Properties Of Graphene Quantum Dots Research Articles

Electronic and magnetic properties of graphene quantum dots doped with alkali metals

The electronic and magnetic properties of armchair-hexagonal (AHEX) and zigzag-triangular (ZTRI) graphene quantum dots doped with alkali metals are investigated using density functional theory. The binding energy confirms the stability of the undoped systems. Although doping decreases stability of single layer structures, in bilayer ones the binding energy between the layers increases. The former is due to the broken bonds and the deformation at the surface, while the later is due to the chemical bonds formation between the layers. We found that the lowest ground state energy for AHEX/AHEX-doped is the singlet/quadruplet spin state. Therefore, AHEX dots experience transformation from diamagnetic to ferromagnetic state after doping. In addition, the optimized spin state for ZTRI/ZTRI-doped is the quintet/sextet spin-polarized state. In few cases, the doped AHEX/ZTRI dots have doublet/quadruplet spin state due to the strong interaction between the flake atoms that passivates some unpaired electrons. Magnetic properties depend also on stacking, for instance pristine bilayer triangular flakes become antiferromagnetic due to pairing between edge states. The energy gap significantly affected by doping, for instance the gap decreases from ~3.7 eV in hexagonal flakes to 1.5 eV when it is doped with Na in the upper position. Electrons from the broken bonds around the doped metal form orbitals loosely bound to the flake consequently decreasing the energy gap. On the other hand, stacking increases the energy gap in bilayer triangular flakes due to the mutual passivation of the reactive edge states from both the layers. The calculated spin up/down density ratio is high in hexagonal flakes leading to high spin polarization (P), for instance P = 0.64 in hexagonal flakes doped with K atom. The found enhanced spin polarization, in addition to the tunable magnetic properties, makes doped graphene flakes promising candidates for spintronic devices.

UV-irradiated hydrothermal synthesis of reduced graphene quantum dots for electrochemical applications

This paper reports the structural and electrochemical properties of graphene quantum dots (GQDs) and reduced graphene quantum dots (rGQDs). The synthesis of GQDs by glucose and obtained the rGQDs by 365 nm UV-source irradiated glucose precursor used in microwave assisted hydrothermal reaction. The modification in chemical interaction was studied by Fourier Transforms Infrared (FT-IR) spectroscopy, and X-ray diffraction (XRD) confirms the structural change due to UV-irradiation effects. The Raman spectra confirms change in the intensity ratio of G and D band, and UV–Visible absorption peak of GQDs observed at 243, 245, 248 and 262 nm for 24, 48, and 72 h UV-irradiated modified rGQDs. Transmission electron microscopy (TEM) images shows reduced the size with increasing UV-irradiation time which confirms the rGQDs. Atomic force microscopy (AFM) employed to know topological surface study. The electrochemical performance of Cyclic Voltammetry from a typical rectangular shape of the integral area suggesting it is exhibits strong electrochemical activity and specific capacity behaviour by redox reaction at different scan rate from 100 to 500 mV/s for GQDs and rGQDs. The current-voltage plot exhibited a gradual increase in current with applied voltage as well as irradiation at different hours.

Ultraviolet‐shielding and water resistance properties of graphene quantum dots/ polyvinyl alcohol composite-based film

The Ultraviolet‐shielding (UV‐shielding) and water resistance properties of graphene quantum dots/polyvinyl alcohol (GQDs/PVA) composite-based film have been investigated. The GQDs/PVA composite-based films were fabricated with different GQDs concentrations of 0, 5, 10, 15, and 20 wt%. The optical property of GQDs was carried out by utilizing fluorescence spectroscopy. Characterizations of GQDs/PVA composite-based films were performed by using FT-IR spectroscopy, and UV-vis spectroscopy. It was found that GQDs exhibited the strongest excitation wavelength in the UV range. GQDs/PVA composite-based films offered an improved UV-shielding capacity when compared to PVA films and glass. Particularly, the GQDs/PVA composite-based film containing 20 wt% GQDs exhibited a UV transmittance of 9.8%, combined with 84% optical transparency. For humidity environment application, the highest contact angle was explored for the 10 wt% GQDs contents suggesting sustainability for humidity environment application. Accordingly, GQDs played an important role in UV-shielding by considering the effect of UV absorption of GQDs and the UV absorption of GQDs can be explained in terms of the photon excitation by UV light. This GQDs/PVA composite could be potentially applied as transparent UV-protective coatings for pharmaceutical packing, food products packing, and UV-shielding or UV filter glass.

