AVS2012 Session AS-TuP: Applied Surface Science Poster Session

In recent years, molecular imaging with submicron lateral resolution has become of more and more interest for characterizing specialized compounds in biological samples. As an example, nanoparticles (NPs) gain great commercial interest in the medical field due to their high mobility in human tissue. Despite broad applications close to the human body, so far, there is only little knowledge of possible toxicity. In this context, the distribution of NPs within cells is of particular interest. Moreover, not only the distribution of NPs but also elemental and molecular cellular distributions such as metabolites and lipids are interesting for medical research.

In this study, we used time-of-flight secondary ion mass spectrometry (ToF-SIMS) and laser-secondary neutral mass spectrometry (Laser-SNMS) to investigate different cells both unexposed and exposed in vitro to silver NPs (AgNPs) and arsenic species. To optimize the analysis, a special silicon wafer sandwich preparation technique was employed; this entails freeze-fracturing and washing of cell cultures that were grown on silicon wafers. The data showed that during freeze-fracturing, the cell membrane is often stripped from the cell, enabling direct analysis of the interior of the cells on one sandwich wafer and the remaining lipid membrane as a mirror image on the opposite wafer. During analysis, the signal from the nutrient materials was observed to diminish the contrast of the molecular signals in the images. By optimizing the preparation and washing procedures, both the contrast and the imaging resolution could be significantly increased due to higher molecular yields and lower background. With these optimization procedures it was possible to detect lipid ions in a higher mass range, especially from those membranes that were stripped from the cells.

Under these optimized conditions, several studies were performed to detect the distributions of trace elements in cells. One study dealt with AgNPs. In this context the uptake of AgNPs of human macrophages was measured with nanometer-scale resolution. 2D and 3D Laser-SNMS images clearly showed that AgNPs are incorporated by macrophages and in part agglomerate to silver aggregates with a diameter of ~3-7 µm. In a similar approach, the distribution of arsenic in cells was measured to obtain more information on the reasons why inorganic arsenic proves carcinogenic in humans. A comparison with ToF-SIMS data showed that especially the high elemental sensitivity of Laser-SNMS makes it possible to detect theses trace elements in cells.

In-depth analysis by secondary ion mass spectrometry (SIMS) is very important for the development of electronic devices using multilayered structures, because the quantity and depth distribution of some elements are critical for the electronic properties. Correct determination of the interface locations is critical for the calibration of the depth scale in SIMS depth profiling analysis of multilayer films. However, the interface locations are distorted from real ones by the several effects due to sputtering with energetic ions.

In this study, we compared the three definitions for the determination of interface locations in SIMS depth profiling of multilayer films. Especially, we investigated the feasibility of 50 atomic % definition for Si/Ge and Si/Ti multilayer films by various SIMS analysis parameters. In 50 atomic % definition, the original SIMS depth profiles are converted into compositional depth profiles by the relative sensitivity factors (RSF) derived from the alloy reference films with well-known compositions determined by Rutherford backscattering spectroscopy (RBS).

The application of the 50% definition determined from the ion intensities was found to be very limited to specific systems showing clear interfaces. The definition of the interface by the dimer ions between the atoms in the two different layers was also difficult to apply due to the small intensity and the unclear variation at the interfaces.

The electrospray droplet impact secondary ion mass spectrometry (EDI/SIMS) has been developed as cluster SIMS[1]. EDI utilizes charged water droplets generated by ambient electrospray. The typical droplet is represented as [(H₂O)_90,000 + 100H]⁺ with mass of ～10⁶ u. The kinetic energy of droplets is about 10⁶ eV with the velocity of 12 km/s. EDI/SIMS has the atomic/molecular level etching abilities[1][2]. EDI/SIMS was found to be applicable to many kinds of inorganic and organic material [1][3]. In spite of the shallow surface etching, relatively high secondary ion yields can be obtained by EDI.

