# Spin-Related Micro-Photoluminescence in Fe\({}^{3+}\) Doped ZnSe Nanoribbons
## 3 Results and Discussion
The black arrow represents the increasing direction of excitation power; **(c)** fitting result of near band-edge emission band by Gaussian functions at excitation power \(P=15\) mW. The black curve 1 is experimental data; the blue 3, green 4 and brown 5 represent three fitting lines of Gaussian function curve (GFC)1, GFC2, and GFC3, respectively; and the red line 2 is the superposition of the three above GFCs; **(d)** Full widths at half maximum (FWHM) of near band-edge emission band changing with excitation power (filled square, red). Plot of intensity (I) versus excitation power (P) for near band edge (filled circles, black, 467 nm), D1 (filled triangles, blue, 553 nm), and D2 (filled pentagons, green, 630 nm) emission peaks. The emission band D1 at 553 nm may be attributed to the internal \({}^{4}\)T\({}_{2}\) (G) \(\rightarrow\)\({}^{6}\)A\({}_{1}\) (S) _d-d_ transition of Fe\({}^{3+}\) doped ZnSe, and the third band D2 may be related to the \({}^{4}\)T\({}_{1}\) (G) \(\rightarrow\)\({}^{6}\)A\({}_{1}\) (S) of single Fe\({}^{3+}\). Furthermore, [PERSON] et al. have studied absorption spectrum of Fe\({}^{3+}\) doped ZnSe nanoparticles, confirming the position of absorption peaks of \({}^{6}\)A\({}_{1}\) (S) \(\rightarrow\)\({}^{4}\)T\({}_{2}\) (G) and \({}^{6}\)A\({}_{1}\) (S) \(\rightarrow\)\({}^{4}\)T\({}_{1}\) (G) at 518 nm and 610 nm, respectively [23]. It is well known that free Fe\({}^{3+}\) as Mn\({}^{2+}\) has \(d^{5}\) configuration with parallel aligned spins (\(\uparrow\uparrow\uparrow\uparrow\uparrow\)) in its ground state S according to [PERSON]'s rule. If _d-d_ transition can happen, it needs to flip at least one d spin to form the first excited state G (\(\uparrow\uparrow\uparrow\uparrow\downarrow\)). In a Fe\({}^{3+}\) doped ZnSe nanoribbon, the strong exchange interactions (_p-d_) between the extended _p_-band electrons of Se and the localized d electrons of Fe are responsible for the change of the forbidden transition (_d-d_ transition), which also existed in FeSe-based superconductors [34]. Therefore, the D1 and D2 emission peaks are assigned to Fe\({}^{3+}\)_d-d_ transition. The emission of \({}^{6}\)A\({}_{1}\) (S) \(\rightarrow\)\({}^{4}\)T\({}_{1}\) (G) is the lowest _d-d_ transition, while \({}^{6}\)A\({}_{1}\) (S) \(\rightarrow\)\({}^{4}\)T\({}_{2}\) (G) is the high-level transition. Usually, the lowest transition can only be detected. In fact, however, D2 is weaker than D1 in the intensity. Moreover, both of their energies shift to the low energy side (518 to 530 nm, and 610 to 630 nm), which clearly indicates that there is _p-d_ hybridization inside. Another effect is the ferromagnetic coupling between neighboring Fe ions if Fe concentration is increased, which leads to a long-lifetime high d level and clear high level _d-d_ transition. This phenomenon is often called \"intra-shell emission\" [30], as the strong emission at 553 nm. It is easy to think that the emission peaks of D1 and D2 are related to structural defects of ZnSe nanoribbons. Many works reported the defect-related emissions between 500 and 700 nm, such as point defect Zn vacancies, Zn interstitials, and structure defects dislocations, stacking faults and so on [35; 36; 37]. In the PL spectrum of Fe\({}^{3+}\) doped ZnSe nanoribbon, the narrow FWHM of D1 and D2 and the sharp near band-edge emission have eliminated the possibility of point defects for the latter often strongly relax the FX emission. The emission of structural defects is dependent on the change of temperatures. With this idea, temperature-dependent PL spectra are shown in Figure 4a. Figure 3d reveals that the intensities of the three emission peaks increase by increasing powers and the intensity of near band-edge emission increases faster than the emissions of D1 and D2 at the same time, indicating a clear character of a low defect lattice. In Figure 4a, the near band-edge emission peak shifts from 450 nm (2.76 eV) to 463 nm (2.68 eV) with temperature increasing from 5 to 300 K, without peak splitting at low temperature.