Force Microscopy In Ultrahigh Vacuum Research Articles

Terphenylthiazole-based self-assembled monolayers on cobalt with high conductance photo-switching ratio for spintronics.

Two new photo-switchable terphenylthiazole molecules are synthesized and self-assembled as monolayers on Au and on ferromagnetic Co electrodes. The electron transport properties probed by conductive atomic force microscopy in ultra-high vacuum reveal a larger conductance of the light-induced closed (c) form than for the open (o) form. We report an unprecedented conductance ratio of up to 380 between the closed and open forms on Co for the molecule with the anchoring group (thiol) on the side of the two N atoms of the thiazole unit. This result is rationalized by Density Functional Theory (DFT) calculations coupled to the Non-Equilibrium Green's function (NEGF) formalism. These calculations show that the high conductance in the closed form is due to a strong electronic coupling between the terphenylthiazole molecules and the Co electrode that manifests by a resonant transmission peak at the Fermi energy of the Co electrode with a large broadening. This behavior is not observed for the same molecules self-assembled on gold electrodes. These high conductance ratios make these Co-based molecular junctions attractive candidates to develop and study switchable molecular spintronic devices.

Time- and Temperature-Dependent Growth Behavior of Ionic Liquids on Au(111) Studied by Atomic Force Microscopy in Ultrahigh Vacuum.

We deposited defined amounts of [C1C1Im][Tf2N] on Au(111) at different temperatures and investigated the morphology and wetting behavior of the deposited films by atomic force microscopy. For multilayer coverages, we observe a drastically different growth behavior when comparing deposition at room temperature (RT) and deposition below 170 K followed by slow annealing to RT. Upon deposition at RT, we find the formation of 2–30 nm high and 50–500 nm wide metastable 3D droplets on top of a checkerboard-type wetting layer. These droplets spread out into stable 2D bilayers, on the time scale of hours and days. The same 2D bilayer structure is obtained after deposition below 170 K and slow annealing to RT. We present a statistical analysis on the time-dependent changes of the shape and volume of the 3D droplets and the 2D bilayers. We attribute the stabilization of the 2D bilayers on the wetting layer and on already formed bilayers to the high degree of order in these layers. Notably, the transformation process from the 3D droplets to 2D bilayer islands is accelerated by tip effects and also X-ray radiation.

Growth of Multilayers of Ionic Liquids on Au(111) Investigated by Atomic Force Microscopy in Ultrahigh Vacuum.

Understanding the growth of ultrathin films of ionic liquids (ILs) on metal surfaces is of highest relevance for a variety of applications. We present a detailed study of the growth of the wetting layer and successive multilayers of 1,3-dimethylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([C1C1Im][Tf2N]) on Au(111). By atomic force microscopy (AFM) in ultrahigh vacuum, we follow the temperature-dependent behavior between 110 and 300 K at defined coverages. We initially observe the formation of a wetting layer with a thickness of ∼0.37 nm with anions and cations arranged in a checkerboard structure. Stable AFM imaging up to 280 K allows us to follow the IL growing on top of the wetting layer in bilayers with an average thickness of ∼0.71 nm, that is, double the height of the wetting layer, in a bilayer-by-bilayer fashion. This growth behavior is independently confirmed from the surface morphology, as deduced from AFM and angle-resolved X-ray photoelectron spectroscopy. High-resolution AFM images at 110 K allow for identifying the molecular surface structure of the bilayers as a striped phase, which is one of the phases also seen for the wetting layer (Meusel, M.; Lexow, M.; Gezmis, A.; Schotz, S.; Wagner, M.; Bayer, A.; Maier, F.; Steinrück, H. P. Atomic Force and Scanning Tunneling Microscopy of Ordered Ionic Liquid Wetting Layers from 110 K up to Room Temperature. ACS Nano2020, 14, 9000–9010).

Polarization-dependent local conductivity and activation energy in KTiOPO4

We study the local conductivity properties of KTiOPO4 using conductive atomic force microscopy in ultrahigh vacuum (UHV). We show that domains with opposite orientations have different conductivity values. We investigate the temperature dependence of the local conductivity and report a difference in the activation energy of 25% between different domains. Furthermore, we show that the local conductivity increases with the number of biased-scans. Finally, it is found that the domain wall conductivity previously observed at ambient conditions vanishes in UHV. Our results are discussed in terms of the screening effects and surface conditions.

