How important is the interfacial chemical bond for electron transport through alkyl chain monolayers?

Molecular Diodes Induced by a Schottky Barrier with a Gold-Silicon Doped Electrode

To create complementary metal oxide semiconductor compatible molecular devices, more insights into the electrode property regarding its metal/semiconductor doping level and creating a functional molecular device are required. In this work, we constructed an EGaIn/alkanethiol/Au-Si molecular diode (with a rectification ratio R of 50.70) induced by Schottky barriers within a gold-silicon doped electrode instead of the functional property of molecules. The relationship between the rectification ratio and the number of methylene units in alkanethiol was analyzed, revealing a gradual increase in the ratio from 3.33 for C6H14S to 50.70 for C16H34S. The rectification ratio of the junction is well modulated by the temperature due to the change in the Schottky barrier. Such a mechanism is explained by the energy band diagrams of the surface space charge region and a combination of density functional theory and Keldysh-Green formalism calculations.

Multichromic Monolayer Terpyridine-Based Electrochromic Materials

The article describes novel electrochromic materials (ECMs) that are based on a monolayer consisting of two or three isostructural metal complexes of 4'-(pyridin-4-yl)-2,2':6',2''-terpyridine simultaneously deposited on surface-enhanced support. The support was made by screen printing of indium tin oxide (ITO) nanoparticles on ITO-glass and has a surface area sufficient for a monolayer to give color visible to the naked eye. The ability to separately electrochemically address the oxidation state of the metal centers on the surface (i.e., Co²⁺/Co³⁺, Os²⁺/Os³⁺, and Fe²⁺/Fe³⁺) provides an opportunity to achieve several distinct color-to-color transitions, thus opening the door for constructing monolayer-based multicolor ECMs.

Charge Transport through Self-Assembled Monolayers of Monoterpenoids

The nature of the processes at the origin of life that selected specific classes of molecules for broad incorporation into cells is controversial. Among those classes selected were polyisoprenoids and their derivatives. This paper tests the hypothesis that polyisoprenoids were early contributors to membranes in part because they (or their derivatives) could facilitate charge transport by quantum tunneling. It measures charge transport across self-assembled monolayers (SAMs) of carboxyl-terminated monoterpenoids (O₂ C(C₉ HX)) and alkanoates (O₂ C(C₇ HX)) with different degrees of unsaturation, supported on silver (Ag^TS ) bottom electrodes, with Ga₂ O₃ /EGaIn top electrodes. Measurements of current density of SAMs of linear length-matched hydrocarbons-both saturated and unsaturated-show that completely unsaturated molecules transport charge faster than those that are completely saturated by approximately a factor of ten. This increase in relative rates of charge transport correlates with the number of carbon-carbon double bonds, but not with the extent of conjugation. These results suggest that polyisoprenoids-even fully unsaturated-are not sufficiently good tunneling conductors for their conductivity to have favored them as building blocks in the prebiotic world.

Harnessing electrostatic catalysis in single molecule, electrochemical and chemical systems: a rapidly growing experimental tool box

Static electricity is central to many day-to-day practical technologies, from separation methods in the recycling of plastics to transfer inks in photocopying, but the exploration of how electrostatics affects chemical bonding is still in its infancy. As shown in the Companion Tutorial, the presence of an appropriately-oriented electric field can enhance the resonance stabilization of transition states by lowering the energy of ionic contributors, and the effect that follows on reaction barriers can be dramatic. However, the electrostatic effects are strongly directional and harnessing them in practical experiments has proven elusive until recently. This tutorial outlines some of the experimental platforms through which we have sought to translate abstract theoretical concepts of electrostatic catalysis into practical chemical technologies. We move step-wise from the nano to the macro, using recent examples drawn from single-molecule STM experiments, surface chemistry and pH-switches in solution chemistry. The experiments discussed in the tutorial will educate the reader in some of the viable solutions to gain control of the orientation of reagents in that field; from pH-switchable bond-dissociations using charged functional groups to the use of surface chemistry and surface-probe techniques. All of these recent works provide proof-of-concept of electrostatic catalysis for specific sets of chemical reactions. They overturn the long-held assumption that static electricity can only affect rates and equilibrium position of redox reactions, but most importantly, they provide glimpses of the wide-ranging potential of external electric fields for controlling chemical reactivity and selectivity.

