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Applied Surface Science 276 (2013) 203– 209

Contents lists available at SciVerse ScienceDirect

Applied Surface Science

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

volution of aluminum surface irradiated by femtosecond laser pulsesith different pulse overlaps

uoqiang Lia, Jiawen Lia,∗, Liang Yanga, Xiaohong Lib, Yanlei Hua,iaru Chua, Wenhao Huanga

Department of Precision Machinery and Precision Instrumentation, University of Science and Technology of China, Hefei 230026, ChinaSchool of Science, Southwest University of Science and Technology, Mianyang 621010, China

a r t i c l e i n f o

rticle history:eceived 17 October 2012eceived in revised form 12 March 2013ccepted 13 March 2013

a b s t r a c t

We report the modification of aluminum surfaces by femtosecond laser pulses with different pulse over-laps in ambient air. Several kinds of structural colors can be generated on the aluminum surfaces byvarying the pulse overlap from −27% to 99%. The SEM images reveal that the structural colors of themodified area depend on the surface morphology. Furthermore, X-ray photoelectron spectroscopy and

vailable online 26 March 2013

eywords:luminumurface modificationemtosecond laser pulse

X-ray diffraction spectra show that the content of Al2O3 increases while that of Al(OH)3 decreases withthe increase of the pulse overlap. Only Al2O3 is formed on the aluminum surfaces when the pulse overlapis larger than 93%.

Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.

tructural colors

. Introduction

The modification of metal surfaces with femtosecond laser irra-iation has become a hot research topic and attracted lots of

nterest in recent years. This novel method has the advantage ofroducing metallic nanostructures with unique properties, whichave potential applications in life sciences [1,2], optical data storage3], microfluidic devices [4,5], powder metallurgy [6] and structuralolors [7–11]. For the successful metal surface processing with fem-osecond laser pulses, it is essential to optimize the laser processingarameters. These parameters include laser fluence, pulse overlap,he distance between adjacent scanning lines, laser incidence anglend so on. It has been reported that aluminum films with gray,olden and black colors can be fabricated by adjusting the laser flu-nce and the number of exposure pulses to induce nanostructures,icroscale cloudy aggregates, and slight periodic marks [12]. The

aser incidence angle can also affect the diffraction grating period13]. Compared to other processing parameters, fewer investiga-ions have been carried out on the evolution of modifying surfacey different pulse overlaps.

Aluminum is a common energetic material, and it has the char-cteristics of low melting point. It can be easily oxidized whenxposed to air. In this work, we modified the aluminum surfaces

∗ Corresponding author. Tel.: +86 551 63601478; fax: +86 551 63601478.E-mail address: jwl@ustc.edu.cn (Jiawen Li).

169-4332/$ – see front matter. Crown Copyright © 2013 Published by Elsevier B.V. All rittp://dx.doi.org/10.1016/j.apsusc.2013.03.067

by scanning the femtosecond laser beam over the surfaces. Theinfluence of the pulse overlap on processing result was studied indetail through observing the induced micro/nano structure, andanalyzing the composition of the modified surface. Results demon-strated that various structural colors can be generated by properlycontrolling the pulse overlap. Such an investigation is supposed tobe able to provide a promising method for producing controllablestructural colors.

2. Experimental setup

A regenerative amplified Ti: Sapphire femtosecond laser sys-tem (Spectra-physics) with the pulse duration of 120 fs, the centralwavelength of 800 nm and the repetition rate of 1 kHz was appliedto process the aluminum surfaces (99.99%, purity). The experimen-tal setup is schematically illustrated in Fig. 1. The pulse energy couldbe continuously varied by using the combination of a �/2 waveplate and a Glan-Taylor polarizer. The average power of the laserpulse is measured by a power meter (Coherent, FM10), which isset behind the polarizer. The laser beam is focused by a biconvexlens with 50 mm focal length, and the diameter of the focal spot isabout 15 �m. The aluminum sheets are mounted horizontally ona high-precision computer controlled linear translational stage. A

series of pulse overlaps in the x-direction on the aluminum surfacesare obtained through changing the speed (v) of the linear transla-tional stage. Before the laser treatment, the aluminum sheets arecleaned with acetone to remove organic dopants on the surface.

ghts reserved.

