C-C bond unsaturation degree in monosubstituted ferrocenes for molecular electronics investigated by a combined near-edge x-ray absorption fine structure, x-ray photoemission spectroscopy, and density functional theory approach

We present the results of an experimental and theoretical investigation of monosubstituted ethyl-, vinyl-, and ethynyl-ferrocene (EtFC, VFC, and EFC) free molecules, obtained by means of synchrotron-radiation based C 1s photoabsorption (NEXAFS) and photoemission (C 1s XPS) spectroscopies, and density functional theory (DFT) calculations. Such a combined study is aimed at elucidating the role played by the C-C bond unsaturation degree of the substituent on the electronic structure of the ferrocene derivatives. Such substituents are required for molecular chemical anchoring onto relevant surfaces when ferrocenes are used for molecular electronics hybrid devices. The high resolution C 1s NEXAFS spectra exhibit distinctive features that depend on the degree of unsaturation of the hydrocarbon substituent. The theoretical approach to consider the NEXAFS spectrum made of three parts allowed to disentangle the specific contribution of the substituent group to the experimental spectrum as a function of its unsaturation degree. C 1s IEs were derived from the experimental data analysis based on the DFT calculated IE values for the different carbon atoms of the substituent and cyclopentadienyl (Cp) rings. Distinctive trends of chemical shifts were observed for the substituent carbon atoms and the substituted atom of the Cp ring along the series of ferrocenes. The calculated IE pattern was rationalized in terms of initial and final state effects influencing the IE value, with special regard to the different mechanism of electron conjugation between the Cp ring and the substituent, namely the σ/π hyperconjugation in EtFC and the π-conjugation in VFC and EFC.     

Symmetry breaking effect in the ferrocene electronic structure by hydrocarbon-monosubstitution: an experimental and theoretical study

We present here the results of a synchrotron radiation-excited UV-photoemission investigation and density functional theory calculations on a structurally related series of organometallic free molecules: ethylferrocene (EtFC), vinylferrocene (VFC), and ethynylferrocene (EFC). This series exemplifies the electronic interactions operating when the C-C substituent group of an aromatic ring is bound to the substrate surface atoms, from a single C-C bond to the double and triple C-C bond pi systems which are still able to preserve substrate-molecule conjugation. A detailed assignment of the gas phase valence photoelectron spectra is discussed, providing new data on the electronic structure of EtFC and EFC and offering a partial reinterpretation of previous assignments on VFC. The broken symmetry of ferrocene caused by the monosubstitution has notable effects on the removal of the molecular orbital (MO) degeneracy which is found to be especially remarkable for the ferrocenelike e(1)' MOs. This effect is ascribed to the interaction between the aromatic cyclopentadyenyl ring and the substituent through sigma/pi hyperconjugation and pi-conjugation mechanisms depending on the nature of the hydrocarbon moiety and its conformational geometry. The vertical ionization energy values of the highest occupied MO for the alkylferrocene and ferrocene free molecules linearly correlate with the redox potential in acetonitrile for ferrocene and the corresponding hybrids obtained by covalently anchoring the free molecule on silicon.     

In situ scanning tunneling microscopy of molecular assemblies of cobalt(II)- and copper(II)-coordinated tetraphenyl porphine and phthalocyanine on Au(100)

Molecules of copper(II) and cobalt(II) 5,10,15,20-tetraphenyl-21H,23H-porphine (CuTPP and CoTPP) and cobalt(II) phthalocyanine (CoPc) are spontaneously adsorbed onto reconstructed Au(100) substrate from a benzene solution containing each individual complex. In situ scanning tunneling microscopy (STM) was used to examine the real-space arrangement and the internal molecular structure of each of the individual molecules in 0.1 M HClO4 under potential control. The adsorption of CuTPP and CoTPP produced the same highly ordered square array with an intermolecular spacing of 1.44 nm on a reconstructed Au(100) surface. These molecular superlattices and the underlying reconstructed Au(100) predominated between 0 and 0.9 V, but lifting of the reconstructed Au(100) surface and elimination of the ordered adlayers occurred at more positive potentials. Molecular resolution STM revealed propeller-shaped admolecule with its center imaged as a protrusion for Co(II) and a depression for Cu(II). In contrast, the spontaneous adsorption of CoPc molecules resulted in a rapid phase transition from the reconstructed Au(100) surface to the (1 x 1) phase, coupled with the production of locally ordered, square-shaped arrays with an intermolecular distance of 1.65 nm. This molecular adlayer and the Au(100)-(1 x 1) remained unchanged when the potential was modulated between 0 and 1.0 V. These results indicate that the subtle variation in the molecular structure of adsorbate influenced not only its spatial arrangement but also the structure of the underlying Au(100) substrate.     

