Spatial Manipulation and Assembly of Nanoparticles by Atomic Force

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Spatial Manipulation and Assembly of Nanoparticles by Atomic Force Microscopy Tip-Induced Dielectrophoresis Peilin Zhou,†,‡,§ Haibo Yu,*,† Wenguang Yang,†,‡ Yangdong Wen,†,‡ Zhidong Wang,†,∥ Wen Jung Li,†,⊥ and Lianqing Liu*,† †

State Key Laboratory of Robotics, Shenyang Institute of Automation, Chinese Academy of Sciences, Shenyang 110016, China University of Chinese Academy of Sciences, Beijing 100049, China § Beijing Advanced Innovation Center for Imaging Technology, Capital Normal University, Beijing 100048, China ∥ Department of Advanced Robotics, Chiba Institute of Technology, Chiba 275-0016, Japan ⊥ Department of Mechanical and Biomedical Engineering, City University of Hong Kong, Kowloon Tong, Hong Kong ‡

ABSTRACT: In this article, we present a novel method of spatial manipulation and assembly of nanoparticles via atomic force microscopy tip-induced dielectrophoresis (AFM-DEP). This method combines the high-accuracy positioning of AFM with the parallel manipulation of DEP. A spatially nonuniform electric field is induced by applying an alternating current (AC) voltage between the conductive AFM probe and an indium tin oxide glass substrate. The AFM probe acted as a movable DEP tweezer for nanomanipulation and assembly of nanoparticles. The mechanism of AFM-DEP was analyzed by numerical simulation. The effects of solution depth, gap distance, AC voltage, solution concentration, and duration time were experimentally studied and optimized. Arrays of 200 nm polystyrene nanoparticles were assembled into various nanostructures, including lines, ellipsoids, and arrays of dots. The sizes and shapes of the assembled structures were controllable. It was thus demonstrated that AFM-DEP is a flexible and powerful tool for nanomanipulation. KEYWORDS: AFM, tip-induced DEP, 3D nanomanipulation, nanoassembly, nanostructure

1. INTRODUCTION Over the last decade, the manipulation and trapping of micro/ nanoparticles have attracted significant attention. Particularly, nanomanipulation is an important enabling technology with wide applications in the fields of nanomaterials,1,2 nanoelectronic devices,3−5 and bionanotechnology.6−8 Owing to the dimension limitations of nanoparticles, there are only a few methods suitable for manipulation of nanoparticles. These methods can be sorted into two categories: noncontact-mode and contact-mode nanomanipulation. Noncontact-mode nanomanipulation typically utilizes interactions between the micro/ nanoparticles and optics, an electric field, or a magnetic field. Several noncontact manipulation techniques have been demonstrated, such as optical tweezers,9,10 magnetic trapping,11,12 and dielectrophoresis (DEP).13−16 Optical tweezers use focused, high-powered laser beams to create an attractive or repulsive force to manipulate microobjects or nanoobjects and have been successful in analyzing a variety of biological systems. However, optical tweezer systems are expensive and usually require the usage of transparent substrates made with glass for biological analysis. Magnetic tweezers can exert force and torque on magnetic particles to control their movement or measure their mechanical properties. This technique has been accepted and used to study biological molecules. Typically, © XXXX American Chemical Society

these biological molecules must be functionalized with magnetic microparticles that can be easily observed through an optical microscope. The complexity of the functionalization process limits the application of magnetic tweezers. DEP technology is a simple, highly efficient method of nanomanipulation through electric field gradients. It has been used in separation,17,18 transportation, and assembly.19−22 The broad application of this technique is supported by its ease of implementation, relevance to both charged and neutral samples, and noncontact manipulation mode, which may reduce sample damage. The sizes, positions, and structures of fixed physical electrodes23−25 typically control the precision, positioning, and shapes produced using conventional DEP but are inflexible and primarily suitable for batch manipulation of nanosamples. Contact-mode nanomanipulation is based on mechanical operation with single or multiple sharp probes, typically using an atomic force microscope (AFM).26−29 The AFM probe plays a key role in imaging and can serve as an end-effector for nanomanipulation or nanomachining. The main advantage of AFM is its ability to accurately control probe movement and Received: March 14, 2017 Accepted: April 28, 2017

