Jiuxiang Dai

A wet chemistry vitrification method is proposed to controllably transform semimetallic gray arsenene nanoflakes into semiconducting vitreous arsenene nanoflakes. Experimental studies and theoretical simulations reveal that the vitrification is attributed to the consumption of arsenic atoms by aqueous HF solution triggered by dissolved oxygen, forming atomic structure disorderliness and arsenic atom defects/vacancies.

Abstract

To manipulate the electrical and optical properties of 2D materials via engineering their phases and crystallinity is of great significance for the construction of nanodevices with versatile functions. Herein, the controllable transformation of semimetallic gray arsenene nanoflakes into semiconducting vitreous arsenene nanoflakes via a wet chemistry vitrification method is reported. Experimental studies and theoretical simulations reveal that the vitrification of gray arsenene nanoflakes is attributed to the consumption of arsenic atoms by aqueous HF via the trigger of dissolved oxygen, resulting in a significant variation of band structure rendered by the formation of atomic structure disorderliness and arsenic atom defects/vacancies. Unlike the semimetallic features of pristine gray arsenene nanoflakes, the as-prepared vitreous arsenene nanoflakes exhibit a strong photoluminescence peak centered at 635 nm corresponding to an optical band gap of 1.95 eV, and the field-effect transistors based on vitreous arsenene nanoflakes also exhibit definitely p-type semiconducting characteristics with a carrier mobility of ≈159.1 cm² V⁻¹ s⁻¹. The wet chemistry induced vitrification of gray arsenene nanoflakes presents an efficient strategy to regulate the electrical and optical properties of arsenene nanoflakes, providing new insights for the interface and band structure engineering of 2D nanomaterials.

h-BN is one of the most promising inorganic materials of this century, with possible applications ranging from aerospace to medicine. It has emerged as an exotic 2D material in the post-graphene era, owing to its exciting optoelectrical properties combined with mechanical robustness, thermal stability, and chemical inertness. An encyclopedic view of the structure, properties, synthesis, and applications of h-BN is provided.

Abstract

Hexagonal boron nitride (h-BN) has emerged as a strong candidate for two-dimensional (2D) material owing to its exciting optoelectrical properties combined with mechanical robustness, thermal stability, and chemical inertness. Super-thin h-BN layers have gained significant attention from the scientific community for many applications, including nanoelectronics, photonics, biomedical, anti-corrosion, and catalysis, among others. This review provides a systematic elaboration of the structural, electrical, mechanical, optical, and thermal properties of h-BN followed by a comprehensive account of state-of-the-art synthesis strategies for 2D h-BN, including chemical exfoliation, chemical, and physical vapor deposition, and other methods that have been successfully developed in recent years. It further elaborates a wide variety of processing routes developed for doping, substitution, functionalization, and combination with other materials to form heterostructures. Based on the extraordinary properties and thermal-mechanical-chemical stability of 2D h-BN, various potential applications of these structures are described.

This review provides a comparative and balanced discussion on various perspectives of graphene quantum dots (GQDs). First, the most exciting properties, synthesis and modification strategies, analytical characterizations, and applications of GQDs are presented. Then, the current challenges and future prospects of these emerging carbon-based nanomaterials are highlighted.

Abstract

Graphene quantum dot (GQD) is one of the youngest superstars of the carbon family. Since its emergence in 2008, GQD has attracted a great deal of attention due to its unique optoelectrical properties. Non-zero bandgap, the ability to accommodate functional groups and dopants, excellent dispersibility, highly tunable properties, and biocompatibility are among the most important characteristics of GQDs. To date, GQDs have displayed significant momentum in numerous fields such as energy devices, catalysis, sensing, photodynamic and photothermal therapy, drug delivery, and bioimaging. As this field is rapidly evolving, there is a strong need to identify the emerging challenges of GQDs in recent advances, mainly because some novel applications and numerous innovations on the ease of synthesis of GQDs are not systematically reviewed in earlier studies. This feature article provides a comparative and balanced discussion of recent advances in synthesis, properties, and applications of GQDs. Besides, current challenges and future prospects of these emerging carbon-based nanomaterials are also highlighted. The outlook provided in this review points out that the future of GQD research is boundless, particularly if upcoming studies focus on the ease of purification and eco-friendly synthesis along with improving the photoluminescence quantum yield and production yield of GQDs.

Author(s): H. Oike, K. Takeda, M. Kamitani, Y. Tokura, and F. Kagawa

We report complex behaviors in the phase evolution of transition-metal dichalcogenide IrTe2 thin flakes, captured with real-space observations using scanning Raman microscopy. The phase transition progresses via growth of a small number of domains, which is unlikely in statistical models that assume...

[Phys. Rev. Lett. 127, 145701] Published Mon Sep 27, 2021

Stable doping by modulating the thickness is realized in 2D layered materials. The decreasing thickness-induced lattice deformation makes defects in PtSSe transit from Pt vacancies in thicker PtSSe to anion vacancies in thinner PtSSe, which leads to controllable doping. Thickness-modulated doping shows great potential in novel electronics and optoelectronics, especially including diodes and photodetectors.

