Multi-walled Carbon Nanotubes Network Research Articles

High-performance and low-cost manganese oxide/multiwalled carbon nanotubes composite as cathode material for aqueous magnesium ion battery

• The λ-MnO 2 /MWCNTs composite was synthesized with feasible solution method and applied as cathode in aqueous magnesium ion battery for the first time. • The λ-MnO 2 particles are distributed in MWCNTs network structure, which increased the specific surface area and conductivity of electrode material. • The λ-MnO 2 /MWCNTs composite showed good electrochemical performances in three electrode and MnO 2 /MWCNTs//AC full cell systems. • The mechanism of magnesium ion insertion/deinsertion in λ-MnO 2 /MWCNTs electrode was investigated in this paper. The aqueous magnesium ion batteries (AMIBs) with the advantages of low cost, intrinsical safety and environmentally friendly have received worldwide attention as promising candidates for green energy storage system. Herein, we report a composite material engineered from λ-MnO 2 particles and multiwalled carbon nanotubes (MWCNTs) as cathode for AMIB. The λ-MnO 2 possesses a spinel-type crystalline structure, which is beneficial for the insertion/deinsertion of Mg 2+ ions. The MWCNTs network shorts ionic diffusion distance and increases the interface of electrode material/electrolyte. Hence, the λ-MnO 2 /MWCNTs composite demonstrates a large capacity of 251 mAh g −1 at 50 mA g −1 in 0.5 M MgCl 2 solution, and high capacity retention of 86.2 % after 1000 cycles at 1000 mA g −1 in MgSO 4 solution. Furthermore, the λ-MnO 2 /MWCNTs//AC system is firstly assembled as full AMIB, which delivers a specific capacity of 84.8 mAh g −1 at 100 mA g −1 . The rate capability test is performed at 300 mA g −1 . This system displays a gradually rise tendency in the specific capacity at initial 100 cycles on account of the activation process. After 1000 cycles, the specific capacity is 56.9 mAh g −1 . This work develops an easily prepared, low-cost and good performance electrode material for AMIB system.

Conducting polymer ink for flexible and printable micro-supercapacitors with greatly-enhanced rate capability

Conducting polymer ink holds great promise for wearable, flexible, and printable high-power-density micro-supercapacitors (MSCs) but chronically suffers from the poor rate capability ascribing to naturally electron-blocked transferring. Herein, we devise an air-stable, easy-fabricating and rapid electron-transferring polyaniline ink by embedding conductive carboxylic multi-walled carbon nanotubes (C-MWCNTs) networks into polyaniline nanosheets. Due to the optimized electron-transferring kinetics, the rate capability of polyaniline ink is significantly increased by 73.7%. Additionally, the large-scale printable MSCs based on this ink deliver remarkable energy density of 2.6 mWh cm −3 , large areal capacitance of 45.4 mF cm −2 and excellent mechanics-electrochemistry stability with 84.6% capacitance retention against 1000 consecutive bending cycles. Evidently, this work provides the polyaniline ink for large-scale, printable, and flexible MSCs, which can underpin the next generation printed electronics in the approaching era of Internet of Things. Conducting polymer ink with greatly enhanced rate performance is successfully devised by embedding carboxylic MWCNTs into polyaniline nanosheets for scalable printing of high-performance micro-supercapacitors. 1. Conducting polymer ink is devised for scalable printing of micro-supercapacitor. 2. Rate capability is enhanced by 73.7% through embedding carboxylic MWCNTs. 3. Micro-supercapacitor with high energy density and good cyclic stability. 4. Capacitive contribution plays a dominant role according to quantitative calculation. 5. Proof-of-concept demonstration of various electronics as practical application.

