3D Nanostructuring of Phase‐Change Materials Using Focused Ion Beam toward Versatile Optoelectronics Applications

In recent years, phase‐change materials have gained importance in nanophotonics and optoelectronics. Sizable optical contrast and added degree of freedom from phase switching drive the use of phase‐change materials in various optical devices with outstanding results and potential for real‐world applications. The local crystallization/amorphization of phase‐change materials and the corresponding reflectance tuning by the crystallized/amorphized region size have potential applications, for example, for future dynamic display devices. Although the resolution is much higher than in current display devices, the pixel sizes in those devices are limited by the locally switchable structure size. Here, the spot sizes are further reduced by using ion beams instead of laser beams, dramatically increasing pixel density, demonstrating superior resolution. In addition, the power to sputter away materials can be utilized in creating nanostructures with relative height differences and local contrast. The experiment focuses on one archetypal phase‐change material, Sb2Se3, prepared by pulsed‐laser deposition on a reflective gold substrate. This study demonstrates that structural colors can be produced and reflectance tuning can be achieved by focused ion beam milling/sputtering of phase‐change materials at the nanoscale. Furthermore, the local structuring of phase‐change materials by focused ion beam can produce high‐pixel‐density display devices with superior resolutions.


Introduction
Optical materials with innovatively engineered properties have the potential to produce attractive functionalities for future applications.One avenue where optical materials have been heavily studied is in nanophotonics and optoelectronics for potential applications in data storage and display devices. [1]The DOI: 10.1002/adma.202303502[4][5] One attractive feature, especially for future display device applications, is how robust the structures are with small reflectance changes with the angle of incidence.Researchers are heavily invested in structural color generation to curtail current mature display devices' power consumption and pixel density scaling problems. [6]11] Even though outstanding results have been reported, moving away from a static system toward dynamically reconfigurable modules seems complicated.[17][18][19] PCMs quickly became the focus for applications in dynamically reconfigurable photonics, including structural color production and future display device applications. [1]The optical contrast between different phases of PCMs and the capability of switching between phases suit the PCM to reflectance-based applications.In addition, the ability to produce partial structural states and tune the reflectance states seems promising for moving forward to a dynamic display device. [15,20,21]Multiple research works have already shown the partial crystallization/amorphization of PCMs by using laser pulses, [15,17,20,22] electrical triggering, [16,18,23] and ion beams. [24]The associated structural changes, and thus reflectance changes, produced multiple states and optical contrasts with significant application potential for future optoelectronics.
In this work, we demonstrate the production of structural colors locally at the surface of reflective heterostructures.We employed the milling/sputtering power of a focused ion beam (FIB) to locally nanostructure an optical reflectance device consisting of a reflective gold substrate and an Sb 2 Se 3 film on top.Although much studied for solar cell applications, [25][26][27][28] Sb 2 Se 3 is one of the low-loss phase change materials that recently attracted growing interest due to its optical properties in both the as-deposited amorphous and the crystalline phases with large optical contrast between them. [19,29,30]Furthermore, the low extinction coefficient in the visible and near-infrared spectral ranges, the high index of refraction contrast between the two phases, and the wide bandgap are promising for coupling the material's phaseswitching properties with optoelectronic devices. [15,31]Here, we experimentally demonstrate local thickness variations and the associated reflectance tuning for producing superhigh-resolution nanoprints with diffraction-limited pixel resolutions.We also show that FIB can create various contrasts throughout the thickness of the active material, adding another degree of freedom for acquiring all accessible structural colors.Our work offers an innovative approach to create nanostructured surfaces of phase change thin films, which can also be partially switched, promising for future reconfigurable optolectronic devices, e.g., displays, superlenses, optical Fourier surfaces, and photonics integrated circuits where surface (and pillar height) manipulations would produce various tailored diffraction signals. [32,33]

Results
Thin films of Sb 2 Se 3 were deposited using pulsed-laser deposition (PLD) on a Si 3 N 4 membrane.For the structural analysis of the as-deposited amorphous and crystalline phases of Sb 2 Se 3 thin films, we used transmission electron microscopy.Figure 1a-c shows the transmission electron microscopy (TEM) images of global (a), local (b) morphologies, and the selected-area electron diffraction (SAED) patterns (c), respectively.The as-deposited films are globally amorphous.The amorphous nature of the asdeposited films is confirmed by SAED, showing diffusive rings as shown in the inset of Figure 1a.In Figure 1b,c, higher resolution images focused on more local areas show the crystalline sample morphology and a diffraction pattern from the SAED shows welldefined spots (Figure 1c).Sb 2 Se 3 crystallizes in an orthorhombic crystal structure (space group Pnma).The overall composition of the deposited film has, on average (in at%), 41 Sb and 59 Se, confirming the production of rather exact stoichiometry.
