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Suspended-Template Electric-Assisted-Nanoimprint for Hierarchical Micro-Nanostructures on Fragile Substrate Chunhui Wang, Jinyou Shao, Dengshui Lai, Hongmiao Tian, and Xiangming Li ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b04031 • Publication Date (Web): 22 Aug 2019 Downloaded from pubs.acs.org on August 25, 2019

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ACS Nano

Suspended-Template Electric-Assisted-Nanoimprint for Hierarchical Micro-Nanostructures on Fragile Substrate Chunhui Wang,† Jinyou Shao,†* Dengshui Lai,† Hongmiao Tian,† and Xiangming Li† †Micro- and Nano-technology Research Center, State Key Laboratory for Manufacturing Systems Engineering , Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China *Corresponding authors: [email protected] (Jinyou Shao)

ABSTRACT

Coating hierarchical micro-nanostructures on the surface of

optoelectronic devices has been demonstrated to improve the overall performance. However, fabricating desired structures on a fragile optoelectronic device substrate is still challenging. A suspended-template electric-assisted-nanoimprint technique is proposed herein to controllably fabricate hierarchical micro-nanostructures on a fragile substrate. The suspension design of the template ensures that it conveniently deforms to fully fit the surface fluctuation of the substrate. The deformation of template and the filling of liquid polymer in the micro-/nano-cavities of the template are both driven by the powerful surface/interface force generated by an electric field

ACS Paragon Plus Environment

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applied between the template and substrate surface, thus protecting the fragile substrate

from

squeezing

damage.

Different

morphologies

of

hierarchical

micro-nanostructures are fabricated by changing the electric field. Based on the suspended-template electric-assisted-nanoimprint, the environmentally adaptable fully-covering hierarchical micro-nanostructures are encapsulated on the surface of flip-film light-emitting diode chips, thus significantly enhancing their light management in complex environments.

KEYWORDS: nanoimprint﹒hierarchical micro-nanostructures﹒suspension design ﹒electric field assisted﹒LED Hierarchical micro-nanostructures inspired by nature exhibit tremendous potential application in fields of energy conservation,1-3 environmental protection,4, artificial intelligence.6,

7

5

and

Especially for high-performance optoelectronic devices,

coating multiscale structures is greatly desired. Surface with hierarchical multiscale design is a significant measure to enhance the light management of an optoelectronic device.8,

9

Energy conversion efficiency of devices from light to electricity or vice

versa can improve significantly by multiple optical effects that are generated or intensified by defined hierarchical micro-nanostructures. The detection field-of-view of photovoltaic devices or the radiation distribution of light emitting devices can also be modulated to satisfy beneficial applications. Additionally, hierarchical surfaces endow optoelectronic devices excellent environmental suitability.10 For example, super-hydrophobicity caused by surface multiscale structuring leaves the device a


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robust and efficient antifouling ability.11 Therefore, by coating predesigned hierarchical micro-nanostructures, the resulting high-photoelectric-performance device can be unattended, and it can function economically in complex environments, such as outdoor and underwater. Multiple and comprehensive performance advantages attract extensive efforts in the manufacturing of multiscale hierarchical structures. Bottom-up manufacturing methods such as self-assembly of colloidal particles12, 13 and chemical synthesis14 are effective for fabricating hierarchical structures, however, structure randomness and inevitability of defects hinder the fabrication of well-defined structures. It is difficult to directly form multiscale hierarchical structures using the traditional top-down processes, therefore, two steps are generally performed. First, the micro-structures are fabricated followed by the nano-structures, or vice versa.15 In these strategies, writing nano-structures by electron beam lithography on prepared micron patterns are not compatible with the mass manufacturing of cost-sensitive optoelectronic devices. Seokwoo Jeon et al. covered an ultrathin flexible nano-phase mask on microstructure substrate and created a “nano-on-micro” pattern by near-field lithography.16 This conformal mask17,

18

skillfully solves the problem of insufficient depth of field in

near-field lithography, but a 3 m thick polyvinylacohol mask is difficult to reuse. Nanoimprint lithography (NIL) incorporated with laser irradiation19 or a sacrificial layer20 is applied to form compound micro-nanostructures. However, a powerful laser or a great embossing force is most likely to damage the substrate, whether hard and brittle, or flexible optoelectronic devices. For these insurmountable stumbling blocks,


