Sol-Gel Processing of Bismuth Germanate Thin-Films

coatings

Article Sol-Gel Processing of Bismuth Germanate Thin-Films

Mihail Secu, Corina Elisabeta Secu, Teddy Tite and Silviu Polosan * National Institute of Materials Physics, P.O. Box MG 7, 077125 Magurele, Romania; msecu@infim.ro (M.S.); cesecu@infim.ro (C.E.S.); teddy.tite@infim.ro (T.T.) * Correspondence: silv@infim.ro; Tel.: +40-214-8-268

Received: 17 February 2020; Accepted: 6 March 2020; Published: 9 March 2020

Abstract: This study aims to obtain uniform and homogeneous bismuth germanate oxides thin films by spin coating and using the sol-gel technique with different precursors, followed by low-temperature annealing at 560 ◦C. By using Bi(NO3)3 precursors, we have obtained transparent, yellowish thin films with a 200 nm thickness. The structural analysis of the initial sol-gel powder has shown the presence of two crystalline structures, the cubic Bi4Ge3O12 (BGO) and monoclinic Bi2GeO5 crystallites, which evolves towards the BGO structure after annealing. The elemental analysis confirmed the composition of the desired compound Bi4Ge3O12 with 60 wt % GeO2 and 40 wt % Bi2O5. On the other hand, by changing the precursor to (Bi(CH3COO)2, the film thickness increased to 500 nm thicker due to the high viscosity of the sol, and a dominant monoclinic Bi2GeO5 crystalline structure appeared. The elemental analysis revealed a nonstoichiometric composition with 38 wt % GeO2 and 62 wt % Bi2O3. Due to the low GeO2 phase content that reacted with metastable Bi2GeO5, we obtained cubic Bi4Ge3O12 as a secondary phase, with Bi2GeO5 as a dominant crystalline phase. The redshifts of both absorptions and emissions spectra peaks confirmed a different disorder structure as an interplay between the cubic Bi4Ge3O12 (BGO) and monoclinic Bi2GeO5 phases.

Keywords: BGO thin ﬁlms; sol-gel; spin coating deposition; slow thermal annealing

1. Introduction

Bismuth germanate oxides, especially Bi4Ge3O12 (BGO), remain on the top of scintillating materials that can be used efficiently as radiation detectors due to some unique properties like high light output and stable and reproducible production [1]. Modern applications in high-energy physics, nuclear medicine, geophysics and today homeland security place more stringent requirements on the energy resolution and efficiency of relatively small detectors. These materials, especially in their crystalline form, are used as imaging calorimeters that measure the energy and trajectory of incident particles and provide effective electron/hadron discrimination based on shower images at the cosmic ray proton spectrum from 40 to 100 TeV [2]. Bismuth germanate has a cubic crystalline structure known as eulityne, and luminescence properties are related to intra-ionic transitions of Bi3+ cations from the 6s6p level to the 6s2 ground level [3]. Bismuth germanate oxides can be used for gamma radiation [4], but the most striking application is in the medicine area, where BGO crystals are used as absorber detectors of a Compton camera for prompt γ-ray imaging during ion beam therapy [5]. In these applications, a scintillator is a streaked block of bismuth germanate oxide (BGO) providing 8 8 pseudo pixels and compared with materials × such as LaBr3:Ce and LYSO:Ce detectors. Concerning the detection efficiency, BGO detectors were determined to be comparable to, and better than LaBr3:Ce and LYSO:Ce, both on account of having no intrinsic radioactivity. In the same area, BGO nanoparticles were used in biolabel applications [6]. The preparation methods of BGO materials vary from BGO single crystals [7] by using the Czochralski method, vacuum thermal evaporation [4,8], glasses and glass-ceramic materials [9–13], pressure-assisted

Coatings 2020, 10, 255; doi:10.3390/coatings10030255 www.mdpi.com/journal/coatings Coatings 2020, 10, 255 2 of 12 combustion [14], and radio frequency (rf) sputtering [15]. A special case of BGO nanocrystals was successfully synthesized using a room-temperature chemical process under atmospheric pressure in an aqueous solution [16]. These BGO nanocrystals were obtained by changing the pH values of solutions between 1 and 9 with different stirring times. However, each of the abovemetioned methods presents specific technical requirements and difficulties. An alternative option is by using the sol-gel route with its advantages: low processing temperature, the ability to control the purity and homogeneity of the final materials on a molecular level, and the large compositional flexibility [17–19]. Several papers describe the obtaining of bismuth germanate oxides by the sol-gel method, but all of them use the calcination method at 900 ◦C to obtain a BGO compound [14,20]. The study aims are to obtain bismuth germanate oxides by the sol-gel technique followed by low-temperature annealing without a calcination process. In our case, the sols obtained by using germanium and bismuth precursors were spin-coated on glass substrates and slowly thermally annealed at 560 ◦C to reach the crystallization of the resulted thin films. This temperature was set in a range of crystallization between 540 and 578 ◦C, determined from BGO non-isothermal crystallization curves with differential thermal analysis (DTA) [21]. The slow thermal annealing avoided the calcination process at 900 ◦C, which created BGO structures by a solid-state reaction between two oxides, Bi2O3 and GeO2 [9] to obtain uniformly and homogeneously deposited thin films that can be used as detectors.

