AB Stacking Research Articles

Wavelength and Stacking Order Dependent Exciton Dynamics in Bulk ReS2

AbstractExciton dynamics in ReS2 are rich and complicated due to its unique lattice and band structures. With femtosecond pump‐probe spectroscopy, the wavelength‐dependent exciton dynamics in bulk ReS2 are studied at room temperature, and the effects from polarization and stacking order are also considered. Signatures from both bright and dark excitons are observed, where the lifetime at bright exciton levels is only several picoseconds, while it is about tens of picoseconds at dark exciton levels. The extremely long up‐/down‐going trend at selective wavelengths is attributed to the transition between different dark exciton levels. Two types of coherent dynamics are observed when the laser wavelength is scanned through different exciton levels: coherent optical stark effect and exciton‐exciton coherent interaction, both occurring within the laser pulse width. Exciton dynamics show a similar trend for the sample with AA stacking order, regardless of polarization. For AB stacking, dark excitons for Exciton II and III are missing when probe pulse polarization is perpendicular to b‐axis, which is attributed to the polarization and stacking‐order dependent band structure. Studying the rich exciton dynamics in bulk ReS2 at room temperature not only has scientific significance, but also facilitates the development of high‐performance excitonic devices.

Sol-gel processing of a covalent organic framework for the generation of hierarchically porous monolithic adsorbents

Sol-gel processing of a covalent organic framework for the generation of hierarchically porous monolithic adsorbents

The effects of substrate and stacking in bilayer borophene

Bilayer borophene has recently attracted much interest due to its outstanding mechanical and electronic properties. The interlayer interactions of these bilayers are reported differently in theoretical and experimental studies. Herein, we design and investigate bilayer beta _{12} borophene, by first-principles calculations. Our results show that the interlayer distance of the relaxed AA-stacked bilayer is about 2.5 Å, suggesting a van der Waals interlayer interaction. However, this is not supported by previous experiments, therefore by constraining the interlayer distance, we propose a preferred model which is close to experimental records. This preferred model has one covalent interlayer bond in every unit cell (single-pillar). Further, we argue that the preferred model is nothing but the relaxed model under a 2% compression. Additionally, we designed three substrate-supported bilayers on the Ag, Al, and Au substrates, which lead to double-pillar structures. Afterward, we investigate the AB stacking, which forms covalent bonds in the relaxed form, without the need for compression or substrate. Moreover, phonon dispersion shows that, unlike the AA stacking, the AB stacking is stable in freestanding form. Subsequently, we calculate the mechanical properties of the AA and AB stackings. The ultimate strengths of the AA and the AB stackings are 29.72 N/m at 12% strain and 23.18 N/m at 8% strain, respectively. Moreover, the calculated Young’s moduli are 419 N/m and 356 N/m for the AA and the AB stackings, respectively. These results show the superiority of bilayer borophene over bilayer hbox {MoS}_2 in terms of stiffness and compliance. Our results can pave the way of future studies on bilayer borophene structures.

Efficient Adsorption of Acetylene over CO2 in Bioinspired Covalent Organic Frameworks

Rational design of covalent organic frameworks (COFs) to broaden their diversity is highly desirable but challenging due to the limited, expensive, and complex building blocks, especially compared with other easily available porous materials. In this work, we fabricated two novel bioinspired COFs, namely, NUS-71 and NUS-72, using reticular chemistry with ellagic acid and triboronic acid-based building blocks. Both COFs with AB stacking mode exhibit high acetylene (C2H2) adsorption capacity and excellent separation performance for C2H2/CO2 mixtures, which is significant but rarely explored using COFs. The impressive affinities for C2H2 appear to be related to the sandwich structure formed by C2H2 and the host framework via multiple host-guest interactions. This work not only represents a new avenue for the construction of low-cost COFs but also expands the variety of the COF family using natural biochemicals as building blocks for broad application.

