion vesicles.

Transmission and scanning electron microscopies are used to examine the epitaxial lateral overgrowth of GaN on GaN nanocolumns grown on AlN/(0001)sapphire by molecular beam epitaxy. Initially, N-rich growth gave a bimodal morphology consisting of defect-free Ga-polar nanocolumns emanating from a compact, highly defective N-polar layer. Under subsequent Ga-rich conditions, the nanocolumns grew laterally to produce continuous Ga-polar overlayers. Threading dislocation (TD) densities in the overlayer were in the range of 108–109cm−2, up to two orders of magnitude less than in the N-polar underlayer. It is proposed that the change in polarity is a key factor controlling the reduction in TD density.

Introduction The semiconductor single-crystal CVD diamond (ob-tained from the gas phase during homoepitaxial deposi-tion) is a wide band gap semiconductor with a gap width of 5.5 eV. CVD diamond has unique characteristics-high mobility of charge carriers, high carrier saturation speed, high electric breakdown field, the greatest thermal conductivity, high radiation and chemical resistance. On a combination of properties the CVD diamond is superior to other wide band gap semiconductors and is considered a promising material for the creation of a new generation of high-power and high-frequency electronic devices. The main difficulty in realization of the potential of CVD diamond as an electronic material is the problem of creating charge carriers inside it. Compared with conventional semiconductors, dopants in diamond have deeper energy levels that significantly impede the activation of the do-pant (the degree of ionization of the dopant at room temperature is less than 1%). Thus, in order to create an acceptable level of conductivity, it is necessary to increase the level of doping, but in case of boron doping this leads to a decrease of carriers (holes) mobility in diamond. To solve the problem of boron doping of CVD diamond, an approach based on delta-doping technology is known. A thin layer of diamond heavily doped with boron (hav-ing a thickness of 1-2 nm and concentration of boron atoms higher than 5×10 20 cm-3) is formed inside an un-doped defect-free diamond of high quality. To achieve high electronic properties (obtaining high hole mobility and conductivity of the layer), it is necessary to realize sharp boundaries between the doped and undoped materials. Recently, this problem has been successfully solved [1, 2]. This report provides an overview of the results of studies on the growth of electronic-quality epitaxial layers of diamond, the production of heavily boron-doped layers and the study of their characteristics. Experiments The novel microwave plasma assisted CVD reactor for growth of nanometric boron delta-doped layers with ultra-sharp interfaces between doped/undoped materials was built in IAPRAS [1]. Fig. 1 shows a schematic of the reactor. The main features of the reactor are: 1) rapid gas switching; 2) laminar gas flow; 3) axial symmetric resonant mode-symmetric discharge; 4) slow growth of diamond 40-100 nm/h. We achieve rapid gas switching from one input gas to another by a home-made electronic switch. The residence time of the reactor is approximately 5 s. In developed reactor the diamond deposition regimes in which one obtains thin doped delta layers with thickness of 1-2 nm with concentrations of boron about 5·10 20 cm-3 were found. Typical parameters of the delta layer under these conditions are given in Fig. 2 for the SS6-1 sample, in which the boron concentration is 4.8·10 20 cm-3 and thickness is 1 nm. Measurement of the boron concentration in the grown samples was carried out by the secondary ion mass spectroscopy (SIMS) method using a time of flight SIMS setup (IONTOF TOF.SIMS-5). T...