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Heat flow control plays a significant role in thermal management and energy conversion processes. Recently, two dimensional (2D) materials with unique anisotropic thermal properties are attracting a lot of attention, as promising building blocks for molding the heat flow. Originated from its crystal structure, in most if not all the 2D materials, the thermal conductivity along the Z direction (kz) is much lower than x-y plane thermal conductivity (kxy). In this work, we demonstrate that 2D nanoplates of vertically grown molybdenum disulfide (VG MoS2) can have anomalous thermal anisotropy, in which kxy (about 0.83W/mK at 300 K) is ~1 order of magnitude lower than kz (about 9.2W/mK at 300 K). Lattice dynamics analysis reveals that this anomalous thermal anisotropy can be attributed to the anisotropic phonon dispersion relations and the anisotropic phonon group velocities along different directions. The low kxy can be attributed to the weak phonon coupling near the x-y plane interfaces. It is expected that this 2D nanoplates of VG MoS2 with anomalous thermal anisotropy and low kxy can serve as a complementary building block for device designs and advanced heat flow control. [ABSTRACT FROM AUTHOR]

We report the thermal conductivity of photoelectrochemically synthesized mesoporous silicon (MPS), with ~20-nm diameter pores and 52%-58% porosity. The thermal conductivity of MPS samples with a thickness of a few microns was measured using the three omega (3ω) differential technique. We experimentally demonstrated that the thermal conductivity of MPS varies between 3 and 7W/m K at room temperature and is dependent on the photoelectrochemical etching times used during the MPS synthesis, which induces a slight change in the MPS porosity. Calculations were conducted using the Boltzmann transport equation in the relaxation time approximation, with the results suggesting that the large thermal conductivity reduction in the MPSs was not entirely explained by the pore boundary scattering. Our findings indicate that elastic softening in the mesoporous structure may be responsible for the reduction in the thermal conductivity. [ABSTRACT FROM AUTHOR]

Spectrally selective absorbers (SSA) with high selectivity of absorption and sharp cut-off between high absorptivity and low emissivity are critical for efficient solar energy conversion. Here, we report the semiconductor nanowire enabled SSA with not only high absorption selectivity but also temperature dependent sharp absorption cut-off. By taking advantage of the temperature dependent bandgap of semiconductors, we systematically demonstrate that the absorption cut-off profile of the semiconductor-nanowire-based SSA can be flexibly tuned, which is quite different from most of the other SSA reported so far. As an example, silicon nanowire based selective absorbers are fabricated, with the measured absorption efficiency above (below) bandgap ∼97% (15%) combined with an extremely sharp absorption cut-off (transition region ∼200 nm), the sharpest SSA demonstrated so far. The demonstrated semiconductor-nanowire-based SSA can enable a high solar thermal efficiency of ≳86% under a wide range of operating conditions, which would be competitive candidates for the concentrated solar energy utilizations. [ABSTRACT FROM AUTHOR]

Spider silk possesses remarkable mechanical properties and can lift weight effectively. Certain kinds of spider silk have unique response to liquid, especially water, because of their hydrophilic proteins, β-sheet characters, and surface structure. The Ornithoctonus huwena (O. huwena) spider is a unique species because it can be bred artificially and it spins silk whose diameter is in nanometer scale. In this work, we report the "shrink-stretch" behavior of the O. huwena spider silk fibers and show how they can be actuated by water to lift weight over long distance, at a fast speed, and with high efficiency. We further rationalize this behavior by analyzing the mechanical energy of the system. The lifting process is energy-efficient and environmentally friendly, allowing applications in actuators, biomimetic muscles, or hoisting devices. Published by AIP Publishing. [ABSTRACT FROM AUTHOR]

