Step-by-step rotation of a molecule-gear mounted on an atomic-scale axis

Mechanically Interlocked Molecular Rotors on Pb(100)

The mechanical coupling between molecules represents a promising route for the development of molecular machines. Constructing molecular gears requires easily rotatable and mutually interlocked pinions. Using scanning tunneling microscopy (STM), it is demonstrated that aluminum phthalocyanine (AlPc) molecules on Pb(100) exhibit these properties. Unlike other phthalocyanines on this substrate, isolated AlPc molecules fluctuate between two azimuthal orientations. Density functional theory (DFT) calculations confirm two stable orientations of single molecules and indicate a relatively low rotation barrier. In STM-constructed dimers and trimers, fluctuations diminish, and various molecular orientations are stabilized. Induced collective rotation of all molecules in the trimers is observed, demonstrating their mechanical interlocking. Potential functions describing angle and distance dependencies of intermolecular and molecule-substrate interactions are derived from DFT calculations of dimers; 52 experimentally determined trimer geometries are reproduced using these potentials. This intuitive approach may prove to be useful in modeling larger structures beyond the scope of quantum mechanical descriptions.

Synthesis of Ce(IV) Heteroleptic Double-Decker Complex with a New Helical Naphthalocyanine as a Potential Gearing Subunit

This paper describes the synthesis of a cerium(IV)-based molecular gear composed of a thioether functionalized phthalocyanine anchoring ligand, and a helical naphthalocyanine rotating cogwheel functionalized with four carbazoles. The naphthalocyanine ligand 9 was obtained after eleven steps (overall yield of 0.2 %) as a mixture of three geometrical isomers, two of which are chiral and exhibit high levels of steric hindrance, as shown by DFT calculations. Their attributions have been made using 1H-NMR based on their different symmetry groups. The ratio of isomers was also determined and the prochiral C4h naphthalocyanine shown to be the major compound (55 %). Its heteroleptic complexation with cerium (IV) and the anchoring phthalocyanine ligand 10 gave the targeted molecular gear in a 16 % yield.

Formation and Manipulation of Diatomic Rotors at the Symmetry-Breaking Surfaces of a Kagome Superconductor

Construction of diatomic rotors, which is crucial for artificial nanomachines, remains challenging due to surface constraints and limited chemical design. Here we report the construction of diatomic Cr-Cs and Fe-Cs rotors where a Cr or Fe atom switches around a Cs atom at the Sb surface of the newly discovered kagome superconductor CsV3Sb5. The switching rate is controlled by the bias voltage between the rotor and scanning tunneling microscope (STM) tip. The spatial distribution of rates exhibits C2 symmetry, possibly linked to the symmetry-breaking charge orders of CsV3Sb5. We have expanded the rotor construction to include different transition metals (Cr, Fe, V) and alkali metals (Cs, K). Remarkably, designed configurations of rotors are achieved through STM manipulation. Rotor orbits and quantum states are precisely controlled by tuning the inter-rotor distance. Our findings establish a novel platform for the controlled fabrication of atomic motors on symmetry-breaking quantum materials, paving the way for advanced nanoscale devices.

Molecular machines working at interfaces: physics, chemistry, evolution and nanoarchitectonics

As a post-nanotechnology concept, nanoarchitectonics combines nanotechnology with advanced materials science. Molecular machines made by assembling molecular units and their organizational bodies are also products of nanoarchitectonics. They can be regarded as the smallest functional materials. Originally, studies on molecular machines analyzed the average properties of objects dispersed in solution by spectroscopic methods. Researchers' playgrounds partially shifted to solid interfaces, because high-resolution observation of molecular machines is usually done on solid interfaces under high vacuum and cryogenic conditions. Additionally, to ensure the practical applicability of molecular machines, operation under ambient conditions is necessary. The latter conditions are met in dynamic interfacial environments such as the surface of water at room temperature. According to these backgrounds, this review summarizes the trends of molecular machines that continue to evolve under the concept of nanoarchitectonics in interfacial environments. Some recent examples of molecular machines in solution are briefly introduced first, which is followed by an overview of studies of molecular machines and similar supramolecular structures in various interfacial environments. The interfacial environments are classified into (i) solid interfaces, (ii) liquid interfaces, and (iii) various material and biological interfaces. Molecular machines are expanding their activities from the static environment of a solid interface to the more dynamic environment of a liquid interface. Molecular machines change their field of activity while maintaining their basic functions and induce the accumulation of individual molecular machines into macroscopic physical properties molecular machines through macroscopic mechanical motions can be employed to control molecular machines. Moreover, research on molecular machines is not limited to solid and liquid interfaces; interfaces with living organisms are also crucial.

