Nanospiral Formation by Droplet Drying: One Molecule at a Time
© Wan et al. 2010
Received: 29 July 2010
Accepted: 9 September 2010
Published: 30 September 2010
We have created nanospirals by self-assembly during droplet evaporation. The nanospirals, 60–70 nm in diameter, formed when solvent mixtures of methanol and m-cresol were used. In contrast, spin coating using only methanol as the solvent produced epitaxial films of stripe nanopatterns and using only m-cresol disordered structure. Due to the disparity in vapor pressure between the two solvents, droplets of m-cresol solution remaining on the substrate serve as templates for the self-assembly of carboxylic acid molecules, which in turn allows the visualization of solution droplet evaporation one molecule at a time.
Patterns formed by solvent evaporation are relevant to various coating processes as well as patterning technology. In capturing the molecular process of an evaporating droplet, this work demonstrates the possibility to further modulate dewetting patterns by amphiphiles capable of self-assembly. Self-assembly as an alternative to lithography has the potential to generate reconfigurable nanostructures [1–3]. Surfactants/amphiphiles are the simplest molecules to self-assemble into complex yet often predictable structures and phases. An interface perturbs and sometimes dominates the self-assembling behavior of amphiphiles. A well-known example of substrate-dominated self-assembly is the epitaxial stripe nanopatterns formed by alkanes and alkane derivatives on highly oriented pyrolytic graphite (HOPG) [4–10]. The 1,3-methylene group distance, 0.251 nm, of all-trans alkyl chains matches the distance of the next nearest neighbor of the HOPG lattice, 0.246 nm, along, e.g., the  crystallographic direction. The head-to-head arrangement gives rise to the stripe nanopattern whose periodicity is 1 × or 2 × the molecular chain length. Such nanopatterns serve as model templates for the study of site-specific adsorption, alignment, assembly, and reaction of small molecules [8, 9, 11, 12] as well as macromolecules [13–16].
In an earlier example, we disrupted the stripe nanopattern of eicosanoic acid (C20A) using mercaptoundecanoic acid capped cadmium sulfide nanoparticles. C20A nanorods with 1.0 nm in thickness and 5.4 nm in width are nucleated directly on the nanoparticle to produce nanoparticle/nanorod hybrid structure . Here, we present another method to perturb the epitaxial interaction between long-chain carboxylic acids and HOPG and to create spiral nanopatterns by adding a co-solvent to the spin coating solution. We propose that the curved nanostructure is formed at the receding solid/liquid/vapor contact line of an evaporating solution droplet, and it traces the entire droplet evaporation process at the molecular scale.
Recently, a number of methods have been reported for making circular nanostructures. Nanorings have been generated by lithography (microcontact printing , electron beam , and AFM tips ), template-based synthesis (using droplets , viruses , and DNA ), self-assembly [24–27], selective dewetting on patterned surfaces [28–30], and evaporation-driven dewetting [27, 31–33]. There have been fewer reports on nanospirals [34–37]. The scientific interests for nanorings range from quantum rings, whose connected geometry at the nanoscale can trap "persistent currents" [38–41], to biomimetic light-harvesting complexes [31, 42, 43] and DNA microarrays for high-throughput DNA mapping [44, 45]. The nanoring structure is also interesting because of its resemblance of the toroid structure of condensed DNA .
Long-chain carboxylic acids including hexadecanoic acid (C16A, Aldrich, 99%), octadecanoic acid (C18A, Fluka, ≥ 99.5%), eicosanoic acid (C20A, Sigma, ≥99%), docosanoic acid (C22A, Aldrich, 99%), tetracosanoic acid (C24A, Fluka, ≥99.0%), and hexacosanoic acid (C26A, Sigma, ≥95%) were used. Solvents used were m-cresol (Aldrich, 97%), methanol (Mallinckrodt Chemicals, 100%), ethanol (Pharmco, 100%), iso-propanol (Fisher Scientific, 100%), and sec-butanol (Fisher Scientific, 99.3%). HOPG (grade ZYB) was purchased from MikroMasch. All chemicals were used as received.
Carboxylic acids were dissolved in a primary alcoholic solvent or a binary solvent of alcohol and m-cresol to yield a final concentration of 0.2–0.4 mM. HOPG was freshly cleaved by adhesive tapes. The spin coating (PM101DT-R485 photoresist spinner, Headway Research) was conducted at room temperature in ambient air with relative humidity <40%. A volume of 100 μL of the solution was dispensed onto HOPG and spun at 3,000 rpm for 60 s. The samples were dried in air for 20 min or longer.
