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Ge1–xSnx nanowires incorporating a large amount of Sn would be useful for mobility enhancement in nanoelectronic devices, a definitive transition to a direct bandgap for application in optoelectronic devices and to increase the efficiency of the GeSn-based photonic devices. Here we report the catalytic bottom-up fabrication of Ge1–xSnx nanowires with very high Sn incorporation (x > 0.3). These nanowires are grown in supercritical toluene under high pressure (21 MPa). The introduction of high pressure in the vapor–liquid–solid (VLS) like growth regime resulted in a substantial increase of Sn incorporation in the nanowires, with a Sn content ranging between 10 and 35 atom %. The incorporation of Sn in the nanowires was found to be inversely related to nanowire diameter; a high Sn content of 35 atom % was achieved in very thin Ge1–xSnx nanowires with diameters close to 20 nm. Sn was found to be homogeneously distributed throughout the body of the nanowires, without apparent clustering or segregation. The large inclusion of Sn in the nanowires could be attributed to the nanowire growth kinetics and small nanowire diameters, resulting in increased solubility of Sn in Ge at the metastable liquid–solid interface under high pressure. Electrical investigation of the Ge1–xSnx (x = 0.10) nanowires synthesized by the supercritical fluid approach revealed their potential in nanoelectronics and sensor-based applications.

Black phosphorus (BP) has extraordinary properties, but its ambient instability remains a critical challenge. Functionalization has been employed to overcome the sensitivity of BP to ambient conditions while preserving its properties. Herein, a simultaneous exfoliation–functionalization process is reported that functionalizes BP flakes during exfoliation and thus provides increased protection, which can be attributed to minimal exposure of the flakes to ambient oxygen and water. A tetrabutylammonium salt was employed for intercalation of BP, resulting in the formation of flakes with large lateral dimensions. The addition of an aryl iodide or an aryl iodonium salt to the exfoliation solvent creates a scalable strategy for the production of functionalized few‐layer BP flakes. The ambient stability of functionalized BP was prolonged to a period of one week, as revealed by STEM, AFM, and X‐ray photoelectron spectroscopy.

Alloying group IV semiconductors offers an effective way to engineer their electronic properties and lattice dynamics. The incorporation of Sn in Ge permits a transition from an indirect to a direct bandgap semiconductor. Here, by combining polarization, laser power-dependent and temperature-dependent micro-Raman spectroscopy we explore the full lattice dynamics of Ge

Alloying group IV semiconductors offers an effective way to engineer their electronic properties and lattice dynamics. The incorporation of Sn in Ge permits a transition from an indirect to a direct bandgap semiconductor. Here, by combining polarization, laser power-dependent and temperature-dependent micro-Raman spectroscopy we explore the full lattice dynamics of Ge1−xSnx (x = 0.01, 0.06 and 0.08) alloy nanowires. In the high Sn content samples (x ≥ 0.06), a low-frequency tail and a high-frequency shoulder are observed which are associated with the F2g optical phonon mode of Ge (Ge–Ge mode). The new modes are assigned to the stretching of Ge–Ge bonds due to Sn-induced lattice relaxation and compression, respectively. The symmetry of the observed Raman modes has been studied by polarization-dependent Raman scattering. Nonlinear fitting of the laser power-dependent intensity of the high-frequency Ge–Ge mode in the Ge1−xSnx alloy nanowires with x = 0.06 and 0.08 suggests the activation of a third-order stimulated Raman scattering process, due to the high intensity localized electric field surrounding the Sn clusters. Finally, from the temperature-dependent Raman study, we have estimated the isobaric Grüneisen parameters for all the observed modes.

We report for the first time the self-catalyzed, single-step growth of branched GeSn nanostructures by a vapor–liquid–solid mechanism. These typical GeSn nanostructures consist of ⟨111⟩-oriented, Sn-rich (∼8 atom %) GeSn “branches” grown epitaxially on GeSn “trunks”, with a Sn content of ∼4 atom %. The trunks were seeded from Au0.80Ag0.20 nanoparticles followed by the catalytic growth of secondary branches (diameter ∼ 50 nm) from the excess of Sn on the sidewalls of the trunks, as determined by high-resolution electron microscopy and energy-dispersive X-ray analysis. The nanowires, with ⟨111⟩-directed GeSn branches oriented at ∼70° to the trunks, have no apparent defects or change in crystal structure at the trunk–branch interface; structural quality is retained at the interface with epitaxial crystallographic relation. The electrochemical performance of these highly ordered GeSn nanostructures was explored as a potential anode material for Li-ion batteries, due to their high surface-to-volume ratio and increased charge carrier pathways. The unique structure of the branched nanowires led to high specific capacities comparable to, or greater than, those of conventional Ge nanowire anode materials and Ge1–xSnx nanocrystals.

