Near-Field Mapping of Localized Plasmon Resonances in Metal-Free

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Near-Field Mapping of Localized Plasmon Resonances in Metal-Free, Nano-membrane Graphene for Mid-Infrared Sensing Applications Mai Desouky, Muhammad R Anisur, Maria Alba, RK Singh Raman, Mohamed A. Swillam, Nicolas H. Voelcker, and Amal Kasry ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01631 • Publication Date (Web): 12 Oct 2018 Downloaded from http://pubs.acs.org on October 14, 2018

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ACS Applied Nano Materials

Near-Field Mapping of Localized Plasmon Resonances in Metal-Free, Nano-membrane Graphene for Mid-Infrared Sensing Applications Mai Desouky1, Muhammad R Anisur2, Maria Alba3&4, RK Singh Raman2,5, Mohamed. A. Swillam6, Nicolas H Voelcker3,4,7&8*, Amal Kasry7& 9* 1

MENTRC, The British University in Egypt, El-Sherouk City, Suez Desert Road, Cairo 11837, Egypt 2

Mechanical & Aerospace Engineering, Monash University, Clayton, VIC 3800, Australia 3

Monash Institute of Pharmaceutical Sciences, Monash University, Parkville, VIC 3052, Australia

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Commonwealth Scientific and Industrial Research Organisation (CSIRO), Clayton, VIC, 3168, Australia 5

6

Chemical Engineering, Monash University, Clayton, VIC 3800, Melbourne, Australia

Department of Physics, The American University in Cairo, AUC Avenue, New Cairo 11835, Cairo, Egypt 7

Melbourne Centre for Nanofabrication, 150 Wellington Rd, Clayton VIC 3168, Australia

ACS Paragon Plus Environment

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INM-Leibniz Institute for New Materials, Campus D2 2, Saarbrücken, 66123, Germany

Faculty of Engineering, The British University in Egypt, El-Sherouk City, Suez Desert Road,

Cairo 11837 KEYWORDS: graphene, plasmonics, mid-infrared, lithography-free, near field

ABSTRACT

Graphene, as an optically transparent material, typically defies any attempt for mid-infrared absorption, which limits its applications in mid-IR biosensing. Although remarkable evidence for mid-IR nanopatterned graphene plasmons has been reported via the induction of free charge carriers, no study so far has investigated plasmonic excitation in nanopatterned graphene without employing induced voltage, high chemical doping, or metallic reflectors. In this work, we show that localized plasmon resonance (LSPR) can be probed in metal free, naturally-doped, nanomembrane graphene (NMG) without induced voltage or using metallic layers. We rely on facile, lithography-free, fabrication methodology to pattern nanoscale holes in a single sheet of graphene using Au nano-islands with hole dimensions as small as 10 nm. We image the LSPR at the graphene membrane edges via scanning near field optical microscopy. Our experimental findings are confirmed by theoretical electromagnetic field mapping at the graphene membrane edges leading to noticeable absorption. We demonstrate the dependence of this absorption wavelength on the hole diameter and inter-hole distance, hence, we present a new avenue to fundamentally boost light harvesting with naturally-doped NMG which is pivotal for mid-IR sensors. We show that our designed NMG can be used as a mid-IR biosensor with theoretically-calculated sensitivity of 850 nm/RIU.


