Showing papers on "Absorption spectroscopy published in 2014"

Organometallic Halide Perovskites: Sharp Optical Absorption Edge and Its Relation to Photovoltaic Performance

Abstract: Solar cells based on organometallic halide perovskite absorber layers are emerging as a high-performance photovoltaic technology. Using highly sensitive photothermal deflection and photocurrent spectroscopy, we measure the absorption spectrum of CH3NH3PbI3 perovskite thin films at room temperature. We find a high absorption coefficient with particularly sharp onset. Below the bandgap, the absorption is exponential over more than four decades with an Urbach energy as small as 15 meV, which suggests a well-ordered microstructure. No deep states are found down to the detection limit of ∼1 cm–1. These results confirm the excellent electronic properties of perovskite thin films, enabling the very high open-circuit voltages reported for perovskite solar cells. Following intentional moisture ingress, we find that the absorption at photon energies below 2.4 eV is strongly reduced, pointing to a compositional change of the material.

Anomalous Band Gap Behavior in Mixed Sn and Pb Perovskites Enables Broadening of Absorption Spectrum in Solar Cells

Abstract: Perovskite-based solar cells have recently been catapulted to the cutting edge of thin-film photovoltaic research and development because of their promise for high-power conversion efficiencies and ease of fabrication. Two types of generic perovskites compounds have been used in cell fabrication: either Pb- or Sn-based. Here, we describe the performance of perovskite solar cells based on alloyed perovskite solid solutions of methylammonium tin iodide and its lead analogue (CH3NH3Sn1–xPbxI3). We exploit the fact that, the energy band gaps of the mixed Pb/Sn compounds do not follow a linear trend (the Vegard’s law) in between these two extremes of 1.55 and 1.35 eV, respectively, but have narrower bandgap (<1.3 eV), thus extending the light absorption into the near-infrared (∼1,050 nm). A series of solution-processed solid-state photovoltaic devices using a mixture of organic spiro-OMeTAD/lithium bis(trifluoromethylsulfonyl)imide/pyridinium additives as hole transport layer were fabricated and studied as a f...

Band gap engineered TiO2 nanoparticles for visible light induced photoelectrochemical and photocatalytic studies

Abstract: Visible light-active TiO2 (m-TiO2) nanoparticles were obtained by an electron beam treatment of commercial TiO2 (p-TiO2) nanoparticles. The m-TiO2 nanoparticles exhibited a distinct red-shift in the UV-visible absorption spectrum and a much narrower band gap (2.85 eV) due to defects as confirmed by diffuse reflectance spectroscopy (DRS), photoluminescence (PL), X-ray diffraction, Raman spectroscopy, electron paramagnetic resonance, transmission electron microscopy, X-ray photoelectron spectroscopy (XPS), electrochemical impedance spectroscopy (EIS) and linear scan voltammetry (LSV). The XPS revealed changes in the surface states, composition, Ti4+ to Ti3+ ratio, and oxygen deficiencies in the m-TiO2. The valence band XPS, DRS and PL results were carefully examined to understand the band gap reduction of m-TiO2. The visible light-responsive enhanced photocatalytic activity of m-TiO2 was demonstrated by degrading methylene blue and brilliant blue G. The EIS and LSV in the dark and under visible light irradiation further support the visible light-induced photocatalytic activities of the m-TiO2 due to a decrease in electron transfer resistance and an increase in photocurrent. This study confirms that m-TiO2 can be used effectively as a photocatalyst and photoelectrode material owing to its enhanced visible light-induced photocatalytic activity.

Tunable optical properties of multilayer black phosphorus thin films

Abstract: Black phosphorus thin films might offer attractive alternatives to narrow-gap compound semiconductors for optoelectronics across mid- to near-infrared frequencies. In this work, we calculate the optical conductivity tensor of multilayer black phosphorus thin films using the Kubo formula within an effective low-energy Hamiltonian. The optical absorption spectra of multilayer black phosphorus are shown to vary sensitively with thickness, doping, and light polarization. In conjunction with experimental spectra obtained from infrared absorption spectroscopy, we also discuss the role of interband coupling and disorder on the observed anisotropic absorption spectra.

