Nitride Clusters Research Articles

Transport properties through hexagonal boron nitride clusters embedded in graphene nanoribbons

First-principles calculations are employed in the study of the electronic and quantum transport properties of hexagonally shaped boron nitride (h-BN) clusters embedded in either zigzag or armchair graphene nanoribbons. Chemical doping of the h-BN cluster was taken into consideration by using carbon atoms to replace either the boron () or the nitrogen () sites in the central ring. While the quantum conductance of the system with zigzag edges is found to be spin-dependent, it was observed that the system with an armchair edge requires an electron imbalance in order to show a spin-dependent conductance. Furthermore, the possibility of molecular adsorption onto these doped systems is studied. The effects of the attached molecules to the quantum conductance shows the potential of these hybrid systems for molecular sensing applications.

The structure evolution process and the electronic properties of armchair boron nitride nanotubes

We report the results of density functional calculations on the structural evolution and electronic properties of armchair boron nitride nanotubes (BNNTs), including SWBNNTs and DWBNNTs. Our results show that the initial structural configuration of the BNNTs was determined by the small boron nitride clusters. The evolution process of the BNNTs is through forming tubular clusters with a global reconstruction from structure of the double-rings. Then, it elongates through the layer-by-layer growth process with local reconstructions. Eventually, the infinite BNNTs can be constructed with corresponding repeat unit, designed by the periodic characteristics on the basis of tubular clusters. From the band structure of armchair BNNTs, it can be found the gap slightly increases with increasing diameter of the tube, decrease with the increasing of the walls. Moreover, the evolution process provides a better way to understand the growth mechanism of armchair BNNTs in atomic-level and guide the production of armchair BNNTs in industrial.

THE RESEARCH OF NONMETALLIC INCLUSIONS IN VACUUM-DEGASSED AUSTENITIC STAINLESS STEEL BY THE METHOD OF SCANNING ELECTRON MICROSCOPY

The non-metallic phases in the two grades 08Cr18Ni10Ti and 03Cr18Ni10 of evacuated austenitic stainless steel have been investigated by scanning electron microscopy and EDX. The data on the composition of nonmetallic inclusions in the metal taken from the ladle, from vacuum treating machine and from CCM, as well as samples of cold rolled sheet of same metal of 1.2 mm thickness have been presented. It has been shown that in samples of 08Cr18Ni10Ti steel taken from CCM caster tundish, the non-metallic phase consists essentially of titanium nitride clusters, located within the crystals, which contain oxides of aluminum and titanium, as well as rare small globules coated with titanium oxides. The inclusions in cold-rolled sheet are arranged substantially over the entire crosssection and consist of small (5 ... 6 microns) titanium nitride crystals. Cast metal sample of titanium-free austenitic 03Cr18Ni10 steel are clean enough from nonmetallic inclusions. Non-metallic phase is represented mainly by small inclusions of calcium aluminium silicate. In the 1.2 mm thick cold-rolled sheet the non-metallic phase is represented extremely rarely by very small globules of 2...3 micron. Results of the present study have been compared with the known composition of non-metallic phase from non-evacuated similar steel grades at the same stage of processing.

ChemInform Abstract: Lanthanides and Actinides: Annual Survey of Their Organometallic Chemistry Covering the Year 2013

AbstractReview: 187 refs.

Structural Ordering and Charge Variation Induced by Cation Substitution in (Sr,Ca)AlSiN3:Eu Phosphor.

