Jan Čechal

Atomic-scale observation of multiconformational binding and energy level alignment of ruthenium-based photosensitizers on TiO2 anatase

Dye-sensitized solar cells constitute a promising approach to sustainable and low-cost solar energy conversion. Their overall efficiency crucially depends on the effective coupling of the photosensitizers to the photoelectrode and the details of the dyes energy levels at the interface. Despite great efforts, the specific binding of prototypical ruthenium-based dyes to TiO2, their potential supramolecular interaction, and the interrelation between adsorption geometry and electron injection efficiency lack experimental evidence. Here we demonstrate multiconformational adsorption and energy level alignment of single N3 dyes on TiO2 anatase (101) revealed by scanning tunnelling microscopy and spectroscopy. The distinctly bound molecules show significant variations of their excited state levels associated with different driving forces for photoelectron injection. These findings emphasize the critical role of the interfacial coupling and suggest that further designs of dye-sensitized solar cells should target a higher selectivity in the dye-substrate binding conformations in order to ensure efficient electron injection from all photosensitizers.

Ultrasmooth metallic foils for growth of high quality graphene by chemical vapor deposition

Synthesis of graphene by chemical vapor deposition is a promising route for manufacturing large-scale high-quality graphene for electronic applications. The quality of the employed substrates plays a crucial role, since the surface roughness and defects alter the graphene growth and cause difficulties in the subsequent graphene transfer. Here, we report on ultrasmooth high-purity copper foils prepared by sputter deposition of Cu thin film on a SiO2/Si template, and the subsequent peeling off of the metallic layer from the template. The surface displays a low level of oxidation and contamination, and the roughness of the foil surface is generally defined by the template, and was below 0.6 nm even on a large scale. The roughness and grain size increase occurred during both the annealing of the foils, and catalytic growth of graphene from methane (≈1000 °C), but on the large scale still remained far below the roughness typical for commercial foils. The micro-Raman spectroscopy and transport measurements proved the high quality of graphene grown on such foils, and the room temperature mobility of the graphene grown on the template stripped foil was three times higher compared to that of one grown on the commercial copper foil. The presented high-quality copper foils are expected to provide large-area substrates for the production of graphene suitable for electronic applications.

Decolorization of organic dyes by gold nanoflowers prepared on reduced graphene oxide by tea polyphenols

Green approaches for chemical syntheses are becoming important in various fields. Here, we report on the formation of 3D branched gold nanoparticles (nanoflowers) on graphene oxide surface (rGO-AuNFs) in the presence of green tea without requiring any additional seeds or surfactants. The morphology, composition, and optical properties of the composite have been characterized by scanning electron microscopy (SEM), Raman spectroscopy, Fourier transformation (FT-IR) infrared spectroscopy, X-ray photoelectron spectroscopy, UV-Vis spectroscopy, and fluorescence spectroscopy. We demonstrate the catalytic efficiency of rGO-AuNFs by measurement of the degradation of three commercial organic dyes – Safranin T, Eosin Y, and Congo red. The observations indicate the enhanced reduction of the mentioned dyes in comparison with uncatalyzed measurements. Contrary to common approaches, no irradiation is required for dye decolorization in the presence of NaBH4. This makes the presented procedure attractive for application in catalyst materials.

Selective growth of Co islands on ion beam induced nucleation centers in a native SiO2 film

We present a straightforward method for fabrication of patterns of metallic nanostructures. The focused ion beam (FIB) lithography has been used to locally modify a native SiO2 layer on a silicon substrate. On the modified areas preferential nucleation of cobalt islands is observed. The cobalt islands formed upon deposition at 400–430 °C combined with an intermediate annealing at 550 °C have a uniform size distribution and their size can be controlled by the distance between the nucleation sites and the amount of deposited material. It is proposed that the island formation at patterned sites is due to reduced surface diffusion of Co atoms in the vicinity of FIB modified areas. The intermediate annealing improves the island morphology since the kinetic diffusion limits are lowered and system reconfigures toward its equilibrium state.

Collagen-grafted ultra-high molecular weight polyethylene for biomedical applications

A novel material for hard tissue implants has been prepared. The ultra-high molecular weight polyethylene (UHMWPE) was grafted with collagen I, to improve its biocompatibility with soft tissue in case of its usage in bone engineering. Before collagen immobilization, commercial grade UHMWPE was treated with air plasma to introduce hydroperoxides onto the surface and subsequently grafted with carboxylic acid to functionalize the surface. Acrylic acid and itaconic acid were used for surface functionalization. After graft polymerization of carboxylic acids, collagen was immobilized covalently through the amide bonds between residual amino and carboxyl groups in the presence of water-soluble carbodiimide/hydroxysuccinimide cross-linking system. Each step of modification was characterized using spectroscopic (EPR, ATR-FTIR, and XPS), microscopic (SEM and CLSM), and contact angle measurement methods. The experimental results showed that plasma treatment led to a generation of free radicals on the UHMWPE surface resulting in the formation of unstable hydroperoxides. These reactive species were used to graft unsaturated carboxylic acids onto UHMWPE. Consequently, collagen was grafted via the-NH2 and-COOH reaction. The obtained experimental data along with microscopic observations confirmed the success of graft poly-merization of itaconic as well as of acrylic acid and collagen immobilization onto the UHMWPE surface.

