Margarita Baitimirova

Roman Viter

Jana Andzane

Arie Van Der Lee

Damien Voiry

Igor Iatsunskyi

Emerson Coy

Lina Mikoliunaite

Saulius Tumenas

Karol Zalęski

Karol Załęski

Zigmas Balevicius

Ieva Baleviciute

Almira Ramanaviciene

Arunas Ramanavicius

Stefan Jurga

Donats Erts
email: donats.erts@lu.lv

Mikhael Bechelany
email: mikhael.bechelany@univ-montp2.fr

Tuning of Structural and Optical Properties of Graphene/ZnO Nanolaminates

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INTRODUCTION

In the past few years both semiconductorszinc oxide (ZnO) and graphene (G, semiconductor with zero eV band gap) have been extensively studied because of their unique physical properties.

ZnO is known as n-type semiconductor with good transparency, high electron mobility, wide band gap of 3.37 eV, high exciton binding energy of 60 meV and strong luminescence at room temperature. [START_REF] Ozgur | A Comprehensive Review of ZnO Materials and Devices[END_REF][START_REF] Bechelany | ZnO Nanotubes by Template-Assisted Sol-Gel Route[END_REF] Due to high photo-sensitivity, chemical stability and non-toxicity, ZnO is perspective material for application in UV [START_REF] Abou Chaaya | ZnO 1D Nanostructures Designed by Combining Atomic Layer Deposition and Electrospinning for UV Sensor Applications[END_REF] , gas [START_REF] Ahn | On-Chip Fabrication of ZnO-Nanowire Gas Sensor with High Gas Sensitivity[END_REF][START_REF] Drobek | MOF-Based Membrane Encapsulated ZnO Nanowires for Enhanced Gas Sensor Selectivity[END_REF] and biological sensors. [START_REF] Viter | Application of Room Temperature Photoluminescence From ZnO Nanorods for Salmonella Detection[END_REF][START_REF] Tereshchenko | Optical Biosensors Based on ZnO Nanostructures: Advantages and Perspectives[END_REF] The sensitivity of ZnO can be enhanced not only by the decrease of its dimensions down to nanostructures, [START_REF] Viter | Tuning of ZnO 1D Nanostructures by Atomic Layer Deposition and Electrospinning for Optical Gas Sensor Applications[END_REF] but also by its combination with carbon-based nanomaterials (carbon nanotubes, graphene) in composites and hybrid structures. [START_REF] Joshi | Enhanced Photocatalytic Performance of Graphene-ZnO Nanoplatelet Composite Thin Films Prepared by Electrostatic Spray Deposition[END_REF][START_REF] Mao | Ultrafast Hydrogen Sensing Through Hybrids of Semiconducting Single-Walled Carbon Nanotubes and Tin Oxide Nanocrystals[END_REF] Investigations of single graphene-ZnO interfaces showed that graphene can act as an excellent electron acceptor and transport material for effectively separate photoinduced excitons formed in ZnO. [START_REF] Dutta | ZnO/Graphene Quantum Dot Solid-State Solar Cell[END_REF][START_REF] Liu | Application of ZnO/Graphene and S6 Aptamers for Sensitive Photoelectrochemical Detection of SK-BR-3 Breast Cancer Cells Based on a Disposable Indium Tin Oxide Device[END_REF] This property makes the combination of graphene-ZnO highly promising for applications in optoelectronic devices, [START_REF] Dutta | ZnO/Graphene Quantum Dot Solid-State Solar Cell[END_REF][START_REF] Son | Emissive ZnO-Graphene Quantum Dots for White-Light-Emitting Diodes[END_REF] gas [START_REF] Singh | ZnO Decorated Luminescent Graphene as a Potential Gas Sensor at Room Temperature[END_REF] and biological sensors, [START_REF] Liu | Application of ZnO/Graphene and S6 Aptamers for Sensitive Photoelectrochemical Detection of SK-BR-3 Breast Cancer Cells Based on a Disposable Indium Tin Oxide Device[END_REF][START_REF] Singh | ZnO Decorated Luminescent Graphene as a Potential Gas Sensor at Room Temperature[END_REF] where graphene can act like a high conducting mesh. In its turn, sensor performance strongly depends on resistance of junction interface. [START_REF] Wang | Metal Oxide Gas Sensors: Sensitivity and Influencing Factors[END_REF] ZnO thin films can be synthesized using various methods including hydrothermal, [START_REF] Alver | Optical and Structural Properties of ZnO Nanorods Grown on Graphene Oxide and Reduced Graphene Oxide Film by Hydrothermal Method[END_REF] Chemical Vapor Deposition (CVD), [START_REF] Faÿ | Zinc Oxide Grown by CVD Process as Transparent Contact for Thin Film Solar Cell Applications[END_REF] magnetron and RF sputtering, [START_REF] Ding | Study on Field Emission and Photoluminescence Properties of ZnO/Graphene Hybrids Grown on Si Substrates[END_REF][START_REF] Ismail | The structural and Optical Properties of ZnO Thin Films Prepared at Different RF Sputtering Power[END_REF] spin-coating, [START_REF] Ding | A Comparison Study Between ZnO Nanorods Coated with Graphene Oxide and Reduced Graphene Oxide[END_REF] ultrasonic spray pyrolysis [START_REF] Lu | Electrochemical Behaviors of Graphene-ZnO and Graphene-SnO2 Composite Films for Supercapacitors[END_REF] and atomic layer deposition (ALD) [START_REF] Lu | Electrochemical Behaviors of Graphene-ZnO and Graphene-SnO2 Composite Films for Supercapacitors[END_REF][START_REF] Whitby | High Spatial Resolution Time-of-Flight Secondary Ion Mass Spectrometry for the Masses: A Novel Orthogonal ToF FIB-SIMS Instrument with In Situ AFM[END_REF][START_REF] Abou Chaaya | Evolution of Microstructure and Related Optical Properties of ZnO grown by Atomic Layer Deposition[END_REF] methods. Among these methods, ALD enables the deposition of high quality and uniform thin films with controlled reaction and growth mechanism at the atomic level and performed at low temperatures (60-100 °C). [START_REF] Whitby | High Spatial Resolution Time-of-Flight Secondary Ion Mass Spectrometry for the Masses: A Novel Orthogonal ToF FIB-SIMS Instrument with In Situ AFM[END_REF][START_REF] Abou Chaaya | Evolution of Microstructure and Related Optical Properties of ZnO grown by Atomic Layer Deposition[END_REF][START_REF] George | Atomic Layer Deposition: An Overview[END_REF] We note here that the ability to synthesize ultrathin film at low temperature offered by ALD is important to prevent the interdiffusion of materials. [START_REF] George | Atomic Layer Deposition: An Overview[END_REF] Graphene can be fabricated by exfoliation from graphite, [START_REF] Hernandez | High-Yield Production of Graphene by Liquid-Phase Exfoliation of Graphite[END_REF] reduction of exfoliated graphene oxide, [START_REF] Loryuenyong | Preparation and Characterization of Reduced Graphene Oxide Sheets via Water-Based Exfoliation and Reduction Methods[END_REF] epitaxial growth using SiC substrates [START_REF] First | Epitaxial Graphenes on Silicon Carbide[END_REF] and CVD on a metal catalytic layer (Cu, Ni). [START_REF] Yao | Controlled Growth of Multilayer, Few-Layer, and Single-Layer Graphene on Metal Substrates[END_REF] Due to economic feasibility and easy fabrication, exfoliated graphene and exfoliated reduced graphene oxide have mostly been used for the fabrication of graphene-ZnO heterostructures. [START_REF] Son | Emissive ZnO-Graphene Quantum Dots for White-Light-Emitting Diodes[END_REF][START_REF] Singh | ZnO Decorated Luminescent Graphene as a Potential Gas Sensor at Room Temperature[END_REF][START_REF] Alver | Optical and Structural Properties of ZnO Nanorods Grown on Graphene Oxide and Reduced Graphene Oxide Film by Hydrothermal Method[END_REF][START_REF] Yao | Controlled Growth of Multilayer, Few-Layer, and Single-Layer Graphene on Metal Substrates[END_REF][START_REF] Kavitha | Glucose Sensing, Photocatalytic and Antibacterial Properties of Graphene-ZnO Nanoparticle Hybrids[END_REF] However, the exfoliation method produces graphene layers with uncontrollable number of monolayers, limited area of coating and poor electronic properties. On the contrary, the CVD method enables the production of high quality monolayer graphene films with large continuous areas and allows as well the transfer of graphene sheets on arbitrary substrates. In a previous report on graphene-ZnO hybrid structures, CVD graphene was used either as a substrate for ZnO deposition, i.e., as a bottom layer, [START_REF] Jiao | Novel ALD-Assisted Growth of ZnO Nanorods on Graphene and its Cu2ZnSn(SxSe1-x)(4) Solar Cell Application[END_REF] or as a top layer. [START_REF] Lin | Hybrid Structure of Graphene Sheets/ZnO Nanorods for Enhancing Electron Field Emission Properties[END_REF] To our knowledge, the design of hybrid material composed of "graphene at the bottom and on the top of ZnO layer", as well as multilayer graphene-ZnO "sandwich"-type nanostructures (G/ZnO nanolaminates) have not been reported yet. Similarly to previously reported Al2O3/ZnO nanolaminates, [START_REF] Viter | Enhancement of Electronic and Optical Properties of ZnO/Al2O3 Nanolaminate Coated Electrospun Nanofibers[END_REF] where Al2O3 layers affected the band gap and photoluminescence of ZnO, influence of graphene on ZnO properties (crystal growth, photoluminescence and band gap), as well as influence of ZnO on graphene properties (strain, charging) are expected in G/ZnO nanolaminates.

