Qiyin Lin

Low thermal conductivity in nanoscale layered materials synthesized by the method of modulated elemental reactants

We report the room-temperature, cross-plane thermal conductivities, and longitudinal speeds of sound of multilayer films [(TiTe2)3(Bi2Te3)x(TiTe2)3(Sb2Te3)y]i (x=1−5,y=1−5) and misfit-layer dichalcogenide films [(PbSe)m(TSe2)n]i (T=W or Mo, m=1–5, and n=1–5) synthesized by the modulated elemental reactants method. The thermal conductivities of these nanoscale layered materials fall below the predicted minimum thermal conductivity of the component compounds: two times lower than the minimum thermal conductivity of Bi2Te3 for multilayer [(TiTe2)3(Bi2Te3)x(TiTe2)3(Sb2Te3)y]i films and five to six times lower than the minimum thermal conductivity of PbSe for misfit-layer dichalcogenides [(PbSe)m(TSe2)n]i. We attribute the low thermal conductivities to the anisotropic bonding of the layered crystals and orientational disorder in the stacking of layered crystals along the direction perpendicular to the surface.

Designed Synthesis, Structure, and Properties of a Family of Ferecrystalline Compounds [(PbSe)1.00]m(MoSe2)n

The targeted synthesis of multiple compounds with specific controlled nanostructures and identical composition is a grand challenge in materials chemistry. We report the synthesis of the new metastable compounds [(PbSe)1.00]m(MoSe2)n using precursors each designed to self-assemble into a specific compound. To form a compound with specific values for m and n, the number of atoms within each deposited elemental layer was carefully controlled to provide the correct absolute number of atoms to form complete layers of each component structural unit. On low-temperature annealing, these structures self-assemble with a specific crystallographic orientation between the component structural units with atomically abrupt interfaces. There is rotational disorder between the component structural units and between MoSe2 basal plane units within the MoSe2 layers themselves. The lead selenide constituent has a distorted rock salt structure exactly m bilayers thick leading to peaks in the off-axis diffraction pattern as a result of the finite size of and rotational disorder between the crystallites. The in-plane lattice parameters of the PbSe and MoSe2 components are independent of the value of m and n, suggesting little or no strain caused by the interface between them. These compounds are small band gap semiconductors with carrier properties dominated by defects and exhibit extremely low thermal conductivity as a result of the rotational disorder. The thermal conductivity can be tuned by varying the ratio of the number of ordered PbSe rock salt layers relative to the number of rotationally disordered MoSe2 layers. This approach, based on controlling the local composition of the precursor and low temperature to limit diffusion rates, provides a general route to the synthesis of new compounds containing alternating layers of constituents with designed nanoarchitecture.

In-plane thermal and thermoelectric properties of misfit-layered [(PbSe)0.99]x(WSe2)x superlattice thin films

The in-plane thermal conductivity is measured to be three times lower in misfit-layered [(PbSe)0.99]x(WSe2)x superlattice thin films than disordered-layered WSe2 because of interface scattering despite a higher cross-plane value in the former than the latter. While having little effect on the in-plane thermal conductivity, annealing the p-type [(PbSe)0.99]2(WSe2)2 films in Se increases the in-plane Seebeck coefficient and electrical conductivity because of decreased defect and hole concentrations. Increasing interface density of the annealed films by decreasing x from 4 to 2 has weak influence on the in-plane thermal conductivity but increases the Seebeck coefficient and decreases the room-temperature electrical conductivity.

Size-Dependent Structural Distortions in One-Dimensional Nanostructures†

Nanoscale materials have been intensely studied since the discovery that the optical properties of semiconductor nanoparticles are size dependent. This and subsequent research has demonstrated that a given physical property of a particle exhibits a size dependence when the size becomes comparable to its characteristic length scale. Examples of relevant length scales include the de Broglie wavelength and/or the mean free path of electrons, phonons, and elementary excitations, all of which typically range from one to a few hundred nanometers. The ability to tune a wide variety of properties by controlling the particle size has spurred the development of novel chemistries for preparing nanostructured elements and compounds with goals of precisely controlling size, shape, and ligand shell. As the size of a nanocrystal decreases, the ratio of bulk to surface atoms decreases. This progression increases the relative contribution of the surface free-energy relative to the volume free-energy of the bulk structure, such that distortions from bulk equilibrium structures might be expected as the nanoparticle size decreases. Unfortunately, while researchers have demonstrated the ability to prepare ordered lattices of nanoparticles, the isolation of lattices of nanoparticles with long-range atomic periodicity is rare. Hence detailed atomic structures and, in turn, the size-structure-property relationships of most nanoparticle systems cannot readily be determined. Recently we reported that the intergrown compounds [(MSe)1+y]m(TSe2)n, with M= {Pb, Bi, Ce} and T= {W, Nb, Ta} self-assemble from designed precursors. The values of m and n represent, respectively, the number of MSe and TSe2 structural units of the unit cell of the superstructure and y describes the misfit between these structural units. As reported herein, the long-range structural order along the modulation direction permits us to determine the atomic structure of these precisely defined one-dimensional (1D) nanolaminate structures as a function of m and n using a combination of scanning transmission electron microscopy (STEM) high-angle annular dark-field (HAADF) imaging and X-ray diffraction (XRD) with Rietveld refinement. STEM-HAADF images of the first five [(PbSe)1.00]m(MoSe2)n compounds in the family where m= n are shown in Figure 1 along with aggregate intensity plots used to quantify the PbSe intraand inter-pair distances. All have a regular periodic structure along the modulated axis with well-defined layers of PbSe and MoSe2. The STEM images show ordered domains of PbSe with characteristic dimensions of a single structural unit along the layering direction and tens of nanometers perpendicular to the layering direction, with random in-plane rotational variants both within a layer and between layers. The orientations of the MoSe2 domains are more difficult to discern from the STEM images, but rotational variants have been observed between individual MoSe2 structural units. The STEM-HAADF images reveal a distortion of the PbSe layers, with the atomic planes grouped into pairs rather than being evenly spaced as expected for the equilibrium (bulk) rock salt structure. The distortion is most evident in the structural variant (m, n)= (2, 2) and decreases in magnitude until it can no longer be observed for (5, 5).