Abstract

For each generation of semiconductors, the issue of doping techniques is always placed at the top of the priority list since it determines whether a material can be used in the electronic and optoelectronic industry or not. When it comes to 2D materials, significant challenges have been found in controllably doping 2D semiconductors into p- or n-type, let alone developing a continuous control of this process. Here, a unique self-modulated doping characteristic in 2D layered materials such as PtSSe, PtS_0.8Se_1.2, PdSe₂, and WSe₂ is reported. The varying number of vertically stacked-monolayers is the critical factor for controllably tuning the same material from p-type to intrinsic, and to n-type doping. Importantly, it is found that the thickness-induced lattice deformation makes defects in PtSSe transit from Pt vacancies to anion vacancies based on dynamic and thermodynamic analyses, which leads to p- and n-type conductance, respectively. By thickness-modulated doping, WSe₂ diode exhibits a high rectification ratio of 4400 and a large open-circuit voltage of 0.38 V. Meanwhile, the PtSSe detector overcomes the shortcoming of large dark-current in narrow-bandgap optoelectronic devices. All these findings provide a brand-new perspective for fundamental scientific studies and applications.

This work for the first time provides direct observation of ultrafast exciton dynamic in mono-, bi-, and tri-layer platinum diselenide single crystals, a rising 2D material star. It is found that photoinduced modulation of excitons and their thickness dependence dominate overall ultrafast transient spectra in the broadband from visible to the near-infrared edge, confirmed by theoretical calculations.

Abstract

Strongly bound excitons are a characteristic hallmark of 2D semiconductors, enabling unique light–matter interactions and novel optical applications. Platinum diselenide (PtSe₂) is an emerging 2D material with outstanding optical and electrical properties and excellent air stability. Bulk PtSe₂ is a semimetal, but its atomically thin form shows a semiconducting phase with the appearance of a band-gap, making one expect strongly bound 2D excitons. However, the excitons in PtSe₂ have been barely studied, either experimentally or theoretically. Here, the authors directly observe and theoretically confirm excitons and their ultrafast dynamics in mono-, bi-, and tri-layer PtSe₂ single crystals. Steady-state optical microscopy reveals exciton absorption resonances and their thickness dependence, confirmed by first-principles calculations. Ultrafast transient absorption microscopy finds that the exciton dominates the transient broadband response, resulting from strong exciton bleaching and renormalized band-gap-induced exciton shifting. The overall transient spectrum redshifts with increasing thickness as the shrinking band-gap redshifts the exciton resonance. This study provides novel insights into exciton photophysics in platinum dichalcogenides.

High-entropy hydrotalcites have been obtained which exhibit superior electrocatalytic performance of oxygen evolution reaction due to the combination of both the dimensional confinement and the high-entropy effects. This study has opened up a dictionary of 2D high-entropy material group and provided a versatile and flexible material platform for a wild range of fields.

Abstract

High-entropy materials (HEMs) with unique configuration and physicochemical properties have attracted intensive research interest. However, 2D HEMs have not been reported yet. To find out unique properties of combining 2D materials and HEMs, a series of 2D high-entropy hydrotalcites (HEHs) is created by coprecipitation method, including quinary, septenary, and even novenary metallic elements. It is found that the fast synthetic kinetics of coprecipitation process conquers the thermodynamically solubility limitation of different elements, which is the prerequisite condition to form HEHs. As the oxygen evolution reaction (OER) electrocatalysts, HEHs show significantly decreased apparent activation energy compared with low-entropy hydrotalcites (LEHs) due to the lattice distortion induced by the multimetallic character of HEHs. This work opens up a new avenue for the development of 2D HEMs, which broadens the family of HEMs and presents a most promising platform for exploring the unknown properties of HEMs.

2D GaN crystals grown on liquid metals exhibit excellent surface-enhanced Raman scattering (SERS) performance both in signal sensitivity, reproducibility and stability. Strong dipole–dipole interaction between 2D GaN and probe molecule, abundant density of states near the Fermi level of 2D GaN and increased charge transfer efficiency give rise to enhanced SERS sensitivity with high signal reproducibility.

Abstract

Surface-enhanced Raman scattering (SERS) based on 2D semiconductors has been rapidly developed due to their chemical stability and molecule-specific SERS activity. High signal reproducibility is urgently required towards practical SERS applications. 2D gallium nitride (GaN) with highly polar Ga–N bonds enables strong dipole–dipole interactions with the probe molecules, and abundant DOS (density of states) near its Fermi level increases the intermolecular charge transfer probability, making it a suitable SERS substrate. Herein, 2D micrometer-sized GaN crystals are demonstrated to be sensitive SERS platforms with excellent signal reproducibility and stability. Strong dipole–dipole interaction between the dye molecule and 2D GaN enhances the molecular polarizability. Furthermore, 2D GaN benefits its SERS enhancement by the combination of increased DOS and more efficient charge transfer resonances when compared with its bulk counterpart.

h-BN is one of the most promising inorganic materials of this century, with possible applications ranging from aerospace to medicine. It has emerged as an exotic 2D material in the post-graphene era, owing to its exciting optoelectrical properties combined with mechanical robustness, thermal stability, and chemical inertness. An encyclopedic view of the structure, properties, synthesis, and applications of h-BN is provided.

