Controlling semiconductor/metal junction barriers by incomplete, nonideal molecular monolayers

We study how partial monolayers of molecular dipoles at semiconductor/metal interfaces can affect electrical transport across these interfaces, using a series of molecules with systematically varying dipole moment, adsorbed on n-GaAs, prior to Au or Pd metal contact deposition, by indirect evaporation or as "ready-made" pads. From analyses of the molecularly modified surfaces, we find that molecular coverage is poorer on low- than on high-doped n-GaAs. Electrical charge transport across the resulting interfaces was studied by current-voltage-temperature, internal photoemission, and capacitance-voltage measurements. The data were analyzed and compared with numerical simulations of interfaces that present inhomogeneous barriers for electron transport across them. For high-doped GaAs, we confirm that only the former, molecular dipole-dependent barrier is found. Although no clear molecular effects appear to exist with low-doped n-GaAs, those data are well explained by two coexisting barriers for electron transport, one with clear systematic dependence on molecular dipole (molecule-controlled regions) and a constant one (molecule-free regions, pinholes). This explains why directly observable molecular control over the barrier height is found with high-doped GaAs: there, the monolayer pinholes are small enough for their electronic effect not to be felt (they are "pinched off"). We conclude that molecules can control and tailor electronic devices need not form high-quality monolayers, bind chemically to both electrodes, or form multilayers to achieve complete surface coverage. Furthermore, the problem of stability during electron transport is significantly alleviated with molecular control via partial molecule coverage, as most current flows now between, rather than via, the molecules.     

Interfacial electrostatics of self-assembled monolayers of alkane thiolates on Au(111): work function modification and molecular level alignments

We have isolated at T < 150 K a weakly adsorbed dimethyl disulfide (DMDS) layer on Au(111) and studied how the vibrational states, S core hole level shifts, valence band photoemission, and work function measurements evolve upon transforming this system into chemisorbed methylthiolate (MT) self-assembled monolayers (SAM) by heating above 200 K. By combining these observations with detailed theoretical electronic structure simulations, at the density functional level, we have been able to obtain a detailed picture of the electronic interactions at the interface between Au and adsorbed thiolates and disulfides. All of our measurements may be interpreted with a simple model where MT is bound to the Au surface with negligible charge transfer. Interfacial dipoles arising from Pauli repulsion between molecule and metal surface electrons are present for the weakly adsorbed DMDS layer but not for the chemisorbed species. Instead, for the chemisorbed species, interfacial dipoles are exclusively controlled by the molecular dipole, its interaction with the dipoles on neighboring molecules, and its orientation to the surface. The ramifications of these results for alignment of molecular levels and interfacial properties of this class of materials are discussed.     

Influence of dipole-dipole interactions on coverage-dependent adsorption: CO and NO on Pt(111)

Density functional theory (DFT) calculations of energetic, geometric, vibrational, and electrostatic properties of different arrangements of CO and NO at quarter and half monolayer coverage on Pt(111) are presented. Differences in the extents of electron back-donation from the Pt surface to these molecules cause the low-coverage adsorbate dipoles to have opposite signs at atop and more highly coordinated bridge or fcc sites. These dipoles of opposite sign occupy adjacent positions in the experimentally observed atop-bridge or atop-fcc high -coverage arrangements, leading to attractive electrostatic interactions and concomitant changes in dipole moments, bond lengths, and vibrational frequencies. The interaction energies are estimated by charge partitioning to extract individual dipoles from the mixed arrangement and by calculations of field-dipole interactions. These estimated dipole interactions contribute significantly (20-60%) to the DFT-calculated relative stability of mixed arrangements over atop-, bridge-, or fcc-only arrangements and thus play an important role in coverage-dependent adsorption. We further extend these analyses to a range of molecules with varying dipole moments and show that the general nature of these interactions is not limited to CO and NO.     

Molecular hydrogen adsorption at surface adatoms

Using electron-energy-loss spectroscopy, we have measured preferential adsorption of molecular hydrogen at Cu and Au adatoms deposited on a cold Cu(100) surface. We show, with particular attention to the D(2)-Au system, that the molecules adsorb at the adatoms, with an enhanced binding energy. The adsorption state is not of chemisorption character, the D(2) rotational and internal vibrational transition energies are close to the corresponding gas phase values, a characteristic property of a physisorbed state. A revealing signature of the D(2)-Au interaction is an induced dipole activity of the rotational transition, which discriminates molecules adsorbed at the adatoms from those adsorbed on the bare substrate surface. The average number of molecules per Au atom depends on the Au coverage and increases at lower coverages, for example, at 4% of an adatom monolayer, there are approximately six D(2) molecules per Au adatom. In this limit, Au monomers prevail, and a cluster of six D(2) around a single Au adatom appears to be an optimal dense two-dimensional configuration.     

