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Reversible Photo-Switching Function in Atomic/Molecular Layer Deposited ZnO-Azobenzene Superlattice Thin Films Aida Khayyami, and Maarit Karppinen Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b01833 • Publication Date (Web): 17 Aug 2018 Downloaded from http://pubs.acs.org on August 22, 2018
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Chemistry of Materials
Reversible Photo-Switching Function in Atomic/Molecular Layer Deposited ZnO-Azobenzene Superlattice Thin Films Aida Khayyami and Maarit Karppinen* Department of Chemistry and Materials Science, Aalto University, P.O. Box 16100, FI-00076 Aalto, Finland *
[email protected] 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+(ZnO2-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 UV-vis spectroscopy; all the films are found 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.
1. INTRODUCTION Certain polymers and organic molecules have the capability to respond to tiny changes in their environment (e.g. temperature, light exposure, electric or magnetic field, humidity or pH) with a measurable change in some intrinsic property, such as color, shape, electrical conductivity or water permeability. This stimuli-responsive behavior is a highly attractive phenomenon for a number of potential applications; in particular, light-stimulated or so-called photoresponsive materials have been investigated for new types of optical switches,1,2 data storage devices,3,4 nanovalves,5 sensors,6 and local drug dispensers.7–9 Thus, the direct noncontact manipulation of photoresponsive materials through light illumination is an attractive phenomenon in itself. In addition, we may imagine hybrid materials where the photoresponsive organic component is embedded in an inorganic matrix in such an intimate way that the photostimulated change in e.g. the size of the photoresponsive organics would switch on an action in the inorganic host material. Such control-embedded hybrid materials could open up exciting new horizons in designing novel functional nanodevices.10–12 The azobenzene moiety C6H5-N=N-C6H5, consisting of a diazene -N=N- bridge between two phenyl groups, is one of the prototype photoresponsive components. It isomerizes reversibly around the N=N double bond upon light ir-
radiation of different wavelengths.13,14 This trans-cis photoisomerization from the more stable trans form to the less stable cis form is accompanied by a remarkable structural change, which has inspired several research groups to challenge azobenzene and its derivatives as photoswitchers for various smart materials.15 The two isomers exhibit distinct absorption bands in the UV-vis region, and also several other differences in physical properties including their different dielectric constants and dipole moments.16 Efforts to incorporate azobenzene molecules into inorganic matrices, such as layered double hydroxides,12,17–19 HfS2,20 Fe3O4,21 GeO2,2 TiO210,22 and ZnO23 have indeed been made, and in particular in the case of the two-dimensional matrices with an anticipation that the weakly polar interlayer galleries would provide the azobenzene molecules with optimal alignment and sufficient free space for the shape change to occur.12,18 For the incorporation of azobenzene moieties in inorganic hosts, various conventional solution-based techniques, such as Langmuir-Blodgett,24 sol gel,10,21 and layer-by-layer electrostatic self-assembly,18 have been used. However, the apparent needs to enhance the intimate interaction between the organic switcher molecules and the inorganic matrix, and to manipulate – preferably with atomic/molecular level accuracy – the organic-to-inorganic ratio and distribution, call for fundamentally novel synthesis approaches.25 In particular, high-end gas-phase
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thin-film techniques could allow not only the precise chemical control of the inorganic-azobenzene interfaces but also the integration of these materials into microelectronics and other advanced technologies. Here we demonstrate for the first time that the state-ofthe-art gas-phase atomic/molecular layer deposition (ALD/MLD) technique26 is superbly suited for the fabrication of precisely layer-engineered thin films where monomolecular azobenzene layers are coherently connected via covalent chemical bonds to the surfaces of accurately thickness-tuned metal oxide layers. This technique is derived from the conventional ALD (atomic layer deposition) thin-film technology,27,28 already used for decades for industrial-scale production of high-quality thin films and coatings of simple inorganic materials. In ALD two (or more) mutually reactive gaseous/evaporated precursors are pulsed into the reactor chamber one after another with intermediate purging, which results in self-limited gas-surface reactions and enables the fabrication of high-quality thin films of well-defined composition and thickness even on large-area substrates. The resultant thin-film coatings are moreover highly conformal, following nearly perfectly various surface architectures down to nanometer precision.28,29 A similar technique, i.e. MLD (molecular layer deposition), for purely organic thin films has also been known – though much less exploited – since 1990s.30,31 Then, in the more recently developed ALD/MLD technique for the hybrid thin films, ALD and MLD cycles are combined to produce alternate inorganic and organic layers with atomic/molecular level precision.32,33 Most interestingly, the combined ALD/MLD technique allows the engineering of carefully designed inorganic-organic multilayer structures as its self-limiting gas-surface reactions enable the precise control of the introduction frequency of the