Chemical Growth Process Research Articles

Quantum chemical calculation and growth process of Ga2O3 grown via TEGa under different oxygen sources in MOCVD

The chemical reaction pathway and growth process of Ga2O3 grown via TEGa and H2O/O2/N2O in MOCVD were investigated.

Selection and Optimisation of Silicon Anodes for All-Solid-State Batteries

In the field of energy storage, Lithium-ion (Li-ion) technology is currently the most widespread on the market. It is widely integrated in portable electronic devices and is gaining interest in the automobile sector due to the development of electrical vehicles. However, its performance are reaching its limits. Solid-state Li-ion battery (SSB) is a very promising technology for next generation energy storage devices due to the promise of higher energy densities and its enhanced safety [1]. SSB are expected to enable a safe use of lithium metal anodes. However, high reactivity of lithium metal with solid electrolyte lead to fast degradation of performances. Another issue is the dendrite growth during cycling [2]. Consequently, silicon has been identified as a promising anode material alternative. Its great abundance, its reasonable working potential of 0,4V vs Li/Li+ and its high theoretical specific capacity of 3579mAh/g are its main advantages. However, due to the formation of an alloy with lithium ions up to the Li15Si4 phase, the silicon will undergo a strong volume expansion of almost 300%. Various techniques are described in the literature to limit this phenomena mainly with liquid electrolytes, such as the use of nano-sized silicon particles [3]–[5]. Moreover, the reactivity of silicon with the solid electrolyte is little studied in all solid-state batteries. In this work, the cyclability and the reactivity of two different silicon materials with sulphide-based solid electrolyte was studied. The silicon materials studied have a different morphology: one is made of commercial micrometric silicon particles (SiMicro) (2-10µm) and the other is silicon nanowires (SiNWs) (10nm) synthesized in the laboratory by a chemical growth process [6].The silicon-based composite consists of 30wt% silicon, 50wt% solid electrolyte (Li6PS5Cl) and 20wt% conductive additive (VGCF). The different powders are mixed by manual grinding for 15 minutes. To study the effect of composite ageing, composite are either used directly for electrochemical characterisation (Fresh Composite) or after a few weeks of storage in the glove box (Aged Composite). The entire cell manufacturing process is carried out in a glove box. The electrochemical tests are performed with Biologic MPG2 and VMP3 at room temperature, between -0.6V and 1V vs. Li-In at a cycling rate of C/20.Normalized capacity as a function of the number of cycles for composites of SiNWs and SiMicro with and without ageing is shown in Figure 1 (a – b). The first striking result is the difference in composite ageing between SiNWs and SiMicro. Indeed, the aged composite causes a strong degradation of the cycling stability for SiMicro compared to the new composite, with 30% of the initial capacity at the 15th cycle versus 60%. On the contrary, no difference is observed for SiNWs between the two types of composites. It is therefore possible to assume that SiMicro has a higher reactivity with the solid electrolyte than SiNWs. To understand what species are formed during storage of the composite and why SiMicro is more reactive than SiNWs in the composite electrode used, XPS analyses were performed. First the raw materials were analysed, then the composite powders with and without ageing. The largest differences between the different powders could be observed with the S2p spectrum presented in Figure 1 (c – d). The presence of sulphur on the nanowires can be explained either by the synthesis conditions or by the storage in a sulphur glove box. The elements observed on the S2p spectrum are therefore very different depending on whether the composite is made with SiMicro or SiNWs because of the difference in the materials alone. With regard to the ageing of the composite powder, the spectra obtained are identical for SiNWs before or after ageing, whereas an evolution is observed with SiMicro. Indeed, a characteristic doublet of oxidised sulphur appears after ageing. It is therefore possible to observe by XPS a degradation of the electrolyte for SiMicro, which is in agreement with the electrochemical characterizations. Future work will involve modifying the surface of SiMicro to compare with the surface of SiNWs obtained after synthesis.[1] A. Kato et al., 2018, doi: 10.1021/acsaem.7b00140.[2] S. Cangaz et al., 2020, doi : 10.1002/aenm.202001320[3] R. Okuno et al., 2020, doi: 10.1149/1945-7111/abc3ff.[4] C. Keller et al., 2021, doi: 10.3390/nano11020307.[5] J. Sakabe et al., 2018, doi: 10.1038/s42004-018-0026-y.[6] O. Burchak et al., 2019, doi: 10.1039/C9NR03749G. Figure 1