Self-assembly of 1-D organic semiconductor nanostructures

This review focuses on the molecular design and self-assembly of a new class of crowded aromatics that form 1-D nanostructures via hydrogen bonding and pi-pi interactions. These molecules have a permanent dipole moment that sums as the subunits self assemble into molecular stacks. The assembly of these molecular stacks can be directed with electric fields. Depending on the nature of the side-chains, molecules can obtain the face-on or edge-on orientation upon the deposition onto a surface via spin cast technique. Site-selective steady state fluorescence, time-resolved fluorescence, and various types of scanning probe microscopy measurements detail the intermolecular interactions that drive the aromatic molecules to self-assemble in solution to form well-ordered columnar stacks. These nanostructures, formed in solution, vary in their number, size, and structure depending on the functional groups, solvent, and concentration used. Thus, the substituents/side-groups and the proper choice of the solvent can be used to tune the intermolecular interactions. The 1-D stacks and their aggregates can be easily transferred by solution casting, thus allowing a simple preparation of molecular nanostructures on different surfaces.     

Molecularly controlled metal-semiconductor junctions on silicon surface: a dipole effect

Silicon surface was chemically modified by covalent attachment of homologous organic molecules having different dipole moments. Surface photovoltage measurements clearly confirm that organic molecules have a profound effect on surface band bending of semiconductor. Metal-molecules--silicon junctions were constituted for molecules belonging to ethynylbenzene series using soft mercury contact. These junctions show a systematic change in the electrical charge transport with dipole moment of molecules. Parameters such as ideality factor, barrier height, and density of interface states of various junctions are estimated to understand the role of organic molecules. These studies offer the prospect to develop molecular electronics, which may find potential applications in solar cells and chemical and biological sensors.     

Surface dipoles and work functions of alkylthiolates and fluorinated alkylthiolates on Au(111)

We study the dipole formation at the surface formed by -CH(3) and -CF(3) terminated short-chain alkylthiolate monolayers on Au(111). In particular, we monitor the change in work function upon chemisorption using density functional theory calculations. We separate the surface dipole into two contributions, resulting from the gold-adsorbate interaction and the intrinsic dipole of the adsorbate layer, respectively. The two contributions turn out to be approximately additive. Adsorbate dipoles are defined by calculating dipole densities of free-standing molecular monolayers. The gold-adsorbate interaction is, to a good degree, determined by the Au-S bond only. This bond is nearly apolar and its contribution to the surface dipole is relatively small. The surface dipole of the self-assembled monolayer is then dominated by the intrinsic dipole of the thiolate molecules. Alkylthiolates increase the work function of Au(111), whereas fluorinated alkylthiolates decrease it.     

Reflection anisotropy spectroscopy study of the adsorption of sulfur-containing amino acids at the Au(110)/electrolyte interface

Protein interactions with surfaces are key to understanding the behavior of implantable medical devices. The optical technique of reflection anisotropy spectroscopy (RAS) has considerable potential for the study of interactions between important biological molecules and surfaces. This study used RAS to investigate the adsorption of S amino acids onto Au(110) in a liquid environment under different conditions of potential and pH. Certain spectral features can be associated with the Au(110), as reported previously, while other features are assigned to bonds between the amino acids and the Au surface. The RA spectra are shown to be influenced by the structure of the amino acid, the solution pH, and the applied electrode potential. This work has assigned the negative feature at 2.5 eV to the Au-thiolate, bond while the positive feature at 2.5 eV is assigned to the disulfide bond. The broad spectral feature at 3.5 eV is attributed to the Au-amino interaction, while the sharper feature at slightly higher energy is associated with the Au-carboxylate interaction. Sulfur-containing amino acids are frequently found on the outside of protein molecules and could be used to anchor the protein to the surface.     

