Elemental analogues of graphene: silicene, germanene, stanene, and phosphorene

Unveiling the potential of graphene and S-doped graphene nanostructures for toxic gas sensing and solar sensitizer cell devices: insights from DFT calculations

CONTEXT

In this work, we explore the potential of 2D materials, particularly graphene and its derivatives, for eco-friendly electricity generation and air pollution reduction. Leveraging the significant surface area of graphene nanomaterials, the susceptibility of these graphene-based nanostructures to hazardous substances and their applicability in clean solar cell (SSC) devices were systematically investigated using density functional theory (DFT), as implemented within Gaussian 5.0 code. Time-dependent DFT (TD-DFT) was employed to characterize the UV-visible spectrum of unstrained nanostructures. Herein, we considered three potentially harmful gases-CO, NH3, and Br2. Adsorption calculations revealed a notable interaction between the pure graphene nanostructure and Br2 gas, while the S-doped counterpart exhibited reduced interaction. Saturated S-doped nanostructures demonstrated an enhanced affinity for NH3 and CO gases compared to their pure S-doped counterparts, attributed to the sulfur (S) atom facilitating gas molecule binding to the nanostructure's surface. Furthermore, simulations of the SSC device architecture indicated the superior performance of the pure graphene nanostructure in terms of light-harvesting efficiency, injection energy, and electron injection into the lower conduction band of CBM titanium dioxide (TiO2). These findings suggest a potential avenue for developing nanostructures tailored for SSC devices and gas sensors, offering a dual solution to address air pollution concerns.

METHODS

Density function theory was used to compute the ground and excited state properties for pure and sulfur-doped graphene nanostructures. The hybrid function B3LYP with a 6-31G* basis set was utilized to describe the exchange correlation. Gauss Sum 2.2 software is used to estimate the density of state (DOS) for all structures under investigation.

An Atomically Resolved Schottky Barrier Height Approach for Bridging the Gap between Theory and Experiment at Metal-Semiconductor Heterojunctions

We propose an atomically resolved approach to capture the spatial variations of the Schottky barrier height (SBH) at metal-semiconductor heterojunctions. This proposed scheme, based on atom-specific partial density of states (PDOS) calculations, further enables calculation of the effective SBH that aligns with conductance measurements. We apply this approach to study the variations of SBH at MoS2@Au heterojunctions, in which MoS2 contains conducting and semiconducting grain boundaries (GBs). Our results reveal that there are significant variations in SBH at atoms in the defected heterojunctions. Of particular interest is the fact that the SBH in some areas with extended defects approaches zero, indicating Ohmic contact. One important implication of this finding is that the effective SBH should be intrinsically dependent on the defect density and character. Remarkably, the obtained effective SBH values demonstrate good agreement with existing experimental measurements. Thus, the present study addresses two long-standing challenges associated with SBH in MoS2-metal heterojunctions: the wide variation in experimentally measured SBH values at MoS2@metal heterojunctions and the large discrepancy between density-functional-theory-predicted and experimentally measured SBH values. Our proposed approach points out a valuable pathway for understanding and manipulating SBHs at metal-semiconductor heterojunctions.

Evolutionary Search and Theoretical Study of Silicene Grain Boundaries' Mechanical Properties

Defects such as grain boundaries (GBs) are almost inevitable during the synthesis process of 2D materials. To take advantage of the fascinating properties of 2D materials, understanding the nature and impact of various GB structures on pristine 2D sheets is crucial. In this work, using an evolutionary algorithm search, we predict a wide variety of silicene GB structures with very different atomic structures compared with those found in graphene or hexagonal boron-nitride. Twenty-one GBs with the lowest energy were validated by density functional theory (DFT), a majority of which were previously unreported to our best knowledge. Based on the diversity of the GB predictions, we found that the formation energy and mechanical properties can be dramatically altered by adatom positions within a GB and certain types of atomic structures, such as four-atom rings. To study the mechanical behavior of these GBs, we apply strain to the GB structures stepwise and use DFT calculations to investigate the mechanical properties of 9 representative structures. It is observed that GB structures based on pentagon-heptagon pairs are likely to have similar or higher in-plane stiffness and strength compared to the zigzag orientation of pristine silicene. However, an adatom located at the hollow site of a heptagon ring can significantly deteriorate the mechanical strength. For all of the structures, the in-plane stiffness and strength were found to decrease with increasing formation energy. For the failure behavior of GB structures, it was found that GB structures based on pentagon-heptagon pairs have failure behavior similar to that of graphene. We also found that the GB structures with atoms positioned outside of the 2D plane tend to experience phase transitions before failure. Utilizing the evolutionary algorithm, we locate diverse silicene GBs and obtain useful information about their mechanical properties.

Emergent Dirac Fermions in Epitaxial Planar Silicene Heterostructure

Silicene, a single layer of Si atoms, shares many remarkable electronic properties with graphene. So far, silicene has been synthesized in its epitaxial form on a few surfaces of solids. Thus, the problem of silicene-substrate interaction appears, which usually depresses the original electronic behavior but may trigger properties superior to those of bare components. We report the direct observation of robust Dirac-dispersed bands in epitaxial silicene grown on Au(111) films deposited on Si(111). By performing in-depth angle-resolved photoemission spectroscopy measurements, we reveal three pairs of one-dimensional bands with linear dispersion running in three different directions of an otherwise two-dimensional system. By combining these results with first-principles calculations, we explore the nature of these bands and point to strong interaction between subsystems forming a complex Si-Au heterostructure. These findings emphasize the essential role of interfacial coupling and open a unique materials platform for exploring exotic quantum phenomena and applications in future-generation nanoelectronics.

Rational Design of Stimuli-Responsive Inorganic 2D Materials via Molecular Engineering: Toward Molecule-Programmable Nanoelectronics

The ability of electronic devices to act as switches makes digital information processing possible. Succeeding graphene, emerging inorganic 2D materials (i2DMs) have been identified as alternative 2D materials to harbor a variety of active molecular components to move the current silicon-based semiconductor technology forward to a post-Moore era focused on molecule-based information processing components. In this regard, i2DMs benefits are not only for their prominent physiochemical properties (e.g., the existence of bandgap), but also for their high surface-to-volume ratio rich in reactive sites. Nonetheless, since this field is still in an early stage, having knowledge of both i) the different strategies for molecularly functionalizing the current library of i2DMs, and ii) the different types of active molecular components is a sine qua non condition for a rational design of stimuli-responsive i2DMs capable of performing logical operations at the molecular level. Consequently, this Review provides a comprehensive tutorial for covalently anchoring ad hoc molecular components-as active units triggered by different external inputs-onto pivotal i2DMs to assess their role in the expanding field of molecule-programmable nanoelectronics for electrically monitoring bistable molecular switches. Limitations, challenges, and future perspectives of this emerging field which crosses materials chemistry with computation are critically discussed.

