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Abstract
The fixed post-manufacturing properties of metal-based photonic devices impose limitations on their adoption in dynamic photonics. Modulation approaches currently available (e.g. mechanical stressing or electrical biasing) tend to render the process cumbersome or energy-inefficient. Here we demonstrate the promise of utilizing magnesium (Mg) in achieving optical tuning in a simple and controllable manner: etching in water. We revealed an evident etch rate modulation with the control of temperature and structural dimensionality. Further, our numerical calculations demonstrate the substantial tuning range of optical resonances spanning the entire visible frequency range with the etching-induced size reduction of several archetypal plasmonic nanostructures. Our work will help to guide the rational design and fabrication of bio-degradable photonic devices with easily tunable optical responses and minimal power footprint.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The employment of metal thin films and micro/nanostructures in photonic devices has opened up tremendous opportunities for the fine control of optical response contingent on material, geometric, and environmental selection. These metallic structures can be used to control the frequency responses of electromagnetic transmission, reflection and scattering spanning a wide frequency range from UV to IR through sub-wavelength interactions with incident light [1]. Traditionally researched optical devices often use coinage metals (e.g. Au, Ag and Cu) as the active components, both in thin film and micro- to nano-structured formats. Successful applications include chemical sensing [2], photocatalysis [3], photovoltaics [4], plasmonic phototherapy [5], and vibrational spectroscopy [6]. Despite the remarkable progress obtained with conventional metals, these elements also present notable challenges and limitations for specific photonic applications. One such constraint is that optical engineering accomplished using these materials is usually fixed at the point of fabrication, offering no or limited capacity for dynamic modification while compared with other state-of-the-art reconfigurable optical systems by virtue of phase-changing materials [7,8], electrical gating/biasing [9], optical or photothermal modulation [10,11], and mechanical stressing [12]. Additionally, these coinage metals are not bio-degradable, which may hinder their sustainable adoption in frontier technologies such as augmented reality [13]. Furthermore, as these photonic devices see greater demand in a world with limited resources, low-cost technologies with cheaper alternatives to coinage and other noble metals will soon become a necessity.
Over recent years, the use of Mg in photonics has spurred rising interest [1,14,15]. The relatively low loss of Mg over a broad wavelength range from NUV-NIR lays the foundation for its applicability in plasmonics [16]. In fact, the optical loss of Mg is lower than Al [17,18], another burgeoning plasmonic material, in optical frequency range [19,20]. Compared with Al, the dynamic transformation of Mg upon exposure to water [1,21] or hydrogen [22,23] with easily achievable experimental conditions (e.g., temperature, pH, etc.) enables an intriguing route towards further optical property modulation post-fabrication, such as in dynamic color printing and display, dynamic holography, advanced cryptography, optical data storage, optical sensing, etc. [24,25]. Compared with coinage metals, Mg is lightweight, low cost, CMOS-compatible, and bio-degradable, all of which are strongly desired characteristics for modern photonic devices [26].
The etching properties of metallic Mg are the basis for the post-manufacturing tunability and biodegradability of Mg-based photonic devices. Mg etches readily and efficiently in water at ambient conditions, i.e. this etching process can occur at room temperature and with neutral pH. Etching leads to a rapid reduction in dimensionality and alters the geometry of structures, allowing for the power-free modification of a photonic device’s properties. In practice, a stable native oxide (a few nanometers thick) is formed on the Mg surface [27], and thus the primary chemical reactions in the etching process are as follows:
The by-products of these reactions are H2 and environmentally non-toxic Mg(OH)2, which is both biodegradable and biocompatible. The use of water as an etching reagent makes Mg-based transient photonic technology instantaneously available for general consumer and laboratory use. Despite the naturally occurring etching in water, being able to control the etch rate is a prerequisite for achieving the precise tuning of device dimensions and the resultant optical properties.