204 G. Li et al. / Applied Surface Science 276 (2013) 203– 209

tion o

Tati(tVBN

3

meotarTids

opsi

dsmnnFs

Fig. 1. Schematic illustra

he experiments are conducted by scanning the laser focused spotcross the aluminum surface in air environment while controllinghe pulse overlap [13,15]. The morphology of the modified surfaces imaged by using a field emission scanning electron microscopyFESEM) (JSM-6700F). The X-ray photoelectron spectrum (XPS) ofhe aluminum surface is measured with ESCALAB 250 (Thermo-G Scientific). The X-ray diffraction spectra are obtained with aruker D8 Advance X-ray diffractometer, which is equipped withi-filtered Cu K�1 radiation at room temperature.

. Results and analysis

Firstly, the possibility of producing structural colors on alu-inum surfaces by altering the pulse overlap is examined. In

xperiments, the average laser fluence is 5.1 J/cm2. The pulseverlap in the y-direction is chosen to be −233% (hatch dis-ance = 50 �m). The aluminum surfaces appear golden, white, graynd black when the overlap is set as −7%, 33%, 93%, and 99%,espectively. The corresponding scan rate is 16, 10, 1, and 0.1 mm/s.he typical results are demonstrated in Fig. 2. It is found that thenduced structural colors become brighter and more vivid with theecrease of the pulse overlap. All these structural colors exhibit theame color under various viewing angles.

To identify how the surface structures are affected by the pulseverlaps, a detailed SEM study is performed on the aluminum sam-le surface. Fig. 3 shows the SEM images of an unexposed aluminumurface. Figs. 4–7 reveal the SEM images of irradiated targets at var-ous pulse overlaps, which are −7%, 33%, 93% and 99%, respectively.

It is clear from Fig. 4(a) that adopting the pulse overlap in x-irection of −7% endows the modified aluminum surface with amooth surface as compared to the untreated surface in Fig. 3. Theore detailed structures can be seen from Fig. 4(b) to (d). Many

anoparticles, whose diameters are in the order of hundreds ofanometers, are embedded on the substrate surface. As shown inig. 4(c), these nanoparticles can be generally classified into twohapes. One is like a flower with diameter of approximately three

Fig. 2. Photograph of modified aluminum: (a

f the experimental setup.

hundred nanometers in diameter, and the other is like a spherewith diameter of approximately one hundred nanometers. Fig. 4(d)confirms that these nanoparticles contain many protrusions andmicro scale aggregates, which are formed by fusing nanoparticlestogether on the surface.

When the pulse overlap increases to 33%, slight stripe structuresdispersed on the substrate surface appear as shown in Fig. 5(a).Nonuniform cracks of different scale are also formed, as evidentlydisplayed in Fig. 5(b). When the surface morphology is magnified20,000 times as shown in Fig. 5(c), block-like structures and grainystructures can be found clearly. This is the evidence that the liquidphase exists in the laser ablation process [14]. Higher pulse overlapmakes more surface materials vaporize, therefore both the quan-tity and the size of spheres decrease and more fluffy structures aregenerated as shown in Fig. 5(d).

Fig. 6(a) represents the surface topography of representativegroove structures generated by scanning a laser beam horizontallyacross a sample surface at a 93% overlap. The period of the grooveis 50 �m, which is determined by the distance of adjacent hori-zontal scanning lines. The width of groove is about 15 �m, whichis approximately equal to the diameter of the focused laser beam.Compared with the case of 33% overlap, the scale formed sphericaland block-like nanoparticle aggregates much bigger as shown inFig. 6(b). More fluffy porous can be generated, which can be foundin Fig. 6(c) and (d).

When the overlap reaches 99%, the processing lines can producedeeper grooves on the aluminum surface as shown in Fig. 7(a) and(b). From the cross-sectional SEM image, Fig. 7(e), the depth of thegrooves is measured to be ranging from 89 to 125 �m. Fig. 7(b), (c)and (d) demonstrate the black aluminum surface has a rich vari-ety of structures including types of nano- and micro-scale voids,nanoprotrusions, microscale aggregates formed by coalescence of

nanoparticles [8].

In order to further study the influence of the pulse overlap onsurface modifications, we also considered the case of the pulseoverlap of 12% (v = 13 mm/s). Under such a condition, another kind

) golden; (b) white; (c) gray; (d) black.

G. Li et al. / Applied Surface Science 276 (2013) 203– 209 205

Fig. 3. SEM images of untreated aluminum surface: (a) 20,000× magnification; (b) 100,000× magnification.