Dual-comb spectroscopy based on quantum-cascade-laser frequency combs

Abstract: Dual-comb spectroscopy performed in the mid-infrared-where molecules have their strongest rotovibrational absorption lines-offers the promise of high spectral resolution broadband spectroscopy with very short acquisition times (μs) and no moving parts. Recently, we demonstrated frequency comb operation of a quantum-cascade-laser. We now use that device in a compact, dual-comb spectrometer. The noise properties of the heterodyne beat are close to the shot noise limit. Broadband (15 cm(-1)) high-resolution (80 MHz) absorption spectroscopy of both a GaAs etalon and water vapour is demonstrated, showing the potential of quantum-cascade-laser frequency combs as the basis for a compact, all solid-state, broadband chemical sensor.

The structure of interfacial water on gold electrodes studied by x-ray absorption spectroscopy

Abstract: The molecular structure of the electrical double layer determines the chemistry in all electrochemical processes. Using x-ray absorption spectroscopy (XAS), we probed the structure of water near gold electrodes and its bias dependence. Electron yield XAS detected at the gold electrode revealed that the interfacial water molecules have a different structure from those in the bulk. First principles calculations revealed that ~50% of the molecules lie flat on the surface with saturated hydrogen bonds and another substantial fraction with broken hydrogen bonds that do not contribute to the XAS spectrum because their core-excited states are delocalized by coupling with the gold substrate. At negative bias, the population of flat-lying molecules with broken hydrogen bonds increases, producing a spectrum similar to that of bulk water.

Electronic Structure of Monoclinic BiVO4

Abstract: A comprehensive approach to understanding the electronic structure of monoclinic scheelite bismuth vanadate (ms-BiVO4), including both valence band (VB) and conduction band (CB) orbital character, is presented Density functional theory (DFT) calculations are directly compared to experimental data obtained via X-ray absorption spectroscopy (XAS), X-ray emission spectroscopy, resonant inelastic X-ray spectroscopy (RIXS), and X-ray photoelectron spectroscopy to provide a complete portrait of the total and partial density of states (DOS) near the bandgap DFT calculations are presented to confirm the VB maximum and CB minimum are comprised primarily of O 2p and V 3d orbitals, respectively Predicted triplet d-manifold splitting of V 3d CB states, arising from lone pair-induced lattice distortions, is quantified by V L- and O K-edge XAS Furthermore, the partial contributions to the total DOS within both the CB and VB, determined by RIXS, are found to be in excellent agreement with DFT calculations Energy le

Broadband ultrafast nonlinear absorption and nonlinear refraction of layered molybdenum dichalcogenide semiconductors

Abstract: A series of layered molybdenum dichalcogenides, i.e., MoX2 (X = S, Se and Te), were prepared in cyclohexyl pyrrolidinone by a liquid-phase exfoliation technique. The high quality of the two-dimensional nanostructures was verified by transmission electron microscopy and absorption spectroscopy. Open- and closed-aperture Z-scans were employed to study the nonlinear absorption and nonlinear refraction of the MoX2 dispersions, respectively. All the three-layered nanostructures exhibit prominent ultrafast saturable absorption (SA) for both femtosecond (fs) and picosecond (ps) laser pulses over a broad wavelength range from the visible to the near infrared. While the dispersions treated with low-speed centrifugation (1500 rpm) have an SA response, and the MoS2 and MoSe2 dispersions after higher speed centrifugation (10 000 rpm) possess two-photon absorption for fs pulses at 1030 nm, which is due to the significant reduction of the average thickness of the nanosheets; hence, the broadening of band gap. In addition, all dispersions show obvious nonlinear self-defocusing for ps pulses at both 1064 nm and 532 nm, resulting from the thermally-induced nonlinear refractive index. The versatile ultrafast nonlinear properties imply a huge potential of the layered MoX2 semiconductors in the development of nanophotonic devices, such as mode-lockers, optical limiters, optical switches, etc.

Estimating Hybridization of Transition Metal and Oxygen States in Perovskites from O K-edge X-ray Absorption Spectroscopy

Abstract: The interaction between the transition metal 3d and the oxygen 2p states via hybridization underpins many of the phenomena in transition metal oxide materials. We report the empirical trend of this interaction using the pre-edge feature of the O K-edge X-ray absorption spectrum. Our assessment method is built on the dipole approximation and the configuration interaction between the transition metal 3d and the oxygen 2p states. We found that hybridization increases with the number of 3d electrons, consistent with the expected electronegativity trend. We support this analysis with density functional calculations, which reveal a systematic increase in the transition metal 3d and the oxygen 2p state mixing with increasing 3d-electron number. Oxidation of the transition metal was also found to increase hybridization, which we believe reflects the reduced transition metal 3d and oxygen 2p energy difference, causing increased covalency. We compare the analysis from the surface-sensitive electron-yield and the bu...

