Nanoscale chemical heterogeneity of 2D materials

Nanoscale spatial resolution infrared spectroscopy has been found to be an excellent tool for the characterization of 2D materials, as it can image optical modes of these materials (see Figure, a) and chemical heterogeneity at nanoscale resolution (see Figure, b) [1, 2]. Currently, there are two approaches to nanoscale mid-IR that yield complimentary information:

  1. Photothermal induced resonance (PTIR, see Figure, d) gives direct, model-free access to local absorption while
  2. scattering scanning near field optical microscopy (s-SNOM, see Figure, c) detects sample dependent changes of scanning probe tip polarizability which can be converted to optical constants through sample geometry dependent models.

As we have recently demonstrated, ideally, PTIR and s-SNOM should be performed on resonant nanoscale structures to fully characterize them. As both techniques pose different requirements on the measurement setup (e.g. low repetition rate pulsed laser vs. continuous wave laser; contact mode AFM for highest sensitivity vs. tapping mode AFM only), thus concurrent use of both techniques is currently not possible.

a) Hyperbolic phonon polariton modes in hexagonal BN [3]. b) s-SNOM spectra of MoS 2 on SiO [1]. c) Sketch of s-SNOM signal generation. d) PTIR signal generation - the local absorption is detected via the cantilever deflection signal [3].

© Bernhard Lendl

a) Hyperbolic phonon polariton modes in hexagonal BN [3]. b) s-SNOM spectra of MoS2 on SiO [1]. c) Sketch of s-SNOM signal generation. d) PTIR signal generation - the local absorption is detected via the cantilever deflection signal [3].


We will develop a new tool for concurrent PTIR and s-SNOM measurements based on a broadly tunable external cavity quantum cascade laser (EC-QCL) operated in pulsed mode. The envisioned advancement in NanoIR technology shall significantly increase spectral coverage (pulsed sources cover a broader range as CW operated sources) and imaging speed. Image fusion of both optical imaging channels will enable facile evaluation of spectral near field images. Our tool will characterize nanoscale devices fabricated by Weber and Eder to determine the presence and composition of defects.


The main challenge in developing the tool itself lies in high speed data processing to extract the pulsed laser s-SNOM signal. FPGA technology will be used to solve this problem. While pulsed laser s-SNOM has been demonstrated before, current approaches require off line data processing and repeat rescan of the sample to acquire the complex tip scattering coefficient.


Nanoscale IR spectroscopy will be used to analyze functionalised MoS2 flakes functionalized by Holzer and Marchetti-Deschmann, as well as structural modifications at flakes patterned by ion impacts (Wilhelm), in particular the adsorption of molecules such as hydrocarbons. We will provide spatially-resolved s-SNOM and PTIR data for catalytic centers to Eder and Parkinson and spatially resolved current maps of memristor devices to Mueller.


Bernhard Lendl is an analytical chemist with a focus on developing new analytical techniques based on vibrational spectroscopy. His group has developed scanning probe based nanoscale imaging techniques. In collaboration with the Centrone group at the National Institute for Standards and Technology (NIST) Lendl demonstrated sensitivity of 2 zmol of cytarabine inside a drug delivery vesicle. Georg Ramer supports this project. He has an established track record in nanoscale IR spectroscopy and general nanoscale material characterization including 2D materials [3], nanoscale characterization of current flow of memristors [4], and thermal and chemical characterization of metall-organic framework.


Group of Prof. Lendl


  1. P. Patoka, G. Ulrich, A. E. Nguyen, L. Bartels, P. A. Dowben, V. Turkowski, T. S. Rahman, P. Hermann, B. Kästner, A. Hoehl, G. Ulm, and E. Rühl. Nanoscale plasmonic phenomena in CVD-grown MoS2 monolayer revealed by ultra-broadband synchrotron radiation based nano-FTIR spectroscopy and near-field microscopy. Optics Express 24, 1154–1164 (2016), DOI: 10.1364/OE.24.001154.
  2. J. J. Schwartz, H.-J. Chuang, M. R. Rosenberger, S. V. Sivaram, K. M. McCreary, B. T. Jonker, and A. Centrone. Chemical Identification of Interlayer Contaminants within van der Waals Heterostructures. ACS Applied Materials & Interfaces 11, 25578–25585 (2019). DOI: 10.1021/acsami.9b06594.
  3. G. Ramer, M. Tuteja, J. R. Matson, M. Davanco, T. G. Folland, A. Kretinin, T. Taniguchi, K. Watanabe, K. S. Novoselov, J. D. Caldwell, and A. Centrone. High-Q dark hyperbolic phonon-polaritons in hexagonal boron nitride nanostructures. Nanophotonics (2020), DOI: 10.1515/nanoph-2020-0048.
  4. J. M. Goodwill, G. Ramer, D. Li, B. D. Hoskins, G. Pavlidis, J. J. McClelland, A. Centrone, J. A. Bain, and M. Skowronski. Spontaneous current constriction in threshold switching devices. Nature Comm. 10, 1–8 (2019). DOI: 10.1038/s41467-019-09679-9.