Control of molecule-based transport for future molecular devices

Location-Selective Work Function Engineering by Self-Assembled Monolayers

Control over specific interfaces in devices represents a key challenge for modern organic electronics and photovoltaics. Such control is frequently gained by the use of self-assembled monolayers (SAMs), which, by selection of a proper anchoring group, are generally discriminative with respect to different materials but are not selective between different areas of the same material. In particular, selective tailoring of the work function may be useful for different functional devices in a circuit. Here we demonstrate an approach for solving this problem, opening a way to function-selective electrostatic engineering of chemically identical areas, such as source and drain electrodes in a specific type of organic transistor and, more importantly, the electrodes in different types of organic devices, such as p- and n-channel transistors, located on the same circuitry board. The approach is based on the ultraviolet-light-promoted exchange reaction of SAMs on gold, a standard electrode material in organic electronics.

Not So Innocent After All: Interfacial Chemistry Determines Charge-Transport Efficiency in Single-Molecule Junctions

Most studies in molecular electronics focus on altering the molecular wire backbone to tune the electrical properties of the whole junction. However, it is often overlooked that the chemical structure of the groups anchoring the molecule to the metallic electrodes influences the electronic structure of the whole system and, therefore, its conductance. We synthesised electron-accepting dithienophosphole oxide derivatives and fabricated their single-molecule junctions. We found that the anchor group has a dramatic effect on charge-transport efficiency: in our case, electron-deficient 4-pyridyl contacts suppress conductance, while electron-rich 4-thioanisole termini promote efficient transport. Our calculations show that this is due to minute changes in charge distribution, probed at the electrode interface. Our findings provide a framework for efficient molecular junction design, especially valuable for compounds with strong electron withdrawing/donating backbones.

Effects of Electrode Materials on Electron Transport for Single-Molecule Junctions

The contact at the molecule-electrode interface is a key component for a range of molecule-based devices involving electron transport. An electrode-molecule-electrode configuration is a prototypical testbed for quantitatively studying the underlying physical chemistry. Rather than the molecular side of the interface, this review focuses on examples of electrode materials in the literature. The basic concepts and relevant experimental techniques are introduced.

High-Strain-Induced Local Modification of the Electronic Properties of VO2 Thin Films

Vanadium dioxide (VO₂) is a popular candidate for electronic and optical switching applications due to its well-known semiconductor-metal transition. Its study is notoriously challenging due to the interplay of long- and short-range elastic distortions, as well as the symmetry change and the electronic structure changes. The inherent coupling of lattice and electronic degrees of freedom opens the avenue toward mechanical actuation of single domains. In this work, we show that we can manipulate and monitor the reversible semiconductor-to-metal transition of VO₂ while applying a controlled amount of mechanical pressure by a nanosized metallic probe using an atomic force microscope. At a critical pressure, we can reversibly actuate the phase transition with a large modulation of the conductivity. Direct tunneling through the VO₂-metal contact is observed as the main charge carrier injection mechanism before and after the phase transition of VO₂. The tunneling barrier is formed by a very thin but persistently insulating surface layer of the VO₂. The necessary pressure to induce the transition decreases with temperature. In addition, we measured the phase coexistence line in a hitherto unexplored regime. Our study provides valuable information on pressure-induced electronic modifications of the VO₂ properties, as well as on nanoscale metal-oxide contacts, which can help in the future design of oxide electronics.

Charge Transport Characteristics of Molecular Electronic Junctions Studied by Transition Voltage Spectroscopy

The field of molecular electronics is prompted by tremendous opportunities for using a single-molecule and molecular monolayers as active components in integrated circuits. Until now, a wide range of molecular devices exhibiting characteristic functions, such as diodes, transistors, switches, and memory, have been demonstrated. However, a full understanding of the crucial factors that affect charge transport through molecular electronic junctions should yet be accomplished. Remarkably, recent advances in transition voltage spectroscopy (TVS) elucidate that it can provide key quantities for probing the transport characteristics of the junctions, including, for example, the position of the frontier molecular orbital energy relative to the electrode Fermi level and the strength of the molecule–electrode interactions. These parameters are known to be highly associated with charge transport behaviors in molecular systems and can then be used in the design of molecule-based devices with rationally tuned electronic properties. This article highlights the fundamental principle of TVS and then demonstrates its major applications to study the charge transport properties of molecular electronic junctions.

