*3.1. Materials*

The experiments were mainly conducted for the deposition of copper and the electrolytes were a mixture of 1 M CuSO4 and 0.005 M H2SO4 in deionized water with ratio 1:1 by volume. A nickel square was also printed to demonstrate the ability to print various materials. The electrolytes for nickel deposition were 113 gL−<sup>1</sup> NiSO4 · 6H2O, 30 gL−<sup>1</sup> NiCl2 · 6H2O and 23 gL−<sup>1</sup> H3BO3 [21].

## *3.2. Apparatus*

Three two-phase stepper motor stages (Zolix, Beijing) were employed to serve as the *x*-, *y*-, and *z*-stages. The stepper motor stages have the screw pith of *L* = 4 mm and step angle of *α* = 1.8◦ with *n* = 20 subdivisions, resulting in the positioning resolution of *Lα*/360*n* = 1 μm in Cartesian coordinates.

Three materials, namely the polished brass plate which was composed of 67%Cu and 33%Zn with the surface roughness Ra = 0.017 μm, the unpolished copper plate with Ra = 0.19 μm, and the polyethylene terephthalate (PET), were tested to serve as the substrate. The surface roughness Ra, which was evaluated by the arithmetic average of the absolute values of the profile height deviations from the mean line within the evaluation length, was measured using a Mitutoyo SJ-210 surface roughness tester with an evaluation length of 1 mm.

As shown in Figure 1a, a syringe was employed as the reservoir and a copper or nickel wire was inserted into the electrolyte to serve as the anode, while the substrate served as the cathode. The electrical potential was applied between the anode and cathode by a signal generator (Tektronix AFG3011C). The syringe with the print head was held onto the *z*-stage and the print head was lowered slowly until the tip touched the substrate where a current signal was recorded by a Keithley digital multimeter DMM7510. Then, the print head retracted 30 μm to form a stable meniscus.

A two-electrode system without the reference electrode was used in the present study. Since the active area of the anode (counter electrode) was far larger than that of the cathode (working electrode), the polarization of the counter electrode was expected to be small [27]. The deposition voltage was tuned to make the corresponding current ∼0.3 mA, which was reported as the optimal deposition current for copper in [22]. After a parametric study, the potential of 25 V between the anode and cathode was selected (the corresponding current recorded was ∼0.3 mA). Both the constant (25 V) and pulsed potential (25 V, 1 kHz, duty cycle = 0.3) were tested for copper deposition. For simplicity, a content potential of 25 V was also employed to print the nickel. It should be noted that the potential value selected was relatively larger than that reported in [25], in which a three-electrode system where a copper bar was used as the reference electrode was employed and a maximum potential of 6 V was applied between the reference electrode and the cathode. In the proposed design, the electrolyte needs to pass through the long and narrow main channel to the tip of the print head. As a result, the transport of the metal ions from the reservoir to the tip of the print head is limited due to the high width to height ratio of the main channel [28]. Therefore, the concentration of the metal ions through the long main channel is expected to be lower than that in the reservoir, which would increase the resistance of the whole circuit.

A commercially obtained fountain pen, as shown in Figure 4, was employed as the print head. The tip of the main channel was machined by a laser engraving machine (Branded Hans Laser) to control the width in the range of 50–300 μm. The width of the slit in the comb structure was ∼400 μm. The material used for the main channel and the comb structure was acrylonitrile butadiene styrene plastic (ABS).

Scanning electron microscopy (SEM) images and energy-dispersive X-ray spectroscopy (EDX) element mapping were obtained at high vacuum with operating voltage of 20 kV. Elastic modulus of the printed structure was measured by the nanoindentation test on an Agilent Nano Indenter G200, and microhardness tests were performed on a Fischerscope HM200.

