Article pubs.acs.org/Langmuir
Spatial Control of Crystallographic Direction in 2D Microarrays of Anisotropic Nanoblocks on Trenched Substrates Riho Matsumoto,† Yoshitaka Nakagawa,† Kazumi Kato,‡ Yuya Oaki,† and Hiroaki Imai*,† †
Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan ‡ National Institute of Advanced Industrial Science and Technology (AIST), Central 1, 1-1-1 Umezono, Tsukuba 305-8560, Japan S Supporting Information *
ABSTRACT: Elaborate two-dimensional (2D) microarrays of tetragonal Mn3 O 4 nanocuboids 10−20 nm in size were constructed with parallel trenches 500 nm wide and 500 nm deep on a silicon substrate. By adjusting the conditions, including the dispersion medium, particle concentration, and evaporation rate, the a-face and c-face 2D arrays were selectively deposited on the upper and lower stages of the trenches, respectively. The crystallographic direction of the tetragonal crystal was alternately switched in the 2D microarrays under these specific conditions at the optimal particle concentration and evaporation rate. Spatial switching of their crystallographic direction was achieved via interaction of the anisotropic nanoblocks and the specifically shaped surfaces.
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INTRODUCTION A number of biominerals possess unique microstructures expressing excellent features, such as high mechanical strength1−6 and specific optical properties.7,8 Especially, the nacres and cross-lamellae comprised of aragonite have received attention as excellent models for lightweight and tough structural materials because of their superior mechanical properties.9,10 In the biogenic microstructures, the phase, size, and direction of the crystals are spatially controlled by organisms. The 90° rotation of the crystallographic direction of the biogenic crystal is frequently observed in the crosslamellae of mollusk shells.9,10 However, the spatial switching of the crystallographic direction has been difficult in nanometerscale artificial systems. Highly ordered architectures of uniform nanometric building blocks are built through a bottom-up approach, such as selfassembly in liquid media.11−15 The crystallographic direction is not controlled in the ordered arrays of spherical building blocks. On the other hand, micrometer- to millimeter-scale arrays of rectangular nanoblocks having the same crystallographic direction are arranged by facing well-defined facets.16−22 The crystallographically orientated assembly of metal nanoblocks and metal oxide nanoblocks is essential for the fabrication of various ordered architectures. The construction of well-organized architectures through the selfassembly of nanoblocks is useful for various technological applications of functional nanometric materials. A wide variety of assemblies consisting of metallic nanoblocks were produced at the air−liquid interface by changing the conditions.23 Diversely shaped one-, two-, and three-dimensional (1D, 2D, © 2017 American Chemical Society
and 3D) arrays were fabricated via the assembly of anisotropic rectangular metal oxide nanoblocks by controlling their crystallographic orientation.24−27 The crystallographic direction of the building blocks in the microarrays has been controlled in previous studies. The next challenge is how to spatially switch the crystallographic direction of nanometric building blocks in 2D and 3D arrays. In previous studies, nanoblocks were arranged in the same crystallographic direction on a substrate. However, spatial control of the direction has not been achieved in the ordered nanoblock arrays. In the present work, direction-switched 2D microarrays of tetragonal nanoblocks are achieved on a trenched silicon substrate 500 nm wide and 500 nm deep (Figure 1a). The regularity of ordered arrays of metal oxide nanocubes was controlled by the interaction of trench walls on a substrate.27 Here, we used tetragonal Mn3O4 nanoblocks as useful anisotropic building blocks. The a-face and c-face 2D arrays were selectively deposited on the upper and lower stages of the parallel trenches (Figure 1b,c) by changing the conditions. Finally, the crystallographic direction of the nanoblocks was alternately switched in 2D microarrays under a suitable condition (Figure 1d). Here, we observed periodical 90° rotation of the direction of the tetragonal crystal. Consequently, the micrometer-scale spatial switching of the crystallographic direction in 2D microarrays of nanoblocks has been achieved in the current study. The control of the Received: September 16, 2017 Revised: November 7, 2017 Published: November 20, 2017 13805
