Air Gap-Based Cavities Dramatically Enhance the True Intrinsic

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Air Gap-Based Cavities Dramatically Enhance the True Intrinsic Spectral Signals of Suspended and Pristine Two-Dimensional Materials Tzu-Yao Lin,† Yang-Chun Lee,† Yu-Wei Lee,† Sih-Wei Chang,† Dai-Liang Ma,§ Bo-Cheng Lin,§ and Hsuen-Li Chen*,†,‡ Department of Materials Science and Engineering and ‡Materials and Electro-Optics Research Division, National Taiwan University, No. 1, Section 4, Roosevelt Road, Taipei 10617, Taiwan § Center of Atomic Initiative for New Materials, National Chung-Shan Institute of Science and Technology, Taoyuan 32546, Taiwan J. Phys. Chem. C Downloaded from pubs.acs.org by WASHINGTON UNIV on 02/28/19. For personal use only.

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S Supporting Information *

ABSTRACT: The properties of two-dimensional (2D) materials are readily affected by their surroundings. Therefore, the underlying substrates and surrounding materials always disturb the pristine properties of 2D materials. Herein, we describe how the pristine properties of suspended 2D materials can be precisely extracted from Raman and photoluminescence (PL) spectra with great signal enhancements by taking advantage of both air gap suspension and nanocavity enhancement effects. The modes of the Raman emission lines were enhanced to almost the same degree when the 2D materials were positioned over the nanocavity: the 2D/G peaks of suspended single-layer graphene (SLG) and the E12g/A1g peaks of MoS2 were significantly enhanced almost equally. Moreover, recording Raman and PL spectra at different positions of the suspended 2D materials was a very powerful tool for observing charge transfer between the pristine 2D materials and the surrounding materials. We also found that the residual holes of the suspended SLG could be neutralized by aluminum (Al) at certain positions. By employing the air cavity structure, we could readily locate the charge neutrality point of the suspended 2D materials. In addition, the PL intensity of MoS2 could be greatly enhanced when using the same nanocavity. The great enhancements in the PL signals from the suspended 2D materials allowed us to further investigate the spectral weights of both the A0 exciton and A− trion peaks when MoS2 was suspended or supported upon various metal films. This approach may open up new doors for techniques allowing precise characterization of abundant information from pristine and suspended 2D materials.

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INTRODUCTION Because two-dimensional (2D) materials possess high specific surface areas, their surface properties dominate their overall properties. Furthermore, the surface properties of 2D materials are readily affected by their supported substrates.1 In other words, the supporting substrates always interfere when studying the properties of 2D materials. Therefore, to avoid the unpredictable influence of the substrates and, thereby, extract the intrinsic properties of pristine 2D materials, it has been necessary to investigate them in suspended states. Because 2D materials can possess interesting and unique properties, they have been applied in light-emitting devices,2 touch panels,3 field-effect transistors,4,5 solar cells,6,7 photodetectors,3,8 transparent conducting films,9 and gas sensors.10 Therefore, a need exists to analyze the intrinsic properties of 2D materials quantitatively and qualitatively. Both Raman11 and photoluminescence (PL)12 spectroscopy techniques can be used to analyze the properties of 2D materials over large areas in a rapid and nondestructive manner. Raman spectroscopy can provide abundant informa© XXXX American Chemical Society

tion about 2D materials, including their structural quality, number of layers,13 doping type,14 and stress distribution.15 Moreover, the band gaps and carrier transport16 of 2D materials can be determined from the emission wavelengths, full widths at half-maximum (FWHMs), and shapes of the emission bands in PL spectra. Because 2D materials contain only one or a few atomic layers, their low degree of optical absorption results in fluorescence of relatively low intensity. Moreover, the low yield of Raman scattering and the atomic thickness of a 2D material will result in weak Raman signals. Therefore, it can be challenging to analyze the detailed features of 2D materials without employing techniques for signal enhancement, such as surface-enhanced Raman scattering,17 interference-enhanced Raman scattering (IERS),18−21 and surface-enhanced fluorescence.22 For example, the light outcoupling of 2D materials can be enhanced by increasing the Received: October 27, 2018 Revised: February 12, 2019

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DOI: 10.1021/acs.jpcc.8b10470 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C surface electric field (E-field) generated through surface plasmon resonance or constructive interference. Although both techniques can enhance the intensity of Raman scattering and fluorescence from a 2D material, the resulting output may be disturbed by the presence of the added structures. For example, when using localized surface plasmon resonance of gold nanoparticles (NPs) to enhance the Raman signals of graphene,23 the direct contact of the graphene with the gold NPs can lead to local tensile stress and p-type doping.24 Moreover, when using IERS to enhance the Raman scattering of graphene,25−28 the graphene must be transferred onto a SiO2 spacer, which would have to be p-type-doped by a SiO2 layer because of the difference in work function between graphene and SiO2.19 On the other hand, Butun et al. found that gold nanodisk arrays could enhance the fluorescence of molybdenum disulfide (MoS2);29 unfortunately, direct contact between the metal and the 2D materials led to quenching and doping effects that distorted the fluorescence spectra. Thus, although the intensity of the signals in Raman and PL spectra can be enhanced when using these techniques, the surrounding substrates will inevitably influence the intrinsic properties of the pristine 2D materials, making it difficult to analyze them accurately. Furthermore, if we were to remove the influence of the substrates, previous studies have suggested that the 2D materials would display their physical properties most optimally when positioned in completely freestanding or suspended states. For example, Bolotin et al.30 discovered that, in a completely freestanding state, the carrier mobility of graphene increased 10 times without the influence of the surface charge and defects of the substrate; Li et al.31 also found that the thermal conductivity of monolayer MoS2 increased significantly in its freestanding state because any phonon oscillation of the substrate hindered the carrier transport of MoS2. Those studies highlight the opportunity for developing new analytical techniques for determining the intrinsic properties of pristine 2D materials. When investigating the intrinsic properties of pristine singlelayer graphene (SLG), Banszerus et al.32 used a contaminationfree dry transfer method, based on van der Waals attraction, to pick up chemical vapor deposition (CVD)-grown graphene directly from copper foil without transferring it using a polymer-based method. In the fabrication process, graphene came into contact only with hexagonal boron nitride and exhibited very high carrier mobilities by avoiding contact with any intermediary, thereby maintaining the characteristics of pristine graphene. In another study, to avoid perturbations arising from a substrate, Berciaud et al.33 suspended graphene upon micrometer-sized trenches. Using this approach, they deduced the quality, doping level, and strain of the suspended SLG by analyzing the intensity, wavenumber, and FWHM of the Raman bands. The hole concentration of SLG supported by a silicon dioxide (SiO2)-covered substrate was greater than that of the suspended SLG. Furthermore, they used the same architecture to analyze the line shape of the 2D Raman peak of the suspended SLG at various carrier concentrations, regulated by a back gate, and found that the 2D Raman peak of the supported SLG was broader and more symmetrical than that of the suspended SLG.34 Similarly, the PL peaks of single-layer and suspended MoS2 were blue-shifted relative to that of single-layer MoS2 on a Si/SiO2 substrate; Scheuschner et al.35 attributed this phenomenon to the spectral weight of the trion signal of MoS2 on a Si/SiO2 substrate being greater than that of the suspended MoS2 because the MoS2 would be n-type-

