Research ArticleChemistry

Infrared vibrational nanocrystallography and nanoimaging

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Science Advances  07 Oct 2016:
Vol. 2, no. 10, e1601006
DOI: 10.1126/sciadv.1601006
  • Fig. 1 IR nanocrystallography.

    (A) Schematic of IR s-SNOM using both synchrotron and quantum cascade laser (QCL) radiation. (B) Lewis structure (top) and crystal structure (bottom) of PTCDA in the β phase. (Bottom right) Crystal axes (red) oriented relative to the substrate, with θ indicating the angle between the a axis of the crystal and the z axis of the surface normal. (C) AFM height of aggregates, depicted schematically (bottom). Scale bar, 500 nm. (D) AFM height of vacuum-deposited film, with schematic (bottom) showing growth of molecules with the a axis parallel to the z axis (blue) and defects with the a axis perpendicular to the z axis (red and green). Scale bar, 200 nm.

  • Fig. 2 Broadband IR s-SNOM of large crystallites.

    (A) Far-field transmission spectrum of dispersed PTCDA particles, fit to calculated reflectance using the Lorentz model. Inset: Schematic of C–H out-of-plane vibrations and C=O stretch of the anhydride. (B) SINS spectra (red) collected at two different nanoscale locations with (green) fit to spherical dipole model with Lorentz oscillators. (C and D) Molecular orientation is measured from SINS spectra in a spatial map across large crystals of PTCDA. Point spectra (circles) show fits of point spectra for θ, represented on a false-color scale. Scale bars, 500 nm.

  • Fig. 3 Correlative analysis of defects.

    AFM topography of vacuum-deposited PTCDA near the film edge (A) and corresponding near-field ΦNF (1777 cm−1) measured on resonance with the carbonyl mode (B). Scale bars, 500 nm. (C) Histogram constructed from the images (A and B) showing correlation between AFM height and ΦNF (1777 cm−1). Distinct subpopulations are identified by their statistical correlation and labeled by the colored borders. (D) Spatial map reconstructed from the correlated populations identified in (C) overlaid on AFM height image. Most of the population (green) has the a axis of PTCDA parallel to the surface normal, with smaller populations uplifted with the a axis parallel to the surface normal (light blue), uplifted regions with the a axis oriented away from the surface normal (red), and regions with less coverage and a axis oriented away from the surface normal (yellow). The substrate is shaded dark blue.

  • Fig. 4 Subdomain nanoscale orientation.

    Topography (A) and ΦNF (C=O) images (B) of PTCDA. Scale bars, 100 nm. (C) Line cut of AFM height and Φ(Embedded Image) along the dashed transect indicated in (A) and (B), showing that molecular orientation is uncorrelated with topographic features. (D) Radial correlation function (red) of the AFM tapping phase, indicating the presence of 3D islands with characteristic size and spacing and Φ(Embedded Image) (blue), which shows an extended correlation length of crystallite orientation. Cor. coeff., correlation coefficient; a.u., arbitrary units.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/2/10/e1601006/DC1

    Description of modeling and calculations for IR vibrational nanocrystallography

    Description of nonresonant signal and signal penetration depth

    fig. S1. Experiment and calculated near- and far-field spectra.

    fig. S2. Orientation dependence of near-field signal.

    fig. S3. Nonresonant near-field signal.

  • Supplementary Materials

    This PDF file includes:

    • Description of modeling and calculations for IR vibrational nano-crystallography
    • Description of nonresonant signal and signal penetration depth
    • fig. S1. Experiment and calculated near- and far-field spectra.
    • fig. S2. Orientation dependence of near-field signal.
    • fig. S3. Nonresonant near-field signal.

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