Effect of dilute doping and non-equilibrium synthesis on the structural, luminescent and magnetic properties of nanocrystalline Zn1-xNixO (x = 0.0025 – 0.03)
Graphical abstract
Introduction
Due to its direct wide band gap (3.37 eV), large exciton energy (60 meV), outstanding electrical and optical as well as piezoelectric properties, ZnO is of great interest for applications in a wide range of technological devices. Zinc oxide being inexpensive makes TM-doped ZnO dilute magnetic semiconductors (DMSs), with Curie temperature (Tc) exceeding room temperature, one of the most attractive candidates among the II-VI semiconductors for applications in spintronics (semiconductor electronics that use both charge and spin) [1], e.g in memories for increase of recording speed, storage density and transmission capacity. Transition metal (TM) doped oxide nanostructures with intrinsic and extrinsic magnetic properties combined with enhanced (adsorption, catalytic) activity can be developed in sensing devices (e.g. in magnetic responsive positioning [2], new luminescence-based detection and magnetic resonance imaging (MRI) contrast agents [3], photocatalytic sensors [4]) for use in (bio)medicine, biology, pollution control, toxicology, energy conversion, etc. The strong excitonic binding energy of ZnO combined with the weakening of the visible emission (which can be induced by TM doping) can find application in photonic devices operating in blue - UV spectral region [5].
The structural, optical and magnetic properties of zinc oxide can be influenced by the dopant type and concentration as well by the methods and conditions of sample preparation [[6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]]. These properties of ZnO are largely associated with native point defects since it naturally exhibits unintentional n-type conductivity [21]. The behavior of the visible emission in photoluminescence (PL) can provide important information (often in combination with electron paramagnetic resonance (EPR)) about native defects in ZnO, including here also the Ni-doped ZnO system. Different interpretations were advanced for this emission as well as for its origin. It is recognized that Ni-doping in ZnO suppresses the green-yellow PL emission [[6], [7], [8],11,12]. However, different reasons were proposed for this suppression, among which the decrease of the concentration of native defects and the increase of non-radiative centers. Regarding structural properties, most articles report that Ni-doping decreases the a- and c-axis lattice constants, as well as the cell volume of ZnO [6,7]. It is claimed that the decrease of the lattice parameters is caused by the substitution of Zn ions with the smaller Ni ions. However, in the condition of low Ni doping levels in ZnO dilute magnetic semiconductors the substitution of the Zn2+ with the slightly smaller Ni2+ could not be the only influential trigger. The different morphologies reported for Ni-doped ZnO systems [6,8,12,13] are considered to be caused mostly by different preparation techniques and synthesis methods used. However, only few studies [13] have been made suggestions on the influence of Ni itself on the thermodynamics of the formation and growth of ZnO, and thereby, on its morphology. Moreover, the theoretical calculations demonstrate that the concentration of certain type of native defects (and thus, the derived properties) of ZnO depends not only on the preparation method, but also on whether the system has reached the thermodynamic equilibrium or not [21,22].
Concerning room temperature ferromagnetism (RTFM), controversial results on the magnetization properties of Ni-doped ZnO systems were reported: e.g. RT saturation magnetization (Ms) can increase [7,12], can decrease or can show a maximum value at certain doping level [7,8,11] with the increase of the Ni concentration. Differences may appear even when using the same method of Ni-doped ZnO synthesis. The sources and mechanism of ferromagnetism in TM-doped ZnO DMSs are still under debate. Some theoretical calculations stated that ferromagnetic state can be the ground state for Ni-doped ZnO system [23], whereas others predicted that the antiferromagnetic (AFM), paramagnetic or spin-glass states of transition metal atoms are preferable to the FM, and that the TM-doped ZnO requires extra carriers for ferromagnetism [24,25]. It was also predicted that in the (Zn,Ni)O system the ferromagnetic state can be stabilized by electron doping and that extra-carriers can be provided by co-doping with n-type elements (e.g. Ga) or by n-type native defects [[24], [25], [26], [27]]. In the latter case, Zn interstitials and oxygen vacancies can act as shallow electron donors [21,28,29] that can supply the carriers in a matrix of localized spins, making (Zn,Ni)O a promising candidate for high-Tc ferromagnetism.
Therefore, there are still many unresolved issues to address in view of understanding and controlling the optical, structural, catalytic or magnetic behaviors of Ni (TM)-doped ZnO required for their practical application. The goal of this work is to identify the influence of the Ni-doping level in the dilute regime, considering as well the specificity of synthesis conditions and the influence of Ni itself on the thermodynamics of ZnO formation and growth, on the morphological, structural, chemical states, photoluminescence and magnetic properties of ZnO nanopowders produced by sol-gel route. The results are explained in terms of formation of a non-equilibrium concentration of donor native defects (Zni and VO) in n-type ZnO and delocalization of carriers supplied by native defects to the Ni2+ sites (assuming as well that Ni doping can enhance the concentration of native defects in their charge state).
Section snippets
Powders preparation
Nanopowders of Zn1-xNixO, x = 0.0025, 0.005, 0.01 and 0.03, were produced by sol-gel synthesis. The solutions prepared from nickel(II) acetate tetrahydrate [Ni(OCOCH3)2 · 4H2O] and zinc acetate dehydrate [Zn(OOCCHH3)2 ⋅ 2H2O] (ACS grade, assay percent range: 98–101.0%, Alfa Aesar) in the propionic acid (CH3CH2COOH) (purity >99%, Merck) were mixed at room temperature (RT) in a 100 ml beaker in order to obtain Zn1-xNixO. This mixture was maintained for 30 min under stirring at 60 °C on a hot
Results
Fig. 1 shows the comparative XRD patterns of the reference and Ni-doped ZnO powders. The XRD measurements reveal a highly crystalline ZnO phase of hexagonal wurtzite structure (space group P63mc) for all powders. XRD data (i.e. a- and c- lattice constant, cell volume, Vc, c/a ratio, crystallite size and microstrain) extracted by Rietveld fitting for reference and Ni-doped ZnO powders are listed in Table 1. The variation of Vc and c-axis constant with the increase of Ni doping level is shown in
Morphology. The influence of Ni doping on the aggregation processes of nanocrystals
The morphology of Zn0.9975Ni0.0025O powder with the lowest doping level and thus, with minimum potential influence of Ni, should be rather determined by the specific preparation conditions. The aggregates of randomly adhered crystals in Zn0.9975Ni0.0025O powder (Fig. 3a1, main panel) denote a high reaction rate during ZnO formation and growth. This rapid or difusionless aggregation kinetic [13] is consistent with the relatively high heating rate used, 4 °C/min (between 200 °C and 400 °C). As
Conclusions
Sol-gel synthesized Ni-doped (0.25–3 at.% Ni) ZnO powders were processed under the non-equilibrium conditions (i.e., short time annealing at 400 °C in air, with high heating and cooling rates). Both the dilute Ni doping level and the non-equilibrium processing conditions were found to modify the morphology, structure, chemical states, photoluminescence and magnetic properties of the nanocrystalline Ni-ZnO powders. The modification of these properties (by Ni doping and processing conditions) can
Acknowledgments
The authors gratefully acknowledge the financial support from the CCDI-UEFISCDI PN-III-P1-1.2-PCCDI-2017-0871: projesct 47PCCDI/2018 and Core Program PN18-110101 of Romanian Ministry of Research and Innovation. The authors thank Dr. I. Pasuk and A. Leca (NIMP, Romania) for their experimental and technical support.
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