Quasi-single Crystalline Research Articles

Reduced internal stress of quasi-single crystalline Na4Fe3(PO4)2P2O7 electrode enhancing sodium-ion kinetics

A NASICON-type quasi-single crystalline Na4Fe3(PO4)2P2O7 (SC) cathode material was successfully synthesized via a freeze-drying method. The as-optimized SC sample displays both outstanding rate performance (107.9 and 88.5 mAhg−1 at 0.1C and 5C, respectively) and excellent cycling stability (88.1 % of capacity retention after 1000 cycles at 1C). Reduced sodium ion migration path of as-prepared SC cathode material effectively enhanced the kinetics of Na+ upon cycling. More importantly, the SC structure could suppress nearly 50 % of internal strain, prodigiously reducing the local stress accumulation accompanied by the inhomogeneous sodium concentration gradient during the charging/discharging processes. The smaller the stress accumulation, the more stable the crystal structure would be obtained, which maintained a more superb mechanical stability of the SC cathode material. At the same time, a high-spin state of Fe in the SC sample was confirmed by both of electron paramagnetic resonance and temperature-dependent magnetization susceptibility, indicating that the eg orbital occupation of Fe2+ could be regulated to optimize the bond strength between the reduction and oxidation processes. As a consequence, the band gap of SC material has decreased from 1.66 to 1.45 eV, and the electronic conductivity has increased from 6.0 to 32.4 μS/cm. It is believed that the SC cathode material could be a competitive candidate material for sodium ion batteries.

Quantifying Degradation Parameters of Single-Crystalline Ni-Rich Cathodes in Lithium-Ion Batteries.

Single-crystal LiNix Coy Mnz O2 (SC-NCM, x+y+z=1) cathodes are renowned for their high structural stability and reduced accumulation of adverse side products during long-term cycling. While advances have been made using SC-NCM cathode materials, careful studies of cathode degradation mechanisms are scarce. Herein, we employed quasi single-crystalline LiNi0.65 Co0.15 Mn0.20 O2 (SC-NCM65) to test the relationship between cycling performance and material degradation for different charge cutoff potentials. The Li/SC-NCM65 cells showed >77 % capacity retention below 4.6 V vs. Li+ /Li after 400 cycles and revealed a significant decay to 56 % for 4.7 V cutoff. We demonstrate that the SC-NCM65 degradation is due to accumulation of rock-salt (NiO) species at the particle surface rather than intragranular cracking or side reactions with the electrolyte. The NiO-type layer formation is also responsible for the strongly increased impedance and transition-metal dissolution. Notably, the capacity loss is found to have a linear relationship with the thickness of the rock-salt surface layer. Density functional theory and COMSOL Multiphysics modeling analysis further indicate that the charge-transfer kinetics is decisive, as the lower lithium diffusivity of the NiO phase hinders charge transport from the surface to the bulk.

Study of the quasi-single crystalline lead sulfide film deposited by magnetron sputtering and its infrared detecting characteristics

Study of the quasi-single crystalline lead sulfide film deposited by magnetron sputtering and its infrared detecting characteristics

A new form of impurity cluster in casting quasi-single crystalline silicon

The casting quasi-single crystalline silicon technology is very promising in the development of high quality and low cost crystals for manufacturing photovoltaic devices. In this study, we find a new type of impurity cluster named as “black spot” in the casting quasi-single crystalline, which is different from the “shadows” caused by SiC/Si3N4 particles discovered in previous research. The “black spot” is systematically studied and characterized using inductively coupled plasma mass spectrometry (ICP-MS), photoluminescence (PL), and infrared (IR) detectors. The test results show that the “black spot” is composed of metal impurities. According to our analysis, the formation mechanism of the “black spot” may be caused by the accumulation of the metal impurities in certain region of the solid/liquid interface. These clustering impurities are not absorbed by enough grain boundaries in the casting quasi-single crystalline after being solidified into the crystal and thus exhibit as “black spot”. An optimization method is proposed in the direction of numerical simulation and verified by experiments to avoid the formation of the “black spot”.

Numerical study on stress control of silicon ingot for photovoltaic applications

When continuous silicon ingot casting is performed using directional solidification—a metallurgical refining method for producing high-purity silicon ingots—the thermal stress and microstructure inside the ingots must be monitored for ensuring the desired yield and purity. This study determined the temperature gradient and thermal stress during silicon ingot casting by applying various controlled parameters to the process simulation. Our results indicate that the temperature gradient influences the thermal stress generated during casting; reducing the temperature gradient can help reduce the thermal stress. The ingot exhibited the lowest thermal stress at an electron beam (E-beam) power of 6 kW, an ingot growth rate of 0.02 mm/min, and a molten silicon level of −10 mm. Quasi-single crystalline silicon ingot was produced via crystal growth simulation under thermal stress-minimizing conditions with single-crystalline seed. The quasi-single crystalline silicon ingot maintained the [001] grain orientation and the purity increased from 2N to 7N6.

