Article pubs.acs.org/Langmuir
Fabrication of Freestanding Nanoparticle Membranes over Wells Changrong Guan,† Li Zhang,† Shuhai Liu,‡ Ying Wang,† Wenhong Huang,† Chaoying Zhang,‡ and Jianhui Liao*,† †
Key Laboratory for the Physics and Chemistry of Nanodevices, Department of Electronics, Peking University, Beijing 100871, China Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Science, Beijing 100083, China
‡
S Supporting Information *
ABSTRACT: Freestanding nanoparticle membranes over circular wells are prepared by utilizing surface engineering. The crucial step of this method is the hydrophobic treatment of the substrate surface, which causes the water droplet to be suspended over wells during drying. Consequently, the nanoparticle monolayer self-assembled at the surface of the water droplet would drape itself over wells instead of being dragged into wells and ruptured into patches after the evaporation of water. This scenario was confirmed by the results of control experiments with changes in the hydrophobicity of the surface and the depth of wells. Moreover, the NaCl crystallization experiment provides additional evidence for the dynamic process of drying. Freestanding nanoparticle membranes with different nanoparticle core sizes and different lengths of ligands have been successfully prepared using the same route. The Young’s modulus of one typical kind of prepared freestanding nanoparticle membrane was measured with force microscopy.
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INTRODUCTION Nanoparticles as building blocks can be arranged into 1D nanoparticle chains,1 2D nanoparticle membranes,2,3 and 3D aggregations.4 These materials have exhibited unique collective optical,5 electronic,6 and other properties that are different from not only the individual particle unit but also the corresponding bulk state. Compared to usual substrate-supported nanoparticle assembles, freestanding nanoparticle arrays provide opportunities to investigate their intrinsic properties without the influence of the substrate.7−9,20,21,23−27 For instance, the optical properties of 1D freestanding nanoparticle chains and the mechanical properties of 2D freestanding nanoparticle membranes have been both studied by taking advantage of the freestanding configuration. The latter structure is becoming a hot research object for its versatile characteristics in optics,10,11 mechanics,12−15 and filtration.16 So far, several methods have been proposed and demonstrated to fabricate freestanding nanoparticle membranes. Generally, a sacrificial layer is required in preparing multilayered freestanding membranes using the layer-by-layer strategy.17,18 As the limiting case, monolayered nanoparticle membranes19 are important and interesting for both basic studies and special applications. One way to prepare such structures is by draping the nanoparticle monolayer, which selfassembles at the water surface,22 over substrates with holes open at both ends. This strategy has been demonstrated on a few special substrates, including copper grids and thin Si3N4 films with through-holes. The requirement of holes open at © 2015 American Chemical Society
both ends not only limits the fabrication and measurement of freestanding nanoparticle membranes (FNMs) but also confines possible applications of FNMs. Herein, we report a method to fabricate FNMs over circular wells in conventional substrates, i.e., silicon. By surface engineering, the substrate surface was modified to be highly hydrophobic. The water droplet placed on the substrate for nanoparticle self-assembly would stay in a Cassie−Baxter state, as predicted by the Laplace model. The water will not contact the bottom of the well until the water evaporates completely. The nanoparticle monolayer could then drape itself over wells to achieve a suspension instead of being dragged into wells. This strategy was also used to prepare large-area FNMs with various nanoparticle core sizes and different ligand molecules. Moreover, the mechanical characterization was demonstrated on a typical kind of FNM by an AFM indentation experiment.
