Conductance of Single Cobalt Chalcogenide Cluster Junctions

Observing the Role of Electron Delocalization in Electronic Transport by Incorporating Actinides into Ligated Metal-Chalcogenide Superatoms

Since delocalization of electronic states is a prerequisite for exerting unique electron transport properties, early actinides (An) with highly delocalized 5f/6d orbitals are natural candidates. However, given the experimental difficulties of such radioactive compounds and the complex relativistic effects in theoretical studies, understanding the electronic structure and bonding of actinides is underdeveloped on the periodic table. A further challenge is the very complicated electronic structures encountered in the confinement of actinides, as vividly illustrated by the weakly radioactive Th(Thorium)-encapsulated metal chalcogenide clusters, Th@Co6Te8L6 (L = PH3, PMe3, PEt3). Here we report the electronic structure and the electron transport properties of the Th@Co6Te8L6 clusters and compare them with those of the hollow Co6Te8L6 clusters using the nonequilibrium Green's function combined with relativistic density functional theory (NEGF-DFT). We found that the equilibrium conductance in Th@Co6Te8(PH3)6 (0.76 G0) has been greatly improved over that in Co6Te8(PH3)6 (0.03 G0), which has also been verified under an applied different bias voltage. The covalent bonding character between 6d (Th) and 3d (Co) atomic orbitals resulting from steric confinement is the source of the performance enhancement and a most important factor governing the accessibility of such 5f/6d orbitals. The results are of significance to the rapidly developing field of molecular nanoelectronics.

High-Spin Superatom Stabilized by Dual Subshell Filling

Quantum confinement in small symmetric clusters leads to the bunching of electronic states into closely packed shells, enabling the classification of clusters with well-defined valences as superatoms. Like atoms, superatomic clusters with filled shells exhibit enhanced electronic stability. Here, we show that octahedral transition-metal chalcogenide clusters can achieve filled shell electronic configurations when they have 100 valence electrons in 50 orbitals or 114 valence electrons in 57 orbitals. While these stable clusters are intrinsically diamagnetic, we use our understanding of their electronic structures to theoretically predict that a cluster with 107 valence electrons would uniquely combine high stability and high-spin magnetic moment, attained by filling a majority subshell of 57 electrons and a minority subshell of 50 electrons. We experimentally demonstrate this predicted stability, high-spin magnetic moment (S = 7/2), and fully delocalized electronic structure in a new cluster, [NEt₄]₅[Fe₆S₈(CN)₆]. This work presents the first computational and experimental demonstration of the importance of dual subshell filling in transition-metal chalcogenide clusters.

Soluble Complexes of Cobalt Oxide Fragments Bring the Unique CO2 Photoreduction Activity of a Bulk Material into the Flexible Domain of Molecular Science

The deposition of metal oxides is essential to the fabrication of numerous multicomponent solid-state devices and catalysts. However, the reproducible formation of homogeneous metal oxide films or of nanoparticle dispersions at solid interfaces remains an ongoing challenge. Here we report that molecular hexaniobate cluster anion complexes of structurally and electronically distinct fragments of cubic-spinel and monoclinic Co₃O₄ can serve as tractable yet well-defined functional analogues of bulk cobalt oxide. Notably, the energies of the highest-occupied and lowest-unoccupied molecular orbitals (HOMO and LUMO) of the molecular complexes, 1, closely match the valence- and conduction-band (VB and CB) energies of the parent bulk oxides. Use of 1 as a molecular analogue of the parent oxides is demonstrated by its remarkably simple deployment as a cocatalyst for direct Z-scheme reduction of CO₂ by solar light and water. Namely, evaporation of an aqueous solution of 1 on TiO₂-coated fluorinated tin oxide windows (TiO₂/FTO), immersion in wet acetonitrile, and irradiation by simulated solar light under an atmosphere of CO₂ give H₂, CO, and CH₄ in ratios nearly identical to those obtained using 20 nm spinel-Co₃O₄ nanocrystals, but 15 times more rapidly on a Co basis and more rapidly overall than other reported systems. Detailed investigation of the photocatalytic properties of 1 on TiO₂/FTO includes confirmation of a direct Z-scheme charge-carrier migration pathway by in situ irradiated X-ray photoelectron spectroscopy. More generally, the findings point to a potentially important new role for coordination chemistry that bridges the conceptual divide between molecular and solid-state science.

