A fullerene molecular tip can detect localized and rectified electron tunneling within a single fullerene-porphyrin pair

Interface Phenomena in Molecular Junctions through Noncovalent Interactions

Noncovalent interactions, both between molecules and at the molecule-electrode interfaces, play essential roles in enabling dynamic and reversible molecular behaviors, including self-assembly, recognition, and various functional properties. In macroscopic ensemble systems, these interfacial phenomena often exhibit emergent properties that arise from the synergistic interplay of multiple noncovalent interactions. However, at the single-molecule scale, precisely distinguishing, characterizing, and controlling individual noncovalent interactions remains a significant challenge. Molecular electronics offers a unique platform for constructing and characterizing both intermolecular and molecule-electrode interfaces governed by noncovalent interactions, enabling the isolated study of these fundamental interactions. Furthermore, precise control over these interfaces through noncovalent interactions facilitates the development of enhanced molecular devices. This review examines the characterization of interfacial phenomena arising from noncovalent interactions through single-molecule electrical measurements and explores their applications in molecular devices. We begin by discussing the construction of stable molecular junctions through intermolecular and molecule-electrode interfaces, followed by an analysis of electron tunneling mechanisms mediated by key noncovalent interactions and their modulation methods. We then investigate how noncovalent interactions enhance device sensitivity, stability, and functionality, establishing design principles for next-generation molecular electronics. We have also explored the potential of noncovalent interactions for bottom-up self-assembled molecular devices. The review concludes by addressing the opportunities and challenges in scaling up molecular electronics through noncovalent interactions.

Enhanced Fe(OH)2-driven reductive Dechlorination via shortened Fe-O bonds and colloidal medium

Fe2+ is usually adsorbed to the surface of iron-bearing clay, and iron (hydr)oxide in groundwater. However, the reductive activity of Fe(OH)2, a prevalent intermediate during the transformation of Fe2+, remains unclear. In this study, high-purity Fe(OH)2 was synthesized and tested for its activity in the degradation of carbon tetrachloride (CT). XRD data confirm that the synthesized material is a pure Fe(OH)2 crystal, exhibiting sharp peaks of (001) and (100) facets. Zeta potential analysis confirms that the off-white Fe(OH)2 is a colloidal suspension with a positive charge of ∼+35-50 mV. FTIR spectra reveal the formation of a coordination compound Fe2+ with OH-/OD-, derived from NaOH/OD. SEM and HRTEM results demonstrate that the Fe(OH)2 crystal has a regular octahedral structure with a size of ∼30-70 nm and average lattice spacings of 2.58 Å. Mössbauer spectrum verifies that the Fe2+ in Fe(OH)2/Fe(OD)2 is hexacoordinated with six Fe-O bonds. XAFS data demonstrate that the Fe-O bonds become shorter as the OH-:Fe(II) ratios increase. DFT results indicate that the (100) crystal face of Fe(OH)2 more readily transfers electrons to CT. In addition to being adsorbed to iron compounds, structural Fe2+ compounds such as Fe(OH)2 could also accelerate the electron transfer from Fe2+ to CT through shortened Fe-O bonds. The rate constant of CT reduction by Fe(OH)2 is as high as 0.794 min-1 when the OH-:Fe(II) ratio is 2.5 in water. This study aims to enhance our understanding of the structure-reactivity relationship of Fe2+ compounds in groundwater, particularly in relation to electron transfer mechanisms.

STM studies on porphyrins and phthalocyanines at the liquid/solid interface for molecular-scale electronics

Porphyrins and phthalocyanines are promising candidates for single-molecule electronics. Among the many characterization tools, scanning tunneling microscopy (STM) represents a very powerful one to gain insight into the electronic properties at the molecular level, by correlating the charge transport behaviours of π-conjugated molecules with ultrahigh resolution imaging. In view of the sophistication of molecular self-assembly in the presence of a solution phase, in this frontier, we focus on STM studies on porphyrins and phthalocyanines at the liquid/solid interface, placing emphasis on the electronic and magnetic properties, as well as the switching behaviour of surface-confined or surface-anchored molecules. Furthermore, we have also addressed the topics of potential that can be exploited in this area.

