Surface Science Network

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Theses

Surface modification and characterization of zinc oxide for use in organic photovoltaic devices, by D. J. Baker

Core-hole clock spectroscopy: Characterization of the method and dynamics of charge transfer at adsorbate metal interfaces, by S. Vijayalakshmi

Molecules coadsorbed on metal surfaces: Investigations using X-ray photoelectron spectroscopy, by S. Vijayalakshmi

Photoelectron Spectroscopic Investigations of Porphyrins and Phthalocyanines on Ag(111) and Au(111): Adsorption and Reactivity, by Y. Bai

Diffusion of Physisorbed Layers and their Connection to MEMS Effectiveness, by B.P. Miller

Preparation and Characterization of Tungsten Tips Suitable for Molecular Electronics Studies, by A.-S. Lucier



With Abstracts

Core-hole clock spectroscopy: Characterization of the method and dynamics of charge transfer at adsorbate metal interfaces, by S. Vijayalakshmi

Abstract In this thesis, charge transfer dynamics has been investigated with the core–hole– clock method (CHC) for both atomic and molecular adsorbates. As a first step, the basic assumption of the core–hole–clock method that the charge transfer time is independent of the reference core hole lifetime has been tested. This is shown for the strongly chemisorbed S atom on the Ru surface for two independent core hole clocks (S(1s), S(2s)). Furthermore, fine changes in surface electronic structure affect the excited adsorbate state lifetime and has been detected by CHC spectroscopy for physisorbed Ar on Cu(111) and Cu(100) surfaces. Combined theoretical computations are consistent with the experimental findings. Finally, charge transfer times from the molecular adsorbate to the underlying substrate have been determined for the molecular adsorbate system (C6F6)(aromatic molecule with equivalent atoms) with core–hole–clock spectroscopy for various adsorbate coverages. There additional vibrational motion typical of the molecular adsorbates complicates the determination of charge transfer times and comparison to gas phase has been used to determine the charge transfer in the adsorbate. The charge transfer times obtained with core–hole–clock spectroscopy for this molecular system have been compared to those from time resolved two-photon photoemission which is an alternative approach to determine charge transfer times and the similarities/and differences between the two spectroscopic methods are discussed.



Molecules coadsorbed on metal surfaces: Investigations using X-ray photoelectron spectroscopy, by S. Vijayalakshmi

Abstract Two aspects related to surface chemistry of molecules have been dealt with in the thesis. The first one pertains to stabilization of molecular methanol on a Zn(0001) surface by coadsorbing water. X-ray photoelectron spectroscopy measurements were carried out after exposing the Zn surface at 80K to the binary vapor from water-methanol liquid mixtures of varying compositions and subsequently warming the surface upto the room temperature. When the surface was exposed to the vapor from a mixture with water molefraction, xw, of 0.5, the proton abstraction and the C-O bond cleavage in methanol leading to methoxy (CH3O) and the hydrocarbon (CHx) species respectively, occured at a much higher temperature of 180K, compared to 120K in the case of pure methanol adsorption. For water rich mixtures (xw = 0.7 and 0.9) molecular methanol was stabilized on the surface upto 200K, beyond which water itself desorbed. For xw = 0.7, virtually no dissociation was observed upto 200K. The increased stability of molecular methanol on Zn(0001) surface is attributed to the surface mediated hydrogen bonds that stabilize a water-methanol complex. The second aspect addresses the nature of adsorption of CS2 on Ni(110) surface in the pure form as well as in mixtures containing oxygen. Pure CS2 adsorbs dissociatively giving rise to graphitic carbon (C1s, 285eV), chemisorbed sulphur (S2p, 164eV) and perhaps a small proportion of CS(a) (C1s, 286eV). A CS2-O2 mixture dilute in oxygen (220:1) gave rise to a new species COS(a) (O1s, 534eV) due to the reaction between the transients, O1-(s) and CS(s). The COS(a) species was also formed in the case of a 15:1 CS2-O2 mixture, the other prominent species on the surface being O1-(a) (531eV). When a mixture with comparable compositions of CS2 and O2 (2:1) was dosed, mainly O2-(oxidic) species (O1s, 530eV) was formed with no evidence of sulphur on the surface. When O2 and CS2 were dosed in a sequential manner, there was some formation of COS(a). On the other hand, when O2 was dosed on a surface predosed with CS2, oxygen adsorption was inhibited.



Photoelectron Spectroscopic Investigations of Porphyrins and Phthalocyanines on Ag(111) and Au(111): Adsorption and Reactivity, by Y. Bai

Abstract Under construction



Diffusion of Physisorbed Layers and their Connection to MEMS Effectiveness, by B.P. Miller

Abstract The aim of this work is to connect the physics of surface diffusion of a lubricant to Micro-Electro-Mechanical System (MEMS) lubrication. Some hurdles must be overcome in order to make this connection. One must have a way to experimentally measure surface diffusivity. Length scales must be taken into account since the mechanism of lubrication varies from the macro scale to the micro scale and even to the nano scale. Lastly, a theoretical model of lubrication that can conform to MEMS geometry is needed for an accurate prediction. In the work presented here, I have used different techniques including a quartz crystal microbalance (QCM), a macroscopic tribometer, a micro tribometer, and an atomic force microscope (AFM) to measure the friction of a lubricant on surfaces relevant to MEMS. The QCM method is different from the others aforementioned since it measures the atomic scale friction of a sliding layer in a contact-free environment directly related to surface mobility. The other three methods are a way to measure lubrication over three different length scales. A surface lubrication model developed by Prof. Donald Brenner incorporates surface diffusion as a mechanism for lubricating a periodic contact. There is a push and pull between the removal of the lubricant from the periodic contact and the replenishment of the lubricant via surface diffusion. A steady-state center concentration can be computed, which is used to determine whether or not lubrication can occur. This model was fit to magnetic hard disc drives (MHDDs), MEMS, and macroscopic industrial machines, but will work for any system with a periodic contact relying on surface diffusion as the replenishment mechanism. Two groups of lubricants were studied in this work. Tricresyl Phosphate (TCP), which is a known high temperature additive to industrial oils, was selected since it possesses a low vapor pressure and has been extensively studied by our group. All of the above mentioned techniques were used to study TCP. The other group of lubricants studied were alcohols, specifically pentanol, ethanol, and trifluoroethanol (TFE). These lubricants were studied exclusively with the QCM technique. Alcohols have been shown to lubricate a MEMS device indefinitely as long as an environment of the alcohol vapor surrounds the contact. The results shown here can be used to directly predict the effectiveness of a lubricant candidate to a MEMS device. Some extra parameters were determined to affect lubrication including contact stress, adhesion, and wetablitiy. These parameters need to be taken into account for future selection of lubricants.



Preparation and Characterization of Tungsten Tips Suitable for Molecular Electronics Studies, by A.-S. Lucier

Abstract A technique for preparing tips with a radius of a few nanometers from tungsten wire is investigated. The sharp shape is obtained by electrochemical etching; further tip processing and characterization is done in ultra-high vacuum. Tips are cleaned through a high temperature annealing process and their sharpness can be quickly estimated from their ?eld emission behaviour. Su?ciently sharp tips are imaged with a ?eld ion microscope; full atomic characterization of the tip apex can be obtained from an analysis of the resulting images and ?eld evaporation can be used to atomically engineer the tip apex into a desired con?guration. Starting from single crystal, (111) oriented tungsten wire, a sharp tip terminating in three atoms can be fabricated; due to its geometry and its stability, this apex con?guration is well suited for applications as an atomically de?ned electrical contact in a single molecule conductivity experiment.