Confocal absorption spectral imaging of MoS2: optical transitions depending on the atomic thickness of intrinsic and chemically doped MoS2

Abstract: We performed a nanoscale confocal absorption spectral imaging to obtain the full absorption spectra (over the range 1.5-3.2 eV) within regions having different numbers of layers and studied the variation of optical transition depending on the atomic thickness of the MoS2 film. Three distinct absorption bands corresponding to A and B excitons and a high-energy background (BG) peak at 2.84 eV displayed a gradual redshift as the MoS2 film thickness increased from the monolayer, to the bilayer, to the bulk MoS2 and this shift was attributed to the reduction of the gap energy in the Brillouin zone at the K-point as the atomic thickness increased. We also performed n-type chemical doping of MoS2 films using reduced benzyl viologen (BV) and the confocal absorption spectra modified by the doping showed a strong dependence on the atomic thickness: A and B exciton peaks were greatly quenched in the monolayer MoS2 while much less effect was shown in larger thickness and the BG peak either showed very small quenching for 1 L MoS2 or remained constant for larger thicknesses. Our results indicate that confocal absorption spectral imaging can provide comprehensive information on optical transitions of microscopic size intrinsic and doped two-dimensional layered materials.

p-type doping of MoS2 thin films using Nb

Abstract: We report on the first demonstration of p-type doping in large area few-layer films of (0001)-oriented chemical vapor deposited MoS2. Niobium was found to act as an efficient acceptor up to relatively high density in MoS2 films. For a hole density of 3.1 × 1020 cm−3, Hall mobility of 8.5 cm2 V−1 s−1 was determined, which matches well with the theoretically expected values. X-ray diffraction scans and Raman characterization indicated that the film had good out-of-plane crystalline quality. Absorption measurements showed that the doped sample had similar characteristics to high-quality undoped samples, with a clear absorption edge at 1.8 eV. Scanning transmission electron microscope imaging showed ordered crystalline nature of the Nb-doped MoS2 layers stacked in the [0001] direction. This demonstration of substitutional p-doping in large area epitaxial MoS2 could help in realizing a wide variety of electrical and opto-electronic devices based on layered metal dichalcogenides.

Highly Asymmetric Phthalocyanine as a Sensitizer of Graphitic Carbon Nitride for Extremely Efficient Photocatalytic H2 Production under Near-Infrared Light

Abstract: Highly asymmetric zinc phthalocyanine derivative (Zn-tri-PcNc) with intense near-IR light (650–800 nm) absorption is utilized as a sensitizer to extend the spectral response region of graphitic carbon nitride (g-C3N4) from ∼450 nm to more than 800 nm. Ultraviolet–visible light (UV-vis) diffuse reflectance absorption spectra (DRS), photoluminescence (PL) spectra, time-resolved photoluminescence spectra (TRPS), and energy band structure analyses are adopted to investigate the photogenerated electron transfer process between Zn-tri-PcNc and g-C3N4 on both thermodynamics and dynamics aspects. After optimizing the photocatalytic condition and adding chenodeoxycholic acid (CDCA) as coadsorbent, Zn-tri-PcNc sensitized g-C3N4 photocatalyst shows a H2 production efficiency of 125.2 μmol h–1 under visible/near-IR-light (λ ≥ 500 nm) irradiation, corresponding to a turnover number (TON) of 5008 h–1 with an extremely high apparent quantum yield (AQY) of 1.85% at 700 nm monochromatic light irradiation. The present work...

Defect correlated fluorescent quenching and electron phonon coupling in the spectral transition of Eu3+ in CaTiO3 for red emission in display application

