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We present ab initio-based electronic transport calculations on the effect of uniaxial and bi-axial stress on the CoSi 2 /n Si interface resistivity for the three main silicon crystallographic directions. For the [001] case, we identify two distinctive low and high bias conduction regimes for both compressive and tensile stress. In these regimes, the current is dominated by electronic transmission pathways near the Γ point for bias up to ~0.1V, while for higher bias it is dominated by transmission at the (±1/2, ±1/2) conduction band valleys of the Brillouin zone, which results in a contact resistivity decrease of up to 30% at 0.2V bias. This effect is less pronounced for the [110] direction, and negligible for the [111] case due to the symmetry of the Si conduction band valleys along these directions. This study provides insight into stress-based optimization pathways for contact resistivity reduction of silicide interfaces in next generation semiconductor devices.

Summary Monolayers of transition metal dichalcogenides (TMDs), such as MoS2 and WS2, have recently triggered worldwide research interest due to their remarkable optical and electronic properties. More fascinatingly is the fact that these monolayers could also adopt various morphologies with exposed edges that include triangular, hexagonal or star-shaped clusters, in addition to nanoribbons. Exciting progress has been recently achieved in the synthesis, characterization, device fabrication and functionalization of these monolayer and few-layer TMDs. This article firstly reviews the properties of bulk and monolayer/few-layer TMDs. The “top-down” and “bottom-up” synthesis routes for different TMDs are then summarized. Raman spectroscopy is now becoming a key tool used to characterize atomically thin TMDs, and this review will show the latest advances using this spectroscopic technique. Here we also summarize the most relevant characterization techniques, optical/electronic device fabrication, functionalization and defect engineering of TMDs. There are numerous opportunities for applications and multiple challenges to overcome, and this review will be instructive and useful to researchers working in the area of 2-dimensional materials, as well as scientists and engineers interested in their applications in electronics, optics, catalysis, energy and many others.

Synthesizing 3D carbon nanotube (CNT) networks with multifunctional characteristics has stimulated the interest from the scientific community since the 1990s. Here, the fabrication of a novel composite material consisting of 3D covalently interconnected multiwalled CNT with silicon carbide (SiC) nano and microparticles is reported. The material is synthesized by a two-step process involving the coating of CNT with silicon oxide (SiO x ) via chemical routes, followed by spark plasma sintering (SPS). SPS enables carbothermal reduction of SiOx and subsequent densification of the material into 3D composite blocks. Covalent interconnections of CNT are facilitated by a carbon diffusion process resulting in SiC formation as SiOx coated CNT are subjected to high temperatures. The presence of SiC in the sintered composite has been confirmed by Raman spectroscopy, as well as through energy filtered transmission electron microscopy maps. Interestingly, the 3D CNT composite exhibits high thermal conductivity (16.72 W m−1 K−1); and also a semiconducting behavior with an electron hopping mechanism associated to a 3D variable range hopping model. These findings demonstrate that it is indeed possible to fabricate SiC–CNT composites with enhanced physical properties that can be used as multifunctional materials.

Quantum molecular dynamics (QMD) and density functional theory are employed in this work in order to study the structural and electronic properties of carbon, boron nitride or hybrid BNC nanoshells. The studied nanoshells can be formed by stacking two zigzag graphene nanoribbons, two zigzag boron nitride nanoribbons or one zigzag graphene nanoribbon on a boron nitride nanoribbon. In all cases only one of the edges of the ribbon is passivated, while the other one is left unpassivated. Our QMD results show that these nanoribbons collapse just a few femtoseconds after the beginning of the simulation, forming a coalesced structure in the shape of a shell. Our band structure calculations revealed that this structures may be metallic or semiconductor, depending on its stoichiometry. Furthermore, a spin splitting for energies near the Fermi level is predicted for both the pure carbon and the hybrid BNC-nanoshell systems. We further show that when a transverse electric field is applied to these systems, the nanoshell structure tends to open up. This effect can lead to the application of these nanoshells for molecular storage. As a proof of concept, We investigate this storage effect for the H2 molecule.

