Article pubs.acs.org/JPCB
Comparison of Plasma Polymerization under Collisional and Collision-Less Pressure Regimes Solmaz Saboohi,*,† Marek Jasieniak,† Bryan R. Coad,† Hans J. Griesser,† Robert D. Short,† and Andrew Michelmore*,†,‡ †
Mawson Institute, and ‡School of Engineering, University of South Australia, Mawson Lakes Campus, Mawson Lakes, Australia 5095 S Supporting Information *
ABSTRACT: While plasma polymerization is used extensively to fabricate functionalized surfaces, the processes leading to plasma polymer growth are not yet completely understood. Thus, reproducing processes in different reactors has remained problematic, which hinders industrial uptake and research progress. Here we examine the crucial role pressure plays in the physical and chemical processes in the plasma phase, in interactions at surfaces in contact with the plasma phase, and how this affects the chemistry of the resulting plasma polymer films using ethanol as the gas precursor. Visual inspection of the plasma reveals a change from intense homogeneous plasma at low pressure to lower intensity bulk plasma at high pressure, but with increased intensity near the walls of the chamber. It is demonstrated that this occurs at the transition from a collision-less to a collisional plasma sheath, which in turn increases ion and energy flux to surfaces at constant RF power. Surface analysis of the resulting plasma polymer films show that increasing the pressure results in increased incorporation of oxygen and lower crosslinking, parameters which are critical to film performance. These results and insights help to explain the considerable differences in plasma polymer properties observed by different research groups using nominally similar processes.
1. INTRODUCTION Plasma polymerization has been the subject of a considerable body of research spanning more than 40 years, and the process is in industrial usage for the fabrication of thin film coatings for a variety of applications. For example, plasma polymers are utilized for protective coatings1 and high-performance surface functionalization applications including surfaces for grafting,2−4 dielectric thin films,5 and nonfouling micropatterned surfaces.6 Yet, while plasma polymerization could be considered a mature technology, there are still many unresolved fundamental questions in regard to the physics of the plasma phase and the chemical reactions that determine the outcome of the process. While many useful coatings have been developed semiempirically, for example for bio applications,7 rationally guided optimization and scale-up from the laboratory to industrially viable processes remain less than straightforward due to insufficient understanding of fundamental physical and chemical processes that occur in the plasma phase of organic, depositing precursor vapors. A further impetus for renewed efforts on improved understanding of plasma polymerization arises from the potential utility of such coatings for new uses at the convergence of emerging technologies on the micro- and nanoscale.8 One reason why a fundamental understanding of plasma polymerization lags behind the detailed understanding that has been achieved for conventional polymerization methods is that there are no agreed standard experimental apparatus and protocols; most laboratories use their own design of plasma reactors. Different designs of the plasma geometry, different pressures, flow rates, applied power levels and frequencies, © 2015 American Chemical Society
impedance matching, and other factors make it very difficult to compare plasma polymerizations performed in different laboratories. This was highlighted in a recent international round robin study, which clearly showed that, despite using nominally the same plasma settings, very different plasma polymers of acrylic acid were produced.9 A key plasma parameter is the operational pressure in the vacuum chamber; it can vary by more than 2 orders of magnitude between laboratories, as recently surveyed by d’Agostino and Palumbo.10 They defined low pressure as being below 70 mTorr (0.09 mbar). An illustrative comparison involves the studies done over 25 years each in the laboratories of Griesser and Short, respectively: for historical reasons, the work of Griesser et al. has been at the higher end of the pressure range (0.13−0.4 mbar), whereas Short et al. have used pressures approximately 1 order of magnitude lower (2−4 × 10−2 mbar). Interestingly, as we show below, theoretical considerations reveal that the collisional behavior of plasma constituents (in the plasma sheaths around surfaces) varies markedly between these ranges: at the higher end, collisions play an important role (collisional regime) in these sheaths, whereas at the lower end, collisions are much less important (collision-less regime, although strictly speaking the collisional frequency is greatly reduced rather than zero). The distinction between collisional and collision-less regimes for sheath processes11 is an important boundary, as the flux and energy Received: July 29, 2015 Revised: November 15, 2015 Published: November 16, 2015 15359
