Graphene-activated Optoplasmonic Nanomembrane Cavities for

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Functional Nanostructured Materials (including low-D carbon)

Graphene-activated Optoplasmonic Nanomembrane Cavities for Photodegradation Detection Yin Yin, Jinbo Pang, Jiawei Wang, Xueyi Lu, Qi Hao, Ehsan Saei Ghareh Naz, Xinxing Zhou, Libo Ma, and Oliver G. Schmidt ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00733 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 9, 2019

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ACS Applied Materials & Interfaces

Graphene-Activated Optoplasmonic Nanomembrane Cavities for Photodegradation Detection Yin Yin†,‡, Jinbo Pang§, Jiawei Wang*,†,⊥,#, Xueyi Lu†, Qi Hao†, Ehsan Saei Ghareh Naz†, Xinxing Zhou∥, Libo Ma*,†, Oliver G. Schmidt†,⊥,# †Institute for Integrative Nanosciences, IFW Dresden, 01069 Dresden, Germany ‡ School

of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, China

§ Institute for Complex Materials, IFW Dresden, 01069 Dresden, Germany ∥ Synergetic Innovation

Center for Quantum Effects and Applications, School of Physics and

Electronics, Hunan Normal University, Changsha 410081, China ⊥ Material Systems for Nanoelectronics, Technische Universität Chemnitz, 09107 Chemnitz, Germany # Research Center for Materials, Architectures and Integration of Nanomembranes (MAIN), Technische Universität Chemnitz, Rosenbergstr. 6, 09126 Chemnitz, Germany Corresponding Authors: *Email: [email protected] and [email protected]. KEYWORDS Graphene, Optoplasmonic sensors, Whispering gallery modes, Photocatalysts, Photodegradation

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT

Graphene, with its excellent chemical stability, biocompatibility and capability of electric field enhancement, has a great potential in optical and optoelectronic applications with superior performances by integrating with conventional optical and plasmonic devices. Here, we design and demonstrate graphene-activated optoplasmonic cavities based on rolled-up nanomembranes, which are employed for in-situ monitoring the photodegradation dynamics of organic dye molecules on the molecular level in real-time. The presence of the graphene layer significantly enhances the electric field of hybrid optoplasmonic modes at the cavity surface, enabling a highly sensitive surface detection. The degradation of Rhodamine 6G molecules on the grapheneactivated sensor surface is triggered by localized laser irradiation and monitored by measuring the optical resonance shift. Our demonstration paves the way for real-time, high-precision analysis of photodegradation by resonance-based optical sensors, which promises the comprehensive understanding of degradation mechanism and exploration of effective photocatalysts.


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Photocatalytic degradation as a fundamental physicochemical process has attracted increasing attention in accelerating pollutant degradation and energy conversion.1-5 In comparison with conventional biological and physical treatments, the photodegradation method is efficient, economical and environment-friendly. Thus, it is of high interest to investigate the degradation technique including the generation, mechanism and detection of the physicochemical process. Conventional methods for detection of degradation dynamics rely on the measurement of absorbance spectra or photocurrent densities.2 However, it is still challenging to accurately monitor the photodegradation of organic compounds on the molecular level and realize spatially resolved information in a sub-micrometer-sized area. Over the past two decades, miniaturized optical sensors have been extensively explored for damage-free, real-time and label-free detection in the biological/chemical analysis.6-13 Among various optical sensors, whispering-gallery-mode (WGM) microcavities have been proven a powerful platform with ultra-high accuracy and resolution down to single molecules/particles.6,

7, 12, 13

Moreover, the strategy of combining plasmonic

nanostructures and WGM microcavities has further improved the sensing performance due to enhanced field localization,14-18 which in turn has enabled detection of single proteins and ions.19, 20