The spin-coated samples were imaged using Nanoscope III Multimode AFM equipped with a piezoelectric scanner with a maximum scan range of 10 μm (x and y) and 2.5 μm (z) from VEECO/Digital Instruments. Height, amplitude, and phase images were obtained in Tapping Mode (oscillation frequency ~ 250–300 kHz) in ambient atmosphere using etched silicon probes (ACT, NanoScience) with nominal radius of curvature <10 nm. The scan rate was 1–3 Hz. Integral and proportional gains were approximately 0.4 and 0.8, respectively. Only flattened height images were shown. The films were usually imaged within minutes of film preparation. However, the nanostructures were unchanged for at least 1 month afterward when stored in ambient environment. The contour length of the stripe was determined using the WSxM 4.0 software.
Contact Angle Measurement
The contact angle was measured by an NRL contact angle goniometer (Model 100, Rame-Hart) in the laboratory atmosphere. One m-cresol droplet of 5 μL was placed on the substrate and contact angles were read on both sides of the droplet. Five droplets were placed at various spots near the center of the substrate, and contact angles were averaged with an error of ±3°.
Results and Discussion
Figure 6c–e shows multiple C20A spirals, partial spirals, and coexisting straight stripes. The spirals of C18A, C20A, and C22A display a center-to-center distance of 5.1, 5.6, and 6.2 nm, respectively, which indicates that the spiral is made of the same head-to-head dimer arrangement as in the epitaxial stripes on HOPG. The sectional height analysis indicates that the spirals have a uniform height of 0.8 ± 0.1 nm. The straight stripes outside the spiral have the same height as the spirals while those inside tend to have a lower height of 0.2–0.4 nm. The lower height value suggests that the structure is templated only by HOPG in which the carboxylic acid carbon plane faces HOPG [4, 53]. The higher height value is consistent with crystalline structure that is not templated by HOPG.
The drying of solution droplets is described by the coffee-stain mechanism [51, 56–59]. The higher evaporation rate at the pinned sessile convex droplet contact edge causes convective capillary flow and precipitation of solute at the edge. The capillary flow goes from the bulk solution to the edge of the droplet in order to maintain the spherical shape to counter evaporative losses . The flow results in solute accumulation at the pinned contact edge as a solid ring. Pinning of the contact line is a "self-pinning" process, which means that the accumulation of the solute at the contact line perpetuates the pinning of the contact line . Multiple rings can result from the solute deposit. An incomplete transfer of solute results in material left inside the ring. Our results show the sequence of this solute deposition for the first time at the molecular scale. The results show that the pinned contact line moves unidirectionally by either a clockwise or counterclockwise inward rotating motion. The process starts with one precipitating H-bonded carboxyl dimer (some spirals have a thicker starting point indicating that sometimes evaporation may start from a cluster of dimers), grows by a crystallization process along a direction normal to the carbon chain and parallel the triple contact line, and terminates with the depletion of either the solute (partial spiral) or solvent (excess deposit of solute as dots inside the spiral).
The length of the spirals provides a measure of droplet concentration at the beginning of droplet evaporation. For example, the total contour length of the spiral in Figure 6b is 272 nm, which corresponds to a total spiral volume of 1.22 × 103 nm3 assuming width and height of 5.6 and 0.8 nm, respectively. The B-form C20A unit cell size is 1.97 nm3 with 4 molecules per unit cell (a = 0.549 nm, b = 0.740 nm, c = 4.855 nm, and β = 90°) . Therefore, the total number of molecules in this spiral is 2.48 × 103. Given an outer diameter of the spiral of 56.5 nm, the droplet volume is 4.7 × 10-21 L (using 15° contact angle). The C20A concentration in the droplet is therefore 0.88 M, a supersaturation of ~ 60 (the C20A solubility in m-cresol is determined to be ~0.015 M at the room temperature).
The molecular packing structure in the spiral is visualized based on the most stable B-form carboxylic acid crystal structure (C18A is used here) . The B-form n-carboxylic acid crystal is described as tablet-shaped plate terminated by (001) and (110) faces with interplanar angle of 75° [61–64]. The spiral width direction corresponds to the  direction with an interplanar spacing same as 2 × chain length. A likely orientation of the spiral face parallel to HOPG is the (110) face whose interplanar spacing is 0.452 nm. The spiral thickness as determined by AFM is larger, which may mean that the crystalline plane of the spiral face is tilted toward the b axis as indicated by the scheme in Figure 7c.
The unique combination of the binary solvent system and the self-assembling tendency of the carboxylic acids at the interface allow the droplet evaporation process to be captured at the molecular scale. The solid/liquid/vapor interface of m-cresol solution droplets serve as templates for the carboxylic acid molecules to self-assemble, which in turn allows the visualization of solution droplet evaporation one molecule at a time. The AFM images show that the pinned contact line moves unidirectionally by either a clockwise or counterclockwise inward rotating motion. The droplet evaporation contributes a new method for the nanospiral formation.
The authors acknowledge partial support from the National Science Foundation (CBET-0553533 and CBET-0755654).
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