Thermoelectric Efficiency of Organometallic Complex Wires via Quantum Resonance Effect and Long-Range Electric Transport Property

Superior long-range electric transport has been observed in several organometallic wires. Here, we discuss the role of the metal center in the electric transport and examine the possibility of high thermoelectric figure of merit (<i>ZT</i>) by controlling the quantum resonance effects. We examined a few metal center (and metal-free) terpyridine-based complexes by first-principles calculations and clarified the role of the metals in determining the transport properties. Quasi-resonant tunneling is mediated by organic compounds, and narrow overlapping resonance states are formed when d<i>-</i>electron metal centers are incorporated. Distinct length (<i>L</i>) and temperature (<i>T</i>) dependencies of thermopower from semiconductor materials or organic molecular junctions are presented in terms of atomistic calculations of <i>ZT</i> with and without considering the phonon thermal conductance. We present an alternative approach to obtain high <i>ZT</i> for molecular junctions by quantum effect

Seebeck Effect in Molecular Wires Facilitating Long-Range Transport

The study of molecular wires facilitating long-range charge transport is of fundamental interest for the development of various technologies in (bio)organic and molecular electronics. Defining the nature of long-range charge transport is challenging as electrical characterization does not offer the ability to distinguish a tunneling mechanism from the other. Here, we show that investigation of the Seebeck effect provides the ability. We examine the length dependence of the Seebeck coefficient in electrografted bis-terpyridine Ru(II) complex films. The Seebeck coefficient ranges from 307 to 1027 μV/K, with an increasing rate of 95.7 μV/(K nm) as the film thickness increases to 10 nm. Quantum-chemical calculations unveil that the nearly overlapped molecular-orbital energy level of the Ru complex with the Fermi level accounts for the giant thermopower. Landauer–Büttiker probe simulations indicate that the significant length dependence evinces the Seebeck effect dominated by coherent near-resonant tunneling rather than thermal hopping. This study enhances our comprehension of long-range charge transport, paving the way for efficient electronic and thermoelectric materials

Thermopower of Benzenedithiol and C₆₀ Molecular Junctions with Ni and Au Electrodes

We have performed thermoelectric measurements of benzenedithiol (BDT) and C₆₀ molecules with Ni and Au electrodes using a home-built scanning tunneling microscope. The thermopower of C₆₀ was negative for both Ni and Au electrodes, indicating the transport of carriers through the lowest unoccupied molecular orbital in both cases, as was expected from the work functions. On the other hand, the Ni–BDT–Ni junctions exhibited a negative thermopower, whereas the Au–BDT–Au junctions exhibited a positive thermopower. First-principle calculations revealed that the negative thermopower of Ni–BDT–Ni junctions is due to the spin-split hybridized states generated by the highest occupied molecular orbital of BDT coupled with <i>s</i>- and <i>d</i>-states of the Ni electrode

Unveiling the Amphiphilic Nature of TMAO by Vibrational Sum Frequency Generation Spectroscopy

By combining heterodyne-detected sum-frequency generation (SFG) spectroscopy, <i>ab initio</i> molecular dynamics (AIMD) simulation, and a post-vibrational self-consistent field (VSCF) approach, we reveal the orientation and surface activity of the amphiphile trimethylamine-<i>N</i>-oxide (TMAO) at the water/air interface. Both measured and simulated C–H stretch SFG spectra show a strong negative and a weak positive peak. We attribute these peaks to the symmetric stretch mode/Fermi resonance and antisymmetric in-plane mode of the methyl group, respectively, based on the post-VSCF calculation. These positive and negative features evidence that the methyl groups of TMAO are oriented preferentially toward the air phase. Furthermore, we explore the effects of TMAO on the interfacial water structure. The O–H stretch SFG spectra manifest that the hydrogen bond network of the aqueous TMAO-solution/air interface is similar to that of the amine-<i>N</i>-oxide (AO) surfactant/water interface. This demonstrates that, irrespective of the alkyl chain length, the AO groups have a similar impact on the hydrogen bond network of the interfacial water. In contrast, we find that adding TMAO to water makes the orientation of the free O−H groups of the interfacial water molecules more parallel to the surface normal. Invariance of the free O–H peak amplitude despite the enhanced orientation of the topmost water layer illustrates that TMAO is embedded in the topmost water layer, manifesting the clear contrast of the hydrophobic methyl group and the hydrophilic AO group of TMAO

Lipid Carbonyl Groups Terminate the Hydrogen Bond Network of Membrane-Bound Water

We present a combined experimental sum-frequency generation (SFG) spectroscopy and <i>ab initio</i> molecular dynamics simulations study to clarify the structure and orientation of water at zwitterionic phosphatidylcholine (PC) lipid and amine <i>N</i>-oxide (AO) surfactant monolayers. Simulated O–H stretch SFG spectra of water show good agreement with the experimental data. The SFG response at the PC interface exhibits positive peaks, whereas both negative and positive bands are present for the similar zwitterionic AO interface. The positive peaks at the water/PC interface are attributed to water interacting with the lipid carbonyl groups, which act as efficient hydrogen bond acceptors. This allows the water hydrogen bond network to reach, with its (<i>up</i>-oriented) O–H groups, into the headgroup of the lipid, a mechanism not available for water underneath the AO surfactant. This highlights the role of the lipid carbonyl group in the interfacial water structure at the membrane interface, namely, stabilizing the water hydrogen bond network

