Influence of Residual Water and Carbon Containing Molecules in Focused Electron Beam Induced Processes

Focused Electron Beam Induced Processing (FEBIP) is a technique used for etching and deposition of micro- and nano-structures using a focused electron beam. FEBIP is commonly carried out in a Scanning Electron Microscope (SEM) with a Gas Injection System (GIS) allowing precursor molecule vapors to be introduced on the substrate where they interact with the focused electron beam. Results of this work shine light on the influence of residual Water and Carbon containing species in the SEM chamber on etching and deposition processes. For etching HOPG (Highly Oriented Pyrolytic Graphite) and PMMA (Polymethyl methacrylate) using water vapor as precursor, it is shown that water participates in the principal chemical reaction for removal of these materials. However, when using Oxygen as precursor for etching these materials, Oxygen has a significantly reduced role in chemical reactions compared to residual water desorbing from surfaces in the FEBIP set-up. Degrees of involvement of Oxygen and Water in chemical reactions are demonstrated by combining experimental results and existing FEBIP Models. The Focused Electron Beam Induced Etching (FEBIE) contribution of Oxygen molecule Gas Phase Ionization prior to adsorption on the substrate surface is determined to be approximately 10% of the etching process for a micron size focused electron beam. A Precursor Gas Preparation System (PGPS) was designed and developed for in-situ fabrication of Metal Fluoride precursors from Xenon di-fluoride, XeF2. PGPS provided means of safe execution of FEBIP experiments without having to store large quantities of various dangerous and toxic materials. Using PGPS, Tungsten Hexafluoride precursor, WF6, was created in-situ and used in FEBIP experiments for deposition of Tungsten containing materials. This precursor allowed the elimination of both Oxygen and Carbon atoms in the precursor molecule in order to study the influence of residual Water (containing Oxygen) and residual Carbon containing species by measuring the amount of these atoms in deposit composition. Installation of a Liquid Nitrogen Cooled Plate over Substrate (without any direct contact with the substrate or GIS in the FEBIP set-up) proved to significantly reduce Oxygen and Fluorine content in deposits by eliminating the contribution of residual molecules from the chamber. Carbon content in deposits was also reduced when using Liquid Nitrogen Cooled Plate over Substrate (LNC-PoS). Non-FEB induced reactions (resulting in build-up of a contamination film on the substrate) are also significantly reduced or eliminated when using LNC-PoS in FEBIP experiments with WF6 precursor. In experiments with LNC-PoS, it is shown that the Carbon content in deposits mostly originates from substrate surface and arrives to FEB location via surface diffusion.

Photocatalytic and Electrocatalytic Reduction of Carbon Dioxide in Pressurized Systems

Patrick Voyame

The depletion of carbon-based fossil fuels and the rise in atmospheric carbon dioxide concentration will force an inevitable change in the future global energy landscape. CO2 reduction presents the advantages of decreasing its atmospheric concentration and storing energy in chemical form in CO2 reduction products. With a predicted conversion to renewable energy such as solar or wind energy, energy storage will become a key process in the near future for buffering the fluctuating energy production. The objective of the present work was to study the efficiency towards CO2 reduction of different molecular catalysts. In particular, conducting experiments in high-pressure and supercritical conditions and observing the effect of CO2 pressure or concentration on the efficiency and selectivity of the reaction. As supercritical CO2 (scCO2) has poor solubilisation capabilities, experiments were conducted in biphasic systems or with addition of an organic co-solvent. To drive the reduction reaction of CO2, a catalyst is needed to overcome the kinetic limitations of the reaction, but an energy input is also necessary. Three different forms for this energy input were used in this work. In a first time a sacrificial product, decamethylferrocene (DMFc), was used to transfer electrons to CO2 in biphasic water/scCO2 systems. Complete oxidation of the DMFc was observed in presence of anion capable of transporting protons from water to the DMFc present in the supercritical phase. A photosensitization cycle was used to supply a water soluble catalyst, NI(II)Cyclam, in electron at the required potential to drive the reduction of CO2 into carbon monoxide in water/scCO2 system. The creation of the interface in the system appeared highly favourable to the efficiency of the catalyst. A second catalyst, a ruthenium polypyridyl carbonyl complex, was used for the photocatalytic reduction of CO2. Pressure had an important impact on the production of one of the two reduction product, carbon monoxide, while the production of formate was unaffected by CO2 pressure. As limitations in productivity in photocatalytic experiments were coming principally from the photosensitizer cycle or the sacrificial electron donor, photosensitizer was replaced by an electrode to provide the catalyst in electrons. Voltammetry in CO2-expanded liquids was described and determination of important parameters such as catalyst concentration and diffusion coefficient as a function of pressure was performed. Electrocatalytic reduction of CO2 by ruthenium and rhenium polypyridyl carbonyl complexes was studied in CO2-expanded liquids. The catalytic mechanisms were observed to be highly influenced by CO2 concentration.

