Photoelectron spectroscopy of chromium-doped silicon cluster anions

Discovery of a series of silicon-based ferrimagnets in CrMnSin (n = 4-20) clusters

Herein, the structural evolution, electronic and magnetic properties of silicon clusters with two different dopants, CrMnSin (n = 4-20) clusters were investigated at density functional theory (DFT) level. Small-sized CrMnSin (n = 4-9) clusters tend to adopt bipyramid-based geometries, while clusters with sizes n = 10 and 11 prefer to opening cage-like structures. For sizes n = 12 to 14, the half-encapsulated structures gradually transform into closed-cage Cr@Sin structures, with the Mn atom exposed outside. Starting from size 15, both the Cr and Mn atoms are completely encapsulated by silicon atoms. Meanwhile, the Cr and Mn atoms in smaller-sized CrMnSin (n = 4-7) clusters tend to be separated, while they prefer to stay together for larger sizes. Cr atom always acts as electron donor, but not for Mn atom. From the average binding energies, one can conclude that it is easier to form larger size clusters. Smaller and larger sized CrMnSin (n = 4-9 and 19-20) clusters prefer to exhibit ferromagnetic Cr-Mn coupling, while sizes n = 10-18 always exhibit ferrimagnetic state. To our knowledge, the CrMnSin clusters is the first kind of neutral transition-metal doped semiconductor clusters that show ferrimagnetic state within a wide size range.

Structural Evolution and Electronic Properties of V2Sin-/0 (n = 7-14) Clusters: Anion Photoelectron Spectroscopy and Theoretical Calculations

A systematic study of the structures and electronic properties of V2-doped silicon clusters, V2Sin-/0 (n = 7-14), was carried out by anion photoelectron spectroscopic experiments combined with theoretical calculations. According to the experimental spectra of V2Sin- (n = 7-14) clusters, the V2Si12- cluster has the highest vertical detachment energy (VDE) of 3.66 eV, while V2Si7- and V2Si14- clusters have lower VDEs of 2.81 and 2.84 eV, respectively. The most stable structure searches find that two V atoms in the V2Sin- clusters with size n = 7 and 8 are located at the surface, while V2Sin- clusters with n ≥ 9 prefer cage-like structures. Based on the analysis of the structural evolution of V2Sin- (n = 9-14) clusters, it can be clearly seen how the antihexagonal prism with one V encapsulated in the cage is gradually built from n = 9 to 12 and further developed from n = 12 to 14 with the extra silicon atoms located at the surface of the Si12 cage. The molecular orbital and the atoms in molecule analysis of the V2Sin- (n = 7-14) anions demonstrate that the strong V-V bond and the delocalized interaction between the V2 moiety and the Sin ligand play a significant role in stabilizing the cluster structures. A strong linear correlation has been found between the Wiberg bond order of the V-V bond and the electron density at the V-V bond critical points.

Making Sense of the Growth Behavior of Ultra-High Magnetic Gd2-Doped Silicon Clusters

The growth behavior, stability, electronic and magnetic properties of the Gd₂Si_n^- (n = 3-12) clusters are reported, which are investigated using density functional theory calculations combined with the Saunders 'Kick' and the Artificial Bee Colony algorithm. The lowest-lying structures of Gd₂Si_n^- (n = 3-12) are all exohedral structures with two Gd atoms face-capping the Si_n frameworks. Results show that the pentagonal bipyramid (PB) shape is the basic framework for the nascent growth process of the present clusters, and forming the PB structure begins with n = 5. The Gd₂Si₅^- is the potential magic cluster due to significantly higher average binding energies and second order difference energies, which can also be further verified by localized orbital locator and adaptive natural density partitioning methods. Moreover, the localized f-electron can be observed by natural atomic orbital analysis, implying that these electrons are not affected by the pure silicon atoms and scarcely participate in bonding. Hence, the implantation of these elements into a silicon substrate could present a potential alternative strategy for designing and synthesizing rare earth magnetic silicon-based materials.

