Superlattice Nanowires Research Articles

On-Wire Design of Axial Periodic Halide Perovskite Superlattices for High-Performance Photodetection.

Precise synthesis of all-inorganic lead halide perovskite nanowire heterostructures and superlattices with designable modulation of chemical compositions is essential for tailoring their optoelectronic properties. Nevertheless, controllable synthesis of perovskite nanostructure heterostructures remains challenging and underexplored to date. Here, we report a rational strategy for wafer-scale synthesis of one-dimensional periodic CsPbCl3/CsPbI3 superlattices. We show that the highly parallel array of halide perovskite nanowires can be prepared roughly as horizontally guided growth on an M-plane sapphire. A periodic patterning of the sapphire substrate enables position-selective ion exchange to obtain highly periodic CsPbCl3/CsPbI3 nanowire superlattices. This patterning is further confirmed by micro-photoluminescence investigations, which show that two separate band-edge emission peaks appear at the interface of a CsPbCl3/CsPbI3 heterojunction. Additionally, compared with the pure CsPbCl3 nanowires, photodetectors fabricated using these periodic heterostructure nanowires exhibit superior photoelectric performance, namely, high ION/IOFF ratio (104), higher responsivity (49 A/W), and higher detectivity (1.51 × 1013 Jones). Moreover, a spatially resolved visible image sensor based on periodic nanowire superlattices is demonstrated with good imaging capability, suggesting promising application prospects in future photoelectronic imaging systems. All these results based on the periodic CsPbCl3/CsPbI3 nanowire superlattices provides an attractive material platform for integrated perovskite devices and circuits.

InAs–InP Superlattice Nanowires with Tunable Phonon Frequencies

AbstractThe control of heat conduction through the manipulation of phonons in solids is of fundamental interest and can be exploited in applications for thermoelectric conversion. In this context, the advent of novel semiconductor nanomaterials with high surface‐to‐volume ratio, e.g. nanowires, offer exciting perspectives, leading to significant leaps forwarding the efficiency of solid‐state thermoelectric converters after decades of stagnation. Beyond the high aspect ratio, the nanowire geometry offers unprecedented possibilities of materials combination and crystal phase engineering not achievable with 2D counterparts. In this work, the growth of long (up to 100 repetitions) wurtzite InAs/InP superlattice nanowires with homogeneous segment thicknesses is reported, with control down to the single digit of nanometer. By means of Raman scattering experiments, clear modifications of the phonon dispersion in superlattice nanowires are found, where both InAs‐like and InP‐like modes are present. The experimentally measured modes are well reproduced by density functional perturbation theory calculations. Remarkably, it is found that the phonon frequencies can be tuned by the superlattice periodicity, opening exciting perspectives for phonon engineering and thermoelectric applications.

GaAs/GaP Superlattice Nanowires for Tailoring Phononic Properties at the Nanoscale: Implications for Thermal Engineering.

The possibility to tune the functional properties of nanomaterials is key to their technological applications. Superlattices, i.e., periodic repetitions of two or more materials in one or more dimensions, are being explored for their potential as materials with tailor-made properties. Meanwhile, nanowires offer a myriad of possibilities to engineer systems at the nanoscale, as well as to combine materials that cannot be put together in conventional heterostructures due to the lattice mismatch. In this work, we investigate GaAs/GaP superlattices embedded in GaP nanowires and demonstrate the tunability of their phononic and optoelectronic properties by inelastic light scattering experiments corroborated by ab initio calculations. We observe clear modifications in the dispersion relation for both acoustic and optical phonons in the superlattices nanowires. We find that by controlling the superlattice periodicity, we can achieve tunability of the phonon frequencies. We also performed wavelength-dependent Raman microscopy on GaAs/GaP superlattice nanowires, and our results indicate a reduction in the electronic bandgap in the superlattice compared to the bulk counterpart. All of our experimental results are rationalized with the help of ab initio density functional perturbation theory (DFPT) calculations. This work sheds fresh insights into how material engineering at the nanoscale can tailor phonon dispersion and open pathways for thermal engineering.

