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Method Article

In Situ SIMS and IR Spectroscopy of Well-defined Surfaces Prepared by Soft Landing of Mass-selected Ions

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DOI:

10.3791/51344

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June 16th, 2014

 ,  , 
1Physical Sciences Division, Pacific Northwest National Laboratory

In This Article

Summary

Soft landing of mass-selected ions onto surfaces is a powerful approach for the highly-controlled preparation of novel materials. Coupled with analysis by in situ secondary ion mass spectrometry (SIMS) and infrared reflection absorption spectroscopy (IRRAS), soft landing provides unprecedented insights into the interactions of well-defined species with surfaces.

Abstract

Soft landing of mass-selected ions onto surfaces is a powerful approach for the highly-controlled preparation of materials that are inaccessible using conventional synthesis techniques. Coupling soft landing with in situ characterization using secondary ion mass spectrometry (SIMS) and infrared reflection absorption spectroscopy (IRRAS) enables analysis of well-defined surfaces under clean vacuum conditions. The capabilities of three soft-landing instruments constructed in our laboratory are illustrated for the representative system of surface-bound organometallics prepared by soft landing of mass-selected ruthenium tris(bipyridine) dications, [Ru(bpy)3]2+ (bpy = bipyridine), onto carboxylic acid terminated self-assembled monolayer surfaces on gold (COOH-SAMs). In situ time-of-flight (TOF)-SIMS provides insight into the reactivity of the soft-landed ions. In addition, the kinetics of charge reduction, neutralization and desorption occurring on the COOH-SAM both during and after ion soft landing are studied using in situ Fourier transform ion cyclotron resonance (FT-ICR)-SIMS measurements. In situ IRRAS experiments provide insight into how the structure of organic ligands surrounding metal centers is perturbed through immobilization of organometallic ions on COOH-SAM surfaces by soft landing. Collectively, the three instruments provide complementary information about the chemical composition, reactivity and structure of well-defined species supported on surfaces.

Introduction

Soft landing of mass-selected ions onto surfaces remains a subject of current research interest due to the demonstrated capabilities of the technique for the highly-controlled preparation of novel materials1-6. Recent efforts have indicated potential future applications of soft landing of mass-selected ions in the preparation of peptide and protein arrays for use in high-throughput biological screening7,8, separation of proteins and conformational enrichment of peptides9-12, covalent attachment of peptides to surfaces9,10,13,14, chiral enrichment of organic compounds15, electrochemical characterization of specific redox-active proteins16-18, production of thin molecular films19,20, processing of macromolecules such as graphene21 and preparation of model catalyst systems through soft landing of ionic clusters22-39, nanoparticles40-48 and organometallic complexes onto support materials19,49-56. The concept of modifying surfaces through soft landing of polyatomic ions was initially proposed by Cooks and co-workers in 197757. In the subsequent years a wide range of instrumental approaches have been developed for the controlled deposition of mass-selected ions from the gas-phase onto surfaces1,4,5. Ions have been produced through processes such as electrospray ionization (ESI)10,58,59, matrix- assisted laser desorption/ionization (MALDI)21, electron impact ionization (EI)60,61, pulsed arc discharge62, inert gas condensation36,63, magnetron sputtering64,65, and laser vaporization25,66,67. Mass selection of gas-phase ions prior to soft landing has been achieved principally employing quadrupole mass filters58,68,69, magnetic deflection devices70, and linear ion trap instruments8,59. A particularly notable advance in ion soft landing methodology occurred recently with the successful implementation of ambient ion soft- and reactive landing by Cooks and co-workers71,72. Using these various ionization and mass-selection techniques, the interactions of hyperthermal (<100 eV) polyatomic ions with surfaces have been studied in order to better understand the factors influencing the efficiency of ion soft landing and the competing processes of reactive and unreactive scattering as well as surface induced dissociation4,73-75.