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DOI: 10.1021/acsami.7b03565 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. Three-dimensional manipulation and assembly of nanoparticles by AFM-DEP. (a) Schematic of the AFM-DEP experimental apparatus. (b) Assembly of nanoparticles using a positive DEP force. (c) Manipulation of nanoparticles using a negative DEP force. (d) Array of nanoparticle dots. (e) Alignment of nanoparticles.

detect the forces produced by interactions between the tip and sample. AFM offers excellent precision in positioning and manipulation, which allows users to image nanosamples during the manipulation process. However, typical AFMs have only a single probe tip as the end-effector, which leads to severely inefficient nanomanipulation.30,31 In addition, contact-mode manipulation using a sharp, hard AFM probe might cause damage to samples and substrates.32,33 Previous researchers have combined several techniques to achieve versatile nanomanipulation. Researchers have investigated the assembly of nanomaterials to AFM probes or sharp tips using tip-induced DEP,34−37 which amply demonstrated that the nanomanipulation based on tip-induced DEP is flexible, efficient, and highly precise. However, compared with nanomaterials assembled to a single probe tip, which is unachievable for removable and multipoint manipulation, researches on manipulation and assembly of micro/nanomaterials at different specific positions of the substrate have received increasing interest; various micro/nanosamples have been three-dimensionally (3D) manipulated at specific positions of the substrate.38,39 In this article, we present a novel method of applying AFM tip-induced DEP (AFM-DEP) for the manipulation and assembly of nanoparticles. A spatial nonuniform electric field can be created by applying an alternating current (AC) voltage between the conductive probe of an AFM and an indium tin oxide (ITO) glass substrate. The AFM probe acted as a movable DEP tweezer for nanomanipulation and assembly of nanoparticles. Theoretical analysis and numerical simulation were performed using a finite element method, which provided a deep understanding of the AFM-DEP mechanism. In the experiments, the effects of different parameters on the assembly of 200 nm polystyrene (PS) nanoparticles were studied. The proposed AFM-DEP method is more flexible and efficient than other nanomanipulation techniques and thus may have more extensive potential applications in many fields of nanotechnology research, such as fast fabrication of nanostructures and nondestructive manipulation of biological nanosamples. For biological applications, for example, drug-loaded nanoparticles or viruses can be transported nondestructively and

released to specific cells to study the interaction and response of cells.

2. MATERIALS AND METHODS 2.1. Experimental Apparatus. The AFM-DEP apparatus consists of four parts: an AFM system with a PC controller, a conductive AFM probe, a microchip, and a signal generator. Figure 1 shows a schematic of the experimental apparatus as well as examples of manipulation and assembly of nanoparticles via AFM-DEP. A commercial AFM system (Dimension 3100; Veeco Instruments, Inc.) was used to control the movement of the AFM probe in the X, Y, and Z directions with nanoscale precision. The sample stage was controlled by a highprecision stepper motor with microscale precision. A charge-coupled device (CCD) camera was used to monitor the relative movement of the AFM probe in three dimensions. The camera was also used to monitor the stability and thickness of the solution in the microchip in real-time. A spatial nonuniform electric field was generated when an AC voltage was applied between the AFM probe and the ITO glass substrate. The AC voltage was created by a signal generator (AFG3022B; Tektronix). As shown in Figure 1b,d, the nanoparticles were assembled to form either a single dot or an array of dots by changing the position of the AFM probe. During assembly, nanoparticles were also adsorbed on the sidewalls of the AFM tip. To remove these nanoparticles, the frequency of the AC voltage was changed to create a negative DEP force so that the nanoparticles were repelled from the AFM tip (Figure 1c). In addition, alignment of these nanoparticles was achieved by controlling the movement of the AFM probe (Figure 1e). 2.2. Methods. A 10 wt % solution of 200 nm diameter PS nanoparticles (Sigma-Aldrich) was mixed with a 1 wt % bovine serum albumin (BSA) solution at a ratio of 1:1 by volume. The BSA was added as the dispersant to decrease the affinity force between the PS nanoparticles and the ITO glass substrate. Then, the mixed solution was diluted to 0.2−5 wt ‰ in deionized water (relative permittivity = 80 and conductivity = 3.2 × 10−4 S/m). Finally, the mixture was ultrasonicated at a frequency of 40 kHz for 20 min to disperse the PS nanoparticles. The conductive AFM probe (SCM-PIT; Veeco Instruments, Inc.) was coated with a thin layer of platinum−iridium. The tip curvature radius, tip height, and probe cantilever length were ∼20 nm and 15 and 225 μm, respectively. The microchip contained three components. The first component was a 30 × 30 × 1 mm3 piece of conductive ITO glass (a glass substrate coated with ITO on one surface). The second component was a microreservoir with an internal floor space of 200 B


