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Wood has a long history as a traditional material which has been widely recognized to be hardly conductive electrically and thermally. Making wood conductive electrically and thermally is technically a new challenge to think outside the box. Here, we demonstrate that wood can be electrically and thermally conductive by dipping wood in the solution where multi-walled carbon nanotube (MWCNT) is dispersed evenly. Our idea is based on the fact that wood usually absorbs water or moisture due to its hygroscopic properties. When walnut wood was dipped in the MWCNT solution, the wood started to absorb water containing MWCNT. As a result, well-networked MWCNTs were homogeneously formed in the structure of wood with natural porosity. Interestingly, superior electrical and thermal conductivity were observed along the cellulose fiber direction of the walnut wood, compared with the tangential direction perpendicular to the fiber. We found that wood has electrically and thermally anisotropic characteristics due to the grain boundary by wood fibers. Furthermore, the degree of conductivity was controllable by varying dipping times. We think that this discovery will open a new avenue to utilize wood as a nanocomposite material having electrical and thermal properties.

Irradiation of MoS

Single-walled carbon nanotube (SWCNT) is generally used as a networked structure in the fabrication of a field-effect transistor (FET) since it is known that one-third of SWCNT is electrically metallic and the remains are semiconducting. In this case, the presence of metallic paths by metallic SWCNT (m-SWCNT) becomes a significant technical barrier which hinders the networks from achieving a semiconducting behavior, resulting in a low on/off ratio. Here, we report on an easy method of controlling the on/off ratio of a FET where semiconducting SWCNT (s-SWCNT) and m-SWCNT constitute networks between source and drain electrodes. A FET with SWCNT networks was simply sonicated under water to control the on/off ratio and network density. As a result, the FET having an almost metallic behavior due to the metallic paths by m-SWCNT exhibited a p-type semiconducting behavior. The on/off ratio ranged from 1 to 9.0 × 104 along sonication time. In addition, theoretical calculations based on Monte-Carlo method and circuit simulation were performed to understand and explain the phenomenon of a change in the on/off ratio and network density by sonication. On the basis of experimental and theoretical results, we found that metallic paths contributed to a high off-state current which leads to a low on/off ratio and that sonication formed sparse SWCNT networks where metallic paths of m-SWCNT were removed, resulting in a high on/off ratio. This method can open a chance to save the device which has been considered as a failed one due to a metallic behavior by a high network density leading to a low on/off ratio.

The balance of electron–hole charge carriers in quantum dot (QD) light-emitting diodes (QLEDs) is an important factor to achieve high efficiency. However, poor interfacial properties between QDs and their adjacent layers are likely to deteriorate the electron–hole charge balance, resulting in the poor performance of a QLED. In this paper, we report an enhanced efficiency in red-emitting inverted QLEDs by modifying the interface properties between QDs and ZnO electron transport layer (ETL) using a thin layer of non-conjugated polymer, poly(4-vinylpyridine) (PVPy). Based on the precise control of the electrical properties with PVPy, the maximum efficiency of the QLED is enhanced by 30% compared to the device without a PVPy layer. In particular, the efficiency at low current density region is significantly increased. We investigate the effect of the PVPy interlayer on the performance of QLEDs and find that this thin layer not only shifts the energy levels of the underlying ZnO ETL, but also effectively blocks the leakage current at the ETL/QD interface.

Black phosphorus (BP) has been proposed as a future optoelectronic material owing to its direct bandgap with excellent electrical performances. However, oxygen (O 2 ) and water (H 2 O) molecules in an ambient condition can create undesired bubbles on the surface of the BP, resulting in hampering its excellent intrinsic properties. Here, we report the electrical degradation pattern of a mechanically exfoliated BP field–effect transistor (FET) in terms of the channel and contact, separately. Various electrical parameters such as the threshold voltage ( V TH ), carrier mobility ( μ ), contact resistance ( R CT ) and channel resistance ( R CH ) are estimated by the Y function method ( YFM ) with respect to time (up to 2000 min). It is found that R CT reduces and then, increases with time; whereas, the behavior of R CH is vice versa in ambient conditions. We attribute these effects to oxygen doping at the contact and the surface oxidation effects on the surface of the BP, respectively.

