A drop of solution containing nonvolatile solute is allowed to evaporate from a sphere-on-flat geometry. Left behind is a striking pattern of gradient concentric rings with unprecedented regularity. The center-to-center distance between adjacent rings, lambda(C-C), and the height of the ring, h(d), are strongly affected by the concentration of the solution and the properties of the solvent. The nature of the formation of regular gradient ring patterns during the course of irreversible solvent evaporation is revealed through theoretical calculations based on the mass conservation of the solution.

Self-assembly of nanoscale materials to form ordered structures promises new opportunities for developing miniaturized electronic, optoelectronic, and magnetic devices. [1][2][3][4] In this regard, several elegant methods based on self-assembly have emerged, [5][6][7][8] for example, self-directed self-assembly, [5] and electrostatic self-assembly.[8] Self-assembly of nanoparticles by irreversible solvent evaporation has been recognized as an extremely simple route to intriguing structures. [9][10][11][12] However, these dissipative structures are often randomly organized without controlled regularity. Herein, we show a simple, onestep technique to produce concentric rings and spokes comprising quantum dots or gold nanoparticles with high fidelity and regularity by allowing a drop of a nanoparticle solution to evaporate in a sphere-on-flat geometry. The rings and spokes are nanometers high, submicrons to a few microns wide, and millimeters long. This technique, which dispenses with the need for lithography and external fields, is fast, cheap and robust. As such, it represents a powerful strategy for creating highly structured, multifunctional materials and devices.Quantum dots (QDs) are highly emissive, spherical, inorganic nanoparticles a few nanometers in diameter. They provide a functional platform for a new class of materials for use in light emitting diodes (LEDs), [13] photovoltaic cells, [14] and biosensors. [15] Because of the quantum-confined nature of QDs such as CdSe, the variation of particle size provides continuous and predictable changes in fluorescence emission. Passivating the vacancies and trap sites on the CdSe surface with higher-band-gap materials, such as ZnS, produces CdSe/ ZnS core/shell QDs that have strong photoluminescence. [16] Two CdSe/ZnS core/shell QDs (4.4 and 5.5 nm in diameter, D) were used as the first nonvolatile solutes in our experiments. The surface of CdSe/ZnS was passivated with a monolayer of tri-n-octylphosphine oxide (TOPO) to impart solubility to the host environment and retain the spectroscopic properties of the materials by preventing them from aggregating. A drop of CdSe/ZnS in toluene was loaded in a confined geometry consisting of a spherical silica lens in contact with an Si substrate (i.e., sphere-on-flat geometry; see Experimental Section), [17][18][19][20][21] which led to the formation of a capillary bridge of the solution as illustrated in Figure 1 a. In situ optical microscopy (OM) revealed two main types of pattern formations, namely, concentric rings and spokes, which depend on whether fingering instabilities of thin film of the evaporating front took place or not.The formation of ringlike deposits in an evaporating drop that contains nonvolatile solutes on a single surface is known as the "coffee-ring" phenomenon. [9,10,22,23] Maximum evaporative loss of the solvent at the perimeter triggers the jamming of the solutes and creates a local roughness (i.e., the contact Figure 1. a) Sphere-on-flat geometry in which a drop of nanoparticle solution is constrai...

