Amorphous GeO2 Research Articles

Amorphous mesoporous GeO x anode for Na-ion batteries with high capacity and long lifespan.

It is recently demonstrated that amorphous Ge anode shows higher reversible Na-ion storage capacity (590 mA h g−1) than crystallized Ge anode (369 mA h g−1). Here, amorphous GeOx anode is prepared by a simple wet-chemistry reduction route at room temperature. The obtained amorphous GeOx shows a porous hierarchical architecture, accompanied with a Brunauer–Emmett–Teller surface area of 159 m2 g−1 and an average pore diameter of 14 nm. This unique structure enables the GeOx anode to enhance the Na-ion/electron diffusion rate, and buffer the volume change. As anode for Na-ion battery, high reversible capacity over 400 mA h g−1, fine rate capability with a capacity of 200 mA h g−1 maintained at 3.0 A g−1 and long-term cycling stability with 270 mA h g−1 even over 1000 cycles at 1.0 A g−1 are obtained.

Germanium oxidation occurs by diffusion of oxygen network interstitials

Density functional modeling is used to show that germanium oxidation occurs by the diffusion of network oxygens across the film as peroxyl bridges, not by molecular O2 interstitials (O2*). The smaller O bond angle of GeO2 leads to lower order rings in the amorphous GeO2 network than in SiO2. This leads to narrower interstitial diffusion channels, and less dilation of the interstitial volume around the transition state. This raises the migration barrier of O2* in GeO2, so that the overall diffusion energy of O2* in GeO2 is now higher than that of a network O interstitial. The low formation energy of the O vacancy in GeO2 leads to GeO2 being O-poor very near the Ge/GeO2 interface, but the lower overall diffusion energy of the O network interstitial than the vacancy leads to the network interstitial dominating diffusion.

GeO2 Thin Film Deposition on Graphene Oxide by the Hydrogen Peroxide Route: Evaluation for Lithium-Ion Battery Anode.

A peroxogermanate thin film was deposited in high yield at room temperature on graphene oxide (GO) from peroxogermanate sols. The deposition of the peroxo-precursor onto GO and the transformations to amorphous GeO2, crystalline tetragonal GeO2, and then to cubic elemental germanium were followed by electron microscopy, XRD, and XPS. All of these transformations are influenced by the GO support. The initial deposition is explained in view of the sol composition and the presence of GO, and the different thermal transformations are explained by reactions with the graphene support acting as a reducing agent. As a test case, the evaluation of the different materials as lithium ion battery anodes was carried out revealing that the best performance is obtained by amorphous germanium oxide@GO with >1000 mAh g-1 at 250 mA g-1 (between 0 and 2.5 V vs Li/Li+ cathode), despite the fact that the material contained only 51 wt % germanium. This is the first demonstration of the peroxide route to produce peroxogermanate thin films and thereby supported germanium and germanium oxide coatings. The advantages of the process over alternative methodologies are discussed.

Yttrium passivation of defects in GeO2 and GeO2/Ge interfaces

Alloying amorphous GeO2 with Y2O3 has been found experimentally to improve its chemical stability and electrical reliability as a gate dielectric in Ge-based field effect transistors. The mechanism is explained here based on density functional calculations. The GeO2 reliability problem is correlated with oxygen deficiency defects, which generate gap states near the band-edges of the underlying Ge. These can be passivated through Y doping. This shifts the defect gap state out of the gap up into the GeO2 conduction band, thus effectively passivating gap states in the GeO2 layer.

Ether-based solvents significantly improved electrochemical performance for Na-ion batteries with amorphous GeOx anodes.

We have previously synthesized highly amorphous GeOx powder by a soft chemical method and demonstrated its performance in electrochemical Na insertion-extraction reactions. However, electrolyte decomposition caused large irreversible capacity decay in the first charge-discharge, leading to poor cycling ability. Electrolyte decomposition was accelerated by the large volume change of amorphous GeOx during charge-discharge. Herein, we report the use of ether-based solvents in Na-ion batteries with amorphous GeOx anodes. XPS measurements were used to evaluate the surface state of the electrode after cycling. The surface layer of the electrolyte decomposition products was thin and mainly composed of Na alkoxide. Thus, the electrochemical performance of this system was greatly improved owing to the significant suppression of electrolyte decomposition. Consequently, electrolytes using ether-based solvents are strong candidates for practical use in Na-ion batteries.

