# High-Resolution Printing of Various Electronic Materials by Electrophotography
## 2 Experimental Details  
Electrophotography is one of the printing methods widely used in office-use laser printers and photocopy machines. Commercial electrophotography for document printing includes: (1) charging a photosensitive drum, (2) image exposure, (3) development (making a latent image appear), (4) transfer, and (5) thermal fixing of toner powder. In the first step, the photosensitive drum is electrically charged and exposed to laser light to draw the printing patterns. The photosensitive drum surface excited by the laser is electrically neutralized by releasing the initially produced electrical charge, producing a latent image of the printed pattern. During the development process, electrically charged toner particles are attracted to the photosensitive drum by the electrical potential difference between the toner reservoir and photosensitive drum and form patterns. The developed toner pattern is transferred to the paper, and the toner particles are thermally fixed on the paper's surface by melting the resin component included in the toner particle. However, this process differs from that in this work in several respects. First, toner particles do not include resin in this study; all toner particles are bare electronic materials and do not include any additives, which is advantageous in the sintering process in the final stage of the printing procedure. The second difference lies in the sintering process. Many electronic materials cannot be thermally fixed at approximately 200 \({}^{\circ}\)C. Therefore, we adopted intense pulsed light (IPL) sintering. Third, we omitted (1) and (2) because this process is technically mature and common to any material. We investigated processes (3)-(5) considering a wide range of electronic materials, such as metals, conductors, semiconductors, and insulators. Figure 1a shows the developer consisting of Ag nanoparticles (NPs) and carrier particles [85; 86]. The carrier particle comprises the ferrite material and has a role in generating electrical charge in the toner particle by frictional charging and by prohibiting natural aggregation of the toner particle. Industrial carrier particles include a surface polymer layer to adjust frictional charging toward the document-printing toner, although the carrier particles we used had no surface polymer coating. Toner particles mixed with the carrier particles remain on the surface of the carrier particle due to van der Waals forces as well as to the image force between the charged particle and the conductive surface, as men  
tioned in ref. [86]. The carrier particles were provided by Powdertech Co., Ltd., Tokyo, Japan All carrier particles used in this study were the same. The Ag nanoparticles were purchased from Sigma-Aldrich (Burlington, MA, USA) and were used as received. The carrier particles and Ag nanoparticles were mixed to form the developer. The dark gray particles include approximately 10% Ag NPs and 90% carrier particles by weight, and the mixed powder had sufficient fluidity for the developer particles. The fluidity of the developer particles is essential for drawing high-resolution electronic circuits, which are evaluated using the repose angle [85]. Figure 1b shows the SEM image of the developer. The Ag NP toner particles were distributed on the surfaces of the carrier particles. In these appropriately mixed particles, the natural aggregation of Ag NPs was effectively prevented. Consequently, the Ag NPs maintained sub-micrometer particle diameters. Figure 2a shows a schematic of the electrophotographic printing equipment. The development system comprised a target electrode, developer reservoir, external voltage supply, and a motorized moving magnet. The developer above the magnet forms a brush-like structure along the magnetic lines of force called a magnetic brush. Figure 2b presents a photograph of a magnetic brush. As shown in the figure, magnetic brushes were formed above each magnet. A motorized mechanism drives the reciprocal motion of the magnetic brush along the developer reservoir.