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. At 90 K above T_{C}, a strong, unanticipated preference for propagation along the substrate [001] azimuth suddenly emerges, which is attributed to presolidification at the liquid-solid interface. The propagation along [001] is accompanied by a distinct change in shape from compact to elongated along [001].

Large Dense Periodic Arrays of Vertically Aligned Sharp Silicon Nanocones.

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 < 3 nm were measured. The silicon nanocones were vertically aligned, baring a height variation of < 5 nm (< 1%) for seven adjacent nanocones, whereas the height inhomogeneity is < 80 nm (< 16%) across the full wafer scale. The height inhomogeneity can be explained by inhomogeneity present in the radii of the initial columnar polymer mask. The presented method might also be applicable to silicon micro- and nanowires derived through other top-down or bottom-up methods because of the combination of ion beam etching pre-shaping and thermal oxidation sharpening. 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 < 3 nm apex radius of curvature tips, at the wafer scale.

Self-Assembled Decanethiolate Monolayers on Au(001): Expanding the Family of Known Phases.

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 ß 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.

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

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 × âˆš3) superstructures. The distance between adjacent ripples is about 18 nm.

Marangoni puffs: dramatically enhanced dissolution of droplets with an entrapped bubble.

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.

Evaporation of Dilute Sodium Dodecyl Sulfate Droplets on a Hydrophobic Substrate.

Evaporation of surfactant-laden sessile droplets is omnipresent in nature and industrial applications such as inkjet printing. Soluble surfactants start to form micelles in an aqueous solution for surfactant concentrations exceeding the critical micelle concentration (CMC). Here, the evaporation of aqueous sodium dodecyl sulfate (SDS) sessile droplets on hydrophobic surfaces was experimentally investigated for SDS concentrations ranging from 0.025 to 1 CMC. In contrast to the constant contact angle of an evaporating sessile water droplet, we observed that, at the same surface, the contact angle of an SDS laden droplet with concentration below 0.5 CMC first decreases, then increases, and finally decreases, resulting in a local contact angle minimum. Surprisingly, the minimum contact angle was found to be substantially lower than the static receding contact angle and decreased with decreasing initial SDS concentration. Furthermore, the bulk SDS concentration at the local contact angle minimum was found to decrease with decrease in the initial SDS concentration. The location of the observed contact angle minimum relative to the normalized evaporation time and its minimum value proved to be independent of both the relative humidity and droplet volume and thus of the total evaporation time. We discuss the observed contact angle dynamics in terms of the formation of a disordered layer of SDS molecules on the substrate at concentrations below 0.5 CMC. The present work underlines the complexity of the evaporation of sessile liquid-surfactant droplets and the influence of surfactant-substrate interactions on the evaporation process.

The Leidenfrost temperature increase for impacting droplets on carbon-nanofiber surfaces.

Droplets impacting on a superheated surface can either exhibit a contact boiling regime, in which they make direct contact with the surface and boil violently, or a film boiling regime, in which they remain separated from the surface by their own vapor. The transition from the contact to the film boiling regime depends not only on the temperature of the surface and the kinetic energy of the droplet, but also on the size of the structures fabricated on the surface. Here we experimentally show that surfaces covered with carbon-nanofibers delay the transition to film boiling to much higher temperatures compared to smooth surfaces. We present physical arguments showing that, because of the small scale of the carbon fibers, they are cooled by the vapor flow just before the liquid impact, thus permitting contact boiling up to much higher temperatures than on smooth surfaces. We also show that as long as the impact is in the film boiling regime, the spreading factor of impacting droplets is consistent with the We(3/10) scaling (with We being the Weber number) as predicted for large We by a scaling analysis.

Building microscopic soccer balls with evaporating colloidal fakir drops.

Evaporation-driven particle self-assembly can be used to generate three-dimensional microstructures. We present a unique method to create colloidal microstructures in which we can control the amount of particles and their packing fraction. To this end, we evaporate colloidal dispersion droplets on a special type of superhydrophobic microstructured surface, on which the droplet remains in Cassie-Baxter state during the entire evaporative process. The remainders of the droplet consist of a massive spherical cluster of the microspheres, with diameters ranging from a few tens up to several hundreds of microns. We present scaling arguments to show how the final particle packing fraction of these balls depends on the dynamics of the droplet evaporation, particle size, and number of particles in the system.

How water droplets evaporate on a superhydrophobic substrate.