Simultaneous oily wastewater adsorption and photodegradation by ZrO2–TiO2 heterojunction photocatalysts

In this work, ZrO2–TiO2 heterojunction photocatalysts were successfully synthesized using sol–gel method. The effect of ZrO2 doped into TiO2 on the retardation of the electron–hole pairs were investigated. The crystalline structure of ZrO2–TiO2 heterojunction photocatalysts was verified through x–ray diffraction (XRD) patterns and the crystallite size were found smaller compared to TiO2 and ZrO2 photocatalysts. Morphological characterization evidenced that the co–doping of ZrO2 into TiO2 has altered the particle size of TiO2 and the shape of the synthesized particle through chemical nucleation and growth process in bulk solution. The small crystallite size of the ZrO2–TiO2 heterojunction recorded the highest surface area with higher incident in the mesopores volume as confirmed by Brunauer–Emmett–Teller (BET) analysis. The adsorption–photodegradation performance of the ZrO2–TiO2 heterojunction photocatalysts on oily wastewater as model pollutant enhanced with the incorporation of small amount of ZrO2 compared to TiO2. The presence of surface adsorbed water peaks and hydroxyl groups as disclosed by Fourier transform infrared (FTIR) supported the finding of the study.

Solution-Processed Titanium Oxide for Rear Contact Improvement in Heterojunction Solar Cells

In this work, we demonstrated a heterojunction Si solar cell utilizing chemically grown titanium oxide (TiOx) as an electron-selective contact layer at its rear surface. With TiOx, the rear surface was passivated to reduce carrier recombination. The reverse saturation current, which is an indicator of carrier recombination, exhibited a 4.4-fold reduction after placing a TiOx layer on the rear surface. With reduced recombination, the open-circuit voltage increased from 433 mV to 600 mV and consequently, the power conversion efficiency (PCE) increased from 9.57 to 14.70%. By X-ray photoemission spectroscopy, the surface passivation was attributed to a silicon oxide interfacial layer formed during the chemical growth process. This passivation results in a 625 cm/s surface recombination velocity for the TiOx-passivated Si surface, which is 2.4 times lower than the sample without TiOx, ensuring the carriers pass through the rear contact without extensive recombination. According to these results, the band alignment for the heterojunction solar cell with and without a TiOx rear contact layer was plotted, the reduced interfacial recombination and the electron and hole blocking structure are the main reasons for the observed efficiency enhancement.

Multimodal spectromicroscopy of monolayer WS2 enabled by ultra-clean van der Waals epitaxy

Van der Waals epitaxy enables the integration of 2D transition metal dichalcogenides with other layered materials to form heterostructures with atomically sharp interfaces. However, the ability to fully utilize and understand these materials using surface science techniques such as angle resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM) requires low defect, large area, epitaxial coverage with ultra-clean interfaces. We have developed a chemical vapor deposition van der Waals epitaxy growth process where the metal and chalcogen sources are separated such that growth times can be extended significantly to yield high coverage while minimizing surface contamination. We demonstrate the growth of high quality 2D WS2 over large areas on graphene. The as-grown vertical heterostructures are exceptionally clean as demonstrated by ARPES, STM and spatially resolved photoluminescence mapping. With these correlated techniques we are able to relate defect density to electronic band structure and, ultimately, optical properties. We find that our synthetic approach provides ultra-clean, low defect density (~1012 cm−2), ~10 μm large WS2 monolayer crystals, with an electronic band structure and valence band effective masses that perfectly match the theoretical prediction for pristine WS2.

