Well-defined Thickness Research Articles

Improved photoelectrical properties of MoS(2) films after laser micromachining.

Direct patterning of ultrathin MoS2 films with well-defined structures and controllable thickness is appealing since the properties of MoS2 sheets are sensitive to the number of layer and surface properties. In this work, we employed a facile, effective, and well-controlled technique to achieve micropatterning of MoS2 films with a focused laser beam. We demonstrated that a direct focused laser beam irradiation was able to achieve localized modification and thinning of as-synthesized MoS2 films. With a scanning laser beam, microdomains with well-defined structures and controllable thickness were created on the same film. We found that laser modification altered the photoelectrical property of the MoS2 films, and subsequently, photodetectors with improved performance have been fabricated and demonstrated using laser modified films.

Photocatalytic and photoelectrochemical properties of sol–gel TiO2 films of controlled thickness and porosity

Nonporous and mesoporous TiO2 films of well-defined thicknesses were synthesized by the sol–gel technique with dip-coating as the deposition method. Due to the potential applications as self-cleaning surfaces and photoanodes in water splitting activities of the prepared TiO2 films were evaluated for the photodegradation of model ink 2,6-dichloroindophenol and water photooxidation in aqueous media containing an inorganic salt. Thickness and mass parameters of the TiO2 films depend linearly on the number of deposition cycles (dip-coating, drying and calcinations). Due to different viscosities of the precursor, thicknesses of a single layer of nonporous and mesoporous titania film were 65nm and 300nm, respectively. The crucial effect of film mass on the decolourization of solid organic films was confirmed. Photocurrent in aqueous electrolyte depends strongly on the type of conductive substrate. It is about five times lower for indium doped tin oxide (ITO) which was confirmed to be due to the increase of resistivity of ITO conductive layer during calcinations. Mesoporous films reveal significantly lower photocurrents than nonporous films which can be explained by insufficient charge transfer in mesoporous films consisting of very small mesopores.

Characterization of a new fluorescence-enhancing substrate for microarrays with femtomolar sensitivity

The demand for high sensitivity in microarray technology has stimulated the research of new labeling strategies, new substrates with increased loading capacity and new approaches to amplify the fluorescence signals. Although these methods improve the sensitivity, they are based on non-routine procedures which limit their widespread usage. A simple way to achieve fluorescence enhancement is the optical interference (OI) coating technology based on the use of substrates with films of well-defined thickness that maximize photoabsorption of the dye molecules in the vicinity of the surface and reflect the emitted light toward the detector. Here we introduce a new substrate for fluorescence enhancement composed of a thin aluminum mirror and a single quarter wave silicon oxide as a dielectric layer. The use of such material allows the development of a substrate suited for parallel production in conventional IC (Integrated Circuit's) technology, which produces a 20-fold enhancement of fluorescence compared to glass, which is the most common material used in microarrays. The new substrate was coated with a copolymer of N,N-dimethylacrylamide, N-acryloyloxysuccinimide, and 3-(trimethoxysilyl)propyl methacrylate, copoly(DMA-NAS-MAPS), which forms, by a simple and robust procedure, a functional nanometric film that covalently binds bio-probes on the surface and efficiently suppresses non-specific adsorption. The performance of the new fluorescence-enhancing substrate in microarray technology was demonstrated for the detection of a panel of inflammation biomarkers pushing detection limits into the femtomolar range.

A Study of Pt Dissolution during Formic Acid Oxidation

The studies on Pt or Pt-alloy based catalysts for formic acid oxidation (FAO) have been focused mostly on their activity with little analysis of durability and long-term performance. While durability of Pt-based FAO catalysts has been associated with CO poisoning, recent studies have brought evidence of correlation between catalyst performance and dissolution of Pt. Aimed at assessing the impact of that correlation, a detailed quantitative durability analysis of Pt ultrathin films grown on Au by surface limited redox replacement of a Pb underpotentially deposited layer is presented in this work. Long-term catalytic performance of those films featuring well-defined thickness and structure has been examined by potential cycling in 2 M HCOOH+0.1 M HClO4 solution at sweep rate of 0.050 V s–1 over an extended potential range limited positively by the onset of Pt oxidation. A clear proportionality between overall life and thickness of the catalyst observed in the tests emphasizes the dissolution of Pt as the ke...