Therefore, it is highly desired to apply the optoplasmonic WGM microcavities for detecting

degradation and pushing down the limit of detection (LOD). Graphene, as a chemically stable and biocompatible two-dimensional (2D) material, has provided a fantastic platform to investigate and exploit optoplasmonic sensing.21-28 In particular, the flat single-atom-layered hexagonal structure with a high charge density is capable of absorbing tiny molecules through strong π-π stacking, which leads to a chemically enhanced sensing capability.21 In addition, the plasmonically enhanced localized electric field can be further



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strengthened by the charge transfer from graphene to the noble metal surface, which facilitates the sensitivity

improvement.23-25

Here,

we

propose

and

demonstrate

graphene-activated

optoplasmonic nanomembrane-based cavities for detecting photodegradation processes of organic dye molecules Rhodamine 6G (R6G). The sensor is formed by successively coating noble metal and graphene layers on rolled-up nanomembrane surface. The presence of the graphene layer enables the formation of pronounced hybrid modes with increased plasmon-type evanescent fields. In this way, ultra-sensitive detection of the R6G molecule degradation under light irradiation is experimentally achieved with a sub-molecular-layer level. This proof-of-concept demonstration opens up a new pathway towards monitoring and analyzing photodegradation by resonance-based optical sensors, which is expected to provide us with deep insight into the degradation mechanisms and allow to explore efficient photocatalytic degradation processes.

RESULTS AND DISCUSSION The optoplasmonic sensor is made by combining graphene layers with a gold-coated nanomembrane cavity, as schematically displayed in Fig. 1. The cavities were fabricated by employing the rolled-up nanotechnology,29-31 as depicted by the schematic diagrams in the bottom panels of Fig. 1(a). In brief, a pre-strained 35 nm SiOx nanomembrane was released from a Ushape pattern to form an array of the microtubular structure with a very high yield (see Experimental Section and Supporting Information Fig. S1). In the following, a 6 nm gold layer was deposited onto the microtubes to form the optoplasmonic cavities, in which strong plasmontype electric fields associated with a high sensitivity to changes in the surrounding environment can be obtained.31 In the present work, high-quality monolayer graphene synthesized by chemical


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vapor deposition (CVD) is introduced on the optoplasmonic cavities as a novel platform to detect photodegradation. Optimal growth protocols have previously led to the preparation of large domains of single crystal monolayer graphene.33-35 Here, large domains of monolayer graphene were removed from Cu foil to a Poly(methyl methacrylate) (PMMA) layer, as sketched in the upper panels of Fig. 1(a). Eventually, the graphene was transferred onto the gold-coated microtubes after dissolving the PMMA layer (see Experimental Section).

Figure 1. (a) Schematic diagrams of the preparation process of a graphene-activated optoplasmonic sensor. (b) A schematic of the designed sensor. Figure 2(a) shows a top-view scanning electron microscopy (SEM) image of a representative graphene coated optoplasmonic nanomembrane cavity, where the graphene pieces enwrapped on top of the microtube can be identified by dark patterns. Raman spectroscopy was applied to examine the quality of the monolayer graphene. Fig. 2(b) shows Raman spectra of the graphene layers before and after being transferred onto the cavity. The G and 2D mode are revealed at 1580 cm-1 and 2700 cm-1, respectively. The intensity ratio of the 2D to G mode is 2.63 and 2.54 in the Raman spectra measured before and after the transfer, confirming the monolayer feature of the graphene.36, 37 In addition, the D band (1350 cm-1) has been adopted to evaluate defects in



graphene.38, 39 Here, the D band is negligible in the measured Raman spectra, implying a high quality of the graphene before and after layer transfer. It is also notable that there is a significant decrease of the Raman peak intensities after the graphene layer has been transferred onto the cavity, which is ascribed to the optical absorption loss in the gold layer.40, 41 Moreover, there is a slight shift of the 2D mode due to the stress release of the graphene layers on the tubular surface.42, 43 To further examine the graphene layer on the tubular cavity, energy dispersive X-ray (EDX) spectroscopy was employed to measure the elemental distribution in the graphene/gold-coated cavity, which agrees well with the SEM image, as shown in Fig. 2(c).

