Accepted Manuscript Linear and nonlinear optical, mechanical, electrical and surface studies of a novel nonlinear optical crystal – Manganese mercury thiocyanate (MMTC) R. Josephine Usha, P. Sagayaraj, V. Joseph PII: DOI: Reference:
S1386-1425(14)00734-3 http://dx.doi.org/10.1016/j.saa.2014.04.161 SAA 12125
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received Date: Revised Date: Accepted Date:
3 January 2014 17 April 2014 23 April 2014
Please cite this article as: R. Josephine Usha, P. Sagayaraj, V. Joseph, Linear and nonlinear optical, mechanical, electrical and surface studies of a novel nonlinear optical crystal – Manganese mercury thiocyanate (MMTC), Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2014), doi: http://dx.doi.org/10.1016/j.saa. 2014.04.161
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Linear and nonlinear optical, mechanical, electrical and surface studies of a novel nonlinear optical crystal – Manganese mercury thiocyanate (MMTC) R. Josephine Ushaa, P. Sagayarajb and V. Joseph b a
Department of Physics, Saveetha School of Engineering, Chennai, Tamil Nadu, India b
Department of Physics, Loyola College, Chennai, Tamil Nadu, India
Abstract The highly efficient nonlinear optical single crystal of manganese mercury thiocyanate has been grown from slow evaporation solvent technique. The second harmonic generation and optical transmittance of the grown crystal are studied by Kurtz and Perry powder technique and spectroscopic absorbance spectrum. Mechanical behaviour is analyzed using Vickers microhardness test. The dielectric response of the grown crystal is studied as a function of the temperature and the results are discussed. Further, electronic properties such as plasma energy, Penngap, Fermi energy and electronic polarizability are evaluated. Third order nonlinear optical studies are performed using by single beam Z-scan technique using Nd: YAG laser and parameters such as nonlinear refractive index n2, absorption co-efficient β and nonlinear optical susceptibility χ(3) are evaluated for the grown crystal. The surface of the grown crystal is analyzed with field emission scanning electron microscope and atomic force microscope analyses. Keywords: X-ray diffraction; Nonlinear optics; Z-scan; Optical transmittance Corresponding author Dr. R. Josephine Usha, Assistant Professor, Department of Physics, Saveetha School of Engineering, Saveetha University, Thandalam, Chennai-602 105. Mobile: 9677108678 E-mail: [email protected]
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1. Introduction A combination of inorganic and organic materials provides a potentially useful approach to more efficient and more stable nonlinear optical (NLO) crystals. Some of the bimetallic complexes of thiocyanate crystallize into non-centrosymmetric space group which confirms the second harmonic generation [1,2]. According to the idea of “combining the inorganic
distorted
polyhedran
with
asymmetric
conjugate
organic
molecules”,
organometallic materials have been attracting a great deal of attention in the nonlinear optical field [3]. Another important advantage of organometallic materials is the high resistance to laser induced damage. The metallic complexes of thiocyanate and their Lewis base have fascinated the material scientists due to their novel nonlinear optical (NLO) and stable physico-chemical properties [4]. The crystals formed with thiocyanate (SCN) ligand show a relatively higher SHG effect than the crystals formed with the other organic ligands. Particularly, bimetallic thiocyanate complexes with AB(SCN)4 (where, A=Mn and Cd; B=Hg and Cd) structure type; MnHg(SCN)4(MMTC) and CdHg(SCN)4 (CMTC) possess favourable properties to become candidate materials for SHG devices. These NLO metal complexes exhibit donor (-conjugate bridge) and acceptor - (D- -A) structures. The most striking features are; the -N=C=S bridges which connect the centre atoms of the infinite three dimensional -A-N=C=S-B- networks. An organic ligand is usually more dominant in the NLO effect. The metal ligand bonding in organometallics gives rise to large molecular hyperpolarizabilities due to the transfer of electron density between the metal atom and the conjugated ligand systems [5,6]. Ligands like thiourea (tu) and thiocyanate (SCN) with S and N donors are capable of combining with metal to form stable complexes through coordinated bonds [7]. The Z-scan study of the crystal confirms the third order NLO behaviour with broad range of potential applications in synthetic, biological, clinical activity, optoelectronics and photonics.
