The use of phosphine chalcogenides in the preparation of cobalt chalcogenides

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.

Electron and Spin Delocalization in [Co6 Se8 (PEt3 )6 ]0/+1 Superatoms

Molecular clusters can function as nanoscale atoms/superatoms, assembling into superatomic solids, a new class of solid-state materials with designable properties through modifications on superatoms. To explore possibilities on diversifying building blocks, here we thoroughly studied one representative superatom, Co6 Se8 (PEt3 )6 . We probed its structural, electronic, and magnetic properties and revealed its detailed electronic structure as valence electrons delocalize over inorganic [Co6 Se8 ] core while ligands function as an insulated shell. 59 Co SSNMR measurements on the core and 31 P, 13 C on the ligands show that the neutral Co6 Se8 (PEt3 )6 is diamagnetic and symmetric, with all ligands magnetically equivalent. Quantum computations cross-validate NMR results and reveal degenerate delocalized HOMO orbitals, indicating aromaticity. Ligand substitution keeps the inorganic core nearly intact. After losing one electron, the unpaired electron in [Co6 Se8 (PEt3 )6 ]+1 is delocalized, causing paramagnetism and a delocalized electron spin. Notably, this feature of electron/spin delocalization over a large cluster is attractive for special single-electron devices.

Phosphinecarboxamide based InZnP QDs - an air tolerant route to luminescent III-V semiconductors

We describe a new synthetic methodology for the preparation of high quality, emission tuneable InP-based quantum dots (QDs) using a solid, air- and moisture-tolerant primary phosphine as a group-V precursor. This presents a significantly simpler synthetic pathway compared to the state-of-the-art precursors currently employed in phosphide quantum dot synthesis which are volatile, dangerous and air-sensitive, e.g. P(Si(CH3)3)3.

Controlling Ligand Coordination Spheres and Cluster Fusion in Superatoms

We show that reaction pathways from a single superatom motif can be controlled through subtle electronic modification of the outer ligand spheres. Chevrel-type [Co₆Se₈L₆] (L = PR₃, CO) superatoms are used to form carbene-terminated clusters, the reactivity of which can be influenced through the electronic effects of the surrounding ligands. This carbene provides new routes for ligand substitution chemistry, which is used to selectively install cyanide or pyridine ligands which were previously inaccessible in these cobalt-based clusters. The surrounding ligands also impact the ability of this carbene to create larger fused clusters of the type [Co₁₂Se₁₆L₁₀], providing underlying information for cluster fusion mechanisms. We use this information to develop methods of creating dimeric clusters with functionalized surface ligands with site specificity, putting new ligands in specific positions on this anisotropic core. Finally, adjusting the carbene intermediates can also be used to perturb the geometry of the [Co₆Se₈] core itself, as we demonstrate with a multicarbene adduct that displays a substantially anisotropic core. These additional levels of synthetic control could prove instrumental for using superatomic clusters for many applications including catalysis, electronic devices, and creating novel extended structures.

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.

Inorganic clusters as metalloligands: ligand effects on the synthesis and properties of ternary nanopropeller clusters

Redox-active multimetallic platforms with synthetically addressable and hemilabile active sites are attractive synthetic targets for mimicking the reactivity of enzymatic co-factors toward multielectron transformations. To this end, a family of ternary clusters featuring three edge metal sites anchored on a [Co6Se8] multimetallic support via amidophosphine ligands are a promising platform. In this report, we explore how small changes in the stereoelectronic properties of these ligands alter [Co6Se8] metalloligand formation, but also substrate binding affinity and strength of the edge/support interaction in two new ternary clusters, M3Co6Se8L6 (M = Zn, Fe; L(-) = Ph2PN(-)iPr). These clusters are characterized extensively using a range of methods, including single crystal X-ray diffraction, electronic absorption spectroscopy and cyclic voltammetry. Substrate binding studies reveal that Fe3Co6Se8L6 resists coordination of larger ligands like pyridine or tetrahydrofuran, but binds the smaller ligand CNtBu. Additionally, investigations into the synthesis of new [Co6Se8] metalloligands using two aminophosphines, Ph2PN(H)iPr (LH) and iPr2PN(H)iPr, led to the synthesis and characterization of Co6Se8LH6, as well as the smaller clusters Co4Se2(CO)6LH4, Co3Se(μ2-PPh2)(CO)4LH3, and [Co(CO)3(iPr2PN(H)iPr)]2. Cumulatively, this study expands our understanding on the effect of the stereoelectronic properties of aminophosphine ligands in the synthesis of cobalt chalcogenide clusters, and, importantly on modulating the push-pull dynamic between the [Co6Se8] support, the edge metals and incoming coordinating ligands in ternary M3Co6Se8L6 clusters.