F ion; (b

ofSeaL

F

ig. 4. SEM images of structures on golden aluminum surface: (a) 500× magnificat

f modified aluminum surfaces are fabricated, which exhibits dif-erent colors from different viewing angles as shown in Fig. 8. The

EM study shows that the entire surface of the aluminum is cov-red with femtosecond laser-induced periodic structures (LIPSS),nd the spatial period of the LIPSS ranges from 500 to 550 nm. TheIPSSs can be considered as a kind of grating structures, which could

ig. 5. SEM images of structures on white aluminum surface: (a) 500× magnification; (b)

) 5000× magnification; (c) 20,000× magnification; (d) 100,000× magnification.

play important roles in modifying the optical properties of metalsurfaces in a very versatile way [12,15–17].

X-ray photoelectron spectroscopy (XPS) and X-ray diffractionspectra measurements are performed to investigate the chemi-cal compositions and surface structures of irradiated aluminumsheets.

5000× magnification; (c) 20,000× magnification; (d) 100,000× magnification.

206 G. Li et al. / Applied Surface Science 276 (2013) 203– 209

Fig. 6. SEM images of structures on gray aluminum surface: (a) 500× magnification; (b) 5000× magnification; (c) 20,000× magnification; (d) 100,000× magnification.

F 500× magnification; (b) 5000× magnification; (c) 20,000× magnification; (d) 100,000×m

iaprltc

Table 1XPS peaks for surface elements.

O1s (ev) C1s (ev) N1s (ev) Al2p (ev)

Untreated aluminum 531.64 284.81 399.84 74.27

ig. 7. The front and side SEM images of structures on black aluminum surface: (a)agnification; (e) side face morphology, 300× magnification.

From the XPS spectra as shown in Fig. 9 and peak table as shownn Table 1, it is found that for untreated aluminum, both the peakst 71.7 eV BE and 74.27 eV BE can be detected, which are from theure aluminum and the oxidized aluminum respectively [14]. This

esult suggests the thickness of the natural oxide film should beess than 10 nm, for the maximum penetration depth of photoelec-ron is about 10 nm in XPS measurement. The peak at 74.27 eV isonsistent with the Al2p binding energy.

Golden aluminum 531.45 284.81 399.6 74.30White aluminum 531.66 284.79 406.98 74.34Gray aluminum 531.71 284.82 407.01 74.42Black aluminum 531.77 284.79 407.18 74.45

G. Li et al. / Applied Surface Science 276 (2013) 203– 209 207

Fig. 8. Color aluminum formed at pulse overlap in x-direction of 12%: (a)

70 72 74 76 78

1000

2000

3000

4000

5000

6000

7000

8000 Untreated Alu minum

Gray Alu minum

Gold en Alu minum

Bla ck Alu minum

Whi te Alu minum

Co

un

ts (

/s)

oadp

bcw

rafodw

lt

TS

Binding Energy (ev)

Fig. 9. XPS spectra for Al.

When the aluminum surfaces are irradiated with the femtosec-nd laser in air, only Al peak at 74.27 eV BE exists on the modifiedluminum surfaces. Results indicate that the thickness of the oxi-ized film will increase more than 10 nm after the irradiationrocess.

All the samples not only have the expected O and Al elements,ut also have a relatively high content of C1s and a small N1s. Theontents of the Al and O increase with the increasing pulse overlap,hich can be concluded from Table 2.

The surface structure of irradiated aluminum is identified by X-ay diffraction spectra. The sharp and high intensity peaks from Alre shown in Fig. 10. In addition, Al2O3 and anorthic Al(OH)3 areound on golden aluminum and white aluminum [18–21], whilenly Al2O3 can be found on the gray and black aluminum, whichemonstrates that the oxidation reaction for aluminum is enhanced

ith the increase of pulse overlap.

The XPS peaks table shows that all the C1s peaks areocated at the same position (284.8 eV), which implicates thathey have come from the same contribution [14]. Carbon is

able 2urface element contents calculated from XPS survey spectra.