DOI: 10.1021/acs.langmuir.7b03264 Langmuir 2017, 33, 13805−13810
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RESULTS AND DISCUSSION
Formation of a-Face and c-Face 2D Arrays. We prepared tetragonal Mn3O4 (a = 0.576 nm and c = 0.944 nm) nanocuboids through a two-phase solvothermal method reported in our previous work.24,27 The Mn3O4 nanocrystals ca. 10 nm wide and ca. 20 nm long exhibited truncated cuboids with four {100} faces and two (001) faces (Figure S1). The nanoblocks were well-dispersed in a low polar dispersion medium because their surfaces were covered with oleic acid molecules. Two types of 2D arrays were produced on a flat substrate through a convective self-assembly method with the evaporation of a dispersion medium (Figure S2).24 The anisotropic Mn3O4 nanoblocks (Figure S2a) were aligned along the a-axis at the air−liquid interface by lateral capillary forces with the evaporation of a low polar dispersion medium (Figure S2b), such as a mixture of toluene and hexane (1:1 in volume). 2D arrays in which the a-faces are parallel to the substrate [a-face 2D arrays (Figure S2c)] were formed through parallel assembly of the linear chains of cuboids (1D arrays) with a decrease in the interface area by evaporation. The c-axis in a-face 2D arrays tends to be parallel to the liquid−solid−air phase boundary, as shown in Figure S2b. Another type of 2D array in a square grid pattern was produced through the same convective selfassembly method from a dispersion of pure hexane. Because the evaporation rate of the medium is higher than that of the mixture of toluene and hexane, 2D clusters of the nanoblocks (Figure S2d) are easily formed through a rapid increase in the particle concentration in the dispersion near the evaporation tip. We thus obtained 2D arrays in which their c-faces are parallel to the substrate [c-face 2D arrays (Figure S2e)] through assembly of the 2D cluster on a substrate after evaporation. Formation of a-Face 2D Arrays on the Upper Stage of Parallel Trenches. When we used parallel trenches 500 nm wide and 500 nm deep (Figure 1a) instead of a flat surface as a substrate, the a-face 2D arrays were selectively deposited on the upper stage of the trenches (Figures 1b and 2). The 2D arrays were prepared on the parallel trenches using a hexane−toluene mixture (1:1 in volume) containing 2.8 × 10−1 g/dm3 nanocuboids via the convective self-assembly method. After evaporation of the dispersion medium at a rate of 3.0 × 10−8 m3/h, we observed ordered arrays on the upper stage (Figure 2a,c) and random arrays on the lower stage (Figure 2a,f). The fast Fourier transform (FFT) pattern of the SEM image (Figure 2d) indicates that the long axes of the nanocuboids are parallel to the surface (Figure 2e). This means that the c-axis and the aface of the tetragonal nanoblocks were parallel to the substrate in the 2D arrays [a-face 2D arrays (Figure S2c)] on the upper stage. On the other hand, the arrangement on the lower stage was confirmed to be random from the FFT pattern (Figure 2g,h). At the air−liquid interface, the a-face 2D arrays are basically formed through lateral stacking of the 1D chains of nanocuboids by lateral capillary forces as explained in the Supporting Information (Figure S2b,c).24−27 We obtain the aface 2D arrays on a flat substrate via deposition of the arrangement produced at the interface after evaporation of the dispersion medium at an optimal evaporation rate of 3.0 × 10−8 m3/h. However, the arrangements formed at the interface are deformed by the ridges of the parallel trenches when the deposition of the arrays occurs on a trenched surface with the evaporation of the medium, as shown in Figure 2i,j. Although
Figure 1. Schematic illustrations of parallel trenches 500 nm wide and 500 nm deep (a) and spatially controlled 2D arrays of anisotropic nanoblocks (b−d). The a-face 2D array deposited on the upper stages (b), the c-face 2D array deposited on the lower stages (c), and the aface and c-face 2D arrays deposited on the lower and upper stages, respectively (d).
arrangements originates from the shapes of the nanoblocks and the surface morphology. On the other hand, the ordered structures are independent of the chemical components of nanoblocks. Thus, this technique would be applicable for highly sophisticated microarchitectures like biominerals.