For the characterization of optical responses in different phases, Sb 2 Se 3 films with varying thicknesses were deposited on thermal SiO 2 /Si substrates for spectroscopic and dynamic ellipsometry characterizations.Figure 1d presents the optical constants (index of refraction n and extinction coefficient k) of the asdeposited and crystalline phases of Sb 2 Se 3 thin films.The characterizations start with a spectroscopic scan of the as-deposited phase in the spectral range of 300-1700 nm, followed by the dynamic ellipsometry where the light intensity was continuously probed while the as-deposited films are annealed with a constant heating rate of 5 °C min −1 .Since the optical properties of the as-deposited and crystalline phases are different, the light intensity will dramatically change precisely at the crystallization temperature T x of the sample.Therefore, accurate crystallization temperature values can be extracted from the first derivative of the intensity plot.As shown in Figure 1f, the crystallization temperature is T x ≈ 175 °C.Subsequently, we use spectroscopic ellipsometry for the crystalline phase.A model describing the sample's heterostructure must be constructed to extract the optical constants from the measured ellipsometry parameters  and Δ.The Tauc-Lorentz oscillator model can explain the optical constants of the amorphous (as-deposited) and crystalline (above T x ) phases of Sb 2 Se 3 films.The dielectric constant values in Figure 1d also correspond well with previously reported results. [34,35]The index of refraction (n) value changes between the phases has maximum values in the visible frequency spectrum, as seen in Figure 1e.This large change can be directly related to a significant reflectance change between amorphous and crystalline phases, attractive for dynamic display devices.
After knowing the ellipsometry optical constants of Sb 2 Se 3 , we can design a heterostructure display device, and the reflectance spectrum can be simulated based on the transfer matrix (TMM) formalism.Figure 2a shows the reflective device design schematics where the active Sb 2 Se 3 material was pulsed laser deposited on a reflective gold substrate.Spacer LAO layers were deposited on the bottom and top of the active material for capping to avoid intermixing between Sb 2 Se 3 with the gold substrate and evaporation during crystallization at high temperatures.It is well known that film thicknesses play a crucial role in tuning the reflectance spectra in the strong interference formalism where an absorbing layer is coated on a highly reflective surface. [2]Figure 2b shows the experimental and simulated reflectance spectra for the Sb 2 Se 3 films with thicknesses of 4-24 nm.The measurements and simulations are given for the as-deposited amorphous and crystalline phases of the Sb 2 Se 3 layer.An excellent agreement is reached by comparing the calculated reflectance spectra with the measured values.Different structural colors, corresponding to Sb 2 Se 3 film thickness variations, are shown in Figure 2c.Multiple shades of structural colors are produced for the as-deposited phase.Another set of variable colors is also made after annealing the samples for 5 min at 250 °C.For comparison, we show, in Figure 2d, the CIE 1931 color space and the respective color coordinates of the reflectance spectra in (b).A color-matching function was used to calculate the x and y chromatic coordinates from the reflectance spectra.The color map shows that both as-deposited amorphous and crystalline phases of the Sb 2 Se 3 layer have sizeable RGB gamut coverages.And the amorphouscrystalline phase transition has significant color differences, especially in the two ends of the spectral range, namely toward both blue and red regions.
In the strong interference formalism, where a lossy thin film is coated on a reflective surface, a slight thickness change leads to a significant shift in the reflectance spectra.Controlling/changing film thickness thus can produce all accessible structural colors.In Figure 2, we showed how a slight change in the thickness of the active Sb 2 Se 3 thin film layer, prepared on a reflective gold substrate, correlates to the production of different structural colors.The produced reflective heterostructure devices are smooth (due to reduced thickness values), and the reflectance is uniform across the device surface.However, additional steps are needed to produce local contrast on the surface.We change the phase of PCM by inducing partial crystallization/amorphization of active layers.By varying the local ordering of the active material with electronic signals or laser pulses, the subdiffraction resolution of images can be nanoprinted.Although, for the contrast, the dynamic range is limited by the maximum reflectance change of the amorphous and crystalline phases of the active PCMs.

