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An attractive feature of nanomaterials is the possibility of tuning their properties through controlling their size and surface morphology, and understanding the effects of various parameters on thermal transport properties of nanostructures has been an active research topic in the past two decades. Through systematic studies of kinked silicon nanoribbons, we show how the kink morphology, a newly recognized degree of freedom for tuning thermal transport in nanostructures, modulates the thermal conductivity of these nanoribbons. For kinked Si nanoribbons that are 34 nm thick and 141 nm wide, the measured thermal conductivity first decreases as the period length reduces from 2 μm to 0.5 μm, reaching a 21% thermal conductivity reduction as compared to that of a straight counterpart at 300 K. However, as the period length drops to a level at which a straight heat transfer channel opens between the heat source and the sink, the thermal conductivity exhibits a steep increasing trend. Moreover, the comparison of thermal conductivity reduction for kinked ribbons along different crystalline directions indicates that phonon focusing could be exploited to further suppress thermal transport in kinked silicon nanoribbons. These results provide important guidelines on modulating heat transfer in nanostructures using kinks, which could be adopted to tune the thermal properties of nanostructures for different applications, such as thermoelectrics, microelectronic device thermal management, and functional thermal regulators. [ABSTRACT FROM AUTHOR]

Understanding thermal transport in nanostructured materials is important for the development of energy conversion applications and the thermal management of microelectronic and optoelectronic devices. Most nanostructures interact through van der Waals interactions, and these interactions typically lead to a reduction in thermal transport. Here, we show that the thermal conductivity of a bundle of boron nanoribbons can be significantly higher than that of a single free-standing nanoribbon. Moreover, the thermal conductivity of the bundle can be switched between the enhanced values and that of a single nanoribbon by wetting the van der Waals interface between the nanoribbons with various solutions. [ABSTRACT FROM AUTHOR]

Selective growth of tungsten oxide nanowires has been achieved using a vapor-solid (VS) process without the assistance of any catalysts. To achieve selective growth, low vapor supersaturation was employed to suppress the spontaneous nucleation of tungsten oxide during the VS process, and patterned tungsten coating was introduced to provide seed nuclei, which promotes the growth of tungsten oxide nanowires and control their growth sites. Patterned tungsten oxide nanowire arrays have been fabricated using simple patterning methods, such as shadow mask and laser-induced nanograting growth. The effects of the source heating temperature and the growth temperature on the nanowire growth have been investigated, showing the morphology of tungsten oxide deposition was sensitive to the vapor supersaturation controlled by both temperatures. [ABSTRACT FROM AUTHOR]

High-yield synthesis of TiO2 one-dimensional (1D) nanostructures was realized by a simple annealing of Ni-coated Ti grids in an argon atmosphere at 950 °C and 760 torr. The as-synthesized 1D nanostructures were single crystalline rutile TiO2 with the preferred growth direction close to [210]. The growth of these nanostructures was enhanced by using catalytic materials, higher reaction temperature, and longer reaction time. Nanoscale tensile testing performed on individual 1D nanostructures showed that the nanostructures appeared to fracture in a brittle manner. The measured Young’s modulus and fracture strength are ~56.3 and 1.4 GPa, respectively. [ABSTRACT FROM AUTHOR]

Catalyst-assisted growth of boron nanowires and novel tube–catalytic particle–wire hybrid boron nanostructures were achieved by pyrolysis of diborane at 820–890°C and ~ 200 mTorr in a quartz tube furnace. Electron microscopy imaging and diffraction analysis reveal that most of the nano-structures are amorphous. Elemental analysis by EELS and EDX shows that the nanostructures consist of boron with a small amount of oxygen and carbon. Possible growth mechanisms for the tube–catalytic particle–wire hybrid boron nanostructures are discussed. [ABSTRACT FROM AUTHOR]

We report on experimental measurements of contact thermal resistance between individual carbon nanotubes. Results indicate that the contact thermal conductance can increase by nearly two orders of magnitude (from 10-8 to 10-6 W/K) as the contact area increases from a cross contact to an aligned contact. Normalization with respect to the contact area leads to normalized contact thermal resistance on the order of 10-9 m2 K/W at room temperature, one order of magnitude lower than that from a molecular dynamics simulation in literature. These results should have important implications in the design of carbon nanotube-polymer composites for tunable thermal properties. [ABSTRACT FROM AUTHOR]

The Young's modulus of single crystalline rutile TiO2 nanoribbons was investigated using nanoindentation. During the experiments, the nanoribbons were laid on three different substrates, including 1 µm thick SiO2 layer on silicon (SiO2/Si), Si(100), and sapphire(0001). Experimental results show the substrates have significant effects on load-indenter displacement curves. To further understand the experimental findings, three-dimensional finite element modeling was carried out to simulate the indentation of nanoribbon-on-substrate systems using ABAQUS. The results show that the receding contact mechanics is a good approximation when describing the contact between the nanoribbon and the substrate. The results also demonstrate that the substrate effect must be carefully considered when performing nanoindentation on one-dimensional nanostructures. Otherwise, the Young's modulus of the nanostructures could either be overestimated or underestimated. The Young's modulus is about 360 GPa, comparable to that of bulk TiO2. [ABSTRACT FROM AUTHOR]

Measurements of thermal transport through contacts between individual multiwall carbon nanotubes show that, contrary to common expectation, the normalized contact thermal conductance per unit area depends linearly on the tube diameter. The result is corroborated with and extended to multilayer graphene nanoribbons through molecular dynamics simulations. Semiquantitative analyses show that these intriguing observations are consistent with an explanation based on an unexpectedly large phonon mean free path in the c-axis direction of graphite, phonon reflection at free surfaces, and phonon focusing in highly anisotropic graphitic materials. [ABSTRACT FROM AUTHOR]

The last two decades have seen tremendous progress in quantitative understanding of several major phonon scattering mechanisms (phonon-phonon, phonon-boundary, phonon-defects), as they are the determinant factors in lattice thermal transport, which is critical for the proper functioning of various electronic and energy conversion devices. However, the roles of another major scattering mechanism, electron-phonon (e-ph) interactions, remain elusive. This is largely due to the lack of solid experimental evidence for the effects of e-ph scattering in the lattice thermal conductivity for the material systems studied thus far. Here we show distinct signatures in the lattice thermal conductivity observed below the charge density wave transition temperatures in NbSe 3 nanowires, which cannot be recaptured without considering e-ph scattering. Our findings can serve as the cornerstone for quantitative understanding of the e-ph scattering effects on lattice thermal transport in many technologically important materials.

Van der Waals (vdW) crystals with covalently bonded building blocks assembled together through vdW interactions have attracted tremendous attention recently because of their interesting properties and promising applications. Compared to the explosive research on two-dimensional (2D) vdW materials, quasi-one-dimensional (quasi-1D) vdW crystals have received considerably less attention, while they also present rich physics and engineering implications. Here we report on the thermal conductivity of exfoliated quasi-1D Ta 2 Pd 3 Se 8 vdW nanowires. Interestingly, even though the interatomic interactions along each molecular chain are much stronger than the interchain vdW interactions, the measured thermal conductivity still demonstrates a clear dependence on the cross-sectional size up to >110 nm. The results also reveal that partial ballistic phonon transport can persist over 13 μm at room temperature along the molecular chain direction, the longest experimentally observed ballistic transport distance with observable effects on thermal conductivity so far. First-principles calculations suggest that the ultralong ballistic phonon transport arises from the highly focused longitudinal phonons propagating along the molecular chains. These data help to understand phonon transport through quasi-1D vdW crystals, facilitating various applications of this class of materials.