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.

Recent advances in magnetism of graphene from 0D to 2D.

Graphene appears as a promising candidate in the spintronic application due to its fascinating electrical properties. A large number of theoretical and experimental studies have demonstrated the accessibility and significance of inducing magnetism in graphene-based systems. This review is dedicated to the overview of the latest five-year advances of studies on graphene's magnetism based on a dimensional viewpoint, including nanoflakes (0D), graphene nanoribbons (1D), graphene sheets and twisted bilayer graphene (2D). Various methods, such as edge engineering, defect engineering, sp3 functionalization, heteroatom adsorption and interlayer rotation, are suggested to induce intriguing magnetic behaviors. Finally, we summarized the challenges and opportunities in the field to provide guidance for future research.

Identifying the effects of oxygen on the magnetism of WS2 nanosheets.

In this paper, the microstructure and magnetic properties of the exfoliated and sulfurized WS2 nanosheets were researched to identify the effects of oxygen on magnetism. The exfoliated WS2 nanosheets were prepared by a liquid exfoliation method, and then the sulfurized WS2 nanosheets were obtained after sulfurization of the exfoliated WS2 nanosheets. The exfoliated WS2 nanosheets show paramagnetism, and contain 1T and 2H phases, sulfur vacancies and some oxygen. The sulfurized WS2 nanosheets with an intrinsic structure exhibit an ordered magnetic signature. A combination of detailed experimental research and first-principles calculation demonstrates that oxygen in the structure of WS2 nanosheets would not induce magnetic moments, which can even suppress the spin-polarized edge states. These results identified the effects of oxygen on the magnetism of WS2 nanosheets and would promote its application in spintronics.

Non-clustering ofsp3fluorine adatoms on pristine graphene surface.

Fluorination can change graphene's properties, and which is theoretically relative to fluorination pattern ofsp3fluorine adatoms on graphene surface. The common view for the pattern is that it can easily form as a large cluster for the low migration barrier of fluorine adatoms on pristine graphene surface. Here, we report thatsp3fluorine adatoms are well-dispersed rather than clustered due to that the intensity ratio of 1.8 for C-CF/CF peaks (R) of fluorinated graphene is much higher thanR≈ 0 for clustered pattern. The low magnetic inducing efficiency of 1 µB/1000F adatoms indicates that the 'nonmagnetic' fluorine pairs rather than 'magnetic' fluorine 'points' dominate the well-dispersedsp3pattern. Our findings introduce a new insight into the fluorination structure properties of fluorinated and othersp3functionalized such as hydrogenated, chlorinated, or hydroxylated graphene and other carbon materials.

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.

Realization of Strong Room-Temperature Ferromagnetism in Atomically Thin 2D Carbon Nitride Sheets by Thermal Annealing.

The presence of the intrinsic band gap of 3.06 eV makes atomically thin carbon nitride sheets (CNs) a promising spin-based semiconductor material. However, the absence of localized spins makes the pristine CNs intrinsically nonmagnetic. Here we report the realization of strong room-temperature (RT) ferromagnetism with a high Curie temperature of ca. 524.2 K in atomically thin 2D CNs by annealing pristine CNs at 700 °C. In particular, the RT saturated magnetization reaches as high as 0.71 emu/g, which is the highest value reported so far in carbon-based materials. The structural characterization combined with density functional theory calculations reveals that (i) the seven C-C bonds per unit cell were formed after annealing and (ii) the C-C bonds can introduce high-density localized spins and realize the long-range ferromagnetic couplings among these spins.

Reversible Thermochromism and Strong Ferromagnetism in Two-Dimensional Hybrid Perovskites.

Two-dimensional (2D) hybrid perovskites have shown many attractive properties associated with their soft lattices and multiple quantum well structure. Herein, we report the synthesis and characterization of two new multifunctional 2D hybrid perovskites, (PED)CuCl4 and (BED)2 CuCl6 , which show reversible thermochromic behavior, dramatic temperature-dependent conductivity change, and strong ferromagnetism. Upon temperature change, the (PED)CuCl4 and (BED)2 CuCl6 crystals exhibit a reversible color change between yellow and red-brown. The associated structural changes were monitored by in situ temperature-dependent powder X-ray diffraction (PXRD). The (BED)2 CuCl6 exhibits superior thermal stability, with a thermochromic working temperature up to 443 K. The conductivity of (BED)2 CuCl6 changes over six orders of magnitude upon temperature change. The 2D perovskites exhibit ferromagnetic properties with Curie temperatures around 13 K.

