1. Introduction

ZnO - based compounds have been attracting wide interest of researchers for many decades. Zinc oxide occurs in three crystal structures; aside from rare cubic structures of rock salt and zinc blende, it most commonly crystallizes in a thermodynamically stable hexagonal wurtzite structure. Hexagonal ZnO is known for outstanding optoelectronic features: wide direct band gap (3.4 eV), extremely large exciton binding energy (60 meV) allowing to measure a strong exciton luminescence at room temperature [1], sub-nanosecond lifetime [2] and low afterglow. Pure zinc oxide may naturally exhibit two kinds of luminescence – weak exciton-related emission in UV and strong defect-related emission in visible spectral range. The origin of visible defect-related emission [3] does not have a unique explanation and it is strongly related to the preparative technology used [4]. Defect-related luminescence is useless for fast scintillators due to its slow decay [5]. The intensity of the excitonic luminescence can be enhanced by doping with donor ions, such as Ga³⁺ or In³⁺ [6] Doping by Ga³⁺ ions and post-preparation heat treatment in reducing atmosphere containing hydrogen [7] are crucial for the enhancement of the narrow excitonic emission at 389 nm and simultaneous suppressing the defect related emission in the visible spectral region [7,8]. Though the explanation of such an effect based on exciton trapping at H_Zn site was proposed [7] the mechanism of defects inactivation and role of hydrogen are unclear and requires further investigation. As shown in [8,9], slow components are completely absent in scintillation decay of ZnO:Ga under the nanosecond pulse soft X-ray excitation.

While the exceptional properties together with high radiation stability predetermine the scintillators based on ZnO:Ga as perspective materials for high-energy particles (HEP) detectors and time-of-flight detectors e.g. in positron emission tomography (TOF-PET), band gap modulation allows to increase the overlap of the emission band of the scintillating nanoparticle and the absorption band of the photosensitizer used for the ¹O₂ production. Therefore, band gap modulated ZnCdO:Ga nanocrystals may be prospective as scintillating material for X-ray induced photodynamic therapy (PDTX). On the other hand, ZnO/ZnMgO quantum wells systems can improve some of the optoelectronic properties, i.g. the efficiency of the scintillation process or photocurrent.

The principle of the band gap modulation in Zn(Cd,Mg)O:Ga is shown in Fig. 1. to shift the bottom edge of the conduction band by affecting the energy levels of 2p orbit of Zn²⁺ and composition of the conduction band (CB) bottom by admixture of specific foreign ions as Cd²⁺ or Mg²⁺ and their energy levels. These foreign ions may occupy the position of Zn²⁺ ions (Fig. 2) in the lattice or the interstitial positions. Specific foreign ions can also affect the valence band in the same manner [10] and this strategy of material modification is known as band-gap engineering. As a result, admixing the ions such as Mg²⁺ or Cd²⁺ leads to the formation of Zn(Cd,Mg)O solid solutions [11] and causes the narrowing or broadening of the band gap. These effects manifest as red or blue shift of the exciton-related emission. Due to very similar crystal radii of Zn²⁺ (0.74 Å) and Mg²⁺ (0.71 Å) and even in the case of bigger Cd²⁺ (0.92 Å), the hexagonal wurtzite structure of ZnO can be preserved. It was observed that the increasing concentration of Cd²⁺ in the ZnO structure over certain limit decreases the intensity of luminescence due to the crystallinity deterioration [12]. Cd concentration in the solid solution, phase purity and luminescence properties strongly depend on the preparative technique. Typically, thin layers or alloyed Zn_1-x(Cd,Mg)_xO films are prepared via sol-gel [13,14], pulsed laser deposition [11,15,16], electrodeposition [17,18], molecular beam epitaxy [19], vapour phase synthesis [20] or chemical vapour deposition [21]. Films are usually grown on sapphire substrates [16]. Reports about free standing powder [22] are very scarce and lack the data about radioluminescence properties and decays.

 

Fig. 1 Scheme of band gap modulation.

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Fig. 2 Wurtzite structure of ZnO with Cd ion in the substitution position.

