Following the conclusion of this thesis, future steps concerning the thesis can be envisaged.
Firstly, experimentation with growth configuration needs to go forwards in order to achieve the goal of wafer-area coverage (up to several inches). In the context of this work, this could be pursued following the configuration proposed in 4.2.4, in which the flow is no longer constrained by the crucible walls. The achieved results with this configuration point towards the need to increase the crystal size, a goal that might be pursued by using growth promoters such as Na.
Regarding the CVD setup, a more accurate pressure measurement system could be implemented in order to control the partial vapour pressure from each of the precursors along the process, allowing greater control over the available amount of reactant. As for the choice of substrate, c-plane sapphire substrates could be considered in order to promote an epitaxial growth (i.e. crystal alignment through minimization of lattice mismatch between grown material and substrate). Direct growth on supported graphene could also be explored, in order to fabricate heterostructures without the need of a transfer step.
In general, however, the transfer step needs to be considered: one of the proposed applications of 2D materials was specifically in flexible electronics. However, both a direct growth on flexible substrate (usually a polymer) and a post-growth transfer process imply the use of high temperatures, which are detrimental to those substrates.
Further, the optimization of the transfer process to place CVD-grown MoSe2 onto transmission electron microscopy (TEM) grids is an interesting research task in order to characterize the material in terms of its lattice structure at atomic resolution. Some methods have been proposed such as coating the TMDC films with a metallic layer, which is then peeled off and transferred to a target substrate, such as reported in reference [66], but so far no consensus has been reached on the optimal approach. Lastly, the fabrication steps required for the fabrication of optoelectronic devices based on 2D TMDCs need further consideration, as these steps were found to deeply affect the device performance, often shadowing the advantages of using the produced material.
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Appendix
Fig. 0.1 First Brillion zone (left) [103]. Electronic band structure in single layer, bulk (right) [23]
Figure 2.1 Scotch tape method [13]
Figure 2.2 Liquid exfoliation. Influence of the choice of solvent in re-aggregation. [104]
Figure 2.3 Lithium intercalation in bulk MoS2
Figure 2.4 Electrochemical exfoliation [105]
Fig. 0.2Metal chalcogenization
Figure 2.5 Schematic view of an MBE growth chamber [67].
Fig. 0.3 Schematic of the CVD process. 2 zone furnace and ampoule
Figure 1 Photography of the microscope and laser from the WiTec Alpha300R system used for Raman spectroscopy measures
Figure 2 Dual Beam FIB-SEM
Figure 3 Scanning Probe Microscope- AFM Dimension Icon
Figure 4 PANalytical X PERT PRO MR X-ray diffraction system
Figure 5 Keithley 6487 Picoammeter/Voltage Source
Figure 6 PVA TEPLA Plasma Asher
Figure 7 Schematic view of the CVD setup showing the configuration of the precursors and substrate in a face-to-face approach.
Figure 8 MARS Hydrogen generator (left) and MKS PR4000B mass flow controller (right)
Figure 9 Furnace flow valves
1. Close all valves. The furnace heating elements should be turned off at this point 2. Switch on vacuum pump
3. Open valve 1 slowly while inspecting pressure gauge. When pressure reaches 0 mbar, close carefully valve 1
4. Open the hydrogen flow line
5. Open the CVD inlet valve and wait for the furnace to fill. When the gauge indicates 1000 mbar, close the inlet valve.
6. Repeat steps 3 and 5 two to five times.
7. After the furnace chamber is filled for the last time, close the inlet valve and turn off the pump.
8. Open valve 1 completely (pressure will drop to 800 mbar) 9. Open inlet valve until the pressure reaches 1000 mbar 10. Close inlet valve
11. Close valve 1 12. Open exhaust valve.
Figure 10 The vacuum pump used was a Pfeiffer Adixen 2005 SD Pascal Rotary Vane Pump (left) and pressure gauge (right)