Solvent-driven self-assembly and polarized emission in nitrogen-doped graphene quantum dots

Abstract

Nitrogen-doped graphene quantum dots (N-GQDs) are tunable nanoscale fluorophores whose photoluminescence (PL) is governed by core states, edge defects, and surface chemistry. Beyond these intrinsic factors, the surrounding medium can fundamentally alter their optical response, yet the influence of solvent-induced self-assembly on polarized emission has remained largely overlooked. Here, we provide the first direct spectroscopic evidence that solvents not only modulate emission intensity but also drive quasi-alignment of emitting dipoles in N-GQDs, producing liquid-crystal-like ordering within colloidal dispersions. Polarization-resolved PL reveals that the three principal emissive pathways of N-GQDs respond differently to solvent environments: non-polar solvents with high positive zeta potential promote tighter dipole alignment and stronger polarization anisotropy, whereas polar solvents broaden orientation distributions and stabilize excited states. These findings establish solvent-induced self-assembly as a critical mechanism for tailoring polarized emission in N-GQDs, opening new directions for solvent-responsive nanomaterials in adaptive light sources, reconfigurable optoelectronics, and next-generation energy-harvesting platforms.

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Introduction

Graphene quantum dots (GQDs) are nanoscale graphene derivatives whose optical properties arise from a complex interplay of quantum confinement, surface and edge configurations, defects and defect distributions, and complex core structures. Nearly all variants of GQDs exhibit room temperature photoluminescence (PL), which is usually attributed to the factors mentioned above, although their precise contributions remain incompletely understood. This makes GQDs a rich system for exploring structure–property relationships at the nanoscale.¹ Nitrogen doping is an effective way to modify GQD characteristics. Nitrogen heteroatoms provide donor levels and improve charge transfer, affecting the bandgap and PL emission. N-GQDs exhibit enhanced PL quantum yield compared to their undoped counterparts, with radiative recombination pathways and exciton dynamics strongly influenced by the bonding configurations of nitrogen atoms, such as pyridinic, pyrrolic, or graphitic.^2,3 These doped sites actively interact with surrounding solvents through dipole–dipole interactions and hydrogen bonding, modulating the local electronic environment and thereby affecting optical transitions.^4,5 While the structural and optical tunability of N-GQDs has been widely explored, the solvent-dependent polarized emission behavior – critical for probing anisotropic electronic states – remains insufficiently studied. Polarization-resolved spectroscopy offers a powerful approach for uncovering orientational and symmetry-related features of emissive states in such complex nanomaterials.

Polarized emission in nanomaterials has consistently been utilized as an investigative tool for understanding the electronic structure, exciton orientation, and spin-related transitions, particularly within semiconductor QDs.⁶ In a pioneering study, Brus and co-workers⁷ employed far-field microscopy to probe single dye molecules at a polymer–air interface, revealing that both fluorescence lifetimes and spectral shifts were strongly correlated with dipole orientation and local dielectric environments. Subsequently, Bawendi and co-workers⁸ demonstrated that far-field polarization microscopy can resolve the 3D orientation of individual nanocrystals with degenerate dipole structures. Building on these works, investigations into II–VI quantum rods demonstrated that elongated nanostructures exhibit linearly polarized emission due to asymmetry in exciton wavefunctions and the alignment of transition dipoles.⁹ Recent investigations into perovskite nanocrystals have revealed solvent-sensitive and geometry-dependent polarized emission in CsPbX₃ perovskite QDs, where crystal field effects and spin–orbit coupling dictate the fine structure of excitons.¹⁰ The application of dipole orientation mapping through advanced super-resolution polarization demodulation techniques has enabled the spatial resolution of emission anisotropy at sub-diffraction scales in fluorophores.¹¹ Such approaches are supported by theoretical models linking PL anisotropy to molecular orientation and local viscosity, as demonstrated in systems ranging from nematic liquid crystals with unusual negative anisotropy behavior to photo-switchable probes where anisotropy provides a direct readout of molecular reconfiguration.¹²

Although significant advancements have been made in comprehending the interactions of polarized light with matter in both inorganic and organic systems, the polarization-resolved spectroscopy of carbon-based QDs across different solvents remains a largely underexplored area. About a decade ago, Ghosh et al.⁴ showed that single carbon dots (CDs), when excited with focused azimuthally polarized light, exhibit fixed, parallel excitation and emission dipole moments, confirming dipole-like single-emitter behavior. A few years later, polarized PL studies concluded that room temperature emission in GQDs originates from the radiative recombination of self-trapped excitons with suppressed carrier relaxation.¹³ Intense polarized PL has also been reported from CD–metal composite nanowires, attributed to plasmonic coupling along the nanowire axis.¹⁴ Jing et al.⁵ utilized polarization-resolved femtosecond spectroscopy and simulations to demonstrate solvent-dependent anisotropic absorption and stimulated emission, concluding that PL of heteroatom-doped CDs arises from electric dipole emission centers. More recently, linearly polarized PL was demonstrated in anisometric CDs aligned uniaxially in a liquid crystal host, highlighting the role of structural anisotropy and controlled alignment in producing polarized emission.¹⁵

The discovery of chiral CDs in 2016 initiated a plethora of studies on circularly polarized emission from graphene and CD systems to investigate their chiroptical properties.^16,17 However, the issue of polarized emission from CDs or GQDs due to self-assembled alignment in solvents has received nearly no attention. It is well established that different solvent polarities can induce self-assembly of suspended nanostructures, which in turn influences their luminescence properties.^18–21 Very recently, Li and Gong demonstrated that solvents of varying polarity can induce self-assembly of dispersed GQDs through AFM studies of GQDs deposited on substrates, which revealed lamellar structures.²² The formation of patterned structures was attributed to solvent-driven self-assembly, highlighting the interplay between the solvent environment, structural ordering, and emission properties. However, the solvent-induced self-assembly of GQD-based systems in colloidal suspensions has remained largely unexplored.

This research endeavor seeks to address these knowledge gaps through a systematic examination of the solvent-dependent polarized emission from N-GQDs. Employing water, IPA, toluene, and chloroform as model solvents, we establish a correlation between variations in polarization anisotropy and the dielectric properties of solvents and their effects on dipole reorientation. The distinct dielectric constants, polarities, and hydrogen-bonding capacities of these solvents make them ideal for probing solute–solvent interactions and their impact on photophysics.^23,24 This study offers a thorough examination of the previously uncharted polarized emission phenomena exhibited by intrinsically structurally-anisotropic N-GQDs in various solvents, illuminating the influence of environmental factors on their optoelectronic characteristics. The results have important implications for the development of N-GQD-based devices for polarized light emission, anisotropic sensing, and solvent-responsive photonic systems.

Experimental

Materials

Citric acid (CA) and ethylenediamine (EDA), both of synthesis grade, were procured from Sigma-Aldrich. Sample purification was carried out using dialysis tubes with molecular weight cut-offs (MWCO) of 500 and 1000. All solutions were prepared using 18.2 MΩ cm deionized (DI) water (Millipore).

