How the microscopic laws of physics –quantum mechanics– manifest themselves at a macroscopic scale has been a topic of great interest and study during the last few decades. Nanoscale electric and magnetic systems are excellent candidates for these studies because they allow one to investigate the transition from the quantum to classical regimes by changing the size of the system under study. One example of this approach is decreasing the size of metallic magnetic systems to nanometers. When the system becomes small enough its quantum properties play an important role in both the magnetic and electric responses to external stimuli. In general, the size, shape, composition, orientation and other properties of the system (like anisotropy or intrinsic interactions), are the sources that govern the way in which a nanoscale system behaves classically or quantum mechanically. A clean understanding of the quantum behavior of nanoscale systems is becoming more and more relevant in view of multiple potential applications in emerging technologies, such as optoelectronics, spintronics or quantum information, and to promote advances in existing and well established technologies, such as combustion, catalysis, solar energy or medical diagnosis, to name a few. The control and manipulation of spin, charge and optical degrees of freedom of nanoscale systems offer exciting possibilities for our future society. In this context, nanometer sized systems combining optical, magnetic and electric properties strictly interrelated are of utmost interest.
Noble metal nanoparticles are also an excellent example of multifunctional nanoscale systems with great potential for biomedical applications, since they could be biocompatible. Perhaps, the clearest example of the combination of optical and electric properties at the nanoscale (i.e., nano-optoelectronics) is presented by noble metal nanoparticles. It is well known that colloidal suspensions of noble metal nanoparticles present a color that is representative of three main particles features: their size, shape and composition. The reason for that is that light is resonantly adsorbed and/or scattered by metallic particles to create plasmons (coherent oscillation of the conduction band electrons).
One of the goals of this project is to study the transport properties of individual noble-metal nanoparticles by means of single-electron transport experiments in the presence of optical irradiation matching the plasmon resonance frequency/ies of the particle. Optical excitation will permit the investigation of the coupling between the conduction electrons of a nanoparticle-based SET and the collective electronic excitations (i.e., surface plasmon resonances) induced by the external irradiation (light). This process has never been studied experimentally in individual metallic nanoscale systems. Single-electron transistors (SETs) based on graphene (instead of gold) will be employed to avoid masking effects due to metallic plasmon resonances in the transistor electrodes. We routinely fabricate graphene-based SETs in our lab. The figure above shows optical and AFM images of such transistors, as well as the electro-burning breaking process used to the achieve the nanogap and the resulting tunneling current as a function of bias.
We are also interested in exploring the interplay between photons and conduction electrons in single-molecule transistors, whether as a direct photon-molecule interaction or through the generation of plasmons in the metallic electrodes from incoming photons or as a result of inelastic charge transport through the junction.
Collaborators in this project:
Eloy Hernandez (UCF, Orlando, USA)
Christian Nijhuis (NUS, Singapore)
Guillem Aromi (UB, Barcelona, Spain)