Group IV alloys have attracted interest in the drive to create Si compatible, direct bandgap materials for implementation in complementary metal oxide semiconductor (CMOS) and beyond CMOS devices. The lack of a direct bandgap in Si and Ge hinders their incorporation into optoelectronic and photonic devices, without the induction of undesirable strain. Alloying of Ge with Sn represents a novel solution to the lack of light emission in group IV compounds, with an indirect-to-direct bandgap transition predicted for Ge at a Sn incorporation greater than 6.5 at. %. Recently, the initiatives on GeSn alloy research has turned its focus on nanoforms to keep track with the miniaturization of Si-related platforms for application in nano/optoelectronics, photonics and energy devices. Here, we review recent advances in the growth and application of Ge1-xSnx nanomaterials. An overview of theoretical band structure calculations for Ge1-xSnx and the effect of band-mixing is briefly explored to highlight the significance of Sn inclusion in Ge for band gap engineering. Different fabrication methods for growing Ge1-xSnx alloy nanostructures are delineated and corelated with thin films growth. This highlight the requirement of low-temperature, kinetically-driven non-equilibrium processess for growing these metastable nanoscale alloys. The optical and electrical properties for both Ge1-xSnx strain-relaxed one dimensional (1D) nanostructures and nanoparticles are reported with additional highlight on the recent key findings on Ge1-xSnx thin films to indicate the potential application of these materials in photonic, nanoelectronic and optotelectronic devices.

Ge1-xSnx alloys form a heterogeneous material system with high potential for applications in both optoelectronic and high-speed electronics devices. The attractiveness of Ge1-xSnx lies in the ability to tune the semiconductor band gap and electronic properties as a function of Sn concentration. Advances in Ge1-xSnx material synthesis have raised expectations recently, but there are considerable problems in terms of device demonstration. Although Ge1-xSnx thin films have been previously explored experimentally, in-depth studies of the electrical properties of Ge1-xSnx nanostructures are very limited, specifically those on nanowires grown via a bottom-up vapor–liquid–solid (VLS) process using metal catalysts. In this study, a detailed electrical investigation is presented of nominally undoped Ge1-xSnx bottom-up-grown nanowire devices with different Sn percentages (3–9 at. %). The entire device fabrication process is performed at relatively low temperatures, the maximum temperature being 440 °C. Device current modulation is performed through backgating from a substrate electrode, achieving impressive on–off current (ION/IOFF) ratios of up to 104, showing their potential for electronic and sensor-based applications. Contact resistance (RC) extraction is essential for proper VLS-grown nanowire device electrical evaluation. Once the RC contribution is extracted and removed, parameter values such as mobility can change significantly, by up to 70% in this work. When benchmarked against other Ge1-xSnx electronic devices, the VLS-grown nanowire devices have potential in applications where a high ION/IOFF ratio is important and where thermal budget and processing temperatures are required to be kept to minimum.

The combination of two active Li-ion materials (Ge and Sn) can result in improved conduction paths and higher capacity retention. Here we report; for the first time; the implementation of Ge1-xSnx alloy nanowires as anode materials for Li-ion batteries. Ge1-xSnx alloy nanowires have been successfully grown via vapor-liquid-solid (VLS) technique directly on stainless steel current collectors. Ge1-xSnx (x = 0.048) nanowires were predominantly seeded from the Au0.80Ag0.20 catalysts with negligible amount of growth was also directly catalysed from stainless steel substrate. The electrochemical performance of the the Ge1-xSnx nanowires as an anode material for Li-ion batteries was investigated via galvanostatic cycling and detailed analysis of differential capacity plots. The nanowire electrodes demonstrated an exceptional capacity retention of 93.4 % from the 2nd to the 100th charge at a C/5 rate, while maintaining a specific capacity value of ~921 mAh/g after 100 cycles. Voltage profiles and differential capacity plots revealed that the Ge1-xSnx nanowires behave as an alloying mode anode material, as reduction/oxidation peaks for both Ge and Sn were observed, however it is clear that the reversible lithiation of Ge is responsible for the majority of the charge stored.

Germanium tin (GeSn) has been proposed as a promising material for electronic and optical applications due to the formation of a direct band-gap at a Sn content >7 at%. Furthermore, the ability to manipulate the properties of GeSn at the nanoscale will further permit the realisation of advanced mechanical devices. Here we report for the first time the mechanical properties of GeSn nanowires (7.1-9.7 at% Sn) and assess their suitability as nanoelectromechanical (NEM) switches. Electron microscopy analysis showed the nanowires to be single crystalline, with surfaces covered by a thin native amorphous oxide layer. Mechanical resonance and bending tests at different boundary conditions were used to obtain size-dependent Young's moduli and to relate the mechanical characteristics of the alloy nanowires to geometry and Sn incorporation. The mechanical properties of the GeSn nanowires make them highly promising for applications in next generation NEM devices.

