Xianghong Niu

An organic-inorganic perovskite ferroelectric with large piezoelectric response

Finding a more flexible mechanical sensor Piezoelectric materials allow conversion between electricity and mechanical stresses. The most efficient piezoelectric materials are ceramics such as BaTiO3 or PbZrO3, which are also extremely stiff. You et al. identified an organic perovskite structured piezoelectric material that is far more pliable yet has a piezoelectric response similar to that of traditional ceramics. This material may be a better option to use as a mechanical sensor for flexible devices, soft robotics, biomedical devices, and other micromechanical applications that benefit from a less stiff piezoelectric material. Science, this issue p. 306 Trimethylchloromethyl ammonium trichloromanganese(II) may be a flexible material competitive for piezoelectric applications. Molecular piezoelectrics are highly desirable for their easy and environment-friendly processing, light weight, low processing temperature, and mechanical flexibility. However, although 136 years have passed since the discovery in 1880 of the piezoelectric effect, molecular piezoelectrics with a piezoelectric coefficient d33 comparable with piezoceramics such as barium titanate (BTO; ~190 picocoulombs per newton) have not been found. We show that trimethylchloromethyl ammonium trichloromanganese(II), an organic-inorganic perovskite ferroelectric crystal processed from aqueous solution, has a large d33 of 185 picocoulombs per newton and a high phase-transition temperature of 406 kelvin (K) (16 K above that of BTO). This makes it a competitive candidate for medical, micromechanical, and biomechanical applications.

Anomalous Size Dependence of Optical Properties in Black Phosphorus Quantum Dots

Understanding electron transitions in black phosphorus nanostructures plays a crucial role in applications in electronics and optoelectronics. In this work, by employing time-dependent density functional theory calculations, we systematically study the size-dependent electronic, optical absorption, and emission properties of black phosphorus quantum dots (BPQDs). Both the electronic gap and the absorption gap follow an inversely proportional law to the diameter of BPQDs in conformity to the quantum confinement effect. In contrast, the emission gap exhibits anomalous size dependence in the range of 0.8-1.8 nm, which is blue-shifted with the increase of size. The anomaly in fact arises from the structure distortion induced by the excited-state relaxation, and it leads to a huge Stokes shift in small BPQDs.

Greatly Enhanced Optical Absorption of a Defective MoS2 Monolayer through Oxygen Passivation

Structural defects in the molybdenum disulfide (MoS2) monolayer are widely reported and greatly degrade the transport and photoluminescence. However, how they influence the optical absorption properties remains unclear. In this work, by employing many-body perturbation theory calculations, we investigate the influence of sulfur vacancies (SVs), the main type of intrinsic defects in the MoS2 monolayer, on the optical absorption and exciton effect. Our calculations reveal that the presence of SVs creates localized midgap states in the bandgap, which results in a dramatic red-shift of the absorption peak and stronger absorbance in the visible light and near-infrared region. Nevertheless, the SVs can be finely repaired by oxygen passivation and are beneficial to the formation of the stable localized excitons, which greatly enhance the optical absorption in the spectral range. The defect-mediated/-engineered absorption mechanism is well understood, which offers insightful guides for improving the performance of two-dimensional dichalcogenide-based optoelectronic devices.

Revealing the underlying absorption and emission mechanism of nitrogen doped graphene quantum dots

Nitrogen-doped graphene quantum dots (N-GQDs) hold promising application in electronics and optoelectronics because of their excellent photo-stability, tunable photoluminescence and high quantum yield. However, the absorption and emission mechanisms have been debated for years. Here, by employing time-dependent density functional theory, we demonstrate that the different N-doping types and positions give rise to different absorption and emission behaviors, which successfully addresses the inconsistency observed in different experiments. Specifically, center doping creates mid-states, rendering non-fluorescence, while edge N-doping modulates the energy levels of excited states and increases the radiation transition probability, thus enhancing fluorescence strength. More importantly, the even hybridization of frontier orbitals between edge N atoms and GQDs leads to a blue-shift of both absorption and emission spectra, while the uneven hybridization of frontier orbitals induces a red-shift. Solvent effects on N-GQDs are further explored by the conductor-like screening model and it is found that strong polarity of the solvent can cause a red-shift and enhance the intensity of both absorption and emission spectra.

Photoabsorption Tolerance of Intrinsic Point Defects and Oxidation in Black Phosphorus Quantum Dots

Black phosphorus quantum dots (BPQDs) exhibit excellent optical and photothermal properties and promising applications in optoelectronics and biomedicine. However, various intrinsic structural defects and oxidation are nearly unavoidable in preparation of BPQDs and how they affect the electronic and optical properties remains unclear. Here, by employing time-dependent density functional theory, we reveal that there are two types of photoabsorption in BPQDs for both point defects and oxidation. A close structure-absorption relation is unraveled: BPQDs are defect-tolerant and show excellent photoabsorption as long as the coordination number (CN) of defective P atoms is 3. By contrast, the unsaturated or oversaturated P atoms with CN ≠ 3 create in-gap-states (IGSs) and completely quench the optical absorption. An effective way to eliminate the IGSs and repair the photoabsorption of defective BPQDs via sufficient hydrogen passivation is further proposed.

