CHAPTER-3 STUDIES ON LINEAR OPTICAL PROPERTIES OF MALONONITRILE DERIVATIVE CRYSTALS

Transcription

1 CHAPTER-3 STUDIES ON LINEAR OPTICAL PROPERTIES OF MALONONITRILE DERIVATIVE CRYSTALS 3.1 INTRODUCTION In these decades, there is an increasing attention on light absorption and structure of matter. There are different types of optical materials including inorganic, organics, polymers, semiconductors, glasses, single crystal, and composites. Nowadays organic materials have been investigated due to their high photo response, easily structure tuneable, and low absorption compared to other materials. The delocalized π- conjugated organic materials could perform effectively in information processing devices. Most of the studies now concentrate on organic materials with π- π*electronic transitions because they are superior to the molecular hyperpolarizabilities. In optical communication system nm range wavelengths are used, even cable television networks where different wavelengths are used for the downstream (1310nm) and upstream (1550nm) sig nals. In every 6 to 12 months, doubling the usage of bandwidth in the internet due to the data based networks. The key components of next generation broadband devices are ultrafast optical switching devices. The materials with low linear loss are required to implement the optical switching devices. An interest of optical switching application has required finding the high third order NLO material, as well as large transmittance or low absorption in near infrared region. Therefore, there is a strong interest to develop new organic nonlinear optical materials with strong absorption of visible and near- IR light. The conjugated π-electron bonding networks found in this molecule is the principal reason for their strong absorption of visible and near-ir light. The most important organic compounds that absorb visible light can be classified into three categories such as polyenes, polymethines and porphyrines ( Kuhn;1949). The candidate molecules polyene and cyanine dyes are one of the major ingredients for the large optical response (Kwon et al;2006). The synthesized polyene type organic compound consist of π-conjugated bridge between dicyanomethylidene acceptor and benzaldehyde donor or acceptor. The linear optical property of synthesized malononitrile crystals and its derivative is 84

2 discussed in this chapter. The optical absorption spectra of malononitrile derivative were recorded in double beam ultraviolet spectrophotometer (ELICO-SL218), in the wavelength range of 190nm-1100nm. The optical constants such as absorption, optical conductivity, extinction coefficient and reflectance are studied. The refractive index of crystals was measured using Abbe s Refractometer ELECTRONIC TRANSITION OF ELECTRONS There are three types of electronic transition, Transition of π, σ, & n electrons, transition of charge transfer electrons and transition of d & f electrons. These transitions are due to the absorption of ultraviolet and visible radiation of outer electrons. The electrons are promoted from their ground state to an excited energy states due to the absorption of atom or molecule. In organic molecules, certain functional groups (chromophores) are restricted to absorption of UV and visible radiation. The continuous absorption band at nm region due to the electronic transition gives an overlapping line, because of the superposition of rotational and vibrational transitions. These chromophores are complex molecules. But malononitrile compound are π-conjugated system, there is a possibility of π-π* transitions. Possible electronic transitions of π, σ, and n electrons are; There are four types of transition n-π* ( nm), n-σ*( nm), π-π*( nm), σ- σ*(125nm) is shown in Fig.3.1. Figure 3.1: The electronic transitions of electrons Mostly organic compounds are based on π-π* electronic transition. This is because the absorption peaks for these transitions fall in an experimentally convenient 85

