Photoluminescence Band Research Articles

Environmentally friendly, highly efficient, and large stokes shift-emitting ZnSe:Mn2+/ZnS core/shell quantum dots for luminescent solar concentrators

In this study, heavy-metal-free orange light-emitting ZnSe:Mn2+/ZnS doped-core/shell (d-C/S) quantum dots (QDs) were synthesized using a nucleation doping strategy. To synthesize high quality d-C/S QDs with high photoluminescence (PL) quantum yield (QY), the Mn2+ concentration was optimized. The resulting ZnSe:Mn2+(5%)/ZnS d-C/S QDs showed a high PL QY of 83.3%. The optical properties of the synthesized QDs were characterized by absorption and PL spectroscopy. Their structural and compositional properties were studied by X-ray diffraction, transmission electron microscopy, and energy dispersive X-ray spectroscopy. After doping Mn2+ into a ZnSe core, the ZnSe:Mn2+/ZnS d-C/S QDs showed a large Stokes shift of 170 nm. The ZnSe:Mn2+/ZnS d-C/S QDs were embedded in a poly(lauryl methacrylate) (PLMA) polymer matrix and the ZnSe:Mn2+/ZnS-based polymer film was fabricated. The fabricated ZnSe:Mn2+/ZnS-PLMA film was highly transparent in the visible spectral region (transmittance > 83.8% for λ ≥ 450 nm) and it exhibited bright orange light under air mass (AM) 1.5G illumination using a solar simulator. The optical path-dependent PL measurement of the ZnSe:Mn2+/ZnS-PLMA film showed no PL band shift and minimal PL decrease under variation of excitation position. These results indicate that the highly efficient and large Stokes shift-emitting ZnSe:Mn2+/ZnS QDs are promising for application to luminescent solar concentrators.

Ultrasensitive Optical Thermometry via Inhibiting the Energy Transfer in Zero-Dimensional Lead-Free Metal Halide Single Crystals.

Self-referencing optical thermometry based on the fluorescence intensity ratio (FIR) have drawn extensive attention as a result of their high sensitivity and non-invasively fast response to temperature. However, it is a great challenge for luminescent materials to achieve simultaneously high absolute and relative temperature sensitivity based on the FIR technique. Herein, we developed a novel optical thermometer by designing hybrid lead-free metal halide (TTPhP)2MnCl4:Sb3+ (TTPhP+ = tetraphenylphosphonium cation) single crystals with multimodal photoluminescence (PL). The large TTPhP+ organic chain resulted in isolated [MnCl4]2- and [SbCl5]2- in the single crystal, which leads to a negligible energy trasfer process within them. Therefore, the two PL bands (band 1 from [MnCl4]2-) with a peak at 518 nm and band 2 (from [SbCl5]2) with a peak at 640 nm exhibit different thermal-quenching effects, which resulted in excellent temperature sensitivity, with the maximum absolute and relative sensitivities reaching 0.236 K-1 and 3.77% K-1 in a temperature range from 300 to 400 K. Both the absolute and relative sensitivities are among the highest values for luminescence thermometry.

Investigation on Physical Properties of IGZO Thin Films under the Conditions of Mixing Oxygen and Argon

Amorphous indium gallium zinc oxide (IGZO) is the most suitable material choice for optoelectronic devices such as thin film transistor (TFT). However, usually, the physical properties of IGZO are of concern rather than the preparation process, which will complicate the control of the main properties of the material. To obtain a simple method of controlling IGZO properties, different proportions of mixed gases of oxygen and argon were added in the process of preparing thin films by pulsed laser deposition (PLD) for studying the fluence of atmosphere on the growth of IGZO. The structure and components of the film are characterized by X-ray diffraction (XRD), which confirmed the amorphous structure. A red-shift of the absorption peak in range of 450–850 nm was generated with the increase in argon concentration. Meanwhile, the transmission spectra showed that the transmittance of the material was lower than 80% in the range of 450–850 nm. Then, different target samples have a wide photoluminescence band at 200–800 nm. Oxygen vacancy defects were considered to be closely related to the photoluminescence behavior. The smallest surface roughness of the films prepared under 50% Ar and 50% O2 and the largest in 100% Ar are proved by atom force microstructure (AFM). Importantly, the greater difference in electrical properties reflects the sensitivity of different oxygen and argon concentrations to material effects. The carrier concentration can be adjusted from 1.08 × 1011 to 1.33 × 1016 by this method. Finally, the IGZO achieved in this work was used in a transistor, which reflected good diode characteristics.

