Nanowire Axis Research Articles

Selective area epitaxy of in-plane HgTe nanostructures on CdTe(001) substrate

Semiconductor nanowires (NWs) are believed to play a crucial role for future applications in electronics, spintronics and quantum technologies. A potential candidate is HgTe but its sensitivity to nanofabrication processes restrain its development. A way to circumvent this obstacle is the selective area growth technique. Here, in-plane HgTe nanostructures are grown thanks to selective area molecular beam epitaxy on a semi-insulating CdTe substrate covered with a patterned SiO2mask. The shape of these nanostructures is defined by the in-plane orientation of the mask aperture along the <110>, <11¯0>, or <100> direction, the deposited thickness, and the growth temperature (GT). Several micron long in-plane NWs can be achieved as well as more complex nanostructures such as networks, diamond structures or rings. A good selectivity is achieved with very little parasitic growth on the mask even for a GT as low as 140 °C and growth rate up to 0.5 monolayer per second. For <110> oriented NWs, the center of the nanostructure exhibits a trapezoidal shape with {111}B facets and two grains on the sides, while <11¯0> oriented NWs show {111}A facets with adatoms accumulation on the sides of the top surface. Transmission electron microscopy observations reveal a continuous epitaxial relation between the CdTe substrate and the HgTe NW. Measurements of the resistance with four-point scanning tunneling microscopy indicates a good electrical homogeneity along the main NW axis and a thermally activated transport. This growth method paves the way toward the fabrication of complex HgTe-based nanostructures for electronic transport measurements.

Detection of an unintentional Si doping gradient in site-controlled GaN nanowires grown using a Si3N4 mask by spatially resolved cathodoluminescence and Raman spectroscopy

Highly uniform arrays of site-controlled GaN nanowires are synthesized by selective area growth using a Si3N4 mask and molecular beam epitaxy. Systematic modulation of the emission along the nanowire axis is observed in spectrally resolved cathodoluminescence linescans. We show that this intensity change is an indicator of unintentional Si incorporation during growth resulting from the interaction between the impinging Ga atoms and the mask material. The gradual reduction of the cathodoluminescence intensity along the nanowire highlights the important role of the growth geometry within the synthesis reactor, with shadowing from the elongating nanowires inhibiting the reaction with the mask. This gradient in Si doping is confirmed by the quenching of the longitudinal optical phonon line measured in Raman spectra along the nanowire axis. The corresponding carrier density is derived from the frequency of the coupled phonon–plasmon mode. The spectroscopic identification of inversion domain boundaries in the majority of the nanowires is also attributed to the Si incorporation. From temperature dependent cathodoluminescence experiments, we derive the activation energy for excitons bound to these defects.

Intrinsic multiferroicity in molybdenum oxytrihalides nanowires

Low-dimensional multiferroics, which simultaneously possess at least two primary ferroic order parameters, hold great promise for post-Moore electronic devices. However, intrinsic one-dimensional (1D) multiferroics with the coexistence of ferroelectricity and ferromagnetism are still yet to be realized, which will be not only crucial for exploring the interplay between low-dimensionality and ferroelectric/ferromagnetic ordering but also significant in rendering application approaches for high density information technologies. Here, we present a theoretical prediction of intrinsic multiferroicity in 1D molybdenum oxytrihalides nanowires, especially focusing on MoOBr3 nanowires which could be readily extracted from experimentally synthesized van der Waals MoOBr3 bulk materials. Due to the spatial inversion symmetry spontaneously broken by Mo atoms’ displacements, MoOBr3 nanowires exhibit 1D ferroelectricity with small coercive electric field and exceptional Curie temperature (~570 K). Additionally, MoOBr3 nanowires also possess 1D antiferroelectric metastable states. On the other hand, both ferroelectric and antiferroelectric MoOBr3 nanowires exhibit ferromagnetic ordering on account of the half-filled Mo-dyz orbitals, a moderate tensile strain (~5%) can greatly boost the spontaneous polarization (~40%) and a mild compress strain (~−2%) may readily switch the magnetic easy axis of ferroelectric MoOBr3 nanowires. Our work holds potential candidates for developing innovative devices that exploit intrinsic multiferroic properties, enabling advancements in novel electronic and spintronic applications.

