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He P, Daaoub AHS, Sangtarash S, Sadeghi H, Yoon HJ. Thermopower in Underpotential Deposition-Based Molecular Junctions. NANO LETTERS 2024; 24:1988-1995. [PMID: 38270106 PMCID: PMC10870761 DOI: 10.1021/acs.nanolett.3c04438] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/16/2023] [Revised: 01/18/2024] [Accepted: 01/18/2024] [Indexed: 01/26/2024]
Abstract
Underpotential deposition (UPD) is an intriguing means for tailoring the interfacial electronic structure of an adsorbate at a substrate. Here we investigate the impact of UPD on thermoelectricity occurring in molecular tunnel junctions based on alkyl self-assembled monolayers (SAMs). We observed noticeable enhancements in the Seebeck coefficient of alkanoic acid and alkanethiol monolayers, by up to 2- and 4-fold, respectively, upon replacement of a conventional Au electrode with an analogous bimetallic electrode, Cu UPD on Au. Quantum transport calculations indicated that the increased Seebeck coefficients are due to the UPD-induced changes in the shape or position of transmission resonances corresponding to gateway orbitals, which depend on the choice of the anchor group. Our work unveils UPD as a potent means for altering the shape of the tunneling energy barrier at the molecule-electrode contact of alkyl SAM-based junctions and hence enhancing thermoelectric performance.
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Affiliation(s)
- Peng He
- Department
of Chemistry, Korea University, Seoul 02841, Korea
| | - Abdalghani H. S. Daaoub
- Device
Modelling Group, School of Engineering, University of Warwick, Coventry CV4 7AL, U.K.
| | - Sara Sangtarash
- Device
Modelling Group, School of Engineering, University of Warwick, Coventry CV4 7AL, U.K.
| | - Hatef Sadeghi
- Device
Modelling Group, School of Engineering, University of Warwick, Coventry CV4 7AL, U.K.
| | - Hyo Jae Yoon
- Department
of Chemistry, Korea University, Seoul 02841, Korea
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Abstract
ConspectusUnderstanding the thermoelectric effects that convert energy between heat and electricity on a molecular scale is of great interest to the nanoscience community. As electronic devices continue to be miniaturized to nanometer scales, thermoregulation on such devices becomes increasingly critical. In addition, the study of molecular thermoelectricity provides information that cannot be accessed through conventional electrical conductance measurements. The field of molecular thermoelectrics aims to explore thermoelectric effects in electrode-molecule-electrode tunnel junctions and draw inferences on how the (supra)molecular structure of active molecules is associated with their thermopower. In this Account, we introduce a convenient and useful junction technique that enables thermovoltage measurements of one molecule thick films, self-assembled monolayers (SAMs), with reliability, and discuss the atomic-detailed structure-thermopower relations established by the technique. The technique relies on a microelectrode composed of non-Newtonian liquid metal, eutectic gallium-indium (EGaIn) covered with a native gallium oxide layer. The EGaIn electrode makes it possible to form thermoelectric contacts with the delicate structure of SAMs in a noninvasive fashion. A defined interface between SAM and the EGaIn electrode allows time-effective collection of large amounts of thermovoltage data, with great reproducibility, efficiency, and reliable interpretation and statistical analysis of the data. We also highlight recent efforts to utilize the EGaIn technique for probing molecular thermoelectricity and structure-thermopower relations. Using the technique, it was possible to unravel quantum-chemical mechanisms of thermoelectric functions, based on the Mott formula, in SAM-based large-area junctions, which in turn led us to set various hypotheses to boost the Seebeck coefficient. By validating the hypotheses again with the EGaIn technique, we revealed that the thermopower of junction increases through the reduction of the energy offset between accessible molecular orbital energy level and Fermi level or the tuning of broadening of the orbital energy level. Such alterations in the shape of energy topography of junction could be achieved through structural modifications in anchoring group and molecular backbone of SAM, and the bottom electrode. Molecular thermoelectrics offers a unique opportunity to build a well-defined nanoscale system and isolate an effect of interest from others, advancing fundamental understanding of charge transport across individual molecules and molecule-electrode interfaces. In the Account, we showed our recent work involving carefully designed molecular system that are relevant to answering the question of how thermopower differs between the tunneling and thermal-hopping regimes. The field of molecular thermoelectrics needs to address practical application-related issues, particularly molecular degradation in thermal environments. In this regard, we summarized the results highlighting the thermal instability of SAM-based junctions based on a traditional thiol anchor group and how to circumvent this problem. We also discussed the power factor (PF)─a practical parameter representing the efficiency for converting heat into electricity─of SAMs, evaluated using the EGaIn technique. In the Conclusion section of this Account, we present future challenges and perspectives.
