IOP PUBLISHING NANOTECHNOLOGY

Nanotechnology 23 (2012) 015304 (7pp) doi:10.1088/0957-4484/23/1/015304

Patterning of periodic nano-cavities onPEDOT–PSS using nanosphere-assistednear-field optical enhancement and laserinterference lithographyDajun Yuan1, Andres Lasagni2, Jeffrey L Hendricks3,David C Martin4 and Suman Das1

1 Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta,GA 30332, USA2 Fraunhofer Institute for Material and Beam Technology, Winterbergstraße 28,Dresden 01277, Germany3 Biotectix LLC, 940 North Main Street, Ann Arbor, MI 48104, USA4 Department of Materials Science and Engineering, University of Delaware, Newark,DE 19716, USA

E-mail: sumandas@gatech.edu

Received 26 August 2011, in final form 17 October 2011Published 8 December 2011Online at stacks.iop.org/Nano/23/015304

AbstractA simple approach for creating periodic nano-cavities and periodic stripes of nano-cavity arrayson poly (3,4-ethylene dioxythiophene)–poly(styrenesulfonate) (PEDOT–PSS) thin films using acombination of optical near-field enhancement through self-assembled silica nanospheres andlaser interference lithography is presented. Monolayers of close-packed silica nanospheres(800, 600, and 430 nm in diameter) are self-assembled on 2 μm thick PEDOT–PSSelectropolymerized films and are subsequently irradiated with 10 ns pulses of 355 nmwavelength laser light. Over areas spanning 2 cm2, circular nano-cavities with central holes ofsize 50–200 nm and surrounding craters of size 100–400 nm are formed in the PEDOT–PSSfilms directly underneath the nanospheres due to strong enhancement (11–18 fold) of theincident light in the near-field, which is confirmed through Mie scattering theory. Predictionsfrom theoretical simulations examining the combined effects of near-field enhancement andinterference are in good agreement with the experimental results. The results illustrate theversatility of the described technique for creating nano-cavity arrays or nano-pores inPEDOT–PSS over large areas with designed periodicity and hole size.

(Some figures may appear in colour only in the online journal)

1. Introduction

Poly(3,4-ethylene dioxythiophene)–poly(styrenesulfonate)(PEDOT–PSS) is an electrochemically stable conjugated poly-mer with high electrical conductivity and moderate trans-parency [1–3]. Integrated circuits based on PEDOT–PSS couldopen the market of low-end, high-volume microelectronicswith application areas of transparent conductors in electrolumi-nescent devices [4], conducting layers in capacitors [5], photo-voltaics [6], and sensors [7], With both electronic and ionic

conductivity, PEDOT–PSS has been recently considered forbioelectronic applications, especially neuronal cell signalingand neural interfaces [8–10].

Porous nanostructures in PEDOT–PSS films play animportant role in improving their performance or in extendingtheir functionality. PEDOT–PSS films with porous structuresare used for sensing nitric oxide (NO) gas [11]. Porosity at bothmicro- and nanometer scales in PEDOT films used as electrodematerials for supercapacitors has enabled the achievementof the highest electrode-specific capacitance amongst all

0957-4484/12/015304+07$33.00 © 2012 IOP Publishing Ltd Printed in the UK & the USA1

Nanotechnology 23 (2012) 015304 D Yuan et al

reported materials [12]. Introduction of nanoporous structureson conjugated polymers has enabled the modification ofsurface wettability [13], and the enlargement of surface area,resulting in improved performance of polymer photovoltaicdevices [14].

