Chemical and Elemental Depth Profiling of Very Thin Organic Layers

Jun 11, 2012 - We present a new synchrotron X-ray photoelectron spectroscopy strategy for surface chemical analysis of materials. Our approach is base...
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Chemical and Elemental Depth Profiling of Very Thin Organic Layers by Constant Kinetic Energy XPS: A New Synchrotron XPS Analysis Strategy Pierre-Luc Girard-Lauriault,*,†,‡ Thomas Gross,† Andreas Lippitz,† and Wolfgang E.S. Unger*,† †

BAM Bundesanstalt für Materialforschung und -prüfung, 12200 Berlin, Germany Department of Chemical Engineering, McGill University, Montréal, H3A 2B2, Canada

‡

S Supporting Information *

ABSTRACT: We present a new synchrotron X-ray photoelectron spectroscopy strategy for surface chemical analysis of materials. Our approach is based on the acquisition of photoelectron spectra at constant kinetic energies with the help of a tunable synchrotron X-radiation source. This ensures both constant and tunable information depth for all elements in a very thin organic layer. Many of the problems known to XPS depth profiling using laboratory equipment are thereby avoided. Using our methodology, the 95% information depth, z95%, can be tuned down to about 0.7 nm in organic materials. The upper limit in our study at the HE-SGM monochromator dipole magnet beamline at the synchrotron radiation source BESSY II is about 4.3 nm. Elemental quantification is achieved through relative sensitivity factors (RSF) specific to the measurement conditions, determined either with the help of calculated photoionization cross sections and inelastic mean free paths or experimentally. The potential of the technique is demonstrated for the in-depth analysis of plasma deposited nitrogen-rich organic thin films used in biomedical applications.

C

complicating the quantification in materials having inhomogeneous subsurface chemical profiles. A widely used strategy to deduce the surface chemistry by XPS is shallow depth profiling by angle-resolved XPS (ARXPS), a technique that has been applied successfully to analyze many materials, including organic layers.1−10 In recent years, a new strategy involving synchrotron radiation, energyresolved XPS (ERXPS), permitting increased surface sensitivity and depth profiling over a wide range of depths, has been developed. The potential of this technique was demonstrated for the fields of catalysis and organic materials.11−18 The improvements brought by ERXPS over existing techniques are numerous and fully justify the use of synchrotron radiation in many cases: it is significantly more surface sensitive, it is much less prone to systematic errors (because the geometry of the experiment remains unchanged), it is less sensitive to surface roughness,12 it is less susceptible to elastic scattering, and the relevant experimental parameters permitting one to calculate depth sensitivity are known with better precision (electron energy in ERXPS versus collection angle in ARXPS). In previous contributions,19,20 we have presented how ERXPS, in combination with ARXPS (AR-ERXPS), can be used to gain an enhanced understanding of the surface chemistry of nitrogen- and oxygen-containing plasma-polymerized films by providing chemical and elemental analysis at

hanging the surface chemistry of a material has been vital to the emergence and the refinement of many technologies. In biomedical applications, for example, only the outermost molecular layer can be “sensed” and “recognized” by cells. The surface chemistry can often differ significantly from that of the bulk material, and control over this parameter is gained through suitable characterization techniques, which vary in near-surface specificity and in the nature of information they can provide. New techniques and strategies permitting a reliable characterization of surfaces are therefore needed to improve our understanding of the surface chemistry effects in multiple applications. The most often used techniques for near-surface characterization include X-ray photoelectron spectroscopy (XPS), timeof-flight secondary ion mass spectrometry (ToF-SIMS), Fourier transform infrared spectroscopy (FTIR), and contact angle goniometry. XPS yields information on elemental composition (all elements except hydrogen and helium) and on the chemical state of the elements composing a material by measuring the intensity and the energy of photoelectrons originating from core-levels. The analysis is, in many cases, nondestructive, and near-surface specificity is due to the small value of the inelastic mean free path (λ, IMFP, a function of kinetic energy) of photoelectrons inside the material; for the case of organic solids, it is in the order of 3.5 nm under the conditions that usually prevail in laboratory XPS instruments that employ Al Kα or Mg Kα X-radiation. However, this value is much higher than the thickness of the layer defining surface properties (a few tenths of nanometer) and varies for different elements which yield photoelectrons of varying kinetic energy, thereby greatly © 2012 American Chemical Society