Hexagonal boron nitride (h-BN) is a ceramic compound which is thermally stable up to 1000 °C in air. Due to this, it is a very challenging task to etch h-BN under air atmosphere at low temperature. In this study, we report that h-BN flakes can be easily etched by oxidation at 350 °C under air atmosphere in the presence of transition metal (TM) oxide. After selecting Co, Cu, and Zn elements as TM precursors, we simply oxidized h-BN sheets impregnated with the TM precursors at 350 °C in air. As a result, microscopic analysis revealed that an etched structure was created on the surface of h-BN flakes regardless of catalyst type. And, X-ray diffraction patterns indicated that the air oxidation led to the formation of Co 3 O 4 , CuO, and ZnO from each precursor. Thermogravimetric analysis showed a gradual weight loss in the temperature range where the weight of h-BN flakes increased by air oxidation. As a result of etching, pore volume and pore area of h-BN flakes were increased after catalytic oxidation in all cases. In addition, the surface of h-BN flakes became highly active when the h-BN samples were etched by Co 3 O 4 and CuO catalysts. Based on these results, we report that h-BN flakes can be easily oxidized in the presence of a catalyst, resulting in an etched structure in the layered structure.

Boron nitride nanotubes (BNNTs) are ceramic compounds which are hardly oxidized below 1000 °C due to their superior thermal stability. Also, they are electrically almost insulators with a large band gap of 5 eV. Thus, it is a challenging task to etch BNNTs at low temperature and to convert their electrical properties to a conductive behavior. In this study, we demonstrate that BNNTs can be easily etched at low temperature by catalytic oxidation, resulting in an electrically conductive behavior. For this, multi-walled BNNTs (MWBNNTs) impregnated with Co precursor (Co(NO3)2·6H2O) were simply heated at 350 °C under air atmosphere. As a result, diverse shapes of etched structures such as pits and thinned walls were created on the surface of MWBNNTs without losing the tubular structure. The original crystallinity was still kept in the etched MWBNNTs in spite of oxidation. In the electrical measurement, MWBNNTs with a large band gap were converted to electrical conductors after etching by catalytic oxidation. Theoretical calculations indicated that a new energy state in the gap and a Fermi level shift contributed to MWBNNTs being conductive.

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.

In this work, graphene field effect transistors (FETs) were fabricated on a trench structure made by carbonized poly(methylmethacrylate) to modify the graphene surface. The trench-structured devices showed different characteristics depending on the channel orientation and the pitch size of the trenches as well as channel area in the FETs. Periodic corrugations and barriers of suspended graphene on the trench structure were measured by atomic force microscopy and electrostatic force microscopy. Regular barriers of 160 mV were observed for the trench structure with graphene. To confirm the transfer mechanism in the FETs depending on the channel orientation, the ratio of experimental mobility (3.6-3.74) was extracted from the current-voltage characteristics using equivalent circuit simulation. It is shown that the number of barriers increases as the pitch size decreases because the number of corrugations increases from different trench pitches. The noise for the 140 nm pitch trench is 1 order of magnitude higher than that for the 200 nm pitch trench.

Irradiation of MoS2 field-effect transistors (FETs) fabricated on Si/SiO2 substrates with electron beams (e-beams) below 30 keV creates electron–hole pairs (EHP) in the SiO2, which increase the interface trap density (Nit ) and change the current path in the channel, resulting in performance changes. In situ measurements of the electrical characteristics of the FET performed using a nano-probe system mounted inside a scanning electron microscope show that e-beam irradiation enables both multilayer and monolayer MoS2 channels act as conductors. The e-beams mostly penetrate the channel owing to their large kinetic energy, while the EHPs formed in the SiO2 layer can contribute to the conductance by flowing into the MoS2 channel or inducing the gate bias effect. The analysis of the device parameters in the initial state and the vent-evacuation state after e-beam irradiation can clarify the effect of the interplay between the e-beam-induced EHPs and ambient adsorbates on the carrier behavior, which depends on the thickness of the MoS2 layer. DC and low frequency noise analysis reveals that the e-beam-induced EHPs increase Nit from 109–1010 to 1011 cm-2⋅eV-1 in both monolayer and multilayer devices, while the interfacial Coulomb scattering parameter αSC increases by three times in the monolayer and decreases to one-tenth of its original value in the multilayer. In other words, an MoS2 layer with a thickness of ~30 nm is less sensitive to adsorbates by surface screening. Thus, the carrier mobility in the monolayer device decreases from 45.7 to 40 cm2V-1s-1, while in the 30 nm-thick multilayer device, it increases from 4.9 to 5.6 cm2V-1s-1. This is further evidenced by simulations of the distribution of interface traps and channel carriers in the MoS2 FET before and after e-beam irradiation, demonstrating that Coulomb scattering decreases as the effective channel moves away from the interface.

