Layered Bulk Materials Research Articles

Exciton-Driven and Layer-Independent Linear and Nonlinear Optical Properties in NbOCl2.

We investigate the electronic structure and linear and nonlinear [second-harmonic generation (SHG)] spectra of the NbOCl2 monolayer, bilayer, and bulk by using a real-time first-principles approach based on many-body theory. First, the interlayer couplings between NbOCl2 layers are very weak, due to the relatively large interlayer distance, saturation of the p orbital of Cl atoms, and high degree of localization of charge density around the Nb atom for both the lowest conduction band and the highest valence band. Second, the quasiparticle gaps and exciton binding energy for the three systems show layer-dependent features and decrease with an increase in layer thickness. Most importantly, the linear and SHG spectra of the NbOCl2 monolayer, bilayer, and bulk are dominated by strong excitonic resonances and exhibit layer-independent features due to the weak interlayer couplings. Our findings demonstrate that excitonic effects should be included in studying the optical properties of not only two-dimensional materials but also layered bulk materials with weak interlayer couplings.

Towards hybrid quantum mechanical/molecular mechanical simulations of Li and Na intercalation in graphite - force field development and DFTB parametrisation.

In this work a previously established QM/MM simulation protocol for the treatment of solid-state interfaces was extended towards the treatment of layered bulk materials enabling for instance investigation of metal intercalation in graphitic carbon materials. In order to study the intercalation of Li in graphite, new density functional tight binding (DFTB) parameters for Li have been created. Molecular dynamics (MD) simulations at constant temperatures (273.15, 298.15 and 323.15 K) have been carried out to assess the performance of the presented DFTB MD simulation approach. The intercalation of variable lithium and sodium content was investigated via z-distribution functions and analysis of the diffusivity in the direction parallel to the graphene plane. Both the calculated diffusion coefficients and the activation energy in case of lithium are in good agreement with experimental data. The comparison of the QM/MM MD simulation results provide detailed insights into the structural and dynamical properties of intercalated metal ions.

Controllable spin splitting in 2D ferroelectric few-layer γ-GeSe

γ-GeSe is a new type of layered bulk material that was recently successfully synthesized. By means of density functional theory first-principles calculations, we systematically studied the physical properties of two-dimensional (2D) few-layer γ-GeSe. It is found that few-layer γ-GeSe are semiconductors with band gaps decreasing with increasing layer number; and 2D γ-GeSe with layer number n ⩾ 2 are ferroelectric with rather low transition barriers, consistent with the sliding ferroelectric mechanism. Particularly, spin–orbit coupling induced spin splitting is observed at the top of valence band, which can be switched by the ferroelectric reversal; furthermore, their negative piezoelectricity also enables the regulation of spin splitting by strain. Finally, excellent optical absorption was also revealed. These intriguing properties make 2D few-layer γ-GeSe promising in spintronic and optoelectric applications.

Graphene oxide bulk material reinforced by heterophase platelets with multiscale interface crosslinking.

Graphene oxide (GO) and reduced GO possess robust mechanical, electrical and chemical properties. Their nanocomposites have been extensively explored for applications in diverse fields. However, due to the high flexibility and weak interlayer interactions of GO nanosheets, the flexural mechanical properties of GO-based composites, especially in bulk materials, are largely constrained, which hinders their performance in practical applications. Here, inspired by the amorphous/crystalline feature of the heterophase within nacreous platelets, we present a centimetre-sized, GO-based bulk material consisting of building blocks of GO and amorphous/crystalline leaf-like MnO2 hexagon nanosheets adhered together with polymer-based crosslinkers. These building blocks are stacked and hot-pressed with further crosslinking between the layers to form a GO/MnO2-based layered (GML) bulk material. The resultant GML bulk material exhibits a flexural strength of 231.2 MPa. Moreover, the material exhibits sufficient fracture toughness and strong impact resistance while being light in weight. Experimental and numerical analyses indicate that the ordered heterophase structure and synergetic crosslinking interactions across multiscale interfaces lead to the superior mechanical properties of the material. These results are expected to provide insights into the design of structural materials and potential applications of high-performance GO-based bulk materials in aerospace, biomedicine and electronics.

