Due to the unique feature of tunable size and surface chemistry, the optical and electronic properties of quantum dots (QDs) can be controlled, which helps to circumvent the challenges to turning these versatile materials into high-performance optoelectronic devices. However, a major bottleneck has prevented further development of devices with superior performance based on QDs, especially for small band QDs. It is the lack of a good understanding of surface chemistry on the electronic properties of QDs that limits their application.
In this thesis, we have precisely controlled the surface chemistry of lead selenide (PbSe) QDs by replacing initial oleate with a group of thiophenol-based organic molecules. Consequently, we are able to successfully engineer the carrier type within PbSe QDs by varying the substitutional group on these thiophenol-based molecules.
Field-effect transistors employing ligand-exchanged QDs are fabricated to study the charge transport properties and trap states within QD films. The results show that ligands with electron-donating substitutional groups give n-channel transistors while electron-withdrawing groups tend to yield p-channel transistors. X-ray photoelectron spectroscopy results confirm that the Fermi level tends to swing n-type for QDs with ligands containing electron-donating groups and p-type for withdrawing groups. This series of ligands shift valence band energy values spanning 2.0 eV for 6.5 nm QDs, which, by far, is the largest energy shift achieved through ligand exchange for PbSe QDs. Models of (111) plane PbSe slabs attached with different ligands are studied by First-principles calculations. The calculation shows a similar trend of band energy shift for semi-infinite 2D slabs passivated with different ligands. These ligands were also applied to 7.3 nm PbSe QDs to fabricate photodetectors used in the extended short-wavelength infrared region (2-3 μm). Preliminary results show that the adjustment of band energies and doping types have great impacts on the charge transport of these devices.
In summary, we have developed a surface modification technique for tailoring the band energy and the doping type of QD films. Although the resulting films are not very conductive, the adjustable energy level with a 2.0 eV window enables these ligands applied for directing charge flow in QD optoelectronic devices.
