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This is the published version of a paper published in Composites Science And Technology.

Citation for the original published paper (version of record):

Fu, Q., Yan, M., Jungstedt, E., Yang, X., Li, Y. et al. (2018)

Transparent plywood as a load-bearing and luminescent biocomposite Composites Science And Technology, 164: 296-303

https://doi.org/10.1016/j.compscitech.2018.06.001

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Contents lists available atScienceDirect

Composites Science and Technology

journal homepage:www.elsevier.com/locate/compscitech

Transparent plywood as a load-bearing and luminescent biocomposite

Qiliang Fu

a

, Min Yan

b

, Erik Jungstedt

a

, Xuan Yang

a

, Yuanyuan Li

a,

, Lars A. Berglund

a,∗∗

aWallenberg Wood Science Center, Department of Fiber and Polymer Technology, KTH-Royal Institute of Technology, SE-10044 Stockholm, Sweden

bDepartment of Applied Physics, School of Engineering Sciences, KTH-Royal Institute of Technology, SE-16440 Kista, Sweden

A R T I C L E I N F O

Keywords:

Transparent wood Nanotechnology Biocomposite Photonics

A B S T R A C T

Transparent wood (TW) structures in research studies were either thin and highly anisotropic or thick and isotropic but weak. Here, transparent plywood (TPW) laminates are investigated as load-bearing biocomposites with tunable mechanical and optical performances. Structure-property relationships are analyzed. The plies of TPW were laminated with controlledfiber directions and predetermined stacking sequence in order to control the directional dependence of modulus and strength, which would give improved properties in the weakest direction. Also, the angular dependent light scattering intensities were investigated and showed more uniform distribution. Luminescent TPW was prepared by incorporation of quantum dots (QDs) for potential lighting applications. TPW can be designed for large-scale use where multiaxial load-bearing performance is combined with new optical functionalities.

1. Introduction

The reported energy consumption in residential and commercial buildings accounts for approximate 40% of the total energy consump- tion [1–3]. In addition, the energy consumption for buildings are pre- dicted to grow even further in the coming decades [4]. In this context, glass is an interesting building material, primarily due to its high op- tical transmittance. It is used for windows and rooftops, to transmit sunlight for lighting purposes, where an additional benefit is the re- duced electrical energy consumption. However, even structural grades of glass are too brittle for large load-bearing structures, and may have problems with glare [5–7]. Moreover, glass has relatively high thermal conductivity, often leading to thermal leakage. These characteristics are limiting the use of glass materials. Alternative building materials are therefore under development, with favorable mechanical properties, sunlight-controlling and even light-emitting properties [8]. Controlled use of daylight in buildings provides both health and energy benefits.

Wood biocomposites have recently become interesting in this context, an“old” material with new functionalities. New wood functionalization approaches have made it possible to combine load-bearing and func- tional properties in biobased wood structures [9–14]. Fink made the transparent wood (TW) for the morphology study [15]. Along this line, nanostructural functionalization has been used successfully to process wood into optically transparent composites with favorable mechanical and optical properties for the engineering purpose [16–18]. Trans- parent wood is a potential candidate for advanced building materials

with merits such as high transmittance, tailored haze, light wave guiding properties, favorable mechanical and thermal insulation prop- erties, etc. [18–20].

In previous work, TW with favorable mechanical properties (90 MPa, at 19 vol% of cellulosic reinforcement), optical transmittance (85%) and haze (71%) was prepared. Delignified wood was im- pregnated by an oligomer mixture based on methyl methacrylate (MMA), and polymerized [16]. The mechanical properties of TW can be tailored by controlling the cellulose volume fraction. Zhu et al. ana- lyzed light guiding characteristics and anisotropic mechanical proper- ties of TW materials [18]. In this work, delignified wood substrates were impregnated with epoxy. Beyond transparency, TW has been considered for applications requiring thermal insulation [21,22], magnetic function [23], luminescence and lasing properties [20,24,25], light diffusion function [26,27] and microfluidic performance [28]. Due to fabrication constraints, TW materials made so far have been rela- tively thin. Thin TWs have load-bearing limitations in terms of design forces and bending moments, in particular in directions transverse to the cellulose orientation direction [18,29]. Oriented cellulose also causes anisotropic light scattering, whereas evenly distributed light is often desired.