DOI: 10.1021/acs.jpcb.5b07309 J. Phys. Chem. B 2015, 119, 15359−15369
Article
The Journal of Physical Chemistry B
1. Briefly, the chamber consisted of an earthed 30 cm diameter, 25 cm high steel cylinder. The RF electrode was 28 cm in
of ions in the deposition process are likely to differ markedly on either side. This is important as the flux of energy to the surface facilitates the chemical processes which lead to deposition; on impact, ions with kinetic energy can directly deposit or create surface radical sites which then enable radical species in the plasma to use chemical energy to deposit. It should be noted that energy may also be provided to the surface by photons from the glow discharge12,13 or high energy electrons.14 While the contribution from these energy sources should not be ignored, the energy provided by ions is orders of magnitude higher and therefore dominates. It is intriguing from literature reports that plasma polymers deposited in the two different regimes also seem to exhibit some consistent differences in properties; plasma polymers deposited under collisional sheath conditions tend to display significant postplasma oxidation upon aging in air,15,16 whereas plasma polymers deposited from collision-less plasmas have been reported to show little postplasma oxidation on storage.17 It also appears from published values that higher pressures tend to lead to higher deposition rates.10 However, the plasma reactors used by Griesser and Short and by other researchers, also comprise significant physical differences, which include reactor design and electrode configuration (internal electrode vs external coil), in addition to the operating variables of power, gas flow rate, and pressure. Though it would need systematic investigation to verify that this putative correlation between pressure, deposition rate and postoxidation applies across a wider range of reactor designs, one can at present surmise that these past reports can be reconciled in that a substantially higher incidence of sheath collisional processes would be expected to lead to a higher density of radicals incorporated into the growing plasma polymer films. In this study, we have investigated the question of how plasma polymerization might differ between collisional and collision-less conditions in the same reactor. For this, we have chosen the precursor ethanol which is attractive for producing plasma polymers with surface hydroxyl groups and good water wettability.18 Plasma polymers with hydroxyl surface groups are also of interest for example for biomaterials applications,7 and there has been considerable effort to make hydroxyl-rich surfaces,19,20 which provides context to the present study. However, while most work on plasma polymerization focuses on the analysis of the resultant coatings and at times their postplasma aging, often by XPS, here we complement surface analyses by in situ measurements of plasma physics parameters (sheath potentials, ion energies and fluxes, and plasma density). We show the remarkable role pressure (alone) plays in determining the nature of the plasma polymer and how differences in the deposited plasma polymers can be interpreted in terms of measurable plasma physics parameters used in the polymerization processes.
Figure 1. Schematic of the steel plasma reactor with interfaced mass spectrometer used. OctIV probe as shown sits external and is connected to the powered electrode. Also shown is the field of view of the camera.
diameter, and was located approximately 1 cm below the top of the chamber. The distance between the RF electrode and the earthed sample stage was 24 cm. The asymmetry of the system, defined as the earthed electrode area/RF electrode area was 4.8. The chamber was pumped down by a two-stage rotary pump to reach a base pressure of 7000 at nominal m/z = 27 amu (C2H3+) was typically achieved. Each sample was characterized by 5 positive and 5 negative ion mass spectra, which were collected from sample areas that did not overlap. All recognizable, clear (i.e., unobscured by overlaps) fragment ions from 2 up to 100 amu range were used in calculations. The peaks were normalized to the total intensity of all selected peaks. Multiple mass spectra were processed with the aid of principal component analysis, PCA.25 PCA was performed using PLS_Toolbox version 3.0 (Eigenvector Research, Inc., Manson, WA) along with MATLAB software v. 6.5 (MathWorks Inc., Natick, MA).
