2D Monoelemental Materials Research Articles

Reaction Mechanism and Performance of Innovative 2D Germanane-Silicane Alloys: SixGe1- xH Electrodes in Lithium-Ion Batteries.

The adjustable structures and remarkable physicochemical properties of 2D monoelemental materials, such as silicene and germanene, have attracted significant attention in recent years. They can be transformed into silicane (SiH) and germanane (GeH) through covalent functionalization via hydrogen atom termination. However, synthesizing these materials with a scalable and low-cost fabrication process to achieve high-quality 2D SiH and GeH poses challenges. Herein, groundbreaking 2D SiH and GeH materials with varying compositions, specifically Si0.25Ge0.75H, Si0.50Ge0.50H, and Si0.75Ge0.25H, are prepared through a simple and efficient chemical exfoliation of their Zintl phases. These 2D materials offer significant advantages, including their large surface area, high mechanical flexibility, rapid electron mobility, and defect-rich loose-layered structures. Among these compositions, the Si0.50Ge0.50H electrode demonstrates the highest discharge capacity, reaching up to 1059 mAh g-1 after 60 cycles at a current density of 75 mA g-1. A comprehensive ex-situ electrochemical analysis is conducted to investigate the reaction mechanisms of lithiation/delithiation in Si0.50Ge0.50H. Subsequently, an initial assessment of the c-Li15(SixGe1- x)4 phase after lithiation and the a-Si0.50Ge0.50 phase after delithiation is presented. Hence, this study contributes crucial insights into the (de)lithiation reaction mechanisms within germanane-silicane alloys. Such understanding is pivotal for mastering promising materials that amalgamate the finest properties of silicon and germanium.

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, CaGe1.5Si0.5, and CaGeSi) in a protic environment. The exfoliated Ge-H, Ge0.75Si0.25H, and Ge0.5Si0.5H 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 Hz1/2 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.

Bandgap‐Engineered Germanene Nanosheets as an Efficient Photodynamic Agent for Cancer Therapy

AbstractTwo‐dimensional (2D) monoelemental materials (Xenes) show considerable potential in bioapplications owing to their unique 2D physicochemical features and the favored biosafety resulting from their monoelemental composition. However, the narrow band gaps of Xenes prevent their broad applications in biosensors, bioimaging and phototherapeutics. In this study, it is demonstrated that 2D germanene terminated with −H via surface chemical engineering, shows a much broadened direct band gap of 1.65 eV, which enables the material to be used as a novel inorganic photosensitizer for the photodynamic therapy of singlet oxygen. Through theoretical analysis and in vitro studies, H‐germanene nanosheets demonstrate a substantially enlarged band gap and favorable biodegradability, demonstrating a substantial cancer treatment capacity. This study demonstrates the feasibility of constructing novel therapeutic photodynamic agents by surface covalent engineering for catalytic tumor therapy.

Bandgap-Engineered Germanene Nanosheets as an Efficient Photodynamic Agent for Cancer Therapy.

Two-dimensional (2D) monoelemental materials (Xenes) show considerable potential in bioapplications owing to their unique 2D physicochemical features and the favored biosafety resulting from their monoelemental composition. However, the narrow band gaps of Xenes prevent their broad applications in biosensors, bioimaging and phototherapeutics. In this study, it is demonstrated that 2D germanene terminated with -H via surface chemical engineering, shows a much broadened direct band gap of 1.65 eV, which enables the material to be used as a novel inorganic photosensitizer for the photodynamic therapy of singlet oxygen. Through theoretical analysis and in vitro studies, H-germanene nanosheets demonstrate a substantially enlarged band gap and favorable biodegradability, demonstrating a substantial cancer treatment capacity. This study demonstrates the feasibility of constructing novel therapeutic photodynamic agents by surface covalent engineering for catalytic tumor therapy.

First principles study of 2D ring-Te and its electrical contact with a topological Dirac semimetal.

In recent years, researchers have manifested their interest in two-dimensional (2D) mono-elemental materials of group-VI elements because of their excellent optoelectronic, photovoltaic and thermoelectric properties. Despite the intensive recent research efforts, there is still a possibility of novel 2D allotropes of these elements due to their multivalency nature. Here, we have predicted a novel 2D allotrope of tellurium (ring-Te) using density functional theory. Its stability is confirmed by phonon and ab initio molecular dynamics simulations. Ring-Te has an indirect band gap of 0.69 eV (1.16 eV) at the PBE (HSE06) level of theories and undergoes an indirect-direct band gap transition under tensile strain. The higher carrier mobility of holes (∼103 cm2 V-1 s-1), good UV-visible light absorption ability and low exciton binding (∼0.35 eV) of ring-Te give rise to its potential applications in optoelectronic devices. Furthermore, the electrical contact of ring-Te with a topological Dirac semimetal (sq-Te) under the influence of an electric field shows that the Schottky barriers and contact types can undergo transition from p-type to n-type Schottky contact and then to ohmic contact at a higher electric field. Our study provides an insight into the physics of designing high-performance electrical coupled devices composed of 2D semiconductors and topological semimetals.

