Microwave plasma assisted reduction synthesis of hexagonal cobalt nanosheets with enhanced electromagnetic performances.

In this study, we employed a microwave plasma assisted reduction (MPAR) method to prepare metallic nanoparticles with desirable morphology. Compared with the hydrogen thermal reduction technique, the MPAR technique could greatly maintain the original morphology of self-sacrificing precursors, as well as proving to be highly efficient, energy-saving and pollution-free. Taking ferromagnetic metallic Co as a forerunner, Co nanosheets with inerratic hexagonal morphology were successfully synthesized on a large scale uniformly. The lateral dimension of the achieved Co nanosheets is in the range of 3∼5 µm with tens of nanometers in thickness. The intact hexagonal flaky shape of Co nanosheets is beneficial for improving dielectric loss by increasing electric channels and interfacial polarization. Consequently, the minimum reflection loss could reach up to -71 dB at a thin thickness of 1.2 mm. Furthermore, the effective bandwidth (RL < -10 dB) could be achieved in a wide range of 2.8∼18 GHz by integrating the thickness from 5.0∼1.0 mm, which provides the possibility for applications in electromagnetic shielding and radar stealth fields. It is believed that the MPAR technique is suitable for designing and preparing novel microwave absorbers on the basis of appropriate precursors, providing new opportunities to acquire high-performance microwave absorbers in the future.

Imaging of water soluble CdTe/CdS core-shell quantum dots in inhibiting multidrug resistance of cancer cells.

Two different colors of water-soluble core-shell quantum dots CdTe/CdS (green and orange red) have been synthesized and characterized in this paper. The formation of core-shell quantum dots not only improves the fluorescence quantum yield, but also reduces the biological toxicity of quantum dots, and improves the fluorescence lifetime. Two novel fluorescent bioprobes, CdTe/CdS (λem = 545 nm)-5-Fu and Bio-CdTe/CdS (λem = 600 nm)-TAM, have been synthesized via the interaction of these two core-shell quantum dots with anticancer drugs (5-Fu) and P-gp inhibitors (TAM), respectively. These two fluorescent probes have been simultaneously used in fluorescence imaging of human breast cancer cells MDA-MB-231/MDR. It can be observed that under the action of P-gp inhibitors distributed on the cell membrane, anticancer drugs can be retained in cancer cells. According to the color of quantum dots on the probe, the visualization results of the action of anticancer drugs and P-gp inhibitors can be obtained. This study shows that to prepare functional bioprobes using core-shell quantum dots CdTe/CdS has great potential in the field of biomedical research such as anticancer drugs.

Effect of Graphene Nanowall Size on the Interfacial Strength of Carbon Fiber Reinforced Composites.

Graphene nanowalls (GNWs) with different sizes (i.e., length and height) were grown directly on the surface of individual carbon fibers (CFs) using a radio frequency plasma-enhanced chemical vapor deposition (RF-PECVD) technique. The size was controlled by varying the deposition time. The GNW-modified CFs were embedded into epoxy resin matrix to prepare a series of carbon-fiber-reinforced composites (CFRCs). The results indicated that GNWs were remarkably effective in improving the interfacial shear strength (IFSS) and interlaminar shear strength (ILSS) of the carbon-fiber-reinforced composites. The enhancement effect on the strength strongly depended on the size of GNWs. It increased with the increase in the GNWs’ size and reached the maximum upon the incorporation of GNWs that were grown for 45 min. Noticeable increases of 222.8% and 41.1% were observed in IFSS and ILSS, respectively. The enhancement mechanism was revealed by means of scanning electron microscope (SEM) fractography analysis. However, further increase of GNW size led to no more improvement in the shear strength. It could result from the increased defect concentration and wrinkle size in the GNWs, which deteriorated the strength.

Hydrothermal synthesis of stable metallic 1T phase WS2 nanosheets for thermoelectric application.