We study the electronic transport properties of graphene nanoribbon barbell systems using a combination of density functional theory and Landauer–Buttiker electronic transport theory to establish the relationship between the conduction gaps of barbell-shaped graphitic heterojunctions and those of their constituting elements. The barbells considered in this study are built from graphene nanoribbons (GNRs) with either a zigzag or armchair mono-hydrogen terminated edge structure. The mechanism of bandgap variations is rationalized on the existence of specific boundary conditions imposed on the spin distribution at the interface, as well as on the property of the junction channel. It is established that the nature of the spin polarized conductance pattern depends significantly on the type of graphene nanoribbon edge and the coupling between the two graphene nanoribbon sublattices. The findings highlight the difficulty of exploiting the intrinsic properties of pristine zigzag GNRs when they are placed between electrodes, even when the contact is seamless and defect-free, since the intrinsic mismatch between the zigzag sectors induce a long-range spin defect state that affects that dictates the details of the conduction gap of the assembly.

High-resolution transmission electron microscopy studies show the dynamics of small graphene platelets on larger graphene layers. The platelets move nearly freely to eventually lock in at well-defined positions close to the edges of the larger underlying graphene sheet. While such movement is driven by a shallow potential energy surface described by an interplane interaction, the lock-in position occurs via edge-edge interactions of the platelet and the graphene surface located underneath. Here, we quantitatively study this behavior using van der Waals density functional calculations. Local interactions at the open edges are found to dictate stacking configurations that are different from Bernal (AB) stacking. These stacking configurations are known to be otherwise absent in edge-free two-dimensional graphene. The results explain the experimentally observed platelet dynamics and provide a detailed account of the new electronic properties of these combined systems.

Graphitic nanowiggles (GNWs) are 1D systems with segmented graphitic nanoribbon GNR edges of varying chiralities. They are characterized by the presence of a number of possible different spin distributions along their edges and by electronic band-gaps that are highly sensitive to the details of their geometry. These two properties promote these experimentally observed carbon nanostructures as some of the most promising candidates for developing high-performance nanodevices. Here, we highlight this potential with a detailed understanding of the electronic processes leading to their unique spin-state dependent electronic quantum transport properties. The three classes of GNWs containing at least one zigzag edge (necessary to the observation of multiple-magnetic states) are considered in two distinct geometries: a perfectly periodic system and in a one-GNW-cell system sandwiched between two semi-infinite terminals made up of straight GNRs. The present calculations establish a number of elementary rules to relate fundamental electronic transport functionality, electronic energy, the system geometry, and spin state.

A first-principles approach is used to establish that substitutional phosphorus atoms within carbon nanotubes strongly modify the chemical properties of the surface, thus creating highly localized sites with specific affinity towards acceptor molecules. Phosphorus-nitrogen co-dopants within the tubes have a similar effect for acceptor molecules, but the P-N bond can also accept charge, resulting in affinity towards donor molecules. This molecular selectivity is illustrated in CO and NH3 adsorbed on PN-doped nanotubes, O2 on P-doped nanotubes, and NO2 and SO2 on both P- and PN-doped nanotubes. The adsorption of different chemical species onto the doped nanotubes modifies the dopant-induced localized states, which subsequently alter the electronic conductance. Although SO2 and CO adsorptions cause minor shifts in electronic conductance, NH3, NO2, and O2 adsorptions induce the suppression of a conductance dip. Conversely, the adsorption of NO2 on PN-doped nanotubes is accompanied with the appearance of an additional dip in conductance, correlated with a shift of the existing ones. Overall these changes in electric conductance provide an efficient way to detect selectively the presence of specific molecules. Additionally, the high oxidation potential of the P-doped nanotubes makes them good candidates for electrode materials in hydrogen fuel cells.

