Exponential Attenuation of Through-Bond Transmission in a Polyene: Theory and Potential Realizations

An exponential falloff with separation of electron transfer and transport through molecular wires is observed and has attracted theoretical attention. In this study, the attenuation of transmission in linear and cyclic polyenes is related to bond alternation. The explicit form of the zeroth Green's function in a Hückel model for bond-alternated polyenes leads to an analytical expression of the conductance decay factor β. The β values calculated from our model (β(CN) values, per repeat unit of double and single bond) range from 0.28 to 0.37, based on carotenoid crystal structures. These theoretical β values are slightly smaller than experimental values. The difference can be assigned to the effect of anchoring groups, which are not included in our model. A local transmission analysis for cyclic polyenes, and for [14]annulene in particular, shows that bond alternation affects dramatically not only the falloff behavior but also the choice of a transmission pathway by electrons. Transmission follows a well-demarcated system of π bonds, even when there is a shorter-distance path with roughly the same kind of "electronic matter" intervening.

Oligoyne Molecular Junctions for Efficient Room Temperature Thermoelectric Power Generation

Understanding phonon transport at a molecular scale is fundamental to the development of high-performance thermoelectric materials for the conversion of waste heat into electricity. We have studied phonon and electron transport in alkane and oligoyne chains of various lengths and find that, due to the more rigid nature of the latter, the phonon thermal conductances of oligoynes are counterintuitively lower than that of the corresponding alkanes. The thermal conductance of oligoynes decreases monotonically with increasing length, whereas the thermal conductance of alkanes initially increases with length and then decreases. This difference in behavior arises from phonon filtering by the gold electrodes and disappears when higher-Debye-frequency electrodes are used. Consequently a molecule that better transmits higher-frequency phonon modes, combined with a low-Debye-frequency electrode that filters high-energy phonons is a viable strategy for suppressing phonon transmission through the molecular junctions. The low thermal conductance of oligoynes, combined with their higher thermopower and higher electrical conductance lead to a maximum thermoelectric figure of merit of ZT = 1.4, which is several orders of magnitude higher than that of alkanes.

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.

Gold nanoparticles assembled with dithiocarbamate-anchored molecular wires

A protocol for the bottom-up self-assembly of nanogaps is developed through molecular linking of gold nanoparticles (AuNPs). Two π-conjugated oligo(phenylene ethynylene) molecules (OPE) with dithiocarbamate anchoring groups are used as ligands for the AuNPs. OPE-4S with a dithiocarbamate in each end of the molecule and a reference molecule OPE-2S with only a single dithiocarbamate end group. The linking mechanism of OPE-4S is investigated by using a combination of TEM, UV-Vis absorption and surface enhanced Raman spectroscopy (SERS) as well as studying the effect of varying the OPE-4S to AuNP concentration ratio. UV-Vis absorption confirms the formation of AuNP aggregates by the appearance of an extended plasmon band (EPB) for which the red shift and intensity depend on the OPE-4S:AuNP ratio. SERS confirms the presence of OPE-4S and shows a gradual increase of the signal intensity with increasing OPE-4S:AuNP ratios up to a ratio of about 4000, after which the SERS intensity does not increase significantly. For OPE-2S, no linking is observed below full coverage of the AuNPs indicating that the observed aggregate formation at high OPE-2S:AuNP ratios, above full AuNP coverage, is most likely of a physical nature (van der Waals forces or π-π interactions).

A theoretical guide for fabricating a conductive molecular wire on a silicon surface via an in situ surface polymerization reaction

