Direct-bandgap emission from hexagonal Ge and SiGe alloys.

Silicon crystallized in the usual cubic (diamond) lattice structure has dominated the electronics industry for more than half a century. However, cubic silicon (Si), germanium (Ge) and SiGe alloys are all indirect-bandgap semiconductors that cannot emit light efficiently. The goal1 of achieving efficient light emission from group-IV materials in silicon technology has been elusive for decades2-6. Here we demonstrate efficient light emission from direct-bandgap hexagonal Ge and SiGe alloys. We measure a sub-nanosecond, temperature-insensitive radiative recombination lifetime and observe an emission yield similar to that of direct-bandgap group-III-V semiconductors. Moreover, we demonstrate that, by controlling the composition of the hexagonal SiGe alloy, the emission wavelength can be continuously tuned over a broad range, while preserving the direct bandgap. Our experimental findings are in excellent quantitative agreement with ab initio theory. Hexagonal SiGe embodies an ideal material system in which to combine electronic and optoelectronic functionalities on a single chip, opening the way towards integrated device concepts and information-processing technologies.

Hydrothermal Synthesis of Monoclinic VO2 Microparticles without Use of Hazardous Reagents: A Key Role for the W-Dopant.

Monoclinic vanadium dioxide (VO2 (M)) is a promising material for various applications ranging from sensing to signature management and smart windows. Most applications rely on its reversible structural phase transition to rutile VO2 (VO2 (R)), which is accompanied by a metal-to-insulator transition. Bottom-up hydrothermal synthesis has proven to yield high quality monoclinic VO2 but requires toxic and highly reactive reducing agents that cannot be used outside of a research lab. Here, we present a new hydrothermal synthesis method using nontoxic and safe-to-use oxalic acid as a reducing agent for V2O5 to produce VO2 (M). In early stages of the process, polymorphs VO2 (A) and VO2 (B) were formed, which subsequently recrystallized to VO2 (M). Without the presence of W6+, this recrystallization did not occur. After a reaction time of 96 h at 230 °C in the presence of (NH4)6H2W12O40 in Teflon-lined rotated autoclaves, we realized highly crystalline, phase pure W-doped VO2 (M) microparticles of uniform size and asterisk shape (ΔH = 28.30 J·g-1, arm length = 6.7 ± 0.4 µm, arm width = 0.46 ± 0.06 µm). We extensively investigated the role of W6+ in the kinetics of formation of VO2 (M) and the thermodynamics of its structural phase transition.

Catalyst-free MBE growth of PbSnTe nanowires with tunable aspect ratio.

Topological crystalline insulators (TCIs) are interesting for their topological surface states, which hold great promise for scattering-free transport channels and fault-tolerant quantum computing. A promising TCI is SnTe. However, Sn-vacancies form in SnTe, causing a high hole density, hindering topological transport from the surface being measured. This issue could be relieved by using nanowires with a high surface-to-volume ratio. Furthermore, SnTe can be alloyed with Pb reducing the Sn-vacancies while maintaining its topological phase. Here we present the catalyst-free growth of monocrystalline PbSnTe in molecular beam epitaxy. By the addition of a pre-deposition stage before the growth, we have control over the nucleation phase and thereby increase the nanowire yield. This facilitates tuning the nanowire aspect ratio by a factor of four by varying the growth parameters. These results allow us to grow specific morphologies for future transport experiments to probe the topological surface states in a Pb1-xSnxTe-based platform.

Quantized Majorana conductance.

Majorana zero-modes-a type of localized quasiparticle-hold great promise for topological quantum computing. Tunnelling spectroscopy in electrical transport is the primary tool for identifying the presence of Majorana zero-modes, for instance as a zero-bias peak in differential conductance. The height of the Majorana zero-bias peak is predicted to be quantized at the universal conductance value of 2e2/h at zero temperature (where e is the charge of an electron and h is the Planck constant), as a direct consequence of the famous Majorana symmetry in which a particle is its own antiparticle. The Majorana symmetry protects the quantization against disorder, interactions and variations in the tunnel coupling. Previous experiments, however, have mostly shown zero-bias peaks much smaller than 2e2/h, with a recent observation of a peak height close to 2e2/h. Here we report a quantized conductance plateau at 2e2/h in the zero-bias conductance measured in indium antimonide semiconductor nanowires covered with an aluminium superconducting shell. The height of our zero-bias peak remains constant despite changing parameters such as the magnetic field and tunnel coupling, indicating that it is a quantized conductance plateau. We distinguish this quantized Majorana peak from possible non-Majorana origins by investigating its robustness to electric and magnetic fields as well as its temperature dependence. The observation of a quantized conductance plateau strongly supports the existence of Majorana zero-modes in the system, consequently paving the way for future braiding experiments that could lead to topological quantum computing.

