Investigation of silica nanoparticles by Auger electron spectroscopy (AES)

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1 ECASIA specil issue pper Received: Novemer 3 Revised: Decemer 3 Accepted: 3 Decemer 3 Pulished online in Wiley Online Lirry (wileyonlinelirry.com) DOI./si.5378 Investigtion of silic nnoprticles y Auger electron spectroscopy (AES) S. Rdes, T. Wirth* nd W. Unger High-priority industril nnomterils like SiO, TiO, nd Ag re eing chrcterized on systemtic sis within the frmework of the EU FP7 reserch project NnoVlid. Silic nnoprticles from n industril source hve een nlyzed y Auger electron spectroscopy. Point, line, nd mp spectr were collected. Mteril specific nd methodologicl spects cusing the specil course of Auger line scn signls will e discussed. Copyright 4 John Wiley & Sons, Ltd. Keywords: nnoprticles; Auger electron spectroscopy; surfce nlysis Introduction By now, huge rnge of nnomterils cn e found in consumer nd technologicl products. Recent progress in nnotechnology development hs een mde in the fields of ntimicroil gents nd iosensors. [] The mnifold industril pplictions led to higher relese of nnoscled chemicls to the environment nd therefore rising humn exposure. On the other hnd, there is lck of stndrd toxicologicl nd physicochemicl chrcteriztion methods for evluting nd mnging the risk of engineered nnoprticles. High-priority industril nnomterils like SiO,TiO, nd Ag re eing chrcterized on systemtic sis within the frmework of the EU FP7 reserch project NnoVlid [] where those issues re eing ddressed. One of the project tsks ims for estlishing reference methods for physicochemicl chrcteriztion nd mesurement. In this context, the identifiction of potentil cndidtes with potentil to e developed to certified reference mteril is closely connected too. In this work, silic industrilly mnufctured y sol gel synthesis hs een investigted. Investigting smll surfce structures typiclly scnning electron microscopy (SEM) or nlyticl electron microscopy (SEM/EDX) is pplied. However, nlyzing nnoprticles, the support mteril will e excited too cused y the electron em penetrting down to the micrometer rnge. In lieu thereof, Auger electron spectroscopy (AES) with n informtion depth of only few nnometers cn e pplied y nlyzing the chemicl composition of nnoprticle surfces. Nnoprticles were nlyzed y Auger spectr, line scns, nd mppings. However, interpreting line scns mteril specific nd methodologicl spects hve to e tken into ccount, which hs een done erlier y Ito et l., too. [5] Different effects cusing specil course of line scn signls will e discussed. Experimentl Silic ws synthesized y sol gel technique with n nionic surfctnt nd Tetrethylorthosilicte (TEOS) s strting mterils. The smple powder ws dispersed in ultr pure wter y soniction. One microlitre ws deposited on conventionl TEM cron coted copper grid. In first step, severl prticles hve een imged nd preselected y SEM. SEM imges re pplied to guide susequent AES mesurements. A PHI 7 Auger Scnning Proe (ULVAC-PHI Inc.) equipped with cylindricl mirror nlyzer ws used. AES of nnoprticles hs to e crried out y pplying very smll electron em dimeter getting good lterl resolution of line scns nd mppings. Therefore, Auger electrons were excited y primry electron em of na or na nd 5 na, respectively. The em size ws estimted to e 9 nd 5 nm, respectively. The primry electron em hit the surfce in perpendiculr direction voiding effects of non symmetric intensity enhncements s descried in Reference [5] too. The reltive energy resolution ΔE/E ws %. Mesuring Auger spectr of low noise y point nlysis, compromise hd to e mde etween higher cquisition time (low em current) nd possile prticle dmge cused y the exciting electron em. [6] Results In the secondry electron imge in Fig., two prticles identified ech y mrker re presented; of which, AES survey spectr in Figs nd re displyed. Anlyzing dielectric mterils like Silic chrging effects cn pper tht influences Auger electrons nd consequently Auger spectr (remrkle shift to higher energies). Drker interiors nd the righter perimeters of the prticles seen in Fig. re hints for chrging. However, looking on the spectr in Figs nd, only smll pek energy shifts pper (: 75 to 74.5 ev, : 5 to 55 ev, Si LVV: 96 to 95 ev, nd : 6 to 69 ev). The energy of the silicon peks shifts towrds lower energies ut tht of the oxygen pek towrds higher energies with respect to vlues well known for SiO. * Correspondence to: T. Wirth, Division 6.8 Surfce Anlysis nd Interfcil Chemistry, Federl Institute of Mterils Reserch nd Testing, Unter den Eichen 87, 5 Berlin, Germny. E-mil: thoms.wirth@m.de Pper pulished s prt of the ECASIA 3 specil issue. Division 6.8 Surfce Anlysis nd Interfcil Chemistry, Federl Institute of Mterils Reserch nd Testing, Unter den Eichen 87, 5 Berlin, Germny Surf. Interfce Anl. (4) Copyright 4 John Wiley & Sons, Ltd.

