D. Ugarte

Quantum Conductance in Silver Nanowires: correlation between atomic structure and transport properties

We have analyzed the atomic arrangements and quantum conductance of silver nanowires generated by mechanical elongation. The surface properties of Ag induce unexpected structural properties, as for example, predominance of high aspect ratio rod-like wires. The structural behavior was used to understand the Ag quantum conductance data and the proposed correlation was confirmed by means of theoretical calculations. These results emphasize that the conductance of metal point contacts is determined by the preferred atomic structures and, that atomistic descriptions are essential to interpret the quantum transport behavior of metal nanostructures.

Spontaneous periodic diameter oscillations in InP nanowires: the role of interface instabilities.

We have observed that thin InP nanowires generated by vapor-liquid-solid growth display spontaneous periodic diameter oscillations when large group III supersaturations are used. Diameter variations are associated with a large number of stacking faults and crystallographic phase changes(wurtzite/zinc-blende); also the axial distance between oscillations depends on the indium precursor flow used during the run. We attribute the morphology changes to a substantial deformation of the triple phase line (vapor-liquid-solid) at the catalyst nanoparticle edge originated from multistep nucleation during growth. The deformation alters the mechanical force balance acting on the nanoparticle during growth in such a way that the particle displaces from the nanowire top and wets the nanowire sidewall. Subsequently, as catalytic growth occurs at the sidewall, the associated increase in diameter will eventually push the NP back to its original wire-top position until the onset of a new instability at the triple phase line.

Kinetic effects in InP nanowire growth and stacking fault formation: the role of interface roughening.

InP nanowire polytypic growth was thoroughly studied using electron microscopy techniques as a function of the In precursor flow. The dominant InP crystal structure is wurtzite, and growth parameters determine the density of stacking faults (SF) and zinc blende segments along the nanowires (NWs). Our results show that SF formation in InP NWs cannot be univocally attributed to the droplet supersaturation, if we assume this variable to be proportional to the ex situ In atomic concentration at the catalyst particle. An imbalance between this concentration and the axial growth rate was detected for growth conditions associated with larger SF densities along the NWs, suggesting a different route of precursor incorporation at the triple phase line in that case. The formation of SFs can be further enhanced by varying the In supply during growth and is suppressed for small diameter NWs grown under the same conditions. We attribute the observed behaviors to kinetically driven roughening of the semiconductor/metal interface. The consequent deformation of the triple phase line increases the probability of a phase change at the growth interface in an effort to reach local minima of system interface and surface energy.

Transmission electron microscopy and molecular dynamics study of the formation of suspended copper linear atomic chains

We report high resolution transmission electron microscopy and classical molecular dynamics simulation results of mechanically stretching copper nanowires conducting to linear atomic suspended chains (LACs) formation. In contrast with some previous experimental and theoretical work in literature that stated that the formation of LACs for copper should not exist our results showed the existence of LAC for the [111], [110], and [100] crystallographic directions, being thus the sequence of most probable occurence.

Enhanced Eshelby twist on thin Wurtzite InP nanowires and measurement of local crystal rotation.

We have performed a detailed study of the lattice distortions of InP wurtzite nanowires containing an axial screw dislocation. Eshelby predicted that this kind of system should show a crystal rotation due to the dislocation induced torque. We have measured the twisting rate and the dislocation Burgers vector on individual wires, revealing that nanowires with a 10-nm radius have a twist up to 100% larger than estimated from elasticity theory. The strain induced by the deformation has a Mexican-hat-like geometry, which may create a tube-like potential well for carriers.

III―V semiconductor nanowire growth: does arsenic diffuse through the metal nanoparticle catalyst?

