Charge transport characteristics of diarylethene photoswitching single-molecule junctions

Toward Practical Single-Molecule/Atom Switches

Electronic switches have been considered to be one of the most important components of contemporary electronic circuits for processing and storing digital information. Fabricating functional devices with building blocks of atomic/molecular switches can greatly promote the minimization of the devices and meet the requirement of high integration. This review highlights key developments in the fabrication and application of molecular switching devices. This overview offers valuable insights into the switching mechanisms under various stimuli, emphasizing structural and energy state changes in the core molecules. Beyond the molecular switches, typical individual metal atomic switches are further introduced. A critical discussion of the main challenges for realizing and developing practical molecular/atomic switches is provided. These analyses and summaries will contribute to a comprehensive understanding of the switch mechanisms, providing guidance for the rational design of functional nanoswitch devices toward practical applications.

Unraveling the Nature of Spin Coupling in a Metal-Free Diradical: Theoretical Distinction of Ferromagnetic and Antiferromagnetic Interactions

Metal-free diradicals based on polycyclic aromatic hydrocarbons are promising candidates for organic spintronics due to their stable magnetism and tunable spin coupling. However, distinguishing and elucidating the origins of ferromagnetic and antiferromagnetic interactions in these systems remain challenging. Here, we investigate the 2-OS diradical molecule sandwiched between gold electrodes using a combined density functional theory and hierarchical equations of motion approach. We find that the dihedral angle between the radical moieties controls the nature and strength of the intramolecular spin coupling, transitioning smoothly from antiferromagnetic to ferromagnetic as the angle increases. Distinct features in the inelastic electron tunneling spectra are identified that can discern the two coupling regimes, including spin excitation steps whose energies directly reveal the exchange coupling constant. Mechanical stretching of the junction is predicted to modulate the spectral line shapes by adjusting the hybridization of the molecular radicals with the electrodes. Our work elucidates the electronic origin of tunable spin interactions in 2-OS and provides spectroscopic fingerprints for characterizing magnetism in metal-free diradicals.

Revisiting Peri-Aryloxyquinones: From a Forgotten Photochromic System to a Promising Tool for Emerging Applications

Emerging applications of photochromic compounds demand new molecular designs that can be inspired by some long-known yet currently forgotten classes of photoswitches. In the present review, we remind the community about Peri-AryloxyQuinones (PAQs) and their unique photoswitching behavior originally discovered more than 50 years ago. At the heart of this phenomenon is the light-induced migration of an aromatic moiety (arylotropy) in peri-aryloxy-substituted quinones resulting in ana-quinones. PAQs feature absorbance of both isomers in the visible spectral region, photochromism in the amorphous and crystalline state, and thermal stability of the photogenerated ana-isomer. Particularly noticeable is the high sensitivity of the ana-isomer towards nucleophiles in solution. In addition to the mechanism of molecular photochromism and the underlaying structure-switch relationships, we analyze potential applications and prospects of aryloxyquinones in optically switchable materials and devices. Due to their ability to efficiently photoswitch in the solid state, PAQs are indeed attractive candidates for such materials and devices, including electronics (optically controllable circuits, switches, transistors, memories, and displays), porous crystalline materials, crystalline actuators, photoactivated sensors, and many more. This review is intended to serve as a guide for researchers who wish to use photoswitchable PAQs in the development of new photocontrollable materials, devices, and processes.

Vertical molecular transistors: a new strategy towards practical quantum devices

Considerable effort has been dedicated to improving molecular devices since they were initially proposed by Aviram and Ratner in 1974. Organic molecules are small and have discrete molecular orbitals. These features can facilitate fascinating quantum transport phenomena, such as single-carrier tunneling, resonant tunneling, and quantum interference. The effective gate modulation of these quantum transport phenomena holds the promise of realizing a new computing architecture that differs from that of current Si electronics. In this article, we review the recent research progress on molecular transistors, specifically vertical molecular transistors (VMTs). First, we discuss the benefits of VMTs for future molecular-scale transistors compared with the currently dominant lateral molecular transistors. Subsequently, we describe representative examples of VMTs, where single molecules, self-assembled monolayers, and isolated molecules are used as transistor channels. Finally, we present our conclusions and perspectives about the use of VMTs for attractive quantum devices.

