AVS2012 Session TF+AS+SS-ThA: Thin Films: Growth and Characterization-III

Thursday, November 1, 2012 2:00 PM in Room 11
Thursday Afternoon

Time Period ThA Sessions | Abstract Timeline | Topic TF Sessions | Time Periods | Topics | AVS2012 Schedule

Start Invited? Item
2:00 PM TF+AS+SS-ThA-1 Atomic Force Microscopy (AFM)-Based Nanografting for the Study of Self-Assembled Monolayer Formation of Organophosphonic Acids on Al2O3 Single Crystal Surfaces
Boray Torun, Berkem Oezkaya, Guido Grundmeier (University of Paderborn, Germany)

The surface chemistry of aluminum oxides plays a crucial role in the field of catalysis, corrosion and adhesion. Alumina (Al2O3) covered aluminum alloys are employed in the construction of lightweight automotive and aerospace parts. In order to protect these materials from environmental factors organic coatings are commonly used. In this context the adhesion between polymer and oxide surfaces is of outmost importance to improve the longevity of industrial parts. Using self-assembled adhesion promoting monolayers the complexity of surface pretreatment processes could be reduced tremendously. Long aliphatic phosphonic acids, such as octadecylphosphonic acid (ODPA), were found to be suitable for forming dense self-assembled monolayers on native oxide covered aluminum substrates. However in contrast to amorphous oxide films, single crystal surfaces provide a much more well-defined experimental and theoretical platform for studies on the adsorption mechanisms and the stability of organophosphonic acids.

In the presented study [1], adsorption, stability, and organization kinetics of organophosphonic acids on single-crystalline alumina surfaces were investigated by means of atomic force microscopy (AFM)-based imaging, nanoshaving, and nanografting. The latter, nano-shaving and -grafting, are rather new techniques to study self-assembly processes. Since they were first reported [2] in 1997, atomic force microscopy based nanografting has been used as a tool to investigate the adsorption of organic monolayers mostly on noble metals, such as gold. [3] Moreover recent studies focused on influences of the confinement between AFM-tip and background monolayer on the adsorption of molecules during the grafting process. [about:blank#_ENREF_1]

AFM friction and phase imaging have shown that chemical etching and subsequent annealing led to heterogeneities on single-crystalline surfaces with (0001) orientation indicating differences in the local surface termination. These findings were supported by angle resolved X-Ray photoelectron spectroscopy (AR-XPS) measurements suggesting a partially hydroxide terminated surface. Self-assembly and stability of ODPA were shown to be strictly dependent upon the observed heterogeneities of the surface termination, where it was locally shown that ODPA can loosely or strongly bind on different terminations of the crystal surface. Furthermore, organization kinetics of ODPA was monitored with nanografting on (0001) surfaces. Supported by measurements of surface wettability and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), it was demonstrated that the lack of organization within the protective adsorbed hexylphosphonic acid (HPA) monolayer on alumina surfaces facilitated the reduced confinement effect during nanografting, such that kinetics information on the organization process of ODPA could be obtained.

[1] Torun, B. et al., Langmuir 2012, 28, (17), 6919-6927.

[2] Xu, S. et al., Langmuir 1997, 13, (2), 127-129.

[3] Yu, J. et al., Langmuir 2008, 24, (20), 11661-11668.

[4] Xu, S. et al., J. Amer. Chem. Society 1998, 120, (36), 9356-9361.

2:20 PM TF+AS+SS-ThA-2 SIMS as a Method for Probing Stability of the Molecule-Substrate Interface in SAMs
Jakub Ossowski, Jakub Rysz (Jagiellonian University, Poland); Andreas Terfort (Goethe University, Germany); Piotr Cyganik (Jagiellonian University, Poland)

Despite the numerous structural studies of Self-Assembled Monolayers (SAMs) available nowadays, the structure and stability of the SAM-substrate interface is still poorly understood and controversial even for the most simple SAM system. As a consequence, the experimental and theoretical analysis of the bonding geometry and the stability of the molecule-substrate interface for technologically relevant, and therefore more complicated SAMs, is extremely difficult.

