Polycrystalline Au Substrates Research Articles

Electronic and Optoelectronic Monolayer WSe2 Devices via Transfer-Free Fabrication Method.

Monolayer transition metal dichalcogenides (TMDs) have drawn significant attention for their potential applications in electronics and optoelectronics. To achieve consistent electronic properties and high device yield, uniform large monolayer crystals are crucial. In this report, we describe the growth of high-quality and uniform monolayer WSe2 film using chemical vapor deposition on polycrystalline Au substrates. This method allows for the fabrication of continuous large-area WSe2 film with large-size domains. Additionally, a novel transfer-free method is used to fabricate field-effect transistors (FETs) based on the as-grown WSe2. The exceptional metal/semiconductor interfaces achieved through this fabrication method result in monolayer WSe2 FETs with extraordinary electrical performance comparable to those with thermal deposition electrodes, with a high mobility of up to ≈62.95 cm2 V-1 s-1 at room temperature. In addition, the as-fabricated transfer-free devices can maintain their original performance after weeks without obvious device decay. The transfer-free WSe2-based photodetectors exhibit prominent photoresponse with a high photoresponsivity of ~1.7 × 104 A W-1 at Vds = 1 V and Vg = -60 V and a maximum detectivity value of ~1.2 × 1013 Jones. Our study presents a robust pathway for the growth of high-quality monolayer TMDs thin films and large-scale device fabrication.

Hole-injection barrier across the intermolecular interaction mediated interfacial DNTT layer

The electronic structures and morphology of dinaphthothienothiophene (DNTT) thin films on highly oriented pyrolytic graphite (HOPG) and polycrystalline Au (pc-Au) substrates have been investigated using photoelectron spectroscopy and atomic force microscopy techniques to better correlate the charge-injection barrier with the interfacial structure. DNTT molecules follow Stranski–Krastanov-type growth mode, i.e., highly-dewetted islands/fibers above a relatively-wetted interfacial layer, on both the substrates, but distinctively different electronic structures. On the HOPG surface, the as-grown and thermal-annealed films show a broad and a sharp highest occupied molecular orbital (HOMO) bands, respectively. On the pc-Au surface, a significantly split HOMO band and a reduced hole-injection barrier are observed in a monolayer thick film, for the first time, which are absent in other thick films. Such splitting indicates a presence of a strong intermolecular interaction in the interfacial layer, presumably due to the frontier π-orbital overlapping arising from partially-covered densely-packed herringbone-like arranged molecules. The dewetted-fibers seem to form with up-right oriented molecules having negligible interaction, which desorb easily by thermal annealing to retain the highly-stable interfacial layer. The reduction of the hole-injection barrier at the DNTT/pc-Au interface, due to the formation of such interfacial layer having strong intermolecular interaction, has an immense importance in increasing the charge-injection efficiency from a practical electrode.

Effect of the Alloying Metal on the Corrosion Resistance of Pd-Rich Binary Alloys with Pt, Rh, and Ru in Sulfuric Acid.

The paper presents the study of the corrosion resistance of electrodeposited Pd and its binary alloys with Pt, Rh, and Ru on a polycrystalline Au substrate. The corrosion resistance was tested in 0.5 M sulfuric acid at room temperature using potentiodynamic polarization and electrochemical impedance spectroscopy techniques. The morphology/composition and work function values were determined by scanning electron microscopy/energy–dispersive X–ray spectroscopy and scanning Kelvin probe, respectively. The obtained results revealed that the Pd electrode is the most resistant to corrosion, whereas the Pd-Ru electrode is the most susceptible to dissolution. It was found that the corrosion resistance of Pd-binary alloys decrease in the following order: Pd > Pd-Pt > Pd-Rh > Pd-Ru. This effect was assigned mainly to the difference in surface roughness factor of tested electrodes.

Evolution of Electronic Structures of Polar Phthalocyanine-Substrate Interfaces.