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fabricating defined hierarchical micro-nanostructures on fragile target optoelectronic devices remains highly challenging. In this paper, we propose a suspended-template electric-assisted-nanoimprint technique. The structure layer of the imprinting template involved is thin and flexible and suspended by discrete and stretchable columns. Driven by an applied electric field between a template and substrate surface, the thin flexible structure layer with low stiffness deforms to fit the surface fluctuation of the substrate until the two are full contact. Meanwhile, pre-sprayed liquid polymer droplets are filled into template cavities and merged into a continuous structural film that is conformally coated on the substrate surface. A substrate covered by prefabricated micro-structures can be well packaged by a template with nanostructures, such that the targeted hierarchical micro-nanostructures can be fabricated after demolding. The suspension design of the thin structure layer achieves a complete contact with badly uneven substrates, which is impossible in conventional NIL process. Additionally, the process is repeatable in the entire imprinting process of loading, holding, demolding, and even aligning. The advantages of NIL are fully inherited in this technique, such as good substrate adaptability of the liquid polymer dispensing,21-23 low cost,24, 25 high resolution,26, 27 and so on. The surface/interface driving force generated by electric field, rather than a mechanical squeeze, protects substrate from fragmentation during the imprinting process. More importantly, the morphology of the fabricated hierarchical micro-nanostructures can be optimized by convenient adjustment of applied electric field. Based on this suspended-template electric-assisted-nanoimprint technique,


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fully-covering artificial compound eye array is directly encapsulated on the flip-film light-emitting diode (LED) chips, which enable the chips to achieve significantly improvement in light management and environmental suitability. RESULTS AND DISCUSSION Fabrication Strategy. Figure 1 shows the schematics of the hierarchical micro-nanostructures fabricated on a fragile substrate by suspended-template electric-assisted-nanoimprint technique. Microstructures are firstly fabricated by electric-assisted-nanoimprint, as shown in figure 1(a). Micro-concave structures are prepared in advance on the template (m-template), and the corresponding micro-convex humps are replicated, as shown in figure 1(b). To fabricate nanostructures, the template is designed as a thin flexible nanohole array layer suspended from a backplane by discrete columns (suspended template). For demonstration purposes, the suspended template is shown upside down in figure 1(c), and more enlarged results are shown in figure S1. Putting the suspended template on the substrate coated with microhumps without any external pressure. Subsequently, a structure layer with a low bending stiffness is driven to wrap the microhumps on the substrate under the applied electric field between them. The columns in the middle of the template with discrete and low tensile moduli can be stretched easily, as shown in figure 1(d). After demolding, hierarchical structures are created by the full coverage of nanopillars on the microhumps array, as shown in figure 1(e). During the two imprinting processes, the liquid polymer is dispensed in the form of liquid droplets to adapt to the uneven substrate surface. The deformation of the suspended template, the


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filling of liquid polymer in the template cavities, and the fusion between droplets are all driven by the external electric field between the substrate surface and template. In the entire process, no mechanical loading occurs on the substrate, thus avoiding extrusion damage.

Figure

1.

Schematic

representation

of

the

suspended-template

electric-assisted-nanoimprint process applied to the fabrication of hierarchical micro-nanostructures. (a) Micro-structures imprinted on substrate surface by electric-assisted-nanoimprint with micro-template (m-template). (b) Imprinted microhump array. (c) Suspended template with nanohole array. (d) Nano-pillars


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imprinted on microhump surface by suspended template. (e) Full coverage of hierarchical micro-nanostructures. Mechanism Analysis of Superior Performance of the Suspended Template. The size of contact area between the imprint template and convex substrate determines the quality of the NIL process.28 Here, a concise energy analysis method is used to study the contact state. The single microconvex shape on the substrate is assumed to be a part of a sphere of radius 𝑅. During the contact process, elastic deformation mainly occurs in the flexible template rather than the rigid substrate. The work performed by the imprint force applied by an external electric field is primarily stored in the flexible template. For the same contact area, a higher elastic potential energy stored in the flexible template implies a greater applied electric field or a larger deformation. This may result in the distortion of the forming structure or the electric breakdown of template. The basic criterion of flexible template design and optimization is to reduce the internal stored energy as much as possible in the case of full contact with the substrate. Two different geometric models of contact pairs between a microconvex unit on the substrate and a conventional tamping template or our suspended template are shown in figures 2(a). Detailed geometric model is shown in figure S2.The driving electric fields are both applied between the fragile and rigid substrate surfaces and the structure layer of the template. In a tamping template, deformation is generated in the middle buffer layer and the structure layer. Bending and stretching are two forms of


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deformation in the thin structure layer of the template, thus, the stored energy contains two parts, i.e., bending energy 𝑈𝑏𝑒 and stretching energy 𝑈𝑠𝑡. The middle layer undergoes complex deformation such as tensile, shear, and compression. The hertz contact model29 is adopted to simplify energy calculations in the tamping middle layer (𝑈ℎ𝑒), which depends on the penetration depth (hpe) of the microconvex in the template. Therefore, the total energy 𝑈𝑡𝑎 stored in the tamping template is calculated by the following equation (1) when in contact with a convex substrate. 𝜋𝐸𝑠𝑡𝑠3

𝑈𝑡𝑎 = 𝑈𝑏𝑒 + 𝑈𝑠𝑡 + 𝑈ℎ𝑒 = 2(1 ― 𝜗 2)𝑠𝑖𝑛2𝜑 + 𝑠

52

8(1 ― 𝑐𝑜𝑠𝜑) 15

(

1 ― 𝜗𝑠𝑢𝑏 𝐸𝑠𝑢𝑏

+

𝜋𝐸𝑠𝑡𝑠(𝑅𝑠𝑖𝑛𝜑)2 𝜑 𝜑0 ∫ 2 𝑐𝑜𝑠𝜑𝑡𝑎𝑛𝜑 0 ( ) 0 2 1 ― 𝜗𝑠

(

1 ― 𝜗𝑚 ―1 𝐸𝑚

)