The use of spontaneous self-assembly as a lithography-and external-fields-free means to construct well-ordered, often intriguing structures, has received much attention owing to the ease of producing complex structures with small feature sizes. [1][2][3] Drying mediated self-assembly of nonvolatile solutes (polymers, nanoparticles, and colloids) through irreversible solvent evaporation of a sessile droplet on a solid substrate (unbound solution) represents one such case. [3][4][5][6][7][8][9][10][11][12][13][14][15][16] However, irregular polygonal network structures (Benard cells) [14,15] and stochastically distributed concentric 'coffee rings' [4][5][6]10] are often observed. The irregular multirings ('coffee rings') are formed via repeated pinning and depinning events (i.e., 'stickslip' motion) of the contact line. [4][5][6]10] The evaporation flux varies spatially, with the highest flux observed at the edge of the drop. Therefore, to form spatially periodic patterns at the microscopic scale, the flow field in an evaporating liquid must be delicately harnessed. In this regard, recently, a few attempts have been made to guide the droplet evaporation in a confined geometry [17][18][19][20] with [17] or without [18][19][20][21][22] the use of external fields. Patterns of remarkably high fidelity and regularity have been produced. [18][19][20][21][22] However, interfacial interactions between nonvolatile solutes and substrates govern the stability of thin films and have not been explored in these studies. The synergy of controlled self-assemblies of solutes, and their destabilization mediated by the interaction between solutes and substrates during the solvent evaporation, can lead to the formation of intriguing, ordered structures. Herein, we report on the spontaneous formation of well-organized mesoscale polymer patterns during the course of solvent evaporation by constraining polymer solutions in a sphere-on-Si geometry, as illustrated in Figure 1 (bound solution, i.e., capillary bridge). Gradient concentric rings and selforganized punch-holelike structures were obtained via mediating interfacial interactions between the polymer and the substrate. This facile approach opens up a new avenue for producing yet more complex patterns in a simple, controllable, and cost-effective manner.Poly(methyl methacrylate) (PMMA), polystyrene (PS), and PS-b-PMMA diblock copolymers were used as nonvolatile solutes to prepare PMMA, PS, and PS-b-PMMA toluene solutions, respectively. The concentration of all the solutions was 0.25 mg mL -1 . The evaporation, in general, took less than 30 min to complete. The pattern formation was monitored in situ by using optical microscopy (OM). After the evaporation was complete, two surfaces (spherical lens and Si) were separated and examined by using OM and atomic force microscopy (AFM). Only the patterns on Si were evaluated. Highly ordered gradient concentric rings of PMMA, persisting toward the sphere/Si contact center, were obtained over the entire surfaces of the sphere and Si except the ...

Poly(ferrocenylsilanes) (PFS) are a novel class of transition metal-containing polymers with a main chain that consists of alternating organosilane and ferrocene units. 1-17 They possess intriguing physical properties that have potential applications in magnetic data storage, 1,2,4-7,10,12,13 photonic device, 1,9 and redox-active materials. 2,5 In addition, they are ideal precursors for producing magnetoceramics whose magnetic properties can be tuned by pyrolysis temperature. 1,2,[4][5][6][7][8][10][11][12][13] For example, pyrolysis of poly(ferrocenyldimethylsilane) at 1000°C turns it into ferromagnetic R-iron (R-Fe) nanoparticles embedded in an amorphous silicon carbide/carbon (SiC/C) matrix. 1,7 It has been demonstrated that the patterned, micron-scale PFS bars, circles, and lines exhibit a significant increase in coercivity in magnetic properties measurements. 12 Furthermore, these patterned PFS can also serve as etch barriers in nano-and microlithographic applications, for example, transferring the patterns into silicon substrate. 11,15-17 PFS is stable in reactive ion etching process compared to common organic polymers due to the presence of iron and silicon in the polymer backbone. 17 However, the creation of these micrometer size patterns involves the preparation and use of either a mask in UV-lithography 11,14 or a stamp in capillary force lithography, 16,17 or requires the use of expensive electron-beam lithography that is not costeffective and operated under high vacuum chamber. 11,12 Moreover, a subsequent step of removal of unexposed PFS is required. 11,12,14 Self-assembly via irreversible solvent evaporation of a droplet containing nonvolatile elements (dyes, nanoparticles, or polymers) represents an extremely versatile way for onestep creation of complex large-scale 18-34 or long-range ordered structures. 35,36 However, the flow instabilities within the evaporating droplet often result in irregular dissipative structures (e.g., convection patterns and fingering instabilities). Therefore, to fully utilize the evaporation as a simple, non-lithography route to produce well-ordered structures that have numerous technological applications, it requires delicately controlling the evaporative flux, the solution concentration, and the interfacial interactions among the solvent, solute, and substrate. To date, a few attempts have been made to control the droplet evaporation in a confined geometry in which self-organized mesoscale patterns are readily obtained. [37][38][39] Recently, patterns of remarkably high fidelity and regularity have been reported. 37 They are formed simply by allowing a drop to evaporate in a confined geometry (1) MacLachlam, M. J.; Ginzburg, M.; Coombs, N.; Coyle, T. W.; Raju, N. P.; Greedan, J. E.; Ozin, G. Cheng, A.; Bartole, A.; Greenberg, S.; Resendes, R.; Coombs, N.; Safa-Sefat, A.; Greedan, J. E.; Stover, H. D. H.; Ozin, G. A.; Manners, I. Resendes, R.; Cheng, A. Y.; Bartole, A.; Safa-Sefat, A.; Coombs, N.; Stover, H. D. H.; Greedan, J. E.; Ozin, G. A.; Manners, I. AdV. Ma...