The electronic structures and core-level spectra of chlorogallium phthalocyanine (ClGaPc) molecules of different thicknesses (submonolayer to multilayer) adsorbed on a polycrystalline Au substrate and a highly oriented pyrolytic graphite (HOPG) substrate, before and after thermal annealing, were investigated using photoelectron spectroscopic techniques for better understanding the charge-transfer properties. The energy-level diagrams (ELDs) of the ClGaPc thin films are found to evolve with film thickness, substrate nature, and thermal annealing. The interfacial dipole moment in the active Au substrate and the molecular dipole moment in the inactive HOPG substrate mainly dictate the ELD. Annealed monolayer films on both the substrates seem to adopt a similar well-ordered Cl-up orientated molecular organization, which is quite interesting, as it certainly indicates a substrate-nature-independent energy minimum configuration. The strong interaction of the active Au substrate gives rise to additional charge transfer and state transfer (of Ga) as evident from the formation of a former lowest unoccupied molecular orbital (F-LUMO) level in the highest occupied molecular orbital (HOMO) region and a low binding energy peak in the Ga 2p3/2 core level. The presence of strong F-LUMO and molecular-dipole-related HOMOd levels in the predicted monolayer of well-ordered Cl-up oriented molecules on the Au and HOPG substrates, respectively, creates the optimum energy-level alignment (ELA) for both the systems, while the opposite shift of the vacuum levels in two different substrates makes the ionization potential (IP) for such a monolayer either minimum (on the Au substrate) or maximum (on the HOPG substrate), which is useful information for tuning the charge injection across the interface in organic semiconductor-based devices.

Controlling Chemical Selectivity in Electrocatalysis with Chiral CuO-Coated Electrodes

This work demonstrates the chiral-induced spin selectivity effect for inorganic copper oxide films and exploits it to enhance the chemical selectivity in electrocatalytic water splitting. Chiral CuO films are electrodeposited on a polycrystalline Au substrate, and their spin filtering effect on electrons is demonstrated using Mott polarimetry analysis of photoelectrons. CuO is known to act as an electrocatalyst for the oxygen evolution reaction; however, it also generates side products such as H2O2. We show that chiral CuO is selective for O2; H2O2 generation is strongly suppressed on chiral CuO but is present with achiral CuO. The selectivity is rationalized in terms of the electron spin-filtering properties of the chiral CuO and the spin constraints for the generation of triplet oxygen. These findings represent an important step toward the development of all-inorganic chiral materials for electron spin filtering and the creation of efficient, spin-selective (photo)electrocatalysts for water splitting.

Growth mechanism and magnetic properties of dendritic nanostructure prepared by pulse electrodeposition

CoCu dendrites were synthesized by pulse direct current electrodeposition (DC-PED) on polycrystalline Au substrate from a Co and Cu-sulphamate-based solution in a three-electrode system. Scanning electron microscopy images showed that the CoCu dendrites consist of a long central backbone with secondary branches, which were composed of many neighbouring cubic-like nanoparticles. Tuning the pulse time was believed to be the key for the formation of a dendritic nanostructure. The X-ray diffraction (XRD) patterns indicated that the dendrite was composed of a solid solution of face-centered cubic CoCu with preferred orientation of {111} planes and a face-centered cubic Co phase. The energy dispersive spectrometry (EDS) demonstrated that Co and Cu element composition distributed uniformly in the dendrite. Our results demonstrate that the co-deposition of Co and Cu ions is possible during the pulse electrodeposition process and which was dominated by the mechanism of thermodynamics and kinetics perspectives. Magnetic measurements showed that the CoCu dendritic films exhibit ferromagnetism and easy-axis direction of the magnetization is perpendicular to the film plane.

