Controlled Self-Organized Positioning of Small Aggregates by

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Controlled Self-Organized Positioning of Small Aggregates by Patterns of (Sub)nanosized Active Sites Rodrigo Perez-Garcia†,‡ and Hans Riegler*,‡ †

Institut für Chemie, Technische Universität Berlin, Strasse des 17. Juni 135, 10623 Berlin, Germany Max Planck Institute of Colloids and Interfaces, Theory and Bio-Systems, Science Park Golm, 14424 Potsdam, Germany

‡

S Supporting Information *

ABSTRACT: Bottom-up structure formation starting from a molecular level, such as solidification, condensation, or precipitation, typically involves nucleation processes. Therefore, nucleation is ubiquitous in nature and technology and has constantly been in the focus of research. Studies on individual nucleation processes are mostly theoretical, because experiments on the very small length scales and short time scales inherent to nucleation are challenging. In particular consecutive investigations onindividually selectedactive nucleation sites have never been performed. We have systematically studied how nanosized, porelike surface modifications repeatedly induce controlled, self-organized positioning of small aggregates through a heterogeneous nucleation and growth scenario. We here prove the nucleation activity of porelike surface modifications at the transition between continuum and molecular length scales. Moreover, it is demonstrated that even very shallow topological featuresbarely exceeding the natural roughness corrugations of a molecularly smooth surfaceinduce the positioning of aggregates, whereas slightly smaller roughness corrugations of the natural surface are not effective.

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INTRODUCTION The bottom-up approach to nanostructure formation at surfaces,1,2 for instance, via localized precipitation,3−5 scratchinduced graphoepitaxial growth,6 or nanoindentation templating7 is typically driven by (heterogeneous) nucleation and growth.8,9 In the latter case “active” sites10,11 may induce localized nucleation and growth through their specific chemical or topological properties, such as a pore (scratch, concavity).12,13 Sites may be active for preferred nucleation through their specific chemical properties (a lower interfacial energy for the attached aggregates/particles) or due to their topological properties. A number of recent theoretical studies on the nucleation properties of single pores or grooves focus on the impact of geometry and size.12,14−20 In fact, pores have for instance been proposed as effective means to increase the rate of (protein) nucleation.21−24 The roughness of a surface has also been analyzed theoretically in some recent reports as a large assembly of nucleation active pores.25−28 In strong contrast to a considerable number of theoretical investigations only rather few experimental data are available on this topic.4,29−33 The reason is rather obvious, in particular, regarding individually reproducible observations on heterogeneous nucleation driven by singled out and locally identified, nanoscale topological artifacts. Such experimental investigations are rather challenging and thus scarce.11 Here we present experimental data on the controlled aggregate positioning driven by arrays of artificially created nanosized concave topological features (“dents”). The data indicate a positioning pathway via “active” sites, viz., via nucleation and growth processes. In contrast to most other © XXXX American Chemical Society

nucleation studies we single out and identify individual nucleation sites. We can repeat the experiment with the same, individual “active” site. It is well accepted that topological artifacts can act as active nucleation sites (a prominent example: the bubble formation in beer at the side walls of the glass).34 Yet, the lower threshold size/dimension of the nucleation active site has never been investigated experimentally up to now. Here we address this point: How much artificial, individual, local topological modification on a nanometer scale is necessary (sufficient) to reproducibly (repeatedly) induce the targeted positioning of adsorbed nanoscale aggregates?

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RESULTS AND DISCUSSION Basic Experimental Approach and Results. Figure 1 presents a synopsis of the experimental approach and of the main observations concerning the coaxed aggregate positioning at specific, prestructured sites on a planar substrate. The lower part of the figure shows an area of a smooth planar silica surface with different arrays of nanosized dents. These arrays of dents were prepared by Atomic Force Microscope (AFM) (see Methods). The spacing between the dents within each array is uniform, typically in the range of 100−300 nm. Different arrays can have different spacing between the dents which will influence the filling of those (see below). The arrays are separated by typically a few micrometers. Right after the preparation sometimes debris can be found next to the dents.

Received: December 15, 2016 Revised: March 14, 2017 Published: March 14, 2017 A

DOI: 10.1021/acs.cgd.6b01840 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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Figure 1. Various arrays of 8 × 8 dents right after their preparation by AFM (the depicted area is only a subsection of a region with more arrays; see Supporting Information). The vertical force for creating the dents was 7.5 μN. Right after preparation there is some debris from the scratching. It is gone after cleaning. (A) depicts an area with an array of dents after cleaning and before C60 deposition. (B) depicts the same area after C60 deposition. The plot shows cross section profiles along the indicated lines. Please note that x and h do not have the same scaling. To reveal the true dimensions, a surface profile across a C60 aggregate is singled out and plotted in scale in the left upper corner of the x-h-plot. Also shown are cartoons of the surface cross section profiles before and after the deposition of C60. These cartoons demonstrate the relative scales of the surface roughness, the dents, the aggregates, and the size of the C60 molecules.

