Solids, Volume 3, Issue 2 (June 2022) – 15 articles

The crystal structure of a copper squarate metal-organic framework is fully determined using first principles methods based in density functional theory. The compressibility of this material is studied by optimizing the structure under different isotropic pressures and uniaxial stresses directed along the direction of minimum compressibility, [1 0 0]. Under isotropic compression, channels become wider along [1 0 0], leading to negative linear compressibility, NLC. Under compression along [1 0 0], the unit-cell volume increases leading to negative volumetric compressibility. Full article

The large surface-to-volume ratio of nanoparticles is understood to be the source of many interesting phenomena. The melting temperature of nanoparticles is shown to dramatically reduce compared to bulk material. Yet, at temperatures below this reduced melting point, a liquid-like atomic arrangement on the surface of nanoparticles is still anticipated to influence its properties. To understand such surface effects, here, we study the coalescence of Au nanoparticles of various sizes using molecular dynamics simulations. Analysis of the potential energy and Lindemann index distribution across the nanoparticles reveals that high-energy, high-mobility surface atoms can enable the coalescence of nanoparticles at temperatures much lower than their corresponding melting point. The smaller the nanoparticles, the larger the difference between their melting and coalescence temperatures. For small enough particles and/or elevated enough temperatures, we found that the coalescence leads to a melting transition of the two nominally solid nanoparticles, here discussed in relation to the heat released due to the surface reduction upon the coalescence and the size dependence of latent heat. Such discontinuous melting transitions can lead to abrupt changes in the properties of nanoparticles, important for their applications at intermediate temperatures. Full article

Nanomaterials based on metal oxides are extensively studied for several applications due to their versatility. Improvements in their performances can be obtained due to specific structural modifications. One possible modification is by doping the crystal structure, which can affect the materials structure and properties, especially in nanosized particles. Electronic features are among the properties that can be modified through the doping process, consequently morphological and optical parameters can also be controlled by this process. In this sense, this review presents some modifications to tin dioxide (SnO₂), one the most studied materials, mainly through the doping process and their impact on several properties. The article starts by describing the SnO₂ structural features and the computational models used to explain the role of the doping process on these features. Based on those models, some applications of doped SnO₂, such as photocatalytic degradation of pollutants, CO₂ reduction, and desulfurization of fossil fuels are presented and discussed. Additionally, the review describes many biological applications related to antimicrobial activity for doped SnO₂ and its nanostructures. Although most of the examples presented in this article are based on the doped SnO₂, it also presents examples related to SnO₂ composites with other nanomaterials forming heterojunctions. The metal oxides SnO₂, doped-SnO₂ and their nanostructures are promising materials, with results reported in many fields presented in this review, such as theoretical and computational chemistry, environmental remediation, nanoparticle morphology control, fossil fuels improvement, and biomedical applications. Although widely explored, there are still fields for innovation and advances with tin dioxide nanostructures, for example, in transparent conducting oxides, in forensics as materials for latent fingerprints visualization, and sensors in medicine for detection of exhaled volatile organic compounds. Therefore, this article aims to be a reference regarding correlating the doping processes and the properties presented by the SnO₂ nanostructures. Full article

Specific heat at constant pressure is traditionally a difficult thermodynamic quantity to obtain from first-principles calculations. While theoretical avenues to $C_p\left(T\right)$ do exist—most notably, the quasi-harmonic approximation—there are many materials for which this approximation is not valid. One of those materials is Ge. In this paper, we demonstrate how a new method—termed the Beyond Quasi-Harmonic method—takes into account all anharmonic vibrations by showing how our results are significantly better than those achieved through using the quasi-harmonic model. In addition, we calculate $C_p\left(T\right)$ for 3C-SiC, a material for which there are surprisingly few experimental results. For 3C-SiC, our results agree well with the available experiments, and for Ge, our results agree very well with the generally accepted values. Full article

A technique to establish electrical contact and perform multi-probe electrical measurements (e.g., four-probe measurements), even at low temperatures, is presented in this work. The natural adhesion contact (NAC) is applicable to the wide range of dimensions of organic crystals. Furthermore, the precise electrode patterns required to carry out multi-probe measurements are guaranteed, in contrast to fine conductive paste painting methods. We demonstrate four-probe electrical measurements of $\alpha$-(BEDT-TTF)${}_2$I${}_3$ (where BEDT-TTF = bis(ethylenedithio) tetrathiafulvalene) at temperatures down to 100 K. The obtained temperature dependence showed a steep meta l–insulator transition and exhibited zero hysteresis throughout several measurement sequences. Full article