In this study the G/ZnO nanolaminates were fabricated by creating alternating ZnO and graphene layers with total nanolaminate thicknesses of 100 nm, which enables structural and optical measurements well above the noise level. [START_REF] Abou Chaaya | Evolution of Microstructure and Related Optical Properties of ZnO grown by Atomic Layer Deposition[END_REF] The fabrication of nanostructures started with the graphene as a bottom layer of a nanolaminate, promoting formation of hexagonal wurtzite structure of ZnO from its first deposited atomic layers. In the next steps ZnO interlayers of different thicknesses were alternated with graphene monolayers. The graphene was chosen also as a top layer of the nanolaminate as it is compatible with most organics, bio-and gas molecules. Therefore, fabricated G/ZnO nanolaminates offer perspective for wide range of applications such as optoelectronic devices, sensors and catalysts.

MATERIALS AND METHODS

G/ZnO nanolaminates were deposited on Si substrates with native silicon oxide (SiO2) layer.

The Si/SiO2 substrates were ultrasonically pre-cleaned with acetone, ethanol and deionized water for 15 min. Monolayer graphene films were synthesized by copper-catalysed low-pressure CVD as reported elsewhere [START_REF] Mattevi | A Review of Chemical Vapour Deposition of Graphene on Copper[END_REF] using a reactor (First Nano Easy tube 101). Graphene monolayers were transferred to the desired substrate by polymer-assisted transfer technique [START_REF] Ghoneim | Simplistic Graphene Transfer Process and its Impact on Contact Resistance[END_REF] followed by the deposition of ZnO layer over it (Figure 1). ZnO layers of the thickness of 10, 25, 50 and 100 nm were synthesized by ALD method using home-made ALD reactor [START_REF] Bachmann | A Practical, Self-catalytic, Atomic Layer Deposition of Silicon Dioxide[END_REF] at the same conditions as described elsewhere. [START_REF] Abou Chaaya | Evolution of Microstructure and Related Optical Properties of ZnO grown by Atomic Layer Deposition[END_REF] In the next step, graphene monolayer was transferred onto the deposited ZnO layer. The sequential deposition of ZnO layer and graphene transfer was repeated until multilayer structure with a total thickness of 100 nm was obtained. For the comparison, 100 nm thick pure ZnO thin film was deposited on Si substrate using the same ALD conditions. Further in the text the samples are named by the formula GxLZnOynm, where x is the number of graphene layers and y is the thickness in nanometers of ZnO single layer in multilayer structure.

The surface topography of fabricated G/ZnO nanolaminates was characterized using atomic force microscopy (AFM, Asylum 3D). The morphology and structure of G/ZnO nanolaminates were studied by X-ray reflectivity (XRR, Bruker D5000), X-ray diffraction (XRD, Bruker D5000 with CuKα radiation, using the Bragg-Brentano symmetric configuration) and high-resolution transmission electron microscopy (HRTEM, JEOL ARM 200F (200 kV). The cross sections and lamellas for TEM investigations were prepared by focused ion beam milling (JEOL JIB-4000) by the method described elsewhere. [START_REF] Iatsunskyi | Study on Structural, Mechanical, and Optical Properties of Al2O3-TiO2 Nanolaminates Prepared by Atomic Layer Deposition[END_REF] The Raman spectra were obtained using Witek ALFA300R Raman spectrometer with 532 nm excitation laser source. Optical properties of G/ZnO nanolaminates were analyzed by UV-VIS reflectance (Shimadzu UV-3600) and photoluminescence (Edinburg Instruments FLS 980) spectroscopy. The excitation of luminescence was performed by a Xe lamp line, centred at 280 nm.

The band gap of ZnO was calculated in absorption edge region according to the procedure, described elsewhere. [START_REF] Hybertsen | First-Principles Theory for the Quasiparticle Energies of Semiconductors and Insulators[END_REF]

RESULTS AND DISCUSSION

In order to understand the effect of graphene incorporation on G/ZnO properties, G/ZnO nanolaminates were fabricated by alternating ZnO and graphene layers with total nanolaminate thickness of 100 nm. The fabrication of nanostructures started with graphene as a bottom layer of the nanolaminate. In the next steps ZnO interlayers of different thicknesses were alternated with graphene monolayers. Graphene was chosen also as a top layer of the nanolaminate (Figure 1). The samples are named by the formula GxLZnOynm, where x is number of graphene layersandy is the thickness in nanometers of ZnO single layer in multilayer structure.

High-resolution cross-section TEM images confirmed the sequence of alternating ZnO and graphene layers throughout all fabricated nanolaminates (Figure 2 a-d). The native silicon oxide (SiO2) layer on the silicon surface was about 2 nm thick (Figure 2a inset). The total thickness of all obtained G/ZnO nanolaminates was about 100 nm. From TEM images, it can be seen that the individual ZnO layers were of constant thickness and well reproducible.

However, the increase of a number of graphene interlayers and accordingly decrease of ZnO interlayer thickness results in worsening of layers smoothness (Figure 2 c,d), most likely related to increase of nanolaminate surface roughness.

The high resolution TEM analysis of G/ZnO nanolaminates showed the presence of ( 100 103) plane is at 2θ = 62.7°). [START_REF] Klingshirn | Introduction. In Zinc Oxide: From Fundamental Properties Towards Novel Applications[END_REF] Peak observed at 2θ = 32.95° is corresponding to (004) reflection plane of silicon substrate.

The above peaks were observed in XRD patterns of all G/ZnO nanolaminates, indicating their high crystalline structure. No characteristic peaks for any impurities as, for example, PMMA, FeCl3, Cu, etc. residuals from graphene transfer process, were observed. Calculated from XRD spectra ZnO lattice constants were equal to a = 0.325 nm and c = 0.519 nm, which fits perfectly with ZnO crystal lattice parameters. [START_REF] Klingshirn | Introduction. In Zinc Oxide: From Fundamental Properties Towards Novel Applications[END_REF] The lattice spacing between the atom stacks calculated from the FFT image of a selected area of ZnO interlayer were in good correlation with XRD data (Figure 2f (inset) and Figure 3a). No significant changes of ZnO lattice parameters were observed with decreasing of thickness of ZnO interlayers in the nanolaminates, which is in good correlation with previous report on lattice parameters of ALD deposited ZnO thin films of different thicknesses. [START_REF] Abou Chaaya | Evolution of Microstructure and Related Optical Properties of ZnO grown by Atomic Layer Deposition[END_REF] No amorphous-to-crystalline growth transition of ZnO was observed in contradiction with previously reported ALD deposited on Si/SiO2 substrates ZnO thin films [START_REF] Abou Chaaya | Evolution of Microstructure and Related Optical Properties of ZnO grown by Atomic Layer Deposition[END_REF] and Al2O3/ZnO nanolaminates, [START_REF] Viter | Enhancement of Electronic and Optical Properties of ZnO/Al2O3 Nanolaminate Coated Electrospun Nanofibers[END_REF] where the amorphous-to-crystalline transition of ZnO occurred for the layers with thickness larger than 25 nm and 10 nm respectively. As it can be seen from the obtained G/ZnO nanolaminates XRD and HRTEM data, even the thinnest (10 nm) ZnO layers have a crystalline nature (Figure 3, G11LZnO10nm, Figure 2f).

From the intensities of the diffraction peaks in XRD spectra (Figure 3a,b) it can be seen that for the 10 and 25 nm thick ZnO interlayers, the [002] crystal growth direction is dominating, while with an increase of ZnO interlayer thickness up to 50 nm, as well as for the 100 nm thick ZnO thin film deposited on Si/SiO2 substrate, the [100] and [101] growth directions significantly dominate over [002] and [110] directions. Presumably, the graphene sublayers promote growth of ZnO crystalline transition layer in [002] direction, which is attributed to very low (< 3%) mismatch between the graphene and the ZnO (002) hexagonal lattices. [START_REF] Xu | Structural and Electronic Properties of Graphene-ZnO Interfaces: Dispersion-Corrected Density Functional Theory Investigations[END_REF] With the increase of ZnO interlayer thickness, [100] and [101], considered as fast ZnO growing directions are prevailing. [START_REF] Xu | Structural and Electronic Properties of Graphene-ZnO Interfaces: Dispersion-Corrected Density Functional Theory Investigations[END_REF] These results suggest that the advantageous role of graphene as template in the formation of highly crystalline ZnO.

The AFM images of the surface of 100 nm thick G/ZnO nanolaminates with various numbers of graphene interlayers indicate that as the number of graphene interlayers increases, the morphology of the top graphene surface became rougher (Figure 4a,b). The calculated root mean square (RMS) roughness of the surface of G/ZnO nanolaminates from AFM data showed linear increase versus the number of graphene layers in the nanolaminates (Figure 4 c). The explanation may be related to the fact that every next graphene layer follows the morphology of the ZnO sublayer surface. The RMS estimation does not account for possible artifacts (strain/elastic resistance of graphene) that might be caused by the pressure of AFM probe during the surface scanning.

The increase of the surface roughness of the G/ZnO nanolaminates as determined is supported by the X-ray reflectivity data (Figure 5). In comparison with pure ZnO thin film (G0LZnO100nm), the number of Kiessig fringes was significantly reduced when increasing number of graphene layers (G2LZnO100nm, G3LZnO50nm, G5LZnO25nm) and then disappeared for G11LZnO10nm, indicating an increase of the G/ZnO nanolaminate surface roughness as well as the interfacial roughness. The ZnO layer without any graphene top or bottom layer (G0LZnO100nm) showed nearly perfect Kiessig fringes, which can be modelled using Parrat's recursive formula [START_REF] Stoev | Recent Theoretical Models in Grazing Incidence X-ray Reflectometry[END_REF] for X-ray reflectivity. The thickness of ZnO layer calculated from the Kiessig fringes was 99.7 nm in perfect agreement with the TEM data. The surface roughness of 1.6 nm was rather close to the RMS surface roughness of 1.9 nm, which was determined by AFM. The gradual extinction of the Kiessig fringes, which is apart from the rather large surface roughness, for larger incident angles--is also observed due to the roughness at the buried interface between the substrate and the film (0.8 nm).

