Electrochemical Reflective Absorption Microscopy for Probing the

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Electrochemical Reflective Absorption Microscopy for Probing the Local Diffusion Behavior in the Electrochemical Interface Yu-Yi Pan, Cheng Zong, Ya-Jun Huang, Rui-Qi Lyu, Yin-Zhen Dai, Lei Wang, and Bin Ren Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b04735 • Publication Date (Web): 21 Jan 2019 Downloaded from http://pubs.acs.org on January 21, 2019

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Analytical Chemistry

Electrochemical Reflective Absorption Microscopy for Probing the Local Diffusion Behavior in the Electrochemical Interface Yu-Yi Pan†, ¶, Cheng Zong†, ¶, Ya-Jun Huang†, Rui-Qi Lyu‡, Yin-Zhen Dai‡, Lei Wang*,‡, Bin Ren*,† †

State Key Laboratory of Physical Chemistry of Solid Surfaces, Collaborative

Innovation Center of Chemistry for Energy Materials, The MOE Key Laboratory of Spectrochemical Analysis & Instrumentation, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China ‡

Department of Mechanical and Electrical Engineering, School of Aerospace

Engineering, Xiamen University, Xiamen 361005, China

ABSTRACT Electrochemical interfaces determine the performance of electrochemical devices, including energy-related systems. The in-depth understanding of the heterogeneous interfaces

requires

in

situ

techniques

with

high

sensitivity,

and

high temporal and spatial resolution. We develop here an electrochemical reflective absorption microscopy (EC-RAM) by using the absorption signals of reacting species with a reasonably good spatial resolution and high sensitivity. We systematically study the response of absorbance (A) and its derivative, i.e. dA/dt, at different positions of electrode surface and at electrodes with different sizes (50 m, 500 m, and 2 mm) both experimentally and theoretically. We find that the derivative cyclic voltabsorptometry (DCVA) frequently used to obtain the local current response in conventional electrochemical optical microscopy techniques is only applicable to reactions of surface species or solution species under the linear diffusion control. For the processes when the radial diffusion cannot be ignored as in the case of a microelectrode or the edge of a large electrode, the DCVA curves show distinct diffusion behaviors for the electroactive species in different regions of the electrode,

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which cannot be directly related to the CV curves. When the radial diffusion dominates the reaction, CVA curves follow the same shape as the CV curves. The developed ECRAM technique can be applied to in situ extract the local response of an electrochemical system during the dynamic reaction processes.

INTRODUCTION Electrochemical interface is a place where the charge transfer, mass transfer, and conversion of energy and materials occur. It determines the performance of electrochemical systems and devices1. The heterogeneity in both morphology and activity of electrode materials and further complication by the diffusion behavior, and the necessity to characterize under the potential control, make it a great challenge to achieve a comprehensive understanding of the electrochemical interfacial structure, reaction process and mechanism. Conventional electrochemical methods have provided important electrochemical dynamic and kinetic information with its high detection sensitivity. However, with the increasing complexity of electrochemical systems, especially in the field of solar energy, lithium ion battery, and corrosion etc., use of the current or potential response of the whole electrode alone cannot meet the request to comprehensively understand the electrochemical system due to the lack of spatial resolution and molecular information. The introduction of scanning probe microscopy (SPM, including scanning tunneling microscopy and atomic force microscopy) into electrochemistry allows the nanometer scale and even atomic scale resolution of the electrode surface to be obtained.2 The discovery of scanning electrochemical microscopy (SECM) offers a unique tool to provide the spatial resolved electrochemical activity for various electrode materials by scanning a microelectrode or nanoelectrode across the electrode surface.3-6 The lack of the molecular fingerprint information of these SPM methods has been overcome recently by tip-enhanced Raman spectroscopy (TERS),7,8 which can provide molecular fingerprint information on the electrode


