Programming crack patterns with light in colloidal plasmonic films

Crack formation observed across diverse fields like geology, nanotechnology, arts, structural engineering or surface science, is a chaotic and undesirable phenomenon, resulting in random patterns of cracks generally leading to material failure. Limiting the formation of cracks or “programming” the path of cracks is a great technological challenge since it holds promise to enhance material durability or even to develop low cost patterning methods. Drawing inspiration from negative phototropism in plants, we demonstrate the capability to organize, guide, replicate, or arrest crack propagation in colloidal films through remote light manipulation. The key consists in using plasmonic photothermal absorbers to generate “virtual” defects enabling controlled deviation of cracks. We engineer a dip-coating process coupled with selective light irradiation enabling simultaneous deposition and light-directed crack patterning. This approach represents a rare example of a robust self-assembly process with long-range order that can be programmed in both space and time.

The authors report on a novel approach for patterning colloidal layers by controlling the propagation and nucleation of cracks during drying of the films.The key idea is to locally vary temperature in the drying films using photo thermal heating effects that can be localized via plasmonic nanoparticles.The local heating changes the evaporation rate and thus distorts the drying front and consequently influences the crack propagation.The authors demonstrate the effect both for droplet drying and in a more defined setting for crack formation in a dip coating scenario.They very convincingly illustrate that the effect can be triggered by homogenous illumination of patterned substrates or by patterned illumination of homogenous films in which either the substate can be heated plasmonically or the plasmonic particles are incorportated in the colloidal suspension.The analysis of the underlying phenomena is compelling.As well as the variety of patterning variations illustrates that this is a versatile method.It is a beautiful and elegant work of high quality.The only point that remains not fully clear to me is how general this mechanism is for other colloidal systems than the particluar one under investigation here.In my experience crack formation in drying colloidal layers typically is not as homogenous and following a strict periodicity as in this example.How is random crack nucleation due to defects suppressed ?Is this behaviour specific for the case of pluronics modification, can it be generalized to other particle types beyond PS particles ?The authors focus very much on manipulation of the crack formation and take the well defined periodic crack pattern for granted.The latter however is the prerequisite for the control.So it would be important to lear more about how easy it is to achieve such well defined crack formation for other colloidal layers.
Reviewer #2 (Remarks to the Author): The manuscript presents a novel and intriguing approach to control the propagation of cracks in colloidal films using light.The experiments are well-designed, and the results are supported by the data presented.The role of gold particles and their plasmonic properties in influencing crack propagation is particularly interesting.However, a more detailed explanation of the relationship between the plasmon wavelength and irradiation, as depicted in Figure 3, would enhance the clarity of the findings.
The role of the PS particles is also somewhat secondary in this study and it would be interesting to know how the size of these particles influences the experiments.It is especially interesting to know what would happen with even larger particles, of a size comparable to the wavelength of the laser used, and much more monodisperse (from the images it seems that the ones used in the experiments are not) since it would give the possibility to study self-assembly phenomena and their possible manipulation.In addition, even larger particles (more than one micron) could resonate with the lasers used (through MIE modes) and introduce even more possibilities.
I would like the authors to discuss this in the final version of the paper and consider the possibilities beyond crack control.
Reviewer #3 (Remarks to the Author): The authors present an interesting study in which they show how the interplay between visible light laser illumination, plasmonic nanoparticles and a colloidal suspension can be harnessed to steer the growth direction of cracks in a forming colloidal thin film on a surface.As the underlying mechanism they propose the local heating via light absorption and dissipation in either Au plasmonic particles on the colloidal film substrate or dispersed directly in the colloidal suspension.In general, this study is well executed with a suite of experiments that test and verify the proposed mechanism in a convincing and careful way.From this perspective, as well as the perspective of novelty this work has the potential to be of interest for the readership of Nature Communications.However, before any decision can be made, the authors should address the following issues: • The English should be improved quite generally throughout the entire text, as well as numerous typos should be eliminated by careful proof-reading.