2. Materials and Methods

Sol-Gel Synthesis of Bismuth Germanate Oxides For the preparation of bismuth germanate oxides, we used the sol-gel method with Germanium(IV) ethoxide (purity: 97%), Ge(OC2H5)4 (Alfa Aesar), citric acid (purity: >99%), C6H8O7 (Alfa Aesar), ethyl alcohol (purity: >99.8%; Sigma-Aldrich), Bismuth(III) nitrate (Bi(NO ) ) 5H O) 98%, Bismuth(III) 3 3 • 2 acetate (purity: 99.99%), Bi(CH3COO)2 (Alfa Aesar), and glacial acetic acid (purity: 99.7%; CH3COOH, Alfa Aesar) as starting materials. In a typical synthesis, aqueous metal salts are mixed with a proper acid to form a liquid sol. The chemical reactions can be formally described by the following three equations ([17] and references therein):

Hydrolysis: Ge-OR + H O Ge-OH + R-OH, ≡ 2 → ≡ Alcohol condensation: Ge-OH + RO-Ge Ge-O-Ge + R-OH, (1) ≡ → ≡ ≡ Water condensation: Ge-OH + HO-Ge Ge-O-Ge + H O with R = -CH -CH . ≡ → ≡ ≡ 2 2 3 For the first synthesis, prepared by a bismuth nitrate precursor, we mixed 0.1 g citric acid, 0.8 mL water, 1.0 mL absolute ethyl alcohol, and 1.0 mL Germanium(IV) ethoxide into a round flask with a plug at room temperature and under continuous magnetic stirring [22]; citric acid was used here as an effective chelating agent. Then, 2.9163 g of stoichiometric Bismuth(III) nitrate pentahydrate was dissolved in the 5.0 mL glacial acetic acid to prevent hydrolysis of Bi3+, and the solution was slowly dropped to the prehydrolyzed solution under constant stirring. For the second synthesis, prepared by a bismuth acetate precursor, Germanium(IV) ethoxide diluted with an equal volume of ethyl alcohol was hydrolyzed under constant stirring, and a solution was obtained. The volume ratio of Ge(OC2H5)4:H2O:CH3COOH was 7:2:1, and glacial acetic acid was used here as a catalyst. Then, the required amount of Bismuth(III) acetate was dissolved in acetic acid with a molar ratio of 1:14, and a second solution was obtained. This solution was dropwise added into the first solution, followed by stirring for 30 min at room temperature. For each of the synthese, a milky white sol was obtained that was used for depositing 250 µL of each solution by spin coating with an Ossila spin-coater with the following parameters: 2000 rpm for 30 s on glass substrates. Both samples with the deposited thin films and solutions were dried at 200 ◦C for 1 hour. The thin films were annealed using a software-controlled furnace Nabertherm at a speed of 3 ◦C/min, until the temperature reached 560 ◦C. Then, they were kept for 1 hour and naturally cooled Coatings 2020, 10, 255 3 of 12 at room temperature. The thicknesses of the obtained films were evaluated by using a Thetametrisis equipment for precise and nondestructive characterization of transparent and semitransparent single films or stacked films. This ellipsometric equipment was used to perform reflectance in the 370–1020 nm Coatings 2020, 10, 255 3 of 12 spectral range and measure the thicknesses from 12 up to 90 µm. The optical constants for BGO (majornaturally phase) cooled thin films at room were temperature. obtained byThe classical thicknes ellipsometryses of the obtained performed films were with evaluated a Woollam by using Variable Anglea SpectroscopicThetametrisis equipment Ellipsometer for (VASE)precise system,and nondestructive equipped with characterization a high-pressure of transparent Xe discharge and lamp incorporatedsemitransparent in an HS-190 single films monochromator or stacked films. [23]. This ellipsometric equipment was used to perform XRDreflectance measurements in the 370–1020nm were performed spectral range on aand Bruker measure D8 Advancethe thicknesses type from X-ray 12 di upff ractometerto 90 μm. The with a copperoptical target constants X-ray tubefor BGO and (major a LynxEye phase) one-dimensionalthin films were obtained detector. by classical The X-ray ellipsometry tube was performed operated at with a Woollam Variable Angle Spectroscopic Ellipsometer (VASE) system, equipped with a high- 40 kV and 40 mA. CuKα1 radiation (λ = 1.54056 Å) was used as an X-ray source. The Crystal Sleuth softwarepressure was Xe used discharge for the lamp analysis incorporat of XRDed data.in an HS-190 monochromator [23]. XRD measurements were performed on a Bruker D8 Advance type X-ray diffractometer with a Absorption and emission spectra were recorded with a Jasco 815 spectrometer while the SEM was copper target X-ray tube and a LynxEye one-dimensional detector. The X-ray tube was operated at performed with a Zeiss EVO 50 scanning electron microscope with a LaB cathode and a Brucker EDX 40 kV and 40 mA. CuKα1 radiation (λ = 1.54056 Å) was used as an X-ray source.6 The Crystal Sleuth system.software Based was on used the SEMfor the images, analysis the of XRD equipment data. may generate secondary electrons, backscattered electrons,Absorption characteristic and X-rays,emission and spectra cathodoluminescence. were recorded with a Jasco 815 spectrometer while the SEM Micro-Ramanwas performed spectrawith a Zeiss were EVO recorded 50 scanning at room electron temperature microscope in awith backscattering a LaB6 cathode configuration and a Brucker using a LabRAMEDX system. HR Evolution Based on spectrometer the SEM images, (Horiba the Jobin-Yvon) equipment equippedmay generate with secondary a confocal electrons, microscope. A He–Nebackscattered laser operating electrons, atcharacteristic 633 nm was X-rays, focused and cathodoluminescence. on the surface of sol-gel glass powders or films with an OlympusMicro-Raman 100 spectraobjective. were The recorded spectrum at room of a referencetemperature BGO in a crystal backscattering was recorded configuration by using an using a LabRAM× HR Evolution spectrometer (Horiba Jobin-Yvon) equipped with a confocal Olympus 10 objective. Accurate calibration was performed by checking the Rayleigh and Si bands at microscope.× A He–Ne laser operating at 633 nm was focused on the surface of sol-gel glass powders 0 and 520.7 cm 1, respectively. The laser excitation power was adjusted to avoid laser-induced heating. or films with− an Olympus 100× objective. The spectrum of a reference BGO crystal was recorded by The scatteredusing an Olympus Raman signal 10× objective. was then Accurate recorded calibrati by usingon was a confocal performed spectrometer, by checking dispersedthe Rayleigh through and an 1800 linesSi bands/mm at di 0ff andraction 520.7 grating cm−1, respectively. and detected The by laser using excitation a CCD power camera. was The adjusted spectral to resolutionavoid laser- of the 1 reportedinduced spectra heating. was The around scattered 0.5 cm Raman− . signal was then recorded by using a confocal spectrometer, Thedispersed bonding through architecture an 1800 lines/mm of glass diffraction powders grating and films and was detected investigated by using a by CCD FTIR camera. with aThe Perkin Elmerspectral Spectrum resolution BX II of spectrophotometer. the reported spectra Thewas around spectra 0.5 were cm− recorded1. in attenuated total reflectance The bonding architecture of glass powders and films was1 investigated by FTIR with a1 Perkin (ATR) mode (PikeMiracle head) in the range of 500–4000 cm− , with a resolution of 4 cm− and a total of 32Elmer scans Spectrum for each sample.BX II spectrophotometer. The spectra were recorded in attenuated total reflectance (ATR) mode (PikeMiracle head) in the range of 500–4000 cm−1, with a resolution of 4 cm−1 and a total 3. Resultsof 32 scans for each sample.

