Diffusion-controlled Growth Process Research Articles

Insights into Perovskite Film Formation Using the Hybrid Evaporation/Spin-Coating Route: An In Situ XRD Study

The hybrid evaporation/spin-coating route has been widely used for fabrication of highly efficient fully textured perovskite silicon tandem solar cells and is a promising route for upscaling. Nevertheless, fundamental aspects of the fabrication process such as the kinetics of perovskite crystallization and the influence of the environmental conditions remain uncertain. In this work, we investigate the individual stages of tandem-relevant 1.66 eV bandgap perovskite formation and map the structural evolution from the precursor to the perovskite phase until the degradation phase. We find that the kinetics of these transitions can be tuned by varying the humidity or the temperature during the annealing treatment. Specifically, increasing the relative humidity up to 50% elevates the reaction rate and results in improved perovskite quality. This is directly reflected in the solar cell power conversion efficiency with a 3%abs increment on average. While high annealing temperatures are found to promote large grain size growth and enhanced crystallinity, we observe that above 150 °C the remnant PbI2 precursor compromises film quality. Furthermore, the perovskite formation kinetics is fitted using the Johnson–Mehl–Avrami–Kolmogorov model. The Avrami constant is found in the range 0.66–0.87, indicating a diffusion-controlled one-dimensional growth process with an associated activation energy of 54.49 kJ/mol. Using in situ XRD, this work gives insights on key process parameters to ensure reproducible synthesis of high-quality perovskite films.

The influence of leveler Brilliant Green on copper superconformal electroplating based on electrochemical and theoretical study

Finding proper additive to achieve copper superconformal electrodeposition is significantly important. Brilliant Green exhibits excellent suppressing ability to the copper electrodeposition according to electrochemical analysis. Meanwhile, 100 mg/L was selected as the optimum value based on the convection-dependent adsorption behavior analysis of Brilliant Green. The interaction among three different additives was also investigated by applying chronopotentiometry as well as Brilliant Green compete to absorb on the cathode surface with SPS. Besides, the introduction of Brilliant Green can improve the transport of cupric ions (Cu2+). The nucleation and growth of copper deposition is 3D diffusion-controlled instantaneous growth process at high overpotential but 3D diffusion-controlled mixing growth process at low overpotential. The synergistic suppressing mechanism was proposed to explain strong suppressing effect of BG on reduction of Cu2+ and the reaction pathways was studied theoretically. Copper interconnect layer with high FP values (80.52 % to 84.38 %) was obtained with low SDT (about 28 μm) after electroplating process was optimized. The surface morphology under the influence of Brilliant Green is compact and uniform and the grain size is reduced by Brilliant Green.

Constitutive and microstructural characteristics of Ti-5Al-2.5Sn alloy during isothermal and non-isothermal multi-stage hot deformation across different phase regions

The flow behaviors and microstructural evolutions during multi-stage hot deformation are essential aspects in the processing of high performance titanium alloys. In the present work, both isothermal multi-stage deformation (IMD) and non-isothermal multi-stage deformation (NMD) over α + β and single β phase regions were employed to study the constitutive behaviors and microstructural evolutions of a near- α Ti-5Al-2.5Sn alloy. The results showed that the flow stress, restoration fraction in α + β phase region of NMD were lower than that of IMD, because that the predominant lamellar microstructures formed during NMD increased α/β boundaries and thereby promoted grain boundaries gliding. In β phase region, the flow behaviors showed slight differences with temperature and deformation passes during both IMD and NMD processes. Clear transitions were observed from the mean flow stress between different phase regions during NMD. In addition, an obvious α + β → β dynamic phase transformation occurred during deformation at 990 °C, which was identified to be a displacive nucleation and diffusion-controlled growth process. In addition, numerous FCC-Ti precipitations were observed during IMD at 960 °C and strain rate of 1 s −1 with the orientation relationship of (0 0 0 1) HCP matrix //{1 1 1} FCC . • Isothermal and non-isothermal multi-stage deformation across different phase regions were performed on a Ti-Al-Sn alloy. • The flow stress during isothermal multi-stage deformation in dual phase region was higher than that of non-isothermal ones. • Clear transitions were observed from the mean flow stress between different phase regions during non-isothermal deformation. • An obvious α→β dynamic phase transformation occurred during isothermal multi-stage deformation at 990 °C. • Numerous FCC-Ti precipitates were observed during isothermal multi-stage deformation at 960 °C.

