Ž .Materials Science and Engineering C 19 2002 95–99www.elsevier.comrlocatermsec

ž /Electronic structure of gold nanoparticles deposited on SiOrSi 100x

G. Petoa,), G.L. Molnara, Z. Pasztia, O. Gesztia, A. Beckb, L. Guczib˝ ´ ´a MTA Research Institute for Technical Physics and Materials Science, P.O. Box 49, H-1525 Budapest, Hungary

b Department of Surface Chemistry and Catalysis, Institute of Isotopes and Surface Chemistry, MTA Chemical Research Center,P.O. Box 77, H-1525 Budapest, Hungary

Abstract

Nanosize gold particles were prepared by Arq ion sputtering of island-like 10-nm-thick film deposited onto SirSiO substrate. ThexŽ .valence band of the gold particles was measured by means of photoelectron spectroscopy and infrared IR absorbance. The size of the

Ž .particles was determined by transmission electron microscopy TEM . The valence band of Au nanoparticles is strongly redistributed withdecreasing size, involving mostly the lowest and the highest binding energy part of the Au 5d valence states. This effect can be attributedmore to the redistribution than to the narrowing of the 5d states.q2002 Elsevier Science B.V. All rights reserved.

Ž .Keywords: Electronic structure; Gold nanoparticles; SiOrSi 100x

1. Introduction

The properties of nanoparticles strongly deviate fromthose of bulk counterparts. The electronic structure is oneof the most important basic parameters which can beexpected to be dependent on the size of the nanoparticles.Although the investigation of the valence band density ofstates of discontinuous films and clusters has a long his-

w xtory 1–7 , there are many conflicting conclusions drawnw xfrom these measurements even now 8–10 . A common

feature of the studied metallic systems is the narrowing ofthe valence band accompanied by the binding energy shift

w xtowards higher values with decreasing cluster size 1–10 .For Cu and Ag, however, we have recently observed aredistribution of the valence band leading to disappearance

w xof the low binding energy d-states 11–13 which disagreewith earlier data.

The noble metal nanoparticles are good candidates forthe investigation because they have special new chemicaland catalytic properties which are probably strongly corre-lated with their electronic structure.

In this work we wish to explore the electronic structureof Au nanoparticles, which were prepared on native oxide

Ž .covered Si 100 . The results will be compared to thoseobtained earlier for other similar systems as Cu and Agw x11,12 .

) Corresponding author. Tel.:q36-1-395-9045; fax:q36-1-395-9154.Ž .E-mail address: peto@mfa.kfki.hu G. Peto .˝

2. Experimental

Gold thin films were deposited by thermal evaporationŽ .in a VT-460 evaporator onto Si 100 wafer covered with

native oxide of nanometer thickness. The substrate waskept at room temperature and the pressure was 10y6 Pa.The gold layer was discontinuous consisting of separateislands with average thickness around 10 nm.

The electronic structure of the Au nanoparticles wasdetermined by measuring the energy distribution of photo-

Ž . Ž .electrons excited by HeI UPS and AlKa XPS radiationin a Kratos ES-300 electron spectrometer. The energyresolution was 0.15 eV in UPS and 0.8 eV in the XPSregion. The samples were cleaned by Arq ion bombard-ment for a few minutes.

The size of the gold islands was altered with furtherArq ion bombardment at 500 eV–2 keV ion energy as it isshown schematically in Fig. 1. By this process the heightas well as the lateral size of the islands is decreased. Thesize reduction process for Au nanoparticles was character-ized by the sputtering time during the in situ Arq ionbombardment and by the AurSi ratio. The native oxidelayer on the Si substrate served as a barrier against theSirAu interaction, but it was thin enough to avoid electri-cal charging.