Enhanced performance of Si/PEDOT: PSS heterojunction solar cells via multi-walled carbons coated with polydopamine

n-Si/PEDOT: PSS heterojunction solar cells are expected to be the alternative for photovoltaic technique for the potentially excellent power conversion efficiency and ease of processing. Herein, a multi-walled carbon nanotubes network coated with polydopamine (MWCNTs&PDA) was applied as an interlayer between silicon substrate and PEDOT:PSS film to enhance the performance of Si/PEDOT:PSS heterojunction device in this study. When the film was applied as an interlayer, the average reflectance of the device within the wavelength range 400–1000 nm dropped from 13.6 % to 8.33 %. The enhanced antireflection ability meant more photon-generated carriers were excited. The introduction of PDA reduced the contact angle from 84.5° to 47° on Si surface, and the minority carrier lifetime (τ) was 2.16 times increase. The high charge separation and improved junction effect of MWCNTs&PDA films incorporated between PEDOT:PSS and Si substrate result in advantageous networks of charge carriers, which is confirmed by the excellent open-circuit voltage (Voc) 637 mV, short-circuit current density (Jsc) Jsc 34.76 mA/cm2 and fill factor (FF) 56.41 %. The power conversion efficiency improved to 12.49 % when the MWCNTs&PDA film was incorporated. The achievements of this study indicated that the MWCNTs coated with polydopamine as an interlayer is a promising material for excellent performance Si/PEDOT:PSS heterojunction solar devices.

Correlation Analysis of Heat Curing and Compressive Strength of Carbon Nanotube–Cement Mortar Composites at Sub-Zero Temperatures

Concrete curing under sub-zero temperatures causes various problems, such as initial cracking and a decrease in mechanical strength. This study investigated the effect of sub-zero ambient temperature and multi-walled carbon nanotube (MWCNT) content on the heat and strength characteristics of heat-cured MWCNT cementitious composites. The experimental parameters were the application of heat curing, MWCNT content, use of an insulation box to achieve a closed system, and ambient temperature. The results showed that the internal temperature change of the MWCNT cementitious composite increased with the ambient temperature and MWCNT content. When an insulation box was installed, the maximum temperature change of the MWCNT cementitious composite during curing increased. Furthermore, heat curing increased the compressive strength of the cementitious composite. Moreover, a microstructure analysis using field-emission scanning electron microscopy verified the formation of a MWCNT network among the cement hydrates.

Engineering the electrospinning of MWCNTs/epoxy nanofiber scaffolds to enhance physical and mechanical properties of CFRPs

A cost-effective approach to improve the physical and mechanical properties of carbon fiber reinforced polymer (CFRP) prepreg composites, where electrospun multiwalled carbon nanotubes (MWCNTs)/epoxy nanofibers were synthesized and incorporated in between the layers of conventional CFRP prepreg composite has been presented. MWCNT-aligned epoxy nanofibers were successfully produced by an optimized electrospinning process. Nanofibers were deposited directly onto prepreg layers to achieve improved adhesion and interfacial bonding, leading to added strength and improvements in other mechanical properties. Thus, interlaminar shear strength (ILSS) and fatigue performance at high-stress regimes increased by 29% and 27%, respectively. Barely visible impact damage (BVID) energy increased significantly by up to 45%. The thermal and electrical conductivities were also enhanced significantly due to the presence of the highly conductive MWCNT networks between the CFRP layers. The presented method was capable of uniformly depositing high contents of MWCNTs at interlaminar ply interface of prepregs to strengthen/enhance CFRP properties, which has not been previously shown to be possible due to high resin viscosity caused by randomly oriented MWCNTs in epoxy system.

Thermoplastic polyurethane/multiwalled carbon nanotubes nanocomposites: Effect of nanoparticle content, shear, and thermal processing

AbstractIn this study, nanocomposites of thermoplastic polyurethane and multiwalled carbon nanotubes (MWCNTs) with varying nanofiller content (ranging from 0 wt% to 1 wt%) were prepared via the melt compounding method. Moreover, the influence of shear field and thermal processing on electrical conductivity has been evaluated. The evaluation of the phase separation degree revealed that with the increase in the nanofiller content from 0% to 0.4%, the phase separation degree increased by 25%. However, a further increase in the nanoparticle content slightly decreased the phase separation degree. Moreover, by increasing the nanofiller content up to 0.4%, the melting temperature and the melting enthalpy of the soft phase as well as the melting temperature of the hard phase increased. With the increase in the nanofiller content to 0.4%, a 3D network of MWCNTs was developed, corroborating the formation of an electrically conductive nanocomposite. The conductivity increased 3750‐fold in the quenched and 5000‐fold in the annealed samples with the increase in the nanofiller content from 0.2% to 1%. In general, the annealed nanocomposites featured lower conductivity than the quenched ones. The effect of the shear on conductivity was nanofiller content‐dependent. Exposure to shear below and above the percolation threshold decreased and increased the electrical conductivity, respectively.

