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We have determined the temperature dependent phase diagram of the square 2D Ising lattice with anisotropic nearest neighbor ((Formula presented.)) and isotropic next-nearest neighbor ((Formula presented.)) interactions. The phase boundaries between the various ordered phases (ferromagnetic, antiferromagnetic and striped antiferromagnetic) and the disordered phase (paramagnetic) are obtained by considering the domain wall free energy. Although the phase boundary equations are not exact, they provide a very accurate description when the nearest neighbor interactions are stronger than the next-nearest neighbor interaction. The square 2D Ising lattice does not exhibit a phase transition when (Formula presented.) if (Formula presented.) and (Formula presented.) have the same sign or (Formula presented.) if (Formula presented.) and (Formula presented.) have opposite signs.

Vanadium dioxide (VO2) is a popular candidate for electronic and optical switching applications due to its well-known semiconductor-metal transition. Its study is notoriously challenging due to the interplay of long and short range elastic distortions, as well as the symmetry change, and the electronic structure changes. The inherent coupling of lattice and electronic degrees of freedom opens the avenue towards mechanical actuation of single domains. In this work, we show that we can manipulate and monitor the reversible semiconductor-to-metal transition of VO2 while applying a controlled amount of mechanical pressure by a nanosized metallic probe using an atomic force microscope. At a critical pressure, we can reversibly actuate the phase transition with a large modulation of the conductivity. Direct tunneling through the VO2-metal contact is observed as the main charge carrier injection mechanism before and after the phase transition of VO2. The tunneling barrier is formed by a very thin but persistently insulating surfacelayer of the VO2. The necessary pressure to induce the transition decreases with temperature. In addition, we measured the phase coexistence line in a hitherto unexplored regime. Our study provides valuable information on pressure-induced electronic modifications of the VO2 properties, as well as on nanoscale metal-oxide contacts, which can help in the future design of oxide electronics., Y.A.B. and K.S. contributed equally. 30 pages, 4 figures, Supplemental Material (28 pages, 16 figures)

The goal of this thesis is to develop an electron injection source based on a cold field emission cathode that ultimately functions as an alternative electron source used during the Birch reduction process. The experimental configuration to conduct an alternative Birch reduction involves the combination of various physical domains, including solid-state physics, physics of interfaces, electrochemistry, and fluid dynamics. The body of this thesis discusses the fabrication and characterization of a silicon nanocone array. It also presents a self-aligned method to incorporate an electronic gate structure that may function as an extractor grid. The emission behavior and transport properties of the silicon nanocones are assessed by scanning tunneling microscopy by performing measurements on flat silicon and silicon nanocone surfaces containing different dopant types and doping concentrations. Furthermore, additional experimental work is conducted to replace a thermally grown silicon dioxide layer with platinum silicide and characterize the electrical properties by means of scanning tunneling microscopy. Lastly, the thesis discusses the fabrication of two plasmonic structures and evaluates the plasmonic modes, surface enhancement, and Raman activity.

A novel fabrication technique is developed to deliver self-aligned gate structures on vertically aligned periodic silicon nanocones with gate aperture radii of 65±3 nm and a surface density of 1.6 billion structures cm-2 across the 100 mm diameter substrate scale. The silicon nanocones were obtained by a combination of ion beam etching and thermal oxidation whereas the gate structures were obtained by a combination of ion beam etching planarization and selective wet chemical etching.

Multi-component fluids with phase transitions show a plethora of fascinating phenomena with rich physics. Here we report on a transition in the growth mode of plasmonic bubbles in binary liquids. By employing high-speed imaging we reveal that the transition is from slow evaporative to fast convective growth and accompanied by a sudden increase in radius. The transition occurs as the three-phase contact line reaches the spinodal temperature of the more volatile component leading to massive, selective evaporation. This creates a strong solutal Marangoni flow along the bubble which marks the beginning of convective growth. We support this interpretation by simulations. After the transition the bubble starts to oscillate in position and in shape. Though different in magnitude the frequencies of both oscillations follow the same power law Image ID:d2sm00315e-t1.gif, which is characteristic of bubble shape oscillations, with the surface tension σ as the restoring force and the bubble's added mass as inertia. The transitions and the oscillations both induce a strong motion in the surrounding liquid, opening doors for various applications where local mixing is beneficial.

Although soil moisture is a key factor of hydrologic and climate applications, global continuous high resolution soil moisture datasets are still limited. Here we use physics-informed machine learning to generate a global, long-term, spatially continuous high resolution dataset of surface soil moisture, using International Soil Moisture Network (ISMN), remote sensing and meteorological data, guided with the knowledge of physical processes impacting soil moisture dynamics. Global Surface Soil Moisture (GSSM1 km) provides surface soil moisture (0–5 cm) at 1 km spatial and daily temporal resolution over the period 2000–2020. The performance of the GSSM1 km dataset is evaluated with testing and validation datasets, and via inter-comparisons with existing soil moisture products. The root mean square error of GSSM1 km in testing set is 0.05 cm3/cm3, and correlation coefficient is 0.9. In terms of the feature importance, Antecedent Precipitation Evaporation Index (APEI) is the most important significant predictor among 18 predictors, followed by evaporation and longitude. GSSM1 km product can support the investigation of large-scale climate extremes and long-term trend analysis.

