Fabrication of Flexible and Transparent Metal Mesh Electrodes Using Surface Energy-Directed Assembly Process for Touch Screen Panels and Heaters.

Transparent conductive electrodes (TCEs) are indispensable components of various optoelectronic devices such as displays, touch screen panels, solar cells, and smart windows. To date, the fabrication processes for metal mesh-based TCEs are either costly or having limited resolution and throughput. Here, a two-step surface energy-directed assembly (SEDA) process to efficiently fabricate high resolution silver meshes is introduced. The two-step SEDA process turns from assembly on a functionalized substrate with hydrophilic mesh patterns into assembly on a functionalized substrate with stripe patterns. During the SEDA process, a three-phase contact line pins on the hydrophilic pattern regions while recedes on the hydrophobic non-pattern regions, ensuring that the assembly process can be achieved with excellent selectivity. The necessity of using the two-step SEDA process rather than a one-step SEDA process is demonstrated by both experimental results and theoretical analysis. Utilizing the two-step SEDA process, silver meshes with a line width down to 2 µm are assembled on both rigid and flexible substrates. The thickness of the silver meshes can be tuned by varying the withdraw speed and the assembly times. The assembled silver meshes exhibit excellent optoelectronic properties (sheet resistance of 1.79 Ω/□, optical transmittance of ≈92%, and a FoM value of 2465) as well as excellent mechanical stability. The applications of the assembled silver meshes in touch screen panels and thermal heaters are demonstrated, implying the potential of using the two-step SEDA process for the fabrication of TCEs for optoelectronic applications.

Directed Assembly of Nanomaterials for Making Nanoscale Devices and Structures: Mechanisms and Applications.

Nanofabrication has been utilized to manufacture one-, two-, and three-dimensional functional nanostructures for applications such as electronics, sensors, and photonic devices. Although conventional silicon-based nanofabrication (top-down approach) has developed into a technique with extremely high precision and integration density, nanofabrication based on directed assembly (bottom-up approach) is attracting more interest recently owing to its low cost and the advantages of additive manufacturing. Directed assembly is a process that utilizes external fields to directly interact with nanoelements (nanoparticles, 2D nanomaterials, nanotubes, nanowires, etc.) and drive the nanoelements to site-selectively assemble in patterned areas on substrates to form functional structures. Directed assembly processes can be divided into four different categories depending on the external fields: electric field-directed assembly, fluidic flow-directed assembly, magnetic field-directed assembly, and optical field-directed assembly. In this review, we summarize recent progress utilizing these four processes and address how these directed assembly processes harness the external fields, the underlying mechanism of how the external fields interact with the nanoelements, and the advantages and drawbacks of utilizing each method. Finally, we discuss applications made using directed assembly and provide a perspective on the future developments and challenges.

High-Rate Printing of Micro/Nanoscale Patterns Using Interfacial Convective Assembly.

Printing of electronics has been receiving increasing attention from academia and industry over the recent years. However, commonly used printing techniques have limited resolution of micro- or sub-microscale. Here, a directed-assembly-based printing technique, interfacial convective assembly, is reported, which utilizes a substrate-heating-induced solutal Marangoni convective flow to drive particles toward patterned substrates and then uses van der Waals interactions as well as geometrical confinement to trap the particles in the pattern areas. The influence of various assembly parameters including type of mixing solvent, substrate temperature, particle concentration, and assembly time is investigated. The results show successful assembly of various nanoparticles in patterns of different shapes with a high resolution down to 25 nm. In addition, the assembly only takes a few minutes, which is two orders of magnitude faster than conventional convective assembly. Small-sized (diameter below 5 nm) nanoparticles tend to coalesce during the assembly process and form sintered structures. The fabricated silver nanorods show single-crystal structure with a low resistivity of 8.58 × 10-5 Ω cm. With high versatility, high resolution, and high throughput, the interfacial convective assembly opens remarkable opportunities for printing next generation nanoelectronics and sensors.

Solution-processed organic field-effect transistors using directed assembled carbon nanotubes and 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT).