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Affiliation(s)
- Jiung Jang
- Department of Chemistry, Korea University, Seoul 02841, Korea
| | - Peng He
- Department of Chemistry, Korea University, Seoul 02841, Korea
| | - Hyo Jae Yoon
- Department of Chemistry, Korea University, Seoul 02841, Korea
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Park S, Jang J, Tanaka Y, Yoon HJ. High Seebeck Coefficient Achieved by Multinuclear Organometallic Molecular Junctions. NANO LETTERS 2022; 22:9693-9699. [PMID: 36441166 DOI: 10.1021/acs.nanolett.2c03974] [Citation(s) in RCA: 6] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/16/2023]
Abstract
This paper describes the thermoelectric properties of molecular junctions incorporating multinuclear ruthenium alkynyl complexes that comprise Ru(dppe)2 [dppe = 1,2-bis(diphenylphosphino)ethane] fragments and diethylnyl aromatic bridging ligands with thioether anchors. Using the liquid metal technique, the Seebeck coefficient was examined as a function of metal nuclearity, oxidation state, and substituent on the organic ligand backbone. High Seebeck coefficients up to 73 μV/K and appreciable thermal stability with thermovoltage up to ∼3.3 mV at a heating temperature of 423 K were observed. An unusually high proximity of the highest occupied molecular orbital (HOMO) energy level to the Fermi level was revealed to give the remarkable thermoelectric performance as suggested by combined experiments and calculations. This work offers important insights into the development of molecular-scale devices for efficient thermoregulation and heat-to-electricity conversion.
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Affiliation(s)
- Sohyun Park
- Department of Chemistry, Korea University, Seoul 02841, Korea
| | - Jiung Jang
- Department of Chemistry, Korea University, Seoul 02841, Korea
| | - Yuya Tanaka
- Laboratory for Chemistry and Life Science, Institute of Innovative Research, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
| | - Hyo Jae Yoon
- Department of Chemistry, Korea University, Seoul 02841, Korea
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He J, Tao L, Xian W, Arbaugh T, Li Y. Molecular self-assembled monolayers anomalously enhance thermal conductance across polymer-semiconductor interfaces. NANOSCALE 2022; 14:17681-17693. [PMID: 36416469 DOI: 10.1039/d2nr04936h] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/16/2023]
Abstract
Thermal issues have become increasingly important for the performance and lifetime of highly miniaturized and integrated devices. However, the high thermal resistance across the polymer/semiconductor interface greatly weakens the fast heat dissipation. In this study, applying the self-assembled monolayer (SAM) technique, organic molecules are employed as heat regulators to mediate interfacial thermal conductance (ITC) between semiconductors (silicon or Si) and polymers (polystyrene or PS). Silane-based SAM molecules with unique functional groups, such as -NH2, -CH3, -SH, and -Cl, are orderly assembled into Si-PS interfaces. Their roles in ITC and the heat transfer mechanism were systematically investigated. Molecular simulations demonstrate that the Si-PS interface decorated with SAM molecules can significantly facilitate heat transfer in varying degrees. Such a difference is primarily due to the different non-bonded interactions and compatibility between SAMs and PS. Compared with the pristine Si-PS interface, the interface incorporated with 3-chloropropyl trimethoxysilane shows the greatest improvement in ITC, about 507.02%. Such improvements are largely attributed to the SAM molecules, as the thermal bridges straighten the molecular SAM chains, develop strong non-bonded interactions with PS, provide the covalent bonding between Si and PS, exhibit a strong coupling effect between two materials' vibrational modes, and eliminate the discontinuities in the temperature field. Eventually, these demonstrations are expected to offer molecular insights to enable effective thermal management through surface engineering for critical-heat transfer materials and microelectronic devices.
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Affiliation(s)
- Jinlong He
- Department of Mechanical Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706-1572, USA.
| | - Lei Tao
- Department of Mechanical Engineering, University of Connecticut, Storrs, Connecticut 06269-3139, USA
| | - Weikang Xian
- Department of Mechanical Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706-1572, USA.
| | - Tom Arbaugh
- Department of Physics, Wesleyan University, Middletown, Connecticut 06459, USA
| | - Ying Li
- Department of Mechanical Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706-1572, USA.
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Park S, Jo JW, Jang J, Ohto T, Tada H, Yoon HJ. Thermopower in Transition from Tunneling to Hopping. NANO LETTERS 2022; 22:7682-7689. [PMID: 36067367 DOI: 10.1021/acs.nanolett.2c03083] [Citation(s) in RCA: 7] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/15/2023]
Abstract
The Seebeck effect of a molecular junction in a hopping regime or tunneling-to-hopping transition remains uncertain. This paper describes the Seebeck effect in molecular epitaxy films (OPIn where n = 1-9) based on imine condensation between an aryl amine and aldehyde and investigates how the Seebeck coefficient (S, μV/K) varies at the crossover region. The S value of OPIn linearly increased with increasing the molecular length (d, nm), ranging from 7.2 to 38.0 μV/K. The increasing rate changed from 0.99 to 0.38 μV·K-1 Å-1 at d = 3.4 nm (OPI4). Combined experimental and theoretical studies indicated that such a change stems from a tunneling-to-hopping transition, and the small but detectable length-dependence of thermopower in the long molecules originates from the gradual reduction of the tunneling contribution to the broadening of molecular orbital energy level, rather than its relative position to the Fermi level. Our work helps to bridge the gap between bulk and nanoscale thermoelectric systems.
