Han, Ali; Aljarb, Areej; Liu, Sheng; Li, Peng; Ma, Chun; Xue, Fei; Lopatin, Sergei; Yang, Chih-Wen; Huang, Jing-Kai; Wan, Yi; Zhang, Xixiang; Xiong, Qihua; Huang, Kuo-Wei; Tung, Vincent; Anthopoulos, Thomas D.; Li, Lain-Jong(Nanoscale Horizons, Royal Society of Chemistry (RSC), 2019)[Article]
Bilayers of two-dimensional (2D) transition metal chalcogenides (TMDs) such as WSe2 have been attracting increasing attention owing to the intriguing properties involved in the different stacking configurations. The growth of bilayer WSe2 by chemical vapor deposition (CVD) has been facilely obtained without proper control of the stacking configuration. Herein, we report the controlled growth of bilayer WSe2 crystals as large as 30 μm on c-plane sapphire by the CVD method. Combining second harmonic generation (SHG), low-frequency Raman and scanning transmission electron microscopy (STEM), we elucidate the as-grown bilayer WSe2 with a 2H stacking configuration. Atomic force microscope (AFM) measurements reveal that the prominent atomic steps provide the energetically favorable templates to guide the upper layer nuclei formation, resembling the “graphoepitaxial effect” and facilitating the second WSe2 layer following the layer-by-layer growth mode to complete the bilayer growth. Field-effect charge transport measurement performed on bilayer WSe2 yields a hole mobility of up to 40 cm2 V−1 s−1, more than 3× higher than the value achieved in monolayer WSe2-based devices. Our study provides key insights into the growth mechanism of bilayer WSe2 crystals on sapphire and unlocks the opportunity for potential bilayer and multilayer TMD electronic applications.
Sun, Xi-xi; Zhang, Junwei; Lv, Xiang; Zhang, Xixiang; Liu, Yao; Li, Fei; Wu, Jiagang(Journal of Materials Chemistry A, Royal Society of Chemistry (RSC), 2019)[Article]
The understanding of high piezoelectricity in potassium sodium niobate (KNN)-based ceramics with a new phase boundary has been limited to unpoled samples. Here, the phase structure, domain structure, and phenomenological theory were studied on both unpoled and poled samples by taking (0.99 − x)(K0.48Na0.52)(Nb0.955Sb0.045)O3–0.01SrZrO3–x(Bi0.5Ag0.5)ZrO3 ceramics as an example. Shifting the phase transition temperatures to room temperature can result in the coexistence of a ferroelectric matrix containing an orthorhombic–tetragonal (O–T) coexisting phase and rhombohedral (R)-related polar nanoregions (PNRs), and then the miniature and nanoscale domain structure can be demonstrated. During the poling process, the R phase-related PNRs can facilitate domain switching and polarization rotation, resulting in a single domain structure and enhanced evidence of the R phase. Therefore, high piezoelectricity originates from a single domain feature as well as the diffused multi-phase coexistence in association with R phase related PNRs. This study provides a systematic approach to understand the physical mechanisms of enhanced piezoelectricity in KNN-based ceramics.
Zeng, Xue; Zhang, Junwei; Si, Mingsu; Cao, Derang; Deng, Xia; Ma, Hongbin; Lan, Qianqian; Xue, Desheng; Zhang, Xixiang; Tao, Kun; Peng, Yong(Nanoscale, Royal Society of Chemistry (RSC), 2019)[Article]
Identifying the dopants and their occupation sites in rare-earth-doped permanent magnets is critical not only to understand the mechanism of tuning their magnetic properties, but also to develop guiding principles to further improve their performance. In this study, we present a direct observation of the preferred atomic sites of La atoms in La-doped M-type SrFe12O19 hexaferrite. Our data solidly clarified that only the Sr2+ cations were replaced by La3+ cations, and the La-doping caused the changes in the valence states of iron cations located at the 4f1 and 2a crystallographic sites. First principles calculations further revealed that after La-doping, the changes in the spin states of the Fe3+ cations located at the 4f1 tetrahedral sites resulted in magnetization enhancement and those of the 2a octahedral sites contributed to electrical neutrality, well matching the experimental atomic-column resolution EELS and magnetic measurement results.
