Surface Science 580 (2005) 11–18

www.elsevier.com/locate/susc

Oxidation and adhesion on the quasicrystallineAlPdMn surface studied by nanolithography

J. Smith a,b,*, G. Torricelli b, F. Marchi a,c, P. Budau a,d,F. Comin b, J. Chevrier a,b,c

a LEPES-CNRS, BP 166, F-38042 Grenoble cedex 9, Franceb ESRF, BP 220, F-38043 Grenoble Cedex, France

c Universite Joseph Fourier, Grenoble, Franced University of Bucharest, Faculty of Physics, Magurele MG-11, Romania

Received 16 June 2004; accepted for publication 20 January 2005

Abstract

We present the novel application of a relatively new technique (nanolithography) to the study of quasicrystalline

surface oxidation. The 5-fold surface of an AlPdMn alloy was oxidized using a metallized AFM tip. The electrochem-

ical nature of this process was confirmed by investigating the influence of humidity and polarity of the applied voltage

on the quasicrystalline oxide. Oxides of different thickness and adhesive properties were created by altering the applied

voltage and the humidity during the lithographic process. The technique can be used in an exhaustive study of prop-

erties of the various types of oxides that form on the AlPdMn surface and the preliminary results of one such study are

reported.

� 2005 Elsevier B.V. All rights reserved.

Keywords: Atomic force microscopy; Adhesion; Oxidation; Surface chemical reaction

0039-6028/$ - see front matter � 2005 Elsevier B.V. All rights reserv

doi:10.1016/j.susc.2005.01.052

* Corresponding author. Present address: National Institute

of Standards and Technology, Polymers Division, 100 Bureau

Drive, Gaithersburg, MD 20899, USA. Tel.: +1 732 322 0316;

fax: +1 732 445 3124.

E-mail address: jrobert44@hotmail.com (J. Smith).

1. Introduction

Since the discovery of quasicrystals in 1982 [1],

the focus of their study has evolved from attempts

to clarify the nature of this solid state and its

surfaces [2] to attempts to reveal several of the

more remarkable quasicrystalline properties ofdirect practical interest. The extremely low adhe-

sive properties, high hardness, good corrosion

ed.

12 J. Smith et al. / Surface Science 580 (2005) 11–18

resistance and low coefficients of friction [3–8] of

quasicrystalline materials make them particu-

larly attractive as wear preventative coatings.

Such coatings have found application in turbine

blades, cookware, razor blades, pistons and cylin-ders [9].

While quasicrystalline oxidation has been stud-

ied by several groups [10–13], very few have stud-

ied the adhesive properties of the oxides that form

on quasicrystalline surfaces [14–16] and none have

done so on the nanometer scale. As such alloys

oxidize in all but the most chemically inert or

reducing environments, properties of the surfaceoxide (i.e. structure, thickness, chemical composi-

tion) determine the adhesive properties of the sur-

face. The goal of the present work is to use the

relatively new technique of AFM-assisted lithogra-

phy to study the adhesive properties of the various

oxides that form on the AlPdMn quasicrystalline

surface.

Both Atomic Force Microscopy (AFM) andScanning Tunneling Microscopy (STM) assisted

lithography have been demonstrated on Si [17–

19]. The process is explained in detail elsewhere

[20]. AFM lithography is an electrochemical pro-

cess characterized by the following brief descrip-

tion. A metallized tip is held close to the sample

surface for a few seconds and a large negative bias

is applied to the tip with respect to the sample. Ifthe environment is sufficiently humid and the

tip–sample distance sufficiently small, a water

meniscus will form between the tip and sample.

The electric field created by the tip bias ionizes

the meniscus and oxidizes the sample surface ex-

actly where the meniscus comes into contact with

it. This process is strongly dependent upon three

factors: (1) the formation of the water meniscusbetween the tip and sample surface (i.e. the humid-

ity and tip–sample distance), (2) the creation of a

field of sufficient strength to ionize the water

meniscus and (3) the duration of the application

of that field. This process has been well character-

ized when performed on Si [17–19]. However, it

has not been studied on any quasicrystalline sur-

face to date.The native oxide of AlPdMn is Al2O3 and stud-

ies of the adhesive properties of the oxidized sur-

face have been performed [9]. It has been shown

that the coefficient of dynamic friction of the 5-

fold surface of AlPdMn decreases by 50% with

the growth of a very thin (<10 A) oxide layer. This

suggests that the coefficient of dynamic friction of

the oxide may decrease with thickness.

