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Challenges on Optical Printing of Colloidal Nanoparticles

While colloidal chemistry provides ways to obtain a great variety of nanoparticles, with different shapes, sizes, material composition, and surface functions, their controlled deposition and combination on arbitrary positions of substrates remains a considerable challenge. Over the last ten years, optical printing arose as a versatile method to achieve this purpose for different kinds of nanoparticles. In this article we review the state of the art of optical printing of single nanoparticles, and discuss its strengths, limitations, and future perspectives, by focusing on four main challenges: printing accuracy, resolution, selectivity, and nanoparticles photostability.


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There is a family of techniques that provide higher flexibility of pattern design by using light to dynamically create regions of the substrate with higher affinity for the colloids.
Optoelectronic Tweezers use light and an electric bias to sculpt a potential landscape on a photosensitive substrate. 5,6 The resulting non-uniform electric field exerts dielectrophoretic forces on NPs that push them towards the illuminated regions. In optothermophoretic printing, 7 absorbing substrates are illuminated to generate a thermal field that drives the NPs towards the hot illuminated regions by means of thermophoretic forces. Bubble-pen lithography 8,9 employs a laser beam to generate microbubbles at the interface of a colloidal suspension and a plasmonic substrate. The bubbles capture and immobilize the NPs. All these techniques allow the serial deposition of individual NPs with high accuracy and versatility of pattern design. However, they require specialized substrates such as highly absorbing or conducting films.
By contrast, optical printing uses optical forces to capture individual NPs from the colloidal suspension, push them towards the substrate, and fix them into predefined locations through van der Waals attraction. This technique is especially suitable for metallic or high-index dielectric NPs, because their plasmon or Mie-like resonances strongly enhance the optical forces, that are otherwise low for sub-micrometer particles. [10][11][12] Optical printing enables the selective immobilization of different kinds of NPs using light of different wavelengths, as long as the NPs present spectrally differentiable resonances. 13,14 In 2010, the groups of Norbert Scherer at the University of Chicago 15  presented by the Scherer group consisted in two steps. First, a single Au NP (40 nm in diameter) was trapped with an optical tweezer working at 817 nm. Then, by manually moving the focus, the optically trapped NP was pushed against the substrate and finally attached to it by van der Waals forces. In the work by the group of Feldmann, single Au NPs (80 nm in diameter) were captured from the colloidal suspension and pushed towards the substrate, without any previous stable trapping step, using a focused laser beam tuned to the plasmon resonance of the NPs. Since then, optical printing has been employed to produce patterns of single 13,14,[17][18][19][20][21] or aggregated [22][23][24] NPs of different sizes, shapes, and materials, including gold, silver, and silicon NPs, onto various substrates.
Here, we review and discuss the advances made to date on optical printing of colloidal NPs, organized in four main challenges: printing accuracy, spatial resolution, selectivity, and photostability of NPs ( Figure 1). First, we briefly describe the fundamentals of the method. Then, the advances made with regard the four challenges are evaluated, including in some cases unpublished data from our group. Finally, outlook and perspectives on the field are presented.

Fundamentals of optical printing
For optical printing, a colloidal suspension of NPs with certain net electric charge is put in contact with a substrate with the same charge sign. In this way, spontaneous deposition of the NPs onto the substrate is avoided by electrostatic repulsion. The basis of optical printing is to overcome the repulsion barrier on specific locations of the substrate by applying optical forces on the NPs. This is achieved using a laser beam tightly focused onto the substrate. 5 The process of optical printing can be summarized in three steps as schematized in Figure   2a.
1) A NP performs free diffusive Brownian motion until it randomly reaches the beam focus. The duration of this diffusive motion depends on NP concentration and laser power, and typically lies in the order of a few seconds. The region of space where optical forces dominate over Brownian motion is defined as the "capture volume", depicted with a green dashed line in Figure 2a. 11 The capture volume can be estimated as the region where the Peclet number is higher than 1, as described in Gargiulo et al. 18 2) Once in the capture volume, optical forces drive the NP towards the substrate. This motion is overdamped (Reynolds number is <<1) and optical force field lines can be interpreted as velocity lines or trajectories. The time scale for this guided motion inside the capture volume is on the order of milliseconds.
