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(a) Band structure of K 8 Al 8 Si 38 with Al on five 6c sites, two 24k sites, and one 16i site (5c2k1i configurations), for samples A (black), B (red), and C (blue), calculated using DFT-PBE. (b) Band structure of K 8 Al 8 Si 38 with Al on six 6c sites, two 16i sites (6c2i configurations) for samples D (black), E (red), and F (blue), calculated using DFT. (c) Band structure of K 8 Al 8 Si 38 (sample A) calculated using DFT-PBE (black) and many body perturbation theory within the G 0 W 0 approximation (red), respectively. Reproduced with permission from He et al., Energy Environ. Sci. 7, 2598 (2014). Copyright 2014 Royal Society of Chemistry.
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Silicon exhibits a large variety of different bulk phases, allotropes and composite structures, such as e. g. clathrates or nanostructures, at both higher and lower densities compared to diamond-like Si-I. New Si structures continue to be discovered. These novel forms of Si offer exciting prospects to create Si based materials, that are non-toxic a...
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... in the real material. Most calculations were con- ducted at the DFT level and one sample (A) was carefully analyzed using many body perturbation theory (MBPT) at the G 0 W 0 level. 34 The position of Al significantly influences the position of the valence bands, while the conduction bands (CBs) are largely unaffected (see Figs. 3(a) and 3(b)). Fig. 3(c) shows the G 0 W 0 corrected band structure of sample A with respect to that obtained at the DFT-PBE level. We found a rigid shift of the uppermost valence band (VB) to lower energy (0.04 eV) and of the lowest conduction band (CB) to higher energy (0.21 eV), yielding a band gap (0.81 eV) increase of approximately 45% with respect to ...
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... of the VBM and CBM of the Si 123 NP embedded in the a-ZnS, as a function of the radial distance from the NP center. The CBM outside the NP is clearly higher than its value inside the NP, by about 0.3 eV. Moreover, the VBM outside the NP is higher than inside by about 0.15 eV. The same type-II offsets were obtained with G 0 W 0 calculations, see Fig. 13(b). We note that the transition between the bands is sharp and well defined for the conduction band, whereas it is more gradual for the valence band. This gradual increase can be attributed to the electronic states of the sulfur shell. The states induced by the sulfur shell are almost energetically degenerate with the VBM states of the ...
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... states induced by the sulfur shell are almost energetically degenerate with the VBM states of the ZnS matrix. Thus, the occupied states of the Si NP are located slightly below the ZnS valence band edge. We verified this interpretation in a graphical manner, plotting the square moduli of the individual wave functions (not shown). The results of Fig. 13 confirm those of Fig. 12, showing that a type-II interface is formed between the Si NPs and the a-ZnS ...
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A decade after their first appearance as components in solar cells, perovskites are still in the center of solar research. Material and device engineering have led to high PCE values...
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... According to recent band calculations, BC-8 Si is predicted to be a semimetal in the bulk form, and a direct gap semiconductor in the nanocrystal form. 8,9) This prediction opens a new possibility of various applications of Si nanocrystals in different fields, e.g. multiple-exciton-generation solar cells, 10) thermoelectric devices, 11) exothermic particles for photothermal therapy, 12) etc. ...
... The tail toward lower energy (<1 eV) is probably due to absorption of amorphous Si which did not crystallize during the vapor condensation and/or absorption of surface suboxides grown in air. According to a recent band structure calculation, 8,9) the bandgap energy of a BC-8 Si nanocrystal E g (nano) in unit of eV is written as ...
Silicon (Si) nanocrystals with diameters of 2–5 nm were produced through non-equilibrium condensation of Si vapor. Electron diffraction analysis indicated that the nanocrystals formed a body-centered cubic (BCC) structure. The optical bandgap energy of the nanocrystals was about 1.6 eV, and photoluminescence (PL) was observed in the region of the bandgap energy. The obtained energy is compared with a theoretically predicted value of the BCC Si crystal of the present size.
... As a result, this leads to the silicon structure having as many allotropes as carbon. By controlling the difference in decompression rate, we can obtain silicon allotropes with different structures and properties (Si-II, Si-XI, Si-V, Si-VI, Si-VII, Si-X phase [6][7][8][9][10]), and, as the pressure increases, the silicon coordination number will also increase, resulting in electron delocalization and increased metal abundance [11]. Therefore, this will cause all these silicon allotropes to exhibit metallic properties at specific temperatures and pressures, and once these conditions change a little, this stable state will no longer be maintained. ...
