Tetragonal phase germanium nanocrystals in lithium ion batteries

Bimetallic Cobalt-Nickel Selenide Nanocubes Embedded in a Nitrogen-Doped Carbon Matrix as an Excellent Li-Ion Battery Anode

Lithium-ion batteries (LIBs) have been widely used for portable electronics and electric vehicles; however, the low capacity in the graphite anode limits the improvement of energy density. Transition-metal selenides are promising anode material candidates due to their high theoretical capacity and controllable structure. In this study, we successfully synthesize a bimetallic transition-metal selenide nanocube composite, which is well embedded in a nitrogen-doped carbon matrix (denoted as CoNiSe₂/NC). This material shows a high capacity and excellent cycling for Li-ion storage. Specifically, the reversible capacity approaches ∼1245 mA h g^-1 at 0.1 A g^-1. When cycled at 1 A g^-1, the capacity still remains at 642.9 mA h g^-1 even after 1000 cycles. In-operando XRD tests have been carried out to investigate the lithium storage mechanism. We discover that the outstanding performance is due to the unique CoNiSe₂/NC nanocomposite characteristics, such as the synergistic effect of bimetallic selenide on lithium storage, the small particle size, and the stable and conductive carbon structure. Therefore, this morphology structure not only reduces the volume change of metal selenides but also produces more lithium storage active sites and shortens lithium diffusion paths, which results in high capacity, good rate, and long cycling.

Growth and analysis of the tetragonal (ST12) germanium nanowires

New semiconducting materials, such as state-of-the-art alloys, engineered composites and allotropes of well-established materials can demonstrate unique physical properties and generate wide possibilities for a vast range of applications. Here we demonstrate, for the first time, the fabrication of a metastable allotrope of Ge, tetragonal germanium (ST12-Ge), in nanowire form. Nanowires were grown in a solvothermal-like single-pot method using supercritical toluene as a solvent, at moderate temperatures (290-330 °C) and a pressure of ∼48 bar. One-dimensional (1D) nanostructures of ST12-Ge were achieved via a self-seeded vapour-liquid-solid (VLS)-like paradigm, with the aid of an in situ formed amorphous carbonaceous layer. The ST12 phase of Ge nanowires is governed by the formation of this carbonaceous structure on the surface of the nanowires and the creation of Ge-C bonds. The crystalline phase and structure of the ST12-Ge nanowires were confirmed by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and Raman spectroscopy. The nanowires produced displayed a high aspect ratio, with a very narrow mean diameter of 9.0 ± 1.4 nm, and lengths beyond 4 μm. The ST12-Ge nanowire allotrope was found to have a profound effect on the intensity of the light emission and the directness of the bandgap, as confirmed by a temperature-dependent photoluminescence study.

Functionalized germanane/SWCNT hybrid films as flexible anodes for lithium-ion batteries

Germanium, with a high theoretical capacity based on alloyed lithium and germanium (1384 mA h g-1 Li15Ge4), has stimulated tremendous research as a promising candidate anode material for lithium-ion batteries (LIBs). However, due to the alloying reaction of Li/Ge, the problems of inferior cycle life and massive volume expansion of germanium are equally obvious. Among all Ge-based materials, the unique layered 2D germanane (GeH and GeCH3) with a graphene-like structure, obtained by a chemical etching process from the Zintl phase CaGe2, could enable storage of large quantities of lithium between their interlayers. Besides, the layered structure has the merit of buffering the volume expansion due to the tunable interlayer spacing. In this work, the beyond theoretical capacities of 1637 mA h g-1 for GeH and 2048 mA h g-1 for GeCH3 were achieved in the initial lithiation reaction. Unfortunately, the dreadful capacity fading and electrode fracture happened during the subsequent electrochemical process. A solution, i.e. introducing single-wall carbon nanotubes (SWCNTs) into the structure of the electrodes, was found and further confirmed to improve their electrochemical performance. More noteworthy is the GeH/SWCNT flexible electrode, which exhibits a capacity of 1032.0 mA h g-1 at a high current density of 2000 mA g-1 and a remaining capacity of 653.6 mA h g-1 after 100 cycles at 500 mA g-1. After 100 cycles, the hybrid germanane/SWCNT electrodes maintained good integrity without visible fractures. These results indicate that introducing SWCNTs into germanane effectively improves the electrochemical performance and maintains the integrity of the electrodes for LIBs.

