Influence of oxide film surface morphology and thickness on the properties of gas sensitive nanostructure sensor

In this study, the gas sensitive metal-oxide semiconductor (MOS) nanostructure sensors based on Ni thin film have been fabricated. The influences of SiO2 film surface morphology and thickness on the response (R%) and electrical properties of the sensors have been investigated at 150 °C. The surface morphology of the SiO2 film has been characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The C-V curves of the MOS nanostructure sensors in pure nitrogen and 2 % hydrogen have been reported as well. For the SiO2 film thicknesses of 14, 65 and 74 nm the measured flat-band voltages (VFB) are 0.7, 1.5 and 2 V, respectively. The responses of different sensors in 2% hydrogen for SiO2 film thicknesses of 14 and 74 nm are 84% and 32%, respectively. The MOS nanostructure sensors exhibited good response to the hydrogen gas, with excellent sensitivity. The MOS nanostructure sensor based on Ni thin film and SiO2 film thickness of 14 nm shows high response and sensitivity

Chemical vapor deposited two-dimensional material based high frequency flexible field-effect transistors

Flexible nanoelectronics have attracted great attention due to interesting concepts such as wearable electronics and internet of things, which requires high speed and low power consumption flexible smart system with functions ranging from sensing, computing to wireless communicating. In this dissertation, transparent and solution processable nanoscale polyimide film for highly flexible gate dielectrics is demonstrated by in-situ opto-electro-mechanical measurement and utilized for two-dimensional nanomaterials based field-effect transistors (FETs). Graphene thin film transistor with the nanoscale polyimide dielectric on flexible glass is operated in extremely high frequency regime and shows the highest experimental saturation velocity (~8.4 × 10⁶ cm/s) in any materials in any flexible transistors. Molybdenum disulfide (MoS₂) based transistors with embedded gate structure on rigid substrate are demonstrated with enhancement mode operation, ON/OFF ratio over 10⁸, the highest transconductance (~ 70 µS/µm) and saturation velocity (~1.8 × 10⁶ cm/s). CVD MoS₂ FETs on flexible plastic substrates are also demonstrated, showing enhancement mode operation, ON/OFF radio over 10¹⁰ and transconductance (~6 µS/µm). The flexible CVD MoS₂ transistors with embedded gate structure were employed to study effects of substoichiometric doping by HfO [subscript 2-x]. After the doping layer, the flexible MoS₂ transistors show ×8 higher source-drain current density as well as more than ×2 mobility improvements. For the another first demonstration, GHz operation and flexibility of graphene and MoS₂ based FETs are realized on commercial available paper substrates, which indicates flexible two-dimensional material based nanoelectronics can be implemented on paper substrates for systems, sensors, and Internet of Things.Electrical and Computer Engineerin

Multiscale Investigation of the Structural, Electrical and Photoluminescence Properties of MoS2 Obtained by MoO3 Sulfurization

In this paper, we report a multiscale investigation of the compositional, morphological, structural, electrical, and optical emission properties of 2H-MoS(2) obtained by sulfurization at 800 °C of very thin MoO(3) films (with thickness ranging from ~2.8 nm to ~4.2 nm) on a SiO(2)/Si substrate. XPS analyses confirmed that the sulfurization was very effective in the reduction of the oxide to MoS(2,) with only a small percentage of residual MoO(3) present in the final film. High-resolution TEM/STEM analyses revealed the formation of few (i.e., 2–3 layers) of MoS(2) nearly aligned with the SiO(2) surface in the case of the thinnest (~2.8 nm) MoO(3) film, whereas multilayers of MoS(2) partially standing up with respect to the substrate were observed for the ~4.2 nm one. Such different configurations indicate the prevalence of different mechanisms (i.e., vapour-solid surface reaction or S diffusion within the film) as a function of the thickness. The uniform thickness distribution of the few-layer and multilayer MoS(2) was confirmed by Raman mapping. Furthermore, the correlative plot of the characteristic A(1g)-E(2g) Raman modes revealed a compressive strain (ε ≈ −0.78 ± 0.18%) and the coexistence of n- and p-type doped areas in the few-layer MoS(2) on SiO(2), where the p-type doping is probably due to the presence of residual MoO(3). Nanoscale resolution current mapping by C-AFM showed local inhomogeneities in the conductivity of the few-layer MoS(2), which are well correlated to the lateral changes in the strain detected by Raman. Finally, characteristic spectroscopic signatures of the defects/disorder in MoS(2) films produced by sulfurization were identified by a comparative analysis of Raman and photoluminescence (PL) spectra with CVD grown MoS(2) flakes

