Pure Wurtzite Phase Research Articles

High Quality Epitaxial Piezoelectric and Ferroelectric Wurtzite Al1- xScxN Thin Films.

Piezoelectric and ferroelectric wurtzite are promising to reshape modern microelectronics because they can be easily integrated with mainstream semiconductor technology. Sc doped AlN (Al1- xScxN) has attracted much attention for its enhanced piezoelectric and emerging ferroelectric properties, yet the commonly used sputtering results in polycrystalline Al1- xScxN films with high leakage current. Here, the pulsed laser deposition of single crystalline epitaxial Al1- xScxN thin films on sapphire and 4H-SiC substrates is reported. Pure wurtzite phase is maintained up to x = 0.3 with ≤0.1 at% oxygen contamination. Polarization is estimated to be 140 µCcm-2 via atomic scale microscopy imaging and found to be switchable via a scanning probe. The piezoelectric coefficient is found to be five times of the undoped one when x = 0.3, making it desirable for high-frequency radiofrequency (RF) filters and 3D nonvolatile memories.

Green synthesis of ZnO nanoparticles by Myrtus communis plant extract with investigation of effect of precursor, calcination temperature and study of photocatalytic performance

An elegant approach was made for the synthesis of ZnO nanoparticles by Myrtus Communis plant extract using two different precursors at variable temperatures (400,600, and 800 °C). The properties of nanoparticles were investigated with PXRD, SEM-EDX, PL, FTIR, and UV–Vis spectrometer. PXRD results inferred that all samples have pure wurtzite phase confirming with JCPDS card No. 36–1451. The UV–Vis spectroscopy exhibited that all estimated bandgaps are a little less than those for bulk ZnO. It was confirmed by EDX spectra that Zn and O atoms are the main compositions of the material. FESEM images displayed hexagonal-like shapes of nanoparticles for all samples prepared with acetate. Hexagonal morphology was observed in the samples having green emission at room temperature PL measurements. It was seen that methylene blue degraded about 90 % after about 20 min of irradiation of UV light for samples calcined at 400 °C. It was also observed that the samples calcined at 400 °C degraded methylene blue by approximately 99 %. This work is the first paper on the morphological utilization and photocatalytic performance of green synthesized ZnO with Myrtus communis plant extract with various calcination temperatures via two different precursor reagents.

Nanostructural Design of ZnO Using an Agro-Waste Extract for a Sustainable Process and Its Photocatalytic Activity.

The use of agro-waste extracts (AWEs) as a sustainable medium for developing cost-effective and ecologically friendly nanomaterials has piqued the interest of current researchers. Herein, waste extracts from papaya barks, banana peels, thumba plants, and snail shells were used for synthesizing ZnO nanostructures via a hydrothermal method, followed by calcination at 400 °C. The crystallinity and pure wurtzite phase formation of ZnO nanostructures were confirmed via X-ray diffraction. ZnO nanostructures with various morphologies such as tight sheet-like, spherical, porous sheet-like, and bracket-shaped, comprising small interconnected particles with a highly catalytically active exposed (0001) facet, were observed via field emission scanning electron microscopy and transmission electron microscopy. The formation mechanism of the various morphologies of the ZnO nanostructures was proposed. Ultraviolet-visible spectra showed different absorption band edges of ZnO nanostructures with a bandgap in the range of 3.17-3.27 eV. Photoluminescence studies showed the presence of various defect states such as oxygen and zinc vacancies and oxygen and zinc interstitials on ZnO nanostructures, which are usually observed in traditionally prepared ZnO. The photocatalytic activity of ZnO nanostructures was evaluated under direct sunlight using rhodamine B (RhB) and Congo red (CR) dyes as probe pollutants. Furthermore, prepared ZnO nanostructures could potentially adsorb anionic dyes (e.g., CR) in the absence of light. Superoxide and hydroxide radicals played a vital role in the photocatalytic activity of ZnO. The photocatalyst could be reused for up to three cycles, indicating its stability. Therefore, this study reports the diverse use of AWEs as cost-effective media for nanomaterial synthesis.

