High-Rate Assembly of Nanomaterials on Insulating Surfaces Using Electro-Fluidic Directed Assembly Cihan Yilmaz,†,§ Asli Sirman,†,§ Aditi Halder,‡ and Ahmed Busnaina*,† †
NSF Nanoscale Science and Engineering Center for High-Rate Nanomanufacturing, Northeastern University, 360 Huntington Ave., 467 Egan Research Center, Boston, Massachusetts 02115, United States ‡ School of Basic Science, Indian Institute of Technology Mandi, Mandi, Himachal Pradesh 175001, India S Supporting Information *
ABSTRACT: Conductive or semiconducting nanomaterials-based applications such as electronics and sensors often require direct placement of such nanomaterials on insulating surfaces. Most fluidic-based directed assembly techniques on insulating surfaces utilize capillary force and evaporation but are diffusion limited and slow. Electrophoretic-based assembly, on the other hand, is fast but can only be utilized for assembly on a conductive surface. Here, we present a directed assembly technique that enables rapid assembly of nanomaterials on insulating surfaces. The approach leverages and combines fluidic and electrophoretic assembly by applying the electric field through an insulating surface via a conductive film underneath. The approach (called electro-fluidic) yields an assembly process that is 2 orders of magnitude faster compared to fluidic assembly. By understanding the forces on the assembly process, we have demonstrated the controlled assembly of various types of nanomaterials that are conducting, semiconducting, and insulating including nanoparticles and single-walled carbon nanotubes on insulating rigid and flexible substrates. The presented approach shows great promise for making practical devices in miniaturized sensors and flexible electronics. KEYWORDS: directed assembly, electrophoresis, fluidic assembly, concentration gradient, nanoparticles, nanotubes
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hours to assemble over a few inches. A convective directed assembly process that utilizes meniscus and evaporation enables precision assembly with single-particle resolution4 takes several hours to assemble over a few millimeters. Although these processes have shown precision assembly with high resolution at the micro and nanoscale, each has serious scalability and throughput shortcomings that must be addressed. Electric-fieldbased directed assembly is among the most commonly used techniques since it provides a fast and scalable way of assembling a wide range of nanomaterials.12,15−17 The electrophoretic force on the particle is applied via an electric field generated using conductive electrodes/surfaces. However, assembly on conductive surfaces is not always feasible, especially for applications where the nanomaterials need to be assembled on an insulated surface such as transistors, sensors, and many other electronic devices. In these cases, typically, nanomaterials are first assembled on a conductive substrate and then transferred onto
irected assembly of nanomaterials such as nanoparticles, carbon nanotubes, nanowires, etc. has been a vast area of study over the last two decades. Various directed assembly techniques have been developed where nanomaterials are assembled into one-, two-, and threedimensional nanostructures onto surfaces by utilizing externally applied electric,1,2 magnetic,3 and convective forces4 and fluidic assembly.5 Assembly using nanomaterials has been used to make functional nanostructures enabling superior device performance and miniaturization in sensors, electronics, optics, energy, and biotechnology applications.2,5−8 For example, dielectrophoretic assembly of nanowires and nanotubes has been conducted to make functional nanostructures and devices.9,10 There are many barriers, however, such as scalability, throughput, yield, repeatability, and integration of different components that have not been fully addressed for commercialization. A few of these barriers have been addressed through using template-guided assembly to assemble nanomaterials to achieve desired nanoscale 2D or 3D architectures.11,12 For example, a fluidic assembly process where capillary force is the dominant assembly mechanism is amenable to a variety of nanomaterials.13,14 However, these processes are diffusion-limited and take several © 2017 American Chemical Society
Received: November 6, 2016 Accepted: July 11, 2017 Published: July 11, 2017 7679
DOI: 10.1021/acsnano.6b07477 ACS Nano 2017, 11, 7679−7689
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Figure 1. (a) Schematic of top and cross-sectional view of the patterned substrate. A SiO2 layer is deposited onto the gold film using PECVD followed by PMMA, which is patterned using e-beam lithography. (b) A schematic of the assembly process in which the patterned substrate and counter electrode are submerged in the solution.