The preparation of well-defined model catalysts for research purposes has been a particularly fruitful application of soft landing of mass-selected ions25,34,35,56,76-81. In the size range of nanoscale clusters, where physical and chemical behavior does not scale linearly with cluster size, it has been demonstrated that the addition or removal of single atoms to or from clusters may drastically influence their chemical reactivity82-84. This nanoscale phenomenon, which results from quantum confinement, was demonstrated convincingly by Heiz and co-workers85 for a model catalyst consisting of soft landed clusters of eight gold atoms (Au8) supported on a defect-rich MgO surface. Several additional studies have provided evidence of the size-dependent reactivity of clusters supported on surfaces34,77,86,87. Moreover, high resolution electron microscopy images indicate that clusters containing as few as ten88 and fifty five89 atoms may be largely responsible for the superior activity of bulk-synthesized gold catalysts supported on iron oxides. Employing soft landing of mass-selected ions, it is possible to prepare stable arrays of size-selected clusters and nanoparticles that do not diffuse and agglomerate into larger structures on the surface of support materials90-92. These previous studies indicate that with continuing development, soft landing of mass-selected clusters and nanoparticles may become a versatile technique for the creation of highly active heterogeneous catalysts that exploit the emergent behavior of large numbers of identical clusters and nanoparticles in extended arrays on surfaces. These extremely well-defined systems may be used for research purposes to understand how critical parameters such as cluster size, morphology, elemental composition and surface coverage influence catalytic activity, selectivity and durability.

Organometallic complexes that are typically used in the solution-phase as homogeneous catalysts also may be immobilized on surfaces through soft landing of mass-selected ions56,80,81. Attaching ionic metal-ligand complexes to solid supports to produce hybrid organic-inorganic materials is currently an active area of research in the catalysis and surface science communities93. The overall goal is to obtain the high selectivity toward a desired product of solution-phase metal-ligand complexes while facilitating an easier separation of products from catalysts and reactants remaining in solution. In this manner, surface immobilized organometallic complexes reap the benefits of both homogeneous and heterogeneous catalysts. Through selection of an appropriate substrate it is possible to maintain or even enhance the organic ligand environment around the active metal center while also achieving strong surface immobilization94. Self-assembled monolayer surfaces (SAMs) on gold may be terminated with a number of different functional groups and are, therefore, ideal systems to investigate the feasibility of tethering organometallic complexes to surfaces through soft landing of mass-selected ions95. Furthermore, ionization methods such as atmospheric pressure thermal desorption ionization (APTDI) have been demonstrated previously to yield gas-phase mixed-metal inorganic complexes that are not accessible through synthesis in solution96. In a similar vein, non-thermal kinetically-limited synthesis and ionization techniques such as magnetron sputtering65, gas aggregation63 and laser vaporization66 also may be coupled with ion soft landing instrumentation to provide a versatile route to novel inorganic clusters and nanoparticles supported on surfaces.

In order to evolve soft landing of mass-selected ions into a mature technology for the preparation of materials, it is critical that informative analytical methods be coupled with soft landing instrumentation to probe the chemical and physical properties of surfaces before, during and after deposition of ions. To date, a multitude of techniques have been applied for this purpose including secondary ion mass spectrometry (SIMS)19,97-100, temperature programmed desorption and reaction50,52, laser desorption and ionization101, pulsed molecular beam reaction102, infrared spectroscopy (FTIR and Raman)98,103,104, surface enhanced Raman spectroscopy103,105, cavity ringdown spectroscopy106, x-ray photoelectron spectroscopy35,107, scanning tunneling microscopy33,108-111, atomic force microscopy112-114, and transmission electron microscopy39. However, to most accurately characterize surfaces prepared or modified by ion soft landing, it is crucial that the analysis be performed in situ without exposure of the substrate to the environment in the laboratory. Previous analyses conducted in situ have provided insight into phenomena such as the reduction of ionic charge of soft landed ions over time37,38,115,116, the desorption of soft landed ions from surfaces52, the efficiency and kinetic energy dependence of ion reactive landing14,81, and the influence of size on the catalytic activity of clusters and nanoparticles deposited onto surfaces117. By way of example, in our laboratory, we have systematically studied the charge reduction kinetics of protonated peptides on the surfaces of different SAMs3. These experiments were performed with a unique soft landing instrument coupled to a Fourier transform ion cyclotron resonance secondary ion mass spectrometer (FT-ICR-SIMS) that enables in situ analysis of surfaces both during and after soft landing of ions97. To expand upon these analytical capabilities, another instrument was constructed that allows in situ characterization of soft landed ions on surfaces using IRRAS104. This surface-sensitive infrared technique enables bond formation and destruction processes as well as conformational changes in complex ions and surface layers to be monitored in real time both during and after soft landing12. For instance, using IRRAS it was demonstrated that ion soft landing may be used to covalently immobilize mass-selected peptides on N-hydroxysuccinimidyl ester functionalized SAMs13,14.

