Amorphous Photonic Structures with Brilliant and Noniridescent Colors via Polymer-Assisted Colloidal Assembly.

Efficient and large area fabrication of amorphous photonic crystals (APCs) with multicolor, angle independency, and fine resolution is always desired owing to their application in color displays, sensors, and pigments. Here, we report a polymer-assisted colloidal assembly (PACA) method to fabricate APCs with brilliant structural colors by the co-assembly of silica colloidal particles, polyvinylpyrrolidone (PVP), and carbon black (CB). PVP is the key to enable the amorphous aggregations of the particles, the uniform and noniridescent structural colors of the APCs. Moreover, multicolor and high-resolution patterns can be prepared through the mask-based brush printing with colloids-PVP-CB precursor solution as ink (named as APCs-ink). The developed printing method can be applied to various substrates with different roughness, curvature, and flexibility such as papers, metals, plastic films, stones, and even curved glasses. PACA is efficient and straightforward for the fabrication of APCs and high-resolution patterns with large area, low cost, and easy operation, which will facilitate their practical applications in the fields of color-related display, green painting, anticounterfeiting, and so on.

Introduction

Colloidal photonic crystals and their bright structural colors are originated from the interference and diffraction or scattering of the visible light by their periodic nano/microstructures. These unique structural colors have attracted considerable interests because of their practical applications in color display,[1−8] printing,[9−12] sensor,[13−22] anticounterfeiting,[23,24] optical devices,[25−27] solar cell,[28−30] and photocatalysis.[31,32] The reflection peaks can be tuned by the particle sizes, the refractive index, or the orientation of photonic crystals, while the angle independence usually depends on the arrangement of particles. Generally, photonic crystals with long-range ordered structures possess angle dependent or iridescent structural colors at varied viewing angles, where angle independent or noniridescent structural colors can be achieved when the colloidal particles are packed into amorphous structures. Although ordered photonic crystals exhibit brighter color saturation, their colors are iridescent, which makes it confuse in the field of color display, sensing, and so on. In this regard, amorphous photonic crystals (APCs) with only short-range ordered structures and noniridescent colors are identical photonic materials in the applications of color displays,[3,33−35] sensors,[20] paintings,[36] and anticounterfeiting.[37,38] However, the fabrication of APCs is still a big challenge because of the strong crystallization tendency of charged particles upon the assembly process.

The key to fabricate APCs is to avoid the colloidal crystallization. Recently, various strategies including spray method,[35] bi-disperse suspension,[39−41] infiltration-assisted colloidal assembly,[42] layer by layer (LBL),[43] coating polymer or particles on the surfaces of particles,[36,44,45] and electrophoretic deposition[46] have been used to prepare APCs. For instance, using the spray method, APC pigments can be obtained because the methanol was rapidly evaporated from the colloidal solutions. This strategy is successful in preparing APCs but subjected to the volatilization speed of solvent. The infiltration-assisted colloidal assembly process requires porous substrates, while the LBL need multisteps to prepare APCs, both of which complicate the fabrication process. For particle modification or other approaches, additional procedures are essential and the amplified fabrication of APCs will be a problem and should be carefully considered. From the above discussion, one can conclude that the developed approaches for fabricating APCs usually require harsh or complicate assembly conditions, and most of them are failure in fabrication of APCs in large area. Therefore, a simple, efficient, and convenient method for fabricating APCs in large area and with a uniform noniridescent structural color is highly desired.

In this work, we report an efficient and straightforward polymer-assisted colloidal assembly (PACA) route for the fabrication of APCs with noniridescent structural colors, which were prepared by the coassembly of silica colloidal particles, polyvinyl pyrrolidone (PVP), and carbon black (CB). The introduction of PVP not only enables the amorphous aggregations of the particles and the angle independent structural colors in the film but also contributes to the uniform structural colors of the APCs. The APC-ink, an ethanol solution consisting of colloidal particles, PVP, and CB can be used for printing patterns with multicolor and high-resolution through maneuverable brush printing with masks. Moreover, the brush printing can be applied to flat, curved, rough, and flexible substrates, including paper, glass, metal, plastic film, and stones. The developed PACA approach and brush printing are quite simple, efficient, and convenient in fabrication of APCs and multicolor patterns with large areas and low cost, which will promote their practical applications in the field of structural color-based display, green painting, or anticounterfeiting.

