Structural defects in a nanomesh of bulk MoS2 using an anodic aluminum oxide template for photoluminescence efficiency enhancement

Two-dimensional (2D) materials beyond graphene have attracted considerable interest because of the zero bandgap drawbacks of graphene. Transition metal dichalcogenides (TMDs), such as MoS2 and WSe2, are the potential candidates for next 2D materials because atomically thin layers of TMDs exhibit unique and versatile electrical and optical properties. Although bulk TMDs materials have an indirect bandgap, an indirect-to-direct bandgap transition is observed in monolayers of TMDs (MoS2, WSe2, and MoSe2). Optical properties of TMD films can be improved by the introduction of structural defects. For example, large-area spatial tuning of the optical transition of bulk MoS2 films is achieved by using an anodic aluminum oxide (AAO) template to induce structural defects such as edge- and terrace-terminated defects in a nanomesh structure. Strong photoluminescence emission peaks with a band gap of 1.81 eV are observed, possibly because of radiative transition at the defect sites. This work shows that the AAO template lithography method has potential for the production of homogenous large-scale nanomesh structures for practical semiconductor processing applications in future MoS2-based electronic and optical devices.

Atomically thin layers of semiconducting transition metal dichalcogenides (TMDs), new semiconducting beyond graphene, have attracted attention for next-generation optoelectronics and electronics owing to their distinctive optical and electrical characteristics, providing tremendous opportunities in the fabrication of nanoscale optical and electrical devices [1][2][3][4][5] . In particular, two-dimensional (2D) monolayer (ML) TMDs have significantly enhanced optical properties owing to their indirect-to-direct band gap transition [6][7][8][9][10][11][12] . Previous studies have reported the indirect-to-direct band gap transition in monolayers of TMDs MX 2 (M = Mo, W and X = S, Se, Te) [i.e. MoS 2 (1.3 to 1.89 eV) 10 , MoSe 2 (1.41 to 1.55 eV) 11 , MoTe 2 (0.88 to 1.02 eV) 9 , WS 2 , (1.43 to 1.97 eV) 12 , and WSe 2 (1.4 to 1.65 eV) 12 ] materials. The synthesis of large scale monolayers of TMDs using chemical vapor deposition (CVD), however, is difficult to achieve due to the problems in controlling the highly uniform atomic monolayer 12,13 . An alternative method for tuning the band gap of the 2D materials is engineering a nanostructural defect using a template patterned with nano-sized holes and a strain-induced using a textured structure and mechanical strain [14][15][16][17] . It is well established that structural defect engineering in graphene can significantly affect its optical and electrical properties 13,14,[18][19][20] . Nanomesh graphene using an anodic aluminum oxide (AAO) template and block copolymer lithography exhibit band gap opening, which overcomes the material's limitation of zero bandgap in graphene 13,14 . However, in TMD materials, structural defect engineering is not straightforward because of their complicated ScIENtIfIc RePoRTS | (2018) 8:6648 | DOI:10.1038/s41598-018-25045-z structure and alloy system. There are only few reports on the effect of structural defects on electrical and optical properties of monolayer MoS 2 21,22 .
Here, a nanomesh-MoS 2 structure was generated using an anodic aluminum oxide (AAO) template. The AAO template is used widely to provide large scale nano-patterned structures. The self-organization process by anodization can lead to a densely packed nano-sized hole array. The hole size and neck width of the AAO template was tuned from 20 nm to 250 nm, resulting from anodization in an oxalic acid solution. Note that bulk MoS 2 employing a nano-hole array structure exhibited remarkable enhancement in luminescence efficiency, because of the presence of structural defects. More importantly, this film exhibited strong photoluminescence (PL) emission peaks with a band gap energy of 1.81 eV, possibly as a result of dominantly radiative recombination excitons at defect sites. This paper reports the potential of the AAO template lithography method for the production of a homogenous large-scale nanomesh structure for practical semiconductor processing applications in future MoS 2 -based electronic and optical devices.

