Laser‐Induced Shockwaves as a Rapid and Non‐Contact Technique for 2D Patterning of Colloidal Monolayer Crystals

A novel approach is reported that harnesses laser‐induced shockwave spallation technique to selectively remove clusters of polystyrene (PS) microspheres from close‐packed monolayers for the creation of 2D colloidal micropatterns. The strategic design of the layer structure by incorporating a thin layer of poly(vinyl alcohol) (PVA) on top of the PS monolayer enables the complete delamination of PS particle clusters. Its use is demonstrated for creating different sizes of circular microscale spallation patterns by regulating the tensile force of the shockwave with laser fluence adjustments. To further control the diameter of the spallation pattern, various shadow masks are utilized to tune the shockwave generation region. The finding reveals that both PVA thickness and spallation force play key roles in adjusting cluster spallation size with complete removal. The study highlights the potential of laser spallation techniques as a rapid and non‐contact method for 2D colloidal crystal patterning by leveraging spatially regulated shockwave spallation forces.


Introduction
2D colloidal crystals, or colloidal monolayers, have gained significant attention due to their unique periodic structure and exceptional properties. [1]These colloidal crystals have found applications in various fields, [1a] including photonics, [2] colloidal lithography, [3] sensing, [4] surface engineering, [5] and fundamental physics studies. [6]A variety of techniques have been developed to generate 2D colloidal monolayers, [1a,7] which can be conceptually classified into two main approaches: liquid interface-mediated assembly [8] and external force-induced assembly methods. [9]Most of these conventional techniques rely on the use of solvent, [8a,b] chemical, [8c,9a] or physical contact, [9d,e] which negatively affect patterning outcomes and limit following processes due to swelling, limited surface chemistry DOI: 10.1002/admi.202301000compatibility, and contamination.As such the downsides emphasize the need to shift focus toward the approaches for achieving contamination-free colloidal monolayer patterning.Besides, to fully exploit the potential of 2D colloidal crystals in future device designs, such as intricate structural pattern designs [8d,10] and metamaterials [11] with novel mechanical behaviors, precise confinement of the colloidal domains onto selected regions of a substrate is indispensable.The most common method for achieving 2D colloidal crystal structural confinement involves "bottomup" approaches, where two main methods are utilized to pattern the substrate surface before assembling colloidal crystals into it: physical patterning [10,12] creates substrate topography via specialized processes like photolithography or laser micromachining, and chemical patterning [9c,e,13] uses surface chemistry and micro-structuring to create regions with differential affinities for colloidal deposition.However, the physical and chemical local pre-treatments require extra processing time and cost and precise control of parameters, such as solution concentration, temperature, and reaction time.Hence, there is a need to develop a technique that allows for the rapid and costeffective patterning of colloidal crystals without the requirement for any pre-treatment processes.
In that context, laser techniques emerge as a promising "topdown" alternative approach for generating 2D patterns of colloidal crystals, leveraging their advantages of a rapid, high-yield, non-contact, and chemical-free process with the ability for subsequent post-processing.Laser-based techniques for surface patterning can be achieved through two techniques: direct laser irradiation, [14] which applies thermal effects to remove the localized material on the surface, and indirect laser patterning, [15] which utilizes laser-induced shockwaves to transfer the mold's pattern to the surface while introducing pressure between the surface and the mold.Direct laser patterning, while efficient and controllable, may lead to material degradation, microstructural changes, and cracks.Indirect surface patterning minimizes these issues by reducing laser thermal effects.Wang et al. explored direct laser irradiation for 2D and 3D colloidal crystal patterns, introducing micropatterning opportunities with high resolution. [16]16b] Laser spallation is an indirect laser patterning technique that employs laserinduced shock waves to trigger the delamination of films (e.g., metals, polymers and composites, ceramics, or biological materials) in substrate-film systems. [17]This method holds theoretical potential as a non-contact approach for colloidal monolayer patterning, as it creates patterns on the rear side of the sample without direct contact with the laser irradiation, solely relying on the tensile force generated by the laser-induced shockwave.Nevertheless, the laser spallation technique has mainly been studied as an adhesion strength quantification technique for films in substrate-film systems, [17] and its potential for creating patterns remains largely unexplored.In 2016, Hiraiwa et al. reported the delamination of polystyrene (PS) microsphere monolayers from an aluminum-coated glass substrate using the laser-induced spallation technique, [18] providing valuable hints into the potential of this technique for PS monolayer patterning.However, the issue of detached PS flakes redeposition onto the substrate, noted in their research as an unavoidable occurrence, continues to pose a significant challenge in utilizing laser spallation techniques in tailored colloidal pattern creation.To address this issue, we hypothesize that tuning layer structures helps minimize PS reattachment and facilitate the formation of distinct patterns.Additionally, we hypothesize that spatial confinement of shockwave pressure enables the reduction of spallation pattern sizes.
