Communication pubs.acs.org/crystal
Etched Glass Self-Assembles into Micron-Size Hollow Platonic Solids Sofiane Boukhalfa† and Sahraoui Chaieb*,‡ †
Department of Materials Science and Engineering, Georgia Institute of Technology, 771 Ferst Drive, J. Erskine Love Building, Atlanta, Georgia, United States ‡ Division of Physical Science and Engineering, King Abdullah University of Science and Technology (KAUST), Thuwal 23955, Saudi Arabia ABSTRACT: The interaction between the spreading of a hydrofluoric acid-based drop on a glass surface and its etching rate gives rise to hollow crystals of various shapes, including cubes, triangles, and icosahedra. These geometries are dependent on their position with respect to the contact line, where a rim forms by agglutination, similar to the formation of a coffee stain. Atomic force microscopy indentation and transmission electron microscopy observations revealed that these crystals are hollow ammonium-fluosilicate-based cryptohalite shells.
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material from the center toward the rim where it is deposited. When the buffered HF etches away the glass, a melt or dispersion composed of ammonium-fluorosilicate nanoparticles is formed and transported by the bubbles that form during the hydrogen release. The nanoparticles agglutinate at the surface of the bubbles. Evidence of this melt can be found in several of the crystals, as shown in Figure 2, where the welding line is indicated with an arrow. Not fully understanding the occurrence of this line connecting the cubes in Figure 2,2 we imaged the etched region using a scanning electron microscope (SEM) and made the following observations: various geometrical objects with sizes ranging from a few up to 100 μm self-assembled as the drop spread and etched away the silicon oxide. The agglutination at the rim of the drop is similar to the coffee-stain effect in which rings encircling dried drops are made of self-assembled colloidal particles.3,4 Outward capillary flow sweeps the particles to the edge forming monolayers of nanocrystals.5 These are assembled at the periphery,6 providing a balance between the convection of the solvent and the diffusion of the nanocrystals to the drop surface. Slow radial flow during the drying drives these objects away from the center toward the rim. Some of them are cubical while others seem to be dodecahedral (see Figure 1). When we move away from the edge toward the center, we encounter various shapes, such as icosahedra, squares, hexagons, and triangles. In the center of the drop, a few large cubes are found. Indeed, there seem to be fewer objects in the center of the drop than closer to the rim of the etched area. As we move away from the
ecause of its importance in research and industry, silicon (Si) has long been the center of the electronics world. During chip manufacturing, a wet etchant such as hydrofluoric acid (HF) is used to strip the photoresist off of the silicon substrate. It is not intended to be used to etch the Si substrate. Such etching may occur unintentionally, however, and the byproducts of this unintentional etching have seldom been studied. Here, we describe crystals that grow as a direct result of the interaction between a silicon oxide substrate and an HFcontaining solution. This interaction is very likely to occur during the silicon etching process. Silicon oxide or simply glass can be etched by pure HF or by a buffered HF solution made of a mixture of nitric acid and HF. The effect of the buffer is to replenish the HF solution when a layer of silicon oxide is etched away. As the pH of the solution rises (HF is removed), the compound breaks down into ammonia and HF, thus replenishing the HF. Furthermore, a secondary reaction takes place in which ammonium-fluosilicate is formed with a release of gaseous hydrogen. In other words:1 NH4F → HF + NH3 HNO3 + NH3 + 6HF + Si → (NH4)2 SiF6 + H 2
We have deposited, at room temperature, a drop of a buffered oxide containing 49% HF and ammonium fluoride (NH4F) as well as nitric acid in stoichiometric proportions. The drop spreads very slowly in a linear fashion, while the precursor thin film etches the silicon oxide or glass. In our table-top experiments, we used a [100] silicon wafer with a 1 μm layer of silicon oxide as well as 171 mm Pyrex coverslips. A diagram of the etching drop and the structure formed is shown in Figure 1a. In Figure 1b, we show how a bubble that results from the chemical reaction carries the © 2012 American Chemical Society
Received: February 26, 2012 Revised: September 5, 2012 Published: September 7, 2012 4692
dx.doi.org/10.1021/cg3002702 | Cryst. Growth Des. 2012, 12, 4692−4695
Crystal Growth & Design
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Figure 1. Morphologies of the self-assembled crystals in a drop of acid on silicon oxide: (a) Diagram of a drop of acid etching away the silicon oxide. The patterns shown around the drop are not to scale. The scale bar corresponds to the SEM images. The size of the drop depended on the quantity of acid that was poured on the substrate, but in most of the cases it reached a centimeter. (b) A scenario of a bubble carrying etched material to the periphery.
