Complex atomic-scale circuits designed in classical, semi-classical and Quantum Hamiltonian Computing approaches using multi-atomic wires interconnects and Au(111) metallic nano-pads contacts discussed in Chapter "The Design of a Surface Atomic Scale Logic Gate with Molecular Latch Inputs" have been investigated to go forward for reaching compact and complex Boolean logic gates systems. The atomic wire electronic band gap, the tunneling intensity decay problems with the length of those atomic-scale interconnects and the practical size of the total atomic-scale circuit are discussed with the future perspective of developing an atomic-scale circuit simulator. Practical problems like the limitation in number of atoms of circuit builders and software limitations in terms of RAM memory and CPU computing time are also discussed for this objective.

To control the logical inputs in a surface atomic scale circuit, we introduce the concept of molecule-latch. The switching of certain molecules on a surface can be used to control the conductance of a very simple atomic scale interferometer that can be constructed atom by atom on an Si(100)H surface by STM atomic manipulation. Among the possible surface switching effects, such as surface rotation, push–pull effect or flip-flop switching, we explored the latter switching effect in detail starting from the acetophenone molecule, which can be switched on the Si(100) surface by an inelastic tunnelling current effect. Since this molecule does not offer a true bistable character on the surface, longer-arm acetophenone-like molecules are introduced, keeping the end phenyl unchanged. Among these molecules, 4-acetylalkylphenyl presents the best bistable character on Si(100)H and can perfectly control the conductance of a Si(100)H atomic scale electronic surface interferometer.

Two-input/one-output atomic scale Boolean logic gates are presented which are supposed to be atom by atom STM constructed on an Si(100)H-passivated surface. For those relatively simple logic gates, semi-classical and quantum Hamiltonian computing (QHC) architectures are compared showing how the QHC design avoids long atomic scale wiring compulsory for any classical or semi-classical “à la Shannon” electric circuit design. For a more realistic approach, the simple double hydrogen switching elements used for the logical inputs were substituted by mechanical molecule latch inputs. This does not transform the logical Boolean truth tables of the surface-implanted logic gates. For both the semi-classical and the QHC designs, the four standard known Boolean logic gates: AND, NOR, NAND and OR can be formally designed with an exception of the OR for the QHC approach. On the surface, AND, NOR, NAND and OR can be implanted in a semi-classical way and NOR and OR in a QHC way. For QHC, the re-emerging of the OR is due to through-surface electronic couplings, which is inducing a dependence between the inputs and the structural parameters of the QHC gates.

Molecular-scale electronics is mainly concerned by understanding charge transport through individual molecules. A key issue here is the charge transport capability through a single--typically linear--molecule, characterized by the current decay with increasing length. To improve the conductance of individual polymers, molecular design often either involves the use of rigid ribbon/ladder-type structures, thereby sacrificing for flexibility of the molecular wire, or a zero band gap, typically associated with chemical instability. Here we show that a conjugated polymer composed of alternating donor and acceptor repeat units, synthesized directly by an on-surface polymerization, exhibits a very high conductance while maintaining both its flexible structure and a finite band gap. Importantly, electronic delocalization along the wire does not seem to be necessary as proven by spatial mapping of the electronic states along individual molecular wires. Our approach should facilitate the realization of flexible 'soft' molecular-scale circuitry, for example, on bendable substrates.

The first experimental demonstration of a controllable rotating molecule gear is presented. A scanning tunneling microscope (STM) is used to construct, manipulate, and observe the rotation of the molecule gear. The appropriate combination of molecule design, molecule manipulation protocol, and surface atomic structure selection leads to the functioning of the molecule gear. Rotation of the molecule gear is done step-by-step and totally under control. The fabrication of solid-state SiO2 nanogears with diameters ranging from 30 nm up to 1 μm and their manipulation using an atomic force microscope tip on a graphite surface is also presented. Ranging in sizes from few tens of nanometers up to submicron diameters, they are going to enable the transmission of mechanical motion from functional mechanical molecule machineries to larger submicron or micron-sized devices through series of solid-state gears and mechanical components compatible with the semiconductor and electronics industry technology.