doped by the substrate. Additionally, there are some groups applying the properties of suspended 2D materials for various nanodevices. For example, Chen et al.36 used the freestanding graphene membranes to fabricate pressure sensors with high sensitivity. They took advantage of the highly reversible elasticity and strength properties of suspended graphene membranes to produce a high-quality and suspended graphene-based pressure sensor. On the other hand, Patil et al.37 used a chemical vapor deposition (CVD)-grown suspended graphene of microribbon to produce a graphene photodetector. The device performed a very high carrier mobility. They demonstrated a suspended graphene photodetector with fourfold photoresponsitivity compared to the substrate-supported graphene microribbons. Although a suspended architecture18,38−41 can prevent perturbations arising from a substrate, the previous studies described above did not consider the enhancement or optimization of the electric field (E-field) within 2D materials. Moreover, the spectra of 2D materials might be distorted24 because the band-to-band ratios of the Raman and PL signals might not be maintained by the enhanced structures when the distinct modes (e.g., G and 2D bands) are generated at different wavelengths.19 Furthermore, the previous methods described above could not significantly enhance the Raman and PL signals of the suspended 2D materials, leading to low signal-to-noise ratios. These shortcomings would lead to a loss of accuracy when analyzing the intrinsic properties of pristine 2D materials in suspended states. In this study, we developed a method to (i) overcome the influence of the substrates when inspecting 2D materials and (ii) enhance the intensity of the signals in the Raman and PL spectra of various 2D materials. We used an optimal air gapbased cavity to support the 2D materials and enhance the Efield within them. We found experimentally that the 2D and G peaks of the suspended SLG were enhanced dramatically; similarly, the intensities of the E12g and A1g peaks of single-layer MoS2 were also enhanced up to several hundred-fold. Furthermore, the extreme broad-band enhancement in Efield provided by the air gap-based cavity could amplify each mode at distinct wavelengths with almost equal degrees, i.e., we could enhance the Raman and PL signals of 2D materials while maintaining the band-to-band ratio, thereby allowing precise investigations into the ratios and relationships between the bands in the Raman and PL spectra. In addition, we found that the suspended SLG was n- or p-type-doped by various metal films, and we could deduce the diffusion distances of the carriers from the metal edge to the SLG. Moreover, the PL intensity of MoS2 was also enhanced dramatically. We could also compare the spectral weights of both the A0 exciton and A− trion peaks for supported and suspended MoS2. Thus, our approach allows the Raman and PL signals of various pristine 2D materials to be enhanced to large degrees without any spectral distortion, potentially opening new doors for the development of novel inspection technologies for various pristine 2D materials.

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EXPERIMENTAL SECTION Sample Preparation and Characterization. Three micron-sized trench array structures were fabricated on a silicon (Si) wafer; the patterning process was performed using electron beam lithography. After development, the width and period of the trench array were observed using an in-line scanning electron microscopy (in-line SEM) system. The

B


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The Journal of Physical Chemistry C following parameters were used for the dry etching of silicon: chamber pressure, 12 mT; chamber temperature, 65 °C; power of bottom/top plates, 120/310 W; gas species/gas flow, HBr/ 125 sccm and Cl2/35 sccm. After the etching process, the structured Si substrate was cleaned by the RCA clean to remove residual photoresist and native oxide. A 50 nm metal (Al, Au, or Ag) film was deposited, using a magnetron sputtering system, onto the structured Si substrate to cover the shallow trench array structure. Graphene and MoS2 flakes were exfoliated mechanically from bulk materials (by using adhesive Scotch tape) and then transferred onto the shallow trench array-based substrate. For comparison, chemical vapor deposition (CVD)-grown single-layer graphene (SLG) was transferred onto the air cavity-based shallow trench array-based substrate through a poly(methyl methacrylate) (PMMA)mediated transfer process. All samples were prebaked to minimize the adsorption of moisture on the 2D materials prior to Raman and PL spectral measurement. The morphology of the air cavity-based trench array structure was determined using thermal field emission scanning electron microscopy (SEM, NOVA NANO 450). The Raman and PL spectra of the 2D materials were recorded using a commercial micro Raman/PL system (UniRAM, UniNanoTech) equipped with an excitation laser operated at a wavelength of 532 nm (WITec, CRM200) and a monochromator having a focal length of 75 cm. To avoid the damage on 2D materials, an attenuator was used to decrease the laser power density to 1% (ca. 0.184 mW/μm2) when we measured the Raman and PL signals of 2D materials. On the other hand, to obtain observable Raman spectral signals of SLG on metal films, we increased the collection time by 100 times (2000 s) compared to the suspension part (20 s). The laser beam was focused by a 100× objective with the numerical aperture of 0.9; the theoretical spot diameter was approximately 720 nm at a wavelength of 532 nm.