High-Performance and Low-Cost Manganese Oxide/Multiwalled Carbon Nanotubes Composite as Cathode Material for Aqueous Magnesium Ion Battery

The aqueous magnesium ion batteries (AMIBs) with the advantages of low cost, intrinsical safety and environmentally friendly have received worldwide attention as promising candidates for green energy storage system. Herein, we report a composite material engineered from λ-MnO2 particles and multiwalled carbon nanotubes (MWCNTs) as cathode for AMIB. The λ-MnO2 possesses a spinel-type crystalline structure, which is beneficial for the insertion/deinsertion of Mg2+ ions. The MWCNTs network shorts ionic diffusion distance and increases the interface of electrode material/electrolyte. Hence, the λ-MnO2/MWCNTs composite demonstrates a large capacity of 251 mA h g-1 at 50 mA g-1 in 0.5 M Mgcl2 solution, and high capacity retention of 86.2% after 1000 cycles at 1000 mA g-1 in MgSO4 solution. Furthermore, the λ-MnO2/MWCNTs//AC system is firstly assembled as full AMIB, which delivers a specific capacity of 84.8 mA h g-1 at 100 mA g-1. The rate capability test is performed at 300 mA g-1. This system displays a gradually rise tendency in the specific capacity at initial 100 cycles on account of the activation process. After 1000 cycles, the specific capacity is 56.9 mA h g-1. This work develops an easily prepared, low-cost and good performance electrode material for AMIB system.

Hydrogen Nanometrology in Advanced Carbon Nanomaterial Electrodes.

A comparative experimental study between advanced carbon nanostructured electrodes, in similar hydrogen uptake/desorption conditions, is investigated making use of the recent molecular beam-thermal desorption spectrometry. This technique is used for monitoring hydrogen uptake and release from different carbon electrocatalysts: 3D-graphene, single-walled carbon nanotube networks, multi-walled carbon nanotube networks, and carbon nanotube thread. It allows an accurate determination of the hydrogen mass absorbed in electrodes made from these materials, with significant enhancement in the signal-to-noise ratio for trace hydrogen avoiding recourse to ultra-high vacuum procedures. The hydrogen mass spectra account for the enhanced surface capability for hydrogen adsorption in the different types of electrode in similar uptake conditions, and confirm their enhanced hydrogen storage capacity, pointing to a great potential of carbon nanotube threads in replacing the heavier metals or metal alloys as hydrogen storage media.

Tin phosphide nanoparticles loaded on multi-walled carbon nanotubes networks as a superior anode material for lithium ion batteries

The development and design of the anode materials has become one of research hotspots for high-performance Lithium ion batteries (LIBs) currently. Sn4P3 has a layer crystal structure, and its special crystal structure endow it superior electrochemical performance, such as high theory capacity, better electron conductivity etc, which is thought to be a promising anode material for LIBs. But the rapid capacity attenuation caused by the huge volume change and tin nanoparticles (NPs) agglomeration during the cycling process greatly limits its application. Herein, tin phosphide/ multi-walled carbon nanotubes (Sn4P3/MWCNTs) is synthesized successfully by a facile in-situ solvothermal method. As the anode for LIBs, the Sn4P3/MWCNTs composite shows a discharge capacity of 783mAhg−1 after 100 cycles at the current density of 100 mA g−1. Even at a current density of 1Ag−1 for 500 cycles, the reversible specific capacity still could be remained at 403mAhg−1. The cycling performance of Sn4P3/MWCNTs at 100 and 500mAg−1 after 100 cycles, which are far superior to that of bare Sn4P3 and MWCNTs. These excellent electrochemical performances are originated from the synergistic effect between MWCNTs and Sn4P3. MWCNTs is help to enhance the conductivity and serve as a framework to disperse Sn4P3 NPs, further it can alleviate the large volume change to improve cycle stability during the charge/discharge process, while Sn4P3 NPs can contribute high specific capacity. In addition, the pseudocapacitance capacity contribution can explain the stable Li+ storage and good rate performance of Sn4P3/MWCNTs composite. Hence, Sn4P3/MWCNTs suggests a huge potential as anode materials for low cost and high- performance LIBs.

