Impact of van der Waals epitaxy on structural and optoelectronic properties of layered solids

Abstract

Emergence of Graphene, Transition metal dichalcogenides (TMDCs) and other van der Waals materials promises to rede fine the building blocks of the future materials and technologies. These materials have gained enormous attention due to their rich electronic, optical and mechanical properties. Combining van der Waals materials to form hybrid heterostructure allows to exploit their individual properties for suitable technological applications. In this context, understanding the interfacial phenomena provides us with a powerful tool to reproducibly design heterostructures with desired physical properties. In a heterostructure, in spite of the weak nature of van der Waals interactions, the two adjacent crystals may not be entirely inert to each other. Interlayer coulomb interaction between MoSe2 and WSe2, has been demonstrated to result in new excitonic states in MoSe2-WSe2 hybrids. Moir e superlattices in Graphene-hBN hybrids leads to massive Dirac fermions and Hofstadter's butterflies. Van der Waals epitaxy can also potentially change the band structures in graphene-TMDC hybrids. Twist angle dependent Van Hove singularities are demonstrated to appear in twisted bilayer graphene. In hBN graphene heterostructures, interlayer strain ( 2 %) is found to be concentrated in periodic domain walls creating soliton like features. Massive interlayer strain ( 5 %) in lattice incommensurate graphene-black phosphorus heterostructures is recently reported. Although the strain can potentially alter the crystal structures of TMDCs, whether van der Waals epitaxy can impact the structural ground state of atomically thin membranes, remains an unaddressed question. In this thesis, we have carried out detailed temperature and electric field dependent Raman spectroscopic, electrical and conducting mode AFM studies of van der Waals dimers of atomically thin MoS2 with graphene/hBN. We observe unusual low frequency Raman modes only from the overlap region of MoS2 and graphene/hBN. These Raman modes, having frequency values 151 cmð€€€1, 227 cmð€€€1, and 330 cmð€€€1 can only be related to the J1, J2 and J3 modes of octahedral (1T/1T0) MoS2 (metallic/low band gap semiconductor). Comparing the intensities of E1 2g Raman mode, on and away from the overlap region, we estimate that 10-15 % octahedral (1T/1T0) MoS2 is distributed in patches/strips in the overall background of 1H phase, only in the overlap region. In our conducting AFM studies also, networks/patches of higher conductance are observed, only in the overlap regions. The areal fraction of octahedral (1T/1T0) MoS2 is further con rmed by electrical transport measurements. Our temperature dependent Raman studies show that this octahedral (1T/1T0) MoS2 is remarkably stable under repeated thermal cycling (T < 500 K), unlike the octahedral (1T/1T0) MoS2 prepared using other methods. This suggests thermodynamic stability of octahedral (1T/1T0) MoS2 in our heterostructures. The study of the time and temperature-dependence of these Raman features suggests the activated nature of 1H to octahedral (1T/1T0) MoS2 phase transition, with an activation energy 830 meV. Similar time-dependent relaxation is also observed in electrical conductance of MoS2 in the overlap region. Using a temperature dependent transfer technique, we have been able to tune the areal fraction of octahedral (1T/1T0) MoS2 dynamically, from 0-25%. The areal fraction increases with decreasing temperature during crystallographic attachment. We attribute the phase transition to van der Waals epitaxy driven stress elds and local charge reorganizations. We have further used the phase engineered MoS2 to create broadband photodetectors. Our graphene/hBN/MoS2 photodetectors show photoresponsivity, R = 5 109 AWð€€€1 in visible range using 640 nm wavelength. In the near infrared region (920 nm) we observed R = 1 109 AWð€€€1, which decreases to R = 3 107 AWð€€€1, at 1720 nm. At 920 nm, and 1720 nm we experimentally found speci c detectivity value, D > 1016 Jones and D > 3 1013 Jones respectively. Unlike in the visible optical wavelength, in the infrared region, the photoresponse is strongly con ned to a back gate voltage window, which may correspond to a convolution of photon energy and the small band gap of the octahedral (1T/1T0) MoS2. In summary, we have demonstrated that van der Waals epitaxy with graphene/hBN can induce 1H to thermodynamically stable octahedral (1T/1T0) structural transition, in atomically thin membranes of MoS2. We have further used the phase engineered MoS2 for broadband photodetection with high fi gures of merit. As the ferroelectric/quantum spin hall insulating features of stable octahedral (1T/1T0) MoS2 remain to be further exploited, our work creates a new formalism in designing novel crystals, naturally unavailable in nature, in van der Waals heterostructures.