Geometry-induced Bloch point domain wall in Permalloy conical frustum nanowires for advanced spintronics applications

In this study, we investigate the pseudo-static magnetic properties of Permalloy conical frustum nanowires using micromagnetic simulations. We thoroughly examine how both the major and minor radii influence the magnetic reversal mechanism when an external magnetic field is applied parallel to the nanowire axis. The obtained results show that under specific geometrical conditions, magnetization reverts though a Bloch point-type domain wall. In these cases, hysteresis curves exhibit two Barkhausen jumps during magnetization reversal, forming a plateau field range in which a Bloch point domain wall nucleates and propagates until its annihilation after the second Barkhausen jump. The nucleation of a Bloch point domain wall in a frustum conical nanowire geometry is reported. These findings highlight the significance of this geometry in nucleating these attractive topological defects for promising applications.

Axial Growth Characteristics of Optically Active InGaAs Nanowire Heterostructures for Integrated Nanophotonic Devices.

III-V semiconductor nanowire (NW) heterostructures with axial InGaAs active regions hold large potential for diverse on-chip device applications, including site-selectively integrated quantum light sources, NW lasers with high material gain, as well as resonant tunneling diodes and avalanche photodiodes. Despite various promising efforts toward high-quality single or multiple axial InGaAs heterostacks using noncatalytic growth mechanisms, the important roles of facet-dependent shape evolution, crystal defects, and the applicability to more universal growth schemes have remained elusive. Here, we report the growth of optically active InGaAs axial NW heterostructures via completely catalyst-free, selective-area molecular beam epitaxy directly on silicon (Si) using GaAs(Sb) NW arrays as tunable, high-uniformity growth templates and highlight fundamental relationships between structural, morphological, and optical properties of the InGaAs region. Structural, compositional, and 3D-tomographic characterizations affirm the desired directional growth along the NW axis with no radial growth observed. Clearly distinct luminescence from the InGaAs active region is demonstrated, where tunable array-geometry parameters and In content up to 20% are further investigated. Based on the underlying twin-induced growth mode, we further describe the facet-dependent shape and interface evolution of the InGaAs segment and its direct correlation with emission energy.

Axially lattice-matched wurtzite/rock-salt GaAs/Pb1−xSnxTe nanowires

We investigate the full and half-shells of Pb1−xSnxTe topological crystalline insulator deposited by molecular beam epitaxy on the sidewalls of wurtzite GaAs nanowires (NWs). Due to the distinct orientation of the IV–VI shell with respect to the III–V core the lattice mismatch between both materials along the nanowire axis is less than 4%. The Pb1−xSnxTe solid solution is chosen due to the topological crystalline insulator properties above some critical concentrations of Sn (x ≥ 0.36). The IV–VI shells are grown with different compositions spanning from binary SnTe, through Pb1−xSnxTe with decreasing x value down to binary PbTe (x = 0). The samples are analysed by scanning transmission electron microscopy, which reveals the presence of (110) or (100) oriented binary PbTe and (100) Pb1−xSnxTe on the sidewalls of wurtzite GaAs NWs.

Symmetry-dependent domain wall propagation in triangular nanowires

Magnetic domain walls (DW) are interesting for several technologies requiring perfect control of their propagation along a nanostructure. Therefore, it is necessary to better understand their dynamics. In this paper, using micromagnetic simulations, we analyze the DW propagation along nanowires (NWs) with triangular cross-section, clarifying the role of cross-section symmetry on the DW velocity. Our results evidence that the lowest DW velocity occurs for equilateral triangles. Such behavior is strongly associated with changes in the magnetic energy of the system during DW rotation around the NW axis. Since fully regular cross-sections cannot be easily fabricated, studying the DW dynamics in NWs with non-regular cross-sections could be useful for experimental works.