MgMn2O4/multiwalled carbon nanotubes composite fabricated by electrochemical conversion as a high-performance cathode material for aqueous rechargeable magnesium ion battery

This work reports a composite material consisting of MgMn2O4 particles and multi-walled carbon nanotubes (MWCNTs) for aqueous magnesium ion batteries (AMIBs), which is prepared with electrochemical conversion method. The electrochemical conversion method has the advantages of easy accessibility, mild condition and small structural failure of electrode material. The obtained MgMn2O4/MWCNTs displays good surface integrity and structure stability, which improves electrochemical performance. Simultaneously, MWCNTs network serves as the pathways of Mg2+ ions and electrons not only shorts the ion migration path but also enlarges the interface of electrode/electrolyte. The MgMn2O4/MWCNTs exhibites the advantages of high conductivity, large charge-discharge capacity and good rate performance. The obtained MgMn2O4/MWCNTs shows a high discharge capacity of 412.9 mAh g−1 at 50 mA g−1, which is close to two times of that for the MgMn2O4 obtained by sol-gel method, and the capacity retention is 73.3% after 1000 cycles at 1000 mA g−1. In addition, the MgMn2O4/MWCNTs//AC system is firstly assembled, which displays a specific discharge capacity of 94.5 mA h g−1 at 50 mA g−1. Furthermore, the relevant electrochemical mechanism of Mg2+ ion insertion/extraction can be explained by the change of manganese valence, which is investigated with ex-situ XRD and XPS measurements. This work expands the compound method of high-performance electrode material for rechargeable aqueous magnesium ion battery.

Performance Enhancement of Inkjet Printed Multi‐Walled Carbon Nanotubes Inks using Synthetic and Green Surfactants

AbstractWhile the efficiency of synthetic surfactants in controlling the dispersion behaviour of nanomaterials in aqueous solution has been well documented, issues on environmental impact remain unsolved. The aim of this study is to synthesize multi‐walled carbon nanotubes (MWCNTs) inks using green surfactants and their chemical, physical, and electrical performances are compared to those of synthetic surfactant inks. The conductive ink is synthesized by dispersing MWCNTs in surfactant solution (gum Arabic [GA], alkali lignin [AL], polyvinylpyrrolidone [PVP] or pluronic F‐127 [PL]) with various concentrations. Among four investigated surfactants, GA/MWCNTs and PVP/MWCNTs inks are selected for further investigation due to their excellent dispersion stability. The printability of both inks is assessed by analyzing the quality of the conductive patterns printed with different inkjet printers theoretically and experimentally. It is found that multi‐nozzle printer is capable of developing printed patterns with lower sheet resistance compared to single‐nozzle printer. Overall, 5 GA/MWCNTs ink recorded as the most stable ink with only 14.4% reduction of absorbance after 30 days and with zeta potential value of −40.3 ± 5.36 mV. Ten layers of 5 GA/MWCNTs ink printed on PVA demonstrate a dense MWCNTs networks, which contribute to the lowest surface resistance of 3.0 kΩ sq−1.

Extreme Energy Dissipation via Material Evolution in Carbon Nanotube Mats.

Thin layered mats comprised of an interconnected meandering network of multiwall carbon nanotubes (MWCNT) are subjected to a hypersonic micro‐projectile impact test. The mat morphology is highly compliant and while this leads to rather modest quasi‐static mechanical properties, at the extreme strain rates and large strains resulting from ballistic impact, the MWCNT structure has the ability to reconfigure resulting in extraordinary kinetic energy (KE) absorption. The KE of the projectile is dissipated via frictional interactions, adiabatic heating, tube stretching, and ultimately fracture of taut tubes and the newly formed fibrils. The energy absorbed per unit mass of the film can range from 7–12 MJ kg−1, much greater than any other material.

Evaluation and modeling of electrical conductivity in conductive polymer nanocomposite foams with multiwalled carbon nanotube networks

Conductive filler/polymer composite foams (CPC) with micro-/nano-scale bubbles have attracted much attention due to their various advantages. Understanding the evolution of electrical behavior in CPC foams is important for their functional application. There have been many experimental investigations of the electrical behavior of CPC foams, but how the electrical conductivity varies with bubble growth is not fully understood. Here, a novel resistive network model based on Monte Carlo simulation to directly predict the electrical conductivity of CPC foams is reported. The model uses multiwalled carbon nanotubes (MWCNTs) as a conductive filler and considers both the intrinsic electrical conductivity of the nanotubes and the electron tunneling effect. The model predictions agreed very well with the experimental data for both the solid and foamed composites. The results showed that the percolation threshold and the conductivity are strongly affected by the relative sizes of MWCNT length and bubble diameter. At relatively low void fractions (up to ~20%), the growth of bubbles with size comparable to the MWCNT length increased the conductivity. Further growth of cells (>30%) resulted in a gradual decrease in the conductivity. This model can be used in the design and optimization of conductive foams.

