Surface Science Poster Session

Tuesday, October 29, 2013 6:00 PM in Room Hall B

SS-TuP-2 Active Sites for CO Oxidation over Au/Al2O3 Model Catalyst
Isao Nakamura, Tadahiro Fujitani (National Institute of Advanced Industrial Science and Technology (AIST), Japan)

Au nanoparticles supported on titanium oxides are known to exhibit high catalytic activity for CO oxidation at low temperature. Numerous investigations have been carried out to elucidate the source of the high catalytic activity for CO oxidation. Recently, some studies have reported that the perimeter interface between Au and TiO2 is the active sites for CO oxidation [1-3]. However, it is not fully understood concerning the role of metal oxide support except for TiO2. In this study, we performed CO oxidation reaction using Au/Al2O3(111)/NiAl(110) surface in order to clarify the active sites for Au/Al2O3 catalyst.

We examined the effect of H2O partial pressure (PH2O) on the CO oxidation over Au/Al2O3(111) at 300 K. No CO2 formation was observed in the absence of H2O. The rate of CO2 formation gradually increased with increasing PH2O. The optimal CO2 formation rate was obtained at PH2O = 1.0 Torr. The H2O promoted CO oxidation reaction over Au/Al2O3(111), as was previously observed for Au/TiO2(110) [3]. However, much water was required to promote the CO oxidation activity for Au/Al2O3(111) compared with Au/TiO2(110).

To clarify the difference of PH2O on CO2 formation rate between Al2O3 and TiO2, we examined the behavior of adsorbed water on the Al2O3 and TiO2 substrates using TPD. As a result, the water adsorbed on TiO2 surface was shown to desorb at higher temperature than that on Al2O3 surface, considering that the coverage of water on TiO2 was higher than that on Al2O3 during CO oxidation reaction at 300 K. Therefore, the optimal PH2O to promote the activity differed between Au/TiO2 and Au/Al2O3 because the reserve capacity of water depends on the support.

Next, we investigated the nature of active sites for CO oxidation over Au/Al2O3 catalyst. The apparent activation energies for CO2 formation over Au/Al2O3(111) was in good agreement with that over Au/TiO2(110) [3], indicative of same active sites. Furthermore, we demonstrated that the activity per total number of Au atoms at the perimeter interface was constant regardless of the Au particle size. These results clearly indicate that the active sites were the perimeter interface between Au nanoparticle and Al2O3 support, which was consistent with the active sites for Au/TiO2.

[1] I. X. Green, W. Tang, M. Neurock, J. T. Yates Jr., Science 333 (2011) 736.

[2] D. Widmann, R. J. Behm, Angew. Chem. Int. Ed. 50 (2011) 10241.

[3] T. Fujitani, I. Nakamura, Angew. Chem. Int. Ed. 50 (2011) 10144.

SS-TuP-3 Scanning Tunneling Microscopy Study of Subsurface Atomic Activity in Heterogeneous Catalysis
Dan Killelea, Jon Derouin, Stacy Heslop, Damian Valencia, Melissa Farmer, Jon Bender (Loyola University Chicago)

Subsurface species are an enigmatic source of energetic reagents in heterogeneous reactions catalyzed by metal surfaces. As absorbed atoms in the selvedge of a metal, they may be metastable with respect to atoms adsorbed to the surface or in the gas-phase, and so can leave the subsurface with excess energy. Furthermore, when subsurface atoms emerge from beneath adsorbed molecules new reaction geometries are enabled that are otherwise inaccessible between reactants co-adsorbed to a surface. Although believed to be important reactive intermediaries, a systematic study of their fundamental chemistry has yet to be undertaken. To address this, we will develop the basis for a more complete understanding by studying the dynamics of subsurface species. We have selected two model systems for study; hydrogen on Ni(111) and oxygen Ag(111) which will provide basic details of subsurface absorption and reactivity, and further provide guidance for utilization of these species to selectively control chemistry. For catalytic transformations in either system, subsurface atoms are key components of catalytic processes, but it remains unclear how they enhance reactions. To reveal the surface dynamics, scanning tunneling microscopy (STM) will image the surface, tracking the movement of individual atoms as they diffuse across and into the metal surfaces. The unique capabilities of STM to selectively energize and manipulate atoms on surfaces will also be used to further investigate the energetics of subsurface incorporation and emergence. To complement STM images, temperature programmed desorption and Auger electron spectroscopy will identify adsorbates and provide thermodynamic information. Our results will show mechanisms for subsurface migration and we will also probe the energetics of subsurface incorporation. Taken together, this new information seeks to narrow the gap our understanding between model and actual catalytic systems and enable chemists to accurately gauge the role of subsurface species in the transformation of plentiful feedstock into energy-rich chemicals over metal catalysts.

SS-TuP-4 Photoelectron Spectroscopy Studies on the Superconducting NbC and MoC
Lazaro Huerta, Raul Escamilla (Universidad Nacional Autónoma de México); Martin Flores (Universidad de Guadalajara, Mexico); L.A. Rivera, Martin Romero (Universidad Nacional Autónoma de México)

X-ray Photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) were used to investigate the binding energies and valence band for the NbC and MoC phases. The NbC and MoC polycrystalline samples superconducting were synthesized by arc melting. X-ray diffraction pattern showed that the samples were essentially single phase. The Nb 3d, Mo 3 d and C 1s core levels associated to the chemical states of NbC and MoC were identified. Comparing the Nb 3d, Mo 3d and C 1s core levels with metallic Nb, Mo and C reference materials, we observed a positive chemical shift for Nb 3d and Mo 3d a negative chemical shift for C 1s in both samples. These results suggest that the charge transfer model based on the concept of electronegativity is applicable on the NbC and MoC compounds.

SS-TuP-5 FTIR Spectroscopy of Water Clusters in Methane Matrix
Nozomi Ozawa, Shinichi Takekuma, Koichiro Yamakawa, Ichiro Arakawa (Gakushuin University, Japan)

Water clusters have been investigated in terms of their geometric structures and electronic states for the last few decades [1]. One of the main purposes of these studies was to obtain information of hydrogen-bond interactions in water and ice. Experimentally, a matrix isolation technique, in which water clusters are prepared in a condensed film of chemically inactive species such as para-H2, and rare gases, has also been a useful method for the cluster study because of high sample density [2][3]. It is worth noting that the cyclic water hexamer which has not been found in the gas phase was observed in para-H2 matrices [4]. In the present study we investigated water clusters in the methane matrix by Fourier Transform Infrared Spectroscopy (FTIRS).

