Haiyuan Chen

Bismuth telluride nanostructures: preparation, thermoelectric properties and topological insulating effect

Bismuth telluride is known to wield unique properties for a wide range of device applications. However, as devices migrate to the nanometer scale, significant amount of studies are being conducted to keep up with the rapidly growing nanotechnological field. Bi2Te3 possesses distinctive properties at the nanometer level from its bulk material. Therefore, varying synthesis and characterization techniques are being employed for the realization of various Bi2Te3 nanostructures in the past years. A considerable number of these works have aimed at improving the thermoelectric (TE) figure-of-merit (ZT) of the Bi2Te3 nanostructures and drawing from their topological insulating properties. This paper reviews the various Bi2Te3 and Bi2Te3-based nanostructures realized via theoretical and experimental procedures. The study probes the preparation techniques, TE properties and the topological insulating effects of 0D, 1D, 2D and Bi2Te3 nanocomposites. With several applications as a topological insulator (TI), the topological insulating effect of the Bi2Te3 is reviewed in detail with the time reversal symmetry (TRS) and surface state spins which characterize TIs. Schematics and preparation methods for the various nanostructural dimensions are accordingly categorized.

Efficiency enhancement in inverted planar perovskite solar cells by synergetic effect of sulfated graphene oxide (sGO) and PEDOT:PSS as hole transporting layer

Inverted planar perovskite solar cells (PSCs) exhibiting a high power conversion efficiency (PCE) have mainly been demonstrated by using poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as the hole transport layer (HTL). As an alternative to the PEDOT:PSS, graphene oxide (GO) is also employed as a HTL in PSCs with decent PCEs. However, the strong acidity and hygroscopicity of PEDOT:PSS and insulting property of GO were the major factors for hindering the fabrication of high-performance PSCs. Here, we demonstrated sulfated graphene oxide (sGO) as a HTL replacing the conventionally used GO and PEDOT:PSS in PSCs, but pristine sGO as simple HTL cannot improve photovoltaic performance of PSCs with a maximum efficiency of 9.9%. Hence, we report a simple solution route for preparing a sGO–PEDOT:PSS composite HTL by combining solution-processable sGO with commercialized PEDOT:PSS solution. The PSC fabricated with 1 : 1 sGO–PEDOT:PSS HTL shows a dramatically enhanced PCE of 13.9%, versus 11.5% for PSC with pristine PEDOT:PSS HTL. This promising strategy could be a critical step toward the ideal HTL design for the advancement of practical perovskite solar cells.

Influence of PEG Stoichiometry on Structure-Tuned Formation of Self-Assembled Submicron Nickel Particles

Self-assembled submicron nickel particles were successfully synthesized via the one-step surfactant-assisted solvothermal method. The impact of surfactant and reducing agent stoichiometry is investigated in this manuscript. Different morphologies and structures of Ni particles, including flower-like nanoflakes, hydrangea-like structures, chain structures, sphere-like structures, and hollow structures were prepared through different processing conditions with two parameters such as temperature and time. Based on scanning electron microscopy (SEM), X-ray diffraction (XRD), thermal gravimetric analysis (TGA) and vibrating sample magnetometry (VSM), the submicron nickel particles show good saturation magnetization and excellent thermal stabilities with a possible growth mechanism for the variety of the structure-tuned formation. Importantly, the microwave absorption properties of the submicron nickel particles were studied. The lowest reflection loss of Ni-P9/T200/H15 with a thin layer thickness of 1.7 mm can reach −42.6 dB at 17.3 GHz.

Excellent microwave absorption of lead halide perovskites with high stability

Given the remarkable progress in physical and structural performances of organic–inorganic lead halide perovskites as a superstar in the photovoltaic field, the study of some of their intrinsic properties is still missing. Herein, we report the microwave absorption performance of MAPbX3 (MA = CH3NH3+, X = I−, Br− or Cl−) perovskite crystals in terms of complex permittivity and permeability. We find that the MAPbI3, MAPbBr3 and MAPbCl3 perovskites possess excellent microwave absorbability, and the optimum reflection losses reach −55.23, −54.70 and −46.44 dB at 16.77, 15.46 and 13.54 GHz with a matching thickness of 1.62, 1.76 and 1.95 mm, respectively. Thereafter, we explore the absorption mechanism and find that the microwave absorption properties can be ascribed to the combination of dielectric loss and magnetic loss, but mainly dielectric loss. In addition, the stability of microwave absorbability is also investigated and it is proved to depend significantly on the decomposition of corresponding perovskites. These results highlight the importance of the discovery of microwave absorbability of organic–inorganic lead halide perovskites. More importantly, our study provides perovskite absorbers as a new variable to be considered in the quest for future microwave devices.

Three-leaf dart-shaped single-crystal BN formation promoted by surface oxygen

Two-dimensional hexagonal boron nitride (h-BN) single crystals with various shapes have been synthesized by chemical vapor deposition over the past several years. Here we report the formation of three-leaf dart (3LD)-shaped single crystals of h-BN on Cu foil by atmospheric-pressure chemical vapor deposition. The leaves of the 3LD-shaped h-BN are as long as 18 {\mu}m and their edges are smooth armchair on one side and stepped armchair on the other. Careful analysis revealed that surface oxygen plays an important role in the formation of the 3LD shape. Oxygen suppressed h-BN nucleation by passivating Cu surface active sites and lowered the edge attachment energy, which caused the growth kinetics to change to a diffusion-controlled mode.

Calculation of Elastic Bond Constants in Atomistic Strain Analysis

Strain analysis has significance both for tailoring material properties and designing nanoscale devices. In particular, strain plays a vital role in engineering the growth thermodynamics and kinetics and is applicable for designing optoelectronic devices. In this paper, we present a methodology for establishing the relationship between elastic bond constants and measurable parameters, i.e., Poisson’s ratio ν and systematic elastic constant K. At the atomistic level, this approach is within the framework of linear elastic theory and encompasses the neighbor interactions when an atom is introduced to stress. Departing from the force equilibrium equations, the relationships between ν, K, and spring constants are successfully established. Both the two-dimensional (2D) square lattice and common three-dimensional (3D) structures are taken into account in the procedure for facilitating, bridging the gap between structural complexity and numerical experiments. A new direction for understanding the physical phenomena in strain engineering is established.