The polarized optical spectra of the ions Ti3+, V3+, Cr3+, Mn3+, Co3+, and Ni3+ in corundum single crystals have been studied at temperatures from 4.2° to 1200°K. A theory of the band strength based on the point‐charge model and p‐d mixing has been developed and applied to the data with results in fair agreement with experiment. The effects of temperature show that the vibrational‐electronic contribution to band strength is quite small at low temperature but may be appreciable at high temperatures. The crystal‐field parameters have been calculated as convergent lattice sums. The observed trigonal‐field parameter has the opposite sign from that calculated by the point‐charge model if the impurity ion is assumed to occupy an Al3+ ion position in the lattice, but has the same sign as calculated for an ion 0.1 A displaced along the c3 axis toward the empty octahedral site. Details of the spectra have been interpreted as showing that the surroundings of an ion are distorted in some electronic states.

Optical spectra of transition-metal ions in corundum

Semiconductor nanowires have attracted extensive interest as one of the best-defined classes of nanoscale building blocks for the bottom-up assembly of functional electronic and optoelectronic devices over the past two decades. The article provides a comprehensive review of the continuing efforts in exploring semiconductor nanowires for the assembly of functional nanoscale electronics and macroelectronics. Specifically, we start with a brief overview of the synthetic control of various semiconductor nanowires and nanowire heterostructures with precisely controlled physical dimension, chemical composition, heterostructure interface, and electronic properties to define the material foundation for nanowire electronics. We then summarize a series of assembly strategies developed for creating well-ordered nanowire arrays with controlled spatial position, orientation, and density, which are essential for constructing increasingly complex electronic devices and circuits from synthetic semiconductor nanowires. Next, we review the fundamental electronic properties and various single nanowire transistor concepts. Combining the designable electronic properties and controllable assembly approaches, we then discuss a series of nanoscale devices and integrated circuits assembled from nanowire building blocks, as well as a unique design of solution-processable nanowire thin-film transistors for high-performance large-area flexible electronics. Last, we conclude with a brief perspective on the standing challenges and future opportunities.

