Graphene-enhanced Raman scattering (GERS), enhancing Raman signals on graphene surface, is an excellent approach to investigate the properties of graphene via the Raman enhancement effect. In the ...present study, we studied the graphene-thickness dependent GERS in detail. First, by keeping molecule density on few-layer graphene using vacuum thermal deposition method, GERS enhancement was found to be the same for all graphene layers (one to six layers). While adsorbing probe molecules by solution soaking, the GERS enhancing factor was different on monolayer and bilayer graphene. By soaking in low concentration solutions, the GERS intensity on bilayer graphene was stronger than that on monolayer graphene, whereas by soaking under high concentration solutions, the GERS intensity difference was much less for that on monolayer and on bilayer. Molecule density, molecular configuration, and GERS enhancing factor are further discussed for molecules on monolayer and bilayer graphene. It was finally concluded that the abnormal graphene-thickness dependence of GERS between monolayer and bilayer graphene was attributed to the different enhancement for GERS on monolayer and bilayer graphene. Monolayer and bilayer graphene have different electronic structure and then doping effect of probe molecules, which shifts the Fermi level of graphenes differently. As a result, monolayer and bilayer graphene have different energy band matching with the probe molecules, yielding different chemical enhancement.
We performed first-principles simulation on the electronic structure and magnetic properties of two-dimensional hexagonal silicene, which was recently synthesized. The results show that the weak ...overlapping between 3p z orbitals of neighbor Si atoms leads to a very reactive surface, resulting in a more energetically stable semiconducting surface upon being fully hydrogenated. Half-hydrogenation breaks the extended π-bonding network of silicene, leaving the electrons in the unsaturated Si atoms localized and unpaired, and thus it exhibits ferromagnetic semiconducting behavior with a band gap of 0.95 eV. The long-range ferromagnetic coupling between Si atoms was also predicted, with a Curie temperature of about 300 K. These results demonstrated that hydrogenation is an efficient route to tune the electronic properties of silicene sheets.
2D covalent organic frameworks (2D COFs) are a unique materials platform that combines covalent connectivity, structural regularity, and molecularly precise porosity. However, 2D COFs typically form ...insoluble aggregates, thus limiting their processing via additive manufacturing techniques. In this work, colloidal suspensions of boronate‐ester‐linked 2D COFs are used as a spray‐coating ink to produce large‐area 2D COF thin films. This method is synthetically general, with five different 2D COFs prepared as colloidal inks and subsequently spray‐coated onto a diverse range of substrates. Moreover, this approach enables the deposition of multiple 2D COF materials simultaneously, which is not possible by polymerizing COFs on substrates directly. When combined with stencil masks, spray‐coated 2D COFs are rapidly deposited as thin films larger than 200 cm2 with line resolutions below 50 µm. To demonstrate that this deposition scheme preserves the desirable attributes of 2D COFs, spray‐coated 2D COF thin films are incorporated as the active material in acoustic sensors. These 2D‐COF‐based sensors have a 10 ppb limit‐of‐quantification for trimethylamine, which places them among the most sensitive sensors for meat and seafood spoilage. Overall, this work establishes a scalable additive manufacturing technique that enables the integration of 2D COFs into thin‐film device architectures.
High‐resolution, large‐scale fabrication of 2D covalent organic framework (COF) thin films is achieved by spray‐coating colloidal inks of these materials. Using this additive manufacturing approach, 2D COF thin films are integrated into acoustic sensing platforms. These sensors achieve superlative detection of volatile amines including a limit‐of‐quantification of 10 ppb for trimethylamine, which is a target analyte for the detection of food spoilage.
Vanadium dioxide (VOsub.2) with semiconductor-metal phase transition characteristics has presented great application potential in various optoelectrical smart devices. However, the preparation of ...doped VOsub.2 film with a lower phase transition threshold on Si substrate needs more investigation for the exploration of silicon-based VOsub.2 devices. In this work, the VOsub.2 films doped with different contents of W element were fabricated on high-purity Si substrate, assisted with a post-annealing process. The films exhibited good crystallinity and uniform thickness. The X-ray diffraction and X-ray photoelectron spectroscopy characterizations illustrated that W element can be doped into the lattice of VOsub.2 and lead to small lattice distortion. In turn, the in situ FT-IR measurements indicated that the phase transition temperature of the VOsub.2 films can be decreased continuously with W doping content. Simultaneously, the doping would lead to largely enhanced conductivity in the film, which results in reduced optical transmittance. This work provides significant insights into the design of doped VOsub.2 films for silicon-based devices.