Molecular Electronics: Creating and Bridging Molecular Junctions and Promoting Its Commercialization

Molecular electronics is driven by the dream of expanding Moore's law to the molecular level for next-generation electronics through incorporating individual or ensemble molecules into electronic circuits. For nearly 50 years, numerous efforts have been made to explore the intrinsic properties of molecules and develop diverse fascinating molecular electronic devices with the desired functionalities. The flourishing of molecular electronics is inseparable from the development of various elegant methodologies for creating nanogap electrodes and bridging the nanogap with molecules. This review first focuses on the techniques for making lateral and vertical nanogap electrodes by breaking, narrowing, and fixed modes, and highlights their capabilities, applications, merits, and shortcomings. After summarizing the approaches of growing single molecules or molecular layers on the electrodes, the methods of constructing a complete molecular circuit are comprehensively grouped into three categories: 1) directly bridging one-molecule-electrode component with another electrode, 2) physically bridging two-molecule-electrode components, and 3) chemically bridging two-molecule-electrode components. Finally, the current state of molecular circuit integration and commercialization is discussed and perspectives are provided, hoping to encourage the community to accelerate the realization of fully scalable molecular electronics for a new era of integrated microsystems and applications.

Dynamical Screening of Local Spin Moments at Metal-Molecule Interfaces

Transition-metal phthalocyanine molecules have attracted considerable interest in the context of spintronics device development due to their amenability to diverse bonding regimes and their intrinsic magnetism. The latter is highly influenced by the quantum fluctuations that arise at the inevitable metal-molecule interface in a device architecture. In this study, we have systematically investigated the dynamical screening effects in phthalocyanine molecules hosting a series of transition-metal ions (Ti, V, Cr, Mn, Fe, Co, and Ni) in contact with the Cu(111) surface. Using comprehensive density functional theory plus Anderson's Impurity Model calculations, we show that the orbital-dependent hybridization and electron correlation together result in strong charge and spin fluctuations. While the instantaneous spin moments of the transition-metal ions are near atomic-like, we find that screening gives rise to considerable lowering or even quenching of these. Our results highlight the importance of quantum fluctuations in metal-contacted molecular devices, which may influence the results obtained from theoretical or experimental probes, depending on their possibly material-dependent characteristic sampling time-scales.

E/Z photoisomerization pathway in pristine and fluorinated di(3-furyl)ethenes

We report an XMCQDPT2 study of the E/Z photoisomerization in a series of fluorinated di(3-furyl)ethenes (3DFEs). Upon excitation, pristine and low-fluorinated 3DFE show conventional behavior of many diarylethenes: unhindered twisting motion toward the pyramidalized zwitterionic state where relaxation to the ground state occurs. However, deep fluorination of 3DFEs can hamper E-to-Z isomerization by giving rise to an alternative excited-state relaxation pathway: an out-of-plane motion of a ring fluorine atom. Importantly, the case of fluorinated 3DFEs reveals serious deficiencies of the popular TDDFT approach. With some commonly used exchange-correlation functionals, the alternative relaxation pathway is not reproduced and, moreover, an irrelevant ring rotation coordinate is predicted instead. Nevertheless, TDDFT remains qualitatively adequate for the E-to-Z twisting coordinate taken alone.

Dual-gated single-molecule field-effect transistors beyond Moore's law

As conventional silicon-based transistors are fast approaching the physical limit, it is essential to seek alternative candidates, which should be compatible with or even replace microelectronics in the future. Here, we report a robust solid-state single-molecule field-effect transistor architecture using graphene source/drain electrodes and a metal back-gate electrode. The transistor is constructed by a single dinuclear ruthenium-diarylethene (Ru-DAE) complex, acting as the conducting channel, connecting covalently with nanogapped graphene electrodes, providing field-effect behaviors with a maximum on/off ratio exceeding three orders of magnitude. Use of ultrathin high-k metal oxides as the dielectric layers is key in successfully achieving such a high performance. Additionally, Ru-DAE preserves its intrinsic photoisomerisation property, which enables a reversible photoswitching function. Both experimental and theoretical results demonstrate these distinct dual-gated behaviors consistently at the single-molecule level, which helps to develop the different technology for creation of practical ultraminiaturised functional electrical circuits beyond Moore’s law.

Conventional silicon-based transistors, which sit at the heart of every computer, are fast approaching the limit of miniaturisation. Here, Meng et al demonstrate a field-effect transistor composed of a single rutheniumdiarylethene molecule with large on/off ratio.

