Nanoscale

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Cite this: Nanoscale, 2018, 10, 18920

Received 15th August 2018,Accepted 11th September 2018

DOI: 10.1039/c8nr06597g

rsc.li/nanoscale

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

Eunah Kim,a Jin-Woo Cho,b Tri Khoa Nguyen,c Trang Thi Thu Nguyen,a

Seokhyun Yoon, a Jun-Hyuk Choi,d Yun Chang Park,e Sun-Kyung Kim,*b

Yong Soo Kim*c and Dong-Wook Kim *a

Heterostructures enable the control of transport and recombina-

tion of charge carriers, which are either injected through electro-

des, or created by light illumination. Instead of full 2D-material-

heterostructures in device applications, using hybrid hetero-

structures 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 anti-

reflection effects and strongly concentrated light near the surface.

In this work, the optical properties of 2D MoS2 monolayers confor-

mally coated on 3D Si-based nanocone (NC) arrays are investi-

gated. Numerical calculations show that the absorption in MoS2monolayers 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.

Intensive research into the use of 2D atomically-thin layeredtransition metal dichalcogenides (TMDCs) for numerous appli-cations has been stimulated by their interesting physical pro-perties, such as high carrier mobility, sizable band-gap energy,

and mechanical flexibility.1–14 As with conventional bulk semi-conductors (SCs), fabrication of TMDC heterostructures allowsfor the control of the transport and recombination of the car-riers, leading to their application in various electronic andoptoelectronic devices.6–8 For bulk covalent-bond SC hetero-structures to have high quality interfaces requires epitaxialgrowth, therefore material selection must be based on match-ing lattice constants and crystal structures. In contrast, theweak van der Waals forces in TMDC heterostructures show anotable tolerance for material combinations forming abruptinterfaces without structural and compositional defects.6–8

However, there are many technical challenges in the fabrica-tion of heterostructures composed solely of TMDC materials,including developing large area growth, establishing reprodu-cible patterning processes, and consideration of thermalbudgets for stacking with other materials.

As an alternative approach, 2D TMDC layers can be stackedon bulk 3D SCs, forming hybrid 2D–3D heterostructures.15–17

The use of 3D SCs can circumvent many of the technicalobstacles to TMDC-based device integration, since well-estab-lished fabrication processes and reliable device architecturesare available for 3D SC devices. Moreover, 3D SCs with surfacenanopatterns offer opportunities to tune the physical pro-perties of the deposited 2D TMDC layers. Li et al.9 and Wanget al.10 demonstrated that nano-patterned SiO2 and sapphiresubstrates could modulate the local strain states and band gapenergies of the TMDC layers on them, respectively.

Among the conventional 3D SCs, Si is one of the mostpromising materials for forming 2D–3D heterostructures. Sican be employed as a platform for on-chip integration in opto-electronic integrated circuits.18 In addition, Si nanostructurescan possess quite distinct optical properties compared withtheir bulk counterparts owing to their high refractive index(HRI). HRI Si nanostructures exhibit broadband omnidirec-tional antireflection effects owing to their graded refractiveindex and strong Mie scattering.16,19–22 Si nanostructures havealso inspired research focused on applications in colour filter-ing,23 wavefront manipulation,24 surface-enhanced spec-

†Electronic supplementary information (ESI) available. See DOI: 10.1039/c8nr06597g

aDepartment of Physics, Ewha Womans University, Seoul 03760, Republic of Korea.

E-mail: dwkim@ewha.ac.krbDepartment of Applied Physics, Kyung Hee University, Yongin 17104,

Republic of Korea. E-mail: sunkim@khu.ac.krcDepartment of Physics and Energy Harvest Storage Research Center,

University of Ulsan, Ulsan 44610, Republic of Korea. E-mail: yskim2@ulsan.ac.krdDepartment of Nano-Manufacturing Technology, Korea Institute of Machinery and

Materials (KIMM), Daejeon 34103, Republic of KoreaeMeasurement and Analysis Division, National Nanofab Center, Daejeon 34141,

Republic of Korea

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troscopy,25 and super focusing.26 TMDC-HRI nanostructurescould be used as nanophotonic platforms, which are useful forvarious applications. It is therefore interesting to investigatethe optical characteristics of TMDC layers directly integratedwith Si-based nanostructures.

