Switchable Induced-Transmission Filters Enabled by Vanadium Dioxide.

An induced-transmission filter (ITF) uses an ultrathin metallic layer positioned at an electric-field node within a dielectric thin-film bandpass filter to select one transmission band while suppressing other bands that would have been present without the metal layer. We introduce a switchable mid-infrared ITF where the metal can be "switched on and off", enabling the modulation of the filter response from a single band to multiband. The switching is enabled by the reversible insulator-to-metal phase transition of a subwavelength film of vanadium dioxide (VO2). Our work generalizes the ITF─a niche type of bandpass filter─into a new class of tunable devices. Furthermore, our fabrication process─which begins with thin-film VO2 on a suspended membrane─enables the integration of VO2 into any thin-film assembly that is compatible with physical vapor deposition processes and is thus a new platform for realizing tunable thin-film filters.

Two-dimensional carbon leading to new photoconversion processes.

Two-dimensional (2D) carbon allotropes, which are atomic thick layers made of network carbon atoms with hexagonal structured lattices, have been neglected until the direct investigation of mechanically exfoliated graphene by Novoselov et al. in 2004. Graphene is a 2D carbon allotrope with a unique structure of hexagonally arranged atoms that give it unparalleled electrical conductivity and carrier mobility, in addition to excellent mechanical flexibility and extremely high specific surface area. Graphene and its derivatives have been extensively studied for photovoltaic and photocatalytic applications due to their inherent nature to extract and transport charges from photon-absorbing semiconductors and conjugated polymers. Graphyne and graphdiyne, 2D carbon allotropes like graphene but containing not only doubly but also triply bonded carbon atoms, are predicted to possess intrinsic semiconductor bandgap and even more superior electrical properties than graphene. The current theoretical understanding and experimental status of graphyne and graphdiyne will be discussed in contrast of graphene, demonstrating those promising competitors to graphene in further lightening a new photoconversion. This review addresses the recent successes and current challenges of graphene, graphyne and graphdiyne, and provides insightful perspectives for the future applications of 2D carbon materials in photoelectric conversion and photocatalysis.

Quintuple-shelled SnO(2) hollow microspheres with superior light scattering for high-performance dye-sensitized solar cells.

Quintuple-shelled SnO2 hollow microspheres are prepared by a hard-template method. DSSCs constructed with SnO2 multi-shell photoanodes show a record photoconversion efficiency of 7.18% due to enhanced light scattering. SnO2 hollow microspheres that are utilized as a scattering layer on top of P25 films increase the DSSC photoconversion efficiency from 7.29% to 9.53%.

Ordered structure rearrangements in heated gold nanocrystal superlattices.

Small-angle X-ray scattering (SAXS) data reveal that superlattices of organic ligand-stabilized gold (Au) nanocrystals can undergo a series of ordered structure transitions at elevated temperature. An example is presented of a body-centered cubic superlattice that evolves into a hexagonal close-packed structure, followed by the formation of binary simple cubic AB13 and hexagonal AB5 superlattices. Ultimately the superlattice decomposes at high temperature to bicontinuous domains of coalesced Au and intervening hydrocarbon. Transmission electron microscopy revealed that the ordered structure transformations result from partial ligand desorption and controlled Au nanocrystal growth during heating, which forces changes in superlattice symmetry. These observations suggest some similarity between organic ligand-coated nanocrystals and microphase-segregated diblock copolymers, where thermally induced nanophase-segregation of Au and organic ligand influences the ordered arrangements in the superlattice.

Colloidal luminescent silicon nanorods.

Silicon nanorods are grown by trisilane decomposition in hot squalane in the presence of tin (Sn) nanocrystals and dodecylamine. Sn induces solution-liquid-solid nanorod growth with dodecylamine serving as a stabilizing ligand. As-prepared nanorods do not luminesce, but etching with hydrofluoric acid to remove residual surface oxide followed by thermal hydrosilylation with 1-octadecene induces bright photoluminescence with quantum yields of 4-5%. X-ray photoelectron spectroscopy shows that the ligands prevent surface oxidation for months when stored in air.

Room temperature hydrosilylation of silicon nanocrystals with bifunctional terminal alkenes.

H-terminated Si nanocrystals undergo room temperature hydrosilylation with bifunctional alkenes with distal polar moieties-ethyl ester, methyl ester, or carboxylic acids-without the aid of light or added catalyst. The passivated Si nanocrystals exhibit bright photoluminescence (PL) and disperse in polar solvents, including water. We propose a reaction mechanism in which ester or carboxylic acid groups facilitate direct nucleophilic attack of the highly curved Si surface of the nanocrystals by the alkene.

Raman Spectroscopy of Oxide-Embedded and Ligand-Stabilized Silicon Nanocrystals.

Oxide-embedded and oxide-free alkyl-terminated silicon (Si) nanocrystals with diameters ranging from 3 nm to greater than 10 nm were studied by Raman spectroscopy. For ligand-passivated nanocrystals, the zone center Raman-active mode of diamond cubic Si shifted to lower frequency with decreasing size, accompanied by asymmetric peak broadening, as extensively reported in the literature. The size dependence of the Raman peak shifts, however, was significantly more pronounced than previously reported or predicted by the RWL (Richter, Wang, and Ley) and bond polarizability models. In contrast, Raman peak shifts for oxide-embedded nanocrystals were significantly less pronounced as a result of the stress induced by the matrix.

Copper selenide nanocrystals for photothermal therapy.