Self-Oscillated Growth Formation of Standing Ultrathin Nanosheets out of Uniform Ge/Si Superlattice Nanowires

Self-oscillation is an intriguing and omnipresent phenomenon that governs a broad range of growth dynamics and formation of nanoscale periodic and delicate heterostructures. A self-oscillating growth phenomenon of catalyst droplets, consuming surface-coating a-Si/a-Ge bilayer, is exploited to accomplish a high-frequency alternating growth of ultrathin crystalline Si and Ge (c-Si/c-Ge) nano-slates, with Ge-rich layer thickness of 14–19 nm, embedded within a superlattice nanowire structure, with pre-known position and uniform channel diameter. A subsequent selective etching of the Ge-rich segments leaves a chain of ultrafine standing c-Si nanosheets down to ∼ 6 nm thick, without the use of any expensive high-resolution lithography and growth modulation control. A ternary-phase-competition model has been established to explain the underlying formation mechanism of this nanoscale self-oscillating growth dynamics. It is also suggested that these ultrathin nanosheets could help to produce ultrathin fin-channels for advanced electronics, or provide size-specified trapping sites to capture and position hetero nanoparticle for high-precision labelling or light emission.

Optoelectronic properties of Ga1−xAlxN superlattice nanowires tuned by multilayer Al composition via first-principles

Considering that the graded structure could generate an electric field, the structure and optoelectronic properties of [Formula: see text][Formula: see text]N superlattice nanowires are considered via first-principles. The structural stability and optoelectronic properties of single-component and component-graded nanowires are discussed. For [Formula: see text][Formula: see text]N superlattice nanowires, the formation energy decreases with increasing Al composition, resulting in a structure that tends to be stable. The [Formula: see text][Formula: see text]N superlattice model is more stable and the bond length changes more dramatically in the superlattice structure with bigger component divergence. The bandgap [Formula: see text] increases with increasing Al composition. The direct bandgap of [Formula: see text][Formula: see text]N superlattice nanowires is also affected by the nanowire sublayers. The absorption coefficient tends to increase with the increase of Al composition in the nanowires. These studies can serve as the basis for the preparation of ideal materials for deep ultraviolet photocathodes and improve the optoelectronic properties of deep ultraviolet photocathodes.

Insight into refractive index modulation as route to enhanced light coupling in semiconductor nanowires.

Recent developments in chemical processes to prepare single-crystalline nanowire (NW) superlattices (SLs) have discovered a range of unique nanophotonic properties. In particular, diameter-modulated silicon NW geometric SLs (GSLs) have shown their ability to produce complex interference effects through which enhanced light manipulation is achieved. Here, we re-imagine the origin of the complex interference effects occurring in shallow-modulated GSLs and present a refractive index modulation as a key deciding factor. We introduce the design of a NW refractive index SL (ISL), a hypothetical uniform-diameter NW in which the refractive index is periodically modulated, and explain the coupling effect between Mie resonance and bound guided state. We apply the ISL concept to other NW SL systems and suggest potential routes to bring substantial enhancements in lasing activities.

GaAs/GaP superlattice nanowires: growth, vibrational and optical properties.

Nanowire geometry allows semiconductor heterostructures to be obtained that are not achievable in planar systems, as in, for example, axial superlattices made of large lattice mismatched materials. This provides a great opportunity to explore new optical transitions and vibrational properties resulting from the superstructure. Moreover, superlattice nanowires are expected to show improved thermoelectric properties, owing to the dominant role of surfaces and interfaces that can scatter phonons more effectively, reducing the lattice thermal conductivity. Here, we show the growth of long (up to 100 repetitions) GaAs/GaP superlattice nanowires with different periodicities, uniform layer thicknesses, and sharp interfaces, realized by means of Au-assisted chemical beam epitaxy. By optimizing the growth conditions, we obtained great control of the nanowire diameter, growth rate, and superlattice periodicity, offering a valuable degree of freedom for engineering photonic and phononic properties at the nanoscale. As a proof of concept, we analyzed a single type of superlattice nanowire with a well-defined periodicity and we observed room temperature optical emission and new phonon modes. Our results prove that high-quality GaAs/GaP superlattice nanowires have great potential for phononic and optoelectronic studies and applications.