3 region of the spectrum ( nm). These transitions need an unsaturated group in the molecule to provide the π electrons CALCULATION OF LINEAR OPTICAL ABSORPTION AND CONSTANTS The linear absorption of the malononitrile crystal has been studied in UV-Vis-NIR spectroscopy. According to the Beer-Lambert law, thus the relation between the absorbance (A), Transmittance (T) and thickness (t) is 1 A log 3.1 T T 10 A 3.2 Optical absorption coefficient ( α) of crystal has been calculated from the relation log T 3.3 d where d is thickness of crystal and T is transmittance of crystal. The optical absorption coefficient is used to study the energy band gap and type of transition of electrons. The optical band gap of crystal is estimated using the relation is αhγ=a(e -hγ) g where, α is absorption coefficient, h is Planck s constant,γ is the frequency of light, Eg is energy band gap and A is slope of Tauc s edge (band tailing parameter). The type of transition of an electron has studied from the plot, absorption coefficient versus wavelength. The four types of transitions are direct allowed transition (n=1/2), indirect allowed transition (n=2), direct forbidden transition (n=3/2) and indir ect forbidden transition (n=3). The graph is drawn between photon energy (hγ) verses (αhγ) 2 or (αhγ) 1/2 based on direct and indirect band gap respectively. The extrapolation of the straight line from the absorption edge to photon energy axis (hγ) gives band gap. Tauc s method has been used to measure the energy band gap of crystals (Tauc;1970). The band gap value of OH1 crystal could be used in optical devices like optical switching and optical attenuators. The reflectance(r) and refractive index (n) of the crystal have been estimated using the relations n

4 1 1 exp() exp() t R 1 exp() t t 3.5 n 2( R 1) 2 ( R 1) 3R 10R 3 Extinction coefficient is a measure of the rate of loss of electromagnetic radiation through scattering and absorption of crystal per unit thickness. The extinction coefficient is calculated in terms of absorption coefficient, K e The optical conductivity (σ op ) or frequency response of the crystal is obtained in terms of the absorption coefficient and refractive index, op c The electrical conductivity ( σ e ) of CDD crystals can also be estimated by optical method using the relation [Ilenikhena et al;2008]. e 2 op The complex dielectric constant is given by 3.9 c re i re n k nk 3.12 i Where, ε re and ε i are real and imaginary constant respectively. The real and imaginary constant are related to extinction coefficient and refractive index (Eya et al;2006) 3.4. ABSORPTION SPECTRUM OF MALONONITRILE DERIVATIVE CRYSTALS The optical absorption spectrum of malononitrile derivative crystal was recorded in the range of nm using ELICO SL 218 double beam UV-Vis-NIR spectrophotometer. It is observed that the absorption edges of OH1, MOT2, Cl1, Br1, OE1 and 3E4HM crystals are at 561,590,610,614,525 and 564nm respectively. The crystals Br1, Cl1, OE1 and MOT2 show the π-π* transition at 380, 379, 398 and 390 is due to the conjugate bond present in the molecules. The absorption coefficient of malononitrile derivative crystal is shown in Fig

5 Figure 3.2: Absorption coefficient of malononitrile derivative crystal The ultraviolet spectroscopy involves the promotion of the electron in π and π* orbitals from the ground state to the higher energy states. MOT2, Cl1, Br1, & OE1 molecule consists of -conjugated bridge between dicyanomethylidene donor and aldehyde acceptor/donor. When a double bonded molecule absorbs light, it undergoes a π-π* transition. Because π-π* energy gaps are narrow than σ-σ* gaps, double bond absorbs light at longer wavelength than molecular hydrogen. The electronic transitions of both molecular hydrogen and double bond are too energetic to be accurately recorded by standard UV spectrophotometers, which generally have a range of nm.It is possible to measure the number of conjugated bonds in molecular structure from the number absorption in UV spectroscopy for new molecules. Sometimes the transition of π-π* bond did not observed in crystal like OH1 and 3E4HM, depends on thickness and absorption ratio of sample. It is observed that OH1 is noncentrosymmetry packing crystal, mostly second harmonic generation compounds did not show the π-π* transitions in the wavelength region. When compare to the spectrum of OH1 and 3E4HM crystals, the presence of ethyl in -meta position of the aldehyde shifts the cut-off wavelength of 3nm. The absorption coefficient of OH1 and 3E4HM crystal varies /mm. Both the molecule did not show the π-π* transition, possibly due to the insufficient of optical energy to the transitions of electrons from lower energy to higher energy and depends upon the 88

6 thickness of the sample. The variation of absorption edge between crystal Cl1 and Br1 is 3nm, but the variation of absorption coefficient is /mm TAUC S METHOD (TO DETERMINE THE OPTICAL BAND GAP) Figure 3.3: Tauc s plot of OH1 and MOT2 crystals The optical absorption coefficient is used to study the energy band gap and type of transition of electrons. Optical absorption coefficient (α) of crystals has been calculated from the given relation 3.3. OH1 crystal has been taken for determination of optical band gap. The optical band gap (Eg) of OH1 crystal has been calculated using the given formula 3.4. The direct allowed transition (n=1/2) is observed in OH1 Figure 3.4: Tauc s plot of Cl1, Br1, OE1, 3E4HM Crystals 89

