Structure and Luminescence of 2D Dilute Magnetic Semiconductors:  Cd₁_-<i>_x</i>Mn<i>_x</i>Se·L_0.5 (L = Diamines)

Abstract

The two-dimensional (2D) dilute magnetic semiconductors Cd 1 - x Mn x Se·L 0.5 (L = ethylenediamine, or en, and 1,6-hexanediamine, or hda, x = 0−0.8) were synthesized in an autoclave at 120 °C. Ab initio structure solution from X-ray powder diffraction reveals the host compound CdSe· hda 0.5 (space group, Pbca, a = 6.8852 Å, b = 6.7894 Å, c = 27.4113 Å) is structurally analogous to CdSe· en 0.5, except for a subtle difference in alignment of aliphatic diamine ligands the hda molecule deflects from the c axis and inclines toward the b axis. CdSe·L 0.5 shows well-defined UV absorption and emission peaks, which is attributed to a 2D exciton band edge transition due to size confinement effect in the c direction and the only photoemission level is the 2D exciton ground state with a long lifetime (7 μs) and intrinsic line width (177 meV) at room temperature. When Cd 2+ is partly substituted by Mn 2+, a strong Mn 2+ -related luminescence peak at 2.12 eV (584 nm) is obtained at room temperature, which can be assigned to Mn 2+ internal transition ( 4 T 1 → 6 A 1 ); its excitation peak overlaps with the photoemission peak of the 2D exciton ground state which indicates that the Mn 2+ emission is driven by the 2D exciton ground-state transition. For x = 0.02, the photoluminescence intensity of Cd 1 - x Mn x Se· hda 0.5 reaches maximum and enhances 28 times compared with that of Cd 1 - x Mn x Se· en . When x < 0.05, the Mn 2+ luminescence is a characteristic single-exponential decay process with a well-defined constant lifetime of 375 μs. Electron spin resonance spectra show that Mn 2+ substitutes Cd 2+ ion and forms a [MnSe 3 N] coordination tetrahedron and that there are isolated Mn 2+ luminescence centers in Cd 1 - x Mn x Se· hda 0.5 ( x < 0.05), which is the key factor for their stronger luminescence character compared to Cd 1 - x Mn x Se· en 0.5 .