Resistance-strain sensitive rubber composites filled by multiwalled carbon nanotubes for structuraldeformation monitoring

In this article, multiwalled carbon nanotube/natural rubber composites with resistance-strain sensitivity were prepared by solution method, when the electrical percolation threshold of multiwalled carbon nanotube is only ∼3.5 wt%. The mechanical properties and resistance-strain response sensitivity were studied and analyzed systematically. The dispersion of multiwalled carbon nanotubes in the natural rubber matrix was characterized by field-emission scanning electron microscope and X-ray diffractometer. The composite exhibits good deformation sensitivity (gauge factor >27), large strain sensing range (>200%), and high signal stability when multiwalled carbon nanotube content was appropriate. The composite is suited to application in strain monitoring of large deformation structures since the resistance-strain response is more stable when strain exceeds 100%. To understand the mechanism of the resistance-strain response, the ‘shoulder peak’ of resistance-strain curve was researched and explained by the digital image correlation method, and an analytical model was developed when considering the effects of electronic tunneling and hopping in multiwalled carbon nanotube networks. Both experiment and analytical results confirm the break-restructure process of multiwalled carbon nanotube networks under applied strain cause the resistance-strain response. Finally, the practical application of the composite to monitoring strain load of rubber isolation bearing was realized.

Carbon nitride quantum dots tethered on CNTs for the electrochemical detection of dopamine and uric acid

A 0D–1D CNQDs/f-CNT architecture composed of 0D CNQDs tethered on a 1D functionalized multiwalled carbon nanotube (f-CNT) network was used for dopamine sensing.

Additive manufacturing of PA12 carbon nanotube composites with a novel laser polymer deposition process

AbstractThe facile manufacture of PA12 MWCNT/silica (50/50 by weight) nanocomposite powders through a high energy mixing process is presented, which are useful to generate 3D objects by a novel Laser Polymer Deposition (LPD) process. The mixing as well as the LPD process led to no discernible changes in the material properties (DSC, SEM, LD) of the core‐shell nanocomposites, enabling the recycling of unconverted powder. The built parts yield ultimate tensile stresses and Young's modulus at 10%–20% of the bulk material. Partially unmolten particles and voids were identified as the main mechanical failure mechanism in the built parts. The mechanical properties are better with low additive content (Young's modulus: 89.8 ± 5.4 MPa; UTS: 12.9 ± 5.3 MPa with 0.25 wt% additives). Electronic conductivity up to the region of moderate conductivity could be achieved by multiwalled carbon nanotube (MWCNT) network formation (8 × 10−4 S cm−1 at 1.25 wt% of additives). A variant of the processing strategy revealed that a higher mechanical strength can be achieved by a laser induced remelting of the traces following their initial construction.

Engineering carbon nanotubes enhanced hydrophobic membranes with high performance in membrane distillation by spray coating

The commonly used commercial hydrophobic membranes are facing great challenges in wetting and low permeation when used in membrane distillation (MD) because of the limited materials and structures. To address these problems, we herein fabricated a kind of carbon nanotubes (CNTs) enhanced membrane with high flux and stable performance in membrane distillation, where a porous hydrophobic network of multi-walled carbon nanotubes (MWCNTs) was spray-coated onto the commercial flat-sheet hydrophobic membrane surface. Then, we systematically investigated the impact of the CNTs loading on the substrate surface properties, membrane structures and MD performance (flux and rejection), and experimentally illustrated the role of CNTs towards improving MD performance. Results illustrate that there is an optimal match of the CNTs spray amount with the substrate pore size and the substrate materials to achieve the maximum specific surface area of membrane, and thus the maximum flux and salt rejection of the resultant CNTs-enhanced membrane, which is governed by the interfacial energy of the CNTs and the substrate, and thus the CNTs packing density. Results find that the CNTs packing density decreases with the decrease of the substrate free surface energy, and the loosest CNTs packing is formed on the polytetrafluoroethylene (PTFE) membrane with the lowest free surface energy. Under the optimized conditions, the CNTs enhanced PTFE membrane exhibited salt rejection of >99.9% and water flux of 33.2 LMH in MD, which was three times as high as that of the pristine PTFE membrane under the temperature difference of 40 °C between hot and cold streams when operated in direct contact membrane distillation. Our research here provides an efficient method to enhance the permeation and stability of commercial membranes by spray-coating CNTs and also supplies some insights on the design of next-generation MD membranes.

