Mechanical, electrical and KPFM investigations of ultrathin membranes of MoSSe alloy

J. Serafińczuk, A. Piejko, M. Tamulewicz-Szwajkowska, K. Król , R. Kudrawiec
Wroclaw University of Science and Technology,

Keywords: MoSSe, KPFM, ultrathin membranes, work function, Young's moduli


2D semiconductors, in particular transition metal dichalcogenides (TMDs), and their alloys are intensively studied due to their wide range of applications. TMDs exhibit new properties that differ from their counterparts. In this work, single atomic layers of these materials were investigated, in particular membranes based on MoSSe crystals. The samples were prepared using mechanical exfoliation and the dry transfer method. The influence of flake-substrate effects on MoSSe layers in terms of their electrical and mechanical properties was investigated. Gold-coated SiO2/Si substrates with etched cavities were used for membrane fabrication. On the basis of contact potential difference (CPD) values obtained from AFM measurements with the Kelvin probe, the work function (WF) of the layers was calculated. The layer work function is determined to be 4.63±0.08, 5.10±0.09, and 5.17±0.11 eV, for suspended MoS2 and MoSe2 and MoS1.44Se0.56 respectively. In addition, a difference of ~0.02 eV was observed between the work function of the supported and suspended areas. The elastic deformation of the freely suspended layers was studied by nanoindentation using AFM. The Young's moduli determined for MoS2, MoSe2 and MoS1.44Se0.56 are 247.96 ± 1.8, 127.60 and 101.23 GPa, respectively. From the analysis of our calculated Young moduli, we find that this parameter is larger for pure crystals. As expected, the obtained E values are much lower than the Young's modulus determined for graphene (about 1 TPa). In the case of a mixed crystal, the Young modulus is lower, which may be due to the intrinsic properties of the crystal and the behavior of the crystal lattice, where there are bonds of different lengths, from sulfur to selenium. Additionally, electrical investigations were carried out for temperatures from 80K to 450K. As the temperature increased, the observed resistance decreased from 40 MΩ for T = 80 K to 630 kΩ for T = 450 K. In addition, we investigated the effect of illuminating the sample with LEDs of different wavelengths on the electrical properties of MoSSe based structures. For T = 80 K, there was a 44% increase in current when the LED was turned on, while for T=450K the current increased by much less, i.e. 6%. The results presented show that MoSSe-based devices can work as photo-detecting sensors.