Tailored Nitrogen-Doped Laser-Induced Graphene on Novel Synthesized Cross-Linked Aromatic Polyimides for Targeted Applications

TL;DR

This paper shows how using specially made polyimides instead of Kapton can create better laser-induced graphene with improved properties for electronics and energy storage.

Contribution

The study introduces new cross-linked polyimides that enable nitrogen-doped LIG with enhanced electrical and structural properties.

Findings

LIG on PI-EDA showed a specific areal capacitance of 3.1 mF/cm², much higher than LIG on Kapton.

PI-APSA-based LIG had the best adhesion and lowest sheet resistance, ideal for wearable electrodes.

PI-Urea-based LIG retained hydrophilicity, offering versatility for different applications.

Abstract

Laser-induced graphene (LIG) is most often produced from commercial Kapton; the properties of LIG are inherently linked to those of the polymer substrate, which results in a limited field of applications for LIG on Kapton. This study demonstrates that tailored properties of LIG, including nitrogen doping, which is favorable for electronic applications, can be achieved by using synthesized cross-linked polyimides (PIs) as substrates for graphene induction. Three amorphous polyimides containing 4-[(4-aminophenyl)sulfonyl]aniline (PI-APSA), 1,2-diaminoethane (PI-EDA), and urea (PI-Urea), as crosslinkers, were prepared from different diamines and maleic anhydride, and subsequently used as substrates to produce in situ nitrogen-doped LIG. The resulting materials were comprehensively characterized and compared with LIG on Kapton. Raman spectroscopy confirmed lower defect densities and higher crystallinity than in LIG on Kapton, while sheet resistance was up to three times smaller. The LIG with PI-EDA showed the highest nitrogen content and a specific areal capacitance of 3.1 mF/cm2, which is more than an order of magnitude higher than that of LIG/on Kapton, highlighting its strong potential for energy storage devices. PI-APSA-based LIG exhibited the best adhesion and lowest sheet resistance, making it suitable for wearable electrodes, whereas PI-urea-based LIG maintained hydrophilicity. Thus, chemically tailored polyimides enable the formation of nitrogen-doped LIG with tunable interfacial properties, higher structural order, and improved electrical and electrochemical performance compared to commercial Kapton.

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Full text

1. Introduction

Graphene, a two-dimensional (2D) sp^2^-hybridized carbon network arranged in a hexagonal lattice, exhibits outstanding electrical, mechanical, and chemical properties that have positioned it as a leading material for next-generation electronic and electrochemical technologies [1,2]. Since its discovery in 2004 [3], extensive efforts have been devoted to developing scalable synthesis routes, including mechanical and chemical exfoliation, chemical vapor deposition, and the reduction of graphene oxide [4].

An important advance in direct graphene patterning for device fabrication was reported in 2014, when Tour and co-workers introduced laser-induced graphene (LIG), enabling one-step conversion and patterning of porous graphene-like networks directly on polymer substrates using a CO_2_ laser [5]. Although the process was initially successful only on polyimide (PI) and poly(etherimide), subsequent studies have expanded LIG formation to a wide range of organic materials, including diverse polymers [6,7,8,9], wood [10], cork [11], paper [12], and even food materials [13]. Therefore, LIG can also be produced from sustainable precursors such as wood, cork, and paper; although these substrates typically yield porous, multilayer, and defect-rich carbon networks, the properties of which are well suited for electrochemistry and sensing, but are generally less suitable for high-performance electronic applications that require large-area uniformity and high carrier mobility comparable to CVD or epitaxial graphene [14]. The method is rapid, cost-effective, and does not require controlled environments, making it highly attractive for the scalable production of flexible electronic components [15] and energy-storage components without complex lithography [16]. Recent reviews have highlighted how laser wavelength, fluence, scan speed, and precursor chemistry collectively govern the quality of graphene, defect density, porosity, and surface functionality, which ultimately determine electrical and electrochemical performance.

During laser irradiation, the localized high-temperature environment leads to cleavage of C–O, C–N, C–H, and C=O bonds, releasing gaseous by-products (CO, CO_2_, N_2_, H_2_, CH_4_, C_2_H_2_) and promoting rearrangement of the remaining carbon into a porous graphene framework [17,18].

Among all substrates tested, aromatic polyimides remain the most efficient and widely used precursors for high-quality LIG because of their excellent thermal stability, chemical resistance, dielectric properties, and mechanical robustness [19,20,21], and they have enabled interdigitated LIG electrodes for flexible micro-supercapacitors and integrated devices [22]. Polyimides, which contain repeating imide groups (–CO–NH–CO–), can be linear (thermoplastic) or cross-linked (thermoset) polymers and can be processed into films, foams, fibers, or aerogels, enabling broad applicability across microelectronics, sensing, and energy-storage applications [23,24,25]. Commercial Kapton^®^, synthesized from pyromellitic dianhydride (PMDA) and 4,4′-oxydianiline (ODA), is the most frequently employed PI for LIG formation due to its high thermal stability and insulating behavior [20,26]. However, Kapton^®^ has a fixed chemical structure, while even minor structural variations in PI monomers can markedly influence the efficiency of laser carbonization and the properties of LIG, driving current interest in synthesizing tailored polyimides as advanced LIG precursors [23].

Despite the broad use of commercial PIs, reports on LIG derived from synthetically designed polyimides are scarce. A recent study by Iqubal et al. demonstrated that benzimidazole-containing copolyimides yield highly conductive and stable LIG suitable for flexible capacitors [27], underscoring the importance of tailored PI design for achieving targeted LIG functionality. Beyond laser-induced graphene systems, chemically engineered functional materials such as electropolymerized benzimidazole-functionalized cobalt phthalocyanine films have also been reported as effective supercapacitor electrodes, demonstrating that tailored molecular and coordination chemistry can deliver competitive capacitive performance [28]. Structural tailoring of polyimides also enables the incorporation of heteroatoms such as N, P, S, B, or F into the graphene lattice during laser induction, significantly enhancing the electrochemical response of LIG [29,30,31]. Nitrogen doping, in particular, improves charge transport, catalytic activity, and defect chemistry due to the formation of pyridinic, pyrrolic, and graphitic N configurations [32,33,34,35]. Nitrogen-doped LIG electrodes have been produced via two-step laser irradiation (ex situ method) utilizing Kapton as the substrate and carbon precursor, with urea as the nitrogen source. This approach involved urea dispersion followed by CO_2_ laser irradiation on LIG [36]. Recently, nitrogen doping has been readily achieved in situ by direct laser writing on N-containing polymer substrates, enabling the fabrication of N-doped LIG (single-step procedure) [27]. Development of one-step processes combining the LIG porous structure with controlled N-doping has remained a challenge.

