Zhizhen Ye

Anisotropic Swelling and Fracture of Silicon Nanowires during Lithiation

We report direct observation of an unexpected anisotropic swelling of Si nanowires during lithiation against either a solid electrolyte with a lithium counter-electrode or a liquid electrolyte with a LiCoO(2) counter-electrode. Such anisotropic expansion is attributed to the interfacial processes of accommodating large volumetric strains at the lithiation reaction front that depend sensitively on the crystallographic orientation. This anisotropic swelling results in lithiated Si nanowires with a remarkable dumbbell-shaped cross section, which develops due to plastic flow and an ensuing necking instability that is induced by the tensile hoop stress buildup in the lithiated shell. The plasticity-driven morphological instabilities often lead to fracture in lithiated nanowires, now captured in video. These results provide important insight into the battery degradation mechanisms.

Ultrafast electrochemical lithiation of individual Si nanowire anodes.

Using advanced in situ transmission electron microscopy, we show that the addition of a carbon coating combined with heavy doping leads to record-high charging rates in silicon nanowires. The carbon coating and phosphorus doping each resulted in a 2 to 3 orders of magnitude increase in electrical conductivity of the nanowires that, in turn, resulted in a 1 order of magnitude increase in charging rate. In addition, electrochemical solid-state amorphization (ESA) and inverse ESA were directly observed and characterized during a two-step phase transformation process during lithiation: crystalline silicon (Si) transforming to amorphous lithium-silicon (Li(x)Si) which transforms to crystalline Li(15)Si(4) (capacity 3579 mAh·g(-1)). The ultrafast charging rate is attributed to the nanoscale diffusion length and the improved electron and ion transport. These results provide important insight in how to use Si as a high energy density and high power density anode in lithium ion batteries for electrical vehicle and other electronic power source applications.

Interfacial Control Toward Efficient and Low‐Voltage Perovskite Light‐Emitting Diodes

High-performance perovskite light-emitting diodes are achieved by an interfacial engineering approach, leading to the most efficient near-infrared devices produced using solution-processed emitters and efficient green devices at high brightness conditions.

p-type conduction in N–Al co-doped ZnO thin films

p-type ZnO thin films have been realized by the N–Al co-doping method. Secondary ion mass spectroscopy demonstrated that the N incorporation was enhanced evidently by the presence of Al in ZnO. The lowest room-temperature resistivity was found to be 57.3Ωcm with a Hall mobility of 0.43cm2∕Vs and carrier concentration of 2.25×1017cm−3 for the N–Al co-doped p-type ZnO film deposited on glass substrate. The results were much better than those for the N-doped p-type ZnO. Moreover, the co-doped film possesses a good crystallinity with c-axis orientation and a high transmittance (90%) in the visible region.

Modeling of silica nanowires for optical sensing

Based on evanescent-wave guiding properties of nanowire waveguides, we propose to use single-mode subwavelength-diameter silica nanowires for optical sensing. Phase shift of the guided mode caused by index change is obtained by solving Maxwells equation, and is used as a criterion for sensitivity estimation. Nanowire sensor employing a wire-assembled Mach-Zehnder structure is modeled. The result shows that optical nanowires, especially those fabricated by taper drawing of optical fibers, are promising for developing miniaturized optical sensors with high sensitivity.

Synthesis of porous rhombus-shaped Co3O4 nanorod arrays grown directly on a nickel substrate with high electrochemical performance

Novel rhombus-shaped Co3O4 nanorod (NR) arrays grown directly on a nickel substrate are successfully synthesized via a fluorine-mediated hydrothermal synthesis approach. The rhombic Co3O4 NRs have an average edge length of 400 nm, an induced edge angle of 50° and a length of 15 μm. The possible formation mechanism of the novel NR arrays was investigated by X-ray diffraction, Raman spectroscopy, scanning and transmission electron microscopies, and X-ray photoelectron spectroscopy. The formation of Co(OH)F plays a crucial role in the formation of the rhombus-shaped NR arrays. When tested as anodes for lithium ion batteries (LIBs) without the addition of other ancillary materials (carbon black and binder), the Co3O4 NR arrays on a nickel substrate exhibit a high reversible capacity (over 1000 mA h g−1) and good cycling performance over 20 cycles in the range of 0.005–3 V at a current rate of 1 C.

