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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Resonant GISAXS at the Sulfur K-edge for MaterialSpecific Investigation of Thin-Film Nanostructures Mihael Coric, Nitin Saxena, Mika Pflüger, Peter Muller-Buschbaum, Michael Krumrey, and Eva M. Herzig J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b01111 • Publication Date (Web): 16 May 2018 Downloaded from http://pubs.acs.org on May 16, 2018

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The Journal of Physical Chemistry Letters

Resonant GISAXS at the Sulfur K-edge for MaterialSpecific Investigation of Thin-Film Nanostructures Mihael Coric1, Nitin Saxena2, Mika Pflüger3, Peter Müller-Buschbaum2, Michael Krumrey3, Eva M. Herzig1,4* 1

Technische Universität München, Munich School of Engineering, Herzig Group, Lichtenbergstr. 4a, 85748 Garching, Germany

2

Technische Universität München, Physik-Department, Lehrstuhl für Funktionelle Materialien, James-Franck-Str. 1, 85748 Garching, Germany 3

4

Physikalisch-Technische Bundesanstalt (PTB), Abbestraße 2-12,10587 Berlin, Germany

Universität Bayreuth, Physikalisches Institut, Dynamik und Strukturbildung – Herzig Group, Universitätsstr. 30, 95447 Bayreuth, Germany

Corresponding Author E-Mail: [email protected]

ACS Paragon Plus Environment

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Scattering techniques are a powerful tool for probing thin film nanomorphologies, but often require additional characterization by other methods. We applied the well-established grazingincidence small angle X-ray scattering (GISAXS) technique for a selection of energies around the absorption edge of sulfur to exploit the resonance effect (grazing incidence resonant tender X-ray scattering - GIR-TeXS) of the sulfur atoms within a P3HT-PC61BM sample to gain information about the composition of the film morphology. With this approach, it is possible to not only identify structures within the investigated thin film but also to link them to a particular material combination.

TOC Graphic

Keywords: polymer thin film, resonant X-ray scattering, scattering contrast, morphology, GIRSAXS, GIRTeXS


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Thin film technologies have become increasingly important over the past decades, with applications in photovoltaics, transistors, light-emitting diodes or thermoelectrics.1–3 In many cases, the functionality strongly correlates with the nanostructure inside or on the surface of the thin film. For a large number of applications nanostructures of organic thin films are of strong interest.4,5 In many approaches, electron sensitive tools are used to probe nanoscale structures. Organic systems often contain more than one component and their electron densities are very similar. Therefore, it is non-trivial to associate observed nanostructures with the known materials present in the sample even when using a combination of complementary methods.6–8 However, it is extremely desirable to have a tool to obtain information on the origin of nanostructures to understand and improve thin film properties. X-ray scattering is a powerful technique to analyze structure in organic and inorganic materials revealing the variation of electron densities on the nanoscale with high statistical relevance. Using measurements in grazing incidence geometry9, results in a large footprint and good statistics. Hence, it is possible to characterize nanomorphologies even for thin organic materials, provided sufficient contrast is available with so called grazing incidence small angle X-ray scattering (GISAXS).10,11 Currently most GISAXS measurements are carried out using a single photon energy in the hard X-ray regime.9 The incoming X-rays interact with the electrons of the examined sample volume, are scattered and the superposition of all scattered X-rays is detected by an area detector. The recorded intensity variations can be analyzed subsequently to resolve nanostructures within the sample volume. This method is sensitive to local electron density differences forming the nanostructures of interest. Since the X-ray scattering relies on the electron density differences, it is often non-trivial to determine the origin of the observed structures. For example, electron differences can occur between air and material or between two different phases within the bulk