Received: March 7, 2012 Accepted: June 11, 2012 Published: June 11, 2012 5984

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Figure 1. Schematic presentation of the constant kinetic energy XPS strategy: (a) Standard laboratory XPS analysis of a homogeneous sample or (b) of an inhomogeneous sample (unreliable quantification model); (c) CKE-XPS analysis of inhomogeneous sample: a better quantification model due to a constant information depth for all elements.

where ϕ is the work function of the spectrometer. The intensity Ii,j of a detected signal originating from an orbital j of an element i, as a function of the angle of emission (θ) and excitation energy (ℏν), is proportional to the photon flux, F, which itself is a function of ℏν according to the transmission characteristics of the monochromator in the synchrotron light beamline, the transmission function of the electron energy analyzer T, a function of the kinetic energy of the photoelectrons, Ekin, of the analysis area, A, of the photoionization cross section of the orbital j of an element i, σij, a function of photon energy ℏν, of the asymmetry of the photoemission of the orbital j of an element i, Lij(β,γ) = [1 + (β/2)((3/2) sin2 γ − 1)], where β is the asymmetry parameter and γ is the angle between the incident X-ray beam and the electron energy analyzer, a constant in our experimental setup, and to the integral over the depth of the product of the concentration depth profile ci(z) of the element i and a decreasing exponential comprising the effects of the inelastic mean free path (λ) and of θ as summarized in eq 2, which, for reason of simplicity, does not contain the effect of polarized photon beam, explained elsewhere.29

varying 95% information depth, z95% (ISO 18115-1:2010, term 5.246). Two approaches for depth profiling of organic materials were proposed, either (i) the evaluation of the component areas in the fitted high resolution spectra with regards to z95%, or (ii) the acquisition and evaluation of photoelectron spectra of all constituents at the same, i.e., constant kinetic energies (hereafter referred to as CKE-XPS). This latter approach was only briefly discussed previously and is the object of the present contribution. In this previous work, we have demonstrated ultimative surface sensitivity of both approaches using a native oxide layer on a silicon wafer and self-assembled monolayers on gold as models for validations of strategies i and ii, respectively. We have also compared synchrotron radiation XPS with laboratory ARXPS, and good agreement between results delivered by both techniques was obtained. We present here a methodology for the depth profiling of elemental compositions in organic materials using CKE-XPS, which is schematized in Figure 1. In addition to providing access to much lower values of z95% than those accessible by laboratory XPS/ARXPS, this methodology ensures constant z95% for different elements, thereby alleviating many of the quantification problems inherent to the analysis of inhomogeneous layers by laboratory equipment. To demonstrate the potential of this technique, we analyze technologically relevant21−23 nitrogen-rich plasma-polymerized films, prepared according to recently established methodologies.24 Primary amino groups in the films were quantified by chemical derivatization with 4-trifluoromethylbenzaldehyde, a widely used technique to characterize films intended for biomedical applications.25−27 We finally show how the information provided by analysis of both derivatized and underivatized films by CKE-XPS allows us to obtain an improved understanding of the films and of the limitations of the derivatization technique used to provide a CF3 marker for primary amines in Chemical Derivatization XPS. Principles of Constant Kinetic Energy XPS. Qualitative XPS spectroscopy is based on the determination of the binding energy, Eb, of electrons originating from an orbital j of an element i, by measurement of the kinetic energy of electrons emitted by photoionization through absorption of a photon of energy ℏν; a basic well-known principle which is presented here in eq 1 for the clarity of the following demonstration:28 E kin = ℏν − E b − ϕ

Iij(θ , ℏν) ≈ F(ℏν)T (E kin)Aσij(E kin)L ij(β , γ )

∫0

∞

c i(z)e−z/λ(E kin)cos θ dz

(2)

In the case where ci can be considered constant, which, while an often flawed assumption, will be the approximation considered in this demonstration due to the impracticality of treating inhomogeneous systems, the integral is easily solved and eq 1 can be rewritten as follows: Iij(θ , ℏν) ≈ F(ℏν)T (E kin)Aσij(E kin)L ij(β , γ )c iλ(E kin)cos θ (3)