(Invited) Nanowires from Dilute Nitride and Dilute Bismide Alloys for Nanophotonics

GaAs-based nanowires (NWs) have a great potential for applications in photonics and optoelectronics, partly owing to flexibility in band structure engineering via alloying, strain and structural polymorphism. Among promising GaAs-based materials for device applications are highly mismatched GaNAs and GaAsBi alloys. The giant bowing in the bandgap energy characteristic for these materials allows easy tuning of emission wavelengths, while other modifications of their electronic structure promise to improve performance of infrared lasers and solar cells. In this talk we will review our recent results related to the electronic properties of novel GaNAs and GaAsBi NWs and provide several examples highlighting their advantages.First of all, we will show that a GaNAs-based mutishell NW can be used as an efficient coherent photon source, where the NW acts both as a miniaturized optical resonator and as a photonic gain medium. Its wavelength can be efficiently tuned by minor changes in N composition, e.g. down to 1 µm with only 2.5 % of nitrogen [1]. The lasing mode is found to arise from the fundamental HE11a mode of the Fabry-Perot cavity from a single NW, exhibiting optical polarization along the NW axis. Moreover, due to superior nonlinear properties of GaNAs, self-frequency conversion of the stimulated emission through second harmonic generation and sum-frequency generation is observed, providing coherent light emission in the cyan-green range.Secondly, we will demonstrate that alloying of GaAs with N and/or Bi leads to the formation of self-assembled quantum dots (QDs) embedded in NWs and, therefore, is an efficient way to fabricate hybrid QD-NW structures with high optical quality. In the case of dilute nitrides, optically active and highly localized QD states are formed due to local fluctuations in N composition [2], whereas in dilute bismides they are caused by Bi segregation at twin planes [3]. Such QDs act as single photon emitters, promising for application in quantum communication technologies.And finally, we will show that favorable band alignment at the interface between a dilute nitride alloy and parental N-free material leads to very efficient energy upconversion due to two-step two-photon absorption in the related NW heterostructures, with the efficiency of this process being among the highest reported in semiconductor nanostructures. Specifically, we show [4] that, in radial GaAs(P)/GaNAs(P) core/shell NW heterostructures, the upconversion efficiency increases by 500 times as compared with that of the constituent materials, even under an excitation power as low as 100 mW/cm2 that is comparable to the one-sun illumination. The upconversion efficiency can be further improved by 8 times through engineering the electric-field distribution of the excitation light inside the NWs so that light absorption is maximized within the desired region of the heterostructure.

Magnetization processes in two-dimensional arrays of iron nanowires

Arrays of iron nanowires (NWs) obtained by template-assisted electrodeposition constitute a promising composite material characterized by a combination of high magnetization in the filler and perpendicular magnetic anisotropy. The properties of these composites arise from the interplay between the behavior of individual NWs and their magnetostatic interactions. In this study, we investigated NW arrays with identical wire diameters but varying spatial arrangements. Major hysteresis loops were studied under various field directions relative to the NW axis. Key parameters such as the slope of the magnetization curve, saturation magnetization, and coercive force were quantified. Additionally, FORC (First Order Reversal Curve) measurements were conducted with the field oriented longitudinally with respect to the NW, offering insights into the inhomogeneity of the demagnetizing field influenced by the NW array's configuration. In the sample with the highest NW density, we observed isotropic behavior of the effective demagnetizing field, and we proposed an explanation for this phenomenon using the effective media approach. Micromagnetic simulations revealed that the magnetic behavior of individual NWs with a 100 nm diameter can be described as an interchange between volumes characterized by vortex and uniform magnetization patterns. Calculations of the demagnetizing field using the effective medium model demonstrated excellent agreement with experimental data across arrays featuring different NW densities. Remarkably, the quantitative consistency of coercive field values obtained from micromagnetic simulations and experimental measurements in the range of angles from 0° to 45° for the studied samples underscores the structural homogeneity of the obtained NWs.