In this context, custom synthesis of novel crosslinked polyimides using nitrogen-containing crosslinkers represents a powerful strategy for engineering the composition, conductivity, and electrochemical functionality of LIG. The aim of this work is to investigate the influence of different crosslinkers in polyimide-based substrates on the structural, mechanical, interfacial, and electrochemical properties of laser-induced graphene formed on these substrates. Motivated by these opportunities, we report the design and characterization of three cross-linked aromatic polyimides derived from N-[3-(2,5-dioxo-2,5-dihydro-1H-pyrrol-1-yl)phenyl]acetamide with different diamine crosslinkers: 4-[(4-aminophenyl)sulfonyl]aniline (PI-APSA), 1,2-diaminoethane (PI-EDA), and urea (PI-Urea), synthesized by high-temperature polymerization in solution. Despite significant progress in laser-induced graphene research, most studies have focused on commercial polyimides or sustainable biomass-derived substrates, while the role of deliberate polyimide chemical design in controlling LIG microstructure, heteroatom incorporation, interfacial adhesion, and electrochemical performance remains insufficiently understood. In this work, we address this gap by systematically tailoring polyimide chemistry and correlating precursor structure with the resulting LIG morphology, composition, mechanical response, adhesion, and capacitive behavior. This approach provides fundamental insight into structure–property relationships governing LIG formation on engineered polyimide substrates and establishes design guidelines for high-performance LIG-based planar electrodes.

2. Materials and Methods

We demonstrate the successful induction of graphene on all three types of synthetic polyimide substrates, as verified with Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX). In addition, the obtained LIG materials were further investigated by a range of complementary techniques, including nanoindentation, sheet resistance measurements, water contact angle analysis, and adhesion testing, while their optical transparency was examined with UV–VIS spectroscopy. The electrochemical performance of the materials was evaluated using cyclic voltammetry (CV). It is shown that N-doped LIG induced on these substrates can have tunable morphology, conductivity, adhesion, wettability, transparency, and electrochemical performance, thereby expanding the design space of LIG beyond commercial Kapton and enabling application-specific material optimization.

2.1. Materials

In this work, for the synthesis of different types of polyimides, the following reactants and reagents were used: maleic anhydride (99%, Thermo Scientific, Budapest, Hungary), 3-aminophenyl amine (99%, Sigma-Aldrich, Darmstadt, Germany), chloroform (>99.8, contains 0.5–1.0% ethanol as a stabilizer, Sigma-Aldrich, Darmstadt, Germany), acetic anhydride (Sigma-Aldrich >99%, Darmstadt, Germany), anhydrous sodium acetate (Fisher Bioreagents, Pittsburgh, PA, USA), sodium bicarbonate (Carlo Erba, Milan, Italy), ethanol (99.8%, Sigma-Aldrich, Darmstadt, Germany), 1,2-diaminoethane (EDA, 99% Merck), 4-[(4-aminophenyl)sulfonyl]aniline (APSA, 97%, Sigma-Aldrich, Darmstadt, Germany), urea (99%, Sigma-Aldrich, Darmstadt, Germany), N-methyl-2-pyrrolidone (NMP, 99%, Thermo Scientific Chemical, Waltham, MA, USA). NMP was purified via vacuum distillation over calcium hydride (Sigma Aldrich, St. Louis, MO, USA). The anhydrous, distilled NMP was stored over molecular sieves (0.4 nm) in a dark glass bottle and used for each PI synthesis. For comparison of results, the commercial polyimide (PI) Kapton^®^ HN tape with a thickness of 122 μm was purchased from DuPont (Wilmington, DE, USA).

2.2. Cross-Linked Polyimide (PI) Synthesis

The synthesis of the PIs was carried out in three phases, improving the procedure reported by Marinović-Cincović et al. [37] concerning the type of polymerization reaction for PI synthesis and some reaction conditions for the preparation of precursors. These authors synthesized PIs by high-temperature bulk polymerization (at 250 °C, 10 h). However, in this study, we synthesized PIs by high-temperature polymerization in solution (at 160–220 °C, 2 h) in order to improve compatibility of the reaction mixture during synthesis and homogeneity of the prepared materials. Among the modified reaction conditions were a longer reaction time (1.5 h) and intensive stirring of the reaction mixture with a mechanical stirrer (250 rpm) for the preparation of precursor I and a longer reaction time (2.5 h) for the preparation of precursor II, as well as using the recrystallization process for both precursors.

2.2.1. Synthesis of Precursor I (I Phase)

In the initial stage, (2Z)-4-[(3-aminophenyl)amino]-4-oxobut-2-enoic acid (precursor I) was synthesized as follows:

Maleic anhydride was dissolved in chloroform at a temperature of 60 °C in a four-necked round-bottom flask. A solution of 3-aminophenylamine was then prepared in chloroform under the same conditions. After cooling both solutions to room temperature, the 3-aminophenylamine solution was added dropwise to the maleic anhydride solution under intensive stirring, maintaining a constant temperature between 10 and 20 °C. A light yellow compound was obtained, filtered off, washed with chloroform, and dried in air. After drying, recrystallization was carried out in chloroform. The yield was 98%, and the melting point was 206 °C (Stuart™ melting point apparatus SMP10). The chemical structure and photograph of precursor I are shown in Figure 1a.

2.2.2. Synthesis of Precursor II (II Phase)

In the second phase of the synthesis, in which N-[3-(2,5-dioxo-2,5-dihydro-1H-pyrrol-1-yl)phenyl]acetamide (precursor II) was obtained, precursor I, acetic anhydride, and anhydrous sodium acetate were mixed in a three-necked round-bottom flask equipped with a vertical condenser. The reaction mixture was heated to 120 °C under continuous stirring and maintained at this temperature throughout the reaction. After a dark brown solution was obtained, stirring continued for 45 min. Cold water with chopped ice was added to the warm solution with stirring, and stirring continued for another 45 min. After leaving for 24 h at ambient temperature, the resulting acidic solution was neutralized using a 5% aqueous sodium bicarbonate solution. The solution was left to stand for 3 h with controlled pH = 7, then filtered and washed with distilled water. The product was dried in air and then recrystallized in ethanol. The yield was 74%, and the melting point was 195 °C. The chemical structure and photograph of precursor II are shown in Figure 1b.

