Jun-jie Shi

First-principles investigation on optical properties of GaN and InGaN alloys

Density functional calculations using the Perdew?Burke?Ernzerhof (PBE) functional, the Heyd?Scuseria?Ernzerhof (HSE) and the local density approximation (LDA)-1/2 methods are performed on GaN and InGaN alloys. Compared with the HSE and LDA-1/2 calculations, we find that the electronic structures and typical features of the dielectric spectra given by the PBE functional are reasonable except for the underestimation of the band gap. The dielectric functions of Ga-rich InxGa1?xN alloys with different In compositions and distributions are discussed in detail. Our results show that both several-atom In?N clusters and short In?N?chains, especially the small In?N clusters, enhance the near-band-edge optical absorption and improve the optical properties of InxGa1?xN alloys. Good agreement with experimental and previous theoretical results is obtained.

Anomalous Light Emission and Wide Photoluminescence Spectra in Graphene Quantum Dot: Quantum Confinement from Edge Microstructure

The physical origin of the observed anomalous photoluminescence (PL) behavior, that is, the large-size graphene quantum dots (GQDs) exhibiting higher PL energy than the small ones and the broadening PL spectra from deep ultraviolet to near-infrared, has been debated for many years. Obviously, it is in conflict with the well-accepted quantum confinement. Here we shed new light on these two notable debates by state-of-the-art first-principles calculations based on many-body perturbation theory. We find that quantum confinement is significant in GQDs with remarkable size-dependent exciton absorption/emission. The edge environment from alkaline to acidic conditions causes a blue shift of the PL peak. Furthermore, carbon vacancies are inclined to assemble at the GQD edge and form the tiny edge microstructures. The bound excitons, localized inside these edge microstructures, determine the anomalous PL behavior (blue and UV emission) of large-size GQDs. The bound excitons confined in the whole GQD lead to the low-energy transition.

Surface polaronic effect on donor-impurity states of a wurtzite nitride nanowire: Two-parameter variational approach

By employing the two-parameter variational approach, the donor-impurity states with surface optical (SO) phonons, also called SO phonon bound polarons in a quasi-one-dimensional (Q1D) wurtzite nanowire (NW) are investigated. Numerical calculations on a GaN NW are performed. The results reveal that the SO phonon contribution to the binding energy of the SO phonon bound polaron in GaN NWs reaches 200 meV, which is one order of magnitude larger than that of GaAs NWs with the same radius. The large contribution of SO phonons to the total binding energy is mainly ascribed to the stronger electron-phonon coupling constant in GaN materials. The calculated results of impurity binding energy are consistent with the recent experimental measurement of the active energy in GaN NW systems. The numerical results also shows that the two-parameter variational approach is necessary and suitable for the description of donor-impurity states in Q1D wurtzite GaN NW structures, especial for the NWs with a relatively small radi...

High-Output-Power Ultraviolet Light Source from Quasi-2D GaN Quantum Structure.

Quasi-2D GaN layers inserted in an AlGaN matrix are proposed as a novel active region to develop a high-output-power UV light source. Such a structure is successfully achieved by precise control in molecular beam epitaxy and shows an amazing output power of ≈160 mW at 285 nm with a pulsed electron-beam excitation. This device is promising and competitive in non-line-of-sight communications or the sterilization field.

Ferromagnetism, adatom effect, and edge reconstruction induced by Klein boundary in graphene nanoribbons

The electronic structure and magnetic characteristics of Klein graphene nanoribbons (KGNRs), as observed by Suenaga and Koshino [K. Suenaga and M. Koshino, Nature 468, 1088 (2010)], are investigated using first-principles calculations. We find three new characteristics induced by the Klein boundary. First, the localized edge states in the KGNRs have a ferromagnetic coupling rather than the antiferromagnetic coupling of the zigzag graphene nanoribbons (ZGNRs). Liebs theorem is no longer applicable in the KGNRs. Second, the marginal single carbon adatom of the ZGNRs can destroy the edge states nearby. The edge states can recover if the length of the zigzag chains is equal to or greater than five times that of the lattice constant. Finally, we show that the pentagon-heptagon edge can be induced from the Klein boundary.