Results and Discussion
Multilayer MoS 2 flakes were obtained by the mechanical exfoliation of bulk MoS 2 crystals and transferred to SiO 2 /Si substrates. Figure 1 shows the process sequence to form a MoS 2 nanomesh. First, a nanoporous Al 2 O 3 membrane was achieved by the anodization of a high purity aluminum disk (99.999%). Details for achieving the nanoporous Al 2 O 3 membrane are provided in the experimental section. The residual aluminum layer was then etched selectively in a mixture solution of CuCl 2 (13 g), HCl (200 mL), and deionization (DI) water (400 mL). In general, the nanopores on top of the Al 2 O 3 layer were rough because of the massive cracks resulting from the continuous growth of the strained Al 2 O 3 layer from the Al-Al 2 O 3 interface 23 . Therefore, the barrier oxide layer at the bottom of the Al 2 O 3 layer was etched slightly using a mixture of H 3 PO 4 (15.15 mL) and DI water (500 mL) to open the pores on the back side of the Al 2 O 3 layer. The yield of the nanoporous Al 2 O 3 membrane was more than 90%, with uniform hole size and density under optimal process conditions. A 30-nm-thick Au layer was deposited on the open pores of the Al 2 O 3 layer. The Al 2 O 3 layer was then removed in a KOH (10 g) and DI water (500 mL) solution to form the Au nanomesh layer. Subsequently, the Au nanomesh was dipped in an aqua regia solution (HCL:HNO 3 :H 2 O = 3:1:2) to smooth the edges of the Au nano-holes. The Au nanomesh layer was then floated on the surface of DI water, and transferred to the MoS 2 /Si substrate by scooping with the substrate. The bulk MoS 2 layer was etched selectively in a reactive ion etch system with CF 4 as the carrier gas. Finally, the nanomesh MoS 2 layer was achieved after removing the Au layer in gold etchant TFA (Transene Company, Inc.).
The pore size of the nanomesh pattern used in this study was controlled by changing the applied bias when forming the AAO template. The pore diameter and neck width of the AAO template increased from 30 to 200 nm and from 40 to 100 nm with increasing anodizing voltage ( Supplementary Fig. S1). The morphology of the nanomesh bulk MoS 2 materials was examined by scanning electron microscopy (SEM) and atomic force microscopy (AFM). When a 10nm-thick Au template with a hole size of 30 nm and a neck width of 40 nm was used as the masking layer (Fig. 2) and CF 4 plasma etching was performed for 6 min, nanomesh MoS 2 with a hole size of 45 nm and a neck width of 30 nm was obtained (Fig. 2a,b). Au was used as a etch mask to fabricate the nanomesh MoS 2 because it is a highly selective etch mask for oxygen plasma and does not form hybrids by a chemical process 24 . In addition, the Au layer has very high flexibility, so it does not crumble easily under the wet-chemical based transfer process, as shown in Fig. 2. The unsuitable etching process conditions (i.e. excessive plasma power and insufficient etching time) restrict the formation of a perfect nanomesh structure (Supplementary Fig. S2). The AFM image, as shown in Fig. 3c, confirmed that a 32 nm-thick nanomesh MoS 2 with a mean hole size of 40 nm was determined, which is in good agreement with that obtained from SEM (Fig. 3b).
Raman spectroscopy was performed to examine the structural defects of the nanomesh MoS 2 with a hole size of 80 nm and a neck width of 10 nm. The bulk MoS 2 flake possesses the general Raman spectrum with two  2g , corresponding to 5 and 6 cm −1 , respectively, as shown in Fig. 4a. Moreover, the E 1 2g mode (corresponding to in-plane vibration mode) is preferentially excited for terrace-terminated films, while the A 1g mode (corresponding to out-of-plane vibration mode) is dominantly excited for edge-terminated films 25 . The red shift of E 1 2g could be attributed to the abundant terrace-terminated defects in the nanomesh film. The red-shifted out-of-plane (A 1g ) located at 402 cm −1 could be explained by the numerous edge-terminated films as well as the doping effects during the process of a MoS 2 nanomesh 26,27 . For solvents (i.e. gold etchant TFA and aqua regia) used and CF4 treatment using reactive-ion etching (RIE) for the fabrication, no obvious PL and Raman shift was observed ( Supplementary  Fig. S3), which suggests no doping and no RIE-induced surface defects of the nanomesh MoS 2 . The Raman spectra peak difference values of 25 and 24 cm −1 between the two Raman modes observed in the MoS 2 flake and nanomesh MoS 2 materials, respectively, were correlated with bulk MoS 2 28 . The Raman spectra mapping and measured at three different areas showed similar peak positions ( Supplementary Fig. S4), indicating the area homogeneity of the optical characteristics of nanomesh MoS 2 .
The optical properties of nanomesh MoS 2 flakes with a hole size of 80 nm and a neck width of 10 nm were determined by micro-PL spectroscopy. The effect of thickness of bulk MoS 2 on PL and Raman spectra can be ignored when the thinkness of bulk MoS 2 is over 6 layers 29,30 . The strain-induced 16,[31][32][33] , structural defects (i.e., grain boundaries, edge, and point defects 21,28,34 ) and doping 26 affect the band structure (i.e., band gap engineering, a direct-to-indirect band gap transition, and semiconductor-to-metal transition) and excitonic optical transition of monolayer and bilayer MoS 2 . Emerging PL was observed in the nanomesh MoS 2 flake (Fig. 4b), indicating a highly efficient optical transition. Although the bulk MoS 2 flake as a reference does not exhibit a prominent PL peak, a remarkable PL peak was located at 1.81 eV, which is a slightly lower value than the band gap of monolayer MoS 2 film at approximately 1.84 eV 10,16,31,32,35 . The broadening of the PL full width half maximum (FWHM) was associated with the radiative transition efficiency. The nanomesh MoS 2 resulted in a significantly narrower PL FWHM of 59.4 nm as compared to that for the bulk MoS 2 flake (FWHM of 103.7 nm), although the FWHM of the former is slightly wider than that of monolayer MoS 2 (FWHM = 36.3 nm)previously obtained by chemical vapor deposition 35 . Although the radiative transition efficiency of nanomesh MoS 2 is relatively lower than that of monolayer MoS 2 , we believe that this behavior provides evidence of defects-related radiative recombination on the nanomesh structure. The repeatability and reproducibility of AAO method was confirmed by optical characteristics of another nanomesh MoS 2 . (Supplementary Fig. S5) Further simulation and microstructural study, including first-principles calculations and scanning transmission electron microscopy will be necessary to fully understand the effect of structural defects on exciton transition in nanomesh bulk MoS 2 .