Carbon Quantum Dot-Titanium Doped Strontium Ferrite Nanocomposite: Visible Light Active Photocatalyst to Degrade Nitroaromatics

The synthesis of carbon quantum dots (CQDs) from agricultural waste is a promising approach for waste valorization. In the present work, CQDs were synthesized using sugarcane bagasse as a carbon precursor. The nanocomposite of CQDs with trimetallic strontium–titanium ferrite was synthesized with an ultrasonication approach. The structural, magnetic and optical features of the synthesized nanocomposite and pristine NPs were studied using different analytical techniques. The TEM micrograph of the nanocomposite reveals the distribution of CQDs (8–10 nm) along with the agglomerated ferrite NPs. To validate the results, the photocatalytic efficiency of the nanocomposite, NPs and CQDs was comparatively studied for the photodegradation of nitroaromatic pollutants viz. p-nitrophenol, martius yellow and pendimethalin under visible-light irradiation. A nanocomposite having a 2:1 w:w ratio of CQDs and Sr0.4Ti0.6Fe2O4.6 displays an excellent photocatalytic performance, with the degradation efficiency ranging from 91.2 to 97.4%, as compared with 65.0–88.3% for pristine NPs and CQDs. These results were supported by band gap and photoluminescence analyses. The promising photocatalytic potential of the nanocomposite over the pristine CQDs and ferrite NPs could be ascribed to the increased specific-surface area (101.3 m2/g), lowering in band gap coupled with fluorescence-quenching which facilitated the transfer of photoinduced charge carriers. The impact of parameters affecting the photocatalytic process viz. pH, catalyst dose and contact time was also investigated. On the basis of quenching and gas chromatography-mass spectrometry (GC-MS) studies, plausible degradation pathways were proposed. The results highlight the broad potential of designing substituted ferrite-CQDs-based nanocomposites as reusable and visible-light-driven photocatalysts.

Permanent optical bleaching in HPHT-diamond via aggregation of C- and NV-centers excited by visible-range femtosecond laser pulses

Red-colored high-pressure, high-temperature synthesized (HPHT) diamond was exposed in its bulk in a number of patterns to millions of 0.3-ps, 515-nm laser pulses focused by a 0.25-NA micro-objective. The resulting arrays of bright micrometer-scale spots produced at variable laser exposure and pulse energy was characterized by optical transmission microspectroscopy, room-temperature dynamic and stationary 3D confocal photoluminescence (PL) microspectroscopy, FT-IR microspectroscopy. These bleached spots exhibit 1) reduced IR-absorbance of C- and A-centers at the pertaining absorbance of interstitials, 2) stronger reduced visible-range absorbance of C centers, comparing to that of NV0 and NV− centers, 3) considerably depth-wise reduced PL intensity of NV0 and NV− centers in favor of the emerging blue-green (480–550 nm) PL band of indistinguishable higher-aggregated H3, H4 centers. The photo-activated aggregation process of nitrogen centers exhibits second-power dependence on laser energy and is apparently related to direct agitation of low-aggregated centers, rather than to photo-generation of interstitial-vacancy pairs.

Photoluminescence properties of anodic aluminium oxide films formed in a mixture of malonic acid and oxalic acid.

In this work, porous anodic aluminium oxide (AAO) films were fabricated by anodization in an electrolyte mixture with various concentration ratios of malonic acid and oxalic acid at room temperature. The photoluminescence (PL) properties of the AAO films before and after annealing from 300°C to 650°C in air or vacuum conditions were investigated, showing a strong PL band in the range 300-550 nm. We observed a weak PL in the AAO film formed in the malonic acid electrolyte, whereas the films fabricated using an electrolyte mixture showed strong PL emissions, exhibiting a maximum. The broad PL band was decomposed into three Gaussian sub-bands, in which the first two sub-bands could be attributed to the luminescence centre oxygen vacancies (F+ and F defect centres), whereas the latter transformed from malonic impurities and oxalic impurities. More interestingly, the redshift of the PL bands occurred with increasing oxalic acid concentration, and the PL wavelength and intensity could be modulated by varying the concentration ratios in the malonic acid and oxalic acid electrolyte mixture.