Coordination-based gold nanoparticle layers

Gold nanoparticle (NP) mono- and multilayers were constructed on gold surfaces using coordination chemistry. Hydrophilic Au NPs (6.4 nm average core diameter), capped with a monolayer of 6-mercaptohexanol, were modified by partial substitution of bishydroxamic acid disulfide ligand molecules into their capping layer. A monolayer of the ligand-modified Au NPs was assembled via coordination with Zr4+ ions onto a semitransparent Au substrate (15 nm Au, evaporated on silanized glass and annealed) precoated with a self-assembled monolayer of the bishydroxamate disulfide ligand. Layer-by-layer construction of NP multilayers was achieved by alternate binding of Zr4+ ions and ligand-modified NPs onto the first NP layer. Characterization by atomic force microscopy (AFM), ellipsometry, wettability, transmission UV-vis spectroscopy, and cross-sectional transmission electron microscopy showed regular growth of NP layers, with a similar NP density in successive layers and gradually increased roughness. The use of coordination chemistry enables convenient step-by-step assembly of different ligand-possessing components to obtain elaborate structures. This is demonstrated by introducing nanometer-scale vertical spacing between a NP layer and the gold surface, using a coordination-based organic multilayer. Electrical characterization of the NP films was carried out using conductive AFM, emphasizing the barrier properties of the organic spacer multilayer. The results exhibit the potential of coordination self-assembly in achieving highly controlled composite nanostructures comprising molecules, NPs, and other ligand-derivatized components.     

The effect of surface dipoles and of the field generated by a polarization gradient on the repulsive force

Double-layer and hydration interactions have been coupled into a single set of equations because both are dependent on the polarization of the water molecules. The coupled equations involve the electric fields generated by the surface charge and surface dipoles, as well as the field due to the neighboring dipoles in water. The dipoles on the surface are generated through the counterions' binding to sites of opposite charge. The equations obtained were employed to explain the restabilization observed experimentally at large ionic strengths for colloidal particles on which protein molecules were adsorbed. Polar molecules adsorbed on a charged surface of colloidal particle can generate a field either in the same direction as that generated by the charge or in the opposite direction. The effect of the sign of the dipole of the adsorbed polar molecules on the interaction between surfaces was also examined.     

Chain-branching control of the atomic structure of alkanethiol-based gold-sulfur interfaces

Density functional theory structure calculations at 0 K and simulations at 300 K of observed high-resolution in situ scanning tunneling microscopy (STM) images reveal three different atomic-interface structures for the self-assembled monolayers (SAMs) of three isomeric butanethiols on Au(111): direct binding to the Au(111) surface without pitting, binding to adatoms above a regular surface with extensive pitting, and binding to adatoms with local surface vacancies and some pitting. Thermal motions are shown to produce some observed STM features, with a very tight energy balance controlling the observed structures. Variation of the degree of substitution on the α carbon is found to significantly change the relative energies for interaction of the different types of adatom structures with the surface, while the nature of the surface cell, controlled primarily by inter-adsorbate steric interactions, controls substrate reorganization energies and adsorbate distortion energies. Most significantly, by manipulating these features, chemical control of the adsorbate can produce stable interfaces with surface pitting eliminated, providing new perspectives for technological applications of SAMs.     

Organic Semiconductor/Gold Interface Interactions: From Physisorption on Planar Surfaces to Chemical Reactions with Metal Nanoparticles

The interaction of gold nanoparticles (AuNPs) with prototypical organic semiconductors used in optoelectronics, namely, tris(8‐hydroxyquinoline)aluminium (Alq₃) and 4,4‐bis[N‐(1‐naphthyl)‐N‐phenylamino]diphenyl (α‐NPD), is investigated in situ by X‐ray photoelectron spectroscopy (XPS). These AuNPs‐on‐molecule experiments are compared with the reversed molecule‐on‐Au cases. The molecules‐on‐Au systems show only weak interactions, and the evolution of the XP spectra is dominated by final‐state effects. In contrast, in the AuNPs‐on‐molecules cases, both initial‐state effects and final‐state effects occur. Spectral features arising for both molecules and metal indicate charge transfer and the formation of organometallic complexes (initial‐state effects). The energy shift in the metal emission underlines the size‐induced nanometric nature of the molecule/Au interaction (final‐state effects). Consequently, the chemical interaction between metals and organic semiconductors likely depends strongly on the deposition sequence in general.     

Dual-scale modeling of benzene adsorption onto Ni(111) and Au(111) surfaces in explicit water.