Effect of edge on nonlinear optical property of graphene quantum dots

Graphene is a two-dimensional material with single-layer honeycomb lattice structure formed by sp<sup>2</sup> hybrid connection of carbon atoms. Graphene has excellent optical, electrical, thermal and mechanical properties, and it is considered to be an ideal material for future flexible optoelectronic devices. In recent years, the nonlinear optical properties and regulation of graphene nanostructures have attracted experimental and theoretical interest. Graphene has good delocalization of π-electrons and its unique plane structure, showing good nonlinear optical properties. Graphene quantum dots can be regarded as small graphene nanoflakes. Their unique electronic structure is closely related to the non-bond orbitals on the boundary/edge. Therefore, it is very important to study the boundary/edge effect on the electronic and optical properties of nanographene. In this paper, effects of the number of edge C=C double bonds and Borazine (B<sub>3</sub>N<sub>3</sub>) doping on the nonlinear optical properties and UV-Vis absorption spectrum of graphene quantum dots are studied by the quantum chemical calculation methods, respectively. It is found that the symmetry of hexagonal graphene quantum dots decreases and the symmetry of charge distribution is broken when C=C double bond is introduced into the armchair edge, which leads the second-order nonlinear optical activity to be enhanced. During the transition from armchair to zigzag edge, the polarizability and the second hyperpolarizability of hexagonal graphene quantum dots and B<sub>3</sub>N<sub>3</sub>-doped graphene quantum dots increase linearly with the number of introduced C=C double bonds incrrasing. In addition, the edge also has an important influence on the absorption spectrum of graphene quantum dots. For graphene quantum dots and B<sub>3</sub>N<sub>3</sub>-doped graphene quantum dots, the introduction of C=C double bond at the armchair edge increases the highest occupied molecular orbital energy level and also reduces the lowest unoccupied molecular orbital energy level, which reduces the energy gap between the frontier molecular orbitals, and thus resulting in the red-shift of the maximum absorption wavelength. The doping of B<sub>3</sub>N<sub>3</sub> ring will increase the energy gap between molecular frontier orbitals of graphene quantum dots, leading the UV-Vis absorption spectrum of graphene quantum dots to be blue-shifted. This study provides theoretical guidance for controlling the nonlinear optical response of graphene quantum dots by edge modification.

Boron regulated dual emission in B, N doped graphene quantum dots

Abstract Graphene quantum dots are promising candidates for LED and solar cell studies with their low toxicity, inexpensive production cost and high luminescence capacity. However, controlling optical properties of graphene quantum dots is a challenge, which can be partially done by heteroatom doping. In this study, we synthesized dual emissive boron and nitrogen doped graphene quantum dots through a bottom-up synthesis method. Dual emission properties of boron and nitrogen doped quantum dots appeared in the presence of high amount of boron precursor in synthesis medium. Dual emissive features of quantum dots were controlled via controlling amount of boron content in quantum dot structure. Boron and nitrogen doped graphene quantum dots showed dual emissive properties at different excitation wavelengths, therefore they were suitable for applications concerning white – LED and solar cells. With their low toxicity, low production cost and controllable dual emission properties, dual emissive boron and nitrogen doped graphene quantum dots can be considered as a powerful alternative to cadmium and indium based quantum dots.