traverse the sheath without collision.22 A simple calculation shows that for an argon atom at a pressure of 0.001 mbar the mean free path is about 50 000 μm, while at 1.3 mbar it is less than 50 μm.11 Taking typical estimates made in argon plasma for ni and Te as 1016 m3 and 2 eV, respectively,26 we would anticipate sheath dimensions in the region of 200−400 μm.11 It should be noted that for ethanol, which is used in this study, the molecular radius is approximately double that of argon which decreases the mean free path even further.27 Therefore, we can readily anticipate the transition from a collision-less to a collisional sheath in the region of 0.05−0.26 mbar. Observation of the plasma glow can yield significant information upon the processes occurring within the plasma, in particular on the plasma density (degree of ionization) and homogeneity of the plasma. Photographs were taken of the glows in the 0.01 mbar and 0.07 mbar ethanol plasmas and are shown in Figure 2.
Figure 2. Glow of 0.01 mbar (left) and 0.07 mbar (right) plasmas of ethanol at 15 W. Arrow indicates the position of the cap of the plasma mass spectrometer.
From Figure 2 it can be seen that the glow within the plasma of ethanol is more intense at 0.01 mbar than at 0.07 mbar. This result fits with theory, as at lower pressure the average electron mean free path is greater and results in higher electron energies (electrons gaining energy from the applied electric field), and consequently on (inelastic) impacts with ethanol molecules (or fragments thereof) this results in greater fragmentation and a higher plasma density. In Figure 2 at 0.07 mbar the cap of the plasma mass spectrometer orifice is labeled (see arrow). Around the cap we can see a much more intense plasma glow at 0.07 mbar. This was also the case at the powered and earthed electrode surfaces. In relative terms, in the 0.07 mbar plasma the difference in the intensity of the plasma glow around immersed surfaces, e.g., mass spectrometer cap and electrodes, versus the bulk plasma is notably greater than that seen in the 0.01 mbar plasma. This phenomenon is well-known and reported where at high plasma pressure (generally at pressures >0.06 mbar) plasma contracts and concentrates around parallel plate electrodes and walls. This is due to the plasma transitioning from α to γ mode;28 at low pressure the mean free path of the electrons is sufficiently large that they can acquire enough energy from the RF field to sustain the plasma through inelastic collisions in the bulk plasma. This is termed α mode and results in a homogeneous bulk plasma. As the pressure increases, the electron mean free path decreases which lowers the average electron energy acquired from the RF field. Thus, while the electron-molecule collision frequency increases, ionization collisions in the bulk plasma decrease in frequency and the bulk plasma density decreases. However, electron-molecule collisions in the sheath also increase in frequency, resulting in free electrons being formed in the sheath. These secondary electrons are then
3. RESULTS 3.1. Plasma Glow and Validating Collisionless/Collisional Regimes. A plasma sheath forms around all objects in contact with the plasma.11 Ions crossing this sheath acquire energy according to the sheath potential, assuming that ions 15361
DOI: 10.1021/acs.jpcb.5b07309 J. Phys. Chem. B 2015, 119, 15359−15369
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The measured ion fluxes at low pressure 50 eV). These fast electrons may then ionize molecules near the sheath, resulting in increased plasma density near electrodes. It has been shown that this breakdown of the sheath at higher pressure results in increasing numbers of fast electrons leaving the sheath and a transition to γ mode of the plasma.29 3.2. Plasma−Surface Interactions. Ion Energy. In Figure 3, the effect of pressure on the ion energy of the (M+H)+ ion
Figure 3. Ion energy distribution (M+H+, m/z 47) as a function of pressure, showing the transition between collision-less and collisional regimes.