... It is worth noting that Si-I can be directly converted to Si-II, but Si-II cannot be directly converted to Si-I. If you want to get Si-I through Si-II, it requires multiple processes and phase transitions (Si-II → Si-XII → Si-III → Si-IV → Si-I) [11]. Since there is a large potential barrier in the transition from the high-pressure phase to the most stable Si-I phase at ambient temperature [12], the product after pressure relief is not the Si-I phase, but some metastable phases of silicon, such as Si-III, Si-XII, Si-IV [13,14]. ...
... We note that this diamond-like state with small displacements is also different from the equilibrium structure at 315 K, which can be seen by the drop of ∼ 8% in the first 200 to 300 fs. The energy difference to an equilibrated system at the same conditions for T e , T I is 0.086 eV/atom [51], comparable to differences between different phases of Si [54,55], and between crystalline and amorphous Si [56,57], which indicates a resemblance to a solid-solid phase transition. Moreover, the pair-correlation function of the system indicates that the short-range order (below 5Å) in this state is comparable to the equilibrium structure, but the long-range order is washed out and loses its fine structure [51]. ...
... In materials with a lower symmetry, i.e., with more degrees of freedom it might be possible to generate stable or metastable phases by solid-to-solid phase transitions [55]. Moreover, the application to more complex systems than silicon has the potential to create novel states in both structure and electronic properties. ...
An intense femtosecond-laser excitation of a solid induces highly nonthermal conditions. In materials like silicon, laser-induced bond-softening leads to a highly incoherent ionic motion and eventually nonthermal melting. But is this outcome an inevitable consequence, or can it be controlled? Here, we performed ab initio molecular dynamics simulations of crystalline silicon after multiple femtosecond-laser pulse excitations with total energy above the nonthermal melting threshold. Our results demonstrate an excitation mechanism, which can be generalized to other materials, that pauses nonthermal melting and creates a metastable state instead, which has an electronic structure similar to the ground state. Our findings open the way for a search for metastable structural and/or electronic transitions in the high-excitation regime. In addition, our approach could be used to switch off nonthermal contributions in experiments, which could be used to obtain reliable electron-phonon coupling constants more easily.
... 52) Since Si-IV is expected to be a semiconductor with an indirect bandgap of 0.95 eV, 17) it will be a possible light absorber for solar energy conversion. 79) These results suggest that the combination of HPT processing and low-temperature annealing can be effective in the formation of novel metastable phases. ...
We report on high-pressure torsion (HPT) processing of Si and related semiconducting materials, and discuss their phase transformations and electrical, thermal, and optical properties. In-situ synchrotron x-ray diffraction revealed that the metastable bc8-structure Si-III and r8-structure Si-XII in the HPT-processed Si samples gradually disappeared and hexagonal-diamond Si-IV appeared during annealing up to 473 K. The formation of Si-III/XII in the samples processed at a nominal pressure of 6 GPa indicated the strain-induced phase transformation from diamond-cubic Si-I to a high-pressure tetragonal Si-II phase during HPT processing, and a following phase transformation from Si-II to Si-III/XII upon decompression. The resistivity decreased with increasing the number of anvil rotations due to the formation of semimetallic Si-III. The thermal conductivity of Si was reduced to ∼3 W m⁻¹K⁻¹ after HPT processing. A weak and broad photoluminescence peak associated with Si-I nanograins appeared in the visible light region after annealing. Metastable bc8-Si0.5Ge0.5 with a semimetallic property was formed by HPT processing of a traveling-liquidus-zone-grown Si0.5Ge0.5 crystal. These results indicate that the application of HPT processing to Si and related semiconductors paves the way to novel devices utilizing nanograins and metastable phases.
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... In contrast to magnesium fluoride [20], the frequencies of coherent phonons in silicon are much more difficult to attribute to certain polymorphic phases. This is caused by, firstly, the presence of a large (more than twelve [33]) number of phases observed at high pressures and, secondly, many frequencies (for example, at 130 cm −1 ) are characteristic of several phases [34]. In addition, the frequencies of phonon vibrations strongly depend on the lattice temperature [34]. ...
