Optical phonons and strong optical nonlinearity of synthesized black phosphorene quantum dots

Here, we prepared ultra-small black phosphorene quantum dots (BPQDs) using a liquid exfoliation method involving sonication. The structural features of BPQDs have been investigated using TEM, AFM, Raman, UV-visible, PL, and FTIR analysis. The morphological analyses show the 1-7 layer of BPQDs with an average size of about 4-5 nm. The spectroscopic analyses of BPQDs imply absorption in the UV wavelength region and band gap energy of about 3.3 eV. Also, in the case of excitation with the wavelength of the UV region, a blue shift emission is seen in the PL spectrum. The P = O, O-P-O and O = P-OH bonds are seen in the FTIR spectrum. Using the FTIR spectrum and Kramers–Kronig (KK) relation, some optical properties of BPQDs, such as refractive index, dielectric function, and frequencies of an optical phonon (transverse (TO) and longitudinal (LO)) were obtained. As well as utilizing the Z-scan technique, the nonlinear refractive index (n2) and nonlinear absorption coefficient (β) were achieved. The closed-aperture (CA) Z-scan testing indicated the self-defocusing of BPQDs. Also, the open aperture (OA) Z-scan traces of the BPQDs with a valley-like pattern expressed the two-photon absorption (2 PA). So, BPQD, as a new material, is a promising candidate for optical and nano-electrical devices.