Ic(m) = Im(plasma on) − Im(plasma off)
(m/z 47) is shown for the fixed plasma power of 15 W and 4 different pressures of 0.01, 0.05, 0.07, and 0.26 mbar. The visual observation of the transition from collision-less to collisional regime is confirmed in Figure 3, where at 0.01 mbar the ion energy distribution is essentially that expected for a collisionless sheath, and by 0.07 mbar the sheath is almost entirely collisional, resulting in average ion energies of 14.5 and 3 eV, respectively. As shown in Figure 3, the transition between collision-less and collisional regimes occurs at around 0.05 mbar. At 0.05 mbar a bimodal ion energy distribution is observed corresponding to ions which traverse the sheath without collision (∼14 eV) and ions which lose kinetic energy due to collisions in the sheath (∼3 eV). This is consistent with previous measurements of oxygen plasmas at slightly higher pressure (0.15 mbar) which showed a broad energy distribution between 0 and 20 eV with specific peaks assigned to charge transfer reactions in the sheath.30 For the chemically complex plasmas used here (see below) these well-defined peaks are absent due to the wide range of reactions which occur in the sheath, each with a different energy loss. Above 0.05 mbar, the high ion energy peak has all but disappeared, and a broad low ion energy distribution is recorded (0.07 and 0.26 mbar). Ion Flux Data. Table 1 shows the ion fluxes recorded across the pressure range of interest at a fixed plasma power of 15 W.
Imonomer(plasma on) Imonomer(plasma off) (1)
The corrected results with plasma are given in Figure 4, and the key ions seen in these mass spectra are given in Table 2. The EI mass spectra (no plasma) at the two pressures are essentially the same, as would be expected. The peak at 28 m/z is assigned to C2H4 or CO, with possibly a small contribution
Table 1. Ion Fluxes Recorded Using OctIV Probe at Fixed Plasma Power (15 W) over the Pressure Range 0.005−0.26 mbar pressure (mbar)
ion flux (1018 ions/m2s)
0.005 0.008 0.01 0.05 0.06 0.26
0.122 0.114 0.110 0.438 0.525 1.605
Figure 4. Mass spectra of the neutral species at 0.01 and 0.07 mbar with plasma after correction for electron impact fragmentation in the spectrometer using eq 1. 15362
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The Journal of Physical Chemistry B Table 2. Peak Assignments for Ethanol Plasma Electron Impact Mass Spectrum peak (m/z)
assignments
19 26 28 31 43 45 46
H3O+ C2H2+ C2H4+, CO+ CH2OH+ C2H3O+ CH3 CH2O+ CH3CH2OH+
from N2 from residual air. Surface analysis presented later showed minimal incorporation of nitrogen species in the plasma polymer film, confirming air contamination was minimal. The 0.07 mbar spectrum compares well to that previously published for ethanol plasmas.34 The EI spectra with plasma closely resemble those with no plasma, and consequently do not show a high level of precursor fragmentation under this power (15 W) and these pressure conditions; this is consistent with low plasma power input at low pressure.34 However, comparing these spectra does reveal a marginally greater level of fragmentation in the starting compound at 0.01 mbar. The degree of dissociation can be calculated using eq 2. dissociation = 1 −
Ion(46) Ioff (46)
(2)
Figure 5. Positive ion mass spectra obtained from plasmas of ethanol at 0.01 and 0.07 mbar. Peaks assigned to oligomers after loss of methyl groups (*) and water (▲).
