Fo

JJ

a

ARR1AA

KNNPWF

1

mcddfiterd

ateeau

r

vR

0h

Applied Surface Science 292 (2014) 717– 725

Contents lists available at ScienceDirect

Applied Surface Science

j ourna l ho me page: www.elsev ier .com/ locate /apsusc

ield emission study of pulsed laser deposition of gold on clean andxidized tungsten tip

. Plsek ∗

. Heyrovsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejskova 3, Prague 8 182 23, Czech Republic

r t i c l e i n f o

rticle history:eceived 2 September 2013eceived in revised form2 November 2013ccepted 6 December 2013vailable online 16 December 2013

a b s t r a c t

Growth of gold overlayers on a clean and oxidized tungsten tip has been studied by Field EmissionMicroscopy. Gold was deposited by conventional vacuum evaporation (VE) and by pulsed laser deposi-tion (PLD) method using a Nd:YAG laser. The effect of the applied deposition method on gold growth wasexamined and the influence of high-kinetic energy particles produced by PLD in vacuum on the morphol-ogy and size of prepared gold nanostructures is discussed. PLD of gold on oxidized tungsten tip resultedin a significant reduction of the oxide layer. Presence of He (∼5 mbar) during PLD decreased the kinetic

eywords:anoparticlesucleation and growthulsed laser depositionork function

energy of the species arriving at the substrate and resulted in gold growth similar to that we observedfor conventional VE.

© 2013 Elsevier B.V. All rights reserved.

ield emission microscopy

. Introduction

Field emission is one of important features of nanostructuredaterials and has attracted considerable attention due to its appli-

ations in displays, electron guns and other micro/nano electronicevices. Compared to conventional technologies nanostructuredevices based on Mo or Si microtip arrays provide compactness,aster turn-on time and sustainability. There is growing interestn the development of field emission cathodes based on nanostruc-ured oxide materials such as ZnO [1,2] and SnO2 [3]. Carbon-basedmitters, namely diamond, amorphous diamond-like carbon andelated films are also very promising candidates for field emissionevices [4].

A significant improvement in field emission performance waschieved very recently by deposition of gold on field emitters con-aining vertical ZnO nanowires [5] or silicon nanocones [6]. It isvident that additional development of new nanostructured fieldmission cathodes requires detailed knowledge of fundamentalspects related to the mechanisms of thin film growth including

nderstanding of interface phenomena.

Pulsed laser deposition (PLD) is a versatile method for prepa-ation of clusters and nanocrystalline films of almost any material

∗ Corresponding author. J. Heyrovsky Institute of Physical Chemistry of the ASCR,. v. i., Department of Low-dimensional Systems, Dolejskova 3 Prague 8, Czechepublic. Tel.: +420 26605 3546.

E-mail address: jan.plsek@jh-inst.cas.cz

169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.apsusc.2013.12.039

[7]. PLD involves a high-power laser beam focused onto a targetmaterial and leads to its vaporization and deposition on the sub-strate material. The ablated material is ejected from the target asa highly forward-directed plume which contains a large fractionof charged ions with kinetic energies between tens and hundredsof eV. This is 2 to 3 orders of magnitude above the particle ener-gies in conventional vacuum evaporation (VE). In spite of the factthat PLD has been subject to extensive development and is usedfor preparation of various kinds of nanomaterials, the growth pro-cess of nanostructures by PLD is rather complex and not yet fullyunderstood.

The major aim of this study is to contribute to this effort by usingfield emission microscope (FEM). This experimental technique withits high resolution (∼2 nm) is well suitable for surface studies ona nanometer scale; to the best of our knowledge it was not yetused to study metal films deposited by PLD. In this study we paidparticular attention to the effect on Au layer growth of depositionmethod and the nature of the support (clean W, WOx). Last butnot least the presented study of the layered field emission cathodemight be useful for application of PLD in field emission devices.