Experimental investigation on kinetic behavior of the deformation-induced α→β transformation during hot working of Ti–6Al–2Zr–1Mo–1V alloy below the β-transus

Hot compression test was performed on Ti–6Al–2Zr–1Mo–1V alloy below the β-transus (Tβ) with the temperature range of 940–970 °C. Microstructural evolution, especially on the equiaxed primary α phase (αp), was characterized by scanning electron microscope (SEM), electron backscatter diffraction (EBSD) and transmission electron microscope (TEM). The results showed that the αp could transform into β during hot deformation at α+β two-phase region, i.e., deformation-induced transformation of α→β (DIT). DIT, as an additional softening mechanism, caused the unusual stress response on the flow curves, which was reflected by the fact of the steady stress (σst)<initial yield stress (σ0.2). The fraction of transformed αp was influenced by various hot working parameters (such as temperature, strain and strain rate). Even complete dissolution of αp was present in some cases, indicating a premature Tβ during straining. The DIT is considered as a displacive nucleation and diffusion-controlled growth process. Its mechanism can currently be divided into two types: internal penetration of αp and α/β interface migration. The αp particles with higher favorable orientation preferentially undergo phase transformation. A kinetic model based on the transformed αp fraction (ωαp(t)) during DIT was developed by taking into account the thermal activation contribution (corresponding to heating duration) and mechanical driving force contribution (corresponding to deformation). The model was fitted to the ωαp(t) as a sigmoidal function of time (t) based on the classical Johnson–Mehl–Avrami (JMA) equation. Such a quantitative kinetic model for DIT can be used to accurately tailor the required balance between αp and β in Ti–6Al–2Zr–1Mo–1V alloy during hot working.

General synthesis of ultrafine metal oxide/reduced graphene oxide nanocomposites for ultrahigh-flux nanofiltration membrane

Graphene-based membranes have great potential to revolutionize nanofiltration technology, but achieving high solute rejections at high water flux remains extremely challenging. Herein, a family of ultrafine metal oxide/reduced graphene oxide (rGO) nanocomposites are synthesized through a heterogenous nucleation and diffusion-controlled growth process for dye nanofiltration. The synthesis is based on the utilization of oxygen functional groups on GO surface as preferential active sites for heterogeneous nucleation, leading to the formation of sub-3 nm size, monodispersing as well as high-density loading of metal oxide nanoparticles. The anchored ultrafine nanoparticles could inhibit the wrinkling of the rGO nanosheet, forming highly stable colloidal solutions for the solution processing fabrication of nanofiltration membranes. By functioning as pillars, the nanoparticles remarkably increase both vertical interlayer spacing and lateral tortuous paths of the rGO membranes, offering a water permeability of 225 L m−2 h−1 bar−1 and selectivity up to 98% in the size-exclusion separation of methyl blue.

The oxidation behavior of Mg-Er binary alloys at 500 ℃

Thermodynamics and kinetics of the oxidation behavior of magnesium-erbium (Mg-Er) alloys at 500 ℃ were studied. The observed protective oxidation behavior, which follows parabolic kinetics, can be attributed to the formation of compact Er2O3 surface layer and continuous dissolution and diffusion of Er from the matrix. The surface film of Mg-8.1Er (wt%) alloy was composed of four sublayers, with the double Er2O3 layers consisting of a compact fine-grain layer and an underlying coarse-columnar-grain layer providing the protective effect. A four-stage oxidation process composed of three transient reaction-controlled oxidation processes and followed by a diffusion-controlled oxides growth process is proposed.

