Kirk H. Schulz

Dependence of the sheet resistance of indium-tin-oxide thin films on grain size and grain orientation determined from X-ray diffraction techniques

ITO thin films (100‐200 nm) are deposited on glass and plastic (PET and polycarbonate) substrates by r.f. sputtering. Process parameters such as oxygen partial pressure, r.f. power, and post deposition annealing parameters are varied to determine the dependence of the sheet resistance on process parameters. The microstructure of these thin films is determined using an X-ray diffractometer (XRD) and a transmission electron microscope (TEM). The experimentally observed dependence of the sheet resistance on the grain size and grain orientation of these films is correlated to the dependence of the electron mobility on grain boundary scattering. Larger grain sizes (<25 nm) in ITO films result in lower sheet resistance (250 V/A). This type of large grain size microstructure is produced with moderate r.f. power (<100 W) and low oxygen partial pressure (<10%). There is a unique correspondence between grain size and grain orientation. ITO films with a strong peak intensity ratio of (400) orientation to all other orientations (<0.35) have the largest grain size (<25 nm) resulting in the lowest sheet resistance (250 V/A) and high transmission ( < 86.7%) at la 550 nm. q 1999 Published by Elsevier Science Ltd. All rights reserved.

Electrical, optical and structural characteristics of indium-tin-oxide thin films deposited on glass and polymer substrates

Abstract The sheet resistance, optical transmittance and microstructure of tin-doped indium oxide (ITO) thin films (50–100-nm thick) rf sputter deposited on polymer substrates are investigated using a four-point probe, spectrophotometer, X-ray diffractometer and a transmission electron microscope (TEM). Sheet resistances vary from 250 Ω/sq. to 170 kΩ/sq. Sheet resistances for the ITO films on polycarbonate substrates are at least an order of magnitude higher than those ITO films deposited on glass substrates at the same time. Annealing ITO films on polycarbonate substrates at 100°C in air for 1 h decreased the sheet resistances significantly (almost by 50%). The X-ray diffraction data indicate polycrystalline films with grain orientations predominantly along (222) and (400) directions. TEM photographs show two distinct regions of growth: a dense growth close to the substrate and a sparse growth away from the substrate. The vertical growth is columnar and rod shaped. Changes in the ITO film sheet resistance either due to the types of substrate used or due to annealing can be correlated to the grain size and grain orientation.

Electrical, optical, and structural properties of indium-tin-oxide thin films deposited on polyethylene terephthalate substrates by rf sputtering

Indium-tin-oxide (ITO) is a transparent conducting material which is deposited as a thin film on glass substrates for use in opto-electronic devices. However, there are several applications for which ITO films on polymeric substrates are desirable. The sheet resistance, optical transmittance, and microstructure of as-deposited ITO thin films on unheated polyethylene terephthalate substrates were studied using rf sputter deposition. During separate deposition runs, the partial pressure of oxygen was varied from 5% to 20% and the deposition time was varied from 15 to 120 min. No significant variations are observed in the sheet resistance with respect to oxygen partial pressure; however, changes in sheet resistance were observed in ITO films deposited on different substrates for short deposition times (15 min). Additionally, the thickness of the film (assumed to be proportional to the deposition time) is shown to have a considerable impact on the sheet resistance and the optical transmittance. The x-ray diff...

A four-point surface conductivity probe suitable for in situ ultrahigh vacuum conductivity measurements

A simple design for a four-point probe suitable for precision surface conductivity measurements is described. Our design makes use of small, commercially available spring contact probes which are mounted in a custom built MACOR ceramic probe head. The design is suitable for use in ultrahigh vacuum applications, and the custom-built parts can be fabricated in any machine shop. Very reproducible values were obtained using this probe for surface conductivity measurements on a MoS2(0001) model catalyst, a sputter deposited indium-tin oxide thin film and a doped silicon wafer.

A virtual instrument approach for automation of temperature programmed desorption

LabVIEW provides a method to create a user friendly automated Virtual Instrument that can be programmed to perform simultaneous display and control functions from several different laboratory instruments. This note describes how LabVIEW was used to create a temperature programmed desorption virtual instrument to facilitate easy control of sample temperature and data collection and handling. The program is easy to use, fairly versatile for different types of catalysis samples, and the data collected can be imported into most spreadsheets. The program as written controls the temperature in a linear fashion with very little deviation from the user defined ramp.

