Buckled Monolayer Research Articles

Spin splitting and spin Hall conductivity in buckled monolayers of group 14: First-principles calculations

Elemental monolayers of the group 14 with a buckled honeycomb structure, namely silicene, germanene, stanene, and plumbene, are known to demonstrate a spin splitting as a result of an electric field parallel to their high symmetry axis which is capable of tuning their topological phase between a quantum spin Hall insulator and an ordinary band insulator. We perform first-principles calculations based on the density functional theory to quantify the spin-dependent band gaps and the spin splitting as a function of the applied electric field and extract the main coefficients of the invariant Hamiltonian. Using the linear response theory and the Wannier interpolation method, we calculate the spin Hall conductivity in the monolayers and study its sensitivity to an external electric field. Our results show that the spin Hall conductivity is not quantized and in the case of silicene, germanene, and stanene degrades significantly as the electric field inverts the band gap and brings the monolayer into the trivial phase. The electric field induced band gap does not close in the case of plumbene which shows a spin Hall conductivity that is robust to the external electric field.

Correlation-induced valley topology in buckled graphene superlattices

Quasi-flat-bands emerging in buckled monolayer graphene superlattices have been recently shown to realize correlated states analogous to those observed in twisted graphene multilayers. Here, we demonstrate the emergence of valley topology driven by competing electronic correlations in buckled graphene superlattices. We show, both by means of atomistic models and a low-energy description, that the existence of long-range electronic correlations leads to a competition between antiferromagnetic and charge density wave instabilities, that can be controlled by means of screening engineering. Interestingly, we find that the emergent charge density wave has a topologically non-trivial electronic structure, leading to a coexistent quantum valley Hall insulating state. In a similar fashion, the antiferromagnetic phase realizes a spin-polarized quantum valley-Hall insulating state. Our results put forward buckled graphene superlattices as a new platform to realize interaction-induced topological matter.

Thermal transport property of novel two-dimensional nitride phosphorus: An ab initio study

Controlling thermal transport property is a crucial issue toward thermal management in two-dimensional (2D) materials. With first-principles calculations and Boltzmann transport equation (BTE), we have explored the thermal transport property of novel 2D nitride phosphorus (NP). We find that, at room temperature, the calculated lattice thermal conductivity (κlat) of novel 2D NP is 25.4 W/mK, being lower than that of buckled monolayer phosphorene, nitrogene, and β-NP. Through the analysis of corresponding phonon–phonon scattering channels (a + a ↔ o and a + o ↔ o), we find that the strong scatterings among acoustic phonons and low-frequency optical phonons are accountable for the low κlat of novel 2D NP. Additionally, we also study the sample size effects of novel 2D NP, which can further decrease κlat until a length of up to 5 μm. These results could provide valuable guidance to design the NP-based thermoelectric nano-devices and potential heat transport applications in other 2D materials.

Atomistic Understanding of the Ferroelectric Properties of a Wurtzite‐Structure (AlN)n/(ScN)m Superlattice

For ferroelectric applications, wurtzite (WZ)‐phase (Al,Sc)N is investigated using ab initio calculations. The key ferroelectric properties, such as the ferroelectric switching barrier and spontaneous polarization, are examined in (AlN)n/(ScN)m superlattice (SL) structures by varying the ScN fraction, and compared with an (Al,Sc)N solid solution (SS). The reduction of the switching barrier is dominated by the local structural instability rather than the flattening of the buckled monolayer composed of metal–nitrogen pairs in WZ‐stacking. This argument is supported by the doping effects of other metal elements as well as the reduction of the switching barrier in the SL, which does not accompany the flattening of the buckled monolayer. The decrease in the spontaneous polarization along the ScN fraction is explained by structural parameters (the in‐plane area and the buckling parameter) and the Born effective charge (BEC). The slight decrease in the spontaneous polarization in the SL is attributed to the larger increase in the in‐plane area rather than the BEC. In contrast, the decrease in the buckling parameter in the SS is suggested to more significantly reduce the spontaneous polarization along the ScN fraction.