The3. Results properties of the obtained bismuth germanate thin films were correlated with the quality of the sol precursorsThe properties that hadof the di obtainedfferent viscosities bismuth germanate and homogeneities. thin films were For correlated example, with the the sample quality prepared of by a bismuththe sol precursors nitrate precursorthat had different became viscosities more homogeneous, and homogeneities. and For the example, obtained the thin sample film prepared was around 200 nmby thicka bismuth due nitrate to a lower precursor viscosity became compare more homo withgeneous, that of the and sample the obtained prepared thin byfilm a was bismuth around acetate precursor,200 nm which thick due was to thicker a lower and viscosity had acompare thickness with of that around of the 500sample nm prepared (Figure1 by). Thisa bismuth thickness acetate result also showedprecursor, the which sample was preparedthicker and by had the a thickness bismuth of nitrate around precursor 500 nm (Figure had good 1). This transparency thickness result and the samplealso prepared showed the by sample the bismuth prepared acetate by the precursor bismuth wasnitrate opaque. precursor had good transparency and the sample prepared by the bismuth acetate precursor was opaque.

(a) (b)

FigureFigure 1. Photos 1. Photos of theof the two two thin thin ﬁlms films samples samples (2.54(2.54 × 2.2.5454 cm) cm) prepared prepared by byspin spin coating coating and andannealing annealing at 560 °C: (a) sample prepared with bismuth nitrate;× (b) sample prepared with bismuth acetate. at 560 ◦C: (a) sample prepared with bismuth nitrate; (b) sample prepared with bismuth acetate.

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3.1. Structural Analysis by XRD 3.1. Structural Analysis by XRD The structural analysis of bismuth germanate oxides obtained by the sol-gel method involved the The structural analysis of bismuth germanate oxides obtained by the sol-gel method involved comparisons of the crystallization processes and elemental analyses in the obtained sol-gel powder the comparisons of the crystallization processes and elemental analyses in the obtained sol-gel after the drying procedure at room temperature and the thermally annealed powder and thin ﬁlms at powder after the drying procedure at room temperature and the thermally annealed powder and 560 C. thin films◦ at 560 °C . TheThe crystallization crystallization features features were were studied by XRD,XRD, andand the the comparison comparison was was done done between between the the two twosamples samples (Figure (Figure2a,b)). 2a,b)). Di ﬀDiffereerent typesnt types of bismuthof bismuth germanate germanate structures structures appeared, appeared, but but the the main main two structures were cubic Bi4Ge3O12 (BGO) and monoclinic Bi2GeO5. two structures were cubic Bi4Ge3O12 (BGO) and monoclinic Bi2GeO5.

(a)

(b)

Figure 2. (a) XRDFigure patterns 2. (b) XRD for the patterns sample for prepared the sample by bismuth prepared nitrate. by bismuth (b) XRD acetate. patterns for the sample prepared by bismuth acetate. In the sample prepared by the bismuth nitrate precursor, even in the as-prepared sol-gel powder, some peaksIn the of sample BGO preparedcan be identified, by the bismuth especially nitrate (310) precursor, at 27.85°, even (321) in theat 31.9°, as-prepared and (400) sol-gel at 34.12°. powder, Thesesome three peaks peaks of BGO defined can a be cubic identified, structure especially with a = b (310) = c = 13.22 at 27.85 Å and◦, (321) α = β at = 31.9γ = 90°.◦, and The (400) as-prepared at 34.12 ◦. α β γ powderThese threehad peakslower definedcrystallinity, a cubic but structure after withthermal a = bannealing= c = 13.22 at Å and56 °C =for =1 hour,= 90◦ .two The crystalline as-prepared powder had lower crystallinity, but after thermal annealing at 56 ◦C for 1 hour, two crystalline structures structures Bi4Ge3O12 (BGO) and Bi2GeO5 were observed as major phases. The presence of Bi2GeO5 was evidencedBi4Ge3O12 by(BGO) the (311) and reflection Bi2GeO5 were peak observed at 28.53°, as but major the phases.dominant The crystalline presence of structure Bi2GeO5 waswas BGO, evidenced for whichby the the (311) (310) reflection reflection peak at 26.85° at 28.53 significantly◦, but the dominant increased crystalline after annealing. structure The was lattice BGO, parameters for which of the the cubic structure (a = b = c = 10.8 Å) were close to the value of 10.52 Å for the BGO monocrystal [24]. In the annealed thin film, the BGO crystalline structure was the dominant one revealed by the (310) Coatings 2020, 10, 255 5 of 12

