Understanding adhesion of gold conductive films on sodium-alginate by photoelectron spectroscopy

Thin Solid Films 690 (2019) 137535

Contents lists available at ScienceDirect

Thin Solid Films journal homepage: www.elsevier.com/locate/tsf

Understanding adhesion of gold conductive films on sodium-alginate by photoelectron spectroscopy

T

Raffaella Capellia,b,c, Piera Maccagnanid, Franco Dinellie, Mauro Murgiaf, Monica Bertoldog, ⁎ Monica Montecchia, Bryan P. Doylec, Emanuela Carleschic, Luca Pasqualia,b,c, a

Dipartimento di Ingegneria E. Ferrari, Università di Modena e Reggio Emilia, Via Vivarelli 10, 41125 Modena, Italy CNR - Istituto Officina dei Materiali, S.S. 14, km 163.5 in Area Science Park, 34012 Trieste, Italy c Department of Physics, University of Johannesburg, PO Box 524, Auckland Park 2006, South Africa d CNR - Istituto per la Microelettronica e Microsistemi, via P. Gobetti 101, 40129 Bologna, Italy e CNR - Istituto Nazionale di Ottica, via G. Moruzzi 1, 56124 Pisa, Italy f CNR - Istituto per lo Studio dei Materiali Nanostrutturati, via P. Gobetti 101, 40129 Bologna, Italy g CNR - Istituto per la Sintesi Organica e la Fotoreattività, via P. Gobetti 101, 40129 Bologna, Italy b

ARTICLE INFO

ABSTRACT

Keywords: Sputtering deposition Thermal evaporation Gold thin films Biopolymer X-ray photoelectron spectroscopy

Ultrathin layers of gold, from 2 to 25 nm of nominal coverage, have been deposited on sodium-alginate biopolymer foils applying two alternative approaches: low power sputtering and thermal evaporation. The morphology of the deposited layers was obtained by means of atomic force microscopy. In the early stages of growth, thermal evaporation gives rise to a top surface resembling the underlying substrate, whereas low power sputtering produces a topography characterized by smoother areas. This indicates that the film growth occurs in different ways. X-ray photoelectron spectroscopy with two photon energies, corresponding to Al Kα and Ag Lα photons, was used to get information on the chemistry at the interface and on the degree of intermixing between Au and sodium-alginate. While no chemical modifications with respect to the bare materials could be detected, the evolution of the intensities of the relevant core levels of Au and sodium alginate (Au 4f and Na 1s in particular) indicated a strong intermixing in the case of films deposited by low power sputtering. This is further supported by optical measurements. The observed behaviour can be correlated with the enhanced adhesion of sputtered films compared to thermally evaporated ones.

1. Introduction Sodium-alginate (SA) is a fascinating material [1]. It is a natural biopolymer derived from marine algae. It is non-toxic, biocompatible and biodegradable, presents good protonic conductivity and has outstanding film-forming characteristics. It has been proposed in tissue engineering and drug delivery [2]. In addition, due to its transparency, chemical and mechanical properties [3] (e.g. it can be easily bent) it is expected to find prominent applications as a dielectric in classes of ‘green’ and flexible electronic devices. In perspective, SA films could flank, or even replace in terms of better overall performance, nanocellulose films and foils [1,4,5]. In this respect, it is necessary to investigate in detail the possibility to integrate SA thin films and foils into electronic circuitry. One initial step in this direction is to form reliable conductive contacts on SA films, which guarantee high conductivity, comparable to existing contacts on dielectrics, and high adhesion combined with flexibility.

⁎

Recently it has been demonstrated [6] that thin gold films deposited by sputtering at very low power onto SA free-standing foils present outstanding adhesion characteristics compared to thin films deposited by ‘conventional’ thermal evaporation. While metallic layers obtained by thermal evaporation can be easily scratched or peeled off, this is not the case for sputtered films. Very smooth and homogenous contacts can be produced, with gold nominal thicknesses ranging from a few nm up to 25 nm. At this thickness the electrical performance of bulk Au is reached. This allows one to obtain highly flexible and robust layers, which maintain high conduction of the gold film even in the ‘bent’ state of the SA foil. This cannot be achieved in Au films produced by thermal evaporation. Interestingly, the two types of deposition methods produced continuous and relatively flat films, as observed by atomic force microscopy. The enhanced adhesion of the sputtered layers compared to the thermally evaporated ones was explained only in qualitative terms, supposing a higher penetration of gold into the SA foil during sputtering at the initial deposition stages, due to the higher kinetic

Corresponding author at: Dipartimento di Ingegneria E. Ferrari, Università di Modena e Reggio Emilia, Via Vivarelli 10, 41125 Modena, Italy. E-mail address: [email protected] (L. Pasquali).

https://doi.org/10.1016/j.tsf.2019.137535 Received 14 May 2019; Received in revised form 9 August 2019; Accepted 2 September 2019 Available online 02 September 2019 0040-6090/ © 2019 Elsevier B.V. All rights reserved.