After cleaning this debris is gone as depicted in frame A. Typically the dents are roundish, concave “pores” with lateral dimensions of 10−20 nm and ∼1.0−1.5 nm deep, as documented by the height profiles (please note: x and h axis have different scales!). To better offer evidence of the dimensions of the topological profile, Figure 1 also shows cartoons in correct scaling of the dent topology and their spacings in relation to the surface roughness (rms roughness ≈ 0.4 nm) and the diameter of a C60 molecule (∼0.9 nm). The dents are quite shallow with radii of curvature ≫10 nm. They are rather weak individual topological features, barely exceeding the surface roughness corrugations. Onto such a surface (frame A) C60 is deposited via precipitation from a toluene solution by spin-casting (see Methods). Frame B depicts the observation after the evaporation of the toluene (frames A and B show identically the same area prior and after to the C60 deposition). Outside of the prestructured area, aggregates of C60 are observed, whose size and lateral distances vary.35 In contrast, in the region that has been prestructured by an array of shallow dents the aggregates are preferentially positioned at the dent sites. The aggregates in the prestructured region are larger and more uniform in size. Their spacing, given by the dent spacing, is

larger than the average spacing outside. In the region of the dent array there are rarely any aggregates, which are not located at the dent sides. If there are any, they are quite small. The height profile indicates that the aggregates fill the dents and stick out. The cartoon on the top shows the topology of the surface roughness in relation to the molecule size, the aggregate size, and the dent spacing. It should be noted that sometimes one can identify a correlation between an aggregate location and a topologically somewhat more pronounced concavity of the natural roughness. Such a case is also indicated in Figure 1. Impact of the Depth of the Dents. Figure 2 shows how the depth of the dents influences the positioning of the C60 aggregates. To this end an array of 9 × 9 dents has been prepared, whose depths vary within the array. The dents within the very left three columns were prepared with a vertical force of 7.5 μN, those within the three columns in the middle 5.5 μN, and those of the three columns on the very right with 3.5 μN. The height profile through the array region taken prior to the C60 deposition along the red vertical line reveals that the three dents from the three columns with the deep scratches can easily be identified. The three dents of the middle columns section are only faintly visible in the AFM image. In the height profile B



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locations of the dents. In the case of the deepest dents every dent holds one aggregate. Dents with intermediate depth also host in most cases an aggregate. The most shallow dents, however, are mostly empty. Still, if there are aggregates in this region, they are preferentially located at the dent position. Within the prestructured region only one rather large aggregate is not positioned at the location of an artificial dent. Its location can be correlated to a pronounced “natural” concavity as indicated in the figure. Variation of the Lateral Dent Spacing. Figure 3 shows aggregate sizes, their locations, and their distances to their next neighbor aggregates for grids with different dent spacings (“Different Grid Spacings”) and also for a surface region without prepatterning (“No Grid”) (The volumes are derived from the surface profile of the aggregates assuming round contact lines and a spherical cap. In particular with small aggregates, the error on the volume data may be quite substantial. Nevertheless, the data clearly reveal the aggregate size (distribution) and the lateral aggregate distances in the case of a variation of the dent spacing respectively without dents.). Dents are prepared with a vertical force of 5.5 μN and are ∼1 nm deep. For the smallest spacing of 130 nm, within the area of the dent array, virtually all the C60 aggregates are located at dent positions. But not every dent holds an aggregate. Some dents appear empty. The aggregate sizes/volumes vary considerably. This is reflected in the plot of the distance between next neighbors as a function of the aggregate volume. Because some dent locations remain unoccupied by aggregates, the distance between aggregates is identical to the grid spacing or larger.

Figure 2. Prepatterned surface section before (A) and after (B) the deposition of C60 (c0 = 3.4 × 10−6M). The surface has been prepatterned with an array of 9 × 9 dents with three different vertical forces. The left three columns of the array were done with 7.5 μN, the middle ones with 5.5 μN, and the right three columns with 3.5 μN. The height profile is taken along the vertical red line. It reveals that the weakest dents are barely discernible in the height profile. The blue circle identifies a rather deep natural dimple in (A), which results in an C60 aggregate positioned in between the aggregates that are located at the dents. The white lines in (B) are a guide to identify the 9 × 9 dent array.

they are still clearly visible. The three columns of dents on the right are rarely discernible in the imaging and in the height profile. After C60 deposition, within the prestructured substrate area, virtually all C60 aggregates are preferentially located at the

Figure 3. Top row: Height profiles taken along the lines depicted in the AFM images below each frame. Middle row: AFM images of C60 aggregates deposited on regions with a variety of regular rectangular arrays of dents of different interdent spacings as shown in the different frames from left to right and a region without any prepatterning (image at the very right). All dents were created with 5.5 μN. Lower row: Average distances between the aggregates as a function of their volume. The horizontal lines show the spacing between the dents. Please note that the lateral scales for the AFM images and the plots with the height profiles vary considerably from left to right as indicated by the black scale bars. C



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Figure 4. Frame A shows the aggregate location resulting from the C60 deposition on a certain substrate area. Frame B shows the aggregate location on the same substrate area under the same conditions after the substrate surface has been cleaned by piranha solution. Frame C depicts an overlay of both findings with those aggregates singled out by green cycles, which are located on the same location in both consecutive deposition processes. The green rectangular frame singles out the top row of an array of 9 × 9 dents, which has been used as a marker to identify the substrate area. The AFM images have been processed such that the aggregates are depicted by red (A) and blue dots (B).

barrier and form stable aggregates (precipitation). If the aggregates wet the substrate interface (contact angle

Controlled Self-Organized Positioning of Small Aggregates by

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