group-IV Elements Research Articles

Accessing the Lithiation/Delithiation Mechanism of a Li-Ion Battery FeSi2/Si/Graphite Composite Negative Electrode and Differentiating the Si/Graphite Contributions By Simultaneous Operando Synchrotron SAXS/Waxs

Combined with renewable energy harvesting technologies, such as solar panels or wind turbines, energy storage is the key to keeping up with the ever-increasing demand for electric energy and reducing the world dependency on fossil fuels. Over the last three decades lithium-ion batteries (LiBs), starting with the first commercialized cell by Sony in 1990 [1], have evolved from the most promising energy-storage technologies [2] to the most widely used nowadays. Not only can LiBs be found in portable devices such as laptops, they are also used in larger scale applications such as electric vehicles. Since the first commercialization of LiBs, graphite (372 mAh.g-1) has been the usual negative electrode material. Nevertheless, the continuous demand, in the automotive industry in particular, for more performant, light, safe and cheap LiBs drives research towards the design and development of new electrode materials characterized in particular by high energy densities and a longer capacity retention during cycling. Among the candidates for high specific gravimetric capacity is silicon (3579 mAh.g-1) [3]. However, two major problems hamper the use of this material as a LiB electrode material. The first is its high volumetric change (297 % [4]) during lithiation, which causes particle cracking and pulverization during cycling which in turn leads to a quick fading of the cell capacity. The second is its high first cycle irreversible capacity. Reducing the particle size to the nanoscale and thus reducing the mechanical strain on the particle by attenuating its volumetric expansion/contraction helps alleviate pulverization. However, because of poor electrode structural stability during cycling this approach alone does not stop capacity degradation [5]. Using a dual-phase composite material that contains an inactive host matrix in which the active material is well-dispersed also alleviates pulverization. The FeSi2/Si/graphite anode material is particularly interesting due to its low-cost, high reversible capacity and good cycling stability [6]. This material is composed of a dual-phase FeSi2 (inactive)/amorphous Si (active) composite and graphite, a conductive matrix to accommodate the Si volumetric expansion, and is seen as well suited for the development of high capacity LiBs. Be that as it may, it is still unclear how this complex material functions and what makes it so performant. With the aim of accessing the lithiation/delithiation mechanism of a FeSi2/Si/graphite negative LiB electrode, we performed simultaneous operando Small and Wide Angle X-Ray Scattering (SAXS and WAXS) measurements on a cycling NMC//FeSi2/Si+graphite pouch-cell filled with a 1 mol.L-1 LiPF6 in FEC/EMC (30/70 wt%) electrolyte at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. Through WAXS we were able to investigate the local structure (10-1 nm) of the cell materials and in particular observe the lithiation/delithiation of the graphite phase by following its corresponding Bragg peak and those of its different lithiated phases (LiC30, LiC24, LiC18, LiC12 and LiC6). Simultaneously, we gained information on the active silicon state of lithiation/delithiation by observing the morphology changes occurring at the nano-scale (1 nm to 10 nm) during the volumetric expansion/contraction of the silicon through SAXS measurements. The combined information give us insight on the lithiation/delithiation mechanism of the negative electrode and enabled us to dissociate the capacity contribution of the silicon phase from that of the graphite phase as a function of potential, time and capacity. By cycling the cell at several current-rates and by repeating the same experiment on an aged cell cycled 300 times, we were also able to enquire into the impact of fast-charging and ageing on the lithiation/delithiation mechanism of the negative electrode. In this talk, we will present those results after proposing a lithiation/delithiation mechanism for the FeSi2/Si/graphite composite electrode. Yoshio Nishi, H.A., Atsuo Omaru Non aqueous electrolyte cell, in US Patent1990, Sony Corp United States of America.Armand, M. and J.M. Tarascon, Building better batteries. Nature, 2008. 451: p. 652.Goriparti, S., et al., Review on recent progress of nanostructured anode materials for Li-ion batteries. Journal of Power Sources, 2014. 257: p. 421-443.Tian, H., et al., High capacity group-IV elements (Si, Ge, Sn) based anodes for lithium-ion batteries. Journal of Materiomics, 2015. 1(3): p. 153-169.Li, H., et al., A High Capacity Nano Si Composite Anode Material for Lithium Rechargeable Batteries. Electrochemical and Solid-State Letters, 1999. 2(11): p. 547-549.Li, T., et al., Cycleable graphite/FeSi6 alloy composite as a high capacity anode material for Li-ion batteries. Journal of Power Sources, 2008. 184(2): p. 473-476. Figure 1