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1. Introduction The mid-IR is an interesting wavelength range for applications in infrared imaging, 1 biomedical diagnostics,2 and chemo- and biosensing applications.3-6 Sensing biomolecules exhibiting weak vibrational/rotational modes in the mid-IR regime is difficult due to the lack of suitable mid-IR absorbing materials as labels.6 Although plasmonics are widely used in optical biosensors, their natural plasmonic resonance occurs mostly in the visible regime.7-8 Researchers have been on a hunt for alternative materials with tunable plasmonic response extending to the mid-IR wavelength range,9-10 which is a useful window for gas and biological sensing applications.2 In the realm of new emerging plasmonic materials, graphene has gained a strong reputation. The intriguing characteristics of graphene surface plasmons (SPs) have made it a remarkable candidate for sensing,2-3, 5, 11 photo detection,12-13 and modulation.10, 14-16 Graphene SPs can sustain long relaxation time17 and can be tuned across the mid-IR wavelength range through different applied voltages.18-19 However, the one-atom-thick layer of graphene does not allow substantial absorbance to take place.19 Previous studies have examined localized SPs in graphene nanostructures whether in the form of nano-patterned graphene20-23 or graphene nanoribbons (GNR).24-27 The momentum mismatch between graphene SPs and the incident light can be addressed by nanopatterning the graphene layer, leading to strong field absorption with sub wavelength mode profiles.19, 23 Nano-meshed graphene (NMG) exhibits a photonic bandgap that undergoes semi metal-semiconductor transformation,3, 28-29 in which localized surface plasmon resonance (LSPR) can be excited by inducing free charge carriers19, 30. So far, the studies reporting doping of NMG or GNR have relied on applying external voltages20-21, 24-25, 30-32 or high chemical doping levels.23,

33-34

A recent study reported mid-IR absorption in highly-doped graphene

nanodisk arrays via an ionic gel with absorption value of 40%.23 Furthermore, other studies have


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reported absorption values as high as 90% in NMG,21 however, they included backside thick metallic layers to suppress graphene transmission which is not favored for CMOS compatible optoelectronic devices. Thus, there is a great need to investigate 2D mid-IR absorbing structures that can harvest light energy to serve as sensing platforms, taking advantage of replacing expensive metals with an atomic thick absorbing graphene layer. In general, engineering NMG to be used as a mid-IR absorber is of particular importance, not only due to the atomic thickness of graphene, but also due to its lower losses, and the possibility to offer selective binding sites in the chemicallyactive holes, which is pivotal for biosensing applications.29 One of the major drawbacks in the nanopatterning of graphene is the utilization of expensive, time consuming techniques, such as electron beam lithography (EBL).24,33 Other techniques like nanosphere lithography have also been widely proposed as alternatives to EBL,28, 34-37 however, nanosphere lithography is not suitable to fabricate nanoscale holes with 10 nm resolution. In our work, we propose a facile, lithography-free, fabrication method for nano-membrane graphene (NMG) preparation, in addition we show theoretically, and experimentally that naturallydoped NMG can produce significant localized surface plasmon resonance (LSPR) in the mid-IR wavelength range without any requirement for heavy doping, external bias, or the application of expensive metallic mirrors. We detected localized plasmon resonances via imaging the NMG using scattering type scanning near field optical microscope (s-SNOM). We further confirmed our experimental findings by theoretical mapping of the electromagnetic field (EM) distribution using finite-difference time-domain (FDTD). We report absorption up to 35% with an enhancement factor of 184 at the resonance wavelength, leveraging for the first time a nm lithography-free fabrication methodology to pattern the NMG with nanoscale holes down to 10 nm in diameter. We theoretically show that the NMG layer can be used as a sensing platform, a shift in the absorption


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wavelength can take place upon changing the refractive index of the surrounding medium. These results open up exciting avenues to realize NMG for mid-IR chemo- and biosensing.

2. Experimental Section 2.1 NMG fabrication First, a graphene monolayer grown on 15 µm Cu foil (Graphenea, USA) was transferred onto a quartz substrate (Figure 1a), using the protocol explained in Kasry et al.38 Second, a 10 nm Au layer was sputtered onto the graphene (using a sputtering system INTVAC AC/DC, Nanochrome) followed by annealing in air for 3 h at 300 ͦ C. Annealing enhances the Au atoms clustering to form Au nano-islands39-40 (Figure 1b). This was followed by the deposition of a 10 nm-thick Cr layer, using the same sputtering system, to cover the exposed graphene areas (Figure 1c). The Au nanoislands were then etched using Au etchant, leaving the underlying graphene exposed, while the rest was still protected by the Cr layer. Finally, an O2 plasma was applied to the holey-Cr-coated graphene (in a plasma cleaner Harrick Plasma, PDC-002 at 50 W for 4 min) to etch the exposed graphene areas. Cr was then removed using Cr etchant, leaving the graphene layer with random holes of the same shape and distribution as those from the Au nano-islands (Figure 1d), albeit with smaller sizes possibly due to the clustering mechanisms of the Au layer.39-40 We refer to this holeygraphene as NMG. It should be mentioned that, although we have not performed any doping while fabricating the NMG, we expect the NMG layer to be doped due to the exposure to various chemicals during the fabrication process, which might have led to inducing free charge carriers to the NMG. Thus, an