Nano-optosensor based on titanium dioxide and graphene quantum dots composited with specific polymer for cefazolin detection

An optosensor using nanocomposite probes was fabricated for the detection of trace cefazolin. The nanoprobes utilized the high affinity of titanium dioxide, the good optical properties of graphene quantum dots and the good selectivity of molecularly imprinted polymer. The integration of these materials produced a rapid, highly sensitive optosensor with excellent selectivity for cefazolin detection. The fluorescence intensity of the nanocomposite probes was quenched when cefazolin re-bound with the imprinted recognition cavities of the nanoprobes. The fabricated nanoprobe exhibited a good linearity for cefazolin from 0.10 to 10.0 μg L−1 with a limit of detection of 0.10 μg L−1. The imprinting factor of the nanoprobe was 10.6 and selectivity for cefazolin was not affected by the analogue structures of cephalexin, cefatriaxone, cephradine, cefaperazone and ceftazidime. This nano-optosensor probe successfully detected cefazolin in milk and recoveries were between 85.0 and 97.4 % with RSDs less than 5.0 %. The results of analysis with nano-optosensor were in good agreement with HPLC analysis. The fabrication strategy of the nanocomposite probe can be modified for the measurement of other toxic compounds.

The effects of various molecular weight of passivator on the photoluminescence properties of graphene quantum dots

Here, we realize manipulation of photoluminescence (PL) intensity of GQDs through various molecular weight of surface passivator. With the increase of passivator molecular weight, only the PL intensity increases. However, the peak positions and half-peak width of spectral line remain unchanged. Our results reveal that surface passivation with larger molecular weight only weakens the chemical activity of graphene quantum dots (GQDs) and the π-π stacking interaction between GQDs, but exerts less impact on GQDs intrinsic energy gap. Therefore, the weaker π-π stacking interaction and chemical activity induce the weaker non-radiative recombination, leading to enhanced fluorescence from radiative recombination.

Electronic and magnetic properties of graphene quantum dots with two charged vacancies

Electronic and magnetic properties of a system of two charged vacancies in hexagonal shaped graphene quantum dots are investigated using a mean-field Hubbard model as a function of the Coulomb potential strength β of the charge impurities and the distance R between them. For β=0, the magnetic properties of the vacancies are dictated by Lieb’s rules where the opposite (same) sublattice vacancies are coupled antiferromagnetically (ferromagnetically) and exhibit Fermi oscillations. Here, we demonstrate the emergence of a non-magnetic regime within the subcritical region: as the Coulomb potential strength is increased to β∼0.1, before reaching the frustrated atomic collapse regime, the magnetization is strongly suppressed and the ground state total spin projection is given by Sz=0 both for opposite and same sublattice vacancy configurations. When long-range electron–electron interactions are included within extended mean-field Hubbard model, the critical value for the frustrated collapse increases from βcf∼0.28 to βcf∼0.36 for R<27Å.

Theoretical investigation of electronic and optical properties of nitrogen doped triangular shaped graphene quantum dots

The electronic and optical properties of graphene quantum dots can be significantly tailored by doping it with heteroatoms, thus extending its potential applications. In this work, we have employed time-dependent density functional theory to systematically explore the effect of introduction of nitrogen atoms in varying concentration at pyridinic and graphitic configuration in armchair and zigzag-edged triangular shaped graphene quantum dots (TQDs) of different sizes. Our results indicate that the electronic band-gap in these N-doped systems can be effectively tuned by varying the configuration as well as concentration of dopants and nature of edge-termination. The variation of electronic band-gap is critically determined by the localized/delocalized nature of molecular orbitals and presence of additional energy levels due to dopant nitrogen atoms. However, the significance of these extra energy levels in modulating the optical properties (appearance of characteristic N-dopant absorption peaks) becomes conspicuous only for specific configuration and concentration of nitrogen atoms. In addition, our studies have attributed the strong dependence of blue/red-shift of absorption spectra and variation in the peak profile to position as well as concentration of dopant atoms and edge-termination pattern. Further, it is observed that the effect of increasing size of TQDs on the strength of most intense absorption peak of pyridinic N-doped TQDs is remarkably different from graphitic N-doped systems. This selective manipulation of optical properties in TQDs due to different N-doping pattern can open up new frontiers for rational design of novel optoelectronic devices.