where Ion(46) and Ioff(46) are the intensities of the peak at 46 m/z with the plasma on and off, respectively. This yielded values of 0.4 and 0.2 at 0.01 mbar and 0.07 mbar, respectively. As expected, when the plasma is ignited the relative intensity of intact precursor peaks (e.g., at m/z = 45 and 46) decrease and fragment peaks (e.g., at m/z = 15, 26, and 28) increase. A very small peak at 2 m/z corresponding to H2+ was also observed as expected after fragmentation of hydrocarbons. As advised by the manufacturer, the sensitivity of the instrument for very low molecular weight species is quite low, particularly at low ion energy and thus this peak is probably underrepresented in the spectra shown. Consistent with published studies on mass spectrometry characterization of plasmas of other precursors there are no significant peaks observed with m/z higher than the mass of the precursor molecule (m/z = 46).35 At the higher pressure (0.07 mbar), the EI mass spectra with and without plasma are almost identical. The difference between low and higher pressure shows greater fragmentation of the precursor at the lower pressure (0.01 mbar). (Note that the electron flux in the EI is a constant.) Therefore, in the plasma there is more fragmentation at lower pressure (where the number of molecules/unit volume is lower). This fits theory as at lower pressure electrons gain more energy, as a result of the greater mean free path between collisions and this increases both the electron density through increased ionization, and the likelihood that any impact will have sufficient energy to break bonds. By switching off the EI source and retuning the optics at the front end of the mass spectrometer, it is possible to focus positive ions into the quadrupole and mass analyze. The positive ion mass spectra for 0.01 mbar and 0.07 mbar plasma of ethanol are shown in Figure 5. The main peaks observed in the positive ion mass spectra at both pressures are at m/z = 47, which corresponds to [M + H]+
and higher-mass peaks separated by mass M; [2M+H]+, [3M +H]+ and [4M+H]+ at m/z = 93, 139, and 185, respectively. These peaks were previously reported by Hazrati.34 These higher mass peaks correspond to formation of oligomeric species. For each oligomer peak, there is also a sequence of lower-mass peaks corresponding to the losses of H2O and CH3. The effect of increasing pressure is similar to that previously reported with power, i.e. a reduction in signal for the higher mass oligomers. The positive ion mass spectrum obtained from the 0.26mbar plasma of ethanol is broadly similar to that of the 0.07 mbar (see the Supporting Information, Figure S2). The fragmentation (and oligomerization) pathways for ethanol in plasma have been previously described by Hazrati et al.34 In both plasmas, oligomerization reactions ( ion−molecule reactions) are seen to give rise to ions in the series [nM+H]+. This phenomenon has been observed widely before in polymerizing plasmas, both in continuous wave36 and pulsed plasma.37 The fragmentation of these oligomeric ions to give further ions of mass greater than the starting compound is also well reported. In ethanol the m/z = 59 signal is particularly interesting. This corresponds to the loss of H2O and CH4 from the ion [2M+H]+. This ion is particularly favored at 0.07 mbar, but not at higher pressure (see the Supporting Information, Figure S2). Why this should be requires further investigation. In the tuning process, the optics of the mass spectrometer were tuned to accept ions at a specific energy (eV). Experimentally, this energy is determined by recording the ion energy spectra for a prominent ion, usually M+H+, and taking the maximum value in the ion energy spread, see Figure 3. In the case of the 0.01 mbar plasma, ions arise from outside the sheath region (that surrounds the mass spectrometer cap) and are therefore diagnostic of the bulk (positive) ion 15363
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The Journal of Physical Chemistry B Table 3. Results of Peak-Fitting the C 1s Coreline Spectra in Figure 6a pressure
C−C/C−H (%) 285 eV
C−O−C/C−OH (%) 286.5 eV
CO (%) 287.9 eV
O−CO (%) 289 eV
O (%)
C (%)
O/C
0.01 mbar 0.07 mbar
71.6 60.1
18.4 22.9
7.2 7.9
1.8 9.2
12.5 19.1
87.5 80.9
0.14 0.24
a These data show a significant increase in the proportion of carbons in more highly oxidized carbon environments and a higher O/C elemental ratio at higher pressure.
chemistry of the plasma.11 In the higher pressure plasma, ions originating from outside of the sheath are likely to have undergone collisions within the sheath region. In the acquisition of mass spectra at 0.01 mbar, because of the narrow ion energy distribution about the peak intensity, it is likely that a good (even a majority) proportion of all the ions falling through the sheath around the entrance of the mass spectrometer are being sampled. However, in the case of the higher pressure plasma (0.07 mbar), because of the spread in ion energies, this is definitely not the case. Based upon this (and the very similar count rates recorded) it is likely that the overall ion flux is substantially greater in the higher pressure plasma. 3.3. Plasma Polymer Surface Analysis. Of particular interest for the plasma processing community is the effect changes in the plasma phase and sheath with pressure have on the chemistry of the resulting plasma polymer. Therefore, we compared plasma polymers deposited from ethanol plasmas at 15 W, but at 0.01 (collision-less) and 0.07 mbar (collisional). XPS Data. The effects of transitioning from the collision-less to collisional regime on the plasma polymer surface chemistries are seen very clearly in Table 3 and Figure 6. Analysis by XPS reveals differences in both the surface elemental composition, expressed as O/C and in the nature of the surface oxygen− carbon functionalities.