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Article
EXPERIMENTAL SECTION
Truncated Mn3O4 nanoblocks as a building block were prepared through a two-phase solvothermal method as described in our previous work.24,27 We dissolved 0.60 mmol manganese(II) chloride and 35 wt % hydrogen peroxide (4.0 cm3) in 31 cm3 of water in a 100 cm3 Teflon container. Oleic acid (3.97 mmol) and tert-butylamine (2.31 mmol) were added to 30 cm3 of toluene. The organic mixture was added to the Teflon container without stirring. At this time, oxygen gas was generated through the decomposition of hydrogen peroxide. When the generation of oxygen gas stopped, the Teflon container was put into a stainless steel autoclave. The autoclave was heated at 115 °C for 12 h. After the reaction, the resultant dark brown liquid (upper phase) was transferred into a glass vial. The resultant dispersion of Mn3O4 nanoblocks was centrifuged at 13 500 rpm for 5 min. The precipitates of 0.7, 2.8, and 5.6 × 10−1 g/ dm3 were redispersed into pure hexane and a hexane−toluene mixture (1:1 in volume) in a 6 cm3 vial by ultrasonication for 30 min. A silicon substrate (7 mm × 16 mm) with and without trenches treated by acetone with ultrasonication for 30 min was put into a 0.5 cm3 dispersion in the vial. Typically, we used parallel trenches 500 nm wide and 500 nm deep on a silicon substrate. The trenches were fabricated on the substrate as shown in Figure 1a. The dispersion was spread on the substrate by its surface tension. We produced the ordered arrays by evaporating liquid media in the vial with a cover. The evaporation rate was controlled by changing the temperature (7 °C and room temperature) and aperture size of the vial. We calculated the evaporation rate from the evaporated volume of the liquid media during a certain period. After removal of organic compounds covering the nanoblocks by calcination in air at 400 °C for 4 h, the morphologies of products were observed via scanning electron microscopy (SEM, FEI Siron operated at 10 kV). We obtained the same structures by the experiments under the same conditions several times. However, the ordered arrangements were not always spread all over the substrate. 13806
DOI: 10.1021/acs.langmuir.7b03264 Langmuir 2017, 33, 13805−13810
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Figure 3. SEM images (a,d,e) and schematic illustrations (b,c,g) of 2D arrays on parallel trenches from a dispersion of pure hexane containing 0.7 × 10−1 g/dm3 nanocuboids. Enlarged images (e) and the FFT pattern (f) on the lower stages. The arrangements of c-face 2D arrays formed in the dispersion in the parallel trenches (g).
faces of the nanoblocks were parallel to the surface and aligned in the same orientation [c-face 2D arrays (Figure S2e)]. On the other hand, the nanocuboids were slightly observed on the upper stage of the trenches. The c-face 2D arrays are basically produced from 2D clusters as explained in the Supporting Information (Figure S2d,e). Thus, we obtain c-face 2D arrays on a substrate via deposition of the clusters during evaporation of the dispersion medium on the flat surface. However, the stacking of the clusters is influenced by the parallel trenches. The 2D arrays are deposited only on the lower stage because the dispersion liquid is confined to the trenches in the progressive stage of evaporation as shown in Figure 3g. Here, we succeeded in spatial control of the c-face 2D arrays using parallel trenches on a silicon substrate. The c-face 2D arrays were formed only on the lower stage of the parallel trenches 200−500 nm wide and 500−1000 nm deep. However, the ordered nanocuboids were also deposited even on the upper stage when the ridges were wider than 1000 nm. The confinement effect for the dispersion liquid would not be remarkably induced by trenches shallower and narrower than 100 nm. Thus, the size and morphology of trenches and ridges are important for the spatial control of the c-face 2D arrays. Spatial Control of a-Face and c-Face 2D Arrays on Upper and Lower Stages of Parallel Trenches. As shown in the Supporting Information (Figure S3), we achieved an alternating formation of a-face and c-face 2D arrays on a flat surface from the toluene−hexane mixture by increasing the particle concentration to 5.6 × 10−1 g/dm3 (×2) and decreasing the evaporation rate to 5 × 10−9 m3/h (×1/6). Figure S3c shows that the a-face 2D array was observed on the c-face 2D array on the substrate. When the concentrated dispersion is slowly evaporated, 2D clusters are formed in bulk with the formation of 1D chains at the air−liquid interface during evaporation (Figure S3a). In this case, the c-face 2D
Figure 2. SEM images (a,c,f,j) and schematic illustrations (b,e,h,i) of 2D arrays on parallel trenches using a hexane−toluene mixture (1:1 in volume) containing 2.8 × 10−1 g/dm3 nanocuboids via the convective self-assembly method after evaporation of the dispersion medium at a rate of 3.0 × 10−8 m3/h. Enlarged images (c,f) and their FFT patterns (d,g) on the upper (c−e) and lower (f−h) stages. The arrangements formed at the interface on the ridges of the parallel trenches (i,j).