Synthesis

N-GQDs were synthesized via a microwave-assisted hydrothermal method. A solution of 2.1 g of CA and 1.8 g of EDA in 30 mL of DI water was prepared and transferred into a Teflon-lined vessel (IPREP). The solution was subjected to microwave irradiation (CEM MARS 6) at 180 °C for 20 minutes. The resulting dark brown colloidal suspension was centrifuged at 15 000 rpm for 15 minutes and then purified by dialysis with 1000 MWCO membranes. After dialysis, the sample was freeze-dried to obtain a powdered form. Solutions were then prepared by dispersing the N-GQDs at a fixed concentration of 1.1 mg mL⁻¹ in all four solvents. All samples were thoroughly ultrasonicated prior to all measurements to ensure uniform colloidal dispersion.

Characterization

High resolution transmission electron microscopy (HR-TEM) was performed using an FEI Titan G² 80–300 microscope operating at 300 kV. High resolution bright-field (BF) and selected area electron diffraction (SAED) modes were utilized. The samples were dispersed in isopropyl alcohol (IPA), sonicated thoroughly, and a drop of each dispersion was deposited onto the appropriate TEM grid. Multiple images were captured for each sample and analyzed using digital measurement software to determine particle sizes. The morphology and local vibrational properties of individual N-GQD particles were investigated using a Vista One photo-induced force microscope (PiFM, Molecular Vista). This technique combines atomic force microscopy with photo-induced force infrared (PiF-IR) spectroscopy, wherein the cantilever tip interacts with pulsed IR radiation at a frequency corresponding to the difference between its resonant vibrational modes. The interaction between the tip and the near-field optical response of the sample induces frequency shifts, which are used to generate both topographical and chemical maps. A platinum-coated tip with a 67 nm radius of curvature was employed, and IR spectra were acquired in the range of 800–1800 cm⁻¹. X-ray diffraction (XRD) measurements were conducted using a Bruker D8 Advance diffractometer with Cu Kα₁ radiation. Raman spectra were collected using a Horiba Xplora Plus system with a 532 nm laser and a Synapse Plus detector, covering the 50–4000 cm⁻¹ spectral range. X-ray photoelectron spectroscopy (XPS) was performed using a Thermo Fisher ESCALAB 250Xi instrument with an Al Kα (1486.6 eV) radiation source. Fourier-transform infrared (FTIR) spectra were recorded using a PerkinElmer Frontier system, while UV-Vis absorption measurements were conducted using a Cary 7000 (Agilent). PL spectra were obtained using an Edinburgh FLS1000 spectrofluorometer. Fluorescence lifetimes were measured using the time-correlated single photon counting (TCSPC) module of the FLS1000 spectrofluorometer, equipped with a 340 nm picosecond pulsed LED (EPLED 340).

Computational

Density functional theory (DFT) calculations were performed to investigate the IR absorption spectra of N-GQDs. All computations were carried out using the ORCA 6.0 quantum chemistry software package. Geometry optimizations were conducted at the CAM-B3LYP/TZVP level of theory. The modelled N-GQD structure consists of 46 carbon, 7 nitrogen, 5 oxygen, and 19 hydrogen atoms. The optimized geometry was confirmed to be a true local minimum, as no imaginary frequencies were found in the vibrational analysis.

Results and discussion

The structural features and bonding architectures of the as-synthesized N-GQDs are summarized in Fig. 1(a–g). The bright-field high resolution transmission electron microscopy (HR-TEM) image presented in Fig. 1(a) reveals a uniform distribution of dark contrast features across the field of view, corresponding to individual N-GQDs. Analyses of several such TEM images reveal a modal lateral dimension of 2.75 nm, with a size distribution ranging from 2 to 4.5 nm, as shown in the inset of Fig. 1(a). The SAED pattern [Fig. 1(b)] exhibits diffraction rings from the (002) and (020) planes with d-spacings of 0.40 nm and 0.25 nm, respectively. The observed (002) spacing, larger than the typical graphitic value of 0.335 nm, reflects nitrogen incorporation, which has been reported to increase the interplanar distance to 0.34–0.45 nm.^25–29 The BF-TEM lattice-resolved image shown in Fig. 1(c) further confirms an interplanar spacing of 0.24 nm, characteristic of the (020) graphitic plane.²⁶ These observations affirm the high crystallinity and structural integrity of the N-GQDs. Fig. 1(d) shows the topography obtained using a photo-induced force microscope (PiFM), revealing that the N-GQDs possess a disc-like morphology and are uniformly distributed across the substrate. The observed lateral sizes range from 1 to 8 nm, with a modal diameter of 2.5 nm, as confirmed by the size distribution histogram [inset, Fig. 1(d)]. The height profiles of the particles vary from 0.3 nm to 1.3 nm, indicating the presence of few-layered structures, typically corresponding to approximately one to five graphene layers. An example of a height profile capturing 3 particles is shown in Fig. 1(e) (more detailed analysis of AFM is presented in Fig. S1). Notably, the size distribution and morphology observed via PiFM are in excellent agreement with the HR-TEM results, confirming the consistency and reliability of the structural characterization. The larger apparent lateral dimension seen in the PiFM images is due to the tip convolution effect, which is a common artifact in scanning probe microscopy. Details regarding the correction method used to estimate the true lateral dimensions are provided in the SI.

Fig. 1 Structure and bonding architecture of the N-GQDs. (a) Bright field HR-TEM micrograph along with the size distribution histogram shown as the inset; (b) SAED pattern of the N-GQDs showing signature of crystallinity; (c) magnified image of a single N-GQD revealing crystalline fringes having an interplanar spacing of ∼0.25 nm corresponding to the (020) graphitic plane; (d) AFM topography of the N-GQDs deposited on cleaved mica; (e) AFM height profile taken over the blue line shown in (d); (f) single particle IR spectra of the particles marked with red, blue and black circles in (b); (g) standard FTIR of the N-GQDs, (h) geometry optimized structure of N-GQDs used for DFT calculations, (i) co-plot of the IR spectrum generated by DFT calculations and the acquired PiF-IR spectrum.

The PiF-IR spectrum [Fig. 1(f)] offers nanoscale-resolved insights into the surface chemistry of N-GQDs, complementing the ensemble-averaged Fourier transform IR (FTIR) data shown in Fig. 1(g). In the single particle spectrum, multiple strong carbonyl (νC=O) bands are observed at 1770, 1742, 1709, 1695, and 1669 cm⁻¹, including specific signals corresponding to carboxylic acid groups (1709 cm⁻¹) and conjugated C=O functionalities, in agreement with the broad carbonyl stretching region observed in the FTIR spectrum [Fig. 1(g)].^30–32 Vibrational features at 1684 and 1649 cm⁻¹ can be attributed to conjugated C=C stretches, consistent with the skeletal vibrations (2160 and 1650 cm⁻¹) seen in FTIR, reflecting sp² carbon domains.^31–33 N–H bending (δNH) and coupled amine/nitrile vibrations (δN–H and νC≡N) appear prominently in the 1591–1567 cm⁻¹ range, mirroring the FTIR signature at 1535 cm⁻¹ and are assigned to δN–H and νC≡N coupling.^32,34,35 Peaks at 1547–1545 cm⁻¹ correspond to νCN and δCHN/C=N modes, further supporting the presence of nitrogen functionalities.^36,37 Additional bands at 1525, 1456 and 1350 cm⁻¹ indicate C=C and C–N/C=N vibrations, aligning with FTIR signals assigned to phenazine-like and aromatic nitrogen species.^31,38,39 The presence of COO⁻ (1419 cm⁻¹), carboxylate (1395 cm⁻¹), and C=N/phenazine (1354 cm⁻¹) groups further corroborates the FTIR evidence of oxygen- and nitrogen-containing functionalities.^40,41 Moreover, the C–O stretching at 1251 cm⁻¹ and symmetric C–O vibration at 1472 cm⁻¹ are consistent with the FTIR bands attributed to epoxy, ether, or alcohol moieties.^42,43 Overall, the strong spectral correlation between PiFM and FTIR confirms a chemically heterogeneous surface populated with oxygen- and nitrogen-functional groups, validating the robustness of the chemical assignments and their relevance to the N-GQDs’ solubility, electronic structure, and reactivity. The details of peak assignments and more elaborate discussions are presented in the SI [Fig. S2].

