Crystals

14 pages, 1094 KiB

Open AccessArticle

Density Functional Investigation of [001] and [111] SiNWs and the Effect of Doping with Boron and Phosphorus

by Nedhal Ali Mahmood Al-Nuaimi, Florian Hilser and Sibylle Gemming

Crystals 2024, 14(7), 585; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst14070585 (registering DOI) - 25 Jun 2024

Abstract

In the present study, we investigate the influence of boron (B) and phosphorus (P) (p- and n-type, respectively) doping on the electronic properties of ultra-thin silicon nanowires (SiNWs) by gradient-corrected density functional calculations with the Perdew–Burke–Ernzerhof (PBE) approximation. In the limit of very [...] Read more.

In the present study, we investigate the influence of boron (B) and phosphorus (P) (p- and n-type, respectively) doping on the electronic properties of ultra-thin silicon nanowires (SiNWs) by gradient-corrected density functional calculations with the Perdew–Burke–Ernzerhof (PBE) approximation. In the limit of very small diameters (5–8 Å), both pristine and highly active unsaturated SiNWs with orientations along the [001] and [111] directions exhibit electronic states around the Fermi level, indicative of conductive properties. Conduction is further enhanced by the introduction of doping atoms, as demonstrated by the relative change in the band structures of SiNWs with and without B and P doping. This investigation provides an important insight into the electronic states of SiNWs, which are candidates for future electronics or sensing applications. Full article

(This article belongs to the Special Issue Advanced Research in Semiconductor Materials and Devices)

18 pages, 1886 KiB

Open AccessArticle

TCAD-Based Design and Optimization of Flexible Organic/Si Tandem Solar Cells

by Marwa S. Salem, Mohamed Okil, Ahmed Shaker, Mohamed Abouelatta, Mostafa M. Salah, Kawther A. Al-Dhlan and Michael Gad

Crystals 2024, 14(7), 584; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst14070584 (registering DOI) - 25 Jun 2024

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Abstract

In order to surmount the Shockley–Queisser efficiency barrier of single-junction solar devices, tandem solar cells (TSCs) have shown a potential solution. Organic and Si materials can be promising candidates for the front and rear cells in TSCs due to their non-toxicity, cost-effectiveness, and [...] Read more.

In order to surmount the Shockley–Queisser efficiency barrier of single-junction solar devices, tandem solar cells (TSCs) have shown a potential solution. Organic and Si materials can be promising candidates for the front and rear cells in TSCs due to their non-toxicity, cost-effectiveness, and possible complementary bandgap properties. This study researches a flexible two-terminal (2-T) organic/Si TSC through TCAD simulation. In the proposed configuration, the organic solar cell (OSC), with a photoactive optical bandgap of 1.78 eV, serves as the front cell, while the rear cell comprises a Si cell based on a thin 70 μm wafer, with a bandgap energy of 1.12 eV. The individual standalone front and bottom cells, upon calibration, demonstrate power conversion efficiencies (PCEs) of 11.11% and 22.69%, respectively. When integrated into a 2-T organic/Si monolithic TSC, the resultant tandem cell achieves a PCE of 20.03%, indicating the need for optimization of the top organic cell to beat the efficiency of the bottom Si cell. To enhance the performance of the OSC, some design ideas are presented. Firstly, the OSC is designed by omitting the organic hole transport layer (HTL). Consequently, through engineering the front contact work function, the PCE is enhanced. Moreover, the influence of varying the absorber defect density of the top cell on TSC performance is investigated. Reduced defect density led to an overall efficiency improvement of the tandem cell to 23.27%. Additionally, the effects of the variation of the absorber thicknesses of the top and rear cells on TSC performance metrics are explored. With the matching condition design, the tandem efficiency is enhanced to 27.60%, with V_OC = 1.81 V and J_SC = 19.28 mA/cm². The presented simulation results intimate that the OSC/Si tandem design can find applications in wearable electronics due to their flexibility, environmentally friendly design, and high efficiency. Full article

(This article belongs to the Special Issue Research on Electrolytes and Energy Storage Materials)

25 pages, 1904 KiB

Open AccessArticle

Double-Helical Tiled Chain Structure of the Twist-Bend Liquid Crystal Phase in CB7CB

by Michael R. Tuchband, Min Shuai, Keri A. Graber, Dong Chen, Chenhui Zhu, Leo Radzihovsky, Arthur Klittnick, Lee Foley, Alyssa Scarbrough, Jan H. Porada, Mark Moran, Joseph Yelk, Justin B. Hooper, Xiaoyu Wei, Dmitry Bedrov, Cheng Wang, Eva Korblova, David M. Walba, Alexander Hexemer, Joseph E. Maclennan, Matthew A. Glaser and Noel A. Clark Show full author list Hide full author list

Crystals 2024, 14(7), 583; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst14070583 (registering DOI) - 25 Jun 2024

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Abstract

The twist-bend nematic liquid crystal phase is a three-dimensional fluid in which achiral bent molecules spontaneously form an orientationally ordered, macroscopically chiral, heliconical winding of a ten nanometer-scale pitch in the absence of positional ordering. Here, the structure of the twist-bend phase of [...] Read more.