Results and Discussion

The procedure for fabricating APC films by PACA is illustrated in Figure a. Monodispersed silica particles, nanosized CB, and PVP were mixed into ethanol to obtain the precursor solution, which was then cast on the glass to form APC films after solvent evaporation. The introduction of CB into the APCs enhanced the color saturation of the film because the CB could absorb the incoherent scattering of light and enhance the coherent scattering of light effectively. The addition of PVP has two advantages on the formation of amorphous structures and uniform structural colors: (1) the PVP chains could be absorbed onto the surfaces of the silica particles, which reduced the repulsion between the silica particles, and caused the aggregation of the particles into amorphous structures; (2) the aggregation tendency of the nanosized CB could be avoided because of strong absorption capacity of the PVP on the surfaces of CB particles, which resulted in the uniform structural colors.

Figure 1

(a) Schematic illustration of the fabrication of APCs. (b) Digital photo, (c) SEM image of APCs, and (d) reflection spectra of ten random points of the fabricated APCs. Edge length of the APCs is 20 * 10 cm. Photos in all figures were taken by Yang Hu, and all the images in this work is free domain.

It is noteworthy that PACA is efficient in fabricating APCs in large area and a short time, as can be seen in Figure b, green APC films with uniform structural colors and a large area of about 200 cm2 were prepared in 5 min by the coassembly of 252 nm silica colloids, 1% CB (in wt, to silica particles), and 2.5% PVP (in wt, to silica particles). The scale-up is remarkably simple and efficient through simple amplification of the amounts of regents, and no additional equipment or procedures are need. The corresponding scanning electron microscopy (SEM) (Figure c) image of the APC films showed that the silica particles were packed very close to each other with short-range ordered but long-range disordered structures. The ring-like pattern (inset in Figure c) of the corresponding 2D fast Fourier-transform images of APCs implied the short arrangement of the silica particles, and the orientation of the short-range ordered structures was equivalent along diverse angles. The APCs exhibit uniform structural colors because the CB particles (red arrows in Figure c) were uniformly distributed into the APCs, which can be further confirmed by the similar reflection intensity of ten random points form APCs (Figure d). Although the reflection intensity is not high enough, APCs still show a bright structural color which can be well observed by eyes and captured by cameras.

APCs showed noniridescent structural colors that can be verified by naked eyes and angle dependent reflectance spectra. The structural color of the green film remained nearly unchanged when it was observed at broad viewing angles from 0 to 80° (Figure a) because of the only short-range ordered structures of APCs. Furthermore, angle resolved reflectance spectra were also used to determine the optical property of the APCs, where the sample was fixed and detector was varied. As shown in Figure b, the reflection peaks located at 550 nm remained constant as the detector changed from 0 to 80°, which was consistent well with the above results. APC films with blue and red structural colors (Figure c,d) could be further fabricated with particles of different sizes through a similar procedure, both of which possessed amorphous structures with only short-ranged order (Figure S1). Owing to the amorphous structures of APCs, the blue and red films showed a noniridescent structural color at various viewing angles.

Figure 2

(a,c,d) Digital photos and (b) reflection spectra of APC films. Edge length of the square is 2.5 cm, and photos were captured under viewing angles (0–80°).

