From Crystalline to Amorphous Germania Bilayer Films at the Atomic Scale: Preparation and Characterization

A new two-dimensional (2D) germanium dioxide film has been prepared. The film consists of interconnected germania tetrahedral units forming a bilayer structure, weakly coupled to the supporting Pt(111) metal-substrate. Density functional theory calculations predict a stable structure of 558-membered rings for germania films, while for silica films 6-membered rings are preferred. By varying the preparation conditions the degree of order in the germania films is tuned. Crystalline, intermediate ordered and purely amorphous film structures are resolved by analysing scanning tunnelling microscopy images

From Crystalline to Amorphous Germania Bilayer Films at the Atomic Scale: Preparation and Characterization

A new two-dimensional (2D) germanium dioxide film has been prepared. The film consists of interconnected germania tetrahedral units forming a bilayer structure, weakly coupled to the supporting Pt(111) metal-substrate. Density functional theory calculations predict a stable structure of 558-membered rings for germania films, while for silica films 6-membered rings are preferred. By varying the preparation conditions the degree of order in the germania films is tuned. Crystalline, intermediate ordered and purely amorphous film structures are resolved by analysing scanning tunnelling microscopy images

Resolving atomic diffusion in Ru(0001)-O(2×2) with spiral high-speed scanning tunneling microscopy

An intermediate state in atomic diffusion processes in the O(2×2) layer on Ru(0001) is resolved with spiral high-speed scanning tunneling microscopy (STM). The diffusion of atomic oxygen in the adlayer has been studied by density functional theory and STM. Transition state theory proposes a migration pathway for the diffusion in the oxygen adlayer. With spiral scan geometries—a new approach to high-speed STM—the oxygen vacancy mobility on the highly covered Ru(0001) surface is determined to be in the range of 0.1 to 1 Hz. Experimental evidence for the intermediate state along the oxygen diffusion pathway is provided in real space and real time

Dynamics in the O(2 × 1) adlayer on Ru(0001): bridging timescales from milliseconds to minutes by scanning tunneling microscopy

The dynamics within an O(2 × 1) adlayer on Ru(0001) is studied by density functional theory and high-speed scanning tunneling microscopy. Transition state theory proposes dynamic oxygen species in the reduced O(2 × 1) layer at room temperature. Collective diffusion processes can result in structural reorientations of characteristic stripe patterns. Spiral high-speed scanning tunneling microscopy measurements reveal this reorientation as a function of time in real space. Measurements, ranging over several minutes with constantly high frame rates of 20 Hz resolved the gradual reorientation. Moreover, reversible fast flipping events of stripe patterns are observed. These measurements relate the observations of long-term atomic rearrangements and their underlying fast processes captured within several tens of milliseconds

Spiral high-speed scanning tunneling microscopy: Tracking atomic diffusion on the millisecond timescale

Scanning tunneling microscopy (STM) is one of the most prominent techniques to resolve atomic structures of flat surfaces and thin films. With the scope to answer fundamental questions in physics and chemistry, it was used to elucidate numerous sample systems at the atomic scale. However, dynamic sample systems are difficult to resolve with STM due to the long acquisition times of typically more than 100 s per image. Slow electronic feedback loops, slow data acquisition, and the conventional raster scan limit the scan speed. Raster scans introduce mechanical noise to the image and acquire data discontinuously. Due to the backward and upward scan or the flyback movement of the tip, image acquisition times are doubled or even quadrupled. By applying the quasi-constant height mode and by using a combination of high-speed electronics for data acquisition and innovative spiral scan patterns, we could increase the frame rate in STM significantly. In the present study, we illustrate the implementation of spiral scan geometries and focus on the scanner input signal and the image visualization. Constant linear and constant angular velocity spirals were tested on the Ru(0001) surface to resolve chemisorbed atomic oxygen. The spatial resolution of the spiral scans is comparable to slow raster scans, while the imaging time was reduced from ~100 s to ~8 ms. Within 8 ms, oxygen diffusion processes were atomically resolved

Continuous network structure of two-dimensional silica across a supporting metal step edge: An atomic scale study