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surface during the electrochemical reaction with a high sensitivity and spatial resolution. However, all these techniques have to rely on scanning. As a result, the data points obtained on an image are indeed acquired at different time, which may distort to some extend the electrochemical information. In comparison, recently developed electrochemical optical microscopies can obtain electrochemical activity of the whole surface with a moderate spatial resolution (even reaches the diffraction limited spatial resolution) in a snapshot way. For instance, Tao and co-workers have developed a plasmon-based electrochemical current microscopy (P-ECM) that couples the surface plasmon resonance (SPR) imaging with the conventional electrochemical methods to reconstruct the localized current as a result of the refractive index change in the vicinity of an SPR active surface during the redox reaction on a nanocatalysts-deposited gold thin film.9-12 Very recently, Vincent and coworkers13 developed an electrochemical surface reflectivity imaging technique to in situ monitor the evolution of a passive film during the CV processes. Willets and coworkers14-16 developed an electrochemical wide-field SERS method using the integrated intensity of the whole SERS spectrum for each point of the surface and reconstruct the current response by SERS signal. In all these SPR, SERS, or surface reflectivity-based methods, the concept of derivative cyclic voltabsorptometry (DCVA) was borrowed to obtain the local current response during the electrochemical reactions. DCVA was first proposed by Bancroft group17 in 1981 by taking the potential derivative of absorbance (dA/dE) and plotting it against potential to obtain the DCVA curve with the same shape as the conventional CV curve.18,19 However, DCVA is only suitable to reactions of adsorbed species or on large planar electrodes. In these systems, the electrochemical reactions are uniform since they rely on the proportional relationship of the absorbance (UV-Vis or IR) or intensity (SPR or SERS) with the total charge of the electrochemical reactions and the relationship between dA/dE or dI/dE and potential follows the 1D diffusion equation. It is highly important to explore the applicability of DCVA for wider systems and possibility to combine with microscopic methods to extract the local electrochemical reaction activity.



In this paper, we developed an electrochemical reflective absorption microscopy (EC-RAM) based on a home-built high temporal-spatial resolution microscopic imaging system. We studied the DCVA and CVA response of electrodes with different sizes to establish a proper approach to reconstruct the electrochemical response from the optical absorption signal of methylviologen. We investigated the peak current and potential distribution on electrodes with different sizes by the EC-RAM imaging. We employed 3D diffusion equation to predict the variation of optical signal with potential for electrodes of different sizes. The method reported here can be conveniently extended to other systems with optical response.

EXPERIMENTAL SECTION: Materials: The working electrode was a polycrystalline gold disk with diameter of 2 mm (Tianjin Ida), 500 m (Tianjin Ida) and 50 m (homemade). Methylviologen dichloride hydrate was from Aldrich (98%). The electrode was polished with 0.3 and 0.05 μm of alumina slurry (Buehler, Ltd.) and then degreased in an ultrasonic bath for 1 min in ethanol, 1 min in acetone, and 3 min in distilled water. High-purity water (Milli-Q, 18.2 MΩ·cm) was used throughout the study. Single point electrochemical reflective absorption spectroscopy The spectra were obtained on a WITec confocal Raman microscopy. We used a LED lamp as the white light source, which was introduced from top and illuminated on the electrode surface via the microscope objective (Olympus, 50×, WD = 8 mm). The reflected light was collected by the same objective, transmitted via an optical fiber, and dispersed by a monochromator with a 600 g/mm grating. The dispersed light was projected onto a spectral EMCCD (DU970-FI, Andor, readout rate of 2.5 MHz with the maximum 649 spectra per second) to obtain the reflected signal. The background was collected at the same point before the electrochemical reaction. The reflected signal and the background were processed with the Beer-Lambert law to obtain the absorption spectrum. Potentiostat (PGSTAT 101, Autolab) was used to perform the electrochemical measurement and control the potential. We used a three-way BNC connector to


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synchronize the potentiostat and EMCCD to obtain the correlated current and spectral data.

Electrochemical reflective absorption microscopy (EC-RAM) A home-built system was used to realize the electrochemical reflective absorption imaging (Figure 1). The light source is a xenon lamp. The light was slightly defocused to illuminate a relative large area of the sample via an objective (in this work, two objectives were used to achieve different field of view, Olympus, 50×, NA = 0.55; Olympus, 10×, NA = 0.3). The reflected light was collected by the same objective and passed through a liquid crystal tunable filter (LCTF, VISR 480-720 nm, CRI Varispec, 0.25 nm bandwidth) to choose the light of the desired wavelength to be imaged, which was finally focused onto an imaging EMCCD (Ixon Ultra 897, Andor, 512 512 pixels, 56-11074 fps) to obtain a single wavelength image. We chose the wavelength at the maximum of the absorption peak of the redox species. The background was first in-situ collected in the same region before applying the potential and then series of single wavelength images were obtained during the electrochemical reaction. The BeerLambert law was applied to every pixel on the image to process the background and signal to obtain the single wavelength absorbance image. The absorbance is contributed by the integral concentration of all electroactive species in the Z direction as described in Section 1.1 of Supporting Information.