The high ionization efficiency may be due to the occurrence of supersonic collision taking place between the droplet and the sample surface. In order to estimate the useful yield (i.e., total ions generated divided by the total molecules desorbed), mass spectra were measured for binary mixtures of C₆₀/Rhodamine B (1:1) and C₆₀/Aerosol OT (1:1). The equimolar samples were crushed and mixed in mortar and deposited on a stainless steel target as thin films using a spatula. This method was adopted from solvent-free MALDI. By assuming that (1) the desorption efficiency is the same for C₆₀, Rhodamine B and Aerosol OT, and (2) the desorbed ionic compounds directly give secondary ion signals, the useful yield was crudely estimated to be ～0.1. This high value explains the high ionization efficiency of EDI/SIMS.

Reference

[1] K. Hiraoka, D. Asakawa, S. Fujimaki, A. Takamizawa, K. Mori, Eur. Phys. J. D38, 225 (2006)

[2] D. Asakawa, K. Mori, K. Hiraoka, Appl. Surf. Sci. 255 (2008) 1217

[3] K. Hiraoka, K. Mori, D. Asakawa, J. Mass. Spectrom. 41 (2006) 894

Latent (that is unintentional) adhesion between organics and inorganic surfaces is a well known phenomenon in many areas of materials science, e.g. the moulding of polymeric components and the storage of coated metal products where a polymeric surface is in intimate contact with the back of another sheet. A complete understanding of the adhesion and adhesion processes that occur at this interface may provide a key to obtaining optimum performance for a particular application.

In this work, we are considered with the effects of migration of organics to the surface of the polymeric host and their role at the polymer/inorganic interface. We have focused on three characteristic organics widely used as additives in a wide range of polymer formulations. In the samples we have studied, characteristic peaks from these additives dominate the ToF-SIMS analysis of the inorganic surface. In addition, surface chemistry of the inorganic surface induces different mechanical performances of the products. The storage period has the potential to play a significant role in the migration of minor components towards the interface under investigation, and data will be presented at two different periods (early and late stages).

In this study we show models using multivariate analysis describing how ToF-SIMS analysis can be applied to understand the surface chemistry of industrial materials. The behaviour of migration from the polymer to the inorganic side of the polymeric assembly induces three characteristic surface chemistries which influence mechanical performance. Our models show good predictions for a validation sample of materials.

We have developed a method to create a cross-section of small samples for Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) utilizing a Focused Ion Beam (FIB) approach. Using this method, a nearly ideal surface for elemental and molecular analysis of the cross-sectional surface is produced, thus providing complimentary information to other microanaltical techniques like scanning elelctron microscopy (SEM) and scanning transmission electron microscopy (STEM).

§Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

Microelectronic devices typically consist of elaborate three-dimensional structures, and careful control of chemical and structural properties is required for effective operation. Structures such as pads and bond pads must be carefully assessed for correct size, composition, layering and contamination levels. Supporting structures and substrates must also have the correct composition, integrity and electronic properties.

X-ray photoelectron spectroscopy (XPS) is a surface-sensitive material characterisation technique that offers sensitive chemical state information, and is already established as a method of choice for the analysis of microelectronic devices. Parallel XPS imaging offers high spatial resolution for observing the lateral distribution of chemical states at the surface of a material, and recent advances in spectroscopic imaging allow the generation of quantitative chemical state images for maximal information content.

Depth profiling, typically with ion beams, allows the gradual removal of material from a sample surface, so that XPS can be used to probe further than its characteristic nanometre-scale analysis depth.

Films of cells on solid substrates are encountered in a variety of biological and biomedical environments, including cells in biofilms that spontaneously colonize medical devices and multilayers of cells filtered from suspensions for analysis. Understanding the chemical properties of cells in such films is important for providing clues about the behavior of the cells or about the effects of treatments that had been applied to the cells.

