Photoinduced Atomic Force Spectroscopy and Imaging of Two-Dimensional Materials

We report on the near-field imaging of atomically thin layers of two-dimensional (2D) materials using photoinduced force mapping. This is accomplished by modifying a traditional atomic force microscopy set up to detect optical forces between a nanoscale tip and a photoexcited sample. Our set up facilitates the imaging of few-layer flakes of ${\mathrm{Mo}\mathrm{S}}_2$ or ${\mathrm{WS}}_2$ and the simultaneous acquisition of optical force spectra, both in ambient and vacuum conditions. The evaluated force spectra in both samples exhibit the characteristic excitonic resonance peaks that are most typically observed in far-field absorption spectroscopy. We also show that nanoscale defect sites and flake edges can be distinguished from the crystalline surfaces with a high spectral resolution. Our results pave the way toward gaining a wholesome understanding of optical interactions and structure-property correlations in 2D materials and their heterostructures.

Figure 1

Schematic of the AFM-based set up used for photoinduced force imaging and spectroscopy.

Figure 2

The topography (a) and photoinduced force maps (b) of a region consisting of two overlapping layers of ${\mathrm{Mo}\mathrm{S}}_2$ on glass, acquired by modulating the laser at f_m = 71 kHz. (c) A representative schematic of the lock-in amplitude as a function of the lock-in detuning frequency when the tip is positioned on top of glass (blue trace) and on top of ${\mathrm{Mo}\mathrm{S}}_2$ (red trace). Similarly, (d) and (e) show the topography and photoinduced force maps acquired at f_m = 69 kHz. (f) same as panel (c) for the case when f_m < f₀. Note that the photoinduced images in (b) and (e) correspond to the optical force gradient maps, as the acquired lock-in signal is based on frequency shifts of the cantilever resonance.

Figure 3

Photoinduced force spectra acquired on a sample consisting of (a) ${\mathrm{Mo}\mathrm{S}}_2$ and (c) ${\mathrm{WS}}_2$ flakes deposited on separate glass substrates. The blue traces are the experimental force signals, retrieved as a function of wavelength. The solid black lines are the Gaussian fits of the measured force spectra and the dashed lines indicate the residual peaks of the corresponding cumulative Gaussian function. The red trace in panel (a) indicates the force spectrum acquired near the same region as the blue trace, shown here to emphasize the reproducibility in experiments. (b) and (d) Far-field absorption spectra obtained on the ${\mathrm{Mo}\mathrm{S}}_2$ and ${\mathrm{WS}}_2$ samples, respectively.

Figure 4

(a) Topography and (b) a representative photoinduced force map acquired on the ${\mathrm{WS}}_2$ sample (at 2.28 eV or λ = 545 nm), indicating regions with varying thicknesses and flake orientations on the neighboring glass substrate. (c) Photoinduced force spectra averaged over regions 1 (red trace) and 2 (green trace), as indicated by the squares in panel (a). (d) Photoinduced force maps acquired at three different wavelengths over the region indicated by the dashed rectangle in panel (a). The dotted yellow rectangles in panel (d) indicate the boundary between two flakes (top panel), defects on the surface or the edges of one multilayer flake (middle panel). The scale bars in panel (d) are 500 nm.