Synthesis, characterization, and exciton dynamics of II-VI semiconducting nanomaterials and ab-initio studies for applications in explosives sensing
- Author(s): Cooper, Jason Kyle
- Advisor(s): Zhang, Jin Z
- et al.
II-VI semiconducting nanostructures have been intensively researched as promising materials in applications including sensing, light emitting diodes (LEDs), lasers, photoelectrochemical (PEC) materials for water splitting and photo-catalysis, and dye or quantum dot sensitized solar cells. On the nanoscale, structural morphology plays a significant role in determining optical, electronic, and physical properties, and thus consequently affects the ultimate device properties.
In zero-dimensional quantum dots (QDs), control of the crystal size can allow for tunability of properties such as the absorption and emission spectrum. As a result, these nanostructures have proven attractive for QD-LEDs, biological labeling, and sensing. Because of the high surface to volume ratio of QDs, the surface quality and structure of these nanocrystals play a significant role in carrier trapping and non-radiative decay processes. QD surfaces can be effectively passivized by increasing the coordination number of surface atoms through coordinating capping ligands. Ligands improve photoluminescence (PL) quantum yield by healing dangling bond bandgap states, but they can also insert electron acceptor states below the conduction band (CB), which can trap photoelectrons and quench QD PL. This latter approach has been investigated for sensing applications such as in explosives detection. Characterization using density function theory DFT and time dependent DFT has been applied to study the acceptor levels in six explosives to help target different II-VI materials as potential fluorescent probes.
In one-dimensional quantum rods or wires, the exciton can be confined in two dimensions, leaving the third available for transport. These structures are promising in light harvesting applications such as PEC water splitting and solar cells. Nanowire (NW) photoanodes allow for efficient collection of photogenerated electrons by providing a direct route to the back contact while minimizing the distance the photohole must travel to reach the solution interface. At the same time, the added surface area increases the surface/electrolyte contact area, as well as improves sensitizer loading, the latter of which has been studied to improve performance in the visible portion of the spectrum. Common sensitizers for metal oxide (MO) anodes, which have large bandgaps and therefore do not absorb visible photons, include organic dyes, QDs, and in some studies the use of metal nanoparticles (NPs) has been proposed. Metal NPs, with their size and shape tunable surface plasmon resonance, can be controlled through the visible-NIR portion of the spectrum, and have been investigated as potential sensitizers on ZnO photoanodes using ultrafast transient absorption spectroscopy.
Enhanced performance of MO photoanodes can also be achieved through various annealing treatments meant to improve crystallinity, decrease intrinsic trapping defects, increase donor concentration by encouraging advantageous defects in n-type semiconductors, or passivate undesirable defects. The effect of air and hydrogen annealing of ZnO NWs on PEC performance was investigated with in-situ TA spectroscopy which showed that the combination of these methods improves PEC water splitting efficiency.
Finally, the application of II-VI materials in three-dimensional thin films has shown great potential in next generation photovoltaics which include the CdS/CdTe solar cell. The n-type CdS window layer, which forms the p-n junction with p-type CdTe, can be deposited by several different techniques but commonly include: DC pulse sputtering (DCPS) and chemical bath deposition (CBD). The dynamics of photoexcited charge carriers in CdS films prepared these methods was investigated with ultrafast transient absorption spectroscopy and the results was analyzed using singular value decomposition global fitting. The CBD sample had significant donor, acceptor, and donor acceptor pair recombination attributed to high oxygen content in the films while the DCPS had intrinsic donor level defects which are expected for n-type CdS. The results suggest that DCPS made thin films should offer improved performance in solar cell applications.