Atomically Defined Nanopropeller Fe3Co6Se8(Ph2PNTol)6: Functional Model for the Electronic Metal-Support Interaction Effect and High Catalytic Activity for Carbodiimide Formation

Atomically defined interfaces that maximize the density of active sites and harness the electronic metal-support interaction are desirable to facilitate challenging multielectron transformations, but their synthesis remains a considerable challenge. We report the rational synthesis of the atomically defined metal chalcogenide nanopropeller Fe₃Co₆Se₈L₆ (L = Ph₂PNTol) featuring three Fe edge sites, and its ensuing catalytic activity for carbodiimide formation. The complex interaction between the Fe edges and Co₆Se₈ support, including the interplay between oxidation state, substrate coordination, and metal-support interaction, is probed in detail using chemical and electrochemical methods, extensive single crystal X-ray diffraction, and electronic absorption and Mössbauer spectroscopy.

Applications of metal selenium/tellurium compounds in materials science

Metal chalcogenides are technologically important materials. Physical, chemical, electrical and mechanical properties of these materials can be fine-tuned by manipulating their shape, size and composition. Although several methods are employed for their synthesis, single-source molecular precursor route has emerged as a versatile strategy for their synthesis and in controlling shape, size and composition of the material under moderate conditions. This chapter gives a brief coverage on the design and development of single-source molecular precursors which have been employed for the preparation of metal selenide/telluride nanocrystals and for deposition of thin films. The discussion includes synthesis of transition-, main group and f-block metal chalcogenolate and/or chalcogenide clusters as precursors and their conversion into metal chalcogenides in the form of thin films and nanostructures. Precursors for ternary metal chalcogenides are also included.

Microporous Battery Electrodes from Molecular Cluster Precursors

Developing novel energy storage materials is critical to many renewable energy technologies. In this work, we report on the synthesis and electrochemical properties of materials composed of porous cobalt selenide microspheres prepared from molecular cluster precursors. The cobalt selenide microspheres excel as Na⁺ ion battery electrode materials, with a specific capacity of ∼550 mA h/g and excellent cycling stability of 85% over 100 cycles, and perform equally well as Li⁺ ion battery electrodes with a specific capacity of ∼600 mA h/g and cycling stability of 80% over 100 cycles. Materials which reversibly store large amounts of Na⁺ ions are uncommon, and these performances represent significant advances in the field. More broadly, this work establishes metal chalcogenide molecular clusters as valuable precursors for creating new, tunable energy storage materials.

Superatom Fusion and the Nature of Quantum Confinement

Quantum confinement endows colloidal semiconducting nanoparticles with many fascinating and useful properties, yet a critical limitation has been the lack of atomic precision in their size and shape. We demonstrate the emergence of quantum confined behavior for the first time in atomically defined Co₆Se₈(PEt₃)₆ superatoms by dimerizing [Co₆Se₈] units through direct fusion. To accomplish this dimerization, we install a reactive carbene on the [Co₆Se₈] core to create a latent fusion site. Then we transform the reactive carbene intermediate into a material with an expanded core, [Co₁₂Se₁₆], that exhibits electronic and optical properties distinct from the parent monomer. The chemical transformation presented herein allows for precise synthetic control over the ligands and size of these clusters. We show by cyclic voltammetry, infrared spectroscopy, single crystal X-ray diffraction, and density functional theory calculations that the resulting fused [Co₁₂Se₁₆] material exhibits strong electronic coupling and electron delocalization. We observe a bandgap reduction upon expanding the cluster core, suggesting that we have isolated a new intermediate in route to extended solids. These results are further corroborated with electronic structure calculations of a monomer, fused dimer, trimer, and tetramer species. These reactions will allow for the synthesis of extended highly delocalized wires, sheets, and cages.

