Materials with competing phases that have similar ground state energies often exhibit a complex phase diagram. Cuprates are a paradigmatic example of such a system that show competition between charge, magnetic, and superconducting orders. The infinite-layer nickelates have recently been revealed to feature similar characteristics. In this paper, I show that these nickelates are additionally near a structural quantum critical point by mapping the energetics of their structural instabilities using first-principles calculations. I first confirm previous results that show a phonon instability in the $P4/mmm$ phase leading to the $I4/mcm$ structure for $R\mathrm{Ni}{\mathrm{O}}_2$ with $R$ = Sm–Lu. I then study the non-spin-polarized phonon dispersions of the $I4/mcm$ phase and find that they exhibit rare-earth size-dependent instabilities at the $X$ and $M$ points for materials with $R$ = Eu–Lu. Group-theoretical analysis was used to enumerate all the isotropy subgroups due to these instabilities, and the distorted structures corresponding to their order parameters were generated using the eigenvectors of the unstable phonons. These structures were then fully relaxed by minimizing both the atomic forces and lattice stresses. I was able to stabilize only five out of the twelve possible distortions. The $Pbcn$ isotropy subgroup with the $M_5^{+}(a,a)$ order parameter shows noticeable energy gain relative to other distortions for the compounds with late rare-earth ions. However, the order parameter of the lowest-energy phase switches first to $X_2^{-}(0,a)+M_5^{+}(b,0)$ and then to $X_2^{-}(0,a)$ as the size of the rare-earth ion is progressively increased. Additionally, several distorted structures lie close in energy for the early members of this series. These features of the structural energetics persist even when antiferromagnetism is allowed. Such a competition between different order parameters that can be tuned by rare-earth ion substitution suggests that any structural transition that could arise from the phonon instabilities present in these materials can be suppressed to 0 K.

In this paper, we investigated the anisotropic thermal transport in two-dimensional (2D) perovskite (phenethylammonium lead iodide) nanolayers through a measurement technique called cross-sectional scanning thermal microscopy. In this method, a target perovskite layer on a substrate was oblique polished with an Ar ion beam to create a low-angle wedge with nanoscale roughness that is followed by high-vacuum scanning thermal microscopy to obtain the thermal conductance map as a function of local thickness. The experimentally obtained data were processed with an analytical model and validated by the finite elemental analysis simulation to quantify the in-plane $\left(k_{l,xy}\right)$ and cross-plane thermal conductivities $\left(k_{l,z}\right)$ of the 2D perovskite from a single set of measurements with nanoscale resolution. We obtained ultralow thermal conductivity $\left(k_l=0.25\pm 0.05\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}\right)$ for the 2D perovskite along with an anisotropy $\left(k_{l,xy}=0.45\pm 0.05\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}\mathrm{and}{k}_{l,z}=0.13\pm 0.05\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}\right)$ linked to the unique structure of the perovskite and different phonon lifetimes and group velocities for in-plane and out-of-plane directions. The results that are available are essential for the thermal management of 2D perovskite-based optoelectronic devices and potential thermoelectric applications of these materials.

Epitaxial thin films of metallic delafossites are a recent topic of intense investigation due to their intriguing electronic states. Using in situ angle-resolved photoemission spectroscopy, we investigated the electronic states of epitaxial ${\mathrm{PdCoO}}_2$ thin films with high crystalline quality and bipolar surface with mixed termination. On this characteristic surface, we observed a surprisingly prominent bulklike single hexagonal large Fermi surface with suppressed surface state. Firstly, observation of a sharp Fermi surface relies on the minimized atomic scale disorder in our high-quality film surface. Additionally, the predominantly two-dimensional bulk electronic state with the Fermi group velocity parallel to the plane of ${\mathrm{PdCoO}}_2$ is expected to make the near-surface bulklike state less sensitive to the scattering by longer length scale random bipolar surface potentials. Furthermore, the origin of the suppressed surface state can be interpreted by screening of the polarity on the surface, which is qualitatively supported by density functional theory calculation. These findings are invaluable for accelerating the search for exotic functionalities in epitaxial ultrathin films and heterostructures of metallic delafossites.