2.2.3. Synthesis of Cross-Linked PIs (III Phase)

In the final, third phase of the synthesis, the polyimides were synthesized by polymerization in solution. The polymerization was carried out with 3 different crosslinkers with terminal amino groups: 4-[(4-aminophenyl)sulfonyl]aniline (APSA), 1,2-diaminoethane (EDA), and urea. These compounds were separately dissolved in N-methyl-2-pyrrolidone (NMP), as was precursor II, mixed in a two-necked round-bottom flask, and stirred at room temperature (25 °C) for 1 h on a magnetic stirrer under a nitrogen atmosphere. The resulting solution was then poured into glass molds and heated stepwise in a ventilated oven from 60 °C (1 h), 80 °C (1 h), 120 °C (2 h), 190 °C (1 h) up to 220 °C (2 h) to obtain polyimides with EDA and APSA, and to 60 °C (1 h), 80 °C (1 h), 120 °C (2 h) and 160 °C (2 h) to obtain polyimides with urea. After cooling, three different polyimides were obtained as sheets (d ≈ 2 cm, t ≈ 0.8 mm). Simplified chemical structures and photographs of the synthesized cross-linked polyimides are shown in Figure 2.

2.3. Graphene Induction and Material Preparation

Graphene was induced with a CO_2_ laser with an emission wavelength of 10.6 μm (DBK FL-350; Radlje ob Dravi, Slovenia). The laser spot diameter was approximately 150 μm. The irradiation was carried out in ambient air. To achieve uniform LIG on the surface of the materials, the laser parameters—including power, scanning speed, and resolution—were carefully optimized (speed: 30, 35, and 40 mm/s; resolution: 900, 1100, and 1300 DPI; at the same power setting of 14%). The optimal laser parameters were: a power setting of 14% (from a maximum of 60 W), a scanning speed of 40 mm/s, and a resolution of 1300 DPI. LIG was induced in a rectangular shape (3 mm × 12 mm). A schematic of the laser-induced graphene pattern is shown in Figure 3.

After successful laser induction, a titanium coating was applied by plasma sputtering (MRC 822 sputtering system, Materials Research Corporation, Orangeburg, NY, USA) to the edges of the material, using a shadow mask, to improve contact adhesion. A copper wire was connected to each material with silver paste (Sigma Aldrich) and insulated with nail polish (Figure 4). The whole procedure was also done with commercial Kapton film, which was used as a reference material for comparison.

2.4. Methods of Characterization

Fourier transform infrared (FTIR) spectra were recorded on a Nicolet 6700 spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). Pure polyimide (PI) samples were measured in attenuated total reflectance (ATR) mode, whereas LIG/PI composites were analyzed using the KBr pellet method after mixing and pressing powdered samples. Spectra were collected from 4000 to 400 cm^−1^ at 4 cm^−1^ resolution with 32 scans.

^1^H and ^13^C NMR spectra were acquired at 25 °C on an Agilent/Varian 400-MR spectrometer (399.74 MHz for ^1^H, 100.53 MHz for ^13^C) in DMSO-d_6_ (Merck, Darmstadt, Germany) containing 0.03% TMS as an internal standard.

Raman spectra of LIG/PI composites were obtained with a DXR Raman microscope (Thermo Fisher Scientific, Waltham, MA, USA) using 532 nm excitation, 2.0 mW laser power, and 10 × 10 s acquisition time. OMNIC software (version 9.8.286) applied automatic fluorescence correction and baseline adjustment. The D/G band intensity ratio (I_D_/I_G_) served to estimate in-plane crystallite size ( $[eqn]$ ) via Equation (1) [38]:

$$[eqn]$$

where $[eqn]$ is the laser wavelength (532 nm).

X-ray diffraction (XRD) patterns were collected on a Rigaku Ultima IV diffractometer (Tokyo, Japan) using Cu Kα radiation (1.54 Å) over 5–70° (2θ) with 0.02° step size.

X-ray photoelectron spectroscopy (XPS) was performed using a SPECS system with an Al Kα source (1486.74 eV, 12.5 kV, 32 mA). Survey scans used 40 eV pass energy (0.5 eV step), while high-resolution C 1s, O 1s, and N 1s spectra used 20 eV pass energy (0.1 eV step). Binding energies were calibrated to C 1s at 284.8 eV; data were processed with SpecsLab (version 2.79-r33432) and CasaXPS software (version 2.3.16Dev52).

Scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) mapping of LIG/PI surfaces were conducted on a Phenom ProX microscope (Phenom, Thermo Fisher Scientific, Waltham, MA, USA) at 15 kV acceleration voltage without prior coating. Images were recorded at magnifications of 500× to 10,000×, and EDX maps at 1000× and 5000×.

Nanoindentation tests employed an Agilent G200 nanoindenter (Santa Clara, CA, USA) with a Berkovich diamond tip. Composite samples (3 × 3 mm) were mounted and indented at 25 points in a 5 × 5 grid (100 μm spacing), applying a maximum load of 30 mN (max. depth ≈ 17 μm).

Sheet resistance was determined using a four-point probe setup (A&M Fell Ltd., London, UK) with gold-plated contacts, a Keithley 224 current source, and a Keysight 34461A multimeter. Values were calculated applying appropriate geometrical correction factors [8,39]. The sheet resistances were calculated using the following equations [8,39]:

$$[eqn]$$ $$[eqn]$$ $$[eqn]$$

where R stands for the resistance of the material [Ω/sq], U refers to the voltage measured between the inner electrodes [V], l represents the current applied through the outer electrodes [A], while k and K refer to geometrical correction factors that depend on the measurement configuration [0.8] [8,39].

Water contact angles were measured via the sessile drop method on an Ossila goniometer (Ossila Ltd L2004A1, Sheffield, UK) following ASTM D7334 [40], using distilled water at room temperature (five measurements per sample).

Adhesion of LIG layers to PI substrates was qualitatively assessed by the Scotch tape test (3M tape, Deutschland GmbH, Neuss, Germany) under consistent manual pressure [7]. The 3M tape test was employed as a qualitative and comparative method to evaluate the relative adhesion stability of LIG layers on different substrates [7,41]. All tests were performed under identical conditions using the same tape, contact area, and consistent manual pressure. The 3M tape is standardized and has a controlled adhesive force, which enables repeatable and comparable results (ASTM D3359) [42]. Electrical resistance was compared before and after tape removal with a digital multimeter (Sanwa Electric Instrument Co., Tokyo, Japan), in line with ASTM D3359 principles [42].