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Conventional growth methods allow for various Cd concentrations, significantly higher than the thermodynamic solubility limit (x = 0.02). The exceptionally high Cd concentration up to 0.25 is mentioned in [11], where thin films were prepared via pulsed laser deposition at low temperature of about 300 °C. The maximum redshift was obtained for a Cd-content of 0.25, with the emission energy of 2.46 eV, which is in good agreement with [23]. Single-phase ZnCdO alloys in [19] fabricated by molecular beam epitaxy in oxygen-rich conditions feature emission energy shift from violet to yellow spectral range. The red shift in PL spectra is assumed to be the indication of localized transitions. However, the energy shift mentioned in this paper means the spectral shift of defect-related emission in the visible spectral range, while the previous works mention the shift of exciton-related emission in UV spectral range.

Reports about ZnMgO synthesis are usually focused on the multiple quantum well systems fabrication and the evaluation of the optoelectronic properties. The principle of quantum wells is based on the capture of electrons moving in the CB at the energy levels of Mg²⁺ states formed above the edge of the CB of undoped ZnO. These energy levels may be introduced in the band structure in the form of doping ion (separated quantum well), core-shell systems or two (or more) thin layers of ZnO and ZnMgO with different width of the band gap (multiple quantum wells). Such multiple quantum well systems feature the increasing intensity of photoluminescence (PL), accompanied by the quantum confinement effect, and the energy can be tuned by the barrier height (may be defined by the Mg content in the ZnMgO solid solution) and layer thickness [24]. The ZnO/(Zn,Mg)O multiple quantum wells prepared by laser molecular beam epitaxy were reported in the detail in [16]. The Mg content used was 12 and 27% to obtain significantly different height of potential barrier between ZnO and ZnMgO layer. Quantum well core-shell heterostructures of ZnO/Zn_(1‑x)Mg_xO with Mg content varying between x = 0.15 and 0.3 grown by metal organic vapour phase epitaxy were reported by [25]. Hydrothermal method for ZnO/ZnMgO core-shell heterostructural nanorods preparation was used in [26]. ZnO nanorods were used as a core and ZnMgO shell was deposited using the atomic layer deposition technique. Hydrothermal method was used also for ZnO-MgO core-shell nanowires preparation in [27]. In this case, MgO layer is deposited to passivate the ZnO surface.

Photochemical precipitation is rare but convenient method to produce large amounts of nanopowders of high quality [28] at low costs and with high production speed. Due to high level of interaction among constituents of the precipitate, it allows for efficient formation of solid solutions [29]. The method is based on the UV-irradiation of aqueous solutions of soluble metal salts and radical scavengers. The irradiation leads to the reaction between the products of photolysis which leads to the precipitation of solid phase. Subsequently, solid phase is separated, dried and heat treated at conditions specific to each kind of product. The possibility of radiation induced preparation of highly efficient ultrafast ZnO:Ga scintillators has been already reported [8] and shows high potential for pushing the boundary of time resolution below 100 ps [30].

The aim of this study was to establish optimum conditions for the photochemical synthesis of ZnMgCdO:Ga solid solutions with maximized Cd or Mg content and to investigate their structural and luminescence properties. Specifically, influence of Ga doping and reduction annealing are studied for such solid solutions to maximize its fast excitonic emission.

2. Experimental

2.1 Synthesis of scintillating powders

Following chemicals were used for the ZnO:Ga, ZnMgO:Ga or ZnCdO:Ga syntheses: zinc oxide, cadmium nitrate tetrahydrate, magnesium nitrate hexahydrate (all chemicals from Aldrich, 99.999% trace metals basis), formic acid (Penta, p.a.) and hydrogen peroxide (Penta, p.a.). Gallium nitrate was obtained by dissolving gallium oxide in nitric acid to obtain precise Ga³⁺ concentration. The preparation of Zn(Mg,Cd)O:Ga scintillators was based on our previous works [8,31], using photochemical synthesis. Irradiation was performed using low-pressure mercury lamps (UMEX, GmbH) with total power 100 W. The solid phase precipitated quantitatively after 100 min of irradiation. Solid phase was decomposed at 250 °C in air. Two-step heat treatment followed: heat treatment at 950 or 1000 °C in air performed in the Clasic 0415 VAC vacuum furnace and additional treatment at 800 °C, using the Setaram LabSys Evo thermoanalyzer under the reducing Ar/H₂ (10:1) atmosphere.

