Low-energy charge transfer excitations in NiO

Abstract

Abstract. Comparative analysis of photoluminescence (PL) and photoluminescence excitation (PLE) spectra of NiO poly-and nanocrystals in the spectral range 2-5.5 eV reveals two PLE bands peaked near 3.7 and 4.6 eV with a dramatic rise in the low-temperature PLE spectral weight of the 3.7 eV PLE band in the nanocrystalline NiO as compared with its polycrystalline counterpart. In frames of a cluster model approach we assign the 3.7 eV PLE band to the low-energy bulk-forbidden p-d (t1g(π)-eg) charge transfer (CT) transition which becomes the allowed one in the nanocrystalline state while the 4.6 eV PLE band is related to a bulk allowed d -d (eg-eg) CT transition scarcely susceptible to the nanocrystallization. The PLE spectroscopy of the nanocrystalline materials appears to be a novel informative technique for inspection of different CT transitions. IntroductionDespite several decades of studies there is still no literature consensus on the detailed electronic structure of the prototype 3d oxide NiO, in particular, the character of the low-energy charge transfer excitations. NiO has long been viewed as a prototype "Mott insulator" [1] with a gap formed by intersite d -d charge transfer (CT) transitions, however, this view was later replaced by that of a "CT insulator" [2] with gap formed by p-d CT transitions. At present we have no comprehensive assignment of intensive spectral features in NiO to different p-d or d -d CT transitions. The main reason for this is that the absorption coefficient rises steeply above 3.5 eV and reaches a giant value of 10 6 cm −1 at and above 4 eV [3] that complicates the detailed absorption and reflection measurements particularly near the fundamental edge. This difficulty can be avoided in part by making use of specific techniques such as electroreflectance measurements [4] or nonlinear absorption measurements [5] that are particularly effective for location of the forbidden transitions.Main goal of our paper is to show with NiO as an example that the photoluminescence excitation (PLE) spectroscopy of the nanocrystalline materials appears to be a novel informative technique for inspection of different charge transfer transitions. Indeed, on the one hand, nanocrystalline state gives rise to a noticeable modification of the optical response of materials, in particular, due to an enhanced role of the surface induced local structural distortions resulting in a shift and splitting of the energy levels, and, mainly, in a liberalization of the

Flexible nanosheets for plasmonic photocatalysis: microwave-assisted organic synthesis of Ni–NiO@Ni2CO3(OH)2 core–shell@sheet hybrid nanostructures

“…The fast component with characteristic time constants of ∼2.1 ns and 2.3 nm for MAS-N70 and pristine N70, respectively, matches with previously reported data for NiO. 26 On the other hand, the slow decay component of 21.5 ns observed for the MAS-N70 sample (Fig. 5d) is very close to literature data, 26 while the 25 ns decay constant obtained for the pristine samples is found to be slightly higher than that reported in the literature.…”

“…26 On the other hand, the slow decay component of 21.5 ns observed for the MAS-N70 sample (Fig. 5d) is very close to literature data, 26 while the 25 ns decay constant obtained for the pristine samples is found to be slightly higher than that reported in the literature. This suggests the presence of more trapping states in the pristine sample in comparison with MAS-N70.…”

“…Based on the literature data we assume that d-d (450 nm) and p-d (410 nm) radiative transitions in NiO have been detected. 26 Luminescence excitation spectra have been recorded for each of these emissions. The comparison of the excitation spectra for both samples is shown in Fig.…”

Charge Transfer Transitions and Circular Magnetooptics in Ferrites

“…It is now believed that the most intensive low-energy electron-hole excitations in insulating 3d oxides correspond to the charge transfer (CT) transitions [7,8,[31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49]. Namely, the CT transitions are considered as a likely source of the optical and magneto-optical response of the 3d metal-based oxide compounds, in particular, ferrites, in a wide spectral range of 1-10 eV, primarily of the fundamental absorption edge.…”

“…These have a long and distinguished history of applications in electron, optical and magnetooptical spectroscopy, magnetism and magnetic resonance. The author and his colleagues has successfully demonstrated the great potential of the cluster model for description of the p-d and d-d charge transfer transitions and their contributions to optical and magnetooptical responses of various 3d oxides, such as ferrites [31][32][33][34][35][36][37][38][39][40], cuprates [41][42][43][44][45], manganites [40,46,47] and nickelates [48,49]. Five Me 3d and eighteen oxygen O 2p atomic orbitals in the octahedral MeO 6 complex with the point symmetry group O h form hybrid Me 3d-O 2p bonding and antibonding e g and t 2g molecular orbitals, and purely oxygen nonbonding a 1g (σ), t 1g (π), t 1u (σ), t 1u (π) and t 2u (π) orbitals (see, e.g., [5,8,29,46]).…”