Nonlinear Optical Response of a Plasmonic Nanoantenna to Circularly Polarized Light: Rotation of Multipolar Charge Density and Near-Field Spin Angular Momentum Inversion.

The spin and orbital angular momentum carried by electromagnetic pulses open new perspectives to control nonlinear processes in light-matter interactions, with a wealth of potential applications. In this work, we use time-dependent density functional theory (TDDFT) to study the nonlinear optical response of a free-electron plasmonic nanowire to an intense, circularly polarized electromagnetic pulse. In contrast to the well-studied case of the linear polarization, we find that the nth harmonic optical response to circularly polarized light is determined by the multipole moment of order n of the induced nonlinear charge density that rotates around the nanowire axis at the fundamental frequency. As a consequence, the frequency conversion in the far field is suppressed, whereas electric near fields at all harmonic frequencies are induced in the proximity of the nanowire surface. These near fields are circularly polarized with handedness opposite to that of the incident pulse, thus producing an inversion of the spin angular momentum. An analytical approach based on general symmetry constraints nicely explains our numerical findings and allows for generalization of the TDDFT results. This work thus offers new insights into nonlinear optical processes in nanoscale plasmonic nanostructures that allow for the manipulation of the angular momentum of light at harmonic frequencies.

Magnetic properties of CoNi nanowires and nanotubes: Insights from micromagnetic simulations and composition-dependent equations

We conducted micromagnetic simulations of CoNi nanowires and nanotubes to study their magnetic properties as a function of their composition. We found that the hysteresis curves for isolated CoNi nanowires exhibit a square trend with an easy axis of magnetization along the nanowire axis. The coercivity and normalized remanence show a U-shaped behavior, with maximum values at Ni and Co and minimum values for the Co40Ni60 alloy. The hysteresis curves for CoNi nanotubes maintain the square trend of nanowires, but with a clear decrease in coercivity and an asymmetric V-shaped behavior for coercivity and normalized remanence. Our research offers valuable and comprehensive insights into the magnetic properties of CoNi nanowires and nanotubes. It underscores the significant influence of composition on magnetic properties, which holds great relevance for potential applications. Our approach, involving the prediction of static magnetic properties through micromagnetic simulations and tailored equations designed for nanostructures, rather than relying on bulk parameters, aligns more closely with the nature of these physical systems. This alignment is validated by the strong agreement with previously reported experimental results. Our work lays a solid foundation for the deliberate design of CoNi nanowires and nanotubes with specific magnetic properties tailored for articular applications. We anticipate that our study will facilitate the development of innovative magnetic nanostructures with controllable properties and enhanced performance.

Omega‐Gate Silicon Nanowire Geometric Diodes with Reconfigurable Self‐Switching Operation and THz Rectification

AbstractGeometric diodes (GDs) represent a relatively unconventional class of diode that produces an asymmetric current response through carrier transport in an asymmetric geometry. Synthesized from the bottom up, Si nanowire‐based GDs are three‐dimensional, cylindrically symmetric nanoscale versions capable of room‐temperature rectification at GHz‐THz frequencies with near zero‐bias turn‐on voltages. Here, by fabricating three‐terminal n‐type Si nanowire GDs with axial contacts and an omega‐gate electrode, a distinct class of reconfigurable self‐switching geometric diodes (SSGDs) is reported. Single‐nanowire SSGD device measurements demonstrate a significant dependence of diode current and polarity on gate potential, where the diode polarity reverses at a gate potential of ≈−1 V under specific grounding conditions. Finite‐element modeling reproduces the experimental results and reveals that the gate potential—in combination with the morphology and dopant profile—produces an asymmetric potential along the nanowire axis that changes asymmetrically with axial bias, altering the effective conductive channel within the nanowire to yield diode behavior. The self‐switching effect is retained in two‐terminal SSGD devices, and modeling demonstrates that both three‐terminal and two‐terminal devices support rectification through THz frequencies. The results reveal a new mechanism of operation for nanowire‐based GDs and characterize a new type of self‐switching diode with reconfigurable polarity.