In this study, we report the development and application of a laser-induced shockwave spallation technique to facilitate the spallation of PS particle clusters, enabling the generation of 2D micropatterns within a close-packed monolayer of PS particles.To achieve complete spallation with well-defined edges, we leverage an additional supportive layer on top of the PS particles for collective delamination.To enhance the resolution of patterns, we reduce pattern size by introducing shadow masks with varying hole sizes to partially block the laser beam.Our results introduce a novel approach for micropattern formation in colloidal particles using a rapid and non-contact method that exploits laser-induced shockwave spallation phenomena.

The Design of a PS Monolayer and Extra Supportive Layers for Clear Spallation
Laser-induced shockwave spallation setup was utilized to selectively spall colloidal microspheres for the formation of micropatterns as shown in Figure 1a.In order to investigate spatially controlled spallation of colloidal particles, we designed a multilayered patterning side consisting of a thin PDMS film, a hexagonally close-packed PS monolayer, and an overlying supportive poly(vinyl alcohol) (PVA) layer, as shown in Figure 1b.The sample structure consists of a BK7 glass substrate, a shock generation side featuring a 400 nm-thick aluminum (Al) film and a Na 2 SiO 3 confining layer, and a patterning side.Following laser-induced shockwave spallation, micro-holes emerged on the surface of the patterning side where the particles were spalled (Figure 1b).For preparing the PS monolayer, we employed the rubbing method, described as a quick and highly reproducible technique for generating a colloidal monolayer on flat or curved substrates.The technique involves the unidirectional rubbing of PS particle powders on a PDMS substrate by a small rubbing piece.In our experiment, we made a modification to the conventional rubbing method by introducing an ultra-thin film of PDMS (<100 nm thick) coated on the glass surface instead of a thicker PDMS film (>10 μm) in the previous report. [19]This PDMS thin film effectively serves as a support layer for PS rubbing, while its minimal thickness ensures a negligible reduction in the laser-induced shockwave pressure during the patterning process.According to the SEM images in Figure 2, the rubbing method successfully produced the hexagonally close-packed PS monolayer on the thin PDMS layer with only a few voids, and this close-packed structure remained intact after the deposition of the PVA thin film via spin coating on top of it.It is worth noting that we also tried using the Langmuir-Blodgett technique to directly assemble the PS monolayer on the glass substrate, as depicted in Figure S1a,b (Supporting Information).However, this approach led to a failure in the sample fabrication, as detailed in Figure S2 (Supporting Information).In the rubbing method, the thin PDMS layer coated on the glass substrate, with its strong affinity for PS particles through rubbing, [19] effectively safeguards the PS structure during the PVA coating process.As presented in Figure 2c, crosssectional SEM images of the multilayer structure consisting of PDMS, PS, and PVA layers on the patterning side of the sample revealed the PDMS layer average thickness of ≈90 nm.Given the challenge of determining PVA thickness via SEM images due to the lack of distinct contrast between PS and PVA, we used alpha step analysis to determine the thickness of PVA on a flat glass substrate instead.We assumed that the thickness of PVA would be consistent whether deposited on glass or on PS particles.The average thicknesses of the PVA layer, as determined by alpha step measurement and summarized in Figure S3 (Supporting Information), were ≈17, 59, 113, 330, and 786 nm, corresponding to PVA concentrations of 1, 2, 3, 5, and 7 wt%, respectively.The fabricated samples were named according to the concentration of PVA solutions as PS-PVA-1, PS-PVA-2, PS-PVA-3, PS-PVA-5, and PS-PVA-7.The sample PS without PVA coating was also fabricated and denoted as PS-PVA-0.
