Transmission Electron Microscopy

5 nm

Fig. 4.13 TEM image of a polycrystalline TAC MoS2film, grown at 750 °C. Red

arrows point to different diffraction patterns from various regions.

In order to better understand the structure of TAC grown films, samples were prepared for TEM. TEM only works for samples thin enough to be electron trans- parent, therefore 2D nature of TMD films makes them very suitable. Furthermore, inorganic TMDs (with strong covalent intralayer bonding) are more resistant to electron dosage than organic or biological samples. The ultra-high vacuum environment of a TEM instrument requires elaborate sample preparation for water- based biological samples. Further, organic molecules are primarily composed of lighter elements, which do not interact strongly with the electron beam and there- fore do not provide as much contrast as the heavier transition metals. The TEM presented here was imaged by Maria O’Brien. Fig. 4.13 shows a TEM image of a typical 1 nm thick MoS2film grown by TAC on a SiO2substrate then transferred

onto a TEM grid. Inspection of these images shows that the domains are typically around tens of nanometres in size, and applying fast Fourier transform processing to several regions shows that the individual domains are highly crystalline, and randomly orientated with respect to the others. FFTs of highly crystalline samples exhibit sharp spots. The location and relative brightness of the spots is related to the symmetry of the sample. Other TEM studies have shown that each grain is highly ordered, with hexagonal symmetry and a mixture of stacking types.101 The size and orientation of the individual domains is dependent on the synthesis conditions (temperature, metal thickness, pressure and substrate). However, most reports describe TAC films with domains ranging from tens to several hundreds of nanometres.111

XTEM was also used to investigate the structure and orientation of TAC grown films, XTEM was performed in collaboration with Dr. Paul Hurley and the Micronano Electronics group at Tyndall National Institute. Fig. 4.14(a) is a XTEM image of MoS2 from 5 nm starting metal thickness, synthesised on

sapphire, with a gold layer deposited above. It has expanded to around 10 nm, a factor of 2, which agrees well with SE and XPS measurements. The film is well aligned, with the the predominant alignment of the basal plane parallel to the substrate.

XTEM of a thicker MoS2 film, also deposited on sapphire, is depicted in

fig. 4.14(b). In this case 10 nm of metal underwent TAC, and the resultant film is not as parallel to the surface as the 5 nm case. The film expanded to around 20 nm, again expanding by a factor of two. Grain boundaries and regions where domains overlap can be seen. The lattice spacing is consistently 0.6 nm, indicating that the layered material is MoS2.

Sapphire Substrate

10 nm

Dissipation layer (gold)

MoS2

(a)

(b)

(b)

(c)

Fig. 4.14 XTEM characterisation of TAC grown MoS2. Starting metal thickness is

(a) 5 nm, (b) 10 nm and (c) 1 nm. Analysis was performed through collaboration with (a), (b) Tyndall National Institute, Cork and (c) SIOM, Shanghai

Additional XTEM studies carried out in collaboration with the group of Prof Jun Wang in SIOM, Shanghai showed that thin MoS2 films grown from 1 nm

Mo are well aligned to the quartz substrate. As is shown in fig. 4.14(c), films are predominantly monolayer, with some bilayer regions. Lattice spacing also confirmed that the film is MoS2. Quartz is more amorphous than crystalline

sapphire, so it is encouraging that MoS2 with monolayer character could be

4.4.4

Atomic Force Microscopy

21 nm

14 nm

10 nm

5 nm

Fig. 4.15 AFM images of MoS2 grown at (a) 700 °C and (b) 1000 °C. (c) re-

lationship between domain size and growth temperature and initial metal film thickness.

AFM studies have identified a strong relationship between the growth tem- perature, thickness and the crystallinity in TAC based TMD films.104,105Statis-

tical analysis of AFM phase maps, such as those presented in fig. 4.15(a) and fig. 4.15(b) shows a clear relationship between domain size and both growth temperature and thickness of the metal precursor. Fig. 4.15(a) shows a MoS2

film from an initial Mo thickness of 10 nm synthesised at 700 °C; the average grain size was 35 nm. Fig. 4.15(b) shows a similar film synthesised at higher temperature (1000 °C). The average grain size is enlarged significantly to 90 nm. A comprehensive study on the film formation reveals that higher crystallinity i.e. larger domains are formed at higher temperatures and when thicker Mo films are converted, a plot of the relationship is shown in fig. 4.15(c). At all temperatures, grain size is smaller for thinner films. This is likely due to the restricted mass transport availability of MoS2 on the solid surface and the limited source of

material. The growth mechanisms of MoS2are still under investigation. Further

improvements in crystallinity have also been seen through the use of more crys- talline substrates such as sapphire. Laskaret al.reported large-area well-aligned TAC MoS2films grown on monocrystalline sapphire substrates.105These results

clearly show that there is considerable scope for optimising the quality of thin TMD films synthesised via the TAC process, by adjusting parameters such as the growth temperature, growth pressure, choice of substrate and starting metal thickness.