Coexistence of polar distortion and conduction in doped 2D group-IV ferroelectrics: SiGe, SiSn, and GeSn

Since the concept of ferroelectric metal predicted in the 1960s has been experimentally realized in the bulk Weyl semimetal WTe2 [Sharma et al 2019 Sci. Adv. 5, eaax5080], it is significant to find the ultrathin polar metal or ferroelectric metal due to the demand of miniature of electronic nanodevices. Here, 2D buckled monolayers composed of group-IV elements such as SiGe, SiSn, and GeSn are selected as prototype. Then, the stability of 2D ferroelectricity in the above monolayers are confirmed based on the results of first-principles calculations. Most interesting, a robustly metallic polar state has been found in the above 2D ferrolectrics under both the electron doping and hole doping, and the polar distortion becomes even more remarkable when the electrons are doped as compared with the undoped system. Thus, the coexistence of polar state and conduction is theoretically verified in the doped group-IV monolayers. We hope the 2D ferroelectric materials can be used as a starting point to look for the polar metals with atomic thickness, and further broaden their applications in 2D electronics or spintronics in the future.

Conflux of tunable Rashba effect and piezoelectricity in flexible magnesium monochalcogenide monolayers for next-generation spintronic devices.

The coupling of piezoelectric properties with Rashba spin-orbit coupling (SOC) has proven to be the limit breaker that paves the way for a self-powered spintronic device (ACS Nano, 2018, 12, 1811-1820). For further advancement in next-generation devices, a new class of buckled, hexagonal magnesium-based chalcogenide monolayers (MgX; X = S, Se, Te) have been predicted which are direct band gap semiconductors satisfying all the stability criteria. The MgTe monolayer shows a strong SOC with a Rashba constant of 0.63 eV Å that is tunable to the extent of ±0.2 eV Å via biaxial strain. Also, owing to its broken inversion symmetry and buckling geometry, MgTe has a very large in-plane as well as out-of-plane piezoelectric coefficient. These results indicate its prospects for serving as a channel semiconducting material in self-powered piezo-spintronic devices. Furthermore, a prototype for a digital logic device can be envisioned using the ac pulsed technology via a perpendicular electric field. Heat transport is significantly suppressed in these monolayers as observed from their intrinsic low lattice thermal conductivity at room temperature: MgS (9.32 W m-1 K-1), MgSe (4.93 W m-1 K-1) and MgTe (2.02 W m-1 K-1). Further studies indicate that these monolayers can be used as photocatalytic materials for the simultaneous production of hydrogen and oxygen on account of having suitable band edge alignment and high charge carrier mobility. This work provides significant theoretical insights into both the fundamental and applied properties of these new buckled MgX monolayers, which are highly suitable for futuristic applications at the nanoscale in low-power, self-powered multifunctional electronic and spintronic devices and solar energy harvesting.

Blue-AsP monolayer as a promising anode material for lithium- and sodium-ion batteries: a DFT study.

Based on first-principle calculations, we proposed a one two-dimensional (2D) blue AsP (b-AsP) monolayer as an ideal anode material for lithium/sodium-ion (Li/Na-ion) batteries for the first time. The b-AsP monolayer possesses thermal and dynamic stabilities. The system undergoes the transition from semiconductor to metal after Li/Na atoms are embedded, which ensures good electric transportation. Most remarkably, our results indicate that the b-AsP monolayer exhibits high theoretical capacities of 1011.2 mA h g-1 (for Li) and 1769.6 mA h g-1 (for Na), low average open circuit voltages of 0.17 eV for Li4AsP and 0.14 eV for Na7AsP systems and ultrafast diffusivity with the low energy barriers of 0.17/0.15 eV and 0.08/0.07 eV of the P/As sides for Li and Na, respectively. Given these exceptional properties, the synthesis of a buckled b-AsP monolayer is desired to achieve a promising electrode material for Li- and Na-ion batteries.