Quasi single-crystalline transformation of porous frameworks accompanied by interlayer rearrangements of hydrogen bonds.

Quasi single-crystal-to-single-crystal transformation of a hydrogen-bonded organic framework (HOF) was accurately revealed and the mechanism was proposed. Interestingly, Br/π interaction allows a snapshot of the intermediate phase of the crystal structure to be solved.

Unravelling the influence of quasi single-crystalline architecture on high-voltage and thermal stability of LiNi0.5Co0.2Mn0.3O2 cathode for lithium-ion batteries

Abstract As a common commercial lithium ion battery cathode material, the development of layered LiNi0.5Co0.2Mn0.3O2 (NCM) is restricted by the relatively unsatisfied energy density, which can be improved by elevating the operating potential range (>4.3 V). However, the conventional secondary particles of LiNi0.5Co0.2Mn0.3O2 (C-NCM) with abundant grain boundaries will suffer from the anisotropic expansion coupled with severe electrode/electrolyte interface parasitic reactions under the high-voltage and elevated-temperature conditions, inevitably resulting in the structural collapse with intergranular cracks and long-term cycling degradation. Herein, the quasi single-crystalline LiNi0.5Co0.2Mn0.3O2 (SC-NCM) with primary particles of 3–6 μm diameter is developed, which exhibits superior cycling performance as well as significantly improved structural integrity, even at elevated-temperature (45 °C) and high-voltage (4.6 V). Remarkably, the SC-NCM||graphite pouch-type full cell maintains 98.7% of its initial capacity at harsh conditions of 45 °C and charged voltage of 4.4 V after 500 cycles with only 0.0026% decay per cycle. More importantly, the fundamental understandings of the quasi single-crystalline architecture on structure stability and phase transformation are clearly unraveled. The results reveal that the SC-NCM primary particles with micron-sizes morphology can effectively prevent the generation of intergranular cracks, thereby alleviating the irreversible structural degradation. Meanwhile, the undesired side interactions are mitigated by the reduced electrode/electrolyte interfaces, thus alleviating the irreversible phase transformation. This work provides an insight into the strategy of developing single-crystalline micron-sized particles, which can maintain the structural stability and improve cycling life of NCM cathodes, even under the harsh operating condition of elevated-temperature and high-voltage.

Fabrication and Characterization of ⟨100⟩-Oriented Quasi-single Crystalline Cu Lines

Nanotwinned copper (nt-Cu) films with ⟨111⟩ crystal orientation were electroplated on Si wafers by pulse plating, with original grain size of ∼1.4 μm. By patterning and annealing the nt-Cu film at ...

A molten-salt protected pyrolysis approach for fabricating a ternary nickel-cobalt-iron oxide nanomesh catalyst with promoted oxygen-evolving performance.

In this work, a highly porous ternary NiCoFe oxide nanomesh with two-dimensional morphology and quasi-single-crystalline (QSC) feature was synthesized via a convenient molten-salt protected pyrolysis approach, which achieves remarkable OER performance with a low overpotential, high current density, improved intrinsic activity and superior operational stability.

Optimization of the controlling recipe in quasi-single crystalline silicon growth using artificial neural network and genetic algorithm

The thermal field in the seeded directional solidification (DS) of quasi-single crystalline (QSC) silicon needs precise control to preserve the seed and maintain a flat solidification interface. Compared with adjusting the furnace configuration and materials properties, optimizing controlling recipe parameters of the DS process is more convenient and efficient in reality for improving the thermal field. However, controlling elements in the furnace interact with each other and the cooperation effect cannot be estimated by qualitatively deducing, which adds difficulties to the design and improvement of controlling recipe. In this study, an optimization system was built to optimize the controlling recipe during crystal growth and an industrial G6 DS furnace was taken as an application example. Flattening the solidification interface, reducing the thermal stress in the growing crystal and shortening the casting time were chosen as three optimization targets. Two independent heaters and a heat gate were the three controlling elements whose recipe parameters were the optimization variables. A 2D heat transfer model along with a thermal stress model was developed and an Artificial Neural Network (ANN) was trained to study the mapping relationship between optimization variables and targets in order to save computing resource and time. Genetic Algorithm (GA) was employed to yield the optimal recipes of the three controlling elements. The results showed that the optimal recipe achieved better crystal quality compared with the original recipe. The effectiveness and efficiency of this optimization system indicates that it can achieve the optimal result that theoretic analysis may hardly access.