UV-Vis spectra of PI films were recorded on a Thermo Fisher EVOLUTION 60 spectrophotometer (Madison, WI, USA, 200–1000 nm). Film thickness was measured with an Iskra NP37 profilometer, and absorption coefficients were derived for comparison [43]. The absorption coefficient was calculated according to Equations (5)–(7) [43]:

$$[eqn]$$ $$[eqn]$$ $$[eqn]$$

where A is defined as absorbance, T as transmittance, a as the absorption coefficient, and L as film thickness.

Electrochemical capacitance was evaluated by cyclic voltammetry on a potentiostat (BioLogic SP-150e, Derbyshire, UK) in a three-electrode setup (working electrode, Pt counter, SCE reference) in 1 M aqueous solution of LiClO_4_ at scan rates of 10–100 mV/s. The preparation of the working electrodes was described in Section 2.3. Specific capacitance (C, in [mF∙cm^2^]) was calculated using the following equation [44]:

$$[eqn]$$

where Q stands for the charge encompassed by a CV curve, ΔU is the potential window [V], and A is the electrode area [cm^2^].

3. Results and Discussion

In this study, three types of polyimides containing different crosslinkers were synthesized by polymerization in solution, and their ability to form N-doped LIG upon laser irradiation was systematically examined. The obtained materials were analyzed in order to compare the effect of each crosslinker on the morphology, chemical composition, and overall quality of LIG. Finally, the goal was to study the properties of the obtained LIG and determine which crosslinker provides LIG with the most suitable characteristics for specific targeted applications, such as electrode materials for supercapacitors or wearable electrodes.

3.1. Monomer Synthesis and Structure

Fourier Transform Infrared Spectroscopy (FTIR) and Nuclear Magnetic Resonance (NMR)

The chemical structure of precursors was investigated by FTIR, ^1^H, and ^13^C NMR. The FTIR spectrum of precursor I (Figure 5a) contained the N–H and O–H bands at 3563 cm^−1^, and the characteristic absorption peaks of the amino groups at 3291 and 3380 cm^−1^, attributed to the symmetric and asymmetric stretching bands of amino groups. The skeletal vibrations of C=C from the aromatic ring appeared at 1629 cm^−1^ and 1441 cm^−1^, while the cis C=C vinyl stretching vibration appeared at 1549 cm^−1^. The C=O stretching band was observed at 1705 cm^−1^, while the stretching vibration of C–N and the deformation vibration of N–H appeared at 1406 cm^−1^. An aromatic C–H stretching vibration was observed at 3090 cm^−1^, while the aromatic C–H out-of-plane deformation vibration was observed at 848 cm^−1^, and the in-plane C–H deformation vibration of the cis vinyl group appeared at 754 cm^−1^.

The ^1^H and ^13^C NMR spectra further confirmed the structure of precursor I, as shown in Figure 6. The peaks at 2.5 ppm and 4.0 ppm were assigned to solvent DMSO-d_6_ and H_2_O from DMSO-d_6_. The peaks at 6.3 and 6.4 ppm were assigned to vinyl protons, while aromatic proton signatures were located within the range of 7.3–8.24 ppm, with attribution of each proton labeled in Figure 6. The peak at 10.45 ppm was assigned to protons of the amino groups.

Carbon atom vibrations in the molecular skeleton of precursor I are marked in Figure 6. The carbon atoms from DMSO-d_6_ appeared at 39.7 ppm. The signals from the carbonyl atoms at δ = 163.4 and 167.2 ppm originated from the amide and carboxyl groups, respectively. The signals from the aromatic carbons appeared at δ = 111.1, 115.4, 129.4, 129.5, and 131.5 ppm. The vinyl carbon atoms appeared at 139.1 ppm.

The FTIR spectrum of precursor II (Figure 5a) exhibited strong absorption peaks at 3107 and 3177 cm^−1^ associated with =C–H stretching vibrations, while the N–H stretching band appeared at 3461 cm^−1^. The C=O stretching vibration was observed at 1716 cm^−1^, while C=C stretching vibrations appeared at 1605, 1593, 1498, and 1454 cm^−1^. The C–O asymmetric and symmetric stretching bands were observed at 1148 and 1074 cm^−1^, respectively. The deformation vibration of aromatic C=C (φ (C=C)) and the C-H out-of-plane deformation vibration (=CH) were observed at 696 and 827 cm^−1^.

Typical ^1^H and ^13^C NMR spectra of precursor II are shown in Figure 7. The ^1^H NMR spectrum showed characteristic signals corresponding to methyl protons of the CH_3_ groups at δ = 1.9 ppm; vinyl protons from maleic anhydride at δ = 7.15 ppm; proton resonances at δ = 7.35, 7.37, 7.39, and 7.60 ppm from aromatic protons; and δ = 10.0 ppm from the –NH protons. The signals at δ = 2.5 and 3.5 ppm appeared from DMSO-d_6_ and H_2_O from DMSO-d_6_.

The shift in the ^13^C NMR spectrum of precursor II at δ = 20.3 ppm was from the carbon atom of the CH_3_ group. The vinyl carbon atoms appeared at δ = 134.9 ppm. The signals from aromatic carbons appeared at δ = 125.2, 126.4, 129.6, and 132.2 ppm. The signals from carbonyl atoms at δ = 170.0 and 185.0 ppm originated from the amide and the maleic anhydride moiety, respectively.

3.2. Polymer Synthesis and Structure

3.2.1. Fourier Transform Infrared Spectroscopy (FTIR)

The chemical structures of the PI sheets and the PI/LIG materials were confirmed by FTIR, as shown in Figure 5a,b. In the FTIR spectra of synthesized PIs, the absorption band at 3340–3370 cm^−1^ was attributed to –NH groups. The bands at 2860, 2935, and 3090 cm^−1^ were assigned to stretching vibrations of C–H and =C–H groups. The C=O stretching bands from amide and maleic anhydride were observed at 1715 and 1780 cm^−1^, respectively. The C–O stretching band from the ether linkage was observed at 1190 and 1300 cm^−1^, while the C=C stretching bands appeared at 1449, 1490, 1605, and 1660 cm^−1^. The bands of δ (N–H) and ν (C–N) appeared at 1549 cm^−1^, while ν (C–N) was observed at 1375 cm^−1^. The deformation vibrations of aromatic C=C bonds (φ (C=C)) were observed at 690 cm^−1^. In FTIR spectra of PI-APSA, the presence of O=S=O symmetric and asymmetric stretching vibrations at 1146 and 1300 cm^−1^, and the O=S=O bending mode at 554 cm^−1^, was evident.

