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Crystalline polymers have attracted significant interest in recent years due to their enhanced mechanical and thermal properties. As one type of organic-inorganic hybrid polymer crystals, polysilsesquioxane can be synthesized by large-scale and inexpensive so-gel processes with two precursors. In this paper, both octylene-bridged and hexylene-bridged PSQ crystals are characterized with infrared spectroscopy and X-ray crystallography to reveal their super high crystallinity. To study the thermal transport in these unique polymer crystals, we use a suspended micro thermal device to examine their thermal properties from 20 K to 320 K and demonstrate their tunable thermal conductivity by varying the length of alkyl chains. We also conduct non-equilibrium molecular dynamics simulations to study the phonon behaviors across the hydrogen bond interface. The simulation results demonstrate good agreement with the experimental results regarding both the value and trend of the PSQ thermal conductivity. Furthermore, from the simulation, we find that the anharmonic phonon scattering and interfacial anharmnic coupling effects across the hydrogen bond interface may explain the experimentally observed thermal properties.

The strongly correlated thermoelectric properties have been a major hurdle for high-performance thermoelectric energy conversion. One possible approach to avoid such correlation is to suppress phonon transport by scattering at the surface of confined nanowire structures. However, phonon characteristic lengths are broad in crystalline solids, which makes nanowires insufficient to fully suppress heat transport. Here, we employed Si-Ge alloy as well as nanowire structures to maximize the depletion of heat-carrying phonons. This results in a thermal conductivity as low as ∼1.2 W/m-K at 450 K, showing a large thermoelectric figure-of-merit (ZT) of ∼0.46 compared with those of SiGe bulks and even ZT over 2 at 800 K theoretically. All thermoelectric properties were "simultaneously" measured from the same nanowires to facilitate accurate ZT measurements. The surface-boundary scattering is prominent when the nanowire diameter is over ∼100 nm, whereas alloying plays a more important role in suppressing phonon transport for smaller ones.

Although it has been qualitatively demonstrated that surface roughness can reduce the thermal conductivity of crystalline Si nanowires (SiNWs), the underlying reasons remain unknown and warrant quantitative studies and analysis. In this work, vapor-liquid-solid (VLS) grown SiNWs were controllably roughened and then thoroughly characterized with transmission electron microscopy to obtain detailed surface profiles. Once the roughness information (root-mean-square, σ, correlation length, L, and power spectra) was extracted from the surface profile of a specific SiNW, the thermal conductivity of the same SiNW was measured. The thermal conductivity correlated well with the power spectra of surface roughness, which varies as a power law in the 1-100 nm length scale range. These results suggest a new realm of phonon scattering from rough interfaces, which restricts phonon transport below the Casimir limit. Insights gained from this study can help develop a more concrete theoretical understanding of phonon-surface roughness interactions as well as aid the design of next generation thermoelectric devices.

Black phosphorus (BP) has emerged as a promising candidate for next-generation electronics and optoelectronics among the 2D family materials due to its extraordinary electrical/optical/optoelectronic properties. Interestingly, BP shows strong anisotropic transport behavior because of its puckered honeycomb structure. Previous studies have demonstrated the thermal transport anisotropy of BP and theoretically attribute this to the anisotropy in both the phonon dispersion relation and the phonon relaxation time. However, the exact origin of such strong anisotropy lacks clarity and has yet to be proven experimentally. Here, the thermal transport anisotropy of BP nanoribbons is probed by an electron beam technique. Direct evidence is provided that the origin of this anisotropy is dominated by the anisotropic phonon group velocity, verified by Young's modulus measurements along different directions. It turns out that the ratio of the thermal conductivity between zigzag (ZZ) and armchair (AC) ribbons is almost same as that of the corresponding Young modulus values. The results from first-principles calculation are consistent with this experimental observation, where the anisotropic phonon group velocity between ZZ and AC is shown. These results provide fundamental insight into the anisotropic thermal transport in low-symmetry crystals.