2. Experimental

Experiments were carried out in a stainless-steel UHV system

(base pressure p < 1 × 10−9 mbar, The Experimental Factory of Sci-entific Engineering, Academy of Sciences, Moscow) equipped withhome-made field emission system described elsewhere [8]. Allexperimental steps (cleaning, oxidation, Au deposition and field

7 Scien

eowtsdWov

Fwd

18 J. Plsek / Applied Surface

mission measurements) were done in one vacuum chamber with-ut exposing the sample to ambient pressure. The tungsten tip, spotelded to a tungsten loop, was prepared by a standard etching

echnique. Before each experiment the tip was always cleaned byhort flashes to about 2500 K in UHV. The tungsten surface was oxi-ized in situ by two different procedures: (i) the sample designated

Ox(RT)/W was exposed to 80 Langmuirs (L) (pO2 ∼1 × 10−6 mbar)

f oxygen at room temperature (RT) and subsequently heated inacuum to 700 K; (ii) the sample designated WOx(1300 K)/W was

ig. 1. FEM images of clean tungsten and Au layers deposited by the PLD and VE techniqork function changes on evaporation time (VE) or number of laser pulses (PLD). Image

ifferent (sharper) tip and therefore the image voltages in this experiment were correspo

ce 292 (2014) 717– 725

exposed to 40 L of oxygen at RT and subsequently heated to 1300 Kin oxygen atmosphere (pO2 ∼1 × 10−6 mbar). The effective thick-nesses of the WOx layers prepared by these procedures on thetungsten foil were determined as 0.20 nm for WOx/W(RT) and0.28 nm for WOx/W(1300) [9]. Details of the oxidation proceduremay be found in ref. [10]. The emitter tip was heated by resistive

heating of the tungsten wire loop and the temperature of the emit-ter tip was determined by means of resistance measurements ofthe apex segment of the loop [11]. Temperature calibration was

ues. Au layer thickness in monolayers was calculated from the dependence of thevoltages for each image are shown. It should be noted that VE at RT was done on andingly lower.

Science 292 (2014) 717– 725 719

bbeIas(w

wf

ϕ

wfFic

cwsit[e

ermt

r

wa

wdfutPpuTgfa

3

3

coftncttbe

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.04.5

4.6

4.7

4.8

4.9

5.0

5.1

5.2

Au on W VE RT VE 600 K PLD RT

ϕ (e

V)

d (ML)

Fig. 2. The work function as a function of gold coverage for gold layers deposited

J. Plsek / Applied Surface

ased on optical pyrometer measurements. Images were acquiredy means of an astronomical CCD camera with very high quantumfficiency, 16 bit digitization and low noise G2CCD-0400 (Moraviannstruments, Czech Republic). Field emission current was measuredt the screen, held at a ground potential. Experimental field emis-ion current-voltage characteristics were plotted as ln(I/U2) vs. 1/UFowler–Nordheim (FN) plot). Slopes and intercepts of FN plotsere obtained by linear interpolation.

In calculating the work function we assumed that the apparentork function of a clean tungsten tip is ϕ0 = 4.54 eV [12]. The work

unction ϕ was calculated as:

= ϕ0

(S

S0

) 23

(1)

here S is the slope estimated from the FN plot, ϕ0 is the workunction of a clean tungsten tip and S0 is the slope estimated fromN plot of the clean tungsten surface. This calculation procedures based on the assumption that the field enhancement factor isonstant during the entire experiment.

Nevertheless, we always discuss work function changes as aonsequence of change of both factors, i.e. changes of the apparentork function and the local field enhancement factor influenced by

urface curvature. We also stress that the calculated work functions used only as a value for comparison of field emission properties ofhe studied samples. We therefore applied the simple FN equation13] without correction factors recently introduced by R. Forbest al. [14].

The radii of curvature of the tips used in the experiments werestimated from the voltage needed to obtain the total emission cur-ent I = 10 �A. This voltage was obtained by extrapolation of theeasured field emission characteristics. The empirical equation for

ungsten tip radius calculation found by M. Dresler [15] is:

= 7.4 × 10−4 × U32

10 (2)

here U10 is the voltage needed for total emission current I = 10 �And r is the tip apex radius of curvature in nm.

