Zhang Yuyang

Transport properties of boron nanotubes investigated by ab initio calculation

We investigate atomic and electronic structures of boron nanotubes (BNTs) by using the density functional theory (DFT). The transport properties of BNTs with different diameters and chiralities are studied by the Keldysh nonequilibrium Green function (NEGF) method. It is found that the cohesive energies and conductances of BNTs decrease as their diameters decrease. It is more difficult to form (N, 0) tubes than (M, M) tubes when the diameters of the two kinds of tubes are comparable. However, the (N, 0) tubes have a higher conductance than the (M, M) tubes. When the BNTs are connected to gold electrodes, the coupling between the BNTs and the electrodes will affect the transport properties of tubes significantly.

Effect of strain on geometric and electronic structures of graphene on a Ru(0001) surface

The atomic and electronic structures of a graphene monolayer on a Ru(0001) surface under compressive strain are investigated by using first-principles calculations. Three models of graphene monolayers with different carbon periodicities due to the lattice mismatch are proposed in the presence and the absence of the Ru(0001) substrate separately. Considering the strain induced by the lattice mismatch, we optimize the atomic structures and investigate the electronic properties of the graphene. Our calculation results show that the graphene layers turn into periodic corrugations and there exist strong chemical bonds in the interface between the graphene N × N superlattice and the substrate. The strain does not induce significant changes in electronic structure. Furthermore, the results calculated in the local density approximation (LDA) are compared with those obtained in the generalized gradient approximation (GGA), showing that the LDA results are more reasonable than the GGA results when only two substrate layers are used in calculation.

Effect of raising geese in cornfield on ecological characteristics of weed community

Weeds have a direct impact on crop quality and yield in agricultural systems. In order to explore a production mode which can control and utilize weeds while maintaining a high weed diversity and a high crop yield, we experimentally compared weeds by raising geese in corn fields (hereinafter referred to as RGICF) and conventionally planting corn fields (CK) in 2013. Weed niche breadth, the structure of community and functional groups and diversity were compared between the periods of BG (before the RGICF treatment was grazed by geese) and AG (after the RGICF treatment was grazed by geese) to observe changes. RGICF is a compound production pattern based on the principle of “Agro-pastoral Integration”, a concept 第 4 期 张宇阳等: 玉米田养鹅措施对杂草群落生态特征的影响 493 proposed in 2011, that uses resources like weeds and the bottom leaves of crops in a tillage system to raise poultry. However, questions remain about the system including its effects on biodiversity, weed community structure, and total biomass. Results indicated that Lolium multiflorum, Medicago sativa, and Chenopodium glaucum had the highest niche breadth during BG in RGICF, and the niche breadths of C. glaucum, Polygonum nepalense, Fragaria moupinensis and Geranium pratense increased to different extents while those of L. multiforum and M. sativa decreased during AG. Additionally, seven weed species were found only during AG. In the CK treatment, Bulbostylis densa had the highest niche breadth during BG, while Gnaphalium affine had the highest during AG. Here, two new weed species were found only during AG. The niche overlap of L. perenne and M. sativa was the highest during BG, and L. perenne overlapped most with F. moupinensis during AG. While in CK, the niche overlap of Bulbostylis densa and Digitaria sanguinalis was greatest during BG, and the niches of B. densa and Gnaphalium affine overlapped the most during AG. In the two treatments, the dicotyledonous annuals (DA) was the dominant functional group during both BG and AG, and the distribution proportion of perennials increased with a more balanced distribution during AG in RGICF. During BG, Shannon-Wiener diversity index and Margalef richness index were significantly higher in RGICF than in CK (P 0.05, n=3). Simpson diversity index of weeds in RGICF was higher than CK during BG but significantly lower than CK (P 0.05, n=3), during both BG and AG. Compared with CK, the corn yield in RGICF was lower (P>0.05, n=3); yields decreased 4%. In summary, although the RGICF production mode reduced corn yields, the practice maintains high biodiversity in agro-ecosystems, and may compensate or even overcompensate yield loss through the production of geese.

