I+D+i

Growth progresses in several stages during electrode deposition. At early times, copper nucleates predominately on top of the platinum electrode. The nuclei grow and quickly form a hard packed film. The hard packed film grows vertically until it reaches a steady thickness dictated by the thickness of the liquid cell (200 nm in this case). Once it mostly occupies the thickness of the channel, the growth becomes essentially two-dimensional. Figure 50 illustrates the growth where gray scale is representative of time with lighter gray value being later times. This view is supported by the bright field intensity images as discussed in then next section.

Figure 49: Schematic of electrode configuration with standard 3-electrode setup labeled with the Working (WE), Reference (RE), and Counter (CE) Electrodes. The gray area represents the nanochannel where the solution is loaded.

Figure 50: Illustration of Copper growth modes. The copper first nucleates at arbitrary sites, then forms a hard packed film which grows vertically until it reached the top of the device where it then grows perpendicular to the electrode in a essentially 2D fashion.

6.3. 2D Growth - Constant Height Assumption

Next, we consider the predominately two-dimensional growth assumption. That is, the copper deposited on the electrode, grows upward to the liquid cell ceiling and then grows out laterally from the electrode with a constant height. We investigate the intensity map of the deposition to establish this fact. We measure the intensity along a line perpendicular to the electrode as illustrated in Figure 51a. The mea- surement is averaged over 10 pixels in the direction transverse to this line (parallel to the electrode) at each time instance during the deposition to minimize noise. The intensity profile is extracted for each frame and plotted in the contour plot of Figure 51b. The lower the intensity (the darker the image), the greater the thickness of the copper deposited. The contour plot supports the hypothesis that the copper reaches a constant thickness and continues to grow in an essentially two-dimensional fashion.

Figure 51cdepicts the normalized intensities as functions of time at a representative location on the electrode and off the electrode. The intensity has been normalized by the background intensity (blue line inFigure 51b) to adjust for drift in the electron beam. The black line, depicting intensity at the electrode location, drops quickly from its initial value and most of the copper is deposited within the first 1 s. The copper continues to thicken at a much slower rate that we take as negligible. The off electrode region has to wait for the interface to progress before its intensity is low- ered by the presence of copper. Once the interface has travelled through the probed location, the signal intensity quickly drops.

6.3.1. Confirming Imaging Sensor is Not Saturated

Since it is possible to saturate the sensor in an imaging system either toward white or black, it is necessary to ensure that intensity values are meaningful. Seeing if

Figure 51: Sample frame from electrode deposition at 400 nA from Appendix Video B.10 indicating (a) image intensity extraction location as red vertical line. White dotted line represents electrode edge location. (b) Contour plot of extracted intensities as a function of distance from the electrode (along the vertical line in (a)) and time. The black line is located on the electrode, the red is off the electrode, the blue line is the background intensity, and the white dotted line represents the electrode edge. (c) Normalized intensities as functions of time for the points indicated inFigure 51b. Black and red points correspond to intensity normalized by the background intensity of each frame. Blue points represent the background intensity normalized by the initial frame intensity to illustrate the decay in the electron beam (due to the thermal emission source).

the intensity measurements in the region analyzed lie between the maximum and minimum intensity level can serve as this check. Figure 52a the time history of the intensity at the four regions indicated in Figure 52a. We plot the mean and standard deviation of each region in Figure 52b. First, we notice the edge of the silicon wafer near the right edge of the image. Second, we see the electrode splitting the scene nearly horizontally, with the darker region in the top half of the image being the Pt electrode. The bottom left portion (the brightest in these bright field images), is a region that consists only of the two silicon nitride windows separated by the solution. We refer to these regions as the window off-electrode, R1 (background), thewindow on-electrode,R2, thesilicon off-electrode,R3, and thesilicon on- electrode,R4. By tracking signal intensities in these regions as functions of time, we find that the region over the electrode and under the silicon edge always has a lower intensity value than any other region, implying that the intensity measures used in the other regions have not saturated the sensor toward its black level (0).

Figure 52: Further probing of intensities to ensure proper white balancing of image system (no saturation of detector). Identification of image regions (a) and their associated mean intensities and standard deviation as error bars (b).