REACTION OF 1,2-ETHANEDITHIOL ON CLEAN, SULFUR-MODIFIED, AND CARBON-MODIFIED MO (110) SURFACES

The reactivity of 1,2-ethanedithiol on the clean Mo (110) and p(4×4)-C/Mo (110) surfaces has been investigated as a function of sulfur coverage using temperature programmed desorption (TPD), Auger electron spectroscopy, and low energy electron diffraction. TPD experiments performed on both surfaces produced similar reaction products, although changes were observed in selectivity. On the clean Mo (110) surface, the major products observed during TPD experiments were acetylene, ethylene, vinyl thiol, and ethanethiol. However, the reaction of ethanedithiol on the p(4×4)-C/Mo (110) surface produced acetylene, ethylene, and ethanedithiol. Product molecules are thought to arise from two distinct types of surface intermediates: (1) a monodentate thiolate species bound to the surface through only one of the ethanedithiol sulfur atoms, and (2) a bidentate organosulfur metallocycle bound to the surface through both of the ethanedithiol sulfur atoms. We propose that vinyl thiol and ethanethiol are produced via C–S bond scission and subsequent hydride elimination of the monodentate thiolate intermediate, and that the bidentate surface metallocycles undergo C–S bond scission to yield acetylene and ethylene. On the carbon-modified surface, complete desulfurization of ethanedithiol occurs upon decomposition, yielding only hydrocarbon products. With increasing sulfur coverage, a decrease in reactivity and a shift in desorption features to lower temperatures is observed for ethanedithiol on the clean and carbon-modified surfaces.The reactivity of 1,2-ethanedithiol on the clean Mo (110) and p(4×4)-C/Mo (110) surfaces has been investigated as a function of sulfur coverage using temperature programmed desorption (TPD), Auger electron spectroscopy, and low energy electron diffraction. TPD experiments performed on both surfaces produced similar reaction products, although changes were observed in selectivity. On the clean Mo (110) surface, the major products observed during TPD experiments were acetylene, ethylene, vinyl thiol, and ethanethiol. However, the reaction of ethanedithiol on the p(4×4)-C/Mo (110) surface produced acetylene, ethylene, and ethanedithiol. Product molecules are thought to arise from two distinct types of surface intermediates: (1) a monodentate thiolate species bound to the surface through only one of the ethanedithiol sulfur atoms, and (2) a bidentate organosulfur metallocycle bound to the surface through both of the ethanedithiol sulfur atoms. We propose that vinyl thiol and ethanethiol are produced via C–S b...

High pressure reaction cell and transfer mechanism for ultrahigh vacuum spectroscopic chambers

A novel high pressure reaction cell and sample transfer mechanism for ultrahigh vacuum (UHV) spectroscopic chambers is described. The design employs a unique modification of a commercial load-lock transfer system to emulate a tractable microreactor. The reaction cell has an operating pressure range of <1×10−4 to 1000 Torr and can be evacuated to UHV conditions to enable sample transfer into the spectroscopic chamber. Additionally, a newly designed sample holder equipped with electrical and thermocouple contacts is described. The sample holder is capable of resistive specimen heating to 400 and 800 °C with current requirements of 14 A (2 V) and 25 A (3.5 V), respectively. The design enables thorough material science characterization of catalytic reactions and the surface chemistry of catalytic materials without exposing the specimen to atmospheric contaminants. The system is constructed primarily from readily available commercial equipment allowing its rapid implementation into existing laboratories.

