Rulong Zhou

Structural and magnetic evolution of bimetallic MnAu clusters driven by asymmetric atomic migration

The nanoscale structural, compositional, and magnetic properties are examined for annealed MnAu nanoclusters. The MnAu clusters order into the L1(0) structure, and monotonic size-dependences develop for the composition and lattice parameters, which are well reproduced by our density functional theory calculations. Simultaneously, Mn diffusion forms 5 Å nanoshells on larger clusters inducing significant magnetization in an otherwise antiferromagnetic system. The differing atomic mobilities yield new cluster nanostructures that can be employed generally to create novel physical properties.

Theoretical and Experimental Characterization of Structures of MnAu Nanoclusters in the Size Range of 1–3 nm

Relative stabilities of MnAu magic-number nanoclusters with 55, 147, 309, and 561 atoms and highly symmetric morphologies (cuboctahedron, icosahedron, onion-like, and core-shell, respectively) are investigated based on density functional theory methods. Through an extensive search, spin arrangements on Mn atoms that give rise to lowest-energy clusters are predicted. The antiferromagnetic spin configurations are found to be the most favorable for all morphologies investigated. The energy rankings among MnAu nanoclusters with the same size and Mn/Au ratio but different morphologies are also determined. The L1(0) structure is found to be increasingly favorable as the size increases from 1.0 to 2.9 nm, consistent with experimental measurements of MnAu nanoparticles in the size range of 1.8-4.6 nm. The decahedron L1(0) morphology is found to be energetically more preferred when the Mn/Au ratio is close to 1:2, whereas the cuboctahedron L1(0) morphology is more preferred when the Mn/Au ratio is close to 1:1. The calculated lattice constants are in excellent agreement with high-resolution TEM measurements for MnAu nanoparticles of similar size. Magnetic states of MnAu nanoclusters are predicted to be stable at room temperature based on estimated Curie or Neél temperature.

[CTi72+]: Heptacoordinate Carbon Motif?

A heptacoordinate carbon motif [CTi7(2+)] is predicted to be a highly stable structure (with D5h point group symmetry) based on ab initio computation. This motif possesses a sizable HOMO-LUMO gap along with the lowest vibrational frequency greater than 95 cm(-1). An investigation of the motif-containing neutral species [CTi7(2+)][BH4(-)]2 further confirms the chemical stability of the heptacoordinate carbon motif. In view of its structural stability, a quasi-one-dimensional (quasi-1D) nanowire [CTi7]n[C16H8]n is built from the carbon motifs. This organometallic nanowire is predicted to be metallic based on density functional theory computation.

Computational Analysis of Stable Hard Structures in the Ti–B System

The lowest energy crystalline structures of various stoichiometric titanium boride (Ti-B) intermetallic compounds are sought based on density functional theory combined with the particle-swarm optimization (PSO) technique. Besides three established experimental structures, i.e., FeB-type TiB, AlB2-type, and Ta3B4-type Ti3B4, we predict additional six metastable phases at these stoichiometric ratios, namely, α- and β-phases for TiB, TiB2, and Ti3B4, respectively. Moreover, we predict the most stable crystalline structures of four new titanium boride compounds with different stoichiometric ratios: Ti2B-PSA, Ti2B3-PSB, TiB3-PSC, and TiB4-PSD. Notably, Ti2B-PSA is shown to have lower formation energy (thus higher stability) than the previously proposed Al2Cu-type Ti2B. The computed convex-hull and phonon dispersion relations confirm that all the newly predicted Ti-B intermetallic crystals are thermodynamically and dynamically stable. Remarkably, the predicted α-TiB2 and β-TiB2 show semi-metal-like electronic properties and possess high Vickers hardnesses (39.4 and 39.6 GPa), very close to the lower limit of superhard materials (40 GPa). Analyses of band structure, density of states, electronic localization function, and various elastic moduli provide further understanding of the electronic and mechanical properties of the intermetallic titanium borides. We hope the newly predicted hard intermetallic titanium borides coupled with desirable electronic properties and high elastic modulus will motivate future experimental synthesis for applications such as high-temperature structural materials.

