Generic nature of long-range repulsion mechanism on a bulk insulator?

Dynamic atomic force microscopy measurements are reported that provide evidence for the presence of long-range repulsion in molecular self-assembly on a bulk insulator surface. We present the structures formed from four different benzoic acid derivatives on the (10.4) cleavage plane of calcite kept in ultra-high vacuum. These molecules have in common that they self-assemble into molecular stripes when deposited onto the surface held at room temperature. For all molecules tested, a detailed analysis of the stripe-to-stripe distance distribution reveals a clear deviation from what would be expected for randomly placed, non-interacting stripes (i.e., geometric distribution). When excluding kinetic effects during growth, this result gives evidence for a long-range repulsion mechanism acting during the assembly of these stripes. The fact that this finding is robust against changes in the molecular structure indicates a generic nature of the observed mechanism, implying a ubiquitous origin such as electrostatic repulsion. Finally, we discuss parameters that might affect the unambiguous observation of this generic repulsion under specific experimental conditions.

Diacetylene polymerization on a bulk insulator surface.

Molecular electronics has great potential to surpass known limitations in conventional silicon-based technologies. The development of molecular electronics devices requires reliable strategies for connecting functional molecules by wire-like structures. To this end, diacetylene polymerization has been discussed as a very promising approach for contacting single molecules with a conductive polymer chain. A major challenge for future device fabrication is transferring this method to bulk insulator surfaces, which are mandatory to decouple the electronic structure of the functional molecules from the support surface. Here, we provide experimental evidence for diacetylene polymerization of 3,3'-(1,3-butadiyne-1,4-diyl)bisbenzoic acid precursors on the (10.4) surface of calcite, a bulk insulator with a band gap of around 6 eV. When deposited on the surface held at room temperature, ordered islands with a (1 × 3) superstructure are observed using dynamic atomic force microscopy. A distinct change is revealed upon heating the substrate to 485 K. After heating, molecular stripes with a characteristic inner structure are formed that excellently match the expected diacetylene polymer chains in appearance and repeat distance. The corresponding density functional theory computations reveal molecular-level bonding patterns of both the (1 × 3) superstructure and the formed striped structure, confirming the assignment of on-surface diacetylene polymerization. Transferring the concept of using diacetylene polymerization for creating conductive connections to bulk insulator surfaces paves the way towards application-relevant systems for future molecular electronic devices.

Modification of a commercial atomic force microscopy for low-noise, high-resolution frequency-modulation imaging in liquid environment.

A key issue for high-resolution frequency-modulation atomic force microscopy imaging in liquids is minimizing the frequency noise, which requires a detailed analysis of the corresponding noise contributions. In this paper, we present a detailed description for modifying a commercial atomic force microscope (Bruker MultiMode V with Nanoscope V controller), aiming at atomic-resolution frequency-modulation imaging in ambient and in liquid environment. Care was taken to maintain the AFMs original stability and ease of operation. The new system builds upon an optimized light source, a new photodiode and an entirely new amplifier. Moreover, we introduce a home-built liquid cell and sample holder as well as a temperature-stabilized isolation chamber dedicated to low-noise imaging in liquids. The success of these modifications is measured by the reduction in the deflection sensor noise density from initially 100 fm/√Hz to around 10 fm/√Hz after modification. The performance of our instrument is demonstrated by atomically resolved images of calcite taken under liquid conditions.

Concept for support and cleavage of brittle crystals.

We report on sample holders for crystals to be cleaved for the preparation of surfaces with large atomically flat terraces. The concept for mounting sample crystals is based on a vicelike clamping mechanism to securely hold the crystal in position while reducing the risk of fragmentation. Sample holders based on this concept and made of suitable materials allow preparation and cleavage of crystals in the ultrahigh vacuum at high or low temperatures. To cleave the crystal, we employ a scalpel blade mounted on a wobble stick to generate a highly localized stress field initiating the cleavage process. The sample holders are used for experiments of highest resolution scanning force microscopy, however, the concept can be transferred to any other system where cleavage faces of crystals are of interest. Exemplarily, scanning force microscopy results demonstrate that (111) cleavage faces of CaF2 crystals can be prepared with steps only a few F-Ca-F triple-layers high and atomically flat terraces extending over areas of several microm2.

How flat is an air-cleaved mica surface?

Muscovite mica is an important mineral that has become a standard substrate, due to its easy cleavage along the {001} planes, revealing a very flat surface that is compatible with many biological materials. Here we study mica surfaces by dynamic atomic force microscopy (AFM) operated in the non-contact mode (NC-AFM) under ultra-high vacuum (UHV) conditions. Surfaces produced by cleaving in UHV cannot be imaged with NC-AFM due to large surface charges; however, cleavage in air yields much less surface charge and allows for NC-AFM imaging. We present highly resolved NC-AFM images of air-cleaved mica surfaces revealing a rough morphology originating from a high density of nanometre-sized particles. Among these particles, we find regularly shaped structures indicating the growth of crystallites on the surface. The contamination layer cannot be removed by degassing in UHV; even prolonged heating at a temperature of 560 K under UHV conditions does not yield an atomically flat surface.

Growth of unidirectional molecular rows of cysteine on Au(110)-(1 x 2) driven by adsorbate-induced surface rearrangements.

Using scanning tunneling microscopy we have studied the nucleation and growth of unidirectional molecular rows upon adsorption of the amino acid cysteine onto the anisotropic Au(110)-(1 x 2) surface under ultrahigh vacuum conditions. By modeling a large variety of possible molecular adsorption geometries using density-functional theory calculations, we find that in the optimum, lowest energy configuration, no significant intermolecular interactions exist along the growth direction. Instead the driving force for formation of the unidirectional molecular rows is an adsorbate-induced surface rearrangement, providing favorable adsorption sites for the molecules.

Anchoring of organic molecules to a metal surface: HtBDC on Cu(110).

The interaction of largish molecules with metal surfaces has been studied by combining the imaging and manipulation capabilities of the scanning tunneling microscope (STM). At the atomic scale, the STM results directly reveal that the adsorption of a largish organic molecule can induce a restructuring of a metal surface underneath. This restructuring anchors the molecules on the substrate and is the driving force for a self-assembly process of the molecules into characteristic molecular double rows.

Busulfan pharmacokinetics in bone marrow transplant patients: is drug monitoring warranted?

Pharmacokinetics were studied in relation to hepatic side-effects in 20 patients (19 adults aged 18-53 years and one child of 11 years) undergoing BMT after conditioning with 1 mg/kg busulfan (every 6 hours for 16 doses). Busulfan was quantitated in plasma samples at 10 time points within the 6 h dosing interval using HPLC before and after dose numbers 1, 2, 5, 13 and 14. For 13 patients data on all five doses are available; for the remaining seven patients three to four doses were studied. Mean maximum concentrations were 1512 ng/ml; mean trough levels for second and subsequent doses were 615 ng/ml. Maxima (Cmax) tended to be lower and times of maxima (Tmax) were later when busulfan was taken with a meal. Correlation of the area under the concentration versus time curve (AUC0-6h) between different doses was low within patients. In several patients problems with compartmental fitting of concentration data were observed mainly caused by the short dosing interval, which made estimates of T1/2 and model derived AUCs unstable. Three patients experienced hepatic veno-occlusive disease; kinetic parameters were not helpful in describing a particulate risk constellation for this subgroup. In our experience, the role of drug monitoring in this setting needs to be defined more clearly.