Francisco Ample

Conductance of a Single Conjugated Polymer as a Continuous Function of Its Length

The development of electronic devices at the single-molecule scale requires detailed understanding of charge transport through individual molecular wires. To characterize the electrical conductance, it is necessary to vary the length of a single molecular wire, contacted to two electrodes, in a controlled way. Such studies usually determine the conductance of a certain molecular species with one specific length. We measure the conductance and mechanical characteristics of a single polyfluorene wire by pulling it up from a Au(111) surface with the tip of a scanning tunneling microscope, thus continuously changing its length up to more than 20 nanometers. The conductance curves show not only an exponential decay but also characteristic oscillations as one molecular unit after another is detached from the surface during stretching.

Voltage-dependent conductance of a single graphene nanoribbon

Graphene nanoribbons could potentially be used to create molecular wires with tailored conductance properties. However, understanding charge transport through a single molecule requires length-dependent conductance measurements and a systematic variation of the electrode potentials relative to the electronic states of the molecule. Here, we show that the conductance properties of a single molecule can be correlated with its electronic states. Using a scanning tunnelling microscope, the electronic structure of a long and narrow graphene nanoribbon, which is adsorbed on a Au(111) surface, is spatially mapped and its conductance then measured by lifting the molecule off the surface with the tip of the microscope. The tunnelling decay length is measured over a wide range of bias voltages, from the localized Tamm states over the gap up to the delocalized occupied and unoccupied electronic states of the nanoribbon. We also show how the conductance depends on the precise atomic structure and bending of the molecule in the junction, illustrating the importance of the edge states and a planar geometry.

Manipulating molecular quantum states with classical metal atom inputs: demonstration of a single molecule NOR logic gate.

Quantum states of a trinaphthylene molecule were manipulated by putting its naphthyl branches in contact with single Au atoms. One Au atom carries 1-bit of classical information input that is converted into quantum information throughout the molecule. The Au-trinaphthylene electronic interactions give rise to measurable energy shifts of the molecular electronic states demonstrating a NOR logic gate functionality. The NOR truth table of the single molecule logic gate was characterized by means of scanning tunnelling spectroscopy.

Imaging Molecular Orbitals by Scanning Tunneling Microscopy on a Passivated Semiconductor

Decoupling the electronic properties of a molecule from a substrate is of crucial importance for the development of single-molecule electronics. This is achieved here by adsorbing pentacene molecules at low temperature on a hydrogenated Si(100) surface (12 K). The low temperature (5 K) scanning tunneling microscope (STM) topography of the single pentacene molecule at the energy of the highest occupied molecular orbital (HOMO) tunnel resonance clearly resembles the native HOMO of the free molecule. The negligible electronic coupling between the molecule and the substrate is confirmed by theoretical STM topography and diffusion barrier energy calculations.

Single Molecular Wires Connecting Metallic and Insulating Surface Areas

Molecular wires are key components for future circuits in molecular electronics. 2] Their utilization in a future electronic device requires the adsorption of the wire on a nonconducting substrate, which makes an electrical characterization on the molecular level challenging. Furthermore, such molecular electronics devices will require ultrathin insulators that electrically separate conducting components, and in particular wires, from each other. Ultrathin films of insulators, such as layers of metal oxides or alkaline halides that are a few atoms thick and grown on conducting substrates retain enough of their insulating properties to act as separators. At the same time, these films are thin enough to ensure for sufficient electron tunneling and thus for imaging of single atoms and molecules by scanning tunneling microscopy (STM). NaCl turns out to be a suitable insulating material; it already exhibits a large band gap as a bilayer, 6] which grows on various metal surfaces in a flat and very homogeneous manner with a low number of defects. The study of single molecules on such films by STM allows to spatially image the orbitals of a single molecule by its reduced and oxidized tunneling resonances as a consequence of the decoupling by the intermediate NaCl film. 18] Single molecules have been studied either on metals or semiconductors or insulating films; that is, on substrates that provide very different environments. However, the adsorption of one and the same molecule on a surface with insulating and conducting areas has never been achieved. Such a planar configuration would provide unique insight into the intramolecular coupling and also access to the molecular wire during charge transport experiments. However, the creation of this particular configuration with large adsorption areas on both materials is an experimentally difficult task. First, it requires very long molecules, which are difficult to synthesize by conventional organic chemistry and can hardly be deposited intact under clean ultra-high vacuum conditions owing to their high molecular weight. To overcome this problem, it is necessary to covalently bind molecular building blocks directly on the surface, which we do by our thermally activated on-surface synthesis. 21] Second, it is even more challenging to make one and the same molecule adsorb simultaneously on different surfaces, as different surface areas exhibit vastly different diffusion properties for the same molecules. For example, molecules are much more mobile on NaCl films than on the metal surface, which requires low sample temperatures to enable adsorption on the NaCl areas upon molecular deposition. However, this condition is in contradiction with the presence of long molecular chains on the surface, because they can only be produced by on-surface synthesis, which requires at least temperatures that allow efficient precursor diffusion, and ideally involves heating of the sample to more than 500 K. The experimental challenge is therefore to connect the two processes, namely organic on-surface synthesis and inorganic crystal growth, which should neither suppress each other nor should they lower the quality of one of the in-situ generated components. Herein, we present a method to adsorb one part of a long molecular wire on a NaCl island whilst the other part of the same wire is located in the metallic area of the surface. We demonstrate how the electronic properties of the wire depend on the local atomic-scale environment. In principle, two preparation steps are required: 1) Covalently bound molecular chains are formed from dibromoterfluorene (DBTF) monomers consisting of three fluorene units carrying lateral methyl groups, and a bromine atom at each end. Very long molecular wires of more than 100 nm length are grown from these molecular precursors on Au(111) at 520 K (Scheme 1). 2) Thermal evaporation of NaCl on clean Au(111) at sample temperatures between 270 and 350 K leads to the formation of crystalline (001)-oriented NaCl islands with rectangular shape, owing to the preferential formation of non-polar step edges. 17] We investigated two methods that differ in the order of these two preparation steps and cause significantly different results; however, both methods yield intact molecules and crystalline NaCl islands (Figure 1). In method I, NaCl growth is initially obtained by evaporation of NaCl on Au(111) held [*] Dr. C. Bombis, L. Lafferentz, Dr. L. Grill Experimental Physics Department, Freie Universit t Berlin and Fritz-Haber-Institut of the Max-Planck-Society 14195 Berlin (Germany) Fax: (+ 49)30-8385-1355 E-mail: leonhard.grill@physik.fu-berlin.de Homepage: www.fhi-berlin.mpg.de/pc