Intrinsic bending flexoelectric constants in two-dimensional materials

Flexoelectricity is a form of electromechanical coupling that has recently emerged because, unlike piezoelectricity, it is theoretically possible in any dielectric material. Two-dimensional (2D) materials have also garnered significant interest because of their unusual electromechanical properties and high flexibility, but the intrinsic flexoelectric properties of these materials remain unresolved. In this work, using atomistic modeling accounting for charge-dipole interactions, we report the intrinsic flexoelectric constants for a range of two-dimensional materials, including graphene allotropes, nitrides, graphene analogs of group-IV elements, and the transition metal dichalcogenides (TMDCs). We accomplish this through a proposed mechanical bending scheme that eliminates the piezoelectric contribution to the total polarization, which enables us to directly measure the flexoelectric constants. While flat 2D materials like graphene have low flexoelectric constants due to weak $\pi-\sigma$ interactions, buckling is found to increase the flexoelectric constants in monolayer group-IV elements. Finally, due to significantly enhanced charge transfer coupled with structural asymmetry due to bending, the TMDCs are found to have the largest flexoelectric constants, including MoS$_{2}$ having a flexoelectric constant ten times larger than graphene.

Phonons and lattice thermal conductivities of graphene family

In the post-graphene era, there has been a significant upsurge of interest on the mechanical and thermal properties of two dimensional graphene-like honeycomb structures made of other group-IV elements. This article deals with application of first principle based density functional theory to investigate the lattice dynamics of the members of this extended graphene family. We explore the changes observed in the lattice thermal conductivity, adopting physical models for estimating phonon lifetimes. We use the Asen-Palmer modified version of Debye-Callaway theory to calculate the lattice thermal conductivity of graphene, as well as of low and double buckled silicene, germanene, and stanene. This allows us to establish a connection between the parameters such as group velocity, Grüneisen parameter, and Debye temperature of the acoustic phonon modes and the lattice thermal conductivity. Our calculations show that the presence of buckling reduces the group velocity and the Debye temperature of the sheets down the group, and hence, reduces their lattice thermal conductivity. However, there is no linear dependence between the buckling height and the observed lowering. An increase in buckling height in sheets with different geometries of the same atomic species, beyond a certain limit, does not lead to change in the group velocity and the Debye temperature of the sheets.

First-principles study of structural, vibrational, and electronic properties of trigonally bonded II-IV-N2

With the first-principles calculations, we probe into the structural, vibrational, and electronic properties of trigonally bonded planar II-IV-N2 (II = Zn, Cd; IV = Si, Ge, Sn), in contrast to their well-known tetrahedrally bonded counterparts. A detailed analysis is presented for the deformed honeycomb-like structures of planar II-IV-N2 in light of atomic radius and electronegativity of different group-II and group-IV elements. Cohesive energy and phonon property are calculated to investigate the stability of monolayer II-IV-N2. Among the six monolayer II-IV-N2 studied, five of them are dynamically stable except for monolayer CdSnN2 which has imaginary phonon frequencies. The band curvatures of trigonally bonded II-IV-N2 reveal distinct difference comparing to their tetrahedrally bonded counterparts and show connections to atomic transmutation and structural distortions. The different variation trends of band gap from tetrahedrally bonded structure to trigonally bonded structure are explored through density of states analysis. The highest valence band energies of monolayer ZnGeN2 and ZnSnN2 at Γ and X point are very close, resulting in a quasi-direct band gap. Tuning of band gap characteristics by uniaxial strain is investigated for these ternary nitrides and it is shown that a slightly change less than 0.1% on lattice constant b can induce indirect-to-direct band gap transition for monolayer ZnSnN2.