Fig. 1(h) presents a model N-GQD structure used for DFT calculations, which comprises a single layer of 46 carbon, 7 nitrogen, 5 oxygen, and 19 hydrogen atoms. The optimized geometry was verified as a true local minimum on the potential energy surface, confirmed by the absence of imaginary frequencies in the vibrational analysis. The models were sized to approximately match the average dimensions of the N-GQDs while remaining computationally tractable. Random N, H, and O functional groups and edge defects were included to reflect the structural heterogeneity observed experimentally. Despite the simplifications of monolayer models with fixed defects, this approach effectively correlates PiF-IR spectra with chemical structure without capturing the full complexity of real particles. Fig. 1(i) compares the vibrational spectra obtained from PiF-IR measurements and the DFT-simulated data. Strong correspondence is observed between experimental and theoretical wavenumbers for key vibrational modes. The carbonyl stretching modes (νC=O) appear at 1770, 1742, 1709, 1695, and 1669 cm⁻¹ in the PiFIR spectrum, with DFT values showing a deviation of less than ±5 cm⁻¹, indicating high predictive accuracy (Table S1). Similarly, peaks assigned to C=C stretching, C–N/C=N vibrations, and amine-related bending modes (δNH₃, δN–H, and νC–N) exhibit close alignment, with most differences falling within 4 to 10 cm⁻¹ (Table S1). Additional bands associated with C–O, COO⁻, and heterocyclic functionalities like phenazine also match well. The consistency between DFT and PiFIR confirms the reliability of the modelled structure and supports its use in interpreting the molecular-level vibrational characteristics of NGQDs.

X-ray diffraction (XRD) of the N-GQDs reveals a broad peak at 21.9°, indicative of disordered graphitic domains with short-range order, typical of quantum-confined sp² carbon systems [Fig. 2(a)]. The Raman spectrum of the N-GQDs [Fig. 2(b)] reveals a comprehensive set of vibrational features that span from 1200 to 2930 cm⁻¹, reflecting their complex structural and chemical nature. A shoulder-like band around 1200 cm⁻¹ is due to graphitic nitrogen (N_g), confirming substitutional nitrogen doping within the carbon lattice.⁴⁴ The 1250–1300 cm⁻¹ region corresponds to pyrene-like aromatic domains, suggesting the presence of large polycyclic aromatic hydrocarbon (PAH) structures.⁴⁴ The D band at 1359 cm⁻¹ indicates structural defects and edge states, while the G band at 1588 cm⁻¹ arises from the in-plane vibration of sp²-bonded carbon atoms.^45,46 A weak band at 1770 cm⁻¹ is attributed to the second-order overtone of the out-of-plane transverse optical (oTO) phonon, which becomes Raman-active due to symmetry breaking from nitrogen doping and finite-size effects.⁴⁷ Notably, a combination mode at 1860 cm⁻¹, corresponding to the iTALO⁻ mode (iTA + LO phonons), further evidences the influence of disorder and doping.⁴⁸ A distinct peak at 2240 cm⁻¹ is assigned to the aromatic nitrile (Ar–C≡N) stretching vibration, suggesting edge functionalization with nitrile groups.⁴⁹ A weak feature around 2350 cm⁻¹ may be linked to the asymmetric stretching mode (ν₃) of CO₂, potentially arising from physisorbed or evolved CO₂ species on the doped surface.⁵⁰ A weak Raman band near 2500 cm⁻¹ (G*) in NGQDs arises from a double-resonance intervalley process involving iTO and LA phonons, indicating structural disorder and edge-related scattering.⁵¹ The 2D band at 2674 cm⁻¹, although broadened and downshifted, indicates the presence of few-layer graphene domains, typical of small, disordered graphene fragments.^46,52 Finally, the D + G combination band at 2930 cm⁻¹ supports the existence of disorder and edge functionalities.^46,53 The intensity ratio I_G/I_D = 1.4 reflects a moderate degree of graphitic ordering with a significant number of defects, while I_2D/I_G = 0.77 is consistent with few-layer graphene-like structures rather than pristine monolayers. Overall, the Raman spectral analysis confirms successful nitrogen doping, aromatic domain formation, and a balance between structural order and nanoscale disorder in the N-GQD system.

Fig. 2 XRD, Raman and surface chemical characterization of N-GQDs. (a) XRD profile showing a broad hump, (b) Raman spectra showing D, G and 2D bands, (c) wide scan XPS spectra showing elemental composition, (d) high resolution C 1s peak showing different carbon bonding states, (e) high resolution N 1s peak, and (f) high resolution O 1s peak showing different oxygen surface states.

The N-GQD system exhibits a balance between structural order and nanoscale disorder. The surface chemical composition of the N-GQDs is further elucidated through a detailed analysis of X-ray photoelectron spectroscopy (XPS) shown in Fig. 2(c–f), corroborating the findings of IR absorption and Raman spectroscopy. The wide scan spectrum shown in Fig. 2(c) confirms the presence of carbon, nitrogen, and oxygen, while high-resolution deconvoluted C 1s [Fig. 2(d)] reveals three distinct components: C=C (284.5 eV) corresponding to sp²-hybridized graphitic carbon domains, C–O/C–N (285.9 eV) and C=O (287.6 eV) indicating functionalization with oxygen and nitrogen heteroatoms.^54–56 The deconvoluted N 1s [Fig. 2(e)] spectrum contains signals at 398.9 eV (pyridinic N), 400.4 eV (amine N), and 401.2 eV (pyrrolic N), indicating that nitrogen atoms are successfully incorporated into both the basal plane and the edge sites of the graphene lattice, which are known to modulate the electronic structure and surface reactivity.^57,58 Similarly, the deconvoluted O 1s spectrum [Fig. 2(f)] shows peaks at 529.7 eV (C–OH), 530.7 eV (C–O–C), and 531.7 eV (C=O), further supporting the presence of hydroxyl, ether, and carbonyl functionalities.^54,59 The overlap of C=O and C=C and nitrogen species signals across XPS, FTIR, and PiF-IR strongly indicates that conjugated carbonyl groups are a dominant surface feature and likely to contribute to the observed optoelectronic and solubility properties of the N-GQDs. Collectively, the structural and surface characterization reveals that the N-GQDs possess a moderately ordered graphitic core decorated with a variety of oxygen- and nitrogen-containing functional groups. These surface states, particularly the conjugated C=O and nitrogen species, are expected to influence charge distribution, electronic transitions, and interaction with solvents or analytes.

