Jiaqi Zhu

Strong and stiff aramid nanofiber/carbon nanotube nanocomposites.

Small but strong carbon nanotubes (CNTs) are fillers of choice for composite reinforcement owing to their extraordinary modulus and strength. However, the mechanical properties of the nanocomposites are still much below those for mechanical parameters of individual nanotubes. The gap between the expectation and experimental results arises not only from imperfect dispersion and poor load transfer but also from the unavailability of strong polymers that can be effectively utilized within the composites of nanotubes. Aramid nanofibers (ANFs) with analogous morphological features to nanotubes represent a potential choice to complement nanotubes given their intrinsic high mechanical performance and the dispersible nature, which enables solvent-based processing methods. In this work, we showed that composite films made from ANFs and multiwalled CNTs (MWCNTs) by vacuum-assisted flocculation and vacuum-assisted layer-by-layer assembly exhibited high ultimate strength of up to 383 MPa and Youngs modulus (stiffness) of up to 35 GPa, which represent the highest values among all the reported random CNT nanocomposites. Detailed studies using different imaging and spectroscopic characterizations suggested that the multiple interfacial interactions between nanotubes and ANFs including hydrogen bonding and π-π stacking are likely the key parameters responsible for the observed mechanical improvement. Importantly, our studies further revealed the attractive thermomechanical characteristics of these nanocomposites with high thermal stability (up to 520 °C) and ultralow coefficients of thermal expansion (2-6 ppm·K(-1)). Our results indicated that ANFs are promising nanoscale building blocks for functional ultrastrong and stiff materials potentially extendable to nanocomposites based on other nanoscale fillers.

Photovoltaic characteristics of amorphous silicon solar cells using boron doped tetrahedral amorphous carbon films as p-type window materials

Boron doped tetrahedral amorphous carbon (ta-C:B) was prepared by filtered cathodic vacuum arc deposition. A band gap of 2.0eV and a conductivity of 1.42×10−7S∕cm were obtained at the doping ratio of 2.13at.%. A device structure was deduced from the conventional amorphous silicon (a-Si:H) solar cell using the ta-C:B window layer. Photovoltaic parameters of the cells were studied by varying the boron content in the ta-C:B films. A roughly 10% relative improvement of conversion efficiency was observed compared to the normal a-Si:H solar cell. The improved cell performance results from the enhancement of short wavelength response.

Dopant-induced band filling and bandgap renormalization in CdO:In films

Published in J. Phys D: Appl. Phys. 46 (2013) 195102. http://dx.doi.org/10.1088/0022-3727/46/19/195102 Dopant-induced band filling and bandgap renormalization in CdO:In films Yuankun Zhu 1 , Rueben J. Mendelsberg 2,3 , Jiaqi Zhu 1 , Jiecai Han 1 and Andre Anders 2 Center for Composite Materials and Structures, Harbin Institute of Technology, Harbin 150080, China Plasma Applications Group, Lawrence Berkeley National Laboratory, Berkeley, California, 94720 Materials Science Division, Argonne National Laboratory, Argonne, Illinois, 60439 Email: zhujq@hit.edu.cn (Jiaqi Zhu) Acknowledgment Research was supported by the LDRD Program of Lawrence Berkeley National Laboratory, by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Building Technology, of the U.S. Department of Energy under U.S. Department of Energy Contract No. DE-AC02-05CH11231. Additional support was provided by the National Natural Science Foundation of China (Grant No.51072039 and 51222205), and the Ph.D. Programs Foundation of the Ministry of Education of China (20112302110036). DISCLAIMER This document was prepared as an account of work sponsored by the United States Government. While this document is believed to contain correct information, neither the United States Government nor any agency thereof, nor The Regents of the University of California, nor any of their employees, makes any warranty, express or implied, or assumes any legal responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by its trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof, or The Regents of the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof or The Regents of the University of California.

Hydrogen Bonding Stabilized Self-Assembly of Inorganic Nanoparticles: Mechanism and Collective Properties.

Developing a simple and efficient method to organize nanoscale building blocks into ordered superstructures, understanding the mechanism for self-assembly and revealing the essential collective properties are crucial steps toward the practical use of nanostructures in nanotechnology-based applications. In this study, we showed that the high-yield formation of ZnO nanoparticle chains with micrometer length can be readily achieved by the variation of solvents from methanol to water. Spectroscopic studies confirmed the solvent effect on the surface properties of ZnO nanoparticles, which were found to be critical for the formation of anisotropic assemblies. Quantum mechanical calculations and all atom molecular dynamic simulations indicated the contribution of hydrogen bonding for stabilizing the structure in water. Dissipative particle dynamics further revealed the importance of solvent-nanoparticle interactions for promoting one-dimensional self-assembly. The branching of chains was found upon aging, resulting in the size increase of the ensembles and network formation. Steady-state and time-resolved luminescent spectroscopes, which probed the variation of defect-related emission, revealed stronger Forster resonance energy transfer (FRET) between nanoparticles when the chain networks were formed. The high efficiency of FRET quenching can be ascribed to the presence of multiple energy transfer channels, as well as the short internanoparticle distances and the dipole alignment.

sp3-rich deposition conditions and growth mechanism of tetrahedral amorphous carbon films deposited using filtered arc

Tetrahedral amorphous carbon (ta-C) films with many superior properties approaching those of diamond crystal were prepared using filtered cathodic vacuum arc technology. To ascertain the sp3-rich deposition condition, the dependence of the film microstructure on the deposition energy was investigated by means of visible Raman spectroscopy, x-ray photoelectron spectroscopy, electron energy loss spectroscopy, x-ray reflectivity, and nanoindentation. The maximum hardness and Young’s modulus are achieved at a bias of −80V, at which the maximum sp3 fraction of about 82% is obtained. Under this condition, the most symmetric Raman line shape, the highest x-ray photoemission C 1s core level position and a π* transition peak with the smallest integral area in the K-edge spectra are simultaneously achieved. The structural properties are found to be strongly correlated with the mass density of the films. At the optimal substrate bias of −80V, the film mass density reaches its maximum value. The cross section of the ...

