Jin-Woo Cho

MoS2 monolayers on Si and SiO2 nanocone arrays: influences of 3D dielectric material refractive index on 2D MoS2 optical absorption

Heterostructures enable the control of transport and recombination of charge carriers, which are either injected through electrodes, or created by light illumination. Instead of full 2D-material-heterostructures in device applications, using hybrid heterostructures consisting of 2D and 3D materials is an alternative approach to take advantage of the unique physical properties of 2D materials. In addition, 3D dielectric nanostructures exhibit useful optical properties such as broadband omnidirectional antireflection effects and strongly concentrated light near the surface. In this work, the optical properties of 2D MoS2 monolayers conformally coated on 3D Si-based nanocone (NC) arrays are investigated. Numerical calculations show that the absorption in MoS2 monolayers on SiO2 NC is significantly enhanced, compared with that for MoS2 monolayers on Si NC. The weak light confinement in low refractive index SiO2 NC leads to greater absorption in the MoS2 monolayers. The measured photoluminescence and Raman intensities of the MoS2 monolayers on SiO2 NC are much greater than those on Si NC, which supports the calculation results. This work demonstrates that 2D MoS2-3D Si nano-heterostructures are promising candidates for use in high-performance integrated optoelectronic device applications.

Broadband omnidirectional absorption enhancement of MoS2 monolayers on sub-100-nm-thick SiO2/Si wafers (Conference Presentation)

MoS2, a representative 2D atomically thin semiconductor, has a sizable band gap leading to intensive research efforts to investigate its unique optical properties and realize a novel optoelectronic device based on MoS2. However, limited optical absorption in extremely thin MoS2 layers is an obstacle for high-efficiency light absorbing devices. In this work, we investigated how reflection and transmission phase-shift at the highly absorbing MoS2 interface could affect the absorption spectra of the MoS2 monolayers on SiO2/Si substrates (SiO2 thickness: 40 ~ 130 nm). Such interface-phase-shift gave rise to interference in MoS2 layer, although the layer thickness was only 0.7 nm, much smaller than the wavelength of the visible light. We compared measured and calculated optical reflection spectra, which showed that aforementioned interface-interference enhanced optical absorption in MoS2 monolayers. Raman intensity of MoS2 monolayers largely varied depending on the SiO2 thickness, which could be well explained by the calculated absorption in MoS2 layers. In addition, the interface-interference enabled omnidirectional absorption enhancement. This work showed that proper choice of the SiO2 thickness could provide us a simple and useful means to improve optical absorption in MoS2 monolayers.