Epitaxy of advanced nanowire quantum devices.

Semiconductor nanowires are ideal for realizing various low-dimensional quantum devices. In particular, topological phases of matter hosting non-Abelian quasiparticles (such as anyons) can emerge when a semiconductor nanowire with strong spin-orbit coupling is brought into contact with a superconductor. To exploit the potential of non-Abelian anyons-which are key elements of topological quantum computing-fully, they need to be exchanged in a well-controlled braiding operation. Essential hardware for braiding is a network of crystalline nanowires coupled to superconducting islands. Here we demonstrate a technique for generic bottom-up synthesis of complex quantum devices with a special focus on nanowire networks with a predefined number of superconducting islands. Structural analysis confirms the high crystalline quality of the nanowire junctions, as well as an epitaxial superconductor-semiconductor interface. Quantum transport measurements of nanowire 'hashtags' reveal Aharonov-Bohm and weak-antilocalization effects, indicating a phase-coherent system with strong spin-orbit coupling. In addition, a proximity-induced hard superconducting gap (with vanishing sub-gap conductance) is demonstrated in these hybrid superconductor-semiconductor nanowires, highlighting the successful materials development necessary for a first braiding experiment. Our approach opens up new avenues for the realization of epitaxial three-dimensional quantum architectures which have the potential to become key components of various quantum devices.

Ceria-Supported Cobalt Catalyst for Low-Temperature Methanation at Low Partial Pressures of CO2.

The direct catalytic conversion of atmospheric CO2 to valuable chemicals is a promising solution to avert negative consequences of rising CO2 concentration. However, heterogeneous catalysts efficient at low partial pressures of CO2 still need to be developed. Here, we explore Co/CeO2 as a catalyst for the methanation of diluted CO2 streams. This material displays an excellent performance at reaction temperatures as low as 175 °C and CO2 partial pressures as low as 0.4 mbar (the atmospheric CO2 concentration). To gain mechanistic understanding of this unusual activity, we employed in situ X-ray photoelectron spectroscopy and operando infrared spectroscopy. The higher surface concentration and reactivity of formates and carbonyls-key reaction intermediates-explain the superior activity of Co/CeO2 as compared to a conventional Co/SiO2 catalyst. This work emphasizes the catalytic role of the cobalt-ceria interface and will aid in developing more efficient CO2 hydrogenation catalysts.

Unveiling Planar Defects in Hexagonal Group IV Materials.

Recently synthesized hexagonal group IV materials are a promising platform to realize efficient light emission that is closely integrated with electronics. A high crystal quality is essential to assess the intrinsic electronic and optical properties of these materials unaffected by structural defects. Here, we identify a previously unknown partial planar defect in materials with a type I3 basal stacking fault and investigate its structural and electronic properties. Electron microscopy and atomistic modeling are used to reconstruct and visualize this stacking fault and its terminating dislocations in the crystal. From band structure calculations coupled to photoluminescence measurements, we conclude that the I3 defect does not create states within the hex-Ge and hex-Si band gap. Therefore, the defect is not detrimental to the optoelectronic properties of the hex-SiGe materials family. Finally, highlighting the properties of this defect can be of great interest to the community of hex-III-Ns, where this defect is also present.

Operando Spectroscopy Unveils the Catalytic Role of Different Palladium Oxidation States in CO Oxidation on Pd/CeO2 Catalysts.

Aiming at knowledge-driven design of novel metal-ceria catalysts for automotive exhaust abatement, current efforts mostly pertain to the synthesis and understanding of well-defined systems. In contrast, technical catalysts are often heterogeneous in their metal speciation. Here, we unveiled rich structural dynamics of a conventional impregnated Pd/CeO2 catalyst during CO oxidation. In situ X-ray photoelectron spectroscopy and operando X-ray absorption spectroscopy revealed the presence of metallic and oxidic Pd states during the reaction. Using transient operando infrared spectroscopy, we probed the nature and reactivity of the surface intermediates involved in CO oxidation. We found that while low-temperature activity is associated with sub-oxidized and interfacial Pd sites, the reaction at elevated temperatures involves metallic Pd. These results highlight the utility of the multi-technique operando approach for establishing structure-activity relationships of technical catalysts.