A contribution to exploring the importance of surface air nucleation in froth flotation – The effects of dissolved air on graphite flotation

The formation of surface microbubbles induced by air nucleation on graphite surfaces and the air diffusion process in oversaturated water play important roles in increasing the recovery of graphite and other valuable minerals in flotation. A microscope equipped with a cuvette, a laser diffraction particle size analyzer, and single bubble pick-up experiments were combined with micro-flotation experiments to clarify these effects. The diffusion-controlled growth process of surface microbubbles was observed with a microscope. It can be shown that higher degrees of dissolved air can improve the probability of surface microbubbles forming on graphite surfaces. Micro-flotation and microscopic experiments confirmed that surface microbubbles occurred selectively on graphite surface but not quartz. Besides, bubble-particle aggregates formed during the conditioning process were observed under the microscope while bubble pick-up experiments indicated that the bubble load increased with the increasing degree of dissolved air. Size distribution analysis also showed that the nucleation microbubbles on graphite surfaces improved the recovery of fine graphite particles due to the formation of microbubble-particle aggregates. Coarser microbubble-particle aggregates induced by surface nucleation bubbles can improve the collision and attachment probability to external carrying bubbles compared to single graphite particles, which is especially relevant for fine particles. This study indicates that nucleation microbubbles on graphite surfaces can significantly promote flotation efficiency, and shows the importance of air nucleation on mineral surfaces in flotation process.

Study of the crystallization kinetics of a Zr57Cu15.4Ni12.6Al10Nb5 amorphous alloy

Abstract The non-isothermal crystallization kinetics with heating rates ranging from 10 K s-1to 80 K s-1and the isothermal crystallization kinetics during annealing from the glass transition temperature to the crystallization onset temperature of a Zr57Cu15.4Ni12.6Al10Nb5 amorphous alloy were studied in detail using X-ray diffraction and differential scanning calorimetry. During non-isothermal crystallization, it is more difficult to nucleate than to grow, and the crystallization resistance increases first and then decreases. During isothermal crystallization of the alloy from 713- 728 K, there are two exothermic peaks corresponding to a diffusion-controlled growth process with decreasing nucleation rate and increasing nucleation rate. From 733- 748 K, only one exothermic peak appears, and the growth process is controlled by the interface with decreasing nucleation rate. Isothermal crystallization is a process in which the crystallization resistance increases. The resistance of isothermal crystallization is less than that of non-isothermal crystallization.

Study of the crystallization kinetics of a Zr57Cu15.4Ni12.6Al10Nb5 amorphous alloy

Abstract The non-isothermal crystallization kinetics with heating rates ranging from 10 K s-1to 80 K s-1and the isothermal crystallization kinetics during annealing from the glass transition temperature to the crystallization onset temperature of a Zr57Cu15.4Ni12.6Al10Nb5 amorphous alloy were studied in detail using X-ray diffraction and differential scanning calorimetry. During non-isothermal crystallization, it is more difficult to nucleate than to grow, and the crystallization resistance increases first and then decreases. During isothermal crystallization of the alloy from 713- 728 K, there are two exothermic peaks corresponding to a diffusion-controlled growth process with decreasing nucleation rate and increasing nucleation rate. From 733- 748 K, only one exothermic peak appears, and the growth process is controlled by the interface with decreasing nucleation rate. Isothermal crystallization is a process in which the crystallization resistance increases. The resistance of isothermal crystallization is less than that of non-isothermal crystallization.

