本篇論文利用第一原理的計算方法模擬分子接面的電子傳輸性 質以及計算出系統的熱電效應。我們模擬了碳鏈以及 Br-Al-Al-Br 兩
44
種系統。
在碳鏈系統中，Seebeck 係數會隨著在電極中間的碳原子的數目 不同其正負號會有奇偶效應。其中當碳鏈中碳數為奇數時 Seebeck 係 數為負的，在碳數為偶數時 Seebeck 係數為正的。這現象在一般塊材 中是很難發現的，一般塊材在同個溫度下只會有一種 Seebeck 係數，
而在碳鏈系統上，不但會因碳數的不同改變 Seebeck 的值，正負號更 會隨著碳數的改變而有所不同。
而奇偶效應的原因為：當每增加一個碳原子時，碳原子提供了四 個自由電子，其中有兩個電子跑到了系統中的 core state，而另外 兩個電子填入了費米能階附近的軌域，進而造成 DOS 在費米能階附 近有全滿半滿的情況，又恰好造成了穿隧方程式在費米能階附近的斜 率會變號。
在 Br-Al-Al-Br 的系統中，我們計算了此系統的熱電效應以及能 量轉換功率。因為這系統在費米能階附近擁有一個較尖銳的峰值，所 以這系統擁有較大的 Seebeck 係數，利用這較大 Seebeck 係數以及溫 差可將此系統視為一個電源供應器。我們另外將系統模擬成電晶體，
發現改變閘極電壓可調控熱電效應所產生的電位差、電流以及熱電轉 換效率。在閘極電壓VG約為-2V 時會有最佳的熱電功率 3.5nW 以及熱 電效率約為 0.09。在無外加閘極偏呀以及TC 300K和 T 60K擁有約
45 Letters, 1974. 29(2): p. 277-283.
[2] Reed, M.A., et al., Conductance of a molecular junction. Science, 1997.
278(5336): p. 252.
[3] Kaun, C.C. and H. Guo, Resistance of alkanethiol molecular wires.
Nano letters, 2003. 3(11): p. 1521-1525.
[4] Di Ventra, M. and N.D. Lang, Transport in nanoscale conductors from first principles. Physical Review B, 2001. 65(4): p. 045402.
[5] Nitzan, A. and M.A. Ratner, Electron transport in molecular wire junctions. Science, 2003. 300(5624): p. 1384.
[6] Wang, W., et al., Inelastic electron tunneling spectroscopy of an alkanedithiol self-assembled monolayer. Nano letters, 2004. 4(4): p.
643-646.
[7] Galperin, M., A. Nitzan, and M.A. Ratner, Inelastic transport in the Coulomb blockade regime within a nonequilibrium atomic limit. Physical Review B, 2008. 78(12): p. 125320.
[8] Jiang, J., et al., First-principles simulations of inelastic electron tunneling spectroscopy of molecular electronic devices. Nano letters, 2005. 5(8): p. 1551-1555.
[9] Yu, L.H., C.D. Zangmeister, and J.G. Kushmerick, Origin of discrepancies in inelastic electron tunneling spectra of molecular junctions. Physical review letters, 2007. 98(20): p. 206803.
46
[10] Paulsson, M., T. Frederiksen, and M. Brandbyge, Inelastic transport through molecules: Comparing first-principles calculations to experiments. Nano letters, 2006. 6(2): p. 258-262.
[11] Solomon, G.C., et al., Understanding the inelastic electron-tunneling spectra of alkanedithiols on gold. The Journal of chemical physics, 2006.
124: p. 094704.
[12] Kushmerick, J.G., et al., Vibronic contributions to charge transport across molecular junctions. Nano letters, 2004. 4(4): p. 639-642.
[13] Chen, Y.C., M. Zwolak, and M. Di Ventra, Inelastic effects on the transport properties of alkanethiols. Nano letters, 2005. 5(4): p.
621-624.
[14] Chen, Y.C., Effects of isotope substitution on local heating and inelastic current in hydrogen molecular junctions. Physical Review B, 2008. 78(23): p. 233310.
[15] Kristensen, I., et al., Inelastic scattering in metal-H2-metal junctions.
Physical Review B, 2009. 79(23): p. 235411.
[16] Djukic, D. and J. van Ruitenbeek, Shot noise measurements on a single molecule. Nano letters, 2006. 6(4): p. 789-793.
[17] Kiguchi, M., et al., Highly conductive molecular junctions based on direct binding of benzene to platinum electrodes. Physical review letters, 2008. 101(4): p. 46801.
[18] Wheeler, P., et al., Shot noise suppression at room temperature in atomic-scale Au junctions. Nano letters, 2010. 10(4): p. 1287-1292.