The Enhancement of the Interface Reaction in LiFePO4 Two-Phase System

LiFePO4 is considered as a high rate capability cathode material for lithium-ion battery. During charging and discharging, the material undergoes a two-phase reaction between Li-rich and Li-poor phases1. The rate-determining step in the phase transition of LixFePO4 is expected to be the nucleation reaction2. Recently, it is reported that surface modification of LiFePO4 with nitrogen improves the rate capability3. However, the origin of the improvement in rate performance still has not been understood well. This is because it is difficult to analyze the interfacial reaction using composite electrodes, which have complex structures of mixture with active material, carbon, and binder. To discuss correlation between the surface modification and reaction kinetics of LiFePO4, we made surface modified LiFePO4 thin-film electrode with nitrogen, and investigated the surface electronic structure of the electrode by total-reflection fluorescence X-ray absorption spectroscopy (TRF-XAS).LiFePO4 thin-film (Bare-LFP) was prepared on a polycrystalline Au substrate by pulsed laser deposition method. The deposition was conducted under Ar atmosphere (0.0001 Pa) at 923K for 30 min. Surface modified LiFePO4 thin-film with nitrogen (N-LFP) was prepared by heating the Bare-LFP under NH3 atmosphere at 923K for 10 min. For the electrochemical analysis, three-electrode cells employing Li metal as a counter and a reference electrode, and 1 M LiClO4 in propylene carbonate as a electrolyte were prepared. Bare-LFP was characterized by cyclic voltammetry and transmission electron microscope (TEM). N-LFP was characterized by N-K edge XAS. The rate capability of Bare-LFP and N-LFP estimated charge/discharge measurements from 1C to 100C.We also discuss the reaction kinetics of LiFePO4 with anion surface modification to enhance rate performance. The reaction kinetics of total process included charge transfer was calculated by cyclic voltammetry (CV) and of phase transition by potentiostatic intermittent titration technique (PITT). The result of CV indicated better kinetics by nitrogen surface modification than by undoped and of PITT corresponded with them.The results support improvement of nucleation kinetics.Here we report a characterization of surfaces of LiFePO4 thin-film model electrodes by TRF-XAS that shows the slightly higher oxidation state of iron by nitrogen surface modification than by undoped. This result indicates that the existence of Fe(2+δ)+at the surface decreases the nucleation energy as well as moderates the lattice strain, leading to better battery performance. The facilitation of phase transition, rather than particle size minimization or intimate carbon contact reforming bulk structures, is a novel approach to enhance reaction kinetics.1. Padhi, A. K.; Nanjundaswamy, K. S.; Goodenough, J. B. J. Electrochem. Soc., 144, 1188,(1997). 2. Delmas, C.; Maccario, M.; Croguennec, L.; Le Cras, F.; Weill, F. Nat Mater, 7, 665,(2008). 3. Park, K. S.; Xiao, P. H.; Kim, S. Y.; Dylla, A.; Choi, Y. M.; Henkelman, G.; Stevenson, K. J.; Goodenough, J. B. Chem Mater, 24, 3212,(2012). This work was partially supported by KAKENHI Grant Number 25249141.

Enhanced Rate Capability in Two Phase Reaction in LiFePO4 By Interfacial Modification Between Cathode and Electrolyte

Olivine-type LiFePO4 is considered as a promising cathode material for high power density lithium-ion battery. Lithium-ion intercalation proceeds thorough a two-phase reaction divided into LiFePO4 and FePO4. During the phase transition between LiFePO4 and FePO4, the nucleation of a new phase occurs at the electrode / electrolyte interface, which is reported to be the rate determining reaction.1 To improve the rate capability of LiFePO4, Park et al performed the surface modification by nitrogen on LiFePO4 particles.2 However, their improved mechanism is not clearly understood due to the difficulty in the observation of electrode / electrolyte interface. In order to analyze the electrode / electrolyte interface, thin-film electrodes and surface sensitive X-ray absorption spectroscopy are powerful tool.3 In this study, we prepared LiFePO4thin-film model electrodes to clarify the reaction kinetics and applied the interfacial modification of nitrogen. Their reaction mechanism is discussed with the information of X-ray absorption spectroscopy. LiFePO4 thin-film was prepared on a polycrystalline Au substrate by pulsed laser deposition method. The deposition was conducted under Ar atmosphere (0.0001 Pa) at 923K for 30 min. The prepared thin films were annealed under ammonia gas at 873 K. The obtained thin-film was characterized by X-ray diffraction and TEM. X-ray absorption spectroscopy at Co K-edge was performed at BL37XU, Spring-8. N K-edge X-ray absorption measurements were performed in Ritsumeikan SR-center. For the electrochemical analysis, three-electrode cells employing Li metal as a counter and a reference electrode, and 1 M LiClO4in propylene carbonate liquid electrolyte were prepared. Annealing of ammonia gas introduces nitrogen into LiFePO4 surface, which is confirmed by N K-edge X-ray absorption spectra. The rate capability of the nitrogen modified LiFePO4 thin-film electrodes is improved. The surface sensitive Fe K-edge X-ray absorption spectra show the increasing of oxidation state in iron, which implies the lattice mismatch is decreased in surface of LiFePO4. Therefore, nucleation reaction is enhanced during two phase reaction between LiFePO4 and FePO4, which is the origin of surface modification effect.

