Photochemical Sciences Ph.D. Dissertations

Title

Synthesis and Optical Properties of Colloidal PbS Nanosheets

Date of Award

2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Photochemical Sciences

First Advisor

Liangfeng Sun (Advisor)

Second Advisor

Hong Lu (Committee Member)

Third Advisor

Mikhail Zamkov (Committee Member)

Fourth Advisor

Michael Zickar (Other)

Abstract

Colloidal semiconductor nanocrystals are emerging materials for next-generation optoelectronic devices including solar cells and light-emitting diodes (LEDs). Colloidal quantum dots have been developed in the past decades to make solar cells and LEDs with decent efficiencies. However, their performances are limited by low conductivity. To address this problem, two-dimensional nanosheets (also known as nanoplatelets, quantum-belts or nanoribbons) have been developed recently. They have a much higher conductivity than quantum-dot films and tunable optical properties. The new method is developed for the synthesis of colloidal PbS nanosheets with a 100% success rate. We confirmed that residual acetate in the reaction mixture influence for the formation of colloidal nanosheets. Therefore, we have introduced an acetate-free method. The energy gap of the nanosheet is mainly determined by its thickness while the lateral size of the nanosheet has a significant effect on its conductivity, electronic coupling with other materials and the rate of light emission. Therefore, tuning the lateral size of the nanosheet is critical for its application in functional devices. Our work has addressed the challenge of tuning the lateral size of colloidal PbS nanosheets by changing reaction parameters, reducing the Pb to S molar ratio, using long-chain chloroalkane and changing the concentration of chloroalkane. Nanosheets with a small lateral size called nanoplatelets to show a narrow light-emission linewidth around 70 mev and an exciton peak. The platelets have a square shape with a typical width and length 20 nm. The exciton binding energy derived from the absorption spectrum is about 25 meV. Efficient electron transfer from photoexcited electrons from colloidal PbS nanosheets to TiO2 nanoparticles is demonstrated. This transfer occurred faster than compared to quantum dots. This will be more useful for fabricating future solar cell devices.

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