Shuo-Wang Yang

One-dimensional iron-cyclopentadienyl sandwich molecular wire with half metallic, negative differential resistance and high-spin filter efficiency properties.

We present a theoretical study on a series of novel organometallic sandwich molecular wires (SMWs), which are constructed with alternating iron atoms and cyclopentadienyl (Cp) rings, using DFT and nonequilibrium Greens function techniques. It is found that that the SMWs are stable, flexible structures having half-metallic (HM) properties with 100% negative spin polarization near the Fermi level in the ground state. Some SMWs of finite size show a nearly perfect spin filter effect (SFE) when coupled between ferromagnetic electrodes. Moreover, their I-V curves exhibit negative differential resistance (NDR), which is essential for certain electronic applications. The SMWs are the first linear molecules with HM, high SFE, and NDR and can be easily synthesized. In addition, we also analyze the underlying mechanisms via the transmission spectra and spin-dependent calculations. These findings strongly suggest that the SMWs are promising materials for application in molecular electronics.

Electron Transport Properties of Atomic Carbon Nanowires between Graphene Electrodes

Long, stable, and free-standing linear atomic carbon wires (carbon chains) have been carved out from graphene recently [Meyer et al. Nature (London) 2008, 454, 319; Jin et al. Phys. Rev. Lett. 2009, 102, 205501]. They can be considered as extremely narrow graphene nanoribbons or extremely thin carbon nanotubes. It might even be possible to make use of high-strength and identical (without chirality) carbon wires as a transport channel or on-chip interconnects for field-effect transistors. Here we investigate electron transport properties of linear atomic carbon wire-graphene junctions by combining nonequilibrium Greens function with density functional theory. For short wires, linear ballistic transport is observed in wires consisting of odd numbers of carbon atoms but not in those consisting of even numbers of carbon atoms. For wires longer than 2.1 nm as fabricated above, however, the ballistic conductance of carbon wire-graphene junctions is independent of the structural distortion, structural imperfections, and hydrogen impurity adsorbed on the linear carbon wires, except for oxygen impurity adsorption under a low bias. As such, the epoxy groups might be the origin of experimentally observed low conductance in the carbon chain. Moreover, double-atomic carbon chains exhibit a negative differential resistance effect.

Charge-Transfer-Based Mechanism for Half-Metallicity and Ferromagnetism in One-Dimensional Organometallic Sandwich Molecular Wires

We present a systematic theoretical study on the mechanism of half-metallicity and ferromagnetism for one-dimensional (1-D) sandwich molecular wires (SMWs) constructed with altering cyclopentadienyl (Cp) and first-row transition metal (Mt). It is unveiled for the first time that, in (MtCp) infinity, one valence electron would transfer from the Mt to the Cp ring, forming Cp (-) and Mt (+) altering structures. This electron transfer not only makes them more stable than the benzene analogues (MtBz) infinity but also leads to completely different half-metallic and ferromagnetic mechanisms. We analyze such unusual half-metallicity and ferromagnetic behaviors and explain each SMW magnetic moment quantitatively. Finally, we indicate that a Peierls transition does not occur in these 1-D SMWs.

The Mechanism of Single-Walled Carbon Nanotube Growth and Chirality Selection Induced by Carbon Atom and Dimer Addition

On the basis of abounding density function calculations, a mechanism is proposed to explain single-walled carbon nanotube (SWCNT) growth and chirality selection induced by single C atom and C(2) dimer addition under catalyst-free conditions. Two competitive reaction paths, chirality change induced by single C atom and nanotube growth through C(2) dimer addition, are identified. The structures of the intermediates and transition states along the potential energy surfaces during the formation of near-armchair (6,5), (7,5), (8,5), and (9,5) caps initiated from the armchair carbon cap (5,5) are elucidated in detail. The results show that the direct adsorptions of C atom or C(2) dimer on growing carbon caps have no energy barrier. Moreover, the incorporations of adsorbed C atom or C(2) dimer display low energy barriers, indicating SWCNT growth and chirality change are thermodynamically and kinetically feasible under catalyst-free growth conditions. In addition, the results also highlight that the concentrations of C atoms and C(2) dimers in the experimental environment would play a critical role in the chiral-selective SWCNT synthesis. Potential opportunities exist in achieving the (n,m) selective growth by delivering single C atom or C(2) dimers at different ratios during different reaction stages.

Theoretical Studies on Structural, Magnetic, and Spintronic Characteristics of Sandwiched EunCOTn+1 (n = 1−4) Clusters

Europium (Eu)-cyclootetatrene (COT = C(8)H(8)) multidecker clusters (Eu(n)COT(n+1), n = 1-4) are studied by relativistic density functional theory calculations. These clusters are found to be thermodynamically stable with freely rotatable COT rings, and their total magnetic moments (MMs) increase linearly along with the number of Eu atoms. Each Eu atom contributes about 7 mu(B) to the cluster. Meanwhile, the internal COT rings have little MM contribution while the external COT rings have about 1 mu(B) MM aligned in opposite direction to that of the Eu atoms. The total MM of the Eu(n)COT(n+1) clusters can thus be generalized as 7n - 2 mu(B) where n is the number of Eu atoms. Besides, the ground states of these clusters are ferromagnetic and energetically competitive with the antiferromagnetic states, meaning that their spin states are very unstable, especially for larger clusters. More importantly, we uncover an interesting bonding characteristic of these clusters in which the interior ionic structure is capped by two hybrid covalent-ionic terminals. We suggest that such a characteristic makes the Eu(n)COT(n+1) clusters extremely stable. Finally, we reveal that for the positively charged clusters, the hybrid covalent-ionic terminals will tip further toward the interior part of the clusters to form deeper covalent-ionic caps. In contrast, the negatively charged clusters turn to pure ionic structures.