First-principles study on the electronic properties of layered Ga2O3/TeO2 heterolayers for high-performance electronic devices

Layered Ga2O3 with high electron mobility and wide bandgap have attracted extensive attention for the applications of optoelectronic and power devices. However, the absence of p-type conducting counterpart restricts its potential. Herein, we propose layered Ga2O3/TeO2 heterolayers to overcome this issue. The structural, electronic properties and carrier mobility of layered Ga2O3/TeO2 heterolayers are investigated by first-principles calculations. All the investigated heterolayers exhibit thermodynamic stability and type-II band alignment characteristic. Both exceptionally high electron and hole mobility are found in the constructed layered Ga2O3/TeO2 heterolayers. For ML Ga2O3/TeO2 heterolayer with AB stacking pattern, the calculated electron and hole mobility can reach 9501 and 11,850 cm2V-1s−1, respectively, which are much superior than pristine ML Ga2O3. The current–voltage curve result of the ML Ga2O3/TeO2 heterolayer channel-based transistor further confirms the enhanced conducting property. Our study applies a new strategy to overcome the p-type conducting issue of layered Ga2O3, and the proposed Ga2O3/TeO2 heterolayers are favorable for high-response detectors and high-frequency power devices.

Interlayer coupling and strain localization in small-twist-angle graphene flakes

Twisted bilayer graphene (TBG) exhibits a wide range of intriguing physical properties, such as superconductivity, ferromagnetism, and superlubricity. Depending on the twist angle, periodic moiré superlattices form in twisted bilayer graphene, with inhomogeneous interlayer coupling and lattice deformation. For a small twist angle (typically < 2°), each moiré supercell contains a large number of atoms (>10,000), making it computationally expensive for first-principles and atomistic modeling. In this work, a finite element method based on a continuum model is used to simulate the inhomogeneous interlayer and intralayer deformations of twisted graphene flakes on a rigid graphene substrate. The van der Waals interactions between the graphene layers are described by a periodic potential energy function, whereas the graphene flake is treated as a continuum membrane with effective elastic properties. Our simulations show that structural relaxation and the induced strain localization are most significant in a relatively large graphene flake at small twist angles, where the strain distribution is highly localized as shear strain solitons along the boundaries between neighboring domains of commensurate AB stacking. Moreover, it is found that there exist many metastable equilibrium configurations at particular twist angles, depending on the flake size. The nonlinear mechanics of twisted bilayer graphene is thus expected to be essential for understanding the strain distributions in the moiré superlattices and the strain effects on other physical properties.

Boosting Electrocatalytic CO 2 Reduction with Conjugated Bimetallic Co/Zn Polyphthalocyanine Frameworks

Boosting Electrocatalytic CO ₂ Reduction with Conjugated Bimetallic Co/Zn Polyphthalocyanine Frameworks

Constructing two-dimensional holey graphyne with unusual annulative π-extension

•A two-dimensional carbon allotrope of holey graphyne (HGY) was designed •1,3,5-tribromo-2,4,6-triethynylbenzene was synthesized for fabrication of HGY •HGY comprised a pattern of six-vertex and highly strained, eight-vertex rings •HGY’s direct p-type semiconductor with high carrier mobility was simulated Here, we report two-dimensional, single-crystalline holey graphyne (HGY) synthesized in an interfacial two-solvent system through a Castro-Stephens-type coupling reaction from 1,3,5-tribromo-2,4,6-triethynylbenzene. As a new type of 2D carbon allotrope, HGY is alternately linked between benzene rings and C≡C bonds and is composed of a pattern of six-vertex and highly strained, eight-vertex rings and an equal percentage of sp2 and sp hybridized carbon atoms. By combining experimental and theoretical studies, we systematically investigated the stability of HGY and its vibrational and optical properties. Density functional theory computations predicted that HGY is a p-type semiconductor that embraces a direct bandgap (∼1.1 eV) with a high carrier mobility. Transmission electron microscopic studies revealed that the synthesized HGY sheets are highly crystalline with AB stacking. Its semiconducting character, nonlinear sp bonding, and special π-conjugated structure endow HGY with promising applications in optoelectronic, energy harvesting, gas separation, catalysis, water remediation, sensor, and energy-related fields. Here, we report two-dimensional, single-crystalline holey graphyne (HGY) synthesized in an interfacial two-solvent system through a Castro-Stephens-type coupling reaction from 1,3,5-tribromo-2,4,6-triethynylbenzene. As a new type of 2D carbon allotrope, HGY is alternately linked between benzene rings and C≡C bonds and is composed of a pattern of six-vertex and highly strained, eight-vertex rings and an equal percentage of sp2 and sp hybridized carbon atoms. By combining experimental and theoretical studies, we systematically investigated the stability of HGY and its vibrational and optical properties. Density functional theory computations predicted that HGY is a p-type semiconductor that embraces a direct bandgap (∼1.1 eV) with a high carrier mobility. Transmission electron microscopic studies revealed that the synthesized HGY sheets are highly crystalline with AB stacking. Its semiconducting character, nonlinear sp bonding, and special π-conjugated structure endow HGY with promising applications in optoelectronic, energy harvesting, gas separation, catalysis, water remediation, sensor, and energy-related fields.