London Dispersion‐Corrected Density Functionals Applied to van der Waals Stacked Layered Materials: Validation of Structure, Energy, and Electronic Properties

AbstractMost density functionals lack to correctly account for long‐range London dispersion interactions, and numerous a posteriori correction schemes have been proposed in recent years. In van der Waals structures, the interlayer distance controls the proximity effect on the electronic structure, and the interlayer interaction energy indicates the possibility to mechanically exfoliate a layered material. For upcoming twisted van der Waals heterostructures, a reliable but efficient and scalable theoretical scheme to correctly predict the interlayer distance is required. Therefore, the performance of a series of popular London dispersion corrections combined with computationally affordable density functionals is validated. As reference data, the experimental interlayer distance of layered bulk materials is used, and corresponding interlayer interaction energies are calculated using the random phase approximation. We demonstrate that the SCAN‐rVV10 and PBE‐rVV10L functionals predict interlayer interaction energies and interlayer distances of the studied layered systems within the range of the defined error limits of 10 meV per atom and 0.12 Å, respectively. Semi‐empirical and empirical dispersion‐corrected functionals show significantly larger error bars, with PBE+dDsC performing best with comparable quality of geometries, but with higher interlayer interaction energy error limits of ≈20 meV per atom.

Photocatalytic properties of anisotropic β-PtX2 (X = S, Se) and Janus β-PtSSe monolayers.

The highly efficient photocatalytic water splitting process to produce clean energy requires novel semiconductor materials to achieve a high solar-to-hydrogen energy conversion efficiency. Herein, the photocatalytic properties of anisotropic β-PtX2 (X = S, Se) and Janus β-PtSSe monolayers were investigated based on the density functional theory. The small cleavage energy for β-PtS2 (0.44 J m-2) and β-PtSe2 (0.40 J m-2) endorses the possibility of mechanical exfoliation from their respective layered bulk materials. The calculated results revealed that the β-PtX2 monolayers have an appropriate bandgap (∼1.8-2.6 eV) enclosing the water redox potential, light absorption coefficient (∼104 cm-1), and exciton binding energy (∼0.5-0.7 eV), which facilitates excellent visible-light-driven photocatalytic performance. Remarkably, the inherent structural anisotropy leads to an anisotropic high carrier mobility (up to ∼5 × 103 cm2 V-1 S-1), leading to a fast transport of photogenerated carriers. Notably, the required small external potential to realize hydrogen evolution reaction and oxygen evolution reaction processes with an excellent solar-to-hydrogen energy conversion efficiency for β-PtSe2 (∼16%) and β-PtSSe (∼18%) makes them promising candidates for solar water splitting applications.

In Situ Fabrication of Freestanding Single-Atom-Thick 2D Metal/Metallene and 2D Metal/ Metallene Oxide Membranes: Recent Developments.

In recent years, two‐dimensional (2D) materials have attracted a lot of research interest as they exhibit several fascinating properties. However, outside of 2D materials derived from van der Waals layered bulk materials only a few other such materials are realized, and it remains difficult to confirm their 2D freestanding structure. Despite that, many metals are predicted to exist as 2D systems. In this review, the authors summarize the recent progress made in the synthesis and characterization of these 2D metals, so called metallenes, and their oxide forms, metallene oxides as free standing 2D structures formed in situ through the use of transmission electron microscopy (TEM) and scanning TEM (STEM) to synthesize these materials. Two primary approaches for forming freestanding monoatomic metallic membranes are identified. In the first, graphene pores as a means to suspend the metallene or metallene oxide and in the second, electron‐beam sputtering for the selective etching of metal alloys or thick complex initial materials is employed to obtain freestanding single‐atom‐thick 2D metal. The data show a growing number of 2D metals/metallenes and 2D metal/ metallene oxides having been confirmed and point to a bright future for further discoveries of these 2D materials.

