Shiyun Xiong

Functionalization mediates heat transport in graphene nanoflakes.

The high thermal conductivity of graphene and few-layer graphene undergoes severe degradations through contact with the substrate. Here we show experimentally that the thermal management of a micro heater is substantially improved by introducing alternative heat-escaping channels into a graphene-based film bonded to functionalized graphene oxide through amino-silane molecules. Using a resistance temperature probe for in situ monitoring we demonstrate that the hotspot temperature was lowered by ∼28 °C for a chip operating at 1,300 W cm−2. Thermal resistance probed by pulsed photothermal reflectance measurements demonstrated an improved thermal coupling due to functionalization on the graphene–graphene oxide interface. Three functionalization molecules manifest distinct interfacial thermal transport behaviour, corroborating our atomistic calculations in unveiling the role of molecular chain length and functional groups. Molecular dynamics simulations reveal that the functionalization constrains the cross-plane phonon scattering, which in turn enhances in-plane heat conduction of the bonded graphene film by recovering the long flexural phonon lifetime.

Large thermal conductivity decrease in point defective Bi2Te3 bulk materials and superlattices

Defective Bi2Te3 structures have been studied with the aim of lowering the thermal conductivity in order to improve the thermoelectric figure of merit. The cross-plane thermal conductivities of structures containing point defects have been computed by means of molecular dynamics techniques, finding a maximum decrease of 70% for a 4% concentration of tellurium atom vacancies. Superlattices with modified stoichiometries have also been considered in order to find the configuration having the lowest thermal conductivity. In this case, a maximum decrease of 70% was also found. These predictions open the way to the design of efficient bulk thermoelectric materials having optimised thermal properties similar to those of superlattices.

Mechanical Tuning of Thermal Transport in a Molecular Junction

Understanding and controlling heat transport in molecular junctions would provide new routes to design nanoscale coupled electronic and phononic devices. Using first-principles full quantum calculations, we tune thermal conductance of a molecular junction by mechanically compressing and extending a short alkane chain connected to graphene leads. We find that the thermal conductance of the compressed junction drops by half in comparison to the extended junction, making it possible to turn on and off the heat current. The low conductance of the off state does not vary by further approaching the leads and stems from the suppression of the transmission of the in-plane transverse and longitudinal channels. Furthermore, we show that misalignment of the leads does not reduce the conductance ratio. These results also contribute to the general understanding of thermal transport in molecular junctions.

Modeling Size and Shape Effects on the Order–Disorder Phase‐Transition Temperature of CoPt Nanoparticles

�b is the cohesive energy per atom of the bulk material and N is the total number of atoms. In general, a NP is enclosed by several face planes. For the i -th plane, its surface area is S i and the corresponding atom density is ρ i . The parameter A i is the ratio between the total number of bonds of a surface atom in the i -th plane and total number of bonds of a core atom. Then

Size‐, Shape‐ and Composition‐Dependent Alloying Ability of Bimetallic Nanoparticles

Based on the surface-area-difference model, the formation enthalpies and the formation Gibbs free energies of bimetallic nanoparticles are calculated by considering size and shape effects. Composition-critical size diagrams were graphed for bulk immiscible bimetallic nanoparticles with the developed model. The results reveal that both the formation enthalpy and formation Gibbs free energy decrease with the decrease of particle size. The effect of rising temperature is similar to the diminishing of particle size on reducing the formation Gibbs free energy. Contrary to the positive formation enthalpy of the bulk immiscible system, a negative formation enthalpy is obtained when the particles are smaller than a critical size. With the decrease of size, the alloying process first takes place in the dilute solute regions, then broadens to the dense solute regions and finally, particles with all compositions can be alloyed. The composition-critical size diagram is classified into three regions by the critical size curves with shape factors of 1 and 1.49, that is, the non-alloying region, alloying region and possible alloying region. The model predictions correspond well with experimental evidences and computer simulation results for Cu-Ag, Au-Ni, Ag-Pt and Au-Pt systems.

Classical to Quantum Transition of Heat Transfer between Two Silica Clusters

Heat transfer between two silica clusters is investigated by using the nonequilibrium Greens function method. In the gap range between 4 Å and 3 times the cluster size, the thermal conductance decreases as predicted by the surface charge-charge interaction. Above 5 times the cluster size, the volume dipole-dipole interaction predominates. Finally, when the distance becomes smaller than 4 Å, a quantum interaction where the electrons of both clusters are shared takes place. This quantum interaction leads to the dramatic increase of the thermal coupling between neighbor clusters due to strong interactions. This study finally provides a description of the transition between radiation and heat conduction in gaps smaller than a few nanometers.

