Ordering of Zn-centered porphyrin and phthalocyanine on TiO2(011): STM studies

Zn(II)phthalocyanine molecules (ZnPc) were thermally deposited on a rutile TiO₂(011) surface and on Zn(II)meso-tetraphenylporphyrin (ZnTPP) wetting layers at room temperature and after elevated temperature thermal processing. The molecular homo- and heterostructures were characterized by high-resolution scanning tunneling microscopy (STM) at room temperature and their geometrical arrangement and degree of ordering are compared with the previously studied copper phthalocyanine (CuPc) and ZnTPP heterostructures. It was found that the central metal atom may play some role in ordering and growth of phthalocyanine/ZnTPP heterostructures, causing differences in stability of upright standing ZnPc versus CuPc molecular chains at given thermal annealing conditions.

Surface science at the PEARL beamline of the Swiss Light Source

The Photo-Emission and Atomic Resolution Laboratory (PEARL) is a new soft X-ray beamline and surface science laboratory at the Swiss Light Source. PEARL is dedicated to the structural characterization of local bonding geometry at surfaces and interfaces of novel materials, in particular of molecular adsorbates, nanostructured surfaces, and surfaces of complex materials. The main experimental techniques are soft X-ray photoelectron spectroscopy, photoelectron diffraction, and scanning tunneling microscopy (STM). Photoelectron diffraction in angle-scanned mode measures bonding angles of atoms near the emitter atom, and thus allows the orientation of small molecules on a substrate to be determined. In energy scanned mode it measures the distance between the emitter and neighboring atoms; for example, between adsorbate and substrate. STM provides complementary, real-space information, and is particularly useful for comparing the sample quality with reference measurements. In this article, the key features and measured performance data of the beamline and the experimental station are presented. As scientific examples, the adsorbate-substrate distance in hexagonal boron nitride on Ni(111), surface quantum well states in a metal-organic network of dicyano-anthracene on Cu(111), and circular dichroism in the photoelectron diffraction of Cu(111) are discussed.

Scanning probe microscopy studies on the adsorption of selected molecular dyes on titania

Titanium dioxide, or titania, sensitized with organic dyes is a very attractive platform for photovoltaic applications. In this context, the knowledge of properties of the titania-sensitizer junction is essential for designing efficient devices. Consequently, studies on the adsorption of organic dyes on titania surfaces and on the influence of the adsorption geometry on the energy level alignment between the substrate and an organic adsorbate are necessary. The method of choice for investigating the local environment of a single dye molecule is high-resolution scanning probe microscopy. Microscopic results combined with the outcome of common spectroscopic methods provide a better understanding of the mechanism taking place at the titania-sensitizer interface. In the following paper, we review the recent scanning probe microscopic research of a certain group of molecular assemblies on rutile titania surfaces as it pertains to dye-sensitized solar cell applications. We focus on experiments on adsorption of three types of prototypical dye molecules, i.e., perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA), phtalocyanines and porphyrins. Two interesting heteromolecular systems comprising molecules that are aligned with the given review are discussed as well.

Morphology Change of C60 Islands on Organic Crystals Observed by Atomic Force Microscopy

Organic-organic heterojunctions are nowadays highly regarded materials for light-emitting diodes, field-effect transistors, and photovoltaic cells with the prospect of designing low-cost, flexible, and efficient electronic devices.1-3 However, the key parameter of optimized heterojunctions relies on the choice of the molecular compounds as well as on the morphology of the organic-organic interface,4 which thus requires fundamental studies. In this work, we investigated the deposition of C60 molecules at room temperature on an organic layer compound, the salt bis(benzylammonium)bis(oxalato)cupurate(II), by means of noncontact atomic force microscopy. Three-dimensional molecular islands of C60 having either triangular or hexagonal shapes are formed on the substrate following a "Volmer-Weber" type of growth. We demonstrate the dynamical reshaping of those C60 nanostructures under the local action of the AFM tip at room temperature. The dissipated energy is about 75 meV and can be interpreted as the activation energy required for this migration process.