X-ray Photoelectron Spectroscopy (XPS), with its combination of chemical selectivity and surface specificity, is an ideal technique for analysing these biofilms and multilayers, but it needs to be combined with profiling to more fully characterise the samples. It is well known that profiling with traditionally used argon monomers results in a high degree of chemical modification for most organic materials. Recent studies have shown, however, that argon cluster beams may be used for depth profiling of organic materials while preserving the chemical information.

This poster will present data from cluster profiling studies of biofilms and biomaterials. The methodology required for optimum profiling of these samples will be discussed, including an evaluation of XPS data acquisition protocols, as well as sputtering conditions.

Graphene has received unprecedented attention since 2010 when the Nobel Prize was awarded to Geim and Novoselov for “groundbreaking experiments regarding the two-dimensional material graphene.” Many graphene-related publications use the C 1s spectrum to demonstrate the existence or formation of graphene, but unique photoemission spectral signatures are difficult due to the inherent thinness of single or even multi-layer graphene. The difficulty is due to the surface sensitivity of XPS and Auger spectra and the analysis of spectra that include graphene, substrate-related carbon and possible adsorbed carbonaceous material on the graphene. This poster explores various XPS and Auger spectral features from studies of graphene grown by the CVD method on Cu (near-single layer) and Ni (multi-layer). In one study, graphene grown on Cu was heated in air to determine if the graphene provided any protection to the underlying Cu substrate. The results indicated not only oxidation (i.e. corrosion) protection, but that there was a time and heat dependence of the protection. To aid in the understanding of graphene vs. substrate contributions to the C 1s XPS spectrum, additional studies were performed on CVD-grown diamond substrates. Diamond substrates (very low oxygen and pure sp3-type carbon) offer an interesting contrast to most substrates that have inherent O and C contributions to the O 1s and C1s spectra. Unique XPS and Auger related features of graphene will be highlighted in this poster.

Alkali migration has been a noted artefact of sputtering with monatomic Ar ion beams and the use of polyatomic ion beams more commonly applied to organic materials has been shown to yield some benefits in reduced migration without incorporation of C into the glass matrix.

In this investigation we compare the results with monatomic Ar ion sputtering and polyatomic (Coronene) for a number of soda-lime-silica glass samples.

The thickness of metal oxide nanolayers can be assessed through X-Ray Photoelectron Spectroscopy (XPS) measurements. This is done by comparing the signal from oxide and metallic XPS peaks. The correct assessment of the oxide layer thickness depends on how accurate the peak areas are quantified through peak fitting. Since the oxide and metallic peaks overlaps in XPS spectra, the calculation of their areas could be tricky. This is specially the case for iron since the Fe 2p peak, which is the iron peak most employed in XPS experiments, is largely asymmetric. In this work, we show that the assessment of the peak areas can carry errors as large as 120% if the traditional Shirley background is employed. The problem is solved by the use of the “active” background removal method, in which the background intensity is optimized during the peak-fitting process. This method is described in detail, as well as its software implementation. The results are supported with High Resolution Transmission Electron Microscopy micrographs.

An auxiliary chamber that can operate at high pressures and elevated temperatures to simulate both catalysis processing and operating conditions has been developed and deployed in EMSL, DOE national scientific user facility. This chamber, which is associated with a gas flow and gas monitoring capability, is connected to the Quantera XPS system. The combination of UHV compatible auxiliary chamber and the main XPS system allows studies to be undertaken of the impact of catalysis processing in highly controlled conditions on the surface composition and chemical state of the elements without exposure of the samples to air after the reactions. Of the many types of studies conducted, we will highlight results from the development of Rh-based/SiO₂ catalysts for ethanol synthesis for use in automobiles and as a potential source of hydrogen for fuel cells and Co/CeO₂-ZrO₂ catalysts forhydrogen production through ethanol steam reforming (ESR). Although precious metals such as Rh, Pt, and Pd have been found to be effective catalysts, an inexpensive alternative using Co/CeO₂-ZrO₂ has attracted much attention recently; their activity for efficient C-C bond cleavage around 400°C has been reported to be similar to that of precious metal catalysts. The combination of side chamber and main XPS system has been particularly valuable to characterize the re-oxidation/re-reduction behavior of these catalysts under various conditions .