7 crystal. The value of energy band gap is calculated from the graph drawn between (αhγ) 2 verses energy axis(hγ) as shown in Fig.3.3 and 3.4. The energy band gap is figured out to extrapolate the linear part from the maximum absorption end to the photon energy axis. The intersecting point on energy axis is band gap energy of OH1 crystal. The energy band gap of OH1 crystal is found to be eV. In UV absorption graph, indirect allowed transition (n=2) was observed for MOT2 crystal. The intersecting point on energy axis is band gap energy of MOT2 crystal. The band gap energy of MOT2 crystal is 2.159eV. Cl 1 absorption spectrum shows an indirect transition and the observed energy band gap is eV. The absorption coefficient of Br1 crystal suggests the occurrence of indirect transitions, and the energy gap is found to be ev. The energy band gap of OE1 and 3E4HM crystal is found to be ev and ev respectively REFLECTANCE Figure 3.5: Reflection spectrum of Cl1, Br1, OE1 and 3E4HM crystal The reflectance is the ratio of the reflected to the incident electromagnetic waves. Reflections occur when light moves to different medium having different refractive 90

8 index. It is an important in the field of telecommunications. It is observed that the reflections are maximum for all crystals at lower wavelength region and minimum at higher wavelength region. MOT2, & OE1 crystal shows the zero reflection on the wavelength region 500nm- 1100nm. 3E4HM and OH1 crystal shows very small reflection nearly zero, but Br1 and Cl1 crystal have more reflection compared to all these crystals. Reflection is the surface property of the crystal; it shows the little opacity of the crystal surface. These materials have high opacity in the UV and Visible region. Reflection spectrum of malononitrile derivative crystal is shown in Fig. 3.6 and 3.7. Figure 3.6: Reflection spectrum of OH1, MOT2 crystals 3.7. EXTINCTION COEFFICIENT Figure 3.7: Extinction coefficient of OH1 and MOT2 crystals 91

9 Extinction coefficient (K e ) explains how easily the electromagnetic radiation passing through the crystal has been calculated using the given relation 3.7. Extinction coefficient is a measure of the rate of loss of electromagnetic radiation through scattering and absorption for crystals per unit thickness. The extinction values of malononitrile derivative crystal are shown in Fig.3.7. & 3.8 Figure 3.8: Extinction coefficient of OE1,3E4HM, Cl1 and Br1 crystals 3.8. OPTICAL CONDUCTIVITY Optical conductivity of malononitrile derivative crystals are shown in Fig.3.9 and 3.10 and it is calculated from the equation 3.8. The frequency responses of crystals are obtained in terms of the absorption coefficient and refractive index. A sizable change in optical constants requires a significant photoinduced population change. This implies that a large fraction of the molecules in the active region of the device must absorb a photon during every binary operation. At proposed bit rates of 10 9 s -1, assuming a conservative quantum yield for photodegradation of a typical organic 92

10 chromophore of 10-8, the device would self-destruct in 10-1 s ( Allen et al;1980) (Greene et al;1990). The increase in optical conductivity of material with respect to increase in photon energy shows the good optical response of the material. The high value of optical conductivity ( s -1 ) shows the very good photoresponse of malononitrile crystals. Figure 3.9: Optical conductivity of OH1 and MOT2 crystals Figure 3.10: Optical conductivity of Cl1, Br1, OE1 and 3E4HM crystals 93

