Electrical, Optical and Structural Properties of CdSe Nanocrystals coupled to Organic π-systems

Abstract

This work focuses on the synthesis and optoelectronic properties of coupled organic-inorganic nanocrystal thin films. The hybrid materials studied herein are composed of CdSe nanocrystals (NCs) coupled with an organic semiconductor (OSC) material. The CdSe NCs were synthesized using a hot injection method, which requires encapsulation of the NCs with insulating organic ligands. These were removed with inorganic ligand (I-) to facilitate the electronic coupling and further incorporate OSCs. The work presented is divided into three parts. The first part talks about the ligand exchange of CdSe NCs with zinc β-tetraaminophthalocyanine linker molecule. The addition of dye molecules increases the optical on-off modulation of the device by 4.5 orders of magnitude in the near-infrared region where CdSe itself has very little absorption. The dye linker also introduces a fast rise time of 74 ± 11 ns upon optical gating with a near-infrared optical pulse. The second topic focuses on the effect of coupling aryleneethylene linker molecules with CdSe NCs. The coupling of linker molecules induces the aggregation-induced emission band of lower energy than the band-edge emission of CdSe NCs. Both the fluorescence band of the coupled system showed randomly induced anticorrelated fluorescence intensity variations. These variations become periodic upon cooling down the sample to ~160 K and the time period of the oscillations showed a strong dependence on laser power intensity. In addition, the coupled system also showed 3 orders of mobilities variation by cooling down the sample to cryogenic temperatures. The third part of the work focuses on the self-assembly of wurtzite and zinc-blende CdSe nanocrystalline systems. NMR spectroscopy was used to quantify the bound and free ligand densities. Small-angle and grazing incidence small-angle X-ray scattering techniques were used to reveal a ~10% increase in the lattice parameters of the self-assembled structure of CdSe NCs by varying the ligand density of nanocrystals.