Nitride phosphors are suitable for white light-emitting diode applications. In this study, the structure of phosphor has been modified through cation substitution to induce charge variation and a rearrangement of neighboring nitride clusters, and consequently enhance its luminescent behavior. Substitution of Ca(2+) by Sr(2+) cations expanded the lattice volume and the bc plane, but shortened the distance between the layers along the a axis. Lattice distortion of the framework introduced high-coordination sites in the Sr/Eu centers and adequate space, thereby facilitating charge variation of activators under reduced atmosphere, as detected through X-ray absorption near-edge structure spectroscopy. As such, the photoluminescent intensity of the phosphors increased by more than 10% and a blue shift occurred. The microstructures of the samples were also analyzed using high-resolution transmission electron microscopy. Cation substitution induced a special change in the anion environment, as indicated in the solid-state Raman spectra. Moreover, typical ordering variations in the SiN4 and AlN4 clusters are generated in the lattice. Meanwhile, neighbor sequence of (Si/Al)N4 around the divalent centers were observed through solid-state nuclear magnetic resonance spectroscopy. The modified ordering distribution resulted in a rigid structure and improved the thermal quenching behavior. Thermal stability has been enhanced by 10% at 473 K when x = 0.9 in SrxCa0.993-xAlSiN3:Eu(2+)0.007 compared with that at x = 0. This study promotes the research of neighbor sequence for selective tetrahedral sites such as Li, Mg, Al, and Si coordinated by N atoms in contact with cation sites.

Non-Chromatographic Isolation of Mixed-Metal Nitride Clusters in Larger Carbon Cages

There is a growing interest in mixed-metal nitride metallofullerenes. Prior to exploring their properties and applications, these mixed-metal endohedrals must first be purified. To date, their isolation has been difficult. Hurdles to overcome include their (1) low abundance in soot extract, (2) their co-synthesis with a complex mixture of many types of fullerenes and metallofullerenes, and (3) their HPLC co-elution with contaminant species. In this presentation, we reveal a separation methodology to minimize and/or avoid HPLC (e.g., recycling HPLC), which is the conventional (i.e., literature) method for isolating mixed-metal nitride endohedral metallofullerenes. We also discuss strategies and details of our purification approach, which exploits differences in their reactivity with Lewis acids and aminosilica for their selective removal from extracts and subsequent isolation.

Isoxazoline-Ring Fused Derivatives of Sc3n@C80 and C60: The Internal Effect on the Exohedral Configuration

Fullerenes are unique among all carbon allotropes as their molecular structures can be fully confirmed by single-crystal X-ray diffraction. With metal atoms or metal complexes encapsulated, endohedral metallofullerenes (EMFs) redistribute electrons between the internal unit and the cage, and thus afford different electronic structures and different chemical properties compared to empty fullerenes. However, because of the relatively low yield of endohedral metallofullerenes, only a limited number of chemical modifications have been reported for them. In this work, an isoxazoline ring was added to Sc3N@C80 for the first time to form only one mono-adduct with high regio-selectivity (2b). An analogous product was obtained for empty cage C60 (2a). X-ray crystallography has revealed their structures, as shown in Fig. 1. The addition site of exohedral moieties of C60 was the [6,6]-bond while it is a [5,6]-bond for Sc3N@C80. Inside the Sc3N@C80, disorder of the Sc atoms represents the dynamic motion of the cluster, and the sum of the three Sc–N–Sc angles (342°) suggests that the trimetallic nitride cluster slightly derives from a coplanar structure, confirming the motion control effect of the exohedral modification. Moreover, it is very interesting to find that the configuration of exohedral moieties differed in 2a and 2b.In detail, the methoxy indicated by the arrow shown in Fig. 1 occurs at opposite sides of the benzene plane. These results reveal the significant internal effect on the exohedral configuration. Figure 1. Drawings of 2a and 2b showing thermal ellipsoids at the 30% probability level. Arrows indicate the special methoxy group that changes conformations when added to different fullerenes. Solvent molecules are omitted for clarity. Figure 1