Molecular Clusters: Nanoscale Building Blocks for Solid-State Materials

The programmed assembly of nanoscale building blocks into multicomponent hierarchical structures is a powerful strategy for the bottom-up construction of functional materials. To develop this concept, our team has explored the use of molecular clusters as superatomic building blocks to fabricate new classes of materials. The library of molecular clusters is rich with exciting properties, including diverse functionalization, redox activity, and magnetic ordering, so the resulting cluster-assembled solids, which we term superatomic crystals (SACs), hold the promise of high tunability, atomic precision, and robust architectures among a diverse range of other material properties. Molecular clusters have only seldom been used as precursors for functional materials. Our team has been at the forefront of new developments in this exciting research area, and this Account focuses on our progress toward designing materials from cluster-based precursors. In particular, this Account discusses (1) the design and synthesis of molecular cluster superatomic building blocks, (2) their self-assembly into SACs, and (3) their resulting collective properties. The set of molecular clusters discussed herein is diverse, with different cluster cores and ligand arrangements to create an impressive array of solids. The cluster cores include octahedral M₆E₈ and cubane M₄E₄ (M = metal; E = chalcogen), which are typically passivated by a shell of supporting ligands, a feature upon which we have expanded upon by designing and synthesizing more exotic ligands that can be used to direct solid-state assembly. Building from this library, we have designed whole families of binary SACs where the building blocks are held together through electrostatic, covalent, or van der Waals interactions. Using single-crystal X-ray diffraction (SCXRD) to determine the atomic structure, a remarkable range of compositional variability is accessible. We can also use this technique, in tandem with vibrational spectroscopy, to ascertain features about the constituent superatomic building blocks, such as the charge of the cluster cores, by analysis of bond distances from the SCXRD data. The combination of atomic precision and intercluster interactions in these SACs produces novel collective properties, including tunable electrical transport, crystalline thermal conductivity, and ferromagnetism. In addition, we have developed a synthetic strategy to insert redox-active guests into the superstructure of SACs via single-crystal-to-single-crystal intercalation. This intercalation process allows us to tune the optical and electrical transport properties of the superatomic crystal host. These properties are explored using a host of techniques, including Raman spectroscopy, SQUID magnetometry, electrical transport measurements, electronic absorption spectroscopy, differential scanning calorimetry, and frequency-domain thermoreflectance. Superatomic crystals have proven to be both robust and tunable, representing a new method of materials design and architecture. This Account demonstrates how precisely controlling the structure and properties of nanoscale building blocks is key in developing the next generation of functional materials; several examples are discussed and detailed herein.

Two-Dimensional Nanosheets from Redox-Active Superatoms

We describe a new approach to synthesize two-dimensional (2D) nanosheets from the bottom-up. We functionalize redox-active superatoms with groups that can direct their assembly into multidimensional solids. We synthesized Co6Se8[PEt2(4-C6H4COOH)]6 and found that it forms a crystalline assembly. The solid-state structure is a three-dimensional (3D) network in which the carboxylic acids form intercluster hydrogen bonds. We modify the self-assembly by replacing the reversible hydrogen bonds that hold the superatoms together with zinc carboxylate bonds via the solvothermal reaction of Co6Se8[PEt2(4-C6H4COOH)]6 with Zn(NO3)2. We obtain two types of crystalline materials using this approach: one is a 3D solid and the other consists of stacked layers of 2D sheets. The dimensionality is controlled by subtle changes in reaction conditions. These 2D sheets can be chemically exfoliated, and the exfoliated, ultrathin 2D layers are soluble. After they are deposited on a substrate, they can be imaged. We cast them onto an electrode surface and show that they retain the redox activity of the superatom building blocks due to the porosity in the sheets.