Following the successful fabrication of the multi-layered structure on the patterning side of the sample, we found the importance of the PVA layer's existence in creating distinct patterns.We compared two samples: one was PS-PVA-5, corresponding to a 330-nm-thick PVA layer on top of PS, while the other remained uncoated PS-PVA-0, with structures nearly identical.Both were tested at a laser fluence of 217 mJ mm −2 .As shown in Figure 3a, the resulting pattern of the PS-PVA-0 sample without PVA coating displayed a significant number of PS microspheres remaining on its surface after the spallation.This phenomenon may be attributed to either the reattachment of PS microspheres, as indicated in other research studies, [18,20] or the existence of non-ablative PS domains.In contrast, the SEM images of the sample PS-PVA-5 (Figure 3b) revealed the formation of a well-defined boundary pattern with a diameter of ≈1.43 ± 0.06 mm.This finding highlights the successful prevention of residual PS microspheres under the shock loading through the application of PVA coating, leading to their complete elimination.To gain a more comprehensive understanding of the shockwave-induced spallation process, we employed a double-sided carbon tape on a plastic substrate, positioned 2 mm away from the sample's patterning side, to collect the spalled PS microspheres.As shown in Figure S4 (Supporting Information), when comparing the resulting patterns of the sample PS-PVA-0 under conditions without and with the use of carbon tape to intentionally collect spalled PS microspheres, no notable distinctions were observed, confirming the carbon tape does not affect the spallation of the PS spheres.The  SEM image of Figure 3a exhibited spalled areas covered with the remained particles without substantial non-uniformity.Reattachment can lead to inhomogeneous distribution of spalled particles.Thus, the residual PS microspheres on the sample PS-PVA-0 after shock loading were predominantly non-ablative PS microspheres.These non-ablative PS domains can be explained by variations in the ease of PS microsphere removal under laserinduced stress.Our rationalization is that weaker points such as grain boundaries and defects facilitate easier PS delamination due to the lower coordination numbers of PS microspheres.In contrast, densely packed regions of PS grains require more energy for delamination, resulting in scattered un-spalled PS particles and flakes.Furthermore, as observed from the SEM images in Figure 4, the spalled material accumulated on the carbon tape surface from the sample PS-PVA-0 consisted of individual PS microspheres and crystal PS domains (Figure 4a), whereas the spalled material collected from the sample PS-PVA-5 was one integrated entity of PS microspheres with the PVA coating (Figure 4b).It is worth noting that under identical substrate acceleration conditions, the tensile force (F = ma) escalates proportionally with the mass of the spalled unit.Therefore, a large PS/PVA unit is subjected to a larger tensile force compared to smaller PS flakes, facilitating its complete spallation.We affirm that the key role of the PVA layer lies in the PVA film's ability to connect PS domains into a large array, thereby creating clear edge patterns without residual PS microspheres.To gain a further understanding of whether the PDMS thin film could be delaminated together with PS and PVA during tensile loading, EDS mapping was conducted at the spalled area.Figure S5 (Supporting Information) reveals a fine distribution of C atoms within the spalled region, affirming that the PDMS layer was intact.
We also investigated the effects of PVA thickness and spallation force on the selective removal of the PS clusters in terms of the formation of well-defined edges.Three samples coated with different PVA concentrations (3, 5, and 7 wt%) were subjected to laser-induced shockwave without a shadow mask.SEM images of the patterns obtained from these samples under different laser fluences, ranging from 129 mJ mm −2 to 217 mJ mm −2 , displayed near-circular holes (Figure S6, Supporting Information).At the same laser fluence, the spalled area of pattern exhibited similar values for all three samples.Notably, at the highest laser fluence of 217 mJ mm −2 , the pattern diameter reached ≈1.43 ± 0.06 mm, which was 1.35 times larger than the laser spot size (1.07 ± 0.03 mm).This enlargement can be attributed to the characteristic of shockwave propagation, which expands outward in a discontinuous spherical shape, named the Mach cone, during its travel. [21]In addition, as the laser fluence decreases, the shockwave pressure and impact area also decrease, [22] resulting in reduced pattern diameters.Notably, although the three samples of PS-PVA-3, PS-PVA-5, and PS-PVA-7 displayed similar pattern sizes under the same laser fluence, the variation in PVA thickness resulted in significant differences in the quality of the created spallation edges.