Effect of R-site substitution and the pressure on stability of RFe12: A first-principles study

We theoretically study the structural stability of RFe12 with the ThMn12 structure (R: rare-earth element, La, Pr, Nd, Sm, Gd, Dy, Ho, Er, Tm, Lu, Y, or Sc, or group-IV element, Zr or Hf) based on density functional theory. The formation energy has a strong correlation with the atomic radius of R. The formation energy relative to simple substances decreases as the atomic radius decreases, except for R= Sc and Hf, while that relative to R2Fe17 and bcc Fe has a minimum for R= Dy. The present results are consistent with recent experimental reports in which the partial substitution of Zr at R sites stabilizes RFe12-type compounds with R= Nd or Sm. Our results also suggest that the partial substitution of Y, Dy, Ho, Er, or Tm for Nd or Sm is a possible way to enhance the stability of the ThMn12 structure. Under hydrostatic pressure, the formation enthalpy decreases up to ≈6 GPa and then starts to increase at higher pressures.

Very small tail state formation in Cu2ZnGeSe4

We find that coevaporated Cu2ZnGeSe4 has an ideal bandgap for solar cells (1.39 ± 0.01 eV) and shows quite reduced tail state absorption with a very low Urbach energy of 28 meV, which is far smaller than those of more studied Cu2ZnSnSe4 and Cu2ZnSnS4. The small tail states in Cu2ZnGeSe4 are found to originate from almost perfect cation ordering, while unusual tail state generation occurs in the Sn-based quaternary compounds by extensive cation substitution. Quite remarkably, the crystal total energy derived from first-principles calculations reveals a unified rule for the cation disordering, confirming that the lighter group-IV element (i.e., Ge) is essential for eliminating the tail state generation induced by cation mixing.

Direct Synthesis of Nanotubular Si-TiO2for Lithium-Ion Batteries Anodes By Anodizing

