Spotlights pubs.acs.org/JPCL
Spotlights: Volume 8, Issue 8
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PHOTOTHERMOELECTRIC EFFECTS AND LARGE PHOTOVOLTAGES IN PLASMONIC Au NANOWIRES WITH NANOGAPS Recently there has been great interest in using structure at the nanoscale to enhance the thermoelectric response of materials for improved energy conversion and photodetection. It is generally thought that electron and lattice temperatures are similar, but this may not be true in optically driven systems, where there is a potential to generate high-energy carriers through the excitation of localized plasmon resonances. To study spatial dependence of the thermoelectric response in plasmonic gold nanowires with and without nanogaps, Zolotavin et al. (10.1021/acs.jpclett.7b00507) used laserscanning microscopy to locally heat the metal nanostructure via excitation of plasmon resonance and direct absorption. In continuous nanowires, photothermoelectric voltages were generated on the microvolt scale, consistent with geometric alterations of the Seebeck coefficient; however, in nanogap structures with highly localized plasmonic modes, photovoltages tens of millivolts in magnitude were produced. The authors applied a model based on the photoexcited hot carrier photocurrent that flows across the nanogap to explain the generation of these large photovoltages. This effect allows for the possibility of engineering the photothermoelectric response in plasmonic photodetectors, which would be useful in plasmonic systems relevant to nanophotonics, photodetection, molecular and atomic junctions, and the study of high-energy carriers at the nanometer scale.
DIRAC NODAL LINES AND TILTED SEMI-DIRAC CONES COEXISTING IN A STRIPED BORON SHEET Graphene has been established as a two-dimensional (2D) Dirac fermion system, with the massless fermions in graphene leading to half-integer/fractional/fractal quantum Hall effects and ultrahigh carrier mobility, among other interesting phenomena. Therefore, it comes as no surprise that researchers are hard at work seeking other 2D Dirac materials, with some shifting their focus from materials with standard Dirac cones to those with Dirac nodal lines and strongly tilted Dirac cones. Boron is one such material being considered, but it is difficult to form Dirac cones in 2D boron monolayers because of the complicated bonding chemistry, and Dirac fermions have not yet been found in monolayer boron sheets. Zhang et al. (10.1021/acs.jpclett.7b00452) set out to determine whether a monolayer boron sheet with Dirac fermions was experimentally feasible, and they identified a new boron monolayer consisting of hexagon as well as rhombus stripes. The boron monolayer, which they have dubbed hr-sB, has exceptional stability and unique Dirac fermions. Dirac nodal lines and tilted semi-Dirac cones coexist around the Fermi level, and the Dirac points in the nodal lines are crossed by two linear bands corresponding to two one-dimensional channels in the hexagon and rhombus stripes, respectively. The tilted semi-Dirac cones are featured at the tilted axis and anisotropic band crossings, which produces a new kind of Dirac fermions. The unique electronic properties induced by special bond characteristics indicate that this boron monolayer may be a good superconductor.
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RELATING CHROMOPHORIC AND STRUCTURAL DISORDER IN CONJUGATED POLYMERS Chromophores are the fragments of a molecule that interacts with light and causes the material’s perceived color. In biological polymers, the chromophores are distinct molecular units incorporated into a sequence of amino acids, and their spatial positions within the structure are fixed. In contrast, in conjugated polymers, the chromophore emerges as a fragment of the chain over which the excitation is localized, and its position and size vary with conformational fluctuations. The static and dynamic variability of these properties constitutes the chromophoric disorder. In their Letter, Simine and Rossky (10.1021/acs.jpclett.7b00290) used quantum−classical simulations to systematically examine regimes of conformational disorder, and they interpret spectroscopic signatures as indications of the conformational disorder and electronic structure in amorphous conjugated polymers. Their findings show that the role of side-chain substituents is fundamental to the behavior, and they contradict conventional wisdom regarding the correlation between planarity and disorder and between exciton delocalization and the size of optical gaps. This finding could aid in the interpretation of spectroscopic data and shed light on the limitations of approximations in popular highly simplified models. © 2017 American Chemical Society
Published: April 20, 2017 1893
DOI: 10.1021/acs.jpclett.7b00883 J. Phys. Chem. Lett. 2017, 8, 1893−1893