• In both introduction and conclusions section, I feel that the authors are somewhat overselling their results by stating relevance for certain types of applications.Quite honestly, it is not obvious to me how the observed effect and approach described in this work really would be relevant/applicable the way it is proposed (e.g. it is not at all obvious to me how this method "opens exciting possibilites for bottom-up device fabrication".What kind of devices?In which area?In which way are cracks important in devices, etc.).Hence, I suggest that the authors either further elaborate in detail how they envision such applications to happen or simply reduce their claims and focus on highlighting the more fundamental -very interesting (!) -aspects of their work.I would argue that the latter should be (more than) enough.Also fundamental curious discoveries deserve to be in the spotlight!• In the context of Figure 1, the general arrangement of the sample could be better explained graphically, e.g., how the plasmonic substrate is localized with respect to the drying front of the colloidal film and how/where exactly the drying droplet is illuminated and imaged.Right now, I am guessing (?) that it is at the edge of the droplet, but it really is not clear.
• Related to the point above, in Figure 1c, where are these images taken?At the edge of the droplet?• The authors are describing the use of "Au spheres".I would argue that such terminology is incorrect because metallic colloidal nanocrystals rarely are spherical but actually facetted crystals.Hence the word "sphere" is misleading.
• When the Au "spheres" and bipyramids are introduced, they are not characterized in terms of their plasmonic properties and it is not motivated why two different types are used.This becomes clear only much later in the text when the different LPSR wavelengths are mentioned and there is referral to figures in SI.It would help the reader to understand this work if, at first mention, the different plasmonic properties of the two types of Au NPs used would be characterized (e.g. by an extinction spectrum of the colloidal solution) and shown, together with a short explanation of what can be achieved by having these two types of particles in terms of light manipulation/interaction.• In in the context of Figure 2g, it is demonstrated that changing the arrangement and size of the plasmonic dots can be used to tune the crack pattern.Then it is also mentioned that the same effect can be obtained using colloidal NPs of two different types, for which laser wavelengths matching their respective LSPRs are used.Here, it would be relevant to showcase also control experiments where the respective samples are illuminated also off-resonance, i.e. the "spheres" with 852 nm and the bipyramids with 532 nm.An even more elegant and convincing experiment that I highly recommend is actually execute a scan across multiple wavelength below, at, and above the LSPR of each type of particle, to really reveal the importance of LSPR excitation.
• In Figures 2 (h) and (i) the small bipyramid symbols are to illustrate different orientations according to the caption but they are oriented in the same way in the two panels?
• The experiments depicted in Figure 3 show how the crack formation in the drying colloidal film is impacted by laser illumination at the LSPR wavelength of Au bipyramids dispersed in the colloidal suspension.While the effect is clear, what I am lacking is an identical control experiment without Au nanoparticles in solution.
• Figure 3 (g) is explained as cross-sectional "micrograph".However, it is not explained how it was obtained.Furthermore, in the caption "hyperspectral analysis" is mentioned.What is meant by this?In which way is this micrograph "hyperspectral"?• Figure 4 d, e, f in particular (but in many other figures as well) font sizes used are very small.Please improve overall readability of figures in this respect • The explanations given in Figure S19 are very important and I therefore suggest to actually include this in the main text.
• On line 286 (as well as in in SI Figure 20) the authors mention "predicted" crack patterns.How are they predicted?Is any kind of model used?This is unclear and nowhere explained?! • On line 295 the authors mention "transparent" electrodes.It is unclear to me in which way this method could be used to make transparent electrodes when they are made from Au lines (which indeed interact with visible light?).Furthermore, what would be the advantage over making such electrodes simply with e.g.photolithography?
We would like to thank the reviewers for the time spent to evaluate our work and for their feedback and constructive comments.Please find hereafter our point-by-point response; in addition the modifications in the manuscript are highlighted in yellow.