Organic-inorganic mixed halide perovskites have been of utmost importance for renewable energy and information technology due to their outstanding properties, but the toxicity of Pb and the instability of the organic component has hindered their applications. Herein, we present detailed mechanical and elastic properties of the family of the Pb-free perovskite-derived two-dimensional (2D) $A_3{B}_2{X}_9$ $(A=\text{K},\text{Rb},\text{Cs}$; $B=\text{Bi},\text{Sb},\text{As},\text{P}$; and $X=\text{Cl},\text{Br},\text{I})$, and demonstrate different physical property tunability of the archetypical ${\mathrm{Cs}}_3{\mathrm{Bi}}_2{\mathrm{I}}_9$ using ab initio simulations. Our calculated properties show that the optoelectronic properties can be tuned efficiently, as evidenced by flat bands in the band structure and the large absorption peaks in the optical spectra. Through strain engineering, we achieve a redshift of the optical absorption towards the visible regime and demonstrate a robust tuning of the optoelectronic properties for device applications.

X-ray magnetic circular dichroism, x-ray powder diffraction and first-principles calculations revealed that dense hydrogenation of a Gd-doped ${\mathrm{SmCo}}_5{\mathrm{H}}_x$ compound $({\mathrm{Sm}}_{0.92}{\mathrm{Gd}}_{0.08}{\mathrm{Co}}_5{\mathrm{H}}_x)$, ranging up to $x\approx 13.5$, flips the direction of Sm $4f$ magnetic moments. Consequently, the magnetic coupling of Sm $4f$ with Co $3d$ moments changes to ferrimagnetic from the conventional ferromagnetic coupling. The ferrimagnetic structure possessed significantly lower Curie temperatures of $T_{\text{C}}\le 131$ K, which is in contrast to the extremely high $T_{\text{C}}\sim 1300$ K of a ${\mathrm{SmCo}}_5$ permanent magnet. Structural parameters of two hydrogenation-induced crystal structures were determined carefully by x-ray powder diffraction. Large volume expansion exceeding $V/V_0=1.3$ indicates that hydrogen occupation in the Sm-3Co tetrahedron occurred above 1 GPa. The hydrogen in the tetrahedron dominates the ferromagnetic coupling between the interatomic Co 3$d$–Sm 5$d$ electron orbitals, which plays an important role in the induction of the ferrimagnetic structure of ${\mathrm{SmCo}}_5{\mathrm{H}}_x$.

Dielectric amorphous multilayers (AMLs) play a critical role in a wide array of technologies such as optical coatings, nanoelectronics, energy harvesting, and recovery devices. However, despite their wide applications, a robust understanding of the effect of the interplay between chemical and structural disorder on the thermal properties of AMLs is still lacking. Therefore, in this paper, we experimentally and numerically investigate the effects of composition and interface density on the sound speed and thermal conductivity of a series of amorphous aluminum nitride and aluminum oxide multilayers grown via plasma-enhanced atomic layer deposition. To systematically change the composition, the oxygen content of the AMLs is proportionally varied with interface density during growth. We find that the longitudinal sound speed of these AMLs is dictated by the oxygen content instead of the number of interfaces. The thermal conductivity, in contrast, is dictated by both interface density and oxygen content. The interfaces act to decrease the thermal conductivity, whereas the oxygen content increases the thermal conductivity. Due to the competing influence of the interfaces and oxygen content, the thermal conductivity of the AMLs remains nearly constant as a function of interface density. Our study provides crucial insights into the effect of the interplay of composition and interfaces on the sound speed and thermal conductivity of AMLs.

We reveal the mixed-mode mechanism of elastocaloric effect in the course of slim-hysteresis superelasticity in a severely deformed NiTi shape memory alloy with tailored (negative/zero/positive) thermal expansion anisotropy. It is shown that in addition to the latent heat of phase transformation, the reversible heat from elastic deformation (known as the thermoelastic effect) plays a significant role in the overall elastocaloric response. The magnitude of the adiabatic temperature change associated with the thermoelastic effect $|\mathrm{\Delta }{T}_{\mathrm{ad}}^{\mathrm{TeE}}|$ can reach 3.1 K, which is comparable to that from the phase transformation latent heat ($|\mathrm{\Delta }{T}_{\mathrm{ad}}^{\mathrm{PT}}|=2.9$ K). The sign and magnitude of the $\mathrm{\Delta }{T}_{\mathrm{ad}}^{\mathrm{TeE}}$ scales with the sign and magnitude of the coefficient of thermal expansion (CTE) along the stressing direction. For the directions with negative CTEs, the $\mathrm{\Delta }{T}_{\mathrm{ad}}^{\mathrm{TeE}}<0$ while for those with positive CTEs the $\mathrm{\Delta }{T}_{\mathrm{ad}}^{\mathrm{TeE}}>0$ upon rapid release of tensile stresses. As such, the cooling performance is strongest when the material is tensioned along the direction of the strongest negative CTE $\left(–14.3\times {10}^{-6}{\mathrm{K}}^{-1}\right)$ owing to the synergistic interplay of $\mathrm{\Delta }{T}_{\mathrm{ad}}^{\mathrm{PT}}$ and $\mathrm{\Delta }{T}_{\mathrm{ad}}^{\mathrm{TeE}}$. The results reveal the importance of thermal expansion property on the elastocaloric effect.