It has been a long-standing goal to make conductive molecular wires or linear polymer chains on traditional semiconductors or insulator substrates to satisfy the ongoing miniaturization in electronic devices. Here, we have proposed a surface in situ polymerization reaction for a pre-absorbed molecule, 4-hydrazinyl-3-(pyridin-4-ylmethyl)-benzaldehyde (HPyMB), to produce a conductive molecular wire on a silicon surface. Our first-principles calculations show that HPyMB molecules can absorb alternatively on the exposed Si atoms created via ultrahigh vacuum scanning tunneling microscopy on a hydrogen passivated H-Si(001)2 × 1 surface along the [110] direction. The adsorption is exothermic and its generated energy is sufficient for the following intermolecular dehydration polymerization reaction to overcome the activation energy barriers and thereafter form a molecular wire on the surface. This polymerized molecular wire is mechanically stable since it is chemically bonded onto the surface. After polymerization, the system becomes conductive due to the charge transfer from the molecule-surface bonds to their pyridine rings. More importantly, by removing 1.1 electrons from the system, the surface polymer chain is the sole conductive channel. Furthermore, its conducting nature remains robust even under a large external electric field. Our findings open a new window for the fabrication of conductive molecular wires or polymer chains on semiconductor surfaces, and provide insights into the mechanism behind the molecular wire conductivity.

The spontaneous formation of single-molecule junctions via terminal alkynes

Herein, we report the spontaneous formation of single-molecule junctions via terminal alkyne contact groups. Self-assembled monolayers that form spontaneously from diluted solutions of 1, 4-diethynylbenzene (DEB) were used to build single-molecule contacts and assessed using the scanning tunneling microscopy-break junction technique (STM-BJ). The STM-BJ technique in both its dynamic and static approaches was used to characterize the lifetime (stability) and the conductivity of a single-DEB wire. It is demonstrated that single-molecule junctions form spontaneously with terminal alkynes and require no electrochemical control or chemical deprotonation. The alkyne anchoring group was compared against typical contact groups exploited in single-molecule studies, i.e. amine (benzenediamine) and thiol (benzendithiol) contact groups. The alkyne contact showed a conductance magnitude comparable to that observed with amine and thiol groups. The lifetime of the junctions formed from alkynes were only slightly less than that of thiols and greater than that observed for amines. These findings are important as (a) they extend the repertoire of chemical contacts used in single-molecule measurements to 1-alkynes, which are synthetically accessible and stable and (b) alkynes have a remarkable affinity toward silicon surfaces, hence opening the door for the study of single-molecule transport on a semiconducting electronic platform.

Nano-fabrication of molecular electronic junctions by targeted modification of metal-molecule bonds

Reproducibility, stability and the coupling between electrical and molecular properties are central challenges in the field of molecular electronics. The field not only needs devices that fulfill these criteria but they also need to be up-scalable to application size. In this work, few-molecule based electronics devices with reproducible electrical characteristics are demonstrated. Our previously reported 5 nm gold nanoparticles (AuNP) coated with ω-triphenylmethyl (trityl) protected 1,8-octanedithiol molecules are trapped in between sub-20 nm gap spacing gold nanoelectrodes forming AuNP-molecule network. When the trityl groups are removed, reproducible devices and stable Au-thiol junctions are established on both ends of the alkane segment. The resistance of more than 50 devices is reduced by orders of magnitude as well as a reduction of the spread in the resistance histogram is observed. By density functional theory calculations the orders of magnitude decrease in resistance can be explained and supported by TEM observations thus indicating that the resistance changes and strongly improved resistance spread are related to the establishment of reproducible and stable metal-molecule bonds. The same experimental sequence is carried out using 1,6-hexanedithiol functionalized AuNPs. The average resistances as a function of molecular length, demonstrated herein, are comparable to the one found in single molecule devices.

Probe-based measurement of lateral single-electron transfer between individual molecules

The field of molecular electronics aims at using single molecules as functional building blocks for electronics components, such as switches, rectifiers or transistors. A key challenge is to perform measurements with atomistic control over the alignment of the molecule and its contacting electrodes. Here we use atomic force microscopy to examine charge transfer between weakly coupled pentacene molecules on insulating films with single-electron sensitivity and control over the atomistic details. We show that, in addition to the imaging capability, the probe tip can be used to control the charge state of individual molecules and to detect charge transfers to/from the tip, as well as between individual molecules. Our approach represents a novel route for molecular charge transfer studies with a host of opportunities, especially in combination with single atom/molecule manipulation and nanopatterning techniques.

