Breitenstein, D.

Bruns, M.

Fartmann, M.

Fuchs, M.

Grehl, T.

Kayser, S.

Koester, R.

Ochs, R.

Schlabach, S.

Szabo, D. V.

English

KITopen

Characterization of core/shell nanoparticle thinfilms for gas analytical applications†M. Fuchs,a D. Breitenstein,b M. Fartmann,b T. Grehl,c S. Kayser,c R. Koester,aR. Ochs,a S. Schlabach,a D. V. Szabóa and M. Brunsa∗Gas analytical microsystems such as the Karlsruhe micro nose (KAMINA) can be improved toward higher sensitivity when usingSnO2 nanoparticle thin films instead of well-established sputtered layers. For achieving long-term stability, such nanoparticlesmust be prevented from growing and agglomerating. A very promising approach is to coat SnO2 nanoparticles with anopen-pored SiO2 shell, simply acting as a spacer, while preserving electrical contact between adjacent core particles. Suchnanoparticulate thin films can easily be realized by the gas-phase Karlsruhe microwave plasma process (KMPP). This providesan all-in-one approach to synthesize particles with diameters less than 10 nm, to coat them in situ with a protective ultrathinSiO2 shell in a downstream step and to finally deposit the core/shell particles solvent-free onto prefabricated micro devices.The present study focuses on the surface analytical characterization of 200 nm thin films consisting of SnO2 core/SiO2 shellnanoparticles with various shell designs. For this purpose, the SiO2 shell thickness was systematically increased while keepingthe SnO2 core size constant. X-ray photoelectron spectroscopy (XPS) provides information concerning chemical binding statesand shell thickness in a nondestructive manner. Angle-resolved XPS together with transmission electron microscopy (TEM)validate the achieved core/shell structure. In the case of the desired ultrathin SiO2 shells, low-energy ion scattering (LEIS) issolely the suitable means to prove that Sn and Si are the constituents of the outermost monolayer of the spherical particles andit demonstrates the attainability of open-pored coatings. Copyright c© 2010 John Wiley & Sons, Ltd.Keywords: core/shell nanoparticles; nanoparticle films; silicon dioxide; stannic oxide; tin dioxide; gas sensors; XPS; LEIS; SEM; TEMIntroductionSputtered SnO2 thin films are well established as gas sensinglayers in analytical microsystems such as the Karlsruhe micronose (KAMINA).[1] To improve the performance toward highersensitivity, a very promising approach is to replace the sputteredthin films with films made of SnO2 nanoparticles. It has recentlybeen shown that microarrays equipped with pure nanocrystallineSnO2 achieve an increase in sensitivity of more than 1 orderof magnitude.[2] However, these crystallites undergo significantgrain growth at operating temperature (300–350 ◦C) and thus theefficiency of the material is lost.The method known as Zener pinning involves restricting growthwith temperature by addition of tiny discrete particles in theinterface regions between the nanocrystals.[3 – 5] In contrast, ourapproach is to coat SnO2 nanoparticles with a protective ultrathinSiO2 shell. This can easily be achieved by the Karlsruhe microwaveplasma process (KMPP), a versatile gas-phase plasma processsuited to synthesize nanoscaled particles with diameters lessthan 10 nm and to coat them in situ in a downstream step.[2,6]Moreover, the final core/shell nanoparticles can be directly andsolvent-free deposited onto prefabricated microdevices. Thesecore/shell nanoparticles are expected to prevent the gas-sensitivecores from growing and agglomerating while maintaining theelectrical conductivity. Therefore, a prerequisite is to achieve anopen-pored SiO2 shell enabling direct contact between adjacentSnO2 nanoparticles, i.e. simply acting as a spacer.In this contribution, we focus on the surface analytical char-acterization of such nanoparticle thin films with a thickness of200 nm made of SnO2 core/SiO2 shell nanoparticles. In particular,X-ray photoelectron spectroscopy (XPS) reveals chemical bindingstates and information on the shell thickness in a nondestructivemanner. For this purpose, the SiO2 shell thickness was systemati-cally increased while keeping the SnO2 core size constant. In thecase of the desired open-pored ultrathin SiO2 shells, low-energyion scattering (LEIS) is solely the suitable means to distinguishbetween Sn and Si as constituents of the outermost monolayerof the spherical particles to prove the attainability of requiredopen-pored coatings.