Molecular Signatures in the Transport Properties of Molecular Wire Junctions: What Makes a Junction “Molecular”?

The simplest component of molecular electronics consists of a single-molecule transport junction: a molecule sandwiched between source and drain electrodes, with or without a third gate electrode. In this Concept article, we focus on how molecules control transport in metal-electrode molecular junctions, and where the molecular signatures are to be found. In the situation where the molecule is relatively short and the gap between injection energy and molecular eigenstates is large, transport occurs largely by elastic tunneling, stochastic switching is common, and the vibronic signature can be found using inelastic electron tunneling spectroscopy (IETS). As the energy gaps for injection become smaller, one begins to see stronger molecular signatures - these include Franck-Condon-like structures in the current/voltage characteristic and strong vibronic interactions, which can lead to hopping behavior at the polaron limit. Conformational changes induced by the strong electric field lead to another strong manifestation of the molecular nature of the junction. We overview some of this mechanistic landscape, focusing on significant effects of switching (both stochastic and controlled by the electric field) and of molecular vibronic coupling.     

Molecularly inherent voltage-controlled conductance switching

Molecular electronics has been proposed as a pathway for high-density nanoelectronic devices. This pathway involves the development of a molecular memory device based on reversible switching of a molecule between two conducting states in response to a trigger, such as an applied voltage. Here we demonstrate that voltage-triggered switching is indeed a molecular phenomenon by carrying out studies on the same molecule using three different experimental configurations-scanning tunnelling microscopy, crossed-wire junction, and magnetic-bead junction. We also demonstrate that voltage-triggered switching is distinctly different from stochastic switching, essentially a transient (time-dependent) phenomenon that is independent of the applied voltage.     

Probing molecular junctions using surface plasmon resonance spectroscopy

The optical absorption spectra of nanometer-thick organic films and molecular monolayers sandwiched between two metal contacts have been measured successfully using surface plasmon resonance spectroscopy (SPRS). The electric field within metal-insulator (organic)-metal (MIM) cross-bar junctions created by surface plasmon-polaritons excited on the metal surface allows sensitive measurement of molecular optical properties. Specifically, this spectroscopic technique extracts the real and imaginary indices of the organic layer for each wavelength of interest. The SPRS sensitivity was calculated for several device architectures, metals, and layer thicknesses to optimize the organic film absorptivity measurements. Distinct optical absorption features were clearly observed for R6G layers as thin as a single molecular monolayer between two metal electrodes. This method also enables dynamic measurement of molecular conformation inside metallic junctions, as shown by following the optical switching of a thin spiropyran/polymer film upon exposure to UV light. Finally, optical and electrical measurements can be made simultaneously to study the effect of electrical bias and current on molecular conformation, which may have significant impact in areas such as molecular and organic electronics.     

25th Anniversary Article: Design of Polymethine Dyes for All‐Optical Switching Applications: Guidance from Theoretical and Computational Studies

All‐optical switching—controlling light with light—has the potential to meet the ever‐increasing demand for data transmission bandwidth. The development of organic π‐conjugated molecular materials with the requisite properties for all‐optical switching applications has long proven to be a significant challenge. However, recent advances demonstrate that polymethine dyes have the potential to meet the necessary requirements. In this review, we explore the theoretical underpinnings that guide the design of π‐conjugated materials for all‐optical switching applications. We underline, from a computational chemistry standpoint, the relationships among chemical structure, electronic structure, and optical properties that make polymethines such promising materials.     