Quantum Plasmonics: Energy Transport Through Plasmonic Gap

At the interfaces of metal and dielectric materials, strong light-matter interactions excite surface plasmons; this allows electromagnetic field confinement and enhancement on the sub-wavelength scale. Such phenomena have attracted considerable interest in the field of exotic material-based nanophotonic research, with potential applications including nonlinear spectroscopies, information processing, single-molecule sensing, organic-molecule devices, and plasmon chemistry. These innovative plasmonics-based technologies can meet the ever-increasing demands for speed and capacity in nanoscale devices, offering ultrasensitive detection capabilities and low-power operations. Size scaling from the nanometer to sub-nanometer ranges is consistently researched; as a result, the quantum behavior of localized surface plasmons, as well as those of matter, nonlocality, and quantum electron tunneling is investigated using an innovative nanofabrication and chemical functionalization approach, thereby opening a new era of quantum plasmonics. This new field enables the ultimate miniaturization of photonic components and provides extreme limits on light-matter interactions, permitting energy transport across the extremely small plasmonic gap. In this review, a comprehensive overview of the recent developments of quantum plasmonic resonators with particular focus on novel materials is presented. By exploring the novel gap materials in quantum regime, the potential quantum technology applications are also searched for and mapped out.

Tuning surface d bands with bimetallic electrodes to facilitate electron transport across molecular junctions

Understanding chemical bonding and conductivity at the electrode-molecule interface is key for the operation of single-molecule junctions. Here we apply the d-band theory that describes interfacial interactions between adsorbates and transition metal surfaces to study electron transport across these devices. We realized bimetallic Au electrodes modified with a monoatomic Ag adlayer to connect α,ω-alkanoic acids (HO₂C(CH₂)_nCO₂H). The force required to break the molecule-electrode binding and the contact conductance G_n=0 are 1.1 nN and 0.29 G₀ (the conductance quantum, 1 G₀ = 2e²/h ≈ 77.5 μS), which makes these junctions, respectively, 1.3-1.8 times stronger and 40-60-fold more conductive than junctions with bare Au or Ag electrodes. A similar performance was found for Au electrodes modified by Cu monolayers. By integrating the Newns-Anderson model with the Hammer-Nørskov d-band model, we explain how the surface d bands strengthen the adsorption and promote interfacial electron transport, which provides an alternative avenue for the optimization of molecular electronic devices.

Side-group-mediated thermoelectric properties of anthracene single-molecule junction with anchoring groups

Charge transfer characteristics of single-molecule junctions at the nanoscale, and consequently, their thermoelectric properties can be dramatically tuned by chemical or conformational modification of side groups or anchoring groups. In this study, we used density functional theory (DFT) combined with the non-equilibrium Green's function (NEGF) formalism in the linear response regime to examine the thermoelectric properties of a side-group-mediated anthracene molecule coupled to gold (Au) electrodes via anchoring groups. In order to provide a comparative inspection three different side groups, i.e. amine, nitro and methyl, in two different positions were considered for the functionalization of the molecule terminated with thiol or isocyanide anchoring groups. We showed that when the anchored molecule is perturbed with side group, the peaks of the transmission spectrum were shifted relative to the Fermi energy in comparison to the unperturbed molecule (i.e. without side group) leading to modified thermoelectric properties of the system. Particularly, in the thiol-terminated molecule the amine side group showed the greatest figure of merit in both positions which was suppressed by the change of side group position. However, in the isocyanide-terminated molecule the methyl side group attained the greatest thermoelectric efficiency where its magnitude was relatively robust to the change of side group position. In this way, different combinations of side groups and anchoring groups can improve or suppress thermopower and the figure of merit of the molecular junction depending on the interplay between charge donating/accepting nature of the functionals or their position.

Femtosecond Charge Transfer Dynamics in Monomolecular Films in the Context of Molecular Electronics