The amorphous structures of APCs are mainly originated from the hinder effect of PVP on the self-assembly of silica particles. For colloids self-assembled in the absence of PVP, the silica particles are randomly moved into the solution because of the Brownian motion of the particles. These silica particles are stabilized by the balance between the repulsive and attractive interactions among the particles, where the former one is arising from good charge separation of the silica particles (particle-OH → particle-O– + H+), as supported by the large value of ζ-potential (−45.3 mV, Figure a) of silica particles. As the solvent gradually evaporated, the average distance between neighboring particles is reduced accordingly; hence, the repulsive and attractive forces between silica particles are progressively enhanced and a new balance is reached, which contributes to the stability of the particles at a high concentration of particles and leads to the long-range ordered structures in the end. In contrast, when appropriate PVP was added into the colloidal solution, PVP chain would be absorbed onto the surfaces of particles because of the strong absorption ability of PVP, which was confirmed by the Fourier transform infrared spectrum (FTIR, Figure S2). The peaks ranged from 2800 to 3100 cm–1 are the typical absorption bands of C–H from PVP, although the particles were washed with ethanol three times with excess ethanol. This result vividly proved the strong absorption of the PVP chain on the surfaces of the silica particles. At this stage, the average size, polydispersity index (PDI), and ζ-potential of the particles remained unchanged (Figure a), indicating such a low mass concentration of PVP had little influence on the repulsions and stability of silica particles. As the solution slightly concentrated over times, more and more PVP were supposed to be absorbed on the surfaces of silica particles, which would hinder the charge separation of particles and cause aggregation of particles. Silica particles dispersed in PVP solution with various mass concentration were prepared to simulate the drying process of the solution consisting of particles (200 mg/mL) and PVP (0.5 mg/mL). As can be seen in Figure , the ζ-potential of the particles decreased from −45.3 to −26.8 mV as the PVP concentration increased from 0.5 to 5 mg/mL. These would lead to the aggregation tendency of the particles, which can be verified by the increased particle size (Figure b) and PDI, accordingly. Therefore, particles would aggregate randomly upon the self-assembly process, which resulted in the amorphous structures of APCs.

Figure 3

Effect of PVP on the (a) potential, PDI and (b) average size of silica particles.

The structures of APCs obtained with a variety of PVP concentration were systematically investigated in order to figure out the effect of PVP on the assembly process. Photonic crystal films with nonuniform structural color were fabricated when the silica particles were assembled in the absence of PVP, owing to the cracks formed upon the assembly process.[47] The particles separated in the film and can be observed from the optical microscopic image (Figure a), where the bright green and the black regions correspond to the crystallization of silica particles and CB aggregation, respectively. The microscopic reflection signal (Figure b) of the crystalline region is much higher than that of black region, which is in agreement with the observation from the optical microscope. Furthermore, the representative SEM images (Figure c,d) also demonstrate the crystalline region with long-range ordered structures and the CB region of particle aggregation.

Figure 4

(a) Microscope image (b) Microscopic reflectance spectra, and corresponding (c,d) SEM images of photonic crystals fabricated with PVP (0 mg) and CB (1%).

When PVP (0.1 mg/mL) was added into the preassembly solution, long-range order still existed in the photonic crystals (Figure d,g), but the particle separation seemed to be reduced (Figure a) compared to that without PVP owing to the adsorption of PVP on the surfaces of CB particles, which helped CB to be distributed uniformly in APCs. Hence, with PVP concentration further increased to 0.5 mg/mL, the APC film with uniform structural color was obtained (Figure b). The appearance of the APCs is quite uniform across the whole film as observed by the naked eye. Ten random points of the film were selected to measure the microscopic reflection intensity in order to determine the uniformity of the APC film. The same reflection intensity (Figure S3) strongly supports the uniformity of the structural color because the CB particles are well distributed into the APCs with the assistance of PVP. The APC film shows constant structural color when the viewing angles are changed from 0 to 80° because of the amorphous structures of the APCs. The corresponding SEM images (Figure e,h) of the APC film show the silica particles were packed closely to each other with only short-range ordered structures, which is the typical characteristic of the amorphous structure. Further increasing the PVP concentration to 1 mg/mL, APC films with angle independent or noniridescent structural colors were obtained because of the amorphous structures (Figure c,f,i). Combining the appearance color and structures of the APCs, we believe the optimized concentration of PVP is about 0.5 mg/mL.

Figure 5

Effect of the PVP amount on the structures of photonic crystals. Microscopic images and SEM images of photonic crystals obtained with different amount of PVP, (a,d,g) 0.01, (b,e,h) 0.05, and (c,f,i) 0.1 mg. Area bars in (c,f), and (i) are 200 , 2 μm, and 500 nm, respectively, and is applied to all the images in the same line.

Beside the PVP concentration, the weight-average molecular weight (Mw) of PVP on the structures of APCs was further examined. All the APC films prepared with different Mws of PVP exhibited a noniridescent structural color (Figure S4) similar to that of original sample, implying the Mw of PVP had little effect on the particle arrangement of APCs.

