High Performance Semiconducting Nanosheets via a Scalable Powder-Based Electrochemical Exfoliation Technique

Abstract

The liquid-phase exfoliation of semiconducting transition metal dichalcogenide (TMD) powders into 2D nanosheets represents a promising route toward the scalable production of ultrathin high-performance optoelectronic devices. However, the harsh conditions required negatively affect the semiconducting properties, leading to poor device performance. Herein we demonstrate a gentle exfoliation method employing standard bulk MoS 2 powder (pressed into pellets) together with the electrochemical intercalation of a quaternary alkyl ammonium. The resulting nanosheets are produced in high yield (32%) and consist primarily of mono-, bi-, triatomic layers with large lateral dimensions (>1 μm), while retaining the semiconducting polymorph. Exceptional optoelectronic performance of nanosheet thin-films is observed, such as enhanced photoluminescence, charge carrier mobility (up to 0.2 cm 2 V −1 s −1 in a multisheet device), and photon-to-current efficiency while maintaining high transparency (>80%). Specifically, as a photoanode for iodide oxidation, an internal quantum efficiency up to 90% (at +0.3 V vs Pt) is achieved (compared to only 12% for MoS 2 nanosheets produced via ultrasonication). Further using a combination of fluorescence microscopy and high-resolution scanning transmission electron microscopy (STEM), we show that our gently exfoliated nanosheets possess a defect density (2.33 × 10 13 cm −2 ) comparable to monolayer MoS 2 prepared by vacuum-based techniques and at least three times less than ultrasonicated MoS 2 nanoflakes. Finally, we expand this method toward other TMDs (WS 2 , WSe 2 ) to demonstrate its versatility toward high-performance and fully scalable van der Waals heterojunction devices.

High-Mobility Flexible Transistors with Low-Temperature Solution-Processed Tungsten Dichalcogenides

“…In Figure e, we characterize the electrochemical MoS 2 transistors using a probe station at atmospheric pressure and in ambient air. The transfer characteristic ( V ds = 1 V) shows n-type behavior, which is typical of MoS 2 for the ⟨ L ⟩ ≈ 1040 nm and ⟨ L ⟩ ≈ 605 nm flake network consistent with previous reports, ,, but ambipolar behavior for the ⟨ L ⟩ ≈ 484 nm flakes, possibly due to doping from residual polymer, solvent, or oxygen edge functional groups, which has been observed previously in MoS 2 flakes . The ambient μ values for each MoS 2 network (Figure f) are calculated to be μ 1040 ≈ 10.7 ± 0.9 cm 2 V –1 s –1 ( N = 9), μ 605 ≈ 4.0 ± 1.3 cm 2 V –1 s –1 ( N = 3) and μ 484 ≈ 0.008 ± 0.002 cm 2 V –1 s –1 ( N = 3) with I on / I off ≈ (2.6 ± 0.4) × 10 3 , (3.3 ± 1.9) × 10 3 , and 28 ± 12 for MoS 2 flakes of ⟨ L ⟩ ≈ 1040, 605, and 484 nm, respectively.…”

Assembling a Photoactive 2D Puzzle: From Bulk Powder to Large-Area Films of Semiconducting Transition-Metal Dichalcogenide Nanosheets

“…However, our lab recently presented a preparation route starting with inexpensive, commercially available powder andusing the electro-intercalation of the large alkylammonium cationsafforded the scalable production of high-quality TMD nanosheets. A schematic of this method is shown in Figure a and can be called electrochemical pellet intercalation (ECPI) . Briefly, commercial TMD powder is pressed into a pellet and then annealed in a sealed container with excess chalcogenide powder.…”

“…Notably any remaining intact portion of the pellet can be collected, dried, and reused in subsequent rounds of ECPI. It should also be stressed that the method is highly adaptable as described in more detail in our publication in ACS Nano …”

“…In that recent work we describe ECPI-made MoS 2 nanosheets in detail and demonstrate their improved optoelectronic performance compared to ultrasonication-produced MoS 2 nanoflakes. It was shown that the ECPI method gives ultrathin nanosheets with high aspect ratios and lateral dimensions frequently surpassing 1 μm as observed in the transmission electron microscope, TEM, images in Figure b,c.…”

“…A schematic of this method is shown in Figure 2a and can be called electrochemical pellet intercalation (ECPI). 30 Briefly, commercial TMD powder is pressed into a pellet and then annealed in a sealed container with excess chalcogenide powder. Annealing conditions depend on the TMD used, for example, 1100 °C for 48 h with 2.3 °C/min ramp and a natural cooling cycle for MoS 2 versus 1000 °C for 12 h with 2.1 °C/min ramp for WSe 2 .…”