Fabrication of gold nanostructures through pulsed laser interference patterningDajun Yuan, Ranadip Acharya, and Suman Das

Citation: Applied Physics Letters 103, 223101 (2013); doi: 10.1063/1.4833548 View online: http://dx.doi.org/10.1063/1.4833548 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/103/22?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Laser nanostructuring of polymers: Ripples and applications AIP Conf. Proc. 1464, 372 (2012); 10.1063/1.4739891 GaAs nanostructuring by self-organized stencil mask ion lithography J. Appl. Phys. 110, 114321 (2011); 10.1063/1.3665693 Technology platform for the fabrication of titanium nanostructures J. Vac. Sci. Technol. B 29, 06FG06 (2011); 10.1116/1.3657517 Angular effects of nanostructure-covered femtosecond laser induced periodic surface structures on metals J. Appl. Phys. 108, 073523 (2010); 10.1063/1.3487934 Patterning of nanostructured thin films by structured light illumination Appl. Phys. Lett. 87, 143103 (2005); 10.1063/1.2061857

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

130.207.50.37 On: Mon, 25 Aug 2014 16:30:02

Fabrication of gold nanostructures through pulsed laser interferencepatterning

Dajun Yuan,1,a),b) Ranadip Acharya,1,b),c) and Suman Das1,2,d)

1Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, USA2School of Materials Science & Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, USA

(Received 5 July 2013; accepted 10 November 2013; published online 25 November 2013)

In this Letter, we report on the experimental development and computational modeling of a simple,

one-step method for the fabrication of diverse 2D and 3D periodic nanostructures derived from

gold films on silicon substrates and over areas spanning 1 cm2. These nanostructures can be

patterned on films of thickness ranging from 50 nm to 500 nm with pulsed interfering laser beams.

A finite volume-based inhomogeneous multiphase model of the process shows reasonable

agreement with the experimentally obtained topographies and provides insights on the flow physics

including normal and radial expansion that results in peeling of film from the substrate. VC 2013AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4833548]

Gold nanostructures have been extensively studied.1

Several important discoveries have been revealed recently,

such as large enhancement of nonlinear optical phenomena,2

surface-enhanced Raman scattering,3 and cell probe.4 A

number of approaches such as E-beam lithography,5 laser

direct-writing,6 nanosphere lithography,7 and nanoimprint

lithography8 have demonstrated successful fabrication of

gold nanostructures. However, they are constrained either by

high cost, low throughput, or limited ability to produce high

aspect ratio nanostructures over large areas relative to nano-

structure dimensions (>1 cm2). Laser interference patterning

(LIP) makes use of the interference of two or more

high-power pulsed laser beams, and involves just a

single-step process. This technique allows the production of

periodic structures with a well-defined long-range order in

length scales ranging from a quarter of the wavelength to

tens of micrometers, simply through the adjustment of the

angles between interfering beams. The structuring process is

based on photo-thermal, photo-physical, or photo-chemical

mechanisms, depending on the type of the material.

A schematic of the experimental setup and the film

structure is shown in Figs. 1(a) and 1(b). Single-shot laser

interference patterning was conducted using a frequency-

tripled, pulsed Nd:YAG laser (Quanta-Ray PRO 290,

Spectra Physics) operating at a wavelength of 355 nm, and

with 10 ns pulse width, 750 mJ average energy per pulse, and

10 mm beam diameter. The 10 mm diameter beam from the

laser was expanded to 30 mm diameter through beam expan-

sion optics. An iris aperture was utilized to spatially filter the

beam down to 20 mm diameter to produce a fluence variation

of less than 15% over the irradiation area. The detailed setup

is described elsewhere.9 Characterization of the fabricated

nanostructures through atomic force microscopy (AFM) and

scanning electron microscopy (SEM) reveals the probable

mechanisms of their formation through pulsed laser-induced

melting and re-solidification, in combination with surface

tension driven effects.

Various 2D nanostructures can be produced on gold

films of thickness less than 50 nm. With two-beam interfer-

ence patterning, gratings can be patterned with different peri-

ods and feature sizes by simply adjusting the angle between

laser beams and the laser fluence. As shown in Fig. 2(a),

with 37.7� angle between two laser beams, gratings with a

period of 550 nm can be achieved over 4 cm2 area. Upon

adjusting the angle between two laser beams to 10.2�, the

pattern period becomes 2 lm, as shown in Fig. 2(b) and upon

adjusting the angle to 17.2�, the pattern period becomes

1.2 lm, as shown in Fig. 2(c).