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The aim of the present work is to report on the growth of manganese mercury thiocyanate (MMTC) single crystals by slow solvent evaporation method (SSEM) and its characterization using single crystal XRD, NLO test, optical studies, Vickers microhardness, dielectric behaviour, surface analysis and thermal studies. Fundamental parameters like valance electron plasma energy, Penngap, Fermi energy and electronic polarizability of the crystal are calculated and reported for the first time. 2. Experimental 2.1. Synthesis and growth of crystal In the present work, manganese mercury thiocyanate (MMTC) was successfully synthesized using commercially available AR grade ammonium thiocyanate, mercury chloride and manganese chloride substances in the stoichiometric ratio 4:1:1 according to the following reaction: 4NH4SCN+MnCl2+HgCl2→ MnHg(SCN)4+4NH4Cl The synthesized salt was dissolved in deionized water. The obtained product was purified by repeated recrystallization before it was used for crystal growth. The solution was constantly stirred for 24 hours to overcome the concentration gradient in the crystallizer. The pH value played a definite role in the crystallization process [8]. The optimum pH value for MMTC was 2.8.The saturated solutions of MMTC of about 250 ml prepared at 35 °C and are allowed for slow evaporation. Self-nucleated seed crystals are allowed to grow on the base of the beakers. The rate of evaporation was controlled by fixing perforated lids on the beakers. Good quality single crystals of dimensions 14 × 12 × 11 mm3 were obtained in a period of 50–60 days (Fig.1a & b).
3
3. Results and discussion
3.1 Single crystal X-ray diffraction analysis Well shaped, transparent single crystal of MMTC was subjected to single crystal X-ray diffraction analysis. Enraf-Nonius CAD4-MV31 single crystal X-ray diffractometer with MoKα (λ=0.71073 Å) radiation was used for single crystal XRD analysis. From single crystal XRD studies, it has been found that the compound crystallizes in the tetragonal crystal system of space group I 4 with the lattice parameters; a=11.274Å, b=11.274Å, c=4.25Å, α=β=γ=90°and V=542(Å)3, which is considered as non-centrosymmetric system and thus
satisfies the requirements for the SHG activity in the crystal. It is observed that lattice parameter values and cell volume of MMTC are in good agreement with the reported values [9]. 3.2 Nonlinear optical test
Quantitative measurement of the conversion efficiency of the crystal was determined using the powder technique developed by Kurtz and Perry [10]. In order to determine SHG efficiency, the crystal was ground into fine powder and then packed in a micro-capillary of uniform bore and exposed to a Q-switched Nd: YAG laser of wavelength 1064 nm. The laser was incident normally on the capillary tube filled with sample and output from the sample was passed through monochromater to collect the intensity of 532 nm component [11]. When a laser input of 2.3 mJ was passed through MMTC, second harmonic signal of 532 nm was produced and the experimental data confirmed the SHG efficiency of the sample nearly 7 times greater than urea.
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3.3 UV–vis–NIR absorbance analysis
Since single crystal is mainly used in optical applications, the optical transmission range and the transparency cut-off wavelength of the material should be known [12]. The optical absorption spectrum for the grown crystal was recorded in the range 200–2500 nm using a VARIAN CARY 5E spectrometer, which covers the entire near-UV, visible, NIR regions. UV-vis-NIR studies also give important structural information because absorption of UV and visible light involves promotion of the electrons in π and n orbital from the ground state to higher energy states. The observed low UV cut-off wavelength of 383 nm (Fig. 2a) indicated that this material is a potential candidate for generating blue-violet light using a diode laser. The optical band gap of the crystal is calculated by using the Tauc’s relation, (1)
α hν = A(hν − E g ) n
where, Eg is the direct transition band gap, n = 1/2 for the direct allowed transition, A is a constant and α is the absorption coefficient. The band gap energy Eg was calculated by the extrapolation of the linear part (Fig. 2b). The band gap is found to be 3.6 eV. As a consequence of the wide band gap, the grown crystal has large transmittance in the entire visible region. 3.4 Vickers microhardness test
It is very important to know the mechanical strength of the specimen before making any device out of the grown crystals [13]. The structure and molecular composition in crystals greatly influence mechanical properties. Hardness of a material is a measure of resistance offered by the lattice for permanent deformation [14]. Microhardness testing is one of the simplest and best methods to understand the strength of the materials.