Thermally-Active Screw Dislocations in Si Nanowires and Nanotubes

that in bulk, dislocations can lead to a signifi cant decrease in κ in the direction perpendicular to the dislocation line. The widely-used theory of Klemens [ 11 ] accounts for this result with perturbation theory, by including scattering of the phonon states (eigenstates of the harmonic crystal) on the linear and non-linear elastic strain regions localized around the dislocation line. By contrast, in SD NWs and NTs, we encounter the unexplored case of thermal transport along the dislocation line. In this work we combine modern theories based on atomistic simulations in order to understand how the thermal properties of Si NWs and NTs accommodating axial SDs with closed and opened cores might differ from the more studied pristine forms. After computing the SD NW and NT structures with objective molecular dynamics [ 12 ] (MD), we used two main methods, the direct method [ 13 ] and the atomistic Green function (AGF) method [ 14 ] (See simulation section), to reveal an important reduction in κ . This fi nding presents signifi cant interest for nanoscale thermoelectricity. We simulated a set of pristine and SD Si NWs and NTs with cubic diamond structure and hexagonal cross sections. The number of 111 layers L in the cross-section was taken to be 12, 16, 20 and 30, so that the radii of the created NWs ranged from 18.8 A to 47.1 A. Next, from the pristine L = 12 NW we created a set of (L,h) NTs, by systematically removing central atomic layers. We label by h the number of 111 inner layers that have been removed. Finally, in all these structures we introduce SDs with the axis located at the center. We considered minimal Burgers vector of magnitude b = 3.8 A and multiples of it, 2b and 3b. In 1b NWs, the created core structure is the Hornstra core, where all atoms remain fourfold coordinated. SDs twist NWs and NTs. This is the Eshelby twist [ 15 ] γ E , which is well known at the macroscale. The presence of γ E creates challenges for the atomistic simulations as it prevents the applicability of the standard periodic boundary conditions. Here, in order to fi nd optimal morphologies (corresponding to minimum energy) we used objective MD [ 12 ]

Native surface oxide turns alloyed silicon membranes into nanophononic metamaterials with ultralow thermal conductivity

Native surface oxide turns alloyed silicon membranes into nanophononic metamaterials with ultra-low thermal conductivity Shiyun Xiong 1,2 , ∗ Daniele Selli 2 , Sanghamitra Neogi 3 , and Davide Donadio 4† arXiv:1705.03143v1 [cond-mat.mtrl-sci] 9 May 2017 1 Functional Nano and Soft Materials Laboratory (FUNSOM) and Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou, Jiangsu 215123 , P.R. China 2 Max Planck Institute for Polymer Research, Ackermannweg 10, 55218 Mainz, Germany 3 Department of Aerospace Engineering Sciences, University of Colorado Boulder, Boulder, Colorado 80309, USA 4 Department of Chemistry, University of California Davis, One Shields Ave. Davis, 95616, CA A detailed understanding of the relation between microscopic structure and phonon propagation at the nanoscale is essential to design materials with desired phononic and thermal properties. Here we uncover a new mechanism of phonon interaction in surface oxidized membranes, i.e., native oxide layers interact with phonons in ultra-thin silicon membranes through local resonances. The local resonances reduce the low frequency phonon group velocities and shorten their mean free path. This effect opens up a new strategy for ultralow thermal conductivity design as it complements the scattering mechanism which scatters higher frequency modes effectively. The combination of native oxide layer and alloying with germanium in concentration as small as 5% reduces the thermal conductivity of silicon membranes to 100 time lower than the bulk. In addition, the resonance mechanism produced by native oxide surface layers is particularly effective for thermal condutivity reduction even at very low temperatures, at which only low frequency modes are populated. Controlling terahertz vibrations and heat transport in nanostructures has a broad impact on several ap- plications, such as thermal management in micro- and nano-electronics, renewable energies harvesting, sensing, biomedical imaging and information and communication technologies [1–8]. Significant efforts have been made to understand and engineer heat transport in nanoscale silicon due to its natural abundance and technological relevance [9–12]. In the past decade researchers ex- plored strategies to obtain silicon based materials with low thermal conductivity (TC) and unaltered electronic transport coefficients, so to achieve high thermoelectric figure of merit and enable silicon-based thermoelectric technology[11–18]. From the earlier studies it was recognized that low- dimensional silicon nanostructures, such as nanowires, thin films and nano membranes feature a largely re- duced TC, up to 50 times lower than that of bulk at room temperature. TC reduction becomes more promi- nent with the reduction of the characteristic dimension of the nanostructures [19–22]. Theory and experiments consistently show that surface disorder and the pres- ence of disordered material at surfaces play a major role in determining the TC of nanostructures [12, 23–25]. However, a comprehensive understanding of the physi- cal mechanisms underlying so large TC reduction is lack- ing. The effect of surface roughness and surface disor- der on phonons has been so far interpreted in terms of phonon scattering [26–30], but scattering would not ac- count for mean free path reduction of long-wavelength low-frequency modes. Recent theoretical work demon- strated that surface nanostructures, such as nanopillars at the surface of thin films or nanowires, can efficiently reduce TC through resonances, a mechanism that is in- trinsically different from scattering [31, 32]. Surface res- onances alter directly phonon dispersion relations by hy- bridizing with propagating modes in the nanostructures, thus hampering their group velocity. In this Communication we unravel the effect of native oxide surface layers on thermal transport in ultra-thin silicon membrane models that closely resemble experi- ments, by atomistic molecular dynamics simulations. We show that the observed low TC in these systems, and plausibly in other oxide coated silicon nanostructures, is predominantly due to resonances analogous to those occurring in nanophononic metamaterials. It is worth stressing that surface oxide layers in low dimensional Si materials grow spontaneously at atmospheric conditions, and do not require any specific processing. Our simu- lations ascertain the occurrence of low frequency reso- nant modes that hybridize with the acoustic branches (ω . 4 THz) of the membrane, effectively suppressing their mean free path (MFP). This discovery opens up the possibility to further optimize the TC of ultra-thin silicon membranes by combining resonances with mass scattering, which affects phonons with higher frequency (ω & 4 THz). We show that alloying the crystalline core of ultra-thin membranes with a small percentage of sub- stitutional germanium atoms brings forth ultra-low TC in silicon membranes with technologically viable thick- ness [33]. All TCs are calculated with equilibrium molecular dy- namics (EMD) simulations at 300 K using LAMMPS [34] with interatomic interactions described by the widely used Tersoff potential [35–37]. The equations of mo- tion are integrated with the velocity Verlet algorithm