2. Experiment

2.1. Initial sample preparation

The purpose of the initial sample preparation

was to create a well-defined surface for AFMlithography and the adhesion studies. The proce-

dure outlined in this section was performed only

one time prior to all AFM work (i.e. it was not re-

peated prior to each experiment).

The quasicrystalline sample involved in this

work was Al70.3Pd20.6Mn9.1. Its 5-fold surface

was polished (in ambient air) to 0.25 lm using

standard SiC and diamond polishing papers. Thisprocess yielded a shiny, mirror-like surface,

whereas the unpolished surface is rough, dull and

relatively non-reflective. It should be noted, how-

ever, that even the polished surface is considerably

less shiny than those resulting from cleaving the

quasicrystal under Ultra High Vacuum (UHV)

conditions.

The surface treatment was performed in a(10�10 Torr) vacuum and consisted of cycles of

Ar ion bombardment (20 min, 1 keV < Ein <

2 keV) and anneal (3 h) at 600 �C. The cycles ofbombardment and anneal were continued until a

5-fold LEED pattern was obtained. Subsequently,

the sample was removed from the chamber and al-

lowed to oxidize in ambient air (density of water

vapor of approximately 7 · 10�6 g/cm2) for severalhours.

2.2. AFM studies

The AFM studies were performed in ambient

air and in flowing gaseous nitrogen at room

temperature using the Digital Instrument NANO-

SCOPE 3100 system. The humidity of the experi-mental environment was measured using a high

sensitivity fast response thermometer-hygrometer

[21].

J. Smith et al. / Surface Science 580 (2005) 11–18 13

Relative adhesive properties of the various oxi-

des were obtained using the AFM tip as the probe

in a nanometer scale probe tack experiment. This

is done by obtaining the ‘‘Force curve’’ [22,23]

(i.e. the deflection of the cantilever vs. the tip–sam-ple separation) as the AFM tip approaches, con-

tacts and then withdraws from the sample

surface. The tip deflection is linearly related to

the force of tip–sample interaction through the

cantilever spring constant. The force of adhesion,

which is the force needed to disengage the tip from

contact with the surface, can be directly extracted

from such a force curve. This force is usually afew nN and is related to the irreversible work of

adhesion. All adhesion measurements presented

were performed with a single non-metallized SiC

tip. Spring constants mentioned in this work are

those supplied by the manufacturer. Normal

deflection spring constants and tip radii are

0.60 N/m and 20 to 50 nm. Our results were all ob-

tained using the same tip in as dry an environmentas possible (h = relative humidity � 19%, corre-

sponding to a density of water vapor of approxi-

mately 3 · 10�6 g/cm3). This was done in order

to minimize the effect of adsorbed moisture on

the measurement.

After the lithography and surface analysis were

performed, the chemistry and depth of the native

oxide on the surface were analyzed. Using Scan-ning Electron Microscopy in conjunction with

depth profile micro-Auger spectroscopy, we mea-

sured the thickness of the native oxide layer on

our sample to be approximately 2 nm. The compo-

sition of the native oxide of AlPdMn was seen to

be essentially Al2O3, which is consistent with the

literature [24].

3. Results

3.1. Lithographic process

The electrochemical nature of the lithographic

process needed to be confirmed as this study

marked the first attempt to apply AFM-assistedlithography to quasicrystalline materials in gen-

eral, and to the AlPdMn system in particular. In

other words, it could not be concluded a priori

that any observed protrusions created on the sur-

face were indeed oxide. These could easily have

been caused by other phenomena due to the high

magnitude (on order of 109 V/m) of the field cre-

ated between the tip and sample. For example, thisfield might cause damage and subsequent deposi-

tion of some part of the AFM tip (e.g. the metallic

outer layer) or direct surface damage. Either phe-

nomenon would result in bumps on the surface

that would be indistinguishable from electrochem-

ically created oxide using only AFM.

The volume of an electrochemically created

oxide, however, should strongly depend on theambient humidity while tip deposition or surface

damage should not. Therefore, the dependence of

the results of the lithographic process on humidity

was verified and taken and as confirmation of the

electrochemical formation of surface oxide. Re-

sults from this experiment appear in Fig. 1. Twelve

patches of oxide, or oxide dots, are shown. Each

dot was created by tapping mode lithographywhile using the same metallized AFM tip. The dots

were formed in succession, starting with the bot-

tom left and finishing with the top right, as indi-

cated by the large white arrow in the figure.