Various methods have previously been explored to createnanoporous structures on PEDOT–PSS films and can bedivided into reactive and non-reactive processes [15, 16].Reactive methods involve radiation to locally initiate achemical reaction in a polymer or precursor-polymerfilm [17, 18], while non-reactive methods involve lithographictechniques to pattern a polymer in its final form without therequirement of further chemical or thermal treatments [16, 19].The difficulties encountered with both the above types ofmethods include low efficiency due to long fabrication timesfor large areas and/or low resolution due to the opticaldiffraction limit. Nanopattern transfer through a combinationof nanosphere self-assembly and reactive ion etching (RIE) canproduce nanostructures in a massively parallel manner [20, 21].However, the requirement of a cleanroom environment canbe cost prohibitive and thus limits the applications. Forindustrial applications, this semiconducting polymer needs tobe patterned using methods that do not add great cost todevice fabrication. Laser-induced nanopatterning with near-field energy enhancement using self-assembled nanospherescan overcome the previously mentioned difficulties through itssimplicity and its capability of massively parallel nano-scalesurface patterning.

In this work, the techniques of laser interference lithog-raphy and nanosphere-assisted near-field optical enhancementare, for the first time, combined with periodic nano-cavitiesand periodic stripes of nano-cavity arrays on PEDOT–PSSfilms. The electromagnetic field surrounding the nanospheresis simulated using Mie scattering theory. The theory predictsstrong enhancement of the optical field by nanospheres inthe near-field region, which enables production of nano-scale features. The experimental results of both singlebeam patterning and laser interference patterning confirm thesimulation results.

2. Experimental details

2.1. Materials

Thin films of poly(3,4-ethylene dioxythiophene)–poly(styrenesulfonate) (PEDOT–PSS) are used as substrates forpatterning. Gold–palladium thin films (100 nm) aresputtered on tin doped indium oxide (ITO) substrates tofacilitate electrochemical deposition and to improve adhesionof conducting polymer films. Subsequently, PEDOT–PSSconducting polymers are electrochemically deposited from asolution of PSS (0.2%, w/v) and ethylene dioxythiophene(EDOT) (0.1%, w/v) in deionized water. The electrochemicalpolymerization [22, 23] is performed in a conventional three-electrode cell. A platinum foil is used as the counterelectrode, a saturated calomel electrode is used as the referenceelectrode, and the probe sites are used as the workingelectrode. The polymerization is carried out galvanostaticallyby applying a current density of 0.75 mA cm−2 using a

Figure 1. The laser interference patterning setup. (1) A 10%reflection beam splitter for monitoring laser power, (2) a highthreshold power meter, (3) a mechanical shutter, (4, 6, 7, 8) arereflection mirrors, (5) a 50/50 beam splitter, and (9) a sample holder.

potentiostat/galvanostat (AutoLab PGStat 12, Eco Chemie,Netherlands). PEDOT–PSS films of different thicknesses arefabricated by varying the deposition time, and thus the totaldeposition charge. Films with thickness of 2000 nm are usedin the experiments described here.

Suspensions comprising 10% wt silica nanospheres inwater (430, 600, and 800 nm diameter, Bangs LaboratoriesInc.) are used for self-assembly. The suspensions are modifiedwith a surfactant, triton-X:methanol = 1:400 (Sigma-Aldrich),in a 2:1 volume ratio to assist the solution in wetting thesubstrate.

2.2. Self-assembly of a silica nanosphere monolayer

Spin coating is a simple and efficient way to achieve well-packed monolayers over areas of 1–2 cm2 [24]. Hexagonallyclosed-packed monolayers of silica nanospheres on PEDOT–PSS surfaces are obtained by spin coating the modifiedsuspensions. The spin speed is varied from 2000 to 2500 rpmin order to obtain a homogeneous monolayer of different sizesof nanospheres.

2.3. Laser-induced near-field patterning

A commercial Nd:YAG laser (Quanta-Ray PRO 290, SpectraPhysics) is used as the light source for the patterningexperiments. The samples are irradiated by the laser at 355 nmwavelength with a pulse width of 10 ns and repetition rate of10 Hz. The 5 mm diameter beam from the laser is expandedto 40 mm diameter through beam expansion optics, allowingpatterning over areas of approximately 2 cm2.