Performance comparison between multiple-quantum-well modulator-based and vertical-cavity-surface-emitting laser-based smart pixels

We compared multiple-quantum-well modulator-based smart pixels and vertical-cavity-surface-emitting laser (VCSEL) based smart pixels in terms of optical switching power, switching speed, and electric-power consumption. Optoelectronic circuits integrating GaAs field-effect transistors are designed for smart pixels of both types under the condition that each pixel has an optical threshold and gain. It is shown that both types perform maximum throughput of ~3 Tbps/cm(2). In regard to design flexibility, the modulator type is advantageous because switching time can be reduced by supplying large electric power, whereas switching time and electric-power consumption are limited to larger than certain values in the VCSEL type. In contrast, in regard to optical implementation, the VCSEL type is advantageous because it does not need an external bias-light source, whereas the modulator type needs bias-light arrays that must be precisely located because the small modulator diameter, <10 μm, is essential to high-speed operation. A bias-light source that increases the total power consumption of the system may offset the advantages of the modulator type.     

Nonlinear Optical Properties of Porphyrins

Porphyrins and phthalocyanines have outstanding chemical and thermal stability. The macrocyclic structure and chemical reactivity of tetrapyrroles offers architectural flexibility and facilitates the tailoring of chemical, physical and optoelectronic parameters. The specific optical properties of the tetrapyrrole macrocycle combined with the synthetic methodologies now available and the already available theoretical and spectroscopic knowledge on their optical behavior make porphyrins a target of choice for this area. They are versatile organic nanomaterials with a rich photochemistry and their excited state properties are easily modulated through conformational design, molecular symmetry, metal complexation, orientation and strength of the molecular dipole moment, size and degree of conjugation of the π‐systems, and appropriate donor‐acceptor substituents. Here we review the structural chemistry and optical properties of recently synthesized porphyrin derivatives that offer potential for nonlinear optical (NLO) applications and complement existing studies on phthalocyanines. Classes of interest include the classic A₄ symmetric tetrapyrroles, while optimized systems include push‐pull porphyrins, oligomeric and supramolecular self‐assembled systems, films and nanoparticle systems, and highly conjugated porphyrin arrays.     

Finite Size Effects on the Switching Dynamics of Spin‐Crossover Thin Films Photoexcited by a Femtosecond Laser Pulse

Using ultrafast optical absorption spectroscopy, the room‐temperature spin‐state switching dynamics induced by a femtosecond laser pulse in high‐quality thin films of the molecular spin‐crossover (SCO) complex [Fe(HB(tz)₃)₂] (tz = 1,2,4‐triazol‐1‐yl) are studied. These measurements reveal that the early, sub‐picosecond, low‐spin to high‐spin photoswitching event, with linear response to the laser pulse energy, can be followed under certain conditions by a second switching process occurring on a timescale of tens of nanoseconds, enabling nonlinear amplification. This out‐of‐equilibrium dynamics is discussed in light of the characteristic timescales associated with the different switching mechanisms, i.e., the electronic and structural rearrangements of photoexcited molecules, the propagation of strain waves at the material scale, and the thermal activation above the molecular energy barrier. Importantly, the additional, nonlinear switching step appears to be completely suppressed in the thinnest (50 nm) film due to the efficient heat transfer to the substrate, allowing the system to retrieve the thermal equilibrium state on the 100 ns timescale. These results provide a first milestone toward the assessment of the physical parameters that drive the photoresponse of SCO thin films, opening up appealing perspectives for their use as high‐frequency all‐optical switches working at room temperature.     

First principle study of the self‐switching characteristics of the guanine based single optical molecular switch using carbon nanotube electrodes

The switching property of an optical single molecular switch based on a single DNA molecule guanine with a single walled carbon nanotube electrode has been investigated using density functional theory along with non‐equilibrium Green''s function based first principle approach. The semi‐empirical model of this single bio‐molecular switch has been operated at an ultra‐high 25 THz frequency in mid‐UV range. This single bio‐molecule comprises switching activity upon UV photo‐excitation. The influence of the highest occupied molecular orbital and lowest unoccupied molecular orbital gap and the quantum ballistic transmission into the switching activity are discussed in detail in this study. It has been observed that the maximum ON–OFF ratio, i.e. 327 is obtained at +0.8 V bias voltage. Theoretical results show that current through the twisted form is sufficiently larger than the straightened form, which recommends that this structure has smart prospective application in the future generation switching nanotechnology.Inspec keywords: molecular electronic states, density functional theory, ab initio calculations, DNA, organic compounds, molecular electronics, Green''s function methods, molecular biophysics, single‐wall carbon nanotubes, optical switches, orbital calculationsOther keywords: nonequilibrium Green''s function, semiempirical model, single bio‐molecular switch, UV photo‐excitation, lowest unoccupied molecular orbital gap, first principle study, single optical molecular switch, switching property, optical single molecular switch, single DNA molecule guanine, single walled carbon nanotube electrode, density functional theory, highest occupied molecular orbital gap, switching nanotechnology     