Generalized Bragg–Williams model for the size-dependent order–disorder transition of bimetallic nanoparticles

Considering the different effects of exterior atoms (face, edge and corner atoms), the Bragg?Williams model is generalized to account for the size, shape and composition-dependent order?disorder transition of bimetallic nanoparticles (NPs) with B2, L10 and L12 ordered structures. The results show that the order?disorder temperatures TC,p are different for different shapes even in the identical particle size. The order of order?disorder temperatures of different shapes varies for different sizes. The long-range order parameter decreases with the increase in temperature in all size ranges and decreases smoothly in large sizes, but drops dramatically in small sizes. Moreover, it is also found that the order?disorder temperature of bimetallic NPs rises with increasing particle sizes and decreases with a deviation from the ideal compositions. The present predictions are consistent with the available literature results, indicating its capability in predicting other order?disorder transition phenomena of bimetallic NPs.

Tunable phase stability and contact resistance of monolayer transition metal dichalcogenides contacts with metal

Monolayer transition metal dichalcogenides/metal (MX2/metal) based transistors have been widely studied. However, further development is hindered by the large contact resistance between MX2 and metal contact. In this paper, we demonstrated that interfacial charge transfer between MX2 and metal is the key for tuning contact resistance. With the lattice misfit criterion applied to screen combination of MX2s and metals, it has been found out that both phase stability of MX2 and contact nature between MX2 and metal will be sensitively affected by interfacial charge transfer. Additionally, we have identified seven MX2/metal systems that can potentially form zero Schottky barrier contacts utilizing phase engineering. On base of interfacial charge calculations and contact resistance analysis, we have presented three types of MX2/metal contacts that can be formed with distinguished contact resistance. Our theoretical results not only demonstrate various choice of MX2/metal designs in order to achieve different amounts of interfacial charge transfer as well as manipulate contact resistance, but also shed light on designing ohmic contacts in MX2/metal systems.Contact engineering: calculations unveil the contact nature of MX 2 /metal structuresInterfacial charge calculations enable the prediction of the contact resistance behaviour of MX2/metal structures. A team led by Bin Ouyang at the University of California Berkeley performed a systematic theoretical investigation of the interplay between interface interactions and phase stability in atomically thin MX2/metal systems, where M is a transition metal and X is a chalcogenide. A combination of interfacial charge calculations and contact resistance analysis allowed the identification of twenty-eight MX2/metal structures that can be further categorised in three groups according to their contact nature. Notably, the first type of contact possesses zero tunnel barrier between MX2 and the metal, whereas the second type enables substantial charge transfer accompanied to a 2H-to-1T’ structural phase transition in MX2. These results highlight viable design routes for contact resistance manipulation in MX2 transistors.