Coherent and incoherent contributions to molecular electron transport

We numerically isolate the limits of validity of the Landauer approximation to describe charge transport along molecular junctions in condensed phase environments. To do so, we contrast Landauer with exact time-dependent non-equilibrium Green’s function quantum transport computations in a two-site molecular junction subject to exponentially correlated noise. Under resonant transport conditions, we find Landauer accuracy to critically depend on intramolecular interactions. By contrast, under nonresonant conditions, the emergence of incoherent transport routes that go beyond Landauer depends on charging and discharging processes at the electrode–molecule interface. In both cases, decreasing the rate of charge exchange between the electrodes and molecule and increasing the interaction strength with the thermal environment cause Landauer to become less accurate. The results are interpreted from a time-dependent perspective where the noise prevents the junction from achieving steady-state and from a fully quantum perspective where the environment introduces dephasing in the dynamics. Using these results, we analyze why the Landauer approach is so useful to understand experiments, isolate regimes where it fails, and propose schemes to chemically manipulate the degree of transport coherence.

Recent Advances in Photochemical Reactions on Single-Molecule Electrical Platforms

The photochemical reaction is a very important type of chemical reactions. Visualizing and controlling photo-mediated reactions is a long-standing goal and challenge. In this regard, single-molecule electrical detection with label-free, real-time and in situ characteristics has unique advantages in monitoring the dynamic process of photoreactions at the single-molecule level. In this Review, we provide a valuable summary of the latest process of single-molecule photochemical reactions based on single-molecule electrical platforms. The single-molecule electrical detection platforms for monitoring photoreactions are displayed, including their fundamental principles, construction methods and practical applications. The single-molecule studies of two different types of light-mediated reactions are summarized as below: i) photo-induced reactions, including reversible cyclization, conformational isomerization and other photo-related reactions; ii) plasmon-mediated photoreactions, including reaction mechanisms and concrete examples, such as plasmon-induced photolysis of S-S/O-O bonds and tautomerization of porphycene. In addition, the prospects for future research directions and challenges in this field are also discussed. This article is protected by copyright. All rights reserved.

Tunable Symmetry-Breaking-Induced Dual Functions in Stable and Photoswitched Single-Molecule Junctions

The aim of molecular electronics is to miniaturize active electronic devices and ultimately construct single-molecule nanocircuits using molecules with diverse structures featuring various functions, which is extremely challenging. Here, we realize a gate-controlled rectifying function (the on/off ratio reaches ∼60) and a high-performance field effect (maximum on/off ratio >100) simultaneously in an initially symmetric single-molecule photoswitch comprising a dinuclear ruthenium-diarylethene (Ru-DAE) complex sandwiched covalently between graphene electrodes. Both experimental and theoretical results consistently demonstrate that the initially degenerated frontier molecular orbitals localized at each Ru fragment in the open-ring Ru-DAE molecule can be tuned separately and shift asymmetrically under gate electric fields. This symmetric orbital shifting (AOS) lifts the degeneracy and breaks the molecular symmetry, which is not only essential to achieve a diode-like behavior with tunable rectification ratio and controlled polarity, but also enhances the field-effect on/off ratio at the rectification direction. In addition, this gate-controlled symmetry-breaking effect can be switched on/off by isomerizing the DAE unit between its open-ring and closed-ring forms with light stimulus. This new scheme offers a general and efficient strategy to build high-performance multifunctional molecular nanocircuits.

Molecular Approach to Engineer Two-Dimensional Devices for CMOS and beyond-CMOS Applications

Two-dimensional materials (2DMs) have attracted tremendous research interest over the last two decades. Their unique optical, electronic, thermal, and mechanical properties make 2DMs key building blocks for the fabrication of novel complementary metal-oxide-semiconductor (CMOS) and beyond-CMOS devices. Major advances in device functionality and performance have been made by the covalent or noncovalent functionalization of 2DMs with molecules: while the molecular coating of metal electrodes and dielectrics allows for more efficient charge injection and transport through the 2DMs, the combination of dynamic molecular systems, capable to respond to external stimuli, with 2DMs makes it possible to generate hybrid systems possessing new properties by realizing stimuli-responsive functional devices and thereby enabling functional diversification in More-than-Moore technologies. In this review, we first introduce emerging 2DMs, various classes of (macro)molecules, and molecular switches and discuss their relevant properties. We then turn to 2DM/molecule hybrid systems and the various physical and chemical strategies used to synthesize them. Next, we discuss the use of molecules and assemblies thereof to boost the performance of 2D transistors for CMOS applications and to impart diverse functionalities in beyond-CMOS devices. Finally, we present the challenges, opportunities, and long-term perspectives in this technologically promising field.