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ACS Applied Materials & Interfaces mm2, which contained the nanoparticle solution. This part was fabricated by a lithography technique that involved application of photoresist (SU-8) on the ITO-coated glass surface. The middle part of the microreservoir was an outdoor microchannel with a length, width, and depth of 10 and 2 mm and 5−30 μm, respectively. The third component consisted of two thin pieces of glass, which were used to cover both ends of the microreservoir and were effective in preventing overquick volatilization of the solution. Finally, the surface of the ITO glass was hydrophilically treated using an O2 plasma so that the solution of PS nanoparticles would form a stable, uniform liquid membrane. When the microchip was placed on the AFM sample stage, the AFM probe could be precisely located above the outdoor microchannel of the microreservoir in the X, Y directions with the aid of the CCD camera. The coordinates of the stage in the Z axis was recorded as an original reference point when the AFM tip touched the ITO substrate. The gap between the tip of the AFM probe and the ITO glass substrate was precisely controlled by adjusting the parameters of the piezoelectric ceramic in the AFM scan head and the stepper motor. A high-precision pipette was used for accurate volume control for the nanoparticle solution. When 2 μL of the PS nanoparticle solution was transferred into the microchip using the high-precision pipette, a stable and uniform liquid membrane with a thickness of 10 μm was formed in the microreservoir.

The relationship between Re[f CM(ω)] and the applied voltage frequency, f, is shown in Figure 2 for 200 nm PS

Figure 2. Plot of Re[f CM(ω)] as a function of the applied voltage frequency for 200 nm PS nanoparticles suspended in a solution medium with different conductivities.

nanoparticles suspended in a solution medium with different conductivities. The dotted line is the boundary between positive and negative DEP. Voltage frequencies of 10 kHz and 10 MHz were applied during our experiment to induce positive and negative AFM-DEP, respectively. 3.2. Simulation. To investigate electrostatic interactions between the AFM probe and the ITO glass, a two-dimensional (2D) finite-element method model of AFM-DEP was established using the commercial software package COMSOL Multiphysics 4.3a to simulate the nonuniform electric field between the AFM probe and the ITO glass. The basic parameters of the tip angle and tip curvature radius were set as 40° and 20 nm, respectively, whereas a 1 μm electrode gap between the tip of the AFM probe and the ITO glass was simulated with a 5 Vpp AC voltage. Figure 3a shows the magnitude of ∇|E|2 between the AFM probe and the ITO glass and the direction (red arrows) of the DEP force. According to the detailed statistics and analysis shown in Figure 3b, the maximum magnitude of ∇|E|2 at the apex of the probe tip can be as high as 4.5168 × 1024 V2/m3. The ∇|E|2 between the AFM probe and the ITO glass decreases rapidly as the distance increases in the x and z directions. Detailed theoretical analyses and numerical simulations were performed to analyze the electric field distribution between the probe tip and the ITO glass. The experimental parameters were set as U = 5 Vpp, r = 20 nm, and θ = 40°. The variation of ∇|E|2 at the apex of the probe tip as a function of the gap distance, d, is shown in Figure 3c. The variations of ∇|E|2 at the apex of the probe tip with the AC voltage, U (with d = 1 μm, r = 20 nm, and θ = 40°); the tip curvature radius, r (with d = 1 μm, U = 5 Vpp, and θ = 40°); and the tip angle, θ (with d = 1 μm, U = 5 Vpp, and r = 20 nm), are shown in Figure 3d−f, respectively. On the basis of Figure 3c− f, we can conclude that the ∇|E|2 increases with U but decreases as d, r, and θ increase. Furthermore, according to the above theory, we can conclude that the DEP force increases as U increases, whereas it decreases as d, r, and θ increase.