Thin Solid Films 690 (2019) 137535

R. Capelli, et al.

energy of the Au. Shallow implantation of Au species in the SA matrix during sputtering is also possible [7]. In this work we tackle this point explicitly. We apply x-ray photoelectron spectroscopy (XPS) at two different photon energies to investigate the possibility of chemical modification of SA following gold deposition and the degree of intermixing at the interface. In particular, high-resolution core levels of Au and of the SA substrate are recorded as a function of the metallic overlayer nominal thickness for films obtained either by sputtering or by thermal evaporation.

significantly, though it is slightly reduced compared to the pristine one and to the evaporated films. The UFM map (Fig. 1e) presents a less pronounced elastic contrast compared to the evaporated one, indicating that the surface is more homogeneous. In Fig. 2 we report the morphology of thicker Au films: 4 nm via thermal evaporation and 5 nm via low power sputtering. The evaporated film (Fig. 2a) still shows elongated grains all over the surface. The morphology and the RMS roughness still closely reproduce those of the bare SA. The UFM contrast (Fig. 2b) is now almost negligible, indicating that the substrate is covered by a uniform overlayer. The sputtered film (Fig. 2c) is even less similar to the pristine surface. We can notice that the smooth areas have expanded, whereas some randomly distributed grains still remain. The RMS roughness is comparable to that of the 2 nm sputtered film. The UFM map (Fig. 2d) indicates that also in this case a homogeneous Au overlayer has formed. A smoothening of the surface roughness in case of deposition via sputtering has previously been reported by some of us [6]. However, AFM does not provide any clear information on the Au/ alginate interface in terms of composition, intermixing, chemical state of the gold aggregates or chemical modifications, i.e., molecular dissociation of the polymer, generation of fragments or reactions induced by sputtering. To tackle these questions, XPS was chosen. Two different energies, namely 1486.71 eV and 2984.3 eV, were used as excitation photons, to vary the electron mean free path of the photoemitted electrons and therefore acquire depth-resolved information on the sub-surface composition. Extended XPS survey scans taken on the same samples as shown in Fig. 2 are reported in Fig. 3. The spectral features reflect the surface composition of the different samples. The overall intensity of carbon and oxygen structures may be influenced by the presence of contaminants at the surface, since no cleaning pre-treatment was applied to the films to avoid chemical modifications of the surface region. However, surface contaminants are not expected to affect the Au levels and Na 1s level (and Na Auger lines). Therefore, these features may be considered as representative of the nature of the deposited overlayer (Au 4f peaks in particular) and of the substrate (Na 1s). From first inspection, it is clear that the 5 nm sputtered film still shows prominent peaks from the SA substrate (Na 1s and Auger lines) that are instead strongly reduced for the 4 nm film obtained by thermal evaporation. In addition, Au core levels appear generally less intense for the sputtered film compared to the evaporated one. Considering the smaller nominal coverage of the thermally evaporated film, this seems to indicate that while the evaporated film tends to form a uniform overlayer, the sputtered gold may penetrate into the SA substrate, either giving rise to pronounced intermixing or to a sub-surface Au layer covered by a thin floating SA layer. Higher resolution spectra of the principal elemental peaks are shown in Figs.4 and 5. The spectra of the Au 4f levels are reported in Fig. 4a for three representative coverages: ultrathin (2 nm), intermediate (4–5 nm) and high (24–25 nm). The spectra always present a single doublet, corresponding to the 7/2–5/2 spin-orbit split components (Δso = 3.6 eV). The line shape does not show variations from low coverage to high coverage. Also, the FWHM = 0.67 eV does not vary with the nominal coverage. This is common to both deposition methods. The absence of satellites and additional shifted components of Au at higher binding energy than the bulk Au 4f7/2 position at 84.0 eV, together with the absence of peak broadening, even at low coverage, seem to exclude that the Au thin films are characterized by a high degree of sparse Au atoms or clusters (Aun) [9–11] on top or diffused into the SA matrix. We may conclude that Au in the matrix aggregates in the form of relatively large nanoparticles and inclusions. The intensity of the Au 4f peaks instead evolves differently for sputtered and evaporated films. At 2 nm of nominal coverage for the evaporated film the Au 4f intensity is already very intense, while for the sputtered film it is much lower, by a factor of almost 20. This trend continues at intermediate coverages of 4–5 nm, with the apparent thickness of the evaporated film, which can be related to the intensity of

