Nanodot array deposition via single shot laser interference pattern using laser-induced forward transfer

Figure 3 contains (a) optical and (b) SEM images of the same Au nanodot array deposited by LIDT with an interference pattern. The shot energy was 97 μJ, and the averaged fluence (1/e²) was 133 mJ cm⁻². The picture was taken to cover the area with the highest concentration of deposited nanodots. It is apparent that nanodots were successfully deposited in an array in a single shot of laser irradiation. 191 nanodots were fabricated on a 12 × 17 matrix at a deposition efficiency rate of 94%.

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Figure 4 provides a schematic explaining the underlying mechanical processes of LIDT using an interference pattern. First, laser energy is induced into the donor film periodically according to the interference pattern, as shown in the top inset. Next, the resulting thermal expansion and vapor pressure creates a nano- sized area of melted metal in a spot in the interference pattern, as represented in figure 4(b). Then, a nanodot is formed due to surface tension, as shown in figure 4(c). In the case of laser processing without a receiver, a nanodrop [19, 21, 23] or nanowhisker [23] forms through a solid–liquid–solid (SLS) mechanism [23, 26] instead of a vapor-liquid-solid (VLS) mechanism [27]. In the case of LIDT, the nanodot detaches from the summit of the nanowhisker and adheres to the receiver film facing the donor film. The nanodot is deposited as the temperature decreases due to thermal diffusion, as shown in figure 4(d). Unlike single LIDT using a focused laser spot, neighboring spots in a matrix with $\Lambda = 3.6\;\mu {\text{m}}$ distance are irradiated simultaneously. It should be noted that the LIDT process starts before the temperature along the horizontal plane of the donor film stabilizes.

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There are some missing and surplus depositions, as shown in figure 3. The former would be due to failure in ejection or deposition, and the latter would be due to dislocation in or after the process or multiple ejections, as explained in the next subsection.

In LIDT, the number of nanodrops in a single shot depends on the parameters. Dr. Narazaki reported that the number of deposited FeSi₂ nanodots increases as a function of fluence [10]. On the other hand, single and multiple nanodots were deposited at different spots in this experiment. In the SEM observation, a structure with multiple nanodots was seen at the center, where the fluence was relatively high. A flattop beam can produce nanodots of uniform size and number [28].

Figures 5(a-1) and (a-2) contain top and bird's eye view images of solo nanodots in the array shown in figure 3. Each red arrow indicates the direction of the observation in the bird's eye view. The shape is a squashed sphere, which indicates that the nanodot was soft and the temperature was high enough at the time of adhesion. The diameter is 538 nm and 484 nm, respectively. The nanodot and nanowhisker formed simultaneously, as shown in figure 4(d), which corresponds with observations from paper research on nanowhisker formation [23]. Similarly, paired nanodots were deposited in a single shot, as shown in figures 5(b-1) and (b-2) in adjoining configuration and in figures 5(c-1) and (c-2) in stacking configuration. The deposition of paired nanodots in a spot means that they are supposed to be ejected from the same spot in an interference pattern. Consecutive formation of nanodots from a single spot may occur when the melted film exists for an extended period. Multiple ejections and depositions were observed in both single spot LIDT [10, 29] and LIDOS [14]. The mechanism of multiple formation may be Rayleigh-instability, which has been observed by time-resolved imaging of liquid behavior [30–32]. Considering the deposition sequence in figures 5(c-1) and (c-2), the subsequent ejection produced a smaller nanodot. The average diameter of the larger base and smaller top nanodots are 592 nm and 389 nm, respectively. The smallest nanodot was 355 nm, as shown in figure 5(c-1).

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The horizontal spacing between nanodots in the case of multiple ejections depends on the direction of each ejection. It is suspected that the direction fluctuates, forming either adjoining or stacking structures. The gap in figures 5(b-1) and (b-2) measure 81.9 nm and 17.2 nm, respectively. A nanogap structure elicits a field enhancement effect, which will be useful for plasmonic applications, such as surface-enhanced Raman scattering.

We compared LIDT with other techniques, including LIDOS, which can also be used to fabricate nanodots [14]. In this technique, nanodots deposit on the source film, as shown in figure 1. The experimental setup was nearly identical to the LIDT setup, except the receiver substrate was excluded. The difference on the side of the deposition would be due to parameters, such as atmospheric conditions, film thickness, fluence, etc. LIDOS experiments took place in atmospheric room-temperature conditions, while LIDT experiments occurred in a vacuum. It is possible that air cerates drag on the ejected particle as it returns to the donor film, in the case of LIDOS. The LIDOS-fabricated nanodots had an average diameter of 376 nm the film thickness was 40 nm, which is similar to the one used this experiment. The pulsed-laser deposition technique can eject multiple droplets from the ablation spot in a single shot, but the size is dispersed [33]. Lithographic techniques can fabricate nano-sized structures, but these techniques are time consuming and expensive [34]. Chemosynthesis methods lack size uniformity and require post processing steps, such as alignment and cleaning. LIDT using an interference pattern has many advantages compared to conventional techniques.

Based on two-dimensional laser-induced fluorescence (2D-LIF) observations from our previous work on single spot LIFT of thin Au film, we concluded that the presence of air significantly limits the velocity and divergence angle of the ejected atoms and particles [3, 35]. LIFT of dye film led to minimal atmospheric deposition [6]. The atmospheric condition may also affect LIDT using interference patterns. For example, the LIDOS experiments, mentioned above, resulted in successful nanodot deposition in atmospheric room-temperature conditions.