Supercapacitor electrode materials: addressing challenges in mechanism and charge storage

In recent years, rapid technological advances have required the development of energy-related devices. In this regard, Supercapacitors (SCs) have been reported to be one of the most potential candidates to meet the demands of human’s sustainable development owing to their unique properties such as outstanding cycling life, safe operation, low processing cost, and high power density compared to the batteries. This review describes the concise aspects of SCs including charge-storage mechanisms and scientific principles design of SCs as well as energy-related performance. In addition, the most important performance parameters of SCs, such as the operating potential window, electrolyte, and full cell voltage, are reviewed. Researches on electrode materials are crucial to SCs because they play a pivotal role in the performance of SCs. This review outlines recent research progress of carbon-based materials, transition metal oxides, sulfides, hydroxides, MXenes, and metal nitrides. Finally, we give a brief outline of SCs’ strategic direction for future growth.

Microwave-Assisted Synthesis of Ge/GeO2-Reduced Graphene Oxide Nanocomposite with Enhanced Discharge Capacity for Lithium-Ion Batteries

Germanium/germanium oxide nanoparticles with theoretically high discharge capacities of 1624 and 2152 mAh/g have attracted significant research interest for their potential application as anode materials in Li-ion batteries. However, these materials exhibit poor long-term performance due to the large volume change of 370% during charge/discharge cycles. In the present study, to overcome this shortcoming, a Ge/GeO2/graphene composite material was synthesized. Ge/GeO2 nanoparticles were trapped between matrices of graphene nanosheets to offset the volume expansion effect. Transmission electron microscopy images revealed that the Ge/GeO2 nanoparticles were distributed on the graphene nanosheets. Discharge/charge experiments were performed to evaluate the Li storage properties of the samples. The discharge capacity of the bare Ge/GeO2 nanoparticles in the first discharge cycle was considerably large; however, the value decreased rapidly with successive cycles. Conversely, the present Ge/GeO2/graphene composite exhibited superior cycling stability.

High specific energy supercapacitor electrode prepared from MnS/Ni3S2 composite grown on nickel foam

Herein, MnS was prepared in situ with Ni3S2 directly on nickel foam to obtain a novel binder-free highly conductive electrode with a superb echinocactus-like morphology.

GeTe-TiC-C Composite Anodes for Li-Ion Storage

Germanium boasts a high charge capacity, but it has detrimental effects on battery cycling life, owing to the significant volume expansion that it incurs after repeated recharging. Therefore, the fabrication of Ge composites including other elements is essential to overcome this hurdle. Herein, highly conductive Te is employed to prepare an alloy of germanium telluride (GeTe) with the addition of a highly conductive matrix comprising titanium carbide (TiC) and carbon (C) via high-energy ball milling (HEBM). The final alloy composite, GeTe-TiC-C, is used as a potential anode for lithium-ion cells. The GeTe-TiC-C composites having different combinations of TiC are characterized by electron microscopies and X-ray powder diffraction for structural and morphological analyses, which indicate that GeTe and TiC are evenly spread out in the carbon matrix. The GeTe electrode exhibits an unstable cycling life; however, the addition of higher amounts of TiC in GeTe offers much better electrochemical performance. Specifically, the GeTe-TiC (20%)-C and GeTe-TiC (30%)-C electrodes exhibited excellent reversible cyclability equivalent to 847 and 614 mAh g⁻¹ after 400 cycles, respectively. Moreover, at 10 A g⁻¹, stable capacity retentions of 78% for GeTe-TiC (20%)-C and 82% for GeTe-TiC (30%)-C were demonstrated. This proves that the developed GeTe-TiC-C anodes are promising for potential applications as anode candidates for high-performance lithium-ion batteries.

Fast and Stable Batteries with High Capacity Enabled by Germanium-Phosphorus Binary Nanoparticles Embedded in a Porous Carbon Matrix via Metallothermic Reduction

Lithium-alloyable materials such as Ge and P have attracted considerable attention as promising anode materials for lithium-ion batteries (LIBs) owing to their high theoretical capacity. However, these materials inevitably undergo capacity attenuation caused by large volume expansion in repeated electrochemical processes. Herein, we propose a facile strategy to synthesize germanium-phosphorus binary nanoparticles embedded in porous carbon (GPBN/C) via metallothermic reduction. As an LIB anode, the GPBN/C electrode exhibits outstanding rate performance (368 mAh g^-1 at 40 A g^-1) and remarkable long-term cycling ability (541 mAh g^-1 at 1.0 A g^-1 after 1000 cycles). Besides, the GPBN/C composite electrode presents an outstanding cycling performance at wide temperature ranges, showing reversible capacities of 1030 and 696 mAh g^-1 at 60 and 0 °C, respectively. Attributed to the formation of highly dispersed Ge-P nanoparticles in a porous carbon matrix, the GPBN/C electrode shows exceptional electrochemical performance. Importantly, our strategy provides an effective way to explore alloy-type electrodes to develop fast and stable high-capacity batteries.

