Youcef A Bioud
email: y.bioud@usherbrooke.ca

Etienne Paradis

Abderraouf Boucherif

Dominique Drouin

Richard Arès
email: richard.ares@usherbrooke.ca

Shape control of cathodized germanium oxide nanoparticles

Keywords: Electrodeposition, crystal growth, GeO2 nanostructures, Cathodoluminescence

In this paper, hexagonal germanium dioxide (GeO2) nanostructures with different morphologies and sizes were synthesized successfully by a simple and fast electrodeposition method. We investigated the electrochemical growth mechanism, the structural and optical properties of the products through Fourier transform infrared spectroscopy (FTIR), dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and cathodoluminescence (CL). The results reveal that the electrodeposited GeO2 nanostructures are pure, dense, and highly crystalline. The XRD analyses indicate that grown GeO2 crystals only shows peaks related to α-quartz structure. CL measurements exhibit strong blue and green lights emissions related to oxygen vacancies in the core of GeO2 crystals. The obtained nanostructures may have potential application in future integrated optical devices.

Introduction

Nanoscale germanium-based materials exhibit a variety of novel properties and benefit a wide range of applications [START_REF] Wu | Germaniumbased nanomaterials for rechargeable batteries[END_REF]- [START_REF] Bioud | Engineering dislocations and nanovoids for high-efficiency III-V photovoltaic cells on silicon[END_REF]. Germanium dioxide (GeO2) nanostructures are versatile materials that have been explored in depth due to their unique thermal, optical and electrical properties. GeO2 is thermally stable, has a high dielectric constant with large band gap energy (5 eV) [START_REF] Zhang | Ultralong germanium oxide nanowires: Structures and optical properties[END_REF], a high mechanical strength [START_REF] Ramana | Optical properties and thermal stability of germanium oxide (GeO2) nanocrystals with α-quartz structure[END_REF] and exhibits a refractive index that is slightly higher than that of SiO2 [START_REF] Chiasera | CO2 Laser irradiation of GeO2 planar waveguide fabricated by rf-sputtering[END_REF], [START_REF] Hsu | Optical, electrical properties and reproducible resistance switching of GeO2 thin films by sol-gel process[END_REF]. GeO2 is also an attractive candidate for lithium-ion batteries (LIBs) due to its high theoretical reversible capacity (1125 mA h g -1 based on 4.25 mol Li per mol Ge) and low operating voltage, as well as long cyclability [START_REF] Ngo | Conducting additive-free amorphous GeO2/C composite as a high capacity and long-term stability anode for lithium ion batteries[END_REF]- [START_REF] Hwang | Mesoporous Ge/GeO2/carbon lithium-ion battery anodes with high capacity and high reversibility[END_REF]. In addition, this nanomaterial shows a good blue photoluminescence behavior, which is highly sought after for optical waveguides and optoelectronic communication devices [START_REF] Armelao | Visible emission from GeO2 nanowires: site-specific insights via X-ray excited optical luminescence[END_REF], especially, electro optical modulators, piezoelectric glass materials, optical fibre materials and non-linear optics [START_REF] Lignie | Growth of piezoelectric water-free GeO2 and SiO2substituted GeO2 single-crystals[END_REF]- [START_REF] Hermet | Density functional theory predictions of the nonlinear optical properties in α-Quartz-type germanium dioxide[END_REF]. Due to this all, so far, different approaches to synthesizing nanostructured GeO2 have been elucidated in the literature, including thermal treatment of Ge [START_REF] Kim | Catalyst-free synthesis of GeO2 nanowires using the thermal heating of Ge powders[END_REF], vapor transport [START_REF] Wu | Germanium nanowire growth via simple vapor transport[END_REF], carbothermal reaction [START_REF] Wu | Preparation and