Reproducible flaws unveil electrostatic aspects of semiconductor electrochemistry

Predicting or manipulating charge-transfer at semiconductor interfaces, from molecular electronics to energy conversion, relies on knowledge generated from a kinetic analysis of the electrode process, as provided by cyclic voltammetry. Scientists and engineers encountering non-ideal shapes and positions in voltammograms are inclined to reject these as flaws. Here we show that non-idealities of redox probes confined at silicon electrodes, namely full width at half maximum <90.6 mV and anti-thermodynamic inverted peak positions, can be reproduced and are not flawed data. These are the manifestation of electrostatic interactions between dynamic molecular charges and the semiconductor’s space-charge barrier. We highlight the interplay between dynamic charges and semiconductor by developing a model to decouple effects on barrier from changes to activities of surface-bound molecules. These findings have immediate general implications for a correct kinetic analysis of charge-transfer at semiconductors as well as aiding the study of electrostatics on chemical reactivity.

Most electrical devices must pass charges across semiconductor interfaces, yet redox-active molecular behavior obscures comprehension of these processes. Here, the authors develop a model to describe redox processes on semiconductor surfaces and gauge these interactions electrochemically.

Molecular Monolayers for Electrical Passivation and Functionalization of Silicon-Based Solar Energy Devices

Silicon-based solar fuel devices require passivation for optimal performance yet at the same time need functionalization with (photo)catalysts for efficient solar fuel production. Here, we use molecular monolayers to enable electrical passivation and simultaneous functionalization of silicon-based solar cells. Organic monolayers were coupled to silicon surfaces by hydrosilylation in order to avoid an insulating silicon oxide layer at the surface. Monolayers of 1-tetradecyne were shown to passivate silicon micropillar-based solar cells with radial junctions, by which the efficiency increased from 8.7% to 9.9% for n⁺/p junctions and from 7.8% to 8.8% for p⁺/n junctions. This electrical passivation of the surface, most likely by removal of dangling bonds, is reflected in a higher shunt resistance in the J-V measurements. Monolayers of 1,8-nonadiyne were still reactive for click chemistry with a model catalyst, thus enabling simultaneous passivation and future catalyst coupling.

Disorder-derived, strong tunneling attenuation in bis-phosphonate monolayers

Monolayers of alkyl bisphosphonic acids (bisPAs) of various carbon chain lengths (C4, C8, C10, C12) were grown on aluminum oxide (AlO(x)) surfaces from solution. The structural and electrical properties of these self-assembled monolayers (SAMs) were compared with those of alkyl monophosphonic acids (monoPAs). Through contact angle (CA) and Kelvin-probe (KP) measurements, ellipsometry, and infrared (IR) and x-ray photoelectron (XPS) spectroscopies, it was found that bisPAs form monolayers that are relatively disordered compared to their monoPA analogs. Current-voltage (J-V) measurements made with a hanging Hg drop top contact show tunneling to be the prevailing transport mechanism. However, while the monoPAs have an observed decay constant within the typical range for dense monolayers, β(mono)  =  0.85  ±  0.03 per carbon atom, a surprisingly high value, β(bis) =  1.40  ±  0.05 per carbon atom, was measured for the bisPAs. We attribute this to a strong contribution of 'through-space' tunneling, which derives from conformational disorder in the monolayer due to strong interactions of the distal phosphonic acid groups; they likely form a hydrogen-bonding network that largely determines the molecular layer structure. Since bisPA SAMs attenuate tunnel currents more effectively than do the corresponding monoPA SAMs, they may find future application as gate dielectric modification in organic thin film devices.