FTIR spectra of LIG/PI revealed that the bands at 1633 cm^−1^ corresponded to C=C stretching vibration and the C=N band hybridized with sp^2^ at 1562 cm^−1^. The bands at 1070 and 1100 cm^−1^ were associated with the C–N stretching vibration and the C–O stretching vibration, respectively. The bands at 2923 cm^−1^ and 2863 cm^−1^ were assigned to –CH stretching vibrations, and a new band appeared at 3440 cm^−1^, corresponding to –OH groups. Similar bands were observed in the FTIR spectra of LIG on Kapton (Figure 5b and Figure S1).

Figure 5 depicts the overlaid FTIR spectra of precursor I, precursor II, and the cross-linked polyimides. In Table S1, we have listed all the vibrations that we have identified from the spectra. The spectra clearly demonstrate the disappearance of precursor-related functional groups (–NH_2_, –OH), and the formation of characteristic C=O groups in amide and imide absorption bands in precursor II and cross-linked PIs (C=O stretching band at 1716 cm^−1^ for precursor II and C=O stretching band at 1715 and 1780 cm^−1^ for PIs). In the PIs, there is also notable formation of a new C–N band at 1375 cm^−1^ and an –NH stretching band at 3340–3370 cm^−1^, as well as SO_2_ vibrations at 1300 and 1146 cm^−1^ in the case of PI-APSA.

3.2.2. Raman Spectroscopy

One of the most powerful techniques for investigating LIG is Raman spectroscopy. Raman spectra provide valuable information about the structure and quality of the obtained LIG. Unlike polyimide, which does not exhibit characteristic peaks, LIG shows three distinct peaks (Figure 8). The D peak, appearing around 1350 cm^−1^, is associated with structural defects and disorder in graphene. The G peak, located near 1580 cm^−1^, corresponds to the in-plane vibrations of sp^2^-bonded carbon atoms, while the 2D peak observed around 2700 cm^−1^ is the second-order overtone of the D band and is particularly useful for evaluating the number of graphene layers and the overall graphitic structure. The intensity ratio of D and G peaks (ID/IG) indicates the degree of disorder in the graphene structure; the higher the ratio, the greater the concentration of defects. Another important parameter is I2D/IG, which is commonly used to evaluate the number of graphene layers. A larger ratio indicates fewer graphene layers [45,46].

The increased noise observed for LIG/PI-APSA and LIG/PI-EDA (Figure 8) can be attributed to substrate interference arising from the polyimide matrix and depends on the type and structure of polyimides (Figure 2). In addition to peak broadening, the positions of the Raman bands provide insight into structural disorder, doping, and strain in the LIG layers. The D-band and 2D-band positions for all LIG materials are slightly shifted to higher wavenumbers relative to LIG/Kapton, which can be attributed to a combination of defect-induced disorder and heteroatom (nitrogen) incorporation [47,48]. The absence of pronounced systematic shifts towards lower wavenumbers indicates that strain effects are limited [49], whereas the observed peak broadening and moderate position variations are consistent with turbostratic, multilayer graphene-like structures typically obtained by laser-induced graphenization.

To obtain information on defect concentration and the number of layers, spectral deconvolution was performed. The first-order peaks were fitted with five functions—two Gaussian and three Voigt, while the second-order peaks were fitted with two Gaussian functions (Figures S2 and S3) [45]. Parameters obtained from the deconvolution are shown in Tables S2–S4.

For LIG on Kapton, the intensity ratio ID/IG is 1.67 (Table 1), whereas for LIG/PI-APSA, LIG/PI-EDA, and LIG/PI-Urea, the values are 0.96, 0.50, and 0.60, respectively. These lower ratios indicate that LIG formed on the synthesized polyimides contains fewer structural defects compared to LIG on Kapton. LIG/PI-EDA stands out with the lowest ID/IG value (0.50), indicating the formation of the LIG with the fewest defects. From ID/IG, one can obtain $[eqn]$ , which represents the crystallite size (lateral crystallinity) of the material and is inversely proportional to ID/IG. Compared to LIG on Kapton, for which $[eqn]$ equals 11.45 nm, our LIG/PI materials exhibit significantly higher values, indicating better crystallinity. Among the studied materials, LIG/PI-EDA has the highest value of $[eqn]$ = 38.45 nm, which is notably higher even compared to results reported in our previous studies [38], where a value of 28 nm was obtained for LIG on PDMS/Triton composites.

LIG/PI-EDA also has a much lower I2D/IG ratio (0.77) compared to LIG on Kapton (2.29). A high value of I2D/IG, as observed for Kapton, is generally associated with few-layer graphene, whereas lower I2D/IG values correspond to graphene structures with a higher number of layers. The same trend is observed for the other synthesized polyimides, with LIG/PI-APSA (1.38) and LIG/PI-Urea (0.48) showing intermediate to low I2D/IG ratios. The lower I2D/IG ratios observed for LIG on the synthesized polyimides compared to Kapton may be attributed to enhanced carbonization during laser treatment. Similar to previous reports [46], where increased local thermal effects promoted the recombination of defects, dopants, and dangling bonds, leading to the formation of thicker and more ordered multilayer graphene, our results suggest that the molecular structure of the synthesized substrates promotes a similar mechanism.

It is also important to emphasize that LIG on Kapton exhibits the largest area of the D peak (57.4%), followed by LIG on PI-APSA (41.2%), while PI-EDA (25.9%) and PI-Urea (23.4%), exhibit the lowest values. This indicates that the defect concentration decreases in the same order, with LIG/PI-EDA and LIG/PI-Urea having the fewest defects. Therefore, it can be concluded that our synthesized polyimides enable the formation of LIG with higher structural quality compared to commercial Kapton.

3.2.3. X-Ray Diffraction (XRD)

X-ray diffractograms display a peak at 2θ = 26°, observed on both LIG/Kapton and LIG/PI (Figure 9). The peak corresponds to diffraction from the (002) plane, confirming the presence of a highly crystalline graphene structure [46,50]. The (002) graphene reflection (~26°) may appear weak for LIG/PI-EDA due to the thin nature of the LIG layer and the dominance of the amorphous PI halo. Applying Bragg’s equation (nλ = 2dsinθ) for the first diffraction order (n = 1), with λ = 1.54056 Å and 2θ = 26°, the interlayer distance is 3.424 Å. This value is consistent with the literature data for graphene [5].

A broad amorphous halo observed at approximately 2θ ≈ 20° is attributed to the polyimide matrices, which indicates an amorphous structure [51,52].

