A main challenge in inkjet printing technology is controlling the amount of material deposited on a substrate. Not only is it essential to minimize the features size of dots via limiting ink spreading on the substrate, but it is also important to control thickness. According to Dimitrijev et al. 2005 and Wood et al. 2000, film thickness is an important factor that effect on functionality of a device. For instance, the performance of a device relies on the thickness of material that deposit on al layers of a thin-film light-emitting device as reported by Adachi et al. 2007; Schrage et al. 2010, Manzoor et al.
2003, Hieronymas et al. 2002, Cho & Cha et a. 2009. The inkjet printer can overcome these issues by ink formulation including wetting and spreading of droplets. Although the size of the droplets determine the amount of the material ejected on the surface, the final shapes and morphologies of the dots is mainly affected by the spreading of droplets on the surface, the density of the deride film, and the amount of solvent evaporation. As we
As we know, the composition of a solvent mixture would affect the particle deposit pattern, but all the ink with varying solvent compositions were not suitable to form stable jets.
5.4. Experimental
The alcohol-based silver ink with 30-35% nanoparticle weight percentage is dispersed by ethylene glycol (EG) as a solvent. It acts as reducing agent [74] to various concentrations to form the ink for testing on the inkjet printer. It was reported that EG is a reducing agent and stabilizer as well [75]. The experiments are carried out under the same conditions and the silver ink is dissolved at room temperature. Different concentrations of silver nanoparticles are presented in Table 5.1.
Table 5.1 Different concentration of silver nanoparticles Sample#1 Sample#2 Sample#3
EG 4ml 19ml 29ml Silver ink 1ml 1ml 1ml
All samples are produced by a Dimatix Materials Printer 2831 (DMP2831) with only a single calibrated nozzle and the ink tank are filled with formulated inks. As we have found out that FDTS coated nanostructured substrates with NCAs is a good candidate, we continued our experiments with theses nanostructured substrates in order to get high resolution colour images.
5.5. Results and discussion
The behavior of diluted colloidal droplets placed on substrates vary according to whether the liquid wets its surface. The formulated inks with varying solid loading produced drops deposited onto substrates. The results have been compared to experimental measurements of the drop profile recorded for different samples. Each droplets undergo different spreading after drying, thereby the density of silver nanoparticles in dried dots differ from each other.
In these section, FDTS is used to obtain a hydrophobic surface on which the silver ink retracts to form a thick silver layer (dark silver mode). We assess the shapes and morphologies of inkjet printed dots by various concentrations of silver nanoparticles.
The morphologies of droplets on polymer substrates are evaluated by an optical microscope and scanning electron microscope (SEM).
5.5.1. low-wettability surface with original silver nanoparticles
Figure 5.1 Experimental results of printing silver ink with 30-35% nanoparticle weight on a polymer nanocone array for green diffractive colour. (a) the optical microscope image shows the printed dots in green pixels. (b) a SEM image (tilted angle at 30°) of two silver dots printed on the nanocone array. (c) a SEM image (tilted at 30°)of center of printed dot. (d) a SEM image (tilted at 30°) of the region 2 of printed dot. (e) an overview of region 1, 2, and 3. (f) a SEM image (tilted at 30°)of unprinted area. Scale bar: (a) 56µm.
The results of the printing of the green colour pattern with the original silver ink is
That is why, no periodic morphology from the nanocones can be preserved into the silver film. This is why, we do not laminate the printed samples. In part (e), it is also observed that the thickness of the printed dots is decreased at the edge of the printed dots.
5.5.2. Low-wettability surface with diluted silver ink (Ag:EG=1:4 ratio)
Figure 5.2 Experimental results of printing silver ink (sample #1) on a polymer nanocone array for green diffractive colour. (a) the optical microscope image shows the printed dots in green pixels. (b) a SEM image (tilted angle at 30°) of single silver dot printed on the nanocone array. (c) a SEM image (tilted at 30°)of center of printed dot. (d) and (e) are inset images to show details in different regions (tilted at 30°). (f) a SEM image (tilted at 30°) of unprinted area. Scale bar: (a) 65µm.
The results of the green colour dots printed on the low-wettability surface are illustrated in Figure 5.2. Optical images showing the dots printed in green pixels
laminated with a transparent index-matching cover film are shown in Figure 5.2 (a). This concentration of ink diluted with ethylene glycol (Ag:EG=1:4 ratio) allows to form a thin silver film conformally coated on the nanocone surface, which assists to preserve the periodic morphology of the printed region as shown in Figure 5.2 (c). This leads to remain printed region active in diffracting light after lamination.
Low-wettability surface with diluted silver ink (Ag:EG=1:19 ratio)
Figure 5.3 Experimental results of printing silver ink (sample #2) on a polymer nanocone array for green diffractive colour. (a) the optical microscope image shows the printed dots in green pixels. (b) a SEM image (tilted angle at 45°) of single silver dot printed on the nanocone array. (c) a SEM image (tilted at 45°) of center of printed dot .(d) and (e) are inset images to show details in different regions (tilted at 45°). (e) a SEM image (tilted at 45°) of unprinted area. Scale bar: (a) 65µm.
We find that the printed samples with silver ink diluted with ethylene glycol (1:20
respectively. The Coffee ring effect that caused by ink transport from the center of dot toward the edge is noticeable, resulting in a decrease the brightness of printed silver dots.
5.5.3. Low-wettability surface with diluted silver ink(Ag:EG=1:29 ratio)
Figure 5.4 Experimental results of printing silver ink (sample #3) on a polymer nanocone array for green diffractive colour. (a) the optical microscope image shows the printed dots in green pixels. (b) a SEM image (tilted angle at 45°) of single silver dots printed on the nanocone array. (c) a SEM image (tilted at 45°)of center of printed dots.(d) and (e) are inset images to show details in different regions (tilted at 45°). (e) a SEM image (tilted at 45°) of unprinted area. Scale bar: (a) 65µm.
In Figure 5.4 (a), a very thin silver film is printed on the nanocones, and silver ink diluted with ethylene glycol (Ag:EG =1:29 ratio) does not provide enough optical contrast to diffract light and operate as desired. By contrast Figure 5.4 and Figure 5.3, in region 2 for both samples, the particles are aggregated, but the brightness of the printed silver dots in printed sample #2 is higher than that in sample #3. This is because dots printed
with silver ink (Ag:EG=1:19 ratio) have a higher diameter ratio due to its higher density.
In fact, more particles transfer to the edge of the dot.