Coatings 2020, 10, 255 5 of 12 (310) reflection at 26.85◦ significantly increased after annealing. The lattice parameters of the cubic structure (a = b = c = 10.8 Å) were close to the value of 10.52 Å for the BGO monocrystal [24]. In the peak, consistent with a cubic structure with a = b = c = 10.48 Å. The crystallite dimensions were annealed thin film, the BGO crystalline structure was the dominant one revealed by the (310) peak, calculated based on the Debye Scherrer equation, assuming there was no stress action on the consistent with a cubic structure with a = b = c = 10.48 Å. The crystallite dimensions were calculated nanocrystallites. Thus, in the initial sol-gel powder, the dimension was estimated as about 20 nm, based on the Debye Scherrer equation, assuming there was no stress action on the nanocrystallites. which rapidly increased in the thermally annealed powder to 48 nm and remained somewhere at Thus, in the initial sol-gel powder, the dimension was estimated as about 20 nm, which rapidly around 25 nm in the annealed thin film. increased in the thermally annealed powder to 48 nm and remained somewhere at around 25 nm in Similar results were obtained in the sample prepared by the bismuth acetate precursor, where the annealed thin film. the intensity of the XRD peaks increased because the film thickness was higher; the BGO structure Similar results were obtained in the sample prepared by the bismuth acetate precursor, where was revealed by the peak at 27.9° assigned to the (310) reflection. However, after thermal annealing, the intensity of the XRD peaks increased because the film thickness was higher; the BGO structure the Bi2GeO5 crystalline phase seemed to be the dominant one, evidenced by the strong peak at 28.5° was revealed by the peak at 27.9 assigned to the (310) reflection. However, after thermal annealing, and accompanied by the BGO ◦crystalline phase marked with two main peaks at 27.65° and 33.9° the Bi2GeO5 crystalline phase seemed to be the dominant one, evidenced by the strong peak at 28.5◦ and assigned to the reflections of (310) and (400) plans. The Bi2GeO5 peaks were enhanced by the thermal accompanied by the BGO crystalline phase marked with two main peaks at 27.65 and 33.9 assigned annealing of the thin film sample compared with those in the BGO structure.◦ Moreover,◦ the to the reflections of (310) and (400) plans. The Bi GeO peaks were enhanced by the thermal annealing nanocrystallites size slightly increased from 362 nm5 in the initial powder to 40 nm in the annealed of the thin film sample compared with those in the BGO structure. Moreover, the nanocrystallites size powder and then 48 nm in the annealed thin film, only taking into account BGO peaks. The cubic slightly increased from 36 nm in the initial powder to 40 nm in the annealed powder and then 48 nm in structure of the thin film was consistent with the lattice parameters values of a = b = c = 10.61, which the annealed thin film, only taking into account BGO peaks. The cubic structure of the thin film was were close to the values of the BGO crystal. In both samples, a peak centered at 26.3° appeared, which consistent with the lattice parameters values of a = b = c = 10.61, which were close to the values of the was not identified, probably related to germanium ethoxide. This peak disappeared after annealing BGO crystal. In both samples, a peak centered at 26.3 appeared, which was not identified, probably in both samples. The (103) peak of bismuth acetate can◦ be seen at 30.8°. related to germanium ethoxide. This peak disappeared after annealing in both samples. The (103) peak3.2. Structural of bismuth Analysis acetate by can Elemental be seen atAnalysis 30.8◦. with the Energy-Dispersive X-Ray Spectroscopy (EDS) Method. 3.2. Structural Analysis by Elemental Analysis with the Energy-Dispersive X-ray Spectroscopy (EDS) Method The elemental analysis performed by using the EDS method enabled the determination of The elemental analysis performed by using the EDS method enabled the determination of composition and the ratio between bismuth and germanium oxides. The procedure was applied to composition and the ratio between bismuth and germanium oxides. The procedure was applied to the the initial sol-gel powder, assuming the same elemental compositions after subsequent thermal initial sol-gel powder, assuming the same elemental compositions after subsequent thermal annealing. annealing. The EDX patterns of the sample prepared by the bismuth nitrate precursor are given in The EDX patterns of the sample prepared by the bismuth nitrate precursor are given in Figure3, Figure 3, together with the elemental analysis in Table 1. together with the elemental analysis in Table1.

Figure 3. Elemental analysis of the sample prepared by the bismuth nitrate precursor. Figure 3. Elemental analysis of the sample prepared by the bismuth nitrate precursor. Table 1. Atomic concentrations of the main elements in the sample prepared by the bismuth nitrateTable precursor.1. Atomic concentrations of the main elements in the sample prepared by the bismuth nitrate precursor.Element Atomic Number Atomic Concentration (wt %) Error (%) OElement Atomic 8 number Atomic concentration 85.92 (wt %) Error (%) 5.5 O 8 85.92 5.5 Ge 32 8.56 0.6 Ge 32 8.56 0.6 Bi 83 5.52 1.2 Bi 83 5.52 1.2 TotalTotal 100 100

The atomic concentration of germanium was 8.56 wt %, while the bismuth concentration was 5.52 wt %, which resulted in 60.8 wt % GeO2 and 39.2 wt % Bi2O5 in the initial powder, preserving the

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CoatingsThe 2020 atomic, 10, 255 concentration of germanium was 8.56 wt %, while the bismuth concentration6 was of 12 5.52 wt %, which resulted in 60.8 wt % GeO2 and 39.2 wt % Bi2O5 in the initial powder, preserving desired composition of 60 wt % GeO2 and 40 wt % Bi2O5 from the chemical synthesis. The EDX the desired composition of 60 wt % GeO2 and 40 wt % Bi2O5 from the chemical synthesis. The EDX patternspatterns of of the the sample sample prepared prepared by by the the bismuth bismuth acetate acetate precursor precursor are are given given in in Figure Figure4, 4, together together with with thethe elemental elemental analysis analysis in in Table Table2. 2.

Figure 4. Elemental analysis of the sample prepared by the bismuth acetate precursor. Figure 4. Elemental analysis of the sample prepared by the bismuth acetate precursor. Table 2. Atomic concentrations of the main elements in the sample prepared by the bismuth acetateTable precursor.2. Atomic concentrations of the main elements in the sample prepared by the bismuth acetate precursor. Element Atomic Number Atomic Concentration (wt %) Error (%) OElement Atomic 8 number Atomic concentration 88.82 (wt %) Error (%) 5.4 O 8 88.82 5.4 Ge 32 4.25 0.3 Ge 32 4.25 0.3 BiBi 8383 6.93 6.93 1.4 1.4 TotalTotal 100 100

In this case, the atomic concentration of germanium was 4.25 wt %, while the bismuth In this case, the atomic concentration of germanium was 4.25 wt %, while the bismuth concentration concentration was 6.93 wt %, which resulted in 38 wt % GeO2 and 62 wt % Bi2O5 in the initial powder. was 6.93 wt %, which resulted in 38 wt % GeO2 and 62 wt % Bi2O5 in the initial powder.