Improvement of n-type conductivity in hexagonal boron nitride monolayers by doping, strain and adsorption

The n-type conductivity of hexagonal boron nitride (h-BN) monolayers has been studied using state-of-the-art first-principles calculations. We adopt three different methods, which are C, S, Si and Si–nO (n = 1, 2, 3) doping, applying strain and alkali metal (AM) atom (Li, Na, K and Rb) adsorption, to improve the n-type conductivity of h-BN monolayers. Three important results are obtained. First, as donor dopants, the activation energies (ED) of CB, SN and SiB are 1.22, 0.50 and 0.86 eV, respectively. The ED of Si can be further reduced via Si–nO codoping with an increasing O-atom number and it decreases to 0.39 eV for Si–3O. Second, ED can be effectively reduced by applying strain. The Si–3O has the lowest activation energy of 0.06 eV under 4% compressive biaxial strain. Finally, there is an obvious charge transfer from adsorbed AM atoms to h-BN monolayers, which results in an enhancement of electron concentration and improvement of n-type conductivity. This charge transfer is insensitive to the strain. The present results are significant for improving the performance of h-BN based two-dimensional optoelectronic nanodevices.

Origin of the wide band gap from 0.6 to 2.3 eV in photovoltaic material InN: quantum confinement from surface nanostructure

The fierce controversy of the band gap of InN from 0.6 to 2.3 eV has been debated for nearly fifteen years. Numerous possible reasons, such as the Moss–Burstein effect, oxygen incorporation and indium : nitrogen stoichiometry, have been postulated to interpret this outstanding issue. Nevertheless, none of them can provide a convincing and comprehensive explanation. Here, we shed new light on this notable debate using state-of-the-art first-principles calculations based on many-body perturbation theory combined with experiments. We demonstrate that the ubiquitous surface nanostructures (NSs) with amazing characteristics, that is, the outgrowth needle- or dot-like nanocrystals of the InN film surface, are vital and significantly alter its electronic structure and optical properties. The valence band inversion in the decreasing order of crystal-field split-off hole and heavy/light hole can occur in these NSs, which leads to an optical transition switch from E ⊥ c in bulk InN to E ‖ c in surface InN NSs. Moreover, the strong surface bound excitons can be induced in InN NSs due to quantum confinement, resulting in the exciton absorption/emission from infrared to visible (green) wavelength. The blue shift of the PL peak in InN quantum dots with decreasing size further provides convincing evidence for the essence of the large variable band gap of InN. The electronic structure, optical polarization properties and especially the strong exciton effect of InN NSs have been investigated systematically and comprehensively and lays the foundation for future applications of InN QD based photovoltaic and light-harvesting devices.

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...

K-Λ crossover transition in the conduction band of monolayer MoS2 under hydrostatic pressure

We experimentally demonstrate the direct-to-indirect bandgap transition of monolayer MoS2 under hydrostatic pressure. Monolayer MoS2 is a promising material for optoelectronics applications owing to its direct bandgap, enhanced Coulomb interaction, strong spin-orbit coupling, unique valley pseudospin degree of freedom, etc. It can also be implemented for novel spintronics and valleytronics devices at atomic scale. The band structure of monolayer MoS2 is well known to have a direct gap at K (K′) point, whereas the second lowest conduction band minimum is located at Λ point, which may interact with the valence band maximum at K point, to make an indirect optical bandgap transition. We experimentally demonstrate the direct-to-indirect bandgap transition by measuring the photoluminescence spectra of monolayer MoS2 under hydrostatic pressure at room temperature. With increasing pressure, the direct transition shifts at a rate of 49.4 meV/GPa, whereas the indirect transition shifts at a rate of −15.3 meV/GPa. We experimentally extract the critical transition point at the pressure of 1.9 GPa, in agreement with first-principles calculations. Combining our experimental observation with first-principles calculations, we confirm that this transition is caused by the K-Λ crossover in the conduction band.