Laser fabrication of 2D and 3D metal nanoparticle ... - OSA Publishing

Laser fabrication of 2D and 3D metal nanoparticle structures and arrays A. I. Kuznetsov,* R. Kiyan, and B. N. Chichkov Laser Zentrum Hannover e.V., Hollerithalle 8, 30419 Hannover, Germany *[email protected]

Abstract: A novel method for fabrication of 2D and 3D metal nanoparticle structures and arrays is proposed. This technique is based on laser-induced transfer of molten metal nanodroplets from thin metal films. Metal nanoparticles are produced by solidification of these nanodroplets. The size of the transferred nanoparticles can be controllably changed in the range from 180 nm to 1500 nm. Several examples of complex 2D and 3D microstructures generated form gold nanoparticles are demonstrated. ©2010 Optical Society of America OCIS codes: (140.3390) Laser materials processing; (220.4241) Nanostructure fabrication; (240.0310) Thin films; (160.3900) Metals.

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Received 30 Jun 2010; revised 28 Jul 2010; accepted 3 Sep 2010; published 22 Sep 2010

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1. Introduction Structures consisting of metal nanoparticles play an important role in modern nanophotonics. A number of nanophotonic devices based on nanoparticles have been theoretically suggested: nanolenses [1,2], “spasers” [3,4], metamaterials with negative refraction in the visible spectral range [5], etc. However, existing technical difficulties in the fabrication of nanoparticle structures significantly limit their practical realization and exploitation. “Spaser” effect has been recently observed on a single core-shell nanoparticle obtained from a solution of nanoparticles prepared by a chemical way [6]. Metamaterials with negative effective permittivity and permeability in the visible spectral range [7] and sensors [8] based on regular arrays of metallic nanoparticles have been recently demonstrated by lithography. Among existing approaches for the fabrication of nanophotonic devices based on metal nanoparticles, only electron and ion beam lithography can provide enough precision and structuring resolution [7–11]. Laser-based methods are usually limited to structures with sizes around 100 nm and were applied for example to fabrication of plasmonic components [12,13]. Other approaches are only able to generate random nanoparticle structures or ordered one dimensional (1D) or two dimensional (2D) arrays through self-assembly [14–20]. One of examples of the latter approach is nanosphere lithography which demonstrated its ability for fabrication of ordered 2D arrays of nanoparticles with different sizes and shape [18–20]. In this case, however, introduction of defects inside the structure or fabrication of structures from particles with different individual properties is not possible. Moreover, the shape of particles fabricated by lithographic approaches is usually close to a half sphere or a disk-like structure. Fabrication of spherical particles, which are required for many nanooptical devices, is problematic by lithographic methods. Among other methods for the fabrication of 2D arrays of metal nanostructures one should mention nanoimprint lithography [21] and electrochemical growth [22], which are relatively inexpensive and promising for plasmonic and sensing applications. Three dimensional (3D) metallic-dielectric structures can be realized only by multi-step lithographic processes which are very complicated, expensive, and time consuming [23,24]. In this paper, a novel method for fabrication of 2D and 3D microstructures consisting of metal micro- and nanoparticles is developed. This technique is based on the laser-induced transfer (LIT) of molten metal nanodroplets recently proposed by our and several other groups [25–30]. Here we show that the LIT process allows fabrication of spherical and spheroidal metal particles with controlled sizes down to 180 nm and their precise positioning in densely

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packed 2D and 3D structures. It is demonstrated that there are no principal limitations on a minimal distance between the LIT fabricated particles. Each new particle can be put in a close proximity to the previous one without any distortion of its position. This technique provides unique possibilities for realization of novel nanophotonic components and metamaterials. 2. Experimental details In our experiments, we use a commercial 1 kHz femtosecond laser system (Tsunami + Spitfire, Spectra Physics) delivering 3 mJ, 30 fs laser pulses at a central wavelength of 800 nm. Gold films with a thickness of 10 to 60 nm, coated onto glass or quartz glass substrates, are fabricated at the Laser Zentrum Hannover (LZH) using Auto 306 Evaporation System (Edwards) or by Layertec GmbH using magnetron sputtering. Possible experimental arrangements are shown in Fig. 1. Depending on experimental conditions, femtosecond laser pulses irradiate the front or the back side of a gold film (donor substrate). Another (receiver) substrate is placed close to the gold coated sample at a distance of ∆z < 10 µm. Droplets are formed and ejected from the surface of a thin gold film towards the receiver substrate due to laser-induced thermal expansion of the molten layer affected by strong surface tension forces. Ejected droplets are attached to the receiver substrate. The two possible process configurations: laser-induced forward [LIFT, Fig. 1(a)] or backward [LIBT, Fig. 1(b)] transfer of gold droplets are further referred to as laser-induced transfer (LIT). Laser-induced processes, leading to the ejection of a liquid droplet from a gold film surface, have been studied in our previous publications [25,26,31]. The generated metal structures on both donor and receiver substrates are analyzed by Scanning Electron Microscopy (SEM). Our investigations show that mechanisms of droplet ejection from metal films thinner than 60 nm are very similar in both LIFT and LIBT configurations. The only difference is in the threshold pulse fluence required for the droplet ejection, which is about 50% higher in case of LIBT configuration.

