Sub-micron scale patterning using femtosecond laser and self-assembled monolayers interaction

Standard positive photoresist techniques were adapted to generate sub-micron scale patterns of gold substrate using self-assembled monolayers (SAMs) and femtosecond laser. Self-assembled monolayers formed by the adsorption of alkanethiols onto gold substrate are employed as very thin photoresists. The process underlying photopatterning of SAMs on gold is well-known at the phenomenological level. Alkanethiolates formed by the adsorption of alkanethiols are oxidized on exposure to UV light in the presence of air to alkylsulfonates. Specifically, it is known that deep UV light of wavelength less than 200 nm is necessary for oxidation to occur. In this study, solid state femtosecond laser of wavelength 800 nm is applied for photolithography. The results show that ultrafast laser of near infrared (NIR) range wavelength can replace deep UV laser source for photopatterning using thin organic films. The essential basis of our approach is the photochemical excitation of specific reactions in a particular functional group (in this case a thiolate sulfur atom) distributed with monolayer coverage on a solid surface. Femtosecond laser photolithography could be applied to fabricate the patterning of surface chemical structure and the creation of three-dimensional nanostructures by combination with suitable etching methods.     

Influence of initial oxygen on the formation of thiol layers

In this study, X-ray photoelectron spectroscopy (XPS) has been used to study thin organic films. For comparison, monolayers were formed on clean and air-exposed metal substrates. Obtained results show that thiols remove contamination oxygen from gold, silver, platinum and copper surfaces. The tightly packed thiolate layers can be formed. In addition, oxygen does not take part in the final bonding of molecules to the surfaces.     

Investigation of the influence of low-concentration hydrogen on the surface potential of thin metallic films for sensor applications

In this paper, a study of the influence of hydrogen (concentrations 6 ppm − 1%) on the work function of thin metallic films at moderately elevated temperatures is presented. The work function was measured indirectly by the observation of the surface potential of dedicated test structures using scanning surface potential microscope. Metallic layers with thicknesses of 10, 20, 30, and 50 nm were deposited on semiconductor substrates as well as on a thick gold layer. The investigations were focused on palladium thin films although a comparison to results obtained for platinum layers was also discussed.     

L'adsorption du bismuth sur des films minces d'or etudiée par variation de résistance electrique

Adsorption of bismuth on gold thin films is studied by electrical resistance variations. At low coverage the increase of gold resistivity due to Bi adatoms is 2.3 ± 0.1 μΩ cm/at% and is independent of temperature and thickness of the Au layers. At 20° C and ?150°C, the shape of the resistance variation curve for increasing Bi coverage indicates that the adatoms form a first monolayer having a higher density at lower temperature. At 85°C two Bi monolayers are made successively: the first is adsorbed on the free surface of the gold film; the second grows on the other face, between the gold and the glass substrate, after migration of the Bi atoms through the grain boundaries or other defects. These results are verified by Auger electron spectroscopy. They are used to show that the initial reflectivities for conduction electrons on the two surfaces are similar in recrystallized gold films; the specularity coefficients P and Q have been evaluated using the Fuchs theory. For clean surface, the electron reflection is large specular (P ? Q ? 0.75 ± 0.05) and becomes entirely diffuse when a monolayer of bismuth is adsorbed on each face of the gold film.     

Physical and electrochemical study of platinum thin films deposited by sputtering and electrochemical methods

In this work platinum thin films deposited by sputtering and electrochemical methods were characterized through physical and electrochemical analysis. The as-grown platinum thin films were characterized through X-ray diffraction (XRD), atomic force microscopy (AFM); scanning electronic microscopy (SEM) and through electrochemical impedance spectroscopy (EIS) measurements. Structural studies indicated that platinum thin films were polycrystalline. Morphological characteristics were significantly affected by the substrate type and synthesis method. Finally the EIS analysis indicated that platinum films were electrochemically stable and present both low resistance of charge transfer and low series resistance; the equivalent circuit of platinum interface has been proposed.     

Crystallization kinetics in ferroelectric thin films: Viability of atomic force microscopy

The pyrochlore to perovskite phase transformation was studied in lead zirconate titanate (PZT) thin films. The films were fabricated on platinum electrodes and annealed by rapid thermal processing (RTP). The phases which formed and their location in the film were analyzed using glancing angle XRD and depth profiling was demonstrated. Grain size and structure, nucleation sites and surface morphology were determined with transmission electron microscop (TEM) and atomic force microscopy (AFM). The role of AFM in this type of transformation study was assessed.

The PZT films crystallized with a (100) orientation which was preferentially nucleated at the platinum/film interface. RTP at 650°C for 15 s was sufficient to complete the transformation. However, columnar grain growth and improvements in the ferroelectric properties were obtained with increased RTP time. A PZT film with RTP at 650°C for 1 min possessed a remanent polarization of 25 μC/cm² and a dielectric constant of ε = 650.     