Local Nanostructuring and Structural Colors
To demonstrate the possibility of reaching high local contrast, we employ the large reflectance tuning from slight thickness changes of the active material.The locally controlled thickness variation on a thin film surface utilized the FIB.The schematic of the FIB-device interaction is illustrated in Figure 3a.A heterostructure reflective device was produced by depositing an about 26 nm-thick Sb 2 Se 3 thin film on a reflective gold substrate.A Ga + ion beam, accelerated to 30 kV, is focused onto the surface of the device.The ion beam is used to locally "sputter away" material, and the amount of removed material depends on the beam intensity and dwelling time.Therefore, calibration is needed to precisely control the FIB-device interaction and the material removal.First, a grayscale image with 25 squares of varying grayscale values (from 10 to 250 on an 8-bit grayscale) was used to pattern the surface of the device.Figure 3b shows the original digital image used to calibrate the FIB power vs thickness.Then, by carefully experimenting with different milling parameters of the ion beam, optimum values for local structuring of the reflective device surface were acquired.More importantly, the beam current, the milling depth, and the beam dwell time are calibrated to allow controlled milling to sub-nm accuracy.Figure 3c shows a scanning electron microscopy (SEM) image where the contrast is created from the collected backscattered elections (BSEs) signal.Each square has a lateral dimension of 10 × 10 μm 2 .The height difference of individual squares can be seen from the SEM image.Since the BSEs intensity scales with the atomic number (Z) of elements, i.e., higher-Z elements scatter electrons more than low-Z elements, the contrast among individual squares in the SEM image indicates the depth profile.The contrast gradient, darker from the top and lighter at the bottom, is due to how close the imaged surface is to the gold (high-Z) substrate.
The precise control of local height is characterized further by atomic force microscopy (AFM).The morphologies of the patterned squares have been investigated using AFM, and the col-lected images are presented in Figure 3d.The local height difference of the individual squares is seen from the AFM line profiling.From rows 1-5, the local contrast changes from lighter to darker, indicating deeper regions.The line profiles over each row are also given in Figure 3e.The line profiles were extracted for each row horizontally as indicated by the corresponding grayscale value in Figure 3e.The height change from the top surface is shown by the steps present in the line scan.Squares in the top rows show a smaller height difference than squares in the bottom rows.From the SEM image in Figure 3c and AFM image and line profiles in Figure 3d,e, the visual changes can be quantitatively correlated to the height based on the grayscale values in the individual squares.Figure 3f shows the relationship between the grayscale values of the original digital image and the nanostructures' milling depth on the film's surface.A linear relation between the grayscale value and the milling depth can be seen.As shown in Figure 2, the local height contrasts correspond to a reflectance change and, thus, to the variable structural colors.The as-prepared structures are optically imaged and are presented in Figure 3g,h for both the as-deposited and crystalline phases of the Sb 2 Se 3 film.A wide variety of colors throughout the film thickness is created from the height variations.Looking at the bottom right square structures in Figure 3g,h, although the 26 nm active Sb 2 Se 3 layer is completely removed, there is an obvious color difference between the as-deposited and crystalline phases.A detailed explanation about the color contrast and the existence of a possible intermixing layer is given in the Supporting Information.The reflectance profiles of individual squares seen in Figure 3g,h have been measured and compared with calculated values.A small gallium ion implantation is expected for the treated surfaces but the reflectance values are unaffected and the reflectance contrast with thickness change persists.More information on the Ga ion implantation and the reflectance results are presented in the Supporting Information (Figure S1, Supporting Information).
Once the milling power of the FIB was calibrated, i.e., the grayscale value to thickness conversion for a given milling/sputtering parameters was obtained, we could nanoprint any image onto the surface of the reflective display device.A heterostructure reflective device similar to what is reported in Figure 3 was used for the proof of concept.The FIB software first loaded a digital image to produce local height variation in nanosized pixel areas.Then, two famous paintings, Girl with a Pearl Earring by Johannes Vermeer and Starry Night by Vincent van Gogh, were nanoprinted on the device.The original digital images are presented in Figure 4a,b.Once the digital pictures were loaded and optimum beam parameters were set, nanostructures of different lateral dimensions were produced.Figure 4c,d shows the BSE-SEM images of the drawn nanostructures.The contrast from the SEM images perfectly resembles the original digital images used for patterning.As discussed earlier, the BSEs signal depends on how deep a region is and how close it is to the gold substrate.In Figure 4c,d, regions closer to the gold substrate are brighter, and regions close to the surface are darker.
