Exploring Molecular and Electronic Property Predictions of Reduced Graphene Oxide Nanoflakes via Density Functional Theory.

In this research, we perform a theoretical interpretation of molecular and electronic properties of reduced graphene oxide (rGO) nanoflakes through the density functional theory. Here, two pristine graphene nanoflake systems were passivated by hydrogen atoms at their edges, armchair (C58H20) and zigzag (C54H20); besides, we implemented 12 rGO systems with a range of low oxide coverage (1, 3, and 4%). Computational calculations were carried out employing the functional hybrid B3LYP and the basis 6-31G(d, p) and 6-311G(d, p) levels of theory. We brought the proposed molecular structures to a stable minimum. We determined the global reactivity descriptors through chemical potential, hardness, softness, and index of electrophilicity. Besides, the maps of electrostatic potential were generated. We found that the hydroxyl and epoxy functional groups dope the graphene molecule in p-type and n-type forms, respectively. In addition, we could attribute the increases of the oxide coverage and the chemical potential to the softness of the molecule. These results suggest that structures with this type of doping can help in developing advanced electronics of sensors and devices.

Introduction

2D materials are nanostructures that present characteristics and phenomena that are not observed in bulk structures.[1] Graphene is composed of a two-dimensional monolayer; it is an allotropic nanomaterial of carbon derived from graphite, which is relatively abundant. It is built by one-atom-thick carbon layers, with sp2 hybridization, and its structure is arranged in the form of a hexagonal ring. Its mechanical, electrical, optical, thermal, and catalytic properties and surface are essential;[2] in addition, this material presents a super-hydrophobicity at nanoscale.[3] Graphene has been the subject of research and it is expected that, with technological advances, this material will be used in various electronic devices of greater capacity and utility.

Despite its short history, graphene has drawn attention for its potential in developing new sensors due to its high electrical conductivity, biocompatibility, and electronic properties. This nanomaterial is also proposed as one solution for the environmental problems currently being faced, which is of great importance. Such is the case of graphene-based absorbents to remove substances like antibiotics, pesticides, and dyes from the environment.[4] However, because its behavior is like a semi-metallic material, to extend graphene applications in electronics of sensors and devices, it is necessary to open band gap energy in the graphene. This has been possible through the use of high electric fields, graphene bilayers, and its doping. In doping graphene with multifunctional oxide groups, graphene oxide (GO) stands out.

Due to its characteristics, GO is of interest in various fields such as biomedicine,[5] physics,[6] electronics,[7] biotechnology,[8] water treatment,[9] and nanotechnology,[10] among others. GO is defined as the carbon layers that are one atom thick, decorated with multifunctional oxides, the groups among which are hydroxyl, carbonyl, carboxyl, and epoxy;[11] these groups give graphene an sp3 configuration,[12] but it also contains aromatic rings;[13] they also give it the property of dispersion in organic solvents and stability in water.[14] Also, GO’s band gap energy and chemical potential can vary depending on the degree of oxidation and working temperature.

Likewise, it is well known that GO is an excellent candidate material for the storage of electrical energy[15] and the development of solid-state ion batteries.[16] Therefore, at present, elucidating the influence of multifunctional oxides on the chemical potential of graphene molecules and the role that this property plays in the storage of electric charge is an open field of research. Therefore, it was necessary to carry out computational calculations using density functional theory (DFT), which allows determination of the chemical potential in molecular structures of reduced GO (rGO) nanoflakes or the range of low concentrations of oxides.

In this regard, it is understood that the DFT studies of graphene and GO have been carried out, which are based on analyzing the chemical reactivity of the species using the descriptors of the conceptual DFT, but not the quantity and influence of multifunctional groups on the chemical potential of the graphene molecule. Various investigations that focus on the reactivity properties of GO have been carried out, such as the work carried out by Roya Ahmadi and Reza Soleymani, which showed that the addition of tyrosine to the GO structure influences energy levels and dipole moment changes.[17] Also, in 2010, Hernández Rosas et al. calculated the forbidden energy band for GO through DFT. In the study on graphene and GO, it was found that the control of these properties is feasible with the meticulous selection of the radical groups that are adsorbed on its surface.[18]

