Ultra-low reflection electromagnetic interference shielding nanofiber film with effective solar harvesting and self-cleaning

It is urgent to develop low-reflection electromagnetic interference shielding material to shield electromagnetic waves (EMW) and reduce their secondary radiation pollution. Herein, an electromagnetic interference shielding nanofiber film is composed of ZnO and carbon nanofiber (CNF) via electrospinning and carbonization approachs, and subsequently coating perfuorooctyltriethoxysilane as a protective layer. On the one hand, ZnO coated by porous carbon, which is derived from ZIF-8, endows the nanofiber film low reflection property through optimizing impedance matching between free space and the nanofiber film. On the other hand, the nanofiber film possesses high electromagnetic interference shielding efficiency, which is beneficial by excellent electrical conductivity of CNF derived from waste leather scraps. Furthermore, the nanofiber film involves abundant interface, which contributes to high interfacial polarization loss. Thus, the nanofiber film with a thickness of 250 μm has electrical conductivity of 53 S/m and shielding efficiency of 50 dB. The reflection coefficient of the nanofiber film is inferior to 0.4 indicates that most of EMW are absorbed inside the materials and the nanofiber film is effective in reducing secondary radiation contamination of electromagnetic waves. Fortunately, the nanofiber film exhibits outstanding solar harvesting performance (106 ℃ at 1 sun density) and good self-cleaning performance, which ensure that the nanofiber film can work in harsh environments. This work supplies a credible reference for fabricating low-reflection electromagnetic shielding nanofiber film to reduce secondary radiation pollution and facilitates the upcycling of waste leather scraps.


Introduction
With the boom of communication instruments and electronic products, the byproduct electromagnetic radiation pollution, which is harmful to human health and the normal operation of precision instruments, has attracted the attention of researchers [1,2].Electromagnetic shielding materials that can dissipate electromagnetic waves effectively via reflection type and absorption type are highly demanded [3,4].Reflection is still the main shielding method in most current works, by introducing materials with high conductivity that can generate impedance mismatching to reflect electromagnetic waves [5][6][7].However, reflection-type electromagnetic shielding materials can cause severe secondary radiation pollution of electromagnetic waves [8,9].Therefore, the development of technology for preparation of low-reflection electromagnetic shielding materials is urgently desired to reduce secondary electromagnetic radiation pollution [10][11][12].
Excellent ultra-low reflection shielding materials should be characterized by low reflection and high electromagnetic interference shielding efficiency (EMI SE) [13].According to the theory of transmission lines, electromagnetic waves in transmitting procedure could be reflected at the interface due to the poor matching between free space with the high impedance (377 Ω) and high electrical conductivity shielding materials with the low impedance.Dielectric materials with low electrical conductivity possess the relatively high impedance value, which ensure that the impedance of shielding materials can match with the impedance of free space as far as possible to decrease reflection [14].The dielectric materials derived from metal-organic frameworks (MOFs) [15][16][17], can be by coated porous carbon via carbonization, which not only decrease reflection but also enlarge the transmission path to dissipate incident electromagnetic waves.For example, Mai et al. fabricated doublelayer composite paper composed of carbonization MOF, MXene and nanocellulose, which exhibits superb absorption performance (SE A /SE T up to 75%) [18].Guo et al. prepared hybrid carbon aerogel with outstanding absorption coefficient (0.72) because of the introduction of MOF derivation [19].Nevertheless, the EMI SE of pure MOF derivatives is relatively poor [20].
The theory of electromagnetic interference shielding demonstrates that shielding efficiency is closely related to electrical conductivity and shielding efficiency of shielding materials increases with electrical conductivity of shielding materials increases [21][22][23].In order to obtain high shielding efficiency, it is feasible to integrate electrical conductivity materials into shielding materials.Carbon nanofiber (CNF) with great electrical conductivity can promote shielding efficiency, which can dissipate