Bulk-Palladium and Palladium-on-Gold Electrocatalysts for the Oxidation of Hydrogen in Alkaline Electrolyte

The commercial feasibility of alkaline-exchange membrane fuel cells and electrolyzers passes by the development of hydrogen oxidation and evolution reaction (HOR/HER) catalysts featuring an activity and/or cost advantage over platinum, which remains the most active metal for these processes. Among these alternatives, Pd appears as a promising candidate, since its price is typically 2–3 fold lower than that of Pt. With this motivation, the first section of this study displays our attempts at quantifying the kinetic parameters of the HOR/HER on bulk Pd in 0.1 M NaOH, which were prevented by the simultaneous absorption of hydrogen into bulk palladium. We succeeded at circumventing this issue by depositing Pd-adlayers on a polycrystalline Au-substrate by galvanic displacement of underpotentially-deposited Cu or by electrochemical plating of Pd2+. The resulting surfaces appear to consist of three-dimensional Pd-structures of an unknown thickness that we believe to scale with the palladium coverage, θPd/Au. This last parameter is inversely proportional to the HOR/HER-activity of the Pd-on-Au surfaces, in agreement with numerous theoretical and experimental studies in acid media that correlate this effect to the tensile strain induced by the Au-substrate on the Pd-lattice.

Formation of Palladium Nanofilms Using Electrochemical Atomic Layer Deposition (E-ALD) with Chloride Complexation

Pd thin films were formed by electrochemical atomic layer deposition (E-ALD) using surface-limited redox replacement (SLRR) of Cu underpotential deposits (UPD) on polycrystalline Au substrates. An automated electrochemical flow deposition system was used to deposit Pd atomic layers using a sequence of steps referred to as a cycle. The initial step was Cu UPD, followed by its exchange for Pd ions at open circuit, and finishing with a blank rinse to complete the cycle. Deposits were formed with up to 75 cycles and displayed proportional deposit thicknesses. Previous reports by this group indicated excess Pd deposition at the flow cell ingress, from electron probe microanalysis (EPMA). Those results suggested that the SLRR mechanism did not involve direct transfer between a Cu(UPD) atom and a Pd(2+) ion that would take its position. Instead, it was proposed that electrons are transferred through the metallic surface to reduce Pd(2+) ions near the surface where their activity is highest. It was proposed that if the cell was filled completely before a significant fraction of the Cu(UPD) atoms had been oxidized then the deposit would be homogeneous. Previous work with EDTA indicated that the hypothesis had merit, but it proved to be very sensitive to the EDTA concentration. In the present study, chloride was used to complex Pd(2+) ions, forming PdCl(4)(2-), to slow the exchange rate. Both complexing agents led to a decrease in the rate of replacement, producing more homogeneous films. Although the use of EDTA improved the homogeneity, it also decreased the deposit thickness by a factor of 3 compared to the thickness obtained via the use of chloride.

Modulation of electrochemical hydrogen evolution rate by araliphatic thiol monolayers on gold

Electroreductive desorption of a highly ordered self-assembled monolayer (SAM) formed by the araliphatic thiol (4-(4-(4-pyridyl)phenyl)phenyl)methanethiol leads to a concurrent rapid hydrogen evolution reaction (HER). The desorption process and resulting interfacial structure were investigated by voltammetric techniques, in situ spectroscopic ellipsometry, and in situ vibrational sum-frequency-generation (SFG) spectroscopy. Voltammetric experiments on SAM-modified electrodes exhibit extraordinarily high peak currents, which differ between Au(111) and polycrystalline Au substrates. Association of reductive desorption with HER is shown to be the origin of the observed excess cathodic charges. The studied SAM preserves its two-dimensional order near Au surface throughout a fast voltammetric scan even when the vertex potential is set several hundred millivolt beyond the desorption potential. A model is developed for the explanation of the observed rapid HER involving ordering and pre-orientation of water present in the nanometer-sized reaction volume between desorbed SAM and the Au electrode, by the structurally extremely stable monolayer, leading to the observed catalysis of the HER.

