The relations between Van Der Waals and covalent forces in layered crystals
Date of Issue2017-01-31
School of Materials Science and Engineering
With the advent of graphene that propels renewed interest in low-dimensional layered structures, it is only prudent for researchers’ world over to develop a deep insight into such systems with an eye for their unique “game-changing” properties. Graphene, being the pioneer of this layered renaissance led the field and displayed exceptional properties such as ultra-high mobility at atomically thin two-dimensional level. An absence of energy band gap in pristine graphene makes it unsuited for logic electronics applications. This led academia to turn their attention towards structurally similar semiconducting counterparts of graphene, layered transition metal dichalcogenides (LTMDs). Although known for decades, these materials developed a special significance when viewed through perspective of low-dimensional atomically thin scale physical properties and phenomena. Prominent in them, MoS2, WSe2 shows tuning of physical properties including band gap with reduced thickness and even an indirect to direct band gap transition when reduced to monolayer. Such direct band gap atomically thin compounds could prove to be very versatile in integrated electronics, optoelectronics applications and for catalysis operations due their extreme case of surface to area ratio. It has been found the interplay of strong and weak bonds plays a very important role in the control of various properties. In general, layered compounds are more sensitive to weak bonds (van der Waals forces) then to strong bonds (covalent bonds). Based on above discussion, thin samples of layered transition metal dichalcogenides are required for both fundamental applications and research as they exhibit special properties. From the perspective of materials science, a novel method for rapid synthesis of thin few layer LTMD crystals has been proposed and developed in-house to complement other existing methods. The synthesis was achieved by plasma that was formed inside the inner sealed quartz tube heated by plasma surrounding the sealed ampoule. Several compounds (MoS2, WS2, MoSe2, WSe2 and ReS2) belonging to layered transition metal dichalcogenides family has been synthesized. Their phase-pure and highly crystalline nature has been confirmed through characterization. The special aspect of the synthesis is the few layer thin nature of the obtained compound that is also confirmed by atomic force microscopy. It has been proposed through this work that such synthesis is practically applicable to other similar metal-nonmetal compounds as long as nonmetal vapor pressure reaches a value required for plasma generation. Another aspect of the current project is the synthesis of large high quality bulk single crystal of layered transition metal that till today proved to be the bed rock of fundamental electronic and optoelectronic research. Several compounds (MoS2, WS2, etc.) and their alloys and similar new layered systems were successfully grown and their growth conditions are optimized for the available setup. As discussed earlier, interplay between strong and weak bonds plays an important role in the physical properties of such compounds, one of them being anisotropy. While traditionally every layered material has out-of-plane anisotropy on account of their sensitivity to van der Waals forces, there is less explored member of LTMD family that has both in-plane and out-of-plane anisotropy as well as insensitivity towards weak forces (vdW forces). This and its covalent bond (strong bond) substitution and resulting alloy properties are investigated in detail. After successful synthesis of the compounds, structural, optoelectronic anisotropy is demonstrated. ReS2 shows highly anisotropic polarization dependent response. Further its FET has an on/off ratio of 105 and mobility of 18 and 40 cm2V-1s-1 at room and low temperature respectively. Its layer independent direct band is utilized for photodetector devices with high responsivity of 103 A/W that depicts its intrinsic optical anisotropy and could be utilized as a linear dichroism media for sensitive detection of polarized light. In addition, the alloy ReSSe is both isostructural and anisotropic like ReS2 besides being direct bad gap. This paves the way for band gap engineering of rhenium dichalcogenides alloys and present them as viable alternative for optoelectronic low cost devices. In the end as an example of real application of that as an electrode of lithium ion battery has been proposed. Earth-abundant dichalcogenides in the form of SnS2 is used as an anode material for lithium-ion battery with LiMn2O4 cathode.