Yi-min Ding

The growth mechanism of silicon nanowires and their quantum confinement effect

Abstract Silicon nanowires (SiNWs) with controlled diameters have been synthesized using a physical evaporation method. The growth mechanism of SiNWs is described based on the vapor–liquid–solid (VLS) model, which can well explain much of the morphology of SiNWs. The quantum confinement effect of SiNWs has been studied by photoluminescence (PL) measurements.

Dependence of the silicon nanowire diameter on ambient pressure

Our present work provides a method to control the diameters of the silicon nanowires. As a dominant experimental parameter, the ambient pressure was controlled between 150 and 600 Torr. It is found that the average size of the silicon nanowires increases with increasing ambient pressure. The mean diameter of the silicon nanowires in our study is proportional to the 0.4 power of ambient pressure. Catalytic nanoparticles and the periodic instability of the nanowires suggest a vapor-liquid-solid growth mechanism. For the growth of nanowires, an explanation of the relationship between the mean diameter of the silicon nanowires and the ambient pressure has been proposed.

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.

Enhancement of TE polarized light extraction efficiency in nanoscale (AlN)m /(GaN)n (m>n) superlattice substitution for Al-rich AlGaN disorder alloy: ultra-thin GaN layer modulation

The problem of achieving high light extraction efficiency in Al-rich AlxGaN is of paramount importance for the realization of AlGaN-based deep ultraviolet (DUV) optoelectronic devices. To solve this problem, we investigate the microscopic mechanism of valence band inversion and light polarization, a crucial factor for enhancing light extraction efficiency, in Al-rich AlxGaN alloy using the Heyd–Scuseria–Ernzerhof hybrid functional, local-density approximation with 1/2 occupation, and the Perdew–Burke–Ernzerhof functional, in which the spin–orbit coupling effect is included. We find that the microscopic Ga-atom distribution can effectively modulate the valence band structure of Al-rich AlxGaN. Moreover, we prove that the valence band arrangement in the decreasing order of heavy hole, light hole, and crystal-field split-off hole can be realized by using nanoscale (AlN)m/(GaN)n (m>n) superlattice (SL) substituting for Al-rich AlxGaN disorder alloy as the active layer of optoelectronic devices due to the ultra-thin GaN layer modulation. The valence band maximum, i.e., the heavy hole band, has px- and py-like characteristics and is highly localized in the SL structure, which leads to the desired transverse electric (TE) polarized (E⊥c) light emission with improved light extraction efficiency in the DUV spectral region. Some important band-structure parameters and electron/hole effective masses are also given. The physical origin for the valence band inversion and TE polarization in (AlN)m/(GaN)n SL is analyzed in depth.

Reducing Mg Acceptor Activation-Energy in Al0.83Ga0.17N Disorder Alloy Substituted by Nanoscale (AlN)5/(GaN)1 Superlattice Using MgGa δ-Doping: Mg Local-Structure Effect

Improving p-type doping efficiency in Al-rich AlGaN alloys is a worldwide problem for the realization of AlGaN-based deep ultraviolet optoelectronic devices. In order to solve this problem, we calculate Mg acceptor activation energy and investigate its relationship with Mg local structure in nanoscale (AlN)5/(GaN)1 superlattice (SL), a substitution for Al0.83Ga0.17N disorder alloy, using first-principles calculations. A universal picture to reduce acceptor activation energy in wide-gap semiconductors is given for the first time. By reducing the volume of the acceptor local structure slightly, its activation energy can be decreased remarkably. Our results show that Mg acceptor activation energy can be reduced significantly from 0.44 eV in Al0.83Ga0.17N disorder alloy to 0.26 eV, very close to the Mg acceptor activation energy in GaN, and a high hole concentration in the order of 1019 cm−3 can be obtained in (AlN)5/(GaN)1 SL by MgGa δ-doping owing to GaN-monolayer modulation. We thus open up a new way to reduce Mg acceptor activation energy and increase hole concentration in Al-rich AlGaN.

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.