The aim of this work is to provide a systematic and comparative study on the material characteristics and electrical contact performance for a germanium-tin (GeSn) alloy with a high percentage of Sn (8%). Thin metal films (10 nm) of Nickel (Ni), Titanium (Ti), or Platinum (Pt) were deposited on Ge 0.92 Sn 0.08 layers and subsequently annealed at different temperatures ranging from 300°C up to 500°C. Various experimental techniques were employed to characterize the metal morphology and the electrical contact behavior, with the intention of identifying the most promising metal candidate, in terms of low sheet resistance and low surface roughness, considering a low formation temperature. The investigations carried out show that for nano-electronic contact applications, nickel-stanogermanide (NiGeSn) turns out to be the most promising candidate among the three different metals analyzed. NiGeSn presents low sheet resistance combined with low formation temperatures, below 400°C; PtGeSn shows better thermal stability when compared to the other two options while, Ti was found to be unreactive below 500°C, resulting in incomplete TiGeSn formation.

Ge1−xSnx alloys with substantial incorporation of Sn show promise as direct bandgap group IV semiconductors. This article reports the influence of growth kinetics on Sn inclusion in Ge1−xSnx alloy nanowires through manipulation of the growth constraints, i.e. temperature, precursor type and catalyst. Ge1−xSnx nanowire growth kinetics were manipulated in a vapour–liquid–solid (VLS) growth process by varying the growth temperature between 425 and 470 °C, using Au and Ag alloys as growth catalysts and different tin precursors such as allyltributytin, tertaethyltin and tetraallyltin. The profound impact of growth kinetics on the incorporation of Sn; from 7 to 9 at%; in Ge1−xSnx nanowires was clearly apparent, with the fastest growing nanowires (of comparable diameter) containing a higher amount of Sn. A kinetically dependent “solute trapping” process was assigned as the primary inclusion mechanism for Sn incorporation in the Ge1−xSnx nanowires. The participation of a kinetic dependent, continuous Sn incorporation process in the single-step VLS nanowire growth resulted in improved ordering of the Ge1−xSnx alloy lattice; as opposed to a randomly ordered alloy. The amount of Sn inclusion and the Sn impurity ordering in Ge1−xSnx nanowires has a profound effect on the quality of the light emission and on the directness of the band gap as confirmed by temperature dependent photoluminescence study and electron energy loss spectroscopy.

One-dimensional nanostructures with controllable morphologies and defects are appealing for use in nanowire devices. This paper details the influence of colloidal magnetite iron oxide nanoparticle seeds to regulate the radial dimension and twin boundary formation in Ge nanowires grown through a liquid-injection chemical vapor deposition process. Control over the mean nanowire diameter, even in the sub-10 nm regime, was achieved due to the minimal expansion and aggregation of iron oxide nanoparticles during the growth process. The uncommon occurrence of heterogeneously distributed multiple layer {111} twins, directed perpendicular to the nanowire growth axis, were also observed in 〈111〉-directed Ge nanowires, especially those synthesized from patterned hemispherical Fe3O4 nanodot catalysts. Consecutive twin planes along 〈111〉-oriented nanowires resulted in a local phase transformation from 3C diamond cubic to hexagonal 4H allotrope. Localized polytypic crystal phase heretostructures were formed along 〈111〉-oriented Ge nanowire using magnetite nanodot catalysts.

In this article we provide a comparative and systematic study on contact formation for germanium-tin (GeSn) thin films containing a high percentage of Sn (8 at.%). 20 nm of Nickel (Ni), Titanium (Ti), or Platinum (Pt) was deposited on Ge0.92Sn0.08 layers grown on Ge substrates, and subsequently annealed between 300 and 500 °C to form stanogermanide alloys. Several experimental techniques were employed to characterize the material and the electrical contact behaviour, with the purpose of identifying the most promising stanogermanide contact candidate, in terms of low sheet resistance, low surface roughness and low formation temperature. Among these three different metals we found that, for nanoelectronic applications, nickel-stanogermanide (NiGeSn) was the most promising candidate based on a low sheet resistance combined with a low formation temperature, below 400 °C. PtGeSn showed better behaviour in terms of thermal stability compared with the other two options, while Ti was found to be relatively unreactive under these annealing conditions, resulting in poor TiGeSn formation. For the lowest resistance stanogermanide contact generated, namely NiGeSn formed at 300 °C, detailed lattice resolution Transmission Electron Microscopy imaging, combined with fast Fourier transformation analysis, identified the formation of the Nix-1(GeSn)y-1 phase.

Here we describe a uniform diameter, direct bandgap Ge1-xSnx alloy nanowires, with a Sn incorporation up to 9%, the fabricated through a conventional catalytic bottom-up growth paradigm employing innovative catalysts and precursors. Optical characterization by means of temperature dependent photoluminescence is used to identify transition point from indirect to direct badgap of GeSn nanowires.