As presented in the articles mentioned above, the investigations were not focused on understanding the influence of the oxidation percentage on the electronic characteristics. Therefore, it is essential to expand the information on the effect of oxides on the chemical potential of a molecular structure of rGO. Therefore, elucidating the influence of multifunctional oxides on GO molecules’ chemical potential is an open field of research. In this research, we determined the chemical potential of rGO molecular structures and the influence of oxide coverage on the chemical potential of rGO molecules for the range of low concentrations of oxides, employing the DFT, for the reason that DFT is one of the most important ab initio methods for calculations in the field of storage and energy conversion[19] and for the study of chemical reactivity, through the analysis of global reactivity indices, such as the electronic chemical potential (μ), chemical hardness (η), and electrophilicity index (ω), and through the frontier molecular orbital theory. Therefore, geometric optimization, calculation of the vibrational frequencies, calculation of the energies (EHOMO, ELUMO, Egap, electronic affinity, and ionization potential), and generation of an electrostatic potential map in the molecular structure of rGO were carried out; the influence of multifunctional oxides on the chemical potential of graphene nanoflakes via computational calculations using the DFT was also estimated.

Theoretical Basis

Through the DFT, it is possible to calculate the reactivity of a molecule with the global reactivity indexes.[20] Parr and co-workers studied the Lagrange multiplier (μ) on the Euler–Lagrange equation, and it was found that μ could be written as the partial derivative of the energy of the system with respect to the number of electrons N in a fixed external potential v(r):[21]where E is a functional of the electron density, ρ.

Applying the derivative to the constant external potential and using the finite difference method to eq , the following expression is obtained:[22]where (I) indicates the ionization potential and (A) is the electronic affinity of the system. Now, considering the Koopmans theorem,[23] these energies can be written as inherent values of higher energy occupied molecular orbital (EHOMO) and lower energy unoccupied molecular orbital (ELUMO), as follows:[24]

Therefore, the electronic chemical potential can be expressed as:[25]

The chemical hardness (η), which expresses the changes in the electronic chemical potential (μ) of the system with respect to the number of electrons N in a potential fixed external v(r), and is written as given below:

Using the finite difference approximation, the following relation can be obtainedwhich indicates that hardness is the energy difference between EHOMO and ELUMO and a large difference corresponds to low-reactive systems; on the other hand, a highly reactive molecule has a small energy difference. The inverse property of hardness is absolute softness and is given by the following relationship:[26]

To quantify the global electrophilic power of a molecule using the relative scale, the term electrophilicity index was included, which is defined as a measure of electrophilic power or the total ability that a chemical species has to attract electrons:[27]

Computational Details

The calculations were performed on DFT at the B3LYP/6-311G(d,p)//6-31G(d,p) level of theory on pristine graphene and rGO nanoflakes employing Gaussian 09W.[28] A pristine armchair graphene nanoflake was modeled using a layer consisting of 58 carbon atoms and zigzag graphene nanoflake of 54 carbon atoms; these structures are based on the Lerf–Klinowski model, which contemplates a random distribution of OH and epoxy groups on the surface of graphene. This model is widely accepted in the scientific community.[29]

The properties of graphene flakes and graphene sheets are very different. To quantify the effect of oxide functionalization on graphene, it is known that the plane-wave DFT and a periodic graphene system can be studied by using the VASP software; however, here, the molecular and electronic property predictions were explored by employing DFT in GO nanoflakes as an alternative way of theoretical study, as reported.[30] We wanted to see the effects of the oxygen-containing groups on the center, the edge, and lateral part of the structures, comparing between armchair and zigzag; hence, 64 structures were calculated using the computational details mentioned with the name positions, in which the edge position is located on the corners of the structures, with at least a carbon atom of separation between the oxygen-containing group and the border passivated by hydrogen atoms; the lateral position is on this passivated border, and the rGO structures that exhibit a systematical chemical potential were selected.

The selection of nanoflakes was carried out by considering the chemical potential criteria only as a possible alternative way to describe a systematic chemical behavior in the GO structures studied here; the 64 systems were established by considering the possible center, edge, and lateral positions of (bi- and tri-) hydroxyl and epoxy groups, as presented in Figures S1–S18.