organic layers within the inorganic matrix.34–36 This has already been demonstrated with various superlattice, nanolaminate and gradient structures, investigated for e.g. gasbarrier37–39 and thermal-barrier40–42 coatings, as well as for tuning the optical properties.43,44 Moreover, from the multiple positive experiences related to the conventional ALD technique for inorganics, we may anticipate that the ALD/MLD processes for novel inorganic-organic thin films should be industry-feasible as well. We chose the ZnO/azobenzene system as our model system because: (i) ZnO has unique (photo)physical properties such as a direct band gap of 3.37 eV along with tunable optical and electronic properties,45–47 (ii) ZnO is one of the prototype ALD thin film materials45,48,49 and also wellbehaving Zn-based ALD/MLD processes have already been developed,50–54 and (iii) azobenzene moieties have been successfully combined with ZnO using conventional synthesis techniques.23,45,55–57 However, while the hybrid film growth principle is exactly the same as with the other already known ALD/MLD processes the challenge here is the
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considerably larger size of the azobenzene moiety compared to the typical organic components so far employed in ALD/MLD. The higher sublimation temperature thus required for the organic precursor restricts the depositions to the higher temperatures and may thus introduce additional concerns for the choice of the inorganic precursor and the substrate materials. We therefore first carefully optimize our ALD/MLD for the selected Zn and azobenzene precursors. We investigate ZnO/azobenzene superlattice (SL) structures within a wide composition range: X-ray reflectivity and infrared spectroscopy are employed to verify the intended superlattice structures, and the reversible photoisomerization of the films is demonstrated through UV-vis spectroscopy observations. 2. EXPERIMENTAL SECTION Thin film depositions. All the thin-film depositions were carried out in a commercial flow-type hot-wall ALD reactor (F-120 by ASM Microchemistry Ltd) using diethyl zinc Zn(C2H5)2 (DEZ; Aldrich, 52 wt% Zn in minimum), water and azobenzene-4,4’-dicarboxylic acid (AzB-DCA; TCI, 95%) as precursors. To reach the precursor vapor pressures required for efficient precursor transport to the substrate, AzB-DCA was placed in an open boat inside the reactor and heated to 310 °C for sublimation, whereas DEZ and H2O were evaporated using external reservoirs held at room temperature. Nitrogen (>99.999%, Schmidlin UHPN 3000 N2 generator) was used as a carrier and purging gas. The depositions were carried out under a 3–4 mbar pressure onto Si(100) and quartz slide substrates; the latter substrates were used for the UV-visible transmittance spectroscopy measurements. The ALD cycle for the ZnO layers was: 1 s DEZ / 1.5 s N2 / 1.5 s H2O / 2 s N2,58 and the ALD/MLD cycle for the (ZnO2-C-C6H4-N=N-C6H4-C-O2) layers was 3 s DEZ / 4 s N2 / 10 s AzB-DCA / 40 s N2, unless otherwise stated for the different precursor pulse lengths investigated. Then, in order to fabricate the superlattice films, the aforementioned ALD cycles for ZnO layers was combined with the ALD/MLD cycles for the Zn-azobenzene layers as follows: {[(DEZ+H2O)m+(DEZ+ s AzB-DCA])n+(DEZ+H2O)m}. This is expected to yield {[(Zn-O)m+(Zn-O2-C-C6H4-N=N-C6H4C-O2)]n+(Zn-O)m} SL structures where single molecular azobenzene layers are sandwiched between wider ZnO layer blocks with the controlled thickness (m cycles); note that n is the number of repetitions of the superlattice cycle, and thus indicates the total number of the monomolecular azobenzene layers in the film. Superlattice thin films with m ranging from 0 to 240 and n ranging from 4 to 150 were deposited through different combinations of the individual DEZ+H2O ALD and DEZ+AzB-DCA ALD/MLD cycles. Characterization techniques. The film thicknesses, roughnesses and densities were determined through X-ray
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Chemistry of Materials
reflectivity (XRR; X’Pert MPD PRO Alfa 1, PANalytical) measurements. The XRR data were fitted by X’Pert Reflectivity software by PANalytical for the film thickness and roughness; the thickness value was further divided by the number of deposition cycles to get the film growth rate, or the so-called growth-per-cycle (GPC) value. The density of the films was calculated from the XRR patterns based on the dependency of the critical angle θc on the mean electron density ρe of the film material, namely ρe =(θc2π)/(λ2re), where λ is the X-ray wavelength and re is the classical electron radius. The degree of crystallinity of the films was investigated by grazing incidence X-ray diffraction (GIXRD; X’Pert MPD PRO Alfa 1, PANalytical; Cu Kα radiation) the incident angle ranging from 20 to 70°. Chemical composition of the films was analyzed using Fourier transform infrared (FTIR; Nicolet Magna 750) spectroscopy; the measurement chamber was continuously purged with purified dry air during the measurements. A background spectrum was collected using an uncoated Si wafer and subtracted from the spectra measured for the samples. Photoisomerization of the films was investigated under irradiation by UV light using a 200 W xenon-doped mercury lamp (Hamamatsu Lightning cure LC8) equipped with a cut-off filter (λ=300–480 nm) from. The photon flux was 3000 mW/cm2 (at the working distance of 10 mm). For these measurements, the samples were grown on quartz substrates. The UV-vis transmittance spectra (Shimadzu UV-2600 spectrometer) were collected for samples in the wavelength range of 200–600 nm. In order to investigate the cis-to-trans back-isomerization reaction the samples were heated in air at 100 °C for 24 hours.