retaining 100% of the precursor’s original structure. However, we acknowledge that ethanol does not polymerize in a conventional sense (ethanol is a fully saturated precursor) and therefore a surface O/C ratio of 0.5 is not achievable. For example, the peak in the neutral species at 28 m/z includes a contribution from CO, which does not become incorporated into the growing plasma polymer and is pumped out of the chamber. This peak is much higher for 0.01 mbar, thus resulting in lower O/C in the plasma polymer film. The greater O/C at higher pressure indicates less loss of oxygen from the precursor, but the rise in more highly oxidized carbons, for example in O− CO at the higher pressure. This most likely implies either a greater fragmentation/rearrangement of the hydroxyl in the plasma, or greater uptake of oxygen post plasma; the latter arising from trapped free radicals reacting with atmospheric oxygen. ToF-SIMS Data. XPS analyses of the plasma deposits of ethanol were supported by time-of-flight secondary ion mass spectrometry (ToF-SIMS). The positive ion mass spectrum for ethanol plasma polymer deposited at 0.07 mbar and 15 W is shown in Figure 7. The survey spectrum (Figure 7a) shows
Figure 6. Unfitted C 1s coreline spectra from plasma polymers of ethanol deposited at two pressures. The higher pressure (0.07 mbar) leads to a distinct broad signal at ca. 289 eV, indicative of more highly oxidized carbon environments.
Figure 7. Positive ToF-SIMS spectrum recorded with ethanol plasma polymer film (15 W, 0.07 mbar). (a) mass range 0−200 amu; (b) high resolution spectrum of the mass range 42−46 amu.
A marked increase in the surface O/C ratio occurs on moving from the collision-less to collisional regime. Peak fitting of the C 1s suggests surface rearrangement of the C−OH functionality to give a greater proportion of more highly oxidized carbons, including a marked increase in the proportion of carbon atoms in the O−CO environment. Based upon the precursor structure, we would expect a surface O/C of 0.5 if the plasma polymer were deposited
peaks that can be assigned to hydrocarbon fragment ions and also some that can be assigned to oxygenated ions. Highresolution spectra are, however, necessary to distinguish between ions that have very close-lying nominal masses, to assign them reliably, and to use them for PCA analysis; an example is shown in Figure 7b for the m/z = 42 to 46 region. It 15364
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Table 4. List of Positive Fragment Ions Present in ToF-SIMS Spectra of Ethanol Plasma Polymer Films Used in PCA m/z
fragment
m/z
fragment
m/z
fragment
m/z
fragment
m/z
fragment
12.00 13.01 14.02 15.02 16.03 25.00 26.01 27.02 28.03 29.04 30.04 37.00 38.01 39.02 40.03 41.04 42.05 43.05 44.06 50.01 51.02 52.03 53.04
C+ CH+ CH2+ CH3+ CH4+ C2H+ C2H2+ C2H3+ C2H4+ C2H5+ C2H6+ C3H+ C3H2+ C3H3+ C3H4+ C3H5+ C3H6+ C3H7+ C3H8+ C4H2+ C4H3+ C4H4+ C4H5+
54.05 55.05 56.06 57.07 58.08 62.02 63.02 64.03 65.04 66.05 67.05 68.06 69.07 70.08 71.09 72.09 74.02 75.02 76.03 77.04 78.05 79.05 80.06
C4H6+ C4H7+ C4H8+ C4H9+ C4H10+ C5H2+ C5H3+ C5H4+ C5H5+ C5H6+ C5H7+ C5H8+ C5H9+ C5H10+ C5H11+ C5H12+ C6H2+ C6H3+ C6H4+ C6H5+ C6H6+ C6H7+ C6H8+