11 3.9. LINEAR REFRACTIVE INDEX The refractive index measurements of malononitrile derivative single crystals were measured using Abbe refractometer connected by digital thermometer. Polished crystal samples have chosen for the study. Each sample independently contact with the central prism and the contact liquid (Methylene iodide or Monobromo naphthalene), which makes the sample close contact to the prism. Table.3.1.Refractive index of malononitrile derivative crystal S.No Crystal Crystal Size mm 3 Contact liquid Temperature K Refractive index 1 OH1 4x3x1 Methylene iodide MOT2 6x4x1 Methylene iodide Cl1 5x2x1 Methylene iodide Br1 4x4x1 Methylene iodide OE1 4x4x1 Monobromonapthalene E4HM 5x1x1 Monobromonapthalene Open the secondary prism completely and stand a milky white reflector against the opposite side of secondary prism so that external light (sodium lamp) reflects on the reflector and it horizontally penetrates the sample. While observing to the eyepiece, gently turn the measurement knob to set the scale indication approximately to the refractive index shown on the test piece. The measured refractive index of malononitrile derivative crystals are shown in Table PHOTOLUMINESCENCE STUDIES Photoluminescence studies are preferred over to detect the lower concentrations of defects. Since the impurity on absorption of light gives rise to the bound excited state, which it returns to its ground state abiding in the analysis of color centre creation mechanism ( Singh et al;2010).photoluminescence (PL) spectrum o f malononitrile crystal was recorded in the wavelength range of 450nm-700nm by using John Yvon- Spex spectrometer-fl3-11 (450W high pressure Xenon lamp). The input excitation wavelength of PL spectrum was given at nm. The excitation wavelength of OH1, MOT2, Cl1, BR1, OE1 &3E4HM are 390nm, 390nm, 382nm, 398nm, 380nm & 390nm respectively. The sharp emission of PL spectrum of high intensity is between (a.u) to (a.u), as shown in Fig This reveals that the 94

12 grown crystals have a red emission property, and it is suitable near IR optical switching application. Figure 3.11: PL spectrum of malononitrile derivative crystal CONCLUSION An optical switch requires a new nonlinear optical material of high transparency or low absorption in near infrared region. As well as optical conductivity of material is important to fastest response of telecommunication switches. The conjugated π- electron bonding networks found in this molecule is the principal reason for their strong absorption of visible and near-ir light. In optical communication systems, the low power optical switching is used for telecommunication bands in the range wavelength nm. The internal efficiency of optoelectronics devices are completely depending on the refractive index, absorption coefficient, optical conductivity, extinction coefficient and reflectance. It is important to determine the linear optical character of materials. The calculated linear optical property of the malononitrile derivative crystals in the wavelength range nm has shown in Table

13 Table 3.2: Comparison of linear optical data of malononitrile derivative crystal Crystal Absorption Bandgap Extinction edge ev coefficient nm Optical PL Linear conductivity Intensity refractive s -1 (a.u) index OH MOT Cl Br OE E4HM The linear optical properties of malononitirle derivative crystal were studied in this chapter. The optical band gap of malononitrile derivative crystals were in the range of 2 to 2.2eV. It reveals that lower optical energy is sufficient to carry the electrons from ground state to excited state. Similarly the optical conductivity of the malononitrile derivative crystals was in the range of /s. The crystals OH1, Cl1, OE1 and 3E4HM have very good photoresponse. Br1 and MOT2 crystals were coming around 10 9 /s. The rate of loss of electromagnetic radiation is depends on crystal properties like transparency, colour of crystal and structure properties. The loss of electromagnetic radiation of MOT2 crystal is lesser than other crystals, possibly because of high transparency compared to other crystals. The extinction coefficient of OH1, Cl1, Br1, OE1 and 3E4HM crystals were coming around The refractive index has been measured using Abbe s refractometer. 3E4HM crystal has low refractive index compared to malononitrile derivative crystals. Photoluminescence intensity is comparatively high for OH1, MOT2 and Br1 crystals. There is an interest to develop the organic compounds for strong absorption in visible region. When compared to all the malononitrile crystals, Br1 and Cl1 is showing the absorption edge at 610 and 614nm respectively, low band gap, good photoresponse and high PL intensity. The presence of chloro and bromo group in aromatic end allows the molecular orbitals to extend like polyene chain. It is effectively increases the conjugation length and decreases of optical band gap. It helps to increases the third order coefficient.the linear optical property of malononitrile derivative crystals shows the suitable for photonic switching application. 96