Designing functional Σ13 grain boundaries at seed junctions for high-quality cast quasi-single crystalline silicon

Dislocation clusters and sub-grain boundaries (Sub-GBs) induced by the misorientation between adjacent seeds are destructive to the quasi-single crystal (QSC) Si ingot quality and the solar cell efficiency. Here, we have designed an artificial Σ13 GB between two adjacent <100>-oriented seeds at the crucible bottom. It is found that the generation of dislocation clusters and sub-GBs from the seed junctions is significantly suppressed owing to the low energy barrier potential of the Σ13 GB. Although some twins could generate from the vertical Σ13 GB, they will not give a bad influence on the ingot quality. The efficiency of solar cells is absolutely high in industrial circles, with an average value of 20.1%. These results pave a new way to the fabrication of high quality QSC-Si ingots for photovoltaic application.

Recombination activity of sub-grain boundaries and dislocation arrays in quasi-single crystalline silicon

Crystallographic defects in quasi-single crystalline (QSC) silicon are seriously detrimental to the performances of solar cells. In this work, we have identified these crystallographic defects as sub-grain boundaries (sub-GBs) and dislocation arrays (DAs). Sub-GBs with misorientation of >0.7° always show strong recombination activity, which are the main cause for the efficiency degradation of solar cells. Moreover, the recombination activity of sub-GBs with misorientation of <0.7° and DAs is strongly dependent on dislocation density and metal contamination level. Phosphorus gettering is an effective way to reduce the effect of metal contamination at the sub-GBs and DAs in QSC silicon.

Reduced metal contamination from crucible and coating using a silicon nitride based diffusion barrier for the growth of cast quasi-single crystalline silicon ingots

During the crystallization of directionally solidified silicon such as multicrystalline or cast quasi-single crystalline silicon, the diffusion of metal impurities from crucible and coating into the solid silicon ingot at high temperatures creates a severe contamination issue in the edge region of the ingot, the so called red zone. We present an effective diffusion barrier which greatly reduces this detrimental metal diffusion. Silicon wafers are coated with a layer system of silicon oxide and silicon nitride by chemical vapor deposition and are placed on the bottom of the crucible. Using this technique, the metal contamination in the red zone can be reduced by about one order of magnitude. This leads to better solar cell performance of wafers fabricated from this ingot area. Also, less ingot material has to be discarded leading to higher wafer yield per ingot.

Wafer scale quasi single crystalline MoS2 realized by epitaxial phase conversion

Vapor–solid phase reaction (VSPR) is a two-step process for synthesizing 2D MoS2. In the first step, a precursor film such as molybdenum oxide is grown on a substrate, followed by a sulfurization process at elevated temperature. This process offers a scalable fabrication of wafer-scale film with feasible control in thickness and uniformity. However, the properties of MoS2 films from this VSPR process often suffer from poor electrical properties. The major reason is their polycrystalline (PC) structure with large concentrations of defects and grain boundaries, which are inherited from the amorphous precursor films. Here, we report a new and scalable VSPR process in which epitaxial MoO2 films (grown over a 2-inch wafer) are used as high-quality precursors, which are converted into quasi-single-crystalline (QSC) MoS2. We demonstrate that the field effect mobility of transistors fabricated using a QSC MoS2 channel is almost 35 times larger, compared to a PC MoS2 channel, also better than most previously reported MoS2 films by other two-step MoS2 formation processes. Our process presents a new approach in which the epitaxial growth of the precursor phase can be used to improve 2D semiconductor and device performance.

X-ray studies of nanoporous gold: Powder diffraction by large crystals with small holes

X-ray diffraction studies of nanoporous gold face the poorly understood diffraction scenario where large coherent crystals are riddled with nanoscale holes. Theoretical considerations derived in this study show that the ligament size of the porous network influences the scattering despite being quasi single crystalline. Virtual diffraction of artificially generated samples confirms the results but also shows a loss of long-range coherency and the appearance of microstrain due to thermal relaxation. Subsequently, a large set of laboratory X-ray investigations of nanoporous gold fabricated by different approaches and synthesis parameters reveal a clear correlation between ligament size and size of the coherent scattering domains as well as extremely high microstrains in samples with ligament sizes below 10 nm.