Acknowledgement - The work reported here was conducted in the Environmental Molecular Sciences Laboratory, a DOE user facility operated by Pacific Northwest National Laboratory for the Office of Biological and Environmental Research of the DOE.

References

[1]. Z. Fang et al., International Journal of Photoenergy 2011 (2011) 297350.

[2]. A. Bosio et al., Progress in Crystal Growth and Characterization of Materials 52 (2006) 247.

[3]. M. Powalla & D. Bonnet, Advances in OptoElectronics 2007 (2007) 97545.

[4]. http://investor.firstsolar.com/releasedetail.cfm?ReleaseID=639463

[5]. D.J. Larson et al., Ultramicroscopy, 79 (1999) 287, M.K. Miller et al., Micro. Microanal., 13 (2007) 428.

[6]. P. P. Choi et al., Microscopy Today, 20 (2012) 18.

[7]. D. J. Larson et al., Microscopy and Microanalysis (2012), in press.

Commercial silicone hydrogel contact lenses (SiHy) are ophthalmic devices designed to correct vision as well as function as an ocular bandage for therapeutic purposes. Surface wettability, modulus, surface topography as well as bulk water content are some of the factors that influence lens comfort and performance. Contact lens surface wettability is believed to be an important factor in comfort as well as overall patient satisfaction. Tear confluence across a lens surface may be improved by the presence of well-chosen biomolecules, capable of retaining moisture. Very recently the application of a natural lubricant hyaluronate (HA), to a daily use multi purpose solution (MPS), Biotrue™ has been reported.

In these studies HA was used in conjunction with a surfactant system present in Biotrue^TM MPS. The exclusive formulation of Biotrue^TM was designed to improve hydration and wettability of various SiHy contact lens materials. Improvements in moisture retention are attributed to the use of high molecular weight HA present in Biotrue™ capable of high water retention. Presence of HA on a lens surface acts as a wetting agent and improves the properties of SiHy contact lens materials. To our knowledge there is no published literature reporting on a visualization method of HA on SiHy contact lenses surfaces.

The purpose of this research was to develop a direct method to demonstrate the presence of a HA network on the surface of SiHy contact lenses using surface chemistry techniques. Senofilcon A^® and balafilcon A^® were chosen to investigate the interaction of HA with SiHy materials. Atomic Force Microscopy (AFM) was applied to examine the topography of both materials in the hydrated and dehydrated state. The visualization of HA chains was done using Confocal Laser Scanning Microscopy (CLSM) and Differential Interference Contrast (DIC) microscopy using a dye selective for HA (Safranin).

Senofilcon A^® and balafilcon A^® were soaked in Biotrue^TM for 4 hours. Lenses were then soaked in 3 mL of Safranin solution. After 3 min the samples were rinsed with DI water for 3 min to remove any unbound dye. Samples were imaged using an Olympus CLSM equipped with a DIC attachment. Individual confocal images were captured using an air objective sequentially across the sample. A large mosaic was generated using fiducial marks stitched together from the individual images. SiHy lenses incubated with Safranin solution but not exposed to Biotrue^TM were characterized as control. Additionally applicability of X-ray Photoelectron Spectroscopy (XPS) for HA mapping on the lenses surface was examined. AFM was used in parallel to study modification of the lens surfaces with HA biopolymer.

The manufacture of modern electronic devices has been longing for the higher control over chemical deposition processes and interface properties, as the electronic devices kept scaling down. As a result, a comprehensive understanding of surface reaction mechanisms between the precursor molecule and surface reactive sites is desired.

Numerous studies have addressed that chemically functionalized Si surfaces are promising solutions.

