Tailoring Palladium Nanoelectrodes for Single Molecule Transistors

L. ARZUBIAGA; F. GOLMAR; R. LLOPIS; F. CASANOVA; L. HUESO

Cuenca

Conferencia; 5th European School on Molecular Nanoscience (ESMolNa 2012); 2012

Considerable advances have been made in the field of single molecule transistors (SMT) in the last few years [1, 2]. The modulation of the current by electrostatic gating through the different orbitals of isolated molecules has already been achieved [3] and a wide variety of SMTs have been prepared using bare hydrocarbon compounds [4], metal-organic coordination complexes [5] and C60 among others [6]. Integrating SMTs in electronic circuits could be an important step in the constant miniaturization of the logic and memory components. However, the transfer of STM technology to the electronics industry is unlikely to happen until we achieve a thorough control and reliability in the fabrication process of SMTs. One of the main drawbacks of devices incorporating a single molecule is the lack of methods for their systematic fabrication. First of all, obtaining the adequate electrode geometry and interelectrode distances for eventually forming a SMT is very challenging. In addition to this, owing to slight differences in the nanometer-scale structure of the created electrode-tips, the environment provided to the attached molecules can be unique in each device. This can alter considerably the properties affecting the operability of the devices, such as gate coupling [2]. In this work we focus in electromigration, which is a widely used technique for fabricating electrodes of molecular dimensions [7]. It consists in opening a breach of a few nanometers in a metallic nanowire by electrical fatigue of the material. The molecule under study is placed in the created nanometric gap, with the two adjoining nanowire-segments acting as source and drain electrodes of the SMT. We present a work in which we study electromigration in palladium nanowires. We are interested in palladium because in addition to its noble-metal nature, it has shown the possibility to prepare ferromagnetic alloys by adding very low percentage of certain metals (about 0.1% Co, Fe, etc.) [8]. This allows establishing a magnetic coupling between the electrodes and the molecule. Moreover, the existence of certain charge-transport features that seem to be intrinsic to bare palladium nanojuntions has been reported [9]. After considering the characteristics of this metal, we are interested in the features that palladium nanoelectrodes might add to the charge transport in SMTs. The aim of this work is first, to obtain a better understanding of the behaviour of palladium during electromigration and second, to gain control on the results of the process, which would eventually lead us to the to the systematic fabrication of nanoelectrodes for SMTs. We tested two different electromigration strategies on palladium nanowires fabricated by electron beam lithography. At the same time, we studied the influence of the initial resistance of the devices in the outcome of the electromigration process. This work shows how we control the conditions at which the onset of electromigration occurs in our devices. In addition to this, we have found the appropriate working conditions for choosing between either palladium nanoconstrictions or nanogap electrodes as the outcome of the process.