2D/3D Metallic Nano-objects Self-Organized in an Organic Molecular Thin Film

We present the fabrication and investigation of the properties of nanocomposite structures consisting of two-dimensional (2D) and three-dimensional (3D) metallic nano-objects self-organized on the surface and inside of organic molecular thin-film copper tetrafluorophthalocyanine (CuPcF4). Metallic atoms, deposited under ultrahigh vacuum (UHV) conditions onto the organic ultrathin film, diffuse along the surface and self-assemble into a system of 2D metallic overlayers. At the same time, the majority of the metal atoms diffuse into the organic matrix and self-organize into 3D nanoparticles (NPs) in a well-defined manner. The evolution of the morphology and electronic properties of such structures as a function of nominal metal content is studied under UHV conditions using transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HR-TEM), and photoelectron spectroscopy (PES) techniques. Using HR-TEM, we have observed the periodicity of atomic planes of individual silver NPs. The steady formation of agglomerates from individual single nanocrystallites with intercrystallite boundaries is observed as well. PES reveals generally weak chemical interactions between silver and the organic matrix and n-doping of CuPcF4 at the initial stages of silver deposition, which is associated with charge transfer from the 2D wetting layer on the basis of core-level spectra shift analysis. Copyright © 2020 American Chemical Society

New insights into the laser-assisted photoelectric effect from solid-state surfaces

Photoemission from a solid surface provides a wealth of information about the electronic structure of the surface and its dynamic evolution. Ultrafast pump-probe experiments are particularly useful to study the dynamic interactions of photons with surfaces as well as the ensuing electron dynamics induced by these interactions. Time-resolved laser-assisted photoemission (tr-LAPE) from surfaces is a novel technique to gain deeper understanding of the fundamentals underlying the photoemission process. Here, we present the results of a femtosecond time-resolved soft X-ray photoelectron spectroscopy experiment on two different metal surfaces conducted at the X-ray Free-Electron Laser FLASH in Hamburg. We study photoemission from the W 4f and Pt 4f core levels using ultrashort soft X-ray pulses in combination with synchronized infrared (IR) laser pulses. When both pulses overlap in time and space, laser-assisted photoemission results in the formation of a series of sidebands that reflect the dynamics of the laser-surface interaction. We demonstrate a qualitatively new level of sideband generation up to the sixth order and a surprising material dependence of the number of sidebands that has so far not been predicted by theory. We provide a semi-quantitative explanation of this phenomenon based on the different dynamic dielectric responses of the two materials. Our results advance the understanding of the LAPE process and reveal new details of the IR field present in the surface region, which is determined by the dynamic interplay between the IR laser field and the dielectric response of the metal surfaces.Comment: 18 pages, 3 figure

Synthesis and study of the correlation between the atomic and electronic structure of low-dimensional carbon-based nanomaterials

Current work is devoted to synthesizing and studying atomic and electronic structure of several low-dimensional carbon-based nanomaterials, which can be of interest for fabrication of advanced electronic devices. All of them exhibit a $\pi$-conjugated structure, which promises high stability and endurance in the case of employment in technological applications. Chapters 1 and 2 serve for a general introduction and description of experimental techniques used to investigate the low-dimensional carbon-based nano-materials in the current work. The first example is mono-, bi-, and tri-layer graphene grown on cubic-SiC(001). Chapter 3 depicts investigation of the atomic and electronic structure of graphene depending on the number of its layers epitaxially grown on SiC substrate. The feasibility of facile CVD growth of cubic-SiC on conventional silicon wafers makes such graphene type compatible with existing silicon technologies. Hence, it is a good candidate for embedding into existing routines of electronic device fabrication. Chapter 4 is dedicated to investigating composite structure obtained via chemical modification of few-layer graphene on cubic-SiC(001) with Neutral red dye derivatives using the diazonium chemistry approach. This results in the synthesis of a layered structure, the surface layer of which exhibits semiconducting properties while buried graphene layers stay semimetallic. Such a system may be of interest for optoelectronic applications. Chapter 5 focuses on study of hybrid organic-inorganic systems consisting of semiconducting thin films of planar tetrafluoro-substituted copper phthalocyanine molecules (CuPcF$_4$) with self-organized indium nanoparticles of variable size at its surface and in the volume. Such thin films can be considered for application as an active layer in resistive random-access memory devices

