Pulsed laser deposition of plasmonic nanostructured gold on flexible transparent polymers at atmospheric pressure

The ablations were performed utilizing a picosecond-pulsed diode-pumped Wedge HF 1064 Nd:YAG laser (BrightSolutions, Italy). Specifications of the laser system used are outlined in table 1. The experimental setup is a simplified form of conventional PLD whereby a transparent substrate is placed either in direct contact with a metallic target or with a spacer in between the target and substrate (figure 1), with the ablation performed under ambient atmospheric conditions. For the results presented here, the target-to-substrate distance was 50 µm with the substrate fixed in place mechanically for the duration of processing.

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For the experiments performed in this study, the laser beam was focused to a spot diameter of 140 µm and unidirectionally rastered using an 2D galvanometer (Raylase SS-12) at a rate of 10 mm s⁻¹ across the target, mapping out a 5  ×  5 mm area in parallel lines with a 50 µm step between each successive scan. The galvanometer was fitted with an ƒ-theta lens with an effective focal length of 108 mm. The current experimental setup allows for depositions over areas up to 100 cm² but can be easily increased through alteration of the optical setup of the laser. To note, this process is differentiated from the similar technique LIRT by the use of longer laser pulse widths allowing for increased plume/laser interaction.

The target material was 99.9% metals basis gold foil (Agar Scientific, UK) with a thickness of 100 µm. The substrate material was 188 µm thickness ZeonorFilm ZF14-188 (Zeon Chemical L.P., Japan) cyclic olefin polymer (COP). The laser beam is incident on the target though the substrate, with the resultant ablation plume travelling towards the substrate. This offers promising opportunity to rapidly deposit nanostructured materials on transparent thin substrates, with enhanced adaptability. The use of flexible substrates also allows for the process to be adapted for high volume roll-to-roll production.

The metrology of the deposited nanomaterial was analysed using scanning electron microscopy (SEM) with an Evo LS50 (Zeiss, Switzerland). The samples were carbon coated to ensure conductivity prior to analysis with an Scancoat Six (Edwards, United Kingdom). Changes to the optical properties of the COP after deposition was examined using a Cary 50 UV–Vis–NIR spectrometer (Agilent, USA) scanning from 200–1100 nm at a rate of 600 nm min⁻¹. Attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR), which has a sampling depth up to 2 µm, was used to examine changes in the surface chemistry of the substrates after deposition using a Spectrum 100 (PerkinElmer, USA). Four scans were taken over 4000–600 cm⁻¹ at resolution of 1 cm⁻¹. The crystalline structure of the AuNPs was investigated using x-ray diffraction (XRD) using an AXS D8 x-ray diffractometer (Brucker, Germany).

Figure 2 shows SEM images of gold nanoparticles (AuNPs) deposited on the COP substrates via CAP at laser fluences of 0.22 J cm⁻² and 0.48 J cm⁻². For the lower fluence depositions (figure 2(a)), a nanoparticle film was evident along with discreet nanostructured meso- and microparticle sized clusters. At higher fluence depositions (figure 2(b)), the nanomaterial film was seen to be more uniform with less micron-sized aggregated clusters than for lower fluence depositions.

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As these larger nanostructured aggregate particles were less evident at the higher fluence depositions they are therefore likely a result of in-plume aggregation due to the colder plasma temperatures at lower ablation fluences [17]. Target fragmentation was ruled out as the mechanism for the microstructure formation, as at higher fluences larger ablated particulates would have sufficient kinetic energy to reach the substrate and would have been more preferentially deposited as fluence increased. As seen for low-pressure confined PLD, the spatial confinement of the plume, by placing the substrate in close proximity of the target, allows for a high plasma temperatures which drives the formation of nanomaterial deposition at the substrate surface [16, 18]. Through varying the target-to-substrate distance and ablation fluence it may be possible to further reduce the size of the deposited nanoparticles.