To improve the luminescence properties of scintillating powders, the preparation procedure described in [8] was followed. The aqueous solutions for irradiation contained total concentration of Zn²⁺ and Cd²⁺ or Mg²⁺ ions = 0.05 M. Concentration of Cd²⁺ ranges between 0.5 to 25 molar %. The concentration dependences after synthesis optimization were investigated in the range of 5-16 mol% of Cd ions (marked as ZCG0-4). The concentration of Ga³⁺ used was 3.4 10⁻⁵ M. In photochemical synthesis, gallium is introduced in the ZnO structure quantitatively, resulting in approximately 0.07 molar % of Ga³⁺ in all samples, the exact value depending on Cd/Mg content. 0.96 M hydrogen peroxide was added to ensure the excess of oxygen. Ammonium formate was added in amount equivalent to the Cd²⁺ ions concentration. These samples were treated as described above.

2.2. Characterization techniques

For structure characterization and phase purity confirmation, X-ray powder diffraction (XRPD) was used. In the case of CdO presence, the weight ratio between ZnO wurtzite phase and CdO cubic rock-salt phase was estimated via RIR method. In the case that only ZnO phase was observed, lattice parameters were calculated and the content of Cd in the ZnO structure was calculated regarding the Vegard’s law and the lattice parameters of hypothetical ZnCdO solid solutions listed in [32]. The combination of these results with X-ray fluorescence elemental analysis (XRF) enables to estimate the Cd content in the Zn_1‑xCd_xO:Ga solid solutions. We reported this method of Cd content estimation already in [31].

Luminescence properties were evaluated by measuring photoluminescence (PL) and radioluminescence emission spectra (RL) using the custom-made spectrofluorometer 5000M (Horiba Jobin Yvon], equipped with nanoLED 339 nm as the excitation source and the X-ray source DEBYFLEX ID3003 (Seifert Gmbh) with tube DX-W 10x1-S 2400 W (tungsten anode).

Detection part of the set-up consists of single grating monochromator and photon counting detector TBX-04 (IBH Scotland). Luminescence decays were evaluated using a convolution procedure (SpectraSolve software package, Ames Photonics).

Cathodoluminescence (CL) emission spectra and decays were measured using time-gated Andor iStar iCCD camera and Hamamatsu R3809U-50 MCP-PMT detector, respectively. The measurements were performed at the laboratory setup [33]. The median energy of polychromatic electron beam was 100 keV, pulse duration <250 ps FWHM and peak electron current on the order of 1-10 A/cm².

3. Results and discussion

3.1 Scintillating powder synthesis

3.1.1 Zn_1-xCd_xO:Ga

UV-irradiation leads to the formation of zinc cadmium peroxide crystalline phase, in whole range of Cd concentrations. There is no separation of the phases because both ZnO₂ and CdO₂ crystallize in the cubic lattice. After heat treatment at 250 °C, only wurtzite phase of ZnCdO was observed, containing up to 10 mol % of Cd. At Cd concentrations higher than 10 mol % in solid phases, the cadmium carbonate phase is noticeable as minor component in XRD spectra. This separation of phases is caused by the addition of ammonium formate in the irradiated solutions, increasing the effectivity of Cd precipitation during irradiation. After irradiation of solutions without ammonium formate, only ZnO phase was observed, but the Cd content in ZnO was independent on Cd content in the irradiated solution and ranges about 12%. After ammonium formate addition into the irradiated solutions, the precipitation of Cd²⁺ ions in the form of ZnCdO₂ is not quantitative, but it is in the correlation with the Cd content in the irradiated solutions. XRF elemental analysis shows that the Cd concentration in the solid phase is significantly lower. The optimal temperature of post-annealing was set at 950 °C, based on TA, XRD and RL measurements. Annealing at higher temperatures in air or in reducing atmosphere leads to the Cd ions evaporation from the sample surface, where the Cd content significantly decreases. The Cd volatility manifests by the CdO unwanted phase formation that is observed in XRD spectra (Fig. 3a). The formation of separate CdO phase after annealing at higher temperatures (>500 °C) was observed only for Cd content above 10 mol%. In the case of the CdO phase presence, the residual content of Cd was calculated for solid ZnCdO solution from XRD data by IR method. We used the combination of overall elemental analysis (XRF) in combination with XRD and Vegard law (using lattice parameters calculation) to estimate the real Cd content incorporated in the ZnO structure, as already shown in [31]. Theoretical lattice parameters for CdO in hypothetical wurtzite structure were used from [32].

 

Fig. 3 XRD data for ZnCdO:Ga solid solutions with various Cd concentration (A) and XRD data for ZnMgO:Ga solid solutions after treatment at various temperatures (B).