During the deposition, the tip was held for 20 s

at a constant bias of �9 V and within several

nanometers of the grounded sample surface. The

partial pressure of nitrogen gas was controlled inorder to change the relative humidity of the exper-

imental environment during the creation of each of

the dots. The map on the right hand side of the fig-

ure gives the humidity during the creation of each

of the dots.

The dots in the lower left were created at max-

imum humidity (�45%) which corresponds to

�10�5 g/cm3 of water vapor. The humidity wasthen decreased continuously (from lower left to

upper right in the image) until two dots were made

at the minimum (�19%). These dots are indicatedin the figure. In the progression, one can easily see

that dot volume decreases substantially as the

humidity decreases in the tested range 45–19%.

This corresponds to a decrease in the density of

water vapor in the experimental environment from10�5 to 3 · 10�6 g/cm3. Finally, the humidity was

again increased to maximum (�45%, �10�5 g/cm3 of water vapor) in order to create the two dots

Fig. 1. (a) AFM tapping mode topographic image of oxide dots made on AlPdMn. Large white arrow indicates the direction of tip

motion in the formation of dots. The dots imaged in (a) are labeled in (b) by the humidity as they were created.

14 J. Smith et al. / Surface Science 580 (2005) 11–18

in the upper right portion of the image. The vol-

ume of these last two dots is nearly identical to

that of the dots created under conditions of maxi-

mum humidity at the beginning of the experiment(lower left of image). This confirms that the ob-

served decrease in dot volume was due to changes

in humidity as opposed to a gradual degradation

in the quality of the tip or to an effect of surface

structure on the lithographic process.

Unexpectedly, the shape and width of these

dots in the plane of the surface does not appear

to change with humidity. During the lithographicprocess, the dot width would be expected to de-

crease with humidity as the volume of the water

meniscus between the tip and surface decreases.

The absence of this effect in the image is perhaps

due to the impact of the tip shape. As the tip dia-

meter is comparable to the lateral dimensions of

the dots, the images of the dots in Fig. 1 result

from a convolution of both the physical shape ofthe dots and the physical shape of the tip. Then,

only the dot dimension perpendicular to the sur-

face could be considered to be measured accurately

by AFM. This dimension is referred to as the rel-

ative ‘‘height’’ of an oxide dot or line. It was mea-

sured by taking the difference between the height

of the feature in the topographic AFM image

and the average height of the adjacent surface over

2 square nm. By this measure, the decrease in rel-

ative humidity from 47% to 24% causes a decrease

in dot height from 25.1 nm to 4.1 nm (Fig. 1). In

other words, a decrease in the humidity from itsmaximum to half this value results in an 84% de-

crease in dot height.

An electrochemical process, as outlined in the

introduction, would require a negative tip bias.

Therefore, the effect of a change in polarity of

the tip bias on the process was also investigated.

A bias of +9 V was applied to the tip as it was

held within a few nanometers of the surface for20 s. The mark created by this process on the sur-

face was then compared to that which was left by

repeating the process on a neighboring region

with a negative tip bias (�9 V). While positivebias �lithography� did induce changes on the sur-face (possibly due to deposition of some part of

the tip), the difference between these changes

and those observed for negative bias lithographywas dramatic. While the former process created

a form on surface less than a few nanometers in

height and width, the latter created an oxide dot

more than 20 nm tall and approximately 20 nm

wide.

The facts that this process is shown to require

the presence of water and a negatively biased tip

are consistent with the process of tip-assisted

0.05.010.015.020.025.030.035.040.0

0.0 2.0 4.0 6.0 8.0 10.0 12.0Magnitude of Voltage Applied to the Tip (V)

Hei

ght o

f Oxi

de L

ine

(nm

)

Fig. 3. Oxide line height as a function of applied voltage on

surface of AlPdMn. Data is averaged over 10 trials.

J. Smith et al. / Surface Science 580 (2005) 11–18 15

oxidation. They are wholly inconsistent with tip

deposition or field-induced surface damage.

3.2. Lithographic parameters

The effect of change in tip bias on lithography

was investigated. For this series of experiments,

lithography was performed in contact mode with

W2C coated tips. Five oxide lines were drawn

under high humidity (�35%) conditions, each witha different tip bias (�10, �8, �6, �4 and �2 V).This experiment was repeated 10 times. It was

found that lithospeed in the formation of linesneeded to be between 10 and 30 nm/s. Below

10 nm/s, the shape of the oxide pattern was clearly

non-linear, while 30 nm/s was too fast to create

substantial amounts of lithographic oxide on the

surface.