Temperature dependent charge transport across tunnel junctions of single-molecules and self-assembled monolayers: a comparative study

In this work we present a comparative study of the temperature behavior of charge current in both single-molecule transistors and self-assembled monolayer-based tunnel junctions with ferrocene molecules.

Probing the nature and resistance of the molecule-electrode contact in SAM-based junctions

It is challenging to quantify the contact resistance and to determine the nature of the molecule-electrode contacts in molecular two-terminal junctions. Here we show that potentiodynamic and temperature dependent impedance measurements give insights into the nature of the SAM-electrode interface and other bottlenecks of charge transport (the capacitance of the SAM (C(SAM)) and the resistance of the SAM (R(SAM))), unlike DC methods, independently of each other. We found that the resistance of the top-electrode-SAM contact for junctions with the form of Ag(TS)-SC(n)//GaO(x)/EGaIn with n = 10, 12, 14, 16 or 18 is bias and temperature independent and hence Ohmic (non-rectifying) in nature, and is orders of magnitude smaller than R(SAM). The C(SAM) and R(SAM) are independent of the temperature, indicating that the mechanism of charge transport in these SAM-based junctions is coherent tunneling and the charge carrier trapping at the interfaces is negligible.

Characterizing the metal-SAM interface in tunneling junctions

This paper investigates the influence of the interface between a gold or silver metal electrode and an n-alkyl SAM (supported on that electrode) on the rate of charge transport across junctions with structure Met(Au or Ag)(TS)/A(CH2)nH//Ga2O3/EGaIn by comparing measurements of current density, J(V), for Met/AR = Au/thiolate (Au/SR), Ag/thiolate (Ag/SR), Ag/carboxylate (Ag/O2CR), and Au/acetylene (Au/C≡CR), where R is an n-alkyl group. Values of J0 and β (from the Simmons equation) were indistinguishable for these four interfaces. Since the anchoring groups, A, have large differences in their physical and electronic properties, the observation that they are indistinguishable in their influence on the injection current, J0 (V = 0.5) indicates that these four Met/A interfaces do not contribute to the shape of the tunneling barrier in a way that influences J(V).

Molecular interfaces for plasmonic hot electron photovoltaics

The use of self-assembled monolayers (SAMs) to improve and tailor the photovoltaic performance of plasmonic hot-electron Schottky solar cells is presented. SAMs allow the simultaneous control of open-circuit voltage, hot-electron injection and short-circuit current. To that end, a plurality of molecule structural parameters can be adjusted: SAM molecule's length can be adjusted to control plasmonic hot electron injection. Modifying SAMs dipole moment allows for a precise tuning of the open-circuit voltage. The functionalization of the SAM can also be selected to modify short-circuit current. This allows the simultaneous achievement of high open-circuit voltages (0.56 V) and fill-factors (0.58), IPCE above 5% at the plasmon resonance and maximum power-conversion efficiencies of 0.11%, record for this class of devices.

Introducing ionic and/or hydrogen bonds into the SAM//Ga2O3 top-interface of Ag(TS)/S(CH2)nT//Ga2O3/EGaIn junctions

Junctions with the structure Ag(TS)/S(CH2)nT//Ga2O3/EGaIn (where S(CH2)nT is a self-assembled monolayer, SAM, of n-alkanethiolate bearing a terminal functional group T) make it possible to examine the response of rates of charge transport by tunneling to changes in the strength of the interaction between T and Ga2O3. Introducing a series of Lewis acidic/basic functional groups (T = -OH, -SH, -CO2H, -CONH2, and -PO3H) at the terminus of the SAM gave values for the tunneling current density, J(V) in A/cm(2), that were indistinguishable (i.e., differed by less than a factor of 3) from the values observed with n-alkanethiolates of equivalent length. The insensitivity of the rate of tunneling to changes in the terminal functional group implies that replacing weak van der Waals contact interactions with stronger hydrogen- or ionic bonds at the T//Ga2O3 interface does not change the shape (i.e., the height or width) of the tunneling barrier enough to affect rates of charge transport. A comparison of the injection current, J0, for T = -CO2H, and T = -CH2CH3--two groups having similar extended lengths (in Å, or in numbers of non-hydrogen atoms)--suggests that both groups make indistinguishable contributions to the height of the tunneling barrier.