An Expanded Family of Dysprosium-Scandium Mixed-Metal Nitride Clusterfullerenes

Metallic nitride clusterfullerene (NCF) is the most rapidly-growing branch of endohedral fullerenes because of its extraordinarily high yield and peculiar electronic structure with six-electron transfer from the encapsulated trimetallic nitride cluster to the outer carbon cage. A large family of dysprosium-scandium (Dy-Sc) mixed-metal nitride clusterfullerenes (MMNCFs), DyxSc3-xN@C2n (x=1, 2, 2n=68, 70, 76 - 86) have been successfully synthesized and isolated, among which the C70 and C82-based MMNCFs represent two new cages that have never been isolated for MMNCFs. Synthesis of DyxSc3-xN@C2n was accomplished by the “selective organic solid” route using guanidinium thiocyanate as the nitrogen source, and their isolation was fulfilled by recycling HPLC. UV-vis-NIR spectroscopic study indicates that almost all DyxSc3-xN@C2n MMNCFs are kinetically stable fullerenes with optical band-gaps beyond 1 eV. This feature is distinctly different to their counterparts Dy3N@C2n (78≤2n≤88), for which those with relatively large cages such as C84 and C86 have optical band-gaps below 1 eV. FTIR spectroscopic study in combination with DFT calculation enables successful assignments of the cage isomeric structures of all DyxSc3-xN@C2n (x=1, 2, 2n=68, 70, 76 - 86) MMNCFs. The carbon cage size distribution of DyxSc3-xN@C2n (2n=68, 70, 76 - 86) is compared to the reported Dy3N@C2n (78≤2n≤88) homogeneous NCF and GdxSc3-xN@C2n (78≤2n≤88) MMNCF families, revealing that the medium-sized Dy metal plays a crucial role on the expanded size distribution of MMNCFs. As a result, DyxSc3-xN@C2n MMNCFs represent the largest MMNCF family reported to date.

Magnetic anisotropy of endohedral lanthanide ions: paramagnetic NMR study of MSc2N@C80-Ih with M running through the whole 4f row.

Paramagnetic and variable temperature 13C and 45Sc nuclear magnetic resonance studies are performed for nitride clusterfullerenes MSc2N@C80 with icosahedral Ih(7) carbon cage, where M runs through all lanthanides forming nitride clusters. The influence of the endohedral lanthanide ions on the NMR spectral pattern is carefully followed, and dramatic differences are found in peak positions and line widths. Thus, 13C lines broaden from 0.01-0.02 ppm in diamagnetic MSc2N@C80 molecules (M = La, Y, Lu) to several ppm in TbSc2N@C80 and DySc2N@C80. Direction of the paramagnetic shift depends on the shape of the 4f electron density in corresponding lanthanide ions. In TmSc2N@C80 and ErSc2N@C80 with prolate 4f-density of lanthanide ions, 13C signals are shifted down-field, whereas 45Sc peaks are shifted up-field versus diamagnetic values. In all other MSc2N@C80 molecules lanthanide ions have oblate-shaped 4f electron density, and the lanthanide-induced shift is negative for 13C and positive for 45Sc peaks. Analysis of the pseudocontact and contact contributions to chemical shifts revealed that the pseudocontact term dominates both in 13C and 45Sc NMR spectra, although contact shifts for 13C signals are also considerable. Point charge computations of the ligand field splitting are performed to explain experimental results, and showed reasonable agreement with experimental pseudocontact shifts. Nitrogen atom bearing large negative charge and located close to the lanthanide ion results in large magnetic anisotropy of lanthanide ions in nitride clusterfullerenes with quasi-uniaxial ligand field.

Metallofullerenes encaging mixed-metal clusters: synthesis and structural studies of Gdx Ho3-x N@C80 and Gdx Lu3-x N@C80.

Metallofullerenes of Gdx Ho3-x N@C80 and Gdx Lu3-x N@C80 encapsulating mixed-metal nitride clusters were synthesized. Spectroscopic characterization of Gdx Ho3-x N@C80 and Gdx Lu3-x N@C80 was employed to reveal their structural and vibrational properties. The structural properties of these species were analyzed by using theoretical calculations. The studies of Gdx Ho3-x N@C80 and Gdx Lu3-x N@C80 laid the foundations for these species to be used as multifunctional molecules.