A review of oligo(arylene ethynylene) derivatives in molecular junctions

Oligo(arylene ethynylene) (OAE) derivatives are the "workhorse" molecules of molecular electronics. Their ease of synthesis and flexibility of functionalisation mean that a diverse array of OAE molecular wires have been designed, synthesised and studied theoretically and experimentally in molecular junctions using both single-molecule and ensemble methods. This review summarises the breadth of molecular designs that have been investigated with emphasis on structure-property relationships with respect to the electronic conductance of OAEs. The factors considered include molecular length, connectivity, conjugation, (anti)aromaticity, heteroatom effects and quantum interference (QI). Growing interest in the thermoelectric properties of OAE derivatives, which are expected to be at the forefront of research into organic thermoelectric devices, is also explored.

Photocontrollable Modulation of Frontier Molecular Orbital Energy Levels of Cyclopentenone-Based Diarylethenes

Photoswitchable diarylethenes provide a unique opportunity to optically modulate frontier molecular orbital energy levels, thereby opening an avenue for the design of electronic devices such as photocontrollable organic field-effect transistors (OFETs). In the present work, the absolute position of the frontier orbital levels of nonsymmetrical diarylethenes based on a cyclopentenone bridge has been studied using cyclic voltammetry and density functional theory (DFT) calculations. It has been shown that varying heteroaromatic substituents make it possible to change the absolute positions of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of both diarylethene photoisomers. The data obtained are used to refine the operation mechanism of the previously developed OFET devices, employing the cyclopentenone-derived diarylethenes at the dielectric/semiconductor interface.

In situ photoconductivity measurements of imidazole in optical fiber break-junctions

We developed a method based on the mechanically controllable break junction technique to investigate the electron transport properties of single molecular junctions upon fiber waveguided light. In our strategy, a metal-coated tapered optical fiber is fixed on a flexible substrate, and this tapered fiber serves as both the optical waveguide and metal electrodes after it breaks. For an imidazole bridged single-molecule junction, two probable conductance values below 1G0 are observed. The higher value shows an approximately 40% enhancement under illumination, while the lower one does not show distinguishable difference under illumination. Theoretical calculations reveal these two conductance values resulting from the imidazole monomer junction and the imidazole dimer junction linked via a hydrogen bond, respectively. In imidazole monomer junctions, the absorption of a single photon strongly shifts the transmission function resulting in optical-induced conductance enhancement. In contrast, the transmission function of imidazole dimer junctions remains at the same level in the bias window despite the light illumination. This work provides a robust experimental framework for studying the underlying mechanisms of photoconductivity in single-molecule junctions and offers tools for tuning the optoelectronic performance of single-molecule devices in situ.

Synthesis and Transport Studies of a Cofacial Porphyrin Cyclophane

Porphyrin cyclophane 1, consisting of two rigidly fixed but still movable cofacial porphyrins and exposing acetate-masked thiols in opposed directions of the macrocycle, is designed, synthesized, and characterized. The functional cyclophane 1, as pioneer of mechanosensitive 3D materials, forms stable single-molecule junctions in a mechanically controlled break-junction setup. Its reliable integration in a single-molecule junction is a fundamental prerequisite to explore the potential of these structures as mechanically triggered functional units and devices.

Photoswitching Molecular Junctions: Platforms and Electrical Properties

Remarkable advances in technology have enabled the manipulation of individual molecules and the creation of molecular electronic devices utilizing single and ensemble molecules. Maturing the field of molecular electronics has led to the development of functional molecular devices, especially photoswitching or photochromic molecular junctions, which switch electronic properties under external light irradiation. This review introduces and summarizes the platforms for investigating the charge transport in single and ensemble photoswitching molecular junctions as well as the electronic properties of diverse photoswitching molecules such as diarylethene, azobenzene, dihydropyrene, and spiropyran. Furthermore, the article discusses the remaining challenges and the direction for moving forward in this area for future photoswitching molecular devices.

Voltage-Controlled Binary Conductance Switching in Gold-4,4'-Bipyridine-Gold Single-Molecule Nanowires

We investigate gold-4,4'-bipyridine-gold single-molecule junctions with the mechanically controllable break junction technique at cryogenic temperature (T = 4.2 K). We observe bistable probabilistic conductance switching between the two molecular binding configurations, influenced both by the mechanical actuation and by the applied voltage. We demonstrate that the relative dominance of the two conductance states is tunable by the electrode displacement, whereas the voltage manipulation induces an exponential speedup of both switching times. The detailed investigation of the voltage-tunable switching rates provides an insight into the possible switching mechanisms.