Hexagonal close-packed arrays of cavities and holes

with feature sizes ranging from 100 nm to 2.5 lm can be cre-

ated with three interfering beams.9 As shown in Fig. 3, with

different angles between laser beams, patterns with period of

(a) 600 nm, (b) 2 lm, and (c) 400 nm can be fabricated. With

the same period, different feature sizes can be achieved by

simply adjusting the laser fluence.

FIG. 1. The setup for two-beam and three-beam LIP, and the sample prepa-

ration. (a) (2) is a 10% reflection beam splitter for monitoring laser power,

(3) is a high threshold power meter, (4) is a mechanical shutter, (1, 5, 7, 9,

10) are reflection mirrors, (6) is a 67% reflection beam splitter, (8) is a 50%

reflection beam splitter, and (11) is a sample holder. (b) Adhesion layers of

titanium with 5 nm to 10 nm thickness and gold films of thickness from

20 nm to 400 nm are deposited by e-beam evaporation on silicon substrates.

a)Email: dajun.yuan@gmail.comb)D. Yuan and R. Acharya contributed equally to this work.c)Email: racharya@gatech.edud)Author to whom correspondence should be addressed. Electronic mail:

sumandas@gatech.edu. Tel.: 404-385-6027.

0003-6951/2013/103(22)/223101/5/$30.00 VC 2013 AIP Publishing LLC103, 223101-1

APPLIED PHYSICS LETTERS 103, 223101 (2013)

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

130.207.50.37 On: Mon, 25 Aug 2014 16:30:02

As the thickness of gold film increases to more than

200 nm, different kinds of 3D structures can be fabricated

over large area. The laser fluence is calculated as follows:

laser fluence (J/cm2)¼ average laser power (w) x exposure

time (s)/the exposure area (cm2). With relatively low laser

fluence, 300 mJ/cm2, micro-dome shape structures can be

formed, as shown in Fig. 4(a). When the laser fluence is

increased to 500 mJ/cm2, micro-mushroom shape structures

can be formed, as shown in Fig. 4(b). The total height of the

structure is around 1.6 lm, the diameter of the ball on the top

is around 500 nm, and the diameter of the neck connecting

the ball to the base is around 200 nm. Millions of

micro-mushroom structures can be formed over 1 cm2 area

with 2.5 lm periodic spacing. When the laser fluence is

increased to 700 mJ/cm2, the stem is pinched off in the

molten state and the sub-micrometer size balls detach from

the base, resulting in sharp needle-shaped structures, as

shown in Fig. 4(c). The diameter of the bottoms of the tips is

around 100 nm, at the tops of the tips is around 10 nm, and

the needle height is around 400 nm. An intermediate stage is

formed with laser fluence of 605 mJ/cm2. Due to slight local

variations in the fluence, some of the sub-micrometer balls

are detached due to stem pinch-off while some of them are

still connected to the stem and base, as shown in Fig. 4(d).

The flow physics of such structure formation processes

has evoked a lot of interest lately. Since noble metals exist

much longer in the molten state, Marangoni convection plays

an important role10 in the associated flow due to laser melt-

ing. The surface tension coefficient decreases with increasing

temperature. The higher surface tension present in colder

regions exerts a pull on the liquid in hotter regions.

Therefore, liquid convection will take place from regions of

higher temperature to regions of lower temperature. The

pressure of evaporation of the solute film is thought to play a

dominant role in the peeling of the film and the formation of

the shell-like structure.11 A model of elasto-plastic flow

combined with the two-temperature model (TTM) in a two-

dimensional approximation has been developed to describe

microbump and nanojet formation.12 Recently, a molecular

dynamics (MD) simulation coupled with TTM model has

been applied to Ni films.13 The MD-TTM simulation result

indicates that nano-bump formation is a four-stage process

comprising of compressive stress generation, peeling of film,

normal and radial expansion. However, the effect of plastic

deformation and pressure of the vaporized film was not

observed in the MD simulation. Nevertheless, the TTM-MD

simulation still required 128 processors and about 1000 h of

calculation for a 250 ps long simulation.13

FIG. 2. 2D gold nanostructures with periods of (a) 550 nm (b) 2 lm, and

(c) 1.2 lm fabricated with two-beam LIP.

FIG. 3. 2D gold nanostructures with periods of (a) 600 nm (b) 2 lm, and

(c) 400 nm fabricated with three-beam LIP.

FIG. 4. 3D gold nanostructures with periods of 2.5 lm fabricated with

three-beam LIP (a) for laser power¼ 300 mJ/cm2, (b) for laser

power¼ 500 mJ/cm2, (c) for laser power¼ 700 mJ/cm2, and (d) for laser

power¼ 605 mJ/cm2.