Fast Water-Assisted Lithium Ion Conduction in Restacked Lithium Tin Sulfide Nanosheets

While two-dimensional (2D) materials may preserve some intrinsic properties of the corresponding layered bulk material, new characteristics arise from their pronounced anisotropy or confinement effects. Recently, exceptionally high ionic conductivities were discovered in 2D materials such as graphene oxide and vermiculite. Here, we report on the water-assisted fast conduction of lithium ions in restacked lithium tin sulfide nanosheets. Li0.8Sn0.8S2 exfoliates spontaneously in water and can be restacked into homogeneous films in which the lithium content is decreased, and a partial substitution of sulfur with hydroxyl groups takes place. Using a recursive supercell refinement approach in reciprocal space along with real-space pair distribution function analysis, we describe restacked lithium tin sulfide as a partially turbostratically disordered material composed of lithium-containing and lithium-depleted layers. In humid air, the material takes up multiple layers of water that coordinate lithium ions in the space between the layers, increasing the stacking distance and screening the interaction between lithium ions and the anionic layers. This results in a 1000-fold increase in ionic conductivity up to 47 mS cm–1 at high humidities. Orientation-dependent impedance spectroscopy suggests a facile in-plane conduction and a hindered out-of-plane conduction. Pulsed field gradient nuclear magnetic resonance spectroscopy reveals a fast, simultaneous diffusion of a majority and a minority species for both 7Li and 1H, suggesting water-assisted lithium diffusion to be at play. This study enlarges the family of nanosheet-based ionic conductors and helps to rationalize the transport mechanism of lithium ions enabled by hydration in a nanoconfined 2D space.

Recent progress of the Computational 2D Materials Database (C2DB)

The Computational 2D Materials Database (C2DB) is a highly curated open database organising a wealth of computed properties for more than 4000 atomically thin two-dimensional (2D) materials. Here we report on new materials and properties that were added to the database since its first release in 2018. The set of new materials comprise several hundred monolayers exfoliated from experimentally known layered bulk materials, (homo)bilayers in various stacking configurations, native point defects in semiconducting monolayers, and chalcogen/halogen Janus monolayers. The new properties include exfoliation energies, Bader charges, spontaneous polarisations, Born charges, infrared polarisabilities, piezoelectric tensors, band topology invariants, exchange couplings, Raman spectra and second harmonic generation spectra. We also describe refinements of the employed material classification schemes, upgrades of the computational methodologies used for property evaluations, as well as significant enhancements of the data documentation and provenance. Finally, we explore the performance of Gaussian process-based regression for efficient prediction of mechanical and electronic materials properties. The combination of open access, detailed documentation, and extremely rich materials property data sets make the C2DB a unique resource that will advance the science of atomically thin materials.

Two‐dimensional materials and synthesis, energy storage, utilization, and conversion applications of two‐dimensional MXene materials

Owing to the first-rate residences of graphene, the research had been achieved on novel two-dimensional (2D) materials. The MXene is a 2D inorganic compound that consists of transition metal carbides, nitrides, and carbonitrides. Ti₃C₂, the primary 2D-layered MXene, was reported in 2011. This material that is a layered bulk material analogous to graphite, changed into derived from its 3D phase, is Ti₃AlC₂ MAX. The start of this review through reviewing the structures and morphologies and their fabrication routes. After this, the discussion converts to the mechanical, electrical, and electronic properties of MXenes. The awareness then turns to their thrilling capacity in energy storage, conversion, and utilization. Energy storage programs encompass electrodes in rechargeable lithium batteries and supercapacitors. In phrases of energy conversion and utilization, photocatalytic hydrogen evolution, solar steam manufacturing, photothermal therapies, photovoltaic devices are presented. Highlights The study shows that transition metal carbides and nitrides (MXenes), own circle of two-dimensional (2D) inorganic compounds. The substances composed of some atomic layers of transition metal carbides, nitrides, or carbonitrides. Ti₃C₂, the primary 2D-layered MXene, were remote in 2011 This material that is a layered material analogous to graphite, changed into derived from its 3D phase, is Ti₃AlC₂ MAX Conversion of mechanical, electrical, and electronic properties of MXenes which turns to their thrilling capacity in energy storage, conversion, and utilization

