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Home Page  Researchers Micha
Asscher
Micha AsscherInstitute of Chemistry Metallic nano particle: characterization and reactivityThe research year of 2011-2012 has been characterized by the following activity: i) Clusters diffusivity on defected surfaces, ii) Sintering resistant catalysts: The role of substrate defects / morphology and electronic structure on anchoring meta-stable nano-alloy clusters of Pd-Au and Pd-Cu (catalysis), iii) Thin TiO2 film has been grown via the reactive layer (amorphous solid water (ASW) ? D2O) assisted deposition (RLAD). We have identified a dewetting process, in which the film aggregates to form rutile/anatase TiO2 nano-crystals, iv) Photo-Induced Desorption (PID) of atoms and molecules within porous silicon has been studied, revealing gigantic enhancement of PID yield at nanometer size tips within the pores. This study was extended to molecules (CO and N2O) in addition to Xe atoms v) Charging of ASW has been characterized using high resolution, low energy electron source at the 1-50 eV range. This system has been studied for its static electric field effect on electron-induced-desorption from top layers of ASW film. Results from selected topics are discussed below: 1. Growth of TiO2 thin film via in-vacuum Sol-Gel like mechanism via Reactive Layer assistance (Liat Zilberberg) The growth mechanism of TiO2 films and their morphology are reported using Reactive Layer Assisted Deposition (RLAD) method under ultra high vacuum conditions. The oxide film formation involves Ti atoms deposition on top of amorphous solid water (ASW) condensed on a SiO2/Si(100) support at 90K. Subsequent annealing leads to desorption of all non-reacted buffer molecules, resulting in deposition of the titanium oxide film. Employing mass spectrometry and using D2O as buffer, evolution of deuterium molecules has been detected during titanium atoms deposition. A solid state sol-gel-like formation mechanism of titanium oxide is proposed based on these observations. The morphology of the oxide films is characterized by AFM as a rather uniform amorphous thin film at room temperature. Upon further annealing above 750K, crystallization of the titanium oxide film has set-in coinciding with a dewetting process of the oxide layer, information obtained from similar growth procedure on amorphous carbon covered TEM grid. It was shown that these films are rather insensitive to the underlying substrate at temperature below 500K.
Figure 1: Evolution of D2 from D2O ASW at various thicknesses vs. time. Ti atoms impingement flux is modulated every 25 sec. In Figure 1 the evolution of D2 molecules is monitored by a mass spectrometer during the modulated deposition of hot Ti atoms. Analysis of the Ti flux and the deuterium pressure evolved suggested that a 1:1 D2 to Ti describes the formation of intermediate Ti(OD)2 fragments on top of the reactive buffer D2O layer. Following surface annealing remaining water molecules desorb and the titania film start to polymerize in a Sol-Gel like mechanism. Eventually, after heating the surface to high temperature (above 1000K), a dewetting process takes place, resulting in the formation of small TiOx nano-crystals, as shown in figure 2. These results were recently accepted for publication in Langmuir.
Figure 2: A) STEM images of Titanium oxide on a-C substrate after annealing to 1100K, for 10 minutes. B) HAADF analysis along the red line in the STEM image. C) EDS test on different regions containing different amount of Ti. D) Focus on one nano-cluster (region 3 in Figure 6A), TEM image revealing a clear lattice structure of the Rutile phase. 2. Low energy positive and negative charging of thick Amorphous Solid Water and its effect on caged molecules (Yonatan Horowitz) The interaction of charged particles with condensed water films has been studied extensively in recent years due to its importance in biological systems, ecology as well as interstellar processes. We have studied low energy electrons (3-25 eV) and positive argon ions (55 eV) charging effects on Amorphous Solid Water (ASW) and ice films, 360? 1080 ML thick, deposited on ruthenium single crystal under ultra high vacuum (UHV) conditions. Charging the ASW films by both electrons and positive argon ions has been measured using a Kelvin probe for contact potential difference (CPD) detection and found to obey plate capacitor physics. The incoming electrons kinetic energy has defined the maximum measurable CPD values by retarding further impinging electrons. L-defects (shallow traps) are suggested to be populated by the penetrating electrons and stabilize them. Low energy electron transmission measurements (currents of 0.4-15 microamperes) have shown that the maximal and stable CPD values were obtained only after a relatively slow change has been completed within the ASW structure. Once the film has been stabilized, the spontaneous discharge was measured over a period of several hours at 103±2K. Finally, UV laser photo-emission study of the charged films has suggested that the negative charges tend to reside primarily at the ASW-vacuum interface, in good agreement with the known behavior of charged water clusters. In Figure 3 the gradual charging of layers of Amorphous Solid Water (ASW) as a result of Ar+ ions (positive charging) and electrons (negative charging) is demonstrated while monitoring the accumulated respective charging by means of contact potential difference (CPD) measurements.