Geometry of tellurene adsorbed on the Si(111)-R30°-Sb surface from first principles calculations.

The 2D form of tellurium, named tellurene, is one of the latest discoveries in the family of 2D mono-elemental materials. In a trilayer configuration, free-standing tellurene was predicted theoretically to acquire two crystallographic forms, the α and β phases, corresponding to either a 1T-MoS2-like geometry or a trilayer slab exposing the Te(101̄0) surface of bulk Te with helical chains lying in-plane and further reconstructed due to the formation of interchain bonds. Either one or the other of the two phases was observed experimentally to prevail depending on the substrate they were grown onto. In the perspective to integrate tellurene on silicon, we here report an ab initio study of the adsorption of tellurene on the Si(111)-R30° surface passivated by antinomy. According to the literature, this substrate is chosen for the growth of several tellurides by molecular beam epitaxy. The calculations reveal that on this substrate the adsorption energy mostly compensates the energy difference between the α and β phases in the free-standing configuration which suggests that the prevalence of one phase over the other might in this case strongly depend on the kinetics effects and deposition conditions.

Chemistry, Functionalization, and Applications of Recent Monoelemental Two-Dimensional Materials and Their Heterostructures.

The past decades have witnessed a rapid expansion in investigations of two-dimensional (2D) monoelemental materials (Xenes), which are promising materials in various fields, including applications in optoelectronic devices, biomedicine, catalysis, and energy storage. Apart from graphene and phosphorene, recently emerging 2D Xenes, specifically graphdiyne, borophene, arsenene, antimonene, bismuthene, and tellurene, have attracted considerable interest due to their unique optical, electrical, and catalytic properties, endowing them a broader range of intriguing applications. In this review, the structures and properties of these emerging Xenes are summarized based on theoretical and experimental results. The synthetic approaches for their fabrication, mainly bottom-up and top-down, are presented. Surface modification strategies are also shown. The wide applications of these emerging Xenes in nonlinear optical devices, optoelectronics, catalysis, biomedicine, and energy application are further discussed. Finally, this review concludes with an assessment of the current status, a description of existing scientific and application challenges, and a discussion of possible directions to advance this fertile field.

Few-layer antimonene electrical properties

Antimonene -a single layer of antimony atoms- and its few layer forms are among the latest additions to the 2D mono-elemental materials family. Numerous predictions and experimental evidence of its remarkable properties including (opto)electronic, energetic or biomedical, among others, together with its robustness under ambient conditions, have attracted the attention of the scientific community. However, experimental evidence of its electrical properties is still lacking. Here, we characterized the electronic properties of mechanically exfoliated flakes of few-layer (FL) antimonene of different thicknesses (∼ 2–40 nm) through photoemission electron microscopy, kelvin probe force microscopy and transport measurements, which allows us to estimate a sheet resistance of ∼ 1200 Ω sq−1 and a mobility of ∼ 150 cm2V−1s−1 in ambient conditions, independent of the flake thickness. Alternatively, our theoretical calculations indicate that topologically protected surface states (TPSS) should play a key role in the electronic properties of FL antimonene, which supports our experimental findings. We anticipate our work will trigger further experimental studies on TPSS in FL antimonene thanks to its simple structure and significant stability in ambient environments.

The Rise of the Xenes: From the Synthesis to the Integration Processes for Electronics and Photonics.

The recent outcomes related to the Xenes, the two-dimensional (2D) monoelemental graphene-like materials, in three interdisciplinary fields such as electronics, photonics and processing are here reviewed by focusing on peculiar growth and device integration aspects. In contrast with forerunner 2D materials such as graphene and transition metal dichalcogenides, the Xenes pose new and intriguing challenges for their synthesis and exploitation because of their artificial nature and stabilization issues. This effort is however rewarded by a fascinating and versatile scenario where the manipulation of the matter properties at the atomic scale paves the way to potential applications never reported to date. The current state-of-the-art about electronic integration of the Xenes, their optical and photonics properties, and the developed processing methodologies are summarized, whereas future challenges and critical aspects are tentatively outlined.

