Approach for Selection of a Synthesis Procedure of GeO<SUB>2</SUB> Ultra-small Nano Particles and Its Characterization

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3. Results and Discussions

Figure 1 Shows the XRD patterns of the samples Ge1-Ge5 as well as Ge7. This substantiate that the nature of the synthesized samples are crystalline having α-quartz type hexagonal structure and of phase P3₂21 (as mentioned in JCPDS data, PDF-85-1519). The peaks at each XRD patterns marked as (100), (011), (110), (102), (111), (200), (112), (202) and (121) are quite similar in broad view. However, the magnified view of these peaks differs in broadening, which increase with the synthesis temperature, similar as reported earlier ^{[18, 19]}.

Figure 1. XRD pattern of samples Ge1-Ge5 and Ge7

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Figure 2. TEM images of samples Ge1-Ge3.

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Figure 3. TEM images of samples Ge4 and Ge5.

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The TEM images as observed for the samples Ge1- Ge5 are shown in Figure 2 and Figure 3. These images shows that the particles developed within each samples are uniform and identical. This arouses us to calculate average particle size from each TEM image with the help of Image-J software. The obtained particle sizes respects to the samples are presented in Table 1. Taking into account the particles of all the samples as observed in the TEM images are spherical in nature we have calculated the volume (V) of particles for each samples by considering the particle size as diameter. Considering the particle size of sample Ge1as initial form of the material, we have calculated the differences in volume (ΔV) of the respective samples (Ge2-Ge5). This allows us to get the volume strain by taking the ratios of ΔV and V (volume of the sample Ge1) for the samples Ge2-Ge5. The volume strains are plotted against the synthesis pressures as shown in Figure 4 and we have recognized this plot similar to well establish stress-strain plot.

Figure 4. Variation of relative volume strain (ΔV/V) corresponding to synthesis pressure. (The solid line guides the eye.)

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After finding this plot, we deliberately go for a synthesis at such pressure that is well above the synthesis pressure of sample Ge5. Figure 5 shows the TEM image of this sample (Ge6).

Figure 5. TEM image of sample Ge6

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From this image, it is observed that a single grain of the material bursts into several small fragments and we have calculated the average grain size of these smaller fragments from Figure 6 (TEM image of Ge6 magnifying the smaller fragments only) with the above-mentioned process as used for the grain size calculations of sample Ge1- Ge5.

Figure 6. TEM image of sample Ge6 (magnifying smaller fragments)

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It is observed that the particle size of sample Ge7 as shown in Table 1 is changed abruptly relative to the previous. Therefore, we suggest the importance of the stress-strain like plot (suggested first as our knowledge is concerned) in synthesis of nano particles, which is for not only GeO₂, but can also be generalized for nano particle synthesis of other materials. However, our aim is to find a standard process to synthesis smallest ever grain of GeO₂. That is why we have prepared a sample (Ge7) at a synthesis pressure far beyond the synthesis pressure of Ge6. We observe from the HRTEM image that the grain size of this sample lowers to the order of 10 nm as shown in Figure 7.

Figure 7. HRTEM image of sample Ge7 with SAED pattern inset

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The SAED pattern as shown inset of Figure 7 confirms the crystalline nature of the sample. The spacing between two successive atomic lines (d_hkl) is calculated from the SAED pattern as 0.224 nm, which is very much comparable to the value of d_hkl as observed in HRTEM image.

The PL characteristics of the samples, Ge1 to Ge5 have already reported ^[20]; therefore, we are interested for photoluminescence characteristics of the sample ‘Ge7’ under different excitation wavelengths (viz. 325nm, 300 nm, 275 nm and 250nm). Figure 8 shows the respective PL spectra for the sample Ge7.

Figure 8. PL Spectra of the sample Ge7at different excitation energies

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From Figure 8, it is observed that the peak at 427nm remains unchanged with the increase of excitation energy. This indicates blue light emission due to the oxygen-vacancies and oxygen-germanium vacancy centre [21-24]^[21]. Another peak is observed at 371 nm for the excitation wavelength 325nm, which shifts towards smaller value of wavelength regions as the excitation wavelength decreases. This phenomenon is being observed probably due to the Raman scattering ^[20]. However, it is to be mentioned here that sample Ge6 is in critical point for transition so we are least interested to study the physical properties of this particular one.

The FTIR spectrum as observed for the sample (Ge7) is shown in Figure 9. A broad peak is observed at 556 cm^-1 instead of three separate peaks in this region as reported due to hexagonal structure of GeO₂ ^{[25, 26, 27, 28]}. The broadness of these three peaks is possibly due to the very small particle sizes of the particles. There is no peak observed at 880 cm^-1. Therefore, the sample does not maintain any layered structure as reported before²¹. The peak observed at 857 cm^-1 supports the existence of oxygen vacancy in the sample ^[29]. The peak at 1645 cm^-1 as well as one of its harmonics at the vicinity of 3450 cm^-1 confirms the presence of hydroxyl (–OH) group ^[27], which is probably responsible for the water absorption properties of the sample as contradictory with the properties of bulk GeO₂.

Figure 9. FTIR Spectrum of sample Ge7

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Figure 10. I-V characteristic of sample Ge7

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A typical voltage-current (V-I) characteristic as obtained across the sample at room temperature is shown in Figure 10. It shows that the current reaches at a peak value of 18.75mA at 0.03 volt and decreases with the voltage and then shows the bulk behaviour. This phenomenon repeats as the polarity of the voltage changes. Presence of oxygen vacancy in the sample is responsible for such incident. An evidence of similar type of phenomenon in Cerium Oxide (CeO₂) has been observed ^[14]. Due to a fair structural similarity of CeO₂ and GeO_2,we propose that the similar phenomena are responsible for the I-V characteristic as mentioned for CeO₂. However, in GeO₂ molecule, Ge atom shares two electrons from ‘4p’ band to oxygen ‘2p’ band. Due to oxygen vacancy, if an oxygen atom leaves its lattice position an electron at ‘4p’ band of Ge atom becomes unpaired, which increase the possibility of electron hopping. Besides this, as proposed by A.V. Shaposhnikov et.al. ^[17] oxygen vacancies remaining in GeO₂ is a type of defect and defects generate a disturbance in the lattice periodicity through the crystal. Such irregularity is responsible for the conducting path between the electrodes ^[30]. Therefore, the carriers produced due to the formation of oxygen vacancy conduct as the voltage is applied. The number of carrier increases with the increasing voltage and reaches at the peak value and subsequently the number of carriers available for conduction drops. Therefore, current is found negligible as comparable to the bulk behaviour, which is shown in Figure 11. The concentration of the oxygen vacancy has been estimated as shown in Table 2. The result is quite similar to the earlier report ^[31].

Figure 11. I -V characteristic of bulk GeO₂

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Table 2. Oxygen vacancy concentration in sample ‘Ge8’ as obtained from I-V curve

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