Graphic abstract
electromagnetic waves by the eddy current effect [24].For instance, Karmal et al. fabricated flexible electromagnetic interference shielding film based on CNF, which exhibited outstanding performance of shielding efficiency (60 dB) [25].
Recently, CNF-based electromagnetic shielding materials derived from biomass materials, such as wood [26] and bamboo [27], have become a research hot topic, which is conducive to green, low-carbon, and sustainable development.In our previous work [28], hydrolysate of waste leather scraps (HWLS) from the leather solid waste in traditional leather industry that possesses a high carbon residue rate after carbonization, emerged superb electrical conductivity and EMI SE, indicating that leather solid waste is beneficial to the preparation of CNF.Therefore, CNF derived from waste leather scraps has good potential for elevating EMI SE, which can realize the resource utilization of waste leather scraps.
Herein, we demonstrate an excellent low-reflection electromagnetic interference shielding nanofiber film composed of ZnO and carbon nanofiber (CNF) via electrospinning and carbonization approachs, and subsequently coating perfuorooctyltriethoxysilane (POTS) as a protective layer.On the one hand, ZnO endows the nanofiber film low reflection property by optimizing impedance matching with free space.On the other hand, the nanofiber film possess high EMI SE, which is attributed to good electrical conductivity of CNF.Furthermore, ample interfaces of the nanofiber film results in strong interfacial polarization to dissipate electromagnetic waves.Consequently, the nanofiber film displays outstanding EMI SE (50 dB with a thickness of 250 μm), excellent solar harvesting (106 ℃ under 1 sun density), and self-cleaning performance.Noteworthy, the reflection coefficient is less than 0.4, suggesting that the nanofiber film can validly reduce the secondary radiation pollution of electromagnetic waves.This work provides an efficient strategy for the preparation of ultra-low reflection electromagnetic shielding nanofiber film with solar harvesting and self-cleaning properties, and facilitates the upcycling of waste leather scraps.

Fabrication of ZIF-8
Firstly, 1.5 g Zn(NO 3 ) 2 ⋅6H 2 O were dissolved in 100 mL methanol solution to form solution A. 3.3 g 2-methylimidazole were dissolved in 50 mL methanol solution to form solution B. Then, the solution A was poured into solution B to form ZIF-8 solution.The ZIF-8 solution was magnetically stirred for 6 h at 25 °C and rinsed with ethanol three times.Finally, ZIF-8 powder was obtained when at 80 °C for 24 h in a vacuum.

Fabrication of HWLS/PAN/ZIF-8 nanofiber film
In our previous study [29], HWLS were prepared using a three-step process.HWLS/PAN(HP) nanofiber film was prepared by electrospinning.Spinning solution with certain viscosity contains 0.4 g HWLS and 0.4 g PAN dissolved in DMF.Electrospinning parameters include that the distance between the needle tip and the roller collector is 20 cm, high voltage is 15 kV and the feed rate of spinning solution is 1 mL/h, respectively.HWLS/PAN/ ZIF-8(HP/ZIF-8) nanofiber film was also fabricated by electrospinning: spinning solution contains 0.4 g of HWLS, 0.4 g of PAN and certain content ZIF-8 (1-5%) dissolved in DMF.Electrospinning parameters are accordance with HP nanofiber film.
The process of fabricating ZnO@CNF nanofiber film is displayed in Fig. 1.ZnO@CNF nanofiber film with multiple layer of structures was obtained by repeating the above steps.

Fabrication of ZnO@CNF@POTS nanofiber film
To obtain a hydrophobic surface, the ZnO@CNF nanofiber film is further coated with a thin layer of POTS through a immersion procedure (15 min in a POTS solution of 40 mg mL −1 ).The final sample, called ZnO@ CNF@POTS nanofiber film, was obtained after drying at 90 °C for 15 min.

Material characterization
Thermal stability of HP and HP/ZIF-8 nanofiber film at N 2 atmosphere heating rate of 10 °C/min was studied utilizing Thermogravimetric analyzer (TGA, Pyris I, PerkinElmer).The surface appearance of nanofiber were measured by scanning electron microscopic (Hitachi S4800 SEM scope).The X-ray diffraction (XRD) and X-ray photoelectron spectra (XPS) were employed to analyze the crystal structure and chemical bond.The graphitization degree of nanofiber film was characterized by Raman spectra (WITecAlpha-300R, Germany).The EMI SE of nanofiber film was investigated on a vector network analyzer (N5230A, Agilent, USA) in the X band (8.2-12.4GHz).The obtained scattering parameters (S 11 and S 21 ) were used to calculate the reflectivity coefficients(R), absorption coefficients(A), transmission coefficients(T), total shielding efficiency (SE T ), reflection loss (SE R ), absorption loss (SE A ) and multiple internal reflection loss (SE M ) (SE M can be negligible when SE T > 15 dB).The calculated equations are as the following: (1)