Formation of gold nanowires through self-assembly during scanning force microscopy

Gold films with a nominal thickness of 5–40 monolayers were grown on dielectric substrates and imaged by scanning force microscopy (SFM). The films originally consisted of well-separated or densely packed clusters. During imaging in contact mode, the morphology of the films changed drastically. At low coverage, i.e. Θ<10 monolayers, the well-known stripes originating from mobile clusters, eventually accumulated into larger aggregates, were observed. In contrast, at larger coverage, highly ordered structures consisting of one-dimensional wires evolved during scanning. They often were parallel with equal separation, i.e. well-defined periodicity, over distances of several μm. Typically, the wires were 5–10 nm high and 50–100 nm wide. Investigations of Au films prepared at varying temperature on different dielectric substrates allow us to suggest a self-assembling mechanism for wire formation in which gold is periodically collected by the SFM tip and redeposited as soon as a critical amount is reached. Received: 22 February 1999 / Accepted: 2 March 1999 / Published online: 7 April 1999     

Enhancement of activity of platinum towards oxidation of ethanol by supporting on titanium dioxide containing phosphomolybdate-modified gold nanoparticles

A new concept of utilization of titanium dioxide matrix in electrocatalysis by admixing it with polyoxometallate modified gold nanoparticles is described here. The approach utilizes Keggin-type phosphododecamolybdate (PMo₁₂O₄₀³⁻) adsorbates capable of modifying, activating and stabilizing Au nanoparticles of the sizes of 30-40 nm. Ultra-thin films of phosphomolybdates on nanostructured gold are characterized by well-defined fast (reversible) multi-electron electrochemical reactions. By dispersing platinum black over the Au-containing TiO₂, the electrocatalytic activity of Pt nanoparticles towards oxidation of ethanol has been enhanced. Remarkable increases of electrocatalytic currents measured under voltammetric and chronoamperometric conditions have been observed. The most likely explanation takes into account improvement of overall conductivity (due to the presence of nanostructured gold) at the electrocatalytic interface (including TiO₂-support), as well as and possibility of specific Pt-TiO₂ or Pt-Au electronic interactions and existence of active hydroxyl groups (on titanium dioxide or polyoxometallate surfaces) in the vicinity of catalytic Pt sites.     

Phosphorylcholine functionalized dendrimers for the formation of highly stable and reactive gold nanoparticles and their glucose conjugation for biosensing

Phosphorylcholine (PC)-functionalized poly(amido amine) (PAMAM) dendrimers were prepared and used as both reducing and stabilizing agents for synthesis of highly stable and reactive gold nanoparticles (Au NPs). Biomimetic PC-functionalized PAMAM dendrimers-stabilized gold nanoparticles (Au DSNPs) were formed by simply mixing the PC modified amine-terminated fifth-generation PAMAM dendrimers (G5-PC) with AuCl₄ ⁻ ions by controlling the pH, no additional reducing agents or other stabilizers were needed. The obtained Au DSNPs were shown to be spherical, with particle diameters ranging from 5 to 12 nm, the sizes and growth kinetics of Au DSNPs could be tuned by changing the pH and the initial molar ratio of dendrimers to gold as indicated by transmission electron microscopy (TEM) and UV–Vis data. The prepared Au DSNPs showed excellent stability including: (1) stable at wide pH (7–13) values; (2) stable at high salt concentrations up to 2 M NaCl; (3) non-specific protein adsorption resistance. More importantly, surface functionalization could be performed by introducing desired functional groups onto the remained reactive amine groups. This was exemplified by the glucose conjugation. The glucose conjugated Au DSNPs showed bio-specific interaction with Concanavalin A (Con A), which induced aggregation of the Au NPs. Colorimetric detection of Con A based on the plasmon resonance of the glucose conjugated Au DSNPs was realized. A limit of detection (LOD) for Con A was 0.6 μM, based on a signal-to-noise ratio (S/N) of 3. These findings demonstrated that the PC modified Au DSNPs could potentially serve as a versatile nano-platform for the biomedical applications.     

Internal mechanical stresses and the thermodynamic and adhesion parameters of the metal condensate-single-crystal silicon system

The kinetics of generation of internal mechanical stresses ??(d) in chromium, copper, gold, and aluminum thin films on single-crystal silicon substrates at different deposition rates has been experimentally investigated using the cantilever method. A two-step character of the variations in internal tensile stresses has been revealed. The regularities of the formation of the maximum level of mechanical stresses in the condensates under investigation have been established. The energy and adhesion parameters of chromium, copper, gold, and aluminum nanolayers on silicon, germanium, and nickel substrates have been studied using the macroscopic methods of surface physics. The interfacial energy, interfacial tension, work of adhesion, interfacial charge, and a new energy characteristic of the interfacial layer, namely, the energy of adhesive bonds, which exceeds the interfacial energy, have been determined.