Insights into Structural, Electronic, and Transport Properties of Pentagonal PdSe2 Nanotubes Using First-Principles Calculations

One-dimensional (1D) novel pentagonal materials have gained significant attention as a new class of materials with unique properties that could influence future technologies. In this report, we studied the structural, electronic, and transport properties of 1D pentagonal PdSe₂ nanotubes (p-PdSe₂ NTs). The stability and electronic properties of p-PdSe₂ NTs with different tube sizes and under uniaxial strain were investigated using density functional theory (DFT). The studied structures showed an indirect-to-direct bandgap transition with slight variation in the bandgap as the tube diameter increased. Specifically, (5 × 5) p-PdSe₂ NT, (6 × 6) p-PdSe₂ NT, (7 × 7) p-PdSe₂ NT, and (8 × 8) p-PdSe₂ NT are indirect bandgap semiconductors, while (9 × 9) p-PdSe₂ NT exhibits a direct bandgap. In addition, under low uniaxial strain, the surveyed structures were stable and maintained the pentagonal ring structure. The structures were fragmented under tensile strain of 24%, and compression of -18% for sample (5 × 5) and -20% for sample (9 × 9). The electronic band structure and bandgap were strongly affected by uniaxial strain. The evolution of the bandgap vs. the strain was linear. The bandgap of p-PdSe₂ NT experienced an indirect-direct-indirect or a direct-indirect-direct transition when axial strain was applied. A deformability effect in the current modulation was observed when the bias voltage ranged from about 1.4 to 2.0 V or from -1.2 to -2.0 V. Calculation of the field effect I-V characteristic showed that the on/off ratio was large with bias potentials from 1.5 to 2.0 V. This ratio increased when the inside of the nanotube contained a dielectric. The results of this investigation provide a better understanding of p-PdSe₂ NTs, and open up potential applications in next-generation electronic devices and electromechanical sensors.

Ultranarrow heterojunctions of armchair-graphene nanoribbons as resonant-tunnelling devices

A systematic investigation is performed on the electronic transport properties of armchair-graphene nanoribbon (AGNR) heterojunctions using spin-polarized density functional theory calculations in combination with the non-equilibrium Green's function formalism. 9-AGNR and 5-AGNR structures are used to form a single-well configuration by sandwiching a 5-AGNR between two 9-AGNRs. At the same time, these 9-AGNRs are matched at the left and right to electrodes, 9 and 5 being the number of carbon dimers as width. This heterojunction mimics an electronic device with two potential barriers (9-AGNR) and one quantum well (5-AGNR) where quasi-bound states are confined. First, we study the ground state properties, and then we calculate the electron transport properties of this device as a function of the well width. We show the presence of electronic tunnelling resonances between the barriers by delocalized electron density inside the well's structure. This is corroborated by transmission curves, localized densities of states (LDOS), current-vs.-bias voltage results, and the trend of the resonances as a function of the well width. This work shows that carbon AGNRs may be used as resonant-tunnelling devices for applications in nanoelectronics.

Origin of structural stability of ScH3 molecular nanowires and their chemical-bonding behavior: Correlation effects of the Sc 3d electrons

A new stable transition-metal trihydride (ScH₃) molecular nanowire was recently reported by Li et al. [J. Am. Chem. Soc. 139, 6290-6293 (2017)]. Of the two typical structures (T-ScH₃ and O-ScH₃), T-ScH₃ is more stable than O-ScH₃. However, the reason why O-ScH₃ is less stable than T-ScH₃ was not known. Using Perdew-Burke-Ernzerhof (PBE), PBE+U, SCAN, and HSE06, as well as crystal orbital Hamilton populations (COHPs), we investigate the orbital-projected band structures and chemical bonding of T-ScH₃ and O-ScH₃. It is found that the energies calculated by PBE, SCAN, and HSE06 indeed reveal that T-ScH₃ is more stable than O-ScH₃, and there is no occupied antibonding state at the Fermi level of the COHP curves of T-ScH₃, supporting the stable Sc-H bonding of T-ScH₃. To the contrary, the Sc-H bonding of O-ScH₃ is unstable because there exist occupied antibonding states at the Fermi level of the COHP curves of O-ScH₃. We found that the results of PBE+U are consistent with those of PBE, SCAN, and HSE06 in the case of U < U_c. However, when U > U_c, the results of PBE+U are opposite to those of PBE, SCAN, and HSE06.