5
The mechanical studies of MMTC single crystals were made by Vickers microhardness test at room temperature. Crystals, free from cracks, with flat and smooth faces, were chosen for the static indentation tests. The crystal was mounted properly on the base of the microscope. Now, the selected faces were indented gently by loads varying from 10 to 100 g for a dwell period of 10 s using Vickers diamond pyramid indenter [15] attached to an incident ray research microscope (Mututoyo MH 112, Japan). The Vickers indented impressions were approximately square in shape. The length of the two diagonals was measured by a calibrated micrometer attached to the eyepiece of the microscope after unloading and the average was found out. For a particular load at least five well defined indentations were considered and the average of all the diagonals (d) was considered. The hardness number (Hv) was calculated using the standard formula [16]: H V = 1.8544 p / d 2
(2)
where, p is the applied load in kg, d is in mm and Hv is in kg mm−2. Fig. 3a shows the variation of Hv as a function of applied load for the MMTC single crystals. It is clear from the figure that Hv increases with increase in load. The mechanical strength of the crystal reveals that it has a good hardness and is useful for any device applications. The Mayer's index number was calculated from the Mayer's law, which relates the load and indentation diagonal length as: P = kd n
(3)
log P = log k + n log d
(4)
where, k is the material constant and ‘n’ is the Meyer's index. In order to find the value of ‘n’, a graph is plotted for log p against log d (Fig.3b) which gives a straight line (after least square fitting). From the slope line, the Meyer's index number ‘n’ was calculated 6
to be 3.476. According to concept of Onitsch, MMTC belongs to soft material category [17]. The above relation indicates that Hv should increase with p if n > 2 and decrease with p when n > 2. 3.5 Dielectric studies
The dielectric properties of the non-conducting optical single crystals are well correlated with electro-optic properties. The different polarization mechanisms in solids such as atomic polarization of lattice, orientation polarization of dipoles, space charge polarization, electric and ionic polarizations can be understood easily by studying the variation of dielectric constant as a function of frequency and temperature. The response of both dielectric loss and dielectric constant with temperature are shown in Fig. 4 (a & b). The dielectric constant is higher at lower frequencies, then decreases sharply with frequency and after that it remains almost constant over the entire higher frequency range. The decrease of dielectric constant in higher frequency region may be due to the fact that the dipoles cannot follow up the fast variation of the applied field. The dielectric constant and loss was evaluated using the relation,
ε ' = Cd / ε 0 A
(5)
ε '' = ε r D
(6)
where, d is the thickness, A is the area of the cross section of the grown crystal and D is dissipation factor. It was found that dielectric constant gradually increased with increase in temperature. This indicates the presence of space charge effect in addition to electronic and atomic conduction in the samples. The characteristic of low dielectric loss at a high frequency range for a given sample suggests that the sample possesses enhanced optical quality with lesser defects and hence this parameter is of vital importance for various NLO materials and 7
their applications. The variation of dielectric constant of MMTC crystal as a function of temperature at different frequencies is shown in Fig. 4c. The valance electron plasma energy is given by, 1/ 2 (7) Zρ ω p = 28 .8 M where, Z is the total number of valance electrons(Z=62), M is the molecular weight
and ρ is the density of MMTC crystal. The dielectric constant at higher frequency is almost constant (ε∞=107.8). The Penn gap (Ep) and Fermi energy (EF) are calculated using the following equations [18],
EP =
ω p
(8)
(ε ∞−1 )1 / 2
(9)
E F = 0 .2948 (ω p ) 4 / 3
The polarizability (α) of the grown material is calculated using the relation,
M (ω p ) 2 S 0 α = × × 0.396 × 10−24 cm −1 (10) 2 2 (ω p ) S 0 + 3E p ρ where, S0 is a constant for a particular material and is given by,
E 1 E S0 = 1 − P + P 4E F 3 4E F
2
(11)
The value of α can also be calculated using Claussius- Mossotti equation [19],
α=
3M ε ∞ −1 4πN A ρ ε ∞ + 2
(12)
8
where, NA is the avagadro number. Table 1 shows the fundamental parameters required for assessing SHG efficiency of MMTC. The SHG efficiency depends upon the polarizability of the medium. Since the polarizability of MMTC is approximately 6.3, the SHG efficiency of MMTC is interpreted as 7 times that of urea. This theoretical prediction of SHG efficiency has been confirmed by our Kurtz and Perry powder SHG test. 3.6 AC conductivity studies
The ac conductivity (σac) was calculated using the relation:
σ ac = ε 0 ε r ω tan δ
(13)
where, ε0 is the permittivity of free space (8.85 × 10 −12 F/m) and (ω = 2πf) is the angular frequency. The temperature dependence of conductivity, σac, for the sample is shown in Fig. 5, as a plot of ln σacT versus 1000/T. It is evident from the graph that the conductivity increases with temperature. The line of best fit for the plot of ln σacT versus 1000/T obeys Arrhenius relationship.
