In 2014, I joined Jiangnan University and set up low-dimensional semiconductor materials and device laboratory. I leaded my students and independently built a planar low-frequency inductively coupled plasma (ICP) system. The plasma system is extraordinary compared to the traditional ICP sources since it can operate in both inductive discharge mode (H-mode) and capacitive discharge mode (E-mode). In H-mode, the electromagnetic field induced by the mutual induction between the coil and plasma dominates the plasma so that the electron/ion density is extremely high (~1012 cm-3), and the positive ions drived by the electromagnetic field can easily bombard onto the treated surface and thus cause excessive damage to the treated sample. In E-mode, the capacitive coupling originating from the radial potential drop across the two ends of the planar induction coil dominates the plasma and produces radial electrostatic field parallel to the substrate surface. At this stage, direct ionization of feedstock gas by radial electrostatic field takes place but most of the generated electrons do not gain sufficient kinetic energy to initiate further ionization collisions. This leads to low ionization rate and low electron/ion density (of the order of magnitude of 108-1010 cm-3) in this regime. As such, the ion bombardment effect and the resultant surface damage are greatly suppressed by both reducing the ion density and restricting the movement direction of positive ions. This is why we call such E-mode discharge as soft plasma. The typical pictures of both H-mode and E-mode discharges are shown in Figure 1.
Such soft plasma is very promising in surface modification, layer thinning and doping of 2D materials with atomic layer thickness. In 2016, we indeed developed a soft, selective and high-throughput atomic-layer-precision etching of 2D transition metal dichalcogenides (TMDs) in SF6+N2 plasmas using such E-mode discharge technology [Scientific Rep. 2016, 6, 19945]. This approach is based on chemical reactive etching by the reactive radicals generated in such plasmas other than ion bombardment etching. After that, my students, group member Dr. Haiyan Nan and I have also developed a facile and reversible phase engineering technique between semiconducting hexagonal (2H) phase and distorted octahedral metallic phase (1T’) phase for MoTe2 nanoflakes based on such soft hydrogen plasma treatment [Nanotechnol. 2019, 30, 034004].
Recently, we are attracted by 2D atomic crystal superlattices which possess a wide range of adjustable electronic properties and thus offer technological opportunities and applications beyond the reach of existing 2D materials. Motivated by the liquid intercalation strategy in which 2D superlattice structures are produced by the intercalation of selected 2D atomic crystals with alkali metal ions or ammonium bromide molecular layers, we believed that plasma intercalation can also produce 2D superlattices as long as the plasma electric field can be kept being parallel to the interlayer space of 2D materials. This feature is highly coinciding with the characteristic of our soft plasma. In 2017, my students and I started to explore the plasma intercalation effect on diverse 2D materials. After many years of intensive investigations, we do develop a soft oxygen plasma intercalation concept and demonstrate the 2D atomic crystal molecular superlattices where monolayer TMDs alternate with oxygen molecular layers. This dry method is effective for both mechanically exfoliated or CVD-grown TMD flakes (including MoS2, WS2, MoSe2, WSe2 and ReS2 etc.) with thicknesses ranging from 2 to 8 layers. By using MoS2 as a model system, we demonstrate that plasma intercalation with oxygen molecular layers produces MoS2[O2]x superlattices in which the interlayer distance increases from 0.6 nm to 0.9 nm compared to pure MoS2, thereby effectively decoupling the MoS2 monolayers. As such, the MoS2[O2]x superlattices display an extremely strong photoluminescence (PL) with the intensity approximately 100 times higher compared to pristine MoS2. The bilayer MoS2[O2]x/WS2[O2]x superlattice lateral heterostructures show much better photoelectric performance than the pristine bilayer MoS2/WS2 lateral heterostructures.
We expect that proper alignment of the plasma generated electric fields may lead to the observation of similar intercalation effects in other types of low-temperature plasmas. Our studies thus provide a potentially universal approach to create such 2D atomic crystal molecular superlattices from pristine 2D nanomaterials and provide a generic platform for fundamental physics research and potential technological applications.
If you are interested in finding out more about this work, please take a look at our paper published in Nature Communications: Xiao, S. Q. et al. 2D atomic crystal molecular superlattices by soft plasma intercalation. Nature Communications (2020, 11, 5960). https://www.nature.com/articles/s41467-020-19766-x
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