Switching Quantum Interference in Phenoxyquinone Single Molecule Junction with Light

Quantum interference (QI) can lead to large variations in single molecule conductance. However, controlling QI using external stimuli is challenging. The molecular structure of phenoxyquinone can be tuned reversibly using light stimulus. In this paper, we show that this can be utilized to control QI in phenoxyquinone derivatives. Our calculations indicate that, as a result of such variation in molecular structure of phenoxyquinone, a crossover from destructive to constructive QI is induced. This leads to a significant variation in the single molecule conductance by a couple of orders of magnitude. This control of QI using light is a new paradigm in photosensitive single molecule switches and opens new avenues for future QI-based photoswitches.

Surface Plasmon-Enhanced Switching Kinetics of Molecular Photochromic Films on Gold Nanohole Arrays

Diarylethene molecules are discussed as possible optical switches, which can reversibly transition between completely conjugated (closed) and nonconjugated (open) forms with different electrical conductance and optical absorbance, by exposure to UV and visible light. However, in general the opening reaction exhibits much lower quantum yield than the closing process, hindering their usage in optoelectronic devices. To enhance the opening process, which is supported by visible light, we employ the plasmonic field enhancement of gold films perforated with nanoholes. We show that gold nanohole arrays reveal strong optical transmission in the visible range (∼60%) and pronounced enhancement of field intensities, resulting in around 50% faster switching kinetics of the molecular species in comparison with quartz substrates. The experimental UV-vis measurements are verified with finite-difference time-domain simulation that confirm the obtained results. Thus, we propose gold nanohole arrays as transparent and conductive plasmonic material that accelerates visible-light-triggered chemical reactions including molecular switching.

Fabrication and functions of graphene-molecule-graphene single-molecule junctions

The past two decades have witnessed increasingly rapid advances in the field of single-molecule electronics, which are expected to overcome the limitation of the miniaturization of silicon-based microdevices, thus promoting the development of device manufacturing technologies and characterization means. In addition to this, they can enable us to investigate the intrinsic properties of materials at the atomic- or molecular-length scale and probe new phenomena that are inaccessible in ensemble experiments. In this perspective, we start from a brief introduction on the manufacturing method of graphene-molecule-graphene single-molecule junctions (GMG-SMJs). Then, we make a description on the remarkable functions of GMG-SMJs, especially on the investigation of single-molecule charge transport and dynamics. Finally, we conclude by discussing the main challenges and future research directions of molecular electronics.

A Suzuki Approach to Quinone-Based Diarylethene Photochromes

Diarylethene photochromes show promise for use in advanced organic electronic and photonic materials with burgeoning considerations for biological applications; however, these compounds typically require UV light for photoswitching in at least one direction, thus limiting their appeal. We here introduce a naphthoquinone-based diarylethene that switches between open and closed forms with visible light. The synthesis of this quinone diarylethene relies on Suzuki methodology, allowing for the inclusion of functional groups not otherwise accessible with current synthetic routes.

Basic enemies of photochromism: irreversible transformation of fluorinated diarylethenes to polyenic enamines and enols

Elongating the π-system of perfluoro-diarylethenes under basic conditions inevitably leads to variable amounts of non-photochromic seven-ring containing byproducts.

Electronic Communication as a Transferable Property of Molecular Bridges?

Electronic communication through molecular bridges is important for different types of experiments, such as single-molecule conductance, electron transfer, superexchange spin coupling, and intramolecular singlet fission. In many instances, the chemical structure of the bridge determines how the two parts it is connecting communicate, and does so in ways that are transferable between these different manifestations (for example, high conductance often correlates with strong antiferromagnetic spin coupling, and low conductance due to destructive quantum interference correlates with ferromagnetic coupling). Defining electronic communication as a transferable property of the bridge can help transfer knowledge between these different areas of research. Examples and limits of such transferability are discussed here, along with some possible directions for future research, such as employing spin-coupled and mixed-valence systems as structurally well-controlled proxies for understanding molecular conductance and for validating first-principles theoretical methodologies, building conceptual understanding for the growing experimental work on intramolecular singlet fission, and developing measures for the transferability of electronic communication as a bridge property.

