Defect Candidates for Single-Photon Emission in Monolayer GaS and GaSe

Imagine a world built not of bricks and mortar, but of single sheets of atoms, so thin they are essentially two-dimensional. In this realm, scientists are searching for tiny imperfections that could act as perfect sources of light. These imperfections, known as defects, are not flaws to be fixed but rather features to be engineered. Just as a single missing brick in a wall might change how sound travels through a room, a missing atom in a crystal sheet can trap electrons and force them to release energy as a single particle of light, called a photon. This ability to emit light one particle at a time is the holy grail for future technologies like ultra-secure communication and quantum computing. The challenge lies in finding the right material and the right defect, because most imperfections either do nothing or release light in a chaotic burst rather than a controlled, single stream.

Two materials have recently caught the attention of researchers for this purpose: monolayer gallium sulfide and monolayer gallium selenide. These are sheets of atoms made from gallium combined with either sulfur or selenium. While they look simple, their internal electronic landscape is complex and sensitive. To understand how these materials behave when an atom is missing or swapped for another, scientists must look at the energy levels where electrons live. If a defect creates a specific energy gap within the material, it might be able to act as a single-photon emitter. However, distinguishing a genuine defect that creates a new energy state from a state that merely shifts slightly due to the presence of a defect is difficult. It requires a very precise way of calculating how electrons interact, especially in these flat, two-dimensional sheets where the usual rules of physics behave differently.

In a recent study, researchers set out to map these hidden energy landscapes in gallium sulfide and gallium selenide. They focused on two types of changes: vacancies, where an atom is simply missing from the sheet, and substitutions, where a foreign atom like silicon or zinc takes the place of a gallium atom. To get a clear picture, they used a sophisticated computer simulation method designed specifically for two-dimensional materials. This method corrects for common errors in how electrons are modeled, ensuring that the energy levels they calculate are as accurate as possible. They built digital models of these atomic sheets, creating large grids of atoms to simulate the environment around a defect, and then relaxed the structures to see how the surrounding atoms would shift to accommodate the missing or swapped piece.

The team examined eight different scenarios, testing four types of defects in each of the two materials. They looked at what happens when a gallium atom is missing, when a sulfur or selenium atom is missing, and when silicon or zinc atoms replace gallium. By analyzing the spin of the electrons—a property that can be thought of as a tiny magnetic orientation—they could see how these defects altered the energy gaps inside the material. The results showed that not all defects are created equal. Some created energy states that were too high in energy to be useful for visible light, while others created states that were perfectly positioned to emit light in the visible spectrum.

The most promising findings came from the gallium vacancies. In the gallium sulfide sheet, when a gallium atom was removed, the surrounding sulfur atoms moved slightly closer to the empty space, creating a specific energy gap of 1.472 electronvolts. This energy difference corresponds to light that could be emitted as a single photon. A similar effect was found in the gallium selenide sheet, where a missing gallium atom created a gap of approximately 1.496 electronvolts. These values are significant because they fall within a range that is highly desirable for optical applications. In contrast, the defects involving missing sulfur or selenium atoms created much larger energy gaps, around 3.465 and 3.385 electronvolts respectively. These higher energies place the potential light emission deep in the ultraviolet range, which is less useful for the specific applications the researchers are targeting.

The study also looked at what happens when silicon or zinc atoms replace gallium. In the gallium sulfide sheet, the zinc substitution created both occupied and unoccupied energy states within the gap, showing a strong dependence on the electron spin. This suggests it could be a candidate for further study, though the gallium vacancy in gallium sulfide emerged as the strongest contender overall. The researchers verified that these energy gaps were not just theoretical numbers by checking the material's response to light. They found that the gallium vacancy in gallium sulfide showed a clear signal at the expected low energy, confirming that the transition between these defect states is optically active.

Ultimately, this work provides a clear roadmap for which defects to pursue. The simulations indicate that the gallium vacancy in monolayer gallium sulfide is the leading candidate for a single-photon emitter, with the gallium vacancy in gallium selenide as a close second. The study rules out the chalcogen vacancies (missing sulfur or selenium) as practical candidates for this specific purpose due to their high energy requirements. While the zinc substitution shows promise, the gallium vacancy offers the most distinct and accessible energy levels. These findings do not yet prove that these materials will work in a real device, but they identify the specific atomic configurations that are worth building and testing in the laboratory. The path forward is now clearer: focus on creating and characterizing these specific missing-atom defects to see if they can indeed become the reliable, single-photon sources needed for the next generation of quantum technology.