Mechanism study of Cu-Zn alloys electrodeposition in deep eutectic solvents

Copper and copper-zinc alloys were electrochemical prepared from Cu2O-ZnO mixed oxides in urea and 1-butyl-3-methylimidazolium chloride deep eutectic solvent at 383 K. The electrochemical reduction and nucleation mechanism of copper and copper-zinc alloys were investigated by cyclic voltammetry and chronoamperometry. Cyclic voltammetry showed that copper species [Cu2ClO·urea]− was reduced to copper at − 0.68 V (vs. Ag), while zinc species [ZnClO·urea]− was reduced to zinc and formed copper-zinc alloy at − 1.8 V (vs. Ag). Chronoamperometry indicated that copper particles were electrodeposited by progressive nucleation with diffusion-controlled growth process in urea-BMIC-Cu2O solutions, and the average diffusion coefficient of copper complex ions was determined to be 2.79 × 10−8 cm2 s−1. On contrast, the electrodeposition of copper-zinc alloy proceeded via combination of 3D instantaneous and progressive nucleation. On the other hand, copper and copper-zinc alloys were electroplated on to a stainless steel substrate and characterized by X-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy.

Thermal and structural analysis of 40La2O3-10Nb2O5-(50-x)Al2O3-xBaO glasses

Aluminate glasses have excellent optical and mechanical properties, in addition to high thermal and chemical stability. However, aluminate glasses require high melting temperatures and exhibit poor glass forming ability, causing difficulties while preparation by the melt-quench method. Therefore, it is necessary to establish novel methods to improve melting conditions and glass forming tendencies for aluminate glasses. 40La2O3-10Nb2O5-(50-x)Al2O3-xBaO (x = 0, 2.5, 5, 7.5, 10) glasses were prepared by the aerodynamic levitation method. XRD, DTA, and Raman and NMR spectroscopic techniques were used to investigate the effects of BaO on the structure and thermal properties—including crystallization behavior—of lanthanum aluminate glass. The results show that the addition of BaO increases the glass transition temperature (Tg) and stability ΔT of the baseline glass, as Al-Ba-7.5 (40Al2O3-10Nb2O5–42.5Al2O3–7.5BaO) glass has the highest ΔT value (151 °C). The 27Al MAS NMR spectrum of a high BaO-content glass (Al-Ba-10) shows that about 80% of [IV]Al and 15% of [V]Al are present in the glass network. Raman scattering spectra indicate that a peak shown around 790 cm−1canbe assigned to the internal stretching vibrations of the Al-non-bridging oxygen (NBO) bonds in depolymerized [AlO4/2] tetrahedrons. The crystallization activation energies were calculated to be 1157.1 kJ/mol and 891.7 kJ/mol (Kissinger method) for Al-Ba-x (x = 0, 7.5) glasses, respectively, which progresses as a diffusion-controlled three-dimensional growth process. The nucleation rate of Al-Ba-0 decreases continuously, while Al-Ba-7.5 increases.

Molecular-Level Understanding of Continuous Growth from Iron-Oxo Clusters to Iron Oxide Nanoparticles.

The formation of inorganic nanoparticles has been understood based on the classical crystallization theory described by a burst of nucleation, where surface energy is known to play a critical role, and a diffusion-controlled growth process. However, this nucleation and growth model may not be universally applicable to the entire nanoparticle systems because different precursors and surface ligands are used during their synthesis. Their intrinsic chemical reactivity can lead to a formation pathway that deviates from a classical nucleation and growth model. The formation of metal oxide nanoparticles is one such case because of several distinct chemical aspects during their synthesis. Typical carboxylate surface ligands, which are often employed in the synthesis of oxide nanoparticles, tend to continuously remain on the surface of the nanoparticles throughout the growth process. They can also act as an oxygen source during the growth of metal oxide nanoparticles. Carboxylates are prone to chemical reactions with different chemical species in the synthesis such as alcohol or amine. Such reactions can frequently leave reactive hydroxyl groups on the surface. Herein, we track the entire growth process of iron oxide nanoparticles synthesized from conventional iron precursors, iron-oleate complexes, with strongly chelating carboxylate moieties. Mass spectrometry studies reveal that the iron-oleate precursor is a cluster comprising a tri-iron-oxo core and carboxylate ligands rather than a mononuclear complex. A combinatorial analysis shows that the entire growth, regulated by organic reactions of chelating ligands, is continuous without a discrete nucleation step.