[7] In addition, scanning electron microscopy(SEM) and transmission electron microscopy (TEM) were used toachieve a comprehensive characterization.ExperimentalSynthesisThe KMPP was used to synthesize SnO2 core/SiO2 shell nanopar-ticles with constant core size and different shell thicknesses. Adetailed description of KMPP is given elsewhere.[2,6]∗ Correspondence to: M. Bruns, Karlsruhe Institute of Technology, Institut fürMaterialforschung III, Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany. E-mail: michael.bruns@kit.edu† Paper published as part of the ECASIA 2009 special issue.a Karlsruhe Institute of Technology, Institut für Materialforschung III, Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germanyb Tascon GmbH, Heisenbergstr. D-15, 48149 Münster, Germanyc ION-TOF GmbH, Heisenbergstr. D-15, 48149 Münster, GermanyTin-IV-chloride (Sigma-Aldrich Chemie GmbH, München, Ger-many) was selected as the precursor material for the SnO2 coreparticles. Different SiO2 shell thicknesses were realized usingsilicon-IV-chloride (Riedel de Haen, Seelze, Germany) with varyingfeed rates. The process gases were in all cases an argon/oxygen80/20 mixture (Ar 5.0, O2 4.8) as the reaction gas and argon (Ar6.0) as the carrier gas for the evaporated precursors (basi SchöberlGmbH & Co. KG, Rastatt, Germany).The substrates used for the nanoparticle film characterizationwere polished single crystal <100> silicon sections (10 × 10 ×0.35 mm3). For gas analytical tests, core/shell nanoparticle filmswere directly and solvent-free deposited onto basic structuresof gas sensor microarrays,[2] which are currently composedof 38 sensor elements on an area of 4 × 8 mm2, and weremanufactured by RF magnetron sputtering, applying a shadowmasking technique.[8]CharacterizationXPS measurements were performed using a K-Alpha XPS spectrom-eter (ThermoFisher Scientific, East Grinstead, UK). Data acquisitionand processing using the Thermo Avantage software is describedelsewhere.[9] All nanoparticle thin films were analyzed using amicrofocused, monochromated Al Kα X-ray source (30–400 µmspot size). Angle-resolved spectra were obtained by acquiring sixangles between 0◦ and 60◦ electron emission using the K-Alpha tiltstage. The K-Alpha charge compensation system was employedduring analysis, using electrons of 8 eV energy, and low-energyargon ions to prevent any localized charge build-up. The spectrawere fitted with one or more Voigt profiles [binding energy (BE)uncertainty: ±0.2 eV], and Scofield sensitivity factors were appliedfor quantification.[10] All spectra were referenced to the C1s peak,assumed to originate from surface hydrocarbon contamination at285.0 eV BE controlled by means of the well-known photoelectronpeaks of metallic Cu, Ag, and Au.LEIS measurements were taken using a Qtac100 spectrometer(ION-TOF GmbH, Münster, Germany). In order to remove hydro-carbon contamination, all samples were cleaned using the atomicoxygen treatment for 40 min at sample temperatures up to 200 ◦C.LEIS spectra were obtained using 3 keV 4He+ scattering. Assuminga sputter yield of 0.1 atoms per He-ion at 6 nA target current,3.2 × 1013 cm−2 atoms were sputtered from the sample surfaceduring the analysis.SEM investigations were performed on a Zeiss Supra 55 scanningelectron microscope (Carl Zeiss SMT AG, Oberkochen, Germany)using magnification up to 3 × 105 at 15 kV accelerating voltage.TEM investigations were performed on a Tecnai F20-ST (FEICompany, Hillsboro, OR USA), operating at 200 keV. For TEMsample preparation, nanoparticles were deposited on 300 meshCu grids covered with holey carbon films. Gas analytical testswere performed with an ARTINOS electronic nose (SYSCA AG,Knittlingen, Germany) using 2-propanol in synthetic air at 50%relative humidity.Results and DiscussionRecently we have demonstrated a significant increase in gasanalytical performance when replacing conventionally sputteredSnO2 with bare SnO2 nanoparticles synthesized by KMPP.[2] Tocounteract the drawback of poor long-term stability due to particlegrowth at 300 ◦C operating temperature, the promising approachFigure 1. SEM images of a SnO2 core/SiO2 shell nanoparticle thin film,deposited directly and solvent-free onto a silicon substrate.is to cover the functional SnO2 particle with an inert open-poredSiO2 spacer. A prerequisite for the core/shell particle developmentis its comprehensive surface analytical characterization. For thispurpose, the SiO2 shell thickness was systematically increasedwhile keeping the SnO2 core size constant. KMPP enables us todeposit nanoparticle films directly and solvent-free onto varioussubstrates. The SEM images in Fig. 1 show the achievement ofa closed primary layer growing up to a columnar film structureof about 200 nm thickness. As