ECASIA special issue paper

Received: 1 November 2013 Revised: 11 December 2013 Accepted: 13 December 2013 Published online in Wiley Online Library

(wileyonlinelibrary.com) DOI 10.1002/sia.5378

Investigation of silica nanoparticles by Augerelectron spectroscopy (AES)†

S. Rades, T. Wirth* and W. Unger

High-priority industrial nanomaterials like SiO2, TiO2, and Ag are being characterized on a systematic basis within the frame-work of the EU FP7 research project NanoValid. Silica nanoparticles from an industrial source have been analyzed by Augerelectron spectroscopy. Point, line, and map spectra were collected. Material specific and methodological aspects causingthe special course of Auger line scan signals will be discussed. Copyright © 2014 John Wiley & Sons, Ltd.

Keywords: nanoparticles; Auger electron spectroscopy; surface analysis

* Correspondence to: T. Wirth, Division 6.8 Surface Analysis and InterfacialChemistry, Federal Institute of Materials Research and Testing, Unter denEichen 87, 12205 Berlin, Germany.E-mail: thomas.wirth@bam.de

† Paper published as part of the ECASIA 2013 special issue.

Division 6.8 Surface Analysis and Interfacial Chemistry, Federal Institute ofMaterials Research and Testing, Unter den Eichen 87, 12205 Berlin, Germany

Introduction

By now, a huge range of nanomaterials can be found in con-sumer and technological products. Recent progress in nanotech-nology development has been made in the fields of antimicrobialagents and biosensors.[1] The manifold industrial applicationslead to a higher release of nanoscaled chemicals to the environ-ment and therefore rising human exposure. On the other hand,there is a lack of standard toxicological and physicochemicalcharacterization methods for evaluating and managing the riskof engineered nanoparticles.

High-priority industrial nanomaterials like SiO2, TiO2, and Ag arebeing characterized on a systematic basis within the framework ofthe EU FP7 research project NanoValid[2] where those issues arebeing addressed. One of the project tasks aims for establishingreference methods for physicochemical characterization and mea-surement. In this context, the identification of potential candidateswith potential to be developed to a certified reference material isclosely connected too. In this work, silica industrially manufacturedby sol–gel synthesis has been investigated.

Investigating small surface structures typically scanningelectron microscopy (SEM) or analytical electron microscopy(SEM/EDX) is applied. However, analyzing nanoparticles, thesupport material will be excited too caused by the electron beampenetrating down to the micrometer range. In lieu thereof, Augerelectron spectroscopy (AES) with an information depth of only afew nanometers can be applied by analyzing the chemicalcomposition of nanoparticle surfaces. Nanoparticles were ana-lyzed by Auger spectra, line scans, and mappings. However,interpreting line scans material specific and methodologicalaspects have to be taken into account, which has been doneearlier by Ito et al., too.[5] Different effects causing a special courseof line scan signals will be discussed.

Experimental

Silica was synthesized by sol–gel technique with an anionicsurfactant and Tetraethylorthosilicate (TEOS) as starting materials.The sample powder was dispersed in ultra pure water by sonica-tion. One microlitre was deposited on a conventional TEM carboncoated copper grid. In a first step, several particles have been

Surf. Interface Anal. (2014)

imaged and preselected by SEM. SEM images are applied toguide subsequent AES measurements. A PHI 700 Auger ScanningProbe (ULVAC-PHI Inc.) equipped with a cylindrical mirror ana-lyzer was used. AES of nanoparticles has to be carried out by ap-plying a very small electron beam diameter getting a good lateralresolution of line scans and mappings. Therefore, Auger electronswere excited by a primary electron beam of 20 keV @ 1 nA or10 nA and 25 KeV @ 1nA, respectively. The beam size wasestimated to be 9 and 15 nm, respectively. The primary electronbeam hit the surface in perpendicular direction avoiding effectsof non symmetric intensity enhancements as described inReference[5] too. The relative energy resolution ΔE/E was 0.5%.Measuring Auger spectra of low noise by point analysis, acompromise had to be made between higher acquisition time(low beam current) and possible particle damage caused by theexciting electron beam.[6]

Results

In the secondary electron image in Fig. 1, two particles identifiedeach by a marker are presented; of which, AES survey spectra inFigs 2a and 2b are displayed. Analyzing dielectric materials likeSilica charging effects can appear that influences Auger electronsand consequently Auger spectra (remarkable shift to higher ener-gies). Darker interiors and the brighter perimeters of the particlesseen in Fig. 1 are hints for charging. However, looking on thespectra in Figs 2a and 2b, only small peak energy shifts appear(C KLL: 275 to 274.5 eV, O KLL: 510 to 515 eV, Si LVV: 96 to95 eV, and Si KLL: 1621 to 1619 eV). The energy of the siliconpeaks shifts towards lower energies but that of the oxygen peaktowards higher energies with respect to values well known for SiO2.