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EXPERIMENTAL SECTION
Materials. A silicon wafer with a 500 nm silicon dioxide layer was used as the substrate. The used thiols (octanethiol, 98.5%; dodecanethiol, 98%; and hexadecanethiol, 95%) were all purchased from Aldrich. The silane molecule (1H,1H,2H,2H-perfluorodecyltriethoxysilane, 97%, hereafter referred to as PFDTES) was purchased from J&K. Ethanol (MOS), NaCl (AR), H2SO4 (98%, MOS), and H2O2 (30%, AR) were all used as purchased. Received: December 16, 2014 Revised: February 12, 2015 Published: March 5, 2015 3738
DOI: 10.1021/la504881n Langmuir 2015, 31, 3738−3744
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Langmuir Fabrication of Circular Wells and Surface Modification. Photolithography (SÜ SS MicroTec, MJB4) and inductively coupled plasma (ICP) etching (TRION) were used to fabricate circular wells on silicon substrates (Supporting Information Figure S2). The openings of circular wells were 1.8 or 2.5 μm in diameter. The depth of the wells can be controlled by the ICP process. For the preparation of FNMs, circular wells with the depth of 500 nm were used. Shallower (300 nm) or deeper (1.5 μm) wells were also used in our study for control experiments. To obtain a surface rich in hydroxy, the substrates were first cleaned in piranha solution (H2SO4 (98%)/ H2O2(30%) = 3:1, hazard, be careful) for 1 h followed by rinsing with deionized water. Subsequently, the substrates were immersed in an ethanol solution of PFDTES (13.5 mMol/L) for 3 h. The substrates were then rinsed with ethanol and dried in a flow of nitrogen. Finally, the silane-treated substrates were held at 140 °C for 1 h in an oven.30 Preparation of FNMs over Circular Wells. Alkanethiol-capped gold nanoparticles were prepared and dispersed in chloroform according to the method presented in the literature.31,32 The mean diameters of the gold nanoparticles used in this work were 8.5, 10.4, and 14.8 nm. A homemade configuration was used for the preparation of FNMs (Supporting Information Figure S3). The configuration consists of two parts. The upper part was made of Teflon (25 mm × 25 mm × 4 mm) containing a cone cave in the center. The diameters of the two terminals of the core were about 2 and 10 mm. The lower part was made of aluminum (25 mm × 25 mm × 5 mm). The silanetreated substrate was sandwiched between these two parts. Then, some ultrapure water (125 μL, 18 MΩ.cm) was infused in the Teflon cave. Freshly prepared gold particle solution was spread on the convex water surface. The nanoparticles would self-assemble into an ordered monolayer at the air/water interface. After the evaporation of water, the nanoparticle monolayer would drape itself over the circular wells and form freestanding nanoparticle arrays. For comparison, the processes were also conducted on substrates without silane treatment. Crystallization Experiment of NaCl on Structured Substrates. We used a 0.5 mol L−1 NaCl solution to characterize the dewetting process on hydrophobic and hydrophilic surfaces. The structured substrate was sandwiched between the homemade configuration mentioned above. The NaCl solution (∼125 μL) was infused into the Teflon cave. After the evaporation of water, NaCl crystals would be generated in the region where salt solution accumulated. Characterization of Samples and Experiment Results. The static contact angle (CA) of substrates was characterized with a contact angle meter (Dataphysics OCA20) at room temperature. The prepared FNMs were characterized with an optical microscope (HCSci, XHC-SV1) and SEM (FEI Quanta 600), which was also used to characterize the structure of wells and the results of the crystallization experiment. AFM (Nanoscope IIIa) was also used to characterize the circular wells and FNMs in tapping mode at room temperature. Force−Indentation Measurements. The force−indentation experiment was conducted with the AFM (Asylum MFP−3D) on FNMs above wells. The wells with a diameter of 1.8 μm were selected to conduct experiments. The measured FNMs are composed of nanoparticles of 14.8 nm diameter and with hexadecanethiols as capping ligands. Tapping mode was used for taking images. Then the AFM was turned to contact mode to perform force−indentation measurements. The spring constant of the AFM tip was about 2.8 N/ m, calibrated before each test. The loading velocity of the AFM tip was about 1 μm/s. The contact point was set to be the center of circular FNMs. The force versus indentation data was recorded and analyzed. More than 10 FNMs were measured. For each FNM, the measurement was repeated 20 times and the data were recorded.
