Iron substitution in CdSe nanoparticles: Magnetic and optical properties

Chemically synthesized, thiol capped undoped, and Fe doped CdSe nanoparticles (NPs) have been investigated using a variety of physicochemical techniques. The electron spin resonance spectra exhibit two distinct signals at $g\sim 4$ and $g\sim 2$ characteristic of ${\text{Fe}}^{3+}$ ions occupying highly asymmetric and nearly symmetric lattice sites, respectively. The room temperature Mössbauer studies show a broad asymmetric doublet with isomer shift $\sim 0.35\text{mm}/\text{sec}$, quadrupole splitting $\sim 0.76\text{mm}/\text{sec}$ characteristic of high spin ferric ions occupying the ${\text{Cd}}^{2+}$ sites with associated changes in local lattice environment. The room temperature photoluminescence spectra show transition from excited state ${\text{Fe}}^{3+}$ (${{}^4\text{T}}_1$, ${{}^4\text{T}}_2$, and ${}^4\text{E}$) to the ground-state ${\text{Fe}}^{3+}$ $\left({{}^6\text{A}}_1\right)$ and also suggest that photoexcited electrons are preferentially transferred to iron ion induced trapping centers in CdSe nanoparticles. Room-temperature ferromagnetism is also observed in thiol capped Fe doped CdSe nanoparticles. Significant changes in saturation magnetization value with increasing iron concentration have been observed. The origin of room temperature ferromagnetism and significant change in ${\text{M}}_{\text{s}}$ value are discussed in terms of $F$-center exchange mechanism (bound magnetic polarons).

Figure 1

(Color online) EDS patterns of CdSe and Fe doped CdSe nanoparticles with different Fe (at. wt. 1.51%, 8.58%, and 16.23%) doping percentages. Insets show the enlarged view of EDS patterns in 5–7.5 KeV region.

Figure 2

(Color online) (a) XRD pattern of pure CdSe NPs, Fe (at. wt. 1.51%) doped CdSe NPs, Fe (at. wt. 8.58%) doped CdSe NPs, and Fe (at. wt. 16.23%) doped CdSe NPs with bulk CdSe zincblende peak position. (b) TEM image of pure thiol capped CdSe nanoparticles. (c) Selective area electron diffraction pattern of pure thiol capped CdSe nanoparticles.

Figure 3

(Color online) UV-vis absorption spectra for different samples, i.e., pure CdSe NPs, Fe (at. wt. 1.51%) doped CdSe NPs, Fe (at. wt. 8.58 %) doped CdSe NPs, and Fe (at. wt. 16.23%) doped CdSe NPs.

Figure 4

(Color online) PL Emission spectra (${\lambda }_{\text{ex}}=360$ and 410 nm) with absorption plot of sample (a) pure CdSe NPs, (b) Fe (at. wt. 1.51%) doped CdSe NPs, (c) Fe (at. wt. 8.58%) doped CdSe NPS, and (d) Fe (at. wt. 16.23%) doped CdSe NPs. (e) Plot of emission (${\lambda }_{\text{ex}}=360$ and 410 nm) intensity vs at. wt. % of Fe in CdSe nanoparticles.

Figure 5

(Color online) Schematic illustration of the photoluminescence spectroscopic analysis of Fe doped CdSe NPs excited by 410 and 360 nm.

Figure 6

(Color online) Valence-band spectra of undoped and Fe doped CdSe nanoparticles with 100 eV photon energy.

Figure 7

(Color online) (a) ESR spectra of pure CdSe and different concentration of Fe (at. wt. 1.52%, 8.58%, and 16.23%) doped CdSe nanoparticles. (b) Fe (at. wt. %) vs $g$ plot of Fe doped CdSe nanoparticles.

Figure 8

(Color online) Mössbauer spectra of (a) pure CdSe NPs, (b) Fe (at. wt. 1.51%) doped CdSe NPs, (c) Fe (at. wt. 8.58%) doped CdSe NPs, and (d) Fe (at. wt. 16.23%) doped CdSe NPs.

Figure 9

(Color online) Room temperature (300 K) magnetization curve after background correction (a) CdSe nanoparticles, (b) Fe (at. wt. 1.51%) doped CdSe nanoparticles, (c) Fe (at. wt. 8.58%) doped CdSe nanoparticles, and (d) Fe (at. wt. 16.23%) doped CdSe nanoparticles. (e) Plot of saturation magnetization vs Fe (at. wt.) doping percentage in CdSe nanoparticles. (f) Magnetization vs temperature curve for 8.58% Fe doped CdSe nanoparticles in the ZFC conditions for the applied field value 1000 Oe.

Figure 10

(Color online) (a) Fe (at. wt. 1.51%) doped CdSe NPs, (b) Fe (at. wt. 8.58%) doped CdSe NPS, and (c) Fe (at. wt. 16.23%) doped CdSe NPs.