title: Energy Transfer in Functionalized II-VI Semiconductor Nanowires and Quantum Dots
title: Energietransfer in funktionalisierten II-VI-Halbleiternanodrähten und kolloidalen Quantenpunkten
Creators: name: Kaiser, Uwe
Publisher: name: Philipps-Universität Marburg
Year: 2014
Subjects: Optical Spectroscopy
Physik
Optische Spektroskopie
Halbleiterphysik

description: Das zeitliche Besetzungsverhalten
      angeregter Zustände ist durch den Energietransfer zu
      benachbarten Energieniveaus bestimmt. In dieser Arbeit wurden
      Halbleiternanodrähte und -quantenpunkte mit Hilfe der
      zeitaufgelösten Photolumineszenzspektroskopie hinsichtlich
      ihres Energietransfers untersucht. Die Abklingkonstanten der
      unterschiedlichen Lumineszenzzentren können im Bereich von
      wenigen Nanosekunden bis hin zu einigen Millisekunden
      bestimmt werden. Der Energietransfer zwischen den hier
      untersuchten Zentren ist, aufgrund der typischen Abstände im
      Bereich von einigen Nanometern, durch die gegenseitige
      Dipol-Dipol-Wechselwirkung bestimmt. Die Beschreibung dieser
      Wechselwirkung und dem damit verbundenen Transfer erfolgt im
      Rahmen des resonanten Energietransfermodells nach Theodor
      Förster. Aufgrund der verringerten Dimensionalität der
      untersuchten Halbleiterstrukturen muss das ursprüngliche
      Transfermodell so modifiziert werden, dass die besondere
      Morphologie der Probenstrukturen berücksichtigt wird. Der
      strahlungslose Energietransfer von einem angeregten Akzeptor
      zu einem Donator führt zu einer verringerten Lebensdauer des
      angeregten Akzeptorzustands. Aufgrund der statistischen
      Verteilung von Akzeptoren und Donatoren in einem Ensemble,
      zeigt das Abklingen der Donatorlumineszenz ein komplexes
      Verhalten, welches durch äußere Einflüsse stark modifiziert
      werden kann. Die korrekte Beschreibung des Energietransfers
      in niedrigdimensionalen Halbleiternanostrukturen führt zu
      einem besseren Verständnis der Dynamik von optisch aktiven
      Zuständen. Zusätzlich können die gewonnenen Erkenntnisse
      dafür genutzt werden bestehende Anwendungen deutlich zu
      verbessern und neue Konzepte zu etablieren.
description: The importance of semiconductor
      nanostructures for technological applications has rapidly
      grown in the last decades. Nowadays, nanostructures with
      highly efficient photoluminescence properties can be
      synthesized. The understanding and control of the
      photoluminescence dynamics in such structures is of major
      importance for potential optical applications. In most
      materials the dynamic of excitation energy is determined by
      an energy transfer from the excited donor to an adjacent
      acceptor. A description of this energy transfer in low
      dimensional nanostructures is given by a modified version of
      the original Förster model. In this work semiconductor
      nanowires as well as quantum dots were investigated by time
      resolved spectroscopy to yield information about the
      population of different energetic states. ZnS nanowires with
      a diameter of 100nm and a length of several micrometers were
      implanted with manganese and terbium, respectively. Both ions
      show a slowly decaying luminescence, which is highly
      non-exponential due to the energy transfer to nonradiative
      centers within the nanowires. A systematical variation of
      doping concentration, ambient temperature and excitation
      power yields a complete understanding of the energy transfer
      in the framework of the modified Förster model. The
      realization of a microscopic time resolved setup enables a
      comparison of the decay characteristics of single nanowires
      with an ensemble of wires. Remarkably, the luminescence decay
      of a single wire is the same as the one of many nanowires due
      to the uniformity of the wires and the high number of
      donor-acceptor pairs within a single wire. Therefore, the
      ensemble measurements can be compared to an especially
      developed Monte Carlo simulation of the microscopic energy
      transfer in a single wire. The results of the simulation are
      in good agreement with the fitting of the modified Förster
      model. In addition to the time resolved measurements
      different optical techniques were used to investigate the
      unique features of the ZnS nanowires. Resonant Raman
      scattering experiments of single nanowires show a high
      angular dependence regarding the orientation of the wire and
      the polarization of the incident and the scattered light.
      This polarization dependence underlines the unique morphology
      of the nanowires. Additionally, photoluminescence excitation
      spectroscopy discloses the exact transitions within the
      variety of terbium states and yields information on the
      energy transfer from the ZnS host material. Colloidal
      semiconductor quantum dots with attached dye molecules were
      investigated by time resolved measurements to gain a better
      understanding of the transfer characteristics in such
      conjugates. Quantum dots show a slowly decaying luminescence
      with a lifetime in the range of 100ns. By manganese doping of
      these quantum dots their lifetime can be enhanced by several
      orders of magnitude, for the manganese luminescence itself
      exhibits a decay time in the millisecond range. For the use
      of colloidal quantum dots in biological sensor applications a
      functionalization with ionsensitive dyes is required. A
      tuning of the spectral overlap between the quantum dot
      emission and the dye absorption leads to an efficient energy
      transfer. This transfer results in a tremendous enhancement
      of the luminescence lifetime of the dye, yielding dye
      molecules with similar luminescence spectra but significantly
      different decay times. Time resolved measurements not only
      enable to distinguish between these different types of dyes,
      but furthermore, allow the determination of the exact ratio
      of several dyes in a given solution, which is the basis of
      temporal multiplexing. Most biological sensing applications
      require the correct knowledge of the quantum dot dye
      conjugate, especially the exact number of dye molecules per
      quantum dot is of great importance for quantitative
      determinations. As the binding of molecules to the polymer
      shell of the quantum dot underlies a certain statistic, this
      statistical distribution has to be taken into account for a
      correct characterization of the energy transfer. The
      consideration of a Poisson distributed number of dye
      molecules per quantum dot leads to a good description of the
      experimentally determined decay of the quantum dot
      luminescence. Additionally, the fitting within the energy
      transfer model results in unique values for the average
      number of dye molecules per quantum dot as well as for the
      transfer rate between the quantum dot and a single dye
      molecule. In this work the importance of the energy transfer
      for semiconductor nanostructures is revealed. A better
      understanding of the transfer characteristics is of great
      significance for an advancement of optical devices.
      Additionally, the energy transfer can be used to enhance
      current sensor concepts, which extends the application area
      of this material class.