[Show abstract][Hide abstract] ABSTRACT: In this work we report the synthesis of copper oxide/tin oxide (CuO/SnO2) mixed nanofibers and their gas sensing properties in terms of H2S gas. The CuO/SnO2 mixed nanofibers were synthesized by electrospinning technique using two needles. Based on the thermogravimetric-differential thermal analysis, the calcination temperature was optimized at 700 °C to acquire both phases of CuO and SnO2. With this method, intermixed nanofibers of SnO2 and CuO were obtained. The sensing properties of the CuO/SnO2 mixed nanofibers to H2S are investigated as functions of operating temperature and gas concentration. The CuO/SnO2 mixed nanofibers were highly sensitive towards H2S with a response 522 for 10 ppm H2S and a response time 1 s at 300 °C. The semiconductormetal transition of CuO due to H2S is likely to the reason of the high H2S response. The results evidently demonstrate that the CuO/SnO2 mixed nanofibers synthesized with double needles are a promising sensor material for detection of H2S.
Journal of Nanoscience and Nanotechnology 11/2015; 15(11). DOI:10.1166/jnn.2015.11505 · 1.56 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this research, we have investigated the instability of P-channel low-temperature polycrystalline silicon (poly-Si) thin-film transistors (LTPS TFTs) with double-layer SiO2/SiN X dielectrics. A negative gate bias temperature instability (NBTI) stress was applied and a turn-around behavior phenomenon was observed in the Threshold Voltage Shift (V th). A positive threshold voltage shift occurs in the first stage, resulting from the negative charge trapping at the SiNX /SiO2 dielectric interface being dominant over the positive charge trapping at dielectric/Poly-Si interface. Following a stress time of 7000 s, the V th switches to the negative voltage direction, which is “turn-around” behavior. In the second stage, the V th moves from −1.63 V to −2 V, overwhelming the NBTI effect that results in the trapping of positive charges at the dielectric/Poly-Si interface states and generating grain-boundary trap states and oxide traps.
Journal of Nanoscience and Nanotechnology 10/2015; 15(10). DOI:10.1166/jnn.2015.11167 · 1.56 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this study, we prepared solution-based In–Ga–ZnO thin film transistors (IGZO TFTs) having a multistacked active layer. The solution was prepared using an In:Zn = 1:1 mole ratio with variation in Ga content, and the TFTs were fabricated by stacking layers from the prepared solutions. After we measured the mobility of each stacked layer, the saturation mobility showed values of 0.8, 0.6 and 0.4 (cm2/Vs), with an overall decrease in electrical properties. The interface formed between the each layers affected the current path, resulting in reduced electrical performance. However, when the gate bias V G = 10 V was applied for 1500 s, the threshold voltage shift decreased in the stack. The uniformity of the active layer was improved in the stacked active layer by filling the hole formed during pre-baking, resulting in improved device stability. Also, the indium ratio was increased to enhance the mobility from 0.86 to 3.47. These results suggest high mobility and high stability devices can be produced with multistacked active layers.
Journal of Nanoscience and Nanotechnology 10/2015; 15(10). DOI:10.1166/jnn.2015.11169 · 1.56 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Networked ZnO nanowires were fabricated via a vapor-phase selective growth method. Pt nanoparticles were functionalized on the networked ZnO nanowires. In this study, for the functioanlization, γ-ray radiolysis was applied. By the method, Pt nanoparticles of ∼10 nm in diameter
were uniformly anchored on the surface of each ZnO nanowire. The sensing properties of the Pt-functionalized, networked ZnO nanowires were investigated in terms of NO2, CO and benzene at 100 °C. The sensing capability of the Pt-functionalized ZnO nanowires at that temperature
supports their potential use in chemical gas sensors.