Effect of morphology on the phonon thermal conductivity in Si/Ge superlattice nanowires

We used nonequilibrium molecular dynamics to investigate the role of morphology in the phonon thermal conductivity of 〈100〉, 〈110〉, 〈111〉 and 〈112〉-oriented Si/Ge superlattice nanowires at 300 K. Such nanowires with 〈112〉 growth direction were found to possess the lowest values of the thermal conductivity [1.6 W/(m·K) for a Si and Ge segment thickness of ∼3 nm] due to the lowest average group velocity and highly effective {113} facets and Si/Ge(112) interface for phonon-surface and phonon-interface scattering, respectively. Comparison with homogeneous and core/shell Si and Ge nanowires showed that the superlattice morphology is the most efficient to suppress the thermal conductivity.

Enhancing the Coherent Phonon Transport in SiGe Nanowires with Dense Si/Ge Interfaces.

The manipulation of phonon transport with coherent waves in solids is of fundamental interest and useful for thermal conductivity design. Based on equilibrium molecular dynamics simulations and lattice dynamics calculations, the thermal transport in SiGe superlattice nanowires with a tuned Si/Ge interface density was investigated by using the core-shell and phononic structures as the primary stacking layers. It was found that the thermal conductivity decreased with the increase of superlattice period lengths (Lp) when Lp was larger than 4 nm. This is because introducing additional Si/Ge interfaces can enhance phonon scattering. However, when Lp<4 nm, the increased interface density could promote heat transfer. Phonon density-of-state analysis demonstrates that new modes between 10 and 14 THz are formed in structures with dense Si/Ge interfaces, which is a signature of coherent phonon transport as those modes do not belong to bulk Si or Ge. The density of the newly generated modes increases with the increase of interface density, leading to an enhanced coherent transport. Besides, with the increase of interface density, the energy distribution of the newly generated modes becomes more balanced on Si and Ge atoms, which also facilitates heat transfer. Our current work is not only helpful for understanding coherent phonon transport but also beneficial for the design of new materials with tunable thermal conductivity.

Controllable synthesis and crystal facet, composition and temperature dependent gas sensing properties of Sn1−xS-CdS superlattice nanowires with ultrafast response

Sn1−xS-CdS superlattice nanowires (NWs) with fast response and good selectivity in low-temperature operating environments for potential applications as gas sensing materials, were designed rationally and synthesized controllably. This work investigated in detail the dual selective gas sensing performance of Sn1−xS-CdS superlattice nanowires that crystal facets change with the tunable composition. The results demonstrated that the Sn0.38S-CdS superlattice NWs with the structure of (0 0 4) lattice plane of SnS growing epitaxially along the [0 0 2] crystallization direction of CdS. And the gas sensors based on Sn0.38S-CdS superlattice NWs displayed the best gas sensing properties, including ultrafast response to acetone with 1 s at 290 °C, and good selectivity to H2S with selectivity value of 29886% at 190 °C. The gas sensing mechanism was carefully investigated by the chemical stability of gas molecules and the adsorption energy on the different crystal planes, as well as the energy band structure of the heterostructure. The study highlights an efficient rout to the development of high-performance multi-functional gas sensors.

Thermal Conductivity of Nanostructure in Microelectronic Equipment and the Enhancement of Its Thermo-Physical Properties

To optimize the heat dissipation performance of microelectronic equipment, optical instrument, and optoelectronic devices, it’s necessary to explore the heat transfer mechanism of the nanostructure in them, however, due to the limitation of the research objects, there isn’t a uniform simulation method for the thermo-physical properties of such nanostructure. Therefore, this paper aims to study the thermo-physical properties of nanostructure in microelectronic equipment. At first, by structuring Si super lattice nanowires in a specific direction and then using the Ge atoms to replace the Si atoms in the nanowires, this paper built a model for the periodic Si/Ge super lattice nanowires. Then, this paper performed integral operation on the dynamic equations of the two types of atoms using the time integration algorithm to attain the trajectories of Si and Ge atoms in the nanowire structure of the microelectronic equipment, and discussed the influence of the size and geometrical shape of the cross section of nanowires on thermal conductivity. After that, this paper simulated the structure of the nano-scale SiC material, selected appropriate cut-off radius, and optimized the structure of the nanostructure model based on the Discover Minimization module. At last, the thermal conductivity of the nanostructure was calculated and its thermo-physical properties were analyzed.