A route to efficient optoelectronic devices using Cd1-xInxSe thin films

In the present work, synthesis of Cd1-xInxSe (0 ≤ x ≤ 0.1) thin films was carried out via a novel chemical growth process. The studies on colorimetric, morphological, electrical properties have been carried out to observe the effect of indium doping into CdSe host. The colorimetric studies supported the physical observations on the change of sample colour. X-ray diffraction analysis confirmed the mixture of hexagonal and tetragonal crystal structures in the as-grown thin films. Doping of In was found to improve the surface morphology and grain structure. The surface topography was assessed via studying different topography parameters. Electrostatic force microscopy (EFM) was used to map local conductance through the detection of charge, potential distribution, and doping profile. Informations on the type of semiconductor, carrier concentration and carrier mobility were studied by performing Hall measurements. The electrochemical photovoltaic (EPV) cells were then fabricated with CdSe and Cd1−xInxSe as the photoelectrodes with a sulfide/polysulfide (0.25 M) redox couple and investigated through the J–V and CV characteristics in dark, power output characteristics and various other photoresponsive characteristics. The boost in conversion efficiency from 0.48% to 1.04% was observed with increased composition parameter, x.

Quaternary schematics for property engineering of CdSe thin films

The synthesis of quaternary Cd1-xZnxSySe1-y (0≤x=y≤0.35) thin films was done through indigenously developed chemical solution growth process. As-obtained thin films were subjected to the physical, chemical, structural and optical characterizations. The nearly hydrophobic nature of the as-deposited films except binary CdSe was observed through the wettability studies. The colorimetric studies supported a change in physical color attributes. The elemental analysis done confirmed the formation of Cd(Zn, S)Se and the chemical states of constituent elements as Cd2+, Zn2+, S2− and Se2−. Structural assessment suggested the formation of the polycrystalline quaternary phase of the hexagonal wurtzite structure. The Raman spectroscopy was also employed for the confirmation studies on Cd1-xZnxSySe1-y thin films. Morphological observations indicated microstructural transformation from an aggregated bunch of nano-sized globular grains into a rhomboid network of petal/flakes like crystallites. The atomic force micrographs (AFM) revealed the enhancement in the hillock structures. From advanced AFM characterizations, we observed that the CdSe thin film has leptokurtic (Sku=3.23) surface, whereas, quaternary Cd(Zn, S)Se films have platykurtic (Sku<3) surface. The orientation of the surface morphology was observed through the angular spectrum studies. The optical absorption studies revealed direct allowed transition for the films with a continuous modulation of the energy bandgap from 1.8eV to 2.31eV.

High-speed photoresponse properties of ultraviolet (UV) photodiodes using vertically aligned Al:ZnO nanowires

Ultraviolet (UV) photodiodes with fast photoresponse properties were fabricated using vertically aligned aluminum doped ZnO nanowires. Stable p-type ZnO have been achieved by doping equimolar concentration of P-N (0.75 at.%) simultaneously in ZnO. The vertically aligned and electrically conducting n-type Al (3 at.%) doped ZnO nanowires were grown by a simple aqueous chemical growth process. The structural, morphological, optical, and electrical properties were investigated. For the fabrication of UV photodiodes, the optimum p-type ZnO layers and n-type ZnO nanowires were stacked upon ITO substrate. A 250 nm thin NiO was deposited as an electron blocking layer (EBL) in between the ZnO p–n junctions. The current density–voltage (J–V) characteristic of the fabricated UV photodiode was measured under dark and UV illumination conditions. Under a reverse bias of 3 V, the device exhibits a high photoresponsivity (R) value of 15.07 (A/W) upon illumination of UV light (λ = 365 nm). The fabricated photodiode exhibits a fast photoresponse switching characteristics with a response and recovery time calculated as 61 ± 11 and 455 ± 41 ms, respectively. The role of vertically aligned nanowires in the formation of oxygen interstitial (Oi) defects and its impact on improving the UV photoresponse properties were investigated.