Journal of Nanoscience and Nanotechnology 09/2015; 15(1). DOI:10.1166/jnn.2015.8368 · 1.56 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We developed a hydrogen sensor of reduced graphene oxide-loaded ZnO nanofibers. An extremely high response of about 866 at a low concentration of 100 ppb was obtained. The combined effect of the presence of rGO nanosheets and hydrogen-induced metallization of ZnO played a crucial role in enhancing the detection behavior.
Chemical Communications 08/2015; DOI:10.1039/C5CC05370F · 6.83 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We have fabricated sensors with reduced graphene oxide (RGO) nanosheets (NSs)-loaded ZnO nanofibers (NFs) via an electrospinning method. The RGO NSs-loaded ZnO NFs were comprised of nanograins with an average diameter of 20 nm. Transmission electron microscopy and X-ray diffraction both revealed the presence of RGO NSs in the ZnO NFs. The sensing properties of RGO NSs-loaded ZnO NFs were examined after exposure to various gases, including O2, SO2, NO2, CO, C6H6, and C2H5OH. The sensor responses showed a bell-shaped behavior with respect to the weight ratio of RGO NSs. It is remarkable that our sensors exhibited significantly higher responses than pure ZnO NFs. We propose a novel hybrid sensing mechanism for the drastic improvement in the sensing behavior that is caused by loading RGO NSs into ZnO NFs. This hybrid sensing mechanism combines the resistance modulation of ZnO/ZnO homointerfaces and RGO-NSs/ZnO heterointerfaces in addition to the radial modulation of the surface depletion layer of ZnO NFs. In the heterointerfaces, the creation of local heterojunctions plays a significant role in raising the sensitivity of RGO-loaded ZnO NFs.
Sensors and Actuators B Chemical 08/2015; DOI:10.1016/j.snb.2015.07.120 · 4.10 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The evolution of low symmetric phases was investigated in epitaxial (Ba0.5, Sr0.5)TiO3 (BST) thin films grown on Si with yttria stabilized zirconia (YSZ). Paraelectric cubic BST thin films, when exposed to tensile strain primarily due to Si, present
low symmetric phases, tetragonal c and monoclinic r phases, depending on the thickness of the BST thin films. When BST films are thin (
[Show abstract][Hide abstract] ABSTRACT: ZnO–CuO composite nanofibers were used for the detection of H2S. Various nanograin sizes were obtained by changing the heat treatment duration. The nanograin size greatly influenced the H2S-sensing performance of the resulting composites. Composite nanofibers consisting of smaller nanograins showed superior sensing properties compared to the composite nanofibers of larger ones. The mechanism is based on the known transition process from the p-CuO-n-ZnO junction to the metallic-CuS-n-ZnO junction in the presence of H2S. This study suggests that the nanograin size needs to be as small as possible to obtain the maximum H2S-sensing properties in CuO-n-type oxide (ZnO or SnO2) composite nanofibers by controlling the heat treatment condition, such as the duration and temperature.
Sensors and Actuators B Chemical 07/2015; 214. DOI:10.1016/j.snb.2015.03.012 · 4.10 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this paper, we report the exceptional toluene-sensing properties of SnO2-ZnO core-shell nanowires (C-S NWs) functionalized with Pt nanoparticles (NPs). A response of 279 was obtained for 100 ppb of toluene, and to the best of our knowledge, this is the highest toluene response to be reported in the literature. The SnO2-ZnO C-S NWs were synthesized via a two-step process: first, a networked core of SnO2 nanowires was prepared via vapor-liquid-solid growth on the patterned electrode layer. Second, the ZnO shell layers were created with atomic layer deposition. The electron-depleted region of the shell layer was further expanded by attaching Pt NPs, which were synthesized using g-ray radiolysis, to the shell layer. In addition to the expansion of the electron-depleted region, the catalytic effect of Pt on toluene greatly improved the toluene-sensing performance of the C-S NWs. The extraordinary toluene-sensing ability of the Pt-functionalized C-S NWs means they have great potential as the sensing platform in exhaled breath sensors used to diagnose lung cancer.