Characterization and Application of Supramolecular Junctions

The convergence of supramolecular chemistry and single-molecule electronics offers a new perspective on supramolecular electronics, and provides a new avenue toward understanding and application of intermolecular charge transport at the molecular level. In this review, we will provide an overview of the advances in the characterization technique for the investigation of intermolecular charge transport, and summarize the experimental investigation of several non-covalent interactions, including π-π stacking interactions, hydrogen bonding, host-guest interactions and σ-σ interactions at the single-molecule level. We will also provide a perspective on supramolecular electronics and discuss the potential applications and future challenges.

Rotational and Vibrational Excitations of a Single Water Molecule by Inelastic Electron Tunneling Spectroscopy

Two low-energy excitations of a single water molecule are observed via inelastic electron tunneling spectroscopy, where a significant enhancement is achieved by attaching the molecule to the tip apex in a scanning tunneling microscope. Density functional theory simulations and quantum mechanical calculations of an asymmetric top are carried out to reveal the origin of both excitations. Variations in tunneling junction separation give rise to the quantum confinement effect on the quantum state of a water molecule in the tunneling junction. Our results demonstrate a potential method for measuring the dynamic behavior of a single molecule confined in a tunneling junction, where the molecule-substrate interaction can be purposely tuned.

Single-molecule level control of host-guest interactions in metallocycle-C60 complexes

Host−guest interactions are of central importance in many biological and chemical processes. However, the investigation of the formation and decomplexation of host−guest systems at the single-molecule level has been a challenging task. Here we show that the single-molecule conductance of organoplatinum(II) metallocycle hosts can be enhanced by an order of magnitude by the incorporation of a C₆₀ guest molecule. Mechanically stretching the metallocycle-C₆₀ junction with a scanning tunneling microscopy break junction technique causes the release of the C₆₀ guest from the metallocycle, and consequently the conductance switches back to the free-host level. Metallocycle hosts with different shapes and cavity sizes show different degrees of flexibility to accommodate the C₆₀ guest in response to mechanical stretching. DFT calculations provide further insights into the electronic structures and charge transport properties of the molecular junctions based on metallocycles and the metallocycle-C₆₀ complexes.

Studying the single-molecule behavior of host-guest complexes can provide fundamental insights into their supramolecular interactions. Here, the authors use the scanning tunneling microscopy break junction technique to show that encapsulation of a C₆₀ molecule significantly enhances the conductance of an organoplatinum metallocycle; mechanical stretching of the junction releases the guest, returning the conductance to free-host level.

Charge transfer complexation boosts molecular conductance through Fermi level pinning

Interference features in the transmission spectra can dominate charge transport in metal-molecule-metal junctions when they occur close to the contact Fermi energy (E F). Here, we show that by forming a charge-transfer complex with tetracyanoethylene (TCNE) we can introduce new constructive interference features in the transmission profile of electron-rich, thiophene-based molecular wires that almost coincide with E F. Complexation can result in a large enhancement of junction conductance, with very efficient charge transport even at relatively large molecular lengths. For instance, we report a conductance of 10-3 G 0 (∼78 nS) for the ∼2 nm long α-quaterthiophene:TCNE complex, almost two orders of magnitude higher than the conductance of the bare molecular wire. As the conductance of the complexes is remarkably independent of features such as the molecular backbone and the nature of the contacts to the electrodes, our results strongly suggest that the interference features are consistently pinned near to the Fermi energy of the metallic leads. Theoretical studies indicate that the semi-occupied nature of the charge-transfer orbital is not only important in giving rise to the latter effect, but also could result in spin-dependent transport for the charge-transfer complexes. These results therefore present a simple yet effective way to increase charge transport efficiency in long and poorly conductive molecular wires, with important repercussions in single-entity thermoelectronics and spintronics.