Building Diatomic and Triatomic Superatom Molecules

In this study, we have developed a method to create Co6Se8 superatoms in which we program the metal-ligand bonds. We exclusively form the Co6Se8 core under simple reaction conditions with a facile separation of products that contain differential substitution of the core. The combination of Co2(CO)8 and PR3 with excess Se gives the differentially and directionally substituted superatoms, Co6Se8(CO)x(PR3)(6-x). The CO groups on the superatom can be exchanged quantitatively with phosphines and isonitriles. Substitution of the CO allows us to manipulate the type and length of chemical bridge between two redox-active superatomic centers in order to modulate intersuperatomic coupling. Linking two superatoms together allows us to form the simplest superatom molecule: a diatomic molecule. We extend the superatom molecule concept to link three superatoms together in a linear arrangement to form acyclic triatomic molecules. These superatom molecules have a rich electrochemical profile and chart a clear path to a whole family of superatom molecules with new and unusual collective properties.

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.

Diorganyl dichalcogenides as useful synthons for colloidal semiconductor nanocrystals

The ability to synthesize colloidal semiconductor nanocrystals in a well-controlled manner (i.e., with fine control over size, shape, size dispersion, and composition) has been mastered over the past 15 years. Much of this success stems from careful studies of precursor conversion and nanocrystal growth with respect to phosphine chalcogenide precursors for the synthesis of metal chalcogenide nanocrystals. Despite the high level of success that has been achieved with phosphine chalcogenides, there has been a longstanding interest in exploring alternate chalcogenide precursors because of issues associated with phosphine chalcogenide cost, purity, toxicity, etc. This has resulted in a large body of literature on the use of sulfur and selenium dissolved in octadecene or amines, thio- and selenoureas, and silyl chalcogenides as alternate chalcogenide precursors for metal chalcogenide nanocrystal synthesis. In this Account, emerging work on the use of diorganyl dichalcogenides (R-E-E-R, where E = S, Se, or Te and R = alkyl, allyl, benzyl, or aryl) as alternate chalcogenide precursors for the synthesis of metal chalcogenide nanocrystals is summarized. Among the benefits of these dichalcogenide synthons are the following: (i) they represent the first and only common precursor type that can function as chalcogen transfer reagents for each of the group VI elements (i.e., to make metal oxide, metal sulfide, metal selenide, and metal telluride nanocrystals); (ii) they possess relatively weak E-E bonds that can be readily cleaved under mild thermolytic or photolytic conditions; and (iii) the organic substituents can be tuned to affect the reactivity. These combined attributes have allowed dichalcogenide precursors to be employed for a wide range of metal chalcogenide nanocrystal syntheses, including those for In2S3, SnxGe1-xSe, SnTe, Cu2-xSySe1-y, ZnSe, CdS, CdSe, MoSe2, WSe2, BiSe, and CuFeS2. Interestingly, a number of metastable phases of compositionally complex semiconductors can be kinetically accessed through syntheses utilizing dichalcogenide precursors, likely as a result of their ability to convert at relatively low temperatures. These include the hexagonal wurtzite phases of CuInS2, CuInSe2, Cu2ZnSn(S1-xSex)4, and Cu2SnSe3 nanocrystals. The discovery of crystal phases on the nanoscale that do not exist in their bulk analogues is a developing area of nanocrystal chemistry, and dichalcogenides are proving to be a useful synthetic tool in this regard. The most recent application of dichalcogenide synthons for semiconductor nanocrystals is their use as precursors for surface ligands. While there is a rich history of using thiol ligands for semiconductor nanocrystals, the analogous selenol and tellurol ligands have not been studied, likely because of their oxidative instability. Dichalcogenides have proven useful in this regard, as they can be reduced in situ with diphenylphosphine to give the corresponding selenol or tellurol ligand that binds to the nanocrystal surface. This chemistry has been applied to the in situ synthesis and ligand binding of selenols to PbSe nanocrystals and both selenols and tellurols to CdSe nanocrystals. These initial studies have allowed the photophysics of these nanocrystal-ligand constructs to be investigated; in both cases, it appears that the selenol and tellurol ligands act as hole traps that quench the photoluminescence of the semiconductor nanocrystals.