Wireless vital signs sensors are increasingly used for remote patient monitoring, but data analysis is often challenged by missing data periods. This study explored the performance of various imputation techniques for continuous vital signs measurements. Wireless vital signs measurements (heart rate, respiratory rate, blood oxygen saturation, axillary temperature) from surgical ward patients were used for repeated random simulation of missing data periods (gaps) of 5–60 min in two-hour windows. Gaps were imputed using linear interpolation, spline interpolation, last observation- and mean carried forwards technique, and cluster-based prognosis. Imputation performance was evaluated using the mean absolute error (MAE) between original and imputed gap samples. Besides, effects on signal features (window’s slope, mean) and early warning scores (EWS) were explored. Gaps were simulated in 1743 data windows, obtained from 52 patients. Although MAE ranges overlapped, median MAE was structurally lowest for linear interpolation (heart rate: 0.9–2.6 beats/min, respiratory rate: 0.8–1.8 breaths/min, temperature: 0.04–0.17 °C, oxygen saturation: 0.3–0.7% for 5–60 min gaps) but up to twice as high for other techniques. Three techniques resulted in larger ranges of signal feature bias compared to no imputation. Imputation led to EWS misclassification in 1–8% of all simulations. Imputation error ranges vary between imputation techniques and increase with gap length. Imputation may result in larger signal feature bias compared to performing no imputation, and can affect patient risk assessment as illustrated by the EWS. Accordingly, careful implementation and selection of imputation techniques is warranted.

The Ge(110) surface reconstructs into ordered and disordered phases, in which the basic unit is a five-membered ring of Ge atoms (pentagon). The variety of surface reconstructions leads to a rich electronic density of states with several surface states. Using scanning tunneling microscopy and spectroscopy, we have identified the exact origins of these surface states and linked them to either the Ge pentagons or the underlying Ge-Ge bonds. We show that even moderate fluctuations in the positions of the Ge pentagonal units induce large variations in the local density of states. The local density of states modulates in a precise manner, following the geometrical constraints on tiling Ge pentagons. These geometry-correlated electronic states offer a vast configurational landscape that could provide new opportunities in data storage and computing applications.

With the goal to delevop the fabrication of a new type of Pt-nanoparticle carbon-support electrocatalyst, we investigate the carbon embedding of Pt cluster superlattices grown on the moiré of a monolayer of hexagonal boron nitride (h-BN) on Ir(111). Our combined scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) study establishes conformal C embedding of the Pt clusters on h-BN/Ir(111) without deterioration of superlattice order, preferential and strong binding of the embedding carbon to the Pt clusters, and upon annealing the formation of a homogeneous amorphous carbon (a-C) matrix. There are indications that while the a-C matrix and the Pt clusters bind strongly to each other, upon annealing both weaken their binding to h-BN.

The need to reduce energy consumption is now recognized worldwide. As a result, researchers have shown an increased interest in topological insulators, because of their potential use in dissipationless electronics. Particularly the quantum spin Hall (QSH) effect has been in the spotlights. This is a quantum state of matter characterized by two topologically protected, spin-polarized metallic states at the edges of a two-dimensional topological insulator. This research focuses on two theoretically predicted quantum spin Hall insulators, namely, germanene and thin film (Bi0.4Sb0.6)2Te3 (BST). The electronic structure of both materials is thoroughly investigated using scanning tunneling spectroscopy. We show that thin film BST displays well-defined topological surface states, surface states caused by the outermost Te atomic layer and quantized bulk energy levels. All together, the states give rise to prominent oscillations in the local density of states. Furthermore, scanning tunneling spectroscopy experiments confirm that the Dirac point is located in close proximity to the bulk valence band, making it difficult to observe signatures of the QSH state. For germanene, we determine a workfunction of 3.8 eV by measuring the image potential states. This is much lower that the workfunction of a regular scanning tunneling microscopy tip, and therefore results in a substantial electric field in the tunnel barrier. Due to the buckled honeycomb structure of germanene, this leads to broken inversion symmetry and charge will shift from one sublattice to the other. With scanning tunneling spectroscopy experiments we show that the size of the gap depends linearly on the application of a perpendicular electric field and that this results in a topological phase transition. Moreover, we show that the robust, topological edge states disappear for a sufficiently high electric field. All together, we have found no indications for the QSH effect in thin film BST. In contrast, we have found compelling evidence for the presence of the QSH effect in germanene on Ge2Pt. Future research could focus on utilizing the topologically protected edge transport for high-performance devices with low energy consumption.

Achieving tunable optoelectronic properties and clarifying interlayer interactions are key challenges in the development of 2D heterostructures. Herein, we report the feasible modulation of the optoelectronic properties of monolayer MoS2 (1L-MoS2) on three different graphene monolayers with varying ability in extracting electrons. Monolayer oxygen-functionalized graphene (1L-oxo-G, a high amount of oxygen of 60%) with a work function (WF) of 5.67 eV and its lowly oxidized reduction product, namely reduced-oxo-G (1L-r-oxo-G, a low amount of oxygen of 0.1%), with a WF of 5.85 eV serving as hole injection layers significantly enhance the photoluminescence (PL) intensity of MoS2, whereas pristine monolayer graphene (1L-G) with a work function (WF) of 5.02 eV results in PL quenching of MoS2. The enhancement in the PL intensity is due to increase of neutral exciton recombination. Furthermore, 1L-r-oxo-G/MoS2 exhibited a higher increase (5-fold) in PL than 1L-oxo-G/MoS2 (3-fold). Our research can help modulate the carrier concentration and electronic type of 1L-MoS2 and has promising applications in optoelectronic devices.

