How clean can polymeric transferred monolayer MoS₂ be?

Contamination-free transfer of monolayer MoS₂ (1L-MoS₂) is extremely beneficial for the fabrication of 1L-MoS₂ based flexible devices and integration into Si technology. Among the existing polymeric film transfer techniques of 1L-MoS₂, the surface-energy-assisted transfer technique is well-known. The technique uses polystyrene (PS) polymer as a carrier polymer, which is advantageous for providing a better mechanical support layer. Also, the transferred film is devoid of any optically visible polymer leftovers, unlike the film transferred by wet chemical etching methods. However, the existence of the microscopic PS residues on the transferred 1L-MoS₂ was not reported to date. In this work, the surface of 1L-MoS₂ was analyzed to establish the topographic modifications induced by the transfer process. The atomic force microscopy images reveal the presence of polymer residues which induces a mean roughness of ~ 5 nm. Additionally, high-resolution transmission electron microscopy analysis also reveals the presence of PS residues on transferred film. The A_1g phonon mode of the transferred film was blue-shifted (2.4 cm^-1), and FWHM (1 cm^-1) was increased. The Raman correlation plot establishes that the surface-energy-assisted transfer technique induces n-type doping to the 1L-MoS₂. The increase in film carrier concentration is because of the electrons transferred from PS polymer to 1L-MoS₂. Our findings are crucial for fabricating any high-performance flexible 2D devices.