3. THEORETICAL ANALYSIS 3.1. Theory. The DEP force acts on an electrically polarizable object that is immersed in a liquid medium and exposed to a spatially nonuniform electric field. The timeaveraged DEP force acting on a spherical particle depends on several physical factors, which are briefly described as follows40 FDEP(ω) = πεmR3 Re(fCM (ω))∇|E|2

(1)

where εm is the real electrical permittivity of the surrounding medium, R is the radius of the particle, ∇|E|2 is the gradient of the square of the applied electric field magnitude, and f CM(ω) is the frequency-dependent Clausius−Mossotti factor for a dielectric uniform sphere and is given by fCM (ω) =

εp*(ω) − εm*(ω) εp*(ω) + 2εm*(ω)

(2)

where ε*p and ε*m are the complex permittivities of the particle and medium, respectively, and are defined by σ ε* = ε − j (3) ω where ε is the permittivity of the particle or medium, σ is the conductivity of the particle or medium, j is −1 , and ω = 2πf, where f is the applied voltage frequency across the liquid medium. For a spherical particle, the conductivity is given by41 σp = σbulk + 2 K s/R

(4)

where σbulk and Ks are the bulk and surface conductivities of the particle material (for PS nanoparticles, σbulk = 1 × 10−16 S/m and Ks = 2 × 10−9 S/m), respectively. As mentioned above, the direction of the DEP force depends on the sign of the real part of the CM factor, Re[f CM(ω)], which has a value between +1.0 and −0.5. The CM factor is governed by the conductivity and real permittivities of the particles and solution medium, as well as by the frequency of the applied AC voltage. The particles can be either attracted to or repelled from the strong electric field, which is called positive DEP (Re[f CM(ω)] > 0) or negative DEP (Re[f CM(ω)] < 0), respectively.

4. RESULTS AND DISCUSSION 4.1. Experiment and Results Characterization of AFMDEP. To demonstrate the ability of AFM-DEP to manipulate nanoparticles in 3D, an assembly experiment was carried out C


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Figure 3. Finite-element method modeling of AFM-DEP. (a) Magnitude of ∇|E|2 and the direction (red arrows) of the DEP force for a 5 V AC voltage and 1 μm electrode gap. (b) Magnitude of ∇|E|2 plotted along the blue line (z direction at x = 0 nm), red line (x direction at z = 0 nm below the tip), green line (x direction at z = 100 nm below the tip), and magenta line (x direction at z = 1000 nm below the tip). (c−f) The variation of ∇| E|2 at the apex of the probe tip in response to various experimental parameters: gap distance, tip radius, AC voltage, and tip angle, respectively.

Figure 4. Experiment of AFM-DEP. (a) Schematic of reversible AFM-DEP. (b) SEM images of the new SCM-PIT probe. (c) SEM images of the SCM-PIT probe taken after tip-induced positive DEP. (d) SEM images of the SCM-PIT probe taken after tip-induced positive DEP and negative DEP sequentially. (e, f) Three-dimensional AFM images of the AFM-DEP results on ITO glass corresponding to (c) and (d), respectively.

nanoparticles are assembled at the tip of the AFM probe and the ITO glass underneath after positive AFM-DEP. After replacing a new SCM-PIT probe, manipulation of the nanoparticles by tip-induced positive and negative DEP was carried out with the same experimental parameters sequentially in another area. According to the corresponding SEM images of the SCM-PIT probe and the AFM image of the result on the ITO glass, as shown in Figure 4d,f, respectively, most of the 200 nm PS nanoparticles were repelled from the tip of the AFM probe and the ITO glass under the tip after negative AFM-

using the following parameters: a solution concentration of 0.5 wt ‰, a solution depth of 10 μm, a gap distance of 0.2 μm, and an AC voltage (U = 10 Vpp at 10 kHz) applied for 10 s. As shown in Figure 4a, according to the illustration of schematic reversible AFM tip-induced DEP, we experimentally analyzed the AFM tip-induced positive DEP and negative DEP. Scanning electron microscopy (SEM) images of the SCM-PIT probe were taken before and after tip-induced positive DEP and are shown in Figure 4b,c, respectively. An AFM image of the results on ITO glass is shown in Figure 4e. Arrays of 200 nm PS D


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Figure 5. High-resolution images and precise characterization of the AFM-DEP experimental results. (a) AFM images of the assembly of a nanoparticle dot using the SCM-PIT probe, after the assembly process. (b) AFM images of the same nanoparticle dot measured using the new TESP probe. Insets in (a) and (b) are SEM images of the corresponding AFM probe tips. (c) AFM scanned profiles corresponding to the cross-sections in (a) and (b).