Dehydrocoupling - an alternative approach to functionalizing germanium nanoparticle surfaces

Surface functionalization is an essential aspect of nanoparticle design and preparation; it can impart stability, processability, functionality, as well as tailor optoelectronic properties that facilitate future applications. Herein we report a new approach toward modifying germanium nanoparticle (GeNP) surfaces and for the first time tether alkyl chains to the NP surfaces through Si-Ge bonds. This was achieved via heteronuclear dehydrocoupling reactions involving alkylsilanes and Ge-H moieties on the NP surfaces. The resulting solution processable RR'2Si-GeNPs (R = octadecyl or PDMS; R' = H or CH3) were characterized using FTIR, Raman, 1H-NMR, XRD, TEM, HAADF, and EELS and were found to retain the crystallinity of the parent GeNP platform.

Level the Conversion/Alloying Voltage Gap by Grafting the Endogenetic Sb2Te3 Building Block into Layered GeTe to Build Ge2Sb2Te5 for Li-Ion Batteries

Many research efforts for advanced Li-ion batteries have been made to design new material with large capacity and long cycle life, but little attention has been paid to regulate the voltage platform until now. Although quite attractive for the binary Ge-based chalcogenides, challenge is that a large potential gap as well as incongruous reaction kinetics is typically found between their conversion step (>1.6 V) and alloying region (<0.4 V). Herein, we propose an endogenetic structural design by grafting Sb₂Te₃ building block into layered GeTe to establish a ternary Ge₂Sb₂Te₅ compound, which can effectively level such a big potential gap. Turning from semiconductive GeTe into metallic conductive Ge₂Sb₂Te₅, the reaction kinetics can be enhanced. The Li_xTe formation step in Ge₂Sb₂Te₅ is found declined to 1.30 V, and the enlistment of Sb (∼0.78 V) bridges the conversion and alloying plateau; thus, the incongruous reaction kinetics and large potential gap between the conversion-alloying step can be alleviated. Furthermore, there is a spatially confined and synergistic effect among Te, Sb, and Ge components, conducting the Li_xTe and Li_xGe processes in a more harmonious and gentle way. Therefore, Ge₂Sb₂Te₅ exhibites much enhanced cyclability and rate performance, with 546 mAh/g remained at 2000 mA/g. This unique design strategy can be leveraged to manipulate the voltage profile of other compounds.

Three-Dimensional Double-Walled Ultrathin Graphite Tube Conductive Scaffold with Encapsulated Germanium Nanoparticles as a High-Areal-Capacity and Cycle-Stable Anode for Lithium-Ion Batteries

The demand for lithium-ion batteries with both high power and high-energy density has attracted widespread attention as energy-storage devices for the increasing demand of consumer electronics, electric vehicles, and grid-scale storage. However, the fabrication of an advanced electrode architecture with high areal capacity, excellent cycling stability, and superior rate performance remains a long-term challenge in the development of advanced electrochemical energy-storage devices. Herein, we design an effective and general strategy to spontaneously encapsulate Ge nanoparticles into a three-dimensional double hydrophilic N-doped ultrathin graphite/void/hydrophobic ultrathin graphite tube network (Ge@3D-DHGT) with control over the position for large specific capacity (1338 mA h g^-1), high rate performance (752 mA h g^-1 at 40 C), and superior cycling stability (up to 1000 cycles). Toward the practical application, the as-prepared Ge@3D-DHGT electrode showed a large areal capacity (10 mA h cm^-2 under 8 mA cm^-2), which provides a highly promising anode with both high capacity and high rate performance. Importantly, this work provides an approach to fabricate high-areal-capacity anodes with long cycling stability and rapid charge-discharge properties with practical applications in advanced rechargeable batteries.

