Molecular orientation in soft matter thin films studied by resonant soft x-ray reflectivity

Abstract

We present a technique to study depth profiles of molecular orientation in soft matter thin films with nanometer resolution. The method is based on dichroism in resonant soft X-ray reflectivity using linear s-and p-polarization. It combines the chemical sensitivity of Near-Edge X-ray Absorption Fine Structure spectroscopy to specific molecular bonds and their orientation relative to the polarization of the incident beam with the precise depth profiling capability of X-ray reflectivity. We demonstrate these capabilities on side chain liquid crystalline polymer thin films with soft X-ray reflectivity data at the carbon K edge. Optical constants of the anisotropic refractive index ellipsoid were obtained from a quantitative analysis using the Berreman formalism. For films up to 50 nm thickness we find that the degree of orientation of the long axis exhibits no depth variation and is independent of the film thickness.

Surface layering and melting in an ionic liquid studied by resonant soft X-ray reflectivity

“…However, standard hard XRR, used in all surface studies to date, is not atom-specific for the low-Z atoms found in ILs; it is only sensitive to the surface-normal total electron density profile, but cannot directly resolve the distribution of the IL's various chemical moieties. In contrast, resonant XRR (8, 9) takes advantage of the strong variation of the atomic scattering factor with X-ray energy near an atom's absorption edge to distinguish between moieties of different atomic composition (10), functional groups (11), and molecular orientation (12,13). Nevertheless, the challenge posed by molecular-resolution resonant soft XRR measurements at liquid surfaces hitherto prohibited such measurement, despite the widely acknowledged need for directly and unambiguously determining the distribution of the chemical moieties near liquid surfaces.…”

Characterization of organic thin films with resonant soft X-ray scattering and reflectivity near the carbon and fluorine absorption edges

“…This dependence has two major consequences: i) "bond-specific" or functional group specific scattering can be achieved in R-SoXS by judicious choice of photon energy [6][7][8], in a manner analogous to NEXAFS microscopy of polymers [ 22,23], ii) in addition, Δδ 2 +Δβ 2 overcomes or at least neutralizes the E 4 factor that would favor use of higher energy photons and orders of magnitude larger scattering intensity can be achieved near the carbon edge relative to photons with E ∼10 keV as used in conventional SAXS [7,11]. This affords the opportunity to get useable scattering intensities in transmission even from rather thin films only 20-200 nm in thickness [7][8][9]24]. More recently, it has been pointed out and demonstrated that R-SoXS furthermore has unique contrast to bond orientation if polarized light and polarization control of the incident photons are available [24].…”

“…This affords the opportunity to get useable scattering intensities in transmission even from rather thin films only 20-200 nm in thickness [7][8][9]24]. More recently, it has been pointed out and demonstrated that R-SoXS furthermore has unique contrast to bond orientation if polarized light and polarization control of the incident photons are available [24].…”

“…This linear dichroism can also be exploited in scattering/reflectivity and the combination of dichroism and reflectivity has been used for the first time to study depth profiles of molecular orientation in soft matter thin films with nanometer resolution. poly(2-{4-[(4-cyanophenoxy)carbonyl]phenoxy}ethyl acrylate), a polymer with liquid-crystalline side chains, has been used in this study [24]. Strong differences in the reflectivity between s and p polarization geometries were recorded in the vicinity of the absorption resonance associated with the phenylene and nitrile π systems present in the polymer.…”

Kramers-Kronig constrained modeling of soft x-ray reflectivity spectra: Obtaining depth resolution of electronic and chemical structure

“…Rather, obtaining the spatial distribution of resonant optical properties directly by simultaneously modeling spectroscopy and structure in fitting experimental data would provide a useful alternative. Fixed-energy determinations of optical constants are common in the x-ray region (generally through fitting of angular resolved reflectivity, R(Q)) [13][14][15][16][17][18][19][20][21] . While this approach has been extended to fitting fixed energy reflectivity data at a number of energies spanning the atomic core level as a means of spectroscopy 22 , full spectroscopic analysis is not generally applied to reflectivity data.…”