Pnictogens Allotropy and Phase Transformation during van der Waals Growth

Pnictogens have multiple allotropic forms resulting from their ns2 np3 valence electronic configuration, making them the only elemental materials to crystallize in layered van der Waals (vdW) and quasi-vdW structures throughout the group. Light group VA elements are found in the layered orthorhombic A17 phase such as black phosphorus, and can transition to the layered rhombohedral A7 phase at high pressure. On the other hand, bulk heavier elements are only stable in the A7 phase. Herein, we demonstrate that these two phases not only co-exist during the vdW growth of antimony on weakly interacting surfaces, but also undertake a spontaneous transformation from the A17 phase to the thermodynamically stable A7 phase. This metastability of the A17 phase is revealed by real-time studies unraveling its thickness-driven transition to the A7 phase and the concomitant evolution of its electronic properties. At a critical thickness of ~4 nm, A17 antimony undergoes a diffusionless shuffle transition from AB to AA stacked alpha-antimonene followed by a gradual relaxation to the A7 bulk-like phase. Furthermore, the electronic structure of this intermediate phase is found to be determined by surface self-passivation and the associated competition between A7- and A17-like bonding in the bulk. These results highlight the critical role of the atomic structure and interfacial interactions in shaping the stability and electronic characteristics of vdW layered materials, thus enabling a new degree of freedom to engineer their properties using scalable processes

Compositional and structural studies of ion-beam modified AlN/TiN multilayers

This work was supported the Serbian Ministry of Education, Science and Technological Development (project ON 171023), ERC Advanced Investigator grant (226470 SILAMPS), and the International Atomic Energy Agency, Vienna (CRP 12024). We thank Petro Parisse and Loredana Casalis of the NanoInnovation Laboratory of Elettra – Sincrotrone Trieste SCpA for the AFM measurements

Eumelanin Graphene-Like Integration: The Impact on Physical Properties and Electrical Conductivity

The recent development of eumelanin pigment-based blends integrating "classical" organic conducting materials is expanding the scope of eumelanin in bioelectronics. Beyond the achievement of high conductivity level, another major goal lays in the knowledge and feasible control of structure/properties relationship. We systematically investigated different hybrid materials prepared by in situ polymerization of the eumelanin precursor 5,6-dihydroxyindole (DHI) in presence of various amounts of graphene-like layers. Spectroscopic studies performed by solid state nuclear magnetic resonance (ss-NMR), x-ray photoemission, and absorption spectroscopies gave a strong indication of the direct impact that the integration of graphene-like layers into the nascent polymerized DHI-based eumelanin has on the structural organization of the pigment itself, while infrared, and photoemission spectroscopies indicated the occurrence of negligible changes as concerns the chemical units. A tighter packing of the constituent units could represent a strong factor responsible for the observed improved electrical conductivity of the hybrid materials, and could be possible exploited as a tool for electrical conductivity tuning

Soft X-ray spectromicroscopy and its application to semiconductor microstructure characterization

The universal trend towards device miniaturization has driven the semiconductor industry to develop sophisticated and complex instrumentation for the characterization of microstructures. Many significant problems of relevance to the semiconductor industry cannot be solved with conventional analysis techniques, but can be addressed with soft x-ray spectromicroscopy. An active spectromicroscopy program is being developed at the Advanced Light Source, attracting both the semiconductor industry and the materials science academic community. Examples of spectromicroscopy techniques are presented. An ALS {mu}-XPS spectromicroscopy project is discussed, involving the first microscope completely dedicated and designed for microstructure analysis on patterned silicon wafers

Addressable graphene encapsulation of wet specimens on a chip for optical, electron, infrared and X-ray based spectromicroscopy studies