Conclusion
In conclusion, this paper reported a straightforward and practical method for the high yield production of large-area nanomesh bulk MoS 2 flake using an AAO template. Optical properties of TMDs films can be improved by introducing structural defects in them. Spatial structural defects including edge-and terrace-terminated defects employing a nanomesh structure can be tuned to yield PL enhancement in a bulk MoS 2 film. Structural defects-induced in bulk MoS 2 flake using a nanomesh structure effectively lead to a highly efficient radiative transition. These results suggest that a structural defect-induced nanomesh bulk MoS 2 film could be suitable for the versatile applications such as the photonic, optoelectronics, and photovoltaic applications.

Method
Preparation of the AAO template: A self-ordered nanomesh Al 2 O 3 template was generated by the anodization of aluminum. A high purity aluminum disk (Goodfellow, Inc.) of 2 cm diameter was prepared. The aluminum disk was dipped in a mixed solution of perchloric acid and ethanol (1:3) at 1 °C with an applied voltage of 20 V to smoothen the substrate surface. The aluminum disk was then anodized in 0.3 mole of oxalic acid. The anodizing voltage was set in the range of 40 to 120 V to control hole size and neck width. The aluminum substrate was maintained at 1 °C using a cooling stage placed in thermal contact with the substrate. Parameters such as anodizing voltage, anodizing solution, anodizing time and solution temperature affected nanohole size and density of the nanomesh structure. The yield of the nanomesh structure produced by the AAO template was very high for the optimal process conditions. The size and density of the nanohole was uniform on the substrate of 2 cm diameter substrate, except the along the edges.
Characterization: The AAO template and nanomesh MoS 2 flakes were examined by SEM (S-4800, HITACHI). The Raman measurements with the excitation laser line of 488 nm were performed using a Renishaw Raman spectroscope integrated. The power of the excitation laser was maintained at 0.9 mW to avoid heating effects. The Raman emission was collected using a Leica 100 × objective (N.A. = 0.8) and 1800 (for the Raman measurements in Fig. 4a) lines mm −1 gratings. The Renishaw Raman spectra with 1200 lines mm −1 grating and 1800 lines mm −1 grating had a spectral resolution of approximately 1 cm −1 . A laser with an excitation wavelength of 488 nm and a spot size of 0.75 µm was used. The Si peak was used for normalization. Steady-state PL (LabRam ARAMIS, Horiba Jobin Yvon) measurements were performed at room temperature using a 514 nm-wavelength laser diode with 100 mW power and a beam size of 1 µm. The morphology, hole size, and neck width were evaluated by AFM (VEECO Dimension 3100 + Nanoscope V(Version 7.0), VEECO). For better quality, an image was obtained using a super sharp silicon tip with a radius of curvature of <10 nm (Appnano). The image was taken over a 5 μm 2 area and a measurement speed of 0.698 Hz.