Industry and the scientific community have been concentrating efforts on the production and development of energy storage systems ESS [1].By the energy density improvements of lithium ion battery electrodes for electronic devices (EDs) and energy vehicles (EVs), and cost of manufacturing must be taken into account, so that new developed materials can with the currently used batteries[2-4].Silicon is an important candidate as anode material for the next generation of lithium ion batteries(LIBs) because of its extremely high specific capacity (4200 mAh/g). However, Si particles degrade during the volumetric change related to lithiation/delithiation processes.Efforts have been made in the last decade in an attempt to minimize the instabilities caused by the large volumetric variation [5,6]. The most effective approaches in this regard include: (a) reduction of particle size to the nanometer size range to lower mechanical stress, (b) formation of a porous structure promoting a stable solid electrolyte interface SEI (c) dispersing Si nanoparticles in a conductive matrix to accommodate the volumetric variations and preserve the mechanical integrity of the electrode, (d) formation of amorphous SiO2 structure in small dimensions, and the(e) formation of intermetallic composite with structure layers to accommodate Si particles[7]. In the other hand, TiO2also incorporates Li(330 mAh/g)and shows a fast lithium intercalation reaction, without major volumetric effects during Li intercalation/deintercalation. In this work the aim is to produce Si nanoparticles embedded in a nanotubular TiO2 matrix, avoiding mechanical stress problems, due to volumetric changes and the formation of SEI directly between Si nanoparticles(SiNP) and the electrolyte. For this, Ti-Si alloys with 2 and 4 at.%Si were prepared by arc melting in an Ar atmosphere. A Si-Ti solid solution phase was obtained by furnace heating to 1200 ºC and rapidly cooling to a hard martensitic phase.Thin sheets of these alloys were cut and anodized in different electrolytes aiming to avoid the dissolution of the Si Particles. The structure and morphology of the Si rich TiO2 nanotubes were investigated by TEM, XPS, SEMand GID/DRX. Cell voltage transients of the anodizing of Ti2Si and Ti4Si indicate different levels of SiNP accumulation. Different electrolyte compositions, organic and inorganic, with and without fluoride were used for the growth of the porous structures. The effect of lithium addition on electrolytes is observed in favor of porous structure. SiNPs embedded in TiO2 were observed by SEM, TEM and characterized by GID/DRX.1% at. Si was measured by EDS onregular tubular TiO2 detached from Ti-2 at.%Si anodized in a fluoride based electrolyte, indicating that around 50% of the Si was not lost to the electrolyte during anodization. In the other hand, metallic Si phase was detected by grazing angle diffraction indicating that Si NP are formed. The developed process shows to be an efficient way for the direct fabrication of self-organized titania nanotubes containing silicon nanoparticles. [1]S.Goriparti, E.Miele, F. De Angelis, E. Di Fabrizio, R. P.Zaccaria, C. Capiglia, Journal of Power Sources, 257 (2014) 421-443. [2]Naoki Nitta, Feixiang Wu, Jung Tae Lee and GlebYushin. Materials Today Volume 18, Number 5 June (2015) [3] Peter G. Bruce, Bruno Scrosati, Jean-Marie TarasconAngewandteChemieAngew. Chem. Int. Ed. (2008), 47, 2930 – 2946 [4] Bruno Scrosati, Jürgen Garche, Lithium batteries: Status, prospects and future, Journal of Power Sources 195 (2010) 2419–2430 [5] Weitian Lia,1, Xianwei Guoa,1, YueLub, Lin Wanga, Ailing Fana, ManlingSuib, HaijunYua, Amorphous nanosized silicon with hierarchically porous structure for highperformance lithium ion batteries. Energy Storage Materials 7 (2017) 203–208 [6] Lucie Leveau , Barbara Laïk , Jean-Pierre Pereira-Ramos, AurelienGohier , Pierre Tran-Van ,Costel-SorinCojocaru. Silicon nano-trees as high areal capacity anodes for lithium-ion batteriesJournal of Power Sources 316 (2016) 1-7. [7] HuajunTian, FengxiaXin, Xiaoliang Wang, Wei He ,Weiqiang HanHigh capacity group-IV elements (Si, Ge, Sn) based anodes for lithium-ion batteries. Journal of Materiomics 1 (2015) 153-169 Figure 1

Vacuum brazing of cubic boron nitride to medium carbon steel with Zr added passive and Ti activated eutectic Ag-Cu alloys

Active element like Ti is usually added to Ag-Cu passive alloy to braze cubic boron nitride (cBN) ceramic particles with steel substrate under high vacuum. Current work focusses on another group-IV element Zr, having more negative Gibbs free energy than Ti, concerning its reaction capability with cBN. In the present study, its effectiveness as an additive to the 72Ag28Cu alloy in brazing of cBN has been investigated. Zr is added to both passive and Ti-activated Ag-Cu alloys. Interestingly, the addition of 2wt% Zr to the composition of eutectic alloy fails to braze cBN, unlike the addition of 2wt% Ti. The underlying science is critically investigated. A new formulation of the alloy with the composition of (72Ag28Cu)-4Zr-4Ti alloy is found to be a beneficial alternative. Wetting of this new alloy on cBN is appreciable and simultaneously an additional benefit of 50% increase in wear resistance is achieved, compared to the (72Ag28Cu)-2Ti alloy. A significantly hard new intermetallic phase, AgCu4Zr is detected in the microstructure which contributes to the enhancement of wear resistance of the new alloy. The joint strength thus achieved in brazing cBN with medium carbon steel using (72Ag28Cu)-2Ti and (72Ag28Cu)-4Zr-4Ti alloys is also compared but found to be compromised in case of the later. The results are analyzed in the light of microstructure, alloy-cBN interfacial chemistry and interfacial fracture under load.

