Zhaoming Fu

Intrinsic negative differential resistance characteristics in zigzag boron nitride nanoribbons

We investigate the charge transport properties of zigzag boron nitride nanoribbons (ZBNNRs) with various hydrogen passivations by employing density functional theory (DFT) combined with the non-equilibrium Greens function (NEGF) formalism. The calculated results reveal that the ZBNNR-based devices exhibit negative differential resistance (NDR) characteristics except those models whose both edges are passivated, due to the mechanism in which the overlap of bands near the Fermi level between the left and right electrodes gets smaller or disappears under a high bias. The NDR characteristics of the perfect ZBNNRs with one or two bare edges are weakly dependent on their widths. This is one intrinsic NDR characteristic of the ZBNNR-based devices, including some defective structures. The intuitive electronic current channels are plotted and analyzed to better understand the charge transport mechanisms. Our results suggest that the ZBNNR-based structures could be favorable candidates for preparing nanoscale NDR devices.

The electronic transport properties of transition-metal dichalcogenide lateral heterojunctions

Two-dimensional (2D) heteromaterials have aroused a new interest in many applications, and have emerged as a unique family of nanomaterials in physics and materials science. Recently, vertical and in-plane heterostructures of MoS2–WS2 monolayers, were successively prepared and observed in experiments [Gong et al., Nat. Mater., 2014, 13, 1135]. Herein, using a first-principles technique, we study the electronic transport properties of several types of zigzag MoS2–WS2 lateral heterojunctions. The results demonstrate that the MoS2–WS2 lateral heterojunctions show an interesting negative differential resistive (NDR) effect, due to owning very similar band structures to that of the pristine MoS2 and WS2 nanoribbons. The electrons always propagate through the heterojunctions along the metal-terminations, while never along the S-termination. The results demonstrate that our proposed transition-metal dichalcogenide (TMD) heterojunctions could become candidates of NDR devices, and have potential applications in nanoelectronics.

The rectifying and negative differential resistance effects in graphene/h-BN nanoribbon heterojunctions

We investigate the electronic transport properties of four types of lateral graphene/h-BN nanoribbon heterojunctions using the non-equilibrium Greens function method in combination with the density functional theory. The results show that the heterojunction displays an interesting rectifying effect when the interface has a left-right type structure, while a pronounced negative differential resistance (NDR) effect when the interface has an up-down type structure. Moreover, when the interface of the heterojunction has a left-bank or right-bank type structure, it presents the rectifying (with a larger rectification ratio) and NDR effects. This work is helpful to further construct and prepare a nanodevice based on the graphene/h-BN heterojunction materials according to the proposed structures.

Width and defect effects on the electronic transport of zigzag MoS2 nanoribbons

Using first-principles methods, we investigate the electronic transport properties of zigzag MoS2 nanoribbons (Z-MoS2NRs). The current?voltage (I?V) curves of Z-MoS2NRs show a negative differential resistive (NDR) effect, and are independent of nanoribbon width. The current flowing through the nanoribbon is mainly along the Mo-edge, with two different local current channels (Mo?????Mo hop current and S?????Mo?????S bond current). The current will be suppressed when introducing a Mo vacancy-defect at the Mo-edge under low biases?while, under high biases, the current through the defected Z-MoS2NRs will increase a little, due to the other S-edge channel being opened.

Spin-dependent electronic transport properties of zigzag silicon carbon nanoribbon

Spin-dependent electronic transport properties of the zigzag silicon carbon nanoribbon (Z-SiCNR) are studied by employing the non-equilibrium Greens function method in the framework of density functional theory. It is found that the Z-SiCNR exhibits a variety of exotic physical properties. While the Z-SiCNR in the metallic FM state presents spin filtering and current-limited effects, it is shown that the abnormal oscillation of spin-polarized currents with spin polarization as high as 100% under a certain bias voltage emerges in the half-metallic AFM state. The results demonstrate that tuning the spin state of the zigzag SiC nanoribbon provides a possible avenue to design next generation spin nanodevices with novel functionalities.