The square-planar nickelates are a class of superconductors analogous to the cuprates and promise to provide insight into the pairing mechanism in high-temperature superconducting oxides. The parent phase of doped superconducting films is ${\mathrm{NdNiO}}_2$, which is prepared by reducing $3^{+}$ Ni in ${\mathrm{NdNiO}}_3$ films. In this paper, we develop an ultrahigh vacuum reduction method using aluminum deposited on top of the $3^{+}$ nickelates and monitor the reduction process in real time through in situ crystal truncation rod measurements and diffraction x-ray absorption near edge spectroscopy measurements across the Ni $K$ edge. We establish a relation between Ni valence and the lattice constant of $\mathrm{NdNi}{\mathrm{O}}_{3-x}$ and show that the process can precisely control the oxygen content in the films. Finally, we extend the reduction process to quintuple square-planar nickelates.

Exchange bias arises from the interfacial exchange coupling in ferromagnet-antiferromagnet bilayers and manifests as a horizontal shift of the magnetic hysteresis loop, constituting a critical component underpinning a broad range of magnetoresistive logic and memory devices. The facile implementation of exchange bias in van der Waals (vdW) magnets would be a key step towards practical devices for emerging vdW spintronics. Here, we report an easy approach to establishing strong exchange bias in the vdW magnet ${\mathrm{Fe}}_5\mathrm{Ge}{\mathrm{Te}}_2$ by a single-step process—atomic layer deposition (ALD) of oxides on ${\mathrm{Fe}}_5{\mathrm{GeTe}}_2$. We successfully created exchange bias of 300–1500 Oe in ${\mathrm{Fe}}_5\mathrm{Ge}{\mathrm{Te}}_2/{\mathrm{Al}}_2{\mathrm{O}}_3, {\mathrm{Fe}}_5\mathrm{Ge}{\mathrm{Te}}_2/\mathrm{ZnO}$, and ${\mathrm{Fe}}_5\mathrm{Ge}{\mathrm{Te}}_2/{\mathrm{V}}_2{\mathrm{O}}_5$ heterostructures, at 130 K. Control experiments showed that increasing the oxidant pulse duration in each ALD cycle or utilizing the stronger oxidant ${\mathrm{O}}_3$ can enhance the exchange bias strength, revealing the key role of the ALD oxidants. Our systematic work elucidates the essential role of ALD-enabled oxidization of ${\mathrm{Fe}}_5\mathrm{Ge}{\mathrm{Te}}_2$ in the formation of exchange bias, and establishes ALD of oxides as a facile, controllable, and generally effective approach to creating giant exchange bias in vdW magnets, representing an integral advance towards practical vdW spintronic devices.

We develop a computational database, website applications (web-apps), and machine-learning (ML) models to accelerate the design and discovery of two-dimensional (2D) heterostructures. Using density functional theory (DFT) based lattice parameters and electronic band energies for 674 nonmetallic exfoliable 2D materials, we generate 226 779 possible bilayer heterostructures. We classify these heterostructures into type-I, -II, and -III systems according to Anderson's rule, which is based on the relative band alignments of the noninteracting monolayers. We find that type II is the most common and type III the least common heterostructure type. We subsequently analyze the chemical trends for each heterostructure type in terms of the Periodic Table of constituent elements. The band alignment data can also be used for identifying photocatalysts and high-work-function 2D metals for contacts. We validate our results by comparing them to experimental data as well as hybrid-functional predictions. Additionally, we carry out DFT calculations of a few selected systems ($\mathrm{Mo}{\mathrm{S}}_2/\mathrm{W}{\mathrm{Se}}_2, \mathrm{Mo}{\mathrm{S}}_2/\text{h-BN}$, and $\mathrm{Mo}{\mathrm{Se}}_2/\mathrm{Cr}{\mathrm{I}}_3$), to compare the band-alignment description with the predictions from Anderson's rule. We develop web-apps to enable users to virtually create combinations of 2D materials and predict their properties. Additionally, we use ML tools to predict band-alignment information for 2D materials. The web-apps, tools, and associated data will be distributed through the jarvis-heterostructure website. Our analysis, results, and the developed web-apps can be applied to the screening and design applications, such as finding alternative photocatalysts, photodetectors, and high-work-function (WF) 2D-metal contacts.