Topographic images were then taken and used

to deduce the relative heights of the lines. The re-

sults from a representative attempt appear inFig. 2. The figure shows an obvious correlation be-

tween the magnitude of the applied bias and line

height. In order to make the comparison quantita-

tive, relative oxide line height vs. applied tip volt-

age is plotted in Fig. 3 as averaged over the 10

trials. The results clearly show a strong and possi-

bly linear dependence of lithographic oxide height

Fig. 2. Oxide lines created by lithography: (a) Topographic image i

lithographic voltage used to create them. The green dotted line mar

(averaged) showing decrease in height of oxide lines in (a) with decr

applied voltage used to create them.

on the magnitude of the applied voltage. Indeed,

the figure shows that changing the voltage from

�4 to �10 V results in a change in line height from6.1 ± 2.2 to 27.2 ± 7.0 nm. Thus, a 6 V increase in

the magnitude of the applied voltage results in an

increase in dot height which is on order of hun-dreds of percent.

Note that the results in Fig. 3 show that the

threshold voltage for lithography on AlPdMn

has been determined to lie within �2 and �4 V.A similar lithographic study on CVD Al metal,

performed under the same conditions and with

the same equipment, showed a threshold also with-

in the range of �2 and �4 V. The threshold isthe same order of magnitude when measured on

n contact mode showing four oxide lines, each labeled by the

ks the section of (a) used to create the plot in (b). (b) Section

ease in magnitude of applied voltage. Lines are labeled by the

16 J. Smith et al. / Surface Science 580 (2005) 11–18

oxidized silicon [25]. These results are consistent

with the hypothesis that the lithographic threshold

depends primarily on the water meniscus formed

between the tip and sample and, so, is relatively

substrate independent.

3.3. Adhesion results

Adhesion measurements show a definite differ-

ence in the characteristics of the lithographic oxide

from those of the native oxide. In total, 360 adhe-

sion measurements were performed on 12 different

lithographic oxides and 240 adhesion measure-ments on 10 different areas of native oxide. Two

representative force curves appear in Fig. 4 and

the force of adhesion is indicated in the figure.

Note that the scales in Fig. 4a and b are identical.

Both force curves were obtained using the same tip

under identical conditions. It is evident from Fig. 4

that the measure of adhesive force presented for

the native oxide is nearly twice that for the litho-graphic oxide. On average, the AFM tip adhesion

to the lithographic oxide surface is only 68% as

strong as the adhesion measured on the native

oxide. This difference was observed for the entire

series of measurements.

4. Discussion

We note that the variance of the measurement

of the force of adhesion on the native oxide was

Fig. 4. Force curves obtained on (a) the native oxide (b) the lithograph

Tip deflection is measured which can be related to tip–sample force, plo

axis is the lateral displacement of the tip from sample where ‘‘0.0 nm’’

in the figure.

21% over 240 measurements. On the lithographic

oxide, the variance in this measurement was clo-

ser to 29% over 360 separate measurements. To

date, we have not measured how the chemical

composition of the oxide varies laterally on thesurface. If this is substantial, it is one possible

source of this error. While chemical inhomogene-

ity in the lithographic and native oxides remains

an issue for further study, we may say with con-

fidence that it will not affect the major conclusion

we draw from the results presented here, i.e. that

the adhesive properties of the lithographic oxide

are significantly weaker than those for the nativeoxide.

Previous studies by Dubois et al. have shown

that simply increasing the thickness of the amor-

phous, native oxide surface layer (below 12 nm)

substantially decreases the adhesive properties of

quasicrystalline surfaces [26,27]. The results pre-

sented here are consistent with those of Dubois

et al. as well as those obtained by Park et al. onthe Al–Ni–Co quasicyrstalline surface [28]. A de-

tailed understanding of the chemical composition

and structure of the oxide produced by nanoli-

thography would help to clarify whether our re-

sults are fully explained by the conclusion of

Dubois et al., i.e. that this effect is due to the influ-

ence of the electronic structure of the underly-

ing quasicrystal. While we have not yetsucceeded in this rather difficult experiment, we

can discuss the results obtained in the context of

the literature.