an insulating surface.18 This process can increase the time and the complexity involved in making such devices. To assemble nanomaterials on insulating surfaces, typically, fluidic assembly techniques11,19−22 are used. In these techniques, nanomaterials are driven to the surface using fluidic forces arise from the capillary, liquid evaporation, and diffusion. Despite their great capabilities, most of these techniques can be slow for large area assembly and integration of nanomaterials. Therefore, there is a need for a versatile assembly technique that is scalable, highrate and allows the assembly of various types of nanomaterials on insulating surfaces such as polymers, oxide films, and others. This could potentially enable various practical device applications including flexible sensors23 and electronics.24 In this paper, we have developed a high-rate directed assembly technique, so-called electro-fluidic directed assembly, which uses a combination of electric field and capillary action at the air− liquid interface to enable assembly of nanomaterials on insulating surfaces. The mechanism of electro-fluidic assembly relies on the application of electrophoresis through an insulating surface combined with a typical fluidic assembly, which yields an assembly process that is 2 orders of magnitude faster compared to fluidic assembly. This approach eliminates the fluidic assembly’s diffusion-limited process by bringing the nanoelements from the bulk solution to the assembly region near the patterned substrate and, consequently, increasing the concentration near the substrate. We demonstrated that the assembly process is largely governed by the applied potential, pH of the suspension, pulling speed of the substrate, and the insulating material thickness. Understanding of assembly mechanism led to the controlled assembly of various types of nanoparticles such as copper, SiO2, CdSe, and single-walled carbon nanotubes (SWCNTs) on polymers and oxides at high rates. To demonstrate the use of the developed electro-fluidic assembly technique in nanoscale device applications, we fabricated a SWCNT-based NO2 gas sensor by directly assembling suspended SWCNTs on SiO2 substrate. The results showed that the developed sensor can detect the change in NO2 concentration from 1 to 5 ppm in air (at 25 °C), showing a promise for highly sensitive sensor applications. In addition, we show the capability of the developed assembly technique in assembling nanomaterials on flexible substrates for potential use in flexible electronics and sensors.
RESULTS AND DISCUSSION Substrate Preparation for Electro-Fluidic Assembly. Figure 1a shows the schematic of the patterned substrate prepared for electro-fluidic assembly and the insulating and conductive layers on the substrate. A thin conducting layer is needed underneath the insulating surface to apply an electric field during assembly, as shown in Figure 1b. A wide variety of insulating films (organic and inorganic) can be utilized for the electro-fluidic assembly. The thickness of the insulating layer could be up to 10 μm or larger depending on the dielectric contact of the insulating substrate. An e-beam resist, poly(methyl methacrylate) (PMMA), is applied onto the insulating layer and lithographically patterned using e-beam, as shown in Figure 1a. The patterned substrate and a counter electrode (a gold film on Si wafer) are then submerged (separated by 5 mm) into a suspension containing nanoelements (Figure 1b). The patterned substrate is then pulled from the suspension at a controlled pulling speed. The electric field is continuously applied, while the substrate is being pulled out from the suspension to provide an additional force to help prevent the detachment of the assembled nanoelements.16 All experiments were conducted in a controlled environment with 30% humidity and at 27 °C temperature. To fully assess the capability or the technique, a baseline was established for fluidic assembly in the absence of an applied electric field. The effect of pulling speed was also considered in establishing the baseline because of its effect on the assembly efficiency (AE). Since the fluidic assembly process depends on the diffusion of the particles toward the substrate, the effect of the pulling speed of the substrate becomes very influential.20 Figure 2 shows that the AE is dependent on the pulling speed. The AE results were quantified using imaging software to analyze the contrast differences in SEM images, where the dark regions indicate particle assembly and the white regions indicate no assembly (see Supporting Information, Figure S1). For example, higher assembly efficiencies are obtained at 0.05 mm/min pulling speed, which yields an AE ∼ 95%. The AE decreases as the pulling speed is increased due to the insufficient nanoparticle concentration in the vicinity of the patterned substrate. The assembly time is calculated by measuring the meniscus travel time over the patterned substrate. For example, a 10 mm × 10 mm substrate at 0.05 mm/min pulling speed will have a total assembly process of 120 min. This time is sufficient for the 7680
DOI: 10.1021/acsnano.6b07477 ACS Nano 2017, 11, 7679−7689
Article
ACS Nano
We first investigated the effect of the applied voltage on the electro-fluidic assembly process. In these experiments, we kept the pulling speed, the charge on the particles, and the thickness of the insulating layer constant and investigated the effect of voltage between 2 and 5 V (see Supporting Information, page 3). The thickness of the insulating film was fixed at 150 nm, and the pH of the suspension was maintained at 10.9 where the zeta potential of the particles was measured as −60 ± 2.1 mV. The objective is to significantly increase the AE at the baseline pulling speed (3 mm/ min) by increasing the electrophoretic force on the particles. Figure 3a shows SEM images and corresponding assembly efficiencies at different voltages. The figures exhibit great enhancement of the assembly of nanoparticles at higher applied electric fields. When the voltage is