3) An interaction between the NP and the substrate appears at separations of a few tens of nanometers, consisting of an electrostatic repulsion and an attractive van der Waals force . The total interaction = + may be obtained by differentiation of the interaction energies between a charged sphere and a charged surface using the Derjaguin-Landau-Verwey-Overbeek (DLVO) formalism 26 . Figure 2b (top) shows an example of DLVO potentials for a 60 nm Au NP near a flat charged surface. As the NP approaches the substrate, the electrostatic repulsion generates a barrier that prevents the NPs from reaching the substrate. There is a minimum axial force (threshold force, ℎ ) that must be applied to the NP to overcome the repulsion barrier. If the NP is pushed over the repulsion barrier, it gets fixed on the substrate by the attractive van der Waals force.
It is then said that the NP has been printed.
Figure 2b (bottom) shows a comparison of for the cases of a Au NP and a Ag NP, both 60 nm in diameter, close to a glass substrate. In this case, the difference in the threshold force ( ℎ ) relies on the different zeta potential (surface charge) of the two types of NPs, as the rest of the parameters are identical.
Implementation and examples of optical printing 6 Figure 2c shows schematically an example setup for optical printing. It basically consists of an upright microscope where the printing laser beam is focused onto the sample by the microscope objective. Usually, a water immersion objective is used in direct contact with the colloidal suspension. In fact, to protect the objective from the adhesion of NPs or other colloid components, a transparent film (food cling film, for instance) may be used to protect the objective lens. Alternatively, the substrate and the colloid may be set in a (flow) chamber, enabling the use of other types of objectives. Various techniques or detection channels may be included to monitor the printing process or characterize the NPs. For example, dark-field imaging is usually employed to monitor the printing process and to visualize the produced patterns of NPs. 16 Confocal detection of the scattering by the NPs provides a useful signal to automate the printing process. 17  Arrays of NPs can be fabricated by sequentially printing single NPs on the desired positions. This is achieved by using a sample or a beam scanner to move the printing laser to the next position once a NP is printed. Figure 2d and 2e show dark-field images of two arrays made by optical printing Au (green) and Ag (blue) NPs, both 60 nm in diameter.
A remarkable property of optically printed NP arrays is their high stability over time. For example, the pattern shown in Figure 2e was fabricated in 2017. A recent inspection shows no significant changes after 4 years. All NPs remained in their original positions.
In between, the sample was dried, manipulated and stored in a drawer at room temperature. In addition, Section S1 of the electronic supplementary information (ESI) shows images of samples that were subjected to vacuum and electron microscopy imaging, also showing high stability.
Alternatively, arrays of NPs may be fabricated by parallel optical printing. Using a spatial light modulator or phase masks, extended foci may be generated to produce optical printing over lines, rings ( Figure 2f) or any other shape. 28 Also, multi-beam arrays may be used to print several NPs in parallel 29 (Figure 2g).

Challenge 1: Accuracy of optical printing
The first challenge is to immobilize the NPs with the maximum possible positional accuracy. Despite being an optical method, already in the first experiments it became clear that the positional accuracy of optical printing was not restricted by the diffraction limit of light. In 2010, Urban et al. 16 printed 80 nm Au NPs with a laser of 532 nm, a wavelength near to the plasmon resonance of the NPs. They printed NPs along a line and estimated the printing precision as the average deviation from the line, which was found to be 50 nm (Figure 3a). They also found that the best accuracy was achieved by using the minimum laser power needed to overcome the electrostatic barrier. In addition, the influence of the electrostatic barrier strength was studied by using substrates coated with increasing numbers of polyelectrolyte layers. It was found that higher laser powers were required for printing when more layers were present, indicating a stronger repulsion. 8 Larger deviations from the target line were observed for the higher number of layers, obtaining an accuracy of ∼100-150 nm.