Recent progress in 2D group IV-IV monochalcogenides: synthesis, properties and applications

Coordination-related, 2D structural phase transitions are a fascinating facet of 2D materials with structural degeneracy. Phosphorene and its new phases, exhibiting unique electronic properties, have received considerable attention. The 2D group IV-IV monochalcogenides (i.e. GeS, GeSe, SnS and SnSe) like black phosphorous possess puckered layered orthorhombic structure. The 2D group IV-IV monochalcogenides with advantages of earth-abundance, less toxicity, environmental compatibility and chemical stability, can be widely used in optoelectronics, piezoelectrics, photodetectors, sensors, Li-batteries and thermoelectrics. In this review, we summarized recent research progress in theory and experiment, which studies the fundamental properties, applications and fabrication of 2D group IV-IV monochalcogenides and their new phases, and brings new perspectives and challenges for the future of this emerging field.

Synthesis of germanium nanocrystals from solid-state disproportionation of a chloride-derived germania glass

Germanium nanocrystals (Ge NCs) have potential to be used in several optoelectronic applications such as photodetectors and light-emitting diodes. Here, we report a solid-state route to synthesizing Ge NCs through thermal disproportionation of a germania (GeOX) glass, which was synthesized by hydrolyzing a GeCl2·dioxane complex. The GeOX glass synthesized in this manner was found to have residual Cl content. The process of nanocrystal nucleation and growth was monitored using powder X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy and Raman spectroscopy. Compared to existing solid-state routes for synthesizing colloidal Ge NCs, this approach requires fewer steps and is amenable to scaling to large-scale reactions.

Molecular-Level CuS@S Hybrid Nanosheets Constructed by Mineral Chemistry for Energy Storage Systems

The transition-metal sulfide, CuS, is deemed a promising material for energy storage, mainly derived from its good chemisorption and conductivity, although serious capacity fading limits its advancement within reversible lithium storage. Learning from the gold extraction method utilizing the lime-sulfur-synthetic-solution, a CuS@S hybrid utilizing CaS _x as both sulfur resource and reductant-oxidant is prepared, which is an efficient approach to apply the metallurgy for the preparation of electrode materials. Regulating the amount of CuCl₂, the CuS@S is induced to reach a molecular-level hybrid. When utilized as an anode within a lithium-ion battery, it presents the specific capacity of 514.4 mA h g^-1 at 0.1 A g^-1 over 200 cycles. Supported by the analyses of pseudo-capacitive behaviors, it is confirmed that the CuS matrix with the suitable content of auxiliary sulfur could improve the durability of the CuS-based anode. Expanding the wider application within lithium-sulfur batteries, the synchronous growth of CuS@S exhibits stronger chemisorption with polysulfides than the mechanical mixture of CuS and S. A suite of in situ electrochemical impedance spectroscopy studies further investigates the stable resistances of the CuS@S within the charge/discharge process, corresponding to the reversible structure evolution. This systematic work may provide a practical fabricating route of metal sulfides for scalable energy storage applications.

Yolk-Shell Germanium@Polypyrrole Architecture with Precision Expansion Void Control for Lithium Ion Batteries

The key properties of yolk-shell architecture in improving electrochemical performance lies in its uniformity and the appropriate void space, which can expand/contract freely upon lithium alloying and leaching without damaging the outer shell, while being achievable with minimal sacrifice of volumetric energy density. Therefore, we developed a highly controllable strategy to fabricate a uniform porous germanium@polypyrrole (PGe@PPy) yolk-shell architecture with conformal Al₂O₃ sacrificial layer by atomic layer deposition (ALD) process. The PGe@PPy yolk-shell anode fabricated with 300 ALD cycles delivers excellent electrochemical performance: high reversible capacity (1,220 mA hr g⁻¹), long cycle performance (>95% capacity retention after 1,000 cycles), and excellent rate capability (>750 mA hr g⁻¹ at 32 A g⁻¹). Electrodes with high areal capacity and current density were also successfully fabricated, opening a new pathway to develop high-capacity electrode materials with large volume expansion.

•

Porous germanium@polypyrrole (PGe@PPy) yolk-shell architecture was developed

•

Precision expansion and void control make PGe@PPy stable during lithiation/delithiation

•

PGe@PPy electrode shows high rate and areal capacity, cycling stability, and current density

•

The full cell shows the stable capacity retention with high energy density

Photoelectric Detectors Based on Inorganic p-Type Semiconductor Materials

Photoelectric detectors are the central part of modern photodetection systems with numerous commercial and scientific applications. p-Type semiconductor materials play important roles in optoelectronic devices. Photodetectors based on p-type semiconductor materials have attracted a great deal of attention in recent years because of their unique properties. Here, a comprehensive summary of the recent progress mainly on photodetectors based on inorganic p-type semiconductor materials is presented. Various structures, including photoconductors, phototransistors, homojunctions, heterojunctions, p-i-n junctions, and metal-semiconductor junctions of photodetectors based on inorganic p-type semiconductor materials, are discussed and summarized. Perspectives and an outlook, highlighting the promising future directions of this research field, are also given.