already known from previousbare particle characterization by means of TEM and electrondiffraction,[2] the SnO2 core particles crystallize in cassiteritestructure and can be adjusted to a size of 4–5 nm with a narrowparticle size distribution The presence of the ultrathin SiO2 shellcannot be well proven by TEM investigations, as its thickness isbelow the resolution limit of the microscope. Additionally, SiO2remains amorphous and provides only low elemental contrast onthe amorphous carbon film. The open-pored structure of the shelltherefore cannot be demonstrated by TEM. This is shown in Fig.2,right part. The upper image shows SnO2 nanoparticles coatedwith a thick SiO2 shell. Here, one has the impression that SnO2nanoparticles (dark dots) are surrounded by an amorphous matrix.The middle image reveals particles with shell, but the cores arenot well separated. In the lowest image, an open-pored shell is notvisible. Hence, this is subject to verification by means of XPS andLEIS.Therefore, XPS measurements were taken in order to obtaininformation on chemical binding states of the core/shell nanopar-ticle films and to estimate the SiO2 shell thickness. The bindingenergies measured for SnO2 (Sn 3d5/2 = 487.3 eV, O 1s=531.4 eV),and SiO2 (Si 2p = 102.9 eV and O 1s = 532.3 eV), as well as fororganic contaminants, are in a good agreement with referencedata and are close to those reported in the literature.[8,11]Angle-resolved XPS data justify the achievement of sphericalcore/shell nanoparticles as the expected identical Sn/Si intensityratio was found for all applied electron emission angles, which is adirect verification of the core/shell structure.[12]Additionally, XPS is suited to measure the thickness ofnanoscaled thin films. The description of the detailed underlyingformalism based on a modified Lambert–Beer equation andleast square fitting recently has been the subject of a series ofcomprehensive studies.[8,9,13,14] In the case of Al (15 nm) core/Alshell nanoparticles, Sánchez-López and Fernández[15] proved thatan assumed cubic shape for the particles, although not in closeFigure 2. O 1s XPS spectra of SnO2 core/SiO2 shell nanoparticle thin films with different shell thicknesses (left side) compared to representative TEMimages (right side). For better visualization, the XPS spectra are normalized to maximum intensity.accordance to the observed morphology, is a reasonable modelfor quantitative XPS analysis for these samples. Therefore, thereported formalism was applied on the present SnO2 core/SiO2shell nanoparticle films using Si 2p as the shell signal and Sn 3d5/2as the core signal, and considering that the XPS information depthcovers nearly the complete particle (Fig.2). Together with TEMresults of the uncoated SnO2 nanoparticles, which reveal sizesof 4–5 nm for the SnO2 core particles, the calculated SiO2 shellthickness from the XPS measurements was found to be in therange 0.05–1.5 nm with a codependency on the precursor feedrate during synthesis (cf Fig. 3).However, measurements and theoretical predictions of attenu-ation lengths may have large uncertainties (>6%) and publishedvalues vary by a factor of 2.[16,17] Together with experimental un-certainties, one can expect an entire deviation of about 20%. Inspite of taking this into account, the XPS results are consistent withthe restricted depth range of about 5 nm.[18,19] Organic contam-inants were neglected because SiO2 shell thickness calculationsusing O1s electrons of both oxides reveal the same results withinthe experimental uncertainty. Additionally, this is justified by LEISresults revealing a maximum adventitious carbon layer of less then0.4 nm thickness for the bare SnO2 particles calculated from theSn onset peak shift during atomic oxygen cleaning.XPS findings alone give reason to assume an open-pored shellin the case of ultrathin coatings when considering the O2− ionicradius of 0.126 nm. The crucial proof, however, succeeded withLEIS measurements and is based on the appearance of their high-energy onset peaks which verify both Sn (2675 eV) and Si (1776 eV)to be constituents of the outermost monolayer of the sphericalparticles, as shown in Fig. 4.