Heat and acid-wash the diamond single crystal particles using aqua regia (concentrated nitric acid + concentrated sulfuric acid) to remove surface contaminants such as metal impurities and oxides, thereby preventing the formation of thermal resistance at the interface due to impurities. After acid-washing, the diamond surface becomes highly polar, making it prone to adsorbing ions and impurities. Multistage ultrasonic cleaning (alkali ultrasonic, deionized water ultrasonic, anhydrous ethanol ultrasonic) is employed to ensure the cleanliness of the diamond single crystal surface.