As PVP show considerable effects on the formation of APCs, one may wonder whether other polyelectrolytes show similar effects when they are introduced upon the assembly process. As shown in Figure S5, when PAA is used to replace the PVP, the corresponding film exhibits intense reflection but with extremely pale colors. Usually, PAA are dissociated with negative charges in solution, which is not favored to be absorbed on the surfaces of particles; hence, the PAA and particles precipitate out separately form the solution. In contrast, muted colors can be achieved as the PEI are used because of the strong absorption of PEI on the surfaces of particles. However, the viscous PEI will induce the low quality of APCs, as can be confirmed by the nonuniform colors and ultralow reflection intensity. Therefore, it is reasonable to conclude that the PVP is the most suitable polyelectrolyte for the fabrication of APCs.

Except for the PVP, the addition of CB particles into APCs plays a crucial role in determining the color visibility and reflection intensity of APCs. When the APC film was prepared in the absence of CB particles, the film exhibited a whitish color (Figure a) caused by the coherent light scattering because of the short-range order of the amorphous structures. From the microscope reflectance spectra of the film, we could see that the background line was very high across the entire visible spectra (Figure d), indicating the incoherent light scattering was strong. The incoherent scattering of light across the whole spectra regions leads to the white appearance of APCs, which is unwanted and should be eliminated. Hence, for the APCs with angle independent structural color, additional material that could reduce the incoherent light scattering efficiently is highly desirable.

Figure 6

Effect of the CB content on the color visibility of APCs. (a–c) Microscopic images and (d–f) reflectance spectra of APCs fabricated with a variety of CB contents. Green, blue, and red structural colors represent the APCs obtained with different particle sizes (a) 252, (b) 197, and (c) 276 nm, respectively.

To achieve this goal, the introduction of black material into APCs that can uniformly absorb light across the entire spectra region is an effective route. CB shows intrinsic advantages of environmentally friendly mass production and is used to absorb the unwanted incoherent scattering light in the APCs. Microscopic reflectance spectra were used to determine the optimal content of CB (Figure d). As 0.5% CB (mt ratio, CB/silica) was added into APC precursor solution, the overall reflection intensity of the obtained APC film was significantly reduced, and a pale green structural color was obtained. With the CB content increased to 1%, most of the incoherent scattering of lights were eliminated, resulting in a saturated green structural color. However, the structural color gradually turns form green to black when the CB content was gradually increased to 4% because excess CB in the APC film would absorb the coherent scattering of light. Except for the green structural color, the blue (Figure b,e) and red (Figure c,f) APC films also showed similar color change tendency with addition of various contents of CB (0–4%). According to reflectance spectra and appearance (Figure S6) of APCs, the optimal content of CB is determined to be about 1%.

Combing the simple, efficient, and large-area fabrication characteristics of APCs in the presented method, a mask-based brush printing technique was used to print multicolor patterns (Figure ). Generally, the brush printing technique is finished by repeated operations including mask covering, brush coating, and drying. The usage of the mask makes it possible to print desired patterns of various structural colors on substrates or any regions of the premade photonic paper. As shown in Figure a, the numerals, mathematical operators, and other patterns with brilliant structural colors could be printed on the papers. Through a similar process, multicolor patterns (Figure b,c,e) with/without colorful background could also be prepared by the brush printing method, in which the using of the mask and the tuning of structural colors with particles of different sizes are performed repeatedly. One can record the change of reflection wavelength (Figure d) across the pattern along the route of white arrows; thus, the information such as the color saturation and particle size of a certain structural color can be acquired. Furthermore, all the colorful patterns showed noniridescent or angle independent structural colors at broad viewing angles (Figure S7). It should be noted that the boundary resolution was determined by the distance between the substrate and the mask, and a reduced distance will be helpful to obtain patterns with high resolution, which was evidenced by the sharp corner of the pattern with the mask tightly contacted with substrate (Figure S8). Besides the simple and convenient operation in creating a multicolor pattern, another advantage of the brush printing is that it could be printed on various substrates including flat and curved glasses, resin, wall, rough stone, flexible paper, plastic film, and tin foil paper (Figure ). These results clearly demonstrated the successful printing of multicolor by the brush printing method in our work. By using delicate masks, high-resolution patterns can be achieved through the brush printing method (Figure ). Here, squares with length of 100 μm and stars with diameter of about 120 μm could be printed on the substrate when stainless steel grids with periodic grooves of square and star were used as masks. The APC squares and stars were uniformly distributed on the substrates, and the point resolution was measured to be 96 and 25 μm (Figure c,d), respectively. The above results suggest that micropattern with noniridescent structural color can be printed using the brush printing method.