In the second seminal work of optical printing, Guffey et al. 15 used light out of resonance (λ = 817 nm) to first trap 40 nm Au NPs, and then bring them into contact with a glass substrate by manually moving the focus. The printing accuracy was found to be around ∼100 nm, which was larger than the standard deviation of the particle position   respectively. On-resonance, the Au NPs were printed at 532 nm (green circles) and the Ag NPs were printed at 405 nm (blue squares). For both, the precision decreases when the laser power increases. Off-resonance, the Au NPs were printed at 640 nm (green circles) and the Ag NPs at 532 nm (blue squares). Interestingly, in this case the 90 was found to be independent of the laser power and larger than the best achieved precision in the resonant case.
As expected from the balance between the optical axial force and the DLVO interaction with the substrate, it was not possible to print particles below a certain threshold of laser power ℎ ( ℎ , Figure 2b). Calculating the optical and the DLVO forces enabled a quantitative prediction of this behaviour. 18 In Figure 3g, at the substrate is compared to ℎ . The region where optical printing is possible is determined by the condition > ℎ which defines a maximum radius depending on the laser power. This maximum radius was called the theoretical "printing radius" . In Figure 3e, is shown along with 90 as a function of the laser power for resonant printing of Ag NPs (blue solid line) and Au NPs (green solid line). The theoretical represents very well the experimental values 90 and correctly predicts the value for the threshold power ℎ , remarkable facts given that there were no free parameters involved in the calculations. On the other hand, for the off-resonance case, it can be seen in Figure 3f that the printing radius does not predict the nearly constant experimental 90 , indicating that a deeper analysis is necessary.
A more detailed discussion is feasible by considering the possible trajectories imposed by the optical forces on the NPs to be printed. Figures 3h and 3i show the trajectories for NPs optically printed on-resonance and off-resonance, respectively, for three laser powers. The achieved with each laser power is depicted by colored boxes at = 0 (i.e., at the substrate position). On-resonance, increasing the laser power leads to more trajectories becoming available for printing that enlarge (i.e., lower precision). By contrast, off-resonance, most trajectories reaching the substrate become available as soon as the laser power surpasses the threshold for printing. Further increments of the laser power only lead to minor or negligible increments of the number of trajectories available 11 for printing, explaining the independence of 90 on laser power for off-resonance printing.
Overall, it can be concluded that the best accuracy for optical printing spherical NPs with Gaussian beams is achieved on resonance and close to threshold power ℎ . At these conditions, 90% of the NPs are printed within 100 nm of the target, and the radial rootmean-square deviation is in the order of 50 nm. These numbers are considerably smaller than the waist of the printing beam and are comparable to the diameter of the NPs. The prediction of the printing accuracy requires two steps: 1) calculating the printing radius from the balance between the optical force and the repulsion by the substrate, and 2) an analysis of the possible trajectories to determine the regions of the printing radius that are likely to be filled.
We note that the above analysis is valid for printing freely diffusing colloidal NPs, where  In summary, it can be concluded that NPs with anisotropic optical properties can be optically printed with preferential orientations using linearly polarized laser beams.

Challenge 2: Lateral resolution
Another important challenge of optical printing concerns its resolution, understood as the minimum distance at which two particles can be printed in a controlled manner. Already in 2013, a limitation to print nanoparticles at short distances was identified. Urban et al. 31,32 observed that "printing of two or more gold nanoparticles in close proximity to each other has been a challenge and separation distances below 300 nm have not been achieved experimentally." A strategy to overcome this problem was presented by Urban et al. 31  the Ag NP is printed first, the repulsion disappears completely, and optical printing is possible and free of interactions for every set distance. Figure 5d shows dark-field and SEM images of exemplary Ag-Au heterodimers, printed with a set distance of 60 nm, equal to the NP diameter. Remarkably, it was possible for the first time to connect a pair of NPs using optical printing. Furthermore, the orientation of the dimer was well controlled.