Figure 6. Analysis of the solution depth. (a) Optical microscopy image of the AFM probe in a nanoparticle solution with a depth of 10 μm. (b) Cross-sectional schematic of an AFM probe in solution. (c, d) Two-dimensional AFM images for the result of positive AFM-DEP with 5 and 30 μm deep solution layers, respectively. (e, f) Three-dimensional AFM images corresponding to (c) and (d), respectively. (g, h) AFM scanned profiles corresponding to the cross-sections in (c) and (d), respectively.

4.2. Relationship between the Morphologies of Nanoparticle Dot and Various Solution Depths. To achieve controllable 3D manipulation and assembly of nanoparticles by AFM-DEP, the influence of solution depth needs to be considered. Therefore, microchannels with depths of 5, 10, 15, 20, and 30 μm were fabricated. The concentration of the nanoparticle solution was 1 wt ‰. The gap distance between the tip and the ITO glass was 1 μm. An AC voltage of 5 Vpp at 10 kHz was applied between the AFM probe and the ITO glass substrate for 10 s. The AFM image (Figure 6c,d) shows the assembly of a nanoparticle dot by AFM-DEP with 5 and 30 μm deep solution layers, respectively. Figure 6e,f shows the 3D AFM images corresponding to Figure 6c,d, respectively. Figure 6g,h shows AFM scanned profiles, which correspond to the cross-sections in Figure 6c,d, respectively. When the solution depth increases from 5 to 30 μm, the heights and widths of the nanoparticle dots vary from 550 to 650 nm and 2.5 to 5.5 μm, respectively. The morphology and structure of the nanoparticle dots varies from inerratic to anomalous. A statistical analysis of the results of nanoparticle dots with different solution depths is shown in Figure 7. The relationships between the sizes (heights, widths) of nanoparticle dots and

DEP. After manipulation of 200 nm PS nanoparticles on the ITO glass by positive AFM-DEP, the probe was cleaned by repelling the PS nanoparticles from the tip by negative AFMDEP. Using the same experimental parameters applied to obtain the results shown in Figure 4e, except that the gap distance was changed to 2 μm, Figure 5a shows AFM images of the assembled nanoparticles, which were obtained using the SCM-PIT probe after the assembly process. As shown in the inset image of the SCM-PIT probe in Figure 5a, although very few nanoparticles stick at the tip, the convolution effect42,43 of the SCM-PIT probe is obvious at nanoscale. To determine the morphology and structure of the nanoassembly results with high resolution and precision, we obtained a scan image of the result using a new TESP probe in the tapping mode with the help of a rapid and automated relocation method for the AFM probe.44 The AFM image of the corresponding result and the SEM image of the new TESP probe are shown in Figure 5b. A comparison of the AFM scanned profiles in Figure 5c, which correspond to the crosssections in Figure 5a,b, demonstrates that more precise characterization of the experimental result could be achieved using the new TESP probe. E


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dramatically increase with the solution depth. Overall, the controllability and consistency of 3D manipulation and assembly by AFM-DEP decreases when the depth of the solution increases. This is caused by the instability in the liquid environment and is exacerbated by spatial nonuniformity of the electric field in deeper solutions. To improve the controllability and consistency of 3D manipulation and assembly and to fabricate better and smaller 3D nanodots, the depth of the solution layer should be limited to no more than 10 μm and the tip should be only just immersing in the solution. 4.3. Relationship between the Morphologies of Nanoparticle Dots and Experimental Parameters. To obtain insight into the influence of experimental parameters on the assembly of PS nanoparticles, we carried out a detailed experimental research. The applied AC voltage was 5 Vpp at a frequency of 10 kHz, the solution concentration was 1 wt ‰, and the AC duration was 10 s. We first analyzed the influence of the gap distance on the 3D manipulation of PS nanoparticles.

