Václav Valeš

Covalent Reactions on Chemical Vapor Deposition Grown Graphene Studied by Surface-Enhanced Raman Spectroscopy.

Graphene is a material of unmatched properties and eminent potential in disciplines ranging from physics, to chemistry, to biology. Its advancement to applications with a specific function requires rational design and fine tuning of its properties, and covalent introduction of various substituents answers this requirement. We challenged the obstacle of non-trivial and harsh procedures for covalent functionalization of pristine graphene and developed a protocol for mild nucleophilic introduction of organic groups in the gas phase. The painstaking analysis problem of monolayered materials was addressed by using surface-enhanced Raman spectroscopy, which allowed us to monitor and characterize in detail the surface composition. These deliverables provide a toolbox for reactivity of fluorinated graphene under mild reaction conditions, providing structural freedom of the species to-be-grafted to the single-layer graphene.

The effect of a thin gold layer on graphene: a Raman spectroscopy study

An understanding of interactions between graphene and its surroundings is crucial for application of graphene in electronic devices. Raman spectroscopy is a convenient and efficient tool to provide information about the doping, stress and defects in graphene; however application of this method is limited to the Si/SiO2 substrate which provides interference enhancement. Here we present a comprehensive Raman study of the single-layer graphene – sapphire – gold system. Due to plasmons generated in the gold-layer the Raman signal of graphene is significantly enhanced. We study the influence of the gold layer thickness and gold particle size on the enhancement. The analysis of the Raman maps showed that graphene on sapphire is only slightly doped and the spatial distribution of doping is quite homogenous. Also no significant strain was generated in graphene sandwiched by sapphire and gold.

Graphene wrinkling induced by monodisperse nanoparticles: facile control and quantification.

Controlled wrinkling of single-layer graphene (1-LG) at nanometer scale was achieved by introducing monodisperse nanoparticles (NPs), with size comparable to the strain coherence length, underneath the 1-LG. Typical fingerprint of the delaminated fraction is identified as substantial contribution to the principal Raman modes of the 1-LG (G and G’). Correlation analysis of the Raman shift of the G and G’ modes clearly resolved the 1-LG in contact and delaminated from the substrate, respectively. Intensity of Raman features of the delaminated 1-LG increases linearly with the amount of the wrinkles, as determined by advanced processing of atomic force microscopy data. Our study thus offers universal approach for both fine tuning and facile quantification of the graphene topography up to ~60% of wrinkling.

Decomposition of Fluorinated Graphene under Heat Treatment

Fluorination modifies the electronic properties of graphene, and thus it can be used to provide material with on-demand properties. However, the thermal stability of fluorinated graphene is crucial for any application in electronic devices. Herein, X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD), and Raman spectroscopy were used to address the impact of the thermal treatment on fluorinated graphene. The annealing, at up to 700 K, caused gradual loss of fluorine and carbon, as was demonstrated by XPS. This loss was associated with broad desorption of CO and HF species, as monitored by TPD. The minor single desorption peak of CF species at 670 K is suggested to rationalize defect formation in the fluorinated graphene layer during the heating. However, fluorine removal from graphene was not complete, as some fraction of strongly bonded fluorine can persist despite heating to 1000 K. The role of intercalated H2 O and OH species in the defluorination process is emphasised.

Crystal structure of defect‐containing semiconductor nanocrystals – an X‐ray diffraction study

Defects of crystal structure in semiconductor nanocrystals embedded in an amorphous matrix are studied by X-ray diffraction and a full-profile analysis of the diffraction curves based on the Debye formula. A new theoretical model is proposed, describing the diffraction from randomly distributed intrinsic and extrinsic stacking faults and twin blocks in the nanocrystals. The application of the model to full-profile analysis of experimental diffraction curves enables the determination of the concentrations of individual defect types in the nanocrystals. The method has been applied for the investigation of self-organized Ge nanocrystals in an SiO2 matrix, and the dependence of the structure quality of the nanocrystals on their deposition and annealing parameters was obtained.