The gold evaporation source for VE and the gold target for PLDere mounted on a rotatable x,y,z - manipulator that made possibleeposition from the front of the tip. VE was done by evaporationrom a gold wire wrapped on a resistively heated tungsten filamentnder ultra high vacuum conditions. The heating current of theungsten filament was kept constant by a stabilized current supply.LD of gold was done by a Q-switched Nd:YAG laser (� = 1064 nm,ulse width 8 ns, energy 45 mJ/pulse, repetition frequency 1/8 Hz)nder ultrahigh vacuum or in the presence of helium (p ∼ 5 mbar).he angle between the incident laser beam and normal to the tar-et surface was approximately 45◦; the distance of the substraterom the target was ∼30 mm. The spot size of the laser beam waspproximately 0.60 mm2.

. Results and discussion

.1. Tungsten-gold system

Fig. 1 shows field emission images of gold layers deposited on alean tungsten tip at RT and for VE deposition also at 600 K. The Auverlayer thickness expressed in monolayers (ML) was determinedrom calibration of dependence of the apparent work function ofhe emitter tip on total evaporation time (VE deposition) or on totalumber of laser pulses (PLD). For the majority of metals adsorbed onlean tungsten it was reported that the saturation of the work func-

ion change as a function of the adsorbate coverage correspondedo one monolayer [16]. The work function of gold layer depositedy VE at RT and 600 K however exhibited a complicated depend-nce on the amount of deposited gold [17–19]. This dependence is

on a clean tungsten tip by PLD at room temperature and VE at room temperatureand at 600 K.

peculiar, with a maximum at a gold layer thickness of 1 ML, fol-lowed by a minimum at 2.5 ML. Saturation is reached at highercoverages (above 4 ML) at work function value of 5.2 eV [17]. Workfunction changes during stepwise VE deposition of gold presentedin Fig. 2 agree well with work function changes published by J. P.Jones [17]; we have therefore assigned 1 ML to evaporation time(or number of laser pulses) at which the apparent work functionreached the maximum at value between 5.1 and 5.2 eV (Fig. 2). Cor-respondence between work function changes of layers prepared byVE and PLD can be only found for coverages below 1 ML. However,there was no minimum in the work function plot for layers pre-pared by PLD (Fig. 2). The saturation point at 5.2 eV was thereforeused for determination of 1 ML of Au deposited by PLD. Certain dif-ferences can be found also on FEM images of gold layers by VE andPLD. The field emission images of gold layers deposited by PLD weremore uniform (there is only little difference between (1 1 1) and(3 1 0) areas) and their changes during stepwise deposition werealso less noticeable than the images captured during VE deposi-tion (Fig. 1). To explain these differences we must first interpretthe work function changes for gold layers deposited by VE.

Many previous studies investigated model Au-W bimetallicsystems in which gold was VE deposited onto various single crys-tal planes of tungsten [20–23]. Similarly as for other thin fccmetal films on bcc metals, superstructures of gold have beenreported [22,24,25]. This phenomenon is mostly explained byadsorbate/substrate atomic size differences and surface stressrelaxation [24]. Stress relaxation influences even several layersof thicker gold films [20]. A critical gold overlayer coverage thatinduces faceting is reported to be slightly greater than one mono-layer [26]. The faceting development in these systems is consideredto be thermodynamically driven but it is kinetically limited. Fromthis reason annealing is usually used to achieve sufficient surfaceatom mobility for mass transport.

For gold on W the annealing temperatures found in theliterature lie within the range 700–1000 K [20,22,27,28]. How-ever on the curved field emission tip we documented goldmigration already at ∼320 K as can be seen in Fig. 3. In thisexperiment we deposited gold on a clean tungsten tip at 78 Kfrom the left side (due to the higher work function of the

Au layer the part of tungsten tip covered by Au appears sub-stantially darker). During heating to ∼320 K we observed rapidmovement of the gold/tungsten boundary (Fig. 3D–H). The

720 J. Plsek / Applied Surface Science 292 (2014) 717– 725

F cleanc , (G) 8

dtdr(

orf

ig. 3. FEM images of diffusion of a Au layer deposited at 78 K from the left side on aaptured from a video sequence while heating to 320 K, (C) 0 s, (D) 3 s, (E) 5 s, (F) 7 s

iffusion started in the direction from the central (1 1 0) planeo (1 2 1) planes (see arrows on Fig. 3D) followed by gold bor-er movement to the (1 1 1) and (3 1 0) regions (Fig. 3G). The lastegion covered by gold was in the vicinity of the (1 0 0) planeFig. 3H).