Rodlike Sb2Se3 Wrapped with Carbon: The Exploring of Electrochemical Properties in Sodium-Ion Batteries

One-dimensional Sb₂Se₃/C rods are prepared through self-assembly by inducing anisotropy, and their corresponding sodium storage behaviors are evaluated, presenting excellent electrochemical performances with superior cycling stability and rate capability. Sb₂Se₃ delivers a high initial charge capacity of 657.6 mA h g^-1 at a current density of 0.2 A g^-1 between 2.5 and 0.01 V. After 100 cycles, the reversible capacity of Sb₂Se₃/C is still retained at 485.2 mA h g^-1. Even at a high rate current density of 2.0 A g^-1, the charge capacity is still retained at 311.5 mA h g^-1. Through the analysis of cyclic voltammetry and in situ electrochemical impedance spectroscopy, the in-depth understanding of high rate performances is explored effectively. Briefly, the sodium storage performance of Sb₂Se₃/C is observably enhanced, benefiting from the 1D structure and the introduction of a carbon layer with robust structure stability and conductivity.

Understanding of multi-level resistive switching mechanism in GeOx through redox reaction in H2O2/sarcosine prostate cancer biomarker detection

Formation-free multi-level resistive switching characteristics by using 10 nm-thick polycrystalline GeO_x film in a simple W/GeO_x/W structure and understanding of switching mechanism through redox reaction in H₂O₂/sarcosine sensing (or changing Ge°/Ge⁴⁺ oxidation states under external bias) have been reported for the first time. Oxidation states of Ge⁰/Ge⁴⁺ are confirmed by both XPS and H₂O₂ sensing of GeO_x membrane in electrolyte-insulator-semiconductor structure. Highly repeatable 1000 dc cycles and stable program/erase (P/E) endurance of >10⁶ cycles at a small pulse width of 100 ns are achieved at a low operation current of 0.1 µA. The thickness of GeO_x layer is found to be increased to 12.5 nm with the reduction of polycrystalline grain size of <7 nm after P/E of 10⁶ cycles, which is observed by high-resolution TEM. The switching mechanism is explored through redox reaction in GeO_x membrane by sensing 1 nM H₂O₂, which is owing to the change of oxidation states from Ge⁰ to Ge⁴⁺ because of the enhanced O^2- ions migration in memory device under external bias. In addition, sarcosine as a prostate cancer biomarker with low concentration of 50 pM to 10 µM is also detected.

Ionic liquid electrodeposition of strain-released Germanium nanowires as stable anodes for lithium ion batteries

With the growing demand for portable and wearable electronic devices, it is imperative to develop high performance Li-ion batteries with long life times. Germanium-based materials have recently demonstrated excellent lithium-ion storage ability and are being considered as the most promising candidates for the anodes of lithium-ion batteries. Nevertheless, the practical implementation of Ge-based materials to Li-ion batteries is greatly hampered by the poor cycling ability that resulted from the huge volume variation during lithiation/delithiation processes. Herein, we develop a simple and efficient method for the preparation of Ge nanowires without catalyst nanoparticles and templates, using ionic liquid electrodeposition with subsequent annealing treatment. The Ge nanowire anode shows improved electrochemical performance compared with the Ge dense film anode. A capacity of ∼1200 mA h g^-1 after 200 cycles at 0.1 C is obtained, with an initial coulombic efficiency of 81.3%. In addition, the Ge nanowire anode demonstrates superior rate capability with excellent capacity retention and stability (producing highly stable discharge capacities of about 620 mA h g^-1 at 5 C). The improved electrochemical performance is the result of the enhanced electron migration and electron transport paths of the nanowires, and sufficient elasticity to buffer the volume expansion. This approach encompasses a low energy processing method where all the material is electrochemically active and binder free. The improved cycling stability and rate performance characteristics make these anodes highly attractive for the most demanding lithium-ion applications.