Reaction of ethanethiol on clean and carbon-modified Mo(110) surfaces as a function of sulfur coverage

Abstract The reactivity of ethanethiol was studied on carbon-modified Mo(110) surfaces using temperature-programmed desorption (TPD), Auger electron spectroscopy (AES), and low-energy electron diffraction (LEED). Ethanethiol TPD experiments performed on clean Mo(110) surfaces yielded ethane and ethylene as reaction products via an ethanethiolate surface intermediate. Relative to clean Mo(110) surfaces, TPD experiments performed on defective p(4×4)-C/Mo(110) surfaces showed no significant differences in reactivity, selectivity, or reaction pathways. Overall, the presence of an ordered carbon overlayer does not appear to affect ethanethiol surface chemistry. Ethanethiol TPD experiments were also conducted on clean and carbided Mo(110) surfaces with adsorbed sulfur. As sulfur coverage is increased, both the Mo(110) and defective p(4×4)-C/Mo(110) surfaces become progressively less reactive towards ethanethiol. Sulfur is thought to bond to adsorption sites and block the formation of the ethanethiolate surface intermediate, resulting in decreased surface reactivity. At sulfur coverages of 0.66 or higher, the ethane reaction pathway shuts down to the point that ethylene is the sole reaction product.

Modeling molybdenum carbide-based hydrodesulfurization (HDS) catalysts using carbon-modified Mo(110) surfaces

Abstract Transition metal carbides, such as Mo 2 C, have been proposed as substitutes for group VIII metal catalysts, since they exhibit similar catalytic properties in some applications. Mo 2 C catalysts have shown potential for commercial use in hydrodesulfurization (HDS) processes and tend to resist sulfur poisoning better than platinum group metals. Although these molybdenum carbide catalysts look encouraging as replacements for MoS 2 -based catalysts, questions remain regarding the fundamental surface chemistry associated with the HDS of organosulfur molecules on carbided and sulfided molybdenum catalyst surfaces. To further investigate the suitability of Mo 2 C for HDS applications, the interaction of sulfur-containing molecules with molybdenum surfaces was examined by utilizing carbon-modified Mo(110) single crystals as model catalysts. Specifically, the reactivity of ethanethiol and 1,2-ethanedithiol were studied on the clean Mo(110), defective p(4×4)-C/Mo(110), and p(4×4)-C/Mo(110) surfaces using temperature programmed desorption (TPD), Auger electron spectroscopy (AES), and low energy electron diffraction (LEED). Ethanethiol and 1,2-ethanedithiol TPD experiments demonstrated that the presence of multiple sulfhydryl (SH) groups influences surface chemistry, given the differences observed in product distribution. Ethanethiol experiments performed on clean Mo(110) surfaces yielded ethane and ethylene as reaction products, while 1,2-ethanedithiol TPD experiments produced acetylene, ethylene, vinyl thiol, and ethanethiol. Ethanethiol TPD experiments showed that no significant differences in reactivity, selectivity, or reaction pathways exist between clean Mo(110) and the defective p(4×4) surfaces. 1,2-Ethanedithiol TPD experiments performed on the clean Mo(110) and p(4×4)-C/Mo(110) surfaces produced similar reaction products, although significant changes were observed in selectivity. On the clean surface, the major desorption products were acetylene, ethylene, vinyl thiol, and ethanethiol. However, the reaction of 1,2-ethanedithiol on the p(4×4)-C/Mo(110) surface produced only acetylene and ethylene. Thus, complete desulfurization of 1,2-ethanedithiol occurs on the p(4×4) surface upon decomposition, yielding only hydrocarbon products.

Deuterium and deuterium sulfide adsorption on MoS2(0001)

Abstract The adsorption of deuterium and deuterium sulfide was studied on the MoS2(0001) basal surface using temperature programmed desorption (TPD) and four-point surface conductivity measurements. Deuterium did not dissociatively adsorb to the basal MoS2(0001) surface upon exposure, and thus was dissociated with a platinum wire during dosing. D2 and D2S were both observed as products following adsorption of dissociated D on the MoS2(0001) surface. The presence of D2S as a desorption product demonstrates that lattice sulfur was removed to form anion vacancies on the MoS2(0001) surface. D2S TPD experiments on an MoS2 surface with anion vacancies showed both D2S and D2 as desorption products. The presence of D2 in the D2S TPD shows that some deuterium sulfide dissociatively adsorbs. Further, the amount of D2 desorbing was found to decrease as a function of exposure, suggesting that D2S was leaving sulfur behind to “heal” anion vacancies. The desorption traces resulting from both deuterium and deuterium sulfide exposures were very broad, and are thought to occur via the recombination of two different desorption states. The possible implications of these experiments to HDS catalytic systems are also discussed.