Electronic State of Oxidized Nanographene Edge with Atomically Sharp Zigzag Boundaries

Combined scanning tunneling microscopy (STM) and density functional theory (DFT) characterizations of the electronic state were performed on the zigzag edge of oxidized nanographene samples. The oxidized zigzag edge with atomically sharp boundaries was prepared by electrochemical oxidation of the graphite surface in aqueous sulfuric acid solution. Bias-dependent STM measurements demonstrated the presence of the edge state at the zigzag edges with local density of states (LDOS) split into two peaks around the Fermi level. Our DFT-based analysis showed that the two-peak structure of the edge state was due to the termination of the zigzag edge by carbonyl functional groups. The LDOS arising from the edge states was slowly dampened in the bulk at the carbonyl-terminated zigzag edges (∼1.5 nm). This result is in clear contrast to the strongly localized edge states at hydrogenated zigzag edges in previous reports. The oxygen atoms in the carbonyl functional groups act as additional π sites at the edges; thus, the topology of the π electron network changes from “zigzag” to “Klein” type, leading to drastic modification of the edge states at the oxidized edges

Fluctuation in Interface and Electronic Structure of Single-Molecule Junctions Investigated by Current versus Bias Voltage Characteristics

Structural and electronic detail at the metal–molecule interface has a significant impact on the charge transport across the molecular junctions, but its precise understanding and control still remain elusive. On the single-molecule scale, the metal–molecule interface structures and relevant charge transport properties are subject to fluctuation, which contain the fundamental science of single-molecule transport and implication for manipulability of the transport properties in electronic devices. Here, we present a comprehensive approach to investigate the fluctuation in the metal–molecule interface in single-molecule junctions, based on current–voltage (<i>I</i>–<i>V</i>) measurements in combination with first-principles simulation. Contrary to conventional molecular conductance studies, this <i>I</i>–<i>V</i> approach provides a correlated statistical description of both the degree of electronic coupling across the metal–molecule interface and the molecular orbital energy level. This statistical approach was employed to study fluctuation in single-molecule junctions of 1,4-butanediamine (DAB), pyrazine (PY), 4,4′-bipyridine (BPY), and fullerene (C₆₀). We demonstrate that molecular-dependent fluctuation of σ-, π-, and π-plane-type interfaces can be captured by analyzing the molecular orbital (MO) energy level under mechanical perturbation. While the MO level of DAB with the σ-type interface shows weak distance dependence and fluctuation, the MO level of PY, BPY, and C₆₀ features unique distance dependence and molecular-dependent fluctuation against the mechanical perturbation. The MO level of PY and BPY with the σ+π-type interface increases with the increase in the stretch distance. In contrast, the MO level of C₆₀ with the π-plane-type interface decreases with the increase in the stretching perturbation. This study provides an approach to resolve the structural and electronic fluctuation in the single-molecule junctions and insight into the molecular-dependent fluctuation in the junctions

Electric Conductance of Single Ethylene and Acetylene Molecules Bridging between Pt Electrodes

We have investigated the conductance and atomic structure of single ethylene and acetylene molecule junctions on the basis of the conductance measurement and vibration spectroscopy of the single molecule junction. Single molecule junctions have a conductance comparable to that of metal atomic junctions (around 0.9<i>G</i>₀: <i><i>G</i></i>₀ = 2<i>e</i>²/<i>h</i>) due to effective hybridization between metal and the π molecular orbital. The ethylene molecules are bound to Pt electrodes via a di-σ bond, while the acetylene molecules are bound to Pt electrodes via di-σ and π bonds. By using the highly conductive single molecule junctions, we investigated the characteristics of vibration spectroscopy of the single molecule junction in an intermediate regime between tunneling and contact. The vibration modes that could modify the conduction orbital were excited for the ethylene and acetylene molecule junctions. The crossover between conductance enhancement and suppression was observed for the single ethylene molecule junction, whereas clear crossover was not observed for the acetylene molecule junction, reflecting the number of conduction orbitals in the single molecule junction