Carbon Electrode-Molecule Junctions: A Reliable Platform for Molecular Electronics

The development of reliable approaches to integrate individual or a small collection of molecules into electrical nanocircuits, often termed "molecular electronics", is currently a research focus because it can not only overcome the increasing difficulties and fundamental limitations of miniaturization of current silicon-based electronic devices, but can also enable us to probe and understand the intrinsic properties of materials at the atomic- and/or molecular-length scale. This development might also lead to direct observation of novel effects and fundamental discovery of physical phenomena that are not accessible by traditional materials or approaches. Therefore, researchers from a variety of backgrounds have been devoting great effort to this objective, which has started to move beyond simple descriptions of charge transport and branch out in different directions, reflecting the interdisciplinarity. This Account exemplifies our ongoing interest and great effort in developing efficient lithographic methodologies capable of creating molecular electronic devices through the combination of top-down micro/nanofabrication with bottom-up molecular assembly. These devices use nanogapped carbon nanomaterials (such as single-walled carbon nanotubes (SWCNTs) and graphene), with a particular focus on graphene, as point contacts formed by electron beam lithography and precise oxygen plasma etching. Through robust amide linkages, functional molecular bridges terminated with diamine moieties are covalently wired into the carboxylic acid-functionalized nanogaps to form stable carbon electrode-molecule junctions with desired functionalities. At the macroscopic level, to improve the contact interface between electrodes and organic semiconductors and lower Schottky barriers, we used SWCNTs and graphene as efficient electrodes to explore the intrinsic properties of organic thin films, and then build functional high-performance organic nanotransistors with ultrahigh responsivities. At the molecular level, to form robust covalent bonds between electrodes and molecules and improve device stability, we developed a reliable system to immobilize individual molecules within a nanoscale gap of either SWCNTs or graphene through covalent amide bond formation, thus affording two classes of carbon electrode-molecule single-molecule junctions. One unique feature of these devices is the fact that they contain only one or two molecules as conductive elements, thus forming the basis for building new classes of chemo/biosensors with ultrahigh sensitivity. We have used these approaches to reveal the dependence of the charge transport of individual metallo-DNA duplexes on π-stacking integrity, and fabricate molecular devices capable of realizing label-free, real-time electrical detection of biological interactions at the single-event level, or switching their molecular conductance upon exposure to external stimuli, such as ion, pH, and light. These investigations highlight the unique advantages and importance of these universal methodologies to produce functional carbon electrode-molecule junctions in current and future researches toward the development of practical molecular devices, thus offering a reliable platform for molecular electronics and the promise of a new generation of multifunctional integrated circuits and sensors.

Platinum(II)-Based Metallo-Supramolecular Polymer with Controlled Unidirectional Dipoles for Tunable Rectification

A platinum(II)-based, luminescent, metallo-supramolecular polymer (PolyPtL1) having an inherent dipole moment was synthesized via complexation of Pt(II) ions with an asymmetric ligand L1, containing terpyridyl and pyridyl moieties. The synthesized ligand and polymer were well characterized by various NMR techniques, optical spectroscopy, and cyclic voltammetry studies. The morphological study by atomic force microscopy revealed the individual and assembled polymer chains of 1-4 nm height. The polymer was specifically attached on Au-electrodes to produce two types of film (films 1 and 2) in which the polymer chains were aligned with their dipoles in opposite directions. The Au-surface bounded films were characterized by UV-vis, Raman spectroscopy, cyclic voltammetry, and atomic force microscopy study. The quantum mechanical calculation determined the average dipole moment for each monomer unit in PolyPtL1 to be about 5.8 D. The precise surface derivatization permitted effective tuning of the direction dipole moment, as well as the direction of rectification of the resulting polymer-attached molecular diodes. Film 1 was more conductive in positive bias region with an average rectification ratio (RR = I(+4 V)/I(-4 V)) ≈ 20, whereas film 2 was more conducting in negative bias with an average rectification ratio (RR = I(-4 V)/I(+4 V)) ≈ 18.