Band flattening in buckled monolayer graphene

The strain fields of periodically buckled graphene induce a periodic pseudomagnetic field (PMF) that modifies the electronic band structure. From the geometry, amplitude, and period of the periodic pseudomagnetic field, we determine the necessary conditions to access the regime of correlated phases by examining the band flattening. As compared to twisted bilayer graphene the proposed system has the advantages that (1) only a single layer of graphene is needed, (2) one is not limited to hexagonal superlattices, and (3) narrower flat bandwidth and larger separation between flat bands can be induced. We, therefore, propose that periodically strained graphene single layers can become a platform for the exploration of exotic many-body phases.

Designing xenes with two-dimensional triangular lattice

Xenes, graphene-like two-dimensional (2D) monoelemental crystals with a honeycomb symmetry, have been the focus of numerous experimental and theoretical studies. In comparison, single-element 2D materials with a triangular lattice symmetry have not received due attention. Here, taking Pb as an example, we investigate the triangular-lattice monolayer made of group-IV atoms employing first-principles density functional theory calculations. The flat Pb monolayer supports a mirror-symmetry-protected spinless nodal line in the absence spin-orbit coupling (SOC). The introduction of an out-of-plane buckling creates a glide mirror, protecting an anisotropic Dirac nodal loop. Both flat and buckled Pb monolayers become topologically trivial after including SOC. A large buckling will make the Pb sheet a 2D semiconductor with symmetry-protected Dirac points below the Fermi level. The electronic structures of other group-IV triangular lattices such as Ge and Sn demonstrate strong similarity to Pb. We further design a quasi-3D crystal PbHfO$_2$ by alternately stacking Pb and 1T-HfO$_2$ monolayers. The new compound PbHfO$_2$ is dynamically stable and retains the properties of Pb monolayer. By applying epitaxial strains to PbHfO$_2$, it is possible to drive an insulator-to-metal transition coupled with an anti-ferroelectric-to-paraelectric phase transition. Our results suggest the potential of the 2D triangular lattice as a complimentary platform to design new type of broadly-defined Xenes.

Auxetic two-dimensional transition metal selenides and halides

Auxetic two-dimensional (2D) materials provide a promising platform for biomedicine, sensors, and many other applications at the nanoscale. In this work, utilizing a hypothesis-based data-driven approache, we identify multiple materials with remarkable in-plane auxetic behavior in a family of buckled monolayer 2D materials. These materials are transition metal selenides and transition metal halides with the stoichiometry MX (M = V, Cr, Mn, Fe, Co, Cu, Zn, Ag, and X = Se, Cl, Br, I). First-principles calculations reveal that the desirable auxetic behavior of these 2D compounds originates from the interplay between the buckled 2D structure and the weak metal–metal interaction determined by their electronic structures. We observe that the Poisson’s ratio is sensitive to magnetic order and the amount of uniaxial stress applied. A transition from positive Poisson’s ratio (PPR) to negative Poisson’s ratio (NPR) for a subgroup of MX compounds under large uniaxial stress is predicted. The work provides a guideline for the future design of 2D auxetic materials at the nanoscale.

Realization of a Buckled Antimonene Monolayer on Ag(111) via Surface Engineering.

The degree of buckling of two-dimensional (2D) materials can have a dramatic impact on their corresponding electronic structures. Antimonene (β-phase), a new 2D material with air stability and promising electronic properties, has been engineered to adopt flat or two-heights-buckling geometries by employing different supporting substrates for epitaxial growth. However, studies of the antimonene monolayer with a more buckled configuration are still lacking. Here, we report the synthesis of an antimonene monolayer with a three-heights-buckling configuration overlaid on SbAg2 surface alloy-covered Ag(111) by molecular beam epitaxy, in which the underlying surface alloy provides interfacial interactions to modulate the structure of the antimonene monolayer. The atomic structure of the synthesized antimonene has been precisely identified through a combination of low-temperature scanning tunneling microscopy and density functional theory calculations. The successful fabrication of a buckled antimonene monolayer could provide a promising way to modulate the structures of 2D materials for future electronic and optoelectronic applications.