Bifurcated Polymorphic Transition and Thermochromic Fluorescence of a Molecular Crystal Involving Three-Dimensional Supramolecular Gear Rotation

The ability of molecules to move and rearrange in the solid state accounts for the polymorphic transition and stimuli-responsive properties of molecular crystals. However, how the crystal structure determines the molecular motion ability remains poorly understood. Here, we report that a three-dimensional (3D) supramolecular gear network in the green-emissive polymorph 1G of a dialkylamino-substituted anthracene-pentiptycene π-system (1) enables an unusual bifurcated polymorphic transition into a yellow-emissive polymorph (1Y) and a new green-emissive polymorph (1G*) via 3D correlated supramolecular rotation. The 90° forward correlated rotation causes the molecular conformation between the octyl and the anthracene units to change from syn to anti, the ladder-like supramolecular columns to constrict, and the gear network to disengage. This cooperative molecular motion is marked by the gradual formation of an intermediate state (1I) across the entire crystal from 170 to 230 °C, which then undergoes bifurcated (forward or backward rotation) and irreversible transitions to form polymorphs 1Y and 1G* at 230-235 °C. Notably, 1G* is similar to 1G but lacks gear engagement, preventing its transformation into 1Y. Nevertheless, 1G can be restored by grinding 1Y or 1G* or fuming with dichloromethane (DCM) vapor. This work illustrates the correlation between the crystal structure and solid-state molecular motion behavior and demonstrates how a 3D molecular gear system efficiently transmits thermal energy to drive the polymorphic transition and induce fluorochromism through significant conformational and packing changes.

Revealing the Orientation Selectivity of Tetrapyridyl-Substituted Porphyrins Constrained in Molecular "Klotski Puzzles"

Understanding and controlling molecular orientations in self-assembled organic nanostructures are crucial to the development of advanced functional nanodevices. Scanning tunneling microscopy (STM) provides a powerful toolbox to recognize molecular orientations and to induce orientation changes on surfaces at the single-molecule level. Enormous effort has been devoted to directly controlling the molecular orientations of isolated single molecules in free space. However, revealing and further controlling molecular orientation selectivity in constrained environments remain elusive. In this study, by a combination of STM imaging/manipulations and density functional theory calculations, we report the orientation selectivity of tetrapyridyl-substituted porphyrins in response to various local molecular environments in artificially constructed molecular "Klotski puzzles" on Au(111). With the assistance of STM lateral manipulations, "sliding-block" molecules were able to enter predefined positions, and specific molecular orientations were adopted to fit the local molecular environments, in which the intermolecular interaction was revealed to be the key to achieving the eventual molecular orientation selectivity. Our results demonstrate the essential role of local molecular environments in directing single-molecule orientations, which would shed light on the design of molecular structures to control preferred orientations for further applications in molecular nanodevices.

A Nanocar and Rotor in One Molecule

Depending on its adsorption conformation on the Au(111) surface, a zwitterionic single-molecule machine works in two different ways under bias voltage pulses. It is a unidirectional rotor while anchored on the surface. It is a fast-drivable molecule vehicle (nanocar) while physisorbed. By tuning the surface coverage, the conformation of the molecule can be selected to be either rotor or nanocar. The inelastic tunneling excitation producing the movement is investigated in the same experimental conditions for both the unidirectional rotation of the rotor and the directed movement of the nanocar.

Extended Tripodal Hydrotris(indazol-1-yl)borate Ligands as Ruthenium-Supported Cogwheels for On-Surface Gearing Motions

This paper reports the synthesis of ruthenium-based molecular gear prototypes composed of a brominated or non-brominated pentaphenylcyclopentadienyl ligand as an anchoring unit and a tripodal ligand with aryl-functionalized indazoles as a rotating cogwheel. Single crystal structures of the ruthenium complexes revealed that the appended aryl groups increase the apparent diameter of the cogwheel rendering them larger than the diameter of the anchoring units and consequently making them suitable for intermolecular gearing motions once the complexes will be adsorbed on a surface.

Inverted Conformation Stability of a Motor Molecule on a Metal Surface

Molecular motors have been intensely studied in solution, but less commonly on solid surfaces that offer fixed points of reference for their motion and allow high-resolution single-molecule imaging by scanning probe microscopy. Surface adsorption of molecules can also alter the potential energy surface and consequently preferred intramolecular conformations, but it is unknown how this affects motor molecules. Here, we show how the different conformations of motor molecules are modified by surface adsorption using a combination of scanning tunneling microscopy and density functional theory. These results demonstrate how the contact of a motor molecule with a solid can affect the energetics of the molecular conformations.