)

2

― 1 𝑑𝜑0 +

𝑅3

(1)

where, 𝐸𝑠, 𝐸𝑚, 𝐸𝑠𝑢𝑏 are the elastic moduli of the structure layer, middle layer, and substrate, respectively, 𝜗𝑠, 𝜗𝑚 are the Poisson ratios of the structure layer and middle layer, respectively, 𝑡𝑠 is the thickness of the structure layer, 𝜑 is the half central angle of the penetration depth corresponding to the arc length. In the suspended template, deformation driven by an external electric field during the contact process with the convex substrate is also generated in the structure layer and middle discrete column layer. The deformation and stored energy in the structure layer of this template is the same as that in the tamping one. The difference between two types of templates is reflected in the middle layer. Ignoring the weak bending deformation of every single column, no shear deformation is found in the independent discrete columns in the suspended template. The external electric field only drives the columns to undergo stretch deformation. Meanwhile, comparing with the tamping one, the effective elastic modulus (𝐸 ∗ ) of the middle layer in our suspended template


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decrease significantly and is calculated by 𝐸 ∗ = 𝑎𝐸 𝑏, where 𝑎 is the diameter of the discrete columns, 𝑏 is the distance between two adjacent columns.30 Therefore, the elastic energy stored in the middle layer 𝑈𝑐𝑜 is lower when the penetration depth by a convex substrate is the same as that in a tamping template. The following equation (2) shows the total energy 𝑈𝑑𝑠 of the suspended template, 𝜋𝐸𝑠𝑡𝑠3

2

𝑈𝑑𝑠 = 𝑈𝑏𝑒 + 𝑈𝑠𝑡 + 𝑈𝑐𝑜 = 2(1 ― 𝜗 2)𝑠𝑖𝑛 𝜑 + 𝑠

𝜋𝐸𝑠𝑡𝑠(𝑅𝑠𝑖𝑛𝜑)2 𝜑 𝜑0 ∫ 2 0 𝑐𝑜𝑠𝜑𝑡𝑎𝑛𝜑0 2(1 ― 𝜗𝑠 )

(

∑(1 ― 𝑐𝑜𝑠𝜑𝑖)2

2

)

― 1 𝑑𝜑0 +

𝐴𝑅2𝐸𝑚𝑖 2𝐻

(2)

The detailed stored energy calculation process for two templates is analyzed in the supporting information from equations (S1) to (S14). Figure 2(b) calculates the changes in 𝑈𝑏𝑒, 𝑈𝑠𝑡, 𝑈ℎ𝑒, and 𝑈𝑐𝑜 during template deformation with the penetration depth. In the calculation, polydimethylsiloxane (PDMS) and ultraviolet PDMS (UV-PDMS) are used as the materials of the middle and structure layers in the template, respectively. The diameter of a single column is 15 m, the height is 25 m, and the duty cycle is 0.3. The thickness of the structure layer is 12 m. The diameter of the microconvex section circle on the substrate is 20 m. Each term of 𝑈𝑡𝑎 or 𝑈𝑑𝑠 increases as the penetration depth increases. In the structure layer, 𝑈𝑠𝑡 is critical. A thin structure layer is more prone to bending rather than stretching. The difference between 𝑈𝑠𝑡 and 𝑈𝑏𝑒 increases with increasing penetration depth. The tamping middle layer is difficult to deform. For a small penetration depth, for example < 10 m, the work performed by the external electric field is mainly used to overcome the deformation of the tamping intermediate buffer.


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The energy required to deform discrete suspended columns is much lower. It is approximately equivalent to the bending energy of the thin structure layer. In fact, in the contact process between the plane of a nonconvex part of a substrate and a flexible template, the designed suspended template consumes less energy than the tamping one. In these areas, stretch is simplified as the only form of deformation, when ignoring the effect of the adjacent complex deformation and the weight of the back plate layer of the template. The discrete columns are much easier to stretch than the entire block. For substrates whose surface contains a microconvex array pattern with a duty cycle of 1, figure 2(c) shows the total energy consumed by the suspended template and the tamping one in full contact with it. The designed flexible suspended template significantly reduced the required energy consumption for a complete contact with microconvex substrate. It is reduced to less than 25% of the total energy expended by the tamping template for penetration depths between 4 and 10 m. In an extreme case, when the column array of the template intermediate layer is of the same layout as the microconvex hump array on the substrate and aligns exactly, the only columns rightly above the microconvex humps will not deform. The external electric field is used completely to drive the thin structure layer deformation. Therefore, lower external driven electric fields and higher fidelity nanostructures can be learned by the designed flexible suspended template. Reliability and durability are factors that have to be considered in suspended design. An experiment is designed to evaluate them. The structure parameter of the