Figure 1. (a) Schematic representation of SLG on an air gap-based nanocavity structure featuring a metal back reflector, an air spacer layer, and SLG. (b) Top-view and cross-sectional SEM images of the air gap-based nanocavity. (c) Distribution of normalized electric field | E| on the surface of a square hole; the laser spot (dashed circle), polarization (arrow) of the laser beam, and scanning direction (green arrow) are also presented.

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RESULTS AND DISCUSSION Two conditions must be met if we are to extract the intrinsic properties of pristine 2D materials from their Raman and PL spectra. First, the supporting substrate must not disturb the intrinsic properties of the 2D materials. That is, the 2D materials should be suspended and in contact only with air. Second, the E-field enhancement generated by the air cavity substrate should be almost the same at each different emission wavelength of the Raman/PL signals from the 2D materials; in other words, the main peaks in the Raman/PL spectra should be enhanced to the same degree such that the band-to-band ratios in the Raman/PL spectra are not changed significantly. To meet these requirements, we developed an air gap-based cavity configuration consisting of a flat metal (Al, Au, or Ag film) as the bottom reflector, an air gap as the cavity or spacer, and a layer of a 2D material (graphene, MoS2) suspended on the top surface of the cavity. Figure 1a displays a schematic representation of the air gap-based cavity that we designed for investigations of suspended 2D materials. Figure 1b presents top-view and cross-sectional SEM images of the air gap-based cavity. The structure featured square holes having a side length of 3 μm and a period of 6 μm in a hexagonal arrangement; the cross-sectional images revealed that the average depth of the trenches and the thickness of the metal film were approximately 125 and 50 nm, respectively. In 2014, Babichev et al.38 suggested that suspended graphene would collapse if the suspended length was greater than 3 μm (i.e., at that point,

the suspended graphene would touch the substrate). Such collapsed 2D materials would cause the E-field within the 2D materials to deviate from the designed and optimized conditions; accordingly, we chose to use square holes having a side length of at most 3 μm. Cross-sectional SEM images also revealed that the metal layer deposited by the sputtering system covered the sidewalls and bottoms of the trenches very well, due to the low aspect ratio of the shallow trenches. Moreover, as displayed in Figure 1c, according to threedimensional finite-difference time-domain method (3DFDTD) simulations, the 2D materials would be suspended over the surface of a square hole (trench) where the amplitude of the E-field was uniform. We used a light source having a wavelength of 532 nm and a spot size of 720 nm to scan the whole area of the square hole. The 40 nm Ag film and 125 nm air cavity were used as the back reflector and spacer, respectively, in the air gap-based cavity. A monitor was placed at the position of suspended graphene to simulate the distribution of the E-field within the graphene. That is, we divided the square hole having a side length of 3 μm into 100 areas and measured the E-field when the light source irradiated each of these areas of the 2D materials. A detailed discussion of the simulation model is presented in the Supporting Information. Figure 1c displays the E-field distribution within the entire hole in a color map of the normalized E-field C


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Figure 2. (a) Calculated amplitudes of E-fields plotted with respect to the penetration depths for 50 nm Al/125 nm air/0.3 nm graphene. (b) Calculated average E-field spectra of 50 nm Al/125 nm air gap/0.3 nm graphene (black line), Si substrate/300 nm SiO2/0.3 nm graphene (red line), and Si substrate/0.3 nm graphene (blue line). (c) Optical microscopy (OM) images of exfoliated SLG flakes on the air gap-based nanocavity. (d) Measured Raman spectra of SLG on the Si substrate, SLG (transferred using the PMMA process) on the air gap-based nanocavity, exfoliated SLG on the air gap-based nanocavity, and SLG (transferred using the PMMA process) on 300 nm SiO2/Si. (e) Experimental and calculated enhancements in the intensities of the G and 2D bands of the SLG on the air gap-based nanocavity and the Si substrate/300 nm SiO2.

intensity. In the edges of the hole, the E-field intensity was slightly lower than at the center. Even so, the differences in the E-field intensities between the edges and the center were insignificant. This uniformity of the E-field intensity indicated that, regardless of the position of the laser spot, there would be almost no variation in the degree of signal enhancement. Therefore, we extracted the amplitude of the E-field within the 50 nm Al/125 nm air gap/0.3 nm graphene structure, relative to the amplitude of the E-field of incident light (Figure 2a). The maximum value of the E-field was located at the position of the graphene. As displayed in Figure 2b, from a plot of the average amplitudes of the E-field within the graphene against the distinct wavelengths for various substrates, the air gap-based cavity appeared very suitable for extracting the intrinsic properties of pristine 2D materials from their Raman/ PL spectra. The black line in Figure 2b is the calculated average E-field (within the graphene) dispersion spectrum for the 50 nm Al/125 nm air gap/0.3 nm graphene structure. Having optimized the thickness of the air spacer layer (i.e., the depth of the air cavity), the maximum E-field (ca. 1.95) within

the graphene appeared at the wavelength of the excitation laser (532 nm), and the E-field in the entire rangefrom the wavelength of excitation to all emission lines (Raman/PL spectra) of graphenealways remained above 1.93 (blue dashed line). As a reference, we also simulated the average Efield (within the graphene) for Si substrate/graphene and 300 nm SiO2/graphene systems, indicated by the blue and red lines, respectively, in Figure 2b. The major advantage of our air gap-based cavity is that it provided superior broad-band and higher E-field enhancements relative to those of the other substrates. Moreover, the ratio of the E-field enhancements for the 2D/G bands (619/ 580 nm) within graphene was very close to 1 (ca. 0.99), and this value was also better than that provided by the 300 nm SiO2/0.3 nm graphene structure (red line). Accordingly, we expected the air gap-based cavity to provide equal enhancement of the distinct signal lines from the Raman/PL spectra, as a method potentially applicable to various 2D materials because of the extreme broad-band enhancement of the E-field. D