In this study, transparent plywood (TPW) is prepared and tailored with respect to mechanical properties and optical performance. In particular, the mechanical properties are enhanced in the transverse direction compared to single ply TW. The structure-property relation- ships for the material were investigated. The plies were laminated in

https://doi.org/10.1016/j.compscitech.2018.06.001

Received 19 January 2018; Received in revised form 30 April 2018; Accepted 2 June 2018

Corresponding author.

∗∗Corresponding author.

E-mail addresses:yua@kth.se(Y. Li),blund@kth.se(L.A. Berglund).

Available online 04 June 2018

0266-3538/ © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).

T

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similar stacking sequences to classical plywood laminates. In-plane quasi-isotropic stiffness properties of TPW can be achieved by appro- priate selection of laminated structures. The angular dependent light scattering through transparent plywood was also studied. Optical properties can be tuned depending on lamination angles between the layers of TPW. Isotropic diffused luminescent TPW was also demon- strated by incorporation of quantum dots (QD) in the material for po- tential lighting applications. TPWs with isotropic load-bearing and light-scattering performance may widen the scope for structural appli- cations of wood nanotechnology. It could be used in buildings, as well as in optical and photonic devices.

2. Experimental

2.1. Materials and chemicals

Tangential veneers of balsa wood (Ochroma pyramidale) with 0.6–0.8 mm thickness were supplied from Wentzels Co. Ltd., Sweden.

Two dimensional sizes (length and width) of 2 cm × 2 cm and 6 cm × 6 cm were dried in an oven at 105 ± 3 °C for 24 h for further use. Sodium chlorite (NaClO2) and methyl methacrylate (MMA) were purchased from Sigma Aldrich. Acetate buffer solution (pH 4.6) was used for chemical extraction process. QDs (CdSe/ZnS with emission peak around 530 nm) were supplied by Mesolight Inc with concentra- tion of 25 mg/mL.

2.2. Delignified wood template fabrication

The wood veneers were chemically extracted by using 1 wt % NaClO2in acetate buffer solution (pH=4.6) for 6 h under 80 °C in order to remove lignin [16]. The delignified wood samples were washed with deionized water three times. Then the samples were freeze-dried. Lignin content of the wood samples was determined according to the standard method: TAPPI T222 om-02 [30]. After totally dried, the delignified wood samples were compressed by 75 kN for 25 min under 25 °C. These compressed delignified wood samples were further used for making TPW.

2.3. Transparent plywood preparation

The pre-polymerized MMA and infiltration processes were per- formed following our previous work [16]. The impregnated delignified five veneers then were assembled with their grains perpendicular (0/

90/0/-90/0, cp-TPW) to each other or laminated with a quasi-isotropic structure by increased 45° (0/45/90/-45/0, qi-TPW). The laminated wood was sandwiched between two glass slides and packaged with aluminum foil. Polymerization process was performed by oven heating at 70 °C for 4 h. For demonstration, the QDs were embedded in trans- parent wood to make luminescent TPW. The preparation of luminescent TPW was done according to our previously protocol [25]. In detail, QDs were first dispersed in pre-polymerized MMA solution. The wood templates were then infiltrated into the QDs dispersed PMMA solution.

After fully infiltration, the wood templates were assembled as cp- and qi-TPW and sandwiched between glasses for further polymerization.

2.4. Morphology characterizations

The morphologies of the specimens were characterized by a Field- Emission Scanning Electron Microscope (FE-SEM, Hitachi S-4800, Japan). The cross-section of the sample was prepared by fracturing in bending after cooling with liquid nitrogen. All samples were coated with platinum−palladium before SEM characterization.

2.5. Mechanical test and analysis

Tensile and bending tests of wood samples were performed in an

Instron 5944 instrument. The Young's modulus of TPW in elastic de- formation region was predicted based on laminate plate theory model (see in supporting information). The numerical software Matlab was used to run the laminate plane program.

