New Approach Synthesizes Large-Scale Silicon Wafer Size, Single Crystalline 2D Materials

How is something made into a crystal? It is not really necessary in the microscopic world that a transparent and glittery gemstone would make it.

When all of its atoms are assembled in agreement with definite mathematical rules, the material is referred to as a single crystal. The natural world has its own distinctive symmetry, for example, snowflakes or honeycombs; similarly, the atomic world of crystals is designed by its own structure and symmetry.

Furthermore, this material structure deeply impacts its physical properties. Particularly, single crystals play a vital role in driving the material’s intrinsic properties to its complete extent. Encountered with the approaching end of the miniaturization process that the silicon-based integrated circuit has enabled up to this point, massive efforts have been devoted to find a single crystalline replacement for silicon.

In a hunt for the imminent transistor, two-dimensional (2D) materials, particularly graphene, have been the topic of intense research worldwide. This 2D version of carbon even incorporates exceptional electricity and heat conductivity since it is thin and flexible and as it has only a single layer of atoms. However, the endeavors in the last 10 years for graphene transistors have been restricted by physical limits; graphene does not allow control over electricity flow because of the lack of band gap. In that case, what about other 2D materials? Several exciting 2D materials have been known to have similar or even better properties. Yet, the lack of understanding in developing appropriate experimental conditions for large-area 2D materials has reduced their maximum size to just a few square millimeters.

Researchers at the Center for Multidimensional Carbon Material (CMCM) within the Institute for Basic Science (IBS) (situated in the Ulsan National Institute of Science and Technology (UNIST)) have demonstrated a new technique to produce silicon wafer size, single crystalline 2D materials in large scale. Prof. Feng Ding and Ms Leining Zhang together with their colleagues in Peking University, China and other institutes have identified a substrate with a lower order of symmetry when compared to that of a 2D material that makes the synthesis of single crystalline 2D materials easy in a large area. “It was critical to find the right balance of rotational symmetries between a substrate and a 2D material,” notes Prof. Feng Ding, one of the corresponding authors of this study.

The scientists have succeeded in synthesizing hBN single crystals of 10 x 10 cm² by using a new substrate: a surface close to Cu (110) that has a lower symmetry of (1) in comparison with hBN with (3).

Then, why is symmetry important? Symmetry, specifically, rotational symmetry, explains the number of times a specific shape fits on to itself during a complete rotation of 360°. The most efficient technique to create large-area and single crystals of 2D materials is to assemble layers over layers of small single crystals and grow them upon a substrate. In this epitaxial growth, it is rather difficult to guarantee that all of the single crystals are oriented in a single direction. The underlying substrate usually affects the orientation of the crystals. With theoretical analysis, the IBS researchers discovered that a hBN island (or a collection of hBN atoms forming a single triangle shape) has two equivalent alignments on the Cu(111) surface that has an extremely high symmetry of (6).

It was a common view that a substrate with high symmetry may lead to the growth of materials with a high symmetry. It seemed to make sense intuitively, but this study found it is incorrect.

Ms Leining Zhang, Study First Author, IBS

Earlier, different substrates like Cu(111) have been employed to produce single crystalline hBN in a huge area; however, none of them were successful. All the attempts came to an end with hBN islands arranging along in many different directions on the surfaces. Swayed by the fact that the key to realize unidirectional alignment is the reduction of the substrate’s symmetry, the scientists made great attempts to acquire vicinal surfaces of a Cu(110) orientation; a surface obtained by cutting a Cu(110) with a small tilt angle. It is similar to creating physical steps on Cu. Since hBN island has the tendency to place in parallel to the edge of each step, it obtains just one preferred alignment. Moreover, the symmetry of the surface is lowered by the small tilt angle.

They ultimately identified that a class of vicinal surfaces of Cu (110) can be employed to promote the growth of hBN with ideal alignment. On a vigilantly chosen substrate with the lowest symmetry—or else the surface will repeat itself only after a 360° rotation—hBN has just one preferred direction of alignment. The teams of scientists under Prof. Kaihui Liu in Peking University has created a distinctive technique to anneal a large Cu foil, up to 10 x 10 cm², into a single crystal with the vicinal Cu (110) surface and, with it, they have realized the production of hBN single crystals of same size.

In addition to ultrathin thickness and flexibility, emerging 2D materials can exhibit amazing properties when they become huge similar to single crystals.

This study provides a general guideline for the experimental synthesis of various 2D materials. Besides the hBN, many other 2D materials could be synthesized with the large area single crystalline substrates with low symmetry. Notably, hBN is the most representative 2D insulator, which is different from the conductive 2D materials, such as graphene, and 2D semiconductors, such as molybdenum disulfide (MoS₂). The vertical stacking of various types of 2D materials, such as hBN, graphene and MoS₂, would lead to a large number of new materials with exceptional properties and can be used for numerous applications, such as high-performance electronics, sensors, or wearable electronics.

Prof. Feng Ding, Study Corresponding Author, IBS