Morphology and structure of HWLS/PAN/ZIF-8 nanofiber film
TEM images and SEM images are used to observe morphology of ZIF-8 and nanofiber.As shown in Fig. 2a, ZIF-8 is clearly a rhombic dodecahedron shape, measuring 100-150 nm. Figure 2b-e shows the SEM images of the nanofiber film surface morphology with different content of ZIF-8.ZIF-8 is embedded in the nanofibers, and the fiber forms a bead-string structure.As the content of ZIF-8 increased, the coating of ZIF-8 in the fiber gradually increased.Among them, when at 3% ZIF-8, the nanoparticles are relatively uniform.When the content is more than 3%, the fiber thins.When at the 5% content, the ZIF-8 agglomerates on the fiber surface.The result demonstrates that when the ZIF-8 content is too excessive, there is severe agglomeration in the spinning solution, which is to the disadvantage of the formation of uniform fibers.
(   obvious diffraction peaks can be seen due to the amorphous state of extracted HWLS.
The tensile strength of HP/ZIF-8 nanofiber film with different ZIF-8 contents is illustrated in Fig. 3b.When the content is only 1%, the tensile strength can reach 8.21 MPa.With the content increasing, the tensile strength of HP/ZIF-8 nanofiber film increased firstly and then decreased.The maximum is 10.58 MPa, when the ZIF-8 content was 3%.The trend emerges is due to that a little of ZIF-8 can improve the rigidity of nanofibers but the agglomeration of superfluous ZIF-8 make nanofibers uneven.Therefore, to obtain high tensile strength, nanofiber film with 3% ZIF-8 content is selected for subsequent experiments.
The TG results of HP and HP/ZIF-8 nanofiber film are depicted in Fig. 3c, which is used to evaluate their thermal stability.Below 200 ℃, the thermal weight loss is 6.5 wt%, mainly due to the release of bound water and DMF solvent in the film.The thermal weight loss at 300 °C-500 °C is caused by PAN cyclization, decomposition, and carbonization and all materials were further carbonized at 600-800 °C.Compared with HP nanofiber film, HP/ ZIF-8 nanofiber film express better thermal stability due to the introduction of ZIF-8.

Morphology and structure of ZnO@CNF nanofiber film
HP/ZIF-8 nanofiber film was carbonized to produce ZnO@CNF nanofiber film, forming the structure of ZnO coated by porous carbon in this process.This structure provides the abundant interface between ZnO and carbon materials, thus contributing to the interface polarization loss.The distribution of ZnO nanoparticles in carbon fibers is uniform as seen by the TEM and EDS mapping in Fig. 4, which reveals the ZnO@CNF nanofiber film consists of carbon nanofibers and ZnO nanoparticles.As can be observed from Fig. 4b-d, the elements C, Zn, and O are uniformly distributed across the sample.
XRD further determined the crystal phases of the ZnO@CNF nanofiber film as displayed in Fig. 5a.As can be seen from Fig. 5a, two obvious characteristic peaks located at 2θ = 31.8°and 36.3°can be assigned to (100) and (101) facets of metallic ZnO (PDF 36-1451) respectively.Figure 5b shows the Raman spectrum of ZnO@CNF nanofiber film with different carbonization temperatures.There are two characteristic peaks associated with D bands and G bands near 1375 cm −1 and 1570 cm −1 .The D band is in connection with the double resonance Raman process of disordered carbon by defects in the carbon structure and the G band illustrates the plane vibration of carbon atoms, which is recognized as the highly oriented structure.As the increase of temperature from 650 to 850 ℃, the peak intensity ratio of the D band to the G band decreased gradually, which demonstrates that the carbonization Fig. 4 Morphological characterization: TEM image of the ZnO@CNFnanofiber film (a); EDS images and the element mapping analysis of ZnO@CNF nanofiber film (b-d) treatment repairs the structural defects and improves the graphitization degree of the nanofiber film.
XPS is used to investigate the bonding configurations and surface chemical states of ZnO@CNF nanofiber film.Figure 5c shows the full survey XPS spectra, which reveal the existence of C, O, Zn, and N elements.The minute information about bond structure of C, Zn, and N elements were shown in Fig. 5d-e.The characteristic peaks of C1s in ZnO@CNF nanofiber film are 284.6,285.8, and 288.6 eV, respectively, corresponding to C-C, C-O, and C = O bonds respectively.Satellite peak of C has demonstrated the existence of graphite C, which is beneficial Fig. 5 XRD spectra of CNF nanofiber film and ZnO@CNF nanofiber film with different carbonization temperature (a).The Raman spectra of CNFs and ZnO@CNF nanofiber film (b).The full survey XPS spectra of ZnO@CNF nanofiber film (c).The XPS spectra of ZnO@CNF nanofiber film and C, Zn, and N elements (d-f).
to improve shielding efficiency.The characteristic peaks of Zn 2p in ZnO@CNF nanofiber film are 1021.9eV and 1045.1 eV corresponding to Zn 2p 3/2 and Zn 2p 1/2 , respectively.The Zn 2p3/2 peak located at 1021.9 eV indicates the existence of ZnO, which can decrease reflection of ZnO@CNF nanofiber film.The characteristic peaks of N 1s in ZnO@CNF nanofiber film are 398.5 eV, 400.1 eV, and 401.2 eV associated with pyridinic N, pyrrolic N, and graphitic N respectively and the graphitic N is conducive to the improvement of electrical conductivity and shielding efficiency.