Characterization of Mg-Zn Layered Bulk Materials Prepared by Powder Metallurgy Method

In this study three-layered materials composed of one zinc layer between two magnesium layers were prepared. Diffusion at the Mg-Zn boundary leads to the formation of thermodynamically more stable, yet mechanically very brittle intermetallic phase. Homogenous distribution of the fine-grained MgZn2 intermetallic phase in magnesium or zinc alloys has a positive effect on strength of these alloys. In a form of continuous thick layer stretching throughout the whole material, the phase may leads to deterioration of mechanical properties. However, the mechanism of fracture has not yet been sufficiently described. The Mg based materials with one layer of Zn were investigated in terms of chemical composition and mechanical properties and fractographic evaluation. The materials with 0.25 mm, 0.5 mm, 1 mm and 2 mm thick layer of Zn were processed via bidirectional hot pressing method at 300 °C and 500 MPa. The phase and chemical composition of prepared materials was characterized by XRD and SEM-EDS methods. The mechanical properties were evaluated based on the results of three-point bend test and fractographic analysis of fracture surface. The results showed formation of MgZn2 intermetallic phase on the interface of Mg and Zn layers and solid solution of Zn in Mg. The results showed that the presence of Zn layer leads to improvement of mechanical properties when compared to pure Mg prepared at the same condition. The strengthening effect of solid solution and intermetallic phase may be the reason of the increase of flexural strength.

Two-dimensional non-layered selenium nanoflakes: facile fabrications and applications for self-powered photo-detector

Two-dimensional (2D) materials exhibit many interesting properties, but most 2D materials are exfoliated from layered bulk materials, limiting the development of the 2D material group. Recently, non-layered 2D materials have aroused great attention due to their excellent catalysis performance, favored compatibility with silicon substrates and high chemical activity. With high photoconductivity, high responsivity and fast response time, non-layered selenium (Se) exhibits important applications in the field of optoelectronics. In this work, we use a simple liquid phase exfoliation method to fabricate 2D Se nanoflakes from bulk Se which possesses a unique chain structure. The thickness of 2D Se nanoflakes was measured to be in the range of 5–10 nm. As-fabricated Se nanoflakes were used in a photodetector by the photoelectrochemical method, showing a high photocurrent density (1.28 μA cm−2) and photoresponsivity (10.45 μA W−1). In addition, the long-term photoelectric measurements indicate that the 2D Se-based photodetector has good time and cycle stability. Our results show that 2D Se has promising potential in liquid-based photo-detectors.

PtTe Monolayer: Two-Dimensional Electrocatalyst with High Basal Plane Activity toward Oxygen Reduction Reaction.

PtTe is a layered bulk material that was discovered in 1897. According to first-principles calculations, it is one of the few layered materials that maintains structure and metallic character when thinned down to the monolayer. Interlayer energy is small enough to allow for chemical exfoliation techniques. Our calculations show that monolayer PtTe is a candidate to substitute Pt electrodes, and we computationally studied its catalytic performance in the oxygen reduction reaction (ORR). Remarkably, the basal plane of a PtTe monolayer exhibits excellent catalytic activity toward ORR, with a positive half-wave potential (∼0.90 V) and a high four-electron reduction pathway selectivity. These characteristics suggest that it outperforms Pt electrodes as catalyst, has a reduced Pt content, high Pt utilization, and a high surface area, and is a promising candidate for fuel cell components.

Interlayer Friction and Superlubricity in Single-Crystalline Contact Enabled by Two-Dimensional Flake-Wrapped Atomic Force Microscope Tips.