The experiments described so far confirm that the resolution of optical printing may be limited by a light-induced repulsion. First, the transitions between non-interacting to interacting regimes occur at set distances comparable to the laser beam waist at the focal plane (~250 nm), i.e., where the already printed NP is illuminated. Second, the magnitude of the repulsion depends on the optical interaction between the first NP and the printing wavelength of the second one. It is strong for Au at 532 nm, weak for Au at 405 nm, and negligible for Ag at 532 nm, consistent with the optical extinctions at each respective wavelength (see Figure S2). If the first printed NP is transparent to the printing wavelength of the second NP, the repulsion is avoided, and optical printing can place NPs at arbitrarily small separations. While scattering and optical binding may generate considerable optical forces in other configurations, they were not the main cause of the observed light-induced repulsion, which was then ascribed to absorption by the printed NP. When a laser focus is set to print a NP near an already printed NP, both NPs absorb light and generate a local temperature increase. In 2017, Gargiulo et al. 19 studied the influence of photothermal heating in the printing process by implementing different strategies to enhance heat dissipation. First, they printed Au-Au 60 nm dimers with different set gaps on a sapphire substrate, a material with higher thermal conductivity (κ ∼ 20 W/m K) than glass (κ ∼ 1 W/m K). Figures 6a and 6b show the calculated temperature increase for a 60 nm Au NP on a glass and a sapphire substrate, respectively. In both cases, the NP is immersed in water and at the center of Gaussian laser beam of 532 nm wavelength, 266 nm waist, and 17 1.2 mW of power. On sapphire, the temperature increase is 7-fold smaller than on glass. Figure 6c shows the measured gaps of dimers of 60 nm Au NPs optically printed on glass and sapphire, versus the set gap. Remarkably, on sapphire, the light induced repulsion is considerably reduced, and optical printing of the second NP is possible at any set gap. An interacting with printing regime for set gaps smaller than 200 nm is observed, leading to experimental gaps as small as 150 nm. On glass, optical printing became impossible at set gaps smaller than 230 nm (horizontal dashed line in Figure 6c). Then, optical printing a Au NP next to a Au nanodisk (ND) was tested. Disks were fabricated on sapphire substrates using top-down lithography. Since they have a larger contact area with the substrate, a better heat dissipation is expected. Figure 6c  substrate, laser power, and distance between the particle and the printing beam. Notably, the temperature increase of the fixed particle is a good predictor of the error when printing a second particle, strongly indicating that the resolution of optical printing is limited by photothermal effects.
Several plausible mechanisms for the photothermal repulsion were evaluated by Gargiulo et al. 19 , including natural convection, Marangoni convection, thermo-osmotic flows, and thermophoresis. They found that a thermophoretic interaction between NPs explains the experimental observations and that the other phenomena only play a minor or negligible role. Thermophoretic forces ℎ were calculated based on temperature maps, using ℎ = −6 μ ∇ with μ the dynamic viscosity, the NP radius, the thermodiffusion coefficient and ∇ the temperature gradient. is typically positive. In that case, NPs move from hot to cold regions. Figure 6e schematically shows the forces active during optical printing of dimers.
In addition to the optical force, thermophoretic repulsion pushes the second NP away from the hot fixed NP towards the cold solution. Figure 6f shows total force fields,  as well as their sorting 13 by size, which had been proposed theoretically a few years before. 39 In this work, Si NPs were optically printed from a polydisperse colloidal suspension using three different wavelengths (405 nm, 532 nm, and 640 nm). Figure 7g shows dark-field images of the Si NPs printed at each wavelength, along with a representative SEM image. Clearly, the Si NPs can be printed in a size selective manner by tuning the laser wavelength. The size selectivity is better observed in the size distributions shown in Figure 7h. In grey, the size distribution of the initial colloid is shown, which includes NPs with radii ranging from 20 nm to 110 nm. Optical printing selects subpopulations of NPs with narrow size distributions with widths of around 10 nm. Individual Si NPs optically printed at the different wavelengths were subjected to correlated single particle scattering spectroscopy and FE-SEM imaging. Figure 7i shows the NP radius ( ) versus the position of the scattering maximum ( ) for 115 optically printed Si NPs. The data follows very well the theoretical prediction for the magnetic dipolar resonance = 2 (with the NP refractive index). The presence of a unique magnetic resonance that predominantly contributes to the optical force for a particular size enables the size-selective optical printing. For a given wavelength, differential optical forces can be applied to the NPs, so that only NPs of a certain size would be capable of surpassing the electrostatic barrier and be printed. It is important to point out that laser power is also a key parameter. If the laser power is too high, the size selectivity is reduced and eventually lost, as shown in Figure S4 of the ESI.