Figure 1 shows a schematic of the laser interferencepatterning setup, which is used for both single beam patterningand interference patterning experiments. The p-polarized laserbeam is split into two beams by a 50/50 beam splitter (5) andthen recombined at a rotation stage (9) for holding sampleswith a designed interference angle (α). The laser power canbe adjusted through the controller of the laser and is monitoredby a high damage threshold power meter (2) together with a10% reflection beam splitter (1). The exposure time (and thusthe number of laser pulses irradiating the sample) is controlledby a mechanical shutter (3) (Uniblitz Electronic VS25S2ZMO)with a temporal resolution of 3.0 ms.

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Nanotechnology 23 (2012) 015304 D Yuan et al

For single beam patterning, the samples are loadedbetween a reflection mirror (4) and beam splitter (5) (notshown in the figure). For two-beam interference patterning,the samples are loaded on the precision rotation stage (9) andexposed to the pulsed interference pattern. All the experimentsare performed in an ambient atmosphere environment.

2.4. Sample characterization

The patterned samples are imaged using a high-resolutionscanning electron microscope (SEM) at an operating voltageof 10 kV (LEO 1530 thermally assisted field emissionmicroscope). All samples are coated with a 2 nmgold/palladium layer to improve the electrical conductivity,prior to imaging.

3. Theoretical calculations

Mie theory, also called Lorenz–Mie theory or Lorenz–Mie–Debye theory is a complete analytical solution of Maxwell’sequations for the scattering of incident electromagneticradiation by spherical particles [25]. The electric and magneticfields inside a sphere satisfy the vector Helmholtz equation:

k2E = 0, �H + k2H = 0 (1)

where k is the wavevector in vacuum, so ks is the wavevector inthe silica sphere. � is a 3D Laplacian. In spherical coordinates,the scattered electric and magnetic field components can bedescribed as follows [26]:

Er = cosφ

(ksr)2

∞∑

l=1

l(l + 1)e Blζl(ksr)P(1)l (cos θ),

Eθ = −cosφ

ksr

∞∑

l=1

[e Blζ

′l (ksr)P

(1)′l (cos θ) sin θ

− im Blζl(ksr)P(1)

l (cos θ)

sin θ

],

Eφ = − sinφ

ksr

∞∑

l=1

[e Blζ

′l (kr)

P(1)l (cos θ)

sin θ

− im Blζl(kr)P(1)′l (cos θ) sin θ

],

Hr =√ε sinφ

(ksr)2

∞∑

l=1

l(l + 1)m Blζl(ksr)P(1)l (cos θ),

Hθ = isinφ

kr

∞∑

l=1

[e Blζ

′l (ksr)

P(1)l (cos θ)

sin θ

+ im Blζ′l (ksr)P

(1)′l (cos θ) sin θ

],

Hφ = −icosφ

kr

∞∑

l=1

[e Blζl(ksr)P

(1)′l (cos θ) sin θ

+ im Blζ′l (ksr)

P(1)l (cos θ)

sin θ

],

(2)

where ζl(x) = ρh(1)l (x), h(1)l (x), is the spherical Hankelfunction, ζ ′

l (x) = ∂ζl(x)∂x . Pm

n (x) are Legendre polynomials,

Pm′n (x) = ∂Pm

n (x)∂x , ε is the dielectric permittivity, and

e Bl = il+1 2l + 1

l(l + 1)

ψ ′l (ksa)ψl(ka)− mψl(ksa)ψ ′

l (ka)

ψ ′l (ksa)ζl(ka)− mψl(ksa)ζ ′

l (ka),

m Bl = il+1 2l + 1

l(l + 1)

mψ ′l (ksa)ψl(ka)− ψl(ksa)ψ ′

l (ka)

mψ ′l (ksa)ζl(ka)− ψl(ksa)ζ ′

l (ka)

(3)

where ψl(ρ) =√πρ

2 Jl+ 12(ρ), Jl+ 1

2(ρ) is the Bessel function

of the first kind, and a is the radius of the sphere. The time-averaged Poynting vector, S, which is the energy flux of anelectromagnetic field, is defined as:

S = I0 Re[E × H∗] (4)

where I0 is the input intensity and H∗ is the complex conjugateof H. In spherical coordinates, the components of the time-averaged Poynting vector are given by:

Sr = I0 Re[Eθ H ∗φ − EφH ∗

θ ]Sθ = I0 Re[EφH ∗

r − Er H ∗φ ]

Sφ = I0 Re[Er H ∗θ − Eθ H ∗

r ].(5)

In Cartesian coordinates, the electromagnetic wave propagatesalong the z direction. The z-component of the Poynting vectoris the optical radiation after near-field enhancement by thesphere and is given by:

Sz = cos θ Sr − sin θ Sθ . (6)

For combined laser interference and near-field enhancement,the input intensity, which is not a constant, can be calculatedunder the assumption of plane waves. The total electric field ofthe interference pattern can be obtained by the superposition ofeach individual beam as follows [27]:

E =n∑

j=1

E j0[cos(−k sinα j (x cos β j − y sinβ j))

+ i sin(−k sinα j (x cos β j − y sin β j))] (7)

where α and β are the angles of the beams with respect tothe vertical and the horizontal axis of the interference planerespectively.

The spatial intensity distribution is given by:

I = cε

2|E |2 (8)

where c is the speed of light.For two-beam interference, the intensity distribution,

which is a line-like pattern, is given by [28, 29]:

I = 2Iinput cos2

(kx sin

(α

2

))(9)

where Iinput is the input intensity and α is the angle between theinterfering laser beams. The interference period is determined

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Nanotechnology 23 (2012) 015304 D Yuan et al

Figure 2. SEM image of the silica sphere monolayer with diameter of800 nm made with the spin-coating method. The scale bar is 1 μm.

by the wavelength λ and the angle α, and is given by theequation:

P = λ

2 sin(α/2). (10)

By substituting I0 in equation (5) with I in equation (9),the z-component of the Poynting vector, Sz , resulting from thecombination of laser interference and near-field enhancementcan be described as:

Sz = 2Iinput cos2

(kx sin

(α

2

))[cos θ Re(Eθ H ∗

φ − EφH ∗θ )

− sin θ Re(EφH ∗r − Er H ∗

φ )] (11)

4. Results and discussion

The creation of monolayers of self-assembled nanospheresthrough spin coating is simple and fast. To achieve well-

packed monolayers over substrate surface areas of 1–2 cm2,the spinning speed is a critical parameter which is determinedby the surface roughness of the substrate. Figure 2 shows ascanning electron micrograph of a self-assembled monolayerof 800 nm silica nanospheres on the PEDOT–PSS film,obtained with a spinning speed of 2200 rpm. In this case,97% of the entire substrate (1.5 cm2 area) is covered by a well-ordered monolayer mixed randomly with a small number ofislands of multiple layers or particle clusters. Monolayers ofsilica spheres with diameters of 600 nm and 430 nm can also beproduced by this method with spinning speeds of 2300 rpm and2500 rpm, respectively. The samples are ready for patterningafter 2 min of spin coating.

The intensity distribution at the interface between thesilica nanospheres and a PEDOT–PSS film obtained througha single incident beam can be calculated from equation (5).Figure 3(a) shows the calculated intensity distributions withsphere diameters of 800, 600, and 430 nm for laser wavelengthof 355 nm and corresponding silica refractive index of 1.48.A strong focusing effect can be observed, with the maximumintensity in the center being enhanced by 18, 13, and 11times the incident intensity, respectively. These results predictthat for a given laser wavelength, the near-field intensityenhancement is directly proportional to nanosphere diameter.Figure 3(b) shows the SEM image of periodic nano-cavitystructures formed by laser-induced near-field patterning with800 nm silica spheres. The inset figure, shown at highermagnification, reveals that the diameter of the cavity centersis about 150 nm. The seven cavities comprise a hexagon,with one cavity in the center, corresponding to the hexagonalpacking arrangement of the self-assembled nanospheres. Thecenter-to-center distance between two adjacent cavities is thediameter of the silica nanosphere used for near-field patterning.The entire surface of 1.5 cm2 area is patterned with periodic

Figure 3. Single laser beam irradiation on the self-assembled nanospheres with different diameters. (a) The calculated intensity distributionson the interface between nanospheres and PEDOT–PSS film with difference nanosphere diameters. (b) SEM image of periodic nanodentstructures formed by laser-induced near-field patterning of 800 nm silica spheres on a PEDOT–PSS thin film with a fluence of 30 mJ cm−2.Scale bar is 2 μm and 500 nm for inset. (c)–(e) are SEM images of nanodent structures patterned with 800, 600, and 430 nm silica spheresand 50 mJ cm−2 laser fluence. Scale bars are 1 μm.