Figure 2. (a) SEM images of a representative gold-coated nanomembrane cavity with graphene coverage. The dark hexagonal pieces denote the graphene layer. A magnified image is shown in the right panel. (b) Raman microscopy spectra of the graphene layer before (on copper foil) and after transferring (on tube). (c) EDX mappings of Au and C of the graphene/gold-coated nanomembrane cavity.


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Tubular nanomembranes support optical WGM resonances along the cross-sectional ring trajectory. Because of the subwavelength-thin cavity wall thickness (~ 100 – 200 nm), a strong evanescent field extends out of the tube surface, which can greatly promote light-matter interactions at the surface and enables various sensing applications and strong coupling systems.4446

Recently, hybrid photon-plasmon modes with largely enhanced evanescent fields have been

demonstrated in metal-coated tubular nanomembrane cavities.32 Besides, it has been reported that a graphene layer can also enhance WGM modes in dielectric microcavities.47-49 Hence, it is desired to develop tubular sensors with enhanced surface fields by introducing both plasmonic and graphene layers. Here, we found that the hybrid modes can be further enhanced by covering monolayer graphene onto an optoplasmonic nanomembrane cavity. Fig. 3(a) displays the transverse magnetic (TM) modes (with the magnetic field parallel to the tube axis) measured on the cavity with a diameter of ~ 8.2 m (in Fig. 2(c)) with (red dot) and without (black dot) a graphene layer. The free spectral range (FSR) of ~8.9 nm suggests a nice consistency with the cavity size. A significant enhancement of the mode intensity is observed at the site with graphene coverage, which is induced by the charge transfer from graphene to the gold layer.25, 50 It has also been reported that the field enhancement decays exponentially with the increase of graphene layers (i.e. bilayer or 3-layer) due to the light absorbed by the graphene itself.25,

51

Thus, a single graphene layer is highly

beneficial for enhanced sensing capability.



Figure 3. (a) Optoplasmonic cavity modes at the site with (red) and without (black) graphene layer. The inset SEM image denotes the measurement sites. “m” stands for the azimuthal mode order. (b) Calculated electric field distributions of the cavity mode without (left) and with (right) a graphene layer. Compared with the resonant spectrum measured at the site without the graphene coverage (black dot), a slight redshift of the resonant modes is discerned at the graphene-coated site (red dot), as indicated by the dashed lines in Fig. 3(a). This redshift is induced by the presence of the graphene layer with positive dielectric permittivity.52, 53 The 2D material possesses an anisotropic dielectric function with in-plane (ε ∥ ) and perpendicular (ε⊥) components because of its singleatom-level thickness, where ε ∥ = 5.04 and ε⊥ = 2.12, respectively, in the visible range.54 The different light polarizations are expected to experience different mode interactions with the graphene layer, i.e., the transverse electric (TE) modes are only influenced by the in-plane