Oxygen Carbon Nitrogen Aluminum

Untreated aluminum 40.38% 43.64% 1.45% 14.54%Golden aluminum 49.92% 32.19% 1.67% 15.22%White aluminum 55.52% 27.21% 1.33% 15.95%Gray aluminum 57.62% 24.21% 1.79% 16.38%Black aluminum 57.65% 22.75% 1.52% 18.08%

viewed at 80◦; (b) viewed at 60◦; (c) SEM image of color aluminum.

a kind of extremely sensitive element therefore it is easy todetect by analyzing the XPS spectra. The origin of the carbonon the aluminum surfaces may come from the accumulatedpollution during pre- and post-irradiation sample handling. Inaddition, we noted that the carbon content decreases signifi-cantly when the pulse overlap increases. This phenomenon canbe explained from the surface topography of modified aluminums. The SEM images in Figs. 5–8 show that the aluminum sheets afterlaser irradiation are covered by particles, grooves and voids, whichallow the carbon element to penetrate deeply into the aluminumsurfaces and meanwhile the content on the surfaces decreases.

As for the N1s, we can see that its content in either the untreatedaluminum surface or the modified ones is very weak, and there isno strong correlation between the N1s content and the pulse over-lap. According to the N1s peak positions in Table 1, the origin of Ncan be attributed to two different contributions. The two peak pos-itions (399.84 eV and 399.6 eV) are very close to the peak position(400 eV) for nitrogen in most organic materials, thus some part of Ncomes from organic matters. The other peak positions (406.98 eV,407.01 eV, 407.18 eV) indicate that the other part of N might existsin the form of some nitrogen compound NO3.

Therefore, colorizing aluminum surfaces can be mainly influ-enced by forming a variety of structures of different size,width/diameter, and/or depth [22]. The surface structures, such asthe microgratings, contribute for changing the optical propertiesof the aluminum surfaces, which result in different colors at dif-ferent viewing angles because of the trapping of light inside theperiodic microstructures and the angular dependence of Fresnelreflections [23–25]. The random microstructures or nanostruc-tures, play significant roles in forming colorful aluminum exhibitone kind of color in different angle of view. Many nanoparticlesof hundreds of nanometer scale in diameter embedded on thesubstrate surface result in yellow color. Block-like structures andgrainy structures endow the aluminum surface with white color.Grooves covered with types of nano- and microscale voids makethe aluminum surface show gray or black color, and so on. Thephenomena of color change shows that the optical properties of thealuminum surface change with control of laser treatment param-eters. Aluminum oxide appears pure white, which enhances thecolor of white, and weakens the other colors on modified aluminumsurfaces.

Based on above investigations, patterned colors on one surfacecan be flexibly produced by altering the pulse overlap. For verifica-tion, blue Olympic rings can be fabricated as shown in Fig. 11(a) by

using an overlap of −27%. A modified aluminum sheet composedof white, black, golden and gray regions as shown in Fig. 11(b) canalso be controllably produced by adopting the pulse overlaps asmentioned.

208 G. Li et al. / Applied Surface Science 276 (2013) 203– 209

20 30 40 50 60 70 80

0

50

100

150

200

Al(O

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20 30 40 50 60 70 80

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Al(

80

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Fig. 10. XRD patterns of femtosecond laser treated aluminum surfaces: (a) golden aluminum; (b) white aluminum; (c) gray aluminum; (d) black aluminum.

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Acknowledgments

Fig. 11. (a) Olympic rings produced at the pulse overlap in x-

. Conclusion

The aluminum surface modified by femtosecond laser irradia-ion with different pulse overlaps is studied. Results indicate thatulse overlap is a very important parameter in the process for mod-

fying aluminum surface. By precisely changing the pulse overlap,ifferent structural colors can be produced, such as golden at −7%verlap, white at 33% overlap, gray at 93% overlap, black at 99%verlap and multicolor at 12% overlap, and so on. These variousinds of structural colors are mainly due to the different kindsf surface micro/nano structures induced by the laser irradiation.

orphological research shows that when adopting 12% overlap,

ubwavelength periodic ripples are formed on aluminum surface.ut when the pulse overlap increases, block-like structures anduffy structures can be induced. When the pulse overlap increases

ion of −27%, (b) combinatorial picture controllably produced.

from −7% to 99%, both the quantity and the size of nanoparticlesin the induced structures decrease, and the laser ablation depthon the processing lines increase. Furthermore, the chemical com-position and contents also vary when changing the pulse overlap.With the increase of the pulse overlap, the Al2O3 increases whileAl(OH)3 decreases. Results imply the enhanced oxidation with thefemtosecond laser irradiation. The laser induced color modificationis controllable, and has a variety of potential applications.

This work is supported by National Science Foundation of China(No. 51275502 and 11204250) and China Postdoctoral ScienceFoundation funded project (No. 2012M511416 and 2012M521245).

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