A van der Waals p-n heterostructure of GaSe/SnS2: a high thermoelectric figure of merit and strong anisotropy

In recent years, van der Waals heterostructures (vdWHs) with controllable and peculiar properties have attracted extensive attention in the fields of electronics, optoelectronics, spintronics and electrochemistry. However, vdWHs with good thermoelectric performance are few due to the complex coupling of thermoelectric coefficients. Here, we employ density functional theory and Boltzmann's transport equation to explore the thermoelectric properties of the p-n vdWH of GaSe/SnS2, which has been experimentally observed to exhibit high performance as an optoelectronic device. We reveal that GaSe/SnS2 possesses strong anisotropy in terms of electronic transport resulting from the anisotropic carrier relaxation time. The longer carrier relaxation time in the y-direction for n-type induces a high power factor of 0.084 W m-1 K-2 at 300 K, while it is only 0.0087 W m-1 K-2) in the x-direction. The strong coupling of low-mid frequency phonon branches and the relatively weak Sn-S bond-induced anharmonicity hinder the phonon transport, which results in the lattice thermal conductivity of GaSe/SnS2 (14.61 and 15.43 W m-1 K-1 along the x- and y-directions at 300 K) being much smaller than the average value of GaSe and SnS2 (43.44 W m-1 K-1 at 300 K). The optimal thermoelectric figure of merit at 700 K for GaSe/SnS2 reaches 2.99, which is significantly higher than those of the constituents of GaSe (0.58) and SnS2 (1.04). The present work highlights the potential thermoelectric applications and the understanding of the thermoelectric transport mechanism for the recently synthesized p-n vdWH of GaSe/SnS2 with a high thermoelectric figure of merit and strong anisotropy.

MXene-based electrochemical devices applied for healthcare applications

The initial part of the review provides an extensive overview about MXenes as novel and exciting 2D nanomaterials describing their basic physico-chemical features, methods of their synthesis, and possible interfacial modifications and techniques, which could be applied to the characterization of MXenes. Unique physico-chemical parameters of MXenes make them attractive for many practical applications, which are shortly discussed. Use of MXenes for healthcare applications is a hot scientific discipline which is discussed in detail. The article focuses on determination of low molecular weight analytes (metabolites), high molecular weight analytes (DNA/RNA and proteins), or even cells, exosomes, and viruses detected using electrochemical sensors and biosensors. Separate chapters are provided to show the potential of MXene-based devices for determination of cancer biomarkers and as wearable sensors and biosensors for monitoring of a wide range of human activities.

Sandwiched Epitaxy Growth of 2D Single-Crystalline Hexagonal Bismuthene Nanoflakes for Electrocatalytic CO2 Reduction

Two-dimensional (2D) bismuthene is predicted to possess intriguing physical properties, but its preparation remains challenging due to the high surface energy constraint. Herein, we report a sandwiched epitaxy growth strategy for the controllable preparation of 2D bismuthene between a Cu foil substrate and a h-BN covering layer. The top h-BN layer plays a crucial role in suppressing the structural transformation of bismuthene and compensating for the charge transfer from the bismuthene to the Cu(111) surface. The bismuthene nanoflakes present a superior thermal stability up to 500 °C in air, attributed to the passivation effect of the h-BN layer. Moreover, the bismuthene nanoflakes demonstrate an ultrahigh faradaic efficiency of 96.3% for formate production in the electrochemical CO2 reduction reaction, which is among the highest reported for Bi-based electrocatalysts. This study offers a promising approach to simultaneously synthesize and protect 2D bismuthene nanoflakes, which can be extended to other 2D materials with a high surface energy.

Optimizing thermoelectric performance of carbon-doped h-BN monolayers through tuning carrier concentrations and magnetic field

The thermoelectric properties of carbon-doped monolayer hexagonal boron nitride (h-BN) are studied using a tight-binding model employing Green function approach and the Kubo formalism. Accurate tight-binding parameters are obtained to achieve excellent fitting with Density Functional Theory results for doped h-BN structures with impurity type and concentration. The influence of carbon doping on the electronic properties, electrical conductivity, and heat capacity of h-BN is studied, especially under an applied magnetic field. Electronic properties are significantly altered by doping type, concentration, and magnetic field due to subband splitting, merging of adjacent subbands, and band gap reduction. These modifications influence the number, location, and magnitude of DOS peaks, generating extra peaks inside the band gap region. Heat capacity displays pronounced dependence on both magnetic field and impurity concentration, exhibiting higher intensity at lower dopant levels. Electrical conductivity is increased by double carbon doping compared to single doping, but is reduced at high magnetic fields because of high carrier scattering. The electronic figure of merit ZT increases with lower impurity concentration and is higher for CB versus CN doping at a given field strength. The power factor can be improved by increasing magnetic field and decreasing doping concentration. In summary, controlling doping and magnetic field demonstrates the ability to effectively engineer the thermoelectric properties of monolayer h-BN.

Unraveling the Emerging Photocatalytic, Thermoelectric, and Topological Properties of Intercalated Architecture MZX (M = Ga and In; Z = Si, Ge and Sn; X = S, Se, and Te) Monolayers

The continuous advancements in studying two-dimensional (2D) materials pave the way for groundbreaking innovations across various industries. In this study, by employing density functional theory calculations, we comprehensively elucidate the electronic structures of MZX (M = Ga and In; Z = Si, Ge, and Sn; X = S, Se, and Te) monolayers for their applications in photocatalytic, thermoelectric, and spintronic fields. Interestingly, GaSiS, GaSiSe, InSiS, and InSiSe monolayers are identified to be efficient photocatalysts for overall water splitting with band gaps close to 2.0 eV, suitable band edge positions, and excellent optical harvest ability. In addition, the InSiTe monolayer exhibits a ZT value of 1.87 at 700 K, making it highly appealing for applications in thermoelectric devices. It is further highlighted that GaSnTe, InSnS, and InSnSe monolayers are predicted to be 2D topological insulators (TIs) with bulk band gaps of 115, 54, and 152 meV, respectively. Current research expands the family of 2D GaGeTe materials and establishes a path toward the practical utilization of MZX monolayers in energy conversion and spintronic devices.

