Nanoscale Heterogeneities in Monolayer MoSe₂ Revealed by Correlated Scanning Probe Microscopy and Tip-Enhanced Raman Spectroscopy

Abstract

Understanding growth, grain boundaries (GBs), and defects of emerging two-dimensional (2D) materials is key to enabling their future applications. For quick, nondestructive metrology, many studies rely on confocal Raman spectroscopy, the spatial resolution of which is constrained by the diffraction limit (∼0.5 μm). Here we use tip-enhanced Raman spectroscopy (TERS) for the first time on synthetic MoSe2 monolayers, combining it with other scanning probe microscopy (SPM) techniques, all with sub-20 nm spatial resolution. We uncover strong nanoscale heterogeneities in the Raman spectra of MoSe2 transferred to gold substrates [one near 240 cm–1 (A1′), and others near 287 cm–1 (E′), 340 cm–1, and 995 cm–1], which are not observable with common confocal techniques and appear to imply the presence of nanoscale domains of MoO3. We also observe strong tip-enhanced photoluminescence (TEPL), with a signal nearly an order of magnitude greater than the far-field PL. Combining TERS with other SPM techniques, we find that GBs can cut into larger domains of MoSe2, and that carrier densities are higher at GBs than away from them.

Recommended Methods to Study Resistive Switching Devices

“…The best quality material is normally achieved by mechanical exfoliation, but this leads to small material flakes (typically <10 µm) with uncontrollable thicknesses, [78] and it requires EBL to pattern the electrodes. [7] CVD can be used to grow high quality graphene, [80] molybdenum disulfide (MoS 2 ), [81] molybdenum diselenide (MoSe 2 ), [82] tungsten disulfide (WS 2 ), [83] tungsten selenide (WSe 2 ), [84] and hexagonal boron nitride (h-BN), [85][86][87] among many others. The two most widespread methods to synthesize 2D materials applied to RS devices are chemical vapor deposition (CVD) and liquid-phase exfoliation.…”

“…The two most widespread methods to synthesize 2D materials applied to RS devices are chemical vapor deposition (CVD) and liquid-phase exfoliation. [89][90][91] A solution commonly employed is to synthesize the 2D material on the most suitable substrates (metallic foils for graphene [80] and h-BN [85][86][87] and SiO 2 or sapphire for 2D transition metal dichalcogenides (TMDs) [81][82][83] ) and transfer it on the desired sample using different methods, [92][93][94] being the wet transfer with the assistance of a polymer scaffold the most used by the RS community. The problem is that the temperature used for the growth is typically >700 °C, which prevents growing the 2D material on wafers with existing integrated circuits due to diffusion problems; the maximum temperature allowed for complementary metal-oxidesemiconductor (CMOS) back-end of line integration is typically 450 °C.…”

“…CVD can be used to grow high quality graphene, molybdenum disulfide (MoS 2 ), molybdenum diselenide (MoSe 2 ), tungsten disulfide (WS 2 ), tungsten selenide (WSe 2 ), and hexagonal boron nitride ( h ‐BN), among many others. The problem is that the temperature used for the growth is typically >700 °C, which prevents growing the 2D material on wafers with existing integrated circuits due to diffusion problems; the maximum temperature allowed for complementary metal‐oxide‐semiconductor (CMOS) back‐end of line integration is typically 450 °C .…”

Advances in Scanning Thermal Microscopy Measurements for Thin Films

“…Another method proposed in 2008 to perform measurements in ambient condition, was proposed using a double scan technique [28]. Most of these contributions can be separated in applications to 1D structures, such as nanowires [29,30] or carbon nanotubes [31,32], 3D materials [33], thin films [34] (which are the objective of the present chapter), and in the last years, the SThM technique has also been successfully applied to the study of the emerging field of 2D materials [35].…”

In-situ Raman analysis of hydrogenation in well-defined ultrathin molybdenum diselenide deposits synthesized through vapor phase deposition

“…Vibrational spectroscopy, e.g. Raman spectroscopy is an effective characterization method for layered materials and correlates with size and thickness of deposits 12 , edge and grain boundary effects 13,14 , and chemisorption and physisorption of active species [15][16][17] .…”