Bene Poelsema

Growth of graphene on Ir(111)

Catalytic decomposition of hydrocarbons on transition metals attracts a renewed interest as a route toward high-quality graphene prepared in a reproducible manner. Here we employ two growth methods for graphene on Ir(111), namely room temperature adsorption and thermal decomposition at 870–1470 K (temperature programmed growth (TPG)) as well as direct exposure of the hot substrate at 870–1320 K (chemical vapor deposition (CVD)). The temperature- and exposure-dependent growth of graphene is investigated in detail by scanning tunneling microscopy. TPG is found to yield compact graphene islands bounded by C zigzag edges. The island size may be tuned from a few to a couple of tens of nanometers through Smoluchowski ripening. In the CVD growth, the carbon in ethene molecules arriving on the Ir surface is found to convert with probability near unity to graphene. The temperature-dependent nucleation, interaction with steps and coalescence of graphene islands are analyzed and a consistent model for CVD growth is developed.

The interaction of hydrogen with platinum(s)−9(111) × (111) studied with helium beam diffraction

Abstract The H 2 Pt(S) −9(111) × (111) interaction system has been studied using helium beam diffraction in the temperature range 250–370 K. Information on the H coverage is obtained from the reduction of coherent scattering upon the presence of disordered adsorbates. The features of this recently proposed new technique are discussed in detail. The incident He beam of thermal energy is essentially non-perturbing and non-penetrating. The diffraction experiment yields surface sensitive information on the interaction of adsorbates (well-suited for H2 too!) with ideal regions of the surface. The method enables discrimination between step edge and terrace properties in a direct geometrical way. So, for instance, isosteric heats of adsorption of 22 and 19 kcal mole have been obtained for the step edges and the terraces, respectively. The obtained preexponentials for desorption have “normal” values for a second order process. The measured isotherms can be fitted well with Langmuir isotherms. The internal consistency of the model description is checked by comparing adsorption and desorption measurements with equilibrium results.

Shape and size effects in the optical properties of metallic nanorods

The influence of size and geometrical shape on the optical properties of randomly oriented metallic nanorods is investigated using the discrete dipole approximation (DDA). Our calculations provide a benchmark for an accurate characterisation of nanorod suspensions by frequently used optical spectroscopic techniques. Our DDA results confirm the longitudinal plasmon resonance to be primarily affected by the nanorod aspect ratio, and also verify that the quasi-static (dipole) approximation for ellipsoidal particles is only valid for very small sizes. For prolate ellipsoidal and cylindrical nanorods with an identical aspect ratio, the latter exhibit a longitudinal resonance at significantly longer wavelengths. The importance of phase retardation and multipole contributions for larger nanorod dimensions is discussed. Also, we investigate the influence on the optical spectra of electron surface scattering, which arises from the limited size of the nanorods in comparison to the electron mean free path.

Germanene: the germanium analogue of graphene

Recently, several research groups have reported the growth of germanene, a new member of the graphene family. Germanene is in many aspects very similar to graphene, but in contrast to the planar graphene lattice, the germanene honeycomb lattice is buckled and composed of two vertically displaced sub-lattices. Density functional theory calculations have revealed that free-standing germanene is a 2D Dirac fermion system, i.e. the electrons behave as massless relativistic particles that are described by the Dirac equation, which is the relativistic variant of the Schrödinger equation. Germanene is a very appealing 2D material. The spin-orbit gap in germanene (~24 meV) is much larger than in graphene (<0.05 meV), which makes germanene the ideal candidate to exhibit the quantum spin Hall effect at experimentally accessible temperatures. Additionally, the germanene lattice offers the possibility to open a band gap via for instance an externally applied electrical field, adsorption of foreign atoms or coupling with a substrate. This opening of the band gap paves the way to the realization of germanene based field-effect devices. In this topical review we will (1) address the various methods to synthesize germanene (2) provide a brief overview of the key results that have been obtained by density functional theory calculations and (3) discuss the potential of germanene for future applications as well for fundamentally oriented studies.

