DOI: 10.1021/cg100493t
Nonfaceted Growth of (111)-Oriented Epitaxial Alkali-Halide Crystals via an Ionic Liquid Flux in a Vacuum
2010, Vol. 10 3608–3611
Shun Kato, Yoko Takeyama, Shingo Maruyama, and Yuji Matsumoto* Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan Received April 14, 2010; Revised Manuscript Received June 3, 2010
ABSTRACT: Nonfaceted growth of (111)-oriented epitaxial KBr crystals on sapphire single crystal substrates has been achieved using a novel vacuum deposition method of flux-mediated epitaxy, involving the use of ionic liquids as a flux. It was found that the ionic liquid could act as a flux to promote the growth of KBr at higher growth temperatures and to stabilize its unstable polar (111) face with no faceting of the crystal surface. Even after evaporation of the remaining ionic liquid to remove it from the KBr crystals, the atomically flat (111) surface could be preserved without refaceting.
1. Introduction Growth of the (111) polar surface of a rock-salt crystal structure has been an elusive goal in the field of surface science and also relevant for material science as new functional properties would be expected for a flat and well-ordered (111) surface. For example, novel catalytic mechanisms might appear, and it would help in constructing two-dimensional electron systems on insulating crystals.1 However, since alternate stacks of cation and anion planes introduce a dipole moment along the Æ111æ direction, the potential energy of a flat (111) crystal surfaces diverges as the crystal thickness increases along the Æ111æ direction. There have been many attempts to achieve the growth of (111) surfaces of rock-salt-type thin films and nanocrystals, for example, for NiO,2-4 MgO,5-8 as well as some alkali halides.9-11 In these cases, either the nonequilibrium nature of a vapor deposition method or surfactant-like effects of surface adsorption, which is not limited to a solution process, have been utilized to achieve the growth of a (111) surface of a rock-salt crystal. With this background in mind, in the present work, we tried to apply our original thin film growth method of fluxmediated epitaxy to the growth of (111)-oriented rock-salt alkali halide films with an atomically flat surface. The fluxmediated epitaxy is a variation of the vapor-liquid-solid (VLS) growth technique12 where an additive flux material is used even though the deposition process takes place in a vacuum. It is useful for accurate control of various growth parameters of multicomponent films and nanostructures,13 as has been demonstrated by showing significant crystallinity improvement in complex oxide films of a high-Tc superconductor NdBa2Cu3O7-δ14 and ferroelectric Bi4Ti3O12,15 and promoting the self-organization of nanoscale oxide structures of Bi4Ti3O12.16 However, the growth control mechanism in flux-mediated epitaxy is quite different from what is typically argued for VLS growth; that is, the growth control mechanism is, as one would expect from the name, much the same as in the well-known flux process in bulk, dominated by thermodynamics rather than kinetics. *To whom correspondence should be addressed. E-mail: matsumoto@ oxide.msl.titech.ac.jp. pubs.acs.org/crystal
Published on Web 06/17/2010
Successful application of flux-mediated epitaxy to thin film growth depends on the ability to find an appropriate flux, as in the bulk process. For the present purpose, the flux material should have the following characteristics: (1) nonvolatile in a vacuum while in the liquid state, (2) polar solvent to dissolve an alkali halide compound and stabilize its (111) surface, and (3) be easily removable after growth. On the basis of these considerations, an ionic liquid (IL) might be a suitable flux material, that is, a molten salt with a melting temperature below 100 C. These materials have recently become very popular and are being intensely studied because of their potential use as a nonvolatile liquid solvent for organic synthesis,17 and applications in some electrochemical devices18 and vacuum processes.19 Most ILs have very low vapor pressures and can therefore be stable even in ultra high vacuum (UHV) and are expected to dissolve alkali halides due to their strong ionicity, which is comparable to ionic solids, and to stabilize the halide (111) polar surface. Furthermore, ILs can be easily removed after growth simply by heating above 150 C to evaporate the flux in UHV. In this paper, we report on the successful growth of nonfaceted (111)-oriented epitaxial KBr nanocrystals on R-Al2O3 (0001) single crystal substrates by flux-mediated epitaxy with ILs in a vacuum and discuss the growth mechanism. 2. Experimental Section A continuous infrared (CW-IR) laser deposition method was employed for thermal evaporation of IL and KBr.20 An 808 nm CW-IR laser beam was introduced into the vacuum chamber (base pressure below 5 10-8 Torr) through a quartz window, irradiating either an IL or KBr target. Since both materials are transparent in this IR region, Si powder was added to the IL and KBr containers for efficient absorption of the IR laser light. Details of the CW-IR laser deposition of ILs and alkali halide materials will be described elsewhere.21 The ionic liquid used in this experiment was 1-butyl-3methylimidazolium hexafluorophosphate ([Bmim][PF6]) from Wako Pure Chemical Industry, Ltd. (purity: 99.9%). It was selected because [Bmim][PF6] can be easily evaporated and is stable in a vacuum in our growth temperature range. Several ILs such as 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([Bmim][TFSA]) and 1-ethyl-3- methylimidazolium ethylsulfate ([Emim][EtSO4]) were tested, but rejected. For example, [Bmim][TFSA] was found to be unstable, gradually evaporating in a vacuum at a high temperature, while [Emim][EtSO4] was likely to decompose during the IR laser deposition process, as confirmed by NMR experiments.21 r 2010 American Chemical Society
