The surface potential measured under the probe is shown in figure 6.3. The area measured by the probe is ∼53 mm2 and thus each measurement overlaps each
other leading to smoother results.
Figure 6.3: Three different surface potential obtained from the voltage probe. The time when the plasma is turned off differs causing different values of the surface potential (a), b), and c)), however, the pattern does not vary.
According to [189] the charge density changes its sign during the period, how- ever, the shape is more or less constant. The charge density results, calculated using equation (4.7), are shown in figure 6.4. The cross section presented at the bottom is made along the y-axis. Figures 6.4a-b show a positively charged profile whereas figure 6.4c shows a negatively charged profile, thus the results presented are in agreement with this article. The profile has a ring-shaped, this is due to the cylindrical configuration of the plasma jet. The profiles are similar to those in [189].
Figure 6.4: Charge density on a acrylic sheet after a helium plasma jet treatment. Top: maps of charge density. Bottom: sections of the charge density crossing the center of deposition. Again, the time when the plasma is turned off differs causing different values of the surface potential (a), b), and c)) whist keeping the same pattern.
Simulation
Simulations of a helium plasma with similar parameters (dimension, pressure, maximum voltage, frequency, etc.) have been done. The surface charge distri- bution of figure 6.5 is taken from one of these simulations where the dielectric is assumed to be positioned at 3 mm distance from the orifice. At this distance, the bullet’s head maintains its annular structure. The simulation was performed
using COMSOL by M. Hasan at the University of Liverpool. This simulation is not detailed here and only serves as an aid to help understand the shape of the charges left on the substrate.
Figure 6.5: Simulation of the charge density section at different time after the end of the pulse. The origin of the x-axis is the center of the plasma plume. Made by M. Hasan, University of Liverpool.
Figure 6.6 shows the difference in the bullet structure for cases where the di- electric is assumed to be 3 mm and 10 mm from the orifice respectively. For 10 mm the structure changed from annular to spherical. The surface charge distribution for 10 mm case is not available at the moment
Figure 6.6: Simulation of electronic density of a helium plasma jet.
Problems and development
The difference of value between the simulation (8×10-5C m-2) and the measure-
ments (1.5×10-6C m-2) is almost 2 orders of magnitude. Several explanations can be made. The first one is the time between the bullet hitting the dielectric and the recording of the charges. In the simulation the charge densities are taken when the bullet hits the surface whereas in experiment, the measurements were taken several minutes after, this can also explain the difference in charge density. The way the plasma jet was switched off: by quickly reducing the high voltage signal, can affect the last plasma bullet by reducing its electron and ion density. Another explanation can be that the simulation used a positive pulse which in- duces a much higher displacement current compare to the continuous wave signal from the experimental plasma. The distance can also influence the result. If the jets is a bit further away from the surface compared to its simulation counterpart, the deposited charge on the surface will be also smaller than expected [189].
6.4
Results and Discussion
The electrostatic charge deposition of a pure helium APPJ was analysed, a ring- shape pattern was observed. This section examines the film composition of acrylic acid deposited by APPJs of different paramters. TOF-SIMS measurement allows detection of patterns in the deposition. Analysis of deposition for different jet parameters indicates their effect on the deposition and the plasma polymerisation.
6.4.1
XPS
The XPS results shown in this section are only a selected few of the analyses performed. Only the first batch of samples have been analysed using XPS. Tables 6.1 and 6.2 and graphs in figure 6.7 are focused on the carbon 1s binding energy. The values are averaged over measurements from three separate films . Table 6.1 corresponding to the values shown on the graph in figure 6.7a) give the percentage of the elementary composition of the carbons. In a pure acrylic or poly(acrylic) acid surface the results would be 33 % for CCOO, COO and C-H/C (see section 4.2.1 for more explanation). The important functional group in acrylic acid is the carboxyl group corresponding to -COOH. As the results show, the XPS can only detect the COO group and cannot tell if any other element is attached to it, be it hydrogen, another atom, or a molecular group. As the carboxyl group is of high interest, the surface was transformed using trifluoroethanol derivatisation (see section 4.2.1). This technique consists of selectively modifying only the carboxyl group (-COOH) into a new compound, -COOCH2CF3. The new surface can be
analysed by XPS where the CF3 is clearly visible (see figure 6.7b). The ratio
between CF3 over COO in the derivatised sample is the percentage of the COOR
detected being COOH, where R represented any molecule or group. The retention of carboxyl group, which is the percentage of measured carboxyl group over the theoretical percentage, is then calculated using the following formula:
[COO]underivatised
33 ·
[CF3]derivatised
[COO]derivatised
The retention was calculated to be 23.43 % using table 6.1 and 6.2. A retention of carboxyl group of ∼25 % is low, most low-pressure plasma polymerisation in the literature obtained a retention of 45 % and above [100]. However, published articles do not always perform derivatisation and often use only the ratio of
COO/33 as the retention assuming that all the COO are coming from the carboxyl group [30,100,103,120,123,169,170]1. One could note that if a similar assumption
was performed here a retention of 67 % could be claimed. Measurement: [%]
C=O / O-C-O CCOO CO COO C-H/C
17.03 0.19 28.86 22.08 31.83
Table 6.1: Values of the underivatised XPS.
Measurement: [%]
C=O / O-C-O CCOO CO COO C-H/C CF3
16.9 1.69 27.08 21.45 25.35 7.51
Table 6.2: Values of the derivatised XPS.
Figure 6.7: XPS results of the C 1s binding energy, with a) underivatised and b) derivatised sample.