ChemInform Abstract: NMR Studies of the Dynamic Motion of Encapsulated Ions and Clusters in Fullerene Cages: A Wheel Within a Wheel

AbstractReview: 37 refs.

Cluster-size dependent internal dynamics and magnetic anisotropy of Ho ions in HoM2N@C80and Ho2MN@C80families (M = Sc, Lu, Y)

The paramagnetic NMR study of HoM2N@C80-Ih and Ho2MN@C80-Ih nitride cluster fullerenes (M = Sc, Lu, Y) reveals strong dependence of Ho-induced paramagnetic shifts (δ(para)) in (13)C NMR spectra on the size of the diamagnetic metal in the cluster. In particular, the δ(para) value in HoY2N@C80 is almost doubled in comparison to that in HoSc2N@C80. X-ray magnetic circular dichroism studies show that all Ho-nitride cluster fullerenes have the same magnetic ground state of Ho(3+). Point-charge ligand-field splitting calculations show that the increase of the M(3+) radius in going from Sc to Y results in a considerable increase of the energy splitting between different Jz states. This leads to a 19% higher magnetic anisotropy of Ho(3+) in HoY2N@C80 than in HoSc2N@C80 at 300 K. Variations of the molecular geometry and cluster dynamics with the size of the cluster are found to have even greater influence on δ(para) values. This work shows that the magnetic properties of the species confined inside the fullerene cages can be tuned using the geometrical factors such as the cluster and the cage size.

Hydrogen adsorption of Be-, Zn-, and Cd-zeolitic imidazolate framework-23: A comparative study

Hydrogen adsorption energies of Be-, Zn-, and Cd-zeolitic imidazolate framework-23 were investigated using the Van der Waals density functional theory implemented in the Quantum ESPRESSO program. From the structural parameters of these systems, we found that the imidazole ligand is unchanged and only the tetrahedral metal nitride cluster is varied. Moreover, our results revealed that the Cd-zeolitic imidazolate framework-23 has the highest electric dipole moment followed by Zn- and Be-zeolitic imidazolate framework-23. On the basis of hydrogen adsorption energy calculations, we found that hydrogen molecules cannot bind with either Be- or Zn-zeolitic imidazolate framework-23 at any of the considered adsorption sites, whereas hydrogen molecules can be trapped on the Cd-zeolitic imidazolate framework-23 with the hydrogen binding energy in the range from 60 to 130 meV. This result suggested that replacing Zn in the metal nitride cluster with a larger metal can enhance the hydrogen adsorption energy of zeolitic imidazolate framework-23.

Clusters Encapsulated in Endohedral Metallofullerenes: How Strained Are They?

Endohedral clusters in metallofullerenes can vary in a broad range of geometrical parameters following the size and shape of the host carbon cage. Obviously, distortions of the cluster may increase its energy and even destabilize the whole clusterfullerene molecule. However, direct evaluation of the magnitude of cluster strain energies has not been done because of the lack of a suitable computational scheme that would allow one to decouple cluster and fullerene distortions and hence estimate individual components. In this work we offer a simple and efficient scheme to calculate cluster distortion energies in endohedral metallofullerenes (EMFs). Using this scheme, we analyze distortions in three classes of EMFs with nitride, sulfide, and carbide clusters and different metal atoms (Sc, Y, Ti).

Endohedral metal or a fullerene cage based oxidation? Redox duality of nitride clusterfullerenes CexM3−xN@C78–88(x = 1, 2; M = Sc and Y) dictated by the encaged metals and the carbon cage size

