Since graphene was synthesized the interest for building new 2D and 3D structures based on the carbon allotropes has been growing every day. One of these 3D structures is know as carbon schwarzites. Schwarzites consist of carbon nanostructures possessing the shape of Triply-Periodic Minimal Surfaces (TPMS), which is characterized by a negative Gaussian curvature introduced by the presence of carbon rings with more than six atoms. Some examples of schwarzite families include: primitive (P), gyroid (G) and diamond (D). Previous studies considering different element species of schwarzites have investigated the mechanical, electrical and thermal properties. In this work, we investigated the stability of germanium (Ge) schwarzites using density functional theory with GGA exchange-correlation functional. We chose one structure of each family (P8bal), (G688) and (D688). It was observed that regions usually flat in carbon schwarzites acquires buckled configurations as previously observed on silicene and germanene monolayers. The investigated structures presented a semiconducting bandgap ranging from $0.13$ to $0.27$ eV. We also performed calculations of optical properties within the linear regime, where it was shown that Ge schwarzites structures absorb light from infrared to ultra-violet frequencies. Therefore, our results open new perspectives of materials that can be used in optelectronics devices application.
Schwarzites are porous structures that present negative Gaussian curvatures. Although initially proposed for carbon, in principle, schwarzites of other elements are possible. In this work, we have carried out a detailed investigation of the effect of element composition (C, Ge, Si, SiC, and BN) on the structural, electronic and optical properties of a gyroid schwarzite structure, the so-called G688. The DFT simulations were carried out using the well-known SIESTA code. Our results showed that formation energy values are in similar range of other related allotrope structures and are thermally s…
We investigated through fully atomistic molecular dynamics simulations, the mechanical behavior (compressive and tensile) and energy absorption properties of two families (primitive (P688 and P8bal) and gyroid (G688 and G8bal)) of carbon-based schwarzites. Our results show that all schwarzites can be compressed (with almost total elastic recovery) without fracture to more than 50%, one of them can be even remarkably compressed up to 80%. One of the structures (G8bal) presents negative Poisson's ratio value (auxetic behavior). The crush force efficiency, the stroke efficiency and the specific e…
Nature-occurring structures exhibiting unique topological features such as complex and gradient porosity has been the basis to create new materials and/or structures. Most studies have been focused on complex periodic porous structures but gradient porous ones have not been yet fully investigated for stable structural designs. In this work, we have proposed and tested a new approach to create cellular griding structures, in which the mass density varies from the center to the borders, i.e, a radial gradient. To create these new structures we exploited the topology of two carbon-based families …
Triply Periodic Minimal Surfaces (TPMS) possess locally minimized surface area under the constraint of periodic boundary conditions. Different families of surfaces were obtained with different topologies satisfying such conditions. Examples of such families include Primitive (P), Gyroid (G) and Diamond (D) surfaces. From a purely mathematical subject, TPMS have been recently found in materials science as optimal geometries for structural applications. Proposed by Mackay and Terrones in 1991, schwarzites are 3D crystalline porous carbon nanocrystals exhibiting the shape of TPMS. Although their …
Core-shell nanowires made of Si and Ge can be grown experimentally with excellent control for different sizes of both core and shell. We have studied the structural properties of Si/Ge and Ge/Si core-shell nanowires aligned along the $[110]$ direction, with diameters up to 10.2~nm and varying core to shell ratios, using linear scaling Density Functional Theory (DFT). We show that Vegard's law, which is often used to predict the axial lattice constant, can lead to an error of up to 1\%, underlining the need for a detailed \emph{ab initio} atomistic treatment of the nanowire structure. We analys…