One-step synthesis of sulfur-incorporated graphene quantum dots using pulsed laser ablation for enhancing optical properties.

To tune the electronic and optoelectronic properties of graphene quantum dots (GQDs), heteroatom doping (e.g., nitrogen (N), boron (B), and sulfur (S)) is an effective method. However, it is difficult to incorporate S into the carbon framework of GQDs because the atomic size of S is much larger than that of C atoms, compared to N and B. In this study, we report a simple and one-step method for the synthesis of sulfur-doped GQDs (S-GQDs) via the pulsed laser ablation in liquid (PLAL) process. The as-prepared S-GQDs exhibited enhanced fluorescence quantum yields (0.8% → 3.89%) with a huge improved absorption band in ultraviolet (UV) region (200 ∼ 400 nm) and excellent photo stability under the UV radiation at 360 nm. In addition, XPS results revealed that the PLAL process can effectively facilitate the incorporation of S into the carbon framework compared to those produced by the chemical exfoliation method (e.g., hydrothermal method). And also, the mechanisms related with the optical properties of S-GQDs was investigated by time-resolved photoluminescence (TRPL) spectroscopy. We believe that the PLAL process proposed in this study will serve as a simple and one-step route for designing S-GQDs and opens up to opportunities for their potential applications.

(Invited) Synthesis and Optical Properties of Graphene Quantum Dots

The outstanding electronic, optical and mechanical properties of graphene strongly inspire the scientific community at both the fundamental and applicative levels. However, the key issue that needs to be addressed is the control and the modification of the electronic properties of graphene, and notably the opening of a sizable bandgap. For the last decade, a great attention has been paid to the size reduction of graphene using conventional top-down approaches (lithography and etching, thermal treatments and oxidation of bulk materials) to fabricate graphene quantum dots (GQDs)1 or graphene nanoribbons (GNRs).2 However, top-down approaches do not permit to manipulate the structure of the material at the atomic scale. In particular, they do not allow a sufficient control of the morphology and oxidation state of the edges, which drastically impact the properties. In order to truly control, with the required level of precision, the morphology and the composition of the materials and of its edges, the bottom-up approach is the relevant way to proceed.3,4 Recently, we reported on the synthesis and single photon emission properties of triangular-shaped GQDs.5 While, this initial report focused on functionalized nanoparticles, we now turn to non-functionalized graphene quantum dots that are in terms of structure closer to real graphene. Here, we described the synthesis, the dispersion and optical properties of a series of rod-shaped particles and we studied the structure-properties relationship in these graphene quantum dots. To this end, we designed a series of GQDs with a given edge type and by changing only one parameter (one dimension, namely the length or the width – Figure 1), we expect to follow simply the evolution of the optical properties.References. 1. Haque, E.; Kim, J.; Malgras, V.; Reddy, K. R.; Ward, A. C.; You, J.; Bando, Y.; Hossain, M. S. A.; Yamauchi, Y. Recent Advances in Graphene Quantum Dots: Synthesis, Properties, and Applications. Small Methods 2018, 2, 1800050.2. Xu, W.; Lee, T.-W. Recent progress in fabrication techniques ofgraphene nanoribbons. Mater. Horiz. 2016, 3, 186-207.3. Narita, A.; Wang, X. Y.; Feng, X.; Müllen, K. New advances in nanographene chemistry. Chem. Soc. Rev. 2015, 44, 6616-6643.4. Narita, A.; Chen, Z.; Chen, Q.; Müllen, K. Solution and on-surface synthesis of structurally defined graphene nanoribbons as a new family of semiconductors. Chem. Sci. 2019, 10, 964-975.5. Zhao, S.; Lavie, J.; Rondin, L.; Orcin-Chaix, L.; Diederichs, C.; Roussignol, P.; Chassagneux, Y.; Voisin, C.; Müllen, K.; Narita, A.; Campidelli, S.; Lauret, J.-S. Single photon emission from graphene quantum dots at room temperature. Nat. Commun. 2018, 9, 3470. Figure 1