As represented in Figure 5, at a laser fluence of 217 mJ mm −2 , the PS-PVA-3 sample, with a thin PVA layer (≈113 nm), produced fracture edge patterns, whereas the PS-PVA-7 sample, with a thick PVA film (≈786 nm), exhibited spalled regions with ripped edges and radical cracks surrounding the hole.The PS-PVA-5 sample, with a PVA thickness of ≈330 nm, generated the most distinct and clear edge patterns.In order to elucidate the aforementioned phenomenon, we undertook a calculation of the tensile force applied at the PS-substrate contact.Once the tensile force surpasses the adhesive force between PS and the substrate, as well as the flexural strength of the PVA film, the PS-PVA film detaches around the borders.The tensile force (F = ma) is determined by the mass (m) of the PS-PVA and the substrate acceleration (a).The substrate acceleration (a) is derived from interferometric data collected from the calibration sample since direct displacement measurements in our PDMS/PS/PVA film were unattainable due to its non-reflective properties.Hence, we assumed that under the same laser fluence, the substrate acceleration remains constant for samples with varying PVA thicknesses.Consequently, the elevated PVA thickness results in a proportional rise in mass, subsequently leading to an increased tensile force.Figure S7 (Supporting Information) summarized the calculated mass values for both PS particles and PVA layers, derived from varying PVA concentrations.This calculation is predicated on the assumption that the PVA film encapsulates half the circumference of a PS microsphere.For PS-PVA-3, PS-PVA-5, and PS-PVA-7 samples, the calculated unit masses of PS-PVA are determined as 5.21 × 10 −12 g, 6.70 × 10 −12 g, and 9.83 × 10 −12 g, respectively.At a laser fluence of 217 mJ mm −2 , surface acceleration, calculated from the test, was ≈390 × 10 9 m s −2 , as shown in Figure 6a.Consequently, the calculated tensile forces for  PS-PVA-3, PS-PVA-5, and PS-PVA-7 samples were 2.0 mN, 2.6 mN, and 3.8 mN, respectively, as manifested in Figure 6b.
In particular, at the laser fluence of 217 mJ mm −2 , the PS-PVA-7 sample provided a 90% increase in the tensile force, while the PS-PVA-5 sample exhibited a 30% increase, as compared with the PS-PVA-3 sample.Our hypothesis is that, despite the PS-PVA-3 sample experienced low tensile force, the lower flexural strength due to its thinner PVA layer has minimal impact on determining delamination at the edges.Instead, tearing in PS-PVA-3 sample is primarily influenced by the PS monolayer crystallinity.Given the imperfect uniformity of PS crystals, tearing at the edges occurs in weaker areas such as grain boundaries and defects.Since the shockwave deforms the surface spherically, [23] the tensile force in the edge regions falls below the threshold required to break the PS crystals but is still sufficient to break the weaker PS areas.This leads to non-uniform delamination of PS/PVA and, as a result, fractured edge patterns.The PS-PVA-7 sample, on the other hand, experienced an increased tensile force; however, its thicker PVA layer, which possessed high flexural strength and durability, exhibited significant resistance to tearing.This resistance resulted in a ripped edge pattern and caused significant tensile stress in the surrounding areas, leading to the development of radial cracks.The PS-PVA-5 sample, featuring a 330 nm PVA thickness, was subjected to a suitably sufficient tensile force, giving rise to a clear edge pattern.