Let us now consider a CKE-XPS experiment, where the photoelectron spectra for the orbitals j and l having different Eb for two elements i and k, are acquired at different ℏν (ℏνj, ℏνl) adjusted in a way to ensure the same Ekin of the detected photoelectrons leading to nearly equal λ parameters. The ratio of the elemental concentrations can be obtained by the ratio of their corresponding eqs 3, which rearranges to eq 4 hereunder. Iij(ℏνj) ck F(ℏνl)σkl(E kin) = × cl F(ℏνj)σij(E kin) Ikl(ℏνl)

(1) 5985

(4)

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electron spectrometer, the detector, and the stability of the sample to hard X-rays. The specific experimental conditions (ℏν) will depend on many factors, but main considerations include range of z95%, limitation of experimental duration, elements analyzed, and overlapping of Auger peaks. Table 1 presents a series of experimental conditions we used for the depth profiling of organic materials containing C, N, O, and F over the range 0.7 nm ≤ z95% ≤ 5.4 nm.

Being constant under both experimental conditions, the factors T, A, L, λ, and cos(θ) cancel their effects. It is noteworthy that L has a constant value because only 1s orbitals (β =2.0 for all elements) are involved and the source-toanalyzer is constant in our experimental setup . In the CKE experiment, concentration ratios can therefore be obtained if σ and F are known for both excitation energies. Values of σ are obtained from tables published by Yeh and Lindau30 while acquiring reliable values of F can be a greater challenge. A possibility is to measure the current of a calibrated GaAs diode temporarily placed in the path of the synchrotron light beam. Another option is to measure the signal intensity originating from the orbital j of an element i in a sample with ci(z) = constant, a sputtered clean solid gold surface for example, at different excitation energies ℏν. The ratio of eq 3 for two different ℏν is presented in eq 5 where A and cos(θ) cancel their effects.

Table 1. Evaluation of the XPS 95% Information Depth, z95%, in Angle and Excitation Energy-Resolved XPS (AR-ERXPS) from IMFPs Calculated by Tanuma et al. for Conventional Polymersa

Iij(θ , ℏν1) F(ℏν1) = F(ℏν2) Iij(θ , ℏν2) ×

a

Spectra were also taken at an emission angle of 60° for a z95% of 1.1 nm.

T (E kin,2)σij(E kin,2)L ij(β , γ )λ(E kin,2) T (E kin,1)σij(E kin,1)L ij(β , γ )λ(E kin,1)

(5)

■

To obtain flux ratios, σij, Lij, λ, and T are required. Values of Lij are constant for 1s orbitals, and values calculated by Yeh and Lindau30 can be used for other orbitals (e.g., Au 4f in this study), λ can be evaluated using the TTP model by Tanuma et al.,31 and T can be determined experimentally following the methodology introduced by Hesse et al.32 This latter method for calculating flux ratios has the advantage of encompassing in the value any other varying experimental parameter such as small variations of the analysis area A. From these calculations, one can derive new relative sensitivity factors, RSFs, applicable to a specific set of experimental conditions. The basic equation which relates I, c, and RSF for two elements is presented below. Iij RSFkl (ℏν) ci = . cl Ikl RSFij(ℏν)

EXPERIMENTAL SECTION Materials. Poly(tetrafluoroethylene) (PTFE, sample from the interlaboratory study of the degradation of organic materials by X-rays in XPS, VAMAS Technical Working Area 2, project A5),33 branched poly(ethyleneimine) (PEI-B, MW 70 000, 30% w/v aqueous solution, Alfa Aesar), linear poly(ethyleneimine) (PEI-L, MW 25 000, Alfa Aesar), poly(acrylonitrile) (PAN, M.W. 150 000, Sigma-Aldrich), poly(acrylic acid) (PAA, MW 450 000, Sigma-Aldrich) were acquired to prepare reference samples for RSF determination. PTFE was transferred on a silicon wafer by rubbing the solid film on the wafer. Thin layers of PEI, PAA, and PAN were obtained by spin coating 3% w/w solutions, aqueous for the three former, N,N-dimethylformamide for the latter, on silicon wafers at 3500 rpm for 45 s. Preparation conditions ensured the formation of thin layers (20−50 nm, monitored by ellipsometry), which was essential to minimize charging effects. Deposition of L-PPE:N Coatings. N-Rich thin films (LPPE:N) were deposited by low pressure capacitively coupled R.F. plasma from mixtures of C2H4 and NH3.. Films of varying composition were obtained by adjusting the gas flow ratio, R (≡ flowNH3/flowC2H4), between 0.75 and 1.5. These films are referred to as L-PPE:N(R) hereafter. All films were deposited on c-Si wafers, and all were examined within 4 to 10 days after deposition, to minimize possible aging effects. Moreover, samples were deposited in a short time frame (1 day), packed in wafer shippers under protective atmosphere immediately after deposition, and overall exposure to ambient air was kept as short as possible during transfers to minimize adventitious hydrocarbon contamination. On the basis of work by TruicaMarasescu et al.24 and Oran et al.,34 we chose to deposit LPPE:N coatings using rather mild plasma conditions (P = 20 W); under these conditions, polymer-like films with maximum nitrogen [N] and amine [NH2] concentrations could be obtained. Further experimental details are given as Supporting Information. Chemical Derivatization with 4-Trifluoromethylbenzaldehyde (TFBA). Derivatizaton experiments were per-

