Structures And Properties Of Water And Cu Nanowires: A Simulation Study

Water, as an unusual liquid due to the network of highly directional hydrogen bonds (H-bonds) connecting adjacent molecules, displays a number of thermodynamic and kinetic anomalies when it is compared to simple molecular and atomic liquids. The crucial role that water plays in chemistry and biology is largely a consequence of its H-bond network. A detailed description of the H-bond network in ambient liquid water (ALW) is the key to understand its unusual properties.Water forms 13 crystalline phases and at least four amorphous states, all of which have been characterized by diffraction studies. Ten out of the 13 of these crystalline phases (ice Ih, ice II, ice III, ice V, ice VI, ice VII, ice VIII, ice IX, ice X, and ice XI) are thermodynamically stable and have a well-defined pressure–temperature (p– T) range of their existence. Three of these (ice Ic, ice IV, and ice XII) are metastable, and are formed from liquid water at their distinctive p– T conditions. Despite that, crystallization of water's high density amorphous has produced at least two metastable phases, ice IV and ice XII, at the same p– T conditions. Crystallization of this amorphous at different p– T conditions has also produced a mixture of several metastable and stable phases of crystalline ices, which have been found to coexist for a long period at low temperatures.Until recently, it was thought that there were only four structurally distinct amorphouss: (i) hyperquenched glassy water and vapor-deposited solid whose annealed states have shown similar diffraction patterns, (ii) high density amorphous(HAD), (iii) low density amorphous (LDA) and (iv)very high density amorphous (VHDA). HDA (density=51.17g/ml at 77 K at 0.1 MPa and 1.31 g/ml at 77 K;1 GPa) is usually produced by pressurizing hexagonal ice (ice Ih) at 77-130 K to a pressure of～0.8-1.5GPa, and it has been occasionally produced by pressurizing cubic ice (ice Ic) to～0.8 GPa at 77-145 K, or less when crystals of ice Ic are small. LDA (density=50.94 g/ml at 77 K, 0.1 MPa) is produced when HDA is heated usually at ambient pressure. In contrast, VHDA (density=1.25 g/ml at 77 K, 0.1 MPa) is produced when HDA is heated to 165 K at a pressure of～0.8–1 GPa. Pressurizing LDA at 77-130 K to 0.3-0.5 GPa also produces HDA.However, there is the limited experimental access to the structure and electronic structure of water although several investigations concerning with the electronic structure of water vapor and ice have been carried out, using photoelectron spectroscopy and x-ray emission spectroscopy. Hence, even the understanding of the properties of pure liquid water remains a challenge. Although the glass transitions are well known, the transition behavior between the amorphous structures and the distinct glass structures is still not clear, especially with regards to the determination of Tg and the specific heatΔCp for the phase transformation from glass to liquid.High pressures P cause high packing densities and unusual structures relative to those at ambient environments, yet different properties away from ambient conditions, which are not yet well understood. Few experiments at extreme thermodynamic conditions, such as high p and high temperature T, have been carried out for structural and electronic properties of water, as well as dissociation mechanism for possible dissociation process. Experimental technique for studying water under high pressure, such as the shock–wave compression method, is hard to find the existence of short-lived H3O+ (<10 ps). Metallic nanowires (NW) have been intensively studied for the fundamental interests in theory and possible applications for molecular electronic devices. Cu as interconnects in microelectronics is the most useful metal while the electronic transport of nanosized interconnects is one of the important characteristics for future microelectronic applications. However, it is hard to obtain the quantum conduction G of Cu NWs and effect of electric field strength V on G of Cu NWs through the limited experimentalComputer simulations can simulate not only experiments with shorter time scales but also experiments that are difficult to realize. In addition, they can provide detailed structural information that is difficult to extract from experimental data. These advantages result in large amount of simulation works to be carried out. In this contribution, first-principles density functional theory (DFT) calculations are performed to determine the structure and electron structure of water, glass transition of water, the effect of P on structure and electron structure of water, effect of V on G of Cu NWs. The detailed contents are listed as follows:1. Electronic structures of hexagonal ice (ice Ih), high-density amorphous ice (HDA), and very high-density amorphous ice (VHDA) are investigated using ab initio density functional theory (DFT) at 77 K under a pressure of 0.1 MPa, focusing on band structure, density of states (DOS), partial density of states (PDOS) and electron density. It is found that the integration intensity of the O-2p bonding band in HDA is 1.53 eV wider than that in the VHDA. Because more 2p electrons in HDA participate the 2p-1s hybridization of O-H. The classical molecular dynamics (MD) method has further been carried out to analyze hydrogen bond network of HDA and VHDA with larger numbers of water molecules under the same temperature, pressure and boundary conditions used as those during the DFT calculation. MD results show that there exist some water molecules with five hydrogen bonds in both HDA (4.1±0.1%) and VHDA (2.8±0.1%), as compared with the LDA, being consistent with the integration intensity results of PDOS. This result can be used to interpret the physical nature of the similar transition temperature of HDA and VHDA to LDA with different heating rates.2. In this work, the glass transition of water was studied with density functional theory. The transition temperature was determined by measuring the heat capacity Cp of low-density amorphous water during rapid heating. This technique ensures that all measurements were implemented without crystallization occurring, which is difficult to be achieved experimentally. The results showed that the glass transition occurs at 171 K, which is much higher than the reported value of 136 K. In addition, the triply hydrogen-bonded water molecules were found when T > 180 K, evidencing the existence of the liquid structure at the higher temperature range.3. A first principle molecular dynamics (FP-MD) simulation for water has been performed to determine electronic structure and dissociation of water as a function of pressure. Our work focuses on effects of covalence bond length, packing density and dissociation on electronic structures of water. It is found that the integration intensity of O-2p bonding band in water decreases with the increase of pressure P and (or) temperature T. Through defining the covalence bond length and analyzing the integral intensity of the partial density of states (PDOS), a new dissociation of H2O→O2-+2H+ under p = 30 GPa and T = 600 K with corresponding electronic structure change is found. After this dissociation, protons (9.376 mol%) hop between diffusive molecular sites while O2-(3.125 mol%) vibrates around their positions.4. As size of liquid water decreases, it freezes to cubic ice (Ic) instead of ordinary hexagonal ice (Ih). This occurrence is thermodynamically considered by determining their size and temperature dependent Gibbs free energies Gh(D,T) and Gc(D,T) where subscripts h and c show Ih and Ic phases, D denotes diameter of a particle, T shows temperature. The size dependences of Gh(D,T) and Gc(D,T) functions are induced by solid-liquid interfacial free energyγand solid-liquid interface stress f. Through letting Gh(D,T) = Gc(D,T), a temperature dependent critical size of water clusters to indentify the forming possibility of Ih or Ic, Dk(T), is determined. The obtained result is consistent with known experimental and theoretical results. In addition, several thermodynamic parameters of Ic are calculated. It is found that melting temperature of Ic is near that of Ih and the entropy difference of Ih and Ic is negligible.5. The ballistic transport properties of Cu nanowires (NW) under electric fields are investigated using first-principles density-function theory for the future application as interconnects in microelectronics. It is found that as electric field strength increases, the amount of quantum conduction of a nonhelical atomic strand decreases, while that of a helical atomic strand is in an opposite tendency. The changes are decided by the changes of atomic layer distance of the NWs and the related electronic distribution along the axis of the NWs.