We present a combined theoretical and experimental study on the electron transport characteristics across several representative interface structures found in back-end-of-line interconnect stacks for advanced semiconductor manufacturing: Cu/Ta(N)/Co/Cu and Cu/Ta(N)/Ru/Cu. In particular, we evaluate the impact of replacing a thin TaN barrier with Ta while considering both Co and Ru as wetting layers. Both theory and experiment indicate a pronounced reduction in vertical resistance when replacing TaN with Ta, regardless of whether a Co or Ru wetting layer is used. This indicates that a significant portion of the total vertical resistance is determined by electron scattering at the Cu/Ta(N) interface. The electronic structure of these nano-sized interconnects is analyzed in terms of the atom-resolved projected density of states and k-resolved transmission spectra at the Fermi level. This work further develops a fundamental understanding of electron transport and material characteristics in nano-sized interconnects.

We present a density functional theory study of the electronic structure, quantum transport and mechanical properties of recently synthesized phosphorus (P) and phosphorus-nitrogen (PN) doped single-walled carbon nanotubes. The results demonstrate that substitutional P and PN doping creates localized electronic states that modify the electron transport properties by acting as scattering centers. Nonetheless, for low doping concentrations (1 doping site per similar to 200 atoms), the quantum conductance for metallic nanotubes is found to be only slightly reduced. The substitutional doping also alters the mechanical strength, leading to a 50% reduction in the elongation upon fracture, while Young's modulus remains approximately unchanged. Overall, the PN- and P-doped nanotubes display promising properties for components in composite materials and, in particular, for fast response and ultra sensitive sensors operating at the molecular level.

Substitutional phosphorus doping in single-wall carbon nanotubes (SWNTs) is investigated by density functional theory and resonance Raman spectroscopy. Electronic structure calculations predict charge localization on the phosphorus atom, generating nondispersive valence and conduction bands close to the Fermi level. Besides confirming sustitutional doping, accurate analysis of electron and phonon renormalization effects in the double-resonance Raman process elucidates the different nature of the phosphorus donor doping (localized) when compared to nitrogen substitutional doping (nonlocalized) in SWNTs.

A detailed characterization, using high resolution electron microscopy/microanalysis (SEM, TEM, HRTEM, and EDX), reveals tubular carbon nanostructures exhibiting complex and fascinating morphologies. The materials were obtained by sulfur-assisted chemical vapor deposition. It is demonstrated that S not only acts on the catalyst, but also can be detected in the carbon lattice of the nanostructures. The experimental data presented here confirms the critical role of S, which is responsible for inducing curvature and therefore influencing the final carbon nanostructure morphology. In particular, different types of covalent Y-junctions of CNTs and even sea urchin-like nanostructures were produced and their experimental conditions are listed and discussed.

We examine the possibility of controlling nanotube growth and simultaneously manipulating the nanotube properties by adding elements in minute amounts (such as nitrogen, phosphorous, and sulfur) that are different from carbon and the metal catalyst during the growth process. This procedure is shown to be capable of producing bamboo-type morphologies, heterodoped carbon nanotubes, and Y-junctions. This also represents a critical step toward tailoring properties and controlling nanotube architectures, thus promoting the development of novel materials with unusual electronic applications. The underlying formation mechanisms that lead to the observed structures and morphologies are elucidated using wide-ranging electronic structure calculations that reveal the fundamentally different nature of nitrogen, phosphorous, and sulfur during carbon nanotube growth. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009

A combination of theoretical techniques, high-resolution microscopy, and energy-dispersive X-ray spectroscopy shows the role sulfur plays in branching phenomena during carbon nanotube (CNT) network growth. A model is proposed in which small amounts of sulfur are enough to trigger the growth of a bud in a CNT, leading to kink formation and subsequent branch growth.

Carbon nanotube growth in the presence of nitrogen has been the subject of much experimental scrutiny, sparking intense debate about the role of nitrogen in the formation of diverse structural features, including shortened length, reduced diameters, and bamboo-like multilayered nanotubules. In this paper, the origin of these features is elucidated using a combination of experimental and theoretical techniques, showing that N acts as a surfactant during growth. N doping enhances the formation of smaller diameter tubes. It can also promote tube closure which includes a relatively large amount of N atoms into the tube lattice, leading to bamboo-like structures. Our findings demonstrate that the mechanism is independent of the tube chirality and suggest a simple procedure for controlling the growth of bamboo-like nanotube morphologies.