Efficient nanostructured quasi-single crystalline silicon solar cells by metal-catalyzed chemical etching

Seed-assisted cast quasi-single crystalline silicon (Qsc-Si) technique allows the production of efficient, low-cost solar cells. However, most of the Qsc-Si wafers still consist of single- and multi-crystalline silicon grains, which lead to difficulties when attempting to achieve high efficiency by using conventional acid or alkali texture processes. This paper highlights the fact that nano-textured Qsc-Si solar cells can reach efficiencies ranging from 18.4% to 18.9% by using the same metal-catalyzed chemical etching technique, along with a depressed color difference. A parallel sub-cell model is proposed to explain how to enhance the performance of Qsc-Si cells.

Solar cell fabrication using two-step textured quasi-single crystalline silicon by alkaline and RIE texturing processes

After alkaline chemical texturing process, SF6/O2 reactive ion etching (RIE) was used for maskless textured Si surface independently from crystal orientation. Additional RIE texturing on alkaline textured Si surface increased the light receiving areas and decreased the surface reflectance by random pyramidal structures with a width of hundreds of nanometers. The weighted average reflectance (WAR) of the textured Si surface by two-step texturing processes was achieved as low as 9.6% in the 300–1200 nm wavelength range. Compared to the alkaline texturing, the alkaline and RIE texturing of front Si surfaces decreased the WAR by 6%. By means of two-step texturing processes for quasi-single crystalline silicon (QSC-Si) surfaces, performance parameters of the solar cell such as conversion efficiency, short circuit current density (Jsc), open circuit voltage (Voc) and fill factor (FF) were examined in 18.2%, 36.6 mA/cm2, 624 mV and 79.7%, respectively.

Magnetostriction of La0.75Sm0.25Mn2Si2 compound

The magnetostriction and magnetization are studied for a quasi-single crystalline sample of the La0.75Sm0.25Mn2Si2 compound which has the spontaneous first-order antiferromagnetic to ferromagnetic (AF–F) phase transition at a temperature of 160 K. For the AF state, the field-induced AF–F transition is observed in magnetic fields directed both along the easy c-axis and in the basal plane. The transition is accompanied by both the volume change ΔV/V = 2.8 × 10−3 and an anisotropic lattice distortion. The anisotropic magnetostriction is negative in magnetic fields applied along the c-axis and positive, in the basal plane. In small magnetic fields, both the volume and anisotropic magnetostrictions are proportional to the square of magnetization. In magnetic fields applied along the c-axis, the field-induced canting of the magnetic moments leads to much larger magnetoelastic deformations than in the basal plane. The values of the magnetovolume coupling constants both along the c-axis and in the basal plane are in agreement with those estimated from the thermodynamic relationships. Very small volume and linear strains are observed for the ferromagnetic state at 250 K. The obtained results suggest that the magnetoelastic effects in these compounds are mainly due to the change of mutual orientations of the Mn magnetic moments of adjacent layers.

Ge Photodetector Monolithically Integrated on Si by Rapid-Melting-Growth Technique

Germanium-based optoelectronic devices have the potential applications in optical communication and detections. In this letter, we demonstrated that high-quality Ge quasi-epilayer can be achieved by rapid-melting-growth and integrated-circuit compatible process. Raman and microscopy techniques revealed that the as-deposited Ge on the silicon substrate became quasi-single crystalline while nanocrystallites embedded within the Ge layer. The Ge/Si heterostructure photodetectors were fabricated with desirable responsivity of 0.22 A/W at −1 V reverse bias and the ratio of photocurrent and dark current is up to 2000 at −0.65 V. This technique demonstrated the possibility of monolithic integration of Ge photodetector for future optical communications at $1.31~\mu \text{m}$ in wavelength with Si electronic circuitry.

Reusability of contaminated seed crystal for cast quasi-single crystalline silicon ingots

Reusing seed crystal is beneficial for reducing the production costs for cast quasi-single crystalline (QSC) silicon ingots. We numerically investigate the reusability of seed crystal in the casting processes with quartz crucible and silicon feedstock of different purities. The reused seed crystal is recycled from the standard QSC ingot and has been highly contaminated by iron impurity. Transient simulations of iron transport are carried out and special attention is paid to the diffusion and distribution characteristics of iron impurity at the ingot bottom. The heights of the bottom iron contaminated region are compared for silicon ingots grown from normal and recycled seed crystals. The results show that the purity of quartz crucible can influence the reusability of seed crystal more significantly than that of the feedstock. The recycled seed crystal with high iron concentration can be reused for casting processes with standard crucible, whereas it is not recommended for reusing for processes with pure crucible.