3.3.3.3. Optical Optical Spectroscopy Spectroscopy TheThe optical optical absorption absorption spectrum spectrum (Figure (Figure5a) 5a) of of the the sample sample prepared prepared by by the the bismuth bismuth nitrate nitrate precursorprecursor with with a a thickness thickness of of around around 200 200 nm nm consisted consisted of of two two absorption absorption peaks, peaks, one one broadband broadband centeredcentered at at 595 595 nm nm and and a a sharper sharper one one at at 320 320 nm. nm. For For the the sample sample prepared prepared by by the the bismuth bismuth acetate acetate precursor,precursor, the the absorption absorption spectrum spectrum was was not not recorded, recorded, because because it it was was not not transparent. transparent. Concerning Concerning the the emissionemission spectra spectra of of both both samples samples excited excited at at 320 320 nm, nm, the the photoluminescence photoluminescence signal signal appeared appeared more more intenseintense at at 520 520 nm nm in in the the ﬁrst first sample sample and and less less intense intense at at 553 553 nm nm in in the the second second sample. sample. The The absorptions absorptions andand emissions emissions were were assigned assigned in in comparison comparison with with those those of of BGO BGO glasses glasses and and crystals crystals [23 [23].].

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((aa)) (b)( b)

FigureFigureFigure 5. 5. (5.a ( )a(a Optical) ) OpticalOptical absorption absorptionabsorption spectrum spectrumspectrum ofof of thethe the samplesample sample preparedprepared prepared by by by the the the bismuth bismuth bismuth nitrate nitrate nitrate precursor precursor precursor andandand emissions emissions emissions spectra spectra spectra of of the the samples samples prepareds prepared prepared byby by thethe the bismuth bismuth nitrate nitrate nitrate and and and bismuth bismuth bismuth acetate acetate acetate precursors. precursors. precursors. (b()b( Reﬂectanceb) )Reflectance Reflectance colors colors colors of ofof the the samples samples prepared preparedprepared by by by thethe the bismuthbismuth bismuth nitrate nitrate and and and bismuth bismuth bismuth acetate acetate acetate precursors precursors precursors ononon the the the CIE CIE CIE 1931 1931 1931 chromaticity chromaticitychromaticity map. map.

FromFromFrom the the the optical opticaloptical reflectance reflectancereflectance spectra, spectra,spectra, we obtained wewe obtained obtained the chromaticity the the chromaticity chromaticity of the annealed of ofthe the annealed thin annealed films thin (Figure thin films 5filmsb). The(Figure(Figure thinner 5b). 5b). sample The The thinner thinner prepared samplesample by the preparedprepared bismuth by by nitratethe the bi bismuth precursorsmuth nitrate nitrate exhibited precursor precursor a yellowexhibited exhibited color, a yellow a suggesting yellow color, color, a predominantsuggestingsuggesting a singlea predominantpredominant phase compared single phasephase with compared thecompared thicker with with one the (samplethe thicker thicker prepared one one (sample (sample by the prepared bismuthprepared by acetate bythe the precursor),bismuthbismuth acetate whichacetate revealed precursor),precursor), a redshift which towards revealedrevealed the a a reds yellow-orangeredshifthift towards towards color the the onyellow-orange yellow-orange the CIE 1931 color chromaticity color on onthe theCIE map. CIE 19311931All chromaticity chromaticity the peak vibrations map.map. identified in the Raman spectra for the initial sample, the powder annealed at 560 ◦AllC,All andthe the thepeakpeak annealed vibrationsvibrations thin identified films were inin compared thethe RamanRaman with spectra spectra those shownfor for the the ininitial theinitial BGO sample, sample, crystal the (Figurethe powder powder6a). 1 Inannealed theannealed sample at at 560 prepared560 °C, °C, andand by thethe the annealed bismuth thinthin nitrate films films precursor, we werere compared compared the first with with observed those those shown peakshown in at thein 63.8 the BGO cm BGO −crystalin crystal the 1 1 BGO(Figure(Figure crystal 6a). 6a). was In In the the broadened samplesample prepared and shifted byby to thethe 73 bismuth bismuth cm− in nitrate nitrate the initial precursor, precursor, powder the the and first first 72 observed cmobserved− in peak the peak annealed at 63.8 at 63.8 cm−1 in the BGO crystal was broadened and shifted to 73 cm−1 in–1 the initial powder and 72 cm−1 in the powder.cm−1 in Forthe theBGO annealed crystal was film, broadened this vibration and appeared shifted to at 73 71 cm cm−1 inand the was initial assigned powder to and the lattice-mode72 cm−1 in the annealed powder. For the annealed film, this vibration appeared at 71 cm–1 and 1was assigned to the vibrationannealed in powder. BGO. The For totally the annealed symmetric film, vibrational this vibration mode appeared A located at at 71 88.5 cm cm–1 and− was was observed assigned in to the the lattice-mode vibration in BGO. The totally symmetric vibrational mode A located at 88.5 cm−1 was samplelattice-mode prepared vibration by the bismuthin BGO. nitrateThe totally precursor symmetric and was vibrational gradually mode decreased A located in the at initial 88.5 powder,cm−1 was observed in the sample prepared by the bismuth nitrate precursor and was gradually decreased in 1 thenobserved appeared in the in thesample annealed prepared sample by and the laterbismuth in the nitrate thin film. precursor The next and peak was vibration gradually at decreased 121.3 cm− in the initial powder, then appeared in the annealed1 sample and later in the thin film. The next peak in the initial powder was shifted to be at 124 cm− in the annealed powder and thin film, while in the the initial powder, then−1 appeared in the annealed sample and later− 1in the thin film. The next peak vibration at 121.3 cm in the initial powder1 was shifted to be at 124 cm in the annealed powder and BGO crystal it appeared−1 at 122.5 cm . This peak had a strong intensity− in1 the initial sample, and its vibration at 121.3 cm in the initial− powder was shifted to− 1be at 124 cm in the annealed powder and thin film, while in the BGO crystal it appeared at 122.5 cm . This peak had a strong intensity in the 1 half width decreased in the annealed powder and thin film. The−1 most striking mode was at 315.5 cm thininitial film, sample, while and in the its halfBGO width crystal decreased it appeared in the at annealed 122.5 cm powder. This andpeak thin had film. a strong The most intensity striking in − the that appeared only in the annealed powder and film and was shifted to be at 360 cm 1 in the BGO initialmode sample, was at 315.5 and itscm half−1 that width appeared decreased only in in the the annealed annealed powder powder and and film thin and film. was Theshifted− most to bestriking at crystal. The peak was related−1 to the total symmetric mode of the Bi–O bond stretching. The peaks from mode360 cm was−1 inat the315.5 BGO cm crystal. that appeared The peak only was inrelated the annealed to the total powder symmetric and film mode and of was the shifted Bi–O bond to be at 1 above 410−1 up to 825 cm were assigned to GeO modes and decreased from the initial sol-gel powder 360stretching. cm in Thethe BGOpeaks −crystal. from above The peak410 up was to 825related4 cm−1 towere the assigned total symmetric to GeO4 modesmode ofand the decreased Bi–O bond tostretching. thefrom annealed the initial The thin peakssol-gel film from powder and powder.above to the 410 annealed up to 825 thin cm film−1 were and powder. assigned to GeO4 modes and decreased from the initial sol-gel powder to the annealed thin film and powder.