While the synthesized polyimides are chemically cross-linked and exhibit an amorphous halo solely, Kapton possesses a semi-crystalline structure. Accordingly, in addition to the amorphous halo, two distinct diffraction peaks can be observed for LIG/Kapton. The peaks at 2θ = 14° and 22° are attributed to the crystalline regions of Kapton, corresponding to the (101) and (010) planes, respectively [53,54].

3.2.4. X-Ray Photoelectron Spectroscopy (XPS)

The XPS spectrum of LIG/Kapton (Figure 10a) exhibits two characteristic graphene peaks, one at ~284 eV and another at ~532 eV, corresponding to C 1s and O 1s signals, respectively. In the spectra of the synthesized materials, an additional peak appears at ~400 eV, corresponding to N 1s. This peak indicates the presence of nitrogen, indicating N-doping of LIG on the synthesized polyimides. The appearance of the N 1s peak is an important result because it shows that nitrogen atoms were incorporated into the graphene structure. Nitrogen doping is often considered beneficial, as it can change the chemical reactivity, surface energy, and electronic structure of graphene. This means that the obtained LIG not only consists of carbon and oxygen, but also contains nitrogen in its lattice, which can provide additional active sites and improve the material’s performance in different applications. Therefore, the presence of nitrogen in the synthesized LIG highlights the advantage of using modified polyimides as substrates for advanced carbon-based materials [27,35,55].

Figure 10b shows the atomic percentages of carbon, oxygen, and nitrogen in each material, as determined by XPS analysis. It can be seen that nitrogen is incorporated into the structure of LIG on the synthesized polymers, at the expense of carbon. All three synthetic materials contain a significant proportion of nitrogen, with LIG/PI-EDA containing the most (13.4%).

Beyond the elemental composition, XPS also provides insight into the bonding configurations of atoms in the graphene lattice. Figure 11 and Figure 12 depict high-resolution XPS spectra of the C 1s and O 1s peaks of LIG/PI. C 1s deconvolution was performed using the reference binding energies: C–C ≈ 284.2 eV, and C–O/C–N ≈ 285.1 eV, C=O ≈ 287.3 eV, O–C=O ≈ 289.6 eV, while the O 1s spectra were fitted with contributions assigned to C-O ≈ 532.6 eV and C–O–C/C–OH ≈ 533.6 eV [27]. Different types of nitrogen doping (Figure 13), such as pyridinic N (at 398.5 eV, 400.9 eV, and 395.5 eV for LIG/PI-APSA, LIG/PI-EDA, and LIG/PI-Urea, respectively), pyrrolic N (at 399.9 eV, 403.9 eV, and 398.4 eV for LIG/PI-APSA, LIG/PI-EDA, and LIG/PI-Urea, respectively), and graphitic N (at 402.4 eV for LIG/PI-APSA and 401.8 eV for LIG/PI-Urea), were detected, each associated with distinct chemical environments. Pyridinic and pyrrolic N are usually located at the edges and defect sites, while graphitic N indicates substitution of nitrogen into the carbon lattice. Comparison reveals that LIG/PI-APSA and LIG/PI-Urea exhibit a distinct presence of graphitic N, whereas LIG/PI-EDA lacks this component, suggesting that different crosslinkers promote different nitrogen doping pathways during laser induction [27,56,57].

3.2.5. Scanning Electron Microscopy with Energy Dispersive X-Ray Spectroscopy (SEM-EDX)

SEM analysis (shown in Figure S4 (1000 $[eqn]$ ), Figure S5 (3000 $[eqn]$ ), Figure 14 (5000 $[eqn]$ ), and Figure S6 (10,000 $[eqn]$ )) revealed a porous structure of the LIG formed on the surface of the PIs. Pore size distribution (Figure S7) and surface porosity (Figure S8) were evaluated from SEM micrographs using ImageJ (software program, version 1.54k). Porosity parameters were determined from images taken at a magnification of 1000 $[eqn]$ , except for LIG/PI-EDA, which was analyzed at 3000 $[eqn]$ due to the fact that the surface of this material contains traces of thermal degradation products from the PI matrix, which occur under laser induction. Magnification of 1000 $[eqn]$ is not sufficient to limit the field of view to the underlying graphene only.

Based on histograms of pore sizes, depicted in Figure 15, pore diameters for LIG/PI-APSA and LIG/PI-Urea are primarily in the range of 5–15 μm. In LIG/PI-EDA, pores are much smaller, with diameters primarily ranging from 0.5 to 3 μm. On Kapton, the pore size distribution is widely distributed between 2 and 12 μm.

Surface porosity of the different materials is depicted in Table S5. All LIG on the synthesized polyimides have higher porosity than LIG on Kapton, which has a porosity of 16%. LIG/PI-APSA and LIG/PI-Urea in particular have porosities of 24% and 29% respectively, whereas LIG/PI-EDA has considerably lower surface porosity (18%).

EDX (Figure S9) indicates N-doping of LIG on the synthesized polymers, consistent with XPS results, with nitrogen atomic percentage content of 17.1%, 13.8%, and 11.4% in LIG/PI-APSA, LIG/PI-EDA, and LIG/PI-Urea, respectively. Higher oxygen content in LIG/PI-EDA, as previously mentioned, is a result of thermal degradation of the PI matrix. Some amounts of additional elements (impurities) were also detected in the materials (Table 2 and Table S6). Minor differences between nitrogen contents obtained by EDX and XPS are expected because XPS probes only the outermost surface of LIG, whereas EDX samples a much larger interaction volume that may include contributions from subsurface PI and embedded impurities. Na detected in EDX for LIG/PI-EDA is attributed to trace inorganic contamination/residual species located in localized domains and/or below the XPS sampling depth; therefore, it may be observed in EDX but not in XPS.

3.2.6. Nanoindentation Measurements

Nanoindentation was employed to evaluate the nanomechanical properties of LIG/PI, specifically focusing on stiffness, hardness, and plasticity. These parameters were analyzed to assess the influence of different chemical modifications and compared to LIG/Kapton. Load–displacement curves for all the materials are depicted in Figure S10. Mechanical properties of the synthesized LIG/PI materials depend strongly on the type of crosslinker used, as reflected in the Young’s modulus, hardness, and plasticity of the materials (Table 3). LIG/PI-Urea has the highest stiffness (6.63 GPa) and hardness (0.46 GPa), with very low plasticity (14.48), which can be explained by strong hydrogen bonds formed by urea groups that create a rigid and tightly connected network. This agrees with previous studies showing that urea linkages increase stiffness and reduce flexibility in polyimides [58]. In contrast, LIG/PI-APSA, despite containing rigid aromatic sulfonyl-phenyl groups, has low stiffness (0.185 GPa) and hardness (0.002 GPa), with high plasticity (92.50). LIG/PI-EDA has intermediate stiffness of 0.379 GPa and hardness of 0.003 GPa, but the highest plasticity (126.33), reflecting the flexible aliphatic structure of ethylenediamine, which allows the material to stretch and bend without breaking [59]. The relatively large uncertainties in measured hardness, particularly for LIG/PI-EDA, originate from the extremely low absolute hardness and the highly heterogeneous, porous morphology of the LIG layer, which results in substantial local variability during indentation measurements.