The twist-bend nematic liquid crystal phase is a three-dimensional fluid in which achiral bent molecules spontaneously form an orientationally ordered, macroscopically chiral, heliconical winding of a ten nanometer-scale pitch in the absence of positional ordering. Here, the structure of the twist-bend phase of the bent dimer CB7CB and its mixtures with 5CB is characterized, revealing a hidden invariance of the self-assembly of the twist-bend structure of CB7CB, such that over a wide range of concentrations and temperatures, the helix pitch and cone angle change as if the ground state for a pitch of the TB helix is an inextensible heliconical ribbon along the contour formed by following the local molecular long axis (the director). Remarkably, the distance along the length for a single turn of this helix is given by 2πR_mol, where R_mol is the radius of bend curvature of a single all-trans CB7CB molecule. This relationship emerges from frustrated steric packing due to the bent molecular shape: space in the fluid that is hard to fill attracts the most flexible molecular subcomponents, a theme of nanosegregation that generates self-assembled, oligomer-like correlations of interlocking bent molecules in the form of a brickwork-like tiling of pairs of molecular strands into duplex double-helical chains. At higher temperatures in the twist-bend phase, the cone angle is small, the director contour is nearly along the helix axis z, and the duplex chains are sequences of biaxial elements formed by overlapping half-molecule pairs, with an approximately 45° rotation of the biaxis between each such element along the chain. Full article

(This article belongs to the Section Liquid Crystals)

14 pages, 2621 KiB

Open AccessArticle

Optimizing the Rolling Process of Lightweight Materials

by Jessica Rawles, Svitlana Fialkova, Kai Hubbard, Zhigang Xu, Christopher Hale and Jagannathan Sankar

Crystals 2024, 14(7), 582; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst14070582 (registering DOI) - 25 Jun 2024

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Abstract

Conventional rolling is a plastic deformation process that uses compression between two rolls to reduce material thickness and produce sheet/plane geometries. This deformation process modifies the material structure by generating texture, reducing the grain size, and strengthening the material. The rolling process can [...] Read more.

Conventional rolling is a plastic deformation process that uses compression between two rolls to reduce material thickness and produce sheet/plane geometries. This deformation process modifies the material structure by generating texture, reducing the grain size, and strengthening the material. The rolling process can enhance the strength and hardness of lightweight materials while still preserving their inherent lightness. Lightweight metals like magnesium alloys tend to lack mechanical strength and hardness in load-bearing applications. The general rolling process is controlled by the thickness reduction, velocity of the rolls, and temperature. When held at a constant thickness reduction, each pass through the rolls introduces an increase in strain hardening, which could ultimately result in cracking, spallation, and other defects. This study is designed to optimize the rolling process by evaluating the effects of the strain rate, rather than the thickness reduction, as a process control parameter. Full article

(This article belongs to the Special Issue Microstructure and Mechanical Behaviour of Structural Materials)

20 pages, 27611 KiB

Open AccessArticle

Effect of Rotational Shear and Heat Input on the Microstructure and Mechanical Properties of Large-Diameter 6061 Aluminium Alloy Additive Friction Stir Deposition

by Xiaohu Zhu, Rui Wang, Lin Wang, Mengmeng Liu and Songmo Li

Crystals 2024, 14(7), 581; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst14070581 (registering DOI) - 25 Jun 2024

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Abstract

Additive friction stir deposition (AFSD), in which molten metal materials are formed into free-form stacked structural parts according to the path design, may have a wide range of applications in high-efficiency mass production. In this study, experiments were conducted for the rotational speed [...] Read more.

Additive friction stir deposition (AFSD), in which molten metal materials are formed into free-form stacked structural parts according to the path design, may have a wide range of applications in high-efficiency mass production. In this study, experiments were conducted for the rotational speed in the AFSD parameters of 6061 aluminium alloy bars to investigate the effects of different rotational shear conditions and heat inputs on the properties of the deposited layer for diameter bars based on the analysis of the micro-morphology, micro-tissue composition, and mechanical properties. The width and thickness of each layer were constant, approximately 40 mm wide and 2.5 mm thick. The particle undulations on the surface of the deposited layer were positively correlated with the AFSD rotational speed. Continuous dynamic recrystallisation in the AFSD process can achieve more than 90% grain refinement. When the rotational speed increases, it causes localised significant orientation and secondary deformation within the recrystallised grains. The ultimate tensile strength of the deposited layer was positively correlated with the rotational speed, reaching a maximum of 211 MPa, and the elongation was negatively correlated with the rotational speed, with a maximum material elongation of 37%. The cross-section hardness of the deposited layer was negatively correlated with the number of thermal cycles, with the lowest hardness being about 45% of the base material and the highest hardness being about 80% of the base material. Full article