A key issue of molecular electronics (ME) is the correlation between the molecular structure and the charge transport properties of the molecular framework. Accordingly, a variety of model and potentially useful molecular systems are designed, to prove a particular function or correlation or to build a prototype device. These studies usually involve the measurements of the static electric conductance properties of individual molecules and their assembles on solid supports. At the same time, information about the dynamics of the charge transport (CT) and transfer in such systems, complementary in the context of ME and of a scientific value on its own, is quite scarce. Among other means, this drawback can be resolved by resonant Auger electron spectroscopy (RAES) in combination with core hole clock (CHC) approach, as described in this Account. The RAES-CHC scheme was applied to a variety of aliphatic and aromatic self-assembled monolayers (SAMs), adsorbed on Au(111) over the thiolate and selenolate docking groups. Electron transfer (ET) from a suitable terminal tail group to the substrate, across the molecular framework, was monitored, triggered by resonant excitation of this group (nitrile in most cases) by narrow-band X-ray radiation. This resulted in the quantitative data for the characteristic ET time, τ_ET, in the femtosecond domain, with the time window ranging from ∼1 fs to ∼120 fs. The derived τ_ET exhibit an exponential dependence on the molecular length, mimicking the behavior of the static conductance and suggesting a common physical basis behind the static CT and ET dynamics. The dynamic decay factors, β_ET, for the alkyl, oligophenyl, and acene molecular "wires" correlate well with the analogous parameters for the static CT. Both τ_ET and β_ET values exhibit a distinct dependence on the character of the involved molecular orbital (MO), demonstrating that the efficiency and rate of the CT in molecular assemblies can be controlled by resonant injection of the charge carriers into specific MOs. This dependence as well as a lack of correlation between the molecular tilt and τ_ET represent strong arguments in favor of the generally accepted model of CT across the molecular framework ("through-bond") in contrast to "through-space" tunneling. Comparison of the SAMs with thiolate and selenolate docking groups suggests that the use of selenolate instead of thiolate does not give any gain in terms of ET dynamics or molecular conductance. Whereas a certain difference in the efficiency of the electronic coupling of thiolate and selenolate to the substrate cannot be completely excluded, this difference is certainly too small to affect the performance of the entire molecule to a noticeable extent. The efficient electronic coupling of the thiolate docking group to the substrate was verified and the decoupling of the electronic subsystems of the substrate and π-conjugated segment by introduction of methylene group into the backbone was demonstrated. No correlation between the molecular dipole or fluorine substitution pattern (at the side positions) and the ET efficiency was recorded. Several representative examples for the resonantly addressable tail groups are given, and perspectives for future research in the context of ET dynamics in molecular assemblies are discussed.

Electron Transfer Dynamics and Structural Effects in Benzonitrile Monolayers with Tuned Dipole Moments by Differently Positioned Fluorine Atoms

To understand the influence of the molecular dipole moment on the electron transfer (ET) dynamics across the molecular framework, two series of differently fluorinated, benzonitrile-based self-assembled monolayers (SAMs) bound to Au(111) by either thiolate or selenolate anchoring groups were investigated. Within each series, the molecular structures were the same with the exception of the positions of two fluorine atoms affecting the dipole moment of the SAM-forming molecules. The SAMs exhibited a homogeneous anchoring to the substrate, nearly upright molecular orientations, and the outer interface comprised of the terminal nitrile groups. The ET dynamics was studied by resonant Auger electron spectroscopy in the framework of the core-hole clock method. Resonance excitation of the nitrile group unequivocally ensured an ET pathway from the tail group to the substrate. As only one of the π* orbitals of this group is hybridized with the π* system of the adjacent phenyl ring, two different ET times could be determined depending on the primary excited orbital being either localized at the nitrile group or delocalized over the entire benzonitrile moiety. The latter pathway turned out to be much more efficient, with the characteristic ET times being a factor 2.5-3 shorter than those for the localized orbital. The dynamic ET properties of the analogous thiolate- and selenolate-based adsorbates were found to be nearly identical. Finally and most importantly, these properties were found to be unaffected by the different patterns of the fluorine substitution used in the present study, thus showing no influence of the molecular dipole moment.

Electrostatic Gate Control in Molecular Transistors

Molecular transistors, in which single molecules serve as active channel components in a three-terminal device geometry, constitute the building blocks of molecular scale electronic circuits. To demonstrate such devices, a gate electrode has been incorporated in several test beds of molecular electronics. The frontier orbitals' alignments of a molecular transistor can be delicately tuned by modifying the molecular orbital energy with the gate electrode. In this review, we described electrostatic gate control of solid-state molecular transistors. In particular, we focus on recent experimental accomplishments in fabrication and characterization of molecular transistors.

Direct single-molecule dynamic detection of chemical reactions

Single-molecule detection can reveal time trajectories and reaction pathways of individual intermediates/transition states in chemical reactions and biological processes, which is of fundamental importance to elucidate their intrinsic mechanisms. We present a reliable, label-free single-molecule approach that allows us to directly explore the dynamic process of basic chemical reactions at the single-event level by using stable graphene-molecule single-molecule junctions. These junctions are constructed by covalently connecting a single molecule with a 9-fluorenone center to nanogapped graphene electrodes. For the first time, real-time single-molecule electrical measurements unambiguously show reproducible large-amplitude two-level fluctuations that are highly dependent on solvent environments in a nucleophilic addition reaction of hydroxylamine to a carbonyl group. Both theoretical simulations and ensemble experiments prove that this observation originates from the reversible transition between the reactant and a new intermediate state within a time scale of a few microseconds. These investigations open up a new route that is able to be immediately applied to probe fast single-molecule physics or biophysics with high time resolution, making an important contribution to broad fields beyond reaction chemistry.