2.6. Optical measurements

An optical characterization setup based on an integration sphere for the measurement of total transmittance and haze has been reported in our previous work [16]. A high-brightness light source with wavelength spanning from 170 nm to 2100 nm was used (EQ-99 from Energetiq Technology Inc.). For the scattering light intensity measurement, a green laser with 550 nm wavelength (4 mm diameter light spot) was used as incidence. The intensity of the scattered light was measured by an optical power meter. The angle of collection is read from a rotating stage on which the power meter is mounted (Fig. S7, in supporting information). The light scattering patterns were taken by using a digital camera (D7000, Nikon). A UV light source with a wavelength of 400 nm was used as incidence. Absolute photoluminescence (PL) quantum yield, luminescence and absorption spectra were measured in an in- tegrating sphere based home-built instrument. The excitation wave- length was 440 nm (6 nm linewidth)filtered by a monochromator after a laser-driven Xe-lamp [31]. Quantum yield was calculated as the ratio of number of emitted photons to the number of absorbed photons ac- cording to the obtained PL luminesce and absorption spectra.

3. Results and discussion

The TPW fabrication process is schematically illustrated inFig. 1, and includes delignification, compression, impregnation, lamination, and polymerization, respectively. Natural wood was used as the raw material for the wood-based porous template and TPW. Lignin was extracted to minimize light-absorbance and refractive index mismatch.

This process also led to an increased the nanoporosity in the wood cell walls. The delignified templates were compressed in order to increase the cellulose volume fraction. After that, the templates were infiltrated with prepolymerized methyl methacrylate oligomers (PMMA), with a refractive index matched with the wood template. The impregnated wood templates were then laminated infive layers with cellulose or- ientation angles in 0/90/0/90/0° (denoted as cross-ply TPW, cp-TPW in abbreviation) or in 0/45/90/-45/0° (denoted as quasi-isotropic TPW, qi-TPW in abbreviation), note that the laminate is not strictly quasi- isotropic (seeFig. S1) [32]. 0 and 90° are defined as longitudinal (L) and transverse (T) directions (Fig. S1). Luminescent TPW is demon- strated by inclusion of QDs in the pre-polymerized MMA solution fol- lowed by lamination and polymerization. The TPW structure, me- chanical and optical properties are then characterized and discussed in the following.

The cross-sectional morphologies of the cell walls are presented in Fig. 2.Fig. 2a illustrates changes in the wood cell wall structure during TW preparation. The cell wall is a composite material with a thickness of micrometers. Cellulose forms afibrillary, nanoscale reinforcement phase embedded in a molecular/nano-scale mixture of lignin and hemicellulose (Fig. 2a left). Cellulose nanofibrils are apparent as white dots in the SEM cross-section (Fig. 2b). No visible larger scale porosity is apparent. The lignin content of balsa wood is typically 20–25% [33].

Cell wall corners and the middle lamella are lignin-rich regions in the wood structure [34]. After delignification, the lignin content was re- duced from 25% to 3%. The original wood turned white, and nano-scale pores (3–90 nm) are apparent in the dried cell walls (Fig. 2c andFig.

S2) due to the removal of lignin (Fig. 2a, center). Note that the drying process and chemical treatment influence the observed pore structure [35,36]. This is in line with earlier work using two-dimensional small- angle X-ray scattering (SAXS) analysis [37,38]. After PMMA infiltra- tion, ply stacking and polymerization, transparent plywood was ob- tained. The inserted images in Fig. 2d and e demonstrate optical

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Fig. 1. Schematic illustration of the preparation processes for transparent plywood (TPW). Compressed delignified wood templates were used. The TPW was fabricated by stacking layers of PMMA impregnated delignified wood followed by polymerization. Luminescent TPW was prepared by mixing quantum dots (QD) in the PMMA oligomer liquid.

Fig. 2. Graphical illustration and SEM images of the microstructure of cell wall. a) Schematic illustration to show: the native cell wall consists of cell wall matrix (lignin/hemicellulose mixture) and cellulose fibrils.

Nano-structured cell wall of: b) native wood; c) de- lignified wood; d) transparent wood (without com- pression); and e) compressed transparent wood. The insets in d) and e) show that both without and with compression single ply TW are transparent.