The pristine graphene nanoflake structures, armchair, and zigzag, as well as rGO structures, with oxidation levels of 1, 3, and 4%, were evaluated through the functional B3LYP and the basis set 6-31G (d, p) to obtain geometric optimization, and then, the B3LYP basis set 6-311G (d, p) was used to obtain energetic optimization. The percentages of oxides (Ox %) was determined by considering the following relation: Ox % = (Total oxygen atoms/Total number of atoms) × 100%. The oxidation values were 1.26, 2.5, and 3.70% for the armchair case and 1.33, 2.63, and 3.89% for the zigzag case.

Vibrational frequencies were evaluated in order to verify the absence of vibrational modes not allowed, and calculation of global descriptors was done to determine the structures with lower and higher chemical potential corresponding to each oxidation level. Consequently, 12 rGO molecular structures were chosen as follows: (i) rGO armchair—hydroxyl edge (1H-E, 1%), epoxy edge (1e-E, 1%), double hydroxyl lateral (2H-LL, 3%), double epoxy center (2e-CC, 3%), triple hydroxyl LLL (3H-LLL, 4%), and triple epoxy center, edge, and lateral (3e-CEL, 4%). (ii) rGO zigzag—hydroxyl edge (1H-E, 1%), epoxy edge (1e-E, 1%), double hydroxyl lateral (2H-LL, 3%), double epoxy edge (2e-EE, 3%), double hydroxyl, epoxy lateral, center, and lateral (2H1e-LCL, 4%), and triple hydroxyl edge (3H-EEE, 4%).

A flowchart that illustrates the computational procedure is shown in Figure .

Figure 1

Computational flowchart for the optimization of graphene and graphene oxide (GO) nanoflake structures using DFT.

Figure presents the GO nanoflakes in the configuration armchair with oxygen-containing groups on the surface at the edge, lateral, and center positions. (a) 1H-E A, (b) 2H-LL, and (c) 3e-CEL. It was possible to observe the different positions considered in our computational calculation and discussed in the Results and Discussion section.

Figure 2

GO nanoflake armchair with oxygen-containing groups on the surface. (a) 1H-E, (b) 2H-LL, and (c) 3e-CEL.

Herein, the molecular electronic properties of pure and oxidized graphene were determined by different functional groups. The finite small cluster model and the details on how to get the lowest-energy isomers of oxidized graphene under different coverages of oxidized groups and a systematic study of HOMO and LUMO of the flake as a function of the position of a single OH group require more systematic computational experiments than what is discussed here. This aspect will be reported in the next work of our group. This study contains the DFT calculation of the electronic properties of rGO functionalized with hydroxyl and epoxy groups and the original contribution to understand the basic properties of graphene-based materials, which is important in the field of nanoscience.

In this section, we have described the methodology carried out to obtain the results, which are shown in detail in the following section.

Results and Discussion

Energetic Properties

Vibrational frequencies were obtained to ensure that optimized molecular structures correspond to energy minimum states, obtaining positive frequencies in all cases. The calculated values of the EHOMO, ELUMO, and band gap energy (Egap) of the rGO armchair and zigzag structures are presented in Table . In addition, it shows the values of the energetic properties for pristine graphene nanoflakes, stabilized by hydrogen, according to type.

Table 1

Energetic Parameters of Hydrogen-Stabilized Graphene Nanoflakes and Molecular Structures of rGO Armchair and Zigzag in the Regime of Low Oxide Coverage

	systems	% O	EHOMO (eV)	ELUMO(eV)	Egap (eV)
armchair	pristine	0	–4.239	–3.401	0.830
	1H-E	1	–4.579	–3.076	1.503
	1e-E	1	–4.544	–3.556	0.988
	2H-LL	3	–4.231	–3.905	0.326
	2e-CC	3	–4.227	–3.596	0.631
	3H-LLL	4	–4.718	–3.592	1.126
	3e-CEL	4	–4.204	–3.260	0.994
zigzag	pristine	0	–4.032	–3.564	0.467
	1H-E	1	–4.589	–2.949	1.640
	1e-E	1	–4.409	–3.666	0.742
	2H-LL	3	–3.866	–3.397	0.469
	2e-EE	3	–4.520	–3.340	1.180
	2H1e-LCL	4	–4.804	–2.752	2.052
	3H-EEE	4	–5.056	–3.349	1.707

Table shows that the molecular structures 2H-LL (armchair, −4.231 eV) and 2H-LL (zigzag, −3.866 eV) present the highest EHOMO values for pristine graphene nanoflakes for both systems, indicating that they are better electron donors. From the table, it is confirmed that these two structures are the ones that present low values of Egap in relation to the other molecular structures of rGO. On the other hand, the lowest ELUMO values belong to the structures 2H-LL (armchair, −3.905 eV) and 1e-E (zigzag, −3.666 eV), which means that they may be the structures that would best accept electrons. These results suggest that hydroxyl groups tend to dope the p-type molecule, while epoxy groups tend to dope the n-type molecule.