Rational design of quinones for high power density biofuel cells

Enzymatic fuel cells (EFCs) are devices that can produce electrical energy by enzymatic oxidation of energy-dense fuels (such as glucose). When considering bioanode construction for EFCs, it is desirable to use a system with a low onset potential and high catalytic current density. While these two properties are typically mutually exclusive, merging these two properties will significantly enhance EFC performance. We present the rational design and preparation of an alternative naphthoquinone-based redox polymer hydrogel that is able to facilitate enzymatic glucose oxidation at low oxidation potentials while simultaneously producing high catalytic current densities. When coupled with an enzymatic biocathode, the resulting glucose/O₂ EFC possessed an open-circuit potential of 0.864 ± 0.006 V, with an associated maximum current density of 5.4 ± 0.5 mA cm^–2. Moreover, the EFC delivered its maximum power density (2.3 ± 0.2 mW cm^–2) at a high operational potential of 0.55 V.     

Molecular adsorption at well-defined electrode surfaces: hydroquinone on Pd(111) studied by EC-STM

The interaction of hydroquinone (H2Q) with well-defined Pd(111) surfaces at preselected potentials in dilute H2SO4 has been studied by molecule-resolved electrochemical scanning tunneling microscopy (EC-STM). H2Q spontaneously undergoes oxidative chemisorption to benzoquinone (Q), which adopts a slightly tilted parallel orientation. Evidently, the surface coordination is through the quinone pi-electron system. At potentials within the double-layer region, a close-packed well-ordered Pd(111)-(3 x 3)-Q adlattice was formed. A potential excursion to 0.7 V, a potential at which the solution-phase Q/H2Q redox reaction takes place, introduced disorder into the organic adlayer; this positive-potential-induced order-to-disorder phase transition is reversible because the ordered (3 x 3)-Q adlattice was regenerated when the potential reverted to 0.4 V. When the potential was poised at 0.2 V, a potential at which hydrogen evolution was initiated, an appreciable fraction of Q was (hydrogenatively) desorbed; the remnant Q molecules were agglomerated in small islands that retained the (3 x 3) symmetry of the full adlayer. Two possible structural models of the Pd(111)-(3 x 3)-Q adlattice are described.     

Orthogonal self-assembly of interconnected one-dimensional inorganic and organic nanostructures on the Si(100) surface

Orthogonal, interconnected inorganic and organic one-dimensional nanostructures have been fabricated by parallel self-assembly on the Si(100) surface and investigated using room temperature ultrahigh vacuum scanning tunneling microscopy. In particular, bismuth nanowires were self-assembled on the clean Si(100)-2 x 1 surface perpendicular to the Si dimer rows, followed by hydrogen passivation of the surrounding Si surface. Styrene molecular chains were then self-assembled on the H-passivated Si(100)-2 x 1 surface to intersect perpendicularly with the Bi nanowires. This general approach can likely be applied to the wide range of inorganic and organic species that spontaneously form one-dimensional nanostructures on the Si(100) surface.     

Si-C(N) sigma linkages and N --> Si dative bonding at pyridine/Si(111)-7 x 7

We experimentally demonstrated that pyridine/Si(111)-7 x 7 can act as an electron donor/acceptor pair as a result of the charge transfer from the electron-rich N atom of pyridine to the electron-deficient adatom of the Si surface, evidenced by the upshift of 1.8 eV (state A) for the N(1s) core level upon the formation of a datively bonded complex compared to physisorbed molecules. Another state (B) whose N(1s) binding energy downshifts by 1.2 eV was assigned to an adduct through Si-C and Si-N covalent linkages, formed via a [4 + 2]-like addition mechanism on Si(111)-7 x 7. Binding molecules through the formation of the dative bond resulted from significant electron transfer opens a new approach for the creation of Si-based molecular architectures and modification of semiconductor interfacial properties with unsaturated organic molecules.     