Gas sensing properties of defective tellurene on the nitrogen oxides: A first-principles study

As the first two dimensional (2D) mono-elemental material of group-VI, tellurene is expected to be a promising gas sensor over the state-of-the-art 2D materials. Based on the density functional theory (DFT) calculation, we mainly focused on improving the adsorption and sensing properties of the tellurene to the nitrogen oxide molecules (NOX) via defect engineering. Three point defects in the tellurene, such as Stone-Wales defect, single vacancy-1 and single vacancy-2, were extensively investigated due to their lower thermodynamic energies. Compared with the pristine tellurene, these defects can enhance the adsorption strength of NOX on the tellurene with the electronic band structures as well as the electronic transport properties (e.g, current-voltage curves, equilibrium transmission spectra) altering sensitively. The changes near the Fermi level of the density of states (DOS) are mainly attributed to the p orbit of N, O and Te atoms. This study can extend the gas sensor family for the nitrogen oxides by providing theoretic reference for the rational design in the experiments.

Tunable Electronic and Optical Properties of 2D Monoelemental Materials Beyond Graphene for Promising Applications

As a new type of two‐dimensional (2D) materials, monoelemental 2D materials have the atomic structure similar to graphene, and their excellent optical and electronic properties have potential applications in many fields. To date, many studies based on monoelemental 2D materials have been reported, and excellent performance has been demonstrated in various fields. The monoelemental 2D materials that have been reported so far are mainly distributed in the group IIIA, IVA, VA, and VIA. Because of their structural similarities to graphene, they are commonly referred to as "Xenes." Here, we have comprehensively reviewed the research progress of monoelemental 2D materials. In this review, we explore the structure, properties, and practical applications of these monoelemental 2D materials. First, the classification, structural features, optical properties, electronic characteristics, and regulating mechanism of these monoelemental 2D materials are introduced. Then, the practical application and research progress of monoelemental 2D materials in various fields are reviewed comprehensively, especially including photoelectric catalysis, solar cells, and other energy fields. This review will give readers a more all‐sided understanding of monoelemental 2D materials and have some guiding significance for their further development.

Bandgap Engineering of Hydroxy-Functionalized Borophene for Superior Photo-Electrochemical Performance.

Two-dimensional (2D) semiconducting boron nanosheets (few-layer borophene) have been theoretically predicted, but their band gap tunability has not been experimentally confirmed. In this study, hydroxy-functionalized borophene (borophene-OH) with tunable band gap was fabricated by liquid-phase exfoliation using 2-butanol solvent. Surface-energy matching between boron and 2-butanol produced smooth borophene, and the exposed unsaturated B sites generated by B-B bond breaking during exfoliation coordinated with OH groups to form semiconducting borophene-OH, enabling a tunable band gap of 0.65-2.10 eV by varying its thickness. Photoelectrochemical (PEC) measurements demonstrated that the use of borophene-OH to fabricate working electrodes for PEC-type photodetectors significantly enhanced the photocurrent density (5.0 μA cm-2 ) and photoresponsivity (58.5 μA W-1 ) compared with other 2D monoelemental materials. Thus, borophene-OH is a promising semiconductor with great optoelectronic potential.

Bandgap Engineering of Hydroxy‐Functionalized Borophene for Superior Photo‐Electrochemical Performance

AbstractTwo‐dimensional (2D) semiconducting boron nanosheets (few‐layer borophene) have been theoretically predicted, but their band gap tunability has not been experimentally confirmed. In this study, hydroxy‐functionalized borophene (borophene‐OH) with tunable band gap was fabricated by liquid‐phase exfoliation using 2‐butanol solvent. Surface‐energy matching between boron and 2‐butanol produced smooth borophene, and the exposed unsaturated B sites generated by B−B bond breaking during exfoliation coordinated with OH groups to form semiconducting borophene‐OH, enabling a tunable band gap of 0.65–2.10 eV by varying its thickness. Photoelectrochemical (PEC) measurements demonstrated that the use of borophene‐OH to fabricate working electrodes for PEC‐type photodetectors significantly enhanced the photocurrent density (5.0 μA cm−2) and photoresponsivity (58.5 μA W−1) compared with other 2D monoelemental materials. Thus, borophene‐OH is a promising semiconductor with great optoelectronic potential.