the 2D arrays are neatly deposited on the upper stage of the trenches, the nanocuboids deposited in narrow trenches are disordered. In consequence, the ordered 2D arrays are formed only on the upper stage of the parallel trenches. Here, we succeeded in spatial control of the a-face 2D arrays using parallel trenches on a silicon substrate. The width and depth of the trenches are important for control of the arrangements. The spatial control was clearly achieved on the parallel trenches 200−500 nm wide and 500−1000 nm deep. However, deformation of the nanocuboid arrangements on the lower stage was not obviously induced by the trenches wider than 1000 nm. Moreover, the trenches narrower or shallower than 100 nm are inferred to be insufficient for the deformation of ordered arrays because bridges of the a-face 2D arrays are easily formed at the edge of the trenches as shown in Figure 2j. Formation of c-Face 2D Arrays on the Lower Stage of Parallel Trenches. As mentioned above, another type of 2D array in a square grid pattern was produced through the same convective self-assembly method from a dispersion of pure hexane containing 0.7 × 10−1 g/dm3 nanocuboids. When we performed the assembly of the nanocuboids at a rate of 1.2 × 10−7 m3/h, the c-face 2D arrays were selectively deposited on the lower stage of parallel trenches (Figure 3). The FFT pattern of the 2D arrays indicates the presence of the square grid pattern of the nanoblocks. This means that the a-axes and c13807
DOI: 10.1021/acs.langmuir.7b03264 Langmuir 2017, 33, 13805−13810
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c-face 2D arrays are deposited only on the lower stage because the dispersion liquid is confined to the trenches as shown in Figure 4. In consequence, we achieved spatial control of the crystallographic direction in 2D microarrays using the trenched surface. The spatially selective deposition of a-face and c-face 2D arrays was clearly achieved with optimal conditions. As shown in Figures 5 and S4, we occasionally observed spatial control of
array is initially deposited on the substrate and the a-face 2D array is then formed on the initial array. Consequently, the specific a-face and c-face bilayer is obtained under this specific condition (Figure S3b). Under the same condition as mentioned above, we prepared the specific a-face and c-face bilayer on the trenched substrate using a hexane−toluene mixture (1:1 in volume). As shown in Figure 4, the a-face and c-face 2D arrays were selectively
Figure 4. Schematic illustration (a,b) and SEM images (c,d,f) of 2D arrays on parallel trenches using a hexane−toluene mixture (1:1 in volume) containing 5.6 × 10−1 g/dm3 nanocuboids at an evaporation rate of 5 × 10−9 m3/h. Enlarged images (d,f) and their FFT patterns (e,g) on the upper (d,e) and lower (f,g) stages. Spatial control of aface and c-face 2D arrays on the upper and lower stages of parallel trenches (a,b).