A vacuum chamber equipped with a liquid helium cryostat was evacuated and baked at 393 K for 24 hours, which resulted in the base pressure of 1 * 10-8 Pa. The methane matrix was deposited on the gold-coated oxygen-free copper substrate which was fixed on the cryostat and maintained at 6 K. Methane and water vapor were dosed from different gas lines through variable leak valves. The mole ratios of water to methane (H2O/CH4) were 0.026-0.053. Infrared spectra were measured in a reflection configuration using a FTIR spectrometer with a compartment of a liquid nitrogen cooled HgCdTe detector. The spectral range was 500-6000 cm-1 and a resolution was set at 4 cm-1 in the present experiment. The incident angle of infrared light on the substrate was 80 . The whole optical path was evacuated to eliminate infrared absorption due to carbon dioxide and water vapor in the atmosphere.

The infrared spectra of water clusters in methane matrices showed adsorption peaks at 3531 cm-1, 3497 cm-1, 3360 cm-1, 3344 cm-1 and 3216 cm-1, all of which are attributed to vibrational modes of bonded OH, which forms a hydrogen bond with an oxygen atom of a neighboring water molecule. The peaks at 3531 cm-1 and 3497 cm-1 are due to the trimer and the peak at 3216 cm-1 to the cage-hexamer. The other peaks at 3360 cm-1 and 3344 cm-1 may be attributed to pentamer and cyclic-hexamer, respectively. In addition to the variety of the absorption peaks, we found that the relative intensities of these peaks depended on the H2O/CH4 ratio, which was likely to reflect the size distribution of the water clusters.

[1] A. Bankura et al., Chem. Phys. 400, 154 (2012).

[2] M. E. Fajardo et al., J. Chem. Phys. 115, 6807 (2001).

[3] S. Hirabayashi et al., J. Chem. Phys. 122, 244501 (2005).

[4] F.Huisken et al., J. Chem. Phys. 104, 17 (1996).
SS-TuP-6 Work Function Variation in Alkali Metal / W(110)
AllanAbraham Padama, WilsonAgerico Diño, Hideaki Kasai (Osaka University, Japan)

In surface systems, work function is an important property that indicates the required energy necessary to remove an electron from solid to vacuum. This property is essential for different applications that require electron emission such as in optoelectronic and thermionic devices and in ion productions for plasma application [1-3]. Low work function system is desired and is usually realized by introducing adsorbate which can modify the electronic properties of the surface under investigation [4-6]. It is therefore fundamental to understand the work function lowering and establish indicators that could possibly predict the changes in work function among materials.

In this present work, we investigated the variation of work function in alkali metal (Li, Na, K, Rb, Cs) / W(110) surface systems at different alkali metal coverage by performing density functional theory-based calculations. For all cases, work function rapidly decreases and reaches minimum value at an optimum coverage and rises again with further increase in alkali metal coverage (Fig. 1). The variation of work function values was analyzed using the density of states projected on the adsorbed alkali metal atoms. For K, Rb, and Cs, we observed that the minimum work function is achieved when the states corresponding to the interaction of the adsorbed alkali metals are partially occupied. Further increase in the coverage shifts the states to the occupied region and signifies the interaction among the adsorbed alkali metals which causes the increase in work function. For Li and Na which have smaller atomic radius, work function starts to increase when these states begin to shift in the occupied region. We conclude that the electronic profile of the system is a simple yet good indicator in predicting work function variation in alkali metal – W(110). The details of our work will be presented in the symposium.

References:

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[2] M. Bacal, Chem. Phys. 398 (2012) 3-6.

[3] M. Lin, R. Jao, W. Lin, J. Vac.Sci. Technol. B 26 (2008) 821-825.

[4] H. Nakanishi, H. Kasai and A. Okiji, Surf. Sci. 197, 515 (1988).

[5] H. Nakanishi, H. Kasai and A. Okiji, Surf. Sci. 216, 249 (1989).

[6] H. Ishida and K. Terakura, Phys. Rev B 36, 4510 (1987).

SS-TuP-7 The Analysis of an Asymmetric Peak of Infrared Absorption due to the Antisymmetric Stretching Vibration of CO2 on TiO2 Nanotubes
Koichiro Yamakawa (Gakushuin University, Japan); Katsuyuki Fukutani (University of Tokyo, Japan)

The origin of asymmetric line shapes appearing in spectroscopy were first discussed in terms of interaction between a discrete state and continuous states by Fano [1]. Thereafter this Fano effect has been observed in a variety of systems including the adsorbate–surface system such as H / W(100), CO / Cu(100), and CO / Cu(111) [2, 3], where metal electronic states of a continuous spectrum interact with the vibrational state of the adsorbate. In the present study we performed Fourier transform infrared spectroscopy (FTIRS) of CO2 adsorbed on TiO2 nanotubes to observe an asymmetric absorption peak in the range of the antisymmetric stretching vibration (v3). In contrast to previous studies on metallic substrates, the asymmetric peak was observed on a semiconductor substrate in this study.

TiO2 nanotubes were synthesized following the way described in the literature [4]. For evaluation of the nanotubes Raman spectroscopy, infrared spectroscopy, scanning electron microscopy and transmission electron microscopy (TEM) were performed. From the TEM images the diameters of the tubes were estimated to be several nanometers. Then, the TiO2 nanotubes dispersed in ethyl alcohol were sprayed on a CaF2(111) surface, which was set on a liquid-nitrogen cooled cryostat in an ultra-high vacuum chamber with a base pressure of 2×10-8 Pa. Infrared spectra were recorded in a transmission configuration, using a FTIR spectrometer at a spectral resolution of 4 cm-1 with a liquid-nitrogen cooled HgCdTe detector. The substrate temperature was kept at 81 K.

