First-principles study of the electronic properties of wurtzite, zinc-blende, and twinned InP nanowires

The electronic properties of zinc-blende, wurtzite, and rotationally twinned InP nanowires were studied using first-principles calculations. The results show that all the simulated nanowires exhibit a semiconducting character, and the band gap decreases with increasing the nanowire size. The band gap difference between the zinc-blende, wurtzite, and twinned InP nanowires and bulk InP can be described by ΔE(g)(wire) = 0.88/D(1.23), ΔE(g)(wire) = 0.79/D(1.22) and ΔE(g)(twin) = 1.3/D(1.19), respectively, where D is the diameter of the nanowires. The valence band maximum (VBM) and conduction band minimum (CBM) originate mainly from the p-orbitals of the P atoms and s-orbitals of the In atoms at the core regions of the nanowires, respectively. The hexagonal (2H) stacking inside the cubic (3C) stacking has no effect on the electronic properties of thin InP nanowires.     

Valence band splitting in wurtzite InP nanowires observed by photoluminescence and photoluminescence excitation spectroscopy

We have investigated individual bulk-like wires of wurtzite InP using photoluminescence, photoluminescence excitation spectroscopy and transmission electron microscopy. Using two different methods we find that the top of the valence band is split, as expected theoretically. This splitting of the valence band is peculiar to wurtzite InP and does not occur in zinc blende InP. We find the energy difference between the two bands to be 40 meV.     

Electrical and optical properties of InP nanowire ensemble p⁺-i-n⁺ photodetectors

We report on a comprehensive study of electrical and optical properties of efficient near-infrared p?-i-n? photodetectors based on large ensembles of self-assembled, vertically aligned i-n? InP nanowires monolithically grown on a common p? InP substrate without any buffer layer. The nanowires have a polytype modulated crystal structure of wurtzite and zinc blende. The electrical data display excellent rectifying behavior with an ideality factor of about 2.5 at 300 K. The ideality factor scales with 1/T, which possibly reflects deviations from classical transport models due to the mixed crystal phase of the nanowires. The observed dark leakage current is of the order of merely ～100 fA/nanowire at 1 V reverse bias. The detectors display a linear increase of the photocurrent with reverse bias up to about 10 pA/nanowire at 5 V. From spectrally resolved measurements, we conclude that the photocurrent is primarily generated by funneling photogenerated carriers from the substrate into the NWs. Contributions from direct excitation of the NWs become increasingly important at low temperatures. The photocurrent decreases with temperature with an activation energy of about 50 meV, which we discuss in terms of a temperature-dependent diffusion length in the substrate and perturbed transport through the mixed-phase nanowires.     

Zigzag zinc blende ZnS nanowires: Large scale synthesis and their structure evolution induced by electron irradiation

Large scale zigzag zinc blende single crystal ZnS nanowires have been successfully synthesized during a vapor phase growth process together with a small yield of straight wurtzite single crystal ZnS nanowires. AuPd alloy nanoparticles were utilized to catalyze a vapor-solid-solid growth process of both types of ZnS nanowires, instead of the more common vapor-liquid-solid growth process. Surprisingly, the vapor-phase grown zigzag zinc blende ZnS nanowires are metastable under high-energy electron irradiation in a transmission electron microscope, with straight wurtzite nanowires being much more stable. Upon exposure to electron irradiation, a wurtzite ZnO nanoparticle layer formed on the zigzag zinc blende ZnS nanowire surface with concomitant displacement damage. Both electron inelastic scattering and surface oxidation as a result of electron-beam heating occur during this structure evolution process. When prolonged higher-voltage electron irradiation was applied, local zinc blende ZnS nanowire bodies evolved into ZnS-ZnO nanocables, and dispersed ZnS-ZnO nanoparticle networks. Random AuPd nanoparticles were observed distributed on zigzag ZnS nanowire surfaces, which might be responsible for a catalytic oxidation effect and speed up the surface oxidation-induced structure evolution.      