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adopted template in this experiment is the same as that in the simulation above. The bottom of the UV-PDMS structure layer is fastened by vacuum, and the top of the polyethylene terephthalate (PET) backplane layer is stretched from one side, as shown in figure 2(d). Fracture is observed on the intermediate PDMS columns rather than on the thin UV-PDMS layer or the bond between the discrete columns and UV-PDMS. Therefore, the tensile strength (Sco) of the discrete PDMS column array is the weakest compared with the tear strength (Ste) of the thin structure layer or the bonding strength (Sbo) between the discrete PDMS columns and UV-PDMS. Kahp-Yang Suh. et al. measured the demolding force by peeling off a flexible template from substrate, and the value was ~0.01–0.03 MPa31 and was significantly lower than tensile modulus (E* ≈ 1 MPa) of the discrete PDMS columns. Therefore, the developed suspended template can be demolded as easily as the conventional flexible template. Both molding and demolding analyses demonstrated that the design of intermediate discrete columns affected little on the durability of the template. According to a study from H. Schmitt, 500 imprints could be performed successfully without any visible degradation using a flexible PDMS template,32 therefore, the lifetime of the dicretelysuspended composite template was more than 500 times.


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Figure 2. Superior properties of the suspended template. (a) Geometric models of contact pairs between a microconvex unit and tamping or suspended template. (b) Analysis of energy composition during contact. (c) Low total energy consumption demonstration of suspended template for different penetration depths. (d) Reliability demonstration by tensile fatigue test.


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Fabrication of Adjustable Morphology Hierarchical Micro-Nanostructures. In our research, the liquid polymer is directly dispensed on the substrate in the form of droplets to accommodate the substrate surface containing a large number of local uplifts. The rheological behavior of the liquid polymer droplets during the imprint process is primarily divided into two processes. Namely, the liquid polymer droplets fill the micro-/nanocavity vertically in the template and flow laterally33, 34 to merge with each other in the gap between the template and substrate. The former has been investigated extensively whether it is driven by natural capillary force,35 mechanical pressure,36 or external electric field.37, 38 The laterally spreading of the liquid droplets to form a thin film is significant and complex to a dispensing-based NIL technology. The driven force to control droplets spreading laterally in our process mainly comes from two aspects including electrostatic attraction pressure Pesa and liquid dielectrophoresis force Fdep. 𝑃𝑒𝑠𝑎 is generated between the electrode pair for the template and substrate,28 𝐸2

𝑃𝑒𝑠𝑎 = 2 𝜀2𝑟 𝜀0

(3)

where, 𝜀𝑟 is the average dielectric constant of the template and polymer layer on the substrate between the two conductive pairs, and 𝜀0 the dielectric constant of vacuum. 𝐹𝑑𝑒𝑝 is generated on the polymer/air interface,37 𝐸2

(4)

𝐹𝑑𝑒𝑝 = 2 (𝜀𝑟 ― 𝜀0)

Both 𝑃𝑒𝑠𝑎 and 𝐹𝑑𝑒𝑝 are correlated positively to the applied voltage. The former squeezes the liquid droplets in the sandwich to flow, and the latter mainly involves wetting along the template or substrate surfaces. Pesa is inversely proportional to the


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square of the distance between the electrode pair, therefore, a positive feedback trend is exhibited during electric field driving contact. Fdep in the air/liquid/solid three-phase contact line increases with the applied electric field before the contact angle saturates. Figure 3 (a i) shows the experimental results of the liquid polymer droplets flowing and merging laterally by the applied electric field. In these experiments, the liquid polymer droplets are initially inkjet dispensed on a wafer surface. Subsequently, the template comes into contact directly with the droplets face to face driven by different applied electric field. Micrographs are captured after curing the polymer and peeling off the template. In the sample without any external load, the natural capillary pressure between the liquid polymer and template/substrate is the primary driving force for spreading droplets regardless of the weak gravity of the thin template. In this case, little coalescence occurs among different droplets beneath the template. With increased applied electric field, the air entrapments encircling owing to the merging of adjacent droplets are removed gradually, and the thickness uniformity of the formed film is improved gradually. Figure 3(a ii) shows that the film thickness uniformity varies with the applied electric field. The peak-to-valley (PV) value of the film surface is measured along the diagonal connection at the center of the initial droplets. hfilm is the average thickness of the generated polymer film. When the applied electric field is larger than 0.3 V/m, air bubbles disappear. A higher applied electric field is helpful to learn a lower ratio of PV/hfilm. When the electric field reaches 1.5 V/m, the ratio decreases to less than 5%. The local bending deformation of the


