Systems at the nanoscale can exhibit distinctive and unexpected properties in electrical, magnetic, mechanical, and chemical aspects. Understanding these properties not only is of importance from the fundamental scientific view but also offers great opportunities for future applications. Theoretical calculations can provide important information to interpret, modify, and predict the novel properties of objects at the nanoscale and therefore play a significant role in the process of exploring the nano world. In this review, six different areas are briefly presented, namely, prediction of new stable structures, modification of properties (especially the electronic structures), design of novel devices for applications, the structures and catalytic effects of clusters, the mechanical and transport properties of gold nanowires, and improvement of materials for hydrogen storage. Based on these examples, we show what can be done and what can be found in the investigations of nanoscale systems with participation of theoretical calculations.

The successful fabrication of single layer graphene has greatly stimulated the progress of the research on graphene. In this article, focusing on the basic electronic and transport properties of graphene nanoribbons (GNRs), we review the recent progress of experimental fabrication of GNRs, the theoretical and experimental investigations of physical properties, and device applications of GNRs. We also briefly discuss the research efforts on the spin polarization of GNRs in relation to the edge states.

The fantastic variation of the physical properties of carbon nanotubes (CNTs) and their bundles under mechanical strain and hydrostatic pressure makes them promising materials for fabricating nanoscale electromechanical coupling devices or transducers. In this paper, we review the recent progress in this field, with much emphasis on our first-principles numerical studies on the structural and vibrational properties of the deformed CNTs under uniaxial and torsional strains, and hydrostatic pressure. The nonresonant Raman spectra of the deformed CNTs are also introduced, which are calculated by the first-principles calculations and the empirical bond polarizability model.

Structural defects in carbon nanotubes (CNTs) have been paid much attention, because they influence the properties of the CNTs to some extent. Among various defects in CNTs, both single vacancies and Stone–Wales (SW) defects are the simple and common ones. In this paper, we review the progress of research in these two kinds of defects in CNTs. For single vacancies, we first address their different structural features in both zigzag and armchair CNTs, and their stabilities in CNTs with different sizes and different symmetries systematically. The presence of the single vacancies in CNTs not only influences the electronic structures of the systems, but also affects the vibrational properties of the tubes. Nevertheless, being active chemically, the single vacancies in the tubes prefer to interact with adsorbates nearby, of which the interaction of the defects with hydrogen atom, hydrogen molecule and some small hydrocarbon radicals (–CH, –CH2 and –CH3) are discussed. The former is associated with H storage and the latter is of merit to improve the local structure of the defect in a CNT. For the Stone–Wales defect, we mainly focus on its stability in various CNTs. The influence of the SW defects on the conductance of CNTs and the identification of such a defect in CNT is described in brief.

An effective central insertion scheme (CIS) that allows to study the electronic structure of nanomaterials at the first principles level is introduced. Taking advantage of advanced numerical methods, such as the implicitly restarted Arnoldi method (IRAM) and spectral transformation, together with efficient parallelization technique, this scheme can provide accurate electronic structures and properties of one-,two-, and three-dimensional nanomaterials with only a fraction of computational time required for conventional quantum chemical calculations. Electronic structures of several nanostructures, such as single-walled carbon nanotubes of sub-100 nm in length, silicon nanoclusters of sub-6.5 nm in diameter and metal doped silicon clusters, calculated at hybrid density functional level are presented.

Single-electron tunneling (SET) and Coulomb blockade (CB) phenomena have been widely observed in nanoscaled electronics and have received intense attention around the world. In the past few years, we have studied SET in carbon nanotube fragments and fullerenes by applying the so-called “Orthodox” theory [28]. As outlined in this review article, we investigated the single-electron charging and discharging process via current-voltage characteristics, gate effect, and electronic structure-related factors. Because the investigated geometric structures are three-dimensionally confined, resulting in a discrete spectrum of energy levels resembling the property of quantum dots, we evidenced the CB and Coulomb staircases in these structures. These nanostructures are sufficiently small that introducing even a single electron is sufficient to dramatically change the transport properties as a result of the charging energy associated with this extra electron. We found that the Coulomb staircases occur in the I-V characteristics only when the width of the left barrier junction is smaller than that of the right barrier junction. In this case, the transmission coefficient of the emitter junction is larger than that of the collector junction; also, occupied levels enter the bias window, thereby enhancing the tunneling extensively.