These two studies clearly show that on-resonance optical printing can be used to print selected subpopulations of NPs from a mixed colloid, according to their differential interaction with light.
Challenge 4: Photostability during optical printing 22 Optical printing was developed with the perspective of utilizing virtually any colloidal NP on solid substrates, circuits, and devices. Achieving that goal would imply that the optical printing process should not alter the properties, morphology, or functionalities of the NPs. However, the high laser power densities employed during the printing impose a challenge on this respect because they could lead to NP deformation or the degradation of surface functionalities. The dark-field images shown in Figure 8a illustrate the harmful effects of high laser power density when printing spherical Ag NPs on-resonance. Most NPs printed at low laser power appear blue, as expected from their plasmon resonance. Copyright 2016 American Chemical Society.

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Anisotropic NPs can undergo morphological changes during optical printing. For example, our group studied the optical printing of Au NRs with an aspect ratio of ~4.4 (longitudinal plasmon resonance at 896 nm) on glass substrates at wavelength of 808 nm, and noticed a length reduction upon printing. Figure 8b shows two FE-SEM images of representative NRs before (blue) and after (orange) printing, as well as length distributions of Au NRs before and after the printing process. The length distribution of printed Au NRs is centered at shorter lengths and with practically no overlap with the length distribution of Au NRs before printing. Thus, it can be concluded that the shorter length of the printed NRs is not due to selective printing, and that reshaping of the NRs is occurring during the printing process. The exact mechanism of this reshaping is yet unclear. Considering the experimental parameters, the temperature of the Au NRs may be increased up 535 K during printing (see Section S5 of the ESI for more details).
Although this value is far below the bulk melting temperature of gold, reshaping can still occur, for example due to activated diffusion of surface atoms. 41 The light-induced morphological changes during printing can potentially be used as a tool to control the shape of the printed nanostructure. In 2016, Babynina et al. demonstrated that it is possible to bend Au NRs (aspect ratio 5.9, longitudinal plasmon resonance at 1064 nm) when printing on-resonance. 25 Furthermore, the bending was more pronounced for higher printing irradiances, as illustrated in Figure 8c. These authors speculated that the bending occurs due to the combination of plasmonic heating with high hydrodynamic pressure working on the tips of the NR as it moves through water towards the substrate, pushed by the optical force. Furthermore, they observed the presence of twin grain boundaries near the bent nanorod vertex, indicating the reorganization of the gold atoms.
Heterogeneous or hybrid NPs are also prone to reshaping during the printing process. For example, the optical printing of silica-gold core-shell NPs (SiO2@Au) was investigated in our group. SiO2@Au NPs (150 nm SiO2 core -20 nm Au shell) were optically printed on a glass substrate using a 640 nm laser beam. The morphology and composition of the printed NPs was not always preserved. Three outcomes were observed, as depicted in SiO2@Au core-shell NP. Furthermore, Figure 9d (blue solid line) shows the scattering spectrum of that individual NP (see more statistics in Section S6 of the ESI), nicely overlapping with the analytical spectrum calculated using Mie theory (dashed black line).
On the other hand, a dimer can be seen in the orange box. The left particle in the dimer has a diameter of (142 ± 4) nm, potentially corresponding to the SiO2 core. The right NP has a diameter of (151 ± 4) nm, roughly matching the volume of the average gold shell in the SiO2@Au NPs. The scattering spectrum of this dimer is shown in Figure 9d, orange solid line. Interestingly, it matches the scattering spectra of a Au sphere with a diameter of 152 nm, indicating that the particle in the right of the dimer is a Au NP. The SiO2 NP has a weak scattering cross section and is not visible by dark-field microscopy or spectroscopy. This is the case for the left particle in the dimer and the NP in the green box. These observations indicate that during the printing process, the energy absorbed by the NP could induce adatom diffusion of the gold shell, producing a gold NP, which may be lost during the printing process, or end up attached to the silica core forming silica-Au NP dimer.
