Abstract:
Atomically thin monolayer two-dimensional transition-metal dichalcogenides
(TMDCs), especially MoS2 has unique electronic, optical, physical and chemical
properties in contrast to their bulk counterpart and other two-dimensional (2D)
materials. MoS2 monolayer has a direct band gap of 1.8 eV which opens up its
applications in optoelectronics, solar cells, gas sensors, water disinfection and FETs
for next generation nano-electronic device applications. Vertically aligned MoS2
flakes have attracted immense attention among research community over inert inplane
MoS2, owing to its superior electrical and optical properties. Therefore, exposing
more edges of MoS2 flakes by controlling its nanostructure allows the formation of
high performance optical and electronic devices. However, thermodynamics favors
the presence of in-plane growth, limiting the number of active sites at the MoS2
surface. Therefore, controlled growth with variable density of vertically aligned MoS2
flakes and understanding their growth mechanism would be one step closer to realize
their great potential in optoelectronics and in gas sensor development. In addition,
scalability, uniformity and reproducible growth remains great challenges, hindering
further progress.
MoS2 has a large specific surface area. The large surface area provides maximum
favorable adsorption sites for the adsorption of gas molecules and to enhance the
surface perturbation in the presence of the gas molecules. MoS2 is very sensitive and
amenable to be used in gas sensing devices. In this context, MoS2 established as the
promising chemical sensing material. The gas molecules adsorption in MoS2 is
position dependent. Thus, engineering the morphology could be a feasible approach
to develop high performance gas sensors. Nevertheless, present gas sensors based on
bare MoS2 flakes suffer from low gas sensing performance. Thus, rigorous approaches
are needed to develop high performance gas sensors based on bare MoS2 working at
room temperature.
Moreover, the in-plane MoS2 film is of only a few nanometres and hence the
absorption of incident light is significantly less. The performance of MoS2 based
photodetector suffered from the absence of high-quality p-n junctions. So, the charge
carrier recombination rate is high in bare MoS2 which compromises the photodetector
performance of MoS2. Integrating MoS2 with other high absorption materials proves to be a viable choice for high-performance PD fabrication. However, no significant
research efforts have been put out in this area. Therefore, the field is still open to fully
realize the promising potential of hybrid heterojunctions for practical applications and
comprehensive understanding of the charge transfer which is substantially crucial for
the devices with multiple functionalities.
Therefore, we explored these challenging issues such as controlled growth of
vertical aligned MoS2 flakes, high performance gas sensors and broadband
photodetectors. To tackle down these challenges, the modified tube in tube
atmospheric chemical vapor deposition technique (APCVD) is adopted to grow MoS2
flakes. The density of vertically aligned MoS2 flakes is controlled by the carrier gas
flow rate. We have optimized the critical APCVD growth parameters. These
experimental finding revealed that gas flow rate is an important parameter to control
the growth of MoS2 flakes. Various characterization techniques are used to analyze
and confirm the structural, morphological and optical properties of synthesized MoS2
flakes. The surface morphology, chemical composition, elemental bonding and
structural properties are determined by using scanning electron microscopy (SEM),
atomic force microscope (AFM), X-ray diffraction (XRD), energy dispersive X-ray
(EDX) and X-ray photoelectron spectroscopy (XPS). The optical properties are
investigated using Raman, photoluminescence (PL) and UV-VIS-NIR spectroscopy.
Based on the structural, morphological and spectroscopic analysis, a detailed growth
mechanism is proposed where in-plane MoS2 is found to work as a seed layer for the
initial growth of vertically aligned MoS2 flakes that finally leads to the growth of
interconnected 3-D network of vertically aligned MoS2 flakes. We fabricated and
developed fast responsive and recoverable sensitive H2 and NO2 gas sensors. We
experimentally demonstrated that the gas adsorption in MoS2 is position-dependent
and understanding of favorable adsorption sites is necessary to develop the highly
sensitive, fast recover RT gas sensors. Considering this, we synthesized morphology
driven H2 and NO2 gas sensors. We synthesized 3-D network of vertically aligned
MoS2 and monolayer pyramid MoS2 structures for H2 gas sensing. We developed NO2
gas sensor based on mixed MoS2 flakes and demonstrated photo-activated NO2 sensor
at room temperature. The mixed MoS2 based sensor shows complete recovery and fast
response in photoactive mode at room temperature and thermally active mode at
moderate temperature, respectively. Lastly, we studied the interaction of vertically aligned MoS2 flakes with light. We developed the high responsive, broadband, fast
photodetector by forming the heterostructures of MoS2. We observed that a change in
the orientation of MoS2 flakes from in-plane MoS2 to vertical MoS2 flakes not only
improved the absorption of gas molecules but also helpful in developing the broadband
PDs.