Zhen-Kun Tang

The stability and electronic properties of novel three-dimensional graphene-MoS2 hybrid structure

Three-dimensional (3D) hybrid layered materials receive a lot of attention because of their outstanding intrinsic properties and wide applications. In this work, the stability and electronic structure of three-dimensional graphene-MoS2 (3DGM) hybrid structures are examined based on first-principle calculations. The results reveal that the 3DGMs can easily self-assembled by graphene nanosheet and zigzag MoS2 nanoribbons, and they are thermodynamically stable at room temperature. Interestingly, the electronic structures of 3DGM are greatly related to the configuration of joint zone. The 3DGM with odd-layer thickness MoS2 nanoribbon is semiconductor with a small band gap of 0.01–0.25 eV, while the one with even-layer thickness MoS2 nanoribbon exhibits metallic feature. More importantly, the 3DGM with zigzag MoS2 nanoribbon not only own the large surface area and effectively avoid the aggregation between the different nanoribbons, but also can remarkably enhance Li adsorption interaction, thus the 3DGM have the great potential as high performance lithium ion battery cathodes.

First-Principles Study of Novel Two-Dimensional (C4H9NH3)2PbX4 Perovskites for Solar Cell Absorbers

Low-dimensional perovskites (A2BX4), in which the A cations are replaced by different organic cations, may be used for photovoltaic applications. In this contribution, we systematically study the two-dimensional (2D) (C4H9NH3)2PbX4 (X═Cl, Br and I) hybrid perovskites by density functional theory (DFT). A clear structures-properties relationship, with the photophysical characteristics directly related to the dimensionality and material compositions, was established. The strong s-p antibonding couplings in both bulk and monolayer (C4H9NH3)2PbI4 lead to low effective masses for both holes (mh*) and electrons (me*). However, mh* increases in proportion to the decreasing inorganic layer thickness, which eventually leads to a slightly shifted band edge emission found in 2D perovskites. Notably, the 2D (C4H9NH3)2PbX4 perovskites exhibit strong optical transitions in the visible light spectrum, and the optical absorption tunings can be achieved by varying the compositions and the layer thicknesses. Such work paves an important way to uncover the structures-properties relationship in 2D perovskites.

Tunable band gap and magnetism of the two-dimensional nickel hydroxide

The electronic structures and magnetic properties of two dimensional (2D) hexagonal Ni(OH)2 are explored based on first-principles calculations. The results reveal that the ground state of the pristine Ni(OH)2 is a direct semiconductor with antiferromagnetic (AFM) coupling between two nearest Ni atoms. Interestingly, the monolayer Ni(OH)2 becomes a ferromagnetic (FM) semiconductor under a biaxial compressive strain of 4%. Furthermore, the band gap of monolayer Ni(OH)2 can be modulated by the different biaxial strains. These tunable electronic structures and magnetic properties of the Ni(OH)2 monolayer make it a promising candidate for applications in 2D spin-devices.

Spatial separation of photo-generated electron-hole pairs in BiOBr/BiOI bilayer to facilitate water splitting

The electronic structures and photocatalytic properties of bismuth oxyhalide bilayers (BiOX1/BiOX2, X1 and X2 are Cl, Br, I) are studied by density functional theory. Briefly, their compositionally tunable bandgaps range from 1.85 to 3.41 eV, suitable for sun-light absorption, and all bilayers have band-alignments good for photocatalytic water-splitting. Among them, heterogeneous BiOBr/BiOI bilayer is the best as it has the smallest bandgap. More importantly, photo-excitation of BiOBr/BiOI leads to electron supply to the conduction band minimum with localized states belonging mainly to bismuth of BiOBr where the H+/H2 half-reaction of water-splitting can be sustained. Meanwhile, holes generated by such photo-excitation are mainly derived from the iodine states of BiOI in the valence band maximum; thus, the O2/H2O half-reaction of water splitting is facilitated on BiOI. Detailed band-structure analysis also indicates that this intriguing spatial separation of photo-generated electron-hole pairs and the two half-reactions of water splitting are good for a wide photo-excitation spectrum from 2–5 eV; as such, BiOBr/BiOI bilayer can be an efficient photocatalyst for water-splitting, particularly with further optimization of its optical absorptivity.

Two-dimensional square-pyramidal VO2 with tunable electronic properties

In order to design the high-performance spintronics, it is rather critical to develop new materials, which can easily regulate the magnetism of nanostructures. In this work, the electronic properties of two dimensional (2D) square-pyramidal vanadium dioxide (S-VO2) are explored based on first-principles calculations. The results reveal that the monolayer S-VO2 is an ideal flexible platform to manipulate the magnetic properties by either biaxial compressive strain or surface modification. Although the ground state of the pristine S-VO2 is a direct semiconductor with antiferromagnetic (AFM) coupling between two nearest V atoms, the monolayer S-VO2 becomes ferromagnetic (FM) under a biaxial compressive strain. Furthermore, the monolayer S-VO2 can be tuned from a nonmagnetic semiconductor to a magnetic semiconductor and even to a half-metal through surface modification. The tunable magnetic properties of the monolayer S-VO2 make it a promising candidate for applications in spin-devices.

Electronic and magnetism properties of two-dimensional stacked nickel hydroxides and nitrides.

