3156 | Energy Environ. Sci., 2019, 12, 3156--3163 This journal is©The Royal Society of Chemistry 2019

Cite this: Energy Environ. Sci.,

2019, 12, 3156

Butylated melamine formaldehyde as a durableand highly positive friction layer for stable, highoutput triboelectric nanogenerators†

Sung Soo Kwak, ‡a Seong Min Kim, ‡a Hanjun Ryu, a Jihye Kim, a

Usman Khan, a Hong-Joon Yoon, a Yo Han Jeongb and Sang-Woo Kim *ab

Mechanical durability and the triboelectric property of the friction layer are crucial for obtaining stable

high power output from triboelectric nanogenerators (TENGs). Here, we introduce butylated melamine

formaldehyde (BMF) as a promising triboelectric material because of both its mechanical durability and

its highly positive triboelectric property. These unique characteristics originate from its functional group,

which contains hydrogen atoms. Kelvin probe force microscopy measurements and density functional

theory calculations confirm that BMF is triboelectrically more positive than are pristine MF and

methylated MF. In addition, the Young’s modulus, calculated by molecular dynamics simulations, of BMF

is six times higher than that of polytetrafluoroethylene (PTFE). Additionally, BMF has demonstrated a

lower wear rate than PTFE in an abrasion test with Cu as the counter friction material. Because of the

superior mechanical and triboelectric characteristics, the root-mean-square output of a rotation-type

BMF based TENG [210 V and 125 mA (24 mA m�2)] is higher than that of the one with PTFE [90 V and

31 mA (5.9 mA m�2)] and remained stable over extended operation of 27 000 cycles. BMF thus offers an

opportunity for fabricating high-performance TENGs with applications in self-powered smart systems.

Broader contextTriboelectric nanogenerators (TENGs), which convert mechanical energy into electricity, have recently attracted much attention as a promising energyharvesting technology, and many advances in TENGs have been made for use as a power source for a variety of applications requiring stable high power for longperiods of operation time. In particular, for TENGs, which are exposed to continuous mechanical friction, the mechanical properties of the triboelectricmaterial must be taken into account. This is because a material with weak mechanical properties can easily wear out even if it has high triboelectric properties,making it difficult to maintain high output. Here, butylated melamine formaldehyde (BMF) was introduced as a strong candidate to be used a triboelectricmaterial for stable high-performance TENGs, because of both its highly positive triboelectric property and mechanical durability derived from a functionalgroup containing many hydrogen atoms. In a rotation-type TENG, where friction is the most severe, it has been shown that BMF can develop high powerwithout a reduction of output from wear, and that high power can remain stable even after 27 000 cycles of operation. Therefore, BMF offers an opportunity forfabricating stable high-performance TENGs with application in self-powered smart systems.

Introduction

Triboelectric nanogenerators (TENGs), which are based on thecoupling of triboelectrification and electrostatic induction,were recently proposed as a promising technology for harvestingabundantly available mechanical energy.1–5 There have beenmany advances in the output power and energy conversion

efficiency of TENGs,6–12 but for practical application, their improvedoutput power must be stable for long periods of time. In particular,TENGs are exposed to continuous mechanical friction and thereforerequire strong mechanical durability of the triboelectric material.Weak durability causes the material to be easily worn out by friction,and the worn particles are transferred to the surface of the oppositetriboelectric material.13–15 This surface contamination createscontact between the same materials rather than between twotriboelectric materials with opposite triboelectric polarities,thus reducing the surface area where triboelectrification shouldoccur. Accordingly, small triboelectric charges are formed onthe surface of the two triboelectric materials, degrading theoutput power. To solve this problem of reduction in output,

a School of Advanced Materials Science and Engineering, Sungkyunkwan University

(SKKU), Suwon, 16419, Republic of Koreab SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University

(SKKU), Suwon, 16419, Republic of Korea. E-mail: kimsw1@skku.edu

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ee01267b‡ S. S. Kwak and S. M. Kim contributed equally to this work.

Received 21st April 2019,Accepted 30th August 2019

DOI: 10.1039/c9ee01267b

rsc.li/ees

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most previous studies have tried to change the device’s structuraldesign so as not to cause severe friction.16–19 These limitedapproaches result in additional processes in the fabrication ofthe device, which makes the device complex and bulky. Since theseTENGs are inefficient and difficult to apply practically due to theirintegrity, an alternative solution to enhance the mechanicalproperties of the material itself is needed. As a result, a stablehigh output of TENGs requires triboelectric materials that have bothhigh triboelectric properties and strong mechanical durability.