To better visualize the local height difference and the origin of contrast in the structured images, we investigated the morphologies by AFM.In Figure 4c, an AFM image showing the patterned region, a 3D view of some local structures (i and ii), and a line profile across the structured region (iii) are presented.The contrast from the AFM image, with local height changes, perfectly resembles the original digital image.However, it is better to have a zoomed-in view of the structured surface to fully unravel the source of the contrast in the SEM and AFM images and, ultimately, in the optical images.Therefore, in Figure 3e (i) and (ii), a closer look and an exaggerated 3D view of the morphology are given for better visualization.The red and green boxes represent the corresponding regions in the AFM image.The 3D views of the structure show the nanosize local height variability.The terrain is comprised of higher peaks and low valleys correlated with the relative grayscale value changes of the original digital image.This change is better visualized in the line profile of the structure presented in (iii).As can be seen, the line profile shows the morphology change with a depth variability of ≈20 nm.The backscattered electrons provide depth profiles.e) A large-area AFM scan of the metasurface after milling the surface.Here, whiter regions are deeper, and darker are close to the surface.In (i) and (ii), the 3D views of some of the regions in the AFM scan are presented to show the depth profile created by the FIB.In (iii), a line profile on the surface of the metastructure is shown.e,f) The optical images of the reflective metasurfaces.The local height difference between individual pixels created high contrast.In addition, phase switching produced another degree of freedom for contrast creation.The optical images of the reflective metasurfaces are presented in (f) and (h).The local height difference between individual pixels created high contrast.In addition, as presented in (g) and (i), phase switching produced another degree of freedom for contrast creation.Girl with a Pearl Earring by Vermeer: Credit 2024@Photo Scala, Florence, Artwork Location: Mauritshuis, The Hague, Netherlands.Starry Night by Van Gogh: Credit 2024@Photo Scala, Florence, Artwork Location: Museum of Modern Art (MoMA), New York, USA.
The local height changes seen in the SEM and AFM images and the corresponding contrast should also translate to the formation of different local structural colors from the thickness changes.Thus, the optical images should produce vivid resolutions and contrast from the reduced individual pixel sizes.Figure 4f-i shows optical images of the fabricated structures for the as-deposited and crystalline phases of the Sb 2 Se 3 thin film layer.Indeed, from the optical images, super-resolution images are visible, with a relatively small region producing extremely large DPI values.For example, the original uploaded digital image, Figure 4a, is close to 2k × 2k pixel size, and the fabricated region in Figure 4f is only 40 × 40 μm 2 .This corresponds to a significant pixels-per-inch (PPI) value of ≈127 000.The high PPI values are achieved because of the relatively small probe size of the ion beam for an accelerating voltage of 30 kV and beam cur-rent of 0.23 nA.This is a crucial point which is discussed in more detail below.