(6)

Another approach is to determine RSFs experimentally, with samples of known and constant ci(z), for all elements constituting the sample. This approach is straightforward and diminishes experimental uncertainty; however, it is more timeconsuming and requires reliable test samples, and RSFs must be reacquired for each new set of experimental conditions. The information depth of the technique can be evaluated by considering the integral in eq 2. Instead of integrating on the full depth, we decided to use the depth, z95%, which corresponds to the sample thickness from which 95% of the detected signal originates. This is described by eq 7 if elastic scattering effects are neglected. z 95% = 3λ cos θ

(7)

This depth is called information depth in the ISO Vocabulary [ISO 18115-1:2010, term 5.246]. The lower bound of obtainable z95% is determined by the minimum value of λ, obtained for a kinetic energy of around 100 eV, and by the maximum value of θ, limited by elastic scattering above approximately 70°. The upper bound is limited by the highest usable kinetic energy which will depend on the beamline, the monochromator of the end station, the energy analyzer of the 5986

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Table 2. Calculated RSF and Experimental RSF Values for N 1s Used for Elemental Quantification at the HE-SGM Beamline of the Synchrotron BESSY II (Berlin, Germany), Valid for the Experimental Conditions Presented Herea experimental RSF constant kinetic energy

constant excitation energy

Ekin = 100 eV Ekin = 200 eV Ekin = 350 eV Ekin = 500 eV hν = 500 eV hν = 620 eV hν = 765 eV

calculated RSF

poly(acrylonitrile)

poly(ethyleneimine) branched

poly(ethyleneimine) linear

0.647 0.665 0.140 0.370 2.111 1.816 1.758

0.647 0.501 0.242 0.218 2.353 1.388 1.780

0.666 0.499 0.253 0.188 2.270 1.439 1.577

0.600 0.523 0.230 0.191 2.227 1.496 1.424

a RSF are normalized to RSFC 1s, which is consequently equal to 1. RSF for both constant kinetic energy and constant excitation energy (akin to standard laboratory XPS) are presented.

Figure 1 schematizes the advantages of this approach for chemical analysis and depth profiling of inhomogeneous samples. A frequently overlooked problem in quantitative elemental XPS analysis is the significant variation of z95% among different photoelectron peaks that are far apart in kinetic energy. In the case of homogeneous samples, this can readily be corrected, but in inhomogeneous samples such those examined in the present study, quantitative elemental analyses suffer from significant systematic error. This shortcoming also applies to depth profiling by ARXPS, where mathematical models that consider these variations rapidly become very complex and where proper reconstruction of a depth profile may, in extreme cases, become impossible. To alleviate these problems, we acquired photoelectron peaks by choosing the excitation energies in such a way that photoelectrons from different emitter elements in a sample would have the same kinetic energy, so that the same values of z95% applied. Examples of such experimental protocols can be found in Table 1. By following boxes identified with the same shades of gray, a C 1s peak taken with 385 eV photons, an N 1s peak taken with 500 eV, and an O 1s peak taken with 620 eV would all yield photoelectrons having kinetic energies near 100 eV, corresponding to a z95% value near 2.1 nm. This z95% value could be further reduced to 1.1 nm, or even 0.7 nm, by increasing the angle of emission to 60° or 70°, respectively. A further advantage of the CKE-XPS approach is that the need to understand the analyzer’s transmission function is rendered superfluous: as a function of kinetic energy, it is rendered constant among the various photoelectron peaks. Determination of Relative Sensitivity Factors for C 1s, N 1s, O 1s, and F 1s. The first step in conducting the CKEXPS experiment was the determination of RSFs, either calculated or experimentally acquired. Values of RSFs presented here are usable for work conducted at the HE-SGM beamline of the synchrotron BESSY II (Berlin, Germany), using the conditions described in the Experimental Section. The methodology described in this contribution, however, can be applied for RSF determination at any adequately equipped beamline and end station. Table 2 presents the RSF values, both calculated and experimental, for N 1s determined for the specific experimental conditions that are detailed in Table 1. Constant excitation energy XPS RSFs, akin to laboratory conditions but using a synchrotron radiation at different energies as a source, are presented here for comparison and were calculated according to the well-established procedure of multiplying T, σ, and λ corresponding to the experimental conditions. A partial validation of the procedure can be obtained by observing calculated concentrations that are close