Designed Synthesis of Well‐Defined Pd@Pt Core–Shell Nanoparticles with Controlled Shell Thickness as Efficient Oxygen Reduction Electrocatalysts

Improving the electrocatalytic activity and durability of Pt-based catalysts with low Pt content toward the oxygen reduction reaction (ORR) is one of the main challenges in advancing the performance of polymer electrolyte membrane fuel cells (PEMFCs). Herein, a designed synthesis of well-defined Pd@Pt core-shell nanoparticles (NPs) with a controlled Pt shell thickness of 0.4-1.2 nm by a facile wet chemical method and their electrocatalytic performances for ORR as a function of shell thickness are reported. Pd@Pt NPs with predetermined structural parameters were prepared by in situ heteroepitaxial growth of Pt on as-synthesized 6 nm Pd NPs without any sacrificial layers and intermediate workup processes, and thus the synthetic procedure for the production of Pd@Pt NPs with well-defined sizes and shell thicknesses is greatly simplified. The Pt shell thickness could be precisely controlled by adjusting the molar ratio of Pt to Pd. The ORR performance of the Pd@Pt NPs strongly depended on the thickness of their Pt shells. The Pd@Pt NPs with 0.94 nm Pt shells exhibited enhanced specific activity and higher durability compared to other Pd@Pt NPs and commercial Pt/C catalysts. Testing Pd@Pt NPs with 0.94 nm Pt shells in a membrane electrode assembly revealed a single-cell performance comparable with that of the Pt/C catalyst despite their lower Pt content, that is the present NP catalysts can facilitate low-cost and high-efficient applications of PEMFCs.

Time-Resolved, in Situ, Small- and Wide-Angle X-ray Scattering To Monitor Pt Nanoparticle Structure Evolution Stabilized by Adsorbed SnCl3– Ligands During Synthesis

Understanding and controlling nanoparticle formation mechanisms is important because of increasing reports of unusual material properties in this critical size region in fields ranging from magnetics, electrocatalysis, optics, and heterogeneous synthesis. Here we use real-time, in situ small-angle and wide-angle X-ray scattering to dynamically monitor the production of Pt critical nuclei from Sn–Pt complexes on a length scale approaching 0.6 nm. A time resolution of 20 s is achieved due to the slow reduction and growth processes of this unique system. The structure and evolution of inorganic Sn ligands on these Pt nanoparticle cores is of particular interest because of the ability of Sn to mitigate poisoning of electrocatalyst surfaces by adsorbed CO intermediates (COads) on the anode of direct alcohol fuel cells. Coherent scattering from the stabilizing ligand shell is identified in the wide-angle scattering data and suggests segregation of Sn ligands at the surface of the nanoparticles synthesized using our approach. We correlate the autoreduction kinetics of precursor complexes with number density, polydispersity, and Pt volume fraction, thereby establishing the mechanism of particle size control at the onset of atomic coalescence. Particle growth is shown to terminate when a Sn shell of well-defined thickness (about 0.6 nm) is completed for a range of Pt core diameters in this critical size range.

Two-Dimensional Growth of CdSe Nanocrystals, from Nanoplatelets to Nanosheets

We report the continuous lateral extension of cadmium selenide nanoplatelets into nanosheets using continuous injection of precursors at high temperature. We show that we can obtain CdSe nanosheets with lateral dimensions up to 700 nm and a well-defined thickness that can be tuned with atomic precision. When the nanosheets’ lateral size increases, they roll on themselves to form nanoscrolls that can unroll upon surface modification. The final geometry of the nanosheets can be tuned to different morphologies using precursors that favor the growth of specific crystal facets. We provide a detailed study of the CdSe nanosheets growth and its optimization for three different thicknesses.