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perturbation (ε∥) while the TM modes are influenced by ε⊥. In this work, however, the TE modes are hardly observable due to the strong shielding effect by the gold coating.32 The enhancement effect is confirmed by calculating the field distributions on the optoplasmonic nanomembrane cavity surfaces with and without graphene coating, as shown in Fig. 3(b). Compared to no graphene coating (left panel), a much stronger electric field is localized in the vicinity of graphene (right panel). The sensor response (i.e., resonance shift) is mainly related to the fraction of the electric field at the sensor surface interacting with the analytes. Thus, a stronger optical field promises a higher sensitivity towards any small perturbations on the cavity surface.55 It has been reported that electrons get transferred from graphene to the Au thin film in order to maintain the continuity of the Fermi levels when they contact with each other, as the work function of Au (5.54 eV) is larger than that of graphene (4.5 eV).50 As such, the charge transfer also plays a key role for the electric field enhancement at graphene-on-Au surface.25 We apply the graphene-enhanced optoplasmonic nanomembrane cavities into detection of the photodegradation of organic molecules. R6G molecules, as a widely used organic dye in textile industries, are introduced onto the graphene-activated optoplasmonic cavity surface and degraded by light irradiation, as schematically shown in Fig. 4(a). In conventional methods, the decolorization in the decomposition of the dye molecules can be recorded by an absorbance spectrum with a UV–Vis absorption spectrometer.2, 27 However, it is difficult to achieve real-time monitoring along with the photodegradation process. In addition, current detection methods require the analyte molecules to be distributed in an aqueous solution which limits the sensing in different degradation environments. Here, the degradation of the R6G molecules is triggered by the laser irradiation (457 nm, 2 mW) and simultaneously monitored by measuring the resonance shift of the optoplasmonic cavity. Owing to the configuration of the graphene-activated



optoplasmonic nanomembrane cavity, a superior sensing response over the conventional optical and optoplasmonic sensor without graphene coating is obtained for resolving the mode shift in the degradation dynamics. The R6G molecule layer was first coated on the tube surface by soaking it into the aqueous solution with different R6G concentrations (from 10-4 to 10-6M) for 2 h and drying in air. The R6G molecule layer (with a refractive index of ~1.656, 57) can be degraded into CO2, H2O and other products which are evaporated easily from the sensor surface, leading to the continuous blueshift of the resonance (see Supporting Information Fig. S2). Fig. 4(a) displays the resonant wavelength shift in response to the laser irradiation time. Perturbation theory was used to calculate the resonance shift against the molecule layer thickness change.45 By fitting the experimental results into the simulation model, the degraded R6G layer thickness can be estimated. For instance, a total blueshift of 3.1 nm was observed for the case of 10-4 M R6G concentration, corresponding to a molecule layer thickness of 4.0 nm. In contrast, the resonant mode shift measured at a nongraphene site under the same concentration condition (e.g., black dot in Fig. 3(a)) is almost neglectable due to the less sensitive detection capability as well as the poor adhesion on a bare Au surface (see Supporting Information Fig. S3). Owing to the graphene-induced field enhancement effect as discussed above, the optoplasmonic cavity coated by a graphene monolayer possesses a higher sensitivity, where the mode shift is much larger than that without graphene coating in both experimental results (see Supporting Information Fig. S4) and numerical calculations (see Supporting Information Fig. S5). In addition, we note that the LOD (usually defined as λresolution/sensitivity) of the optoplasmonic sensor can be further improved by introducing a sharp resonant peak with a narrow linewidth. This can be obtained by optimizing the tubular axial confinement in the future.17


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It has been reported that there is a negative thermal expansion coefficient in graphene58 and the thermo-optic effect in R6G-doped microcavities59 which may influence the effective refractive index of the optoplasmonic cavity and contribute to the extra mode shift. Here the thermal effect induced extra mode shift can be negligible due to the relatively low laser power (2 mW) and good thermal conductivity of ultra-thin tube wall.

Figure 4. (a) Measured resonant mode shift upon different R6G concentrations. Measurement data (scattered symbols) are fitted by exponential decay curves (solid lines). The inset shows the schematic of R6G photodegradation detected on the platform of the graphene-activated optoplasmonic cavity. The corresponding R6G layer thickness is identified based on simulation results. (b): Calculated mode shift in response to the change of R6G molecule layer thickness upon degradation on the graphene-activated optoplasmonic cavity. Insets: a schematic of resonant light field interacting with the R6G layer on graphene (top-right) and resonant mode blueshift in response to the R6G layer change (left-bottom). The resonant mode shift can be well fitted by an exponential decay curve, except for the case of the coated multilayer R6G (e.g., 10-4 M R6G concentration). As shown by the red squares in Fig. 4(a), for the multilayer case the mode shift first experiences a slow and linear shift due to