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transparency of cp- and qi-TPWs, respectively.Fig. 2d is the cross-sec- tional SEM image of TW containing PMMA but without pre-compres- sion of the wood template. Nanopores are revealed in the cell wall, indicating very limited, if any PMMA presence in the cell wall. Some debond gaps are observed at the interface between cell walls and PMMA in lumen space (Fig. S3a). Gel state shrinkage of PMMA during polymerization is an important reason. In contrast, when the delignified wood template was pre-compressed before by PMMA infiltration (Fig. 2e), the porosity in the cell wall appeared to be reduced. The volume of the lumen space for TW (Fig. S3a, without compression) is obviously much larger than for compressed TPW plywood (Fig. S3b).

The thickness of the compressed cell walls in TPW appears to be smaller than for TW (without compression) with cell wall nanoporosity (Fig.

S3).

Micrographs of the hierarchical structure and interfacial morphol- ogies of TPW are presented inFig. 3. The different orientations of the layers are apparent in the TPW. Fig. 3a illustrates that plies within layers are arranged at millimeter scale; wood tracheids are arranged along the longitudinal direction in each layer with diameters at the 10 micron scale; the cellulose nanofibrils in the cellulose with diameters at nanometer scale and are highly aligned [39–41]. By tuning the or- ientation of TW plies, nanocellulosefibrils are preferentially aligned so that mechanical and optical properties can be tailored.Fig. 3b shows the SEM image of a single ply of TW, where the wood structure is sandwiched between two PMMA layers.Fig. 3c shows a fracture surface from the cp-TPW withfive layers. In qi-TPW, the stacking sequence is [0, 45, 90,−45 0] (Fig. 3d). Interface regions between layers for cp- TPW (Fig. 3e) and qi-TPW (Fig. 3f) show that no apparent micro-scale defects are present, indicating good interfacial bonding between layers.

This is due to the fact that PMMA (oligomers and monomers) is

infiltrated into delignified wood to bond wood veneers. Peeling of the plies to expose the cell wall (Fig. 3g) revealed aligned cellulose nano- fibrils. Around 85 vol% of the secondary wood cell wall consists of the S2layer with cellulose nanofibrils oriented at a fairly small microfibril angle with respect to the longitudinal cell axis [42].

The tensile stress-strain curves in longitudinal and transverse di- rections are shown in Fig. 4. The test directions are illustrated by drawings inFig. 4a and4b. The ultimate strength for single ply TW increases linearly with cellulose volume fraction (Fig. S4). The ultimate strength and elastic modulus of single ply TW with 12 vol% cellulose are 62.5 MPa and 4.3 GPa, respectively. The strength and elastic mod- ulus are around 40 MPa and 2.3 GPa for the pure PMMA (Fig. 4). The longitudinal ultimate strength are 50.1 MPa for cp-TPW and 45.4 MPa for qi-TPW. Compared with PMMA, the elastic modulus increases to 4.1 GPa for cp-TPW and 3.9 for qi-TPW GPa with 10 vol% cellulose (Fig. 4c andTable 1). The increase is due to the reinforcement effect from the delignified cellulosic wood template.

Low mechanical properties in the transverse direction are often a limitation for load-bearing applications. The data from transverse ten- sile tests are presented inFig. 4d andTable 1. The ultimate transverse strength and elastic modulus were 14.6 MPa and 2.4 GPa for single ply TW. After lamination, the ultimate strengths for the two types of TPWs with 10 vol% cellulose are 42 MPa for qi-TPW and 45 MPa for cp-TPW, respectively. Compared to single ply TW, the “transverse” elastic modulus in T direction increases from 2.4 GPa (single ply TW) to 3.5 for qi-TPW and 3.9 GPa for cp-TPW, respectively. This increase is due to the stacking orientation of the laminated structure (Fig. 3). The bending strength values of TPWs (68 MPa for cp-TPW and 82 MPa for qi-TPW) are higher than that of single ply TW and PMMA (43 MPa and 57 MPa) inFig. S5. The improvement of bending strength for TPW is also due to Fig. 3. Micrographs of TPW plywood. a) Sketch of the hierarchical structure for the TPW with laminated veneers, nanofibril bundles and cellulose molecules. SEM images of: b) single ply TW; c)five layer transparent plywood in cp-TPW; d) five layer transparent plywood in qi-TPW; e) interlaminar interface in cp-TPW; and f) interlaminar interface in qi-TPW. High magnification SEM image of g) highly aligned nanocellulose fibrils (green arrows). (For interpretation of the references to colour in thisfigure legend, the reader is referred to the Web version of this article.)