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of a binary film if the two phases have different electron densities. These differences are sometimes rather small, especially for hard X-rays when working with organic materials. Therefore, resonant soft X-ray scattering (R-SoXS) became a well-established method for the examination of organic thin film materials in the past years, exploiting the changes in dispersion properties. In this work, we present some advantages and limitations using intermediate photon energies for grazing incidence X-ray scattering measurements. X-ray resonances are strongly dependent on the molecular bonds present in the examined material. Organic materials have various different bonds involving carbon (C=C, -C-H, C-H2,..). RSoXS is therefore mainly applied to energies around the carbon edge making measurements sensitive to these carbon moieties. Typical absorption spectra in this photon energy range vary strongly as a function of energy because of the energetic fine structure of the various different bonds in which the absorbing atom participates. Additionally the absorbing atom is present as a large overall fraction of the material. Powerful measurements can be made by choosing contrast conditions (i.e. particular energies) where pairs of materials of multi-component systems have matching absorption and dispersion properties or strongly differing ones.12–15 This way, the material origin of the involved structures can be resolved.16 The drawback of these high absorption coefficients, however, makes it only feasible to examine very thin films in transmission12 while in grazing incidence measurements only a limited penetration depth can be reached.17 Turning to higher photon energies (here around the sulfur K absorption edge), the absorbing atom is typically present in a significantly smaller fraction, the changes in absorption and dispersion properties are therefore much weaker and conditions where the contrast of material pairs match is not necessarily possible. However, due to the weak absorption properties, grazing incidence methods are well suited for nanostructure characterization of the complete film volume. Furthermore, the


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resolution limit increases with photon energy, which can be of advantage. Using the model system P3HT:PC61BM (poly(3-hexylthiophene-2,5-diyl):phenyl-C61-butyric acid methyl ester), we show how grazing incidence, resonant SAXS (GIRSAXS) or more specifically GIR-TeXS (in the tender X-ray energy range) can be used to learn more about the nanostructure than in conventional GISAXS measurements with single energy X-rays. P3HT:PC61BM is a binary semiconducting thin film system widely employed in organic photovoltaics.18 The efficiency of light conversion into electrical energy is strongly linked with the nanomorphology within such a thin film. It is therefore of strong interest to determine which origin the observed nanostructures have.19–24 P3HT, a hole-conducting polymer, contains a sulfur atom in each of its thiophene rings along the backbone, while PC61BM contains no sulfur, neither does the solvent (toluene) used for spincoating of the film. Single energy GISAXS at 2465 eV (an energy away from the absorption edge, to avoid any absorption influenced scattering) of such a P3HT:PC61BM film reveals the presence of two dominant structure sizes of (13±4) nm and (210±10) nm, while a broad distribution of intermediate structure sizes are also present (figure S1). This is in agreement with similar studies25 where P3HT crystallites and PCBM aggregates are found embedded in an amorphous, mixed matrix. Since small changes in processing (e.g. change of solvent26) can have a strong impact on the nanostructure of the material, it is not necessarily valid to simply relate literature observations to the examined system here. Therefore, with single energy X-rays alone it is not possible to distinguish between the following scenarios when only electron density differences are examined: Scenario a) both structure sizes are embedded in the matrix, scenario b) the small structure sizes are dominantly at the surface while the large structures are dominantly embedded in the film, scenario c) the large structure sizes are dominantly at the surface while the small


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structures are dominantly embedded in the film. Furthermore, it is undeterminable whether the structures consist of P3HT or PC61BM. To determine the material origin of the observed structure sizes requires for example time-resolved measurements to see the components grow in 10

combination with material specific crystallization properties

or a series of large samples for

neutron measurements.27 Using the signature of absorption properties of P3HT around the sulfur K-edge with energy, i.e. recording scattering patterns as a function of photon energy 𝐸!" below, at and above the absorption edge of sulfur, we extract the material type and environment of the structure sizes. To explain this measurement approach in more detail, we first look at the relation between absorption properties of the materials and observed intensity at the detector. The X-rays incident on the sample will interact with the electrons present in their pathway. The resulting scattering of the X-rays is recorded at the detector as a two-dimensional intensity map. The recorded intensity 𝐼!"# is directly related to the modulation of the incident beam 𝐼! by the differential scattering cross section 𝑑𝜎 of the sample along a given direction into the solid angle 𝑑Ω via28 𝐼!"# ∝ 𝐼!