In the spectral range of v3 an absorption peak grew around 2340 cm-1 first, as the exposure of CO2 was increased. After this peak was saturated, a new absorption peak grew at 2350 cm-1. Saturation of the latter peak was followed by formation of a CO2 multilayer. Noting that the former peak appeared to be only a slight shoulder at the saturation of the latter peak, the former was assigned to v3 of CO2 on oxygen vacancy sites, while the latter to that on terrace sites. Whereas the absorption peak at 2350 cm-1 exhibited a normal absorption feature, the peak at 2340 cm-1 revealed an asymmetric shape. Adsorption on defect sites is thought to play an important role for appearance of the asymmetric peak.

[1]U. Fano, Phys. Rev. 124, 1866 (1961).

[2]Y. J. Chabal, Phy. Rev. Lett. 55, 845 (1985).

[3]C. J. Hirschmugl et al., J. Electron Spectrosc. Relat. Phenom. 54, 109 (1990).

[4]T. Kasuga et al., Adv. Mater. 11, 1307 (1999).

SS-TuP-8 Deposition of the Transparent and Conductive ZnO Film by an Atmospheric Pressure Plasma Jet
Chun-Ming Hsu, Shau-Tzu Lien, Yao-Jhen Yang, Jian-Zhang Chen, I-Chun Cheng, Cheng-Che Hsu (National Taiwan University, Taiwan, Republic of China)

In this work, deposition of transparent and conductive ZnO thin films using an atmospheric pressure plasma jet (APPJ) is presented. The film properties, namely morphology, crystal structure, conductivity, and transmittance, are examined using SEM, XRD, 2-probe measurement, and UV-Vis spectrometer, respectively. The APPJ is sustained by a pulsed power source with a repetitive frequency up to 25 kHz using N2 or O2 as plasma gases. Zinc chloride solution is used as the precursor and is nebulized and then sprayed into the downstream of the plasma jet to deposit thin films on glass or Si wafer substrates. It is found that upon exposure of the precursor to the plasma jet, sheet-like zinc hydroxide chloride is first formed, and is then converted to zinc oxide if the jet temperature is sufficiently high. Under an optimal condition, ZnO films with the resistivity of 0.9 Ohm-cm and the average transmittance between 400 and 800 nm of 80% can be obtained. This demonstrates a one-step and fast process without the need of post-annealing steps to fabricate transparent and conductive ZnO films.

SS-TuP-9 Effect of Isothermal Aging on the Interfacial Reactions between Sn-58Bi Solder and ENEPIG
Sang-Min Lee, Seung-Boo Jung (Sungkyunkwan University, Republic of Korea)

The interfacial reaction between the Sn-58wt.% Bi solder and a surface finish of ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold) were studied during isothermal aging at 125 ℃ . The thin uppermost layer of immersion gold was resolved into solders bulk and the intermetallic compound (IMC) as known as PdSn4 was created between tin of solder and Pd-P layer, while the solder was reflowing. As aging time increased, the new IMC creation of Ni3Sn4 between Ni-P layer and tin of solder below PdSn4 became dominent. As aging time increased more, the IMC of PdSn4 was resolved out into solder bulk and the IMC of Ni3Sn4 remained. As aging time went further, Ni-P layer was consumed and pits happened on Ni-P layer made paths Ni3Sn4 through Cu layer, and the IMC of Ni3Sn4 was discontinuously growing. As result of Cu penetration through Ni-P layer, the IMC became into (Ni,Cu)3Sn4, and the new IMC of (Ni,Cu)6Sn5 under (Ni,Cu)3Sn4 was created and growing. As aging time increased over 500 hour, the main IMC is (Ni,Cu)6Sn5 and (Ni,Cu)3Sn4 between ENEPIG and 58wt.% Bi solder. In addition to Ni-P layer, presence of phosphorous made such as Ni3P layer and NiSnP layer. Result of this study showed that the surface finish of ENEPIG layer acts as a diffusion barrier and be able to expect the IMC behavior for long term as low temperature solder.

SS-TuP-10 Adsorption Properties of Ammonia on Defective Graphite Surfaces
Riku Shibuya, Masataka Sakurai, Yoshihiko Saito, Keiichi Kawarai (University of Tsukuba, Japan); Takahiro Kondo (University of Tsukuba and PREST JST, Japan); Junji Nakamura (University of Tsukuba, Japan)

The graphite surface consists of the p conjugated system which originates from the resonance of the 2pZ electron orbital of the carbon atoms and it is well known as an inert surface. However, because of the perturbation of π conjugated system, the non-bonding π electronic states of the graphite-related materials such as graphene sheets and graphite are known to appear as the localized states at the carbon atoms around the zigzag edge,1,2 metal cluster,3 defects,4 and dopants. We have reported that localized non-bonding electronic states near the Fermi level in the unoccupied region at carbon atoms around point defects of graphite by scanning tunneling microscopy/ spectroscopy (STM/STS). This localized state propagates with threefold symmetry perpendicular to the zigzag edges at the point defect. Considering that the carbon atoms with the localized state may act as an acid site, we have examined the ammonia adsorption properties on graphite surface with point defects. In this work, we have prepared the defect induced HOPG surface (Ar+/HOPG) by Ar ion bombardment (ion energy is 140 eV and coverage of defect is 1%). We investigated the ammonia adsorption properties on Ar+/HOPG by Temperature programmed desorption (TPD). Helium atom scattering (HAS), XPS, and STM/STS. From TPD and HAS, we found that ammonia weekly chemisorbed near the defects at 110 K.The detail of adsorption features will be discussed in the presentation with our STM /STS and XPS results .

1. M. Fujita, et al., J. Phys. Soc. Jpn., 65 (1996) 1920.

2. Y. Niimi, et al., Appl. Surf. Sci., 241 (2005) 43; Y. Kobayashi, et al., Phys. Rev. B 71 (2005) 193406 .

3. T. Kondo, Y. Iwasaki, Y. Honma, Y. Takagi, S. Okada and J. Nakamura, Phys. Rev. B 80 (2009) 233408.

4. T. Kondo, Y. Honma, T. Machida, J. Oh, J. Nakamura, Phys. Rev. B 82 (2010) 153414

SS-TuP-11 Photo-functional Property of TiO2/Cu2O and TiO2/TiN/Cu2O Thin Films by Reactive Magnetron Sputtering
Takumi Nakajima, Ichiro Takano (Kogakuin University, Japan)

Characteristics based on photo-inducement of TiO2 have attracted various interests in many fields. TiO2 has been known as one of a promising photocatalyst and is already used in various practical applications, such as the degradation of environmental pollutants and the self-cleaning of glasses. Furthermore, the most important characteristic as a photocatalyst of TiO2 is well known that photo-excited state is very stable and does not cause self-decomposition. Therefore, the electrolysis of water is performed under ultra-violet (UV) light irradiation to TiO2. However UV energy accounts for only a small fraction (~5%) of the sun’s energy compared to the visible region (45%).

