The influence of two-dimensional organic adlayer thickness on the ultralow frequency Raman spectra of transition metal dichalcogenide nanosheets

Abstract

Recently, it has been reported that physisorbed adsorbates can be trapped between the bottom surface of twodimensional (2D) materials and supported substrate to form 2D confined films. However, the influence of such 2D confined adsorbates on the properties of 2D materials is rarely explored. Herein, we combined atomic force microscopy (AFM), Kelvin probe force microscopy (KPFM) and Raman spectroscopy especially the ultralow frequency (ULF) Raman spectroscopy to explore the influence of 2D confined organic adlayer thickness on the ULF breathing modes of few-layer MoS 2 and WSe 2 nanosheets. As the thickness of organic adlayers increased, red shift, coexistence of blue and red shifts as well as blue shift of ULF breathing mode was observed. KPFM measurement confirmed the enhanced n-doping and p-doping behaviors of organic adlayers as their thickness increased, respectively. Our results will provide new insights into the interaction between 2D confined adsorbates and bottom surface of 2D nanosheets, which could be useful for modulating properties of 2D materials.

Thick Two-Dimensional Water Film Confined between the Atomically Thin Mica Nanosheet and Hydrophilic Substrate

“…Adlayers of small organic molecules (tetrahydrofuran and cyclohexane) confined between the graphene nanosheet and substrate have been imaged by atomic force microscopy (AFM) . Previously, we have also found that acetone and ethanol molecules can form adlayers with thicknesses varying from 0.3 to more than 3 nm when they were confined between a few-layer MoS 2 nanosheet and SiO 2 /Si substrate . Therefore, it is possible for acetone and ethanol molecules to form layered structure in the confined environment between mica nanosheet and SiO 2 /Si substrate.…”

“…Besides water, volatile organic molecules such as acetone and ethanol have also been reported to be trapped between the hydrophobic–hydrophilic and hydrophilic–hydrophilic interfaces. ,,, In order to investigate the influence of organic molecules on the stability of the water film, the stabilized water films under the mica nanosheets were incubated in two sealed boxes containing acetone and ethanol at room temperature for 1 h. As shown in Figure a,c, the thickness of the water film confined between the mica and substrate was measured as 1.88 nm. Meanwhile, 37% of the area of the mica nanosheet was occupied by the water film (Figure i).…”

“…22 Previously, we have also found that acetone and ethanol molecules can form adlayers with thicknesses varying from 0.3 to more than 3 nm when they were confined between a few-layer MoS 2 nanosheet and SiO 2 /Si substrate. 55 Therefore, it is possible for acetone and ethanol molecules to form layered structure in the confined environment between mica nanosheet and SiO 2 /Si substrate. In this work, the water molecules could be exchanged with the ethanol molecules when the thick water film confined between the mica nanosheet and SiO 2 /Si substrate was exposed to an ethanol vapor.…”

Layer dependence of stacking order in nonencapsulated few-layer CrI3

“…A complete understanding of the low-temperature lattice structure and stacking order is crucial for 2D vdW ferromagnetic materials. Raman spectroscopy is a powerful tool to study the crystal structures, lattice vibration and shift and interlayer coupling [22][23][24][25][26][27][28]. In this study, we investigate the layer, polarization and helicity dependencies of Raman features of non-encapsulated 2-5 layer and bulk CrI 3 at 10 K, demonstrating that few-layer and bulk CrI 3 are rhombohedral phase at low temperature, including antiferromagnetic bilayer CrI…”

Observation of room‐temperature out‐of‐plane switchable electric polarization in supported 3R‐MoS2 monolayers

“…With the advantages of high sensitivity and spatial resolution, KPFM has been widely used to probe the surface potential and work function of 2D materials. Therefore, KPFM is a unique and powerful tool to reveal the existence of trapped water/ice molecules, as the contact potential difference (CPD) and the work function of MoS 2 monolayers are sensitive to the molecules absorbed at the surface 20,42 . Figure 5A,C shows the topographic images of 3R‐stacked MoS 2 layers prepared under dry and ambient conditions, respectively.…”