dc.description.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 cm1, 227 cm1, and 330 cm1 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 AW1 in
visible range using 640 nm wavelength. In the near infrared region (920 nm) we observed
R = 1 109 AW1, which decreases to R = 3 107 AW1, 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. | en_US |