We present a density-functional theory (DFT) study of the structural, electronic, and chemical bonding behaviour in germanium (Ge)-doped vanadium dioxide (VO$_2$). Our motivation is to explain the reported increase of the metal-insulator transition temperature under Ge doping and to understand how much of the fundamental physics and chemistry behind it can be captured at the conventional DFT level. We model doping using a supercell approach, with various concentrations and different spatial distributions of Ge atoms in VO$_2$. Our results suggest that the addition of Ge atoms strongly perturbs…
We present ab initio calculations of electronic and optical properties of perturbed hexagonal germanium and demonstrate that it is a superior material for active optoelectronic devices in the infrared spectral region. It is known that perfect lonsdaleite Ge is a pseudodirect semiconductor, i.e., with direct fundamental band gap but almost vanishing oscillator strength for the lowest-energy optical transitions. Perturbing the system by replacing a Ge atom in the unit cell with a Si atom boosts of the oscillator strength at the minimum direct gap by orders of magnitude, with a concurrent blue sh…
High-quality defect-free lonsdaleite Si and Ge can now be grown on hexagonal nanowire substrates. These hexagonal phases of group-IV semiconductors have been predicted to exhibit improved electronic and optical properties for optoelectronic applications. While lonsdaleite Si is a well-characterized indirect semiconductor, experimental data and reliable calculations on lonsdaleite Ge are scarce and not consistent regarding the nature of its gap. Using ab initio density-functional theory, we calculate accurate structural, electronic, and optical properties for hexagonal Ge. Given the well-known …
The response to compression of the clathrate type-II structure Ge(cF136) is investigated by means of \textit{ab initio} small-cell metadynamics at different temperatures and pressures. At lower pressure $p$=2.5 GPa the metastable metallic bct-5 phase competes against $β$-Sn Ge(tI4), which forms at higher pressures. On lowering temperature, the presence of amorphous intermediates obtained from Ge$_{136}$ is more pronounced and is instrumental to the formation of denser structural motifs, from which metallic bct-5 can form. Therein, anisotropic box fluctuations promote phase formation. The metad…
In spite of remarkable developments in the field of advanced materials, silicon remains one of the foremost semiconductors of the day. Of enduring relevance to science and technology is silicon's nanomechanical behaviour including phase transformation, amorphization and dislocations generation, particularly in the context of molecular dynamics and materials research. So far, comprehensive modelling of the whole cycle of events in silicon during nanoscale deformation has not been possible, however, due to the limitations inherent in the existing interatomic potentials. This paper examines how w…
Depth-Resolved Lattice Distortions in a Silicon-Germanium Qubit Host
Q-NEXT, Argonne National Laboratory（Q-NEXT，阿贡国家实验室）；Materials Science Division, Argonne National Laboratory（材料科学部，阿贡国家实验室）；Pritzker School of Molecular Engineering, University of Chicago（普里茨克分子工程学院，芝加哥大学）；Department of Physics, University of Wisconsin-Madison（物理学系，威斯康星大学麦迪逊分校）；Center for Nanoscale Materials, Argonne National Laboratory（纳米材料中心，阿贡国家实验室）
Owing to the advances in computational techniques and the increase in computational power, atomistic simulations of materials can simulate large systems with higher accuracy. Complex phenomena can be observed in such state-of-the-art atomistic simulations. However, it has become increasingly difficult to understand what is actually happening and mechanisms, for example, in molecular dynamics (MD) simulations. We propose an unsupervised machine learning method to analyze the local structure around a target atom. The proposed method, which uses the two-step locality preserving projections (TS-LP…
