Contact resistance in metal-molecule-metal junctions based on aliphatic SAMs: effects of surface linker and metal work function

Using conducting probe atomic force microscopy (CP-AFM), we have formed molecular tunnel junctions consisting of alkanethiols and alkane isonitrile self-assembled monolayers sandwiched between gold, platinum, silver, and palladium contacts. We have measured the resistance of these junctions at low bias (dV/dI |V=0) as a function of alkane chain length. Extrapolation to zero chain length gives the contact resistance, R0 . R0 is strongly dependent on the type of metal used for the contacts and decreases with increasing metal work function; that is, R0,Ag > R0,Au > R0,Pd > R0,Pt. R0 is approximately 10% smaller for Au junctions with isonitrile versus thiol surface linkers. We conclude that the Fermi level of the junction lies much closer to the HOMO than to the LUMO.     

Formation of alkanethiol and alkanedithiol monolayers on GaAs(001)

The formation of alkanethiol (H-(CH2)n-SH, n = 8-18) and 1,8-octanedithiol (HS-(CH2)8-SH) monolayer films on n-type GaAs(001) has been systematically studied. We observed a nonlinear dependence of the film thickness on molecular length, which is drastically different from monolayer films of the same molecules on metals. For 8 < or = n < or = 14, the films are only 3-4.5 A thick, significantly smaller than the corresponding molecular length. For n = 16 and 18, the measured film thicknesses were 9 and 11 A, respectively, consistent with molecules orienting with a tilt angle of approximately 60 degrees from the surface normal. Unlike the alkanethiols, the thickness of the 1,8-octanedithiol monolayer is almost the same as its molecular length, indicating that dithiol molecules orient vertically with only one thiol end group bound to the GaAs surface. Additional support for this conclusion comes from the fact that X-ray photoelectron spectroscopy of the 1,8-octanedithiol monolayer clearly resolves two types of S atoms in the monolayer: those bound to the GaAs surface and those existing as free thiols. A suggestion was made on the mechanisms for alkanethiol and alkanedithiol monolayer formation.     

Bottom-contact poly(3,3'-didodecylquaterthiophene) thin-film transistors with gold source-drain electrodes modified by alkanethiol monolayers

A series of alkanethiol monolayers (CH 3(CH 2) n-1 SH, n = 4, 6, 8, 10, 12, 14, 16) were used to modify gold source-drain electrode surfaces for bottom-contact poly(3,3'-didodecylquaterthiophene) (PQT-12) thin-film transistors (TFTs). The device mobilities of TFTs were significantly increased from approximately 0.015 cm (2) V (-1) s (-1) for bare electrode TFTs to a maximum of approximately 0.1 cm (2) V (-1) s (-1) for the n = 8 monolayer devices. The mobilities of devices with alkanethiol-modified Au electrodes varied parabolically with alkyl length with values of 0.06, 0.1, and 0.04 cm (2) V (-1) s (-1) at n = 4, 8, and 16, respectively. Atomic force microscopy investigations reveal that alkanethiol electrode surface modifications promote polycrystalline PQT-12 morphologies at electrode/PQT-12 contacts, which probably increase the density of states of the PQT-12 semiconductor at the interfaces. The contact resistance of TFTs is strongly modulated by the surface modification and strongly varies with the alkanethiol chain length. The surface modifications of electrodes appear to significantly improve the charge injection, with consequent substantial improvement in device performance.     