(a) (b) FigureFigure 6. (6.a) Raman(a) Raman spectra( aspectra) of the of samplethe sample prepared prepared by the by bismuth the bismuth nitrate nitrate precursor; precursor;(b) (b) Raman (b) Raman spectra of thespectra sample of the prepared sample byprepared the bismuth by the citratebismuth precursor. citrate precursor. Figure 6. (a) Raman spectra of the sample prepared by the bismuth nitrate precursor; (b) Raman

spectra of the sample prepared by the bismuth citrate precursor.

Coatings 2020, 10, 255 8 of 12 CoatingsCoatings 2020 2020, ,10 10, ,255 255 88 ofof 1212

InIn thethe samplesample preparedprepared byby thethe bismuthbismuthbismuth citratecitrate precursorprprecursorecursor (Figure(Figure6 6b),b),6b), the thethe peaks peakspeaks was waswas similar similarsimilar to toto 1 thosethose inin thethe the samplesample sample preparedprepared prepared byby by thethe the bismuthbism bismuthuth acetateacetate acetate precursor,precursor, precursor, exceptexcept except atat at 269269 269 cmcm cm−−−11, , whichwhich waswas quitequite intenseintense in in the the sol-gel sol-gel annealed annealed powderpowder powder andand and cannotcannot cannot bebe be seenseen seen inin in thethe the annealedannealed annealed thinthin thin film.film. film. TheThe FTIR FTIRFTIR spectra spectraspectra recorded recordedrecorded on onon both bothboth the thethe annealed annealedannealed powder powderpowder and andand thin thinthin film filmfilm are depictedareare depicteddepicted in Figure inin FigureFigure7a,b. In7a,b.7a,b. the InIn sample thethe samplesample prepared preparedprepared by the by bismuthby thethe bismuthbismuth nitrate nitrat precursor,nitratee precursor,precursor, two main twotwo peaks mainmain can peakspeaks be observed cancan bebe observedobserved at 553 and atat 1 1 584553553 cmandand− 584584in the cmcm− initial−11 inin thethe sol-gel initialinitial powder sol-gelsol-gel and powderpowder shifted andand to shiftedshifted 548 and toto 589 548548 cm andand− 589in589 the cmcm sol-gel−−11 inin thethe powder sol-gelsol-gel and powderpowder film 1 annealed at 560 C. Less visible were the next two peaks at 742 and 778 cm that can−1 be seen not only andand film film annealed annealed◦ at at 560 560 °C. °C. Less Less visible visible were were the the next next two two peaks peaks at at 742 742 and and− 778 778 cm cm−1 that that can can be be seen seen innotnot the only only annealed in in the the annealed powder,annealed butpowder, powder, also in but but the also also annealed in in the the annealed thinannealed film asthin thin one film film broadband as as one one broadband broadband (Figure7a). (Figure (Figure 7a). 7a).

((aa)) (b(b))

FigureFigure 7.7. ( a(a)) FTIR FTIR spectraspectra spectra ofof of thethe the samplesample sample prepared prepared prepared by by by the the the bismuth bismuth bismuth nitrate nitrate nitrate precursor; precursor; precursor; (b )( (b FTIRb)) FTIR FTIR spectra spectra spectra of theofof the the sample sample sample prepared prepared prepared by by theby th th bismuthee bismuth bismuth acetate acetate acetate precursor. precursor. precursor.

1 TheThe mostmost intense intenseintense peak peak cancan bebe be seenseen seen atat at 861861 861 cmcm cm−−11 − inin in thethe the initialinitial initial sol-gelsol-gel sol-gel powderpowder powder andand and shiftedshifted shifted toto 868868 to 1 868cmcm−−1 cm1 inin− thethein theannealedannealed annealed thinthin thin film.film. film. TheThe The lastlast last groupgroup group ofof of peakspeaks peaks thatthat that arosearose arose fromfrom the thethe bismuth bismuth germanategermanategermanate 1 1 structurestructure appearedappeared atatat 940 940940 and andand 998 998998 cm cmcm−−−11,, , and andand thethe otherother ones,ones, which whichwhich appeared appearedappeared at atat 1320 13201320 and andand 1425 14251425 cm cmcm−−−11,, , werewere assigned assigned toto to organicorganic organic residues,residues, residues, becausebecause because theythey they disappeareddisappeared disappeared afterafter after thermalthermal thermal annealing.annealing. annealing. SimilarSimilar assignmentsassignments can can be be mademade forfor thethe samplesample preparedprepared byby thethe bismuthbismuth acetateacetate precursorprecursor (Figure(Figure7 7b), b),7b),but but but thethe the peakspeaks peaks werewere were muchmuch much betterbetter better resolvedresolved resolved duedue due toto to thethe the higherhigher higher thicknessthickness thickness ofof of thethe the obtained obtained film.film. film. TheThe opticaloptical microscopymicroscopy imageimage ofof thethe samplesample preparedprpreparedepared byby thethe bismuthbismuth nitratenitrate precursorprecursor takentaken duringduring thethe RamanRaman measurementsmeasurements revealedrevealed aa smooth,smooth, homogeneous homogeneous and and uniform uniform distribution distribution ofof the the BGOBGO thinthin filmfilmfilm after after slow slow thermalthermal thermal annealingannealing annealing (Figure(Figure (Figure8 8). ).8).

