Inkjet printers have been developed to be capable of printing various materials and devices; which includes OLEDs, conducting polymers, ceramics and so forth. In this research, patterned colloidal nano-particles could be formed by one step process via micro-chemical react-ions between reactive ink-drops from inkjet printer having more than two inkjet heads. Each of the two inkjet heads prints different inks, which are in a complete solution state, and these inks react with each other to be synthesized to form colloidal nano-particles after they are dropped on the exact position. This is the reason why we call this novel scheme; Drop and Synthesis (DAS.) Unlike conventional inkjet printing, colloidal nano-particles do not exist in any of the inks, thus the probability of inkjet head clogging can be significantly reduced.
Based on the method described above, ZnS:Mn, a well-known phosphor studied in the past decades, is presented. Experimental results show that these two inks are synthesized very well and the synthesized nano-particles emit orange-yellow light when excited by 324 nm UV light.
Scanning electron microscopy (SEM) images show the colloidal nano-particles having a size of average 20 nm. At the same time, the layered structure from the SEM pictures can be observed. Meanwhile, X-ray diffraction pattern identifies the colloidal ZnS:Mn nano-particles as nearly stoichiometric cubic. We can also verify additional XRD peaks apart from Wurtzite-2H (ZnS) reference peaks are attributed to the layered structure formed by residual chemicals and PVP.
Photoluminescence and cathodoluminescence spectra results indicate that the three inkjet-printed ZnS:Mn samples with different amount of PVP have two peaks at 590 nm and 430 nm. The magnitude of the peaks at 590 nm and 430 nm decreases and increases respectively as the amount of added PVP increases. Such difference is discussed in conjunction with $^4T_1$ to $^6A_1$ transition of $Mn^{2+}$ ions and the “self-activated” colloidal ZnS due to sulphur vacancies in the lattice.
In addition, inkjet-printed poly(3,4-ethylenedioxythiophene) (PEDOT) using the proposed DAS method is presented. Spin-coated PEDOT confirms the existence and promising properties. Inkjet-printing PEDOT using DAS method itself is a success, but further improvements are required such as enhancing the surface coverage rate.
