Jiuxiang Dai

A simple, scalable strategy is attempted using a direct gas-phase fluorination technique to fluorinate hexagonal boron nitride (hBN). The nature of fluorine bonding to the hBN lattice and their chemical coordination are evaluated using various analytical techniques and theoretical models. Interestingly, the derived F-hBN displays significant improvement in its thermal and friction properties and displays a semiconducting behavior.

Abstract

Hexagonal boron nitride (hBN) has received much attention in recent years as a 2D dielectric material with potential applications ranging from catalysts to electronics. hBN is a stable covalent compound with a planar hexagonal lattice and is relatively unreactive to most chemical environments, making the chemical functionalization of hBN challenging. Here, a simple, scalable strategy to fluorinate hBN using a direct gas-phase fluorination technique is reported. The nature of fluorine bonding to the hBN lattice and their chemical coordination are described based on various characterization studies and theoretical models. The fluorine functionalized hBN shows a bandgap reduction and displays a semiconducting behavior due to the fluorination process. Additionally, the fluorinated hBN shows significant improvement in its thermal and friction properties, which could be substantial in applications such as lubricants and thermal fluids. Theory and simulations reveal that the enhanced friction properties of fluorinated hBN result from reduced inter-planar interaction energy by electrostatic repulsion of intercalated fluorine atoms between hBN layers without significant disruption of the in-plane lattice. This technique paves the way for the fluorination of several other 2D structures for various applications such as magnetism and functional nanoscale electronic devices.

Nature, Published online: 29 September 2021; doi:10.1038/s41586-021-03867-8

Extremely anisotropic thermal conductors based on large-area van der Waals thin films with random interlayer rotations are reported here.

npj 2D Materials and Applications, Published online: 30 September 2021; doi:10.1038/s41699-021-00263-8

Photoluminescence as a probe of phosphorene properties

Two new metal organic chemical vapor deposition processes for the fabrication of dense, homogenous, and phase pure cobalt and nickel ferrite thin films are developed. To this end, molecularly designed precursors are chosen and the films thoroughly analyzed. Additionally, synchrotron-based X-ray photoelectron spectroscopy is used for cation disorder analysis for the first time. The resulting films are proven to be applicable as potential electrodes in supercapacitors.

Abstract

Transition metal ferrites, such as CoFe₂O₄ (CFO) and NiFe₂O₄ (NFO), have gained increasing attention as potential materials for supercapacitors. Since chemical vapor deposition (CVD) offers advantages like interface quality to the underlying substrates and the possibility for coverage of 3D substrates, two CVD processes are reported for CFO and NFO. Growth rates amount to 150 to 200 nm h⁻¹ and yield uniform, dense, and phase pure spinel ferrite films according to X-ray diffraction (XRD), Raman spectroscopy, Rutherford backscattering spectrometry and nuclear reaction analysis (RBS/NRA) and scanning electron microscopy (SEM). Atom probe tomography (APT) and synchrotron X-ray photoelectron spectroscopy (XPS) give insights into the vertical homogeneity and oxidation states in the CFO films. Cation disorder of CFO is analyzed for the first time from synchrotron-based XPS. NFO is analyzed via lab-based XPS. Depositions on conducting Ni and Ti substrates result in electrodes with pseudocapacitive behavior, as evidenced by cyclovoltammetry (CV) experiments. The interfacial capacitances of the electrodes are up to 185 µF cm⁻².

In addition to P-Pb coordination, there is a hydrogen bond between F⁻ and MA⁺/FA⁺ as well as an ionic bond between F⁻ and Pb²⁺ for perovskite/fluorinated black phosphorene (F-BP), thereby achieving high PCE (22.06%). Significantly, F-BP devices exhibit improved humidity and shelf-life stabilities due to the excellent ambient stability of F-BP nanosheets, resulting from antioxidation and antihydration behavior of fluorine adatoms.