Fabrication and characterization of metal-molecule-metal junctions by conducting probe atomic force microscopy

Metal-molecule-metal junctions were fabricated by contacting Au-supported alkyl or benzyl thiol self-assembled monolayers (SAMs) with an Au-coated atomic force microscope (AFM) tip. The tip-SAM microcontact is approximately 15 nm(2), meaning the junction contains approximately 75 molecules. Current-voltage (I-V) characteristics of these junctions were probed as a function of SAM thickness and load applied to the microcontact. The measurements showed: (1) the I-V traces were linear over +/-0.3 V, (2) the junction resistance increased exponentially with alkyl chain length, (3) the junction resistance decreased with increasing load and showed two distinct power law scaling regimes, (4) resistances were a factor of 10 lower for junctions based on benzyl thiol SAMs compared to hexyl thiol SAMs having the same thickness, and (5) the junctions sustained fields up to 2 x 10(7) V/cm before breakdown. I-V characteristics determined for bilayer junctions involving alkane thiol-coated tips in contact with alkane thiol SAMs on Au also showed linear I-Vs over +/-0.3 V and the same exponential dependence on thickness. The I-V behavior and the exponential dependence of resistance on alkyl chain length are consistent with coherent, nonresonant electron tunneling across the SAM. The calculated conductance decay constant (beta) is 1.2 per methylene unit ( approximately 1.1 A(-)(1)) for both monolayer and bilayer junctions, in keeping with previous scanning tunneling microscope and electrochemical measurements of electron transfer through SAMs. These measurements show that conducting probe-AFM is a reliable method for fundamental studies of electron transfer through small numbers of molecules. The ability to vary the load on the microcontact is a unique characteristic of these junctions and opens opportunities for exploring electron transfer as a function of molecular deformation.     

Length dependence of charge transport in nanoscopic molecular junctions incorporating a series of rigid thiol-terminated norbornylogs

Four tetrathiol-terminated norbornane homologues were synthesized and self-assembled monolayers (SAMs) of these molecules were formed on Au via adsorption from CH2Cl2. SAMs were characterized structurally via spectroscopic ellipsometry (SE), reflection-absorption infrared spectroscopy (RAIRS), Rutherford backscattering spectrometry (RBS), and X-ray photoelectron spectroscopy (XPS). Results of these analyses show that the rigid norbornylogs form monolayers that have a surface coverage slightly lower than that of alkanethiols, and that they exhibit a nonmonotonic dependence of film thickness on molecular length. Nanoscale molecular junctions incorporating these SAMs were formed and characterized electrically using conducting probe atomic force microscopy (CP-AFM). The resistances of these junctions scale exponentially with the contour length of the molecules, with beta = 0.9 A(-1), consistent with a nonresonant tunneling mechanism. Further, the resistance of norbornyl SAMs correlates well with the resistance of alkanedithiol SAMs of similar length, suggesting that the norbornyl molecules form sulfur-metal bonds on both ends of the junction.     

Tunneling characteristics of Au-alkanedithiol-Au junctions formed via nanotransfer printing (nTP)

Construction of permanent metal-molecule-metal (MMM) junctions, though technically challenging, is desirable for both fundamental investigations and applications of molecule-based electronics. In this study, we employed the nanotransfer printing (nTP) technique using perfluoropolyether (PFPE) stamps to print Au thin films onto self-assembled monolayers (SAMs) of alkanedithiol formed on Au thin films. We show that the resulting MMM junctions form permanent and symmetrical tunnel junctions, without the need for an additional protection layer between the top metal electrode and the molecular layer. This type of junction makes it possible for direct investigations into the electrical properties of the molecules and the metal-molecule interfaces. Dependence of transport properties on the length of the alkane molecules and the area of the printed Au electrodes has been examined systematically. From the analysis of the current-voltage (I-V) curves using the Simmons model, the height of tunneling barrier associated with the molecule (alkane) has been determined to be 3.5 ± 0.2 eV, while the analysis yielded an upper bound of 2.4 eV for the counterpart at the interface (thiol). The former is consistent with the theoretical value of ~3.5-5.0 eV. The measured I-V curves show scaling with respect to the printed Au electrode area with lateral dimensions ranging from 80 nm to 7 μm. These results demonstrate that PFPE-assisted nTP is a promising technique for producing potentially scalable and permanent MMM junctions. They also demonstrate that MMM structures (produced by the unique PFPE-assisted nTP) constitute a reliable test bed for exploring molecule-based electronics.     