[19] Chen, Y.C. and M. Di Ventra, Effect of electron-phonon scattering on shot noise in nanoscale junctions. Physical review letters, 2005. 95(16):
p. 166802.
[20] Liu, Y.S. and Y.C. Chen, Counting statistics in nanoscale junctions.
Physical Review B, 2011. 83(3): p. 035401.
[21] Chen, Y.C., M. Zwolak, and M. Di Ventra, Local heating in nanoscale conductors. Nano letters, 2003. 3(12): p. 1691-1694.
[22] Huang, Z., et al., Measurement of current-induced local heating in a single molecule junction. Nano letters, 2006. 6(6): p. 1240-1244.
[23] Di Ventra, M., S. Pantelides, and N. Lang, The benzene molecule as a molecular resonant-tunneling transistor. Applied Physics Letters, 2000. 76: p. 3448.
[24] Ma, C.L., D. Nghiem, and Y.C. Chen, Alkanethiol-based
single-molecule transistors. Applied Physics Letters, 2008. 93: p.
222111.
47
[25] Solomon, P.M. and N.D. Lang, The Biphenyl Molecule as a Model Transistor. ACS nano, 2008. 2(3): p. 435-440.
[26] Lang, N.D. and P.M. Solomon, Gating of a Three-Leg Molecule. ACS nano, 2009. 3(6): p. 1437-1440.
[27] Song, H., et al., Observation of molecular orbital gating. Nature, 2009.
462(7276): p. 1039-1043.
[28] Ahn, C., et al., Electrostatic modification of novel materials. Reviews of modern physics, 2006. 78(4): p. 1185.
[29] Lindsay, S.M. and M.A. Ratner, Molecular transport junctions:
Clearing mists. Advanced Materials, 2007. 19(1): p. 23-31.
[30] Ludoph, B. and J. Van Ruitenbeek, Thermopower of atomic-size metallic contacts. Physical Review B, 1999. 59(19): p. 12290.
[31] Reddy, P., et al., Thermoelectricity in molecular junctions. Science, 2007. 315(5818): p. 1568.
[32] Baheti, K., et al., Probing the chemistry of molecular heterojunctions using thermoelectricity. Nano letters, 2008. 8(2): p. 715-719.
[33] Malen, J.A., et al., Identifying the length dependence of orbital alignment and contact coupling in molecular heterojunctions. Nano letters, 2009. 9(3): p. 1164-1169.
[34] Malen, J.A., et al., Fundamentals of energy transport, energy conversion, and thermal properties in organic–inorganic heterojunctions.
Chemical Physics Letters, 2010. 491(4): p. 109-122.
[35] Taylor, J., H. Guo, and J. Wang, Ab initio modeling of quantum transport properties of molecular electronic devices. Physical Review B, 2001. 63(24): p. 245407.
[36] Di Ventra, M., S. Pantelides, and N. Lang, First-principles calculation of transport properties of a molecular device. Physical review letters, 2000. 84(5): p. 979-982.
[37] Lang, N. and P. Avouris, Oscillatory conductance of carbon-atom wires. Physical review letters, 1998. 81(16): p. 3515-3518.
[38] Lang, N., Negative differential resistance at atomic contacts. Physical Review B, 1997. 55(15): p. 9364.
[39] Liu, Y.S., H.T. Yao, and Y.C. Chen, Atomic-Scale Field-Effect Transistor as a Thermoelectric Power Generator and Self-Powered Device. The Journal of Physical Chemistry C, 2010.
[40] Lang, N., Resistance of atomic wires. Physical Review B, 1995. 52(7):
p. 5335.
[41] Hohenberg, P. and W. Kohn, Inhomogeneous electron gas. Physical
48
Review, 1964. 136(3B): p. B864.
[42] Kohn, W. and L.J. Sham, Self-consistent equations including exchange and correlation effects. Phys. Rev, 1965. 140(4A): p.
A1133-A1138.
[43] Paulsson, M. and S. Datta, Thermoelectric effect in molecular electronics. Physical Review B, 2003. 67(24): p. 241403.
[44] Zheng, X., et al., Thermoelectric transport properties in atomic scale conductors. The Journal of chemical physics, 2004. 121: p. 8537.
[45] Wang, B., et al., Oscillatory thermopower of carbon chains:
First-principles calculations. Physical Review B, 2005. 71(23): p.
233406.
[46] Pauly, F., J. Viljas, and J. Cuevas, Length-dependent conductance and thermopower in single-molecule junctions of dithiolated oligophenylene derivatives: A density functional study. Physical Review B, 2008. 78(3): p. 035315.
[47] Dubi, Y. and M. Di Ventra, Thermoelectric Effects in Nanoscale Junctions. Nano letters, 2008. 9(1): p. 97-101.