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flexible suspended template results in the thickness difference of the formed film sandwich during contact with the droplets on the substrate. Owing to the spacing sensitivity of Pesa, the inconsistency of the film thickness can be restrained by a higher electric field. It is noteworthy that the electric field applied is higher than the strength required by the liquid polymer to fill the micro-/nanocavities of the template.39 Once the uniform liquid film is formed, the polymer micro-/nanopatterns are also generated. Figure 3(b i–iv) shows the different imprint results when applying different external electric fields between the suspended template and the same type of microconvex substrate. The entire surface of different substrates is successfully covered with nanopillar arrays. Figure 3(b v) further shows the variation in the total height of the formed hierarchical micro-nanostructures with the applied electric field. With increased applied electric field, the microconvex humps on the substrate become more prominent gradually. During fabrication, the liquid thin film that merged from the liquid polymer droplets is squeezed and flowed along the substrate surface to reproduce the microconvex shape as it is driven by high electric field. Therefore, the flexible suspended template can be used to imprint composite micro-nanostructures with fully covered nanostructures and an adjustable height of the microconvex humps only by changing the applied electric field. As a comparison, the imprint result from fabrication by the tamping template under a 3.5 V/m applied electric field is shown in figure S3. Two deficiencies are presented. First, the formed nanostructures areas remain isolated from each other rather than fused into a continuous film. The ups and downs of the substrate hinder the fusion among adjacent liquid polymer droplets. In


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addition, only the top parts of the polymer-covered microconvex humps resurfaced, and the total height of the formed composite micro-nanostructures is limited by the deformation ability of the tamping template. By doubling the amount of the liquid polymer droplets, bubble-free polymer films are formed, as shown in figure S4, but the total height of the imprint composite micro-nanostructures is still limited. The comparison results demonstrate that the developed flexible discretely suspended template ensures that the liquid polymer droplets form a continuous film without air bubbles, and facilitates the formation of a wraparound contact with the microconvex structures

on

the

substrate

surface.

The

proposed

suspended-template

electric-assisted-nanoimprint is an effective and controllable technique to fabricate hierarchical micro-nanostructures. Three different shapes of micro-nanostructures are also fabricated by the proposed suspended-template electric-assisted-nanoimprint technique as shown in figure 3(c). In figure 3(c i), the nanohole-array replicated from the above nanopillar-array are covered on the micro particle contamination with a diameter of ~5 μm on a plate surface. Figure 3(c ii) shows that the closely-arranged square holes, which are 500 nm in period, are fabricated on the striped structures with a period of 10 μm. Figure 3(c iii) shows that nano-gratings with a width of 200 nm cover the surface of micro-straight-line with a width of 7 μm and a height of 1.5 μm. During these fabrication processes, the driven electric fields are all 2 V/m. It is reasonable to believe that more micro-nanostructures with diverse forms and different functions can be fabricated by optimizing the template and process.


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Figure 3. Suspended-template electric-assisted-nanoimprint in inkjet-dispensed manner. (a) Electroinduced liquid polymer droplets’ merging behavior. i) Micrographs of droplet fusion under different electric field. ii) Fusion film thickness uniformity controlled by electric field. (b) Imprinting by different electric fields. i–iv) Different morphologies. v) Total height variation of the formed structures. (c) The fabricated different micro-nanostructures. i) Nano-holes on the micro-particle contamination. ii) Square nano-holes on the micro-striped structures. iii) Nano-gratings on the micro-straight-line.


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Encapsulating the Fragile Flip-Film LED Chip for High Performance. Strong environmental suitability is one of the main purposes of coating the surface of functional devices with hierarchical micro-nanostructures. The wettability of different surface structures is characterized by the contact angle 𝜃 of deionized water, as shown in figure 4(a). The contact angle 𝜃𝑓 of polymer flat surface is ~98°, the contact angle 𝜃𝑚 of the microhump only substrate surface is ~120°, and the contact angle 𝜃𝑛 of the nanopillar only surface is ~142°. Both the micro and nanostructures surfaces would endow the functional device a certain hydrophobic performance (𝜃 > 90°). Amazingly, the fabricated hierarchical micro-nanostructures significantly improved the non-wettability of the surface. The contact angle 𝜃𝑚𝑛 of the hierarchical micro-nanostructure surface increases to 171°. This superhydrophobic (𝜃 > 150°)40 surface protects functional devices from water more effectively, such as those that are used outdoors in rainwater. Figure 4(b) shows the change in contact angle with the total height of composite micro-nanostructures. Different test surfaces were fabricated by different applied electric fields. According to the above experimental results, higher external electric fields help create composite micro-nanostructures of larger total height until the initial height of the microhumps on the substrate is fully exposed. When the applied electric field is small (0.4 V/m), the contact angle (144°) of the fabricated micro-nanostructure surface is close to that of the nanostructure only surface. The contact angle increases with the total height of the surface composite structure and becomes saturated. Therefore, the surface energy or wettability of a substrate can be improved by conveniently adjusting the applied


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external electric field during fabrication. In a functional device, the proposed suspended-template

electric-assisted-nanoimprint

is

a

controllable

surface-modification method to fabricate an antifouling surface by optimizing the template structure and process parameters. Water repellency enables functional structure surfaces to exhibit excellent self-cleaning performance. Figure 4(c) shows that the sliding angle of deionized water is only ~4° of the hierarchical micro-nanostructure sample whose static contact angle is 169°. Extremely slippery water droplets provide an excellent driving mechanism for the cleaning of contamination particles on a substrate surface, especially for functional

devices

used

outdoors.