Data analysis The detailed data processing scheme to obtain single pixel CVA and DCVA from CCD images was introduced in Section 1.2 in the Supporting Information. All the imaging data were processed using a home-written scripts.



Figure 1. Scheme of the electrochemical reflective absorption microscopy (EC-RAM). A xenon lamp was used as the white light source, and the white light images were acquired by the imaging EMCCD using the LCTF as a wavelength selector and the potential was controlled by an Autolab potentiostat, and more details are given in the main text.

RESULTS AND DISCUSSION Transient electrochemical reflective absorption spectroscopy


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Figure 2. (a) Cyclic voltammograms obtained in 1 mM methylviologen + 0.1 M KCl solution using a 500 m gold working electrode; (b) Evolution of absorption spectra of methylviologen during the cyclic voltammetry experiment obtained at a scan rate of 10 mV/s, 20 mV/s, 50 mV/s, 70 mV/s and 100 mV/s; (c) DCVA curves obtained using absorbance at 605 nm during the electrochemical reaction at different scan rates as indicated in the figure; (d) A plot of anodic peak current (in black) and (dA/dt)p (in red) with 1/2, both of which show a linear response, indicating the reaction is related to the solution species. We aim at establishing an EC-RAM for providing the absorption information over the whole illuminated area. However, whether such a method can be effectively used to derive the electrochemical information highly depends on the establishment of a quantitative relation between the absorbance and concentration of redox species. For this purpose, we used methylviologen, a model reversible redox system in electrochemistry. The cyclic voltammograms (Figure 2 (a)) of this species obtained at different scan rates show a pair of redox peaks. The peak separation did not change with the scan rate and the anodic peak currents followed a linear dependence on the square root of the scan rate (Figure 2 (d)), indicating the reaction is a diffusioncontrolled reversible processes. The diffusion coefficient is estimated to be 8.41×106cm2·s-1

according to the Randles-Sevcik equation.

The corresponding time-dependent absorption spectra (from 532 nm to 667 nm, calculated using the reflection spectrum and the background spectrum) during the CV process (in the potential range of -0.4 V~-0.9 V) are given in Figure 2 (b). The peak at 605 nm is the absorption feature of MV+·, and MV2+ does not show absorption peak in this range. Therefore, this figure shows a clear transformation between MV+· and MV2+ during the cyclic voltammetric process. We then used the intensity of the 605 nm peak to derive the electrochemical reaction information. Figure 2 (c) shows the corresponding DCVA curves of methylviologen at five scan rates of 10 mV/s, 20 mV/s, 50 mV/s, 70 mV/s, and 100 mV/s. Very impressively, these curves show almost the



same response as the CV curves shown in Figure 2 (a). The similarity between the DCVA and CV curves can be clearly seen by plotting them in a same figure, see Figure S6 for the scan rate of 10 mV/s. Although we did see a slightly more positive anodic peak and negative cathodic peak in DCVA than CV, the agreement is more satisfactory than most previous DCVA methods even when we did not consider the diffusion behavior. Since the reaction involves the diffusion of solution species, we plot the peak values of dA/dt, i.e., (dA/dt)p, versus the square root of the scan rate in Figure 2 (d). The plot shows a good linear relationship as the conventional CV for solution species, indicating the reliability of using reflection spectra to derive the quantitative electrochemical information.20,21

Electrochemical reflective absorption microscopic study

Figure 3. Spatial distribution of the peak potentials (in volt) for the anodic (a) and cathodic (b) peaks of CV curves and the peak intensities for the anodic (c) and cathodic (d) peaks of dA/dt curve on a 500 m gold electrode during the CV process in 1 mM methylviologen + 0.1 M KCl solution observed with EC-RAM. The potential and intensity values were extracted from DCVA curves using the potential dependent absorption data obtained at each pixel of the electrode surface. Scan rate: 5 mV/s. Objective: 10×, NA=0.3.