Figure 7. Heights and widths of the nanodots vs various solution depths.

various solution depths are determined. We can conclude that the heights and widths of the 3D micro/nanodots increase with the depth of the solution. In addition, the standard deviation and the ratio of the standard deviation to the mean size

Figure 8. Results of assembly of nanodots with different experimental parameters. (a−c) AFM images of nanodots with gap distances of 0.5, 2, and 8 μm, respectively. (d) Cross-sections of (a)−(c). (e−g) AFM images of nanodots with AC voltages of 0.2, 1, and 10, respectively. (h) Cross-sections of (e)−(g). (i−k) AFM images of nanodots with solution concentrations of 0.2, 1, and 5 wt ‰, respectively. (l) Cross-sections of (i)−(k). (m−o) AFM images of nanodots with applied AC durations of 2, 10, and 30 s, respectively. (p) Cross-sections of (l)−(n). F


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Figure 9. Statistics and analysis of nanodots synthesized with different experimental parameters. (a−d) Heights and widths of the nanodots vs various gap distances, AC voltage amplitudes, solution concentrations, and AC voltage durations, respectively.

To do this, we varied the gap distance between 0.5 and 8 μm. Figure 8a−d shows AFM scan images of nanoparticle dots assembled with gap distances of 0.5, 2, and 8 μm, respectively. When the gap distance increases from 0.5 to 8 μm, the heights and widths of the nanoparticle dots vary from 654 to 210 nm and 2.86 to 0.67 μm, respectively. These results were consistent with the theoretical analysis that the DEP force decreases as the gap distance increases; a weaker DEP force led to less nanoparticles being trapped and assembled under the same experimental conditions. Therefore, the size of the 3D dots decreases as the gap distance increases. Figure 9a shows the relationship between the sizes (heights, widths) of the nanoparticle dots and the gap distance. To analyze the effect of the AC voltage, the gap distance, solution concentration, and AC duration were kept at 1 μm, 1 wt ‰, and 10 s, respectively. Figure 8e−h shows AFM scan images of nanoparticle dots assembled with amplitudes of AC voltage of 0.2, 1, and 10 Vpp, respectively. When the AC voltage increases from 0.2 to 10 Vpp, the heights and widths of the nanoparticle dots vary from 217 to 1059 nm and 1.57 to 3.31 μm, respectively. The results are consistent with the theoretical analysis that the DEP force increases as the AC voltage increases; a stronger DEP force led to more nanoparticles being trapped and assembled under the same experimental conditions. As a consequence, the size of the 3D dots increases as the AC voltage increases. Relationships between AC voltages and the sizes of nanoparticle dots are shown in Figure 9b. To study the effect of the nanoparticle solution concentration, the gap distance, AC voltage, and AC duration were fixed at 1 μm, 5 Vpp at 10 kHz, and 10 s, respectively. Figure 8i−l shows AFM scan images of nanoparticle dots assembled with solution concentrations of 0.2, 1, and 5 wt ‰, respectively. The nanoparticle dots become bigger as the solution concentration increases. The relationship between the sizes of the nanoparticle dots and the solution concentration is shown in Figure 9c. In additional, the effect of the duration time of AFM-DEP was also analyzed. The gap distance and AC voltage were fixed at 1 μm and 5 Vpp at 10 kHz, respectively. Figure 8m−p shows

AFM scan images of nanoparticle dots synthesized with AC voltage durations of 2, 10, and 30 s, respectively. According to the experimental results, we found that the nanoparticle dots become larger with longer applications of the AC voltage, as shown in Figure 9d. On the basis of the analysis of nanoparticle dots assembled with different experimental parameters, we can conclude that the heights and widths of the 3D dots decrease as the gap distance increases and increase with the AC voltage, solution concentration, and AC duration. From the morphologies and structures of the results in Figure 8 and the standard deviations of the plots in Figure 9, we can conclude that the shape and size of the 3D dots become irregular and nonuniform when the gap distance is too high or the AC voltage is too low. This phenomenon is caused by unstable nanomanipulation by a weak DEP force. The shape and size of the 3D dot also become irregular and nonuniform when the solution concentration and the AC voltage duration increase. This is because the influence of Brownian motion on the DEP process is non-negligible,45 and higher solution concentrations exacerbate Brownian movement46 and solution volatilization. In addition, a longer duration time leads to an increase in the solution concentration. Moreover, as observed in Figure 8, the size of the assembled nanoparticles is large even when the optimized experimental conditions are applied. On the one hand, the resolution of assembly is related to the size of the nanoparticles; for 200 nm PS nanoparticles, the resolution of assembly is about 300 nm. On the other hand, the size of the AFM tip has a significant influence on the size of the deposited nanoparticles. Currently, the entire AFM tip is coated with a thin conductive layer. To improve the assembly precision, we propose to use an insulatorcoated AFM tip with only the conductive tip-point exposed in future experiments. In this case, the induced spatial electric field can be confined to a small range between the AFM tip and the substrate. 4.4. Fabrication of Various Nanostructures. On the basis of the above research, different nanostructures were fabricated in a controlled manner. Nanoparticles were G