P-Superdoped Graphene: Synthesis and Magnetic Properties.

Phosphorus (P)-doping in vacancies of graphene sheets can significantly change graphene's physical and chemical properties. Generally, a high level for P-doping is difficult due to the low concentration of vacancy but is needed to synthesize graphene with the perfect properties. Herein, we synthesized the P-superdoped graphene with the very high P content of 6.40 at. % by thermal annealing of fluorographite (FGi) in P vapor. Moreover, we show that the P-doping level can be adjusted in the wide range from 2.86 to 6.40 at. % by changing the mass ratio of red phosphorus to FGi. The magnetic results show that (i) P-doping can effectively create localized magnetic moments in graphene; (ii) the higher the doping level of sp3-type POx groups, the higher the magnetization of P-superdoped graphene is; and (iii) the high P-doping levels can lead to the coexistence of antiferromagnetic and ferromagnetic behavior. It is proposed that the sp3-type POx groups are the major magnetic sources.

Increasing Solar Absorption of Atomically Thin 2D Carbon Nitride Sheets for Enhanced Visible-Light Photocatalysis.

Atomically thin 2D carbon nitride sheets (CNS) are promising materials for photocatalytic applications due to their large surface area and very short charge-carrier diffusion distance from the bulk to the surface. However, compared to their bulk counterpart, CNS' applications always suffer from an enlarged bandgap and thus narrowed solar absorption range. Here, an approach to significantly increase solar absorption of the atomically thin CNS via fluorination followed by thermal defluorination is reported. This approach can greatly increase the visible-light absorption of CNS by extending the absorption edge up to 578 nm. The modulated CNS loaded with Pt cocatalyst as a photocatalyst shows a superior photocatalytic hydrogen production activity under visible-light irradiation to Pt-CNS. Combining experimental characterization with theoretical calculations shows that this approach can introduce cyano groups into the framework of CNS as well as the accompanied nitrogen vacancies at the edges, which leads to both narrowing the bandgap and changing the charge distribution. This study will provide an effective strategy to increase solar absorption of carbon-nitride-based photocatalysts for solar energy conversion applications.

Realization of Ambient-Stable Room-Temperature Ferromagnetism by Low-Temperature Annealing of Graphene Oxide Nanoribbons.

Graphene oxide nanoribbons (GONRs) annealed at the low temperature of 400 °C (aGONRs-400) are developed as an excellent room-temperature (RT) ferromagnet. The saturated magnetization ( Ms) of aGONRs-400 is high, up to 0.39 emu/g at room temperature, and the RT ferromagnetism (FM) exhibits excellent ambient stability with Ms preserved for over half a year. The preferential distribution of the magnetic phenolic hydroxyl toward the edges, which contributes to the long-range ferromagnetic couplings, was confirmed by X-ray photoemission spectroscopy measurement and gradient annealing analysis. The approach of low-temperature annealing is proved to be efficient both to remove the prominent nonmagnetic epoxy groups on the basal plane of GONRs or transform them to magnetic hydroxyl groups and to preserve the magnetic phenolic hydroxyl at the edges to realize a strong and ambient stable FM.

Review of the Electronic, Optical, and Magnetic Properties of Graphdiyne: From Theories to Experiments.

Graphdiyne (GDY), a two-dimensional artificial-synthesis carbon material, has aroused tremendous interest because of its unique physical properties. The very high activity affords the possibility to chemically dope GDY with metal atoms or lightweight elements such as hydrogen and halogen and so on. Chemical doping has been confirmed to be an effective method to lead to various GDY derivatives with useful physical properties. Thus, this review is intended to provide an overview of the electronic, optical, and magnetic properties of pristine GDY and its derivatives reported from theories to experiments. Because of the importance of pristine GDY and its derivatives in real applications, we also summarize the main physical applications of GDY and its derivatives reported in recent years in this review. We believe that the review will be valuable to all those interested in GDY.

