Te-Huan Liu

Decoupling of CVD graphene by controlled oxidation of recrystallized Cu

Large-area graphene grown by chemical vapour deposition (CVD) is promising for applications; however, the interaction between graphene and the substrate is still not well understood. In this report, we use a combination of two non-destructive characterization techniques, i.e., electron backscatter diffraction (EBSD) and Raman mapping to locally probe the interface between graphene and copper lattices without removing graphene. We conclude that the crystal structure of the Cu grains under graphene layers is governed by two competing processes: (1) graphene induced Cu surface reconstruction favoring the formation of Cu(100) orientation, and (2) recrystallization from bulk Cu favoring Cu(111) formation. The underlying Cu grains, regardless of reconstruction or recrystallization, induce a large hydrostatic compression to the graphene lattice. Interestingly, the strong interaction could be decoupled by allowing the intercalation of a thin cuprous oxide interfacial-layer. The Cu2O layer is mechanically and chemically weak; hence, graphene films can be detached and transferred to arbitrary substrates and the Cu substrates could be re-used for graphene growth.

Anisotropic thermal conductivity of MoS2 nanoribbons: Chirality and edge effects

Previous studies of the thermal transport in MoS2 are limited to the 0° (zigzag) and 30° (armchair) chiralities. We investigate the anisotropic thermal transport properties of MoS2 nanoribbons with various crystal chiralities by employing the full-band phonon dispersion relations obtained from first-principle calculations. The ribbons with chiralities other than 0° and 30° always have lower thermal conductivity, yet a local maximum at 19.1°. In addition, the thermal conductivity can be further decreased by increasing the edge roughness due to the largely degraded longitudinal phonons. These findings suggest possibilities of obtaining a higher thermoelectric efficiency in MoS2 nanoribbons.

Low-temperature grown graphene films by using molecular beam epitaxy

Complete graphene film is prepared by depositing carbon atoms directly on Cu foils in a molecular beam epitaxy chamber at 300 °C. The Raman spectrum of the film has indicated that high-quality few-layer graphene is obtained. With back-gated transistor architecture, the characteristic current modulation of graphene transistors is observed. Following the similar growth procedure, graphitization is observed at room temperature, which is consistent with the molecular dynamics simulations of graphene growth.

First-principles mode-by-mode analysis for electron-phonon scattering channels and mean free path spectra in GaAs

We present a first-principles framework to investigate the electron scattering channels and transport properties for polar material by combining the exact solution of linearized electron-phonon (e-ph) Boltzmann transport equation in its integral-differential form associated with the e-ph coupling matrices obtained from polar Wannier interpolation scheme. No ad hoc parameter is required throughout this calculation, and GaAs, a well-studied polar material, is used as an example to demonstrate this method. In this work, the long-range and short-range contributions as well as the intravalley and intervalley transitions in the e-ph interactions (EPIs) have been quantitatively addressed. Promoted by such mode-by-mode analysis, we find that in GaAs, the piezoelectric scattering is comparable to deformation-potential scattering for electron scatterings by acoustic phonons in EPI even at room temperature and makes a significant contribution to mobility. Furthermore, we achieved good agreements with experimental data for the mobility, and identified that electrons with mean free paths between 130 and 210 nm contribute dominantly to the electron transport at 300 K. Such information provides deeper understandings on the electron transport in GaAs, and the presented framework can be readily applied to other polar materials.

Seeded growth of boron arsenide single crystals with high thermal conductivity

Materials with high thermal conductivities are crucial to effectively cooling high-power-density electronic and optoelectronic devices. Recently, zinc-blende boron arsenide (BAs) has been predicted to have a very high thermal conductivity of over 2000 W m−1 K−1 at room temperature by first-principles calculations, rendering it a close competitor for diamond which holds the highest thermal conductivity among bulk materials. Experimental demonstration, however, has proved extremely challenging, especially in the preparation of large high quality single crystals. Although BAs crystals have been previously grown by chemical vapor transport (CVT), the growth process relies on spontaneous nucleation and results in small crystals with multiple grains and various defects. Here, we report a controllable CVT synthesis of large single BAs crystals (400–600 μm) by using carefully selected tiny BAs single crystals as seeds. We have obtained BAs single crystals with a thermal conductivity of 351 ± 21 W m−1 K−1 at room ...

PIXE ANALYSIS OF ANCIENT CHINESE CHANGSHA PORCELAIN

Abstract In this work, proton induced X-ray emission (PIXE) method was applied for the analysis of ancient Chinese Changsha porcelain produced in the Tang dynasty (AD 618–907). A collection of glazed potsherds was obtained in the complex of the famous kiln site at Tongguan, Changsha city, Hunan province. Studies of elemental composition were carried out on ten selected Changsha potsherds. Minor and trace elements such as Ti, Mn, Fe, Co, Cu, Rb, Sr, and Zr in the material of the porcelain glaze were determined. Variation of these elements from sample to sample was investigated. Details of results are presented and discussed.