Redox behavior of endohedral metallofullerenes, in particular their oxidation process, can be classified as a fullerene-based or endohedral species-based process according to the mechanism of the electron transfer. Here we report on the phenomenon of the strain-driven electrochemical behavior achieved by encapsulating the cerium-containing clusters into a series of carbon cages ranging from C78 to C88. The Ce-based mixed metal nitride clusterfullerenes CexM3-xN@C2n (x = 1, 2; M = Sc or Y; 2n = 78-88) were synthesized and characterized. The magnitude of the inherent strain caused by the limited inner space of the carbon cage for the relatively large nitride clusters can be varied by choosing different scaffold metals (Sc, Lu, or Y) to tailor the size of the encaged CexM3-xN cluster and by matching the nitride cluster with different fullerene cages in the size range from C78 to C88. The redox properties of CexM3-xN@C2n were investigated by cyclic and square wave voltammetry. The mechanism of the electrochemical oxidation of Ce-based mixed metal nitride clusterfullerenes, in particular whether the fullerene-based oxidation or the Ce(III) → Ce(IV) process is observed, is found to be dependent on the scaffold metal and the size of the fullerene cage. The endohedral oxidation of Ce(III) to Ce(IV) was observed for a number of compounds as revealed by the negative shift of their oxidation potentials with respect to the values measured for the non-Ce analogues. Experimental studies are supported by DFT calculations. We conclude that the prerequisites for the Ce-based endohedral oxidation process are suitable inherent cluster-cage strain and sufficiently high oxidation potential of the fullerene cage.

NMR Studies of the Dynamic Motion of Encapsulated Ions and Clusters in Fullerene Cages: A Wheel Within a Wheel

Nuclear magnetic resonance (NMR) has provided an important approach to investigate the structure and dynamics of encapsulated metal ions and clusters in endohedral metallofullerenes (EMFs). In this paper, we review NMR studies of the environment and dynamics of dimetallic, trimetallic nitride, metal carbide and metal cyanide clusters encapsulated in EMFs. The NMR chemical shielding parameter is a sensitive probe for monitoring the carbon cage (13C) and encapsulated clusters (13C, 14N, 139La, 45Sc, and 89Y). Future NMR EMF studies could be very fruitful and help elucidate coupled motion between the carbon cage and encapsulated cluster or other “wheel within a wheel” motional processes.

Structural and electronic properties of [formula omitted] nanoribbons: A first principles study

We have performed an extensive ab initio study on the energetic stability of hydrogen passivated BxCyNz nanoribbons and at the electronic structure and magnetic properties of BC2N ribbons with different widths and configurations. In particular, it was investigated that BC2N ribbons composed of boron–nitride clusters surrounded by carbon atoms are showing armchair and zigzag edges. It was seen that the zigzag and armchair BC2N ribbons can be small gap semiconductors or metallic according to the ribbons width. Also, magnetic behavior is observed for these structures, for all the considered widths, while the armchair ones do not show any magnetization.

CuCl2 for the Isolation of a Broad Array of Endohedral Fullerenes Containing Metallic, Metallic Carbide, Metallic Nitride, and Metallic Oxide Clusters, and Separation of Their Structural Isomers

A typical arc-synthesis generates many types of fullerenes and endohedrals. Resulting soot extracts contain a complex mixture of >50 types of fullerenes, metallofullerenes, and their structural isomers. Prior to application development, novel separation methods are required to fractionate this rich array of metallic, metallic carbide, metallic nitride, and metallic oxide endohedrals, all of which can be present in a single, soot extract. Herein, we report the discovery of CuCl2 as a Lewis acid that will selectively precipitate only the more reactive members of each of these endohedral families. The more reactive Sc4O2@Ih-C80, Sc3C2@Ih-C80, and Sc3N@D3h-C78 endohedrals are quickly removed from extracts to greatly decrease the number of endohedrals present in a sample. Experiments indicate that enrichment factors of several orders of magnitude can be achieved within minutes of reaction time. CuCl2 also has sufficient selectivity to resolve and separate structural isomers, as demonstrated with Er2@C82 (isomer I, Cs(6)-C82 versus isomer III). The selective complexation of CuCl2 with fullerenes can be correlated to their first oxidation potential. We estimate a significantly lower threshold of precipitation for CuCl2 (<0.19 V) compared to stronger Lewis acids. Fullerenes and metallofullerenes having first oxidation potentials above 0.19 V tend to remain unreacted in solution. In contrast, species with first oxidation potentials below 0.19 V (vs Fc/Fc(+)) precipitate via complexation, and are easily decomplexed. CuCl2 is compared to Lewis acids having higher precipitation thresholds (e.g., FeCl3) in our goal to predict a priori which endohedrals would remain in solution versus which endohedral species would complex and precipitate. The ability to predict endohedral precipitation a priori is beneficial to the design of purification strategies for metallofullerenes.

