Optical Properties of Malachite Green Dye Doped SiO 2 Glasses : Effect of Transition Metal ( FeI ) Used as a Codopant

Enhanced luminescence properties of Malachite Green (MG) (oxalate) in Fe-MG codoped SiO 2 glasses compared to its values in MG doped SiO 2 glasses are reported here. The enhancement is chiefly attributed to a resonance nonradiative energy transfer betweenFe andMG.Thequantumyield ofMalachiteGreen (MG), in presence of Iron, trapped in sol-gel derived SiO 2 glass increases by an order of ∼10 compared to that in low viscous solvent while a lifetime of 3.29 ns is reported.


Introduction
Different groups of elements used as dopant and/or codopant often redefine and modify material properties of chemically compatible solid matrices.Tuning of band gap in materials for application in solar cells, inducing extrinsic conductivity in semiconductors, evolution of critical temperature TC in superconductivity or colossal magneto resistivity [1], and enhancement in luminescence properties of lanthanide doped materials [2] are few examples.Amongst various elements used as dopant and/or codopant, transition metal (TM) elements offer specific advantage.Codoping of TM improves photocatalytic activity of -type metal oxide semiconductors [3], changes optical properties and photoactivity of nanoparticulates [4], influences the broad band luminescence excitation mechanism in rare earth-doped glasses [5], quenches lanthanide (Nd 3+ ) transitions in glasses [6], and so forth.TM oxide semiconductors are codoped with other elements to stabilize their magnetic properties [7].Besides the effect of codopant elements, luminescence properties of materials also depend on the host structure and other chemical constituents of the host.Luminescence intensity and quantum yield of organic molecules and chromophore dyes when trapped in porous solid matrices are increased by several orders compared to their values in liquid solvent.As such, laser dyes doped in solid porous matrices are extensively used in the development of solid state dye lasers [8].But, to our knowledge, very few works are reported on codopant effect on luminescence properties of dyes under confinement of solid matrices and none with transition metal as codopant.The study of energy transfer in silica xerogel codoped with Coumarin-120 dye and lanthanide ions, namely, Eu 3+ and Tb 3+ [9], and in thorium phosphate xerogel codoped with Coumarin-460 dye and Tb 3+ [10] and Eu 3+ [11] is an example of some of such reported works.It is, therefore, rightly said that the potential of doping/codoping of sol-gel derived glasses and similar materials with both lanthanide and transition metal elements is yet to be fully explored [12].
In this paper, the effect of codoping transition metal (Fe-I) on optical properties of Malachite Green (MG) oxalate dye trapped in the pores of sol-gel derived silica glasses via doping is investigated with emphasis on photoluminescence properties.MG (oxalate) dye is a member of the triarylmethane group with low quantum yield, which is increased by ∼10 3 times the values reported [13] in low-viscous liquid solvents when doped in solid silica matrices.

Instruments and Methods.
IR spectra in the range 4000-400 cm −1 were recorded using KBr technique with a Jasco FT/IR 300E spectrophotometer.VIS absorption spectra of the glasses in the range of 400-700 nm were recorded in UV-VIS-NIR spectrophotometer (CARY-5E) and their photoluminescence spectra were taken in a JY Fluorolog-3-11 spectrofluorometer, excited by 445 nm wavelength of a 450 W Xenon lamp, keeping slit width at 5 nm, with integration time 0.1 s and step size 1 nm.Fluorescence lifetime measurements of four glasses, namely, M1, M4, F3M1, and F3M4 at  em = 573 nm, 585 nm, 628 nm, and 624 nm, respectively, were performed in FluoroCube -a lifetime measurement system from HORIBA.The instrument response function (IRF) of the instrument was 1.263 ns obtained using LUDOX SM-30 colloidal suspension in water.The excitation source for lime time measurement was a 460 nm LED.TEM micrographs of four MG doped SiO 2 glass matrices with and without Fe-I codopant were recorded in a JEOL JEM-2100 ultrahigh resolution TEM.All measurements and recordings were done in room temperature (25-27 ∘ C).