223101-2 Yuan, Acharya, and Das Appl. Phys. Lett. 103, 223101 (2013)

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

130.207.50.37 On: Mon, 25 Aug 2014 16:30:02

Nanostructure formation driven by the hydrodynamic

effect in laser irradiation processes has been simulated using

finite difference approaches in earlier works.14,15 However,

these approaches were limited to the consideration of hydro-

dynamic and surface tension forces only. In our work, a com-

plete inhomogeneous multiphase model is solved and the

effect of interfacial drag between air and the liquefied metal

is modeled. Furthermore, the effect of the substrate is incor-

porated using a conjugate heat transfer model. The Navier-

Stokes equation is employed with the surface tension force

modeled as volume force term concentrated at the interface16

to investigate the formation of nano-bumps on gold films

due to interaction with laser beam. The key physics that have

been modeled include: (a) phase change phenomenon

(melting and solidification), (b) interfacial drag force

between liquid gold and air, (c) surface tension between liq-

uid gold and air, (d) buoyancy (due to density differences

between the different phases), and (e) conjugate heat transfer

between the gold film and the Si Substrate. In the model, the

following assumptions were made: (1) superheating of the

liquid gold film is not implemented and is treated as if it is in

isothermal condition.14 (2) The liquid gold film is modeled

as dispersed phase with diameter of 10 nm. The air and solid

gold are modeled as continuous phases. (3) The flow is lami-

nar. (4) Thermal radiation is not included. (5) The thermal

interaction between the atmosphere and the gold film is not

included. (6) Property values are constant.15 The surface ten-

sion value is averaged over the temperature range.17

The computational fluid dynamics (CFD) model is

solved in ANSYS CFX to predict the topography of the

nanostructure formed. For the 0.2 lm film, a domain size of

11 lm is chosen in the z-direction to ensure little/no bound-

ary interaction. This domain extent size is sufficient to

accommodate the nano-structure formed. A 1 lm thick Si

substrate is modeled in the z-direction. Due to symmetry,

2.5 lm of the film region in the x and y directions is mod-

eled, which corresponds to one wavelength of the laser inten-

sity profile. The entire simulation is solved in an Intel Xeon

workstation and took 85 h of calculation to solve for 20 ns

long simulation while running on 2 processors.

The continuity equation18 for each phase is solved. The

momentum equation is solved for each phase a. The interfa-

cial drag between air and liquid gold is modeled using parti-

cle model and the drag coefficient is obtained using the

Schiller-Naumann19 model. The drag force between air and

solid gold is modeled using mixture model and a drag coeffi-

cient of 0.01 is assumed. However, one solidification sink in

the momentum model is used to keep the solid gold velocity

at a zero value.

For each phase present, the thermal energy equation

is solved.18 The two-resistance model is used at the solid

gold-liquid gold interface to model the interphase mass

transfer. For the liquid side, infinite fluid specific heat trans-

fer coefficient or zero resistance is used while for the solid

side, the Ranz-Marshall20 correlation is used. Fig. 5 shows

the mesh of the multi-domain model with boundary condi-

tions imposed on them. Here, the gravity vector is oriented

in the negative z-direction.

The entire set of equations along with the necessary

boundary conditions is solved in ANSYS CFX, a finite element-

based finite volume CFD software. The solidification sink

and the transient source term are implemented through CFX

Expression Language (CEL). The flow in each case is simu-

lated for 20 ns with a time step of 10�11 s, while the source

is operational for 10 ns.

The CFD result matches reasonably well with the MD

simulation results. In Fig. 6, red color indicates higher value

of gold mass fraction and blue color indicates lower gold and

higher air mass fraction. The velocity vector at the top is ori-

ented primarily in the y-direction in Figs. 6(a) and 6(b) that

explains the normal expansion of the film. In Figs. 6(c) and

6(d), the mass fraction plot with the velocity vector is pre-

sented at t¼ 8 and t¼ 16 ns. The velocity vector clearly

shows trend of radial expansion.

The quantification of the velocity vector is done on the

clipped gold mass fraction profile as shown in Fig. 7(a). The ve-

locity ranges observed show a reasonable match with prior

FIG. 5. Model mesh showing different boundary and interfaces.

FIG. 6. Velocity vector overlapped on the mass fraction contour of gold for

(a) t¼ 0.2 ns, (b) t¼ 0.8 ns, (c) t¼ 8 ns, and (d) t¼ 16 ns.

FIG. 7. (a) Velocity contour on clipped gold mass fraction profile, Volume

fraction plot showing (b) peeling of the film at t¼ 4 ns, and (c) shell like

structure formation at t¼ 16 ns.

