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

Pulse radiolytically generated (SCN)2•- radicals bind strongly to the gold nanoparticle surface, with an apparent equilibrium constant of 4.7 × 103 M-1. The resultant complex has a strong absorption band at 390 nm which quickly undergoes chemical changes to yield an oxidation product, [Au(SCN)2]-, with an absorption maximum at 320 nm. The spectral changes associated with the chemical interaction between SiO2/Al2O3-stabilized gold nanoparticles and (SCN)2•- are elucidated using time-resolved transient absorption spectroscopy.

Complexation of Gold Nanoparticles with Radiolytically Generated Thiocyanate Radicals ((SCN)2

Photoelectrochemistry in colloidal systems: Interfacial electron transfer between colloidal TiO2 and thionine in acetonitrile

Fluorescence and photoelectrochemical studies of chlorophyll a (Chl a) adsorbed on nanocrystalline SnO2 film were carried out. The results of fluorescence and incident photon to current conversion efficiency (IPCE) as a function of applied bias suggest that the fluorescence quenching and the photocarrier generation are interrelated. Fluorescence quenching has thus been utilized to determine the photogeneration efficiency, η(e), of charges in a SnO2/Chl a based photoelectrochemical cell. A value of 0.75 was obtained for η(e) for unbiased cells. With an IPCE of 13%, η(e) of 75%, and a light harvesting efficiency of 70%, the charge collection efficiency of ∼23% was evaluated. These results suggest that the losses due to the charge recombination are a major factor that limit the efficiency of the cells.

Fluorescence and photoelectrochemical behavior of chlorophyll a adsorbed on a nanocrystalline SnO2 film

Small CdSe colloids (particle diameter 30-50 A) that exhibit quantization effects are prepared in acetonitrile. The authors show intensive transient photobleaching in the 370-500-nm region and transient absorption around 300 and 500-600 nm immediately after bandgap excitation of the semiconductor. The nature and kinetic behavior of the transient photobleaching, which is attributed to the trapped charge carriers, are elucidated by investigating the influence of laser dose and various hole and electron scavengers.

Prashant V. Kamat

Papers

Complexation of Gold Nanoparticles with Radiolytically Generated Thiocyanate Radicals ((SCN)2

Photoelectrochemistry in colloidal systems: Interfacial electron transfer between colloidal TiO2 and thionine in acetonitrile

Fluorescence and photoelectrochemical behavior of chlorophyll a adsorbed on a nanocrystalline SnO2 film

Transient photobleaching of small cadmium selenide colloids in acetonitrile. Anodic decomposition

Photochemistry on surfaces: triplet-triplet energy transfer on colloidal TiO2 particles