In this Article, we elucidate the etching behavior of Mg thin films and microstructures to establish their potentiality for dynamic optical modulation. We determine how temperature, deposition method, and geometry affect the rate of Mg etching. Etch rate is found to increase monotonically with escalating temperatures (up to 60 °C), with the rate change differing with thin-film deposition method (radio frequency sputtering and electron-beam evaporation). Additionally, the etching rates along different dimensions of Mg microdisks differ enormously depending on their geometric aspect ratios. We also investigate the chemical changes at Mg structure surfaces before and after etching through X-ray Photoelectron Spectroscopy (XPS), revealing the generation of several primary Mg-derived compounds. To illustrate the potential of using Mg as a fundamental building block for dynamic photonic devices, we numerically demonstrate the optical responses of various nanostructures, all of which exhibit appreciable transitions of the scattering resonance frequency in the visible frequency regime due to reduced structural dimensions. This directly translates to the controllable tuning of the hue of color printing devices and the sensitivity of optical sensors. The influence of the MgO capping oxide on the optical behavior of the structures is also examined for practical considerations. Our results provide critical information on the dynamic properties of Mg and their projected impact on the optical characteristics of relevant devices. In turn, these results will guide the rational design and fabrication of novel Mg-photonic devices such as transient color pixels, plasmonic sensors, and antennas with tunable optical responses.
2. Sample preparation and characterization
2.1 Fabrication of Mg structures and films
All radio-frequency sputtered (RFS) films were deposited using a 3” diameter Mg target and an AJA ATC 1800 Sputtering unit. Films fabricated through E-beam evaporation (EBE) were created using a custom-built Denton E-beam/thermal evaporator with 99.95% pure Mg pellets. These thin film depositions were conducted at room temperature, with deposition rates of 44.4 and 5.0 Å/sec respectively. Thin films were deposited on 1 × 1 squared inch glass substrates cleaned with acetone, methanol, isopropanol, and de-ionized H2O prior to fabrication. Election-Beam Lithography (EBL) was used to fabricate the Mg micro- and nanostructures using Microchem 495 PMMA A4 as a positive E-beam resist. Resist layers targeting 190 nm in thickness were spin-coated at 4000 RPM for 45 seconds. EBL was conducted using a Raith e-LINE EBL system with an accelerating voltage of 30 keV. Liftoff was done through immersion in 1:3 MIBK:IPA solution for 40 seconds. Deposition was completed using EBE with an average deposition rate of 5.0 Å/sec and the E-beam resist was removed through ultrasonication in acetone for 180 seconds.
2.2 Temperature-dependent etching experiments
Spectroscopic ellipsometry (SE) measurements were performed in air for wavelengths ranging from 193 to 1688 nm using an M-2000 Variable-Angle Spectroscopic Ellipsometer (VASE) manufactured by J.A. Woollam. Measurements were taken before etching and after all subsequent etching steps. All etching was completed using de-ionized H2O. Before every measurement, the samples were dried with lab-grade N2 gas. The resultant data for each temperature/etch time thickness measurement is an average of three distinct locations on the samples separated by a distance sufficient to ensure no overlap. Reflection data was recorded with incident angles between 55° and 75° measured in increments of 5°, with initial sample tilt alignment done at 65°. Transmission data was recorded at 0° with respect to the normal of the substrate. Baseline measurements were taken in air before each transmission measurement to isolate the optical properties of the thin films. Experimental temperatures at and above 40 °C were regulated using a hot plate. Etching done at 10 °C required a cool thermal bath to maintain the testing environment. The de-ionized H2O used for this low-temperature etching was regulated using ice packs and a larger, insulated bath. All etching was monitored to ensure that the etching environment did not diverge from the target temperatures by more than 2 °C.
2.3 X-ray photoelectron spectroscopy
The XPS measurements were acquired using a Kratos AXIS 165 model XPS instrument using 1486.6 eV Al Kα X-Ray sources. Charging effects were corrected by setting the C 1s peak at 286 ± 0.1 eV. The peak fitting of components was executed on ESCApe software after performing Shirley background subtraction.