Local Milling with Depth Modulation
The use of FIB for nanostructuring a reflective device's surface has numerous advantages.For one, the small probe size means we can produce height differences with nanosized lateral dimensions producing vivid structural colors with very high PPI values.However, the functionality and advantage of using FIB extend beyond what is reported in Figure 4.The milling/sputtering power of the FIB, correlated to the grayscale values uploaded, is linearly related to the milling depth or the local height profiles created.While keeping the relative height differences Since the reflectance state depends highly on the film thickness, producing the metastructure heterostructure reflective states at varying depth levels makes different contrast states.b) By linearly increasing the milling/sputtering power of the FIB, contrast can be produced at a specific location throughout the film thickness.In (i), four different contrast states are seen for the as-deposited phase of the Sb 2 Se 3 film on gold.Here, the exact "contrast formation depths" and the relative depth variations are specified.The BSE-SEM image in (ii) shows the differences in contrast since the distance to the gold differs for the four states.(iii) Crystallization also induces another contrast state.In (c), the ability of the FIB milling to produce contrast in smaller lateral dimensions is shown.(i) The contrast modulation on a smaller lateral dimension (down to 10 μm width) is presented.(ii) BSE-SEM images of the metastructure reflective device for varying lateral dimensions is presented.The milling power used for the deepest structure in (c) (i.e., the structure to the right with a "red" background) was used to produce the smaller dimension structures in (i) and (ii).Girl with a Pearl Earring by Vermeer: Credit 2024@Photo Scala, Florence, Artwork Location: Mauritshuis, The Hague, Netherlands.constant, we can produce structured surfaces and various contrasts throughout the film thickness by varying the absolute depth.Figure 5a shows the schematic description of how the structures could be made at different depth levels throughout the film thickness.For instance, we create an image with a depth variation of plus or minus 1.5 nm at a depth of 5 nm, ±3 nm at a depth of 10 nm, ±4.5 nm at a depth of 15 nm, and ±6 nm at a depth of 20 nm. Figure 5b shows variable contrasts of a nanostructured surface created at different depth levels of the Sb 2 Se 3 thin film layer in this way.The nanostructures and the contrasts in Figure 5b(i) are produced by increasing the milling/sputtering power of the FIB by a small amount starting from the top.Varying shades of contrasts with superb resolutions can be seen by accessing all different thickness ranges of the active material.This is also another crucial point discussed in more detail below.
An SEM image from the BSEs signal is presented in Figure 5b(ii).The depth profile change is visualized in more detail from the contrast change of the produced nanostructures going from left to right.The structure from the left is placed close to the surface, and the BSEs signal is relatively low compared with the structure from the right.Milling/sputtering done in a relatively deeper region and thus close to the gold substrate have a more intense BSEs signal and appears brighter.Furthermore, the crystallization produced another set of contrasts from the optical parameter changes.Figure 5b(iii) shows the structured surfaces after crystallization (using hotplate annealing) and makes another set of contrasts different from what was seen for the asdeposited amorphous phase of the Sb 2 Se 3 thin film.Accessing all available depth levels throughout the film's thickness and the added degree of freedom from crystallization will immensely increase the functionality when moving toward a dynamic display device.In Figure 4, we showed that nanoprinting digital images could achieve extremely high PPI values on a reflective device surface, but the PPI value can be even higher.
Figure 5c shows the production of the same structures in a series of smaller lateral dimensions.It is worth noting that the spot size for an ion beam with an accelerating voltage of 30 kV with the applied advanced optics is well below 10 nm (although extremely complicated to know the exact value as various factors contribute to the final spot size).It is even smaller for best operating conditions and optimized practices (i.e., choice of beam current, working distance, and objective aperture).However, for small and fine structures, crosstalk between neighboring pixels is a problem.Indeed we expect some averaging of pixels and crosstalk in the small structures presented in Figure 5c.Nevertheless, looking at these structures milled in Figure 5c, where we would expect some crosstalk between neighboring pixels, the contrast, with rapid local variations as in the original image, still persists and is comparable to the larger structures.Moreover, since we are talking about pixel sizes close to the spot size of the ion beam, we can still claim that super-resolution printing is maintained.Executing designs in reduced dimensions offers the possibility of producing nanostructured surfaces with wide size tunability, which is crucial for future functionality.Additional examples of nanoprinting and depth-modulated contrast formation of the Starry Night by Vincent van Gogh are given in the Supporting Information (Figure S2, Supporting Information).

Discussions
In this work, we demonstrated the possibility of producing a super-high-resolution reflective nanostructured surface using FIB.The heterostructure device consists of a reflective gold substrate and a lossy PCM, Sb 2 Se 3 .By correlating the grayscale values of digital images to the milling/sputtering power of the FIB, local nanostructuring with sub-nm height variations control was achieved.Since small thickness changes of a thin film in the strong interference formalism translate to considerable reflectance changes, variable structural colors can be produced locally, thus creating contrast.Furthermore, the small probe size of the FIB is ideal for creating nanosized height contrasts in a relatively small lateral dimension creating vivid contrast with very high PPI values.Moreover, we showed our ability to prepare depth-modulated super-resolution nanostructure formation at any depth level throughout the thickness of the film, adding another degree of freedom for contrast tuning.