Ordered molecular assemblies of substituted bis(phthalocyaninato) rare earth complexes on Au(111): in situ scanning tunneling microscopy and electrochemical studies

Substituted bis(phthalocyaninato) rare earth complexes ML2 (M = Y and Ce; L = [Pc(OC8H17)8]2, where Pc = phthalocyaninato) were adsorbed onto single crystalline Au(111) electrodes from benzene saturated with either YL2 or CeL2 complex at room temperature. In situ scanning tunneling microscopy (STM) and cyclic voltammetry (CV) were used to examine the structures and the redox reactions of these admolecules on Au(111) electrodes in 0.1 mol dm(-3) HClO4. The CVs obtained with YL2- and CeL2-coated Au(111) electrodes respectively contained two and three pairs of redox peaks between 0 and 1.0 V (versus reversible hydrogen electrode). STM molecular resolution revealed that YL2 and CeL2 admolecules were imaged as spherical protrusions separated by 2.3 nm, which suggests that they were oriented with their molecular planes parallel to the unreconstructed Au(111)-(1 x 1). Both molecules when adsorbing from approximately micromolar benzene dosing solutions produced mainly ordered arrays characterized as (8 x 5 radical3)rect (theta = 0.0125). The redox reactions occurring between 0.2 and 1.0 V caused no change in the adlayer, but they were desorbed or oxidized at the negative and positive potential limits. The processes of adsorption and desorption at the negative potentials were reversible to the modulation of potential. Electrochemical impedance spectroscopy (EIS) and CV measurements showed that YL2 and CeL2 adlayers could block the adsorption of perchlorate anions and mediating electron transfer at the Au(111) electrode, leading to the enhancement of charge transfer for the ferro/ferricyanide redox couple.     

Thermal chemistry of styrene on Si(100)2x1 and modified surfaces: electron-mediated condensation oligomerization and posthydrogenation reactions

The room-temperature (RT) adsorption and surface reactions of styrene on Si(100)2x1 have been investigated by thermal desorption spectrometry, low-energy electron diffraction, and Auger electron spectroscopy. Styrene is found to adsorb on Si(100)2x1 at a saturation coverage of 0.5 monolayer, which appears to have little effect on the 2x1 reconstructed surface. The chemisorption of styrene on the 2x1 surface primarily involves bonding through the vinyl group, with less than 15% of the surface moiety involved in bonding through the phenyl group. Except for the 2x1 surface where molecular desorption is also observed, the adsorbed styrene is found to undergo, upon annealing on the 2x1, sputtered and oxidized Si(100) surfaces, different thermally induced processes, including hydrogen abstraction, fragmentation, and/or condensation oligomerization. Condensation oligomerization of styrene has also been observed on Si(100)2x1 upon irradiation by low-energy electrons. In addition, large postexposure of atomic hydrogen to the chemisorbed styrene leads to Si-C bond cleavage and the formation of phenylethyl adspecies. Hydrogen therefore plays a decisive role in stabilizing and manipulating the processes of different surface reactions by facilitating different surface structures of Si.     

Kinetics of redox polymer-mediated enzyme electrodes

Oxygen-reducing enzyme electrodes are prepared from laccase of Trametes versicolor and a series of osmium-based redox polymer mediators covering a range of redox potentials from 0.11 to 0.85 V. Experimentally obtained current density generated by the film electrodes is analyzed using a one-dimensional numerical model to obtain kinetic parameters. The bimolecular rate constant for mediation is found to vary with mediator redox potential from 250 s(-1) M(-1) when mediator and enzyme are close in redox potential to 9.4 x 10(4) s(-1) M(-1) when the redox potential difference is large. The value of the bimolecular rate constant for the simultaneously occurring laccase-oxygen reaction is found to be 2.4 x 10(5) s(-1) M(-1). The relationship between mediator-enzyme overpotential and bimolecular rate constant is used to determine the optimum mediator redox potential for maximum power output of a hypothetical biofuel cell with a planar cathode and a reversible hydrogen anode. For laccase of T. versicolor (E(e)(0) = 0.82), the optimum mediator potential is 0.66 V (SHE), and a molecular structure is presented to achieve this result.     