2 Cu LMM N KLL Si LMM N KLL Cl KLL Si LMM S. Rdes, T. Wirth nd W. Unger Figure. Secondry electron imge providing n overview on silic prticles distriuted cross TEM grid showing two selected points used for chemicl nlysis y AES. Therefore, it cn e concluded tht negtive chrge flows off vi the cron film (refer lso to Ito et l. [5] ). Both prticle surfces consist of SiO (silic). Furthermore, cron, nitrogen, fluorine, nd iron were detected, the ltter three only t trce levels. Additionlly, smll copper pek on prticle ppered in the spectrum. Looking on the AES mppings in Figs 3 nd 3, oxygen nd silicon signls originting from the silic prticles pper. The AES mpping of cron in Fig. 3c shows tht the silic prticles re covered y cron contmintion, nd the higher cron intensities mesured in the surroundings of ech prticle originte from cron contmintion nd the cron film underneth. Oxygen seems to e lmost uniformly distriuted on the prticle res. However, for those two prticles nlyzed y point nlyses (Fig. ), some depletion in intensity ecomes visile. Regrding the trces of iron, fluorine, nd nitrogen oserved in the spectr (Fig. ), it cn e ssumed tht these elements exist uniformly distriuted in contmintion lyer on top or in the ulk of the silic prticles nd do not originte from other prticulr mtter djcent to the silic prticles. The x 4 dn(e)/de Kinetic Energy (ev) x 4 dn(e)/de Kinetic Energy (ev) Figure. Selected point AES spectr tken from silic prticles () nd () shown in Fig.. [ (75 ev), N KLL (389 ev), F KLL (659 ev), Fe L3M,3 M,3 (6 ev), Fe L3M,3 M4,5 (654 ev), Fe L3M4,5 M4,5 (75 ev), Cu LMM (9 ev), Si LVV (96 ev), nd (6 ev)]. wileyonlinelirry.com/journl/si Copyright 4 John Wiley & Sons, Ltd. Surf. Interfce Anl. (4)

Chrcteriztion of nnoprticles y Auger electron spectroscopy. µm. µm. µm. µm. µm. µm c Figure 3. AES mps of (), (), nd (c) signls tken from the re shown in Fig.. copper signl in the spectrum in Fig.

The nd intensities increse, rech plteu, nd decrese when the primry electron em moves cross the prticle, wheres the signl chnges in reciprocl mnner.

In Figs 5, 5, 6, nd 6, line scns cross other silic prticles re presented. The silicon nd oxygen line scns show principlly.5 x 5.5...3.4.6.7.8 Distnce (µm) Figure 4. () Detil of Fig.

3 Chrcteriztion of nnoprticles y Auger electron spectroscopy. µm. µm. µm. µm. µm. µm c Figure 3. AES mps of (), (), nd (c) signls tken from the re shown in Fig.. copper signl in the spectrum in Fig. is most proly cused y the copper grid. In Figs 4 nd 4, line scns cross nother silic prticle lredy shown in Fig. (upper right corner) re presented. The nd intensities increse, rech plteu, nd decrese when the primry electron em moves cross the prticle, wheres the signl chnges in reciprocl mnner. Moreover, the signls of silicon nd oxygen show trnsition effect, tht is, slight increse when the trnsition region projected prticle edge cron foil on TEM grid is crossed. In Figs 5, 5, 6, nd 6, line scns cross other silic prticles re presented. The silicon nd oxygen line scns show principlly.5 x Distnce (µm) Figure 4. () Detil of Fig. (SEM) with scn line cross nnosized silic prticle (~ nm dimeter). () AES line scns of (top), (middle), nd (ottom) signls..5 x Distnce (µm) Figure 5. () Detil of Fig. (SEM) with scn line cross nnosized silic prticle (~6 nm dimeter). () AES line scns of (top), (middle), nd (ottom) signls. Surf. Interfce Anl. (4) Copyright 4 John Wiley & Sons, Ltd. wileyonlinelirry.com/journl/si

S. Rdes, T. Wirth nd W. Unger.5 x 5.5...3.4 Distnce (µm) Figure 6. () SEM picture of three prticles with scn line cross the nnosized prticle (~8 nm dimeter) in the middle of the two others.

Considering the cron line scns, quite different results re otined. Wheres the cron line scns shown in Figs 5 nd 4 qulittively coincide rther well, the cron line scn in Fig.

