Received: 22 March 2019 | Revised: 10 June 2019 | Accepted: 18 June 2019 DOI: 10.1002/ces2.10015 ORIGINAL ARTICLE Effects of processing parameters on 3D structural ordering and optical properties of inverse opal photonic crystals produced by atomic layer deposition Heloisa G. Campos1 | Kaline P. Furlan2,3 | Daniel E. Garcia1 | Robert Blick3 | Robert Zierold3 | Manfred Eich4,5 | Dachamir Hotza1 | Rolf Janssen2 1Graduate Program in Materials Science and Engineering (PGMAT), Federal Abstract University of Santa Catarina (UFSC), Vertical convective self‐assembly has been extensively used for the preparation of Florianópolis, Brazil direct photonic crystals, which can be later infiltrated with a more stable material, 2 Institute of Advanced Ceramics, Hamburg such as oxide ceramics, by atomic layer deposition. However, the relationship be- University of Technology (TUHH), Hamburg, Germany tween the self‐assembly parameters of the direct photonic crystals and the optical 3Institute of Nanostructure and Solid‐ properties of the inverse opal photonic crystals remains elusive. In this work, the State Physics and Center for Hybrid effect of different experimental parameters on the 3D structure and the density of Nanostructures, Universität Hamburg, Hamburg, Germany defects of polystyrene direct photonic crystals produced by vertical convective self‐ 4Institute for Optical and Electronic assembly was assessed. Self‐assembly was investigated using deionized water as Materials, Hamburg University of media with polymer particles’ concentrations up to 2 mg/mL; temperatures of 40, Technology, Hamburg, Germany 50, and 80°C; and relative humidity of 45%, 70%, and 90%. The 3D structure of the 5Institute of Materials Research, Helmholtz‐ resultant direct photonic materials was characterized by the combination of scanning Zentrum Geesthacht, Geesthacht, Germany electron microscopy and image analysis, and their optical properties were assessed Correspondence by reflectance measurements. These results were correlated with the performance of Rolf Janssen, Institute of Advanced Ceramics, Hamburg University of oxide‐based inverse opal photonic crystals produced by the controlled infiltration of Technology, Hamburg 21073, Germany. the former direct photonic crystals by atomic layer deposition (ALD). It was found Email: [email protected] that the thickness increased with the concentration of polystyrene particles, while the Funding information photonic structure ordering is dependent on the synergy between humidity and tem- SFB 986 Hamburg University of perature. Results also showed higher defects population with increasing evaporation Technology, Grant/Award Number: temperature and decreasing relative humidity. DXX/3264/SFB/1986/M09/C5 KEYWORDS 3D structure, optical properties, photonic materials, vertical convective self‐assembly 1 | INTRODUCTION weak forces such as van der Walls, electrostatic, hydrogen bonds, hydrophobic, and stacking interactions as well as ste- Self‐assembly is a process whereby individual components ric forces.1 Application of self‐assembly techniques requires arrange themselves into an ordered structure. In a self‐as- highly ordered photonic crystals, such as optical fibers, la- sembled structure, the building blocks are usually linked by sers, photovoltaic cells, and other optoelectronic devices.2‒6 This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. © 2019 The Authors. International Journal of Ceramic Engineering & Science published by Wiley Periodicals, Inc. on behalf of American Ceramic Society 68 | wileyonlinelibrary.com/journal/ces2 Int J Ceramic Eng Sci. 2019;1:68–76. CAMPOS ET AL. | 69 Colloidal crystals can be self‐assembled from a colloidal 2 MATERIALS AND METHODS suspension by evaporation of the liquid solvent or by grav- | ity sedimentation in a variety of approaches,7 such as spin Monodisperse polystyrene (PS) particles (microparticles), coating,8 vertical deposition,9,10 electrophoresis,11,12 centrif- with diameter of (0.76 ± 0.02) µm, were self‐assembled ugation,13 freezing,14 physical confinement,15 or convective on single crystal sapphire substrates (25 × 30 × 0.53 mm, self‐assembly16 to name a few of them. Crystec). Vertical convective self‐assembly (VCSA) was Vertical deposition is the method most extensively ap- performed inside a humidity chamber (HCP 108, Memmert) plied to produce outsized scale colloidal crystals. Novel for 120 hours, or until the complete evaporation of the solvent functional nanomaterials such as photonic materials with using Teflon beakers containing PS suspensions in filtered tailor‐made thickness have been obtained using a bottom‐up deionized water. The angle between the substrate normal and