Presolidification in Eutectic Droplets.

Evidence of presolidification, the counterpart to premelting, is reported. Near the eutectic temperature, T_{C}, the propagation direction of thermal gradient driven motion of eutectic Ge-Pt droplets on Ge(110) is determined by presolidification. Well above T_{C}, the micron-sized droplets move towards the hottest location at the substrate, irrespective of crystalline direction.

Microscopic Study of the Spinodal Decomposition of Supported Eutectic Droplets During Cooling: PtGe/Ge{110}.

We embarked on an 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.

Temperature-Controlled Rotational Epitaxy of Graphene.

When graphene is placed on a crystalline surface, the periodic structures within the layers superimpose and moiré superlattices form. Small lattice rotations between the two materials in contact strongly modify the moiré lattice parameter, upon which many electronic, vibrational, and chemical properties depend. While precise adjustment of the relative orientation in the degree- and sub-degree-range can be achieved via careful deterministic transfer of graphene, we report on the spontaneous reorientation of graphene on a metallic substrate, Ir(111).

Pressure-Induced Melting of Confined Ice.

The classic regelation experiment of Thomson in the 1850s deals with cutting an ice cube, followed by refreezing. The cutting was attributed to pressure-induced melting but has been challenged continuously, and only lately consensus emerged by understanding that compression shortens the O:H nonbond and lengthens the H-O bond simultaneously. This H-O elongation leads to energy loss and lowers the melting point.

Chemical vapor deposition growth of bilayer graphene in between molybdenum disulfide sheets.

Direct growth of flat micrometer-sized bilayer graphene islands in between molybdenum disulfide sheets is achieved by chemical vapor deposition of ethylene at about 800°C. The temperature assisted decomposition of ethylene takes place mainly at molybdenum disulfide step edges. The carbon atoms intercalate at this high temperature, and during the deposition process, through defects of the molybdenum disulfide surface such as steps and wrinkles.

Defect Dominated Charge Transport and Fermi Level Pinning in MoS/Metal Contacts.

Understanding the electronic contact between molybdenum disulfide (MoS) and metal electrodes is vital for the realization of future MoS-based electronic devices. Natural MoS has the drawback of a high density of both metal and sulfur defects and impurities. We present evidence that subsurface metal-like defects with a density of ∼10 cm induce negative ionization of the outermost S atom complex.

Graphene Visualizes the Ion Distribution on Air-Cleaved Mica.

The distribution of potassium (K) ions on air-cleaved mica is important in many interfacial phenomena such as crystal growth, self-assembly and charge transfer on mica. However, due to experimental limitations to nondestructively probe single ions and ionic domains, their exact lateral organization is yet unknown. We show, by the use of graphene as an ultra-thin protective coating and scanning probe microscopies, that single potassium ions form ordered structures that are covered by an ice layer.

Structure and Dynamics of Confined Alcohol-Water Mixtures.

The effect of confinement between mica and graphene on the structure and dynamics of alcohol-water mixtures has been studied in situ and in real time at the molecular level by atomic force microscopy (AFM) at room temperature. AFM images reveal that the adsorbed molecules are segregated into faceted alcohol-rich islands on top of an ice layer on mica, surrounded by a pre-existing multilayer water-rich film. These faceted islands are in direct contact with the graphene surface, revealing a preferred adsorption site.

Electrochemically Induced Nanobubbles between Graphene and Mica.

We present a new method to create dynamic nanobubbles. The nanobubbles are created between graphene and mica by reducing intercalated water to hydrogen. The nanobubbles have a typical radius of several hundred nanometers, a height of a few tens of nanometers and an internal pressure in the range of 0.

Visualization of steps and surface reconstructions in Helium Ion Microscopy with atomic precision.

Helium Ion Microscopy is known for its surface sensitivity and high lateral resolution. Here, we present results of a Helium Ion Microscopy based investigation of a surface confined alloy of Ag on Pt(111). Based on a change of the work function of 25meV across the atomically flat terraces we can distinguish Pt rich from Pt poor areas and visualize the single atomic layer high steps between the terraces.

Latent heat induced rotation limited aggregation in 2D ice nanocrystals.

The basic science responsible for the fascinating shapes of ice crystals and snowflakes is still not understood. Insufficient knowledge of the interaction potentials and the lack of relevant experimental access to the growth process are to blame for this failure. Here, we study the growth of fractal nanostructures in a two-dimensional (2D) system, intercalated between mica and graphene.

Desorption of oxygen from alloyed Ag/Pt(111).

We have investigated the interaction of oxygen with the Ag/Pt(111) surface alloy by thermal desorption spectroscopy (TDS). The surface alloy was formed during the deposition of sub-monolayer amounts of silver on Pt(111) at 800 K and subsequent cooling to 300 K. The low-temperature phase of the surface alloy is composed of nanometer-sized silver rich stripes, embedded within platinum-rich domains, which were characterized with spot profile analysis low energy electron diffraction.

Electronically stabilized nanowire growth.

Metallic nanowires show unique physical properties owing to their one-dimensional nature. Many of these unique properties are intimately related to electron-electron interactions, which have a much more prominent role in one dimension than in two or three dimensions. Here we report the direct visualization of quantum size effects responsible for preferred lengths of self-assembled metallic iridium nanowires grown on a germanium (001) surface.

Digging gold: keV He(+) ion interaction with Au.