Ordered heteromolecular overlayers formed by metal phthalocyanines and porphyrins on rutile titanium dioxide surface studied at room temperature

Molecular heterostructures are formed from meso-tetraphenyl porphyrins-Zn(II) (ZnTPP) and Cu(II)-phthalocyanines (CuPc) on the rutile TiO2(011) surface. We demonstrate that ZnTPP molecules form a quasi-ordered wetting layer with flat-lying molecules, which provides the support for growth of islands comprised of upright CuPc molecules. The incorporation of the ZnTPP layer and the growth of heterostructures increase the stability of the system and allow for room temperature scanning tunneling microscopy (STM) measurements, which is contrasted with unstable STM probing of only CuPc species on TiO2. We demonstrate that within the CuPc layer the molecules arrange in two phases and we identify molecular dimers as basic building blocks of the dominant structural phase.

Single-Molecule Tribology: Force Microscopy Manipulation of a Porphyrin Derivative on a Copper Surface

The low-temperature mechanical response of a single porphyrin molecule attached to the apex of an atomic force microscope (AFM) tip during vertical and lateral manipulations is studied. We find that approach-retraction cycles as well as surface scanning with the terminated tip result in atomic-scale friction patterns induced by the internal reorientations of the molecule. With a joint experimental and computational effort, we identify the dicyanophenyl side groups of the molecule interacting with the surface as the dominant factor determining the observed frictional behavior. To this end, we developed a generalized Prandtl-Tomlinson model parametrized using density functional theory calculations that includes the internal degrees of freedom of the side group with respect to the core and its interactions with the underlying surface. We demonstrate that the friction pattern results from the variations of the bond length and bond angles between the dicyanophenyl side group and the porphyrin backbone as well as those of the CN group facing the surface during the lateral and vertical motion of the AFM tip.

Large area scanning probe microscope in ultra-high vacuum demonstrated for electrostatic force measurements on high-voltage devices

BACKGROUND

The resolution in electrostatic force microscopy (EFM), a descendant of atomic force microscopy (AFM), has reached nanometre dimensions, necessary to investigate integrated circuits in modern electronic devices. However, the characterization of conducting or semiconducting power devices with EFM methods requires an accurate and reliable technique from the nanometre up to the micrometre scale. For high force sensitivity it is indispensable to operate the microscope under high to ultra-high vacuum (UHV) conditions to suppress viscous damping of the sensor. Furthermore, UHV environment allows for the analysis of clean surfaces under controlled environmental conditions. Because of these requirements we built a large area scanning probe microscope operating under UHV conditions at room temperature allowing to perform various electrical measurements, such as Kelvin probe force microscopy, scanning capacitance force microscopy, scanning spreading resistance microscopy, and also electrostatic force microscopy at higher harmonics. The instrument incorporates beside a standard beam deflection detection system a closed loop scanner with a scan range of 100 μm in lateral and 25 μm in vertical direction as well as an additional fibre optics. This enables the illumination of the tip-sample interface for optically excited measurements such as local surface photo voltage detection.

RESULTS

We present Kelvin probe force microscopy (KPFM) measurements before and after sputtering of a copper alloy with chromium grains used as electrical contact surface in ultra-high power switches. In addition, we discuss KPFM measurements on cross sections of cleaved silicon carbide structures: a calibration layer sample and a power rectifier. To demonstrate the benefit of surface photo voltage measurements, we analysed the contact potential difference of a silicon carbide p/n-junction under illumination.

Characterization of individual molecular adsorption geometries by atomic force microscopy: Cu-TCPP on rutile TiO2 (110)

Functionalized materials consisting of inorganic substrates with organic adsorbates play an increasing role in emerging technologies like molecular electronics or hybrid photovoltaics. For such applications, the adsorption geometry of the molecules under operating conditions, e.g., ambient temperature, is crucial because it influences the electronic properties of the interface, which in turn determine the device performance. So far detailed experimental characterization of adsorbates at room temperature has mainly been done using a combination of complementary methods like photoelectron spectroscopy together with scanning tunneling microscopy. However, this approach is limited to ensembles of adsorbates. In this paper, we show that the characterization of individual molecules at room temperature, comprising the determination of the adsorption configuration and the electrostatic interaction with the surface, can be achieved experimentally by atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM). We demonstrate this by identifying two different adsorption configurations of isolated copper(ii) meso-tetra (4-carboxyphenyl) porphyrin (Cu-TCPP) on rutile TiO2 (110) in ultra-high vacuum. The local contact potential difference measured by KPFM indicates an interfacial dipole due to electron transfer from the Cu-TCPP to the TiO2. The experimental results are verified by state-of-the-art first principles calculations. We note that the improvement of the AFM resolution, achieved in this work, is crucial for such accurate calculations. Therefore, high resolution AFM at room temperature is promising for significantly promoting the understanding of molecular adsorption.