Label-free spectromicroscopy methods offer the capability to examine complex cellular phenomena. Electron and X-ray based spectromicroscopy methods, though powerful, have been hard to implement with hydrated objects due to the vacuum incompatibility of the samples and due to the parasitic signals from (or drastic attenuation by) the liquid matrix surrounding the biological object of interest. Similarly, for many techniques that operate at ambient pressure, such as Fourier transform infrared spectromicroscopy (FTIRM), the aqueous environment imposes severe limitations due to the strong absorption of liquid water in the infrared regime. Here we propose a microfabricated multi-compartmental and reusable hydrated sample platform suitable for use with several analytical techniques, which employs the conformal encapsulation of biological specimens by a few layers of atomically thin graphene. Such an electron, X-ray, and infrared transparent, molecularly impermeable and mechanically robust enclosure preserves the hydrated environment around the object for a sufficient time to allow in situ examination of hydrated bio-objects with techniques operating under both ambient and high vacuum conditions. An additional hydration source, provided by hydrogel pads lithographically patterned in the liquid state near/around the specimen and co-encapsulated, has been added to further extend the hydration lifetime. Note that the in-liquid lithographic electron beam-induced gelation procedure allows for addressable capture and immobilization of the biological cells from the solution. Scanning electron microscopy and optical fluorescence microscopy, as well as synchrotron radiation based FTIR and X-ray fluorescence microscopy, have been used to test the applicability of the platform and for its validation with yeast, A549 human carcinoma lung cells and micropatterned gels as biological object phantoms

Adventures of III-V Semiconductor Surfaces

Tailoring the surface composition and morphology of materials to enable new electronic devices is important for a wide range of applications such as quantum computing or spintronics. A fundamental understanding of the changes induced in the surface during different process steps can help to establish new synthesis routes as well as devices. This thesis focuses on the manipulation of III-V semiconductor compounds, in particularthe surfaces of InAs and GaAs crystals.By implementing lithographically defined metal islands on the InAs surface, we push the boundaries of substrate annealing temperatures inhibiting the formation of In droplets locally. The employed Pd layer acts as a sink for freely diffusing In atoms above the congruent melting temperature. Here, As atoms go into gas phase at a higher rate compared to In due to the difference in vapor pressure. This lateral control over the concentration of In on the surface was investigated via scanning electron, atomic force as well as X-ray photoemission electron microscopy (XPEEM), and opens new pathways for epitaxy and the synthesis of heterostructures. Furthermore, theoretical studies have shown that the implementation of Bi atoms into the lattice of III-V compound semiconductors can facilitate band gap reduction and increased spin-orbit coupling desirable for fabricating of topological insulators. Particularly, the interaction of group III elements with Bi has attracted great interest. However, manufacturing these diluted Bismides is not trivial, since most approaches like molecular beam epitaxy, synthesis from the melt or metal organic vapor deposition suffer from limited and inhomogeneous Bi incorporation into the crystal.By following the approach of depositing Bi atoms onto a III-V sample and subsequent annealing, this thesis aims to synthesize and characterize heterostructures displaying III-V bulk properties and a surface made of III-As-Bi compounds. Different sample preparation routes were explored focusing on GaAs and InAs substrates with zinc blende (ZB) and wurtzite (WZ) crystal structure. The latter is only achievable in low-dimensional materials and will be employed in the form of InAs nanosheets. Part of this study focusses on the investigation of Bi-induced structural and chemical changes in the surface of the III-V compounds by utilizing surface sensitive techniques such as scanning tunneling microscopy, X-ray photoemission spectroscopy, low energy electron diffraction and XPEEM. Our results show that the mechanism of Bi incorporation is highly dependent on the underlying crystal structure, as well as process parameters such as time and substrate temperature. Additionally, first band structure measurements of InAs WZ crystal nanosheets collected via averaging angleresolved photoemission spectroscopy (ARPES) are presented. In contrast to other ZB crystal facets, a 2D electron gas (2DEG) is already detected after removing the native oxide and diminished after Bi deposition. We attribute the origin of the 2DEG to unique step and edge morphologies found on the WZ nanosheets. The thesis concludes with an ARPES study on InAs(111)B substrates presenting new electronic states inside the band gap based on the interaction of Bi and As atoms