Lattice chain theories for dynamics of acoustic flexural phonons in nonpolar nanomaterials

Knowledge of lattice dynamics of acoustic flexural phonons is critically important in understanding the mechanical, electrical, and thermal properties of nanomaterials. However, the general nature of acoustic flexural phonons for the span of possible nanomaterials and the dependence of these on microscopic material properties remain unclear. Therefore, we develop general lattice chain theories to understand acoustic flexural vibrations in covalent and van der Waals lattices. The analytic theories suggest that the flexure branch has quadratic dispersion for low dimensional lattices in the long-wavelength limit, quite different from linear branches in bulk van der Waals bonded lattices like graphite. Surprisingly, interlayer van der Waals interactions in multilayer lattices do not determine the behavior of the flexural dispersion. The bonds of covalent interactions in multilayer and buckled monolayer lattices play different roles in determining the flexural dispersion with those in flat monolayer lattices, e.g., graphene. Further, based on the correspondence between microscopic and continuum dynamics, we provide a universal and effective approach to characterize the intrinsic bending rigidity of low dimensional nanomaterials using dispersion data.

Prediction of giant and ideal Rashba-type splitting in ordered alloy monolayers grown on a polar surface.

A large and ideal Rashba-type spin-orbit splitting is desired for the applications of materials in spintronic devices and the detection of Majorana fermions in solids. Here, we propose an approach to achieve giant and ideal spin-orbit splittings through a combination of ordered surface alloying and interface engineering, that is, growing alloy monolayers on an insulating polar surface. We illustrate this unique strategy by means of first-principle calculations of buckled hexagonal monolayers of SbBi and PbBi supported on Al2O3(0001). Both systems display ideal Rashba-type states with giant spin-orbit splittings, characterized with energy offsets over 600 meV and momentum offsets over 0.3 Å−1, respectively. Our study thus points to an effective way of tuning spin-orbit splitting in low-dimensional materials to draw immediate experimental interest.

Superhigh flexibility and out-of-plane piezoelectricity together with strong anharmonic phonon scattering induced extremely low lattice thermal conductivity in hexagonal buckled CdX (X = S, Se) monolayers

Although CdX (X = S, Se) has been mostly studied in the field of photocatalysis, photovoltaics, their intrinsic properties, such as, mechanical, piezoelectric, electron and phonon transport properties have been completely overlooked in buckled CdX monolayers. Ultra-low lattice thermal conductivity [1.08 W m−1 K−1 (0.75 W m−1 K−1)] and high p-type Seebeck coefficient [1300 μV K−1 (850 μV K−1)] in CdS (CdSe) monolayers have been found in this work based on first-principles DFT coupled to semi-classical Boltzmann transport equations, combining both the electronic and phononic transport. The dimensionless thermoelectric figure of merit is calculated to be 0.78 (0.5) in CdS (CdSe) monolayers at room temperature, which is comparable to that of two-dimensional (2D) tellurene (0.8), arsenene and antimonene (0.8), indicating its great potential for applications in 2D thermoelectrics. Such a low lattice thermal conductivity arise from the participation of both acoustic [91.98% (89.22%)] and optical modes [8.02% (10.78%)] together with low Debye temperature [254 K (187 K)], low group velocity [4 km s−1 (3 km s−1)] in CdS (CdSe) monolayers, high anharmonicity and short phonon lifetime. Substantial cohesive energy (∼4–5 eV), dynamical and mechanical stability of the monolayers substantiate the feasibility in synthesizing the single layers in experiments. The inversion symmetry broken along the z direction causes out-of-plane piezoelectricity. |d33| ∼ 21.6 pm V−1, calculated in CdS monolayer is found to be the highest amongst structures having atomic-layer thickness. Superlow Young’s modulus ∼41 N m−1 (31 N m−1) in CdS (CdSe) monolayers, which is comparable to that of planar CdS (29 N m−1) and TcTe2 (34 N m−1), is an indicator of its superhigh flexibility. Direct semiconducting band gap, high carrier mobility (∼500 cm2 V−1 s−1) and superhigh flexibility in CdX monolayers signify its gigantic potential for applications in ultrathin, stretchable and flexible nanoelectronics. The all-round properties can be synergistically combined together in futuristic applications in nano-piezotronics as well.