Two-dimensional materials have gained great attention as a promising thermoelectric (TE) material due to their unique density of state with confined electrons and holes. Here, we synthesized 1T phase tungsten disulfide (WS2) nanosheets with high TE performance via the hydrothermal method. Flexible WS2 nanosheets restacked thin films were fabricated by employing the vacuum filtration technique. The measured electrical conductivity was 45 S cm-1 with a Seebeck coefficient of +30 µV K-1 at room temperature, indicating a p-type characteristic. Furthermore, the TE performance could be further improved by thermal annealing treatment. It was found the electrical conductivity could be enhanced 2.7 times without sacrificing the Seebeck coefficient, resulting in the power factor of 9.40 µW m-1 K-2. Moreover, such 1T phase WS2 nanosheets possess high phase stability since the TE properties maintained constant at least half one year in the air atmosphere. Notably, other kinds of 1T phase transitional metal dichalcogenides (TMDCs) with excellent TE performance also could be imitated by using the procedure in this work. Finally, we believe a variety of materials based on 1T phase TMDCs nanosheets have great potential as candidate for future TE applications.

Triethanolamine doped multilayer MoS2 field effect transistors.

Chemical doping has been investigated as an alternative method of conventional ion implantation for two-dimensional materials. We herein report chemically doped multilayer molybdenum disulfide (MoS2) field effect transistors (FETs) through n-type channel doping, wherein triethanolamine (TEOA) is used as an n-type dopant. As a result of the TEOA doping process, the electrical performances of multilayer MoS2 FETs were enhanced at room temperature. Extracted field effect mobility was estimated to be ∼30 cm2 V-1 s-1 after the surface doping process, which is 10 times higher than that of the pristine device. Subthreshold swing and contact resistance were also improved after the TEOA doping process. The enhancement of the subthreshold swing was demonstrated by using an independent FET model. Furthermore, we found that the doping level can be effectively controlled by the heat treatment method. These results demonstrate a promising material system that is easily controlled with high performance, while elucidating the underlying mechanism of improved electrical properties by the doping effect in a multilayered scheme.

Electrical percolation characteristics of metallic single-walled carbon nanotube networks by vacancy evolution.

In the present study, we demonstrate the effect of vacancy evolution on high-pure metallic single-walled carbon nanotube (m-SWCNT) networks by observing the electrical characteristics of the networks on the field-effect transistor (FET). By catalytic oxidation using Co catalyst, vacancy evolution was gradually realized in high-pure m-SWCNT formed as networks between source-drain electrodes of FET. The evolution of vacancy defects in the m-SWCNT networks gradually proceeded by heating FET several times at 250 °C in air. Atomic force microscopic images showed the presence of the Co catalyst nanoparticles, which were evenly formed in the m-SWCNT networks between the electrodes of FET. Vacancy evolution was confirmed by monitoring the D- and G-bands in the Raman spectra measured from the networks after every step of the catalytic oxidation. With vacancy evolution in the networks, the D-band gradually increased, and the transconductance of m-SWCNT networks drastically decreased. In addition, the metallic behaviour of the m-SWCNT networks was converted into a semiconducting one with an on/off ratio of 2.7.

Low-frequency noise in multilayer MoS2 field-effect transistors: the effect of high-k passivation.

Diagnosing of the interface quality and the interactions between insulators and semiconductors is significant to achieve the high performance of nanodevices. Herein, low-frequency noise (LFN) in mechanically exfoliated multilayer molybdenum disulfide (MoS2) (~11.3 nm-thick) field-effect transistors with back-gate control was characterized with and without an Al2O3 high-k passivation layer. The carrier number fluctuation (CNF) model associated with trapping/detrapping the charge carriers at the interface nicely described the noise behavior in the strong accumulation regime both with and without the Al2O3 passivation layer. The interface trap density at the MoS2-SiO2 interface was extracted from the LFN analysis, and estimated to be Nit ~ 10(10) eV(-1) cm(-2) without and with the passivation layer. This suggested that the accumulation channel induced by the back-gate was not significantly influenced by the passivation layer. The Hooge mobility fluctuation (HMF) model implying the bulk conduction was found to describe the drain current fluctuations in the subthreshold regime, which is rarely observed in other nanodevices, attributed to those extremely thin channel sizes. In the case of the thick-MoS2 (~40 nm-thick) without the passivation, the HMF model was clearly observed all over the operation regime, ensuring the existence of the bulk conduction in multilayer MoS2. With the Al2O3 passivation layer, the change in the noise behavior was explained from the point of formation of the additional top channel in the MoS2 because of the fixed charges in the Al2O3. The interface trap density from the additional CNF model was Nit = 1.8 × 10(12) eV(-1) cm(-2) at the MoS2-Al2O3 interface.