4 S. Rdes, T. Wirth nd W. Unger.5 x Distnce (µm) Figure 6. () SEM picture of three prticles with scn line cross the nnosized prticle (~8 nm dimeter) in the middle of the two others. () AES line scns of (top), (middle), nd (ottom) signls. similr shpes. However, trnsition phenomen do not occur for the silicon nd oxygen line scns shown in Fig. 5 ut occur gin in Fig. 6. Considering the cron line scns, quite different results re otined. Wheres the cron line scns shown in Figs 5 nd 4 qulittively coincide rther well, the cron line scn in Fig. 6 shows strong signl oscilltions when crossing the prticle. Line scns cross nnoscled silic prticle of out 5 nm dimeter nd ig silic prticle of out 5 nm dimeter re presented in Figs 7, 7, 8, nd 8, respectively. The shpes of the oxygen nd silicon line scns in Fig. 7 re similr to those shown in Figs 4 nd 6 ut re showing higher noise levels. The cron line scn shows only smll dip t the left trnsition region projected prticle edge cron foil on TEM grid support. Results otined with the ig 5 nm prticle, which shows complex irregulr morphology in the SEM picture, re displyed in Fig. 8. Multiple trnsition effects re oserved for the line scns for ll three elements investigted. Discussion As mentioned erlier, the two prticles nlyzed y point mesurements show drk spots in the center. The silicon peks in selected point AES spectr shown in Figs nd show slight shift to lower pek energies. The peks resemle n oxide (Si +4 ) like shpe, ut the pek shpes of the Si LVV signls tend to e typicl for n elementl one (Si ). Therefore, it cn e concluded tht these spots re cused y the exiting primry em vi electron em stimulted ond reking nd oxygen desorption. Such em dmge hs een discussed in the work of Tnum et l. [6] for thin silicon oxide lyers on Si wfer, too. The informtion depth of the Si LVV pek is out nm, nd the one of the pek is out nm. Electron em stimulted desorption of oxygen occurred in the top most tomic lyers cusing the elementl like pek shpe of the Si LVV signl wheres the Si +4 like KLL signl origintes from less dmged deeper lyers of the silic prticle. The vlleys of the cron signl t the prticle rims in Figs 6 nd 8 re cused y the prticle tht shields the cron Auger electrons of the cron film. The enhncement of the cron signl t the projected prticle rims cnnot e ttriuted to higher cron concentrtions there ut to well known edge nd topogrphicl effects. [3 5] This holds true lso for the intensity enhncements of silicon nd oxygen Auger emission t the projected prticle rims in Figs 4, 6, 7, nd 8. Auger electrons re not only excited within the spot re of the electron em.5 x Distnce (µm) Figure 7. () SEM picture with scn line cross nnosized prticle (~5 nm dimeter). () AES line scns of (top), (middle), nd (ottom) signls. wileyonlinelirry.com/journl/si Copyright 4 John Wiley & Sons, Ltd. Surf. Interfce Anl. (4)

Chrcteriztion of nnoprticles y Auger electron spectroscopy 3 x 4.5.5.5 Distnce (µm) Figure 8. () SEM picture with scn line cross prticle (~5 nm dimeter) with complex shpe.

5 Chrcteriztion of nnoprticles y Auger electron spectroscopy 3 x Distnce (µm) Figure 8. () SEM picture with scn line cross prticle (~5 nm dimeter) with complex shpe. () AES line scns of (top), (middle), nd (ottom) signls. ut lso in regions next to it cused y electron scttering nd/or ckscttering. This results in severl distortion effects in AES line scns like the intensity enhncement t rims oserved in our lines cns. However, there re oviously cses where no trnsition effects re oserved, for exmple, for line scns of ll three elements in Fig. 5 or the cron line scn shown in Fig. 4. The ltter oservtion could hve different resons: () A ring of cronceous contmintion mteril existing round the deposited prticle levels out edge effects. Nnoprticles were deposited y drop csting onto the cron film of the TEM grid support from wter dispersion. Therefore, inevitly hydrocrons were deposited, too. () The prticles re prtilly emedded in the cron foil of the TEM grid (Fig. 5). Physicl resons for the rtifcts re sctter processes of primry, secondry, nd Auger electrons. These rtifcts cn strengthen, compenste, or weken ech other. Some methodologicl work hs to e performed to explin these effects in detil, which re importnt for dt interprettion of AES mesurements of nnoprticles. Acknowledgements The reserch leding to these results hs received funding from the Europen Union Seventh Frmework Progrmme (FP7/7 3) under grnt greement no (NnoVlid Development of reference methods for hzrd identifiction,risk ssessment,nd LCA of engineered nnomterils). Thnks re due to Dr R. H. Lrdor (Nnologic AB, Stockholm, Sweden) for providing silic smples. References [] A. Nrynn, P. Shrm, B. M. Moudgil, KONA Powder Prt. J. 3, 3,. [] [3] S. Bumgrtl, F. Leier, T. Wirth, Entwicklung von Verfhren zur Direktestimmung unterschiedlicher Nitride, Cronitride und Cride in Sthl, Amt für mtliche Veröffentlichungen der Europäischen Gemeinschften, Luxemurg, Luxemurg, 996. [4] Y.Li,S.Mo,Z.Ding,Monte Crlo Simultion of SEM nd SAM Imges, Applictions of Monte Crlo Method in Science nd Engineering (Ed: S. Mordechi), InTech,. [5] H. Ito, M. Ito, Y. Mgtni, F. Soed, Appl. Surf. Sci., 996, /, 5. [6] S. Tnum, T. Kimur, K. Nishid, S. Hshimoto, M. Inoue, T. Ogiwr, M. Suzuki, K. Miur, Appl. Surf. Sci., 5, 4,. Surf. Interfce Anl. (4) Copyright 4 John Wiley & Sons, Ltd. wileyonlinelirry.com/journl/si

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