Noncontact AFM and differential reflectance spectroscopy joint analyses of bis-pyrenyl thin films on bulk insulators: Relationship between structural and optical properties

The combined characterization of structural and optical properties of organic interfaces adsorbed on inorganic solid substrates down to the molecular scale is crucial from a fundamental point of view, but also if one tackles efficient applied devices. In this work, a set of joint structural and optical analyses of self-assemblies of $\ensuremath{\pi}$-conjugated bis-pyrene derivatives upon adsorption on two alkali halides bulk insulators is reported. The structural analysis is performed by means of noncontact atomic force microscopy in ultrahigh vacuum either at room or at the liquid-nitrogen temperature with molecular resolution. The surface coverage ranges from the submonolayer (ML) regime up to 5 ML. In situ optical spectroscopy is performed by means of differential reflectance (DR) spectroscopy. A thorough fitting methodology of the DR spectra allows us to derive the complete dielectric function of the molecular adlayers treated in an anisotropic formalism, albeit restricted to an uniaxial approximation. Conclusions regarding the process of condensation of the molecules into H aggregates from its early stages up to the solid molecular phase are drawn. This work highlights three main reasons to bridge high-resolution structural and optical characterization of the molecular layers, which all point towards the necessity to constrain the fitting process, namely, (i) characterizing the growth mode of the molecules, (ii) identifying the structural order of the resulting assemblies, and (iii) discriminating their constitutive phases by means of molecular resolution imaging.

Electrical properties of Molecular Beam Epitaxy grown Barium Titanate probed by conductive Atomic Force Microscopy

The electrical properties of Barium Titanate 60nm thick layers grown by Molecular Beam Epitaxy has been probed at the nanoscale by Conductive-Atomic Force Microscopy in Ultra-High Vacuum and in air, and the results compared to electrical measurements obtained on large electrodes. The role of the atmosphere of measurement is clearly highlighted by the absence of resistive memory effect when the measurements are operated in air. Considering the possibly high concentration of oxygen vacancies in such samples, we conclude that the difference might be explained by the annihilation of the oxygen vacancies by the oxygen of the environment. This emphasises the need to control precisely the atmosphere of measurement when probing the electrical properties of such thin dielectric films by atomic force microscope.

Contrast in nanoscale friction between rotational domains of graphene on Pt(111)

The nanoscale lubrication properties of graphene depend on chemical functionalization, adsorbants, and bonding to the substrate. The dependence of friction on the rotational orientation of graphene on a Pt(111) surface was studied by high-resolution friction force microscopy in ultra-high vacuum and interpreted through complementary simulations. Lateral forces reveal an atomic-scale stick-slip motion with the periodicity of the graphene structure. Additionally, the lateral forces were modulated by a Moiré pattern, which depends on the rotation of the respective domain. Comparing the experimental results to simulations of the friction process based on the Prandtl-Tomlinson model and to atomistic simulations of the tip-sample interactions, it was found that the modulation of lateral forces originated from a structural mismatch between Pt(111) and graphene domains that were locally pinned to the substrate. Domains with preferred orientations, such as the R1° and the R24°, appeared to exhibit lower average friction than those with orientations less frequently observed, such as the R10° domain.

Nc-AFM Imaging and Manipulation of a Triphenylene Derivative on KBr(001)

The adsorption of single hexamethoxyltriphenylene molecules on KBr(001) at low temperature (T < 10 K) was studied by low-temperature (T = 5 K) noncontact atomic force microscopy in ultrahigh vacuum. Images of the frequency shift, measured at constant height, show intramolecular features that are interpreted on the basis of molecular force-field calculations. The main conclusion is that the experimental contrast originates from the electrostatic interaction between a negatively terminated ionic tip and the atomic charges of the molecule. The main trends of the experimental observations of lateral manipulations could also be reproduced by the same method, giving some insight into the manipulation mechanisms. In particular, it is shown that the strong interaction of the oxygen atoms of the molecule with positive ions of the tip plays a major role by facilitating the partial lifting of the molecule from the substrate during the intermediate stages of these manipulation processes.

Crossing Thermal Lubricity and Electronic Effects in Friction: Vanadium Dioxide under the Metal–Insulator Transition

The remarkable turnover of friction on a vanadium dioxide (VO2) surface driven by the metal–insulator transition is revealed using temperature‐variable atomic force microscopy in ultrahigh vacuum. Phononic and electronic contributions are known as two major components in mediating friction energy dissipation. Here, a VO2 thin film is prepared on a silicon wafer with preferential orientations of (100) and (120) in the monoclinic phase using pulsed laser deposition. Corresponding friction and conductivity images show that friction decreases below the critical temperature, above which two trends are seen as the temperature increases: less friction on the insulating domains and higher friction on the metallic domains. This distinct temperature dependence of friction is attributed to the combined effects of thermal lubricity and electronic contributions. This study indicates the promising potential for vanadium oxide to tune friction in the electric regime as well as with temperature.