**Figure 4.** The dimensions of the print head.

#### **4. Results and Discussions**

#### *4.1. Printing Speed*

To determine the maximum appropriate printing speed for the proposed ECAM system that can be achieved with a relatively high resolution and uniform morphology, lines printed at various printing speeds on the brass substrate were investigated. For simplification and convenience regarding the influence of frequency and duty cycle of the pulsed potential, a constant potential was simply applied. An optical microscope was employed to measure the printed line width. Figure 5a shows the printed copper line width as a function of printing speed. It can be observed that the printed line width decreases in an approximately linear fashion as speed increases, which indicates that a higher speed leads to a higher resolution of the printed line. However, a printing speed that is too high results in a non-uniform printed line width along the printing direction and serrated boundaries. For instance, as shown in Figure 5a, the line printed at 500 μm/s is much more uniform than that printed at 1000 μm/s. Further increasing the printing speed resulted in non-continuous printed lines, as shown in Figure 5b for the speed of 3000 μm/s. On the other hand, a printing speed that is too low not only impairs the deposition rate but also allows sufficient time for solvent (water) evaporation. As a result, the solute material (for instance, CuSO4 for copper deposition) readily precipitates on the substrate, such as for the line printed at 100 μm/s. Consequently, in the present study, 500 μm/s was chosen as the optimal speed.

For the speed of 500 μm/s, taking the diameter of the meniscus having the same dimension, i.e., 500 μm, and the current to be ∼ 0.3 mA, then the corresponding current density was 0.15A/cm2, which was comparable to the values reported in [3,21,29] (0.02–0.64 A/cm2).

**Figure 5.** (**a**) Printed line width as a function of printing speed. (**b**) An optical microscope image of the printed line with the speed of 3000 μm/s.

#### *4.2. Printing a Copper Square*

A copper square with the length of 2 mm was printed on the brass substrate. Two printing strategies, as shown in Figure 6a,b, with the constant and pulsed potential were investigated. The distance between two adjacent printing lines was 100 μm. For Printing Strategy 1 (see Figure 6a), the print head moves along the *x*-axis for all the layers; Ra*x* in Figure 6a is the surface roughness measured along the *x*-direction, which is parallel to the printing direction, while Ra*y* is the surface roughness measured along the *y*-direction, which is perpendicular to the printing direction. For Printing Strategy 2 (see Figure 6b), the printing direction rotates 90◦ after printing each layer, and the surface roughness Ra is measured for both the *x*- and *y*-directions. The Ra values shown in Figure 6c were averaged by measuring three samples, and, for each sample, at least five different locations were analyzed.

**Figure 6.** The average surface roughness, Ra, was investigated for two printing strategies. (**a**) Printing Strategy 1, where the print head moves along the *x*-axis for all layers. (**b**) Printing Strategy 2, where the printing direction rotates 90◦ after printing each layer. (**c**) Ra as a function of the number of deposition layers and printing strategies under pulsed and constant potential.

Figure 6c shows that Ra increases with the increasing number of deposition layers. For Printing Strategy 1, Ra*x* is slightly smaller than Ra*y*, which suggests that the surface is smoother along the direction parallel to the printing direction than that along the perpendicular direction. In contrast, the square obtained by Printing Strategy 2 with a pulsed potential has the smallest Ra, which indicates that the surface roughness can be improved by applying the pulsed potential with varying the printing directions during the process. The advantage of Printing Strategy 2 compared to Printing Strategy 1 is

that the upper-layer material can be filled into the gap and hole of the lower layer during the printing process, therefore some uneven areas can be repaired and the surface flatness is improved. The surface roughness shown in Figure 6c was measured using a Mitutoyo SJ-210 surface roughness tester with an evaluation length of 1 mm.

Figure 7a shows the printed square by depositing two layers on the brass substrate with Printing Strategy 2 and the pulsed potential. The yellow part at the boundary is believed that some electrolytes were accumulated at the boundary during printing and the zinc element in the brass substrate was dissolved in the electrolytes and then deposited on the substrate. This was confirmed by using an unpolished copper substrate and, repeating the same process for printing the square, the yellow part was no longer observed, as shown in Figure 7b. The proposed ECAM system also worked well on both the polished and unpolished substrates, and the stability of the meniscus was not affected. Further measurements showed that the square printed on the polished substrate resulted in a smoother printed surface (Ra = 0.036 μm) than that on the unpolished substrate (Ra = 0.233 μm). This is mainly because the actual surface area of the rough surface is larger than that of the smooth surface, therefore the current density on the rough surface is relatively small. If the local current density were not sufficient for the deposition of metal ions, there would be no metals deposited on that region. Hence, due to the worse uniformity of current density on the rough surface, the corresponding roughness of the printed structures is higher.