(Invited) Optical Properties of Graphene Nanoribbons and Quantum Dots

Graphene is a key material for nanoelectronics. Nevertheless, its zero gap makes it unsuitable for applications needing semiconductors with sizeable energy gaps. One way to open a gap in graphene is to use size reduction effects. The reduction of one dimension leads to carbon nanotubes and graphene nanoribbons that are 1D carbon nanostructures. Reducing one more dimension leads to 0D graphene quantum dots. The optical properties of carbon nanotubes have been investigated for approximately 20 years, while the study of graphene quantum dots and nanoribbons is at its infancy.Among potential application of these sp2 carbon nanostructures, the use of graphene nanoribbons and quantum dots as light emitters attracts a lot of attention. Here, I will present our recent results on the optical properties of graphene quantum dots synthesized by bottom-up chemistry [1].

Electrochemiluminescence revealing that HNO3-oxidized single-walled carbon nanotubes are essentially tubular graphene quantum dot-nanoassemblies

Graphene quantum dots (GQDs) as recently emerging 0-D graphitic nanomaterials, have attracted much attention due to their unique optical, electrical and catalytic properties. However, some properties of GQDs, such as very small size, low density, and excellent water-solubility, make it difficult to be separated and immobilized. This may limit the recycle and purification of GQDs, and thus their applications in catalysis and sensing. In this work, we prepared tubular GQD-nanoassemblies (t-GQD-NAs) by etching single-walled carbon nanotubes (SWCNTs) with concentrated HNO3. The synthesized t-GQD-NAs were characterized by TEM, SEM, XPS, Raman spectroscopy and electrochemiluminescence (ECL) in details. t-GQD-NAs were much shorter and slimmer in morphology, bore more oxygen-containing groups, and had a higher surface defect density compared with SWCNTs. t-GQD-NAs not only maintained good UV absorption property of SWCNTs but also showed strong infrared ECL emission and broad ECL spectrum, verifying that t-GQD-NAs were assembled from GQDs of various sizes and some graphene nanoribbons. This is for the first time that 0-D GQDs were reported to be assembled into 1-D tubular carbon structures. The t-GQD-NAs with abundant surface states and good ECL activity could be easily separated, purified, and immobilized, suggesting their promising applications in ECL sensing and catalysis.

Optical properties of graphene quantum dots: the role of chiral symmetry

We analyse the electronic and optical properties of graphene quantum dots (GQD) using accurate ab initio many-body GW and Bethe-Salpeter calculations. We show that most pristine GQD, including structures with irregular shapes, are characterized by dark low energy singlet excitations that quench fluorescence. We rationalize this property by exploiting the chiral symmetry of the low energy electronic states in graphene. Edge sp3 functionalization is shown to efficiently brighten these low lying excitations by distorting the sp2 backbone planar symmetry. Such findings reveal an original indirect scenario for the influence of functionalization on the photoluminescence properties.