Hole polarons and p -type doping in boron nitride polymorphs

Boron nitride polymorphs hold great promise for integration into electronic and optoelectronic devices requiring ultrawide band gaps. We use first-principles calculations to examine the prospects for $p$-type doping of hexagonal ($h\ensuremath{-}\mathrm{BN}$), wurtzite ($wz\ensuremath{-}\mathrm{BN}$), and cubic ($c\ensuremath{-}\mathrm{BN}$) boron nitride. Group-IV elements (C, Si) substituting on the N site result in a deep acceptor, as the atomic levels of the impurity species lie above the BN valence-band maximum. On the other hand, group-II elements (Be, Mg) substituting on the B site do not give impurity states in the band gap; however, these dopants lead to the formation of small hole polarons. The tendency for polaron formation is far more pronounced in $h\ensuremath{-}\mathrm{BN}$ compared to $wz\ensuremath{-}\mathrm{BN}$ or $c\ensuremath{-}\mathrm{BN}$. Despite forming small hole polarons, Be acceptors enable $p$-type doping, with ionization energies of 0.31 eV for $wz\ensuremath{-}\mathrm{BN}$ and 0.24 eV for $c\ensuremath{-}\mathrm{BN}$; these values are comparable to the Mg ionization energy in GaN.

Tunnelling characteristics of Stone–Wales defects in monolayers of Sn and group-V elements

Topological defects in ultrathin layers are often formed during synthesis and processing, thereby strongly influencing the electronic properties of layered systems. For the monolayers of Sn and group-V elements, we report the results based on density functional theory determining the role of Stone–Wales (SW) defects in modifying their electronic properties. The calculated results find the electronic properties of the Sn monolayer to be strongly dependent on the concentration of SW defects, e.g. defective stanene has nearly zero band gap (≈0.03 eV) for the defect concentration of 2.2 × 1013 cm−2 which opens up to 0.2 eV for the defect concentration of 3.7 × 1013 cm−2. In contrast, SW defects appear to induce conduction states in the semiconducting monolayers of group-V elements. These conduction states act as channels for electron tunnelling, and the calculated tunnelling characteristics show the highest differential conductance for the negative bias with the asymmetric current–voltage characteristics. On the other hand, the highest differential conductance was found for the positive bias in stanene. Simulated STM topographical images of stanene and group-V monolayers show distinctly different features in terms of their cross-sectional views and distance-height profiles. These distinctive features can serve as fingerprints to identify the topological defects in experiments for the monolayers of group-IV and group-V elements.

Quasiparticle self-consistentGWelectronic band structure ofCd-IV-N2compounds

Quasiparticle self-consistent $GW$ calculations of the electronic band structures of ${\text{Cd-IV-N}}_{2}$ with group-IV elements, Si, Ge, and Sn, are reported. The lattice parameters in the $Pna{2}_{1}$ crystal structure are obtained both in the local density approximation and generalized gradient approximation (GGA) and provide respectively an underestimate and overestimate of the lattice constants for these until now not synthesized materials. The Wyckoff positions are obtained by structural minimization. At the GGA lattice constant, which is expected to be the most reliable, ${\mathrm{CdSiN}}_{2}$ is found to have an indirect gap $(U\ensuremath{-}\mathrm{\ensuremath{\Gamma}})$ of 2.72 eV, which is 0.55 eV lower than the direct gap at $\mathrm{\ensuremath{\Gamma}}$ of 3.27 eV. In ${\mathrm{CdGeN}}_{2}$, the indirect gap of 2.01 eV is only 0.1 eV smaller than the direct gap of 2.11 eV, and in ${\mathrm{CdSnN}}_{2}$ the gap is direct and equals 0.64 eV close to that of InN, as expected. The direct/indirect nature of ${\mathrm{CdGeN}}_{2}$ is found to be sensitive to shear strain. The conduction band effective masses decrease from ${\mathrm{CdSiN}}_{2}$ to ${\mathrm{CdGeN}}_{2}$ to ${\mathrm{CdSnSn}}_{2}$. The energies of formation indicate these materials to be stable with the possible exception of ${\mathrm{CdSnN}}_{2}$.