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Figure 1. Schematic illustration of the process of fabricating freestanding nanoparticle monolayers over circular wells in SiO2. (a) The silicon substrate with circular wells modified with silane molecules to make the surface highly hydrophobic. The circular wells were fabricated by photolithography and ion-coupling etching. (Inset) The molecular structure of silane molecule 1H,1H,2H,2H-perfluorodecyltriethoxysilane (PFDTES). (b) Some ultrapure water was gently placed on top of the treated silicon substrate. A proper amount of gold nanoparticle solution was spread on the water surface. (c) Formation of a close-packed nanoparticle monolayer self-assembled at the air/ water interface after the evaporation of the solvent. (d) The nanoparticle monolayer draped itself over wells after the evaporation of water, forming freestanding nanoparticle sheets.
substrate was cleaned in piranha solution (H2SO4 (98%)/ H2O2(30%) = 3:1). The treatment of piranha solution makes the silicon surface completely hydrophilic so that the contact angle is unmeasurable. More importantly, the surface would be rich in hydroxyl groups, which are ready to react with silane molecules. The inset in Figure 1a shows the molecular structure of the PFDTES silane molecule. The cleaned substrate was then immersed in a solution of PFDTES. The PFDTES molecules would form a self-assembled monolayer on the surface. Because PFDTES molecules have strongly hydrophobic terminal groups, the surface of the substrate would be hydrophobic after the PFDTES treatment. Note that baking the substrate at 140 °C for 1 h after selfassembly is necessary to help form chemical bonds. The contact angle of the silicon substrate measured after this modification was 115.0 ± 1.4°. This confirms the self-assembly of PFDTES molecules on the substrate and indicates that the modifying process can increase the hydrophobicity of the substrate effectively. After the modification, some water was infused to cover the substrate, as shown in Figure 1b. Several drops of nanoparticle solution were spread on the water surface. Nanoparticles would self-assemble into 2D nanoparticle arrays (Figure 1c). After the evaporation of the water, the nanoparticle monolayer would drape itself over wells and form FNMs (Figure 1d). The hydrophobic modification process is crucial for the preparation of FNMs over circular wells. The main effect is to keep the water above the substrate from touching the bottom of the wells, as shown in Figure 1b. It had been demonstrated that the lack of such a modifying process would lead to the failure of achieving FNMs (Supporting Information Figure S7). We contribute this failure to the dragging of nanoparticle
RESULTS AND DISCUSSION
Procedures to Prepare FNMS. Figure 1 shows the schematic process for the preparation of FNMs. A silicon wafer with circular wells was used as a substrate. This structured substrate was fabricated by photolithography. Before used, the 3739
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Figure 2. Characterization of freestanding nanoparticle monolayers. (a) SEM image of the area covered with nanoparticle monolayers. The holes without bright circles in the SEM image were covered with freestanding nanoparticle sheets. Note that large-area freestanding nanoparticle sheets could be prepared using our method. (b) SEM image with high magnification of a single freestanding nanoparticle sheet at the edge of the well. There is no obvious difference in the nanoparticle monolayer between the suspended part and the supported part. (c) 3D AFM image of suspended nanoparticle sheets over two circular wells with diameters of 2.5 and 1.8 μm, respectively. (d) The height profile across a single hole along the white dashed line in c. The step height at the edge of the monolayer is about 30 nm.
FNMs in a area of 10 μm × 10 μm. Two wells covered by FNMs with diameters of 1.8 and 2.5 μm can be identified in the image. The homogeneous color of the image indicates the uniformity of the nanoparticle monolayer of the whole area. The height profile along the white dashed line in Figure 2a is shown in Figure 2b. The height difference between the supported nanoparticle monolayer and the freestanding nanoparticle monolayer above the well is only about 30 nm (3 times the nanoparticle diameter). Note that the depth of wells is about 500 nm (Supporting Information Figure S8). The small height difference provides direct evidence for the successful preparation of FNMs. Our experimental results indicate that our method possesses three advantages. First, FNMs can be prepared over one-endopen holes. This not only avoids complex fabrication processes of through holes but also enables more measurements and applications of FNMs. Moreover, our process is compatible with traditional silicon process technology. Second, our method has high efficiency. We can obtain hundreds of FNMs over a large area from one preparation process. Third, FNMs prepared with our method have good quality. For each single FNM, the nanoparticle array is no different from that supported by the substrate. Dynamic Process of the Formation of FNMs. The success of our strategy can be explained by the classic theory, describing the state of a water droplet on a substrate with hydrophobic wells. When the droplet is placed gently on the structured hydrophobic substrate, the curvature of the droplet recessing into a well is governed by the Laplace equation28 for every individual well. Note that the curvature is the same at the