Dielectric nanostructures are able to tune the physical pro-perties of TMDC layers directly integrated with them, as thereduced dimensionality and weak dielectric screening canenhance the Coulomb interactions in the atomically thinlayers.14 It should also be noted that thermal oxidation of Sican significantly change the refractive index (n): n of SiO2 ismuch lower than that of Si. Oxidized Si nanostructures there-fore exhibit wide tunability of the optical spectra.22,23 Thesereports suggest that the comparative optical characterizationof TMDC layers on Si and SiO2 nanostructures will lead toimportant and interesting findings.

In this work, we prepared MoS2 monolayers conformallycoated on two kinds of NC arrays: Si NC and SiO2 NC. Thephotoluminescence (PL) and Raman intensities of the MoS2monolayers on SiO2 NC were significantly enhanced compared

with those on Si NC. Optical simulations suggested that therefractive indices of the dielectric materials modified the elec-tric field distribution and the resulting optical absorption inthe 2D MoS2 monolayers on the 3D dielectric nanostructures.

Fig. 1a and b show the cross-sectional scanning electronmicroscopy (SEM) images and schematic diagrams of the twokinds of NC array structures. Both the structures consist ofcrystalline Si wafers with surface NC arrays patterned byoptical lithography and dry etching (for details see theExperimental section). After patterning, thermal oxidation ofthe samples was carried out. One sample, hereafter denoted as“Si NC”, had a 50 nm-thick thermally grown SiO2 layer(Fig. 1a). The other sample, hereafter denoted as “SiO2 NC”,had completely oxidized NCs and the thickness of the SiO2

layer in the flat regions between neighbouring NCs was up to340 nm (Fig. 1b).

MoS2 monolayers were grown directly on the NC arraysusing the metal organic chemical vapour deposition (MOCVD)technique. Growth conditions for the MoS2 layer can be foundin the Experimental section. Fig. 2a shows a top-view SEMimage of the MoS2 monolayer on Si NC, which shows the for-mation of the continuous MoS2 monolayer on the high aspectratio Si NC. Transmission electron microscopy measurementsof the sample also confirmed the conformal growth of theMoS2 monolayer on the NC array, as shown in Fig. 2b and c.The source materials used in the MOCVD process were sup-plied in the gas phase and their reaction could form uniformMoS2 layers on the high aspect ratio NCs.27 The top surfacelayer of the substrate can affect the quality of the MoS2 layerand such influences complicate comparative studies of MoS2monolayers on different substrates. To avoid these influences,both Si NC and SiO2 NC were oxidized under the same con-ditions. For Si NC, the SiO2 layer thickness was chosen to be50 nm, which can avoid serious reduction of the opticalabsorption in the MoS2 layer due to the optical interference inthe MoS2/SiO2/Si structure.

11 Detailed physical characteristics,including typical surface roughness and morphology, of theMOCVD-grown MoS2 monolayers will be publishedelsewhere.28

Fig. 3a and b show the calculated and experimental opticalreflectivity spectra of the bare and MoS2-coated NC arrays,respectively. The optical reflectivity of the bare Si NC array isvery low across the whole visible wavelength range. The bare

Fig. 1 Cross-sectional SEM images and schematic diagrams of (a) Si NCand (b) SiO2 NC. SiO2 layers were grown by thermal oxidation of the pat-terned Si wafers. The light and dark grey areas above the NCs areregions containing epoxy with diffused Pt and epoxy used for the speci-men preparation, respectively.

Fig. 2 (a) A top-view SEM image of the MOCVD-grown MoS2 monolayer on Si NC. (b)–(c) Cross-sectional transmission electron microscopyimages of MoS2 monolayers, conformally coated on NCs.