Ambient Degradation-Induced Spin Paramagnetism in Phosphorene.

The sizeable direct bandgap, high mobility, and long spin lifetimes at room temperature offer black phosphorus (BP) potential applications in spin-based semiconductor devices. Toward these applications, a critical step is creating a magnetic response in BP, which is arousing much interest. It is reported here that ambient degradation of BP, which is immediate and inevitable and greatly changes the semiconducting properties, creates magnetic moments, and any degree of degradation leads to notable paramagnetism. Its Landau factor g measured is ≈1.995, revealing that the magnetization mainly results from spin rather than orbital moments. Such magnetism most likely results from the unsaturated phosphorus in the vacancies which are stabilized by O adatoms. It can be tuned by changing any one of the ambient factors of ambient temperature, humidity, and light intensity, and can be stabilized by exposing BP in argon. The findings highlight the importance of evaluating the effect of ambient degradation-induced magnetism on BP's spin-based devices. The work seems an essential milestone toward the forthcoming research upsurge on BP's magnetism.

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.

Half-metallicity in a honeycomb-kagome-lattice Mg3C2 monolayer with carrier doping.

To obtain high-performance spintronic devices with high integration density, two-dimensional (2D) half-metallic materials are highly desired. Herein, we proposed a stable 2D material, i.e., the Mg3C2 monolayer, with a honeycomb-kagome lattice based on the particle-swarm optimization algorithm and first-principles calculations. This monolayer is an anti-ferromagnetic (AFM) semiconductor in its ground state. We have also demonstrated that a transition from an AFM semiconductor to a ferromagnetic half-metal in this 2D material can be induced by carrier (electron or hole) doping. The half-metallicity arises from the 2pz orbitals of the carbon (C) atoms for the electron-doped system and from the C 2px and 2py orbitals in the case of hole doping. Our findings highlight a new promising material with controllable magnetic and electronic properties towards 2D spintronic applications.

Ultrasmall and Monolayered Tungsten Dichalcogenide Quantum Dots with Giant Spin-Valley Coupling and Purple Luminescence.

Monolayered tungsten dichalcogenide quantum dots (WS2 QDs) have various potential applications due to their large spin-valley coupling and excellent photoluminescence (PL) properties. What is expected is that with the decrease in lateral size of QDs, the stronger quantum confinement effect will dramatically strengthen the spin-valley coupling and widen the band gap. However, ultrasmall monolayered WS2 QDs prepared by ion intercalation unavoidably undergo the problem of structural defects, which will create defect levels and significantly change their properties. In this study, we report that by annealing defective monolayered WS2 QDs in sulfur vapor, pristine monolayered WS2 QDs with an ultrasmall lateral size of ca. 1.8-3.8 nm can be obtained. The results show that the ultrasmall monolayered WS2 QDs exhibit a giant spin-valley coupling of ca. 821 meV. Moreover, the pristine ultrasmall monolayered WS2 QDs show purple PL centered at 416 nm, and the defect PL peaks in defective WS2 QDs can be effectively removed by annealing. All of these results afford the ultrasmall monolayered QDs various applications such as in optoelectronics, spintronics, valleytronics, and so on.

Nitrogen-Superdoped 3D Graphene Networks for High-Performance Supercapacitors.

An N-superdoped 3D graphene network structure with an N-doping level up to 15.8 at% for high-performance supercapacitor is designed and synthesized, in which the graphene foam with high conductivity acts as skeleton and nested with N-superdoped reduced graphene oxide arogels. This material shows a highly conductive interconnected 3D porous structure (3.33 S cm-1 ), large surface area (583 m2 g-1 ), low internal resistance (0.4 Ω), good wettability, and a great number of active sites. Because of the multiple synergistic effects of these features, the supercapacitors based on this material show a remarkably excellent electrochemical behavior with a high specific capacitance (of up to 380, 332, and 245 F g-1 in alkaline, acidic, and neutral electrolytes measured in three-electrode configuration, respectively, 297 F g-1 in alkaline electrolytes measured in two-electrode configuration), good rate capability, excellent cycling stability (93.5% retention after 4600 cycles), and low internal resistance (0.4 Ω), resulting in high power density with proper high energy density.