With the advances in the separation and purification of carbon nanotubes (CNTs), the use of highly pure metallic or semiconducting CNTs has practical merit in electronics applications. When highly pure CNTs are applied in various fields, CNT networks are preferred to individual CNTs. In such cases, the presence of an electrical path becomes crucial in the network. In this study, we report on the electrical percolation thresholds of semiconducting single-walled carbon nanotube (s-SWCNT) networks, and their electrical characteristics in field-effect transistors (FET). Using the Monte Carlo method, s-SWCNT networks were randomly generated in the channels defined by the source-drain electrodes of the FET. On the basis of percolation theory, the percolation thresholds of s-SWCNT networks were obtained at different channel lengths (2, 6, and 10 μm) by generating random s-SWCNT networks 100 times. The network density corresponding to the electrical percolation threshold was theoretically gained at each channel length. As a result, the network densities at the percolation thresholds for the channel lengths of 2, 6, and 10 μm were 6.8, 9.0, and 9.9 tube μm(-2), respectively. In addition, SPICE calculations were performed for each s-SWCNT network, constituting an electrical path between the source and the drain electrodes of the FET. In all channel lengths, the on/off ratio of the s-SWCNT networks was enhanced with increasing network density. Finally, we found a power law relationship between the on/off ratio of the s-SWCNT networks and the network density at the percolation threshold.

Doping effects in devices based on two-dimensional (2D) materials have been widely studied. However, detailed analysis and the mechanism of the doping effect caused by encapsulation layers has not been sufficiently explored. In this work, we present experimental studies on the n-doping effect in WSe

International audience; The thermoelectric power (TEP) of single walled carbon nanotube (SWCNT) thin films in pure metallic SWCNT (m-SWCNT) and pure semiconducting SWCNT (s-SWCNT) networks as well as in m- and s-SWCNT mixtures is investigated. The TEP measured on the pure s-SWCNT film (≈88 μV/K) was found to be almost 7 times higher than that of the m-SWCNTs (≈13 μV/K). Moreover, a quasilinear increase of TEP of the mixed SWCNT networks was observed as the fraction of s-SWCNTs is increased. The experimentally determined relationship between TEP and the fraction of s-SWCNTs in the mixture allows fast and simple quantitative analysis of the s:m ratio in any as-prepared heterogeneous SWCNT network. Furthermore, a semiempirical model analyzing the effect of the intertube junctions is proposed and applied to describe the thermoelectric behavior of the prepared SWCNT networks. The results of calculations match well with the experimental data and clearly demonstrate that the measured TEP of thin SWCNT films is principally controlled by the intertube junctions. The fundamental role of junctions in generating thermoelectric power is not limited to only SWCNT networks as discovered here, but also could be applied to systems where nanoparticles/nanotube form percolating paths in thin films and composite materials.

International audience; Two-dimensional materials have outstanding scalability due to their structural and electrical properties for the logic devices. Here, we report the current fluctuation in multilayer WSe2 field effect transistors (FETs). In order to demonstrate the impact on carrier types, n-type and p-type WSe2 FETs are fabricated with different work function metals. Each device has similar electrical characteristics except for the threshold voltage. In the low frequency noise analysis, drain current power spectral density (SI) is inversely proportional to frequency, indicating typical 1/f noise behaviors. The curves of the normalized drain current power spectral density (NSI) as a function of drain current at the 10 Hz of frequency indicate that our devices follow the carrier number fluctuation with correlated mobility fluctuation model. This means that current fluctuation depends on the trapping-detrapping motion of the charge carriers near the channel interface. No significant difference is observed in the current fluctuation according to the charge carrier type, electrons and holes that occurred in the junction and channel region.

