Structural and luminescence characterization of β-Ga2O3 nanopowders obtained via high-energy ball milling

β-Ga2O3 nanocrystals have been successfully obtained by the high-energy ball milling method. The obtained nanocrystalline powders have been characterized using X-ray diffraction, transmission electron microscopy, energy-dispersive X-ray spectroscopy, and luminescent spectroscopy. X-ray diffraction patterns show a single gallium oxide structure formed in the milling process with a small amount of α-Ga2O3 phase. The grains of irregular shape were identified with the TEM technique. HRTEM images confirmed the formation of the β-Ga2O3 nanocrystals with a size distribution of 50–80 nm. Three luminescence bands peaked at about 2.52, 2.91, and 3.26 eV were observed in photoluminescence spectra. The gradual blue shift of the emission maxima at the excitation in the fundamental absorption edge under the different milling conditions was detected.


Introduction
Gallium oxide (β-Ga 2 O 3 ) is wide band gap semiconductor, which has attracted significant interest in the last decade, primarily due to various practical applications in particular in power electronics, photodetectors, gas sensors, electrodes, and many others (Pearton et al. 2018;Higashiwaki et al. 2016;Luchechko et al. 2019). This material is transparent in UV and visible spectral regions and exhibits optimal luminescence efficiency. It is therefore a candidate for the next generation of different devices due to its high thermal and longer-term chemical stability.
Nowadays, various preparation methods for nanostructured materials are known. Numerous studies have been provided on the synthesis and investigations of the characteristics of β-Ga 2 O 3 low-dimensional structures obtained mainly by sol-gel, chemical vapor deposition, and hydrothermal methods (Liang et al. 2001;Gopal et al. 2018;Yu et al. 2020;Rafique et al. 2017;Zhang et al. 2005). This made it possible to develop new materials and expand the areas of gallium oxide applications. The combination of different phases of wide band gap semiconductor materials, especially β-Ga 2 O 3 , GaN, SnO 2 as well as graphene makes heterogeneous materials with double or/and triple junctions suitable for advanced nano-and micro-sensors, radiation photodetectors, and other devices (Lupan et al. 2015;Ai et al. 2017).
Among these various methods, the high-energy ball milling method is of particular interest, as it is possible to obtain nanomaterials without contamination with chemical components used during the synthesis because the process is performed under solvent-free conditions. This method has been widely exploited for the synthesis of various nanomaterials (Baláž et al. 2013), in particular nanograins, nanocomposites, and nano quasi-crystalline materials. High-energy ball milling is governed by many parameters, such as milling speed, size and size distribution of the balls, dry or wet milling, the temperature of milling, and the duration of milling (Benjamin 1990;Yadav et al. 2012). Β-Ga 2 O 3 has been synthesized using high-energy ball milling in the past (Swamy et al. 2013).
In this work, structural and luminescent properties of β-Ga 2 O 3 powders prepared by high-energy ball milling were investigated using the methods of X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray (EDX) spectroscopy, and luminescent spectroscopy.

Experimental details
Undoped β-Ga 2 O 3 phosphors were synthesized using a highenergy ball milling method. The high-purity grade Ga 2 O 3 (Sigma Aldrich, 99.999%) was used. The high-energy ball milling process was performed in a Pulverisette 7 Premium line planetary ball mill (Fritsch, Idar-Oberstein, Germany) under the following conditions: air atmosphere, 15 tungsten carbide (WC) milling balls (10 mm diameter), ball-topowder ratio 37, milling speed 300 rpm. The milling time and overall mass of the sample were changed from 1 to 3 h and from 3 to 7 g, respectively (see Table 1), which were the most optimal for obtaining nanomaterials with high luminescence yield.
X-ray diffraction (XRD) patterns were recorded using a D8 Advance X-ray diffractometer (Bruker, Germany) in the Bragg-Brentano geometry, working with a CuKα (λ = 0.15418 nm) radiation. The operating voltage and current were 40 kV and 40 mA, respectively. All samples were scanned in the range from 20° to 50° 2θ with 0.04° step. The microstructure and morphology of obtained nanopowders were visualized using the transmission electron microscopy (S/TEM) FEI Tecnai Osiris microscope equipped with a field emission gun (FEG), operating at an accelerating voltage of 200 kV. Samples were examined in classical transmission (Bright field); high resolution (HRTEM) and selected area diffraction (SAED) modes. A chemical composition mapping was performed by a super energy-dispersive X-ray spectrometer (EDS) coupled with TEM.
Photoluminescence characterization of the samples was carried out on SM 2203 spectrofluorometer at room temperature. More details about the used experimental techniques are described in the works (Kravets et al. 2019;Luchechko et at. 2020a).