Geometry Dependent I−V Characteristics of Silicon Nanowires

The current-voltage (I-V) characteristics of small-diameter hydrogenated and pristine silicon nanowires (SiNWs) are calculated by nonequilibrium Greens function combined with density functional theory. We show that the I-V characteristics depend strongly on length, growth orientation, and surface modification of the SiNWs. In particular, a length of 3 nm is suggested for the nanowires to retrieve its intrinsic conducting properties from the influences of both the electrodes and metal/semiconductor mismatched surface contact; surface reconstruction would enhance the conductance in hydrogenated SiNW, which is explained by the extra conducting eigenchannel found in the transmission spectrum, suggesting possible surface conducting channel. Discussions with available experimental data are given.

Graphene layers on Cu and Ni (111) surfaces in layer controlled graphene growth

The properties of graphene strongly depend on its thickness. It is important to understand the graphene–metal interaction to control its thickness during its growth on metal surfaces. Here, we used the DFT-D2 method of Grimme, which includes the critical long-range van der Waals forces in the graphene–metal interaction, to study the interfaces between mono-, bi-, and trilayer graphene and Cu and Ni (111) surfaces. Our results show the adsorption energy increases with the increase of graphene layers on Ni (111); in contrast, it decreases on Cu (111). Charge density and partial density of states analyses show that monolayer graphene adsorbed on Ni (111) is more reactive than that on Cu (111). Another graphene layer can be easily formed on top of monolayer or bilayer graphene adsorbed on Ni (111); but not on Cu (111). These findings provide a useful guide for achieving precise layer controlled graphene growth and designing graphene based devices.

Reactive Sites for Chiral Selective Growth of Single-Walled Carbon Nanotubes: A DFT Study of Ni55–Cn Complexes

The physical and electronic properties of single-walled carbon nanotubes (SWCNTs) are determined by their chirality. The chirality selection mechanism in SWCNT growth is not fully understood. In this study, the interaction between near-armchair (n,5), where n = 6, 7, 8, and 9, zigzag (9,0), and armchair (5,5) nanotubes and a fully relaxed Ni(55) metal cluster during the early stage of growth is studied by density functional theory calculations. We found that kink sites at the end edge of (n,5) nanotubes are more reactive than other sites based on the charge transfer analysis at the Ni-C interface. The frontier orbitals of the (6,5) and (7,5) caps are localized on their kink-step sites, which stretch outward from the carbon cap surface, having typical 2p(z) orbital feature of carbon atom with high reactivity. Such favorable frontier orbital spatial orientation and location is ideal to incorporate more carbon species. These reactive sites may lead to the faster growth rate, resulting in the chirality selectivity toward the (6,5) and (7,5) nanotubes. In contrast, the frontier orbitals of (8,5) and (9,5) caps spread over the entire carbon cap surface. Adding carbon species at these sites may lead to the chirality change or formation of other carbon structures. Our results showed that the spatial distribution and orientation of frontier orbitals is useful in explaining the chiral selectivity. Engineering catalyst clusters to control these reactive sites has high potential to further improve chirality control in SWCNT synthesis.

A theoretical mechanistic study on electrical conductivity enhancement of DMSO treated PEDOT:PSS

Conductive polymers have been attracting attention for decades due to their promising applications in photovoltaic cells and thermoelectrics. Among them, poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is the most extensively studied one with the features of high water dispersibility, transparency and thermal stability as well as having relatively high electrical conductivity (EC). Nevertheless, the EC of as-prepared PEDOT:PSS is still unsatisfactory for real applications. Experimental studies on PEDOT:PSS have showed that its low EC could be elevated by more than 3 to 4 orders of magnitude by polar solvent treatment. However, the mechanism of this enhancement remains unclear. In this work, dimethyl sulfoxide (DMSO) treated PEDOT:PSS polymers are studied using multiscale molecular modeling, including density functional theory (DFT) calculations and molecular dynamics (MD) simulations. We elucidate the mechanism of EC enhancement at the molecular level, demonstrating that DMSO dissolves the PSS shell to release the conductive PEDOT in the core for self-aggregation, leading to subsequent phase separation of PEDOT and PSS by charge screening. These findings are important for the selection of alternative solvents for further EC enhancement of PEDOT:PSS in thermoelectric applications.

Orbital-Engineering based Screening of π-Conjugated d8 Transition-Metal Coordination Polymers for High-Performance n-Type Thermoelectric Applications

Extraordinary progress has been achieved in polymer-based thermoelectric materials in recent years. New emerging π-conjugated transition-metal coordination polymers are one of the best n-type polymer-based thermoelectric materials. However, the microscopic descriptions on geometric structures, orbital characteristics, and most importantly, thermoelectric properties remain elusive, which has seriously hampered the experimentalists to draw a straightforward design strategy for new n-type polymer-based thermoelectric materials. Herein, we assess the n-type thermoelectric properties of 20 π-conjugated d8 metal center coordination polymers and rationalize their thermoelectric properties in terms of molecular geometry, orbital nature, and electron-phonon coupling based on first-principles calculations. An explicit screening rule for high-performance n-type π-conjugated transition-metal coordination polymeric thermoelectric materials was found, i.e., smaller metal center d orbital component ratio in the conduction band minimum, weaker electron-phonon coupling, higher intrinsic mobility, and thereby higher thermoelectric power factor can be achieved. Guided by this rule, poly(Pd-C2S4) and poly(Ni-C2Se4) show very high power factors. We built a map of high-performance π-conjugated transition-metal coordination polymers for n-type thermoelectric applications, which will help to accelerate the screening and design of innovative n-type thermoelectric polymers.