Effect of metal oxide additions to quality on Ge/GeO2 interfaces

Alloying amorphous GeO2 with Y2O3 or related group IIIA oxides is known experimentally to improve its properties as a gate dielectric in field effect transistors. The mechanism of this is studied here by density functional calculations. The metal site coordination is found to be 6–7, by increasing the oxygen coordination to 3 or higher. The alloying is found to increase the bulk modulus. Alloying also increases the diffusion energy of the oxygen vacancies in GeO2 next to the metal and also increases the vacancy formation energy of oxygens that are second neighbors of the metal sites. In this way, a relatively small metal concentration can reduce the O vacancy diffusion rate and thereby the GeO evolution rate. Oxygen vacancies at the Ge/GeO2 interface next to a metal site are found to divide into two types, those which rebond across the vacancy (La, Hf) and those without rebonding (Y, Sc, Al), the latter being preferable as they do not give rise to interfacial gap states.

Electrospinning of GeO2–C fibers and electrochemical application in lithium-ion batteries

Abstract GeO2–C fibers were successfully synthesized using electrospinning homogeneous sol and subsequent calcination in an inert atmosphere. The spinnable sol was prepared by adding polyacrylonitrile (PAN) and polyvinylpyrrolidone (PVP) in a weight ratio of 1:1 into a mixture with white precipitate produced by dropping GeCl4 into DMF. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed to characterize the as-obtained fibers, and electrochemical tests were conducted to measure electrochemical performance of the electrode. The electrospun fibers have uniform diameters of ∼300 nm. After being calcined at 600 °C for 2 h in Ar, they transform to amorphous GeO2–C fibers with the same morphologies. The GeO2–C fibers exhibit excellent cycling stability with a high reversible capacity of 838.93 mA h g−1 after 100 cycles at a current density of 50 mA g−1, indicating the composite fibers could be promising anode candidates for lithium-ion batteries.

Size-controllable synthesis of amorphous GeOx hollow spheres and their lithium-storage electrochemical properties

Size controllable synthesis of GeOx hollow spheres was achieved using a solvothermal reaction. The GeOx hollow spheres exhibit excellent lithium storage properties.

Electrochemical performance of highly amorphous GeOx powders synthesized in different alcohols for use in Na- and Li-ion batteries

The large particle size of amorphous powders deteriorated the cycle performance of a Na-ion cell more than that of a Li-ion cell, due to large decomposition of the electrolyte.

Electronic structure and optical properties of amorphous GeO2 in comparison to amorphous SiO2

Amorphous germania (a-GeO2) is an excellent glass former of great industrial and scientific importance. However, in comparison with a-SiO2, its structure and fundamental properties were less well studied. Using a large near-perfect continuous random network (CRN) model with 1296 atoms and no over- or under-coordinated atoms, we have investigated the structural, electronic, and optical properties of a-GeO2 glass. Our results show that the bond length and bond angle distributions in a-GeO2 are larger than in a-SiO2. The gross features of the electronic density of states in a-GeO2 are similar to a-SiO2, but the GeO bonds are weaker than SiO bonds as reflected in the lower calculated total bond order density. The average tetrahedral angle (θ) and bridging angle (φ) are smaller in a-GeO2 than in a-SiO2. The calculated optical absorption spectrum shows two distinctive peaks in excellent agreement with experiment. The calculated refractive index of a-GeO2 (n=1.69) is also in close agreement with the measured value. In contrast to a-SiO2, there is no clear evidence of excitonic peak in a-GeO2. These a-GeO2 and a-SiO2 models could be used as a prototype for other investigations of these glasses or their mixtures containing defects, substitutional impurities and in the form of vitreous nano-particles.

Te implantation in Ge(001) for n-type doping applications

5×1015 Te+ ionscm−2 were implanted in an Ge(001) substrate using an industrial implanter with a Te+ beam energy of 180keV. In addition to usual implantation-mediated defects observed in Ge with usual dopants, Te implantations lead to the formation of amorphous surface GeO clusters exhibiting micrometer scale sizes, as well as deep extended defects. Implantation defects promote the formation of two distributions of dislocation loops and clusters located at two different depths in the Ge substrate during annealing. No interactions between Te atoms and dislocation loops were observed. However, the formation of non-equilibrium Te–Ge clusters, probably mediated by Ge self-interstitials, was found to prevent the Te solubility to exceed ~5×1019cm−3 in Ge. The regular implantation method is shown to be ineffective for the production of high level n-type Ge doping using Te, due to the important Ge damage caused by Te implantation.