[7]In the first attempt, pulsed gas exposure experiments on open-pored SnO2 core/SiO2 shell nanoparticle films demonstrate anoverall improvement of the gas analytical properties compared tosputtered SnO2 thin films and till now reveal long-term stabilityfor more than 3 months. In particular, the dynamic range ofthe array is increased by a factor of 10 indicating a significantimprovement in sensitivity. The response time remains similarto that of sputtered SnO2 which obviously is due to the open-pored and columnar structure of the nanoparticle films (cf Fig. 1).However, the recovery time is slightly increased by a factor of1.5 based conditionally upon the increase of the effective surfacearea.Nevertheless, further shell optimization is necessary for theextension of lifetime while preserving the excellent gas sensingproperties. In particular, the long-term behavior at elevatedtemperatures is currently under investigation.Figure 3. SiO2 shell thickness dependency on Si/Sn concentration ratioscalculated from XPS data (bars indicate ±5% error).Figure 4. LEIS spectra of SnO2 core/SiO2 shell nanoparticle thin films withdifferent shell thicknesses after atomic oxygen cleaning.ConclusionsNovel SnO2 core/SiO2 shell nanoparticle films for gas analyticalapplication are attainable using the KMPP. Independent tech-niques such as SEM and TEM, XPS, and LEIS must be combinedfor a comprehensive particle and film characterization. Herewiththe SnO2 core size, the achieved SiO2 shell, and the respectivechemical composition could be substantiated. The achievementof homogenous and crack-free films with columnar morphologyof about 200 nm thickness was proved by SEM analysis. Finally,the desired open-pored ultrathin spacer shell, necessary for re-stricting the growth of the functional SnO2 particles at elevatedtemperatures and to ensure electrical contact, was verified by LEISmeasurements.In addition, first gas analytical tests proved the significantincrease of the sensor performance when replacing conventionallysputtered SnO2 with SnO2 core/SiO2 shell nanoparticle thin films.AcknowledgementsThe authors thank Udo Geckle for performing the SEM measure-ments. C. Natzeck, SYSCA AG, Knittlingen, Germany is gratefullyacknowledged for testing the gas analytical performance.References[1] J. Goschnick, Microelectron. Eng. 2001, 57, 693.[2] B. Schumacher, R. Ochs, H. Troeße, S. Schlabach, M. Bruns,D. V. Szabó, J. Haußelt, Plasma Process. Polym. 2007, 4, 865.[3] L. A. O’Dell, S. L. P. Savin, A. V. Chadwick, M. E. Smith, Nanotechnol-ogy 2005, 16, 1836.[4] Y. Al-Angry, S. L. P. Savin, K. E. Rammutla, M. J. Pooley, E. R. H. VanEck, A. V. Chadwick, Radiat. Eff. Defects Solids 2003, 158, 209 .[5] N. L. Wu, S. Y. Wang, I. A. Rusakova, Science 1999, 285, 1375.[6] D. V. Szabó, S. Schlabach, R. Ochs, B. Schumacher, M. Bruns, CeramForum Int. 2007, 84, 7.[7] H. H. Brongersma, M. Draxler, M. de Ridder, P. Bauer, Surf. Sci. Rep.2007, 62, 63.[8] V. Trouillet, H. Troesse, M. Bruns, E. Nold, R. G. White, J. Vac. Sci.Technol., A 2007, 25, 927.[9] K. L. Parry, A. G. Shard, R. D. Short, R. G. White, J. D. Whittle,A. Wright, Surf. Interface Anal. 2006, 38, 1497.[10] J. H. Scofield, J. Electron Spectrosc. Relat. Phenom. 1976, 8, 129.[11] G. P. López, D. G. Castner, B. D. Ratner, Surf. Interface Anal. 1991, 17,267.[12] I. Tunc, S. Suzer, M. A. Correa-Duarte, L. M. Liz-Marzan, J. Phys. Chem.B 2005, 109, 7597.[13] M. P. Seah, S. J. Spencer, Surf. Interface Anal. 2002, 33, 640.[14] M. P. Seah, R. White, Surf. Interface Anal. 2002, 33, 960.[15] J. C. Sánchez-López, A. Fernández, Surf. Interface Anal. 1998, 26,1016.[16] S. Tanuma, C. J. Powell, D. R. Penn, Surf. Interface Anal. 1991, 17, 927.[17] Z. H. Lu, J. P. McCaffrey, B. Brar, Appl. Phys. Lett. 1997, 71, 2764.[18] P. J. Cumpson, M. P. Seah, Surf. Interface Anal. 1997, 25, 430.[19] S. Tanuma, in Surface Analysis by Auger and X-ray PhotoelectronSpectroscopy (Eds: D. Briggs, J. T. Grant), IM Publications and SurfaceSpectra Limited: Chichester, 2002.      Repository KITopen  Dies ist ein Postprint/begutachtetes Manuskript.  Empfohlene Zitierung:  Fuchs, M.; Breitenstein, D.; Fartmann, M.; Grehl, T.; Kayser, S.; Koester, R.; Ochs, R.; Schlabach, S.; Szabo, D. V.; Bruns, M. Characterization of core/shell nanoparticle thin films for gas analytical applications. 2010. Surface and interface analysis.  doi:10.5445/IR/110079781     Zitierung der Originalveröffentlichung:  Fuchs, M.; Breitenstein, D.; Fartmann, M.; Grehl, T.; Kayser, S.; Koester, R.; Ochs, R.; Schlabach, S.; Szabo, D. V.; Bruns, M. Characterization of core/shell nanoparticle thin films for gas analytical applications. 2010. Surface and interface analysis, 42, 1131–34.  doi:10.1002/sia.3223               Lizenzinformationen: KITopen-Lizenz 

Characterization of core/shell nanoparticle thin films for gas analytical applications

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Characterization of core/shell nanoparticle thin films for gas analytical applications

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