Reversible Photoswitching Function in Atomic/Molecular-Layer-Deposited ZnO:Azobenzene Superlattice Thin Films

Abstract

We report new types of reversibly photoresponsive ZnO:azobenzene superlattice thin films fabricated through atomic/molecular-layer deposition (ALD/MLD) from diethylzinc, water, and 4,4'-azobenzene dicarboxylic acid precursors. In these ultrathin films, crystalline ZnO layers are interspersed with monomolecular photoactive azobenzene dicarboxylate layers. The thickness of the individual ZnO layers is precisely controlled by the number (m) of ALD cycles applied between two subsequent MLD cycles for the azobenzene layers; in our {[(Zn-O) _m +(Zn-O₂-C-C₆H₄-N=N-C₆H₄-C-O₂)] _n +(Zn-O) _m } samples, m ranges from 0 to 240. The photoresponsive behavior of the films is demonstrated with ultraviolet-visible spectroscopy; all the films are found to be photoreactive upon 360 nm irradiation, the kinetics of the resultant trans-cis photoisomerization somewhat depending on the superlattice structure. The reversibility of the photoisomerization reaction is then confirmed with a subsequent thermal treatment. Our work thus provides proof-of-concept evidence of the suitability of the ALD/MLD technology for the implementation of photoactive moieties such as azobenzene within an inorganic matrix as an attractive new methodology for creating novel light-switchable functional materials.

Figure 1

(a) Structure of the trans and cis isomers of the AzB-DCA precursor and schematic two-dimensional illustrations of (b) the bonding structure in the hybrid m = 0 film and (c) the {[(Zn–O)_m+(Zn–O₂–C–C₆H₄–N=N–C₆H₄–C–O₂)]_n+(Zn–O)_m} superlattice structure with m = 3.

Figure 2

Optimization of ALD/MLD parameters: (a) GPC as a function of the precursor pulse lengths of DEZ (AzB-DCA for 10 s) and AzB-DCA (DEZ for 3 s) at 320 °C, (b) GPC and surface roughness values at different deposition temperatures (pulse sequence of DEZ for 3 s, N₂ for 4 s, AzB-DCA for 10 s, and N₂ for 40 s), and (c) film thickness vs the number of ALD/MLD cycles at 320 °C. In panels a and b, the number of ALD/MLD cycles was 300.

Figure 3

(a) FTIR spectra for a representative (92 nm) hybrid m = 0 film (deposited on silicon) and also for the AzB-DCA precursor powder (mixed with KBr) for reference. (b) UV–vis absorption spectra of hybrid films with varying numbers of ALD/MLD cycles. The green line shows the spectrum of AzB-DCA in aqueous solution. In the inset, the absorbance of the films at λ_max is plotted vs the number of cycles.

Figure 4

(a) XRR and (b) GIXRD patterns and (c) FTIR spectra for selected {[(Zn–O)_m+(Zn–O₂–C–C₆H₄–N=N–C₆H₄–C–O₂)]_n+(Zn–O)_m} superlattice thin films with varying m [and n (see Table 1)].

Figure 5

Changes in UV–vis spectra upon UV irradiation of as-deposited {[(Zn–O)_m+(Zn–O₂–C–C₆H₄–N=N–C₆H₄–C–O₂)]_n+(Zn–O)_m} films. The top insets illustrate the kinetics of the changes.

Figure 6

UV–visible absorption spectra of {[(Zn–O)₁₀+(Zn–O₂–C–C₆H₄–N=N–C₆H₄–C–O₂)]₆₀+(Zn–O)₁₀}: (a) the trans isomer, (b) the same sample after irradiation with UV light, and (c) the recovered trans isomer after it had been heated at 100 °C in a furnace.

Figure 7

XRR patterns for hybrid (m = 0, n = 400) and superlattice (m = 90, n = 10) thin films. The XRR results are shown for both as-deposited films and the same films after irradiation with UV light at 365 nm for 1 h.