How does the porphyrin-like vacancy affect the spectral properties of graphene quantum dots? A theoretical study

By processing graphene quantum dot, an ideal semiconductor material with suitable band gap and higher electron mobility can be obtained. Thus, it has a broad prospect in the application of photoelectric response materials. Here, a graphene defect with porphyrin-like structure is selected to achieve the controlable light absorption. The double five-membered-ring parallel vacancy are based on self-healing properties of popular graphene defects. Aimed to separate exciton and hole more effectively and achieve higher photoelectric conversion efficiency, the occupied orbital and unoccupied orbital of the quantum dot with objected defect structure is taken as orderly dispersion to form an obvious charge separation state under the demonstration with first principles calculation. Most importantly, a real time real space charge separation is calculated by time-dependent ab-initio quantum dynamics based on numerical atomic basis sets. The result shows the specific graphene defects can form an efficient pure graphene photoelectric response medium like porphyrin skeleton, and the vacancy will induce to adjust and control the specific wavelength of the response light and charge separated state manipulably with odevity of number of peripheral carbon rings by the calculation of ultrafast process.

Quantitative Understanding of Charge-Transfer-Mediated Fe3+ Sensing and Fast Photoresponse by N-Doped Graphene Quantum Dots Decorated on Plasmonic Au Nanoparticles.

The formation of a heterostructure with plasmonic nanoparticles drastically alters the optoelectronic properties of graphene quantum dots (GQDs), resulting in exceptional properties. In the present work, we prepare nitrogen-doped GQDs decorated on gold nanoparticles (Au@N-GQDs) by a one-step green reduction method and study its extraordinary fluorescence and photoresponse characteristics. The as-prepared Au@N-GQDs show more than one order of magnitude enhancement in the fluorescence intensity as compared to the bare N-GQDs, which is attributed to hot electron generation and improved absorption in N-GQDs by local field enhancement and the modification of the edge functional groups. Because of the selective coordination to Fe3+ ions, the Au@N-GQDs exhibit extraordinary quenching of fluorescence, with ultrahigh sensitivity for the detection of Fe3+ (<1 nM). A new model for the charge-transfer dynamics is developed involving the Langmuir's law of adsorption to explain the unusual quenching, which strongly deviates from the known models of static/dynamic quenching. The proposed sensor is successfully implemented for the ultrasensitive detection of Fe3+ ions in human serum and Brahmaputra river water samples, representing its high potential applications in clinical as well as environmental diagnosis. Additionally, because of its high absorption in the UV-vis-NIR region and high charge density with long life excitons, the Au@N-GQDs are utilized as photodetectors with ∼104 times faster response than that of bare N-GQDs. The Au@N-GQD-based photodetector possesses a high responsivity of ∼1.36 A/W and a remarkably high external quantum efficiency of ∼292.2%, which is much superior to the GQD-based photodetectors reported till date. The underlying mechanism of ultrafast photoresponse is ascribed to the transfer of hot electrons along with the tunneling of the electrons from Au NPs to N-GQDs as well as the defect reduction of N-GQDs by the incorporation of Au NPs. Without the use of any charge transporting layer, the outstanding performance of N-GQD-based plasmonic photodetector opens up unique opportunities for future high-speed optoelectronic devices.

Revealing the role of nitrogen dopants in tuning the electronic and optical properties of graphene quantum dots via a TD-DFT study.

Graphene quantum dots (GQDs) have been suggested to have a wide range of applications due to their unique electronic and optical properties. Moreover, heteroatom doping has become a viable way to fine-tune the properties of GQDs. However, the working principle of the doping strategy is still not conclusive. In this study, the effects of size, configuration of the nitrogen dopant, and N/C ratio on the electronic and optical properties of GQDs have been carefully examined. First, the variation of the adsorption wavelength of pristine GQDs was evaluated for which a linear relation is established against different diameters. Moreover, it is found that both the configuration and content of nitrogen dopants have a significant impact on the adsorption wavelength and band gap of GQDs. In particular, different nitrogen species could have exactly opposite effects on the adsorption behavior. The origin of the nitrogen doping effect is calibrated from orbital localization, charge analysis, natural transition orbitals, and atomic contribution towards excitation. It is noted that nitrogen doping can simultaneously reduce both light adsorption energy and emission energy compared with the pristine one. This study provides an insightful explanation for the electronic and optical properties of GQDs and consolidates the theory base of the doping strategy.

