Controllable Synthesis and Morphology-Dependent Light Emission Efficiency of Zn2GeO4 Nanophosphors

1. Introduction

III-V semiconductors are widely recognized as some of the most significant basic materials for the development of light-emitting devices worldwide, due to their exceptional capabilities.[1] Nevertheless, semiconductor industry is searching for novel light emitting materials due to the high toxicity of current semiconductors and some of their alternatives, especially those based on arsenic,[2] gallium,[3] or lead.[4] In addition, the production of indium, which is a common element appearing in III-V semiconductors, is highly dependent on the fluctuations of other metals markets as it is only obtained as a by-product of other metal ores processing.[5] Apart from that, rare earth elements, which appear in the composition of current phosphors, have been classified as critical raw materials by several international organizations, which have recommended lessening their usage in the industry.[6,7] All of these factors have intensified the search for novel semiconductor compounds, which may overcome most of these drawbacks.

Ternary oxides, particularly those based on ZnO and TiO₂, have emerged as some of the most promising candidates to replace current light-emitting semiconductors, fulfilling all previous requirements. Furthermore, various synthesis methods can be employed to prepare oxide nanophosphors with an up to standard purity and crystallinity with controlled morphologies and particle sizes, which can be easily tuned by simply modifying synthesis conditions.[8,9,10,11]

Among these materials, bulk Zn₂GeO₄ emits in the blue-green region, peaking at 2.39 eV. Nevertheless, its photoluminescence can be considered rather complex as it is the contribution of three different signals, whose maxima occur at 2.28, 2.38 and 2.73 eV respectively.[12] DFT calculations attribute the luminescence of Zn₂GeO₄ to Zn 3d orbitals and the presence of native defects such as oxygen vacancies and zinc interstitials.[13,14] However, Zn₂GeO₄ nanoparticles emit different wavelengths depending on their morphology and particle size.[15] In particular, hexagonal microrods emit at 2.03, 2.40 and 2.86 eV,[16] nanowires at 1.89, 2.11 and 2.34 eV,[17] whilst small nanoparticles present luminescence signals at 2.4, 2.7 and 3.1 eV.[18] Therefore, such remarkable variations imply that morphology may affect Zn₂GeO₄ luminescence. Nevertheless, information about the optical properties of more types of undoped Zn₂GeO₄ particles is fairly limited. Covering a broader range of nanoparticle shapes and sizes may help to correlate the emitted light with different characteristic features of Zn₂GeO₄ crystallites with the ultimate goal of generating toxic-free nanophosphors with tunable properties.

Several studies have reported the preparation of Zn₂GeO₄ nanoparticles with a myriad of morphologies employing diverse synthetic methods. Solvothermal and hydrothermal procedures stand out among these methodologies due to the fine morphological and particle size control they provide. In particular, numerous articles report the solvothermal synthesis of Zn₂GeO₄ nanorods, commonly applied as photocatalysts,[19,20] for biosensing,[21] and for lithium batteries.[22,23] Apart from that, Zn₂GeO₄ ultrathin nanoribbons [24] can also be obtained following a hydrothermal method, while other procedures can be employed to prepare Zn₂GeO₄ with different morphologies: CVD (nanowires),[25,26,27,28] ceramic (polycrystals),[29] template-assisted method (hollow spheres),[30] etc.

In this article we report the reproducible hydrothermal synthesis of short Zn₂GeO₄ nanorods for light-emitting applications. An exhaustive structural, morphological and luminescence characterization was conducted, which allowed us to correlate the optoelectronic properties of this material with its morphological aspects, which is of maximum importance for its possible technological applications.

2. Materials and Methods

2.1. Synthesis

Zn₂GeO₄ nanoparticles were synthesized following a conventional hydrothermal method. The stoichiometric amounts GeO₂ (Sigma-Aldrich, 99.9%) were dissolved in a 0.33 M NaOH solution (Sigma-Aldrich, 98%) under continuous stirring. The corresponding amount of a Zn(CH₃COO)₂·2H₂O (Merck, 99.5%) solution was added dropwise and the resulting mixture was poured into a Teflon-line stainless steel autoclave. The autoclave was introduced in an oven for 12h at 100^oC. The as-obtained white precipitate was centrifuged, washed several times with water until pH = 7, and dried overnight. The synthesis was carried out several times to verify the reliability of the results obtained.

2.2. Structural characterization

X-ray diffraction (XRD) measurements were performed in a Bruker D8 ADVANCE A25 diffractometer using a Cu tube in Bragg-Brentano optics with fixed slits, Ni filter and a position sensitive LynxEye SSD160-2 detector. Patterns were recorded within the 2θ range of 5−80°, using a step size of 0.01° and a collection time of 1 s per step.