EPFL2016

Oxygen assisted focused electron beam induced deposition of silicon dioxide

Alexandre Perentes

Focused Electron Beam Induced Deposition (FEBID) is a rapid prototyping technique for the investigation, production and modification of 2 and 3-D nanostructures. The process takes place at room temperature, in the high-vacuum chamber of a Scanning Electron Microscope (SEM). The principle is based on the local FEB decomposition of molecules injected in the chamber and adsorbed on a surface. These molecules, called the precursors, are specifically chosen to contain the element of interest to be deposited. However, the electron induced decomposition results in contaminated materials containing additional parasitic elements from the precursor molecule. These contaminants are incorporated with the element of interest, and downgrade the properties of the materials and prevent therefore their use for many applications. Gas-assisted FEBID, the subject of this thesis, is a promising evolution and solution to the problem that was never extensively studied in literature. It consists in injecting a reactive gas simultaneously to the precursors, which will react in real time with the deposited contaminants to form volatile products, leaving ideally only the interesting material on the surface. This work focused on the Oxygen assisted FEBID of Silicon dioxide (SiO2) and on the understanding of the deposition process. SiO2 it is a high performance material for nano-optic and electronic applications, widely used in research and industry. Modifications were brought to an existing SEM to allow the simultaneous injection of two different and controlled gases, and to allow the in-situ control of growth of optical materials. Si-precursors of three families (alkoxy-, alkyl- and isocyanato-silanes) were compared. The FEBID material could be gradually converted from contaminated Si-materials to pure SiO2, above a [O2]/[precursor] ratio specific to each precursor chemistry and precursor flow. The SiO2 deposited was stoichiometric, C and OH–free, had an estimated density of 2.2, was amorphous and had optical properties close to that of fused silica. Optical structures for nano-optics and plasmonics were produced. Compared to traditional FEBID, O2 assisted FEBID required much lower energy to induce a decomposition reaction, and secondary electrons achieved 90 % of the deposition process in our setup. The electron efficiency could be multiplied by 20 compared to conventional FEBID. O2 assisted FEBID was insensitive to electron density in our setup. The conditions for no C and large deposition rates during O2 assisted FEBID were high molecule reactivity to O2, low sticking coefficient, low acceleration voltage and dwell time (< 6 µs), and large replenishment time (> 60 µs). As function of the precursor chemistry and flow, additional O2 influenced the deposition rate, which could be increased it by a factor of 7. The O2 and precursor molecules co-adsorption resulted in a surface coverage competition, which determined the deposition efficiencies. Limitations due to the residual water in the SEM were demonstrated and characterized. They appeared as Oxygen incorporation in the deposited materials, which influenced the deposition process. Residual water impinging flow was demonstrated to be responsible of the FEBID process deposition efficiency when no Oxygen was injected.

EPFL2007

Cluster-Surface Interaction and Catalytic Activity

Simon Bonanni

This thesis is devoted to the study of the catalytic activity of small supported metal clusters. The TiO2(110)-(1×1) supported platinum cluster activity towards CO oxidation, its correlation with the cluster morphology, support properties, and the cluster stability are studied. For this purpose a new ultra-high vacuum (UHV) compatible reactor has been developed, which, in combination with a low temperature scanning tunneling microscope, a size selected cluster source and standard techniques for sample preparation, constitutes a unique experimental setup that allows for in situ investigations of the morphology and catalytic properties of size-selected cluster based model catalysts. First, large platinum clusters (100 to 200 atoms per cluster), grown from deposited atoms, were investigated. This system initially shows a low catalytic activity, due to the formation of a TiOx (0 < x < 2) encapsulation layer on the clusters upon annealing, known as strong metal-support interaction (SMSI) state. We observed that a soft sputtering and a second annealing to 1100 K give rise to a highly active catalyst for CO oxidation. We attribute the activation of the catalytic activity to the destruction of the encapsulation layer by the soft sputtering and to the absence of its reformation during the second annealing. This procedure could in principle be used to activate other initially passive SMSI systems. Second, the effect of the reduction state of the TiO2 support on the catalytic activity of the supported Pt clusters has been analyzed. It is demonstrated that small platinum clusters (in this case Pt7) are two orders of magnitude more active toward CO oxidation when they are supported on slightly reduced TiO2 crystals than when they are supported on strongly reduced ones. This is attributed to the consumption of the oxygen absorbed on the surface by the crystal interstitial titanium atoms, which are present in higher concentration in strongly reduced crystals than in weakly reduced ones. The effect could take place also for other catalytic processes involving oxygen on TiO2 supported metal clusters and allow for the tuning of oxidation and de-oxidation reactions. Platinum cluster size effects on the catalytic CO oxidation have been investigated depositing Pt3, Pt7 and Pt10 clusters on TiO2(110)-(1×1). From Pt3 clusters a CO2 production, per platinum atom, two times bigger than the one from Pt7 and Pt10 has been observed. The cluster morphology was strongly modified by the exposure to the reaction environment (temperatures up to 600 K and CO and O2 doses). This highlights the necessity to study the stability of small platinum clusters as a function of the temperature and gas exposure. Finally, as suggested by the preceding measurements, the stability of Pt7 clusters on TiO2 has been investigated as a function of the temperature, from 300 K to 600 K, and of the exposure to gases. Pt7 clusters are proved to be stable in UHV up to 430 K on the TiO2(110)-(1×1) surface. If the annealing is performed while exposing the sample to gases (CO, O2 and a mixture of the two) important cluster sintering takes place already at 430 K. This effect can be attributed to a modification of the cluster-support interaction induced by the adsorption of gases and to the additional energy injected in the system by the CO combustion taking place on the clusters when both CO and O2 are dosed.

EPFL2012