Supporting Information

Transcription

1 Supporting Information Enhanced thermoelectricity in High-temperature β-phase Copper (I) Selenides embedded with Cu 2 Te nanoclusters Sajid Butt, a,b, * Wei Xu, c,f, * Muhammad U. Farooq, d Guang K. Ren, a Qinghua Zhang, a Yingcai Zhu, c Sajid U. Khan, b Lijuan Liu, c Meijuan Yu, e Fida Mohmed, a Yuanhua Lin, a and Ce-Wen Nan a, * a State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing , People s Republic of China b Department of Materials Science and Engineering, Institute of Space Technology, Islamabad 44000, Pakistan c Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing , P.R. China d School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing , People s Republic of China e Department of Physics, Zhengzhou University, No.100 Science Avenue, Zhengzhou, Henan, , P.R. China f RICMASS, Rome International Center for Materials Science Superstripes, Via dei Sabelli 119A, Rome, Italy *Corresponding authors: cwnan@tsinghua.edu.cn (C.W. Nan), xuw@mail.ihep.ac.cn (W. Xu) and sajidarif@hotmail.com (S. Butt) S-1

2 Contents 1. Structural aspects 2. Electronic structure 2.1 XPS 2.2 Electronic partial density of states (DOS) 3. Cluster calculations for hexagonal Cu 2 Te 4. Electrical transport properties S-2

3 1. Structural aspect Figure S1: Atomic scheme of hexagonal Cu 2 Te cluster for XANES simulation, in which each Te atom is coordinated with six Cu atoms in a hexagonal geometry Figure S2: The high temperature XRD patterns of pure and 10% Te-doped Cu 2 Se recorded at 473K in which both the pure and the doped specimens exhibit high-temperature β-phase with cubic symmetry (indexed by PDF#: ) S-3

4 Figure S3: The synchrotron X-ray diffraction patterns (SXRD) of (a) pure Cu 2 Se and (b) 10% Te doped Cu 2 Se, at elevated temperatures and the theoretical diffraction patterns of monoclinic Cu 2 Se, cubic Cu 2 Se and hexagonal Cu 2 Te phases. * refers to the peaks appearing by the grazing reflections of window in furnace. S-4

5 2. Electronic structure 2.1 XPS Figure S4: (a) The experimental Cu 2p XPS spectra (black dots) deconvoluted to Cu 1+ (blue) and Cu 2+ (yellow) and (b) Te 3d spectra, in which black dots represent the experimental values and the red solid line is the fitted curve. 2.2 Electronic partial density of states (DOS) As shown in Figure S5, the XANES spectra for Te L 3 -edge of hexagonal Cu 2 Te are compared with the projected density of states. Apparently, the pre-edge A originates from the hybridized Te d states and Cu p states in the vicinity of Fermi energy. The three distinctive features A, B S-5

6 and C are mainly due to the empty Te d states while feature A and C contains small contribution from Cu p states. Owing to directional distribution of Te d orbitals and the planar distribution of the Cu p orbitals, feature B is merely contributed by Te d states. By referring to the geometry of the TeCu 6 pyramids, the orbital hybridization is unlikely to occur along the z axis; therefore, in some directions, the spectral features contain no hybridized states but Te d states. The electronic configuration and charge transfer are listed in Table S2 for comparison. Figure S5: Comparison of the Te L 3 -edge XANES spectra with the electronic partial density of states (DOS) projected on (a) Te atoms and (b) Cu atoms 3. Cluster calculations for hexagonal Cu 2 Te The multiple scattering theory allows us to investigate the structural effects on the spectral features. It is known that the locations of the coordinated atoms surrounding the central absorber are on the certain circles; therefore, the coordinates at different distances from the central absorber are called shells, as given in Table S3. The shell-by-shell calculations can be performed S-6

7 by increasing shells, i.e. coordinates at equal distances, while keeping the potential unchanged. This allows us to study the pure geometric effects as imposed upon the spectral features. As shown in Figure S6(a), broaden feature appears when only two shells are involved; whereas the features a and b arise as the atoms in third shell participate in the scattering of photoelectrons. Figure S6: (a) Comparison of the Te L3-edge as calculated with increasing sizes of clusters, where the number refers to the number of shells included in the cluster, full is the largest cluster with 200 atoms. The geometric scheme of atomic clusters with (b) 3 shells and 4 shells, and (c) with 9 shells and 10 shells As shown in Figure S6(b), by adding six Te atoms at the central layer of the absorber, the features a, b and d become more pronounced. When 10 shells are involved, the entire spectral features, a, b, c and d are emerged. In Figure S6(c), the feature c pops up with the addition of six Te atoms in the bottom layers. This means that the feature c is due to the long distance scattering and the symmetrized resonant scattering between the bottom and the top layers of atoms. The S-7

8 1st derivative of XANES spectral features become much sharper as more atoms are involved. Therefore, as for nano-sized clusters, the spectral features appear at the certain sizes of the cluster while remain broadened until the size increases to the critical size for constructing a bulk feature Te L 3 -edge EXP Theory: Hexagonal Cu 2 Te Theory: Te@Se of monoclinic Cu 2 Se Theory: pure Te E-E 0 (ev) Figure S7: Comparison of the first derivative Te L 3 -edge XANES of experiment, theoretical calculations of hexagonal Cu 2 Te phase, substitution of Se by Te in monoclinic Cu 2 Se and pure Te phase, respectively S-8

9 4. Electrical transport properties Figure S8: The room temperature carrier s concentration (n) and carrier s mobility (µ) for pure and Te-doped specimens Table S1: The Chemical compositions, abbreviated names and lattice parameters of pure and doped specimens. Composition Abbreviation a (Å) b (Å) c (Å) Cu 2 Se Cu 2 Se Cu 2.1 SeTe 0.05 Te-5% Cu 2.2 SeTe 0.10 Te-10% Cu 2.3 SeTe 0.15 Te-15% Cu 2.4 SeTe 0.20 Te-20% S-9

10 Table S2: The charge transfer (CT) and the electronic configuration of hexagonal Cu 2 Te as calculated from FEFF code, s, p and d refer to the core-electron orbitals, charge transfer is the differences between electronic configurations calculated via self-consistent field method and the neutron state electronic configurations and stoichiometry refers to the atomic ratio in the atomic cluster in calculation. s p d CT Stoichiometry Te (excited) Cu Te (ground) Table S3: Atomic coordinates of shells in the hexagonal Cu 2 Te around Te absorber; bond distance is the distance between absorber and the coordinate atoms; coordinates contain information about the elements and the number of atoms. No Bond distance (Å) Coordinates No Bond distance (Å) Coordinates Cu Cu Te Te Cu Cu Te Te Te Cu Cu S-10

11 Table S4: The coordination numbers of Te in Cu 2 Te, Cu 2 Se and Te, bond distance is the distance between absorber and the coordinate atoms; coordinates contain information about the types and numbers of atoms. Coordination Hexagonal-Cu 2 Te Monoclinic-Cu 2 Se Te 1 6Cu, 2.46 Å 4Cu, Å, unequal 2Te, 2.89 Å 2 1Te, 2.60Å 4Cu, Å, unequal 4Te, 3.50Å 3 6Cu, 3.85 Å 3Se, Å, unequal 8Te, 4.50Å 4 6Te, 3.90Å 2Cu+6Se, Å, unequal 4Te, 4.90Å S-11