Abstract: This paper reports on the defect correlated self-quenching and spectroscopic investigation of calcium titanate (CaTiO3) phosphors. A series of CaTiO3 phosphors doped with trivalent europium (Eu3+) and codoped with potassium (K+) ions were prepared by the solid state reaction method. The X-ray diffraction results revealed that the obtained powder phosphors consisted out of a single-phase orthorhombic structure and it also indicated that the incorporation of the dopants/co-dopants did not affect the crystal structure. The scanning electron microscopy images revealed the irregular morphology of the prepared phosphors consisting out of μm sized diameter particles. The Eu3+ doped phosphors illuminated with ultraviolet light showed the characteristic red luminescence corresponding to the 5D0→7FJ transitions of Eu3+. As a charge compensator, K+ ions were incorporated into the CaTiO3:Eu3+ phosphors, which enhanced the photoluminescence (PL) intensities depending on the doping concentration of K+. The concentration quenching of Eu3+ in this host is discussed in the light of ion-ion interaction, electron phonon coupling, and defect to ion energy transfer. The spectral characteristics and the Eu-O ligand behaviour were determined using the Judd-Ofelt theory from the PL spectra instead of the absorption spectra. The CIE (International Commission on Illumination) parameters were calculated using spectral energy distribution functions and McCamy's empirical formula. Photometric characterization indicated the suitability of K+ compensated the CaTiO3:Eu3+ phosphor for pure red emission in light-emitting diode applications.

Air stable p-doping of WSe2 by covalent functionalization.

Abstract: Covalent functionalization of transition metal dichalcogenides (TMDCs) is investigated for air-stable chemical doping. Specifically, p-doping of WSe2 via NOx chemisorption at 150 °C is explored, with the hole concentration tuned by reaction time. Synchrotron based soft X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) depict the formation of various WSe2–x–yOxNy species both on the surface and interface between layers upon chemisorption reaction. Ab initio simulations corroborate our spectroscopy results in identifying the energetically favorable complexes, and predicting WSe2:NO at the Se vacancy sites as the predominant dopant species. A maximum hole concentration of ∼1019 cm–3 is obtained from XPS and electrical measurements, which is found to be independent of WSe2 thickness. This degenerate doping level facilitates 5 orders of magnitude reduction in contact resistance between Pd, a common p-type contact metal, and WSe2. More generally, the work presents a platform for man...

Crystal Structures, Optical Properties, and Effective Mass Tensors of CH3NH3PbX3 (X = I and Br) Phases Predicted from HSE06.

Abstract: The crystal structures are successfully established for tetragonal and orthorhombic CH3NH3PbX3 (X = I and Br). The equilibrium lattice parameters are computed by the DFT+D2 method, and the results are compared to experimental values. The band dispersions and electronic densities of states are calculated by HSE06, showing that their band gaps are in the range from 1.63 to 2.3 eV. Although the calculated dielectric functions of MAPbX3 compounds are similar to other semiconductors, the absorption spectra of their bulk crystals are drifted away from visible light spectrum. The effective mass tensors of holes and electrons are also evaluated in three principal directions at the Γ point. The anisotropies in the effective masses of the hole and electron are illustrated for two orthorhombic phases.

Highly Thermally Stable Single-Component White-Emitting Silicate Glass for Organic-Resin-Free White-Light-Emitting Diodes

Abstract: Thermal management is still a great challenge for high-power phosphor-converted white-light-emitting diodes (pc-WLEDs) intended for future general lighting. In this paper, a series of single-component white-emitting silicate SiO2–Li2O–SrO–Al2O3–K2O–P2O5: Ce3+, Tb3+, Mn2+ (SLSAKP: Ce3+, Tb3+, Mn2+) glasses that simultaneously play key roles as a luminescent convertor and an encapsulating material for WLEDs were prepared via the conventional melt-quenching method, and systematically studied using their absorption spectra, transmittance spectra, photoluminescence excitation and emission spectra in the temperature range 296–498 K, decay curves, and quantum efficiency. The glasses show strong and broad absorption in 250–380 nm region and exhibit intense white emission, produced by in situ mixing of blue-violet, green, and orange-red light from Ce3+, Tb3+, and Mn2+ ions, respectively, in a single glass component. The quantum efficiency of SLSAKP: 0.3%Ce3+, 2.0%Tb3+, 2.0%Mn2+ glass is determined to be 19%. More ...

Operando X-ray Absorption Study of the Redox Processes Involved upon Cycling of the Li-Rich Layered Oxide Li1.20Mn0.54Co0.13Ni0.13O2 in Li Ion Batteries

Abstract: Operando X-ray absorption spectroscopy investigations have been carried out to follow changes in the atomic and electronic local structures of all three transition metals for the Li1.20Mn0.54Co0.13Ni0.13O2 layered oxide during the first and second charges and discharges of lithium batteries. The experiments were performed using a Quick-XAS monochromator on the SAMBA beamline at Synchrotron SOLEIL to record the three K-edges by edge-jumping between two energy ranges ([Mn, Co] and [Co, Ni]) every 3 min during the cycling of the battery. The results obtained especially at the Mn K-edge fully support the participation of oxygen in the reversible charge–discharge reaction of this Li- and Mn-rich layered material as a redox center and not only with oxygen loss, as was proposed previously.