Multi-Sensing Platform Based on 2D Monoelement Germanane

Covalently functionalized germanane is a novel type of fluorescent probe that can be employed in material science and analytical sensing. Here, a fluorometric sensing platform based on methyl-functionalized germanane (CH3 Ge) is developed for gas (humidity and ammonia) sensing, pH (1-9) sensing, and anti-counterfeiting. Luminescence (red-orange) is seen when a gas molecule intercalates into the interlayer space of CH3 Ge and the luminescence disappears upon deintercalation. This allows for direct detection of gas absorption via fluorometric measurements of the CH3 Ge. Structural and optical properties of CH3 Ge with intercalated gas molecules are investigated by density functional theory (DFT). To demonstrate real-time and on-the-spot testing, absorbed gas molecules are first precisely quantified by CH3 Ge using a smartphone camera with an installed color intensity processing application (APP). Further, CH3 Ge-paper-based sensor is integrated into real food packets (e.g., fish and milk) to monitor the shelf life of perishable foods. Finally, CH3 Ge-based rewritable paper is applied in water jet printing to illustrate the potential for secret communication with quick coloration and good reversibility by water evaporation.

Compression-induced hexa-to-tetra phase transition of confined germanene

Via molecular dynamics (MD) simulations we find the existence of the new allotrope of two-dimensional (2D) germanene, i.e. 2D tetra-germanene (tetra-Ge) which contains entirely tetragons. We compress 2D hexa-germanene (hexa-Ge) step by step over a broad density range at constant temperature and hexa-tetra Ge phase transition occurs. We find that the compression of hexa-Ge at 2000 K (not far above the melting point of hexa-Ge) leads to the formation of tetra-Ge with the highest quality. Atomic structure of the obtained tetra-Ge at 300 K is analyzed in details. Although fraction of tetragons in the tetra-Ge is very high (larger than 0.99), some defects are found in addition to the skew tetragons. Due to containing almost entirely tetragons, tetra-Ge may exhibit new behaviors unlike those of the hexa-Ge. Subsequent studies in this direction for 2D tetra-Ge. In addition, first-principles calculations under density functional theory confirm the existence of stable tetra-Ge.

Reveal Ultrafast Electron Relaxation across Sub-bands of Tellurium by Time- and Energy-Resolved Photoemission Microscopy

Exploring ultrafast carrier dynamics is crucial for the materials' fundamental properties and device design. In this work, we employ time- and energy-resolved photoemission electron microscopy with tunable pump wavelengths from visible to near-infrared to reveal the ultrafast carrier dynamics of the elemental semiconductor tellurium. We find that two discrete sub-bands around the Γ point of the conduction band are involved in excited-state electron ultrafast relaxation and reveal that hot electrons first go through ultrafast intra sub-band cooling on a time scale of about 0.3 ps and then transfer from the higher sub-band to the lower one on a time scale of approximately 1 ps. Additionally, theoretical calculations reveal that the lower one has flat-band characteristics, possessing a large density of states and a long electron lifetime. Our work demonstrates that TR- and ER-PEEM with broad tunable pump wavelengths are powerful techniques in revealing the details of ultrafast carrier dynamics in time and energy domains.

A simple and efficient process for the synthesis of 2D carbon nitrides and related materials

We describe here a new process for the synthesis of very high quality 2D Covalent Organic Frameworks (COFs), such a C2N and CN carbon nitrides. This process relies on the use of a metallic surface as both a reagent and a support for the coupling of small halogenated building blocks. The conditions of the assembly reaction are chosen so as to leave the inorganic salts by-products on the surface, to further confine the assembly reaction on the surface and increase the quality of the 2D layers. We found that under these conditions, the process directly returns few layers material. The structure/quality of these materials is demonstrated by extensive cross-characterizations at different scales, combining optical microscopy, Scanning Electron Microscopy (SEM)/Transmission Electron Microscopy (TEM) and Energy Dispersive Spectroscopy (EDS). The availability of such very large, high-quality layers of these materials opens interesting perspectives, for example in photochemistry and electronics (intrinsic transport properties, high gap substrate for graphene, etc...).

Tuning Gaps and Schottky Contacts of Graphene/Phosphorene Heterostructures by Vertical Electric Field and Strain

We present a comprehensive study of the structural and electronic properties of a graphene/phosphorene (G/P) heterostructure in the framework of density functional theory, including van der Waals interaction in the exchange-correlation functional. While the G(4 × 1)/P(3 × 1) superlattice usually used in the literature is subject to a strain as high as about 7%, the in-plane strain could be drastically reduced to under 1% in the G(4 × 13)/P(3 × 12) heterostructure investigated here. Adapting the lattice constants of the rectangular lattices, the equilibrium configuration in the xy plane of phosphorene relative to the graphene layer is optimized. This results in an equilibrium interlayer distance of 3.5 Å and a binding energy per carbon atom of 37 meV, confirming the presence of weak van der Waals interaction between the graphene and the phosphorene layers. The electronic properties of the heterostructure are evaluated under different values of interlayer distance, strain and applied vertical electric field. We demonstrate that G/P heterostructures form an n-type Schottky contact, which can be transformed into p-type under external perturbations. These findings, together with the possibility to control the gaps and barrier heights, suggest that G/P heterostructures are promising for novel applications in electronics and may open a new avenue for the realization of innovative optoelectronic devices.

Nanobio Interface Between Proteins and 2D Nanomaterials

Two-dimensional (2D) nanomaterials have significantly contributed to recent advances in material sciences and nanotechnology, owing to their layered structure. Despite their potential as multifunctional theranostic agents, the biomedical translation of these materials is limited due to a lack of knowledge and control over their interaction with complex biological systems. In a biological microenvironment, the high surface energy of nanomaterials leads to diverse interactions with biological moieties such as proteins, which play a crucial role in unique physiological processes. These interactions can alter the size, surface charge, shape, and interfacial composition of the nanomaterial, ultimately affecting its biological activity and identity. This review critically discusses the possible interactions between proteins and 2D nanomaterials, along with a wide spectrum of analytical techniques that can be used to study and characterize such interplay. A better understanding of these interactions would help circumvent potential risks and provide guidance toward the safer design of 2D nanomaterials as a platform technology for various biomedical applications.