Inspired by the successful synthesis of bulk ScSI in a recent work [Ferrenti et al., Chem. Mater. 34, 5443 (2022)], we have systematically investigated the mechanical and electronic properties of $\text{Sc}X\mathrm{I}$ $(X=\mathrm{S},\text{Se})$ monolayers and their heterostructures by using first-principles calculations. Our calculations verify the experimental speculation that the bulk ScSI is readily exfoliatable and the monolayers of $\text{Sc}X\mathrm{I}$ $(X=\mathrm{S},\text{Se})$ are stable. The Young's moduli with strong anisotropy $\left(50.2\text{–}91.5\mathrm{N}{\mathrm{m}}^{-1}\right)$ of $\text{Sc}X\mathrm{I}$ monolayers are comparable to those of phosphorene $\left(26\text{–}105\mathrm{N}{\mathrm{m}}^{-1}\right)$, but smaller than those of isotropic graphene $\left(349\mathrm{N}{\mathrm{m}}^{-1}\right)$, $\mathrm{Mo}{\mathrm{S}}_2$ $\left(122.3\mathrm{N}{\mathrm{m}}^{-1}\right)$, and $h\text{−}\mathrm{BN}$ $\left(276\mathrm{N}{\mathrm{m}}^{-1}\right)$, indicating their lower stiffness. In addition, ScSI/ScSeI monolayers show good flexibility with critical strain of 29%/33%. Application of strain can effectively regulate the band gap $\left(E_g\right)$ and band edge of $\text{Sc}X\mathrm{I}$ $(X=\mathrm{S},\text{Se})$ monolayers. For instance, the $E_g$ of the ScSeI monolayer is reduced from 1.83 to 1.59 eV and the band gap type is changed from indirect to direct band gap when a compressive strain of 6% is applied along the $x$ direction, which is attributed to the orbital hybridization between the $d$ orbital of Sc and $p$ orbital of the elements at the $X$ and I sites. More importantly, $\text{Sc}X\mathrm{I}$ $(X=\mathrm{S},\text{Se})$ monolayers can form type II vertical heterostructure with typical two-dimensional semiconductors due to the deeper energy levels of their valence band maximum and conduction band minimum. In addition, $\text{Sc}X\mathrm{I}$ $(X=\mathrm{S},\text{Se})$ monolayers can also be used to form type I lateral heterostructure with the ScSeBr monolayer. The excellent ductility, strain-tuned electronic properties, and heterostructure design make $\text{Sc}X\mathrm{I}$ $(X=\mathrm{S},\text{Se})$ monolayers promising candidates for the application of flexible electronic devices.

Despite countless experimental probes into magnetite's electronic structure across the Verwey transition ${\mathrm{Fe}}_3{\mathrm{O}}_4$, the exact origin of this archetypical metal-insulator transition remains a puzzle. Advanced x-ray diffraction techniques have mostly resolved the monoclinic structure of the insulating phase, including interatomic bond lengths, but the complexity of the charge-orbitally ordered state is difficult to disentangle. We combined resonant elastic x-ray scattering and x-ray photon correlation spectroscopy to probe charge-orbital fluctuations in the insulating state of magnetite. By accessing the Bragg forbidden ${\left(00\frac{1}{2}\right)}_{\mathrm{c}}$ peak at the oxygen $K$-edge, we complement our previous study on the iron $L_{\mathit{3}}-\mathrm{edge}$ to reveal the dynamics of the iron $3d$ and oxygen $2p$ orbital domains. Our new results reveal a decoupling of the orbital correlation lengths between the oxygen $2p$ states and site-specific iron $3d$ states, and we further show charge-orbital domain fluctuations at the iron $t_{2g}$ orbital sites of trimeron chains. These results also demonstrate an experimental method capable of distinguishing electronic dynamics between the oxygen ligands and the transition metal that underpins emergent behaviors in complex oxides.