the strong π-π interactions between the multilayered dye molecules which potentially slow down the degradation process under laser irradiation.27 When the molecule layer decreases as thin as one monolayer (~1 nm), an exponential decay of the R6G layer is observed, where the dynamic degradation mechanism is related to the linear-driving force (LDF) model60 for dealing with molecular layer changes. As expected, the thinner R6G layer exhibits a faster degradation time under the same laser irradiation power, as shown in Fig. 4(a). Benefitting from the excellent chemical stability and compatibility of the graphene layer and reliable structural stability of the rolled-up microtube cavities, there is no changes on the sensing performances as the same preparation and sensing steps have been repeated several times, revealing the good reusability and stability of the sensing system. Apart from the sensing application demonstrated in this work, it would also be possible to integrate photocatalysts in the present platform by functionalizing the cavity surface by coating candidate nanomembrane materials such as TiO2 and ZnO which have already been widely used in plasmon-enhanced photocatalysis.61-64 Previous studies have demonstrated the photoactivity of the functionalized TiO2 microtubes as micromotors and swimmers under UV light irradiation.65 In addition, more sensitive degradation detection beyond organic dyes, such as gas molecules like NO2 and CO2, is feasible as the Q-factor of the optoplasmonic sensor becomes further improved. With investigations on the present sensing platform, the basic mechanisms underlying the degradation process, especially in the presence of a photocatalyst, may be discovered through analyzing the optical signals such as resonance shift, mode splitting and linewidth broadening.6, 12, 66


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CONCLUSION In conclusion, a real-time high-precision detection scheme for detection of photodegradation dynamics has been demonstrated based on a graphene-activated optoplasmonic nanomembrane cavity. By coating monolayer graphene onto the optoplasmonic cavity, the electric field of hybrid optoplasmonic mode at the sensor surface is significantly enhanced, which allows for highly sensitive detection of the degradation at the hybrid cavity surface. The degradation of R6G molecules on the curved nanomembrane surface was triggered by laser irradiation and monitored by measuring the resonance shift of the optoplasmonic cavity. The results of this work pave the way towards further investigations of photodegradation detected by resonant light which may push catalytic-mechanism-oriented studies to the few-molecule level.

EXPRIMENTAL SECTION Fabrication of graphene/gold-coated nanomembrane cavities. Cavities were fabricated by rolling prestrained nanomembranes into three-dimensional tubular structures. Photoresists (ARP3510, Allresist GmbH) serving as the sacrificial layer was firstly patterned on Si wafer by standard photolithography process (MJB4, SÜSS Microtec). A SiOx layer with the thickness of 35 nm was afterwards deposited onto the photoresist layer using electron beam evaporation (Edwards Auto500 e-beam). The deposition rate was changed from 6 Å/s to 0.5 Å/s for creating differential strained layers. The sacrificial layer was etched away by acetone to roll up the nanomembranes. To avoid the structural collapse, a critical point dryer (931 GL, Tousimis CPD) was used to dry the microtubes. An HfO2 layer with a thickness of 30 nm was grown on the microtube surfaces to mechanically strengthen the structure by ALD. A 6 nm gold layer was deposited on the microtubes



