Research ArticleChemistry

Nanoscale simultaneous chemical and mechanical imaging via peak force infrared microscopy

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Science Advances  23 Jun 2017:
Vol. 3, no. 6, e1700255
DOI: 10.1126/sciadv.1700255
  • Fig. 1 Operational scheme of PFIR microscopy.

    (A) Operation diagram for the PFIR apparatus. Detailed operation mechanisms are described in Materials and Methods. (B) Gate-averaged traces of vertical deflections of the cantilever with the laser interaction (red curve) and without the laser interaction (blue curve). The timing of the infrared laser pulse (black curve) is chosen to be within the contact regime of the peak force tapping cycle. Subtraction of the two vertical deflection traces is used to obtain the PFIR trace. (C) The PFIR trace of PTFE at the infrared frequency of 1160 cm−1 is shown. The laser interaction leads to three types of behaviors: (i) the contact resonance oscillation of the cantilever, (ii) the baseline offset Δ, and (iii) the slope of the baseline. (D) PFIR spectra of PTFE from the oscillation amplitude (osc. amp.) (blue curve) and the baseline offset (red curve) across its vibrational resonances. The FTIR spectrum for a bulk sample is included as a reference (black dashed curve). AU, arbitrary units. (E) The power dependence of the PFIR oscillation amplitudes for the PTFE sample versus the pulse energy of the infrared laser. A linear dependence is observed.

  • Fig. 2 PFIR imaging and spectroscopy on P2VP.

    (A) Topography of nanoscale P2VP polymer islands on a gold substrate. (B) The PFIR image of the same area with an infrared frequency of 1433 cm−1. Islands of P2VP show increased PFIR response. (C and D) The measured modulus and adhesion maps of the same area. The mechanical responses are simultaneously collected with the PFIR image. (E) PFIR spectra based on the oscillation amplitude of contact resonance (blue curve) and baseline offset (red curve) from the marked locations (black circle) in (B). The FTIR spectrum (dashed curve) is included as a reference.

  • Fig. 3 PFIR imaging of nanophase separation of a PS-b-PMMA block copolymer.

    (A) Topography of the PS-b-PMMA block copolymer. (B) PFIR image at the infrared frequency of 1492 cm−1, resonant with one of the vibrational modes of polystyrene. Therefore, the domains of polystyrene provide high PFIR signals. (C) PFIR at the infrared frequency of 1725 cm−1. The infrared frequency is resonantly exciting the vibrational mode of PMMA; therefore, the domains of PMMA provide high PFIR signals. (D) PFIR image at the infrared frequency of 1620 cm−1. The frequency is off-resonant with both PMMA and polystyrene, so a low PFIR response is observed. (E and F) Simultaneously registered modulus map (E) and adhesion map (F) of the same region of block copolymer. (G) PFIR spectra taken at the two locations marked by the white arrows in (A). The spectrum (red curve) is taken from the ridge of the topography and corresponds to the PMMA domains. The spectrum (blue curve) is taken from the valley of the topography and corresponds to the polystyrene (PS) domain. (H) The reference FTIR spectra from the bulk sample of PMMA (red curve), polystyrene (blue curve), and the PS-b-PMMA block copolymer (inset). The PFIR spectra show good agreement with the reference FTIR spectra from the bulk. inten., intensity. (I) Estimation of the spatial resolution from a section profile in (B) shows a spatial resolution of 10 nm, from the width between 90 and 10% of the edge height of the PFIR signal.

  • Fig. 4 PFIR imaging and spectroscopy on the perovskite crystal.

    (A) The AFM topography of the surface of the CH3NH3PbBr3 perovskite crystal. A 500-nm × 300-nm nanocrystal is observed on the large perovskite crystal surface. (B) The PFIR image obtained of the region with the infrared frequency of 1585 cm−1. (C and D) The co-registered modulus and adhesion images of the region on the perovskite crystal. Clear inhomogeneity within the small nanocrystal can be identified from the infrared absorption map, modulus, and adhesion map. (E) PFIR spectra from three individual locations [marked as 1, 2, and 3 in (A) on the perovskite nanocrystal]. The FTIR spectrum of bulk perovskite crystals is shown as a reference (black dashed curve). (F) A section profile of the PFIR response from the perovskite crystal from the marked red line in (B). It demonstrates a spatial resolution of 12 nm, estimated from the width between 90 and 10% of the height of the signal.

  • Fig. 5 PFIR microscopy studies on BNNT.

    A) The topography of a BNNT on a gold substrate. (B) The FTIR spectrum of bulk BNNTs shows the phonon resonances but not the SPhP resonances that would be present above 1380 cm−1 marked in the gray box. (C) The PFIR image at the infrared frequency of 1390 cm−1. (D) The PFIR image at the infrared frequency of 1430 cm−1. (E and F) The s-SNOM images taken at 1390 cm−1 (E) and 1430 cm−1 (F) of the same region of BNNT are shown as references. A similar spatial distribution of the infrared response is observed. (G) PFIR spectra taken from a series of locations along the BNNT at 100-nm increments from the tube terminal show position-dependent variations with clear SPhP resonances. The spectra are vertically offset for clarity and were taken along the black dashed line in (A) starting 50 nm away from the terminal. (H) A possible SPhP excitation scheme for PFIR. (I) The SPhP excitation and detection scheme in s-SNOM. (J) Estimation of spatial resolution in the PFIR measurements from a cut profile of the BNNT [white line in (C)] is found to be 9.5 nm from the width between 90 and 10% of the height of the PFIR signal.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/6/e1700255/DC1

    fig. S1. Fourier transform of the PFIR trace.

    fig. S2. Power dependence of the baseline offset amplitude.

    fig. S3. Mechanical behaviors and power dependence of PMMA.

    fig. S4. Comparison between the cantilever oscillation amplitude and baseline offset in PFIR microscopy on the PS-b-PMMA block copolymer.

    fig. S5. Deformation map of perovskite from peak force tapping microscopy.

    fig. S6. Detailed PFIR image of a 400-nm × 400-nm region of the PS-b-PMMA block copolymer.

    fig. S7. Procedure to calibrate the tip radius for PFIR microscopy.

    fig. S8. PFIR images with different peak force set points.

    fig. S9. The spectra of PTFE with different peak force set points.

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. Fourier transform of the PFIR trace.
    • fig. S2. Power dependence of the baseline offset amplitude.
    • fig. S3. Mechanical behaviors and power dependence of PMMA.
    • fig. S4. Comparison between the cantilever oscillation amplitude and baseline offset in PFIR microscopy on the PS-b-PMMA block copolymer.
    • fig. S5. Deformation map of perovskite from peak force tapping microscopy.
    • fig. S6. Detailed PFIR image of a 400-nm × 400-nm region of the PS-b-PMMA block copolymer.
    • fig. S7. Procedure to calibrate the tip radius for PFIR microscopy.
    • fig. S8. PFIR images with different peak force set points.
    • fig. S9. The spectra of PTFE with different peak force set points.

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