Rapid Formation of a Dense Sulfur Layer on Gold through Use of Triphenylmethane Sulfenyl Chloride as a Precursor

The use of triphenylmethane sulfenyl chloride as a new precursor leads to the efficient deposition of sulfur on polycrystalline gold and Au(111) substrates. The modified surfaces are characterized using X-ray photoelectron spectroscopy (XPS), electrochemistry and scanning tunneling microscopy (STM). The XPS data shows the rapid deposition of polymeric sulfur within very short times. Electrochemical stripping cyclic voltammetry (CV) confirms the rapid deposition and shows that high coverage values are achieved. STM imaging shows the formation of a wide range sulfur layer and production of the well-known etch pits. High-resolution STM images confirm the high density of the sulfur layers and show formation of a long-range phase consisting of rhombus structures close to the previously described rectangular structures along with other parallelograms and partial parallelograms. The present results do not show the initial formation of any organic self-assembled monolayer (SAM) indicating that the formation of polymeric sulfur does not result from the decomposition of an initial SAM as previously observed with alkyl and aryl thiolate-based SAMs. The suggested mechanism involves an initial reductive process similar to the one reported for thiocyanates and sulfenyl chlorides. This is followed by the dissociation of the Ph(3)C-S bond, leaving only sulfur on the surface, through a process leading to the recombination of the remaining fragments to yield triphenylmethyl chloride.

HCOOH oxidation on thin Pd layers on Au: Self-poisoning by the subsequent reaction of the reaction product

The oxidation of HCOOH has been investigated on thin Pd layers ranging from 1 to 17 ML equivalents electrodeposited on polycrystalline Au substrate (Pd@Au). The results are compared to those on Pd black. Potentiodynamic polarization curves suggest that on both types of catalyst HCOOH oxidizes through dehydrogenation path. The high current densities achieved on 4–17 ML Pd@Au surpass more than three times the activity of Pd black. Chronoamperometric test reveals that the Pd@Au electrodes lose their activity faster than Pd black. In the stripping experiment performed after the chronoamperometry COads is detected on all the surfaces. On the 4 ML Pd@Au, which exhibits the highest deactivation rate, the COads coverage is the largest. After the chronoamperometric test repeated in the CO2 saturated supporting electrolyte, the anodic stripping shows that COads is present on all the electrodes, again with the highest coverage on 4 ML Pd@Au. It is concluded that deactivation of Pd surfaces is caused by the COads formed in the electrochemical reduction of CO2, which is the product of HCOOH oxidation. Electronic modification of Pd by Au substrate causes stronger interactions of Pd with HCOOH and CO, which increases HCOOH oxidation rate, but also accelerates the poisoning by COads.

Influence of Surface Morphology and Substrate on Thermal Stability and Desorption Behavior of Octanethiol Self-Assembled Monolayers: Cu, Ag, and Au

The formation and thermal desorption behaviors of octanethiol (OT) SAMs on single crystalline Au(111) and polycrystalline Au, Ag, and Cu substrates were examined by X-ray photoelectron microscopy (XPS), thermal desorption spectroscopy (TDS), and contact angle (CA) measurements. XPS and CA measurements revealed that the adsorption of OT molecules on these metals led to the formation of chemisorbed self-assembled monolayers (SAMs). Three main desorption fragments for dioctyl disulfide (C8SSC8+, dimer), octanethiolate (C8S+), and octanethiol (C8SH+) were monitored using TDS to understand the effects of surface morphology and the nature of metal substrates on the thermal desorption behavior of alkanethiols. TDS measurements showed that a sharp dimer peak with a very strong intensity on single crystalline Au(111) surface was dominantly observed at 370 K, whereas a broad peak on the polycrystalline Au surface was observed at 405 K. On the other hand, desorption behaviors of octanethiolates and octanethiols were quite similar. We concluded that substrate morphology strongly affects the dimerization process of alkanethiolates on Au surfaces. We also found that desorption intensity of the dimer is in the order of Au ≫ Ag > Cu, suggesting that the dimerization process occurs efficiently when the sulfur–metal bond has a more covalent character (Au) rather than an ionic character (Ag and Cu). The relative desorption intensity of the octanethiolates to the octanethiols follows the order of bond strength, Cu > Au > Ag. Alkanethiolates may be a dominant desorption product as metal–sulfur bonds become stronger. In this study, we clearly demonstrate that the thermal stability and desorption behaviors of alkanethiol SAMs are strongly influenced by the surface morphologies of metal substrates, bonding character of the sulfurs, the bond strength of metal–sulfur, and van der Waals interactions. Our results provide new insights into understanding the thermal stability and desorption behaviors of alkanethiols on Au, Ag, and Cu surfaces.