Black phosphorus attracts enormous attention as a promising layered material for electronic, optoelectronic and thermoelectric applications. Here we report large anisotropy in in-plane thermal conductivity of single-crystal black phosphorus nanoribbons along the zigzag and armchair lattice directions at variable temperatures. Thermal conductivity measurements were carried out under the condition of steady-state longitudinal heat flow using suspended-pad micro-devices. We discovered increasing thermal conductivity anisotropy, up to a factor of two, with temperatures above 100 K. A size effect in thermal conductivity was also observed in which thinner nanoribbons show lower thermal conductivity. Analysed with the relaxation time approximation model using phonon dispersions obtained based on density function perturbation theory, the high anisotropy is attributed mainly to direction-dependent phonon dispersion and partially to phonon–phonon scattering. Our results revealing the intrinsic, orientation-dependent thermal conductivity of black phosphorus are useful for designing devices, as well as understanding fundamental physical properties of layered materials., Understanding the flow of heat in materials just one or a few atoms thick is vital for harnessing them in compact electronic devices. Here, the authors present the temperature-dependent thermal conductivity of black phosphorus ribbons and demonstrate an intrinsic orientation dependence.

Strong field enhancement and confinement in plasmonic nanostructures provide suitable conditions for nonlinear optics in ultracompact dimensions. Despite these enhancements, second-harmonic generation (SHG) is still inefficient due to the centrosymmetric crystal structure of the bulk metals used, e.g., Au and Ag. Taking advantage of symmetry breaking at the metal surface, one could greatly enhance SHG by engineering these metal surfaces in regions where the strong electric fields are localized. Here, we combine top-down lithography and bottom-up self-assembly to lodge single rows of 8 nm diameter Au nanoparticles into trenches in a Au film. The resultant "double gap" structures increase the surface-to-volume ratio of Au colocated with the strong fields in ∼2 nm gaps to fully exploit the surface SHG of Au. Compared to a densely packed arrangement of AuNPs on a smooth Au film, the double gaps enhance SHG emission by 4200-fold to achieve an effective second-order susceptibility χ((2)) of 6.1 pm/V, making it comparable with typical nonlinear crystals. This patterning approach also allows for the scalable fabrication of smooth gold surfaces with sub-5 nm gaps and presents opportunities for optical frequency up-conversion in applications that require extreme miniaturization.

It is reported that electron doped n-type SnSe2 nanoplates show promising thermoelectric performance at medium temperatures. After simultaneous introduction of Se deficiency and Cl doping, the Fermi level of SnSe2 shifts toward the conduction band, resulting in two orders of magnitude increase in carrier concentration and a transition to degenerate transport behavior. In addition, all-scale hierarchical phonon scattering centers, such as point defects, nanograin boundaries, stacking faults, and the layered nanostructures, cooperate to produce very low lattice thermal conductivity. As a result, an enhanced in-plane thermoelectric figure of merit ZTmax of 0.63 is achieved for a 1.5 at% Cl doped SnSe1.95 pellet at 673 K, which is much higher than the corresponding in-plane ZT of pure SnSe2 (0.08). NRF (Natl Research Foundation, S’pore) ASTAR (Agency for Sci., Tech. and Research, S’pore) MOE (Min. of Education, S’pore)

We have developed a new and accurate technique to measure temperature dependent in-plane Seebeck coefficient and electrical conductivity of organic and inorganic thin films. The measurement device consists of one heater, two thermometers, and a four-probe configuration which is patterned on a substrate of choice using a simple shadow mask. The high resolution in temperature measurements and repeatability of resistance thermometry is leveraged while enabling simple implementation using only a shadow mask for patterning. We calibrate the technique using nickel and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) thin films. The error bar for the Seebeck coefficient is less than 1%, almost 10 times better than complementary techniques for thin films. Moreover, our method enables high-throughput characterization of thermoelectric properties of a variety of different large area inorganic and organic thin films that can be prepared by spin coating, drop casting, evaporation, sputtering, or any other growth technique and hence has potential for wide usage in the thermoelectrics and nanoscale transport community to study thin films.

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 (3 ω) differential technique. We experimentally demonstrated that the thermal conductivity of MPS varies between 3 and 7 W/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.

We present that two-dimensional (2D) bilayer molybdenum disulfide (MoS2) exhibits an enhanced Seebeck coefficient over its three-dimensional (3D) counterpart arising from dimensionality confinement. It has been predicted that quantum confinement enhances thermoelectric performance but no studies have focused on a single material to present a theoretical and experimental comparison, which would illustrate the enhancement of thermoelectric performance. Layered MoS2 provides an opportunity to verify this hypothesis and in this work, we extensively study the Seebeck coefficient, S, the electrical conductivity, σ, and the thermoelectric powerfactor, S2σ of 2D monolayer and bilayer MoS2 using theoretical Boltzmann Transport Equation calculations and compare the results to well-characterized experimental data. We conclude that dimensional confinement indeed enhances the Seebeck coefficient by up to ∼50% in 2D bilayer MoS2 over 3D MoS2 under similar doping concentrations because of the discretization of density o...