Controlling the Dimensions of the Patterns by Applying Various Shadow Masks
We further explored ways to achieve smaller spallation patterns by employing circular shadow masks to partially obstruct the pulsed laser, thus spatially constraining shockwave generation (Figure 7a).The shadow masks are positioned closely to the surface of the shock generation side of the sample, with no gaps.Our hypothesis is that a shadow mask, partially blocking the laser, creates smaller shockwaves.As illustrated in Figures S8a-c (Supporting Information), with an ideal flat-top laser beam, the laser energy was determined by the shaded area within the shadow mask hole diameter.We calculated the percentage of laser energy transmitted through the shadow masks in relation to the shadow mask hole diameter.As demonstrated in Figure S8d (Supporting Information), the proportion of transmitted laser energy tended to decrease as the shadow mask hole size decreased.As such, we conducted calibration measurements in order to comprehend the characteristics of the laser-induced shockwave in the presence of shadow masks.The surface accelerations of samples with shadow masks featuring hole diameters of 200 μm (SM-200), 400 μm (SM-400), and 600 μm (SM-600) were compared to a control sample without any shadow mask.As shown in Figure 6a, under the same laser fluence of 217 mJ mm -2 , the samples employing SM-600, SM-400, and SM-200 exhibited surface accelerations of 275 × 10 9 m s −2 , 230 × 10 9 m s −2 , and 93 × 10 9 m s −2 , respectively.These values correspond to 70.5%, 59.0%, and 23.8% of the surface acceleration observed in the sample without shadow mask (390 × 10 9 m s −2 ).The decline in these proportion values aligned with the laser energy percentages transmitted through the shadow masks as determined by the ideal flat-top laser beam profile (Figure S8d, Supporting Information).Variations in the surface acceleration proportion values in our experiment, in comparison to the expected percentage of laser energy transmission through the shadow masks, may be attributed to possible deviations from an ideal flat-top profile in our laser system.However, this evidence supported our hypothesis that the use of shadow masks reduces the transmitted laser energy, leading to a reduction in the overall surface acceleration.
We effectively generated a smaller 2D pattern of PS monolayer by harnessing shadow masks in the laser-induced shockwave spallation technique, strategically confining pulsed laser irradiated areas.Because PVA 5 wt% was determined as the opti-mal condition in the shadow mask-free laser test, we selected it for subsequent laser testing with the application of the shadow mask SM-600.Interestingly, SEM images in Figure 7b revealed a ripped edge pattern in the sample PS-PVA-5 with SM-600.As shown in Figure 7c, the use of shadow masks resulted in a significant decrease in tensile force.Particularly, when laser fluences were set at 194 mJ mm −2 and 217 mJ mm −2 , the tensile forces in the presence of SM-600 exhibited a noticeable reduction of 34% and 32%, respectively, compared to cases where a shadow mask was not employed.Therefore, the ripped edge in the pattern of the sample PS-PVA-5 with SM-600 can be attributed to the tensile force exerted on the sample.This force may have led to PS delamination, but it was insufficient to fracture the PVA film.It is important to highlight that an increase in PVA film thickness led to higher tensile forces (as shown in Figure 6b), promoting the delamination of PS/PVA.Nevertheless, thicker PVA films also possess greater fracture strength and are more resistant to breaking, which in turn impedes the formation of a well-defined edge pattern.Hence, when adjusting the sizes of shadow masks, fine-tuning the PVA thickness to attain a well-defined boundary pattern under specific laser fluence conditions is necessary.
By leveraging three shadow masks of varying sizes and optimizing the corresponding PVA layer thickness, we demonstrated the potential to reduce the pattern size to below 200 μm.We discovered the appropriate thickness of the PVA layer for each type of shadow mask.As detailed in Figures S9,S10, and S11 (Supporting Information) and summarized in Figure 8a-d, the PS-PVA-5 sample is the optimal sample for the laser spallation test without a shadow mask; however, when using shadow masks SM-600 and SM-400, the PS-PVA-2 sample proves to be the most suitable.Interestingly, when the SM-200 is employed, PVA coating is unnecessary to create the best pattern quality.The pattern diameter was largely regulated by adjusting the laser fluence and employing various types of shadow masks, as demonstrated in Figure 8e.The laser spallation tests employing the SM-600 created patterns ranging from 700 μm to 1010 μm in diameter, depending on the applied laser fluence.Similarly, with the SM-400, patterns vary from 407 μm to 635 μm, and with the SM-200, patterns range from 142 μm to 376 μm in diameter.This highlights the capability of fine-tuning the pattern size by using the laser-induced shockwave spallation technique.Figure S12 (Supporting Information) depicted arrays of circular patterns successfully generated on the PS-PVA-0 sample using the laser spallation technique with SM-200 at various laser fluences.It is worth noting that, in this study, we aimed to demonstrate colloidal crystal patterning feasibility with the laser spallation method.For that purpose, we used a simple circular design to verify this.Although experiments with other shapes showed potential for varied patterns (Figure S13, Supporting Information), detailed findings will be presented in a separate article to maintain focus and length.After the patterning process, the removal of the PVA layer is crucial to obtain a pure 2D colloidal crystal pattern, enhancing its applications in optical devices, sensors, or catalysts.Therefore, we also investigated the potential of hot-water washing to eliminate the PVA coating.As illustrated in Figure S14 (Supporting Information), the adherence of PS particles to the PDMS layer ensures the preservation of the 2D pattern's quality after PVA layer removal, suggesting the possibility of removing the PVA layer without adversely impacting the quality and stability of the 2D patterns.