Low Energy Twist Defects in Pentatwinned Silver Nanowires

Transmission electron microscopy investigation of networks of pentatwinned silver nanowires has identified the presence of nanoscale defects within individual wires, showing narrow regions of banded contrast normal to the nanowire axis (bamboo faults). Structural analysis using machine learning decomposition of scanning precession electron diffraction data identifies these bamboo faults as a pair of twist boundaries normal to the wire axis creating low energy coincident site lattices between the faulted region and the parent nanowire. This leads to a conservation of the relative misorientation of the twinned structure within the faulted region, leading to an apparent local rotation of the nanowire. Their presence after spraying and nonthermal processing suggests that they may form during nanowire synthesis. However, examination of the networks before and after cyclic straining finds an increase in the density of the defects, indicating that they may also form as a result of mechanical deformation.

Fabrication of transparent TiO2-GO nanocomposites deposited on vertically aligned ZnO nanowire arrays as hybrid nanostructured photo-anode for dye-sensitized solar cells

In this article, we report the fabrication of hybrid nanostructures comprised of vertically aligned Zinc Oxide (ZnO) nanowires (NWs) coated with titanium dioxide-graphene oxide (TiO2-GO) nanocomposites by easy, cost-effective, chemical method and demonstrate their utilization as photo-anode for dye-sensitized solar cells (DSSCs). A thin layer of transparent TiO2 with varying GO amount is deposited on top of vertical ZnO NW arrays by sol-gel spin coating method. A conventional DSSC structure is fabricated by using hybrid structure as photo-anode, ruthenium based N3 dye as photosensitizers, iodine solution (I−/I3+) as electrolyte and fluorine-doped tin oxide (FTO) coated glass substrate as counter electrode. The fabricated hybrid nanostructure based DSSC exhibits enhanced power conversion efficiency (η) compared to that of pristine ZnO NW photo-anode. Also, other characteristic parameters of DSSCs like open-circuit voltage (Voc), short-circuit current density (Jsc) and fill factor (FF) are estimated from the current density-voltage (J-V) relation measured under illumination of 1 sun (100 mW/cm2) solar spectrum for ZnO NW/TiO2-GO hybrid nanostructures. Improvement in these obtained parameters is observed with increasing GO amount in the TiO2-GO nanocomposite layers deposited on top of ZnO NW arrays. Best performance is delivered by the hybrid nanostructure consists of ZnO NW/TiO2-GO (10%)with optimized value of η ∼0.72% along with Voc ∼491 mV, Jsc ∼2.19 mA/cm2 and FF ∼0.66. This betterment in performance is attributed to the combined effects of improvement in charge transport through the axis of ZnO NWs, increment in contact area and modulation of conductivity of TiO2 by inclusion of conducting GO network. The modulation in charge transport properties of hybrid nanostructures with varying GO amount is studied by complex impedance spectroscopic analyses. The study indicates that particular amount of GO significantly modifies the interfacial effects in fabricated hybrid nanostructures to facilitate better charge transportation.