σ ac = σ 0 exp(−Eac / kT )
(14)
Therefore, the sample exhibits Arrhenius type conductivity behaviour in the temperature range of investigation. Activation energies were estimated using the slope of the above line plots (E = −(slope)k × 1000). The activation energy of MMTC, for the conduction process, calculated from the plot is found to be 0.2211 eV. Thus, the lower value of activation energy suggests the crystal contains less number of defects.
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3.7 Impedance spectroscopy analysis
Due to the conductivity phenomena, the blocking contact effect may play an important role in the dielectric spectra. To separate the volume polarization and electrode polarization due to ionic and electronic conductivity, the impedance representation of the measured results was used for analysis. The frequency dependence of the complex impedance was obtained from the dielectric spectra according to formula: Z * = 1 / jωε *ε 0
(15)
where, ε* is the complex dielectric permittivity of the sample, ε0 is the dielectric constant, ω = 2πν and ν is measurement frequency [20-25]. The dependence of imaginary part of the impedance versus the real part on sample is presented in Fig. 6 (a, b & c). Electrical conductivity depends on thermal treatment of the crystal. The value of conductivity ln σdc is found to increase with temperature. From the plot of ln σdcT versus 1000/T, activation energy (Edc) is calculated (Fig. 6d) to be 1.85 eV. The low activation energies confirm that the trapping levels originating probably from the defect states are responsible for conduction in the region. 3.8 Thermal studies
The combined thermo gravimetric (TG) and differential thermal analysis (DTA) of MMTC crystal was recorded in the range from room temperature to 700 °C using NETZSCH
STA 449F3 under nitrogen atmosphere, with a heating rate of 10 °C/min. The recorded TG/DTA curves of MMTC crystal are shown in Fig. 7. It is clear from the DTA curve that there is no phase transition before melting. The sharpness of the peak demonstrated good crystallinity and purity of the sample. From the figure, it is observed that there are three stages of decomposition. TG curve of MMTC implies that the sample is stable up to 200 °C 10
and starts to melt around 340 °C. The first step is an exothermic process in the temperature range of 344-357 °C. It implies the loss of 2C (2CO2), 2S (2SO2) and 4N (2N2) and second step involves the loss of one HgS. The SCN group attached with the metal in which sulphur lies in between the metal and C N and not in the terminal position. The last step is the decomposition of MnSO4 and the final product is pure Mn3O4.
3.9 Third-order nonlinear optical properties (Z-scan technique)
To study the third order NLO properties a single beam Z-scan technique was employed, which readily provides the magnitude and sign of nonlinearity. The Z-scan technique is based on the principle of spatial beam distortion and offers simplicity as well as very high sensitivity for distinguishing the contribution of the real part (nonlinear refraction) and the imaginary part (nonlinear absorption) of third-order nonlinear susceptibility χ(3). For measuring the refractive nonlinear property, an aperture was placed in front of the detector and the transmittance was recorded as a function of the sample position on the Z-axis (closed aperture Z-scan). For measuring the nonlinear absorption, the sample transmittance was measured without the aperture as a function of sample position (open aperture Z-scan) [26]. In this experiment, Nd: YAG laser beam of wavelength 532 nm was used. The sample was fixed across the focal region along the axial direction that is the direction of the propagation laser beam. The transmission of the beam through an aperture placed in the far field is measured using photo detector fed to the digital power meter. For an open aperture Z-scan, a lens to collect the entire laser beam transmitted through the sample replaced the aperture. The sample causes an additional focusing or defocusing, depending on whether nonlinear refraction is positive or negative.