**Figure 7.** (**a**) A copper square printed on the polished brass substrate for two layers. (**b**) A copper square printed on the unpolished copper substrate for two layers. (**c**) SEM image of the side view of a copper square printed on the brass substrate for 32 layers. (**d**) Elastic modulus and Vickers hardness for the printed copper square. (**e**) EDX of the printed copper square.

The side view for the square printed on the brass substrate for 32 layers is shown in Figure 7c, illustrating the dense morphology and height of ∼12 μm. As the printing speed was 500 μm/s with 22 passes (each pass was 2 mm) for printing each layer, the total time for printing every layer was 88 s. Hence, Figure 7c suggests the thickness for each pass is ∼375 nm. This implies that the printing process has an exceptionally high *z*-height resolution, making it potentially useful for fabricating functional electronics such as thin-film sensors.

The elastic modulus *E* and Vickers hardness HV of the printed copper square on the polished brass substrate were measured by the nanoindentation and microhardness tests, respectively. Six points were tested and the results are shown in Figure 7d. It can be observed that the average elastic modulus of the printed copper is ∼75 GPa, while the average Vickers hardness is ∼200 HV. As a reference, Chen et al. [25] reported that the hardness of their printed copper structures ranged from 184 to 228 HV and the hardness of copper used in circuit bonding wires can range from 50 to 176 HV [30]. The processes for obtaining the elastic modulus and Vickers hardness are explained in Appendix A. The formation of pure elemental copper for the printed square on the polished brass substrate was confirmed by EDX, as shown in Figure 7e. The deposited copper of the test point can be as high as 96.83 wt% with 3.17 wt% of oxygen. Further SEM study showed the grain size of the printed square was as small as ∼1 μm.

#### *4.3. Printing Various Copper Structures*

Three letters, "S", "Y", and "U", were separately printed from corresponding computer-aided design (CAD) models with pulsed potential and the image of a combination of the printed letters featuring the pattern "SYSU" is shown in Figure 8a. As the letters were printed separately, to make a uniform background, the substrate is removed and only the letters are shown in Figure 8a. A typical SEM image of the letter "U" is shown in Figure 8b, illustrating the dense copper morphology without noticeable dendrites. Figure 8c is a close-up of the white boxed area in Figure 8b and clearly shows that the printed letter has a smooth surface. Multiple passes were applied for printing each letter and the printing time was ∼45 min. SEM images of the printed letter "Y" (Figure 7d) indicates that the printed copper structures are polycrystalline with the grain size as small as ∼1 μm. The letter "Y" shown in the boxed area in Figure 7d is an unedited image without removing the substrate.

**Figure 8.** (**a**) Images of printed structures featuring the letters "SYSU." Only the letters without the substrate are shown to make an uniform background. (**b**) A typical SEM image of the letter "U"; the zoomed-in view of the white boxed area is shown in (**c**). (**d**) SEM image of the white boxed area in letter "Y".

The letter "U" printed under pulsed and constant potential are also compared in Figure 9a,b. The SEM images show that the pulsed potential results in denser morphologies with sharp boundaries (see Figure 9a), whereas the structures printed under the constant potential have a porous morphology (see

Figure 7b). This is mainly because the pulsed potential can provide a higher current density, which favors the initiation of grain nuclei and leads to finer-grained deposition [31]. Again, The two letters "U" shown in the boxed area of Figure 9a,b are unedited images without removing the substrate.

