Theoretical Studies On The Spin Hamiltonian Parameters For D^5,7,9 Ions In Nanocrystals

Nanocrystals doped with transition-metal impurities usually show unique optical, luminescent and magnetic performances, which may closely correlate to the local structures and electronic energy levels of the dopants. Electron paramagnetic resonance( EPR) is an effective method to investigate transitions of unpaired electrons, impurity occupation(at the core or surface of nanocrystals) and local structures. By analyzing the spin Hamiltonian parameters( SHPs, i.e., g factors, hyperfine structure constants and zero-field splittings) of EPR spectra for transition-metal ions in nanocrystals, important microscopic information(e.g., impurity occupation and local structures) of the transition-metal impurities in nanocrystals can be obatined and provide the reliable theory basis for the investigations on the performances and local structures of the nanocrystals with transition-metal dopants.53d, 73 d and 93 d ions are very important systems in the transition-metal groups. Despite the abundant EPR experimental data for these ions in nanocrystals(especially II- VI semiconductor nanocrystals), theoretical explanations and analyses about the above experimental results are relatively scarece. 1) The previous investigations were largely confined to transition-metal ions in either nanocrystals or bulk materials solely. For the same transition-metal ions(e.g., 2Mn(10)) in both nanocrystals and bulk materials, few comparitive investigations were stressed on the local behaviors and spectroscopic performances. 2) Quite different spectroscopic performances(e.g., significant difference of hyperfine structure constants) were found for the dopants(e.g., 2Mn(10)) at the core or surface of nanocrystals. However the quantitative criterion of occupation(at the core or surface) has not been established for 2Mn(10) in II- VI nanocrystals up to now, accompanied by some wrong judgements for occupation of impurities(e.g., 2Mn(10)) in II- VI nanocrystals. 3) In the previous theoretical analyses of anisotropic g factors and hyperfine structure constants of the transition-metal dopants(e.g., 2Cu(10)) in nanocrystals, the local structures of impurities were not quantitativly connected with the EPR analysis but often treated with various adjustable tetragonal field parameters. Thus, the previous investigations are difficult to acquire quantitative information of local structures around the impurities. 4) In spite of strong covalency for transition-metal ions in II- VI semiconductors, only the central ion orbital and spin-orbit coupling contributions were considered in the previous investigations based on the conventional crystal-field model, which is not suitable to the transition ions in II- VI systems. Thus, the contributions containing the ligand orbital and spin-orbit coupling interactions are normaly important for these systems and should be taken into account.In order to get over the defects of the previous investigations, in this work, the perturbation formulas of the SHPs for 3d~5, 73 d and 93 d ions in trigonally or tetragonally distorted octahedra(or tetrahedra) are used for these impurities in II- VI nanocrystals. By analyzing II- VI nanocrystals containing53d(2Mn(10)),73d(Co2+) and93d(2Cu(10)) ions systematically, the EPR results are satisfactorily interpreted for these systems, and the local structural information of impurities are obtained. Especially, the quantitative criterion of occupation(at the core or surface) for 2Mn(10)in the Cd X and Zn X(X = S, Se, Te) nanocrystals is presented for the first time.(1) The criterion of occupation for 2Mn(10) in Cd X and Zn X nanocrystals is presented for the first time. In view of the different hyperfine structure constants for distinct occupations(at the core or surface) of 2Mn(10) in Cd X and Zn X nanocrystals, the quantitative criterion is presented from the inequations of hyperfine structure constants for 2Mn(10) in these nanocrystals, and the previous assignments of signals SI as 2Mn(10)at the core of Cd S and Zn Te nanocrystals are renewed by those as 2Mn(10) at the surface.(2) By using the high order perturbation formulas of SHPs for trigonally distorted tetrahedral 53 d clusters containing the ligand orbital and spin-orbit coupling contributions, the EPR spectra and local structures are systematically studied for 2Mn(10) in Zn X(X= O and S) nanocrystals and bulks materials in a uniform way. For 2Zn O : Mn(10)bulk materials, the local tension is induced arising from size mismatch substitution of smaller 2Zn(10) by larger 2Mn(10), which forces 2Mn(10) to shift along the 3C axis by about 0.042 ? away from the oxygen triangles to remove the local tension. For the 2Zn O : Mn(10) nanocrystals, the impurity 2Mn(10) is found to undergo along the 3C axis by about 0.036 ? away from the oxygen triangles because of the internal stress and size mismatch substitution. Compared with the bond angle(q ≈ 109.47o) of the cubic 2Mn(10) center in Zn S bulk materials, the 2 2Mn O(10)-- bond angle shows an angular increase of 0.39° for the trigonal Mn2(10) center in Zn S nanocrystals.(3) Based on the perturbation formulas for a 93 d ion in tetragonally distorted tetrahedra and tetragonally elongated octahedra, the SHPs are studied for 2Cu(10) in the Zn O and Zn S nanocrystals as well as Cd S thims with different impurity concentrations. The calculated SHPs based on tetragonally distorted tetrahedra indicate better coincidence with the experimental values for all the above systems than those based on tetragonally elongated octahedra. Due to the Jahn- Teller effect, the angular deviations(35)q are 2.47°, 1.68° and 1.54°, respectively, for Cu2(10) in Zn O and Zn S nanocrystals and Cd S thims, inducing tetragonally compressed tetrahedra. And the previous improper assignments of occupations of impurity 2Cu(10) are also corrected. Second, the isotropic g factors for 2Cu(10) in X = O, S and Zn X( Se) nanocrystals are investigated, and the isotropic g factor is predicted for 2Cu(10) in Zn S nanocrystals. The angular deviations(35)q are found to be about 1.26°, 1.24° and 1.07° for 2Cu(10) in Zn O, Zn S and Zn Se nanocrystals, respectively, due to the dynamical Jahn- Teller effect. As for the isotropic g factors for 2Cu(10) in the Zn O nanocrystal systems I and II with different impurity concentrations, the angular deviations(35)q are found to be about 1.5°, and the different concentration dependences of g factors and(35)q are also discussed in view of the dissimilar preparation conditions for the two systems.(4) From the perturbation formulas of SHPs for a tetragonally elongated octahedral 93 d cluster, the two 2Cu(10) centers I and II in Zn O nanocrystals are theoretically studied, and the experimental d- d transition bands and EPR spectra are satisfactorily interpreted. Centers I and II demonstrate 3.4% and 3.2% tetragonal elongation distortions, respectively, due to the Jahn- Teller effect. On the other hand, the measured Knight shifts for 2Cu(I)(10) site in 2 4Hg Ba Cu O(10)dare reasonably explained from the perturbation formulas of Knight shifts for a tetragonally elongated octahedral 93 d cluster, and the anisotropic g factors are also theoretically predicted.(5) In light of the perturbation formulas of SHPs for trigonally distorted tetrahedral 73 d clusters, the d- d transition optical and EPR spectra are theoretically analyzed for 2Co(10) in Zn O nanocrystals, with the ligand contributions included. The calculated d- d transition bands and SHPs show reasonable coincidence with the experimental data, and the local angular deviation is found to be about 0.51°.