In this presentation we report extensive static secondary ion mass spectrometry (SIMS) studies1 on homologic series of thiols (BPnS, CH3‑C6H4‑C6H4-(CH2)n-S-Au(111), n = 2-6) and selenols (BPnSe, CH3‑C6H4‑C6H4-(CH2)n-Se-Au(111), n = 2-6) where structure and stability of molecule-substrate interface was systematically modified as verified by our previous experiments2-5. Correlating SIMS data with previous microscopic2, spectroscopic3 and very recent neutral mass spectrometry studies4,5 we show that SIMS can be successfully applied to monitor fine changes in the molecule-substrate interface stability of these model SAMs. Further, to demonstrate general applicability of SIMS for such analysis, we report use of this method for monitoring influence of S versus Se substitution in purely aliphatic (heksadecanethiol/selenol) and aromatic (anthracenethiol/selenol) SAMs on Au(111). In summary our experiments show that a new approach for probing the stability of molecule-substrate interface in SAMs can be proposed by using SIMS. Importantly, this technique is relatively fast and can be applied for virtually all complicated and technologically relevant SAMs.

References

(1) J. Ossowski, P. J. Rysz, A. Terfort and P. Cyganik in preparation.

(2) P. Cyganik, K. Szelagowska-Kunstman, et al. J. Phys. Chem. C 2008, 112, 15466.

(3) K. Szelagowska-Kunstman, P. Cyganik, et al. Phys. Chem. Chem. Phys. 2010, 12, 4400.

(4) S. Wyczawska, P. Cyganik, A. Terfort, P. Lievens, ChemPhysChem (communication) 2011, 12, 2554.

(5) F. Vervaecke, S. Wyczawska, P. Cyganik, et al. ChemPhysChem (communication) 2011, 12, 140.

2:40 PM TF+AS+SS-ThA-3 Wet Chemical Surface Modification of Silicon Oxide and Oxide Free Silicon by Aluminum Oxide
Peter Thissen, Abraham Vega, Tatiana Peixoto, Yves Chabal (University of Texas at Dallas)

Wet chemical surface modification is a powerful method to change the chemical properties of surfaces. Although it has been used extensively, there are still many issues that limit a the applicability of these reactions. Substrate dip coating in aqueous solutions is particularly useful to facilitate both organic and inorganic layer functionalization. For instance, the bonding of phosphonic acid to silicon oxide is weak in water because the Si-O-P bond is easily hydrolyzed. We demonstrate here that this problem is alleviated by the addition of an ultra-thin aluminum oxide layer to the silicon oxide surface via dip-coating a silicon substrate in an aqueous solution of aluminum chloride. The growth kinetics of the aluminum oxide layer are characterized by several surface sensitive techniques and found to follow a Stranski-Krastanov mechanism. Once the aluminum oxide layer is in place, a self assembled monolayer (SAM) of octadecylphosphonic acid (ODPA) is attached by the “tethering by aggregation and growth” (T-BAG) method performed in a controlled environment. We demonstrate that this ODPA layer grafted on the aluminum oxide interlayer remains stable in water. We also show that, following the same wet chemical approach, we are able to attach aluminum hydroxyl directly on oxide-free silicon surfaces previously functionalized with 1/3 monolayer OH. [1] Finally, we show that our approach can easily be transferred to other metal oxides and discuss the most influencing parameters.

[1] Michalak, D. J.; Amy, S. R.; Aureau, D.; Dai, M.; Esteve, A.; Chabal, Y. J. Nat. Mater.2010, 9, 266-271.