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the enhancements of the G and 2D peaks of the exfoliated SLG on the air gap-based cavity were ca. 470 and 490 times compared to those of the SLG on the Si substrate, respectively. We suspect that the enhancement of the 2D band was greater than that of the G band because of the low carrier concentration in the near-pristine SLG transferred using mechanical exfoliation. In 2008, Das et al.47 discovered that a lower carrier concentration in graphene would result in a higher 2D/G ratio in its Raman spectrum. That is, the Raman intensity ratio of the 2D/G bands of the PMMA-transferred SLG was lower than that of the exfoliated SLG because of the residual polymer on the SLG during the transfer process, resulting in a higher hole concentration in the PMMAtransferred SLG (we discuss this phenomenon in further detail in the following sections). Even so, we could still enhance the intensities of both the G and 2D bands of the SLG by almost 500-fold, while maintaining the band-to-band ratio of the G and 2D bands, when using the air gap-based cavity, regardless of whether the transfer occurred using PMMA or mechanical exfoliation, with the enhancement being far greater than that provided by the 300 nm SiO2/Si substrate (green star in Figure 2e).49 To demonstrate that the experimental results for probing suspended SLG were not affected by the metal film of the cavity, we tested other metal filmssilver (Ag) and aluminum (Al)as reflectors for the air gap-based cavities. The inset to Figure 3a displays a schematic representation of the setup for probing the suspended SLG, with the laser spot (green circle dot) located at the center of the cavity-based square hole. Because the work function of Al (4.24 eV)50 is lower than that of graphene (4.56 eV),51 graphene would be n-doped (electron-doped) by Al,52 and the 2D and G bands would

To confirm the broad-band enhancement ability of the air gap-based cavity, we measured and compared the Raman spectra of SLG on the air gap-based substrate and a bare Si substrate. Figure 2c displays an optical microscopy (OM) image of graphene flakes that had been transferred, through mechanical exfoliation, onto the air gap-based cavity; the area highlighted by the white dashed lines is a region in which the graphene was present in both single-layered and suspended situations. The red and blue lines in Figure 2d are the Raman spectra measured from the SLG on the identical cavity structure, but prepared using a polymer [poly(methyl methacrylate), PMMA]-mediated transfer method and mechanical exfoliation, respectively. The G and 2D bands of SLG were the two major Raman peaks that we observed. Compared to the SLG on the Si substrate, the intensities of the G and 2D bands for the PMMA-transferred SLG on the air gap-based cavity were enhanced by factors of 485 and 460, respectively. The enhancement of each band was close to the value calculated using the equation42−44 enhancement =

2 Eairgapnanocavity,532 2 ESi,532

×

2 Eairgapnanocavity,Raman 2 ESi,Raman

(1)

where Eair gap cavity is the amplitude of the average E-field within the SLG on the air gap-based cavity and ESi is the amplitude of the average E-field within the SLG on the Si substrate. We took into account both the wavelength of the excitation laser light (λex = 532 nm) and the emission wavelength of the Raman scattering light. Because we considered the average E-field within the SLG in terms of the excitation and emission wavelengths, the calculated enhancement factors for the signals of the G and 2D bands in the Raman spectra were 593 and 565, respectively. Presumably, the difference between the measured and calculated values arose from the imperfection of the fabrication process. The Raman peak positions of SLG are also affected by its carrier concentrations.45,46 As indicated in Figure 2d, the peak positions of the 2D/G bands for the SLG on the Si substrate (2673.6/1577 cm−1) shifted obviously away from those of the PMMA-transferred suspended SLG (2665/1575.2 cm−1), due to p-type (hole) doping from the Si substrate to the SLG (i.e., the wavenumbers of the 2D and G bands increased by 8.6 and 1.8 cm−1, respectively). This phenomenon is consistent with a previous study that found that the peak positions of the 2D and G bands shifted to larger wavenumbers when graphene was p-type-doped by an electron acceptor.47 Therefore, if we could exclude the influence of doping, we would be able to extract the precise intrinsic properties of pristine graphene from its Raman spectra. Compared to the PMMA-transferred suspended SLG, the mechanically exfoliated suspended SLG did not feature any polymer residue and, therefore, was presumably less influenced by doping.48 Thus, removed from the effects of doping induced by the substrate and polymer residue, the mechanically exfoliated suspended SLG (blue line in Figure 2d) provided the lowest Raman wavenumbers for the 2D/G bands (2663/1574 cm−1) compared to that on Si, 300 nm SiO2, and suspended SLG (transfer by PMMA). We emphasize that this Raman spectrum was generated completely from pristine graphene. As displayed in Figure 2e, the enhancements of the G and 2D peaks of SLG were 88 and 84 times on 300 nm SiO2/Si relative to the SLG on the Si substrate, respectively. However,

Figure 3. (a) Raman spectra of the SLG on the air gap-based nanocavity (Ag and Al as reflectors); inset: schematic representation of the experimental scheme for probing suspended SLG using a 532 nm excitation laser. (b) Raman shifts of the 2D and G bands of the SLG on the air gap-based nanocavity (Ag and Al as reflectors). E


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Figure 4. (a) Schematic representation of the experimental scheme for positioning the SLG using an X−Y scanning stage. (b) Raman spectra recorded at various positions of the SLG on the air gap-based cavity (Au as reflector). (c) Raman spectra recorded at different positions of the SLG on the air gap-based cavity (Al as reflector). (d) Ratio of the intensities of the 2D and G peaks in the Raman spectra recorded at different positions. (e) Estimated electron concentration within the SLG at different positions. (f) The Lorentz fitting of G bands of the SLG on the air gap-based cavities measured at the center and edge positions. F


