Effects of coffee ring via inkjet printing seed layers on field emission properties of patterned ZnO nanorods

Abstract

Inkjet printing, a fast, simple, efficient graphical deposition technology, is first applied to achieve high-quality emission arrays. Patterned arrays of ZnO nanorods have been successfully synthesized via hydrothermal method after inkjet printing the ZnO seed layer. During printing, different substrate temperatures were found to affect the morphology, microstructure and field emission (FE) properties of ZnO arrays. The results showed that the FE performance was improved when the coffee ring effect was eliminated by raising the substrate temperature due to higher aspect ratio of the nanorods. Both the compensating flow characteristics inside the droplets and the mechanism of regulating the rheological behavior of the solution during inkjet printing were analyzed to inhibit the effect of coffee ring, which played an important role in the later patterning electrode construction of emission arrays. The selective growth of the emitter material can be easily realized by introducing the direct patterning technology of inkjet printing in the preparation of field emission electron source.

Introduction

As is known, the patterned array of emission materials can improve the emission performance of electron source and facilitate the large-scale integration of electron source devices. There are many problems in traditional electronic source patterning preparation processes, such as microfabrication and vacuum deposition. These processes are complex, polluted and impurities are introduced during processing, which may destroy the material properties, resulting in emission current instability. In order to find a lower cost, more simple manufacturing process, we count on the direct pattern of technology, i.e. inkjet printing [1], [2], [3], [4]. Inkjet printing [5], an additive process, compared to the traditional semiconductor material processing technology, can make the cost of large-scale process more acceptable. Because of its full data-driven and maskless process, it is more versatile than other direct printing methods. The material can be deposited on the substrate in a carrier solution by piezoelectric driving. This solution process also makes the selection of deposited materials and substrates more flexible [6]. Patterned printing can be applied to any types of substrates (e.g. flexible polymer [7]) and used in large quantities of production or roll-to-roll processing of large substrates. Since inkjet printing can directly achieve patterned processing on the flexible and large area of the substrate, the substrate is not selective and the process is pollution-free. It has attracted wide interests in low-cost flexible electronics. Therefore, this paper is devoted to the application of inkjet printing to the preparation of field emission electron source. With the help of this technology, the patterning electrode construction of electron source and the selective growth of the emitter material can be easily achieved.

ZnO one-dimensional nanomaterials [8], which have a low turn-on field and high emission current density due to their wide band gap about 3.37 eV and high exciton binding energy around 60 meV, have become one of the most promising materials for field emitters. The traditional patterned self-assembling of ZnO is commonly performed by photolithography [9] or electron beam lithography [10] followed by selective etching by sputter deposition of zinc metal [11] or by chemical vapor deposition [12]. However, these technologies are complex and may introduce impurities during processing, which may destroy the field emission properties. In recent years, other methods of realizing the patterned ZnO seed layer have been reported, such as self-assembly monolayer by hydrophobic/hydrophilic interaction [13], [14], microcontact printing [15], atomic layer epitaxy (ALE) lithography [16]. The seed layers are subsequently grown to form the patterned ZnO nanoarrays in the processes of thermal oxidation [17], thermal evaporation [18] or hydrothermal reaction [9], [11], [19]. Nevertheless, these methods have many common shortcomings, including complicated steps, very time-consuming, high cost, low yield, strict requirements for preparation conditions (high temperature and pressure) and so on. Some methods are also subjected to practical limitations, such as the need for photolithography photomask or the mold for contact printing. And the lithography cannot be applied to plastic substrate which is heat sensitive or corrosive chemical sensitive. Therefore, the development of a fast, simple and efficient graphical deposition technology with accurate positioning is of great significance to achieve ZnO emission arrays with high-performance.

Inkjet printing has opened up a new field of research for the selective growth of nanomaterials because it can directly pattern the nanomaterials in the predetermined area to obtain the controlled morphology of the nanoparticles [4], [20] and nanowires [21], [22]. Besides, the growing position of ZnO nanoarrays may be easily changed during printing. Recently, Ko [23] has reported the formation of ZnO nanowires in direct local regions by inkjet printing of ZnO nanoparticles as seed layers. However, direct inkjet printing of nanoparticles or nanowires has a common drawback, that is, nozzle clogging, and the choice of ink in terms of concentration and viscosity is limited. In this regard, Kwon [24] proposed inkjet printing of zinc acetate precursor to get ink patterning instead of printing ZnO nanoparticles, followed by local hydrothermal growth of ZnO nanowires, which not only avoids the traditional method of multi- process, but also eliminates the frequent nozzle clogging.

In this paper, we reported the rapid patterning method of ZnO emitter arrays in the specific location of the field emission electron source device assisted by the precise deposition of ZnO seed layer on the cathode by inkjet printing. The effects of the coffee ring during printing the seed layers on the filed emission properties of patterned ZnO arrays were further studied. Meanwhile the presented method may break through the unstable emission problems caused by impurity in the traditional manufacturing process of electronic source and is expected to be applied to flexible substrates and large-scale industrial production.