Construction and physical properties of low-dimensional structures for nanoscale electronic devices

Over the past decades, construction of nanoscale electronic devices with novel functionalities based on low-dimensional structures, such as single molecules and two-dimensional (2D) materials, has been rapidly developed. To investigate their intrinsic properties for versatile functionalities of nanoscale electronic devices, it is crucial to precisely control the structures and understand the physical properties of low-dimensional structures at the single atomic level. In this review, we provide a comprehensive overview of the construction of nanoelectronic devices based on single molecules and 2D materials and the investigation of their physical properties. For single molecules, we focus on the construction of single-molecule devices, such as molecular motors and molecular switches, by precisely controlling their self-assembled structures on metal substrates and charge transport properties. For 2D materials, we emphasize their spin-related electrical transport properties for spintronic device applications and the role that interfaces among 2D semiconductors, contact electrodes, and dielectric substrates play in the electrical performance of electronic, optoelectronic, and memory devices. Finally, we discuss the future research direction in this field, where we can expect a scientific breakthrough.

A nanographene disk rotating a single molecule gear on a Cu(111) surface

On Cu(111) surface and in interaction with a single hexa-tert-butylphenylbenzene molecule-gear, the rotation of a graphene nanodisk was studied using the large-scale atomic/molecular massively parallel simulator molecular dynamics simulator. To ensure a transmission of rotation to the molecule-gear, the graphene nanodisk is functionalized on its circumference bytert-butylphenyl chemical groups. The rotational motion can be categorized underdriving, driving and overdriving regimes calculating the locking coefficient of this mechanical machinery as a function of external torque applied to the nanodisk. The rotational friction with the surface of both the phononic and electronic contributions is investigated. For small size graphene nanodisks, the phononic friction is the main contribution. Electronic friction dominates for the larger disks putting constrains on the experimental way of achieving the transfer of rotation from a graphene nanodisk to a single molecule-gear.

Effect of lubricants on the rotational transmission between solid-state gears

Lubricants are widely used in macroscopic mechanical systems to reduce friction and wear. However, on the microscopic scale, it is not clear to what extent lubricants are beneficial. Therefore, in this study, we consider two diamond solid-state gears at the nanoscale immersed in different lubricant molecules and perform classical MD simulations to investigate the rotational transmission of motion. We find that lubricants can help to synchronize the rotational transmission between gears regardless of the molecular species and the center-of-mass distance. Moreover, the influence of the angular velocity of the driving gear is investigated and shown to be related to the bond formation process between gears.

Molecular Gears: From Solution to Surfaces

This review highlights the major efforts devoted to the development of molecular gears over the past 40 years, from pioneering covalent bis-triptycyl systems undergoing intramolecular correlated rotation in solution, to the most recent examples of gearing systems anchored on a surface, which allow intermolecular transmission of mechanical power. Emphasis is laid on the different strategies devised progressively to control the architectures of molecular bevel and spur gears, as intramolecular systems in solution or intermolecular systems on surfaces, while aiming at increased efficiency, complexity and functionality.

Addressing a lattice of rotatable molecular dipoles with the electric field of an STM tip

Functional molecular groups mounted on specific foot structures are ideal model systems to study intermolecular interactions, due to the possibility to separate the functionality and the adsorption mechanism. Here, we report on the rotational switching of a thioacetate group mounted on a tripodal tetraphenylmethane (TPM) derivative adsorbed in ordered islands on a Au(111) surface. Using low temperature scanning tunnelling microscopy, individual freestanding molecular groups of the lattice can be switched between two bistable orientations. The functional dependence of this rotational switching on the sample bias and tip-sample distance allows us to model the energy landscape of this molecular group as an electric dipole in the electric field of the tunnelling junction. As expected for the interaction of two dipoles, we found states of neighbouring molecules to be correlated.

Orientation Switching of Single Molecules on Surface Excited by Tunneling Electrons and Ultrafast Laser Pulses

We investigate the orientation switching of individual azobenzene molecules adsorbed on a Au(111) surface using a laser-assisted scanning tunneling microscope (STM). It is found that the rotational motion of the molecule can be regulated by both sample bias and laser wavelength. By measuring the switching rate and state occupation as a function of both bias voltage and photon energy, the threshold in sample bias and the minimal photon energy are derived. It has been revealed that the tip-induced local electrostatic potential remarkably contributes to the reduction in hopping barrier. We also find that the tunneling electrons and photons play distinct roles in controlling rotational dynamics of single azobenzene molecules on the surface, which are useful for understanding dynamic behaviors in similar molecular systems.