Analysis of the causes of variance in resistance measurements on metal-molecule-metal junctions formed by conducting-probe atomic force microscopy

Alkanethiol tunnel junctions were studied using conducting-probe atomic force microscopy to determine causes of variability in measured resistance behavior. Measurements were made on Au/decanethiol/Au monolayer junctions, and effects of substrate roughness, tip chemistry, presence of solvent, extensive tip usage, applied load, and tip radius were examined. Resistance measurements yielded log-normal distributions under a variety of conditions, indicating that the origin of the variance is likely to be either changes in tunneling length or electronic overlap. Spreads in resistance values for a given tip were much less when flat, template-stripped Au substrates were used rather than rough, evaporated Au substrates. Chemical modification of tips with ethanethiol (C2) or butanethiol (C4) and performing measurements under cyclohexane were also found to reduce variance by a factor of about 2-4. Experiments performed with unmodified tips showed an increase in junction resistance over the course of hundreds of consecutive measurements, whereas junctions made with modified tips or under cyclohexane did not. Attempts to ascribe variance between tips to varying tip radii failed; however, decreases in resistance with increasing applied load on the tip contact were observed and could be interpreted in terms of conventional contact mechanics models.     

Attachment of Amine‐ and Maleimide‐Containing Ferrocene Derivatives onto Self‐Assembled Alkanethiol and Alkanedithiol Monolayers: Voltammetric Evaluation of Cross‐Linking Efficiencies and Surface Coverage of Electroactive Groups

Ferrocene derivatives containing primary amines and maleimide groups were attached covalently onto N‐hydrosuccinimidyl (NHS)‐terminated alkanethiol self‐assembled monolayers (SAMs) and SAMs of alkanedithiol. The surface coverage and efficiencies of the two cross‐linking reactions were evaluated with cyclic voltammetry. All the ferrocene derivatives attached onto the alkanethiol or alkanedithiol SAMs exhibit reversible redox waves. The surface coverage of the aminated ferrocene groups was compared to that of N‐hydrosuccinimidyl (NHS)‐terminated alkanethiol SAM. The covalent attachment of β‐ferrocenylethylamine onto a 11,11′‐dithio‐bis(succinimidylundecanoate) SAM yielded an efficiency as high as 63.1%. The cross‐linking efficiency of this reaction was found to increase with the nucleophilicity of the amino groups. SAMs of longer alkyl chains favor the attachment of a greater number of ferrocene derivatives. As for the Michael‐type electrophilic addition between the sulfhydryl groups of the alkanedithiol SAMs and the ferrocenyl maleimide, the cross‐linking efficiencies were found to range from 6.5% to 25.7%, depending on the alkanedithiol chain length. The difference in the efficiencies between the two types of cross‐linking reactions might be partially attributable to the steric hindrance imposed by the SAMs and the relative sizes of the functional groups.     

A Solid‐State Protein Junction Serves as a Bias‐Induced Current Switch

A sample‐type protein monolayer, that can be a stepping stone to practical devices, can behave as an electrically driven switch. This feat is achieved using a redox protein, cytochrome C (CytC), with its heme shielded from direct contact with the solid‐state electrodes. Ab initio DFT calculations, carried out on the CytC–Au structure, show that the coupling of the heme, the origin of the protein frontier orbitals, to the electrodes is sufficiently weak to prevent Fermi level pinning. Thus, external bias can bring these orbitals in and out of resonance with the electrode. Using a cytochrome C mutant for direct S?Au bonding, approximately 80 % of the Au–CytC–Au junctions show at greater than 0.5 V bias a clear conductance peak, consistent with resonant tunneling. The on–off change persists up to room temperature, demonstrating reversible, bias‐controlled switching of a protein ensemble, which, with its built‐in redundancy, provides a realistic path to protein‐based bioelectronics.     