223101-3 Yuan, Acharya, and Das Appl. Phys. Lett. 103, 223101 (2013)

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

130.207.50.37 On: Mon, 25 Aug 2014 16:30:02

literature.13 Since density of gold is much higher than surround-

ing air, the mass fraction contour alone cannot provide informa-

tion about the hollow nano-structure and the peeling of the

film. Hence the volume fraction contour is drawn for t¼ 4 ns

and t¼ 16 ns in Figs. 7(b) and 7(c). In Fig. 7(c), the presence of

air in the film region explains the peeling of the film from the

substrate and the formation of a hollow nano-structure.

The lowest laser fluence (300 mJ/cm2) resulted in the for-

mation of micro-domes on the film with diameter ranging from

500 nm to 3 lm and height ranging from 50 to 200 nm. An in-

termediate laser fluence (500 mJ/cm2) resulted in the formation

of mushroom structures with diameter ranging from 200 nm to

700 nm and height ranging from 1 to 2 lm. The highest laser

fluence (700 mJ/cm2) allowed a portion of the liquid metal to

separate during recoil and pinch-off, and consequently, sharp

conical peaks were formed with base diameters in the range

100–300 nm, tip heights in the range 1–2.5 lm, and tip diame-

ters around 10 nm. The simulated topography is presented in

Fig. 8. As shown, the final shape of the micro-dome,

mushroom, and micro-needle structure matches well with the

experimentally obtained nano-structure in Figs. 4(a)–4(c),

respectively. Additional simulation till 40 ns shows presence

of liquid Au in the structure but the topography did not show

any significant variation.

The CFD model also allows us to view the systematic

evolution of the nano-needle formation and the pinch-off

effect caused due to the surface tension. The effect of air

drag and the effect of laser power density are taken into con-

sideration. Fig. 4(c) shows the needle structure obtained with

higher power density and the separated droplet. Fig. 9 shows

the systematic growth of the needle and separation phenom-

enon as predicted by simulation. Previous literature on

pulsed laser irradiation of metal films also revealed the hol-

low shell-like structure.11,13,21 Similar observations can be

found for each case simulated where air entrapment is

observed inside the metal layer.

In conclusion, the LIP technique allows the production

of periodic structures with a well-defined long-range order in

length scales ranging from a quarter of the wavelength to tens

of micrometers, simply through the adjustment of the angles

between interfering beams. In the current work, a finite vol-

ume model is presented to predict the evolution of the experi-

mentally obtained nanostructures. The model solves the

inhomogeneous multiphase problem considering the effect of

the interfacial air drag and the conjugate heat loss to the sub-

strate and hence, it is more general in nature compared to the

MD simulation. The effect of surface tension and air drag is

studied in terms of the resultant nanostructure formation. The

velocity, mass fraction, and volume fraction contours explain

the normal and radial expansion of the gold film and subse-

quent peeling from the substrate, which is consistent with the

predictions of the much more computationally expensive MD

simulation.13 The separation phenomenon observed at higher

laser power is explained by the pinch-off effect and topo-

graphical match between the experimental results and simu-

lated mass fraction contours are obtained. The CFD model

thus considers the effect of atmosphere for the process, takes

significantly less time to solve and predicts physical phenom-

ena consistent with the atomistic simulation.

1C. M. Cobley, J. Chen, E. C. Cho, L. V. Wang, and Y. Xia, Chem. Soc.

Rev. 40(1), 44 (2011).2D. C. Marinica, A. K. Kazansky, P. Nordlander, J. Aizpurua, and A. G.

Borisov, Nano Lett. 12(3), 1333 (2012); W. Husinsky, A. Ajami, P.

Nekvindova, B. Svecova, J. Pesicka, and M. Janecek, Opt. Commun.

285(10), 2729 (2012); P. Genevet, J. P. Tetienne, E. Gatzogiannis, R.

Blanchard, M. A. Kats, M. O. Scully, and F. Capasso, Nano Lett. 10(12),

4880 (2010).3R. A. Alvarez-Puebla, A. Agarwal, P. Manna, B. P. Khanal, P.

Aldeanueva-Potel, E. Carb�o-Argibay, N. Pazos-P�erez, L. Vigderman, E.