Elastic bilayer phononic crystal

Inspired by twisted bilayer graphene, we present an elastic bilayer consisting of a plate with two honeycomb lattices of mechanical resonators on both sides of the plate. Surface acoustic waves (SAW), which can interact depending on the thickness of the plate, are supported by each side of the plate. Beyond twisting a lattice with respect to the other, the coupling strengths between the two sides can be controlled via the plate thickness, which strongly affects the dispersion of elastic waves in the bilayer structure. The dispersion relationship of the SAW is theoretically characterized by calculating the band structure for the cases of AA and AB stacking configurations, as well as for special cases of twisting angles that produce sublattices with even and odd symmetries. Furthermore, we explore the topological characteristics of the bands and uncover a possible mechanism of creating topological elastic Valley states. The proposed bilayer plate could constitute a promising platform for manipulating mechanical waves and exploring quantum analog phenomena which could open routes toward innovative mechanical and optomechanical devices at the microscale, for instance.

Local Density of States Modulated by Strain in Marginally Twisted Bilayer Graphene

In marginally twisted bilayer graphene, the Moiré pattern consists of the maximized AB (BA) stacking regions, minimized AA stacking regions and triangular networks of domain walls. Here we realize the strain-modulated electronic structures of marginally twisted bilayer graphene by scanning tunneling microscopy/spectroscopy and density functional theory (DFT) calculations. The experimental data show four peaks near the Fermi energy at the AA regions. DFT calculations indicate that the two new peaks closer to the Fermi level may originate from the intrinsic heterostrain and the electric field implemented by back gate is likely to account for the observed shift of the four peaks. Furthermore, the dI/dV map across Moiré patterns with different strain strengths exhibits a distinct appearance of the helical edge states.

Electro-lubrication in Janus transition metal dichalcogenide bilayers

Lubrication induced by a vertical electric field or bias voltage is typically not applicable to two-dimensional (2D) van der Waals (vdW) crystals. By performing extensive first-principles calculations, we reveal that the interlayer friction and shear resistance of Janus transition metal dichalcogenide (TMD) MoXY (X/Y = S, Se, or Te, and X ≠ Y) bilayers under a constant normal force mode can be reduced by applying vertical electric fields. The maximum interlayer sliding energy barriers between AA and AB stacking of bilayers MoSTe, MoSeTe, and MoSSe decrease as the positive electric field increases because of the more significant counteracting effect from the electric field energy and the more significant enhancement in interlayer charge transfer in AA stacking. Meanwhile, the presence of negative electric fields decreases the interlayer friction of bilayer MoSTe, because the electronegativity difference between Te and S atoms reduces the interfacial atom charge differences between AA and AB stacking. These results reveal an electro-lubrication mechanism for the heterogeneous interfaces of 2D Janus TMDs.

Tunable electronic properties of the GeC/MoS2 heterostructures: A first-principles study

First-principle calculations based on the density functional theory are carried out to investigate the structural and electronic properties of the GeC/MoS2 hetero-structures (including superlattice, hetero-bilayer and hetero-trilayer). Among all six GeC/MoS2 superlattice systems considered here, the AB stacking model (the Ge atoms are aligned on the Mo atoms, all S and C atoms are above the center of the hexagonal ring) is the most stable structure. The AB stacking GeC/MoS2 hetero-bilayer system possesses a type-II band alignment with an intrinsic direct band gap of 0.73 eV (GGA-PBE) to 1.18 eV (HSE06), which can effectively separate the photogenerated electron-hole pairs. Our results revealed that the bandgap could be modulated effectively by applying in-plane biaxial compressing/stretching (range from −3% to 3%) while maintaining the type-II band alignment. Furthermore, due to the influence of van der Waals interaction within atomic layers of heterostructures, the controllable band gaps could also be realized in trilayer MoS2/GeC/MoS2 and GeC/MoS2/GeC. This control over the band structure suggests the potential application of GeC/MoS2 hetero-structures to switching functions in future atomicscale electronic devices.