X-ray induced electrostatic graphene doping via defect charging in gate dielectric

Graphene field effect transistors are becoming an integral part of advanced devices. Hence, the advanced strategies for both characterization and tuning of graphene properties are required. Here we show that the X-ray irradiation at the zero applied gate voltage causes very strong negative doping of graphene, which is explained by X-ray radiation induced charging of defects in the gate dielectric. The induced charge can be neutralized and compensated if the graphene device is irradiated by X-rays at a negative gate voltage. Here the charge neutrality point shifts back to zero voltage. The observed phenomenon has strong implications for interpretation of X-ray based measurements of graphene devices as it renders them to significantly altered state. Our results also form a basis for remote X-ray tuning of graphene transport properties and X-ray sensors comprising the graphene/oxide interface as an active layer.

Preparation of CuO/ZnO nanocomposite and its application as a cysteine/homocysteine colorimetric and fluorescence detector

Cysteine and homocysteine play a crucial role in many biological functions but abnormal levels of these amino acids may lead to various forms of pathogenesis. Therefore, selective and easy-to-use methods for the detection of cysteine and homocysteine are essential for the early diagnosis of developing diseases. In this paper we report on a rapid, straightforward and highly selective method for the detection of cysteine (Cys) and homocysteine (Hcy) which uses a CuO/ZnO nanocomposite as a dual colorimetric and fluorometric assay. The presence of Cys and Hcy in a solution of these nanorods (NRs) induces a change in its color from light blue to dark grey which is visible to the naked eye. This is accompanied by a blue shift in the absorption spectra from 725 nm to 650 nm and a decrease in the intensity of CuO/ZnO nanocomposite emission. These changes are ascribed to the reduction of Cu(II) to Cu(0), and the oxidation of cysteine (homocysteine) and subsequent formation of the disulfide bond. This novel assay method does not respond to any other amino-acid which is present in living organisms; therefore the selective determination of cysteine (homocysteine) with a lower analyte limit of 40 μM (4.8 μg mL(-1)) can be carried out in aqueous solutions without the need for any sophisticated instrumentation, fluorophore molecules or complicated procedures.

Real-time observation of self-limiting SiO2/Si decomposition catalysed by gold silicide droplets

The thermal decomposition of thin SiO2 layers on silicon substrates draws significant attention due to its high technological importance in the semiconductor industry and in all relevant fields where silicon is employed as a substrate or part of an active device. Understanding of the underlying processes on silicon surfaces is therefore of fundamental importance. Here we show that the presence of gold silicide (AuSi) catalytically enhances the decomposition of SiO2 layers on a Si substrate, which proceeds via void nucleation under the positions of Au nanoparticles and subsequent lateral growth of the void. Our real-time secondary electron microscopy data reveal that the presence of a AuSi droplet within the void enhances the reaction rate due to an increased pre-exponential factor of the rate limiting step (i.e., SiO desorption at temperatures beyond 700 °C). While the SiO2 is decomposed the silicon surface in the open voids is covered by an Au monolayer. Consequently, as the void grows, the AuSi droplet is depleted of gold and the reaction rate enhancement is terminated when the supply of gold stops. Hence, the size of the pits is determined by the initial size of the Au nanoparticle. Our work thus provides insight into Au-enhanced SiO2 decomposition and its self-limiting nature offers a way for the preparation of nanoscale features with nanometer precision.

Simple device for the growth of micrometer-sized monocrystalline single-layer graphene on SiC(0001)

The thermal decomposition of SiC wafers has proven to be a reliable method to obtain epitaxial graphene. However, the sublimation of Si induced by annealing of SiC substrates is notoriously difficult to control. To tackle the problem, the authors developed a fairly simple apparatus for the growth of micrometer-scale homogeneous single- and bilayer graphene in Ar atmosphere. The device is a furnace based on a considerably improved version of a directly heated element, and can achieve the desired sample quality reproducibly and efficiently. The authors characterize the samples prepared using this device by atomic force microscopy, low energy electron diffraction, Raman spectroscopy, scanning tunneling microscopy, x-ray photoemission spectroscopy, and near-edge x-ray absorption spectroscopy.

Aerosol-assisted Chemical Vapor Deposition of Metal Oxide Structures: Zinc Oxide Rods

Whilst columnar zinc oxide (ZnO) structures in the form of rods or wires have been synthesized previously by different liquid- or vapor-phase routes, their high cost production and/or incompatibility with microfabrication technologies, due to the use of pre-deposited catalyst-seeds and/or high processing temperatures exceeding 900 °C, represent a drawback for a widespread use of these methods. Here, however, we report the synthesis of ZnO rods via a non-catalyzed vapor-solid mechanism enabled by using an aerosol-assisted chemical vapor deposition (CVD) method at 400 °C with zinc chloride (ZnCl2) as the precursor and ethanol as the carrier solvent. This method provides both single-step formation of ZnO rods and the possibility of their direct integration with various substrate types, including silicon, silicon-based micromachined platforms, quartz, or high heat resistant polymers. This potentially facilitates the use of this method at a large-scale, due to its compatibility with state-of-the-art microfabrication processes for device manufacture. This report also describes the properties of these structures (e.g., morphology, crystalline phase, optical band gap, chemical composition, electrical resistance) and validates its gas sensing functionality towards carbon monoxide.