The Pennsylvania State University The Graduate School Department of Physics PRESSURE TUNING OF PNICOGEN CHALCOGENIDE THERMOELECTRICS A Thesis in Physics by Thomas J. Scheidemantel c 2004 Thomas J. Scheidemantel ° Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy August 2004 The thesis of Thomas J. Scheidemantel has been reviewed and approved∗ by the following: John V. Badding Associate Professor of Chemistry Thesis Co-Adviser Co-Chair of Committee Gerald D. Mahan Distinguished Professor of Physics Thesis Co-Adviser Co-Chair of Committee Peter E. Schiffer Professor of Physics Jorge O. Sofo Associate Professor of Physics James B. Anderson Evan Pugh Professor of Chemistry Jayanth R. Banavar Professor of Physics Head of the Department of Physics ∗Signatures are on file in the Graduate School. iii Abstract The thought of a cooling device or power generator with no moving parts seems almost a part of science fiction. Such devices do exist. They are used in a small niche of applications ranging from the cooling of individual solid state components to generating power aboard deep space telescopes. In this document, we present research that attempts to further improve and understand presently used and potentially new thermoelectric materials. To accomplish this we will use the well established technique of pressure tuning, along with first-principles calculations to study the effects of arsenic on currently used room temperature thermoelectric materials. The small size of arsenic may provide extra degrees of freedom in these currently used alloys. We found that pressure provides a route to the phase of As Te that is isostruc- 2 3 tural with the rhombohedral (R3m) structures of Bi Te and Sb Te . The ambient 2 3 2 3 pressure phase of As Te is monoclinic. We also found that As Te is more soluble in 2 3 2 3 group V-VI alloys containing Sb Se than in Bi Te or Sb Te alone. 2 3 2 3 2 3 We also present a new host for low-dimensional thermoelectric structures. Some thermoelectric properties are enhanced by lowering the dimensionality of some materials. Current research though uses hosts and techniques that are expensive and not feasible for scaling up to commercial levels. We present a host and some techniques that may make this up-scaling a reality. iv Table of Contents List of Tables ...................................... viii List of Figures ..................................... ix Acknowledgments ................................... xiv Chapter 1. Introduction ................................ 1 1.1 History................................... 1 1.2 EfficiencyofThermoelectricDevices . 3 1.3 Present Efficiencies of Thermoelectric Devices . .... 6 1.4 CurrentResearch............................. 7 1.5 GoalofthisThesis ............................ 8 1.5.1 Arsenic .............................. 8 1.5.2 Low-DimensionalThermoelectrics . 11 Chapter 2. Experimental Methods in the Diamond Anvil Cell .......... 12 2.1 Introduction................................ 12 2.2 SamplePreparation............................ 13 2.3 MeasurementofThermoelectricPower . 16 2.4 Powder X-ray Diffraction and Raman Spectroscopy . 17 2.5 ResultsforStandards........................... 20 2.6 Conclusions ................................ 20 v Chapter 3. Transport Coefficients from First-Principles Calculations ....... 24 3.1 Introduction................................ 24 3.2 The Transport Distribution and Transport Coefficients . ..... 25 3.3 Implementation.............................. 29 3.4 Application to Bi Te .......................... 31 2 3 3.4.1 Computational Details . 31 3.4.2 Results .............................. 32 3.4.2.1 BandStructure. 32 3.4.2.2 Transport Coefficients of Bi Te ........... 36 2 3 3.4.2.3 Analysis of Transport Coefficients . 38 3.5 Conclusions ................................ 43 Chapter 4. Electronic Structure and Transport Properties of α and β As Te . 45 − 2 3 4.1 Introduction................................ 45 4.2 Method .................................. 48 4.3 ResultsandDiscussion.......................... 50 4.3.1 BandStructures ......................... 50 4.3.1.1 α As Te ....................... 50 − 2 3 4.3.1.2 β As Te ....................... 53 − 2 3 4.3.2 TransportProperties. 57 4.4 Conclusions ................................ 59 Chapter 5. Thermoelectric Power of As Te and Alloys Containing As Te Under 2 3 2 3 Pressure .................................. 61 vi 5.1 Introduction................................ 61 5.1.1 As Te .............................. 61 2 3 5.1.2 Alloys ............................... 62 5.2 ExperimentalDetails........................... 64 5.2.1 As Te .............................. 64 2 3 5.2.2 Alloys ............................... 66 5.3 ResultsandDiscussion.......................... 