Stabilizing the heavily-doped and metallic phase of MoS₂ monolayers with surface functionalization

Abstract

Monolayer molybdenum disulfide (MoS2) is one of the most studied two-dimensional (2D) transition metal dichalcogenides that is being investigated for various optoelectronic properties, such as catalysis, sensors, photovoltaics, and batteries. One such property that makes this material attractive is the ease in which 2D MoS2 can be converted between the semiconducting (2H) and metallic/semi-metallic (1T/1T’) phases or be heavily n-type doped 2H phase with ion intercalation, strain, or excess negative charge. Using n-butyl lithium (BuLi) immersion treatments, we achieve 2H MoS2 monolayers that are heavily n-type doped with shorter immersion times (10 – 120 mins) or conversion to the 1T/1T’ phase with longer immersion times (6 – 24 h); however, these doped/converted monolayers are not stable and promptly revert back to the initial 2H phase upon exposure to air. To overcome this issue and maintain the modification of the monolayer MoS2 upon air exposure, we use BuLi treatments plus surface functionalization p-(CH3CH2)2NPh-MoS2 (Et2N-MoS2)—to maintain heavily n-type doped 2H phase or the 1T/1T’ phase, which is preserved for over 2 weeks when on indium tin oxide (ITO) or sapphire substrates. We also determine that the low sheet resistance and metallic-like properties correlate with the BuLi immersion times. These modified MoS2 materials are characterized with confocal Raman/photoluminescence, absorption, X-ray photoelectron spectroscopy as well as scanning Kelvin probe microscopy, scanning electrochemical microscopy, and four-point probe sheet resistance measurements to quantify the differences in the monolayer optoelectronic properties. We will demonstrate chemical methodologies to control the modified monolayer MoS2 that likely extend to other 2D transition metal dichalcogenides, which will greatly expand the uses for these nanomaterials.

Scaling up Simultaneous Exfoliation and 2H to 1T Phase Transformation of MoS2

“…Also, it is well discussed in the recent literature that the thermodynamically unstable 1T‐MoS 2 was dynamically stable due to the increase in charge density caused by compressive strain. [ 56,57 ]…”

Scanning electrochemical probe microscopy investigation of two-dimensional materials

“…This map suggested that the phase conversion appears to proceed from the outside of the flake inwards and along straight lines, which was likely caused by a sliding of the S atomic plane. Zhang et al demonstrated the stabilization of heavily n-type doped 2H and 1T MoS 2 monolayers with a low reversion to the initial phase [117]. The nbutyl lithium immersion treatment converts the 2H phase to n-type 2H/1T', while surface functionalization stabilizes the phase.…”

Atomic and structural modifications of two-dimensional transition metal dichalcogenides for various advanced applications

“… 125 Furthermore, covalent functionalization of 1T′-MoS 2 with Et 2 NPh improves the stability of electronic transport properties for at least two weeks under atmospheric conditions. 126 …”