DNA-triggered asymmetric ZnO nanoparticles

Abstract DNA/ZnO nanoparticles were obtained by using a biopolymer-assisted chemical bath growth process. A dynamic light scattering (DLS) technique was used to investigate the size and the growth kinetics of the nanoparticles and scanning electron microscopy (SEM) was used to characterize their morphology. The obtained nanoparticles were larger than the original DNA molecules indicating that DNA acted as a template for the growth of asymmetric DNA/ZnO nanoparticles. The growth kinetics and the particle size were tuned with the solution temperature and composition. Thus, the DNA characteristics can be used to build up different supra-molecular objects by covering with nano ZnO the different polynucleotide structures present in solution. The implications of this process have several positive benefits for building nano-scale device.

Growth and characterization of near white light emitting Al-Ga:ZnO nanowires

Aluminum and gallium dual doped ZnO (AGZO) thin film seed layers are deposited by doping equal concentration of Al and Ga ranging from 0.5–1.0 at% with a step of 0.1 at%. Structural analysis reveals hexagonal wurtzite structure of the AGZO films with a preferential orientation along (002) plane. Morphological analysis shows uniform distribution of crystallites. An average of 80% optical transmittance is observed for AGZO seed layers. The optimum doping concentration is found to be 0.8:0.8 at% of Al:Ga, which exhibits higher electron carrier concentration of 5.83 × 1020 cm−3 and lowest resistivity value of 1.72 × 10−2 Ω cm. The as-deposited conducting AGZO seed layers are eventually treated with an aqueous chemical growth (ACG) process with varying time duration, such as 5, 10 and 15 h in order to obtain one dimensional nanostructures. Morphological analysis shows that the as-grown nanowires are vertically aligned with higher growth density. Enhanced near white light emission has been observed for the as-grown AGZO nanowires with an increase in ACG process duration.

Novel one-dimensional ZnO nanorods synthesized through a two-step post-treatment for efficiency enhancement of dye-sensitized solar cells

Novel 1D irregular ZnO nanorods (ZNR) with a rough surface and a partially hollow structure (IZRH) was synthesized through an aqueous chemical growth process, followed by a two-step post-treatment. ZNR was first treated using an innovative hydrothermal method, followed by freezing at subzero temperatures to form IZRH. The as-prepared IZRH was compounded with hierarchical ZnO nanoparticles (ZNP) synthesized through low-temperature solid-phase method. The mixture of IZRH and ZNP was characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy; it was used as photoanode in dye-sensitized solar cells (DSSCs). The photovoltaic performance of DSSCs was investigated. Result shows that after freezing post-treatment, IZRH can significantly enhance the conversion efficiency of DSSCs. The maximum power conversion efficiency reaches 7.08%, which is 33% and 45% higher than those of only hydrothermal post-treatment and without post-treatment, respectively.

Facile fabrication of ZnO nanorods/ZnO nanosheet–spheres hybrid photoanode for dye-sensitized solar cells

Zinc oxide ( ZnO ) nanorods (ZNRs) and hierarchical ZnO nanosheet–spheres (ZNSs) were prepared through a simple aqueous chemical growth process and a low-temperature solid-phase method, respectively. The prepared ZNRs and ZNSs were mixed to obtain a composite structure by using a circumference oscillator. After structure and morphology characterizations via X-ray diffraction and scanning electron microscopy, the mixture of ZNRs and ZNSs was used as a photoanode in dye-sensitized solar cells (DSSCs). Photovoltaic performance and optimal mixture ratio were investigated. The results indicated that the photovoltaic properties of DSSCs depended on the microstructures, morphologies and mixture ratios of the electrodes. In addition, the mixture of ZNRs and ZNSs (molar ratio of 1:12) yielded an overall light conversion efficiency of 6.02%, with a fill factor of 65.0%, a short-circuit current of 13.49 mA/cm2, and an open-circuit voltage of 0.69 V. These values are higher than those of pure ZNRs or pure ZNSs.