Achieving low-cost fabrication of organic field-effect transistors (OFETs) has long been pursued in the semiconductor industry. Solution-based process allows the fabrication of OFETs cost-effective because of its merit of vacuum-free and room temperature operation. Here, we show a facile and scalable fabrication of solution-processed OFETs using carbon nanotube (CNT) as source/drain electrodes and 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) as semiconducting layer on silicon as well as on flexible and transparent polyethylene terephthalate (PET) substrates. The CNT electrodes and the C8-BTBT film are fabricated using a dip coating-based directed assembly process, and two dip coating parameters, the pulling speed and the solution concentration, are carefully chosen so that the thickness of the C8-BTBT film is close to that of the CNT electrodes. The fabricated OFET devices show typical p-channel behavior. Low-cost, ease of processing, wafer level scalability and good compatibility with various substrates make the fabrication process presented in this paper well suited for next-generation electronics and sensors.

Scalable Directed Assembly of Highly Crystalline 2,7-Dioctyl[1]benzothieno[3,2- b][1]benzothiophene (C8-BTBT) Films.

Assembly of organic semiconductors with ordered crystal structure has been actively pursued for electronics applications such as organic field-effect transistors (OFETs). Among various film deposition methods, solution-based film growth from small molecule semiconductors is preferable because of its low material and energy consumption, low cost, and scalability. Here, we show scalable and controllable directed assembly of highly crystalline 2,7-dioctyl[1]benzothieno[3,2- b][1]benzothiophene (C8-BTBT) films via a dip-coating process. Self-aligned stripe patterns with tunable thickness and morphology over a centimeter scale are obtained by adjusting two governing parameters: the pulling speed of a substrate and the solution concentration. OFETs are fabricated using the C8-BTBT films assembled at various conditions. A field-effect hole mobility up to 3.99 cm2 V-1 s-1 is obtained. Owing to the highly scalable crystalline film formation, the dip-coating directed assembly process could be a great candidate for manufacturing next-generation electronics. Meanwhile, the film formation mechanism discussed in this paper could provide a general guideline to prepare other organic semiconducting films from small molecule solutions.

Directed assembly-based printing of homogeneous and hybrid nanorods using dielectrophoresis.

Printing nano and microscale three-dimensional (3D) structures using directed assembly of nanoparticles has many potential applications in electronics, photonics and biotechnology. This paper presents a reproducible and scalable 3D dielectrophoresis assembly process for printing homogeneous silica and hybrid silica/gold nanorods from silica and gold nanoparticles. The nanoparticles are assembled into patterned vias under a dielectrophoretic force generated by an alternating current (AC) field, and then completely fused in situ to form nanorods. The assembly process is governed by the applied AC voltage amplitude and frequency, pattern geometry, and assembly time. Here, we find out that complete assembly of nanorods is not possible without applying both dielectrophoresis and electrophoresis. Therefore, a direct current offset voltage is used to add an additional electrophoretic force to the assembly process. The assembly can be precisely controlled to print silica nanorods with diameters from 20-200 nm and spacing from 500 nm to 2 µm. The assembled nanorods have good uniformity in diameter and height over a millimeter scale. Besides homogeneous silica nanorods, hybrid silica/gold nanorods are also assembled by sequentially assembling silica and gold nanoparticles. The precision of the assembly process is further demonstrated by assembling a single particle on top of each nanorod to demonstrate an additional level of functionalization. The assembled hybrid silica/gold nanorods have potential to be used for metamaterial applications that require nanoscale structures as well as for plasmonic sensors for biosensing applications.

Life cycle impacts and benefits of a carbon nanotube-enabled chemical gas sensor.

As for any emerging technology, it is critical to assess potential life cycle impacts prior to widespread adoption to prevent future unintended consequences. The subject of this life cycle study is a carbon nanotube-enabled chemical gas sensor, which is a highly complex, low nanomaterial-concentration application with the potential to impart significant human health benefits upon implementation. Thus, the net lifecycle trade-offs are quantified using an impact-benefit ratio (IBR) approach proposed herein, where an IBR < 1 indicates that the downstream benefits outweigh the upstream impacts. The cradle-to-gate assessment results indicate that the midpoint impacts associated with producing CNTs are marginal compared with those associated with the other manufacturing stages. The cumulative upstream impacts are further aggregated to units of disability-adjusted life years (DALYs) using ReCiPe end point analysis method and quantitatively compared with the potential downstream DALY benefits, as lives saved, during the use phase. The approach presented in this study provides a guiding framework and quantitative method intended to encourage the development of nanoenabled products that have the potential to realize a net environmental, health, or societal benefit.