Eshelby Twist and Magic Helical Zinc Oxide Nanowires and Nanotubes

Twisted zinc oxide nanowires and nanotubes were recently synthesized by screw-dislocation growth. We show theoretically that once their diameter increases above a critical size of the order of a few atomic spacings, the existence of these structures can be rationalized in terms of the energetics of surfaces and veritable Eshelby's twist linear elasticity mechanics supplemented by a nonlinear core term. For Burgers vector larger than the minimum allowed one, a twisted nanotube with well-defined thickness, rather than a nanowire, is the most stable nanostructure. Results are assistive for designing ultrathin nanostructures made out of nonlayered materials.

Room temperature synthesis of samarium oxide nanotubes using cost-effective electroless deposition method

Nanotubes consisting of samarium oxide (SmNT) were fabricated by electroless deposition using ion track-etched polycarbonate templates. To achieve nanotubes with thin walls and small surface roughness, the tubes were generated by a procedure containing several steps under aqueous conditions. The approach reported below will process open-ended nanotubes with well-defined outer diameter and wall thickness.

Pulsed laser deposition growth of heteroepitaxial YBa2Cu3O7/La0.67Ca0.33MnO3superlattices on NdGaO3and Sr0.7La0.3Al0.65Ta0.35O3substrates

Heteroepitaxial superlattices of [YBa2Cu3O7(n)/ La0.67Ca0.33MnO3(m)]x, where n and m are the number of YBCO and LCMO monolayers and x the number of bilayer repetitions, have been grown with pulsed laser deposition on NdGaO3 (110) and Sr0.7La0.3Al0.65Ta0.35O3 (LSAT) (001). These substrates are well lattice matched with YBCO and LCMO and, unlike the commonly used SrTiO3, they do not give rise to complex and uncontrolled strain effects due to structural transitions at low temperature. The growth dynamics and the structure have been studied in-situ with reflection high energy electron diffraction (RHEED) and ex-situ with scanning transmission electron microscopy (STEM), x-ray diffraction, and neutron reflectometry. The individual layers are found to be flat and continuous over long lateral distances with sharp and coherent interfaces and with a well-defined thickness of the individual layer. The only visible defects are antiphase boundaries in the YBCO layers that originate from perovskite unit cell height steps at the interfaces with the LCMO layers. We also find that the first YBCO monolayer at the interface with LCMO has an unusual growth dynamics and is lacking the CuO chain layer while the subsequent YBCO layers have the regular Y-123 structure. Accordingly, the CuO2 bilayers at both the LCMO/YBCO and the YBCO/LCMO interfaces are lacking one of their neighboring CuO chain layers and thus half of their hole doping reservoir. Nevertheless, from electric transport measurements on asuperlattice with n=2 we obtain evidence that the interfacial CuO2 bilayers remain conducting and even exhibit the onset of a superconducting transition at very low temperature. Finally, we show from dc magnetization and neutron reflectometry measurements that the LCMO layers are strongly ferromagnetic.

Electrochemical Atomic Layer Deposition (E-ALD) of Palladium Nanofilms by Surface Limited Redox Replacement (SLRR), with EDTA Complexation

Atomic-scale control in the formation of Pd thin films is being developed using electrochemical atomic layer deposition (E-ALD) via surface limited redox replacement (SLRR). Pd has unique hydrogen storage properties. To study hydrogen storage capacity, hydrogen charging and discharging kinetics and its catalytic properties at the nanoscale will require films with well-defined thickness and structure. SLRR is the use of underpotential deposition (UPD) to form a sacrificial atomic layer of a less noble metal, such as Cu or Pb, and to exchange it at open circuit potential (OCP) for a more noble metal (Pd) via galvanic displacement. The deposits were grown using an automated electrochemical flow cell system which allowed sequential variation of solutions and potentials. Electron probe microanalysis (EPMA) revealed excess growth at the flow cell ingress, suggesting that the SLRR mechanism involved electron transfer from substrate to Pd2+ ions, rather than direct electron exchange from sacrificial metal atom(s) to Pd2+ ions. Ethylenediaminetetraacetic acid (EDTA) was used to slow the galvanic displacement by complexing the Pd2+, in an attempt to form more uniform Pd deposits. The resulting films were more homogeneous and displayed the expected Pd voltammetry in H2SO4. The charge for UPD remained constant from cycle to cycle, indicating no roughening of the surface. Ways of optimizing complexing agent properties, as well as the flow cell design and deposition parameters are discussed.