Simulation of GaAs nanowire annealing

The GaAs nanowire (NW) morphology transformation during a high temperature annealing of densely packed wires was analyzed by Monte Carlo simulation. For long densely spaced NWs, the arsenic evaporation with its removal from the system occurs only in the upper NW part at such distance that As2 molecules can overcome due to the channeling between adjacent NWs before reaching vacuum. In the lower NW part, two oppositely directed processes proceed: destruction and restoration of the NW side wall due to the readsorbed material. Thus, the lower part of wire evaporation is close to a quasi-equilibrium process, and the upper part is close to free evaporation into vacuum. An excess of gallium primarily occurs at the top part of the crystal, since it is in the conditions close to a free arsenic evaporation. An excess gallium gathering leads to the Ga droplet formation. Droplets in the middle part of the NW side walls are formed later. A droplet-free region is formed on the sidewall of sufficiently long NWs with a length>345 nm and the distance of 28 nm between the wires after the annealing at T = 890 K. That means that the congruent evaporation temperature varies along the NW axis due to the variation of arsenic pressure, resulting in the congruent evaporation from the lower NW region and incongruent evaporation from the upper wire region. Simulation results predict the dendrite (thin nanowire branching off original wires) growth during a prolonged annealing under incongruent conditions.

Analysis of electromechanical couplings and nonlinear carrier transport in flexoelectric semiconductors

In recent years, a linearization method has been extensively employed to investigate the electromechanical fields and carrier distribution in flexoelectric semiconductors, where the assumption of a small perturbation of carrier concentration is adopted. However, this method fails to accurately describe the realistic physical process in which a considerable variation of carrier concentration takes place. Based on fully coupled nonlinear equations, this paper presents a finite element approach to study the electromechanical couplings and nonlinear carrier transport in flexoelectric semiconductors. This method is applied to calculate the electrostatic potential in a bent piezoelectric semiconductive nanowire (NW) going beyond simple considerations and to simulate the nonlinear current–voltage (I–V) characteristics of a mechanically loaded flexoelectric p–n junction. The results indicate that the inherently nonlinear drift of carriers gives rise to the asymmetric distribution of the electric potential relative to the NW axis in the upper body. Flexoelectricity brings about a remarkable enhancement in output voltage and is responsible for the linear variation of electric potential along the length direction of the NW unless close to two ends. Furthermore, the barrier height and I–V relations of a flexoelectric p–n junction can be effectively tuned by mechanical forces due to the flexoelectric effect, the effect of which relies on the size of the p–n junction configuration. This work is a good starting point to comprehend the coupling of flexoelectricity and nonlinear carrier transport in static and dynamic cases, and offers an effective approach to numerically deal with the issues involved in flexoelectronics and piezoelectronics at the nanoscale.

Effect of electromigration on onset of morphological instability of a nanowire

Solid cylindrical nanowires are vulnerable to a Rayleigh–Plateau-type morphological instability. The instability results in a wire breakup, followed by formation of a chain array of spherical nanoparticles. In this paper, a base model of a morphological instability of a nanowire on a substrate in the applied electric field directed along a nanowire axis is considered. Exact analytical solution is obtained for $$90^\circ $$ contact angle and, assuming axisymmetric perturbations, for a free-standing wire. The latter solution extends the 1965 result by Nichols and Mullins without electromigration effect (Nichols and Mullins in Surface-(Interface-) and volume-diffusion contributions to morphological changes driven by capillarity. Trans Metall Soc AIME 233:1840–1848, 1965). For general contact angles the neutral stability is determined numerically. It is shown that a stronger applied electric field (a stronger current) results in a larger instability growth rate and a decrease of the most dangerous unstable wavelength; in experiment, the latter is expected to yield more dense chain array of nanoparticles. Also it is noted that a wire crystallographic orientation on a substrate has larger impact on stability in a stronger electric field and that a simple switching of the polarity of electrical contacts, i.e. the reversal of the direction of the applied electric field, may suppress the instability development and thus a wire breakup would be prevented. A critical value of the electric field that is required for such wire stabilization is obtained.