photoluminescence properties of crystalline GeO2 nanowires[END_REF], carbon nanotube confined reaction of Ge [START_REF] Zhang | Synthesis of GeO2 nanorods by carbon nanotubes template[END_REF], chemical vapor deposition [START_REF] Mathur | Germanium nanowires and core-shell nanostructures by chemical vapor deposition of [Ge (C5H5) 2][END_REF], supercritical fluidliquid-solid synthesis [START_REF] Hanrath | Supercritical fluid-liquid-solid (SFLS) synthesis of Si and Ge nanowires seeded by colloidal metal nanocrystals[END_REF], electro-spinning [START_REF] Kim | Preparation and morphology of germanium oxide nanofibers[END_REF], e-beam evaporation [START_REF] Saikiran | Formation of Ge nanocrystals from ion-irradiated GeO2 nanocrystals by swift Ni ion beam[END_REF], [START_REF] Krupanidhi | Electrical characterization of amorphous germanium dioxide films[END_REF], sol-gel deposition [START_REF] Zhang | Concave micro-lens arrays fabricated from the photo-patternable GeO2/ormosils hybrid sol-gel films[END_REF], [START_REF] Zhang | Monolithic germanium oxide aerogel with the building block of nano-crystals[END_REF], radio frequency (RF) magnetron sputtering [START_REF] Terakado | Photo-induced phenomena in sputtered GeO2 films[END_REF], [START_REF] Rana | Noise reduction of a-Si1-xGexOy microbolometers by forming gas passivation[END_REF], and laser ablation [START_REF] Lauhon | Epitaxial core-shell and coremultishell nanowire heterostructures[END_REF]. These techniques allow the synthesis of various GeO2 shapes with tuned size such as nanoparticles, nanofibers, nanosheets [START_REF] Nejaty-Moghadam | Facile synthesis of GeO2 nanostructures and measurement of photocatalytic, photovoltaic and photoluminescence properties[END_REF], nanocubes, nanospindles, nanocapsules [START_REF] Wu | Preparation of monodisperse GeO2 nanocubes in a reverse micelle system[END_REF], [START_REF] Adachi | Formation of GeO2 nanosheets using water thin layers in lamellar phase as a confined reaction field-in situ measurement of SAXS by synchrotron radiation[END_REF], nanowires, nanorods [START_REF] Hidalgo | High aspect ratio GeO2 nano-and microwires with waveguiding behaviour[END_REF], [START_REF] Kim | Room temperature synthesis of germanium dioxide nanorods and their in vitro photocatalytic application[END_REF], and porous structures [START_REF] Wei | Morphology memory but reconstructing crystal structure: porous hexagonal GeO2 nanorods for rechargeable lithium-ion batteries[END_REF], [START_REF] Zou | A mesoporous germanium oxide with crystalline pore walls and its chiral derivative[END_REF]. However, GeO2 nanoparticles fabricated from electrochemical processes have some distinctive advantages over nanoparticles fabricated by other methods mainly because (i) they are conveniently synthesized potentiostatic control, enabling electrophoretic deposition from the electrolyte, (ii) high density of nanocrystals, (iii) pure material (free of contaminants and chemical residues), (iv) high specific surface area, (v) coupled optical properties, (vi) can be transformed to powder by grinding and dispersed in a solution. In addition, the electrochemical process is standard, scalable that is already well integrated within an industrial fully automatic environment. Such a process meets the criteria, which provide an attractive solution for some specific applications. This paper reports the successful synthesis of hexagonal GeO2 nanostructures (nano-bundle aggregates and nanowires) by a simple and fast electrodeposition method with a plausible growth mechanism. The prepared GeO2 nanostructures are characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), energy-dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) measurements. Finally, we focus on performing a comprehensive optical characterization of GeO2 nanostructures by cathodoluminescence (CL).

Experimental:

GeO2 nanostructures synthesis was accomplished using a conventional three-electrode single-compartment electrochemical cell. GeO2 nanostructures were potentiostatically deposited on p + type Ge wafers, (100) oriented with a nominal resistivity of 0.025 Ω cm (working electrode WE). A platinum sheet was used as counter-electrode (CE) and saturated calomel electrode (SCE) as reference electrode. All solutions were prepared from analytical grade reagents and double distilled water. As germanium tetrachloride (GeCl4) is most widely used germanium source for germanium electrodeposition in ionic liquids, electrodeposition of GeO2 was conducted in a hydrogen peroxide solution containing 10% of germanium tetrachloride solution (GeCl4:H2O2, with the ratio of 1:9 by volume) at room temperature (20 °C). A wide range of GeO2 morphology as a function of applied potentials was studied and a controlled structure, morphology and phase composition was sought. All potentials are reported with reference to the SCE scale. As surface conditions and activation are critical to electrocrystallisation nucleation and growth, Ge wafers were immersed in hydrofluoric acid (49%) to remove the native oxide before anodization. All deposition processes were carried out under stirring (700 rpm) using a cylindrical PTFEcoated magnetic stirrer bar of 12 mm long and 4.5 mm diameter with a pivot ring of 6 mm diameter, placed between the WE and the CE. At the end of the deposition process, all the wafers were rinsed with deionized water and dried under a high purity nitrogen gas flow.

The chemical composition of the exposed surface is verified by energy-dispersive X-ray (EDX) spectroscopy. Powder X-ray diffractograms have been measured with a Philips X'Pert diffractometer in the 2θ range from 10 to 65•. The cathodoluminescence (CL) spectra and images are acquired at room temperature, in the same SEM setup as the one used for imaging the surface. Our CL system, in association with a spectrometer, allows monochromatic CL (GATAN MonoCL2) imaging as well as acquisition of CL spectra on localized spots of a sample with a spectral resolution of 0.5 nm. The accelerating voltage used in the CL characterization is 20 keV.

Results:

 Electrochemical characterization and morphology:

The electrochemical characterizations such as cyclic voltammetry measurements help us to identify oxidation reduction processes potentially undergone by the system of interest and to choose an appropriate potential [START_REF] Mahenderkar | Electrodeposited germanium nanowires[END_REF], [START_REF] Laidoudi | Growth and characterization of electrodeposited Cu2O thin films[END_REF]. The electrochemical window of this ionic liquid was found to be 3.5 V (-2.0 V to 1.5 V) on platinum. Figure 1(a) shows cyclic voltammetry scan performed between -1.5 V to +0.5 V at a scan rate of 20 mV s -1 . The spectrum reveals a large reduction wave on the cathodic scan corresponds to two reduction reactions for Ge 4+ ions: one leads to Ge 2+ ions and the other to Ge [START_REF] Wu | Electrodeposition of germanium from the ionic liquid 1-butyl-1-methylpyrrolidinium dicyanamide[END_REF]. The reduction waves at -0.6 and -1.1 V versus SCE were assigned to the reduction of Ge 4+ to Ge 2+ and Ge 2+ to Ge, respectively [START_REF] Meng | Electrodeposition of 3D ordered macroporous germanium from ionic liquids: a feasible method to make photonic crystals with a high dielectric constant[END_REF]:

Ge 4+ + 2e -→ Ge 2+
(1)

Ge 2+ + 2e -→ Ge (2)
The nucleation and growth of GeO2 crystals are chemical steps and depend on the pH [START_REF] Pourbaix | Atlas of electrochemical equilibria in aqueous solution[END_REF].

The nucleation started when the produced Ge 2+ ions react with OH -ions in the solution to form GeO2:

Ge 2+ + 2OH -→ GeO 2 ↓ +H 2 (3)
The current-time transients of the deposits produced at the -0. Their density increases by increasing the cathodic potential. A closer observation of an individual nano-bundle shows different forms and shapes: (i) a nanorod -like morphology at low deposition potential (-0.8 V), with a particle size of few nanometers (Fig. 1c), (ii) a mixt of nanowires and quartz-like at deposition potential of -1 V (Fig. 1d), and (iii) a pure quartz-like morphology at high deposition potential of -1.2 V (Fig. 1e). Similar GeO2 shapes have been obtained by heating of Ge powders [START_REF] Kim | GeO2 nanostructures fabricated by heating of Ge powders: Pt-catalyzed growth, structure, and photoluminescence[END_REF]. Figure 2(a) shows a defined edge of the formed GeO2 with the rest of the substrate, indicating that the nanostructures are correctly and orderly electrodeposited on the emerged part of the substrate.

In order to understand the effect of current density on the growth mechanism, the deposition time is increased from 20 s to 300 s under the same applied potentials of -0.8 V, -1 V, and -1.2 V. The resulting forms are presented in Figs. 2b, 2c and 2d, respectively.

