Varlei Rodrigues

Evidence for spontaneous spin-polarized transport in magnetic nanowires.

The exploitation of the spin in charge-based systems is opening revolutionary opportunities for device architecture. Surprisingly, room temperature electrical transport through magnetic nanowires is still an unresolved issue. Here, we show that ferromagnetic (Co) suspended atom chains spontaneously display an electron transport of half a conductance quantum, as expected for a fully polarized conduction channel. Similar behavior has been observed for Pd (a quasimagnetic 4d metal) and Pt (a nonmagnetic 5d metal). These results suggest that the nanowire low dimensionality reinforces or induces magnetic behavior, lifting off spin degeneracy even at room temperature and zero external magnetic field.

Origin of Anomalously Long Interatomic Distances in Suspended Gold Chains

The discovery of long bonds in gold atom chains has represented a challenge for physical interpretation. In fact, interatomic distances frequently attain 3.0-3.6 A values, and distances as large as 5.0 A may be occasionally observed. Here we studied gold chains by transmission electron microscopy and performed theoretical calculations using cluster ab initio density functional formalism. We show that the insertion of two carbon atoms is required to account for the longest bonds, while distances above 3 A may be due to a mixture of clean and one C atom contaminated bonds.

Observation of the smallest metal nanotube with a square cross-section

Understanding the mechanical properties of nanoscale systems requires a range of measurement techniques and theoretical approaches to gather the relevant physical and chemical information. The arrangements of atoms in nanostructures and macroscopic matter can be different, principally due to the role of surface energy, but the interplay between atomic and electronic structure in association with applied mechanical stress can also lead to surprising differences. For example, metastable structures such as suspended chains of atoms and helical wires have been produced by stretching metal junctions. Here, we report the spontaneous formation of the smallest possible metal nanotube with a square cross-section during the elongation of silver nanocontacts. Ab initio calculations and molecular simulations indicate that the hollow wire forms because this configuration allows the surface energy to be minimized, and also generates a soft structure capable of absorbing a huge tensile deformation.

Metal nanowires: atomic arrangement and electrical transport properties

We have studied the atomic arrangement and defect formation in metal nanowires (NWs) generated by mechanical elongation using in situ high resolution transmission electron microscopy. It has been observed that the narrowest constriction of gold and platinum NWs is crystalline and defect-free; in particular, gold NWs adopt only three kinds of atomic arrangement. A model correlating these gold structures and the quantum conductance behaviour is proposed, which showed a remarkable agreement with ultrahigh-vacuum mechanically controllable break junction electrical transport measurements.

Size limit of defect formation in pyramidal Pt nanocontacts.

We report high resolution transmission electron microscopy and ab initio calculation results for defect formation in sharp pyramidal Pt nanocontacts. Our results show that there is a size limit to the existence of twins (extended structural defects). These defects are always present but blocked away from the tip axes. They may act as scattering planes, influencing the electron conductance for Pt nanocontacts at room temperature and Ag/Au nanocontacts at low temperature (<150 K).

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.

Structural Study of Metal Nanowires

The structural and mechanical properties of nanometric metal wires (nanowires—NWs) represent a fundamental issue for the understanding of different phenomena such as friction, fracture, adhesion, etc. [1]. Nanometric systems are formed by a reduced number of atoms and the majority of them are located at the surface; this may lead to new and different attributes (structural, optical, etc.) when compared to their macroscopic counter part.

New experimental setup for metallic clusters production based on hollow cylindrical magnetron sputtering

Nanoscale structures have been widely studied because their properties differ greatly from their bulk counterpart systems, raising both a fundamental and technological interest. Despite the great advances that have been made, the domain still presents great challenges. The development of dedicated instrumentation, in particular, is an essential issue since the well established techniques used for atomic size and for macroscopic systems are often not suited for the study of nanoaggregates. In this article, the authors present the development of a new cluster source aimed to produce pure or alloy metallic clusters ranging from two up to thousands atoms in a controllable way. The setup is based on the design of Haberland et al. [J. Vac. Sci. Technol. 12, 2925 (1994)] with the implementation of an hollow cylindrical sputtering as atoms source that enhances the control over the production of alloy clusters and also improves target usage.

Development of a quartz tuning-fork-based force sensor for measurements in the tens of nanoNewton force range during nanomanipulation experiments

Understanding the mechanical properties of nanoscale systems requires new experimental and theoretical tools. In particular, force sensors compatible with nanomechanical testing experiments and with sensitivity in the nN range are required. Here, we report the development and testing of a tuning-fork-based force sensor for in situ nanomanipulation experiments inside a scanning electron microscope. The sensor uses a very simple design for the electronics and it allows the direct and quantitative force measurement in the 1-100 nN force range. The sensor response is initially calibrated against a nN range force standard, as, for example, a calibrated Atomic Force Microscopy cantilever; subsequently, applied force values can be directly derived using only the electric signals generated by the tuning fork. Using a homemade nanomanipulator, the quantitative force sensor has been used to analyze the mechanical deformation of multi-walled carbon nanotube bundles, where we analyzed forces in the 5-40 nN range, measured with an error bar of a few nN.

Low-cost nanomanipulator for in situ experiments in a SEM.

Here, we describe the development of an inexpensive and versatile manipulation system for in situ experiments in a field emission scanning electron microscope based on a parallel-guiding plate-spring mechanism and low cost materials. The system has been tested for a wide range of applications, such as collecting, moving, and positioning particles, fabricating atomic force microscopy tips based on carbon nanotubes, and characterizing individual nanobjects. The nanomanipulation results demonstrate that there are many opportunities for the use of physical manipulation in the bottom-up approach to fabrication of nanodevices.