Plasmon resonances in silicon nanowires: geometry effects on the trade-off between dielectric and metallic behaviour

Surface plasmons (SP) arising from nanometer silicon objects allow control and manipulation of light at the nanoscale exhibiting significant advantages in a plethora of applied research areas such as nanophotonic, environment, energy, biology, and medicine. These SP can achieve more significant potential, thanks to the industrial scalability and low cost offered by silicon compared with other metals and semiconductor nanosized materials. However, as they have not yet been fully understood and exploited, silicon’s plasmon mechanisms need to be thoroughly studied. In particular, the influence of nanowire shape on surface plasmon behavior and the existence of physical constraints for surface plasmon excitation remains to be fully understood. In a previous study, we have demonstrated that thanks to their anisotropic one-dimensional shape, silicon nanowires sustain two types of plasmon resonances, the longitudinal ones along the main nanowire axis, with harmonic behavior and the transversal resonance, which takes place along the diameter. We demonstrated our data on a particular set of sizes, 30 nm for the diameter and about 400 nm for the length. Here we show how the resonances change when the diameter is smaller than 30 nm and the length is smaller than 400 nm. We use electron energy loss spectroscopy to map the several plasmonic modes from the fundamental one to the higher orders, with the goal of understanding how the SP resonances change when the diameter and length are smaller than 30 nm and 400 nm, respectively. We then use modeling to support the experimental findings. According to the mode order, the study illustrates the various locations inside the nanowires where discrete resonance spots can be found. Another important finding of this work is the disappearance of the surface plasmon modes for nanowires shorter than a predetermined threshold for any diameter in the range investigated, showing that the nanowire length is a key factor in maintaining electron oscillations. With this finding, a crucial physical limit for this phenomenon in silicon is established.

Crystalline texture of cobalt nanowire arrays probed by the switching field distribution and FORC diagrams

The crystalline texture of arrays of low diameter Co nanowires (NWs) synthesized by electrodeposition using electrolytes with different acidities (pH in the range 2.0–6.6) was studied by the switching field distribution (SFD) and first order reversal curve (FORC) diagrams. Particularly, the SFD determined as the derivative of the descending part of the major hysteresis loop has proven to be a reliable and powerful method for the identification of different crystalline textures in the NWs and the quantification of their corresponding texture percentages. The presence of the face centered cubic-like texture at low pH values and hexagonal close packed textures with the c-axis perpendicular and parallel to the NWs axis at higher pH values have been identified by performing multiple Gaussian fits to the SFD by virtue of their different magnetic behavior observed during reversal of the magnetization. The field position and size of each curve in the multiple Gaussian fit provide information about the corresponding magnetic contribution and volumetric texture percentage of each crystalline texture in the NWs, respectively. The analysis of the SFD has been complemented and validated with FORC diagram measurements, showing that the width of the coercive field distribution is in good agreement with the width of the SFD. Also, it has been found that the different branches observed in the FORC diagrams along the interaction axis provide further insight on the interaction between magnetocrystalline fields. This work provides a novel methodology for the use of magnetometry as a reliable technique for the study of the interplay between the microstructure and magnetic behavior of arrays of NWs.

Stacking Fault Induced Symmetry Breaking in van der Waals Nanowires.

While traditional ferroelectrics are based on polar crystals in bulk or thin film form, two-dimensional and layered materials can support mechanisms for symmetry breaking between centrosymmetric building blocks, e.g., by creating low-symmetry interfaces in van der Waals stacks. Here, we introduce an approach toward symmetry breaking in van der Waals crystals that relies on the spontaneous incorporation of stacking faults in a nonpolar bulk layer sequence. The concept is realized in nanowires consisting of Se-rich group IV monochalcogenide (GeSe1-xSx) alloys, obtained by vapor-liquid-solid growth. The single crystalline wires adopt a layered structure in which the nonpolar A-B bulk stacking along the nanowire axis is interrupted by single-layer stacking faults with local A-A' stacking. Density functional theory explains this behavior by a reduced stacking fault formation energy in GeSe (or Se-rich GeSe1-xSx alloys). Computations demonstrate that, similar to monochalcogenide monolayers, the inserted A-layers should show a spontaneous electric polarization with a switching barrier consistent with a Curie temperature above room temperature. Second-harmonic generation signals are consistent with a variable density of stacking faults along the wires. Our results point to possible routes for designing ferroelectrics via the layer stacking in van der Waals crystals.