We report the synthesis, crystal structure, and physical properties of two new high-entropy silicides (HESs), namely ${\left({\mathrm{Nb}}_{0.1}{\mathrm{Mo}}_{0.3}{\mathrm{W}}_{0.3}{\mathrm{Re}}_{0.2}{\mathrm{Ru}}_{0.1}\right)}_5{\mathrm{Si}}_3$ and ${\left({\mathrm{Nb}}_{0.2}{\mathrm{Mo}}_{0.3}{\mathrm{W}}_{0.3}{\mathrm{Re}}_{0.1}{\mathrm{Ru}}_{0.1}\right)}_5{\mathrm{Si}}_3$. Structural analysis indicates that both HESs consist of a (nearly) single tetragonal ${\mathrm{W}}_5{\mathrm{Si}}_3$-type phase (space group $I4/mcm$) with a disordered cation distribution. Electrical resistivity, magnetic susceptibility, and specific heat measurements show that ${\left({\mathrm{Nb}}_{0.1}{\mathrm{Mo}}_{0.3}{\mathrm{W}}_{0.3}{\mathrm{Re}}_{0.2}{\mathrm{Ru}}_{0.1}\right)}_5{\mathrm{Si}}_3$ and ${\left({\mathrm{Nb}}_{0.2}{\mathrm{Mo}}_{0.3}{\mathrm{W}}_{0.3}{\mathrm{Re}}_{0.1}{\mathrm{Ru}}_{0.1}\right)}_5{\mathrm{Si}}_3$ are weakly coupled bulk superconductors, which represent the first superconducting high-entropy nonoxide ceramics. In particular, these HESs have higher $T_{\mathrm{c}}$ values (3.2–3.3 K) compared with those of the binary counterparts and their $B_{\mathrm{c}2}\left(0\right)/T_{\mathrm{c}}$ ratios are the largest among superconductors of the same structural type.

Donor spin states in silicon are a promising candidate for quantum information processing. One possible donor spin readout mechanism is the bound exciton transition that can be excited optically and creates an electrical signal when it decays. This transition has been extensively studied in the bulk, but in order to scale towards localized spin readout, microfabricated structures are needed for detection. As these electrodes will inevitably cause strain in the silicon lattice, it will be crucial to understand how strain affects the exciton transitions. Here we study the phosphorus donor bound exciton transitions in silicon using hybrid electro-optical readout with microfabricated electrodes. We observe a significant zero-field splitting as well as mixing of the hole states due to strain. We can model these effects assuming the known asymmetry of the hole $g$ factors and the Pikus-Bir Hamiltonian describing the strain. In addition, we describe the temperature, laser power, and light polarization dependence of the transitions. Importantly, the hole mixing should not prevent donor electron spin readout, and using our measured parameters and numerical simulations, we anticipate that hybrid spin readout on a silicon-on-insulator platform should be possible, allowing integration into silicon photonics platforms.

Using an all-electron, first-principles, Landau-type theory, we study the nature of short-range order and compositional phase stability in equiatomic refractory high-entropy alloys, NbMoTa, NbMoTaW, and VNbMoTaW. We also investigate selected binary subsystems to provide insight into the physical mechanisms driving order. Our approach examines the short-range order of the solid solutions directly, infers disorder/order transitions, and also extracts parameters suitable for atomistic modeling of diffusional phase transformations. We find a hierarchy of relationships between the chemical species in these materials which promote ordering tendencies. The most dominant is a relative atomic size difference between the $3d$ element, V, and the other $4d$ and $5d$ elements which drives a $B32$-like order. For systems where V is not present, ordering is dominated by the difference in filling of valence states; pairs of elements that are isoelectronic remain weakly correlated to low temperatures, while pairs with a valence difference present $B2$-like order. Our estimated order-disorder transition temperature in VNbMoTaW is sufficiently high for us to suggest that SRO in this material may be experimentally observable.

We use a generative neural network model to create thousands of new one-dimensional (1D) materials. The model is trained using 508 stable one-dimensional materials from the Computational 1D Materials Database (C1DB) database. More than 500 of the new materials are shown with density-functional theory calculations to be dynamically stable and with heats of formation within 0.2 eV of the convex hull of known materials. Some of the new materials could also have been obtained by chemical element substitution in the training materials, but completely new classes of materials are also produced. The band structures, electronic densities of states, work functions, effective masses, and phonon spectra of the new materials are calculated, and the data are added to the C1DB.