formed by exposing freshly prepared samples to TFBA vapors for a period of 180 min in a small vacuum chamber (ca. 300 mL) described previously.35 A detailed description of this experimental procedure is provided as Supporting Information. Angle- and Excitation Energy-Resolved XPS (ARERXPS) Using Synchrotron Radiation. Synchrotron-based measurements were carried out at the HE-SGM monochromator dipole magnet beamline at the synchrotron radiation source BESSY II (Berlin, Germany), which can deliver incident radiations ℏν between ∼150 and 900 eV. The monochromator was used with grid 1, and both slits were set to 0.2 μm. The end station is equipped with a Scienta R3000 analyzer, used in transmission mode at a pass energy of 50 eV. The source-toanalyzer angle, γ, was 45°, and the electron emission angle was either 0°, or 60° or 70°, for angle-resolved ERXPS (ARERXPS). The relative intensity, IB0, of the X-ray beam entering the analysis chamber was evaluated before and after each measurement by measuring the current produced by a gold mesh placed where the light beam passes through using a picoampere meter. Experimental RSFs were derived from peak areas, Iij, obtained from the spectra of films of PTFE, PEI-L, PEI-B, and PAN and using stoichiometric concentrations, ci, by dividing Iij by cj and normalizing all RSF to RSFC 1s (which is set to 1). Reference analysis beam intensities, IB0R, were measured for each RSF. Relative X-ray flux data were obtained by using Au 4f intensities measured with a freshly sputtered gold surface at all used excitation energies. The relative fluxes, F(ℏν), are determined by dividing IAu4f by T, σ, and λ, in accordance with eq 5; reference analysis beam intensities, IB0R, were measured for each F(ℏν). Calculated RSF are obtained by multiplying F(ℏν) and σιφ in accordance with eq 4; all RSF for a specific set of experimental conditions are normalized to RSFC 1s in those conditions. The calculation of elemental concentration for a given data set obtained from a sample of unknown constituent concentrations was performed by dividing Iij by RSFij to obtain a set of ci which were normalized. To account for the decay of intensity of synchrotron radiation vs time, each RSF used in a specific calculation was multiplied by the factor IB0/IB0R.

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RESULTS AND DISCUSSION Constant Kinetic Energy XPS (CKE-XPS). CKE-XPS uses an approach significantly different than standard laboratory conditions for elemental quantification. In a previous contribution,36 we have demonstrated the ultrashallow surface sensitivity (z95%) of the technique by using model materials such as undecanthiol self-assembled monolayers and silicon with its native oxide. The information depth can be tuned down to 0.7 nm, something that is unattainable using laboratory XPS. 5987