Herein, we illustrate the capabilities of three unique custom-built instruments located at the Pacific Northwest National Laboratory that are designed for in situ TOF-SIMS, FT-ICR-SIMS, and IRRAS analysis of substrates produced through soft landing of mass-selected ions onto surfaces. As a representative system, we present results for soft landing of mass-selected organometallic ruthenium tris(bipyridine) dications [Ru(bpy)3]2+ onto carboxylic acid terminated SAMs (COOH-SAMs) to prepare immobilized organometallic complexes. It is shown that in situ TOF-SIMS offers the advantages of extremely high sensitivity and large overall dynamic range which facilitates identification of low abundance species including reactive intermediates that may only be present for short periods of time on the surfaces. TOF-SIMS also provides insight into how the removal of a ligand from an organometallic ion in the gas-phase, prior to soft landing, influences its efficiency toward immobilization on surfaces and its chemical reactivity towards gaseous molecules. Complementary characterization using in situ FT-ICR-SIMS provides insights into the charge reduction, neutralization and desorption kinetics of the doubly charged ions on the surface while in situ IRRAS probes the structure of the organic ligands surrounding the charged metal centers, which may influence the electronic properties and reactivity of the immobilized ions. Collectively, we illustrate how soft landing of mass-selected ions combined with in situ analysis by SIMS and IRRAS provides insight into the interactions between well-defined species and surfaces which have implications for a broad range of scientific endeavors.

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Protocol

1. Preparation of COOH-SAM Surfaces on Gold for Soft Landing of Mass-selected Ions

  1. Obtain flat gold substrates on silicon (Si) or mica backing materials. Alternatively, prepare gold films on Si or mica surfaces according to procedures described in the literature118,119. Note: Use surfaces that have the following specifications: 1 cm2 or circular and 5 mm in diameter, 525 µm thick Si layer, 50 Å thick Ti adhesion layer, 1,000 Å Au layer.
  2. Place fresh gold-on-silicon surfaces into glass scintillation vials and immerse in pure (non-denatured) ethanol.
  3. Place scintillation vials containing gold surfaces immersed in ethanol into an ultrasonic cleaner and wash for 20 min to remove any surface debris. Note: Do not ultrasonically wash gold on mica surfaces as this will detach the gold film from the mica backing material.
  4. Remove washed gold surfaces from vials and dry with a stream of pure N2 to prevent the formation of any residual spots from the ethanol.
  5. Place the dried gold surfaces face up in an ultraviolet (UV) cleaner and irradiate for 20 min to remove surface organic matter.
  6. In glass scintillation vials, prepare 5 ml of 1 mM solutions of 16-mercaptohexadecanoic acid (COOH-SAM) in non-denatured ethanol.
  7. Add hydrochloric acid to a final concentration of 1% HCl in ethanol to ensure protonation of the carboxylic acid groups of the molecules.
  8. Place the washed, dried and UV-cleaned gold surfaces face up into the COOH-SAM solutions ensuring that the entire gold surface is fully immersed in each vial. Allow the monolayer surfaces on gold to assemble for at least 24 hr in the dark (wrap vials in foil).
  9. Remove the surfaces from the COOH-SAM solutions and place in new scintillation vials containing 5 ml of 1% HCl in ethanol. Ultrasonically wash the SAM surfaces for 5 min to remove any physisorbed molecules from the monolayer surface.
  10. Remove the washed surfaces from the vials and rinse with several 1 ml aliquots of 1% HCl in ethanol. Dry the COOH-SAM surfaces under a stream of N2.
  11. Using clean metal forceps and wearing gloves place a SAM surface onto one of three metal sample mounts that are compatible with each soft landing instrument taking care not to touch the front facing gold surface in the process. Ensure that the surface is fixed tightly in place and that there is strong electrical contact between the back side of the surface and the metal sample mount.
  12. Employing the load-lock sample introduction capabilities of the soft landing instruments (each is slightly different), ensure that the gate valve separating the sample introduction region of the instrument from the soft landing chamber is closed. Bring the sample introduction chamber up to atmospheric pressure by turning off the turbomolecular vacuum pump and ionization pressure gauge and closing the valve to the foreline mechanical vacuum pump.
  13. When the sample introduction chamber reaches atmospheric pressure open the sample door and secure the sample holder firmly to the manipulator (xyz-stage or z-translator) inside of the instrument. Close the door and open the valve to the foreline mechanical vacuum pump. When the sample introduction chamber reaches a pressure of 10-3 Torr, turn on the turbomolecular vacuum pump and ionization pressure gauge.
  14. When the sample introduction chamber reaches a pressure of 10-5 Torr, open the gate valve to the soft landing chamber. Use the magnetic manipulator or xyz-stage to position the SAM surface in line with the ion beam to begin soft landing.