Light-induced organic monolayer modification of iodinated carbon electrodes

We report the modification of carbon electrodes formed from pyrolyzed photoresist films (PPF) via plasma iodination followed by the organic monolayer modification of these surfaces. The iodinated surfaces were characterized using cyclic voltammetry, atomic force microscopy, and X-ray photoelectron spectroscopy to enable the optimization of the iodination while preserving the stability and smoothness of the carbon surface. Subsequently, the C-I surface was further modified with molecules that possess an alkene or alkyne at one end through light activation with low energy (visible range λ 514 nm). The versatility of the modification reaction of the C-I surfaces is shown by reactions with undecylenic acid, 1,8-nonadiyne, and S-undec-10-enyl-2,2,2-trifluoroethanethioate (C11-S-TFA). Modification with 1,8-nonadiyne allows further modification via "click" chemistry with azido-terminated oligo(ethylene oxide) molecules demonstrated briefly to alter the hydrophilicity of the surface after attachment of ethylene oxide moieties. Furthermore, patterning of C11-S-TFA was demonstrated using a simple photolithography technique. Deprotection of the C11-S-TFA gave a free thiol allowed patterning of gold nanoparticles on the surface as verified using scanning electron microscopy (SEM). These results demonstrate that plasma iodination to form C-I is a versatile, simple, and modular approach to functionalize the carbon surface.

Effect of Molecule-Surface Reaction Mechanism on the Electronic Characteristics and Photovoltaic Performance of Molecularly Modified Si

We report on the passivation properties of molecularly modified, oxide-free Si(111) surfaces. The reaction of 1-alcohol with the H-passivated Si(111) surface can follow two possible paths, nucleophilic substitution (S_N) and radical chain reaction (RCR), depending on adsorption conditions. Moderate heating leads to the S_N reaction, whereas with UV irradiation RCR dominates, with S_N as a secondary path. We show that the site-sensitive S_N reaction leads to better electrical passivation, as indicated by smaller surface band bending and a longer lifetime of minority carriers. However, the surface-insensitive RCR reaction leads to more dense monolayers and, therefore, to much better chemical stability, with lasting protection of the Si surface against oxidation. Thus, our study reveals an inherent dissonance between electrical and chemical passivation. Alkoxy monolayers, formed under UV irradiation, benefit, though, from both chemical and electronic passivation because under these conditions both S_N and RCR occur. This is reflected in longer minority carrier lifetimes, lower reverse currents in the dark, and improved photovoltaic performance, over what is obtained if only one of the mechanisms operates. These results show how chemical kinetics and reaction paths impact electronic properties at the device level. It further suggests an approach for effective passivation of other semiconductors.

Wet chemical functionalization of III-V semiconductor surfaces: alkylation of gallium arsenide and gallium nitride by a Grignard reaction sequence