We present a multiscale modeling approach for studying interactions of organic molecules with metal surfaces in explicit water. The approach is based on combining adsorption energies of isolated molecules on transition metal surfaces calculated by ab initio density functional methods and classical molecular dynamics simulations using atomistically detailed force fields. The interaction of benzene with Ni(111) and Au(111) surfaces was studied. It is shown that a strong affinity of water for the hydrophilic surfaces makes benzene adsorption on Au thermodynamically unfavorable, while on Ni there is no preference. The work presented here serves as a first step in modeling the interactions of larger organic molecules with metal surfaces.     

Determination of the electronic structure of self-assembled L-cysteine/Au interfaces using photoemission spectroscopy

The electronic and chemical structure of the interface between the amino acid L-cysteine and Au was determined by photoemission spectroscopy (PES). L-cysteine was deposited by repeatedly dipping Au substrates into solutions of L-cysteine in methanol with various concentrations. To enable repeat deposition without significant contamination, the dipping procedure was performed in a glovebox directly connected to the ultrahigh vacuum (UHV) chamber in a N2 atmosphere. X-ray photoemission spectroscopy (XPS) measurements between deposition steps allowed to characterize the chemical interaction at the interface to be characterized. Ultraviolet photoemission spectroscopy (UPS) measurements yielded the orbital line-up at the interface as well as the highest occupied molecular orbital (HOMO) structure of L-cysteine. The charge injection barrier between the L-cysteine HOMO and the Au Fermi level was found to be 3.0 eV. The interface dipole between the Au substrate and the L-cysteine overlayer was determined to be 1.03 eV. The results also indicate the formation of an interface state approximately 1.5 eV above the HOMO of the L-cysteine.     

Entropic barriers in nanoscale adhesion studied by variable temperature chemical force microscopy

Intermolecular interactions drive the vast majority of condensed phase phenomena from molecular recognition to protein folding to particle adhesion. Complex energy barriers encountered in these interactions include contributions from van der Waals forces, hydrogen bonding, and solvent medium. With the spectacular exception of hydrophobic interactions, contributions from the medium are usually considered secondary. We report a variable temperature force microscopy study of the interactions between several hydrogen bonds in different solvents that challenges this point of view. Surprisingly, we observed an increase in the strength of the interaction between carboxylic acid groups in ethanol as the temperature increased. Moreover, when we switched to a nonpolar solvent we observed the opposite behavior: The binding force decreased as the temperature increased. Kinetic model of bond dissociation provided quantitative interpretation of our measurements. We attributed the observed phenomena to a large entropic contribution from the ordered solvent layers that are forming on the probe and sample surfaces upon detachment. The observed reversal in the force vs temperature trend is a manifestation of a transition between thermodynamic and kinetic regimes of unbinding predicted by the model. Our results indicate that entropic barriers dominated by the interactions of solvent molecules with the surface exist in a much wider variety of systems than previously thought.     

Design of biomolecular interface for detecting carbohydrate and lectin weak interactions

A hybrid functional biomolecular interface designed at a molecular size level is very effective at capturing an analyte with high sensitivity even if the interaction is very weak, as when detecting proteins with carbohydrate. We designed and processed a protein (lectin) recognition molecular interface taking the following points into consideration: (1) the height (molecular length) difference between the capturing and spacer molecules; (2) the ratio of capturing molecules in the recognition interface. When the height difference between the maltoside part (Concanavalin A (Con A) recognition group) and the OH group terminated spacer molecules exceeded (>(CH(2))(6)), the association rate constant (k(a)) became larger (k(a)(1/Ms): ～2.6 times) and the dissociation constant (K(D)) became much smaller (K(D)(M): 1.0 × 10(-6): ～0.17 times) compared with the similar heights (lengths) of both molecular interfaces. With regard to maltoside density, a 100% maltoside monolayer was unsuitable for detecting Con A. We constructed a nanostructured recognition site with a maltoside part of 10%, which was the most suitable ratio for Con A detection. The binding interaction between Con A and the maltoside group was changed from monovalent binding to bivalent binding when the maltoside part was diluted in the recognition interface. From electrochemical measurements, even though there was a small amount of maltoside component on the suitable recognition monolayer, quality similar to that of 100% maltoside was observed.     

Competition between dipolar and steric interactions in swallow-tailed compounds

A new class of swallow-tailed mesogen with the longitudinal dipole moment in the direction of the non-branched 'head' is presented. Mixtures of these compounds with 'conventional' swallow-tailed substances were investigated by polarization microscopy and dielectric measurements. In the mixtures, dipolar and steric interactions favour opposite arrangements of the molecules in the short range order.     