We present a scanning tunneling microscopy study of decanethiol on Au(001) in the low coverage regime. As expected, the hex reconstruction is lifted, however, no ordered decanethiol phases form. We observe large areas free of Au islands and covered with a disordered decanethiol phase. I(t) spectroscopy measurements suggest that this disordered phase is dynamic and most likely comprises diffusing Au adatoms, decanethiol molecules, and/or Au-adatom-decanethiol molecule complexes. We have performed density functional theory calculations and show that the activation barrier for diffusion is lower when Au-adatom-molecule complexes are considered in comparison to the case of bare molecule. These findings suggest that although no vacancy pits form on Au(001), Au-adatoms expelled during the lifting of the hex reconstruction may be still important for the diffusion of thiol molecules on this surface.

We have studied decanethiolate self-Assembled monolayers on the Au(001) surface. Planar and striped phases, as well as disordered regions, have formed after exposing the Au surface to a decanethiol solution. The planar phases that we observe have a hexagonal symmetry and have not been previously reported for thiols on the Au(001) surface and have lower coverage compared to that of the other known thiol planar phases such as the square α phase. The striped phases that we observe are similar to the previously reported β phase but still feature unit cells that cannot be modeled as the archetype, and the coverage is also somewhat lower. The disordered decanethiolate regions are more dynamic compared to the ordered phases, confirmed with I(t) spectroscopy. This suggests that in these regions the coverage is too low in order to form ordered decanethiolate phases. Our findings contribute to the growing family of possible decanethiol formations on the Au(001) surface, for which still less is known compared to the extensive overview present for the Au(111) surface.

We embarked on an in situ low-energy electron microscopy, photo-electron emission microscopy, and selected area low-energy electron diffraction study during the cooling of huge eutectic droplets through the critical stages of the eutectic transition. On this journey through uncharted waters, we revealed an expected initial shrinking of the exposed area of the droplet, followed by an unanticipated expansion. We attribute this behavior to an initial fast amorphization of the interface between the droplet and surface, followed by the recrystallization of Ge expelled from the droplet at the interface. As a major surprise, we discovered the emergence of extensive "spaghetti"-like patterns, which are rationalized in terms of parallel Ge ripples oriented along, mainly, [-554] and [-55-4] directions. They emerge during spinodal decomposition when passing the eutectic temperature of the system. Their sides are defined by Ge{111} and Ge{11-1} vicinals covered with Pt-modified (√3 × √3) superstructures. The distance between adjacent ripples is about 18 nm.

Germanium sulfide (GeS) is a layered monochalcogenide semiconductor with a band gap of about 1.6 eV. To verify the suitability of GeS for field-effect-based device applications, a detailed understanding of the electronic transport mechanisms of GeS-metal junctions is required. In this work, we have used conductive atomic force microscopy (c-AFM) to study charge carrier injection in metal-GeS nanocontacts. Using contact current-voltage spectroscopy, we identified three dominant charge carrier injection mechanisms: thermionic emission, direct tunneling, and Fowler-Nordheim tunneling. In the forward-bias regime, thermionic emission is the dominating current injection mechanism, whereas in the reverse-bias regime, the current injection mechanism is quantum mechanical tunneling. Using tips of different materials (platinum, n-type-doped silicon, and highly doped p-type diamond), we found that the Schottky barrier is almost independent of the work function of the metallic tip, which is indicative of a strong Fermi-level pinning. This strong Fermi-level pinning is caused by charged defects and impurities.

Convex cylindrical silicon nanostructures, also referred to as silicon nanocones, find their value in many applications ranging from photovoltaics to nanofluidics, nanophotonics, and nanoelectronic applications. To fabricate silicon nanocones, both bottom-up and top-down methods can be used. The top-down method presented in this work relies on pre-shaping of silicon nanowires by ion beam etching followed by self-limited thermal oxidation. The combination of pre-shaping and oxidation obtains high-density, high aspect ratio, periodic, and vertically aligned sharp single-crystalline silicon nanocones at the wafer-scale. The homogeneity of the presented nanocones is unprecedented and may give rise to applications where numerical modeling and experiments are combined without assumptions about morphology of the nanocone. The silicon nanocones are organized in a square periodic lattice, with 250 nm pitch giving arrays containing 1.6 billion structures per square centimeter. The nanocone arrays were several mm2 in size and located centimeters apart across a 100-mm-diameter single-crystalline silicon (100) substrate. For single nanocones, tip radii of curvature Graphic abstract A novel method is presented where argon ion beam etching and thermal oxidation sharpening are combined to tailor a high-density single-crystalline silicon nanowire array into a vertically aligned single-crystalline silicon nanocones array with

We studied the energy-level alignment at interfaces between various transition-metal dichalcogenide (TMD) monolayers, MoS2, MoSe2, WS2, and WSe2, and metal electrodes with different work functions (WFs). TMDs were deposited on SiO2/silicon wafers by chemical vapor deposition and transferred to Al and Au substrates, with significantly different WFs to identify the metal-semiconductor junction behavior: oxide-terminated Al (natural oxidation) and Au (UV-ozone oxidation) with a WF difference of 0.8 eV. Kelvin probe force microscopy was employed for this study, based on which electronic band diagrams for each case were determined. We observed the Fermi-level pinning for MoS2, while WSe2/metal junctions behaved according to the Schottky-Mott limit. WS2 and MoSe2 exhibited intermediate behavior.

The wetting properties of multi-component liquids are crucial to numerous industrial applications. The mechanisms that determine the contact angles for such liquids remain poorly understood, with many intricacies arising due to complex physical phenomena, for example due to the presence of surfactants. Here, we consider two-component drops that consist of mixtures of vicinal alkane diols and water. These diols behave surfactant-like in water. However, the contact angles of such mixtures on solid substrates are surprisingly large. We experimentally reveal that the contact angle is determined by two separate mechanisms of completely different nature, namely Marangoni contraction (hydrodynamic) and autophobing (molecular). It turns out that the length of the alkyl tail of the alkane diol determines which mechanism is dominant, highlighting the intricate coupling between molecular physics and the macroscopic wetting of complex fluids.