Supramolecular Systems and Chemical Reactions in Single-Molecule Break Junctions

The major challenges of molecular electronics are the understanding and manipulation of the electron transport through the single-molecule junction. With the single-molecule break junction techniques, including scanning tunneling microscope break junction technique and mechanically controllable break junction technique, the charge transport through various single-molecule and supramolecular junctions has been studied during the dynamic fabrication and continuous characterization of molecular junctions. This review starts from the charge transport characterization of supramolecular junctions through a variety of noncovalent interactions, such as hydrogen bond, π-π interaction, and electrostatic force. We further review the recent progress in constructing highly conductive molecular junctions via chemical reactions, the response of molecular junctions to external stimuli, as well as the application of break junction techniques in controlling and monitoring chemical reactions in situ. We suggest that beyond the measurement of single molecular conductance, the single-molecule break junction techniques provide a promising access to study molecular assembly and chemical reactions at the single-molecule scale.

Interactions between two C60 molecules measured by scanning probe microscopies

C60-functionalized tips are used to probe C60 molecules on Cu(111) with scanning tunneling and atomic force microscopy. Distinct and complex intramolecular contrasts are found. Maximal attractive forces are observed when for both molecules a [6,6] bond faces a hexagon of the other molecule. Density functional theory calculations including parameterized van der Waals interactions corroborate the observations.

Electron transfer through coordination bond interaction between single molecules: conductance switching by a metal ion

Metal-coordination bond interaction within molecular junctions was revealed to significantly facilitate electron transfer between single molecules. Such facilitation was utilized to construct bistable molecular switches activated by a single metal ion.

Synthesis, electron transport, and charge storage properties of fullerene–zinc porphyrin hybrid nanodiscs

A MIS-type diode (Ag/C₆₀–ZnTANP circular discs/p-Si/Ag) has been fabricated and its charge storage properties have been explored.

High-resolution molecular orbital imaging using a p-wave STM tip

Individual pentacene and naphthalocyanine molecules adsorbed on a bilayer of NaCl grown on Cu(111) were investigated by means of scanning tunneling microscopy using CO-functionalized tips. The images of the frontier molecular orbitals show an increased lateral resolution compared with those of the bare tip and reflect the modulus squared of the lateral gradient of the wave functions. The contrast is explained by tunneling through the p-wave orbitals of the CO molecule. Comparison with calculations using a Tersoff-Hamann approach, including s- and p-wave tip states, demonstrates the significant contribution of p-wave tip states.

Laserlike vibrational instability in rectifying molecular conductors

We study the damping of molecular vibrations due to electron-hole pair excitations in donor-acceptor (D-A) type molecular rectifiers. At finite voltage additional nonequilibrium electron-hole pair excitations involving both electrodes become possible, and contribute to the stimulated emission and absorption of phonons. We point out a generic mechanism for D-A molecules, where the stimulated emission can dominate beyond a certain voltage due to the inverted position of the D and A quantum resonances. This leads to current-driven amplification (negative damping) of the phonons similar to laser action. We investigate the effect in realistic molecular rectifier structures using first-principles calculations.

Preparation of tips coated with poly(dimethylsiloxane) for scanning tunneling microscopy in aqueous solutions

A simple and reliable method for preparing tips coated with poly(dimethylsiloxane) (PDMS) for scanning tunneling microscopy (STM) in aqueous solutions is reported. Au STM tips were dip-coated with PDMS and subsequently cured. The coated tips were characterized by scanning electron microscopy, energy dispersive x-ray spectroscopy, and voltammetry. We show that the PDMS layer smoothly covers the tip surface except for its very apex. These tips enable atomically resolved imaging of a sample surface and measurement of single-molecule conductance below 1.0 nS with STM in aqueous solutions. Because of the superior compatibility of PDMS with organic solvents, the PDMS-coated tips would be useful for preparing molecular tips via their chemical modification in such solvents, enabling chemically selective imaging of a single molecule in water.

Passing current through touching molecules

The charge flow from a single C(60) molecule to another one has been probed. The conformation and electronic states of both molecules on the contacting electrodes have been characterized using a cryogenic scanning tunneling microscope. While the contact conductance of a single molecule between two Cu electrodes can vary up to a factor of 3 depending on electrode geometry, the conductance of the C(60)-C(60) contact is consistently lower by 2 orders of magnitude. First-principles transport calculations reproduce the experimental results, allow a determination of the actual C(60)-C(60) distances, and identify the essential role of the intermolecular link in bi- and trimolecular chains.

