growth of chromium on the structured surface of al2o3/nial(100)

Ž .Applied Surface Science 142 1999 152–158

Growth of chromium on the structured surface ofž /Al O rNiAl 1002 3

Javier Mendez ) ,1, Horst Niehus´Institut fur Physik, Oberflachenphysik und Atomstoßprozesse, Humboldt-UniÕersitat zu Berlin, InÕalidenstrasse 110, Berlin D-10115,¨ ¨ ¨

Germany

Abstract

The properties of chromium clusters on a thin nanostructured Al O surface have been investigated by scanning2 3Ž . Ž . Ž .tunneling microscopy STM and spectroscopy STS . Oxidation of NiAl 001 leads to the formation of thin ordered Al O2 3

stripes. Upon chromium evaporation small clusters with a mean size of ;3 nm are preferentially formed on top of the oxidestripes. The STS measurements indicate a metallic behavior of these chromium clusters. q 1999 Elsevier Science B.V. Allrights reserved.

PACS: 61.16.Ch; 61.46.qw; 73.20.Dx; 73.61.At

Keywords: Chromium; Aluminum oxide; Scanning tunneling microscopy; Scanning tunneling spectroscopy; Barrier height; Cluster

1. Introduction

Metal oxides play an important role in materialw xresearch 1,2 . The surfaces of aluminum oxide have

been extensively studied due to their importance incatalysis. Formation of well ordered Al O films has2 3

been observed on the low indexed NiAl surfacesw x3–10 . These epitaxial films can be used as supportfor evaporated metal clusters in catalytic systemw x11–13 .

) Corresponding author. Tel.: q49-30-8413-5121; Fax: q49-30-8413-5106; E-mail: [email protected]

1 Permanent address: Laboratorio de Nuevas Microscopıas, De-´partamento de Fısica de la Materia Condensada, Universidad´Autonoma de Madrid, E-28049 Spain. E-mail:´[email protected]

Ž .Upon oxidation of NiAl 001 nanostructuredAl O oxide can be formed, characterized by or-2 3

² :dered oxide stripes running along the 100 direc-w xtions 10,14 . Such a structured metal-oxide surface

might serve as a substrate for metal adsorption andstudying properties of spatially confined material. Inthe following the formation of clusters will be dis-cussed.

2. Experimental

The experiments were performed in an UHVchamber with a base pressure -10y10 mbar, de-

w xscribed previously 15 . The set-up includes surfacetechniques as LEED optics, a double CMA Augerspectrometer, a Beetle-type scanning tunneling mi-croscope, a sputter ion gun, and a Cr evaporator. The

0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0169-4332 98 00666-7

( )J. Mendez, H. NiehusrApplied Surface Science 142 1999 152–158´ 153

Ž .NiAl 100 single-crystal sample is mounted in amolybdenum helix, and can be prepared by sputter-ing and heating at 1400 K until the Auger spectrumis free of signals for impurities. The LEED pattern ofthe clean surface after such preparation exhibit theŽ .c 62=362 R458 superstructure, corresponding to a

missing row reconstruction of the partially aluminumw xterminated surface 16 . This surface was exposed to

Ž .oxygen between 2 and 10 L O at 300 K and2

subsequent annealing at 1000 K. The resulting sur-face consists of both, ordered oxide stripes andamorphous oxide. In order to get rid of the amor-phous oxide, the sample was flashed up to 1500 K.As a result, the LEED pattern shows a combination

Ž .of the c 62=362 R458 structure, characteristic forthe clean surface, and intensity stripes along the

Ž . Ždirection of the 1=1 spots see Fig. 1 in Ref.w x.14 , indicating formation of ordered oxide stripes

² :along the 100 directions. It is also possible toobtain this partially oxidized surface by oxidation at1000 K and 10 times smaller oxygen exposure.

Chromium was evaporated with a rate of 0.3Armin as determined by a quartz microbalance.

Ž .After Cr evaporation the LEED c 62=362 R458

superstructure has disappeared and chromium peaksoccur in the Auger spectrum.

The Beetle-type STM has been controlled by aw xSPM-UAM digital electronic and software 17 , in-

cluding spectroscopy capabilities. The system wasŽused to perform STM images, CITS current image

. w xtunneling spectroscopy 18 and barrier height im-ages.

In order to achieve reproducible spectroscopicmeasurements, it was absolutely necessary to cleanthe W tip directly before a scanning tunneling spec-

Ž .troscopy STS experiment. The following procedurewas used: the STM tip is grounded through a pi-

Ž .coamperemeter and a positive voltage 1–2 kV wasapplied to a Ta plate less than 1 mm apart from thetip. The field emission current from an unpreparedtip appears rather unstable at the range of 30 nA.With this current, a pressure of 5=10y5 mbar of Arwas introduced in the vacuum chamber, and throughionization of the Ar in front of the tip by means ofthe emission current, the tip was sputtered. Afterapproximately 30 min of sputtering the current is

Ž .stabilized at a higher value ;100 nA otherwise theprocess was repeated with a higher starting current

for the field emission. With a tip-to-plate distance of;0.5 mm and after complete Fowler–Nordheim tip

w xanalysis 19 , we found that the half of the thresholdvoltage as measured in volts may serve as a good

˚Ž .estimation for the actual tip radius in angstrom A˚ ¨w x20,21 .

