General characteristics of exfoliated hBN. Figure shows a typical AFM image of the exfoliated hBN sheets with a lateral size of a few m. Each sheet has a relatively smooth surface with a thickness t of 24 nm. Considering that the interlayer distance of hBN is ~0.33 nm, we can estimate that each nanosheet consists of ~515 BN layers. We show in Fig. a low-magnification annular dark field scanning TEM (ADF-STEM) image of the exfoliated hBN, showing a random stacking of micrometer-sized hBN layers. A high-magnification ADF-STEM image of non-twisted hBN layers is given in Fig. , illustrating 2D honeycomb structures of BN with asymmetric ADF scattering intensities. One may expect that three boron and three nitrogen atoms in each hexagonal ring are responsible for this asymmetric contrast, which can be in principle distinguished by their intensity based on the Z-(atomic number) contrast principle. However, we should note that our exfoliated hBN nanosheets are likely to have more than 5 AA-stacked BN layers, which are formed by stacking many anti-aligned layers B to N and N to B. In such layers, the expected Z-contrast will be averaged out to yield symmetric ADF scattering intensities. We hence assume that observed asymmetric ADF contrast results from a slight specimen tilt. To confirm the assumption, we performed simulations by changing x-tilt angle φ (for details, see Fig. ) using a comprehensive multipurpose crystallographic program Recipro. After several try-and-error attempts, we found that the specimen of t = ~8 nm and φ = ~1° yields the asymmetric contrast which is most comparable to that of the observed ADF-STEM image (see Fig. ). Although the thickness estimated from the simulation is somewhat greater than that obtained from an AFM image shown in Fig. , these estimated values are reasonably consistent with each other. Thus, we can conclude that the thickness of our exfoliated hBN sheets lies in the range from ~3 to ~8 nm, which will be thin enough to satisfy the weak phase object approximation (WPOA) especially for the samples consisting of light elements such as B and N. Under WPOA, the TEM image operated in the phase contrast mode at optimum focus directly reveals the projected potential. HRTEM images shown in Fig. ,b demonstrate that the resulting layers have a clean and adsorbates-free area extending several tens of nanometers (see also Fig. ). The corresponding fast Fourier transform (FFT) image yields six spots of 0.21, 0.12 and 0.11 nm spacings attributed, respectively, to 101 ̅ 0, 112 ̅ 0 and 202 ̅ 0 Bragg peaks (see the inset in Fig. ). Figure is the PPA strain mapping obtained for the HRTEM image shown in Fig. . As shown in Fig. , the strain tensor is estimated to be as low as 1 % over the entire region of interest (see Fig. for more details of the PPA analysis).