Jiuxiang Dai

Out-of-plane chemical order in layered T2 phase Ti₄MoSiB₂ (o-MAB) is theoretically predicted and experimentally verified. Chemical exfoliation of the compound is shown by derivation of a 2D material, TiO _x Cl _y . The 2D sheets have a direct bandgap of ≈4.1 eV and display characteristics suitable for supercapacitor applications.

Abstract

Exploratory theoretical predictions in uncharted structural and compositional space are integral to materials discoveries. Inspired by M ₅SiB₂ (T2) phases, the finding of a family of laminated quaternary metal borides, M′₄ M″SiB₂, with out-of-plane chemical order is reported here. 11 chemically ordered phases as well as 40 solid solutions, introducing four elements previously not observed in these borides are predicted. The predictions are experimentally verified for Ti₄MoSiB₂, establishing Ti as part of the T2 boride compositional space. Chemical exfoliation of Ti₄MoSiB₂ and select removal of Si and MoB₂ sub-layers is validated by derivation of a 2D material, TiO _x Cl _y , of high yield and in the form of delaminated sheets. These sheets have an experimentally determined direct band gap of ≈4.1 eV, and display characteristics suitable for supercapacitor applications. The results take the concept of chemical exfoliation beyond currently available 2D materials, and expands the envelope of 3D and 2D candidates, and their applications.

Plastically bendable crystals hold untapped potential for all-organic smart photonics and optoelectronics. By using micromanipulation, the building of crystal-based mechanically complaint optical microcircuits is showcased.

Abstract

With the rising concerns about global cybersecurity, safe data transduction that would be impervious to cyber attacks necessitates an immediate shift from electron-based to light-based devices. Here, the construction of silicon-free, all-organic photonic integrated circuits by micromanipulation of organic crystals of two mechanically different materials with complementary optical properties, one of which is plastically bendable, is described. The resulting optical circuits are endowed with mechanical reconfiguration at two levels: first, the individual components can be processed into arbitrary shape before integration into the circuit, and second, the circuit itself is reconfigurable even after it is fabricated. The results do not only demonstrate the infinite structural variations in optical microstructures one can build by using organic crystals, but they also show that deformable light-transducive organic crystals carry an untapped potential for lightweight all-organic optical minicircuitry.

Strong interlayer transition in few layers of InSe/PdSe₂ heterojunction is presented. Interlayer transition at the interface is certified both theoretically (density functional theory) and experimentally (photoluminance spectroscopy and Kelvin probe force microscopy). Owing to the strong interlayer transition, the heterojunction shows a wide spectrum response (532 to 1650 nm) with ultra-high photoresponsivity 58.8 A W⁻¹ at 1650 nm.

Abstract

Near infrared (NIR) photodetectors based on 2D materials are widely studied for their potential application in next generation sensing, thermal imaging, and optical communication. Construction of van der Waals (vdWs) heterostructure provides a tremendous degree of freedom to combine and extend the features of 2D materials, opening up new functionalities on photonic and optoelectronic devices. Herein, a type-II InSe/PdSe₂ vdWs heterostructure with strong interlayer transition for NIR photodetection is demonstrated. Strong interlayer transition between InSe and PdSe₂ is predicted via density functional theory calculation and confirmed by photoluminance spectroscopy and Kelvin probe force microscopy. The heterostructure exhibits highly sensitive photodetection in NIR region up to 1650 nm. The photoresponsivity, detectivity, and external quantum efficiency at this wavelength respectively reaches up to 58.8 A W⁻¹, 1 × 10¹⁰ Jones, and 4660%. The results suggest that the construction of vdWs heterostructure with strong interlayer transition is a promising strategy for infrared photodetection, and this work paves the way to developing high-performance optoelectronic devices based on 2D vdWs heterostructures.

Carbon is one of the most fascinating elements due to its structurally diverse allotropic forms stemming from its bonding varieties (sp, sp², and sp³). Exploring new forms of carbon has always been the eternal theme of scientific research. Herein, we report the amorphous (AM) carbon materials with high fraction of sp³ bonding recovered from compression of fullerene C₆₀ under high pressure and high temperature previously unexplored. Analysis of photoluminescence and absorption spectra demonstrates that they are semiconducting with a bandgap range of 1.5–2.2 eV, comparable to that of widely used amorphous silicon. Comprehensive mechanical tests demonstrate that the synthesized AM-III carbon is the hardest and strongest amorphous material known so far, which can scratch diamond crystal and approach its strength. The produced AM carbon materials combine outstanding mechanical and electronic properties, and may potentially be used in photovoltaic applications that require ultrahigh strength and wear resistance.