Interrelation of micro-strain, energy band gap and PL intensity in Ce doped ZnS quantum structures

Three-dimensional (3-D) quantum structures (QSs) of ZnS and Cerium (Ce) doped ZnS were synthesized via chemically affordable sol-gel process. Influence of Ce doping and thus induced micro-strain on the structural, morphological, and optical characteristics was explored. XRD confirmed the formation of single-phase zinc blende ZnS. Estimated average crystal size corresponding to highest intensity XRD peak (111) varied within 1.65–4.65 nm which are comparable with Bohr radius of ZnS. Due to the size mismatch between Ce and Zn, micro-strain and vacancies were found to be developed in host matrix of ZnS. Thermodynamic calculations validated an expansion and contraction in lattice parameter due to Ce doping. FTIR spectra confirmed the presence of different functional groups related to Zn and S. Photoluminescence (PL) emissions observed at 420, 461, 509 and 560 nm are related to the defect states such as interstitial sulfur, zinc interstitial, sulfur vacancies and zinc vacancies respectively. Rise of another emission peak in doped ZnS at 600 nm was due to 5d → 4f energy level transitions in Ce3+ ions. Evolved micro-strain profile, PL intensity and energy band gap variation were analogous to each other with respect to doping concentration. Microscopic images confirmed the structural transformation to cuboidal shaped ZnS QSs with increase in doping concentration. EDX and XPS supported the elemental analysis along with oxidation states of the available elements such as Zn, S and Ce.

Photoluminescence from Vacancy‐Containing Defects in GaN

Photoluminescence (PL) bands in GaN associated with point defects involving nitrogen or gallium vacancy (VNor VGa) are reviewed. The VN‐containing defects, including the isolated VNand its complexes with acceptors, are often observed in PL from semi‐insulating GaN and are responsible for the green (GL2) and red (the RL2 family) bands. The complexes of the VGawith hydrogen and oxygen are abundantly formed in n‐type GaN grown by the ammonothermal method. Some of these complexes are responsible for PL bands in the red‐yellow region of the PL spectrum.

Low Temperature Photoluminescence Properties of α‐CsPbI 3 Nanocrystals with High Quantum Yield

Temperature-dependent photoluminescence (PL) properties of high quality cubic phase α-CsPbI3 nanocrystals are investigated, and thermal transfer from free excitons to trapped excitons is observed. Free exciton recombination is the main origin of the high PL quantum yield (97%) at room temperature. Little change of PL peak energy is observed with decreasing temperature from 280 to 200 K, and it shifts from 1.84 to 1.81 eV in the region of 200 and 80 K. The PL peak intensity decreases with decreasing temperature first and then increases, and the minimum value is observed at 180 K. Two PL bands are obtained by Gaussian fittings in the PL spectra detected at different temperatures, and the high energy band is attributed to the emission of free excitons, while that for the low energy band is related to the transition of trapped excitons. The PL integrated intensity ratio of trapped exciton and free exciton transition increases from 0.18 to 2.10 with decreasing the temperature, which suggests that thermal transfer from free exciton to trapped exciton states leads to the dominant optical emission of trapped exciton recombination in the low temperature region.

Defect engineered blue photoluminescence in ZnO:Al/TiO2 heterostructures

Tailoring the blue photoluminescence (PL) in Al-doped ZnO (AZO)/TiO2 heterostructures is demonstrated by a controlled induction of shallow defect centers by 50 keV Ar+-ions. This is established by a combination of temperature dependent PL and electron paramagnetic resonance spectroscopy. The dominant blue-violet PL in an as-grown sample comprises a near band-edge emission, along with a peak associated with a radiative recombination of the electrons in shallow donor levels (Zn interstitials) and the holes from the valence band. However, the evolution of an additional yellow-green PL band at a fluence of 1 × 1015 ions/cm2 is governed by deep donor levels, particularly ionized oxygen vacancies. Irradiation at 1 × 1016 ions/cm2 further leads to the formation of Zn vacancies (shallow acceptors) owing to the development of an O-rich surface. The structural modifications of these samples have been investigated by field-emission scanning electron microscopy , transmission electron microscopy, and Rutherford backscattering. While small micro-cracks are found at a fluence of 2 × 1016 ions/cm2, the formation of graded layers is obtained at the highest fluence of 5 × 1016 ions/cm2 owing to ballistic intermixing and diffusion of the constituents. Detailed investigation suggests that a significant amount of Ti atoms is diffused in AZO by a complete deterioration of the AZO/TiO2 matrix at the highest fluence.