Hydrogen-Terminated Two-Dimensional Germanane/Silicane Alloys as Self-Powered Photodetectors and Sensors

2D monoelemental materials, particularly germanene and silicene (the single layer of germanium and silicon), which are the base materials for modern electronic devices demonstrated tremendous attraction for their 2D layer structure along with the tuneable electronics and optical band gap. The major shortcoming of synthesized thermodynamically very unstable layered germanene and silicene with their inclination toward oxidation was overcome by topochemical deintercalation of a Zintl phase (CaGe₂, CaGe_1.5Si_0.5, and CaGeSi) in a protic environment. The exfoliated Ge-H, Ge_0.75Si_0.25H, and Ge_0.5Si_0.5H were successfully synthesized and employed as the active layer for photoelectrochemical photodetectors, which showed broad response (420-940 nm), unprecedented responsivity, and detectivity on the order of 168 μA W^-1 and 3.45 × 10⁸ cm Hz^1/2 W^-1, respectively. The sensing capability of exfoliated germanane and silicane composites was explored using electrochemical impedance spectroscopy with ultrafast response and recovery time of less than 1 s. These positive findings serve as the application of exfoliated germanene and silicene composites and can pave a new path to practical applications in efficient future devices.

Intervalley scattering induced significant reduction in lattice thermal conductivities for phosphorene

The thermal transport properties of buckled phosphorene (β-P) and antimonene (β-Sb) are investigated using first-principles methods. The large acoustic-optical phonon gaps of 3.8 THz and 2.2 THz enable the four-phonon interaction to play an important role in phonon scattering for both β-P and β-Sb. Considering the electron-phonon coupling, the lattice thermal conductivity can further undergo 84% decrease to 4.9 W mK^-1 for p-type β-P at n = 5 × 10¹³ cm^-2. By quantitatively describing the scattering probability of electrons in different paths combined with electron-phonon coupling matrix element analysis, it is found that multi-valley features of electronic band structure and strong electron-phonon coupling strength make electrons have strong intervalley scattering behavior in β-P. The former plays an important role in the energy conservation condition of the scattering process, and the latter determines the selection rule. Our work elucidates the contribution of higher-order phonon interactions as well as electron-phonon coupling effects to lattice thermal conductivity, and provides a new idea for finding materials with low lattice thermal conductivity induced by intervalley scattering.

Single-Atomic-Layer Stanene on Ferromagnetic Co Nanoislands with Topological Band Structures

Introducing magnetism to two-dimensional topological insulators is a central issue in the pursuit of magnetic topological materials in low dimensionality. By means of low-temperature growth at 80 K, we succeeded in fabricating a monolayer stanene on Co/Cu(111) and resolving ferromagnetic spin contrast by field-dependent spin-polarized scanning tunneling microscopy (SP-STM). Increases of both remanence to saturation magnetization ratio (M_r/M_s) and coercive field (H_c) due to an enhanced perpendicular magnetic anisotropy (PMA) are further identified by out-of-plane magneto-optical Kerr effect (MOKE). In addition to ultraflat stanene fully relaxed on bilayer Co/Cu(111) from density functional theory (DFT), characteristic topological properties including an in-plane s-p band inversion and a spin-orbit coupling (SOC) induced gap about 0.25 eV at the Γ̅ point have also been verified in the Sn-projected band structure. Interfacial coupling of single-atomic-layer stanene with ferromagnetic Co biatomic layers allows topological band features to coexist with ferromagnetism, facilitating a conceptual design of atomically thin magnetic topological heterostructures.

Pentagonal CmXnY6-m-n (m = 2, 3; n = 1, 2; X, Y = B, N, Al, Si, P) Monolayers: Janus Ternaries Combine Omnidirectional Negative Poisson Ratios with Giant Piezoelectric Effects

Two-dimensional (2D) materials composed of pentagon and Janus motifs usually exhibit unique mechanical and electronic properties. In this work, a class of ternary carbon-based 2D materials, C_mX_nY_6-m-n (m = 2, 3; n = 1, 2; X, Y = B, N, Al, Si, P), are systematically studied by first-principles calculations. Six of 21 Janus penta-C_mX_nY_6-m-n monolayers are dynamically and thermally stable. The Janus penta-C₂B₂Al₂ and Janus penta-Si₂C₂N₂ exhibit auxeticity. More strikingly, Janus penta-Si₂C₂N₂ exhibits an omnidirectional negative Poisson ratio (NPR) with values ranging from -0.13 to -0.15; in other words, it is auxetic under stretch in any direction. The calculations of piezoelectricity reveal that the out-of-plane piezoelectric strain coefficient (d₃₂) of Janus panta-C₂B₂Al₂ is up to 0.63 pm/V and increases to 1 pm/V after a strain engineering. These omnidirectional NPR, giant piezoelectric coefficients endow the Janus pentagonal ternary carbon-based monolayers as potential candidates in the future nanoelectronics, especially in the electromechanical devices.

DFT based investigations for the structural and electronic properties of coved zigzag BP nanoribbons

The peculiar properties of 2-D nano-materials have always inspired the research community for the further discovery of novel materials. Although III-V nitrides have been extensively explored for various remarkable phenomena, phosphides of the same group are yet to be explored. Taking a step in this direction, here we report the structural and electronic properties of zigzag BP nanoribbons (ZBPNR) with coved edge defects. The effect of sp² and sp³ edge passivation has also been compared to reveal interesting findings. The position of coved defect is considered in a number of different possibilities. It is observed that all the structures are energetically stable and maintain planar geometries. The H-passivated ribbons exhibit a semiconductor behavior with their band gap reciprocal to the ribbon width. However, for coved edge nanoribbons, a semiconductor as well as pure metallic nature has been predicted depending upon the site of the coved defect. Furthermore, the nature of the band gap is direct in H-passivated nanoribbons whereas for coved edges a direct to indirect alternation has been observed. The obtained wide range of electronic band gap (0.15 eV to 1.34 eV) indicates that ZBPNR could be useful for designing beyond silicon semiconductor devices.

2D Functionalized Germananes: Synthesis and Applications

In the realm of 2D layered materials, the monoelemental group 14 Xene, germanene, as the germanium analog of graphene, has emerged as the next prospective candidate. Preceded by silicon, germanium is widely used in the semiconductor industry; thus, germanene is deemed compatible with existing semiconductor technologies. Germanene consists of mixed sp² -sp³ -hybridized networks in a buckled hexagonal honeycomb structure. Chemical exfoliation of Zintl phases, such as CaGe₂ , specifically the topotactical deintercalation in acidic media, removes the alkaline earth metal ions Ca²⁺ , giving rise to layered germanane (germanene with the Ge centers covalently saturated with terminal hydrogen atoms). Diverse variants of functionalized germananes (with covalent group(s) termination) can be obtained by varying the topotactical deintercalation precursors, elevating the game with limitless functionalization possibilities for customizable properties or new functionalities. The preparation of Zintl phases to the details of functionalized and modified germananes and their properties, and the additional exfoliation step to achieve mono- or few-layer germananes, are comprehensively covered. The progress and challenges of 2D functionalized germananes in optoelectronics, catalysis, energy conversion and storage, sensors, and biomedical areas are reviewed. This review provides insight into designing and exploring this class of atomically thin semiconductors in realizing future nanoarchitectonics.