3D nitrogen-doped graphene foam with encapsulated germanium/nitrogen-doped graphene yolk-shell nanoarchitecture for high-performance flexible Li-ion battery

Flexible electrochemical energy storage devices have attracted extensive attention as promising power sources for the ever-growing field of flexible and wearable electronic products. However, the rational design of a novel electrode structure with a good flexibility, high capacity, fast charge–discharge rate and long cycling lifetimes remains a long-standing challenge for developing next-generation flexible energy-storage materials. Herein, we develop a facile and general approach to three-dimensional (3D) interconnected porous nitrogen-doped graphene foam with encapsulated Ge quantum dot/nitrogen-doped graphene yolk-shell nano architecture for high specific reversible capacity (1,220 mAh g⁻¹), long cycling capability (over 96% reversible capacity retention from the second to 1,000 cycles) and ultra-high rate performance (over 800 mAh g⁻¹ at 40 C). This work paves a way to develop the 3D interconnected graphene-based high-capacity electrode material systems, particularly those that suffer from huge volume expansion, for the future development of high-performance flexible energy storage systems.

The development of materials for energy storage hinges on the design of electrodes with large capacity, flexibility, fast charge–discharge rate and long cycling lifetime. Here, the authors develop electrodes based on nitrogen doped graphene with encapsulated Ge quantum dots with yolk-shell architecture.

Graphene boosted Cu2GeS3 for advanced lithium-ion batteries

Ternary Cu₂GeS₃ (CGS) serves as lithium ion battery anode materials for the first time, whose electrochemical performance is significantly improved by the introduction of reduced graphene oxide.

Highly Reversible and Superior Li-Storage Characteristics of Layered GeS2 and Its Amorphous Composites

A layered GeS₂ material was assessed as an electrode material in the fabrication of superior rechargeable Li-ion batteries. The electrochemical Li insertion/extraction behavior of the GeS₂ electrode was investigated from extended X-ray absorption measurements as well as by cyclic voltammetry and differential capacity plots to better understand its Li insertion/extraction behavior. Using the Li insertion/extraction reaction mechanism of the GeS₂ electrode, an interesting amorphous GeS₂-based composite was developed and tested for use as a high-performance electrode. Interestingly, the amorphous GeS₂-based composite electrode exhibited highly reversible discharging and charging reactions, which were attributed to a conversion/recombination reaction. The amorphous GeS₂-based composite electrode exhibited highly reversible and outstanding electrochemical performances, a highly reversible capacity (first charge capacity: 1298 mAh g^-1) with a high first Coulombic efficiency (83.3%), rapid rate capability (ca. 800 mAh g^-1 at a high current rate of 700 mA g^-1), and long capacity retention over 180 cycles with high capacity (1100 mAh g^-1) thanks to its interesting electrochemical reaction mechanism. Overall, this layered GeS₂ and its amorphous GeS₂/C composite are novel alternative anode materials for the potential mass production of rechargeable Li-ion batteries with excellent performance.

Nanostructured germanium deposited on heated substrates with enhanced photoelectric properties

Obtaining high-quality materials, based on nanocrystals, at low temperatures is one of the current challenges for opening new paths in improving and developing functional devices in nanoscale electronics and optoelectronics. Here we report a detailed investigation of the optimization of parameters for the in situ synthesis of thin films with high Ge content (50 %) into SiO₂. Crystalline Ge nanoparticles were directly formed during co-deposition of SiO₂ and Ge on substrates at 300, 400 and 500 °C. Using this approach, effects related to Ge-Ge spacing are emphasized through a significant improvement of the spatial distribution of the Ge nanoparticles and by avoiding multi-step fabrication processes or Ge loss. The influence of the preparation conditions on structural, electrical and optical properties of the fabricated nanostructures was studied by X-ray diffraction, transmission electron microscopy, electrical measurements in dark or under illumination and response time investigations. Finally, we demonstrate the feasibility of the procedure by the means of an Al/n-Si/Ge:SiO₂/ITO photodetector test structure. The structures, investigated at room temperature, show superior performance, high photoresponse gain, high responsivity (about 7 AW^-1), fast response time (0.5 µs at 4 kHz) and great optoelectronic conversion efficiency of 900% in a wide operation bandwidth, from 450 to 1300 nm. The obtained photoresponse gain and the spectral width are attributed mainly to the high Ge content packed into a SiO₂ matrix showing the direct connection between synthesis and optical properties of the tested nanostructures. Our deposition approach put in evidence the great potential of Ge nanoparticles embedded in a SiO₂ matrix for hybrid integration, as they may be employed in structures and devices individually or with other materials, hence the possibility of fabricating various heterojunctions on Si, glass or flexible substrates for future development of Si-based integrated optoelectronics.

