Two dimensional materials with various structure designs: synthesis, characterizations and applications
Doctor of Philosophy
Two-dimensional (2D) materials have attracted both academic and industrial interests and attentions in the past decade, because they exhibit novel electrical, mechanical, chemical, and thermal properties which are rare in their bulk counterparts. Since the discovery of graphene in 2004, various other 2D materials have been synthesized successfully in recent years, including h-BN, TMDs, MXene and many more in the rapidly growing 2D family. However, most of the reported 2D materials and devices are based on single-crystal flakes, or their stacked heterostructures. Structure difference in 2D materials will dramatically change properties of pristine materials and devices performance. Exploring various structures in 2D materials will not only deepens our understanding about the relationship of structure-properties when materials thickness is limited to a few nanometers, but also promotes and diversifies practical applications of 2D materials. In chapter 3, we develop a lateral monolayer MoSe2-WSe2 PN junctions by one-step direct CVD method. The PN junctions show excellent diode behavior. The rectification ratio could reach as high as 1000 without any gate tuning. Moreover, a record-high open-circuit voltage of 0.72 V is observed in 2D-materials based PN junctions, originating from the large built-in potential across the WSe2-MoSe2 junction. Furthermore, the corresponding self-powered photodetectors exhibit fast response time, down to 6 ms, which is several magnitudes faster than photodetectors with single TMDs. In chapter 4, we develop an effective method to grow high quality, uniform, large-area and continuous h-BN films on industry stainless steel directly to protect stainless steel from oxidization and corrosion. The elastic modulus and hardness of h-BN coated stainless steel is several times higher than that of bare stainless steel. The sliding friction coefficient is only about 0.2, which is about one third of that of polished stainless steel. Bare stainless steel begins to be oxidized in the air when the temperature is higher than 600 ℃; while no obvious changes are observed for h-BN coated stainless steel even at the temperature of 800 ℃. h-BN also revealed excellent protection performance of stainless steel against corrosion. The corrosion current was less than one tenth of that of the bare stainless steel at both room temperature and high temperature and pressure conditions. In chapter 5, we report a new method to create multiple triangular nanopores easily on CVD-grown monolayer MoS2 by oxidization at high temperature and subsequent etching. The sizes of triangular nanopores are easily tunable with oxidization temperature and change from about 24 nm to 107 nm for oxidization from 300 to 450 ℃. Plenty of edge sites, which are the active sites toward HER, are exposed on porous MoS2. Basal plane activity of monolayer porous MoS2 are significantly improved compared with pristine MoS2. In chapter 6, we demonstrate monolayer Janus is an effective and universal SERS-active substrate for biomolecules sensing. Multiple characteristic Raman peaks of adsorbed biomolecules, e.g. glucose and dopamine, are clearly observed in their Raman spectra. The estimated Raman enhancement factor is higher than 105, which is several magnitudes higher than that of other 2D materials. The C-C stretching peak at 1360 cm-1 is used to indicate glucose concentration and its integrated-peak intensity increases linearly with glucose concentration in the range of 1- 10 mM. In summary, novel properties of 2D materials could be achieved by exploring various structures in 2D materials, releasing huge practical applications potentials at the same time. In this thesis, four structures of 2D materials, including monolayer lateral MoSe2-WSe2 PN junctions, continuous h-BN films on stainless steel, monolayer porous MoS2 and monolayer Janus as SERS-active substrate for biomolecules sensing, are synthesized and their properties are investigated accordingly. These provides us essential knowledge and opportunities for future applications based on 2D materials.