2. Experimental details Sodium-alginate (SA) was purchased from Sigma-Aldrich with a viscosity of 20 ± 5 cps and used in ultrapure water solution (resistivity: 18.2 MΩ/cm at 298 K - Millipore Direct-Q® 3 UV purification system) at a concentration of 2% wt at room temperature. A solution of 14 ml was then cast on a 90 mm polystyrene Petri dish. After complete evaporation of water at room temperature over several days, dry transparent films of 20 ± 5 μm were peeled off from the dish. Au was deposited either by sputtering in a MRC 8622 RF system, applying a power of 20 W [6], or by thermal evaporation with a Al2O3 coated Mo crucible at a base pressure of 8.0 × 10−5 Pa. The nominal thickness of gold films was evaluated with a Sycon stc 200 quartz microbalance. The evaporation rate was 0.15 nm/s, with film thicknesses ranging from 2 to 25 nm. A SPECS PHOIBOS 150 electron analyser in tandem with a monochromatised Al Kα (1486.71 eV) and Ag Lα (2984.3 eV) dual photon xray source was used to collect the XPS data. A low energy electron flood gun was used to counteract charging of the sample (electron energy = 2.5 eV, emission current = 20 μA). Survey scans were taken with Al Kα photons with a resolution of 1.0 eV. High resolution spectra were taken with a resolution of 0.6 eV, for Al Kα photons, and 1.3 eV, for Ag Lα photons. The spectra were recorded at normal emission. Atomic force microscopy (AFM) was performed using a hybrid system made of a commercial head (SMENA, NT-MDT), home-built electronics and a digital lock-in amplifier (Zurich HF2LI). The setup was operated either in intermittent contact mode (ICM) or in ultrasonic force mode (UFM). The cantilevers employed are commercially available: from MikroMasch (HQ:NSC35) for ICM and from NT-MDT (CSG30) for UFM. UFM allows one to simultaneously map the morphology and the elasticity of the surface investigated. The depth of elastic sensitivity is in the range of a few nm from the surface, depending on the normal load applied [8]. Optical transmission and specular reflectivity measurements were performed with linearly polarized light in s-scattering geometry (electric field perpendicular to the scattering plane) in the 500–700 nm wavelength range using an Ocean Optics DH-20000-BAL light source and an Ocean Optics USB2000-XR1 grating monochromator equipped with CCD detectors. Reflectivity was measured at 60° of grazing incidence with respect to the surface. 3. Results and discussion Au thin films of different nominal thicknesses from 2 to 25 nm were deposited onto free-standing SA flat foils, either by sputtering or by thermal evaporation. In Fig. 1 representative AFM images of the bare substrate and of 2 nm Au layers are shown. The SA foil (Fig. 1a) presents a flat surface characterized by elongated domains separated by shallow grooves. The root mean square (RMS) roughness measured is 5.6 nm. The topography of the thermally evaporated film (Fig. 1b) also shows elongated grains all over the surface, similar to the pristine substrate. The UFM (Fig. 1c) map presents areas with different elastic contrast, indicating that the Au overlayer is not completely homogeneous. On the other hand, the morphology of the sputtered film (Fig. 1d) is quite different from the pristine surface: smooth regions are surrounded by small round domains. The RMS roughness does not vary 2

Thin Solid Films 690 (2019) 137535

R. Capelli, et al.

Fig. 1. a) AFM topography image of the pristine SA foil obtained in ICM; b) topography and c) UFM maps of a 2 nm Au film deposited via thermal evaporation; d) topography and e) UFM maps of a 2 nm Au film deposited via low power sputtering.

