**4. Conclusions**

High-quality Bi2Se3 thin films have been prepared on a mica substrate via vdW epitaxial growth. The film exhibited a highly c-axis-oriented growth with an extraordinary crystallinity, reflected by a narrow XRD rocking curve FWHM of ~317 arc seconds. At the same time, fence-like planes with Bi2Se3 (015) parallel to the mica (001) surface emerged, which was explained by using the interface model associated with the CSLU concept developed especially for vdWE. To eliminate the fence-like planes, an amorphous Bi2Se3 buffer layer prepared at the substrate temperature of room temperature was applied, prior to the Bi2Se3 crystal growth. As a result, only the Bi2Se3 (001) growth direction with vdW gaps parallel to the substrate showed up, with a smoother surface than that without the amorphous buffer layer. However, this suppression in the fence-like planes sacrifices to a certain degree the crystal lateral growth alignment, mirrored by a broadening phi scan peak FWHM. A high Hall mobility of 1761 cm<sup>2</sup>/(Vs) for Bi2Se3 on mica was obtained at ~12 K, and 697 cm<sup>2</sup>/(Vs) at room temperature with a nearly constant sheet electron density of ~1.0 × 10<sup>14</sup> cm<sup>−</sup>2. By applying an amorphous Bi2Se3 buffer layer, the carrier density was reduced to about half that without the buffer layer. The mobility at room temperature was boosted to 726 cm<sup>2</sup>/(Vs), and linearly increased to 1469 cm<sup>2</sup>/(Vs) as the temperature dropped to 12 K. An optoelectronic device consisting of a Bi2Se3/mica TCE, organic absorber, and Ca/Al metal back electrode was designed and prepared, generating a short-circuit photocurrent of 2.75 mA/cm<sup>2</sup> and an open-circuit voltage of 697 mV under one sun irradiation. The results demonstrate the grea<sup>t</sup> potential of 2D Bi2Se3 along with mica for flexible optoelectronic applications.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2079-4991/10/9/1653/s1, Figure S1: AFM images of 90 nm-thick Bi2Se3 film with an rms roughness of 17.8 nm, Figure S2: In-plane phi scan of Bi2Se3 on mica with buffer layer, Figure S3: UPS spectrum of Bi2Se3.

**Author Contributions:** Conceptualization, S.W. and A.N.; methodology, S.W., A.N. and Q.H.; validation, Y.L.; formal analysis, Y.L.; investigation, Y.L., Q.Z., X.L. and H.L.; resources, Q.H.; data curation, S.W. and A.N.; writing—original draft preparation, S.W.; writing—review and editing, A.N.; supervision, S.W.; project administration, S.W. and A.N.; funding acquisition, S.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Wuhan University of Technology-Tibet University Joint Innovation Fund, gran<sup>t</sup> number LZJ2020003, and the Reform and Development Funds for Local Region Universities from China Government in 2020, gran<sup>t</sup> number ZCKJ 2020-11.

**Conflicts of Interest:** The authors declare no conflict of interest.
