Nazir P. Kherani

Large‐Scale Synthesis of Ultrathin Bi2S3 Necklace Nanowires

One of the most fascinating areas of nanoscience is the study of one-dimensional nanostructures. From our materials chemistry viewpoint the idea of bringing the size of nanowires down to a point where nanoscale colloidal analogues of polymers can be studied is most stimulating. This direction has been the subject of intense study that promises to develop a new class of materials in which the topological properties of polymers can be coupled with quantum size effects and inorganic crystalline materials. Our strategy goes in a singular direction: instead of connecting nanocrystals by ligand chemistry or dipole interactions we aim to synthesize colloidal nanowires thin enough to display polymer-like behavior. Colloidal chemistry has in recent years shown that virtually any composition can be obtained as colloidal nanocrystals of the most diverse shapes. Much work though still needs to be done in the area of pnictide chalcogenides for which very few syntheses are available for obtaining colloidally stable products and none to our knowledge has shown fully demonstrated quantum size effects. Pnictide chalcogenides (Bi2S3 in particular) are in fact known to show extremely wide changes in the bandgap energy and conductivity with changes in stoichiometry. In the present work we report a “low”-temperature, gramscale route to Bi2S3 nanowires of unprecedented thickness (< 2 nm), with a necklace architecture, strong excitonic features never before seen in bismuth chalcogenides, and extremely high extinction coefficients. The wires are colloidally stable for months even after extensive purification. The nanowires were obtained by injecting a solution of sulfur in oleylamine into a saturated solution of bismuth citrate in oleylamine at 130 8C (see Supporting Information). The nanowires start nucleating immediately after injection and are shown in Figure 1a. The wires are below 2 nm in diameter and display remarkable size uniformity as well as a lack of extensive branching (even though branching points can be seldomly observed). The length of the wires could not be measured rigorously due to their melting under e-beam irradiation, but light scattering measurements indicate their length can be in the order of several microns. Strikingly, the diameter of the wires does not change during growth (Supporting Information), as it will be later shown. In Figure 1b we show a Z-contrast TEM image, which indicates the melting the wires undergo upon e-beam irradiation. The inhomogeneity in the droplet spacing is a hint to the nanowire9s texture that will be highlighted later. The HRTEM image obtained from the wires (Figure 1c) shows some slightly elongated nanocrystals. The fact that only certain parts of the wires show lattice fringes at a given time indicates their polycrystalline microstructure. The lattice spacing that is evidenced in Figure 1c corresponds to the (021) planes of the Bi2S3 structure. The reaction is highly scalable due to the high concentration of the reagents and its high yield (> 60%): multigram quantities can be routinely obtained in a laboratory environment. Figure 1d depicts over 17 grams of nanowires produced in the course of a single reaction (ca. 350 mL). Unlike for single-crystalline ultrathin nanowires, the powder X-ray diffraction (XRD) pattern for the Bi2S3 nanowires (Figure 2a) is devoid of relatively sharp features, which seems to indicate a rather isotropic nanocrystalline component. In ultrathin nanowires, the different coherence lengths along the different crystallographic directions, which are due to the high aspect ratio and small size, give rise to sharp peaks on an broad background. In this case, the XRD pattern is more consistent with a spherical nanocrystal system: Rietveld refinement of the XRD pattern was successfully accomplished by using the Bi2S3 unit cell as well as a spherical particle model with a 1.6 nm size, consistent with the microscopy data (Supporting Information). It is important here not to overestimate the accuracy of Rietveld refinement when it comes to nanocrystal shape determination but it is safe to say that we are not dealing with a one-dimensional single crystal. [*] L. Cademartiri, R. Malakooti, Dr. S. Petrov, Prof. G. A. Ozin Lash Miller Chemical Laboratories Department of Chemistry, University of Toronto 80 St. George Street, Toronto, ON, M5S 3H6 (Canada) Fax: (+1)416-971-2011 E-mail: gozin@chem.utoronto.ca Homepage: http://www.chem.toronto.edu/staff/GAO/group.html

High-efficiency photonic crystal solar cell architecture

Thin silicon solar cells suffer from low light absorption compared to their thick counterparts, especially in the near infra-red regime. In order to obtain high energy conversion efficiency in thin solar cells, an efficient light trapping scheme is required. In this paper, we theoretically demonstrate significant enhancement in efficiency of thin crystalline silicon solar cells by using photonic crystals as the light absorbing layer. In particular, a relative increase of 11.15% and 3.87% in the energy conversion efficiency compared to the optimized conventional design is achieved for 2 microm and 10 microm thicknesses, respectively.

Cross-linking Bi2S3 ultrathin nanowires: a platform for nanostructure formation and biomolecule detection.

This paper describes the use of chemical cross-linking of ultrathin inorganic nanowires as a bottom-up strategy for nanostructure fabrication as well as a chemical detection platform. Nanowire microfibers are produced by spinning a nanowire dispersion into a cross-linker solution at room temperature. Nanomembranes with thicknesses down to 50 nm were obtained by injecting the nanowire dispersion at the cross-linker-solution/air interface. Furthermore, the sensitivity of the nanowire to amine cross-linkers allowed development of a novel sensing platform for small molecules, like the neurotransmitter serotonin, with detection limits in the picomolar regime.

Comparison of uranium and zirconium cobalt for tritium storage

Abstract The utility of ZrCo for tritium storage has been studied and a comparison with uranium has been made. Loading, unloading, and delivery operations typically required in tritium handling loops were conducted using two identical beds: one containing 25.0 g of ZrCo and the other 25.8 g of uranium powder. Hydrogen was the working gas. The two beds have similar performance characteristics although the bed containing uranium has faster loading kinetics and can attain a lower vacuum at room temperature. Particulates are transported into the loop by both beds and appear to be more difficult to contain in the ZrCo case. The temperature ramp rate to the unloading temperature for ZrCo must be controlled if operating at high H/M ratios. A transformation of the ZrCo alloy into a phase which has inferior loading rate characteristics may be induced by rapid temperature ramping.

