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

The singlet and triplet excited states of C{sub 60} and C{sub 70} have been characterized in poly(methyl methacrylate) (PMMA) films by using picosecond and nanosecond laser flash photolysis techniques. The absorption properties and lifetimes of these excited states are very similar to those observed in solutions. When complexed with dimethylaniline (DMA) in the PMMA films, these fullerenes exhibit decreased lifetimes in their excited states. Photoinduced electron transfer between C+{sub 60} (or C{sub 70}) and DMA in the PMMA film was carried out with 532-nm laser pulse excitation. The polymeric environment facilitated the stabilization of electron-transfer products. 38 refs., 10 figs.

Photochemistry of fullerenes. Excited-state behavior of C60 and C70 and their reduction in poly(methyl methacrylate) films

Progress in Perovskite Photocatalysis

Thin films of SnO{sub 2}, nanocrystallites have been surface-modified with thionine, methylene blue, and oxazine 170 by adsorption from the corresponding dye solutions. The strong electrostatic interaction between the cationic dye and the negatively charged semiconductor nanocrystallites results in close packing of the dye on the semiconductor surface. These closely packed H-aggregates of the adsorbed dye are active both electrochemically and photoelectrochemically. Electron transfer from semiconductor nanocrystallites into the adsorbed dye aggregates leads to bleaching of the colored film. The extent of dye bleaching which is readily controlled by the applied potential, has been probed by spectroelectrochemical measurements. The photocurrent action spectra of these dye-modified SnO{sub 2} films indicate charge injection from excited dye aggregate into the semiconductor nanocrystallites with an incident photon-to-photocurrent efficiency of < 1 %.

Electrochemically Active Nanocrystalline SnO2 Films: Surface Modification with Thiazine and Oxazine Dye Aggregates

ConspectusThe optical and electronic properties of metal halide perovskites provide insight into the operation of solar cells as well as their long-term operational stability Halide mobility in perovskite films is an important factor influencing solar cell performance One can visualize halide ion migration through halide exchange between two nanocrystal suspensions or between physically paired films of two different metal halide perovskites The ability to tune band gap by varying halide ratios (Cl:Br or Br:I) allows the synthesis of mixed halide perovskites with tailored absorption and emission across the entire visible spectrum Interestingly, mixed halide (eg, MAPb(Br05I05)3) films undergo phase segregation to form Br-rich and I-rich sites under steady state illumination Upon halting illumination, segregated phases mix to restore original mixed halide compositions Introducing multiple cations (Cs, formamidinium) at the A site or alloying with Cl greatly suppresses halide mobilities Long-term irradiation of MAPb(Br05I05)3 films also cause expulsion of iodide leaving behind Br-rich phases Hole trapping at I-rich sites in MAPb(Br05I05)3 is considered to be an important step in inducing halide mobility in photoirradiated films This Account focuses on halide ion migration in nanocrystals and nanostructured films driven by entropy of mixing in dark and phase segregation under light irradiation

Halide Ion Migration in Perovskite Nanocrystals and Nanostructures.

The goal of this study was to assess the ability of squaraine-rotaxanes to generate singlet oxygen for potential application in photodynamic therapy (PDT). Specifically, we compare the aggregation and photophysical properties of an iodinated squaraine dye and an iodinated squaraine-rotaxane. Even under strongly aggregating conditions, the absorption spectra of both remain relatively sharp. An X-ray crystal structure of the iodinated squaraine dye shows that it adopts perpendicular, end-to-face orientations in the solid state. Singlet oxygen generation efficiency was measured by trapping with 1,3-diphenylisobenzofuran. The triplet state of the rotaxane was characterized using laser flash photolysis. The results of this study suggest that heavily halogenated squaraine-rotaxanes have potential as singlet oxygen photosensitizers for PDT.

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Prashant V. Kamat

Papers

Photochemistry of fullerenes. Excited-state behavior of C60 and C70 and their reduction in poly(methyl methacrylate) films

Progress in Perovskite Photocatalysis

Electrochemically Active Nanocrystalline SnO2 Films: Surface Modification with Thiazine and Oxazine Dye Aggregates

Halide Ion Migration in Perovskite Nanocrystals and Nanostructures.

Singlet oxygen generation using iodinated squaraine and squaraine-rotaxane dyes.