Conclusion
In summary, we demonstrated a novel technique by selectively detaching PS clusters to fabricate 2D patterns of PS monolayer using laser spallation technique.Under laser-induced shockwave loading, PS monolayers supported by extra PDMS and PVA layers were selectively spalled, resulting in 2D circular micropatterns.Our thorough investigation provided a deep understanding of the specific contributions made by each constituent layer: PDMS, PS, and PVA to the pattern formation process.The PVA layer plays a key role in preventing un-spalled PS, leading to the complete delamination of the PS clusters and forming clear micro-patterns.In addition, the diameter of these micro-patterns was controlled by adjusting laser fluences or by using shadow masks with different hole sizes to partially block the laser beam.Moreover, our research revealed that utilizing a shadow mask with a diminished diameter leads to a significant reduction of surface acceleration and, consequently, a decrease in tensile force.Therefore, a precise reduction in the thickness of the PVA film is needed to enhance its susceptibility to tearing, facilitating the creation of patterns with distinct borders.We found that the PS-PVA-5 sample is optimal for laser tests without a shadow mask, while PS-PVA-2 is ideal for SM-600 and SM-400; for SM-200, no PVA coating yields the highest pattern quality.Our study strongly supports the potential of laser-induced shockwave spallation as a non-contact method for surface colloidal patterning, delivering advantages in terms of speed, cost-effectiveness, and minimal chemical intervention.This patterned colloidal crystal films offer versatile applications, including microfluidics for precise fluid manipulation, biotechnology applications, such as cell culture substrates, serving as templates for nanostructure fabrication, and potential use in electronic devices.However, further research is required to customize these films and patterns to meet specific application requirements.
Sample Preparation: The 0.5 mm-thick BK7 glass substrate was thoroughly cleaned in an ultrasonic bath with acetone, ethanol, and deionized water, followed by drying in an oven.UV-ozone cleaning was performed to remove residual organics and achieve a hydrophilic glass surface.A 400 nm-thick aluminum (Al) film was deposited as an energy absorbing layer on the UV-ozone-treated glass surface using electron beam evaporation.Subsequently, a 7 μm-thick Na 2 SiO 3 film was spin-coated on top of the Al film as a confining layer to enhance bulk longitudinal wave amplitudes in the substrate.This side was labelled as the shock generation side.
Calibration Sample: In addition to the deposition of the 400 nm-thick Al film on the shock generation side, a 200 nm-thick Al reflective layer was deposited using an e-beam evaporator onto the bare surface of the BK7 glass, positioned on the opposite side.This calibration sample is used to determine the displacement at the free surface, therefore providing information on the substrate stress profile.
Samples for Laser-Induced Patterning Testing: For the laser-induced 2D patterning testing, a PS monolayer was prepared on the bare surface of the BK7 glass situated opposite the shock generation side by rubbing method.
Preparation of the Thin PDMS-Coated BK7 Glass Substrate: A mixture of the PDMS prepolymer and curing agent (10:1 ratio) was diluted with hexane at a 1:40 (PDMS: hexane) ratio and stirred for at least 1 h.The resulting PDMS-hexane solution was spin-coated three times (2000 rpm 1 min -1 ) onto the bare surface of the BK7 glass, and then cured at 80 °C for 5 h.
Preparation of the PDMS Rubbing Piece: A mixture of the PDMS prepolymer and curing agent (10:1 ratio) was poured onto a polystyrene petri dish with a uniform thickness of 1.5 mm.The cast liquid was cured at 80 °C for 5 h.Then the PDMS film was cut into pieces with a dimension of 4 × 4 cm 2 .