Group-IV (Si, Ge, and Sn)-doped AgAlTe2 for intermediate band solar cell from first-principles study

Earlier studies of chalcopyrites as the absorber for intermediate band solar cells (IBSCs) mainly focused on Cu-based compounds, whose intermediate band is usually empty due to its intrinsic p-type conductivity. This is not beneficial to the two sub-bandgap absorptions. In this paper, we demonstrate that the intermediate bands in group IV (Si, Ge, and Sn) doped AgAlTe2 are delocalized and mainly contributed by the anti-bonding state of group-IV elements s state and Te-p state. Overall, we suggest that Sn-doped AgAlTe2 should be a promising absorber candidate for IBSCs based on the theoretical efficiency and defect stability.

Controlling Thermodynamic Properties of Ferromagnetic Group-IV Graphene-Like Nanosheets by Dilute Charged Impurity

Using the Kane-Mele Hamiltonian, Dirac theory and self-consistent Born approximation, we investigate the effect of dilute charged impurity on the electronic heat capacity and magnetic susceptibility of two-dimensional ferromagnetic honeycomb structure of group-IV elements including silicene, germanene and stanene within the Green’s function approach. We also find these quantities in the presence of applied external electric field. Our results show that the silicene (stanene) has the maximum (minimum) heat capacity and magnetic susceptibility at uniform electric fields. From the behavior of theses quantities, the band gap has been changed with impurity concentration, impurity scattering strength and electric field. The analysis on the impurity-dependent magnetic susceptibility curves shows a phase transition from ferromagnetic to paramagnetic and antiferromagnetic phases. Interestingly, electronic heat capacity increases (decreases) with impurity concentration in silicene (germanene and stanene) structure.

Plasmon properties and hybridization effects in silicene

The plasmonic character of monolayer silicene is investigated by time-dependent density functional theory in the random phase approximation. Both the intrinsic (undoped) and several extrinsic (carrier doped or gated) conditions are explored by simulating injection of a probe particle (i.e., an electron or a photon) of energy below 20 eV and in-plane momentum smaller than $1.1\phantom{\rule{0.16em}{0ex}}\AA{}{}^{\ensuremath{-}1}$. The energy-loss function of the system is analyzed, with particular reference to its induced charge-density fluctuations, i.e., plasmon resonances and corresponding dispersions, occurring in the investigated energy-momentum region. At energies larger than 1.5 eV, two intrinsic interband modes are detected and characterized. The first one is a hybridized $\ensuremath{\pi}$-like plasmon, which is assisted by competing one-electron processes involving $s{p}^{2}$ and $s{p}^{3}$ states, and depends on the slightest changes in specific geometric parameters, such as nearest-neighbor atomic distance and buckling constant. The second one is a more conventional $\ensuremath{\pi}\ensuremath{-}\ensuremath{\sigma}$ plasmon, which is more intense than the $\ensuremath{\pi}$-like plasmon and more affected by one-electron processes involving the $\ensuremath{\sigma}$ bands with respect to the analogous collective oscillation in monolayer graphene. At energies below 1 eV, two extrinsic intraband modes are predicted to occur, which are generated by distinct types of Dirac electrons (associated with different Fermi velocities at the so-called Dirac points). The most intense of them is a two-dimensional plasmon, having an energy-momentum dispersion that resembles that of a two-dimensional electron gas. The other is an acoustic plasmon that occurs for specific momentum directions and competes with the two-dimensional plasmon at mid-infrared energies. The strong anisotropic character of this mode cannot be explained in terms of the widely used Dirac-cone approximation. As in mono-, bi-, and few-layer graphene, the extrinsic oscillations of silicene are highly sensitive to the concentration of injected or ejected charge carriers. More importantly, the two-dimensional and acoustic plasmons appear to be a signature of the honeycomb lattice, independently of the chemistry of the group-IV elements and the details of the unit-cell geometry.

