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@PHDTHESIS{Grellmann:185638,
      author       = {Grellmann, Thomas},
      title        = {{C}ryogenic {B}reak-{J}unction {C}haracterization of
                      {S}ingle {O}rganic {M}olecules},
      volume       = {98},
      school       = {Universität Köln},
      type         = {Dr.},
      address      = {Jülich},
      publisher    = {Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag},
      reportid     = {FZJ-2014-07064},
      isbn         = {978-3-95806-015-9},
      series       = {Schriften des Forschungszentrums Jülich. Reihe
                      Schlüsseltechnologien / Key Technologies},
      pages        = {VI, 86 S.},
      year         = {2014},
      note         = {Universität Köln, Diss., 2014},
      abstract     = {In this thesis, a cryogenic mechanically controllable
                      break-junction (MCBJ) setup is developed and used for the
                      characterization of single organic molecules at different
                      temperatures. Molecules are the building blocks of life and
                      among others responsible for charge transfer in biological
                      processes,e.g. photosynthesis in plants and metabolism in
                      humans. Since these processes are not yet completely
                      understood, single molecules are highly interesting systems
                      to study. Furthermore, due to their astonishing properties
                      they might be used as electronic components in future
                      technologies. This approach is called “molecular
                      electronics”, existing representatives that use thin or
                      thick layers of organic molecules are for instance light
                      emitting diodes (OLED) or liquid crystal displays (LCD).
                      However, single molecule devices are still not in use, and
                      in order to build such systems the single molecules have to
                      be understood, especially their electronic properties. Since
                      the size of single molecules is typically of the order of
                      nm, their characterization requires an appropriate setup. In
                      this thesis a MCBJ is developed that traps single molecules
                      between two electrodes, and thus allows to characterize its
                      electronic features. The first aim of this thesis is the
                      construction and development of a cryogenic MCBJ setup,
                      consisting mainly of three parts: (i) the sophisticated
                      sample holder, suitable for measurements inside the
                      helium-flow cryostat, including a piezo-positioner to bend
                      the sample, (ii) an automated setup to control the
                      measurement parameters and acquire the data (e.g.
                      temperature, applied voltage, measured current,
                      piezo-position), and (iii) the development of suitable
                      break-junction samples consisting mainly of a
                      lithographically prepared Au bridge (with nm size
                      dimensions). The three parts together allow measurements of
                      molecules with a position control in the pm regime, currents
                      ranging from below pA to mA, and temperatures ranging from 4
                      K to room temperature. The second aim is to develop and to
                      establish adequate procedures for break-junction
                      measurements, regarding mounting of the sample, deposition
                      of molecules and measurement techniques. For the latter one,
                      conductance–position characteristics (CPCs),
                      current–voltages characteristics(IVC), standard CPC
                      histograms and contour histograms are developed, automated
                      and tested in reference measurements without molecules. The
                      third aim is the measurement of simple “test-bed”
                      molecules. Hexanedithiol and benzenedithiol are chosen as
                      representatives for simple alkanes and conjugated molecules,
                      respectively. CPCs of the rod-like hexanedithiol show a
                      distinct peak in agreement with literature values, while
                      BeDT exhibits a more complex behavior. IVCs are performed on
                      the molecules and molecular levels are obtained. The curves
                      demonstrate the quality of the measurement techniques and
                      the analysis methods. Finally, the fourth aim is the
                      measurement of the temperature dependent electronic
                      properties of the more complex molecules terphenyldithiol
                      (TPT) and porphyrine (TPyP). The latter one plays an
                      important role in biological processes and represents a
                      promising candidate for molecular electronics. We find, that
                      TPT exhibits a distinct and strongly temperature dependent
                      conductance peak, while the level of the molecular orbital
                      is independent of temperature. It is shown that for TPT a
                      transition from direct tunneling to “hopping” mechanism
                      takes place around a temperature of 100K. The TPyP displays
                      unusual CPCs. Only tilted plateaus are observed, that are
                      indicative for clustering of the molecule. Contour
                      histograms demonstrate the presence of these clusters in the
                      break-junction. IVCs of TPyP reveal a temperature dependent
                      electronic or vibrational mode. It shifts towards higher
                      voltages with increasing temperature and is therefore only
                      detectable belowT $\lesssim$ 180K. Above this temperature
                      reordering of the molecules and the molecule-Au bonds leadto
                      large noise in the IVC at large voltages. The MCBJ
                      represents an elegant way to detect interesting electronic
                      properties of single molecules. Here, a major aspect is the
                      great stability of the metal-molecule-metal system that
                      allows extensive characterization even in the case of
                      complex molecules. In this thesis it is shown, that lower
                      temperatures improve the performance of the MCBJ, and, even
                      more crucial, temperature dependent measurements allow a
                      deeper insight in the charge transfer of single molecules.},
      keywords     = {Dissertation (GND)},
      cin          = {PGI-8 / ICS-8 / JARA-FIT},
      cid          = {I:(DE-Juel1)PGI-8-20110106 / I:(DE-Juel1)ICS-8-20110106 /
                      $I:(DE-82)080009_20140620$},
      pnm          = {423 - Sensorics and bioinspired systems (POF2-423)},
      pid          = {G:(DE-HGF)POF2-423},
      typ          = {PUB:(DE-HGF)11},
      url          = {https://juser.fz-juelich.de/record/185638},
}