偶氮苯自组装单分子膜中长程电子转移机理

利用自组装技术在金电极表面构造了具有不同前端健长度偶氮苯功能化的单分子膜体系：Au／S（CH2）nNHCO-N＝N-OCH2CH3（n＝2，3，4，6）．研究结果表明，仍氮苯到金电极的表现电子转移速率随它们之间的距离长度的增加而呈指数性的下降趋势．基于Marcus电子隧穿理论，得到了此自组装膜体系的长程电子隧穿系数ρ＝（1．35±0．2）／CH2在和可逆电活性分子自组装膜体系及理论计算相比较的基础上，从偶氮苯分子自组装膜结构与电子转移过程的关系角度对这一结果进行了分析和说明．     

Mechanical and charge transport properties of alkanethiol self-assembled monolayers on a au(111) surface: the role of molecular tilt

The relationship between charge transport and mechanical properties of alkanethiol self-assembled monolayers (SAMs) on Au(111) films has been investigated using an atomic force microscope with a conductive tip. Molecular tilts induced by the pressure applied by the tip cause stepwise increases in film conductivity. A decay constant beta = 0.57 +/- 0.03 A-1 was found for the current passing through the film as a function of tip-substrate separation due to this molecular tilt. This is significantly smaller than the value of approximately 1 A-1 found when the separation is varied by changing the length of the alkanethiol molecules. Calculations indicate that, for isolated dithiol molecules S-bonded to hollow sites, the junction conductance does not vary significantly as a function of molecular tilt. The impact of S-Au bonding on SAM conductance is discussed.     

On the Hopping Efficiency of Nanoparticles in the Electron Transfer across Self‐Assembled Monolayers

Redox reactions of solvated molecular species at gold‐electrode surfaces modified by electrochemically inactive self‐assembled molecular monolayers (SAMs) are found to be activated by introducing Au nanoparticles (NPs) covalently bound to the SAM to form a reactive Au–alkanedithiol–NP–molecule hybrid entity. The NP appears to relay long‐range electron transfer (ET) so that the rate of the redox reaction may be as efficient as directly on a bare Au electrode, even though the ET distance is increased by several nanometers. In this study, we have employed a fast redox reaction of surface‐confined 6‐(ferrocenyl) hexanethiol molecules and NPs of Au, Pt and Pd to address the dependence of the rate of ET through the hybrid on the particular NP metal. Cyclic voltammograms show an increasing difference in the peak‐to‐peak separation for NPs in the order Au<Pt<Pd, especially when the length of the alkanedithiol increases from octanedithiol to decanedithiol. The corresponding apparent rate constants, k_app, for decanedithiol are 1170, 360 and 14 s^?1 for NPs of Au, Pt and Pd, respectively, indicating that the efficiency of NP mediation of the ET clearly depends on the nature of the NP. Based on a preliminary analysis rooted in interfacial electrochemical ET theory, combined with a simplified two‐step view of the NP coupling to the electrode and the molecule, this observation is referred to the density of electronic states of the NPs, reflected in a broadening of the molecular electron/NP bridge group levels and energy‐gap differences between the Fermi levels of the different metals.     

Nanotube-metal junctions: 2- and 3-terminal electrical transport

We address the quality of electrical contact between carbon nanotubes and metallic electrodes by performing first-principles calculations for the electron transmission through ideal 2- and 3-terminal junctions, thus revealing the physical limit of tube-metal conduction. The structural model constructed involves surrounding the tube by the metal atoms of the electrode as in most experiments; we consider metallic (5,5) and n-doped semiconducting (10,0) tubes surrounded by Au or Pd. In the case of metallic tubes, the contact conductance is shown to approach the ideal 4e2/h in the limit of large contact area. For three-terminals, the division of flux among the different transmission channels depends strongly on the metal material. A Pd electrode has nearly perfect tube-electrode transmission and therefore turns off the straight transport along the tube. Our results are in good agreement with some recent experimental reports and clarify a fundamental discrepancy between theory and experiment.     