Single Molecular Resistive Switch Obtained via Sliding Multiple Anchoring Points and Varying Effective Wire Length

A single molecular resistive (conductance) switch via control of anchoring positions was examined by using a molecule consisting of more than two same anchors. For this purpose, we adopted the covered quaterthiophene (QT)-based molecular wire junction. The QT-based wire consisted of two thiophene ring anchors on each side; thus, shift of anchors was potentially possible without a change in the binding modes and distortion of the intramolecular structure. We observed three distinct conductance states by using scanning tunneling microscope-based break junction technique. A detailed analysis of the experimental data and first-principles calculations revealed that the mechanism of the resistive switch could be explained by standard length dependence (exponential decay) of conductance. Here, the length is the distance between the anchoring points, i.e., length of the bridged π-conjugated backbone. Most importantly, this effective tunneling length was variable via only controlling the anchoring positions in the same molecule. Furthermore, we experimentally showed the possibility of a dynamic switch of anchoring positions by mechanical control. The results suggested a distinct strategy to design functional devices via contact engineering

Aggregation-Induced Emission Enhancement from Disilane-Bridged Donor–Acceptor–Donor Luminogens Based on the Triarylamine Functionality

Six novel donor–acceptor–donor organic dyes containing a Si–Si moiety based on triarylamine functionalities as donor units were prepared by Pd-catalyzed arylation of hydrosilanes. Their photophysical, electrochemical, and structural properties were studied in detail. Most of the compounds showed attractive photoluminescence (PL) and electrochemical properties both in solution and in the solid state because of intramolecular charge transfer (ICT), suggesting these compounds could be useful for electroluminescence (EL) applications. The aggregation-induced emission enhancement (AIEE) characteristics of <b>1</b> and <b>3</b> were examined in mixed water/THF solutions. The fluorescence intensity in THF/water was stronger in the solution with the highest ratio of water because of the suppression of molecular vibration and rotation in the aggregated state. Single-crystal X-ray diffraction of <b>4</b> showed that the reduction of intermolecular π–π interaction led to intense emission in the solid state and restricted intramolecular rotation of the donor and acceptor moieties, thereby indicating that the intense emission in the solid state is due to AIEE. An electroluminescence device employing <b>1</b> as an emitter exhibited an external quantum efficiency of up to 0.65% with green light emission. The emission comes solely from <b>1</b> because the EL spectrum is identical to that of the PL of <b>1</b>. The observed luminescence was sufficiently bright for application in practical devices. Theoretical calculations and electrochemical measurements were carried out to aid in understanding the optical and electrochemical properties of these molecules

Graphene Layer Encapsulation of Non-Noble Metal Nanoparticles as Acid-Stable Hydrogen Evolution Catalysts

Acid-stable, non-noble catalysts are promising for hydrogen evolution reaction (HER); however, they get easily damaged when used in acidic electrolytes, thus reducing the HER lifetimes. Moreover, completely blocking catalysts from acidic electrolytes degrades HER performance. To achieve a balance between the HER lifetime and performance, we vary the number of N-doped graphene layers (1–2, 2–3, and 3–5 layers) encapsulating NiMo nanoparticles as efficient HER catalysts and obtain the optimal number of protective layers. Our data show that 3–5 graphene layers achieved the best balance, with a stable current density of 100 mA cm^–2 for 25 h in 0.5 M H₂SO₄. Density functional theory calculations are performed to show the effect of encapsulating graphene layer number on the catalytic activity and protection of non-noble NiMo in acidic electrolytes

Cooperation between holey graphene and NiMo alloy for hydrogen evolution in an acidic electrolyte

The development of noble-metal-free hydrogen evolution reaction (HER) materials for electrochemical water splitting is the key to achieving low-cost and efficient electrocatalysis that drives electrochemical hydrogen evolution. However, the electrocatalytic activities of most non-noble metals decrease in acidic electrolytes. Here, we have fabricated non-noble-metal electrodes using a bicontinuous and open porous NiMo alloy covered by nitrogen-doped (N-doped) graphene with nanometer-sized holes. This noble-metal-free HER catalyst exhibits performance almost identical with that of a Pt/C electrode, while its original catalytic activity is preserved even in acidic electrolytes. Density functional theory calculations indicate that the interfacial fringes between the nanoholes and NiMo surface induce charge transfer and promote hydrogen adsorption and desorption. The nanometer-sized holes simultaneously provide minimal surface area for chemical reactions, while delaying NiMo dissolution in excessive amounts of acidic electrolyte. Our method for the fabrication of the NiMo alloy provides a route to a promising class of electrochemical hydrogen-producing electrodes