Photothermal-reaction-assisted two-photon lithography of silvernanocrystals capped with thermally cleavable ligands

Won Jin Kim, Xavier Vidal, Alexander Baev, Hong Sub Jee, Mark T. Swihart, andParas N. Prasada�

Department of Chemistry, Institute for Lasers, Photonics, and Biophotonics, University at Buffalo,The State University of New York, Buffalo, New York 14260, USA

�Received 21 January 2011; accepted 19 February 2011; published online 30 March 2011�

We report an alternative approach to produce micropatterns of metallic nanoparticles usingphotothermal-reaction-assisted two-photon direct laser writing. The patterns are achieved using afacile surface treatment of silver nanoparticles �Ag NPs� functionalized with thermally cleavableligands; N-�tert-butoxycarbonyl�-L-cysteine methyl ester. The ligand cleavage initiated by pulsedlaser-induced thermal reaction results in a significant change in dispersiblility of the nanocrystals,thereby enabling a solvent-selective development process after photopatterning. We demonstratedthat Ag NP patterns with submicron linewidths can be achieved using near infrared pulsed laserillumination. © 2011 American Institute of Physics. �doi:10.1063/1.3565245�

Metallic nanoparticles �MNPs� have attracted much at-tention due to their unique light-matter interactions.1–7 Exci-tation of the surface plasmon band of MNPs by ultrafast laserpulses is known to induce strong local heating.8–10 Moreover,“hot” electrons generated under ultrafast laser excitation canexist in a nonequilibrium state �i.e., non-Fermi distribution�within the lattice of MNPs. Thermal relaxation processes arebelieved to proceed as follows. Initially, the MNPs absorbthe light, then the electron gas thermalizes internally byelectron–electron coupling to produce a hot electron distri-bution. The temperature of the electron distribution can bevery high because of the small value of the heat capacity ofelectrons. The hot electrons then equilibrate with the latticeby electron–phonon coupling which is reportedly the mostdominant mechanism for external heat conversion byMNP.11–14 Ahmadi et al., for example, reported that the sur-face plasmon band absorbance of gold nanoparticles isbleached under ultrafast laser illumination that results inelectron–electron and electron–phonon coupling. The absor-bance then recovers on a picosecond time scale. Above acertain threshold of laser fluence, however, gold nanopar-ticles start melting and ablation occurs.15 Interestingly, a“hot” non-Fermi electron energy distribution within Agnanoparticles can also be generated by two-photon laser ex-citation at 800 nm.14 The heat released by electron–phononrelaxation following this excitation can be used to initiatephotothermal reactions of the ligands surrounding MNP, pro-viding a basis for photopatterning the MNPs for direct-writedevice fabrication.

Our study is focused on two-photon lithography ofMNPs assisted by hot-electron-induced photothermal reac-tion to achieve subwavelength laser writing using a near in-frared �NIR� laser. To demonstrate this approach, we devel-oped photopatternable materials that are composed of MNPand thermally cleavable functional ligands. An example of athermally cleavable group is tert-butoxycarbonyl �known ast-BOC� which possesses not only short alkyl groups but alsosolubilizing moieties that enable low cost solution-basedprocessing.16–18 We have selected a t-BOC containing ligand

for the MNP, so that the thermally cleavable groups can beremoved by photothermal reaction to produce gas-phasebyproducts.

Here, we demonstrate that thermal relaxation of hot elec-trons can lead to thermal cleavage of functional ligands onthe MNP assemblies. In this technique, two-photon absorp-tion of the closely packed silver NP film leads to local heat-ing and subsequent thermal cleavage of the solubilizing por-tion of the functional ligands on their surface. Unexposedregions remain dispersible and can be removed by simplewashing while irradiated regions remain on the substrate�Scheme 1�. The photothermal reaction-assisted two-photonlithography offers a pathway toward fabricating subwave-length structures without extrinsic additives. The shortligands that remain after removal of the solubilizing t-BOCgroups provide densely packed MNPs in solid state films byefficiently decreasing the interparticle gap.