Here, we introduce butylated melamine formaldehyde(BMF) as a strong candidate to be used as a triboelectric frictionlayer for the stable high-output performance of TENGs. BMF offersa high Young’s modulus (Y), which can potentially give higherabrasion or wear resistance.20–22 In addition, it can provide strongtriboelectricity, because its functional group contains hydrogenatoms. In fact, the H atoms can donate more electrons to thecounter material’s surface during the triboelectrification, therebyleaving its surface positively charged.23–27 BMF, therefore, is astrong positive triboelectric material. The triboelectric propertiesof BMF were analyzed using Kelvin probe force microscopy(KPFM), density functional theory (DFT) and electrical outputpower measurements. Molecular dynamics (MD) simulationssuggest that BMF’s many H atoms, which are used for cross-linking to form a complex structure,28,29 allow BMF to have a Y sixtimes higher than that of PTFE. Because of the strong tribo-electric property and mechanical strength in terms of higher Y,the TENGs based on BMF can potentially give stable and betteroutput performance. Under similar frictional conditions, arotational sliding-mode TENG with BMF has produced a root-mean-square (RMS) output voltage and current [210 V and125 mA (24 mA m�2)] that are 2.3 and 4.2 times, respectively,higher than those of PTFE [90 V and 31 mA (5.9 mA m�2)]. Inaddition, the output of BMF is highest compared to reportedpositive triboelectric materials.30–32 The TENG remained stableand fully functional even after 27 000 cycles of rotational friction.

Results and discussion

Triboelectrification is a phenomenon in which the surfaces oftwo contacting, distinct materials are electrically charged bycharge transfer at the interface. The amount and sign of thecharge on the surface of each material depend on the relativedifference of the work function DF of the contactingmaterials.6–12 Generally, the surface triboelectric charge densitys of a material is directly proportional to the output current I33–35

(see Supplementary Note 1, Fig. S1, and Table S1, ESI†)

I(t) B s (1)

To show the importance of the mechanical durability oftriboelectric materials and how it affects the triboelectriccharge density and the output current of TENGs, we haveplotted the behavior of the triboelectric charge as a functionof time depending on the difference in mechanical durability, inFig. 1a. For a triboelectric material with high mechanical durability,as shown in the plot (a, dynamic curve as a function of time),

the triboelectric charge is saturated at a certain time, t0, duringsuccessive contacts and separations (triboelectrification). Thetriboelectric charge density typically saturates over time with thecontinuous frictional contact of the triboelectric layers.36 Afterthat, the slope approaches zero, indicating a constant lineparallel to the time axis, because the highly durable materialis hardly worn, as in eqn (2):

IðtÞ �

QHigh Sattribo

t0� tA

t � t0ð Þ

QHigh Sattribo

At4 t0ð Þ

8>>>><>>>>:

(2)

For a triboelectric material with low mechanical durability,the continuous wear even during saturation can reduce thetriboelectric charging area, which can contribute to a lower slopeuntil t0

0 and even a negative slope k0 because of the continuingwear; k0 is a degradation gradient caused by reduction of theeffective contact area, as expressed by eqn (3):

IðtÞ �

QHigh Sattribo � k0t

� �

t00 � t

Aðt � t0

0 Þ

1

AQLow Sat

tribo � k0 t� t00� �� �

ðt4 t00 Þ

8>>>><>>>>:

(3)

As a result, even materials with high triboelectric propertiescannot generate high outputs because of the wear of the materialby continuous friction unless they have strong mechanical durability.

We introduced butylated melamine formaldehyde (BMF) asa triboelectric material. It is a thermosetting polymer, knownfor its excellent durability, low cost, excellent heat resistance,and its flame retardancy.37,38 The chemical structure of thesingle unit of BMF is schematically described in Fig. S2 (ESI†),where the molecules of the functional group butyl areencircled. To characterize BMF as a triboelectric material, PTFEis chosen for comparison, because it is one of the most negativematerials in the triboelectric series, is widely used for high-output TENGs,39–41 but has poor mechanical durability; itssurface easily becomes worn and damaged.13–15 We have experi-mentally demonstrated how the different mechanical durabilityof BMF and PTFE affects the surface of opposite frictionmaterials. Fig. 1b (inset) shows the optical images of the Cusurface before and after rotational friction with PTFE and withBMF. For BMF, there is almost no contamination from wornparticles. However, for friction with PTFE, worn particles areobserved on the Cu surface that could potentially reduce theamount of triboelectric charge by reducing the contact areabetween Cu and PTFE. In addition, by using KPFM, we alsomeasured the surface potential of three different Cu surfaces:pristine, after friction with BMF, and after friction with PTFE;the results are shown in Fig. 1b. The surface potential of the Curemains essentially the same even after friction with BMF; incontrast, the surface potential is significantly reduced afterfriction with PTFE because of the worn PTFE particles. Thereduction in the surface can potentially reduce the triboelectric

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output because of the corresponding reduction in the differencein the work function/surface potential of the friction layers.