Elastic and Piezoelectric Properties of Zincblende and Wurtzite Crystalline Nanowire Heterostructures

The elastic and piezoelectric properties of zincblende and wurtzite crystalline InAs/InP nanowire heterostructures have been studied using electro‐elastically coupled continuum elasticity theory. A comprehensive comparison of strains, piezoelectric potentials and piezoelectric fields in the two crystal types of nanowire heterostructures is presented. For each crystal type, three different forms of heterostructures—core‐shell, axial superlattice, and quantum dot nanowire heterostructures—are considered. In the studied nanowire heterostructures, the principal strains are found to be insensitive to the change in the crystal structure. However, the shear strains in the zincblende and wurtzite nanowire heterostructures can be very different. All the studied nanowire heterostructures are found to exhibit a piezoelectric field along the nanowire axis. The piezoelectric field is in general much stronger in a wurtzite nanowire heterostructure than in its corresponding zincblende heterostructure. Our results are expected to be particularly important for analyzing and understanding the properties of epitaxially grown nanowire heterostructures and for applications in nanowire electronics, optoelectronics, and biochemical sensing.     

Kinetic effects in InP nanowire growth and stacking fault formation: the role of interface roughening

InP nanowire polytypic growth was thoroughly studied using electron microscopy techniques as a function of the In precursor flow. The dominant InP crystal structure is wurtzite, and growth parameters determine the density of stacking faults (SF) and zinc blende segments along the nanowires (NWs). Our results show that SF formation in InP NWs cannot be univocally attributed to the droplet supersaturation, if we assume this variable to be proportional to the ex situ In atomic concentration at the catalyst particle. An imbalance between this concentration and the axial growth rate was detected for growth conditions associated with larger SF densities along the NWs, suggesting a different route of precursor incorporation at the triple phase line in that case. The formation of SFs can be further enhanced by varying the In supply during growth and is suppressed for small diameter NWs grown under the same conditions. We attribute the observed behaviors to kinetically driven roughening of the semiconductor/metal interface. The consequent deformation of the triple phase line increases the probability of a phase change at the growth interface in an effort to reach local minima of system interface and surface energy.     

Probing the wurtzite conduction band structure using state filling in highly doped InP nanowires

We have grown InP nanowires doped with hydrogen sulfide, which exhibit sulfur concentrations of up to 1.4%. The highest doped nanowires show a pure wurtzite crystal structure, in contrast to bulk InP which has the zinc blende structure. The nanowires display photoluminescence which is strongly blue shifted compared with the band gap, well into the visible range. We find evidence of a second conduction band minimum at the gamma point about 0.23 eV above the band edge, in excellent agreement with recent theoretical predictions. Electrical measurements show high conductivity and breakdown currents of 10(7) A/cm(2).     

Correlated micro-photoluminescence and electron microscopy studies of the same individual heterostructured semiconductor nanowires

To correlate optical properties to structural characteristics, we developed a robust strategy for characterizing the same individual heterostructured semiconductor nanowires (NWs) by alternating low temperature micro-photoluminescence (μ-PL), low voltage scanning (transmission) electron microscopy and conventional transmission electron microscopy. The NWs used in this work were wurtzite GaAs core with zinc blende GaAsSb axial insert and AlGaAs radial shell grown by molecular beam epitaxy. The series of experiments demonstrated that high energy (200 kV) electrons are detrimental for the optical properties, whereas medium energy (5-30 kV) electrons do not affect the PL response. Thus, such medium energy electrons can be used to select NWs for correlated optical-structural studies prior to μ-PL or in NW device processing. The correlation between the three main μ-PL bands and crystal phases of different compositions, present in this heterostructure, is demonstrated for selected NWs. The positions where a NW fractures during specimen preparation can considerably affect the PL spectra of the NW. The effects of crystal-phase variations and lattice defects on the optical properties are discussed. The established strategy can be applied to other nanosized electro-optical materials, and other characterization tools can be incorporated into this routine.     

