Orientationally correlated colloidal polycrystals without long-range positional order

Abstract

We probe the local and global structure of spin-coated colloidal crystals via laser diffraction measurements and scanning electron and atomic force microscopies, and find that they are unique three-dimensional orientationally correlated polycrystals, exhibiting short-range positional order but long-range radial orientational correlations, reminiscent of-but distinct from-two-dimensional colloidal hexatic phases. Thickness and symmetries are controllable by solvent choice and spin speed. While the polycrystallinity of these colloidal films limits their applicability to photonics, we demonstrate their feasibility as templates to make crack-free magnetic patterns. DOI: 10.1103/PhysRevE.77.050402 PACS number͑s͒: 82.70.Dd, 64.70.pv, 64.75.Yz The self-assembly of colloidal microspheres has been used to address the fundamental questions of how materials crystallize ͓1-6͔ or fail to crystallize ͓7-9͔. Micrometer-scale colloidal crystals can be used as a template that, using further processing methods, can be used to create photonic materials ͓10-12͔, optical sensors ͓13͔, and antireflection coatings ͓14͔. However, the high density of missing-sphere defects and cracks in photonic crystals produced via self-assembly ͓15͔ remains a serious limitation, and thus the study of colloidal defects ͓16͔ is an active area of research. Spin-coating of colloidal suspensions is the quickest and most reproducible method to make large-area colloidal crystals. While spin-coating has been proposed to fabricate single crystals for photonic applications ͓17,18͔, the symmetric radial optical interference patterns observed are unexpected for single crystals. We find here that spin-coated colloidal films are indeed neither single crystals nor powder polycrystals, but are in fact a unique polycrystal phase. While true singledomain sizes are ϳ10 m, there is orientational correlation on the centimeter scale. Our results demonstrate a novel crystal packing strategy by which long-range orientational order develops in the absence of long-range positional order, reminiscent of two-dimensional colloidal hexatic phases ͓19,20͔, and leading to crack-free crystals. Distinct from colloidal hexatic phases, our polycrystals exhibit centimeterscale orientational order, which arises due to the spinning axis and can be produced with fourfold, sixfold, or mixed symmetries for a range of thicknesses as a function of spin speed. The electrodeposition of magnetic material through colloidal polycrystals demonstrates their feasibility for material templating applications.The standard technique to make large-area close-packed crystals is controlled ͑vertical͒ drying, utilizing capillary forces ͓21-23͔ to direct self-assembly. Other external shear ͓24͔, electric ͓25͔, electrohydrodynamic ͓26,27͔, and gravitational forces ͓28͔ have also been used. Making dried colloidal crystals with these methods is slow, taking from hours to days. Spin-coating has been shown to be a robust technique ͓17,18,29,30͔ to make large-area colloidal crystals in minutes. In this work, we ...

Micromagnetic modeling of experimental hysteresis loops for heterogeneous electrodeposited cobalt films

“…In this context, we and others have investigated the viability of crack-free polycrystalline colloidal templates prepared by a very rapid spin coating method. [16][17][18][19] For our work, Co was electrodeposited on indium tin oxide (ITO) coated glass slides (Delta Technologies) using an electrolyte containing 0.1M H 3 BO 3 (EM Science, 99.5% pure) and 0.1M CoSO 4 (EM Science, 97.0% pure) with application of a constant potential (Hokuto Denko HA 501 potentiostat/galvanostat, À1.1 V to À1.25 V vs. saturated calomel) for times up to 10 min to yield deposits with average thicknesses $100 nm. To produce patterned films, we fabricated porous silica templates by spin-coating silica spheres, and then we electroplated cobalt in the voids between the particles.…”

“…Template thickness was controlled via the substrate rotation rate, as described elsewhere. 18 In order to fabricate the porous templates, we utilized silica spheres (250 nm or 500 nm diameter, Fiber Optic Center Inc.) that were dried for at least 3 h at 150 C to remove absorbed water. Spheres were then mixed with methyl ethyl ketone (MEK) at a ratio of 7.2 ml of MEK for each gram of silica (equivalent to a mass fraction of 5.8 MEK to silica).…”

Quantifying disorder in colloidal films spin-coated onto patterned substrates

“…Numerous techniques have been used to obtain well-ordered dried colloidal deposits [4][5][6][7][8][9][10][11][12][13][14][15][16][17]. However, it is challenging to obtain defect-free colloidal crystals over large areas.…”

“…It is very fast, reproducible, simple, and needs less material [20]. However, the axial symmetry of spin-coating makes the resultant colloidal films polycrystalline in a peculiar way, i.e., the orientationally correlated polycrystal (OCP) [11].…”

“…Spin-coating has recently been introduced in colloid science as a far-from-equilibrium modality to fabricate colloidal crystals [11,[19][20][21][22]. It is very fast, reproducible, simple, and needs less material [20].…”