Zhang, Junli; Zhu, Shimeng; Li, Hongli; Zhu, Liu; Hu, Yang; Xia, Weixing; Zhang, Xixiang; Peng, Yong; Fu, Jiecai(Nanoscale, Royal Society of Chemistry (RSC), 2018)[Article]
Discovering how the magnetization reversal process is governed by the magnetic anisotropy in magnetic nanomaterials is essential and significant to understand the magnetic behaviour of micro-magnetics and to facilitate the design of magnetic nanostructures for diverse technological applications. In this study, we present a direct observation of a dynamical magnetization reversal process in single NiFe2O4 nanowire, thus clearly revealing the domination of shape anisotropy on its magnetic behaviour. Individual nanoparticles on the NiFe2O4 nanowire appear as single domain states in the remanence state, which is maintained until the magnetic field reaches 200 Oe. The magnetization reversal mechanism of the nanowire is observed to be a curling rotation mode. These observations are further verified by micromagnetic computational simulations. Our findings show that the modulation of shape anisotropy is an efficient way to tune the magnetic behaviours of cubic spinel nano-ferrites.
Graphene oxide (GO) membranes have been widely explored for their excellent physical and chemical properties, and abundant functional groups. In this work, we report the improvement of the perpendicular magnetic anisotropy (PMA) of CoFeB thin films by applying a coating of GO membranes. We observe that the PMA of the CoFeB/MgAl–O stacks is strongly enhanced by the coating of GO membranes and even reaches 0.6 mJ m−2 at room temperature after an annealing process. The critical thickness of the membrane-coated CoFeB for switching the magnetization from the out-of-plane to the in-plane axis exceeds 1.6 nm. First-principle calculations are performed to investigate the contribution of the GO membranes to the magnetic anisotropy energy (MAE). Due to changes in the hybridization of 3d orbitals, varying the location of the C atomic layer with Co changes the contribution of the Co–C stacks to PMA. Thus, the large PMA achieved with GO membranes can be attributed to the orbital hybridization of the C and O atoms with the Co orbitals. These results provide a comprehensive understanding of the PMA and point towards opportunities to achieve multifunctional graphene-composite spintronic devices.
Liu, He; Zhang, Haitao; Fei, Linfeng; Ma, Hongbin; Zhao, Guoying; Mak, CheeLeung; Zhang, Xixiang; Zhang, Suojiang(Nanoscale, Royal Society of Chemistry (RSC), 2017-07-26)[Article]
Fe-doped HTaWO6 (H1-3xFexTaWO6, x = 0.23) nanotubes as highly active solid acid catalysts were prepared via an exfoliation-scrolling-exchange process. The specific surface area and pore volume of undoped nanotubes (20.8 m2 g-1, 0.057 cm3 g-1) were remarkably enhanced through Fe3+ ion-exchange (>100 m2 g-1, 0.547 cm3 g-1). Doping Fe ions into the nanotubes endowed them with improved thermal stability due to the stronger interaction between the intercalated Fe3+ ions and the host layers. This interaction also facilitated the preservation of effective Brønsted acid sites and the generation of new acid sites. The integration of these functional roles resulted in Fe-doped nanotubes with high acidic catalytic activities in the Friedel-Crafts alkylation of anisole and the esterification of acetic acid. Facile accessibility to active sites, generation of effective Brønsted acid sites, high stability of the tubular structure and strong acid sites were found to synergistically contribute to the excellent acidic catalytic efficiency. Additionally, the activity of cycled nanocatalysts can be easily recovered through annealing treatment.