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We then performed electrochemical reflective absorption microscopy (EC-RAM) study of methylviologen redox reaction at 500 μm gold electrode. For this purpose, we first acquired a series of potential dependent reflective images from the gold electrode surface, using white light as the excitation source and centering the LCTF at 605 nm during the cyclic voltammetric processes. Then we obtained the DCVA curves for every pixel at the electrode surface by extracting the reflective intensity from the series of images. We then extracted the peak potentials and peak currents (in fact they are the maximal or minimal dA/dt, but we term it as current for simplicity of expression) for both the reduction and oxidation processes at each pixel. The potential and current distribution images can then be obtained by plotting the currents and potentials with the position. The detailed data processing is shown in Section 1.2 and the raw CCD imaging data is shown in Figure S7 of the Supporting Information. In this way, we are able to obtain the spatial distribution of various electrochemical parameters over the whole surface, and such technique is termed EC-RAM. The images are shown in Figure 3. It can be seen from Figure 3 (a) that the “reductive peak” potential distributes over the range of -0.700 V to -0.718 V with a more negative potential at the edge. The reduction peak intensity (reflect to some extend the reaction current) shown in Figure 3 (c) reveals that the dA/dt is larger at the center. This response is contradictory to our understanding of the fast radial diffusion behavior of MV2+ (reactant) and a large local current at the edge of the electrode surface. Such a behavior may be understood by the fact that dA/dt is influenced by both the accelerated generation of MV+• (increase in the concentration) and the fast radial diffusion of MV+• out of the electrode region (decrease in the MV+• concentration). The generated MV+ at the edge diffuses rapidly out of the detection range of EC-RAM by the radial diffusion. However, The generated MV+• at the center diffuse linearly, remaining within the detection range. Therefore, dA/dt at the center of the electrode will not be affected by diffusion of MV+•.



Figure 4. Distribution of various MV+• concentration-related parameters over surface position and time during the cyclic voltammetry process: (a) the changing rate of the integral concentration in the Z direction, (b) the surface concentration (mol/m3), and (c) the integral concentration (mol/m3) in the Z direction. R=0 μm represents the center of electrode; R=250 μm represents the edge of electrode. Scan rate: 5 mV/s. To verify the above experimental results, we used COMSOL to simulate the cyclic voltammetry process of methylviologen molecules on the 500 m gold electrode. First, we obtained the spatial distribution of MV+• concentration during the electrochemical process. We then integrated the concentration along the Z direction to obtain the species contributing to the experimentally measured absorbance. The detailed geometry model, electrochemical parameters and data processing method can be found in Section 2 in the Supporting Information. Figure 4 (a) shows the changing rate of the integral concentration of MV+• in the Z direction with time (dcintegral-z/dt) from the center (R=0 m) to the edge (R=250 m) of the electrode during the electrochemical process. The data for a single position on the electrode is shown in Figure S4. It can be seen that the absolute values of oxidation and reduction peaks of dcintegral-z/dt decrease gradually from the center electrode to the edge, which agrees well with our experiment results in Figures 3 (c) and 3 (d) Figures 4 (b) and 4 (c) show the variation of the surface concentration and integral concentration in the Z direction of MV+• from the center to the edge of the electrode during the cyclic voltammetry process. As can be seen from simulated results in Figures 4 (b) and 4 (c), the surface and integral concentrations of MV+• in the center are larger than that at the edge during the positive sweeping process (100 - 200 s). Therefore the cathodic current density at the center is larger than that at the edge. This result can be used to interpret the result in Figure 3 (d). We also performed an EC-RAM imaging of the methylviologen reaction in the center and at the


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edge (both about 70 m 70 m) of a 2 mm gold electrode (raw CCD imaging data in the center of electrode are shown in Figure S8) to further verify the above results. In the center, the reductive and oxidative peak current and potential are uniformly distributed (see Figure S9), and the local dA/dt could be directly related to the local current of conventional CV as the system is dominated by the linear diffusion. However, at the edge, the peak potentials and currents (See Figure S10) show a similar inhomogeneous response to that in Figure 3. dA/dt will be distorted and can no longer be correlated to the current in the conventional CV as the diffusion is now dominated by the radial diffusion.