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Figure 10. Nanoparticles were assembled into different 3D nanostructures by applying various experimental parameters. (a) Two-dimensional AFM image of a basin-shaped nanostructure and (b, c) the corresponding 3D AFM image and AFM scanned profile, respectively. (d−f) AFM images and AFM scanned profiles of a linear nanostructure, respectively. (g−i) AFM images and AFM scanned profiles of an ellipsoidal nanostructure, respectively. (j−l) AFM images and AFM scanned profiles of an array of nanodots, respectively.

= 10 Vpp at 10 kHz) was applied for 5 s, a single nanodot was fabricated. Subsequently, another three nanodots were fabricated at specific positions using the same parameters.

assembled into various 3D nanostructures at specific positions on the ITO glass by AFM-DEP. The first nanostructure to be fabricated was in the shape of a basin, as shown in Figure 10a− c. The solution concentration was 1 wt ‰, the solution depth was 10 μm, and the gap distance was 1 μm. After an AC voltage (U = 5 Vpp at 10 kHz) was applied for 2 s, the probe moved down with a velocity of 1 μm/s in the Z direction for 0.9 μm, whereas the AC voltage was maintained for 3 s. The 2D AFM scan image of the nanobasin revealed a width, height, and basin depth of 5 μm and 290 and 270 nm, respectively. The next nanostructure to be fabricated was linear, as shown in Figure 10d−f. The solution concentration was fixed at 0.2 wt %, the solution depth was 5 μm, and the gap distance was 0.5 μm. After an AC voltage (U = 2 Vpp at 10 kHz) was applied for 2 s, the probe moved in the direction of the white arrow (Figure 10d) with a velocity of 1 μm/s during 3 s of tip-induced DEP. The width, height, and length of the nanostructure were 550 nm, 2.7 μm, and 257 nm, respectively. As shown in Figure 10g−i, an ellipsoidal shape was fabricated. The solution concentration, solution depth, and gap distance were 1 wt ‰ and 10 and 1 μm, respectively. After an AC voltage (U = 10 Vpp at 10 kHz) was applied for 2 s, the probe moved along the white arrow track (Figure 10g) and reciprocated four times at a velocity of 2 μm/s during tip-induced DEP. The width, height, and length of the nanostructure were 2.014, 0.904, and 3.835 μm, respectively. The final nanostructure fabricated was an array of dots, as shown in Figure 10j−l. The solution concentration, solution depth, and gap distance were fixed at 2 wt ‰ and 20 and 1 μm, respectively. After an AC voltage (U

5. CONCLUSIONS In this article, we have demonstrated a new method for manipulation and assembly of nanoparticles by AFM-DEP. Compared with nanomanipulation based solely on AFM or DEP, this method overcomes the low manipulation efficiency and contact manipulation mode of AFM, as well as the low precision and inflexibility of DEP. It extends the scope of application to fields in which the precise positioning and accurate manipulation ability of AFM and the effective manipulation ability of DEP are needed. The analysis and majorization of nanomanipulation by AFM-DEP with different experimental parameters were carried out. By adjusting the relative movement between the AFM probe and the ITO glass with different experimental parameters, 200 nm PS nanoparticles were assembled into 3D nanostructures of various sizes and shapes at specific positions on the ITO glass. The method of AFM-DEP has shown its ability for potential applications in many fields of nanotechnology research, such as the fabrication of nanostructures and arrays, assembly of nanodevices, and nondestructive manipulation of biological nanoparticles.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.Y.). H


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ACS Applied Materials & Interfaces *E-mail: [email protected] (L.L.).

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ORCID

Peilin Zhou: 0000-0002-8521-2285 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research work was partially supported by the National Natural Science Foundation of China (project no. 61475183, 61503258 and U1613220), the CAS FEA International Partnership Program for Creative Research Teams, and the Youth Innovation Promotion Association CAS.

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REFERENCES

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