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annealing process was performed for NMG before carrying out the optical imaging to exclude the chemical dopants effect.41 This, however, did not leave the NMG completely undoped as shown in the results. A Dimension Icon atomic force microscope (AFM) was used to image the surface of the graphene and the NMG samples as shown in Figure 1(e-g). The imaging is performed in Tapping Mode using a SCANASYST-AIR cantilever. Graphene and NMG samples were characterized using a Raman spectrometer (Renishaw Invia micro-Raman Spectrometer, UK) equipped with 632 nm wavelength red laser operating at 10% of laser power with 1 μm spot size under a 100x objective. More details about the NMG characterizations are in the supporting information. 2.2 Optical Imaging We visualized the localized graphene plasmons by s-SNOM using a metallized tip of an AFM illuminated by an IR laser source as depicted in Figure 2a. Imaging was conducted using a scattering-type scanning near field optical microscope neaSNOM (Neaspec GmbH, Germany), more details are in the supporting Information. This highly localized field confined at the s-SNOM tip acts as an optical antenna to excite graphene plasmons. The metallized tip forms a wave that can radially propagate outwards and excite the plasmons in the NMG. Due to the interaction between the tip-derived wave and NMG plasmons, IR scattering signal entails information about phase and amplitude of the probed plasmons and the plasmon resonance. 2.3 Image analysis Near field optical microscope images were analyzed using Gwyddion software. To compare the plasmon signal, minimum and maximum color ranges in all images were unified. To compare the


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plasmon enhancement at the edges, image profile was extracted using the extract profile function in Gwyddion. The statistical function in the same software was used to calculate the different holes’ populations (more details about the holes percentages are in the supporting information). 2.4 Theoretical simulations We used Finite-Difference Time-Domain (FDTD) simulations to confirm our experimental findings and to shed further light on the EM field distribution within the NMG. Lumerical software was used to conduct the FDTD, which employs the Green’s function42 to calculate the surface conductivity of graphene upon certain applied voltage or doping concentration. The monolayer of graphene sheet was modeled as 0.3 nm thick layer with a dynamical surface conductivity of the intra- and inter-band transitions as follows:

 (,  c, , T ) =  intra (,  c, , T ) +  inter(,  c, , T )

Equation 1

 intra ( ,  c, , T ) =

 f d ( ) f d (− ) −ie2  ( − )d 2 ћ ( + i 2) 0  

Equation 2

 inter ( ,  c, , T ) =

 f d (− ) − f d ( ) ie2 ( + i 2)  d 2 0 ( + i 2) 2 − 4( / ) 2 

Equation 3

1

Fd ( ) = exp

 − c (

K bT

)

Equation 4

+1


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Figure 1. Schematic showing the fabrication process of the NMG. (a) Graphene layer on quartz substrate; (b) Au nano-islands formed after annealing, at 300 ͦ C for 3 h, of a 10 nm Au layer sputtered on the graphene sheet; (c) a thin sputtered Cr layer on top of the formed Au nano-islands; (d) NMG formed after Au nano-islands etching followed by O2 plasma treatment to etch the exposed graphene, then the holey-Cr layer is dissolved; (e) AFM image of the graphene sheet before any treatment; (f) SEM image of the Au nano-islands formed after annealing; and (g) AFM image of the resulting NMG.

Where, ω is angular frequency, Γ is a scattering term, µc is chemical potential, e is electronic charge, ħ is reduced Plank’s constant, Kb is Boltzmann constant and Fd() is the Fermi-Dirac


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distribution. The intra-band transition term is computed analytically while the interband transition is solved numerically.