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Lattice parameters were calculated from XRD data (Fig. 4). It is evident that the lattice parameters a and c are in correlation with the Cd concentration in the ZnO (calculated values from XRF and XRD analyses were used) and increasing with Cd content up to the ~5% of Cd in the ZnO phase. At higher contents of Cd, the lattice parameters are suppressed by distortion of the ZnO crystal lattice caused by the CdO separate phase formation. These effects may be associated with a decrease in crystallite size, observed already in [11,16]. The ratio of parameters a and c does not change throughout the concentration range, indicating that the lattice deformation is not directionally oriented.

 

Fig. 4 Lattice parameter a and c in dependence on Cd concentration estimated from XRD and XRF or Cd concentration calculated from Vegard law.

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3.1.2 Zn_1-xMg_xO:Ga

Zn_1-xMg_xO compounds were prepared to evaluate the effect of Mg on the luminescence spectra. Magnesium oxide MgO has cubic structure and tetrahedral effective ionic radius of Mg²⁺ (0.57 Ǻ) is similar to that of Zn²⁺ (0.60 Ǻ). Due to the mixed crystal Zn_1-xMg_xO has similar lattice constants to those of ZnO. It was found that MgO cannot be prepared by the direct photochemical preparation, because Mg(OH)₂ is formed. But magnesium peroxide with cubic structure can be prepared as in the case of CdO₂. ZnMgO precursor (zinc-magnesium-peroxide) can be calcined at higher temperature (1000-1100 °C) than ZnCdO and XRD analysis show no separation of phases (Fig. 3b). It enables preparation of perfect crystals without evident defect luminescence. The Mg concentration in the irradiated solutions have been significantly higher (the molar ratio Zn:Mg 2:1 or 1:1) to incorporate the sufficient amount of Mg ions into the ZnO. The lattice parameters a = 3.2526 and c = 5.2028 were calculated from XRD data and show slight deviation from tabulated values a = 3.2504 and c = 5.2063 caused by the distortion of the crystal lattice by Mg incorporation. The actual Mg content in the ZnO structure is difficult to determine because of the deformation of crystal lattice and the complexity of the Vegard law application, as shown in [34]. The combination of XRD and XRF method used for Zn_1-xCd_xO is inappropriate because of low detection limits for MgO. A shift of diffraction peaks together with strong band gap modulation is demonstrable evidence of successful incorporation of Cd or Mg in the ZnO structure (Fig. 5).

 

Fig. 5 Diffraction peaks with a noticeable shift for ZnCdO:Ga, ZnO:Ga and ZnMgO:Ga scintillators.

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3.2 Luminescence and scintillation properties

The Cd concentrations up to 1% do not affect the position of UV emission, but induce an increasing intensity of luminescence at 550 nm. At higher Cd concentrations, the samples of ZnCdO:Ga treated only at 950 °C feature strong defect luminescence in visible spectral range. Its maximum position in green spectral region is long-wavelength-shifted with increasing Cd concentration, as shown in (Fig. 6(a)). We assume that the spectral shift and the increase in the intensity are caused by the more localized recombination, as proposed in [19]. After heat treatment in reducing atmosphere, emission in visible spectral range was totally suppressed, consistently with processes described in detail in [8], and the emission in UV area was red-shifted to lower energies, which is caused by the narrowing the band gap (Fig. 6(b).)

 

Fig. 6 RL emission spectra of ZnCdO:Ga samples in visible range (A), in UV range after heat treatment in reducing atmosphere (B) and RL emission manifesting red (Zn_0.87Cd_0.13O:Ga) and blue (ZnMgO:Ga) shift in the range of 376 nm to 411 nm (C).

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On the other hand, samples of ZnMgO:Ga feature the blue shift to the higher energies. After optimizing the preparative techniques and treatment, only UV emission with sufficient intensity and significant red and blue shift was observed. The most promising results are shown in Fig. 6(c). The dependence of the emission peak shift on the Cd concentration in ZnO phase is shown in Fig. 7. The total RL intensity strongly depends on the crystal phase purity and Cd content in ZnO lattice. We assumed that up to the certain Cd concentration (5‑6%), the quality of crystal structure is not affected, because Cd²⁺ substituted Zn²⁺ positions (Fig. 2). It is possible that above this concentration, Cd ions introduce lattice disorder, as shown in [35], leading to the deterioration in crystallinity and decreasing in the RL intensity. Another explanation can be that negligible amount of CdO is formed (below the XRD detection limit).

 

Fig. 7 The spectral position of UV emission peak as a function of Cd concentration in the solid solution.