Designing of Te-doped ZnO and S-g-C3N4 /Te-ZnO nano-composites as excellent photocatalytic and antimicrobial agents

Today, it is crucial but challenging to develop a narrow bandgap photo-catalyst that can eliminate contaminants when exposed to visible light. In this work, we synthesized the ZnO, Te-doped ZnO, and s-g- C3N4 assisted Te-doped ZnO nanoparticles by a susceptible and economical co-precipitation method. Different techniques were employed to characterize the compounds like XRD confirms the formation of pure wurtzite phase present within ZnO nanoparticles. The morphology of produced nanoparticles was studied using FESEM, which reveals that ZnO nanoparticles develop into spherical particles whereas doped particles take the form of nano-sheets with a regular hexagonal pattern. The FT-IR and UV visible spectrum was used to measure the functional groups and optical properties of compounds. The doped compound shows the maximum efficiency of degradation against methylene blue dye under sunlight exposure. Furthermore, E. coli and S. aureus were used as test subjects for the antibacterial activity of these synthetic compounds. On the other hand, the greatest antifungal activity of S-g-C3N4/Te-ZnO against C. albicans was calculated to be 44.6 mm, and 26.5 mm against E. salmonicolor. Te-doped ZnO nanoparticles with the aid of S-g-C3N4 showed remarkable photocatalytic and antimicrobial activities.

Synthesis of Size-Tunable Indium Nitride Nanocrystals

Indium nitride (InN) is an air stable III-V semiconductor with a small band gap of 0.7 eV and as such is of interest as a key material in near-infrared (NIR) LEDs, photodetectors, and multijunction solar cells. Conventionally, InN has been synthesized through physical deposition techniques which involve extreme temperatures and harsh conditions and yield nonluminescent materials. Here we report a new low temperature, solution-based synthetic route to colloidal InN nanocrystals that produces phase-pure wurtzite InN with tunable photoluminescence. The luminescence is attributed to the presence of InN surface states.

High-performance two-photon absorption optical limiter and stabilizer based on phase-pure thick-shell CdSe/CdS core/shell quantum dots

To achieve high-sensitivity two-photon absorption (2PA) optical limiters and stabilizers, two-photon active materials need to have large 2PA cross-sections. Phase-pure wurtzite (WZ) CdSe/CdS core/shell quantum dots (QDs) with a shell thickness of 11 CdS monolayers were prepared by a high-temperature pyrolysis method, which possess a large volume and nearly defect-free core/shell interfaces. The 2PA cross-section of QDs is measured to be as large as 1.5 × 105 GM by the nonlinear transmittance method. Due to the large 2PA cross-section of phase-pure thick-shell WZ CdSe/CdS core/shell QDs, we successfully explored their application in the field of optical limiting and stabilization. Finally, we successfully fabricated a high-sensitivity optical stabilizer made of a polymer (polymethylmethacrylate) matrix comprising phase-pure thick-shell WZ CdSe/CdS core/shell QDs, which can reduce the amplitude fluctuation by ∼67%. In addition, the device reduces the input energy by ∼40%, indicating that the device can also be applied as an optical limiter. This work promotes the application of optical limiters and stabilizers based on QDs in practical work to a certain extent.

Green synthesized ZnO nanoparticles using Ganoderma lucidum: Characterization and In Vitro Nanofertilizer effects

Nanomaterials are one of the most popular topics of technological applications in the last decade, thus it is crucial to synthesize these materials with eco-friendly methods. In the present study, ZnO nanoparticle has been biologically synthesized by using Ganoderma lucidum extract with various concentrations, and its structural, optical, morphological, and elemental characteristics are determined with XRD, UV–visible spectrometer, and SEM-EDS analyses. As well, nanofertilizer properties on Lepidium sativum (garden cress) have been investigated. It is found that structural properties of ZnO NPs have pure wurtzite phase with p63 mc space group. Optical bandgaps of nanoparticles which have not differed with the increasing extract concentration is about 3.24 eV. In SEM-EDS analysis, three different hexagonal shapes (discs, rods, and pyramidals) of nanoparticles have been observed with stoichiometric Zn and O ratios. The radicle, plumule lengths, fresh and, dry weights contents of L. sativum have been increased at rates of 45 %, 41 %, 16 %, 33 % at 250 ppm of green synthesized ZnO NPs with 25 ml extract concentration, respectively. This study is the first report on the utilization of green synthesized ZnO with G. lucidum extracts as a nano-nutrient. • Effects of extract concentration on ZnO NPs were investigated. • 25 ml Ganoderma lucidum extract concentration presents the highest growth parameters on Lepidium sativum. • The production mechanism of green synthesized ZnO NPs is discussed, as well as its future potential as nano fertilizer. • ZnO NPs were an almost uniformed and hexagonal disk, rod, and pyramidal shape with different extract concentrations.