2. Soft Landing of Mass-selected Ru(bpy)32+ onto COOH-SAM Surfaces

  1. Obtain tris(2,2'-bipyridyl)dichloro-ruthenium(II) hexahydrate solid. Dissolve the red crystals in pure methanol to create stock solutions with a concentration of 10-3 M. Dilute the stock solutions by a factor of either 10 or 100 with methanol to achieve optimum electrospray ion current of mass-selected Ru(bpy)32+ m/z = 285.
  2. Load the diluted solutions into 1 ml glass syringes. Use a syringe pump to infuse the solution through a 360 µm outer diameter 80 µm inner diameter fused silica capillary that is biased between +2 to +3 kV to generate positive ions. Adjust the syringe pump flow rate between 20-40 µl/hr to obtain optimum ion current and stability at the surface.
  3. Adjust the quadrupole mass-filter to the mass of the Ru(bpy)32+ ion m/z = 285 to prevent soft landing of species other than Ru(bpy)32+ onto the surface. Using a high-resistance electrometer connected to the substrate through a vacuum electrical feed-through, adjust the voltage settings of the ion optics and radiofrequency ion guides to maximize the ion current and stability of Ru(bpy)32+ measured at the SAM surface. Allow the experiment to run for a selected period of time to achieve the desired coverage of ions on the surface of the COOH-SAM.
  4. Increase the potential gradient in the high-pressure collision quadrupole region of the soft landing instruments to create the harsh conditions that enable gas-phase ligand stripping from the organometallic ion through collision induced dissociation. Note: Examine the schematic diagram of one of the three soft landing instruments, which is also representative of the early stages of the other two instruments, which is presented in Figure 1. The fragmentation of the Ru(bpy)32+ ion occurs in region 4. Increase the voltage applied to the back plate of the electrodynamic ion funnel to remove one bipyridine ligand from Ru(bpy)32+ m/z = 285 producing gas-phase Ru(bpy)22+ m/z = 207 in region 4 of the instrument81. Mass select the highly reactive undercoordinated fragment ion using the quadrupole mass filter in region 6 of the instrument and soft land onto COOH-SAM surfaces to examine how the extent of ligation influences the properties of supported organometallic ions.
  5. Adjust the surrounding ion optics, including the DC voltages applied to the quadrupole rods as well as the conductance limit to maximize the current of mass-selected Ru(bpy)22+ fragment ions at the surface.