ic oxide using a SiC in a dry (19%, 3 · 10�6 g/cm2) environment.

tted on the y-axis, through the spring constant of the tip. The x-

denotes contact. The measure of the force of adhesion is labeled

J. Smith et al. / Surface Science 580 (2005) 11–18 17

All studies to date have shown that oxidizing the

surface of the AlPdMn quasicrystal under a variety

of conditions yields a surface layer that is nearly

chemically identical to the native oxide of aluminum

metal, i.e. Al2O3 [14–16,26,27]. Then, the simplestplausible description of the lithographic oxide is to

consider it is an amorphous layer of Al2O3. How-

ever, we do expect the precise composition and

structure of the oxide to depend somewhat on the

fabrication process, as has been observed in the case

of the oxidation of Si(100) by nanolithography [29].

Indeed, our experimental observations indicate an

oxidation mechanism that depends upon experi-mental details such as the precise level of humidity,

the tip quality as well as parameters such as the ap-

plied voltage and normal force. To a first approxi-

mation, however, it is reasonable to consider the

oxide surface in contact with the AFM tip as the

same as the surface of the native oxide layer.

In this case, our results show that the thickness

of the oxide layer is inversely related to the adhe-sive properties of the oxide (Fig. 4). That this dif-

ference cannot be ascribed to changes in the lateral

dimension of the oxide (i.e. changes from that of

the surface covering native oxide with its effec-

tively infinite lateral extent to the finite litho-

graphic oxide), can be assumed because the

lateral dimension of the lithographic oxide (300–

500 nm) is greater than the radius of the tip (20–50 nm) by at least an order of magnitude. Then,

even the lithographic oxide has a large lateral

dimension with respect to the tip. The nanometer

scale changes in oxide thickness (Figs. 1–3) are

known be of the scale over which substantial vari-

ations in Van der Waals interactions occur in any

materials system. As the extraordinary adhesive

properties of quasicrystals are suspected to lie intheir peculiar Van der Waals interactions [26,27],

our results show that nanolithography provides a

practical method to probe changes in adhesive

properties quasicrystalline oxides in detail.

The work also shows that it would be useful to

measure the adhesive properties of the quasicrys-

talline surface in the absence of oxidation, i.e. in

UHV just subsequent to the surface preparationdescribed in the introduction. In such an experi-

ment, adhesive properties can be quantified via

the non-contact interaction between a tip and the

surface at distances varied from a few nanometers

up to 500 nm. The analysis of such results, using

the Lifschitz formula for the non-retarded Van

der Waals interaction at distance shorter than

the plasma length, will lead to accurately quantify-ing the adhesive force between the tip and the sam-

ple. This work is currently in progress.

5. Conclusions and future directions

The primary result of this study is to show the

first successful use of AFM-assisted lithographyto create various oxides on the quasicrystalline

AlPdMn surface. We have proved the electrochem-

ical nature of this process by demonstrating the ef-

fect of humidity and the polarity of applied voltage

on it. We also have determined the ‘‘lithographic

range’’ (i.e. the range necessary to perform lithog-

raphy on this surface) of for several key parame-

ters. These include the humidity (23–47%), tipbias: �(4–11 V) and lithospeed in the formation

of lines (10–30 nm/s). We have studied effect of

changing several of the lithographic parameters

on oxide thickness. First, we determined that a de-

crease in humidity to half its maximum value de-

creases oxide height by 84%. Next, we observed

that the increase in the magnitude of the tip bias

by 6 V (from �4 V) introduces an increase in oxideheight that is on order of hundreds of percent.

We have also demonstrated the utility of this

technique in the study of the adhesive properties

of quasicrystals. Our results show that the adhe-

sion of the thicker lithographic oxide to a SiC

probe is 32% weaker than that measured if the

same experiment is performed on the native oxide

of AlPdMn. This result along with the size scale ofthe difference in the thickness of these layers are

consistent with the hypothesis Van der Waals

interactions play a key role in the peculiarity of

the adhesive properties of these surfaces. While it

is difficult to extend this analysis further without

a better knowledge of the oxide layer, this raises

an issue that merits future study.

Finally, these results suggest that it may bepossible to diminish the already phenomenally

weak adhesive properties of AlPdMn by increasing

the thickness of its oxide. If such a procedure is

18 J. Smith et al. / Surface Science 580 (2005) 11–18

developed it would have a very significant impact

in the use of quasicrystalline materials as wear-

resistant coatings.

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