LAYER TRANSFER TECHNIQUE AND ANALYSIS THEREOF

When a thin TMDC layer is synthesized, it is important for fundamental and applied research for it to be transferred to an arbitrary substrate (Gurarslan et ah, 2014; Lee et ah, 2013). As the growth temperature of TMDC MLs are relatively high, temperature-sensitive substrates (such as polymer- based substrates) cannot be used in the synthetic process, while their use is essential for flexible electronics. It is thus essential to develop a transfer technique to implement large-area TMDC on different substrates. One such technique that maintains the quality of the as-grown ML has been reported (Lee et ah, 2013). The as-grown MoS, sample was cut into three pieces and treated for 30 s with DI water, isopropyl alcohol, and acetone, respectively. The surface of the as-grown ML is hydrophobic, so that isopropyl alcohol and acetone were found to spread out on MoS₂, whereas water remained as a droplet. The treatment of MoS, MLs with DI within those 30 s resulted in the breakdown of floating cut-pieces into smaller ones, indicating that the as-grown MoS, ML can easily be detached from the substrate with the help of DI water. A surface-energy-assisted process has been reported that allowed the perfect transfer of centimeter-scale ML and few'-layer TMDC films from original grow'th substrates onto arbitrary substrates with no observable wrinkles, cracks, or polymer residues. The unique strategies used in this process included leveraging the penetration of water between hydro- phobic TMDC films and hydrophilic growth substrates to lift off the films and diy transferring the film after the lift-off. Scalable transfer of suspended TMDC layers on nanoscale patterned substrates (varying from polymers to Si to metals) using a capillary-force-free wet-contact printing method was demonstrated in (Li et al., 2015). As a proof-of-concept, a photodetector of suspended MoS, was fabricated using this method. As an advantage of this approach, the authors note the possibility of directly suspending a TMDC layer on nanoscale mterdigitated electrodes.

COMPARISON OF EXFOLIATED AND CVD-GROWN TMDCS

While mono- and few-layer sheets can be obtained by both exfoliation and CVD growth, the samples are not perfectly identical. The issue of comparing exfoliated and CVD-grown samples has been discussed, (Plech- inger et al., 2014) where MoS, single-layer samples obtained along these two pathways were compared using optical spectroscopy. In Figure 1.13a, individual spectra measured at different positions of the CVD sample are shown (the positions P1-P3 are separated by 100 pm). The vertical lines serve as a guide to the eye to mark maximum and minimum A exciton peak positions. The spectral shift indicates that the growth conditions, and the corresponding microscopic properties of the MoS, layer, vary in different parts of the layer, which may be associated with different strains (Plechinger et al., 2014). CVD-grown and exfoliated layers were also compared (Li et al., 2014). Figure 1.14b shows a comparison of the reflectance spectra for exfoliated and CVD-grown MoS, MLs. Shifts in the A and В excitonic peaks of ~40 meV are observed, although the overall dielectric function is very similar. For comparison, in Figure 1.14a, reflectance spectra for two different exfoliated samples of MoS, are shown. In addition to the shift of the A exciton peak position, it was also found that the FWHM of the peaks changed as a function of position, with values between 80 and 60 meV, while in an exfoliated single-layer MoS₂ flake has a significantly lower FWHM of about 37 meV, indicating a larger inhomogeneous broadening of the A exciton transition in the CVD-grown sample. In exfoliated MoS₂ flakes at low temperatures, a second, lower-energy PL peak was observed, which was previously associated with localized excitons bound to surface adsorbates (Plechinger et al., 2012). The difference was also observed in the temperature dependence of the PL emission (Figure 1.13b). While the maximum of the A exciton emission redshifts by 35 meV in the temperature range from 4 to 300 К in CVD samples, in exfoliated MoS, flakes, the authors observed a spectral redshift of the A exciton peak by 72 meV in the same temperature range.

FIGURE 1.13 (a) Normalized PL spectra measured on different positions of a CYD film and on an exfoliated flake at liquid-helium temperature; (b) The A exciton peak position as a function of temperature for the CYD-grown sample (black dots) and an exfoliated MoS, flake (red hexagons) (c) Raman spectra measured on different positions of the CYD-grown film and on exfoliated monolayer (ML) and bulk-like flakes. All spectra are normalized to the amplitude of the A_lf mode. The vertical lines mark the positions of A,, and E hi an exfoliated monolayer and serve as a guide to the eye.

Source: Reprinted with permission from Plechinger et al. (2014).

According to an earlier report, shifting of the output spectra towards the red region suggests a temperature-induced reduction of the bandgap, resulted from the thermal expansion of the crystal lattice. Hence, the fact that the redshift is far less pronounced in the CVD-grown sample shows that the CVD-grown MoS, film strongly adheres to the SiO, substrate, which has a veiy small thermal expansion coefficient (Plechinger et al., 2014). Finally, Figure 1.13c shows four Raman spectra collected at different positions on the CVD film compared with the Raman spectrum from exfoliated flakes. All spectra were normalized to the A_lg mode amplitude. In the CVD-grown sample, the E_g amplitude was lower than the A_lg, while in the exfoliated flakes, the opposite was observed. Additionally, the line widths for both Raman modes in the CVD-grown film were larger than in the exfoliated flake, and the E_g mode was asymmetric. It was argued that these results suggest that the carrier density in the CVD-grown film may be significantly smaller than in the exfoliated flake (Plechinger et al., 2014). It should also be noted that stoichiometry variation dining the CVD growth may have an effect on the optical and electrical properties of the grown layers. This issue has already been addressed for the case of MoS, (Kim et al., 2014).

FIGURE 1.14 (a) Comparison of the reflectance spectra of two different exfoliated MoS, monolayers, (b) Comparison of the reflectance spectra of exfoliated (red) and CYD-grown (blue) MoS, monolayers.

Source: Reprinted with permission from Li et al. (2014). © American Chemical Society.