On the other hand, the molecular structures of rGO with armchair and zigzag borders can be ordered from smallest to largest, according to the value of Egap as follows: 2H-LL (3%) < 2e-CC (4%) < 3e-CEL (4%) < 1e-E (1%) < 3H-LLL (4%) < 1H-E (1%) and 2H-LL (3%) < 1e-E (1%) < 2e-EE (3%) < 1H-E (1%) < 3H-EEE (4%) < 2H1e-LCL (4%). Thus, the molecular structures of rGO 1H-E (armchair; Egap = 1.503 eV) and 2H1e-LCL (zigzag; Egap = 2.052 eV) exhibit the highest band gap energy, and the molecular structures of rGO 2H-LL (armchair; Egap = 0.326 eV) and 2H-LL (zigzag; Egap = 0.469 eV) exhibit the lowest band gap energy.

Therefore, the 1H-E armchair structures and 2H1e-LCL zigzag are the molecules least likely to respond against any external disturbance,[31] which generates low reactivity in the system, as expected; whereas, the 2H-LL armchair and zigzag structures exhibit the greatest softness and the greatest reactivity.

Additionally, it is known that materials with band gap energies between 0.1 and 5 eV exhibit the behavior of a semiconductor material,[32] and the molecules studied in this work present values of band gap energy in the range reported. On the other hand, a material with a band gap energy lower than 0.1 eV is referred to as conductive, although it is known that graphene without impurities is considered a zero-gap semiconductor.[33] In this case, as they are pristine quantum dot graphene molecules, the molecular structures armchair (C58H20) and zigzag (C54H20) exhibit band gap energies of Egap = 0.830 eV and Egap = 0.467 eV, respectively. Therefore, they can be considered semiconductor materials, like the previously studied rGO structures.[34]

Global Reactivity Indexes

The global reactivity descriptors, such as chemical potential (μ), chemical hardness (η), softness (S), and the electrophilicity index (ω), are used in the characterization of the molecular systems under study. Table shows the values of these reactivity indicators in the molecular systems studied, which were determined by expressions , 678, described on a theoretical basis. As oxide coverage increases, chemical potential and hardness increase, while softness and electrophilicity decrease.

Table 2

Global Reactivity Indexes of Molecular Structures of Graphene Nanoflakes and rGO Armchair and Zigzag, with Different Oxidation Levels: Hardness (η), Softness (S), and Electrophilicity Index (ω)

	systems	%O	η (eV)	S (eV–1)	ω (eV)
armchair	pristine	0	0.419	2.387	17.417
	1H-E	1	0.751	1.331	9.747
	1e-E	1	0.494	2025	16.606
	2H-LL	3	0.163	6.121	50.690
	2e-CC	3	0.315	3.170	24.255
	3H-LLL	4	0.563	1.776	15.333
	3e-CEL	4	0.472	2.118	14.754
zigzag	pristine	0	0.234	4.279	30.868
	1H-E	1	0.820	1.220	8.664
	1e-E	1	0.371	2.694	21.958
	2H-LL	3	0.234	4.268	28.142
	2e-EE	3	0.590	1.695	13.089
	2H1e-LCL	4	1.026	0.974	6.956
	3H-EEE	4	0.854	1.171	10.345

According to the data shown in Table , the molecular structure of the rGO 1H-E armchair is the system with the highest hardness, and therefore, it is characterized by its low softness (S = 1.331 eV–1), which would indicate little tendency to give or receive electrons, because hardness is associated with the resistance of a system to change in its electronic distribution. There is a relation between softness and electric dipole polarizability; both properties depend on the valence electrons, and according to the HSAB principle, a soft base is one with a donor atom of high polarizability and a soft acid should have an acceptor atom with several easily excited outer electrons. The alternative definition of softness given by Fuentealba et al.[35] requires the polarizability to be computed directly; however, Yang and co-workers[36] proposed to know the global reactivity indexes by employing the HOMO and LUMO bands and their relation with the softness; it simplifies the correlation of the results of the theoretical and experimental band gap energies independent of polarizability, as presented in expression .

