Rewritable color nanoprints in antimony trisulfide films

Abstract

Materials that exhibit large and rapid switching of their optical properties in the visible spectrum hold the key to color-changing devices. Antimony trisulfide (Sb₂S₃) is a chalcogenide material that exhibits large refractive index changes of ~1 between crystalline and amorphous states. However, little is known about its ability to endure multiple switching cycles, its capacity for recording high-resolution patterns, nor the optical properties of the crystallized state. Unexpectedly, we show that crystalline Sb₂S₃ films that are just 20 nm thick can produce substantial birefringent phase retardation. We also report a high-speed rewritable patterning approach at subdiffraction resolutions (>40,000 dpi) using 780-nm femtosecond laser pulses. Partial reamorphization is demonstrated and then used to write and erase multiple microscale color images with a wide range of colors over a ~120-nm band in the visible spectrum. These solid-state, rapid-switching, and ultrahigh-resolution color-changing devices could find applications in nonvolatile ultrathin displays.

Fig. 1. Rewritable thin-film device.

Schematic of the proposed rewritable device consisting of antimony trisulfide PCM switched between (A) crystalline (C-PCM) and (B) amorphous (A-PCM) states. The thin film consists of Si₃N₄ (5 nm)/Sb₂S₃ (t nm)/Si₃N₄ (5 nm)/Al (100 nm). The reflected colors of the sample are determined by the thickness (t) and the state of the PCM. With t = 20 nm here, the crystalline sample (purple) is amorphized using femtosecond laser pulses (high-intensity pulses with short duration) using a Nanoscribe GmbH two-photon lithography (TPL) system, while the amorphous sample (yellow) is crystallized using a thermal annealing process (effectively similar to providing low-intensity pulses with long duration). Optical micrograph of Vincent van Gogh’s self-portrait demonstrating that intermediate states (neither yellow nor purple) can be written on the device by varying exposure power of femtosecond laser pulses and erased via thermal annealing.10 μm

Fig. 2. Phase transitions of Sb₂S₃.

(A) Optical micrographs of the device with 20-nm-thick Sb₂S₃ in amorphous (i) and crystalline (ii) states. (iii) The crystalline sample can be reamorphized to varying degrees to its original orange color in square areas exposed with increasing excitation power of femtosecond laser pulses from 6.0 to 12.0 mW. (iv) The reamorphized color patches were switched back to the crystalline state after the thermal annealing process. Reflectance (B and D) and Raman (C and E) spectra of the amorphous sample, the crystalline sample, and the reamorphized color patches. (F) Cyclability measurement for the device with 20-nm-thick Sb₂S₃. (G) Reflectance resonant wavelength (valley) as a function of switching cycles. a.u., arbitrary units.

Fig. 3. Measured refractive indices of Sb₂S₃.

(A and B) Measured refractive indices n and extinction coefficients k of 20-nm-thick Sb₂S₃ in the four-layer sample in amorphous, crystalline, and reamorphized states. Crystalline states 1 and 2 correspond to the birefringent orientations of the crystal grains seen as purple and blue, respectively, in Fig. 2A. Scale bar, 10 μm. (C to E) Optical micrographs of the anisotropic Sb₂S₃ crystals under unpolarized and mutually perpendicular polarizations. The crystals at areas (i) and (iii) appear blue under P0 and shift to purple color under P90. The opposite is true for areas (ii) and (iv). The two bright particles are used as the markers for positioning. Scale bars, 5 μm.

Fig. 4. Color palettes of the rewritable devices in amorphous and crystalline states for different thicknesses of Sb₂S₃.

(A and B) Amorphous (left) and crystalline (right) color palettes with Sb₂S₃ thickness varying from 10 to 70 nm. Scale bars, 100 μm. (C) Reflectance spectra from representative color patches. Solid lines represent the amorphous state, and short dashed lines represent the crystalline state. With increasing thickness of Sb₂S₃, the fundamental (first resonance dip, as indicated) red-shifts out into the infrared and the second harmonic resonance appears within the visible spectrum. (D) Color coordinates from measured spectra plotted on the CIE 1931 chromaticity diagram of the devices with the thickness of the Sb₂S₃ film varying from 10 to 70 nm.

Fig. 5. Superresolution printing via laser switching of PCM.

(A) Optical micrograph of resolution test dot arrays in 20-nm-thick Sb₂S₃ in pulse mode. Identical patterns written in 15-nm-thick Sb₂S₃ in pulse mode (B) and grating lines in continuous mode (C). Pitch (center-to-center distance) of neighboring dots and gratings are marked as p and d, respectively. (D) Measured average color intensities of four neighboring dots in the x direction with d = 0.6, 0.7, 1.0, and 1.4 μm in (A) (intensities normalized). Calculated temperature distribution within the xy plane of Sb₂S₃ layer (E) and cross section (F) of the device.

Fig. 6. Rewritable chalcogenide color microprints.

(A) Optical micrographs of the same region of sample with 20-nm-thick Sb₂S₃ showing (i) the self-portrait painting of Vincent van Gogh, (ii) the erased sample in crystalline state after thermal annealing, (iii) a print of the Vase with Fourteen Sunflowers of Vincent van Gogh as the second image patterned on the same area, and (iv) a second self-portrait painting of Vincent van Gogh written as the third image after erasing the sample a second time. (B) Optical microscopy images of the amorphous, crystalline, and the reamorphized color patches of the sample with 15-nm-thick Sb₂S₃. The color patches with laser power from 2.0 to 8.0 mW correspond to intermediate states. The color patch with power of 10.0 mW is switched back to amorphous state. (C) Rewritable color prints on the same area of the sample with 15-nm-thick Sb₂S₃. Micrograph of Girl with a Pearl Earring (Johannes Vermeer, 1665) written as the first color print (left). Erased sample with thermal annealing (middle). Mona Lisa (Leonardo da Vinci) written as the second painting on the same area (right). Scale bars, 20 μm.