Nanomaterial Coating on Micro-Hotplate By EHD Inkjet Printing with in-Site Infrared Laser Heating

Abstract

Introduction EHD inkjet printing was applied in the fabrication of micro gas sensors recently [1-3]. In most cases, the evaporation of the ink solvent is slow, which makes thickness and uniformity of the film difficult to control. This work introduces an IR laser heating source to the EHD printing system of previous work [2], which accelerates the evaporation of the as-deposited ink solvent on the MHP, so that thick films and even columnar structures can be deposited without the use of special ink solvent.This paper reports a novel in-situ infrared (IR) laser aided electrohydrodynamic (EHD) inkjet printing technique which can deposit hierarchical micro/nano-structured gas-sensitive materials on micro-hotplate (MHP) for micro gas sensor fabrication. Gas-sensitive materials can be deposited quickly on an MHP to form flatten thin film, thick film, and 3D columnar structures. Method The diagram of the printing system is shown in Figure 1. An 808 nm IR laser source with adjustable output power is mounted on the EHD inkjet printing system, focusing at the printing spot with a diameter of about 1 mm. The freestanding MHP with metal electrodes embedded in SiO2 membrane can be heated up to 200oC in seconds by adsorption of the IR power. After EHD printing of the ink onto the 100 µm * 100 µm area of the MHP, the IR laser is turned on with optimized output power and time interval to heat the MHP to a temperature that the solvent evaporated rapidly and the dry gas-sensitive materials are uniformly covered on the surface of the MHP. By repeating the EHD printing and IR heating process, thickness of the film increases regularly and thick film can be prepared. In the preparation of columnar structure, by shrinking the amount of ink droplet and accelerating the IR heating, a tip appears on the surface of the deposited materials. The electric field intensity is larger at the tip since it shortened the distance to the needle, therefore, the electric force drives the ink droplet to deposit on the tip and the tip continues to grow, forming a 3D columnar structure. Results and Conclusions The temperature of the MHP changes with the power of the IR laser in a linear relationship (Figure 2). The flower-like tin oxide powder doped with 3 at% palladium was prepared as the hierarchical-structured gas-sensitive material with a hydrothermal method (Figure 3). The ink was prepared by mixing the powder, DI water and surfactant in a proper proportion, and it was EHD printed on the surface of the MHP (Figure 4). Without IR heating, the solvent did not evaporate completely even after 600 s at room temperature, making it not suitable to print again. By turning on the IR laser with 450 mW for 5 s, the ink was dried and uniform thin film with thickness dfilm<1 µm was coated on the MHP as shown in figure 5 (b). Repeat the printing-heating cycle for 4 times only took 100 s, and thick film of dfilm ≈ 40 µm was obtained as shown in Figure 5 (c). Figure 6 compares the power consumption of the MHP before and after printing, which shows that the MHP with thick gas sensitive film remains good thermal isolation property. The printed 3D columnar structure with height of about 120 µm is shown in figure 5 (d). Increasing the power of the laser can accelerate the evaporation. However, when the IR laser power is more than 500 mW, the droplet on the MHP boil violently and nanomaterials splashes all around.For the first time, the IR aided EHD printing allows the printing of flatten thick gas sensitive film composed of hierarchical-structured nanomaterial on MHP. Also, the technique can further be applied in fabrication of other MEMS devices.This work is supported by the Nature Science Foundation of China (61874018 and 61274076), the Fundamental Research Funds for the Central Universities, and the Nature Science Foundation of Liaoning (20180550923).