Fig. 3. Survey scans taken with a monochromatized Al Kα source (hν = 1486.71 eV) on bare SA, on a gold film of 5 nm deposited by sputtering on SA and on a gold film of 4 nm deposited by thermal evaporation on SA. Peaks have been labelled according to their binding energy.

shown as a function of nominal coverage. Dashed lines in Fig. 4b represent a best-fit power law to the experimental points and serve as guides to the eye, to highlight the overall trends. Considering that the inelastic mean free path (IMFP) for Au 4f photoelectrons excited with 1486.71 eV photons in a gold matrix is about 1.8 nm [12] and considering that 95% of the photoelectron signal originates from a region which is about 3 times the IMFP, the behavior of the evaporated films is perfectly compatible with a metallic overlayer covering the SA substrate from the initial deposition stages. Instead, for the sputtered film the increase of the Au intensity is much slower and seems to support some enhanced diffusion of the metal into the SA. In Fig. 5a the C 1s spectra are shown for the bare alginate and for the 4 nm evaporated and 5 nm sputtered films. Apart from the intense structure at low binding energy, associated to CeC and CeH bonding,

Fig. 2. a) Topography and b) UFM maps of a 4 nm Au film deposited via thermal evaporation; c) topography and d) UFM maps of a 5 nm Au film deposited via low power sputtering.

the gold features, higher than the thickness of the sputtered film. In Fig. 4b the Au 4f intensity normalized to the intensity of the bulk (which is assumed to be reached at 24–25 nm for both methods [6]) is 3

Thin Solid Films 690 (2019) 137535

R. Capelli, et al.

Fig. 4. a) High resolution XPS spectra of Au 4f levels taken at normal emission with a monochromatized Al Kα source at increasing nominal thickness of the deposited film; b) Normalized intensity of Au 4f as a function of nominal thickness; dashed lines serve only as a guide to the eye.

that partly can be ascribed to contributions from contaminants at the surface, all components of SA show up at the expected binding energy of the bulk polymer [13,14]. No relevant traces of reaction at the interface or molecular dissociation can be detected from the spectra. This is also confirmed for O 1s (Fig. 5b) and Na 1s (Fig. 5c) levels. A slight smearing of the O 1 s structure for the 4 nm evaporated film may again be associated to contributions from contaminants. The most relevant remark regards the intensity of the features: while for the sputtered film the intensity of the structures related to SA remains practically unaffected, supporting the idea of Au diffusion into the polymer, their intensity is reduced for the evaporated film. This is consistent with the formation of a compact gold overlayer. To get more specific information on the possible formation of an intermixed interface on top of SA we followed the intensity evolution of the Au 4f and Na 1s peaks at different photon energies. Since the presence of C-rich (or O-rich) contaminants can lead to an overestimation of the polymeric component, we have chosen Na 1s as the more representative structure for the SA substrate. Au 4f and Na 1s peaks were measured at 1486.71 eV of photon energy (monochromatized Al Kα source) and at 2984.3 eV (monochromatized Ag Lα source). The intensity ratio is plotted in Fig. 6a and b respectively, as a function of the nominal coverage for both types of

Fig. 5. High resolution XPS spectra of (a) C 1s, (b) O 1s and (c) Na 1s levels taken at normal emission with a monochromatized Al Kα source on the bare SA foil and at intermediate thickness of the deposited film. Assignation of the features is consistent with refs [13,14].

deposition methods. It can be observed that the evaporated films immediately show a strong damping of Na with respect to gold, which suggests the formation of an Au overlayer on top of SA from the initial deposition stages. At lower photon energy (Fig. 6a) the Na 1s/Au 4f ratio remains practically constant for all nominal coverages. In the hypothesis of a compact continuous metallic layer on top of the SA polymer, the behavior of the Na 1s/Au 4f intensity ratio would follow a law of the type: 4

