טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentFreund-Koren Shiri
SubjectOptical Properties of Semiconductor Nanocrystals and their
Hybrid Structures
DepartmentDepartment of Chemistry
Supervisor Professor Efrat Lifshitz
Full Thesis textFull thesis text - English Version


Abstract

The chemical and physical properties of nanocrystals (NCs) of metallic and semiconducting compounds differ distinctly from those of the corresponding bulk solids, due to their small dimensions and thus quantum confinement of the electronic states.

The extensive study of individual nanocrystals stimulated preliminary work in the development of ordered or disordered arrays of nanocrystals.  The arrays show new collective effects.  Thus, appropriate engineering of nanocrystals arrays offer the opportunity to design devices with the benefits of individual nanocrystals and the collective properties of the arrays.

The first part of the thesis focused on the spectroscopic properties of CdTe nanocrystals. A series of nearly monodispersed crystalline CdTe nanoparticles were isolated. The particles show strong band edge photoluminescence tunable in the visible spectral range and have been characterized by powder X-ray diffraction (XRD), high resolution transmittance electron microscopy (HR-TEM), absorption (Abs), photoluminescence (PL), fluorescence line narrowing (FLN) and photoluminescence excitation (PLE) spectroscopy.

A photoluminescence quantum efficiency of up to 85% is observed at room temperature. Moreover, the CdTe NCs fabricated by this method possess very high chemical and spectroscopic stability.

The CdTe NCs synthesized by a newly developed colloidal method are compared to CdTe NCs synthesized in an alternative surfactant mixture.

The PL spectra, PLE and FLN measurements showed a PL band that is composed of two components, and both show typical quantum size effect of the individual nanocrystals. The Stokes shift was calculated and shown to be size dependent-as the size of the nanocrystallite decreases, the Stokes shift becomes larger.

            The second part of the thesis focused on the spectroscopic properties of molecular-wire/nanocrystalline mixture.  The molecular-wires consisted of conjugated switchable chromophores.  The nanocrystalline component consisted of lead selenide (PbSe). 

Emission of both open and closed forms of the conjugated switchable chromophores was measured. Irradiating the intense absorption bands of the open form resulted in the closed form, emitting at longer wavelength; however, the conversion was not complete. Irradiating the intense absorption bands of the closed form resulted in complete conversion to the open form.

      Appropriate volumes of separate NCs and chromophore solutions in toluene were mixed to create films of conjugated molecules and NCs. The photoluminescence spectra of the mixture showed quenching of the emission of both the NCs and the open form of the chromophore. This can be attributed to either energy transfer or charge transfer. Further investigation of this phenomenon should be carried out in the future.