The construction and structure-property manipulation of “small-molecule machines”

Constructing low-dimensional quantum devices and manipulating their properties is one of the key research directions in physics. Due to their intrinsic structural diversities and ingenious functions as small building blocks, small molecules are a significant resource to build quantum machines for various applications, ranging from nano-rotors to ultrahigh-density information storage. The research in this project focuses on the design, construction, and structure-property manipulation of “small molecule machines” using scanning tunneling microscopy/spectroscopy (STM/STS). The primary discoveries include the following: (1) We construct a large-scale, highly-ordered array of anchored, single-molecule rotors with well-defined axis on gold surfaces using tetra- tert -butyl zinc phthalocyanine (( t -Bu)4-ZnPc) molecules. Experiments and theoretical calculations reveal the role of gold adatoms as the well-defined contact between the molecule and the surface. We further verified the manipulation of molecular rotors by putting the molecule at different regions of the surface and by changing the functional groups of the molecules. We also develop a new methodology to determine molecular configurations of a large molecular complex in a dynamical process on a metal surface by combining time-resolved tunneling spectroscopy ( I - t spectra) and density functional theory calculations (DFT). (2) We demonstrate a reversible conductance transition on the molecular scale (0.6 nm) in a complex of 3-nitrobenzal malononitrile and 1,4-phenylenediamine, by the application of local electric field pulses. Macroscopic and local current-voltage ( I / V ) measurements confirm an excellent electrical-bistability behavior. We also suggest that the conductance transition may arise from the disorder-to-crystalline transition in the thin film. (3) We investigate the structure and conductance of rotaxane molecules. For the first time, we reveal that the conductance switching in rotaxane-based devices is an inherent property of rotaxane molecules. The results suggest that the conductance switching should be attributed to movements of the cyclobis ring along the rod section of the dumbbell-shaped backbone of the rotaxane molecule. (4) By using hydrogen-atom adsorption on and desorption from manganese phthalocyanine (MnPc) molecules on Au(111) surfaces, we demonstrate reversible control of a Kondo resonance and a single-spin state in individual magnetic- molecule machines. We also reveal that hydrogen-atom adsorption leads to redistribution of charge within the 3d orbitals, which directly contributes to the Kondo resonance disappearance. This process is reversed by thermal annealing or applying local voltage pulse to desorb the hydrogen atom. A prototype of extremely high-density information storage (>40 TB/cm2) is achieved. (5) By applying an “atomic operation” on MnPc molecules, we create a new quantum system, dehydrogenated MnPc. In dehydrogenated MnPc molecules on Au(111) substrates, by using low-temperature, high-magnetic-field scanning tunneling microscopy, we achieved measurements of the Lande g -factor with intramolecular resolution. The magnetic-field dependence of the extended Kondo effect at different sites of the molecule reveals an inhomogeneous distribution of Lande g -factors and opens up a new route to access local spin properties within a single molecule. The above achievements provide insights for physics in quantum systems and have significant scientific value for the design of new-generation small-molecule machines.

The effects of four transplantation methods on five coral species at the Sanya Bay

Coral transplantation is considered as one of the major tools to increase coral abundance for degraded coral reefs. To investigate the effects of various methods and coral species in transplantation, coral fragments (n=902) of five coral species were transplanted by four methods at Luhuitou, the Sanya Bay, Hainan Province, China, where the reef has been over-exploited and is still threatened by human activities and natural disasters. Ten months after the transplant, the average survivorship of the transplanted corals was 45.5%. Methodologies had different effects on the transplanted corals, but none of them was efficacious for all coral species. Methodology could not change the decreasing trend for Montipora foliosa and Acropora hyacinthus, although it did slow down their decline. All transplants of A. hyacinthus and M. foliosa had high mortalities and significant decrease on survival area, while Porites andrewsi and Galaxea fascicularis had lower mortalities and partial mortalities. Only one method had significant effect on increasing survival area of G. fascicularis, same as P. andrewsi. Out of the five transplanted coral species, Pocillopora damicornis was the only species that had living tissue area increase in all applied methods, while the others had decreased live tissue area in one or more methods. The results of this study suggested that performing coral transplantation in a highly threatened area was not efficient unless the threats were diminished or erased. Moreover, proper species selection for coral transplantation is crucial, especially in a disturbed environment. Methodology, although having limited effects on improving results of coral transplantation, cannot compensate the maladjustment of vulnerable species to the stresses on the Luhuitou Reef. Coral transplantation on Luhuitou Reef should not be performed unless the stresses are under controlled, and corals with good tolerance to the environment should be considered first.

Electric dipolar interaction assisted growth of single crystalline organic thin films

We report on a forest-like-to-desert-like pattern evolution in the growth of an organic thin film observed by using an atomic force microscope. We use a modified diffusion limited aggregation model to simulate the growth process and are able to reproduce the experimental patterns. The energy of electric dipole interaction is calculated and determined to be the driving force for the pattern formation and evolution. Based on these results, single crystalline films are obtained by enhancing the electric dipole interaction while limiting effects of other growth parameters.