Molybdenum oxide (MoO.sub.x) films had been grown by using plasma-enhanced atomic layer deposition (PEALD) with Mo(CO).sub.6 precursor and O.sub.2 plasma reactant in a substrate temperature range of ...150-275 °C. The effect of substrate temperature on the chemical, optical, surface morphological, and structural properties of the MoO.sub.x thin films was explored systematically. The substrate temperature performed a significant role in depositing MoO.sub.x films and three kinds of different precursor reaction mechanisms of PEALD-MoO.sub.x thin films handled by substrate temperature were presented firstly and illustrated comprehensively. The growth of amorphous MoO.sub.x film was observed between 150 and 175 °C. Moreover, an obvious transition to polycrystalline deposition was demonstrated for the deposition temperatures at 225 °C and higher. Both Mo.sup.6+ and Mo.sup.5+ valence states existed in all prepared MoO.sub.x films, which inferred the deficient lattice oxygen in the films. And the proportion of non-lattice oxygen reduced with the increasing deposition temperature. The elaboration of deposition mechanism of PEALD-MoO.sub.x films provides a guideline for the preparation of high-quality MoO.sub.x films.
In the advancement of complementary metal-oxide-semiconductor device technology, SiO2 was used as an outstanding dielectric and has dominated the microelectronics industry for the last few decades. ...However, with the recent size downscaling, ultrathin SiO2 is no longer suitable. ZrO2 has been introduced as a high-k dielectric to replace SiO2. This paper reviews recent progress of ZrO2 thin films as dielectric layers for volatile dynamic random access memory (DRAM) applications and as a gate dielectric for CMOS devices. Materials and electrical properties of ZrO2 films obtained by different deposition methods are compared. The effects of different top and bottom electrodes, and different doping elements, on ZrO2 dielectric properties are described. Applications discussed include the use of ZrO2 in Ge and SiGe nanocrystal-embedded nonvolatile flash memory devices. ZrO2 films as charge trapping layers in SOZOS (poly-Si/SiO2/ZrO2/SiO2/Si) and TAZOS (TaN/Al2O3/ZrO2/SiO2/Si) based nonvolatile flash memory stacks, and bipolar, unipolar, and nonpolar ZrO2-based resistive switching memory devices are also briefly discussed. The impact of electrode materials, metal nanocrystals, metal implantation, metal doping, metal layers, and oxide ion conductor buffer layer on resistive switching properties and switching parameters of emerging ZrO2-based resistive switching memory devices for high speed, low power, nanoscale, nonvolatile memory devices are briefly reviewed. A roadmap of the applications of ZrO2 thin film in future low power, nanoscale microelectronic device applications is realized from this review.
The discovery of high mobility p-type two-dimensional beta-tellurite (beta-TeO.sub.2) has led to an increasing interest in tellurium oxide-related polymorphs. Bulk TeO.sub.2 is known to exist in ...three polymorphs (alpha-, beta-, and gamma-TeO.sub.2), all of which exhibit wide bandgaps. By utilizing the interfacial interactions, we successfully synthesized a new polymorph of TeO.sub.2 with a rutile structure on a tetragonal FeTe surface via soft surface oxidation. Irrespective of the film thickness, the rutile TeO.sub.2 exhibited a metallic Fermi surface, as revealed by low-temperature scanning tunneling spectroscopy and further confirmed by our theoretical calculations. Striped wrinkles with an apparent lattice shift were observed on large rutile TeO.sub.2 monolayer islands due to lattice distortion. The density of states around the Fermi level accordingly shift from a U-shaped gap to a V-shaped feature. Our synthesis and observation of rutile TeO.sub.2, based on interface engineering and in situ tunneling spectroscopy, can help tune the electronic properties of tellurium oxide in reduced dimensions.
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