Figure 7

Series of photonic patterns (a–c) with controlled structural colors that were printed on (a,c) the substrate or (b) photonic paper of blue background. (d) Reflectance spectra of the sample in (c) along the white arrow. Digital photos of multicolor patterns captured at (e) 0 and (f) 45°. Area bars in (a,b) and (c,e) are 1 and 2 cm, respectively.

Figure 8

Square pattern was printed on various substrates through a brush printing method. Length of the square is 2 cm in all images.

Figure 9

Optical microscopic images of photonic prints of “squares” and “star”.

Conclusions

In summary, a PACA method is reported to fabricate APCs with noniridescent structural colors by the co-assembly of silica colloidal particles, PVP, and CB into amorphous structures. The concentration of PVP and CB were optimized to realize bright structural colors, which can be controlled using particles with different diameters. PVP plays a key role in the formation of only short-range ordered structures of APCs through suppressing the crystallization of colloidal particles upon assembly process, while the introduction of CB is crucial for the color visibility of APCs by absorbing the incoherent scattering of light. Such silica–PVP–CB mixed solution can be used as APC-ink directly for photonic printing, as only brush and masks are necessary for the developed brush printing. Multicolor and high-resolution APC patterns with noniridescent structural colors can be easily fabricated by using the mask-based brush printing method, which can be also applied to flat, curved, rough, and flexible substrates, such as paper, glasses, metal, plastic film, and stones. The fabrication method together with APC-ink and the brush printing method investigated in the presented work may disclose a potentially useful material system for fabricating noniridescent structural color patterns in a more simple, efficient, and economical way.

Experimental Section

Materials

PVPs of different molecules (Mw: 40 000, 55 000, 30 000, and 1300 000) were purchased from Sigma-Aldrich. CB (30 nm) was purchased from Aladdin. Tetraethyl orthosilicate (98%), ethanol (EtOH, 99%), and aqueous ammonia (28%) were purchased from Sinopharm Chemical Reagent Co. Ltd. All the chemicals are used as received without further purifications.

Fabrication of APCs

Silica particles of tunable sizes were synthesized by the well-known Stöber method. After purification, silica particles were dispersed in ethanol with a concentration of 200 mg/mL. For the fabrication of APCs, CB and PVP with the desired amount were mixed with the silica colloidal dispersion (0.1 mL), which was then dropped onto the glass. The glass was then heated to 50 °C for 2 min, and APCs with bright and noniridescent structural colors were obtained.

Brush Printing of Patterned APCs

In brief, a mask with a designed groove pattern was prepared using the craft punch mold, and it was placed onto the photonic paper or the substrate. Then, APC-ink with silica particles (2 g), PVP (0.5 mg), and CB (20 mg) dispersed in ethanol (10 mL) were brushed onto the substrate to prepare patterns using a brush bought in the market. After the solvent was evaporated, the mask was removed and noniridescent structural color pattern was obtained.

Characterization

The assembly structures of APCs were investigated by using the HITACHI SEM-SU8010. FT-IR spectra were obtained on a Thermofisher Nicolet IS50 FT-IR spectrometer ranging from 4000 to 400 cm–1. The optical microscope images and microscopic reflectance spectra were obtained on an Olympus BXFM reflection-type microscope operated in a darkfield mode. The reflectance and backscattering spectra at different angles were measured by using a NOVA spectrometer (Hamamatsu, S7031). The reflection of the APC film at various angles was detected by using the angle resolved detector (R1) coupled with the spectra (NOVA). The ambient light was used as the light source, and the detect angle was later changed from 0 to 80° relative to the normal to the surface of the sample. The zeta-potential and the kinetic particle size was measured in the presence of ethanol by using Malvern ZS 90.