Thermal conductivity of two stable bilayer phosphorene stackings: A computation study

We report the thermal conductivity of two dynamically stable bilayer phosphorene stackings, i.e., a twisted phase with a twist angle of ∼70.5° (or 2O-tαP phase) and a shifting phase with half of the lattice constants (or AB phase). This was achieved by using the first-principles-driven lattice dynamics calculations and a fully iterative solver of the Boltzmann transport equation, the latter including an anharmonic phonon–phonon scattering effect. At room temperature, the thermal conductivity of the 2O-tαP phase is 146 and 108 W/mK along its two orthogonal lattice basis vectors, respectively, larger than that along the armchair direction (69 W/mK) of the AB phase, while smaller than that along the AB zigzag direction (164 W/mK); with an increasing temperature, the conductivity decreases along the basis vectors of the 2O-tαP and AB stackings, and the anisotropy lessens for both stackings. The thermal transport anisotropies for the two kinds of bilayer stacking can be attributed to the different proportions of their acoustic branches along different directions. In particular, the phonon mean free path showed that the in-plane transverse acoustic branch is the main contribution of the thermal conductivity along the short lattice constant direction of the 2O-tαP phase due to the twist angle extending the propagation path of transverse acoustic waves in the direction. Finally, the thermal conductivity accumulation was revealed to increase in the form of a hyperbolic tangent with mean free path of the phonons, which can be used to evaluate the size effect of the stacking materials in practice.

Direct Detection of Inhomogeneity in CVD-Grown 2D TMD Materials via K-Means Clustering Raman Analysis.

It is known that complex growth environments often induce inhomogeneity in two-dimensional (2D) materials and significantly restrict their applications. In this paper, we proposed an efficient method to analyze the inhomogeneity of 2D materials by combination of Raman spectroscopy and unsupervised k-means clustering analysis. Taking advantage of k-means analysis, it can provide not only the characteristic Raman spectrum for each cluster but also the cluster spatial maps. It has been demonstrated that inhomogeneities and their spatial distributions are simultaneously revealed in all CVD-grown MoS2, WS2 and WSe2 samples. Uniform p-type doping and varied tensile strain were found in polycrystalline monolayer MoS2 from the grain boundary and edges to the grain center (single crystal). The bilayer MoS2 with AA and AB stacking are shown to have relatively uniform p-doping but a gradual increase of compressive strain from center to the periphery. Irregular distribution of mode in WS2 and mode in WSe2 is revealed due to defect and strain, respectively. All the inhomogeneity could be directly characterized in color-coded Raman imaging with correlated characteristic spectra. Moreover, the influence of strain and doping in the MoS2 can be well decoupled and be spatially verified by correlating with the clustered maps. Our k-means clustering Raman analysis can dramatically simplify the inhomogeneity analysis for large Raman data in 2D materials, paving the way towards direct evaluation for high quality 2D materials.

Control of electric properties of silicene heterostructure by reversal of ferroelectric polarization