The solvent-dependent optical properties of N-GQDs were systematically investigated using UV-Vis absorption, PL spectroscopy, PL excitation (PLE), and time-resolved fluorescence decay measurements across four solvents: water, IPA, chloroform, and toluene. As illustrated in Fig. 3(a), the UV-Vis spectra of N-GQDs in all solvents exhibit two characteristic absorption bands: a prominent high-energy peak which is usually attributed to π–π* electronic transitions within the graphitic sp² domains, and a broad low-energy shoulder arising from n–π* transitions associated with surface-bound oxygen and nitrogen functional groups.³² It should be noted here that the absorption spectra of N-GQDs in different solvents show noticeable variation, particularly in chloroform and toluene, due to their strong intrinsic absorption and steep refractive-index dispersion near their respective UV cutoffs (∼245 nm for chloroform and ∼285 nm for toluene).^60,61 These solvent effects reduce transmitted intensity and distort the baseline, leading to apparent differences in absorbance compared to water and isopropanol. Consequently, the absorption data in chloroform and toluene are interpreted qualitatively rather than quantitatively, serving primarily to reveal general trends that are supported by polarized emission and complementary spectroscopic analyses in our subsequent analyses. The trend in the UV-vis spectra reflects reduced stabilization of the electronic states in less polar environments. In polar solvents (e.g. water), strong solute–solvent interactions, particularly hydrogen bonding and dipolar stabilization, enhance the stabilization of both ground and excited states, resulting in higher-energy absorption.⁶² In contrast, weaker interactions in non-polar solvents like toluene lead to a smaller energy gap and lower-energy (red-shifted) absorption. This systematic shift emphasizes the critical role of the solvent environment in modulating the electronic structure and optical transitions of N-GQDs. The lower energy, n–π* absorption band also redshifted from water (358 nm) to toluene (373 nm). This shift can be attributed to the inability of toluene, a non-polar and non-hydrogen-bonding solvent, to stabilize the lone-pair orbitals involved in n–π* transitions. In contrast, polar solvents effectively stabilize these non-bonding orbitals through dipole–dipole and hydrogen-bonding interactions, leading to lower-energy transitions.^24,63

Fig. 3 Solvent-dependent optical properties of colloidal N-GQDs. (a) UV-visible absorption spectra; (b) PL excitation (PLE) spectra; (c) normalized PL emission spectra; and (d) time-resolved PL decay profiles in four solvents. (e–h) 2-D contour maps of excitation-dependent normalized PL emission spectra of the N-GQDs in water, IPA, chloroform and toluene, respectively.

The PLE spectra, shown in Fig. 3(b), exhibit a similar solvent-dependent redshift in the high-energy region as seen in the UV-Vis spectra, demonstrating that solvent polarity governs the photoexcitation energetics. All the PLE peaks are consistently red-shifted compared to the corresponding UV-Vis absorption peaks. This difference arises because PLE reflects both the probability of photon absorption and the efficiency of radiative recombination, with the emission originating from lower energy relaxed excited states. The presence of distinct high- and low-energy PLE bands confirms the existence of at least two emissive transitions in N-GQDs, both modulated by the surrounding solvent.⁶² This consistent behaviour from water (most polar) to toluene (least polar) underscores how solvent interactions tune both the initial excitation and subsequent radiative decay channels in these systems.

Contrastingly, the PL emission spectra, shown in Fig. 3(c), show an inverse trend with solvent polarity, a progressive blue shift in emission from water to toluene [450–410 nm]. We propose that this reversal arises from variations in structural relaxation, referring to the reorganization of the excited-state electronic structure following photon absorption. In polar solvents like water, strong solvent–solute interactions promote effective structural relaxation, allowing the excited N-GQDs to reorganize and emit from a lower-energy, fully relaxed state, resulting in red-shifted emissions. In non-polar solvents such as toluene, the absence of such stabilizing interactions leads to incomplete relaxation, causing emission from higher-energy, partially relaxed states, and thus blue-shifted PL.⁶³ This interpretation is corroborated by the fluorescence lifetime data shown in Fig. 3(d) [details of lifetime data shown in Table S2]. N-GQDs exhibit the longest lifetimes in water, followed by IPA and chloroform, and the shortest in toluene. The longer lifetimes in polar solvents indicate slower relaxation and efficient stabilization of the excited state, whereas the shorter lifetimes in non-polar solvents point to faster transitions due to inefficient relaxation. These lifetime dynamics confirm that solvent polarity critically influences the structural relaxation process and, consequently, the emission properties of N-GQDs.⁶⁴

Further evidence of this polarity-dependent relaxation is provided by the excitation-dependent PL emission shown in Fig. 3(e–h). In water [Fig. 3(e)] and IPA [Fig. 3(f)], N-GQDs exhibit excitation-independent emission up to excitation wavelengths of ∼390 nm, indicating emission from energetically well-defined, homogeneous states that are fully relaxed due to strong solvent stabilization. Beyond these thresholds (λ_th), the emission becomes excitation-dependent, with progressive red shifts at longer excitation wavelengths. This behaviour suggests the activation of additional, less-relaxed emissive states associated with surface traps or localized defects.^65,66 In chloroform [Fig. 3(g)], the onset of excitation-dependent PL occurs at a higher excitation energy (372 nm), while in toluene [Fig. 3(h)], there appears to be a less well-defined λ_th; first, we see a point of inflection around 310 nm, followed by a second at ∼350 nm, indicating reduced stabilization of excited states. The weaker solute–solvent interactions in these media limit structural relaxation, resulting in a broader distribution of emissive states that vary with excitation wavelength. Toluene's minimal polarity and lack of hydrogen bonding lead to highly heterogeneous emission, dominated by poorly relaxed, energetically diverse states. To evaluate the threshold wavelength separating excitation dependent and independent behaviour, the variation of PL peak energy with excitation energy [Fig. S3] was utilized. Each dataset was segmented and fitted with linear regressions, with the intersection defining the threshold excitation wavelength beyond which surface or trap states dominate.

In addition, zeta potential measurements (SI, Table S3) provide a crucial electrokinetic link to these photophysical observations. N-GQDs exhibit highly negative surface potentials in polar solvents (−16.6 mV in water and −13.9 mV in IPA), indicative of strong solvation and electrostatic stabilization.⁶⁷ In contrast, significantly positive zeta potentials are observed in less polar solvents (+42.6 mV in chloroform and +85.0 mV in toluene), reflecting reduced surface ionization and diminished solvent screening.⁶⁷ These shifts in surface charge align with PL behaviour: polar solvents enhance colloidal and electronic stabilization, promoting red-shifted, long-lived emission, while non-polar solvents facilitate surface trap exposure, leading to blue-shifted, short-lived, and excitation-dependent PL. The combined results from UV-Vis, PLE, PL emission, lifetime decay, excitation–emission mapping, and zeta potential measurements collectively establish a coherent photophysical framework: solvent polarity dictates the extent of structural relaxation in N-GQDs, which in turn determines the energy, lifetime, and excitation dependence of their emission. Polar solvents enable effective solvation, surface passivation, and excited-state stabilization, resulting in homogeneous, red-shifted emission. Conversely, non-polar solvents limit these interactions, inducing heterogeneous, blue-shifted, and excitation-sensitive PL. This solvent-controlled optical tunability underscores the critical role of the solvation environment in designing and optimizing N-GQD-based materials for applications.

