Second harmonic generation has helped to find defects in two-dimensional materials

Brazilian and American scientists have invented a simple and fast way to find defects in two-dimensional materials. To do this, they combined the method of microscopy using second harmonic generation microscopy with a dark field. Prior to this, the main instrument for testing single-layer structures were considered expensive and large electronic microscope is unsuitable for mass application in production. A study published in the journal Nano Letters.

In recent years, the electronics industry is looking for ways to use materials of a single layer of atoms – two-dimensional materials (the most famous of them, perhaps, graphene). Using such materials, the designers of the processors will be able to reduce the size of devices and their energy consumption. However, mass production of complex components impossible without a fast and reliable way to identify the marriage.

Until recently, defects in two-dimensional material took a lot of time and require serious equipment and high qualification, but a group of researchers from the United States and Brazil under the leadership of Bruno Carvalho (Bruno R. Carvalho) from the Federal University of Rio Grande (Brazil) proposed a method of fault detection that can be applied quickly and massively.

There is a physical phenomenon, called the generation of the second optical harmonic generator (SHG). Photons, passing through some materials are combined to form new photons with twice the energy, i.e. double the frequency. In other words, the object fluoresces in a different color compared to the fact that it is covered. This effect is enhanced in places where it disrupts the structure of atoms in a two-dimensional material. Emitting laser light at a fixed frequency and recording it on twice, you get the picture of these violations of the structure, which will indicate the defects.

The problem is that visible to such image defects are not very bright in the background light, which emits due to SHG of the rest of the material without defects. In search of ways to solve the problem, the researchers studied under the electron microscope the atomic structure of single-layer diselenide molybdenum, and their calculations showed that the defect structure should emit light at an increased angle compared to the homogeneous material. To test this, the researchers adapted the technology of darkfield microscopy. By placing the sample in the center, they blocked the Central part of the lens and began to capture only those rays which fall on the edges at a large angle.

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