Lateral and flexural phonon thermal transport in graphene and stanene bilayers.

Stanene, a low thermal conductivity two-dimensional (2D) sheet composed of group-IV element Sn, is a prototype material with novel properties such as 2D topological insulating behavior and near-room-temperature quantum Hall effects. Monolayer graphene, on the other hand, possesses unusual thermal properties, but has a zero bandgap. By stacking stanene and graphene monolayers vertically into a hetero-bilayer, an indirect bandgap can be obtained, making the hetero-bilayer a good candidate for special applications. In this work, the in-plane thermal conductivity (κ) and out-of-plane interfacial thermal resistance (R) in the hetero-bilayer are systematically investigated using non-equilibrium molecular dynamics and transient pump-probe methods. Effects of dimension, system temperature and van der Waals coupling strength on the thermal properties are examined. The predicted in-plane thermal conductivity of the graphene/stanene hetero-bilayer is 311.1 W m-1 K-1, higher than most 2D materials such as phosphorene, hexagonal boron nitride (h-BN), MoS2 and MoSe2. Phonon power spectra are recorded for graphene and stanene individually to help the explanation of their κ difference. The inter-layer thermal resistance between graphene and stanene hetero-bilayers is predicted to be 2.13 × 10-7 K m2 W-1, which is on the same order of magnitude as several other 2D bilayer structures.

A generalized model on the evaluation of entropy and entropy of mixing of liquid Na-Sn alloys

Recently proposed theory of entropy of mixing of the structurally inhomogeneous binary liquid alloys of alkali metals and group-IV elements is applied successfully to the liquid Na-Sn alloy. This alloy indicates chemical short range ordering (CSRO) i.e. exhibits partially salt like characteristics due to strong tendencies to compound formation, in the solid as well as in the liquid state. So, the generalized model for entropy of charged-hard-spheres mixture of arbitrary charge and size is employed to evaluate entropies of mixing, treating the sample as partially charge transfer system. The computed entropies of mixing are in excellent agreement with the experimental data.

Recent progress in the study of stanene

Inspired by the successfully experimental realization of graphene, other ultrathin materials consisting of group-IV elements Si, Ge and Sn have been attracted plenty of interests due to their unprecedented electronic properties. Among them, two-dimensional low-buckled Sn-based stanene and its derivatives present novel quantum properties due to its large spin-orbital coupling, such as a quantum spin Hall insulating behaviour with a very large band gap, and the capability to support enhanced thermoelectric performance, topological superconductivity and the near-room-temperature quantum anomalous Hall effect and so on. Therefore, stanene shows the great application potentials in the dissipationless electric conduction at room temperature, high performance thermoelectricity, spintronics and topological quantum computation. This review will briefly introduce the recent progress in the study of stanene both in theory and in experiment.