Compared with previously reported results for future dynamic display devices, using structured nanopillars [19,36] or through partial crystallization/amorphization of PCMs by electrical [16][17][18]23] or laser [15,17,20,22] signals for creating contrast, our ability to accurately reconfigure the height profile of neighboring "pixels" with any lateral dimensions comes with obvious advantages. Creting the nanostructured by FIB is relatively easy, especially compared with classical methods like electron beam lithography (EBL), where creating local height variations is virtually impossible or requires multiple iterations.In contrast, this work shows that FIB can produce structures with relatively high lateral resolutions with extremely small pixel sizes. On the other and, the ability of FIB to: 1) create localized height variations and 2) access all depth profiles throughout the film's thickness to create contrasts is of significant importance when moving forward to a functioning dynamic display device.The above-mentioned abilities could only be achieved by FIB and are complex problems for other methods like EBL.
Another important point to note is the added value of the initially nanostructured surfaces to combine them in the future with known partial crystallization/amorphization techniques.In principle, one can produce a pixelated structured surface where neighboring pixels have different reflectance profiles, thus creating various colors (RGB, for example), mimicking traditional display devices and combinations that present different colors.Furthermore, this color mixing scheme could be achieved by partially tuning the reflectance of the individual pixel so that a dynamic display could be produced.Finally, at first sight, the current "height-variations approach" might suggest that the structural colors would not work for grazing angle viewing.However, a closer look at the aspect ratio of nm scale height variations versus pixels of the order of a micrometer still makes it evident that the surface remains highly smooth and can still be observed well under grazing angle.On the contrary, the current approach using a strong interference effect is, in this respect, clearly advantageous compared to using Fabry-Pérot type interference.

Conclusion
This work demonstrates a local nanostructure of phase-change thin films to create structural colors and vivid contrasts for highresolution dynamic image formation using a focused ion beam.Through a controlled milling/sputtering of materials, we showed we could create height differences with various lateral dimensions.Furthermore, we can generate structural colors locally by accessing the reflectance changes with the thickness of a heterostructure reflective device consisting of a phase-changing Sb 2 Se 3 film on a reflective gold layer.We showed that structural color changes could produce a contrast with extremely small pixel sizes, producing high-resolution images with extremely high PPI values.Moreover, by linearly changing the milling power of the FIB, contrast can be modulated throughout the thickness of the film.This way, any available combinations of contrast levels and structural colors can be achieved.We believe our work will open up research on future dynamic display devices as a prime example here, but our approach can be used for a large variety of reconfigurable optoelectronic applications like superlenses, optical Fourier surfaces, and photonics integrated circuits.

Experimental Section
Materials and Characterizations: Sb 2 Se 3 thin films were pulsed laser deposited on various substrates.A laser fluence of 1 J cm −2 , a processing gas (Ar) of 0.12 mbar, and a repetition rate of 1 Hz were used for the depositions.The PLD system has a KrF excimer laser operating at 248 nm wavelength.Sb 2 Se 3 thin films were deposited on Si 3 N 4 membranes for elemental composition analysis and local imaging by TEM.A TEM (JEOL 2010) operating at 200 kV accelerating voltage and an energy-dispersive X-ray (EDX) detector was used to analyze the as-deposited and crystalline Sb 2 Se 3 thin films.
Spectroscopic Ellipsometry Measurement and Data Fitting: Spectroscopic ellipsometry (J.Woollam UV-VIS) was used to extract the optical constants of Sb 2 Se 3 thin films in the spectrum range of 300-1700 nm.For ellipsometry, multiple samples of Sb 2 Se 3 with varying thicknesses were deposited on thermal SiO 2 substrates.A spectroscopic scan was done within the spectral range.For both amorphous and crystalline Sb 2 Se 3 samples, measurements of  and Δ were collected from multiple angles of incidence (65°, 70°, 75°) for increased data fitting accuracy and reduced parameter correlations.The data fitting/analysis of the measured ellipsometry data was done with the WVASE software.A heterostructure model was built, and the Tauc-Lorentz optical oscillator was used to model the optical properties of the Sb 2 Se 3 thin films.During data fitting, the Tauc-Lorentz oscillator parameters and the film thickness were allowed to vary, and the refractive index (n) and extinction coefficient (k) were extracted.The crystallization temperature (T x ) of the as-deposited Sb 2 Se 3 thin films was extracted from dynamic ellipsometry (DE) measurements.A heating stage (HTC-100) was attached to the variable-angle spectroscopic ellipsometry (VASE) setup, and the TempRampVase software controller was used for temperature ramping.A 70°incidence angle and a heating rate of 5 °C min −1 were used for all samples.