Figure 3: Upper section: CPD (V) values as a function of ASW film thickness (ML) and incident electron kinetic energies (3 ? 25 eV) are presented. Lower section: CPD evolution as a function of ASW film thickness (ML) after Ar+ low energy ion beam (LEIB) at constant kinetic energy (?55 eV). All ASW films were initially annealed to 120K, cooled to 100 K and subsequently exposed to either ion or electron irradiation for 6 minutes. We have demonstrated that electrons can be accumulated only up to a certain amount of charge that develops exactly the voltage equivalent to the incident electrons kinetic energy. This is shown in Figure 4 as a retardation plot, showing the maximum CPD that can be developed for each incident electron energy.
Figure 4: The measured CPD (V) and its spontaneous decay for a 480 ML thick ASW film. The same film was exposed to consecutive periods of 6 minute electron irradiation at increasing e- - beam energy intervals of 5 eV. The natural decay of the charge was recorded for the subsequent 10 minutes. Note the dashed curve (starting at ? 3000 sec.) where 20 eV electrons exposure hits when the films? CPD is higher than 20 V. This exposure resulted in no further accumulation of charge, i.e., the CPD continues to decay, unaffected. This system of charged ASW films behaves practically like a nano-capacitor, as demonstrated when calculating the accumulated charge vs. the film thickness, shown in Figure 5. This charge develops electric fields of the order of 106 ? 107 Volts/cm, close to the discharge threshold
Figure 5: Dashed curve: The theoretical charge (Q) accumulation in an ideal parallel plate capacitor (see eq. 1) as a function of ASW thickness with a steady CPD of 25 V and ?(T) = 3.2. Dots: The calculated charge according to experimental CPD values and ASW film thickness while the temperature was kept constant at 100 K, at this temperature ?(T) = 3.2 . The question where the electrons reside while trapped and solvated within the solid water is important in order to understand where interaction with trapped molecules may be anticipated. We have addressed this question by exposing the negatively charged film (480 monolayers thick) to UV light generated by an excimer laser at 248nm (5.0 eV). The electrons were photo-emitted by the light as shown in Figure 6. When forming a sandwich of electrons between two layers of ASW, the results of figure 6 have not changes. We have concluded that the trapped electrons prefer to reside at the ASW-vacuum interface. This is a similar behavior to that reported for water nano-clusters, but here it reflects the behavior on a macroscopic 2D film that is nanometer thick.
Figure 6: KrF Excimer laser (5.0 eV) photo-emitted electrons from ASW ? vacuum interface vs. the number of photons. The dashed line is a guide to the eye. The above ASW charging results were recently published in J. Chem. Phys. List of publications in Nanoscience and Nanotechnology (2010-2012)
Cooperation with industries and defence authorities:
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Students and co-workersPost docs: Dr. Arumugam MurugadossM.Sc. students: Elishama Sorek, Shay Bar-Nachum, Roey Sagi, Jonatan Ankri Graduate students:Ori Stein, Yonatan Horowitz, Gil Toker, Liat Gilat |
