Electrical-field tuned thermoelectric performance of graphene nanoribbon with sawtooth edges.

By using first-principle calculations combined with the non-equilibrium Green's function approach, we report that a vertical electrical field can modulate the thermoelectric performance of a graphene nanoribbon with sawtooth edges. The results show that the sawtooth graphene nanoribbon exhibits the spin-dependent Seebeck effect under the temperature gradients, and is strengthened by increasing the width of the sawtooth graphene nanoribbon. When the vertical electrical field is applied to the device, the spin-dependent Seebeck effect can also be enhanced. The vertical electrical field can modulate the device transferring from hole-conducting to electron-conducting. This opens up the possibility of tuning the thermal transport properties of the device by the electrical field.

Monolayer MXenes: promising half-metals and spin gapless semiconductors.

Half-metals and spin gapless semiconductors are promising candidates for spintronic applications due to the complete (100%) spin polarization of electrons around the Fermi level. Based on recent experimental and theoretical findings of graphene-like monolayer transition metal carbides and nitrides (also known as MXenes), we demonstrate using first-principles calculations that monolayers Ti2C and Ti2N exhibit nearly half-metallic ferromagnetism with the magnetic moments of 1.91 and 1.00µB per formula unit, respectively, while monolayer V2C is a metal with unstable antiferromagnetism, and monolayer V2N is a nonmagnetic metal. Interestingly, under a biaxial strain, there is a phase transition from a nearly half-metal to truly half-metal, spin gapless semiconductor, and metal for monolayer Ti2C. Monolayer Ti2N is still a nearly half-metal under a suitable biaxial strain. Large magnetic moments can be induced by the biaxial tensile and compressive strains for monolayer V2C and V2N, respectively. We also show that the structures of these four monolayer MXenes are stable according to the calculated formation energy and phonon spectrum. Our investigations suggest that, unlike monolayer graphene, monolayer MXenes Ti2C and Ti2N without vacancy, doping or external electric field exhibit intrinsic magnetism, especially the half-metallic ferromagnetism and spin gapless semiconductivity, which will stimulate further studies on possible spintronic applications for new two-dimensional materials of MXenes.

Unusually large spin polarization and magnetoresistance in a FeMg8-FeMg8 superatomic dimer.

Electronic transport across a FeMg8 magnetic superatom and its dimer has been investigated using a density functional theory combined with Keldysh nonequilibrium Green's-function formalism. For a single cluster, our studies for the cluster supported in various orientations on a Au(100) surface show that the transport is sensitive to the contact geometry. Investigations covering the cases where the axes of Mg square antiprism are 45°, perpendicular, and parallel to the transport direction, show that the equilibrium conductance, transferred charge, and current polarizations can all change significantly with orientation. Our studies on the transport across a magnetic superatom dimer FeMg8-FeMg8 focus on the effect of electrode contact distance and the support. The calculated I-V curves show negative differential resistance behavior at larger electrode-cluster contact distances. Further, the equilibrium conductance in ferromagnetic state shows an unusually high spin polarization that is about 81.48% for specific contact distance, and a large magnetoresistance ratio exceeding 500% is also found. The results show that the superatom assemblies can provide unusual transport characteristics, and that the spin polarization and magnetoresistance can be controlled via the contact geometry.

Controlling the band gap energy of cluster-assembled materials.

Cluster-assembled materials combine the nanoscale size and composition-dependent properties of clusters, which have highly tunable magnetic and electronic properties useful for a great variety of potential technologies. To understand the emergent properties as clusters are assembled into hierarchical materials, we have synthesized 23 cluster-assembled materials composed of As7(3-)-based motifs and different countercations and measured their band gap energies. We found that the band gap energy varies from 1.09 to 2.21 eV. In addition, we have carried out first principles electronic structure studies to identify the physical mechanisms that enable control of the band gap edges of the cluster assemblies. The choice of counterion has a profound effect on the band gap energy in ionic cluster assemblies. The top of the valence band is localized on the arsenic cluster, while the conduction band edge is located on the alkali metal counterions. Changing the counterion changes the position of the conduction band edge, enabling control of the band gap energy. We can also vary the architecture of the ionic solid by incorporating cryptates as counterions, which provide charge but are separated from the clusters by bulky ligands. Higher dimensionality typically decreases the band gap energy through band broadening; however band gap energies increased upon moving from zero-dimensional (0D) to two-dimensional (2D) assemblies. This is because internal electric fields generated by the counterion preferentially stabilize the adjacent lone pair orbitals that mark the top of the valence band. Thus, the choice of the counterion can control the position of the conduction band edge of ionic cluster assemblies. In addition, the dimensionality of the solid via internal electric fields can control the valence band edge. Through covalently linking arsenic clusters into composite building blocks, we have also been able to tune the band gap energy. We used a theoretical description based on cluster orbital theory to provide microscopic understanding of the electronic character of the composite building blocks and the observed variations in the band gap energy. Also, we have shown how dimeric linkers can be used to control the band gap energy. Lastly, we also investigated the effects of charge transfer complexes of M(CO)3 on the band gap energy.

Synthesis, structure and band gap energy of covalently linked cluster-assembled materials.