Silver nanoparticles �Ag NPs� were prepared by a typicalsolvothermal method.19 The dodecanoic acid or other in situgenerated ligands from that synthesis were then replacedwith N-�tert-butoxycarbonyl�-L-cysteine methyl ester �Ald-rich, 97%� by a simple ligand exchange. This thiol-terminated ligand binds more strongly to the silver surfacethan carboxylic acids or amines, making ligand exchangestraightforward.18 Note that N-�tert-butoxycarbonyl�-L-cysteine methyl ester protected Ag NPs, and its depro-tected form of Ag NPs are denoted as t-BOC Ag NPs andt-BOC deprotected Ag NPs, respectively �Scheme 1�. Thet-BOC Ag NPs were characterized by absorption spectros-copy and transmission electron microscopy �TEM� as shownin Fig. 1. The TEM image of the Ag NPs shows a sphericalshape with a mean diameter of 9.3�0.9 nm. The absorptionspectrum �Fig. 1� shows a distinct surface plasmon resonanceband at 410 nm in chloroform. However, this plasmonic ab-sorption is shifted toward the red and the band is broadenedin solid particle films. This is a common behavior of MNPsdue to strong near-field interactions between NPs.20,21 In-deed, these closely packed MNPs assemblies offer efficientlight trapping which may enhance the photopatterning of sil-ver nanoparticles using NIR laser.

Absorbance measurement showed that the Ag NP filmsamples do not have noticeable plasmonic absorption beyond

a�Author to whom correspondence should be addressed. Electronic mail:pnprasad@buffalo.edu.

APPLIED PHYSICS LETTERS 98, 133110 �2011�

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700 nm. Thus, a NIR �800 nm� writing wavelength was se-lected to achieve two-photon lithography. As a proof-of-concept experiment, a top-down approach was used to pat-tern the Ag NP film �Scheme 1�. To photo-pattern the filmsby two-photon lithography, the t-BOC protected Ag NPswere dispersed in chloroform �50 mg/ml�, and spin coatedonto a glass plate to produce a thin film ��150 nm thick-ness, measured using a profilometer�. The as-prepared filmwas then irradiated with an 800 nm mode-locked Ti:sapphirelaser �Mira 900 from Coherent� pumped by a diode-pumpedsolid state laser �Verdi-V10, Coherent�, with 130 fs pulses ata repetition rate of 76 MHz. The laser beam was introducedinto an inverted optical microscope �Nikon Eclipse TE 200�and tightly focused onto the Ag NP film through the objec-tive lens. The spin-coated film was scanned in the�x ,y�-plane in the focus of the beam by a computer-controlled piezo stage. Finally, the written patterns were im-mersion developed �4:1 chloroform:ethanol by volume�.

Figure 2 shows examples of patterns produced withvarying laser input power and numerical aperture �NA� ob-

jectives under the same scanning speed �0.05 �m /ms�. Sev-eral conditions were tested by varying the laser power �from1 to 35 mW� with a fixed scanning speed in order to inves-tigate laser power-dependent correlations. Subsequently, theNA of objective lens was varied systematically to obtain thebest image resolution. The scanning speed of 0.05 �m /mswith 1 mW input power level was sufficient to obtain thepatterned film, and larger NA �1.0 in this study� was found toproduce the optimal result. Patterns with features as small as500 nm can be readily defined by this process. Scanningelectron microscope �SEM� images show resulting represen-tative patterns of Ag NPs after lithography �Fig. 2�a�� andelemental energy dispersive x-ray spectroscopy �EDS� ofpatterned Ag NPs also clearly reveals a distinct peak ataround 3 keV due to the K� fluorescence x-rays of Ag NPs�Fig. 2�c��.

The Ag NP pattern was also imaged via dark-field scat-tering of incoherent light. The dark-field images were re-corded using an upright Nikon Eclipse 800 microscope witha high numerical dark-field condenser �NA 1.20–1.43, oilimmersion� under white light �tungsten lamp�, and resultingdark-field images were captured using a QImaging Micro-publisher 3.3 RTV color camera. A 0.5� lens was placed infront of the camera to provide a larger field of view. Imageacquisition was conducted using the QCAPTURE softwarefrom the camera manufacturer. A representative image is dis-played in Fig. 2�b�, showing that the Ag NPs remain only inthe laser-exposed areas while the unexposed areas arewashed out completely by the developing process.

SCHEME 1. Schematic illustration of photothermal reaction-assisted two-photon lithography using surface functionalized silver nanoparticles.

FIG. 1. �Color online� Absorption spectra of a t-BOC Ag NP film �solidline� and of a t-BOC Ag NP dispersion in chloroform �dashed line�. Theinset displays a TEM image of Ag NPs; the scale bar indicates 10 nm.

