High-energy exciton transitions in quasi-two-dimensional cadmium chalcogenide nanoplatelets

Abstract

Semiconductor nanoparticles of cadmium chalcogenides are known to exhibit pronounced thickness-dependent E0 series of exciton transitions at the Γ point of the Brillouin zone (BZ). In this work, we report an experimental evidence for high-energy series of exciton transitions, which originates from BZ points different from the Γ point, in the family of cadmium chalcogenide quasi-2D nanoplatelets (NPLs). Intensive UV absorption bands demonstrating a pronounced size effect are observed for CdTe, CdSe, and CdS NPLs in addition to the E0 exciton bands in the visible region. These new bands are attributed to transitions analogous to the E1, E1 + ∆1, and E2 series observed in bulk crystals. First-principles DFT calculations of the electronic structure and absorption spectra support this explanation and show that the main contribution to these optical transitions comes from X and M points of the 2D BZ, which originate from L and X points of the 3D BZ. At the same time, the E0 series of transitions at the Γ point is well described by the multiband effective-mass model. The observation of the UV exciton bands reveals tunable optical properties of cadmium chalcogenide NPLs in UV spectral region, which may be interesting for practical applications.

Lattice dynamics of quasi-two-dimensional CdSe nanoplatelets and their Raman and infrared spectra

“…As shown in Ref. 22, energetically the most favorable position of these surface atoms are the bridge positions in which the halogen atoms enter the positions of the missing selenium atoms in the ZB structure ( Fig. 1).…”

Nanosilicon stabilized with ligands: Effect of high‐energy electron beam on luminescent properties

“…In this case, the influence of the surface microrelief and the dielectric properties of the environment on the characteristics of excitons localized in the near‐surface region can be described by introducing the effective permittivity, for example, according to the effective medium model 19 . And the increase in the binding energy of the surface exciton can be due to an increase in the ratio between the kinetic and potential energies of the quasiparticles, namely, the vanishing of the kinetic energy of the quasiparticle in one, two, or three coordinates and, thus, an increase in the potential energy of the particle with a dimensional limitation of its displacements 19,20 …”

“…19 And the increase in the binding energy of the surface exciton can be due to an increase in the ratio between the kinetic and potential energies of the quasiparticles, namely, the vanishing of the kinetic energy of the quasiparticle in one, two, or three coordinates and, thus, an increase in the potential energy of the particle with a dimensional limitation of its displacements. 19,20 F I G U R E 3 Photoluminescence spectra of (A) Bu m -nc-Si n (I) and (B) (NHC) m -nc-Si n (II): 1, dry nanoparticles before treatment; 2, dry nanoparticles after 5-min treatment by 7-MeV electron beam (Ф 1 = 1.25Á1,015 cm −2 ); 3, dry nanoparticles after 48-min treatment by 7-MeV electron beam (Ф 2 = 1.2Á1,016 cm −2 ); and 4, nanoparticles in suspension without treatment…”

Negative thermal expansion in CdSe quasi-two-dimensional nanoplatelets

“…In Ref. 5 it was shown that the most energetically favorable position of terminating atoms is the bridge position, in which the atoms enter the positions of missing Se atoms.…”

“…These properties are promising for various applications in nano-and optoelectronics (see Ref. 5 and references therein). It is known that the thermodynamically stable modification of bulk CdSe is the hexagonal wurtzite structure.…”