The absolute value of the size of the nanoparticles wasŽ .determined with transmission electron microscopy TEM

after the final ion bombardment on the same sample whichwas investigated by photoemission. The samples for TEMinvestigation were prepared by extraction replica method

0928-4931r02r$ - see front matterq2002 Elsevier Science B.V. All rights reserved.Ž .PII: S0928-4931 01 00449-0

( )G. Peto et al.rMaterials Science and Engineering C 19 2002 95–99˝96

Fig. 1. Size modification process induced by Arq ion bombardment. Theangle of ion beam was 508. The process was carried out in situ in theES-300 photoelectron spectrometer. The current density was 1mArcm2.

w x14 . The particles were stripped off the substrate surfaceby means of collodion, which was covered by C film. Thecollodion was dissolved after C deposition. The TEMinvestigation was carried out by a Philips CM20 electronmicroscope, and X-ray microanalysis was made by a ger-manium detector NORAN EDS attached to the electronmicroscope.

A MIDAC 210-HFTIR spectrometer was used to mea-Ž .sure the infrared IR absorption spectra of the samples in

Fig. 2. Photoemission from 4f atomic level of Au after surface cleaningŽ . Ž . Ž .a , 15-min b and 30-min c ion bombardment.

the 4500–1000-cmy1 wave number range. The resolutionwas 4 cmy1.

3. Results and discussion

Fig. 2 shows the Au 4f emission of the nanoparticles atdifferent steps of the sputtering. The as-cleaned spectrumŽ Ž .. w xcurve a is equivalent to the published data 15 . TheAurSi ratio was around 3.3 showing that the gold film isdiscontinuous. With further ion bombardment, the positionof the 4f emission is shifted to higher binding energy,

Ž Ž .while the AurSi ratio decreased to 2 and 1.3 curves bŽ . .and c , respectively , showing that the coverage of the Au

film is decreased together with the size of the Au islands.The shift of the Au 4f level is another indication of thesize decrease of Au nanoparticles similarly as it was

w xobserved for Cu and Ag 11,12 .The size dependence of the valence band is shown on

Fig. 3. The spectra were obtained with AlKa excitation. InŽ . Ž . Ž .Fig. 3, the curves a , b and c represent the same

samples as those in Fig. 2. In spite of the relatively lowresolution, it is obvious that the Au 5d peak at 4 eV

Ž .binding energy BE strongly decreases or even disappearsŽ Ž . Ž ..with decreasing particle size curves b and c compared

w xto the well known bulk spectrum 16,17 given by curveŽ .a .

Ž .Fig. 3. Energy distribution curves of photoelectrons EDCs excited byŽ . Ž . Ž .AlK a radiation. The curves a , b and c were obtained after the same

sputtering as in Fig. 2.

( )G. Peto et al.rMaterials Science and Engineering C 19 2002 95–99˝ 97

Size dependence of the valence band density of statesŽ .DOS can be investigated with much better resolution byultraviolet photoemission using HeI excitation. The resultis shown in Fig. 4. The valence band spectra of the sampleat the beginning of the sputtering is identical with that

Ž Ž .. w xmeasured for bulk gold curve a 16,17 but with longersputtering, i.e. with decreasing of the size of nanoparticlesthe valence band 5d states are redistributed. Firstly thepeak at around 2–3 eV BE starts to decrease, and when ithas more or less disappeared, the other peak at 6–7 eV BEdecreases. Finally, the whole d valence states are redis-tributed below a certain size of Au nanoparticles. Themodification of the d states at lower BE is similar to that

w xobserved for Cu and Ag 11,12 , but change at higher BEis characteristic of Au only.

A clear size dependence is visible at the Fermi levelalthough this change is observable only at smaller parti-cles. The detailed size dependence at the Fermi level isshown in Fig. 5. Although the Fermi cut off is readily

Ž Ž ..detectable even at the smallest particle size curve c ,there is change near to the Fermi level in 0-0.6 eV BEregion. The separate emission from the SirSiO substratex

shows that the observed effect can only be correlated withAu emission. However, emission from the substrate shouldbe taken also into consideration.