Tuning the reactivity of graphene by surface phase orientation

Tuning the local reactivity of graphene is a subject of paramount importance. Among the available strategies, the activation/passivation of graphene by copper substrate is very promising because it enables the properties of graphene to be influenced without any transfer procedure, since graphene can be grown directly on copper. Herein, it is demonstrated that the reactivity of graphene towards fluorination is strongly influenced by the face of the surface of the copper substrate. Graphene on the copper foil was probed and grain orientations were identified. The results of the reactivity were evaluated by means of X-ray photo electron and Raman spectroscopy. Graphene on the grains with a surface orientation close to the (111) face is the most reactive, whereas graphene on the grains close to the (110) surface is least reactive. The long-term stability test showed that the decomposition of fluorinated graphene was slowest on the grains with a surface orientation close to the (111) face. The results are consistent with the variation of the mechanical strain of graphene on different faces of copper. In contrast, no clear correlation of the graphene reactivity with doping induced by different facets was found.

Production of three-dimensional quantum dot lattice of Ge/Si core–shell quantum dots and Si/Ge layers in an alumina glass matrix

We report on the formation of Ge/Si quantum dots with core/shell structure that are arranged in a three-dimensional body centered tetragonal quantum dot lattice in an amorphous alumina matrix. The material is prepared by magnetron sputtering deposition of Al2O3/Ge/Si multilayer. The inversion of Ge and Si in the deposition sequence results in the formation of thin Si/Ge layers instead of the dots. Both materials show an atomically sharp interface between the Ge and Si parts of the dots and layers. They have an amorphous internal structure that can be crystallized by an annealing treatment. The light absorption properties of these complex materials are significantly different compared to films that form quantum dot lattices of the pure Ge, Si or a solid solution of GeSi. They show a strong narrow absorption peak that characterizes a type II confinement in accordance with theoretical predictions. The prepared materials are promising for application in quantum dot solar cells.

Doping of C70 fullerene peapods with lithium vapor: Raman spectroscopic and Raman spectroelectrochemical studies

Raman spectroscopy and in situ Raman spectroelectrochemistry were applied to study the lithium vapor doping of C70@SWCNTs (peapods). A strong degree of doping was proved by the vanishing of the single walled carbon nanotubes (SWCNTs) radial breathing mode (RBM) and by the attenuation of the tangential (TG) band intensity. In contrast to potassium vapor doping, the strong downshift of the frequency of the TG band has not been observed for Li-doping. The Li vapor treated peapods remained partly doped even if they were exposed to humid air. This has been reflected by a reduced intensity of the nanotube and the fullerene modes and by the change of the shape of the RBM band as compared to that of the undoped sample. The modes of the intratubular fullerene were almost unresolved after the contact of the Li-doped sample with water. A lithium insertion into the interior of a peapod and its strong interaction with the intratubular fullerene is suggested to be responsible for the air-insensitive residual doping. This residual doping was studied by spectroelectrochemical measurements. The TG band of the Li doped peapods is partly upshifted during the anodic doping, which points to the different state of C70@SWCNTs and C60@SWCNTs studied previously.

Co nanocrystals in amorphous multilayers – a structure study

The structure of magnetron-sputtered Co/SiO2 multilayers has been investigated using grazing-incidence small-angle X-ray scattering, X-ray diffraction, transmission electron microscopy and ion scattering techniques. A theoretical description of diffuse X-ray scattering from three-dimensional self-assembled ensembles of nanoparticles is also presented. The data revealed that Co-rich nanoparticles self-organize in a three-dimensional lattice and a dependence of the lattice parameters as well as the mean particle size on the nominal layer thickness was observed. Originally amorphous Co-rich layers crystallize readily during deposition, creating both pure Co and Co oxide particles. The results presented are important for controlled production and reliable characterization of metallic nanoparticles in solid amorphous matrices, aiming to obtain a well ordered monodisperse ensemble of nanoparticles.

Growth of a three-dimensional anisotropic lattice of Ge quantum dots in an amorphous alumina matrix

Simple processes for the preparation of semiconductor quantum dot lattices embedded in dielectric amorphous matrices play an important role in various nanotechnology applications. Of particular interest are quantum dot lattices with properties that differ significantly in different directions parallel to the material surface. Here, a simple method is demonstrated for the fabrication of an anisotropic lattice of Ge quantum dots in an amorphous Al2O3 matrix by a self-assembly process. A specific deposition geometry with an oblique incidence of the Ge and Al2O3 adparticles was used during magnetron sputtering deposition to achieve the desired anisotropy. The observed Ge quantum dot ordering is explained by a combination of directional diffusion of adparticles from the Ge and Al2O3 targets and a shadowing process which occurs during deposition as a result of the specific surface morphology. The prepared material shows a strong anisotropy of the electrical conductivity in different directions parallel to the sample surface.