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SiO2 NC array exhibits relatively high optical reflectivity,although it has a high aspect ratio geometrical shape. Suchdistinction originates from the difference in the refractiveindices of Si and SiO2. The resonant modes of the HRI Si NCenable enhanced light absorption in the sample.19–22 Coatingthe NC surface with the MoS2 monolayer somewhat reducesthe optical reflectivity of the NC array. Despite the sub-nmthickness of the MoS2 monolayer, the large optical absorption

coefficient could reduce the optical reflectivity of the MoS2-coated NC arrays.1,11 The finite-difference time-domain (FDTD)calculation (for details see the Experimental section) resultsreproduce the experimental data well, which suggests that theoptical constants of the MOCVD-grown MoS2 monolayers arecomparable to those reported for exfoliated flakes.12

The optical properties of the MoS2 layers grown on the NCarrays were investigated using PL and Raman spectroscopy, asshown in Fig. 4a and b. In the PL spectra (Fig. 4a), strong emis-sion peaks appear at wavelength (λ) ∼670 nm for both thesamples. The peak positions of our MoS2 layers on the NC sub-strates show good agreement with those of the MoS2 mono-layers reported in the literature.1,7–9 There was no bandgapreduction in our MoS2 layers grown on the NC substrates, com-pared with unstrained MoS2 layers on planar substrates [seeESI, Fig. S1†].9,10 As shown in Fig. 4b, the Raman spectra ofboth samples have peaks at 382 and 404 cm−1, which can beassigned to the in-plane (E12g) and out-of-plane (A1g)vibrational modes of the MoS2 monolayers, respectively.2,9

Both the PL and Raman signal intensities of the sample onSiO2 NC (MoS2–SiO2 NC) are much higher than those for thesample on Si NC (MoS2–Si NC), although Si NC exhibits muchmore notable antireflection effects than SiO2 NC (Fig. 3b). ThePL intensity of MoS2–SiO2 NC is 10 times greater than that ofMoS2–Si NC. These experimental results show that the notablelight trapping capabilities of HRI Si NC cannot guarantee theenhanced optical absorption of the deposited MoS2 mono-layer. The absorption of the MoS2 monolayer (AMoS2) at λ =470 nm (PL excitation wavelength) and 532 nm (Raman exci-tation wavelength) was calculated using the FDTD method, asshown in Fig. 4c and d. The sample dependence of the calcu-lated AMoS2 explains the difference in the PL and Raman peakintensities.

Fig. 3 Calculated (symbols) and measured (lines) optical reflectivityspectra of (a) bare and (b) MoS2-coated Si NC and SiO2 NC.

Fig. 4 (a) PL and (b) Raman spectra of MoS2 monolayers on Si NC and SiO2 NC. The calculated optical absorption of the MoS2 monolayers at (c) λ =470 nm (PL excitation source) and (d) λ = 530 nm (Raman excitation source).

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Fig. 5a and b show the FDTD-simulated AMoS2 spectra andthe cross-sectional distributions of the electric field (E-field)intensity (|E/E0|

2, where E0 represents the magnitude of theE-field of the incident light) in the two samples. In the simu-lations we used the optical constants of the exfoliated MoS2monolayer, which exhibit three local maxima, at λ = 430, 620,and 660 nm, originating from the C, B, and A excitonic reso-nances of MoS2, respectively.1,12 AMoS2 of MoS2–SiO2 NC ismuch larger than that of MoS2–Si NC over the entire visiblewavelength range. Fig. 5b shows the E-field intensity distri-bution near the surface of MoS2–Si NC and MoS2–SiO2 NC atλ = 430 nm, where AMoS2(λ) is the local maximum. Si NC con-fines the incoming light in the NCs much more strongly thanSiO2 NC, owing to the geometrical optical resonance in HRInanostructures.19–22 The light confinement effect in the NCs isless notable in MoS2–SiO2 NC than MoS2–Si NC, since the realpart n of SiO2 is much less than that of Si. The weak light con-finement results in a relatively large E-field intensity in theMoS2–SiO2 NC surface, which can increase AMoS2. It shouldalso be noted that the absorption of Si (ASi) in MoS2–Si NC ishigher than that in MoS2–SiO2 NC [see ESI, Fig. S2†].Enhanced scattering and absorption cross-section of the HRISi NC can increase ASi, as discussed above.

