Thermopower and conductance of single-molecule junctions and atomic contacts

Thermoelectric effects in molecular junctions are of great interest from fundamental and applied point of view. Indeed, organic thermoelectric materials are believed to be one of the potential solutions for key energy problems like the problem of waste heat recovery (e.g. from transportation vehicles) or the heat dissipation problem (e.g. in microelectronics). Present day inorganic thermoelectric materials, despite the good performance, are already globally limited, relatively difficult to process (energetically expensive and toxic), heavy and brittle for use in everyday life. Organic thermoelectric materials are promising alternatives since they are light, flexible and potentially cheap, although their present efficiency still needs to be improved. A strategy for enhancing the thermoelectric performance is the introduction of nanostructures and multiple interfaces [See2010]. Thus, one of the most important open problems in nanoscience concerns the understanding and optimization of thermoelectricity in organic thermoelectric materials at the nanoscale [Zhang2014]. Single-molecule junctions formed using scanning probe techniques constitute an excellent model system to study the processes occurring at the organic-inorganic interface at a fundamental level. In most of the experiments in molecular junctions, the electronic conductance is typically the only magnitude measured. Quite recently, the possibility of measuring the thermopower to give further insight into the transport process has been demonstrated [Reddy2007] and is currently used by just a few groups [Baheti2008; Widawsky2011; Yee2011]. The main goal of this thesis has been to study experimentally the thermopower and conductance of single-molecule junctions using a scanning tunneling microscope (STM) in ambient conditions. An important part of this work is the construction of a new STM head specifically designed for these measurements and the development of a novel powerful technique for measuring simultaneously the thermopower and conductance of single-molecule junctions, making a complete characterization of the molecular junction possible. This is detailed in chapter 4. In chapter 5, this new technique is used to measure the thermopower of C60 molecules and demonstrate the possibility of engineering the thermopower of a molecular junction by molecular scale manipulation, in particular, the enhancement of thermopower by forming a C60 dimer is shown. The thermoelectric properties of atomic nanocontacts of gold and platinum are explored in chapter 6. As contact size dimensions are reduced, a crossover from bulk to quantum behaviour involving a change of sign of the thermopower takes place. Interestingly, quantum oscillations are observed in gold atomic-size contacts, whereas in platinum they are totally absent. This difference between gold and platinum is traced back to the different electronic structure of these two metals. In chapter 7 the effect of lateral chains on the thermopower of OPE derivatives is examined. The addition of lateral chains is found to increase the thermopower as it brings the Fermi level closer to molecular resonances. An enhancement of thermopower with stretching of the molecule is also observed. Finally, in chapter 8 the use of C60 as a linker in molecular junctions is explored by forming single-molecule junctions of dumbbell molecules, consisting of two fullerenes joined by a conjugated backbone. References [Baheti2008] Baheti, K. , J. A. Malen, P. Doak, P. Reddy, S. -Y. Jang, et al. (2008). "Probing the Chemistry of Molecular Heterojunctions Using Thermoelectricity. " Nano Letters 8(2): 715-719. [Reddy2007] Reddy, P., S. -Y. Jang, R. A. Segalman and A. Majumdar (2007). "Thermoelectricity in Molecular Junctions. " Science 315(5818): 1568-1571. [See2010] See, K. C. , J. P. Feser, C. E. Chen, A. Majumdar, J. J. Urban, et al. (2010). "Water-Processable Polymer? Nanocrystal Hybrids for Thermoelectrics. " Nano Letters 10(11): 4664-4667. [Widawsky2011] Widawsky, J. R. , P. Darancet, J. B. Neaton and L. Venkataraman (2011). "Simultaneous Determination of Conductance and Thermopower of Single Molecule Junctions. " Nano Letters 12(1): 354-358. [Yee2011] Yee, S. K. , J. A. Malen, A. Majumdar and R. A. Segalman (2011). "Thermoelectricity in Fullerene? Metal Heterojunctions. " Nano Letters 11(10): 4089-4094. [Zhang2014] Zhang, Q. , Y. Sun, W. Xu and D. Zhu (2014). "Organic Thermoelectric Materials: Emerging Green Energy Materials Converting Heat to Electricity Directly and Efficiently. " Advanced Materials 26(40): 6829-6851.