For investigating the superior mechanical durability of BMF,we experimentally measured the wear rate of BMF while itoperates as a friction layer in a rotation-type TENG and com-pared the results with that of PTFE. The wear rate of bothmaterials was calculated by measuring the lost weight of thetwo films after an extended number of rotations; the results areshown in Fig. 1c. BMF has a wear rate much slower than that ofPTFE; for example, after 5000 rotations, the wear rate of BMF isabout four times lower than that of PTFE.

The H atoms in the functional group with ramiform shapesare attached to the ends of MF, resulting in BMF. The inter-actions between H atoms (possibly van der Waals forces becauseof a large surface area and polarizability around the ramiform Hatoms in the amorphous BMF) can construct a robust and morecomplex polymeric structure that affects Y more than in thesimple structure of PTFE. In order to investigate the mechanicalstrength of BMF, its Y was calculated by MD simulations (usingthe Forcite package with the COMPASS II force field; for detailson the MD simulations, see Methods) and compared with thereported Y of PTFE (see Fig. 1d).42,43 The amorphous cells of theBMF polymer obtained by geometric optimization are shown in

Fig. S3 (ESI†). Considering a randomly entangled structure,three different BMF amorphous cells were selected in order tomake a reliable estimation of Y. The induced strain wasmeasured while increasing the pressure applied on the amorphouscell, and the Y values along the x, y, and z axes of the three differentamorphous cells were calculated. Accordingly, an average Y of2.98 GPa was obtained for BMF, which is about six times higherthan Y of PTFE (0.5 Gpa) (see Fig. 1d). The hardness of bothmaterials was also analysed, using the pencil hardness tester. On ahardness scale ranging from 9B to 9H, BMF is placed at 5Hcompared to the 7B of PTFE (see Fig. 1e).

A functional group containing many H atoms in BMF notonly strengthens it but also acts to improve the triboelectricproperties. We have employed three types of MF polymers:pristine, methylated, and butylated. Fig. S2 (ESI†) schematicallydescribes the chemical structure of the single unit of each. BMFhas the highest number of H atoms, which make it triboelectricallymore positive than is pristine MF or methylated MF (MMF).23–27 Inorder to evaluate the triboelectric property of the three types ofMFs, we measured the surface potential of each MF using KPFM,as shown in Fig. 2a. All three types of MF have positive surfacepotentials and, as expected, BMF has the most positive surfacepotential because there are more H atoms in its functional group

Fig. 1 (a) Proposed plot of triboelectric charge as a function of time for two different cases (high and low mechanical durability). (b) Change of thesurface potential of Cu initially and after friction with BMF and PTFE. (inset) Optical images of the Cu surface after friction with BMF and PTFE with a300 mm scale bar. (c) Wear rate of BMF and PTFE measured by an abrasion test with Cu as the counterpart friction material. (d) Young’s modulus of BMF,calculated by MD simulation, and PTFE. (e) Hardness range of the pencil hardness tester and measured hardness of BMF and PTFE.

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than in MF or MMF. Since the difference in the work functionsof the friction materials can greatly affect the triboelectricoutput,15,44,45 the work function (jmaterial) of the MFs wasobtained using the measured surface potential by eqn (4),

jmaterial = jProbe_Pt � eVCPD (4)

where jProbe_Pt is the work function of the probe, VCPD is themeasured contact potential difference (CPD), and e is theelectronic charge, as shown in Fig. 2b. With Cu as the secondfriction layer, BMF demonstrates a higher difference in workfunction than do MF and MMF (see Fig. 2b). Additionally, wemeasured the triboelectric output voltage of three verticalcontact-separation-mode TENGs using MF/BMF, MF/MMF,MMF/BMF and MF/nylon as the pairs of friction layers (seeFig. S4, ESI†); the peak direction (i.e., a negative peak whencontacting) further confirms that BMF is triboelectrically themost positive material among the three MFs and it is morepositive than nylon, which is well known as one of the mostpositive materials in the triboelectric series.46,47