Facile and Novel Route for the Preparation of ZnO Nanoparticles with Different Cr Loadings for Opto-Photocatalysis Applications

The current article deals with the facile yet novel route to prepare zinc oxide (ZnO) nanoparticles with different weight percentages of chromium as a dopant. The impact of such dopant into the ZnO host lattice is explored in terms of the structural, vibrational, optical, and photocatalytic characteristics. The Bragg reflections in the X-ray diffraction displayed a phase pure wurtzite ZnO hexagonal system. The morphology reflects spherical-shaped ZnO particles in all the systems. The optical analysis ensured a good ultraviolet light absorption and a bandgap energy in the range of 3.30–3.24 eV. The principal Raman vibrations ensured the presence of the wurtzite ZnO crystal structure. The decolorization experiment of methyl green dye with pristine and various chromium-doped ZnO nanoparticles was conducted under the illumination of visible light. The obtained results showed that the incorporation of Cr in the framework significantly improved the photocatalytic performance of ZnO.

Evaluation of the Photocatalytic Activity of Distinctive-Shaped ZnO Nanocrystals Synthesized Using Latex of Different Plants Native to the Amazon Rainforest.

ZnO nanocrystals with three different morphologies have been synthesized via a simple sol-gel-based method using Brosimum parinarioides (bitter Amapá) and Parahancornia amapa (sweet Amapá) latex as chelating agents. X-ray diffraction (XRD) and electron diffraction patterns (SAED) patterns showed the ZnO nanocrystals were a pure hexagonal wurtzite phase of ZnO. XRD-based spherical harmonics predictions and HRTEM images depicted that the nanocrystallites constitute pitanga-like (~15.8 nm), teetotum-like (~16.8 nm), and cambuci-like (~22.2 nm) shapes for the samples synthesized using bitter Amapá, sweet Amapá, and bitter/sweet Amapá chelating agent, respectively. The band gap luminescence was observed at ~2.67–2.79 eV along with several structural defect-related, blue emissions at 468–474 nm (VO, VZn, Zni), green emissions positioned at 513.89–515.89 (h-), and orange emission at 600.78 nm (–). The best MB dye removal efficiency (85%) was mainly ascribed to the unique shape and oxygen vacancy defects found in the teetotum-like ZnO nanocrystals. Thus, the bitter Amapá and sweet Amapá latex are effective chelating agents for synthesizing distinctive-shaped ZnO nanocrystals with highly defective and remarkable photocatalytic activity.

A rapid microwave-assisted method of synthesizing wurtzite Cu2ZnSnS4 in a novel solvent and its controlled phase evolution

Stable pure wurtzite Cu2ZnSnS4 (CZTS) microparticles are successfully synthesized and the phase control evolution is studied, where a mixture of ethylene glycol (EG) and triethylenetetramine (TETA) is used in conjunction with a rapid and low-cost microwave-assisted method to control the crystalline phases of the particles. The CZTS phase is mainly controlled by changing the reaction time, reaction power, surfactant vs. raw material ratio. The particles are characterized with X-ray diffraction, Raman spectroscopy, scanning electron microscope, transmission electron microscope and UV–vis spectrophotometer. The results show that when EG and TETA are mixed in a ratio of 1:6 and the reaction power is 800 W, CZTS in pure wurtzite phase is obtained rapidly when the raw material ratio is Cu: Zn: Sn: S = 2 : 1.25: 1: 6 and the reaction time is 120 s. The phase transition can be effectively controlled by controlling the reaction conditions such as raw material ratio, reaction power and reaction time.