Fan, Qihui; Liu, Ruchuan; Jiao, Yang; Tian, Chunxiu; Farrell, James D.; Diao, Wenwen; Wang, Xiaochen; Zhang, Fengrong; Yuan, Wei; Han, Haibo; Chen, Jinfeng; Yang, Yue; Zhang, Xixiang; Ye, Fangfu; Li, Ming; Ouyang, Zhongcan; Liu, Liyu(Lab Chip, Royal Society of Chemistry (RSC), 2017-07-10)[Article]
A 3-D microfluidic system consisting of microchamber arrays embedded in a collagen hydrogel with tuneable biochemical gradients that mimics the tumour microenvironment of mammary glands was constructed for the investigation on the interactions between invasive breast cancer cells and stromal cells. The hollow microchambers in collagen provide a very similar 3-D environment to that in vivo that regulates collective cellular dynamics and behaviour, while the microfluidic channels surrounding the collagen microchamber arrays allow one to impose complex concentration gradients of specific biological molecules or drugs. We found that breast epithelial cells (MCF-10A) seeded in the microchambers formed lumen-like structures similar to those in epithelial layers. When MCF-10A cells were co-cultured with invasive breast cancer cells (MDA-MB-231), the formation of lumen-like structures in the microchambers was inhibited, indicating the capability of cancer cells to disrupt the structures formed by surrounding cells for further invasion and metastasis. Subsequent mechanism studies showed that down regulation of E-cad expression due to MMPs produced by the cancer cells plays a dominant role in determining the cellular behaviour. Our microfluidic system offers a robust platform for high throughput studies that aim to understand combinatorial effects of multiple biochemical and microenvironmental factors.
Cheung, Ming Sin; Fan, Hongsheng; Xu, Yingying; Wang, Rongming; Zhang, Xixiang(Nanoscale, Royal Society of Chemistry (RSC), 2017-08-24)[Article]
Hollow Co2P nanoflowers (Co2P HNF) are successfully prepared via a one-step, template-free method. Microstructure analysis reveals that Co2P HNF is assembled by nanorods, possesses abundant mesopores and a amorphous carbon shell. Density functional theory calculation and electrochemical measurements demonstrate the high electrical conductivity of Co2P. Benefiting from the unique nanostructures, when employed as electrode material for supercapacitors, Co2P HNF exhibits a high specific capacitance, an outstanding rate capability, and an ultralong cycle stability. Furthermore,. the constructed Co2P HNF//AC ASC yields a high energy density of 30.5 Wh kg-1 at a power density of 850 W kg-1, along with an superior cycling performance (108.0% specific capacitance retained after 10000 cycles at 5 A g-1). These impressive results make Co2P HNF a promising candidate for supercapacitor applications.
Zhang, Xuan; Zheng, Xiujun; Zhang, Hong; Zhang, Junli; Fu, Jiecai; Zhang, Qiang; Peng, Chaoyi; Bai, Feiming; Zhang, Xixiang; Peng, Yong(J. Mater. Chem. C, Royal Society of Chemistry (RSC), 2017-08-10)[Article]
Here we demonstrate that the building blocks of semiconductor WO3 nanowires can be controllably soldered together by a novel nano-soldering technique of in situ SEM-FIB thermal soldering, in which the soldering temperature can precisely remain in an optimal range to avoid a strong thermal diffusion.
Low-dimensional spinel ferrites have recently attracted increasing attention because their tunable magnetic properties make them attractive candidates as spin-filtering tunnel barriers in spintronic devices and as magnetic components in artificial multiferroic heterostructures. Although we know that the distribution of cations (Fe3+ and Co2+) in a spinel structure governs its magnetic properties, their distribution in the so-called ideal inverse spinel structure of a ferrite, CoFe2O4, has not yet been imaged with sub-ångstrom resolution. In this work, we fill this gap in evidence by reporting a direct observation of the distribution of cations in an ideal inverse spinel structure of CoFe2O4 nanofibres using aberration-corrected transmission electron microscopy (TEM). The ordering of Co2+ and Fe3+ at the octahedral sites imaged along either ,  or [-112] orientation was identified as 1 : 1, in accordance with the ideal inverse spinel structure. The saturation magnetisation calculated based on the crystal structure as determined from the TEM image is in good agreement with that measured experimentally on the spinel CoFe2O4 nanofibres, further confirming results from TEM.
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