High-Throughput Computational Screening of Two-Dimensional Semiconductors

Two-dimensional (2D) materials have attracted great attention mainly due to their unique physical properties and ability to fulfill the demands of future nanoscale devices. By performing high-throughput first-principles calculations combined with a semiempirical van der Waals dispersion correction, we have screened 73 direct- and 183 indirect-gap 2D nonmagnetic semiconductors from nearly 1000 monolayers according to the criteria for thermodynamic, mechanical, dynamic, and thermal stabilities and conductivity type. We present the calculated lattice constants, formation energy, Young's modulus, Poisson's ratio, shear modulus, anisotropic effective mass, band structure, band gap, ionization energy, electron affinity, and simulated scanning tunnel microscopy for each candidate meeting our criteria. The resulting 2D semiconductor database (2DSdb) can be accessed via the Web site https://materialsdb.cn/2dsdb/index.html. The 2DSdb provides an ideal platform for computational modeling and design of new 2D semiconductors and heterostructures in photocatalysis, nanoscale devices, and other applications. Further, a linear fitting model was proposed to evaluate band gap, ionization energy, and electron affinity of 2D semiconductors from the density functional theory (DFT) calculated data as initial input. This model can be as precise as hybrid DFT but with much lower computational cost.

Feature-Rich Electronic Properties of Sliding Bilayer Germanene

This study employs first-principles calculations to elucidate the properties of sliding bilayer germanene (BLGe). The buckled structure of germanene can afford a greater number of metastable stacking configurations than planar graphene and enrich the electronic properties. Herein, a detailed analysis of the structural variety, shift-dependent energy bands, and spatial charge densities of BLGe is presented. The projected density of states (PDOS) reveals diverse structures such as plateaus, dips, symmetric/asymmetric peaks, and shoulders. The low-lying ones of the prominent structures could correspond to single or multiorbital hybridization, depending on the stacking configuration.

Combined effect of stacking and magnetic field on the electrical conductivity and heat capacity of biased trilayer BP and BN

In this paper, the Kubo-Greenwood formula based on the tight-binding model is used to investigate the effects of the bias voltage and magnetic field on the electrical conductivity and heat capacity of the trilayer BP and BN with energy-stable stacking structures. The results show that electronic and thermal properties of the selected structures can be significantly modified by external fields. The position and intensity of DOS peaks and the band gap of selected structures are affected by the external fields. When external fields increases above critical value, the band gap decreases to zero and semiconductor-metallic transition occurs. The results show that the thermal properties of the BP and BN structures are zero in TZ temperature region and increase by temperature above TZ. The increasing rate for thermal properties depends on the stacking configuration and changes with the bias voltage and magnetic field. In the presence of the stronger field, the TZ region decreases below 100 K. Compared to the BP structures, the BN types with larger band gap has smaller electrical conductivity which can be increased in order to 3L-BP by applying the stronger magnetic field or bias voltage. These results are interesting for the future development of nanoelectronic devices.

Synthetic Nanoarchitectonics of Functional Organic-Inorganic 2D Germanane Heterostructures via Click Chemistry

Succeeding graphene, 2D inorganic materials made of reactive van der Waals layers, like 2D germanane (2D-Ge) derivatives, have attracted great attention because their physicochemical characteristics can be entirely tuned by modulating the nature of the surface substituent. Although very interesting from a scientific point of view, almost all the reported works involving 2D-Ge derivatives are focused on computational studies. Herein, a first prototype of organic-inorganic 2D-Ge heterostructure has been synthesized by covalently anchoring thiol-rich carbon dots (CD-SH) onto 2D allyl germanane (2D-aGe) via a simple and green "one-pot" click chemistry approach. Remarkably, the implanted characteristics of the carbon nanomaterial provide new physicochemical features to the resulting 0D/2D heterostructure, making possible its implementation in yet unexplored optoelectronic tasks-e.g., as a fluorescence resonance energy transfer (FRET) sensing system triggered by supramolecular π-π interactions-that are inaccessible for the pristine 2D-aGe counterpart. Consequently, this work builds a foundation toward the robust achievement of functional organic-inorganic 2D-Ge nanoarchitectonics through covalently assembling thiol-rich carbon nanoallotropes on commercially available 2D-aGe.

Effect of hydrogen coverage on elastic and optical properties of silicene: a first-principle study

The structural, electronic, and optical properties of hydrogenated silicene have been investigated using first-principles DFT calculations. Compared to pristine silicene, hydrogenated silicene exhibits high stability, reduced anisotropy, and less birefringence. Hydrogenated silicene shows a constant refractive index in the visible region, increasing exponentially in silicene. The elastic and optical parameters such as Young's modulus (Y), Poisson's ratio (ν), bulk modulus (B), shear modulus (G), dielectric constant ε(0), refractive index n(0), conductivity threshold (E_th), birefringence Δn(0), and plasmon energy (ħω_p) were calculated for the first time for different hydrogen coverage on silicene, which is crucial in the applications of linear and non-linear optoelectronic devices. The estimated parameters agree well with the available experimental and reported values.

Site-selective growth of two-dimensional materials: strategies and applications

Over the years, there have been major advances in two-dimensional (2D) materials on account of their excellent and unique properties. Among the various strategies for 2D material fabrication, chemical vapor deposition (CVD) is considered as the most promising method to achieve large-area and high-quality 2D film growth. Furthermore, to realize the potential applications of 2D materials in different fields, the integration of 2D materials into functional devices is essential. However, the materials made by common CVD are randomly distributed on substrates, which is disadvantageous for fabricating arrays of devices. To solve this problem, a site-selective growth method was developed to meet the requirement of batch production for practical applications because it achieves control over the locations of products and benefits the subsequent direct integration. Herein, state-of-the-art methods for site-selective synthesis, including seeded growth and patterned growth, are reviewed. Then, the electronic and optoelectronic applications of the as-grown 2D materials are also reviewed. Finally, the remaining challenges and future prospects regarding site-selective methods and applications are discussed.