monolayers into the wells by water in the ending stage of water evaporation. We would discuss this issue in detail later to describe the whole dynamic process during the preparation. Note that the evaporation rate is another key factor in the preparation of FNMs. In fact, we conducted control experiments at different evaporation rates. For most cases, a relatively slow evaporation rate would get better results. For a relatively higher evaporation rate, more ruptured monolayer above wells appeared. Therefore, it is believed that a higher evaporation rate may cause a violent dewetting process in the end and tear the nanoparticle monolayer. In our experiments, we found that the best FNMs could be prepared when the evaporation rate was controlled to be about 0.42 μL/min. Characterization of FNMs. Figure 2a shows a typical SEM image of prepared FNMs over a large area. The wells in the SEM image exhibit two states. Some are circled with bright rings, and others are not. By closely checking the wells in highmagnification SEM images (Supporting Information Figure S7), we find that the wells with bright circles are without FNMs. In contrast, wells without bright circles are covered by FNMs. This provides an easier way to check the results of prepared FNMs quickly. In Figure 2a, most of the circular wells are covered with FNMs, indicating the efficiency of our method. Figure 2b shows a high-magnification SEM image of an FNM. Note that no obvious differences can be found between the freestanding nanoparticle monolayer above the well and that supported by the substrate. Moreover, we do not observe ruptures or wrinkles in the monolayer. FNMs were also characterized by tapping-mode atomic force microscopy (AFM). Figure 2c shows a typical AFM image of 3740
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Langmuir top and bottom of the droplet.34 Figure 3a shows a water droplet placed on a hydrophobic substrate with circular wells
The consistency between the experimental results and the expectations from the model indicates the validity of the model. The crucial requirement is to keep the water in the Cassie− Baxter state and not to touch the bottom of the wells. By fulfilling this requirement, the nanoparticle monolayer will not experience a downward dragging force by the water in the final stage of evaporation, which enables us to succeed in the preparation of FNMs. To demonstrate this scenario further, we performed the following salt crystallization experiments. Salt Crystallization Experiments. Salt crystallization experiments were carried out to trace the behavior of the water droplet,29 particularly for the final stage of evaporation. In a salt solution, crystallization will start when the concentration is higher than the saturation concentration. Using this phenomenon, we can identify the static state of the water droplet on a hydrophobic substrate. If salt crystals are found in the wells, then that means that the droplet is in the Wenzel state. Otherwise, the droplet is in the Cassie−Baxter state. The crystallization experiments were conducted by adding 125 μL of a 0.5 mol L−1 NaCl water solution to the setup mentioned above. After the evaporation of water, SEM was used to characterize the locations of crystals. The results are shown in Figure 4. For the substrate treated with PFDTES, no salt crystals were found in the wells, as shown Figure 4a,b. This suggests that the water droplet was in the Cassie−Baxter state so that at the end of evaporation the water film above the substrate would be dragged away, instead of depositing in the wells. However, for the substrate untreated with PFDTES, cubic NaCl crystals are solely in wells (Figure 4c,d), implying that the solution filled the wells and crystallization took place there. Note that the NaCl crystals are the gray bulk pieces in Figure 4c,d. These experimental results provide solid evidence for the droplet above the substrate treated with PFDTES staying in the Cassie−Baxter state for the whole preparation process. On the basis of the above salt crystallization experiments, we propose a possible dynamic process for the formation of FNMs. When placed on the substrate with wells, the water droplet would stay in the Cassie−Baxter state and would not touch the bottom of the wells because of the highly hydrophobic surface. During the evaporation of water, the droplet would remain in the Cassie−Baxter state and finally turn into a thin film. Then the water would be dragged away horizontally, leaving the upper nanoparticle monolayer to drape itself over wells and form FNMs. In this process, the nanoparticle monolayer would not experience a violent down-drag force, which guarantees the successful preparation of FNMs. In contrast, the water would fill and deposit in wells in a hydrophilic substrate. Therefore, the nanoparticle monolayer would be dragged into the wells and broken into patches at the bottom of the wells (Supporting Information Figure S7). Fabrication of FNMs with Different Ligands and Different Core Sizes. Our method can be used to prepare FNMs composed of nanoparticles with different diameters and different ligands. In our demonstration, we tried nanoparticles with three different diameters, i.e., 8.5, 10.4, and 14.8 nm. Three alkanethiols were used as capping ligands, including octanethiol, dodecanethiol, and hexadecanethiol. An optical microscope was used to quickly characterize prepared FNMs.33 By comparing optical microscope images with SEM images, we found that good FNMs would be yellow in optical microscope images (Supporting Information Figure S9). Figure 5a−c shows three typical optical microscope images of FNMs. In particular,
Figure 3. Schematic illustration of the analysis model of a droplet on a hydrophobic well substrate. (a) Representation of droplet placed on a hydrophobic well substrate. R is the curvature of the droplet, σ is the recessing of the droplet into a single well, H is the height of the well, and D is the diameter of well. (b) Droplet in the Cassie−Baxter state. (c) Droplet in the Wenzel state.