Amorphous GeOx-Coated Reduced Graphene Oxide Balls with Sandwich Structure for Long-Life Lithium-Ion Batteries

Amorphous GeOx-coated reduced graphene oxide (rGO) balls with sandwich structure are prepared via a spray-pyrolysis process using polystyrene (PS) nanobeads as sacrificial templates. This sandwich structure is formed by uniformly coating the exterior and interior of few-layer rGO with amorphous GeOx layers. X-ray photoelectron spectroscopy analysis reveals a Ge:O stoichiometry ratio of 1:1.7. The amorphous GeOx-coated rGO balls with sandwich structure have low charge-transfer resistance and fast Li(+)-ion diffusion rate. For example, at a current density of 2 A g(-1), the GeOx-coated rGO balls with sandwich and filled structures and the commercial GeO2 powders exhibit initial charge capacities of 795, 651, and 634 mA h g(-1), respectively; the corresponding 700th-cycle charge capacities are 758, 579, and 361 mA h g(-1). In addition, at a current density of 5 A g(-1), the rGO balls with sandwich structure have a 1600th-cycle reversible charge capacity of 629 mA h g(-1) and a corresponding capacity retention of 90.7%, as measured from the maximum reversible capacity at the 100th cycle.

Temperature dependent photoluminescence and electroluminescence characteristics of core-shell Ge–GeO2 nanowires

Ge nanowires of several micrometers long and an average diameter of ∼70 nm have been grown by the vapour–liquid–solid technique. Different structural characteristics indicate the presence of a Ge core along with a thin (~10 nm) amorphous GeO2 shell layer. The signature of direct as well as indirect band gap emission is observed from the temperature dependent photoluminescence measurements of Ge nanowires. Room temperature near infrared electroluminescence has been achieved from the metal–insulator–semiconductor structure consisting of Ge nanowires. The phonon mediated indirect transition dominates the electroluminescence characteristics above 100 K, whereas a no phonon direct transition at low temperatures makes the device attractive for optical emitters.

Paramagnetic centers in amorphous GeO2

In this work we investigate paramagnetic centers in amorphous germania (a-GeO2). We provide an analysis of the Fermi contacts and of the g-tensors calculated by first-principles for the E′-Ge, Ge–Ge and Ge forward-oriented (GeFO) configurations. The electron paramagnetic resonance (EPR) parameters calculated for the Ge–Ge configurations, where the unpaired electron is shared between two Ge nearest neighbors, do not support such a model of the E′-Ge∗, nor the latter can be simply regarded as an E′-Ge center with a distorted geometry. A fair agreement with the experimental EPR parameters supports the assignment of the Ge(2) center in a-GeO2 to GeFO configurations. Furthermore we show that electronic levels induced in the bandgap by GeFO configurations are considerably deeper (∼1eV) with respect to those related to the E′-Ge centers.

CO2 Laser irradiation of GeO2 planar waveguide fabricated by rf-sputtering

We present a fabrication protocol combining radio frequency sputtering technique and CO2 laser annealing for GeO2 based planar optical waveguides. The effects of pulsed CO2 laser irradiation on the optical and structural properties of pure GeO2 planar waveguide are evaluated by different techniques as m-line and micro-Raman spectroscopy and AFM measurements. Amorphous GeO2 planar waveguide was fabricated by Radio Frequency magnetron sputtering system on v-SiO2 substrate. An increase of the refractive index of approximately 0.04 at 1.5 μm and a decreasing of the attenuation coefficient from 0.9 to 0.5 dB/cm at 1.5 μm have been observed after pulsed CO2 laser annealing. Raman spectroscopy and AFM results showed that after an adapted pulsed CO2 laser annealing, the resulting materials showed a crystalline environment in which the phase of the crystalline GeO2 varies with varying irradiation time.