Surface grafting and reactivity of calixarene-based receptors and pseudorotaxanes on Si(100)

The first report is given here on the anchoring on H-Si(100) of calix[4]arenes and calix[6]arene-based pseudorotaxanes, versatile building blocks for molecular devices. Covalent functionalization on Si was reached through a wet chemistry recipe, by making use of an extra-mild photochemical activation via visible light of C=C terminated anchoring arms. Our approach largely preserves the integrity of the molecular substrate, also allowing for a full monolayer of pseudorotaxane to be formed on Si(100). Molecular adhesion has been demonstrated by the presence and quantitation of XPS signals from specific elements in the molecules. AFM measurements performed on Si(100)/calix[4]arenes have revealed structures 2.3 nm high, consistent with the length of the molecule. The availability of the calix[4]arene cavity to host further species after anchoring on Si has been demonstrated by the successful complexation reaction with Cs(+) ions, resulting in a 1:1 calix/Cs(+) ratio, and with N-methyl pyridinium iodide. A pseudorotaxane species, composed of a calix[6]arene wheel derivatised with N-phenylureido groups on the upper rim and a viologen (4,4'-bipyridinium) containing axle, has been anchored on Si(100) via the C=C termination of the axle. We demonstrated the self-assembling of this pseudorotaxane covalently bound by use of XPS.     

Controlled fabrication of 1D molecular lines across the dimer rows on the Si(100)-(2 x 1)-H surface through the radical chain reaction

Current interest in the fabrication of organic nanostructures on silicon surface is focused on the self-directed growth of 1D molecular lines with predefined position, structure, composition, and the length on the H-terminated Si(100)-(2 x 1) surface. To date, no studies have succeeded in growing the molecular line across the dimer rows on Si(100)-(2 x 1)-H, which is highly desirable. Using scanning tunneling microscopy (STM), we studied a new molecular system (allyl mercaptan, CH2=CH-CH2-SH) that undergoes chain reaction across the dimer row on the Si(100)-(2 x 1)-H surface at 300 K and produces covalently bonded 1D molecular lines. In combination with the previous findings of chain reaction along the rows, the present observations of self-directed growth of 1D molecular lines across the dimer rows on the Si(100)-(2 x 1)-H surface provide a means to connect any two points (through molecular lines) on a 2D surface.     

Reaction mechanism of cis-1,3-butadiene addition to the Si(100)-2 x 1 surface

A set of 40 finite temperature ab initio molecular dynamics trajectories is employed to investigate the distribution of addition products and underlying microscopic mechanism of the addition of 1,3-butadiene to the Si(100)-2 x 1 surface. The product yields are in good agreement with recent STM measurements and include a Diels-Alder [4 + 2] adduct with a surface dimer acting as the dienophile, a [4 + 2]-like adduct that bridges two dimers within a row, a [4 + 2]-like adduct that bridges two dimers in adjacent rows, and an interdimer [2 + 2]-like adduct. The trajectories indicate that a common mechanism underlies the distribution and is predominantly a nonconcerted stepwise mechanism that proceeds via an intermediate zwitterion composed of a carbocation bonded to a negatively charged surface dimer.     

Measurements of electron-transfer rates of charge-storage molecular monolayers on Si(100). Toward hybrid molecular/semiconductor information storage devices

