Anthony Kipkorir

Chemistry and Biochemistry

Faculty Advisor: Prashant Kamat

Designing AgInS2-CdS Heterostructure with improved charge separations

Curious look into the future of energy demands points to the need utilize sustainable renewable energy to replace fossil fuels that have polluted the environment. Among many semiconductors, ternary quantum dots (QDs) particularly AgInS2 have gained popularity in recent years because of their potential applications in photovoltaic electricity generation and photocatalysis. Besides, such ternary QDs do not contain elements like lead that are harmful to environment. AgInS2 and its derivatives have bandgaps matching well to the solar spectrum. This feature along with composition-based band gap tunability and high absorption coefficients across the visible spectrum has given ternary quantum dots a niche in ongoing research.  

The main drawback with this material is the existence of surface defects (trap states) which provide  non-radiative recombination sites. These trap states originate from the non-bonding orbitals of the under-coordinated surface atoms. Use of ligands alone to stabilize the surface of QDs is not enough to address this problem. Recent research proposes modification of the surface AgInS2 QDs by coupling it with a higher band gap material to form a shell that eliminate the trap states (e.g., ZnS and CdS). Preliminary results from these studies shows that this surface modification significantly improve the QDs photophysical properties.

The band position of AgInS2 core and CdS shell allows for the design of quasi-type II heterostructure. This band alignment will allow the photogenerated charge carries to be separated thus allowing easy harvesting and utilization of the same. On the other hand, having a thick shell can hamper the rate of electron transfer as the electrons will have to tunnel through a longer distance. Therefore, understanding the role of shell thickness as a function of PLQY and electron transfer (ET) is crucial to optimizing the benefits that the shell bring. In this work, I will investigate how photoluminescence quantum yield (PLQY) and electron transfer (ET) process are affected with increasing shell thickness. 

Project Objectives

Objective 1: The proposed study will begin with designing and synthesizing AgInS2-CdS heterostructure with varying CdS shell thickness. The presence of CdS shell and shell thickness will be probed using high resolution transmission electron microscopy (HR-TEM) (in the Notre Dame Integrated Imaging Facility (NDIIF)).

Objective 2: Absolute PLQY of the synthesized heterostructures will be determined using the integrated system available in Kamat laboratory. This information will help in tracking how the radiative recombination pathway is activated as the shell size is increased.

Objective 3: Excited state behavior of these material will be probed by employing ethyl viologen (EV2+) as a model electron acceptor. With the aid of photoluminescence spectroscopy, preliminary information on the interaction of the electron shuttler (EV2+) and AgInS2-CdS heterostructure will be obtained.

Objective 4: Consequently, insights on electron transfer processes will be obtained by using transient absorption spectroscopy technique (available in Kamat lab). The rates of electron transfer with varying shell thickness will be elucidated by monitoring how the EV2+ deactivates the excited states of the respective materials. These insights will be pivotal in designing efficient solar cell device or a photocatalyst.