Nanowire Electronics: From Nanoscale to Macroscale

Liquid-junction dye-sensitized solar cells (DSSCs) based on nanocrystalline titania (TiO2) electrodes constitute a potentially low-cost alternative to traditional inorganic siliconbased photovoltaics and have been studied extensively over the past two decades. Liquid-junction DSSCs now show high short-circuit photocurrent densities (Jsc) and good fill factors (FF) owing to improvements made in the photosensitizer and the titania electrodes. Despite these improvements, a remaining issue of critical importance is the relatively low open-circuit photovoltage (Voc) obtained. The Voc of a liquid-junction DSSC is determined by the energy difference between the quasi-Fermi level (QFL) of the semiconductor and the potential of the redox couple in the electrolyte. For n-type TiO2, the injection of electrons from photoexcited dye molecules raises the QFL towards the conduction band (CB). 5] Thus, the maximum achievable Voc would correspond to the case of a degenerate semiconductor and would therefore equal the energy difference between the TiO2 CB edge and the widely used tri-iodide redox level; this theoretical maximum achievable value of Voc for n-TiO2based DSSCs is 0.95 V. Nevertheless, Voc values of 0.7–0.8 V are typically obtained in reported DSSCs, with the deviation from the theoretical maximum commonly explained by interfacial recombination at the TiO2–dye or TiO2–electrolyte interfaces. Substantial efforts have been made to improve the photovoltage obtained by retarding the recombination losses. For example, a thin overcoat of different insulting metal oxides, such as Nb2O5 and Al2O3, have frequently been used to modify the TiO2 electrode by making a core/shell structure. Several kinds of organic molecular additives, such as deoxycholic acid, 4-guanidinobutyric acid, and 4-tertbutylpyridine (TBP) have also been used in the redox electrolyte. However, these approaches have been found to yield only approximately 50 mV improvement in the open-circuit photovoltage. The intentional incorporation of atomic impurities into semiconducting materials is a common approach for tailoring properties such as band gap or electric conductivity for specific applications. Doping is routinely performed with bulk semiconductors and has recently been extended to nanoscale materials as well. Among nanostructured materials, semiconducting nanowires are widely studied because of their special electrical and optical properties and because it is possible to use them as components in functional devices such as solar cells. Unlike bulk materials, one-dimensional nanowires are usually prepared under non-equilibrium conditions, and it has proved challenging to dope them homogeneously; high-temperature vapor-phase approaches are commonly employed in their synthesis, which are limited in regards to homogeneous doping and alloying because of the high growth temperatures. In contrast, low-temperature hydrothermal synthesis approaches possess an inherent advantage over vapor-phase routes for doping purposes. There have recently been reports on the synthesis of aligned rutile TiO2 nanowire arrays on transparent conducting oxide (TCO) substrates by hydrothermal synthesis; however, no reports of anisotropic transition-metal-doped TiO2 nanowires grown on TCO substrates in the solution phase exist. Herein, we present the synthesis of titania nanowire arrays homogeneously doped with tantalum and prepared under hydrothermal conditions. The synthetic process presented herein should readily be extendable to allow doping of nanowires with different transition metals (e.g., Fe, W, Cr); however, this report is limited to the Ta-doped system. Further, we have translated this advance in materials synthesis into enhanced device performance by demonstrating dye-sensitized solar cells with a very high open-circuit photovoltage of 0.87 V, strikingly close to the theoretical maximum. Figure 1a, b shows field-emission scanning electron microscopy (FESEM) top-surface images of a typical assynthesized nanowire array sample at both low and high magnification. A highly uniform and densely packed array of nanowires is obtained, with an average wire width of approximately 20 nm. Figure 1 c is a cross-sectional view of the same film with a thickness of approximately 3.6 mm, indicating that the nanowires grow almost perpendicularly from the substrate. This finding is confirmed by the X-ray diffraction (XRD) pattern, which shows a remarkably enhanced (002) peak (Figure 1d). XRD patterns indicate the absence of peaks corresponding to the Ta2O5 phase. Owing to the low doping concentration detected by energydispersive X-ray spectroscopy (EDX; 0.83 at%) and to the comparable ionic radii of tantalum (0.064 nm) and titanium ions (0.061 nm), no peak shift was detected after tantalum doping. Figure 1e is a high-resolution TEM (HRTEM) image of the as-prepared nanowire sample, showing the nanowires to be highly crystalline (rutile). The nanowires grow in the (001) direction with the [110] crystal plane parallel to the [*] Dr. X. J. Feng, Dr. K. Shankar, M. Paulose, Prof. C. A. Grimes Materials Research Institute, The Pennsylvania State University University Park, PA 16802 (USA) E-mail: cgrimes@engr.psu.edu

Tantalum-doped titanium dioxide nanowire arrays for dye-sensitized solar cells with high open-circuit voltage.

We report a colloidal synthesis of gallium oxide (Ga(2)O(3)) nanocrystals having metastable cubic crystal structure (gamma phase) and uniform size distribution. Using the synthesized nanocrystal size series we demonstrate for the first time a size-tunable photoluminescence in Ga(2)O(3) from ultraviolet to blue, with the emission shifting to lower energies with increasing nanocrystal size. The observed photoluminescence is dominated by defect-based donor-acceptor pair recombination and has a lifetime of several milliseconds. Importantly, the decay of this phosphorescence is also size dependent. The phosphorescence energy and the decay rate increase with decreasing nanocrystal size, owing to a reduced donor-acceptor separation. These results allow for a rational and predictable tuning of the optical properties of this technologically important material and demonstrate the possibility of manipulating the localized defect interactions via nanocrystal size. Furthermore, the same defect states, particularly donors, are also implicated in electrical conductivity rendering monodispersed Ga(2)O(3) nanocrystals a promising material for multifunctional optoelectronic structures and devices.