... However, to identify new phases in silicon, one should pay attention to the time delays when the changes in the phonon spectrum occurred. Because the pressures at which the phase transitions occurred were relatively well-defined, at least for stationary conditions [33], and the pressure at the shock wave front decayed exponentially [39], it was possible to plot the points on axes (time-pressure), where the x-axis represented the delay at which the phase transitions occurred, and the y-axis represented the pressure at which the phase transitions should have occurred. If these points formed an exponential relationship (pressure decay), then the phase change was correctly restored. ...
In this study, we reconstructed the dynamics of the impact of mid-IR-range (4.6 μm) femtosecond laser pulses on bulk silicon under tight focusing conditions (NA = 0.5). Our experimental results show that under this impact, the deposited energy density (DED) reaches approximately 4 kJ/cm3 (at an energy slightly above the plasma-formation threshold). Initially, the femtosecond pulse energy is absorbed by the laser-induced plasma, with a lifetime of approximately 160–320 fs (depending on the laser pulse energy). The energy transfer from the plasma to the atomic subsystem occurs on a sub-ps timescale, which generates a shock wave and excites coherent phonons on a sub-ps scale. The shift of atoms in the lattice at the front of the shock wave results in a cascade of phase transitions (Si-X => Si-VII => Si-VI => Si-XI => Si-II), leading to a change in the phonon spectra of silicon.
... 20 Recently, Si-III nanoparticles have been reported to be promising for future solar energy conversion utilizing multiexciton generation processes. 21 These metastable phases may, thus, become potential candidates for novel Si devices. ...
We report on electric, thermal, and optical properties of Si subjected to severe plastic deformation. Single-crystalline Si wafers were processed by high-pressure torsion (HPT) under a nominal pressure of 6 GPa. The HPT-processed samples consisted of metastable body-centered-cubic Si-III and rhombohedral Si-XII as well as diamond-cubic Si-I and amorphous phases. The metastable phases increased with increasing the number of anvil rotations ( N). The resistivity of the single-crystalline Si (20 Ω cm) increased to 50 Ω cm after HPT processing for N = 10 and then it decreased to ∼0.7 Ω cm when increasing N to 100. Such an increase and a subsequent decrease in resistivity were attributed to the grain refinement and the increase in the volume fraction of semimetallic Si-III, respectively. The thermal conductivity was reduced by two orders of magnitude (∼3 W m ⁻¹ K ⁻¹ ) after HPT processing for N ≥ 50. A weak broad photoluminescence peak originating from Si-I nanograins appeared in the visible light region after annealing at 600 °C. These results indicate that the resistivity, thermal conductivity, and photoluminescence of the HPT-processed Si strongly depend on the formation of metastable phases and grain refinement, which are induced by shear strain under high pressure.
... Despite dramatic differences in various properties [21], bulk and monolayer XS 2 share the same semiconducting behavior with wide and indirect band gaps [22]. However, the characteristics of the indirect band gap may affect the efficiency of light absorption [23]; therefore, it is worthwhile to find a polymorph of XS 2 with the direct band gap desired for the photoelectric field. ...
Transition metal dichalcogenides (TMDs) have attracted intense interest due to their potential applications as electronic and optical devices. Here, we propose two XS2 (X = Zr and Hf) compounds within orthorhombic Pnma phase, denoted as Pnma-ZrS2 and Pnma-HfS2 as promising candidates for thin-film solar cells and nanoelectronics. Both Pnma-ZrS2 and Pnma-HfS2 are found to be semiconductors with desired direct band gaps of 1.07 eV and 1.24 eV, respectively, which lead to large visible-light absorption. The absorption coefficients of the first absorption peak for the Pnma-XS2 are about twice larger than the corresponding one for P-3m1 phase with a 1T-CdI2 like structure. Such Pnma-XS2 polymorphs are predicted to be mechanically, dynamically, and thermodynamically stable up to 500 K, and they are highly realizable in the experiment since a phase transition from the P-3m1 phases to the Pnma phases could occur under moderate temperature based on our calculations.
... The lattice structure of MgF2 is presented in Fig. 1. It is essential to mention that under high pressure, the phase transition should occur in a cascade manner, as it is demonstrated for the shock-wave induced phase transitions in Si: α-diamond ⇒ β-Sn ⇒ Imma ⇒ simple hex [24][25][26] . The possible paths of atom movement during solid-to-solid phase transitions are also marked in Fig. 1. ...