2.4 Mg structure etching/atomic force microscopy
Mg microstructure etching was conducted using de-ionized H2O at ambient temperature. The microstructures were dried with N2 gas before etching and after each subsequent etch step. Atomic force microscopy (AFM) was conducted using an Asylum MFP-3D Infinity System with an acoustic cancellation hood. Topographical images were taken using AC tapping mode in air. Uncoated silicon AFM probes with 7 nm tip radii were used for these measurements.
2.5 Finite-difference-time-domain (FDTD) simulations
Optical scattering of the isolated nanostructures was derived from FDTD simulations using the Lumerical FDTD Solver. The dielectric function of Mg and MgO used in the simulations were obtained from previous literature [15,28]. A total-field scattered-field (TFSF) light source was used with a broadband emission set for wavelengths from 350 to 850 nm. The simulation region was set up such that Perfect Matching Layer (PML) boundaries were applied to the boundaries perpendicular to the source injection, whereas symmetric/anti-symmetric boundaries were applied in other directions for faster simulation. To ensure simulation accuracy, a mesh override region was added on top of the nanostructures. A box power monitor was used to enclose the nanostructure for collecting and calculating the scattered power.
3. Results and discussion
3.1 Etching behavior of Mg thin films
An understanding of the reaction between Mg and water as a function of both etching temperature and material deposition method is paramount in predicting and controlling the resultant dynamic optical responses. Figures 1(a) and 1(b) show how temperature affects the rate of Mg thin film reduction for samples fabricated using EBE and RFS, respectively. Both sets of Mg thin films are subjected to etch steps of a specified duration, with the same time target used for all points in each specific etch temperature/deposition method set. The film thickness values are derived using Spectroscopic Ellipsometry (SE) data from three separate area spots acquired after each etch step. Least-squares linear fits of Mg film thickness/etch time coordinate pairs are generated for each of the four experimental etch temperatures: 10, 20, 40, and 60 °C. Note we plot the thickness starting from 60 s because in most samples the film thicknesses increase initially up until 60 s before being etched away. This is due to the drastic change in surface chemicals upon immediate exposure to water (see Tables S1 and S2 in Supplement 1). We will discuss the surface chemistry in the next section. The reduction of the Mg films thicknesses behaves according to a linear fit with R2 values equal to or greater than 0.909 and 0.972 for RFS and EBE, respectively. Therefore, the etch rate for each temperature can be calculated as the slope of the thickness reduction between etch steps. These etch rates are plotted for EBE and RFS Mg thin films in Fig. 1(c). RFS films have a greater capacity for the tuning of etch rate with raised temperature. The etch rate for the RFS films is ∼140% faster and the etch rate for EBE films is increased by more than 90% as etching temperature increases from 10 °C to 60 °C. This significant increase shows that the etch rate can be varied by a substantial degree through etching temperature, which is crucial for precise control of the size and the consequent dynamic optical response of Mg structures, regardless of the fabrication method used.
Fig. 1. Etching behavior of Mg thin films fabricated by (a) electron-beam evaporation (EBE) and (b) radio-frequency sputtering (RFS) since the first etch step with least squares linear fit for individual temperatures. In both graphs the Mg film thickness has been normalized to the maximum height value for each temperature. The error bars represent thickness analysis errors associated with spectroscopic ellipsometry data. (c) Etch rate vs. temperature with least squares linear fit for both physical deposition methods. The error bars represent the least square fitting error for the slopes at each temperature.