81.07 82.08 83.09 84.09 85.10 89.04 90.05 91.05 93.07 94.08 95.09 96.09 97.10 19.02 29.00 30.01 31.02 42.01 43.02 44.03 45.03 46.04 47.01
C6H9+ C6H10+ C6H11+ C6H12+ C6H13+ C7H5+ C7H6+ C7H7+ C7H9+ C7H10+ C7H11+ C7H12+ C7H13+ H3O+ CHO+ CH2O+ CH3O+ C2H2O+ C2H3O+ C2H4O+ C2H5O+ C2H6O+ CH3O2+
54.01 55.02 56.03 57.03 58.04 59.05 60.02 60.06 61.03 68.03 69.03 70.04 71.01 71.05 72.02 72.06 73.03 73.06 74.04 74.07 75.04 79.02 81.03
C3H2O+ C3H3O+ C3H4O+ C3H5O+ C3H6O+ C3H7O+ C2H4O2+ C3H8O+ C2H5O2+ C4H4O+ C4H5O+ C4H6O+ C3H3O2+ C4H7O+ C3H4O2+ C4H8O+ C3H5O2+ C4H9O+ C3H6O2+ C4H10O+ C3H7O2+ C5H3O+ C5H5O+
82.04 83.05 84.06 85.03 86.07 87.04 87.08 88.05 89.06 92.05 94.04 95.05 96.06 97.03 97.06 98.07 99.04 99.08 100.05 100.09 18.03 30.03 44.05
C5H6O+ C5H7O+ C5H8O+ C4H5O2+ C5H10O+ C4H7O2+ C5H11O+ C4H8O2+ C4H9O2+ C3H8O3+ C6H6O+ C6H7O+ C6H8O+ C5H5O2+ C6H9O+ C6H10O+ C5H7O2+ C6H11O+ C5H8O2+ C6H12O+ NH4+ CH4N+ C2H6N+
shows that the peak at m/z = 43 in the survey spectrum consists of two close-lying but distinct peaks. The spectrum for ethanol plasma polymer film formed at 0.01 mbar and 15 W was qualitatively identical and is therefore shown in the Supporting Information (Figure S3). The difference between the spectra was in the relative peak intensities. Visual interpretation and comparison of these spectra could lead to erroneous conclusions due to subjective selection of a limited number of peaks; it is much preferable to interpret differences in the spectra by means of statistical analysis methods such as PCA. In Table 4 are listed 115 observed positively charged fragment ions in the mass range 0− 100 amu that were used for comparing ethanol plasma polymer films. Most of the observed peaks can be assigned to hydrocarbon and hydrocarbon plus oxygen ions, as expected for an ethanol plasma polymer and in agreement with the XPS data. There are also three peaks, albeit with low intensity, that are assignable to nitrogen-containing species, which presumably arose from N embedded in the plasma polymers due to the minor presence of residual air in the plasma reactor. Using multivariate analysis (PCA) to compare the ToF-SIMS spectra, the scores plot (Figure 8a) demonstrates statistically significant differences in the positive ion ToF-SIMS spectra for ethanol plasma polymer films fabricated at 0.01 and 0.07 mbar, respectively. The data, recorded on 8 separate areas of each sample, form two well-separated clusters along Principal Component 1 (PC1), which captures 93.43% of the original data variance and thus suffices to construct a model for the spectral differences. PC2 arises mainly from one outlier and at 3.65% can be neglected, as can higher PCs. Accordingly, the chemical differences between the two plasma polymers are adequately described by the peaks that load onto PC1. This loadings plot (Figure 8b) shows the positively charged ions whose intensity differs most between the two plasma polymers; peaks that are more intense for the polymer deposited at 0.07
Figure 8. PCA of positive ion ToF-SIM spectra for 0.01 and 0.07 mbar ethanol plasma polymers: (a) scores plot on PC1 and PC2; (b) loadings plot on PC1.