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As stated in [7] the explanation of the defect emission suppression and exciton emission enhancement can be based on exciton trapping at H_Zn site. Speculating further, using gallium nitrate as the doping agent an eventual nitrogen anion N^3- embedding in ZnO can easily create a hole capture site, analogously as H_Zn site proposed for that in ref. 7. Captured hole in both cases can create an anchor for trapped exciton formation in agreement with ref. 7.

From the application point of view, red shift of the excitonic emission enables to increase the spectral overlap between absorption peak of photosensitizer used for PDTX (i.e. protoporphyrin IX, PpIX) and emission peak of ZnCdO:Ga (Fig. 8). This may improve the efficiency of composite scintillators for PDTX, despite the decrease in RL intensity to about 20% of ZnO:Ga.

 

Fig. 8 The overlap of the PpIX absorption spectrum, ZnO:Ga and Zn_0.88Cd_0.12O:Ga emission spectrum.

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Photoluminescence (PL) (Fig. 9(a,b)) and cathodoluminescence (CL) (Fig. 10(a,b)) decays were measured and decay times determined for both ZnCdO:Ga and ZnMgO:Ga powder samples (see fit functions I(t) in figures). Despite of subnanosecond decay time values of the fastest component, given the large dynamic range of our measurements (at least two orders of magnitude in cathodoluminescence and more than three orders of magnitude in photoluminescence decays) the extracted values of decay times are reliable and also their trends are clearly shown. PL decay of ZnCdO:Ga powder shows, besides the extremely fast component about 220 ps, minor slower components (< 1% of total intensity) with estimated decay times about 1.2 ns and 5.6 ns. In comparison, ZnMgO:Ga exhibit only fast decay about 500 ps, without any slower component. CL decays for ZnCdO:Ga in Fig. 9(a) and 10(a) show comparable values of fast component decay times taking into account the error of their determination (20-30%). The reason for much shorter value in CL decay of ZnMgO:Ga compared to photoluminescence one is not clear and needs further study. ZnCdO:Ga sample at longer time scale provided also weak slow component with decay time on the order of few tens of ns (not shown here), but relative yield lower than 1-2%, which prevented further investigation. Furthermore, its time course is obscured by some afterpeak at instrumental response (not shown). Its total intensity (relative to fast component) becomes somewhat higher compared to photoluminescence decay which point to further delays in transfer stage of scintillation mechanism and presence of charge traps in the structure.

 

Fig. 9 PL decay of ZnCdO:Ga (A) and ZnMgO:Ga (B) powder. Decay data given by symbols, instrumental response (IR) by blue line and convolution of IR and function I(t) by red line.

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Fig. 10 CL decay of ZnCdO:Ga (A) and ZnMgO:Ga (B) powder. Decay data given by symbols, instrumental response (IR) by blue line and convolution of IR and function I(t) by red line.

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The above reported luminescence characteristics show that also in the mixed ZnMgO and ZnCdO materials it is possible to achieve total suppression of slow visible luminescence by Ga doping and annealing in a reduction atmosphere following the procedures described for pure ZnO [7]. We also demonstrate that the spectral shift of fast excitonic luminescence in near UV-violet spectral due to alloying Mg or Cd does not affect the timing characteristics of this luminescence and preserves its superfast character. Thus, it enables the use of these materials for superfast scintillators e.g. in time-of-flight based techniques in medical and high energy physics fields [36].

4. Conclusions

Photo-induced precipitation is a convenient wet-chemical method for the synthesis of band-gap modulated scintillating ZnMgO:Ga and ZnCdO:Ga powders. A shift of diffraction peaks together with strong band gap modulation provides firm evidence of successful incorporation of Cd or Mg in the ZnO structure. Significant blue or red shift of UV emission maxima was obtained with the maximal admixture of 13 mol % of Cd or Mg ions, without any emission in visible spectral range. It was manifested that the UV excitonic emission is able to shift in the range of 376-425 nm at the cost of the decrease in total luminescence intensity. Nevertheless, both types of solid solutions feature excellent luminescence properties, especially the exciton luminescence decay dominated with extremely fast subnanosecond component with very minor content of slower processes and without any emission in visible spectral range. Emission spectrum of Zn_0.89Cd_0.11O:Ga shows practically perfect overlap with absorption spectrum of PpIX used as photosensitizer for ¹O₂ production. Thus, Zn(Cd,Mg)O:Ga-based scintillators show good potential in scintillation applications requiring fast timing and in photo/radiotherapy.