In this work, an easy method to etch monolayer graphene is shown by catalytic oxidation in the presence of ZnO nanoparticles (NPs). The catalytic etching of monolayer graphene, which was transferred to the channel of field-effect transistors (FETs), was performed at low temperature by heating the FETs several times under an inert gas atmosphere (ZnO + C → Zn + CO or CO2). As the etching process proceeded, diverse etched structures in the shape of nano-channels and pits were observed under microscopic observation. To confirm the evolution of etching, current–voltage characteristics of monolayer graphene were measured after every step of etching by catalytic oxidation. As a result, the conductance of monolayer graphene decreased with the development of etched structures. This decrease in conductance was analyzed by percolation theory in a honeycomb structure. Finally, well-patterned graphene was obtained by oxidizing graphene under air in the presence of NPs, where Al was deposited on graphene as a mask for designed patterns. This method can substitute graphene etching via carbon hydrogenation using H2 at high temperature.

We report that noise level was reduced by passivating WSe2 field effect transistors (FETs) with high-k dielectric material. Low frequency noise and I-V characteristics of the device were measured from WSe2 FETs before and after the passivation with ZrO2 to confirm the effect of passivation by high-k dielectric material. As a result, there was no significant change in the I–V characteristics. In the low frequency noise analysis, our device showed 1/f noise behaviors, in agreement with the carrier number fluctuation (CNF) model. The passivation process contributed to the reduction of trap sites at the top surface, leading to the decrease of noise level at the low current regime. Extracted volume trap densities were reduced from 2.0 × 1020 cm−3eV−1 to 8.7 × 1019 cm−3eV−1.

Doping effects in devices based on two-dimensional (2D) materials have been widely studied. However, detailed analysis and the mechanism of the doping effect caused by encapsulation layers has not been sufficiently explored. In this work, we present experimental studies on the n-doping effect in WSe2 field effect transistors (FETs) with a high-k encapsulation layer (Al2O3) grown by atomic layer deposition. In addition, we demonstrate the mechanism and origin of the doping effect. After encapsulation of the Al2O3 layer, the threshold voltage of the WSe2 FET negatively shifted with the increase of the on-current. The capacitance-voltage measurements of the metal insulator semiconductor (MIS) structure proved the presence of the positive fixed charges within the Al2O3 layer. The flat-band voltage of the MIS structure of Au/Al2O3/SiO2/Si was shifted toward the negative direction on account of the positive fixed charges in the Al2O3 layer. Our results clearly revealed that the fixed charges in the Al2O3 encapsulation layer modulated the Fermi energy level via the field effect. Moreover, these results possibly provide fundamental ideas and guidelines to design 2D materials FETs with high-performance and reliability.

Oxygen-plasma treatment has been introduced for improving electrical characteristics and low-temperature processing of sol-gel derived Pb(Zr x , Ti l m x )O 3 (PZT) thin films. The ferroelectric PZT films with room temperature oxygen-plasma treatment showed enhanced polarization and decreased leakage current densities, as well as much reduced electrical fatigues. Besides, oxygen-plasma treatment at 200°C, following a low-temperature annealing in oxygen at 450°C, gave rise to ferroelectric hysteresis loops, which was explained by the perovskite phase formation starting from the surface layers.

The electrical characteristics of hydrogen passivated polycrystalline-silicon p-channel metal-oxide-semiconductor thin-film transistors (poly-Si PMOS TFTs) for high density memory devices have been investigated. A lightly doped offset (LDO) structure was adopted to reduce the electric field induced in the drain region. Compared with the on-current, the off-current was reduced significantly with an LDO length. As a result, the TFT with an LDO presented the enhanced on/off current ratio. However, it still has a drawback of reducing the on-current due to increased parasitic resistance. To reduce this effect, a source overlap structure was introduced. Although the on current was slightly improved at a longer source overlap length due to channel length reduction, the dependence of the on/off current ratio on a source overlap length was small compared with that on an LDO length. In the measurement of leakage currents, the lower channel and gate breakdown voltages were obtained at the longer source overlap structure.

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.