Crystalline gallium arsenide (GaAs) (111)A and gallium nitride (GaN) (0001) surfaces have been functionalized with alkyl groups via a sequential wet chemical chlorine activation, Grignard reaction process. For GaAs(111)A, etching in HCl in diethyl ether effected both oxide removal and surface-bound Cl. X-ray photoelectron (XP) spectra demonstrated selective surface chlorination after exposure to 2 M HCl in diethyl ether for freshly etched GaAs(111)A but not GaAs(111)B surfaces. GaN(0001) surfaces exposed to PCl(5) in chlorobenzene showed reproducible XP spectroscopic evidence for Cl-termination. The Cl-activated GaAs(111)A and GaN(0001) surfaces were both reactive toward alkyl Grignard reagents, with pronounced decreases in detectable Cl signal as measured by XP spectroscopy. Sessile contact angle measurements between water and GaAs(111)A interfaces after various levels of treatment showed that GaAs(111)A surfaces became significantly more hydrophobic following reaction with C(n)H(2n-1)MgCl (n = 1, 2, 4, 8, 14, 18). High-resolution As 3d XP spectra taken at various times during prolonged direct exposure to ambient lab air indicated that the resistance of GaAs(111)A to surface oxidation was greatly enhanced after reaction with Grignard reagents. GaAs(111)A surfaces terminated with C(18)H(37) groups were also used in Schottky heterojunctions with Hg. These heterojunctions exhibited better stability over repeated cycling than heterojunctions based on GaAs(111)A modified with C(18)H(37)S groups. Raman spectra were separately collected that suggested electronic passivation by surficial Ga-C bonds at GaAs(111)A. Specifically, GaAs(111)A surfaces reacted with alkyl Grignard reagents exhibited Raman signatures comparable to those of samples treated with 10% Na(2)S in tert-butanol. For GaN(0001), high-resolution C 1s spectra exhibited the characteristic low binding energy shoulder demonstrative of surface Ga-C bonds following reaction with CH(3)MgCl. In addition, 4-fluorophenyl groups were attached and detected after reaction with C(6)H(4)FMgBr, further confirming the susceptibility of Cl-terminated GaN(0001) to surface alkylation. However, the measured hydrophobicities of alkyl-terminated GaAs(111)A and GaN(0001) were markedly distinct, indicating differences in the resultant surface layers. The results presented here, in conjunction with previous studies on GaP, show that atop Ga atoms at these crystallographically related surfaces can be deliberately functionalized and protected through Ga-C surface bonds that do not involve thiol/sulfide chemistry or gas-phase pretreatments.

Electrochemical impedance study of GaAs surface charge modulation through the deprotonation of carboxylic acid monolayers

Modifications to the space charge region of p+ and p-GaAs due to surface charge modulation by the pH-induced deprotonation of bound carboxylic acid terminal monolayers were studied by electrochemical impedance spectroscopy and correlated to flat-band potential measurements from Mott-Schottky plots. We infer that the negative surface dipole formed on GaAs due to monolayer deprotonation causes an enhancement of the downward interfacial band bending. The space charge layer modifications were correlated to intermolecular electrostatic interactions and semiconductor depletion characteristics.

Control of molecule-based transport for future molecular devices

In this review, possibilities to modify intentionally the electronic transport properties of metal/molecule/metal devices (MMM devices) are discussed. Here especially the influence of the metal work function, the metal-molecule interface, the molecule dipole and different tunneling mechanisms are considered. A route to evaluate the effective surface work function of metal-molecule systems is given and, based on experimental results, an exemplary estimation is performed. The electron transport across different metal-molecule interfaces is characterized by relating transmission coefficients extracted from experimentally derived molecular conductances, decay constants or tunneling barrier heights. Based on the reported results the tunneling decay constant can be assumed to be suitable to characterize intrinsic molecular electron transport properties, while the nature of the metal-molecule contacts is properly described by the transmission coefficient. A clear gradation of transmission efficiencies of metal-anchoring group combinations can be given.

