2D Materials in Lego Style: Synthesis, Characterizations and Applications
Ajayan, Pulickel M.
Doctor of Philosophy
Recently, the emergence and development of 2D materials with various optical and electrical properties has opened up new routes for electronic and optoelectronic device fabrication based on atomically thin layers. For example, graphene behaves as a semi-metal with extremely high mobility, hexagonal boron nitride (h-BN) is a good insulator and monolayer TMDs such as MoS2, MoSe2 and WSe2 are semiconductors with direct band gap. This diversity offers the opportunity to construct atomically thin electronics based entirely on 2D materials. One of the most promising applications is to get 2D integrated circuits to replace the traditional silicon based ones, which will be much thinner and faster. 2D materials can be considered to be analogous to Lego blocks. The Lego game is to use different Lego blocks to get a complicated Lego building. Similarly, we can use different 2D materials to get the corresponding integrated circuits or devices for energy related applications. Based on this purpose, we need different 2D blocks, which are the most fundamental parts in the 2D world, 2D materials with tunable properties, and different strategies to combine the 2D materials together. Chapter 1 focuses on synthesis, characterization and applications of pristine 2D materials, which are the fundamental blocks for the 2D world. In this part, we synthesized different 2D materials such as insulator (h-BN), metal (graphene) and semiconductors (MX2, M = metal and X = chalcogen) for different applications. There are two directions in this part: one is to explore new 2D materials and the other one is to improve the growth of 2D materials to push them closer to their real applications. Moreover, semiconductors with different band gap (from 1.1 eV to 2.8 eV) and different type (p type and n type) have been developed. Furthermore, we improved the growth of different 2D materials to get their millimeter-scale single crystals or even continuous film. In the coming Chapter 2, we focused on the 2D alloys. The purpose of alloying 2D materials is to engineer the phase and band gap by changing the composition in the alloys. By this, we can tune the optical and electrical properties in 2D materials very easily. The first project in this part is about h-BNC system, which can open a band gap in graphene system, resulting in both high mobilities and high ON-OFF ratio in their transistors. Then we developed the MoS2-xSex (x, 0-2) alloys, in which the band gap can be continuously tuned from 1.50 eV to 1.84 eV. At last, RexMo1-xS2 (x, 0-1) system is developed to study the phase transition with different x. In Chapter 3, heterostructures based on different 2D materials are developed by different strategies. For example, we can get h-BN/h-BNC/graphene lateral heterostructure by combing a conversion method and lithography. We also developed the heterostructures based on MoS2/WS2 and MoSe2/WSe2 by a one-step growth method and two-step growth method, respectively. In both of them, we can get the in-plane and vertical heterostructures. The interface of the in-plane interface is atomically seamless and sharp and the bilayer heterostructures have fixed stacking orientations, which are more advantageous than other methods. At last, we developed more complicated heterostructures, which can be composed by 3 or 4 different 2D materials. In Chapter 4, we further developed several different 3D structures constructed by 2D materials for energy storage and conversion. Basically, this part is inspired by graphene aerogel with porous 3D structure. The porous structure enables the access of electrolyte very easily and the graphene network has very good electrical conductivity, advantageous to work as electrochemical applications. In this part, we developed several different structures for different applications, including MoS2/GO as the anode for lithium ion battery, VO2/GO as the cathode for lithium ion battery and h-BNC as ORR catalyst. For the lithium ion battery, the structures developed here have better performance than the commercial ones with higher capacity, better stability and much higher charge and discharge rate. H-BNC aerogel can even beat the performance of commercial Pt/C as the ORR catalyst. In summary, the research based on 2D materials is like the Lego game, including exploring the Lego blocks (pristine 2D materials and their alloys) and combining them together to form the functional devices (2D heterostructures and 3D porous structure from 2D materials).