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通过低温超高真空扫描隧道显微镜及其谱学方法研究并展示了分子配体在调控表面吸附的单个八乙基钴卟啉(CoOEP)分子的电子态和输运性质中的重要作用. 通过单分子剪裁可以脱去该分子外围的甲基, 并在中心钴原子的微分电导谱中观察测到d轨道共振到近藤共振的演变. 实验结果结合第一性原理的理论计算研究表明, 在脱去甲基前后中心钴原子的化学环境和磁矩均未发生显著变化, 这一演变可以通过一个简化模型来阐释并被归结为脱去甲基后分子配体与衬底成键改变了体系隧穿参数所导致. 此外, 实验结果表明CoOEP分子配体的输运性质可受到分子间距离和范德华相互作用的显著调控. 在CoOEP低聚体中位于分子之间的乙基被抬高, 同时在其微分电导谱谱中00.8 V区域内新出现一个强的共振峰. 这一新的共振峰表现出等间距的多峰细节, 其峰间距与卟啉环和乙基之间的CC键伸缩模式能量符合. 这一新共振峰的出现被归结为由于分子局部与衬底耦合减弱形成双结隧穿体系所导致的振子态隧穿峰.We demonstrate that the molecular ligands play important roles in controlling the electronic states and electron transport properties of single cobalt (Ⅱ) octaethylporphyrin (CoOEP) molecule adsorbed on Au(111) surface by using low-temperature scanning tunneling microscopy and spectroscopy. Single CoOEP molecule adsorbed on Au(111) surface has eight methyl groups pointing out of the surface plane. A peak located at -50 mV in dI/dV spectrum measured on the Co atom of CoOEP is identified as a d-orbital mediated resonance. We find that the methyl groups in CoOEP can be removed in a stepwise manner, and finally lead to a fully-demethylized CoOEP. The d-orbital mediated resonance gradually evolves into a sharp Kondo resonance located right at Fermi level in the demethylization process. Both experimental and theoretical results indicate that the chemical environment and magnetic moment of the central Co atom change slightly in the fully-demethylized CoOEP: the Co atom is slightly lifted by 0.15 and the magnetic moment increases from 0.5 B to 0.6 B. The emergence of Kondo effect is qualitatively explained with a simple model by consideringthe change in the tunneling parameters of the ligands upon demethylization. We also show that the transport properties of the CoOEP can be dramatically controlled by weak intramolecular van der Waals interaction. In CoOEP closely-packed dimers and trimers where CoOEP molecules are introduced close enough to each other, the ethyl groups in the neighboring area are found to be strongly lifted by 0.4 . More surprisingly, a pronounced resonance shows up at 00.8 V in the dI/dV spectra of the lifted ethyl groups. High resolution spectra show that the new resonance consists of multiple peaks with equal spacing of 137 8 mV. The spacing energy coincides with the vibrational energy of stretching mode of CC bond between ethyl group and the brim carbon atom of the porphyrin ring. Therefore the newly-emerged resonance is attributed to the vibronic states originating from the intramolecular CC bond stretching mode excited by tunneling electrons. A model considering the local formation of a barrier between the lifted part of the molecule and the substrate is employed to explain the experimental observations. Our findings show that the electron transport properties in single molecules can be intensely tuned by controlling the chemical properties of the molecular ligands.
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Keywords:
- Scanning tunneling microscopy /
- single molecule /
- Kondo effect /
- vibronic states
[1] Nazin G V, Qiu X H, Ho W 2003 Science 302 77
[2] Qiu X H, Nazin G V, Ho W 2004 Phys. Rev. Lett. 92 206102
[3] Wu S W, Nazin G V, Chen X, Qiu X H, Ho W 2005 Phys. Rev. Lett. 93 236802
[4] Zhao A D, Li Q X, Chen L, Xiang H J, Wang W H, Pan S, Wang B, Xiao X D, Yang J L, Hou J G, Zhu Q S 2005 Science 309 1542.
