*3.2. ZnO Ripple Pattern*

Based on the idea that ZnO ripple pattern growth mechanism is analogous to the coffee ring effect [27,29,30] and the solvent annealing technique [31], ZnO films were dried in the air at ambient temperature over various time periods (from 0 to 3 h) before annealing. After that, they were placed on a hot plate for thermal treatment over 10 minutes at 125, 150, 175, and 200 ◦C, respectively. Peak-to-valley roughness values (RPtoV) of patterned ZnO films, measured by AFM, are summarized in Table 2. The highest RPtoV values for each annealing temperature were as follows: 110 nm at 125 ◦C, 122 nm at 150 ◦C, and 113 nm at 175 ◦C, achieved when the drying time was 60, 10, and 5 min, respectively. Devices with a short drying time require a high annealing temperature for the optimum pattern, i.e., the highest RPtoV value. This result is reasonable, because short dried samples has more remaining solvent [32]. The sample will need a higher temperature to remove the remaining solvent, and consequently, to form the pattern. During the drying process, the films will partially dry, and small ZnO particles (cross-linked ZnO precursors) begin to form inside the film. These particles will travel, following the convection current of the solvent during thermal annealing of the film. Changing the drying time affects the initial size of seed crystal particles and the amount of the remaining solvent, which modifies the convection flow and the coffee ring effect [29,32]. The device with the highest RPtoV was achieved at 150 ◦C after 10 min of drying time. The RPtoV values were abruptly decreased with a long drying time (3 h) and a high annealing temperature (200 ◦C). The lack of remaining solvent due to the long drying process or fast evaporation at high temperature hinders the convection flow, and consequently, the rearrangement of ZnO precursor to form a 3D pattern.

The AFM images of the best nano-ripple pattern fabricated by a low temperature process, and the pattern obtained by ramping method, are shown in Figure 3. Pattern sizes with the novel low temperature method (Figure 3a) showed similar RPtoV value to that achieved by the ramping method (Figure 3b). RPtoV value of ZnO film fabricated by ramping method was 135 and 120 nm for drying

time of 10 min followed by annealing temperature of 150 ◦C during 10 min. Finally, we succeed on the fabrication of 3D nano-patterned ZnO film, without using a high temperature, by introducing the drying step in the process.


**Table 2.** RPtoV value of ZnO films with different drying times and annealing temperatures (units in nm).