Heavy fermion behavior in the quasi-one-dimensional Kondo lattice CeCo2Ga8

Dimensionality plays an essential role in determining the anomalous non-Fermi liquid properties in heavy fermion systems. So far most heavy fermion compounds are quasi-two-dimensional or three-dimensional. Here we report the synthesis and systematic investigations of the single crystals of the quasi-one-dimensional Kondo lattice CeCo2Ga8. Resistivity measurements at ambient pressure reveal the onset of coherence at T * ≈ 20 K and non-Fermi liquid behavior with linear temperature dependence over a decade in temperature from 2 to 0.1 K. The specific heat increases logarithmically with lowering temperature between 10 and 2 K and reaches 800 mJ/mol K2 at 1 K, suggesting that CeCo2Ga8 is a heavy fermion compound in the close vicinity of a quantum critical point. Resistivity measurements under pressure further confirm the non-Fermi liquid behavior in a large temperature–pressure range. The magnetic susceptibility is found to follow the typical behavior for a one-dimensional spin chain from 300 K down to T *, and first-principles calculations predict flat Fermi surfaces for the itinerant f-electron bands. These suggest that CeCo2Ga8 is a rare example of the quasi-one-dimensional Kondo lattice, but its non-Fermi liquid behaviors resemble those of the quasi-two-dimensional YbRh2Si2 family. The study of the quasi-one-dimensional CeCo2Ga8 family may therefore help us to understand the role of dimensionality on heavy fermion physics and quantum criticality.Heavy fermion materials: evidence for a quasi-one-dimensional Kondo latticeEvidence is provided for a heavy fermion material that is an unusual example of a quasi-one-dimensional Kondo lattice system. Heavy fermion materials are compounds with localised magnetic moments that interact strongly with the surrounding conduction electrons, causing a rich variety of exotic behaviour to arise. And when these moments form a periodic array, a Kondo lattice can be formed —where scattering off the moments becomes coherent. An international team of researchers led by Yi-feng Yang and Youguo Shi from the Institute of Physics, Chinese Academy of Sciences now present an unusual example of a Kondo lattice system that is quasi-one-dimensional in nature. Using a combination of experiments and first-principles calculations they show that the cerium atoms in single crystals of CeCo2Ga8 form chains, leading to quasi-one-dimensional Kondo lattice behaviour and unusual quantum critical scalings that are not expected in the conventional quantum critical theory.

Importance of oxygen spillover for fuel oxidation on Ni/YSZ anodes in solid oxide fuel cells

Using first principles simulations and the Monte Carlo method, the optimal structure of the triple-phase boundaries (TPB) of the Ni/Yttria-Stabilized Zirconia (YSZ) anode in solid oxide fuel cells (SOFCs) is determined. Based on the new TPB microstructures we reveal different reaction pathways for H2 and CO oxidation. In contrast to what was believed in previous theoretical studies, we find that the O spillover from YSZ to Ni plays a vital role in electrochemical reactions. The H2 oxidation reaction can proceed very rapidly, by means of both the H and O spillovers, whereas the CO oxidation can only proceed through the O spillover pathway. Further understanding of the roles of defects and dopants allows us to explain puzzling experimental observations and to predict ways to improve the catalytic performance of SOFCs.

The magnetism and spin-dependent electronic transport properties of boron nitride atomic chains

Very recently, boron nitride atomic chains were successively prepared and observed in experiments [O. Cretu et al., ACS Nano 8, 11950 (2015)]. Herein, using a first-principles technique, we study the magnetism and spin-dependent electronic transport properties of three types of BN atomic chains whose magnetic moment is 1 μB for BnNn-1, 2 μB for BnNn, and 3 μB for BnNn+1 type atomic chains, respectively. The spin-dependent electronic transport results demonstrate that the short BnNn+1 chain presents an obvious spin-filtering effect with high spin polarization ratio (>90%) under low bias voltages. Yet, this spin-filtering effect does not occur for long BnNn+1 chains under high bias voltages and other types of BN atomic chains (BnNn-1 and BnNn). The proposed short BnNn+1 chain is predicted to be an effective low-bias spin filters. Moreover, the length-conductance relationships of these BN atomic chains were also studied.

Effects of Sm doping content on the ionic conduction of CeO2 in SOFCs from first principles

Sm-doping effects on ionic conduction of the CeO2 electrolyte in solid oxide fuel cells (SOFCs) are investigated using the first-principles calculations. We focus on the influence of the Sm content on ionic conductivity in Sm-doped ceria (SDC). In previous studies, the Sm-doping effects are attributed to the increase in the oxygen vacancies induced by Sm3+. However, our investigations reveal that Sm doping contents play multiple roles in affecting the ionic conductivity. First, the activity of oxygen migration can be controlled by the Sm concentration. Second, the association energy between the dopant and oxygen vacancies, which is very important for O conductivity in SDC, can also be tuned by changing the dopant content. In addition, oxygen-rich and oxygen-poor conditions will significantly modify the band structures of SDC. Our work is helpful to understand the mechanism of high ionic conductivity in the electrolyte of Sm-doped ceria in SOFCs.

The growth modes of graphene in the initial stage of a chemical vapor-deposition process

Using first-principles techniques, the growth modes of carbon clusters including chain and island structures on the Ni (111) surface are investigated, which is crucial to understand the graphene growth in the initial stage. One of the interesting findings is that both the chains and islands of carbon have higher mobility than the single carbon atom on the Ni substrate. More importantly, it is found that there exists two different growth modes, i.e., the bonding of the carbon cluster with a carbon atom, and the bonding between two clusters. The former corresponds to the preferred growth mode of a one-dimensional carbon chain; and the latter tends to happen between two-dimensional carbon island clusters. In addition, we discussed the relationship between the strong migration ability of grapheme islands and the defect formation according to our simulation results. At last, based on the calculated Bronsted–Evans–Polanyi relation, the catalytic properties of Ni substrate for graphene growth can be well described quantitatively.