Multi-Functional Properties of MWCNT/PVA Buckypapers Fabricated by Vacuum Filtration Combined with Hot Press: Thermal, Electrical and Electromagnetic Shielding.

The applications of pure multi-walled carbon nanotubes (MWCNTs) buckypapers are still limited due to their unavoidable micro/nano-sized pores structures. In this work, polyvinyl alcohol (PVA) was added to a uniform MWCNTs suspension to form MWCNT/PVA buckypapers by vacuum infiltration combined with a hot press method. The results showed an improvement in the thermal, electrical, and electromagnetic interference (EMI) shielding properties due to the formation of dense MWCNTs networks. The thermal and electrical properties rose from 1.394 W/m·k to 2.473 W/m·k and 463.5 S/m to 714.3 S/m, respectively. The EMI performance reached 27.08 dB. On the other hand, ABAQUS finite element software was used to simulate the coupled temperature-displacement performance. The electronic component module with buckypapers revealed a homogeneous temperature and thermal stress distribution. In sum, the proposed method looks promising for the easy preparation of multi-functional nanocomposites at low-cost.

Using a Novel Approach to Estimate Packing Density and Related Electrical Resistance in Multiwall Carbon Nanotube Networks.

In this work, we use contrast image processing to estimate the concentration of multi-wall carbon nanotubes (MWCNT) in a given network. The fractal dimension factor (D) of the CNT network that provides an estimate of its geometrical complexity, is determined and correlated to network resistance. Six fabricated devices with different CNT concentrations exhibit D factors ranging from 1.82 to 1.98. The lower D-factor was associated with the highly complex network with a large number of CNTs in it. The less complex network, having the lower density of CNTs had the highest D factor of approximately 2, which is the characteristic value for a two-dimensional network. The electrical resistance of the thin MWCNT network was found to scale with the areal mass density of MWCNTs by a power law, with a percolation exponent of 1.42 and a percolation threshold of 0.12 g/cm. The sheet resistance of the films with a high concentration of MWCNTs was about six orders of magnitude lower than that of less dense networks; an effect attributed to an increase in the number of CNT–CNT contacts, enabling more efficient electron transfer. The dependence of the resistance on the areal density of CNTs in the network and on CNT network complexity was analyzed to validate a two-dimension percolation behavior.

Impedance spectroscopy of short multiwalled carbon nanotube networks deposited on a paper substrate: tracking the evolution of in-plane and thru-plane electronic properties

Multiwalled carbon nanotubes (MWNT) were deposited via dropcasting and dried using vacuum filtration. Because of the slightly larger pore size (5–10 µm) of the paper substrate used as compared to the length of the nanotubes (0.5–2 µm), a variety of MWNT networks were formed both on the surface and through the thickness of the paper. Using a combination of impedance spectroscopy, equivalent circuit modeling, and microscopy techniques, it was possible to describe in detail how the electrical properties change as a function of how the MWNTs are distributed on the porous substrate by varying the number of deposited layers (1–20) as well as the dispersion concentration (0.1–5 mg/mL). In the in-plane, four different electrical responses were observed and modeled: (1) a substrate dominated spectrum representing unconnected MWNTs, (2) one that included bundle and junction responses as well as some inductances representing sparsely distributed MWNT networks, (3) followed by a parallel RL circuit for partially connected MWNT networks, and (4) finally a series RL circuit for fully connected MWNT networks. In the thru-plane, only two different electrical responses were observed and modeled. The results for the in-plane and thru-plane properties were used to generate percolation curves that show that electrical conductivity can change as much as 10 orders of magnitude for the same exact MWNTs. These results indicate that not only do the characteristics of the nanotubes themselves play a role but also the structure of the underlying substrate and the details of how the films are deposited.