Redox switchable self-assembled monolayers of functional ruthenium(III/II) complexes on optically transparent platinum electrodes

Self-assembled monolayers (SAMs) of functional Ru^II complexes on optically transparent ultrathin Pt films (10-nm nominal thicknesses) have been prepared. We have demonstrated that reversible Ru^III/II redox switching of these SAMs can be achieved by using appropriate oxidizing or reducing agents. Such a switching in molecular properties has been simply detected by transmission spectroscopy with a conventional spectrophotometer following the appearance/disappearance of the optical absorption metal-to-ligand charge-transfer (MLCT) band in the visible region. In light of their potential multifunctional properties, these SAMs on an optically transparent metal electrode are appealing candidates in the perspective of integrated molecular switch nanodevices.     

Reversible electrical switching of spin polarization in multiferroic tunnel junctions

Spin-polarized transport in ferromagnetic tunnel junctions, characterized by tunnel magnetoresistance, has already been proven to have great potential for application in the field of spintronics and in magnetic random access memories. Until recently, in such a junction the insulating barrier played only a passive role, namely to facilitate electron tunnelling between the ferromagnetic electrodes. However, new possibilities emerged when ferroelectric materials were used for the insulating barrier, as these possess a permanent dielectric polarization switchable between two stable states. Adding to the two different magnetization alignments of the electrode, four non-volatile states are therefore possible in such multiferroic tunnel junctions. Here, we show that owing to the coupling between magnetization and ferroelectric polarization at the interface between the electrode and barrier of a multiferroic tunnel junction, the spin polarization of the tunnelling electrons can be reversibly and remanently inverted by switching the ferroelectric polarization of the barrier. Selecting the spin direction of the tunnelling electrons by short electric pulses in the nanosecond range rather than by an applied magnetic field enables new possibilities for spin control in spintronic devices.     

Towards an optimal model for a bistable nematic liquid crystal display device

Bistable liquid crystal displays offer the potential for considerable power savings compared with conventional (monostable) LCDs. The existence of two stable field-free states that are optically distinct means that contrast can be maintained in a display without an externally applied electric field. An applied field is required only to switch the device from one state to the other, as needed. In this paper we examine a theoretical model of a possible bistable device, originally proposed by Cummings and Richardson (Euro J Appl Math 17:435–463 2006), and explore means by which it may be optimized, in terms of optical contrast, manufacturing considerations, switching field strength, and switching times. The compromises inherent in these conflicting design criteria are discussed.     

Fast Transient Current Response to Switching Events in Short Chains of Molecular Islands

We consider current flow between two metallic leads joined by tunneling junctions to a molecular island. The junctions are assumed to be “wiggly,” that is, switching on and off intermittently. The resulting sequence of current transients overlaps in dependence on the interplay between the switching times and the decay time of “initial” correlations induced by each switching event. The process is described in terms of nonequilibrium Green’s functions.     

Novel electrical switching behaviour and logic in carbon nanotube Y-junctions

Carbon-nanotube-based electronics offers significant potential as a nanoscale alternative to silicon-based devices for molecular electronics technologies. Here, we show evidence for a dramatic electrical switching behaviour in a Y-junction carbon-nanotube morphology. We observe an abrupt modulation of the current from an on- to an off-state, presumably mediated by defects and the topology of the junction. The mutual interaction of the electron currents in the three branches of the Y-junction is shown to be the basis for a potentially new logic device. This is the first time that such switching and logic functionalities have been experimentally demonstrated in Y-junction nanotubes without the need for an external gate. A class of nanoelectronic architecture and functionality, which extends well beyond conventional field-effect transistor technologies, is now possible.     

