A Systems Approach to Molecular Electronics

Molecular electronics is an area of research in which molecules are employed to yield the active and passive device components (switches, diodes, resistors) of an electronic circuit or integrated circuit. Consider the crossbar circuits of nanowires in the electron micrograph at the left [1]. The smallest (100 element) crossbar in this image is patterned at a density approach 10^12/cm^2, and the wire diameter is approximately 8 nm. At a doping level (with species like Boron or Arsenic) of 10^18/cm^3, a similar 8 nm diameter, micrometer-long segment of silicon wires would have 20-30 dopant atoms, and a junction of two crossed wires would contain approximately 0.1-0.2 dopant atoms. Thus, field-effect transistors fabricated at these wiring densities might exhibit non-statistical, and perhaps non-predictable behavior. Related arguments, such as the gate oxide thickness, power consumption, (just from leakage currents through the gate oxide), fabrication costs, and others also highlight the difficulty of scaling standard electronics materials to molecular dimensions [2]. The point is that at device areas of a few tens of square nanometers, molecules have a certain fundamental attractiveness because of their size, because they represent the ultimate in terms of atomic control over physical properties, and because of the diversity of properties, such as switching, dynamic organization and recognition that can be achieved through such control.Molecular electronics circuits based on crossbar architectures can be utilized for both logic and memory applications [3], but in order to realize such applications, many things must be simultaneously considered. These include the design of the molecule, the molecule electrode interface, electronically configurable and defect tolerant circuit architectures, methods for bridging the nanometer-scale densities of these circuits to the sub-micrometer densities achievable with lithography, etc. [4] In this talk I will treat such circuits as a system, and discuss how all of these various properties are interrelated. I will also present experimental results of working random-access memory and configurable logic circuits, and FET-based multiplexers capable of bridging length scales.In these circuits the active device elements are molecular mechanical complexes organized at each of the junctions within the crossbar, as shown at left in the drawing. The molecules are switched via 1 or 2 electron process that results in a mechanical isomerization of the molecule, and thereby a change in the tunneling characteristics of the junction. Detailed electrical measurements on single molecule, three-terminal devices are revealing substantial information concerning how these types of devices can be better designed and optimized, and such measurements will also be presented and discussed

Growth and characterisation of organic semiconductors at metal surfaces

This thesis is a report of work carried out at the Jeremiah Horrocks Institute at the University of Central Lancashire, the Stephenson Institute For Renewable Energy at the University of Liverpool and the Center for Nanoscale Materials at Argonne National Lab. The focus of study are systems of interest to developing molecular electronics and systems that facilitate the synthesis of graphene patterned with ordered defects. The approach taken to developing the latter is via the deposition of precursor molecules atop symmetry conflicting substrates

Four-Step Domino Reaction Enables Fully Controlled Non-Statistical Synthesis of Hexaarylbenzene with Six Different Aryl Groups*

Hexaarylbenzene (HAB) derivatives are versatile aromatic systems playing a significant role as chromophores, liquid crystalline materials, molecular receptors, molecular-scale devices, organic light-emitting diodes and candidates for organic electronics. Statistical synthesis of simple symmetrical HABs is known via cyclotrimerization or Diels–Alder reactions. By contrast, the synthesis of more complex, asymmetrical systems, and without involvement of statistical steps, remains an unsolved problem. Here we present a generally applicable synthetic strategy to access asymmetrical HAB via an atom-economical and high-yielding metal-free four-step domino reaction using nitrostyrenes and α,α-dicyanoolefins as easily available starting materials. Resulting domino product—functionalized triarylbenzene (TAB)—can be used as a key starting compound to furnish asymmetrically substituted hexaarylbenzenes in high overall yield and without involvement of statistical steps. This straightforward domino process represents a distinct approach to create diverse and still unexplored HAB scaffolds, containing six different aromatic rings around central benzene core. © 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH Gmb

Assessing the reversed exponential decay of the electrical conductance in molecular wires: the undeniable effect of static electron correlation

An extraordinary new family of molecular junctions, inaccurately referred to as "anti-Ohmic" wires in the recent literature, has been proposed based on theoretical predictions. The unusual electron transport observed for these systems, characterized by a reversed exponential decay of their electrical conductance, might revolutionize the design of molecular electronic devices. This behavior, which has been associated with intrinsic diradical nature, is reexamined in this work. Since the diradical character arises from a near-degeneracy of the frontier orbitals, the employment of a multireference approach is mandatory. CASSCF calculations on a set of nanowires based on polycyclic aromatic hydrocarbons (PAHs) demonstrate that, in the frame of an appropriate multireference treatment, the ground state of these systems shows the expected exponential decay of the conductance. Interestingly, these calculations do evidence a reversed exponential decay of the conductance, although now in several excited states. Similar results have been obtained for other recently proposed candidates to "anti-Ohmic" wires. These findings open new horizons for possible applications in molecular electronics of these promising systems.Xunta de Galicia | Ref. GRC2019/24Agencia Estatal de Investigación | Ref. PGC2018-095953-B-I0

Theoretical Principles of Single-Molecule Electronics: A Chemical and Mesoscopic View

Exploring the use of individual molecules as active components in electronic devices has been at the forefront of nanoelectronics research in recent years. Compared to semiconductor microelectronics, modeling transport in single-molecule devices is much more difficult due to the necessity of including the effects of the device electronic structure and the interface to the external contacts at the microscopic level. Theoretical formulation of the problem therefore requires integrating the knowledge base in surface science, electronic structure theory, quantum transport and device modeling into a single unified framework starting from the first-principles. In this paper, we introduce the theoretical framework for modeling single-molecule electronics and present a simple conceptual picture for interpreting the results of numerical computation. We model the device using a self-consistent matrix Green's function method that combines Non-Equilibrium Green's function theory of quantum transport with atomic-scale description of the device electronic structure. We view the single-molecule device as "heterostructures" composed of chemically well-defined atomic groups, and analyze the device characteristics in terms of the charge and potential response of these atomic groups to perturbation induced by the metal-molecule coupling and the applied bias voltage. We demonstrate the power of this approach using as examples devices formed by attaching benzene-based molecules of different size and internal structure to the gold electrodes through sulfur end atoms.Comment: To appear in International Journal of Quantum Chemistry, Special Issue in memory of J.A. Pople. 13 pages, 9 figure

Towards next generation time-domain diffuse optics devices

Diffuse Optics is growing in terms of applications ranging from e.g. oximetry, to mammography, molecular imaging, quality assessment of food and pharmaceuticals, wood optics, physics of random media. Time-domain (TD) approaches, although appealing in terms of quantitation and depth sensibility, are presently limited to large fiber-based systems, with limited number of source-detector pairs. We present a miniaturized TD source-detector probe embedding integrated laser sources and single-photon detectors. Some electronics are still external (e.g. power supply, pulse generators, timing electronics), yet full integration on-board using already proven technologies is feasible. The novel devices were successfully validated on heterogeneous phantoms showing performances comparable to large state-of-the-art TD rack-based systems. With an investigation based on simulations we provide numerical evidence that the possibility to stack many TD compact source-detector pairs in a dense, null source-detector distance arrangement could yield on the brain cortex about 1 decade higher contrast as compared to a continuous wave (CW) approach. Further, a 3-fold increase in the maximum depth (down to 6 cm) is estimated, opening accessibility to new organs such as the lung or the heart. Finally, these new technologies show the way towards compact and wearable TD probes with orders of magnitude reduction in size and cost, for a widespread use of TD devices in real life