While the solvatochromic behaviour and solvent-dependent electronic stabilization of GQDs have been discussed in the literature, our objective here is not to revisit these well-established effects. Instead, we report a previously unobserved phenomenon – solvent interactions induce a quasi-alignment of N-GQDs within colloidal suspensions, resulting in polarized PL emission. This solvent-mediated orientational ordering represents a novel form of anisotropic optical response in otherwise isotropic dispersions of GQDs and opens new avenues for understanding and controlling direction-dependent emission in nanocarbon-based fluorophores.

To probe the anisotropic nature of the emissive states and explore potential orientational ordering in colloidal suspensions, we conducted polarization-resolved PL measurements across a range of solvent environments. We observe striking features in the polarization-resolved PL behaviour of the N-GQDs dispersed in the four different solvents – water, isopropanol, chloroform, and toluene – offering deeper insight into their emission mechanisms and the influence of the solvent environment on emissive states. To exclude the possibility of excitation-induced fluorescence anisotropy, all measurements were performed under unpolarized broadband excitation from a xenon source, and control experiments with varied excitation polarization confirmed that the emission polarization was independent of the excitation state. Additionally, polarization-resolved PL detection was carried out with corrections for Woods’ anomaly and polarizer wavelength response, as accounted for by the instrument calibration (SI).

The introduction of a polarizer in the emission detection arm resulted in a consistent reduction in PL intensity relative to the unpolarized (X–X) configuration, with the emission intensity exhibiting a periodic angular dependence across all solvents [Fig. 4(a) and Fig. S4]. Notably, the integrated intensity consistently peaked near 10° and reached a minimum around 100°, indicative of partially polarized emission and reflecting an underlying anisotropy in the orientation of emitting dipoles. This anisotropy is likely a consequence of solvent-induced quasi-alignment of the N-GQDs within the colloidal suspension. Even by visual inspection of the polarization-resolved PL spectra [Fig. 4(a)], a consistent and periodic variation in integrated intensity, along with an apparent shift in peak position with the analyzer angle, is readily discernible. These spectral modifications, present across all solvents, suggest that the polarizer introduces angularly dependent features in the emission profile. Fig. 4(b) presents the influence of solvent environment on the polarization-resolved emission profiles of the samples, measured at four distinct analyzer orientations (X-10, X-40, X-70, X-100). The normalized emission spectra reveal that, for each solvent and analyzer configuration, the PL profiles exhibit clear signs of spectral splitting and the emergence of distinct features that evolve periodically as the analyzer angle is varied. The observed periodic variations arise from an interplay between solvent-induced alignment effects and the inherent anisotropy of the emitting species, which becomes evident only when the full set of analyzer configurations is considered together. However, the individual profiles, when examined in isolation, appear irregular and lack an immediately discernible systematic trend. A clearer picture emerges when we interpret the net PL emission as a convolution of three distinct emissive pathways. The PL spectra across all solvents are well described by a superposition of three Gaussian components, each corresponding to a discrete emissive transition. A few representative deconvoluted spectra for the N-GQDs in water are shown in Fig. 4(c); the complete set is presented in SI Fig. S5. The three-band emission structure is consistent with the observed absorption features, excitation-dependent PL behaviour, and the tri-exponential decay kinetics (Table S2) revealed in time-resolved PL (TRPL) measurements. Collectively, these results strongly support the existence of three electronically and structurally independent emissive centres: (i) π–π* recombination from conjugated sp² carbon domains, (ii) nitrogen-related surface states, and (iii) oxygen-associated trap states.

Fig. 4 (a) Variation of emission intensity of N-GQDs in water, IPA, chloroform and toluene at different analyzer angles; (b) comparison of the normalized PL spectra of N-GQDs in different solvents at different polarizer angles: 10° (maximum), 40°, 70° and 100° (minimum); (c) deconvoluted PL spectra of N-GQDs in water at analyzer angles of 10°, 40°, 70°, and 100°, illustrating the relative contributions of distinct emissive components at different polarization orientations. The complete set of deconvoluted spectra for all solvents and analyzer configurations is provided in the SI; (d) polarization map of the synthesized N-GQDs with experimental data shown as scatter plots and fitted curves shown in solid lines in different solvents: water, IPA, chloroform, and toluene.

Importantly, the polarization sensitivity and angular modulation of individual Gaussian components underscore their electronic and structural decoupling, suggesting emission from spatially or energetically distinct sites within the N-GQDs. Although the presence of these emissive centres is consistent across all solvents, their relative contributions and polarization behaviour vary depending on the solvent-specific interactions, revealing a solvent-tunable emission landscape. Polarization-resolved deconvolution of the PL spectra as a function of analyzer angle reveals a pronounced and regular sinusoidal variation in the integrated intensities of both the high- and mid-energy peaks across all solvents as evident from the polarization maps shown in Fig. 4(d) and SI Fig. S6. This consistent angular dependence strongly supports the partially aligned and anisotropic nature of these emissive centres, likely to originate from well-defined conjugated domains and nitrogen doping configurations that interact directionally with the electric field vector of the emission.

In contrast, the low-energy peak assigned to oxygen-related trap states shows a less predictable and irregular variation with the analyzer angle [scattered red triangles in Fig. 4(d)]. This behaviour can be attributed to the heterogeneous nature of oxygen functional groups on the GQD surface. Unlike the relatively uniform π-conjugated domains or nitrogen dopant configurations, oxygen functionalization introduces a diverse array of chemical environments, including carboxyl (–COOH), hydroxyl (–OH), epoxy (C–O–C), and carbonyl (C=O) groups. The presence of this diverse population of trap states is confirmed by vibrational spectroscopic analysis – both FTIR and PiFIR spectra reveal multiple oxygen-containing moieties, each contributing differently to the local electronic structure and thus generating broad, energetically disordered emissive sites.³² These oxygen-associated traps are likely distributed with random spatial orientation and variable electronic coupling to the GQD core, leading to incoherent or weakly polarized emission that lacks a systematic angular dependence. The apparent arbitrariness in the low-energy peak's polarization response therefore reflects the structural and electronic disorder inherent to oxygen-related emissive states, distinguishing them from the more anisotropic and well-coupled transitions associated with π–π* and nitrogen states.