deposited on the upper and lower stages of the trenches, respectively. After evaporation of the dispersion medium, we observed the a-face arrays on the upper stage (Figure 4c,d) and c-face 2D arrays on the lower stage (Figure 4f). The FFT pattern (Figure 4e) indicates that the long axis of the nanocuboids is aligned in the same orientation. Thus, the caxes of the nanoblocks are parallel to the surface in the 2D arrays on the upper stage. The a-axes of the nanoblocks on the lower stage are parallel to the surface and aligned in the same orientation because the FFT pattern (Figure 4g) of the SEM image (Figure 4f) shows the presence of the square grid pattern. Here, we successfully switched the crystallographic direction spatially on the trenched substrate. Figure 4a,b illustrates the spatial control of a-face and c-face 2D arrays on the upper and lower stages of parallel trenches. The 2D arrangements formed at the air−liquid interface are deformed with the ridge when the deposition is performed on a trenched surface. The a-face 2D arrays are deposited on the upper stage of the trenches. When the nanoblocks are partially concentrated with the progress of evaporation, we also obtain the c-face 2D arrays on a substrate via deposition of the 2D clusters during slow evaporation of the dispersion medium. The
Figure 5. SEM images (a−f) of 2D arrays on parallel trenches from a hexane−toluene mixture (1:1 in volume) containing 2.8 × 10−1 g/dm3 nanocuboids at an evaporation rate of 3.0 × 10−8 m3/h. Enlarged images (e,f) and their FFT patterns (g,h) on the upper (e,g) and lower (f,g) stages, respectively. We observed the a-face arrays on the upper stage (e) and c-face 2D arrays on the lower stage (f). 13808
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Langmuir a-face and c-face 2D arrays on a trenched surface even from a hexane−toluene mixture (1:1 in volume) containing 2.8 × 10−1 g/dm3 nanocuboids at an evaporation rate of 3.0 × 10−8 m3/h. After evaporation of the dispersion medium, the a-face and cface 2D arrays were selectively deposited on the upper and lower stages of the trenches, respectively (Figure 5e,f). The specific structures would be produced in the progressive stage of the deposition by decreasing the dispersion medium. This means that the deposition occurred upon increasing the nanocuboid concentration. Moreover, the evaporation rate would decrease upon increasing the vapor concentration in the container. Here, we also achieved spatial switching of the crystallographic direction on the trenched substrate using a concentrated dispersion. The micrometer-scale spatial switching of the crystallographic direction in 2D microarrays of nanoblocks has been achieved in the present study. The spatial control originates from the shapes of the nanoblocks and the surface morphology. On the other hand, the chemical components of the nanoblocks are not essential for the control because their surfaces are covered with oleic acid. Because the wettability of the liquid medium, such as toluene and hexane, is also essential for the fabrication of the ordered arrays through spreading of the dispersion, the silicon surface is not necessary for the spatial control. Thus, the controlled assembly using a trenched surface is applicable for the fabrication of ordered arrays of other anisotropic nanoblocks on various sorts of substrate materials. As mentioned in the previous sections, the size and morphology of trenches are important for the spatial control. Parallel trenches 200−500 nm wide and 500−1000 nm deep are deduced to be suitable for control of the arrangements of nanocuboids 10−20 nm in size. On the other hand, the trenches wider than 1000 nm or narrower than 100 nm would not be effective for the control of the ordered arrays.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Yuya Oaki: 0000-0001-7387-9237 Hiroaki Imai: 0000-0001-6332-9514 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partially supported by Grant-in-Aid for Challenging Exploratory Research (15K14129), and Grant-inAid for Scientific Research (A) (16H02398) from the Japan Society for the Promotion of Science.
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REFERENCES
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CONCLUSIONS We performed spatial control of the crystallographic direction in 2D microarrays consisting of tetragonal crystal blocks. Elaborate 2D microarrays of tetragonal Mn3O4 nanocuboids 10−20 nm in size were constructed with parallel trenches 500 nm wide and 500 nm deep on a silicon substrate. The a-face and c-face 2D arrays were selectively deposited on the upper and lower stages of the trenches, respectively. An alternating formation of the a-face and c-face 2D arrays of the nanoblocks was achieved by adjusting the conditions, especially at the optimal particle concentration and evaporation rate. Because the spatial switching originates from the shapes of the nanoblocks and the surface shapes, controlled assembly using a trenched surface is a potential technique for the fabrication of highly sophisticated microarchitectures.
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selective deposition of a-face on the upper stage and cface 2D arrays on the lower stage of a trenched surface (Figure S4) (PDF)
ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b03264. Schematic illustration and transmission electron microscopy images of Mn3O4 nanocuboids (Figure S1), explanation of formation routes from nanocuboids to a-face and c-face 2D arrays (Figure S2), explanation of alternate formation of a-face and c-face 2D arrays on a flat surface (Figure S3), another example of spatially 13809
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