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ISSN 1054-660X, Laser Physics, 2007, Vol. 17, No. 12, pp. 13821388.

MAIK Nauka /Interperiodica (Russia), 2007.Original Text Astro, Ltd., 2007.

INTRODUCTION

In laser ablation, most of the absorbed energy is car-ried off with the ejected material, so that there is littleor no thermal damage to the surrounding substrate area.The absorption of laser radiation by solids first convertsthe electromagnetic energy into electronic excitationand, then, into thermal, chemical, and mechanical ener-gies to cause evaporation, ablation, excitation, andplasma formation [1]. Plasma is transient in nature withcharacteristic parameters that evolve quickly and arestrongly dependent on the irradiation conditions, suchas incident-light laser intensity, laser wavelength, irra-diation spot size, ambient gas composition, and ambi-ent pressure [25]. Plume hydrodynamics can be inves-tigated using different methods, but the easiest forplume propagation is fast photography employingICCD [6, 7]. The surface morphology of laser-irradi-ated gold may be explored using many methods, butSEM micrograph study is the simplest. Macrolevelquantitative analysis of the crystllinity of laser-irradi-ated gold is done by studying XRD patterns obtainedfrom an X-ray diffractometer. Much work has been per-formed using the novel technique of laser ablation,since laser ablation reveals various parameters to sup-port material processing [810].

Ted D. Bennett et al. [11] studied the pulsed-lasersputtering of gold at near-threshold fluences (materialremoval rates

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LASER IRRADIATION EFFECTS ON GOLD 1383

a computer-controlled gated ICCD camera contributetowards a better understanding of the hydrodynamics ofplume propagation obtained from gold plasma. Mor-phological studies explain the fundamental mecha-nisms of laser ablation and heat conduction. The studyof the XRD patterns leads toward a better comprehen-sion of the structural information [1417].

EXPERIMENTAL SETUPGold samples were annealed under a vacuum

~10

6

Torr, at a temperature of 355

C for a duration of8 h. The samples were irradiated by Q-switchedNd:YAG lasers for ICCD imaging and morphologicaland structural analysis. The target was kept on a rota-tion to minimize the local heating and drilling. Gold-plume imaging was accomplished using gated ICCD. Adigital pulse generator was used to control the delaybetween the laser pulse and the imaging system. Tocontrol the saturation for the camera light, neutral-den-sity (ND) filters were used between the glass window ofthe vacuum chamber and the ICCD camera. The othersamples were irradiated at an angle of 45

with the nor-mal to the surface for 50 shots. The irradiated surfacewas then analyzed by using SEM for surface morphol-ogy. Macroscopic structural analysis was carried out

employing an X-ray diffractometer. The scheme of theexperimental setup is shown in Fig. 1.

RESULTS AND DISCUSSION

ICCD Imaging

Time-resolved ICCD images of the gold plume areshown in Fig. 2 with a gating time of 10 ns. Each imageis obtained from a single laser pulse. These are two-dimensional images of the gold-plasma plume that pro-vide an orthogonal view of the plume expansion withrespect to the target surface. If the laser intensity isabove the ablation threshold of the target, then evapora-tion, ionization, and melting of the target material takesplace to create a plasma plume. The motion of the spe-cies present in the plume obeys forward peaking [18,19]. The accelerated ions in the expanding plasmaplume may attain an energy value of around a few hun-dred electronvolts. The production of ions alsoincreases at high fluences as a result of the fast interac-tion between the laser pulse and the rapidly producedplasma [20]. As the laser photon reaches the target sur-face, plasma emission immediately begins. With thepassage of time, the emission can be resolved into twocomponents. Light emission very close to the target

PTG

ICC

D

Nd:YAGlaser

Six portchamber

Vacuumgauge

Target

Laser induced plasma

Focusinglens

Fig. 1.

Scheme of the experimental setup.

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KHALEEQ-UR-RAHMAN et al.

surface is present even after 500 ns. The stationaryemission region very near to the target surface is a con-sequence of the gas collision between the plumeejectants in the high-pressure region of the initialexpansion. This results in the Knudsen layer with theejected material along with the backward-movingmaterial. The second component of the plasma plumeexpands very rapidly in the forward direction along thenormal to the surface of the target [21].

Initially, the plume front is found to be in a sphericalform, but, afterwards, it becomes sharpened andexpands at an extremely high rate. Normally, thehigher-energy particles are emitted closer to the normaltarget surface. Plasma ions are angularly distributed

with respect to energy and charged states due to themutual interaction and collisions with normal atomions or with nanoparticles [22]. The ion flux is found tobe high in the axial direction and reduces in the radialdirection. The maximum flux and energy of the ions isin the forward direction. This is known as forwardpeaking [19, 23].