The lifetime $\tau$ of an isothermal and purely diffusively dissolving droplet in a host liquid scales as $\tau \sim R_0^2$ with its initial radius $R_0$ [Langmuir, Phys. Rev. 12, 368 (1919)]. For a droplet dissolving due to natural convection driven by density differences, its lifetime scales as $\tau\sim R_0^{5/4}$ [Dietrich et al., J. Fluid Mech. 794, 45 (2016)]. In this paper we experimentally find and theoretically derive yet another droplet dissolution behavior, resulting in $\tau \sim R_0^4$. It occurs when the dissolution dynamics is controlled by local heating of the liquid, leading to a modified solubility and a thermal Marangoni flow around the droplet. The thermal gradient is achieved by plasmonic heating of a gold nanoparticle decorated sample surface, on which a sessile water droplet immersed in water-saturated 1-butanol solution is sitting. The resulting off-wall thermal Marangoni flow and the temperature dependence of the solubility determine the droplet dissolution rate, resulting in a shrinkage $R(t) \sim (\tau -t )^{1/4}$ of the droplet radius and thus in $\tau \sim R_0^{4}$., Comment: 15 pages, 6 figures

Periodic structures of silicon are of interest in quantum-dot-based applications because of their unique optical and electronic properties. We report on the fabrication of stable quasi-ordered Si nanocluster arrays on the moiré of a hexagonal boron nitride (h-BN) monolayer on Ir(111). The h-BN monolayer promotes the growth of regular Si nanoclusters at 130 K and electronically decouples the clusters from the underlying metallic substrate. Using scanning tunneling microscopy and spectroscopy, we have investigated the cluster binding sites, their electronic structure, and their thermal stability. We find that the clusters display a size-dependent bandgap and that they are stable up to 577 K, after which cluster coalescence degrades the arrays.

In this paper, we obtain maps of the spatial tunnel barrier variations in self-assembled monolayers of organosulfurs on Au(111). Maps down to the sub-nanometer scale are obtained by combining topographic scanning tunneling microscopy images with dI/dz spectroscopy. The square root of the tunnel barrier height is directly proportional to the local work function and the dI/dz signal. We use ratios of the tunnel barriers to study the work function contrast in various decanethiol phases: the lying-down striped β phase, the dense standing-up φ phase, and the oxidized decanesulfonate λ phase. We compare the induced work function variations too: the work function contrast induced by a lying-down striped phase in comparison to the modulation induced by the standing-up φ phase, as well as the oxidized λ phase. By performing these comparisons, we can account for the similarities and differences in the effects of the mechanisms acting on the surface and extract valuable insights into molecular binding to the substrate. The pillow effect, governing the lowering of the work function due to lying-down molecular tails in the striped low density phases, seems to have quite a similar contribution as the surface dipole effect emerging in the dense standing-up decanethiol phases. The dI/dz spectroscopy map of the nonoxidized β phase compared to the map of the oxidized λ phase indicates that the strong binding of molecules to the substrate is no longer present in the latter. Fil: Tsvetanova, Martina. University of Twente; Países Bajos Fil: Oldenkotte, Valent J. S.. University of Twente; Países Bajos Fil: Bertolino, María Candelaria. University of Twente; Países Bajos. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigaciones en Físico-química de Córdoba. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas. Instituto de Investigaciones en Físico-química de Córdoba; Argentina Fil: Gao, Yuqiang. University of Twente; Países Bajos Fil: Siekman, Martin H.. University of Twente; Países Bajos Fil: Huskens, Jurriaan. University of Twente; Países Bajos Fil: Zandvliet, Harold J. W.. University of Twente; Países Bajos Fil: Sotthewes, Kai. University of Twente; Países Bajos

Die thermische Zersetzung von Graphenoxid (GO) ist auf atomarer Ebene ein komplexer Prozess und noch nicht vollstandig verstanden. In dieser Untersuchung wurde eine Unterklasse von GO verwendet, oxofunktionalisiertes Graphen (oxo‐G), um die thermische Disproportionierung zu untersuchen. Wir stellen den Einfluss des Erhitzens auf die elektronischen Eigenschaften einer einschichtigen oxo‐G‐Flocke vor und korrelieren die chemische Zusammensetzung und die Topographie durch Zwei‐Kontakt‐Transportmessungen, XPS, TEM, FTIR und STM. Uberraschenderweise fanden wir heraus, dass oxo‐G, das auf 300 °C erhitzt wurde, neben Graphendomanen und Lochern auch C‐C‐sp3‐Bereiche und moglicherweise C‐O‐C‐Bindungen enthalt. Auffallend ist, dass diese C‐O‐C/C‐C‐sp3‐getrennten sp2‐Bereiche mit wenigen Nanometern Durchmesser Halbleitereigenschaften mit einer Bandlucke von etwa 0.4 eV aufweisen. Wir schlagen vor, dass sp3‐Bereiche konjugierte sp2‐C‐Atomgruppen einschliesen, was zu den lokalen Halbleitereigenschaften fuhrt. Dementsprechend ist Graphen mit sp3‐C in Doppelschichtbereichen eine potentielle Klasse von Halbleitern und ein potentielles Ziel fur zukunftige chemische Modifikationen.

