Jin-Cheng Zheng

Strain engineering of thermal conductivity in graphene sheets and nanoribbons: a demonstration of magic flexibility

Graphene is an outstanding material with ultrahigh thermal conductivity. Its thermal transfer properties under various strains are studied by reverse nonequilibrium molecular dynamics. Based on the unique two-dimensional structure of graphene, the distinctive geometries of graphene sheets and graphene nanoribbons with large flexibility and their intriguing thermal properties are demonstrated under strains. For example, the corrugation under uniaxial compression and helical structure under light torsion, as well as tube-like structure under strong torsion, exhibit enormously different thermal conductivity. The important robustness of thermal conductivity is found in the corrugated and helical configurations of graphene nanoribbons. Nevertheless, thermal conductivity of graphene is very sensitive to tensile strain. The relationship among phonon frequency, strain and thermal conductivity are analyzed. A similar trend line of phonon frequency dependence of thermal conductivity is observed for armchair graphene nanoribbons and zigzag graphene nanoribbons. The unique thermal properties of graphene nanoribbons under strains suggest their great potentials for nanoscale thermal managements and thermoelectric applications.

On the origin of increased phonon scattering in nanostructured PbTe based thermoelectric materials.

We have investigated the possible mechanisms of phonon scattering by nanostructures and defects in PbTe-X (X = 2% Sb, Bi, or Pb) thermoelectric materials systems. We find that among these three compositions, PbTe-2% Sb has the lowest lattice thermal conductivity and exhibits a larger strain and notably more misfit dislocations at the precipitate/PbTe interfaces than the other two compositions. In the PbTe-Bi 2% sample, we infer some weaker phonon scattering BiTe precipitates, in addition to the abundant Bi nanostructures. In the PbTe-Pb 2% sample, we also find that pure Pb nanoparticles exhibit stronger phonon scattering than nanostructures with Te vacancies. Within the accepted error range, the theoretical calculations of the lattice thermal conductivity in the three systems are in close agreement with the experimental measurements, highlighting the important role of misfit dislocations, nanoscale particles, and associated interfacial elastic strain play in phonon scattering. We further propose that such particle-induced local elastic perturbations interfere with the phonon propagation pathway, thereby contributing to further reduction in lattice thermal conductivity, and consequently can enhance the overall thermoelectric figure of merit.

Recent advances on thermoelectric materials

By converting waste heat into electricity through the thermoelectric power of solids without producing greenhouse gas emissions, thermoelectric generators could be an important part of the solution to today’s energy challenge. There has been a resurgence in the search for new materials for advanced thermoelectric energy conversion applications. In this paper, we will review recent efforts on improving thermoelectric efficiency. Particularly, several novel proof-of-principle approaches such as phonon disorder in phonon-glass-electron crystals, low dimensionality in nanostructured materials and charge-spin-orbital degeneracy in strongly correlated systems on thermoelectric performance will be discussed.

Role of Sodium Doping in Lead Chalcogenide Thermoelectrics

The solubility of sodium and its effects on phonon scattering in lead chalcogenide PbQ (Q = Te, Se, S) family of thermoelectric materials was investigated by means of transmission electron microscopy and density functional calculations. Among these three systems, Na has the highest solubility limit (~2 mol %) in PbS and the lowest ~0.5 mol %) in PbTe. First-principles electronic structure calculations support the observations, indicating that Na defects have the lowest formation energy in PbS and the highest in PbTe. It was also found that in addition to providing charge carriers (holes) for PbQ, Na introduces point defects (solid solution formation) and nanoscale precipitates; both reduce the lattice thermal conductivity by scattering heat-carrying phonons. These results explain the recent reports of high thermoelectric performance in p-type PbQ materials and may lead to further advances in this class of materials.

Pressure‐volume‐temperature relations in MgO: An ultrahigh pressure‐temperature scale for planetary sciences applications

[1] In situ crystallography based on diamond anvil cells have been extended to the multimegabar regime. Temperatures in these experiments have crossed the 2500 K mark. Yet, current high pressure-temperature (PT) standards of calibration produce uncertainties that inhibit clear conclusions about phenomena of importance to planetary processes, e.g., the postperovskite transition in Earth’s mantle. We introduce a new thermal equation of state (EOS) of MgO which appears to be predictive up to the multimegabar and thousands of kelvin range. It is obtained by combining first principles local density approximation quasi-harmonic (QHA) calculations with experimental lowpressure data. This EOS agrees exceptionally well with shock compression data. The postspinel and postperovskite phase boundaries recalculated using our EOS match seismic observations. The latter, in particular, supports the idea that postperovskite transforms back to perovskite before the core-mantle boundary. The recalculated experimental Clapeyron slope of the postperovskite transition is also more consistent with those obtained by first principles calculations. Citation: Wu, Z., R. M. Wentzcovitch, K. Umemoto, B. Li, K. Hirose, and J.-C. Zheng (2008), Pressure-volume-temperature relations in MgO: An ultrahigh pressure-temperature scale for planetary sciences applications, J. Geophys. Res., 113, B06204,