Tiny 2D silicon quantum sheets: a brain photonic nanoagent for orthotopic glioma theranostics

We report that atomically thin two-dimensional silicon quantum sheets (2D Si QSs), prepared by a scalable approach coupling chemical delithiation and cryo-assisted exfoliation, can serve as a high-performance brain photonic nanoagent for orthotopic glioma theranostics. With the lateral size of approximately 14.0 nm and thickness of about 1.6 nm, tiny Si QSs possess high mass extinction coefficient of 27.5 L g−1 cm−1 and photothermal conversion efficiency of 47.2% at 808 nm, respectively, concurrently contributing to the best photothermal performance among the reported 2D mono-elemental materials (Xenes). More importantly, Si QSs with low toxicity maintain the trade-off between stability and degradability, paving the way for practical clinical translation in consideration of both storage and action of nanoagents. In vitro Transwell filter experiment reveals that Si QSs could effectively go across the bEnd.3 cells monolayer. Upon the intravenous injection of Si QSs, orthotopic brain tumors are effectively inhibited under the precise guidance of photoacoustic imaging, and the survival lifetime of brain tumor-bearing mice is increased by two fold. Atomically thin Si QSs with strong light-harvesting capability are expected to provide an effective and robust 2D photonic nanoplatform for the management of brain diseases.

Structures, properties and application of 2D monoelemental materials (Xenes) as graphene analogues under defect engineering

• As graphene analogues, 2D monoelemental materials composed of elements from the group-IIIA to the group-VIA have rich and unique electronic and optical properties similar to and different from graphene. • As a method that is easy to be introduced experimentally, defect doping is also used to effectively regulate the electronic and optical properties of the system in the simulation calculation of materials. • Intrinsic defects as an inevitable means in the process of experiment and external defects as an artificial controllable control means, all of them have significant influence on the properties of single element two-dimensional materials, whether these systems are based on experimental or theoretical calculations. • Defective 2D Xenes materials excellent optoelectronic properties, so they have profound applications in optoelectronic devices, energy storage and conversion and catalysis. They are also expected to play positive and important role in many other fields. From the group-IIIA (borophene), to the group-IVA (silicene, germanene and stanine), as well as the group-VA (phosphorene, arsenene, antimonene and bismuthene) and even the group-VIA (selenene and tellurene nanosheets), these two-dimensional (2D) monoelemental materials are highly similar to graphene in lattice structure, and they can also be collectively referred to as Xenes. However, the unique specific atomic arrangement of these materials gives rise to a vast diversity of properties, and these single element 2D materials as graphene analogues are the most chemically tractable materials for synthetic exploration, thus attracting widespread attention and research. Intrinsic defects, as an defects inevitably introduced in the of experimental preparation, and extrinsic defects, as an artificially introduced and controllable way, both of them can effectively improve the environmental stability of single element materials and regulate the properties of Xenes. Herein, a review of the historical work about the properties and application of defective 2D monoelemental materials is presented. Based on the structure and properties of original single element 2D materials, the change of electronic and optoelectronic properties of these materials from different main groups under the action of different defects are classified and compared. In addition, the existing methods of realizing defects of 2D Xenes-based materials are summarized, and the application of the defects engineering in the field of optoelectronic devices, energy storage and conversion, catalysts as well are also comprehensively searched and reviewed. Finally, this review provides the prospect and challenge for the preparation and application of defective 2D monoelemental materials on the basis of the current research and development.

Recent progress in 2D group-V elemental monolayers: fabrications and properties

A large number of two-dimensional (2D) monoelemental materials with huge application potentials have been developed, since graphene was reported as a monoelemental material with unique properties. As cousins of graphene, 2D group-V elemental monolayers have gained tremendous interest due to their electronic properties with significant fundamental bandgap. In this review, we extensively summarize the latest theoretical and experimental progress in group-V monoelemental materials, including the latest fabrication methods, the properties and potential applications of these 2D monoelementals. We also give a perspective of the challenges and opportunities of 2D monoelemental group-V monolayer materials and related functional nanodevices.

