Transition from semiconductor to conductor of a Mg2N electride induced by strain.

Electrides are a class of materials in which electrons are not bound to atoms but are similar to anions in crystals. To date, there are more than 300 electrides that have been discovered by first-principles. Alkaline-earth metal nitrides (AE2N, AE = Be, Mg, Ca, Sr, and Ba) are an important component of electride materials. Ca2N, Sr2N, and Ba2N structures have been identified and synthesized in previous research studies. Furthermore, the structures of Be2N (R3̄m symmetry) and Mg2N (R3m symmetry) were recently identified. For Mg2N, it has zero-dimension (0D) interstitial localized electrons and band structure with semiconductor properties, which is significantly different from the other AE2N structures (two-dimension electrides and metal properties). Consequently, Mg2N was systematically studied in this work. We found that the pristine Mg2N was an indirect band gap semiconductor with a band gap of 0.243 eV. It transitioned to a metal when 2% stretch stress was applied to the c-axis. Moreover, at 5% stretch stress, the structure exhibited 2D interstitial localized electrons with the superconducting transition temperature (Tc) of 0.3 K. These studies thus provide a deeper understanding of the physicochemical properties of Mg2N as an electride.

Tunable Schottky and ohmic contacts in the Ti2NF2/α-Te van der Waals heterostructure.

Two dimensional α-Te holds great promise in optoelectronic devices because of its high mobility and excellent environmental stability. In this study, the electronic structures and interfacial contact characteristics of the Ti2NF2/α-Te van der Waals heterostructure are investigated by means of first-principles calculations. It is found that p-type Schottky contacts with a Schottky barrier (SB) of 0.21 eV are formed at the Ti2NF2/α-Te interface. By applying external electric fields or controlling the interlayer coupling between the Ti2NF2 and α-Te monolayers, the SB height can be effectively tuned, and all the n-type Schottky, p-type Schottky, n-type ohmic and p-type ohmic contacts can be achieved. Such an extremely high tunability is further found to be closely associated with the charge transfer at the interface, as well as the interface dipole and the potential step. Our results provide an avenue for the design of future α-Te-based electronic devices with high performance.

Shape-Dependent Linear Dichroism Spectra of Colloidal Semiconductor Nanocrystals.

Semiconductor nanocrystals are normally dispersed in the solvent for property studies as well as practical applications. However, rare attention has been paid to their orientation status in the colloidal solution. Herein, with the help of linear dichroism (LD) spectroscopy, we demonstrate that isotropic NCs of high symmetry (i.e., quantum dots, QDs) and anisotropic NCs (e.g., quantum rods, QRs and nanoplates, NPLs) but under diluted concentration are randomly dispersed without any preferential orientation. Meanwhile, anisotropic NCs under a high concentration can behave with some net orientation along a certain direction. For example, CdSe quantum rods (QRs) and nanoplatelets (NPLs) both show an obviously preferred orientation along the vertical direction in solution when their solution absorbances increase to certain values. An in-depth analysis of QRs' LD spectrum shows that the first excitonic transition of QRs is strongly quantumly confined while its higher-energy excitonic transitions are weakly quantumly confined. In contrast, the NPLs' LD spectrum indicates that their excitonic transitions are isotropic in the spatial space. This work provides a new viewpoint of the real status of anisotropic semiconductor NCs in solution.

Curcumin attenuates surgery-induced cognitive dysfunction in aged mice.

Post-operative cognitive dysfunction (POCD) is associated with elderly patients undergoing surgery. However, pharmacological treatments for POCD are limited. In this study, we found that curcumin, an active compound derived from Curcuma longa, ameliorated the cognitive dysfunction following abdominal surgery in aged mice. Further, curcumin prevented surgery-induced anti-oxidant enzyme activity. Curcumin also increased brain-derived neurotrophic factor (BDNF)-positive area and expression of pAkt in the brain, suggesting that curcumin activated BDNF signaling in aged mice. Furthermore, curcumin neutralized cholinergic dysfunction involving choline acetyltransferase expression induced by surgery. These results strongly suggested that curcumin prevented cognitive impairments via multiple targets, possibly by increasing the activity of anti-oxidant enzymes, activation of BDNF signaling, and neutralization of cholinergic dysfunction, concurrently. Based on these novel findings, curcumin might be a potential agent in POCD prophylaxis and treatment.

New insight into the helium-induced damage in MAX phase Ti3AlC2 by first-principles studies.

In the present work, the behavior of He in the MAX phase Ti3AlC2 material is investigated using first-principle methods. It is found that, according to the predicted formation energies, a single He atom favors residing near the Al plane in Ti3AlC2. The results also show that Al vacancies are better able to trap He atoms than either Ti or C vacancies. The formation energies for the secondary vacancy defects near an Al vacancy or a C vacancy are strongly influenced by He impurity content. According to the present results, the existence of trapped He atoms in primary Al vacancy can promote secondary vacancy formation and the He bubble trapped by Al vacancies has a higher tendency to grow in the Al plane of Ti3AlC2. The diffusion of He in Ti3AlC2 is also investigated. The energy barriers are approximately 2.980 eV and 0.294 eV along the c-axis and in the ab plane, respectively, which means that He atoms exhibit faster migration parallel to the Al plane. Hence, the formation of platelet-like bubbles nucleated from the Al vacancies is favored both energetically and kinetically. Our calculations also show that the conventional spherical bubbles may be originated from He atoms trapped by C vacancies. Taken together, these results are able to explain the observed formation of bubbles in various shapes in recent experiments. This study is expected to provide new insight into the behaviors of MAX phases under irradiation from electronic structure level in order to improve the design of MAX phase based materials.