Copyright © 2014 John Wiley & Sons, Ltd.

Figure 1. Secondary electron image providing an overview on silica par-ticles distributed across a TEM grid showing two selected points used forchemical analysis by AES.

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Figure 2. Selected point AES spectra taken from silica particles 1 (a) andL3M2,3M2,3 (600 eV), Fe L3M2,3M4,5 (654 eV), Fe L3M4,5M4,5 (705 eV), Cu L

S. Rades, T. Wirth and W. Unger

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Therefore, it can be concluded that negative charge flows off viathe carbon film (refer also to Ito et al.[5]).

Both particle surfaces consist of SiO2 (silica). Furthermore,carbon, nitrogen, fluorine, and iron were detected, the latterthree only at trace levels. Additionally, a small copper peak onparticle 1 appeared in the spectrum.

Looking on the AES mappings in Figs 3a and 3b, oxygen andsilicon signals originating from the silica particles appear. TheAES mapping of carbon in Fig. 3c shows that the silica particlesare covered by carbon contamination, and the higher carbonintensities measured in the surroundings of each particleoriginate from carbon contamination and the carbon film under-neath. Oxygen seems to be almost uniformly distributed on theparticle areas. However, for those two particles analyzed by pointanalyses (Fig. 1), some depletion in O KLL intensity becomesvisible. Regarding the traces of iron, fluorine, and nitrogenobserved in the spectra (Fig. 2), it can be assumed that theseelements exist uniformly distributed in a contamination layeron top or in the bulk of the silica particles and do not originatefrom other particular matter adjacent to the silica particles. The

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2 (b) shown in Fig. 1. [C KLL (275 eV), N KLL (389 eV), F KLL (659 eV), FeMM (922 eV), Si LVV (96 eV), and Si KLL (1621 eV)].

4 John Wiley & Sons, Ltd. Surf. Interface Anal. (2014)

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Figure 3. AES maps of (a) O KLL, (b) Si KLL, and (c) C KLL signals taken from the area shown in Fig. 1.

Characterization of nanoparticles by Auger electron spectroscopy

copper signal in the spectrum in Fig. 2a is most probably causedby the copper grid.

In Figs 4a and 4b, line scans across another silica particlealready shown in Fig. 1 (upper right corner) are presented. TheSi KLL and O KLL intensities increase, reach a plateau, and decreasewhen the primary electron beam moves across the particle,

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Figure 4. (a) Detail of Fig. 1 (SEM) with a scan line across a nanosized silica paand Si KLL(bottom) signals.

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Figure 5. (a) Detail of Fig. 1 (SEM) with a scan line across a nanosized silica pand Si KLL (bottom) signals.

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whereas the C KLL signal changes in a reciprocal manner. More-over, the signals of silicon and oxygen show a transition effect, thatis, a slight increase when the transition region ‘projected particleedge’ – ‘carbon foil on TEM grid’ is crossed.

In Figs 5a, 5b, 6a, and 6b, line scans across other silica particlesare presented. The silicon and oxygen line scans show principally

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rticle (~100 nm diameter). (b) AES line scans of C KLL (top), O KLL (middle),

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article (~60 nm diameter). (b) AES line scans of C KLL (top), O KLL (middle),

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Figure 6. (a) SEM picture of three particles with scan line across the nanosized particle (~80 nm diameter) in the middle of the two others. (b) AES linescans of C KLL (top), O KLL (middle), and Si KLL (bottom) signals.

S. Rades, T. Wirth and W. Unger

similar shapes. However, transition phenomena do not occur forthe silicon and oxygen line scans shown in Fig. 5b but occur againin Fig. 6b. Considering the carbon line scans, quite different resultsare obtained. Whereas the carbon line scans shown in Figs 5b and4b qualitatively coincide rather well, the carbon line scan in Fig. 6bshows strong signal oscillations when crossing the particle.Line scans across a nanoscaled silica particle of about 25 nm

diameter and a big silica particle of about 500 nm diameter arepresented in Figs 7a, 7b, 8a, and 8b, respectively. The shapes ofthe oxygen and silicon line scans in Fig. 7b are similar to thoseshown in Figs 4b and 6b but are showing higher noise levels.The carbon line scan shows only a small dip at the left transitionregion ‘projected particle edge’ – ‘carbon foil on TEM gridsupport’. Results obtained with the big 500 nm particle, whichshows a complex irregular morphology in the SEM picture, aredisplayed in Fig. 8a. Multiple transition effects are observed forthe line scans for all three elements investigated.

