Enhancing Visible-Light Absorption of 2D Carbon Nitride by Constructing 2D/2D van der Waals Heterojunctions of Carbon Nitride/Nitrogen-Superdoped Graphene.

Carbon nitride sheets (CNs) down to the two-dimensional (2D) limit have been widely used in photoelectric conversion due to their inherent band gap and extremely short charge-carrier diffusion distance. However, the utilization of visible light remains low due to the rapid recombination of photogenerated electron-hole pairs and enlarged band gap. Here, atomically thin 2D/2D van der Waals heterojunctions (vdWHs) of N-superdoped graphene (NG) and CNs (CNs/NG) are fabricated via a facile electrostatic self-assembly method. Our results revealed that the vdWHs can increase the visible-light absorption of CNs by extending the absorption edge from 455 to up to 490 nm. The recombination of photogenerated electron-hole pairs is inhibited because superdoped N in CNs/NG facilitates the transmission of photogenerated carriers in the melon chain. This study opens a new avenue for narrowing the band gap and promoting photoexcited carrier separation in carbon-nitride-based materials.

Exfoliable and self-healable two-dimensional materials from wurtzite zinc chalcogenides as building blocks of nanodevices.

With the advent of graphene, two-dimensional (2D) materials have emerged as promising candidates for next-generation electronic and optoelectronic applications. The most anticipated 2D materials have been synthesized and exploited for novel applications. Multilayered zinc chalcogenides (ZnX) are the best precursors for obtaining atomic layer two-dimensional materials by exfoliation. Therefore, we carry out a detailed density functional theory-based study to achieve an exfoliation process of ZnX non-van der Waals sheets by straining and provide a microscopic understanding of the ferroelectric, optic, and spin behaviors of ZnX systems and the corresponding self-healable two-dimensional ZnX devices. The results revealed that 2D ZnX sheets can be obtained when strain is 14% for ZnS and ZnSe, and the peak values of exfoliation energy have a similar order of magnitude to those of traditional 2D materials, indicating the possibility of obtaining 2D ZnX monolayers. For intrinsic 2D ferroelectric materials with in-plane electric polarization, the direction of ZnX sheets can be reversed using an electric field with an energy barrier of ∼0.175 eV per atom for ZnSe, offering a promising functional basis for their application in ferroelectric nanodevices. The first absorption of photons for polarization perpendicular to the monolayer plane occurs in a high energy range of photons, facilitating their application in LEDs. The spin splitting in non-centrosymmetric ZnX crystals exhibits a Rashba spin-texture according to first-principles calculations. The self-healable two-dimensional nanodevices have a smooth curve from -0.5 to 0.5 eV. This work indicates the potential value of non-van der Waals ZnX 2D materials for their application in photoelectric and spintronic nanodevices.

Universal Fluorination-Created Edge C-F Groups in Networks of Multidimensional Carbon Materials.

Fluorination can significantly change the physical and chemical properties of carbon materials (CMs). Common sense for the fluorination mechanism for CMs indicates that one basal-plane C-F group (CF group) can form as one fluorine atom bonded to one carbon atom along the out-of-plane carbon networks without creating edge C-F groups (including CF2 and CF3 groups) at vacancies in carbon networks. We report that fluorination can generally create edge C-F groups in multidimensional CMs such as graphite, graphene, carbon nanotubes, and fullerene, and the concentration of edge C-F groups is dependent on both the crystallinity of starting CMs and the fluorination pressure and temperature. As an example, we show the significant differences in the band gap opening, photoluminescence, and magnetic properties between two half-fluorinated graphenes with different concentrations of edge C-F groups. Our findings highlight the importance of fluorination in creating edge C-F groups in the structure and properties and introduce new insight into fluorinated CMs.

Spin-filtering and tunneling magnetoresistance effects in 6,6,12-graphyne-based molecular magnetic tunnel junctions.

In the present study, by cutting 6,6,12-graphyne along vertical and horizontal directions, two kinds of 6,6,12-graphyne nanodots (6,6,12-GYNDs) with different sizes are obtained. Using these 6,6,12-GYNDs, we theoretically designed two kinds of 6,6,12-graphyne-based molecular magnetic tunnel junctions (MMTJs) and investigated their spin-dependent transport properties. Depending on the orientation of the 6,6,12-GYNDs and the connection of the 6,6,12-GYNDs to electrodes, our results show that the two MMTJs have novel transport behaviors. Two different net spin currents can be obtained by tuning the spin configurations and the maximal order of magnitudes of tunneling magnetoresistance values of the two MMTJs reaches 106%. The high spin-filtering ratio and large tunneling magnetoresistance value provide high sensitivity for practical applications. Therefore, the spin-filtering and tunneling magnetoresistance effects enable 6,6,12-graphyne-based MMTJs to be used as spintronic devices.

Electronic properties of α-graphyne on hexagonal boron nitride and α-BNyne substrates.

The upsurge in the research of α-graphyne (α-GY) has occurred due to the existence of a Dirac cone, whereas the absence of band gap impedes its semiconductor applications. Here, the electronic properties of α-GY on hexagonal boron nitride (h-BN) and α-BNyne (α-BNy) monolayers are investigated using first-principles calculations. Through engineering heterostructures, the band gap opening can be achieved and has different responses to the substrate and stacking sequence. Intriguingly, the band gap of α-GY/α-BNy with Ab1 stacking mode is up to 77.5 meV in the HSE06 functional, which is distinctly greater than K B T at room temperature. The characteristic Dirac band of α-GY is preserved on the α-BNy substrate, while it changes into a parabolic band on the h-BN substrate. Additionally, we also find that changing the interlayer distance is an alternative strategy to realize the tunable band gap. Our results show that by selecting a reasonable substrate, the linear band structure and thus the high carrier mobility as well as the distinct band gap opening could coexist in α-GY. These prominent properties are the key quantity for application of α-GY in nanoelectronic devices.