In traditional solar cells one photon absorbed can lead to at most one electron of current. Singlet fission, a process in which one singlet exciton is converted to two triplet excitons, provides a potential improvement by producing two electrons from each photon of sufficient energy. The literature contains several reports of singlet fission in various systems, but the mechanism of this process is poorly understood. In this paper we examine a two-step mechanism with a charge transfer state intermediate, applicable when the initial excited state is localized. Density matrix theory is used to examine how various molecular properties such as orbital energies and electronic couplings affect singlet fission yield in the regime of fast, coherent electron transfer. Several promising chromophores are discussed and density functional theory is used to predict fission yield for each in the context of this mechanism. Finally, implications for chromophore design are discussed, and future experiments are suggested.

Entering a new phase: Tin sulfide (SnS) nano‐ and microcrystals were synthesized by a seedless, solution procedure. Three‐dimensional tetrahedral particles crystallized in the zinc blende (ZB) structure (see SEM image) while two‐dimensional platelike particles crystallized in the orthorhombic phase. The optical properties of the orthorhombic plates were similar to bulk SnS, but the absorption edge of the ZB tetrahedra was blue‐shifted by around 300 nm compared to that of bulk SnS.

Singlet exciton fission, a process that converts one singlet exciton to a pair of triplet excitons, has the potential to enhance the efficiency of both bulk heterojunction and dye-sensitized solar cells and is understood in crystals but not well understood in molecules. Previous studies have identified promising building blocks for singlet fission in molecular systems, but little work has investigated how these individual chromophores should be combined to maximize triplet yield. We consider the effects of chemically connecting two chromophores to create a coupled chromophore pair and compute how various structural choices alter the thermodynamic and kinetic parameters likely to control singlet fission yield. We use density functional theory to compute the electron transfer matrix element and the thermodynamics of fission for several promising chromophore pairs and find a trade-off between the desire to maximize this element and the desire to keep the singlet fission process exoergic. We identify promising molecular systems for singlet fission and suggest future experiments.

with these topologies are difficult to fabricate with other lithographic techniques, such as photolithography.[10] Combined with our previous work, [5] which described methods to mold patterned composites of gels, the techniques discussed here allow the formation of structures that incorporate distinct populations of cells within or on the surface of a gel. These lithographic techniques thus enable the formation of micrometer-scale tissues in vitro that contain separate epithelial and mesenchymal compartments. ExperimentalStamps were cast from patterned lithographic masters using polydimethylsiloxane (PDMS, Sylgard 184, Dow Corning), as described previously [20]. Stamps were coated by adsorption of a monolayer of bovine serum albumin (fatty acid-poor BSA, Calbiochem; 1 % in PBS, > 1 h) to allow distortion-free detachment of molded gels [5]. To fabricate collagen gels with defined cavities, we first added liquid precursors of Matrigel (BD Biosciences;~0.2±0.4 lL mm ±2) to the patterned surface of a treated stamp, and centrifuged the stamp (for isolated features, 200 g, 10 min, 4 C; for interconnected features, 50 g, 5 min, 4 C) to remove excess liquid Matrigel precursor from the raised regions of the stamp. The stamp was brought vertically into contact with a flat layer (~100 lm thick) of collagen gel (rat tail collagen type I, BD Biosciences, 3.66 mg mL ±1 ; pre-cooled to 4 C) on a Petri dish or a glass cover slip, and heated to 37 C at 100 % humidity (40±60 min) to gel the molded Matrigel. We removed the stamp by carefully adding excess PBS; the surface tension of PBS caused the stamp to detach spontaneously from the underlying gels. We then slowly flushed the surrounding PBS with liquid collagen precursors two to three times until the molded Matrigel was completely immersed in liquid collagen precursors; heating the sample to 37 C at 100 % humidity (30 min) allowed the added collagen precursors to gel and thereby encase the Matrigel. Immersion of the composite of gels in dispase (GIBCO; 2±2.5 U mL ±1 in PBS or culture media, 1.5±2.5 h, 37 C) digested the Matrigel. The digestion was stopped by washing with fresh PBS or media.To incorporate iron powder in Matrigel, iron powder (Polysciences, average size of iron particles~10 lm) was coated with BSA (1 %, 1 h), and mixed with liquid Matrigel precursors at~1:5 volume ratio. To incorporate cells in Matrigel, cells were trypsinized, washed with PBS, concentrated by centrifugation (300 g, 2 min), and mixed with liquid Matrigel precursors at~1:2 volume ratio (~10 7 cells mL ±1 ). The iron particles or cells in liquid Matrigel precursors were allowed to settle in the relief features of the stamp (5±10 min, 4 C) before centrifugation and removal of excess liquid Matrigel precursors. To generate fluorescent features of Matrigel, Alexa Fluor 488-conjugated goat IgG (Molecular Probes, 200 lg mL ±1 ), and Oregon Green 488-conjugated human collagen type IV (Molecular Probes, 250 lg mL ±1 ) were mixed with liquid Matrigel precursors.Bovine pulmonary artery endothelial cells (...

This communication describes a new synthetic approach to one- (1D) and two-dimensional (2D) NbSe2 nanoscale materials using soft chemical methods. Our one-pot synthesis provides a direct route to control the morphology of nanostructures that can exhibit complex electronic properties, and can produce layered, nanocrystalline materials in high yield.