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can be considered analogous to their previously described counterparts, it must be acknowledged that custom built plasma systems are unique and so are the films prepared using them. Previous extensive characterizations have partly elucidated the complex chemistry of this family of materials by permitting the quantitative and semiquantitative analysis of various functional groups such as amines, nitriles, isonitriles, imines, and alkenyls.24,26,27,37 Of particular interest is the technique used to quantify primary amines. As described in numerous previous contributions,25−27 primary amines react selectively with aldehydes to form imine bonds. This property is used to bind a chemical marker to primary amino groups, hence permitting their quantification. In the case presented here, we have used 4trifluoromethylbenzaldehyde, which permits quantification by determination of the chemically bound fluorine by XPS. Different calculations permit concentration values to be obtained from the elemental quantifications on a derivatized sample. Because quantification of amines is not the object of the present contribution, we will only discuss elemental concentrations here. XPS analysis is otherwise particularly useful in providing information on the chemistry of L-PPE:N films, both using elemental quantification and analysis of high resolution spectra. For coatings of these types, curve fitting of the C 1s peak (presented as Supporting Information, Figure S1) generally involves four components (or subpeaks), here named cc, c1, c2, and c3. Several different attributions for these peaks can be found in the literature,27,38,39 but none appear to be completely satisfactory, on account of plasma polymerized materials’ high degree of structural complexity. Typically, cc (∼285.0 eV) is assigned to species such as C−C, CC, and C−H, c1 (∼286.2 eV) is assigned to C−N (amino groups), CN, and C−O, c2 (∼287.2 eV) is generally assigned to CN, and c3 (∼288.4 eV) is assigned to N−C−O, N−CO, and CO. For derivatized films, a fifth component, named cf3, is typically found around 293 eV and attributed to the trifluoromethyl group. These assignments, which are mainly deduced from comparisons with chemical shifts in standard polymers, cannot be considered reliable for quantitative analyses. Even though considerable efforts were already deployed earlier,27 the exact chemical nature of these materials remains elusive; they may even contain complex groups not encountered in traditional organic synthesis, which are therefore extremely difficult to identify. The relative areas of the components in a curve fit can give nevertheless an indication of the relative abundance of functional groups in the materials. While cc, c1, c2, and c3 are considered unspecific, cf3 can, in derivatized samples, be used to quantify primary amino groups. The analysis of high resolution spectra can also be used to create a depth profile by noting the variation of component intensity with z95%. These are used to confirm the observations made by depth profiling of elements by CKE-XPS, which will be the subject of the following section. X-ray Degradation. An often brought up concern for synchrotron radiation XPS of organic materials is the possibility of significant X-ray degradation, a well-known and often discussed issue of laboratory XPS, which would be exacerbated by the high intensity of the synchrotron beam.40,41 To evaluate X-ray degradation, we have measured C 1s and F 1s core-level spectra under identical conditions at the beginning and at the end of the approximately 2 h long measurement session of

to the expected concentrations (presented as Supporting Information, Table S1). Calculated CKE-XPS RSFs were obtained according to eqs 4−6, derived above, while their experimental counterparts were obtained directly from eq 6 using expected elemental concentrations taken from the chemical formula of the analyzed polymers. The latter has the advantage of encompassing all known and eventually unknown experimental parameters in a one single calculation, thereby limiting experimental uncertainties. Higher uncertainties are inherent to calculated RSF based on the combination of values (F, T, σ, λ) obtained through multistep calculation involving theoretical or experimental models. Moreover, it has been difficult to precisely establish a transmission function for low kinetic energies as the available procedures32 were not optimized for this energy range. Experimental RSF calculated using different reference materials show reasonable agreement for all energies and experimental conditions used. Also, they show good general agreement with the calculated values for the lower values of kinetic energy but significantly less so at higher energies. Spectra used to calculate the RSF at 350 and 500 eV kinetic energy were noisy and broad due to low X-ray fluxes and higher band widths delivered by the monochromator in this energy range. This results in an increased uncertainty of the data obtained in these energy ranges. In the future, the development of CKE-XPS specialized equipment should help alleviate this problem. It is generally acknowledged that XPS is a precise technique yielding results consistent to a fraction of a percent; however, systematic errors are typically estimated to often a minimum of 10−15% of real concentration values when calculated RSFs are used. It is therefore suggested that experimental RSFs should be used whenever possible. Given the above explained complexity and condition-dependent nature of uncertainties in CKE-XPS, we have decided not to include error bars on the graphs in the present contribution because we could not obtain calculations in which we have more confidence than the above given generally accepted numbers (10−15%). An important concern is the quality, mainly chemical homogeneity, of the samples used to determine experimental RSF. Unfortunately, no other characterization technique has the capability to determine the chemistry of the near-surface layer in the range available to CKE-XPS. To evaluate the homogeneity of our material in the near-surface region, we have used CKE-XPS with calculated RSF. Elemental concentrations in PEI-L were calculated with these RSF for different experimental conditions corresponding to varying z 95% (presented as Supporting Information, Table S1). The results demonstrate the expected chemical homogeneity within a very shallow surface layer for 0.7 nm ≤ z95% ≤ 4.2 nm. This evaluation is highly reliable in the range 0.7 nm ≤ z95% ≤ 2.1 nm since only the angle of emission is varied. Application: L-PPE:N Films. Plasma-prepared surfaces are ideal candidates for CKE-XPS experiments because they are susceptible to contain chemical depth profiles in the near surface region, something that has direct implications on applications. Out of many potential candidates, we have chosen to analyze nitrogen-rich plasma polymers deposited from a mixture of ammonia and ethylene, a material that has demonstrated significant potential for biomedical applications.24,26,37 The films presented here are prepared in a plasma deposition apparatus different than in our previous contributions by using a similar methodology. While these new films 5988