**Figure 9.** SEM images of the letter "U" printed under (**a**) pulsed and (**b**) constant potential.

A few attempts were also made to print free standing copper pillars, which are shown in Figure 10. Figure 10a,b shows the printed copper pillars with slopes of tan *α* = 1 and 2, respectively, where *α* is the angle with respect to the *x*-axis. Figure 10c is a printed vertical pillar followed by a pillar with the slope tan*α* = 3, and Figure 10d is a mixed structure with the overhanging pillars. The red arrows indicate the movement paths of the print head, and the numbers in Figure 10d represent the printing path sequence. The printed pillars with slopes tan *α* in Figure 10a–c were achieved by alternately moving the print head a distance of *d* = 3 μm along the *x*-axis and *d* · tan *α* along the *z*-axis. The movement paths in Figure 10d were also realized by controlling the movement distances of the *x*- and *z*-stages via a computer program. The speed of the print head was controlled to a low value (3 μm/s) for both the *x*- and *z*-stages. The diameter of the printed pillars in Figure 10 is ∼50 μm, and dendritic morphologies can be observed in all structures. As explained in [25], when printing structures such as those in Figure 8, movement of the print head within the *<sup>x</sup>*–*y* plane results in mechanical removal of the dendrites. However, for the structures shown in Figure 10, without the aid of mechanical removal, dendrites are easily formed due to preferential deposition at the center of the meniscus, which could be improved by applying an appropriate type of potential [4,25]. Nevertheless, Figure 10 demonstrates the ability of the proposed ECAM system to print structures with small overhanging angles.

**Figure 10.** Images of 3D-printed Cu structures with different shapes. Red arrows indicate the movement paths of the print head. (**a**) A tilted pillar with slope tan*α* = 1, where *α* is the angle with respect to the *x*-axis. (**b**) A tilted pillar with slope tan*α* = 2. (**c**) A vertical pillar followed by a tilted pillar with slope tan*α* = 3. (**d**) Mixed structure with overhanging features. The numbers indicate the sequence of the printing path.

#### *4.4. Printing a Copper Circuit on the PET Substrate*

A circuit printed on the PET substrate was attempted to present a potential application with the proposed ECAM system. As the PET substrate was non-conductive, a thin copper film was first deposited on the PET substrate as the seed layer through the vacuum vapor plating technology. The thickness of the seed layer was 150 nm. This thickness ensured good chemical and mechanical properties of the deposited copper [32] and good adhesion between the substrate and the seed layer [24]. The proposed printing process was then performed on the seed layer to fabricate the designed patterns. Finally, a chemical etching process was employed to remove the seed layer and clean the substrate. The etching solution was composed of 0.4 M hydrochloric acid and 0.07 M FeCl3 and the etching process lasted 10 s. Before depositing the seed layer, the substrate was cleaned by rinsing with ethanol and ultrapure water and subsequent 10 min plasma. As shown in Figure 11, a "pulse signal" pattern was printed on the PET substrate. Multiple passes were performed for printing the "pulse signal" pattern and the printing time was ∼1 h. This example is shown as a proof-of-concept. Further investigations are needed to improve the quality of the printed circuit by optimizing the processing parameters and testing the corresponding electrical and mechanical properties.

**Figure 11.** A "pulse signal" pattern printed on the PET substrate.

#### *4.5. Printing a Nickel Square*

A nickel square was finally printed with the same configurations as printing the copper. The square was printed using Printing Strategy 2 (see Figure 6b) for 16 layers. The SEM image of the printed nickel square is shown in Figure 12a. The formation of the nickel was further confirmed by testing a small region of the printed square using the EDX element mapping, as shown in Figure 12b. The printed nickel was 86.47 wt%. The elements of copper and zinc tabulated in Figure 12b were believed to come from the brass substrate. The surface roughness Ra of the printed nickel square was 0.141 μm. Figure 12 demonstrates various materials can be printed by the proposed ECAM system. Future studies may incorporate the electrolytes for printing the nickel and copper and control the depositing sequence of different elements by manipulating the applied potential.

**Figure 12.** (**a**) The printed nickel square. (**b**) EDX element mapping for a small region of the printed nickel square and the corresponding wt.% of the detected elements.