Active heat flow control is essential for broad applications of heating, cooling, and energy conversion. Like electronic devices developed for the control of electric power, it is very desirable to develop advanced all-thermal solid-state devices that actively control heat flow without consuming other forms of energy. Here we demonstrate temperature-gated thermal rectification using vanadium dioxide beams in which the environmental temperature actively modulates asymmetric heat flow. In this three terminal device, there are two switchable states, which can be regulated by global heating. In the "Rectifier" state, we observe up to 28% thermal rectification. In the "Resistor" state, the thermal rectification is significantly suppressed (

0D transition metal phosphides (TMPs) nanocrystals (NCs)–2D ultrathin black phosphorus (BP) heterostructure (Ni2P@BP) have been synthesized via a facile sonication-assisted exfoliation followed by a solvothermal process. Compared with the bare BP, the specially designed Ni2P@BP architecture can enhance the electrical conductivity (from 2.12 × 102 to 6.25 × 104 S m–1), tune the charge carrier concentration (from 1.25 × 1017 to 1.37 × 1020 cm–3), and reduce the thermal conductivity (from 44.5 to 7.69 W m–1 K–1) at 300 K, which can be considered for multiple applications. As a result, the Ni2P@BP exhibits excellent Li storage properties and high hydrogen evolution reaction electrocatalytic activities. The Ni2P@BP shows improved Li diffusion kinetics (e.g., the Li ions diffusion coefficient increases from 1.14 × 10–14 cm2 s–1 for pure BP nanosheets to 8.02 × 10–13 cm2 s–1 for Ni2P@BP). In addition, the Ni2P@BP electrode sustains hydrogen production with almost unchanged activity over 3000 cycles, which indicates its good chemical stability when operating under strong reducing environment.

The thermal conductivity of individual single-crystalline Bi nanowires grown by the on-film formation of nanowires (ON-OFF) has been investigated. We observed that the thermal conductivity of single-crystalline Bi nanowires is highly anisotropic. Thermal conductivity of nanowires (diameter ∼100 nm) in the off-axis [102] and [110] directions exhibits a difference of ∼7.0 W/m·K. The thermal conductivity in both growth directions is diameter-dependent, which indicates that thermal transport through the individual Bi nanowires is limited by boundary scattering of both electrons and phonons. This huge anisotropy in thermal conductivities of Bi nanowires suggests the importance of direction-dependent characterization of charge, thermal transport, and thermoelectric properties of Bi nanowires.

Phonons in low-dimensional structures with feature sizes on the order of the phonon wavelength may be coherently scattered by the boundary. This may give rise to a new regime of heat conduction, which can impact thermal energy transport and conversion. Traditional methods used to investigate phonon transport in one-dimensional structures suffer from uncertainty due to contact resistance, defects, and limited control over sample dimensions. We have developed a new batch-fabrication technique for suspended microdevices with integrated silicon nanowires from silicon-on-insulator (SOI) wafers. The nanowires are defect-free and have extremely high aspect ratios (length/critical dimension >2000). The nanowire dimensions (length and critical dimension) can be precisely controlled during fabrication. With these novel devices, phonon transport in silicon nanowires is systematically investigated. The room temperature thermal conductivity of nanowires with critical width around 80 nm is about 20 W/(m K) and much lower than that in smooth VLS wires. This suggests that the surface morphology of the structures has a significant effect on the thermal conductivity, but this phenomenon is not currently understood. This fabrication technique can also be used for thermal transport investigation in a wide-range of low-dimensional structures.

We measured thermal conductivity of an individual single-crystalline Bi nanowire grown by a spontaneous growth method. The measured thermal conductivity (к) for 98 nm diameter Bi nanowire is about 1.6 W/m·K at 300 K, which is much lower than that of bulk Bi (к for bulk Bi≈ 8 W/m-K at 300 K). This indicates that the thermal conductivity of Bi nanowires is suppressed by the strong phonon boundary scattering.