Ligand-stabilized copper selenide (Cu(2-x)Se) nanocrystals, approximately 16 nm in diameter, were synthesized by a colloidal hot injection method and coated with amphiphilic polymer. The nanocrystals readily disperse in water and exhibit strong near-infrared (NIR) optical absorption with a high molar extinction coefficient of 7.7 × 10(7) cm(-1) M(-1) at 980 nm. When excited with 800 nm light, the Cu(2-x)Se nanocrystals produce significant photothermal heating with a photothermal transduction efficiency of 22%, comparable to nanorods and nanoshells of gold (Au). In vitro photothermal heating of Cu(2-x)Se nanocrystals in the presence of human colorectal cancer cell (HCT-116) led to cell destruction after 5 min of laser irradiation at 33 W/cm(2), demonstrating the viabilitiy of Cu(2-x)Se nanocrystals for photothermal therapy applications.

Enantioselectivity of polychlorinated biphenyl atropisomers in sediment and biota from the Turtle/Brunswick River estuary, Georgia, USA.

To investigate the potential for enantioselective transformation and accumulation, the enantiomer distributions of seven polychlorinated biphenyl (PCB) atropisomers were measured in the sediment and biota from a sub-tropical estuary heavily contaminated with Aroclor 1268, a technical mixture of highly chlorinated PCB congeners. Enantiomer fractions (EFs) of PCBs 91, 95, 136, 149, 174, 176, and 183 in marsh sediment, invertebrate, forage and predatory fish species, and bottlenose dolphins were determined. Non-racemic EFs greater than 0.75 were found in sediments for PCBs 136 and 174, likely the result of microbial dechlorination. Although enantiomer fractions in grass shrimp (Palaemonetes spp.) mirrored those of sediment, fish species had EFs that differed significantly from sediment or grass shrimp. Similarly, bottlenose dolphins were also found to contain non-racemic quantities of PCBs 91, 136, 174, 176, and 183. Non-racemic EFs in these biota were likely a result of both uptake of non-racemic proportions of PCBs from the diet and enantioselective biotransformation.

Alkyl passivation and amphiphilic polymer coating of silicon nanocrystals for diagnostic imaging.

A method to produce biocompatible polymer-coated silicon nanocrystals for medical imaging is shown. Silica-embedded Si nanocrystals are formed by HSQ thermolysis. The nanocrystals are then liberated from the oxide and terminated with Si-H bonds by HF etching, followed by alkyl monolayer passivation by thermal hydrosilylation. The Si nanocrystals have an average diameter of 2.1 nm ± 0.6 nm and photoluminesce with a peak emission wavelength of 650 nm, which lies within the transmission window of 650-900 nm that is useful for biological imaging. The hydrophobic Si nanocrystals are then coated with an amphiphilic polymer for dispersion in aqueous media with the pH ranging between 7 and 10 and an ionic strength between 30 mM and 2 M, while maintaining a bright and stable photoluminescence and a hydrodynamic radius of only 20 nm. Fluorescence imaging of polymer-coated Si nanocrystals in biological tissue is demonstrated, showing the potential for in vivo imaging.

Gold seed removal from the tips of silicon nanorods.

A chemical method was developed to remove the gold (Au) seed particles from the tips of solution-liquid-solid (SLS) grown silicon (Si) nanorods. The nanorods are capped with hydrophobic ligands during the synthesis, which made it necessary to perform the Au etching in an aqua regia and chloroform emulsion. Preliminary etching experiments revealed that a thin Si shell coated the Au seeds and prevented Au removal. Therefore, a rapid thermal quench of the reaction mixture was needed to crack this shell and provide etchant access to the Au seed. More than 95% of the Au seeds could be removed from the tips of thermally quenched samples without damaging the crystalline Si nanorods.

Colloidal silicon nanorod synthesis.

The colloidal synthesis of crystalline silicon (Si) nanorods with diameters of 5 to 10 nm and lengths of 15 to 75 nm is demonstrated. Trisilane was decomposed in a hot solvent in the presence of dodecylamine and gold (Au) nanocrystals. Nanorods form by Au-seeded solution-liquid-solid growth with dodecylamine serving as capping ligands that stabilize the nanorod dispersion. Post-synthesis etching of the Au seeds from the nanorod tips is also demonstrated.

Synthesis and photoluminescent properties of size-controlled germanium nanocrystals from phenyl trichlorogermane-derived polymers.

We report the preparation of luminescent oxide-embedded germanium nanocrystals (Ge-NC/GeO2) by the reductive thermal processing of polymers derived from phenyl trichlorogermane (PTG, C6H5GeCl3). Sol-gel processing of PTG yields air-stable polymers with a Ge:O ratio of 1:1.5, (C6H5GeO1.5)n, that thermally decompose to yield a germanium rich oxide (GRO) network. Thermal disproportionation of the GRO results in nucleation and initial growth of oxide-embedded Ge-NC, and subsequent reaction of the GeO2 matrix with the reducing atmosphere results in additional nanocrystal growth. This synthetic method affords quantitative yields of composite powders in large quantities and allows for Ge-NC size control through variations of the peak thermal processing temperature and reaction time. Freestanding germanium nanocrystals (FS-Ge-NC) are readily liberated from Ge-NC/GeO2 composite powders by straightfoward dissolution of the oxide matrix in warm water. Composites and FS-Ge-NC were characterized using thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), selected area electron diffraction (SAED), energy dispersive X-ray spectroscopy (EDX), and photoluminescence (PL) spectroscopy.