Nanostructured Graphene on β-SiC/Si(001): Atomic and Electronic Structures, Magnetic and Transport Properties (Brief Review)

The studies of the properties of graphene synthesized on the surface of epitaxial films of cubic single-crystal silicon carbide preliminarily grown on Si(001) wafers have been reviewed. These studies were supported by the Russian Foundation for Basic Research, project no. 17-02-01139. The results of these studies demonstrate that graphene layers synthesized on β-SiC/Si(001) substrates have the atomic structure and electronic properties of a quasi-freestanding graphene sheet. Continuous graphene layers with a preferential direction of nanodomain boundaries, which is determined by the orientation of steps on the initial surface, can be synthesized on vicinal SiC(001) substrates. The possibility of controlled growth of mono-, bi-, and trilayer graphene on β-SiC/Si(001) wafers has been demonstrated. The studies have shown the opening of a transport gap and a high positive magnetoresistance in a parallel magnetic field in an ordered system of graphene nanoribbons on the vicinal SiC(001) surface. It has been shown that the functionalization of graphene with organic compounds changes the electronic properties of graphene on SiC(001), modifying it to a semiconductor with given properties, which allows applications in modern micro- and nanoelectronics

In-situ study of multi-phase indium nanoparticle growth on/into CuPcF4_4 organic thin film in ultra-high vacuum conditions

We study the in-situ growth of a nanocomposite material consisting of a thin CuPcF4 film and multiphase/multidimensional indium nanoparticles, self-organizing on the surface and in the bulk, at various stages of thermal deposition of metal on an organic film under ultrahigh vacuum conditions. The analysis of high-resolution transmission electron microscopy (HR-TEM) images provided valuable information about the evolution of morphology, size, density, and distribution of indium nanoparticles upon indium deposition. These 2D/3D ultra-small nano-objects turned out to have not only body-centered tetragonal (bct) crystal structure, typical for bulk indium, but also unusual face-centered cubic (fcc) one. Using a synchrotron facility, the study of the electronic structure of the hybrid nanocomposite on variable stages of metal deposition was performed by XPS and NEXAFS. Core-level spectra related to the organics indicated reasonably weak chemical interaction of indium with CuPcF4 molecules, which is not the case for a number of metal/organic semiconductor systems, while valence band spectra have shown a considerable change of the material electronic properties. The energy level diagrams, derived from the experiment, can be applied for the creation of new prototypes of metal-organic memory devices

2D/3D Metallic Nano-objects Self-Organized in an Organic Molecular Thin Film

We present the fabrication and investigation of the properties of nanocomposite structures consisting of two-dimensional (2D) and three-dimensional (3D) metallic nano-objects self-organized on the surface and inside of organic molecular thin-film copper tetrafluorophthalocyanine (CuPcF$_4$). Metallic atoms, deposited under ultrahigh vacuum (UHV) conditions onto the organic ultrathin film, diffuse along the surface and self-assemble into a system of 2D metallic overlayers. At the same time, the majority of the metal atoms diffuse into the organic matrix and self-organize into 3D nanoparticles (NPs) in a well-defined manner. The evolution of the morphology and electronic properties of such structures as a function of nominal metal content is studied under UHV conditions using transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HR-TEM), and photoelectron spectroscopy (PES) techniques. Using HR-TEM, we have observed the periodicity of atomic planes of individual silver NPs. The steady formation of agglomerates from individual single nanocrystallites with intercrystallite boundaries is observed as well. PES reveals generally weak chemical interactions between silver and the organic matrix and n-doping of CuPcF$_4$ at the initial stages of silver deposition, which is associated with charge transfer from the 2D wetting layer on the basis of core-level spectra shift analysis