Controlled switching of single-molecule junctions by mechanical motion of a phenyl ring

Mechanical methods for single-molecule control have potential for wide application in nanodevices and machines. Here we demonstrate the operation of a single-molecule switch made functional by the motion of a phenyl ring, analogous to the lever in a conventional toggle switch. The switch can be actuated by dual triggers, either by a voltage pulse or by displacement of the electrode, and electronic manipulation of the ring by chemical substitution enables rational control of the on-state conductance. Owing to its simple mechanics, structural robustness, and chemical accessibility, we propose that phenyl rings are promising components in mechanical molecular devices.

Electrical characterization of single molecule and Langmuir-Blodgett monomolecular films of a pyridine-terminated oligo(phenylene-ethynylene) derivative

Monolayer Langmuir-Blodgett (LB) films of 1,4-bis(pyridin-4-ylethynyl)benzene (1) together with the "STM touch-to-contact" method have been used to study the nature of metal-monolayer-metal junctions in which the pyridyl group provides the contact at both molecule-surface interfaces. Surface pressure vs area per molecule isotherms and Brewster angle microscopy images indicate that 1 forms true monolayers at the air-water interface. LB films of 1 were fabricated by deposition of the Langmuir films onto solid supports resulting in monolayers with surface coverage of 0.98 × 10(-9) mol·cm(-2). The morphology of the LB films that incorporate compound 1 was studied using atomic force microscopy (AFM). AFM images indicate the formation of homogeneous, monomolecular films at a surface pressure of transference of 16 mN·m(-1). The UV-vis spectra of the Langmuir and LB films reveal that 1 forms two dimensional J-aggregates. Scanning tunneling microscopy (STM), in particular the "STM touch-to-contact" method, was used to determine the electrical properties of LB films of 1. From these STM studies symmetrical I-V curves were obtained. A junction conductance of 5.17 × 10(-5) G 0 results from the analysis of the pseudolinear (ohmic) region of the I-V curves. This value is higher than that of the conductance values of LB films of phenylene-ethynylene derivatives contacted by amines, thiols, carboxylate, trimethylsilylethynyl or acetylide groups. In addition, the single molecule I-V curve of 1 determined using the I(s) method is in good agreement with the I-V curve obtained for the LB film, and both curves fit well with the Simmons model. Together, these results not only indicate that the mechanism of transport through these metal-molecule-metal junctions is non-resonant tunneling, but that lateral interactions between molecules within the LB film do not strongly influence the molecule conductance. The results presented here complement earlier studies of single molecule conductance of 1 using STM-BJ methods, and support the growing evidence that the pyridyl group is an efficient and effective anchoring group in sandwiched metal-monolayer-metal junctions prepared under a number of different conditions.

Electrical properties and mechanical stability of anchoring groups for single-molecule electronics

We report on an experimental investigation of transport through single molecules, trapped between two gold nano-electrodes fabricated with the mechanically controlled break junction (MCBJ) technique. The four molecules studied share the same core structure, namely oligo(phenylene ethynylene) (OPE3), while having different aurophilic anchoring groups: thiol (SAc), methyl sulfide (SMe), pyridyl (Py) and amine (NH2). The focus of this paper is on the combined characterization of the electrical and mechanical properties determined by the anchoring groups. From conductance histograms we find that thiol anchored molecules provide the highest conductance; a single-level model fit to current-voltage characteristics suggests that SAc groups exhibit a higher electronic coupling to the electrodes, together with better level alignment than the other three groups. An analysis of the mechanical stability, recording the lifetime in a self-breaking method, shows that Py and SAc yield the most stable junctions while SMe form short-lived junctions. Density functional theory combined with non-equlibrium Green's function calculations help in elucidating the experimental findings.

Polydiacetylene stabilized gold nanoparticles – extraordinary high stability and integration into a nanoelectrode device

A new diacetylene containing photopolymerizable ligand molecule was developed, and tailored for applications in nanoelectronic devices based on gold nanoparticles.