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Nanotechnology 23 (2012) 015304 D Yuan et al

Figure 4. (a)–(c) are SEM images of nanodent structures patterned with 800 nm silica spheres and laser fluence of 50 mJ cm−2, 35 mJ cm−2,and 20 mJ cm−2 respectively. (d)–(f) are SEM images of nanodent structures patterned with 600 nm silica spheres and laser fluence of50 mJ cm−2, 35 mJ cm−2, and 20 mJ cm−2 respectively. Scale bars are 500 nm.

Figure 5. Laser interference irradiation on the self-assembled nanospheres with different diameters. (a) and (b) are simulated intensity andexperimental results of interference patterning on 800 nm self-assembled silica spheres (laser fluence is 30 mJ cm−2). (c) and (d) are simulatedintensity and experimental results of interference patterning on 600 nm silica spheres (laser fluence is 30 mJ cm−2). (e) and (f) are simulatedintensity and experimental results of interference patterning on 430 nm silica spheres (laser fluence is 50 mJ cm−2). Scale bars are 2 μm.

nanostructures using one pulse of 30 mJ cm−2 exposure doselasting 10 ns. Considering the near-field intensity enhancementfrom the nanospheres, the effective maximum input laserfluence is 540 mJ cm−2. The reported threshold damagefluence for PEDOT–PSS films thicker than 820 nm is 255 ±19 mJ cm−2 [30]. Simple calculations from the intensity plotdepicted in figure 3(a) allow the prediction of the diameter ofthe regions surpassing this threshold to be about 150–180 nm,which is in good agreement with the experimentally obtaineddiameter of the formed cavities (∼150 nm, figure 3(b)).

Moreover, the conical shape of the cavities can be predictedfrom the intensity distribution and the widely accepted formof Beer’s law: d(F) = 1/αeff ln(F/Fth). The ablationdepth (d(F)) is thus a function of the fluence (F), and thethreshold fluence (Fth), and the effective absorption coefficient(αeff) [31]. All of the nanospheres in the patterned area wereejected simultaneously due to the vapor pressure generatedfrom the ablation of PEDOT–PSS underneath each nanosphere.Figures 3(c)–(e) show the nano-cavity structures patternedwith the same laser fluence (50 mJ cm−2) but using self-

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Nanotechnology 23 (2012) 015304 D Yuan et al

Figure 6. Laser interference irradiation of self-assembled 800 nm spheres with different laser fluences. (a) and (b) are simulated intensity andexperimental results of the input fluence of 30 mJ cm−2. (c) and (d) are simulated intensity and experimental results of the input fluence of10 mJ cm−2. Scale bars are 10 μm and scale bars for insets are 1 μm.

assembled nanospheres of different sizes: 800 nm, 600 nm, and430 nm, respectively. As the center-to-center distance betweentwo adjacent cavities is determined by the size of the silicananosphere, smaller diameters of self-assembled nanospheresresult in a higher areal density of nano-cavity structures.