Electric-Field-Controllable Conductance Switching of an Overcrowded Ethylene Self-Assembled Monolayer

Molecular isomerism has been discussed from the viewpoint of the tiniest switch and memory elements in electronics. Here, we report an overcrowded ethylene-based molecular conductance switch, which fulfills all the essential requirements for implementation into electronic devices, namely, electric-field-controllable reversible conductance change with a molecular-level spatial resolution, robust conformational bistability under ambient conditions, and ordered monolayer formation on electrode surfaces. The conformational state of this overcrowded ethylene, represented by a folded or twisted conformer, is susceptible to external environments. Nanoscopic measurements using scanning tunneling microscopy techniques, together with theoretical simulations, revealed the electronic properties of each conformer adsorbed on Au(111). While the twisted conformer prevails in the molecularly dispersed state, upon self-assembly into a monolayer, a two-dimensional network structure of the folded conformer is preferentially formed due to particular intermolecular interaction. In the monolayer state, folded-to-twisted and its reverse isomerization can be controlled by the modulation of electric fields.

Six state molecular revolver mounted on a rigid platform

The rotation of entire molecules or large moieties happens at 100 ps time scales and the transition process itself is experimentally inaccessible to scanning probe techniques. However, the reversible switching of a molecule between more than two metastable states allows to assign a rotational switching direction. Rotational switching is a phenomenon that is particularly interesting with regard to possible applications in molecular motors. In this work, single tetraphenylmethane molecules deposited on a Au(111) surface were studied in a low temperature scanning tunneling microscope (STM). These molecules comprise rotational axes mounted on a tripodal sulfur-anchored stand and with the STM tip, we were able to induce transitions between six rotational states of the molecular motif. We were able to identify critical parameters for the onset of rotational switching and to characterize the influence of the local environment. The subtle difference between fcc and hcp stacking and the rotational state of neighboring molecules clearly influence the population of the rotational states.

Unidirectional Real-Time Photoswitching of Diarylethene Molecular Monolayer Junctions with Multilayer Graphene Electrodes

We fabricate and characterize vertical molecular junctions consisting of self-assembled monolayers of diarylethene (DAE) contacted by a multilayer graphene (MLG) electrode on the top and gold on the bottom. The DAE molecular junctions show two stable electrical states, a closed state (high conductance) or an open state (low conductance), which are created upon illumination with UV or visible light, respectively. For the Au-DAE-MLG junction structure, we observe that the current levels between the two conductance states are separated by 2 orders of magnitude. However, in a real-time measurement, we observe only unidirectional switching behavior from the open to the closed state.

Sub-5 nm Metal Nanogaps: Physical Properties, Fabrication Methods, and Device Applications

Sub-5 nm metal nanogaps have attracted widespread attention in physics, chemistry, material sciences, and biology due to their physical properties, including great plasmon-enhanced effects in light-matter interactions and charge tunneling, Coulomb blockade, and the Kondo effect under an electrical stimulus. These properties especially meet the needs of many cutting-edge devices, such as sensing, optical, molecular, and electronic devices. However, fabricating sub-5 nm nanogaps is still challenging at the present, and scaled and reliable fabrication, improved addressability, and multifunction integration are desired for further applications in commercial devices. The aim of this work is to provide a comprehensive overview of sub-5 nm nanogaps and to present recent advancements in metal nanogaps, including their physical properties, fabrication methods, and device applications, with the ultimate aim to further inspire scientists and engineers in their research.

Conductance Photoswitching of Metal-Organic Frameworks with Embedded Spiropyran

Conductive metal-organic frameworks (MOFs) as well as smart, stimuli-responsive MOF materials have attracted considerable attention with respect to advanced applications in energy harvesting and storage as well as in signal processing. Here, the conductance of MOF films of type UiO-67 with embedded photoswitchable nitro-substituted spiropyrans was investigated. Under UV irradiation, the spiropyran (SP) reversibly isomerizes to the open merocyanine (MC) form, a zwitterionic molecule with an extended conjugated π-system. The light-induced SP-MC isomerization allows for remote control over the conductance of the SP@UiO-67 MOF film, and the conductance can be increased by one order of magnitude. This research has the potential to contribute to the development of a new generation of photoelectronic devices based on smart hybrid materials.