Tungsten ditelluride (WTe2) is a transition metal dichalcogenide (TMD) with physical and electronic properties that make it attractive for a variety of electronic applications. Although WTe2 has been studied for decades, its structure and electronic properties have only recently been correctly described. We experimentally and theoretically investigate the structure, dynamics and electronic properties of WTe2 and verify that WTe2 has its minimum energy configuration in a distorted 1T structure (Td structure), which results in metallic-like transport. Our findings unambiguously confirm the metallic nature of WTe2, introduce new information about the Raman modes of Td-WTe2 and demonstrate that Td-WTe2 is readily oxidized via environmental exposure. Finally, these findings confirm that, in its thermodynamically favored Td form, the utilization of WTe2 in electronic device architectures such as field effect transistors may need to be reevaluated.

The utilization of graphene nanoribbons for next generation nanoelectronics is commonly expected to depend on the controlled synthesis that yields a low density of defects. Edge roughness and vacancies have been shown to have a large impact on the performance of graphene nanoribbon transistors. In contrast, we show how certain defects can be used to enhance the electronic and magnetic properties of graphene nanoribbons. We explore the properties of hybrid graphene nanoribbons with armchair and zigzag features joined by an array of pentagon–heptagon structural defects. The graphene nanoribbons display an increased density of states at the Fermi level, and half metallicity in absence of external fields.

The electronic transport properties in multiterminal graphene oxide framework (GOF) materials are investigated using a combination of theoretical and computational methods. GOFs make up four-terminal $[origin=c]90H$-shaped GNR-$L$-GNR junctions where sandwiched boronic acid molecules (L) are covalently linked to two graphene nanoribbons (GNRs) of different edge chiralities. The transport properties are governed by both tunneling and quasiresonant regimes. We determine how the presence of linker molecules affects the transport properties and establish that the through-molecule transport properties can be tuned by varying the chemical composition of the pillar molecules but are not significantly modified when changing the type of electrodes from zigzag GNRs to armchair GNRs. In addition, we find that in multilinker systems containing two parallel molecules in the device area, the coupling between the molecules can lead to both constructive and destructive quantum interferences. We also examine the inability of the classical Kirchhoff's superposition law to account for electron flow in multilinker GOF nanonetworks.

The magnetic properties of itinerant electrons and the effects of the electron-correlations are studied on a tetrahedral cluster surrounded by nonmagnetic metallic atoms in the framework of the Hubbard model. This model takes into account the kinetic energy which describes the electronic hopping between nearest neighbor atoms and the on-site Coulomb repulsion only on the atoms of the tetrahedral cluster. The ground-state energy and the wave function are obtained exactly using the numerical diagonalization method. Results demonstrate that for low electron concentration, the tetrahedral cluster shows antiferromagnetic order due to the fact that the electrons prefer to stay in the host metal to avoid the double occupations. However, when the number of the electrons is large, the tetrahedral cluster exhibits a ferromagnetic ground-state.

Nitrogen-doped graphitic nanoribbons (Nx-GNRs), synthesized by chemical vapor deposition (CVD) using pyrazine as a nitrogen precursor, are reported for the first time. Scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) reveal that the synthesized materials are formed by multilayered corrugated GNRs, which in most cases exhibit the formation of curved graphene edges (loops). This suggests that during growth, nitrogen atoms promote loop formation; undoped GNRs do not form loops at their edges. Transport measurements on individual pure GNRs exhibit a linear I–V (current-voltage) behavior, whereas Nx-GNRs show reduced current responses following a semiconducting-like behavior, which becomes more prominent for high nitrogen concentrations. To better understand the experimental findings, electron density of states (DOS), quantum conductance for nitrogen-doped zigzag and armchair single-layer GNRs are calculated for different N doping concentrations using density functional theory (DFT) and non-equilibrium Green functions. These calculations confirm the crucial role of nitrogen atoms in the transport properties, confirming that the nonlinear I–V curves are due to the presence of nitrogen atoms within the Nx-GNRs lattice that act as scattering sites. These characteristic Nx-GNRs transport properties could be advantageous in the fabrication of electronic devices including sensors in which metal-like undoped GNRs are unsuitable., M.T. thanks JST-Japan for funding the Research Center for Exotic NanoCarbons, under the Japanese regional Innovation Strategy Program by the Excellence. M.T. is grateful to the Penn State Center for Nanoscale Science (MRSEC; NSF grant number DMR-0820404), for a seed grant on “Defect Engineering in Layered Materials”. H.T. acknowledges support of CAPES, Brazil, through its Foreign Scientist Invited program. F.J.R.M., F.L.U., and E.M.S. acknowledge CONACYT (México) grants CB-2008-SEP-107082, 60218-F1 and 48300 S-3907, respectively. X.J. and M.S.D. acknowledge the MURI grant ONR-N00014-09-1-1063. R.M.G. was supported by MCINN, project number FIS2009-12721-C04-01 and scholarship AGAUR “FI-DGR 2011”. This work was supported by CONACYT Ph.D. scholarships 223807 (J.O.M.) and 223824 (M.L.G.B.), as well as financial research support from PSU. J.O.M. thanks complementary support from the Graduate Complementary Scholarship program (DGRI-SEP, México). B.G.S. was supported by the Center for Nanophase Materials Sciences (CNMS), sponsored at Oak Ridge National Laboratory by the Division of Scientifi c User Facilities, U.S. Department of Energy.