Additionally, to further confirm the simulation results,nano-cavity structures are created with different laser fluencesusing silica nanospheres of diameter 800 and 600 nm. Thediameter of the formed nano-cavities decreases with decreasinglaser fluence. Figures 4(a)–(c) show the nano-cavity structurespatterned using 800 nm silica spheres with laser fluencesof 50 mJ cm−2, 35 mJ cm−2, and 20 mJ cm−2 respectively;figures 4(d)–(f) show the nano-cavity structures patternedusing 600 nm silica nanospheres and with the same laserfluences. Even though the laser fluences utilized for thesamples shown in figures 4(c) and (f) are identical, dueto the different levels of intensity enhancement in 800 and600 nm nanospheres, the effective maximum input laserfluences are 360 mJ cm−2 for figure 4(c) and 260 mJ cm−2 forfigure 4(f). While nano-cavity structures are clearly observablein figure 4(c), the patterns can barely be seen in figure 4(f).This indicates that the laser fluence should be very close tothe threshold fluence, which matches the previous result verywell [30].

The intensity distribution of two interfering laser beamsis given by equation (9). When this periodic line-like patternis used instead of a single laser beam to irradiate the self-assembled monolayer of nanospheres, the near-field enhancedintensity distribution is given by equation (11). This type ofintensity distribution can create periodic stripes of nano-cavityarrays on the PEDOT–PSS film. Figures 5(a), (c), and (e) showthe simulated intensity distributions of interference patterningon 800, 600, and 430 nm self-assembled silica spheres, andfigures 5(b), (d), and (e) show the corresponding experimentalresults. The angle between the laser beams is approximately4◦, resulting in a period of 5.1μm (equation (10)). Six to seven

spheres of 800 nm diameter can fit within one period dependingon the orientation between the self-assembled spheres and theline-like interference pattern. Similarly, eight to nine spheresof 600 nm diameter and eleven to twelve spheres of 430 nmdiameter can fit within one period. However, the number ofspheres that are involved in patterning depends on the inputfluence. For example, only four to five spheres are involvedin patterning in figure 5(b). One to two spheres remain onthe substrate since the laser fluence at their locations is belowthe damage threshold even after the near-field enhancement atthat location. In figure 5(d), five to six spheres of 600 nmin diameter are involved in patterning leaving three to fourspheres on the substrate. Similarly, seven to eight spheres of430 nm in diameter are involved in patterning, while four tofive spheres remain on the substrate.

Figure 6 shows both the simulated intensity distributionand experimental results of laser interference irradiation on theself-assembled 800 nm spheres with different laser fluences.Taking into account that the ablation threshold fluence for thePEDOT–PSS is 255 ± 19 mJ cm−2 (indicated by the horizontalline in figures 6(a) and (c)), for a 30 mJ cm−2 input fluence,three to five nanospheres should be involved in patterning, withone to two nanospheres remaining on the PEDOT–PSS surface(figures 6(a) and (b)). For an input fluence of 10 mJ cm−2,one to three spheres are involved in the patterning process(figures 6(c)–(d)) with four to five remaining on the surface.Experimental results match the simulations very well, showingthat the width of the patterned areas can be precisely controlledby controlling the laser fluence.

5. Summary and conclusions

Analytical and experimental investigations on the featurescreated on PEDOT–PSS thin films by the irradiation ofnanospheres on the substrate surface with a 355 nm wavelengthpulsed laser are presented. Monolayers of silica particles with

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Nanotechnology 23 (2012) 015304 D Yuan et al

different diameters (430, 600, and 800 nm) are successfullyself-assembled on PEDOT–PSS films by spin coating atrotation speeds between 2200 and 2500 rpm. Nano-cavity structures with central holes of size 50–200 nm andsurrounding craters of size 100–400 nm are formed in thePEDOT–PSS films with a single shot exposure, removing thenanospheres totally even at low laser fluences (<20 mJ cm−2).The features indicate an enhancement of the incident intensityin the near-field due to the presence of the spheres. Theintensity is calculated for a single sphere on a substrate usingMie theory. From the calculations at a laser wavelength of355 nm, the intensity enhancement is 18, 13, and 11 timesfor particle diameters of 800 nm, 600 nm, and 430 nm,respectively. Periodic stripes of nano-cavity arrays arealso patterned through a combination of laser interferencepatterning and laser-induced near-field enhancement. Theobtained results demonstrate the feasibility of integrating bothtechnologies for the rapid fabrication of complex nano-scalegeometries.

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