Temperatures used for VE deposition are too low to developrdered pyramidal superstructures at nanometer scale as waseported by Sendecki et al. [28]. However field emission imagesrom RT deposition and much more for 600 K deposition are

tungsten tip. (A) a clean W, (B) Au layer deposited from the left side, (C–H) images s, and (H) 12 s. The image voltages are shown for each image.

indicating bright areas in (1 1 1) and (3 1 0) regions (see Fig. 1, cov-erage ∼2 ML). This brightening is probably caused by a decreaseof calculated work function due to a change of the local surfaceroughness of the tip. We thus deduce that even at RT some shortdistance diffusion of gold atoms can take place to produce less

developed ordered superstructures. One can thus conclude thatthe decrease of the measured work function displayed in Fig. 2 iscaused by an increase of local surface roughness of the gold layer,which increases the local field enhancement factor. Considering the

J. Plsek / Applied Surface Science 292 (2014) 717– 725 721

Fig. 4. FEM images of Au layers deposited by VE and PLD on a tungsten tip oxidized at RT (Au/WOx(RT)/W). The images in the first row represent clean W and in the secondrow the W surface after exposure to oxygen for individual deposition series. The image voltages are shown for each image.

722 J. Plsek / Applied Surface Science 292 (2014) 717– 725

Fig. 5. FEM images of Au layers deposited by VE and PLD on a tungsten tip oxidized at 1300 K (Au/WOx(1300)/W). Images in the first row represent clean W and those in thesecond row oxidized W surface for each deposition series. The image voltages are shown for each image.

Science 292 (2014) 717– 725 723

eow

tcbwiehaatWsst

3

tacs(TrO(ddgit

PtofucItttaifntTaTnFeost(rtso

0 1 2 33.0

3.5

4.0

4.5

5.0

5.5

6.0

Au on WOx(1300)/W VE PLD

ϕ (e

V)

d (ML)

0 1 2 33.5

4.0

4.5

5.0

5.5

6.0

Au on WOx(RT)/W VE PLD PLD in He

ϕ (e

V)

d (ML)

J. Plsek / Applied Surface

mployed calculation procedure of the work function the increasef the local field enhancement factor resulted in a decrease of theork function.

On the other hand, we didn’t observe the above-mentioned fea-ures for gold layers prepared by PLD. Explanation of this behaviouran be found in the absence of overlayer superstructures producedy VE deposition of thin gold layers on tungsten. Provided that PLDas carried out in vacuum the main difference between PLD and VE

s in the kinetic energy of particles arriving at the surface. Kineticnergies of particles during PLD can be 2 or 3 orders of magnitudeigher than the energies of particles produced by VE. Impingingtoms or ions with higher energy can thus cause resputtering oftoms from the top monolayer [29]. We can therefore concludehat in PLD the development of ordered Au superstructures on the

tip is substantially suppressed by resputtering. The absence ofignificant surface roughness of Au layers prepared by PLD is con-equently reflected in the measured constant work function forhickness above one monolayer (Fig. 2).

.2. Tungsten-tungsten oxide-gold system

Au layers of various thickness deposited by VE and PLD onungsten surface oxidized in situ by two different procedures werelso investigated (Figs. 4 and 5). We used the same oxidation pro-edures as in our previous study of Pd layer growth in Pd/WOx

ystems [10]. The tungsten surface oxidized at room temperatureWOx(RT)/W) mainly affected the vicinity of (1 0 0) planes (Fig. 4).he higher emission from this area was attributed to incorpo-ation of oxygen and to surface reconstruction of this area [30].xidation at 1300 K (WOx(1300 K)/W) furthermore influences the

1 1 1) plane. On tungsten foil oxidized by both of above proce-ures oxidized W2+ and W4+ species were identified [10]. Besidesifferent features in FEM images oxidation at 1300 K results in areater extent of oxidation, apparent in higher oxide thickness andncreased concentration of W2+ surface species in comparison withhe WOx(RT)/W samples [9].

