Research Lab News: RLE and MTL Exploration in Flatland by Jing Kong, Professor, Research Laboratory of Electronics and Microsystems Technology Laboratories Graphene, a single layer of carbon atoms arranged in a honeycomb lattice, has attracted tremendous attention during the past decade. Historically it was called “monolayer graphite”, and has been mostly studied on metallic substrates. The explosion of graphene research started in 2004 when Geim and Novoselov demonstrated that such a single atomic layer can be isolated on an insulating substrate, and exhibit remarkable properties. Since then, many further unique properties and great potential have been demonstrated with this fascinating material. More recently, the rapid advance in graphene has also inspired the investigations of other two-dimensional (2D) materials with many distinct and useful properties. Such possibilities have opened our eyes to an entire world of 2D crystals. In fact, materials with confined dimensions (such as quantum dots, nanowires or nanotubes) have been a main focus of scientific investigations for several decades. The restrained dimensionalities bring many interesting properties to these systems that their bulk counterpart cannot offer. Among them, 2D materials bear the unique characteristic that they can be considered both macroscopic (in-plane dimension, thus easier for integration) and microscopic (out-of-plane dimension). Although a wealth of knowledge has been accumulated for thin film systems, the science of a truly 2D system has been a relatively unexplored topic. With the enormous interest and need for these 2D materials, it is highly desirable to obtain them with high quality and in a controlled manner. The research focus of the nano-materials and electronics group led by Prof. Jing Kong has been on the synthesis and characterization of graphene and related 2D materials, using a chemical vapor deposition (CVD) method. Figure 1, above, illustrates the CVD synthesis of graphene using metallic substrates. In a typical CVD synthesis, the metal substrate is placed inside a (quartz) tube furnace, which is heated up to high temperature (e.g. 1000°C). Hydrocarbon gas is introduced into the growth chamber and catalytically decomposed on the metal surface, leading to the growth of graphene. In order to isolate the graphene from the metallic substrate, a transfer technique has been developed and widely used: a polymer (Poly(methyl methacrylate), PMMA) layer is coated on the graphene first as a protective layer, followed by removal of the metal substrate via wet chemical etching. After thorough rinsing, the graphene/PMMA layer is trans-
30
www.eecs.mit.edu
Figure 1. Illustration of the CVD synthesis of graphene using metallic substrates.
ferred onto the target substrate, and lastly the PMMA is removed either by using acetone or via thermal annealing. Figure 2a on the next page shows graphene that is transferred onto an Si substrate (with SiO2) (left) and optical absorption of graphene in the UV-vis regime (right). This was among the very early samples taken when the synthesis and transfer techniques were developed in 2008. At present, rollto-roll processing and production of much larger areas (~100 cm2) have been demonstrated. Monolayer hexagonal Boron Nitride (hBN) is the second member of the 2D family that entered the scene. It has a very similar honeycomb structure as graphene, but each hexagon is composed of alternating Boron and Nitrogen atoms. It has been called “white graphene” because apart from structure, in many aspects it has similarly remarkable properties as those of graphene, such as high mechanical strength and chemical stability. But “white graphene” is a direct bandgap (~6eV) semiconductor while graphene has no bandgap. It also has a wide range of applications based on its extraordinary properties, such as being a deep ultraviolet emitter, acting as a transparent membrane or dielectric layer, and able to be used for protective coatings. Particularly, it has been demonstrated as a superior substrate for graphene based devices — the on-substrate mobility of graphene on exfoliated single crystalline hBN is comparable to that of suspended graphene due to its ultra-flat and impurity-free charge surface. A giant flexoelectric effect was also predicted for monolayer hBN, suggesting its potential for ambient agitation energy harvesting. The CVD synthesis of hBN has also been under investi-
Exploration in Flatland, continued gation. Compared to graphene, hBN is more challenging to grow. Nevertheless, there have already been some encouraging results. Figure 2b (right, middle panel) shows the optical image of monolayer hBN transferred on Si substrate (with SiO2) and on quartz substrate (inset). The right panel shows transmission electron microscope images of the hBN layer on the edge. Both monolayer and bilayer hBN can be seen here. The synthesis of hBN is also very similar to graphene — a metallic substrate is used and the synthesis is at high (~1000°C) temperature — except that a different precursor (containing both B and N) is used. Afterwards hBN can be transferred to desired substrates using the same method as shown in Figure 1 (step 3). Although graphene has many outstanding characteristics, it lacks a bandgap, making it very challenging for a graphene transistor to be turned off. Ways to overcome this problem have been proposed and are under investigation, such as applying an electric field across the two layers of AB-stacked bilayer graphene to open up a bandgap. In the meantime, layered transition metal dichalcogenides (LTMD) (such as MoS2, WSe2) in the 2D material family have attracted extensive attention as atomically thin semiconductors. More interestingly, due to quantum confinement, the bandgaps of these semiconductors depends on their layer number, and they change from indirect bandgap to direct bandgap in the transition to monolayer. Thus the LTMD monolayers, being considered as the thinnest semiconductor, exhibit great potential both for advanced short-channel devices and optoelectronic devices. Furthermore, the broken inversion symmetry of the monolayer and the strong spin-orbit coupling lead to a fascinating interplay between spin and “valley” (i.e., valley in the electronic band structure) physics, enable simultaneous control over the spin and valley degrees of freedom, and create an avenue toward the integration of spintronics and valleytronics applications. The CVD method has also been applied to the synthesis of the LTMD materials. Interestingly, it was found that it is easier to directly use insulating substrates, while most metallic substrates will have a severe sulfurization problem. By using organic aromatic-based small molecules as “seeds”, mm2 to cm2 sized monolayer MoS2 has been grown on Si substrates with SiO2. Figure 2c, bottom right, shows the optical image of individual triangular MoS2 flakes grown on Si (with SiO2) substrates, and the photoluminescence spectrum on the right shows a peak at ~1.8 eV from monolayer MoS2.
Figure 2: (a top) Photograph of 2 x 2cm2 graphene trasferred onto a Si/SiO2 substrate (left) and optical absorption of graphene, monolayer hBN. (b middle) Optical image of monolayer hBN transferred onto a Si/SiO2 substrate (left) with hBN transferred to a quartz substrate (left inset) and transmission microscope image of monolayer (1L, middle) and bilayer (2L) hBN. (c bottom) optical image of triangular monolayer MoS2 flakes directly grown on Si/SiO2 substrates (left) and photoluminescence spectrum from monolayer MoS2(right).
The investigative efforts on 2D materials have seen rapid growth in the past few years. Many exciting possibilities with new phenomena remain to be discovered and novel devices are yet to be seen. There has been constant need for the development of better synthesis methods and processes in order to provide high quality materials. Better capabilities will greatly facilitate the explorations in this Flatland.
MIT EECS Connector — Spring 2014
31