Size-Tunable Phosphorescence in Colloidal Metastable γ-Ga2O3 Nanocrystals

Defect assisted improved room temperature ferromagnetism in Ce doped SnO2 nanoparticles

Multiferroics, materials that exhibit coupling between spontaneous magnetic and electric dipole ordering, have significant potential for high-density memory storage and the design of complex multistate memory elements. In this work, we have demonstrated the solvent-controlled synthesis of Cr3+-doped BaTiO3 nanocrystals and investigated the effects of size and doping concentration on their structure and phase transformation using X-ray diffraction and Raman spectroscopy. The magnetic properties of these nanocrystals were studied by magnetic susceptibility, magnetic circular dichroism (MCD), and X-ray magnetic circular dichroism (XMCD) measurements. We observed that a decrease in nanocrystal size and an increase in doping concentration favor the stabilization of the paraelectric cubic phase, although the ferroelectric tetragonal phase is partly retained even in ca. 7 nm nanocrystals having the doping concentration of ca. 5%. The chromium(III) doping was determined to be a dominant factor for destabilization...

Interplay between Size, Composition, and Phase Transition of Nanocrystalline Cr3+-Doped BaTiO3 as a Path to Multiferroism in Perovskite-Type Oxides

Colloidal free-standing Cr3+-doped In2O3 nanocrystals were synthesized in oleylamine from indium (III) and chromium (III) acetylacetonate precursors. The nanocrystals were treated with trioctylphosphine oxide to remove surface-bound dopant ions and ensure internal doping. The lattice resolved transmission electron microscopy images reveal that nanocrystals are faceted and highly crystalline, with no evidence of a secondary phase formation. The average doping concentration estimated with energy dispersive X-ray spectroscopy at the single nanocrystal level agrees with the average doping concentration from the analogous nanocrystal ensemble measurement. Ligand-field electronic absorption spectroscopy suggests that Cr3+ dopants are preferentially substituted for In3+ ions in their trigonally distorted octahedral (b) sites in In2O3 nanocrystals. Nanocrystalline films, prepared under mild conditions using colloidal Cr3+-doped In2O3 nanocrystals as building blocks, exhibit robust room temperature ferromagnetism....

Colloidal Chromium-Doped In2O3 Nanocrystals as Building Blocks for High-TC Ferromagnetic Transparent Conducting Oxide Structures

We report the first synthesis and characterization of cobalt- and chromium-doped GaN nanowires (NWs), and compare them to manganese-doped GaN NWs. Samples were synthesized by chemical vapor deposition method, using cobalt(II) chloride and chromium(III) chloride as dopant precursors. For all three impurity dopants hexagonal, triangular, and rectangular NWs were observed. The fraction of NWs having a particular morphology depends on the initial concentration of the dopant precursors. While all three dopant ions have the identical effect on GaN NW growth and faceting, Co and Cr are incorporated at much lower concentrations than Mn. These findings suggest that the doping mechanism involves binding of the transition-metal intermediates to specific NW facets, inhibiting their growth and causing a change in the NW morphology. We discuss the doping concentrations of Mn, Co, and Cr in terms of differences in their crystal-field stabilization energies (ΔCFSE) in their gas-phase intermediates and in substitutionally...

General control of transition-metal-doped GaN nanowire growth: toward understanding the mechanism of dopant incorporation.

Manganese-doped SnO2 nanocrystals and nanowires with diameters below SnO2 Bohr radius were synthesized by solution methods. X-ray absorption studies reveal that dopant ions are substitutionally incorporated as Mn2+ and Mn3+. Mn2+ is the dominant species at low doping levels, but the fraction of Mn3+ increases with doping concentration. Room-temperature ferromagnetism with the saturation moment of 0.27 μB/Mn is observed for nanocrystalline films containing high fraction of Mn2+ dopant, which is associated with hybridization of Mn2+ d-levels with a donor-impurity band. These results imply the possibility of manipulating magnetic interactions via dopant electronic structure and quantum confinement of the host lattice.

Electronic structure and magnetic properties of sub-3 nm diameter Mn-doped SnO2 nanocrystals and nanowires

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Interplay between Size, Composition, and Phase Transition of Nanocrystalline Cr3+-Doped BaTiO3 as a Path to Multiferroism in Perovskite-Type Oxides

Colloidal Chromium-Doped In2O3 Nanocrystals as Building Blocks for High-TC Ferromagnetic Transparent Conducting Oxide Structures

General control of transition-metal-doped GaN nanowire growth: toward understanding the mechanism of dopant incorporation.

Electronic structure and magnetic properties of sub-3 nm diameter Mn-doped SnO2 nanocrystals and nanowires