The network structure of a silica bilayer film at a monolayer-bilayer transition and across a supporting metal step edge was studied at the atomic scale by scanning tunneling microscopy. The ring size distribution, ring-ring distances, and height profiles are analyzed across the step edge region. Density functional theory proposes two models to explain the observed network structure: a pinning of the lower layer to the substrate and a carpetlike mode. The results indicate a continuous coverage of the silica bilayer film across the step edge

Structure and registry of the silica bilayer film on Ru(0001) as viewed by LEED and DFT

Silica bilayers are stable on various metal substrates, including Ru(0001) that is used for the present study. In a systematic attempt to elucidate the detailed structure of the silica bilayer film and its registry to the metal substrate, we performed a low energy electron diffraction (I/V-LEED) study. The experimental work is accompanied by detailed calculations on the stability, orientation and dynamic properties of the bilayer at room temperature. It was determined, that the film shows a certain structural diversity within the unit cell of the metal substrate, which depends on the oxygen content at the metal-bilayer interface. In connection with the experimental I/V-LEED study, it became apparent, that a high-quality structure determination is only possible if several structural motifs are taken into account by superimposing bilayer structures with varying registry to the oxygen covered substrate. This result is conceptually in line with the recently observed statistical registry in layered 2D-compound materials

A high-speed variable-temperature ultrahigh vacuum scanning tunneling microscope with spiral scan capabilities

We present the design and development of a variable-temperature high-speed scanning tunneling microscope (STM). The setup consists of a two-chamber ultra-high vacuum system, including a preparation and a main chamber. The preparation chamber is equipped with standard preparation tools for sample cleaning and film growth. The main chamber hosts the STM that is located within a continuous flow cryostat for counter-cooling during high-temperature measurements. The microscope body is compact, rigid, and highly symmetric to ensure vibrational stability and low thermal drift. We designed a hybrid scanner made of two independent tube piezos for slow and fast scanning, respectively. A commercial STM controller is used for slow scanning, while a high-speed Versa Module Eurocard bus system controls fast scanning. Here, we implement non-conventional spiral geometries for high-speed scanning, which consist of smooth sine and cosine signals created by an arbitrary waveform generator. The tip scans in a quasi-constant height mode, where the logarithm of the tunneling current signal can be regarded as roughly proportional to the surface topography. Scan control and data acquisition have been programmed in the experimental physics and industrial control system framework. With the spiral scans, we atomically resolved diffusion processes of oxygen atoms on the Ru(0001) surface and achieved a time resolution of 8.3 ms per frame at different temperatures. Variable-temperature measurements reveal an influence of the temperature on the oxygen diffusion rate

Mesoscopic Structures and Coexisting Phases in Silica Films

Silica films represent a unique two-dimensional film system, exhibiting both crystalline and vitreous forms. While much scientific work has focused on the atomic-scale features of this film system, mesoscale structures can play an important role for understanding confined space reactions and other applications of silica films. Here, we report on mesoscale structures in silica films grown under ultrahigh vacuum and examined with scanning tunneling microscopy (STM). Silica films can exhibit coexisting phases of monolayer, zigzag, and bilayer structures. Both holes in the film structure and atomic-scale substrate steps are observed to influence these coexisting phases. In particular, film regions bordering holes in silica bilayer films exhibit vitreous character, even in regions where the majority film structure is crystalline. At high coverages mixed zigzag and bilayer phases are observed at step edges, while at lower coverages silica phases with lower silicon densities are observed more prevalently near step edges. The STM images reveal that silica films exhibit rich structural diversity at the mesoscale

Growth and Atomic‐Scale Characterization of Ultrathin Silica and Germania Films: The Crucial Role of the Metal Support

The present review reports on the preparation and atomic‐scale characterization of the thinnest possible films of the glass‐forming materials silica and germania. To this end state‐of‐the‐art surface science techniques, in particular scanning probe microscopy, and density functional theory calculations have been employed. The investigated films range from monolayer to bilayer coverage where both, the crystalline and the amorphous films, contain characteristic XO4 (X=Si,Ge) building blocks. A side‐by‐side comparison of silica and germania monolayer, zigzag phase and bilayer films supported on Mo(112), Ru(0001), Pt(111), and Au(111) leads to a more general comprehension of the network structure of glass former materials. This allows us to understand the crucial role of the metal support for the pathway from crystalline to amorphous ultrathin film growth