Figure 3. Concentric lines that expand radially in a hexagon- and square-faced crystal. The scale bar is 10 μm.
for the shape selection. However, in the case of large droplets, we did observe the absence of small cubes. The radial flow becomes faster with bigger drops and eventually overtakes the etching process. The best fit of the X-ray diffraction results of these objects revealed an underlying structure similar to that of barrarite (NH4SiF6),14,7 a gem found naturally in volcanoes (Figure 4).8 The structure was cubic with an average lattice constant equal to 8.352 Å. The interatomic distance was about 4.8 Å in the [111] direction. Due to the cubic nature of this crystal structure, we identified it as cryptohalite. The mechanism leading to the formation of such large crystals remained unclear, but we speculate that they transport the hydrogen released during the etching reaction. In nature, similar crystals are found in volcanoes, where, it is believed, they are formed under high pressure and temperature.8 (We should note, however, that the crystals found in volcanoes are not hollow.) Remarkably, etching a glass slide under ambient temperature and pressure formed the same, albeit hollow, crystals during our table-top experiment. X-ray photoelectron (XPS), Raman, and Fourier transform infrared (FTIR) spectroscopy revealed that the structure was that of diammonium-fluosilicate. Auger spectroscopy was not possible because the electron beam damaged the crystals before any data could be collected. (The structure was unstable.) We investigated the structure in the [100−500 nm] range using small-angle X-ray scattering (SAXS) and did not observe
Figure 2. SEM image of a cluster of cubes self-assembled from a melt of etched silicon oxide. The red arrow indicates the solder that welds the two cubes together. Note the smoothness of the cubes.
center of the drop, there is a preponderance of small cubes. This might be due to an interaction between the etching rate and the slow spread of the drop. While the polyhedra at the rim are smooth, the ones closest to the center of the drop exhibit internal concentric lines that expand radially from the center of the crystal (Figure 3). These crystals are isolated and not connected to any neighbors, unlike those shown in Figure 2. We note that the lines do not originate at the center of the polyhedra (right-hand side of Figure 3). We suggest that the growth of these crystals is different from the growth of the crystals at the rim. These crystals grow exponentially (kinetically with a time constant of several minutes) from a supersaturated solution while the polyhedra at the rim are transported by hydrogen bubbles. We varied the volume of the drop from 1 μL to 100 μL, but we did not observe any change in the size of the crystals nor in their distribution throughout the droplet. This suggests that another mechanism rather than surface energy is responsible 4693
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of the crystal we indented. This nonlinearity is due to geometry and not due to a nonlinear stress strain relation, although this boundary cannot be drawn very clearly.13 Larger crystals had thicker shells. Our transmission electron microscopy (TEM) observations at 120 kV of one of the crystals did not permit us to probe the structure at the intermediate scale because the crystals could not withstand the power of the electron beam and melted away as soon as we started the observation, leaving behind a membranous structure enclosing an aggregate of nanoparticles, which were themselves in smaller aggregates. Figure 5b shows a picture of what was left after one of the crystals (a hexagon) was hit with a 120 kV electron beam. These results suggest that the structure of the crystals is similar to a membrane that encapsulates etched nanoparticles such as a viral capsule.14 We measured the reflectance from a white light source perpendicular to the substrate and noticed a strong reflection in the red (626 nm) and near-infrared (765 nm) regions (Figure 6). There seemed to be absorption in the 540 and 700 nm regions. These dips in the intensity might induce an upreconversion, which would result in the strong emission at 626 nm. Although photonic crystals are “machined” like opals that trap part of the wavelength, the crystals on hand were not designed in such a manner but had a nonperiodic structure in the 200−800 nm (visible) range. However, they had inherent properties that might be used in future self-assembled photonic band gap materials such as icosahedral quasicrystals.15 In our preliminary table-top experiment, no precursor was necessary except a glass substrate and several drops of etchant. Furthermore, we were able to observe various shapes and sizes of crystals. We are currently designing experiments to sort them by size by varying the ratio of the etchant in the solution and the speed of the spreading droplet. From our preliminary observations, it appears that controlling the velocity of the
Figure 4. X-ray diffraction spectrum versus the diffraction angle. The bottom peaks are a best fit to the (NH4)2SiF6 structure. All important peaks of the fundamental structure are present.
