Influence of Gas Adsorption and Gold Nanoparticles on the Electrical Properties of CVD-Grown MoS₂ Thin Films

Molybdenum disulfide (MoS₂) has increasingly attracted attention from researchers and is now one of the most intensively explored atomic-layered two-dimensional semiconductors. Control of the carrier concentration and doping type of MoS₂ is crucial for its application in electronic and optoelectronic devices. Because the MoS₂ layers are atomically thin, their transport characteristics may be very sensitive to ambient gas adsorption and the resulting charge transfer. We investigated the influence of the ambient gas (N₂, H₂/N₂, and O₂) choice on the resistance (<i>R</i>) and surface work function (WF) of trilayer MoS₂ thin films grown via chemical vapor deposition. We also studied the electrical properties of gold (Au)-nanoparticle (NP)-coated MoS₂ thin films; their <i>R</i> value was found to be 2 orders of magnitude smaller than that for bare samples. While the WF largely varied for each gas, <i>R</i> was almost invariant for both the bare and Au-NP-coated samples regardless of which gas was used. Temperature-dependent transport suggests that variable range hopping is the dominant mechanism for electrical conduction for bare and Au-NP-coated MoS₂ thin films. The charges transferred from the gas adsorbates might be insufficient to induce measurable <i>R</i> change and/or be trapped in the defect states. The smaller WF and larger localization length of the Au-NP-coated sample, compared with the bare sample, suggest that more carriers and less defects enhanced conduction in MoS₂

Band Alignment at Au/MoS₂ Contacts: Thickness Dependence of Exfoliated Flakes

We investigated the surface potential (<i>V</i>_surf) of exfoliated MoS₂ flakes on bare and Au-coated SiO₂/Si substrates using Kelvin probe force microscopy. The <i>V</i>_surf of MoS₂ single layers was larger on the Au-coated substrates than on the bare substrates; our theoretical calculations indicate that this may be caused by the formation of a larger electric dipole at the MoS₂/Au interface leading to a modified band alignment. <i>V</i>_surf decreased as the thickness of the flakes increased until reaching the bulk value at a thickness of ∼20 nm (∼30 layers) on the bare and ∼80 nm (∼120 layers) on the Au-coated substrates, respectively. This thickness dependence of <i>V</i>_surf was attributed to electrostatic screening in the MoS₂ layers. Thus, a difference in the thickness at which the bulk <i>V</i>_surf appeared suggests that the underlying substrate has an effect on the electric-field screening length of the MoS₂ flakes. This work provides important insights to help understand and control the electrical properties of metal/MoS₂ contacts

Asymmetrically Coupled Plasmonic Core and Nanotriplet Satellites

Here, we report asymmetrical multiple electromagnetic coupling of plasmonic core and nanotriplet satellites. Within the plasmonic core and nanotriplet satellites, an enhanced local field is generated which expands across the core due to multiple electromagnetic coupling between a core and nanotriplets. Based on 3D simulations of our plasmonic nanosystem, the overall local field enhancement reaches to over 10⁴ times, compared with that of a single nanoparticle array. A strong local field distribution across the core to nanotriplets as well as the critical role of the plasmonic core is demonstrated through the 3D simulations. It is proposed that a self-assembled nanotriplet array is completed through two stages of dewetting of a gold thin film on an anodic aluminum oxide (AAO) template. Formation of the core–nanotriplet satellites is significantly influenced by geometrical parameters (i.e., the pore diameter and depth) of the AAO template. Our experimental results show that the local field of our plasmonic nanostructures is amplified up to ∼110 times by adopting a core into the nanotriplet satellites, compared with that of the nanotriplets array without a core. This approach offers a promising strategy for creating an advanced nanoplasmonic platform with strong local field distribution and high-throughput production