Graphene-nanotube 3D networks: intriguing thermal and mechanical properties

Carbon-based nanomaterials have drawn strong interest for potential applications due to their extraordinary stability and unique mechanical, electrical and thermal properties. For the minimization of microelectronics/micromechanics circuits, bridging the low dimensional microscopic structure and mesoscopic modeling is indispensable. Graphene and carbon nanotubes are suggested as ideal ‘building blocks’ for the bottom-up strategy, and recently the integration of both materials has stimulated research interests. In this work we investigated the thermal and mechanical performance in the pillared-graphene – constructed by combining graphene sheets and carbon nanotubes to create a three-dimensional nano network. Reverse non-equilibrium molecular dynamics simulations were carried out to analyze the thermal transport behavior. The obtained thermal conductivities are found to be possibly isotropic in two specific directions or highly anisotropic for certain structure configurations. In the mechanical performance analysis, tensile deformations are loaded along graphene plane and along tube axis. The elongation responses and stress-strain relations are observed to be nearly linear, and the calculated strength, fracture strain and Youngs moduli are lower than the pristine graphene or carbon nanotubes. The alterations in the thermal and mechanical performances are ascribed to the bond conversion on the junctions.

Tuning the indirect-direct band gap transition of SiC, GeC and SnC monolayer in a graphene-like honeycomb structure by strain engineering: A quasiparticle GW study

We have calculated the electronic properties of graphene and SiC, GeC and SnC monolayers in a two-dimensional graphene-like honeycomb structure under various strained conditions using first principles calculations based on density functional theory and the quasiparticle GW approximation. Our results show that the indirect–direct band gap transition of group-IV carbides can be tuned by strain, which indicates a possible new route for tailoring the electronic properties of ultrathin nanofilms through strain engineering.

Physical and chemical mechanisms in oxide-based resistance random access memory

In this review, we provide an overview of our work in resistive switching mechanisms on oxide-based resistance random access memory (RRAM) devices. Based on the investigation of physical and chemical mechanisms, we focus on its materials, device structures, and treatment methods so as to provide an in-depth perspective of state-of-the-art oxide-based RRAM. The critical voltage and constant reaction energy properties were found, which can be used to prospectively modulate voltage and operation time to control RRAM device working performance and forecast material composition. The quantized switching phenomena in RRAM devices were demonstrated at ultra-cryogenic temperature (4K), which is attributed to the atomic-level reaction in metallic filament. In the aspect of chemical mechanisms, we use the Coulomb Faraday theorem to investigate the chemical reaction equations of RRAM for the first time. We can clearly observe that the first-order reaction series is the basis for chemical reaction during reset process in the study. Furthermore, the activation energy of chemical reactions can be extracted by changing temperature during the reset process, from which the oxygen ion reaction process can be found in the RRAM device. As for its materials, silicon oxide is compatible to semiconductor fabrication lines. It is especially promising for the silicon oxide-doped metal technology to be introduced into the industry. Based on that, double-ended graphene oxide-doped silicon oxide based via-structure RRAM with filament self-aligning formation, and self-current limiting operation ability is demonstrated. The outstanding device characteristics are attributed to the oxidation and reduction of graphene oxide flakes formed during the sputter process. Besides, we have also adopted a new concept of supercritical CO2 fluid treatment to efficiently reduce the operation current of RRAM devices for portable electronic applications.

Photoluminescence studies of SiC nanocrystals embedded in a SiO2 matrix

Abstract The dependence of the photoluminescence (PL) from SiC nanocrystals embedded in a SiO 2 matrix on annealing is presented. Blue-green PL has been observed at room temperature from annealed SiC–SiO 2 composite films. The intensity of the single emission band at 460 nm (2.7 eV) shows a strong dependence on the annealing temperature. The combination of high-resolution transmission electron microscopy (HRTEM), Fourier transform infrared (FTIR) transmission spectra and PL results suggest that SiC nanocrystals have been incorporated into the SiO 2 matrix and O-deficient defects were formed. The origin of luminescence is attributed to the creation of defects in silicon oxide.

Strong Phonon Scattering by Layer Structured PbSnS2 in PbTe Based Thermoelectric Materials

The incorporation of PbSnS(2) in PbTe results in a tremendous reduction of the lattice thermal conductivity to 0.8 W/mK at room temperature, a reduction of almost 60% over bulk PbTe. Transmission electron microscopy reveals very high density displacement layers, misfit dislocations, and phase boundaries. Our thermal transport calculations based on modified Debye-Callaway model, well in agreement with the experimental measurements, reveal that the layer structured PbSnS(2) plays an important role in reducing the lattice thermal conductivity.