We note here that due to the gas barrier properties of the graphene CVD thin films, [START_REF] Seo | Tailored CVD Graphene Coating as a Transparent and Flexible Gas Barrier[END_REF] the porosity of our thin films could not be evaluated using common techniques such as Surface acoustic wave (SAW) /BET technique or Ellipsometry Porosimetry.

The reflectivity curve of G/ZnO nanolaminates with two graphene layers G2LZnO100nm showed some short-period Kiessig fringes at very small angles due to the thickness of the ZnO layer and a long-period fringe with a (first) minimum at 2θ = 0.9. Although these longperiod Kiessig fringes at very small angles could be attributed to the bottom graphene layer, they are more coming from the native silica layer of the substrate (Figure 5). The same longperiod fringe was seen slightly shifted to larger angles in the reflectivity curve of nanostructures with three graphene layers G3LZnO50nm. The fringes between 2Θ=0.5-0.9° correspond to the 50 nm thick ZnO interlayers and fringes below 2Θ = 0.5° appeared due to the total thickness of the G/ZnO multilayer nanostructure. The reflectivity curve of G/ZnO nanostructures with five graphene layers G5LZnO25nm showed different fringe periodicities and did not allow to identify the thickness of ZnO interlayers. The reflectivity curve of G/ZnO nanostructures with eleven graphene layers G11LZnO10nm cannot be interpreted qualitatively or quantitatively because of its very high RMS surface roughness (Figure 4).

The top graphene layer cannot be detected in the reflectivity curves, because the RMS surface roughness of G/ZnO nanolaminates is much larger than the supposed thickness of the graphene layer. Additionally it is worth noting that because of the quenching of the Kiessig fringes, caused by the relatively high surface and interfacial roughness of G/ZnO nanolaminates, the reflectivity curves of all fabricated G/ZnO structures cannot be analyzed quantitatively, but only qualitatively.

In order to evaluate possible strain and charging effects of graphene in G/ZnO nanolaminates, the Raman spectra of the fabricated structures were investigated (Figure 6a).

Two intensive peaks at 1583 cm -1 and 2671 cm -1 , corresponding to the G band induced by the vibration of sp 2 -hybridized carbons and to the 2D band of second-order process induced Raman feature, respectively, [START_REF] Malard | Raman Spectroscopy in Graphene[END_REF] were observed in all the obtained spectra of the G/ZnO nanolaminates. Both G and 2D peaks were sharp, indicating that no oxidation of graphene to graphene oxide [START_REF] Kudin | Raman Spectra of Graphite Oxide and Functionalized Graphene Sheets[END_REF] occurred during the formation of ALD ZnO layers and thus suggesting that the high quality of graphene is largely preserved.

The ratio of intensities of the 2D band to the G band (I2D/IG) of single CVD graphene layers used for the G/ZnO nanolaminates fabrication was above 2 (see Table 1, G1LZnO0nm), confirming the monolayer structure of this graphene. [START_REF] Wu | Control of Thickness Uniformity and Grain Size in Graphene Films for Transparent Conductive Electrodes[END_REF] The observed low intensity of D peak at 1350 cm -1 in this spectrum (Figure 6a, Table 1) indicated the presence of insignificant amount of structural defects in the monolayer, which is typical for CVD-grown on Cu substrate graphene. [START_REF] Luo | Effect of Substrate Roughness and Feedstock Concentration on Growth of Wafer-Scale Graphene at Atmospheric Pressure[END_REF] When increasing the number of graphene interlayers in the nanolaminates, the ID/IG ratio moderately increased from 0.1 for a single graphene monolayer up to ~0.22 for the G/ZnO nanolaminates containing from 2 to 11 graphene layers regardless the number of the graphene layers in the nanolaminate (Table 1). Thereby, in contradiction with sputtering techniques, where significant increase in the ID/IG peak intensity ratio up to 1 is typically observed after ZnO deposition on the graphene substrate, [START_REF] Shin | High Quality Graphene-Semiconducting Oxide Heterostructure for Inverted Organic Photovoltaics[END_REF] the ALD deposition of the ZnO on graphene surface does not induce severe structural damage in it.

The I2D/IG ratio in the Raman spectra of G/ZnO nanolaminates decreased from 2.68 for the nanolaminate containing two graphene layers G2LZnO100nm down to 1.33 and 1.20 for G3LZnO50nm and G5LZnO25nm structures containing 3 and 5 graphene layers, respectively, and further down to 0.87 for the G11LZnO10nmnanolaminate containing 11 graphene layers (Figure 6 a-c, Table 1). These changes in I2D/IG intensity ratio may be related to such processes occurring in graphene layers of nanolaminates as charging [START_REF] Beams | Raman Characterization of Defects and Dopants in Graphene[END_REF] and/or mechanically induced strain. [START_REF] Si | Strain Engineering of Graphene: a Review[END_REF] However, the typical redshifts for uniaxial or biaxial graphene strain of either G or 2D peaks by 30-50 cm -1 per 0.5% of strain [START_REF] Si | Strain Engineering of Graphene: a Review[END_REF] were not observed. This suggests that graphene interlayers in the G/ZnO nanolaminates do not experience significant strain, and the changes in positions and intensity ratios of G and 2D peaks are mostly related to graphene charging. In spite of absence of the significant damage, the minor strain of graphene may take place in G/ZnO nanolaminates. The first layer of graphene of all prepared structures may have insignificant strain due to van der Waals interaction between the graphene and silicon substrate during transfer. The next transferred graphene layers of G/ZnO nanolaminates may be exposed to strain forming from van der Waals force existing at the graphene-ZnO (002) interface of ZnO-graphene hybrid structures [START_REF] Xu | Structural and Electronic Properties of Graphene-ZnO Interfaces: Dispersion-Corrected Density Functional Theory Investigations[END_REF] that may result in graphene strain due to lattices mismatch, as well as to the increase of roughness of the nanolaminates (Figure 4).

The evaluation of G-band position of nanolaminates showed that it blue-shifts with the increase of number of graphene interlayers and respectively decreases of thickness of ZnO interlayers (Figure 6 a,b,c). Blue-shift of G-band is indicative for the graphene charge. [START_REF] Iqbal | Modification of the Structural and Electrical Properties of Graphene Layers by Pt Adsorbates[END_REF] As in ZnO-graphene hybrid structures, the electron transfer occurs from ZnO to graphene as a result of the difference in their work functions, [START_REF] Xu | Structural and Electronic Properties of Graphene-ZnO Interfaces: Dispersion-Corrected Density Functional Theory Investigations[END_REF] the above changes in Raman spectra of G/ZnO nanolaminates may be attributed to the negative charge of graphene by electron transfer from ZnO layer. Presumably, this electron transfer from ZnO to graphene results in the formation of depletion layer in ZnO near the graphene surface, and facilitates accumulation of holes in it. For quantifying graphene charge in the G/ZnO nanolaminates, the estimation of concentration of excess electrons per graphene layer was performed based on data reported on interdependence between Raman shift of G-band and the increase of concentration of electrons in monolayer graphene. [START_REF] Beams | Raman Characterization of Defects and Dopants in Graphene[END_REF] It was found that the charge per graphene layer linearly decreases with the increase of number of graphene layers and respectively with decrease of thickness of ZnO interlayers (Figure 6 c). The reason for graphene charge to decrease may be related to the interaction and the overlapping of ZnO depletion layers with the decrease of ZnO interlayer thickness, as well as to the change of defect concentration in ZnO interlayers related to change of prevailing crystallographic ZnO growth directions with the decrease of interlayer thickness (Figure 3), and requires further clarification. However, this linear dependence of graphene charge on number of graphene layers in G/ZnO nanolaminates may be useful in charge-related applications, for example, for tuning graphene charge by varying the number of graphene interlayers in the nanolaminates for selective adsorption of positive charged target molecules on the graphene surface.

The number of graphene interlayers and thickness of ZnO layers in G/ZnO nanolaminates has strong influences on their optical properties. The shift of ZnO absorption edges from 360 up to 400 nm is clearly seen on reflectance spectra recorded for different G/ZnO nanolaminates (Figure 7a). The shift of absorption edges of G/ZnO nanolaminates relative to absorption edge position of pure ZnO may be related to the formation of optically active defects at the graphene-ZnO interface [START_REF] Abou Chaaya | Tuning Optical Properties of Al2O3/ZnO Nanolaminates Synthesized by Atomic Layer Deposition[END_REF] and ZnO grain boundaries. The band gap of G/ZnO nanolaminates was calculated from the assumption that the reflectance in the absorption edge is related to the band gap values due to the following equations [START_REF] Abou Chaaya | Evolution of Microstructure and Related Optical Properties of ZnO grown by Atomic Layer Deposition[END_REF] :

       R 100 ln D     g 2 E hv hv D  
Where R, D, hv and Eg are reflectance, optical density, photon energy and band gap, respectively. The Eg values were calculated from the intersection of linear part of the curve with hv-axis. The fitting was performed automatically using Origin 7.0 software (Figure 7b).

