Shunfang Li

First-order magnetic and structural phase transitions in Fe1+ySexTe1-x

We use bulk magnetic susceptibility, electronic specific heat, and neutron scattering to study structural and magnetic phase transitions in Fe1+ySexTe1-x. Fe1.068Te exhibits a first-order phase transition near 67 K with a tetragonal-to-monoclinic structural transition and simultaneously develops a collinear antiferromagnetic (AF) order responsible for the entropy change across the transition. Systematic studies of the FeSe1-xTex system reveal that the AF structure and lattice distortion in these materials are different from those of FeAs-based pnictides. These results call into question the conclusions of present density-functional calculations, where FeSe1-xTex and FeAs-based pnictides are expected to have similar Fermi surfaces and therefore the same spin-density wave AF order.

Substrate co-doping modulates electronic metal–support interactions and significantly enhances single-atom catalysis

Transitional metal nanoparticles or atoms deposited on appropriate substrates can lead to highly economical, efficient, and selective catalysis. One of the greatest challenges is to control the electronic metal-support interactions (EMSI) between the supported metal atoms and the substrate so as to optimize their catalytic performance. Here, from first-principles calculations, we show that an otherwise inactive Pd single adatom on TiO2(110) can be tuned into a highly effective catalyst, e.g. for O2 adsorption and CO oxidation, by purposefully selected metal-nonmetal co-dopant pairs in the substrate. Such an effect is proved here to result unambiguously from a significantly enhanced EMSI. A nearly linear correlation is noted between the strength of the EMSI and the activation of the adsorbed O2 molecule, as well as the energy barrier for CO oxidation. Particularly, the enhanced EMSI shifts the frontier orbital of the deposited Pd atom upward and largely enhances the hybridization and charge transfer between the O2 molecule and the Pd atom. Upon co-doping, the activation barrier for CO oxidation on the Pd monomer is also reduced to a level comparable to that on the Pd dimer which was experimentally reported to be highly efficient for CO oxidation. The present findings provide new insights into the understanding of the EMSI in heterogeneous catalysis and can open new avenues to design and fabricate cost-effective single-atom-sized and/or nanometer-sized catalysts.

Net W monolayer: A high-performance electrode material for Li-ion batteries

“Two-dimensional materials as electrodes” is believed to be one of the key solutions for the development of future battery technologies. Based on the first-principles calculations, we predict that a metallic carbon allotrope (net W), with high electrochemical performance, can be served as an anode material for Li-ion batteries (LIBs). The net W exhibits metallic conductivity, and the conductivity is excellently maintained after Li adsorption. Specifically, upon Li intercalation, only slight lattice variations (<1.5%) occur, which ensures a good cycling stability. The low diffusion barriers of 0.4 eV and the moderate average open circuit voltage of 0.42 V are in between those of the currently used anodes, graphite, and TiO2. Most remarkably, the storage capacity can be up to 1675 mA h g−1, which is about 4.5 times larger than that of the commercial graphite anode. The present findings identify that net W could be an excellent anode material for the application in LIBs.

Sub-surface alloying largely influences graphene nucleation and growth over transition metal substrates

Sub-surface alloying (SSA) can be an effective approach to tuning surface functionalities. Focusing on Rh(111) as a typical substrate for graphene nucleation, we show strong modulation by SSA atoms of both the energetics and kinetics of graphene nucleation simulated by first-principles calculations. Counter-intuitively, when the sub-surface atoms are replaced by more active solute metal elements to the left of Rh in the periodic table, such as the early transition metals (TMs), Ru and Tc, the binding between a C atom and the substrate is weakened and two C atoms favor dimerization. Alternatively, when the alloying elements are the late TMs to the right of Rh, such as the relatively inert Pd and Ag, the repulsion between the two C atoms is enhanced. Such distinct results can be well addressed by the delicately modulated activities of the surface host atoms in the framework of the d-band theory. More specifically, we establish a very simple selection rule for optimizing the metal substrate for high quality graphene growth: the introduction of an early (late) solute TM in the SSA lowers (raises) the d-band center and the activity of the top-most host metal atoms, weakening (strengthening) the C-substrate binding, meanwhile both energetically and kinetically facilitating (hindering) the graphene nucleation, and simultaneously promoting (suppressing) the orientation disordering the graphene domains. Importantly, our preliminary theoretical results also show that such a simple rule is also proposed to be operative for graphene growth on the widely invoked Cu(111) catalytic substrate.

Kinetic pathways towards mass production of single crystalline stanene on topological insulator substrates

As a highly appealing new member of the two-dimensional (2D) materials family, stanene was first epitaxially grown on a three-dimensional topological insulator of Bi2Te3; yet, to date, a standing challenge is to drastically improve the overall quality of such stanene overlayers for a wide range of potential applications in next-generation quantum devices. Here we use state-of-the-art first-principles approaches to explore the atomistic growth mechanisms of stanene on different Bi2Te3(111)-based substrates, with intriguing discoveries. We first show that, when grown on experimentally studied Te-terminated Bi2Te3, stanene would follow an unusual partial-layer-by-partial-layer growth mode, characterized by short-range repulsive pairwise interactions of the Sn adatoms; the resultant stanene overlayer is destined to contain undesirable grain boundaries. More importantly, we find that stanene growth on Bi2Te3(111) pre-covered with a Bi bilayer follows a highly desirable nucleation-and-growth mechanism, strongly favoring single crystalline stanene. We further show that both systems exhibit pronounced Rashba spin-orbit couplings, while the latter system also provides new opportunities for the potential realization of topological superconductivity in 2D heterostructures. The novel kinetic pathways revealed here will be instrumental in achieving the mass production of high-quality stanene with emergent physical properties of technological significance.