Synthesis of cadmium telluride quantum wires and the similarity of their band gaps to those of equidiameter cadmium telluride quantum dots

High-quality colloidal CdTe quantum wires having purposefully controlled diameters in the range of 5-11 nm are grown by the solution-liquid-solid (SLS) method, using Bi-nanoparticle catalysts, cadmium octadecylphosphonate and trioctylphosphine telluride as precursors, and a TOPO solvent. The wires adopt the wurtzite structure, and grow along the [002] direction (parallel to the c axis). The size dependence of the band gaps in the wires are determined from the absorption spectra, and compared to the experimental results for high-quality CdTe quantum dots. In contrast to the predictions of an effective-mass approximation, particle-in-a-box model, and previous experimental results from CdSe and InP dot-wire comparisons, the band gaps of CdTe dots and wires of like diameter are found to be experimentally indistinguishable. The present results are analyzed using density functional theory under the local-density approximation by implementing a charge-patching method. The higher-level theoretical analysis finds the general existence of a threshold diameter, above which dot and wire band gaps converge. The origin and magnitude of this threshold diameter is discussed

Spectroscopic properties of colloidal indium phosphide quantum wires

Colloidal InP quantum wires are grown by the solution-liquid-solid (SLS) method, and passivated with the traditional quantum dots surfactants 1-hexadecylamine and tri-n-octylphosphine oxide. The size dependence of the band gaps in the wires are determined from the absorption spectra, and compared to other experimental results for InP quantum dots and wires, and to the predictions of theory. The photoluminescence behavior of the wires is also investigated. Efforts to enhance photoluminescence efficiencies through photochemical etching in the presence of HF result only in photochemical thinning or photo-oxidation, without a significant influence on quantum-wire photoluminescence. However, photo-oxidation produces residual dot and rod domains within the wires, which are luminescent. The results establish that the quantum-wire band gaps are weakly influenced by the nature of the surface passivation, and that colloidal quantum wires have intrinsically low photoluminescence efficiencies

Chemical Fabrication Used to Produce Thin-Film Materials for High Power-to- Weight-Ratio Space Photovoltaic Arrays

The key to achieving high specific power (watts per kilogram) space solar arrays is the development of a high-efficiency, thin-film solar cell that can be fabricated directly on a flexible, lightweight, space-qualified durable substrate such as Kapton (DuPont) or other polyimide or suitable polymer film. Cell efficiencies approaching 20 percent at AM0 (air mass zero) are required. Current thin-film cell fabrication approaches are limited by either (1) the ultimate efficiency that can be achieved with the device material and structure or (2) the requirement for high-temperature deposition processes that are incompatible with all presently known flexible polyimide or other polymer substrate materials. Cell fabrication processes must be developed that will produce high-efficiency cells at temperatures below 400 degrees Celsius, and preferably below 300 degress Celsius to minimize the problems associated with the difference between the coefficients of thermal expansion of the substrate and thin-film solar cell and/or the decomposition of the substrate

Ambipolar conduction in transistors using solution grown InAs nanowires with Cd doping

Nanowire field effect transistors have been fabricated using Cd doped InAs nanowires synthesized using a solution-liquid-solid technique. Both n-channel and p-channel characteristics have been observed, which implies that the surface Fermi level is not pinned in the conduction band. The observation of a p channel is attributed to the passivation of surface states by surface ligands introduced during nanowire synthesis and to the effects of heavy acceptor doping. Devices in which the surface ligands are removed by O-2 plasma treatment exhibit only n-channel conduction, which would be consistent with surface Fermi level pinning in the conduction band. (c) 2007 American Institute of Physics

Halometallates Bind as Z‑Type Ligands on Wurtzite CdSe Nanoplatelets

Two-phase ligand exchange of amine-ligated, wurtzite, [CdSe(n-octylamine)0.43(oleylamine)0.07] nanoplatelets with ammonium salts (NH4Cl or [NH4]2[ZnCl4]) or the Lewis acids (ZnCl2 or CdCl2) in N-methylformamide/hexane afford NMF-ligated [CdSe(NMF)0.18(n-octylamine)0.09] nanoplatelets. Subsequent ligand exchanges with the intermediate NMF-ligated nanoplatelets give halometallate-ligated CdSe nanoplatelets having CdX3– or ZnX42– ligands (X = Cl, Br, and I). Two-phase ligand exchange of [CdSe(n-octylamine)0.43(oleylamine)0.07] nanoplatelets with ammonium salts [NH4][CdX3] in N,N-dimethylformamide afford halometallate ligation directly. Analysis of the absorption spectra and X-ray diffraction data of the halometallate-ligated nanoplatelets establish that the halometallates bind as intact, Z-type ligands

Silver Chloride as a Heterogeneous Nucleant for the Growth of Silver Nanowires

Various additives are employed in the polyol synthesis of silver nanowires (Ag NWs), which are typically halide salts such as NaCl. A variety of mechanistic roles have been suggested for these additives. We now show that the early addition of NaCl in the polyol synthesis of Ag NWs from AgNO₃ in ethylene glycol results in the rapid formation of AgCl nanocubes, which induce the heterogeneous nucleation of metallic Ag upon their surfaces. Ag NWs subsequently grow from these nucleation sites. The conclusions are supported by studies using <i>ex situ</i> generated AgCl nanocubes

Crystal-Phase Control by Solution–Solid–Solid Growth of II–VI Quantum Wires

A simple and potentially general means of eliminating the planar defects and phase alternations that typically accompany the growth of semiconductor nanowires by catalyzed methods is reported. Nearly phase-pure, defect-free wurtzite II–VI semiconductor quantum wires are grown from solid rather than liquid catalyst nanoparticles. The solid-catalyst nanoparticles are morphologically stable during growth, which minimizes the spontaneous fluctuations in nucleation barriers between zinc blende and wurtzite phases that are responsible for the defect formation and phase alternations. Growth of single-phase (in our cases the wurtzite phase) nanowires is thus favored

Reversible Exchange of L‑Type and Bound-Ion-Pair X‑Type Ligation on Cadmium Selenide Quantum Belts

CdSe quantum belts of composition {CdSe­[<i>n</i>-octylamine]_0.53} and protic acids HX (X = Cl, Br, NO₃, acetate (OAc), and benzoate (OBz)) react to exchange the L-type amine ligation to bound-ion-pair X-type ligation. The latter ligation has X^– anions bound to the nanocrystal surfaces and closely associated LH⁺ counter-cations (protonated <i>n</i>-octylamine or tri-<i>n</i>-octylphosphine (TOP) to balance the surface charges. The compositions of the exchanged QBs are {CdSe­[Br]_0.44[<i>n</i>-octylammonium]_0.41}, {CdSe­[NO₃]_0.10[TOPH]_0.12}, {CdSe­[OBz]_0.08[<i>n</i>-octylammonium]_0.02[TOPH]_0.06}, and {CdSe­[OAc]_0.16[<i>n</i>-octylammonium]_0.02[TOPH]_0.14}. (The HCl-exchanged QBs are insufficiently stable for elemental analysis.) The bound-ion-pair X-type ligation is fully reversed to L-type <i>n</i>-octylamine ligation in the cases of X = NO₃, acetate, and benzoate. The ligand exchanges are monitored by absorption spectroscopy, and the exchanged, bound-ion-pair X-type ligated nanocrystals are characterized by a range of methods