Multi-technique Characterization of Self-assembled Carboxylic Acid Terminated Alkanethiol Monolayers on Nanoparticle and Flat Gold Surfaces

Gold nanoparticles (AuNPs) with 14, 25 and 40nm diameters were functionalized with different chain length (C6, C8, C11 and C16) carboxylic acid terminated alkanethiol self-assembled monolayers (COOH-SAMs). X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) were used to examine the changes in surface chemistry as both AuNP diameter and SAM chain length were varied. COOH-SAMs on flat gold surfaces were also examined and compared to the COOH-SAM on AuNP results. For a given surface, as the COOH-SAM chain length increased the XPS C/Au atomic ratio increased due to an increased number of carbon atoms per molecule in the overlayer and an increased attenuation of the Au substrate signal. For the C16 COOH-SAMs, as the size of AuNPs decreased the XPS C/Au atomic ratio and the apparent SAM thickness increased due to the increased curvature of the smaller AuNPs. The C16 COOH-SAMs on the flat Au had the lowest XPS C/Au atomic ratio and apparent SAM thickness of any C16 COOH-SAM covered Au surface. The effective take-off angles of the COOH-SAMs were also calculated by comparing the apparent thickness of COOH-SAMs with literature values. The effective take-off angle for C16 COOH-SAM on 14nm, 25nm and 40nm diameter AuNPs and flat Au were found to be 57°, 53°, 51° and 39°, respectively, for data acquired in a mode that collects a wide range of photoelectron take-off angles. The effective take-off angle for C16 COOH-SAM on 14nm AuNP and flat Au decreased to 52° and 0°, respectively, for data acquired in a mode that collects a narrow range of photoelectron take-off angles. The ToF-SIMS results showed similar changes in surface chemistry with COOH-SAM chain length and AuNP size. For example, the ratio of the sum of the C(1-4)H(x)O(y) positive ion intensities to the sum of the Au-containing positive ions intensities increased with decreasing AuNP size and increasing COOH-SAM chain length. Fourier transform IR spectroscopy in the attenuated total reflectance mode (FTIR-ATR) was used to characterize the crystallinity of the COOH-SAMs. The CH(2) stretching frequencies decreased with increasing COOH-SAM chain length on flat Au. The C16 COOH-SAM on the 14nm AuNPs exhibited a crystalline-like CH(2) stretching frequency. The size, size distribution, shapes and solution stability of AuNPs were investigated with transmission electron microscopy (TEM) and UV/VIS spectroscopy. As the average diameter of the AuNPs decreased the size distribution became narrower and the shape became more spherical.     

Simultaneous nanoindentation and electron tunneling through alkanethiol self-assembled monolayers

Electrical tunnel junctions consisting of alkanethiol molecules self-assembled on Au-coated Si substrates and contacted with Au-coated atomic force microscopy tips were characterized under varying junction loads in a conducting-probe atomic force microscopy configuration. Junction load was cycled in the fashion of a standard nanoindentation experiment; however, junction conductance rather than probe depth was measured directly. The junction conductance data have been analyzed with typical contact mechanics (Derjaguin-Müller-Toporov) and tunneling equations to extract the monolayer modulus (approximately 50 GPa), the contact transmission (approximately 2 x 10(-6)), contact area, and probe depth as a function of load. The monolayers are shown to undergo significant plastic deformation under compression, yielding indentations approximately 7 Angstroms deep for maximum junction loads of approximately 50 nN. Comparison of mechanical properties for different chain lengths was also performed. The film modulus decreased with the number of carbons in the molecular chain for shorter-chain films. This trend abruptly reversed once 12 carbons were present along the backbone.     