Thin Solid Films 690 (2019) 137535

R. Capelli, et al.

possible to access the substrate SA at 2–4 nm of nominal coverage. Apparently, the surface inhomogeneities observed by UFM in Fig. 1c for the 2 nm coverage do not appreciably affect the overall effective thickness of the film probed by XPS. The situation is schematized in the graphic of Fig. 6, where the thermally evaporated films are represented by compact layers progressively increasing their thickness with nominal coverage. The different sampling depths of the two photon energies are also qualitatively shown (in light and dark orange for Al Kα and Ag Lα photons, respectively). The sputtered films present a different behavior. Na 1s intensity reduction is much slower with the increasing nominal thickness, as noticed above. The observed trend at the two photon energies cannot be simply reproduced by assuming the validity of Eq. (1). We tried to modify the model by including an additional SA overlayer on top of Au, with Au in a ‘sandwich’-like configuration between the SA substrate and a thin SA overlayer, but we could not reach consistent results at the two photon energies. We therefore tried to interpret the data at intermediate nominal coverage (that is at 4–5 nm) in terms of an intermixed Au-Alginate layer of effective thickness deff, as qualitatively represented in the graphic of Fig. 6, second-most from the right. In this case, if f is the average fractional composition of Au in the layer (the remaining (1 − f) being SA), a simplified model for the attenuation of the intensities of gold and alginate core levels at intermediate coverage would lead to:

INa 1s (deff ) IAu 4f (d eff )

Fig. 6. Na 1s / Au 4f7/2 intensity ratio as a function of nominal thickness, taken with hν = 1486.71 eV (a) and 2984.3 eV (b). Inset: sketch of the different type of interfaces which form depending on the deposition method and different sampling depths that can be achieved at the two photon energies.

IAu 4f (d eff )

d eff

e

=C

Au (Ek, Na1s ) deff

1

e

Au (Ek , Au4f )

d eff aver (Ek , Na1s )

+ (1

f) 1

=C f 1

e

e

deff aver (Ek, Au 4f )

d eff aver (Ek , Na1s )

(2)

where deff = dnominal/f, λaver(Ek, Na1s) = fλAu(Ek, Na1s) + (1 − f)λAlginate(Ek, and λaver(Ek, Au4f) = fλAu(Ek, Au4f) + (1 − f)λAlginate(Ek, Au4f) are the average IMFPs at Na 1s and Au 4f kinetic energies in the intermixed layer of f fractional gold composition. In turn, λAlginate(Ek,Na1s-Al Kα) = 1.5 nm, λAlginate(Ek,Au4f-Al λAlginate(Ek,Na1s-Ag Kα) = 3.9 nm, Lα) = 5.2 nm, λAlginate(Ek,Au4f-Ag Lα) = 7.1 nm are the IMFPs of Na 1s and Au 4f photoelectrons in the alginate organic matrix at the two photon energies [12]. The model assumes that there are no empty spaces in the Au-SA intermixed film, i.e. it is a compact solid. All relevant parameters and result from Eqs. (1) and (2) are given in Table 1. By fitting this expression with the experimental values obtained at 4–5 nm of nominal coverage (sputtered films) we obtained for Al Kα photons (Fig. 6a) a value of deff = 15 ± 3 nm for the intermixed layer and an average gold fraction f = 0.28 ± 0.02 over the sampled region. To be noted is that the effective thickness deff exceeds by more than three times the IMFPs, which indicates that the intermixed region extends over the whole XPS sampling depth. At higher photon energy (Ag Lα photons – Fig. 6b), for the same nominal thickness of 4–5 nm, we obtained a best fit value of deff = 30 ± 3 nm for the intermixed layer and an average gold fraction f = 0.15 ± 0.02. Due to the larger sampling depth, it can be deduced that some Au penetrates deep into the SA matrix, over distances larger than 20 nm. Comparing the f values obtained at the two photons, it can be inferred that the concentration of Au presents a graded concentration, which progressively decreases with depth, as schematically shown in the in the rightmost graphic of Fig. 6. At higher coverage, namely at 24 nm of nominal coverage, the intensity ratio has reached the same values obtained for the evaporated films at both photon energies. This indicates that a continuous compact layer of bulk-like gold has finally formed, with an overall thickness that exceeds the XPS sampling depth. These findings seem to indicate that the intermixing region is relatively thick in the case of sputtering, with a gold concentration that progressively decreases with depth. The formation of a relatively thick intermixed layer, extending over tens of nm, compared to the nominal Na1s)