[Show abstract][Hide abstract] ABSTRACT: Networked p-CuO nanowires were grown on patterned-electrode pads by the thermal oxidation of Cu layers. Vertically aligned CuO nanowires grown on adjacent round-shape electrode pads were entangled, eventually forming nanowire–nanowire junctions. The sensing properties of the networked CuO nanowires were examined for a range of oxidizing gases, such as NO2, SO2 and O2, and reducing gases, such as CO, C6H6, C7H8, and H2, and compared with those of networked n-SnO2 nanowires. The gas responses of the networked CuO nanowires to the tested oxidizing gases were inferior to those of networked n-SnO2 nanowires. In contrast, for reducing gases, the networked CuO nanowires showed comparable gas responses to the networked n-SnO2 nanowires. The results suggest that the networked CuO nanowires are more promising for the detection of reducing gases rather than oxidizing gases.
Sensors and Actuators B Chemical 06/2015; 212. DOI:10.1016/j.snb.2014.12.081 · 4.10 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report the synthesis of SnO2?Cu2O n?p core?shell nanowires (C?S NWs) and their use as chemiresistive sensors for detecting trace amounts of gas. The n?p C?S NWs were synthesized by a two-step process, in which the core SnO2 nanowires were prepared by the vapor growth technique and subsequently the Cu2O shell layers were deposited by atomic layer deposition. A systematic investigation of the sensing capabilities of the n?p C?S NWs, particularly as a function of shell thickness, revealed the underlying sensing mechanism. The radial modulation of the hole-accumulation layer is intensified under shells thinner than the Debye length. On the other hand, the contribution of volume fraction to resistance modulation is weakened. By the combination of these two effects, an optimal sensing performance for reducing gases is obtained for a critical p-shell thickness. In contrast, the formation of p-shell layers deteriorates the NO2-sensing performance by blocking the expansion of the hole-accumulation layer due to the presence of p?n heterointerface.
[Show abstract][Hide abstract] ABSTRACT: Bandgap engineering of atomically thin 2D crystals is critical for their applications in nanoelectronics, optoelectronics, and photonics. Here, we report a simple but rather unexpected approach for bandgap engineering of muscovite-type mica nanosheets (KAl3Si3O10(OH)2) via controlled molecular thickness. Through density functional calculations, we analyze electronic structures in 2D mica nanosheets and develop a general picture for tunable bandgap narrowing induced by controlled molecular thickness. From conducting atomic force microscopy, we observe an abnormal bandgap narrowing in 2D mica nanosheets, contrary to well-known quantum size effects. In mica nanosheets, decreasing the number of layers results in reduced bandgap energy from 7 to 2.5 eV, and the bilayer case exhibits a semiconducting nature with ∼2.5 eV. Structural modeling by transmission electron microscopy and density functional calculations reveal that this bandgap narrowing can be defined as a consequence of lattice relaxations as well as surface doping effects. These bandgap engineered 2D mica nanosheets open up an exciting opportunity for new physical properties in 2D materials and may find diverse applications in 2D electronic/optoelectronic devices.
Chemistry of Materials 06/2015; 27(12):150610090301009. DOI:10.1021/cm504802j · 8.35 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We have realized a p-type-like conduction in initially n-type SnO2 nanowires grown using a vapor-liquid-solid method. The transition was achieved by irradiating n-type SnO2 nanowires with a high-energy electron beam, without intentional chemical doping. The nanowires were irradiated at doses of 50 and 150 kGy, and were then used to fabricate NO2 gas sensors, which exhibited n-type and p-type conductivities, respectively. The tuneability of the conduction behavior is assumed to be governed by the formation of tin vacancies (under high-energy electron beam irradiation), because it is the only possible acceptor, excluding all possible defects via density functional theory (DFT) calculations. The effect of external electric fields on the defect stability was studied using DFT calculations. The measured NO2 sensing dynamics, including response and recovery times, were well represented by the electron-hole compensation mechanism from standard electron-hole gas equilibrium statistics. This study elucidates the charge-transport characteristics of bipolar semiconductors that underlie surface chemical reactions. The principles derived will guide the development of future SnO2-based electronic and electrochemical devices.