The sizes of bright spots on FEM images of Au prepared byLD are smaller and their number is higher than for VE deposi-ion. With increasing Au coverage the number of emitting spotsn Au layers prepared by VE decreases and for 2 ML there are onlyew dominating spots on FEM images (Figs. 4 and 5). The grad-ally increasing emission from bright spots with increasing Auoverage is demonstrated by the decrease of the image voltage.t should be noted that the image voltage was set to avoid satura-ion of the camera from the areas on the screen which exhibitedhe highest image brightness. Since the emission current exponen-ially rises with the applied voltage, areas with higher emissionre preferentially imaged and field emission image of the oxides in this case completely suppressed. The increase of emissionrom a few bright spots can be interpreted as a 3D growth of Auanoparticles, accompanied with decreasing local surface curva-ure of nanoparticles connected with increasing local electric field.he last images of the series on Figs. 4 and 5 corresponding to

coverage of 2 ML of Au are displayed also on logarithmic scale.he field emission current according to FN equation depends expo-entially on the electric field and linearly on the emission area.EM images on logarithmic scale thus better illustrate the fieldmission area which can be attributed to gold coverage. Imagesn log scale shown in Figs. 4 and 5 demonstrate that VE depo-ition induces growth of fewer particles with a larger diameterhan PLD. Since the distance between the centres of two opposite2 1 1)-type planes on the FEM image of clean W is equal to the

adius of curvature of the tip; we can scale FEM images by calcula-ion of the tip radius [15]. By measuring the diameter of individualpots on log scale images we can then roughly calculate the sizef deposited clusters. The main inaccuracy of this approach can be

Fig. 6. The work function as a function of gold coverage for gold layers depositedby PLD and VE at room temperature on WOx(RT)/W and WOx(1300 K)/W.

attributed to the lens effect of conducting nanoparticles on the tipsurface [13]; we therefore overestimate the size of the nanoparti-cles. The mean values of the nanoparticle diameter were obtainedas ∼5.5 nm for VE and ∼3.6 nm for PLD. The higher island den-sity connected with their smaller size for PLD can be explainedby higher energy and instantaneous flux of the impinging atomsor ions. This finding agrees with the conclusion made by Schmidet al. for homoepitaxy of Pt by PLD [31]. Resputtering of gold atomsfrom the top monolayer can also contribute to growth of flat goldnanoparticles.

Work function changes of gold layers during stepwise deposi-tion on oxidized tungsten tip are shown in Fig. 6. The differencebetween VE and PLD is clearly visible.

Our previous adsorption experiments with gold depositedon WOx surfaces didn’t reveal any work function changesafter CO dosing (1 min, pCO ∼ 5 × 10−7 mbar) at RT. We canthus neglect work function changes due to adsorption frombackground gases. The different character of the FN plotsseems to be easily explained exclusively by changes inlocal surface curvature. However, the decrease of the work

function is very strong for VE and was already observed alsofor “compact” gold layers [32]. The exceptional enhancement offield emission was tentatively interpreted as a consequence

724 J. Plsek / Applied Surface Science 292 (2014) 717– 725

F f He (∼o

oetl[

Peoncearltftslt

daoapoopddsfPi

ig. 7. FEM images of Au layers deposited by PLD on WOx(RT)/W in the presence of 0.3, 0.6 and 0.9 ML deposited by PLD in He.

f a resonance-enhanced tunnelling mechanism involvingmission from underlying tungsten through the oxide poten-ial barrier and empty adatom level. This hypothesis wasater corroborated by field electron energy spectroscopy33].

