Effect of contaminations and surface preparation on the work function of single layer MoS₂

Abstract

SummaryThinning out MoS2 crystals to atomically thin layers results in the transition from an indirect to a direct bandgap material. This makes single layer MoS2 an exciting new material for electronic devices. In MoS2 devices it has been observed that the choice of materials, in particular for contact and gate, is crucial for their performance. This makes it very important to study the interaction between ultrathin MoS2 layers and materials employed in electronic devices in order to optimize their performance. In this work we used NC-AFM in combination with quantitative KPFM to study the influence of the substrate material and the processing on single layer MoS2 during device fabrication. We find a strong influence of contaminations caused by the processing on the surface potential of MoS2. It is shown that the charge transfer from the substrate is able to change the work function of MoS2 by about 40 meV. Our findings suggest two things. First, the necessity to properly clean devices after processing as contaminations have a great impact on the surface potential. Second, that by choosing appropriate materials the work function can be modified to reduce contact resistance.

Structural, optical and electrostatic properties of single and few-layers MoS 2 : effect of substrate

“…For the entire MoS 2 flake, V CPD value is significantly lower than that of the gold substrate, where V CPD value of a single layer is notably the lowest. The V CPD value increases with the layer thickness, consistent with literature [24], though the bulk value (as well as a part of the flake) is compromised by decoration of the surface by environmental adsorbates (also seen as bright clusters in figure 1(a)), leading to a decrease in surface potential of the MoS 2 layers [24]. With this exception, distribution of the surface potential within each layer is relatively homogeneous within the 20 mV accuracy.…”

“…In both cases the obtained screening length is significantly larger than in graphene, where a strong-coupling regime is achieved already at ∼1-2 nm, albeit strongly depending on the initial charge density [18,19]. However, in a recent experiment the screening length of ∼2.96 nm was reported [24], making it similar to the one in graphene despite their significant differences in the conductivity and the electronic structure. Thus, the observed electrostatic properties of layered MoS 2 were attributed to a weakcoupling regime leading to the reduced screening properties.…”

“…∼100 mV increase as compared to the main part of a single flake. This is most likely due to suppressed charge transfer for the suspended monolayer compared to the supported one [22,24]. The effect of the trench on the absolute value of the enhanced surface potential is significantly less pronounced in the case of thicker (5 L and 8 L) flakes.…”

Unconventional electroabsorption in monolayer MoS 2

“…In particular, if the latter two change by different amounts, this results in a field-induced change in the distance between the exciton and trion peaks. On the other hand, the spectral properties of MoS 2 are sensitive to the dielectric environment [29,30]. In the momentum-resolved photoluminescence study MoS 2 was sandwiched between microscope oil and quartz, both dielectrics with relatively low ε r .…”

Tuning the optical emission of MoS2 nanosheets using proximal photoswitchable azobenzene molecules

“…Therefore, heterostructures made by sandwiching SAMs between a metallic film and a monolayer of a 2D crystal are expected not only to tune the electronic properties of the 2D materials, as demonstrated previously by Margapoti et al, 22 but also to optically modulate their spectroscopic properties. Therefore, keeping in mind that the optoelectronic properties of MoS 2 are influenced by substrate, 23,24 it is expected that the use of photochromic molecules as electron donors/acceptors should result in the photo-tuning and -switching of the MoS 2 optical properties. In this work, we report photoswitchable optical properties of 1L-and two-layer (2L)-MoS 2 and the tunability of the MoS 2 optical properties by means of a proximal mixed selfassembled monolayers (mSAMs) of photochromic azobenzene molecules.…”