Helium ion microscopy

Helium Ion Microcopy (HIM) based on Gas Field Ion Sources (GFIS) represents a new ultra high resolution microscopy and nano-fabrication technique. It is an enabling technology that not only provides imagery of conducting as well as uncoated insulating nano-structures but also allows to create these features. The latter can be achieved using resists or material removal due to sputtering. The close to free-form sculpting of structures over several length scales has been made possible by the extension of the method to other gases such as Neon. A brief introduction of the underlying physics as well as a broad review of the applicability of the method is presented in this review.

Temperature dependency of the initial sticking probability of H2 and CO on Pt(111)

The initial sticking probability of the reactive gases H2 and CO on a nearly defect free Pt(111) surface is studied in the temperature range 90–300 K by means of Thermal Energy Atom Scattering (TEAS). By means of TEAS relative initial sticking probabilities can he measured with great accuracy. H2/Pt(111): The initial sticking probability is found to increase with increasing surface temperature. The important role in the chemisorption process played by defects, even at concentrations < 10−3 is emphasized. A two-stage model is proposed to explain these results. CO/Pt(111): The initial sticking probability is found to decrease with increasing surface temperature. This observation is explained with a precursor model.

In?situ observation of stress relaxation in epitaxial graphene

Upon cooling, branched line defects develop in epitaxial graphene grown at high temperature on Pt(111) and Ir(111). Using atomically resolved scanning tunneling microscopy we demonstrate that these defects are wrinkles in the graphene layer, i.e. stripes of partially delaminated graphene. With low energy electron microscopy (LEEM) we investigate the wrinkling phenomenon in situ. Upon temperature cycling we observe hysteresis in the appearance and disappearance of the wrinkles. Simultaneously with wrinkle formation a change in bright field imaging intensity of adjacent areas and a shift in the moire spot positions for micro diffraction of such areas takes place. The stress relieved by wrinkle formation results from the mismatch in thermal expansion coefficients of graphene and the substrate. A simple one-dimensional model taking into account the energies related to strain, delamination and bending of graphene is in qualitative agreement with our observations.

Quantitative Analysis of Gold Nanorod Alignment after Electric Field-Assisted Deposition

We have studied the alignment of colloidal gold nanorods, deposited from solution onto well-defined substrates in the presence of an AC electric field generated by micrometer spaced electrodes. The field strengths employed in our experiments are sufficiently large to overcome Brownian motion and induce accumulation and alignment of the nanorods in the region near the electrodes with their long axis parallel to the field. However, despite the large fields, we find that the degree of alignment is considerably smaller than what was previously reported for field-induced nanorod alignment in suspension. We show that hydrodynamic interactions and capillary effects during drying, as well as friction of nanorods on the substrate surface, to not play a major role. The limited alignment of nanorods is ascribed to the different experimental configuration and the correspondingly larger density of nanorods. The mutual interactions of nanorods give rise to a disturbance of the local electric field and therewith their orientation. For sufficiently large field strengths, these interactions lead to the formation of nanorod chains that ultimately bridge the electrode gap. Furthermore, for small electrode spacing, the nanorods accumulate on the electrode surface, and the screening of their mutual interactions results into considerably improved alignment.

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.

Helium scattering as a probe of the clean and adsorbate covered Pt(111) surface

Abstract Thermal energy (16–165 meV) helium nozzle beam scattering has been used as a probe of the physical structure of the nominal Pt(111) surface and for the consequential effects of the adsorption of gases including CO, H 2 , O 2 and C 2 H 2 . By careful crystal alignment and polishing techniques a Pt(111) surface with terrace widths greater than 3000 A was achieved as judged by the helium scattering. The profile of the scattered peak was immeasurably affected by the presence of various adsorbates while the intensity decreases monotonically with coverage. The sensitivity of the scattered intensity to adsorbates is, in general, a function of the energy and angle of incidence of the helium beam. The scattering cross-section of the adsorbate can be, for some cases, much larger than its Van der Waals diameter and apparently increases with the degree of perfection of the surface.