The most interesting finding is that gold layers deposited byLD on oxidized tungsten tip didn’t exhibit any significant fieldmission enhancement. This can be explained by the flat shapef gold deposits and by the absence of resonance-enhanced tun-elling. Since phenomena of enhancement of field emission wereonfirmed for several metal-oxide-metal systems [32] we canxplain the absence of resonance-enhanced tunnelling only by

decreased thickness of the oxide layer after PLD of gold. Theesonance-enhanced tunnelling actually strongly depends on oxideayer thickness and a very thin oxide layer doesn’t meet the condi-ions for resonance tunnelling [32]. Thinning of the potential barrierormed by the oxide layer is a direct consequence of oxide reduc-ion due to preferential sputtering of oxygen [34,35] by energeticpecies originating from the PLD process. With regard to the oxideayer thickness we cannot exclude even gold atom penetration ofhe oxide layer.

It is well known that ablated energetic particles can be slowedown by collisions with inert gases (He, Ne, Ar). Helium is a suit-ble gas because it minimizes scattering of the deposited particlesut of axis between the target and the substrate [36]. To prove thebove-mentioned effect of high kinetic energy of deposited goldarticles we deposited gold on WOx(RT)/W by PLD in the presencef He (Fig. 7). Based on published measurements of the velocitiesf laser-produced Au(I) species [37] we expected that He pressureHe∼5 mbar would be sufficient to decrease the kinetic energy ofeposited species well below 1 eV at the distance of 3 cm (i.e. ouristance between the target and the sample). Comparing the image

hapes, applied image voltages (Fig. 7) and the values of the workunction of the deposited gold layer (Fig. 6) one can conclude thatLD in the presence of He, at a pressure of about pHe∼5 mbar, resultsn Au growth very similar to that we obtained with VE.

5 mbar). (A) clean W, (B) oxidized W, (C–E) Au layers with estimated thicknesses

4. Conclusions

A distinct influence of deposition by PLD in vacuum on Au layergrowth on clean and oxidized W surface has been demonstrated.High energy particles produced during PLD in vacuum caused spe-cial features of the deposited gold nanostructures which can besummarized as follows:

1. Au layers on clean W were flat and the superstructure formationobserved for layers deposited by VE was suppressed.

2. Compared with VE, Au growth by PLD on oxidized W resultedin smaller size of deposited particles and more homogeneouscoverage of the tip surface.

3. The oxide interlayer was significantly reduced due to sputter-ing during deposition. As a consequence of the oxide reductionwe didn’t observed any field emission enhancement previouslyreported for many other metal-oxide-metal systems preparedby VE.

PLD in He atmosphere (pHe∼5 mbar) sufficiently reduced thekinetic energy of the deposited particles and induced similar Augrowth as that we observed for VE.

Acknowledgements

This work was supported by Czech Ministry of Education, Youthand Sports (ERC-CZ: (LH13022). The author would like to expresshis thanks to Dr. V. Nikolajenko and Dr. D. Zhukov for their technicalassistance during the field emission microscopy experiments. I alsothank Dr. Zdenek Bastl for his careful and thoughtful reading of themanuscript.

References

[1] C.X. Xu, X.W. Sun, Appl. Phys. Lett. 83 (2003) 3806.[2] D. Banerjee, S.H. Jo, Z.F. Ren, Adv. Mater. 16 (2004) 2028–2032.

Scien

[[[[[

[[[[[[

[[

[

[

[

[[[[

[

[

[[

J. Plsek / Applied Surface

[3] J.H. He, T.H. Wu, C.L. Hsin, K.M. Li, L.J. Chen, Y.L. Chueh, L.J. Chou, Z.L. Wang,Small 2 (2006) 116–120.