To understand the role of n in the NC region, we calculatedthe E-field intensity distributions and AMoS2 for MoS2 mono-layers coated on dielectric (DE) NC arrays (hereafter denoted as“MoS2–DE NC”). The geometrical configuration of MoS2–DENC is almost identical to that of MoS2–Si NC, as illustrated inFig. 6a. The only difference is that the Si material that makes

up the NC region (the grey region in Fig. 6a) is replaced withnon-absorbing DE materials with n = 2 and 3 (n of SiO2 and Siis 1.46 and 3.94 at λ = 600 nm, respectively). Fig. 6b clearlyshows that the light confinement in the NC region of MoS2–DENC with n = 2 is less notable compared with that withn = 3. Consequently, the surface E-field intensity of MoS2–DENC with n = 2 is higher than that of MoS2–DE NC withn = 3. Fig. 6c shows that the AMoS2 spectra vary greatly depend-ing on the n of the NC, as expected from the surface E-fielddistribution. AMoS2 for the MoS2–DE NC with n = 2 is higherthan that with n = 3 over the visible wavelength range. Morestrongly confined light in the MoS2–DE NC with a higher ngives rise to a smaller AMoS2.

AMoS2 for MoS2–DE NC with n = 3 is similar to that forMoS2–Si NC in the range 400 nm < λ < 500 nm, although nSi(4.3–5.6) is much greater than 3 in this wavelength range.29

This suggests that the real part of n alone cannot determineAMoS2. At short wavelengths, Si has an imaginary part of n, andhence the complex Fresnel coefficients can induce a phaseshift of the light reflected and transmitted at the interface.30

As a result, not only the propagation length in media, but alsothe refractive indices of the absorbing media, cause light inter-ference and determine the absorption spectra.11 This couldexplain the similar AMoS2 for MoS2–DE NC with n = 3 andMoS2–Si NC at short wavelengths. In contrast, AMoS2 for MoS2–Si NC is smaller than that for MoS2–DE NC with n = 2 and 3 atλ > 500 nm. Since the imaginary part of n for Si becomessmaller with increasing λ, the real part of n dominantly deter-

Fig. 6 (a) Schematic diagram of MoS2-coated DE NC with refractiveindex n. (b) The FDTD-calculated electric field intensity (|E/E0|

2) distri-butions in the coated DE NC samples with n = 2 and n = 3 at λ =430 nm. The incident light was linearly polarized and E0 indicates themagnitude of the electric field of the incident light. (c) The FDTD-calcu-lated absorption spectra of the MoS2 monolayers on DE NC with n = 2and n = 3 as, and Si NC.

Fig. 5 FDTD-calculated (a) absorption spectra of the MoS2 monolayersand (b) electric field intensity (|E/E0|

2) distributions at λ = 430 nm forMoS2–Si NC and MoS2–SiO2 NC. The incident light was linearly polar-ized and E0 indicates the magnitude of the electric field of the incidentlight.

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mines the surface E-field intensity and resulting AMoS2.Consequently, AMoS2 for MoS2–DE NC with n = 3 is higher thanthat for MoS2–Si NC at λ > 500 nm.