Polystyrene‐based Macromolecular Ammonium Halides for Tuning Color and Exchange Kinetics of Perovskite Nanocrystals

AbstractPost‐synthesis anion exchange of all‐inorganic cesium lead halide perovskite nanocrystals (CsPbX3 NCs, where X=Cl, Br, and/or I) provides a rapid and simple means of tuning their band gap and photoluminescence emission wavelengths. Here we report color‐shifting of CsPbX3 nanocrystals induced by a macromolecular source of halide ions, specifically using polystyrene with ammonium halides as pendent groups. This strategy for introducing new halides to the perovskite nanocrystals gave access to perovskite‐polymer hybrid materials as solutions, thin films, or free‐flowing powders. Spectroscopic measurements of the halide‐exchanged nanocrystal products revealed high photoluminescence quantum yields across the visible spectrum, with exchange kinetics that were tunable based on the solution environment, suggesting an aggregation‐inhibited exchange process that affords access to multi‐colored solutions and films.

Polystyrene-based Macromolecular Ammonium Halides for Tuning Color and Exchange Kinetics of Perovskite Nanocrystals.

Post-synthesis anion exchange of all-inorganic cesium lead halide perovskite nanocrystals (CsPbX3 NCs, where X=Cl, Br, and/or I) provides a rapid and simple means of tuning their band gap and photoluminescence emission wavelengths. Here we report color-shifting of CsPbX3 nanocrystals induced by a macromolecular source of halide ions, specifically using polystyrene with ammonium halides as pendent groups. This strategy for introducing new halides to the perovskite nanocrystals gave access to perovskite-polymer hybrid materials as solutions, thin films, or free-flowing powders. Spectroscopic measurements of the halide-exchanged nanocrystal products revealed high photoluminescence quantum yields across the visible spectrum, with exchange kinetics that were tunable based on the solution environment, suggesting an aggregation-inhibited exchange process that affords access to multi-colored solutions and films.

Optical Property and Stability of Lead‐Free Cs3Sb2Br9 Perovskite Microplatelets Prepared by a Solution Method

Herein, a solution method is used to synthesize lead‐free Cs3Sb2Br9 perovskite microplatelets. The freshly prepared microplatelets exhibit photoluminescence (PL) peaks at about 420 and 600 nm, when the excitation wavelengths are 325 and 532 nm, respectively. The 420 nm PL band is attributed to the band edge excitonic recombination while the 600 nm PL band originates from radiative defect state. The 600 nm emission is stronger than that of the 420 nm emission, implying the high density of defects and relatively poor crystal quality. Moreover, 2D PL and Raman mappings indicate the presence of defects at the edges. These features suggest the crystal quality, so as the structural stability of the Cs3Sb2Br9 microplatelets obtained by solution method is relatively inferior. During the storage, the Cs3Sb2Br9 microplatelets significantly degrade and the PL rapidly quenches. To improve the stability, a surface capping strategy by oleic acid (OA) is developed. The PL intensity retains more than 80% and the morphology maintains after storage for about one month, demonstrating the improved stability of the OA‐capped Cs3Sb2Br9 microplatelets.

Germanium Nanoparticles Prepared by Laser Ablation in Low Pressure Helium and Nitrogen Atmosphere for Biophotonic Applications.

Due to particular physico-chemical characteristics and prominent optical properties, nanostructured germanium (Ge) appears as a promising material for biomedical applications, but its use in biological systems has been limited so far due to the difficulty of preparation of Ge nanostructures in a pure, uncontaminated state. Here, we explored the fabrication of Ge nanoparticles (NPs) using methods of pulsed laser ablation in ambient gas (He or He-N2 mixtures) maintained at low residual pressures (1–5 Torr). We show that the ablated material can be deposited on a substrate (silicon wafer in our case) to form a nanostructured thin film, which can then be ground in ethanol by ultrasound to form a stable suspension of Ge NPs. It was found that these formed NPs have a wide size dispersion, with sizes between a few nm and hundreds of nm, while a subsequent centrifugation step renders possible the selection of one or another NP size fraction. Structural characterization of NPs showed that they are composed of aggregations of Ge crystals, covered by an oxide shell. Solutions of the prepared NPs exhibited largely dominating photoluminescence (PL) around 450 nm, attributed to defects in the germanium oxide shell, while a separated fraction of relatively small (5–10 nm) NPs exhibited a red-shifted PL band around 725 nm under 633 nm excitation, which could be attributed to quantum confinement effects. It was also found that the formed NPs exhibit high absorption in the visible and near-IR spectral ranges and can be strongly heated under photoexcitation in the region of relative tissue transparency, which opens access to phototherapy functionality. Combining imaging and therapy functionalities in the biological transparency window, laser-synthesized Ge NPs present a novel promising object for cancer theranostics.