SiO2@Au NPs exhibit broad optical resonances over a large region of the visible and IR spectrum, as shown in the single particle scattering spectra of Figure 9d, meaning that they can be printed with many different wavelengths. In such cases, a strategy to minimize photothermal damage of NPs during the printing is to find a wavelength that maximizes the axial optical force while minimizing the absorption. The ratio between the radiation pressure efficiency and the absorption efficiency as a function of the wavelength, for the SiO2@Au NPs, is shown in Figure 9e (black curve). 42 Interestingly, the efficiency ratio / has a value of 1.5 at 640 nm, but peaks at 8.3 at 900 nm. A detailed analysis of and indicates that, in comparison to 640 nm, a printing beam of 900 nm would double the optical force while reducing the photogenerated heat by a factor of 2.8 ( Figure S7). Therefore, printing with 900 nm would reduce photothermal damage significantly and could be the key to print intact SiO2@Au NPs. On the contrary, considering that optical printing of bare SiO2 NPs is challenging due to their weak optical response, using a wavelength that maximizes / , e.g. around 500 nm, could enable the printing of bare SiO2 NPs using the SiO2@Au NPs as precursors. It is interesting to remark that this strategy is not always available. For instance, the / vs.
wavelength for 60 nm spherical Au NP (Figure 9e, red curve) presents only minor variations. The preservation of the surface functionality of colloidal NPs is another key challenge.
During the printing process, the temperature of NPs can increase several hundreds of degrees, destabilizing their surface functionalization. For example, due to their high affinity for gold 44 Figure 6a.
However, it should be noted that the high temperatures during the printing process are only reached during a short time of a few milliseconds or less, 18 so the stability of the functionalizing groups will depend on the particular kinetics of each reaction.
The possibility of printing functionalized Au NP was experimentally tested by Do et al. 43 They printed 60 nm Au NPs surface-functionalized with DNA-origami 46 sheets (hereafter Au@DNA-origami) as illustrated in Figure 9f. The Au NPs were functionalized with a thiolated chain of 24 thymine bases, to which the DNA sheets were attached by DNA hybridization. As DNA origami is also temperature sensitive, 47,48 they performed a so called soft optical deposition consisting of trapping the Au@DNA-origami NPs with a laser off-resonance, and subsequently approaching them to a positively charged substrate, i.e., having the opposite charge than the negatively charged NPs. The elimination of the substrate repulsion allowed them to use sufficiently low laser powers to significantly decrease the absorption and managed to print intact Au@DNA-origami NPs (Figure 9g).
However, this approach has the important disadvantage that NPs may attach to the substrate spontaneously in random positions due to the electrostatic attraction. The authors minimized this by using highly diluted colloids, which in turn made the printing process very slow. Increasing the trapping power to 229 mW, which corresponded to NP temperatures of 329 K, led to degradation of the 2D DNA sheets (Figure 9g). A later report by Goodman et al. 49 found similar results when illuminating SiO2@Au NPs coated with double-stranded DNA. These results indicate that, when performing optical printing of DNA-modified NPs, it is more likely to reach the melting temperature of the DNA duplex than breaking the Au-S bond. 50 The different examples presented in this section demonstrate that light absorption and heating during the printing process may alter the NP composition, shape, or surface functionalization. Nonetheless, there are clear avenues to optically print intact NPs, such as looking for wavelengths that maximize the ratio between optical forces and absorption, or finely tuning the NP-substrate repulsion to minimize the laser power required for printing. In addition, minimizing unnecessary exposure to the laser beam could be achieved by automating the printing process to rapidly turn off the laser upon printing of a NP. 27

Conclusions and perspectives
Optical printing is a nanofabrication technique that allows the positioning, orientation, combination, and selection of individual NPs from a colloidal suspension into arbitrary patterns on a variety of substrates. Here, we have reviewed the advances and contributions to the field since its introduction one decade ago and discussed them in the context of four challenges: accuracy, resolution, selectivity, and photostability.
