Our current research interests are directed towards the understanding of the electronic properties of novel semiconductor nanostructures, such as quantum dots, quantum rings and superlattices. We are interested in the electronic excitations of molecular agreggates, including DNA and J-aggregates. More recently, we were also involved in the electronic and transport properties of graphene and spintronics based on organic molecules.

 

Semiconductor nanostructures

 

heteroestructuraOver the last two decades, artificial structures have become the cutting edge of semiconductor physics. These structures can be grown by a variety of methods, such as molecular beam epitaxy or metal-organic vapour deposition. They consist of semiconductor layers with interface geometry, doping level and chemical composition defined with atomic-scale precision. The characteristic features of the heterostructures are interfaces between different materials which restric the motion of the electrons to two, one or zero dimensions. They can be separated into two classes, namely

  • heterojunctions and quantum wells
  • superlattices

We have been working in transport properties of semiconductor superlattices driven by electric fields. As growth defects always appear in superlattices, we have developed some models to include interface roughness scattering. Recently we have been involved with time dependent phenomena in dc fields (Bloch oscillations) and in ac-dc fields (Rabi oscillations).

 

Graphene nanostructures

 

grapheneGraphene is the name given to a flat monolayer of carbon atoms tightly packed into a two-dimensional (2D) honeycomb lattice, and is a basic building block for graphitic materials of all other dimensionalities. From the point of view of its electronic properties, graphene is a zero-gap semiconductor, in which low-energy quasiparticles can formally be described by a Dirac-like Hamiltonian. The Dirac equation is a direct consequence of graphene's crystal symmetry. Its honeycomb lattice is made up of two equivalent carbon sublattices A and B, and cosine-like energy bands associated with the sublattices intersect at zero E near the edges of the Brillouin zone, giving rise to conical sections of the energy spectrum for |E| < 1 eV.

graphene ringOur group has proposed a quantum interference device based on a graphene nanorings. The superposition of the electron wavefunction propagating from the source to the drain along the two arms of the nanoring gives rise to interesting interference effects. A side-gate voltage applied across the ring allows for control of the interference pattern at the drain. The electron current between the two leads can therefore be modulated by the side gate. The latter manifests itself as conductance oscillations as a function of the gate voltage. The proposed device operates as a quantum interference transistor with high on/off ratio.

 

Molecular systems

 

dnaOptical properties of molecular aggregates have been an active research area for over 50 years. The special interest in the optical response of these systems arises from collective effects that are caused by the interaction between the molecules within the aggregate. The study of such effects has recently attracted considerable interest in a much broader class of systems known as nanostructures, which include not only molecular aggregates but also polymers and semiconductor nanostructures.

Most of linear optical response of molecular aggregates can successfully be studied by means of the Frenkel exciton. The molecular aggregate is then modeled as a linear chain ot two-level absorber, whose electronic states are described by the Frenkel Hamiltonian. It has been shown that that the low-temperature absorption properties of molecular aggregates embedded in a glassy host can be explained quantitatively by takin into account Gaussian disorder of the transition frequencies of the absorbers within each aggregate (inhomogeneity).

Our group has been mainly concerned with optical properties of disordered mlecular aggregates when defects present some kind of spatial correlations. We have considered two rather different kind of spatial correlations, namely short-range correlations (random dimer lattices) and long-range correlations (aperiodic lattices).