Summary form only given. Amorphous Hafnium-Indium-Zinc Oxide(a-HIZO) semiconductor materials are used to replace amorphous Gallium-Indium-Zinc Oxide(a-GIZO) as channel layer of thin film transistors (TFTs) to improve the stability of the electrical characteristics. However, research activities of a-HIZO etching process for the fabrication of TFTs have not been reported. In this work, we present the etching characteristics of HIZO stacked with photoresist using Cl2/Ar chemistry. Etching behaviours of HIZO have been investigated as a function of source power, bias power and pressure. As the Cl2 concentration increased from pure Ar, the etch rate of HIZO film was slightly different from the trend of GIZO film. Atomic Force Microscopy(AFM), Depth profile of the HIZO film by Auger Electronspectroscopy(AES) and X-ray Photoelectron Spectroscopy(XPS) of the etched surface was carried to investigate the etching mechanism as Cl2 increased. Additionally we tried to compare the etching mechanism of HIZO film with GIZO film to confirm the difference of chemical bonds caused by the influence of hafnium doping, estimated that the low chemical reaction between hafnium and Cl2 based plasma suppress the reactive ion etching.

Modulation of electrical properties in MoS2 flakes is an attractive issue from the point of view of device applications. In this work, we demonstrate that an ambipolar behavior in MoS2 field effect transistors (FETs) can be easily obtained by heating MoS2 flakes under air atmosphere in the presence of cobalt oxide catalyst (MoS2 + O2 → MoOx + SOx). The catalytic oxidation of MoS2 flakes between source-drain electrodes resulted in lots of MoOx nanoparticles (NPs) on MoS2 flakes with thickness reduction from 64 nm to 17 nm. Consequently, N-type behavior of MoS2 FETs was converted into ambipolar transport characteristics by MoOx NPs which inject hole carriers to MoS2 flakes.

We studied effects of oxygen plasma treatment on the ferroelectric Pb(Zrx,Ti1−x)O3 (PZT) films prepared by a sol-gel method. Electrical characteristics of the films were found to be improved considerably by exposure to the O2 plasma, with enhanced remanent polarization and decreased leakage current densities. Chemical bonding analysis by means of x-ray photoelectron spectroscopy in the Pb 4f region indicated that the intensity from the perovskite PZT phase increased considerably after the O2 plasma treatment, giving rise to the improved performances of the PZT films.

International audience; We report an investigation of the effects of low-temperature annealing on the electrical properties of amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs). X-ray photoelectron spectroscopy was used to characterize the charge carrier concentration, which is related to the density of oxygen vacancies. The field-effect mobility was found to decrease as a function of the charge carrier concentration, owing to the presence of band-tail states. By employing the transmission line method, we show that the contact resistance did not significantly contribute to the changes in device performance after annealing. In addition, using low-frequency noise analyses, we found that the trap density decreased by a factor of 10 following annealing at 120 °C. The switching operation and on/off ratio of the a-IGZO TFTs improved considerably after low-temperature annealing.

International audience; Various plasma treatment effects such as oxygen (O2), nitrogen (N2), and argon (Ar) on amorphous indium gallium zinc oxide thin-film transistors (a-IGZO TFTs) are investigated. To study oxygen stoichiometry in a-IGZO TFTs with respect to various plasma environments, X-ray photoelectron spectroscopy was employed. The results showed that oxygen vacancies were reduced by O2 and N2 plasmas while they were increased after Ar plasma treatment. Additionally, the effects of plasma treatment on trap distribution in bulk and surface channels were explored by means of low-frequency noise analysis. Details of the mechanisms used for generating and restoring traps on the surface and bulk channel are presented.

We report the etching characteristics of a stacked hafnium–indium–zinc oxide (HIZO) with a photoresist using the gas mixture of chlorine and argon (Cl2/Ar). The etching behaviors of HIZO have been investigated in terms of a source power, a bias power and a chamber pressure. As the concentration of Cl2 was increased compared to pure Ar, the etch rate of HIZO film was found slightly different from that of indium–zinc oxide (IZO) film. Moreover, to investigate the etching mechanism systematically, various inspections were carried out such as atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) depending on the portion of Cl2. Additionally, we compared the etching mechanism of HIZO film with IZO film to confirm the difference of chemical bonds caused by the influence of hafnium doping.