On the interface between biomaterials and two-dimensional materials for biomedical applications

Two-dimensional (2D) materials have garnered significant attention due to their ultrathin 2D structures with a high degree of anisotropy and functionality. Reliable manipulation of interfaces between 2D materials and biomaterials is a new frontier for biomedical nanoscience and combining biomaterials with 2D materials offers a promising way to fabricate innovative 2D biomaterials composites with distinct functionality for biomedical applications. Here, we focus exclusively on a summary of the current work in the interface investigation of 2D biomaterials. Specifically, we highlight extraordinary features that make 2D materials so desirable, as well as the molecular level interactions between 2D materials and biomaterials that have been studied thus far. Furthermore, the approaches for investigating the interface characteristics of 2D biomaterials are presented and described in depth. To capture the emerging trend in mass manufacturing of 2D materials, we review the research progress on biomaterial-assisted exfoliation. Finally, we present a critical assessment of newly developed 2D biomaterials in biomedical applications.

Single-Element 2D Materials beyond Graphene: Methods of Epitaxial Synthesis

Today, two-dimensional materials are one of the key research topics for scientists around the world. Interest in 2D materials is not surprising because, thanks to their remarkable mechanical, thermal, electrical, magnetic, and optical properties, they promise to revolutionize electronics. The unique properties of graphene-like 2D materials give them the potential to create completely new types of devices for functional electronics, nanophotonics, and quantum technologies. This paper considers epitaxially grown two-dimensional allotropic modifications of single elements: graphene (C) and its analogs (transgraphenes) borophene (B), aluminene (Al), gallenene (Ga), indiene (In), thallene (Tl), silicene (Si), germanene (Ge), stanene (Sn), plumbene (Pb), phosphorene (P), arsenene (As), antimonene (Sb), bismuthene (Bi), selenene (Se), and tellurene (Te). The emphasis is put on their structural parameters and technological modes in the method of molecular beam epitaxy, which ensure the production of high-quality defect-free single-element two-dimensional structures of a large area for promising device applications.

Engineering van der Waals Materials for Advanced Metaphotonics

The outstanding chemical and physical properties of 2D materials, together with their atomically thin nature, make them ideal candidates for metaphotonic device integration and construction, which requires deep subwavelength light-matter interaction to achieve optical functionalities beyond conventional optical phenomena observed in naturally available materials. In addition to their intrinsic properties, the possibility to further manipulate the properties of 2D materials via chemical or physical engineering dramatically enhances their capability, evoking new science on light-matter interaction, leading to leaped performance of existing functional devices and giving birth to new metaphotonic devices that were unattainable previously. Comprehensive understanding of the intrinsic properties of 2D materials, approaches and capabilities for chemical and physical engineering methods, the resulting property modifications and novel functionalities, and applications of metaphotonic devices are provided in this review. Through reviewing the detailed progress in each aspect and the state-of-the-art achievement, insightful analyses of the outstanding challenges and future directions are elucidated in this cross-disciplinary comprehensive review with the aim to provide an overall development picture in the field of 2D material metaphotonics and promote rapid progress in this fast emerging and prosperous field.

Tracing the structural evolution of quasi-freestanding germanene on Ag(111)

In the last decade, research on 2D materials has expanded massively due to the popularity of graphene. Although the chemical engineering of two-dimensional elemental materials as well as heterostructures has been extensively pursued, the fundamental understanding of the synthesis of 2D materials is not yet complete. Structural parameters, such as buckling or the interface structure of a 2D material to the substrate directly affect its electronic characteristics. In order to proceed the understanding of the element-specific growth and the associated ability of tuning material properties of two-dimensional materials, we performed a study on the structural evolution of the promising 2D material germanene on Ag(111). This study provides a survey of germanium formations at different layer thicknesses right up to the arising of quasi-freestanding germanene. Using powerful surface analysis tools like low-energy electron diffraction, x-ray photoelectron spectroscopy, and x-ray photoelectron diffraction with synchrotron radiation, we will reveal the internal and interfacial structure of all discovered germanium phases. Moreover, we will present models of the atomic and chemical structure of a \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {Ag}_2\hbox {Ge}$$\end{document}Ag2Ge surface alloy and the quasi-freestanding germanene with special focus on the structural parameters and electronic interaction at the interface.

Band Polarization Effect on the Kondo State in a Zigzag Silicene Nanoribbon

Using the Numerical Renormalization Group method, we study the properties of a quantum impurity coupled to a zigzag silicene nanoribbon (ZSNR) that is subjected to the action of a magnetic field applied in a generic direction. We propose a simulation of what a scanning tunneling microscope will see when investigating the Kondo peak of a magnetic impurity coupled to the metallic edge of this topologically non-trivial nanoribbon. This system is subjected to an external magnetic field that polarizes the host much more strongly than the impurity. Thus, we are indirectly analyzing the ZSNR polarization through the STM analysis of the fate of the Kondo state subjected to the influence of the polarized conduction electron band. Our numerical simulations demonstrate that the spin-orbit-coupling-generated band polarization anisotropy is strong enough to have a qualitative effect on the Kondo peak for magnetic fields applied along different directions, suggesting that this contrast could be experimentally detected.

Novel hybrid monolayers SixGeySn1-x-y: first principles study of structural, electronic, optical, and electron transport properties with NH3 sensing application

The structural, electronic, optical, and electron transport properties of three different atomically thin novel hybrid monolayers comprising of Si, Ge, and Sn atoms in varying proportions are studied using first principles calculations within the framework of density functional theory. The fabrication of similar hybrid materials is practically realizable but the different properties of these novel monolayers are yet to be explored. The proposed hybrid buckled honeycomb monolayers with sp²-sp³ like orbital hybridization are mechanically and dynamically stable, confirmed by the analysis of in-plane elastic constants, phonon dispersion curve and cohesive energy of the monolayers. The electronic band structures of these hybrid two-dimensional (2D) monolayers, namely Ge_0.25Sn_0.25Si_0.50, Si_0.25Ge_0.25Sn_0.50, and Sn_0.25Si_0.25Ge_0.50, show a considerable direct energy bandgap ranging from 120 meV to 283.8 meV while preserving the linear energy-momentum relation at the K point of the Brillouin zone. The calculated significantly low effective mass (0.063-0.101m₀) and very high acoustic phonon limited mobility (∼10⁶ cm² V^-1 s^-1) of the charge carriers inside the hybrid monolayers ensure the presence of relativistic-massless Dirac fermions. In order to further investigate the electronic properties, we have calculated the atom projected density of states and differential charge density. Optical properties, e.g. dielectric function, electron loss function, absorption coefficient, refractive index, reflectivity, and optical conductivity, are also explored for parallelly and perpendicularly polarized incident light. These hybrid monolayers show anisotropic optical response for parallel and perpendicular polarization as a function of frequency of the incident light. Polarization tunable plasma frequency, high absorption coefficient over a wide range of frequency, and high refractive indices suggest these hybrid monolayers as potential candidates for optoelectronic applications. We have also designed three different armchair nanoribbons to study the effect of the adsorption of NH₃ molecules on these hybrid nanoribbons. Our calculated electron transport properties ensure the applications of these nanoribbons as an NH₃ sensor at the molecular level. Thus, our results suggest that the proposed Si_xGe_ySn_1-x-y hybrid monolayers can be a potential candidate for nanoelectronics, optoelectronics and sensor based applications.