The obtained band gap values determined from the reflectance spectra of ZnO decrease with the increase of the number of graphene interlayers and the decrease of thickness of ZnO interlayers in nanolaminates from the initial value of 3.42 eV, obtained for pure ZnO thin film, down to value of 3.10-3.13 eV for nanolaminates containing 5 and more graphene layers respectively (Figure 7c). The levelling of the band gap at these values may be related to the domination of ZnO [002] crystallographic growth direction over other directions at ZnO interlayer thicknesses below 25 nm as confirmed by XRD data (Figure 3), which may reduce the concentration of optically active defects.

In the photoluminescence (PL) spectra of G/ZnO nanolaminates (Figure 8a), the peaks with maximum at ~3.3 eV (375 nm) related to excitation transitions in ZnO [START_REF] Viter | Tuning of ZnO 1D Nanostructures by Atomic Layer Deposition and Electrospinning for Optical Gas Sensor Applications[END_REF][START_REF] Abou Chaaya | Evolution of Microstructure and Related Optical Properties of ZnO grown by Atomic Layer Deposition[END_REF] were observed for all samples. Peaks related to deep level transitions at energies about 2.4 -2.5 eV (oxygen and zinc vacancies and zinc interstitials), [START_REF] Ozerov | Enhancement of Exciton Emission from ZnO Nanocrystalline Films by Pulsed Laser Annealing[END_REF] as well as at 2.0 eV, related to interstitial oxygen in ZnO films [START_REF] Ozerov | Enhancement of Exciton Emission from ZnO Nanocrystalline Films by Pulsed Laser Annealing[END_REF] were not observed for both ZnO thin film deposited on SiO2 surface and for G/ZnO nanolaminates. From the Figure 8 it is clearly seen that the intensity of PL signals decreased with the increase of number of graphene layers and the corresponding decrease of the thickness of ZnO interlayers in the G/ZnO nanolaminates. PL emission may be quenched because of several reasons such as negative charge of graphene surface, with concomitant screening effect in graphene, [START_REF] Reynolds | Polariton and Free-Exciton-Like Photoluminescence in ZnO[END_REF] transfer and trapping of photo-excited electrons in graphene, [START_REF] Dutta | ZnO/Graphene Quantum Dot Solid-State Solar Cell[END_REF][START_REF] Liu | Application of ZnO/Graphene and S6 Aptamers for Sensitive Photoelectrochemical Detection of SK-BR-3 Breast Cancer Cells Based on a Disposable Indium Tin Oxide Device[END_REF][START_REF] Xu | Structural and Electronic Properties of Graphene-ZnO Interfaces: Dispersion-Corrected Density Functional Theory Investigations[END_REF] and similarly to ZnO/Al2O3nanolaminates the increase of the total depletion regions relative to total thickness of ZnO (100 nm) when decreasing the ZnO layer thickness [START_REF] Viter | Enhancement of Electronic and Optical Properties of ZnO/Al2O3 Nanolaminate Coated Electrospun Nanofibers[END_REF][START_REF] Abou Chaaya | Tuning Optical Properties of Al2O3/ZnO Nanolaminates Synthesized by Atomic Layer Deposition[END_REF] .

These factors result in the decrease of exciton radiative recombination compared to nonradiative processes in the G/ZnO nanolaminates with the increase of the number of graphene interlayers and the decrease of thickness of ZnO interlayers. The presumable thickness of ZnO layer with 100%-quenched PL near the single surface of a graphene interlayer was estimated by empirical linearization of PL luminescence intensity as function of summed all quenched ZnO layer thicknesses in the G/ZnO nanolaminates. The closest to linear curve was obtained assuming that the thickness of a ZnO layer with quenched PL was 10 nm near every graphene/ZnO interface (Figure 8 b). However, as it can be seen from the Figure 8 a-b, PL signals are still detected when the presumed total thickness of ZnO layers in between two graphene layers is 10 nm (Figure 8 b, point G11LZnO10nm). As a consequence, it can be concluded that PL quenching mechanism in the nanolaminates may not be only based on the dimension of architecture and thus a more complex model is required to fully understand the optical properties of such graphene/ZnO layered films.

CONCLUSION

In summary, new nanomaterials of G/ZnO nanolaminates have been developed and investigated. Combination of CVD and ALD offers a precise control of the thickness and composition of the films. The number of alternating graphene and ZnO interlayers and the thickness of ZnO interlayers influenced the structure and the optical properties of the fabricated nanolaminates. During the ALD deposition of ZnO nanolayers, the bottom graphene used as a substrate stimulated the formation of ZnO crystalline structure, thus avoiding formation of amorphous transition layer between the graphene and the crystalline ZnO thin film. [START_REF] Viter | Enhancement of Electronic and Optical Properties of ZnO/Al2O3 Nanolaminate Coated Electrospun Nanofibers[END_REF][START_REF] Abou Chaaya | Tuning Optical Properties of Al2O3/ZnO Nanolaminates Synthesized by Atomic Layer Deposition[END_REF][START_REF] Raghavan | Nanocrystalline-to-Amorphous Transition in Nanolaminates Grown by Low Temperature Atomic Layer Deposition and Related Mechanical Properties[END_REF][START_REF] Viter | Optical Properties of Ultrathin Al2O3/ZnO Nanolaminates[END_REF] We found that ALD deposition of ZnO thin films on surface of graphene layers did not introduce severe structural damage. Minor strain and charge transfer from ZnO to graphene were observed in fabricated G/ZnO nanolaminates. The charge transfer resulted in negative charging of graphene as observed by blue-shift of G-band of Raman spectra of the nanolaminates. These results are meaningful for exploration of devices based on Graphene and ZnO films and the conclusion obtained in this study could be commonly used for other Graphene/metal oxide nanolaminates. The ZnO PL intensity was found to be dependent on the number of graphene layers in G/ZnO nanolaminate. Presumably, the underlying reason for the PL decrease is the partial suppression of radiative recombination occurring at graphene-ZnO interface. The tuning of the graphene charge together with the ZnO PL intensity in G/ZnO nanolaminates has prospects in applications for FET gate, biosensors, chemical sensors and catalysts. The variable negative charging of graphene layers might provide a selective adsorption of the positive charged target molecules on its surface.

Optical properties of G/ZnO nanolaminates can be used in optical transducer during sensor/biosensor development. Finally, multilayer structure synthesized by alternating graphene and metal oxide layers could be very attractive for a wide range of applications such as the realization of mid-infrared graphene hyperbolic metamaterials, [START_REF] Chang | Realization of Mid-infrared Graphene Hyperbolic Metamaterials[END_REF] encapsulation materials with high impermeability to moisture and air [START_REF] Seo | Laminated Graphene Films for Flexible Transparent Thin Film Encapsulation[END_REF] or to improve the thermal conductivity of polymers. [START_REF] Malekpour | Thermal Conductivity of Graphene Laminate[END_REF] These new properties of the G/ZnO nanolaminates materials are under investigations.

  ), (002), (101), (110) and (103) fringes in the selected area of ZnO layer (Figure 2 e, f). The fringe separation of 0.275 nm determined from the HRTEM image (Figure 2 e) corresponds to (100) plane of wurtzite crystal structure of ZnO. Obtained XRD spectra of the ZnO layers of G/ZnO nanolaminates (Figure 3 a) demonstrated the diffraction peaks at 2θ = 31.80°, 34.54°, 36.16° and 56.32°, corresponding to (100), (002), (101) and (110) reflection planes of hexagonal wurtzite structure of ZnO respectively 8 in perfect agreement with TEM results. The peak related to (103) plane of ZnO crystal presented in FFT image (Figure 2 f) was not observed as it was out of XRD measurement range (diffraction peak position of (

FIGURESFigure 1 .

 1 FIGURES

Figure 2 .

 2 Figure 2.TEM images of cross-section of pure ZnO layer (a) and Graphene/ZnO

Figure 3 .Figure 4 .

 34 Figure 3.X-ray diffraction patterns of ZnO thin film and Graphene/ZnO nanolaminates (a)

Figure 5 .

 5 Figure 5. X-ray reflectivity profiles of ZnO thin film and Graphene/ZnO nanolaminates.

Figure 6 .Figure 7 .

 67 Figure 6.Raman spectra of graphene and Graphene/ZnO nanolaminates (a), close view of G

Figure 8 .Table 1 .

 81 Figure 8.Photoluminescence spectra of ZnO thin film and Graphene/ZnO nanolaminates (a),

ACKNOWLEDGEMENTS:This work was supported by Latvian National Research program IMIS-2 and partially supported by the EU under BIOSENSORS-AGRICULT, Contract PIRSES-GA-2012-318520 'Development of Nanotechnology-based Biosensors for Agriculture', Scientific grant of The Research Council of Lithuania (application registration No. VIZ-TYR-126, 2015), by Campus France (PHC GILIBERT 2015, PROJET N° 32978PG and PHC OSMOSE 2016, PROJET N° 35056UB) and by the ANR project ANR-14-CE07-0011 "BONALD". Z.B., I.B. and A.R. research was funded by a grant (No. TAP LZ-3/2015) from the Research Council of Lithuania. K.Z. acknowledges support from Polish National Science Center Grant DEC-2014/15/B/ST3/02927.


Roman Viter
email: roman.viter@lu.lv

Igor Iatsunskyi

Viktoriia Fedorenko

Saulius Tumenas

Zigmas Balevicius

Arunas Ramanavicius

Sebastien Balme

Mateusz Kempiński

Grzegorz Nowaczyk

Stefan Jurga

Mikhael Bechelany
email: mikhael.bechelany@univ-montp2.fr

Enhancement of Electronic and Optical Properties of ZnO/Al2O3 Nanolaminate Coated Electrospun Nanofibers

Keywords: Nanolaminates, atomic layer deposition, electrospinning, Work function, photoluminescence

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INTRODUCTION

Nanolaminates are new class of promising nanomaterials with well-tailored properties.