Preparation of covalently bonded polyaniline nanofibers/carbon nanotubes supercapacitor electrode materials using interfacial polymerization approach

In this paper, the covalently bonded polyaniline (PANI) nanofiber/multi-walled carbon nanotubes (MWCNT) composites were synthesized via interfacial polymerization of aniline with para-phenylenediamine functionalized MWCNT at the interface of oil/water system. Owing to the diffusion-controlled growth process of PANI, PANI with uniform fiber structure were obtained. The morphology analysis showed that the diameter of PANI nanofiber decreased with the increasing of MWCNT loading amount. Impedance analysis showed that the charge-transfer resistances of the composites were reduced also with the increasing of MWCNT loading amount. The decreasing of charge-transfer resistances and change of morphology resulted in enhanced capacitive properties. Electrochemical tests showed that the specific capacitance of PANI, PANI/MWCNT-10% and PANI/MWCNT-20% were 405, 641 and 764 F·g-1, respectively. As comparison with pure PANI nanofiber, the specific capacitance of the composites increased by 58% and 88.6%, respectively.

Electrochemical Behavior of Co(II) Reduction for Preparing Nanocrystalline Co Catalyst for Hydrogen Evolution Reaction from 1-ethyl-3-methylimidazolium Bisulfate and Ethylene Glycol System

The electrochemical behavior of Co(II) reduction and Co nucleation/growth process on glassy carbon (GC) electrode in 1-ethyl-3-methylimidazolium bisulfate ([EMIM]HSO4) ionic liquid (IL) and ethylene glycol (EG) system is investigated. Cyclic voltammetry (CV) measurements indicate that Co(II) reduction occurs by a one-step process, Co(II) to Co(0), and it is an irreversible reaction. The diffusion coefficient of Co(II) is 2.24 × 10−6 cm2 s−1 at 323 K in the system. Chronoamperometry measurements show that the growth/nucleation process of Co on GC electrode in [EMIM]HSO4-EG is a three-dimensional (3D) progressive nucleation at lower overpotentials and instantaneous nucleation at higher overpotentials under diffusion controlled growth process. These effects of electrodepositing potential, current density and temperature on Co coating thickness are also investigated. The Co coatings are observed by energy dispersive spectrometer (EDS), scanning electron microscope (SEM) and X-ray diffractometer (XRD). SEM micrographs confirm that the Co coatings are relatively loose with a fibrous surface morphology. XRD pattern of the prepared coating reveals the characteristic peak of crystalline Co with a preferred orientation direction and the average size of Co grains is 11 nm. The nanocrystalline Co coating exhibits an excellent catalytic activity and stability for hydrogen evolution reaction (HER) in alkaline medium.

Synthesis and Evaluation of RhxSy Catalyst with High Affinity for Nafion Ionomer for HOR/HER in a H2-Br2 Regenerative Fuel Cell

Synthesis and evaluation of RhxSy with high mass-specific electrochemical surface area (ECSA/mass) and high affinity to Nafion polymer are reported. Besides the previously reported supporting carbon functionalization, the nanoparticle size was optimized by a diffusion-controlled growth process to control Rh2S3 monomer concentration, which reduced the average particle size from 13.2 to 5.2 nm and increased the ECSA/mass from 9.1 to 36.5 m2/g-Rh. The affinity issue between the surface ketone functional group and the Nafion ionomer in the catalyst ink was resolved by using the Baeyer-Villiger reaction and ester hydrolysis to convert it to the Nafion-friendly carboxylic group with no adverse effect on the precipitated catalysts. The H2-Br2 fuel cell tests confirmed that the high mass transfer issue observed with the RhxSy catalysts on ketone-dominated carbon substrate was resolved with the new catalysts, validating the hypothesis that the low affinity to Nafion polymer of the surface ketone group was the cause for the formation of thick Nafion polymer layer on the catalyst surface and high mass transportation resistance. Using the new catalyst, the discharge performance of the H2-Br2 fuel cell improved by 5.6 times (0.15 vs. 0.027 A/cm2 at 0.2 V below OCV) over that of the fuel cell with the RhxSy/untreated carbon catalyst.