Anion photoelectron spectroscopy and density functional theory study of TM2Sin- (TM = V, Cr; n = 14-20) clusters

We investigated the structural evolution and electronic properties of medium-sized silicon cluster anions doped with two transition metal atoms, TM₂Si_n^- (TM = V, Cr; n = 14-20), by using mass-selective anion photoelectron spectroscopy combined with density functional theory (DFT) calculations. Putative ground state structures of these clusters were obtained by using a genetic algorithm coupled with the DFT calculations. It was found that the two TM atoms tend to form a TM-TM bond, which - except for V₂Si₁₉^- - is shorter than the nearest neighbour distance in the crystalline state of the respective metals. The V₂Si_n^- clusters with n = 14 to 17 exhibit structures based on a silicon hexagonal antiprism, while the larger ones exhibit more fullerene-like cage structures. Cr₂Si_n^- clusters follow the same trend, although with a silicon hexagonal prism structure for n = 14 and 15, and the transition to fullerene-like structures occurring at n = 17. Among these clusters, TM₂Si₁₈^- have the largest average binding energy and second order differences in energy, therefore the highest relative stability. All of the clusters possess total magnetic moment of 1 μB, but with very different contributions from the doped TM atoms. Especially in the Cr doped clusters there is a tendency towards an anitiferromagnetic arrangement of the magnetic moments of the two Cr atoms.

Structures and electronic properties of VSin- (n = 14-20) clusters: a combined experimental and density functional theory study

We present a systematic study of the structures and electronic properties of vanadium-doped silicon cluster anions, VSi_n^- (n = 14-20), by combining photoelectron spectroscopy (PES) measurements and density functional theory (DFT) based theoretical calculations. High resolution PES of low temperature (10 K) clusters are acquired at a photon wavelength of 248 nm. Low-lying structures of VSi_14-20^- are obtained by a genetic algorithm based global minimum search code combined with DFT calculations. Excellent agreement is found between the measured PES and the simulated electron density of states of the putative ground-state structures. We conclude that clusters with sizes n = 14 and n = 15 prefer cage-like structures, with the encapsulated vanadium atom bonding with all silicon atoms, while a fullerene-like motif is more favorable for n ≥ 16. For the sizes n = 16 to 19, the structures consist of a V@Si₁₄ with two, three, four, and five Si atoms on the surface of the cage. For n = 20 the structure consists of a V@Si₁₅ with five Si atoms on the surface of the cage. VSi₁₄^- has the highest stability and stands out as a simultaneous closing of electronic and geometrical shells.

Electronic States of CoSin-/0/+ (n = 1-3) Clusters from Density Matrix Renormalization Group-CASPT2 Calculations

Density matrix renormalization group-CASPT2 (DMRG-CASPT2), CASPT2, and density functional theory are employed to describe the complicated geometrical and electronic structures of CoSi_n^-/0/+ (n = 1-3) clusters. The active spaces of DMRG-CASPT2 are extended to 23 orbitals. The DMRG-CASPT2 method with such large active spaces is reasonable to provide highly accurate relative energies of the electronic states. The pure BP86, PBE, and TPSS functionals appear to be suitable to compute the relative energies of the electronic states of cobalt-doped silicon clusters. The leading configurations, bond distances, vibrational frequencies, normal modes, and relative energies of the electronic states are reported. The electron detachment energies of the removals of one electron from the anionic and neutral clusters are estimated. All six bands in the photoelectron spectrum of CoSi₃^- are interpreted based on the computed electron detachment energies and Franck-Condon factor simulations.

Appearance of V-encapsulated tetragonal prism motifs in VSi10- and VSi11- clusters

The structural and electronic properties of V-doped silicon clusters, VSi10-/0 and VSi11-/0, were investigated by using mass-selected anion photoelectron spectroscopy in combination with theoretical calculations. Photoelectron spectroscopy of VSi10- and VSi11- clusters with spectral similarity reveals that the two cluster structures resemble each other. Interestingly, theoretical calculation studies provide definitive evidence of the global minima for the two clusters to be V-encapsulated tetragonal prism motifs with extra Si atoms bicapped and tricapped, respectively. The enhanced stability of the tetragonal prism unit in VSi10- and VSi11- is due to the strong interactions between 3d (V) and 3p (Si) orbitals, and more charge transfers from the Sin framework to the encapsulated V atom. The tetragonal prism unit possessed by both the VSi10- and VSi11- clusters is observed for the first time in the current work, and may offer new ideas in developing components for Si-based nanodevices.