Giant reduction of thermal conductivity and enhancement of thermoelectric performance in twinning superlattice InAsSb nanowires

Semiconductor nanostructures hold great promise for high-efficiency waste-heat recovery exploiting thermoelectric energy conversion. They could significantly contribute to the implementation of environmentally friendly energy sources and to the realization of self-powered biomedical wearable devices. A crucial thermoelectric material requirement is a reduced thermal conductivity together with good electrical transport properties. In this work we demonstrate a drastic reduction of the thermal conductivity in III-V semiconductor nanowires as a result of the introduction of periodic crystal-lattice twin planes during growth. The electrical and thermal transport of these nanostructures, known as twinning superlattice nanowires, are probed and compared with their twin-free counterparts, showing a one order of magnitude decrease of thermal conductivity while maintaining unaltered electrical-transport properties and Seebeck coefficients. This leads to tenfold enhancement of the thermoelectric figure of merit, ZT. Our study reports for the first time the complete experimental measurement of electrical and thermal properties in twinning superlattice nanowires, demonstrating their emergence as a novel class of nanomaterials of great potential for high-efficiency thermoelectric-energy harvesting.

Thermal stability and mechanical properties of Si/Ge superlattice nanowires having inclination interfaces from simulations at atomic scale

Thermal stability and mechanical properties of Si/Ge superlattice nanowires having inclination interfaces from simulations at atomic scale

Self-organization and tunable characteristic lengths of two-dimensional hexagonal superlattices of nanowires directly grown on substrates

The organization of nano-objects on macroscopic surfaces is a key challenge for the technological improvement and implementation of nanotechnologies. For achieving operational functions, it is required to assemble nano-objects as controllable building blocks in highly ordered superstructures. Herein, we demonstrate the growth and self-organization of metallic nanowires on surfaces into hexagonal superlattices with tunable characteristic lengths depending of the stabilizing surfactants employed. Starting from a reacting mixture containing a Pt(111) substrate, a Co organometallic precursor, an amine, and an acid dissolved in a solvent, we quantify the structural evolution of superlattices of vertical single-crystalline Co nanowires on Pt, using a combined analysis of small angle neutron scattering, transmission, and scanning electron microscopies. We show the concerted steps of a spontaneous growth and self-organization of the nanowires into two-dimensional (2D) hexagonal lattice on Pt, at intervals starting from a few hours of reaction to a highly ordered superlattice at longer times. Furthermore, it is shown that apart from long-chain acid and long-chain aliphatic amine pairs used as stabilizers, the combination of a long-chain aliphatic and a short-chain aromatic ligand in the synthesis can also be employed for the nanowire superlattices development. Interestingly, the possibility to employ different pairs allows quantitative modulation of the nanowire arrays, such as the interwire distance and the packing fraction.

Giant thermoelectric figure of merit in fluorine-doped single walled-carbon nanotubes

Herein, we report on a giant thermoelectric figure of merit (ZT) of a non-degenerate fluorine-doped single-walled carbon nanotube (FSWCNT) using a tractable analytical approach and the phonon lattice Boltzmann model (LBM). We investigate the influence of the doping concentration (no), and overlapping integrals (Δs and Δz) on ZT. ZT and the temperature range of operation can be tuned using the doping (impurity) concentration and the overlapping integrals, respectively. The lattice thermal conductivity obtained using the phonon LBM was calculated to be κℓ=107.2W/mK, which yielded a ZT greater than 6. Interestingly, the ZT obtained is higher than what has been reported for superlattices (ZT≈1.4) and superlattice nanowire (ZT≈4) at 300K, making FSWCNT a potential candidate for thermoelectric applications such as chip level electronics cooling, remote telecommunication power production, and temperature control systems in solid state lasers.

Elastic behavior of metal-assisted etched Si/SiGe superlattice nanowires containing dislocations

We systematically investigate structural parameters, such as shape, size, elastic strain, and relaxations, of metal-assisted etched vertically modulated Si/SiGe superlattice nanowires by using electron microscopy, synchrotron-based x-ray diffraction, and numerical linear elasticity theory. A vertical Si/Ge superlattice with atomically flat interfaces is grown by using molecular beam epitaxy on Si-buffered Si(001) substrates. The lattice constants for Si and Ge are 5.43 and 5.66 Å, respectively, which indicate a lattice mismatch of 4.2%. This results in a strained layer in the boundary between Si and Ge leading to dislocations. These substrates serve as the starting material for nanostructuring the surface by using metal-assisted etching. It is shown that the high quality crystalline structure is preserved in the fabrication process, while the lattice mismatch is partially relieved by dislocation formation. Despite this highly effective relaxation path, dislocations present in the parent superlattice do not vanish upon nanostructuring for wires with diameters of down to at least 80 nm. We relate these observations to the applicability of silicon-based nanowires for high-performance thermoelectric generators.