At very early stages, the plume expansion is linearfor all background gas pressures. The shock model,given by the equation [18]

(1)explains the explosive release of energy at the earlierstages of plume expansion, where

0

is a constant whichdepends on

Y

, the ratio of the specific heat capacity ofthe expanding gas, whereas, at the later stages, the dragmodel, given by the equation [18]

(2)is used to describe the plume expansion, where

R

0

is thestopping distance of the plume and

is the slowingcoefficient.

From the above discussion, we can say that ICCDimaging of the plasma plume provides a significantunderstanding about the dynamics of the various spe-cies present in the gold plume [24].

SEM Analysis

The exposure of the metal surface results in changesin the surface morphology, hardness, and phase. Figure 3is a SEM micrograph of irradiated gold at a 300

mag-nification. It shows the nonsymmetric heat conductionand ablation of material exposed for 50 shots. Theconelike structures show hydrodynamic sputtering and

R 0 E0/0( )1/5t2/5,=

R R0 1 t( )exp( )=

10 ns 20 ns 50 ns

1000 ns 1500 ns 5000 ns

Fig. 2.

Time-resolved gold-plume dynamics of ICCD imaging in a vacuum.

100

m

MAG = 300

EHT = 10.00 kVDETECTOR = SE1Date: 26 Jul 2005

Fig. 3.

SEM micrograph of laser-irradiated gold with 50shots of an Nd:YAG laser at a magnification of 300

.

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LASER IRRADIATION EFFECTS ON GOLD 1385

splashing. The ablation of the laser-irradiated metal tar-get explained using a dimensional hydrodynamicsmodel describes the absorption of laser radiation, elec-tron-heat conduction, and electronphonon and elec-tronion energy exchanges, which result in materialmotion and expansion [14]. Asperities show cones dueto a laser effect. These cones develop on the surface dueto repetitive laser shots, where, at some points, materialejection is fast while at some other points, it is slow.Pulsed-laser irradiation leads to the development of asurface topography characterized by droplet and ridgeformations and the liberation of micron-sized dropletsinto the plume. The majority of the surface topographyhas been studied by hydrodynamic sputtering, alongwith the ejection of neutral gold atoms [11]. This non-uniform ejection develops asperities, turbulent features,and cones. These irregularities are due to surface impu-rities, crystal imperfections, and the TEM modes of thelaser used for irradiation [25], since the target is irradi-ated with a multimode Nd:YAG laser. The Material isalso splashed out from the irradiated-zone boundary.An indication of the resolidification of the splashed

material is also present. All of these phenomena are ingood agreement with previous investigations [15].

XRD Analysis

The crystallographic or structural study was carriedout by the analysis of X-ray diffractometer patterns.The XRD pattern of gold before laser irradiation showsa homogenous and crystalline structure [26]. Eight ofthe nine peaks match the standard pattern for gold. Peak[111] at 2

= 38.231

is due to pladium impurity. Acomparison of the two patterns shows that the irradia-tion of gold samples causes an increase in the intensityas indicated by the peaks at angles 44.422

, 77.645

,

110.842

, and 115.269

, whereas, for some peaks, adecrease is also observed after the laser exposure asshown by the peak at angles 38.231

, 64.659

, 81.766

,

and 98.157

, respectively. A plot of 2

versus the rela-tive intensity, shown in Fig. 6, indicates that, up to anangle of 80

, the intensity shows a Gaussian fit,whereas, afterwards, a polynomial-fit trend is observedfor the unexposed sample. At the same time, a plot

11010090807060504030

1600

900

400

100

Cou

nts

120 Jul_Au

Position, 2

, deg

Fig. 4.

XRD pattern of unexposed gold.

11010090807060504030

1600

900

400

100

Cou

nts

129 Oct_Abl

Position, 2

, deg

Fig. 5.

XRD pattern of exposed gold.

0

0

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KHALEEQ-UR-RAHMAN et al.

between the same quantities for the exposed samplesrepresents a LorentzB fit up to an angle of 65

; after-wards, a decrease and increase in the intensity isobserved. Physical aspects that can change the intensitymay include the crystal imperfections caused by non-uniform strains imposed by laser irradiation on thesamples surface. Diffraction and scattering effects canalso cause the variation in intensity. The peak [111] at2

= 38.497

indicates the maximum intensity for theexposed gold. When recrystallization occurs, the peaksattain their maximum sharpness. Sometimes, the reflec-tion from the crystal surface also causes incrementalchanges in the intensity of the radiation. The thermalstresses due to laser irradiation can disturb the latticevibrations of the atoms, thus causing a variation in theintensity. Mostly, the energy absorbed by the atoms isconducted in a nonuniform way. Thus, a change in theintensity is observed [27, 28].

No significant change in the values of 2

is observedin the XRD patterns of gold as shows in Figs. 4 and 5.

The grain size of the irradiated simples is calculatedby using the formula [29]

(3)where

D

is the grain size,

is the wavelength of theX rays used,

B

is the FWHM, and

is the Braggsangle.