To examine the chemical composition changes at the sample surfaces upon etching, we analyze the XPS spectra of a pristine and a 60-second etched film, based on Mg 2p signal range. As displayed in Fig. 2(a), the spectrum of the pristine film is dominated by metallic Mg, along with naturally occurring MgO, Mg(OH)2 and MgCO3 due to sample being handled in air [21,29]. No other elemental compositions have been identified as a result of exposure to air, as indicated by the XPS. The Mg/O ratios of these species do not change upon etching, meaning the stoichiometry has maintained [30]. However, when the film is exposed to water, the hydroxide contribution dramatically increases (see Fig. 2(b)), and the intensity ratios of the Mg(OH)2 to MgO in O 1s region (O[Mg(OH)2]/O[MgO]) increase from 1.26 to 1.43. This result indicates that most of the oxide layer on the upper surface of the Mg etches away and hydroxides from the water bond on the surface to form Mg(OH)2 instead. The binding energy difference between Mg[MgO] and Mg[Mg(OH)2] peaks decrease from 0.72 eV to 0.02 eV, which also agrees with previous studies [31]. The full width of half maximum (FWHM) of the Mg[MgO] peak increases from 1.56 eV to 1.83 eV, which also supports that the bonding character of the surface oxygen has changed [32]. These results are in agreement with prior ones in the literature that have shown that porous Mg(OH)2 forms spontaneously on the native oxide after immersion in water [31]. The XPS spectra of the films after drying from the etchant can be found in Fig. S3(b) in Supplement 1 and the intensity ratio of the O[MgOH] to O[MgO] decrease to 1.07, indicating loss of surface hydroxides upon dehydration.
Fig. 2. XPS evolution of pristine (a) O 1s, (b) Mg 2p and etched (60 s) (c) O 1s, (d) Mg 2p narrow spectra of Mg thin films deposited by E-beam evaporation, including the contributions of the constituents: metallic bulk Mg, surface oxide, Mg(OH)2, and MgCO3.
While the etching of Mg thin films reveals informative chemistry between water and Mg/MgO, the etching behavior of Mg-based micro- and nano-structures are more pertinent to the optical performance of photonic devices as the geometry and size of these structures regulate the linear etch rate along different dimensions. For instance, in a Mg micro/nanodisk the etch rate would notably differ in the lateral and normal directions depending on the geometric aspect ratio, which allows for anisotropic reduction in dimensions and provides an additional design parameter. Changes in the shape of micro/nanostructures have been shown previously to drastically alter their optical behaviors [33,34], and the insightful definition of initial structure geometries and sizes will provide fine control of their optical properties during etching in water.
3.2 Etching of 3D Mg structures
We perform a detailed analysis of Mg etching behavior in 3D microstructures and find an anisotropic etch rate. Figures 3(a) and 3(b) show AFM images and line profiles acquired from a Mg microdisk fabricated via EBE and etched in water for various durations. The roughness of the rods are unchanged upon etching, despite the accumulation of MgO and Mg(OH)2 at the structure surfaces as previously discussed. The sequence of line profiles acquired using the very same Mg rods shows the anisotropic shrinkage of the structures. On average, a disk’s height etches by more than 75% of its original value over the course of 45 seconds. By defining the height as the average value of thickness of the profile points along the plateaus of the disks and the diameters as the one at half of their heights, we quantify the etch rate along both dimensions. As presented in Fig. 3(c), the average height and diameter etch at a rate of about 90 nm/min and 720 nm/min, respectively. The diameter (rod side walls) etches ∼810% faster than the height (see detailed analysis in Supplement 1). Similar behavior is observed in micro-/nano rods of different diameters (see Fig. S4 in Supplement 1). This result bodes well for the utilization of Mg 3D structures for on-demand destructible optical devices in a no-power environment, such as transient modulators and sensors.
Fig. 3. Etching behavior of 3 µm diameter Mg disks. (a) Sequence of atomic force microscopy (AFM) images and (b) AFM line profiles of a representative Mg disk as a function of etching time in water at room temperature. (c) Average Mg disk height and diameter with respect to etch time since the first etch step. This data is acquired from AFM line profiles of four 3 µm diameter Mg disks deposited on the same Si substrate as they are etched concurrently in water. The average height for each individual disk is calculated from the plateau section of the disk, and diameter is estimated at half the height. The error bars represent the standard deviation in average Mg disk height and diameter between all four samples.