In this study, to improve the photo-functional property of TiO2 the double layer films were fabricated by the constitution of the TiO2 layer of an n-type semiconductor and the Cu2O layer of a p-type semiconductor. Each arrangement of the film with TiO2/Cu2O was also investigated by optical permeability. Furthermore, to prevent diffusion of Cu from the Cu2O layer to the TiO2 layer, the TiN layer was inserted between the TiO2 layer and the Cu2O layer. As the following study, this Cu2O layer was replaced to the Ag2O layer. Those layers were fabricated by reactive magnetron sputtering. Composition and microstructure of these films were investigated by X-ray photoelectron spectroscopy and X-ray diffraction, respectively. Chromatic change of a methylene blue solution was applied for the photocatalytic evaluation. Light irradiation to the sample in a methylene blue solution was carried out using a commercial sterilizing lamp as ultraviolet light and an artificial sun lamp as visible light. Transmittance of a methylene blue solution was measured by a spectrophotometer after irradiation for 6 hours by each lamp.

The XRD pattern of the TiO2/Cu2O thin film showed the strong peak of the anatase-rutile TiO2 from an upper layer and the weak peak of Cu2O from a lower layer. The suitable photocatalytic effect was obtained by the constitution of TiO2 with 300 nm and Cu2O with 200 nm in thickness, when the photocatalytic effect showed about 31 % under an artificial sun lamp and about 90 % under a sterilization lamp. In the case of the TiO2/TiN/Cu2O film it was estimated that the diffusion of Cu atoms was almost prevented by inserting TiN from the XRD pattern, when the photocatalytic effect showed about 39 % under an artificial sun lamp and about 89 % under a sterilization lamp.

SS-TuP-12 Characterization of Bimetallic Pt-Co Clusters and Surface Composition Effect on Catalyst Performance for Proton Exchange Membrane
AliSems Ahsen (Gebze Institute of Technology, Turkey); OguzKaan Ozdemir (Yildiz Technical University, Turkey); Osman Ozturk (Gebze Institute of Technology, Turkey); Hui Yan, RandimaPiyumalie Galhenage, Donna Chen (University of South Carolina)

Pt and Pt-Co bimetallic alloy clusters were grown both on commercial hydrophobic carbon paper substrate and on TiO2(110) crystal. Pt and Pt-Co bimetallic surfaces were deposited on carbon paper substrate using dc magnetron sputtering to investigate electrocatalytic properties of Pt-Co interaction toward the oxygen reduction reaction in 0.5M H2SO4 solution by means of cyclic voltammetry (CV) and linear sweep voltammetry (LSV) on rotating disc electrode (RDE). Pt‐Co alloy catalysts showed significant improvement on catalytic activity against the pure platinum catalysts. The increase of Co content leads to rise in the electrochemical active surface area (EASA) and improved Pt utilization. The improvement is closely related with the electronic interaction of Pt with Co and modification of the surface electronic structure using bimetallic alloy clusters. Pt-Co alloy surface as a catalyst improved also polymer electrode membrane fuel cell (PEM) performance and deceased Pt usage compared to pure Pt catalyst. Bimetallic clusters are investigated on TiO2 crystal surface to clarify the Pt-Co interaction.

The growth, nucleation and chemical activity of bimetallic Pt-Co clusters on TiO2(110) crystal have been investigated by using scanning tunneling microscopy (STM) and low energy ion scattering (LEIS). STM results demonstrate that the mobility of Pt atoms on TiO2(110) crystal is higher than mobility of Co atoms. For equivalent coverage of Pt and Co the number of cluster density of Pt is higher than for Co. For clusters with a total coverage of 0.25 ML, the cluster density increases as the fraction of Co increases. Next, the surface compositions of Pt on Co and Co on Pt bimetallic clusters are studied using low energy ion scattering (LEIS). The bulk composition is ranging from 100% Pt to 75%, 50%, 25% Pt and 100% Co within total metal coverage of 0.25 ML. For all coverages the cluster surfaces are covered by Pt, which is relatively mobile compared to Co. This explains also the electrochemical improvement. After annealing to 900K all clusters become encapsulated by titania but for the bimetallic clusters the interaction of Co with Pt can be observed up to 600K.

SS-TuP-14 Improvement of PIN-Type Amorphous Silicon Thin Film Solar Cell Performance with Phosphorus Light Doping in Intrinsic Layer
Won-Ho Son, Sang-Kwon Lee, TaeGu Kim, Si-Hun Lee, Sie-Young Choi (Kyungpook National University, Republic of Korea)
In this paper, by investigating the effect of phosphorus light doping on an i-layer of the pin-type hydrogenated amorphous silicon (a-Si:H) solar cells, we confirmed that the incorporation of phosphorus in a-Si:H films has produced superior i-layer material for pin-type thin-films solar cells than the undoped i-layer material. The doped a-Si:H films with various phosphorus concentrations were fabricated by mixing phosphine (PH3), silane (SiH4)with hydrogen (H2) during the i-layer deposition. All films of amorphous materials were deposited by 13.56 MHz plasma enhanced chemical vapor deposition (PECVD) method. The PH3 flow rate in the reactive gas was modified from 0 to 4.2 sccm (standard cubic centimeter per minute). The phosphorus concentration of the i-layer was measured by secondary ion mass spectroscopy (SIMS). It has been shown from the experiments that a-Si:H film doped with about 2×1018 atoms/cm3 of phosphorus shows an n--type property at a flow rate of 0.9 sccm. The optical properties of the films were measured with UV-VIS-NIR spectrophotometer in the range of 380 - 2000 nm. The various values of open-circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF) and conversion efficiency (Eff) were measured under 100 mW/cm2 (AM 1.5) solar simulator irradiation. Finally, by applying this process, we were able to improve the all the electrical parameters (Voc, Jsc and FF).
SS-TuP-15 Experimental and Theoretical Investigation of Stress-induced Near-surface Compositional Redistribution on Si0.8Ge0.2 Substrates for 2D Array Growth of Ge Quantum Dots
Swapnadip Ghosh, Jose Bonilla (University of New Mexico); Daniel Kaiser, Talid Sinno (University of Pennsylvania); Sang M. Han (University of New Mexico)

