Yu-lang Cen

Modulation of electronic and magnetic properties in InSe nanoribbons: edge effect

Quite recently, the two-dimensional (2D) InSe nanosheet has become a hot material with great promise for advanced functional nano-devices. In this work, for the first time, we perform first-principles calculations on the structural, electronic, magnetic and transport properties of 1D InSe nanoribbons with/without hydrogen or halogen saturation. We find that armchair ribbons, with various edges and distortions, are all nonmagnetic semiconductors, with a direct bandgap of 1.3 (1.4) eV for bare (H-saturated) ribbons, and have the same high electron mobility of about 103 cm2V-1s-1 as the 2D InSe nanosheet. Zigzag InSe nanoribbons exhibit metallic behavior and diverse intrinsic ferromagnetic properties, with the magnetic moment of 0.5-0.7 μ B per unit cell, especially for their single-edge spin polarization. The edge spin orientation, mainly dominated by the unpaired electrons of the edge atoms, depends sensitively on the edge chirality. Hydrogen or halogen saturation can effectively recover the structural distortion, and modulate the electronic and magnetic properties. The binding energy calculations show that the stability of InSe nanoribbons is analogous to that of graphene and better than in 2D InSe nanosheets. These InSe nanoribbons, with novel electronic and magnetic properties, are thus very promising for use in electronic, spintronic and magnetoresistive nano-devices.

Tuning the electronic and optical properties of hexagonal boron-nitride nanosheet by inserting graphene quantum dots

It is difficult to integrate two-dimensional (2D) graphene and hexagonal boron-nitride (h-BN) in optoelectronic nanodevices, due to the semi-metal and insulator characteristic of graphene and h-BN, respectively. Using the state-of-the-art first-principles calculations based on many-body perturbation theory, we investigate the electronic and optical properties of h-BN nanosheet embedded with graphene dots. We find that C atom impurities doped in h-BN nanosheet tend to phase-separate into graphene quantum dots (QD), and BNC hybrid structure, i.e. a graphene dot within a h-BN background, can be formed. The band gaps of BNC hybrid structures have an inverse relationship with the size of graphene dot. The calculated optical band gaps for BNC structures vary from 4.71 eV to 3.77 eV, which are much smaller than that of h-BN nanosheet. Furthermore, the valence band maximum is located in C atoms bonded to B atoms and conduction band minimum is located in C atoms bonded to N atoms, which means the electron and hole...

Promising photovoltaic and solid-state-lighting materials: two-dimensional Ruddlesden-Popper type lead-free halide double perovskites Csn+1Inn/2Sbn/2I3n+1 (n=3) and Csn+1Inn/2Sbn/2Cl3n+1/Csm+1Cum/2Bim/2Cl3m+1 (n=3, m=1)

As a newly-rising member in the perovskite family of solar cell absorbers, the lead-free halide double perovskites Cs2M+M3+X6 (X = Cl, Br, I), with suitable bandgaps in the visible-light region and intrinsic thermodynamic stability, pave a new way in designing environmentally friendly perovskite solar cell devices. Here, for the first time, we optimize 24 kinds of layered Ruddlesden–Popper (RP) type n = 1–3 Csn+1Mn/2+Mn/23+X3n+1 (M+: In+, Cu+, Ag+, Au+; M3+: Bi3+, Sb3+) and investigate their electronic, optical and transport properties based on powerful first-principles and GW-BSE calculations. Surprisingly, we find that Cs4In3/2Sb3/2I10 (n = 3) has a direct bandgap of 1.29 eV, a very suitable value for visible-light absorption and emission. In order to modulate the bandgap and charge distribution, we further construct 7 two-dimensional halide double perovskite heterostructures with direct bandgaps and type-II band alignment, leading to a remarkable spatial separation of photo-generated carriers and a noticeable reduction of the carrier recombination rate, which is beneficial for photovoltaic devices. Typically, the Cs4In3/2Sb3/2Cl10/Cs2Cu1/2Bi1/2Cl4 heterostructure has a direct bandgap of 1.65 eV with a great electron–hole spatial separation. Both the different layered CsInSbI perovskites and CsInSbCl/CsCuBiCl-based heterostructures have an ultrahigh absorption coefficient of ∼105 cm−1 in the visible-light region, which is comparable with bulk MAPbI3. Moreover, their electron and hole mobilities are three/four orders of magnitude larger than MAPbI3 up to 102–103 cm2 V−1 s−1. The novel properties of the suitable bandgap, spatial separation of carriers, excellent absorption coefficient and extremely high carrier mobility suggest that the 2D layered RP-type CsInSbI- and CsInSbCl/CsCuBiCl-based perovskites have great potential as promising lead-free solar cell absorbers and solid-state-lighting materials.