We successfully grew bulk single crystals of ${\mathrm{VO}}_x$ and ${\mathrm{TiO}}_x$ with a sodium-chloride structure by the floating-zone method, and we performed optical reflectivity measurement and pump-probe spectroscopy to clarify the electronic structure of these series of compounds and their $x$ dependence. We found that optical spectra can be discussed on the basis of the Mott-Hubbard model, although a disorder arising from the deficiencies of Vi or Ti and O plays an important role in the electronic structure as well as their $x$ dependence.

The band offsets at the interfaces between ($\overline{2}01$) $\beta -{\mathrm{Ga}}_2{\mathrm{O}}_3$ and ${\mathrm{Al}}_2{\mathrm{O}}_3$ polymorphs are calculated through hybrid functional calculations. For alumina, we consider four representative phases, i.e., $\alpha , \kappa , \theta$, and $\gamma -{\mathrm{Al}}_2{\mathrm{O}}_3$. We generate realistic slab models for the interfaces, which satisfy electron counting rules. The O atoms bridge the $\beta -{\mathrm{Ga}}_2{\mathrm{O}}_3$ and the ${\mathrm{Al}}_2{\mathrm{O}}_3$ slabs and the dangling bonds at the interfaces are saturated. The band offsets are obtained through an alignment scheme, which requires separate bulk and interface calculations. The calculated band offsets are useful for the design of devices based on the $\beta -{\mathrm{Ga}}_2{\mathrm{O}}_3/{\mathrm{Al}}_2{\mathrm{O}}_3$ heterojunctions, particularly $\beta -{\mathrm{Ga}}_2{\mathrm{O}}_3$ metal-oxide semiconductor field effect transistors.

Infinite-layer (IL) nickelates are an emerging family of superconductors whose similarities and differences to cuprate superconductors are under intense debate. To date, the IL phase of nickelates can only be reached via topotactic oxygen reduction of the perovskite phase, using ${\mathrm{H}}_2$ gas or reducing agents such as ${\mathrm{CaH}}_2$. While the topotactic reduction method has been widely employed on thin film and polycrystalline powder samples, the reduction of ${\mathrm{La}}_{1-x}{\mathrm{Ca}}_x{\mathrm{NiO}}_3$ single crystals with lateral dimensions up to 150 $\mu \mathrm{m}$ was achieved only recently, using an indirect contact method with ${\mathrm{CaH}}_2$. Here we report the topotactic transformation of much larger ${\mathrm{LaNiO}}_3$ crystals with lateral dimensions of more than one millimeter, via direct contact with ${\mathrm{CaH}}_2$. We characterize the crystalline, magnetic, and electronic properties of the obtained IL ${\mathrm{LaNiO}}_2$ crystals by powder and single-crystal x-ray diffraction (XRD), magnetometry, electrical transport, and x-ray photoelectron spectroscopy (XPS) measurements. The amount of incorporated topotactic hydrogen due to the reduction process is determined by a gas extraction method. In addition, we investigate the evolution of the lattice parameters under hydrostatic pressure up to 12 GPa, using high-resolution synchrotron XRD. Furthermore, we provide a direct comparison of several physical properties of the ${\mathrm{LaNiO}}_2$ crystals to their powder and thin film counterparts.

Graphene has attracted significant attention as a pioneer of two-dimensional zero gap semiconductors, but the development of new two-dimensional materials with a finite band gap has been actively pursued. In this study, the structural stability of double bilayers (DBs) of group II-V compounds (II: Be, Zn, and Cd; V: P, As, and Sb) has been systematically investigated using first-principles calculations based on density functional theory. The thermodynamic calculations have confirmed that BeP, BeAs, ZnP, and ZnAs can be produced through exothermic reactions from their constituent bulk systems. It has also been confirmed that all the compounds have phonon dynamical stabilities. Only CdP and CdAs have been found to have an AB-stacked DB structure with threefold symmetry, while the other compounds have ${\text{AB}}^{\prime }$-stacked DB structure with broken symmetry. The difference in atomic radii between group II and group V results in the so-called size effect, which determines the stacking pattern. The structural stability of II-V DB thin films is explained by analogy with the surface structural stability of compound semiconductors: The change in the atomic arrangement of the DB structure alters the electronegativity of the surface orbitals of the II-V thin film, which does not result in any unsaturated bonds, i.e., no metallic bands across the Fermi level appear. The various DB II-V compounds proposed in this study will join the ranks of atomic-level 2D semiconductor materials.