Low-bias negative differential resistance in junction of a benzene between zigzag-edged phosphorene nanoribbons

We study the electron transport properties through the junction of a benzene molecule in conjunction with two monolayer zigzag-edged phosphorene nanoribbon (ZPNR) electrodes by applying the nonequilibrium Green's functions in combination with the density functional theory. We find that the molecular junction with two phosphorus-carbon bonds exhibits an interesting low-bias negative differential resistance effect with a peak-to-valley ratio of 29, which originates from the edge states in ZPNR due to the anisotropic band structure of phosphorene. Importantly, the performance of the junction can be tuned via the molecule-ZPNR interface bonding. The findings may be useful in sensitive-device applications. Furthermore, the physical mechanisms are revealed and discussed in terms of the electronic transmission spectrum, the evolution of the frontier molecular orbitals, the local device density of states around the Fermi level, and the projected density of states.

Quantum Transport through a Single Conjugated Rigid Molecule, a Mechanical Break Junction Study

This Account provides an overview of our recent efforts to unravel charge transport characteristics of a metal-molecule-metal junction containing an individual π-conjugated molecule. The model system of our choice is an oligo(phenylene-ethynylene) consisting of three rings, in short OPE3, which represents a paradigmatic model system for molecular-scale electronics. Members of the OPE family are among the most studied in the field thanks to their simple and rigid structure, the possibility of chemically functionalizing them, and their clear transport characteristics. When investigating charge transport in molecular systems, two general directions can be distinguished: one in which assemblies composed of many molecules contacted in parallel are studied, while in the other a single molecule is investigated at a time. In the former approach, molecule-molecule interactions and ensemble-averaged quantities may play a role, thereby introducing broadening of spectral features and hindering the study of the behavior of individual molecules making it more difficult to deconvolute local and intrinsic molecular effects from collective ones. In contrast, single-molecule experiments directly probe individual molecular features and, when they are repeated many times, allow build up of a statistical representation of the changes introduced by, e.g., different junction configurations. Especially in recent years, experimental techniques have advanced such that now large sets of individual events can be measured and analyzed with statistical tools. To study individual single-molecule junctions, we use the break junction technique, in which two sharp movable electrodes are formed by breaking a thin metallic wire and used to contact a single or few molecules. By probing thousands of single-molecule junctions in different conditions, we show that their creation involves independent events justifying the statistical tools that are used. By combining room- and low-temperature data, we show that the dominant transport mechanism for electrons through the OPE3 molecule is off-resonant tunneling. The simplest model capturing transport details in this case is a single-level model characterized by three parameters: the level alignment of the frontier orbital with the Fermi energy of the leads and the electronic couplings to the leads. Variations in these parameters give a broad distribution (1 order of magnitude) in the observed conductance values, indicating that at the microscopic level both the hybridization with the metallic electrodes and the molecular electronic configuration can fluctuate. The low-temperature data show that these variations are due to abrupt changes in the configuration of the molecule in the junction leading to changes in either one of these parameters or both at the same time. The complementary information gained from different experiments is needed to build up a consistent and extended picture of the variability of molecular configurations, omnipresent in single-molecule studies. Knowledge of this variability can help one to better understand the behavior of molecules at the atomic level and at the metal-molecule interface in particular.

Inelastic electron tunneling spectroscopy of difurylethene-based photochromic single-molecule junctions

Diarylethene-derived molecules alter their electronic structure upon transformation between the open and closed forms of the diarylethene core, when exposed to ultraviolet (UV) or visible light. This transformation results in a significant variation of electrical conductance and vibrational properties of corresponding molecular junctions. We report here a combined experimental and theoretical analysis of charge transport through diarylethene-derived single-molecule devices, which are created using the mechanically controlled break-junction technique. Inelastic electron tunneling (IET) spectroscopy measurements performed at 4.2 K are compared with first-principles calculations in the two distinct forms of diarylethenes connected to gold electrodes. The combined approach clearly demonstrates that the IET spectra of single-molecule junctions show specific vibrational features that can be used to identify different isomeric molecular states by transport experiments.

Effects of Photochromic Furan-Based Diarylethenes on Gold Nanoparticles Aggregation

The photochromic properties of furan-based diarylethenes and their interaction with citrate-capped gold nanoparticles were investigated by ultraviolet/visible absorption spectroscopy and transmission electron microscopy. We identified the optimal concentration of diarylethenes in water-ethanol mixture required for stability of colloidal systems. Nanoparticles coupled with diarylethene derivatives exhibit a new surface plasmon resonance band coming from their aggregation. We analyzed the effects of functional side-chain groups on aggregation process. These results can be considered as a basis for further designing of novel hybrid nanomaterials and optoelectronic elements.