Optical Heterostructure Device and Reflectance Measurements: Reflectance profiles of multiple reflective heterostructure devices were calculated by using the transfer-matrix algorithm.The ellipsometry data fitting for thickness before and after crystallization show density change between the as-deposited amorphous and crystalline phases of Sb 2 Se 3 (a 6%-10% thickness reduction is common for most phase-change materials).For all reflectance calculations, the thickness reductions upon crystallization was accounted for.The ellipsometry-extracted values for the optical constants of Sb 2 Se 3 were used for the calculations.Sb 2 Se 3 thin films with varying thicknesses were deposited on a reflective gold (Au) layer to produce reflective devices.A 100 nm thick Au layer was first deposited on a Si substrate using an e-beam evaporator (Temescal FC2000).A Ti layer, 5 nm thick, was first deposited before the gold layer to increase its adhesion.To avoid intermixing between the Au layer and the Sb 2 Se 3 thin film, a 10 nm spacer LaAlO x (LAO) layer was deposited first on top of the Au layer.In some samples, a 10 nm capping LAO layer was deposited on top of the Sb 2 Se 3 layers.
Local Nanostructuring and Controlled Surface Milling: The local structuring of the Sb 2 Se 3 was done by a focused ion beam (FIB of FEI Helios G4 CX Dual Beam system) operating at 30 kV accelerating voltage.The grayscale value of an image was initially calibrated to the milling power of the Ga + ion beam.Then, an image was loaded, and the corresponding pattern was created by ion beam milling (sputtering away materials).Finally, the morphologies of the completed structures were imaged by SEM (Helios G4 CX) and by AFM (Dimension Icon, Bruker).The reflectance spectra of the locally nanostructured areas (for both the as-deposited and crystalline phases) were measured using a home-built system to collect intensity from <10 μm regions.The as-deposited phase was thermally annealed at 250 °C for 5 min to induce crystallization.The setup uses white light and is focused by a 100× objective with a numerical aperture of 0.9 (see schematic in Figure S1, Supporting Information).The collected light intensity was analyzed by a spectrum analyzer and normalized to a reference signal.

Figure 1 .
Figure 1.Structural and optical analysis and measurement of pulsed-laser-deposited Sb 2 Se thin films.a) TEM characterization of an as-deposited phase of Sb 2 Se 3 thin film on Si 3 N 4 membrane.The as-deposited phase is entirely amorphous, as confirmed by the FFT pattern in the inset.b,c) TEM image and associated SAED pattern of a crystalline Sb 2 Se 3 thin film.The as-deposited phase was thermally annealed at 250 °C for 5 min to induce crystallization.d) Spectroscopic ellipsometry measurements of as-deposited and annealed Sb 2 Se 3 thin films deposited on thermally grown SiO 2 substrates were used to extract refractive index (n) and extinction coefficients (k) for a wide spectral range of 300-1700 nm.e) A significant difference in n and k values between the as-deposited amorphous and crystalline samples is seen.f) The precise crystallization temperature (T x ) of the as-deposited Sb 2 Se 3 thin films is extracted from dynamic ellipsometer measurements.

Figure 2 .
Figure 2. A reflective heterostructure design based on Sb 2 Se 3 thin films on reflective gold substrates.a) The schematics of the heterostructure stack comprise an active Sb 2 Se 3 , a reflective gold substrate, a capping, and thin spacer LAO layers.b) Measured and simulated normal incidence angle reflectivity spectra of the heterostructure device for varying the Sb 2 Se 3 film thickness.Both measured and simulated reflectivity spectra contain results for the as-deposited amorphous and crystalline phases of the Sb 2 Se 3 layer.The dashed black lines indicate how the cavity mode wavelength increases systematically with increasing layer thickness in the same way for measured and simulated reflectance spectra.c) Structural colors produced by the reflective heterostructure design.The made colors are for the as-deposited and crystalline phases of Sb 2 Se 3 , with thickness varying from 4 to 24 nm.The crystalline phase was achieved by a hotplate annealing of the as-deposited samples.All images' dimensions are the same, and the scale bar represents 200 μm.d) The corresponding color coordinates for the reflectance values in (b) are plotted on a chromatic diagram for both the as-deposited amorphous and crystalline thin films of Sb 2 Se 3 .A gamut area index, using the RGB primary colors, is included to the color map for a better illustration of the RGB gamut coverage.