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Figure 1. Out-of-plane θ-2θ XRD patterns of KBr films grown at various temperatures between RT and 110 C (a) without a flux and (b) with an ionic liquid flux. Table 1. Growth Conditions for ILs and KBr [Bmim][PF6] KBr
growth temp (C)
pulse width (ms)
repetition rate (Hz)
deposition rate (nm/min)
thickness (nm)
RT RT, 50, 80, 110
16-17 20
4 2
0.6 0.5
50 35
The deposition conditions for IL and KBr are summarized in Table 1. In this study, lattice mismatch would play a critical role in growing (111)-oriented KBr films on R-Al2O3 (0001). Since the KBr(111) surface has a hexagonal lattice consisting of K or Br atoms with a unit cell parameter of 0.66005/21/2 = 0.4668 nm (JCPDS 36-1471), the lattice mismatch with R-Al2O3 (0001) is as low as -2.4%, considering a unit cell parameter of 0.4785 nm (JCPDS 46-1212). In some experiments, in order to directly examine the effect of IL on the growth of KBr, an IL layer was deposited at room temperature (RT) on only one-half of a substrate by shielding the rest of the substrate with a physical mask and leaving it uncoated for comparison. KBr was then deposited on the whole substrate surface at various growth temperatures between RT and 110 C. The two halves of a substrate thus provided two different samples of KBr films for each growth temperature, one grown with an IL flux and the other without. The crystallinity and crystal orientation of the KBr samples were examined by conventional powder X-ray diffraction (XRD). The crystal morphology was observed with a tapping-mode atomic force microscope (AFM). The evaporation process of IL from KBr crystal samples was observed in a vacuum with a UHV-laser microscope.22
3. Results and Discussion Figure 1 shows the out-of-plane θ-2θ XRD patterns of KBr films grown at various temperatures between RT and 110 C. When KBr was directly deposited on R-Al2O3 (0001) without predeposition of IL, only a pattern consistent with the (100) plane of KBr was present, and none of the diffraction peaks corresponding to (111) planes were found (Figure 1a). In contrast, when KBr was deposited on IL-covered R-Al2O3 (0001) substrates, both (100) and (111) planes were always observed (Figure 1b). The XRD peak intensities for the (100) and (111) planes are plotted as a function of the growth temperature in Figure 2. For deposition at RT, IL seems to impede the growth of KBr. A strong crystalline peak of the (100) plane was observed only when KBr was directly deposited on sapphire without the IL flux. In contrast, at growth temperatures above RT, strong diffraction peaks corresponding to both (100) and (111)
Figure 2. XRD peak intensities for the (100) and (111) planes plotted as a function of the growth temperature.
planes appeared when KBr was deposited on IL-covered substrates, reversing the trend of dropping peak intensities with increasing growth temperature for direct growth on sapphire. The (111) orientation became dominant at higher growth temperatures. A similar enhancement of (111) intensity was seen for thicker IL layers (not shown). A set of tapping-mode AFM topography images is shown in Figure 3 for sample surfaces obtained when KBr was deposited without the IL flux at different temperatures. Nucleation is observed homogeneously all over the surface, and this tendency is not significantly affected by changes in growth temperature. This result indicates that the diffusion of KBr precursors is very limited on R-Al2O3(0001) in the present growth temperature range, resulting in the growth of polycrystalline KBr. According to published powder XRD data for polycrystalline KBr (JCPDS 36-1471), the (200) reflection has the strongest intensity, while the (111) nearly vanishes. It is due to the polycrystalline nature of the KBr films grown without ILs on R-Al2O3(0001) that we observed only the (200) reflection, as shown in Figure 1a.
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Figure 3. Tapping-mode AFM topographs of KBr films deposited without flux at different temperatures: (a) RT, (b) 50 C, (c) 80 C, and (d) 110 C, respectively. AFM image sizes are 10 10 μm.
Figure 5. (a, b) UHV-laser microscope images of KBr islands before and after heating in UHV at 300 C to remove the IL flux. (c, d) Tapping-mode AFM topographs of (111)-oriented KBr islands grown with and without the IL flux, together with the corresponding cross-sectional profiles.
Figure 4. Surface morphology of KBr films deposited on ILcovered substrates for different growth temperatures: (a-d) tappingmode AFM topographs at RT, 50 C, 80 C, 110 C, respectively, and (e-h) the corresponding phase images of (a-d). (i) A schematic illustration of KBr growth in IL droplets. AFM image sizes are 10 10 μm.