Transmission electron microscopy (TEM) experiments were conducted in a JEOL JEM 2100 microscope, located in the facilities of the National Center of Electron Microscopy (ICTS-CNME). The spatial resolution achieved operating at 200 kV in High Resolution Transmission Electron Microscopy mode (HRTEM) is 0.25 nm.

2.3. Photoluminescence measurements

A Horiba Jobin Ybon LabRaman Hr800 confocal microscope using a 325nm He-Cd laser as excitation source was used for photoluminescence (PL) measurements at room temperature (RT). An Edinburgh Instruments FLS1000 system, equipped with a 450W Xe lamp and a helium cryostat, was employed for acquiring photoluminescence-photoluminescence excitation (PL-PLE) spectra from 4K up to RT.

3. Results and discussion

3.1. Structural and morphological chacacterization

XRD was carried out in the as-synthesized nanoparticles, whose diffraction pattern is displayed in Figure 1. All the diffraction peaks were perfectly indexed on the rhombohedral space group R-3 basis of Zn₂GeO₄ (ICSD Collection Code: 68382), with cell parameters a = b = 14.28 Å and c =9.55 Å. This diffractogram clearly shows that nanocrystals possess a willemite-like structure[31] compared to the Zn₂GeO₄ reference, where the structure can be described as formed of tetrahedrally coordinated zinc and germanium atoms, sharing corners.[32] The absence of additional reflections, which could be attributed to impurities, is indicative of the remarkable purity of the Zn₂GeO₄ sample, even without the need for high-temperature thermal treatments. The relative wide maxima suggest the nanoparticle nature of the material.

The average crystal size has been calculated from the values of the form factor K, the wavelength of the incident radiation λ, the full width at half maximum B, of the diffraction maximum {210}, and the diffraction angle θ, along with Scherrer formula: D = K·λ/(B·cosθ). The mean value is 17 ± 0.02 nm.

To confirm these results, transmission electron microscopy studies have been carried out. The low-magnification TEM image depicted in Figure 2a evidence that most nanoparticles possess a short rod-like morphology. Particle length and diameter distributions are presented in Figure 2b and c, revealing their small size. Specifically, particle length ranges between 24 and 54 nm with an average length of 37 ± 7 nm, whilst their mean diameter stands at 13 ± 3 nm, varying from 7 to 19 nm.

In addition, the study by high-resolution electron microscopy (HRTEM) confirms the structure of the Zn₂GeO_4. Figure 2d and f depict the electron micrograph top-view images of representative agglomerates of Zn₂GeO₄ nanocrystals oriented along the [111] zone axis. As suggested by these images, the radial morphology of these rod-like nanoparticles is greatly varied, but numerous cross-sections present 60^o and 120^o angles. Therefore, some of these nanoparticles may derive from distorted hexagonal and truncated triangular prismatic nanocrystals.

The digital diffraction pattern (DDP), extracted from one of the crystals (Figure 2e) clearly show the perfect willemite-like lattice of the nanocrystals. The reflections distances and angles measured are 3.6 Å at 60^o, corresponding to {-220}, {02-2} and {20-2} planes oriented along a [111] zone axis. On the other hand, the characteristic tunnels, inherent to the willemite structure and parallel to the [111]-axis direction, can be readily discerned in Figure 2g, which depicts a magnified region from Figure 2f. The experimental diameter of the tunnels has been estimated using HRTEM images, and the measured values are approximately 7.3 Å, in accordance with the structure described for this oxide. [31]

3.2. Optical characterization of the nanoparticles

An in-depth study of the luminescence emission of the as-synthesized Zn₂GeO₄ nanoparticles was carried out. An analysis of the emission provides information about the electronic recombinations between conduction and valence bands or between the electronic levels caused by native defects within the bandgap. As a first step, room temperature luminescence of these Zn₂GeO₄ nanocrystals has been assessed. Figure 3 shows the room temperature luminescence spectrum acquired with an He-Cd laser (λ= 325 nm). A broad visible band covering practically the whole visible range is observed. PL spectra agree with the complex nature of the luminescence band involving several radiative centers. The broad luminescence emission can be deconvoluted intro three components (2.18 eV, 2.41 eV and 2.81 eV), as previously reported in other Zn₂GeO₄ nanoparticles growth by a different synthesis method.[18] The green-yellow broad band whose maximum is centered at 2.41 eV is the dominant contribution to the luminescence of this material. Li and co-workers [33] have suggested that this complex band is related to Ge centers (bands peaked at 2.18 eV and 2.41 eV) and oxygen defects (band peaked at 2.8 eV). These emission contributions are very close to the ones reported for microrods,[16] and rather distinct to Zn₂GeO₄ nanoparticles, nanowires and Zn₂GeO₄ in bulk form. [15,17,18] Hence, our results evidence that particles with similar morphologies, but clearly different particle sizes (nanorods and microrods) possess an analogous phosphor emission, which reinforces the relevant effect of morphology on the light emission efficiency of Zn₂GeO₄.