3.3 Numerical simulations of Mg nanostructures
Next, we tap into the unique vantage point of Mg compared to all other commonly used metals, i.e. the excellent and easy controllability of its structural sizes through etching in water, to demonstrate its potential in transient optical devices and to help the rational design of the geometry and sizes of these device structures. We numerically examine and compare the optical behaviors of several archetypal nanostructure configurations (including trimer, heptamer, bowtie and metal-insulator-metal (MIM) tandem disk) with varying dimensions to showcase the versatility of this concept. As Fig. 4 shows, all of the structures exhibit pronounced blueshift of their scattering peak frequency with decreased sizes (i.e. due to the etching process of Mg in water). These resonance shifts beget a plethora of applications in photonic devices. For example, it can produce a smooth transition of hues in color display devices for dynamic color tuning. This effect also allows for the on-demand tailoring of sensing wavelengths for transient optical sensors. In addition, the range of available LSPR resonances for these structures spans the entire visible frequency range with reasonable structure diameters (sub-200 nm). This result indicates that by choosing appropriate structural dimensions and etching parameters, the entire sRGB gamut may be traversed. It should be noted that the resonance excitation mechanisms for the listed nanostructures are different, hence their different features. Structure choice should therefore be based on the desired photonic device application. As an example, the scattering peaks in nanodisk trimers, bowties, and MIM tandem disks arise from the electric dipolar responses and their couplings, whereas the scattering dips for the heptamers are attributed to the coupling of a super-radiant mode and a sub-radiant mode formed between the inner disk and the outer ring and the associated Fano-like interference [35] which inherently yields a narrower resonance. This is a desirable feature for better color saturation in displays [36,37] and for narrow-band optical sensors. With a similar size reduction, the trimer exhibits a more pronounced resonance shift (∼3.5 $\Delta \lambda /\mathrm{\Delta }D$) compared to the MIM disk ((∼2.4 $\Delta \lambda /\mathrm{\Delta }D$), and therefore one can choose one over the other based on the requirements for device tuning sensitivity.
Fig. 4. FDTD-simulated field enhancement profiles at resonance frequencies and scattering spectra of several archetypal Mg-based nanostructures: (a) trimer, (b) heptamer, (c) bowtie, and (d) tandem M-I-M disk. The thicknesses of the nanostructures in (a-c) are H = 40 nm, and the top and bottom disks In the MIM structure have a thickness H = 30 nm. The middle row shows the strong LSPR-field enhancement in the vicinity of the nanostructure surfaces. In the bottom row, an evident resonance frequency blueshift can be observed for all configurations as the dimensions of the nanostructures diminish.
Lastly, it should be emphasized that even a high purity Mg surface is usually covered by an ultrathin capping layer of MgO that passivates the film and protects the metal from undesired chemical reactions, as seen in the prior section. This capping layer is conducive to a delayed dynamic response, which is particularly suitable for implantable temporary biomedical devices that are expected to operate over longer timeframes [38]. We also note here this passivation/protection may fail in environment where sulfur or fluorine compounds are present [39]. On the other hand, the presence of these MgO capping layers also intrinsically exerts an influence on the optical properties of Mg-comprised components. We therefore numerically simulate the optical responses of an Mg nanodisk as both a singular metallic structure and as a structure with a 3-nm native MgO layer in Fig. 5. The optical response of a larger nanodisk (diameter 110 nm) features a slightly redshifted localized surface plasmon resonance (LSPR) peak, but its resonance magnitude is barely attenuated. In contrast, the LSPR peak shifting and amplitude damping are much more significant for a smaller Mg disk (diameter 50 nm), with a much broader resonance profile as a consequence. On the principle of the effective medium theory [40], the Mg disk nanostructure can be viewed as a composite material with an effective polarizability controlled by the volume fraction of the oxide, the larger of which would yield an effectively lower plasma frequency, hence red-shifting the LSPR more substantially. At the same time, the surface oxide also dampens the resonance amplitude as the density of free electrons is effectively reduced. In practice, the optimal size of an Mg structure is a trade-off between the diminishing influence of the native oxide and the increasing radiative damping typical of a larger-sized structure. Moreover, additional factors (e.g. the presence of Mg(OH)2 and other impurities at the nanostructure surface, roughness, etc.) with varying fabrication and etching conditions such as deposition method and etching temperature also need to be taken into account to accurately compute the optical properties for comparison with experimental results and/or predicting nanostructures’ behavior upon illumination.