FigureFigure 8. 8. The TheThe optical opticaloptical image imageimage of of the the sample sample prepar preparedprepareded by by the the bismuth bismuth nitrate nitrate precursor.precursor.

4.4. Discussion Discussion

TheThe annealingannealing procedureprocedure at at 560560 ◦°C °CC was was performed performed in in the the region region of of the thethe BGO BGOBGO crystallization crystallizationcrystallization peak, peak,peak, determineddetermined for forfor the thethe glass glass samples samples obtained obtained obtained by by by the the the melt melt melt quenching quenching quenching procedure procedure procedure at at at556 556 556 °C °C◦ (829 C(829 (829 K) K) K)[21]. [21]. [21 In In]. Infact,fact, fact, thethe the annealingannealing annealing pointpoint point waswas was overlappedoverlapped overlapped withwith the thethe second-phasesecond-phase BiBiBi222GeOGeO1212 crystallizationcrystallization peakpeak determineddetermined by by DTA DTA at at 568 568 °C °C (840 (840 K). K). Therefore, Therefore, abov abovee this this temperature, temperature, the the bulk bulk crystallization crystallization of of

Coatings 2020, 10, 255 9 of 12

determined by DTA at 568 ◦C (840 K). Therefore, above this temperature, the bulk crystallization of the Bi4Ge3O12 (BGO) phase was completed by the formation of intermediary-metastable-phase Bi2GeO5. The structural analyses of the initial sol-gel powder and the thermally annealed powder and thin film with a thickness of 200 nm confirmed the coexistence of two crystalline phases before annealing, cubic Bi4Ge3O12 (BGO) and monoclinic Bi2GeO5 crystallites, which evolved towards a BGO crystalline structure in the annealed thin film. The most dominant structure seemed to be the BGO crystalline phase, evidenced by the (310) peak, which was consistent with a cubic structure having a = b = c = 10.48, close to the crystallography of BGO monocrystals. The elemental analysis of this thin sample prepared by the bismuth nitrate precursor confirmed the composition of the desired Bi4Ge3O12 crystalline compound with 60 wt % GeO2 and 40 wt % Bi2O5. In the case of the sample prepared by the bismuth acetate precursor, with higher thickness, we observed the coexistence of the same cubic Bi4Ge3O12 (BGO) and monoclinic Bi2GeO5 crystallites, even after thermal annealing. This fact was marked by the elemental analysis, in which Bi2O3 was predominant in comparison with the GeO4 structures in the second sample, where 38 wt % GeO2 and 62 wt % Bi2O3 co-existed. The thermal annealing of 40 wt % Bi2O3 and 60 wt % GeO2 at 556 ◦C, produced both phases, cubic and monoclinic, but the monoclinic one was metastable. The reaction between GeO2 and Bi2GeO5 led to Bi4Ge3O12 (BGO), shown as following [21]: 2Bi GeO +GeO Bi Ge O (2) 2 5 2 → 4 3 12 However, according to the elemental analysis, the amount of GeO2 was much lower in the sample prepared by the bismuth acetate precursor, compared with that of Bi2O5, and the above reaction produced BGO as the secondary phase with the dominant one being the monoclinic Bi2GeO5 phase. The broad peak from 595 nm from the absorption spectrum was assigned to the Mie scattering on small crystallites with dimensions comparable with the wavelength of the light scattering. More than that, the excitation into this band did not give photoluminescence. A possible explanation might be related to the small thickness of the film and the small contact angle during the spin coating procedure (also connected with the viscosity of the solution) and the glass substrate, which increased the nucleation processes [24]. This smaller contact angle induced lower surface energy, increasing the nucleation rate. Because the GeO4 tetrahedra still existed in solutions, as can be seen in the vibrational spectra (Figure7), the nucleation rate should be assigned to the clusterization of these GeO 4 tetrahedra, inducing a scattering of light [25]. The explanation is based on the turbidity effect proportional to the size parameter of the cluster, which took into account the refractive index of the glass [26]. The second absorption band centered at 320 nm was similar to the absorption band centered 3 1 3+ at 290 nm in BGO crystals, which was assigned to the P1– S0 transition of Bi ions. In the case of the sample prepared by the bismuth nitrate precursor, the absorption at 290 nm in BGO crystals was shifted to 320 nm similar to the previous bismuth germanate melt-quenched samples. In fact, this 320 nm absorption band was not given by the radiation absorption of Bi3+ ions [27]. The oxygen atoms, around the Bi3+ ions, absorbed the energy on p-electrons and transferred it to Bi p-electrons. Since the annealed film was formed from two networks Bi2O3 and GeO2, there were GeO4 tetrahedra that absorbed the radiation energy and transferred it to Bi3+ ions [28]. The redshifts of both absorptions and emissions spectra were consistent with the disordered structure that appeared between the cubic Bi4Ge3O12 (BGO) and monoclinic Bi2GeO5 phases. The pure Bi4Ge3O12 single crystal belonged to the I4− 3m space group with four formula units in crystallographic unit cells [29]. Each unit cell contained 38 symmetrically nonequivalent atoms with respect to the inner vectors of translations: 6 Ge, 8 Bi, and 24 O atoms. More relevant was the existence of isolated GeO4 tetrahedra with S4 symmetry, leading to a deformed octahedral environment of oxygen around Bi3+ ions, but these isolated tetrahedra appeared in the amorphous structure during the formation of bismuth germanate thin films. There were 46 vibrational modes, but only 27 were Raman actives and some of them can be compared with those obtained by sol-gel thin film deposition and precursors. The theoretical calculations of Raman-active modes indicated two vibrational modes 1 at 64 and 66 cm− as transversal (TO) and longitudinal (LO) Bi, O, and Ge vibrations, which can be Coatings 2020, 10, 255 10 of 12