[Show abstract][Hide abstract] ABSTRACT: In this study, n-ZnO nanofibers (NFs) were electronically sensitized by optimal loading of reduced graphene oxide nanosheets (RGO NSs). The RGO NSs-loaded ZnO NFs were comprised of nanograins with an average diameter of 20 nm. The sensing characteristics of RGO NSs-loaded ZnO NFs were examined with respect to various oxidizing and reducing gases, (O2, SO2, NO2, CO, C6H6, and C2H5OH). The sensor responses exhibited a bell-shaped behavior as a function of the weight ratio of RGO NSs. It is remarkable that gas-sensing characteristics were significantly improved and responses were higher than pure n-ZnO NFs. We propose a novel hybrid sensing mechanism for the drastic improvement of sensing behavior by loading RGO NSs into n-ZnO NFs. This hybrid sensing mechanism combines the resistance modulation of both the ZnO-ZnO homointerfaces and that of p-RGO NSs-n-ZnO heterointerfaces in addition to the radial modulation of the surface depletion layer of ZnO NFs.
The spring meeting of Materials Research Society of Korea, GumiCo (Gumi Convention and Exhibition Center); 05/2015
[Show abstract][Hide abstract] ABSTRACT: SnO2-ZnO composite nanofibers fabricated using an electrospinning method exhibited exceptional hydrogen (H2) sensing behavior. The existence of tetragonal SnO2 and hexagonal ZnO nanograins was confirmed by an analysis of the crystalline phase of the composite nanofibers. A bifunctional sensing mechanism of the composite nanofibers was proposed in which the combined effects of SnO2-SnO2 homointerfaces and ZnO-SnO2 heterointerfaces contributed to an improvement in the H2 sensing characteristics. The sensing process with respect to SnO2-ZnO heterojunctions is associated not only with the high barrier at the junctions, but also the semiconductor-to-metallic transition on the surface of the ZnO nanograins upon the introduction of H2 gas.
[Show abstract][Hide abstract] ABSTRACT: In this paper we investigate O2 sensing dynamics in BiFeO3 (BFO) nanofibers at various concentrations and temperatures, by using a combined experiment and computer simulation approach. Samples of pristine BFO, Ni-doped BFO, and Pb-doped BFO nanofibers were prepared. By incorporating Ni and Pb, additional acceptor states are introduced in BFO. Density functional theory calculations show that Ni prefers to substitute Fe site while Pb substitutes Bi site, resulting in a new deep donor originating from Ni interstitial defects, along with oxygen vacancies (Vo). We find that both the sensing response and recovery time are shorter in samples made of pristine BFO nanofibers than in Ni- and Pb-doped nanofiber samples. We interpret the observed sensing dynamics through charge transport theory of the major (acceptors) and minor (donors) carriers, and found that the minor carrier compensation plays a significant role in determining the response and recovery time of the sensor device. This minor carrier compensation charge transport mechanism will provide new insights into more robust sensor development strategies, and into the research of ion-electron coupling in chemical dynamics of semiconductors.
[Show abstract][Hide abstract] ABSTRACT: We report, and propose a mechanism for, the exceptional hydrogen gas (H2) sensing ability of ZnO nanofibers. In comparison to SnO2 nanofibers, ZnO nanofibers show outstanding H2 gas response and unmistakable H2 selectivity. Different from the reducing gas effect observed in SnO2 nanofibers, a semiconductor-to-metal transition that occurs in the presence of H2 gas molecules is responsible for the exceptional response and selectivity of ZnO nanofibers to H2. Notably, the presence of nanograins within nanofibers further intensifies the resistance modulation observed due to this transition.