Basic materials physics of transparent conducting oxides

Materials displaying the remarkable combination of high electrical conductivity and optical transparency already from the basis of many important technological applications, including flat panel displays, solar energy capture and other opto-electronic devices. Here we present the basic materials physics of these important materials centred on the nature of the doping process to generate n-type conductivity in transparent conducting oxides, the associated transition to the metallic (conducting) state and the detailed properties of the degenerate itinerant electron gas. The aim is to fully understand the origins of the basic performance limits of known materials and to set the scene for new or improved materials which will breach those limits for new-generation transparent conducting materials, either oxides, or beyond oxides.     

Microwave plasma synthesis of lanthanide zirconates from microwave transparent oxides

Lanthanide zirconate phases Ln(2)Zr(2)O(7) and Ln(4)Zr(3)O(12) (Ln = Y, La, Gd, Dy, Ho, Yb) have been prepared using a microwave induced plasma methodology, which allows rapid synthesis using materials which do not couple directly with microwaves at room temperature. We describe the measurement of heating profiles of the precursor binary metal oxides which can be used to identify conditions conducive to the synthesis of more complex oxides. Uncontrolled heating which can be a feature of microwave synthesis of ceramics is not observed, allowing reproducible synthesis. Conventionally these phases are prepared at >1400 °C over hours or days and are being investigated for applications including the immobilisation of nuclear waste where rapid processing is important. Using the microwave plasma method, phase-pure materials have been prepared in minutes. Furthermore, it is clear that Ln(2)Zr(2)O(7) and Ln(4)Zr(3)O(12) also exhibit significant plasma-promoted dielectric heating (e.g. >2200 °C for Dy(4)Zr(3)O(12)) which is typically greater than either of the respective precursors, thus providing a driving force to rapidly complete the reaction.     

Toward transparent molecular wires: electron and energy transfer in transition metal derivatized conducting polymers

A great deal of research has concentrated on long range electron and energy transport in transition metal-based systems, including molecular donor-acceptor assemblies, electron and energy transfer cascades, dendrimers, and derivatized polymer systems. In an effort to improve efficiencies for electron and energy transport over large distances, several groups have now turned to conjugated systems. Several challenges exist to incorporating conducting materials/polymers in the study of photoinduced electron and energy transfer: solubility and processibility of the materials, thermal stability and limitations on direct spectroscopic characterization due to band gap absorptions. We have prepared a new series of conducting materials that provides for direct incorporation of chromophores and electrophores within the backbone of a conducting polymer. Energy transfer dynamics between conducting polymer bridges and porphyrin or metal-to-ligand charge transfer (MLCT) chromophores can be controlled through intermolecular interactions in solid vs solution samples. We have also developed a methodology to incorporate transmissive benzothiophene-type polymers such as polyisothianaphthene (PITN) within a copolymer assembly. These new materials are now being used to investigate long range electronic coupling and have potential applications that range from artificial photosynthesis to light emitting diodes.     

Low-temperature emission of singlet oxygen from the surface of transition metal oxides

Low-temperature emission of the singlet form of dioxygen ¹O₂ from the surface of the individual and mixed V and Mo oxides was detected by thermal desorption at 20–350°C. The amount of the desorbing ¹O₂ and the temperature range of ¹O₂ emission were found to depend on the composition of the catalyst; the low-temperature ¹O₂ form can be regenerated after its contact with oxygen.     

Hybrid photoactive assemblies: electron injection from host-guest complexes into semiconductor nanoparticles

Novel ternary assemblies consisting of fully encapsulated host-guest complexes (hemicarceplexes) and wide band gap semiconductor nanoparticles were investigated. The water-soluble amphiphilic host (octacarboxyhemicarcerand) traps the hydrophobic chromophore within its cavity and binds to the surface of metal oxide nanoparticles. Fluorescence quenching and fast charge injection, kforward >/= 7 x 109 s-1, from the S2 state of encapsulated azulene were observed. Charge recombination proceeds at a much lower rate of 2 x 107 s-1. Interestingly, the recombination kinetics is homogeneous, suggesting that electron tunneling through the wall of the "molecular container" is the rate-limiting step of the process.     