Regarding the electrophilicity index, this molecular structure exhibits the lowest value of the molecular structures of rGO and that of pristine armchair graphene nanoflakes, which suggests that this system has little acquisition of electronic charge and could be an excellent candidate for nucleophiles.

Additionally, it was found that the position of the hydroxyl group could directly influence the electrophilicity index of the 2H-LL molecule; this is possibly attributed to the transfer of electrons from graphene to the −OH group because graphene can lose electrons and thus increase the electrophilic character of the molecule.[37] The functionalization with two hydroxyl groups on the sides allows graphene nanoflake to lose electrical charge and generate sites conducive to the attraction of electrons on peripheral carbon atoms, unlike the molecular structure of rGO 1H-E. Likewise, it causes the carbons close to the functional group −OH to show an increase in the donor character. Therefore, the molecular structure of rGO 2H-LL armchair presents a value of ω = 50.690 eV with reference to pristine armchair graphene nanoflakes, with a value of ω = 17.417 eV, which suggests that this molecule presents a high value of chemical potential and a low value of hardness, as shown in Table , and it can be considered a good electrophile.[38]

When observing the molecular structures of rGO zigzag, it is shown that the multifunctional groups exhibit a lower electrophilicity index compared to the pristine graphene molecule, which could be associated with the pristine graphene zigzag being more reactive (S = 4.279 eV–1), and consequently, it is possible that it accepts additional charge from its environment[39] and generates little resistance to the transfer of electrical charge (η = 0.234 eV).[40] A large decrease occurs by adding an epoxy group on the surface; such is the case of the molecular structure 2H1e-LCL (ω = 6.956 eV), which could be attributed to the fact that the epoxy group tends to donate its electrons from HOMO and therefore induces n-type doping of the rGO molecule. In addition, the epoxy on the pristine graphene nanoflake surface can promote the displacement of electrons from the carbon atoms (close to the epoxy group) to the oxygen, and consequently, reduce the loss of electrical charge of the other carbon atoms present in the graphene molecule. In this way, the high electrical charge transfers of carbon atoms, bonded to oxygen, can cause the graphene sheet to corrugate around carbons near the epoxy group and stretch the C–C bonds. Regarding hardness, this system presents the highest value (η = 1.026 eV), which could be due to the opposition imposed by the molecular structure of rGO to the change in its electronic distribution.

Figure shows the influence of oxide coverage on the chemical potential of the molecular structures of rGO armchair and zigzag borders obtained by computational calculations. It is observed that the higher the level of oxidation in the rGO structures, the greater the chemical potential, except for the rGO zigzag molecular structures 1H-E (1%) and 2H-LL (3%).

Figure 3

Influence of oxide coverage on the chemical potential of pristine graphene nanoflakes and rGO armchair and zigzag structures.

The systems with the lowest chemical potential are the molecular structures of rGO 2H-LL zigzag at the 3% oxidation level and 1H-E armchair at the 1% oxidation level, which indicate a lower reactivity. Although the value of the chemical potential in the armchair 1H-E structure is higher than that of the pristine armchair graphene nanoflake, it is a fairly small difference and its reactivity is verified with the other global descriptors.

The armchair system with the highest chemical potential is 3H-LLL with a value of 4.155 eV, and for the zigzag system, it is the 3H-EEE molecular structure with 4.203 eV, which shows a greater reactivity.[41]

With the analysis of the chemical potential, it was found that rGO has a greater chemical potential than pristine graphene molecular structures, which suggests that rGO is more reactive than pristine graphene nanoflakes and therefore exhibits a greater inclination to take part in chemical reactions, which is consistent with the results reported by Boukhvalov.[42] Regarding the strength of the influence of the position on the GO nanoflakes, it was revealed that the influence was strong in the lateral and edge position and weak in the center position, possibly attributed to the passivation effect by hydrogen atoms in the edge position that lead to interactions between the oxygen-containing groups.