3. RESULTS AND DISCUSSION We deposited a series of {[(Zn-O)m+( Zn-O2-C-C6H4-N=NC6H4-C-O2)]n+(Zn-O)m} thin films from diethylzinc, water and azobenzene dicarboxylic acid precursors, see Table 1 and Figure 1; the depositions yielded visually homogeneous thin films for all m and n values investigated. In the following we first discuss the optimization of the ALD/MLD process parameters of the DEZ+AzB-DCA process for the hybrid (Zn-O2-C-C6H4-N=N-C6H4-C-O2)n films with m=0, then investigate the possibility to combine this process with the well-known DEZ+H2O ALD process for different SL structures with m>0, and finally characterize the resultant thin films for the basic chemical and structural properties and also for their photoresponse behaviors. ALD/MLD process parameters. We first confirmed the fulfilment of the surface saturation criterion for both precursors in our DEZ+AzB-DCA process by gradually increasing the pulse length of one of the precursors at the time while keeping the pulse length for the other precursor fixed, and then monitoring the possible changes in the growth-per-cycle (GPC) value accordingly, see Figure 2(a). These experiments were carried out at 320 0C, which is the lowest feasible deposition temperature for the DEZ+AzBDCA process, defined by the relatively high evaporation temperature of AzB-DCA (310 0C). Based on Figure 2(a), the surface reactions reach saturation in 2 s for DEZ and in 10 s for AzB-DCA. For the rest of the experiments we fixed the pulse/purge lengths as follows: 3 s DEZ / 4 s N2 / 10 s AzBDCA / 40 s N2.
Table 1. The {[(Zn-O)m+ (Zn-O2-C-C6H4-N=N-C6H4-C-O2)]n+ (Zn-O)m} film structures deposited in this work at 320 oC.
Film type
n
m
Expected ZnO-layer thickness (nm)
n x (m+1) + m
Total film thickness (nm)
GPC (Å/cycle)
ZnO SL SL SL SL SL SL SL Hybrid Hybrid Hybrid Hybrid
0 4 6 10 15 30 60 150 200 300 400 600
1000 240 171 90 37 19 10 4 0 0 0 0
130 31 22 12 5 2.5 1.3 0.52 0 0 0 0
1000 1200 1197 990 592 589 610 604 200 300 400 600
130 143 137 122 92 103 120 164 60 92 125 192
1.30 1.20 1.15 1.25 1.55 1.75 1.95 2.70 3.00 3.00 3.10 3.20
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Chemistry of Materials
Figure 1. (a) Structure of the trans and cis isomers of the AzB-DCA precursor, and schematic 2D 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.
7
5 4 3 2
5 4 3 2 1
1 0
GPC Roughness
(b)
6
Roughness (nm)
AzB-DCA DEZ
(a)
6
GPC (Å/ cycle)
7
0 0
5
10
320
15
330
340
350
360
370
380
Deposition temperature (C)
Pulse length (s) 240
(c)
200
Thickness (nm)
GPC (Å/ cycle)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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160
R2= 0.99996
120 80 40 0
0
100
200
300
400
500
600
Number of cycles
Figure 2. Optimization of ALD/MLD parameters: (a) GPC as a function of the precursor pulse lengths of DEZ (10 s AzB-DCA) and AzBDCA (3 s DEZ) at 320 0C, (b) GPC and surface roughness values at different deposition temperatures (pulse sequence: 3 s DEZ / 4 s N2 / 10 s AzB-DCA / 40 N2), and (c) film thickness versus the number of ALD/MLD cycles at 320 0C. In (a) and (b), the number of ALD/MLD cycles was 300.