Fig. 1. (a) Laser-induced forward transfer (LIFT) scheme. (b) Laser-induced backward transfer (LIBT) scheme.

3. Results and discussion Single-pulse femtosecond laser irradiation of a thin gold film induces melting of the irradiated area and formation of a jet-like structure with a droplet on its top [Fig. 2(a)]. The jet height and shape depend on the laser fluence [25]. In this experiment, 30 fs laser beam with the Gaussian intensity profile and diameter of 8 mm was focused onto the front surface of a 60 nm gold film by a 20 mm focus lens. The structure shown in Fig. 2(a) is produced at laser pulse energy of 70 nJ. At laser pulse energies higher than a certain threshold value (in this case ≥75 nJ), the droplet is ejected from the jet-like structure in the direction perpendicular to the film surface. In these experimental conditions, transfer of the liquid gold droplet onto a receiver substrate can be realized. After the transfer process, the gold droplets assume a precise spherical shape due to high surface tension of the molten metal and after solidification form metal particles on the receiver substrate.

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Fig. 2. (a) Side-view SEM image of a jet-like structure fabricated on the surface of a 60 nm gold film by a single 30 fs laser pulse with Gaussian intensity profile. Laser beam with a diameter of 8 mm is focused onto the sample surface by a 20 mm focus lens. Laser pulse energy is 70 nJ. (b) Side-view SEM image of an array of 800 nm spherical gold particles transferred onto a glass substrate by subsequent 30 fs laser pulses in the LIBT configuration. The focusing conditions are the same as in (a). Laser pulse energy is 75 nJ. (c) Side-view SEM image of a 180 nm gold particle transferred in the LIFT configuration from a 20 nm gold film using 100 × oil-immersed microscope objective (NA 1.4). Laser pulse energy is 3.3 nJ. All images are taken at an angle of 45°.

Moving from pulse to pulse the laser focus position over the sample surface allows fabrication of different structures consisting of spherical gold particles on the receiver substrate [Fig. 2(b)]. The size of transferred droplets depends on the volume of laser-molten material and can be varied by changing laser focusing conditions and the gold film thickness. Transfer of particles with the sizes between 220 nm and 1500 nm has been recently demonstrated by our group [25]. The smallest particle size has been achieved using a 0.8 NA microscope objective for laser beam focusing. Here, it is shown that even smaller particles can be controllably transferred by LIT using immersion oil optics with higher numerical aperture. An oil-immersion objective with 1.4 NA (Zeiss, Plan APOCHROMAT 100 × , working distance is of 170 µm) has been applied to focus laser beam onto a gold film surface in the LIFT configuration [Fig. 1(a)]. For this purpose a gold film with 20 nm thickness has been evaporated onto a thin (150 µm) glass substrate. A drop of immersion oil has been placed onto the top side of the substrate on the opposite side relative to the gold film [Fig. 1(a)]. The laser beam was focused through the substrate onto the gold film surface. In this configuration, the beam focus size of a few hundreds of nanometers on the sample surface was realized. Figure 2(c) demonstrates an SEM image of a particle controllably transferred from a 20 nm gold film at laser pulse energy of 3.3 nJ. It has a spheroidal shape with sizes 180nm × 180nm × 140nm, which are significantly smaller than previously reported values [25,28]. In this experiment, the receiver substrate has been covered with a 60 nm gold film. Presence of this additional metal film significantly improves attachment of the transferred particles to the receiver substrate. However, in this case, the particle shape changes from spherical to spheroidal one (see discussions further in this paper). In typical LIT experiments, the donor and receiver substrates are positioned at the same mechanical stages. However, in case of LIT-induced generation of nanodroplets, this significantly limits the minimum distance between the LIT-generated nanoparticles. In this case, the minimum distance is determined by the size of a laser-molten area on a sample surface, which is larger than the generated particle size (for example, a 800 nm gold particle is generated from a 3 µm laser-molten area of a 60 nm thin gold film [25]). For possible applications of this approach in plasmonics and metamaterials it is necessary to decrease the distance between the neighbour particles down to several or even few hundreds of nanometers in order to assure strong interparticle interaction. This can be realized if the donor and receiver substrates are positioned independently on different mechanical stages. In this case, #130957 - $15.00 USD

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the donor substrate can be shifted between the laser pulses and every new pulse can irradiate a fresh sample surface. The minimum distance between the neighbouring particles is then determined by the resolution of the positioning system used in the experiment and can be below 100 nm. Examples of structures fabricated from gold particles are shown in Fig. 3. In this experiment, each particle was transferred onto a glass substrate by a single 30 fs laser pulse with the Gaussian intensity profile in the LIFT configuration. Laser beam with a diameter of 8 mm was focused onto the sample surface by a 20 mm focus lens. Laser pulse energy was 50 nJ. These examples demonstrate abilities of the LIT approach to fabricate densely packed 2D structures. One can see that there are no principal limitations on a minimal distance between the LIT fabricated particles. Each new particle can be put in a close proximity to the previous one without any distortion of its position. It is remarkable that all particles have a spherical shape and similar sizes.