The nanorods shape obtained at -0.8 V are transformed into a criss-cross network of nanowires with an average diameter of 200 nm and an average length of 3 microns. The mixed structure obtained at -1.0 V become denser with packed wire-like particles surrounding the spherical shapes. The dimensions of the nanowires are smaller than those at -0.8 V with a diameter ranging from 50 nm to 100 nm. The quartz-like shapes obtained at -1.2 V appears to be faceted or hexagonal submicrometer crystallites with typical diameters up to 1µm. The images show that the density and the dimension of the formed nanostructures increase with the deposition time, while the different morphology obtained at each applied potential reflects a growth mechanism differs. In fact, the growth often occurs according to the 1D growth mode at low potential (-0.8V) and to the 3D growth mode at high potential (-1.2V). However, the full process could be confirmed and better understood with further experimental work.

 Chemical and structural Analyses

In order to verify the chemical composition of the formed nanostructures, the samples were investigated and characterized through Fourier transform infrared spectroscopy (FTIR), energy-dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD). Figure 3(a) shows the absorbance obtained by FTIR of bulk Ge sample after immersion in HF 49% solution. The dominant feature of this spectrum is at 2050 cm -1 , which corresponds to the stretching mode of the GeHx bonds [START_REF] Park | Modification of H-terminated Ge surface in hydrochloric acid[END_REF], [START_REF] De Gryse | Determination of oxide and morphology in silicon and germanium using infra-red absorption spectroscopy[END_REF]. GeHx bending modes are also present and centered at 840 cm -1 [START_REF] Rivillon | Hydrogen passivation of germanium (100) surface using wet chemical preparation[END_REF]. Figure 3(b) shows the absorbance after anodization at -1.2 V during 300 s. The large band at 780 cm -1 is attributed to the vibration of Ge-O, assuming an oxidized surface and/or GeO2 precipitates [START_REF] De Gryse | Determination of oxide and morphology in silicon and germanium using infra-red absorption spectroscopy[END_REF], [START_REF] Rivillon | Hydrogen passivation of germanium (100) surface using wet chemical preparation[END_REF]. Vibrational measurements show clear evidence that following the anodization process; the hydrogen-terminated Ge surface is completely converted to contain GeOx bonds. (A°) and angles of (90°, 90°, 120°). No reflection peaks from impurities, such as unreacted Ge or other germanium oxides (tetragonal with a structure of rutile), were observed, within the detection limit of our technique (2-2.5 nm), indicating a high purity of the product [START_REF] Jing | Morphology controls of GeO2 particles precipitated by a facile acid-induced decomposition of germanate ions in aqueous medium[END_REF].

The observed diffraction peaks from (100), ( 101), (110), and (102) planes can be indexed to those of the α-quartz GeO2 (space group P3221) [START_REF] Jing | Morphology and crystal phase evolution of GeO2 in liquid phase deposition process[END_REF]. The intensity of the (101) peak is very strong and its width at half maximum is relatively narrow, indicating a good crystallization state.

 Light emission from electrodeposited GeO2 nanostructures

To investigate the optical properties, GeO2 nanostructures were examined by roomtemperature cathodoluminescence spectroscopy. The morphology and CL characteristics of the electrodeposited samples at -0.8 V and -1.2 V are reported on the spatial mapping (Fig. 6 a-d) which reveal luminescence only from the GeO2 nanostructures. Fig. 6e shows the typical room temperature emission spectra from both samples. Two distinct emission peaks are observed in CL spectra, i.e. at 411 nm and 560 nm. The strong and dominated peak appeared at 560 nm (2.2 eV) can be related with the green emission while the presence of a broad peak at 411 nm (3.0 eV) can be referred as blue emission. Both blue and green light emissions are correlated to radiative recombination of defects in quartz-like GeO2 crystals [START_REF] Armelao | Visible emission from GeO2 nanowires: site-specific insights via X-ray excited optical luminescence[END_REF], [START_REF] Wu | Preparation and photoluminescence properties of crystalline GeO2 nanowires[END_REF], [START_REF] Jawad | Synthesis, structural, and optical properties of electrochemically deposited GeO2 on porous silicon[END_REF]. In fact, oxygen vacancies and germanium-related oxygen vacancies act as luminescence center and contribute to the observed luminescence from GeO2 nanostructures such as single oxygen vacancy (OV) or double oxygen vacancy (DOV) [START_REF] Zacharias | Blue luminescence in films containing Ge and GeO2 nanocrystals: the role of defects[END_REF]- [START_REF] Jiang | GeO2 nanotubes and nanorods synthesized by vapor phase reactions[END_REF]. No violet emission has been observed in the CL spectra from tetragonal GeO2 crystals [START_REF] Kim | GeO2 nanostructures fabricated by heating of Ge powders: Pt-catalyzed growth, structure, and photoluminescence[END_REF]. Because of their characteristic emission properties, these GeO2 nanostructures may have potential application in future integrated optical devices.