(This article belongs to the Special Issue Additive and Subtractive Manufacturing of Metallic Materials: Process-Structure-Property-Performance Relationships)

12 pages, 3883 KiB

Open AccessArticle

Tuning the Superspin Dynamics in Inverse Spinel Ferrite Nanoparticle Ensembles via Indirect Cation Substitution

by Cristian E. Botez and Alex D. Price

Crystals 2024, 14(7), 580; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst14070580 - 22 Jun 2024

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Abstract

We used magnetic and synchrotron X-ray diffraction measurements to investigate the possibility of tuning the strength of magnetic interparticle interactions in nanoparticle ensembles via chemical manipulation. Our main result comes from temperature-resolved in-phase ac-susceptibility data collected on 8 nm average-diameter Ni_0.25Zn [...] Read more.

We used magnetic and synchrotron X-ray diffraction measurements to investigate the possibility of tuning the strength of magnetic interparticle interactions in nanoparticle ensembles via chemical manipulation. Our main result comes from temperature-resolved in-phase ac-susceptibility data collected on 8 nm average-diameter Ni_0.25Zn_0.75Fe₂O₄ (Ni25) and Ni_0.5Zn_0.5Fe₂O₄ (Ni50) nanoparticles at different frequencies, χ′ vs. T|_f. We found that the relative peak temperature variation per frequency decade,

ϕ = \frac{∆ T}{T \cdot ∆ l o g (f)}

—a known measure of interparticle interaction strength—exhibits a four-fold increase, from ϕ = 0.04 in Ni50 to ϕ = 0.16 in Ni25. This corresponds to a fundamental change in the nanoparticles’ superspin dynamics, as proven by the fit of phenomenological models to magnetic relaxation data. Indeed, the Ni25 ensemble exhibits superparamagnetic behavior, where the temperature dependence of the superspin relaxation time, τ, is described in the Dorman–Bessais–Fiorani (DBF) model:

τ (T) = τ_{r} \exp (\frac{E_{B} + E_{a d}}{k_{B} T}),

with parameters τ_r = 4 × 10⁻¹² s, and (E_B + E_ad)/k_B = 1473 K. On the other hand, the nanoparticles in the Ni50 ensemble freeze collectively upon cooling in a spin-glass fashion according to a critical dynamics law:

τ (T) = \frac{τ_{0}}{{[\frac{T}{T_{g}} - 1]}^{z ν}}

, with τ₀ = 4 × 10⁻⁸ s, T_g = 145 K, and zν = 7.2. Rietveld refinements against powder X-ray diffraction data reveal the structural details that underlie the observed magnetic behavior: an indirect cation replacement mechanism by which non-magnetic Zn ions are incorporated in the tetrahedral sites of the inverse spinel. Full article

(This article belongs to the Special Issue Exploring the Magnetic World: Advances in Synthesis, Characterization, and Revolutionary Applications of Magnetic Nanoparticles)

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15 pages, 5750 KiB

Open AccessReview

Pyroelectric Properties and Applications of Lithium Tantalate Crystals

by Jiashun Si, Xuefeng Xiao, Yan Zhang, Yan Huang, Shuaijie Liang, Qingyan Xu, Huan Zhang, Lingling Ma, Cui Yang and Xuefeng Zhang

Crystals 2024, 14(7), 579; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst14070579 - 22 Jun 2024

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Abstract

Lithium tantalate crystals, as a type of pyroelectric material, stand out from many other pyroelectric materials due to the advantages of high Curie temperature, large pyroelectric coefficient, high figure of merits, and environmental friendliness. Due to the pyroelectric effect caused by their spontaneous [...] Read more.

Lithium tantalate crystals, as a type of pyroelectric material, stand out from many other pyroelectric materials due to the advantages of high Curie temperature, large pyroelectric coefficient, high figure of merits, and environmental friendliness. Due to the pyroelectric effect caused by their spontaneous polarization, lithium tantalate crystals have broad application prospects in wide spectral bandwidth and uncooled pyroelectric detectors. This article reviews the pyroelectric properties of lithium tantalate crystals and evaluates methods for pyroelectric properties, methods for modulating pyroelectric properties, and pyroelectric detectors and their applications. The prospects of lithium tantalate thin films, doped lithium tantalate crystals, and near stoichiometric lithium tantalate crystals as response components for pyroelectric detectors are also discussed. Full article

(This article belongs to the Section Inorganic Crystalline Materials)

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17 pages, 3322 KiB

Open AccessArticle

Exploring the Interplay between Structure and Electronic Behavior across Pressure-Induced Isostructural and Structural Transitions in Weyl-Type Semimetal NbAs

by João E. F. S. Rodrigues, Emin Mijit, Angelika D. Rosa, Laura Silenzi, Nodoka Hara, Catalin Popescu, José A. Alonso, Tetsuo Irifune, Zhiwei Hu and Andrea Di Cicco

Crystals 2024, 14(7), 578; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst14070578 - 21 Jun 2024

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Abstract

NbAs is a Weyl semimetal and belongs to the group of topological phases that exhibit distinct quantum and topological attributes. Topological phases have a fundamentally different response to external perturbations, such as magnetic fields. To obtain insights into the response of such phases [...] Read more.