Respective Role of Fe and Mn Oxide Contents for Arsenic Sorption in Iron and Manganese Binary Oxide: An X-ray Absorption Spectroscopy Investigation

Abstract: In our previous studies, a synthesized Fe-Mn binary oxide was found to be very effective for both As(V) and As(III) removal in aqueous phase, because As(III) could be easily oxidized to As(V). As(III) oxidation and As(V) sorption by the Fe-Mn binary oxide may also play an important role in the natural cycling of As, because of its common occurrence in the environment. In the present study, the respective role of Fe and Mn contents present in the Fe-Mn binary oxide on As(III) removal was investigated via a direct in situ determination of arsenic speciation using X-ray absorption spectroscopy. X-ray absorption near edge structure results indicate that Mn atoms exist in a mixed valence state of +3 and +4 and further confirm that MnOx (1.5 < x < 2) content is mainly responsible for oxidizing As(III) to As(V) through a two-step pathway [reduction of Mn(IV) to Mn(III) and subsequent Mn(III) to Mn(II)] and FeOOH content is dominant for adsorbing the formed As(V). No significant As(III) oxidation by pure FeOOH had been observed during its sorption, when the system was exposed to air. The extended X-ray absorption fine structure results reveal that the As surface complex on both the As(V)- and As(III)-treated sample surfaces is an inner-sphere bidentate binuclear corner-sharing complex with an As-M (M = Fe or Mn) interatomic distance of 3.22-3.24 angstrom. In addition, the MnOx and FeOOH contents exist only as a mixture, and no solid solution is formed. Because of its high effectiveness, low cost, and environmental friendliness, the Fe-Mn binary oxide would play a beneficial role as both an efficient oxidant of As(III) and a sorbent for As(V) in drinking water treatment and environmental remediation.

Fitting of calibration-free scanned-wavelength-modulation spectroscopy spectra for determination of gas properties and absorption lineshapes.

Abstract: The development and initial demonstration of a scanned-wavelength, first-harmonic-normalized, wavelength-modulation spectroscopy with nf detection (scanned-WMS-nf/1f) strategy for calibration-free measurements of gas conditions are presented. In this technique, the nominal wavelength of a modulated tunable diode laser (TDL) is scanned over an absorption transition to measure the corresponding scanned-WMS-nf/1f spectrum. Gas conditions are then inferred from least-squares fitting the simulated scanned-WMS-nf/1f spectrum to the measured scanned-WMS-nf/1f spectrum, in a manner that is analogous to widely used scanned-wavelength direct-absorption techniques. This scanned-WMS-nf/1f technique does not require prior knowledge of the transition linewidth for determination of gas properties. Furthermore, this technique can be used with any higher harmonic (i.e., n>1), modulation depth, and optical depth. Selection of the laser modulation index to maximize both signal strength and sensitivity to spectroscopic parameters (i.e., gas conditions), while mitigating distortion, is described. Last, this technique is demonstrated with two-color measurements in a well-characterized supersonic flow within the Stanford Expansion Tube. In this demonstration, two frequency-multiplexed telecommunication-grade TDLs near 1.4 μm were scanned at 12.5 kHz (i.e., measurement repetition rate of 25 kHz) and modulated at 637.5 and 825 kHz to determine the gas temperature, pressure, H2O mole fraction, velocity, and absorption transition lineshape. Measurements are shown to agree within uncertainty (1%–5%) of expected values.

Profiling the nanoscale gradient in stoichiometric layered cathode particles for lithium-ion batteries

Abstract: Chemical and structural evolution in battery materials influences properties relevant to ionic and electronic transport and ultimately impacts the battery performance. Although chemical and structural gradients have been observed in several cathode materials, the origin(s) of these phenomena are poorly understood. Via high-throughput core-level spectroscopies {i.e., X-ray absorption spectroscopy (XAS), depth-profiled X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS)}, as well as scanning transmission electron microscopy (STEM), the present study seeks to achieve mechanistic understanding for these phenomena in a stoichiometric Rm layered cathode material (e.g., LiNixMnxCo1−2xO2, NMC). We observed that the surfaces of particles in the composite electrode are complicated by the presence of a surface reaction layer resulting from electrolyte decomposition. In large particle ensembles, the global nickel oxidation state switches between Ni2+ and Ni2+x (x = 1–2) during charging/discharging processes, and hole states are also created at the O2p level due to the TM3d–O2p hybridization states. In primary particles, the surface is less oxidized than the bulk counterpart of the same particle whenever the particle has been cycled. This is partially attributed to the reconstruction from an Rm structure to an Fmm structure at the surfaces of NMC particles. This work provides a unique insight into correlating crystal structures with charge compensation mechanisms and performance fading in stoichiometric layered cathode materials.