Effects of oxygen and water vapor on the formation and growth of silica layer beneath barium strontium aluminosilicate coatings

Barium strontium aluminosilicate (BSAS)-coated C/SiC composites were corroded under environments containing varying H2O/O2 ratios at elevated temperatures. After corrosion, continuous thermally grown silica oxide (silica-TGO) was formed at the BSAS coating/SiC bond coat interface, and the thickness of this layer increased with the corrosion time prolonged. The growth kinetics of the silica-TGO layer formed beneath the BSAS coating upon corrosion with a 50%H2O–50%O2 flowing gas mixture at elevated temperatures were examined. The results revealed that this silica-TGO layer beneath BSAS grew following a parabolic behavior, in line with a diffusion-controlled growth process. Based on the calculated growth rates and the activation energies of the silica-TGO layer, oxygen ion was identified as the main oxidant. The corrosion experiments under flowing gas mixtures containing varying H2O/O2 ratios revealed that water vapor was also involved in the growth of the silica-TGO layer.

The isokinetic behavior in diffusion controlled growth processes

The isokinetic behaviour of crystallization processes controlled by diffusion is analysed under both isothermal and continuous cooling/heating conditions. The kinetic function is computed employing an approximate expression for the so called exponential integral. Diffusion controlled 3D growth of homogeneously nucleating crystalline grains is an isokinetic reaction if the nucleation and diffusion activation energies are identical. In the non isokinetic range, the kinetic functions in both continuous heating and isothermal transformations are the same except for a factor which depends on the nucleation and diffusion activation energies ratio. This development is applied to the crystallization of AgGeSe glasses. The primary crystallization kinetic of glasses with compositions (Ge25Se75)100−y Agy (with y = 10, 15, 20 and 25 at. %) was studied in a previous work using differential scanning calorimetry and X-ray diffraction. The analysis is grounded on the Kolmogorov-Johnson-Mehl-Avrami model generalized to account for the compositional changes of the parent phase, responsible for the decreasing of both the nucleation frequency and the growth rate of the primary grains. The kinetic study of the crystallization process from continuous heating calorimetric data has been performed applying the master curve method. The primary crystallization product is the ternary phase γ-Ag8GeSe6. The values of apparent activation energy Ea, obtained are in the range: 2.0 eV/at < Ea < 2.6 eV/at. A model of diffusion controlled 3D growth with decreasing homogeneous nucleation and soft impingement has been developed that reproduces the rate of transformation obtained experimentally for the studied alloys.

Electrodeposition of Zn and Cu–Zn alloy from ZnO/CuO precursors in deep eutectic solvent

The electrodeposition of Zn and Cu–Zn alloy has been investigated in choline chloride (ChCl)/urea (1:2 molar ratio) based deep eutectic solvent (DES). Cyclic voltammetry study demonstrates that the reduction of Zn(II) to Zn is a diffusion-controlled quasi-reversible, one-step, two electrons transfer process. Chronoamperometric investigation indicates that the electrodeposition of Zn on a Cu electrode typically involves three-dimensional instantaneous nucleation with diffusion-controlled growth process. Micro/nanostructured Zn films can be obtained by controlling the electrodeposition potential and temperature. The electrodeposited Zn crystals preferentially orient parallel to the (101) plane. The Zn films electrodeposited under more positive potentials and low temperatures exhibit improved corrosion resistance in 3wt% NaCl solution. In addition, Cu–Zn alloy films have also been electrodeposited directly from CuO–ZnO precursors in ChCl/urea-based DES. The XRD analysis indicates that the phase composition of the electrodeposited Cu–Zn alloy depends on the electrodeposition potential.

