One‐Step Room Temperature Synthesis of Printable Carbon Quantum Dots Ink for Visual Encryption and High‐Performance Photodetector

Abstract Carbon quantum dots (CQDs) have emerged as promising materials for optoelectronic applications and have garnered much interest as potential competitors to conventional inorganic or hybrid semiconductor quantum dots because of carbon's intrinsic merits of high stability, low cost, and environment‐friendliness. The ability of easy formulation of functional ink of CQDs is necessary for the development of industrial‐scale, reliable, inexpensive printing/coating processes, for its full exploitation in the ever‐growing class of applications in sensors, optoelectronics, and energy storage and conversion. Here a facile one‐step room‐temperature synthesis of printable, fluorescent CQD ink is demonstrated. The as‐synthesized fluorescent CQD ink is used for invisible fingerprint stamps, printing of micro‐patterns, and soft lithographic patterning with a resolution down to 1.5 µm. This functional CQD ink is also used to fabricate a high‐performance CQD‐ZnO heterojunction ultraviolet (UV) photodetector with a photo‐responsivity of 3.85 A W⁻¹, detectivity of 6.78 × 10¹⁰ Jones, and an external quantum efficiency (EQE) of 15.3%. The enhanced device performance can be attributed to CQD's high photocurrent generation efficiency and rational combination of the asymmetric electrode materials. This work enables a high‐temperature stable CQD fluorescent ink synthesis method to fulfill the processing requirements of printing and soft lithographic patterning for visual encryption and optoelectronics.