Hard Superconducting Gap and Diffusion-Induced Superconductors in Ge-Si Nanowires.

We show a hard superconducting gap in a Ge-Si nanowire Josephson transistor up to in-plane magnetic fields of 250 mT, an important step toward creating and detecting Majorana zero modes in this system. A hard gap requires a highly homogeneous tunneling heterointerface between the superconducting contacts and the semiconducting nanowire. This is realized by annealing devices at 180 °C during which aluminum interdiffuses and replaces the germanium in a section of the nanowire. Next to Al, we find a superconductor with lower critical temperature (TC = 0.9 K) and a higher critical field (BC = 0.9-1.2 T). We can therefore selectively switch either superconductor to the normal state by tuning the temperature and the magnetic field and observe that the additional superconductor induces a proximity supercurrent in the semiconducting part of the nanowire even when the Al is in the normal state. In another device where the diffusion of Al rendered the nanowire completely metallic, a superconductor with a much higher critical temperature (TC = 2.9 K) and critical field (BC = 3.4 T) is found. The small size of these diffusion-induced superconductors inside nanowires may be of special interest for applications requiring high magnetic fields in arbitrary direction.

Ballistic Phonons in Ultrathin Nanowires.

According to Fourier's law, a temperature difference across a material results in a linear temperature profile and a thermal conductance that decreases inversely proportional to the system length. These are the hallmarks of diffusive heat flow. Here, we report heat flow in ultrathin (25 nm) GaP nanowires in the absence of a temperature gradient within the wire and find that the heat conductance is independent of wire length. These observations deviate from Fourier's law and are direct proof of ballistic heat flow, persisting for wire lengths up to at least 15 µm at room temperature. When doubling the wire diameter, a remarkably sudden transition to diffusive heat flow is observed. The ballistic heat flow in the ultrathin wires can be modeled within Landauer's formalism by ballistic phonons with an extraordinarily long mean free path.

Large area, patterned growth of 2D MoS2 and lateral MoS2-WS2 heterostructures for nano- and opto-electronic applications.

The patterned growth of transition metal dichalcogenides (TMDs) and their lateral heterostructures is paramount for the fabrication of application-oriented electronics and optoelectronics devices. However, the large scale patterned growth of TMDs remains challenging. Here, we demonstrate the synthesis of patterned polycrystalline 2D MoS2 thin films on device ready SiO2/Si substrates, eliminating any etching and transfer steps using a combination of plasma enhanced atomic layer deposition (PEALD) and thermal sulfurization. As an inherent advantage of ALD, precise thickness control ranging from a monolayer to few-layered MoS2 has been achieved. Furthermore, uniform films with exceptional conformality over 3D structures are obtained. Finally, the approach has been leveraged to obtain in-plane lateral heterostructures of 2D MoS2 and WS2 thin films over a large area which opens up an avenue for their direct integration in future nano- and opto-electronic device applications.

High Mobility Stemless InSb Nanowires.

High aspect-ratio InSb nanowires (NWs) of high chemical purity are sought for implementing advanced quantum devices. The growth of InSb NWs is challenging, generally requiring a stem of a foreign material for nucleation. Such a stem tends to limit the length of InSb NWs and its material becomes incorporated in the InSb segment. Here, we report on the growth of chemically pure InSb NWs tens of microns long. Using a selective-area mask in combination with gold as a catalyst allows complete omission of the stem, thus demonstrating that InSb NWs can grow directly from the substrate. The introduction of the selective-area mask gives rise to novel growth kinetics, demonstrating high growth rates and complete suppression of layer deposition on the mask for Sb-rich conditions. The crystal quality and chemical purity of these NWs is reflected in the significant enhancement of low-temperature electron mobility, yielding an average of 4.4 × 104 cm2/(V s), compared to previously studied InSb NWs grown on stems.

Phonon Engineering in Twinning Superlattice Nanowires.