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Figure 4d,e, plotting the intensity ratio and electron concentration with respect to the distance to the edge allowed us to estimate a value of approximately 600 nm for the diffusion distance of the hole from the edge of the Au film to the SLG. The estimated carrier concentration of holes in the SLG on the Au film was 1.57 × 1012 cm−2, a value that became increasingly lower as the laser spot moved away from the edge of the Au film. From analysis of the Raman spectra in Figure 4e, when the distance between the laser spot and the cavity edge (Au film) was greater than 900 nm, we could deduce that the carrier concentration of the suspended SLG was not affected by the Au film and that the essentially undoped SLG had a residual carrier (hole) concentration of 2.1 × 1011 cm−2. That is, the three points (center, position 1, position 2) marked by the black dashed box in Figure 4e contained essentially undoped (pristine) graphene that exhibited p-type properties in air.33 Next, we used an Al film as the back reflector in the air gapbased cavity and performed an analysis similar to that described above. In contrast to the Au film, which had a high work function, the Al film would donate electrons into the graphene (n-type doping) because the work function of Al is lower than that of graphene.53,57 Figure 4c presents the normalized Raman spectra of the SLG above the Al-based cavity at various positions. When the laser spot irradiating the SLG gradually approached the edge of the cavity (Al film), the first three Raman spectra of the SLG (center, position 1, position 2) were not affected by the presence of the Al film. Starting from position 3, we found that the Raman spectra of the SLG began to be influenced by the Al film. As seen in Figure S3b in the Supporting Information, the 2D/G bands shifted to lower/higher wavenumbers as the laser spot reached closer to the Al film. The shift of the 2D/G bands might have been caused by n-doping and tensile stress of the SLG on the Al-based cavity. The 2D/G bands would, however, move to lower/higher wavenumbers as a result of n-doping, while the tensile stress would shift both the 2D and G bands to lower wavenumbers. Therefore, we deduce that the change in the 2D/G bands was dominated by the effect of the carrier concentration of the SLG. From the red dots in Figure 4d,e, we estimated that the diffusion distance of the electrons from the cavity edge of the Al film to the SLG was approximately 900 nm, i.e., it was larger than the diffusion distance of the holes from the Au film (ca. 600 nm). From the intensity ratio of the 2D/G bands, we estimated the carrier concentration of electrons in the SLG on the Al film-based cavity to be 9 × 1011 cm−2, a value that decreased as the laser spot moved away from the Al edge. Because the residual carrier (hole) concentration in the SLG would be neutralized by n-doping (electrons) from Al,58 we expected a charge neutrality point (CNP) to exist on the SLG somewhere between the center of the cavity and the Al edge. As indicated in the blue dashed box in Figure 4d, position 3 was closest to the CNP, where all of the charges in the volume would be neutralized to zero;59 the carrier concentration of holes at position 3 (900 nm away from the center), extracted from Figure 4e, was 1.7 × 1011 cm−2. The Raman intensity ratio of the 2D and G bands of the SLG would reach a maximum when the material possessed the intrinsic structure having equal electron and hole densities. On the other hand, strain is also an important issue for SLG. According to previous studies, both strain and doping concentrations would affect the peak positions of G and 2D bands of SLG. If we want to distinguish the spectral response

shift to lower and higher wavenumbers, respectively, if graphene were to come into direct contact with Al. Figure 3a,b reveals, however, that the peak positions in the Raman spectra were coincident for the suspended SLG on the Al filmand Ag (work function: 4.74 eV) film-based cavities. In other words, the Raman signals were not affected by the doping effect from the metal layer of the reflector in the air gap-based cavities. Thus, the spectra were generated completely from the suspended pristine SLGotherwise, the 2D/G bands would have shifted further to higher wavenumbers if graphene were to come into direct contact with a metal having a work function higher than that of Al, such as Ag (4.74 eV).50 To further investigate the properties of the suspended SLG, we used a scanning stage to accurately position the suspended SLG for surface mapping of the Raman signals. Figure 4a displays a schematic representation of the setup for analyzing the SLG at various positions above the cavity. The distance from the center of the cavity to the edge was 1500 nm; we used the laser spot as a probe to measure the Raman signals of the SLG at 300 nm increments. The green solid circles in Figure 4a indicate the positions of the laser spot, and the names of the six positions (center, positions 1−4, metal film) appear in the marked spectra. Figure 4b presents the Raman spectra of the SLG on the Au-coated cavity at the various positions, with the intensity of the Raman signals normalized. As displayed in the top spectra of Figure 4b,c, we obtained the observable spectra by increasing the collection time by 100 times to get the spectra on metal films. Except for the SLG on a flat Au film (the first line from top in Figure 4b), the other five Raman spectra were measured from the SLG suspended above the Au-based cavity. When the laser spot irradiated the positions of the SLG away from the center of the cavity, the Raman spectra collected from the first three positions (center, position 1, and position 2) of the SLG were almost identical and clearly not affected by the doping effect from the Au film. Position 3 was 900 nm away from the center of cavity; here, we could observe that the Raman spectrum of the SLG began to be influenced by the Au film, as seen in Figure S3a in the Supporting Information, the 2D/G bands of SLG both shifting to higher wavenumbers as the laser spot became closer to the Au film. This phenomenon was due to the difference in work functions between Au (5.3 eV)50 and graphene such that the Au film would p-dope (hole-doping) the SLG.53−55 The Raman peak positions of the 2D and G bands of the SLG at the various positions both shifted to higher wavenumbers when the laser spot reached closer to the edge of the cavity (Au film). Because the 2D/G bands of the SLG could be affected by both p-doping56 and tensile stress,15 the Au film might have had these effects on the SLG for two reasons: because of the difference in work function and because of the surface roughness. Therefore, we could not use Figure S3a to determine the exact diffusion distance of the hole from the Au edge to the SLG. Fortunately, the Raman intensity ratio between the 2D and G bands of the SLG (I2D/IG) would be influenced only by the carrier concentration so that we could use that ratio to examine the charge transfer between the SLG and the metal film. In 2008, Das et al.47 demonstrated that the Raman intensity ratio of the 2D and G bands of SLG would decrease upon increasing the carrier concentration. Therefore, we could use the value of I2D/IG to estimate the carrier concentration at the various positions. A detailed discussion of the estimated process of carrier concentration is presented in the Supporting Information. As revealed by the black dots in G