Lower Electric Field-Driven Magnetic Phase Transition and Perfect Spin Filtering in Graphene Nanoribbons by Edge Functionalization

Perfect spin filtering is an important issue in spintronics. Although such spin filtering showing giant magnetoresistance was suggested using graphene nanoribbons (GNRs) on both ends of which strong magnetic fields were applied, electric field controlled spin filtering is more interesting due to much easier precise control with much less energy consumption. Here we study the magnetic/nonmagnetic behaviors of zigzag GNRs (zGNRs) under a transverse electric field and by edge functionalization. Employing density functional theory (DFT), we show that the threshold electric field to attain either a half-metallic or nonmagnetic feature is drastically reduced by introducing proper functional groups to the edges of the zGNR. From the current-voltage characteristics of the edge-modified zGNR under an in-plane transverse electric field, we find a remarkable perfect spin filtering feature, which can be utilized for a molecular spintronic device. Alteration of magnetic properties by tuning the transverse electric field would be a promising way to construct magnetic/nonmagnetic switches.

Enhanced oscillatory rectification and negative differential resistance in pentamantane diamondoid-cumulene systems

We propose a new functionality for diamondoids in nanoelectronics. Based on the nonequilibrium Green's function formalism and density functional theory, we reveal that when attached to gold electrodes, the pentamantane-cumulene molecular junction exhibits large and oscillatory rectification and negative differential resistance (NDR) - depending on the number of carbon atoms in cumulene (Cn). When n is odd rectification is greatly enhanced where the rectification ratio can reach ∼180 and a large negative differential resistance peak current of ∼3 μA. This oscillatory behavior is well rationalised in terms of the occupancy of the carbon 2p states in Cn. Interestingly, different layers of C atoms in the pentamantane molecule have different contributions to transmission. The first and third layers of C atoms in pentamantane have a slight contribution to rectification, and the fifth and sixth layers have a stronger contribution to both rectification and NDR. Thus, our results suggest potential avenues for controlling their functions by chemically manipulating various parts of the diamondoid molecule, thus extending the applications of diamondoids in nanoscale integrated circuits.

Neutral and Anionic Gold Decamers: Planar Structure with Unusual Spatial Charge-Spin Separation

We have investigated the issue of two-dimensional (2D) versus three-dimensional (3D) structures for neutral-state Au10 and clarified the lowest-energy structure among a few 2D Au10(-) isomers. Though almost all previous works were based on density functional theory (DFT), we here carried out not only extensive DFT calculations but also high levels of ab initio calculations of Möller-Plesset second order perturbation theory (MP2), and coupled cluster theory with single and double excitations (CCSD) including perturbative triple excitations [CCSD(T)]. While DFT favors 2D structures, MP2 and CCSD(T) favor 3D structures for moderate-sized basis sets. However, we note that the basis-set superposition error (BSSE) corrections make the ab intio results favor 2D structures too. The near-degeneracy (driven by relativistic effects) of 5d and 6s orbitals of gold helps stabilize acute apex gold atoms, resulting in 2D structures. The planar triangular structures of a local minimum Au10 (triplet) and the global minimum Au10(-) show remarkable spatial charge-spin separation due to their singly occupied molecular orbital(s). By the same reason, Au10(-) shows much larger vertical detachment energy than other even-numbered gold cluster anions.

Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems

We present the science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems, targeting an evolution in technology, that might lead to impacts and benefits reaching into most areas of society. This roadmap was developed within the framework of the European Graphene Flagship and outlines the main targets and research areas as best understood at the start of this ambitious project. We provide an overview of the key aspects of graphene and related materials (GRMs), ranging from fundamental research challenges to a variety of applications in a large number of sectors, highlighting the steps necessary to take GRMs from a state of raw potential to a point where they might revolutionize multiple industries. We also define an extensive list of acronyms in an effort to standardize the nomenclature in this emerging field.

Clean coupling of unfunctionalized porphyrins at surfaces to give highly oriented organometallic oligomers

The direct coupling of complex, functional organic molecules at a surface is one of the outstanding challenges in the road map to future molecular devices. Equally demanding is to meet this challenge without recourse to additional functionalization of the molecular building blocks and via clean surface reactions that leave no surface contamination. Here, we demonstrate the directional coupling of unfunctionalized porphyrin molecules--large aromatic multifunctional building blocks--on a single crystal copper surface, which generates highly oriented one-dimensional organometallic macromolecular nanostructures (wires) in a reaction which generates gaseous hydrogen as the only byproduct. In situ scanning tunneling microscopy and temperature programmed desorption, supported by theoretical modeling, reveal that the process is driven by C-H bond scission and the incorporation of copper atoms in between the organic components to form a very stable organocopper oligomer comprising organometallic edge-to-edge porphyrin-Cu-porphyrin connections on the surface that are unprecedented in solution chemistry. The hydrogen generated during the reaction leaves the surface and, therefore, produces no surface contamination. A remarkable feature of the wires is their stability at high temperatures (up to 670 K) and their preference for 1D growth along a prescribed crystallographic direction of the surface. The on-surface formation of directional organometallic wires that link highly functional porphyrin cores via direct C-Cu-C bonds in a single-step synthesis is a new development in surface-based molecular systems and provides a versatile approach to create functional organic nanostructures at surfaces.

Oxidative biodegradation of single- and multi-walled carbon nanotubes

In this study we compare the biodegradation of both single-walled (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) using two different oxidative conditions. In particular, we demonstrate that oxidized multi-walled carbon nanotubes are highly degraded, although not to completeness when treated with horseradish peroxidase (HRP) in the presence of hydrogen peroxide.

The finite-size effect on the transport properties in edge-modified graphene nanoribbon-based molecular devices

The size-dependence on the electronic and transport properties of the molecular devices of the edge-modified graphene nanoribbon (GNR) slices is investigated using density-functional theory and Green's function theory. Two edge-modifying functional group pairs are considered. Energy gap is found in all the GNR slices. The gap shows an exponential decrease with increasing the slice size of two vertical orientations in the two edge terminated cases, respectively. The tunneling probability and the number of conducting channel decreases with increasing the GNR-slices size in the junctions. The results indicate that the acceptor-donor pair edge modulation can improve the quantum conductance and decrease the finite-size effect on the transmission capability of the GNR slice-based molecular devices.

Doping single-walled carbon nanotubes through molecular charge-transfer: a theoretical study

We study the effect of the molecular charge transfer on the electronic structure of metallic (5,5) and semiconducting (8,0) single-walled carbon nanotubes (SWNTs) induced by surface adsorption of various organic donor-acceptor molecules of different affinities using ab initio density functional theory. Our results, obtained from first-principles spin-polarized calculations show that the adsorption of molecules with different affinities reflects the difference in interaction strength that measure the overall energy of adsorption. Moderate values of the binding energy of these surface adsorbed molecular charge-transfer complexes suggest that the nature of interaction is in the physisorption regime, and mainly governs by Coulombic forces. We also find that the large band gap of semiconducting (8,0) SWNT can be tuned through the surface adsorption of selective organic molecules which gives rise to mid-gap localized molecular levels near the Fermi energy with tuning of band gap region. Interestingly, we find that the metallic (5,5) SWNT and semiconducting (8,0) SWNT turn into semiconducting and metallic nanotubes respectively in presence of selective surface adsorbed molecules, corroborating recent experimental findings. We also suggest that these charge transfer effect can be probed through optical conductivity measurement, as the low-frequency profiles are affected by charge transfer.

