The strength of metal crystals is reduced below the theoretical value by the presence of dislocations or by flaws that allow easy nucleation of dislocations. A straightforward method to minimize the number of defects and flaws and to presumably increase its strength is to increase the crystal quality or to reduce the crystal size. Here, we describe the successful fabrication of high aspect ratio nanowhiskers from a variety of face-centered cubic metals using a high temperature molecular beam epitaxy method. The presence of atomically smooth, faceted surfaces and absence of dislocations is confirmed using transmission electron microscopy investigations. Tensile tests performed in situ in a focusedion beam scanning electron microscope on Cu nanowhiskers reveal strengths close to the theoretical upper limit and confirm that the properties of nanomaterials can be engineered by controlling defect and flaw densities.Single crystalline metal wires, or metal whiskers, with diameters larger than one micrometer have been routinely fabricated 1 and used for experimentally examining mechanical, 2 ferromagnetic, 3 superconductive, 4 and electronic 5 properties. Interest in metal whiskers became intense once it was observed that they could exhibit strengths close to theoretical (ideal) values. 2,6 Whiskers have been grown by the vapor liquid solid method 7 (VLS) or metal halide reduction, 6 the latter of which have demonstrated high strength. 6 This has been attributed to the near-equilibrium nature of the growth process and the resultant absence of defects and flaws in the samples. Extending the fabrication of high quality metal whiskers to submicrometer diameters, as routinely produced for semiconductors, 7-9 is clearly desired but has been largely elusive. However, single crystalline metal nanowires have only occasionally been successfully fabricated. 10 Here we describe the first time to our knowledge the fabrication of metal single crystalline nanowhiskers (NWs) with diameters as small as 20 nm and no defects, as evidenced by their strengths near the theoretical upper limit. Such nanostructures have the potential to serve as model systems for elucidating intrinsic properties in tiny structures, such as quantum effects, and to be used as building blocks in nanotechnological applications where unique functionalities are required.In this study, free-standing, high aspect ratio, single crystalline nanowhiskers of a variety of different materials (copper, gold, silver, aluminum, and silicon) have been successfully grown from partially C-coated, oxidized and nonoxidized Si (100), (110), and (111) substrates under molecular beam epitaxy (MBE) conditions. Both elevated substrate temperatures (on the order of 0.65 T M of the deposited species) and the partial C layer are necessary to achieve nanowhisker growth. In the remainder of this publication we will focus on results from Cu nanowhiskers. Figure 1 shows scanning electron micrographs of two Cu samples with nominal Cu thicknesses of 45 and 200 nm, respectively. In addi...

Whether a solid material can take part in an electrochemical reaction is, besides thermodynamic constraints, determined by kinetic factors, such as the rate of charge transfer and transport within the bulk and across the interface. In view of low electronic and ionic conductivity, Li 2 O has been regarded as an electrochemical inactive material at room temperature. However, recent studies of Tarascon's group show that if Li 2 O is dispersed with transition metal elements (M = Fe, Co, Ni, Cu) on a nanoscale, Li can be electrochemically extracted from M/Li 2 O nanocomposites. [1,2] This surprising and technologically highly relevant finding may be due to the reduced transport pathways, the interface chemistry and/ or size effects on these properties. [3,4] It is known that discharging a Li/MO x cell can transform the metal oxide in situ into a M/Li 2 O nanocomposite.[5±6] In this way, Li can be reversibly stored in lithium batteries by forming and decomposing Li 2 O using transition metal oxides as precursors: [1,2,7,8] MOIn view of technological application, it is significant to note that the nanocomposite need not be pre-fabricated but forms during the insertion of lithium into the transition metal oxide.Like Li 2 O, LiF is also known as a poor electronic and ionic conductor at room temperature, [9] and particularly strong bonds are formed between lithium and fluorine (the electronegativity difference between lithium and fluorine is 3, compared to 2.5 for Li 2 O). Hence it has never been reported to be electrochemically active in a Li cell. It is naturally very relevant to find out whether transition metal fluorides can show a similar electrochemical behavior and whether the very stable LiF can be reversibly decomposed if dispersed with transition metals on an atomic or nanometer scale.The electrochemical reaction of metal fluorides with Li and its reverse reaction may be written as:The emf values of above reactions are calculated for various metal fluorides (M = Ti, V, Mn, Fe, Co, Ni, Cu, Zn, Ag, Sn) according to Nernst equation (interfacial energy has been neglected) from thermodynamic data [10] and listed in Table 1.All the metal fluorides considered here naturally exhibit a lower reaction voltage than the decomposition voltage of 6.1 V for pure LiF, but higher values, by about 1 V, than the corresponding metal oxides. In view of thermodynamics, it is still possible to investigate the above reactions in common nonaqueous electrolyte systems with an electrochemical window of a width of 5 V. In addition, according to Table 1, most transition metal fluorides show theoretical gravimetric capacities which are comparable to the transition metal oxides, but much higher than for graphite (< 372 mA h g ±1 ), which has been used as anode material in commercial Li ion batteries.Hence, we found it relevant to investigate the electrochemical reactions of MF n (M = Ti, V, Mn, Fe, Co, Ni, Cu, Zn, Ag, Sn; n = 2 or 3) with lithium at room temperature. In this communication, we report mainly the results of TiF 3 and VF 3 in vie...

Linear defects in crystalline materials, known as dislocations, are central to the understanding of plastic deformation and mechanical strength, as well as control of performance in a variety of electronic and photonic materials. Despite nearly a century of research on dislocation structure and interactions, measurements of the energetics and kinetics of dislocation nucleation have not been possible, as synthesizing and testing pristine crystals absent of defects has been prohibitively challenging. Here, we report experiments that directly measure the surface dislocation nucleation strengths in high-quality 〈110〉 Pd nanowhiskers subjected to uniaxial tension. We find that, whereas nucleation strengths are weakly size- and strain-rate-dependent, a strong temperature dependence is uncovered, corroborating predictions that nucleation is assisted by thermal fluctuations. We measure atomic-scale activation volumes, which explain both the ultrahigh athermal strength as well as the temperature-dependent scatter, evident in our experiments and well captured by a thermal activation model.

There has been relatively little study on time-dependent mechanical properties of nanowires, in spite of their importance for the design, fabrication and operation of nanoscale devices. Here we report a dislocation-mediated, time-dependent and fully reversible plastic behaviour in penta-twinned silver nanowires. In situ tensile experiments inside scanning and transmission electron microscopes show that penta-twinned silver nanowires undergo stress relaxation on loading and complete plastic strain recovery on unloading, while the same experiments on single-crystalline silver nanowires do not exhibit such a behaviour. Molecular dynamics simulations reveal that the observed behaviour in penta-twinned nanowires originates from the surface nucleation, propagation and retraction of partial dislocations. More specifically, vacancies reduce dislocation nucleation barrier, facilitating stress relaxation, while the twin boundaries and their intrinsic stress field promote retraction of partial dislocations, resulting in full strain recovery.