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Fig. 8a depicts the Z-scan data for closed aperture setup which shows the peak followed by a valley-normalized transmittance which is the signature for negative nonlinearity i.e., self-defocusing [27]. The nonlinear absorption coefficient was determined by fitting the experimental open aperture (Fig. 8b) Z-scan data. Similarly, the nonlinear refractive index was obtained by fitting the closed aperture Z-scan experimental data. The n2, βeff, Imχ(3) and Reχ(3) of MMTC crystal were found to be -5.23X10 -11 esu, 2.71X10-6cmW−1, 2.77X10-8m2/V2 and 0.823X10 -9 m2/V2, respectively (Table 2). It was observed that the intercept of βeff with I0 on the vertical axis was non-zero suggesting that a higher order effect, such as excited state absorption (ESA) via two photon absorption contributing to nonlinear absorption (NLA) was present. Thus, the present study confirms the enhancement in the third order nonlinearities due to the presence of SCN ligand. 3.10 Field emission scanning electron microscope analysis
Field emission scanning electron microscopy (FESEM) observations of MMTC sample were carried out using an instrument model SU 6600. The as-grown crystals with well defined planes and of smooth surface were selected for this analysis and no polishing was done. Fig. 9 (a & b) shows the morphology of MMTC crystal. It is clear from the FESEM micrographs that the crystal surface contains voids of irregular size. With increased magnification, the elongated voids are clearly visible and these patterns are possibly caused by the fluctuations of Mn and Hg metal ligands when SCN bridges them to form the three dimensional network. The compositional analysis of grown crystal MMTC was made using energy dispersive X-ray analysis (EDAX). The electron image of the crystal is shown in Fig. 9c. This EDAX spectrum (Fig. 9d) of MMTC shows that the presences of manganese, mercury, sulphur, carbon and nitrogen. 12
3.11 Atomic force microscope analysis
AFM is extensively used for the investigation of the surface properties with nanometre resolution. Freshly cleaved MMTC crystal was taken and the results were obtained by XE-70-Parksystems, Korea. Fig.10 (a & b) shows AFM images recorded on the flat surface of MMTC for different magnifications using micro fabricated cantilever. Spring constant and resonance frequency of the cantilever are estimated to be 0.2 N m−1, 23 KHz respectively. It is observed from the 3D image (Fig. 10c) that crystal surface possesses valleys and hillocks. It is well known that a large number of peaks and valleys in an image significantly affect the average roughness (S a) and root mean square (S q) values [28]. The estimated roughness values, ten point height (S z), surface skewness (Ssk) are given in Table 3.
Conclusion
Single crystals of MMTC were grown by slow evaporation method at room temperature. Single crystal XRD analysis revealed the structure of the crystal as tetragonal. Kurtz and Perry powder SHG test confirms the frequency doubling of the grown crystal and its efficiency is found to be higher than that of urea. The optical absorption studies reveal that the crystal has UV cut-off wavelength around 383 nm. The electronic polarizability of the crystal is found to be 6.3 from the dielectric response. Hardness analysis proved that the MMTC crystal belongs to soft material category. The electrical conductivity studies were carried out and the activation energy is determined. Thermal studies revealed that the crystal is thermally stable up to 200 °C. Closed aperture Z-scan studies revealed the negative nonlinearity in the crystal and the open aperture Z-scan depicted the saturation absorption. The third order nonlinear properties confirm its suitability for nonlinear optical devices.