Structural Analysis
IR Spectra: Structure-Composition Relation of Matrix.IRabsorption spectra of MG-Fe doped sol-gel glass F3M1 are shown in Figures 1(a) and 1(b), respectively.IR absorption spectroscopy using KBr technique is successful in study of structure-composition relation in glasses and ceramics [16].Moreover, it can be used to identify low concentration impurities, such as water and hydroxyl ions in glasses [17].
The fingerprint region of the IR spectra in Figures 1(a) and 1(b) is predominated by bands associated with network vibrational modes of Si-O-Si and Si-O groups.Strong and intense band due to Si-O-Si asymmetric stretching appears around 1088 cm −1 and due to its bending mode at around 464 cm −1 (with a shoulder at around 549 cm −1 ).The weak band at around 797 cm −1 may be assigned to Si-O-Si symmetric stretching of the bridging oxygen.The band around 969 cm −1 may be assigned to Si-O stretching with one or two nonbridging oxygens [18].Besides, at around 1384 cm −1 a strong and intense band is attributed to vibration of TEOS, ethoxy group (due to CH 3 symmetric bending) [19].This band is very unstable and disappears on application of heat.

TEM-M1
TEM-diffraction of M1    in presence of 0.03 M of Fe-I are red shifted by ∼6 nm and ∼10 nm, respectively, with a corresponding increase in absorbance by 73.3% and 90.9%, respectively.In FM1 ( = 1-4) and FM4 ( = 1-4) glasses the absorbance of Band I increases linearly with Fe concentration up to 0.03 M and accounts for an additional increase of 34 M −1 cm −1 and 27 M −1 cm −1 in the molar extinction coefficient () of MG in FM1 ( = 1-4) and FM4 ( = 1-4) glasses, respectively.Similar variation in absorbance with Fe concentration is absent for Band II.where ∫ () ≈ () × Δ and () is the molar absorption coefficient at frequency (cm −1 ) and Δ is the band width at 1/2() measured directly from absorption spectra (FWHM) "Origin-8.5"software.These values of MG-Fe doped glasses are much reduced compared to their corresponding values in M1 and M4 glasses.This is due to a reduction in its Δ values of the bands in codoped glasses.The calculated oscillator strength of the absorption bands together with absorbance maxima wavelength ( (abs) ) and FWHM of the bands are compiled in Table 1 for comparison.Presence of small quantities of Fe-I in vicinity of MG molecules in the matrix can lower site symmetry around the molecule and could have resulted in increased absorbance in these codoped glasses.Codopants like Al and P are known to cause such enhancement in absorbance of lanthanide transitions in doped glasses [21].A possible cause of deviation of absorption from Beer-Lambert law and a subsequent decrease in absorbance in some cases at higher concentration of Fe-I (0.04 M) may be attributed to shadow effect, where the absorbing dye molecules are effectively screened from the incident radiation by presence of large quantity of Fe-I codopants in its path.Addition of 0.01 M-0.03 M Fe-I significantly improves the luminescence properties of MG in these glasses.The photoluminescence bands of MG in SiO 2 glasses in presence of Fe-I under 445 nm excitation become narrower in width, sharper in profile, and with peak centres red shifted by ∼ 55 nm and ∼39 nm in FM1 ( = 1-4) and FM4 ( = 1-4) glasses, respectively.Photoluminescence intensity of MG with 0.03M concentration of Fe-I increases 4.8 and 1.72 times in FM1 ( = 1-4) and FM4 ( = 1-4) glasses, respectively, and intensities above it are saturated in F4M4 and quenched in F4M1 glasses, respectively.
Subsequently, measured values of quantum yield and radiative and nonradiative transition probabilities of MG in F3M1 and F3M4 glasses-where enhancement in luminescence intensity was best-are improved compared to its values in M1 and M4 glasses.The radiative (  ) and nonradiative ( nr ) transition probabilities are calculated using the following relations [17]: where  and  are measured fluorescence quantum yield (QY) and intrinsic decay time, respectively, of MG in the TEOS silica matrix used in this study.Fluorescence quantum yield () is measured by comparison to a reference fluorophore [riboflavin] of known QY using the single point method by using the following equation: In (3),   is the quantum yield of the reference fluorophore.