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the laminated material design.

The modulus of both TPWs were predicted from laminate theory (in supporting information) [43,44]. The predictions for TPW plywood are based on properties of a single ply TW. The predicted data are rea- sonably close to the experimental data (Table 1). Deviations may due to underestimated value for in-plane shear modulus of single ply TW. The laminate plate theory could be extended to the TPW with different thicknesses and stacking sequences. A comparison of mechanical properties between current work and other published TWs is summar- ized in Table S1. The present material exhibits higher mechanical properties than comparable TWs. In particular, the TPW obviously has better mechanical properties in the transverse direction. For some en- gineering applications in buildings with in-plane multiaxial loads, the TPW with quasi-isotropic mechanical in-plane properties is attractive.

The isotropic structure of a material leads to isotropic optical properties. Data for the optical properties of TWP are shown inFig. 5.

Both TPW laminates exhibit high transmittances: 83% for cp-TPW and 75% for qi-TPW at the thickness of 3–3.5 mm (Fig. 5a). PMMA (3 mm thickness) and single ply TW (0.8 mm thickness) both display an optical transmittance of up to 90–95%. Single ply TW has a haze around 50–60% while PMMA has a haze less than 5% haze. The refractive-

index-matched PMMA was infiltrated into micro-pores of lumens (Fig.

S3) to reduce the refractive index contrast. The incident light travels through the cellulose-PMMA matrix with scattering from material in- homogeneities, such as wood-PMMA interface gaps [45]. The inherent light attenuation of wood from nanoporosity may have been reduced by high-pressure compression (Fig. 2e). In addition to high total trans- mittance, the TPWs also showed high optical haze, about 80% for both types of TPW (Fig. 5b). A“KTH” logo can be clearly observed behind the single ply TW, or both TPWs, when the samples are placed directly on the logo (Fig. S6a). However, the logo became visually blurred when the samples are 5 mm above the logo due to the haze (Fig. S6b); this blurring was higher for the two TPWs than for single ply TW.

In order to study the scattering behavior of TPW, a setup was de- signed and built for the measurement of transmitted light intensity (Fig.

S7). The scattering patterns and light intensities at different angles are presented inFig. 5c–f,Fig. S9 and S10. A squeezed parallelogram-like scattering pattern in the L direction of the single ply TW is presented in Fig. 5c. The scattered light distributions are different between L and T directions because of the anisotropic structure of wood (Fig. 5e). In contrast, a homogeneous, nearly circularly scattered beam pattern was observed for the qi-TPW (Fig. 5d). This pattern is significantly different Fig. 4. Mechanical properties of TPW. Sketches show the loading in a) longitudinal (L) direction and b) transverse (T) direction, respectively. Typical stress- strain curves of the tensile test in c) longitudinal direc- tion and d) transverse direction, respectively. The me- chanical performance values were calculated from ten- sile test with advanced video extensometer (AVE).

Table 1

Summary of the mechanical properties of single ply and TPW, and the Young's modulus predictions from laminate theory.

Wood species Tensile test direction Cellulose volume fraction (%) Size Mechanical properties*

l × w × t (mm3) E (GPa) Ea(GPa) σ(MPa) εc≈ (%)

Single ply TW L 12 60 × 5 × 0.8 4.3 ± 0.4 62.5 ± 2.7 1.5

T 12 2.4 ± 0.2 14.6 ± 1.2 0.7

cp-TPW L 10 60 × 5 × 3.5 4.1 ± 0.3 3.7 50.1 ± 2.6 1.2

T 10 3.9 ± 0.1 3.3 44.9 ± 1.3 1.3

qi-TPW L 10 60 × 5 × 3.5 3.9 ± 0.2 3.4 45.4 ± 2.9 1.2

T 10 3.5 ± 0.3 3.0 42 ± 1.8 1.4

‘-’: No data available. ‘l × w × t’: Length, width and thickness, respectively. ‘E’: The Young's modulus from the experimental data. Ea: The predicted Young's modulus is determined based on laminate plate theory (in supporting information).‘σ’: Ultimate strength. ‘εc’: Failure at break. ‘*’: T The mechanical performance values were calculated from tensile test with AVE.