A novel photoresponsive and fully conjugated N-heterocyclic carbene (NHC) has been synthesized that combines the excellent photophysical properties of arylazopyrazoles (AAPs) with an NHC that acts as a robust surface anchor (AAP-BIMe). The formation of self-assembled monolayers (SAMs) on gold was proven by ToF-SIMS and XPS, and the organic film displayed a very high stability at elevated temperatures. This stability was also reflected in a high desorption energy, which was determined by temperature-programmed SIMS measurements. E-/Z-AAP-BIMe@Au photoisomerization resulted in reversible alterations of the surface energy (i.e. wettability), the surface potential (i.e. work function), and the conductance (i.e. resistance). The effects could be explained by the difference in the dipole moment of the isomers. Furthermore, sequential application of a dummy ligand by microcontact printing and subsequent backfilling with AAP-BIMe allowed its patterning on gold. To the best of our knowledge, this is the first example of a photoswitchable NHC on a gold surface. These properties of AAP-BIMe@Au illustrate its suitability as a molecular switch for electronic devices.

Particle (monolayer) assembly is essential to various scientific and industrial applications, such as the fabrication of photonic crystals, optical sensors, and surface coatings. Several methods, including rubbing, have been developed for this purpose. Here, we report on the serendipitous observation that microparticles preferentially partition onto the fluorocarbon-coated parts of patterned silicon and borosilicate glass wafers when rubbed with poly(dimethylsiloxane) slabs. To explore the extent of this effect, we varied the geometry of the pattern, the substrate material, the ambient humidity, and the material and size of the particles. Partitioning coefficients amounted up to a factor of 12 on silicon wafers and even ran in the 100s on borosilicate glass wafers at zero humidity. Using Kelvin probe force microscopy, the observations can be explained by triboelectrification, inducing a strong electrostatic attraction between the particles and the fluorocarbon zones, while the interaction with the noncoated zones is insignificant or even weakly repulsive.

Two-dimensional (2D) barrier materials such as graphene, boron nitride, and molybdenum disulfide hold great promise for important applications such as DNA sequencing, desalination, and biomolecular sensing. The 2D materials commonly span pores through an insulating membrane, and electrical fields are applied to drive cross-barrier transport of charged solvated species. While the low-voltage transmembrane transport is well-understood and controllable, high-voltage phenomena are uncontrolled and result in the apparent breakdown of the 2D material's critical insulating properties. Here we use suspended graphene over a 50 nm silicon nitride nanopore as a model system and show that delamination of the 2D material occurs at higher voltages and can directly cause a number of the puzzling high-voltage transport observations. We confirm the occurrence of delamination and observe via atomic force microscopy measurement a micron-scale delaminated patch in a system using chemical vapor deposition graphene. Furthermore, we show that the conductivity of the same system is strongly correlated to the area of delamination via coincident current measurements and optical imaging of the delaminated area. Finally, we demonstrate that delamination alone can cause a dramatic breakdown of barrier function through observation of a reversible increase in conductance of samples prepared with pristine defect-free graphene. These findings should have a great impact on the design and interpretation of 2D barrier material for both experiments and applications.

We present a curious effect observed during the dissolution process of water-immersed long-chain alcohol drops with an entrapped air bubble. These droplets dissolve while entrapping an air bubble pinned at the substrate. We qualitatively describe and explain four different phases that are found during the dissolution of this kind of system. The dissolution rate in the four phases differ dramatically. When the drop-water interface and the air bubble contact each other, rapid cyclic changes of the morphology are found: The breakage of the thin alcohol layer in between the bubble and the water leads to the formation of a three phase contact line. If the surface tension of the water-air interface supersedes those of the alcohol-water and alcohol-air interfaces, alcohol from the droplet is pulled upwards, leading to a closure of the air-water interface and the formation of a new thin alcohol film, which then dissolves again, leading to a repetition of the series of events. We call this sequence of events Marangoni puffing. This only happen for alcohols of appropriate surface tension. The Marangoni puffing is an intermediate state. In the final dissolution phases the Marangoni forces dramatically accelerate the dissolution rate, which then becomes one order of magnitude faster than the purely buoyancy-convective driven dissolution. Our results have bearing on various dissolution processes in multicomponent droplet systems.

We report on the fabrication and modification of a top-down nanofabrication platform for enormous parallel silicon nanowire-based devices. We explain the nanowire formation in detail, using an additive hybrid lithography step, optimising a reactive ion etching recipe for obtaining smooth and vertical nanowires under a hybrid mask, and embedding the nanowire in a dielectric membrane. The nanowires are used as a sacrificial template, removal of the nanowires forms arrays of well-defined nano-pores with a high surface density. This platform is expected to find applications in many different physical domains, including nanofluidics, (3D) nanoelectronics, as well as nanophotonics. We demonstrate the employment of the platform as field emitter arrays, as well as a state-of-the-art electro-osmotic pump.

peer-reviewed Semithio-bambusurils are a unique family of anion-binding host macrocycles that form self-assembled monolayers (SAMs) on Au(111). SAMs of semithio-bambusijn]uril homologs with different cage sizes (1: n = 4; 2: n = 6) have been investigated using electrochemistry, X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), and molecular dynamics (MD) simulations. Electrochemical measurements showed that electron transfer occurs via tunneling through the SAMs, and the low resistivity indicated an open layer architecture. XPS confirmed that thiocarbonyl sulfur atoms are chemisorbed to the Au(111) surface, and STM revealed the formation of ordered domains in a rectangular lattice for 1 and a highly ordered triangular/hexagonal lattice for 2. MD simulations substantiated the STM data by quantifying the balance between molecule–surface bonding, molecular conformations, and supramolecular packing that drive the formation of SAMs that maximize their surface coverage within the limits of conformational strain. SFI