Realizing Zinc Blende GaAs/AlGaAs Axial and Radial Heterostructure Nanowires by Tuning the Growth Temperature

Vertical zinc blende GaAs/AlGaAs heterostructure nanowires were grown at different temperatures by metalorganic chemical vapor deposition via Au-assisted vapor-liquid-solid mechanism.It was found that radial growth can be enhanced by increasing the growth temperature.The growth of radial heterostructure can be realized at temperature higher than 500℃,while the growth temperature of axial heterostructure is lower than 440℃.The room temperature photoluminescence properties of the nanowires were investigated and the relevant growth mechanism was discussed.     

The effects of oxygen on the surface passivation of InP nanowires

The effects of surface passivation on the electronic and structural properties of InP nanowires have been investigated by first-principles calculations. We compare the properties of nanowires whose surfaces have been passivated in several ways, always by H atoms and OH radicals. Taking as the initial reference nanowires that are fully passivated by H atoms, we find that the exchange of these atoms at the surface by OH radicals is always energetically favorable. A nanowire fully passivated by OH radicals is about 2.5?eV per passivated dangling bond more stable than a nanowire fully passivated by H atoms. However, the energetically most stable passivated surface is predicted to have all In atoms bonded to OH radicals and all P atoms bonded to H atoms. This mixed passivation is 2.66?eV per passivated dangling bond more stable than a nanowire fully passivated by H atoms. Our results show that, in comparison with the fully H-saturated nanowire, the fully OH-saturated nanowire has a smaller energy band gap and localized states near the energy band edges. Also, more interestingly, concerning optical applications, the most stable H+OH passivated nanowire has a well-defined energy band gap, only 10% smaller than the H-saturated nanowire energy gap, and few localized states always close to the valence band maximum.     

Faceting, composition and crystal phase evolution in III-V antimonide nanowire heterostructures revealed by combining microscopy techniques

III-V antimonide nanowires are among the most interesting semiconductors for transport physics, nanoelectronics and long-wavelength optoelectronic devices due to their optimal material properties. In order to investigate their complex crystal structure evolution, faceting and composition, we report a combined scanning electron microscopy (SEM), transmission electron microscopy (TEM), and scanning tunneling microscopy (STM) study of gold-nucleated ternary InAs/InAs(1-x)Sb(x) nanowire heterostructures grown by molecular beam epitaxy. SEM showed the general morphology and faceting, TEM revealed the internal crystal structure and ternary compositions, while STM was successfully applied to characterize the oxide-free nanowire sidewalls, in terms of nanofaceting morphology, atomic structure and surface composition. The complementary use of these techniques allows for correlation of the morphological and structural properties of the nanowires with the amount of Sb incorporated during growth. The addition of even a minute amount of Sb to InAs changes the crystal structure from perfect wurtzite to perfect zinc blende, via intermediate stacking fault and pseudo-periodic twinning regimes. Moreover, the addition of Sb during the axial growth of InAs/InAs(1-x)Sb(x) heterostructure nanowires causes a significant conformal lateral overgrowth on both segments, leading to the spontaneous formation of a core-shell structure, with an Sb-rich shell.     

Semiconductor nanowire heterostructures

Recent progress on the synthesis and characterization of semiconductor nanowire heterostructures is reviewed. We describe a general method for heterostructure synthesis based on chemical vapour deposition and the vapour-liquid-solid growth of crystalline semiconducting nanowires. We then examine examples of nanowire heterostructures for which physical properties have been measured, considering the effects of synthetic conditions on the heterointerfaces as well as the electrical and optical characterization measurements that reveal heterointerface formation and quality. Finally, we identify areas of technical and conceptual progress that can contribute to the development of functional nanowire heterostructures.     