Figure 5. |dA/dt| images on a 500 μm gold electrode surface at potentials indicated in the figure during the linear negative scan of potential at 5 mV/s for the reductive reaction of methylviologen observed with EC-RAM. Solution:1 mM methylviologen + 0.1 M KCl. Objective: 10×, NA=0.3. To further understand the above peculiar imaging result using DCVA signals, we used the absolute |dA/dt| values at 605 nm as the imaging signal and obtained a series of images at different potentials during the negative scan. It can be seen from Figure 5 that from -0.40 V to -0.50 V, |dA/dt| values are almost zero, suggesting no reaction on the electrode surface. From -0.50 V to -0.70 V, |dA/dt| values increase with the negative shift of potential and decrease uniformly from center to edge of the electrode. This phenomenon is the same as that in Figure 3. However, at -0.80 V, |dA/dt| distribution becomes obviously disordered. Then, from -0.80 V to -0.90 V, |dA/dt| values decrease



with the further negative shift of potential and increase from center to edge. This inverse phenomenon compared to that at potentials positive than -0.80 V can be understood by different diffusion behaviors at the center and edge of electrode. At potentials where the reduction reaction of MV2+ reaches the diffusion limit in the center of the electrode, the reaction at the edge has not reached the limit because the radial diffusion process can provide extra reactants. Under such a condition, the current density in the center gradually dropped and at the edge kept increasing with the further negative shift of potential, leading to the increased |dA/dt| from center to edge of electrode. From Figures 3 to 5, we have already shown that in regions where the diffusion is controlled by the radial diffusion process, dA/dt cannot be correlated to the local current. For a 500 m electrode, the overall dA/dt response of the electrode is a combined contribution of linear and radial diffusion processes. It would be interesting to explore the effect of the electrode on the response of absorption signal (see Figure S11).

The effect of the electrode size on the absorption response To understand the diffusion behavior on the absorption response, we proposed a simple model in Figure S12, where the yellow disk represents the gold electrode, the arrows stand for the diffusion direction of MV+•generated from the reduction of MV2+, and the blue region indicates the detection range of the reflective absorption signal. Therefore, the diffusion direction could be vectorially decomposed into the linear diffusion perpendicular to the electrode surface and the radial diffusion along the electrode surface. In our experimental setup, only the generated MV+• that diffuses along the vertical direction can be detected. We then used three-dimensional diffusion equation to describe the motion of MV+• on considering that both the linear and radial diffusion are possible during the electrochemical reaction on the electrode surface. We performed the polar coordinate transformation to manifest the difference between the linear diffusion and the radial diffusion as follows:[22]


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c  D 2C t

2 

(1)

2 2 2   x y z

(2)

X  r cos 

(3)

Y  r sin 

(4)

Zz

(5)

Combining Eq. 2, 3, 4 and 5 into Eq. 1,, we obtain:

1 c 1  c 1  2c  2c   (r )  2 D t r r r r  2 z 2 Due to the symmetry of a disk electrode,

(6)

 2c 0  2

1 c  2 c 1 c  2 c    D t r 2 r r z 2

(7)

where c is the concentration of MV+• at the electrode surface, r is the radius of electrode, z is the Z direction coordinate, and D is the diffusion coefficient of MV+•. The left part of Equation 7 represents the total diffusion rate of MV+•. The first two terms in the right express the radial diffusion of the generated MV+• along the electrode surface, and the last term indicates the linear diffusion of generated MV+• perpendicular to the electrode surface. It is clear from Equation 6 that the radial diffusion increases with the decreased electrode size. For the electrode that tends to be infinitesimal, Equation (7) can be simplified as: 1 c 1 c  D t r r

(8)

In this case, MV+• generated at time t would diffuse outside the detection range at time t+Δt (Δt→0) due to the decreased detection range. That means MV+• generated at time t could only be detected at that time. Therefore, the amount of MV+• (proportional to absorbance) generated at time t was directly proportional to the respective current at that time. We can then obtain: At  I t

(9)



This expression predicts that it is the CVA curve instead of DCVA curve that will follow the same shape CV curve for a microelectrode.

The absorption response of a microelectrode

Figure 6. (a) CV curve for the reaction on a 50 μm electrode during the CV process in 1 mM methylviologen + 0.1 M KCl solution. CVA (b), DCVA (c) curve for the reaction on a 50 μm electrode during the CV process in 1 mM methylviologen + 0.1 M KCl solution. Scan rate: 50 mV/s. Objective: 50×, NA=0.55.