Figure 3 .
Figure 3. Thin film thickness vs FIB milling/sputtering power calibration with digital grayscale values.a) A schematic of the reflective heterostructure device produced by pulsed-laser deposition of Sb 2 Se 3 thin film on a reflective gold substrate.A Ga + ion beam is used to create a nanostructured metasurface on the surface of the reflective heterostructure device.b) A digital image with 25 squares of varying 8-bit grayscale values from 10 to 250.The image was used to calibrate the milling/sputtering power of the focused ion beam (FIB) for different milling/sputtering parameters.c) An SEM image obtained from backscattered electrons (BSEs).The contrast between individual squares is due to how close the squares are to the bottom gold substrate.d) An AFM scan of the square structures and e) the line profiles of individual rows showing the local height differences.The line profiles are extracted from the AFM image in (d) where lines were drawn horizontally for each row.The associated grayscale values are added for the first and last row of the squares for better correlation of each line profile with the associated squares in (d).f) The milling/sputtering depth change with grayscale value.The relationship has been extracted from the AFM image and the associated line profiles shown in (d) and (e).g,h) Optical images of the structured squares with multiple structural colors have been produced due to local thickness variations of the Sb 2 Se 3 thin film for the as-deposited (g) and crystalline (h) thin film.

Figure 4 .
Figure 4. Super-resolution nanoprinting of a grayscale image using a focused ion beam (FIB).a) The original grayscale image, Girl with a Pearl Earring by Johannes Vermeer and b) Starry Night by Vincent van Gogh, to be printed on the Sb 2 Se 3 surface.The milling rate of the FIB was calibrated beforehand so that the sputtering rate is directly related to the grayscale value of each pixel.c,d) Backscattered electrons' SEM images of the fabricated metasurfaces.The backscattered electrons provide depth profiles.e) A large-area AFM scan of the metasurface after milling the surface.Here, whiter regions are deeper, and darker are close to the surface.In (i) and (ii), the 3D views of some of the regions in the AFM scan are presented to show the depth profile created by the FIB.In (iii), a line profile on the surface of the metastructure is shown.e,f) The optical images of the reflective metasurfaces.The local height difference between individual pixels created high contrast.In addition, phase switching produced another degree of freedom for contrast creation.The optical images of the reflective metasurfaces are presented in (f) and (h).The local height difference between individual pixels created high contrast.In addition, as presented in (g) and (i), phase switching produced another degree of freedom for contrast creation.Girl with a Pearl Earring by Vermeer: Credit 2024@Photo Scala, Florence, Artwork Location: Mauritshuis, The Hague, Netherlands.Starry Night by Van Gogh: Credit 2024@Photo Scala, Florence, Artwork Location: Museum of Modern Art (MoMA), New York, USA.

Figure 5 .
Figure5.The added degree of freedom for super-resolution image formation at multiple depth levels and contrast formation at variable lateral dimensions.a) Schematic description of the concept of depth modulation for contrast variation.Since the reflectance state depends highly on the film thickness, producing the metastructure heterostructure reflective states at varying depth levels makes different contrast states.b) By linearly increasing the milling/sputtering power of the FIB, contrast can be produced at a specific location throughout the film thickness.In (i), four different contrast states are seen for the as-deposited phase of the Sb 2 Se 3 film on gold.Here, the exact "contrast formation depths" and the relative depth variations are specified.The BSE-SEM image in (ii) shows the differences in contrast since the distance to the gold differs for the four states.(iii) Crystallization also induces another contrast state.In (c), the ability of the FIB milling to produce contrast in smaller lateral dimensions is shown.(i) The contrast modulation on a smaller lateral dimension (down to 10 μm width) is presented.(ii) BSE-SEM images of the metastructure reflective device for varying lateral dimensions is presented.The milling power used for the deepest structure in (c) (i.e., the structure to the right with a "red" background) was used to produce the smaller dimension structures in (i) and (ii).Girl with a Pearl Earring by Vermeer: Credit 2024@Photo Scala, Florence, Artwork Location: Mauritshuis, The Hague, Netherlands.