Piezoelectricity-enhanced multifunctional applications of hydrothermally-grown p-BiFeO3–n-ZnO heterojunction films

This paper reports the fabrication of p-BiFeO3 (BFO)–n-ZnO composite films on tin-doped indium oxide substrates through a facile hydrothermal method and the tuning of various functional properties of single-crystalline BFO and ZnO for piezoelectricity-enhanced multiapplications. BFO microplates were embedded in ZnO nanorod arrays; the clear interfaces indicated the robust formation of the heterojunction, which was also confirmed through X-ray photoelectron spectroscopy. The conductivity type of the BFO and ZnO was determined through Mott–Schottky, open-circuit potential, and photoelectrochemical measurements. Moreover, the induced piezopotential distributions of the samples were theoretically simulated, and the piezotronics, piezophototronics, and Schottky behavior of the composites were determined. The composite-based piezoelectric nanogenerators exhibited durable output and excellent sensitivity, enabling practical sensor applications. Excellent piezophotodegradation with a rate constant of approximately 3 × 10−2 min−1 for the composite was attributable to predominant ·O2− radicals. The maximum applied bias photon-to-current efficiency and piezophotoelectrochemical current density were approximately 0.86% (at 0.63 V vs. Ag/AgCl) and 1.4 mA cm−2, respectively. The multiapplications of the p–n junction composites were primarily attributable to the enhanced piezoelectric coefficient (d33 ≈ 27.3 pm·V−1), favorable electrochemical surface area (≈69.8 mF cm−2 mg−1), prolonged charge carrier lifetime, weak photoluminescence, and suitable band positions and piezopotential-induced band bending.

Structural and temperature-dependent luminescence of Terbium doped YAl3(BO3)4 phosphor synthesized by the combustion method

A series of Y1-xAl3(BO3)4:x Tb3+ (x = 0.5 to 7 wt%) phosphors synthesized by a gel combustion method have been systemically investigated by X-ray diffraction (XRD), Fourier transform infrared (FTIR), energy dispersive spectroscopy (EDS), and photoluminescence (PL) as a function of temperature from 300 K to 10 K and 300 K–550 K. An XRD analysis confirms that the phosphors crystallized, and its crystal structure was analysed. The synthesized phosphor matches the XRD pattern provided in the ICSD File No 96-152-6006. The FTIR analysis indicates that nitrates and organic matter have been completely removed and the BO3 groups are present. The broad PL band peaked at 420 nm with a shoulder circa 460 nm of YAl3(BO3)4 is associated with hydrous components which attached to the sample in environmental conditions after synthesis. The PL spectra of YAl3(BO3):Tb3+ phosphors exhibit a bright and narrow green main emission peak at 543 nm corresponding to the 5D4 →7F5 transition under 359 nm excitation. The PL intensity increases with increasing Tb3+ ion concentration up to 5 wt %, followed by evidence for concentration quenching. There is a possibility that higher concentration quenching could be from confinement effects of localised resonant energy transfer. PL data revealed that activation energies for thermal quenching at 485 nm and 543 nm were found to be 0.659 and 0.092 eV, and 0.585 and 0.087 eV, respectively.