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.

Structural and spectroscopic characterization of prepared Ag2S nanoparticles with a novel sulfuring agent

Ag2S nanoparticles were prepared by a solvothermal process via reaction of Ag(NO3) and a new sulfuring agent from class of thio Schiff-base (2-(benzylidene amino) benzenethiol (C13H11NS)) in presence of various solvents. Spectra such as X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SEAD), ultraviolet-visible (UV-Vis) spectroscopy, thermo gravimetric-differential thermal analysis (TG-DTA), and Fourier transform Infrared (FT-IR) employed to characterize the synthesized products. Results of this paper indicate that shape and size of the silver sulfide can be controlled by means of setting certain reaction parameters such as the reaction temperature, presence of surfactant, and type of solvent. Silver sulfide nanoparticles with different morphology and size successfully prepared. In addition, the growth mechanisms of the Ag2S nanoparticles discussed preliminarily.

Fluorescent asymmetrically cobalt-tipped CdSe@CdS core@shell nanorod heterostructures exhibiting room-temperature ferromagnetic behavior

A colloidal two-step seeded-growth approach has been devised to selectively synthesize three-component magnetic/semiconductor hybrid nanocrystals (HNCs) with a matchstick-like profile and tunable geometric parameters. The newly developed heterostructures individually comprise a single metallic Co head connected to either apexes of one rod-shaped section made of a CdSe core eccentrically embedded in a CdS shell. The specific topological arrangement realized arises from the peculiar anisotropic reactivity of the noncentrosymmetric CdSe@CdS core@shell nanorods that have been used as substrates to seed heterogeneous nucleation of Co in a surfactant-free environment from an organometallic precursor. The HNCs retain appreciable fluorescent emission in spite of photoexcited charge transfer from the semiconductor to the metal domain and exhibit unusual ferromagnetic-like behavior at room temperature.

Mechanistic Study of Precursor Evolution in Colloidal Group II−VI Semiconductor Nanocrystal Synthesis

The molecular mechanism of precursor evolution in the synthesis of colloidal group II-VI semiconductor nanocrystals was studied using 1H, 13C, and 31P NMR spectroscopy and mass spectrometry. Tri-n-butylphosphine chalcogenides (TBPE; E = S, Se, Te) react with an oleic acid complex of cadmium or zinc (M-OA; M = Zn, Cd) in a noncoordinating solvent (octadecene (ODE), n-nonane-d20, or n-decane-d22), affording ME nanocrystals, tri-n-butylphosphine oxide (TBPO), and oleic acid anhydride ((OA)2O). Likewise, the reaction between trialkylphosphine selenide and cadmium n-octadecylphosphonic acid complex (Cd-ODPA) in tri-n-octylphosphine oxide (TOPO) produces CdSe nanocrystals, trialkylphosphine oxide, and anhydrides of n-octadecylphosphonic acid. The disappearance of tri-n-octylphosphine selenide in the presence of Cd-OA and Cd-ODPA can be fit to a single-exponential decay (kobs = (1.30 +/- 0.08) x 10-3 s-1, Cd-ODPA, 260 degrees C, and kobs = (1.51 +/- 0.04) x 10-3 s-1, Cd-OA, 117 degrees C). The reaction approaches completion at 70-80% conversion of TOPSe under anhydrous conditions and 100% conversion in the presence of added water. Activation parameters for the reaction between TBPSe and Cd-OA in n-nonane-d20 were determined from the temperature dependence of the TBPSe decay over the range of 358-400 K (deltaH++ = 62.0 +/- 2.8 kJ.mol-1, deltaS++ = -145 +/- 8 J.mol-1.K-1). A reaction mechanism is proposed where trialkylphsophine chalcogenides deoxygenate the oleic acid or phosphonic acid surfactant to generate trialkylphosphine oxide and oleic or phosphonic acid anhydride products. Results from kinetics experiments suggest that cleavage of the phosphorus chalcogenide double bond (TOP=E) proceeds by the nucleophilic attack of phosphonate or oleate on a (TOP=E)-M complex, generating the initial M-E bond.