Electrochemical properties of carbon nanodiscs

Carbon nanodiscs are new bottom-up prepared nanomaterials with well-defined diameter and thickness (of 1–2 μm and 20–50 nm, respectively). We investigate their electrochemical performance towards oxidation and reduction of small inorganic molecules as well as biomarkers. We have found that carbon nanodiscs exhibit faster observable heterogeneous electron transfer rates than graphite. However, this can be due to nanographitic impurities within the nanodiscs sample.

Horizontal structures of velocity and temperature boundary layers in two-dimensional numerical turbulent Rayleigh-Bénard convection

We investigate the structures of the near-plate velocity and temperature profiles at different horizontal positions along the conducting bottom (and top) plate of a Rayleigh-Bénard convection cell, using two-dimensional (2D) numerical data obtained at the Rayleigh number Ra = 108 and the Prandtl number Pr = 4.4 of an Oberbeck-Boussinesq flow with constant material parameters. The results show that most of the time, and for both velocity and temperature, the instantaneous profiles scaled by the dynamical frame method [Q. Zhou and K.-Q. Xia, “Measured instantaneous viscous boundary layer in turbulent Rayleigh-Bénard convection,” Phys. Rev. Lett. 104, 104301 (2010)] agree well with the classical Prandtl-Blasius laminar boundary layer (BL) profiles. Therefore, when averaging in the dynamical reference frames, which fluctuate with the respective instantaneous kinematic and thermal BL thicknesses, the obtained mean velocity and temperature profiles are also of Prandtl-Blasius type for nearly all horizontal positions. We further show that in certain situations the traditional definitions based on the time-averaged profiles can lead to unphysical BL thicknesses, while the dynamical method also in such cases can provide a well-defined BL thickness for both the kinematic and the thermal BLs.

Colloidal nanoplatelets with two-dimensional electronic structure

The syntheses of strongly anisotropic nanocrystals with one dimension much smaller than the two others, such as nanoplatelets, are still greatly underdeveloped. Here, we demonstrate the formation of atomically flat quasi-two-dimensional colloidal CdSe, CdS and CdTe nanoplatelets with well-defined thicknesses ranging from 4 to 11 monolayers. These nanoplatelets have the electronic properties of two-dimensional quantum wells formed by molecular beam epitaxy, and their thickness-dependent absorption and emission spectra are described very well within an eight-band Pidgeon-Brown model. They present an extremely narrow emission spectrum with full-width at half-maximum less than 40 meV at room temperature. The radiative fluorescent lifetime measured in CdSe nanoplatelets decreases with temperature, reaching 1 ns at 6 K, two orders of magnitude less than for spherical CdSe nanoparticles. This makes the nanoplatelets the fastest colloidal fluorescent emitters and strongly suggests that they show a giant oscillator strength transition.