The amorphous iron-germanium system ($a$-Fe$_x$Ge$_{1-x}$) lacks long-range structural order and hence lacks a meaningful Brillouin zone. The magnetization of \aFeGe is well explained by the Stoner model for Fe concentrations $x$ above the onset of magnetic order around $x=0.4$, indicating that the local order of the amorphous structure preserves the spin-split density of states of the Fe-$3d$ states sufficiently to polarize the electronic structure despite $\mathbf{k}$ being a bad quantum number. Measurements reveal an enhanced anomalous Hall resistivity $ρ_{xy}^{\mathrm{AH}}$ relative to cry…
Semiconductor superlattices have been extensively investigated for thermoelectric applications, to explore the effects of compositions, interface structures, and lattice strain environments on the reduction of thermal conductivity, and improvement of efficiency. Most studies assumed that their electronic properties remain unaffected compared to their bulk counterparts. However, recent studies demonstrated that electronic properties of silicon (Si)/germanium (Ge) superlattices show significant variations depending on compositions and growth substrates. These studies used a constant relaxation t…
Using first principle calculations, we study the structural, optical and electronic properties of two-dimensional silicene-like structures of CSi7 (carbosilicene) and GeSi7 (germasilicene) monolayers. We show that both CSi7 and GeSi7 monolayers have different buckling that promises a new way to control the buckling in silicene-like structures. Carbon impurity decreases the silicene buckling, whereas germanium impurity increases it. The CSi7 has semiconducting properties with 0.25 eV indirect band gap, but GeSi7 is a semimetal. Also, under uniaxial tensile strain, the semiconducting properties …
Inorganic metal-halide cubic perovskite semiconductors have become more popular in industrial applications of photovoltaic and optoelectronic devices. Among various perovskites, lead-free materials are currently most explored due to their non-toxic effect on the environment. In this study, the structural, electronic, optical, and mechanical properties of lead-free cubic perovskite materials FrBX3 (B = Ge, Sn; X = Cl, Br, I) are investigated through first-principles density-functional theory (DFT) calculations. These materials are found to exhibit semiconducting behavior with direct bandgap ene…
Thermodynamically stable lithium silicides and germanides from density-functional theory calculations
用密度泛函理论（DFT）对Li-Si和Li-Ge体系进行了计算。锂化硅和锗，包括它们的亚稳相，作为锂离子电池（LIB）阳极发挥着重要的技术作用。计算包括对通过将原子种类从国际晶体结构数据库中的X-Y结构交换为Li-Si和Li-Ge而获得的晶体结构的结构优化，其中X={Li，Na，K，Rb，Cs}并且Y={Si，Ge，Sn，Pb}。为了在各种Li-Si和Li-Ge化学计量下对此进行补充，还进行了从头算随机结构搜索（AIRSS）。在基态化学计量之间，包括最近发现的Li $_{17}$ Si $_{4}$相，计算了平均电压，表明锗可能是LIB中硅阳极的更安全的替代品，因为其具有更高的锂插入电压。计算预测高密度Li $_1$ Si $_1$和Li $_1$ Ge $_1$ $P4/mmm$层状相，其分别在2.5和5 GPa以上变为基态，并揭示硅和锗在Li $_x$ Si，$x=2.33-3.25$化学计量范围内形成哑铃的倾向。DFT预测了Li $_{11}$ Ge $_6$ $Cmmm$、Li $_{12}$ Ge $_7$ $Pnma$和Li $_7$ Ge $_3$ $P32_12$相以及几种新的Li-Ge化合物的稳定性，化学计量为Li $_5$ Ge $_2$、Li $_{13}$ Ge $_5$、Li $_8$ Ge $_3$和Li $_{13}$ Ge $_4$。
We report the effect of germanium as n-type dopant on the electrical and optical properties of AlxGa1-xN layers grown by plasma assisted molecular-beam epitaxy. The Al content has been varied from x = 0 to 0.66, confirmed by Rutherford backscattering spectrometry, and the Ge concentration was increased up to [Ge] = 1E21 cm-3. Even at these high doping levels Ge does not induce any structural degradation in AlxGa1-xN layers with x below 0.15. However, for higher Al compositions, clustering of Ge forming crystallites were observed. Hall effect measurements show a gradual decrease of the carrier …
High Photovoltaic Efficiency in Bulk-Stacked One-Dimensional GeSe$_{2}$ van der Waals Crystal
Kyung Hee University（庆熙大学）；Korea Institute of Science and Technology Information (KISTI)（韩国科学技术信息研究院）；Korea Institute for Advanced Study (KIAS)（韩国高等科学研究院）；Kangwon National University（江原国立大学）；Hongik University（弘益大学）
Optoelectronic properties and performance optimization for photovoltaic applications of R3m-RbGeX$_3$ (X = Cl, Br, I) perovskites: A combined DFT and SCAPS-1D study