In

summary,

suspended-template

electric-assisted-nanoimprint is an effective technique to cover designed hierarchical micro-nanostructures on the surface of functional devices and yield strong environmental suitability.


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Figure 4. Assessment of surface wettability. (a) Contact angle (CA) of deionized water for polymer flat surface, micro-humps, nano-pillars, and hierarchical micro-nanostructure surface. (b) Variation in contact angle with morphology of hierarchical micro-nanostructure surface. (c) Sliding angle (SA) test of hierarchical micro-nanostructure surface. Environmentally adaptable hierarchical micro-nanostructures with high total height were fabricated on the surface of a flip-film LED chip by suspended-template electric-assisted-nanoimprint to encapsulate the chip. During imprinting, an external driven electric field was applied between the top layer of the n-type gallium nitride semiconductor of the chip substrate and the flexible suspended template. Figure 5 (a i) shows the schematics of the flip-film LED chip covered with hierarchical micro-nanostructures. The comparison of electroluminescence emission images of LED chips covered without or with hierarchical micro-nanostructures at the same injection currents are shown in figures 5(a ii) and (a iii). It was found that the chips covered with micro-nanostructures exhibited brighter light emission. Optimization design of the micro-nanostructures is very important to enhance the properties of the LED chips. In our fabrication, the shape of the micro-nanostructures is adjustable by changing the driving electric field, and the structural size is interrelated. The total height and the duty ratio of the micro-nanostructures are designated to optimize the light extraction efficiency of the packaged LED chips. Figure 5(b) shows the numerical analysis process. As shown in figure 5(b i), when the


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total height is higher than 4 μm (duty ration is fixed at 0.5), the light extraction efficiency exceeds 74% and fluctuates within a small range. Sparse or too tight arrangement of the micro-nanostructures is not conductive to improve the light extraction efficiency as shown in figure 5(b ii). When the duty ratio is between 0.4 and 0.7 (total height is fixed at 5 μm), the light extraction efficiency is higher than 74%. The refractive index of the selected material has a great influence on the light efficiency as shown in figure 5(b iii). Higher refractive indices are better for light extraction. Unfortunately, nanometer-formable material with high refractive indices and low light absorption is very rare. In our application, the refractive index is 1.516 at 450 nm (peak wavelength of LED chip). The used micro-nanostructures in our chip packaging test experiment is fabricated under the driving electric field of 3 V/m, and its total height is ~5 m, and the duty ratio is 0.5. The photoelectric properties of the original chip and that of the chip covered with different structures, i.e., microconvex hump array, nanopillars, and composite micro-nanostructures, are compared and analyzed. Figure 5(c) gathers the angle-resolved photoluminescence (PL)41 characters of different package chips. Compared to the original chip, the encapsulated chips generate a higher PL intensity and broader emission angle in the free space. From high to low, the PL intensity is the sample

encapsulated

by

composite

micro-nanostructures,

nanopillars

and

microhumps. By introducing a layer of patterning polymer at the initial GaN-Air interface of the LED chips, a gradient refractive index interface is formed and replaces the abrupt change, thereby resulting in a part of the light entering the air,


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which is otherwise impossible. The fabricated hierarchical micro-nanostructures allow for more light from GaN to enter the gradient refractive index layer compared to only micro or nanostructures, thus resulting in a higher PL intensity. For the same reason, the full-width at half-maximum divergence angle of the sample covered with hierarchical micro-nanostructures is widened to 140.34° (19.32°-159.66°) from 126.27° (27.54°-153.81°) of the unpacked chips. The light output characteristics with increase in input current (L-I) further demonstrated higher light extraction efficiency by patterning encapsulation, as shown in figure 5(d). The electro optical efficiency of a device is indicated from the slope of the L-I curve. A higher efficiency than that of the original bare chip is shown by patterning encapsulation. The chip covered by hierarchical micro-nanostructures yields the highest efficiency. At the input current of 200mA, the electro optical efficiencies of the sample encapsulated by composite micro-nanostructures, nanopillars, microhumps and bare chips are 70.31%, 66.30%, 63.98%, and 54.53%, respectively. Figure 5(e) compares the current voltage (I-V) characteristics of different chips. The forward voltage measured at the input current of 200 mA is close to 2.81 V for different types of chips. Whether or not the chip surface is coated, the type of coating structure has little effect on the series resistances. The suspended-template electric-assisted-nanoimprint technique and the structures fabricated using it induced almost no effect on the electrical properties of the chips, thus strongly demonstrating its excellent process compatibility for encapsulating fragile optoelectronic devices. Therefore, hierarchical micro-nanostructure encapsulation fabricated by the


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suspended-template electric-assisted-nanoimprint technique improves the optical performance of a flip-film LED chip while maintaining its electrical properties.