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Affiliation(s)
- Sohyun Park
- Department of Chemistry, Korea University, Seoul 02841, Korea
| | - Jeong Woo Jo
- Department of Chemistry, Korea University, Seoul 02841, Korea
| | - Jiung Jang
- Department of Chemistry, Korea University, Seoul 02841, Korea
| | - Tatsuhiko Ohto
- Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan
| | - Hirokazu Tada
- Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan
| | - Hyo Jae Yoon
- Department of Chemistry, Korea University, Seoul 02841, Korea
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Liu E, Lei L, Ye R, Deng D, Xu S. Improved relative temperature sensitivity of over 10% K -1 in fluoride nanocrystals via engineering the interfacial layer. Chem Commun (Camb) 2022; 58:9076-9079. [PMID: 35876695 DOI: 10.1039/d2cc02548e] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Real-time in situ temperature sensing is of significance in the bio-medical field; however, the low relative temperature sensitivity Sr is one of the major obstacles in the development of nanothermometers. Herein, we provide an effective route that engineers the interfacial layer in a core/shell/shell nanostructure to enlarge the temperature-dependent luminescence intensity ratio (LIR) variations followed by an improved Sr. The CaF2 interlayer is employed to inhibit the interaction between the core and outer shell, and increase the interfacial phonon energy to enhance the negative thermal quenching effect (TQE) of Nd3+ ions in the outer shell and positive TQE of Er3+ ions in the core layer. Based on the temperature-dependent LIR variations of Er (650 nm) to Nd (800 nm), the maximum Sr of 10.01% K-1 and minimum Sr of % 2.56% K-1 are achieved.
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Affiliation(s)
- Enyang Liu
- Key Laboratory of Rare Earth Optoelectronic Materials and Devices of Zhejiang Province, Institute of Optoelectronic Materials and Devices, China Jiliang University, Hangzhou, 310018, People's Republic of China.
| | - Lei Lei
- Key Laboratory of Rare Earth Optoelectronic Materials and Devices of Zhejiang Province, Institute of Optoelectronic Materials and Devices, China Jiliang University, Hangzhou, 310018, People's Republic of China. .,Department of Physics, Zhejiang Normal University, Jinhua, Zhejiang, 321004, China
| | - Renguang Ye
- Key Laboratory of Rare Earth Optoelectronic Materials and Devices of Zhejiang Province, Institute of Optoelectronic Materials and Devices, China Jiliang University, Hangzhou, 310018, People's Republic of China.
| | - Degang Deng
- Key Laboratory of Rare Earth Optoelectronic Materials and Devices of Zhejiang Province, Institute of Optoelectronic Materials and Devices, China Jiliang University, Hangzhou, 310018, People's Republic of China.
| | - Shiqing Xu
- Key Laboratory of Rare Earth Optoelectronic Materials and Devices of Zhejiang Province, Institute of Optoelectronic Materials and Devices, China Jiliang University, Hangzhou, 310018, People's Republic of China.
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Park S, Kang S, Yoon HJ. Thermopower of Molecular Junction in Harsh Thermal Environments. NANO LETTERS 2022; 22:3953-3960. [PMID: 35575639 DOI: 10.1021/acs.nanolett.2c00422] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/15/2023]
Abstract
Molecular junctions can be miniaturized devices for heat-to-electricity conversion application, yet these operate only in mild thermal environments (less than 323 K) because thiol, the most widely used anchor moiety for chemisorption of active molecules onto surface of electrode, easily undergoes thermal degradation. N-Heterocyclic carbene (NHC) can be an alternative to traditional thiol anchor for producing ultrastable thermoelectric molecular junctions. Our experiments showed that the NHC-based molecular junctions withstood remarkably high temperatures up to 573 K, exhibiting consistent Seebeck effect and thermovoltage up to approximately |1900 μV|. Our work advances our understanding of molecule-electrode contact in the Seebeck effect, providing a roadmap for constructing robust and efficient organic thermoelectric devices.
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Affiliation(s)
- Sohyun Park
- Department of Chemistry, Korea University, Seoul 02841, Korea
| | - Seohyun Kang
- Department of Chemistry, Korea University, Seoul 02841, Korea
| | - Hyo Jae Yoon
- Department of Chemistry, Korea University, Seoul 02841, Korea
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