One-pot synthesis of highly fluorescent boron and nitrogen co-doped graphene quantum dots for the highly sensitive and selective detection of mercury ions in aqueous media

The doping of heteroatoms can radically alter the electronic properties of graphene quantum dots (GQDs), thereby realizing the adjustment of optical properties of GQDs, and further significantly improving quantum yield (QY) and other issues. In this paper, a simple, low-consumption and pollution-free synthesis strategy of boron and nitrogen co-doped graphene quantum dots (BGQDs) were developed by pyrolysis carbonization of citric acid (CA), boric acid and urea. The B,N-GQDs synthesized by this method showed bright blue fluorescence with a relatively high QY of 17.16%. The selective specificity of B,N-GQDs for Hg2+ was mainly attributed to the strong electron affinity of Hg2+ itself, which enabled it to perform kinetic chelation with boron, nitrogen, oxygen and other functional groups in B,N-GQDs more quickly. Therefore, the fluorescence of B,N-GQDs aqueous solution can be significantly quenched by aqueous solution containing Hg2+ or true water samples. B,N-GQDs fluorescent probe had high sensitivity to Hg2+. Moreover, it can respond quickly in a wide range of Hg2+ concentrations from 1 to 300 μM. The linear relationship was best at a concentration of 0∼4 μM, (R2 = 0.99753), and the minimum detection limit can achieve 4.3 nM (S/N = 3). The recoveries of Hg2+ in true water samples were between 99.6% and 104%, which indicated that B,N-GQDs can be successfully applied to the detection of Hg2+ in true water samples.

TiO2/Graphene Quantum Dots core-shell based photo anodes with TTIP treatment- A perspective way of enhancing the short circuit current

Solar Cell with TiO2-Graphene Quantum core/shell heterostructure synthesized by surface functionalization route is used for the present study. The structural properties of Graphene Quantum Dots (GQDs) coated TiO2 are confirmed by XRD, Raman, SEM, TEM and mapping analysis. Energies of the Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) of the GQDs against vacuum are determined using cyclic voltammetry technique and ultraviolet photo electron spectroscopy. When compared to the bare devices, J-V characteristics of the GQDs incorporated solar cell show an enhancement in efficiency around 38% and Jsc of 48%. Moreover, TiO2-GQD (TG 10) device pre and post treated with Titanium Tetra Isopropoxide (TTIP) shows the highest power conversion efficiency (PCE) of 5.1% which is ~65% enhancement when compared to untreated TG 10 based cell. This enhancement is due to higher light harvesting capacity, reduction in charge transfer resistance (Rct) at the interface, increase in life time of carriers and increase in charge transfer rates.

Theoretical insights into tunable optical and electronic properties of graphene quantum dots through phosphorization

Doping heteroatoms or covalent bonding with specific groups is an effective route to modify optical and electronic properties of graphene quantum dots (GQDs). In this work, effects of five phosphorous (P) bonding configurations (i.e., C2P, C2PO, C2PO2, C3P, and C3PO) on optical and electronic properties of GQDs are investigated using density functional theory (DFT) and time-dependent DFT (TD-DFT). Optical absorption spectra, HOMO–LUMO gaps, and excited states of GQDs doped with different P bonding configurations at various locations are calculated to reveal electron transition processes. Due to the availability of vacant third orbitals, Ρ can have various stereochemistry and bonding. It is demonstrated that P bonding configurations with different geometrical configurations have various influences on the spectra of doped GQDs, and the existence of PO double bond in C2PO2 and C3PO configurations can induce multiple absorption peaks in P-doped GQDs. The calculated HOMO–LUMO gap has a larger gap reduction when P exhibits the sp3 hybridization. According to excited state analysis, P-doping with tetrahedral-like configurations has more evident effect on electronic structure of P-doped GQDs, while pyramidal-like configurations have noticeable charge transfer ability in the absorption process.