For a theoretical understanding of the triboelectric propertiesof each MF, the highest occupied molecular orbital (HOMO)distributions of the three types of MFs with different functionalgroups of H atoms were calculated after geometric optimizationusing DFT simulation, as shown in Fig. 2c. The orbital state iscalculated as the square of the wave function and is thedistribution of the probability of the space in which electronscan exist. Depending on the sign of the wave function, the colorsof the orbital state are divided into blue (+) and yellow (�). TheHOMO is an orbital state that is filled with electrons and so caneasily donate electrons to a counterpart material during tribo-electrification. In principle, therefore, the presence of H atomsin a molecule/unit cell/functional group can give rise to HOMOstates. On the other hand, the lowest unoccupied molecular

orbital (LUMO) distributions of the three MFs, which are theelectron acceptor states, were also calculated using DFT simula-tion, as shown in Fig. S5 (ESI†).27,48 When there is contactelectrification between two materials, the charge can potentiallybe transferred from the HOMO of one material, making itpositive, to the LUMO of the counterpart, making that negative.It can be seen (see Fig. 2c) that the HOMO is distributed aroundthe central region in each MF. However, there are additionalHOMO distributions around the functional groups in MMF andBMF, because they contain additional H atoms. Therefore, allthe three MFs have basically a strong tendency to have positivetriboelectricity, whereas the positive triboelectricity in BMFcan potentially be further increased because of the additionalHOMO states.

Fourier-transform infrared spectroscopy (FTIR) was used toanalyze the presence of the higher number of H atoms in theBMF film than in the MMF and MF films; the results are shownin Fig. 2d. For the functionalized MF films (i.e., MMF andBMF), there is an intense peak positioned at 1100 cm�1 becauseof the C–O stretching that does not occur in the pristine MF.Besides, there is a difference in the absorption intensity of thethree types of MFs around 2900 cm�1 because of C–H stretchingand, in particular, BMF has the highest intensity peak becauseof the C–H bonding.29

To experimentally evaluate the triboelectric output of thethree types of MFs and PTFE, our proposed TENG employs afreestanding rotational design with the most severe frictionamong the working modes of TENGs, as schematically describedin Fig. 3a. It consists of a stator and rotator and is fabricated usingprinted circuit board (PCB) technology. The stator, which consistsof patterned Cu electrodes on the substrate, is covered with apolymer, such as MFs or PTFE. The patterned Cu sectors are 10 cmin diameter and are divided into groups (A and B), where the

Fig. 2 (a) Surface potential of the three MFs measured using KPFM. (b) Work function and difference of the work function from Cu of MF, MMF, and BMFcalculated using the KPFM results for the surface potential. (c) HOMO distribution of the three MFs calculated using DFT simulations. (d) FTIR spectrum ofthe three types of MF.

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sectors of the same group are mutually connected (through wiring).On the other hand, the rotator also consists of patterned (i.e., withthe same central angle) Cu sectors on the epoxy substrate, but theycorrespond to a single group of the Cu electrodes of the stator. TheCu sectors on the rotator act as the counter friction layer to the MFsor PTFE film. The stator and rotator are superimposed, making acontact between the two friction materials.49–51

In order to experimentally demonstrate that the functiona-lized MF (i.e., MMF and BMF), because of the higher number ofH atoms in the functional groups, can generate a higherelectrical output from the TENG, we compared the voltageoutput (at a load resistance of 40 MO) and current output (ata load resistance of 100 O) of the three rotation type-TENGswith MF, MMF, and BMF as the friction layers; the results areshown in Fig. 3b. At a constant rotating speed of 320 rpm, thevoltage and current output of the TENGs with a functionalizedMF frictional layer are higher than those from the one with apristine MF friction layer. The TENG with a BMF friction layerhas the highest output in terms of voltage and current, and theoutput of BMF is the highest compared to the TENGs ofreported positive triboelectric materials.23–27,30–32 The KPFMresults (Fig. 2a), DFT calculations (Fig. 2c), and the comparisonof electrical outputs (Fig. 3b) show that, among the three types ofMFs, BMF is the strongest candidate as a friction layer for TENGs.