Growth of GaInAs/AlInAs heterostructure nanowires for long-wavelength photon emission

We investigated the growth of GaInAs/AlInAs heterostructure nanowires on InP(111)B and Si(111) substrates in a metalorganic vapor phase epitaxy reactor. Au colloids were used to deposit Au catalysts 20 and 40 nm in diameter on the substrate surfaces. We obtained vertical GaInAs and AlInAs nanowires on InP(111)B surfaces. The GaInAs nanowires capped with GaAs/AlInAs layers show room-temperature photoluminescence. The peak exhibits a blue-shift when the Ga content in the core GaInAs nanowire is increased. For the GaInAs/AlInAs heterostructure growth, it is possible to change the Ga content sharply but Al also exists in the GaInAs layer regions. We also found that the ratios of Ga and Al contents to In content tend to increase and the axial growth rate to decrease along the nanowire toward the top. We were also able to make vertical GaInAs nanowires on Si(111) surfaces after a short growth of GaP and InP.     

GaAs/AlGaAs heterostructure nanowires studied by cathodoluminescence

In this report we explore the structural and optical properties of GaAs/A1GaAs heterostructure nanowires grown by metalorganic vapour phase epitaxy using gold seed-particles. The optical studies were done by low-temperature cathodo- luminescence （CL） in a scanning electron microscope （SEM）. We perform a systematic investigation of how the nanowire growth-temperature affects the total photon emission, and variations in the emission energy and intensity along the length of the nanowires. The morphology and crystal structures of the nanowires were investigated using SEM and transmission electron microscopy （TEM）. In order to correlate specific photon emission characteristics with variations in the nanowire crystal structure directly, TEM and spatially resolved CL measurements were performed on the same individual nanowires. We found that the main emission energy was located at around 1.48 eV, and that the emission intensity was greatly enhanced when increasing the GaAs nanowire core growth temperature. The data strongly suggests that this emission energy is related to rotational twins in the GaAs nanowire core. Our measurements also show that radial overgrowth by GaAs on the GaAs nanowire core can have a deteriorating effect on the optical quality of the nanowires. Finally, we conclude that an in situ pre-growth annealing step at a sufficiently high temperature significantly improves the optical quality of the nanowires.     

Synthesis of tetrahedral quasi-type-II CdSe-CdS core-shell quantum dots

Synthesis of colloidal nanocrystals of II-VI semiconductor materials has been refined in recent decades and their size dependent optoelectronic properties have been well established. Here we report a facile synthesis of CdSe-CdS core-shell heterostructures using a two-step hot injection process. Red-shifts in absorption and photoluminescence spectra show that the obtained quantum dots have quasi-type-II alignment of energy levels. The obtained nanocrystals have a heterostructure with a large and highly faceted tetrahedral CdS shell grown epitaxially over a spherical CdSe core. The obtained morphology as well as high resolution electron microscopy confirms that the tetrahedral shell have a zinc blende crystal structure. A phenomenological mechanism for the growth and morphology of the nanocrystals is discussed.     

Determination of band offsets in heterostructured colloidal nanorods using scanning tunneling spectroscopy

The ability to tailor the properties of semiconductor nanocrystals through creating core/shell heterostructures is the cornerstone for their diverse application in nanotechnology. The band-offsets between the heterostructure components are determining parameters for their optoelectronic properties, dictating for example the degree of charge-carrier separation and localization. So far, however, no method was reported for direct measurement of these factors in colloidal nanocrystals and only indirect information could be derived from optical measurements. Here we demonstrate that scanning tunneling spectroscopy along with theoretical modeling can be used to determine band-offsets in such nanostructures. Applying this approach to CdSe/CdS quantum-dot/nanorod core/shell nanocrystals portrays its type I band structure where both the hole and electron ground state are localized in the CdSe core, in contrast to previous reports which predicted electron delocalization. The generality of the approach is further demonstrated in ZnSe/CdS nanocrystals where their type II band alignment, leading to electron-hole separation, is manifested.     