Figure 5. Application in encapsulation of flip-film LED chip. (a) i) Schematics of flip-film

LED

chip

encapsulated

by

hierarchical

micro-nanostructures.

ii)

Electroluminescence images of bare LED chips. ii) Electroluminescence images with hierarchical micro-nanostructures covered. (b) Effect of micro-nanostructures parameters on light extraction efficiency of the encapsulated LED chips. (c) Angle-resolved photoluminescence characters testing of chips encapsulated by


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micro-nanostructures (m-n), only nanopillars (nano), only microhumps (micro), and none. (d) Comparison of light output characteristics. (e) Current voltage characteristics of different chips. CONCLUSION In summary, suspended-template electric-assisted-nanoimprint was proposed and demonstrated to imprint full coverage deterministic hierarchical micro-nanostructures on a fragile substrate. The suspension design of a flexible template was the prerequisite for enabling an optimum fit with the prepared microconvex substrate. The driven load generated by an electric field applied between the template and substrate surface prevented the damage of the fragile substrate. The morphology of the formed hierarchical micro-nanostructures could be controlled conveniently by changing the driven electric field. Hierarchical environment-friendly micro-nanostructures were encapsulated

on

the

flip-film

LED

chips

by

suspended-template

electric-assisted-nanoimprint, thus enhancing its electro optical efficiency and broadening its emission angle. MATERIALS AND METHODS Preparation of template. The fabrication process of the suspended-template is shown in Figure 6. UV-PDMS (KER-4690 from Micro Resist Technology GmbH) is used to fabricate the thin flexible structure layer due to its high resolution and low shrinkage of 0.02%. To begin with, 10-12 m liquid UV-PDMS thin layer are spun coated on the silicon master with nano-structures and cured by UV light irradiation at


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2000 mJ/cm2 for 10 minutes as show in figure 6(a). This mold is pillars-array on a hexagonal lattice with the period 600 nm, diameter of 300 nm, height of 500 nm. Then the integrated transparent conductive electrode with high mechanical stability consisted by indium tin oxide (ITO) and silver nanowire (Ag NW) network42 is coated on the surface of cured UV-PDMS, and 30 nm of SiO2 is sputtered by Denton Vacuum Explorer14 sputter as the adhesion layer as shown in figure 6(b). The sheet resistance of the integrated conductive electrode is 50 ohms∙sq-1. On the other hand, a PDMS (Sylgard 184 from Dow Corning) discrete columns layer is molded as shown in figure 6(c), and the backplane PET layer is bonding to PDMS layer as shown in figure 6(d). PDMS is cured in 10 minutes at 100 C. Thickness of PET is 120 m. This mold is holes-array on a rectangular lattice with the period 50 m, diameter of 15 m, depth of 25 m. As shown in figure 6(e), the composite PET/PDMS column is further bonding to conductive UV PDMS layer by oxygen plasma treatment.43, 44 The plasma treatment process to SiO2 and PDMS surface is carried out on the Plasma System V6-G from PiNK GmbH, the flow rate of 150 ml/min, microwave power of 200 W, process time of 30 seconds. In the end, the whole suspended template is peeled off from the silicon mold. Suspended-templates used to fabricate different shapes of micro-nanostructures are prepared by the same process, except that the nano-molds are different. Nano-holes-array mold replicated from above pillars-array. Square-hole-array mold and nano-gratings mold are fabricated by electron beam lithography and etching process, respectively using the CABL-9000C from Crestec Inc. and ICP-180 from


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Oxford Instruments. The depth of square-hole is 250 nm, and the thickness of wall is 200 nm. The height of nano-gratings is 200 nm, and the duty ratio is 1:1.

Figure 6. Preparation of suspended template. (a) Spin coating thin UV-PDMS structure layer. (b) Coating integrated transparent conductive electrode on the back of UV-PDMS. (c) Spin coating PDMS columns. (d) Bonding PET backplane and PDMS columns. (e) Bonding PET/PDMS columns and UV-PDMS layer. The m-template stacks are 30 m PDMS/100 nm ITO-Ag NWs/120 m PET. Micro concaves pattern on PDMS layer are replicated from micro-humps mold. The conductive layer and PET layer are successively stacked on the back of PDMS layer as the fabrication of suspended template. The micro-humps mold is fabricated by melting straight micro-pillars on a glass substrates. The initial pattern of the micro-pillars is on a hexagonal lattice with the period 30 m, diameter of 15 m, height of 12 m, and the material is mr-NIL 6000E from Micro Resist Technology GmbH. Hold the sample at 35 C for 6 minutes, the micro-humps structure are formed with the height ~7 m and the diameter of bottom section circle ~20 m. The striped