3. Results and Discussions 3.1 NMG characterizations Figure 1g shows an AFM image of the NMG after the patterning process (Figure 1(a-d)) compared to a bare graphene sheet shown in Figure 1e. The structure resembles a membrane with nonperiodic holes of variable dimensions, hence the term nano-membrane graphene (NMG). Both graphene and NMG were compared using the Raman spectroscopy showing characteristic holes’ peak as indicated in the Supporting Information (Figure S1). The holes diameters varied from 10 to 25 nm whereas the center-to-center distances varied from 30 to nearly 60 nm. This is due to the random sizes of the Au nano-islands, as revealed by the scanning electron microscopy (SEM) image in Figure 1f. Raman spectroscopy measurements indicate the difference between the bare graphene and the NMG (Supporting Information).

3.2 Near-field imaging Figure 2b shows the s-SNOM phase images of graphene at the wavelength of 11.1 µm and NMG at multiple wavelengths: 10.7, 10.9, 11.1, and 11.3 µm, respectively. These images clearly indicate that for bare graphene sheet of zero bandgap, no plasmons can possibly be excited, as expected.


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Figure 2. Nano-IR imaging of confined plasmons in NMG. (a) Schematic of a nano-IR imaging of the NMG using the s-SNOM technique, where a metalized AFM tip is illuminated by an IR laser source in order to excite plasmons in the NMG; (b) the s-SNOM optical phase images of graphene at 11.1 µm and NMG at discrete wavelengths as indicated for each image; in case of graphene, no optical signal is observed, but a gradual change in the optical signal intensity and distribution are obvious in case of the NMG, according to the excitation wavelength. The color bar on the right indicates the false color scale used here. However, a majority of holes with specific dimensions (a and d) dominates the fabricated NMG, which results in this high pronounced optical signal at a wavelength of 11.1 µm. Other holes with different dimensions (which are minority in this case) can still resonate with the incident light, thus, we can still observe an optical signal in case of 10.9 and 11.3 µm.


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However, for NMG, an optical signal starts to appear at wavelength of 10.9 µm, whereas the highest recorded optical signal is at 11.1 µm. Moving further to longer wavelength, the optical signal starts to fade at 11.3 µm. We interpret these results as follows: light is not likely to couple to bare graphene sheet due to the momentum mismatch between graphene SPs and light. For NMG, the introduced membranelike structure acts as a momentum matching network that facilitates the LSPR excitation in the NMG sheet. However, at certain wavelengths, graphene plasmons are excited due to holes with specific dimensions (diameter “d” and center to center distance “a”). In other words, holes with certain dimensions can break the momentum mismatch, leading to coupling between the wave vectors of graphene plasmons and that of the incident light, attributed as resonance, appearing as a pronounced high optical signal at the resonating wavelength, in this case 11.1 m. This hypothesis is discussed in more details in the theoretical section below. Our fabricated NMG structure contains a combination of multiple holes of different dimensions due to the randomness of the clustered Au nano-islands; therefore, we can expect multiple LSPR probed at different wavelengths but with different optical intensities, as can be seen in Figure 2b. Figure 3a and b show the nearfield optical images of NMG and their corresponding optical signal profile at two different wavelengths: 10.9 µm and 11.1 µm, respectively. The extracted profiles, corresponding to the blue line on the optical images, indicate the localized plasmon enhancement at the edges of the holes. At 11.1 m (Figure 3b), the optical signal is higher as compared to that at 10.9 m (Figure 3a), confirming the field enhancement at 11.1 m due to the resonance triggered by the majority of holes of certain dimensions (a and d). The peaks in the extracted profile represent the field enhancement at the edges, and they correspond to the edge-to-edge-distances.


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Figure 3. The optical phase images of the NMG and the corresponding extracted line profiles showing the optical enhancement of the field intensity at the edges of the holes at (a) 10.9 µm and (b) 11.1 µm, respectively. The peaks indicate the enhanced signal at the holes’ edges for both resonances. The color scale resembles the plasmon resonance, where the peaks indicates the enhanced signal at the edges of the holes.

3.3 Theoretical analysis Based on our experimental results, we proceeded with further theoretical analysis to investigate the LSPR excitation at 11.1 µm, and to provide a clear understanding of the effect of holes’ dimensions on the LSPR in the naturally-doped NMG.