REVIEWER COMMENTS
Reviewer #1 (Remarks to the Author): The authors report on a novel approach for patterning colloidal layers by controlling the propagation and nucleation of cracks during drying of the films.The key idea is to locally vary temperature in the drying films using photo thermal heating effects that can be localized via plasmonic nanoparticles.The local heating changes the evaporation rate and thus distorts the drying front and consequently influences the crack propagation.The authors demonstrate the effect both for droplet drying and in a more defined setting for crack formation in a dip coating scenario.They very convincingly illustrate that the effect can be triggered by homogenous illumination of patterned substrates or by patterned illumination of homogenous films in which either the substate can be heated plasmonically or the plasmonic particles are incorportated in the colloidal suspension.The analysis of the underlying phenomena is compelling.As well as the variety of patterning variations illustrates that this is a versatile method.It is a beautiful and elegant work of high quality.
Response: we thank the referee for the positive feedback.Below we address the remaining questions.
The only point that remains not fully clear to me is how general this mechanism is for other colloidal systems than the particluar one under investigation here.In my experience crack formation in drying colloidal layers typically is not as homogenous and following a strict periodicity as in this example.How is random crack nucleation due to defects suppressed ?Is this behaviour specific for the case of pluronics modification, can it be generalized to other particle types beyond PS particles ?The authors focus very much on manipulation of the crack formation and take the well defined periodic crack pattern for granted.The latter however is the prerequisite for the control.So it would be important to lear more about how easy it is to achieve such well defined crack formation for other colloidal layers.
Response: we agree with the point raised by the referee regarding the generality of the crack self-ordering process and its applicability to other colloidal systems.In principle, the crack self-ordering process can be generalized to other colloidal solutions with different compositions, sizes and solvents.To support this statement, we provide several micrographs showing oriented crack patterns in colloidal films made of YAG:Ce, SiO 2 and PMMA colloids, obtained by drying an aqueous colloidal droplet.
In these cases, we observe radially aligned cracks similar to those shown for PS in Figure 1 and Figure S3.However, we also need to specify that the overall quality of the periodic patterns and the presence of defects vary significantly from one system to another.At this stage, to the best of our knowledge, there is no general rule, and the characteristics of the colloidal solution (concentration, stabilizer, etc.) need to be optimized on a case-by-case basis.Based on that, PS particles give the best results in terms of quality and ordering that this is the reason why, we focused on this system.It is indeed important to discuss this point.
ACTION TAKEN To reflect the above considerations, we have included in this revised version: • a discussion at page 14 on how the crack self-ordering process can be generized to other colloidal systems • the micrographs with YAG:Ce, SiO 2 and PMMA in SI as Figure S24 230 nm SiO 2 colloids 300 nm PMMA colloids The manuscript presents a novel and intriguing approach to control the propagation of cracks in colloidal films using light.The experiments are well-designed, and the results are supported by the data presented.
Response: we thank the referee for the positive feedback.Below we address the remaining questions.
The role of gold particles and their plasmonic properties in influencing crack propagation is particularly interesting.However, a more detailed explanation of the relationship between the plasmon wavelength and irradiation, as depicted in Figure 3, would enhance the clarity of the findings.
Response: we thank the reviewer for the suggestion.To clarify the relationship between plasmon wavelength and irradiation we have taken two actions.
ACTIONS TAKEN: 1. We added Figure 1(e) to illustrate the ideal relationship between plasmon wavelength and laser wavelength.In addition the following sentences have been added in page 6: " We attributed the origin of the light directed self-ordering to evaporation driven photo-thermal effect.To achieve high efficient photo-heating, the laser wavelength ideally needs to match the absorbing specific wavelengths on the plasmonic nanoparticles as illustrated in Figure 1(e). 33When the nanoparticles are exposed to laser irradiation at their resonance frequency, a maximun of energy from the absorbed photons is converted into heat.Irradation at different resonances results in lower photothermal heating."To assess the photothermal heating effect, we irradiated the colloidal system with lasers at different wavelengths (532 and 852 nm) while maintaining the same power of 3.39 W/cm².As shown in Figure (b), irradiation at 852 nm led to the maximum heating, reaching temperatures exceeding 100°C.In contrast, irradiation at 532 nm was less efficient, resulting in a temperature of approximately 40°C.This reduced efficiency can be attributed to the weaker transverse absorption peak and the mismatch with the resonance wavelength of the transverse absorption peak.