Electrospray deposition of organic molecules on bulk insulator surfaces

Large organic molecules are of important interest for organic-based devices such as hybrid photovoltaics or molecular electronics. Knowing their adsorption geometries and electronic structures allows to design and predict macroscopic device properties. Fundamental investigations in ultra-high vacuum (UHV) are thus mandatory to analyze and engineer processes in this prospects. With increasing size, complexity or chemical reactivity, depositing molecules by thermal evaporation becomes challenging. A recent way to deposit molecules in clean conditions is Electrospray Ionization (ESI). ESI keeps the possibility to work with large molecules, to introduce them in vacuum, and to deposit them on a large variety of surfaces. Here, ESI has been successfully applied to deposit triply fused porphyrin molecules on an insulating KBr(001) surface in UHV environment. Different deposition coverages have been obtained and characterization of the surface by in-situ atomic force microscopy working in the non-contact mode shows details of the molecular structures adsorbed on the surface. We show that UHV-ESI, can be performed on insulating surfaces in the sub-monolayer regime and to single molecules which opens the possibility to study a variety of complex molecules.

Chain-like structure elements in Ni40Ta60 metallic glasses observed by scanning tunneling microscopy

The structure of metallic glasses is a long-standing question because the lack of long-range order makes diffraction based techniques difficult to be applied. Here, we used scanning tunneling microscopy with large tunneling resistance of 6 GΩ at low temperature in order to minimize forces between probe and sample and reduce thermal fluctuations of metastable structures. Under these extremely gentle conditions, atomic structures of Ni40Ta60 metallic glasses are revealed with unprecedented lateral resolution. In agreement with previous models and experiments, icosahedral-like clusters are observed. The clusters show a high degree of mobility, which explains the need of low temperatures for stable imaging. In addition to icosahedrons, chain-like structures are resolved and comparative density functional theory (DFT) calculations confirm that these structures are meta-stable. The co-existence of icosahedral and chain-like structures might be an key ingredient for the understanding of the mechanical properties of metallic glasses.

Transformations of PTCDA structures on rutile TiO2 induced by thermal annealing and intermolecular forces

Transformations of molecular structures formed by perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) molecules on a rutile TiO2(110) surface are studied with low-temperature scanning tunnelling microscopy. We demonstrate that metastable molecular assemblies transform into differently ordered structures either due to additional energy provided by thermal annealing or when the influence of intermolecular forces is increased by the enlarged amount of deposited molecules. Proper adjustment of molecular coverage and substrate temperature during deposition allows for fabrication of desired assemblies. Differences between PTCDA/TiO2(110) and PTCDA/TiO2(011) systems obtained through identical experimental procedures are discussed.

Local detection of nitrogen-vacancy centers in a nanodiamond monolayer

Nitrogen-vacancy defect centers (NV) contained in nanodiamonds (NDs) are a promising candidate in quantum information processing and single photon sources due to the capability of controlling their assembly on various surfaces. However, their detection with traditional optical techniques becomes challenging when probing high NV densities at the nanometer scale. Here, we combine scanning probe techniques to characterize in a monolayer the structural and electronic properties of bucky-diamonds with sizes below 10 nm. We further observe by light-assisted Kelvin- and scanning tunneling spectroscopy a clear signature of negatively charged subsurface NV centers in NDs at the nanoscale where conventional techniques are limited.

Obtaining detailed structural information about supramolecular systems on surfaces by combining high-resolution force microscopy with ab initio calculations

State-of-the art experimental techniques such as scanning tunneling microscopy have great difficulties in extracting detailed structural information about molecules adsorbed on surfaces. By combining atomic force microscopy and Kelvin probe force microscopy with ab initio calculations, we demonstrate that we can obtain a wealth of detailed structural information about the molecule itself and its environment. Studying an FFPB molecule on a gold surface, we are able to determine its exact location on the surface, the nature of its bonding properties with neighboring molecules that lead to the growth of one-dimensional strips, and the internal torsions and bendings of the molecule.