Molecular computing: paths to chemical Turing machines

In this perspective, we highlight some of the recent advances in the development of molecular and biomolecular systems for performing logic operations and computing. We also present a blueprint of a chemical Turing machine using a processive catalytic approach.

To comply with the rapidly increasing demand of information storage and processing, new strategies for computing are needed. The idea of molecular computing, where basic computations occur through molecular, supramolecular, or biomolecular approaches, rather than electronically, has long captivated researchers. The prospects of using molecules and (bio)macromolecules for computing is not without precedent. Nature is replete with examples where the handling and storing of data occurs with high efficiencies, low energy costs, and high-density information encoding. The design and assembly of computers that function according to the universal approaches of computing, such as those in a Turing machine, might be realized in a chemical way in the future; this is both fascinating and extremely challenging. In this perspective, we highlight molecular and (bio)macromolecular systems that have been designed and synthesized so far with the objective of using them for computing purposes. We also present a blueprint of a molecular Turing machine, which is based on a catalytic device that glides along a polymer tape and, while moving, prints binary information on this tape in the form of oxygen atoms.

Cooperative Gold Nanoparticle Stabilization by Acetylenic Phosphaalkenes

Acetylenic phosphaalkenes (APAs) are used as a novel type of ligands for the stabilization of gold nanoparticles (AuNP). As demonstrated by a variety of experimental and analytical methods, both structural features of the APA, that is, the P=C as well as the C≡C units are essential for NP stabilization. The presence of intact APAs on the AuNP is demonstrated by surface-enhanced Raman spectroscopy (SERS), and first principle calculations indicate that bonding occurs most likely at defect sites on the Au surface. AuNP-bound APAs are in chemical equilibrium with free APAs in solution, leading to a dynamic behavior that can be explored for facile place-exchange reactions with other types of anchor groups such as thiols or more weakly binding phosphine ligands.

Conical Gradient Junctions of Dendritic Viologen Arrays on Electrodes

The three-dimensional construction of arrays of functional molecules on an electrode surface, such as organic semiconductors and redox-active molecules, is a considerable challenge in the fabrication of sophisticated junctions for molecular devices. In particular, well-defined organic layers with precise molecular gradients are anticipated to function as novel metal/organic interfaces with specific electrical properties, such as a space charge layer at the metal/semiconductor interface. Here, we report a strategy for the construction of a three-dimensional molecular array with an electrical connection to a metal electrode by exploiting dendritic molecular architecture. Newly designed dendritic molecules consisting of viologens (1,1′-disubstituted-4,4′-bipyridilium salts) as the framework and mercapto groups as anchor units form unique self-assembled monolayers (SAMs) on a gold surface reflecting the molecular design. The dendritic molecules exhibit a conical shape and closely pack to form cone arrays on the substrate, whereas, in solution, they expand into more flexible conformations. Differences in the introduction position of the anchor units in the dendritic structure result in apical- and basal-type cone arrays in which the spatial concentration of the viologen units can be precisely configured in the cones. The concentration in apical-type SAMs increases away from the substrate, whereas the opposite is true in basal-type SAMs.

Single grain boundary break junction for suspended nanogap electrodes with gapwidth down to 1-2 nm by focused ion beam milling

Single grain boundary junctions are used for the fabrication of suspended nanogap electrodes with a gapwidth down to 1-2 nm through the break of such junctions by focused ion beam (FIB) milling. With advantages of stability and no debris, such nanogap electrodes are suitable for single molecular electronic device construction.

Light-Induced Switching of Tunable Single-Molecule Junctions

A major goal of molecular electronics is the development and implementation of devices such as single-molecular switches. Here, measurements are presented that show the controlled in situ switching of diarylethene molecules from their nonconductive to conductive state in contact to gold nanoelectrodes via controlled light irradiation. Both the conductance and the quantum yield for switching of these molecules are within a range making the molecules suitable for actual devices. The conductance of the molecular junctions in the opened and closed states is characterized and the molecular level E0, which dominates the current transport in the closed state, and its level broadening Γ are identified. The obtained results show a clear light-induced ring forming isomerization of the single-molecule junctions. Electron withdrawing side-groups lead to a reduction of conductance, but do not influence the efficiency of the switching mechanism. Quantum chemical calculations of the light-induced switching processes correlate these observations with the fundamentally different low-lying electronic states of the opened and closed forms and their comparably small modification by electron-withdrawing substituents. This full characterization of a molecular switch operated in a molecular junction is an important step toward the development of real molecular electronics devices.