Electron transport properties of PAl12-based cluster complexes

The electronic transport properties of PAl12-based cluster complexes are investigated by density functional theory (DFT) in combination with the non-equilibrium Green's function (NEGF) method. Joining two PAl12 clusters via a germanium linker creates a stable semiconducting complex with a large HOMO-LUMO gap. Sequential attachment of an electron-donating ligand, N-ethyl-2-pyrrolidone, to one of the two linked clusters results in the shifting of the electronic spectrum of the ligated cluster while the energy levels of the unligated cluster are mostly unchanged. Using this approach, one can eventually align the HOMO of the ligated cluster to the LUMO of the non-ligated cluster, thereby significantly reducing the HOMO-LUMO gap of the complex. As a result, the transport properties of the complex are highly dependent on the number of attached ligands. Although a single ligand is observed to generally decrease the current, the inclusion of two or more ligands shows a significant increase in the amount of current at most voltages. The resulting increase of the current can be attributed to two factors, first the reduction in the HOMO-LUMO gap due to ligand attachment which has moved the transmission orbitals into the bias window. Secondly, when two or more ligands are attached to the complex, the HOMOs become delocalized across the scattering region, and this significantly enhances the currents.

Interfacial magnetism in a fused superatomic cluster [Co6Se8(PEt3)5]2

An isolated Co₆Se₈(PEt₃)₆ cluster is non-magnetic; however, we find that a magnetic unit can be formed by fusing two Co₆Se₈(PEt₃)₅ superatoms into a [Co₆Se₈(PEt₃)₅]₂ dimer. Theoretical studies indicate that the dumbbell-shaped [Co₆Se₈(PEt₃)₅]₂ dimer has a spin moment of 2μ_B, and the spin density is primarily localized at the interfacial Co-sites where two clusters are fused into a dimer. The dimer has a low ionization energy of 4.17 eV, allowing the dimer to donate charge to C₇₀ during the formation of a cluster assembled material, as seen in recent experiments by Nuckolls and co-workers. The donation of charge causes the dimer's magnetic moment to drop from 2μ_B to 1μ_B. We hypothesize that adding electrons to the dimer, such as doping impurities to the crystal lattice, may enhance the magnetic moment by neutralizing the charged cluster. This reveals a strategy for stabilizing magnetic moments in ligated cluster assemblies.

The superatomic state beyond conventional magic numbers: Ligated metal chalcogenide superatoms

The field of cluster science is drawing increasing attention due to the strong size and composition-dependent properties of clusters and the exciting prospect of clusters serving as the building blocks for materials with tailored properties. However, identifying a unifying central paradigm that provides a framework for classifying and understanding the diverse behaviors is an outstanding challenge. One such central paradigm is the superatom concept that was developed for metallic and ligand-protected metallic clusters. The periodic electronic and geometric closed shells in clusters result in their properties being based on the stability they gain when they achieve closed shells. This stabilization results in the clusters having a well-defined valence, allowing them to be classified as superatoms-thus extending the Periodic Table to a third dimension. This Perspective focuses on extending the superatomic concept to ligated metal-chalcogen clusters that have recently been synthesized in solutions and form assemblies with counterions that have wide-ranging applications. Here, we illustrate that the periodic patterns emerge in the electronic structure of ligated metal-chalcogenide clusters. The stabilization gained by the closing of their electronic shells allows for the prediction of their redox properties. Further investigations reveal how the selection of ligands may control the redox properties of the superatoms. These ligated clusters may serve as chemical dopants for two-dimensional semiconductors to control their transport characteristics. Superatomic molecules of multiple metal-chalcogen superatoms allow for the formation of nano-p-n junctions ideal for directed transport and photon harvesting. This Perspective outlines future developments, including the synthesis of magnetic superatoms.