Long-range, ordered, three-dimensional micro-/nano-structures of Si/Ge heteroepitaxial systems possess unique electronic properties for a wide range of applications, including quantum computers, photodetectors, and solar cells. Herein, we use simulation to predict and experiment to demonstrate the compositional redistribution of Si and Ge in the near-surface region of Si0.8Ge0.2 substrates by applying a spatially structured compressive stress to the substrate and thermally annealing the substrate under stress. The stress is applied by a mechanical assembly that presses a 2D array of Si pillars (40 nm diameter and 400 nm pitch) fabricated on a Si substrate against the Si1-xGex substrate. Based on energy dispersive x-ray spectroscopy, the compressed region shows Ge depletion (8%), while the surrounding areas show an increase in Ge concentration (3%). This compositional variation in turn can be used to selectively grow a 2D array of Ge quantum dots upon Ge exposure. When the annealing temperature is above 950 °C, the compressive stress under each pillar (> 1 GPa) is found to generate plastic deformation, while no plastic deformation is observed below 950 °C. The depth of the plastically deformed regions increases from 15 to 30 nm as the annealing temperature, held constant for each annealing experiment, increases from 950 to 1000 °C. We attribute the plastic deformation to (1) the localized pressure applied to the substrate under the contact area (4.3 to 15 GPa), (2) the near-surface substrate stiffness decreasing with increasing substrate temperature, and (3) the tensile biaxial stress (86 to 135 GPa) under the compressed region due to different thermal expansion rates of Si vs. Si0.8Ge0.2. In particular, the biaxial stress exceeds the reduced Young’s modulus (75.5 GPa) at the annealing temperature range. The compositional redistribution of Ge under purely elastic deformation conditions is also predicted, using a kinetic Monte Carlo simulation that accounts for the influence of composition, temperature, and stress on the diffusion kinetics of Ge in SiGe alloy. We will compare the prediction with experimental results in this presentation.

SS-TuP-16 Characterization and Chemical Activity of Pt-Ru Clusters on HOPG: Vapor Deposition and Electroless Deposition
RandimaPiyumalie Galhenage, Kangmin Xie, John Tengco, John Monnier, Donna Chen (University of South Carolina)

Bimetallic Pt-Ru clusters on highly oriented pyrolytic graphite (HOPG) have been studied as model systems for direct methanol fuel cell catalysts. Metal clusters deposited via vapor-deposition in ultrahigh vacuum have been compared with clusters prepared via electroless deposition (ED) of Ru onto existing Pt seed clusters. Cluster sizes and distributions are characterized by scanning tunneling microscopy (STM), while surface compositions are examined by low energy ion scattering (LEIS) and X-ray photoelectron spectroscopy (XPS). For the growth of 0.25 ML of pure Pt on HOPG, STM images show that large Pt clusters (average height 33 Å) are formed, and these clusters are located almost exclusively at the step edges. Ar ion sputtering was used to introduce defects and create nucleation sites for Pt clusters on the terraces; this treatment resulted in smaller cluster sizes (12 Å ) and higher cluster densities (0.12 x 1012 cm-2 vs. 2.25 x 1012 cm-2 ) compared to growth on the unmodified HOPG surface. When 0.5 ML of Ru was vapor-deposited onto 0.5 ML Pt clusters, the number of clusters did not increase, indicating that bimetallic clusters are formed by incorporation of Ru into the Pt seed clusters. Electroless deposition of Ru onto 0.5 ML Pt clusters on HOPG was carried out in solution using Ru(NH3)6Cl3 with formic acid as a reducing agent at pH 3. LEIS experiments confirmed that Ru was deposited onto the Pt seed clusters, whereas no Ru was deposited on HOPG in the absence of the Pt seed clusters. STM images of the Pt-Ru clusters grown by ED indicate that the cluster densities and sizes are comparable to that of the vapor-deposited clusters. Methanol oxidation experiments will be carried out to study the activity of the Pt-Ru/HOPG surfaces in a recirculating loop reactor coupled to the vacuum chamber.

SS-TuP-18 Molecular Control of Doping at a Topological Insulator Surface: F4-TCNQ on Bi2Se3
Jingying Wang, Andrew Hewitt, Dan Dougherty (North Carolina State University)
The simplest 3D topological insulator is Bi2Se3, with a single almost perfectly-conical topological surface state. A challenge in the quest to characterize charge and spin transport in this unique surface state band is that samples are almost always heavily n-doped by Se vacancies when grown from the binary melt. Recently, the strong electron acceptor, F4-TCNQ has been succesfully employed to control doping in exfoliated Bi2Se3 and allowed access to surface transport properties [1]. This poster reports our characterization of F4-TCNQ film growth on Bi2Se3 by combinig Atomic Force microscopy, X-ray photoelectron spectroscopy, and angle-resolved UV photoelectron spectroscopy. We report evidence for a Stranski-Krastanov film growth mode and interfacial charge transfer effects. [1] D. Kim et al., Nat. Phys. 8, 460 (2012).
SS-TuP-19 Tunneling Electron-induced Hopping Motion of CO on Ag (110) Surface
Junepyo Oh, Yousoo Kim (RIKEN, Japan)

Inelastic electron tunneling (IET) process can induce motion and reaction of a single molecule at metal surfaces. CO hopping phenomena on Pd(110) and Cu(110) surfaces have been achieved with tunneling electrons from the tip of a scanning tunneling microscope (STM) [1]. This phenomenon is initiated by the excitation of a high-frequency (HF) vibrational mode (C-O stretching mode, ~240 meV) with inelastic tunneling electrons from the tip of scanning tunneling microscopy. The CO hopping, however, cannot be induced on Cu(110), even though the hopping barrier is lower than that on Pd(110). This result ascribed as anharmonic coupling between low-frequency (LF, ~25 meV) modes and the HF mode combined with electron-hole pair creation. W. Ho et al. reported that CO molecules on Ag(110) surface always move when the tip-substrate bias voltage reached beyond 240 meV [2]. It is really interesting because Cu and Ag are very similar to each other in terms of vibrational mode coupling [3].