Compared to LIG/Kapton, which has intermediate mechanical properties (stiffness 0.452 GPa, hardness 0.008 GPa, plasticity 56.50), the new LIG/PI materials outperform it either in flexibility (LIG/PI-APSA and LIG/PI-EDA) or in stiffness (LIG/PI-Urea). Although aromatic rings generally increase polymer chain rigidity, the stiffness and hardness measured for LIG/PI materials are dominated by the microstructure of the LIG layer. In PI-APSA, laser induction yields a more porous LIG structure, which lowers the effective load-bearing fraction during indentation and thus reduces the apparent stiffness and hardness. Hence, the indentation response is controlled primarily by LIG porosity rather than the aromaticity of the pristine PI backbone.

These results demonstrate that by selecting an appropriate crosslinker, it is possible to tailor the mechanical properties of the material. The synthesized polyimides provide a broader spectrum of mechanical properties than commercial Kapton, enabling improved control over stiffness, hardness, and flexibility.

3.2.7. Sheet Resistance

Compared to the reference LIG/Kapton material (89.62 Ω/sq), both LIG/PI-APSA (29.37 Ω/sq) and LIG/PI-Urea (30.23 Ω/sq) have significantly lower sheet resistance (Table 4). In contrast, LIG/PI-EDA has the highest sheet resistance (218.43 Ω/sq), suggesting less effective carbonization and a more disrupted conductive framework.

A crucial factor underlying these differences is nitrogen doping, which plays an important role in tuning the electronic properties of graphene. Nitrogen atoms can be incorporated into the carbon lattice in several bonding configurations, each exerting a distinct influence on conductivity. Among the bonding configurations, graphitic N is particularly beneficial, as in that configuration nitrogen substitutes a carbon atom within the hexagonal graphene lattice and contributes additional electrons (n-type doping). Because N integrates seamlessly into the structure, this configuration preserves lattice order and allows electrons to move with minimal scattering, thereby lowering electrical resistance. In contrast, pyridinic and pyrrolic N typically occupy edge sites or induce local distortions in the lattice. These configurations may disrupt electronic pathways, which reduces carrier mobility and increases resistance [56,57].

XPS analysis confirmed that LIG/PI-EDA did not contain graphitic nitrogen, whereas LIG/PI-APSA and LIG/PI-Urea did. This provides an explanation for the much higher sheet resistance of the EDA-based LIG. By contrast, the APSA- and urea-based LIG benefits from the presence of graphitic nitrogen doping. The reference LIG/Kapton entirely lacks nitrogen doping, which results in higher sheet resistance than in the APSA- and urea-based LIG.

3.2.8. Water Contact Angle (WCA)

Figure 16 depicts photographs of water droplets on the surfaces of LIG/PI. Measurements of water contact angle were also performed on the surfaces of the PI materials without LIG. The results are summarized in Table 5. All the PI materials are initially hydrophilic, turning hydrophobic upon graphene induction, except for LIG/PI-Urea, which remains hydrophilic [8,60]. While exhibiting the highest standard deviation due to surface non-uniformity, this material demonstrates pronounced hydrophilicity. This retention of hydrophilicity is particularly notable, since graphene is typically hydrophobic. Maintaining a hydrophilic surface can be advantageous for applications such as biosensing, electrochemical sensors, or other devices requiring good wetting and strong interface interactions with aqueous media [61,62].

These findings demonstrate that the surface character of LIG/PI is influenced not only by the properties of graphene itself, but also by the chemical structure of the precursor polyimide. In particular, the choice of crosslinker plays a decisive role in governing whether the resulting surface exhibits hydrophobic or hydrophilic behavior. This provides an important insight, indicating that through careful design of the polyimide matrix and its crosslinking chemistry, it is possible to tailor the surface wettability of the obtained LIG.

3.2.9. Adhesion Testing

Adhesion is crucial for the practical use of LIG because it determines how stable and reliable the graphene layer is during use. Weak adhesion leads to detachment and loss of functionality, while strong adhesion results in durable and functional layers.

After the adhesive tape test (Figure 17), LIG/PI-APSA exhibited outstanding stability, showing only a negligible change in resistance and thus confirming superior adhesion between LIG and the substrate compared to the reference LIG on Kapton film. In contrast, the other two substrates, LIG/PI-Urea and LIG/PI-EDA, demonstrated noticeably weaker adhesion, as reflected in larger resistance changes after the tape test (Table S7).

The differences in adhesion can be explained by considering the molecular structure of the employed crosslinkers. The superior adhesion observed for LIG/PI-APSA, evidenced by the negligible resistance increase after the tape test (64 to 80 Ω), is primarily governed by chemical affinity and interfacial interactions between the laser-induced carbon layer and the underlying PI. In PI-APSA, the presence of aromatic sulfonyl–phenyl groups promotes stronger interchain interactions and enhanced interfacial bonding, which stabilizes the LIG layer against delamination. It should be noted that interfacial adhesion and nanoindentation-derived stiffness/hardness probe different material characteristics. While strong LIG/PI adhesion in PI-APSA is governed by chemical affinity and interchain interactions at the interface, the indentation response is dominated by the porosity and densification of the laser-induced carbon layer and the near-surface heat-affected zone. Consequently, superior adhesion does not necessarily correlate with higher surface stiffness or hardness. These findings are consistent with computational studies showing that aromatic polyimides, owing to their rigid molecular backbones and enhanced interchain interactions, exhibit significantly stronger adhesion to substrates compared to aliphatic counterparts [63]. In contrast, polyimides with aliphatic crosslinkers, such as ethylenediamine or urea, exhibited large resistance increases after the tape test (from 0.170 to 13,000 kΩ and from 0.160 to 200 kΩ, respectively), reflecting weaker interfacial interactions and greater susceptibility to failure under mechanical stress. For LIG/Kapton, the resistance increase was nearly two orders of magnitude (from 0.357 to 13.6 kΩ), suggesting moderate adhesion that is inferior to the adhesion of LIG to PI-APSA, but superior to the aliphatic-based PIs. This comparison highlights the crucial role of substrate structure—particularly aromaticity and backbone rigidity—in determining the adhesion strength of LIG. Our results demonstrate that this interfacial binding strength can be controlled and tuned through appropriate substrate design, which represents an important advantage for future use of these materials.