Molecular Electrostatic Potential

On the molecular electrostatic potential (MEP), the red color corresponds to a region with high electron density (negative), which produces n-type doping in the molecule, and therefore, this is the preferred site to carry out an electrophilic attack; blue color on the surface means a minimum concentration of electrons (positive), which generates p-type doping. Therefore, it is a preferred surface for nucleophilic attack. The colors belonging to the electron density shown on the surface of a molecule increase in the following order: blue < green < yellow < orange < red.[43]

Regarding the structures studied, Figures and 5 show the MEP for armchair and zigzag pristine graphene nanoflakes, in which a uniform charge density distribution is observed, and in the passivated edges, little electronic concentration is shown.

Figure 4

MEP of pristine graphene nanoflake and rGO armchair type. (a) Pristine graphene, (b) 1H-E (1%), (c) 1e-E (1%), (d) 2H-LL (3%), (e) 2e-CC (3%), (f) 3H-LLL (4%), and (g) 3e-CEL (4%).

Figure 5

MEP of pristine graphene nanoflake and rGO zigzag type. (a) Pristine graphene, (b) 1H-E (1%), (c) 1e-E (1%), (d) 2H-LL (3%), (e) 2e-EE (3%), (f) 3H-EEE (4%), and (g) 2H1e-LCL (4%).

Additionally, Figure illustrates the reactive sites of the armchair rGO molecular structures. In Figure , it is observed that the −OH groups on the surface of the rGO structures, 1H-E, 2H-LL, and 3H-LLL armchair, favor the formation of electric dipoles. It is well known from the physics and chemical concepts that in a general manner, an electrical dipole configuration can be formed by the difference of electrical charge configuration; then, by assuming the electrical charge distribution in an −OH multifunctional group as a dipole configuration, it is possible to think in an storage of electrical field, as a localized manner, due to atomic scale exhibit by hydroxyl groups presence in graphene nanoflakes, and this behavior of localized electrical field in hydroxyl groups suggest a possible great interest in the future development of advanced electronics of sensors, devices, and energy electrical storage. Therefore, the −OH group, being an acceptor impurity, tends to dope the molecules mentioned above in a p-type way;[44] so that the number of holes would exceed the number of free electrons, and consequently, the holes would be the majority carriers and the free electrons the minority ones.

On the other hand, the epoxy group for the molecular structures of rGO, 1e-E, 2e-CC, and 3e-CEL, tends to exhibit a behavior of doping the n-type molecule because it is a donor, and they redistribute the charge toward the interior of the molecular structure of graphene nanoflakes, accepting more electrons than holes, and consequently, the structures exhibit a red color on the epoxy group and a yellow color around the carbons that surround it.

Similar cases occur with the molecular structures of rGO zigzag. As shown in Figure , the −OH group confers a p-type doping to the molecules: 1H-E, 2H-LL and 3H-EEE. As in armchair structures, in the zigzag, the epoxy group produces an n-type doping in the molecular structures of rGO: 1e-E, 2e-EE, and 2H1e-LCL.

Conclusions

DFT calculations were analyzed to provide valuable information about the influence of the low oxide coverage on rGO global reactivity indexes. The geometric optimization procedure was carried out on rGO molecules, achieving the calculation of the energies (EHOMO, ELUMO, Egap, electronic affinity, and ionization potential) and the vibrational frequencies of the molecular structures of graphene stabilized by hydrogens and of oxidized graphene in the low oxide coverage regime.

The influence of oxide coverage on chemical potential, hardness, softness, and electrophilicity index was studied in oxidized graphene molecules for the regime of low oxide coverage using computational calculations based on the DFT; it was found that with greater oxidation, the potential and hardness values tend to increase, while the softness and the electrophilicity index tend to decrease, which could be attributed to the molecular polarizability induced by the presence of the epoxy and hydroxyl groups. The distribution of electric charge density in the proposed molecular structures was analyzed using MEP, and it was found that the epoxy and hydroxyl groups tend to dope n-type and p-type rGO molecules, respectively.

The geometric optimization procedure was carried out on rGO molecules, achieving the calculation of the energies (EHOMO, ELUMO, Egap, electronic affinity, and ionization potential) and the vibrational frequencies of the molecular structures of graphene stabilized by hydrogens and of oxidized graphene in the low oxide coverage regime. These results suggest a possible electrical field storage at the atomic scale due to the hydroxyl group’s presence in graphene nanoflakes, which could benefit the future novel advanced electronics of sensors, devices, batteries, and supercapacitors.