Development of two-dimensional transition metal dichalcogenides for electronic and optoelectronic applications
Chow, Wai Leong
Date of Issue2016-05-30
School of Electrical and Electronic Engineering
This thesis presents research work on transition metal dichalcogenides (TMDCs), a promising class of 2D materials in the post-graphene era. TMDCs are in the form of X-M-X, where M is a transition metal element from groups 4-7 and group 10 and X is a chalcogen (S, Se or Te). Due to the vast number of possible combinations, more than over 40 members can exist in the TDMC family. Here, only two of the members – MoS2 and PdSe2 – are discussed. The significance of this thesis can be broadly classified into the following three parts: 1. As one of the pioneer 2D materials to be in the TDMC family, MoS2 has proven itself to be a stellar material in electronic, optoelectronic and valleytronic applications. However, due to its large specific surface area, MoS2 is susceptible to absorbing oxygen species, which can have a tremendous influence on its properties. Previous works have mainly focussed on physisorbed oxygen, yet few studies have considered oxygen chemisorption on MoS2. To address this, a simple oxygen plasma process is used to introduce oxygen chemisorption on MoS2. Using Raman spectroscopy, the frequency of A1g mode is observed to be pinned while that of E12g mode redshifts. This is attributed to the softening of in-plane Mo-S force constants when oxygen atoms are chemisorbed onto the sulphur sites. Furthermore using photoluminescence spectroscopy, a redshift in the photoluminescence peak is observed and is attributed to the reduction of band gap in oxygen-chemisorbed MoS2. 2. In principle, any bulk crystals with interlayers bonded by van der Waals force can be thinned down to monolayer. This is the concept and motivation behind van der Waals heterostructures. Existing works have demonstrated the stacking of MoS2 with various materials such as graphene, hBN, WSe2, BP and CNTs, which are mostly inorganic based. To investigate further, an inorganic-organic heterostructure is presented, in which rubrene is used to stack with MoS2. Here, different applications are realised by changing the stacking configuration of the MoS2-rubrene heterostructure. First, a photodetector with fast photoresponse time (< 5 ms) coupled with high photoresponsivity (510 mA W-1 at 532 nm) is demonstrated. Second, an ambipolar FET with μe = 0.36 cm2V−1s−1 and μh = 1.27 cm2V−1s−1, coupled with an on/off ratio of 103 is achieved. Lastly, an inverter with good voltage gain of 2.3 has been realised. 3. Due to the rich chemistry of TMDCs, identifying promising new 2D materials will be instrumental towards creating next-generation 2D electronics. In this contribution, PdSe2, a promising TMDC, is introduced into the 2D family. The PdSe2 is grown by melting stoichiometric amount of Pd and Se powders where their crystal structure, phase, purity and composition are confirmed with XRD, EDS and TEM. Intrinsically, PdSe2 FETs are found to exhibit ambipolar carrier transport (μe (avg) = 17 cm2V−1s−1 and μh (avg) = 7 cm2V−1s−1, with on/off ratio of 102). Interestingly, high performance n-type PdSe2 FETs (μe = 216 cm2V−1s−1 with on/off ratio of 103) can be realised after vacuum annealing. Lastly, progressive p-doping in PdSe2 is also demonstrated using F4-TCNQ.
DRNTU::Engineering::Electrical and electronic engineering