fabrication approach.17‒19 However, depending on the type the beaker base was kept at 81° to 84°. and the extent of the defects, colloidal crystals may suffer Colloidal crystal films were obtained by controlled evap- from reduced reflectance capability or no photonic band gap oration of the liquid and different combinations of PS con- making them unusable for applications.20 Typical defects centration (1, 1.5, 2 mg/mL), temperature (40, 55, 80°C), and may include line defects, point defects, drying cracks, stack- relative humidity (RH) (45%, 70%, 90%). ing faults, and random variations in the sphere positions.21,22 After vertical colloidal self‐assembly, thickness and struc- Thus, the creation of defect‐free three‐dimensional (3D) tural morphology of the resulting direct photonic crystals photonic crystals remains still challenging, although several were investigated by scanning electron microscopy (SEM, reports for the production of photonic crystals by colloi- Leo 1530). The self‐assembly defects were evaluated by dal self‐assembly have been published. During the self‐as- image analysis (ImageJ, 1.51p22) in a minimum of 20 SEM sembly process, the colloidal crystal growth is affected by images taken at lower magnification (500×). For elimina- particle concentration, liquid evaporation rate, and the in- tion of noise (coming from the shadowing in between the terparticle forces in the suspension.5 As analyzed by Kuai spheres), a Gaussian filter was applied, with no loss of infor- et al,22 evaporation temperature and relative humidity—both mation concerning the defects. After binarization, the area of parameters influence the crystal growth rate by changing the the defect was analyzed by an automated threshold definition. evaporation rate of the solvent—have a strong influence on Atomic layer deposition (ALD) of Al2O3 using the pre- the film thickness, defects, and cracks. Those parameters cursors trimethylaluminum (TMA, Sigma‐Aldrich) and de- were also analyzed by Liu et al,5 who observed that lower- ionized water was performed in a reactor (Savannah 100, ing the evaporation temperature and increasing the relative Ultratech/Cambridge Nanotech) for infiltrating the direct humidity during the assembly process lead to the formation photonic crystals (PS templates) at 95°C. The cycle was of high‐quality crystals. performed under exposure mode (0.2/60/90 seconds) with The effect of the suspension concentration on defective 3 nitrogen as carrier gas (30 sccm or cm /min). After ALD, area is well reported in the literature. Yan et al,15 Xiao et the polymeric template was calcined in air at 500°C for al,1 and Weiss et al23 showed that the density of defects 30 minutes, generating the inverse opal photonic crystal increases for higher polymer concentration. Moreover, structure. Zhang et al21 and Voitchovsky et al24 reached similar The optical response of both direct and inverse opal pho- results when studying the correlation between the type tonic crystals was evaluated by specular reflectance mea- of liquid used and the amount of defects. Additionally, surements (UV‐vis‐NIR spectrometer, PerkinElmer, Lambda Voitchovsky et al24 revealed that the density of defects 1050) from 900 to 1850 nm and at an incident angle of 8°. can be decreased by using higher polar solvents to lower the evaporation rate. In this paper, direct photonic crys- tals were produced on sapphire substrates via vertical 3 RESULTS AND DISCUSSION convective self‐assembly of polystyrene particles from | aqueous suspensions. A systematic study of self‐assembly 3.1 Visual characteristics parameters, namely suspension concentration, processing | temperature, and relative humidity, was performed. The The direct photonic crystals fabricated by vertical convective influence of these factors on the defect density and pho- self‐assembly showed an angle‐dependent reflectance behav- tonic structure ordering was assessed by scanning electron ior when interacting with visible light, as expected. As seen microscopy and spectroscopy techniques, respectively. in Figure 1, reflected light changes from blue‐green to red as 25 These results were correlated with the performance of consequence of the incident angle. oxide‐based inverse opal photonic crystals produced by All produced samples showed characteristic self‐assem- controlled infiltration of the former direct photonic crys- bly stripes, which were expected from the vertical depo- 3,26 tals by atomic layer deposition. sition, as described elsewhere. The orientation of the 70 | CAMPOS ET AL. (A) (B) FIGURE 1 Photograph (A) and scanning electron microscopy (SEM) image (B) of a polystyrene (PS) direct photonic crystal self‐assembled on a sapphire substrate