For reference, we compared these temperature values with the temperature of a system without Au bipyramids, which did not exhibit significant heating.When applied to the light-induced dip-coating experiment for deviating the cracks, 532nm irradiation results in a lower deviation compared to the 852nm, as shown in Figure (c).In summary, we confirm that exciting plasmonic particles off-resonance reduces photon absorption, heat conversion, and, consequently, crack deviation.
The role of the PS particles is also somewhat secondary in this study and it would be interesting to know how the size of these particles influences the experiments.It is especially interesting to know what would happen with even larger particles, of a size comparable to the wavelength of the laser used, and much more monodisperse (from the images it seems that the ones used in the experiments are not) since it would give the possibility to study self-assembly phenomena and their possible manipulation.In addition, even larger particles (more than one micron) could resonate with the lasers used (through MIE modes) and introduce even more possibilities.I would like the authors to discuss this in the final version of the paper and consider the possibilities beyond crack control.
Response: We thank the reviewer for this suggestion.We agree that using larger particles of a size comparable to the wavelength of the laser would open up very interesting possibilities for generating additional optical features.
For instance, as mentioned by the reviewer, larger monodispersed colloids can be self-assembled reversibly into 2D or 3D photonic crystals, giving rise to structural colors (J.Am.Chem.Soc. 2021, 143, 30, 11535-11543).As an intriguing perspective for our work, the colloidal self-assembly process and the crack formation could be monitored by tracking the optical evolution of the photonic structure.In addition, using even larger-sized colloids would enable even more sophisticated interactions between light and the colloidal assembly, such as multiple scattering, optical amplification of light resulting reconfigurable random lasing (Nat.Phys. 2022, 18, 939-944).We anticipate that these are indeed some of our short-term perspectives for a follow-up article.
ACTION TAKEN These perspectives are now addressed in the revised manuscript on page 16.
Reviewer #3 (Remarks to the Author): The authors present an interesting study in which they show how the interplay between visible light laser illumination, plasmonic nanoparticles and a colloidal suspension can be harnessed to steer the growth direction of cracks in a forming colloidal thin film on a surface.As the underlying mechanism they propose the local heating via light absorption and dissipation in either Au plasmonic particles on the colloidal film substrate or dispersed directly in the colloidal suspension.In general, this study is well executed with a suite of experiments that test and verify the proposed mechanism in a convincing and careful way.From this perspective, as well as the perspective of novelty this work has the potential to be of interest for the readership of Nature Communications.
Response: we are delighted that the referee believes our work is of potential interest for publication in Nature Communications and are thankful for the constructive comments.Below we address the remaining questions.
However, before any decision can be made, the authors should address the following issues: • The English should be improved quite generally throughout the entire text, as well as numerous typos should be eliminated by careful proof-reading.
Response: thank you for the suggestion, we have revised our manuscript to remove typos and rephrase some sentences.
• In both introduction and section, I feel that the authors are somewhat overselling their results by stating relevance for certain types of applications.Quite honestly, it is not obvious to me how the observed effect and approach described in this work really would be relevant/applicable the way it is proposed (e.g. it is not at all obvious to me how this method "opens exciting possibilites for bottom-up device fabrication".What kind of devices?In which area?In which way are cracks important in devices, etc.).Hence, I suggest that the authors either further elaborate in detail how they envision such applications to happen or simply reduce their claims and focus on highlighting the more fundamental -very interesting (!) -aspects of their work.I would argue that the latter should be (more than) enough.Also fundamental curious discoveries deserve to be in the spotlight!Response: thank you very much for the comment; we couldn't agree more with the reviewer's view.Indeed, even though this method may have implications for the fabrication of patterned surfaces, our work is fundamentally driven by scientific curiosity.

ACTION TAKEN
In line with the reviewer's suggestion, we have removed comments about applications from the abstract, introduction, and conclusions.
• In the context of Figure 1, the general arrangement of the sample could be better explained graphically, e.g., how the plasmonic substrate is localized with respect to the drying front of the colloidal film and how/where exactly the drying droplet is illuminated and imaged.Right now, I am guessing (?) that it is at the edge of the droplet, but it really is not clear.
