We present exact analytical and numerical results for the electronic spectra and the Friedel oscillations around a substitutional impurity atom in a graphene lattice. A chemical dopant in graphene introduces changes in the on-site potential as well as in the hopping amplitude. We employ a T -matrix formalism and find that disorder in the hopping introduces additional interference terms around the impurity that can be understood in terms of bound, semi-bound, and unbound processes for the Dirac electrons. These interference effects can be detected by scanning tunneling microscopy.

We present a theoretical model and Monte Carlo simulation results that naturally explain all the features of the thermally activated photoluminescence upconversion effect ͓also known as anti-Stokes photoluminescence ͑ASPL͔͒ observed in ensembles of colloidal semiconductor nanocrystal quantum dots ͑QDs͒. The proposed ASPL mechanism includes the following principal ingredients: ͑i͒ optical-phonon-assisted absorption of an incident photon in a relatively large dot in the ensemble, ͑ii͒ emission of a higher-energy photon from the zero-phonon exciton-polaron state, with an upconversion equal to one optical-phonon energy, and ͑iii͒ cascade reabsorption and re-emission processes involving QDs of successively smaller sizes within the sample, rendering the experimentally observed large anti-Stokes shift of the energy of the photon that finally leaves the sample. The results obtained by the Monte Carlo modeling based on the proposed mechanism reproduce all the experimentally observed ASPL trends in colloidal QD solutions.

We present the theory of the thermally activated photoluminescence upconversion, also known as anti‐Stokes photoluminescence (ASPL), based on the quantum dot (QD) polaron effect and explaining all the principal features of the ASPL in colloidal solutions of chemically grown nanocrystals (NCs). The results obtained by Monte‐Carlo simulations reproduce all the experimentally observed ASPL trends in colloidal NC solutions and are being extended to dense QD systems, such as films composed by layers of closely packed NCs (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)