with diameter D and depth H. The curvature radius of the droplet is R. The largest recessing δ is found in the center of the well, expressed as ⎛ 1 ⎞⎟ δ = r ⎜tan θ − ⎝ cos θ ⎠
where r is the radius of the well and θ is the contact angle of water on the substrate. When the droop is greater than the depth of the well, the droplet will contact the bottom of the wells, resulting in the transition of the droplet above wells from the Cassie−Baxter state (Figure 3b) to the Wenzel state (Figure 3c).35 For a hydrophilic substrate, the droplet will fill every well and stay in the Wenzel-like state. In our experiment, the contact angle θ for the hydrophobic substrate with wells is about 115°. The radii of the wells used are 1.25 and 0.9 μm, respectively. According to the model, the maximum droop is about 276 and 199 nm for two cases. That means that the water would stay in the Cassie−Baxter state if wells are deeper than these critical depths. In our case, the typical depth of wells is about 500 nm, much deeper than both critical depths. This implies that the water droplet would remain in the Cassie−Baxter state and be suspended above the well without contacting the bottom during evaporation. In fact, deeper wells can also be used to apply our strategy (Supporting Information Figure S5). However, when the depth of wells (180 nm) is less than the critical depths, the droplet would be in the Wenzel state, leading to the failure to prepare FNMs (Supporting Information Figure S10). Moreover, for hydrophilic wells untreated with PFDTES, the droplet would stay in the Wenzel-like state and fail in the preparation of FNMs (Supporting Information Figure S7). 3741
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Figure 4. Crystallization of NaCl solution on a substrate with circular wells. (a) SEM image of the substrate with a hydrophobic surface by treating with PFDTES after drying the NaCl solution. No NaCl crystals were found in the holes. (b) High-magnification SEM image of the same sample as in a. Only a few NaCl crystals were found outside the hole, indicating that the water was outside the hole in the final stage of the drying process. (c) SEM image of the untreated substrate with a hydrophilic surface, showing one NaCl crystal in each hole after the drying of the NaCl solution. (d) High-magnification SEM image of the same sample as in c. In each hole, a NaCl crystal was found, suggesting that the NaCl solution remained in the hole until drying was finished. Note that the NaCl crystals are the gray bulk pieces in c and d.
Figure 5. Large-area fabrication of freestanding nanoparticle monolayers with different ligands and different core sizes. (a) Optical image of largearea freestanding nanopartical sheets, with a nanoparticle size of 10.4 nm in diameter using dodecanethiols as ligands. (b) Optical image of FNMs, with a nanoparticle size of 10.4 nm in diameter using hexadecanethiols as ligands. (c) Optical image of a freestanding nanopartical sheet, with a nanoparticle size of 14.8 nm in diameter using hexadecanethiols as ligands. (d) Optical image of FNMs with double-layer thickness by repeating the fabrication process twice. The area with a crack for the second layer was selected to clearly show the monolayer (dark region) and the double layer (bright region).