Conclusions

In summary, tuning size and morphology of GeO2 nanocrystals are made possible by electrodeposition. The formed nanostructures were characterized in detail in terms of their morphological, structural, and compositional properties which revealed that they are pure, grown in very high density, and highly crystalline. XRD analysis confirms the crystalline nature of the electrodeposited GeO2 having α-quartz type hexagonal structure. The CL measurements showed that the structures have a strong blue and green light emissions assigned to radiative recombination of defects in GeO2 crystals. This study demonstrates that the electrodeposition technique can be used for the synthesis of large quantity GeO2 nanostructures which could be a promising candidate for the fabrication of cost-effective optoelectronics devices.

  8, -1 and -1.2 V for the first 20 s were shown in figure1(b). The shape of the curve can clearly indicate the nucleationgrowth mechanism. At the beginning of the applied potentials, a high cathodic current was observed for a short time of 2 s compared to the deposition of 20 s which indicates the formation of GeO2 on the surface of Ge substrate. The curves i(t) show also a normal dependence with the applied potential. The increase of the current density with the applied potential indicates that the process of the electrodeposition becomes faster. Figure1(c-e) shows planar-view SEM images of the samples obtained at the -0.8, -1 and -1.2 V during 20s. The nanoparticles accumulate mostly on the side of the Ge wafer oriented toward the counter electrode during deposition. For all cathodic deposition potentials, the grains are assembled into a network of rounded agglomeration so-called "nano-bundle aggregates".

Figures

  Figures4a-d and 4e-h show the EDX mapping of the electrodeposited samples at -0.8 V

Fig. 1 :

 1 Fig. 1: (a) Cyclic voltammogram recorded in a GeCl4:H2O2 solution, with the ratio of 1:9 by volume. The potential scan rate is 20 mV s -1 . (b) Current transients for GeO2 nanostructures deposited at different applied potentials. Planar-view SEM images of GeO2 nano-structures electrodeposited at -0.8 V (c), -1 V (d) and -1.2 V (e) on Ge substrate during 20 s.

Fig. 2 :

 2 Fig. 2: Planar-view SEM images shown the emerged substrate limit in the electrolyte (a) of GeO2 nano-structures electrodeposited at -0.8 V (b), -1 V (c) and -1.2 V (d) on Ge substrate during 300 s. The inset shows high-resolution SEM image on the nano-assembled structures.

Figure 3 :

 3 Figure 3: Typical transmission-FTIR spectra of the Ge surface before (a) and after electrodeposition of GeO2 nanostructures at -1.2 V during 300 s (b), which is dominated by a large band correlated to the vibration of Ge-O bonds.

Fig. 4 :Fig. 5 :

 45 Fig. 4: EDX mapping of GeO2 nanowires (a) and quartz structures (e) electrodeposited at 0.8 V and 1.2 V respectively. The top view SEM image and the EDX element maps for (b,c) Ge, (f,g) O elements, (d,h) a mixture of Ge and O and (i) EDX spectra acquired in SEM at the top view sample.

Fig. 6 :

 6 Fig. 6: SEM and CL micrographs taken from the top surface of GeO2 nanowires (a,b) and quartz structures (c, d) electrodeposited at 0.8 V and 1.2 V respectively. (e) The corresponding CL spectra.