Nature Materials, Published online: 05 August 2021; doi:10.1038/s41563-021-01058-4

MoS2/graphite and MoS2/h-BN interfaces are shown to have ultra-low friction coefficients, whereas edges and interface steps mainly contribute to the friction force.

A deep neural network based generative model for high-throughput generation of cubic materials is proposed. By creating 20 million virtual materials and applying machine learning and DFT based fast screenings, their model has not only rediscovered most of known cubic materials, but generated large number of new cubic structures with new prototypes: 506 new-prototype stable materials have been verified computationally.

Abstract

High-throughput screening has become one of the major strategies for the discovery of novel functional materials. However, its effectiveness is severely limited by the lack of sufficient and diverse materials in current materials repositories such as the open quantum materials database (OQMD). Recent progress in deep learning have enabled generative strategies that learn implicit chemical rules for creating hypothetical materials with new compositions and structures. However, current materials generative models have difficulty in generating structurally diverse, chemically valid, and stable materials. Here we propose CubicGAN, a generative adversarial network (GAN) based deep neural network model for large scale generative design of novel cubic materials. When trained on 375 749 ternary materials from the OQMD database, the authors show that the model is able to not only rediscover most of the currently known cubic materials but also generate hypothetical materials of new structure prototypes. A total of 506 such materials have been verified by phonon dispersion calculation. Considering the importance of cubic materials in wide applications such as solar panels, the GAN model provides a promising approach to significantly expand existing materials repositories, enabling the discovery of new functional materials via screening. The new crystal structures discovered are freely accessible at www.carolinamatdb.org.

Measured resonance Raman optical activity (RROA) often did not agree with theoretical predictions. We uncover a new form of chiral Raman spectroscopy, eCP-Raman, that explains some of these inconsistencies. This combination of electronic circular dichroism and circularly polarized Raman has also immense potential as a sensitive technique for applications in chemistry or biology.

Abstract

Resonance Raman optical activity (RROA) is commonly measured as the difference in intensity of Raman scattered right and left circularly polarized light, I_R−I_L, when a randomly polarized light is in resonance with a chiral molecule. Strong and sometimes mono-signate experimental RROA spectra of several chiral solutes were reported previously, although their signs and relative intensities could not be reproduced theoretically. By examining multiple light-matter interaction events which can occur simultaneously under resonance, we show that a new form of chiral Raman spectroscopy, eCP-Raman, a combination of electronic circular dichroism and circularly polarized Raman, prevails. By incorporating the finite-lifetime approach for resonance, the experimental patterns of the model chiral solutes are captured theoretically by eCP-Raman, without any RROA contribution. The results open opportunity for applications of eCP-Raman spectroscopy and for extracting true RROA experimentally.

Nanoengineered Heterostructures

In article number 2100503, Zhinan Guo, Han Zhang, Paras N. Prasad, and co-workers report a nanoengineered heterostructure for the first time demonstration of an nanointerface using an inserted graphene layer between black phosphorus (BP) and InSe which inhibits interlayer recombination and greatly improved photodetection performances.

In article number 2101170, Bilu Liu, Hui-Ming Cheng, and co-workers synthesize atomically thin nanoribbons to exploit the property of materials with the physical limits of both thickness and width. The electronic and optoelectronic measurements of nanoribbons possess decent performance in terms of mobility, on/off ratio, and photoresponsivity, suggesting a platform to study the properties and applications of such nanoribbon materials at thickness limits.

In article number 2102006, Jason D. Slinker and co-workers leverage CsPbBr_3−xCl_x perovskites, polyelectrolytes, and a salt additive to demonstrate pure blue emission from single-layer light-emitting electrochemical cells. The electrolytes transport the ions from salt additives, enhancing charge injection and stabilizing the inherent perovskite emissive lattice for highly pure and sustained blue emission. The resulting pure blue emission meets the US NTSC blue standard benchmark.