We used the self-consistent method-based density functional theory (DFT) and non-equilibrium Green’s function (NEGF) to simulate molecular transport. Our numerical calculations for the organic molecular measurement made by Reichert et al. (Phys. Rev. Lett., 2002, 88: 176804) and for the alkanedithiols measurement made by Xu et al. (Science, 2003, 301: 1221) met the related experimental values quite well. This means that the first-principles calculations based on DFT and NEGF can well explain the conduction measurements of some large molecules. The numerical study reveals the fact that molecular conduction does not obey the classic law; in stead it illustrates the quantum behavior. We designed active molecular transistors controlled by the gate bias with high working frequency. They may be the next generation electronic devices.

This review deals with the high-throughput field in surface catalysis and adsorption. Special focus is placed on advanced methods for knowledge discovery such as density functional theory (DFT) simulations. An inventory of successful cases on several elements in Group I-B and VIII is reported, including the relevant data and knowledge management, which are very important in chemical industry, fuel cell, and environment protection, for both scientific and economical reasons.

In this review, we present our recent first principles studies on the sequential H₂ dissociative chemisorption and H desorption on the Pt_n and Pd_n clusters (n=2-9, 13). Upon full saturation by H atoms, the calculated H₂ dissociative chemisorption energy and H desorption energy on Pt₁₃ and Pd₁₃ clusters are similar to the corresponding values on smaller close-packed clusters. Indeed, the catalytic performances of these subnano clusters do not vary significantly with the particle sizes or shapes. Instead, they are dependent on the surface metal atoms which can be accessed by H atoms. In addition to the coverage dependency of the H₂dissociative chemisorption and H sequential desorption energies, the phase transition of both Pt₁₃ and Pd₁₃from the icosahedral to fcc-like structures at certain H coverage was also investigated.

Motivated by recent studies of graphenen nanoribbons (GNRs), we explored electronic properties of pure and chemically modified boron nitride nanoribbons (BNNRs) using the density functional theory method. Pure BNNRs with both edges fully saturated by hydrogen are semiconducting with wide band gaps. Values of the band gap depend on the width and the type of edge. The chemical decoration of BNNRs’ edges with four different functional groups, including –F, –Cl, –OH, and –NO₂, was investigated. The band-gap modulation by chemical decoration may be exploited for nanoelectronic applications.

An armchair graphene nanoribbon switch has been designed based on the principle of the Klein paradox. The resulting switch displays an excellent on–off ratio performance. An anomalous tunneling phenomenon, in which electrons do not pass through the graphene nanoribbon junction even when the conventional resonance condition is satisfied, is observed in our numerical simulations. A selective tunneling rule is proposed to explain this interesting transport behavior based on our analytical results. Based on this selective rule, our switch design can also achieve the confinement of an electron to form a quantum qubit.

The preferable configuration and electronic structure of several types of free radical functionalized boron nitride nanotubes (BNNTs) were investigated by using density functional theory computations. All the free radicals have strong interaction with B atom in the tube, in spite of the electroaffinity of the radicals. However, though a large charge is transferred from tubes to NH₂, OH or CN radicals, little change happens to the electronic structure of BNNTs, while COOH and COCl radicals introduce halffilled impurity levels around the Fermi level. Higher functionalization concentration leads to multiple impurity states around the Fermi level, and makes BNNTs p-type semiconductors.

The electronic properties of boron nanotubes with axial strain are investigated by first principle calculations. The band gaps of the (3, 3) and (5, 0) boron nanotubes are found to be modified by axial strain significantly. We find that the semiconductor–metal transition occurs for the (3, 3) boron nanotubes with both compressive and tensile strain. While for the (5, 0) boron nanotubes, only the tensile strain induces the semiconductor–metal transition. These boron nanotubes have the largest gaps under compressive strain.

We have studied the thermal conductivity of single-walled carbon nanotubes (SWCNTs) using the NEMD method. The results indicate that the thermal conductivity values are not profoundly influenced by the specific simulation-technique used in the MD simulations. Some possible reasons, which could be responsible for the discrepancy on thermal conductivity values of SWCNTs in the literatures, are discussed.