Revealing the nano-structures of low-dimensional germanium on Ag(1 1 0) using XPS and XPD

In this work, we present a structural investigation of sub-monolayer films of germanium on Ag(1 1 0) by means of photoelectron spectroscopy (XPS) and diffraction (XPD), as well as low-energy electron diffraction (LEED). Since the rising progress in the synthesis of various kinds of nanoribbons, also germanium nanoribbons (Ge-NR) have been synthesized on Ag(1 1 0), recently. Here, we focus on their structural evolution and found the formation of two different phases of germanium at coverages of $${0.5}\, \hbox {ML}$$ 0.5 ML and $${0.7}\,\hbox {ML}$$ 0.7 ML , differing fundamentally from predicted nanoribbon structures. By means of LEED measurements, we obtained evidence for germanium superstructures which are not aligned along the $$[{\overline{1}}\;1\;0]$$ [ 1 ¯ 1 0 ] -direction, as expected for nanoribbon growth. Using synchrotron-based high-resolution XPS and XPD experiments of the Ge 3d and Ag 3d core-levels, we resolved the local chemical and atomic order of the germanium films. Thus, the strong internal bonding of the buckled germanium film and a weak Van-der-Waals interaction between silver and germanium were discovered. Moreover, XPD-simulations delivered a detailed model of the structural arrangement of the preliminary nanoribbon phase, which also provided an approach to identify the origin of the two chemically shifted components in the Ge 3d signal by applying a component-wise decomposition of the XPD data.

DNA/RNA sequencing using germanene nanoribbons via two dimensional molecular electronic spectroscopy: an ab initio study

Developing fast, reliable, and cost effective, yet practical DNA/RNA sequencing methods and devices is a must. In this regard, motivated by the recently introduced two-dimensional electronic molecular spectroscopy (2DMES) technique for molecular recognition, and the compatibility of 2D layers of group IV elements with the current technology of manufacturing electronic devices, we investigate the capability of germanene nanoribbons (GeNRs) as a feasible, accurate, and ultra-fast sequencing device under the application of 2DMES. We show that by employing 2DMES, not only can GeNRs unambiguously distinguish different nucleobases to sequence DNA/RNA, they are also capable of recognizing methylated nucleobases that could be related to cancerous cell growth. Our calculations indicate that, compared to frequently used graphene layers, germanene provides more distinct adsorption energies for different nucleobases which implies its better ability to recognize various molecules unambiguously. By calculating the conductance sensitivity of the system for experimental purposes, we also show that the introduced sequencing device possesses a high sensitivity and selectivity characteristic. Thus, our proposed system would be a promising device for next-generation DNA sequencing technologies and would be realizable using the current protocols of fabricating electronic devices.

Few-layer Mg-deficient borophene nanosheets: I2 oxidation and ultrasonic delamination from MgB2

By using I₂ as an oxidant and CH₃CN as a reaction medium, few-layer Mg-deficient borophene nanosheets (FBN) with a stoichiometric ratio of Mg_0.22B₂ are prepared by oxidizing MgB₂ in a mixture of CH₃CN and HCl for 14 days under nitrogen protection and followed by ultrasonic delaminating in CH₃CN for 2 h. The prepared FBN possess a two-dimensional flake morphology, and they show a clear interference fringe with a d-spacing of 0.251 nm corresponding to the (208) plane of rhombohedral boron. While maintaining the hexagonal boron networks of MgB₂, the FBN have an average thickness of about 4.14 nm (four monolayer borophene) and a lateral dimension of 500 nm, and the maximum Mg deintercalation rate can reach 78%. The acidity of the reaction system plays an important role; the HCl reaction system not only facilitates the oxidation of MgB₂ by I₂, but also increases the deintercalation ratio of Mg atoms. Etching of the Mg atom layer with HCl, the negative charge decrease of the boron layer by I₂ oxidation, and the Mg chelating effect from CH₃COOH due to the hydrolysis of CH₃CN in an HCl environment led to a high deintercalation rate of the Mg atom. Density functional theory (DFT) calculations further support the result that the maximum deintercalation rate of Mg atoms is about 78% while maintaining the hexagonal layer structure of boron. This research solves the problems of low Mg atom deintercalation rate and hexagonal boron structure destruction when using the precursor MgB₂ to produce borophene nanosheets, which is of great significance for large-scale novel preparation and application of borophene nanosheets.

Strain-tuned mechanical, electronic, and optoelectronic properties of two-dimensional transition metal sulfides ZrS2: a first-principles study

Two-dimensional semiconductor material zirconium disulfide (ZrS₂) monolayer is a new promising material with good prospects for nanoscale applications. Recently, a new zirconium disulfide (ZrS₂) monolayer with a space group of 59_Pmmn has been successfully predicted. Using first-principles calculations, this new monolayer ZrS₂ structure is obtained with stable indirect bandgaps of 0.65 eV and 1.46 eV at the DFT-PBE (HSE06) functional levels, respectively. Strain engineering studies on the ZrS₂ monolayer show effective bandgap modulation. The bandgap shows a nearly linear regularity from narrow to wide under strain (ranged from - 6 to + 8%). Young's modulus of elasticity of ZrS₂ along the tensile directions (x-axis and y-axis) is 83.63 (N/m) and 63.61 (N/m) with Poisson's ratios of 0.09 and 0.07, respectively. The results of carrier mobility show that the electron mobility along the y-axis can reach 1.32 × 10³ cm² V^-1 s^-1. Besides, the order of magnitude of the light absorption coefficient in the ultraviolet spectral region is calculated to reach 2.0 × 10⁵ cm^-1 for ZrS₂ monolayers. Moreover, the bandgap and band edge position of Pmmn-ZrS₂ can satisfy the redox potentials of photocatalytic water splitting by strain regulating. The results indicate that the new two-dimensional Pmmn-ZrS₂ monolayer is a potential material for photovoltaic devices and photocatalytic water decomposition.