Investigation on P-N dual acceptor doped p-type ZnO thin films and subsequent growth of pencil-like nanowires

Phosphorous and nitrogen dual acceptor doped p-type ZnO (PNZO) have been deposited by spray pyrolysis method on glass substrates. An equimolar doping concentration of P and N were varied from 0.25–1.25 at% with a step of 0.25 at%. Preferred orientation along (002) planes with hexagonal wurzite structure was observed from structural analysis. Morphological analysis reveals uniform distributions of grains. Electrical studies showed dual acceptor doping of P and N in ZnO results in p-type behavior. The optimum doping concentration of P and N was found to be 0.75 at% which exhibited hole concentration of 4.48 × 1018 cm−3 and low resistivity value of 9.6 Ω.cm. Photoluminescence (PL) studies revealed that, as-deposited films exhibit strong UV emission at 383 nm of the spectrum. The surface morphology of the optimum PNZO (0.75 at%) samples were further modified in the form of vertically aligned pencil-like nanowires by modified aqueous chemical growth (ACG) process. During ACG process, more acceptor related defects such as oxygen interstitials (Oi) were formed in the PNZO nanopencils. These acceptor defects induce enhanced emission in the visible region (400 nm to 700 nm) and also promote stable p-type characteristics.

Size-tunable synthesis of monodisperse thorium dioxide nanoparticles and their performance on the adsorption of dye molecules

We report a facile method to synthesize monodisperse, uniform and size-controllable thorium dioxide nanoparticles via a hydrothermal precursor synthesis and a subsequent calcination treatment. Highly uniform spherical precursor nanoparticles with a tunable diameter of 38–274 nm and monodisperse irregular precursor nanoparticles with a size range of 20–45 nm can be successfully synthesized using thorium nitrate pentahydrate, urea and glycerol as the metal source, nucleation agent, and size-controlling agent, respectively. The chemical composition, growth process, and size control of the precursor nanoparticles have been systematically investigated, and a probable formation mechanism of these nanoparticles was also proposed. Calcination of precursor nanoparticles at 800 °C leads to the formation of uniform thorium dioxide nanoparticles without any morphological deformation. Furthermore, the size and shape effects of thorium dioxide nanoparticles on the uptake of dye molecules have been studied. The corresponding adsorption behaviors of as-prepared samples can be elucidated very well by the nitrogen adsorption–desorption analysis.

Low temperature growth of ZnO nanowire arrays with enhanced high performance photocatalytic activity and reusability

Aligned ZnO nanowire arrays have been grown on the silicon substrates with ZnO seed layer by an aqueous chemical growth process at low reaction temperature. The volumes of ammonium hydroxide solution are not only controlled by the dimension and density of ZnO nanowires, but are also influenced by their optical emission property. The ZnO nanowire arrays exhibit very strong and broad green emission from defect in the cathodoluminescence spectrum. In addition, the ZnO nanowires grown on the silicon substrates can serve as effective and convenient recyclable photocatalysts. Only a very slight decrease in the photodecomposition rate was observed after ten cycles of the photocatalysis experiment.

Enhanced visible emission from vertically aligned ZnO nanostructures by aqueous chemical growth process

ZnO nanostructures consisting of nanowires and nanorods have been grown by aqueous chemical growth (ACG) process on the ZnO seed layers. ACG process has been carried out for 5h and 15h to obtain the one dimensional nanostructures. This ZnO nanostructure comprises mixture of vertically aligned nanorods and nanowires. Structural analysis reveals that the crystal orientation is along (002) plane exhibiting a hexagonal wurtzite structure. The crystallinity of ZnO nanostructures increases after aqueous chemical treatment with respect to the growth duration. Optical studies revealed that when the duration of aqueous chemical treatment is increased, the transmittance of the films decreases considerably. The as-deposited ZnO seed layers exhibit electron carrier concentration of 4.69×1014cm−3 and the value increases to 9.44×1015cm−3 after 5h and 1.53×1016cm−3 after 15h of aqueous chemical treatment. It has been observed that ZnO nanostructures exhibit enhanced luminescence properties in visible region which covers the entire visible region of the spectrum.