Electric-field control of the tautomerization and metal ion binding reactivity of 8-hydroxyquinoline immobilized to an electrode surface

Surface-enhanced Raman scattering (SERS) spectra of a metal-complexing ligand, immobilized to a silver electrode surface, exhibits significant structural changes upon application of modest potentials. A detailed spectroscopic investigation shows that the potential applied to the electrode surface governs the tautomerization equilibrium of the immobilized ligand, p-((8-hydroxyquinoline)azo)benzenethiol (SHQ). Potential-dependent SERS spectra reveal that SHQ exists predominantly in a keto-hydrazone tautomeric form at applied potentials that are negative of -300 mV (vs Ag/AgCl), while the enol-azo tautomer is strongly favored at potentials positive of this value. The observed switching of the tautomer population occurs within a narrow range of applied potentials, approximately 200 mV (Ag/AgCl). Electrical control over the tautomerization equilibrium of immobilized SHQ governs the reactivity of the ligand toward metal ion complexation, where the enol-azo tautomer exhibits much greater affinity for metal ion binding compared to its keto-hydrazone counterpart. Accordingly, the potential applied to the electrode can be used to influence metal ion binding of immobilized SHQ through control over the tautomerization equilibrium, to produce an electrically switchable surface for metal ion complexation. Large differences in the electric dipole moment of the two tautomers, estimated from density functional theory calculations, suggested a model where the potential dependence arises from the interaction of the ligand dipole with electric fields that exist at a polarized electrode surface. This model accurately predicts the relative tautomer populations versus applied potential, at interfacial electric fields that are consistent with previous measurements of the vibrational Stark effect at polarized interfaces. Potential applications of this technology to several areas of analytical chemistry are considered.     

Optical switching of porphyrin-coated silicon nanowire field effect transistors

We study porphyrin derivative coated silicon nanowire field effect transistors (SiNW-FETs), which display a large, stable, and reproducible conductance increase upon illumination. The efficiency and the kinetics of the optical switching are studied as a function of gate voltage, illumination wavelength, and temperature. The decay kinetics from the high- to the low-conductance state is governed by charge recombination via tunneling, with a rate depending on the state of the SiNW-FET. The comparison to porphyrin-sensitized carbon nanotube FETs allows the environment- and molecule-dependent photoconversion process to be distinguished from the charge-to-current transducing effect of the semiconducting channel.     

Real-Time Sensing of the Thermal Diffusivity for Dynamic Control of Anisotropic Heat Conduction of Liquid Crystals

Molecular orientational order can be used to characterize the anisotropic behavior in mechanical, optical, and thermophysical properties. The creation of appropriate molecular orientation has the potential for producing a novel material or thermal switching device, which can control anisotropic heat conduction. Liquid crystals, which are widely used in display elements, have anisotropy not only in their optical, but also in their thermophysical properties, under given molecular orientational alignment conditions; this material can be a variable device with anisotropic heat conduction by controlling the molecular alignment. In the present study, a real-time sensing system for thermal diffusivity using the forced Rayleigh scattering (FRS) method was developed to investigate the transient behavior in the thermal anisotropy of nematic liquid crystals. This technique can be used to measure the in-plane thermal diffusivity perpendicular to the transient thermal grating created by interfering pulsed laser beams, and the thermal anisotropy of the sample can be determined using this non-contact method. The present FRS system can provide continuous measurements of the thermal diffusivity with subsecond time resolution, allowing evaluation of the dynamic behavior of anisotropy in the thermal diffusivity even during a transient process. In this article, the anisotropy of the in-plane thermal diffusivity of 4-4′-pentyl-4-biphenylcarbonitrile (5CB) with molecular alignment induced by either a rubbed substrate or an electric field has been measured. Also, the time evolution of the anisotropic thermal diffusivity in real-time under a dynamically controlled external electric field has been measured. The experimental results demonstrate the capability of dynamic anisotropic control of heat conduction by molecular alignment variations.     