Ultrathin Semiconducting Bi2Te2S and Bi2Te2Se with High Electron Mobilities

High carrier mobility and moderate band gap are two key properties of electronic device applications. Two ultrathin two-dimensional (2D) semiconductors, namely, Bi2Te2S and Bi2Te2Se nanosheets, with novel electronic and optical properties are predicted based on first-principles calculations. The Bi2Te2S and Bi2Te2Se monolayers own moderate band gaps (∼0.7 eV) and high electron mobilities (∼20 000 cm2 V-1 s-1), and they can absorb sunlight efficiently through the whole incident solar spectrum. Meanwhile, layer-dependent exponential decay band gaps are also unveiled. The relatively low interlayer binding energies suggest that these monolayers can be easily exfoliated from bulk structures. Their high dynamical and thermal stabilities are further verified by phonon dispersion calculations and ab initio molecular dynamics simulations. The exceptional properties render Bi2Te2X (X = S, Se) monolayers promising candidates in future high-speed (opto)electronic devices.

Electronic structures and optical properties of arsenene and antimonene under strain and an electric field

Using density functional and many-body perturbation theories, we explore the strain and electric field effects on the electronic structures and optical properties of hexagonal arsenene (β-As) and antimonene (β-Sb). The calculations show that they can transform from indirect into direct bandgap semiconductors, and even semimetals under biaxial tensile strain and an electric field perpendicular to the layer. In particular, under a stronger electric field, their bandgaps gradually close owing to the field-induced motion of nearly free electron states. More interestingly, increasing the strain can significantly red-shift the optical absorption spectra and even enhance the optical absorption in the energy region of 1.2–2.2 eV (including infrared and partial visible light). Under a stronger electric field, their optical absorptions are enhanced and a large exciton binding energy can be retained. Such dramatic characteristics in the electronic structures and optical properties suggest great potential of β-As and β-Sb for novel electronic and optoelectronic devices.

Arsenene-Based Heterostructures: Highly Efficient Bifunctional Materials for Photovoltaics and Photocatalytics

Constructing suitable type II heterostructures is a reliable solution for high-efficient photovoltaic and photocatalytic materials. Arsenene, as a rising member of monoelemental two-dimensional materials, shows great potential as a building block of heterostructures because of its suitable band gap, high carrier mobility, and good optical properties. On the basis of accurate band structure calculations by combining the many-body perturbation GW method with an extrapolation technique, we demonstrate that arsenene-based heterostructures paired with molybdenum disulfide, tetracyano-quinodimethane, or tetracyanonaphtho-quinodimethane can form type II band alignments. These arsenene-based heterostructures cannot only satisfy all the requirements as photocatalysts for photocatalytic water splitting but can also show an excellent power conversion efficiency of ∼20% as potential photovoltaics.

Photo-Oxidative Degradation and Protection Mechanism of Black Phosphorus: Insights From Ultrafast Dynamics

The environmental instability and protection of black phosphorus (BP) is one of the most attractive hotspots in two-dimensional materials. The generation of superoxide is believed to be the key culprit, while the photogenerated electron dynamics is yet to be known. In this work, we carry out time domain ab initio nonadiabatic molecular dynamics to understand the photogenerated electron dynamics at the molecule/BP interface. It is found that oxygen can trap the photogenerated electrons of BP rapidly owing to strong electron-phonon (e-p) coupling and becomes an active superoxide under light illumination. A good protection layer, such as perylene diimide (PDI), has comparable capabilities of trapping photogenerated electrons to oxygen, which can efficiently prevent the formation of superoxide and thus suppress the degradation of BP. Moreover, PDI can enhance the separation of photogenerated electron-hole pairs of BP by prolonging the time of holding photogenerated electrons of BP. In contrast, 7,7,8,8-tetracyano- p-quinodimethane (TCNQ) has weak e-p coupling and a large band edge offset between trapping and donor states and thereby poor trapping ability for photogenerated electrons, leading to poor protection efficiency. This study provides the first in-depth understanding of the BP degradation and protection mechanism from excited-state dynamics and can be applicable to other 2D photo-oxidative degradation.

Electronic, photocatalytic, and optical properties of two-dimensional boron pnictides

By employing first-principles calculations, we investigate the stabilities, quasi-particle band structures, and photocatalytic and optical properties of monolayer boron pnictides. Calculations indicate that monolayer boron pnictides have highly thermal stabilities verified by molecular dynamics, appreciable direct bandgaps, and good optical absorptions in the visible and near-infrared ranges. In addition, the relatively small exciton binding energies are also observed in the three systems, facilitating the separation of photogenerated electrons and holes. More interestingly, monolayer boron phosphide satisfies the criteria of photocatalyst for water splitting, and its photocatalytic performance can be further enhanced by applying biaxial tensile strain. Our researches provide valuable insight for finding monolayer boron pnictides applied in optoelectronics and photocatalytic water splitting.