Interlayer friction between the atomic planes of 2D materials and heterostructures is a promising probe of the physics in their interlayer couplings and superlubricity. However, it is still challenging to measure the interlayer friction between well-defined 2D layers. We propose an approach of thermally assisted mechanical exfoliation and transfer to fabricate various 2D flake-wrapped atomic force microscopy (AFM) tips and to directly measure the interlayer friction between 2D flakes in single-crystalline contact. First, superlubricity between different 2D flakes and layered bulk materials is achieved with a friction coefficient as low as 10-4. The rotation angle dependence of superlubricity is observed for friction between graphite layers, whereas it is not observed between graphite and h-BN because of the incommensurate contact of the mismatched lattices. Second, the interlayer lateral force map between ReS2 layers is measured with atomic resolution, showing hexagonal patterns, as further verified by theoretical simulations. The tribological system constructed here offers an experimental platform to study interlayer couplings and friction between 2D flakes and layered bulk materials.

Nafion-Mediated Liquid-Phase Exfoliation of Transition Metal Dichalcogenides and Direct Application in Hydrogen Evolution Reaction

Owing to its mass-production capability and simple process, liquid-phase exfoliation (LPE) of layered bulk materials is commonly used for synthesizing two-dimensional materials, such as graphene or transition metal dichalcogenides (TMDs). Surfactants are often used in LPE process to improve the exfoliation efficiency and dispersion stability of target materials. However, during the surfactant-mediated exfoliation process, the as-applied surfactant typically forms strong covalent/noncovalent bonds with target materials, thereby deteriorating the properties of starting materials. Rinsing the surfactants can also be problematic because the decreased dispersion of the exfoliated flakes leads to restacking. Therefore, the contradictory phenomenon between maintaining the dispersion stability and preserving the material property has been a demanding issue in the LPE process of two-dimensional materials. Herein, a surfactant-assisted LPE of molybdenum disulfide (MoS2) using Nafion as the surfactant in environment...

Fluorine‐Free Synthesis of High‐Purity Ti3C2Tx (T=OH, O) via Alkali Treatment

AbstractMXenes, 2D compounds generated from layered bulk materials, have attracted significant attention in energy‐related fields. However, most syntheses involve HF, which is highly corrosive and harmful to lithium‐ion battery and supercapacitor performance. Here an alkali‐assisted hydrothermal method is used to prepare a MXene Ti3C2Tx (T=OH, O). This route is inspired from a Bayer process used in bauxite refining. The process is free of fluorine and yields multilayer Ti3C2Tx with ca. 92 wt % in purity (using 27.5 m NaOH, 270 °C). Without the F terminations, the resulting Ti3C2Tx film electrode (ca. 52 μm in thickness, ca. 1.63 g cm−3 in density) is 314 F g−1 via gravimetric capacitance at 2 mV s−1 in 1 m H2SO4. This surpasses (by ca. 214 %) that of the multilayer Ti3C2Tx prepared via HF treatments. This fluorine‐free method also provides an alkali‐etching strategy for exploring new MXenes for which the interlayer amphoteric/acidic atoms from the pristine MAX phase must be removed.

Fluorine-Free Synthesis of High-Purity Ti3 C2 Tx (T=OH, O) via Alkali Treatment.

MXenes, 2D compounds generated from layered bulk materials, have attracted significant attention in energy-related fields. However, most syntheses involve HF, which is highly corrosive and harmful to lithium-ion battery and supercapacitor performance. Here an alkali-assisted hydrothermal method is used to prepare a MXene Ti3 C2 Tx (T=OH, O). This route is inspired from a Bayer process used in bauxite refining. The process is free of fluorine and yields multilayer Ti3 C2 Tx with ca. 92 wt % in purity (using 27.5 m NaOH, 270 °C). Without the F terminations, the resulting Ti3 C2 Tx film electrode (ca. 52 μm in thickness, ca. 1.63 g cm-3 in density) is 314 F g-1 via gravimetric capacitance at 2 mV s-1 in 1 m H2 SO4 . This surpasses (by ca. 214 %) that of the multilayer Ti3 C2 Tx prepared via HF treatments. This fluorine-free method also provides an alkali-etching strategy for exploring new MXenes for which the interlayer amphoteric/acidic atoms from the pristine MAX phase must be removed.