Endohedral Metallofullerenes Containing Lanthanides: A Robust Yet Simple Computational Approach

Endohedral metallofullerenes (EMFs) containing lanthanides are thoroughly analyzed using density functional theory. Our methodology, which uses planes waves as basis functions and pseudopotentials and takes into account the on-site Coulomb repulsion via the Hubbard-like U parameter, is able to reproduce the electronic structure and the main geometrical parameters for this family of compounds that presents unpaired f electrons. In addition, the relative abundances of lanthanide EMFs observed in chromatograms as well as the preference of a nitride cluster for a given fullerene are properly predicted. Cluster–cage interactions are optimal when the cluster fits perfectly within the available hollow space of the carbon cage. Except for cerium nitride fullerenes, f electrons do not play a significant role in the electrochemical properties of lanthanide EMFs. If one is only interested in a qualitative prediction of the structure, reactivity, and electronic properties, then calculations that do not explicitly consider the unpaired f electrons can be acceptable.

Carbide Cluster Metallofullerenes: Structure, Properties, and Possible Origin

Endohedral metallofullerenes (EMFs) are hybrid molecules with different metallic species encapsulated inside the fullerene cages. In addition to conventional EMFs that contain only metal ions, researchers have constructed novel compounds that encapsulate metallic clusters of nitride, carbide, oxide, cyanide, and sulfide. Among these structures, carbide cluster metallofullerenes (CCMFs) are unique because their synthesis requires only graphite and the metal source. As a result the molecular structures of CCMFs are particularly difficult to characterize. Two carbon atoms are encapsulated inside the cage, but they do not participate in constructing the cage framework. Recent X-ray crystallographic studies of EMFs have allowed researchers to unambiguously identify CCMFs (MxC₂@C2n). Previously most of these structures had been described as conventional EMFs Mx@C2n+2. Most of these species are scandium-containing compounds such as Sc3C₂@Ih(7)-C₈₀ [not Sc₃@C3v(7)-C₈₂], Sc₂C₂@C2v(5)-C₈₀ [not Sc₂@C₈₂], Sc₂C₂@Cs(6)-C₈₂ [not Sc₂@Cs(10)-C₈₄], Sc₂C₂@C2v(9)-C₈₂ [not Sc₂@C2v(17)-C₈₄], Sc₂C₂@C3v(8)-C₈₂ [not Sc₂@D2d(23)-C₈₄], and Sc₂C₂@D2d(23)-C₈₄ [not Sc₂@C₈₆]. Additional examples of CCMFs include Gd₂C₂@D₃(85)-C₉₂, Sc₂C₂@C2v(6073)-C₆₈, Ti₂C₂@D3h(5)-C₇₈, M₂C₂@C3v(8)-C₈₂, M₂C₂@Cs(6)-C₈₂ (M = Y, Er, etc.), Y₂C₂@C₈₄, Y₂C₂@D₃(85)-C₉₂, Y₂C₂@D₅(450)-C₁₀₀, and Lu₃C₂@D₂(35)-C₈₈. The existence of so many CCMF species reminds us that the symbol '@' (which denotes the encapsulation status of EMFs) should be used with caution with species whose molecular structures have not been determined unambiguously. This Account presents a detailed summary of all aspects of CCMFs, including historically erroneous assignments and corrected structural characterizations, along with their intrinsic properties such as electrochemical and chemical properties. We emphasize structural issues, features that are fundamental for understanding their intrinsic properties. Finally, we discuss the formation mechanism and possible origin of cluster EMFs, not just CCMFs.