All of the peaks of the exposed samples indicate adecrease in the grain size. Figure 7 shows a monotonicdecrease in the grain size, whereas a sharp increase isobserved at the end of the graph for the unexposed sam-ple. The other graph for the irradiated sample indicatesa monotonic increase. A rapid decrease is observed atthe start and endpoints of the graph, which is exactlyopposite when compared with that of the unexposedsample. The hardness of the material and the recovery

D 0.9/B ,cos=

processes are the two fundamental phenomena thatmostly affect the grain size [30]. Thermal stresses alsoplay a vital role in the change or variation of the grainsize. The grain growth implies a redistribution of thegrain orientation by the laser irradiation of the targetmaterial [31].

Figure 8 shows that the laser irradiation on gold hasa negligible effect on the

d-

spacing of the sample whenplotted against the 2

values. A trend of exponentialdecay of the third order is found for both the unexposedand exposed samples. Quite weak stresses are imposedby laser irradiation, indicating no significant distur-bances in the planes of the target samples. Also, no

0

30

Relative intensity

2

, deg

20

40

60

80

100

40 50 60 70 80 90 100 110 120

Un-exposedExposed

Fig. 6.

XRD pattern of 2

vs. relative intensity.

0.530

Grain size,

2

, deg

2.0

3.0

4.5

5.5

7.5

40 50 60 70 80 90 100 110 120

1.01.5

2.5

3.54.0

5.0

7.06.56.0

ExposedUn-exposed

Fig. 7.

XRD pattern of 2

vs. grain size.

0.8

30

d

-Spacing,

2

, deg

1.0

1.4

1.6

2.0

2.4

40 50 60 70 80 90 100 110 120

1.2

1.8

2.2

Un-exposedExposed

Fig. 8.

XRD pattern of 2

vs.

d

-spacing.

Trend: Exponential decay of third order

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LASER IRRADIATION EFFECTS ON GOLD 1387

change occurs in the (hkl) values used to verify theseeffects.

Thus, we can say that the intensity changes theredistribution of the grain orientation and adegradationof crystalline quality has occurred after laser irradiationof the gold samples.

CONCLUSIONSThe dynamics of the gold-plasma plume concludes

that, initially, the plume is spherical, but, later, it issharpened, indicating the forward peaking of theejected species. At earlier stages, plume expansion islinear and obeys the shock model, whereas, at laterstages, it is nonlinear and obeys the drag model. Mor-phological studies show that heat conduction is nonuni-form and nonsymmetric. The SEM micrograph indi-cates a cone formation and micron-sized particulatesare produced by repetitive laser shots. Hydrodynamicsputtering, splashing, and redeposition are observedalong with the neutrals of gold. The XRD patternsexhibited a variation in the intensity of the diffractedpeaks due to laser irradiation. The intensity variationobeys a Gaussian fit and a polynomial fit for the unex-

posed samples, whereas Lorentz_B fits for the plots ofthe exposed sample. The grain size decreases monoton-ically. Negligible effects are produced by laser irradia-tion on the

d-spacing and this follows the exponentialdecay of the third order. There is no change in the [hkl]values.

ACKNOWLEDGMENTSThe authors acknowledge the Higher Education

Commission of Pakistan for sponsoring this PhDproject. Also, the authors acknowledge the Governmentof Punjab, Education Department, for granting studyleave to K.A. Bhatti in order to pursue PhD studies.

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Table 1. Peak list of unexposed gold

Peak no. h k l d, 2, deg I, % FWHM, degGrainsize, Phase

1 1 1 1 2.35404 38.231 100 = x 0.295 4.843 Gold2 2.20778 40.876 4 Palladium3 2 0 0 2.03941 44.422 24.64 0.295 4.943 Gold4 2 2 0 1.44157 64.659 27.38 0.394 4.054 Gold5 3 1 1 1.22934 77.645 27.59 0.394 4.398 Gold6 2 2 2 1.17787 81.766 20.63 0.492 3.629 Gold7 4 0 0 1.02029 98.157 9.90 0.394 3.484 Gold8 3 3 1 0.93635 110.842 8.14 0.394 6.037 Gold9 4 2 0 0.91198 115.269 11.38 0.360 7.005 Gold

Table 2. Peak list of exposed gold

Peak no. h k l d, 2, deg I, % FWHM, degGrainsize, Phase

1 1 1 1 2.3385 38.497 100 = x 0.394 3.629 Gold2 2.20111 41.005 4.4082 Palladium3 2 0 0 2.02963 44.648 34.3165 0.590 2.473 Gold4 2 2 0 1.43701 64.890 18.4369 0.590 2.711 Gold5 3 1 1 1.22714 77.841 32.9474 0.590 2.94 Gold6 2 2 2 1.1750 82.001 7.5918 0.590 3.031 Gold7 4 0 0 1.01933 98.282 3.9451 0.787 2.622 Gold8 3 3 1 0.9350 111.081 22.4486 0.590 4.04 Gold9 4 2 0 0.9108 115.492 22.5706 0.960 0.720 Gold

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