Ultrahigh-strain ferroelasticity in two-dimensional honeycomb monolayers: from covalent to metallic bonding

We propose a possible ferroelastic switching pathway of two-dimensional (2D) honeycomb lattice (including graphene, BN, stanene, etc.) that may swap its armchair and zigzag direction, reversing an unprecedented strain of 73.2%. Our ab initio calculations reveal that such pathway cannot work in covalent systems like graphene and BN; for monolayer with metallic bonds like stanene, stanane and InBi that have all been synthesized, however, such pathway can be feasible with a low switching barrier (<0.15 eV) and stress (<graphene upon 1% tensile strain), also with the highest energy/stress point in the elastic region. Their distinct behaviors are attributed to the different feature of covalent bonds and metallic bonds: the former is rigid with directionality, while the latter is malleable with ductility. A general trend of linear decrease in switching barrier with uprising metallicity for the same group compounds is revealed. Similar behaviors can be extended to bulk zinc-blended or wurtzite structure that can be deemed as multilayer stacking of buckled monolayer. Binary compounds like InBi monolayer are even multiferroics with both in-plane and vertical ferroelectricity as well as nontrivial topological properties.

β-As Monolayer: Vibrational Properties and Raman Spectra.

Recently, semiconducting and other extraordinary properties of the monolayer of the V-group element have attracted a broad interest and attention. The success of experimentally growing antimonene and black phosphorus makes the arsenic monolayer a reasonable candidate for two-dimensional semiconductors. By using DFT calculation, we investigate the vibrational properties and Raman spectra of the buckled honeycomb monolayer of arsenic (β-As) for four commonly used laser lines. By calculating Raman tensor of each active modes of the β-As monolayer, we obtained polarization angle-dependent Raman intensities when the wave vector of incident light is parallel and perpendicular with the plane of the β-As monolayer. We found that the nonresonant Raman spectra have two peaks at 235 and 305 cm–1 that correspond to the in-plane vibrating mode Eg and out-of-plane vibrating mode A1g, which is similar to germanene, blue phosphorene, and β-Sb monolayer Raman spectra. There are two (four) minima and two (four) maxima when the polarization direction of scattered light is parallel (perpendicular) to that of the incident light and the wave vector of the incident light is parallel to the β-As monolayer. The Raman intensities of neither parallel polarization configuration nor perpendicular polarization configuration depend on the polarization direction when the wave vector of incident light is perpendicular to the β-As monolayer. The relation between shapes of the polar plots and relative values of Raman tensor elements is found. The Raman intensities decrease with increasing wavelength of incident laser lines in most cases. Our results will help experimentalists to identify the existence and the orientation of the β-As monolayer while they are growing the β-As monolayer.

Spin-polarized magneto-electronic properties in buckled monolayer GaAs

We develop the generalized tight-binding model to fully explore the magneto-electronic properties of monolayer GaAs, where the buckled structure, multi-orbital chemical bondings, spin-orbit coupling, electric field, and magnetic field are considered simultaneously. The diverse magnetic quantization covers three groups of spin-polarized Landau levels (LLs) near the Fermi level, with the unique initial energies, LL degeneracy, energy spacings, magnetic-field-dependence, and spin splitting. Furthermore, the Landau state probabilities exhibit specific oscillation patterns, being composed of the localization centers, node regularities, and energy-dependent variations of the dominating orbitals. The density of states directly reflects the main features of the LL energy spectra in the form, height, number, and frequency of the spin-split delta-function-like prominent peaks. The electric field leads to the monotonous/nonmonotonous LL energy dispersions, LL crossing behavior, gap modulation, phase transition and enhancement of spin splitting. The complex gap modulations and even semiconductor-semimetal transitions are attributed to the strong competition among the intrinsic interactions, magnetic field, and electric field. Such predicted magneto-electronic properties could be verified by scanning tunneling spectroscopy and are helpful in designing the top-gated and phase-change electronic devices.