Ab initio study of electron mean free paths and thermoelectric properties of lead telluride

Last few years have witnessed significant enhancement of thermoelectric figure of merit of lead telluride (PbTe) via nanostructuring. Despite the experimental progress, current understanding of the electron transport in PbTe is based on either band structure calculation using first principles with constant relaxation time approximation or empirical models, both relying on adjustable parameters obtained by fitting experimental data. Here, we report parameter-free first-principles calculation of electron and phonon transport properties of PbTe, including mode-by-mode electron-phonon scattering analysis, leading to detailed information on electron mean free paths and the contributions of electrons and phonons with different mean free paths to thermoelectric transport properties in PbTe. Such information will help to rationalize the use and optimization of nanosctructures to achieve high thermoelectric figure of merit.

Unusual high thermal conductivity in boron arsenide bulk crystals

Moving the heat aside with BAs Thermal management becomes increasingly important as we decrease device size and increase computing power. Engineering materials with high thermal conductivity, such as boron arsenide (BAs), is hard because it is essential to avoid defects and impurities during synthesis, which would stop heat flow. Three different research groups have synthesized BAs with a thermal conductivity around 1000 watts per meter-kelvin: Kang et al., Li et al., and Tian et al. succeeded in synthesizing high-purity BAs with conductivities half that of diamond but more than double that of conventional metals (see the Perspective by Dames). The advance validates the search for high-thermal-conductivity materials and provides a new material that may be more easily integrated into semiconducting devices. Science, this issue p. 575, p. 579, p. 582; see also p. 549 Boron arsenide has an ultrahigh thermal conductivity, making it competitive with diamond for thermal management applications. Conventional theory predicts that ultrahigh lattice thermal conductivity can only occur in crystals composed of strongly bonded light elements, and that it is limited by anharmonic three-phonon processes. We report experimental evidence that departs from these long-held criteria. We measured a local room-temperature thermal conductivity exceeding 1000 watts per meter-kelvin and an average bulk value reaching 900 watts per meter-kelvin in bulk boron arsenide (BAs) crystals, where boron and arsenic are light and heavy elements, respectively. The high values are consistent with a proposal for phonon-band engineering and can only be explained by higher-order phonon processes. These findings yield insight into the physics of heat conduction in solids and show BAs to be the only known semiconductor with ultrahigh thermal conductivity.

Phonon Hydrodynamic Heat Conduction and Knudsen Minimum in Graphite

In the hydrodynamic regime, phonons drift with a nonzero collective velocity under a temperature gradient, reminiscent of viscous gas and fluid flow. The study of hydrodynamic phonon transport has spanned over half a century but has been mostly limited to cryogenic temperatures (∼1 K) and more recently to low-dimensional materials. Here, we identify graphite as a three-dimensional material that supports phonon hydrodynamics at significantly higher temperatures (∼100 K) based on first-principles calculations. In particular, by solving the Boltzmann equation for phonon transport in graphite ribbons, we predict that phonon Poiseuille flow and Knudsen minimum can be experimentally observed above liquid nitrogen temperature. Further, we reveal the microscopic origin of these intriguing phenomena in terms of the dependence of the effective boundary scattering rate on momentum-conserving phonon-phonon scattering processes and the collective motion of phonons. The significant hydrodynamic nature of phonon transport in graphite is attributed to its strong intralayer sp2 hybrid bonding and weak van der Waals interlayer interactions. More intriguingly, the reflection symmetry associated with a single graphene layer is broken in graphite, which opens up more momentum-conserving phonon-phonon scattering channels and results in stronger hydrodynamic features in graphite than graphene. As a boundary-sensitive transport regime, phonon hydrodynamics opens up new possibilities for thermal management and energy conversion.

Large thermoelectric power factor from crystal symmetry-protected non-bonding orbital in half-Heuslers

Modern society relies on high charge mobility for efficient energy production and fast information technologies. The power factor of a material—the combination of electrical conductivity and Seebeck coefficient—measures its ability to extract electrical power from temperature differences. Recent advancements in thermoelectric materials have achieved enhanced Seebeck coefficient by manipulating the electronic band structure. However, this approach generally applies at relatively low conductivities, preventing the realization of exceptionally high-power factors. In contrast, half-Heusler semiconductors have been shown to break through that barrier in a way that could not be explained. Here, we show that symmetry-protected orbital interactions can steer electron–acoustic phonon interactions towards high mobility. This high-mobility regime enables large power factors in half-Heuslers, well above the maximum measured values. We anticipate that our understanding will spark new routes to search for better thermoelectric materials, and to discover high electron mobility semiconductors for electronic and photonic applications.The intrinsic origin of high-power factors observed in half-Heusler alloys remains elusive, limiting the design of new thermoelectric materials. In this work, the authors reveal it is due to weakened electron–acoustic phonon coupling, originating from crystal symmetry protection of non-bonding orbitals.