The synthesis of III-V semiconductor nanowires (NWs) is based on the delivery of atoms from a vapor phase to a catalytic metal nanoparticle (NP). Although there has been extensive work on such systems, the incorporation pathways of group V atoms remain an open issue. Here, we have performed a detailed structural and chemical analysis of the catalyst NP in NWs where we switch the V atomic element during growth (heterostructured InP/InAs/InP NWs). Our experimental results indicate a group V pathway where these atoms actually diffuse through the catalytic NP by formation of a stable phase containing As under growth conditions. We have observed distinct NW growth behavior within a narrow temperature range (30 degrees C) suggesting a transition between vapor-liquid-solid and vapor-solid-solid growth modes.

Temperature effects on the occurrence of long interatomic distances in atomic chains formed from stretched gold nanowires

The origin of long interatomic distances in suspended gold atomic chains formed from stretched nanowires remains the object of debate despite the large amount of theoretical and experimental work. Here, we report new atomic resolution electron microscopy observations acquired at room and liquid-nitrogen temperatures and theoretical results from ab initio quantum molecular dynamics on chain formation and stability. These new data are suggestive that the long distances are due to contamination by carbon atoms originating from the decomposition of adsorbed hydrocarbon molecules.

Correlation between quantum conductance and atomic arrangement of atomic-size silver nanowires

We have studied the effect of thermal effects on the structural and transport response of Ag atomic-size nanowires (NWs) generated by mechanical elongation. Our study involves both time-resolved atomic resolution transmission electron microscopy imaging and quantum conductance measurement using an ultra-high-vacuum mechanically controllable break junction. We have observed drastic changes in conductance and structural properties of Ag nanowires generated at different temperatures (150 and 300 K). By combining electron microscopy images, electronic transport measurements, and quantum transport calculations, we have been able to obtain a consistent correlation between the conductance and structural properties of Ag NWs. In particular, our study has revealed the formation of metastable rectangular rod-like Ag wire (3/3) along the [001] crystallographic direction, whose formation is enhanced. These results illustrate the high complexity of analyzing structural and quantum conductance behaviour of metal atomic...

Characterization of interface abruptness and material properties in catalytically grown III–V nanowires: exploiting plasmon chemical shift

We have studied the assessment of chemical composition changes in III-V heterostructured semiconductor nanowires (NWs) with nanometric spatial resolution using transmission electron microscopy methods. These materials represent a challenge for conventional spectroscopy techniques due to their high sensitivity to electron beam irradiation. Radiation damage strongly limits the exposure time to a few (5-10) s, which reduces the sensitivity of the traditionally used x-ray spectroscopy. The rather low counting statistics results in significant errors bars for EDS chemical quantification (5-10%) and interface width determination (few nanometers). Plasmon chemical shift is ideal in this situation, as its measurement requires very short exposure times (approximately 100 ms) and the plasmon peak energy can be measured with high precision (approximately 20 meV in this work). This high sensitivity allows the detection of subtle changes (1-2%) in composition or even the detection of a small plasmon energy (33 +/- 7) meV change along usually assumed pure and homogeneous InAs segments. We have applied this approach to measure interface widths in heterostructure InAs/InP NWs grown using metal catalysts and also to determine the timescale (approximately 10 s) in which beam irradiation induces material damage in these wires. In particular, we have detected small As concentrations (4.4 +/- 0.5)% in the final InP segment close to the Au catalyst, which leads to the conclusion that As diffuses through the metal nanoparticle during growth.

Surface effects on the mechanical elongation of AuCu nanowires: De-alloying and the formation of mixed suspended atomic chains

We report here an atomistic study of the mechanical deformation of AuxCu(1−x) atomic-size wires (nanowires (NWs)) by means of high resolution transmission electron microscopy experiments. Molecular dynamics simulations were also carried out in order to obtain deeper insights on the dynamical properties of stretched NWs. The mechanical properties are significantly dependent on the chemical composition that evolves in time at the junction; some structures exhibit a remarkable de-alloying behavior. Also, our results represent the first experimental realization of mixed linear atomic chains (LACs) among transition and noble metals; in particular, surface energies induce chemical gradients on NW surfaces that can be exploited to control the relative LAC compositions (different number of gold and copper atoms). The implications of these results for nanocatalysis and spin transport of one-atom-thick metal wires are addressed.