Desymmetrised pentaporphyrinic gears mounted on metallo-organic anchors

Mastering intermolecular gearing is crucial for the emergence of complex functional nanoscale machineries. However, achieving correlated motion within trains of molecular gears remains highly challenging, due to the multiple degrees of freedom of each cogwheel. In this context, we designed and synthesised a series of star-shaped organometallic molecular gears incorporating a hydrotris(indazolyl)borate anchor to prevent diffusion on the surface, a central ruthenium atom as a fixed rotation axis, and an azimuthal pentaporphyrinic cyclopentadienyl cogwheel specifically labelled to monitor its motion by non-time-resolved Scanning Tunneling Microscopy (STM). Desymmetrisation of the cogwheels was first achieved sterically, i.e. by introducing one tooth longer than the other four. For optimal mechanical interactions, chemical labelling was also investigated as a preferential way to induce local contrast in STM images, and the electronic properties of one single paddle were modulated by varying the porphyrinic scaffold or the nature of the central metal. To reach such a structural diversity, our modular synthetic approach relied on sequential cross-coupling reactions on a penta(p-halogenophenyl)cyclopentadienyl ruthenium(ii) key building block, bearing a single pre-activated p-iodophenyl group. Chemoselective Sonogashira or more challenging Suzuki-Miyaura reactions allowed the controlled introduction of the tagged porphyrinic tooth, and the subsequent four-fold cross-couplings yielded the prototypes of pentaporphyrinic molecular gears for on-surface studies, incorporating desymmetrised cogwheels over 5 nm in diameter.

Nanocars with Permanent Dipoles: Preparing for the Second International Nanocar Race

With the desire to synthesize surface-rolling molecular machines that can be translated and rotated with extreme precision and speed, we have synthesized a series of five nanocars. Each structure features a permanent dipole moment, generated by an N,N-dimethylamino- moiety on one end of the car coupled with a nitro group on the other end. These cars are designed to be stimulated with an electric field gradient from a scanning probe microscopy tip. The nanocars all possess unexplored combinations of structural features: tert-butyl wheels, short alkyne chassis, and combination sets of wheels including one set of tert-butyl wheels and another set of larger adamantane wheels on the same car. Each of these features needs to be assessed as preparation for the second International Nanocar Race that is taking place in 2022.

Transmitting Stepwise Rotation among Three Molecule-Gear on the Au(111) Surface

The realization of a train of molecule-gears working under the tip of a scanning tunneling microscope (STM) requires a stable anchor of each molecule to the metal surface. Such an anchor can be promoted by a radical state of the molecule induced by a dissociation reaction. Our results, rationalized by density functional theory calculations, reveal that such an open radical state at the core of star-shaped pentaphenylcyclopentadiene (PPCP) favors anchoring. Furthermore, to allow the transmission of motion by STM manipulation, the molecule-gears should be equipped with specific groups facilitating the tip-molecule interactions. In our case, a tert-butyl group positioned at one tooth end of the gear benefits both the tip-induced manipulation and the monitoring of rotation. With this optimized molecule, we achieve reproducible and stepwise rotations of the single gears and transmit rotations for up to three interlocked units.

Rotation of Ethoxy and Ethyl Moieties on a Molecular Platform on Au(111)

Molecular rotors have attracted considerable interest for their prospects in nanotechnology. However, their adsorption on supporting substrates, where they may be addressed individually, usually modifies their properties. Here, we investigate the switching of two closely related three-state rotors mounted on platforms on Au(111) using low-temperature scanning tunneling microscopy and density functional theory calculations. Being physisorbed, the platforms retain important gas-phase properties of the rotor. This simplifies a detailed analysis and permits, for instance, the identification of the vibrational modes involved in the rotation process. The symmetry provided by the platform enables active control of the rotation direction through electrostatic interactions with the tip and charged neighboring adsorbates. The present investigation of two model systems may turn out useful for designing platforms that provide directional rotation and for transferring more sophisticated molecular machines from the gas phase to surfaces.

Star-Shaped Ruthenium Complexes as Prototypes of Molecular Gears

The design and synthesis of two families of molecular-gear prototypes is reported, with the aim of assembling them into trains of gears on a surface and ultimately achieving controlled intermolecular gearing motion. These piano-stool ruthenium complexes incorporate a hydrotris(indazolyl)borate moiety as tripodal rotation axle and a pentaarylcyclopentadienyl ligand as star-shaped cogwheel, equipped with five teeth ranging from pseudo-1D aryl groups to large planar 2D paddles. A divergent synthetic approach was followed, starting from a pentakis(p-bromophenyl)cyclopentadienyl ruthenium(II) complex as key precursor or from its iodinated counterpart, obtained by copper-catalyzed aromatic Br/I exchange. Subsequent fivefold cross-coupling reactions with various partners allowed high structural diversity to be reached and yielded molecular-gear prototypes with aryl-, carbazole-, BODIPY- and porphyrin-derived teeth of increasing size and length.

