Harold J.W. Zandvliet

Surface Bubble Nucleation Stability

Recent research has revealed several different techniques for nanoscopic gas nucleation on submerged surfaces, with findings seemingly in contradiction with each other. In response to this, we have systematically investigated the occurrence of surface nanobubbles on a hydrophobized silicon substrate for various different liquid temperatures and gas concentrations, which we controlled independently. We found that nanobubbles occupy a distinct region of this parameter space, occurring for gas concentrations of approximately 100%-110%. Below the nanobubble region we did not detect any gaseous formations on the substrate, whereas micropancakes (micron wide, nanometer high gaseous domains) were found at higher temperatures and gas concentrations. We moreover find that supersaturation of dissolved gases is not a requirement for nucleation of bubbles.

Self-organized, one-dimensional Pt nanowires on Ge(001)

Pt atoms adsorbed onto Ge(001) surface form extremely well-ordered nanowire arrays by self-organization after high-temperature annealing. Using scanning tunneling spectroscopy/microscopy, it is shown that they are metallic and defect free. They are only 0.4 nm thick with a spacing of 1.6 nm in between, and have aspect ratios up to 1000. Their formation can be discussed in terms of a relativistic property possessed by heaviest 5d elements, and the pathway to their formation can be explained by dimer breakup on Ge(001) surface at elevated temperatures followed by a surface polymerization reaction

Electrolytically Generated Nanobubbles on Highly Orientated Pyrolytic Graphite Surfaces

Electrolysis of water is employed to produce surface nanobubbles on highly orientated pyrolytic graphite (HOPG) surfaces. Hydrogen (oxygen) nanobubbles are formed when the HOPG surface acts as negative (positive) electrode. Coverage and volume of the nanobubbles enhance with increasing voltage. The yield of hydrogen nanobubbles is much larger than the yield of oxygen nanobubbles. The growth of the individual nanobubbles during the electrolysis process is recorded in time with the help of AFM measurements and correlated with the total current. Both the size of the individual nanobubbles and the total current saturate after typical 1 minute; then the nanobubbles are in a dynamic equilibrium, meaning that they do not further grow, in spite of ongoing gas production and nonzero current. The surface area of nanobubbles shows a good correlation with the nanobubble volume growth rate, suggesting that either the electrolytic gas emerges directly at the nanobubbles’ surface, or it emerges at the electrode’s surface and then diffuses through the nanobubbles’ surface. Moreover, the experiments reveal that the time constants of the current and the aspect ratio of nanobubbles are the same under all conditions. Replacement of pure water by water containing a small amount of sodium chloride (0.01 M) allows for larger currents, but qualitatively gives the same results.

Smart Design of Stripe-Patterned Gradient Surfaces to Control Droplet Motion

The motion of droplets under the influence of lithographically created anisotropic chemically defined patterns is described and discussed. The patterns employed in our experiments consist of stripes of alternating wettability: hydrophobic stripes are created via fluorinated self-assembled monolayers, and for hydrophilic stripes, the SiO(2) substrate is used. The energy gradient required to induce the motion of the droplets is created by varying the relative widths of the stripes in such a way that the fraction of the hydrophilic area increases. The anisotropic patterns create a preferential direction for liquid spreading parallel to the stripes and confine motion to the perpendicular direction, giving rise to markedly higher velocities as compared to nonstructured surface energy gradients. Consequently, the influence of the distinct pattern features on the overall motion as well as suggestions for design improvements from an application point of view are discussed.

Parallel Electron-Hole Bilayer Conductivity from Electronic Interface Reconstruction

The perovskite SrTiO3-LaAlO3 structure has advanced to a model system to investigate the rich electronic phenomena arising at polar oxide interfaces. Using first principles calculations and transport measurements we demonstrate that an additional SrTiO3 capping layer prevents atomic reconstruction at the LaAlO3 surface and triggers the electronic reconstruction at a significantly lower LaAlO3 film thickness than for the uncapped systems. Combined theoretical and experimental evidence (from magnetotransport and ultraviolet photoelectron spectroscopy) suggests two spatially separated sheets with electron and hole carriers, that are as close as 1 nm.

Transport through a Single Octanethiol Molecule

Octanethiol molecules adsorbed on Pt chains are studied with scanning tunneling microscopy and spectroscopy at 77 K. The head of the octanethiol binds to a Pt atom and the tail is lying flat down on the chain. Open-loop current time traces reveal that the molecule wags its tail and attaches to the scanning tunneling microscopy-tip resulting in a dramatic increase of the current. We measured a single molecule resistance of 100-150 Mohms.

Dynamic Dewetting through Micropancake Growth

We experimentally investigate the dynamics of nanometer-high, micrometer-wide gassy layers at the interface between a hydrophobic solid and bulk water. These micropancakes grow laterally in time, on the timescale of an hour, leading to partial dewetting of the solid. The growth is directional, mediated by chemical roughness on the substrate, and transient, occurring within the first hour after liquid deposition. We use circularity to measure the roundness of a micropancake (circularity C = 2(piA)(1/2)/L, where A is the surface area and L is the perimeter). The growth is anisotropic, as demonstrated by a decrease in circularity with time. However, once a micropancake reaches size saturation, its bulk rearranges its shape in order to minimize the length of its three-phase line. We interpret this combination of growth followed by bulk rearrangement as dynamic dewetting.

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

Summary 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. We have characterized the changes caused by a focused He+ ion beam at normal incidence to the Au{111} surface as a function of ion fluence and energy. Under the influence of the beam a periodic surface nanopattern develops. The periodicity of the pattern shows a power-law dependence on the ion fluence. Simultaneously, helium implantation occurs. Depending on the fluence and primary energy, porous nanostructures or large blisters form on the sample surface. The growth of the helium bubbles responsible for this effect is discussed.

Channeling in helium ion microscopy: Mapping of crystal orientation

Summary 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. In addition, we will show how it can be used to obtain crystallographic information. We will discuss the origin of channeling contrast in secondary electron images, illustrate this with experiments, and develop a simple geometric model to predict channeling maxima. Conclusion: Channeling plays an important role in helium ion microscopy and has to be taken into account when trying to achieve maximum image quality in backscattered helium images as well as secondary electron images. Secondary electron images can be used to extract crystallographic information from bulk samples as well as from thin surface layers, in a straightforward manner.

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

Summary 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. We demonstrate how thin organic and inorganic layers as well as self-assembled monolayers can be visualized on heavier element substrates by changes in the backscatter yield. Thin layers of light elements on heavy substrates should have a negligible direct influence on backscatter yields. However, using simple geometric calculations of the opaque crystal fraction, the contrast that is observed in the images can be interpreted in terms of changes in the channeling probability. Conclusion: The suppression of ion channeling into crystalline matter by adsorbed thin films provides a new contrast mechanism for HIM. This dechanneling contrast is particularly well suited for the visualization of ultrathin layers of light elements on heavier substrates. Our results also highlight the importance of proper vacuum conditions for channeling-based experimental methods.