Organic light-emitting diode microcavities from transparent conducting metal oxide photonic crystals.

We report herein on the integration of novel transparent and conducting one-dimensional photonic crystals that consist of periodically alternating layers of spin-coated antimony-doped tin oxide nanoparticles and sputtered tin-doped indium oxide into organic light emitting diode (OLED) microcavities. The large refractive index contrast between the layers due the porosity of the nanoparticle layer led to facile fabrication of dielectric mirrors with intense and broadband reflectivity from structures consisting of only five bilayers. Because our photonic crystals are easily amenable to large scale OLED fabrication and simultaneously selectively reflective as well as electronically conductive, such materials are ideally suited for integration into OLED microcavities. In such a device, the photonic crystal, which represents a direct drop-in replacement for typical ITO anodes, is capable of serving two necessary functions: (i) as one partially reflecting mirror of the optical microcavity; and (ii) as the anode of the diode.

Selectively Transparent and Conducting Photonic Crystals

[*] Prof. N. P. Kherani, Dr. A. Chutinan The Edward S. Rogers Sr. Department of Electrical and Computer Engineering University of Toronto 10 King’s College Road, Room GB254B Toronto, ON M5S 3G4 (Canada) E-mail: kherani@ecf.utoronto.ca Prof. G. A. Ozin, D. P. Puzzo, L. D. Bonifacio Materials Chemistry Research Group, Department of Chemistry University of Toronto 80 St. George Street, Toronto, ON M5S 3H6 (Canada) E-mail: gozin@chem.utoronto.ca P. G. O’Brien Department of Materials Science and Engineering University of Toronto 184 College Street Room 140, Toronto, ON M5S 3E4 (Canada)

Photoexcited Surface Frustrated Lewis Pairs for Heterogeneous Photocatalytic CO2 Reduction

In this study we investigated, theoretically and experimentally, the unique photoactive behavior of pristine and defected indium oxide surfaces providing fundamental insights into their excited state properties as well as an explanation for the experimentally observed enhanced activity of defected indium oxide surfaces for the gas-phase reverse water gas shift reaction, CO2 + H2 + hν→ CO + H2O in the light compared to the dark. To this end, a detailed excited-state study of pristine and defected forms of indium oxide (In2O3, In2O3-x, In2O3(OH)y and In2O3-x(OH)y) surfaces was performed using time dependent density functional theory (TDDFT) calculations, the results of which were supported experimentally by transient absorption spectroscopy and photoconductivity measurements. It was found that the surface frustrated Lewis pairs (FLPs) created by a Lewis acidic coordinately unsaturated surface indium site proximal to an oxygen vacancy and a Lewis basic surface hydroxide site in In2O3-x(OH)y become more acidic and basic and hence more active in the ES compared to the GS. This provides a theoretical mechanism responsible for the enhanced activity and reduced activation energy of the photochemical reverse water gas shift reaction observed experimentally for In2O3-x(OH)y compared to the thermochemical reaction. This fundamental insight into the role of photoexcited surface FLPs for catalytic CO2 reduction could lead to improved photocatalysts for solar fuel production.

Tailoring the Electrical Properties of Inverse Silicon Opals ‐ A Step Towards Optically Amplified Silicon Solar Cells

electrical conductivity of i-Si-o, and how these material properties correlate with the optical properties of i-Si-o. To reach our objectives, we fabricated i-Si-o by infiltrating the voids of a silica-opal template with Si using chemical vapor deposition (CVD), and subsequently etching the silica spheres with hydrofluoric acid (HF). [19] Accurate measurements of the dc electrical dark conductivities (sd) require electrodes and electrical contacts on the i-Si-o films. Therefore, instead of using glass substrates, which are most commonly employed, we prepared all samples on sapphire substrates due to their high stability against HF etching. Note that the i-Si-o fabricated on glass substrates usually gets detached and becomes free-standing after HF etching, while high- quality intact i-Si-o films can be achieved on sapphire substrates.

High-quality surface passivation of silicon using native oxide and silicon nitride layers

We report on the attainment of high quality surface passivation of crystalline silicon using facile native oxide and plasma enhanced chemical vapour deposition SiNx. Using systematic measurements of excess carrier density dependent minority carrier lifetime, it is observed that the inferred interface defect density decreases with increasing native oxide thickness while the interface charge density remains unchanged with thickness, which ranges from 0.2 A to 10 A. A surface recombination velocity of 8 cm/s is attained corresponding to a native oxide layer thickness of ∼10 A. Similar chemically grown oxide layer followed by SiNx deposition is shown to yield comparable passivation, indicating practical viability of the passivation scheme.

Photonic crystal intermediate reflectors for micromorph solar cells: a comparative study

Wave-optics analysis is performed to investigate the benefits of utilizing Bragg-reflectors and inverted ZnO opals as intermediate reflectors in micromorph cells. The Bragg-reflector and the inverted ZnO opal intermediate reflector increase the current generated in a 100 nm thick upper a-Si:H cell within a micromorph cell by as much as 20% and 13%, respectively. The current generated in the bottom muc-Si:H cell within the micromorph is also greater when the Bragg-reflector is used as the intermediate reflector. The Bragg-reflector outperforms the ZnO inverted opal because it has a larger stop-gap, is optically thin, and due to greater absorption losses that occur in the opaline intermediate reflectors.