NbAs is a Weyl semimetal and belongs to the group of topological phases that exhibit distinct quantum and topological attributes. Topological phases have a fundamentally different response to external perturbations, such as magnetic fields. To obtain insights into the response of such phases to pressure, we conducted a comprehensive study on the pressure-induced electronic and structural transitions in NbAs. We used micro-X-ray diffraction (XRD) and micro-X-ray spectroscopy (XAS) techniques to elucidate the changes at different atomic and electronic length scales (local, medium, and bulk) as combined with theoretical calculations. High-pressure XRD measurements revealed a rather common compression behavior up to ~12 GPa that could be fitted to an equation of state formalism with a bulk modulus of

⟨K_{0}⟩ =

179.6 GPa. Complementary Nb K-edge XAS data unveiled anomalies at pressure intervals of ~12–15 and ~25–26 GPa in agreement with previous literature data from XRD studies. We attribute these anomalies to a previously reported topological Lifshitz transition and the tetragonal-to-hexagonal phase transition, respectively. Analysis of EXAFS results revealed slight changes in the mean next-nearest neighbor distance Nb–As₍₁₎ (~2.6 Å) at ~15 GPa, while the second nearest neighboring bond Nb–Nb₍₁₎ (~3.4 Å) shows a pronounced anomaly. This indicates that the electronic changes across the Lifshitz transition are accommodated first in the medium-range atomic structure and then at the local range and bulk. The variances of these bonds show anomalous but progressive evolutions close to the tetragonal-to-hexagonal transition at ~25 GPa, which allowed us to derive the evolution of vibration properties in this material. We suggest a prominent displacive character of the

I 4_{1} m d \to P \bar{6} m 2

transition facilitated by phonon modes. Full article

(This article belongs to the Special Issue Properties of Transition Metals and Their Compounds at Extreme Conditions (2nd Edition))

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16 pages, 3768 KiB

Open AccessArticle

Sustainable Production and Physicochemical Characteristics of Calcium Sulfate Dihydrate Prepared from Waste Eggshells

by Somkiat Seesanong, Chaowared Seangarun, Banjong Boonchom, Nongnuch Laohavisuti, Wimonmat Boonmee, Pesak Rungrojchaipon and Phairat Phimsirikul

Crystals 2024, 14(7), 577; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst14070577 - 21 Jun 2024

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Abstract

Gypsum products (calcium sulfate dihydrate, CaSO₄·2H₂O) were synthesized through an eco-friendly and low-cost process by two different renewable calcium carbonate sources (CaCO₃), hen and duck eggshell wastes, with product yields obtained of 84.73 and 87.74%, respectively. The [...] Read more.

Gypsum products (calcium sulfate dihydrate, CaSO₄·2H₂O) were synthesized through an eco-friendly and low-cost process by two different renewable calcium carbonate sources (CaCO₃), hen and duck eggshell wastes, with product yields obtained of 84.73 and 87.74%, respectively. The X-ray fluorescence results indicated that calcium oxide (CaO) and sulfur trioxide (SO₃) are the major elemental components of CaSO₄·2H₂O prepared from both calcium sources. The Fourier transform infrared results confirmed the vibrational characteristics of SO₄²⁻ and H₂O functional groups in the chemical structure of the prepared samples. The X-ray diffraction patterns of CaSO₄·2H₂O prepared from both calcium sources confirmed the sample’s crystal structure as well as the chemical formula, after comparing them to the standard powder diffraction file. The crystallite sizes of CaSO₄·2H₂O products were calculated from the experimental diffraction peak through the Scherrer equation and found to be 19–20 nm. The positive preferential growth (Pg) value highlighted the excellent stability of the synthesized CaSO₄·2H₂O. The scanning electron microscopic results showed the agglomeration particles of hen- and duck-CaCO₃ raw agents, whereas plate-like particles were observed for hen- and duck-CaSO₄·2H₂O products but the particle sizes were different. Full article

(This article belongs to the Special Issue Synthesis, Processing and Characterization of Micro- and Nanostructured Functional Materials for Advanced Applications)

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Crystals, Volume 14, Issue 7 (July 2024) – 9 articles

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