Structural and optical characterization of strained free-standing InP nanowires

The structural and optical properties of high-quality crystalline strained InP nanowires are reported in this article. The nanowires were produced by the vapor-liquid-solid growth method in a chemical-beam epitaxy reactor, using 20 nm gold nanoparticles as catalysts. Polarization-resolved photoluminescence experiments were carried out to study the optical properties of the InP nanowires. These experiments revealed a large blue shift of 74 meV of the first electron-to-heavy hole optical transition in the nanowires, which cannot be solely explained by quantum size effects. The blue shift is mainly attributed to the presence of biaxial compressive strain in the inward radial direction of the InP nanowires. High-resolution transmission electron microscopy Electron and selected area electron diffraction experiments show that the nanowires have high crystal quality and grow along a [001] axes. These experiments also confirmed the presence of 1.8% compressive radial strain and 2% tensile longitudinal strain in the nanowires. A simple theoretical model including both quantum confinement and strain effects consistently describes the actual energy position of the InP nanowires optical emission.     

Sb Incorporation in Wurtzite and Zinc Blende InAs1−xSbx Branches on InAs Template Nanowires

The physical properties of material largely depend on their crystal structure. Nanowire growth is an important method for attaining metastable crystal structures in III–V semiconductors, giving access to advantageous electronic and surface properties. Antimonides are an exception, as growing metastable wurtzite structure has proven to be challenging. As a result, the properties of these materials remain unknown. One promising means of accessing wurtzite antimonides is to use a wurtzite template to facilitate their growth. Here, a template technique using branched nanowire growth for realizing wurtzite antimonide material is demonstrated. On wurtzite InAs trunks, InAs_1?xSb_x branch nanowires at different Sb vapor phase compositions are grown. For comparison, branches on zinc blende nanowire trunks are also grown under identical conditions. Studying the crystal structure and the material composition of the grown branches at different x_v shows that the Sb incorporation is higher in zinc blende than in wurtzite. Branches grown on wurtzite trunks are usually correlated with stacking defects in the trunk, leading to the emergence of a zinc blende segment of higher Sb content growing parallel to the wurtzite structure within a branch. However, the average amount of Sb incorporated within the branch is determined by the vapor phase composition.     

Modeling of InAs-InSb nanowires grown by Au-assisted chemical beam epitaxy

Interesting phenomena during the Au-assisted chemical beam epitaxy of InAs-InSb nanowire heterostructures have been observed and interpreted within the framework of a theoretical model. An unusual, non-monotonous diameter dependence of the InSb nanowire growth rate is demonstrated experimentally within a range of deposition conditions. Such a behavior is explained by competition between the Gibbs-Thomson effect and different diffusion-induced material fluxes. Theoretical fits to the experimental data obtained at different flux pressures of In and Sb precursors allow us to deduce some important kinetic coefficients. Furthermore, we discuss why the InAs nanowire stem forms in the wurtzite phase while the upper InSb part has a pure zinc blende crystal structure. It is hypothesized that the 30° angular rotation of nanowire when passing from InAs to the InSb part is driven by the lowest surface energy of (1100) wurtzite and (110) zinc blende facets.     

Scalable fabrication of novel SiC nanowire nonwoven fabric

Flexible papers constructed by one-dimensional nanowires have attracted much attention due to their various applications. Herein, a novel nonwoven fabric with paper-like qualities composed of zinc blende SiC (β-SiC) nanowires was fabricated by a scalable rolling process. The SiC nanowires were synthesized by the carbothermal reduction reaction of the carbon fiber and carbonaceous silica xerogel. The crystal phase, morphology and microscopic structure of the as-prepared SiC nanowires were characterized by field emission scanning electron microscope, X-ray diffraction and high-resolution transmission electron microscopy. The nanowire vapor–solid growth mechanism and preparation process for SiC nanowire nonwoven fabric were also discussed. The freestanding SiC nanowire nonwoven fabric exhibited high flexibility, high mechanical strength, excellent refractory performance and thermal stability. With high flexibility, high mechanical strength, superior nonflammability and thermal stability, the flexible paper-like 3C-SiC nanowire nonwoven fabric materials would be expected to have some potential applications, such as ceramic matrix composites, metal matrix composites, energy storage, catalyst supports, radiation-proof fabric, filtration and separation.