Tjeerd Rogier Johannes Bollmann

Hydration layers trapped between graphene and a hydrophilic substrate

Graphene is mechanically exfoliated on (111) under ambient conditions. We demonstrate the formation of a several monolayer thick hydration layer on the hydrophilic substrate and its response to annealing at temperatures up to 750 K in an ultra-high vacuum environment. Upon heating, water is released, however, it is impossible to remove the first layer. The initially homogeneous film separates into water-containing and water-free domains by two-dimensional Ostwald ripening. Upon severe heating, thick graphene multilayers undergo rupture, while nanoblisters confining sealed water appear on thinner sheets, capable of the storage and release of material. From modeling the dimensions of the nanoblisters, we estimate the graphene/(111) interfacial adhesion energy to be , thereby viable for polymer-assisted transfer printing.

Quantum size effects on surfaces without a projected bandgap: Pb/Ni(111)

We have studied the initial growth of Pb on Ni(111) using low-energy electron microscopy (LEEM) and selective area low-energy electron diffraction (μLEED). First, a one-layer-high wetting layer develops that consists of small (7 × 7) and (4 × 4) domains. For larger coverages, Pb mesas are formed that are embedded in the wetting layer. In spite of the absence of a projected bandgap on clean Ni(111), we observe distinct quantum size effect (QSE)-driven preferred heights. These are apparent from a variety of frequently occurring island height transitions during growth, both on wide terraces and across substrate steps. Also, the average island heights that evolve during deposition at 422 and 474 K show a clear signature of QSE-driven preferred heights. These distinctly include five, seven and nine layers and thus correspond nicely to the values obtained in the key examples of QSE: Pb films on Si(111) and Ge(111). We suggest that the Pb-induced surface modification of Ni(111) shifts the Fermi level into the gap of the interface projected Ni bulk bands, thereby effectively causing decoupling of the Pb states with the bulk Ni states.

Hole-doping of mechanically exfoliated graphene by confined hydration layers

By the use of non-contact atomic force microscopy (NC-AFM) and Kelvin probe force microscopy (KPFM), we measure the local surface potential of mechanically exfoliated graphene on the prototypical insulating hydrophilic substrate of CaF2(111). Hydration layers confined between the graphene and the CaF2 substrate, resulting from the graphene’s preparation under ambient conditions on the hydrophilic substrate surface, are found to electronically modify the graphene as the material’s electron density transfers from graphene to the hydration layer. Density functional theory (DFT) calculations predict that the first 2 to 3 water layers adjacent to the graphene hole-dope the graphene by several percent of a unit charge per unit cell.

Routes to rupture and folding of graphene on rough 6H-SiC(0001) and their identification

Summary Twisted few layer graphene (FLG) is highly attractive from an application point of view, due to its extraordinary electronic properties. In order to study its properties, we demonstrate and discuss three different routes to in situ create and identify (twisted) FLG. Single layer graphene (SLG) sheets mechanically exfoliated under ambient conditions on 6H-SiC(0001) are modified by (i) swift heavy ion (SHI) irradiation, (ii) by a force microscope tip and (iii) by severe heating. The resulting surface topography and the surface potential are investigated with non-contact atomic force microscopy (NC-AFM) and Kelvin probe force microscopy (KPFM). SHI irradiation results in rupture of the SLG sheets, thereby creating foldings and bilayer graphene (BLG). Applying the other modification methods creates enlarged (twisted) graphene foldings that show rupture along preferential edges of zigzag and armchair type. Peeling at a folding over an edge different from a low index crystallographic direction can result in twisted BLG, showing a similar height as Bernal (or AA-stacked) BLG in NC-AFM images. The rotational stacking can be identified by a significant contrast in the local contact potential difference (LCPD) measured by KPFM.

Determining the energetics of vicinal perovskite oxide surfaces

The energetics of vicinal SrTiO3(001) and DyScO3(110), prototypical perovskite vicinal surfaces, has been studied using topographic atomic force microscopy imaging. The kink formation and strain relaxation energies are extracted from a statistical analysis of the step meandering. Both perovskite surfaces have very similar kink formation energies and exhibit a similar triangular step undulation.

Studying Pulsed Laser Deposition conditions for Ni/C-based multi-layers

Abstract Nickel carbon based multi-layers are a viable route towards future hard X-ray and soft γ-ray focusing telescopes. Here, we study the Pulsed Laser Deposition growth conditions of such bilayers by Reflective High Energy Electron Diffraction, X-ray Reflectivity and Diffraction, Atomic Force Microscopy, X-ray Photoelectron Spectroscopy and cross-sectional Transmission Electron Microscopy analysis, with emphasis on optimization of process pressure and substrate temperature during growth. The thin multi-layers are grown on a treated SiO substrate resulting in Ni and C layers with surface roughnesses (RMS) of ≤0.2 nm. Small droplets resulting during melting of the targets surface increase the roughness, however, and cannot be avoided. The sequential process at temperatures beyond 300 °C results into intermixing between the two layers, being destructive for the reflectivity of the multi-layer.

Imaging pulsed laser deposition oxide growth by in situ atomic force microscopy

To visualize the topography of thin oxide films during growth, thereby enabling to study its growth behavior quasi real-time, we have designed and integrated an atomic force microscope (AFM) in a pulsed laser deposition (PLD) vacuum setup. The AFM scanner and PLD target are integrated in a single support frame, combined with a fast sample transfer method, such that in situ microscopy can be utilized after subsequent deposition pulses. The in situ microscope can be operated from room temperature up to 700 °C and at (process) pressures ranging from the vacuum base pressure of 10-6 mbar up to 1 mbar, typical PLD conditions for the growth of oxide films. The performance of this instrument is demonstrated by resolving unit cell height surface steps and surface topography under typical oxide PLD growth conditions.

Formation of nanowires and their interaction with atomic steps during growth of Bi on Ni(111)

Using low-energy electron microscopy (LEEM) and selective area low-energy electron diffraction, we have characterized both the (7×7) wetting layer and the BiNi9 [2 0 −2 5] nanowires that form during the growth of Bi on Ni(111). The 60 ± 20 nm wide nanowires have lengths up to 10 μm and a height of 4–6 atomic layers. After the formation of the wetting layer and nanowires, quantum size effect driven growth ensues, accompanied by the gradual disappearance of the nanowires and resulting meandering of the substrate steps. The displacements of substrate steps, directly imaged with LEEM, can be traced back to dealloying.

Escape from Flatland: strain and quantum size effect driven growth of metallic nanostructures

In this thesis, we show the influence of and subtle balance between QSE and strain stabilizing interactions on the growth, as well as on structural and electronic properties of nanostructures. We present a LEEM and µLEED study in combination with Tensor LEED calculations illustrating the relevance of the QSE for quantization of island heights and ultimate film structures for Bi/Ni(111). Alloying results in BiNi9 nanowires, that lead to meandering steps during dealloying. We show distinct QSE driven preferred heights (3,5,7 and 9) for the growth of Pb/Ni(111), apparent from height transitions. Pb mesas of 40 layers in height decay within 1-10 ms at about 526 K to form compact 3D structures. Their decay is induced by a thermal instability in the bi-domain wetting layer, leading to an energetic balance that is tipped in favor of the surface free energy. As an example of strain dominated growth, we studied the morphology and structure of Cu on W(100), showing strong 3D growth of hut shaped Cu crystallites with steep facets. The boundary between the Cu crystallite and the pseudomorphic adlayer can be explained by the complete cancellation of shear stress.