In this work, we have measured the hopping probability of CO molecule on Ag(110) surface by injecting tunneling electrons from STM tip as a function of sample bias (Vs) up to 320 meV. The probability shows sudden increases at 80 meV, even though there are no vibrational modes correspond to this energy. As one of possibilities, it is considered that direct excitation of either a frustrated rotation or translation mode causes the CO hopping on the Ag(110) surface. The detail of hopping mechanism at the low bias range will be discussed in the presentation.

References;

[1] T. Komeda, Y. Kim, M. Kawai, B.N.J. Persson, H. Ueba, Science295(2002)2055

[2] H.J. Lee, W. Ho, Science286(1999)1719

[3] N. Lorente, H. Ueba, Eur. Phys. J. D35(2005)341

SS-TuP-20 Properties of Metal Coating Films on the Biodegradable Resin Irradiated by Ion Beam
Ryosuke Tan, Ichiro Takano (Kogakuin University, Japan)

Recently, biodegradable resin attracts attention as one of the effective use of resources on environmental measures. PGA (Polyglycolic acid) used in this study is categorized to a kind of polyester resin and is composed of hydrogen, carbon and oxygen. PGA shows a high gas barrier property, a high hydrolysis property and high mechanical strength. These characteristics are applied for sutures of surgery or multi-layer PET bottles, while there is hardly application in an electronic field. The usage of PGA in electronic parts such as a printed circuit board has the important role in environmental measures, however, there are some problems that have to be overcome.

In this study, the surface of PGA was modified by using an ion beam so that the durability of the PGA coated with metal films was improved. The ion beam cut off the bonding of molecules and as a result the surface of PGA turned to a carbon layer which was stable against heating or humidity. Metal films of Ti or Cu were deposited on the modified PGA by vacuum evaporation. The ion beam irradiation and vacuum evaporation were performed using the high current ion implanter with an electron beam evaporator. The ion beam was extracted from the bucket type ion source with multi-aperture electrodes.

The Ar+ ion irradiation conditions were controlled at a current density of 20 μA/cm2, an acceleration voltage of 1 kV and irradiation time of 0-70 s. The deposition conditions of Ti and Cu were kept at deposition rate of 0.3 nm/s and a film thickness of 200 nm. The prepared samples were the metal coated PGA sample, the ion irradiated PGA sample and the metal coated sample on the irradiated PGA. The sample hardness was measured by a load-unloading method using micro-hardness tester with Knoop indenter. Friction coefficient and wear measurements were evaluated by reciprocating sliding test with load of 0.198N. The water contact angle was measured by using the θ/2 method. Electrical conductivity of metal films was calculated from V-I characteristics measured using the four probe method. Hydrolysis test was evaluated in a distilled water of 100 ml of 30℃. Surface chemical-bonding state was investigated by X-ray photoelectron spectroscopy.

Knoop hardness of Ti or Cu coated samples on the Ar+ ion irradiated PGA showed the maximum value around 50s in irradiation time. Mechanical properties of these samples were improved as compared with the metal coated PGA without Ar+ ion irradiation. The measurement results of electrical conductivity suggested the possibility of PGA used as a printed circuit board.

SS-TuP-21 Atomic Force Microscopy Studies of Charge Transport Properties of Strained and Indented Topological Insulator
JinHeui Hwang, Sangku Kwon (KAIST, Republic of Korea); Joonbum Park (Pohang University of Science and Technology, Republic of Korea); Jhinhwan Lee (KAIST, Republic of Korea); JunSung Kim (Pohang University of Science and Technology, Republic of Korea); Ho-Ki Lyeo (Korea Research Institute of Standards and Science, Republic of Korea); JeongYoung Park (KAIST, Republic of Korea)
We investigated the correlation between electrical transport and mechanical stress in Bi2Te2Se by using a conductive probe atomic force microscopy in an ultra-high vacuum environment. Uniform distribution of measured friction and current were observed over a single quintuple layer terrace, which is an indication of the uniform chemical composition of the surface. By measuring the charge transport of Bi2Te2Se surface as a function of the load applied by a tip to the sample, we found that the current density varies with applied load. The variation of current density was explained in light of the combined effect of the changes in the in-plane conductance and spin-orbit coupling that were theoretically predicted. We suppose that the local density of states is modified by tip-induced strain, but topological phase still remains. We exposed a clean topological insulator surface by tip-induced indentation. The surface conductance on the indented Bi2Te2Se surface was studied, and the role of surface oxide on the surface conductance is discussed.
SS-TuP-22 Silicon Rich Oxide Films Deposited with Different Flows: An Approximation of its Kinetics Growth
Karim Monfil, Diana Vázquez, Godofredo Garcia, José Luna (Benemérita Universidad Autónoma de Puebla, Mexico); Alfredo Morales (Centro de Investigación en Materiales Avanzados, Mexico); Jesus Carrillo, Alfredo Benítez (Benemérita Universidad Autónoma de Puebla, Mexico)
Recently silicon rich oxides have been a material important for optical functions. Various techniques can be used as Hot-fire Chemical Vapor Deposition (HFCVD). The SiOx has shown to have a very good photoluminescence response and is also compatible with silicon technology. Therefore, in this works, SiOx Films were obtained by hot fire chemical vapor deposition (HFCVD) with different deposit conditions and an approximation of its kinetics grown was realized. To realize this approximation with experimental data, the hydrogen flows were changing of: 50, 75, 100, and 125 and 150 sccm, and the range of deposit temperature were from 800 to 1000°C. This approximation was realized with Arrhenius´s equation. The activation energies for SiOx films were determined by Arrhenius’s plot using the differences between weights before and after of the deposits. The slope of the plot describes us that our deposits have been due at the transport of the precursors. A more detailed analysis of the results is presented to confirm this.
SS-TuP-23 Ultrasound Influence on the Si-SiO2 System Defects Structure
Daniel Kropman (Tallinn University of Technology, Estonia)
Effect of ultrasonic treatment on the defect structure of the Si-SiO2 system. The effect of ultrasonic treatment (UST) on the defect structure of the Si–SiO2 system by means of electron spin resonance (ESR), metallography, MOS capacitance technique and secondary ions mass-spectroscopy (SIMS) is presented. The non-monotonous dependence of the defect densities on the US wave intensity has been observed. The influence of the UST frequency on the ESR signal intensity of the defect centres depended on the defects type and structure and may be caused by vibrational energy dissipation which are a function of defect centers type. The influence of the UST on the Si–SiO2 interface properties depends on the oxide thickness and crystallographic orientation.The density of point defects and absorbed impurities at the Si–SiO2 interface can be reduced and its electrical parameters improved by an appropriate choice of the UST and oxidation condition.
SS-TuP-24 A Review of the Current State and Usage of SIMS Libraries
Michaeleen Pacholski (The Dow Chemical Company)
There are several databases and libraries of static SIMS data, however their utility in general problem solving is unclear. Many of these libraries are older, and use ion sources with comparatively low yields to today’s sources. We have initiated a survey of the community and will present the results at the meeting. The questions probe which resources are used, approaches to interpretation of unknowns, how many people add their own data to the libraries, ideas for improvement to the tools and types of samples analyzed.
SS-TuP-27 Stabilization and Transformation of Nanoalloy Janus-type Configurations Predicted by Adapted FCEM/CBEV Method
Micha Polak, Leonid Rubinovich (Ben-Gurion University of the Negev, Israel)