Charge transport and rectification in arrays of SAM-based tunneling junctions

This paper describes a method of fabrication that generates small arrays of tunneling junctions based on self-assembled monolayers (SAMs); these junctions have liquid-metal top-electrodes stabilized in microchannels and ultraflat (template-stripped) bottom-electrodes. The yield of junctions generated using this method is high (70-90%). The junctions examined incorporated SAMs of alkanethiolates having ferrocene termini (11-(ferrocenyl)-1-undecanethiol, SC(11)Fc); these junctions rectify currents with large rectification ratios (R), the majority of which fall within the range of 90-180. These values are larger than expected (theory predicts R <or= 20) and are larger than previous experimental measurements. SAMs of n-alkanethiolates without the Fc groups (SC(n-1)CH(3), with n = 12, 14, 16, or 18) do not rectify (R ranged from 1.0 to 5.0). These arrays enable the measurement of the electrical characteristics of the junctions as a function of chemical structure, voltage, and temperature over the range of 110-293 K, with statistically large numbers of data (N = 300-800). The mechanism of rectification with Fc-terminated SAMs seems to be charge transport processes that change with the polarity of bias: from tunneling (at one bias) to hopping combined with tunneling (at the opposite bias).

Infrared spectroscopy of the organic monolayer sandwiched between a Hg electrode and a Si substrate

We have successfully observed the vibrational spectra of organic monolayers sandwiched between a liquid Hg electrode and a Si substrate by means of a newly developed reflection absorption (RA) device. The vibrational spectra of organic monolayers between two electrodes can be observed under a certain bias voltage. The monolayers were fabricated by the reaction of hydrogen-terminated Si(111) with 1-octadecene. A metal/insulator/semiconductor structure was prepared using liquid Hg as a metal electrode and the organic monolayer as an insulator. Infrared (IR) light entered from the Si substrate side with an incident angle of 75 degrees. The reflected IR light from the metallic Hg was detected by a mercury-cadmium-telluride detector. We obtained RA spectra using a bare H-Si(111) substrate as a reference. The absorbance of the RA spectrum was comparable with that of the transmission spectrum for the octadecyl-terminated Si(111) without Hg. The C-H stretching modes in the CH(2) group show blueshifts, and the C-H antisymmetric stretching modes in the CH(3) are broadened in comparison with the transmission spectrum. Under a certain bias voltage, we observed changes in band shape. We concluded that the variation was due to the temperature increase by resistive heating of the substrate.

Dipole tuning of charge transport in molecular junctions

Molecules with different magnitude and direction of dipole moments are sandwiched between mercury (Hg) and p(+)-Si to form Hg-molecules-(p(+)) Si junctions. The importance of the dipole moment of molecules in controlling the symmetry of current-voltage curves is shown by carrying out charge transport measurements at these molecular junctions. Junction parameters are obtained by considering charge transport across the junction as a combination of tunneling and Schottky emission. An interesting effect due to dipole reversal resulting in rectification of junction curves in the opposite quadrants of the current-voltage axis is observed and analyzed. The junction curves also exhibit a different degree of rectification with increasing magnitude of the dipole moment of the sandwiched molecules.

Molecules on si: electronics with chemistry

Basic scientific interest in using a semiconducting electrode in molecule-based electronics arises from the rich electrostatic landscape presented by semiconductor interfaces. Technological interest rests on the promise that combining existing semiconductor (primarily Si) electronics with (mostly organic) molecules will result in a whole that is larger than the sum of its parts. Such a hybrid approach appears presently particularly relevant for sensors and photovoltaics. Semiconductors, especially Si, present an important experimental test-bed for assessing electronic transport behavior of molecules, because they allow varying the critical interface energetics without, to a first approximation, altering the interfacial chemistry. To investigate semiconductor-molecule electronics we need reproducible, high-yield preparations of samples that allow reliable and reproducible data collection. Only in that way can we explore how the molecule/electrode interfaces affect or even dictate charge transport, which may then provide a basis for models with predictive power.To consider these issues and questions we will, in this Progress Report, review junctions based on direct bonding of molecules to oxide-free Si.describe the possible charge transport mechanisms across such interfaces and evaluate in how far they can be quantified.investigate to what extent imperfections in the monolayer are important for transport across the monolayer.revisit the concept of energy levels in such hybrid systems.