Plasmonic nanostructures have been widely applied in various types of solar cells for improving light absorption and therefore energy conversion efficiency. In this work, we demonstrate that Au@SiO2 core-shell nanorods with finely tuned aspect ratios are highly beneficial for the CH3NH3PbI3 perovskite solar cell, with the simultaneous enhancement of solar absorption and external quantum efficiency across a broad range of wavelength, which can contribute to the increased cross-sectional scattering and spectrally absorbing energy density. Therefore, a 16.1% improvement (from 12.4% to 14.4%) of the maximal external quantum efficiency can be achieved by such structures, accompanied with a 13.5% improvement (from 20.0 to 22.7 mA/cm2) of the maximal short-circuit current density and little improvement of the open-circuit voltage and fill factor. Our findings also provide a general guideline to design solar cell structures with thinner absorber layers and improve the absorption in other poorly light-absorbing devices like lead free perovskite solar cells as well. [ABSTRACT FROM AUTHOR]

Perovskite-based optoelectronic devices have shown great promise for solar conversion and other optoelectronic applications, but their long-term performance instability is regarded as a major obstacle to their widespread deployment. Previous works have shown that the ultralow thermal conductivity and inefficient heat spreading might put an intrinsic limit on the lifetime of perovskite devices. Here, we report the observation of a remarkably efficient thermal conductance, with a conductivity of 11.2±0.8W m–1 K–1 at room temperature, in densely packed perovskite CH3NH3PbI3 films, via noncontact time-domain thermal reflectance measurements. The temperature-dependent experiments suggest the important roles of organic cations and structural phase transitions, which are further confirmed by temperature-dependent Raman spectra. The thermal conductivity at room temperature observed here is over one order of magnitude larger than that in the early report, suggesting that perovskite device performance will not be limited by thermal conductance. [ABSTRACT FROM AUTHOR]

van der Waals (vdW) heterojunctions formed by two-dimensional (2D) materials have attracted tremendous attention due to their excellent electrical/optical properties and device applications. However, current 2D heterojunctions are largely limited to atomic crystals, and hybrid organic/inorganic structures are rarely explored. Here, we fabricate the hybrid 2D heterostructures with p-type dioctylbenzothienobenzothiophene (C8-BTBT) and n-type MoS2. We find that few-layer C8-BTBT molecular crystals can be grown on monolayer MoS2 by vdW epitaxy, with pristine interface and controllable thickness down to monolayer. The operation of the C8-BTBT/ MoS2 vertical heterojunction devices is highly tunable by bias and gate voltages between three different regimes: interfacial recombination, tunneling, and blocking. The pn junction shows diode-like behavior with rectifying ratio up to 105 at the room temperature. Our devices also exhibit photovoltaic responses with a power conversion efficiency of 0.31% and a photoresponsivity of 22 mA/W. With wide material combinations, such hybrid 2D structures will offer possibilities for opto-electronic devices that are not possible from individual constituents. [ABSTRACT FROM AUTHOR]

Heat flow control plays a significant role in thermal management and energy conversion processes. Recently, two dimensional (2D) materials with unique anisotropic thermal properties are attracting a lot of attention, as promising building blocks for molding the heat flow. Originated from its crystal structure, in most if not all the 2D materials, the thermal conductivity along the Z direction (kz) is much lower than x-y plane thermal conductivity (kxy). In this work, we demonstrate that 2D nanoplates of vertically grown molybdenum disulfide (VG MoS2) can have anomalous thermal anisotropy, in which kxy (about 0.83W/mK at 300 K) is ~1 order of magnitude lower than kz (about 9.2W/mK at 300 K). Lattice dynamics analysis reveals that this anomalous thermal anisotropy can be attributed to the anisotropic phonon dispersion relations and the anisotropic phonon group velocities along different directions. The low kxy can be attributed to the weak phonon coupling near the x-y plane interfaces. It is expected that this 2D nanoplates of VG MoS2 with anomalous thermal anisotropy and low kxy can serve as a complementary building block for device designs and advanced heat flow control. [ABSTRACT FROM AUTHOR]