Persistent Luminescence Hole-Type Materials by Design: Transition-Metal-Doped Carbon Allotrope and Carbides

Electron traps play a crucial role in a wide variety of compounds of persistent luminescence (PL) materials. However, little attention has been placed on the hole-trap-type PL materials. In this study, a novel hole-dominated persistent luminescence (PL) mechanism is predicted. The mechanism is validated in the night pearl diamond (NPD) composed of lonsdaleite with ultralong persistent luminescence (PL) (more than 72 h). The computed band structures suggest that the Fe ion dopant in lonsdaleite is responsible for the luminescence of NPD due to the desired defect levels within the band gap for electronic transition. Other possible impurity defects in lonsdaleite, such as K, Ca, Mg, Zn, or Tl dopants, or C vacancy can also serve as the hole-trap centers to enhance the PL. Among other 3d transition-metal-ion dopants considered, Cr and Mn ions are predicted to give rise to PL property. The predicted PL mechanism via transition-metal doping of lonsdaleite offers an exciting opportunity for engineering new PL materials by design.

Role of vacancies to p-type semiconducting properties of SiGe nanowires

Many experiments have shown that both composition-randomly-distributed Si1−xGex nanowires (NW) and the Ge/Si core/shell NW possess excellent p-type semiconducting properties without relying on any doping strategy. Vacancies in both NW are believed to play a key role in the p-type semiconducting properties. To gain deeper insights into the role of vacancies, we performed first-principle calculations to systematically study the effects of single Si or Ge vacancies in four distinct SiGe NW, namely, randomly-distributed triangular-prism (RTP) NW, fused triangular-prism (FTP) NW, the GecoreSishell NW and SicoreGeshell NW. We find that the tendency for vacancy formation depends strongly on the structures of the NW. The defective RTP, FTP and GecoreSishell NW show promising p-type semiconducting properties while the defective SicoreGeshell NW does not. The Si vacancies in the inner region are attributed to the p-type properties of the RTP NW, and both the Si and Ge vacancies at the core/shell interfaces are attributed to the p-type properties of the FTP and the GecoreSishell NW. Our results show how the vacancies affect the electronic structures and the semiconducting properties of different SiGe NW, and offer an explanation of why the synthesized Si1−xGex and GecoreSishell NW possess excellent p-type semiconducting properties without relying on any doping strategy.

Experimental and theoretical studies of hydroxyl-induced magnetism in TiO nanoclusters.

A main challenge in understanding the defect ferromagnetism in dilute magnetic oxides is the direct experimental verification of the presence of a particular kind of defect and distinguishing its magnetic contributions from other defects. The magnetic effect of hydroxyls on TiO nanoclusters has been studied by measuring the evolution of the magnetic moment as a function of moisture exposure time, which increases the hydroxyl concentration. Our combined experiment and density-functional theory (DFT) calculations show that as dissociative water adsorption transforms oxygen vacancies into hydroxyls, the magnetic moment shows a significant increase. DFT calculations show that the magnetic moment created by hydroxyls arises from 3d orbitals of neighboring Ti sites predominantly from the top and second monolayers. The two nonequivalent hydroxyls contribute differently to the magnetic moment, which decreases as the separation of hydroxyls increases. This work illustrates the essential interplay among defect structure, local structural relaxation, charge redistribution, and magnetism. The microscopic differentiation and clarification of the specific roles of each kind of intrinsic defect is critical for the future applications of dilute magnetic oxides in spintronic or other multifunctional materials.

