Upconverting Nanoengineered Surfaces: Maskless Photolithography for Security Applications

The two complementary technologies of colloidal upconverting nanoemitters and maskless photolithography are exploited to fabricate nanoengineered optically active surfaces for anticounterfeiting applications based on the multiphoton absorption phenomenon in lanthanide nanocomposites with a visualization wavelength in the NIR. It is demonstrated that the unique optical, thermal, and temporal characteristics of these versatile upconverting surface distinguishes them from their counterparts. A unique behavior that is captured is the ability to actively tune their emission color by modifying the pumping power, temperature, and excitation frequency. A new low-cost negative photoresist is employed for implementation of maskless photolithography of single- and double-color labels using two efficient upconverting nanocomposites based on NaYF4:Yb3+, Er3+ and NaYF4:Yb3+, Tm3+ nanoemitters. In addition, it is shown that the detectability of the proposed anticounterfeiting approach can be carried out using just a smartphone. Each of the emission peaks of the upconversion nanoparticles is associated with a different multiphoton absorption mechanism and their thermosensitivity varies from one peak to another. Furthermore, their photoluminescent color changes by scanning the excitation beam impinging on the surfaces composed of both upconversion nanoparticles doped in the UV-curable resist. Long-term photostability of these surfaces under continuous excitation by a high power laser makes them a promising nanoemitters for the next generation of anticounterfeiting labels.