High-performance junction field-effect transistor based on black phosphorus/β-Ga2O3 heterostructure

Black phosphorous (BP), an excellent two-dimensional (2D) monoelemental layered p-type semiconductor material with high carrier mobility and thickness-dependent tunable direct bandgap structure, has been widely applied in various devices. As the essential building blocks for modern electronic and optoelectronic devices, high quality PN junctions based on semiconductors have attracted widespread attention. Herein, we report a junction field-effect transistor (JFET) by integrating narrow-gap p-type BP and ultra-wide gap n-type β-Ga2O3 nanoflakes for the first time. BP and β-Ga2O3 form a vertical van der Waals (vdW) heterostructure by mechanically exfoliated method. The BP/β-Ga2O3 vdW heterostructure exhibits remarkable PN diode rectifying characteristics with a high rectifying ratio about 107 and a low reverse current around pA. More interestingly, by using the BP as the gate and β-Ga2O3 as the channel, the BP/β-Ga2O3 JFET devices demonstrate excellent n-channel JFET characteristics with the on/off ratio as high as 107, gate leakage current around as low as pA, maximum transconductance (gm) up to 25.3 µS and saturation drain current (IDSS) of 16.5 µA/µm. Moreover, it has a pinch-off voltage of –20 V and a minimum subthreshold swing of 260 mV/dec. These excellent n-channel JFET characteristics will expand the application of BP in future nanoelectronic devices.

Germanene-Based Theranostic Materials for Surgical Adjuvant Treatment: Inhibiting Tumor Recurrence and Wound Infection

Summary Germanene (Ge) is a newly discovered two-dimensional (2D) monoelemental material whose biomedical applications remain largely unexplored. Here, we provide the proof-of-principle evidence of a 2D Ge-based strategy for surgical adjuvant treatment. The exfoliated Ge nanosheets (NSs) exhibited high drug-loading capacity, multiresponsive (pH- and near-infrared [NIR]-sensitive) drug-release behavior, NIR-triggered deep tumor penetration, good biocompatibility, and excellent multimodal imaging-guided treatment. When combined with hydrogel (composed of clinically approved agarose and chitosan), the developed drug-loaded Ge@hydrogel was coated on the postoperative wound surface after tumor removal. Followed by NIR irradiation, the generated local hyperthermia and NIR-triggered drug release achieved immediate residual tumor-eliminating and bacteria-killing effects, while the hydrogel served as a therapeutic reservoir for long-lasting antitumor and bactericidal effects. In summary, this is the first application of 2D Ge-based surgical adjuvant treatment, potentially marking a jumping-off point for future studies on biomedical applications of Ge.

Passively Q-switched and mode-locked erbium-doped fiber lasers based on tellurene nanosheets as saturable absorber

Various two-dimensional (2D) materials show unique optical properties and excellent performance in acting as saturable absorber (SA) for demonstrating all-fiber ultra-fast lasers. Tellurene, as a new-fashioned few-layer 2D monoelemental material, was designed as an excellent saturable absorber to achieve Q-switched and mode-locked operations within erbium-doped fiber (EDF) lasers in our experiment. High-quality tellurene-based SA with a modulation depth of 0.97% was obtained by blending few-layer tellurene nanosheet solution prepared by liquid phase exfoliation method and the polyvinyl alcohol (PVA) solution. Inserting the SA into the EDF laser cavity by sandwiching the tellurene-PVA film between two fiber ferrules, either the passively Q-switched or the passively mode-locked operations can be obtained. The repetition rate varies from 15.92 to 47.61 kHz, and the pulse duration decreases from 8.915 to 5.196 µs in the passively Q-switched operation. To the best of our knowledge, this is the first demonstration focusing on the modulation application of tellurene in designing Q-switched pulsed laser operations. Additionally, mode-locked operations were also achieved by adjusting the polarization state. The obtained results fully indicate that tellurene can be developed as an efficient SA for pulsed fiber lasers.

2D tellurene/black phosphorus heterojunctions based broadband nonlinear saturable absorber

Abstract Two-dimensional (2D) mono-elemental materials (Xenes) show remarkable potential in the fields of fundamental science and technology, have been regarded as a wide range of building blocks for electronic technologies due to their unique chemical, physical, electrical, and optical properties. Here, 2D tellurene/black phosphorus (Te/BP) heterojunctions are successfully fabricated through liquid-phase exfoliation (LPE) method. Their nonlinear optical absorption properties at 1.0, 2.0, and 2.8 μm have been studied by an open-aperture Z-scan method. The results revealed the excellent broadband saturable absorption responses of the prepared BP/Te heterojunctions, which are further confirmed by using them as saturable absorbers (SAs) for passively Q-switched all-solid-state lasers operating at 1.0, 2.0, and 2.8 μm, respectively. In particular, 1.04 μm continuous-wave (CW) mode-locked lasers with a pulse width of 404 fs is realized for the first time, to our best knowledge. Our work indicates that 2D Xenes especially 2D Xenes based heterojunctions have great potential in the fields of pulsed laser generation/modulation and other optoelectronic and photonic devices.