3.2.10. Ultraviolet-Visible Spectroscopy (UV-VIS)

Figure S11 depicts UV-VIS spectra, from which absorption coefficients were calculated using Equation (7), based on measured transmittance (T) (Table S8). Equation (7) takes into account the different thicknesses of the materials, making the absorption coefficient directly comparable. PI-EDA exhibits the lowest transparency, with the largest absorption coefficient of 63.5 cm^−1^. This is significantly higher than in the other synthesized materials, as well as in Kapton (23.5 cm^−1^). In contrast, PI-APSA has the smallest absorption coefficient (12.7 cm^−1^), indicating the highest transparency among the tested materials.

3.2.11. Capacitance Measurements

Figure 18 and Figure S12 depict the cyclic voltammograms of LIG/Kapton and the synthesized LIG/PI materials. Measurements were performed at different scan rates, ranging from 10 to 100 mV/s. As shown in Table 6, the specific capacitance of electrodes based on the synthetic materials is up to 15 times higher than on Kapton (Table S8). On the synthesized polyimides, the specific areal capacitance is inversely proportional to the scan rate, which is not the case on Kapton. LIG/PI-EDA performs as the best electrode for the electrochemistry of the studied materials. At a scan rate of 10 mV/s, the specific areal capacitance of this material reached 3.1 mF/cm^2^. Such a high specific capacitance could be correlated with the small pore size observed in this material. Both the small pore size and nitrogen doping contribute to improved charge transfer and overall electrochemical performance [27]. XPS indicates that LIG/PI-EDA has the highest surface nitrogen content (13.4%), followed by LIG/PI-APSA (10%) and LIG/PI-Urea (8.5%) (Table 7 and Table S10). This higher nitrogen content correlates well with the observed differences in specific capacitance [64], particularly when compared to LIG/Kapton, which contains no nitrogen, therefore having significantly lower specific capacitance. It is possible that the overall higher porosity of the LIG on synthetic polymers plays a role in the higher specific areal capacitance compared to LIG on Kapton, although evidently the nitrogen content plays a dominant role. Although LIG/PI-EDA exhibits a higher charge-transfer resistance (Rct), it delivers the highest areal capacitance due to its highly porous and defect-rich carbon architecture, which maximizes electrolyte-accessible surface area and promotes surface-controlled charge storage. In addition, nitrogen-containing functionalities contribute to pseudocapacitive effects, compensating for slower charge-transfer kinetics. Hence, areal capacitance in these LIG electrodes is governed primarily by accessible surface area and surface chemistry rather than by Rct alone.

Although the Kapton-derived LIG exhibits a higher I2D/IG ratio, indicative of fewer graphene layers, this does not directly translate into a higher surface area for charge storage. In fact, LIG, derived from the synthesized polyimides, characterized by lower I2D/IG values, consists of multilayer graphene with increased structural disorder and turbostratic stacking. Such structures favor the development of hierarchical porosity, enlarged interlayer spacing, and edge-rich domains, which enhance electrolyte accessibility and ion adsorption. Consequently, the effective electrochemically active surface area of LIG on our synthetic polyimides is likely higher than it is on Kapton.

Deviations of CV curves from an ideal rectangular shape likely originate from hindered ion transport and wetting of inner parts of the LIG layer, which are also the cause of the capacitance drop at higher potential sweep rates. Overall, the findings suggest that LIG on these synthetic polyimide materials holds strong potential for electrochemical applications, especially for microcapacitors, where low electrical resistance and good capacitive behavior are essential (Table 7 and Table S10).

4. Conclusions

In this study, we have shown that synthesized amorphous cross-linked polyimides are effective substrates for N-doped laser-induced graphene that provide clear advantages over commercial semi-crystalline polyimide Kapton. By incorporating three different crosslinkers—4-[(4-aminophenyl)sulfonyl]aniline, 1,2-diaminoethane, and urea—polyimides with distinct structural features were obtained, enabling controlled formation of LIG and tuning of its properties. The properties of the LIG depend on the chosen crosslinker (Table 7).

All N-doped LIG/PI materials exhibited improved graphene structural order (smaller ID/IG and larger $[eqn]$ ) and larger specific areal capacitance compared to LIG/Kapton, and in some cases, smaller sheet resistance and better adhesion. The choice of crosslinker strongly influenced LIG morphology, wettability, adhesion, and electrical behavior.

Among the synthesized materials, LIG/PI-EDA has the highest structural order (as evidenced by the largest ID/IG) and the best electrochemical performance, with specific areal capacitance reaching 3.1 mF/cm^2^ at 10 mV/s, making this material the most suitable candidate for energy-storage applications. LIG/PI-APSA has the smallest sheet resistance, high hydrophobicity, the best adhesion, and the smallest stiffness and hardness, making it highly suitable for applications as an electrode in wearable electronics. LIG/PI-Urea has intermediate values of sheet resistance, large stiffness, and a maintained hydrophilicity, which is advantageous for sensing or interfacial electrochemistry (Table 7 and Table S10).

Overall, the results confirm that chemical modification of the polyimide substrate and nitrogen incorporation offer a powerful strategy for tuning graphene structure and functionality. Each crosslinker provides distinct performance benefits, enabling targeted design of LIG for specific technological applications.

Among the investigated systems, LIG/PI-EDA exhibited the highest structural order, as evidenced by the lowest ID/IG ratio of 0.50 and the largest in-plane crystallite size ( $[eqn]$ ≈ 38.5 nm), as well as the best electrochemical performance with a specific areal capacitance of 3.1 mF/cm^2^ at a scan rate of 10 mV/s. In contrast, LIG/PI-APSA showed the lowest sheet resistance of approximately 26 Ω/sq, together with the most robust interfacial adhesion, as confirmed by minimal material removal by adhesive tape. LIG/PI-Urea maintained a hydrophilic surface character, with water contact angle values remaining below 90°, while still providing stable electrical performance. Compared to commercial Kapton, which exhibited a significantly higher ID/IG ratio (1.67) (and consequently a smaller $[eqn]$ ≈ 11.5 nm), and inferior electrochemical response, all chemically tailored polyimide substrates enabled substantial improvements in the structural, electrical, and functional properties of laser-induced graphene, thereby demonstrating that chemical design of polyimide precursors is an effective strategy for engineering high-performance LIG for flexible electronic and electrochemical applications.