Helical [110] gold nanowires make longer linear atomic chains

Surface Effect on Young's Modulus of Sub-Two-Nanometer Gold [111] Nanocontacts

The surface bond nature of face centered cubic metals has been controversial between hardening and softening theoretically because of the lack of precise measurement. Here, we precisely measured the size dependence of Young's modulus of gold [111] nanocontacts with a clean surface by our in situ TEM-frequency modulation force sensing method in ultrahigh vacuum at room temperature. Young's modulus gradually decreased from ca. 80 to 30 GPa, as the nanocontact width decreased below 2 nm, which could be explained by surface softening; Young's modulus of the outermost atomic layer was estimated to be approximately 22 GPa, while that of the other part was almost the same with the bulk.

First-principles simulations on suspended coinage-metal nanotubes composed of different atomic species

Substitutional doping of gold and copper atoms in a (4, 4) silver single-wall nanotube has been investigated using first-principles simulations. It is found that the Au- and Cu-substitutional doping of the tip-suspended (4, 4) Ag tube can maintain the hollow tubular structure at different alloy compositions due to the existence of a local minimum in the string tension variation with their unit cell lengths. The bonding energy differences between the mono-elements and hetero-elements and string tension may play important roles in suppressing the "self-purification" effects so that the nanoalloy tubes can be formed. Analysis of the band structure suggests that the number of conduction channels of the Ag-Au alloy tubes may lie between the pure (4, 4) Ag and Au tubes.

Coinage metal (4, 4) nanotubes, simulated by first-principles calculations

The structural stability of coinage metal nanotubes with a square cross-section has been investigated by the first-principles numerical simulations. In addition to the reported (4, 4) silver tube, it is found that the hollow (4, 4) copper and gold nanotubes can also be formed by applying an appropriate stress to an 8(A)/8(B) fcc wire. The stability of these coinage metal (4, 4) nanotubes, formed by tip-stretching the wires, has been explained by a local minimum in the string tension variation with their tube lengths. Interestingly, we have explained why a low-stress stretching is needed to obtain the (4, 4) Cu tube in contrast to a higher one for both the (4, 4) Ag and Au tubes due to the larger stiffness coefficient of copper than those of silver and gold, which could be proved by future experiments.

Coupled effects of size and uniaxial force on phase transitions in copper nanowires

By means of molecular dynamics simulations, we investigate solid-solid phase transitions between the regular crystalline structure and the unbuckled atomic-sized one in ultrathin copper nanowires under uniaxial elongation and compression. The interatomic potential employed has been verified to be capable of describing the structural and mechanical properties of ultrathin copper nanowires. The coupled effects of size, uniaxial force and relaxing temperature on the transitions have been revealed. Both the reported phase transition from the crystalline structure to the helical one and the unexpected inverse behavior are found. At a relaxing temperature of 900 K, helical structures are dominant for effective diameters less than 0.8 nm, while the uniaxial force may lead to helical-crystalline phase transitions for thicker ones. We also observe that the transition from a helical 12-7-1 nanowire to a crystalline [110] nanowire is much easier than the inverse transition. Our results demonstrate the structural phase transitions between the crystalline structures and the unbuckled atomic-sized ones of the force-suspended metal nanowires.

Domain boundary formation in helical multishell gold nanowires

Helical multishell gold nanowires are studied theoretically for the formation mechanism of the helical domain boundary. Nanowires with a wire length of more than 10 nm are relaxed by quantum mechanical molecular dynamics simulation with a tight-binding form Hamiltonian. In the results, non-helical nanowires are transformed into helical ones with the formation of atom pair defects at the domain boundary, where the defective atom pair is moved from an inner shell. Analysis of local electronic structure shows a competitive feature of the energy gain of reconstruction on the wire surface and the energy loss of the defect formation. A simple energy scaling theory gives a general explanation of domain boundary formation.