Silicene is a kind of two-dimensional material composed of a honeycomb arrangement of silicon atoms. Compared with the structure of graphene, the buckled structure of silicene weakens the &lt;inline-formula&gt;&lt;tex-math id="M5"&gt;\begin{document}$\pi—\pi$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20220815_M5.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20220815_M5.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; overlaps and turns the hybrid orbitals from &lt;inline-formula&gt;&lt;tex-math id="M6"&gt;\begin{document}$\rm sp^2$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20220815_M6.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20220815_M6.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; to &lt;inline-formula&gt;&lt;tex-math id="M7"&gt;\begin{document}$\rm sp^3$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20220815_M7.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20220815_M7.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt;, which enhances the spin-orbit coupling strength but still preserves the Dirac cone near &lt;i&gt;K&lt;/i&gt; or &lt;i&gt;K&lt;/i&gt;&lt;i&gt;'&lt;/i&gt;. Owing to its buckled structure, silicene is susceptible to external parameters like electric field and substrate, which draws lots of attention both experimentally and theoretically. Recent progress of ferroelectricity in two-dimensional (2D) van der Waals materials found that the spontaneous ferroelectric polarization can be preserved even above room temperature, which inspires us to investigate how to tune the electric properties of silicene through the spontaneous polarization field of 2D ferroelectric substrate. &lt;inline-formula&gt;&lt;tex-math id="M8"&gt;\begin{document}${\rm In_{2}}X_3$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20220815_M8.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20220815_M8.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; (&lt;i&gt;X&lt;/i&gt; = Se,S,Te) Family recently were found to have single ferroelectric monolayers with reversible spontaneous electric polarization in both out-of-plane and in-plane orientations, and the lattice mismatch between silicene and &lt;inline-formula&gt;&lt;tex-math id="M9"&gt;\begin{document}$\rm In_{2}S_3$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20220815_M9.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20220815_M9.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt;is negligible. Therefore, we investigate the stacking and electric properties of silicene and monolayer &lt;inline-formula&gt;&lt;tex-math id="M10"&gt;\begin{document}$\rm In_{2}S_3$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20220815_M10.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20220815_M10.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; heterostructure by the first-principles calculations. The spontaneous polarization field of &lt;inline-formula&gt;&lt;tex-math id="M11"&gt;\begin{document}$\rm In_{2}S_3$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20220815_M11.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20220815_M11.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; is calculated to be 1.26 &lt;inline-formula&gt;&lt;tex-math id="M12"&gt;\begin{document}$\rm μC {\cdot} cm^{-2}$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20220815_M12.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20220815_M12.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt;, comparable to the experimental results of &lt;inline-formula&gt;&lt;tex-math id="M13"&gt;\begin{document}$\rm In_{2}Se_3$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20220815_M13.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20220815_M13.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt;. We compare the different stacking order between silicene and &lt;inline-formula&gt;&lt;tex-math id="M14"&gt;\begin{document}$\rm In_{2}S_3$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20220815_M14.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20220815_M14.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt;. The calculated results shown that the &lt;i&gt;AB&lt;/i&gt; stacking is the ground state stacking order, and the reversal of the ferroelectric polarization could tune the band structure of heterostructure. When the polarization direction of &lt;inline-formula&gt;&lt;tex-math id="M15"&gt;\begin{document}$\rm In_{2}S_3$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20220815_M15.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20220815_M15.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; is upward, the layer distance between silicene and &lt;inline-formula&gt;&lt;tex-math id="M16"&gt;\begin{document}$\rm In_{2}S_3$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20220815_M16.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20220815_M16.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; is 3.93 Å, the polarization field and substrate interaction together break the&lt;i&gt; AB&lt;/i&gt; sublattice symmetry and induce a 1.8 meV band gap near the Dirac point of &lt;i&gt;K&lt;/i&gt; and &lt;i&gt;K'&lt;/i&gt;, while the Berry curvature around &lt;i&gt;K &lt;/i&gt;and &lt;i&gt;K' &lt;/i&gt;have opposite signs, corresponding to valley Hall effect. When the polarization is downward, the layer distance decreases to 3.62 Å and the band gap around &lt;i&gt;K&lt;/i&gt; and&lt;i&gt; K'&lt;/i&gt; both increase to 30.8 meV. At the same time a 0.04&lt;i&gt;e&lt;/i&gt; charge transfer makes some bands move across the Fermi energy, corresponding to metal state. Our results pave the way for studying the ferroelectric tuning silicene heterostructures and their potential applications in information industry.

Bidirectional Light-Driven Ion Transport through Porphyrin Metal–Organic Framework-Based van der Waals Heterostructures via pH-Induced Band Alignment Inversion

Bidirectional Light-Driven Ion Transport through Porphyrin Metal–Organic Framework-Based van der Waals Heterostructures via pH-Induced Band Alignment Inversion

Commensurate Stacking Phase Transitions in an Intercalated Transition Metal Dichalcogenide.