Well-ordered self-assembled monolayers created via vapor-phase reactions on a monolayer template

The reaction of vapor-phase alkyl isocyanates (O=C=N-(CH2)n-1CH3) with OH-terminated alkanethiol template monolayers on Au produces well-organized self-assembled monolayers, containing intrachain carbamate linkages (Au/S(CH2)16O(C=O)NH(CH2)n-1CH3, where n = 1-8, 11, and 12). X-ray photoelectron spectroscopy, contact angle goniometry, and reflection absorption infrared spectroscopy suggest that the template surface completely reacts with the isocyanates yielding a monolayer that contains an interchain hydrogen-bonded carbamate network. Spectroscopic data indicates that the alkyl underlayer remains well ordered following reaction with the isocyanates. The order of the overlayer and the hydrogen-bonding interactions between adjacent chains increase as a function of the alkyl isocyanate chain length, n. The overlayer appears to be well ordered for n > or = 5.     

Electron transport and redox reactions in carbon-based molecular electronic junctions

A unique molecular junction design is described, consisting of a molecular mono- or multilayer oriented between a conducting carbon substrate and a metallic top contact. The sp2 hybridized graphitic carbon substrate (pyrolyzed photoresist film, PPF) is flat on the scale of the molecular dimensions, and the molecular layer is bonded to the substrate via diazonium ion reduction to yield a strong, conjugated C-C bond. Molecular junctions were completed by electron-beam deposition of copper, titanium oxide, or aluminium oxide followed by a final conducting layer of gold. Vibrational spectroscopy and XPS of completed junctions showed minimal damage to the molecular layer by metal deposition, although some electron transfer to the molecular layer resulted in partial reduction in some cases. Device yield was high (>80%), and the standard deviations of junction electronic properties such as low voltage resistance were typically in the range of 10-20%. The resistance of PPF/molecule/Cu/Au junctions exhibited a strong dependence on the structure and thickness of the molecular layer, ranging from 0.13 ohms cm2 for a nitrobiphenyl monolayer, to 4.46 ohms cm2 for a biphenyl monolayer, and 160 ohms cm2 for a 4.3 nm thick nitrobiphenyl multilayer. Junctions containing titanium or aluminium oxide had dramatically lower conductance than their PPF/molecule/Cu counterparts, with aluminium oxide junctions exhibiting essentially insulating behavior. However, in situ Raman spectroscopy of PPF/nitroazobenzene/AlO(x)/Au junctions with partially transparent metal contacts revealed that redox reactions occurred under bias, with nitroazobenzene (NAB) reduction occurring when the PPF was biased negative relative to the Au. Similar redox reactions were observed in PPF/NAB/TiO(x)/Au molecular junctions, but they were accompanied by major effects on electronic behavior, such as rectification and persistent conductance switching. Such switching was evident following polarization of PPF/molecule/TiO2/Au junctions by positive or negative potential pulses, and the resulting conductance changes persisted for several minutes at room temperature. The "memory" effect implied by these observations is attributed to a combination of the molecular layer and the TiO2 properties, namely metastable "trapping" of electrons in the TiO2 when the Au is negatively biased.     

Molecular rectification in a metal-insulator-metal junction based on self-assembled monolayers

An electrical junction formed by mechanical contact between two self-assembled monolayers (SAMs)--a SAM formed from an dialkyl disulfide with a covalently linked tetracyanoquinodimethane group that is supported by silver (or gold) and a SAM formed from an alkanethiolate SAM that is supported by mercury-rectifies current. The precursor to the SAM on silver (or gold) was bis(20-(2-((2,5-cyclohexadiene-1,4-diylidene)dimalonitrile))decyl)) disulfide and that for the SAM on mercury was HS(CH(2))(n-1)CH(3) (n = 14, 16, 18). The electrical properties of the junctions were characterized by current-voltage measurements. The ratio of the conductivity of the junction in the forward bias (Hg cathodic) to that in the reverse bias (Hg anodic), at a potential of 1 V, was 9 +/- 2 when the SAM on mercury was derived from HS(CH(2))(15)CH(3). The ratio of the conductivity in the forward bias to that in the reverse bias increased with decreasing chain length of the alkanethiol used to form the SAM on mercury. These results demonstrate that a single redox center asymmetrically placed in a metal-insulator-metal junction can cause the rectification of current and indicate that a fixed dipole in the insulating region of a metal-insulator-metal junction is not required for rectification.     