Train of Single Molecule-Gears

Two molecule-gears, 1.2 nm in diameter with six teeth, are mounted each on a single copper adatom separated exactly by 1.9 nm on a lead surface using a low-temperature scanning tunneling microscope (LT-STM). A functioning train of two molecule-gears is constructed complete with a molecule-handle. Not mounted on a Cu adatom axle, this ancillary molecule-gear is mechanically engaged with the first molecule-gear of the train to stabilize its step-by-step rotation. Centered on its Cu adatom axle, the rotation of the first gear of the train step by step rotates the second similar to a train of macroscopic gears. From the handle to the first and to this second molecule-gear, the exact positioning of the two Cu adatom axles on the lead surface ensures that the molecular teeth-to-teeth mechanics is fully reversible.

How to control single-molecule rotation

The orientation of molecules is crucial in many chemical processes. Here, we report how single dipolar molecules can be oriented with maximum precision using the electric field of a scanning tunneling microscope. Rotation is found to occur around a fixed pivot point that is caused by the specific interaction of an oxygen atom in the molecule with the Ag(111) surface. Both directions of rotation are realized at will with 100% directionality. Consequently, the internal dipole moment of an individual molecule can be spatially mapped via its behavior in an applied electric field. The importance of the oxygen-surface interaction is demonstrated by the addition of a silver atom between a single molecule and the surface and the consequent loss of the pivot point.

Surface manipulation of a curved polycyclic aromatic hydrocarbon-based nano-vehicle molecule equipped with triptycene wheels

With a central curved chassis, a four-wheeled molecule-vehicle was deposited on a Au(111) surface and imaged at low temperature using a scanning tunneling microscope. The curved conformation of the chassis and the consequent moderate interactions of the four wheels with the surface were observed. The dI/dV constant current maps of the tunneling electronic resonances close to the Au(111) Fermi level were recorded to identify the potential energy entry port on the molecular skeleton to trigger and control the driving of the molecule. A lateral pushing mode of molecular manipulation and the consequent recording of the manipulation signals confirm how the wheels can step-by-step rotate while passing over the Au(111) surface native herringbone reconstructions. Switching a phenyl holding a wheel to the chassis was not observed for triggering a lateral molecular motion inelastically and without any mechanic push by the tip apex. This points out the necessity to encode the sequence of the required wheels action on the profile of the potential energy surface of the excited states to be able to drive a molecule-vehicle.

Modification of the Potential Landscape of Molecular Rotors on Au(111) by the Presence of an STM Tip

Molecular rotors on solid surfaces are fundamental components of molecular machines. No matter whether the rotation is activated by heat, electric field or light, it is determined by the intrinsic rotational potential landscape. Therefore, tuning the potential landscape is of great importance for future applications of controlled molecular rotors. Here, using scanning tunneling microscopy (STM), we demonstrate that both tip-molecule distance and sample bias can modify the rotational potential of molecular rotors. We achieve the potential energy difference variations of ∼0.3 meV/pm and ∼18 meV/V between two configurations of a molecular rotor, a tetra- tert-butyl nickel phthalocyanine molecule on Au(111) substrate. Further analysis indicates that the mechanism of modifying the rotational potential is a combination of the van der Waals interaction and the interaction between the molecular dipole and an electric field. This work provides insight into the methods used to modify the effective rotational potential energy of molecular rotors.

Highly ordered molecular rotor matrix on a nanopatterned template: titanyl phthalocyanine molecules on FeO/Pt(111)

Molecular rotors, motors and gears play important roles in artificial molecular machines, in which rotor and motor matrices are highly desirable for large-scale bottom-up fabrication of molecular machines. Here we demonstrate the fabrication of a highly ordered molecular rotor matrix by depositing nonplanar dipolar titanyl phthalocyanine (TiOPc, C₃₂H₁₆N₈OTi) molecules on a Moiré patterned dipolar FeO/Pt(111) substrate. TiOPc molecules with O atoms pointing outwards from the substrate (upward) or towards the substrate (downward) are alternatively adsorbed on the fcc sites by strong lateral confinement. The adsorbed molecules, i.e. two kinds of molecular rotors, show different scanning tunneling microscopy images, thermal stabilities and rotational characteristics. Density functional theory calculations clarify that TiOPc molecules anchoring upwards with high adsorption energies correspond to low-rotational-rate rotors, while those anchoring downwards with low adsorption energies correspond to high-rotational-rate rotors. A robust rotor matrix fully occupied by low-rate rotors is fabricated by depositing molecules on the substrate at elevated temperature. Such a paradigm opens up a promising route to fabricate functional molecular rotor matrices, driven motor matrices and even gear groups on solid substrates.