Enhancement of intrinsic green emission in phase pure ZnO

The Phase pure wurtzite structured ZnO phosphor was prepared by sintering three different precursors viz. zinc acetate, zinc nitrate, and ZnO at 800 °C, 900 °C, and 1000 °C in a reducing atmosphere for 1 h. The prepared ZnO phosphor samples were characterized by powder X-ray diffraction and the Rietveld refinement analysis in order to study their structural information that confirms their wurtzite hexagonal phase. Scanning electron microscopy (SEM) pictures exhibited irregular spherical morphology with an average particle size between 1 and 3 μm. The corresponding EDAX spectrum confirmed the presence of Zn and O species. Dependence of sintering temperature on the photoluminescence (PL) properties of ZnO samples and their enhancement on deep-level emission were studied. Deep level green emission around 505 nm was observed for the ZnO samples sintered in the reducing atmosphere without adding any impurities. Among three samples, the maximum PL emission intensity and better CIE colour coordinates under 380 nm excitation were obtained for the ZnO sintered at 900 °C and 1000 °C. Maximum Entropy Method (MEM) were used to determine the electron distributions in the unit cell which supports the PL investigation.

Controlling electrical and optical properties of wurtzite CdxZn1−xO with high Cd contents via native defects manipulation by low-temperature annealing

Bandgap energies in wurtzite (WZ) structured CdxZn1−xO alloys are known to decrease with increasing Cd content (x). Our previous work demonstrated that WZ-CdxZn1−xO alloys with a high Cd content of x ∼ 0.6 and a low gap of 2 eV can be stabilized by oxygen interstitials when grown in an O-rich environment. However, such O-rich WZ-CdxZn1−xO alloys have poor electrical properties due to compensating native defects. In this work, we synthesized pure WZ phase CdxZn1−xO thin films with different Cd contents by magnetron sputtering in an oxygen-rich environment. Changes in structural, electrical, and optical properties of these O-rich wurtzite CdxZn1−xO after rapid thermal annealing were investigated. While alloys with a low Cd composition of 0.2 can maintain a pure wurtzite structure up to 500 °C, phase separation occurs at a lower annealing temperature of ∼400 °C for Cd-rich (x = 0.6) films. Isochronal and isothermal annealing studies reveal the kinetics of native defects in these alloys. Highly mobile hydrogen interstitial donor defects, oxygen interstitials, and more stable cation vacancies outdiffuse sequentially as the annealing temperature increases from <300 to >400 °C. By exploiting the difference in the energy barrier between acceptor defects removal and phase separation, a pure wurtzite phase alloy with a low bandgap of 2 eV and decent electrical properties was realized by annealing O-rich WZ-Cd0.6Zn0.4O at 300 °C with an extended annealing duration of >100 s. These results demonstrate a practical way to obtain low-gap oxide semiconductors with strong optical absorption and controllable electrical conductivities.

Nitrogen stabilizes the wurtzite polymorph in ZnSe1−xTex thin films

Phase-pure wurtzite structure is observed in ZnSe1−xTex thin films doped by flowing molecular nitrogen during growth. A combination of factors help stabilize this phase and the result opens the door to new polymorph engineering in II–VI materials.

Probing strain in wurtzite InP-InAs core-shell nanowires with Raman spectroscopy

We used Raman spectroscopy to investigate phonon modes of single InP-InAs core-shell nanowire with a pure wurtzite phase to reveal the embedded strained state. Raman spectra show that the InP core exhibits tensile strain and the InAs shell exhibits compressive strain due to the radial heterostructure growth. Using different polarization excitation configurations, we identified distinct ${A}_{1}$ (TO) and ${E}_{1}$ (TO) modes, atomic vibrations along and perpendicular to the [0001] direction, respectively. We also identified ${E}_{2h}$ phonon modes in wurtzite InP and InAs, lattice vibration along with the [0001] direction, a fingerprint to distinguish a wurtzite phase from a zinc-blende phase. The Raman spectra changes reveal the resonance effects of InP and InAs between the ${E}_{0}+{\mathrm{\ensuremath{\Delta}}}_{0}$ energy level and the ${E}_{1}$ gap by varying excitation energy, which indicates ${\mathrm{\ensuremath{\Gamma}}}_{7\mathrm{c}}\text{\ensuremath{-}}{\mathrm{\ensuremath{\Gamma}}}_{6\mathrm{v}}$ transition consists of a split-off valence band and a band-gap transition along [0001] directions of the Brillouin zone. The polar patterns show the Raman ${A}_{1}$ (TO) mode of InP only predominates in the $x(yy)\overline{x}$ configuration, indicating the InP ${A}_{1}$ (TO) phonon is sensitive to excitation polarization, suggesting the resonant Raman spectroscopy with specific polarization offers a direct way for characterizing wurtzite InP. Our findings deepen the understanding of strain in wurtzite-phase core-shell nanowire, providing a significant reference for engineering strain in core-shell nanowire to control optical and electric properties.