[4] N.S. Xu, S.E. Huq, Mater. Sci. Eng. R: Rep. 48 (2005) 47–189.[5] S.-Y. Lin, S.-J. Chang, T.-J. Hsueh, ECS J. Solid State Sci. Technol. 2 (2013)

N149–N151.[6] Y.-M. Chang, P.-H. Kao, H.-M. Tai, H.-W. Wang, C.-M. Lin, H.-Y. Lee, J.-Y. Juang,

Phys. Chem. Chem. Phys. 15 (2013) 10761–10766.[7] D.B. Chrisey, G.K. Hubler, Pulsed Laser Deposition of Thin Films, John Wiley &

Sons, Inc., New York, 2006.[8] Z. Knor, S. Biehl, J. Plsek, L. Dvorák, Ch. Edelmann, Vacuum 51 (1998)

11–19.[9] I. Jirka, J. Plsek, F. Sutara, V. Matolín, V. Cháb, K.C. Prince, J. Phys. Chem. B 110

(2006) 23837–23844.10] J. Plsek, I. Jirka, V. Nikolajenko, Z. Knor, Surf. Sci. 600 (2006) 3943–3949.11] C.J. Workowski, J. Phys. E 10 (1977) 538–543.12] E.W. Muller, J. Appl. Phys. 26 (1955) 732–737.13] R. Gomer, Surf. Sci. 299-300 (1994) 129–152.14] R.G. Forbes, A. Fischer, M.S. Mousa, J. Vacuum. Sci. Technol. B Microelectron.

Nanometer Struct. 31 (2013) 02B103.15] M. Drechsler, E. Henkel, Z. Angew. Phys. 6 (1954) 341–346.

16] Z. Szczudło, A. Ciszewski, Y.B. Losovyj, Appl. Surf. Sci. 174 (2001) 138–147.17] A. Cetronio, J.P. Jones, Surf. Sci. 44 (1974) 109–128.18] J.P. Jones, E.W. Roberts, Thin Solid Films 48 (1978) 215–228.19] J.P. Jones, J. Solid State Chem. 104 (1993) 149–159.20] M.L. Hildner, K.E. Johnson, R.J. Wilson, Surf. Sci. 388 (1997) 110–120.

[[[[

ce 292 (2014) 717– 725 725

21] C.Z. Dong, Springer Ser. Surf. Sci. 24 (1991).22] J.J. Kołodziej, K. Pelhos, I.M. Abdelrehim, J.W. Keister, J.E. Rowe, T. Madey, Prog.

Surf. Sci. 59 (1998) 117–134.23] T.E. Madey, J. Guan, D. Cheng-Zhi, S.M. Shivaprasad, Surf. Sci. 287-288 (1993)

826–830.24] K. Pelhos, I.M. Abdelrehim, C.-H. Nien, T.E. Madey, J. Phys. Chem. B 105 (2001)

3708–3717.25] T.E. Madey, C.-H. Nien, K. Pelhos, J.J. Kolodziej, I.M. Abdelrehim, H.-S. Tao, Surf.

Sci. 438 (1999) 191–206.26] J. Guan, R.A. Campbell, T.E. Madey, Surf. Sci. 341 (1995) 311–327.27] S. Mróz, E. Bauer, Surf. Sci. 169 (1986) 394–404.28] S. Sendecki, B. Barwinski, Appl. Surf. Sci. 134 (1998) 243–246.29] R. Eason, Pulsed Laser Deposition of Thin Films, John Wiley & Sons, Inc., Hobo-

ken, NJ, USA, 2006.30] J. Sotola, V. Savkin, Z. Knor, Collect. Czech. Chem. Commun. 57 (1992)

2481–2493.31] M. Schmid, C. Lenauer, A. Buchsbaum, F. Wimmer, G. Rauchbauer, P. Scheiber,

G. Betz, P. Varga, Phys. Rev. Lett. 103 (2009) 076101–076104.32] J. Plsek, Z. Knor, Appl. Surf. Sci. 171 (2001) 157–174.33] A. Cörper, G. Bozdech, N. Ernst, T. Klüner, H.-J. Freund, Phys. Status Solidi B 242

(2005) 2462–2467.34] J.B. Malherbe, S. Hofmann, J.M. Sanz, Appl. Surf. Sci. 27 (1986) 355–365.35] R. Kelly, Surf. Sci. 100 (1980) 85–107.36] T. Scharf, J. Faupel, K. Sturm, H.-U. Krebs, Appl. Phys. A 79 (2004) 1587–1589.37] E. Irissou, B. Le Drogoff, M. Chaker, D. Guay, J. Appl. Phys. 94 (2003) 4796.