The advent of free-electron lasers opens new routes for experimental high-pressure physics, which allows studying dynamics of condensed matter with femtosecond resolution. A rapid compression, that can be caused by laser-induced shock impact, leads to the cascade of high-pressure phase transitions. Despite many decades of study, a complete understanding of the lattice response to such a compression remains elusive. Moreover, in the dynamical case (in contrast to quasi-static loading) the thresholds of phase transitions can change significantly. Using the third harmonic pump-probe technique combined with molecular dynamics to simulate the terahertz (THz) spectrum, we revealed the dynamics of ultrafast laser-induced phase transitions in MgF 2 in all-optical experiment. Tight focusing of femtosecond laser pulse into the transparent medium leads to the generation of sub-TPa shock waves and THz coherent phonons. The laser-induced shock wave propagation drastically displaces atoms in the lattice, which leads to phase transitions. We registered a cascade of ultrafast laser-induced phase transitions (P42/mnm ⇒ Pa-3 ⇒ Pnam) in magnesium fluoride as a change in the spectrum of coherent phonons. The phase transition has the characteristic time of 5-10 ps, and the lifetime of each phase is on the order of 40-60 ps. In addition, phonon density of states, simulated by molecular dynamics, together with third-harmonic time-resolved spectra prove that laser-excited phonons in a bulk of dielectrics are generated by displacive excitation (DECP) mechanism in plasma mediated conditions.
... Photovoltaic solar collectors allow direct conversion of solar energy into electricity with conversion efficiencies that have increased in parallel with the development in the field of solar cells. Silicon-based solar cells were found to be the most effective with high efficiencies exceeding 20% [3]. Moreover, other inorganic materials have also been investigated in various solar cell architectures. ...
... Various natural compounds extracted from plants such as natural pigments [2] were experienced in the development of economically competitive solar panels for direct sunlight conversion. Nonetheless, solar cells based on these natural dyes are characterized by small values of conversion efficiency, which mostly falls below 1.5% [3], when compared to conventional solar cells. Despite this drawback, natural dye-based solar cells remain very attractive, particularly from an economical point of view. ...
The use of naturally extracted compounds as dye sensitizers is a very promising alternative for the manufacture of low-cost solar cells. These directly convert solar energy to electricity. In the present study, Aloe Latex Solid (ALS), which is an orange-yellow solid compound extracted from Aloe Vera leaves, was deposited on a TiO2 thin film (TiO2/ALS) for the construction of two different solar cell configurations. The UHPLC-DAD-ESI-MS analysis, UV–Vis and FTIR spectroscopic studies were performed for the prepared dye sensitizer. In fact, the performance of the TiO2/ALS composite was investigated in a heterojunction dye sensitized solar cell (HJ-DSSC) and a liquid electrolyte-based dye sensitized solar cell (LE-DSSC) to identify the architecture with highest efficiency of sunlight conversion. The solar cells photovoltaic performance in terms of short-circuit current, open circuit voltage, fill factor and energy conversion efficiency was tested with photocurrent density–voltage measurements. Interesting solar conversion efficiencies were obtained for both architectures with a maximum value of about 1.17% corresponding to the LE-DSSC configuration.
... Moreover, depending on the amplitude and time duration of loading, the final phase of Si is varied from amorphous to Ibam, BC8, hex. diamond (Si-IX), etc. [13,14]. The presumptive tree of phase transitions in Si primarily depends on the velocity of pressure release and do not take into account the role of temperature that would probably arise in the experiments, both in the case of laser ablation (when the laser pulse is directly focused onto the sample's surface [15,16]) or shock-wave impact. ...
We demonstrate an ultrafast (<0.1 ps) reversible phase transition in silicon (Si) under ultrafast pressure loading using molecular dynamics. Si changes its structure from cubic diamond to β-Sn on the shock-wave front. The phase transition occurs when the shock-wave pressure exceeds 11 GPa. Atomic volume, centrosymmetry, and the X-ray-diffraction spectrum were revealed as effective indicators of phase-transition dynamics. The latter, being registered in actual experimental conditions, constitutes a breakthrough in the path towards simple X-ray optical cross-correlation and pump-probe experiments.