Preparation of the PS Monolayer: The PS latex microspheres were washed two times with ethanol to remove water-soluble surfactants and salts.The PS microspheres were then dried in a convection oven at 60 °C overnight, yielding PS powder.The PS powder was rubbed onto the thin PDMS-coated BK7 glass substrate using the PDMS rubbing piece, resulting in the formation of the PS monolayer.
PVA Coating Process: PVA solutions with different concentrations (1, 2, 3, 5, or 7 wt% PVA) were prepared by dissolving PVA powder in DI water with magnetic stirrer at 85 °C for 5 h.These PVA solutions were then spincoated onto the PS monolayer at 2000 rpm for 1 min.
Characterization: The surface morphologies, cross-sectional images, and energy-dispersive X-ray spectroscopy (EDS) analysis of samples were examined using scanning electron microscopy (SEM) (JCM-7000, Neo-Scope, Jeol Ltd., Japan) with an acceleration voltage of 15 kV.The sample surfaces were sputtered with a thin platinum layer to avoid specimen charging.The PVA thickness was characterized by a stylus profiler (Alpha-step D-500, KLA Tencor, USA).Samples for measuring the PVA thickness were prepared by spin-coating PVA solutions with different concentrations (1, 2, 3, 5, or 7 wt%) onto glass substrates at 2000 rpm for 1 min.
Laser Spallation Experiment Setup: A typical laser spallation experimental setup is schematically illustrated in Figure S15 (Supporting Information).In this setup, high energy lamp pumped Q-switched laser pulse ( = 1064 nm, CNI lasers LPS 1064-L 700 mJ) was incident onto the Al absorbing layer.A high-energy focusing lens was placed 150 mm (lens focal length) in front of the specimen.Because of the confinement of the Na 2 SiO 3 confining layer, the rapid expansion of the Al generated a longitudinal stress wave, also known as shock wave.The compressive stress wave propagated through the glass substrate. [24]After reaching the free surface, the stress wave reflected back, loading the Al reflective layer and glass substrate in tension and causing the Al surface displacement.To measure the out-of-plane displacement, interferometric measurements were performed at the surface of the reflective 200 nm-thick Al film using a Michelson interferometer.A diode pumped continuous laser ( = 532 nm, Cobolt Samba) was incident upon a beam-splitting cube.One beam was focused on the Al-200 nm film, while the second beam was directed toward a stationary mirror.The two beams were then recombined in the beam splitter and directed onto the photodiode detector (EOT ET-2030).Interferometer voltage signals were measured with the photodetector and digitized on a 2.5 GHz oscilloscope (Tektronix DPO 7254) at a sample rate of 40 GS s −1 .
Calibration Tests and Measuring Principles: Due to the non-reflective nature of the PDMS/PS/PVA film surface, direct measurements of displacement were not feasible.Instead, calibration measurements were conducted on the calibration samples to obtain interference signals as voltage traces under varied conditions: without a shadow mask and with different shadow masks (SM-600, SM-400, or SM-200) applied onto the shock generation side of the sample.The calibration test was conducted three times at each laser fluence ranging from 129 to 217 mJ mm −2 to average the values.The pulsed laser spot size was maintained at 1 mm for all tests.The temporal interference pattern was recorded as a voltage trace, V(t), given by where V max and V min are the maximum and minimum measured interferometric signals, ∅ is the phase angle,  0 is the wavelength of the probe laser, and u(t) is the surface displacement.Surface displacements were calculated from the interference equation.24b,25] Figure S16 (Supporting Information) represented the interferometric data obtained from the calibration sample without using a shadow mask, including photodetector fringe data, surface displacement, and surface acceleration.The tensile stress that applies the film/substrate interface after the reflection at the free surface, delaminating the film from the substrate, was given by: 24b] The PVA-coated PS monolayer prevents the measurement of the thickness of the film and the film density.Therefore, instead of looking at interface stress, the force acting at the PS particle-substrate interface as an alternative is calculated. [18]As suggested by Hiraiwa et al., a simple model is employed that accounts for