Evaporation of water droplets on a superhydrophobic substrate, on which the contact line is pinned, is investigated. While previous studies focused mainly on droplets with contact angles smaller than 90°, here we analyze almost the full range of possible contact angles (10°-150°). The greater contact angles and pinned contact lines can be achieved by use of superhydrophobic carbon nanofiber substrates. The time evolutions of the contact angle and the droplet mass are examined. The experimental data are in good quantitative agreement with the model presented by Popov [Phys. Rev. E 71, 036313 (2005)], demonstrating that the evaporation process is quasistatic, diffusion-driven, and that thermal effects play no role. Furthermore, we show that the experimental data for the evolution of both the contact angle and the droplet mass can be collapsed onto one respective universal curve for all droplet sizes and initial contact angles.

Atomic seesaws.

The dynamics of two types of atomic seesaws are studied by open feedback loop scanning tunneling microscopy. The first type of atomic seesaw is a regular Ge dimer of the dimer reconstructed Ge(001) surface and the second type of atomic seesaw is a dimer located on the ridges of Au induced nanowires on a Ge(001) surface. On the bare Ge(001) surface the flip-flop motion of the dimers is induced by phasons, which perform a one-dimensional random walk along the substrate dimer rows. The phasons on the Au induced nanowires ridges are pinned and therefore only a limited number of dimers exhibit flip-flop behavior for the Au/Ge(001) system.

Playing pinball with atoms.

We demonstrate the feasibility of controlling an atomic scale mechanical device by an external electrical signal. On a germanium substrate, a switching motion of pairs of atoms is induced by electrons that are directly injected into the atoms with a scanning tunneling microscope tip. By precisely controlling the tip current and distance we make two atom pairs behave like the flippers of an atomic-sized pinball machine. This atomic scale mechanical device exhibits six different configurations.

Self-lacing atom chains.

The structural and electronic properties of self-lacing atomic chains on Pt modified Ge(001) surfaces have been studied using low-temperature scanning tunnelling microscopy and spectroscopy. The self-lacing chains have a cross section of only one atom, are perfectly straight, thousands of atoms long and virtually defect free. The atomic chains are composed of dimers that have their bonds aligned in a direction parallel to the chain direction. At low temperatures the atomic chains undergo a Peierls transition: the periodicity of the chains doubles from a 2 × to a 4 × periodicity and an energy gap opens up. Furthermore, at low temperatures (T<80 K) novel quasi-one-dimensional electronic states are found. These quasi-one-dimensional electronic states originate from an electronic state of the underlying terrace that is confined between the atomic chains.

Scanning tunneling spectroscopy.

The scanning tunneling microscope (STM) has revolutionized our ability to explore and manipulate atomic-scale solid surfaces. In addition to its unparalleled spatial power, the STM can study dynamical processes, such as molecular conformational changes, by recording current traces as a function of time. It can also be employed to measure the physical properties of molecules or nanostructures down to the atomic scale. Combining STM imaging with measurement of current-voltage (I-V) characteristics [i.e., scanning tunneling spectroscopy (STS)] at similar resolution makes it possible to obtain a detailed map of the electronic structure of a surface. For many years, STM lacked chemical specificity; however, the recent development of STM-IETS (inelastic electron tunneling spectroscopy) has allowed us to measure the vibrational spectrum of a single molecule. This review introduces and illustrates these recent developments with a few simple scholarly examples.

Spatial mapping of the electronic states of a one-dimensional system.

Using low-temperature scanning tunneling microscopy and spectroscopy, we have recorded spatial maps of confined electronic states in the troughs between self-organized Pt nanowires on Ge(001) that are spaced 2.4 nm apart. Two sub-bands are resolved, which correspond to the lowest energy levels of a quantum mechanical particle in a box. As expected, the spatial dI/dV maps exhibit a maximum and a minimum in the middle of the troughs for the n = 1 and n = 2 states, respectively.

Dynamics and energetics of Ge(001) dimers.

The dynamic behavior of surface dimers on Ge(001) has been studied by positioning the tip of a scanning tunneling microscope over single flip-flopping dimers and measuring the tunneling current as a function of time. We observe that not just symmetric, but also asymmetric appearing dimers exhibit flip-flop motion. The dynamics of flip-flopping dimers can be used to sensitively gauge the local potential landscape of the surface. Through a spatial and time-resolved measurement of the flip-flop frequency of the dimers, local strain fields near surface defects can be accurately probed.

Quantum confinement between self-organized Pt nanowires on Ge(001).

The existence of one-dimensional (1D) electronic states between self-organized Pt nanowires spaced 1.6 or 2.4 nm apart on a Ge(001) surface is revealed by low-temperature scanning tunneling microscopy. These perfectly straight Pt nanowires act as barriers for a surface state (located just below the Fermi level) of the underlying terrace. The energy positions of the 1D electronic states are in good agreement with the energy levels of a quantum particle in a well. Spatial maps of the differential conductivity of the 1D electronic states conclusively reveal that these states are exclusively present in the troughs between the Pt nanowires.