The effect of permanent magnetic field on photoluminescence of nanopowder oxides produced by pulsed electron beam evaporation
Introduction
Nanopowders (NP) have unique properties related to the dimensional factor, as well as other features of the internal structure, particularly the defects of composition, structure and shape. This is especially the case for NP produced by physical methods — evaporation by a laser [1] or plasma synthesis [2]. At the same time, from all known methods of production of NP using the evaporation–condensation principle, the method of evaporation with pulse electron beam (PEBE) in vacuum allows to obtain NP with the largest number of defects of various types [3].
Powders produced by PEBE have the following features [3].
(1) Morphologically they consist of small particles (5–10 nm) which are connected into three-dimensional agglomerate structures (20–600 nm) with fractal structure (Fig. 1).
(2) Agglomerates contain pores of various sizes, mostly mesopores (Fig. 2), which can be filled with water, air and various gases which are almost completely removed when heated to 400 °C. As an example, nitrogen adsorption–desorption isotherms and pore volume distribution for NP CeO2 are given in Fig. 3.
(3) The particles often have a crystalline core and an amorphous shell. Images of typical core–shell nanoparticles Cu-doped ZnO and Gd2O3@SiO2 are shown in Figs. 4a and 4b, respectively.
(4) The results of energy dispersion X-ray analysis show severe stoichiometry disorder in NP, usually excess metal in oxide or fluoride forms [4].
(5) In this work was used NP Al2O3, which consisted of three crystalline phases (, and ) and an amorphous component. Relative content of crystalline phases in NP is given in Fig. 5a. The diffractogram of amorphous crystalline NP containing the cubic phase CeO2 is shown in Fig. 5b.
The presence of room temperature ferromagnetism associated with defective structure of NP, including d0 magnetism of non-magnetic oxides and fluorides in bulk state [5].
There is currently no generally accepted explanation for d0 ferromagnetism of oxides [6], although this phenomenon is observed in various chemical compounds [7], [8]. That is why the study of the nature of this phenomenon is relevant.
In photoluminescence (PL), the energy of excitation radiation is transferred to radiation, including that arising due to various types of defects [9], which makes this diagnosis one of the most important ways to study defects. In addition, the PL spectrum depends on particle size and porosity [10], which is particularly important for mesoporous NP produced by PEBE.
It has long been known [11] that an increase in the intensity of PL correlates with an increase in electronic defects located in the band gap, since the intensity of radiation is a function of the concentration of electronic defects. Considerable interest in establishing correlation of magnetic and luminescent properties has been manifested in recent years [12], [13]. In particular, in work [14] was showed that a constant magnetic field affects the microstructure and homogeneity of the ultrasmall superparamagnetic iron oxide hydrocolloids, them photodynamic activity and fluorescent properties. The result of the work [14] is in agreement with previous findings in which aggregation of NPs has been shown to impact the fluorescence emittance; aggregated metallic NPs displayed significantly enhanced PL compared to not aggregated ones [15], [16].
The purpose of the present work was to study the change of photoluminescent properties of nanopowders of cerium and aluminum oxides in a permanent magnetic field.
Section snippets
Experimental technique
The PEBE technique and the results of the study of the properties of mesoporous nanocrystalline powders CeO2 and Al2O3 are described in [17], [18], respectively. Note that the investigated NP had similar textural and magnetic properties (Table 1), the structure of NP contained both crystalline and amorphous constituents. The difference between the NP was that the magnetism of the CeO2 was ferromagnetic and that of Al2O3 was a superposition of ferro and diamagnetic constituents. In addition, NP
Results and discussion
Properties of the NP and the obtained measurement results are given in Table 1 and Fig. 2, Fig. 3. Spectrum 2 in Fig. 2 was recorded using cable P1-630A-FS-1 and spectrum 2 in Fig. 3 using a P1-460B-FS-1 cable. As can be seen from the obtained data, measurements through the fiber optic cable, when the magnetic field is superimposed, significantly decrease the amplitude of the signal. To eliminate the noise, the recorded spectra were smoothed (Fig. 2).
For the first time, it has been discovered
Conclusion
Thus, for the first time, the effect of a permanent magnetic field on shift of photoluminescent peaks of nanopowder oxides produced by pulsed electron beam evaporation in vacuum has been discovered.
It has been found that when a magnetic field of 0.154 T is superimposed, shifts in PL spectrum maxima are observed for Al2O3 in the yellow region (14 nm) and CeO2in the red region (50 nm).
In the above references [14], [15], [16], the authors linked the increase in the intensity of PL mainly to the
CRediT authorship contribution statement
S.Yu. Sokovnin: Conceptualization, Methodology, Project administration, Writing - review & editing. V.G. Ilves: Investigation, Resources, Writing - review & editing. M.G. Zuev: Investigation, Resources.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The author would like to thank Dr. D.R. Emlin (Institute of Electrophysics UB RAS) for magnetic measurements.
This work was supported in part by the Russian Foundation for Basic Research and Sverdlovsk region within the framework of project No. 20-48-660019 p_a.
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