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fluorine-containing derivatized L-PPE:N samples, known to be particularly susceptible to X-ray degradation. In all cases, we have found that both the total peak intensities and the relative area of the cf3 component of the C 1s peak fit had maximum variations of 5%, thereby ensuring that X-ray degradation was, in our experimental conditions, limited. This observation is in conformity with our expectation because (i) even if the synchrotron white light is particularly intense, the monochromatization process implies an important reduction in intensity, something that is dependent on the monochromator’s performance at various wavelengths, and (ii) higher intensities imply shorter measurement times and therefore less opportunity of X-ray damage. In our particular experiment was found that X-ray fluxes varied from slightly higher than those of laboratory sources at 385 eV to much lower at 765 and 880 eV. Application: Depth Profiling of L-PPE:N Films by CKEXPS. The main interest in CKE-XPS is chemical depth profiling the near-surface region and L-PPE:N films are showcased here to demonstrate its capabilities and better understand the chemistry of the plasma-prepared materials. Figures 2, 3, and 4 present the depth profiling of L-PPE:N films of different gas flow ratios ((a) R = 1.5, (b) R = 1, (c) R = 0.75), both underivatized (Figure 2) and derivatized (Figures 3 and 4) using elemental quantification by CKE-XPS (Figures 2 and 3)

Figure 3. CKE-XPS elemental depth analyses of derivatized L-PPE:N films made with different gas flow ratios: (a) R = 1.5; (b) R = 1, (c) R = 0.75. Evolutions of the concentrations of constituent elements: C (■, □), N (●, ○), O (▲, △) F(▼, ▽), obtained using experimental (full symbols, plain lines) RSF (from Tables 2 and S2).

and analysis of the high resolution C 1s peak fit components (Figure 4). Both approaches offer complementary and independent information, which can be evaluated for consistency, thus providing validation of the technique. Figure 2 presents the evolution of elemental (C, N, and O) composition evaluated for varying values z95%. . It should be emphasized here that, as shown by eqs 2 and 7, z95% is not a depth at which a concentration is punctually determined but rather a layer thickness from which a certain percentage of the detected photoelectrons originate. Elemental compositions computed from both experimental and calculated RSF show good agreement; however, the use of experimental RSFs yields a higher consistency, especially for higher z95% where very low incident photon fluxes increase experimental uncertainties. The results, which show near constant elemental compositions for all these three films, point to a slight enrichment of the near surface region in C. These observations are in contrast to previous analysis of analogous materials prepared in slightly different conditions where it was clearly demonstrated that films prepared at elevated R values show a strong surface enrichment of C and depletion of N.20 The uncovered differences in the chemical profile of very closely related materials, something that is impossible to assess by standard laboratory techniques, has important implications for our understanding of plasma-prepared surfaces and emphasizes their complexity.

Figure 2. CKE-XPS elemental depth analyses of L-PPE:N films made with different gas flow ratios: (a) R = 1.5; (b) R = 1, (c) R = 0.75. Evolutions of the concentrations of constituent elements: C (■, □), N (●, ○), O (▲, △), obtained using experimental (full symbols, plain lines) and calculated (open symbols, dotted lines) RSF (from Tables 2 and S2). 5989

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information originated from a layer of thickness of 0.13 nm, which corresponds to the length of a C−F bond.