Electrical properties and EMI shielding performances
of ZnO@CNF nanofiber film

Single layer of ZnO@CNF nanofiber film
As is known to all, high electrical conductivity is very important for EMI shielding performance.The electrical conductivity performance of ZnO@CNF nanofiber film at various carbonization temperatures is exhibited in Fig. 6a.As the temperature of carbonization increases, the conductivity increases firstly and then decreases.The maximum conductivity was 53 S/m at a carbonation temperature of 800 °C, which was consistent with the Raman results.This is due to the fact that the increasing of the carbonization temperature result in the improvement of graphitization degree, which generates additional graphitic carbon and ordered structure inside the fiber.Therefore its conductivity at this moment reaches the peak.However, as the temperature increases to 850 °C, the conductivity decreases marginally due to the growing number of pores and defects on the fiber surface that break the crosslinking point between nanofibers and hinder the formation of a complete conductive network.Furthermore, when the carbonization temperature was 800 °C, the ZnO@CNF nanofiber film was connected to a circuit and loaded with a voltage of 3 V to try to light up a small bulb with a power of 3W (insert Fig. 6a).This confirmed the ZnO@CNF nanofiber film possesses the high electrical conductivity, ensuring its great shielding properties.
Figure 6b demonstrates the electromagnetic interference characteristics of ZnO@CNF nanofiber films at different carbonization temperatures in the x-band.The electromagnetic shielding efficiency of the nanofiber film is up to 27.4 dB at a carbonization temperature of 800 °C.To reveal the electromagnetic shielding mechanism, SE T , SE A , and SE R of the ZnO@CNF nanofiber film in the x-band are analyzed.As displayed in Fig. 6c, it can be seen that the SE T , SE R , and SE A of the ZnO@CNF nanofiber film are ~ 27.4 dB, ~ 2.3 dB, and ~ 25.1 dB, respectively (−86% SE T at 8.4 GHz).As the carbonization temperature increases, the SE A of the ZnO@CNF nanofiber film is all higher than the SE R , proving that absorption is the main contributor to shielding electromagnetic waves.
Furthermore, the reflectivity (R), transmittance (T), and absorption coefficient (A) of the CNF and ZnO@ CNF nanofiber film under carbonization temperature of 800 °C were calculated to demonstrate the type of shielding.As shown in Fig. 6d, the A values of CNF and ZnO@CNF nanofiber film are 0.32 and 0.60, respectively.When the ZIF-8 is converted into ZnO nanoparticles/carbon composites, the A value of the ZnO@ CNF nanofiber film was significantly increased, indicating the high EM waves absorption capacity of the ZnO@CNF nanofiber film.Compared to pure CNF nanofiber film, the improvement in A of the ZnO@CNF nanofiber film can be attributed to the interface polarization between ZnO and carbon materials.