Considering its higher wear resistance and highly positivetriboelectric properties, BMF is a strong candidate for creatingdurable, high-output TENGs. As a practical demonstration, wehave compared the triboelectric power output (RMS value) of

the BMF-based rotational-type TENG with that of a PTFE-basedrotational-type TENG during continuous operation. Underrotation of 320 rpm for 800 s, the electrical output of thePTFE-based TENG initially rises to 150 V and 55 mA, but,because of poor wear resistance, it eventually declines to about90 V and 30 mA (see Fig. 3c and d). On the other hand, because ofhigher wear resistance, the electrical output of the BMF-basedTENG rises to its peak electrical output of 210 V and 125 mA andthereafter tends to remain stable (see Fig. 3c and d). Though thedifference in the surface potential and work function for PTFE–Cuis larger than the difference in the surface potential and workfunction for BMF–Cu, PTFE has a lower electrical output becauseof its weak wear resistance (see Fig. S7, ESI†). PTFE can generate ahigher peak triboelectric output, but the possible wearing of thesurface under the continuous strong friction during rotation bothreduces the peak output and causes a decline. We confirmed thatthe experimental output results are consistent with the effect onthe triboelectric charge, which depends on the difference inmechanical durability predicted in Fig. 1a. In order to investigatethe durability and stability of the output, Fig. 3e shows thetriboelectric output of the BMF-based TENG for 27 500 rotations.The output voltage remains stable over such an extended numberof rotations, demonstrating that BMF is a strong friction materialcandidate for a stable, high-output TENG.

The electrical output of the rotation-type BMF-based TENGcan be increased by increasing the segmentation of the Cuelectrodes and by stacking multiple TENGs. Fig. 4a shows theoutput voltage and current as a function of the number of segments.

Fig. 3 (a) Schematic description of the freestanding rotation-type MF-TENG design. (b) Voltage and current output of the rotation-type TENGs based onMF, MMF, and BMF; the rotation speed was 320 rpm; peak values measured at load resistances of 40 M O and 100 O. (c and d) Output voltage and currentof the TENGs based on BMF and PTFE at a rotation speed of 320 rpm; peak values measured at load resistances of 40 MO and 100 O, respectively.(e) Output voltage of the rotation-type BMF based TENG for 27 500 cycles at a speed of 320 rpm and load resistance of 40 MO.

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The segmentation significantly increases the current outputof the TENG because of the corresponding increase in thefrequency of the charge transfer. The TENG’s output alternatesbetween a high level of voltage and a low level of current, andtherefore has to be rectified and transformed to a low-voltagedirect current (DC) output to make it suitable for poweringelectronics. We therefore used a power-management integratedcircuit (PMIC) (DC 2151A_LTC3331 from Linear Technology,Inc.) in order to convert the TENG output into a DC output of5 V (see Fig. 4b). The PMIC first rectifies the AC output of theTENG and charges its built-in supercapacitor to 18 V; thesupercapacitor is thereafter discharged until the chargingvoltage decays to 5 V; and the discharged energy is processedinto an output voltage of 5 V through a DC/DC converter.Fig. 4c–e and Fig. S8 (ESI†) show the input and output voltageof the PMIC when it is supplied by stacked BMF TENGs; theTENG’s stacking is from 1 to 6, and the electrode segmentationis 96; the load resistance is 6.16 kO. It can be seen that thestacking of only five TENGs could not provide a constant DCof about 5 V, but stacking of six TENG devices provided analmost constant 5 V DC, which shows that device stacking caneffectively increase the TENG’s output. The equivalent galvano-static current Ieg can be calculated as beyond 191.75 mA, whichis a much higher effective output current than previouslyreported TENGs51–55 (see Supplementary Note 2, ESI†). The poweroutput of the six stacked BMF TENGs, after conversion fromPMIC, was also used to charge a 700 mA h battery (EFL700A39from STMicroelectronics) to 4 V in about 17 min (see Fig. 4f). It ismuch faster than that of the rotating-type PTFE-based TENGreported by our group.51 The results in Fig. 4 demonstrate thatthe BMF TENG could be used as a power source with a stable andhigh electrical output for practical applications.

