There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Two organolead halide perovskite nanocrystals, CH3NH3PbBr3 and CH3NH3PbI3, were found to efficiently sensitize TiO2 for visible-light conversion in photoelectrochemical cells. When self-assembled on mesoporous TiO2 films, the nanocrystalline perovskites exhibit strong band-gap absorptions as semiconductors. The CH3NH3PbI3-based photocell with spectral sensitivity of up to 800 nm yielded a solar energy conversion efficiency of 3.8%. The CH3NH3PbBr3-based cell showed a high photovoltage of 0.96 V with an external quantum conversion efficiency of 65%.

Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells

Single-layer metal dichalcogenides are two-dimensional semiconductors that present strong potential for electronic and sensing applications complementary to that of graphene.

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Electronics and optoelectronics of two-dimensional transition metal dichalcogenides.

Although gold is the subject of one of the most ancient themes of investigation in science, its renaissance now leads to an exponentially increasing number of publications, especially in the context of emerging nanoscience and nanotechnology with nanoparticles and self-assembled monolayers (SAMs). We will limit the present review to gold nanoparticles (AuNPs), also called gold colloids. AuNPs are the most stable metal nanoparticles, and they present fascinating aspects such as their assembly of multiple types involving materials science, the behavior of the individual particles, size-related electronic, magnetic and optical properties (quantum size effect), and their applications to catalysis and biology. Their promises are in these fields as well as in the bottom-up approach of nanotechnology, and they will be key materials and building block in the 21st century. Whereas the extraction of gold started in the 5th millennium B.C. near Varna (Bulgaria) and reached 10 tons per year in Egypt around 1200-1300 B.C. when the marvelous statue of Touthankamon was constructed, it is probable that “soluble” gold appeared around the 5th or 4th century B.C. in Egypt and China. In antiquity, materials were used in an ecological sense for both aesthetic and curative purposes. Colloidal gold was used to make ruby glass 293 Chem. Rev. 2004, 104, 293−346

Gold nanoparticles: assembly, supramolecular chemistry, quantum-size-related properties, and applications toward biology, catalysis, and nanotechnology.

To use solar irradiation or interior lighting efficiently, we sought a photocatalyst with high reactivity under visible light. Films and powders of TiO 2- x N x have revealed an improvement over titanium dioxide (TiO 2 ) under visible light (wavelength 2 has proven to be indispensable for band-gap narrowing and photocatalytic activity, as assessed by first-principles calculations and x-ray photoemission spectroscopy.

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Visible-Light Photocatalysis in Nitrogen-Doped Titanium Oxides

Publisher Summary This chapter discusses some important features of chemically synthesized semiconductor heterostructures. By controlling the methods of preparation it is possible to tailor the photocatalytic property of these composite semiconductor nanoclusters. Their ability to carry out redox processes, with greater efficiency and selectivity than the single component systems has made them potential candidates for the conversion and storage of solar energy, and in the mineralization of chemical pollutants. These systems have potential applications in designing novel microheterogeneous assemblies, microelectronics, and chemical sensors. The feasibility of synthesizing composite semiconductor nanoclusters, by chemical precipitation or electrochemical deposition, opens up a wide array of possibilities, in utilizing these materials for chemical sensors, electrooptics, microelectronics, imaging technology, and photovoltaics. The chapter classifies composite semiconductor nanoclusters into two categories— namely, capped and coupled type heterostructures. The capped nanoclusters essentially have a coreshell geometry, while in a coupled system two semiconductor nanoclusters are in contact with each other. Photo-induced charge transfer processes in single and multicomponent semiconductor nanostructures are discussed in the chapter.

Composite semiconductor nanoclusters

Photoinduced Phase Segregation in Mixed Halide Perovskites: Thermodynamic and Kinetic Aspects of Cl–Br Segregation

The squaraine dye (SQ) anchored onto CdS nanowires serves as a photosensitizing dye and a hole acceptor. This dual role of the squaraine dye has been successfully exploited in a nanowire solar cell to improve the photoconversion efficiency. Electrophoretic deposition of CdS NWs and CdS NWs+SQ composite onto conducting glass electrodes was performed to obtain robust photoanodes and evaluate the photovoltaic performance of nanowire solar cells (NWSCs). Whereas the sensitization property of the SQ extends the response of CdS NWSCs into the near-IR (NIR) region, its redox property facilitates shuttling of holes to the electrolyte and suppressing the charge recombination process. Transient absorption measurements confirm the formation of cation radical of the dye arising from these two processes. The dual role of the squaraine dye has enabled us to improve the power conversion efficiency of NWSCs by a factor of ∼20. Photoelectrochemical, spectroelectrochemical, and spectroscopic measurements provide insight in...

CdS Nanowire Solar Cells: Dual Role of Squaraine Dye as a Sensitizer and a Hole Transporter

Newly synthesized ortho- and para-substituted fullerene−aniline dyads exhibit orientation-dependent electron transfer under photoexcitation. Minimum energy conformation based on molecular mechanics calculations yield folded conformations for ortho-substituted dyads and extended ones for the para-substituted dyads. 1H NMR studies also provided evidence for the folding of the anilinic group in the case of ortho-substituted dyads. Through-space charge transfer processes in these dyads were investigated using steady state fluorescence and lifetime measurements. The decrease in the spatial distance between the donor and acceptor moieties of the ortho-substituted dyads facilitated an efficient electron transfer. The marked increase in the singlet excited state deactivation rate constants and quantum yields of charge separation observed in the case of the ortho-substituted dyads, in polar solvents, further support a topographically controlled electron transfer process.

Orientation-Dependent Electron Transfer Processes in Fullerene−Aniline Dyads

The kinetics and mechanism of the free radical oxidation of the azo dye, Acid Yellow 9, by sulfate radical anions and hydroxyl radicals have been studied using pulse radiolysis and product analysis. Sulfate radicals react via one electron oxidation, generating the dye radical cation, which has an absorption maximum centered on 470 nm. In basic solutions, the radical cation mainly undergoes deprotonation to yield the aminyl radical which has a strong absorption with a maximum centered on 370 nm, and the pKa for the radical cation is 5.5. Formation of a sulfated product, 1, is indicative of a coupling reaction between the radical cation and sulfate radical anions. Studies also indicate that the hydroxyl radicals react with the dye by both electron transfer as well as by adduct formation.

Prashant V. Kamat

Papers

Composite semiconductor nanoclusters

Photoinduced Phase Segregation in Mixed Halide Perovskites: Thermodynamic and Kinetic Aspects of Cl–Br Segregation

CdS Nanowire Solar Cells: Dual Role of Squaraine Dye as a Sensitizer and a Hole Transporter

Orientation-Dependent Electron Transfer Processes in Fullerene−Aniline Dyads

Free radical induced oxidation of the azo dye Acid Yellow 9