Massive dipoles across the metal-semiconductor cluster interface: towards chemically controlled rectification

An interface between a metallic cluster (MgAl₁₂) and a semiconducting cluster (Re₆Se₈(PMe₃)₅) is shown to be marked by a massive dipole reminiscent of a dipolar layer leading to a Schottky barrier at metal-semiconductor interfaces. The metallic cluster MgAl₁₂ with a valence electron count of 38 electrons is two electrons short of 40 electrons needed to complete its electronic shells in a superatomic model and is marked by a significant electron affinity of 2.99 eV. On the other hand, the metal-chalcogenide semiconducting cluster Re₆Se₈(PMe₃)₅, consisting of a Re₆Se₈ core ligated with five trimethylphosphine ligands, is highly stable in the +2 charge-state owing to its electronic shell closure, and has a low ionization energy of 3.3 eV. The composite cluster Re₆Se₈(PMe₃)₅-MgAl₁₂ formed by combining the MgAl₁₂ cluster through the unligated site of Re₆Se₈(PMe₃)₅ exhibits a massive dipole moment of 28.38 D resulting from a charge flow from Re₆Se₈(PMe₃)₅ to the MgAl₁₂ cluster. The highest occupied molecular orbital (HOMO) of the composite cluster is on the MgAl₁₂ side, which is 0.53 eV below the lowest unoccupied molecular orbital (LUMO) localized on the Re₆Se₈(PMe₃)₅ cluster, reminiscent of a Schottky barrier at metal-semiconductor interfaces. Therefore, the combination can act as a rectifier, and an application of a voltage of approximately 4.1 V via a homogeneous external electric field is needed to overcome the barrier aligning the two states: the HOMO in MgAl₁₂ with the LUMO in Re₆Se₈(PMe₃)₅. Apart from the bias voltage, the barrier can also be reduced by attaching ligands to the metallic cluster, which provides chemical control over rectification. Finally, the fused cluster is shown to be capable of separating electron-hole pairs with minimal recombination, offering the potential for photovoltaic applications.

Single-atom control of electrical conductance and thermopower through single-cluster junctions

The control of single atoms offers fundamental insight into understanding the charge transport through single clusters, and the atomic precision of the clusters provides the opportunity to manipulate the charge transport even at the single-atom level. Herein, we designed and investigated the electrical conductance and thermopower of Anderson-type polyoxometalate (POM) clusters with single-atom variation using the scanning tunneling microscopy break-junction (STM-BJ) technique. Our results show the electrical conductance of single clusters can be changed by an order of magnitude by substituting different center-metal atoms, and the electrical conductance of clusters shows different bias-dependence. Furthermore, the Seebeck coefficients of the POM clusters also can be significantly changed by the center-metal atoms. The non-equilibrium quantum transport calculations reveal that the electrostatic potential profile is non-uniformly dependent on the center-metal atoms. This leads to gating of electrical conductance by bias voltage. This supports the tuning of thermopower and bias dependent transmission spectra. This work provides the fundamental understanding of single-atom control of charge transport in POM single-cluster junctions.

Single-cluster electronics

This article reviews the scope of inorganic cluster compounds interrogated in single-molecule break-junction measurements. This body of work lies at the intersection between the fields of inorganic cluster chemistry and single-molecule electronics, where discrete inorganic cluster molecules are used as the active components in molecular electronic circuitry. We explore the breadth of transition metal and main group cluster compounds that have been studied in single-cluster junctions, largely within the context of scanning tunnelling microscopy break-junction (STM-BJ) measurements. Our discussion centers on how the structure and bonding of inorganic cluster compounds give rise to desirable quantum transport effects such as room-temperature current blockade, sequential tunneling, voltage-gated conductance switching, destructive quantum interference, and high thermoelectric currents.

A ligand-induced homojunction between aluminum-based superatomic clusters

A superatomic molecule formed by joining two metallic clusters linked by an organometallic bridge can behave like a semiconductor and the addition of ligands can induce a significant energy level shift across an inter-cluster homojunction. This shift is induced by the N-ethyl-2-pyrrolidone ligands, and the placement of the ligands strongly affects the direction of the dipole moment, including the case where the dipole moment is parallel to the cluster interface. This computational study provides an alternative strategy for constructing nanometer-scale electronic interfaces between clusters mimicking semiconductor motifs. The semiconducting features in the PAl12 clusters emerge from the grouping of the quantum states in a confined nearly free electron gas that creates a substantial energy gap. An organometallic Ge(CH3)2(CH2)2 bridge links the clusters while maintaining the cluster's electronic shell structure. The amount of level shifting between the bridged clusters can be changed by controlling the number of ligands. Attaching multiple ligands can result in a broken gap energy alignment in which the HOMO level of one cluster is aligned with the LUMO level of the other bridged cluster. Furthermore, the singly ligated bridged superatomic molecule is found to exhibit promising features to separate the electron-hole pairs for photovoltaic applications.