Multiple layers of ZnO@CNF nanofiber film
Figure 7a presents the EMI shielding efficiency of ZnO@ CNF nanofiber film substantially increases with the total thickness, which is consistent with the theory of EMI shielding.As seen from Fig. 7b, the SE A values of ZnO@ CNF nanofiber film are always superior to SE R regardless of the number of layers, suggesting the shielding mechanism for ZnO@CNF nanofiber film is primarily determined by absorption.Figure 7c, d further highlight the outstanding advantages of ZnO@CNF nanofiber film.The EMI shielding efficiency and the thickness of ZnO@ CNF nanofiber film are compared with similar materials recently reported in Fig. 7c.Notably, the ZnO@CNF nanofiber film exhibits better EMI shielding efficiency with thinner thickness than recent reports of EMI shielding composites, such as MXene/FeCo [30], TPU/MXene [31], C@CoFe [32], cot@PDA/CNTs [33], LSW/PVA/ PANI [34], Silicone rubber/Ag [35], CNF/Ag [36], MWC-NTs-PAN [37].And Fig. 7d exhibits the thin thickness (left) and the light weight (right) of ZnO@CNF nanofiber film.

EMI shielding mechanism of ZnO@CNF nanofiber film
For the sake of analyzing the potential EMI shielding mechanism of the EM absorption behavior of ZnO@CNF nanofiber film, the complex permittivity (ε r = ε′−jε″) and complex permeability (μ r = μ′−jμ″) in the x-band are measured.Thereinto, the real part (ε′, μ′) stands for the storage capabilities of electric and magnetic energy, and the imaginary part (ε″, μ″) stands for the loss capabilities of electric and magnetic energy [38,39].Figure 8a, b displays the relative complex permittivity and permeability in the x-band.Definitely, the ε′ and ε′′ of the ZnO@ CNF are mainly distributed in the range of 24-35, which is beneficial to achieve EMI shielding.The μ′′ of ZnO@ CNF is 0, which proves that the electromagnetic shielding material is not magnetic.As we all know, the dielectric tangent loss (tan δε = ε″/ε′) and magnetic tangent loss (tan δµ = µ″/µ′) are the dominant principles for assessing the dielectric property.As seen from Fig. 8c, the film possesses the excellent tan δε value, implying its extraordinary dielectric loss and effective capacity of energy conversion [40].As noted above, the absorption of electromagnetic waves by ZnO@CNF nanofiber film is through the effect of dielectric loss.
Figure 8d displays the morphology of single fiber, which reveals ZnO nanoparticles were coated by carbon fibers.And the electromagnetic shielding mechanism of ZnO@CNF nanofiber film is shown in Fig. 8e.
When the incident EM waves reach the surface of ZnO@ CNF nanofiber film, most of EM waves can be absorbed into the film due to the introducing of ZnO, which can optimize the impedance matching with free space and decrease reflection property.EM waves in the ZnO@CNF nanofiber film can be dissipated by the conductive loss of CNF, the dielectric loss of ZnO, and the polarization loss at pore interface.Meanwhile, the conductivity network composed of one dimension nanofiber and porous structure can provide transmission route for EM waves to dissipate EM waves.This further manifests that the ZnO@ CNF nanofiber film can be suitable for the preparation of electromagnetic shielding-related products.

Solar harvesting performances of ZnO@CNF nanofiber film
ZnO @ CNF nanofiber film has good self-heating properties under low-temperature conditions and has anti-freezing, anti-ice, and anti-fog effects in practical application.The photothermal performance of the ZnO@CNF nanofiber film is also explored.The ZnO@ CNF film is cut into a circle with a diameter of 2 cm and placed under xenon lamp.The change of its surface temperature is recorded by K-type thermocouple (SMART AS887, China), and infrared images are taken by thermal IR imaging device (DS-2TPH16-6AVF/W, China).As shown in Fig. 9a, when ZnO@CNF nanofiber film comes Fig. 8 Real part and imaginary part of the permittivity, permeability, tan ε and tan μ vs frequency of ZnO@CNF nanofiber film (a-c).TEM image of ZnO@CNF nanofiber film (d).The EMI-shielding mechanism of the ZnO@CNF nanofiber film (e) into light at 1 sun irradiation (100 mW/cm 2 ) for 160 s, the ZnO@CNF nanofiber film both exhibited excellent photothermal properties at temperatures ranging from room temperature to 106 °C within 50 s.The ZnO@ CNF nanofiber film has the superb absorbing capacity to photon energy through coherent oscillations of surface electrons [41,42].Furthermore, cycling stability of solar harvesting is tested to evaluate the thermal shock resistance of ZnO@CNF nanofiber film, the reproducible heating and cooling profiles are almost identical, as evidenced in Fig. 9b, suggesting ZnO@CNF nanofiber film possesses excellent photothermal cycle stability.In summary, the integrated excellent photothermal properties will further provide a broad potential application for ZnO@ CNF nanofiber film with high electromagnetic interference shielding performance.Figure 9c demonstrates the schematic diagram of the photothermal experiment.The ZnO@CNF nanofiber film can capture the photometric energy of the xenon lamp, and then convert it into heat energy, which can ensure the self-heating capability without electricity.

