Formation routes and structural details of the CaF1 layer on Si(111) from high-resolution noncontact atomic force microscopy data

We investigate the CaF1/Si(111) interface using a combination of high-resolution scanning tunnelling and noncontact atomic force microscopy operated at cryogenic temperature as well as x-ray photoelectron spectroscopy. Submonolayer CaF1 films grown at substrate temperatures between 550 and 600 ◦C on Si(111) surfaces reveal the existence of two island types that are distinguished by their edge topology, nucleation position, measured height, and inner defect structure. Our data suggest a growth model where the two island types are the result of two reaction pathways during CaF1 interface formation. A key difference between these two pathways is identified to arise from the excess species during the growth process, which can be either fluorine or silicon. Structural details as a result of this difference are identified by means of high-resolution noncontact atomic force microscopy and add insights into the growth mode of this heteroepitaxial insulator-on-semiconductor system

Measuring the reactivity of a silicon-terminated probe

It is generally accepted that the exposed surfaces of silicon crystals are highly reactive due to the dangling bonds which protrude into the vacuum. However, surface reconstruction can not only modify the reactivity of bulk silicon crystals, but plays a key role in determining the properties of silicon nanocrystals. In this study we probe the reactivity of silicon clusters at the end of a scanning probe tip by examining their interaction with closed shell fullerene molecules. Counter to intuitive expectations, many silicon clusters do not react strongly with the fullerene cage, and we find that only specific highly oriented clusters have sufficient reactivity to break open the existing carbon-carbon bonds

NC-AFM contrast formation on the calcite (1014) surface

Rahe P, Schütte J, Kühnle A. NC-AFM contrast formation on the calcite (1014) surface. Journal of Physics : Condensed Matter. 2012;24(8): 84006.Calcite, the most stable polymorph of calcium carbonate, is one of the most abundant simple salts in the geological environment. Consequently, its natural (10 (1) over bar4) cleavage plane has been studied extensively by a wide range of surface-sensitive techniques, giving indications for two reconstructions, namely a (2 x 1) and a so-called 'row-pairing' reconstruction. The existence of the (2 x 1) reconstruction has been discussed controversially in the literature, but is now confirmed as a true surface property. In contrast, a comprehensive discussion on the existence of the row-pairing reconstruction is lacking so far. Here, we present a non-contact atomic force microscopy (NC-AFM) study of the (10 (1) over bar4) calcite surface performed in an ultra-high vacuum. We discuss a broad variety of different NC-AFM contrasts and present a comprehensive classification scheme. This scheme encompasses a total of 12 different contrast modes. Atomically resolved NC-AFM images are shown, giving experimental evidence for 10 of these contrast modes. In particular, some of these modes allow for identification of the two surface reconstructions while others do not. This variety in appearances provides an explanation for the seemingly contradicting observations in the literature. Based on a detailed investigation of the influence of tip termination and interaction regime, we further analyse the existence of the row-pairing reconstruction

Molecular self-assembly on an insulating surface: interplay between substrate templating and intermolecular interactions

Kittelmann M, Rahe P, Kühnle A. Molecular self-assembly on an insulating surface: interplay between substrate templating and intermolecular interactions. Journal of Physics : Condensed Matter. 2012;24(35): 354007.We report on molecular self-assembly of biphenyl-4,4'-dicarboxylic acid (BPDCA) on CaCO3(10 (1) over bar4) under ultra-high vacuum conditions. Two-dimensional, ordered islands are obtained upon deposition at room temperature, coexisting with a streaky structure that is ascribed to individual, mobile molecules forming a two-dimensional gas-like phase. High-resolution non-contact atomic force microscopy (NC-AFM) images of the molecular islands reveal an ordered inner structure that is dominated by rows of molecules aligned side by side running along the [(42) over bar 61] crystallographic direction. A detailed analysis of these rows exhibits inter-row distances that are multiples of the calcite unit cell dimension along the [01 (1) over bar0] direction, clearly demonstrating the templating effect of the substrate. Our results indicate that an excellent size match of the molecular structure with respect to the underlying substrate results in an increased binding of the BPDCA molecules to the surface. In between the rows, a different molecular structure is coexisting with the molecules aligning head to tail. This structure is explained by intermolecular hydrogen bond formation very similar to the BPDCA bulk structure. The coexistence of the bulk-like structure with the row structure suggests a close balance of intermolecular and molecule-surface interactions to be responsible for the observed structure formation

Contrast inversion in non-contact atomic force microscopy imaging of C-60 molecules

Loske F, Rahe P, Kühnle A. Contrast inversion in non-contact atomic force microscopy imaging of C-60 molecules. Nanotechnology. 2009;20(26):264010.Non-contact atomic force microscopy (NC-AFM) was applied to study C-60 molecules on rutile TiO2(110). Depending on the tip-sample distance, distinctly different molecular contrasts are observed. Systematically decreasing the tip-sample distance results in contrast inversion that is obtained reproducibly on the C-60 islands. This change in contrast can be related to frequency shift versus distance (d f (z)) curves at different sample sites, unraveling crossing points in the d f (z) curves in the attractive regime. We have performed simulations based on a simple Morse potential, which reproduce the experimental results. This combined experimental and simulation study provides insight into the mechanisms responsible for molecular contrast in NC-AFM imaging. Moreover, this work demonstrates the importance of distance-dependent measurements for unambiguously identifying molecular positions within a molecular island using NC-AFM