As of yet, the upper limit of film thicknesses deposited via the CAP technique has not been found though work on gold films deposited via laser-induced reverse transfer found that the process window for deposition was quite small (at pulse energies from 36 to 40 nJ) [12]. Depositions via the CAP technique were performed at larger pulse energies and over a larger range (up to 78 mJ) allowing for higher material removal from the target as well as higher plume velocities, which may allow ultimately higher film thicknesses to be deposited.

As native COP is noted for its high transparency from the NIR to Mid-UV wavelengths [19], UV–Vis–NIR transmission spectroscopy was performed to examine changes in the optical properties of the COP substrate after deposition. The transmission spectra of the COP before and after deposition is shown in figure 3. After deposition, a broadband decrease in transmission of between 10% and 80% across all wavelengths was observed with this decrease becoming higher at longer wavelengths.

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The plasmonic behavior of the film was also analyzed using UV–Vis–NIR absorbance spectroscopy. The appearance of a broad natured absorbance band after deposition, centered at 598 nm (inset figure 3), suggests a large nanoparticle size dispersion, while the larger portion of the band being towards longer wavelengths indicates the dispersity being weighted towards larger sized nanoparticles as evident from figure 2 [20]. The peak position of 598 nm is also indicative that the peak nanoparticle size is approximately 100 nm [21]. Tailoring of both the optical transmission and the plasmonic response of the thin film may possible through alteration of the thickness of the deposited material and the deposited nanoparticle size respectively. Through this it may also be possible to remove the larger, aggregated particles with sizes outside of the nanoscale. This highlights the flexibility of the CAP process in the fabrication of optically active gold ultrathin films.

Changes in the surface chemistry of the polymer film as a result of the laser deposition process were examined using ATR-FTIR spectroscopy (figure 4). Samples processed at the maximum fluence (0.48 J cm⁻²) were chosen for analysis as any thermal degradation of the substrate would be more evident than when compared to lower fluence ablations.

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Previous work on the thermal degradation of COC has seen the formation of spectral features such as increased C=O stretching at 1710 cm⁻¹, C=H stretching (2900–2800 cm⁻¹) and the formation of a large band at 4000–3000 cm⁻¹ (indicative of C=C–H) [22, 23]. As the C=H and C=O bonds are at similar intensities before and after deposition, there has been limited oxidation and dehydrogenation of the surface due to the laser deposition process. Further evidence of this is the absence of an increased C=C signal at 1645–1445 cm⁻¹ indicating no carbonization of the surface. Though some reduction in peak intensity was evident, this was attributed to the deposited gold film reducing the sampling depth into the COP bulk. Though no significant degradation was seen over the range of fluences examined in this work, previous studies have demonstrated substrate melting at fluences of 0.34 J cm⁻² and substrate warping at fluences of 0.79 J cm⁻² [23]. Neither warping or melting was evident from the morphological examination, however it is possible that damage may become evident if a larger laser fluence range is examined. It may be further possible to mitigate this damage through the use of focusing optics (allowing for a smaller beam waist at the target than the substrate) or through increasing target-to-substrate distance.

XRD analysis was performed to examine the crystalline structure of the substrate and deposited gold nanoparticles. A large broadband emission from the amorphous COP substrate from 10° to 30°. The presence of four characteristic peaks corresponding to the (1 1 1), (2 0 0), (2 2 0) and (3 1 1) peaks of face-centered cubic gold are shown in figure 5. A ratio of 1.185 of the (2 0 0) and (1 1 1) peaks indicates that the nanoparticles are deposited in a random orientation. An average crystallite size of 6.60 Å was estimated using the Scherrer equation [24]. The peak positions for the principle Bragg reflections are similar to that reported previously for AuNPs synthesized via wet chemical and biological techniques [25–27]. The peak at 25° was attributed to the adhesive tape used to affix the sample within the XRD sample stage and is consistent with other data using the same method of XRD sample preparation [28].

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