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.

Phone: þ82 2 3290 3851, Fax: þ82 2 953 3780, Web: http://flexiblenano.korea.ac.kr Carbon nanotube networks showing superior electric properties, high chemical stability, strong mechanical properties, and flexibility are also known to exhibit thermoelectric effects. However, the experimental thermoelectric figure of merit, ZT, of pristine carbon nanotubes is typically in the range of 10 À3 -10 À2 , which is still not attractive for thermal energy conversion applications. In this work, we show possible ways to improve the thermoelectric properties of single walled carbon nanotubes (SWCNTs) by means of chemical treatments. In this study, we primarily investigated the effect of chemical treatment on the electrical conductivity and thermoelectric power (TEP) of the entangled network of nanotubes, also, known as ''buckypaper''. This chemical treatment increased the electrical conductivity due to p-type doping, thereby, showing a decrease in the TEP given by the Seebeck coefficient, whereas the n-type dopants changed the sign and value of the TEP from about 40 to À40 mV K À1 . Neutral polymers, in terms of doping, such as PVDF, PMMA, PVA, PS, and PC, were expected to hinder phonon transport through the nanotube network, increasing the Seebeck coefficient up to ca. 50 mV K À1 . Our results reveal the importance of chemical doping determining the sign and the magnitude of the TEP, and role of the polymer matrix in the development of more effective thermoelectric composites based on carbon nanotubes.

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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 (WS) nanosheets with high TE performance via the hydrothermal method. Flexible WS nanosheets restacked thin films were fabricated by employing the vacuum filtration technique. The measured electrical conductivity was 45 S cm with a Seebeck coefficient of +30 μV K 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 K. Moreover, such 1T phase WS 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.

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 (MoS) 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 MoS FETs were enhanced at room temperature. Extracted field effect mobility was estimated to be ∼30 cm V s 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.