Effective Mg:Ag / MoO3 recombination zone for tandem organic photovoltaic devices

ABSTRACTWe demonstrate an effective recombination zone consisting of Mg:Ag (1:3) alloy and MoO3 layers with 0.8 nm and 3 nm respectively for application in tandem organic photovoltaic devices based on zinc phthalocyanine (ZnPc) donor and fullerene C60 acceptor. The Mg:Ag layer ensures an optimum electron selectivity, while MoO3 layer effectively selects holes. A conversion efficiency of 2.2% has been achieved under an illumination of 100 mW/cm2 at room temperature. The open circuit voltage of 810 mV is close to the sum of the open circuit voltages of the constituent single cells. The recombination Mg:Ag-MoO3 layer system is investigated with regard to the requirements of high optical transparency, work function compatibility, and facilitation of light absorption. The respective characterizations were carried out by UV-Visible spectroscopy, Kelvin probe force microscopy in ultrahigh vacuum, current-voltage and external quantum efficiency methods.

Minimum Threshold for Incipient Plasticity in the Atomic-Scale Nanoindentation of Au(111)

The formation of the smallest permanent indentation in a Au(111) surface is studied by scanning tunneling microscopy and atomic force microscopy in ultrahigh vacuum. The 9.5nm radius W(111) indenter was characterized insitu by field ion microscopy. Elastic and plastic indentations are identified both in the residual impression image and by features in their force-displacement curves such as the sink-in depth, pop-ins, and hysteresis energy. Plasticity is best identified quantitatively in the force-displacement curves by the sink-in depth. The minimum of plastic damage producible in the substrate is associated with an energy budget of ∼70 eV.

Elastic Response of Graphene Nanodomes

The mechanical behavior of a periodically buckled graphene membrane has been investigated by noncontact atomic force microscopy in ultrahigh vacuum. When a graphene monolayer is grown on Ru(0001), a regular arrangement of 0.075 nm high nanodomes forming a honeycomb lattice with 3 nm periodicity forms spontaneously. This structure responds in a perfectly reversible way to relative normal displacements up to 0.12 nm. Indeed, the elasticity of the nanodomes is proven by realistic DFT calculations, with an estimated normal stiffness k∼40 N/m. Our observations extend previous results on macroscopic graphene samples and confirm that the elastic behavior of this material is maintained down to nanometer length scales, which is important for the development of new high-frequency (terahertz) electromechanical devices.

Systematic study of the dolomite (104) surface by bimodal dynamic force microscopy in ultra-high vacuum

We have investigated the morphology and structure of dolomite MgCa(CO3)2(104) surfaces by bimodal dynamic force microscopy with flexural and torsional resonance modes in ultra-high vacuum at room temperature. We found that the surface slowly decomposes by degassing CO2 in a vacuum and becomes covered by amorphous clusters, presumably MgO and CaO. By choosing an optimal sample preparation procedure (i.e. cleaving in a vacuum and mild annealing for stabilizing clusters for a short time), atomically clean surfaces were obtained. The complex tip–sample interaction, arising from carbonate groups and Mg and Ca atoms of the surface, induces a large variety of atomic-scale imaging features.

Quantitative multichannel NC-AFM data analysis of graphene growth on SiC(0001)

Noncontact atomic force microscopy provides access to several complementary signals, such as topography, damping, and contact potential. The traditional presentation of such data sets in adjacent figures or in colour-coded pseudo-three-dimensional plots gives only a qualitative impression. We introduce two-dimensional histograms for the representation of multichannel NC-AFM data sets in a quantitative fashion. Presentation and analysis are exemplified for topography and contact-potential data for graphene grown epitaxially on 6H-SiC(0001), as recorded by Kelvin probe force microscopy in ultrahigh vacuum. Sample preparations by thermal decomposition in ultrahigh vacuum and in an argon atmosphere are compared and the respective growth mechanisms discussed.