Conclusions

We fabricated integrated nanostructures of MOCVD-grown 2DMoS2 monolayers on 3D Si and SiO2 NC arrays. The PL andRaman intensities of the MoS2 monolayer on SiO2 NC werehigher than those on Si NC, although Si NC exhibited muchlower optical reflectivity in the visible wavelength range. TheFDTD calculations showed that the strongly confined light inSi NC prevented a large E-field from developing at the NCsurface. Instead, the weak light confinement in low refractiveindex SiO2 led to a large surface E-field intensity and increasedthe absorption of the MoS2 monolayers on SiO2 NC. The calcu-lated absorption of the MoS2 monolayer on SiO2 NC was up to0.48 at λ = 430 nm. For 2D–3D heterostructure fabrication, Sihas the advantage of established integrated device fabricationtechniques, as well as the unique optical benefits of the HRInanostructures. Moreover, tuning the optical spectral responseof the 2D MoS2 layers can be achieved simply by thermal oxi-dation of the Si nanostructures. The 2D MoS2-3D Si nano-heterostructures could provide a useful means of realizinghigh-performance multifunctional optoelectronic devices.

Experimental sectionFabrication of NC arrays

The NC arrays were fabricated using photolithography and dryetching on 8-inch Si wafers. A photoresist was spin-coatedonto the wafers, which were then exposed using a KrF stepper(NSR-S203B, Nikon). Inductively coupled plasma reactive ionetching (TCP-9400DFM, Lam Research) was performed to formNCs. The patterned Si wafers were then thermally oxidized.The resulting Si NC had a 50 nm thick SiO2 layer and SiO2 NChad a 340 nm thick SiO2 layer between neighboring NCs. Thedetailed geometrical parameters of Si NC and SiO2 NC can befound in Fig. S3.†

Fabrication of atomically-thin MoS2 covered NC arrays

The conformal atomically-thin MoS2 covered nanocone surfacewas achieved using MOCVD (Dada), where molybdenum hexa-carbonyl (MHC), diethyl sulfide (DES), argon (Ar), and hydro-gen (H2) were employed as the chemical precursor for Mo,chemical precursor for S, carrier gas, and reduction gas,respectively.28 The as-synthesized NC arrays were horizontallylocated in the center of the reaction chamber to grow atomic-ally-thin MoS2 under the following optimal growth conditions:Reaction temperature, 600 °C; reaction pressure, 60 torr; reac-tion time, 8 h; Ar flow-rate, 30 sccm; H2 flow-rate, 3 sccm; DESflow rate, 1.5 sccm; and MHC freely flowing. The schematicillustration of the MOCVD system set up is provided inFig. S4.†

Optical simulations and characterization

Reflectivity spectra, absorption spectra, and electric-field inten-sity distributions for the MoS2 monolayers on NC substrateswere obtained by conducting home-built FDTD simulations. Anormally incident, broadband (λ = 400–800 nm) plane wavewas used as the light source for all of the FDTD simulations.The size of a unit cell was 0.4 × 0.4 × 0.4 nm for the x-, y-, andz-axes. The n of SiO2 was set as constant (1.46),29 while thedielectric function of the MoS2 monolayer was strongly disper-sive, which was available from the experimental data reportedby Li et al.12 The proper mesh size could be determined basedon the comparison of the numerical simulation data and theanalytic solution reported in our earlier work (ref. 11). Thereflectivity spectra were obtained by using a broadband (λ =400–800 nm) Xe lamp and spectrophotometer equipped withan integrating sphere. Room-temperature Raman scatteringspectra were obtained using a spectrometer (207, McPherson)equipped with a nitrogen-cooled charge coupled device arraydetector (Princeton Instruments). The samples were excitedwith a 10 mW 532 nm DPSS laser. Low powers were used toensure that the samples did not decompose as a result of loca-lized laser heating. Optimal results were obtained with 20 sintegration for the NC array. PL spectra were obtained using amicro-photoluminescence system (MonoRa500i, DongwonOptron) with an excitation wavelength of 473 nm, an outputpower of 5 mW, and a spot size of ∼1 μm2.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was supported by the National ResearchFoundation of Korea (2009-0093818, 2014R1A4A1071686,2016R1D1A1A09917491, 2016R1D1A1B01009032, 2017R1E1A1A01075350, and 2018R1A6A1A03025340).

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