The mechanism of emerging catalytic activity of gold nano-clusters on rutile TiO2(110) in CO oxidation reaction

This paper reveals the fact that the O adatoms (O(ad)) adsorbed on the 5-fold Ti rows of rutile TiO(2)(110) react with CO to form CO(2) at room temperature and the oxidation reaction is pronouncedly enhanced by Au nano-clusters deposited on the above O-rich TiO(2)(110) surfaces. The optimum activity is obtained for 2D clusters with a lateral size of ～1.5 nm and two-atomic layer height corresponding to ～50 Au atoms∕cluster. This strong activity emerging is attributed to an electronic charge transfer from Au clusters to O-rich TiO(2)(110) supports observed clearly by work function measurement, which results in an interface dipole. The interface dipoles lower the potential barrier for dissociative O(2) adsorption on the surface and also enhance the reaction of CO with the O(ad) atoms to form CO(2) owing to the electric field of the interface dipoles, which generate an attractive force upon polar CO molecules and thus prolong the duration time on the Au nano-clusters. This electric field is screened by the valence electrons of Au clusters except near the perimeter interfaces, thereby the activity is diminished for three-dimensional clusters with a larger size.     

巨大芽孢杆菌D01吸附金(AU3+)的谱学表征

对休眠的巨大芽孢杆菌(Bacillus megatherium)D01菌体吸附Au^3 的作用过程进行了谱学表征．运用AAS考察了pH、时间和温度对D01菌体吸附Au^3 过程的化学动力学和热力学相关参数的影响．D01菌粉中硫元素含量的EDX分析说明该菌体中对Au^3 具有还原作用的L-半胱氨酸和蛋氨酸的含量极少;D01菌体水解后葡萄糖含量的UV-vis测定说明该菌体水解产物中含有一定量的还原糖,空白的和吸附Au^3 的D01菌体的FFIR检测表明该菌体细胞壁肽聚糖层糖类化合物的羟基和肽链侧链氨基酸残基离子化羧基为吸附Au^3 的活性基团;肽聚糖层部分多糖的水解产物低聚糖、二糖及单糖等还原糖的半缩醛羟基游离态醛基为电子供体,将Au^3 原位还原成Au^0．葡萄糖和Au^3 相互作用的XRD和FFIR表征证明Au^3 是在还原糖的醛基上直接被还原成Au^0．     

In situ STM imaging and direct electrochemistry of Pyrococcus furiosus ferredoxin assembled on thiolate-modified Au111 surfaces

We have addressed here electron transfer (ET) of Pyrococcus furiosus ferredoxin (PfFd, 7.5 kDa) in both homogeneous solution using edge plane graphite (EPG) electrodes and in the adsorbed state by electrochemistry on surface-modified single-crystal Au111 electrodes, This has been supported by surface microscopic structures of PfFd monolayers, as revealed by scanning tunneling microscopy under potential control (in situ STM). Direct ET between PfFd in phosphate buffer solution, pH 7.9, and EPG electrodes is observed in the presence of promoters. Neomycin gives rise to a pair of redox peaks with a formal potential of ca -430 mV (vs SCE), corresponding to [3Fe-4S]1+/0. The presence of an additional promoter, which can be propionic acid, alanine, or cysteine, induces a second pair of redox peaks at approximately -900 mV (vs SCE) arising from [3Fe-4S]0/1-. A robust neomycin-PfFd complex was detected by mass spectrometry. The results clearly favor an ET mechanism in which the promoting effect of small organic molecules is through formation of promoter-protein complexes. The interaction of PfFd with small organic molecules in homogeneous solution offers clues to confine the protein on the electrode surface modified by the same functional group monolayer and to address diffusionless direct electrochemistry, as well as surface microstructures of the protein monolayer. PfFd molecules were found to assemble on either mercaptopropionic acid (MPA) or cysteine-modified Au111 surfaces in stable monolayers or submonolayers. Highly ordered (2 radical 3 x 5)R30 degrees cluster structures with six MPA molecules in each cluster were found by in situ STM. Individual PfFd molecules on the MPA layer are well resolved by in situ STM. Under Ar protection reversible cyclic voltammograms were obtained on PfFd-MPA/Au111 and PfFd-cysteine/Au111 electrodes with redox potentials of -220 and -201 mV (vs SCE), respectively, corresponding to the [Fe3S4]1+/0 couple. These values are shifted positively by 200 mV relative to homogeneous solution due to interactions between the promoting layers and the protein molecules. Possible mechanisms for such interactions and their ET patterns are discussed.