Au3Si4- and Au4Si4: Electronically Equivalent but Different Polarity Superatoms

A series of Au_xSi₄^- cluster anions (x = 1-4) were generated most abundantly by laser ablation of a Au₄Si alloy target. Photoelectron spectroscopy and density functional theory (DFT) calculation of Au_xSi₄^- (x = 1-4) revealed that Au₃Si₄^- can be viewed as an electronically closed superatom and is composed of a Si₄ unit whose three adjacent edges of a single facet are bridged by three Au atoms. Such phase-segregated structure is facilitated by aurophilic interaction between the three Au atoms and results in a large permanent dipole moment (4.43 D). DFT calculations on an electronically equivalent superatom Au₄Si₄ predicted a new structure in which the uncoordinated Si atom of Au₃Si₄^- is bonded by Au⁺. This Au₄Si₄ is much more stable than a cubic structure previously reported and has a large HOMO-LUMO gap (1.68 eV) and a small permanent dipole moment (0.41 D).

Structural and electronic properties of exohedrally doped neutral silicon clusters LnSin (n = 5, 10; Ln = Sm, Eu, Yb)

Lanthanide-doped silicon clusters have been extensively studied in the fields of optoelectronics, magnetism and nanomaterials during the last decade. Herein, systematic structure searches for typical neutral clusters of lanthanide-doped silicon clusters LnSin (n = 5, 10; Ln = Sm, Eu, Yb) have been performed by means of density functional theory coupled with the "stochastic kicking" global search technique. It is found that the Ln atom in LnSin prefers to locate on the surface of Sin to form an exohedral structure, and this exohedral configuration may dominate the nascent structure of LnSin. The spin density and Mulliken population analyses indicate that LnSin clusters possess remarkable magnetic moments (except for YbSin), which are mainly supplied by the Ln 4f electrons (except for Yb). Density of states visually shows the significant spin polarization for open-shell structures of SmSin and EuSin. As for the YbSin (n = 5, 10) system, it has a closed-shell electronic structure with a large HOMO-LUMO gap of 2.72 eV. Bonding analysis, including localized orbital locator and electron density difference, shows that the Si-Si covalent interaction and Sm-Si electrostatic interaction are important for the structural stability of LnSin.

Synthesis and structure of a family of rhodium polystannide clusters [Rh@Sn10]3-, [Rh@Sn12]3-, [Rh2@Sn17]6- and the first triply-fused stannide, [Rh3@Sn24]5

Through relatively subtle changes in reaction conditions, we have been able to isolate four distinct Rh/Sn cluster compounds, [Rh@Sn10]3-, [Rh@Sn12]3-, [Rh2@Sn17]6- and [Rh3@Sn24]5-, from the reaction of K4Sn9 with [(COE)2Rh(μ-Cl)]2(COE = cyclooctene). The last of these has a hitherto unknown molecular topology, an edge-fused polyhedron containing three Rh@Sn10 subunits, and represents the largest endohedral Group 14 Zintl cluster yet to have been isolated from solution. DFT has been used to place these new species in the context of known cluster chemistry. ESI-MS experiments on the reaction mixtures reveal the ubiquitous presence of {RhSn8} fragments that may play a role in cluster growth.

Structural evolution and electronic properties of CoSin− (n = 3–12) clusters: mass-selected anion photoelectron spectroscopy and quantum chemistry calculations

Experimental measurements and theoretical calculations show that CoSi₁₀⁻ has the highest vertical detachment energy among all the CoSi_n⁻ (n = 3–12) clusters, implying CoSi₁₀⁻ has special stability.

Quantum chemical models for the absorption of endohedral clusters on Si(111)-(7 × 7): a subtle balance between W–Si and Si–Si bonding

The absorption of endohedral clusters on Si(111)-7 × 7 generates a new bond between W and a surface silicon adatom.