These materials are obtained by alternating layers with an individual thickness on the nanometer scale of different materials. [START_REF] Abou Chaaya | Tuning Optical Properties of Al2O3/ZnO Nanolaminates Synthesized by Atomic Layer Deposition[END_REF] They manifest outstanding advanced mechanical, 2 electrical and optical properties [START_REF] Abou Chaaya | Tuning Optical Properties of Al2O3/ZnO Nanolaminates Synthesized by Atomic Layer Deposition[END_REF] which depend from the constituent materials forming these nanolaminates. Zinc Oxide (ZnO) which can be incorporated in the nanolaminate material is a n-type semiconductor with wide band gap value (3.3 eV) and high exciton binding energy (0.06 eV). It also exhibited room temperature photoluminescence in UV-Vis region. [START_REF] Abou Chaaya | Tuning Optical Properties of Al2O3/ZnO Nanolaminates Synthesized by Atomic Layer Deposition[END_REF][START_REF] Abou Chaaya | Evolution of Microstructure and Related Optical Properties of ZnO Grown by Atomic Layer Deposition[END_REF] Among the different techniques to elaborate oxides based nanolaminates, Atomic Layer Deposition (ALD) is a powerful tool to fabricate different oxide materials. [START_REF] Marichy | Atomic Layer Deposition of Nanostructured Materials for Energy and Environmental Applications[END_REF][START_REF] Bechelany | Atomic Layer Deposition of Biobased Nanostructured Interfaces for Energy, Environmental and Health Applications[END_REF] It is based on two self-limiting surface reactions which allow growing continuous, smooth, and conformal ultrathin film on high aspect ratio and complex substrates such as the 1D nanotructures with a precise dimensional control.

This paper deals with the investigation of electronic and optical Properties of ZnO/Al2O3

Nanolaminate Coated Electrospun Nanofibers.

The design of Al2O3/ZnO nanolaminates as well as ZnO thin film doped with Al was widely investigated by ALD. [START_REF] Raghavan | Nanocrystalline-to-Amorphous Transition in Nanolaminates Grown by Low Temperature Atomic Layer Deposition and Related Mechanical Properties[END_REF][START_REF] Abou Chaaya | Enhanced Ionic Transport Mechanism by Gramicidin a Confined inside Nanopores Tuned by Atomic Layer Deposition[END_REF][START_REF] Cabello-Aguilar | Slow Translocation of Polynucleotides and Their Discrimination by Alpha-Hemolysin inside a Single Track-Etched Nanopore Designed by Atomic Layer Deposition[END_REF][START_REF] Elam | ZnO/Al2O3 Nanolaminates Fabricated by Atomic Layer Deposition: Growth and Surface Roughness Measurements[END_REF][START_REF] Karvonen | Enhancement of the Third-Order Optical Nonlinearity in ZnO/ Al2O3 Nanolaminates Fabricated by Atomic Layer Deposition[END_REF] The ZnO deposited on Al2O3 thin films in the Al2O3/ZnO nanolaminates demonstrated a different growth direction in comparison to single ZnO thin film. [START_REF] Banerjee | Structural, Electrical, and Optical Properties of Atomic Layer Deposition Al-Doped Zno Films[END_REF] ZnO demonstrates as well a change from amorphous to nanocrystalline with the increase of the bilayers thickness where Al2O3 layer remains amorphous. [START_REF] Raghavan | Nanocrystalline-to-Amorphous Transition in Nanolaminates Grown by Low Temperature Atomic Layer Deposition and Related Mechanical Properties[END_REF] The change of the growth direction, the crystallinity and the Al doping influence the optical, [START_REF] Abou Chaaya | Tuning Optical Properties of Al2O3/ZnO Nanolaminates Synthesized by Atomic Layer Deposition[END_REF][START_REF] Viter | Optical Properties of Ultrathin Al2O3/ZnO Nanolaminates[END_REF] electrical [START_REF] Moret | Atomic Layer Deposition of Zinc Oxide for Solar Cell Applications[END_REF] and mechanical properties [START_REF] Raghavan | Nanocrystalline-to-Amorphous Transition in Nanolaminates Grown by Low Temperature Atomic Layer Deposition and Related Mechanical Properties[END_REF] of the deposited thin films. Recently we investigated the optical and the structure properties of ZnO/Al2O3 nanolaminates deposited on Si substrates by Atomic Layer Deposition (ALD). [START_REF] Abou Chaaya | Tuning Optical Properties of Al2O3/ZnO Nanolaminates Synthesized by Atomic Layer Deposition[END_REF] We demonstrated the amorphous-to-crystalline phase transitions of ZnO in ZnO/Al2O3 nanolaminates as well as the quantum confinement and size dependent room temperature photoluminescence. [START_REF] Abou Chaaya | Tuning Optical Properties of Al2O3/ZnO Nanolaminates Synthesized by Atomic Layer Deposition[END_REF] It was shown a correlation between structure (grain size and layer thickness) and optical (extinction coefficient and refractive index), pointing to the influence of growth conditions on the fundamental properties of the nanolaminates. [START_REF] Abou Chaaya | Tuning Optical Properties of Al2O3/ZnO Nanolaminates Synthesized by Atomic Layer Deposition[END_REF] Different 1D nanostructures such as Carbon Nanotubes (CNTs) and polymer fibers have been investigated in order to design core/shell nanostructures using ALD of metal oxides. [START_REF] Kayaci | Polymer-Inorganic Core-Shell Nanofibers by Electrospinning and Atomic Layer Deposition: Flexible Nylon-Zno Core-Shell Nanofiber Mats and Their Photocatalytic Activity[END_REF][START_REF] Kemnade | Non-Destructive Functionalisation for Atomic Layer Deposition of Metal Oxides on Carbon Nanotubes: Effect of Linking Agents and Defects[END_REF][START_REF] Lin | Atomic Layer Deposition of Zinc Oxide on Multiwalled Carbon Nanotubes for UV Photodetector Applications[END_REF][START_REF] Oldham | Atomic Layer Deposition on Polymers: Applications to Physical Encapsulation of Electrospun Nylon Nanofibers[END_REF][START_REF] Stano | Conformal Atomic Layer Deposition of Alumina on Millimeter Tall, Vertically-Aligned Carbon Nanotube Arrays[END_REF] Different studies were performed for the growth of oxides of Carbon nanotubes. Stafona et al.

reported the influence of CNTs type and the defect concentration on the ALD coating. [START_REF] Stano | Conformal Atomic Layer Deposition of Alumina on Millimeter Tall, Vertically-Aligned Carbon Nanotube Arrays[END_REF] For low defect concentration, ALD coating cannot be realized. The functionalization of CNTs surface is needed to promote the nucleation of the oxide film. Carboxyl and hydroxyl groups are formed on the CNTs surface to initiate the ALD growth. Kemnade et al. showed that conformal growth of ALD ZnO layer on CNTs requested the modification of the CNTs with aromatic linking agents. [START_REF] Kemnade | Non-Destructive Functionalisation for Atomic Layer Deposition of Metal Oxides on Carbon Nanotubes: Effect of Linking Agents and Defects[END_REF] Lin et al. reported on the importance of CNT functionalization before deposition of ZnO nanolayers by ALD. [START_REF] Lin | Atomic Layer Deposition of Zinc Oxide on Multiwalled Carbon Nanotubes for UV Photodetector Applications[END_REF] Kayaci et al. reported that ALD ZnO layers provided a conformal coating of nylon fibers. [START_REF] Kayaci | Polymer-Inorganic Core-Shell Nanofibers by Electrospinning and Atomic Layer Deposition: Flexible Nylon-Zno Core-Shell Nanofiber Mats and Their Photocatalytic Activity[END_REF] However, Oldham et al. demonstrated that TMA and DEZ precursors could chemically react with polyamide-6 (PA-6) nylon nanofibers destroying their physical properties. [START_REF] Oldham | Atomic Layer Deposition on Polymers: Applications to Physical Encapsulation of Electrospun Nylon Nanofibers[END_REF] Therefore, the conformal and uniform ALD coating on 1D nanostructures could be achieved by the right choice of the core chemical composition and surface state of the 1D nanostructures. PAN nanofibers are good chemically stable material for 1D core/shell nanostructure design. [START_REF] Shi | Electrospun Polyacrylonitrile Nanofibers Loaded with Silver Nanoparticles by Silver Mirror Reaction[END_REF] PAN nanofibers have been successfully used for chemical deposition of different metal nanoparticles on their surface. It was shown that PAN nanofibers have good mechanical and thermal stability. The adsorption sites present on the surface of the PAN fibers would facilitate the nucleation of the metal oxide layers by ALD. Thus, integration of metal oxide ALD technique and PAN nanofibers has a good perspective.

Previously, we have reported on the fabrication of three-dimensional polyacrylonitrile /ZnO material prepared by the combination of electrospinning and atomic layer deposition (ALD) as new material with large surface area to enhance the optical properties. [START_REF] Abou Chaaya | Zno 1D Nanostructures Designed by Combining Atomic Layer Deposition and Electrospinning for UV Sensor Applications[END_REF][START_REF] Viter | Tuning of Zno 1D Nanostructures by Atomic Layer Deposition and Electrospinning for Optical Gas Sensor Applications[END_REF] . The study of room temperature photoluminescence (PL) spectrum exhibits peak intensity which enhance by a factor of 2000 compared to a flat Si substrate. [START_REF] Viter | Tuning of Zno 1D Nanostructures by Atomic Layer Deposition and Electrospinning for Optical Gas Sensor Applications[END_REF] In the present work, we report on the enhancement of electronic and optical properties of 1D organic core with ZnO/Al2O3 nanolaminates shell nanostructures. The influence of structure parameters of ZnO/Al2O3 nanolaminates (thickness, grain size, number of bilayers, defect concentration) on the main electronic (Work Function) and optical (photoluminescence and band gap) properties will be discussed. The enhancement of electronic and optical properties will allow application in different fields such sensors and biosensors.