1 seen in the BGO crystal [30]. The totally symmetric vibrational mode A was calculated at 97 cm− and was assigned as significantly strong peak to the Bi and Ge vibrations, but in the pure BGO crystal 1 appeared at 90 cm− . The presence of this peak at the same wavenumber in all samples and the shift of the first peak suggested fixed positions for Bi and Ge ions and a rearranging mode of oxygen ions in all samples towards a pure BGO structure. 1 Regarding the theoretical vibration of mode E, it was calculated at 115–126 cm− and was 1 assigned to the Bi–O–Ge bond bending observed at 123 cm− in horizontal polarization laser mode 1 3+ and 126 cm− in vertical mode. For the totally symmetric mode of the (GeO4)3 against Bi bond 1 stretching, the theoretical vibration of mode E was calculated at 315–318 cm− and was assigned to the transversal mode. However, in the case of the BGO crystal, this vibration showed a quite large shifting. This fact supported the idea of a relaxed BGO structure in thin films and a distorted one in bulk BGO samples. The theoretical calculations for GeO4 vibrations indicated two classes of vibrational modes, 1 1 assigned to the O–Ge–O bond bending from 407 to 469 cm− and from 716 to 797 cm− assigned to 1 the Ge–O bond stretching. In BGO crystals, these modes appeared at 406, 409, and 445 cm− for the 1 O–Ge–O bond bending and from 703 to 820 cm− for the Ge–O bond stretching. Similar vibrational peaks were observed in the melt-quenched BGO samples, and their corresponding devitrified the counterparts [12]. For the Raman spectra of the sample prepared by the bismuth acetate precursor, the peaks were more intense, because the sample was thicker than the first sample. Some additional peaks were related to the presence of the bismuth germanate phase, Bi2GeO5, as can be seen from the XRD patterns (Figure2). This phase appeared during the crystallization process, but in our case, it appeared in the as-prepared sol-gel powder. The spectra pattern was consistent with the Raman spectra analysis. A number of 25 active modes were identified from the infrared spectroscopy, but due to the experimental constraints (all spectra were 1 measured in the ATR mode), the low-frequency peaks at 92, 203, and 363 cm− appeared as overtones or second-order vibrations at 536 and 567 cm–1 in the BGO crystals. These vibrations were assigned 1 to ν2 GeO4 lattice vibrations as the A1 transition type and calculated at 536 cm− [31]. Another two 1 1 bands at 740 and 778 cm− or at 721 and 775 cm− [32] were assigned to ν1 and ν3 vibrations of GeO4 1 tetrahedra in BGO crystals and are in good agreement with the experimental one at 742 and 778 cm− 1 in our case. In a similar manner with Bordum, the most intense band appeared at 862 cm− , which 1 was measured at 863 cm− in the BGO crystals. This band appeared as overtones between 567 and 1 294 cm− (not seen in our spectra) with the coupling between two transitions A1 and F2. The 940 and 1 4 998 cm− peaks were assigned to the stretching vibrations of isolated (GeO4) − tetrahedra.

5. Conclusions Transparent and homogeneous bismuth germanate thin films were obtained by using the sol-gel method. Their physical properties were strongly influenced by initial raw materials, which induced different viscosities of sol precursors and wettability. This method aimed to avoid the calcination procedure at higher temperatures, which induced a solid-state reaction between oxides and proposed a slow thermal annealing technique for BGO thin-film crystallization. The structure, morphology, and optical properties of the films were influenced by the precursors Bi(NO3)3 or Bi(CH3COO)2. The initial sol-gel powder showed the presence of two crystalline structures, the cubic Bi4Ge3O12 (BGO) and monoclinic Bi2GeO5 crystallites. By using bismuth nitrate as a precursor instead of bismuth acetate, the sol-gel processing method led towards the desired BGO compound after annealing, having a dominant phase of thin films deposited on glass substrates and a crystallization point below that of BGO. Thin films with a 200 nm thickness containing cubic Bi4Ge3O12 particles were obtained by using a Bi(NO3)3 precursor. Using bismuth acetate as a starting material, the obtained thin films were thicker and less homogeneous and revealed Bi2GeO5 as a dominant phase but with BGO (around 40%) as a secondary phase after slow thermal annealing. Coatings 2020, 10, 255 11 of 12

The optical absorption and luminescence spectra showed redshifts assigned to different disorder structures within the cubic Bi4Ge3O12 and monoclinic Bi2GeO5 crystalline phases. The obtaining of the desired BGO compound led to better spectroscopic parameters, especially photoluminescence, which fundamentally contributed to a better scintillating process for high-energy radiation detectors. The sol-gel procedure of bismuth germanate thin films may induce oxygen defects during crystallization processes, which exhibits a magnetic moment, and hence these thin films with oxygen defects are known as diluted magnetic materials. Similarly, thin films of bismuth germanate glass and glass-ceramic materials can possess such magnetic properties by the sol-gel method as well [33].

Author Contributions: Conceptualization, S.P.; methodology S.P.; writing -original draft preparation, S.P.; writing—review and editing, M.S.; synthesis, C.E.S.; investigation, S.P. and T.T. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by a grant from the Romanian Ministry of Research and Innovation DEXMAV 12PFE/2018 and Core Program PN19-03 (contract no. 21 N/08.02.2019). Acknowledgments: The authors are grateful to Core Program PN19-03 (contract no. 21 N/08.02.2019) and Project DEXMAV (contract no. 12PFE/2018). Conﬂicts of Interest: The authors declare no conﬂicts of interest.

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