a specific structural signature at this intensity. We believe that the smaller crystals that are made of crystalline cryptohalite are packed in an amorphous fashion into a larger-scale structure in the 1−100 μm range, suggesting that the growth is not epitaxial. Using the same method as that of Ivanovska et al.9−12 for atomic force microscopy (AFM) indentation, we measured the force versus the indentation of the crystals and found that they behaved like clamped membranes under a point load (Figure 5a). The self-assembled crystals were hollow structures. From the linear behavior and up to a displacement of 0.07 μm, we extracted the Young’s modulus, and from the full fit, which included the nonlinear bending term, we deduced that the film thickness was on the order of 21−30 nm, depending on the size
Figure 5. Force measurements and a TEM image revealing the hollow structure of the crystals. (a) Force versus displacement using a calibrated AFM. The red curve is the best fit to the force due to the spring contribution of the crystal (linear) and the nonlinear buckling regime (cubic). The green line indicates the linear regime from which we retrieved the spring constant of the crystal. From the cubic contribution, we determined the thickness of the crystal shell. (b) A TEM image of the hexagon from the pattern shown in part a. The hexagon did not withstand the power of the electron beam and melted away. A membrane surrounds the aggregate. We believe that the particles are enclosed in this membrane. 4694
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(6) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Self-organization of CdSe nanocrystallites into three-dimensional quantum dot superlattices. Science 1995, 270, 1335. (7) Schlemper, E. O.; Hamilton, W. C.; Rush, J. J. Structure of cubic ammonium fluosilicate: neutron-diffraction and neutron-inelasticscattering studies. J. Chem. Phys. 1966, 44, 2499. (8) Lapham, D. M.; Barnes, J. H.; Downey, W. F., Jr.; Finkelman, R. B. Mineralogy associated with burning anthracite deposits of Eastern Pennsylvania: Pennsylvania Geological Survey. Min. Res. Rep. 1980, 79, 44. (9) Ivanovska, I. L.; et al. Bacteriophage capsids: Tough nanoshells with complex elastic Properties. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 7600. (10) Dinsmore, A. D.; et al. Colloidosomes: Selectively permeable capsules composed of colloidal particles. Science 2002, 298, 1006. (11) Sun, Y.; Xia, Y. Shape-controlled synthesis of gold and silver nanoparticles. Science 2002, 298, 2176−2179. (12) Dubois, M.; et al. Self-assembly of regular hollow icosahedra in salt-free catanionic Solutions. Nature 2001, 411, 672. (13) Biot, M. A. Mechanics of incremental deformations; John Wiley & Sons; 1965. (14) Coulibaly, F.; et al. The molecular organization of cypovirus polyhedra. Nature 2007, 446, 97. (15) Man., W.; Megens, M.; Steinhardt, P. J.; Chaikin, P. M. Experimental measurement of the photonic properties of icosahedral quasicrystals. Nature 2005, 496, 993.
Figure 6. Reflectance versus wavelength of the reflected light. The curves in blue and red are the background reflectance and incident light, respectively. The curve in green is the reflected light spectrum collected from the rim region. While there is strong activity at 626 nm, we notice absorption at 540 and 700 nm.
spread affects the thickness of these crystals but not their size distribution. Our preliminary experiment raises a variety of questions such as why these crystals form and what causes their forms to be selected given the flow boundary conditions in the spreading droplet.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS S.C. thanks R. Haasch for XPS assistance, M. Sardela for his comments on the X-ray spectra, B. Cunningham for lending us the reflectance measurement equipment, and S. Salapaka for the AFM measurements. These experimental characterizations were carried out in the Center for Microanalysis of Materials, University of Illinois, which is partially supported by the U.S. Department of Energy under Grant DEFG02-91-ER45439. S.B. is an undergraduate at the department of material science and engineering.
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REFERENCES
(1) Shih, S. Photoluminescence and formation mechanism of chemically etched silicon. Appl. Phys. Lett. 1992, 60, 1863. (2) We believe that the line shown in the figure is not due to a capillary effect during drying. The width of the line suggests otherwise as well as the absence of this line in other geometries. (3) Deegan, R. B.; et al. Capillary flow as the cause of ring stains from dried liquid drops. Nature 1997, 398, 827. (4) Bigioni, T.; et al. Kinetically driven self assembly of highly ordered nanoparticle monolayers. Nat. Mater. 2006, 5, 265. (5) Lin, X. M.; Jaeger, H. M.; Sorensen, C. M.; Klabunde, K. J. Formation of long range-ordered nanocrystal superlattices on silicon nitride substrates. J. Phys. Chem. B 2001, 105, 3353. 4695
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