Soft Adsorption of Densely Packed Layers of DNA-Plasmid·1,3-Diaminopropane Complexes onto Highly Oriented Pyrolitic Graphite Designed To Erode in Water

In this Article, we report a simple and effective method to build up self-assembled and well-calibrated layers of plasmid DNA·1,3-diaminopropane complexes onto highly oriented pyrolitic graphite (HOPG). The method is based on the self-assembly of the poly electrolytes onto HOPG in an excess of positively charged protonated diamines (Dap2+) in comparison to the negatively charged phosphate moieties of the DNA backbone in solution. Although short distortions in the helical parameters of DNA (maximum 12% hypochromicity) are revealed by UV–vis absorption spectrometry, the native B form of the plasmids is conserved. By fixing the excess of positive charges arising from Dap2+ cations, it is possible to construct assemblies of a well-defined thickness ranging typically from 1 monolayer (ML) of DNA to 10 ML; 1 ML has a thickness of 2.2 ± 0.5 nm. Adding TRIS-EDTA (TE) buffer lowers considerably the damage rate observed when plasmids are mixed with Dap2+ in pure water. The thickness of the first dense monolayer mat...

Microstructural and domain effects in epitaxial CoFe2O4 films on MgO with perpendicular magnetic anisotropy

CoFe2O4 (CFO) epitaxial thin films of various thicknesses were grown on MgO substrates using the pulsed electron-beam deposition technique. The films have excellent in-plane coherence with the substrate, exhibit layer-by-layer growth and have well-defined thickness fringes in x-ray diffraction measurements. Atomic force microscopy (AFM) measurements indicate that misfit dislocations form in thicker films and the critical thickness for the dislocation formation is estimated. Perpendicular magnetic anisotropy in CFO due to epitaxial in-plane tensile strain from the substrate was found. A stripe-like domain structure in the demagnetized state is demonstrated using magnetic force microscopy (MFM), in agreement with previous predictions. Coercivity increased in thicker films, which is explained by domain wall pinning due to misfit dislocations at the CFO/MgO interface.

Preparation of hollow silica microspheres with controlled shell thickness in supercritical fluids

This article presents a novel route to prepare hollow silica microspheres with well-defined wall thickness by using cross-linked polystyrene (PS) microspheres as templates with the assistance of supercritical carbon dioxide (SC-CO2). In this approach, the cross-linked PS templates can be firstly prepared via emulsifier-free polymerization method by using ethylene glycol dimethacrylate or divinylbenzene as cross-linkers. Then, the silica shell from the sol–gel process of tetraethyl orthosilicate (TEOS) which was penetrated into the PS template with the assistance of SC-CO2 was obtained. Finally, the hollow silica spheres were generated after calcinations at 600 °C for 4 h. The shell thickness of the hollow silica spheres could be finely tuned not only by adjusting the TEOS/PS ratio, which is the most frequently used method, but also by changing the pressure and aging time of the SC-CO2 treatment. Fourier transform infrared spectroscopy, transmission electron microscopy, and scanning electron microscope were used to characterize these hollow silica spheres.

Far field coherent thermal emission from a bilayer structure

Recent years, there has been an increased interest in the conception of micro/nanostructures with unusual radiative properties, far away from those of blackbody, especially thermal sources with temporal and/or spatial coherent emission. Such structures are indeed extremely interesting for energy conversion systems, radiative cooling devices, etc. The present study numerically investigates temporal coherent emission from a very simple structure composed of one layer of germanium and one of silicon carbide. Our investigation shows that, for well-defined thicknesses, this two-layer structure is able to emit in narrow spectral peak.

Cerium (IV) oxide nanotubes prepared by low temperature deposition at normalpressure

This paper reports the synthesis of cerium dioxide nanotubes (CeNTs) by electrolessdeposition using ion-track-etched polycarbonate templates. To achieve nanotubes with thinwalls and small surface roughness the tubes were generated by a several-step-containingprocedure under aqueous conditions. The approach reported below will process open endnanotubes with well-defined outer diameter and wall thickness.

Fabrication of Zinc Oxide Nanotubes by Chemical Bath Deposition Using Ion Track-Etched Templates

Nanotubes consisting of zinc oxide were fabricated by electroless deposition, using ion track-etched polycarbonate templates. To achieve nanotubes with thin walls and small surface roughness, the tubes were generated by a several steps containing procedure under aqueous conditions. The reported approach that is described shortly will process open-end nanotubes with well-defined outer diameter and wall thickness.