Dopant ion size and electronic structure effects on transparent conducting oxides. Sc-doped CdO thin films grown by MOCVD

A series of Sc-doped CdO (CSO) thin films have been grown on both amorphous glass and single-crystal MgO(100) substrates at 400 degrees C by MOCVD. Both the experimental data and theoretical calculations indicate that Sc3+ doping shrinks the CdO lattice parameters due to its relatively small six-coordinate ionic radius, 0.89 angstroms, vs 1.09 angstroms for Cd2+. Conductivities as high as 18100 S/cm are achieved for CSO films grown on MgO(100) at a Sc doping level of 1.8 atom %. The CSO thin films exhibit an average transmittance >80% in the visible range. Sc3+ doping widens the optical band gap from 2.7 to 3.4 eV via a Burstein-Moss energy level shift, in agreement with the results of band structure calculations within the sX-LDA (screened-exchange local density approximation) formalism. Epitaxial CSO films on single-crystal MgO(100) exhibit significantly higher mobilities (up to 217 cm2/(V x s)) and carrier concentrations than films on glass, arguing that the epitaxial CSO films possess fewer scattering centers and higher doping efficiencies due to the highly textured microstructure. Finally, the band structure calculations provide a microscopic explanation for the observed dopant size effects on the structural, electronic, and optical properties of CSO.     

Using Prussian blue analogue nanoparticles confined into ordered mesoporous silica monoliths as precursors of oxides

Powdered Prussian blue analogues (PBAs) and PBAs confined in ordered mesoporous silica monoliths were used as oxide precursors through thermal treatment under an oxidizing atmosphere. The study focuses on the transformation of the alkali cation-free CoCo PBA of chemical formula K_0.1Co^II₄[Co^III(CN)₆]_2.7·20 H₂O. The compounds were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), IR spectroscopy and small-angle X-ray scattering (SAXS), and the magnetic properties of the calcined samples were investigated. In both cases, powdered and confined PBAs, the coordination polymers are transformed into well-crystallized Co₃O₄ spinel oxide. In the case of the confined PBA, isolated Co₃O₄ single crystals confined within the ordered mesoporosity of the monoliths were evidenced by HRTEM. A preliminary study shows an effect of particle size and confinement on the magnetic properties of the confined oxide particles.     

Reductive electron injection into duplex DNA by aromatic amines

An assay based on photoinduced reaction and subsequent cleavage of duplex DNA containing a bromodeoxyuridine ((Br)U) residue and an abasic site was developed to screen aromatic amines for their ability to initiate charge transfer by reductive electron donation. Two candidates, N,N,N',N'-tetramethyl-1,5-diaminonaphthalene (TMDN) and 1,5-diaminonaphthalene (DAN), expressed the desired activity, and an oligodeoxynucleotide-TMDN conjugate was subsequently prepared to identify additional variables affecting the efficiency of electron injection and transfer into DNA. This system demonstrated only mild sensitivity to molecular oxygen but was strongly inhibited by high concentrations of 2-mercaptoethanol. The nucleobase counter to the attached TMDN strongly modulated charge transfer as evident by a 60-fold decrease in reduction of the distal (Br)U when the counterbase A was substituted for C. An inverse relationship between this reduction and quenching of TMDN fluorescence by the counterbase was also discovered and is consistent with a competition between radical recombination and electron migration away from the initial site of its injection into DNA.     

Evolution of water vapor from indium-tin-oxide transparent conducting films fabricated by dip coating process

Tin-doped indium oxide In₂O₃ (indium-tin-oxide) transparent conducting films were fabricated on silicon substrates by a dip coating process. The thermal analysis of the ITO films was executed by temperature-programmed desorption (TPD) or thermal desorption spectroscopy (TDS) in high vacuum. Gas evolution from the ITO film mainly consisted of water vapor. The total amount of evolved water vapor increased on increasing the film thickness from approx. 25 to 250 nm and decreased by increasing the preparation temperature from 365 to 600°C and by annealing at the same temperature for extra 10 h. The evolution occurred via two steps; the peak temperatures for 250 nm thick films were approx. 100-120 and 205-215°C. The 25 nm thick films evolved water vapor at much higher temperatures; a shoulder at approx. 150-165°C and a peak at approx. 242°C were observed. The evolution temperatures increased by increasing the preparation and the annealing temperatures except in case of the second peak of the 25 nm thick films. The evolution of water vapor at high temperature was tentatively attributed to thermal decomposition of indium hydroxide, In(OH)₃, formed on the surface of the nm-sized ITO particles. This revised version was published online in August 2006 with corrections to the Cover Date.     