One of the current challenges in nanoscience is tailoring the phononic properties of a material. This has long been a rather elusive task because several phonons have wavelengths in the nanometer range. Thus, high quality nanostructuring at that length-scale, unavailable until recently, is necessary for engineering the phonon spectrum. Here we report on the continuous tuning of the phononic properties of a twinning superlattice GaP nanowire by controlling its periodicity. Our experimental results, based on Raman spectroscopy and rationalized by means of ab initio theoretical calculations, give insight into the relation between local crystal structure, overall lattice symmetry, and vibrational properties, demonstrating how material engineering at the nanoscale can be successfully employed in the rational design of the phonon spectrum of a material.

Hexagonal silicon grown from higher order silanes.

We demonstrate the merits of an unexplored precursor, tetrasilane (Si4H10), as compared to disilane (Si2H6) for the growth of defect-free, epitaxial hexagonal silicon (Si). We investigate the growth kinetics of hexagonal Si shells epitaxially around defect-free wurtzite gallium phosphide (GaP) nanowires. Two temperature regimes are identified, representing two different surface reaction mechanisms for both types of precursors. Growth in the low temperature regime (415 °C-600 °C) is rate limited by interaction between the Si surface and the adsorbates, and in the high temperature regime (600 °C-735 °C) by chemisorption. The activation energy of the Si shell growth is 2.4 ± 0.2 eV for Si2H6 and 1.5 ± 0.1 eV for Si4H10 in the low temperature regime. We observe inverse tapering of the Si shells and explain this phenomenon by a basic diffusion model where the substrate acts as a particle sink. Most importantly, we show that, by using Si4H10 as a precursor instead of Si2H6, non-tapered Si shells can be grown with at least 50 times higher growth rate below 460 °C. The lower growth temperature may help to reduce the incorporation of impurities resulting from the growth of GaP.

Spin-Orbit Interaction and Induced Superconductivity in a One-Dimensional Hole Gas.

Low dimensional semiconducting structures with strong spin-orbit interaction (SOI) and induced superconductivity attracted great interest in the search for topological superconductors. Both the strong SOI and hard superconducting gap are directly related to the topological protection of the predicted Majorana bound states. Here we explore the one-dimensional hole gas in germanium silicon (Ge-Si) core-shell nanowires (NWs) as a new material candidate for creating a topological superconductor. Fitting multiple Andreev reflection measurements shows that the NW has two transport channels only, underlining its one-dimensionality. Furthermore, we find anisotropy of the Landé g-factor that, combined with band structure calculations, provides us qualitative evidence for the direct Rashba SOI and a strong orbital effect of the magnetic field. Finally, a hard superconducting gap is found in the tunneling regime and the open regime, where we use the Kondo peak as a new tool to gauge the quality of the superconducting gap.

Twofold origin of strain-induced bending in core-shell nanowires: the GaP/InGaP case.

Nanowires have emerged as a promising platform for the development of novel and high-quality heterostructures at large lattice misfit, inaccessible in a thin film configuration. However, despite core-shell nanowires allowing a very efficient elastic release of the misfit strain, the growth of highly uniform arrays of nanowire heterostructures still represents a challenge, for example due to a strain-induced bending morphology. Here we investigate the bending of wurtzite GaP/In x Ga1-x P core-shell nanowires using transmission electron microscopy and energy dispersive x-ray spectroscopy, both in terms of geometric and compositional asymmetry with respect to the longitudinal axis. We compare the experimental data with finite element method simulations in three dimensions, showing that both asymmetries are responsible for the actual bending. Such findings are valid for all lattice-mismatched core-shell nanowire heterostructures based on ternary alloys. Our work provides a quantitative understanding of the bending effect in general while also suggesting a strategy to minimise it.

Single-Crystalline Hexagonal Silicon-Germanium.

Group IV materials with the hexagonal diamond crystal structure have been predicted to exhibit promising optical and electronic properties. In particular, hexagonal silicon-germanium (Si1-xGex) should be characterized by a tunable direct band gap with implications ranging from Si-based light-emitting diodes to lasers and quantum dots for single photon emitters. Here we demonstrate the feasibility of high-quality defect-free and wafer-scale hexagonal Si1-xGex growth with precise control of the alloy composition and layer thickness. This is achieved by transferring the hexagonal phase from a GaP/Si core/shell nanowire template, the same method successfully employed by us to realize hexagonal Si. We determine the optimal growth conditions in order to achieve single-crystalline layer-by-layer Si1-xGex growth in the preferred stoichiometry region. Our results pave the way for exploiting the novel properties of hexagonal Si1-xGex alloys in technological applications.