High-performance planar heterojunction perovskite solar cells: preserving long charge carrier diffusion lengths and interfacial engineering

AbstractWe demonstrate that charge carrier diffusion lengths of two classes of perovskites, CH3NH3PbI3−xClx and CH3NH3PbI3, are both highly sensitive to film processing conditions and optimal processing procedures are critical to preserving the long carrier diffusion lengths of the perovskite films. This understanding, together with the improved cathode interface using bilayer-structured electron transporting interlayers of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM)/ZnO, leads to the successful fabrication of highly efficient, stable and reproducible planar heterojunction CH3NH3PbI3−xClx solar cells with impressive power-conversion efficiencies (PCEs) up to 15.9%. A 1-square-centimeter device yielding a PCE of 12.3% has been realized, demonstrating that this simple planar structure is promising for large-area devices.

Controlling the Lithiation-Induced Strain and Charging Rate in Nanowire Electrodes by Coating

The advanced battery system is critically important for a wide range of applications, from portable electronics to electric vehicles. Lithium ion batteries (LIBs) are presently the best performing ones, but they cannot meet requirements for more demanding applications due to limitations in capacity, charging rate, and cyclability. One leading cause of those limitations is the lithiation-induced strain (LIS) in electrodes that can result in high stress, fracture, and capacity loss. Here we report that, by utilizing the coating strategy, both the charging rate and LIS of SnO(2) nanowire electrodes can be altered dramatically. The SnO(2) nanowires coated with carbon, aluminum, or copper can be charged about 10 times faster than the noncoated ones. Intriguingly, the radial expansion of the coated nanowires was completely suppressed, resulting in enormously reduced tensile stress at the reaction front, as evidenced by the lack of formation of dislocations. These improvements are attributed to the effective electronic conduction and mechanical confinement of the coatings. Our work demonstrates that nanoengineering the coating enables the simultaneous control of electrical and mechanical behaviors of electrodes, pointing to a promising route for building better LIBs.

Transparent and flexible thin films of ZnO-polystyrene nanocomposite for UV-shielding applications

A solution casting approach was developed to obtain flexible and self-supporting ZnO-polystyrene (PS) nanocomposite thin films (ca. 360 μm) which were highly transparent in the visible region and exhibited excellent UV-absorbing properties. The nanocomposite films were prepared from homogeneous solutions of ligand-modified ZnO nanocrystals and PS. UV-Vis spectra, transmission electron microscopy (TEM), thermogravimetric analysis (TGA), X-ray diffraction (XRD) and photoluminescence (PL) characterization techniques were employed to study optical and structural properties, as well as thermal stabilities of the nanocomposite films. Results revealed the high UV-shielding efficiency of the composites: for a film containing 1.0 wt. % of ZnO nanocrystals, over 99% of UV light at wavelengths between 200 and 360 nm was absorbed while the optical transparency in the visible region was slightly below that of a neat PS film. Minute amounts of organic ligands minimized aggregation of the ZnO nanocrystals, leading to the homogeneous blend solutions and eventually the well dispersed ZnO-PS nanocomposite films with stable optical properties. The present work is of interest for developing transparent UV-shielding materials and should help in the understanding and design of inorganic-polymer nanocomposites with desired properties.

3D graphene foams decorated by CuO nanoflowers for ultrasensitive ascorbic acid detection

When the in vitro research works of biosensing begin to mimic in vivo conditions, some certain three-dimensional (3D) structures of biosensors are needed to accommodate biomolecules, bacteria or even cells to resemble the in vivo 3D environment. To meet this end, a novel method of synthesizing CuO nanoflowers on the 3D graphene foam (GF) was first demonstrated. The 3DGF/CuO nanoflowers composite was used as a monolithic free-standing 3D biosensor for electrochemical detection of ascorbic acid (AA). The 3D conductive structure of the GF is favorable for current collection, mass transport and loading bioactive chemicals. And CuO nanoflowers further increase the active surface area and catalyze the redox of AA. Thus, all these features endows 3DGF/CuO composite with outstanding biosensing properties such as an ultrahigh sensitivity of 2.06 mA mM(-1) cm(-2) to AA at 3 s response time.