66 5.3.1 As Te .............................. 66 2 3 5.3.2 Alloys ............................... 70 5.4 Conclusions ................................ 72 Chapter 6. Low-Dimensional Structures in Silica Hosts .............. 76 6.1 Introduction................................ 76 6.2 ExperimentalMethods .......................... 80 6.2.1 Deposition............................. 80 6.2.1.1 Germanium....................... 81 6.2.1.2 Silicon.......................... 81 6.2.1.3 Arsenic ......................... 81 6.2.2 Analysis.............................. 82 6.3 ResultsandDiscussion.......................... 82 6.3.1 Hosts ............................... 82 6.3.2 Deposition............................. 84 6.3.2.1 Germanium....................... 84 vii 6.3.2.2 Silicon.......................... 84 6.3.2.3 Arsenic ......................... 89 6.4 Conclusions ................................ 89 Chapter 7. Conclusions ................................ 91 References ........................................ 94 viii List of Tables 1.1 Groups III, IV, V, and VI of the periodic table . .... 9 1.2 Summary of group V-VI compounds . 9 6.1 First-order Raman peaks of arsenic [55]. 90 ix List of Figures 1.1 A schematic example of a thermoelectric cooler. The majority carriers of each leg carry their thermal energy away from the heat source in response totheflowingcurrent. ............................ 4 2.1 The piston-cylinder assembly (a) of the Mao-Bell diamond anvil cell used in our lab. The assembly is shown in the cell body (b). 14 2.2 A view of the sample assembly looking down the axis of the diamond (a) and from the side (b) is shown. 15 2.3 DAC arrangement for x-ray diffraction experiments. ..... 19 2.4 Thermoelectric power of CePd versuspressure.. 21 3 2.5 The powder diffraction pattern for silver inside the DAC at zero pressure is plotted versus d-spacing. The distance from the sample to the film is then calculated using the known d-spacing. 22 3.1 The electronic structure of Bi Te is shown with and without SO. The 2 3 figure shows the importance of SO in Bi Te by moving band edges 2 3 furtherfromtheFermienergyatΓ.. 33 3.2 The energy difference between the two states, ε and ε , is shown ΓΥ ΓZ versus the upper limit of the energy window used. An upper limit above 8.0Ry issufficienttoproducereliableresults. 35 x 3.3 Calculated Seebeck coefficients with experimental data from reference [33]. 37 3.4 Calculated electrical conductivity with experimental data from refer- ence[20]. ................................... 39 3.5 Estimated values of ZT with data from reference [33]. 40 3.6 The (a) TD, (b) density of states, (c) Seebeck coefficient, (d) PF, and (e) ZT are all plotted versus chemical potential. Lines depicting the size of Bi Te ’s energy gap span the plot. Lines indicating a reasonable doping 2 3 20 3 range (n,p < 10 cm− ) are shown with the PF (d). 42 4.1 (a) Layered structure and (b) Brillouin zone of α As Te . ....... 47 − 2 3 4.2 The (a) rhombohedral β-As Te unit cell with the three inequivalent 2 3 atoms labeled, and (b) the Brillouin zone indicating special ~k-points are shown...................................... 47 4.3 The band structure of α As Te with and without spin orbit coupling − 2 3 includedisshown. .............................. 51 4.4 The density of states of α As Te ...................... 52 − 2 3 4.5 Contours of the (a) HVB and (b) LCB in the mirror plane containing Γ, U, and Z are shown. Dark areas indicate maxima and light indicate minima. The band edges are labeled m. The contours are separated by 0.1eV ..................................... 54 xi 4.6 The band structure of β-As Te (solid line) is plotted for comparison 2 3 with Bi Te (dotted line). β-As Te ’s band edges are located at m and 2 3 2 3 bismuth telluride’s at b............................ 55 4.7 The density of states of β-As Te . The inset shows a closer view near 2 3 theFermienergy................................ 58 4.8 The transport distribution of β As Te is shown along with that of − 2 3 Bi Te . .................................... 58 2 3 14 4.9 The calculated power factors, using τ = 1.0 10− s, of β As Te are × − 2 3 shown along with those of Bi Te . ..................... 60 2 3 5.1 High pressure and temperature phase diagram of As Te . The authors 2 3 studied the structure up to only 1.7GP a. ................. 63 5.2 The thermoelectric power of pure As Te is plotted versus pressure. 68 2 3 5.3 The logarithm of the resistance with respect to atmospheric pressure resistance of As Te is plotted against pressure. 68 2 3 5.4 X-ray diffraction patterns at 4 GP a and 8 GP a. ............. 69 5.5 Diffraction pattern of (Sb Te ) (Bi Te ) (Sb Se ) (As Te ) . Re- 2 3 72 2 3 25 2 3 2 2 3 1 finement yielded cell parameters of a = 4.2907A˚ and c = 30.4624A˚.