This thesis is devoted to the study of collective phenomena on metallic surfaces. Great part of the research was done on molecular self-assembled monolayers (SAMs), which we studied with scanning tunnelling microscopy (STM). These films are very exciting to measure at the nanoscale. We focused on the dynamics at the boundary between molecular phases, on mapping the nanoscale properties of phases, and on trying to understand the mechanisms for the SAM formation by exploring less-well studied surfaces and molecular coverage regimes. Both Au(111) and Au(001) substrates were deployed. We used the decanethiol molecule as the SAM building block. When STM was not sufficient, density functional theory (DFT) calculations were also performed. Our analysis on the phase boundary dynamics was also incorporated in a more general anisotropic triangular lattice framework. Additionally, we wished to extend our collective phenomena framework by deploying low energy electron microscopy (LEEM) to study dynamics on the surface. However, SAMs are prone to electron-induced damage. Therefore, we alternatively looked at the Bi-modified Ni(111) surface. Bi deposition on the Ni(111) and Cu(111) surfaces has already shown to lead to a variety of dynamic processes. Therefore, the Bi/Ni system was a perfect playground for our study, with the added value that we can further expand our knowledge of its surface phase diagram. We obtained an ultrathin 3-fold symmetric Bi wetting layer and demonstrated a reproducible phase transition between an ordered and a disordered state, which occurs when heating-up the system. Upon cooling down, the original structure could be restored.

The authors regret to inform that they would like to change the corresponding author in the Elsevier system from Gert Desmet to Ignaas Jimidar. All authors have given their consent to make this change. See earlier correspondence and collected signatures from the authors. The authors would like to apologise for any inconvenience caused.

This thesis is devoted to the study of the Pt/Ge(110) system. The Pt/Ge(110) system hosts interesting physics, since after the deposition of Pt on the intrinsically anisotropic Ge(110) surface and subsequent annealing at sufficiently high temperatures, the formation of the 2D material germanene, the germanium analogue of graphene, is observed. Germanene possesses, just like graphene, two sub-lattices and a honeycomb like structure with (partially) overlapping 2pz orbitals. Consequently the band structure of germanene also exhibits a linear dispersion relation and the electrons in germanene can be also best described as massless Dirac fermions [1-2]. The formation of germanene in the Pt/Ge(110) system is connected to the formation of a Pt-Ge eutectic droplet at elevated temperatures and the subsequent spinodal decomposition upon cooling down.

High quality thin films of the topological insulator (Bi$_{0.4}$Sb$_{0.6}$)$_2$Te$_3$ have been deposited on SrTiO$_3$ (111) by molecular beam epitaxy. Their electronic structure was investigated by in situ angle-resolved photoemission spectroscopy and in situ scanning tunneling spectroscopy. The experimental results reveal striking similarities with relativistic ab-initio tight binding calculations. We find that ultrathin slabs of the three-dimensional topological insulator (Bi$_{0.4}$Sb$_{0.6}$)$_2$Te$_3$ display topological surface states, surface states with large weight on the outermost Te atomic layer, and dispersive bulk energy levels that are quantized. We observe that the bandwidth of the bulk levels is strongly reduced. These bunched bulk states as well as the surface states give rise to strong peaks in the local density of states., Comment: 8 pages, 4 figures, 4 pages of supplemental material

Thermally stimulated motion of micron-sized eutectic PtGe droplets on Ge(110) has been studied in situ mainly by photoemission electron microscopy, low-energy electron microscopy, and spatially resolved low-energy electron diffraction. In line with earlier studies of eutectic AuSi, PtSi, AuGe, and PtGe clusters on, respectively, various Si and Ge substrates we find that the motion toward regions at higher temperature is driven by the entropy gain of substrate atoms which become constituents of the droplet during its journey. At ∼1100 K, i.e., well above the bulk eutectic temperature, the direction is governed solely by the local thermal gradient, irrespective of eventual crystalline preferences. Access to the diffusivity of the host material (in this case Ge) inside the eutectic droplets shows that this is well above one order of magnitude higher than expected if it was rate limiting for the velocity of the moving droplets. This excludes a significant gradient of the (Ge) concentration inside the droplet and disqualifies dissolution-diffusion-deposition flow as the driving force for motion of the droplets on the surface, as assumed widely hitherto, to explain surface diffusion of eutectic droplets on surfaces. In addition, the interface between the droplet and the surface appears flat and we find no indication for "endotaxy."The droplets make direct contact with the flat Ge substrate through atomic steps, which are abundantly present at the interface. The droplets are surrounded by a PtGe3 wetting layer with an ordered (2 × 1) structure. Dissolution of the edges of the wetting layer at the leading edge of the droplet with an activation energy of 2.2 eV is identified as the rate limiting step for its motion.