!" !! !"!

! ∝ 𝐼! 𝐸!! 𝛼(𝐸!" ) Δ𝛿(𝐸!" )! + Δ𝛽 𝐸!"

!

𝑁

!" !! !"#

(1)

The total scattering cross section in turn is related to the number 𝑁 and the scattering cross section

!" !! !"#

of individual objects. A contrast between objects in or on top of a thin film

arises because each component (object and surrounding thin film medium as well as air or vacuum at the top interface) has a refractive index 𝑛∗ . The complex refractive index 𝑛∗ is given by 𝑛∗ = 1 − 𝛿 + 𝑖𝛽 where 𝛿 denotes the dispersion and 𝛽 the absorption of the material respectively. The scattering contrast29 is given by (Δ𝛿 ! + Δ𝛽! ), with Δ denoting the difference of the scattering object to the surrounding medium. It is essential to note that 𝛿 as well as 𝛽 depend


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on the photon energy 𝐸!" as shown in figure 1. The scattered intensity depends also on the incident field intensity throughout the film. This can often be neglected when comparing intensities at a single photon energy. However, since the electric field intensity changes strongly around the absorption edge due to the variation of the optical properties (absorption, dispersion) of the film, this effect needs to be taken into account in our analysis of comparing scattering intensities at different energies. Therefore, we introduce the relative correction factor 𝛼(𝐸!" ), which accounts for the changes in electric field intensity in the thin film and is calculated as 𝛼 𝐸!" ∝

!

!"#$

𝐸(𝐸!" , 𝑧) 𝑑𝑧. The electric field 𝐸(𝐸!" , 𝑧) depends on the refractive index 𝑛∗

and therefore on the photon energy 𝐸!" . The scattering cross section of the individual objects further contains information on the shape (form factor 𝐹(𝑞)) and arrangement (structure 𝑆(𝑞)) of these objects and can be expressed with !" !! !"#

where

= 𝐹 𝑞

𝐹 𝑞 !

!

𝑆(𝑞)

𝑆(𝑞)

!

(2)

!

denotes the average of the examined domain. Focusing on horizontal

intensity cuts at the Yoneda region of the 2D GISAXS data and using the local monodisperse approximation30,31 we model our measured scattering intensity arising from the sum of differently sized objects 𝑖, weighted with an energy dependent prefactor 𝑊! (𝐸!" ) that depends on the contrast between each object and its respective surrounding medium: 𝐼!"# 𝐸!" , 𝑞 ∝ Σ! 𝑊! 𝐸!! 𝐹! 𝑞 with

!

𝑆! 𝑞

! 𝑊! 𝐸!" = 𝐼! 𝐸!" 𝛼 𝐸!" Δ𝛿(𝐸!" )! + Δ𝛽(𝐸!" )! 𝑁!

(3)

The morphology of the static sample does not change during measurements at different energies. This implies constant form and structure factors for such a series of measurements (eq 3). However, the weighing prefactor 𝑊! (𝐸!" ) is a unique function of energy depending on the


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material absorption properties of the structure involved. 𝑊! (𝐸!" ) can therefore be used to identify which material gives rise to a particular object or structure size 𝑖. 𝐼! and 𝐸!" are fixed by the measurement conditions, while 𝛼 𝐸!" depends on the vertical film material composition and the photon energy but not on the scattering vector 𝑞. It is the contrast contribution (Δ𝛿 ! + Δ𝛽! ) that will vary with energy differently for changing 𝑞 depending on the materials involved and therefore needs to be obtained to reveal the material species of the examined structure sizes. To determine which of the discussed structure scenarios is valid and which materials form which structure size, GIR-TeXS experiments were carried out with a photon energy around the K-shell binding energy of sulfur (sulfur K edge at 2473 eV). To measure in that energy range, a grazing incidence scattering set-up in vacuum is necessary to avoid air absorption of the low energy X-rays. Furthermore, the photon energy needs to be accurately tunable from 2465 eV to 2480 eV. Therefore, the experiment was performed at the four-crystal monochromator beamline of PTB at BESSY II.32 The experiment itself consists of two measurements: near edge X-ray absorption fine structure (NEXAFS)33,34 and grazing incidence resonant tender X-ray scattering (GIR-TeXS)3,35 using an in-vacuum hybrid pixel PILATUS detector.36 Details on the measurements and sample preparation are given in the Supporting Information.