Porous graphene is synthesized starting from oxo-functionalized graphene, a type of graphene oxide with few lattice defects. A certain control over pore diameters is achieved and the average pore diameter can be adjusted by etching time. Diameters of hundreds of nanometers can be realized, opening up membrane applications.

Abstract

Wet-chemical generation of pores in graphene is a challenging synthetic task. Although graphene oxide is available in large quantities and chemically diverse, extended lattice defects already present from synthesis hamper the controlled growth of pores. However, membrane, energy, or nanoelectronic applications essentially require uniform pores in applications. Here, oxo-functionalized graphene (oxoG), a type of graphene oxide with a controlled density of vacancy defects, is used as starting material. Pores in graphene are generated from potassium permanganate treated oxoG and heating from room temperature to 400 °C. With etching time, the size of pores increases and pore-diameters of, for example, 100–200 nm in majority become accessible. The experiments are conducted on the single-layer level on Si/SiO₂ wafers. Flakes remain stable on the µm scale and do not fold. The process leads to rims of pores, which are functionalized by carbonyl groups in addition to hydroxyl and carboxyl groups. In addition, it is found that heterostructures with intrinsically n-doped MoS₂ can be fabricated and photoluminescence (PL) measurements reveal a 10-fold increased PL. Thus, graphene with pores is a novel highly temperature-stable electron-accepting 2D material to be integrated into van der Waals heterostructures.

npj 2D Materials and Applications, Published online: 04 August 2021; doi:10.1038/s41699-021-00251-y

Efficient Ohmic contacts and built-in atomic sublayer protection in MoSi₂N₄ and WSi₂N₄ monolayers

A 1D van der Waals Ta₂Ni₃Se₈ is synthesized by controlling the reaction composition and temperature. After obtaining high-quality bulk crystals, a few nanometer-scale Ta₂Ni₃Se₈ nanowire is isolated by mechanical exfoliation to be used in the fabrication of field-effect transistor. The Ta₂Ni₃Se₈ field-effect transistor (FET) exhibits ambipolar response with mobilities of 20.3 and 3.52 cm² V^-1 s^-1 for electrons and holes, respectively.

Abstract

In this study, high-purity and centimeter-scale bulk Ta₂Ni₃Se₈ crystals are obtained by controlling the growth temperature and stoichiometric ratio between tantalum, nickel, and selenium. It is demonstrated that the bulk Ta₂Ni₃Se₈ crystals could be effectively exfoliated into a few chain-scale nanowires through simple mechanical exfoliation and liquid-phase exfoliation. Also, the calculation of electronic band structures confirms that Ta₂Ni₃Se₈ is a semiconducting material with a small bandgap. A field-effect transistor is successfully fabricated on the mechanically exfoliated Ta₂Ni₃Se₈ nanowires. Transport measurements at room temperature reveal that Ta₂Ni₃Se₈ nanowires exhibit ambipolar semiconducting behavior with maximum mobilities of 20.3 and 3.52 cm² V⁻¹ s⁻¹ for electrons and holes, respectively. The temperature-dependent transport measurement (from 90 to 295 K) confirms the carrier transport mechanism of Ta₂Ni₃Se₈ nanowires. Based on these characteristics, the obtained 1D vdW material is expected to be a potential candidate for additional 1D materials as channel materials.

The channel current measured in the PtSe₂ field-effect transistor under switching light shows positive photoconductivity at low pressure that converts into negative photoconductivity at atmospheric pressure. Experimental observations and density functional theory calculations demonstrate that such behavior is caused by light-induced oxygen desorption.

Abstract

Platinum diselenide (PtSe₂) field-effect transistors with ultrathin channel regions exhibit p-type electrical conductivity that is sensitive to temperature and environmental pressure. Exposure to a supercontinuum white light source reveals that positive and negative photoconductivity coexists in the same device. The dominance of one type of photoconductivity over the other is controlled by environmental pressure. Indeed, positive photoconductivity observed in high vacuum converts to negative photoconductivity when the pressure is raised. Density functional theory calculations confirm that physisorbed oxygen molecules on the PtSe₂ surface act as acceptors. The desorption of oxygen molecules from the surface, caused by light irradiation, leads to decreased carrier concentration in the channel conductivity. The understanding of the charge transfer occurring between the physisorbed oxygen molecules and the PtSe₂ film provides an effective route for modulating the density of carriers and the optical properties of the material.