Intercalation and stacking-order modulation are two active ways in manipulating the interlayer interaction of transition metal dichalcogenides (TMDCs), which lead to a variety of emergent phases and allow for engineering material properties. Herein, the growth of Pb-intercalated TMDCs-Pb(Ta1+x Se2 )2 , the first 124-phase, is reported. Pb(Ta1+x Se2 )2 exhibits a unique two-step first-order structural phase transition at around 230 K. The transitions are solely associated with the stacking degree of freedom, evolving from a high-temperature (high-T) phase with ABC stacking and R3m symmetry to an intermediate phase with AB stacking and P3m1, and finally to a low-temperature (low-T) phase again with R3msymmetry, but with ACB stacking. Each step involves a rigid slide of building blocks by a vector [1/3, 2/3, 0]. Intriguingly, gigantic lattice contractions occur at the transitions on warming. At low-T, bulk superconductivity with Tc ≈ 1.8 K is observed. The underlying physics of the structural phase transitions are discussed from first-principle calculations. The symmetry analysis reveals topological nodal lines in the band structure. The results demonstrate the possibility of realizing higher-order metal-intercalated phases of TMDCs and advance the knowledge of polymorphic transitions, and may inspire stacking-order engineering in TMDCs andbeyond.

First-principal insight of the gold-metal interaction to bilayer MoSe2 of AB and AA stacking order

The two-dimensional (2D) bilayer (2L) semiconductors are interesting for next-generation electronic device applications due to their additional degree of freedom such as stacking order. However, the report on the interaction of metals to the 2L semiconductor of different stacking orders is not reported yet. Here, we compare the interaction of gold (Au) metal to 2L-MoSe2 of AB and AA stacking order using density functional theory (DFT) calculations. In 2L-MoSe2, the most stable structures have the 2H and the 3R phases, which corresponds to the AB and AA stacking order. To unveil the nature of the interaction between Au and 2L- (AB and AA) MoSe2 semiconductor, the Schottky barrier height (ΦSB, n), tunnel barrier (ΦTB, n), and orbital overlap are calculated. We find that due to significantly enhanced carrier injection (lower ΦSB, n, and ΦTB, n), and higher orbital overlap, the Au/2L-AA MoSe2 resulted as a better alternative, compared to Au/2L-AB MoSe2. The findings provide theoretical guidance for the selection of stacking order for 2L materials for high-performance 2D-electronic devices.

Highly adjustable piezoelectric properties in two-dimensional LiAlTe2 by strain and stacking

Two-dimensional (2D) piezoelectric materials have attracted wide attention because they are of great significance to the composition of piezoelectric nanogenerators. In this work, we have systematically studied the piezoelectric properties of 2D LiAlTe2 by using the first-principles calculation and found the 2D LiAlTe2 monolayer exhibits both large in-plane piezoelectric coefficient d 11 (3.73 pm V−1) and out-of-plane piezoelectric coefficient d 31 (0.97 pm V−1). Moreover, the piezoelectric coefficients of 2D LiAlTe2 are highly tunable by strain and stacking. When different uniaxial strains are applied, d 11 changes dramatically, but d 31 changes little. When 2% stretching is applied to 2D LiAlTe2 monolayer along the x-axis, d 11 reaches 7.80 pm V−1, which is twice as large as the previously reported 2D piezoelectric material MoS2. Both AA stacking and AB stacking can enhance the piezoelectric properties of 2D LiAlTe2, but they have different effects on in-plane and out-of-plane piezoelectric coefficients. AA stacking can greatly increase d 31 but has little impact on d 11. In the case of four-layer AA stacking, the d 31 reaches 3.32 pm V−1. AB stacking can both increase d 11 and d 31, but d 11 grows faster than d 31 as the number of layers increases. In the case of four-layer AB stacking, d 11 reaches 18.05 pm V−1. The excellent and highly tunable piezoelectric performance provides 2D LiAlTe2 greater potential for the application of piezoelectric nano-generators and other micro-nano piezoelectric devices.

Growing twisted bilayer graphene at small angles

Growing twisted bilayer graphene at small angles