Redox kinetics were measured for two electroactive molecules attached to Si(100) surfaces, a ferrocene (Fc-BzOH) and a Zn(II) trimesitylporphyrin (Por-BzOH). Each molecule was derivatized with a benzyl alcohol linker for attachment to the Si surface via the formation of a Si-O bond. A complete protocol was developed for the preparation of stable Si(100) surfaces derivatized with the electroactive molecules. The redox-kinetic measurements were performed on the resulting Fc-BzOH and Por-BzOH monolayers to probe (1) the rate of electron transfer (k0) for oxidation in the presence of applied potentials and (2) the rate of charge dissipation after the applied potential is disconnected (in the form of a charge-retention half-life t1/2). The k0 values for the two types of monolayers were found to be similar to one another as were the t1/2 values. Perhaps more importantly, the electron-transfer rates for both the Fc-BzOH and the Por-BzOH monolayers differ from the charge-dissipation rates by approximately 6 orders of magnitude and are strongly dependent on the surface concentration of the electroactive species. For the Por-BzOH monolayers on Si(100), the k0 and t1/2 values and their trends as a function of surface coverage were determined to be similar to those previously measured for the analogous thiol-derivatized molecule assembled on Au(111). In contrast, the Fc-BzOH monolayers on Si(100) were found to exhibit much slower electron-transfer and charge-dissipation rates than those in the corresponding thiol-Au(111) case. Two alternative hypotheses are advanced to explain both the diminution in rates with increased surface coverage and the contrasting behavior with the analogous thiols on Au, one based on space-charge effects at the monolayer-solution interface, and a second relying on changes in distance of the redox centers from the surface as modulated by the orientation of the linking chains. Collectively, the ability to prepare and study stable, electroactive molecular media on Si(100) is likely to be key in the development of hybrid molecular/semiconductor devices.     

Difficulty for oxygen to incorporate into the silicon network during initial O2 oxidation of Si(100)-(2x1)

First principles calculations and scanning tunneling microscopy studies of the oxidation of Si(100)-(2x1) surfaces by molecular oxygen reveal that the surface silanone (O)(Si=O) species is remarkably stable, constituting the key intermediate for initial oxidation. The propensity for oxygen to remain within the top surface layer as opposed to incorporating within Si-Si backbonds is surprisingly high. This resistance to incorporation into a cubic lattice even at higher coverages could be a factor to facilitate surface amorphization in subsequent steps.     

Vinyltrimethylsilane (VTMS) as a probe of chemical reactivity of a TiCN diffusion barrier-covered silicon surface

This paper presents the first molecular level investigation of chemical reactivity of a surface of an amorphous diffusion barrier film deposited on a Si(100)-2 x 1 single crystal. Vinyltrimethylsilane (VTMS) is chosen as a probe molecule because of its chemical properties and because of its role as a ligand in a common copper deposition precursor, hexafluoroacetylacetonato-copper-vinyltrimethylsilane, (hfac)Cu(VTMS). The surface chemistry of vinyltrimethylsilane on titanium carbonitride-covered Si(100)-2 x 1 has been investigated using multiple internal reflection Fourier transform infrared spectroscopy (MIR-FTIR), Auger electron spectroscopy (AES), thermal desorption mass spectrometry, and computational analysis. On a film with nominal surface stoichiometry TiC(x)N(y) (x approximately y approximately 1) preannealed to 800 K, VTMS adsorbs molecularly at cryogenic temperatures even at submonolayer coverages; the major pathway for its temperature-programmed evolution is desorption. Adsorption at room temperature leads to chemisorption via a double-bond attachment. A set of computational models was designed to investigate the possible adsorption sites for a VTMS molecule on a TiCN-covered Si(100)-2 x 1 surface. A comparison of the computational predictions for a variety of possible adsorption sites with the results of thermal desorption and infrared measurements suggests that approximately 90% of the adsorbed VTMS is chemisorbed along the Ti-C bond while approximately 10% is chemisorbed on a Ti corner atom, the minority site of the surface. The Ti-N bond is not participating in the chemisorption process.     

Benzidine Derivatives: A Class of High Redox Potential Molecules for Aqueous Organic Flow Batteries

The development of water-soluble redox-active molecules with high potentials is one of the effective ways to enhance the energy density of aqueous organic flow batteries (AOFBs). Herein, a series of promising N-substituted benzidine analogues as water-soluble catholyte candidates with controllable redox potentials (0.78–1.01 V vs. standard hydrogen electrode (SHE)) were obtained by the molecular engineering of aqueous irreversible benzidines. Theoretical calculations reveal that the redox potentials of these benzidine derivatives in acidic solution are determined by their electronic structure and alkalinity. Among these benzidine derivatives, N,N,N′,N′-tetraethylbenzidine(TEB) shows both high redox potential (0.82 V vs. SHE) and good solubility (1.1 M). Pairing with H₄[Si(W₃O₁₀)₄] anolyte, the cell displayed discharge capacity retention of 99.4 % per cycle and a high coulombic efficiency (CE) of ∼100 % over 1200 cycles. The stable discharge capacity of 41.8 Ah L⁻¹ was achieved at the 1.0 M TEB catholyte with a CE of 97.2 % and energy efficiency (EE) of 91.2 %, demonstrating that N-substituted benzidines could be promising for AOFBs.     