An Epidermis-like Hierarchical Smart Coating with a Hardness of Tooth Enamel

We overcome the fundamental dilemma in achieving hard materials with self-healing capability by integrating an epidermis-like hierarchical stratified structure with attractive mechanical and barrier properties of graphene oxide and show that such biomimetic design enables a smart hierarchical coating system with a synergetic healing effect and a record-high stiffness (31.4 ± 1.8 GPa)/hardness (2.27 ± 0.09 GPa) among all self-healable polymeric films even comparable to that of tooth enamel. A quasi-linear layer-by-layer (LBL) film with constituent graphene oxide is deposited on top of an exponential LBL counterpart as a protective hard layer, forming a hierarchical stratified assembly mimicking the structure of epidermis. The hybrid multilayers can achieve a complete restoration after scratching thanks to the mutual benefit: The soft underneath cushion can provide additional polymers to assist the recovery of the outer hard layer, which in turn can be a sealing barrier promoting the self-healing of the soft layer during stimulated polymer diffusion. The presenting hybridization mode of LBL assembly represents a promising tool for integrating seemingly contradictory properties in artificial materials with potential performances surpassing those in nature.

Improved work function of preferentially oriented indium oxide films induced by the plasma exposure technique

The preferentially oriented In2O3 thin films were prepared on glass substrates by conventional magnetron sputtering with Ar+ plasma exposure at room temperature. Based on the x-ray diffraction, x-ray photoelectron spectroscopy, and UV photoelectron spectroscopy results, it was found that the Ar+ plasma exposure not only enhanced the low-temperature crystallization of In2O3 thin films, but also led to a dramatic improvement in the work function. Furthermore, it demonstrated that the shift mechanism of the work function in In2O3 thin films mainly combined with theelimination of oxygen defects and the change of the preferential orientation of In2O3 film surface.

Structural Characteristics and Electrode Activities of Phosphorus Incorporated Tetrahedral Amorphous Carbon Films

Studies on structural properties and electrochemical behaviors of conductive phosphorus incorporated tetrahedral amorphous carbon (ta-C:P) films deposited using filtered cathodic vacuum arc technique with PH3 as the dopant source are presently reported. The structural characteristics of the films are characterized by X-ray photoelectron spectroscopy, Raman spectroscopy and Fourier transform infrared spectroscopy. The electrochemical reactivity of ta-C:P films are evaluated by cyclic voltammetry and differential pulse voltammetry. We find that phosphorus implantation enhances the clustering of sp2 sites dispersed in sp3 skeleton, develops the densities of π and π * states and improves the electrical and electrochemical behaviors of the films. It has been established that the inorganic films with amorphous structure appear excellent electrical conductivity and electrochemical activity similar to boron-doped diamond or nitrogen-doped amorphous carbon electrodes. These characteristics demonstrate greatly potential application of ta-C:P films as electrode materials in terms of its large electrochemical potential window, low background current, and a considerable electrochemical activity towards ferricyanide reduction and metal ions analysis.

Growth of three-dimensional diamond mosaics by microwave plasma-assisted chemical vapor deposition

We realized the growth of a novel type of diamond mosaic crystal by chemical vapor deposition of a diamond layer on tightly placed oriented seed substrates, using a combination of seeds of different heights to form a three-dimensional structure. Here, a simple T-shaped mosaic is demonstrated as a proof of the principle. The 3D mosaic was epitaxially grown by microwave plasma CVD on (100)-oriented 3 × 3 × 1 mm3 type Ib HPHT diamond substrates arranged horizontally and vertically. Very straight and sharp junctions between the vertical and horizontal parts were observed and characterized with SEM. Confocal Raman spectroscopy mapping of the CVD diamond layer revealed only a narrow (∼20 μm wide) zone of enhanced defect abundance and/or non-uniform stress, if any, around the junction. The Raman and photoluminescence mapping of the mosaic’s cross-section gave further information on the spatial distribution of the epilayer with a thickness of up to 200 μm. The work demonstrates diamond growth on both sides of the vertically positioned seed plate, with a film thickness variation of less than 25%, thus doubling the diamond mass uptake achieved with the CVD process. The developed technique clears the way for the design and growth of various complex diamond shapes.

Diamond Deposition on Graphite in Hydrogen Microwave Plasma

Hydrogen plasma etching of graphite generates radicals that can be used for diamond synthesis by chemical vapor deposition (CVD). We studied the etching of polycrystalline graphite by a hydrogen microwave plasma, growth of diamond particles of the non-seeded graphite substrates, and characterized the diamond morphology, grain size distribution, growth rate, and phase purity. The graphite substrates served simultaneously as a carbon source, this being the specific feature of the process. A disorder of the graphite surface structure reduces as the result of the etching as revealed with Raman spectroscopy. The diamond growth rate of 3 – 5 μm/h was achieved, the quality of the produced diamond grains improving with growth time due to inherently nonstationary graphite etching process. Received on 29-12-2017 Accepted on 05-03-2018 Published on 16-08-2018