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Figure caption
Fig. 1a Seed crystals of MMTC Fig. 1b As grown single crystal of MMTC Fig. 2a UV-vis-NIR absorbance spectrum of MMTC Fig. 2b Tauc’s plot of MMTC Fig. 3a Variation of Vickers microhardness number Hv with applied load Fig. 3b Plot of log of indentation diagonal length versus log of applied load Fig. 4a Variation of dielectric loss with log frequency at different temperatures Fig. 4b Variation of dielectric constant with log frequency at different temperatures Fig. 4c Variation of dielectric constant of MMTC crystal as a function of temperature at different frequencies Fig. 5 Plot of ac conductivity of MMTC versus temperature Fig. 6 (a,b&c) The real and the imaginary part of the impedance spectra of MMTC crystal at different temperatures Fig. 6d Arrhenious plot of MMTC crystal Fig. 7 TG-DTA traces of MMTC Fig. 8 (a&b) Closed and open aperture Z-scan curve of MMTC
Fig. 9 (a&b) FESEM micrographs of MMTC Fig. 9 (c&d) Electron image and EDAX spectrum of MMTC Fig. 10 (a&b) AFM image, (c) 3D image of MMTC
17
Table Table 1. Fundamental parameters for assessing SHG efficiency of MMTC crystal Table 2. Third order optical nonlinearity data of MMTC Table 3. Surface parameters of MMTC measured by AFM
18
(a)
(b)
Fig. 1a Seed crystals of MMTC
Fig. 1b As-grown single crystal of MMTC
19
Fig. 3a Variation of Vickers microhardness number Hv with applied load.
22
Fig. 3b Plot of log of indentation diagonal length versus log of applied load
23
Fig. 4aVariation of dielectric loss with log frequency at different temperatures
24
Fig. 4b Variation of dielectric constant with log frequency at different temperatures
25
Fig.4c Variation of dielectric constant of MMTC crystal as a function of temperature at different frequencies
26
Fig. 5 Plot of ac conductivity of MMTC versus temperature
27
(a)
(b)
Fig. 8 (a&b) Closed and open aperture Z-scan curve of MMTC
31
(a)
(b)
Fig. 9 (a&b) FESEM micrographs of MMTC
32
(c)
(d)
Fig. 9 (c&d) Electron image and EDAX spectrum of MMTC
33
(b)
(a)
(c)
Fig. 10 (a&b) AFM image, (c) 3D image of MMTC
34
Fig. 2a UV-vis-NIR absorbance spectrum of MMTC
Fig. 2b Tauc’s plot of MMTC
Fig. 6 (a,b&c) The real and the imaginary part of the impedance spectra of MMTC crystal at different temperatures
Fig. 6d Arrhenious plot of MMTC crystal
Fig. 7 TG-DTA traces of MMTC
Table 1. Fundamental parameters for assessing SHG efficiency of MMTC crystal
S. No 1 2 3 4
5 6
Parameters Density (g/cm3) Plasma energy (eV) Penn gap (eV) Fermi gap (eV) Polarizability using plasma energy value (cm3) Polarizability using Claussius-Mossotti equation
40
Values for MMTC 2.98 18.694 1.809 14.629
6.2567×10-23 6.311×10-23
Table 2. Third order optical nonlinearity data of MMTC S.No 1 2 3 4 5 6
Parameters ∆φ n2 (cm2/W) β (cm/W) Im χ(3) (esu) Re χ(3) (esu) χ(3) (esu)
MMTC 0.11 -63.3 2.71X10 -6 2.77X10 -8 0.823X10-9 3.13X10 -8
41
Table 3. Surface parameters of MMTC measured by AFM S.NO. Parameter MMTC 1 Roughness average (Sa) (nm) 7.0754 2 Root mean square (Sq) (nm) 10.1859 3 Surface skewness (Ssk) 2.10411 4 Surface kurtosis (Sku) 12.3428 5 Ten-point height (Sz) (nm) 193.247 6 Density of summits (Sds) (1/µm²) 40.3
42
Highlights • • • •
MMTC crystallizes in tetragonal crystal system. Plasma energy, Penngap and electronic polarizability have been calculated for the first time. Z-scan study confirms the enhancement in the third order nonlinearities due to the presence of SCN ligand. Surface parameters like average roughness (Sa), root mean square (Sq) and surface skewness (Ssk) were calculated by using AFM analysis.
43
Graphical Abstract
It is observed from the AFM image that crystal surface possesses valleys and hillocks. It is well known that a large number of peaks and valleys in an image significantly affect the average roughness (Sa) and root mean square (Sq) values.