Templated syntheses of heterometal tungsten-cobalt-sulfur clusters with different nuclearities

A series of new W-Co-S clusters have been synthesized utilizing the complex [Et₄N][(Tp*)WS₃] (Tp* = hydridotris(3,5-dimethylpyrazol-1-yl)borate(1-)) as the synthetic template, by rational control of the stoichiometry of reactants and by employment of a reductant or precipitant as an auxiliary reagent. By solubility control or addition of a precipitant to avoid the involvement of Cl^- in the reaction, the cluster [(Tp*)WS₃]₂Co (2) could be obtained. With different ratios of the Co source to the template, together with the use of an appropriate amount of reducing reagent, clusters [{(Tp*)WS₃}₂Co]^1- (3) and [{(Tp*)WS₃}₂Co₂Cl]^1- (4) could be synthesized as their Et₄N⁺ salts, and [{(Tp*)WS₃}₂Co₄Cl₂(DMF)]^1- (5) could be isolated as its [Co(DMF)₆]²⁺ salt. A common structural feature of these clusters is that they all have a pair of [(Tp*)WS₃] templates chelating different numbers of Co atoms to form W-Co-S clusters with distinct nuclearities. The clusters [(Tp*)WS₃]₂Co (2) and [{(Tp*)WS₃}₂Co]^1- (3) both have one Co atom chelated by two [(Tp*)WS₃] templates, and the difference lies in not only the oxidation states but also the fact that the Co atom in 2 adopts an octahedral coordination environment while that in 3 bears a tetrahedral coordination sphere, which means that the [(Tp*)WS₃] template is tri-dentate in 2 and bi-dentate in 3. The cluster [{(Tp*)WS₃}₂Co₂Cl]^1- (4) has two Co atoms chelated by a pair of [(Tp*)WS₃] templates, in which one of the Co atoms is also terminally bound by a Cl^- ligand. There are four Co atoms chelated by two [(Tp*)WS₃] templates in the cluster [{(Tp*)WS₃}₂Co₄Cl₂(DMF)]^1- (5), two of which are bound terminally by the Cl^- ligands. Studies of redox properties by cyclic voltammetry indicate the existence of two reversible redox pairs related to clusters 2 and 3, respectively.

Superatomic molecules with internal electric fields for light harvesting

Traditional p-n junctions used for photovoltaics require an interface where a light induced electron-hole pair is separated by an electric field. Developing alternative strategies for forming strong internal electric fields for electron-hole pair separation offers the possibility for better performance. We demonstrate that fusing two superatomic clusters with donor/acceptor ligands on opposite sides of the cluster leads to such a strong internal electric field. In two fused metal-chalcogenide Re₆S₈Cl₂(L)₄ clusters with donor PMe₃ ligands and acceptor CO ligands on the opposite sides of the fused clusters, the electronic levels undergo shifts analogous to band bending in traditional p-n junctions. The fused cluster has a large dipole moment, and an optical spectrum that strongly absorbs excitation above the HOMO-LUMO gap of the fused clusters, but is optically very weak for the lowest energy excitation that can lead to electron-hole pair recombination. This is because the electron is localized on the CO portion of the fused cluster, while the electron-hole pair is localized on the PMe₃ side of the cluster. It is shown that the electronic states localized on each side of the cluster can be aligned/misaligned by applying a voltage in different directions, offering diode like characteristics.