Demonstration of optically controlled data routing with the use of multiple-quantum-well bistable and electro-optical devices

We present experimental results on a 1-to-64-channel free-space photonic switching demonstration system based on GaAs/GaAlAs multiple-quantum-well active device arrays. Two control schemes are demonstrated: data transparent optical self-routing usable in a packet-switching environment and direct optical control with potential signal amplification for circuit switching. The self-routing operation relies on the optical recognition of the binary destination address coded in each packet header. Address decoding is implemented with elementary optical bistable devices and modulator pixels as all-optical latches and electro-optical and gates, respectively. All 60 defect-free channels of the system could be operated one by one, but the simultaneous operation of only three channels could be achieved mainly because of the spatial nonhomogeneities of the devices. Direct-control operation is based on directly setting the bistable device reflectivity with a variable-control beam power. This working mode turned out to be much more tolerant of spatial noises: 37 channels of the system could be operated simultaneously. Further development of the system to a crossbar of N inputs and M outputs and system miniaturization are also considered.     

Thin films of porphyrin molecules imaged with the atomic force microscope

The assembly of two-dimensional molecular structures of zinc porphyrin molecules arising from the dewetting of a porphyrin solution on mica and graphite is investigated using atomic force microscopy. Both a near equilibrium nucleation and growth process, and a far from equilibrium spinodal dewetting process are observed. Nucleation and growth around pre-existing surface defects on mica produces single layer disks, ∼ 10 μm in diameter, of densely packed molecules. Spinodal dewetting gives rise to the formation of much smaller, single layer, molecular islands of various sizes on both mica and graphite.     

Memristive switching mechanism for metal/oxide/metal nanodevices

Nanoscale metal/oxide/metal switches have the potential to transform the market for nonvolatile memory and could lead to novel forms of computing. However, progress has been delayed by difficulties in understanding and controlling the coupled electronic and ionic phenomena that dominate the behaviour of nanoscale oxide devices. An analytic theory of the 'memristor' (memory-resistor) was first developed from fundamental symmetry arguments in 1971, and we recently showed that memristor behaviour can naturally explain such coupled electron-ion dynamics. Here we provide experimental evidence to support this general model of memristive electrical switching in oxide systems. We have built micro- and nanoscale TiO2 junction devices with platinum electrodes that exhibit fast bipolar nonvolatile switching. We demonstrate that switching involves changes to the electronic barrier at the Pt/TiO2 interface due to the drift of positively charged oxygen vacancies under an applied electric field. Vacancy drift towards the interface creates conducting channels that shunt, or short-circuit, the electronic barrier to switch ON. The drift of vacancies away from the interface annilihilates such channels, recovering the electronic barrier to switch OFF. Using this model we have built TiO2 crosspoints with engineered oxygen vacancy profiles that predictively control the switching polarity and conductance.     

Nano-architectures by covalent assembly of molecular building blocks

The construction of electronic devices from single molecular building blocks, which possess certain functions such as switching or rectifying and are connected by atomic-scale wires on a supporting surface, is an essential goal of molecular electronics. A key challenge is the controlled assembly of molecules into desired architectures by strong, that is, covalent, intermolecular connections, enabling efficient electron transport between the molecules and providing high stability. However, no molecular networks on surfaces 'locked' by covalent interactions have been reported so far. Here, we show that such covalently bound molecular nanostructures can be formed on a gold surface upon thermal activation of porphyrin building blocks and their subsequent chemical reaction at predefined connection points. We demonstrate that the topology of these nanostructures can be precisely engineered by controlling the chemical structure of the building blocks. Our results represent a versatile route for future bottom-up construction of sophisticated electronic circuits and devices, based on individual functionalized molecules.