Results and discussion
The present experiments were set up in such a way to show whether milling time or changing overall mass will significantly alter the phase composition, and subsequently the photoluminescence properties.
XRD patterns for the samples obtained under different milling conditions are shown in Fig. 1. All diffraction peaks are in good agreement with the data for β-Ga 2 O 3 reported by (Liang et al. 2001;Lupan et al. 2015;Yu et al. 2020) and in card of Powder Diffraction Standards. Dominant diffraction peaks of (402), (202), (111), and (311) crystallographic planes are located at about 30.4, 31.7, 35.2, and 38.4° 2 theta positions and correspond to the monoclinic β-phase of gallium oxide. Besides the main β-Ga 2 O 3 phase, a small amount of additional α-Ga 2 O 3 phase was revealed in all samples, which means a phase transformation takes place during milling. This phenomenon has been reported to occur for many different phases like CaCO 3 (Baláž 2021;Baláž et al. 2015), PbO (Dachille and Roy 1960) or ZnS (Baláž 2008). The content of α-Ga 2 O 3 phase seems to be quite similar for all three samples, thus it cannot be stated whether its formation is favored by either longer milling time (sample G3) or using the lower sample mass (sample G1). Figure 2 shows the TEM image of the typical β-Ga 2 O 3 powder (G1). It can be seen that G1 powder consists of agglomerated particles with an average particle diameter of about 60-90 nm (Fig. 2a). The TEM images of samples G2 and G3 were very similar. The atomic-scale HRTEM image in Fig. 2b revealed that the formed β-Ga 2 O 3 nanocrystals possess a good crystallinity with an interplanar distance of about ~ 0.27 nm. This distance corresponds to the ( 111 -) crystallographic plane of β-Ga 2 O 3 with a monoclinic structure (Bae et al 2018).
The selected electron diffraction pattern of G1 powders is shown in Fig. 3. This figure demonstrates typical patterns with sharp reflections of the monoclinic β-phase of gallium oxide. As there are isolated dots (not concentric rings), it seems that the crystallinity of the sample is quite good.
The HAADF-STEM image of β-Ga 2 O 3 grains for sample G1 is shown in Fig. 4a. The observed grains of irregular shape are a common feature after mechanically obtaining powders by high-energy ball milling (Baláž 2008). Figure 4 also shows the typical EDS mapping of the G1 sample. The results of elemental mapping of Ga (Fig. 4b) and O (Fig. 4c) show that both elements are homogeneously distributed in the sample, and they perfectly match each other.
The photoluminescence spectra of β-Ga 2 O 3 powders treated under different milling conditions are shown in  (Villora et al. 2003). As can be seen in Fig. 5, the photoluminescence band of β-Ga 2 O 3 powders is broad and extends from 350 to 700 nm with a maximum of emission at about 425 nm. There is a shift of this emission maximum in the region of larger wavelengths when the excitation wavelength of powders was changed from 250 to 280 nm. Moreover, as can be seen from Fig. 5a, the change in milling conditions also leads to a shift of the emission maximum of G1-G3 powders to the shorter wavelengths at the excitation of 250 nm. The use of lower mass and shorter milling time (sample G1) results in luminescence at a greater wavelength, whereas longer milling with a higher amount of sample (G3) shows the luminescence maximum at a shorter wavelength. However, the position of the maximum does not change when changing the milling conditions of powders excited at 280 nm (Fig. 5b). As the α-Ga 2 O 3 phase does not show any emission in the visible spectral region, its presence will not be considered in the text below. In the energy coordinates, the luminescence spectrum of the G3 nanopowder was decomposed into three elementary Gaussian curves (Fig. 6). The half-width of the elementary luminescence bands at room temperature was about The UV band, the maximum of which is at 3.26 eV at room temperature, has an intensity of approximately 75% of the blue band. The maximum green luminescence band is located in the region of larger wavelengths (~ 490 nm). It should be noted, that the position of the radiation maxima of elementary bands coincides well with the position of the known bands of intrinsic luminescence of β-Ga 2 O 3 crystals (Vasil'tsiv et al. 1988;Binet et al. 1998;Onuma et al. 2013).