• Related to the point above, in Figure 1c, where are these images taken?At the edge of the droplet?Response: Thank you for your comments; we will address both questions in a single response.The reviewer's observation is accurate -the images were indeed captured at the edge of the droplet.

ACTION TAKEN
To improve clarity, we have made changes to Figure 1 by including Figure 1(c), which provides a top-view illustration of the drying colloidal droplet, the plasmonic substrate (covering the entire substrate), and the position of the laser beam at the droplet's edge.Furthermore, we have revised the accompanying text to explicitly mention that the micrographs were taken at the droplet's edge.We believe that this visual representation will aid the reader in better comprehending the sequence of micrographs in Figure 1(d).
• The authors are describing the use of "Au spheres".I would argue that such terminology is incorrect because metallic colloidal nanocrystals rarely are spherical but actually facetted crystals.Hence the word "sphere" is misleading.
Response: We agree with the reviewer, the term "sphere" is not accurate.
ACTION TAKEN Consequently, we have amended the text, and we now refer to these materials as "dewetted Au nanoparticles" as has been previously employed in the literature (ACS Appl. Nano Mater. 2019, 2, 5, 3238-3245).
• When the Au "spheres" and bipyramids are introduced, they are not characterized in terms of their plasmonic properties and it is not motivated why two different types are used.This becomes clear only much later in the text when the different LPSR wavelengths are mentioned and there is referral to figures in SI.It would help the reader to understand this work if, at first mention, the different plasmonic properties of the two types of Au NPs used would be characterized (e.g. by an extinction spectrum of the colloidal solution) and shown, together with a short explanation of what can be achieved by having these two types of particles in terms of light manipulation/interaction.Response: We appreciate the reviewer's suggestion and agree that the justification for choosing these two plasmonic systems should be provided earlier in the text.

ACTION TAKEN
To address this, we have incorporated an explanation at the end of the introduction with the following sentences."We engineer various colloidal plasmonic systems to spatially and temporally program the heating zones, acting as "virtual defects".More specifically, we investigated two systems: gold dewetted nanoparticles and gold bipyramids (Au BPs), which convert light into heat in the visible or near-infrared range, respectively.Moreover, we chose these two plasmonic systems to explore two different strategies: integrating the photothermal heaters on the substrate or within the colloidal solution."With these sentences, we aid the reader by anticipating the optical properties and the reasons for using the two plasmonic systems.However, we also believe that, for the sake of clarity and to avoid back-and-forth reading, the detailed optical characterization of each plasmonic system is better integrated into the specific discussion of each experiment.
• In in the context of Figure 2g, it is demonstrated that changing the arrangement and size of the plasmonic dots can be used to tune the crack pattern.Then it is also mentioned that the same effect can be obtained using colloidal NPs of two different types, for which laser wavelengths matching their respective LSPRs are used.Here, it would be relevant to showcase also control experiments where the respective samples are illuminated also offresonance, i.e. the "spheres" with 852 nm and the bipyramids with 532 nm.An even more elegant and convincing experiment that I highly recommend is actually execute a scan across multiple wavelength below, at, and above the LSPR of each type of particle, to really reveal the importance of LSPR excitation.
Response: We agree with reviewer regarding scanning multiple wavelengths, which would indeed be very interesting and elegant.However, this presents significant technical challenges, as it would require the use of numerous (powerful and costly) lasers at different wavelengths that are currently beyond our reach.Nevertheless, in response to the reviewer's request, we conducted experiments where we excited the plasmonic particles with laser wavelengths that do not match their respective main Localized Surface Plasmon Resonance (LSPR).It's essential to note that when exciting plasmonic particles off-resonance, we expect a reduced or negligible absorption of photons and conversion into heat.
We characterized the temperature increase using an IR camera and analyzed crack deviation through image analysis.In the case of the dewetted particles irradiated with an 852nm laser, the IR camera showed no temperature increase, confirming that off-resonance irradiation does not induce photothermal heating.The Au bipyramids, irradiated with the different lasers, displays a wavelength depend crack deviation as shown in the newly added Figure S18.To assess the photothermal heating effect, we irradiated the colloidal system with lasers at different wavelengths (532 and 852 nm) while maintaining the same power of 3.39 W/cm².As shown in Figure (b), irradiation at 852 nm led to the maximum heating, reaching temperatures exceeding 100°C.In contrast, irradiation at 532 nm was less efficient, resulting in a temperature of approximately 40°C.This reduced efficiency can be attributed to the weaker transverse absorption peak and the mismatch with the resonance wavelength of the transverse absorption peak (515 vs 532 nm).