Abstract

Extraordinary electronic and photonic features (e.g., tunable direct bandgap, high ambipolar carrier mobility) render few-layer black phosphorus (BP) nanosheets/quantum dots an important optoelectronic material. However, most of the BP applied in metal halide perovskite solar cells (PSCs) are produced by sonication-assisted liquid exfoliation, which inevitably brings inferior electronic properties, thus leading to limited beneficial effects. Furthermore, this study uncovers that the intrinsic instability of BP nanosheets sandwiched between (CsFAMA)Pb(BrI)₃ perovskite and spiro-OMeTAD has a deleterious effect on the performance stabilization of PSCs. To address the above constraints, a feasible strategy herein is developed by introducing high-quality fluorinated BP (F-BP) nanosheets synthesized by one-step electrochemical delamination. In addition to P-Pb coordination, there is a strong hydrogen bond between F⁻ and MA⁺/FA⁺ as well as an ionic bond between F⁻ and Pb²⁺ for the perovskite/F-BP interface, thus leading to fewer interfacial traps than perovskite/BP, which is responsible for the highest power conversion efficiency (22.06%) of F-BP devices. More importantly, F-BP devices exhibit significantly improved humidity and shelf-life stabilities due to the excellent ambient stability of F-BP, resulting from the antioxidation and antihydration behavior of fluorine adatoms. Overall, the findings provide a promising strategy to simultaneously enhance the photovoltaic performance and long-term stability of BP-based PSCs.

Highly controllable 2D resistive random access memory (RRAM) based on Bi₂Se₃ nanosheet is achieved by a damage-less ion implantation technology using ultralow-energy plasma. The transport mechanism, resistive switching mechanism, memory window, and working voltage in RRAM are successfully controlled by oxygen plasma injection. The memristors demonstrate excellent properties of high resistive switching ratio, outstanding cycling endurance, and retention performance.

Abstract

Resistive random access memory (RRAM) based on ultrathin 2D materials is considered to be a very feasible solution for future data storage and neuromorphic computing technologies. However, controllability and stability are the problems that need to be solved for practical applications. Here, by introducing a damage-less ion implantation technology using ultralow-energy plasma, the transport mechanisms of space charge limited current and Schottky emission are successfully realized and controlled in RRAM based on 2D Bi₂Se₃ nanosheets. The memristors exhibit stable resistive switching behavior with a high resistive switching ratio (>10⁴), excellent cycling endurances (300 cycles), and great retention performance (>10⁴ s). The reliability and controllability of Bi₂Se₃ memory endowed by oxygen plasma injection demonstrate the great potential of this ultralow-energy ion implantation technology in the application of 2D RRAM.

2D Mo₂CT _x MXene flakes are successfully synthesized and studied on their chemiresistive effect, demonstrating an enhanced sensitivity toward water vapor at room temperature in dry air. The low-noise resistance signal allows the detection of H₂O down to 10 ppm, with humidity suppressing the Mo₂CT _x response to organic analytes due to the blocking of adsorption active sites.

Abstract

2D transition metal carbides and nitrides (MXenes) open up novel opportunities in gas sensing with high sensitivity at room temperature. Herein, 2D Mo₂CT _x flakes with high aspect ratio are successfully synthesized. The chemiresistive effect in a sub-µm MXene multilayer for different organic vapors and humidity at 10¹–10⁴ ppm in dry air is studied. Reasonably, the low-noise resistance signal allows the detection of H₂O down to 10 ppm. Moreover, humidity suppresses the response of Mo₂CT _x to organic analytes due to the blocking of adsorption active sites. By measuring the impedance of MXene layers as a function of ac frequency in the 10⁻²–10⁶ Hz range, it is shown that operation principle of the sensor is dominated by resistance change rather than capacitance variations. The sensor transfer function allows to conclude that the Mo₂CT _x chemiresistance is mainly originating from electron transport through interflake potential barriers with heights up to 0.2 eV. Density functional theory calculations, elucidating the Mo₂C surface interaction with organic analytes and H₂O, explain the experimental data as an energy shift of the density of states under the analyte's adsorption which induces increasing electrical resistance.