Collapse behavior and forces of multistack nanolines.

Two types of multistack nanolines (MNLs), Si-substrate (Si)/siliconoxynitride (SiON)/amorphous Si (a-Si)/ SiO(2) and Si/ SiO(2) /polycrystalline Si (poly-Si)/ SiO(2) were used to measure the collapse force and to investigate their collapse behavior by an atomic force microscope (AFM). The Si/SiON/a-Si/ SiO(2) MNL showed a larger length of fragment in the collapse patterns at a smaller collapse force. The Si/ SiO(2) /poly-Si/ SiO(2) MNL, however, demonstrated a smaller length of fragment at a higher applied collapse force. The collapse forces increased by the square of the linewidth in both Si/SiON/a-Si/ SiO(2) and Si/SiO(2) /poly-Si/ SiO(2) MNLs. Once an AFM tip touches an Si/SiON/a-Si/ SiO(2) line, which is a softer MNL, it was delaminated first at the Si/SiON interface. One end of the delaminated line was first broken and then the other end was bent until it was broken. A harder MNL, Si/ SiO(2) /poly-Si/ SiO(2), however, was broken at two ends simultaneously after the delamination occurred at the Si/ SiO(2) /poly-Si interface. The different collapse behaviors were attributed to the magnitude of adhesion forces at the stack material interfaces and the mechanical strength of MNLs.

Convective assembly and dry transfer of nanoparticles using hydrophobic/hydrophilic monolayer templates.

A convective directed-assembly process on a flat substrate that does not require motion and is followed by a dry-transfer process of nanoparticles is presented. The convective assembly process was achieved using Au nanoparticles on hydrophobic/hydrophilic-surface-patterned Si substrates as functions of temperature, gap height, and particle size. An investigation of the particle assembly mechanism showed that the effects of temperature, gap height, and particle size were responsible for controlling the evaporation time, the evaporation length, and the assembly speed, respectively. To ensure conformal contact during the dry-transfer process, chemically patterned hybrid templates with elastic and flexible properties were fabricated and used. The hybrid templates provided conformal contact with a target silicon substrate coated with MPTMS (3-mercaptopropyltrimethoxysilane) and successfully transferred Au particles to the target substrates.

Directed assembly of high density single-walled carbon nanotube patterns on flexible polymer substrates.

We report an effective technique for the controlled assembly of single-walled carbon nanotubes (SWNTs) and demonstrate organized high density network architectures on soft polymeric substrates. We utilize the surface energy differential between a plasma treated (hydrophilic) parylene-C surface and a photoresist (hydrophobic) surface to create microscale patterns of SWNT networks on a 10 microm thick parylene-C substrate. The large scale fabrication of patterned SWNT structures presented is achieved by performing site-selective fluidic assembly of SWNTs. Electrically continuous nanotube network micro-arrays as small as 4 microm wide that are up to 1500 microm long with controlled separation have been fabricated by dissolving the photoresist after assembly. Electrical and mechanical characterization of nanotube networks on the flexible substrate in both static and dynamic modes indicates that the structure can handle both compressive and tensile deformations with no hysteresis. The technology presented has immediate applications in making thin film transistors, interconnects and sensors on flexible substrates.

Nanoparticle scanning and detection on flat and structured surfaces using fluorescence microscopy.

A new technique is proposed for the scanning and detection of nanoparticles on flat substrates and three-dimensional structures using fluorescence microscopy. This technique is utilized for particle removal measurements especially in semiconductor and hard disk manufacturing. This fluorescent particle scanning technique enables nanoscale particle detection. The technique shows that single particles down to 63 nm could be detected and counted. The technique is also capable of detecting particles in trenches that are as deep as 500 microm.