In these measurements riboflavin with   = 0.3 is taken as the reference."" is the integrated fluorescence intensity and "OD" is the optical density.Subscript "" refers to the reference fluorophore.Lifetime () of excited MG states in SiO 2 glasses with and without codopants is estimated by double exponent iteration of the recorded decay curves.Its values do not show much variation on codoping with Fe-I.Decay curves recorded for M1, M4, F3M1, and F3M4 glasses are presented in Figure 7. Photoluminescence properties derived using (2)-(3) for glasses F3M1, F3M4, M1, and M4 along with measured lifetime () of excited states of MG, peak emission wavelength (  ), and effective band width (Δ eff ) of luminescence bands are compiled in Table 2.
Confinement of organic molecules within rigid pores of a solid matrix restricts nonradiative routes for deactivation of excited molecules, resulting in an increase in photoluminescence intensity and quantum yield.Rigidity of the pores reduces space for rotational motion of the molecules and thereby limits nonradiative relaxation channels of excited trapped molecules.Codoping the confined organic chromophores/molecules [MG] with Fe-I leads to further filling up the pores, reducing free space, and increasing rigidity of the porous matrix and thereby photoluminescence intensity [22].However, change in the matrix rigidity due to incorporation of Fe-I cannot solely account for an increase in photoluminescence intensity as in the present case.A significant contribution is from a resonance nonradiative energy transfer from Fe to MG.A strong spectral overlap observed between a 5 P 3 → a 5 D 4 emission band of Fe-I at 567 nm (sensitizer) and S 0 → S 1 MG absorbance band at 561 nm (activator) accompanied by increasing photoluminescence intensity of activator with donor concentration is indicative of such energy transfer.This Förster-Dexter [23,24] kinetics governs resonance energy transfer and assumes random International Journal of Optics  and uniform distribution of sensitizer and activator where sensitizer-sensitizer interaction is considered to be absent.
The energy transfer depends strongly on spectral overlap, emission cross-section of sensitizer, and absorption crosssection of activator expressed by critical distance Schematic representation of possible routes of energy transfer in Fe-Mg codoped SiO 2 glass under 445 nm excitation is presented in Figure 8.The transfer is accompanied by nonradiative loss to the matrix site that may have been expressed as vibration or heat because of which photoluminescence intensity increases nonlinearly with Fe-I concentration.
Further, TEM micrographs in Figure 2 along with diffraction patterns depict a transformation in silica matrix in presence of Fe-I and are indicative of a modification in the host ligand field acting on the vibrational levels of the trapped molecules causing a red shift [25] in the photoluminescence bands of Fe-MG codoped glasses.The comparatively homogeneous spectral features of these red shifted bands may be attributed to the changes in matrix structure.

Conclusion
A significant enhancement in photoluminescence intensity in Fe-MG codoped SiO 2 sol-gel glasses is observed which is attributed primarily to a resonance energy transfer from Fe to MG apart from increased rigidity of the pores, in which MG molecules are entrapped, due to codoping.Routes of energy transfer along with observed variations in transitions are also analysed.Intensity of the photoluminescence band in the MG doped SiO 2 glass with its peak centred (  ) at 573 nm and an effective bandwidth (Δ eff ) of 133 nm is enhanced 4.8 times in presence of 0.03 M of Fe-I.Further, (  ) of the band is shifted towards red by 55 nm and (Δ eff ) becomes narrower by 48 nm.The quantum yield of MG in presence of Fe-I is increased 2.16 times.The improvement in various radiative parameters needs to be examined in context of the effect of codopants from other groups, like Al-a commonly used codopant to sensitize luminescence efficiency in order to understand the effectiveness of Fe-I as a sensitizer of MG luminescence.

aFigure 8 :
Figure 8: (a) Spectral overlap between a 5 P 3 → a 5 D 4 emission band of Fe-I and S 0 → S 1 MG absorbance band.(b) Schematic representation of energy transfer from Fe-I to MG dye in SiO 2 glass under 445 nm excitation.