FIG. 2. �Color online� SEM image of two-photon patterned Ag NP filmsproduced �a� using NA1.0 lens with 13 mW power, with 30 �m scale barand �b� its dark-field image with 10 �m scale car. �c� EDS spectrum of thefishnet structure, which confirmed the existence of silver NPs. Inset displaysSEM image of 0.5 �m lines. �d� Relationship between the laser power andlinewidth of the patterned Ag NPs under different objective lenses. �e� SEMimages showing dependence of the morphology of patterned Ag lines oninput power.

133110-2 Kim et al. Appl. Phys. Lett. 98, 133110 �2011�

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As shown in Fig. 2�d�, linewidth increases in proportionto the input power, suggesting that higher laser power leadsto the photothermal reaction occurring over a wider area. Asa result, both objective lenses �NA 1.0 and NA 0.8� exhibitstrong input-power-dependence linewidth tunability for theAg NPs patterns with slopes of 0.294 �NA 0.8�, and 0.238�NA 1.0�, respectively. In case of NA 1.0 objective lens, forexample, linewidth is gradually reduced from 2.8 to 0.5 �mwith the decrease in the input power. The NA 0.8 objectivelens shows the same trend, with wider linewidth. On theother hand, when the particles were exposed to a higherpower, melting was observed. Figure 2�e� shows that the filmmorphology varied with laser input power at a fixed scanningspeed of 0.05 �m /ms. As shown in Fig. 2�e�, Ag NPs startmelting and create large spheroidal silver clusters at above20 mW input power. Koda and co-workers have reported thatslow heat release of the deposited laser energy into the sur-roundings is responsible for the melting of the MNPs.22 Forthe same reason, these Ag NPs are also melted by two-photon absorption from the NIR laser light even when thepeak fluence is not so high. The threshold fluence for meltingof the Ag NPs by two-photon lithography was estimated tobe 35 mJ /cm2, based on the pulse energy, duration, and fo-cal spot size.

Fourier transform infrared �FT-IR� spectroscopy wasused to confirm the chemical structure change on the surfaceof the Ag nanoparticles �Fig. 3�a��. The FT-IR studies wereconducted at room temperature. A sodium chloride �NaCl�disk was used as a blank where the sample of dispersed AgNPs in chloroform was drop-cast. After recording the IRspectra of t-BOC protected Ag NPs, the film was exposed tothe laser using the same method as described for the photo-lithography to obtain t-BOC deprotected Ag NPs. Figure 3�b�displays representative IR spectra, before and after deprotec-tion. Significant changes are clearly observed between thespectrum of the t-BOC protected Ag NPs and the spectrumof the deprotected Ag NPs. Specifically, the strong carbonylstretching peak at 1702 cm−1 and methyl stretching peak at

2975 cm−1 are drastically decreased after deprotection of thet-BOC moiety, suggesting that the t-BOC groups are selec-tively deprotected using near-IR laser through the photother-mal reaction. It is clear that the FT-IR results agree well withpreviously mentioned photothermal reaction of Ag NPs as-semblies. As a result, these studies verified that the thermalrelaxation of hot electrons leads to thermal cleavage of func-tional ligands on the Ag NP assemblies in the given condi-tions.

In summary, an approach is presented to pattern silvernanoparticles using photothermal-reaction-assisted two-photon lithography with submicron resolution. We havedemonstrated that the low energy �NIR� excitation leads tophotothermal cleavage of a t-BOC group from the ligandsattached on Ag NPs, thereby allowing them to form a sub-wavelength NP pattern. The simple and straightforwardlithographic approach demonstrated here with silver nano-particles should be useful in the fabrication of MNP-basedoptoelectronic devices and is extendable to other metallicNPs. Key advantages of two-photon lithography over com-peting approaches include the ability to write arbitrary pat-terns without use of masks, with subwavelength resolution,and the ability to pattern three-dimensional structures.

This work was supported in part by a grant from theAir Force Office of Scientific Research �Grant No.FA95500610398�.

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FIG. 3. �Color online� �a� Schematic of photothermal reaction, and �b� rep-resentative IR spectra of t-BOC protected �circle� and deprotected �triangle�silver NPs using 800 nm pulsed laser �13 mW�.

133110-3 Kim et al. Appl. Phys. Lett. 98, 133110 �2011�

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