Toward ubiquitous environmental gas sensors-capitalizing on the promise of graphene

Atomically thin sheets of carbon known as "graphene" have captured the imagination of much of the scientific world during the past few years. Although these single sheets of graphite were under our noses for years-within technologies ranging from the humble pencil, which has been around since at least 1565 (Petroski, H. The Pencil: A History of Design and Circumstance; Alfred A. Knopf: New York, 1993), to modern nuclear reactors-graphene was merely considered as part of graphite's crystal structure until 2004, when Novoselov, Geim, and colleagues (Science 2004, 306, 666-669) first presented some of the surprising electrical properties of graphene layers they had isolated by mechanically peeling sheets off graphite crystals. Today, graphene's unique electronic structures and properties, bolstered by other intriguing properties discovered in the intervening years, threaten the dominance of carbon nanotubes, a more mature allotrope of carbon, in potential applications from electronics to sensors. In this review, we will consider the promise of graphene for producing small-scale gas sensors for environmental monitoring.

Tuning molecular orbitals in molecular electronics and spintronics

With the advance of nanotechnology, a variety of molecules, from single atoms to large-scale structures such as graphene or carbon nanotubes, have been investigated for possible use as molecular devices. Molecular orbitals (MOs) are a key ingredient in determining the transport properties of molecules, because they contain all the quantum mechanical information of molecular electronic structures and offer spatial conduction channels for electron transport. Therefore, the delicate modulation of the MOs enables us to tune the performance of electron transport through the molecule. Electric and magnetic fields are powerful and readily accessible means for that purpose. In this Account, we describe the effects of external fields on molecular electronic and spintronic devices. Quantum transport through a molecule that connects source and drain electrodes depends strongly on the alignment of molecular energy levels with respect to the chemical potentials at both electrodes. This dependence results from the energy levels being exploited in resonant tunneling processes when the molecule is weakly coupled to the electrodes in the molecular junction. Molecular energy levels can be shifted by the Stark effect of an external electric field. For a molecule with no permanent dipole moment, the polarizability is the primary factor determining the energy shift of each MO, according to the second-order Stark effect; more polarizable MOs undergo a larger energy shift. Interestingly, even a small shift may lead to a completely nontrivial result. For example, we show a magnetic on-off switching phenomenon of a molecule controlled by an electric field. If a molecule has a nonmagnetic ground state but a highly polarizable magnetic excited state with an energy slightly above the ground state, the magnetic excited state can have lower energy than the ground state under a sufficiently strong electric field. A magnetic field is normally used to control spin orientation in a ferromagnetic system. Here we show that the magnetic field can also be used to control MOs. A graphene nanoribbon with zig-zag-shaped edges (ZGNR) has a ferromagnetic spin ordering along the edges, and the spin states have unique orbital symmetries. Both spin polarizations and orbital symmetries can simultaneously be controlled by means of an external magnetic field. The ZGNR spin-valve devices incorporating this effect are predicted to show an extreme enhancement (compared with conventional devices) of magnetoresistance due to the double spin-filtering process. In such a system, spins are filtered not only by spin matching-mismatching between both electrodes as in normal spin-valve devices, but also by the orbital symmetry matching-mismatching. Thus, a new type of magnetoresistance, and with extremely large values, so-called super-magnetoresistance (distinct from the conventional tunneling or giant magnetoresistance), is available with this method. MOs are at the heart of understanding and tuning transport properties in molecular systems. Therefore, investigating the effects of external fields on MOs is important not only for understanding fundamental quantum phenomena in molecular devices but also for practical applications in the development of interactive devices.