Single Tripyridyl–Triazine Molecular Junction with Multiple Binding Sites

We present an electronic characterization of a single molecular junction of 2,4,6-tris­(2′,2″,2‴-pyridyl)-1,3,5-triazine (TPTZ) with multiple metal–molecule binding sites using scanning tunneling microscopy-based break junction method under ambient conditions. The TPTZ molecule consists of a centered triazine moiety and surrounding three 2-pyridyl groups. The benzene rings containing a N atom in TPTZ act as molecular binding sites for bridging a gap between two Au electrodes to form a single molecular junction. Because the N atom at the <i>ortho</i>-position in the 2-pyridyl groups is spatially hidden from the electrode surfaces, the single molecular junction forms via direct metal−π couplings. We demonstrated that the single TPTZ molecular junctions exhibit highly conductive character up to 10^–1 <i>G</i>₀ (<i>G</i>₀ = 2<i>e</i>²/<i>h</i>), which is due to the effect of the direct metal−π coupling. We found three preferential conductance states of ca. 10^–1, 10^–2, and 10^–4 <i>G</i>₀, which suggests that the single TPTZ molecular junctions have three charge transport paths depending on the molecular anchoring sites on the Au electrodes. Analysis of electrode–gap distance in the molecular junction revealed that effective gap length is 0.5, 0.9, and 1.2 nm for the high, medium, and low conductance states, respectively. By combining the results of the measured conductance and the estimated electrode–gap distance, we proposed models of junction-structures for the observed three conductance states. This study demonstrates that a molecular junction consisting of multiple metal−π binding sites provides high and tunable conductance behavior based on the multiple charge transport paths within a molecule

Single Molecular Bridging of Au Nanogap Using Aryl Halide Molecules

Single molecular junctions of benzene dihalide molecules (<i>para</i>-X–(C₆H₄)–X, X = Cl, Br, I) binding to Au electrodes were systematically studied by using the scanning tunneling microscopy break junction (STM-BJ) technique. The STM-BJ characterization revealed that the single molecular junction was formed only with 1,4-diiodobenzene, which was due to its ability to form particularly stable halogen bonds with Au electrodes for the iodide anchoring group. The conductance and strength of the metal–molecule bond of the single 1,4-diiodobenzene molecular junction were compared with that of 1,4-benzenediamine (<i>para-</i>H₂N–(C₆H₄)–NH₂). The conductance of a single 1,4-diiodobenzene molecular junction was 3.6 × 10^–4 <i>G</i>₀ (<i>G</i>₀ = 2e²/h), which was smaller than 1 × 10^–2 <i>G</i>₀ measured for 1,4-benzenediamine. The distances to break single molecular junctions were 0.05 and 0.03 nm for single 1,4-diiodobenzene and 1,4-benzenediamine molecular junctions, respectively. The longer breakdown distance of the single 1,4-diiodobenzene molecular junctions indicated that the Au–I bond was stronger than that of the Au–NH₂ bond. The present work demonstrates that an iodide group can be utilized as an anchoring group for the single molecular junction

Formation of a Chain-like Water Single Molecule Junction with Pd Electrodes

Atomic scale interaction between the water molecule and the Pd electrodes was investigated by the mechanically controllable break junction technique at cryogenic temperature. The interaction between the water molecule and the atomic scale Pd electrodes and the resultant formation of the single-molecule junction of the water molecule bridging the gap between the Pd electrodes were confirmed by vibrational spectroscopy where the water–Pd vibrational mode of 70 meV was identified. We found that no water dissociation occurred on the atomic scale Pd electrodes. The electronic transport measurement revealed that the water single molecule junction carried the electronic current in the ballistic transport regime and the conductance was determined to be 1 <i>G</i>₀ where <i>G</i>₀ is the conductance quantum. The length analysis and current-bias voltage measurement of the junction suggest that the single water molecule is connected to Pd atomic chain