Synthesis and characterization of ZnO nanoparticles using aqueous extract of Becium grandiflorum for antimicrobial activity and adsorption of methylene blue

Nanotechnology is a recent field of modern research dealing with synthesis, strategy and manipulation of particle’s structure in size range of 1–100 nm. This study introduces one of the methods of synthesis of nanoparticles, i.e., green synthesis of ZnO NPs using aqueous leaf extract of Becium grandiflorum (AM: ‘Yedegamentisie’). The biomolecules of the plant extract (such as phenols, flavonoids, saponins, glycosides, steroids, tannins and alkaloids) were used as capping and reducing agent during synthesis of ZnO NPs. Response surface methodology coupled with Box-Behnken design (RSM-BBD) was used to optimize the synthesis of ZnO NPs and adsorption studies of the as-synthesized ZnO NPs. Then, ZnO NPs was characterized using different spectroscopic and microscopic instruments such as UV–Vis spectroscopy, FTIR, XRD and SEM–EDS to consider its purity, shape and crystallinity. UV–Vis analysis showed peaks in the range 305–312 nm due to synthesis of ZnO NPs. FTIR analysis showed the availability of different phytochemicals in the plant extract and synthesis of ZnO NPs at 490 cm−1. Powder XRD patterns confirmed formation of phase pure wurtzite structures of ZnO NPs. The synthesized ZnO NPs were used to remove MB dye from aqueous solution by acting as a photocatalyst and adsorbent as well as, it also showed antimicrobial activity against two gram positive (Staphylococcus epidermidis, Staphylococcus aureus) and three gram negative (Escherichia coli, Klebsiella pneumonia, Pseudomonas aeruginosa) bacteria.

Wurtzite phase control for self-assisted GaAs nanowires grown by molecular beam epitaxy

The accurate control of the crystal phase in III–V semiconductor nanowires (NWs) is an important milestone for device applications. Although cubic zinc-blende (ZB) GaAs is a well-established material in microelectronics, the controlled growth of hexagonal wurtzite (WZ) GaAs has thus far not been achieved successfully. Specifically, the prospect of growing defect-free and gold catalyst-free wurtzite GaAs would pave the way towards integration on silicon substrate and new device applications. In this article, we present a method to select and maintain the WZ crystal phase in self-assisted NWs by molecular beam epitaxy. By choosing a specific regime where the NW growth process is a self-regulated system, the main experimental parameter to select the ZB or WZ phase is the V/III flux ratio. Using an analytical growth model, we show that the V/III flux ratio can be finely tuned by changing the As flux, thus driving the system toward a stationary regime where the wetting angle of the Ga droplet can be maintained in the range of values allowing the formation of pure WZ phase. The analysis of the in situ reflection high energy electron diffraction evolution, combined with high-resolution scanning transmission electron microscopy (TEM), dark field TEM, and photoluminescence all confirm the control of an extended pure WZ segment, more than a micrometer long, obtained by molecular beam epitaxy growth of self- assisted GaAs NWs with a V/III flux ratio of 4.0. This successful controlled growth of WZ GaAs suggests potential benefits for electronics and opto-electronics applications.