3. Results and discussion

We investigated the four different surfaces: cleanNiAl, Al O rNiAl, and after evaporation of typi-2 3

˚cally 0.5 A of chromium, CrrNiAl, and CrrAl O rNiAl.2 3

3.1. STM inÕestigation

STM images of the clean and oxidized NiAlw xsurfaces are already reported 10 . Similar images are

here included to emphasize the difference with re-spect to the Cr evaporated surfaces.

In Fig. 1 four different STM topographic imagesare presented corresponding to the different surfaces.

Ž . Ž .The clean NiAl 100 surface in Fig. 1a shows twoterraces separated by a 0.3-nm high step. This stepheight corresponds to the aluminum–aluminum sepa-

² :ration in 001 direction. After Cr evaporation, theŽ .surface exhibit island formation Fig. 1b . On a

larger scale also step roughening can be recognized.ŽSTM images of the partially oxidized surface in

.Fig. 1c show the ordered oxide structure with stripes² :along the two 100 directions. After Cr evaporation

we observe preferential growth on the oxide stripesŽ .Fig. 1d , characterized with Cr clusters of similar

Ž .sizes ;3 nm wide . Between the stripes, clusters ofsmaller size can be seen.

3.2. Barrier height images

The barrier height images can be obtained byrecording at every point the tunneling current as afunction of the distance, and calculating the barrierheight value from the exponential increase of thecurrent. Fig. 2a–b displays the images obtained overa surface where two successive oxidation procedures

Ž .were performed. The topography Fig. 2a shows

( )J. Mendez, H. NiehusrApplied Surface Science 142 1999 152–158´154

Ž . 2Fig. 1. Topographic STM images corresponding to: a the bare NiAl surface; the image displays a flat area 360 nm with a step of 0.3 nm˚ 2Ž .height across. Vs756 mV, Is0.03 nA; b CrrNiAl: after 0.5 A chromium evaporation on the NiAl surface, 60 nm . Island formation

Ž . ² :and surface roughening; Vs468 mV, Is0.01 nA; c Al O rNiAl: partially oxidized NiAl. Ordered Al O stripes along the 1002 3 2 32 ˚Ž . Ž .directions and NiAl regrowth between them. 130 nm , Vs2.65 V, Is0.06 nA; d CrrAl O rNiAl: evaporated chromium 0.5 A on the2 3

partially oxidized surface. 100 nm2, Vs2.78 V, Is0.018 nA.

oxide stripes and clean areas at several heights, whileŽ .the barrier height image Fig. 2b discriminate both

Ž .areas: the oxide regions with low -1 eV and theŽ .clean areas with high barrier height values 4–5 eV .

Additional contrast at stripes can be associated withdifferent oxide thickness.

After chromium evaporation onto the oxidizedsurface, the barrier height over the clusters at the


Ž .Fig. 2. Topographic and corresponding barrier height images over: a–b the Al O rNiAl surface: the oxide areas exhibit a smaller barrier2 32 Ž .height value, which also depends on the thickness of the oxide; 60 nm , Vs193 mV, Is0.11 nA. c–d CrrAl O rNiAl: clusters on the2 3

˚ 2oxide stripes after evaporation of 0.5 A Cr. These clusters display values up to 12 eV in the barrier height. 45 nm , Vs1.9 V, Is0.015nA.

stripes expose high values, even higher than usualŽ .5–12 eV , possibly due to interactions between tipand sample. 2 In Fig. 2c–d the topographic and

2 We observe that the current deviates from the expectedexponential dependence when the tip is approached to the clusters.

barrier height images exhibit contrast for the clusterson the stripes and between them.

3.3. CITS–STS measurements

In order to obtain spectroscopic information, wew xperformed I–V plots at every point of an image 18 .

The spectroscopy potentially introduces a more com-


plete information than the topography images. InFig. 3a a topographic image is displayed which is

ŽŽ .obtained on a partially oxidized surface A for theŽ . .oxide stripe and B for the clean NiAl . In the center

Ž .of the image there is a hole C , ;10 nm wide and0.3 nm deep. In Fig. 3c the derivative of the tunnel-

Ž . Žing current d IrdV over the averaged areas as.indicated in Fig. 3a are shown. The data obtained

over the oxide stripes exhibit a diode like character-Ž .istic in the d IrdV curve cf. black line in Fig. 3b .

