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CBB Student works to gain theoretical understanding of experimental observations in photocathodes

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Gihan Dodanduwa Waduge, right, is a rising senior at the City College of New York. He contributed to the building of better photocathodes as part of his Center for Bright Beams REU program at Cornell this summer. Kevin Nangoi, left, and Jai-Kwan Bai, center, mentored Gihan through the program.

Photocathodes are used to produce a beam of electrons using the photoelectric effect, in which photons from electromagnetic waves knock electrons out of atoms of metals or semiconductors. For the same energy/intensity, some materials will emit more electrons than others, thereby having a higher quantum efficiency (QE). The electrons from some will also form a more focused beam, resulting in a lower mean transverse energy (MTE). For example, caesium antimonide (Cs3Sb), a semiconductor, has both one of the highest QEs and lowest MTEs, and is therefore desirable as photocathode material.

Every material has a photoemission threshold which is at the minimum energy required to remove an electron from an atom. For Cs3Sb, the observed photoemission threshold is lower than its expected one, which means that electrons can be removed with photons of lower energy than expected. While the QE of the electron beam produced this way is not very high, it does have a low MTE. "These are very interesting experimental results, but the theory behind is still not well understood," says Kevin Nangoi, one of Dodanduwa Waduge's mentors.

The expected threshold is based on ideal Cs3Sb, where there are no defects in the material. At threshold, electrons are excited from occupied electronic states to unoccupied electronic states with energies just high enough to get the electrons out of the material. Between the highest-energy occupied state and the lowest-energy unoccupied state, there is an energy gap where there are no electronic states, which is expected in ideal semiconducting or insulating materials. However, in an imperfect semiconductor, there can be occupied electronic states in this gap region, usually called the "mid-gap states". The lower experimental threshold in Cs3Sb could be because the photocathode material used is not perfect, and so the electrons may be excited from the mid-gap states instead of from the usual occupied states that have lower energies. As the MTE depends on where the electrons are excited from, mid-gap states can also help to understand why the observed MTE from Cs3Sb near threshold is low. "The understanding [of mid-gap states] may allow us to make better photocathodes in the future," says Jai Kwan Bae, who also mentored Dodanduwa Waduge

Dodanduwa Waduge's project was to help understand why the observed photoemission threshold of Cs3Sb is lower than expected and why it is associated with such a low MTE. He did this computationally, employing a method called the density functional theory (DFT), which is primarily used to evaluate the electronic structure of solid state materials. He calculated and compared the electronic structures of pure Cs3Sb and defective Cs3Sb - for example, with atoms missing or impurities present - so that researchers could gain insight into Cs3Sb's mid-gap states and, consequently, its low observed MTE near threshold.

This research allowed Dodanduwa Waduge to work on programming in a professional setting and the experience he got with scientific writing made it easier for him to understand research papers. He further accumulated knowledge on his research area. "It was quite surprising that even though my project was mostly about computational analysis, there was a lot of theoretical knowledge on solid state physics that I got to know at each step," he says.