Fine-tuning of single-molecule conductance by tweaking both electronic structure and conformation of side substituents

Herein, we describe a method to fine-tune the conductivity of single-molecule wires by employing a combination of chemical composition and geometrical modifications of multiple phenyl side groups as conductance modulators embedded along the main axis of the electronic pathway. We have measured the single-molecule conductivity of a novel series of phenyl-substituted carotenoid wires whose conductivity can be tuned with high precision over an order of magnitude range by modulating both the electron-donating character of the phenyl substituent and its dihedral angle. It is demonstrated that the electronic communication between the phenyl side groups and the molecular wire is maximized when the phenyl groups are twisted closer to the plane of the conjugated molecular wire. These findings can be refined to a general technique for precisely tuning the conductivity of molecular wires.

Graphene-DNAzyme Junctions: A Platform for Direct Metal Ion Detection with Ultrahigh Sensitivity

Many metal ions are present in biology and in the human body in trace amounts. Despite numerous efforts, metal sensors with ultrahigh sensitivity (< a few picomolar) are rarely achieved. Here, we describe a platform method that integrates a Cu2+-dependent DNAzyme into graphene-molecule junctions and its application for direct detection of paramagnetic Cu2+ with femtomolar sensitivity and high selectivity. Since DNAzymes specific for other metal ions can be obtained through in vitro selection, the method demonstrated here can be applied to the detection of a broad range of other metal ions.

Correlation of breaking forces, conductances and geometries of molecular junctions

Electrical and mechanical properties of elongated gold-molecule-gold junctions formed by tolane-type molecules with different anchoring groups (pyridyl, thiol, amine, nitrile and dihydrobenzothiophene) were studied in current-sensing force spectroscopy experiments and density functional simulations. Correlations between forces, conductances and junction geometries demonstrate that aromatic tolanes bind between electrodes as single molecules or as weakly-conductive dimers held by mechanically-weak π - π stacking. In contrast with the other anchors that form only S-Au or N-Au bonds, the pyridyl ring also forms a highly-conductive cofacial link to the gold surface. Binding of multiple molecules creates junctions with higher conductances and mechanical strengths than the single-molecule ones.

Synthesis, characterization and solid-state photoluminescence studies of six alkoxy phenylene ethynylene dinuclear palladium(II) rods

A rare family of six discrete binuclear [PdCl(PEt3)2] phenylene ethynylene rods with alkoxy side chains (methoxy, ethoxy and heptoxy) have been developed, and their solid-state photoluminescence results have been presented and discussed. The shorter bridging ligands are of the general formula H-C≡C-C6H2(R)2-C≡C-H, where R = H, OCH3, OC2H5, and OC7H15, whereas the longer ones are based on H-C≡C-C6H4-C≡C-C6H2(R)2-C≡C-C6H4-C≡C-H, where R = OCH3, OC2H5. These ligands display increasing length in both the main dimension (backbone length) as well as the number of carbons in the side chains (R, alkoxide side chain) that stem from the central phenylene moiety. The X-ray crystal structures of two of the prepared complexes are reported: one corresponds to a shorter rod, 1,4-bis[trans-(PEt3)2ClPd-C≡C]-2,5-diethoxybenzene (6c), while the second one is associated with a longer rod, the binuclear complex 1,4-bis[trans-(PEt3)2ClPd-4-(-C≡C-C6H4-C≡C)]-2,5-diethoxybenzene (7c). All new compounds were characterized by NMR spectroscopy ((1)H, (13)C{(1)H} and (31)P{(1)H}) as well as ESI-MS(TOF), EA, FTIR, UV-Vis, cyclic voltammetry and solid-state photoluminescence. Our work shows the influence of the alkoxy side chains on the electronic structure of the family of binuclear Pd rods by lowering its oxidation potential. In addition to this, the increase of the length of the bridge results in a higher oxidation potential. Solid state photoluminescence results indicate that Pd complexes are characterized by a marked decrease in both the emission intensity and the fluorescence lifetime values as compared to their ligands. This behaviour could be due to some degree of ligand-to-metal charge transfer.