Medium-sized [Formula: see text] (n = 14-20) clusters: a combined study of photoelectron spectroscopy and DFT calculations

Size-selected anionic silicon clusters, [Formula: see text] (n  =  14-20), have been investigated by photoelectron spectroscopy and density functional theory (DFT) calculations. Low-energy structures of the clusters are globally searched for by using a genetic algorithm based on DFT calculations. The electronic density of states and vertical detachment energies have been simulated by using ten DFT functionals and compared to the experimental results. We systematically evaluated the DFT functionals for the calculation of the energetics of silicon clusters. CCSD(T) single-point energies based on MP2 optimized geometries for selected isomers of [Formula: see text] are also used as benchmark for the energy sequence. The HSE06 functional with aug-cc-pVDZ basis set is found to show the best performance. Our global minimum search corroborates that most of the lowest-energy structures of [Formula: see text] (n  =  14-20) clusters can be derived from assembling tricapped trigonal prisms in various ways. For most sizes previous structures are confirmed, whereas for [Formula: see text] a new structure has been found.

Synthesis and Characterization of Metal-Encapsulating Si16 Cage Superatoms

Nanoclusters, aggregates of several to hundreds of atoms, have been one of the central issues of nanomaterials sciences owing to their unique structures and properties, which could be found neither in nanoparticles with several nanometer diameters nor in organometallic complexes. Along with the chemical nature of each element, properties of nanoclusters change dramatically with size parameters, making nanoclusters strong potential candidates for future tailor-made materials; these nanoclusters are expected to have attractive properties such as redox activity, catalysis, and magnetism. Alloying of nanoclusters additionally gives designer functionality by fine control of their electronic structures in addition to size parameters. Among binary nanoclusters, binary cage superatoms (BCSs) composed of transition metal (M) encapsulating silicon cages, M@Si₁₆, have unique cage structures of 16 silicon atoms, which have not been found in elemental silicon nanoclusters, organosilicon compounds, and silicon based clathrates. The unique composition of these BCSs originates from the simultaneous satisfaction of geometric and electronic shell-closings in terms of cage geometry and valence electron filling, where a total of 68 valence electrons occupy the superatomic orbitals of (1S)²(1P)⁶(1D)¹⁰(1F)¹⁴(2S)²(1G)¹⁸(2P)⁶(2D)¹⁰ for M = group 4 elements in neutral ground state. The most important issue for M@Si₁₆ BCSs is fine-tuning of their characters by replacement of the central metal atoms, M, based on one-by-one adjustment of valence electron counts in the same structure framework of Si₁₆ cage; the replacement of M yields a series of M@Si₁₆ BCSs, based on their superatomic characteristics. So far, despite these unique features probed in the gas-phase molecular beam and predicted by quantum chemical calculations, M@Si₁₆ have not yet been isolated. In this Account, we have focused on recent advances in synthesis and characterizations of M@Si₁₆ BCSs (M = Ti and Ta). A series of M@Si₁₆ BCSs (M = groups 3 to 5) was found in gas-phase molecular beam experiments by photoelectron spectroscopy and mass spectrometry: formation of halogen-, rare-gas-, and alkali-like superatoms was identified through one-by-one tuning of number of total valence electrons. Toward future functional materials in the solid state, we have developed an intensive, size-selected nanocluster source based on high-power impulse magnetron sputtering coupled with a mass spectrometer and a soft-landing apparatus. With scanning probe microscopy and photoelectron spectroscopy, the structure of surface-immobilized BCSs has been elucidated; BCSs can be dispersed in an isolated form using C₆₀ fullerene decoration of the substrate. The intensive nanocluster source also enables the synthesis of BCSs in the 100-mg scale by coupling with a direct liquid-embedded trapping method into organic dispersants, enabling their structure characterization as a highly symmetric "metal-encapsulating tetrahedral silicon-cage" (METS) structure with Frank-Kasper geometry.

The structural landscape in 14-vertex clusters of silicon, M@Si14: when two bonding paradigms collide

The structural chemistry of the title clusters has been the source of controversy in the computational literature because the identity of the most stable structure appears to be pathologically dependent on the chosen theoretical model. The candidate structures include a D_3h-symmetric 'fullerene-like' isomer with 3-connected vertices (A), an 'arachno' architecture (B) and an octahedral isomer with high vertex connectivities typical of 'closo' electron-deficient clusters (C). The key to understanding these apparently very different structures is the fact that they make use of the limited electron density available from the endohedral metal in very different ways. Early in the transition series the favoured structure is the one that maximises transfer of electron density from the electropositive metal to the cage whereas for later metals it is the one that minimises repulsions with the increasingly core-like d electrons. The varying role of the d electrons across the transition series leads directly to strong functional dependency, and hence to the controversy in the literature.