Rational synthesis of carbon-coated hollow Ge nanocrystals with enhanced lithium-storage properties

High-capacity anode materials based on alloy-type group IV elements always have large volume expansion during lithiation when they are used in lithium-ion batteries. Designing hollow structures is a well-established strategy to accommodate the volume change because of sufficient internal void space. Here we report a facile template-free route to prepare hollow Ge nanospheres without using any templates through a quasi-microemulsion method. Ge nanocrystals are preferably self-assembled along the interface of liquid vesicles between water and tetrahydrofuran, and well-defined hollow architectures of ∼50 nm in diameter are formed. Both the wall thickness and hollow interiors can be easily tuned. After subsequent carbon coating via pyrolysis of acetylene, the as-formed Ge@C nanocomposite with hollow interiors exhibits a highly reversible capacity of about 920 mA h g(-1) at 200 mA g(-1) over 50 cycles, and excellent rate capability. The small size and the high structural integrity of hollow Ge@C structures contribute to the superior lithium-storage performances.

Chiral nematic porous germania and germanium/carbon films

We report our extensive attempts and, ultimately, success to produce crack-free, chiral nematic GeO2/cellulose nanocrystal (CNC) composite films with tunable photonic properties from the controlled assembly of germanium(iv) alkoxides with the lyotropic liquid-crystalline CNCs in a mixed solvent of water/DMF. With different pyrolysis conditions, the photonic GeO2/CNC composites can be converted into freestanding chiral nematic films of amorphous GeO2, and semiconducting mesoporous GeO2/C and Ge/C replicas. These new materials are promising for chiral separation, enantioselective adsorption, catalysis, sensing, optoelectronics, and lithium ion batteries. Furthermore, the new, reproducible synthesis strategies developed may be applicable for constructing other composites and porous materials with chiral nematic ordering.

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.

Mesoporous Ge/GeO2/Carbon Lithium-Ion Battery Anodes with High Capacity and High Reversibility

We report mesoporous composite materials (m-GeO2, m-GeO2/C, and m-Ge-GeO2/C) with large pore size which are synthesized by a simple block copolymer directed self-assembly. m-Ge/GeO2/C shows greatly enhanced Coulombic efficiency, high reversible capacity (1631 mA h g(-1)), and stable cycle life compared with the other mesoporous and bulk GeO2 electrodes. m-Ge/GeO2/C exhibits one of the highest areal capacities (1.65 mA h cm(-2)) among previously reported Ge- and GeO2-based anodes. The superior electrochemical performance in m-Ge/GeO2/C arises from the highly improved kinetics of conversion reaction due to the synergistic effects of the mesoporous structures and the conductive carbon and metallic Ge.

Probing lithium germanide phase evolution and structural change in a germanium-in-carbon nanotube energy storage system

Lithium alloys of group IV elements such as silicon and germanium are attractive candidates for use as anodes in high-energy-density lithium-ion batteries. However, the poor capacity retention arising from volume swing during lithium cycling restricts their widespread application. Herein, we report high reversible capacity and superior rate capability from core-shell structure consisting of germanium nanorods embedded in multiwall carbon nanotubes. To understand how the core-shell structure helps to mitigate volume swings and buffer against mechanical instability, transmission electron microscopy, X-ray diffraction, and in situ (7)Li nuclear magnetic resonance were used to probe the structural rearrangements and phase evolution of various Li-Ge alloy phases during (de)alloying reactions with lithium. The results provide insights into amorphous-to-crystalline transition and lithium germanide alloy phase transformation, which are important reactions controlling performance in this system.

Ge@C core–shell nanostructures for improved anode rate performance in lithium-ion batteries

The Ge@C core–shell nanostructures exhibit excellent cycling performance and rate capability as an electrode material for lithium ion batteries.

Synthesis and structure of free-standing germanium quantum dots and their application in live cell imaging

Colloidally synthesized free-standing Ge qdots with a unique core–shell structure were demonstrated to be a viable bio-imaging probe.

Ionic liquid electrodeposition of Ge nanostructures on freestanding Ni-nanocone arrays for Li-ion battery

With the growing demand for portable and wearable electronic devices, it is imperative to develop high-performance Li-ion batteries with long lifetimes.

A synchronous approach for facile production of Ge–carbon hybrid nanoparticles for high-performance lithium batteries

A new synchronous approach for facile production of uniform Ge–carbon hybrid nanoparticles from a germanium chelate complex is designed and described.

