Fig. 2: Temperature-dependent near-field maps of nano-THz response of WTe2 micro-crystals.
From: Terahertz response of monolayer and few-layer WTe2 at the nanoscale
The scale bars in all panels are 3 µm. a Near-field images of the normalized amplitude contrast \({S}_{1}/{S}_{1}^{{{{{\mathrm{sub}}}}}}\) of nearly identical regions at 6 different temperatures between 250 K and 70 K. The 44 K image includes only 2L and 3L regions and the 1L region is shown in Fig. 2c. Broadband THz signal utilized in these images shows intensity peaked at 0.6 THz. The signal due to the SiO2/Si substrate (\({S}_{1}^{{{{{\mathrm{sub}}}}}}\)) has negligible temperature dependence in the THz range studied here. The horizontal dashed arrow in the panel at 200 K indicates the scanning line-cut used to construct the plot in Fig. d. Micrometer-sized dark spots also visible in the topographic AFM contrast can be attributed to bubbles in the encapsulated structures. b Near-field \({S}_{2}/{S}_{2}^{{{{{\mathrm{sub}}}}}}\) images taken simultaneously with \({S}_{1}/{S}_{1}^{{{{{\mathrm{sub}}}}}}\). c Enlarged images zoomed at the interface between 1 L and 2 L, 1 L and the substrate. d \({S}_{1}/{S}_{1}^{{{{{\mathrm{sub}}}}}}\) line-cut (averaged over 5 neighboring pixels) at 250 K and 70 K. The line-cut corresponds to the arrow in Fig. 2a. e Normalized \({S}_{2}\) signals averaged in the regions indicated in the \({S}_{2}\) images (white dashed boxes) for the substrate and for 1L, 2L, 3L regions of WTe2. The filled squares are experimental data. The error-bars are the standard deviations of the extracted data. The dashed line is a model calculation using the dielectric properties of the bulk material. The magnitude of the signal for bulk dielectric properties is rescaled by a factor of 0.67 to be comparable with 3L WTe2.