Conclusions

We have demonstrated that BMF, because of its mechanicaldurability and highly positive triboelectric property, is a pro-mising candidate material for fabricating stable, high-outputTENGs. DFT simulations showed that, among pristine MF andthe functionalized MFs, BMF has the highest HOMO states,arising from the larger number of H atoms. Besides, KPFMmeasurements demonstrated that BMF has a more positivesurface potential (2.879 V) than do pristine MF (1.818 V) orMMF (1.181 V) and correspondingly the highest work-functiondifference from Cu. BMF possesses strong mechanical durabil-ity, because its wear rate, measured through an abrasion test,and Y, calculated using MD simulations, are about four timeslower and about six times higher, respectively, than those ofPTFE, a well-known material for high-output TENGs. ThoughPTFE has a higher work function difference from Cu, a rotation-type TENG with BMF–Cu as the friction pair has produced ahigher electrical output [210 V and 125 mA (24 mA m�2)] thanhas the PTFE–Cu-based TENG [90 V and 31 mA (5.9 mA m�2)]after a short time. Furthermore, the output of the BMF TENGremained stable over 27 000 cycles of rotation. Six stacked BMFTENGs with the Cu electrodes segmented into 96, after AC-to-DC conversion and processing through a PMIC, produced analmost constant 5 V DC and successfully charged a 700 mA hbattery to 4 V in about 17 min.

ExperimentalFabrication of the TENGs with three types of MFs

MF was synthesized using melamine powder (99% purity,Sigma Aldrich) and formaldehyde solution (36.5–38% in H2O,

Fig. 4 (a) Output voltage and current as a function of the number of segments of the Cu electrode. (b) Schematic of the power management circuit systemwith the TENG, oscilloscope (OSC), and battery. (c–e) Charging voltage of the supercapacitor and voltage output from the PMIC when supplied by stackedBMF based TENGs; stacking of 2, 4, and 6 TENGs. (f) Charging of a 700 mA h battery by the output of six stacked TENGs after conversion by the PMIC.

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Sigma Aldrich) by following the method outlined by Ullah.37

The synthesized pristine-MF, MMF (CAS No. 68002-20-0, SigmaAldrich) and BMF (CAS No. 68002-25-5, BOC Sciences) werecoated on the surface of the stator, fabricated by PCB technology,using an automatic bar coater; the coating was followed byannealing at 180 1C for 2 h in an oven. For the rotation typeTENG, a positive photo-reactive (PR) coated PCB was preparedand a mask with the grating designs of the stator and rotator wasplaced on the PCB. It was exposed to ultra violet (UV) and the PRexposed to UV was developed. Then, the exposed copper of thePCB is etched using ferric chloride solution and the remainingPR was removed using acetone. In the case of the rotator, a cross-shaped hole was formed in the center to match the shape of therotating shaft. In the case of the stator, a circular hole was alsoformed in the center and its diameter was larger than that of therotating shaft. Finally, the MFs were coated or PTFE film wasattached on the surface of the stator.

Measurements

FT-IR (Bruker IFS-66/S, TENSOR27) measurements of the MFswere carried out to confirm the H atoms in the functional groupof each MF. KPFM measurements (Park Systems XE-100 withPT/Cr-coated silicon tips) were conducted to characterize thesurface potential of the three types of MFs. An oscilloscope(Tektronix DPO 3052) and a low-noise current preamplifier(SR570, Stanford Research Systems) were used for electricalmeasurements.

DFT simulations

In order to compute the HOMO and LUMO energy levels of thethree MFs, DFT calculations were conducted using Dmol3 underthe generalized gradient approximation with the Becke–Lee–Yang–Parr exchange–correlation energy function. The k-pointwas set to gamma (1 � 1 � 1) and a double-numerical qualitybasis set with polarization functions was used for all atoms tooptimize the geometry. For the boundary conditions, the toleranceof energy was 1 � 10�5 Ha, gradient was 2 � 10�3 Ha Å�1,displacement convergence was 5 � 10�5, global orbital cut-off was3.7 Å and self-consistent field (SCF) tolerance was 1.0 � 10�6. Toincrease the speed of SCF convergence, direct inversion in theiterative subspace was used.

MD simulations

First, the optimization for obtaining a stable configuration ofBMF was carried out in the following manner: (1) Ten monomersof BMF were linked to form each BMF chain and the five BMFchains were then randomly distributed in a periodic boundarycondition to produce three different BMF amorphous cells.(2) The amorphous cells were annealed and cooled between300 K and 500 K with five annealing and cooling cycles, fiveheating ramps per cycle, and 100 dynamics steps per ramp usingan isochoric-isothermal ensemble (NVT). (3) After the geometricoptimization using a smart algorithm (a combination of steepestdescent, adjusted basis set Newton–Raphson, and quasi-Newtonmethods), a MD simulation at 298 K was performed for 500 pswith NVT to achieve a stable condition.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the Basic Science ResearchProgram (NRF-2018R1A2A1A19021947, NRF-2018R1D1A1B07050868and NRF-2017R1A2B4010642) through a National ResearchFoundation (NRF) of Korea Grant funded by the Ministry ofScience and ICT.

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