Two-dimensional (2D) layered materials receive a lot of attention because of their outstanding intrinsic properties and wide applications. In this work, the structural, electronic and magnetic properties of nickel hydroxides (Ni(OH)2) and nitrides XN (X = B, Al, and Ga) heterostructures are studied by first-principles calculations. The results show that the pristine monolayer Ni(OH)2 owns no macro magnetism with antiferromagnetic (AFM) coupling between two nearest Ni atoms, the electronic structure can be modulated through the heterostructures. The Ni(OH)2-GaN and Ni(OH)2-AlN heterostructures retain the AFM coupling, while Ni(OH)2-BN heterostructure have a larger magnetic moment with ferromagnetic (FM) coupling. The complete electron–hole separation is found in the Ni(OH)2-GaN heterostructure. The tunable electronic and magnetic properties of the Ni(OH)2-XN heterostructures open a new door to design the spintronic devices in the 2D stacked nanostructures.

Two-dimensional Ni(OH)2-XS2 (X = Mo and W) heterostructures

Two-dimensional (2D) van der Waals (vdW) heterostructures have received a lot of attention because of their wide applications in electronics and optoelectronics. In this work, the electronic structures and optical properties of nickel hydroxides (Ni(OH)2) and transition metal dichalcogenides (XS2, X = Mo, W) heterostructures are studied by hybrid density functional theory. The results reveal that all the considered Ni(OH)2-XS2 heterostructures are indirect semiconductors with a band gap of 0.040–0.825 eV. Additionally, the AB stacked Ni(OH)2-XS2 heterostructures are more stable than the AA stacked one. Interestingly, the complete electron–hole separation is found in the Ni(OH)2-XS2 heterostructure, and its conduction band minimum and valence band maximum are located on the XS2 and Ni(OH)2 layers, respectively. Besides, the optical absorption peaks of Ni(OH)2-XS2 heterostructures are mainly located within the visible light region. These fascinating electronic structures and optical absorption of the Ni(OH)2-XS2 heterostructures make them promising candidates for applications in 2D optoelectronics.

Direct observation of multiple rotational stacking faults coexisting in freestanding bilayer MoS 2

Electronic properties of two-dimensional (2D) MoS2 semiconductors can be modulated by introducing specific defects. One important type of defect in 2D layered materials is known as rotational stacking fault (RSF), but the coexistence of multiple RSFs with different rotational angles was not directly observed in freestanding 2D MoS2 before. In this report, we demonstrate the coexistence of three RSFs with three different rotational angles in a freestanding bilayer MoS2 sheet as directly observed using an aberration-corrected transmission electron microscope (TEM). Our analyses show that these RSFs originate from cracks and dislocations within the bilayer MoS2. First-principles calculations indicate that RSFs with different rotational angles change the electronic structures of bilayer MoS2 and produce two new symmetries in their bandgaps and offset crystal momentums. Therefore, employing RSFs and their coexistence is a promising route in defect engineering of MoS2 to fabricate suitable devices for electronics, optoelectronics, and energy conversion.

A novel three dimensional semimetallic MoS2

Transition metal dichalcogenides (TMDs) have many potential applications, while the performances of TMDs are generally limited by the less surface active sites and the poor electron transport efficiency. Here, a novel three-dimensional (3D) structure of molybdenum disulfide (MoS2) with larger surface area was proposed based on first-principle calculations. 3D layered MoS2 structure contains the basal surface and joint zone between the different nanoribbons, which is thermodynamically stable at room temperature, as confirmed by first principles molecular dynamics calculations. Compared the two-dimensional layered structures, the 3D MoS2 not only owns the large surface areas but also can effectively avoid the aggregation. Interestingly, although the basal surface remains the property of the intrinsic semiconductor as the bulk MoS2, the joint zone of 3D MoS2 exhibits semimetallic, which is derived from degenerate 3d orbitals of the Mo atoms. The high stability, large surface area, and high conductivity make ...

Enhanced optical absorption via cation doping hybrid lead iodine perovskites

The suitable band structure is vital for perovskite solar cells, which greatly affect the high photoelectric conversion efficiency. Cation substitution is an effective approach to tune the electric structure, carrier concentration, and optical absorption of hybrid lead iodine perovskites. In this work, the electronic structures and optical properties of cation (Bi, Sn, and TI) doped tetragonal formamidinium lead iodine CH(NH2)2PbI3 (FAPbI3) are studied by first-principles calculations. For comparison, the cation-doped tetragonal methylammonium lead iodine CH3NH3PbI3 (MAPbI3) are also considered. The calculated formation energies reveal that the Sn atom is easier to dope in the tetragonal MAPbI3/FAPbI3 structure due to the small formation energy of about 0.3 eV. Besides, the band gap of Sn-doped MAPbI3/FAPbI3 is 1.30/1.40 eV, which is considerably smaller than the un-doped tetragonal MAPbI3/FAPbI3. More importantly, compare with the un-doped tetragonal MAPbI3/FAPbI3, the Sn-doped MAPbI3 and FAPbI3 have the larger optical absorption coefficient and theoretical maximum efficiency, especially for Sn-doped FAPbI3. The lower formation energy, suitable band gap and outstanding optical absorption of the Sn-doped FAPbI3 make it promising candidates for high-efficient perovskite cells.