Nanowires are filamentary crystals with a tailored diameter that can be obtained using a plethora of different synthesis techniques. In this review, we focus on the vapor phase, highlighting the most ...influential achievements along with a historical perspective. Starting with the discovery of VLS, we feature the variety of structures and materials that can be synthesized in the nanowire form. We then move on to establish distinct features such as the three-dimensional heterostructure/doping design and polytypism. We summarize the status quo of the growth mechanisms, recently confirmed by in situ electron microscopy experiments and defining common ground between the different synthesis techniques. We then propose a selection of remaining defects, starting from what we know and going toward what is still to be learned. We believe this review will serve as a reference for neophytes but also as an insight for experts in an effort to bring open questions under a new light.
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Two-dimensional (2D) materials are a new type of materials under intense study because of their interesting physical properties and wide range of potential applications from nanoelectronics to ...sensing and photonics. Monolayers of semiconducting transition metal dichalcogenides MoS2 or WSe2 have been proposed as promising channel materials for field-effect transistors. Their high mechanical flexibility, stability, and quality coupled with potentially inexpensive production methods offer potential advantages compared to organic and crystalline bulk semiconductors. Due to quantum mechanical confinement, the band gap in monolayer MoS2 is direct in nature, leading to a strong interaction with light that can be exploited for building phototransistors and ultrasensitive photodetectors. Here, we report on the realization of light-emitting diodes based on vertical heterojunctions composed of n-type monolayer MoS2 and p-type silicon. Careful interface engineering allows us to realize diodes showing rectification and light emission from the entire surface of the heterojunction. Electroluminescence spectra show clear signs of direct excitons related to the optical transitions between the conduction and valence bands. Our p–n diodes can also operate as solar cells, with typical external quantum efficiency exceeding 4%. Our work opens up the way to more sophisticated optoelectronic devices such as lasers and heterostructure solar cells based on hybrids of 2D semiconductors and silicon.
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We show that optically active quantum dots (QDs) embedded in MBE-grown GaAs/AlGaAs core–shell nanowires (NWs) are coupled to the NW mechanical motion. Oscillations of the NW modulate the QD emission ...energy in a broad range exceeding 14 meV. Furthermore, this opto-mechanical interaction enables the dynamical tuning of two neighboring QDs into resonance, possibly allowing for emitter–emitter coupling. Both the QDs and the coupling mechanism, i.e. material strain, are intrinsic to the NW structure and do not depend on any functionalization or external field. Such systems open up the prospect of using QDs to probe and control the mechanical state of a NW, or conversely of making a quantum nondemolition readout of a QD state through a position measurement.
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In semiconductor nanowires, the coexistence of wurtzite and zinc-blende phases enables the engineering of the electronic structure within a single material. This presupposes an exact knowledge of the ...band structure in the wurtzite phase. We demonstrate that resonant Raman scattering is a important tool to probe the electronic structure of novel materials. Exemplarily, we use this technique to elucidate the band structure of wurtzite GaAs at the Γ point. Within the experimental uncertainty we find that the free excitons at the edge of the wurtzite and the zinc-blende band gap exhibit equal energies. For the first time we show that the conduction band minimum in wurtzite GaAs is of Γ7 symmetry, meaning a small effective mass. We further find evidence for a light-hole–heavy-hole splitting of 103 meV at 10 K.
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Self-assembled nanowire (NW) crystals can be grown into nearly defect-free nanomechanical resonators with exceptional properties, including small motional mass, high resonant frequency and low ...dissipation. Furthermore, by virtue of slight asymmetries in geometry, a NW's flexural modes are split into doublets oscillating along orthogonal axes. These characteristics make bottom-up grown NWs extremely sensitive vectorial force sensors. Here, taking advantage of its adaptability as a scanning probe, we use a single NW to image a sample surface. By monitoring the frequency shift and direction of oscillation of both modes as we scan above the surface, we construct a map of all spatial tip-sample force derivatives in the plane. Finally, we use the NW to image electric force fields distinguishing between forces arising from the NW charge and polarizability. This universally applicable technique enables a form of atomic force microscopy particularly suited to mapping the size and direction of weak tip-sample forces.