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Figure 1. a) Absorption (𝛽) and b) dispersion (𝛿) of P3HT and PCBM as a function of X-ray energies are depicted. Only the sulfur containing P3HT shows a significant change in the absorption as well in the dispersion curve with energy. Figure 1c) shows the scattering contrast for P3HT-vacuum and P3HT-PCBM. The absorption measurements show the spectral response upon the change of the photon energy for the individual materials used in the experiment. Figure 1a shows the measured absorption (𝛽) and figure 1b the dispersion (𝛿) values, which were obtained from the absorption measurements with the Kramers-Kronig relation using the KKCalc software.37 As expected, the values for P3HT vary strongly at the sulfur K-edge, while they exhibit only a minor slope over the examined range for PCBM. In figure 1c possible material combinations giving rise to


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scattering contrasts are shown as a function of energy: scattering contrasts (Δ𝛿 ! + Δ𝛽! ) between P3HT and vacuum (circles) as well as between P3HT and PC61BM (triangles). The shape of the scattering contrast function with photon energy of these two material combinations differs clearly for both cases, as seen in figure 1c, and is proportional to 𝑊! (𝐸!" )/𝛼! (𝐸!" ) as shown in eq 3. Structure sizes arising from P3HT objects in vacuum (e.g. surface structures on film) or PC61BM objects in a P3HT containing matrix (or vice versa) can therefore be distinguished by the inspection of 𝑊! (𝐸!" )/𝛼! (𝐸!" ). To obtain 𝑊! (𝐸!" )/𝛼! (𝐸!" ) experimentally, GISAXS measurements on P3HT:PC61BM thin films are carried out at several energies below, at and above the sulfur absorption edge. Examples of the 2D data at several energies are shown in figure S2 in the Supporting Information.


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Figure 2. Vertical line cuts of the 2D GISAXS data measured at different photon energies around the sulfur edge. The curves are shifted vertically for clarity. The beamstop is held by suture in front of the detector and is therefore also visible as a gap in the vertical line cuts. The changes visible by eye are the appearance and disappearance of fringes in 𝑞! direction. Figure 2 shows vertical line cuts (𝑞! direction) from the 2D data for all measured energies depicting this effect. Fringes clearly visible at 𝑞! lower than the specular peak are strongly reduced at the absorption edge (2473 eV) and also vary in position with changing energy. Both effects can be understood from electric field intensity (EFI) simulations38, which are shown in figure 3.

Figure 3. Normalized electric field intensity (EFI) calculations including polarization effects for a) a pure P3HT film and b) a P3HT:PCBM 1:1 homogeneously mixed film for the energy values used in this work. The calculations were done in FitGISAXS38 with a two-layer system consisting of a glass substrate and a 130 nm pure P3HT (a) or mixed P3HT:PCBM film (b)


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respectively with a surface roughness of 5 nm. Calculations use absorption and dispersion values measured in the framework of the here presented study. To obtain 𝛼(𝐸!" ) the EFI values for the mixed film are summed up throughout the full depth of the film for each energy and normalized to 2465 eV. The calculations show that for energies around the resonance, the EFI depth distribution flattens compared to pre-edge energies reducing the fringes on the detector. The shifting of the fringes can likewise be understood from the shifting of the EFI depth distribution with changing 𝐸!" . Additionally, the existence of a significant EFI throughout the complete depth of the film shows that the film is fully penetrated by the X-rays for all investigated photon energies, so that we obtain structural information on the full film from all our GISAXS measurements in the energy series. As described above, we use the EFI calculations to calculate 𝛼(𝐸!" ), the relative correction factor. Using the integrated EFI to calculate 𝛼(𝐸!" ), we take into account absorption, dispersion39 and waveguiding effects40 within the film. After having shown that the full film is examined for all applied energies, we use the horizontal line cuts at the Yoneda peak41 of P3HT (figure S3) to extract 𝑊! (𝐸!" ). The two clearly identifiable shoulders at small and largest 𝑞 show that a large and small set of nanostructures dominate in the sample. 𝑊! (𝐸!" ) can now be obtained by tracking the intensity changes with energy for the different scattering contributions. This can be achieved by different routes. In route 1, we model the data with constant form and structure factors for differently sized objects (see figure S1) and track the weighting contributions of the objects with energy (single free parameter). In route 2 (model independent approach), we integrate regions of interest in Kratky plots for the different energies (see figure S4) and extract the integrated intensities which are also proportional to 𝑊! (𝐸!" ).