Dangling-bond logic gates on a Si(100)-(2 × 1)-H surface.

Atomic-scale Boolean logic gates (LGs) with two inputs and one output (i.e. OR, NOR, AND, NAND) were designed on a Si(100)-(2 × 1)-H surface and connected to the macroscopic scale by metallic nano-pads physisorbed on the Si(100)-(2 × 1)-H surface. The logic inputs are provided by saturating and unsaturating two surface Si dangling bonds, which can, for example, be achieved by adding and extracting two hydrogen atoms per input. Quantum circuit design rules together with semi-empirical elastic-scattering quantum chemistry transport calculations were used to determine the output current intensity of the proposed switches and LGs when they are interconnected to the metallic nano-pads by surface atomic-scale wires. Our calculations demonstrate that the proposed devices can reach ON/OFF ratios of up to 2000 for a running current in the 10 µA range.

Single OR molecule and OR atomic circuit logic gates interconnected on a Si(100)H surface

Electron transport calculations were carried out for three terminal OR logic gates constructed either with a single molecule or with a surface dangling bond circuit interconnected on a Si(100)H surface. The corresponding multi-electrode multi-channel scattering matrix (where the central three terminal junction OR gate is the scattering center) was calculated, taking into account the electronic structure of the supporting Si(100)H surface, the metallic interconnection nano-pads, the surface atomic wires and the molecule. Well interconnected, an optimized OR molecule can only run at a maximum of 10 nA output current intensity for a 0.5 V bias voltage. For the same voltage and with no molecule in the circuit, the output current of an OR surface atomic scale circuit can reach 4 µA.

Synthesis and STM Imaging of Symmetric and Dissymmetric Ethynyl‐Bridged Dimers of Boron–Subphthalocyanine Bowl‐Shaped Nanowheels

The futures wheel: A new class of wheels, based on subphthalocyanine fragments, for future incorporation in functional nanovehicles is reported (see figure). The syntheses of a symmetric wheel, a nitrogen-tagged wheel, and their ethynyl-bridged homodimers are presented. Theoretical calculations and STM imaging demonstrate the advantage of a bowl-shaped structure and the efficiency of the tag for STM imaging.

STM manipulation of a subphthalocyanine double-wheel molecule on Au(111).

A new class of double-wheel molecules is manipulated on a Au(111) surface by the tip of a scanning tunneling microscope (STM) at low temperature. The double-wheel molecule consists of two subphthalocyanine wheels connected by a central rotation carbon axis. Each of the subphthalocyanine wheels has a nitrogen tag to monitor its intramolecular rolling during an STM manipulation sequence. The position of the tag can be followed by STM, allowing us to distinguish between the different lateral movements of the molecule on the surface when manipulated by the STM tip.

[11]Anthrahelicene on InSb(001) c(8×2): A Low‐Temperature Scanning Probe Microscopy Study

The adsorption of individual [11]anthrahelicene molecules and their self-assembly into monolayer islands on an InSb(001) c(8×2) reconstructed surface is studied with low-temperature scanning probe microscopy. A racemic mixture is deposited on atomically flat terraces of InSb at room temperature. At lower coverage, the molecules tend to decorate atomic step edges of the substrate. At higher coverage, [11]anthrahelicene molecules form 2D islands. A quasi-hexagonal ordering of molecules within the layer is identified. Furthermore, it is shown that molecules adsorb with the helical axis almost perpendicular to the substrate. Interference between tunneling through the molecular layer and directly through space is reported. Finally, experimental results are compared to those of theoretical calculations.