The mechanical switching of a single pentacene molecule chemisorbed in a planar configuration along a dimer row of the Si(100)-(2 × 1) surface was performed experimentally using the tip apex of a scanning tunneling microscope. The mechanical switching reaction path was identified theoretically on the ground state potential energy surface of the pentacene/Si(100)-(2 × 1) system. A low-temperature scanning tunneling microscope as well as semiempirical ASED+ molecular mechanical and elastic scattering quantum chemistry (ESQC) calculations were employed to perform the studies. Pushing with the STM tip apex and at zero bias voltage exactly at the center of the chemisorbed pentacene molecule induces a mechanical conformation change of the pentacene from its metastable to its surface stable conformation on the Si(100)-(2 × 1) surface.

Scanning tunneling microscopy is a very suitable instrument for the local probing and spectroscopic characterization of individual molecules, in our case narrow graphene nanoribbons. The electronic properties of a graphene nanoribbon can be controlled by its edge structure and width. Bottom-up approaches like on-surface synthesis allow the formation of extended conjugated electronic systems. Moreover, they lead to atomically defined edges which are important as structural defects have been predicted to modify the electronic structure. We have used low temperature scanning tunneling microscopy to investigate the formation, adsorption properties, and electronic structure of single graphene nanoribbons. 10,10′-Dibromo-9,9′-bianthryl molecules were used as molecular building blocks to form graphene nanoribbons after linking of the monomers and subsequent cyclodehydrogenation. In addition to intact ribbons, the influence of various defects on the electronic states is also investigated.

Electron transport through atomic-scale circuits made of dangling-bond (DB) wires is discussed using the N-ESQC technique taking also into account the contacting nanopads on the circuits. The band structure of the Si(001)-(2x1):H surface is analyzed together with the ones with infinite dangling-bond wires. The exponential decay in the conductance when tunneling between two atomic wires through the hydrogenated Si(001) surface shows two different rates depending of the tunneling direction. Taking advantage of this difference in the decay rate, an efficient OR DB logic gate is presented. When the DB is connected to metallic nanopads, the circuit is behaving like a quantum box with states resonating as captured by its electronic transmission spectrum. The through-surface leakage current between the nanopads is also evaluated. Finally and in the tunneling regime, the DB electronic circuit rules are discussed using series and parallel surface circuits.

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.

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(1,2). 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.

The future's 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.

Atomic-scale dangling-bond Boolean logic gates with two inputs and one output are designed on a Si(001)-(2x1):H surface. The dangling-bond logic gates are connected to the macroscopic scale by metallic nano-electrodes physisorbed on the Si(100)-(2x1):H surface. The logic inputs are provided by saturating and unsaturating surface Si dangling bonds, which can, for example, be achieved by adding and extracting two hydrogen atoms per input. Quantum-transport calculations were used to investigate the operation of the proposed dangling-bond logic gates interconnected to the metallic nano-electrodes by surface dangling-bond wires. Our calculations indicate that the proposed dangling-bond logic devices can reach ON/OFF ratios up to 2000.

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.

The detailed fabrication and manipulations of solid state nano gears up to 350 nm in diameter is reported. Atomic force microscopy (AFM) and ultra high vacuum (UHV) scanning tunneling microscopy (STM) are used to maneuver the gears. The aim is to bridge the gap between the current solid state gears and the now available nanoscale gears. As in many technology integrations, miniaturization is a way to boost efficiency and an opening to new applications.

The design of artificial molecular machines often takes inspiration from macroscopic machines. However, the parallels between the two systems are often only superficial, because most molecular machines are governed by quantum processes. Previously, rotary molecular motors powered by light and chemical energy have been developed. In electrically driven motors, tunnelling electrons from the tip of a scanning tunnelling microscope have been used to drive the rotation of a simple rotor in a single direction and to move a four-wheeled molecule across a surface. Here, we show that a stand-alone molecular motor adsorbed on a gold surface can be made to rotate in a clockwise or anticlockwise direction by selective inelastic electron tunnelling through different subunits of the motor. Our motor is composed of a tripodal stator for vertical positioning, a five-arm rotor for controlled rotations, and a ruthenium atomic ball bearing connecting the static and rotational parts. The directional rotation arises from sawtooth-like rotational potentials, which are solely determined by the internal molecular structure and are independent of the surface adsorption site.

A process is presented to fabricate solid-state nano-gears down to a 60 nm outer diameter with six teeth, where the 350 nm diameter ones already have 24 teeth. The small gears are free to move on a polycrystalline gold surface. The gears can be manipulated one by one, using an atomic force microscope (AFM) tip, to construct a train of gears where mechanical motion can be transmitted from one gear to another by mastering the surface friction. This is a first step on the way to bridge the fabrication gap between microfabricated and molecule gears.

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.

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.