The use of molecules to build electronic circuits was historically motivated by the rapid size reductions of conventional electronic components in line with Moore’s Law. Single-molecule devices represent the limit in component miniaturization, with work over the past two decades demonstrating that molecules can indeed function as nanoscale wires, resistors, switches, and diodes. However, photolithographic technologies have since advanced to a point where they can produce circuits with feature sizes approaching molecular dimensions. Rather than working towards immediate technological applications, present-day efforts in molecular-scale electronics focus on the novel functionality of these materials, distinct from their solid-state counterparts, as well as on fundamental structure-property relationships. Routine, reproducible studies of atomically precise "single-molecule junctions" (below) are now possible using scanning probe microscope-based (SPM) methods, where junctions are typically formed via the self-assembly of components from solution between nanogap separated electrodes under ambient conditions.
Figure 1. A schematic of a molecular junction comprising 1,7-diaminoheptane.
Single-molecule conductance measurements using the STM-BJ method:
The scanning tunneling microscope-based break-junction (STM-BJ) method was introduced in 2003 by Xu and Tao and has been extensively developed since (see "other useful references," below). In a typical approach, gold point contacts are repeatedly formed and broken between an STM tip and substrate (Fig. 2a) while recording the conductance (G = current/voltage = I/V) as a function of tip-substrate displacement. Each conductance-displacement trace shows step features around integer multiples of the conductance quantum (G0, Fig 2b-(i)). These are attributed to the formation of nanowires of integer atom thicknesses. After breaking the gold nanocontact, additional steps are observed at lower conductance (Fig 2b-(ii) in the presence of (linker-functionalized) molecules that can bridge the electrodes. These molecules are introduced, for example, as a solution in 1,2,4-trichlorobenzene. Further tip-substrate displacement results in rupture of the single-molecule junction (Fig 2b-(iii)). By compiling thousands of conductance-displacement traces into conductance histograms, individual step features add up to form peaks representing the most probable conductance values (Fig. 2c). In the coherent transport limit (see "other useful references" (3), below), we can use a simple model to understand the key factors affecting charge transport (strictly, the probability of transmission) through a molecular junction. We observe a higher junction conductance with improved energetic alignment (smaller ∆E) between the highest occupied or lowest unoccupied molecular orbitals (HOMO or LUMO) and the metal Fermi energy (EF), or with greater broadening of HOMO/LUMO resonances due to increased electronic coupling (𝛤) with the continuum of levels in the electrode. ∆E and 𝛤 can be rationally tuned through chemical modifications to the molecular backbone, linkers, or electrode material, or by using experimental techniques that shift the molecular levels relative to EF.
Figure 2. Overview of the STM-BJ method.
OUR INITAL RESEARCH DIRECTION(S):
(1) Inorganic and redox-active circuit components. While previous studies have predominantly used organic, redox-inactive, compounds, we are interested in systematically investigating metal-containing and (multi-site) redox-active molecules as electronic components. We wish to probe and establish chemical structure-electronic property relationships involving d-orbitals. We also seek to explore how oxidation or reduction events can influence the movement of charge carriers through molecular materials on the nanoscale. Such redox events can significantly modify molecular electronic structures and may facilitate charge transport processes via mechanisms other than single step tunneling.
(2) Air-free conductance measurements. Our group has recently built a working STM-BJ setup inside of an inert atmosphere glovebox. This allows us to probe air-sensitive molecules and utilize non-Au metal electrode materials that are otherwise difficult or impossible to study in air. We are excited about the potential of this unusual instrument to expand the experimental scope of the field and have multiple establishing projects underway. Several other projects are immediately available for interested students/postdocs. Such projects span fundamental investigations of molecular charge transport to studies of in situ synthesis/surface catalysis where molecular conductance is used as a signature of reactivity.
(3) Molecular junctions inspired by ordered polymers. We are interested in exploring possible relationships between the conductance of single-molecules and the conductivity of bulk molecular-based materials such as ordered 2D and 3D polymers (also known as covalent organic and metal-organic frameworks). While the underlying transport mechanisms in each case are distinct (tunneling vs. band/hopping transport), these processes are all strongly influenced by the degree of electronic coupling between molecular orbitals. By studying the conductance of appropriate single-molecule models with polymer-inspired structures, we aim to identify molecular structure-electronic coupling trends that may help to inform electronic structure engineering in extended materials. This work has significant synergies with our research on Ordered 2D and 3D Polymers.
Key techniques (* = project dependent):
Scanning tunneling microscope-based break junction (STM-BJ) measurements
Air-free chemistry (glovebox, Schlenk line)*
Relevant Inkpen lab publications:
(1) "Charge transport across dynamic covalent chemical bridges" Z. Miao, T. Quainoo, T. M. Czyszczon-Burton, N. Rotthowe, J. M. Parr, Z.-F. Liu,* and M. S. Inkpen,* Nano Lett., 2022, accepted [article] [ChemRxiv]
Other useful references:
(1) B. Xu, N.J. Tao, "Measurement of single-molecule resistance by repeated formation of molecular junctions" Science, 2003, 301, 1221–1223. (2) L. Venkataraman, J.E. Klare, I.W. Tam, C. Nuckolls, M.S. Hybertsen, M.L. Steigerwald, "Single-Molecule Circuits with Well-Defined Molecular Conductance" Nano Lett., 2006, 6, 458–462. (3) T.A. Su, M. Neupane, M.L. Steigerwald, L. Venkataraman, C. Nuckolls, "Chemical principles of single-molecule electronics" Nature Rev. Mater., 2016, 1, 16002.