Junkai Deng

Enhanced Charge Carrier Mobility in Two-Dimensional High Dielectric Molybdenum Oxide

We demonstrate that the energy bandgap of layered, high-dielectric α-MoO(3) can be reduced to values viable for the fabrication of 2D electronic devices. This is achieved through embedding Coulomb charges within the high dielectric media, advantageously limiting charge scattering. As a result, devices with α-MoO(3) of ∼11 nm thickness and carrier mobilities larger than 1100 cm(2) V(-1) s(-1) are obtained.

Mechanism of the shape memory effect in martensitic alloys: an assessment

The (one-way) shape memory effect is a phenomenon that when a martensitic alloy is deformed in a martensitic state it recovers its original shape upon heating to the parent phase. This is a universal effect for certain martensitic alloys. We will assess the mechanism of the effect critically and select the essential factors which govern the effect. We try to understand it from a unified view, invoking the group–subgroup symmetry relation between the parent and martensite phase, along with analysis of reversible twinning modes in martensite. By such an assessment, we will show why typical shape memory alloys, such as Ti–Ni, Cu–Al–Ni etc., exhibit good shape memory characteristics, while others, such as ferrous alloys, do not. Thus, we will show that most of the shape memory characteristics of various martensitic alloys can be understood consistently from such an approach.

Band engineering of Ni1−xMgxO alloys for photocathodes of high efficiency dye-sensitized solar cells

Density functional theory calculations were carried out for Ni1−xMgxO alloys using both GGA+U method and hybrid exchange-correlation functional HSE06. We find that the band gap of Ni1−xMgxO is a nonlinear function of MgO concentration with a strong bowing behavior at high Mg content. Band edge alignment is determined using heterojunction superlattice models. The valence-band-maximum of Ni1−xMgxO is shown to be tunable within a range of 0.90 eV. By comparing with the highest-occupied-molecular-orbital levels of some of the most widely used dye molecules, we propose that Ni1−xMgxO is a promising alternate to replace NiO photocathode in dye-sensitized solar cells with an enhanced open-circuit voltage and transparency of cathode films.

Two-dimensional shape memory graphene oxide

Driven by the increasing demand for micro-/nano-technologies, stimuli-responsive shape memory materials at nanoscale have recently attracted great research interests. However, by reducing the size of conventional shape memory materials down to approximately nanometre range, the shape memory effect diminishes. Here, using density functional theory calculations, we report the discovery of a shape memory effect in a two-dimensional atomically thin graphene oxide crystal with ordered epoxy groups, namely C8O. A maximum recoverable strain of 14.5% is achieved as a result of reversible phase transition between two intrinsically stable phases. Our calculations conclude co-existence of the two stable phases in a coherent crystal lattice, giving rise to the possibility of constructing multiple temporary shapes in a single material, thus, enabling highly desirable programmability. With an atomic thickness, excellent shape memory mechanical properties and electric field stimulus, the discovery of a two-dimensional shape memory graphene oxide opens a path for the development of exceptional micro-/nano-electromechanical devices.

Electric Field Induced Reversible Phase Transition in Li Doped Phosphorene: Shape Memory Effect and Superelasticity

Phosphorene, the single-layer form of black phosphorus, as a new member of atomically thin material family, has unique puckered atomistic structure and remarkable physical and chemical properties. In this paper, we report a discovery of an unexpected electromechanical energy conversion phenomenon-shape memory effect-in Li doped phosphorene P4Li2, using ab initio density functional theory simulations. Two stable phases are found for the two-dimensional (2D) P4Li2 crystal. Applying an external electric field can turn on or off the unique adatom switches in P4Li2 crystals, leading to a reversible structural phase transition and thereby the shape memory effect with an tunable strain output as high as 2.06%. Our results demonstrate that multiple temporary shapes are attainable in one piece of P4Li2 material, offering programmability that is particularly useful for device designs. Additionally, the P4Li2 displays superelasticity that can generate a pseudoelastic tensile strain up to 6.2%. The atomic thickness, superior flexibility, excellent electromechanical strain output, the special shape memory phenomenon, and the programmability feature endow P4Li2 with great application potential in high-efficient energy conversion at nanoscale and flexible nanoelectromechanical systems.

Enhanced lithium adsorption and diffusion on silicene nanoribbons

For developing lithium (Li)-ion batteries with large energy densities and high rate capabilities, it is crucial that electrode materials show good reactivity with charge carrying Li ions, and simultaneously allow for their fast transport. Using first principles density functional theory calculations, here we investigate the interaction of Li with the edges of monolayer as well as multilayer silicene and identify the low energy Li binding sites and the pathways for Li diffusion. Both Li binding and diffusion are found to be significantly controlled by the morphology of the silicene edges. We show that among known structural forms of silicon, monolayer silicene edges provide the strongest binding with Li. The energy barriers for Li diffusion on monolayer silicene nanoribbons are generally very low (0.14–0.26 eV), and in particular, zigzag edges can allow for up to 80 times faster diffusion than on a silicene monolayer. Our results indicate that monolayer silicene nanoribbons terminated with zigzag edges can provide promising candidate materials for the negative electrodes of Li-ion batteries.

Edge stresses of non-stoichiometric edges in two-dimensional crystals

The elastic properties of edges are among the most fundamental properties of finite two-dimensional (2D) crystals. Using a combination of the first-principles density functional theory calculations and a continuum elasticity model, we present an efficient technique to determine the edge stresses of non-stoichiometric orientations in multicomponent 2D crystals. Using BN and MoS2 as prototypical examples of 2D binary monolayers with threefold in-plane symmetry, we unambiguously compute unique edge stresses of commonly observed non-stoichiometric edges. Our results show that the edge stresses for these structurally distinct orientations can differ significantly from the average values that have been typically reported to date.

Fabrication of Phosphorus Nanostructures/TiO 2 Composite Photocatalyst with enhancing Photodegradation and Hydrogen Production from Water under Visible Light

As the world faces serious environmental pollution and energy shortage, developing Vis-light-driven photocatalysts for water splitting is highly attractive in clean energy utilization. Fabricating heterostructures has been proposed to be an efficient system to enhance the photocatalytic activity. However, synthesizing heterostructures with good contact and understanding charge transfer dynamics are still unresolved issues. In this work, a facile calcination approach was used to synthesize red phosphorus (RP) nanostructures/TiO2 heterostructured composites. The RP nanostructures were directly grown on the TiO2 nanoparticles with an intimate surface contact. By adjusting the molar ratio of amorphous RP to TiO2 and the synthesizing temperature, thin nanorod-like RP nanostructures with a large exposed surface and a good surface contacting with TiO2 were obtained. The synergetic effect of heterostructured RP/TiO2 composites leads to an enhanced charge separation and transfer, and a better utilization of visible-light. As expected, the RP/TiO2-700 °C composites exhibit good photocatalytic activity of degrading RhB and the optimal H2 evolution rate. This work not only provides a method to prepare earth abundant elemental phosphorus well-contacted heterostructures, expand the well-known UV-active TiO2 photocatalyst to visible active, but also deepens understanding of charge transfer dynamics in heterostructured photocatalyst.

Cation ordering induced polarization enhancement for PbTiO 3 − SrTiO 3 ferroelectric-dielectric superlattices

In this paper, an efficient computational material design approach (cluster expansion) is employed for the ferroelectric

Evidence for short-time limit of martensite deaging in shape-memory alloys: Experiment and atomistic simulation

PbTiO_3