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best of our knowledge, no approach for enhancing Raman scattering for suspended monolayer 2D materials has been reported previously. Moreover, our study is the first to achieve great signal enhancementsas high as 500-foldfrom suspended 2D materials, in an approach that could be used to study the intrinsic properties of 2D materials while avoiding any disturbance from a substrate. In addition to graphene, we found that the intrinsic properties of pristine monolayer MoS2 could also be extracted from the enhanced Raman and PL spectra when using the air gap-based cavity. The MoS2 samples were prepared through mechanical exfoliation of bulk MoS2 and transferred onto the air gap-based cavity using the Scotch tape method. We also deposited a MoS2 sample on a Si substrate as a reference. Figure 5a,b presents OM images of the exfoliated MoS2 flakes on the Si substrate and air gap-based cavity, respectively. To locate the monolayer MoS2, we used Raman spectroscopy to identify the number of layers of each MoS2 flake. For the sample displayed in Figure 5c, the two characteristic E12g and A1g Raman peaks were separated by approximately 20 cm−1 indicative of a monolayer of MoS2.61 In Figure 5b, the area within the white dashed lines indicates a region in which the MoS2 was simultaneously in monolayer form and suspended above the cavity; the inset provides a schematic representation of the sample. Figure 5c presents the measured Raman spectra of the monolayer MoS2 on the Si substrate and above the air gap-based cavity, recorded from the areas bordered by the white dashed lines in Figure 5a,b, respectively. First, we observed that both the characteristic E12g and A1g peaks moved to higher wavenumbers when the monolayer MoS2 was suspended. Because the Si-supported monolayer MoS2 would have been n-type-doped by the Si substrate, the suspended MoS2 had a lower electron concentration, and its A1g and E12g phonon lines were shifted by 6 and 5 cm−1, respectively.62 Second, when using the air gap-based cavity, the average amplitude of the E-field within the monolayer of MoS2 was enhanced by 4.4 and 1.65 times relative to the amplitude of the E-field on the Si substrate and 300 nm SiO2/Si, respectively. In contrast to the 2D/G bands of the SLG, the two characteristic peaks of the monolayer MoS2 were both located at much lower wavenumbers; therefore, the wavelengths of the Stokes shifts were also very close to 532 nm. According to eq 1, the enhancement could be approximated as E4. As displayed in Figure 5c, the measured enhancements of the A1g and E2g peaks of MoS2 on 300 nm SiO2/Si were both 67 times those on Si substrate. Notably, both the A1g and E2g peaks on the air gap-based cavity were enhanced by 350 times relative to MoS2 on the Si substrate. Thus, we could enhance the Raman signals of various kinds of 2D materials by using the same air gap-based cavity. In addition to enhancing the Raman signals, we expected that the air gap-based cavity structure might also enhance the PL signals of 2D materials. The ultra-broad-band E-field enhancement above the air gap-based cavity can not only enhance both the absorption and emission within MoS2, but also prevent the emission spectra from exhibiting signal distortion. We could extract the intrinsic properties of pristine MoS2 from the PL spectrum with great magnification of the signal intensity. For the PL spectral regime, the black line in Figure 6a displays the calculated average E-field within the MoS2 just above the cavity. For a reference, indicated by the blue and red lines in Figure 6a, we also calculated the E-field for the MoS2 on a 300 nm SiO2/Si and Si substrate,

of Raman peaks affected by strain and doping effects, people generally should fix one factor and change the other one. Mohiuddin and Huang et al. transferred the SLGs on different flexible substrates to control and adjust the strain on the SLG.15,60 By excluding the disturbance from doping effect, both studies found that strain on SLG would lead to broadening and shift of both the G and 2D bands of Raman signals to lower wavenumber. Moreover, the G band would also split into two G sub-bands, namely, G− and G+, under increasing strain, and the shifting ratios of strain effect were 12.5 and 5.6 cm−1/% strain for the G− and G+ bands, respectively. On the other hand, Das et al. adjusted the doping concentration by using the top-gate setup to investigate the relationship between doping concentration and Raman signals without the disturbance of strain effect.47 They demonstrated how the peak positions of the Raman signals were affected by the carrier concentration. According to the study of Das et al., n-type doping would shift the G and 2D bands to higher and lower wavenumbers, respectively, while p-type doping would shift both the G and 2D bands to higher wavenumbers.47 In our study, as shown in Figure S3a, for the Au base cavity (p-type doping in SLG), both the G and 2D peaks shift to higher wavenumbers. However, as displayed in Figure S3b, for the Al base cavity (n-type doping in SLG), the G and 2D peaks shift to higher and lower wavenumbers, respectively. According to previous study, the strain on SLG would induce both the G and 2D bands shifting to lower wavenumber. In this study, the conditions of preparing and transferring processes of Au- and Al-based cavities are the same; however, the shift trends of G and 2D bands for the Au and Al cases are totally different. That is, in this study, the trend of peak shifting of G and 2D bands of SLG on the air gap-based cavities seems to be dominated by the doping effect but not strain effect. Moreover, when the SLG has strain, the strain-induced asymmetry of G band will be introduced, which means that the G band will split into two sub-bands.60 However, as shown in Figure 4f, we try to fit the G band data at the center and edge positions of the Au- and Al-based air gap-based cavities by Lorentzian functions. We found that all of the G bands did not split and showed the symmetry profiles. Based on the above discussion, in this study, we suggest that the behaviors of Raman signals of SLGs on the air gap-based cavities are dominated by the doping effect. The broad-band enhancement of the E-field provided by the air gap-based cavity played a vital role, allowing us to obtain information from the band-to-band intensity ratio in the Raman spectra because the signal intensities were enhanced dramatically while maintaining the band-to-band ratio of the signals. For example, the enhancement ratio of the E-field intensity for the 2D over the G band within the SLG was approximately 1. Our experimental results demonstrate that the intrinsic pristine properties of pristine suspended graphene could be identified readily from its enhanced Raman spectra when using the air gap-based cavity. Using the cavity, the Raman scattering signals were enhanced up to nearly 500 times for either the PMMA-transferred or mechanically exfoliated suspended SLG. In a previous study, Lee et al. recorded the Raman spectra of multilayer (3−18 layers) MoS2 suspended over circular holes (diameter: 2−7 μm; depth: 3 μm) in SiO2/ Si substrates; they found that the Raman intensity of the multilayer MoS2 was enhanced only slightly because of multiple reflections within the multilayered MoS2.39 To the H