Helium ion microscopy (HIM) was used to investigate the interaction of a focused He(+) ion beam with energies of several tens of kiloelectronvolts with metals. HIM is usually applied for the visualization of materials with extreme surface sensitivity and resolution. However, the use of high ion fluences can lead to significant sample modifications.

Decoupling of the copper core in a single copperphthalocyanine molecule.

Here, we show how a copper atom in a copperphthalocyanine (CuPc) molecule can be decoupled from its environment. This is realized by trapping the CuPc molecule between two adjacent nanowires that are 1.6 nm apart.

Formation and decay of a compressed phase of 4,4'-biphenyldicarboxylic acid on Cu(001).

The molecular arrangement of 4,4'-biphenyldicarboxylic acid (BDA) on Cu(001) has been studied at high coverage and relatively high temperature (~400 K) using Low Energy Electron Microscopy, LEEM, and selected area diffraction, μLEED. Next to the previously reported c(8 × 8) structure, we also observe a compressed phase with a [structure: see text] superstructure in matrix notation. All four equivalent (rotational and mirror) domains are equally populated.

Growth anomalies in supramolecular networks: 4,4'-biphenyldicarboxylic acid on cu(001).

We have used low energy electron microscopy to demonstrate how the interaction of 4,4'-biphenyldicarboxylic acid (BDA) molecules with (steps on) the Cu(001) surface determines the structure of supramolecular BDA networks on a mesoscopic length scale. Our in situ real time observations reveal that steps are permeable to individual molecules but that the change in crystal registry between different layers of the Cu substrate causes them to be completely impermeable to condensed BDA domains. The resulting growth instabilities determine the evolution of the domain shape and include a novel Mullins-Sekerka-type growth instability that is characterized by high growth rates along, instead of perpendicular to, the Cu steps.

Size fluctuations of near critical nuclei and Gibbs free energy for nucleation of BDA on Cu(001).

We present a low-energy electron microscopy study of nucleation and growth of BDA on Cu(001) at low supersaturation. At sufficiently high coverage, a dilute BDA phase coexists with c(8×8) crystallites. The real-time microscopic information allows a direct visualization of near-critical nuclei, determination of the supersaturation and the line tension of the crystallites, and, thus, derivation of the Gibbs free energy for nucleation.

Imaging ultra thin layers with helium ion microscopy: Utilizing the channeling contrast mechanism.

Background: Helium ion microscopy is a new high-performance alternative to classical scanning electron microscopy. It provides superior resolution and high surface sensitivity by using secondary electrons.

Results: We report on a new contrast mechanism that extends the high surface sensitivity that is usually achieved in secondary electron images, to backscattered helium images.

Channeling in helium ion microscopy: Mapping of crystal orientation.

Background: The unique surface sensitivity and the high resolution that can be achieved with helium ion microscopy make it a competitive technique for modern materials characterization. As in other techniques that make use of a charged particle beam, channeling through the crystal structure of the bulk of the material can occur.

Results: Here, we demonstrate how this bulk phenomenon affects secondary electron images that predominantly contain surface information.

A colloidal route to fabricate hierarchical sticky and non-sticky substrates.

We present a facile and inexpensive bottom-up colloidal route to prepare sticky superhydrophobic surfaces and non-sticky ones. Either spin coating to assemble silica microspheres into random multilayered arrays or irreversible adsorption of gold nanoparticles is used to manufacture substrates with a single length scale roughness. Hierarchical roughness with multiple length scales is achieved by decorating the silica spheres with gold nanoparticles.

Directional liquid spreading over chemically defined radial wettability gradients.

We investigate the motion of liquid droplets on chemically defined radial wettability gradients. The patterns consist of hydrophobic fluorinated self-assembled monolayers (SAMs) on oxidized silicon substrates. The design comprises a central hydrophobic circle of unpatterned SAMs surrounded by annular regions of radially oriented stripes of alternating wettability, i.

The influence of substrate temperature on growth of para-sexiphenyl thin films on Ir{111} supported graphene studied by LEEM.

The growth of para-sexiphenyl (6P) thin films as a function of substrate temperature on Ir{111} supported graphene flakes has been studied in real-time with Low Energy Electron Microscopy (LEEM). Micro Low Energy Electron Diffraction (μLEED) has been used to determine the structure of the different 6P features formed on the surface. We observe the nucleation and growth of a wetting layer consisting of lying molecules in the initial stages of growth.

Interplay of wrinkles, strain, and lattice parameter in graphene on iridium.

Following graphene growth by thermal decomposition of ethylene on Ir(111) at high temperatures we analyzed the strain state and the wrinkle formation kinetics as function of temperature. Using the moiré spot separation in a low energy electron diffraction pattern as a magnifying mechanism for the difference in the lattice parameters between Ir and graphene, we achieved an unrivaled relative precision of ±0.1 pm for the graphene lattice parameter.

Quantum size effect driven structure modifications of Bi films on Ni(111).

The quantum-size effect (QSE) driven growth of Bi film structures on Ni(111) was studied in situ using low energy electron microscopy and selective area low energy electron diffraction (μLEED). Domains with a (3×3), [(3)(1)(-1)(2)], and (7×7) film structure are found with a height of 3, 5, and 7 atomic layers, respectively. A comparison of I/V-μLEED curves with tensor LEED calculations shows perfectly accommodated Fermi wavelengths, indicative that not only the quantized height, but also the film structure is driven by QSE.