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With the continued maturation of III–V nanowire research, expectations of material quality should be concomitantly raised. Ideally, III–V nanowires integrated on silicon should be entirely free of ...extended planar defects such as twins, stacking faults, or polytypism, position-controlled for convenient device processing, and gold-free for compatibility with standard complementary metal–oxide–semiconductor (CMOS) processing tools. Here we demonstrate large area vertical GaAs x Sb1–x nanowire arrays grown on silicon (111) by molecular beam epitaxy. The nanowires’ complex faceting, pure zinc blende crystal structure, and composition are mapped using characterization techniques both at the nanoscale and in large-area ensembles. We prove unambiguously that these gold-free nanowires are entirely twin-free down to the first bilayer and reveal their three-dimensional composition evolution, paving the way for novel infrared devices integrated directly on the cost-effective Si platform.
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Carbon nanostructures that feature two-dimensional extended nanosheets are important components for technological applications such as high-performance composites, lithium-ion storage, photovoltaics ...and nanoelectronics. Chemical functionalization would render such structures better processable and more suited for tailored applications, but typically this is precluded by the high temperatures needed to prepare the nanosheets. Here, we report direct access to functional carbon nanosheets of uniform thickness at room temperature. We used amphiphiles that contain hexayne segments as metastable carbon precursors and self-assembled these into ordered monolayers at the air/water interface. Subsequent carbonization by ultraviolet irradiation in ambient conditions resulted in the quantitative carbonization of the hexayne sublayer. Carbon nanosheets prepared in this way retained their surface functionalization and featured an sp(2)-rich amorphous carbon structure comparable to that usually obtained on annealing above 800 °C. Moreover, they exhibited a molecularly defined thickness of 1.9 nm, were mechanically self-supporting over several micrometres and had macroscopic lateral dimensions on the order of centimetres.
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Nanowire (NW) crystal growth via the vapour-liquid-solid mechanism is a complex dynamic process involving interactions between many atoms of various thermodynamic states. With increasing speed over ...the last few decades many works have reported on various aspects of the growth mechanisms, both experimentally and theoretically. We will here propose a general continuum formalism for growth kinetics based on thermodynamic parameters and transition state kinetics. We use the formalism together with key elements of recent research to present a more overall treatment of III-V NW growth, which can serve as a basis to model and understand the dynamical mechanisms in terms of the basic control parameters, temperature and pressures/beam fluxes. Self-catalysed GaAs NW growth on Si substrates by molecular beam epitaxy is used as a model system.
Diamond is one of the most promising materials for high power and extreme conditions electronics. For this to become a reality, incorporation of active dopants needs to be understood. In this article ...we demonstrate how cathodoluminescence spectroscopy can be used to quantify and spatially map the boron concentration in diamond. We achieve this by probing exciton dynamics outside the steady state using ultrafast electron pulses and time-resolved spectroscopy. The capture lifetime of free excitons by boron allows us to measure the impurity concentration directly, and is selectively sensitive to electrically active dopants. In addition, we update the value of the Auger lifetime of boron-bound excitons in diamond to 185±2 ps. We study different regimes of free and bound excitons dynamics by characterizing different growth sectors of the crystal, with boron concentration varying between 2.8⋅1016 and 4.7⋅1017cm−3. At higher doping levels, a regime of free-exciton-diffusion-limited relaxation is reached. Overall, this study provides new prospects for the characterization of doping in diamond, by allowing to study impurities incorporation and activation at the microscale and the impact of crystalline defects on the electrical properties, non-destructively and in a self-consistent way.
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Topological qubits based on Majorana Fermions have the potential to revolutionize the emerging field of quantum computing by making information processing significantly more robust to decoherence. ...Nanowires are a promising medium for hosting these kinds of qubits, though branched nanowires are needed to perform qubit manipulations. Here we report a gold-free templated growth of III–V nanowires by molecular beam epitaxy using an approach that enables patternable and highly regular branched nanowire arrays on a far greater scale than what has been reported thus far. Our approach relies on the lattice-mismatched growth of InAs on top of defect-free GaAs nanomembranes yielding laterally oriented, low-defect InAs and InGaAs nanowires whose shapes are determined by surface and strain energy minimization. By controlling nanomembrane width and growth time, we demonstrate the formation of compositionally graded nanowires with cross-sections less than 50 nm. Scaling the nanowires below 20 nm leads to the formation of homogeneous InGaAs nanowires, which exhibit phase-coherent, quasi-1D quantum transport as shown by magnetoconductance measurements. These results are an important advance toward scalable topological quantum computing.
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