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ACS Nano

structures are also fabricated by the same melting process. The initial pattern of micro-gratings is width of 6 m, height of 6 m, and period of 10 m. The height of stripe after melting is ~2.5 m. Details of process. The liquid polymer purchased from Micro Resist Technology GmbH (with the commercial name Ormostamp®). The initial viscosity of the liquid polymer is 0.41 Pa·s at room temperature. In experimental, acetone is mixed with liquid polymer in a 1:1 ratio to decrease the viscosity. In the visible light range, the optical loss is less than 0.1 dB∙cm-1. Dispensing of the liquid polymer on microconvex substrate is used the DMP-3000 from FUJIFILM Dimatix whose stage is set to 70 C to accelerate volatilization of acetone. Liquid polymer droplets with volume 1 pL are initially inkjet-dispensed on a substrate surface with the diameter about 10 m. To fabricate nanopillars, the liquid polymer is dispensed on 45  45 m square array. To fabricate micro-humps, the liquid polymer is dispensed by EVG 101CS. The holding time of template contact with the droplets before UV curing is around 6 seconds. The driving electric field is applied by TREK610E HV. The liquid polymer cures by exposing it to UV irradiation for 12 seconds at 250 mW/cm2. All of the samples are deposited anti-adhesion of the fluorocarbon (C4F8) layer in an ICP-CVD chamber before measuring the wettability. The simulation on light extraction of LED chips is performed by commercial finite-difference time-domain (FDTD) software (FDTD Solutions). The light source is dipole. The bottom boundary is highly reflective, and the other are perfect matching layers. The refractive index of the luminous medium is 2.42.


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Characterization and evaluation. All of the SEM images are obtained using a HITACHI S-3000N SEM. The micrographs are obtained by KEYENCE VH-8000 microscope. Total height of the formed hierarchical micro-nanostructures is obtained by LSCM (Olympus OLS4000). Contact angle is measured by OCA15EC (Data Physics). Thickness of polymer film is measured by ellipsometer (PZ2000 from Jobin Yvon S.A.S). The flip film LED chips are supplied by Nationstar Corporation. I-V characteristic of the chips are measured by semiconductor device analyzer (KEYSIGHT B500A). The absolute light output power is collected by IS236A from Thorlabs at the temperature of 25 C. The angle-resolved PL is tested from 0°–180° by QEpro system from Ocean optics Co. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.*******. Additional theoretical calculation, SEM images, and experimental sample photos. The authors declare no competing financial interest. AUTHOR INFORMATION Corresponding Author Jinyou Shao, [email protected]; Present Addresses


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Micro- and Nano-technology Research Center, State Key Laboratory for Manufacturing Systems Engineering , Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. J. Shao and C. Wang conceptualized the idea and designed the experiments. C. Wang and S. Lai developed the suspended template and performed the imprinting experiments. C. Wang and H. Tian analyzed the superior properties of the suspended template. C. Wang and X. Li tested the performances of the fabricated hierarchical micro-nanostructures. All of the authors discussed the whole paper. ACKNOWLEDGMENT This work is financed by the National Key R&D Program of China (grant no. 2017YFB1102900) and NSFC Funds (grant no. 51805422). REFERENCES 1. Zhai, S.; Zhao, Y.; Zhao, H. High-Efficiency Omnidirectional Broadband Light-Management Coating Using the Hierarchical Ordered-Disorder Nanostructures with Ultra Mechanochemical Resistance. ACS Appl. Mater. Interfaces 2019, 11, 12978-12985. 2. Chen, M.; Zhang, Y.; Xing, L.; Liao, Y.; Qiu, Y.; Yang, S.; Li, W. Morphology-Conserved Transformations of Metal-Based Precursors to Hierarchically


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ACS Nano

39. Liang, X.; Zhang, W.; Li, M.; Xia, Q.; Wu, W.; Ge, H.; Huang, X.; Chou, S. Y. Electrostatic Force-Assisted Nanoimprint Lithography (EFAN). Nano Lett. 2005, 5, 527-530. 40. Genzer, J.; Efimenko, K. Recent Developments in Superhydrophobic Surfaces and Their Relevance to Marine Fouling: a Review. Biofouling 2006, 22, 339-360. 41. Zhuang, Z.; Guo, X.; Liu, B.; Hu, F.; Dai, J.; Zhang, Y.; Li, Y.; Tao, T.; Zhi, T.; Xie, Z. Great Enhancement in the Excitonic Recombination and Light Extraction of Highly Ordered InGaN/GaN Elliptic Nanorod Arrays on a Wafer Scale. Nanotechnology 2015, 27, 015301. 42. Wang, C.; Shao, J.; Tian, H.; Li, X. Protective Integrated Transparent Conductive Film with High Mechanical Stability and Uniform Electric-Field Distribution. Nanotechnology 2019, 30, 185303. 43. McDonald, J. C.; Duffy, D. C.; Anderson, J. R.; Chiu, D. T.; Wu, H.; Schueller, O. J.; Whitesides, G. M. Fabrication of Microfluidic Systems in Poly (Dimethylsiloxane). Electrophoresis 2000, 21, 27-40. 44. Bhattacharya, S.; Datta, A.; Berg, J. M.; Gangopadhyay, S. Studies on Surface Wettability of Poly (Dimethyl) Siloxane (PDMS) and Glass under Oxygen-Plasma Treatment and Correlation with Bond Strength. J. Microelectromech. S. 2005, 14, 590-597.


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