Layer-by-Layer Graphene Growth on β-SiC/Si(001)

The mechanism of few-layer graphene growth on the technologically relevant cubic-SiC/Si(001) substrate is uncovered using high-resolution core-level and angle-resolved photoelectron spectroscopy, low-energy electron microscopy, and microspot low-energy electron diffraction. The thickness of the graphitic overlayer supported on the silicon carbide substrate and related changes in the surface structure are precisely controlled by monitoring the progress of the surface graphitization in situ during high-temperature graphene synthesis, using a combination of microspectroscopic techniques. The experimental data reveal gradual changes in the preferential graphene lattice orientations at the initial stages of the few-layer graphene growth on SiC(001) and can act as reference data for controllable growth of single-, double-, and triple-layer graphene on silicon carbide substrates

Active Sites of Te-hyperdoped Silicon by Hard X-ray Photoelectron Spectroscopy

Multiple dopant configurations of Te impurities in close vicinity in silicon are investigated us- ing photoelectron spectroscopy, photoelectron diffraction, and Bloch wave calculations. The samples are prepared by ion implantation fol- lowed by pulsed laser annealing. The dopant concentration is variable and high above the sol- ubility limit of Te in silicon. The configurations in question are distinguished from isolated Te impurities by a strong chemical core level shift. While Te clusters are found to form only in very small concentrations, multi-Te configurations of type dimer or up to four Te ions surrounding a vacancy are clearly identified. For these con- figurations a substitutional site location of Te is found to match the data best in all cases. For isolated Te ions this matches the expecta- tions. For multi-Te configurations the results contribute to understanding the exceptional ac- tivation of free charge carriers in hyperdopingof chalcogens in silicon

From graphene oxide towards aminated graphene: facile synthesis, its structure and electronic properties

International audienceAbstract In this paper we present a facile method for the synthesis of aminated graphene derivative through simultaneous reduction and amination of graphene oxide via two-step liquid phase treatment with hydrobromic acid and ammonia solution in mild conditions. The amination degree of the obtained aminated reduced graphene oxide is of about 4 at.%, whereas C/O ratio is up to 8.8 as determined by means of X-ray photoelectron spectroscopy. The chemical reactivity of the introduced amine groups is further verified by successful test covalent bonding of the obtained aminated graphene with 3-Chlorobenzoyl chloride. The morphological features and electronic properties, namely conductivity, valence band structure and work function are studied as well, illustrating the influence of amine groups on graphene structure and physical properties. Particularly, the increase of the electrical conductivity, reduction of the work function value and tendency to form wrinkled and corrugated graphene layers are observed in the aminated graphene derivative compared to the pristine reduced graphene oxide. As obtained aminated graphene could be used for photovoltaic, biosensing and catalysis application as well as a starting material for further chemical modifications

Surface functionalization of few-layer graphene on β -SiC(001) by Neutral Red dye

Few-layer graphene on -SiC(001) functionalized with phenazine dye Neutral Red by means of diazonium chemistry has been studied using X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure, photoemission electron microscopy, scanning tunneling microscopy, and density functional theory calculations. The experimental data reveal the formation of a composite phenazine dye/graphene structure with a large energy gap. The molecules in this structure can be oriented both parallel and perpendicular to the graphene surface. According to scanning tunneling spectroscopy and theoretical calculations, the density of electron states in different surface areas depends on the local short-range order and the molecules’ environment. On the other hand, the photoemission spectroscopy study shows that the bottom layers of the few-layer graphene remain intact, which inherently makes the synthesized layered composite a low-dimensional metal/semiconductor heterostructure. In addition, photoemission electron microscopy imaging shows a high homogeneity of the dye-modified graphene on a micrometer scale