Epitaxial growth of vertically aligned highly conducting ZnO nanowires by modified aqueous chemical growth process

We report the fabrication of one dimensional conducting Al doped ZnO nanowires by spray pyrolysis and subsequent aqueous chemical growth (ACG) process. Al doped ZnO (AZO) thin film seed layers have been prepared with the Al concentration varying from 1 to 5at% by spray pyrolysis technique. The structural analysis shows that the as grown films have hexagonal wurtzite crystal structure with a preferential orientation of (002). The AZO seed layers have an average optical transmittance of 80%. It is observed that 3at% is the optimum value of Al doping concentration in ZnO, showing lowest electrical resistivity of 3.51×10−2Ωcm with a carrier concentration of 1.52×1019cm−3 and exhibiting n-type behavior. This optimum AZO (3at%) films are used as a seed layers for the growth of vertically aligned one dimensional highly conducting ZnO nanowires by a modified aqueous chemical growth (ACG) process. The optical transmittance is found to decrease to the order 50% after the AZO nanowires grown upon AZO (3at%) films. As grown ZnO nanowires exhibit an increased carrier concentration of 2.87×1020cm−3 and a lower resistivity of 3.00×10−2Ωcm in comparison to AZO 3at% seed layers. Photoluminescence studies reveal that AZO seed layers and nanowires exhibit strong UV emission at 378nm. Moreover, AZO nanowires show enhanced luminescent properties in the visible region ranging from 400 to 700nm.

Morphology and crystallinity control of wet chemically grown ZnO nanorods

The controlled growth of ZnO nanorods on technologically relevant substrates is essential for their practical integration into next generation optoelectronic and piezoelectric devices. In this report, highly ordered and uniform vertical ZnO nanorod arrays were synthesized using a facile, low temperature selective area wet chemical growth process. The nanorods were grown through nucleation windows that were patterned in a PMMA mask using electron beam lithography. At first, the technique was demonstrated on `ideal' ZnO single crystal substrates, where the geometrical parameters of the highly uniform and crystallographically coherent nanorods were dictated by the nucleation pattern, the polarity of the substrate, and the growth conditions. The obtained geometry was then compared to 4 further arrays corresponding to different ZnO seed layers deposited on Si and sapphire substrates. Scanning electron microscopy showed that the crystal orientation and the alignment of the nanorods were determined by the underlying seed layer. The piezoresponse force microscopy revealed that the d_{33} piezoelectric tensor component of the wet chemically grown nanorods was comparable (6-12 pm/V) to that of the highest value measured on ZnO single crystal (12.4 pm/V). The presented nanorod arrays have several potential applications, from nanorod based light emitting devices to CMOS compatible piezoelectric nanoforce sensors.

Light transmission enhancement from hybrid ZnO micro-mesh and nanorod arrays with application to GaN-based light-emitting diodes

A hybrid ZnO micro-mesh and nanorod arrays (MMNR) was fabricated as a light output window for GaN-based light-emitting diodes (LEDs) to enhance the light extraction efficiency. The light output power of GaN-based LEDs with the ZnO MMNR is improved by 95% compared to the original planar LEDs. The ZnO MMNR is manufactured by photolithography techniques and a two-step wet chemical growth process. The incident angle-resolved light transmission of the ZnO MMNR beyond the critical angle of total internal reflection is greatly enhanced. The light diffraction pattern of the ZnO MMNR shows that it possesses both the two-dimensional diffraction grating effect of a ZnO micro-mesh and the light scattering effect of a ZnO nanorod array. LEDs with the ZnO MMNR have greater light extraction efficiency than those with only a ZnO micro-mesh or a ZnO nanorod array. The local optical field patterns of the ZnO micro-mesh and the ZnO MMNR are investigated using confocal scanning electroluminescence microscopy. The microscopic light extraction mechanism of the ZnO MMNR is analyzed in-depth.

A Cu/ZnO Nanowire/Cu Resistive Switching Device

Abstract A new device has been realized using flip-chip joining two printed circuit boards (PCBs) on which zinc oxide (ZnO) nanowires were synthesized. Energy dispersive X-ray measurement has been conducted for the ZnO nanowires, confirming that Cu elements have been diffused into the nanowires during the chemical growth process. From I–V measurements, this Cu/ZnO nanowire/Cu structure exhibits a resistive tuning behaviour, which varies greatly with the frequency of the applied sinusoidal source.