mbar load positively onto PC1, whereas peaks more intense for the polymer deposited at 0.01 mbar load negatively. Thus, the surface of the ethanol plasma polymer film prepared at 0.07 mbar and 15 W is characterized by more intense oxygen-containing fragments such as C2H3O+, C2H5O+, C3H7O+, and C4H9O+, and also by saturated hydrocarbon ions 15365
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The Journal of Physical Chemistry B with 3 or more carbon atoms (e.g., C3H7+, C4H9+, C5H9+, and C5H11+). This increased intensity of oxygen-containing ions accords with the XPS data. In contrast, the surface of the film prepared at 0.01 mbar and 15 W contains more intense unsaturated aliphatic hydrocarbon fragments such as C2H3+, C3H5+, C4H5+, and C5H5+, and also aromatics as evidenced by C6H5+ and C7H7+. The nitrogen-containing fragment ions did not differ significantly between these samples. Hence we can conclude that the composition of ethanol plasma polymers varies significantly with pressure; the collisional regime (0.07 mbar) leads to higher oxygenation and a larger proportion of saturated hydrocarbon structural elements, while the collisionless regime (0.01 mbar) leads to less oxygenation and more unsaturated/cross-linked hydrocarbon structural elements. To estimate the extents of cross-linking in the two ethanol plasma polymer films we applied a method used previously38−40 that is based on information provided by PCA, with the extent of cross-linking expressed by the C/H ratios derived from the average formulas calculated for the two sets of fragment ions that load positively and negatively, respectively, on PC1 (again, higher PCs can be neglected given the dominance of PC1). The average chemical composition CxHy was calculated for both sets. For oxygenated molecules, each oxygen atom was replaced by a CH2 group. Using this analysis, higher values of C/H indicate a higher extent of cross-linking. All fragment ions listed in Table 4 except H3O+, NH4+, CH4N+, and C2H6N+ were used in the calculations. Thus, for the ethanol plasma polymer coating produced at 0.07 mbar a C/H ratio of 0.53 was obtained, while the polymer deposited at 0.01 mbar gave a value of 0.82. Thus, the collisional plasma regime results in plasma polymer films which are higher in both oxygen and hydrogen and possess a lower degree of cross-linking. This is supported by density calculations of the plasma polymers, which showed that at lower pressure (higher cross-linking) the density was lower (1.4 g cm−3 compared to 1.7 g cm−3) due to cross-linking causing less ordered packing.
confirms the fundamentally different nature of the deposition processes in collisional and collision-less regimes. Comparison of these results with other groups is somewhat fraught, as other factors such as reactor geometry, pumping speed, RF power used, and coupling efficiency confound any comprehensive analysis. However, some general observations are possible. For example, in deposition of hard hydrocarbon films Peter et al.41 showed that increasing pressure resulted in lower ion energy, higher ion flux, and higher hydrogen content, consistent with the findings here. Similarly, Peppas and Hopwood42 showed that increasing ion energy decreased the retention of hydrogen, while simultaneously increasing hardness, presumably due to sputtering and abstraction of hydrogen and cross-linking of the film. Ruiz et al. deposited amine functionalized surfaces at high pressure (0.8 mbar) which showed good retention of functionality of primary amines but exhibited low stability in water, both of which could be explained as being due to low cross-linking.43 Differences in the aging of plasma polymers have also been observed. For example, aminated plasma polymer surfaces fabricated at high pressure have been shown to uptake oxygen significantly for up to 1 year postplasma,15,44 while a similar surface produced at low pressure was remarkably stable over the same time period.45 While we have been aware of the different levels of initial surface oxidation (and subsequent rates of oxidation) in plasma polymers, this was often thought to be due to reactor geometry; it has never previously been possible to draw out so clearly the effect of pressure. According to d’Agostino and Palumbo,10 who have conducted a retrospective study of papers published on plasma polymerization, over the pressure range