Discussion

As mentioned earlier, the two particles analyzed by point mea-surements show dark spots in the center. The silicon peaks in

a

Figure 7. (a) SEM picture with scan line across a nanosized particle (~25 nm(bottom) signals.

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selected point AES spectra shown in Figs 2a and 2b show a slightshift to lower peak energies. The Si KLL peaks resemble an oxide(Si+4) like shape, but the peak shapes of the Si LVV signals tend tobe typical for an elemental one (Si0). Therefore, it can beconcluded that these spots are caused by the exiting primarybeam via electron beam stimulated bond breaking and oxygendesorption. Such beam damage has been discussed in the workof Tanuma et al.[6] for thin silicon oxide layers on a Si wafer,too. The information depth of the Si LVV peak is about 0.5 nm,and the one of the Si KLL peak is about 2 nm. Electron beam stim-ulated desorption of oxygen occurred in the top most atomiclayers causing the elemental like peak shape of the Si LVV signalwhereas the Si+4 like KLL signal originates from less damageddeeper layers of the silica particle.

The valleys of the carbon signal at the particle rims in Figs 6band 8b are caused by the particle that shields the carbon Augerelectrons of the carbon film. The enhancement of the carbonsignal at the projected particle rims cannot be attributed tohigher carbon concentrations there but to well known edgeand topographical effects.[3–5] This holds true also for the inten-sity enhancements of silicon and oxygen Auger emission at theprojected particle rims in Figs 4b, 6b, 7b, and 8b. Auger electronsare not only excited within the spot area of the electron beam

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diameter). (b) AES line scans of C KLL (top), O KLL (middle), and Si KLL

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Figure 8. (a) SEM picture with scan line across a particle (~500 nm diameter) with complex shape. (b) AES line scans of C KLL (top), O KLL (middle), andSi KLL (bottom) signals.

Characterization of nanoparticles by Auger electron spectroscopy

but also in regions next to it caused by electron scattering and/orbackscattering. This results in several distortion effects in AES linescans like the intensity enhancement at rims observed in ourlines cans. However, there are obviously cases where notransition effects are observed, for example, for line scans of allthree elements in Fig. 5b or the carbon line scan shown in Fig. 4b.The latter observation could have different reasons:

(1) A ring of carbonaceous contamination material existingaround the deposited particle levels out edge effects.Nanoparticles were deposited by drop casting onto the carbonfilm of the TEM grid support from water dispersion. Therefore,inevitably hydrocarbons were deposited, too.

(2) The particles are partially embedded in the carbon foil of theTEM grid (Fig. 5b).

Physical reasons for the artifacts are scatter processes ofprimary, secondary, and Auger electrons. These artifacts canstrengthen, compensate, or weaken each other. Some methodo-logical work has to be performed to explain these effects indetail, which are important for data interpretation of AESmeasurements of nanoparticles.

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Acknowledgements

The research leading to these results has received funding from theEuropean Union Seventh Framework Programme (FP7/2007–2013)under grant agreement no. 263147 (NanoValid –Development of ref-erencemethods for hazard identification, risk assessment, and LCA ofengineered nanomaterials). Thanks are due to Dr R. H. Labrador(Nanologica AB, Stockholm, Sweden) for providing silica samples.

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30, 221.[2] http://www.nanovalid.eu/[3] S. Baumgartl, F. Leiber, T. Wirth, Entwicklung von Verfahren zur

Direktbestimmung unterschiedlicher Nitride, Carbonitride und Carbidein Stahl, Amt für amtliche Veröffentlichungen der EuropäischenGemeinschaften, Luxemburg, Luxemburg, 1996.

[4] Y. Li, S. Mao, Z. Ding, Monte Carlo Simulation of SEM and SAM Im-ages, Applications of Monte Carlo Method in Science and Engineering(Ed: S. Mordechai), InTech, 2011. www.intechopen.com

[5] H. Ito, M. Ito, Y. Magatani, F. Soeda, Appl. Surf. Sci., 1996, 100/101, 152.[6] S. Tanuma, T. Kimura, K. Nishida, S. Hashimoto, M. Inoue, T. Ogiwara,

M. Suzuki, K. Miura, Appl. Surf. Sci., 2005, 241, 122.

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