For reference, we compared these temperature values with the temperature of a system without Au bipyramids, which did not exhibit significant heating irradiated at 852 nm.When applied to the light-induced dip-coating experiment for deviating the cracks, 532nm irradiation results in a lower deviation compared to the 852nm, as shown in Figure (c).In summary, we confirm that exciting plasmonic particles off-resonance reduces photon absorption, heat conversion, and, consequently, crack deviation.

ACTION TAKEN
The Figure is now added in SI as Figure S18 and the discussion is integrated in the manuscript in page 13.
• In Figures 2 (h) and (i) the small bipyramid symbols are to illustrate different orientations according to the caption but they are oriented in the same way in the two panels?
Response: We appreciate the reviewer for pointing this out.We now realize that the caption and description may have been misleading.The small bipyramid symbols are not intended to represent pattern orientation; they are simply used to distinguish samples made of dewetted Au nanoparticles from those made of Au BPs.

ACTION TAKEN
2.In line with what was also requested by Reviewer 3, we conducted an experiment to demonstrate the wavelength-dependent crack deviation, especially for the Au BPs system.When exciting plasmonic resonance, we basically expect a reduced or negligible absorption of photons and conversion into heat.We characterized the temperature increase using an IR camera and analyzed crack deviation through image analysis.The Au bipyramids, irradiated with the different lasers, exhibit a wavelength depend crack deviation as shown in the following Figure (S18).

Figure
Figure (a) illustrates the absorbance spectrum of colloidal Au bipyramids, which exhibit two absorption peaks.The spectrum displays a prominent longitudinal absorption peak at around 850 nm and a weaker transverse absorption peak at approximately 515 nm.To assess the photothermal heating effect, we irradiated the colloidal system with lasers at different wavelengths (532 and 852 nm) while maintaining the same power of 3.39 W/cm².As shown in Figure(b), irradiation at 852 nm led to the maximum heating, reaching temperatures exceeding 100°C.In contrast, irradiation at 532 nm was less efficient, resulting in a temperature of approximately 40°C.This reduced efficiency can be attributed to the weaker transverse absorption peak and the mismatch with the resonance wavelength of the transverse absorption peak.For reference, we compared these temperature values with the temperature of a system without Au bipyramids, which did not exhibit significant heating.When applied to the light-induced dip-coating experiment for deviating the cracks, 532nm irradiation results in a lower deviation compared to the 852nm, as shown in Figure(c).In summary, we confirm that exciting plasmonic particles off-resonance reduces photon absorption, heat conversion, and, consequently, crack deviation.

Figure
Figure (a)  illustrates the absorbance spectrum of colloidal Au bipyramids, which exhibit two absorption peaks.The spectrum displays a prominent longitudinal absorption peak at around 850 nm and a weaker transverse absorption peak at approximately 515 nm.To assess the photothermal heating effect, we irradiated the colloidal system with lasers at different wavelengths (532 and 852 nm) while maintaining the same power of 3.39 W/cm².As shown in Figure(b), irradiation at 852 nm led to the maximum heating, reaching temperatures exceeding 100°C.In contrast, irradiation at 532 nm was less efficient, resulting in a temperature of approximately 40°C.This reduced efficiency can be attributed to the weaker transverse absorption peak and the mismatch with the resonance wavelength of the transverse absorption peak (515 vs 532 nm).For reference, we compared these temperature values with the temperature of a system without Au bipyramids, which did not exhibit significant heating irradiated at 852 nm.When applied to the light-induced dip-coating experiment for deviating the cracks, 532nm irradiation results in a lower deviation compared to the 852nm, as shown in Figure(c).In summary, we confirm that exciting plasmonic particles off-resonance reduces photon absorption, heat conversion, and, consequently, crack deviation.