Interlocking Mechanism between Molecular Gears Attached to Surfaces

While molecular machines play an increasingly significant role in nanoscience research and applications, there remains a shortage of investigations and understanding of the molecular gear (cogwheel), which is an indispensable and fundamental component to drive a larger correlated molecular machine system. Employing ab initio calculations, we investigate model systems consisting of molecules adsorbed on metal or graphene surfaces, ranging from very simple triple-arm gears such as PF₃ and NH₃ to larger multiarm gears based on carbon rings. We explore in detail the transmission of slow rotational motion from one gear to the next by these relatively simple molecules, so as to isolate and reveal the mechanisms of the relevant intermolecular interactions. Several characteristics of molecular gears are discussed, in particular the flexibility of the arms and the slipping and skipping between interlocking arms of adjacent gears, which differ from familiar macroscopic rigid gears. The underlying theoretical concepts suggest strongly that other analogous structures may also exhibit similar behavior which may inspire future exploration in designing large correlated molecular machines.

Tuning the Electronic and Dynamical Properties of a Molecule by Atom Trapping Chemistry

The ability to trap adatoms with an organic molecule on a surface has been used to obtain a range of molecular functionalities controlled by the choice of the molecular trapping site and local deprotonation. The tetraphenylporphyrin molecule used in this study contains three types of trapping sites: two carbon rings (phenyl and pyrrole) and the center of a macrocycle. Catching a gold adatom on the carbon rings leads to an electronic doping of the molecule, whereas trapping the adatom at the macrocycle center with single deprotonation leads to a molecular rotor and a second deprotonation leads to a molecular jumper. We call "atom trapping chemistry" the control of the structure, electronic, and dynamical properties of a molecule achieved by trapping metallic atoms with a molecule on a surface. In addition to the examples previously described, we show that more complex structures can be envisaged.

Conformation Manipulation and Motion of a Double Paddle Molecule on an Au(111) Surface

The molecular conformation of a bisbinaphthyldurene (BBD) molecule is manipulated using a low-temperature ultrahigh-vacuum scanning tunneling microscope (LT-UHV STM) on an Au(111) surface. BBD has two binaphthyl groups at both ends connected to a central durene leading to anti/syn/flat conformers. In solution, dynamic nuclear magnetic resonance indicated the fast interexchange between the anti and syn conformers as confirmed by density functional theory calculations. After deposition in a submonolayer on an Au(111) surface, only the syn conformers were observed forming small islands of self-assembled syn dimers. The syn dimers can be separated into syn monomers by STM molecular manipulations. A flat conformer can also be prepared by using a peculiar mechanical unfolding of a syn monomer by STM manipulations. The experimental STM dI/dV and theoretical elastic scattering quantum chemistry maps of the low-lying tunneling resonances confirmed the flat conformer BBD molecule STM production. The key BBD electronic states for a step-by-step STM inelastic excitation lateral motion on the Au(111) are presented requiring no mechanical interactions between the STM tip apex and the BBD. On the BBD molecular board, selected STM tip apex positions for this inelastic tunneling excitation enable the flat BBD to move controllably on Au(111) by a step of 0.29 nm per bias voltage ramp.

Single-Molecule Rotational Switch on a Dangling Bond Dimer Bearing

One of the key challenges in the construction of atomic-scale circuits and molecular machines is to design molecular rotors and switches by controlling the linear or rotational movement of a molecule while preserving its intrinsic electronic properties. Here, we demonstrate both the continuous rotational switching and the controlled step-by-step single switching of a trinaphthylene molecule adsorbed on a dangling bond dimer created on a hydrogen-passivated Ge(001):H surface. The molecular switch is on-surface assembled when the covalent bonds between the molecule and the dangling bond dimer are controllably broken, and the molecule is attached to the dimer by long-range van der Waals interactions. In this configuration, the molecule retains its intrinsic electronic properties, as confirmed by combined scanning tunneling microscopy/spectroscopy (STM/STS) measurements, density functional theory calculations, and advanced STM image calculations. Continuous switching of the molecule is initiated by vibronic excitations when the electrons are tunneling through the lowest unoccupied molecular orbital state of the molecule. The switching path is a combination of a sliding and rotation motion over the dangling bond dimer pivot. By carefully selecting the STM conditions, control over discrete single switching events is also achieved. Combined with the ability to create dangling bond dimers with atomic precision, the controlled rotational molecular switch is expected to be a crucial building block for more complex surface atomic-scale devices.