Influence of charge transfer doping on the morphologies of C60islands on hydrogenated diamond C(100)-(2×1)

The adsorption and island formation of C${}_{60}$ fullerenes on the hydrogenated C(100)-($2\ifmmode\times\else\texttimes\fi{}1$):H diamond surface is studied using high-resolution noncontact atomic force microscopy in ultrahigh vacuum. At room temperature, C${}_{60}$ fullerene molecules assemble into monolayer islands, exhibiting a hexagonally close-packed internal structure. Dewetting is observed when raising the substrate temperature above approximately 505 K, resulting in two-layer high islands. In contrast to the monolayer islands, these double-layer islands form extended wetting layers. This peculiar behavior is explained by an increased molecule-substrate binding energy in the case of double-layer islands, which originates from charge transfer doping. Only upon further increasing the substrate temperature to approximately 615 K, the wetting layer desorbs, corresponding to a binding energy of the charge transfer-stabilized film of 1.7 eV.

Domain structures of single layer graphene imaged with conductive probe atomic force microscopy

We report nanoscale domain boundaries on single layer graphene probed with conductive probe atomic force microscopy in ultrahigh vacuum. Graphene was prepared using the inductively coupled plasma chemical vapor deposition technique on a copper substrate. Current mapping revealed domains 50–100 nm in size on the single layer graphene. Stick slip images revealed a hexagonal structure of the graphene layer in the middle of the domain. The conductance on the domain boundary is lower than that inside the domain structure, which is associated with disorder on the boundary, while friction and adhesion measurements on the domain boundaries did not show any contrast. We found that the conductance contrast is prominent at high loads, suggesting the domain boundary plays a significant role in the charge transport properties of graphene under mechanical deformation. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Pre-breakdown negative differential resistance in thin oxide film: Conductive-atomic force microscopy observation and modelling

Negative differential resistance in thin nitrated silicon oxide and silicon oxide films is observed during current voltage measurements using conductive atomic force microscopy in ultra-high vacuum if the voltage sweep direction is reversed just before the dielectric breakdown voltage is reached. Current voltage characteristics are discussed using a trap assisted tunneling model. The resulting concentration of traps needed in order to reproduce the experimental characteristic leads to the consideration of a filamentlike conduction model, rather than a trap assisted tunneling model in which traps are randomly generated. The model reproduces well the effect of the voltage ramp parameters on the experimental current voltage characteristics, including the negative differential resistance. The effect of the growth dynamic parameters on the current voltage characteristics and the occurrence of the pre-breakdown negative differential resistance are discussed with the help of the model.

Probing nanoscale conductance of monolayer graphene under pressure

The correlation between charge transport and mechanical deformation of the single layer graphene layer was studied with conductive probe atomic force microscopy/friction force microscopy in ultra-high vacuum. By measuring the current and friction on a graphene layer that is under mechanical stress, we identify crossover of two regimes in the current density that depend on the applied pressure. We suggest that the difference in work function under mechanical deformation as well as a change in the density of state and formation of a dipole field are responsible for this crossover behavior.

Correlation of adhesion force and electrical conductivity in Magnéli-type vanadium oxides and highly oriented pyrolytic graphite

A correlation of adhesion force and electrical conductivity is established for a vanadium oxide and highly oriented pyrolytic graphite (HOPG). Adhesion forces were determined by analysis of pull-off forces obtained from force–distance curves with atomic force microscopy in ultrahigh vacuum, on clean surfaces and with defined contact conditions. The investigated samples include (i) different stoichiometries of Magnéli-type vanadium oxides (V3O5, V4O7, V6O11, and V7O13), which undergo metal-insulator-transitions as a function of temperature, (ii) the (0001) basal plane and the (10–10) prism plane of highly oriented pyrolytic graphite (HOPG), which differ in their respective perpendicular-plane electrical conductivities by several orders of magnitude, and (iii) the (100) surfaces of pure metal single crystals, i.e., silver (Ag), copper (Cu), iron (Fe) and gold (Au). It is shown, that the vanadium oxides as well as the graphite exhibit significantly lower adhesion forces in their electrical conductive state than in their non-electrical conductive state. The values obtained for the electrical conductive states are quantitatively confirmed by the measurements on the single crystal metals.

Molecular Order and Disorder in the Frictional Response of Alkanethiol Self-Assembled Monolayers

Molecular processes in the frictional response of an alkanethiol monolayer, self-assembled on a Au(111) surface, are studied by means of high-resolution friction force microscopy in ultrahigh vacuum. With increasing load, three regimes are observed on defect-free domains of the monolayer: smooth sliding with negligible friction, regular molecular stick-slip motion with increasing friction, and the onset of wear in the monolayer. Molecular contrast in the lateral force is found for inequivalent molecules within the unit cell of the c(4 × 2) superstructure. Significant differences in the frictional response are found between defect-free domains and areas including a domain boundary. Friction increases by an order of magnitude on domain boundaries in connection with irregular stick-slip motion. This increased friction at domain boundaries is observed at loads below the onset of wear.