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Figure 4. FDTD simulation results performed with P-polarized light normally incident on NMG, with hole diameter “d” and center-to-center distance “a”. (a) absorption of NMG at 0.22 V (we refer to that as naturally -doped), and graphene sheet on quartz substrate at the same potential; (b) absorption of NMG on quartz with different “a” at fixed “d” (20 nm), (c) absorption of NMG on quartz with different “d” at fixed “a” (60 nm), (d) absorption of low doped NMG on quartz at different incident angles (0˚ to 60˚); (e) |E|2 for low doped NMG at wavelength 11.1 µm with a field enhancement maximum value of 36789; (f) |E|2 for low doped NMG at wavelength 14.0 µm with a field enhancement maximum value of 199 (The color scale in e and f represents the field intensity).

Figure 4a shows the FDTD simulation results for NMG with diameter (d) 20 nm and center-tocenter distance (a) 60 nm, at 0.22 eV potential, which yields an absorption peak at 11.1 µm, while graphene, at the same applied potential, yields zero absorption at the same wavelength. In these simulations, the frequency-dependent permittivity of the quartz across the mid IR wavelength range was extracted from Kischkat et al.

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This absorption wavelength (11.1 µm), is the same

wavelength previously depicted by the s-SNOM phase image in Figure 2b. Also, since our structure is not strictly periodic, we investigated the theoretical effect of the variation in hole dimensions (a and d) on the LSPR of NMG. Figure 4b shows the variation of the LSPR peak upon changing “a”. It can be clearly seen that as the distance between the holes is increased, the coupling strength between the plasmons of the NMG is reduced; therefore, the LSPR peak experiences a red shift. Similarly, when the distance between two holes is reduced, the energy of confinement is increased which results in blue shifting of the LSPR peak. However, when the distance between two holes is further reduced to 40 nm, there is no observable LSPR peak which indicates that there


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is an optimum center-to-center distance that achieves coupling between light and graphene plasmons. When two holes lie at close proximity (40 nm or less), damping becomes very high and the plasmon coupling cannot be identified. As previously mentioned, our fabricated NMG displays holes with variable “a” where the fraction of holes with specific dimension is what determines the LSPR resonance wavelength. For example, the LSPR at 11.1 µm is the result of holes of a = 60 nm. Other resonating wavelengths can be expected at various “a” as indicated in Figure 4b. The same concept is adopted for holes with variable diameters; Figure 4c shows the variation of the LSPR peak upon changing “d”, where reducing the hole diameter red shifts the absorption peak. However, further increase in diameter results in blue shift. At 40 nm diameter, damping rises dramatically, and the plasmon resonance cannot be observed. Although theoretically the change in “d” results in a change in the resonance wavelength, we argue that the variation of “a” has a more pronounced effect on altering the absorption for two reasons: the first is that, the change in “d” dramatically alters the LSPR peak shift on a broader wavelength range whereas the change in “a” alters the LSPR peak on a narrower wavelength range (Figure 4(b-c)). Also, by comparing Figure 4b and 2b, we observe that the resonance at 10.7 µm can be achieved with holes of a=63 nm, whereas by comparing Figure 4c and 2b, holes of “d” exceeding 25 nm are required to produce LSPR at the same wavelength, however, this diameter is not likely to be found in our fabricated NMG (Figure 1g). The second reason is that, as depicted from the AFM image in Figure 1g, the variation in “d” is small compared to “a”. Most of the holes seem to have “d” varying only between 10 to 25 nm (Supporting Information). We further investigated if the excited NMG plasmon is a localized plasmon or rather a surface wave. This aspect was tested in our theoretical analysis through the excitation of plasmonic


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response at all angles of incidence. Figure 4d shows that this absorption peak is reproducible at normal and oblique incidence from 0 to 60˚, thus corroborating the finding that the observed peak is indeed due to localized plasmons that can be excited upon breaking the momentum mismatch between the incident light and NMG plasmons. Mapping the localizations of the EM field distribution in NMG at the resonance wavelength of 11.1 µm confirms that the plasmonic response is localized at the graphene membrane edges with field enhancement factor of 184, as compared to the NMG at wavelength of 14.0 µm, where the NMG exhibits no absorption (Figure 4(e-f)). The field enhancement factor is calculated from the magnitude of the absolute electric field component using the following relation:

Enhancement factor =

E11.1 E14

2

2

Equation 5

The EM field is enhanced at the membrane edges whilst it degrades away as the plasmons propagate further from the edges on both sides. Figure 5a shows the EM field distribution between two holes patterned in NMG (d = 20 nm, a = 60 nm) at the resonating wavelength of 11.1 µm. It can be clearly seen that there is a decaying field in the graphene itself (at and around the edges), which indicates that the NMG can act as a plasmonic network where its peak intensity is at the membrane edges. Figure 5b shows a comparison between the experimental s-SNOM optical signal for graphene sheet and NMG at the resonance wavelength (11.1 µm) with respect to the theoretical electric field distribution at the NMG edges. This optical signal features two distinct peaks corresponding to the distance between the NMG edges, while for the pristine graphene sheet, the optical signal is flat, lacking an excited plasmon. According to Figure 5a, the theoretical edge-toedge distance for this resonating wavelength is 40 nm. However, the measured distance between


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Figure 5. (a) FDTD simulations showing the EM field between two holes in NMG (d = 20 nm, a = 60 nm) at 11.1µm, indicating the decay of the field intensity around the edges. (b) Optical signal profile of NMG (same as in figure 3b) and graphene at 11.1 µm, clearly indicating the enhancement at the hole’s edges in case of the NMG, whereas the flat profile in case of the non-patterned graphene indicates the absence of plasmon resonance.


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the optical signal peaks at the edges in Figure 5b is 60 nm. We are ascribing this discrepancy to the broadness of the peaks, the variation in the dimensions of the structure, and to the resolution limitation of the s-SNOM which makes it difficult to distinguish the edges at this scale of 20 nm. It is important to point out that the holes’ edge-to-edge distances are the major factor in altering the plasmonic response at a particular wavelength. Therefore, by controlling the hole dimensions (a and d), the LSPR response can be tuned.

3.3.1. Effect of doping Plasmon resonance can be achieved when matching between the frequency of incident light and that of plasmons is fulfilled. In case of our NMG, patterning the graphene into nanoholes introduces a photonic bandgap19, 28, 29 creating empty states (air dielectric regions) and filling other states (membrane edges) allowing for possible electron-hole excitations in which excited plasmons can couple to. Theoretically, there must be a minimum applied potential that needs to be induced for plasmon resonance to be excited, we claim that these charge carriers have been naturally induced. Even though annealing was performed to remove the chemical dopants introduced in NMG, minor doping would still be expected from the exposure of active pore sites to air, thus referred as naturally-doped NMG. In order to confirm such a claim, we conducted FDTD simulations on NMG with the same studied dimensions (d=20 nm and a= 60 nm) at different applied potentials. Figure 6a shows the absorption spectrum of NMG at different applied potentials: 0.15, 0.20, 0.22, 0.25 and 0.30 eV. At a potential of value 0.20 eV, a weak localized plasmon response is observed. However, at a


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Figure 6. (a) FDTD simulations of NMG (d=20 nm and a=60 nm) showing the effect of applied potential on the absorption spectrum. (b) Normalized absorption spectrum of highly doped nonpatterned graphene, and NMG without any induced potential, showing that both cases do not exhibit any absorption.

potential of 0.22 eV, the localized plasmon response is seen at a wavelength of 11.1 µm, which corresponds to the same wavelength previously demonstrated by the s-SNOM phase image (Figure 2b). A further increase in the applied potential shifts the localized plasmon resonance peak to lower wavelength. Increasing the potential increases the number of induced charge carriers, which increases the coupling efficiency between the plasmons and the excited electron-hole pairs. Therefore, strengthening the plasmon energy will allow the absorption peak to experience a blue shift. Comparing the experimental results to Figure 6a indicates that the potential (0.22 eV) corresponds to the NMG natural doping effect that causes LSPR to be excited.