A single-crystal is a solid object in which an orderly three-dimensional arrangement of the atoms is repeated throughout the entire volume. A polished single-crystal makes for a very well-defined substrate. Therefore, crystals are used extensively in surface physics, catalytic chemistry, and the investigation of material properties. Improvements in single-crystal fabrication have historically enabled leaps in technological advancements and opened new paradigms of scientific understanding. The growth of elemental metal single-crystals is usually achieved through classic growth techniques such as the Czochralski or floating zone methods. Drawbacks of these techniques are the susceptibility to contamination from the crucible and thermal stress-induced defects due to contact with the ambient. These drawbacks can be mitigated by growing in a containerless environment, which can be achieved by using electromagnetic levitation (EML). EML relies on the Lorentz force that acts between a levitation coil and the metal sample. In this thesis we report on the development of a novel crystal growth apparatus that employs electromagnetic levitation in a vacuum to grow metal single-crystals of superior quality and purity. This apparatus enables two growth modes: containerless undercooled crystallization and levitation-based Czochralski growth. The system can levitate a wide variety of metals while having precise control over the sample temperature in both the liquid and solid state within 1 K. As a proof of concept, we show the successful containerless growth of copper single-crystals. These crystals are at least equal in quality to commercially available single-crystals grown using the traditional Czochralski technique. As such, we made an essential contribution towards improving the currently available state-of-the-art crystals.

Electromagnetic levitation (EML) enables the handling of samples in a containerless manner, facilitating more precise measurements or manipulation of materials. Currently, EML coil design is based on two-dimensional (2D) mathematical models combined with empirical data and experience. We propose that employing a truly 3D model enables a more accurate and rational coil design. This approach shows significant differences in magnetic field, and the resulting lift and heating properties, when comparing a 2D and a 3D coil. A 3D coil, resembling the geometry of a 2D coil, requires more current to generate the same lift. Reversing the situation, a 3D coil producing the exact same magnetic field as a 2D coil has a vastly different design. Furthermore, we show that the assumption that the magnetic field is homogeneous in the axial plane within the sample volume is invalid, even for small samples, in the upper and lower regions of the coil. Using a 3D model, we design a coil capable of levitating and melting an iridium sphere with a diameter of 6 mm.

Pt-Ge eutectic alloy droplets are scrutinized by microscopic analysis of electrons excited by 4.9-eV photons. Using only the work function and contact angle as fitting parameters, we determine the exact droplet shape that reproduces the experimental electron emission profile. The local inclination of the droplets' surface plays a decisive role in the generated emission profile. Intensity variations at the illuminated side of the eutectic droplets are a consequence of standing waves in the electromagnetic field responsible for excitation. Intensity variations at the nonilluminated side are ascribed to a diffraction pattern of the photons after their interaction with the droplet.

Bubbles and droplets are ubiquitous on our planet. They are involved in many natural phenomena and industrial processes. In general, in practical situations, the formation of bubbles and droplets not only occurs in pure liquids, but also in binary or ternary liquids. In multicomponent liquids, diverse forces and effects play a role, such as thermophoresis, body forces originated from a density gradient. For the bubbles/droplets at the micro scale, solutal Marangoni forces become relevant as well. Moreover, the behavior of bubbles and droplets becomes even more complex if phase transitions occur in such systems, e.g., evaporation or the nucleation of a new phase, such as in the Ouzo effect. Over the past several years, bubbles/droplets in multicomponent systems received increasing attention due to their relevance for various applications and the occurrence of numerous counterintuitive phenomena. This thesis is focused on bubbles induced by plasmonic excitation and droplets generated through the Ouzo effect. We present a detailed study of the dynamics of their nucleation and growth in multicomponent systems.

In this thesis we have studied several 2D materials as well as a few van der Waals heterostructures. The most famous and probably also most promising 2D material is graphene. Graphene occurs in Nature and the material is stable against ambient conditions, which is extremely beneficial for technological applications. Unfortunately, graphene also has one severe drawback: it is gapless and therefore pristine graphene cannot be used for field-effect based device applications. As an alternative for graphene we have studied in this proposal germanene. Germanene, the germanium analogue of graphene, is also a 2D Dirac material that hosts Dirac fermions, but the honeycomb lattice is buckled. This buckling allows the opening of bandgap by applying an external transversal electric field. This electric field results in a shift of charge from one triangular sub-lattice to the other sub-lattice. In Chapter 3 we focus our attention on the synthesis and characterization of germanene on a molybdenum disulfide substrate (MoS2). This is an interesting system because MoS2 has a substantial bandgap of about 1 eV, which allows to keep the key electronics states of the germanene layer, which are located in the vicinity of the Fermi level, intact. The germanene layer will be grown using molecular beam epitaxy. In Chapter 4 we study the electronic properties of the germanene layer. Special attention will be paid to formation of charge puddles. In Chapter 5 we elaborate on the possibility to alter the quantum state of matter of a 2D Dirac material with a buckled honeycomb lattice using an external transversal electric field. We will show that beyond a critical transversal electric field the 2D Dirac material undergoes a topological phase transition from a 2D topological insulator to a normal band insulator. In Chapter 6 we present a detailed study of the structural and electronic properties of germanium selenide (GeSe). GeSe is a transition metal chalcogenide with a bandgap that is very comparable to the bandgap of MoS2. The structure of both 2D materials is, however, different: GeSe has a rectangular unit cell, whereas MoS2 has a hexagonal unit cell. Chapter 7 deals with the structural and electronic properties of graphene on a MoS2 substrate. A graphene flake is placed on the substrate via the solvent transfer technique. In this chapter we also present a new technique to measure the Fermi velocity of graphene. This new technique relies on an analysis of the voltage dependent inverse decay length of the electrons that tunnel from tip to substrate, or vice versa. This is interesting as electron-electron interactions in conventional 2D electron gases result in a decrease of the Fermi velocity. We believe our newly developed technique is also applicable to many other 2D van der Waals heterostructure systems.