Chemical ordering patterns in 201 atom Pt-Ir truncated octahedron (TO) nanoparticles (NPs) have been studied using energetics based on coordination-dependent bond-energy variations (CBEV1), combined with the statistical-mechanical free-energy concentration expansion method (FCEM2). The latter had to be adapted for handling Janus-type asymmetric configurations by removing the restriction of central-symmetry (which was imposed in our previous study of 923 atom Pt-Ir cuboctahedrons3) in favour of cylindrical-symmetry with respect to the [111] or [100] NP axes.

The phase-separated Janus (or quasi-Janus) structures are found to be more stable at low temperatures as compared to core-shell or onion-like Tos, and the significant role of CBEV in their stabilization is elucidated. In particular, due to its preferential bond strengthening with the segregated Pt surface, Ir starts to populate mainly (111) subsurface sites, serving as a driving force for quasi-Janus formation via the quite large Pt-Ir demixing/separation tendency. The high efficiency of the FCEM enables to compute the complete temperature dependence of chemical ordering, revealing a sharp transformation from the Janus type to symmetric structures. The corresponding transition temperatures vs. the NP overall composition (together with results to be computed for 38 and 586 atom TO NPs) can furnish the first Pt-Ir nanophase diagrams.

Such chemical ordering patterns, especially the subsurface site-specific composition, can be highly relevant to the reactivity and selectivity of Pt-Ir NPs in catalytic reactions.

1. M. Polak and L. Rubinovich, Surf. Sci. Rep.,38, 127-194 (2000).

2. L. Rubinovich and M. Polak, Phys. Rev.B80, 045404 (2009).

3. H. Tigger, L. Rubinovich and M. Polak, J. Phys. Chem. C, 116, 26000-26005 (2012).

SS-TuP-28 Ion-Modified MOS Structures
Radhey Shyam, Endu Srinadhu, Jim Harriss, Rod Harrell, Chad Sosolik (Clemson University)

As-grown SiO2 layers (50 nm to 200 nm) were exposed to incident beam of Na+ ions with energies in the range of 100 eV to 10 keV. The oxide is analyzed post exposure by encapsulating the irradiated and pristine region under top metallic (Aluminum) contacts or within a finished MOS device. Characterization of the resulting ion-modified and pristine MOS devices involves the standard techniques of bias-temperature stress and high and low frequency capacitance-voltage (C-V) measurements. Our high frequency C-V data show flatband voltage shifts and changes in slope of the irradiated devices that are beam energy dependent. The flatband voltage shifts are greater than those expected for mobile charges only, implying an irradiation-dependent effect. Additionally, the interface trap density, extracted from the high and low frequency C-V measurements, increases by one order of magnitude over our incident beam energy. We model these effects with standard formalisms for MOS capacitance in the presence of interface defects, where SRIM is used to include ion implantation depth and damage within the oxide.