Passivation of Ring Defects in Czochralski-Grown Silicon Using Magnesium Fluoride Films

This work investigates the potential application of magnesium fluoride (MgFx) films and compares its effectiveness to conventional silicon nitride (SiNx:H) for passivating ring defects, which sometimes occur in n-type Czochralski-grown silicon wafers subjected to high-temperature processes. We find that the passivation of ring defects in MgFx coated samples is fully activated only at 600 °C, whereas passivation in SiNx:H coated samples can be activated by 5 min anneals at 500 °C. Consistent with the passivation activation temperature, energy-dispersive X-ray shows that MgFx films significantly lose fluorine at temperature ≥ 600 °C. Further, subsequent annealing of the previously passivated samples reveals that the ring defects reappear and become recombination-active again. However, this depassivation of the ring defects occurs significantly faster in SiNx:H passivated samples than in MgFx passivated samples at temperatures ≤ 500 °C, whereas the depassivation happens at a rate similar to that at temperatures ≥ 600 °C. Furthermore, in situ monitoring of the change in band–band photoluminescence intensity during annealing also confirms that the passivation and depassivation of ring defects is faster in SiNx:H passivated samples. Based on these observations, we suggest the possibility that the dominant passivating agent in the MgFx and SiNx:H films is fluorine and hydrogen, respectively. Finally, minority carrier lifetime measurements indicate that both the passivation methods improve the bulk lifetime significantly.

Photoluminescent Silica Nanostructures and Nanohybrids.

The complicated photophysics of wide variety of defects existing in silica (SiO2 ) layer of nanometer thickness determines widespread photoluminescence bands of Si/SiO2 based system as well as SiO2 nanoparticles (NPs) for their applications in photovoltaic and optoelectronic devices. This review attempts to summarize different photophysical processes in pure SiO2 NPs. Moreover, these NPs also act as scaffolds for various guest molecules to perform their specific functions. Guest fluorophore molecules when trapped inside pores of SiO2 NPs exhibit a different photodynamics than free state, which opens up several important applications of hybrid SiO2 NPs in artificial photosynthesis, sensing, biology and optical fiber.

MoS2 monolayer quantum dots on a flake: Efficient sensitization of exciton and trion photoluminescence via resonant nonradiative energy and charge transfers

MoS2 monolayer quantum dots (1L-QDs) are hybridized on 1L MoS2 on SiO2/Si. The exciton and trion photoluminescence (PL) of the 1L-QD/MoS2 hybrid is studied in comparison to bare 1L MoS2. The 1L-QDs have a round-like shape with an average lateral size of 3 nm, which results in interband absorption and PL within 2.3–3.5 eV. The bare flake shows the A and B exciton PL bands, overlapped with a weak trion band. The 1L-QD/MoS2 hybrid comprises the emission bands so that the 1L-QD PL intensity is not altered, but the flake PL is tremendously enhanced. This effect of sensitization depends on the PL excitation energy: 6.4 times under UV excitation involving the transitions between the valence band and the excited states in the conduction band of the 1L-QDs and 2.7 times under blue light excitation involving interband absorption between the 1L-QD quantum states. The strongest effect is explained by an efficient nonradiative energy transfer (NRET), similar to Förster resonance energy transfer, and possible charge transfer from the resonant levels in the donor 1L-QDs to the levels in the acceptor MoS2 flake. On the contrary, exciting e-h pairs between the 1L-QD quantum states, they rapidly recombine there, weakening NRET.

Absorption and Emission of Chemically and Electrochemically Doped Graphene Nanoribbons

Graphene nanoribbons (GNRs) are narrow stripes of graphene that exhibit unique optical and electronic properties depending on their width and edge type. Recent advances in the bottom-up synthesis of GNRs have enabled precise control over their structure and thus their bandgap. However, the systematic optical and electrical characterization of GNRs obtained from solution-mediated reactions has so far been prevented by the poor stability and processability of GNR dispersions. Here we employ liquid cascade centrifugation (LCC) to obtain size-selected and stable dispersions of bottom-up synthesized 9-aGNRs in toluene and tetrahydrofuran and investigate their intrinsic optical properties. Dispersions and films of these 9-aGNRs show well-defined absorption and photoluminescence bands between 800 and 1000 nm, with relative intensities depending on the LCC-fraction. The best GNR dispersions show photoluminescence quantum yields of up to 70 %. Trions are charged excitons with red-shifted emission that can be created in doped low-dimensional semiconductors, such as carbon nanotubes. Theoretical studies predict large trion binding energies (> 300 meV) in graphene nanoribbons (depending on their width). Hence their optical properties upon doping and possible new features are of interest. Chemical doping of our 9-aGNR dispersions with the strong molecular p-dopant F4-TCNQ and electrochemical doping of GNR thin films in ambipolar electrolyte-gated field-effect transistors results in the expected bleaching of the main absorption bands but also the emergence of new redshifted, charge-induced absorption peaks that may indicate the existence of trions in GNRs.