Si-Based Ge and GeSn Material Epitaxy and Thermal Stability Characterization

_In the past decade, Si photonics has become one of the hottest research topics in the world since it holds great promise for maintaining the performance roadmap known as Moore’s law. The application of silicon photonics technologies to requires the development of high quality Si-based materials for photonics devices monolithically integrated to a Si platform. Germanium (Ge), due to its large absorption coefficient at near-infrared frequencies, and its lower cost and compatibility of parallel processing with silicon compared to III-V semiconductors, is perceived as the best candidate for on-chip photodetectors. Meanwhile, Ge is a pseudo-direct bandgap material because its direct Γ-valley CB is just 136.5 meV above the lowest L-valley CB at room temperature . This energy difference can be effectively reduced by incorporating Sn, another group-IV element, into Ge to modulate the band structure. GeSn alloy has been proven to be another promising material for the Si-based laser and PDs covering all telecommunication windows. In this presentation, Ge 、GeSn and GeSn/Ge MQW have been grown on Si, Ge/Si and Ge. The crystallinity and growth quality of the Ge and GeSn films were investigated through material characterization methods including x-ray diffraction, scanning electron microscopy, energy-dispersive x-ray spectroscopy, and transmission electron microscopy. In order to grow GeSn on Si and Ge subtrate,the growth is performed at low temperature, but the devices fabrication process will be carried out at high temperature, so the stability study is important. The effect of rapid thermal annealing on the conversion of metastable elastically strained GeSn layers into a plastically relaxed state is also examined. the results show that GeSn alloy have good thermal stability at 500℃if Sn contents are as low as ~3%. The stability of the GeSn/Ge MQW structure is temperature-dependentand and the temperature over 400℃ should be avoided to guarantee the structural integrity of the MQWs.

Electronic band structure trends of perovskite halides: Beyond Pb and Sn to Ge and Si

The trends in electronic band structure are studied in the cubic $AB{X}_{3}$ halide perovskites for $A=\text{Cs}$; $B=\text{Pb}$, Sn, Ge, Si; and $X=\text{I}$, Br, Cl. The gaps are found to decrease from Pb to Sn and from Ge to Si, but increase from Sn to Ge. The trend is explained in terms of the atom $s$ levels of the group-IV element and the atomic sizes which changes the amount of hybridization with $X\text{\ensuremath{-}}p$ and hence the valence bandwidth. Along the same series spin-orbit coupling also decreases and this tends to increase the gap because of the smaller splitting of the conduction band minimum. Both effects compensate each other to a certain degree. The trend with halogens is to reduce the gap from Cl to I, i.e., with decreasing electronegativity. The role of the tolerance factor in avoiding octahedron rotations and octahedron edge sharing is discussed. The Ge containing compounds have tolerance factor $tg1$ and hence do not show the series of octahedral rotation distortions and the existence of edge-sharing octahedral phases known for Pb and Sn-based compounds, but rather a rhombohedral distortion. ${\mathrm{CsGeI}}_{3}$ is found to have a suitable gap for photovoltaics both in its cubic (high-temperature) and rhombohedral (low-temperature) phases. The structural stability of the materials in the different phases is also discussed. We find the rhombohedral phase to have lower total energy and slightly larger gaps but to present a less significant distortion of the band structure than the edge-sharing octahedral phases, such as the yellow phase in ${\mathrm{CsSnI}}_{3}$. The corresponding silicon based compounds have not yet been synthesized and therefore our estimates are less certain but indicate a small gap for cubic ${\mathrm{CsSiI}}_{3}$ and ${\mathrm{CsSiBr}}_{3}$ of about $0.2\ifmmode\pm\else\textpm\fi{}0.2$ eV and $0.8\ifmmode\pm\else\textpm\fi{}0.6$ eV for ${\mathrm{CsSiCl}}_{3}$. The intrinsic stability of the Si compounds is discussed.

Piezoelectricity of Functionalized Graphene: A Quantum-Mechanical Rationalization

A large out-of-plane piezoelectricity can be induced in graphene by carbon substitution. Several simple substitutions are considered where C atoms are replaced by heavier group-IV elements (Si, Ge, and Sn). A more complex functionalization (namely, pyrrolic N-doped graphene) is also investigated where different functional groups, such as F, Cl, H3C, and H2N, are studied. Piezoelectric and elastic response properties of all systems are determined quantum mechanically at the ab initio level of theory. A rationalization of the physical and chemical parameters which most affect the out-of-plane piezoelectricity of functionalized graphene is reported, which reveals the dominant character of the nuclear over electronic contribution. The combination of an out-of-plane symmetry-breaking defect and a soft infrared-active phonon mode, with a large cell-deformation coupling, is shown to constitute the necessary prerequisite to induce a large out-of-plane piezoelectric response into functionalized graphene.