We investigate the fundamental electronic and optical properties of novel semiconductor nanostructures. Our taks is to ellucidate the main physical mechanism ruling their behavior to eliminate the less important ones. Then we develope computational methods to simulate, understand and predict their behavior. Our main areas of research at present are:

OPTICS OF QUANTUM DOTS

Quantum dots are a sort of artificial atoms with tunable size, shape and composition. One way to trap carriers in these 'atoms' is photexcitation. Through laser excitation different aggregates of electrons and holes become confined inside the dot. After a short time electron and holes recombine emitting photons in the near infrarred and visible range of the spectrum. These makes optics of quantum dots a technologically relevant field. We simulate optical spectra of different kinds of quantum dots and quantum dot molecules, paying spetial attention to the role of Coulomb correlation among carriers, the influence of the spin-orbit interaction of holes and the effect of external fields.

A few selected publications

1. M.F. Doty, J.I. Climente, M. Korkusinski, M. Scheibner, A.S. Bracker, P. Hawrylak, and D. Gammon,Phys. Rev. Lett. 102, 047401 (2009). “Antibonding ground states in quantum dot molecules”


2. F. Rajadell, J.I. Climente, J. Planelles and A. Bertoni, J. Phys. Chem. C 113, 11268 (2009).
   “Excitons, biexcitons and trions in CdSe nanorods”.
   


3. J.I. Climente, Appl. Phys. Lett. 93, 223109 (2008).
   “Tuning the tunnel coupling of quantum dot molecules with longitudinal magnetic fields”.
   


4. J.I. Climente, M. Korkusinski, P. Hawrylak, and J. Planelles, Phys. Rev. B 71, 125321 (2005).
   “Voltage control of the magnetic properties of charged semiconductor quantum dots containing magnetic ions”.
   


5. J.I. Climente, J. Planelles and W. Jaskoslki, Phys. Rev. B 68, 075307 (2003).
   “Magneto-optical transitions in nanoscopic rings”.

NANOCRYSTALS IN DIELECTRICALLY INHOMOGENOUS ENVIRONMENTS

Chemically synthesized quantum dots are usually embedded in non-semiconductor environments (water, polymeric solvants, air). The different dielectric constant of the quantum dot and the surroundings strongly modifies the electrical properties of carriers confined in the semiconductor. We have developed a coupled Poisson equation – Schrödinger Hamiltonian solver which allows us to study the behavior of interacting electrons and excitons in realistic conditions. We can also obtain information about hybrid metal-semiconductor nanocrystals. These are a new class of nanosystems where one expects to take advantadge of the combination of the properties of different materials, and even produce new ones which are not present for the separate systems.

A few selected publications

1. J. I. Climente, M. Royo, J .L. Movilla and J. Planelles, Phys. Rev. B 79, R161301 (2009).
   “Coulomb correlation amplification by dielectric mismatch in semiconductor nanorods”. ‏


2. J. Planelles, F. Rajadell and M. Royo, J. Appl. Phys. 104, 014313 (2008).
   “Dielectric control of spin in semiconductor spherical quantum dots”.
   


3. J. L. Movilla, G. Garcia-Belmonte, J. Bisquert and J. Planelles, Phys. Rev. B 72, 153313 (2005).
   “Calculation of electronic density of states induced by impurities in TiO2 quantum dots”.
   


4. J. L. Movilla and J. Planelles, Phys. Rev. B 71, 075319 (2005).
   “Off-centering of hydrogenic impurities in quantum dots”.

CHARGE AND SPIN DYNAMICS IN QUANTUM DOTS

Quantum dots have a discrete energy levels, just like atoms. Yet, exploiting the excited states of quantum dots is a difficult because, unlike atoms, quantum dots are not isolated. Rather, they are coupled to a solid state environment which provides sources of fast relaxation to carriers confined in excited states.
For electrons in the excited orbitals of quantum dots, coupling to phonons limits the lifetime to some nanoseconds. We investigate how to control electron-phonon coupling in order to enhance such lifetimes. A promising alternative is to use excited states whose spin is different from that of the ground state. In this case lifetimes can be as long as miliseconds, because the transition is forbidden by spin selection rules, which makes this system appealing for quantum information storage and computation devices. The limiting factors in this case are spin-orbit coupling and hyperfine interaction between the quantum dot carriers and the atoms in the semiconductor lattice. We study how these factors influence the system and how to minimize their impact.

A few selected publications

1. J. I. Climente, A. Bertoni, G. Goldoni, M. Rontani and E. Molinari, Phys. Rev. B 75, R081303 (2007).
   “Magnetic field dependence of triplet-singlet relaxation in quantum dots with spin-orbit coupling”.


2. A. Bertoni, J .I. Climente, M. Rontani, G. Goldoni and U. Hohenester, Phys. Rev. B 76, 233303 (2007).
   “Signatures of molecular correlations in few-electron dynamics of coupled quantum dots”


3. J. I. Climente, A. Bertoni, G. Goldoni and E. Molinari, Phys. Rev. B 74, 035313 (2006).
   “Phonon-induced electron relaxation in weakly confined single and coupled quantum dots”.

 

COLLABORATORS

Iwan Moreels (Istituto Italiano di Tecnologia, Italy)‏
Manuel Barranco (Universitat de Barcelona, Spain)‏
Marti Pi (Universitat de Barcelona, Spain)‏
Andrea Bertoni (INFM S3, Italy)‏
Guido Goldoni (University of Modena, Italy)‏
Pawel Hawrylak (IMS National Research Council, Canada)‏
Wlodzimierz Jaskólski (University Nicholas Copernicus, Torun, Poland)‏
Matt Doty (University of Delaware, USA)‏
Dan Gammon (Naval Research Laboratory, USA)‏
Shun-Jen Cheng (National Chiao Tung University, Taiwan)‏
Garnett Bryant (NIST, Washington, USA)
Juan Martínez (Universitat de València, Spain)‏

COLLABORATORS IN PREVIOUS FIELDS

Josef Paldus (University of Waterloo, Canada)
Jacek Karwowski (University Nicholas Copernicus, Torun, Poland)
Carmela Valdemoro (CSIC, Madrid, Spain)
Avel·lí Corma
Claudio Zicovich-Wilson