INa 1s (deff )

e

(1)

where deff is the effective thickness of the gold layer on top of SA, in this case corresponding with the nominal thickness, C is a constant depending on the ratio of the Na 1s and Au 4f intensities for the two bare materials, i.e. on, the relative atomic scattering factors, λAu is the IMFP of electrons in gold with kinetic energies corresponding to the Na 1s levels and Au 4f levels. For Al Kα photons, λAu(Ek,Na1s-Al Kα) = 0.75 nm and λAu(Ek,Au4f-Al Kα) = 1.8 nm [12]. At the nominal coverages investigated here, the experimental trend (almost flat line) agrees with this relation. At higher photon energy (2984.3 eV – Fig. 6b) for the evaporated film, it can be noticed that at low coverage the Na 1s level is more intense than at higher coverage. Also in this case, this behavior can be modelled with a compact continuous film covering SA, following Eq. (1). At this photon energy the IMFP is larger, λAu(Ek,Na1s-Ag Lα) = 2.3 nm and λAu(Ek,Au4f-Ag Lα) = 3.2 nm [12], thus making it 5

Thin Solid Films 690 (2019) 137535

R. Capelli, et al.

Table 1 Model parameters and XPS peaks ratios as used in eq. 1 and 2. See text for the meaning of the parameters. IMFPs - λmatrix(core

level-excitation photon)

λAu (Na1s-AlKα)

λAu (Au4f-AlKα)

λAu(Na1s-AgLα)

λAu(Au4f-AgLα)

λAlg(Na1s-AlKα)

λAlg(Au4f-AlKα)

λAlg(Na1s-AgLα)

λAlg(Au4f-AgLα)

0.75 nm

1.8 nm

2.3 nm

3.2 nm

1.5 nm

3.9 nm

5.2 nm

7.1 nm

Thermal evaporation films (Eq. (1)) Nominal thickness Na1s/Au4f ratio (AlKα) deff f Na1s/Au4f ratio (AgLα) deff f

2 nm 0.06 2 nm – 0.43 2 nm –

4 nm 0.05 4 nm – 0.22 4 nm –

Sputtered films (Eq. (2)) 25 nm 0.08 25 nm – 0.11 25 nm –

2 nm 1.95 1.68

4 nm 0.23 15 ± 3 nm 0.28 ± 0.02 0.68 30 ± 3 nm 0.15 ± 0.02

5 nm 0.27

6 nm 0.12

24 nm 0.13

0.50

0.27

0.19

chemical reaction involving either gold nano-domains or SA, in fact the two materials preserve their bulk XPS line shape. Instead, the enhanced adhesion has mechanical origin, being ascribed to the fine diffusion of gold interconnected domains into the SA matrix. 4. Conclusions High resolution x-ray photoelectron spectroscopy at different photon energies, flanked by AFM images, was used to obtain depth-resolved information of the interaction between a gold overlayer and a sodium alginate polymeric foil. The study was stimulated by the enhanced adhesion displayed by sputtered Au films over thermally evaporated ones. XPS showed that the chemical nature of both gold and sodium-alginate do not change depending on the deposition method and the two materials maintain their bulk properties. Smoother surfaces and an enhanced diffusion of Au into the sodium-alginate matrix are instead observed for the sputtered films, while thermal evaporation produces thin films that do not show pronounced interface intermixing. In both cases one finally obtains a continuous and compact bulk-like gold conducting layer, but due to the stronger intermixing, sputtered films adhere more strongly as observed in ref. [6], which can be fruitfully applied in the fabrication of flexible contacts on sodium-alginate. Fig. 7. Transmission (continuous lines) and reflectivity (broken lines) of gold/ SA films of 4–5 nm of nominal coverage grown by thermal evaporation and low power sputtering.

Acknowledgements The authors acknowledge Giulio Pizzochero for the technical support in the sputtering deposition of gold on sodium-alginate films.

thickness values, is also consistent with the SEM images reported in ref. [6]. SEM proved that the sputtered films are formed by nano-domains, which progressively interconnect when increasing the deposition time, giving rise to denser films and thus favoring conduction. No conjecture was initially given for the possibility of intermixing. SEM cross-sections did however show the presence of gold domains in layers which appeared sizably thicker than the nominal thickness values. This can now be explained in terms of the XPS results. For an independent and qualitative test on the different nature of the interfaces in case of sputtering and thermal evaporation we measured the optical transmission and reflectivity of the films at 4–5 nm of nominal thickness. Results are reported in Fig. 7. A quantitative interpretation of the spectral line-shapes is outside the scope of the present work. It can be clearly seen that the transmission is generally lower for the thermally evaporated film with respect to the sputtered one, in spite of a slightly lower nominal thickness in the first case. Accordingly, the reflectivity of the evaporated film is higher. This suggests the formation of a more compact gold layer in the case of thermal evaporation and supports the XPS observation of an increased intermixing for the sputtered film. The high degree of intermixing shown for sputtered films with respect to the evaporated ones provides suitable explanation for the enhanced adhesion of the metallic layers. This is not related to any