MATERIALS AND METHODS

Polyacrylonitrile (PAN) nanofibers have been elaborated by electrospinning technique to

be a template for the nanolaminates design by Atomic Layer Deposition (ALD).

Electrospinning of PAN nanofibers

Polyacrylonitrile PAN (Mw = 150000) and Dimethylformamide (ACS reagent, ≥ 99.8% ) were purchased from Sigma Aldrich. Polyacrylonitrile (10 wt% PAN) was dissolved in dimethylformamide (DMF). The electrospinning solution was maintained under agitation for 1 hour and then heated in an oil bath at 80°C for 10 min. The used electrospinning machine was a home-made device using an HPx 600 605 generator (physical instruments) and a KDS 100 syringe pump. The polymer solution was electrospuned in ambient air atmosphere under an applied voltage of 25 kV with a flow rate of 3 mL h -1 using a 0.7 mm diameter syringe connected to the positive output of the generator. A fix collector was placed 25 cm from the tip of the syringe and was related to the negative output of the generator. The droplet was transformed to a Taylor cone due to the electrostatic field applied between the syringe and the collector. The evaporation of the solvent during the process induces the formation of a net of submicron fiber.

ALD deposition of nanolaminates

Diethyl Zinc (DEZ) (Zn(CH2CH3)2, 95% purity, CAS: 557-20-0) and Trimethylaluminum (TMA) ((CH3)3Al) 98% purity, CAS: 75-24-1) were purchased from Sigma Aldrich. A custom made ALD reactor was used for the synthesis of ultrathin Al2O3/ZnO nanolaminates. ALD was performed using sequential exposures of TMA (DEZ) and H2O separated by a purge of Argon with a flow rate of 100 sccm. The deposition regime for ZnO and Al2O3 consisted of 0.2 s pulse of TMA (DEZ), 30 s of exposure to TMA (DEZ), 30 s of purge with Argon followed by 2 s pulse of H2O, 30 s of exposure to H2O and finally 40 s purge with Argon. Al2O3/ZnO ultrathin nanolaminates with different numbers of cycles were deposited on electrospun PAN nanofibers (Table 1). The deposition temperature was fixed to 100 °C. We note here that in contrast to the deposition of thin films on flat surfaces, the reactants had to diffuse into the high-aspect-ratio structures. Therefore an exposure time should be added and adapted in order to assure the conformal coating of the PAN nanofibers. GPC of ~0.2 nm/cycle has been observed for Al2O3 and ZnO. These values are consistent with what has been reported elsewhere by Karvonen et al. [START_REF] Karvonen | Enhancement of the Third-Order Optical Nonlinearity in ZnO/ Al2O3 Nanolaminates Fabricated by Atomic Layer Deposition[END_REF] on ZnO/Al2O3 nanolaminates. The difference between the GPC of Al2O3 reported on single Al2O3 layers and the ZnO/Al2O3 nanolaminates has been attributed to the higher surface area of the rough crystalline ZnO interfaces. 9

Structural, chemical and Optical characterizations

Structural and chemical compositions of all PAN-nanolaminates were analyzed by transmission electron microscopy (TEM). Images were obtained with a JEOL ARM 200F highresolution transmission electron microscope (200 kV) with an EDX analyzer (JED2300, at least 30 accumulations, matrix 512x512 points in STEM mode). Dark and bright field detectors were used.

FTIR spectra were obtained with the Bruker VERTEX 70v vacuum spectrometer using the RT-DLaTGS detector (spectral resolution of 4 cm -1 ). X-Ray diffraction measurements were performed with PANAlytical Xpert-PRO diffractometer equipped with a X'celerator detector using Ni-filtered Cu-radiation. Surface structural and chemical properties of PAN-nanolaminates were analyzed by means of the XPS/UPS technique mounted within the Omicron Nanotechnology multiprobe UHV system, using a monochromatized Al-Kα X-ray as the excitation source (hν = 1486.6 eV) and choosing C 1s (284.6 eV) as the reference line. XPS was conducted under ultrahigh vacuum (10 -7 Pa). CasaXPS software was used to analyze the XPS data. Optical properties of ultrathin nanolaminates have been studied by photoluminescence spectroscopy (in the spectral range of 370-800 nm). The excitation of luminescence was performed by a solid state laser source (Nd:YAG, 355 nm, 13 mW cm -2 ). The registration of the emitted spectra was provided by an experimental setup described elsewhere. [START_REF] Abou Chaaya | Tuning Optical Properties of Al2O3/ZnO Nanolaminates Synthesized by Atomic Layer Deposition[END_REF][START_REF] Abou Chaaya | Evolution of Microstructure and Related Optical Properties of ZnO Grown by Atomic Layer Deposition[END_REF][START_REF] Viter | Tuning of Zno 1D Nanostructures by Atomic Layer Deposition and Electrospinning for Optical Gas Sensor Applications[END_REF] RESULTS AND DISCUSSION

Chemical and structural characterizations

TEM images of PAN -Al2O3/ZnO nanostructures are presented in Figure 1 (TEM image of the end face of PAN fiber and TEM/EDX mapping in Supporting Information (Figure SI1 and SI2)). The TEM images confirm the conformal coating of the PAN nanofibers by ALD. It is clearly seen the nanolaminates formation (ZnO (dark stripes) and Al2O3 (light stripes)). The total thickness of nanolaminates layer is 25 nm (Figure 1d). Lattice fringes seen in high-resolution TEM images reveal ZnO nanocrystals with (101) and (002) planes (Figure 1a and 1b insets).

Using ImageJ software, the grain size and the interplane distances for (101) and (002) lattice planes were calculated (Insets of Figure 1 a,b,c). The grain size was calculated using elliptical shape fitting and the longer axis was used as the grain size. The average grain size for 20 nm, 10 nm and 5.2 nm bilayer thicknesses were 9.8±2.5 nm, 6.5±1.5 nm and 5.3±2.1 nm, respectively. For 2 and 1.2 bilayer thicknesses, the grain size was not determined due to the quality of the image and the resolution of TEM. We suppose that the in case of 20 nm, 10 nm and 5.2 nm bilayer thicknesses, the grains were elongated and limited by the layer thickness. The TEM analysis provided evidence that the Al2O3 layers are amorphous for all samples. The ZnO layers are in a polycrystalline phase. The TEM images demonstrate the ability to produce highly uniform layers of nanolaminates covered PAN fibers by the ALD processes.

The Fourier-transform infrared (FTIR) absorbance spectrum is presented in Figure 2a.

The FTIR data reveals a presence of the main vibration bands of polyacrylonitrile (PAN) in investigated samples. The bands manifest themselves in the ranges of 2260 -2240 cm -1 (C≡N stretch) and 1485 -1035 cm -1 (C-O and C-C stretch and aliphatic CH-group vibrations of different modes in CH, CH2 and CH3). Very sharp and strong peak at 2243 cm -1 is assigned to nitrile band. After deposition of Al2O3/ZnO nanolaminates, two new bands appears in the 1670 -1600 cm -1 spectral range which are attributed to carbonyl group vibrations. [START_REF] Fauzi Ismaila | Effect of Blending Temperature on the Characteristics of Modified Polyacrylonitrie Homopolymer[END_REF] In the spectral range 600 -380 cm -1 , the peak at 407 cm -1 is visible, which is related to ZnO E1,TO phonon mode. With a decrease of the nanolaminate thickness, the ZnO phonon related absorption decreases, and at the 1.2 nm bilayer thickness it is undetectable. This tendency implies the transition from the polycrystalline to amorphous phase of the nanolaminates with low bilayer thicknesses. Similar tendency is show by XRD spectra recorded from nanolaminates with the same bilayer thicknesses (Figure 2b). The ZnO related XRD peaks essentially decrease with a decrease of bilayer thickness. It is reasonable to assume that amorphous nature of ultrathin nanolaminates required minimum thickness to allow crystallization in a thin film. [START_REF] Viter | Optical Properties of Ultrathin Al2O3/ZnO Nanolaminates[END_REF] The proposed crystallization model for ultrathin layers and superlattices is taking into account the interface energies, the thickness of the layers, the melting point of the system, and the bulk amorphous crystallization temperature. [START_REF] Zacharias | Crystallization in the Limit of Ultra Thin Layers-a New Crystallization Model[END_REF] It was shown that exponential increase of the crystallization temperature leads to the decrease of the minimal thickness at which crystallization occurs. From XRD (Figure 2b), HRTEM and previous studies, [START_REF] Raghavan | Nanocrystalline-to-Amorphous Transition in Nanolaminates Grown by Low Temperature Atomic Layer Deposition and Related Mechanical Properties[END_REF][START_REF] Viter | Optical Properties of Ultrathin Al2O3/ZnO Nanolaminates[END_REF] this bilayer thickness value was shown to be 2.5 nm for Al2O3/ZnO nanolaminates. So, we assume that close to this value is the point at which the crystallization process starts in Al2O3/ZnO nanolaminates. The ZnO grain size decreases with decreasing the bilayer thickness. The amorphous Al2O3 layer play the role of stopping layer for ZnO growth. The ZnO layer is forced to renucleate on the underlying amorphous Al2O3 layer inducing the decrease of the grain size when the bilayer thickness decreases.