3. Analysis by In Situ TOF-SIMS Before and After Exposure to Reactive Gases

  1. Turn off the syringe pump and high voltage to the ESI emitter. Open the gate valve that separates the two regions of the instrument during operation. Use the magnetic manipulator to move the prepared surface from the soft landing chamber to the analysis stage inside the TOF-SIMS part of the instrument.
  2. Disengage the manipulator from the sample and retract it fully from the SIMS analysis chamber. Close the gate valve between the soft landing and SIMS parts of the instrument because the TOF-SIMS operates at a much lower pressure than the soft landing region of the instrument.
  3. To conduct the TOF-SIMS experiment, load the instrument control file in the software and ensure that the Ga+ source is producing a sufficiently stable current of primary ions. Note: Employ 15 keV primary gallium ions (Ga+, 500 pA, 5 nsec pulse width, 10 kHz repetition rate) to induce desorption of soft landed material from the surfaces. Extract secondary ions ejected from the surface into the mass analyzer, which consists of three separate electrostatic sectors.
  4. Acquire x- and y-axis line profiles across the surface to determine the center of the deposited spot of ions on the substrate (typically at the center of the surface and 3 mm in diameter). Position the surface so that the Ga+ primary ion beam is incident on the center of the deposited spot of ions. Acquire a TOF-SIMS mass spectrum for 5 min.
  5. Turn off the primary Ga+ ion beam and high voltages of the TOF-SIMS. Use the magnetic manipulator to move the sample back into the soft landing part of the instrument. Make sure that the gate valve separating the two chambers is closed before proceeding further.
  6. Use a high-vacuum leak valve on the soft landing chamber to introduce a controlled flow of ultra-high purity oxygen (O2) gas from a cylinder into the instrument. Use the adjustable gate valve in front of the turbomolecular vacuum pump to throttle the pumping speed of the pump to achieve a steady-state pressure of 10-4 Torr of O2 inside the soft landing chamber.
  7. Following exposure of the surface to O2 for 30 min close the leak valve, open the gate valve over the turbomolecular vacuum pump and allow the remaining O2 to pump away. After the pressure in the chamber has decreased, open the gate valve to the SIMS part of the instrument and use the magnetic manipulator to position the surface on the analysis platform for a second round of TOF-SIMS analysis.
  8. After the second TOF-SIMS spectrum is obtained as described in sections 3.3-3.4, open the gate valve and position the surface back in the soft landing chamber for exposure to 10-4 Torr of C2H4 for 30 min. Perform a SIMS analysis again as described above.

4. Analysis by In Situ FT-ICR-SIMS During and After Soft Landing

  1. Prepare the SAM surfaces for experiments with the in situ FT-ICR-SIMS instrument in a manner similar to that described in section 1 but on circular substrates 5 mm in diameter. Note: Use substrates that are laser cut from a gold coated silicon wafer (5 nm chromium adhesion layer and 100 nm of polycrystalline vapor-deposited gold). Be aware that the most notable difference is that in the FT-ICR-SIMS instrument the surface is positioned inside the bore of a superconducting magnet. The presence of the magnet necessitates that the surfaces be placed at the end of a 5 ft manual z-translator to enable them to be positioned safely and adjustably at the rear plate of the ICR cell.
  2. Using the load-lock interface, position a circular SAM surface at the rear trapping plate of the ICR cell located inside of the 6 Tesla magnet. Note: Be aware that this instrument is a specially designed 6-Tesla FT-ICR mass spectrometer configured for studying ion-surface interactions97,120.
  3. Operate the ion soft landing portion of the FT-ICR-SIMS instrument in a manner similar to that described in section 2.
  4. Use a cesium ion source to create a continuous beam of 8 kV Cs+ primary ions to sputter the surface during and after ion soft landing.
  5. Utilize the ESI source positioned at 90° toward the main instrument axis to generate ions for soft landing. Focus the ions through a 90° bending quadrupole120. Note: Be aware this instrument geometry facilitates simultaneous soft landing of Ru(bpy)32+ and transmission of the primary Cs+ ion beam to the surface thereby enabling the monitoring of the soft landing process both during and after ion deposition.
  6. Trap and analyze the secondary ions ejected from the surface using FT-ICR-MS. Note: Employ static SIMS conditions corresponding to a total ion flux of about 1010 ions/cm2 (current 4 nA, duration 80 µsec, spot diameter 4.6 mm, 10 shots per spectrum, ~200 data points) for these experiments that last for about 7 hr. Average each SIMS spectrum over 10 shots corresponding to an acquisition time of ~10 sec.
  7. Acquire kinetics data by sampling the SAM surface every 4 min for approximately 7 hr during and after ion deposition.
  8. Perform data acquisition and instrument control using an automated modular data control system described in the literature121.