Photoelectron spectroscopic study on the interaction of l-cysteine with the silver-based dental alloy

The electronic structure of the silver-based dental alloy MC12 and its constituent metals and their interaction with l-cysteine have been investigated by ultraviolet photoelectron spectroscopy (UPS) using synchrotron radiation. The UPS spectra of l-cysteine on polycrystalline Ag, Cu, Pd, and Au substrates have been measured to understand the interaction of l-cysteine with the dental alloy surface. It was found that the electronic states of MC12 originate dominantly from Cu 3d states and Pd 4d states around the top of the valence bands, while the 4–7-eV electronic structure of MC12 originates from the Ag 4 d5/2 and Ag 4 d3/2 states. For l-cysteine, it was found that a new peak at 2 eV is observed for the thin films on Ag, Cu, and Au, while the structure around 2 eV on MC12 is similar to that on Pd. The shift of the 5-eV peak is observed for the thin films on Cu, Pd, and Au, but not on Ag and MC12. These results indicate that the interaction of l-cysteine with MC12 may be dominantly due to the Pd-S, Cu-S, and Ag-O bonds, while the contribution of the Ag-S bond is small.

The ethanol electrooxidation at Pt layers deposited on polycrystalline Au

The ethanol electro-oxidation reaction was evaluated using a polycrystalline Au substrate modified with two different amounts of Pt using the galvanic exchange methodology. FTIR results suggest that Pt deposits have a greater ability to break the C-C bond present in the ethanol molecule. However, under potentiostatic conditions both modified Au surfaces undergo faster deactivation in comparison with polycrystalline platinum as indicated by the chronoamperometric results. XPS results indicate the presence of two phases depending on the Pt content. These are: (i) Pt-Au alloy and (ii) segregated Pt. The structural and electronic properties of these phases were related to the differences observed in the catalytic activity.

Thermodynamic Studies of PEG (Mw 20,000) Adsorption onto a Polycrystalline Gold Electrode

Cyclic voltammetry, differential capacity, and chronocoulometry were employed in a quantitative study of polyethylene glycol (Mw 20000) (PEG20000) adsorption onto a polycrystalline Au substrate. The surface tension, film pressure, relative Gibbs surface excess, and the Gibbs energy of adsorption were determined as a function of the electrode potential. The results indicated that, in the potential interval studied (−0.9 to 0.35 V vs saturated calomel electrode), PEG20000 adsorbed onto the polycrystalline Au surface via its oxygen atoms as a flat submonolayer of rodlike molecules. The adsorbed mass increased considerably with the increasing PEG20000 concentration and more positive potential. In addition, the surface tension decreased as the quantity of PEG20000 adsorbed on the surface rose with the increasing PEG20000 concentration, an effect characteristic of surface active agents. The maximum surface concentration was found to be 4.59 × 10−10 moloxygen cm−2 for the highest PEG20000 concentration studied (100 μM). The Gibbs energy of adsorption (ΔGads) was determined using the Henry adsorption isotherm. On the basis of the ΔGads values obtained, it was concluded that the PEG20000 molecules were weakly chemisorbed on the polycrystalline Au surface.