Structural assignment, and electronic and magnetic properties of lanthanide metal doped silicon heptamers Si7M0/− with M = Pr, Gd and Ho

The ground state geometries of neutral and anionic lanthanide-metal-doped silicon clusters Si₇M^0/− with M = Pr, Gd and Ho were determined by quantum chemical (DFT) computations and the previous experimental photoelectron spectra were assigned.

Biradical character in the ground state of [Mn@Si12]+: a DFT and CASPT2 study

DFT and CASPT2 calculations reveal that the ground state of [Mn@Si₁₂]⁺is a biradical, quite unlike isoelectronic and isostructural Cr@Si₁₂.

On the structural landscape in endohedral silicon and germanium clusters, M@Si12 and M@Ge12

Amongst the endohedral clusters of the tetrel elements, M@En, the 12-vertex species are unique in that three completely different geometries, the icosahedron (Ih, [Ni@Pb12](2-)), the hexagonal prism (HP, Cr@Si12) and the bicapped pentagonal prism (BPP, [Ru@Ge12](3-)) have been identified in stable molecules. We explore here the origins of this structural diversity by comparing stability patterns across isovalent and isoelectronic series, M@Si12, M@Ge12 and [M@Ge12](3-). The BPP structure dominates the structural landscape for high valence electron counts (57-60) while the HP has a rather narrower window of stability around the 54-56 count. Moreover the preference for an HP structure is unique to silicon: in no case is a rigorously D6h-symmetric structure the global minimum for M@Ge12. Distortions from the high-symmetry limits, where present, can be traced to degeneracies or near-degeneracies in the frontier orbital domains. In all cases the structure adopted is that which maximizes the delocalization of electron density between the metal and the cluster cage, such that both components attain stable electronic configurations.

Discovery of a silicon-based ferrimagnetic wheel structure in V(x)Si(12)(-) (x = 1-3) clusters: photoelectron spectroscopy and density functional theory investigation

Our studies show that VSi(12)(-) adopts a V-centered hexagonal prism with a singlet spin state. The addition of the second V atom leads to a capped hexagonal antiprism for V(2)Si(12)(-) in a doublet spin state. Most interestingly, V(3)Si(12)(-) exhibits a ferrimagnetic, bicapped hexagonal antiprism wheel-like structure with a total spin of 4 μ(B).

Probing the valence orbitals of transition metal-silicon diatomic anions: ZrSi, NbSi, MoSi, PdSi and WSi

Evolution of electronic properties and the nature of bonding of the 4d-transition metal silicides (ZrSi, NbSi, MoSi and PdSi) are discussed, revealing interesting trends in the transition metal-silicon interactions across the period. The electronic properties of select transition metal silicide diatomics have been determined by anion photoelectron imaging spectroscopy and theoretical methods. The electron binding energy spectra and photoelectron angular distributions obtained by 2.33 eV (532 nm) photons have revealed the distinct features of these diatomics. The theoretical calculations were performed at the density functional theory (DFT) level using the unrestricted B3LYP hybrid functional and at the ab initio unrestricted coupled cluster singles and doubles (triplets) (UCCSD(T)) methods to assign the ground electronic states of the neutral and anionic diatomics. The excited electronic states were calculated by the DFT (TD-DFT)/UB3LYP method. We have observed that the valence molecular orbital configuration of the ZrSi and NbSi anions are significantly different from that of the MoSi and PdSi anions. By combining our experimental and theoretical results, we report that the composition of the highest occupied molecular orbitals shift from a majority of transition metal s- and d-orbital contribution in ZrSi and NbSi, to mainly silicon p-orbital contribution for MoSi and PdSi. We expect these observed atomic scale transition metal-silicon interactions to be of increasing importance with the miniaturization of devices approaching the sub-nanometer size regime.