This paper reports an approach for the generation of molybdenum disulfide nanostructures by the sulfidation of patterned sub-300 nm features of molybdenum metal. Our method can be used to pattern arbitrary shapes of MoS 2 nanostructures with independent control over their width, height, and length. In addition, we can control the orientation of the crystals by placing the patterned substrates at different locations in the quartz tube furnace. These nanostructures can be fabricated with variable pitch, over large areas (cm 2 ), and on a range of insulating and conducting substrates (e.g., sapphire, fused silica, and silicon). This work provides a general strategy for patterning nanoscale crystalline structures on surfaces-in particular, metal sulfide nanomaterials-by combining topdown nanoscale patterning techniques with bottom-up chemical methods.MoS 2 is a layered semiconducting material that has shown promise in chemical sensors, [1] in solar cells, [2] in catalysis, [3][4][5] and for low-friction surfaces. [6][7][8] Recent studies have suggested that reducing the size of the MoS 2 crystals can improve their lubrication properties in bearings, O-rings, or other heavy-wear applications.[9] The ability to pattern MoS 2 nanostructures and other metal-sulfide materials on surfaces with specific sizes and shapes has the potential to optimize and improve their usefulness. MoS 2 ribbons have successfully been grown on the step edges of highly oriented pyrolytic graphite by electrochemical methods. [10,11] Although the heating of MoO 2 nanowires in H 2 S for several days could achieve increased lateral dimensions of MoS 2 ribbons, control of other aspects of this system, such as the height, the spacing, and the overall length of the ribbons, remains a challenge. There are two strategies for organizing nanostructures on surfaces: i) synthesis of the nanomaterials followed by assembly into architectures, or ii) direct growth of the nanostructures at predefined locations. The former approach relies on assembly methods such as fluidic-based assembly, [12,13] electric-and magnetic-field-mediated assembly, [14][15][16] electrostatic assembly, [17,18] and template-based assembly. [19,20] Serial lithographic techniques, including scanning-probe and electronbeam (e-beam) writing, can easily pattern functional structures with sub-100 nm features; however, their slow write speeds and small write areas can be a drawback. [21][22][23] Examples of parallel patterning methods are nanosphere lithography and laser-assisted embossing, which can generate sub-100 nm patterns of simple geometries. [24,25] We and others have developed a suite of soft-lithographic nanopatterning tools that can generate small (sub-30 nm) features over relatively large areas (> 1 cm 2 ) in a parallel process. [26,27] These tools have been used to create Si and GaAs nanostructures by chemical etching [28,29] and to direct the growth of arrays of ZnO nanowires and carbon nanotubes. [30][31][32] Here we report an important variant of the directed-growth method: ...

netic stirrer and dissolved completely at 60 C. This solution was heated to 190±200 C at a rate of 5 C min ±1 , and began to turn black indicating the formation of Pt nanoparticles. Iron pentacarbonyl (50 lL) were added into the mixture after reaction at this temperature for 5 min. The solution was then refluxed at~290 C for 30 min before being cooled to room temperature. The nanoparticles were separated using hexane and ethanol, and stored in hexane. The solidstate conversion of nanoparticles was done on silicon wafers under a flow of gas mixture of H 2 (5 vol.-%) in Ar at 450 C for 30 min.To prepare a Langmuir film, a suspension of Pt@Fe 2 O 3 nanoparticles in hexane (500 lL) was washed with ethanol (1.50 mL). The particles were separated using centrifuge (VWR Scientific, Model V) at 6000 rpm. The precipitate was re-dispersed in hexane (500 lL) and precipitated out using ethanol (1.5 mL) as an anti-solvent. This suspension was diluted in hexane and used as the stock solution. In a typical procedure, the stock solution (250 lL) was separated with ethanol (1.75 mL) and dispersed in hexane (2 mL) at a particle concentration of 1.0 mg mL ±1 to make the spreading solution. LB films of Pt@Fe 2 O 3 nanoparticles were made using a KSV 3000 Langmuir trough in a Class 10 000 clean room. Nanoparticle suspension in hexane was spread drop-wise on top of deionized water (Barnstead Nanopure II, 16.7 MX) using a microsyringe. Compression of Langmuir films was done at a rate of 5 mm ±1 min ±1 after the hexane was evaporated (~5 min). LB films were prepared at a surface pressure of 50 mN m ±1 and lifted onto patterned PDMS stamps at a rate of 1 mm min ±1 . The PDMS stamps of micrometer-scale antidots and lines were replicated from the original masters of photoresist on silicon wafer following the standard procedure [29]. Silicon wafers used in these experiments were cleaned with acetone and methanol using a sonication bath (Branson 2510), dried with a stream of nitrogen gas, and finally freshly cleaned using a plasma cleaner (Harrick PDC-32G) prior to contact printing. To convert Pt@Fe 2 O 3 nanoparticles to FePt thin films, pLB films of Pt@Fe 2 O 3 nanoparticles on silicon wafer were transferred into a tube furnace (Lindberg/Blue). The temperature of the furnace was first raised to 300 C at 150 C h ±1 , annealed at this temperature for 2 h, then raised to 450 C at 100 C h ±1 , and maintained at this temperature for 30 min. Throughout the entire annealing process, the sample was under the atmosphere of forming gas, and covered with a small piece of quartz to reduce the drifting of particles.Field-emission SEM images were obtained using a LEO 982 microscope. Tapping-mode AFM and MFM images were collected using a Nanoscope IIIa microscope from Digital Instrument. The magnetic tips (MFMR) were purchased from Nanosensors. These cantilevers are coated with cobalt alloy (40 nm thick) on the tip side and aluminum (30 nm thick) on the detector side. The tip radii are typically less than 50 nm. The magnetic properties of the patterned...

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