How to obtain and understand in situ GIWAXS data is highlighted, and recent results of in situ GIWAXS studies on versatile perovskite photovoltaic systems that elucidate crystallization and film formation mechanisms in terms of material compositions, film deposition methods, and film treatment procedures are summarized and assessed.

Abstract

Metal halide perovskites are of fundamental interest in the research of modern thin-film optoelectronic devices, owing to their widely tunable optoelectronic properties and solution processability. To obtain high-quality perovskite films and ultimately high-performance perovskite devices, it is crucial to understand the film formation mechanisms, which, however, remains a great challenge, due to the complexity of perovskite composition, dimensionality, and processing conditions. Nevertheless, the state-of-the-art in situ grazing-incidence wide-angle X-ray scattering (GIWAXS) technique enables one to bridge the complex information with device performance by revealing the crystallization pathways during the perovskite film formation process. In this review, the authors illustrate how to obtain and understand in situ GIWAXS data, summarize and assess recent results of in situ GIWAXS studies on versatile perovskite photovoltaic systems, aiming at elucidating the distinct features and common ground of film formation mechanisms, and shedding light on future opportunities of employing in situ GIWAXS to study the fundamental working mechanisms of highly efficient and stable perovskite solar cells toward mass production.

Light: Science & Applications, Published online: 05 October 2021; doi:10.1038/s41377-021-00640-4

Research progress of full electroluminescent white light-emitting diodes based on a single emissive layer

Two-dimensional (2D) materials with puckered layer morphology are promising candidates for next-generation optoelectronics devices owing to their anisotropic response to external perturbations and wide band gap tunability with the number of layers. Among them, palladium diselenide (PdSe 2 ) is an emerging 2D transition-metal dichalcogenide, with a band gap ranging from ##IMG## [http://ej.iop.org/images/2053-1583/8/4/045036/tdmac255aieqn1.gif] {$\sim1.3$} eV in the monolayer to a predicted semimetallic behaviour in the bulk. Here, we use angle-resolved photoemission spectroscopy to explore the electronic band structure of PdSe 2 with energy and momentum resolution. Our measurements reveal the semiconducting nature of the bulk. Furthermore, constant binding-energy maps of reciprocal space display a remarkable site-specific sensitivity to the atomic arrangement and its symmetry. Supported by density functional theory calculations, we ascribe...

Two-dimensional (2D) palladium ditelluride (PdTe 2 ) and platinum ditelluride (PtTe 2 ) are two Dirac semimetals, which have fascinating quantum properties such as superconductivity, magnetism and topological order and show the promising applications in future nanoelectronics and optoelectronics. However, the synthesis of PdTe 2 and PtTe 2 monolayers (MLs) is hindered by a strong interlayer coupling and orbital hybridization. In this study, we demonstrate an efficient synthesis of PdTe 2 and PtTe 2 MLs Large-area and high quality MLs of PdTe 2 and patterned PtTe 2 were epitaxially grown on the Pd(111) and Pt(111) surfaces by direct telurization in ultra-high vacuum. This was confirmed by x-ray photoelectron spectroscopy, low energy electron diffraction and scanning tunnelling microscopy. PdTe 2 ML demonstrated high thermal stability showing no decomposition sign after annealing at 470 °C. A w...

We study photoluminescence (PL) of MoS 2 monolayers in optical cavities that can be tuned in operando. Technically, we use the recently developed squeezable nanojunction (SNJ). It is a versatile mechanical setup that has been useful to study thermoelectric effects at electronic tunneling distances. Here, we emphasize on a cavity with 0–3 micrometer distance with optical access. Due to the tunable cavity, we see strong distortions of PL spectra. By an analysis of the ensemble, we identify a normalization protocol that gives access to disentangling the contributions from excitation, gating and emission. The systematic evolution of data reconfirms the drastic influence of the local electromagnetic mode budget on the spectral properties. The experiment further underscores the broadband application range of the SNJ technique, able for combining (nano-) electronic functionality with optical access and a tunable light-matter interface.