Vertical and lateral drift corrections of scanning probe microscopy images

Rahe P, Bechstein R, Kühnle A. Vertical and lateral drift corrections of scanning probe microscopy images. Journal of Vacuum Science & Technology B. 2010;28(3):C4E31.A procedure is presented for image correction of scanning probe microscopy data that is distorted by linear thermal drift. The procedure is based on common ideas for drift correction, which the authors combine to a comprehensive step-by-step description of how to measure drift velocities in all three dimensions and how to correct the images using these velocities. The presented method does not require any knowledge about size or shape of the imaged structures. Thus, it is applicable to any type of scanning probe microscopy image, including images lacking periodic structures. Besides providing a simple, ready-to-use description of lateral and vertical drift correction, they derive all formulas needed from the model of linear drift. (C) 2010 American Vacuum Society. [DOI: 10.1116/1.3360909

Organisation and ordering of 1D porphyrin polymers synthesised by on-surface Glaser coupling

One-dimensional polymer chains consisting of π-conjugated porphyrin units are formed via Glaser coupling on a Ag(111) surface. Scanning probe microscopy reveals the covalent structure of the products and their ordering. The conformational flexibility within the chains is investigated via a comparision of room temperature and cryogenic measurements

Direct Visualization of Molecule Deprotonation on an Insulating Surface

Kittelmann M, Rahe P, Gourdon A, Kühnle A. Direct Visualization of Molecule Deprotonation on an Insulating Surface. ACS Nano. 2012;6(8):7406-7411.Elucidating molecular-scale details of basic reaction steps on surfaces is decisive for a fundamental understanding of molecular reactivity within many fields, including catalysis and on-surface synthesis. Here, the deprotonation of 2,5-dihydroxybenzoic acid (DHBA) deposited onto calcite (101;4) held at room temperature is followed in situ by noncontact atomic force microscopy. After deposition, the molecules form two coexisting phases, a transient striped phase and a stable dense phase. A detailed analysis of high-resolution noncontact atomic force microscopy images indicates the transient striped phase being a bulk-like phase, which requires hydrogen bonds between the carboxylic acid moieties to be formed. With time, the striped phase transforms into the dense phase, which is explained by the deprotonation of the molecules. In the deprotonated state, the molecules can no longer form hydrogen bonds, but anchor to the surface calcium cations with their negatively charged carboxylate group. The deprotonation step is directly confirmed by Kelvin probe force microscopy images that unravel the change in the molecular charge

Frequency-modulated atomic force microscopy operation by imaging at the frequency shift minimum: The dip-df mode

Rode S, Schreiber M, Kühnle A, Rahe P. Frequency-modulated atomic force microscopy operation by imaging at the frequency shift minimum: The dip-df mode. Review of Scientific Instruments. 2014;85(4):43707.In frequency modulated non-contact atomic force microscopy, the change of the cantilever frequency (Delta f) is used as the input signal for the topography feedback loop. Around the Delta f(z) minimum, however, stable feedback operation is challenging using a standard proportional-integral-derivative (PID) feedback design due to the change of sign in the slope. When operated under liquid conditions, it is furthermore difficult to address the attractive interaction regime due to its often moderate peakedness. Additionally, the Delta f signal level changes severely with time in this environment due to drift of the cantilever frequency f(0) and, thus, requires constant adjustment. Here, we present an approach overcoming these obstacles by using the derivative of Delta f with respect to z as the input signal for the topography feedback loop. Rather than regulating the absolute value to a preset setpoint, the slope of the Delta f with respect to z is regulated to zero. This new measurement mode not only makes the minimum of the Delta f(z) curve directly accessible, but it also benefits from greatly increased operation stability due to its immunity against f(0) drift. We present isosurfaces of the Delta f minimum acquired on the calcite CaCO3(1014) surface in liquid environment, demonstrating the capability of our method to image in the attractive tip-sample interaction regime. (C) 2014 AIP Publishing LLC

Combined NC-AFM and DFT study of the adsorption geometry of trimesic acid on rutile TiO2(110)

Greuling A, Rahe P, Kaczmarski M, Kühnle A, Rohlfing M. Combined NC-AFM and DFT study of the adsorption geometry of trimesic acid on rutile TiO2(110). Journal of Physics : Condensed Matter. 2010;22(34): 345008.The adsorption behavior of trimesic acid (TMA) on rutile TiO2(110) is studied by means of non-contact atomic force microscopy (NC-AFM) and density-functional theory (DFT). Upon low-coverage adsorption at room temperature, NC-AFM imaging reveals individual molecules, centered above the surface titanium rows. Based on the NC-AFM results alone it is difficult to deduce whether the molecules are lying flat or standing upright on the surface. To elucidate the detailed adsorption geometry, we perform DFT calculations, considering a large number of different adsorption positions. Our DFT calculations suggest that single TMA molecules adsorb with the benzene ring parallel to the surface plane. In this configuration, two carboxylic groups can anchor to the surface in a bidentate fashion with the oxygen atoms binding to surface titanium atoms while the hydrogen atoms approach oxygen atoms within the bridging oxygen rows. The most favorable adsorption position is obtained in the presence of a hydroxyl defect, allowing for additional binding of the third carboxylic group