Evaporation‐Induced Self‐Assembly of Nanoparticles from a Sphere‐on‐Flat Geometry

Abstract

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...

Dynamical Model for the Formation of Patterned Deposits at Receding Contact Lines

“…The model incorporates wettability, capillarity, evaporation, convective transport of the solution and diffusion of the solute and has been derived employing a long-wave approximation. We find that a strong nonlinear dependence of viscosity (i.e., the front mobility) on concentration triggers, in an intricate interaction with evaporation and diffusion, the deposition of periodic and aperiodic line patterns as observed in experiments for many different materials and settings [2,[5][6][7][8][9][10][11]30]. We believe that the model explains a basic mechanism for the formation of regular line patterns.…”

Modelling the formation of structured deposits at receding contact lines of evaporating solutions and suspensions

“…This changes when the experiments are performed in a controlled way on smaller scales: Recently, both polymer solutions [21][22][23] and (nano)particle suspensions [24][25][26] have been employed in various small-scale geometries where one is able to exercise greater control over the contact line as it recedes due to evaporation. As a result, strikingly regular line patterns are created, where the deposited structures show typical distances ranging from 10-100µm.…”

Mesoscale Patterns Formed by Evaporation of a Polymer Solution in the Proximity of a Sphere on a Smooth Substrate: Molecular Weight and Curvature Effects

“…8 In our previous work, we reported that concentric rings of electrically conducting polymer and organometallic polymer of high regularity were formed naturally and spontaneously via controlled, repetitive "stick-slip" motion of the three-phase contact line when a drop of polymer solution was confined either between two crossed cylindrical mounts covered with single crystals of mica sheets 10 or between a spherical lens made of silica and a Si substrate (sphere-on-flat geometry), resulting in a capillary-held polymer solution (i.e., capillary bridge). [11][12][13][14][15][16][17] The evaporation in this geometry was restricted to the edge of the droplet, and the "stick-slip" cycles resulted in hundreds of concentric rings with regular spacing, very much resembling a miniature archery target. Each ring was nanometers high and several microns wide.…”