Fullerene for organic electronics

This tutorial review surveys and highlights the integration of different molecular wires-in combination with chromophores that exhibit (i) significant absorption cross section throughout the visible part of the solar spectrum and (ii) good electron donating power-into novel electron donor-acceptor conjugates. The focus is predominantly on charge transfer and charge transport features of the most promising systems.

Single-molecule chiral recognition on a surface by chiral molecular tips

Chiral surfaces attract increasing interest due to their vital role in a variety of scientific fields, such as chiral separation and heterogeneous enantioselective catalysis. The most urgent issue in research on such two-dimensional chirality is a lack of methodologies that recognize molecular chirality on a surface. Here we show that the chiral molecular tips enable for the first time discrimination of enantiomers on a single-molecule basis. The chiral selectivity is attributed to favorable chemical interactions that the molecular tips form with only one of two enantiomers. The stereoselective observation reveals spatial distribution of the enantiomers on a surface at the molecular level. The chiral molecular tips open a way for control of organization of enantiomers toward the advanced functionality of these chiral surfaces through knowledge on pivotal roles of chirality on molecular assemblies as shown here.

Designing functionalized porphyrins capable of pseudo-2D self-assembly on surfaces

A crystallography-instructed strategy to highly ordered layering of porphyrins with different topologies on HOPG is developed based on meso-tetraarylporphyrins bearing 2-ethoxyethanol side chains as "sticky ends". These sticky ends are capable of displaying directive hydrogen bonding motifs with the inherent D4h symmetry of the porphyrins. Solvent effects are shown to have an important role in fabricating the adsorption. Metalation and subsequent axial ligation was a key controlling factor in the topology of the layer, leading to pseudo-2D structures on HOPG.

Magnetic field enhanced nano-tip fabrication for four-probe STM studies

We present an improved method for fabricating tungsten STM tips for measuring nanoscale objects using a multiprobe STM. The tips were prepared by a 'drop-off' technique combined with a magnetic field. For magnetic field strengths above 150 Oe the electrolyte was seen to rotate about the tungsten anode. This rotation reduced the adhesion of bubbles and led to a reduction in the cone angle of the tips produced. This property is advantageous for multiprobe measurements as it allows one to bring multiple tips into close proximity.

Charge-Transfer Effect at the Interface of Phthalocyanine−Electrode Contact Studied by Scanning Tunneling Spectroscopy

Scanning tunneling microscopy (STM) and spectroscopy (STS) are used in this work to investigate the charge-transfer effect at the molecule-substrate interface of substituted metal phthalocyanines. STS results revealed that the apparent energy gaps for both fluorinated phthalocyanines and unsubstituted phthalocyanines are essentially the same, which agree with the hybrid density functional calculations. More interestingly, there is a systematic shift of the energy level of valence bands, possibly as the result of charge-transfer effect at the molecule-substrate interface.

Complementary base-pair-facilitated electron tunneling for electrically pinpointing complementary nucleobases

Molecular tips in scanning tunneling microscopy can directly detect intermolecular electron tunneling between sample and tip molecules and reveal the tunneling facilitation through chemical interactions that provide overlap of respective electronic wave functions, that is, hydrogen-bond, metal-coordination-bond, and charge-transfer interactions. Nucleobase molecular tips were prepared by chemical modification of underlying metal tips with thiol derivatives of adenine, guanine, cytosine, and uracil and the outmost single nucleobase adsorbate probes intermolecular electron tunneling to or from a sample nucleobase molecule. We found that the electron tunneling between a sample nucleobase and its complementary nucleobase molecular tip was much facilitated compared with its noncomplementary counterpart. The complementary nucleobase tip was thereby capable of electrically pinpointing each nucleobase. Chemically selective imaging using molecular tips may be coined "intermolecular tunneling microscopy" as its principle goes and is of general significance for novel molecular imaging of chemical identities at the membrane and solid surfaces.