To understand better the origin of these emission bands, PL and PLE spectra were acquired at RT and at low temperature (4K). Figure 4 shows two PL-PLE maps: Figure 4a recorded at RT, and Figure 4b at 4K. In both cases, spectra were collected varying the excitation wavelength between 250-285 nm (4.96 – 4.35 eV) in order to ensure an excitation energy over, or close to the bandgap energy to be used, while the light emitted was recorded between 320-650 nm (3.88 – 1.9 eV).

Analyzing the data from these emission maps, a broadening of the emission spectra occurs as temperature increases. Furthermore, it can be observed that the maximum emission of the sample occurs with an excitation in the range of 265-270 nm for 4K, and that this range increases to 260-280 nm when the sample is at RT. Using an excitation with an energy above the gap of the material produces an UV emission increase centered between 320 and 400 nm. Additionally, it is possible to observe how this emission decreases due to a reduction in the gap of the material when temperature rises.

Figure 5 shows emission spectra at RT and at 4K, acquired on the sample from 320 to 650 nm with an excitation wavelength of 250 nm in order to ensure an excitation energy over the bandgap energy.[18] At this excitation energy, a relatively intense emission centered at 3.4 eV is observed, which has been previously attributed to a recombination of V_O level electrons, acting as donor centers, with valence bands holes.[12] Therefore, the intensity of this band is directly related to oxygen vacancies. The broad feature observed in Figure 5a is not composed by a unique emission band as it is shown in Figure 5b. Deconvolution of the PL spectrum at 4K shows that it is composed of five gaussian emissions: three in the visible range (2.19 eV, 2.53 eV and 2.80 eV) and two gaussian emissions in the ultraviolet range (3.35 eV and 3.59 eV). The intensity evolution of the PLE spectrum of these two UV bands allows us to calculate the optical band gap of the nanoparticles.

In that sense, Figure 6 shows the intensity emission of the nanoparticles at 360nm (3.44 eV) varying the excitation wavelength from 250 nm to 285 nm. The extrapolation of the sharp PLE decay at the shortest wavelengths to a linear function provides the value of the energy bandgap. The fitting linear curve of this extrapolation is plotted by the green discontinuous line the Figure 6. The calculated optical band gap of the nanoparticles is estimated at 4.75 eV at 4K, which is analogous to the reported for microwires (4.76 eV) and slightly larger than Zn₂GeO₄ long nanowires (4.68 eV).[34] Nevertheless, it greatly differs from the DFT-calculated bulk Zn₂GeO₄ (4.4 eV), and is smaller than the experimental Zn₂GeO₄ thin layers band gap (4.9 ± 0.1 eV).[35]

These results clearly evidence the influence of crystallite morphology in the optoelectronic properties of Zn₂GeO₄ as nanoparticles with radically different external shapes present rather distinctive optoelectronic responses.

4. Conclusions

Zn₂GeO₄ short nanorods were successfully prepared by a reproducible hydrothermal synthesis. Nanoparticles are homogeneous in both particle size and morphology, with no impurities. PL spectra collected at both room temperature and at 4K are rather wide, covering almost all the visible range being centered in the green-yellow region. Spectra can be deconvoluted in several emission contributions, which are attributed to native vacancies. These contributions are comparable to the ones reported for Zn₂GeO₄ microrods, revealing the clear influence of Zn₂GeO₄ morphology on its luminescence. Zn₂GeO₄ nanorods bandgap was estimated (4.75 eV), which is similar to the one obtained for microwires and nanowires, and clearly different from bulk Zn₂GeO₄. This strongly reinforces that morphology is a crucial parameter in the emission of Zn₂GeO₄. Further analyses will be performed to analyze oxygen and germanium vacancies in these nanorods to compare with other Zn₂GeO₄ morphologies with the aim to shed light onto their relationship with particle size and morphology, and their contribution to the variations in the as-observed Zn₂GeO₄ optoelectronic properties.