The surface composition and chemical state polymer fiber coated with different bilayers thicknesses of ZnO/Al2O3 nanolaminates were studied by XPS (Figure 3). Samples depict zinc (Zn), aluminum (Al), oxygen (O) and carbon (C) components. It is worth to say that no peaks, related to Al were observed for nanolaminates with 10 and 20 nm bilayer thicknesses (Figure 3 d ande ). It is related to the limit of the penetration depth of the excitation X-ray photons. As was shown in our previous work, the Auger parameter is a useful tool to determine the stoichiometry and the phase of obtained structures. [START_REF] Iatsunskyi | Structural and XPS Studies of PSi/TiO2 Nanocomposites Prepared by ALD and Ag-Assisted Chemical Etching[END_REF] The main peak of the Zn (LMM) Auger series occurs at kinetic energy of 988±0.5 eV. From this values and Zn 2p3/2 binding energy, we calculated the modified Auger parameter α ' (α ' =BE(Zn 2p3/2)+KE(Zn LMM), where BE and KE are a binding energy of the core level 2p3/2 and a kinetic energy of the Auger transition involving electrons from L3, M45 and M45 core levels) as defined by Wagner. For all samples, our calculations lead to a value of 2009.9±0.5 eV corresponding to ZnO (Wurtzite). [START_REF] Briggs | Handbook of X-Ray Photoelectron Spectroscopy[END_REF] This indicates the formation of stoichiometric ZnO in all samples.

As the studied samples had a single ZnO layer thickness from 0.6 to 10 nm, their fundamental properties would change from the bulk materials to nanoscale. [START_REF] Mosquera | Exciton and Core-Level Electron Confinement Effects in Transparent Zno Thin Films[END_REF] In order to understand the surface role and quantum confinement effects, the energy ranges related to Zn and O lines were rescaled and analyzed. Figure 4 shows the core level of the Zn 2p spectra. The core level binding energies of Zn 2p3/2 and Zn 2p1/2 are about 1022.5 ± 0.1 eV and 1045.6 ± 0.1 eV (20 nm bilayer thickness). These peaks indicate the presence of ZnO phase. [START_REF] Das | Micro-Raman and Xps Studies of Pure ZnO Ceramics[END_REF][START_REF] De La Rosa | Controlling the Growth and Luminescence Properties of Well-Faceted ZnO Nanorods[END_REF][START_REF] Wang | Microstructural and Optical Characteristics of Solution-Grown Ga-Doped ZnO Nanorod Arrays[END_REF][START_REF] Wang | The Effects of Oxygen Partial Pressure on the Microstructures and Photocatalytic Property of Zno Nanoparticles[END_REF] The binding energy difference (duplet) between these components is about 23.1±0.1 eV which could be attributed to the Zn 2+ ions in ZnO. From Figure 4 we can see the shift of peak positions to higher energies with the decrease of bilayer thickness. However, the difference of 23.1 eV in all samples indicates the presence of the normal state of Zn 2+ in the ZnO. The oxygen core level peak behaves similarly (Figure 4 bottom image). The observed phenomenon of XPS size shift can be explained, by growth of surface to volume atoms relation and correspondingly relative increase of dangling bonds on the surface. This leads to change of electronic structure in nanocrystals, resulted in increase of binding energies. [START_REF] Iatsunskyi | Structural and XPS Studies of PSi/TiO2 Nanocomposites Prepared by ALD and Ag-Assisted Chemical Etching[END_REF][START_REF] Mosquera | Exciton and Core-Level Electron Confinement Effects in Transparent Zno Thin Films[END_REF][START_REF] Kovalev | Size Shift of Xps Lines Observed from Pbs Nanocrystals[END_REF] The XPS peaks were analyzed by de-convolution of peaks using CasaXPS software (Supporting information; Figure SI3). It was found, that Zn 2p1/2 peak of 1.2 nm bilayer thickness sample was well fitted with one peak, centered at 1045 eV, whereas other samples showed 2 peaks fitting of the corresponding Zn 2p1/2 peak. The obtained peaks P1 and P2 were in the range of 1045±0.6 eV and 1044±0.6 eV, respectively. The ratio of Zn 2p1/2 (P2)/(P1) increased with a growth of a bilayer thickness, pointing to the decrease of surface-to-volume aspect ratio. The analysis of Zn 2p1/2 peaks showed similar results: the main peak was shifted towards lower energies and additional peak at 1022.3±0.6 eV has appeared at the spectra, pointing to the presence of Zn species (interstitials) or Zn-OH in ZnO. [START_REF] Sharma | Effect of Surface Groups on the Luminescence Property of ZnO Nanoparticles Synthesized by Sol-Gel Route[END_REF] Analysis of O 1S peak has been fitted with 3 peaks, centered at 529.6±0.5 (P1), 531.3±0.2 (P2) and 532.1±0.4 (P3) eV. The present oxygen at XPS spectrum peaks corresponded to chemisorbed surface oxygen and water (P1), surface adsorbed OH and/or oxygen vacancy (P2) and O 2-(P3) in ZnO lattice. [START_REF] Zhang | Effect of Aspect Ratio and Surface Defects on the Photocatalytic Activity of ZnO Nanorods[END_REF] It is worth to say that ratio P2/P3 decreased with the increase of the bilayer thickness, pointing to the improvement of nanolaminates crystallinity and the decrease of the defect concentration.

Band gap energy determination

It is well known, that the difference between the onset of inelastic electron losses (Eloss) energy and the core level peak (e.g. oxygen peak -EO 1s) allows one to estimate the energy gap (Eg) of the material. [START_REF] Nichols | Measurement of Bandgap Energies in Low-K Organosilicates[END_REF] The intersection of the linear-fit line of the measured electron loss spectra near the core level peak and the background "zero" level gives us the onset of inelastic losses occurring in Al2O3. [START_REF] Iatsunskyi | Structural and Xps Characterization of ALD Al2O3 Coated Porous Silicon[END_REF] Then we can calculate the band gap energy of Al2O3 using the formula:

O loss g E E E  
. Applying this theory, we have calculated Eg of Al2O3 for PAN-Nanolaminates samples (Figure 5). Following values were obtained: 5.37±0.06, 5.78±0.09, 5.88 and 6.01±0.08 eV for PAN-nanolaminates samples with 0.6, 1, 2.6 and 5 nm Al2O3 layer thickness, respectively. Unfortunately, it was impossible to calculate the Eg of Al2O3 for PAN-Al2O3/ZnO (10 nm Al2O3 layer thickness) due to the thick top layer of ZnO under the Al2O3 layer. Our obtained values of about 6 eV is low compared to the one reported for amorphous Al2O3 (8.8 eV). [START_REF] Nichols | Measurement of Bandgap Energies in Low-K Organosilicates[END_REF] This observation might be explained by the highly defected structure of Al2O3 layer in nanolaminates. Upon increasing the Al2O3 layer thickness in PAN-nanolaminates, the Eg gradually increases and tends to bulk value.

Work function determination

The electronic structure of PAN-ZnO/Al2O3 nanolaminates as a function of the bilayers thicknesses was studied with UV photoelectron spectroscopy (UPS) (He I emission 21.22 eV).

The The change of Φ could be explained by band bending, formed at the upper layer of the nanolaminates (ZnO-air interface) (Figure 7). Electron affinity of ZnO and Al2O3 were taken χZnO=4.37 eV and χAl2O3=1.35 eV respectively. [START_REF] Ozawa | Angle-Resolved Photoelectron Spectroscopy Study of the Anion-Derived Dangling-Bond Band on ZnO(1010)[END_REF][START_REF] Ren | High-Density NiSi Nanocrystals Embedded in Al2O3/SiO2 Double-Barrier for Robust Retention of Nonvolatile Memory[END_REF] Fermi level EF position in each component of the nanolaminates was estimated:

    F E (Equation 1)
where Φ is a work function.

The conductance band and valence band gap offsets cold be calculated as (Table 2):

1 2 c E      (Equation 2) c 2 g 1 g v E E E E      (Equation 3)
where Eg1,2 and χ1,2 are band gap values of Al2O3 and ZnO and electron affinity values of ZnO and Al2O3, respectively.

The built in potential Vbi between two altering ZnO and Al2O3 layers was calculated due to (Table 2): 4) where q is an electron charge, EF1 and EF2 are Fermi level positions in Al2O3 and ZnO, correspondently.

1 F 2 F bi E E q V    (Equation
To estimate potential drops in ZnO and Al2O3, the Debye lengths in the altering materials were estimated: 5)

q N T k L d 0 D        (Equation
where k ,T, ε and ε0 are the Boltzmann's constant, absolute temperature, dielectric permittivity of ZnO (ε=8.5) and electric constant (ε0=8.85•10 -12 F m -1 ), respectively. The donor concentration in ZnO and Al2O3 were taken 10 23 m -3 and 10 16 m -3 , respectively. 37-38 The estimated Debye lengths were 10 -8 m and 5•10 -5 m for ZnO and Al2O3, respectively.

The potential drops in ZnO and Al2O3 could be calculated as:

ZnO bi 3 O 2 Al DZnO 3 O 2 DAl bi DZnO ZnO V V V L L V L V    
(Equation 6)

Based on equations ( 5) and ( 6) the potential drop in ZnO was 10 -4 eV. Therefore, the band bending in ZnO at the interface between ZnO and Al2O3 is significantly small and the potential drop will be mostly faced in Al2O3 layer.