Analysis of Electrodeposited Nickel-Iron Alloy Film Composition Using Particle-Induced X-Ray Emission

The elemental composition of electrodeposited NiFe thin films was analyzed with particle-induced X-ray emission (PIXE). The thin films were electrodeposited on polycrystalline Au substrates from a 100 mM NiSO4, 10 mM FeSO4, 0.5 M H3BO3, and 1 M Na2SO4solution. PIXE spectra of these films were analyzed to obtain relative amounts of Ni and Fe as a function of deposition potential and deposition time. The results show that PIXE can measure the total deposited metal in a sample over at least four orders of magnitude with similar fractional uncertainties. The technique is also sensitive enough to observe the variations in alloy composition due to sample nonuniformity or variations in deposition parameters.

Interface doping of conjugated organic films by means of diffusion of atomic components from the surfaces of semiconductors and of metal oxides

The paper reports the results on the interface formation of 5–10 nm thick conjugated layers of Cu-phthalocyanine (CuPc) with a number of solid surfaces: polycrystalline Au, (SiO 2) n–Si, ZnO ( 0 0 0 1 ¯ ) , Si(1 0 0), Ge(1 1 1), CdS ( 0 0 0 1 ¯ ) and GaAs(1 0 0). The results were obtained using Auger electron spectroscopy (AES) and low-energy target current electron spectroscopy (TCS). The organic overlayers were thermally deposited in situ in UHV onto substrate surfaces. The island-like organic deposits were excluded from the analysis so that only uniform organic deposits were considered. In the cases of polycrystalline Au, Si(1 0 0) and Ge(1 1 1) substrates the AES peaks of the substrate material attenuated down to the zero noise level upon the increase of the CuPc film thickness of 8–10 nm. The peaks corresponding to oxygen atoms in the case of SiO 2 substrate, and to atoms from the ZnO, GaAs and CdS substrates were clearly registered in the AES spectra of the 8–10 nm thick CuPc deposits. The relative concentration of the substrate atomic components diffused into the film was different from their relative concentration at the pure substrate surface. The concentration of the substrate dopant atoms in the CuPc film was estimated as one atom per one CuPc molecule. Using the target current electron spectroscopy, it was shown that the substrate atoms admixed in the CuPc film account for the appearance of a new peak in the density of unoccupied electronic states. Formation of intermediate TCS spectra until the CuPc deposit reaches 2–3 nm was observed in the cases of GaAs(1 0 0), ZnO ( 0 0 0 1 ¯ ) , Ge(1 1 1) surfaces. The intermediate spectra show a less pronounced peak structure different from the one typical for the CuPc films. It was suggested that the intermediate layer was formed by the CuPc molecules fully or partially decomposed due to the interaction with the relatively reactive semiconductor surfaces.

Studies on the interactions between bovine β-lactoglobulin and chitosan at the solid–liquid interface

Chitosan ultrathin films have been formed on polycrystalline Au substrates using the LbL technique with the purpose of studying its interaction with bovine β-lactoglobulin (β-LG) at the solid–liquid interface. The immobilization of chitosan was followed by Quartz Crystal Microbalance with energy dissipation (QCM-D), Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS). The behavior of the chitosan films in the presence of β-LG solutions with different bulk concentrations ([β-LG]), ionic strength ( I), and pH has been investigated using the same techniques plus Atomic Force Microscopy (AFM). The results showed that for pHs lower than protein's p I, weak intermolecular forces (H bonding, Van der Waals, hydrophobic, etc.) are established between β-LG and chitosan (especially close to the p I) leading to low coverage nonspecific adsorption. On the contrary when pH > p I, strong ionic bonding through attractive electrostatic interactions lead to high coverage adsorbed phases composed of large β-LG aggregates. The adsorption process was shown to consist of a relatively fast step (in which these interactions are predominant) which is followed, once the β-LG monolayer is exceeded, by the slow formation of thicker and increasingly viscoelastic films through β-LG self-aggregation. QCM-D and AFM experiments unveiled the role of [β-LG] and I on the formation of these aggregates. The adsorption isotherm built from impedance data in the medium-low [β-LG] range (0.001–0.3 mg mL −1), showed good fitting to the Langmuir model confirming that the formation of one β-LG monolayer is achieved in this concentration range.