SS-TuP-29 Adsorbate-induced Atomic Segregation Phenomena and Morphological Stability of Size- and Shape-Selected Octahedral PtNi Nanoparticles
Mahdi Ahmadi, Farzad Behafarid (University of Central Florida); Chunhua Cui, Peter strasser (Technical University Berlin, Germany); Beatriz Roldan Cuenya (University of Central Florida)
A study of the morphological and chemical stability of shape-selected octahedral PtNi nanoparticles (NPs) supported on highly-oriented pyrolytic graphite (HOPG) is presented. In situ X-ray photoelectron spectroscopy (XPS), ex situ atomic force microscopy (AFM) and transmission electron microscopy (TEM) measurements were used to monitor atomic segregation phenomena in PtNi NPs under vacuum, H2, and O2 atmospheres. The initial oxidation state of the as-prepared NPs was found to play a pivotal role in the final surface composition of the NPs after the different thermal treatments. In particular, for these ex-situ synthesized NPs, Ni segregation to the NP surface was observed in all environments until all NiOx species were reduced. Enhanced Ni surface segregation was observed in the presence of oxygen, while Pt surface segregation took place in hydrogen and vacuum above 270°C.
SS-TuP-31 Surface Plasmon Mediated Chemical Solution Deposition of Gold Nanoparticles at Room Temperature
WeiDavid Wei (University of Florida)
Utilizing intrinsic surface properties to direct and control nanostructure growth on a large-scale surface is fundamentally interesting and holds great technological promise. We have developed a novel “bottom-up” approach to fabricating sub-10 nm Au nanoparticles on a nanostructured Ag surface via a chemical solution deposition by using localized surface plasmon resonance (SPR) excitation. The Au nanoparticle sizes are tunable from 3 to 10 nm by easily adjusting the deposition time. Unlike the nanoparticles synthesized through wet chemistry methods, these Au nanoparticles are thermally stable and their surfaces are free of organic ligand protection. Moreover, the fabricated Au nanoparticles show high efficiency for CO oxidation in the presence of visible light.
SS-TuP-32 Point Defects Interaction with Impurities in the Si-SiO2 System and its Influence on the Interface Properties
Edvin Dauksta (Riga Technical University, Latvia); Daniel Kropman (Tallinn University of Technology, Estonia)
Point defects at Si surface and interface with dielectric affect on the minority charg carriers live time and are responsible for solar cells efficiency. PD density diminishing and its interaction with impurities (hydrogen) allow to improve solar cells efficiency Hydrogen content in SiO2 were varied by Si3N4 deposition. Si3N4 is not transparent for H and provide it storige in SiO2. It was establised that Si3N4 deposition on Si-SiO2 improve the interface reliability.Interface properties modification is possible achiev by ultrasonic treatment.
SS-TuP-33 Non-Thermal Oxidation and Reduction Reactions of CO Trapped in Amorphous Solid Water Films
Greg Kimmel, Nikolay Petrik (Pacific Northwest National Laboratory)
Non-thermal reactions in aqueous systems are important in several areas ranging from radiation chemistry and biology to astrochemistry. However, a detailed understanding of the relevant physical and chemical processes often remains elusive. We have investigated the non-thermal reactions of CO buried in amorphous solid water (ASW) films using infrared reflection absorption spectroscopy (IRAS), electron-stimulated desorption (ESD), and temperature programmed desorption (TPD) in ultrahigh vacuum at temperatures from 20 K to 120 K. The non-thermal reactions, which were initiated with 100 eV electrons, lead to reduction and/or oxidation of the CO depending on the conditions of the experiments. The reduction occurs through successive hydrogenation reactions eventually producing methanol, while oxidation leads to the formation of CO2. For CO trapped within ~40 monolayers (ML) of the ASW/vacuum interface, both reduction and oxidation are observed. However for CO trapped deeper than 40 ML in the ASW films, only reduction reactions are observed. The results indicate that the energetic electrons create both hydrogen atoms and hydroxyl radicals within ~40 ML of the ASW/vacuum interface. The H and OH are primarily responsible for the subsequent reactions. However, the hydrogen atoms can subsequently diffuse through the ASW accounting for the different spatial profiles of the reduction and oxidation reactions within the ASW films.
SS-TuP-34 Spin Crossover Transition in a Molecular Adsorbate
Xin Zhang, Sumit Beniwal (University of Nebraska Lincoln); Tatiana Palamarciuc, Patrick Rosa, Jean-François Létard (Université de Bordeaux, France); Jing Liu (Northeastern University); Eduardo Vega Lozada, Fernand Torres, Luis Rosa (University of Puerto Rico – Humacao); Bernard Doudin (Université Louis Pasteur Strasbourg, France); Axel Enders, Peter Dowben (University of Nebraska Lincoln)
We investigated the occupied and unoccupied electronic structure of ultra thin films of the spin crossover [Fe(H2B(pz)2)2(bipy)] complex (with H2B(pz)2= bis(hydrido)bis(1H-pyrazol-1-yl)borate and bipy = 2,2’-bipyridine) by ultraviolet photoelectron spectroscopy (UPS), inverse photoemission (IPES) and X-ray absorption spectroscopy (XAS). The XAS spectra clearly shows the change of iron L edge spectra associated with thermal induced spin crossover and the Raman spectra are consistent with the intact molecule – so the molecule can be evaporated and will form a thin molecule film on a variety of substrates. The shift of the unoccupied density of states seen in inverse photoemission is consistent with the thermally induced spin crossover transition for molecules deposited on the organic ferroelectric copolymer polyvinylidene fluoride with trifluoroethylene (PVDF-TrFE) as well as gold. Perhaps more significant is the fact that the spin crossover transition, and certainly the unoccupied electronic structure, is influenced by the ferroelectric polarization direction of PVDF-TrFE substrates at temperatures in the vicinity of the thermally driven spin cross-over transition, if the [Fe(H2B(pz)2)2(bipy)] complex film is very thin 25 molecular layers or less. Combining the STM studies of different adsorbate coverage with the thickness dependent IPES results of the molecular adsorbate on gold substrates, we understand that the molecular thin film spin-states transition may well be affected by thickness of the film which significantly change the magnetic moment of the thin film based on the SQUID measurements.
SS-TuP-35 Evaluating Influencing Factors on Nanofiltration and Reverse Osmosis Membrane Processes: A Computational Chemistry Approach to Modeling
Alison Kennicutt, James Kilduff, Lisa Morkowchuck, Curt Breneman (Rensselaer Polytechnic Institute)

We have developed quantitative structure-property relationship (QSPR) models to assess the efficacy of water treatment processes for removal of endocrine disrupting compounds, pharmaceuticals, and personal care products (EDC/PPCPs). Data sets were developed from literature sources reporting nanofiltration and reverse osmosis membrane rejection. QSPRs were developed and implemented by relating compound properties to their removal in membrane water treatment processes. Properties were coded by descriptors capturing physical, chemical, and electronic molecular features. Descriptors were selected maximize model accuracy, sensitivity, and pre dictive ability. Individual descriptor importance was assessed using stability of descriptor weights across observations and sensitivity of model predictions across perturbations in descriptor values. Models were developed using Partial Least Squares and Support Vector Machine techniques, and were cross-validated and y-scrambled to verify that results were not serendipitous.

Initial modeling work was done to primarily investigate influential characteristics of the contaminant compound. Descriptor sets were then expanded to attempt to capture membrane influences, including membrane surface chemistry, as well. Modeling was also broadened to develop predictive models based on mechanistic data, instead of a process-specific percent rejection value. Mechanistic parameters of EDC/PPCP fate and transport in membrane processes can be determined from a combined film and hydrodynamic model, a pore surface diffusion model (PSDM) for adsorption, and a steric, electric, and dielectric exclusion (SEDE) model.