Incorporation of Znq2 complexes into mesoporous silica and their transparent polymer luminescent nanocomposites

Znq₂-functionalized colloidal mesoporous silicas (Znq₂-CMS)/polymer transparent nanocomposites were prepared by in situ bulk polymerization. CMS nanoparticles or nanorods with hydroxyl-, mercapto- and sulfonic-functionalized interiors were obtained by different synthetic routes in the nanosize dimensions between 50 and 500 nm. The luminescent Znq₂ complex was successfully introduced in the pores of different mesoporous silicas by chemical adsorption as the driving force. The different internal circumstances of mesoporous silicas had an obvious effect on the luminescence and lifetime of Znq₂ complex. The transparent fluorescent nanocomposites were fabricated from different Znq₂-CMS and suitable monomers. The Znq₂-CMS were uniformly dispersed in the polymer matrix without evident aggregation. The photoluminescence properties of Znq₂-CMS in the transparent matrix exhibited a dependence on the inner surrounding of CMS due to the interaction between Znq₂-CMS and polymers. The maximum emission peak of the nanocomposites had a red-shift of 28 nm as compared to pure Znq₂-CMS.     

Subpicosecond photoinduced charge injection from "molecular tripods" into mesoporous TiO2 over the distance of 24 angstroms

Extended rigid tripodal sensitizers were used to investigate the rate of long-distance photoinduced charge transfer from the MLCT excited states of RuII-based chromophores into mesoporous TiO2 films. The distance between the RuII center and the surface of the semiconductor was 24 A. Rapid biexponential charge injection with a major subpicosecond component as fast as 240 fs was observed upon femtosecond laser excitation of the tripods bound to the TiO2 surface. This rate exceeds the typical rates of vibrational cooling and thus strongly supports the possibility of "hot electron injection" occurring at very large donor-to-semiconductor distances.     

A comparative study on photo-induced electron transfer from fluorescein to anthraquinone and injection into colloidal TiO2

A dyad-anthraquinone-methyl ester of fluorescein-and its model compound-butyl ester-were synthesized. The effects of photo-induced electron transfer from fluorescein to an organic anthraquinone acceptor and injection into inorganic colloidal TiO(2) were studied respectively. It is found that the photo-induced electron transferring to an organic acceptor is much faster than injecting into inorganic colloidal particles when fluorescein was excited by visible light. While inorganic colloidal TiO(2) was excited by UV, the electron of fluorescein will inject into TiO(2).     

Photoacoustic measurement of electron injection efficiencies and energies from excited sensitizer dyes into nanocrystalline TiO2 films

Time-resolved photoacoustic calorimetry is used to measure the energy released upon injection of an electron from an electronically excited dye adsorbed to nanocrystalline TiO2 into the conduction band of this material. More energy is released when the environment of the dye is made less polar, because the energy of the dye-oxidized state has a more pronounced solvent dependence than the edge of the conduction band of the TiO2 semiconductor. Such energy dependences should be considered in the design of more efficient dye-sensitized solar cells.     

Inhomogeneity of electron injection rates in dye-sensitized TiO2: comparison of the mesoporous film and single nanoparticle behavior

As it has been shown by pump-probe experiments electron injection at the interface between a dye molecule and mesoporous TiO2 proceeds with rates exceeding 1 x 10(13) s(-1). However, similar dye-TiO2 systems exhibit residual dye emission with lifetimes extending into the long nanosecond range. To address this inhomogeneity of injection rates time-correlated single photon counting microscopy was used to compare the emission behavior of dye-sensitized mesoporous films of TiO2 with that of individual anatase nanoparticles that had undergone extensive dialysis. The sensitized films produce intense residual emission with multiexponential decay components as long as 220 ns. The channels of mesoporous films contain physisorbed and trapped dye, which is the dominant source of the emission. It is likely that the wide range of lifetimes reflects the distribution of mean free paths experienced by the loose dye molecules diffusing within the film prior to undergoing oxidative quenching. In contrast, the intensity of emission from individual nanoparticles from which the loose dye was removed by dialysis is orders of magnitude lower. The lifetimes obtained from such particles are much shorter, with the primary component on a sub-nanosecond time scale. The presence of residual emission with a 230 ps lifetime shows that even on the surfaces of dialyzed nanoparticles there is a fraction of sensitizer molecules that do not inject electrons with the same high rate as is observed in ultrafast pump-probe experiments on films. Since the physisorbed dye was removed from these samples by dialysis, the residual emission is likely to originate from dye molecules bound to surface defects. Unusual collective emission bursts were observed in some of the measurements on sensitized nanoparticles. We attribute this behavior to stimulated emission from individual nanocrystallites.     

The double window for electron beam injection into the flue gas process vessel

The double window configuration for electron beam injection into the flue gas treatment process vessel applied in Polish Pilot Plant was described. The effectiveness of such a system was discussed and flue gas dosimetry results were presented. Approximately 64% of total beam power with initial electron energy 0.7 MeV was delivered into the gas phase due to the losses in double window (two 50 μm titanium foils and 70 mm air gap between them) and process vessel definite diameter 1.6 m.