Transmission mechanism and quantum interference in fused thienoacenes coupling to Au electrodes through the thiophene rings

So far, quantum interference in molecular devices where the anchors are inseparable parts of the whole molecule has been seldom discussed. In this article, we perform first-principles calculations on the electronic transmission properties of a series of Au-thienoacene-Au junctions where the molecule interacts with electrodes through the S atoms in thiophene rings in a fused-ring system. The calculated binding energy of the Au-S interaction is highly dependent on the substitution sites of the thiophene rings, which agrees with the experimental report that the Au-S interaction is too weak to form a junction for some molecules. The electronic coupling at the molecule-electrode contact is also affected by the molecular structure. To distinguish the coupling mechanism, we show the importance of investigating the electron distribution of frontier molecular orbitals in combination with the percentage of the π system in the partial density of states on the S atoms. Because of the difference in electronic coupling, comparison between molecules should be done with care. On the other hand, conductance suppression due to the destructive quantum interference originating from the molecular topology is demonstrated by comparing the properties of isomers with similar molecule-electrode coupling.

Patterning Superatom Dopants on Transition Metal Dichalcogenides

This study describes a new and simple approach to dope two-dimensional transition metal dichalcogenides (TMDCs) using the superatom Co6Se8(PEt3)6 as the electron dopant. Semiconducting TMDCs are wired into field-effect transistor devices and then immersed into a solution of these superatoms. The degree of doping is determined by the concentration of the superatoms in solution and by the length of time the films are immersed in the dopant solution. Using this chemical approach, we are able to turn mono- and few-layer MoS2 samples from moderately to heavily electron-doped states. The same approach applied on WSe2 films changes their characteristics from hole transporting to electron transporting. Moreover, we show that the superatom doping can be patterned on specific areas of TMDC films. To illustrate the power of this technique, we demonstrate the fabrication of a lateral p-n junction by selectively doping only a portion of the channel in a WSe2 device. Finally, encapsulation of the doped films with crystalline hydrocarbon layers stabilizes their properties in an ambient environment.

Rigid multipodal platforms for metal surfaces

In this review the recent progress in molecular platforms that form rigid and well-defined contact to a metal surface are discussed. Most of the presented examples have at least three anchoring units in order to control the spatial arrangement of the protruding molecular subunit. Another interesting feature is the lateral orientation of these foot structures which, depending on the particular application, is equally important as the spatial arrangement of the molecules. The numerous approaches towards assembling and organizing functional molecules into specific architectures on metal substrates are reviewed here. Particular attention is paid to variations of both, the core structures and the anchoring groups. Furthermore, the analytical methods enabling the investigation of individual molecules as well as monomolecular layers of ordered platform structures are summarized. The presented multipodal platforms bearing several anchoring groups form considerably more stable molecule-metal contacts than corresponding monopodal analogues and exhibit an enlarged separation of the functional molecules due to the increased footprint, as well as restrict tilting of the functional termini with respect to the metal surface. These platforms are thus ideally suited to tune important properties of the molecule-metal interface. On a single-molecule level, several of these platforms enable the control over the arrangement of the protruding rod-type molecular structures (e.g., molecular wires, switches, rotors, sensors) with respect to the surface of the substrate.

Solvent-dependent conductance decay constants in single cluster junctions

We study single cluster electrical transport in a series of metal chalcogenide molecular clusters using scanning tunneling microscope-based break-junction measurements.

Single-molecule conductance measurements have focused primarily on organic molecular systems. Here, we carry out scanning tunneling microscope-based break-junction measurements on a series of metal chalcogenide Co₆Se₈ clusters capped with conducting ligands of varying lengths. We compare these measurements with those of individual free ligands and find that the conductance of these clusters and the free ligands have different decay constants with increasing ligand length. We also show, through measurements in two different solvents, 1-bromonaphthalene and 1,2,4-trichlorobenzene, that the conductance decay of the clusters depends on the solvent environment. We discuss several mechanisms to explain our observations.

A cyclic dodecanuclear cobalt cluster based on a derivative of the rhodamine 6G dye with unusual magnetization

A novel cyclic dodecanuclear cluster [Co^II₁₂(L)₆(OH)₆(Ac)₆]·2DMF (1) decorated by a rhodamine 6G derivative (H₂L) presents dominant intracluster antiferromagnetic interactions with S-shaped M vs. H plots.

Synthesis and investigation of cobalt chalcogenide clusters with thienyl phosphine ligands as new acceptor materials for P3HT

In progress towards a covalently linked organic–inorganic structure, cobalt chalcogenide clusters were synthesized with thienyl phosphines ligands. Results indicate the clusters have potential as new acceptor materials for P3HT.