3:00 PM TF+AS+SS-ThA-4 Static and Dynamic Depth Profiling of Thin Films with Low Energy Ion Scattering (LEIS)
Rik ter Veen, Michael Fartmann (Tascon GmbH, Germany); Thomas Grehl (ION-TOF GmbH, Germany); Birgit Hagenhoff (Tascon GmbH, Germany)

With the ever increasing demand of thinner and better defined thin layer structures, good depth resolution becomes more and more critical in depth profiling techniques. Low Energy Ion Scattering (LEIS) is known as the most surface sensitive chemical analysis technique (see [1] for a review of LEIS technique). It is considerably less known that LEIS can also be applied for so called "static depth profiling" by interpreting the backgrounds on the low energy side of the LEIS peaks. The energy that the particles lose while travelling through the sample is a measure for the depth of the scattering atom, in a way similar to Rutherford Back Scattering (RBS) but for a much smaller depth range. New models have been developed to understand the process that gives rise to these backgrounds and that contains the information from layers below the surface up to depths of 10 nm. These models will be presented.

The models for this static depth profiling can be verified by dynamic (sputter) depth profiling. After each sputter step a full LEIS spectrum is recorded, which contains the surface information as well as the static depth profile at that point in the dynamic depth profile. In this way, the static depth profile can forecast the dynamic depth profile. This technique will be demonstrated for an Si/SiO2/W/Al2O3 system.

LEIS is particularly suited for dynamic depth profiling. Since LEIS is so surface specific, the depth resolution is excellent, as long as the sputter conditions are chosen with care. Furthermore, LEIS can be quantified easily, in many cases - such as in depth profiles - without the use of references. However, any dynamic depth profile suffers from artifacts, such as preferential sputtering and ion beam mixing. By combining the dynamic depth profiling with static depth profiling there is an independent check on these artifacts. Furthermore, it will be shown how static depth profiling can give relevant information also at shallow depths where in a dynamic depth profile sputter equilibrium will not have been reached yet.