Controllable Synthesis and Crystal Facet, Composition and Temperature Dependent Gas Sensing Properties of Sn1-Xs-Cds Superlattice Nanowires with Ultrafast Response

Controllable Synthesis and Crystal Facet, Composition and Temperature Dependent Gas Sensing Properties of Sn1-Xs-Cds Superlattice Nanowires with Ultrafast Response

Thermal transport in twinning superlattice and mixed-phase GaAs nanowires.

With continuing advances in semiconductor nanowire (NW) growth technologies, synthesis of tailored crystal structures is gradually becoming a reality. Mixtures of the bulk zinc blende (ZB) and wurtzite (WZ) phase can be achieved in III-V NWs under various growth conditions. Among the possible crystal structures, the twinning superlattice (TSL) has attracted particular interest for tuning photonic and electronic properties. In this work, we investigated the mechanisms underlying thermal transport in pristine, TSL, and disordered polytypic GaAs NWs, using non-equilibrium molecular dynamics and transmission spectra obtained from the atomistic Green's function method. We found that a TSL period of 50 Å minimizes the thermal conductivity and determine a phonon coherence length of about 20 to 50 nm, depending on the NW diameter. Our findings indicate strong dependence of the thermal conductivity on the NW surface and internal structure at a given diameter, critical for thermoelectric optimization.

Time-resolved cathodoluminescence investigations of AlN:Ge/GaN nanowire structures

Light emitting diodes represent a key technology that can be found in many areas of everydays life. Therefore, the improvement of the efficiency of such structures offers a high economic and ecological potential. One approach is electrostatic screening of the quantum-confined Stark effect (QCSE) in polar III-V heterostructures by n-type doping in order to increase the oscillator strength of electronic transitions in quantum structures. In this study, we analyzed the cathodoluminescene (CL) spectra of different functional parts of individual AlN/GaN nanowire superlattices and studied their decay characteristics with sub-nanosecond time resolution. This allows us to extract information about strain and electric fields in such heterostructures with an overall spatial resolution <100 nm. The samples, which were investigated in a temperature range from 10 to 300 K by using time-integrated cathodoluminescence spectroscopy (TICL) and time-resolved cathodoluminescence spectroscopy (TRCL) consist of GaN bottom and top layer and a 40-fold stack of GaN nanodiscs, embedded in AlN barriers that were doped with Ge. We show, that the QCSE is reduced with increasing doping concentration due to a screening of the internal electric fields inside GaN nanodiscs, resulting in a reduction of the carrier lifetimes and a blue shift of the emitted light. Due to the small diameter of the electron excitation beam CL offers the possibility to individually analyze the different functional parts of the nanowires.

4D-STEM at interfaces to GaN: Centre-of-mass approach & NBED-disc detection

4D-scanning transmission electron microscopy (4D-STEM) can be used to measure electric fields such as atomic fields or polarization-induced electric fields in crystal heterostructures. The paper focuses on effects occurring in 4D-STEM at interfaces, where two model systems are used: an AlN/GaN nanowire superlattice as well as a GaN/vacuum interface. Two different methods are applied: First, we employ the centre-of mass (COM) technique which uses the average momentum transfer evaluated from the intensity distribution in the diffraction pattern. Second, we measure the shift of the undiffracted disc (disc-detection method) in nano-beam electron diffraction (NBED). Both methods are applied to experimental and simulated 4D-STEM data sets. We find for both techniques distinct variations in the momentum transfer at interfaces between materials: In both model systems, peaks occur at the interfaces and we investigate possible sources and routes of interpretation. In case of the AlN/GaN superlattice, the COM and disc-detection methods are used to measure internal polarization-induced electric fields and we observed a reduction of the measured fields with increasing specimen thickness.