Bowl Inversion and Electronic Switching of Buckybowls on Gold

Bowl-shaped π-conjugated compounds, or buckybowls, are a novel class of sp²-hybridized nanocarbon materials. In contrast to tubular carbon nanotubes and ball-shaped fullerenes, the buckybowls feature structural flexibility. Bowl-to-bowl structural inversion is one of the unique properties of the buckybowls in solutions. Bowl inversion on a surface modifies the metal–molecule interactions through bistable switching between bowl-up and bowl-down states on the surface, which makes surface-adsorbed buckybowls a relevant model system for elucidation of the mechano-electronic properties of nanocarbon materials. Here, we report a combination of scanning tunneling microscopy (STM) measurements and ab initio atomistic simulations to identify the adlayer structure of the sumanene buckybowl on Au(111) and reveal its unique bowl inversion behavior. We demonstrate that the bowl inversion can be induced by approaching the STM tip toward the molecule. By tuning the local metal–molecule interaction using the STM tip, the sumanene buckybowl exhibits structural bistability with a switching rate that is two orders of magnitude faster than that of the stochastic inversion process

Formation of Single Cu Atomic Chain in Nitrogen Atmosphere

We study the conductance and geometry of the Cu atomic junction in the presence of N₂, through combination of experimental measurement and theoretical calculation. A mechanically controllable break-junction measurement at low temperature reveals N₂ molecules stabilize a Cu atomic junction, and reduce its conductance value. The length analysis about the Cu atomic junction indicates that it is elongated with the length of a few atoms, although it is not elongated without molecules. We investigate Cu atomic junction’s geometry by calculating the conductance and total energies of the several models by changing the separation between two Cu electrodes. Through combination of experimental and theoretical study, we show that the Cu linear atomic chain is formed with the support of N₂ molecule, and N₂ molecule attached on the Cu linear atomic chain. The formation of Cu linear atomic chain is explained that attached N₂ molecules reduce the surface energy of the Cu atomic junction or N₂ molecule directly supports the Cu–Cu bond

“Doping” of Polyyne with an Organometallic Fragment Leads to Highly Conductive Metallapolyyne Molecular Wire

Exploration of highly conductive molecules is essential to achieve single-molecule electronic devices. The present paper describes the results on single-molecule conductance study of polyyne wires doped with the organometallic Ru­(dppe)₂ fragment, X(CC)_<i>n</i>Ru­(dppe)₂(CC)_<i>n</i>X. The metallapolyyne wires end-capped with the gold fragments (X = AuL) are subjected to single-molecule conductance measurements with the STM break junction technique, which reveal the high conductance (10^–3–10^–2 <i>G</i>₀; <i>n</i> = 2–4) with the low attenuation factor (0.25 Å^–1) and the low contact resistance (33 kΩ). A unique “‘doping’” effect of Ru­(dppe)₂ fragment was found to lead to the high performance as suggested by the hybrid density functional theory-nonequilibrium green function calculation

Effect of the Molecule–Metal Interface on the Surface-Enhanced Raman Scattering of 1,4-Benzenedithiol

The influence of the number of molecule–metal interactions on the surface-enhanced Raman scattering (SERS) spectroscopy of 1,4-benzenedithiol (BDT) was investigated. For this purpose, a series of SERS-active samples were prepared featuring one or two molecule–metal interfaces. Molecules were adsorbed on the surface of a rough Au substrate, or sandwiched between Au nanoparticles (NPs) and a flat Au(111) substrate in a “sphere–plane” disposition. In the presence of the Au surface(s), vibrational energy and intensity of the SERS spectra differs significantly from the bulk. Molecule–metal charge transfer upon chemisorption weakens intramolecular bonds, resulting in the observed red shift of the breathing and CC stretching modes. This effect was found to be more pronounced for samples with multiple molecule–metal interfaces. In addition, the SERS spectra of BDT featured additional <i>b</i>₂ signals not present in the bulk spectra. Chemical enhancement of the <i>b</i>₂ modes takes place by means of photoinduced charge transfer from an occupied molecular orbital to an unoccupied metal orbital. Analysis of the normalized SERS intensity revealed a larger scattering enhancement for the samples with a sphere–plane disposition arising from the stronger electromagnetic enhancement effect via plasmonic localization of optical fields. Complementary studies on 4-aminobenzenethiol support these findings