Kelvin probe force microscopy of nanocrystalline TiO2 photoelectrodes

Dye-sensitized solar cells (DSCs) provide a promising third-generation photovoltaic concept based on the spectral sensitization of a wide-bandgap metal oxide. Although the nanocrystalline TiO2 photoelectrode of a DSC consists of sintered nanoparticles, there are few studies on the nanoscale properties. We focus on the microscopic work function and surface photovoltage (SPV) determination of TiO2 photoelectrodes using Kelvin probe force microscopy in combination with a tunable illumination system. A comparison of the surface potentials for TiO2 photoelectrodes sensitized with two different dyes, i.e., the standard dye N719 and a copper(I) bis(imine) complex, reveals an inverse orientation of the surface dipole. A higher surface potential was determined for an N719 photoelectrode. The surface potential increase due to the surface dipole correlates with a higher DSC performance. Concluding from this, microscopic surface potential variations, attributed to the complex nanostructure of the photoelectrode, influence the DSC performance. For both bare and sensitized TiO2 photoelectrodes, the measurements reveal microscopic inhomogeneities of more than 100 mV in the work function and show recombination time differences at different locations. The bandgap of 3.2 eV, determined by SPV spectroscopy, remained constant throughout the TiO2 layer. The effect of the built-in potential on the DSC performance at the TiO2/SnO2:F interface, investigated on a nanometer scale by KPFM measurements under visible light illumination, has not been resolved so far.

Energy loss triggered by atomic-scale lateral force

We perform bimodal atomic force microscopy measurements on a Br-doped NaCl (001) surface to investigate the mechanisms behind frequency shift and energy dissipation contrasts. The peculiar pattern of the dissipated energy in the torsional channel, related to frictional processes, is increased at the positions of Br impurities, otherwise indistinguishable from Cl ions in the other measured channels. Our simulations reveal how the energy dissipates by the rearrangement of the tip apex and how the process is ultimately governed by lateral forces. Even the slightest change in lateral forces, induced by the presence of a Br impurity, is enough to trigger the apex reconstruction more often, thus increasing the dissipation contrast; the predicted dissipation pattern and magnitude are in good quantitative agreement with the measurements.

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(CO(3))(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 CO(2) 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.

Measuring electric field induced subpicometer displacement of step edge ions

We provide unambiguous evidence that the applied electrostatic field displaces step atoms of ionic crystal surfaces by subpicometers in different directions via the measurement of the lateral force interactions by bimodal dynamic force microscopy combined with multiscale theoretical simulations. Such a small imbalance in the electrostatic interaction of the shifted anion-cation ions leads to an extraordinary long-range feature potential variation and is now detectable with the extreme sensitivity of the bimodal detection.

Directed rotations of single porphyrin molecules controlled by localized force spectroscopy

Directed molecular repositioning is a key step toward the build up of molecular machines. To artificially generate and control the motion of molecules on a surface, excitations by light, chemical, or electrical energy have been demonstrated. Here, the application of local mechanical forces is implemented to achieve directed rotations of molecules. Three-dimensional force spectroscopy with sub-Ångström precision is used to characterize porphyrin derivatives with peripheral carbonitrile groups. Extremely small areas on these molecules (≈ 100 × 100 pm(2)) are revealed which can be used to control rotations. In response to the local mechanical forces, the molecular structure elastically deforms and then changes its conformation, which leads to its rotation. Depending on the selection of one of four submolecular areas, the molecule is either rotated clockwise or counterclockwise.

Design and performance of a combined secondary ion mass spectrometry-scanning probe microscopy instrument for high sensitivity and high-resolution elemental three-dimensional analysis

State-of-the-art secondary ion mass spectrometry (SIMS) instruments allow producing 3D chemical mappings with excellent sensitivity and spatial resolution. Several important artifacts however arise from the fact that SIMS 3D mapping does not take into account the surface topography of the sample. In order to correct these artifacts, we have integrated a specially developed scanning probe microscopy (SPM) system into a commercial Cameca NanoSIMS 50 instrument. This new SPM module, which was designed as a DN200CF flange-mounted bolt-on accessory, includes a new high-precision sample stage, a scanner with a range of 100 μm in x and y direction, and a dedicated SPM head which can be operated in the atomic force microscopy (AFM) and Kelvin probe force microscopy modes. Topographical information gained from AFM measurements taken before, during, and after SIMS analysis as well as the SIMS data are automatically compiled into an accurate 3D reconstruction using the software program "SARINA," which was developed for this first combined SIMS-SPM instrument. The achievable lateral resolutions are 6 nm in the SPM mode and 45 nm in the SIMS mode. Elemental 3D images obtained with our integrated SIMS-SPM instrument on Al/Cu and polystyrene/poly(methyl methacrylate) samples demonstrate the advantages of the combined SIMS-SPM approach.