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CONCLUSION We present a new XPS strategy named constant kinetic energy XPS (CKE-XPS), which can alleviate a well-known drawback of traditional XPS. Using CKE-XPS, the analytical information for all elements using CKE-XPS originates from the same depth, improving greatly the XPS quantitative chemical analysis of inhomogeneous samples. In addition, analysis of ultrashallow surface layers is made possible. Because the availability of synchrotron radiation is increasing, we believe that the conditions are favorable for the emergence of this new method. The need for characterization techniques permitting an enhanced understanding of the chemistry of the near-surface region is clearly established. We have presented CKE-XPS as a strategy to depth profile the elemental composition of materials in the range z95% ≥ 0.7 nm. We presented the determination of condition-specific RSFs, either by calculation or by using reference samples. The latter were found to be preferable, whenever their determination is possible because they eliminate important sources of uncertainties inherent to the calculated values. We have demonstrated the capacities of this new technique for the analysis of new L-PPE:N materials, both derivatized and underivatized. The existence of a near-surface chemical depth profile in derivatized samples has demonstrated how CKE-XPS can reveal otherwise unattainable information about the surface chemistry. The improvements described above should permit significant advances for many surface-driven applications. For example, using algorithms to determine the chemical depth profile (CDP), it will be possible to determine the chemistry of the surface-relevant atomic layers, thus allowing meaningful comparison of inhomogeneous surfaces. Moreover, applying the same methodology to very thin self-assembled monolayers will aid the understanding of their structure and orientation. Finally, in conjunction with the new “noninvasive” sputtering technologies available with argon gas cluster ion beam sources, we foresee the application of this method to the XPS analysis of cell-surface interfaces.

Figure 4. ERXPS depth-resolved analyses of L-PPE:N films corresponding to different gas flow ratios: (a) R = 1.5; (b) R = 0.75. Evolution of the different component-peaks of the highresolution C 1s spectra, cx: cc (■, □), c1 (●, ○), c2 (▲, △) c3 (▼, ▽) and cf3 (◆, ◇). Attributions are detailed in the main text.

Similar results are presented for derivatized L-PPE:N films in Figure 3, where fluorine was also investigated. In this case, however, the CKE-XPS analysis shows a strong surface enrichment of F, something that eluded analyses at higher z95%. The strong enrichment is observable in L-PPE:N(1.5) and L-PPE:N(1) while a much milder effect is seen in LPPE:N(0.75), thereby highlighting differences in samples that would have intuitively considered similar. The observations are further confirmed in Figure 4, where the cf3 component of the C 1s peak fit is shown to be enriched toward the surface. Interestingly, elemental composition and analysis of components at z95% = 0.7 nm are close to the stoichiometry of the pure 4-trifluoromethylbenzaldehyde molecule. The exact reason for the surface enrichment in fluorine is not fully clear at this point, but differences between samples renders the hypothesis of a nonchemically bound fluorine-rich layer at the surface, composed of the nonvolatile 4-trifluoromethylbenzoic acid for example, improbable. A possible explanation is that LPPE:N(1.5) and L-PPE:N(1) have inhomogeneous amino group densities in the near-surface region, while having an homogeneous elemental profile. Another proposed hypothesis is the enrichment of the surface by postderivatization displacements (or alignment) of C−F species toward the surface, assisted by macromolecular motions that are thermodynamically driven by surface-energy reduction. Differences between samples could be explained by various degrees of cross-linking, which govern the potential extent of these macromolecular motions. The subtle changes in the profile of the first monolayers of the surface are only detectable in the most surface sensitive conditions: for z95% = 0.7 nm, 40% of the

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ASSOCIATED CONTENT

S Supporting Information *

Additional information as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], fax: 514-3986678; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.-L. Girard-Lauriault is grateful to the Adolf Martens Fellowship Program of BAM and to the Fonds Québécois de la Recherche sur la Nature et les Technologies (FQRNT) for postdoctoral fellowship support. We also acknowledge support from the BESSY II synchrotron radiation facility and its team, as well as A. Nefedov (Karlsruhe Institute of Technology, KIT) from the HE-SGM CRG at BESSY. 5990

dx.doi.org/10.1021/ac300585q | Anal. Chem. 2012, 84, 5984−5991

Analytical Chemistry

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Article

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dx.doi.org/10.1021/ac300585q | Anal. Chem. 2012, 84, 5984−5991