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 (B27N24C3) or the nitrogen (B27N24C3) 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.

Graphene nanowiggles (GNWs) are periodic repetitions of graphene nanoribbon (GNR) junctions resulting in quasi-one-dimensional wiggle-edged structures. They are synthesized using a surface-assisted bottom-up chemical approach and have been predicted to possess unusual electromagnetic properties. Here we show that GNWs also possess superior thermoelectric properties compared to their straight GNR counterparts. We employ a combination of density-functional theory and semiempirical approaches to demonstrate that the presence of wigglelike edges dramatically degrades thermal conductance due to phonons but preserves excellent electronic conduction, resulting in significant enhancement of the thermoelectric figure of merit $ZT$. We show that the resonant tunneling effect between alternate parallel and oblique sectors contributes to maintaining GNR electronic transport properties. We also present a systematic study for a large set of nanowiggle structures to establish how geometry and spin states influence $ZT$ at room temperature, thereby providing a road map for guiding the design and synthesis of specific GNWs for targeted thermoelectric applications.

Recent experiments have demonstrated a viable bottom-up strategy to produce narrow and highly ordered nanoribbons, including complex segmented structures called graphitic nanowiggles (GNWs). These defect-free systems are made of successive repetitions of finite-sized graphitic nanoribbons (GNRs) regularly connected at a given angle. Theoretical calculations have shown that these systems exhibit emergent and versatile properties at a level even higher than that found in their basic GNR constituents. Their main structural characteristic is the presence of multiple edge-dependent domains. This atomic structure has a marked influence on their physical properties since GNWs with at least one zigzag sector were shown to accommodate multiple magnetic states. The present detailed study shows how these properties vary with the details of the geometry. We also provide a quantum-mechanics-based explanation for the origin of GNW's multiple magnetic states. We find that the electronic structure of a GNW is sensitively dependent on the specific way its basic sectors are assembled, as well as on the details of the spin alignment along its edges. As a result, GNWs provide a new means to tune and design systems with desired electronic structure.

Theoretical and computational chemical physics and materials science offers great opportunity toward helping solve some of the grand challenges in science and engineering, because structure and properties of molecules, solids, and liquids are direct reflections of the underlying quantum motion of their electrons. With the advent of semilocal and especially nonlocal descriptions of exchange and correlation effects, density functional theory (DFT) can now describe bonding in molecules and solids with an accuracy which, for many classes of systems, is sufficient to compare quantitatively to experiments. It is therefore becoming possible to develop a semiquantitative description of a large number of systems and processes. In this chapter, we briefly review DFT and its various extensions to include nonlocal terms that are important for long-range dispersion interactions that dominate many self-assembly processes, molecular surface adsorption processes, solution processes, and biological and polymeric materials. Applications of DFT toward problems relevant to energy systems, including energy storage materials, functional nanoelectronics/optoelectronics, and energy conversion, are highlighted.