R. Zubarev, N. A. Kotov, and L. M. Liz-Marz�an, Proc. Natl. Acad. Sci.

U.S.A. 108(20), 8157 (2011); K. L. Wustholz, A. Henry, J. M. McMahon,

R. G. Freeman, N. Valley, M. E. Piotti, M. J. Natan, G. C. Schatz, and R.

P. Van Duyne, J. Am. Chem. Soc. 132(31), 10903 (2010); E. C. Dreaden,

R. D. Near, T. Abdallah, M. H. Talaat, and M. A. El-Sayed, Appl. Phys.

Lett. 98, 183115 (2011).4S. D. Brown, P. Nativo, J. Smith, D. Stirling, P. R. Edwards, B.

Venugopal, D. J. Flint, J. A. Plumb, D. Graham, and N. J. Wheate, J. Am.

Chem. Soc. 132(13), 4678 (2010); G. Wang, X. Zheng, P. P. Shum, C. Li,

H. Ho, and L. Tong, presented at the 9th International Conference onOptical Communications and Networks (ICOCN 2010), Nanjing, China,

2010.

FIG. 8. Simulated topography of (a) micro-dome structure, (b) mushroom

structure, and (c) micro-needle structure.

FIG. 9. Growth of needle-like structure and separation (Multimedia view).

[URL: http://dx.doi.org/10.1063/1.4833548.1]

223101-4 Yuan, Acharya, and Das Appl. Phys. Lett. 103, 223101 (2013)

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

130.207.50.37 On: Mon, 25 Aug 2014 16:30:02

5W. Yue, Z. Wang, Y. Yang, L. Chen, A. Syed, K. Wong, and X. Wang,

J. Micromech. Microeng. 22(12), 125007 (2012); Y. Lin, Y. Zou, Y. Mo,

J. Guo, and R. G. Lindquist, Sensors 10(10), 9397 (2010).6G. J. Leggett, ACS Nano 5(3), 1575 (2011); Y. Yang, D. Lin, and G. J.

Cheng, Appl. Phys. Lett. 101, 091911 (2012).7Z. A. Lewicka, W. Y. William, and V. L. Colvin, paper presented at theSPIE NanoScienceþEngineering, San Diego, 2011; D. Lerose, M.

Bechelany, L. Philippe, J. Michler, and S. Christiansen, J. Cryst. Growth

312(20), 2887 (2010).8C. Liang, M. Liao, W. Chen, T. Cheng, W. Chang, and C. Lin, Opt.

Express 19(5), 4768 (2011); G. Barbillon, Micromachines 3(1), 21 (2012).9D. Yuan, W. Lin, R. Guo, C. P. Wong, and S. Das, Nanotechnology

23(21), 215303 (2012); D. Yuan and S. Das, paper presented at the MRSFall Meeting, Symposium FF, Boston, 2011; A. Lasagni, D. Yuan, J. L.

Hendricks, D. C. Martin, and S. Das, Nanotechnology 23(1), 015304 (2012).10F. Korte, J. Koch, and B. N. Chichkov, Appl. Phys. A 79(4), 879 (2004).11Y. Nakata, T. Okada, and M. Maeda, Jpn. J. Appl. Phys., Part 2 42(12A),

L1452 (2003).

12Y. P. Meshcheryakov and N. M. Bulgakova, Appl. Phys. A 82(2), 363

(2006).13D. Ivanov, B. Rethfeld, G. O’Connor, T. Glynn, A. Volkov, and L.

Zhigilei, Appl. Phys. A 92(4), 791 (2008).14G. B. N. Seifert, Appl. Surf. Sci. 133, 189 (1998).15N. Seifert, G. Betz, and W. Husinsky, Appl. Surf. Sci. 103, 63 (1996).16J. U. Brackbill, D. B. Kothe, and C. Zemach, J. Comput. Phys. 100, 335

(1992).17P. Burn, A. Holmes, A. Kraft, D. Bradley, A. Brown, R. Friend, and R.

Gymer, Nature 356, 47 (1992).18INC ANSYS, in ANSYS CFX Release (ANSYS, INC, Canonsburg, 2010),

Vol. 11, pp. 69; S. V. Patankar, Numerical Heat Transfer and Fluid Flow(Taylor & Francis Group, New York, 1980).

19L. Schiller and A. Naumann, Vdi Ztg. 77, 318 (1935).20W. E. Ranz and W. R. Marshall, Chem. Eng. Prog. 48(3), 141 (1952).21A. Kuznetsov, J. Koch, and B. Chichkov, Appl. Phys. A 94(2), 221 (2009);

J. P. Moening, D. G. Georgiev, and J. G. Lawrence, J. Appl. Phys. 109(1),

014304 (2011).

223101-5 Yuan, Acharya, and Das Appl. Phys. Lett. 103, 223101 (2013)

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

130.207.50.37 On: Mon, 25 Aug 2014 16:30:02