Solid state effects in the NEXAFS spectra of alkane-based van der Waals crystals: Breakdown of molecular model

Experimental data conclusively shows that the weak van der Waals interactions between alkane molecules, <1.5% the strength of covalent bonding on a per carbon atom basis, modify the carbon 1s near edge X-ray absorption fine structure (NEXAFS) spectrum significantly in ordered solids, both in intensity, spectral shape, peak position, and dichroic signature. This constitutes a further breakdown of the ‘building block’ model, or, more precisely, even a molecular model in interpreting NEXAFS spectra. These observations have significant implications for the interpretation and use of NEXAFS spectra from any crystalline or semi-crystalline macromolecules, small molecules, or other weakly interacting systems. 2006 Elsevier B.V. All rights reserved. Over the last two decades, near edge X-ray absorption fine structure spectroscopy (NEXAFS) has evolved into a powerful characterization tool for a number of materials and systems. The NEXAFS of the carbon 1s absorption edge is particularly rich in spectroscopic features [1–3], a result of the many different bonding configurations carbon can have with itself and with heteroatoms such as nitrogen, oxygen, and other elements. The interpretation and use of NEXAFS for polymers and large molecules in the condensed phase are generally based on small-molecule analogues and calculations based on isolated small molecules [1,2]. Implicit in this approach is the assumption that the character of the bonding and antibonding orbitals in molecules are dominating the NEXAFS spectra and that intermolecular interactions and matrix effects are negligible. Indeed, the relative strength of covalent C–H orbitals and van der Waals forces in alkane crystals is 98.5% covalent. It is thus not surprising that the small-molecule analogue approach (‘building-block principle’) in combination with 0009-2614/$ see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.08.110 * Corresponding author. E-mail address: [email protected] (H. Ade). 1 Present address: Infineon Technologies, IFD P300 CP, Königsbrücker Straße 180, D-01099 Dresden, Germany. 2 Present address: Advanced Light Source, Berkeley National Laboratory, Berkeley, CA 94720, United States. calculations has yielded considerable insight for the interpretation of NEXAFS spectra of complex and large molecules, including polymers [4–7]. In the case of completely saturated molecules such as alkanes and related materials, the originally envisioned building block was the C–C unit with a single bond between carbon atoms [8]. However, analysis of the angular dependence in self assembled alkyl monolayers showed that the transition dipole moment (TDM) is not along the individual bond direction, but along the backbone direction [9]. A breakdown of the ‘building block’ model was pronounced. For extensive delocalized orbitals in p-conjugated systems, the ‘building block’ necessary to properly describe the NEXAFS spectrum was also shown to be much larger than the individual bonds, requiring a chemical moiety large enough to comprise and properly represent any delocalized orbital [10]. For polymers and weakly interacting molecules in the solid state, intermolecular interactions were generally not observed or expected and hence neglected. Only few theoretical and experimental studies examined the influence of van der Waals forces and intermolecular interactions on NEXAFS spectra of organic materials. Theoretical consideration by Weiss et al. suggested small energy shifts, but no intensity changes, as a function of 288 Y. Zou et al. / Chemical Physics Letters 430 (2006) 287–292 molecular separation [11]. No effect on conformation was observed in theory and thus spectral changes between ordered and disordered Langmuir–Blodgett (LB) films of Ca-archidate were interpreted to arise from density, i.e. intermolecular, changes rather than conformational, i.e. intramolecular, changes [11]. A study of ethylene-1-alkene copolymers with controlled composition showed relatively large changes in intensity that correlated with the crystallinity of the ethyle-1-alkene copolymer. In contrast to the prediction of Weiss et al., no discernable energy shifts could be observed in these ethylene-1-alkene copolymers. Theoretical calculations by Schöll et al. suggested that the observed changes in intensity are due to intermolecular interaction rather than difference in conformations of a polymer chain [12]. Although solid state effects have thus previously been observed [11–13], the extent and control of order in the materials investigated