The estimated values of ZnO work function are strongly affected by the surface band bending in the upper ZnO layer. The later could be estimated from equation 7: 7)

bl 20 bl 1 s V     (Equation
Where Φ1bl Φ20bl are work function of ZnO in 1 bilayer and 20 bilayer samples respectively. The obtained Vs value was 0.64 eV. The depletion width in ZnO could be calculated from the equation 8:

39 d 0 s N q V 2 W       
(Equation 8)

The estimated W value was around 50 nm. Therefore, the upper ZnO layer will be fully depleted and a double barrier structure will be formed on the borders of the ZnO layers, pointing to the confinement of the electrons in ZnO along Z-axis. The observed change of work function in ZnO layers results from the presence of hydroxyl and oxygen groups, adsorbed on ZnO surface. This is corroborated by the band at 3500 cm -1 observed in FTIR and XPS analysis. The change of the work function with the increase of the bilayer numbers is affected by 'screening' of the induced electric field on the surface of the middle Al2O3 layers. The part of the depleted ZnO layers will decrease and therefore, the average value of the work function will decrease.

Photoluminescence of PAN-Al2O3/ZnO nanolaminates

The photoluminescence of the PAN-Al2O3/ZnO nanolaminates samples showed intense broad visible emission (Figure 8). The deconvolution of the peaks was performed using Gaussian fitting (Supporting Information; Figure SI4). Peaks at 473, 520 and 580 nm have been found. In

ZnO, PL appears due to the photogenerated holes and electrons as the equilibrium holes concentration is too low. PL depends on the defects concentration. The visible PL intensity increases with the decrease of the layer thickness due to the growth of defects number. It was shown that PAN, as a template, induces the nucleation of pure ZnO during the growth affecting the optical properties. [START_REF] Viter | Tuning of Zno 1D Nanostructures by Atomic Layer Deposition and Electrospinning for Optical Gas Sensor Applications[END_REF] However, in the present work PAN template did not influence the PL of ZnO as since Al2O3 layer was the first layer deposited over PAN in all the analyzed samples. It is known that PL of ZnO is sensitive to surface band bending and grain size dimensions. [START_REF] Abou Chaaya | Evolution of Microstructure and Related Optical Properties of ZnO Grown by Atomic Layer Deposition[END_REF] Due to the depleted layer, no exciton emission was observed in the samples with 20, 10 and 5.2 nm bilayers thicknesses. Moreover, the strong visible emission has been observed for all samples.

The intensity of the measured emission was 1000 folds higher than the PL emission of the nanolaminates, deposited on Si substrates. [START_REF] Abou Chaaya | Tuning Optical Properties of Al2O3/ZnO Nanolaminates Synthesized by Atomic Layer Deposition[END_REF][START_REF] Viter | Tuning of Zno 1D Nanostructures by Atomic Layer Deposition and Electrospinning for Optical Gas Sensor Applications[END_REF] The significant increase of the PL was found with the increase of the bilayer numbers. It has been reported that visible PL in ZnO nanostructures is resulted from defects, mostly from Zn and O vacancies and interstitials. [START_REF] Abou Chaaya | Tuning Optical Properties of Al2O3/ZnO Nanolaminates Synthesized by Atomic Layer Deposition[END_REF][START_REF] Abou Chaaya | Evolution of Microstructure and Related Optical Properties of ZnO Grown by Atomic Layer Deposition[END_REF][START_REF] Viter | Tuning of Zno 1D Nanostructures by Atomic Layer Deposition and Electrospinning for Optical Gas Sensor Applications[END_REF] According to that, the observed peaks at 473, 520 and 580 nm could be assigned to neutral, single charged and double charged oxygen vacancies, respectively. [START_REF] Abou Chaaya | Tuning Optical Properties of Al2O3/ZnO Nanolaminates Synthesized by Atomic Layer Deposition[END_REF][START_REF] Abou Chaaya | Evolution of Microstructure and Related Optical Properties of ZnO Grown by Atomic Layer Deposition[END_REF][START_REF] Viter | Tuning of Zno 1D Nanostructures by Atomic Layer Deposition and Electrospinning for Optical Gas Sensor Applications[END_REF] However, the peak at 473 nm could also be related to Zn vacancies. [START_REF] Abou Chaaya | Tuning Optical Properties of Al2O3/ZnO Nanolaminates Synthesized by Atomic Layer Deposition[END_REF][START_REF] Abou Chaaya | Evolution of Microstructure and Related Optical Properties of ZnO Grown by Atomic Layer Deposition[END_REF][START_REF] Viter | Tuning of Zno 1D Nanostructures by Atomic Layer Deposition and Electrospinning for Optical Gas Sensor Applications[END_REF] Measuring only room temperature luminescence, we cannot discriminate the observed peak at 473 nm between two different defect states. However this fact does not change the nature of the emission.

According to XPS data, the number of defects decreased with an increase of the bilayer thickness. Therefore, the 1. The work function of ZnO is related to a width of the space charge and band bending. 40- 41 The change of WF can point to the adsorption/desorption, interface forming and other changes on the surface. In our case, WF demonstrates that the band bending decreases when more Al2O3 layers are introduced in the nanolaminates what induced electron-rich area in ZnO. It is one of the factors which influences PL value.

In our previous works, 1, 3 we observed similar results when no excitonic peaks were found for thin layers due to band bending and low grain size. This will result from the high concentration of defects, the low grain size (compatible with exciton Bohr radius for ZnO (2.6 nm)) and the surface band bending. Therefore, domination of defect level emission and low rate of exciton formation are observed.

The ZnO layers were polycrystalline with small grain size. We suppose that the grain boundaries will have the following effect in the optical properties of the nanolaminates: (i) Quantum PL intensity (a.u.)

 (nm)

Al2O3/ZnO 20(0.5 nm/0.5 nm) Al2O3/ZnO 10(1 nm/1 nm) Al2O3/ZnO 4(2.5 nm/2.5 nm) Al2O3/ZnO 2(5 nm/5 nm) Al2O3/ZnO 1(10 nm/10 nm) Reference

TABLES

  work function (Φ) of the ZnO could be estimated by subtracting the secondary electron cutoff value from the He excitation source of 21.22 eV (Φ = 21.22 eV -ESEC).

  Figure 6 shows the UPS spectra of PAN-ZnO/Al2O3 nanolaminates. The calculated values of Φ are shown in the image. The Φ of ZnO decreases gradually from 5.86 eV (20 nm bilayer thickness) to 5.19 eV (1.2 nm bilayer thickness), which is possibly due to the morphology changes of the top ZnO layers in nanolaminates. The structure of ZnO layer in the 20 nm bilayer thickness sample is polycrystalline (TEM results; Figure 1), thus, the energy barriers presenting at the border of disorientated ZnO nanocrystallites increases the value of the Φ. The small peak corresponding to Al2O3 is also present in the Figure 6. The average value of WF for Al2O3 is approximately 4.7 eV and it changes insignificantly. Work functions of ZnO and Al2O3 layers are plotted in Figure 6. The work function of Al2O3 was insignificantly changed whereas the ZnO work function significantly decreased with the bilayer thickness.

  photoluminescence. The work function measurements of the samples showed the significant

FIGURESFigure 1 .

 1 Photoluminescence of the nanolaminates corresponded to emission bands in ZnO nanolayers.

Figure 2 .Figure 3 .

 23 Figure 2. a) Absorbance spectra and b) XRD spectra of PAN-Al2O3/ZnO nanolaminates at

Figure 4 .

 4 Figure 4. XPS survey spectra of ZnO/Al2O3 nanolaminates coated polymer fiber with different

Figure 5 .Figure 6 .x 4

 564 Figure 5. O 1s Energy-loss spectra for band gap determination of Al2O3 of ZnO/Al2O3

Figure 7 .Figure 8 .

 78 Figure 7. Zone diagram of Al2O3/ZnO Nanolaminates formation: a) the single layers before

Table 1 .

 1 Al2O3/ZnO ultrathin nanolaminates synthesis by ALD

		Cycles	Cycles	Numbers	Bilayer thickness
	Samples
		of Al2O3	of ZnO	of Bilayers	(nm)
	Al2O3/ZnO 20 (0.6/0.6 nm)	3	3	20	1.2
	Al2O3/ZnO 10 (1/1 nm)	5	5	10	2
	Al2O3/ZnO 4 (2.6/2.6 nm)	13	13	4	5.2
	Al2O3/ZnO 2 (5/5 nm)	25	25	2	10
	Al2O3/ZnO 1 (10/10 nm)	50	50	1	20

Table 2 .

 2 Electronic parameters of Al2O3/ZnO ultrathin nanolaminates synthesis by ALD; The numbers in square parenthesis are references.

		Electron	Band gap	Work function
				ΔEc (eV)	ΔEv (eV)	Vbi (eV)
		affinity (eV)	(eV)	(eV)
	Al2O3	1.35 [36]	6±0.1	4.8±0.1
				3	0.3±0.1	2±0.1
	ZnO	4.35 [35]	3.3 [35]	5.8±0.1

ACKNOWLEDGMENTS. This work was partially supported by the EU under BIOSENSORS-AGRICULT, Contract PIRSES-GA-2012-318520 'Development of Nanotechnology-based Biosensors for Agriculture', 'Nanomaterials and their application to biomedicine' (contract number PBS1/A9/13/2012), and 'FOTONIKA-LV-FP7-REGPOT-CT-2011' (contract number 285912). "The authors would like to acknowledge the "Institut Européen des Membranes (IEM)-UMR CNRS 5635" which partially supports this study through the project "Axe-health/P1S-2014". This study was partially supported by the ANR project ANR-14-CE07-0011 "BONALD".

Supporting Information Available. Deconvolution of XPS and PL peaks, TEM and EDX/TEM mapping are supplied as Supporting Information. This information is available free of charge via the Internet at http://pubs.acs.org.