Fig. 5. The influence of a native oxide layer through FDTD simulation on the scattering properties of a Mg nanodisk with different diameter D and thickness H: (a) D = 110 nm, H = 17.4 nm and (b) D = 50 nm, H = 3.6 nm. Note that Qscat denotes the scattering cross-section normalized by the geometrical cross-section, and is therefore dimensionless. The insets show the field enhancement in the vicinity of the nanodisk surfaces with and without the native oxide layer, with borders color-coded to their respective LSPR peaks. For the small disk, the 3 nm native oxide layer significantly attenuates the plasmonic oscillation at the surface, whereas for the larger disk the oxide layer merely redshifts the resonance as a result of an effectively larger permittivity, i.e. less metallicity, in the composite disk.
Here we report the remarkable etching property of Mg as a simple yet efficient means to potentially achieving transient photonic modulation. We have demonstrated that through the combinatorial influences of fabrication method, etching temperature and structural geometry, the etching dynamics of Mg structures can be well controlled and tailored. Further, our calculations have shown a profound tunability of Mg-comprising nanostructures in the optical frequency range, pointing to great promises for future experimental realization of various optical sensing and color-printing devices. Overall, our results of the controllable etching behaviors usher a novel concept for cost-effective and bio-degradable dynamic optics and photonics.
4. Conclusion
In conclusion, the reaction between Mg/MgO and water is key to inexpensive, biodegradable photonics with prospective applications in dynamic color displays, biological research, pharmaceuticals, and visible light signal modification. We have explored the etching behavior of Mg as a function of temperature, deposition method, and structural geometry for its future adoption in dynamic photonic devices. We have determined that the etch rate of Mg thin films in water increases linearly with respect to temperature, and that this change is substantial enough to double the etching rate over less than a 50°C difference. The etching behavior is also dependent upon the deposition method, which should be taken into consideration for fabrication of Mg-based optical components. An anisotropic etch rate is observed in the fabricated microdisks based on their geometry, implying that this geometry offers an additional tuning knob for the dynamic optical properties of future Mg structures. The etching of Mg/MgO in water would directly translate to the modulation of optical response in real time, and we have predicted substantial tunability of optical resonances spanning the entire visible light range of the electromagnetic spectrum with the reduction of dimensions in several classic plasmonic nanostructures commonly used with other metals. Despite the limitation inherent to this approach due to the irreversibility of this process, the simplicity of operation and its cost-effectiveness would nonetheless herald its utilization in applications where reversible dynamical behaviors are not a necessity, in contrast with several aforementioned dynamic modulation mechanisms. To summarize, our work sheds light on the great potential of utilizing Mg for viable control of dynamic optical responses and provides relevant information for effectively engineering low cost and environmentally friendly Mg-based photonic devices with tunable interactions with visible light in power-free environments.
Funding
National Science Foundation, DMR (16-09414, 20-16617).
Acknowledgments
The authors acknowledge Dr. Karen Gaskell from UMD for fruitful discussions regarding the XPS measurements and the support of the Maryland NanoCenter. MSL thanks the financial support from the National Science Foundation (awards #16-09414 and #20-16617) and UC Davis. TGF acknowledges the A. James Clark fellowship from the Clark School of Engineering at the University of Maryland.
Author Contributions: TGF and TG contributed equally to this manuscript as co-first authors. All authors were involved in the discussion and analysis of the data, and in the preparation of this manuscript.
Disclosures
The authors declare no conflicts of interest.
Supplemental document
See Supplement 1 for supporting content.
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