Introduction

Gallium oxide (β-Ga2O3) 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-Ga2O3 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-Ga2O3, GaN, SnO2 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-Ga2O3 has been synthesized using high-energy ball milling in the past (Swamy et al. 2013).

In this work, structural and luminescent properties of β-Ga2O3 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-Ga2O3 phosphors were synthesized using a high-energy ball milling method. The high-purity grade Ga2O3 (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-to-powder 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.

Table 1 Milling parameters for studied β-Ga2O3 powders
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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-Ga2O3 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-Ga2O3 phase, a small amount of additional α-Ga2O3 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 CaCO3 (Baláž 2021; Baláž et al. 2015), PbO (Dachille and Roy 1960) or ZnS (Baláž 2008). The content of α-Ga2O3 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).

Fig. 1
figure 1

Experimental XRD patterns of β-Ga2O3 nanocrystals for different milling conditions (the theoretical Bragg reflections of corresponding phases are shown for comparison at the bottom)

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Figure 2 shows the TEM image of the typical β-Ga2O3 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-Ga2O3 nanocrystals possess a good crystallinity with an interplanar distance of about ~ 0.27 nm. This distance corresponds to the (\(\overline{1 }11\)-) crystallographic plane of β-Ga2O3 with a monoclinic structure (Bae et al 2018).

Fig. 2
figure 2

STEM and high-resolution TEM (HRTEM) images of β-Ga2O3 nanocrystals milled for 1 h using sample mass 3 g (sample G1)

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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.

Fig. 3
figure 3

Electron diffraction Debye–Scherrer rings distribution for β-Ga2O3 (sample G1) powder

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The HAADF-STEM image of β-Ga2O3 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.

Fig. 4
figure 4

HAADF-STEM image (a) of β-Ga2O3 grains (sample G1), with corresponding chemical composition EDS maps (b,c) which represent the location of each element in the grain

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The photoluminescence spectra of β-Ga2O3 powders treated under different milling conditions are shown in Fig. 5. According to the literature, undoped β-Ga2O3 powders should exhibit luminescence upon excitation with light from the area of fundamental absorption edge (the wavelength range 230–265 nm) or in front of the absorption edge (the band 280 nm) (Villora et al. 2003). As can be seen in Fig. 5, the photoluminescence band of β-Ga2O3 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-Ga2O3 phase does not show any emission in the visible spectral region, its presence will not be considered in the text below.

Fig. 5
figure 5

Photoluminescence spectra of β-Ga2O3 powders excited by 250 nm (a) and 280 nm (b)

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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 0.5 eV. The emission maxima of the elementary bands are located in the UV ~ 3.26 eV (~ 380 nm), blue ~ 2.91 eV (~ 425 nm), and green ~ 2.52 eV (~ 490 nm) regions of the spectrum. The blue luminescence band with a maximum of 2.91 eV was the most intense among the elementary bands. 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-Ga2O3 crystals (Vasil’tsiv et al. 1988; Binet et al. 1998; Onuma et al. 2013).

Fig. 6
figure 6

Deconvolution of the photoluminescence spectrum of β-Ga2O3 powders (G3) into elementary bands under 250 nm excitation

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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.

Fig. 7
figure 7

Luminescence excitation spectra of β-Ga2O3 powders at the emission registration on 470 nm (a-total excitation intensity; b-normalized excitation)

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UV luminescence band of β-Ga2O3 is independent of impurities and only decreases during doping, in particular with Cr3+ ions (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-Ga2O3 nanocrystalline powders with a small amount of α-Ga2O3 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-Ga2O3 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-Ga2O3 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.