Azide reactions for controlling clean silicon surface chemistry: benzylazide on Si(100)-2 x 1

A combination of experimental and computational studies presents direct proof of a novel reaction pathway that delivers aromatic compounds onto a Si(100)-2 x 1 substrate. Benzylazide chemisorbs on a Si(100)-2 x 1 surface, and this chemisorption is followed by nitrogen elimination, leading to a stable surface adduct based on a Si-Si-N cyclic entity. This reaction occurs via a stable surface intermediate with the surface-bound nitrogen molecule stabilized by the presence of a neighboring aromatic group, which eventually releases nitrogen into the gas phase and forms the final product.     

Structural and electron-transfer characteristics of O-, S-, and Se-tethered porphyrin monolayers on Si(100)

Monolayers of two classes of Zn porphyrins have been prepared and examined on Si(100). These molecules, designated as ZnPBzX- and ZnPCH2X-, contain either a benzyl (-Bz-) or a methylene (-CH2-) unit terminated with a Group VI atom (X = O, S, Se) appended to a meso-position of the porphyrin, with the nonlinking meso-substituents consisting of either mesityl (-Bz- class) or p-tolyl and phenyl (-CH2- class) units. The two series of ZnPBzX- and ZnPCH2X- monolayers on Si(100) were examined using a variety of techniques, including X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and various electrochemical methods. The studies reveal the following characteristics of the ZnPBzX- and ZnPCH2X- monolayers. (1) Surface binding can be readily achieved to Si(100) with both relatively short (-Bz-) and very short (-CH2-) tethers regardless of the nature of the Group VI anchoring atom (O, S, Se). (2) The longer -Bz- tether affords monolayers with the porphyrin ring in a somewhat more upright orientation with respect to the surface than the shorter -CH2- tether. The more upright adsorption geometry of the porphyrins bearing the former type of linker leads to a higher packing density and more homogeneous redox thermodynamics. (3) The kinetics of electron transfer does not depend on the type of Group VI atom used for anchoring to the Si(100) surface. On the other hand, the type of linker does affect the electron-transfer rates, with the monolayers bearing the -CH2- linker exhibiting systematically faster rates than those bearing the -Bz- linker. Collectively, the studies reported herein provide a detailed picture of how the anchor atom and the linker type influence the structural and electron-transfer characteristics of these general classes of monolayers.     

Elucidation of the redox behavior of 2,5-dimercapto-1,3,4-thiadiazole (DMcT) at poly(3,4-ethylenedioxythiophene) (PEDOT)-modified electrodes and application of the DMcT-PEDOT composite cathodes to lithium/lithium ion batteries

The redox reactions of DMcT at PEDOT-modified glassy carbon electrodes (GCEs) in acetonitrile (AN) have been investigated via cyclic voltammetry (CV) and the electrochemical quartz crystal microbalance (EQCM) in order to elucidate the redox reaction mechanism. A redox couple at -0.29 V versus Ag/Ag+ was assigned to the dimerization process of singly protonated DMcT (DMcT-1H), and a second couple observed at +0.42 V was assigned to the polymerization process of the protonated DMcT dimer. Our investigations revealed further that the anodic current response at +0.55 V (polymerization process) has a shoulder at +0.38 V ascribed to the dimerization process of doubly protonated DMcT (DMcT-2H), indicating that the redox couple at +0.42 V is the overlapping response of the polymerization of the protonated DMcT dimer and the dimerization of the DMcT-2H monomer. It was also confirmed that the dimerization process of DMcT-1H at -0.29 V proceeded not only at the surface of a PEDOT film but also inside the film as previously suggested. Moreover, the thermodynamics of these redox reactions at PEDOT-modified GCEs are dependent on the basicity (or acidity) of the solution, as anticipated and previously shown at unmodified GCEs. The oxidation of DMcT occurs at less positive potentials and the reduction occurs at more negative potentials in the presence of base. On the basis of the results obtained, the full redox reaction scheme for DMcT at a PEDOT-modified GCE is proposed.