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Figure 4. Intensity dependencies for the a) large and b) small model structures extracted from the model (red squares) as well as from the Kratky plots (blue triangles) as a function of the Xray energies used in the experiment. Plotted are also the scattering contrast function for P3HTvacuum and P3HT-PCBM (grey rings). In figure 4 the behavior of 𝑊! (𝐸!" ) with energy for the two dominant structure sizes and both extraction approaches is shown (blue and red curves) after division through the structure independent factor 𝛼(𝐸!" ). Additionally, the scattering contrast functions obtained from the absorption measurements are plotted for P3HT-vaccum (figure 4a) and for P3HT-PCBM (figure 4b). 𝑊! (𝐸!" ), the intensity weight function of the large structures present in the film correlates well with the scattering contrast of P3HT-vacuum as a function of energy. This indicates that the scattering intensity of the large structures originates dominantly from P3HT objects located at the surface (interface to vacuum). In contrast, 𝑊! (𝐸!" ) of the small structures behaves clearly


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differently and shows an energy dependency that follows the scattering contrast of P3HTPC61BM implying that these objects are mostly embedded within the film. Since the film matrix contains a mixture of P3HT-PC61BM, the small structure contain dominantly PC61BM. We conclude that large structures arise from P3HT surface structures of the polymer thin film, while the small structures occur within the film and are formed by PCBM aggregates. The existence of (210±10 nm) large surface structures can also be confirmed by AFM measurements and surface sensitive GISAXS measurements (figure S5), supporting our GIR-TeXS findings. We can therefore determine that scenario c) is applicable to the P3HT:PC61BM film spincoated from toluene and determine the materials of the structures present in the film. In summary, we have shown that it is possible to use GIR-TeXS at X-ray absorption edges for material-sensitive investigations of nanostructures. A binary system in which only one material contains a specific atomic species was successfully analyzed, for the model system P3HT:PCBM for GIR-TeXS at the sulfur edge. The obtained structural information is supported by AFM topography measurements, surface and full thin film GISAXS measurements and literature results. This technique can also be applied to various alternative material systems and to different elemental absorption edges, depending on the accessibility of the required X-ray energies and resulting experimental challenges and limitations. This method will be particularly powerful in the future if applied to all-polymer multi-component systems or systems containing nonfullerene acceptors, especially in the context of organic thin film systems, like organic photovoltaics.


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Supporting Information -

file type: PDF

-

contains the experimental details of the samples used in this work as well as additional graphs that are referenced in the text

The authors acknowledge financial support by the Bavarian State of Ministry of Education, Science and the Arts via the project “Energy Valley Bavaria”, by TUM.solar in the context of the Bavarian Collaborative Research Project “Solar Technologies Go Hybrid” (SolTech), by the GreenTech Initiative (Interface Science for Photovoltaics-ISPV) of the EuroTech Universities and by the Nanosystems Initiative Munich (NIM). MC is grateful for the ALS Doctoral Fellowship in Residence from the Advanced Light Source (ALS) at the Lawrence Berkeley National Laboratory (LBNL). We also thank Dr. Cheng Wang from the ALS at the LBNL for fruitful discussions and Jan Wernecke, Stefanie Langner and Levent Cibik from the Physikalisch-Technische Bundesanstalt for support during the measurements.


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