Water oxidation on hematite photoelectrodes: Insight into the nature of surface states through in situ spectroelectrochemistry

Abstract: Uniform planar films of hematite (α-Fe2O3), deposited by atomic layer deposition, were examined using in situ spectroelectrochemistry during photoinduced water oxidation. A change in the absorption spectrum of hematite electrodes during water oxidation was measured under illumination and applied potentials. The absorption was correlated to a charge measured by cyclic voltammetry and with a capacitance measured by impedance spectroscopy. Modification of the hematite surface with alumina reduced the absorption feature and the associated capacitance, suggesting that these features are associated with the surface. Comparing the spectral change of hematite to absorption features of molecular analogues allowed us to tentatively assign the absorbance and capacitive features to the oxidation of a low valent iron-aqua or iron-hydroxyl species to a high valent iron-oxo chemical species at the surface.

Efficiency Improvement of Solution-Processed Dithienopyrrole-Based A-D-A Oligothiophene Bulk-Heterojunction Solar Cells by Solvent Vapor Annealing

Abstract: The extension of a series of dithienopyrrole containing A-D-A oligothiophenes for application in solution-processed bulk heterojunction solar cells is described. Using solvent vapor annealing, power conversion efficiencies up to 6.1% are obtained. Exposure of the photoactive layer to chloroform vapor results in increased absorption and ordering of the donor:acceptor blend, as is evident from UV-vis absorption spectroscopy, X-ray diffraction (XRD) spectroscopy, and atomic force microscopy (AFM). The type and position of the solubilizing alkyl chains influences the dissolution, optical, and packing properties of the oligomers. However, despite subtle differences in molecular structure, all electron donors could be implemented in solar cells demonstrating power conversion efficiencies between 4.4 and 6.1%. Upon further optimization of these in-air, processed devices, it is expected that additional improvements in photovoltaic performance can be achieved.

Fullerene modified C3N4 composites with enhanced photocatalytic activity under visible light irradiation

Abstract: Fullerene modified C3N4 (C60/C3N4) composites with efficient photocatalytic activity under visible light irradiation were fabricated by a simple adsorption approach. The as-prepared C60/C3N4 composites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflectance absorption spectra (DRS), Fourier transform infrared spectroscopy (FTIR) and photoluminescence spectra (PL). The photocatalytic degradation of rhodamine B (RhB) by the C60/C3N4 composites was investigated and optimized, suggesting that the optimal amount of C60 in the composites was 1 wt%. The significantly enhanced photocatalytic activity could be attributed to the efficient separation of photogenerated electrons and holes in the C60/C3N4 composites. A possible mechanism of C60/C3N4 composites as photocatalysts was proposed.

DFT study of optical properties of pure and doped graphene

Abstract: Ab-initio calculations based on density functional theory (DFT) have been performed to study the optical properties of pure graphene and have been compared to that of individual boron (B), nitrogen (N) and BN co-doped graphene sheet. The effect of doping has been investigated by varying the concentrations of dopants from 3.125% (one atom of the dopant in 32 host atoms) to 18.75% (six dopant atoms in 50 host atoms) for individual B and N doping and from 6.25% (one B/N pair in 32 host atoms) to 75% for BN co-doping. Positions of the dopants have also been varied for the same concentration of substitution doping. The dielectric matrix has been calculated within the random phase approximation (RPA) using VASP (Vienna ab-initio Simulation Package) code. The dielectric function, absorption spectrum and energy loss-function of single layer graphene sheet have been calculated for light polarization parallel and perpendicular to the plane of graphene sheet and compared with doping graphene. The calculated dielectric functions and energy-loss spectra are in reasonable agreement with the available theoretical and experimental results for pure graphene. It has been found that individual B and N doping does not significantly affect the imaginary dielectric function and hence the absorption spectra. However, significant red-shift in absorption towards the visible range of the radiation at high doping is found to occur for the B/N co-doping. The results can be used to tailor the optical properties of graphene in the visible region.