Electrochemical behavior of gold colloidal alkyl modified silicon surfaces

Herein, we report on the production of nanoelectrode arrays by attaching colloidal gold on silicon-bound mixed self-assembled monolayers of TFA-protected alkenylthiol (C(11)-S-TFA) and undecylenic acid (acid). Effective modification of the surface, tethering of the nanoparticles, and the direct influence of the deprotected alkenylthiol (C(11)-SH) /acid ratio on the number of adherent particles were demonstrated using X-ray photoelectron spectroscopy, electrochemistry, and atomic force microscopy. Cyclic voltammetry showed that the enhancement of electron transfer to the silicon surface by the presence of nanoparticles is influenced by the number of tethered nanoparticles.

Silicon (100) electrodes resistant to oxidation in aqueous solutions: an unexpected benefit of surface acetylene moieties

Here we report on the functionalization of alkyne-terminated alkyl monolayers on highly doped Si(100) using "click" reactions to immobilize ferrocene derivatives. The reaction of hydrogen-terminated silicon surfaces with a diyne species was shown to afford very robust functional surfaces where the oxidation of the underlying substrate was negligible. Detailed characterization using X-ray photoelectron spectroscopy, X-ray reflectometry, and cyclic voltammetry demonstrated that the surface acetylenes had reacted in moderate yield to give surfaces exposing ferrocene moieties. Upon extensive exposure of the redox-active architecture to oxidative environments during preparative and characterization steps, no evidence of SiOx contaminants was shown for derivatized SAMs prepared from single-component 1,8-nonadiyne, fully acetylenylated, monolayers. An analysis of the redox behavior of the prepared Si(100) electrodes based on relevant parameters such as peak splitting and position and shape of the reduction/oxidation waves depicted a well-behaved redox architecture whose spectroscopic and electrochemical properties were not significantly altered even after prolonged cycling in aqueous media between -100 and 800 mV versus Ag|AgCl. The reported strategy represents an experimentally simple approach for the preparation of silicon-based electrodes where, in addition to close-to-ideal redox behavior, remarkable electrode stability can be achieved. Both the presence of a distal alkyne moiety and temperatures of formation above 100 degrees C were required to achieve this surface stabilization.

Contacting organic molecules by soft methods: towards molecule-based electronic devices

Can we put organic molecules to use as electronic components? The answer to this question is to no small degree limited by the ability to contact them electrically without damaging the molecules. In this Account, we present some of the methods for contacting molecules that do not or minimally damage them and that allow formation of electronic junctions that can become compatible with electronics from the submicrometer to the macroscale. In "Linnaean" fashion, we have grouped contacting methods according to the following main criteria: (a) is a chemical bond is required between contact and molecule, and (b) is the contact "ready-made", that is, preformed, or prepared in situ? Contacting methods that, so far, seem to require a chemical bond include spin-coating a conductive polymer and transfer printing. In the latter, a metallic pattern on an elastomeric polymer is mechanically transferred to molecules with an exposed terminal group that can react chemically with the metal. These methods allow one to define structures from several tens of nanometers size upwards and to fabricate devices on flexible substrates, which is very difficult by conventional techniques. However, the requirement for bifunctionality severely restricts the type of molecules that can be used and can complicate their self-assembly into monolayers. Methods that rely on prior formation of the contact pad are represented by two approaches: (a) use of a liquid metal as electrode (e.g., Hg, Ga, various alloys), where molecules can be adsorbed on the liquid metal and the molecularly modified drop is brought into contact with the second electrode, the molecules can be adsorbed on the second electrode and then the liquid metal brought into contact with them, or bilayers are used, with a layer on both the metal and the second electrode and (b) use of preformed metal pads from a solid substrate and subsequent pad deposition on the molecules with the help of a liquid. These methods allow formation of contacts easily and rapidly and allow many types of monolayers and metals to be analyzed. However, in their present forms such approaches are not technologically practical. Direct in situ vacuum evaporation of metals has been used successfully only with bifunctional molecules because it is too invasive and damaging, in general. A more general approach is indirect vacuum evaporation, where the metal atoms and clusters, emitted from the source, reach the sample surface in an indirect line of sight, while cooled by multiple collisions with an inert gas. This method has clear technological possibilities, but more research is needed to increase deposition efficiency and find ways to characterize the molecules at the interface and to prevent metal penetration between molecules or through pinholes, also if molecules lack reactive termination groups. This Account stresses the advantages, weak points, and possible routes for the development of contacting methods. This way it shows that there is at present no one ideal soft contacting method, whether it is because of limitations and problems inherent in each of the methods or because of insufficient understanding of the interfacial chemistry and physics. Hopefully, this Account will present the latter issue as a research challenge to its readers.