Selective Growth of Metal-Free Metallic and Semiconducting Single-Wall Carbon Nanotubes.

A major obstacle for the use of single-wall carbon nanotubes (SWCNTs) in electronic devices is their mixture of different types of electrical conductivity that strongly depends on their helical structure. The existence of metal impurities as a residue of a metallic growth catalyst may also lower the performance of SWCNT-based devices. Here, it is shown that by using silicon oxide (SiOx ) nanoparticles as a catalyst, metal-free semiconducting and metallic SWCNTs can be selectively synthesized by the chemical vapor deposition of ethanol. It is found that control over the nanoparticle size and the content of oxygen in the SiOx catalyst plays a key role in the selective growth of SWCNTs. Furthermore, by using the as-grown semiconducting and metallic SWCNTs as the channel material and source/drain electrodes, respectively, all-SWCNT thin-film transistors are fabricated to demonstrate the remarkable potential of these SWCNTs for electronic devices.

Metal-free spin and spin-gapless semiconducting heterobilayers: monolayer boron carbonitrides on hexagonal boron nitride.

The interfaces between monolayer boron carbonitrides and hexagonal boron nitride (h-BN) play an important role in their practical applications. Herein, we respectively investigate the structural and electronic properties of two metal-free heterobilayers constructed by vertically stacking two-dimensional (2D) spintronic materials (B4CN3 and B3CN4) on a h-BN monolayer from the viewpoints of lattice match and lattice mismatch models using density functional calculations. It is found that both B4CN3 and B3CN4 monolayers can be stably adsorbed on the h-BN monolayer due to the van der Waals interactions. Intriguingly, we demonstrate that the bipolar magnetic semiconductor (BMS) behavior of the B4CN3 layer and the spin gapless semiconductor (SGS) property of the B3CN4 layer can be well preserved in the B4CN3/BN and B3CN4/BN heterobilayers, respectively. The magnetic moments and spintronic properties of the two systems originate mainly from the 2pz electrons of the carbon atoms in the B4CN3 and B3CN4 layers. Furthermore, the BMS behavior of the B4CN3/BN bilayer is very robust while the electronic property of the B3CN4/BN bilayer is sensitive to interlayer couplings. These theoretical results are helpful both in understanding the interlayer coupling between B4CN3 or B3CN4 and h-BN monolayers and in providing a possibility of fabricating 2D composite B4CN3/BN and B3CN4/BN metal-free spintronic materials theoretically.

Creation of localized spins in graphene by ring-opening of epoxy derived hydroxyl.

Creation of high-density localized spins in the basal plane of graphene sheet by introduction of sp(3)-type defects is considered to be a potential route for the realization of high-magnetization graphene. Theoretical and experimental studies confirmed that hydroxyl can be an effective sp(3)-type candidate for inducing robust magnetic moment. However, the artificial generation of hydroxyl groups for creating high-density spins on the basal plane of graphene sheet is very scarce. Here we demonstrate that high-content hydroxyl groups can be generated on the basal plane of graphene oxide (GO) sheet by ring opening of epoxy groups. We show that by introduction of 10.74 at.% hydroxyl groups, the density of localized spins of GO can be significantly increased from 0.4 to 5.17 µB/1000 C. Thus, this study provided an effective method to obtain graphene with high-density localized spins.

Elemental superdoping of graphene and carbon nanotubes.

Doping of low-dimensional graphitic materials, including graphene, graphene quantum dots and single-wall carbon nanotubes with nitrogen, sulfur or boron can significantly change their properties. We report that simple fluorination followed by annealing in a dopant source can superdope low-dimensional graphitic materials with a high level of N, S or B. The superdoping results in the following doping levels: (i) for graphene, 29.82, 17.55 and 10.79 at% for N-, S- and B-doping, respectively; (ii) for graphene quantum dots, 36.38 at% for N-doping; and (iii) for single-wall carbon nanotubes, 7.79 and 10.66 at% for N- and S-doping, respectively. As an example, the N-superdoping of graphene can greatly increase the capacitive energy storage, increase the efficiency of the oxygen reduction reaction and induce ferromagnetism. Furthermore, by changing the degree of fluorination, the doping level can be tuned over a wide range, which is important for optimizing the performance of doped low-dimensional graphitic materials.