Regardless of the milling conditions, all three elementary bands of host luminescence were observed in the investigated gallium oxide powders G1-G3. The relative intensity of the elementary bands' emission changed. The total emission intensity, as well as the intensity of the UV and blue luminescence bands, were the highest in G3 powder, thus it seems that using a higher sample mass and longer milling time seems to be more favorable. Figure 7 shows the excitation spectra for different gallium oxide powders. For all three samples, the excitation spectrum covers the wavelength range (energies) of the excitation light from 230 to 320 nm (3.8-5.5 eV), extending from the fundamental absorption region (230-270 nm) to the transparency region (270-450 nm). The main excitation maximum is located near the edge of the fundamental absorption of 254 nm (Luchechko et al. 2018). In the region of transparency of gallium oxide in front of the edge of the fundamental absorption is another excitation band at 4.4 eV (280 nm). In addition to the main excitation bands in the longer wavelength region of the spectrum in the energy range of 2.75-3.75 (wavelengths 350-450 nm) a wide low-intensity excitation band of luminescence with a maximum near 350-370 nm is observed. The excitation spectra of luminescence differ in the ranges of 230-300 and 300-400 nm, depending on the milling conditions of powders. The maximum excitation intensity of luminescence is observed for G3 powder at a wavelength of about 254 nm (Fig. 7a). To see the change in the relative intensity of the different excitation bands of G1-G3 powders, the spectra were normalized (Fig. 7b). For powder G1, there is an increase in the relative intensity of the excitation bands at 280 and 365 nm, which are located in the transparency region of gallium oxide.  (Luchechko et al. 2020b;Vasyltsiv et al. 2021). UV luminescence of gallium oxide is characterized by a significant Stokes shift, high quantum yield, broad elementary bands, and decay time of the order of 10 -6 s and is most often attributed to radiative recombination of autolocalized excitons or electrons with holes through donor-acceptor pairs (Onuma et al. 2018;Frodason et al. 2020). The autolocalization of excitons occurs as a result of the deformation interaction of electronic excitations with the acoustic oscillations of the lattice. The process of autolocalization is characteristic of phosphors with strong electron-phonon interaction and is highly dependent on temperature. In the case of gallium oxide, the increase in temperature is always accompanied by temperature quenching of UV luminescence. The energy of thermal quenching of UV luminescence coincides with the energy of some of their phonons 0.08 eV and may be due to the thermal destruction of autolocalized states.
The blue luminescence band observed is most often (Vasil'tsiv et al. 1988;Binet et al. 1998;Onuma et al. 2013) attributed to the recombination of electrons with holes through the donor-acceptor pairs. Since the donors and acceptors in gallium oxide can be of different types and different depths (Luchechko et al. 2020c;Usseinov et al. 2021;Vasyltsiv et al. 2021), it can be expected that the position of the maximum radiation can vary from UV to green parts of the spectrum.
With an increase in the intensity of milling, the average size of nanoparticles decreases, and accordingly, the effective surface of grains increases. The main defects that are formed during milling are concentrated precisely on the surface. Such defects are primarily oxygen vacancies and interstitial gallium. The oxygen evaporates into the atmosphere, which leads to the formation of an excess of gallium. Enhancing the number of interstitial gallium ions, which is a shallow donor, leads to an increase in the concentration of donor-acceptor pairs, that are responsible for blue luminescence.

Conclusions
Monoclinic gallium oxide β-Ga 2 O 3 nanocrystalline powders with a small amount of α-Ga 2 O 3 admixture phase have been successfully obtained using the high-energy ball milling method. XRD, SAED and HRTEM confirmed that the nanocrystals have a monoclinic structure. The α-Ga 2 O 3 phase did not show any influence on the luminescent properties. The gradual blue shift of the emission maximum was observed when using higher sample mass and longer milling time. The relative intensity of the low energy emission band at 2.52 eV decreases under these conditions. The increase in the yield of blue luminescence in milled samples with milling time increasing can be explained by lattice distortion during mechanical processing, which causes the formation of oxygen vacancies and, as a consequence, the formation of excess gallium. The main models for explaining blue emission are DAP luminescence associated with these defects.
Synthesis of β-Ga 2 O 3 nanocrystals by high-energy ball milling is a new way to operate the properties of this material, which gives the possibility to obtain phosphors based on low-dimensional structures.