In standard molecular junctions, a molecular structure is placed between and connected to metal leads. Understanding how mechanical tuning in such molecular junctions can change heat conductance has interesting applications in nanoscale energy transport. In this work, we use nonequilibrium molecular dynamics simulations to address the effect of stretching on the phononic contribution to the heat conduction of molecular junctions consisting of single long-chain alkanes and various metal leads, such as Ag, Au, Cu, Ni, and Pt. The thermal conductance of such junctions is found to be much smaller than the intrinsic thermal conductance of the polymer and significantly depends on the nature of metal leads as expressed by the metal–molecule coupling and metal vibrational density of states. This behavior is expected and reflects the mismatch of phonon spectra at the metal molecule interfaces. As a function of stretching, we find a behavior similar to what was observed earlier [M. Dinpajooh and A. Nitzan, J. Chem. Phys. 153, 164903 (2020)] for pure polymeric structures. At relatively short electrode distances, where the polyethylene chains are compressed, it is found that the thermal conductances of the molecular junctions remain almost constant as one stretches the polymer chains. At critical electrode distances, the thermal conductances start to increase, reaching the values of the fully extended molecular junctions. Similar behaviors are observed for junctions in which several long-chain alkanes are sandwiched between various metal leads. These findings indicate that this behavior under stretching is an intrinsic property of the polymer chain and not significantly associated with the interfacial structures.
- Award ID(s):
- 2010875
- NSF-PAR ID:
- 10438063
- Date Published:
- Journal Name:
- Nanoscale
- Volume:
- 15
- Issue:
- 30
- ISSN:
- 2040-3364
- Page Range / eLocation ID:
- 12586 to 12597
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
-
NA (Ed.)Gold-dithiol molecular junctions have been studied both experimentally and theoretically. However, the nature of the gold-thiolate bond as it relates to the solvent has been seldom investigated. It is known that solvents can impact the electronic structure of single molecule junctions, but the correlation between the solvent and dithiol-linked single-molecule junction conductance is not well understood. We study molecular junctions formed with thiol terminated phenylenes from both 1-chloronaphthalene and 1-bromonaphthalene solutions. We find that the most probable conductance and the distribution of conductances are both affected by the solvent. First-principles calculations show that junction conductance depends on the binding configurations (adatom, atop, bridge) of the thiolate on the Au surface as has been shown previously. More importantly, we find that brominated solvents can restrict the binding of thiols to specific Au sites. This mechanism offers new insight into the effects of the solvent environment on covalent bonding in molecular junctions.more » « less
-
more » « less
Abstract We report that the single‐molecule junction conductance of thiol‐terminated silanes with Ag electrodes are higher than the conductance of those formed with Au electrodes. These results are in contrast to the trends in the metal work function Φ(Ag)<Φ(Au). As such, a better alignment of the Au Fermi level to the molecular orbital of silane that mediates charge transport would be expected. This conductance trend is reversed when we replace the thiols with amines, highlighting the impact of metal–S covalent and metal–NH2dative bonds in controlling the molecular conductance. Density functional theory calculations elucidate the crucial role of the chemical linkers in determining the level alignment when molecules are attached to different metal contacts. We also demonstrate that conductance of thiol‐terminated silanes with Pt electrodes is lower than the ones formed with Au and Ag electrodes, again in contrast to the trends in the metal work‐functions.
-
more » « less
Abstract We report that the single‐molecule junction conductance of thiol‐terminated silanes with Ag electrodes are higher than the conductance of those formed with Au electrodes. These results are in contrast to the trends in the metal work function Φ(Ag)<Φ(Au). As such, a better alignment of the Au Fermi level to the molecular orbital of silane that mediates charge transport would be expected. This conductance trend is reversed when we replace the thiols with amines, highlighting the impact of metal–S covalent and metal–NH2dative bonds in controlling the molecular conductance. Density functional theory calculations elucidate the crucial role of the chemical linkers in determining the level alignment when molecules are attached to different metal contacts. We also demonstrate that conductance of thiol‐terminated silanes with Pt electrodes is lower than the ones formed with Au and Ag electrodes, again in contrast to the trends in the metal work‐functions.
-
We demonstrate that imidazole based π–π stacked dimers form strong and efficient conductance pathways in single-molecule junctions using the scanning-tunneling microscope-break junction (STM-BJ) technique and density functional theory-based calculations. We first characterize an imidazole-gold contact by measuring the conductance of imidazolyl-terminated alkanes ( im-N-im , N = 3–6). We show that the conductance of these alkanes decays exponentially with increasing length, indicating that the mechanism for electron transport is through tunneling or super-exchange. We also reveal that π–π stacked dimers can be formed between imidazoles and have better coupling than through-bond tunneling. These experimental results are rationalized by calculations of molecular junction transmission using non-equilibrium Green's function formalism. This study verifies the capability of imidazole as a Au-binding ligand to form stable single- and π-stacked molecule junctions at room temperature.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/d3nr00505d