Ultrafast Acoustofluidic Exfoliation of Stratified Crystals.

While the remarkable properties of 2D crystalline materials offer tremendous opportunities for their use in optics, electronics, energy systems, biotechnology, and catalysis, their practical implementation largely depends critically on the ability to exfoliate them from a 3D stratified bulk state. This goal nevertheless remains elusive, particularly in terms of a rapid processing method that facilitates high yield and dimension control. An ultrafast multiscale exfoliation method is reported which exploits the piezoelectricity of stratified materials that are noncentrosymmetric in nature to trigger electrically-induced mechanical failure across weak grain boundaries associated with their crystal domain planes. In particular, it is demonstrated that microfluidic nebulization using high frequency acoustic waves exposes bulk 3D piezoelectric crystals such as molybdenum disulphide (MoS2 ) and tungsten disulphide (WS2 ) to a combination of extraordinarily large mechanical acceleration (≈108 m s-2 ) and electric field (≈107 V m-1 ). This results in the layered bulk material being rapidly cleaved into pristine quasi-2D-nanosheets that predominantly comprise single layers, thus constituting a rapid and high throughput chip-scale method that opens new possibilities for scalable production and spray coating deposition.

Two-dimensional semiconductors ZrNCl and HfNCl: Stability, electric transport, and thermoelectric properties

Searching for novel two-dimensional (2D) semiconducting materials is a challenging issue. We investigate novel 2D semiconductors ZrNCl and HfNCl which would be isolated to single layers from van der Waals layered bulk materials, i.e., ternary transition-metal nitride halides. Their isolations are unquestionably supported through an investigation of their cleavage energies as well as their thermodynamic stability based on the ab initio molecular dynamics and phonon dispersion calculations. Strain engineering is found to be available for both single-layer (1L) ZrNCl and 1L-HfNCl, where a transition from an indirect to direct band gap is attained under a tensile strain. It is also found that 1L-ZrNCl has an excellent electron mobility of about 1.2 × 103 cm2 V−1 s−1, which is significantly higher than that of 1L-MoS2. Lastly, it is indicated that these systems have good thermoelectric properties, i.e., high Seebeck coefficient and high power factor. With these findings, 1L-ZrNCl and 1L-HfNCl would be novel promising 2D materials for a wide range of optoelectronic and thermoelectric applications.

Bulk β-Te to few layered β-tellurenes: indirect to direct band-Gap transitions showing semiconducting property

Herein we report a prediction of a highly kinetic stable layered structure of tellurium (namely, bulk β-Te), which is similar to these layered bulk materials such as graphite, black phosphorus, and gray arsenic. Bulk β-Te turns out to be a semiconductor that has a band gap of 0.325 eV (HSE06: 0.605 eV), based on first-principles calculations. Moreover, the single-layer form of the bulk β-Te, called β-tellurene, is predicted to have a high stability. When the bulk β-Te is thinned to one atomic layer, an indirect semiconductor of band gap is changed to 1.265 eV (HSE06: 1.932 eV) with a very high kinetic stability. Interestingly, an increase of the number of the β-tellurene layers from one to three is accompanied by a shift from an indirect to direct band gap. Furthermore, the effective carrier masses, the optical properties and phonon modes of few-layer β-tellurenes are characterized. Few-layer β-tellurenes strongly absorb the ultraviolet and blue-violet visible lights. The dramatic changes in the electronic structure and excellent photo absorptivities are expected to pave the way for high speed ultrathin transistors, as well as optoelectronic devices working in the UV or blue-green visible regions.