5. Analysis by In Situ IRRAS During and After Soft Landing

  1. Prepare the SAM surfaces for experiments with the in situ IRRAS instrument in a manner similar to that described in section 1. Note: Be aware that the largest difference with the IRRAS instrument results from the precise positioning of the surface with the z-translator that is necessary to locate the substrate at the focal point of the parabolic mirrors and in line with the beam of ions. Maximize the overlap between the infrared beam and the spot of deposited ions on the surface.
  2. Conduct the IRRAS experiments in a grazing-incidence geometry employing an FTIR spectrometer equipped with a liquid nitrogen cooled mercury-cadmium-telluride (MCT) detector.
  3. Utilize a gold-coated flat mirror to direct the light exiting the FTIR spectrometer onto a parabolic gold mirror. Reflect the light from the parabolic mirror through a mid-infrared wire grid polarizer and into the vacuum chamber through a viewport.
  4. Direct the infrared light from the spectrometer onto the COOH-SAM surface positioned inside the vacuum chamber. Note: The vacuum chamber is held at a pressure of 10-5 Torr during ion soft landing.
  5. Position the reflective SAM on gold surface inside the vacuum chamber at the focal point of the first parabolic mirror using the motor-driven z-translator.
  6. Reflect the IR light entering the vacuum chamber from the surface of the SAM and out of the chamber through a second viewport. Use a second parabolic gold mirror to focus the reflected light from the surface onto an MCT detector.
  7. Purge the pathway of the IR beam outside of the vacuum chamber with N2.
  8. Acquire spectra at set intervals during the deposition.

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Results

1. Investigating the Reactivity of Ru(bpy)32+ on COOH-SAMs Using In Situ TOF-SIMS

Soft landing of mass-selected organometallic ions onto functionalized SAMs is first illustrated using in situ TOF-SIMS to provide maximum sensitivity toward detection of adducts formed between the deposited ions and the individual molecules in the monolayers as well as any products of chemical reactions following exposure of the surfaces to reactive gases. The doubly charged ...

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Discussion

Soft landing of mass-selected ions is generally conducted employing unique custom-built instrumentation that exists in several laboratories around the world that are specially equipped for these experiments. Modifications are constantly being made to these instruments to facilitate the ionization of a wider array of compounds, to achieve larger ion currents and shorter deposition times, to multiplex soft landing and thereby achieve simultaneous deposition of several species at different locations on the surface, and to a...

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Disclosures

The authors have nothing to disclose.

Acknowledgements

This research was funded by the Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences of the U.S. Department of Energy (DOE). GEJ acknowledges support from the Linus Pauling Fellowship and the Laboratory Directed Research and Development Program at the Pacific Northwest National Laboratory (PNNL). This work was performed using EMSL, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at PNNL. PNNL is operated by Battelle for the U.S. DOE.

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Materials

List of materials used in this article
NameCompanyCatalog NumberComments
Gold on Silicon Substrates 1 cm2Platypus TechnologiesAu.1000.SL1custom 
Gold on Silicon Substrates 4.8 mm diameter circularSPI Supplies4176GSW-AB 
Glass Scintillation VialsFisher Scientific03-337-14 
Non-denatured EthanolSigma-Aldrich459836-1L 
Ultraviolet CleanerBoekel Scientific 
16-Mercaptohexadecanoic AcidSigma-Aldrich448303-5G 
Hydrochloric AcidSigma-Aldrich320331-500ML 
Aluminum FoilSigma-AldrichZ185140-1EA 
Metal Forceps/TweezersWiha49185 
Nitrile GlovesFisher ScientificS66383 
Tris(2,2′-bipyridine)dichlororuthenium(II) hexahydrateSigma-Aldrich224758-1G 
MethanolSigma-Aldrich322415-1L 
1 ml Gas Tight Glass SyringeHamilton 
Syringe PumpKD Scientific100 
360 μm ID Fused Silica CapillaryPolymicro TechnologiesTSP075375 
High Resistance ElectrometerKeithley6517A 
Commercial TOF-SIMS InstrumentPhysical ElectronicsTRIFT 
Ultra High Purity OxygenMathesonG1979175 
Research Purity EthyleneMathesonG2250178 
Cesium Ion SourceHeat Wave Labs101502 
Commercial FTIR SpectrometerBrukerVertex 70 

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In Situ SIMS and IR Spectroscopy of Well-defined Surfaces Prepared by Soft Landing of Mass-selected Ions
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