[5] Gao L, Ji W, Hu Y B, Cheng Z H, Deng Z T, Liu Q, Jiang N, Lin X, Guo W, D S X, Hofer W A, Xie X C, Gao H J 2007 Phys. Rev. Lett. 99 106402
[6] Fu Y S, Ji S H, Chen X, Ma X C, Wu R, Wang C C, Duan W H, Qiu X H, Sun B, Zhang P, Jia J F, Xue Q K 2007 Phys. Rev. Lett. 99 256601
[7] Chen L, Hu Z P, Zhao A D, Wang B, Luo Y, Yang J L, Hou J G 2007 Phys. Rev. Lett. 99 146803
[8] Zhao A D, Hu Z P, Wang B, Xiao X D, Yang J L, Hou J G 2009 J. Chem. Phys. 128 234705
[9] Tsukahara N, Noto K, Ohara M, Shiraki S, Takagi N, Takata Y, Miyawaki J, Taguchi M, Chainani A, Shin S, Kawai M 2009 Phys. Rev. Lett. 102 167203
[10] Wang Y F, Kröger J, Berndt R, Vázquez H, Brandbyge M, Paulsson M 2010 Phys. Rev. Lett. 104 176802
[11] Franke K J, Schulze G, Pascual J I 2011 Science 332 940
[12] Mugarza A, Krull C, Robles R, Stepanow S, Ceballos G, Gambardella P 2011 Nat. Commun. 2 490
[13] Strózecka A, Soriano M, Pascual J I, Palacios J J 2012 Phys. Rev. Lett. 109 147202
[14] Minamitani E, Tsukahara N, Matsunaka D, Kim Y, Takagi N, Kawai M 2012 Phys. Rev. Lett. 109 086602
[15] Liu L W, Yang K, Jiang Y H, Song B Q, Xiao W D, Li L F, Zhou H T, Wang Y L, Du S X, Ouyang M, Hofer W A, Castro Neto A H, Gao H J 2013 Sci. Rep. 3 1210
[16] Krull C, Robles R, Mugarza A, Gambardella P 2013 Nat. Mater. 12 337
[17] Heinrich B W, Braun L, Pascual J I, Franke K J 2013 Nat. Phys. 9 765
[18] Pan S, Fu Q, Huang T, Zhao A D, Wang B, Luo Y, Yang J L, Hou J G 2009 Proc. Natl. Acad. Sci. USA 106 15259
[19] Chiappe G, Louis E 2006 Phys. Rev. Lett. 97 076806
[20] Aguiar-Hualde J M, Chiappe G, Louis E, Anda E V, Simonin J 2009 Phys. Rev. B 79 155415
[21] Kondo J 1964 Prog. Theor. Phys. 32 37
[22] Hewson A C 1993 The Kondo Problem to Heavy Fermions (Cambridge: Cambridge University Press)
[23] Schlcker S, Koster J, Nissum M, Popp J, Kiefer W 2001 J. Phys. Chem. A 105 9482
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[1] Nazin G V, Qiu X H, Ho W 2003 Science 302 77
[2] Qiu X H, Nazin G V, Ho W 2004 Phys. Rev. Lett. 92 206102
[3] Wu S W, Nazin G V, Chen X, Qiu X H, Ho W 2005 Phys. Rev. Lett. 93 236802
[4] Zhao A D, Li Q X, Chen L, Xiang H J, Wang W H, Pan S, Wang B, Xiao X D, Yang J L, Hou J G, Zhu Q S 2005 Science 309 1542.
[5] Gao L, Ji W, Hu Y B, Cheng Z H, Deng Z T, Liu Q, Jiang N, Lin X, Guo W, D S X, Hofer W A, Xie X C, Gao H J 2007 Phys. Rev. Lett. 99 106402
[6] Fu Y S, Ji S H, Chen X, Ma X C, Wu R, Wang C C, Duan W H, Qiu X H, Sun B, Zhang P, Jia J F, Xue Q K 2007 Phys. Rev. Lett. 99 256601
[7] Chen L, Hu Z P, Zhao A D, Wang B, Luo Y, Yang J L, Hou J G 2007 Phys. Rev. Lett. 99 146803
[8] Zhao A D, Hu Z P, Wang B, Xiao X D, Yang J L, Hou J G 2009 J. Chem. Phys. 128 234705
[9] Tsukahara N, Noto K, Ohara M, Shiraki S, Takagi N, Takata Y, Miyawaki J, Taguchi M, Chainani A, Shin S, Kawai M 2009 Phys. Rev. Lett. 102 167203
[10] Wang Y F, Kröger J, Berndt R, Vázquez H, Brandbyge M, Paulsson M 2010 Phys. Rev. Lett. 104 176802
[11] Franke K J, Schulze G, Pascual J I 2011 Science 332 940
[12] Mugarza A, Krull C, Robles R, Stepanow S, Ceballos G, Gambardella P 2011 Nat. Commun. 2 490
[13] Strózecka A, Soriano M, Pascual J I, Palacios J J 2012 Phys. Rev. Lett. 109 147202
[14] Minamitani E, Tsukahara N, Matsunaka D, Kim Y, Takagi N, Kawai M 2012 Phys. Rev. Lett. 109 086602
[15] Liu L W, Yang K, Jiang Y H, Song B Q, Xiao W D, Li L F, Zhou H T, Wang Y L, Du S X, Ouyang M, Hofer W A, Castro Neto A H, Gao H J 2013 Sci. Rep. 3 1210
[16] Krull C, Robles R, Mugarza A, Gambardella P 2013 Nat. Mater. 12 337
[17] Heinrich B W, Braun L, Pascual J I, Franke K J 2013 Nat. Phys. 9 765
[18] Pan S, Fu Q, Huang T, Zhao A D, Wang B, Luo Y, Yang J L, Hou J G 2009 Proc. Natl. Acad. Sci. USA 106 15259
[19] Chiappe G, Louis E 2006 Phys. Rev. Lett. 97 076806
[20] Aguiar-Hualde J M, Chiappe G, Louis E, Anda E V, Simonin J 2009 Phys. Rev. B 79 155415
[21] Kondo J 1964 Prog. Theor. Phys. 32 37
[22] Hewson A C 1993 The Kondo Problem to Heavy Fermions (Cambridge: Cambridge University Press)
[23] Schlcker S, Koster J, Nissum M, Popp J, Kiefer W 2001 J. Phys. Chem. A 105 9482
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