Molecular tunnel junctions based on π-conjugated oligoacene thiols and dithiols between Ag, Au, and Pt contacts: effect of surface linking group and metal work function

The tunneling resistance and electronic structure of metal-molecule-metal junctions based on oligoacene (benzene, naphthalene, anthracene, and tetracene) thiol and dithiol molecules were measured and correlated using conducting probe atomic force microscopy (CP-AFM) in conjunction with ultraviolet photoelectron spectroscopy (UPS). Nanoscopic tunnel junctions (~10 nm(2)) were formed by contacting oligoacene self-assembled monolayers (SAMs) on flat Ag, Au, or Pt substrates with metalized AFM tips (Ag, Au, or Pt). The low bias (<0.2 V) junction resistance (R) increased exponentially with molecular length (s), i.e., R = R(0) exp(βs), where R(0) is the contact resistance and β is the tunneling attenuation factor. The R(0) values for oligoacene dithiols were 2 orders of magnitude less than those of oligoacene thiols. Likewise, the β value was 0.5 per ring (0.2 ?(-1)) for the dithiol series and 1.0 per ring (0.5 ?(-1)) for the monothiol series, demonstrating that β is not simply a characteristic of the molecular backbone but is strongly affected by the number of chemical (metal-S) contacts. R(0) decreased strongly as the contact work function (Φ) increased for both monothiol and dithiol junctions, whereas β was independent of Φ within error. This divergent behavior was explained in terms of the metal-S bond dipoles and the electronic structure of the junction; namely, β is independent of contact type because of weak Fermi level pinning (UPS revealed E(F) - E(HOMO) varied only weakly with Φ), but R(0) varies strongly with contact type because of the strong metal-S bond dipoles that are responsible for the Fermi level pinning. A previously published triple barrier model for molecular junctions was invoked to rationalize these results in which R(0) is determined by the contact barriers, which are proportional to the size of the interfacial bond dipoles, and β is determined by the bridge barrier, E(F) - E(HOMO). Current-voltage (I-V) characteristics obtained over a larger voltage range 0-1 V revealed a characteristic transition voltage V(trans) at which the current increased more sharply with voltage. V(trans) values were generally >0.5 V and were well correlated with the bridge barrier E(F) - E(HOMO). Overall, the combination of electronic structure determination by UPS with length- and work function-dependent transport measurements provides a remarkably comprehensive picture of tunneling transport in molecular junctions based on oligoacenes.     

Structure and conductance of aromatic and aliphatic dithioacetamide monolayers on Au(111)

The structure and electrical properties of self-assembled monolayers of cyclic aromatic and aliphatic dithioacetamides (1,4-bis(mercaptoacetamido)benzene and 1,4-bis(mercaptoacetamido)cyclohexane) and of mixed dithioacetamide/alkanethiol monolayers are characterized by X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM) and contact angle goniometry. Both dithioacetamides are found to pack densely on Au(111), however the monolayers are poorly ordered as a result of hydrogen bond formation between the amide groups. The coassembly and the insertion method are compared for the formation of mixed dithioacetamide/alkanethiol monolayers. By coassembly, islands of dithioacetamides in a dodecanethiol matrix can only be obtained at a low dithioacetamide/dodecanethiol concentration ratio in solution (1/10) and by thermal annealing of the resulting monolayers. Small and well defined dithioacetamide domains are realized by insertion of dithioacetamides into defect sites of closely packed octanethiol monolayers. These domains are used to determine the molecular conductance by means of STM height profiles and molecular lengths resulting from density functional theory (DFT) calculations. The difference in the tunneling decay constant beta measured for aromatic dithioacetamides (beta = 0.74-0.76/A) and for aliphatic dithioacetamides (beta = 0.84-0.91/A) highlights the influence of the conjugation within the cyclic core on molecular conductance.