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In order to confirm that the excited localized plasmons is due to the combined effect of graphene patterning into NMG and natural doping, we investigated the absorption of highly doped graphene on quartz with potential of 0.6 eV without any patterning, and NMG without any induced potential. Figure 6b shows the normalized absorption of highly doped unpatterned graphene and that of NMG without applied potential (both normalized with respect to the quartz substrate), which indicates zero absorption, as expected, thus confirming that only through patterning, together with minimum level of doping, a localized plasmon resonance can be excited and detected (More details are in the supporting information, Figure S4).

3.4 NMG as a biosensor Mid-IR, label-free biosensors are of great importance in studying the structural signature of some proteins, where the vibrational fingerprints of a protein might have conformational information that can help in understanding its function in some diseases.5, 44-45 Graphene plasmonic structures offer strong potential for the development of mid-IR biosensors.6, 46-47 We show here that NMG has the potential to be a sensitive mid-IR biosensor. We theoretically investigate the shift in resonance due to the change in refractive index of the medium surrounding the NMG, this is theoretically performed in Lumerical FDTD simulations by changing the background index above the NMG and inside the holes. We used different values of refractive indices (n) that represent the increase in “n” with increasing the biological target concentrations, with a background refractive index of water (1.33). Figure 7a shows the shift in the absorption wavelength upon changing “n”, Figure 7b shows that the dependence of the wavelength on the refractive index is linear with a slope of 825, revealing a sensitivity of 825 nm/RIU in a dynamic range of 0.12 RIU.


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Figure 7. FDTD simulations of (a) Absorption at different refractive indices of the NMG surrounding medium, and (b) the resonance wavelength as a function of the refractive index of the surrounding medium, showing a shift in the resonance wavelength upon increasing the refractive index of the surrounding medium.

This sensitivity is slightly higher than other nanostructured sensors based on silicon cavities or gold nanoslits48-49. There might be a window for enhancing the sensitivity even further by controlling the holes’ diameters and center-to-center distances. Although these theoretical results are based on using a regular structure, which is not the case in the fabricated one, we believe that the results might reflect the experimental sensitivity, as the majority of holes have the regularity of 20 nm hole size and 60 nm center-to-center distance, the calculations of the holes’ population is explained in the supporting information. As previously mentioned, besides having the advantages of exciting LSPR without the need of external voltage or metallic layer, NMG offers another advantage which is the possibility to functionalize the chemically-active holes. This can be a platform for higher surface area as well as higher specificity, which are two optimum requirements for a highly-sensitive biosensor.


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4. Conclusion In our study, we described a methodology to design a graphene nano-membrane structure prepared using simple and inexpensive, lithography free protocol by patterning nanoscale holes with dimensions down to 20 nm. We report, for the first time, the excitation of LSPR in NMG without the need of induced potential, external chemical doping, or even thick reflective metallic layers. LSPR was optically detected via s-SNOM technique in the mid-IR wavelength range. We report an absorption in NMG reaching 35% at normal incidence and 39% at oblique incidence in the midIR wavelength range. These results refute the previously held belief that induced potential, modifying graphene edges into a zigzag or arm chair edges, or high doping levels are necessary to excite graphene plasmons. Finally, in terms of practical applications, our results engender new paradigms to utilize mid-IR absorption for biosensing, where a sensitivity of 825 nm/RIU is theoretically demonstrated.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website. The supporting information contain description of the optical imaging set up, the used characterization techniques, as well as details of the calculating the holes’ percentages. Details of the theoretical analysis is also explained.


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AUTHOR INFORMATION Corresponding Author *Email: [email protected], *Email: [email protected]

Notes The authors declare no competing financial interests.

Author Contributions AK has fabricated the NMLG, imaged, and analyzed the NMLG plasmonics at the Melbourne Centre for Nanofabrication under the supervision of NHV. MD has performed the theoretical modeling supervised by AK at the BUE and MAS at the AUC. All other authors were involved in the characterizations like Raman spectroscopy, SEM, and AFM, as well as analyzing the results and editing the manuscript. MD and AK have written the manuscript, and it was edited and revised by all other authors.

ACKNOWLEDGMENT This work was performed in part at the Melbourne Centre for Nanofabrication (MCN) in the Victorian Node of the Australian National Fabrication Facility (ANFF), and with the support of the British University in Egypt (BUE). The authors would like to thank Ahmed Maarouf for the comprehensive discussions.


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