Quantitative analysis of polarization anisotropy further substantiates the distinct origins and solvent sensitivity of the three emissive channels. Among them, the high-energy peaks consistently exhibit the highest degree of polarization across all solvents, reaching values as high as 0.98 in chloroform, strongly indicating that these transitions originate from intrinsically ordered and structurally embedded π-domains within the sp² carbon lattice.⁶⁸ In contrast, the mid-energy peaks showed moderate DOP values ranging from 0.3 to 0.84 and were markedly more responsive to solvent polarity, a trend consistent with the solvent-dependent shifts in PLE maxima and the broadening of decay components observed in TRPL measurements. We may recall that in solvents such as water and IPA, the PL decay profiles exhibited longer lifetimes [Fig. 3(e)], supporting the role of solvent-mediated relaxation and solvation dynamics around surface functional groups, particularly nitrogen and oxygen moieties. This behaviour reflects the interaction of these solvents with dipolar or charged emissive sites, leading to dynamic stabilization and redistribution of excited-state populations. The variation in DOP in different solvents also correlates with zeta potential measurements (SI, S3), which provide insight into the surface charge characteristics and solvent-particle interfacial interactions. In solvents, where the zeta potential is strongly positive (+42.6 mV and +85.0 mV), the reduced dielectric screening may promote closer interparticle interactions or quasi-alignment through dipolar attraction or electrostatic stabilization, resulting in higher DOP, particularly in the π–π* emission channel.⁶⁷ Conversely, in solvents with moderate negative zeta potentials, the solvation shell partially inhibits such alignment, leading to lower overall DOP, especially in solvent-sensitive mid- and low-energy emission bands.⁶⁷

To understand how solvent polarity and surface charge influence the self-assembly and optical anisotropy of the NGQDs, we analyzed the DOP and the dipole orientation angles (θ₀) of emission in the four solvents. It should be noted here that the observed polarization anisotropy of the N-GQDs in different solvents is governed by the combined effects of polarity, π-solvation, and interfacial charge (ζ potential), which act in a competitive manner. As summarized in Table S3, water (dipole moment ≈ 3 D, ζ = −16.6 mV) shows a high DOP (0.866) for the high-energy peak and a moderate DOP (0.406) for the medium-energy peak, with an average dipole orientation angle of ∼9°.^69,70 IPA, with lower polarity (1.66 D) and zeta potential (−13.9 mV), yields an even higher DOP (0.94) for the high-energy peak, though the DOP for the medium-energy peak decreases to 0.305. Chloroform, a low-polarity solvent (1.15 D) with positive ζ (+42.6 mV), shows the highest DOP (0.987) at the high-energy peak and a significantly increased DOP (0.701) at the medium-energy peak, suggesting tighter, more anisotropic dipolar packing.⁷⁰ In contrast, toluene, with the lowest polarity (0.375–0.43 D) and highest ζ potential (+85 mV), shows a strong DOP at the medium-energy peak (0.84) but a reduced DOP (0.735) at the high-energy peak.^67,70 From water to chloroform, the DOP of the high-energy π–π* band increases with decreasing polarity but then drops markedly in toluene, revealing a non-monotonic trend. This reversal is likely due to toluene's strong π-solvation of sp² domains combined with its very high positive ζ (+85 mV), which stabilizes dispersion, maintains larger interparticle separation, and broadens dipole orientation distributions, thereby suppressing alignment despite its low polarity.^67,70 In contrast, chloroform, with lower ζ and weaker π-solvation, allows tighter quasi-alignment, resulting in a higher DOP. Interestingly, the medium-energy N-related emission shows the opposite behaviour, reaching its maximum DOP in toluene. This suggests that, unlike π-domains, N-related states couple more strongly to the local solvent shell than to long-range interparticle packing. In toluene, the weak hydrogen bonding, low dielectric screening, and electrostatic stabilization from the high positive ζ may orient or rigidify these surface states at the particle–solvent interface, enhancing their anisotropy even under conditions where π-domains are less aligned. Similarly, the N-related medium-energy peak shows a slightly higher DOP in water (0.406) than in IPA (0.305), deviating from the general trend of increasing DOP with decreasing solvent polarity. This behaviour can be attributed to stronger hydrogen bonding and a more negative ζ potential in water, which promote greater ordering and rigidity of the N-related surface states at the particle–solvent interface. In contrast, IPA's weaker hydrogen bonding and less negative ζ potential lead to reduced interfacial ordering, despite its lower polarity. In summary, the polarization behaviour of N-GQDs arises from the dynamic balance between solvent polarity, π-solvation strength, and interfacial charge – three competitive factors that together dictate the alignment and anisotropy of distinct emissive states. The definitive contribution of each factor, however, warrants further targeted experimentation to isolate and quantify their individual effects. A simplified schematic of the solvent-dependent polarized emission due to the alignment of N-GQDs and the proposed PL mechanism consistent with the observations are summarized in Fig. 5.

Fig. 5 Solvent-driven alignment modulates polarized emission from N-GQDs. Schematic illustrating the partially and fully polarized emission components arising from solvent-induced alignment of emissive dipoles, along with the proposed PL mechanism in colloidal N-GQDs.

In polar solvents such as IPA and water, the N-GQDs exhibit quasi-alignment, as depicted in the upper blue box. This partial ordering results in partially polarized high- and mid-energy PL emissions and unpolarized low-energy emissions. The quasi-aligned N-GQDs allow the high- and mid-energy emissive centres corresponding to π–π* recombination in conjugated sp² carbon domains and N-related surface states to couple anisotropically with the emission's electric field vector, yielding measurable polarization degrees but with some orientational distribution. The low-energy emission, associated with oxygen-related trap states, remains largely unpolarized due to the heterogeneous and randomly oriented oxygen functional groups on the surface. Conversely, in less polar solvents such as chloroform and toluene, the figure shows more aligned N-GQDs (bottom green box). This stronger alignment, promoted by higher positive zeta potentials and reduced solvation shell effects, tightens the packing of dipoles, resulting in higher degrees of polarization at both high- and mid-energy emission peaks. The emission becomes more directionally coherent, indicated by higher DOP (0.987 for high-energy peaks). Notably, the low-energy emission remains unpolarized, consistent with the persistent disorder of oxygen-related trap states, which do not couple strongly to the alignment. The rightmost part of the figure summarizes the energy level transitions underpinning these observations. The PL emission arises from radiative transitions involving the π–π* states, nitrogen-related states (N-states), and oxygen trap states (O-states), while non-radiative transitions occur within the conduction and valence bands. The anisotropic emission polarization results from the directional coupling of these discrete emissive centers to the excitation and emission electric fields, modulated by the solvent-induced alignment or quasi-alignment of N-GQDs.

Our findings demonstrate that solvent polarity and interfacial charge profoundly modulate the anisotropic PL emission from N-GQDs by influencing the alignment and emissive pathways of distinct electronic states. Future studies integrating time-resolved polarization dynamics and controlled assembly strategies could further elucidate the interplay between solvent-mediated organization and optical anisotropy in colloidal N-GQDs.