Bubbles are widely present in numerous natural and industrial processes. Understanding the nucleation mechanism and dynamics of the bubbles is essential for exploiting relevant applications in material engineering, microfluidics, catalysis and electrolysis and many others. In this thesis, we have focused on two categories of bubbles: (i) Surface micro/nanobubbles on structured surfaces. We studied the entrapment of nanobubbles at the solid-liquid interface on nanostructured hydrophobic surfaces. Results show that the morphology and density of the trapped surface nanobubbles depends on the type of nanostructures. This verifies that the pinning effect of surface structures contributes to surface nanobubble nucleation and stability. Furthermore, the entrapment and evolution of microbubbles on a hydrophobic structured surface with microwells are investigated. We find that after nucleation, the microbubbles formed on the microwells dissolve in a constant contact angle mode. We also quantify how a high gas undersaturation enhances the dissolution process by varying the gas undersaturation level. (ii) Plasmonic microbubbles. We studied the gas effect on plasmonic bubble dynamics in water and bubble formation in binary and ternary liquid system. It is found that in pure water, the bubble nucleation temperature depends on the gas concentration and the long-term bubble growth is governed by gas diffusion. In binary liquids of water-ethanol mixtures, we find that plasmonic bubbles exhibit ethanol concentration-dependent behavior. We reveal the mechanism of the formation of the unstable bubbles in binary liquids at low ethanol concentration, and the sudden bubble shrinkage during stable bubble growth in liquids with high ethanol ratios. Finally, plasmonic bubble nucleation in ternary liquids consisting of ethanol, water, and trans-anethole oil is investigated. It is observed that oil (trans-anethole) droplet plumes are produced around the growing plasmonic bubbles. The nucleation and self-organization of droplet plumes depends on the ethanol-water relative weight ratio and hence Marangoni number.

Graphene nanoribbons (GNRs) are considered as major building blocks in future carbon-based electronics. The electronic performance of graphene nanostructures is essentially influenced and determined by their edge termination and their supporting substrate. In particular, semi-conducting, as well as metallic GNRs, can be fabricated by choosing the proper template which is favorable for device architecture designs. This study highlights the impact of microscopic details of the environment of the GNRs on the charge transport in GNRs. By means of lateral force, conductive atomic force and nanoprobe measurements, we explore the charge propagation in both zig-zag and armchair GNRs epitaxially grown on SiC templates. We directly image transport channels on the nanoscale and identify SiC substrate steps and nano-instabilities of SiC facets as dominant charge scattering centers. [Figure not available: see fulltext.].

[Image: see text] Here we report on a novel, noninvasive route for operando tailoring of the charge transport properties of metal/WS(2) contacts without the negative impacts to two-dimensional materials arising from conventional doping methods. The doping level of thin WS(2) flakes supported on insulating mica is susceptible to local charge variations induced by the presence of a hydration layer between mica and WS(2). We demonstrate, via the use of several complementary scanning probe techniques, that the direct control of the state and thickness of this intercalated water film controls the charge injection properties of Pt/WS(2) nanocontacts. A switch from unipolar to ambipolar transport was achieved by environmentally controlling the thickness of the intercalated water. We show that the effect persists even for multilayer flakes and that it is completely reversible, opening a new route toward the realization of novel electronics with environmentally controllable functionalities.

[Image: see text] The friction of graphene on mica was studied using lateral force microscopy. We observed that intercalation of alcohol molecules significantly increases the friction of graphene, as compared to water. An increase of 1.8, 2.4, and 5.9 times in friction between the atomic force microscopy tip and single-layer graphene was observed for methanol, ethanol, and 2-propanol, respectively. Moreover, the friction of graphene is found to be higher for single-layer graphene than for multilayer graphene. We attribute the increase in friction to the additional vibrational modes of alcohol molecules. The significant variation of the frictional characteristics of graphene at the nanoscale by altering the intercalant could open up applications for the next-generation nanolubricants and nanodevices.

The journal of physical chemistry / C C, Nanomaterials and interfaces 123(9), 5411-5420 (2019). doi:10.1021/acs.jpcc.8b10971, Published by American Chemical Society, Washington, DC

The spatial dependence of the local density of states at the Fermi level of a stepped Au(111) surface is studied by thermovoltage scanning tunneling microscopy. The periodicity of the standing waves is not given by ${\ensuremath{\lambda}}_{\text{F}}/2=1.8$ nm, as expected based on the band diagram of Au(111), but rather varies between 1.5 and 2.1 nm depending on the exact width of the terraces. This counterintuitive result can be understood by considering the superposition of incident and reflected electron waves, which have a periodicity of ${\ensuremath{\lambda}}_{\text{F}}/2$. The change in periodicity is a direct consequence of the decaying nature of the Friedel oscillations.

The physicochemical hydrodynamics of bubbles and droplets out of equilibrium, in particular with phase transitions, displays surprisingly rich and often counterintuitive phenomena. Here we experimentally and theoretically study the nucleation and early evolution of plasmonic bubbles in a binary liquid consisting of water and ethanol. Remarkably, the submillimeter plasmonic bubble is found to be periodically attracted to and repelled from the nanoparticle-decorated substrate, with frequencies of around a few kHz. We identify the competition between solutal and thermal Marangoni forces as origin of the periodic bouncing. The former arises due to the selective vaporization of ethanol at the substrate's side of the bubble, leading to a solutal Marangoni flow towards the hot substrate, which pushes the bubble away. The latter arises due to the temperature gradient across the bubble, leading to a thermal Marangoni flow away from the substrate which sucks the bubble towards it. We study the dependence of the frequency of the bouncing phenomenon from the control parameters of the system, namely the ethanol fraction and the laser power for the plasmonic heating. Our findings can be generalized to boiling and electrolytically or catalytically generated bubbles in multicomponent liquids., Comment: 12 pages, 3 figures