the inertia of the PVA-coated PS microspheres and considers the PVA-coated PS microspheres to be rigid bodies that follow the substrate surface motion until detachment. [18]The tensile force was calculated by the Newton's second law: F = ma, where m is the mass of a unit consisting of one PS microsphere and a PVA thin film coated on top of the PS microsphere, and a is the measured substrate surface acceleration from the calibration sample. [18,20]aser-Induced Shockwave Patterning: The samples PS-PVA-0, PS-PVA-1, PS-PVA-2, PS-PVA-3, PS-PVA-5, and PS-PVA-7 were prepared for laserinduced shockwave patterning.Thirty-six laser pulses struck a single specimen to examine 36 spallation patterns.Each strike is separated 2.5 mm from the center of the nearest neighbors, facilitated by a motion-controlled actuator system on the specimen, while leaving the pulsed laser path unchanged.The pattern spallation occurs instantly upon pressing the laser button to activate the laser beam, with a processing time within 1 s.Once the pulsed laser beam is emitted from the laser source, it reaches the PS array within 1 μs and spallation would occur around tens of nanoseconds since the acceleration of the patterning surface lasts only for 10-20 ns.To further investigate the spallation insight, plastic pieces covered with carbon tape were positioned 2 mm away from the patterning side of the PS-PVA-0 and PS-PVA-5 samples to gather their spalled flakes.

Figure 1 .
Figure 1.Schematic illustrations of laser-induced shockwave spallation technique and specimen structure: a) The laser spallation experiment for generating 2D patterns of PS monolayer, and b) illustrations of the sample structure before and after the spallation process.

Figure 2 .
Figure 2. Structure of PS monolayer and extra supportive layers.Top view SEM images of (a) PS monolayer prepared by the rubbing method and (b) PVA-coated PS monolayer, the insets of (a) and (b) are the magnified images; and (c) cross-section images of the patterning side's structure of the PVA-coated PS monolayer on the glass substrate, the magnified images for closer examination.

Figure 3 .
Figure 3. Impact of PVA supportive layer on clear edge pattern formation.SEM images showing spalled regions at the patterning side of the samples under a laser fluence of 217 mJ mm −2 , without using a shadow mask: a) Sample PS-PVA-0; and b) sample PS-PVA-5.

Figure 4 .
Figure 4. Comparison of the dimensions and shapes of fragmented PS particle clusters spalled from the samples PS-PVA-0 and PS-PVA-5.SEM images of (a) detached PS particles and flakes from the patterning side of the PS-PVA-0 sample, and (b) a spalled PS/PVA cluster from the patterning side of the PS-PVA-5 sample, both on a carbon tape surface.

Figure 6 .
Figure 6.Comparative analysis of surface acceleration and tensile force under varied shadow masks.a) Surface acceleration as a function of laser fluence for calibration samples tested without a shadow mask and with shadow masks SM-600, SM-400, and SM-200 and (b) the calculated tension force as a function of PVA thickness without a shadow mask and with various types of shadow masks (SM-600, SM-400, and SM-200), at a laser fluence of 217 mJ mm −2 .

Figure 7 .
Figure 7. Impact of shadow mask application on tensile forces and pattern formation.a) Illustration of the laser spallation experiment setup for creating 2D patterns of PS monolayer with a shadow mask; b) SEM images show the spallation of the sample PS-PVA-5 under the shadow mask-free laser testing and the laser test with shadow mask SM-600, at laser fluences of 194 mJ mm −2 and 217 mJ mm −2 ; and c) tensile forces measured at the interface between the substrate and PS particles, as a function of laser fluence for various samples, including those without a shadow mask and those tested with shadow masks SM-200, SM-400, and SM-600.The calculation of tensile forces utilized the parameters of the PS-PVA-5 sample.

Figure 8 .
Figure 8. Precise control of pattern diameter through laser fluence and shadow masks.SEM images showing clear spallation depending on PVA thickness: a) PS-PVA-5 sample undergoes shadow mask-free laser test; (b) and (c) sample PS-PVA-2 undergo laser tests with SM-600 and SM-400, respectively; and (d) sample PS-PVA-0 undergoes a laser test with SM-200.(e) Summary of variation in pattern diameters depending on the laser fluence and shadow mask types.