Conclusion

In summary, this work demonstrates how solvent polarity and interfacial charge modulate the anisotropic PL of N-GQDs, providing the first systematic evidence of solvent-induced quasi-alignment of emitting dipoles in colloidal suspensions in graphene dot-based systems. Polarization-resolved PL spectroscopy across four solvents of varying polarity reveals periodic angular modulations in emission intensity and spectra, indicative of partially aligned, anisotropic emissive centres. Spectral deconvolution identifies three distinct pathways: high-energy π–π* transitions from conjugated sp² domains, mid-energy nitrogen-related surface states, and low-energy oxygen-associated trap states. The π–π* and nitrogen-related transitions exhibit strong, solvent-sensitive polarization anisotropy, reaching degrees of polarization up to 0.98 in low-polarity, high-ζ solvents like chloroform. In contrast, oxygen-related trap states show weak, irregular polarization, reflecting their chemical heterogeneity. Low-polarity solvents and positive surface charge favour tighter dipole packing and enhanced anisotropy, while polar solvents stabilize excited states, broaden dipole orientations, and extend PL lifetimes. These observations highlight the interplay between solvent-mediated interactions, excited-state dynamics, and structural anisotropy. Our findings establish a mechanistic framework for solvent-tunable polarized emission in N-GQDs, linking molecular alignment to distinct emissive pathways. Beyond providing fundamental insight, they suggest routes for engineering GQD-based materials as solvent-responsive polarized light sources, anisotropic sensors, and photonic components. Future work combining time-resolved polarization dynamics, field-assisted alignment, and single-particle polarized PL mapping can further elucidate how nanoscale organization shapes optical anisotropy, advancing the design of functional carbon-based nanomaterials.

Conflicts of interest

There are no conflicts of interest to declare.

Data availability

The data that support the findings of this study are available online and can be accessed through the link https://osf.io/swygk/files/osfstorage/68b0cb21105708719bf98ebb.

Supplementary information (SI): (1) details of photoinduced force microscopy, (2) details of FTIR analysis, (3) additional optical characterization results, (4) zeta potential analysis, (5) Wood's anomaly and polarizer response, and (6) solvent polarity, zeta potential and the variation of DOP. See DOI: https://doi.org/10.1039/d5nr03664j.

Acknowledgements

MR acknowledges the funding provided by SECIHTI (formerly CONAHCyT) in the form of scholarships as a member of the National System of Researchers (SNI 1047863) and acknowledges the support from the Federico Baur Endowed Chair in Nanotechnology