Next we investigated the growth rate at different deposition temperatures, see Figure 2(b). It was found that the GPC value of ca. 3 Å/cycle at 320 0C gradually increases
with increasing deposition temperature, up to ca. 7 Å/cycle at 380 0C, possibly due to some CVD type growth. Also revealed from Figure 2(b) is that, the surface roughness increases with increasing deposition temperature; at 390 0C and higher temperatures the films were so rough that they could not be reliably studied by XRR anymore. For the rest of the experiments we fixed the deposition temperature to 320 0C. To confirm the ideal ALD/MLD type growth at this temperature we finally deposited a series of hybrid films by applying different numbers of DEZ+AzB-DCA cycles. Figure 2(c) shows that the film thickness increases linearly with increasing number of cycles, which indicates an excellent atomic/molecular level control over the film thickness. Characterization of m=0 hybrid films. From GIXRD measurements, all the hybrid m=0 films deposited via the DEZ+AzB-DCA process in the temperature range from 320 to 380 0C were found amorphous. The film density of the hybrid film deposited at 320 0C was determined from the XRR data to be ca. 1.5 g/cm3, which is less than one third of the ideal density of bulk wurtzite-structured ZnO (5.6 g/cm3) and the density determined for our crystalline ZnO film deposited at 320 0C (5.0 g/cm3); this is what one could expect as the films are composed of zinc ions in combination with light and spacious organic molecules instead of single negatively charged oxide ions. All the films independent of the deposition temperature were homogeneous
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in appearance and stable under ambient conditions. A representative film deposited at 320 0C was re-analyzed after one-month storage and found to be unaltered. In Figures 3(a) and 3(b) we display FTIR and UV-vis spectra for our hybrid m=0 film deposited at 320 0C to confirm the presence of the azobenzene moieties in the film. Also given are the spectra recorded for the 4,4´-azobenzene dicarboxylic acid precursor for comparison. The disappearance of the sharp signal at 1690 cm-1 in the FTIR spectrum of the thin film sample which is characteristic of the C=O stretching of free carboxylic acids confirms the bonding of the organic precursor to zinc cation through this unit upon the hybrid thin film formation. At the same time, the fact that the absorption band seen at 3200–3500 cm-1 in the spectrum of the precursor due to the OH group is missing from the spectrum of the hybrid thin film indicates that the azobenzene moiety is bonded to Zn through the oxygen atom from the carboxylate OH group as well to form the (Zn-O2-C-C6H4-N=N-C6H4-C-O2)n hybrid film, as shown in Figure 1(b). Finally we note that the splitting between the dominant absorption peaks at 1413 and 1598 cm−1 due to the symmetric and asymmetric stretchings of the carboxylate group, respectively, is 185 cm−1, which confirms that the carboxylate group is indeed in a bridging position between two zinc cations.59,60
Transmittance (arb. units)
(a) AzB-DCA+KBr
1540 1413
1690 1598
Thin Film (m=0)
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1)
2.0 n=600 1.5 400 1.0
2.0
333 nm
(b)
Absorbance
2.5
Absorbance
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Chemistry of Materials
1.6 1.2
R2=0.998
0.8 0.4 0.0 0
200
400
600
Number of cycles
The UV-vis absorption spectra for the AzB-DCA precursor (in aqueous solution) and our hybrid m=0 films with various thicknesses corresponding to ALD/MLD cycle numbers of n = 200, 300, 400 and 600 are depicted in Figure 3(b). The strong absorption band seen at ca. 320 nm for the AzB-DCA precursor is due to the π-π* transition in the azobenzene trans-isomer.18,24 For our hybrid (Zn-O2-CC6H4-N=N- C6H4-C-O2)n thin films this transition shows a 10-nm blue shift compared to the absorption band of AzBDCA (shown by the green line) which indicates that the azobenzene units pack as so-called H-aggregates, with parallel orientations of the molecules.24,61–63 As displayed in the inset of Figure 3(b), the absorbance at 320 nm increases linearly with the number of cycles, indicating a stepwise and regular film growth. Finally, it should be mentioned that all the aforementioned FTIR and UV-vis observations are in line with the schematic structure presented in Figure 1(b) for the m=0 hybrid films. Superlattice m>0 films. In Figure 4(a) we show XRR patterns for representative {[(Zn-O)m+(Zn-O2-C-C6H4N=N-C6H4-C-O2)]n+(Zn-O)m} thin films deposited with m ALD cycles of DEZ+H2O to control the individual ZnOlayer thickness between monomolecular azobenzene layers deposited with a single ALD/MLD cycle. The superlattice structures can be confirmed by the presence of welldefined Kiessig fringes as well as SL peaks (demonstrated by dash lines in Figure 4(a)) which are periodically repeated and correspond to the expected superlattice repeat units.64 It is also possible to fit the XRR data for the individual layer thicknesses; we did this for the n=4, m=240 SL film to yield the total film thickness at 141 nm and the individual ZnO-layer thickness at 30 nm in excellent agreement with the expected values given in Table 1. Figure 4(b) shows GIXRD patterns for selected SL thin films and for a reference ZnO film. The diffraction peaks are well explained by the hexagonal wurtzite structure of ZnO and are in Figure 3(b) indexed accordingly. For m
