טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentGrinbom Gal
SubjectScanning Tunneling Spectroscopy of PbSe/PbS Core-Shell
Nanocrystals Quantum Dots
DepartmentDepartment of Chemistry
Supervisor Professor Efrat Lifshitz
Full Thesis textFull thesis text - English Version


Abstract

The IV-VI (PbSe, PbS, PbTe) semiconductor nanocrystal quantum dots (NQDs) have attracted much attention because of their applications in near-infrared (NIR) optoelectronics, nonlinear optics, and biology. Good quality PbSe NQDs with excellent homogeneity were grown by colloidal synthesis. In recent years, Lifshitz et

al. [1,2] developed the synthesis of PbSe/PbS core-shell NQDs, exhibiting chemical robustness over months and years and emission quantum efficiency up to 80%.

The research described in this paper focused on the characterization of the PbSe/PbS core-shell structures using X-ray photoelectron spectroscopy (XPS), as well as energy dispersive X-ray spectroscopy (EDX), to confirm the stoichiometric composition of the samples. In addition, transmission electron microscopy (TEM) measurements yielded the core and the shell diameters. Interestingly, the first publication discussing the electronic configuration of the PbSe/PbS core/shell NQDs suggests that they can form type-I or type-II configurations, depending on the energy offsets between the core and the shell materials on the ratio between the core and the shell diameters. In type-I configuration, the band-edges of the shell wrap the core, leading to a confinement of the carriers' wave functions into the core regime. Regarding type-II, band-edges of the constituents have a staggered configuration, and thus, electrons and holes are spatially separated, occupying different parts of the hetero-structure. Recent efforts in using the IV-VI NQDs in solar cells [3] and NIR light sources [1] pointed to an existing obstacle, associated with an Auger recombination that quenches an efficient multi-carriers accumulation or inversion of population. It should be noted that the IV-VI NQDs could offer an opportunity for a reduction of carrier repulsion and the Auger process and thus, be beneficial for the applications mentioned. The current work presents our attempts to characterize the electronic configuration of the PbSe/PbS core-shell structures, using scanning tunneling spectroscopy (STS). For that purpose, various NQDs with different diameter core/shell ratios were attached to a gold (111) substrate via hexanedithiol linking molecules, while the measurements were carried out in ultra high vacuum at 20K. Current voltage curves were measured and enabled determination of the density of states of the samples discussed.