[1] H.H. Brongersma et al, Surf. Sci. Rep. 62 (2007)63

3:20 PM BREAK
3:40 PM Invited TF+AS+SS-ThA-6 Paul Holloway Award Talk: Surface Chemistry and Structure of Alloy Thin Films under Reaction Conditions and their Correlations to Catalytic Performances of CO2 Conversion and Methane Partial Oxidation
Franklin Tao (University of Notre Dame)
Formation of alloys is one of the important approaches todesign of new catalysts with high activity and selectivity as a second metalcould tune electronic structure of the first metal or/and create thermodynamicallyfavorable sites for an ideal reaction channel. Co-Ru alloys are activecatalysts for conversion of CO2 into fuel molecules CH4.Pd-based alloys are important catalysts for methanol partial oxidation toproduce hydrogen. Thin films of model catalysts of alloys Co-Ru and Pd-Co wereprepared through e-beam evaporation in UHV. In-house ambient pressure X-rayphotoelectron spectroscopy using monochromatic Al Kα were used to examine the evolutionof surface compositions of alloy catalysts and the oxidation states of theconstituting elements under reaction conditions and during catalysis incontrast to those before or after a reaction; Surface chemistry (compositionand oxidation state) of active phases of Co-Ru and Pd-Co was revealed. High pressure STM providedvisible information of surface structure at nano and atomic scale underreaction conditions. These studies clearly suggest a modification of Coelectron state through coordinating Ru atoms and thus tuning the adsorptionenergy of intermediates on Co in CO2 conversion, whichenhances the selectivity to production of CH4. The formed Co0.85Ru0.15alloy exhibits 100% selectivity to the production of CH4 and aconversion of 40% which is higher than both pure Co and pure Ru. The promotioneffect of the alloy film for CO2 conversion was rationalized byelectronic effects of Ru to Co in the alloy thin films under reactionconditions. In terms of Pd-Co alloy catalysts, segregation of Co to surfaceunder reaction conditions was observed. Through the measurement of surfacecomposition using AP-XPS and the coordination of Pd on surface visualized withSTM, a correlation between surfacechemistry and structure of Pd-Co alloy surface under reaction conditions and the corresponding catalyticperformances were built. Themodification of Co to the catalytic behaviors of Pd was identified.
4:20 PM TF+AS+SS-ThA-8 Time-resolved and Surface Plasmon Resonance Studies in Metal-Insulator Phase Transition in VO2 Thin Films
Lei Wang, César Clavero, Kaida Yang, Elizabeth Radue, Matt T. Simons, Irina Novikova, Rosa Alejandra Lukaszew (College of William and Mary)
Vanadium dioxide (VO2) is a prominent example for a material exhibiting a metal-insulator transition (MIT) as a function of temperature with a phase transformation around 340 K from a low-temperature insulator state to a high-temperature conducting state. During the MIT the lattice structure of VO2 transforms from a monoclinic (insulator) to a tetragonal structure (conductor). Whether these structural changes are solely responsible for the nature of the transition or whether correlation effects also play a role, has been a subject of much debate. Two mechanisms have been generally considered to explain the origin of the MIT in VO2. The Mott-Hubbard mechanism suggests that electron-electron correlation drives the first-order MIT whereas the Peierls mechanism proposes that a strong electron-lattice interaction leads to the MIT. In order to have a better understanding of the phase transition mechanism and the optical properties of this material across the MIT, we present our research studies on epitaxial VO2 thin films. We have investigated the optical transmission of a VO2 thin film during the thermally induced MIT in two different optical spectral regions, with cw THz light and low power (1 mW) IR light (1520 nm HeNe), to identify different mechanisms at play. We have found that the transmission of the THz light starts to decrease at higher temperature than that of the IR light thus probing different stages during the thermally induced MIT. We also investigated surface plasmon polariton excitation in VO2 thin films in the IR region, and observed a clear trend from non-absorption in the insulator phase to a high absorption in the metallic phase while changing the VO2 temperature . Our studies are aimed at helping to understand the evolution of the metallic phase in VO2 thin films after the MIT and relaxation back to the insulator phase upon the MIT which is of paramount importance for ultra-fast switch applications. Finally, we note that Cavalleri et al. [i] reported that the light-induced phase transition happens in less than half a pico-second thus hinting at electronic processes, although they also found that it strongly depended on pump-laser power which is suggestive of lattice interactions. We will compare out time-resolved measurements also using pump-probe techniques but with the sample held at low-temperature vs. room-temperature to illustrate the role of the pump-power on the photo-induced MIT.
 
[i] A. Cavalleri, Cs. Tóth, C.W. Siders, J. A. Squier, F. Ráksi, P. Forget and J. C. Kieffer, Phys. Rev. Lett. 87 (23), 237401 (2001).
  
 
4:40 PM TF+AS+SS-ThA-9 Growth, Microstructure and Optical Properties of Sputter-Deposited Gallium Oxide Thin Films
Sampath Samala, Chintalapalle Venkata Ramana (The University of Texas at El Paso)

Gallium oxide (β-Ga2O3), which is a stable oxide of gallium, is a wide band gap material. The high melting point coupled with stable structure makes of β-Ga2O3 the best candidate for high temperature sensing. β-Ga2O3 thin films can be used for developing oxygen sensors operating at higher temperatures (≥ 900 oC). This feature opens the possibility of developing the integrated of β-Ga2O3 based oxygen sensors for power generation systems. The present work was performed on the analysis of growth behavior, microstructure, and optical properties of of β-Ga2O3 films grown by sputter deposition. Ga2O3 thin film were deposited on Si(100) and quartz substrate by varying the growth temperature from room temperature to 800 oC. The characteristic analysis of the samples was performed employing grazing incidence X-ray diffraction (GIXRD), scanning electron microscopy (SEM), and spectrophotometry measurements. GIXRD analyses indicate that the samples grown at lower temperatures were amorphous while those grown at ≥ 400 oC. SEM results indicate that the morphology evolution is dependent on the temperature. The characteristic shape of the grains changed from triangular to square and finally to spherical morphology with increasing temperature. Optical characterization indicates that the band gap varies from 4.1 to 5.1 eV as a function of increasing temperature. The correlation between growth conditions, microstructure and band gap is established.

