Optical properties due to electronic transitions in two-dimensional semiconductors (CnH2

Abstract

Optical spectra in the visible and uv regions are investigated in layer-type perovskite compounds (C H2 + &NH3)PbI4 with n =4, 6, 8, 9, 10, and 12. The spacing between the PbI4 layers changes from 15.17 A for n =4 to 24.51 A for n =12. In spite of these different spacings, the optical spectra are almost the same for these compounds, which means that the interaction between the layers is weak. The lowest exciton is located at 2.56 eV at 1.6 K, and its oscillator strength and binding energy are 0.7 per formula unit and 320 meV, respectively. These values are very large compared with those in a three-dimensional analog PbI2. The large oscillator strength and binding energy can be explained by the small dielectric constant of the alkylammonium "barrier layer, " which strengthens the Coulomb interaction between an electron and a hole.

Bandgap and exciton binding energies in lead-iodide-based natural quantum-well crystals

“…1. Optical absorption spectra of (a) (C 6 absorption spectra of bilayer and trilayer spin-coated films at 5 K. In these optical absorption spectra, any abrupt leaps of the optical absorption spectra with decreasing temperature like those of (C 10 H 21 NH 3 ) 2 PbI 4 [3] are not observed. The optical absorption spectrum of the 3D thin film [17] is shown in Fig.…”

“…In fact, we have shown recently that the excitons in (C 6 H 13 NH 3 ) 2 PbI 4 are Wannier-type ones with comparatively strong 2D character [12]. Another unique feature of this crystal is that the dielectric constant of the barrier layer is much smaller than that of the well layers (the dielectric constants of the well and the barrier are about 6.1 and 2.1 [3], respectively), in which the effective Coulomb interaction between electrons and holes in the well layers is expected to be enhanced (so called image charge effect [13,14]). We have shown recently that the image charge effect definitely play important roles in the excitonic properties in this crystal [15].…”

“…We first prepared n-alkylamine C n H 2nþ1 NH 3 I ðn ¼ 1; 6Þ by reacting C n H 2nþ1 NH 2 with a stoichiometric amount of HI aqueous solution in a flask cooled with water. The obtained solutions were evaporated to remove water followed by washing with diethylether to obtain white powder of C n H 2nþ1 NH 3 2 . It was found that, however, trilayer were most efficiently prepared when these materials were reacted in the following non-stoichiometric condition: C 6 H 13 NH 3 I:CH 3-NH 3 I:PbI 2 ¼ 2:3:3 in molar ratio [18].…”

“…The solid line is the calculated line using the effective-mass approximation along the c-axis so that the bandgap of bilayer, trilayer, and tetralayer were well reproduced. The parameters we have used in the calculation are summarized as follows: (1) the bandgap of the well and the barrier are 1.70 eV [17] and 5.50 eV [3], respectively. The band offset are set to be 0.9 and 2.8 eV.…”

“…In addition, it had been shown that there exists a family of '2D -3D' crystals (C n H 2nþ1 NH 3 [16]. The three-dimensional (3D) analogue CH 3 NH 3 PbI 3 [17] corresponds to the case m ¼ 1: Since the thickness of the inorganic layers can be controlled by changing m; systematic studies of the excitonic and electronic properties between the 2D and 3D crystals can be performed.…”