A major challenge in molecular electronics is to control the nature of the coupling between the molecule and the electrodes, which is necessary to optimize electronic performance. Our poor understanding of the organic-electrode interfaces arises as the major impediment towards the execution of reliable functional devices. Coupling the molecular frontier orbital (i.e., the molecular orbital responsible for conduction) to an electrode is key to generating optimal device performance for a broad range of applications, including molecular-based rectification, spin-transport and memory. In particular, a strong electronic coupling ensures that the frontier orbital efficiently follows the electrode’s Fermi-level under electrical bias, but also results in hybridization of orbitals and in a corresponding broadening of the molecular energy levels and large leakage currents. On the contrary, a weak electronic coupling prevents hybridization and secures narrow molecular energy levels, enabling access to internal degrees of freedom within the molecule (e.g., vibrational modes, spin…). However, this is detrimental for device performance in response to applied bias for some specific applications (e.g., poor rectification ratios). Hence, many applications require a delicate balance between the molecule-electrode coupling, to control electronic functionality, and the localization of the molecular orbitals within the molecule (low broadening). In this context, non-covalent molecule-electrode interactions can be strong enough to form molecular junctions, but only rarely have been shown to generate an efficient device performance.
Particularly interesting applications within molecular electronics are the rectification of both charge and spin currents flowing through molecular junctions. A molecular charge rectifier (or molecular diode) allows the flow of electrical current in only one direction of bias, while electrical conduction in a molecular spin rectifier is governed by the polarization of the conduction electrons. In such devices, the control of the molecule-electrode coupling constitutes the basic building block for the realization of the desired functionality. In collaboration with Christian Nijhuis, at the Graphene Research Centre of the National University of Singapore, have produced exciting results demonstrating that large rectification ratios can be obtained by means of non-covalent interactions in molecular junctions based on self-assembled monolayers (SAMs) of long alkane chains with a ferrocene (Fc) unit (see figure above and reference).
The broad goal of this project is to investigate charge and spin current rectification in molecular junctions in a wide range of temperatures (T = 0.2–300 K). For this, we employ three-terminal single-electron transistors (SETs) to develop molecular electronic devices that allow for the study of conduction through individual molecular junctions. Our studies at UCF are complemented by transport measurements in macroscopic SAM-based junctions by our collaborator Nijhuis in Singapore
Recent Works:
Ran Liu, Yingmei Han, Feng Sun, Gyan Khatri, Jaesuk Kwon, Cameron Nickle, Lejia Wang, Chuan-Kui Wang, Damien Thompson, Zong-Liang Li, Christian A. Nijhuis, and Enrique del Barcoa
“Stable Universal 1- and 2-Input Single-Molecule Logic Gates”
Advanced Materials (2022) https://doi.org/10.1002/adma.202202135
Xiaoping Chen, Bernhard Kretz, Francis Adoah, Cameron Nickle, Xiao Chi, Xiaojiang Yu, Enrique del Barco, Damien Thompson, David A. Egger, and Christian A. Nijhuis
“A Single Atom Change Turns Insulating Saturated Wires into Molecular Conductors”
Nature Communications 12, 3432 (2021)
Yingmei Han, Cameron Nickle, Maria Serena Maglione, Senthil Kumar Karuppannan, Javier Casado-Montenegro, Dongchen Qi, Xiaoping Chen, Anton Tadich, Bruce Cowie, Marta Mas-Torrent, Concepció Rovira, Jérôme Cornil, Jaume Veciana, Enrique del Barco, and Christian A. Nijhuis
“Bias-Polarity Dependent Direct and Inverted Marcus Charge Transport Affecting Rectification in a Redox-Active Molecular Junction”
Advance Science, 210055 (2021)
Senthil Kumar Karuppannan, Rupali Reddy Pasula, Tun Seng Herng, Jun Ding, Xiao Chi, Enrique del Barco, Stephan Roche, Xiaojiang Yu, Nikolai Yakovlev, Sierin Lim and Christian A. Nijhuis
“Room-Temperature Tunnel Magnetoresistance across Biomolecular Tunnel Junctions Based on Ferritin”
Journal of Physics: Materials 4, 035003 (2021)
Damien Thompson, Enrique del Barco, and Christian A. Nijhuis
“Design principles of dual-functional molecular switches in solid-state tunnel junctions”
Appl. Phys. Lett. 117, 030502 (2020)
Yingmei Han, Cameron Nickle, Ziyu Zhang, Hippolyte P. A. G. Astier, Thorin J. Duffin, Dongchen Qi, Zhe Wang, Enrique del Barco, Damien Thompson and Christian A. Nijhuis
“Electric-field-driven dual-functional molecular switches in tunnel junctions”
Nature Materials 19, 843–848 (2020)
Li Yuan, Lejia Wang, Alvar R. Garrigues, Li Jiang, Harshini Venkata Annadata, Marta Anguera Antonana, Enrique Barco & Christian A. Nijhuis
“Transition from direct to inverted charge transport Marcus regions in molecular junctions via molecular orbital gating”
Nat. Nanotechnology, doi:10.1038/s41565-018-0068-4 (2018)
See highlights on this work in the News & Views of the journal by Joshua Hihath “Charge transport in the inverted Marcus region” Nat. Nanotechnology News & Views, doi:10.1038/nnano.2017.123 (2018)
X. Chen, M. Roemer, L. Yuan, W. Du, D. Thompson, E. del Barco, and C. A. Nijhuis
“Large-Area Molecular Tunnel Junctions with Giant Rectification of Electrical Current”
Nat. Nanotechnology, doi:10.1038/nnano.2017.110 (2017)
See highlights on this work in the News & Views of the journal by Nicolas Clement and Akira Fujiwara “Molecular diodes: Breaking the Landauer limit” Nat. Nanotechnology News & Views, doi:10.1038/nnano.2017.123 (2017)
Alvar R. Garrigues, Li Yuan, Lejia Wang, Simranjeet Singh, Enrique del Barco and Christian A. Nijhuis
“Temperature dependent charge transport across tunnel junctions of single-molecules and self-assembled monolayers: a comparative study”
Dalton Trans. 45, 17153-17159 (2016)
A. R. Garrigues, L. Wang, E. del Barco, and C. A. Nijhuis
“Electrostatic Control over Temperature-Dependent Tunneling across a Single Molecule Junction”
Nat. Commun. 7, 11595 (2016)
A. R. Garrigues, L. Yuan, L. Wang, E. R. Mucciolo, D. Thompson, E. del Barco, and C. A. Nijhuis
“A Single-Level Tunnel Model to Account for Electrical Transport through Single Molecule- and Self-Assembled Monolayer-based Junctions”
Sci. Rep. 6, 26517 (2016)
Li Yuan, Nisachol Nerngchamnong, Cao Liang, Hicham Hamoudi, Enrique del Barco, Max Roemer, Ravi K. Sriramula, Damien Thompson, and Christian A. Nijhuis
“Turning Around a Diode at the Molecular Level”
Nature Comm. 6, 6324 (2015)
Other publications by the group
Collaborators in this project:
Physics:
David Sanchez (Universtat Illes Balears, SPAIN)
Eduardo Mucciolo (UCF, Orlando, USA)
Chemistry:
Christian Nijhuis (NUS, Singapore)