Self-cleaning and environmental stability of ZnO@ CNF@POTS nanofiber film
Humidity in the external environment can negatively affect the performance of the electronic equipment.Consequently, the hydrophobicity of the surface of ZnO@CNF@POTS nanofiber films was examined.Seen from Fig. 10a, b, the ZnO@CNF nanofiber film has the water contact angles of 145° and the ZnO@CNF@POTS nanofiber film is 150.5°,respectively.The POTS layer can further augment the hydrophobicity of ZnO@CNF@ POTS nanofiber film surface due to low surface energy.In addition, self-cleaning performance was tested in Fig. 10c.ZnO@CNF@POTS nanofiber film was placed in a clean slide, coffee powder as the simulated dirt was on the surface of the nanofiber film, and then rinsed with a steady current.It can be observed that coffee powder was washed off the surface of the nanofiber film, which demonstrates that ZnO@CNF@POTS nanofiber film has an outstanding self-cleaning performance.
To evaluate environmental stability, shielding efficiency and contact angles were tested after soaking the nanofiber film in deionized water (neutral surroundings), 1 M NaCl solution (body sweat), HCl solution of pH = 1 (strong acid), and NaOH solution of pH = 14 (strong basic) for different days.From Fig. 10d-e, it can be seen that after soaking in different surroundings for 7 days, the contact angle of ZnO@CNF@POTS nanofiber film is kept above 140° and the electromagnetic shielding efficiency is more than 20 dB that satisfy the standard of business electromagnetic shielding materials, which demonstrates that the ZnO@CNF@POTS nanofiber film has excellent environmental stability.

Conclusions
In summary, a concise and expedient preparation tactic to fabricate electromagnetic shielding film with ultra-low reflection, via electrospinning and carbonization, which incorporates EMI SE, solar harvesting, and self-cleaning properties.On the one hand, ZnO coated by porous carbon, which is derived from ZIF-8, endows the nanofiber film low reflection property through optimizing impedance matching between free space and the nanofiber film.On the other hand, the nanofiber film possesses high electromagnetic interference shielding efficiency, which is beneficial by excellent electrical conductivity of CNF derived from waste Fig. 9 The photothermal response of ZnO@CNF nanofiber film in environmental situations under light irradiation of 100 mW/cm 2 (a).Insets are the corresponding IR images.Temperature-time curve of ZnO@CNF nanofiber film under repeated heating-cooling cycles (b).Schematic diagram of heat generation by the solar harvesting effect of ZnO@CNF nanofiber film under light irradiation (c) leather scraps.Furthermore, the nanofiber film involves abundant interface, which contributes to high interfacial polarization loss.The ZnO@CNF nanofiber film offers outstanding EMI SE (50 dB), ultra-low reflectivity (R < 0.4), excellent solar harvesting (106 ℃ at 1 sun), and self-cleaning performances.It is believed that the electromagnetic shielding film provides a pathway for decreasing the secondary radiation pollution of electromagnetic waves.

Fig. 1
Fig. 1 Schematic diagram of the fabrication of ZnO@CNF nanofiber film

Fig. 6
Fig. 6 Electrical conductivity (a), EMI shielding efficiency (b) and average SE (SE T , SE A , SE R ) (c) of ZnO@CNF nanofiber film at various carbonization temperatures.Comparison of the R, T, A values of composite film at X-band (d)

Fig. 7
Fig. 7 EMI shielding efficiency (a) and SE R , SE A and SE T (b) of ZnO@CNF nanofiber film with different layers.Comparison of EMI shielding efficiency and thickness of composite film in published work (c).The image representation of thickness and weight (d)