Abstract

Hexagonal boron nitride (h-BN) has emerged as a strong candidate for two-dimensional (2D) material owing to its exciting optoelectrical properties combined with mechanical robustness, thermal stability, and chemical inertness. Super-thin h-BN layers have gained significant attention from the scientific community for many applications, including nanoelectronics, photonics, biomedical, anti-corrosion, and catalysis, among others. This review provides a systematic elaboration of the structural, electrical, mechanical, optical, and thermal properties of h-BN followed by a comprehensive account of state-of-the-art synthesis strategies for 2D h-BN, including chemical exfoliation, chemical, and physical vapor deposition, and other methods that have been successfully developed in recent years. It further elaborates a wide variety of processing routes developed for doping, substitution, functionalization, and combination with other materials to form heterostructures. Based on the extraordinary properties and thermal-mechanical-chemical stability of 2D h-BN, various potential applications of these structures are described.

3D graphene (3DG) is repeatedly crushed and reshaped to upgrade its overall performance like “Phoenix Nirvana.” 3DG is reborn to a higher self as NvG, making it emerge with unprecedented advantages in areas such as capacitive deionization and becoming a new engine to initiate the development of innovative 3D structures for carbon materials.

Abstract

The constructing of 3D materials with optimal performance is urgently needed to meet the growing demand of advanced materials in the high-tech sector. A distinctive 3D graphene (3DG) is designed based on a repeated rebirth strategy to obtain a better body and performance after each round of rebirth, as if it is Phoenix Nirvana. The properties of reborn graphene, namely 3DG after Nirvana (NvG), has been dramatically upgraded compared to 3DG, including high density (3.36 times) together with high porosity, as well as better electrical conductivity (1.41 times), mechanical strength (32.4 times), and ultrafast infiltration behavior. These advantages of NvG make it a strong intrinsic motivation for application in capacitive deionization (CDI). Using NvG directly as the CDI electrode, it has an extremely high volumetric capacity of 220 F cm⁻³ at 1 A cm⁻³ and a maximum salt absorption capacity of 8.02~9.2 mg cm⁻³ (8.9–10.2 times), while the power consumption for adsorption of the same mass of salt is less than a quarter of 3DG. The “Phoenix Nirvana”-like strategy of manufacturing 3D structures will undoubtedly become the new engine to kick-start the development of innovative carbon materials through an overall performance upgrade.

npj 2D Materials and Applications, Published online: 24 September 2021; doi:10.1038/s41699-021-00261-w

Author(s): Alex Smolyanitsky and Binquan Luan

Nanopores in 2D materials are highly desirable for DNA sequencing, yet achieving single-stranded DNA (ssDNA) transport through them is challenging. Using density functional theory calculations and molecular dynamics simulations we show that ssDNA transport through a pore in monolayer hexagonal boron...

[Phys. Rev. Lett. 127, 138103] Published Fri Sep 24, 2021

A molten-salt-assisted crystallization (MSAC) strategy is developed to improve the grain growth of all-inorganic perovskite films. MSAC enables more active mass transfer and interaction among precursor colloids. Devices based on the MSAC strategy show much increased efficiency to as high as 19.83% with open-circuit voltage as high as 1.2 V.

Abstract

Dynamic manipulation of crystallization is pivotal to the quality of polycrystalline films. A molten-salt-assisted crystallization (MSAC) strategy is presented to improve grain growth of the all-inorganic perovskite films. Compared with the traditional solvent annealing, MSAC enables more intensive mass transfer by means of convection and diffusion, which is beneficial to the interaction among the precursor colloids and to inducing in-plane growth of perovskite grains, resulting in the formation of high-quality perovskite films with suppressed pinhole and crack formation. Additionally, the introduction of molten salt alters the intermediate phases, and thus changes the crystallization pathways by reducing the energy barrier to produce films with desired optical and electrical properties. As a result, the MSAC strategy endows the devices with champion steady-state output efficiency of 19.83% and open-circuit voltage (V _oc) as high as 1.2 V, among the highest for this type of solar cell, thanks to its effectively reduced V _oc deficit.

Nature Communications, Published online: 24 September 2021; doi:10.1038/s41467-021-25941-5

Nature Communications, Published online: 24 September 2021; doi:10.1038/s41467-021-25922-8

Author(s): C. Elias, G. Fugallo, P. Valvin, C. L’Henoret, J. Li, J. H. Edgar, F. Sottile, M. Lazzeri, A. Ouerghi, B. Gil, and G. Cassabois

Dispersionless energy bands in k space are a peculiar property gathering increasing attention for the emergence of novel electronic, magnetic, and photonic properties. Here, we explore the impact of electronic flat bands on the light-matter interaction. The van der Waals interaction between the atom...

[Phys. Rev. Lett. 127, 137401] Published Thu Sep 23, 2021