Methane detection scheme based upon the changing optical constants of a zinc oxide/platinum matrix created by a redox reaction and their effect upon surface plasmons

We detect changes in the optical properties of a metal oxide semiconductor (MOS), ZnO, in a multi-thin-film matrix with platinum in the presence of the hydrocarbon gas methane. A limit of detection of 2% by volume with concentrations from 0 to 10% and maximum resolution of 0.15% with concentrations ranging from 30% to 80% at room temperature are demonstrated along with a selective chemical response to methane over carbon dioxide and the other alkane gases. The device yields the equivalent maximum bulk refractive index spectral sensitivity of 1.8 × 105 nm/RIU. This is the first time that the optical properties of MOS have been monitored to detect the presence of a specific gas. This single observation is a significant result, as MOS have a potentially large number of target gases, thus offering a new paradigm for gas sensing using MOSs

WTC2005-63892 EXPERIMENTAL STUDY ON SUPERLUBRICITY OF AG NANOMETER-THICK-LAYERS BY SLIDING ON A MACROSCOPIC SYSTEM

ABSTRACT The experimental study on the Ag film was carried out using a diamond pin-on-plate type tribometer under ultrahigh vacuum (UHV) conditions. The coefficient of friction varied with the film morphology in nanometric scale up to 170 nm, and superlubricity as minimum coefficient of friction 0.007 was obtained on 5-nm Ag film with network structure. RHEED and STM observation of the film showed that the film morphologies changed drastically during rubbing, and that the superlubricity of this system is attributed to the lamella gliding of Ag INTRODUCTION The concept of a superlubricity was theoretically discussed by Hirano and Shinjo [1]. Martin et al. [2] showed extraordinary low friction coefficient of molybdenum disulfide (MoS 2 ), which was less than 0.002 (milli-range), in ultrahigh vacuum (UHV) condition. They pointed out that the millirange coefficient of friction was attributed to the mechanism of the superlubric state as Hirano et al. discussed [1]. The sliding plane of MoS 2 is c-plane. Recently, Goto et al. [3][4][5] showed that the milli-range friction of epitaxial Ag film, and concluded that the low friction coefficient was attributed to inter-layer shearing between Ag (111) planes parallel to the sliding direction. This paper provides another example different from the superlubricity of MoS 2 , because the interaction between MoS 2 layers is Van der Waals interaction, whereas the interaction between Ag (111) planes is metallic bond

Efficient Charge Separation in 2D Janus van der Waals Structures with Build-in Electric Fields and Intrinsic p-n Doping

Janus MoSSe monolayers were recently synthesised by replacing S by Se on one side of MoS$_2$ (or vice versa for MoSe$_2$). Due to the different electronegativity of S and Se these structures carry a finite out-of-plane dipole moment. As we show here by means of density functional theory (DFT) calculations, this intrinsic dipole leads to the formation of built-in electric fields when the monolayers are stacked to form $N$-layer structures. For sufficiently thin structures ($N<4$) the dipoles add up and shift the vacuum level on the two sides of the film by $\sim N \cdot 0.7$ eV. However, for thicker films charge transfer occurs between the outermost layers forming atomically thin n- and p-doped electron gasses at the two surfaces. The doping concentration can be tuned between about $5\cdot 10^{12}$ e/cm$^{2}$ and $2\cdot 10^{13}$ e/cm$^{2}$ by varying the film thickness. The surface charges counteract the static dipoles leading to saturation of the vacuum level shift at around 2.2 eV for $N>4$. Based on band structure calculations and the Mott-Wannier exciton model, we compute the energies of intra- and interlayer excitons as a function of film thickness suggesting that the Janus multilayer films are ideally suited for achieving ultrafast charge separation over atomic length scales without chemical doping or applied electric fields. Finally, we explore a number of other potentially synthesisable 2D Janus structures with different band gaps and internal dipole moments. Our results open new opportunities for ultrathin opto-electronic components such as tunnel diodes, photo-detectors, or solar cells