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Figure 6. (a) PL spectrum of exfoliated monolayer MoS2 flakes on the air gap-based nanocavity and calculated average E-field spectra on the 50 nm Al/125 nm air/1 nm MoS2, Si substrate/1 nm MoS2, and 300 nm SiO2/Si/ MoS2. (b) PL spectra of exfoliated monolayer MoS2 flakes on the air gap-based nanocavity, Si substrate, and 300 nm SiO2/ Si.

the MoS2 on the Si substrate, 300 nm SiO2/Si, and the air gapbased cavity, respectively; therefore, the presence of the cavity enhanced the absorption of the MoS 2 by factors of approximately 20 and 3 for Si substrate and 300 nm SiO2/Si, respectively (because the degree of light absorption is proportional to E2). On the other hand, at the wavelength of the PL emission peak (670 nm), the cavity also significantly increased the average E-field, and thus the spontaneous emission should have been enhanced. Figure 6b displays the PL spectra of monolayer MoS2 flakes measured at the same positions as the Raman spectra, as indicated in Figure 5c. The PL intensity of MoS2 on 300 nm SiO2/Si substrate was enhanced by approximately 43-fold relative to that on the Si substrate. Notably, the intensity of PL peaks on the air gap-based cavity was enhanced 180 times relative to that on the Si substrate. This significant enhancement was due to the aforementioned reasons, as well as the concentration of electrons in the monolayer MoS2. The MoS2 transferred on the Si substrate should possess an electron concentration higher than that of the suspended monolayer MoS2 above the cavity because MoS2 would be n-doped by the Si substrate. Through the injection of excess electrons from Si, MoS2 would produce a more negative trion, and therefore, its light-emitting efficiency would decrease65 because the efficiency of the PL signal generated from exciton recombination would be greater than that generated from trion recombination. In addition to the suppression of PL from trion recombination, this phenomenon would also cause the emission wavelength to red-shift slightly, as indicated by the green arrow in Figure 6b.66 We discuss the effect of the trion on the PL of the MoS2 in the following paragraph. In contrast, previous studies have revealed that light out-coupling of 2D materials was enhanced through plasmonic nanostructures or

Figure 5. (a, b) OM images of exfoliated MoS2 flakes on the (a) Si substrate and (b) air gap-based nanocavity (scale bar: 3 μm); inset to (b): schematic representation of MoS2 flakes on the air gap-based nanocavity. (c) Raman spectra of exfoliated monolayer MoS2 flakes on the Si substrate, air gap-based nanocavity and 300 nm SiO2/Si. (d) Calculated amplitudes of E-fields, plotted with respect to penetration depths, for the 50 nm Al/125 nm air/1 nm MoS2 and Si substrate.

respectively. The PL enhancement originated from both an increase in light absorption that enhanced the light−matter interaction63 and the spontaneous emission rate resulting from the Purcell effect.64 At the wavelength of the excitation laser (532 nm), the average E-fields were 0.388, 1.01, and 1.67 for I


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generated from the doping effect in MoS2.67 As displayed in Figure 7b, the intensities of the PL peaks dropped significantly, to only 1.8 and 9.5% of those of the suspended MoS2, when the MoS2 was in contact with the Al and Ag films, respectively. The average E-fields within the MoS2 on the Al film were 0.33/ 0.26 at the absorption/emission wavelengths; these values are significantly lower than the values of 0.6/0.47 and 1.67/1.74 on the Ag film and the air gap-based cavity, respectively. The low E-field on the Al film would directly decrease the absorption and emission of the monolayer MoS2; this phenomenon is one of the main reasons why the lowest PL intensity of MoS2 in this study was that on the Al film. Furthermore, we analyzed the effect of negative trions on the PL spectra of the monolayer MoS2. When the monolayer MoS2 was in contact with n-type dopants (Ag and Al films; Si substrate), it would obtain excess electrons from the substrates (Ag/Al/Si) to generate negative trions (the bound state of two electrons to a hole).67 Because the work function of Ag (4.74 eV) is greater than those of Al (4.24 eV) and Si (4.6 eV),50 the number of electrons in MoS2 (5.15 eV)68,69 coming from the Al and Si substrates would be greater than that coming from Ag. That is, the number of trions in MoS2 supported by an Al film or a Si substrate would be greater than that in MoS2 supported by a Ag film. Figure 8 reveals that the PL spectra of the four samples could be fitted to two peaks (under the assumption that both

photonic crystals, where the Raman/PL signals were not those from pristine or suspended 2D materials. In this study, the intrinsic properties of the pristine suspended 2D materials were not influenced by the presence of the air gap-based cavity, but these materials still exhibited great enhancements in both their Raman and PL signals. We used an X−Y scanning stage to measure the PL spectra of monolayer MoS2 at various positions. Figure 7a displays a

Figure 7. (a) PL spectra recorded at different positions of exfoliated monolayer MoS2 flakes on the air gap-based nanocavity; inset: schematic representation of the experimental scheme for positioning the MoS2 using an X−Y scanning stage. (b) PL intensity recorded at different positions of the exfoliated monolayer MoS2 flakes on the air gap-based nanocavity.