Phononic Stability Analysis of Two-Dimensional Carbon Nitride Monolayers

Abstract:In this study we examined the structural, dynamical stability and electronical properties of carbon nitrides monolayers as C6N6 and C6N8. We found that buckled form of C6N8 monolayer is dynamically stable instead of planar C6N8, which is many times studied in the literature. While planar C6N8 has negative optical phonon modes, with proper created buckling in the structure can dissappear these imaginarities and makes the system dynamically stable. This buckled C6N8 has 2.05 eV direct band gap, which falls in the visible region. Other investigated carbon nitride is C6N6 and as is known in the literature planar C6N6 monolayer is stable, while created buckling results instability for the structure. We believe that with this study, confliction on the stability of carbon nitride structures will annihilate and investigation can focus on the planar C6N6 and buckled C6N8 monolayers.

Surface Engineering of Two-Dimensional Hydrogenated Silicon Nanosheets for Tailored Applications

Approaching the predicted limits by Moor’s law, which come with the shrinking size of electronics, science needs to find alternatives for the fabrication of novel devices. Within the last decades, more and more hybrid materials have been developed based on 0D- 1D- and also 2D nanomaterials. This development is also driven by the search for fast, cheap, reproducible and environmental-friendly low-cost devices. Hydrogenated silicon nanosheets (SiNSs) consist of just a buckled monolayer of sp3-hybridized silicon atoms, which are decorated with hydrogen atoms. The sheets show green and consistent photoluminescence at ~510 nm and are predicted show semiconducting behaviour. SiNSs may be used as an alternative for not only bulk silicon, but also already known two dimensional materials. Well-studied and established nanosilicon surface chemistry offers solid ground for precise surface engineering, leading to control of the nanomaterial for reproducible device fabrication. New applications and processing techniques can thus be developed, which lead to facile fabrication of enhanced sensors and (opto)electronics. In this regard, research pursues additionally the combination of organic materials’ properties with those of the nanomaterials, building the basis for new devices. One example is the combination of the widely used organic semiconductor poly(3-hexylthiophene-2,5-diyl) with the precisely modified SiNSs. Again, surface engineering plays the key role in the hybrid materials’ synthesis and subsequent application. This gives the opportunity to tune the properties of the hybrid material in the desired way. In this context, we present different ways of surface engineering with the modification of SiNSs as an example for the subsequent application in various (opto)electronic devices (e.g., SGFETs and photonic sensors).

Exceptional Optical Absorption of Buckled Arsenene Covering a Broad Spectral Range by Molecular Doping.

Using density functional theory calculations, we demonstrate that the electronic and optical properties of a buckled arsenene monolayer can be tuned by molecular doping. Effective p-type doping of arsenene can be realized by adsorption of tetracyanoethylene and tetracyanoquinodimethane (TCNQ) molecules, while n-doped arsenene can be obtained by adsorption of tetrathiafulvalene molecules. Moreover, owing to the charge redistribution, a dipole moment is formed between each organic molecule and arsenene, and this dipole moment can significantly tune the work function of arsenene to values within a wide range of 3.99–5.57 eV. Adsorption of TCNQ molecules on pristine arsenene can significantly improve the latter’s optical absorption in a broad (visible to near-infrared) spectral range. According to the AM 1.5 solar spectrum, two-fold enhancement is attained in the efficiency of solar-energy utilization, which can lead to great opportunities for the use of TCNQ–arsenene in renewable energy. Our work clearly demonstrates the key role of molecular doping in the application of arsenene in electronic and optoelectronic components, renewable energy, and laser protection.