THz modulators based on 2D materials are attracting attention due to their potential applications in THz communication, sensing, and biomedical diagnostics. The authors present recent progress on 2D materials with different photonic structures, fundamental of THz modulation, and THz operation principles. This work provides useful guidance for the design strategy of 2D materials based THz modulators. (DOI: 10.1002/inf2.12236)

THz modulators based on 2D materials are attracting attention due to their potential applications in THz communication, sensing, and biomedical diagnostics. The authors present recent progress on 2D materials with different photonic structures, fundamental of THz modulation, and THz operation principles. This work provides useful guidance for the design strategy of 2D materials based THz modulators. (DOI: 10.1002/inf2.12236)

Metal chlorides intercalated in the van der Waals gap of bilayer graphene are found to have polymorphic phases. New phases and phase transformation of AlCl₃, as well as new hybrid alloy of quasi-1D AlCl₃ and CuCl₂ are directly observed by scanning transmission electron microscopy.

Abstract

Unprecedented 2D metal chloride structures are grown between sheets of bilayer graphene through intercalation of metal and chlorine atoms. Numerous spatially confined 2D phases of AlCl₃ and CuCl₂ distinct from their typical bulk forms are found, and the transformations between these new phases under the electron beam are directly observed by in situ scanning transmission electron microscopy (STEM). The density functional theory calculations confirm the metastability of the atomic structures derived from the STEM experiments and provide insights into the electronic properties of the phases, which range from insulators to semimetals. Additionally, the co-intercalation of different metal chlorides is found to create completely new hybrid systems; in-plane quasi-1D AlCl₃/CuCl₂ heterostructures are obtained. The existence of polymorphic phases hints at the unique possibilities for fabricating new types of 2D materials with diverse electronic properties confined between graphene sheets.

Tunable wetting membranes of roughness hierarchy are fabricated through TiO₂ nanofibers deposited by plasma-assisted methodologies going from superhydrophobic/oleophilic behavior to superomniphilic one under UV illumination. Surface functionalization of nanostructured surfaces provides a superomniphobic grade stable against aging and external factors. Membrane modification by this plasma-assisted protocol outputs from microfilter to microfluidic applications, extendable to other functionalities.

Abstract

Fabrication of tunable wetting surfaces is sought for the last years given its importance on energy, biomaterials and antimicrobials, water purification, microfluidics, and smart surfaces. Liquid management on surfaces mainly depends on the control at the micro- and nanoscale of both roughness and chemical composition. Herein, the combination of a soft-template method and plasma-enhanced chemical vapor deposition is presented for the synthesis of TiO₂ nanofibers on porous substrates such as cellulose and stainless-steel membranes. The protocol, carried out under mild conditions, produces 3D nanomembranes with superhydrophobicity and oleophilicity that are tested as microliter water/oil filters. Photoactivation of TiO₂ by UV illumination provides a straightforward approach for wetting tunability that converts the surface into amphiphilic. A final chemical modification of the TiO₂ nanofibers by embedding them in an elastomeric polymeric shell and by fluorine-based grafting opens the path toward the formation of superomniphobic and self-cleaning surfaces with long-lasting lifetimes. Thus, a reliable procedure is demonstrated for the fabrication of TiO₂ nanofibers, which allows the modification of porous supports and provides an innovative route for the development of 3D nanomembranes with under design wetting. This protocol is extendable to alternative metal oxides, metals, and core@shell nanoarchitectures with potential multifunctionalities.

Nature Chemistry, Published online: 07 October 2021; doi:10.1038/s41557-021-00795-y

Spin-crossover nanoparticles have been covalently grafted onto a semiconducting MoS2 layer to form a self-strainable heterostructure. Under light or thermal stimulus, the nanoparticles switch between their high- and low-spin states, in which they have different volumes. This generates a reversible strain over the MoS2 layer and, in turn, alters the electrical and optical properties of the heterostructure.