The elemental 2D materials beyond graphene potentially used as hazardous gas sensors for environmental protection

The intrinsic and electronic properties of elemental two-dimensional (2D) materials beyond graphene are first introduced in this review. Then the studies concerning the application of gas sensing using these 2D materials are comprehensively reviewed. On the whole, the carbon-, nitrogen-, and sulfur-based gases could be effectively detected by using most of them. For the sensing of organic vapors, the borophene, phosphorene, and arsenene may perform it well. Moreover, the G-series nerve agents might be efficiently monitored by the bismuthene. So far, there is still challenge on the material preparation due to the instability of these 2D materials under atmosphere. The synthesis or growth of materials integrated with the technique of surface protection should be associated with the device fabrication to establish a complete process for particular application. This review provides a complete and methodical guideline for scientists to further research and develop the hazardous gas sensors of these 2D materials in order to achieve the purpose of environmental protection.

Hexatetra-Carbon: A Novel Two-Dimensional Semiconductor Allotrope of Carbon

Employing first-principles calculations based on density functional theory (DFT), we designed a novel two-dimensional (2D) elemental monolayer allotrope of carbon called hexatetra-carbon. In the hexatetra-carbon structure, each carbon atom bonds with its four neighboring atoms in a 2D double layer crystal structure, which is formed by a network of carbon hexagonal prisms. Based on our calculations, it is found that hexatetra-carbon exhibits a good structural stability as confirmed by its rather high calculated cohesive energy −6.86 eV/atom, and the absence of imaginary phonon modes in its phonon dispersion spectra. Moreover, compared with its hexagonal counterpart, i.e., graphene, which is a gapless material, our designed hexatetra-carbon is a semiconductor with an indirect band gap of 2.20 eV. Furthermore, with a deeper look at the hexatetra-carbon, one finds that this novel monolayer may be obtained from bilayer graphene under external mechanical strain conditions. As a semiconductor with a moderate band gap in the visible light range, once synthesized, hexatetra-carbon would show promising applications in new opto-electronics technologies.

Two-Dimensional Sb/InS van der Waals Heterostructure for Electronic and Optical Related Applications

Stable configurations with excellent optical adsorption are crucial for the photovoltaics or photocatalysis. Two-dimensional materials with intrinsic electric-field have been proposed suitable for electric and optical device. Here, we have...

Solution-processed two-dimensional materials for next-generation photovoltaics

In the ever-increasing energy demand scenario, the development of novel photovoltaic (PV) technologies is considered to be one of the key solutions to fulfil the energy request. In this context, graphene and related two-dimensional (2D) materials (GRMs), including nonlayered 2D materials and 2D perovskites, as well as their hybrid systems, are emerging as promising candidates to drive innovation in PV technologies. The mechanical, thermal, and optoelectronic properties of GRMs can be exploited in different active components of solar cells to design next-generation devices. These components include front (transparent) and back conductive electrodes, charge transporting layers, and interconnecting/recombination layers, as well as photoactive layers. The production and processing of GRMs in the liquid phase, coupled with the ability to "on-demand" tune their optoelectronic properties exploiting wet-chemical functionalization, enable their effective integration in advanced PV devices through scalable, reliable, and inexpensive printing/coating processes. Herein, we review the progresses in the use of solution-processed 2D materials in organic solar cells, dye-sensitized solar cells, perovskite solar cells, quantum dot solar cells, and organic-inorganic hybrid solar cells, as well as in tandem systems. We first provide a brief introduction on the properties of 2D materials and their production methods by solution-processing routes. Then, we discuss the functionality of 2D materials for electrodes, photoactive layer components/additives, charge transporting layers, and interconnecting layers through figures of merit, which allow the performance of solar cells to be determined and compared with the state-of-the-art values. We finally outline the roadmap for the further exploitation of solution-processed 2D materials to boost the performance of PV devices.

Tunable 2D Group-III Metal Alloys

Chemically stable quantum-confined 2D metals are of interest in next-generation nanoscale quantum devices. Bottom-up design and synthesis of such metals could enable the creation of materials with tailored, on-demand, electronic and optical properties for applications that utilize tunable plasmonic coupling, optical nonlinearity, epsilon-near-zero behavior, or wavelength-specific light trapping. In this work, it is demonstrated that the electronic, superconducting, and optical properties of air-stable 2D metals can be controllably tuned by the formation of alloys. Environmentally robust large-area 2D-In_x Ga_{1- x} alloys are synthesized byConfinement Heteroepitaxy (CHet). Near-complete solid solubility is achieved with no evidence of phase segregation, and the composition is tunable over the full range of x by changing the relative elemental composition of the precursor. The optical and electronic properties directly correlate with alloy composition, wherein the dielectric function, band structure, superconductivity, and charge transfer from the metal to graphene are all controlled by the indium/gallium ratio in the 2D metal layer.

Emerging beyond-graphene elemental 2D materials for energy and catalysis applications

Elemental two-dimensional (2D) materials have emerged as promising candidates for energy and catalysis applications due to their unique physical, chemical, and electronic properties. These materials are advantageous in offering massive surface-to-volume ratios, favorable transport properties, intriguing physicochemical properties, and confinement effects resulting from the 2D ultrathin structure. In this review, we focus on the recent advances in emerging energy and catalysis applications based on beyond-graphene elemental 2D materials. First, we briefly introduce the general classification, structure, and properties of elemental 2D materials and the new advances in material preparation. We then discuss various applications in energy harvesting and storage, including solar cells, piezoelectric and triboelectric nanogenerators, thermoelectric devices, batteries, and supercapacitors. We further discuss the explorations of beyond-graphene elemental 2D materials for electrocatalysis, photocatalysis, and heterogeneous catalysis. Finally, the challenges and perspectives for the future development of elemental 2D materials in energy and catalysis are discussed.

Magnetism in Au-Supported Planar Silicene

The adsorption and substitution of transition metal atoms (Fe and Co) on Au-supported planar silicene have been studied by means of first-principles density functional theory calculations. The structural, energetic and magnetic properties have been analyzed. Both dopants favor the same atomic configurations with rather strong binding energies and noticeable charge transfer. The adsorption of Fe and Co atoms do not alter the magnetic properties of Au-supported planar silicene, unless a full layer of adsorbate is completed. In the case of substituted system only Fe is able to produce magnetic ground state. The Fe-doped Au-supported planar silicene is a ferromagnetic structure with local antiferromagnetic ordering. The present study is the very first and promising attempt towards ferromagnetic epitaxial planar silicene and points to the importance of the substrate in structural and magnetic properties of silicene.