Profiling nanowire thermal resistance with a spatial resolution of nanometers.

We report a new technique to profile the thermal resistance along a nanowire with a spatial resolution of better than 20 nm. Using this technique, we mapped the thermal conductivity along a Si0.7Ge0.3/NiSi0.7Ge0.3 heterostructured nanowire. We also measured the interfacial thermal resistance (ITR) across the Si/NiSi2 interface embedded in Si/NiSi2 heterostructured nanowires. The ITR does not change even for adjacent interfaces as close as ∼ 50 atomic layers.

Diameter-dependent thermal transport in individual ZnO nanowires and its correlation with surface coating and defects.

A systematic study of the thermal transport properties of individual single-crystal zinc oxide (ZnO) nanowires (NWs) with diameters in the range of ∼50-210 nm is presented. The thermal conductivity of the NWs is found to be dramatically reduced by at least an order of magnitude compared to bulk values, due to enhanced phonon-boundary scattering with a reduction in sample size. While the conventional phonon transport model can qualitatively explain the temperature dependence, it fails to account for the diameter dependence. An empirical relationship for assessing diameter-dependent thermal properties is observed, which shows an approximately linear dependence of the thermal conductivity on the cross-sectional area of the NWs in the measured diameter range. Furthermore, it is found that an amorphous-carbon layer coating on the NWs does not perturb the thermal properties of the NW cores, whereas 30 keV Ga(+) ion irradiation at low dose (∼4 × 10(14) cm(-2)) leads to a remarkable reduction of the thermal conductivity of the ZnO NWs.

Thermal transport in suspended and supported few-layer graphene.

We report thermal conductivity (κ) measurements from 77 to 350 K on both suspended and supported few-layer graphene using a thermal-bridge configuration. The room temperature value of κ is comparable to that of bulk graphite for the largest flake, but reduces significantly for smaller flakes. The presence of a substrate lowers the value of κ, but the effect diminishes for the thermal transport in the top layers away from the substrate. For the suspended sample, the temperature dependence of κ follows a power law with an exponent of 1.4 ± 0.1, suggesting that the flexural phonon modes contribute significantly to the thermal transport of the suspended graphene. The measured values of κ are generally lower than those from theoretical studies. We attribute this deviation to the phonon-boundary scattering at the graphene-contact interfaces, which is shown to significantly reduce the apparent measured thermal conductance of graphene.

Controllable epitaxial crystallization and reversible oriented patterning of two-dimensional colloidal crystals.

We demonstrate a reliable and highly efficient epitaxial templating approach for the formation of two-dimensional (2D) colloidal crystals. By applying an alternating electric field (AEF), one-dimensional colloidal lines are used as an epitaxial template to site specifically initiate 2D colloidal crystallization and control the orientation of the 2D colloidal crystals. The kinetics of the crystallization and structure ordering is precisely and conveniently manipulated by the external AEF. The well-defined artificial linear defects are embedded inside the 2D colloidal crystals by means of heteroepitaxy, whereas the unwanted existing defects are controllably relaxed via an electrically induced annealing process. This epitaxial templating approach is fast, reversible, and amenable for large-area oriented patterning of colloidal crystals, providing a new way for the creation of novel materials and devices with functions and properties that can be reversibly changed, such as electrically tunable photonic waveguides and e-papers.

Precise fabrication of point defects in self-assembled three-dimensional macroporous photonic crystals.

We demonstrate a new and simple method of precisely fabricating defects in three-dimensional (3D) CdS macroporous photonic crystals (PCs) with a variable pressure scanning electron microscope. Well-defined point defects, not only vacancies but also an impurity (a reduced-size sphere), were directly fabricated by electron-beam irradiation under a gas atmosphere. This provides a convenient and straightforward method of introducing various designed defects into 3D PCs for photonic-band-gap-based applications.

Low-temperature growth of uniform ZnO particles with controllable ellipsoidal morphologies and characteristic luminescence patterns.

Uniform ellipsoidal ZnO particles have been synthesized in an aqueous solution in the presence of triethonalamine (TEA) mediated by sonication at the temperature below 80 degrees C. Scanning electron microscopy observations reveal that the ellipsoidal particles are highly uniform with a hexagonal cross-section. The morphologies of the ZnO particles can be systematically controlled from elongated rugby ball-like ellipsoidal to half-ellipsoidal by increasing the TEA concentration. Spatial resolved cathodoluminescence measurements at room temperature show that the ellipsoidal ZnO particles are intrinsically encoded with barcode-like ultraviolet luminescence patterns, which are of either a wide stripe or a narrow stripe perpendicular to the length at the core of the particles depending on the growth temperature. Moreover, the luminescence spectra of the ellipsoidal particles can be tuned by heat treatments at elevated temperatures, while maintaining the luminescence patterns. We believe that the well-defined uniform ellipsoidal ZnO particles embedded with unique luminescence characteristics hold great potential for use in bioengineering and photonics, such as biological labeling and optical probes.

Length-dependent thermal conductivity in suspended single-layer graphene.

Graphene exhibits extraordinary electronic and mechanical properties, and extremely high thermal conductivity. Being a very stable atomically thick membrane that can be suspended between two leads, graphene provides a perfect test platform for studying thermal conductivity in two-dimensional systems, which is of primary importance for phonon transport in low-dimensional materials. Here we report experimental measurements and non-equilibrium molecular dynamics simulations of thermal conduction in suspended single-layer graphene as a function of both temperature and sample length. Interestingly and in contrast to bulk materials, at 300 K, thermal conductivity keeps increasing and remains logarithmically divergent with sample length even for sample lengths much larger than the average phonon mean free path. This result is a consequence of the two-dimensional nature of phonons in graphene, and provides fundamental understanding of thermal transport in two-dimensional materials.

Perfectly dissolved boron nitride nanotubes due to polymer wrapping.

We report for the first time that boron nitride nanotubes (BNNTs) may be dissolved in organic solvents by wrapping them with a polymer. Transmission electron microscopy and cathodoluminescence studies indicate the strong pi-pi interactions between BNNTs and the polymer. A band gap ranging from 5.2 to 5.5 eV was documented for the BNNTs independent of their geometrical characteristics by using ultraviolet-visible absorption experiments on composite films and thin BNNT films prepared from solutions.