Structural changes in a Schiff base molecular assembly initiated by scanning tunneling microscopy tip

We report the controlled self-organization and switching of newly designed Schiff base (E)-4-((4-(phenylethynyl) benzylidene) amino) benzenethiol (EPBB) molecules on a Au (111) surface at room temperature. Scanning tunneling microscopy and spectroscopy (STM/STS) were used to image and analyze the conformational changes of the EPBB molecules. The conformational change of the molecules was induced by using the STM tip while increasing the tunneling current. The switching of a domain or island of molecules was shown to be induced by the STM tip during scanning. Unambiguous fingerprints of the switching mechanism were observed via STM/STS measurements. Surface-enhanced Raman scattering was employed, to control and identify quantitatively the switching mechanism of molecules in a monolayer. Density functional theory calculations were also performed in order to understand the microscopic details of the switching mechanism. These calculations revealed that the molecular switching behavior stemmed from the strong interaction of the EPBB molecules with the STM tip. Our approach to controlling intermolecular mechanics provides a path towards the bottom-up assembly of more sophisticated molecular machines.

Simultaneous and coordinated rotational switching of all molecular rotors in a network

A range of artificial molecular systems has been created that can exhibit controlled linear and rotational motion. In the further development of such systems, a key step is the addition of communication between molecules in a network. Here, we show that a two-dimensional array of dipolar molecular rotors can undergo simultaneous rotational switching when applying an electric field from the tip of a scanning tunnelling microscope. Several hundred rotors made from porphyrin-based double-decker complexes can be simultaneously rotated when in a hexagonal rotor network on a Cu(111) surface by applying biases above 1 V at 80 K. The phenomenon is observed only in a hexagonal rotor network due to the degeneracy of the ground-state dipole rotational energy barrier of the system. Defects are essential to increase electric torque on the rotor network and to stabilize the switched rotor domains. At low biases and low initial rotator angles, slight reorientations of individual rotors can occur, resulting in the rotator arms pointing in different directions. Analysis reveals that the rotator arm directions are not random, but are coordinated to minimize energy via crosstalk among the rotors through dipolar interactions.

Direct imaging of rotating molecules anchored on graphene

There has been significant research interest in controlling and imaging molecular dynamics, such as translational and rotational motions, especially at a single molecular level. Here we applied aberration-corrected transmission electron microscopy (ACTEM) to actuate and directly image the rotational motions of molecules anchored on a single-layer-graphene sheet. Nanometer-sized carbonaceous molecules anchored on graphene provide ideal systems for monitoring rotational motions via ACTEM. We observed the preferential registry of longer molecular axis along graphene zigzag or armchair lattice directions due to the stacking-dependent molecule-graphene energy landscape. The calculated cross section from elastic scattering theory was used to experimentally estimate the rotational energy barriers of molecules on graphene. The observed energy barrier was within the range of 1.5-12 meV per atom, which is in good agreement with previous calculation results. We also performed molecular dynamics simulations, which revealed that the edge atoms of the molecule form stably bonds to graphene defects and can serve as a pivot point for rotational dynamics. Our study demonstrates the versatility of ACTEM for the investigation of molecular dynamics and configuration-dependent energetics at a single molecular level.

Current-Driven Supramolecular Motor with In Situ Surface Chiral Directionality Switching

Surface-supported molecular motors are nanomechanical devices of particular interest in terms of future nanoscale applications. However, the molecular motors realized so far consist of covalently bonded groups that cannot be reconfigured without undergoing a chemical reaction. Here we demonstrate that a platinum-porphyrin-based supramolecularly assembled dimer supported on a Au(111) surface can be rotated with high directionality using the tunneling current of a scanning tunneling microscope (STM). Rotational direction of this molecular motor is determined solely by the surface chirality of the dimer, and most importantly, the chirality can be inverted in situ through a process involving an intradimer rearrangement. Our result opens the way for the construction of complex molecular machines on a surface to mimic at a smaller scale versatile biological supramolecular motors.

Controllable Scission and Seamless Stitching of Metal-Organic Clusters by STM Manipulation

Scanning tunneling microscopy (STM) manipulation techniques have proven to be a powerful method for advanced nanofabrication of artificial molecular architectures on surfaces. With increasing complexity of the studied systems, STM manipulations are then extended to more complicated structural motifs. Previously, the dissociation and construction of various motifs have been achieved, but only in a single direction. In this report, the controllable scission and seamless stitching of metal-organic clusters have been successfully achieved through STM manipulations. The system presented here includes two sorts of hierarchical interactions where coordination bonds hold the metal-organic elementary motifs while hydrogen bonds among elementary motifs are directly involved in bond breakage and re-formation. The key to making this reversible switching successful is the hydrogen bonding, which is comparatively facile to be broken for controllable scission, and, on the other hand, the directional characteristic of hydrogen bonding makes precise stitching feasible.