Electroburning of few-layer graphene flakes, epitaxial graphene, and turbostratic graphene discs in air and under vacuum

Graphene-based electrodes are very promising for molecular electronics and spintronics. Here we report a systematic characterization of the electroburning (EB) process, leading to the formation of nanometer-spaced gaps, on different types of few-layer graphene (namely mechanically exfoliated graphene on SiO2, graphene epitaxially grown on the C-face of SiC and turbostratic graphene discs deposited on SiO2) under air and vacuum conditions. The EB process is found to depend on both the graphene type and on the ambient conditions. For the mechanically exfoliated graphene, performing EB under vacuum leads to a higher yield of nanometer-gap formation than working in air. Conversely, for graphene on SiC the EB process is not successful under vacuum. Finally, the EB is possible with turbostratic graphene discs only after the creation of a constriction in the sample using lithographic patterning.

Revealing the direct effect of individual intercalations on DNA conductance toward single-molecule electrical biodetection

The effect of individual intercalations on DNA conductance is revealed electrically at the single-molecule level by using DNA-functionalized molecular junctions.

Towards single molecule switches

Scanning tunneling microscope (STM) controlled reversible switching of a single-dipole molecule imbedded in hydrogen-bonded binary molecular networks on graphite.

Transition-metal-free controlled polymerization for poly(p-aryleneethynylene)s

A catalytic amount of fluoride anions promoted the polymerization of 1-pentafluorophenyl-4-[(trimethylsilyl)ethynyl]benzene, providing a high-molecular-weight polymer in a chain-growth-like manner.

A transition-metal-free controlled polymerization for the attainment of poly(p-aryleneethynylene)s is developed. The polymerization of 1-pentafluorophenyl-4-[(trimethylsilyl)ethynyl]benzene with a catalytic amount of fluoride anions proceeds in a chain-growth-like manner to afford polymers with controlled molecular weights and low polydispersity indexes. The mechanism involves a pentacoordinated fluorosilicate as a key intermediate. The anionic “living” nature of this process is applied to block copolymerization and also surface-terminated polymerization.

Multi-responsive photo- and chemo-electrical single-molecule switches

Incorporating molecular switches as the active components in nanoscale electrical devices represents a current challenge in molecular electronics. It demands key requirements that need to be simultaneously addressed including fast responses to external stimuli and stable attachment of the molecules to the electrodes while mimicking the operation of conventional electronic components. Here, we report a single-molecule switching device that responds electrically to optical and chemical stimuli. A light pointer or a chemical signal can rapidly and reversibly induce the isomerization of bifunctional spiropyran derivatives in the bulk reservoir and, consequently, switch the electrical conductivity of the single-molecule device between a low and a high level. The spiropyran derivatives employed are chemically functionalized such that they can respond in fast but practical time scales. The unique multistimuli response and the synthetic versatility to control the switching schemes of this single-molecule device suggest spiropyran derivatives as key candidates for molecular circuitry.

Breaking Force and Conductance of Gold Nanojunctions: Effect of Humidity

Forces acting on elongated gold nanojunctions and their electric conductance were simultaneously measured by current-sensing force spectroscopy in an atmosphere with controlled humidity. The breaking force of "thick" nanojunctions with conductance >20G0 is not affected by the environmental humidity. The presence of ambient water stabilizes "thin" nanojunctions with conductance <15G0, whose breaking force of 10-15 nN was higher than that in a dry atmosphere due to the capillary forces. The observed effect of humidity would not be possible to distinguish by techniques measuring only forces or only conductance in nanojunctions.