The fluorination and hydrogenation reactions on (6, 6) and (10, 0) single-walled carbon nanotubes (SWCNTs) have been examined via computing the reaction energy for the chemisorption. The examined nanotubes have comparable lengths and diameters, with or without Stone–Wales defects on the sidewall. The two fluorine or hydrogen atoms are anchored to the external walls of the SWCNTs. The computed chemisorption energies of these virtual reactions reveal that the fluorination and hydrogenation of the nanotubes are moderately sensitive to the nanotube chirality and the sidewall topology, and the (10, 0) SWCNT with Stone–Wales defect can be easily fluorinated and hydrogenated.

The conductance of a family of ruthenium-quasi cumulene-ruthenium molecular junctions including different numbers of carbon atoms, both in even numbers and odd numbers, are investigated using a fully self-consistent ab initio approach which combines the non-equilibrium Green’s function formalism with density functional theory. Our calculations demonstrate that although the overall transport properties of the Ru-quasi cumulene-Ru junctions with an even number of carbon atoms are different from those of the junctions with an odd number of carbon atoms, the difference between the corresponding currentvoltage (I-V) characteristics of these molecular junctions declines to lesser than 16% when the voltage goes up. In each group, the molecular junctions give a large transmission around the Fermi level since the Ru-C πbonds can extend the π conjugation of the carbon chains into the Ru electrodes, and their I-Vcharacteristics are almost linear and independent of the chain length, illustrating potential applications as conducting molecular wires in future molecular electronic devices and circuits.

A systematic density functional theory (DFT) study has been performed to investigate the electronic and magnetic properties of one-dimensional sandwich polymers constructed with benzene (Bz) and the second-row transition metal (TM= Y, Zr, Nb, Mo, and Tc). Within the framework of generalized gradient approximation (GGA), [Tc(Bz)]_∞ is a ferromagnetic half-metal, and [Nb(Bz)]_∞ is a ferromagnetic metal. With the on-site Coulomb interaction for 4d TM atoms being taken into account, [Tc(Bz)]_∞ keeps a robust half-metallic behavior, while [Nb(Bz)]_∞ becomes a spin-selective semiconductor. The stability of the half-metallic [Tc(Bz)]_∞ polymer is discussed based on magnetic anisotropy energy (MAE). Compared with 0.1 meV per metal atom in [Mn(Bz)]_∞, the calculated MAE for [Tc(Bz)]_∞ is 2.3 meV per metal atom. Such a significantly larger MAE suggests that Tc(Bz)]_∞ is practically more promising than its first-row TM equivalent.

We have investigated the structural and magnetic properties of small bimetallic Co_n-1Cr (n = 2 - 9) clusters by means of spin-polarized density functional theory approach. We found the ground state structures of Co_n-1Cr were very similar to those of pure Co_nexcept n = 3,6. The clusters were of high stability at n = 6. Magnetic moments showed an interesting size-dependent variation: the magnetic moments of Co_n-1Cr can be obtained by minus 7μ_B for n = 2, 4, 6-9 or by plus 1_μB for n = 5 from those of the Co_n counterparts. The different magnetic behavior stems from ferromagnetic alignment or ferrimagnetic alignment of Co and Cr atoms in Co_n-1Cr, associated with their longer or shorter interatomic distances.

The inelastic electron tunneling spectroscopy (IETS) of semifluorinated hexadecanethiol junctions is theoretically studied. The numerical results show that the C–F vibration modes of semifluorinated alkanethiol series can not be detected, and the C–H stretching mode in IETS is related to the CH2 vibration. It is demonstrated that the Raman modes are preferred over IR modes in IETS, which is in good agreement with the experimental measurements presented by Beebe et al. [Nano Lett., 2007, 7(5): 1364].

Using the scattering matrix method, we investigate the thermal conductance associated with ballistic phonons at low temperatures in asymmetric quantum structures. The results show that when the structure is an ideal quantum wire, the universal value π2kB2/(3h) can be observed at very low temperatures. However, for asymmetric quantum structure, the thermal conductance is less than the universal value π2kB2/(3h), even at T → 0. The results also show that the thermal conductance is strongly dependent on the transport direction. The rectification effect can be observed in the asymmetric structure and can be adjusted by changing the structural parameters. A brief analysis of these results is given.