Influence of AlN buffer layer on molecular beam epitaxy growth of wurtzite Al1−xScxN thin films

Wurtzite–Al1−xScxN thin films deposited by solid-state alloying of AlN with ScN exhibit high piezoelectric coefficient and large band gap that makes it a promising material for a variety of applications in piezo-electronics, electronic, acoustoelectric devices, etc. Research on epitaxial Al1−xScxN growth in wurtzite crystal structure is still at an early stage and achieving high scandium (Sc) concentrations in epitaxial films without any phase separation or secondary phase formation is still a critical challenge. Moreover, as most of the reports of wurtzite–Al1−xScxN growth thus far relies on low-vacuum growth techniques, such as magnetron sputtering that are prone to large impurities and contaminants detrimental for device applications, high-vacuum deposition techniques, such as molecular beam epitaxy method needs to be developed. In this paper, we report the epitaxial growth of wurtzite–Al1−xScxN on sapphire (Al2O3) substrates under different Sc fluxes using ultra-high vacuum plasma-assisted molecular beam epitaxy. To prevent ScN phase separation, a 30 nm AlN buffer layer is deposited in situ on GaN epilayers as well as Al2O3 substrates that result in phase-pure wurtzite–Al1−xScxN thin films without any phase separation or secondary phase formation. The structural and compositional analyses performed with high-resolution X-ray diffraction (HRXRD) and secondary ion mass spectroscopy (SIMS), reveal epitaxial wurtzite–Al1−xScxN growth with 0001 orientations on (0001) Al2O3 substrates and the presence of cubic ScN. Demonstration of phase-pure Al1−xScxN on AlN buffer layers will enable the development of devices with improved efficiencies.

Phase-Pure Wurtzite GaAs Nanowires Grown by Self-Catalyzed Selective Area Molecular Beam Epitaxy for Advanced Laser Devices and Quantum Disks

The control of the crystal phase in self-catalyzed nanowires (NWs) is one of the central remaining open challenges in the research field of III/V semiconductor NWs. While several groups analyzed an...

Molecular beam epitaxy and characterization of wurtzite ScxAl1−xN

We demonstrate the growth of pure wurtzite phase ScxAl1−xN with a Sc composition as high as x = 0.34 on GaN and AlN templates using plasma-assisted molecular beam epitaxy. The wurtzite structure is well maintained even at high growth temperatures up to 900 °C for Sc0.2Al0.8N. Smooth surface morphology (root mean square roughness less than 1 nm) and excellent crystal quality [(002) plane rocking curve full-width at half maximum below 450 arc sec] are achieved over the range of x ≤ 0.34. Optical absorption studies indicate a decreasing bandgap with increasing Sc with a linear relationship of Eg(x) = 6.1 − 3.39x, which is in good agreement with the theoretical prediction. A monotonically tunable refractive index between AlN and GaN is further measured for ScxAl1−xN with various Sc compositions. This work provides a viable path for the epitaxy of wurtzite ScxAl1−xN with high Sc compositions. The distinct effect of substitutional Sc on bandgap and refractive index could be used in designing high-performance optoelectronic, electronic, and piezoelectric devices, and III-nitride integrated photonics and optical cavities.

Phase Selection in Self-catalyzed GaAs Nanowires.

Crystal phase switching between the zincblende and wurtzite structures in III-V nanowires is crucial from the fundamental viewpoint as well as for electronic and photonic applications of crystal phase heterostructures. Here, the results of in situ monitoring of self-catalyzed vapor-liquid-solid growth of GaAs nanowires by molecular beam epitaxy inside a transmission electron microscope are presented. It is demonstrated that the occurrence of the zincblende or wurtzite phase in self-catalyzed nanowires is determined by the sole parameter, the droplet contact angle, which can be finely tuned by changing the group III and V fluxes. The zincblende phase forms at small (<100°) and large (>125°) contact angles, whereas pure wurtzite phase is observed for intermediate contact angles. Wurtzite nanowires are restricted by vertical sidewalls, whereas zincblende nanowires taper or develop the truncated edge at their top. These findings are explained within a dedicated model for the surface energetics. These results give a clear route for the crystal phase control in Au-free III-V nanowires. On a more general note, in situ growth monitoring with atomic resolution and at the technological-relevant growth rates is shown to be a powerful tool for the fine-tuning of material properties at the nanoscale.