References

1. Y. Yan, J. Gong, J. Chen, Z. Zeng, W. Huang, K. Pu, J. Liu and P. Chen, Adv. Mater., 2019, 31, 1808283.
2. L. Chen, S. Yang, Y. Li, Z. Liu, H. Wang, Y. Zhang, K. Qi, G. Wang, P. He and G. Ding, Adv. Funct. Mater., 2024, 34, 2401246.
3. M. T. Hasan, R. Gonzalez-Rodriguez, C. Ryan, N. Faerber, J. L. Coffer and A. V. Naumov, Adv. Funct. Mater., 2018, 28, 1804337.
4. S. Ghosh, A. M. Chizhik, N. Karedla, M. O. Dekaliuk, I. Gregor, H. Schuhmann, M. Seibt, K. Bodensiek, I. A. T. Schaap, O. Schulz, A. P. Demchenko, J. Enderlein and A. I. Chizhik, Nano Lett., 2014, 14, 5656–5661.
5. P. Jing, D. Han, D. Li, D. Zhou, L. Zhang, H. Zhang, D. Shen and S. Qu, Adv. Opt. Mater., 2017, 5, 1601049.
6. D. M. Jameson and J. A. Ross, Chem. Rev., 2010, 110, 2685–2708.
7. J. J. Macklin, J. K. Trautman, T. D. Harris and L. E. Brus, Science, 1996, 272, 255–258.
8. S. A. Empedocles, R. Neuhauser and M. G. Bawendi, Nature, 1999, 399, 126–130.
9. J. Hu, L. Li, W. Yang, L. Manna, L. Wang and A. P. Alivisatos, Science, 2001, 292, 2060–2063.
10. D. Wang, D. Wu, D. Dong, W. Chen, J. Hao, J. Qin, B. Xu, K. Wang and X. Sun, Nanoscale, 2016, 8, 11565–11570.
11. K. Zhanghao, L. Chen, X.-S. Yang, M.-Y. Wang, Z.-L. Jing, H.-B. Han, M. Q. Zhang, D. Jin, J.-T. Gao and P. Xi, Light:Sci. Appl., 2016, 5, e16166.
12. N. Ojha, K. H. Rainey and G. H. Patterson, Nat. Commun., 2020, 11, 21.
13. L. Xiao, Y. Wang, Y. Huang, T. Wong and H. Sun, Nanoscale, 2017, 9, 12637–12646.
14. Y. Kamura and K. Imura, J. Phys. Chem. C, 2020, 124, 7370–7377.
15. S. Subedi, A. K. Rella, L. G. Trung, V. Kumar and S.-W. Kang, ACS Nano, 2022, 16, 6480–6492.
16. N. Suzuki, Y. Wang, P. Elvati, Z.-B. Qu, K. Kim, S. Jiang, E. Baumeister, J. Lee, B. Yeom, J. H. Bahng, J. Lee, A. Violi and N. A. Kotov, ACS Nano, 2016, 10, 1744–1755.
17. A. Ghosh, B. Parasar, T. Bhattacharyya and J. Dash, Chem. Commun., 2016, 52, 11159–11162.
18. Z. Liu, W. Li, W. Sheng, S. Liu, R. Li, Q. Li, D. Li, S. Yu, M. Li, Y. Li and X. Jia, J. Am. Chem. Soc., 2023, 145, 5310–5319.
19. P. Jonkheijm, P. van der Schoot, A. P. H. J. Schenning and E. W. Meijer, Science, 2006, 313, 80–83.
20. H. Li, X. Qian, H. Mohanram, X. Han, H. Qi, G. Zou, F. Yuan, A. Miserez, T. Liu, Q. Yang, H. Gao and J. Yu, Nat. Nanotechnol., 2024, 19, 1141–1149.
21. T. Gao, T. Wang, W. Wu, Y. Liu, Q. Huo, Z. A. Qiao and S. Dai, Adv. Mater., 2019, 31, 1806254.
22. J. Li and X. Gong, Adv. Funct. Mater., 2025, 2403394.
23. X. Liu, N. Feng, X. Sun and H. Li, Adv. Opt. Mater., 2025, 2403394.
24. B. A. Larsen, P. Deria, J. M. Holt, I. N. Stanton, M. J. Heben, M. J. Therien and J. L. Blackburn, J. Am. Chem. Soc., 2012, 134, 12485–12491.
25. Y. Tang, X. Wang, J. Chen, X. Wang, D. Wang and Z. Mao, Carbon, 2021, 174, 98–109.
26. W. Chen, D. Li, L. Tian, W. Xiang, T. Wang, W. Hu, Y. Hu, S. Chen, J. Chen and Z. Dai, Green Chem., 2018, 20, 4438–4442.
27. M. Srivastava, A. K. Das, P. Khanra, Md. E. Uddin, N. H. Kim and J. H. Lee, J. Mater. Chem. A, 2013, 1, 9792.
28. S. Zhang, H. Liu, J. Yu, B. Li and B. Ding, Nat. Commun., 2020, 11, 5134.
29. L. Qu, Y. Liu, J.-B. Baek and L. Dai, ACS Nano, 2010, 4, 1321–1326.
30. C. Yang, H. Aslan, P. Zhang, S. Zhu, Y. Xiao, L. Chen, N. Khan, T. Boesen, Y. Wang, Y. Liu, L. Wang, Y. Sun, Y. Feng, F. Besenbacher, F. Zhao and M. Yu, Nat. Commun., 2020, 11, 1379.
31. F. Rigodanza, M. Burian, F. Arcudi, L. Đorđević, H. Amenitsch and M. Prato, Nat. Commun., 2021, 12, 2640.
32. S. Kundu, A. B. Siddique, I. F. G. González, K. A. R. Mireles, M. I. P. Valverde, N. A. U. Castillo, M. Reghunathan, D. I. G. Gutiérrez, E. M. Guerra and M. Ray, Nanoscale, 2025, 17, 17647–17657.
33. Y. Yamada, M. Kawai, H. Yorimitsu, S. Otsuka, M. Takanashi and S. Sato, ACS Appl. Mater. Interfaces, 2018, 10, 40710–40739.
34. B. Sui, Y. Zhu, X. Jiang, Y. Wang, N. Zhang, Z. Lu, B. Yang and Y. Li, Nat. Commun., 2023, 14, 6782.
35. F. S. Ruggeri, G. Longo, S. Faggiano, E. Lipiec, A. Pastore and G. Dietler, Nat. Commun., 2015, 6, 7831.
36. V. Ţucureanu, A. Matei and A. M. Avram, Crit. Rev. Anal. Chem., 2016, 46, 502–520.
37. R. Shi, L. Liu, Y. Lu, C. Wang, Y. Li, L. Li, Z. Yan and J. Chen, Nat. Commun., 2020, 11, 178.
38. P. K. Srivastava, P. Yadav and S. Ghosh, Nanoscale, 2016, 8, 15702–15711.
39. J. Guo, Y. Xu, S. Jin, L. Chen, T. Kaji, Y. Honsho, M. A. Addicoat, J. Kim, A. Saeki, H. Ihee, S. Seki, S. Irle, M. Hiramoto, J. Gao and D. Jiang, Nat. Commun., 2013, 4, 2736.
40. M. Wang, R. Wang, H. Yao, Z. Wang and S. Zheng, RSC Adv., 2016, 6, 63365–63372.
41. A. B. Siddique, S. M. Hossain, A. K. Pramanick and M. Ray, Nanoscale, 2021, 13, 16662–16671.
42. A. Ngqalakwezi, D. Nkazi, G. Seifert and T. Ntho, Catal. Today, 2020, 358, 338–344.
43. X. He, J. M. Larson, H. A. Bechtel and R. Kostecki, Nat. Commun., 2022, 13, 1398.
44. M. Ayiania, E. Weiss-Hortala, M. Smith, J.-S. McEwen and M. Garcia-Perez, Carbon, 2020, 167, 559–574.
45. F. Tuinstra and J. L. Koenig, J. Chem. Phys., 1970, 53, 1126–1130.
46. E. Dervishi, Z. Ji, H. Htoon, M. Sykora and S. K. Doorn, Nanoscale, 2019, 11, 16571–16581.
47. Y. Jiang, S. Chowdhury and R. Balasubramanian, J. Colloid Interface Sci., 2019, 534, 574–585.
48. R. Rao, R. Podila, R. Tsuchikawa, J. Katoch, D. Tishler, A. M. Rao and M. Ishigami, ACS Nano, 2011, 5, 1594–1599.
49. A. Mueller, M. G. Schwab, N. Encinas, D. Vollmer, H. Sachdev and K. Müllen, Carbon, 2015, 84, 426–433.
50. W. Gao, G. Shi, Z. Jin, J. Shu, Q. Zhang, R. Vajtai, P. M. Ajayan, J. Kono and Q. Xu, Nano Lett., 2013, 13, 3698–3702.
51. Y. Wang, X. Zhou, Y. Jin, X. Zhang, Z. Zhang, Y. Wang, J. Liu, M. Wang, Y. Xia, P. Zhao, Z. Zhang and H. Wang, Phys. Rev. B, 2019, 100, 241407.
52. R. Ye, C. Xiang, J. Lin, Z. Peng, K. Huang, Z. Yan, N. P. Cook, E. L. G. Samuel, C.-C. Hwang, G. Ruan, G. Ceriotti, A.-R. O. Raji, A. A. Martí and J. M. Tour, Nat. Commun., 2013, 4, 2943.
53. E. Martins Ferreira, M. V. Moutinho, F. Stavale, M. M. Lucchese, R. B. Capaz, C. A. Achete and A. Jorio, Phys. Rev. B:Condens. Matter Mater. Phys., 2010, 82, 125429.
54. L. Zhang, W. Yi, J. Li, G. Wei, G. Xi and L. Mao, Nat. Commun., 2023, 14, 6318.
55. J. Cao, Y. Hu, L. Chen, J. Xu and Z. Chen, Int. J. Hydrogen Energy, 2017, 42, 2931–2942.
56. Y.-X. Wang, M. Rinawati, W.-H. Huang, Y.-S. Cheng, P.-H. Lin, K.-J. Chen, L.-Y. Chang, K.-C. Ho, W.-N. Su and M.-H. Yeh, Carbon, 2022, 186, 406–415.
57. H. Ul Hassan, M. W. Iqbal, S. M. Wabaidur, A. M. Afzal, M. A. Habila and E. Elahi, Int. J. Hydrogen Energy, 2023, 48, 31531–31549.
58. D. Kurniawan and W.-H. Chiang, Carbon, 2020, 167, 675–684.
59. R. K. Vayalakkara, C.-L. Lo, H.-H. Chen, M.-Y. Shen, Y.-C. Yang, A. Sabu, Y.-F. Huang and H.-C. Chiu, J. Controlled Release, 2022, 345, 417–432.
60. H. S. Nalwa, Handbook of organic electronics and photonics, American Scientific Pub., 2008.
61. A. Jordan, P. Stoy and H. F. Sneddon, Chem. Rev., 2021, 121, 1582–1622.
62. G. J. Brealey and M. Kasha, J. Am. Chem. Soc., 1955, 77, 4462–4468.
63. M. Montalti, L. Prodi, N. Zaccheroni and G. Falini, J. Am. Chem. Soc., 2002, 124, 13540–13546.
64. F. Terenziani, A. Painelli, C. Katan, M. Charlot and M. Blanchard-Desce, J. Am. Chem. Soc., 2006, 128, 15742–15755.
65. J. Hoche, A. Schulz, L. M. Dietrich, A. Humeniuk, M. Stolte, D. Schmidt, T. Brixner, F. Würthner and R. Mitric, Chem. Sci., 2019, 10, 11013–11022.
66. A. B. Siddique, S. M. Hossain, A. K. Pramanick and M. Ray, Nanoscale, 2021, 13, 16662–16671.
67. D. Kurniawan, R.-C. Jhang, K. K. Ostrikov and W.-H. Chiang, ACS Appl. Mater. Interfaces, 2021, 13, 34572–34583.
68. M. Born and E. Wolf, Principles of Optics, Pergamon Press, 4th edn, 1970.
69. J. K. Gregory, D. C. Clary, K. Liu, M. G. Brown and R. J. Saykally, Science, 1997, 275, 814–817.
70. C. Baker and J. L. Gole, ACS Sens., 2016, 1, 235–242.

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