schematic representation of the setup. In great contrast to the Raman spectra of the SLG in Figure 4b,c, the shape and intensity of the PL peaks of the suspended monolayer MoS2 were unaffected by the position of the light, until the sample reached an area in direct contact with the metal film. As displayed in Figure 7a, upon moving from the center to position 4, the position, intensity, and shape of the peak in the PL spectra of the suspended monolayer MoS2 remained almost unchanged. Thus, the suspended MoS2 was not readily doped by the metal filmno matter how close the two becameas long as the MoS2 was not in direct contact with the metal film. In great contrast to graphene, the charge-carrier mobility of MoS2 is very low, resulting in a very short diffusion distance for charge carriers under zero bias. Figure 7b reveals that the intensity of the PL emission peak remained constant at positions from 0 (center) to 1200 nm (edge, position 4); this finding suggests that the fluorescence from the suspended MoS2 was not readily quenched through direct exciton ionization, regardless of the distance between the excitation laser spot and the cavity edge. When the MoS2 was in contact with the metal film, the peaks in the PL spectra of the monolayer MoS2 underwent significant changes in their position, shape, and intensity, due to the low surface E-field within MoS2 and the negative trions

Figure 8. Analysis of the PL spectral shapes for (a) suspended MoS2, (b) MoS2 on Si substrate, (c) MoS2 on Ag film, and (d) MoS2 on Al film. The peaks in the PL spectra were decomposed into two overlapping peaks with assumed Lorentzian functions, corresponding to A0 exciton and A− trion peaks.

are Lorentzian), which we assigned as having originated from radiative recombination of the A0 exciton (ca. 1.87 eV; λ = 663 nm) and the A− trion (ca. 1.79 eV; λ = 693 nm).65 The PL emission peak could be decomposed (Figure 8a) into an A0 exciton peak and an A− trion peak when the monolayer MoS2 was suspended.35 As displayed in Figure 8b−d, when the monolayer MoS2 was supported by Si, Ag, and Al, respectively, the spectral weight of the A− trion peak was greater than that of the suspended MoS2. The increase in the number of trions may have been the reason for the red shift of the signal in the PL spectra of the MoS2 on the Al film (λ = 700 nm) by nearly J


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as a result of the broad-band enhancement provided by the cavity. Furthermore, we found that the suspended MoS2 was not readily doped by the metal film from its edges. The great enhancement in the intensities of the PL signals of the suspended 2D materials allowed us to further investigate the spectral weights of both the A0 exciton and A− trion peaks when the MoS2 was suspended or supported by metal films. We hope that our investigation paves the way for the development of new methods for characterizing the precise and abundant properties of a variety of pristine and suspended 2D materials, with great signal enhancements.

30 nm, relative to that of the suspended MoS2 (λ = 672 nm). In contrast to the spectrum of the MoS2 suspended above the cavity, the spectral shape of the peak for the MoS2 on the Al film was obviously symmetrical (Figure 8d) because the emission peak was dominated by the A− trion. In general, negative trions in MoS2 lower the PL intensity, due to the excessive lifetime. Thus, we observed lower PL intensities for the MoS2 on the Al film and Si substrate compared to those when suspended above the cavity or on an Ag film. In summary, we have developed an air gap-based cavity structure to extract the intrinsic properties of various pristine 2D materials; here, the use of air as a spacer avoided the unpredictable influence of the substrates. The air gap-based cavity provided tremendous enhancements in both the Raman and PL signals of 2D materials, with several hundred-fold enhancement factors. In particular, the recording of spectra at different positions appears to be a powerful technique for monitoring the charge transfer behavior between the pristine 2D materials and their surrounding substrates.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b10470. • Simulation model of three-dimensional finite-difference time-domain method (3D-FDTD); optimization of the surface electric field; Raman shifts of the 2D and G bands; and estimated process of carrier concentration (PDF)

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CONCLUSIONS Because the surface properties of 2D materials are readily affected by their surroundings, underlying substrates and surrounding materials always disturb any examination of the intrinsic properties of pristine 2D materials. In this study, we found that the intrinsic properties of pristine 2D materials could be extracted from Raman and PL spectra, with great signal enhancements, through the effects of both air gap suspension and cavity enhancement. First, we investigated the interference-enhanced effect of the air gap-based cavity by using optical thin-film theory and 3D-FDTD methods. We found that the average E-fields within various 2D materials could be enhanced dramatically, due to constructive interference over a very broad-band spectral width. Each mode of the Raman emission lines obtained at different wavelengths from different 2D materials was enhanced to almost the same degree when employing an optimized air gapbased cavity. As a result, we found experimentally that the intensities of the 2D and G peaks of mechanically exfoliated and suspended SLG were enhanced almost equally; similarly, the intensities of the E12g and A1g peaks of MoS2 were largely coenhanced by up to same degree. The Raman peak positions of 2D materials are affected significantly by the carrier concentration. The mechanically exfoliated and suspended SLG was not disturbed by any polymer residue or any effects arising from doping by the substrates; therefore, this suspended SLG provided Raman spectral signals at the lowest wavenumbers (2663/1574 cm−1 for 2D/G bands) among those of our tested materials. Furthermore, recording the spectra at different positions appears to be a powerful technique for monitoring charge transfer between pristine 2D materials and their surrounding media. We found that the SLG was n-doped (p-doped) by the Al (Ag) film, with a diffusion distance for electrons (holes) from the Al (Ag) edge to the suspended SLG of approximately 900 (600) nm. We also noted that the residual holes in the suspended SLG could be neutralized by Al at a position approximately 600 nm away from the cavity edge. By using the air cavity structure, we could readily determine the CNP in suspended 2D materials. In addition, we experimentally verified that the PL intensity of MoS2 could also be enhanced, 180- and 4.2-fold relative to that on a Si substrate and 300 nm SiO2/Si, respectively. Both the light absorption and spontaneous emission of MoS2 increased

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(PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +886-2-3366-3240. ORCID

Hsuen-Li Chen: 0000-0002-7569-572X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the Ministry of Science and Technology, Taiwan, for supporting this study under contracts MOST 106-2221-E-002-158-MY3, MOST 106-2221-E-002105-MY3, and NTU-107L9008.

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