The asymmetry of d IrdV might be a consequenceof the formation of an effective dipole layer resultingfrom the metal-oxygen bonding in the surface. Incontrast, the d IrdV curves obtained on the clean

Ž .NiAl areas B, C expose metallic characteristics ascan be deduced from the more symmetric, almostparabolic shape in the spectrum. The strong overalldecrease close to zero voltage is due to a knowneffect resulting from the transmission function of thetunneling system. In addition, a new feature in the

Ž .spectrum obtained over the center hole C can berecognized, namely a peak structure around 0.9 V.

Ž . Ž .Keeping in mind, that both curves B and C areobtained at the clean surface, we infer the peakstructure to an electronic localization effect due tothe 10-nm wide hole.

Using the spectroscopic differences between ox-ide and clean NiAl, it is possible to obtain chemical

Ž . 2 Ž .Fig. 3. Spectroscopy over the Al O rNiAl surface. a Topographic image 100 nm , Vs2.3 V, Is0.03 nA, with oxide stripes A , clean2 3Ž . Ž . Ž .NiAl B and a ;10-nm wide and 0.3-nm deep hole C . b d IrdV image at Vsy2.3 V showing chemical contrast between the oxide

Ž . Ž .and the clean surface. c d IrdV of the different areas selected. The data obtained over the oxide stripes A display a diode-like behavior.Ž . Ž .The plot over the hole C differs from the one over the clean surface B basically by the occurrence of a peak at 0.9 V.


Ž .contrast images, as the conductance image d IrdVobtained at y2.3 V presented in Fig. 3b.

The spectroscopy data obtained over the surfaceswith evaporated chromium indicate special character-istics for the clusters on the stripes. Fig. 4 presentsthe topography and spectroscopy of the CrrAl O r2 3

Ž . Ž .NiAl 001 system. The topographic image Fig. 4ashows the characteristic clusters above the oxide

Ž . Ž X.stripes B , and two terraces without oxide A, A .Ž .The derivative d IrdV in Fig. 4b, indicates that the

complete surface recover a metallic behavior, charac-terized with V-shape plots. It might be noted that the

Ž X.terraces A, A present a similar behavior as com-Ž .pared with the CrrNiAl system not shown here .

The lower conductance close to zero voltage for theŽ .clusters on the stripes B , might be assigned to the

different tip-to-sample separations above the clustersand the terraces. Such a difference appears alsoplausible from the barrier height image in Fig. 2c–d.

ŽAfter plotting the normalized derivative d Ir. Ž . w x Ž .dV r IrV 22,23 Fig. 4c , clearly a higher den-

sity of states can be deduced close to the Fermi levelfor the clusters areas. Thus, chromium clusters onthe aluminum oxide are marked by the introductionof both, occupied and unoccupied electronic statesclose to the Fermi energy. The strong peak occurringat about y0.4 V can be compared with a similarpeak structure measured for the system of chromiumon iron where also a peak below the Fermi level has

w xbeen reported 24 . The nature of the peak structureis not fully clear yet, at least in the case of clusterformation on the Al O stripes electronic confine-2 3

Ž . Ž 2 .Fig. 4. Spectroscopy over the CrrAl O rNiAl surface. a Topographic image 70 nm , Vs1.5 V, Is0.1 nA with clusters on the oxide2 3Ž . Ž . Ž .and a step along the area between. b d IrdV over the clusters B and the terrace A showing a V-shape characteristic, indicative of a

Ž . Ž . Ž .metallic behavior. c Normalized derivative d IrdV r IrV , with a higher density of states for the clusters in the proximity of the FermiŽ .level "0.4 V .


ment might be responsible for the effect. The strongŽ .differences in the d IrdV data Fig. 3c and Fig. 4b

demonstrate also the ability to use the method forŽ .imaging the surface with chemical contrast Fig. 3b

w xin accordance with other STM data 24–26 .

4. Conclusions

Ž .The nanostructured Al O surface on NiAl 1002 3

has been used as a substrate for chromium evapora-tion. The resulting surface structure was character-ized by STM and STS. The STM topographic datashow after Cr evaporation a preferential formation of

Ž .small Cr clusters of a similar size ;3 nm on theAl O stripes. The clean, oxidized and Cr covered2 3

surfaces are marked by distinct differences in thespectroscopic data, which might be used for STMexperiments with chemical contrast. Peak features inthe d IrdV spectra have been determined above smallholes on NiAl as well as above Cr clusters on theAl O areas. The Cr clusters on the Al O stripes2 3 2 3

are marked by the introduction of electronic statesclose to the Fermi energy, and a metallic behavior ofthe clusters can be deduced.

Acknowledgements

Discussion with R.P. Blum, D. Ahlbehrendt andG. Gilarowski are gratefully acknowledged. J.M.acknowledge the Spanish Ministerio de Educacion´for financial support. Also the financial support ofthe DFG via the SFB290 is gratefully acknowledged.

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growth of chromium on the structured surface of al2o3/nial(100)

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