5:00 PM TF+AS+SS-ThA-10 Optical and Structural Properties of Hafnium Oxide Thin Films Prepared Using Different Deposition Techniques
Lirong Sun, Neil Murphy, John Grant, John Jones, Rachel Jakubiak (Air Force Research Laboratory)
The high dielectric constant and optical transparency of hafnium oxide makes it a useful component in leading-edge integrated circuitry and optical coatings. The optical and structural properties of stoichiometric HfO2 films vary significantly depending on the deposition mechanism. We prepared 200 nm thick films of HfO2 on silicon (100) substrates derived from DC magnetron sputtering (DCMS), high power impulse magnetron sputtering (HiPIMS) and pulsed laser deposition (PLD). Analysis of x-ray diffraction data revealed that films deposited via PLD are amorphous, while those deposited using the magnetron sputtering methods had peaks at 2θ of 28.3°, 31.3°, 34.3° and 50.0° indicative of polycrystalline monoclinic HfO2. This is further supported by the FT-IR data collected in the far-IR regime where absorption bands at 258, 341, 410 and 514 cm-1 were present. AFM and SEM images indicate that the sputtered samples had rougher surface morphology and larger grain sizes than the PLD films where the surface was uniform and smooth (RMS surface roughness less than 0.1nm). The degree of surface roughness and grain size is inversely proportional to the refractive index. At 632 nm PLD films had an index of refraction of 2.10 while the index of the sputter coated films was 1.98, presumably due to presence of voids. The high refractive index and homogeneity of the PLD films indicate that they were highly packed without voids during growth. Additionally, the influence of the O2/Ar ratio, working pressure, HiPIMS pulse profile and duty cycle on optical properties, surface roughness, particle size and structural properties of the HfO2 thin films were characterized and evaluated.
5:20 PM TF+AS+SS-ThA-11 Nitrogen Induced Changes in the Structure and Electronic Properties of WO­ Thin Films
Chintalapalle Venkata Ramana, Rama Sesha Vemuri (The University of Texas at El Paso); Mark Engelhard, Suntharampillai Thevuthasan (Pacific Northwest National Laboratory)

Tungsten oxide (WO3) is a wide band gap semiconductor (~ 3.2 eV), which exhibits excellent physical, chemical and electronic properties. WO3 thin films have been widely used in electrochromics and chemical sensors. Recently, the band gap modification with anionic and cationic doping of WO3 was gained importance to utilize these materials in photo-catalysis for energy production and utilization. The present work was performed on nitrogen incorporated WO3 (N-WO3)films to explore the options to engineer the microstructure and electronic properties. Specifically, the effect of nitrogen incorporation and processing parameters on the microstructure evolution and band gap of WO3 thin films is investigated. The samples were grown using reactive RF magnetron sputtering where the nitrogen concentration in the films is varied by varying partial pressure of nitrogen during deposition while keeping all other process parameters constant. Quantitative measurements employing X-ray photoemission spectroscopy indicate the nitrogen content increases with increasing nitrogen partial pressure. Structural analysis employing grazing incidence X-ray diffraction demonstrated that the nitrogen atoms embedded in WO3 crystal matrix changes the crystal-texturing and thus induce changes in the physical properties. Optical spectrophotometry analysis on the N-WO3 films revealed a shift in the fundamental absorption edge which is in linear relation with the corresponding nitrogen concentration. The correlation between microstructure, dopant profile, dielectric constant and band gap in WO3 films will be presented and discussed.

Time Period ThA Sessions | Abstract Timeline | Topic TF Sessions | Time Periods | Topics | AVS2012 Schedule