(Invited) High Performance Deep Ultraviolet Nanowire Light Emitting Diodes and Lasers

Deep ultraviolet light sources are important for a broad range of applications including water purification, disinfection, and medical diagnostics. The currently reported AlGaN planar light emitting diodes (LEDs) and lasers generally exhibit poor performance, due to the presence of large densities of defects and dislocations and the inefficient p-type doping. In this context, we have investigated the molecular beam epitaxial growth and characterization of nearly defect-free AlGaN nanowire heterostructures grown directly on Si substrate. We have demonstrated AlGaN nanowire LEDs that can operate in the UV-B (280-320nm) and UV-C (<280 nm) bands, which exhibit excellent current current-voltage characteristics. We have further demonstrated electrically injected AlGaN nanowire lasers on Si that can exhibit relatively low threshold current densities (~ 100 A/cm2). In this work, catalyst-free AlGaN nanowires were grown directly on Si substrate by radio-frequency plasma-assisted molecular beam epitaxy. Due to the diffusion controlled growth process, such self-organized AlGaN nanowires exhibit unique core-shell structures, with the presence of Al-rich AlGaN shell. As a consequence, the nonradiative surface recombination can be greatly suppressed. By varying the Al/Ga flux ratio during the growth process, the emission wavelengths can be controllably varied from ~ 210 nm to >340 nm. The nanowire heterostructures exhibit relatively high internal quantum efficiency (~ 80%) at room-temperature in the UV-C band. We have further identified that Mg-dopant incorporation can be significantly enhanced in AlGaN nanowires, compared to their planar counterparts, due to the reduced formation energy for Al(Ga)-substitutional Mg-doping. The resulting AlN nanowire LEDs can exhibit excellent current-voltage characteristics, with a turn-on voltage of 6 V. We have also demonstrated that self-organized AlGaN nanowire arrays can function as a high Q optical cavity, due to the Anderson localization of light. The AlGaN nanowire lasers consist of n-AlGaN cladding layer, i-AlGaN active region, and p-AlGaN cladding layer. The devices were fabricated using standard photolithography and metallization techniques. Under continuous wave biasing conditions, the devices can exhibit very low threshold current density. For devices operating in the UV-AII (~ 320-340 nm) band, the threshold current density is ~ 12 A/cm2 at 6 K. The threshold current density increases slightly to ~ 200 A/cm2 for operation wavelengths in the UV-C band. The measured threshold current densities are nearly two to three orders of magnitude lower, compared to conventional AlGaN quantum well lasers under either optical pumping or electrical injection.

Electrochemical Mechanism of Cr(III) Reduction for Preparing Crystalline Chromium Coatings Based on 1-Ethyl-3-methylimidazolium Bisulfate Ionic Liquid

The electrochemical mechanism of Cr(III) reduction and chromium nucleation/growth process on a glassy carbon (GC) electrode in 1-ethyl-3-methylimidazolium bisulfate ([EMIM]HSO4) ionic liquid are studied. These results from the cyclic voltammetry (CV) show that the reduction of Cr(III) occurs by a two-step process, Cr(III) to Cr(II) and Cr(II) to Cr(0), respectively, the first step is an irreversible process with a diffusion coefficient of Cr(III) in solution of 2.60 × 10−7 cm2 s−1 at 303 K and 15.99 × 10−7 cm2 s−1 at 358 K. The reduction has been verified to be a two-step process by electrochemical impedance spectroscopy (EIS) study. In the EIS measurements, the preferable equivalent circuit is also made to fit the experimental data. These effects of electrodeposition potential, current density and electrolyte temperature on the coating thickness are studied. These results show that the electrodeposition potential, current density and temperature have great effect on the coating thickness. The chromium deposits are studied by SEM, EDS and XRD. XRD pattern of the electrodeposited layer reveals the characteristic peak of crystal Cr. Chronoampererometry experiments reveal that the growth/nucleation mechanism of chromium in [EMIM]HSO4 on GC electrode is more progressive in character at lower overpotentials and a three-dimensional instantaneous nucleation behavior under diffusion controlled growth process at higher overpotentials.