NIR Luminescence from Sol-Gel Er3+Doped SiO2:GeO2Transparent Gels, Nanostructured Powders and Thin Films for Photonic Applications

Samples of (1 – x)SiO2:xGeO2 compositions, containing x = 10 and 20, and doped with Er3+, were prepared by a simple sol-gel route. Homogeneous and transparent gels were synthesized and xerogels were obtained by gel annealing from 700 oC to 1100 oC. Films exhibiting high transmittance in the visible and near infrared were deposited by spin-coating technique using the colloidal precursors. The materials were characterized structurally and microstructurally by some techniques. Spherical completely amorphous and tetragonal GeO2 nanocrystals from 3.7 nm to 25.0 nm in diameter embedded in silica-rich amorphous phase materials were obtained. The optical properties were studied by transmission spectra from ultraviolet to near infrared region, photoluminescence measurements in the infrared region, and the average lifetime of the metastable state 4I13/2 of Er3+ ions were determined. The 1 mol% Er3+ doped host with x = 20 have an infrared emission 4.9 times higher than the compound containing x = 10, and the Er3+4I13/2 level present lifetime between 15.0 ms and 7.5 ms. The optical band gap and refractive index values were determined, as well as the Sellmeier parameters. Finally, rib channel waveguides using femtosecond laser etching technique on silica substrates were obtained with width of 15 mm. The 1 mol% Er3+ doped 80SiO2:20GeO2 compounds can be applicable in integrated optical systems.

Conducting additive-free amorphous GeO2/C composite as a high capacity and long-term stability anode for lithium ion batteries.

In this study, a novel method has been proposed for synthesizing amorphous GeO2/C composites. The amorphous GeO2/C composite without carbon black as an electrode for Li-ion batteries exhibited a high specific capacity of 914 mA h g(-1) at the rate of C/2 and enhanced rate capability. The amorphous GeO2/C electrode exhibited excellent electrochemical stability with a 95.3% charge capacity retention after 400 charge-discharge cycles, even at a high current charge-discharge of C/2. Furthermore, a full cell employing the GeO2/C anode and the LiCoO2 cathode displayed outstanding cycling performance. The superior performance of the GeO2/C electrode enables the amorphous GeO2/C to be a potential anode material for secondary Li-ion batteries.

Pressure-induced structural transition in amorphous GeO2: a molecular dynamics simulation

We studied the structural and dynamical properties of amorphous germanium dioxide (GeO2) from low to high pressure by means of the classical molecular dynamics technique. The simulations were done in the micro-canonical ensemble, with systems at densities ranged from 3.16 to 6.79 g/cm3, using a pairwise potential. The network topology of the systems is analyzed at atomic level through partial pair correlations, coordination number and angular distributions. The dynamic properties were characterized by means of the vibrational density of states. According the density increases, a structural transformation from a short-range order, defined by a building block composed by a basic (GeO4) tetrahedron, to a basic (GeO6) octahedron is observed. The vibrational density of states also presents important changes when the density increases, with a low frequency band lessened, and a high density band wider and flatter.

Theoretical investigations of the spin Hamiltonian parameters and local tetragonal distortions for Cu2+ in crystalline and amorphous TeO2 and GeO2

The spin Hamiltonian parameters (i.e., anisotropic g factors and hyperfine structure constants) and local tetragonal distortions for Cu2+ in crystalline and amorphous TeO2 and GeO2 are theoretically investigated using the high-order perturbation formulas of these parameters for a tetragonally elongated octahedral 3d9 cluster. The impurity Cu2+ occupying the octahedral sites are found to experience the relative tetragonal elongation ratios of about 11.4% and 9.5% for crystalline TeO2 and GeO2 and 10.8% and 6.6% for amorphous TeO2 and GeO2, respectively, along the C4 axis due to the Jahn–Teller effect. This reveals the larger tetragonal elongation distortions for the Cu2+ centres in crystalline than amorphous systems (especially TeO2). The theoretical spin Hamiltonian parameters show good agreement with the experimental data. The results are discussed.

Storage of Lithium in Hydrothermally Synthesized GeO2 Nanoparticles

Amorphous GeO2 nanoparticles were prepared via a surfactant-assisted hydrothermal process. The effect of the reaction temperature and the surfactant concentration on the morphology of GeO2 particles were investigated. Particles of less than 300 nm were obtained when using 1,2-diaminopropane surfactant in a synthesis carried out at 180(◦)C. The synthesized germanium oxide nanoparticles were evaluated for their utility as the active anode material in Li-ion batteries. The electrode prepared with this material exhibited a stable capacity ∼600 mAh g(-1) at 0.2 C rate for up to 150 cycles in a conventional electrolyte containing ethylene carbonate (EC). The cyclability of the GeO2 nanoparticle electrode was further improved by using a fluorinated ethylene carbonate (FEC) based electrolyte, which showed capacities greater than 600 mAh g(-1) and retained more than 96% of their capacity after 500 cycles at 0.2 C rate. The effect of different electrolyte systems was studied by using electrochemical impedance spectroscopy and electron microscopy.