An A2 B2 O7 -Type High-Entropy Oxide for Efficient Photoelectrochemical Photodetector with Excellent Long-Term Stability.

Optoelectronics with excellent long-term stability is meaningful for practical applications. Herein, for the first time, an A2 B2 O7 type high-entropy oxide of (La0.2 Ce0.2 Nd0.2 Gd0.2 Bi0.2 )2 Ti2 O7 (ATO) is synthesized and applied for photoelectrochemical photodetection. The lattice distortion, highly dispersed metal composition, and exposed active sites of ATO are beneficial for the fast separation and transmission of photogenerated electron/hole pairs, endowing ATO-based devices with good photodetection performance. Both the density functional theory calculations and the nondegenerate transient absorption spectroscopy demonstrate the good optoelectronic properties of ATO. The systematic experimental studies reveal the tunable photodetection capability of ATO-based photodetector (PD) in the visible region. A photocurrent of 772.00 nA cm-2 and a responsivity of 4.02 µA W-1 can be achieved as the PD in 1.0 m KOH with the bias potential of 0.6 V. Importantly, the robust and reproducible ON/OFF signals of the PD can be verified and there is only ≈5.00% attenuation in photocurrent even after 6 months, revealing the great potential of high- entropy oxides for practical applications.

van der Waals integration of mixed-dimensional CeO2@Bi heterostructure for high-performance self-powered photodetector with fast response speed.

Heterostructures have been extensively investigated for optoelectronic devices owing to their fantastic physicochemical properties. Herein, a mixed-dimensional van der Waals heterostructure (vdWH) CeO2@Bi, 1D ceria (CeO2) loaded with 0D bismuth quantum dots (Bi QDs), is synthesized through a facile hydrothermal bottom-up method. It is found that the fabricated CeO2@Bi-based photoelectrochemical (PEC)-type photodetector (PD) shows self-powered photodetection capability with a fast photoresponse speed of 0.02 s. Besides, a photocurrent of 2.00 µA cm-2 and a photoresponsivity of 888.89 µA W-1 under 365 nm illumination are obtained. Furthermore, good long-term cycle stability is also observed after 1 month in a harsh environment, indicating the great potential for practical applications. These results are further supported by density functional theory (DFT) calculations. We believe that the presented work is expected to provide a new pathway for the future utilization of vdWHs for high-performance optoelectronics.

Electrical Switch of Poisson's Ratio in IV-VI Monolayers via Pseudophase Transitions.

The recent discovery of negative Poisson's ratio (NPR) in two-dimensional (2D) atomic crystals has stimulated extensive research interest in their intriguing physical properties. Here, via density functional theory (DFT) calculations, we reveal in the family of 2D IV-VI semiconductors that an iso-symmetry structure variation concerning the switch of the cation (IV) versus anion (VI) in the outermost layers leads to the change of sign of Poisson's ratio. Such iso-symmetry structural pseudo-phase transition can be induced by external strains, as well as electric fields, realizing the possibility of an electrically switchable Poisson effect. The phase transition process could involve a dynamic intermediate state with an alternative cation/anion switch in the frequencies of 2-3 THz according to the real-time time-dependent DFT (rt-TDDFT) calculations. The results open the way for studying pseudophases in 2D materials associated with sharply different physical properties, such as Poisson's ratio, for electromechanical and optoelectronic applications.

Synthesis and optoelectronics of mixed-dimensional Bi/Te binary heterostructures.

Mixed-dimensional binary heterostructures, especially 0D/2D heterostructures, have attracted significant attention due to their unique physical properties. In this contribution, 0D bismuth quantum dots (Bi QDs, VA) are immobilized onto 2D Te nanosheets (Te NSs, VIA) to prepare Bi QDs/Te NSs binary heterostructures (Bi/Te) through a facile and cost-effective hydrothermal method. The results from both experiments and density functional theory (DFT) calculations demonstrate the enhanced photo-response behaviors of Bi/Te-based photoelectrochemical (PEC)-type photodetectors (PDs). The as-prepared PDs exhibit a high photocurrent of 18.21 µA cm-2, significantly higher than those of previously reported pristine Bi QD and Te NS-based PDs. The PDs are further demonstrated to have excellent self-power capability and long-term stability over 30 days. Additionally, the obtained 786 fs pulse output signal in the telecommunications band reveals the great potential of Bi/Te for ultrafast photonic devices. It is believed that such VA/VIA binary heterostructures provide opportunities for developing multifunctional optoelectronic devices for nano-science applications.