was insufficient to allow complete understanding. We thus used NEXAFS microscopy to study exceptionally well defined materials in the form of single alkane crystals with defined orientations. The experiments show that even just fractional changes in the weak van der Waals interactions, <1.5% the strength of covalent bonding on a per carbon atom basis, modify the NEXAFS spectrum of organic van der Waals solids significantly in intensity, spectral shape, peak position, and dichroic signature. Thin crystals of n-tetracontane C40H82 (TC, Tm = 80 C) and n-nonadecane C19H40 (ND, Tm = 33 C) were prepared by solution-casting. TC (98.8%, Supelco) and ND (99%, Aldrich) were dissolved in toluene and acetonitrile (99%, Fluka), respectively, with a mass/volume ratio Fig. 1. (a) VLM image of crystallites from n-tetracontane (top) and n-nonad crystalline alkanes and polyethylene as viewed from the [001] direction. The ob C–C backbone plane with the a-axis is indicated. h0 48 , based on crystalli contains the carbon–carbon backbone. of 0.5 mg/ml and stirred at 50 C. A drop of the solution was placed onto a 100 nm thin silicon nitride (Si3N4) window. The crystallization during the rapid solvent evaporation took place at room temperature for TC. The ND solution was cast at 8 C. The crystallites produced were characterized with a Nikon Labophot II visual light microscope (VLM) equipped with a SPOT CCD attachment. Although some irregular crystallites formed, numerous crystallites with straight edges and well defined angles and shapes could be found for both alkanes. Acute and obtuse angles of 68 and 112 , respectively, were measured for crystallites with regular diamond or parallelogram shapes and even thickness (see Fig. 1a). These angles are in complete accordance with {110} terminated orthorhombic unit cells (See Fig. 1b) derived from X-ray diffraction studies [14]. Well formed, 100–200 nm thick crystals were investigated with scanning transmission X-ray microscopes (STXM) at advanced light source (ALS) beamlines 5.3.2 (BL5.3.2) and 11.0.2 (BL11.0.2), respectively, with a spatial resolution of 40 nm and energy resolution R > 2000 [15]. With proper optics alignment, the BL5.3.2 STXM delivers X-rays linearly polarized in the horizontal plane. At the BL11.0.2-STXM, an elliptical undulator provides complete control over the polarization of the X-ray beam. Images and spectra were recorded with the Si3N4 membrane mounted perpendicular to the optical axis [15]. At BL5.3.2, this sample geometry puts the polarization direction in the plane of the membrane. The spectra from crystallites of different thickness were compared to exclude the possibility of any thickness related effect. Additionally, the ecane (bottom) after casting from solution. (b) Orthorhombic unit cell of tuse angle spanned by {110} planes is labeled as a. The angle h0 of the C– ne structure shown in [25]. (c) Symmetry plane of propylene unit, which Y. Zou et al. / Chemical Physics Letters 430 (2006) 287–292 289 samples were rotated in the plane of the Si3N4 membrane in small angular steps to acquire dichroic spectra from the same crystal. The radiation damage was monitored after the acquisition of spectra by checking for mass loss and for the occurrence of a 285.1 eV resonance that is due to formation of C@C double bonds caused by X-ray radiation damage [16]. The X-ray beam was defocused for the acquisition of spectra to effectively eliminate the possibility of radiation damage. The spectra were carefully normalized to the incident intensity. In order to remove any sensitivity to orientation and acquire average reference spectra, circularly polarized light at the BL11.0.2-STXM was utilized. The energy scale for all the spectra was calibrated using characteristic vibronic peaks of gaseous CO2 [17]. Fig. 2 shows the carbon 1s NEXAFS spectra of typical TC crystallites and STXM images of such crystals acquired at a photon energy of 288.0 eV. Crystallites imaged with STXM are also well-defined parallelograms with obtuse angle a = 112 ± 1 , agreeing with the VLM and the derived calculated angle between the two long diagonals of the orthorhombic unit cell. The shape analysis strongly suggests that the alkyl chains of TC are perpendicular to the substrate. This is directly confirmed by NEXAFS spectra from various crystallites. Following the assignment by Stöhr et al. [18], the dominant features in the NEXAFS spectra of alkanes are distinctive C 1s ! r C–H=Rydberg peaks around 288 eV and a broad C 1s ! r C–C feature at 291 eV. Conventional NEXAFS dichroism spectra of n286 288 290 292 294 no rm .in t[ a. u] photon energy C1s NEXAFS