NaBH₄ is a very cheap and hydrogen-rich material, as well as a potential hydrogen store. However, the high temperature of its thermal decomposition (above 530 °C) renders it inapplicable in practical use. Here, we studied the effect of addition of diverse V-containing catalysts on thermal hydrogen desorption. It turns out that mechanochemical doping of NaBH₄ with vanadium metal, its oxides, or nanoparticles lowers the temperature of pyrolysis significantly. Notably, NaBH₄ milled for 3 h with 25 wt.% V₂O₅ or VO₂ releases ca. 70% of stored hydrogen in the temperature range of ca. 370–450 °C. On the other hand, precursors and solvents used to prepare rather uniform vanadium nanoparticles (~4 nm) suspended in THF or less uniform and larger ones (~15 nm) in o- difluorobenzene have adverse effect on the purity of hydrogen evolved. Full article

So far, diesel particulate filters (DPFs) have been widely used to collect diesel particulates including soot in the exhaust after-treatment. However, as the soot is continuously collected in the porous filter, the exhaust pressure (pressure drop) increases. To optimize the filter design for reducing its pressure drop, we need a numerical simulation. In this study, we simulated the particle-laden flow across the DPF. Structure of SiC-DPF was obtained by an X-ray CT technique. We conducted the numerical simulation by changing the soot aggregation diameter (simply called soot size), and evaluated the time-variation of the pressure drop. For discussing the soot deposition process, the contributions of the Brownian diffusion and the interception effect were separately estimated. Especially, we focused on the soot deposition region which could affect the pressure drop, together with the soot cake permeability and the soot packing density. Results show that, as the soot size is smaller, more soot is trapped. As a result, the shift from the depth filtration to the surface filtration is observed earlier. Therefore, for discussing the pressure drop, it is important to consider where the soot deposition occurs as well as the deposited soot mass in the filter. Full article

Polymethylmethacrylate (PMMA) is a material of choice for many biomedical coating applications. However, such applications are limited due to the toxicity of the traditional solvents used for the solution processing of PMMA coatings and composites. This problem is addressed using an isopropanol-water co-solvent, which allows for the dissolution of high molecular mass PMMA and the fabrication of coatings by a dip-coating method from concentrated PMMA solutions. The use of the co-solvent offers a versatile strategy for PMMA solubilization and coating deposition, despite the insolubility of PMMA in water and isopropanol. Composite coatings are obtained, containing hydroxyapatite, silver oxide, zinc oxide, micron size silica and nanosilica. Such coatings are promising for the manufacturing of implants with enhanced biocompatibility, bioactivity and antimicrobial properties and the fabrication of biosensors. Ibuprofen, tetracycline and amoxicillin are used as model drugs for the fabrication of PMMA-drug composite coatings for drug delivery. The microstructure and composition of the coatings are analyzed. The versatile dip-coating method of this investigation provides a platform for various biomedical applications. Full article

HfS₂ has recently emerged as a promising 2D semiconductor, but the lack of a reliable method to produce continuous films on a large scale has hindered its spreading. The atomic layer deposition of the material with the precursor tetrakis-dimethylamino-hafnium with H₂S is a relatively novel solution to this problem. This paper shows that it is a facile approach to synthesizing homogeneous and smooth HfS₂ layers in a controlled and reproducible manner. The deposition is examined at different temperatures and layer thicknesses, exploring the ALD window of the deposition and the chemical, morphological and electronic properties of the films. The method yielded films with wafer-sized uniformity and controlled properties and is, thus, a promising way to prepare this important transition metal dichalcogenide material. Full article

A new solid-state ${}^{19}$F magic-angle spinning NMR signal at an isotropic ${}^{19}$F chemical shift of −53 ppm is measured from graphite fluoride synthesized by reaction of graphite with F${}_2$ at temperatures above 750 K with no catalyst. Two-dimensional NMR suggests the −53 ppm ${}^{19}$F NMR signal originates from covalent fluoromethanetriyl groups belonging to ordered (C${}_y$F)${}_n$ bulk domains composited with the major (CF)${}_n$ domains. Quantitative ${}^{19}$F and ${}^{13}$C NMR find $y=4.32\pm 0.64$. DFT calculations of NMR chemical shifts for unsaturated fluorographene models show that a (C${}_4$F)${}_n$ phase with fluorine bound covalently to a single side of the carbon layer best explains the observed NMR chemical shifts. We assign the new phase to this (C${}_4$F)${}_n$ structure, which constitutes up to 15% of the carbon in our graphite fluoride composites. The (C${}_4$F)${}_n$ content of the composite affects bulk electrochemical properties in a manner similar to graphite fluorides produced by conventional, catalyzed fluorination processes. Full article