using electron beam evaporation. Next, CVD grown monolayer graphene (ACS Material, LLC) was transferred from the copper foil to the microtube according to a transfer protocol.67, 68 In brief, the graphene monolayer on copper foil was spin-coated with a PMMA layer, and then detached from the copper foil by using an ammonium persulfate aqueous solution and then cleaned in distilled water. Next, the graphene is fished to the gold-coated microtubes in isopropanol medium. The PMMA was removed by dissolving in acetone for 5 times. After drying in air for 3h, the graphene-coated cavities have been readily prepared eventually. Materials characterization. The morphologies and structures of the graphene/gold-coated cavities were characterized by scanning electron microscopy (SEM, Zeiss DSM982) operated at 5 kV. Raman spectroscopy (LabRAM HR Evolution, Horiba) was employed to check the quality of graphene monolayer. Optical sensing of R6G degradation. Graphene/gold-coated cavities were immersed into the R6G aqueous solution (Sigma-Aldrich) with different concentration of 10-4, 10-5 and 10-6 M for 2 h, to absorb the molecules on the sensor surface. A 457 nm continuous-wave diode-pumped laser (Cobolt) with a moderate power of 2 mW was used in our measurement for exciting the optical WGM resonances and simultaneously triggering the degradation. Meanwhile, the resonances from the cavity were recorded every 1 min to monitor the degradation process. Numerical simulations. Simulations of the electric field of the microtube sensors were performed by finite element method (COMSOL Multiphysics) on a workstation (see Supporting Information Fig. S6). A circular calculation domain with a diameter of 25 µm was set and the mesh size was set as 5 nm. Fine meshing was applied to regions around the metal and graphene region with the size of less than 1 nm. Afterwards, the corresponding scattering boundary conditions and perfect


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matching layers were set to simulate the mode field distribution. The dipole excitation source was put inside the tube wall to simulate the pumping light. A far-field monitor was applied for measuring the resonances in the visible range.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the Internet at http://pubs.acs.org. An overview SEM image of the cavities on Si wafer (Figure S1); Mode shift in response to the laser irradiation induced molecules degradation performed by the optoplasmonic sensor with graphene (Figure S2); Mode shift in response to the laser irradiation induced molecules degradation performed by the optoplasmonic sensor without graphene (Figure S3); Resonance shift in response to the R6G molecules degradation with and without graphene layer (Figure S4); Calculated mode shift in response to the change of R6G molecule layer thickness upon degradation on the optoplasmonic cavity with and without graphene layer (Figure S5); Schematic showing the setting for the two-dimensional numerical simulations and the simulated mode field intensity profile (Figure S6).

Notes The authors declare no competing ﬁnancial interest.



ACKNOWLEDGMENT The authors thank R. Engelhard, S. Harazim, B. Eichler and S. Baunack for technical support. This work was supported by the German Research Foundation DFG (Grant FOR 1713, SCHM 1298/221 and Leibniz program).


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Figure 1. (a) Schematic diagrams of the preparation process of a graphene-activated optoplasmonic sensor. (b) A schematic of the designed sensor. 125x46mm (300 x 300 DPI)

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Figure 2. (a) SEM images of a representative gold-coated nanomembrane cavity with graphene coverage. The dark hexagonal pieces denote the graphene layer. A magnified image is shown in the right panel. (b) Raman microscopy spectra of the graphene layer before (on copper foil) and after transferring (on tube). (c) EDX mappings of Au and C of the graphene/gold-coated nanomembrane cavity. 142x127mm (300 x 300 DPI)


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Figure 3. (a) Optoplasmonic cavity modes at the site with (red) and without (black) graphene layer. The inset SEM image denotes the measurement sites. “m” stands for the azimuthal mode order. (b) Calculated electric field distributions of the cavity mode without (left) and with (right) a graphene layer. 87x108mm (300 x 300 DPI)



Figure 4. (a) Measured resonant mode shift upon different R6G concentrations. Measurement data (scattered symbols) are fitted by exponential decay curves (solid lines). The inset shows the schematic of R6G photodegradation detected on the platform of the graphene-activated optoplasmonic cavity. The corresponding R6G layer thickness is identified based on simulation results. (b): Calculated mode shift in response to the change of R6G molecule layer thickness upon degradation on the graphene-activated optoplasmonic cavity. Insets: a schematic of resonant light field interacting with the R6G layer on graphene (top-right) and resonant mode blueshift in response to the R6G layer change (left-bottom). 128x47mm (300 x 300 DPI)


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Graphene-activated Optoplasmonic Nanomembrane Cavities for

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