10:00 AM - MS02.09.03
Controllably Strained Layered Transition Metal Dichalcogenides Grown on Single-Crystal Quartz Substrates via Chemical Vapor Deposition
Fangze Liu1,Oleg Kozlov1,Vladimir Sayevich1,Igor Fedin1,Hsinhan Tsai1,Wanyi Nie1,Victor Klimov1
Los Alamos National Laboratory1
Show Abstract
Strain engineering is widely used for tuning the properties of materials and thereby realizing high performance electronic and optoelectronic devices. In bulk three-dimensional materials, the strain-controlled tunability is, however, limited by low values of ‘failure strain’ which triggers formation of lattice defects. On the other hand, two-dimensional (2D) materials such as graphene and layered transition metal dichalcogenides (TMDs) can tolerate much greater strain, and thus are perfect materials for practically implementing the ideas of strain engineering. [1] Among many different methods to introduce strain in TMDs, mismatch between coefficients of thermal expansion (CTE) of a growth substrate and a TMD layer is one of the most promising method mainly due to two reasons: (1) large-area strained TMDs can be directly prepared through bottom-up synthesis such as chemical vapor deposition (CVD) or molecular beam epitaxy (MBE); (2) the induced strain is maintained intrinsically without any external force or treatment since the as-grown TMD is anchored to the substrate at the fabrication stage. Previous reports on CTE mismatch of CVD-grown TMDs have been only able to achieve tensile strain or low compressive strain (<0.2%). [2] While tensile strain normally decreases the band gap of TMDs, compressive strain tends to increase the band gap. Moreover, recent theoretical and experimental studies [3, 4] have shown that a large compressive strain can be used to stabilize a nominally unstable 1T TMD phase, which is attractive for applications including hydrogen evolution and low-contact-resistance electronic devices. [5]
Here, we use single crystal quartz substrates, which have large CTE compared with MoS2, to achieve compressive strain up to 1%. The strong built-in strain is indicated by the increased photoluminescence and optical absorption energies as well as ‘stiffening’ of characteristic A1g and E12g Raman modes. Owing to the different CTE of single crystal quartz along the a- and c-axes, MoS2 grown on different facets experiences different levels of strain. Specifically, the layers grown on the c-plane exhibit higher strain compared to those prepared on the x-plane which is direct consequence of the higher CTE. Additionally, the MoS2 layers grown on the AT-cut quartz are highly aligned due to the strongly anisotropic nature of the underlying quartz surface. [6]
The ability to tune electronic properties of TMD layers by engineered strain can be especially useful for controlling interfacial interactions in complex hetero-structures comprising TMD materials as electronically and/or optically active components. Presently, we are exploring this capability for manipulating charge and energy flows in multi-dimensional materials assembled from TMDs and colloidal quantum dots.
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