To demonstrate the above prediction, we performed electrochemical absorbance study on a 50 μm microelectrode. For such a size, the diffusion behavior is now mainly controlled by the radial diffusion process. We found a very uniform peak potential distribution and small difference in the peak current distribution over the electrode surface (Figure S13). It indicates that difference in the diffusion behavior between edge and center of electrode is very small at this scale. As a result, we can directly use the absorption response of the whole electrode to reconstruct electrochemical current for the electrode with a size of 50 μm or smaller. The corresponding CV, CVA, and DCVA curves for methylviologen reaction at 50 m electrode obtained at a scan rate of 50 mV/s are shown in Figure 6. The CV curve is of sigmoidal shape, which is a typical response of a microelectrode, as seen in Figure 6(a). On the other hand, DCVA curve is very different from the CV curve of a microelectrode. However, the CVA curve (Figure 6 (b)) on a 50 μm electrode is very different from peak shape observed on the 500 μm electrode at a same scan rate (Figure S11 a) but similar to the sigmoidal shape obtained at a low scan rate of 5 mV/s (see Figure S11 c).


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The dropping of the CVA curves during the consecutive scan is due to the accumulation of MV+• in the detection region, which increases the absorption. For the 50 μm microelectrode, MV+• generated during the negative scan ( MV2+ + e- → MV+• ) can quickly diffuse to the infinity due to the dominant radial diffusion of MV+•. The released MV+• will accumulate in the optical path for measuring the absorption, which leads to the continuous increase of absorbance (shift to the negative value). The above results demonstrate that the electrochemical current can be directly reconstructed via absorbance as far as the diffusion process is controlled by the radial diffusion for the microelectrodes or electrodes with smaller size. This conclusion is especially meaningful for some practical systems which is challenging to obtain its electrochemical CV. The ongoing project in the group is now able to reconstruct the CV of single nanoparticles via their CVA curve. Such a current reconstruction method can also be applied to SERS and SPR as their intensities represent the surface concentration rather than that of the species in the bulk solution, so that we can obtain a better quantitative relationship between the electrochemical current density and I SERS and ISPR. Such a quantitative relationship is especially important for a broader application in the electrochemical imaging. Indeed, we can even see some fine structures in the EC-RAM imaging of the 50 μm microelectrode in Figure S13.

CONCLUSIONS In summary, we have developed an electrochemical reflective absorption microscopy (EC-RAM) on an upright microscope, which allows us to investigate the electrochemical reaction on a non-transparent electrode. This method shows unique advantage of absorption spectroscopy with a monolayer and sub-monolayer sensitivity[23]. For the 500 m electrode or the edge of a large electrode (2 mm), where the radial diffusion could not be neglected, the DCVA method is not able to reconstruct the optical CV response. In the extreme cases like microelectrode (radial diffusion dominates the reaction), CVA curve follows the same shape as the CV curve. Whereas, in the center part of the large electrodes, the DCVA can well reproduce the



electrochemical CV response. We have further been able to obtain heterogeneous spatial distribution of the peak intensity (reflecting the reacting current) and peak potential during the CV process via EC-RAM imaging by either using CVA or DCVA methods, which clearly reveals the different diffusion behavior at electrodes. Such a current reconstruction method can also be applied to other electrochemical microscopy like P-ECM and electrochemical wide-field SERS, to reveal the electrochemical reactivity on some fine structures at electrode surfaces. It can be further extended to study more systems like cells, lithium ion batteries, and fuel cells, which has farreaching significance for nano-spectroelectrochemistry. ASSOCIATED CONTENT Supporting Information： Additional information as noted in the text. This material is available free of charge via the Internet at http:// pubs.acs.org. AUTHOR INFORMATION Corresponding Author * [email protected], [email protected] Author Contributions ¶These authors contributed equally. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT We acknowledge support from the National Natural Science Foundation of China (21633005, 21790354, 21711530704 and 21621091) and the Ministry of Science and Technology of China (2016YFA0200601) others for any contributions.

REFERENCES

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for TOC only


Electrochemical Reflective Absorption Microscopy for Probing the

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