On the other hand, the surface morphology of KBr films deposited on IL-covered substrates is strongly dependent on the growth temperature, as shown in Figure 4a-d. It is important to note that all the KBr crystals are found in the IL droplets, even though there is some free space between the IL droplets at higher growth temperatures. In order to more clearly see the morphology of KBr crystals in the IL droplets, the simultaneous phase images are also shown in Figure 4e-h. The phase contrast in the tapping-mode AFM often reflects differences in the properties of individual components of a heterogeneous materials system: in the present case the dark contrast corresponds to solid substances, KBr and Al2O3, while the bright contrast corresponds to the softer IL droplets. With increasing growth temperature each individual crystal became larger while the nucleation density decreased, finally
exhibiting the expected hexagonal or triangular shapes seen on most (111)-oriented crystal surface, as is schematically illustrated in Figure 4i. All of these hexagonal or triangular KBr crystals have the same in-plane orientation relative to the substrate, indicating that their growth on the R-Al2O3 (0001) substrate is epitaxial. These results suggest that the solubility of KBr in the IL increases at higher growth temperatures and thereby the apparent diffusion rate of KBr precursors is enhanced through repeated cycles of dissolution and crystallization in the IL flux. As a consequence, the crystal growth of KBr was promoted by the IL above RT. Furthermore, a reversible process like this should allow the system to find an equilibrium state if the growth rate is slow enough. In this case, for a rather low deposition rate of 0.5 nm/min, we did indeed find that the growth of (111)-oriented KBr crystals dominated the process, owing to the more favorable lattice matching with the substrate than would be possible for (100) crystallites on an R-Al2O3 (0001) substrate. We emphasize the critical role that that lattice matching plays in growing a (111)-oriented KBr film from the viewpoint of thermodynamics. In fact, when the deposition rate was increased to 2 nm/min., so that the kinetic factor became dominant, the (100) orientation reappeared. Furthermore, a similar enhancement of KBr growth in the presence of an IL flux above RT was observed when an R-Al2O3 (1102) substrate was used instead, but the (100) and (111) orientations still coexisted even at high temperatures (not shown). This is understandable, because for this substrate orientation, both crystal planes have a large lattice mismatch and there is thermodynamically little difference in the stability between the two orientations. The growth experiments proved that an IL can act as a flux to promote the crystal growth of KBr in a vacuum deposition system. We now discuss the possibility of stabilizing the (111) polar surface. In order to
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examine the shape of KBr crystals, we tried to remove the IL flux after crystal growth by heating a sample in a vacuum above 300 C. Two laser microscope images, taken before and after heating in a vacuum, are shown in Figure 5a,b. When the IL flux was completely removed from the sample surface, there was almost no change in the density of KBr crystals or their shapes, with individual crystallites becoming clearly visible, even by the laser microscope. A three-dimensional image of a hexagonal KBr crystal after the removal of the IL flux is shown in Figure 5c, together with its cross-section profile. The top of the KBr crystal surface is very flat, even after heating the sample above 300 C, which would normally induce a refacetting of a (111) surface if the IL flux is not used. Experiments also showed that while it is occasionally possible to fabricate (111)-oriented KBr crystals without an IL flux at 110 C at a much higher deposition rate of 2.1 nm/min, such growth is not reproducible. A three-dimensional image of a triangular KBr crystal grown without an IL flux is shown in Figure 5d, together with its cross-sectional profile for comparison. The top of the crystal (111) surface is far from being flat, faceting into sets of (100) faces. This result is clear evidence of the beneficial effect of the IL flux in stabilizing the KBr (111) surface. Unfortunately, although flat KBr (111) microcrystals were obtained, they did not coalesce into a single continuous film. As can be seen in Figure 4, the IL droplets are not likely to spread over the whole surface and the crystal growth is confined within individual IL droplets. As a consequence, the KBr crystals continue to grow along the out-ofplane direction within an IL droplet, but cannot expand along the lateral directions. In order to obtain a single continuous film, wettability control of an IL flux would be important, and is a remaining challenge for our future work. 4. Conclusions We have presented a new method for nonfaceted growth of (111)-oriented epitaxial KBr crystals on sapphire single crystal substrates by flux-mediated epitaxy. The method involves the use of an ionic liquid as a flux. Above RT, the ionic liquid can promote the growth of KBr crystals and stabilize the polar (111) face. Consequently, we are able to obtain relatively large (111)-oriented KBr single crystals with an atomically flat (111) surface, epitaxially grown on sapphire single crystal substrates.
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In further studies, we expected to be able to apply this method to other epitaxial rock-salt films and nanocrystals with (111) polar surfaces. Acknowledgment. This work was supported by a Grant-inAid for Scientific Research from Ministry of Education, Culture, Sports, Science and Technology of Japan (No. 21656010).
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