New phases of 3d-transition metal–cerium binary compounds: an extensive structural search

We perform a comprehensive study to explore the low-energy crystalline phases of 3d transitional metal–cerium (TM–Ce) binary compounds using an unbiased structural search method coupled with first-principles optimization. For Ce–Sc, Ce–Ti, Ce–V, Ce–Cr and Ce–Mn binary systems, no stable crystalline phases are found from the structural search, offering an explanation for why none of these binary compounds have been observed in experiments. For Ce–Fe, Ce–Co, Ce–Ni, Ce–Cu and Ce–Zn binary systems, in addition to the previously known experimental structures, we also find several new low-energy crystalline phases. The computed electronic structures show that Ce atoms are in different states in the predicted binary compounds. In the Ce–Fe, Ce–Co and Ce–Ni compounds, the Ce 4f electrons are partially itinerant so that Ce atoms tend to adopt intermediate valence states between Ce+4 and Ce+3 due to the hybridization among Ce-4f, Ce-5d states and 3d states of TM. In the Ce–Cu and Ce–Zn binary compounds, the Ce-4f states are more localized with the charge state of Ce being close to 3+. In particular, the ferromagnetic metal (FM)-rich phases of the Ce–Fe, Ce–Co and Ce–Ni compounds tend to exhibit FM ordering in their ground states, owing to the strong exchange interaction among metal elements, whereas the non-magnetic states are usually preferred for FM-deficient phases. Magnetic orderings are also found in some other TM-rich phases of Ce–Cu and Ce–Zn compounds, where the magnetic moments are located on the Ce atoms due to the Kondo effect. Mechanic properties of these compounds are also computed based on density functional theory methods. This systematic study offers significantly new data for Ce-based alloys and will be useful to understand the intriguing behavior of the Ce-4f electron, thereby calling for future experimental confirmation of the newly predicted phases of Ce–TM compounds.

Mechanistic study of pressure and temperature dependent structural changes in reactive formation of silicon carbonate

The discovery of the silicon carbonate through chemical reaction between porous SiO2 and gaseous CO2 addressed a long-standing question regarding whether the reaction between CO2 and SiO2 is possible. However, the detailed atomic structure of silicon carbonate and associated reaction mechanism are still largely unknown. We explore structure changes of silicon carbonate with pressure and temperature based on systematic ab initio molecular dynamics simulations. Our simulations suggest that the reaction proceeds at the surface of the porous SiO2. Increasing number of CO2 molecules can take part in the reaction by increasing either the pressure or temperature. The final product of the reaction exhibits amorphous structures, where most C atoms and Si atoms are 3-fold and 6-fold coordinated, respectively. The fraction of differently coordinated C (Si) atoms is pressure dependent, and as a result, the structure of the final product is pressure dependent as well. When releasing the pressure, part of the reaction product decomposes into CO2 molecules and SiO2 tetrahedrons. However more than 50% of C atoms are still in 3-fold coordination, implying that stable silicon carbonate may be obtained via repeated annealing under high pressure. The mechanism underlying this chemical reaction is predicted with two possible reaction pathways identified. Moreover, the reaction transition curve is obtained from the extensive simulation, which can be useful to guide the synthesis of silicon carbonate from the reaction between SiO2 and CO2.

Efficient electron and hole doping in compositionally abrupt Si/Ge nanowires

Efficient doping in semiconductor nanowires can be a challenging task in materials science. In this study, we explore effects of various dopant elements (P, N, Al, B, and O) on the electronic properties of three types of compositionally abrupt SiGe nanowires (NWs), namely, the core-shell Ge(core)/Si(shell) and Si(core)/Ge(shell) NWs, and the fused triangular-prism SiGe NW. Based on the density-functional theory calculations, we find that the substitution of Ge by the pentavalent P at the interfacial region between the core and shell of Ge/Si NWs leads to an easy injection of high-density free-electron-like carriers, whereas the substitution of Si by trivalent Al or B at the interfacial region leads to an easy injection of high-density free-hole-like carriers. However, the introduction of the pentavalent N has little effect on the conductivity of the three types of SiGe NWs. For the divalent O dopant, only the substitution of Si by O in the fused triangular-prism SiGe NW can result in high-density free-hole-like carriers at low temperature. This comprehensive study demonstrates, for the first time, that the doping efficiency not only depends on the type of dopant element (which is well-known) but also on the interfacial geometry of the Si/Ge domains within the compositionally abrupt NWs. The study can offer guidance to the synthesis of novel compositionally abrupt SiGe NWs through a tailored interfacial geometry and controlled interfacial doping.