Synchrotron X-ray based characterization of technologically relevant III-V surfaces and nanostructures

Innovative design and materials are needed to satisfy the demand for efficient and scalable devices for electronic and opto-electronic applications, such as transistors, LEDs, and solar cells. Nanostructured III-V semiconductors are an appealing solution, combining the excellent functional properties of III-V materials with the flexibility typical of nanostructures, such as the nanowires (NWs) studied here. However, there are a number of open challenges, that currently hinder the performance of III-V nanostructure devices: first, the surface quality of III-V materials is still one of their main limiting factors. Other problems specific of III-V NWs are the control of dopant incorporation - crucial for their functionalization -, and of their structural inhomogeneity (e.g. lattice strain and tilt), that can affect opto-electronic performance. These problematics require a set of non-trivial cutting-edge characterization tools: here an approach based on a combination of X-ray synchrotron techniques is demonstrated.Synchrotron based X-ray photoelectron spectroscopy (XPS) has been used to study the surface chemistry of III-V model systems and to monitor industrially relevant processing on them. A new passivation process improving the surface quality of InAs substrates used for electronics has been investigated: the surface structure and composition resulting from thermal oxidation followed by ex situ deposition of a high-k material via atomic layer deposition (ALD) has been assessed with XPS. The implementation of this passivation approach in gate stacks showed improvements in performance, that were attributed to the specific stoichiometry of the thermal oxide. The dynamics of the ALD process on InAs was also studied in situ with ambient pressure XPS: it was observed that the chemisorption of the precursor is an important step to ensure a good quality of the high-k oxide deposition.Dopant evaluation in NWs is challenging due to their small dimensions. Here, a first approach to this problem was to perform XPS to study the effects of Zn dopant incorporation on the surface of GaAs NWs, used for solar cells. High doping conditions during growth were found to form a Zn layer on the outside of the NW that suppresses the native oxides, which are generally a cause of poor passivation of III-V surfaces. In another experiment, XPS scanning microscopy was used to study surface Zn doping in an InP NW with an axial pn junction, also used for solar cells. The surface potential drop along the junction was monitored in operando, while applying a bias to the NW device, and it was found smaller than what expected for the bulk. Finally, a quantitative evaluation of Zn dopants incorporation in III-V NWs was studied for the first time with nano-focused X-ray fluorescence, due to the excellent combination of low detection limits and spatial resolution. Dopant gradients and memory effects were noted along InP and InGaP NWs, showing complex dopant incorporation mechanisms during the growth. The structural inhomogeneity in InGaN nano-pyramids for next generation LEDs was also investigated. The influence of different processing parameters on lattice and strain were studied with full field X-ray diffraction microscopy. This imaging technique uses Bragg diffraction intensity as contrast mechanism and has a large field of view, useful for imaging at once large areas patterned with pyramids, giving valuable statistical consistency. The growth parameters providing the best lattice quality and homogeneity were assessed.This thesis shows how cutting edge synchrotron characterization methods can provide useful information for improving III-V surfaces and nanostructures for next generation devices. Moreover, in most cases advances in the characterization methods are achieved, that can be relevant also in other and broader scientific fields

In situ observation of oscillatory redox dynamics of copper

How a catalyst behaves microscopically under reaction conditions, and what kinds of active sites transiently exist on its surface, is still very much a mystery to the scientific community. Here we present an in situ study on the red-ox behaviour of copper in the model reaction of hydrogen oxidation. Direct imaging combined with on-line mass spectroscopy shows that activity emerges near a phase boundary, where complex spatio-temporal dynamics are induced by the competing action of simultaneously present oxidizing and reducing agents. Using a combination of in situ imaging with in situ X-ray absorption spectroscopy and scanning photoemission microscopy, we reveal the relation between chemical and morphological dynamics and demonstrate that a static picture of active sites is insufficient to describe catalytic function of redox-active metal catalysts. The observed oscillatory redox dynamics provide a unique insight on phase-cooperation and a convenient and general mechanism for constant re-generation of transient active sites