During the last years, a detailed description of the printing mechanism was built, and the most relevant physical parameters were identified. The positional accuracy of optical printing is determined by the balance between the DLVO repulsion and the axial optical force at the substrate, and by the shape of the optical force field at the printing wavelength.
For Au and Ag NPs, using lasers tuned to the plasmon resonance of each type of NP and employing low powers led to higher positional accuracy, in the range of 50 to 150 nm.
The resolution of optical printing is compromised by repulsive thermophoretic forces that arise from absorption and heating by the fixed NPs. It must be noted that this fact, usually discussed as a limitation, is beneficial to print arrays of isolated single NPs. On the other hand, if the goal is to connect NPs, different strategies can be used. One approach is to reduce the optical interaction between the already printed NPs and the printing wavelength. This strategy was proven useful to connect two distinct NPs. However, it has limited capabilities for the fabrication of dimers consisting of alike NPs or arrays of three or more connected NPs. The second approach is to implement heat management strategies to reduce the temperature increase during printing. In specific cases, such as using a sapphire substrate covered with a reduced graphene oxide layer, this strategy totally suppresses the thermophoretic repulsion. Further research in this direction may lead to practical configurations of more general use. A third, rather unexplored strategy is to reduce thermophoretic interactions by lowering the magnitude (or inverting the sign) of the thermo-diffusion coefficient . Even though important advances on the understanding of thermophoretic interactions have been recently achieved, 51 a clear insight regarding heat conductive NPs is still missing, and further studies would be required in that direction. 28 First studies of optical printing were made with spherical metallic (Au and Ag) NPs. More recently, optical printing was applied to differently shaped Au NPs, multi-component inorganic NPs, inorganic-organic hybrid nanosystems, and even 2d materials. 52 In these cases, ensuring the stability of the NPs during the printing process is the main challenge.
Here, a few guidelines were summarized to tackle this, such as finding printing wavelengths that maximize optical forces while minimize the generated heat and reducing the electrostatic barrier with the substrate. In the future, we envision the optical printing of plasmonic and hybrid nanoparticles with a variety of morphologies and surface supramolecular modifications, by correctly selecting the optical printing conditions of these materials. When monodisperse colloids are not available, on-resonance optical printing can be used to selectively print subpopulations of NPs, as long as they present differentiable spectral responses. In this case, calculations of the optical force are particularly useful to predict the printing selectivity as a function of the wavelength.
Most demonstrations of optical printing used glass substrates. However, optical printing can also incorporate colloidal NPs into specific locations of more complex substrates. It has already been shown that Au NPs can be printed into photonic crystals, 53 living cells, 54 or 2D carbon materials. 19 We envision further integration of plasmonic NPs into photonic systems such as 2D planar waveguides, 55 hollow-core 3D waveguides, 56 microstructures, 57 optical fibers, 58 or microcavities. 59 The progress reviewed here expands the field of application of optical printing as a template-free and versatile nanofabrication strategy for positioning colloidal NPs on a solid surface. The insights into the physical mechanism of optical printing provide clear guidelines for devising new, optimized optical printing strategies with high accuracy, resolution, selectivity, and photostability. We envision the fabrication of complex functional nanostructures and circuits based on connected NPs, and their hierarchical integration to on-chip photonic systems.
Finally, optical printing can accelerate new research too. Having an ordered array of equivalent NPs facilitates the implementation of automated characterization routines and the accumulation of larger datasets. For example, it has been used to study plasmonassisted chemical reactions on individual NPs, 20 and the photothermal response of single Au NPs on different substrates. 27 The detailed knowledge about the mechanism of optical 29 printing enables its use to investigate other phenomena at the nanoscale, such as thermophoresis and heat dissipation.