Visualization and thermodynamic encoding of single-molecule partition function projections

Ensemble averaging of molecular states is fundamental for the experimental determination of thermodynamic quantities. A special case occurs for single-molecule investigations under equilibrium conditions, for which free energy, entropy and enthalpy at finite temperatures are challenging to determine with ensemble averaging alone. Here we report a method to directly record time-averaged equilibrium probability distributions by confining an individual molecule to a nanoscopic pore of a two-dimensional metal-organic nanomesh, using temperature-controlled scanning tunnelling microscopy. We associate these distributions with partition function projections to assess real-space-projected thermodynamic quantities, aided by computational modelling. The presented molecular dynamics-based analysis is able to reproduce experimentally observed projected microstates with high accuracy. By an in silico customized energy landscape, we demonstrate that distinct probability distributions can be encrypted at different temperatures. Such modulation provides means to encode and decode information into position-temperature space.

A single-molecular twin rotor: correlated motion of two pyrimidine rings coordinated to copper

A single molecular twin rotor comprising a copper complex exhibits the correlation of the motions of two coordinated pyrimidine rings.

Solid-state SiO₂ nano-gears AFM tip manipulation on HOPG

On a native graphite surface, 15 nm-thick solid-state nanogears are nanofabricated with a 30 nm outer diameter and six teeth. The nanogears are manipulated one at a time by the tip of an atomic force microscope using the sample stage displacements for the manipulation and recording of the corresponding manipulation signals. For step heights below 3.0 nm, nanogears are manipulated up and down native graphite surface step edges. In the absence of a central shaft per nanogear, gearing between nanogears is limited to a few 1/12 turns for six teeth. When the graphite step is higher than 3 nm, a rack-and-pinion mechanism was constructed along the edge with a 90 nm nanogear pinion.

Topological dynamics in supramolecular rotors

Artificial molecular switches, rotors, and machines are set to establish design rules and applications beyond their biological counterparts. Herein we exemplify the role of noncovalent interactions and transient rearrangements in the complex behavior of supramolecular rotors caged in a 2D metal-organic coordination network. Combined scanning tunneling microscopy experiments and molecular dynamics modeling of a supramolecular rotor with respective rotation rates matching with 0.2 kcal mol(-1) (9 meV) precision, identify key steps in collective rotation events and reconfigurations. We notably reveal that stereoisomerization of the chiral trimeric units entails topological isomerization whereas rotation occurs in a topology conserving, two-step asynchronous process. In supramolecular constructs, distinct displacements of subunits occur inducing a markedly lower rotation barrier as compared to synchronous mechanisms of rigid rotors. Moreover, the chemical environment can be instructed to control the system dynamics. Our observations allow for a definition of mechanical cooperativity based on a significant reduction of free energy barriers in supramolecules compared to rigid molecules.

Nanometer thick elastic graphene engine

Significant progress has been made in the construction and theoretical understanding of molecular motors because of their potential use. Here, we have demonstrated fabrication of a simple but powerful 1 nm thick graphene engine. The engine comprises a high elastic membrane-piston made of graphene and weakly chemisorbed ClF3 molecules as the high power volume changeable actuator, while a 532 nm LASER acts as the ignition plug. Rapid volume expansion of the ClF3 molecules leads to graphene blisters. The size of the blister is controllable by changing the ignition parameters. The estimated internal pressure per expansion cycle of the engine is about ∼10(6) Pa. The graphene engine presented here shows exceptional reliability, showing no degradation after 10,000 cycles.

Atom-by-atom assembly

Atomic manipulation using a scanning tunneling microscope (STM) tip enables the construction of quantum structures on an atom-by-atom basis, as well as the investigation of the electronic and dynamical properties of individual atoms on a one-atom-at-a-time basis. An STM is not only an instrument that is used to 'see' individual atoms by means of imaging, but is also a tool that is used to 'touch' and 'take' the atoms, or to 'hear' their movements. Therefore, the STM can be considered as the 'eyes', 'hands' and 'ears' of the scientists, connecting our macroscopic world to the exciting atomic world. In this article, various STM atom manipulation schemes and their example applications are described. The future directions of atomic level assembly on surfaces using scanning probe tips are also discussed.

One-way rotation of a molecule-rotor driven by a shot noise

The shot noise of a tunneling current passing through a molecule-motor can sustain a one-way rotation when populating the molecular excited states by tunneling inelastic excitations. We demonstrate that a ratchet-like ground state rotation potential energy curve is not necessary for the rotation to occur. A relative shift in energy difference between the maxima of this ground state and the minima of the excited states is the necessary condition to get to a unidirectional rotation. The rotor speed of rotation and its rotation direction are both controlled by this shift, indicating the necessity of a careful design of both the ground and excited states of the next generation of molecule-motors to be able to generate a motive power at the nanoscale.