Fig. 4. Size dependence of the UPS valence band spectra from AuŽ . Ž . Ž .nanoparticles. The sputtering times on curves a , b and c are the same

as in Fig. 2 earlier. Data on curves b and b were recorded after 20- and1 2

25-min ion bombardments, respectively. The SiO curve represents thex

emission from the SirSiO substrate after removing the gold film.x

Fig. 5. Size dependence of the photoemission data for Au nanoparticles atŽ . Ž . Ž . Ž .the Fermi level a , b , b and c , respectively. Curve SiO represents1 2 x

the emission from the substrate only.

The FT IR absorption may offer further information onthe density of states at the Fermi level. The result of themeasurement is shown in Fig. 6. The effect of the absorp-tion of the SirSiO substrate was eliminated by usingx

references, namely, an uncovered SirSiO substrate andx

the SirSiO substrate after totally removing the Au filmx

with Arq ion bombardment. These references were usedŽ Ž ..for the as-deposited sample curve a and for sputtered

Ž Ž ..Au layer curve b in Fig. 6.It is seen that the absorbance of the as-deposited sample

is larger than that after sputtering, and there is differencein the spectrum when Au nanoparticles were detected bythe photoemission measurements. This result indicates thatsome changes are anticipated in the DOS near the Fermilevel in the gold nanoparticles. Although the observedeffect in IR absorbance is quite little, as it is independentexperimental fact, it strongly supports the above UPS dataindicating the modification of the DOS near the Fermilevel. On the other hand, the metal–nonmetal transitioncan be excluded at this size range.

The size of the gold nanoparticles was measured byTEM on the same sample that was used for photoemissionand IR measurement. Two types of gold features arevisible on the TEM micrograph. The smaller particles witharound 5-nm diameter are isolated particles. Their sizes arein agreement with the expected measures. The other fea-tures — at least 200 nm in diameter — seem to be too

( )G. Peto et al.rMaterials Science and Engineering C 19 2002 95–99˝98

Ž . Ž . Ž Ž . .Fig. 6. a Infrared absorption of Au particles deposited onto the SiOrSi substrate and b after the final ion bombardment curve c in Figs. 2–5 . Thex

contribution of the SirSiO substrate was removed using an uncovered substrate and a substrate after totally removing the gold film with Arq ionxŽ Ž . Ž . .bombardment for curves a and b , respectively .

large to exhibit size effect. These, however, consist ofparticles nearly as small as the isolated ones. These aggre-gates of small particles could be in agreement with thephotoemission data if they were separated from each otherŽ .Fig. 7 .

Fig. 7. TEM micrograph of Au nanoparticles after the final ion bombard-ment step. The inserted diffraction pattern is related to Au. According toX-ray microanalysis, the features on the micrograph contain only Au. Thesmall features are separate nanoparticles with 5-nm size. The largerstructures are aggregates of nanoparticles with 5–10-nm size.

4. Conclusion

Size dependence of the valence band of Au nanoparti-cles is similar to that observed for Cu and Ag in the lowestbinding energy region of valence d states. The highestbinding energy part of the valence d states is size depen-dent in the case of gold which is opposite to Cu or Ag. Thesize dependence cannot be explained by the existing mod-els, i.e. by assumption of narrowing or broadening of thevalence band d states. A new model including some struc-tural changes induced modification in atomic potential orin s-p-d hybridization would be needed to explain theobserved experimental data. Although there is size depen-dent modification in the DOS near to the Fermi level,metal-nonmetal transition is not detectable at this sizerange of nanoparticles.

According to recent observations, nanosize Au particlesturned out to be promising catalysts in certain circum-stances. As their metallic character seems to be stronglyrelated to the catalytic activity, further experiments involv-ing infrared absorption spectroscopy are in progress inorder to determine the size range of the expected metal–nonmetal transition.

Acknowledgements

This work was supported by the National Science andŽ .Research Grant Grants T30427 and T034920 .

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