We have synthesized a series of cluster assembled materials in which the building blocks are As(7)(3-) clusters linked by group 12 metals, Zn, Cd and Hg, to investigate the effect of covalent linkers on the band gap energy. The synthesized assemblies include zero dimensional assemblies of [Zn(As(7))(2)](4-), [Cd(As(7))(2)](4-), [Hg(2)(As(7))(2)](4-), and [HgAsAs(14)](3-) in which the clusters are separated by cryptated counterions, and assemblies in which [Zn(As(7))(2)](4-), [Cd(As(7))(2)](4-) are linked by free alkali atoms into unusual three-dimensional structures. These covalently linked cluster-assembled materials have been characterized by elemental analysis, EDX and single-crystal X-ray diffraction. The crystal structure analysis revealed that in the case of Zn and Cd, the two As(7)(3-) units are linked by the metal ion, while in the case of Hg, two As(7)(3-) units are linked by either Hg-Hg or Hg-As dimers. Optical measurements indicate that the band gap energy ranges from 1.62 eV to 2.21 eV. A theoretical description based on cluster orbital theory is used to provide a microscopic understanding of the electronic character of the composite building blocks and the observed variations in the band gap energy.

On the stability of an unsupported mercury-mercury bond linking group 15 Zintl clusters.

The dimeric Zintl ion [Hg(2)(As(7))(2)](4-) has been synthesized with high crystalline yield from the reaction of an ethylendiamine solution of the intermetallic Zintl phase K(3)As(7) with diphenyl mercury. Single crystal X-ray diffraction of [K(2,2,2-crypt)](4)[Hg(2)As(14)], 1 (2,2,2-crypt = 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]-hexacosane), reveals that the cluster anion exhibits a Hg-Hg bond and the compound has been further characterized using Raman spectroscopy, cyclic voltammetry and its band gap energy was measured. Theoretical studies provide a microscopic understanding of the bonding in this unusual compound.

The Zintl ion [As7]2-: an example of an electron-deficient As(x) radical anion.

[K(2,2,2-crypt)](2)[As(7)]·THF, 1 (2,2,2-crypt = 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane) is the first well characterized seven-atom radical anion of group 15. UV-Vis spectroscopy confirms the presence and electronic structure of [As(7)](2-). Cyclic voltammetry in DMF solution shows the As(7)(3-)/As(7)(2-) redox couple as a one-electron reversible process. Theoretical investigations explore the bonding and properties of compound 1.

Controlling band gap energies in cluster-assembled ionic solids through internal electric fields.

Assembling ionic solids where clusters are arranged in different architectures is a promising strategy for developing band gap-engineered nanomaterials. We synthesized a series of cluster-assembled ionic solids composed of [As(7)-Au(2)-As(7)](4-) in zero-, one-, and two-dimensional architectures. Higher connectivity is expected to decrease the band gap energy through band broadening. However, optical measurements indicate that the band gap energy increases from 1.69 to 1.98 eV when moving from zero- to two-dimensional assemblies. This increase is a result of the local electric fields generated by the adjacent counterions, which preferentially stabilize the occupied cluster electronic states.

Cluster-assembled materials: toward nanomaterials with precise control over properties.

One pathway toward nanomaterials with controllable band gaps is to assemble solids where atomic clusters serve as building blocks, since the electronic structures of clusters vary with size and composition. To study the role of organization in cluster assemblies, we synthesized multiple architectures incorporating As(7)(3-) clusters through control of the countercations. Optical measurements revealed that the band gaps vary from 1.1-2.1 eV, even though the assemblies are constructed from the identical cluster building block. Theoretical studies explain this variation as being a result of altering the lowest unoccupied molecular orbital levels by changing the countercations. Additional variations in the gap are made by covalently linking the clusters with species of varying electronegativity to alter the degree of charge transfer. These findings offer a general protocol for syntheses of nanoassemblies with tunable electronic properties.

[Te2As2]2-: a planar motif with "conflicting" aromaticity.

[K(18-crown-6)]2[Te2As2] is the first four-membered ring Zintl anion of elements from groups XV and XVI. The anion has an unexpected triplet aromatic ground state.

From designer clusters to synthetic crystalline nanoassemblies.

Clusters have the potential to serve as building blocks of materials, enabling the tailoring of materials with novel electronic or magnetic properties. Historically, there has been a disconnect between magic clusters found in the gas phase and the synthetic assembly of cluster materials. We approach this challenge through a proposed protocol that combines gas-phase investigations to examine feasible units, theoretical investigations of energetic compositional diagrams and geometrical shapes to identify potential motifs, and synthetic chemical approaches to identify and characterize cluster assemblies in the solid state. Through this approach, we established As7(3-) as a potential stable species via gas-phase molecular beam experiments consistent with its known existence in molecular crystals with As to K ratios of 7:3. Our protocol also suggests another variant of this material. We report the synthesis of a cluster compound, As7K1.5(crypt222-K)1.5, composed of a lattice of As7 clusters stabilized by charge donation from cryptated K atoms and bound by sharing K atoms. The bond dimensions of this supercluster assembled material deduced by X-ray analysis are found to be in excellent agreement with the theoretical calculations. The new compound has a significantly larger band gap than the hitherto known solid. Thus, our approach allows the tuning of the electronic properties of solid cluster assemblies.