A gate controlled molecular switch based on picene-F₄TCNQ charge-transfer material

We show that the recently synthesized charge-transfer material picene-F₄TCNQ can be used as a gate-voltage controlled molecular switch. The picene-F₄TCNQ system is compared with the extensively characterized anthraquinone-based molecular system, which is known to exhibit large switching ratios due to quantum interference effects. In the case of picene-F₄TCNQ we find switching ratios larger by one order of magnitude. Further, our calculations reveal that the picene-F₄TCNQ system resembles remarkably well the I-V characteristics of a classical diode. The reverse-bias current of this molecular diode can be increased two orders of magnitude by an external gate voltage. Based on density-functional theory calculations we show that the hybrid states formed by the picene-F₄TCNQ system play the key role in determining transport properties. We further conclude that the tuning of quantum transport properties through hybrid states is a general concept which opens a new route towards functional materials for molecular electronics.

Molecule-electrode interfaces in molecular electronic devices

Understanding charge transport of single molecules or a small collection of molecules sandwiched between electrodes is of fundamental importance for molecular electronics. This requires the fabrication of reliable devices, which depend on several factors including the testbed architectures used, the molecule number and defect density being tested, and the nature of the molecule-electrode interface. On the basis of significant progresses achieved in both experiments and theory over the past decade, in this tutorial review, we focus on new insights into the influence of the nature of the molecule-electrode interface, the most critical issue hindering the development of reliable devices, on the conducting properties of molecules. We summarize the strategies developed for controlling the interfacial properties and how the coupling strength between the molecules and the electrodes modulates the device properties. These analyses should be valuable for deeply understanding the relationship between the contact interface and the charge transport mechanism, which is of crucial importance for the development of molecular electronics, organic electronics, nanoelectronics, and other interface-related optoelectronic devices.

Manipulating transport through a single-molecule junction

Molecular Electronics deals with the realization of elementary electronic devices that rely on a single molecule. For electronic applications, the most important property of a single molecule is its conductance. Here we show how the conductance of a single octanethiol molecule can be measured and manipulated by varying the contact's interspace. This mechanical gating of the single molecule junction leads to a variation of the conductance that can be understood in terms of a tunable image charge effect. The image charge effect increases with a decrease of the contact's interspace due to a reduction of the effective potential barrier height of 1.5 meV/pm.

Electronic transport through zigzag/armchair graphene nanoribbon heterojunctions

The electronic transport properties of a graphene nanoribbon (GNR) are known to be sensitive to its width, edges and defects. We investigate the electronic transport properties of a graphene nanoribbon heterojunction constructed by fusing a zigzag and an armchair graphene nanoribbon (zGNR/aGNR) side by side. First principles results reveal that the heterojunction can be either metallic or semiconducting, depending on the width of the nanoribbons. Intrinsic rectification behaviors have been observed, which are largely sensitive to the connection length between the zGNR and aGNR. The microscopic origins of the rectification behavior have been revealed. We find that the carrier type can alter from electrons to holes with the bias voltage changing from negative to positive; the asymmetrical transmission spectra of electrons and holes induced by the interface defects directly results in the rectification behavior. The results suggest that any methods which can enhance the asymmetry of the transmission spectra between holes and electrons could be used to improve the rectification behavior in the zGNR/aGNR heterojunction. Our findings could be useful for designing graphene based electronic devices.

Controlled transport through a single molecule

We demonstrate how an electrode-molecule-electrode junction can be controllably opened and closed by careful tuning of the contacts' interspace and voltage. The molecule, an octanethiol, flips to bridge a ~1 nm interspace between substrate and scanning tunnelling microscope tip when an electric field exceeds a threshold (switch 'on'). Reducing the field below this threshold value leads to the reproducible detachment of the octanethiol (switch 'off'). Once contacted, a further reduction of the contacts' interspace leads to an increase of the conductance of the molecule.

Stepwise preparation and characterization of molecular wires made of zinc octaethylporphyrin complexes bridged by 4,4'-bipyridine on HOPG

Molecular-scale devices can be made using a step-by-step procedure, in a controllable and highly versatile way. In this report, we describe the growth of molecular wires (MW) from zinc (II) octaethylporphyrin (ZnOEP) assembled on highly oriented pyrolytic graphite (HOPG) by a step-by-step approach using 4,4'-bipyridine (BP) to bridge the porphyrin units, via coordination of the nitrogen atom to zinc. In order to gain an insight into the molecular self-organization of these wires, we carried out a detailed scanning tunnelling microscopy (STM) analysis of each monolayer, using a solid/liquid interface technique, up to a complete ZnOEP/BP/ZnOEP/BP/ZnOEP-assembled structure. The electrical properties of the MWs were assessed by scanning tunnelling spectroscopy (STS) and by current-sensing atomic force microscopy (CS-AFM), showing an increase of electrical resistance with the length of the MW.