[48] Markussen, T., A.P. Jauho, and M. Brandbyge, Surface-decorated silicon nanowires: a route to high-ZT thermoelectrics. Physical review letters, 2009. 103(5).
[49] Ke, S.H., et al., Thermopower of Molecular Junctions: An ab Initio Study. Nano letters, 2009. 9(3): p. 1011-1014.
[50] Finch, C., V. Garcia-Suarez, and C. Lambert, Giant thermopower and Figure of merit in single-molecule devices. Physical Review B, 2009.
79(3): p. 033405.
[51] Markussen, T., A.P. Jauho, and M. Brandbyge, Electron and phonon transport in silicon nanowires: Atomistic approach to thermoelectric properties. Physical Review B, 2009. 79(3): p. 035415.
[52] Bergfield, J. and C. Stafford, Thermoelectric Signatures of Coherent Transport in Single-Molecule Heterojunctions. Nano letters, 2009. 9(8):
p. 3072-3076.
[53] Liu, Y.S., Y.R. Chen, and Y.C. Chen, Thermoelectric Efficiency in Nanojunctions: A Comparison between Atomic Junctions and Molecular Junctions. ACS nano, 2009. 3(11): p. 3497-3504.
[54] Dubi, Y. and M. Di Ventra, Colloquium: Heat flow and thermoelectricity in atomic and molecular junctions. Reviews of modern physics, 2011. 83(1): p. 131.
[55] Adachi, H., et al., Gigantic enhancement of spin Seebeck effect by
49
phonon drag. Applied Physics Letters, 2010. 97(25): p.
252506-252506-3.
[56] Galperin, M., A. Nitzan, and M.A. Ratner, Inelastic effects in molecular junction transport: scattering and self-consistent calculations for the Seebeck coefficient. Molecular Physics, 2008. 106(2-4): p.
397-404.
[57] Entin-Wohlman, O., Y. Imry, and A. Aharony, Three-terminal thermoelectric transport through a molecular junction. Physical Review B, 2010. 82(11): p. 115314.
[58] Hsu, B.C., et al., Seebeck coefficients in nanoscale junctions: Effects of electron-vibration scattering and local heating. Physical Review B, 2011. 83(4): p. 041404.
[59] Jorn, R. and T. Seideman, Implications and Applications of Current-Induced Dynamics in Molecular Junctions. Accounts of chemical research, 2010. 43(9): p. 1186-1194.
[60] Esposito, M., K. Lindenberg, and C. Van den Broeck, Universality of efficiency at maximum power. Physical review letters, 2009. 102(13): p.
130602.
[61] Liu, Y.S. and Y.C. Chen, Seebeck coefficient of thermoelectric molecular junctions: First-principles calculations. Physical Review B, 2009. 79(19): p. 193101.
[62] Dubi, Y. and M. Di Ventra, Thermospin effects in a quantum dot connected to ferromagnetic leads. Physical Review B, 2009. 79(8): p.
081302.
[63] Galperin, M., et al., Cooling mechanisms in molecular conduction junctions. Physical Review B, 2009. 80(11): p. 115427.
[64] Liu, Y.S., B.C. Hsu, and Y.C. Chen, Effect of Thermoelectric Cooling in Nanoscale Junctions. The Journal of Physical Chemistry C, 2011.
115(13): p. 6111-6125.
[65] Hirjibehedin, C.F., C.P. Lutz, and A.J. Heinrich, Spin coupling in engineered atomic structures. Science, 2006. 312(5776): p. 1021.
[66] Lyo, I.W. and P. Avouris, Negative differential resistance on the atomic scale: Implications for atomic scale devices. Science, 1989.
245(4924): p. 1369.
[67] Yu, M.L., et al., New Evidence for Localized Electronic States on Atomically Sharp Field Emitters. Physical review letters, 1996. 77(8): p.
1636-1639.
[68] Bergfield, J.P., M.A. Solis, and C.A. Stafford, Giant Thermoelectric
50
Effect from Transmission Supernodes. ACS nano, 2010. 4(9): p.
5314-5320.
[69] Lang, N., Field-induced transfer of an atom between two closely spaced electrodes. Physical Review B, 1992. 45(23): p. 13599.
[70] Thygesen, K.S. and A. Rubio, Conserving GW scheme for nonequilibrium quantum transport in molecular contacts. Physical Review B, 2008. 77(11): p. 115333.
[71] Sai, N., et al., Dynamical corrections to the DFT-LDA electron conductance in nanoscale systems. Physical review letters, 2005.
94(18): p. 186810.
[72] Chen, Y.C. and M. Di Ventra, Shot noise in nanoscale conductors from first principles. Physical Review B, 2003. 67(15): p. 153304.