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from the transmitted spot of light going through PMMA (Fig. S8a and S8b), but similar to the cp-TPW plywood, due to the isotropic structural design of the TPW (Fig. S8c and S8d). The intensities of the scattered light beam show Gaussian-like distributions in both L and T directions for qi- and cp-TPWs (Fig. 5e,f, andFigs. S8 and S9). The TPW can also spread an incident laser beam from a side facet to the whole sample (Fig. S10) due to its isotropic light scattering properties. This char- acteristic could be of interest for an efficient lighting panel, for the purpose of indoor environment illumination.

For an incident laser beam (green) normal to the surface, the scat- tered light intensities for TPW spread within a broad range of angles ranging from−60° to 60° (Fig. 5f andFig. S8d). Thefive-layer qi-TPW

appears to create a similar isotropic light scattering effect as for transparent nanopaper based on random-in-the-plane oriented cellulose nanofibrils. This can be ascribed to fiber orientations perpendicular or in off-axis angles as shown inFig. 3andFig. S1. This is different from the light scattering for transparent nanopaper, where the nanofibrils were randomly distributed in the plane [46]. This effect implies that TPW, when used for construction panels in buildings or rooftops/ceil- ings, can efficiently convert incoming sunlight into evenly diffusive indoor light. Such optically isotropic material can also be applied in many other applications, including decorative panels, furniture, art design, and even optical and photonic devices.

As a demonstration, a uniformly diffusive luminescent TPW for Fig. 5. Optical properties of the TPW. a) Total transmittance of single ply TW, pure PMMA and TPW. b) Haze for cp- and qi-TPW. Photographs of scattering pattern for single ply TW c) and qi-TPW d). Light intensity distributions in L e) and T f) directions. g) TPW and TPW@QD exposed in vertically incident UV light. h) Single ply TW@QD and TPW@QD in UV light.

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lighting application is demonstrated. For this purpose, CdSe/ZnS QDs were embedded in the TW resulting in a lighting panel which emits green light when excited by UV light. The wood templates impregnated by QD and PMMA oligomers were then laminated into luminescent TPW with isotropic light emitting characteristics, as shown inFig. 5g,h, and Fig. S12. The light absorption of TPW@QD occurred at around 420 nm, and the emission band peak around 550 nm with a quantum yield of 22% (Fig. S11). Isotropic luminescence was observed over the whole TPW@QD under a UV light source pumping with 90° incident angle (Fig. 5g, right). In the samefigure, “regular” TPW is obviously not luminescent under the same UV exposure (Fig. 5g, left). For a single- layer anisotropic TW@QD, the luminescence is not uniformly dis- tributed over the sample (Fig. 5h, left). In contrast, the luminescent light from the transparent plywood“TPW@QD” shows highly scattered uniform illumination (right side inFig. 5h, andFig. S12).

4. Conclusions

In summary, transparent plywood (TPW) with tunable mechanical and optical properties has been prepared by controlling lamination angles and cellulose content. The ultimate strength in“transverse” di- rection was increased from 15 to around 45 MPa, due to lamination.

Light is transmitted through TWP and subjected to isotropic angular spreading. Incorporation of QDs, a luminescent TPW was obtained.

From the engineering point of view, the TPW can be designed with higher strength, and could be prepared in large dimensions. Mechanical modeling based on laminate plate theory was applied to predict and explain the elastic modulus for various TPW with different stacking.

Together with relatively simple processing technique, this class of material can be used as a next-generation load-bearing building mate- rial with tailorable control of daylight and even light-emitting possi- bilities. The concept could be used in applications such as veneer structures, engineering decorations, photonics, light sensor and energy storage devices, to name just a few.

Acknowledgements

We acknowledge the funding from the European Research Council Advanced Grant No. 742733, Wood NanoTech. Ilya Sychugov is ac- knowledged for support with photoluminescence experiments. Q. Fu is grateful to China Scholarship Council (CSC) for supporting his PhD study.

Appendix A. Supplementary data

Supplementary data related to this article can be found athttp://dx.

doi.org/10.1016/j.compscitech.2018.06.001.

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