Enhanced sodium-ion battery performance by structural phase transition from two-dimensional hexagonal-SnS2 to orthorhombic-SnS

Structural phase transitions can be used to alter the properties of a material without adding any additional elements and are therefore of significant technological value. It was found that the hexagonal-SnS2 phase can be transformed into the orthorhombic-SnS phase after an annealing step in an argon atmosphere, and the thus transformed SnS shows enhanced sodium-ion storage performance over that of the SnS2, which is attributed to its structural advantages. Here, we provide the first report on a SnS@graphene architecture for application as a sodium-ion battery anode, which is built from two-dimensional SnS and graphene nanosheets as complementary building blocks. The as-prepared SnS@graphene hybrid nanostructured composite delivers an excellent specific capacity of 940 mAh g(-1)and impressive rate capability of 492 and 308 mAh g(-1) after 250 cycles at the current densities of 810 and 7290 mA g(-1), respectively. The performance was found to be much better than those of most reported anode materials for Na-ion batteries. On the basis of combined ex situ Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and ex situ X-ray diffraction, the formation mechanism of SnS@graphene and the synergistic Na-storage reactions of SnS in the anode are discussed in detail. The SnS experienced a two-structural-phase transformation mechanism (orthorhombic-SnS to cubic-Sn to orthorhombic-Na3.75Sn), while the SnS2 experienced a three-structural-phase transformation mechanism (hexagonal-SnS2 to tetragonal-Sn to orthorhombic-Na3.75Sn) during the sodiation process. The lesser structural changes of SnS during the conversion are expected to lead to good structural stability and excellent cycling stability in its sodium-ion battery performance. These results demonstrate that the SnS@graphene architecture offers unique characteristics suitable for high-performance energy storage application.

Dense Si(x)Ge(1-x) (0 < x < 1) materials landscape using extreme conditions and precession electron diffraction

High-pressure and -temperature experiments on Ge and Si mixtures to 17 GPa and 1500 K allow us to obtain extended Ge-Si solid solutions with cubic (Ia3) and tetragonal (P4(3)2(1)2) crystal symmetries at ambient pressure. The cubic modification can be obtained with up to 77 atom % Ge and the tetragonal modification for Ge concentrations above that. Together with Hume-Rothery criteria, melting point convergence is employed here as a favored attribute for solid solution formation. These compositionally tunable alloys are of growing interest for advanced transport and optoelectronic applications. Furthermore, the work illustrates the significance of employing precession electron diffraction for mapping new materials landscapes resulting from tailored high-pressure and -temperature syntheses.

Nitrogen/carbon atomic ratio-dependent performances of nitrogen-doped carbon-coated metal oxide nanocrystals for anodes in lithium-ion batteries

We report the hydrothermal synthesis of the N-doped carbon-coated NiO nanocrystals (N-C-NiO NCs) with tunable N/C atomic ratios using the nitrogen-containing ionic liquids (ILs) as new carbon precursor, and the N-doped carbon layer composition-dependent performances of N-C-NiO NCs anode for lithium-ion batteries (LIBs). The results indicate that the N-doped carbon coating can significantly enhance the electronic conductivity, effectively avoid the problems of cracking or pulverization of the NiO, and prevent the aggregation of the active materials upon cycling. These properties make the synthesized material a promising anode material for LIBs. The N-C-NiO NCs with the N/C atomic ratio of 21.2% in the N-doped carbon layer show a high specific capacity of ∼710 mAh g(-1) at a current rate of 0.3 C (very closed to the theoretical capacity of 718 mAh g(-1) for NiO), a high rate capability (still able to deliver a discharge capacity of ∼430 mAh g(-1) at a current density of 10 C), and good capacity retention upon cycling (maintains at 710 mAh g(-1) at least up to the 50th cycle) compared with those of pristine NiO nanoparticles. Moreover, the electrochemical performances of the N-C-NiO NCs depend on the composition (N/C atomic ratios) in the N-doped carbon layer and are enhanced with increasing of the N/C ratios. Our approach offers an effective and convenient technique to improve the specific capacities and rate capabilities of highly insulating electrode materials for batteries and may also provide general and effective approach toward the synthesis of other metal oxides coated with N-doped carbon layer.

Composition-tuned SnxGe1−xS nanocrystals for enhanced-performance lithium ion batteries

Complete composition-tuned Sn_xGe_1−xS alloy nanocrystals exhibit excellent cycling performances in lithium ion batteries, with the greatest rate capability for Sn-rich compositions.