Fig. 3. Side-view SEM images of gold particle structures with interparticle distance of 1.5 µm (a)&(b) and 1µm (c)&(d). Each particle is transferred by a single 30 fs laser pulse with the Gaussian intensity profile in the LIFT configuration. Laser beam with a diameter of 8 mm is focused onto the sample surface by a 20 mm focus lens. Laser pulse energy is 50 nJ. Between the laser pulses, the donor substrate is shifted relative to the receiver substrate. All images are taken at an angle of 45°.

By independent positioning of donor and receiver substrates on different mechanical stages one can place LIT-generated particles on top of each other. This can be applied for the fabrication of 3D structures consisting of metal particles. Typical examples of structures formed on a glass receiver substrate by ejection of two gold droplets on top of each other are shown in Fig. 4. Each droplet is ejected under the same experimental conditions as in Fig. 3. It can be seen that the second droplet is splashed on the top of the first droplet. This process strongly depends on the heat conductivity of the receiver substrate, which affects the solidification time of the molten droplets. In this case, the splashing of the droplet happens before the solidification. This can be avoided if the receiver substrate has significantly higher heat conductivity. Figure 5 shows examples of structures fabricated on a glass receiver substrate covered by a 60 nm gold film. Each particle was transferred by a single 30 fs laser pulse with 35 nJ pulse energy. Laser focusing conditions were the same as in Fig. 3. As can be seen in Fig. 5, presence of a metal coating on the receiver substrate results in fast cooling of transferred droplets and their solidification before the splashing. Similar results are obtained when other substrates with high thermal conductivity, such as silicon or sapphire, are used as receivers. These examples demonstrate that the LIT approach can be applied to the fabrication of 2D and 3D structures consisting of metal particles as elementary building blocks.

Fig. 4. Side-view SEM images of gold structures formed on a glass receiver surface in case when a second gold particle is ejected on top of a first one. Laser irradiation conditions are the same as in Fig. 3. The donor substrate is shifted relative to the receiver substrate between the laser pulses. All images are taken at an angle of 45°.

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Fig. 5. Side view SEM images of gold particle structures fabricated on a glass receiver substrate covered with a 60 nm gold film. The particles are ejected on top of each other by subsequent 30 fs laser pulses with the Gaussian intensity profile. The donor substrate is shifted relative to the receiver substrate between the laser pulses. Laser beam with a diameter of 8 mm is focused onto the sample surface by a 20 mm focus lens. Laser pulse energy is 35 nJ. All images are taken at an angle of 45°.

Particles shown in Fig. 5 have the oblate spheroid shape with the aspect ratio of about 1.17. This small deviation from the spherical shape can be explained by the impact of liquid droplets onto the receiver surface and their fast cooling and solidification. The solidification proceeds faster than the surface tension can restore a perfect spherical shape. Results presented in this paper are obtained for gold particles. However, our investigations show that similar structures can also be fabricated from silver and copper particles. In general, it can be expected that this approach can be applied to all materials, which could be melted by laser irradiation. 4. Conclusion In conclusion, a novel method for fabrication of 2D and 3D microstructures consisting of metal micro- and nanoparticles has been demonstrated. This technique is based on laserinduced forward or backward transfer of molten metal nanodroplets ejected from thin donor metal films towards a receiver substrate. The transferred droplets attach to the receiver substrate and solidify producing particles of spherical or spheroidal shape. The size of the generated particles depends on the laser focusing conditions and metal film thickness and can be varied in the range of 180 to 1500 nm. It is shown that there are no principal limitations on a minimal distance between the LIT fabricated particles. Each new particle can be put in a close proximity to the previous one without any distortion of its position. Together with the use of receiver substrates with high heat conductivity this allowed the demonstration of complex 3D nanoparticle structures directly written by lasers for the first time. Particles can be produced from different donor materials, which can be melted by laser irradiation. This technique provides unique possibilities for realization of novel nanophotonic components and metamaterials. Acknowledgments The authors acknowledge financial support from the Schwerpunktprogramm SPP1391 “Ultrafast Nanooptics” of the Deutsche Forschungsgemeinschaft (DFG) and the Centre for Quantum Engineering and Space-Time Research (QUEST).

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