Light-induced photoisomerization of a diarylethene molecular switch on solid surfaces

Diarylethenes are molecular switches, the state of which can efficiently be controlled by illumination with ultraviolet or visible light. To use the change in the molecular properties when switching between the two states for a specific function, direct contact with solid surfaces is advantageous as it provides immobilization. Here we present a study of a diarylethene derivate (T-DAE, 1,2-bis(5-methyl-2-phenylthiazol-4-yl)cyclopent-1-ene) in direct contact with highly ordered graphite as well as with semimetallic Bi(1 1 1) surfaces by x-ray photoelectron spectroscopy, x-ray absorption spectroscopy and simulated spectra based on density functional theory. On both surfaces, the molecule can be switched from its open to its closed form by 325-475 nm broadband or ultraviolet illumination. On the other hand, back isomerization to the ring-open T-DAE was not possible.

Vertical resonant tunneling transistors with molecular quantum dots for large-scale integration

Quantum molecular devices have a potential for the construction of new data processing architectures that cannot be achieved using current complementary metal-oxide-semiconductor (CMOS) technology. The relevant basic quantum transport properties have been examined by specific methods such as scanning probe and break-junction techniques. However, these methodologies are not compatible with current CMOS applications, and the development of practical molecular devices remains a persistent challenge. Here, we demonstrate a new vertical resonant tunneling transistor for large-scale integration. The transistor channel is comprised of a MOS structure with C₆₀ molecules as quantum dots, and the structure behaves like a double tunnel junction. Notably, the transistors enabled the observation of stepwise drain currents, which originated from resonant tunneling via the discrete molecular orbitals. Applying side-gate voltages produced depletion layers in Si substrates, to achieve effective modulation of the drain currents and obvious peak shifts in the differential conductance curves. Our device configuration thus provides a promising means of integrating molecular functions into future CMOS applications.

Disentangling electron- and electric-field-induced ring-closing reactions in a diarylethene derivative on Ag(1 1 1)

Using scanning tunneling microscopy and spectroscopy we investigate the adsorption properties and ring-closing reaction of a diarylethene derivative (C5F-4Py) on a Ag(1 1 1) surface. We identify an electron-induced reaction mechanism, with a quantum yield varying from 10^-14-10^-9 per electron upon variation of the bias voltage from 1-2 V. We ascribe the drastic increase in switching efficiency to a resonant enhancement upon tunneling through molecular orbitals. Additionally, we resolve the ring-closing reaction even in the absence of a current passing through the molecule. In this case the electric-field can modify the reaction barrier, leading to a finite switching probability at 4.8 K. A detailed analysis of the switching events shows that a simple plate-capacitor model for the tip-surface junction is insufficient to explain the distance dependence of the switching voltage. Instead, describing the tip as a sphere is in agreement with the findings. We resolve small differences in the adsorption configuration of the closed isomer, when comparing the electron- and field-induced switching product.

High-Yield Functional Molecular Electronic Devices

An ultimate goal of molecular electronics, which seeks to incorporate molecular components into electronic circuit units, is to generate functional molecular electronic devices using individual or ensemble molecules to fulfill the increasing technical demands of the miniaturization of traditional silicon-based electronics. This review article presents a summary of recent efforts to pursue this ultimate aim, covering the development of reliable device platforms for high-yield ensemble molecular junctions and their utilization in functional molecular electronic devices, in which distinctive electronic functionalities are observed due to the functional molecules. In addition, other aspects pertaining to the practical application of molecular devices such as manufacturing compatibility with existing complementary metal-oxide-semiconductor technology, their integration, and flexible device applications are also discussed. These advances may contribute to a deeper understanding of charge transport characteristics through functional molecular junctions and provide a desirable roadmap for future practical molecular electronics applications.

Thermally Activated Tunneling Transition in a Photoswitchable Single-Molecule Electrical Junction

Exploring the charge transport process in molecular junctions is essential to the development of molecular electronics. Here, we investigate the temperature-dependent charge transport mechanism of carbon electrode-diarylethene single-molecule junctions, which possess photocontrollable molecular orbital energy levels due to reversible photoisomerization of individual diarylethenes between open and closed conformations. Both the experimental results and theoretical calculations consistently demonstrate that the vibronic coupling (thermally activated at the proper temperature) drives the transition of charge transport in the junctions from coherent tunneling to incoherent transport. Due to the subtle electron-phonon coupling effect, incoherent transport in the junctions proves to have different activation energies, depending on the photoswitchable molecular energy levels of two different conformations. These results improve fundamental understanding of charge transport mechanisms in molecular junctions and should lead to the rapid development of functional molecular devices toward practical applications.