High indium content InGaAs is a leading candidate for n-channel devices in future generations of complementary metal-oxide-semiconductor (CMOS) technology due to its high electron mobility and high saturation velocity. In recent years significant progress has been made in improving the electrical quality of the high-k dielectric InGaAs interface by the atomic layer deposition (ALD) of high-k materials on passivated surfaces. An additional technological issue which needs to be addressed for metal oxide semiconductor field effect transistors (MOSFETs) fabrication is the relatively high source/drain (S/D) contact resistance which results from poor dopant activation in III-V semiconductors. One proposed solution to this issue is to fabricate metal S/D Schottky-barrier MOSFET devices which requires control over the barrier height at the metal-InGaAs interface. It has recently been reported that the insertion of an ultrathin layer dielectric layer at the contact interface between the metal and the semiconductor can help in releasing the Fermi level to obtain a rectifying contact.

In this study we investigate the effectiveness of the insertion of an ultrathin ALD deposited Al₂O₃ dielectric layer on the Schottky barrier formed at the interface between the metal and the InGaAs. Schottky contacts were fabricated on 1nm and 2nm Al₂O₃ layers deposited on native oxide and sulphur passivated In_0.53Ga_0.47As for both n and p doped substrates. To investigate the dependence of Schottky barrier height (SBH) on metal work function (WF), both low (Al~4.30 eV) and high (Pt~5.65 eV) WF metals were deposited on these surface.

Rectifying behaviour was observed for the p-type substrates for the Al-InGaAs and Al/ Al₂O₃/InGaAs junctions and the SBH was measured to be ~0.60eV. Ohmic behaviour was observed on the Pt-InGaAs and Pt-Al₂O₃-InGaAs junctions regardless of the dielectric thickness. The Al₂O₃/InGaAs interfacial chemistry of these surfaces was investigated with x-ray photoelectron spectroscopy and no arsenic oxide was found on the Al₂O₃- native and sulphur treated InGaAs surfaces which suggests that on the native oxide InGaAs surface, Al₂O₃ depositionresulted in the consumption of the interfacial oxide.

Ohmic behaviour was observed on the all n-type metal/InGaAs and metal/Al₂O₃/InGaAs junctions regardless of the metal WF or thickness of the dielectric layer which suggests that the Fermi level is pinned near to the top of the conduction band for these InGaAs samples.

The Naval Research Laboratory (NRL) will be constructing a SIMS Single Stage Accelerator Mass Spectrometer (SSAMS) instrument starting at the end of 2013 for use with nuclear forensics, cosmology, and other applications. The instrument will enable analysis of both positive and negative ions, and will have a molecular destruction capability. These features will address our goal to improve sensitivity and precision of select species, broaden the range of elements and isotopes to measure, and ease sample chemical pre-processing requirements.

To provide these features, the front portion of a Cameca IMS 6f will be combined with an NEC SSAMS system. The NEC SSAMS system will include a bipolar 300-kV air insulated single stage accelerator, custom multi-port 90° high mass resolution injection magnet (ME/Z² = 2.6 amu-MeV), 90° double focusing analysis magnet (ME/Z² = 75 amu-MeV), electrostatic spherical analyzer, and a molecular ion dissociator. High-speed electrostatic switching is also included in both magnets to allow high efficiency and precision of measurements of small sample particles. The multi-port injector magnet enables nearly continuous matrix normalization over a large mass range without having to change the magnetic field of the injector. The bipolar power supply for the accelerator allows measurement of both electropositive and electronegative elements, while the molecule destruction feature minimizes molecular interferences. Access to electropositive elements should provide improved sensitivity for rare earth elements, Uranium, and Plutonium. Before NRL can apply the instrument, the fundamentals of the instrument must be established.

The fundamentals include establishing molecular destruction cross sections of anticipated molecular ions, charge state distributions, overall transmission, and molecule fragment patterns. Upon establishing the performance characteristics of the instrument, the NRL SIMS-SSAMS will be unique tool able to better understand the constituents of an unknown material in nuclear forensics, cosmology, and other applications.