Probing molecules in integrated silicon-molecule-metal junctions by inelastic tunneling spectroscopy

Molecular electronics has drawn significant attention for nanoelectronic and sensing applications. A hybrid technology where molecular devices are integrated with traditional semiconductor microelectronics is a particularly promising approach for these applications. Key challenges in this area include developing devices in which the molecular integrity is preserved, developing in situ characterization techniques to probe the molecules within the completed devices, and determining the physical processes that influence carrier transport. In this study, we present the first experimental report of inelastic electron tunneling spectroscopy of integrated metal-molecule-silicon devices with molecules assembled directly to silicon contacts. The results provide direct experimental confirmation that the chemical integrity of the monolayer is preserved and that the molecules play a direct role in electronic conduction through the devices. Spectra obtained under varying measurement conditions show differences related to the silicon electrode, which can provide valuable information about the physics influencing carrier transport in these molecule/Si hybrid devices.

Functionalization of acetylene-terminated monolayers on Si(100) surfaces: a click chemistry approach

In this article, we report the functionalization of alkyne-terminated alkyl monolayers on Si(100) using "click" chemistry, specifically, the Cu(I)-catalyzed Huisgen 1,3-dipolar cycloaddition reaction of azides with surface-bound alkynes. Covalently immobilized, structurally well-defined acetylene-terminated organic monolayers were prepared from a commercially available terminal diyne species using a one-step hydrosilylation procedure. Subsequent derivatization of the alkyne-terminated monolayers in aqueous environments with representative azide species via a selective, reliable, robust cycloaddition process afforded disubstituted surface-bound [1,2,3]-triazole species. Neither activation procedures nor protection/deprotection steps were required, as is the case with more established grafting approaches for silicon surfaces. Detailed characterization using X-ray photoelectron spectroscopy and X-ray reflectometry demonstrated that the surface acetylenes had reacted in moderate to high yield to give surfaces exposing alkyl chains, oligoether anti-fouling moieties, and functionalized aromatic structures. These results demonstrate that click immobilization offers a versatile, experimentally simple, chemically unambiguous modular approach to producing modified silicon surfaces with organic functionality for applications as diverse as biosensors and molecular electronics.

Thiol-terminated monolayers on oxide-free Si: assembly of semiconductor-alkyl-S-metal junctions

Self-assembled monolayers formed by thermal hydrosilylation of a trifluoroacetyl-protected alkenylthiol on Si-H surfaces, followed by removal of the protecting groups, yield essentially oxide-free monolayers suitable for the formation of Si-C11H22-S-Hg and Si-C11H22-S-Au junctions in which the alkyl chains are chemically bound to the silicon surface (via Si-C bonds) and the metal electrode (via Hg-S or Au-S bonds). Two barriers to charge transport are present in the system: at low bias the current is temperature activated and hence limited by thermionic emission over the Schottky barrier in the silicon, whereas as at high bias transport is limited by tunneling through the organic monolayer. The thiol-terminated monolayer on oxide-free silicon provides a well-characterized system allowing a careful study of the importance of the interfacial bond to the metal electrode for current transport through saturated molecules.