Interference‐Enhanced Broadband Absorption of Monolayer MoS₂ on Sub‐100 nm Thick SiO₂/Si Substrates: Reflection and Transmission Phase Changes at Interfaces

Abstract

bandgap energy of TMDCs is in the visible range, and thus intensive research efforts have been devoted to realize highefficiency TMDC-based optoelectronic devices. [1][2][3][4][5][6][7] TMDC materials have usually been prepared on SiO 2 /Si wafers by mechanical exfoliation from the bulk or thin film deposition for physical characterization and device fabrication. After TMDC sample preparation, Raman and photoluminescence (PL) spectroscopic measurements are usually performed to identify the thickness and bandgap of the sample. [8][9][10][11][12][13][14][15][16][17][18][19][20][21] The optical absorption and emission signals of the sample become larger as the electric field (E-field) intensity of the incident light in the sample increases. In the SiO 2 /Si substrates, the light undergoes multiple reflections in the SiO 2 layer, leading to interference between its partial waves. Local maxima of the E-field intensity appear when the SiO 2 thickness is equal to (2m − 1)λ/4n SiO2 , where m is a positive integer, λ is the light wavelength, and n SiO2 is the refractive index of SiO 2 , which are identified as Fabry-Perot (FP) resonances. [9][10][11][12][13] At every FP resonance, TMDC layers on SiO 2 /Si substrates can produce very strong Raman and PL intensities. To take advantages of the FP resonance, the dielectric layer is at least a quarter-wave in thickness and most of the researchers have used SiO 2 /Si substrates with ≈300 nm thick SiO 2 layers. [9][10][11][12][13] The light-matter interactions in extremely thin TMDC layers can be strengthened using plasmonic nanoantennae, photonic nanostructures, and the FP cavities. [11,[13][14][15][16][17][18][19][20] Generally, the absorption enhancement is observed only within narrow ranges of the wavelength and angle of the incident light. [9][10][11][12][13][14][15][16][17][18][19][20] To aid material characterization, we can choose an optimal nanostructure to maximize the absorption of a TMDC sample at a specific wavelength. However, such wavelength-and anglesensitive absorption is inappropriate for some applications, including photovoltaics, photodetection, and photocatalysis. [5][6][7] As an alternative approach, Jariwala et al. [7] demonstrated nearunity, broadband absorption in <15 nm thick TMDC layers on Ag and Au back-reflecting substrates. Because TMDC materials usually have a complex refractive index n n ik  = + with an extremely high extinction coefficient (k) in the visible spectrum, The optical characteristics of MoS 2 monolayers on SiO 2 /Si substrates with an SiO 2 thickness ranging from 40 to 130 nm are investigated. The measured Raman and optical reflection spectra of the MoS 2 monolayers vary considerably depending on the SiO 2 thickness. The Raman peak intensity of the MoS 2 monolayer on the substrate with an 80 nm thick SiO 2 layer is four times larger than those in the cases of 40-and 130 nm thick SiO 2 layers, indicating a significant difference in the absorption at the excitation wavelength. The incident light undergoes anomalous phase changes upon r...

MoS2 Monolayers on Au Nanodot Arrays: Surface Plasmon, Local Strain, and Interfacial Electronic Interaction

“…The surface morphology (Figure a,d) observed by AFM suggested that the area fraction of the suspended MoS 2 region near the nanodots was very small, as reported in a tip-enhanced Raman spectroscopy study . Wrinkles were detected on the MoS 2 surface, which may have formed during the wet transfer process of the relatively large MoS 2 layers (about 1 cm 2 ) . Except for the nanodot boundaries and wrinkles, the MoS 2 layers followed the surface morphology of the AuNA and AO-AuNA substrates.…”

“…The overall PL intensity of MoS 2 /AuNA was larger than that of MoS 2 /AO-AuNA, although the PL intensity showed some spatial variation for both samples. The spatial resolution of the confocal PL measurement was about 550 nm, which is larger than the size of the nanodots. Thus, the light confinement effects near the nanodots were difficult to observe in the PL maps.…”

“… Hereafter, the bare Au nanodot arrays and Al 2 O 3 -coated Au nanodot arrays are denoted as AuNAs and AO-AuNAs, respectively. MoS 2 monolayers were grown on 300-nm-thick SiO 2 /Si substrates using chemical vapor deposition and then transferred onto both kinds of nanodot arrays using a wet transfer process …”

MoS2/Graphene Composites as Promising Materials for Energy Storage and Conversion Applications

“…This unique material, with its special layered structure, has attracted significant interest in energy-based research. [27][28][29][30] It has many advantages: (1) extraordinary crystal, physical, chemical, electronic and photosensitive properties; [18,[31][32][33][34][35][36][37][38] (2) bandgap-adjustable properties (direct band-gap of ∼1.8 eV), potentially to be applied in electronic devices, which promotes applications containing energy conversion and storage, catalysts, and electronic devices; [31,[39][40][41][42] (3) the 2D nanostructure of MoS 2 offers a large number of active sites. These sites originate from the under-coordinated sulphur atoms at the edges, thus leading to platinum-like catalysis activity.…”

Atomically Thin Semiconducting Transition-Metal Dichalcogenides: From Synthesis to Electro-Optical Properties

“…This gives rise to two peaks in the optical absorption spectra of MoS 2 , identified as A (1.85 eV) and B (2 eV) excitons [11]. Considerable efforts have been made to study the absorption and transmission properties of these materials [40][41][42].…”