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Langmuir the nanoparticle diameters are 10.4 nm for Figure 5a,b but 14.8 nm for Figure 5c. The ligands are dodecanthiol for Figure 5a but hexadecanethiol for Figure 5b,c. The optical microscope images show that changing the nanoparticle diameter or ligand does not influence the quality of FNMs prepared using our method. This confirms the universality of our strategy. FNMs made from more combinations between nanoparticle core sizes and ligands were also prepared (Supporting Information Figure S6). Moreover, double-layered FNMs could also be prepared by carrying out the preparation process on monolayered FNMs. Figure 5d shows the optical microscope image of doublelayered FNMs. To clearly see the difference between monolayered FNMs with double-layered FNMs, a region with a crack in the second layer was selected. In Figure 5d, the double-layered FNMs exhibit a brighter yellow color. In principle, thicker multilayered FNMs can also be achieved by conducting the preparation process repeatedly. Force−Indentation Experiments. The mechanical properties of FNMs are interesting. Therefore, it is important to demonstrate that the FNMs prepared by our method can be used for mechanistic studies. To do this, we tested the mechanical characteristics of one typical kind of prepared FNMs made from 14.8 nm nanoparticles capped with hexadecanethiols. Force−indentation experiments on FNMs were performed using AFM probes. The schematic measurement process is illustrated in Figure 6a. An AFM tip was induced to carry out indention−retraction cycles with respect to the FNM while the force−indentation curves were recorded. Ten FNMs (diameter of 1.8 μm) at different positions were randomly chosen for the study. For each FNM, data of 20 indentation−retraction cycles were recorded. Figure 6b shows a typical trace of the force−indentation curve. All indentation curves could be analyzed using the classical circle plate theory.36 In this model, force can be expressed as a function of indentation: F = aw3 + bw, where w is the indentation caused by applied force F, E is the Young’s moduli, h is the thickness of the membrane, and R is the radius of the FNM. Prefactor a can be described as a = πEh/(3R2). In our experiments, h can be expressed as h = r + 2l, in which r represents the mean nanoparticle diameter (14.8 nm) and l represents the length of alkylthiol ligands (2.3 nm).37 And the radius of FNMs was 0.9 μm. By fitting the indentation curves with the model, Young’s moduli E can be determined. Figure 6c shows the histogram of the Young’s moduli for 10 FNMs. The average value of moduli E is 0.91 ± 0.70 GPa, which is comparable to reported moduli values of FNMs made from dodecanethiol-capped 6 nm Au nanoparticles.24
Figure 6. Force response measurements of freestanding nanoparticle sheets. (a) Schematic of the force measurement using AFM indentation. Proper force was exerted on the FNM through the AFM tip to acquire the force−indention curve. (b) Typical curve of force versus indention. The suspending sheet was composed of 14.8 nm particles encapsulated with hexadecanethiols. (c) Young’s moduli measured from 10 FNMs over circular wells.
GPa. Our strategy paves the way for the facile preparation of FNMs over one-end-open holes, which may make the study on FNMs easier and open possibilities of new applications for FNMs.
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ASSOCIATED CONTENT
S Supporting Information *
CONCLUSIONS A new strategy to prepare large-area FNMs over circular wells is reported. The success of the method relies on the hydrophobic modification of the surface by PFDTES, which keeps the water droplet in the Cassie−Baxter-like state so that the nanoparticle monolayer will not experience the downward dragging force in the final stage of water evaporation. Optical microscopy, SEM, and AFM were used to characterize the prepared FNMs. A classic Laplace model and salt crystallization experiment were used to demonstrate the Cassie−Baxter-like state of the water droplet. This strategy was successfully applied to other nanoparticles with different sizes and ligands. Moreover, the Young’s moduli of a typical FNM was measured using the AFM indentation method, giving a mean Young’s modulus of 0.91
Characterization of gold nanoparticales. Processes to fabricate wells in the substrate. Preparation procedures of freestanding nanoparticle membranes over wells. Surface modification of the silicon substrate with silane molecules. SEM images of freestanding nanoparticle sheets over deep wells. Optical images of freestanding nanoparticle arrays with different nanoparticle sizes and ligands. Preparation of freestanding nanoparticle membranes using surface-treated and untreated substrates. AFM characterization of the depth of wells. Comparison between the optical image and the SEM image of freestanding nanoparticle arrays. The results for preparing FNMs using wells with a depth of 180 nm. A representative fitting curve to the AFM force curve. This material is available free of charge via the Internet at http://pubs.acs.org. 3743
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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 This work was financially supported by the Ministry of Science and Technology of the People's Republic of China (No. 2011CB933001, 2012CB932702) and the National Natural Science Foundation of China (no. 60971001).
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DOI: 10.1021/la504881n Langmuir 2015, 31, 3738−3744