Photoluminescence studies reveal three C_N-related luminescence bands in GaN doped with carbon: the YL1 band at 2.17 eV caused by electron transitions via the −/0 level of the C_N, the BL_C band at 2.85 eV due to transitions via the 0/+ level of the C_N and the BL2 band at 3.0 eV attributed to the C_NH_i complex. The BL_C band studied here has the zero-phonon line at 3.17 eV and a phonon-related fine structure at low temperatures. The 0/+ level of the C_N is found at 0.33 ± 0.01 eV above the valence band, in agreement with recent theoretical predictions. These results will help to choose an optimal correction scheme in hybrid functional calculations. Full article

A molecular dynamics simulation was used to investigate the effect of applied strain on the formation of primary defects and the probability of interstitial dislocation loops (IDLs) formation of tungsten (W) during a collision cascade event. The research investigated primary knock-on atom energies of 1, 6, 10, and 14 keV, applied on a deformed W structure (form −1.4~1.6%). The peak and surviving number of Frenkel pairs (FPs) increased with increasing tension; however, these increases were more pronounced under higher strain due to the formation of IDLs. For 10 self-interstitial atoms (SIA) lengths, the strain effect reduces the clustering energy of the IDLs by about 7 eV. In general, the current findings suggest that strain effects should be carefully considered in radiation-damaged environments, particularly in low-temperature, high-radiation-energy environments. The compressed condition may advantage materials used in high-radiation-damage devices and power systems. Full article

We extend our previous semi-empirical model for quantum transport in a conventional nano-MOSFET to FDSOI transistors. In ultra-thin-body and -BOX (UTBB) FDSOI transistors, the electron channel can be treated as an electron waveguide. In the abrupt transition approximation, it is possible to derive an analytical approximation for the potential seen by the charge carriers. With these approximations we calculate the threshold voltage and the transfer characteristics, finding remarkably good agreement with experiments in the OFF-state given the relative simplicity of our model. In the ON-state, our theory fails because Coulomb interaction between the free charge carriers and the device heating is neglected in our approach. Full article

Raman Spectroscopy is a well-known method for identifying molecules by their spectroscopic “fingerprint”. In Surface Enhanced Raman Scattering (SERS), the presence of nanometallic surfaces in contact with the molecules enormously enhances the spectroscopic signal. Raman enhancing surfaces are often fabricated lithographically or chemically, but the throughput is low and the equipment is expensive. In this work a SERS layer was formed by the self-assembly of silver nanospheres from a hexane suspension onto an imprinted thermoplastic sheet (PET). In addition, the SERS layer was transferred and securely bonded to other surfaces. This is an important attribute for probes into solid specimen. Raman spectra were obtained with Rhodamine 6G (R6G) solution concentrations ranging from 1 mm to 1 nm. The methods described here produced robust and sensitive SERS surfaces with inexpensive equipment, readily available materials, and with no chemical or lithographic steps. These may be critical concerns to laboratories faced with diminishing funding resources. Full article

Recent efforts towards developing novel lead electrodes involving carbon and lead composites have shown potential for increasing the cycle life of lead–acid (LA) batteries used to store energy in various applications. In this study, first-principles calculations are used to examine the structural stability, defect formation energy, and migration barrier of C in Pb for LA batteries. Density functional theory with the GGA-PBE functional performed the best out of various functionals used for structural stability calculations. Furthermore, with the complete incorporation of C in the Pb matrix, the results show that C is energetically preferred to be at the octahedral interstitial ($C_i^{Octa}$) site in the FCC structure of Pb. Additionally, climbing-image nudged elastic band calculations show a minimum energy pathway for C diffusing from a stable octahedral site to the adjacent octahedral site assisted by a tetrahedral intermediate site. Therefore, the minimum energy pathway for C migration is envisioned to be $C_i^{Octa}\to C_i^{Tetra}\to C_i^{Octa}$, where the total energy barrier is observed to be ~90% and more than 100% lower than the $C_i^{Tetra}\to C_i^{Tetra}$ and $C_i^{Octa}\to C_i^{Octa}$ barriers, respectively. Full article