Funding This work was partially supported by the National Research Foundation (NRF) of South Africa [grant numbers 85364, 90698 and 93205]. References [1] B.H.A. Rehm, M.F. Moradali (Eds.), Alginates and their Biomedical Applications, Springer Singapore, Singapore, 2018, , https://doi.org/10.1007/978-981-106910-9. [2] K.Y. Lee, D.J. Mooney, Alginate: properties and biomedical applications, Prog. Polym. Sci. 37 (2012) 106–126, https://doi.org/10.1016/J.PROGPOLYMSCI.2011. 06.003. [3] G.I. Olivas, G.V. Barbosa-Cánovas, Alginate–calcium films: water vapor permeability and mechanical properties as affected by plasticizer and relative humidity, LWT Food Sci. Technol. 41 (2008) 359–366, https://doi.org/10.1016/J.LWT.2007. 02.015. [4] H. Zhu, Z. Fang, C. Preston, Y. Li, L. Hu, Transparent paper: fabrications, properties, and device applications, Energy Environ. Sci. 7 (2014) 269–287, https://doi.org/ 10.1039/C3EE43024C. [5] S. Li, P.S. Lee, Development and applications of transparent conductive nanocellulose paper, Sci. Technol. Adv. Mater. 18 (2017) 620–633, https://doi.org/10. 1080/14686996.2017.1364976. [6] P. Maccagnani, M. Bertoldo, F. Dinelli, M. Murgia, C. Summonte, L. Ortolani, G. Pizzocchero, R. Verucchi, C. Collini, R. Capelli, Flexible conductors from Brown

6

Thin Solid Films 690 (2019) 137535

R. Capelli, et al.

[7]

[8]

[9] [10]

algae for green electronics, Adv. Sustain. Syst. (2019) 1900001, , https://doi.org/ 10.1002/adsu.201900001. R. Chodun, K. Nowakowska-Langier, K. Zdunek, P. Konarski, S. Okrasa, Characteristic STATE of substrate and coatings interface formed by impulse plasma deposition method, Thin Solid Films 663 (2018) 25–30, https://doi.org/10.1016/J. TSF.2018.08.009. F. Dinelli, M.R. Castell, D.A. Ritchie, N.J. Mason, G.A.D. Briggs, O.V. Kolosov, Mapping surface elastic properties of stiff and compliant materials on the nanoscale using ultrasonic force microscopy, Philos. Mag. A. 80 (2000) 2299–2323, https:// doi.org/10.1080/01418610008216474. T. Miyama, Y. Yonezawa, Aggregation of photolytic gold nanoparticles at the surface of chitosan films, Langmuir. 20 (2004) 5918–5923, https://doi.org/10.1021/ la040002n. K.S. Kim, N. Winograd, X-ray photoelectron spectroscopic binding energy shifts due to matrix in alloys and small supported metal particles, Chem. Phys. Lett. 30 (1975)

91–95, https://doi.org/10.1016/0009-2614(75)85505-9. [11] M.G. Mason, Electronic structure of supported small metal clusters, Phys. Rev. B 27 (1983) 748–762, https://doi.org/10.1103/PhysRevB.27.748. [12] M.P. Seah, An accurate and simple universal curve for the energy-dependent electron inelastic mean free path, Surf. Interface Anal. 44 (2012) 497–503, https://doi. org/10.1002/sia.4816. [13] A. Jejurikar, X.T. Seow, G. Lawrie, D. Martin, A. Jayakrishnan, L. Grøndahl, Degradable alginate hydrogels crosslinked by the macromolecular crosslinker alginate dialdehyde, J. Mater. Chem. 22 (2012) 9751, https://doi.org/10.1039/ c2jm30564j. [14] S.K. Tam, J. Dusseault, S. Polizu, M. Ménard, J.-P. Hallé, L. Yahia, Physicochemical model of alginate–poly-l-lysine microcapsules defined at the micrometric/nanometric scale using ATR-FTIR, XPS, and ToF-SIMS, Biomaterials. 26 (2005) 6950–6961, https://doi.org/10.1016/J.BIOMATERIALS.2005.05.007.

7