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Nanoscale chemical imaging by photoinduced force microscopy

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Science Advances  25 Mar 2016:
Vol. 2, no. 3, e1501571
DOI: 10.1126/sciadv.1501571
  • Fig. 1 Schematic of IR PiFM experiment.

    The incident mid-IR laser is electrically triggered to pulse at fm = f1f0, where f0 and f1 are the first and second mechanical eigenmode resonances of the cantilever. The topography of the sample is recorded by the AFM feedback system at f1, and the PiFM is concurrently recorded at f0 by the feedback laser position-sensitive detector and lock-in electronics. The sample is raster-scanned under the tip to generate the image. The incident light is polarized along the tip axis to maximize the signal coupling of the dipole-dipole force along the vertical direction of the cantilever vibration.

  • Fig. 2 The PiF signal and cantilever amplitude as a function of tip-sample distance on 10-nm-thick PS-r-PEDCPMA brush prepared on Si and excited with 1733 cm−1.

    The black curve is the amplitude curve of the second resonance, which probes the topography in a dynamic AFM mode. The green curve is the amplitude of the first mechanical resonance, used for probing the photoinduced force with direct-drive modulation to measure both the localized and nonlocalized forces. The blue curve is the amplitude of the first mechanical resonance, used for probing the photoinduced force with sideband modulation to measure mostly the localized force behavior. The orange curve is the amplitude at the contact resonance frequency, which measures the sample dilation once the tip is in physical contact with the surface. The three curves (blue, green, and red) are normalized by the maximum signal obtained in the sideband excitation. a.u., arbitrary unit.

  • Fig. 3 PiFM and FTIR spectra of homopolymers PS, PMMA, and P2VP.

    (A to C) A comparison of PiFM-generated spectra (black lines) and FTIR spectra (red lines) of homopolymers (A) PS, (B) PMMA, and (C) P2VP. The PiFM spectrum is normalized against a Si substrate. The fine structure in the PiFM resonance peaks is due to water absorption in the laser pathway. Each PiFM spectrum was acquired in 1-cm−1 steps and dwelling 50 ms for each wave number.

  • Fig. 4 Chemically selective PiFM imaging of PS-b-PMMA.

    (A to E) Fingerprint regions of PS-b-PMMA observed by PiFM at 1492 cm−1 (A and D) and 1733 cm−1 (B and E) corresponding to the absorption bands for PS and PMMA, respectively. The topography (C) and PiFM (A or B) signal are obtained simultaneously. (D and E) PiFM resolves the expected ~20-nm-wide domains of the self-assembled BCP with a spatial resolution of <10 nm. (F) Normalized point spectra show contributions from the convolution of multiple lamellae due to the tip diameter.

  • Fig. 5 Fingerprint region of PS-b-P2VP.

    (A to E) PiFM imaging at (A) 1447 cm−1, (B) 1452 cm−1, (C) 1469 cm−1, (D) 1492 cm−1, and (E) 1589 cm−1. The PiFM point spectra taken from the locations shown in (E) were normalized against a Si background. The different spectral response (green curve) of one of the three nodules clearly visible in (A) suggests that a material other than PS and P2VP may be present.

  • Fig. 6 DSA region of PS-b-P2VP.

    (A to D) PiFM images at (A) 1492 cm−1 and (C) 1589 cm−1 selectively image PS and P2VP domains, respectively. Topographic images (B) and (D) were taken simultaneously with images (A) and (C), respectively. Black lines at the bottom of (A) and (B) indicate where PiFM responds to PS lines, whereas white lines at the bottom of (C) and (D) indicate the location of P2VP lines. These colored indicators show that PS domains in the BCP lie directly on the top of topographic ridges, which are attributed to the known topography of XPS lines in the underlying guiding pattern. (E) Fabrication steps for the DSA of PS-b-P2VP (details are discussed in Materials and Methods). EUV-IL, extreme ultraviolet interference lithography.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/2/3/e1501571/DC1

    Text

    Fig. S1. Schematic for thermal calculation: A gold-coated probe with end radius a in the vicinity of PS on a silicon substrate.

    Movie S1. Demonstration of hyperspectral photoinduced force imaging on a BCP composed of P2VP and PS.

    References (45, 46)

  • Supplementary Materials

    This PDF file includes:

    • Text
    • Fig. S1. Schematic for thermal calculation: A gold-coated probe with end radius a in the vicinity of PS on a silicon substrate.
    • Legend for movie S1
    • References (45, 46)

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    Other Supplementary Material for this manuscript includes the following:

    • Movie S1 (.mp4 format). Demonstration of hyperspectral photoinduced force imaging on a BCP composed of P2VP and PS.

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