44
S1386-1425(14)00734-3 http://dx.doi.org/10.1016/j.saa.2014.04.161 SAA 12125
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received Date: Revised Date: Accepted Date:
3 January 2014 17 April 2014 23 April 2014
Please cite this article as: R. Josephine Usha, P. Sagayaraj, V. Joseph, Linear and nonlinear optical, mechanical, electrical and surface studies of a novel nonlinear optical crystal – Manganese mercury thiocyanate (MMTC), Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2014), doi: http://dx.doi.org/10.1016/j.saa. 2014.04.161
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Linear and nonlinear optical, mechanical, electrical and surface studies of a novel nonlinear optical crystal – Manganese mercury thiocyanate (MMTC) R. Josephine Ushaa, P. Sagayarajb and V. Joseph b a
Department of Physics, Saveetha School of Engineering, Chennai, Tamil Nadu, India b
Department of Physics, Loyola College, Chennai, Tamil Nadu, India
Abstract The highly efficient nonlinear optical single crystal of manganese mercury thiocyanate has been grown from slow evaporation solvent technique. The second harmonic generation and optical transmittance of the grown crystal are studied by Kurtz and Perry powder technique and spectroscopic absorbance spectrum. Mechanical behaviour is analyzed using Vickers microhardness test. The dielectric response of the grown crystal is studied as a function of the temperature and the results are discussed. Further, electronic properties such as plasma energy, Penngap, Fermi energy and electronic polarizability are evaluated. Third order nonlinear optical studies are performed using by single beam Z-scan technique using Nd: YAG laser and parameters such as nonlinear refractive index n2, absorption co-efficient β and nonlinear optical susceptibility χ(3) are evaluated for the grown crystal. The surface of the grown crystal is analyzed with field emission scanning electron microscope and atomic force microscope analyses. Keywords: X-ray diffraction; Nonlinear optics; Z-scan; Optical transmittance Corresponding author Dr. R. Josephine Usha, Assistant Professor, Department of Physics, Saveetha School of Engineering, Saveetha University, Thandalam, Chennai-602 105. Mobile: 9677108678 E-mail: [email protected]
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1. Introduction A combination of inorganic and organic materials provides a potentially useful approach to more efficient and more stable nonlinear optical (NLO) crystals. Some of the bimetallic complexes of thiocyanate crystallize into non-centrosymmetric space group which confirms the second harmonic generation [1,2]. According to the idea of “combining the inorganic
distorted
polyhedran
with
asymmetric
conjugate
organic
molecules”,
organometallic materials have been attracting a great deal of attention in the nonlinear optical field [3]. Another important advantage of organometallic materials is the high resistance to laser induced damage. The metallic complexes of thiocyanate and their Lewis base have fascinated the material scientists due to their novel nonlinear optical (NLO) and stable physico-chemical properties [4]. The crystals formed with thiocyanate (SCN) ligand show a relatively higher SHG effect than the crystals formed with the other organic ligands. Particularly, bimetallic thiocyanate complexes with AB(SCN)4 (where, A=Mn and Cd; B=Hg and Cd) structure type; MnHg(SCN)4(MMTC) and CdHg(SCN)4 (CMTC) possess favourable properties to become candidate materials for SHG devices. These NLO metal complexes exhibit donor (-conjugate bridge) and acceptor - (D- -A) structures. The most striking features are; the -N=C=S bridges which connect the centre atoms of the infinite three dimensional -A-N=C=S-B- networks. An organic ligand is usually more dominant in the NLO effect. The metal ligand bonding in organometallics gives rise to large molecular hyperpolarizabilities due to the transfer of electron density between the metal atom and the conjugated ligand systems [5,6]. Ligands like thiourea (tu) and thiocyanate (SCN) with S and N donors are capable of combining with metal to form stable complexes through coordinated bonds [7]. The Z-scan study of the crystal confirms the third order NLO behaviour with broad range of potential applications in synthetic, biological, clinical activity, optoelectronics and photonics.
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The aim of the present work is to report on the growth of manganese mercury thiocyanate (MMTC) single crystals by slow solvent evaporation method (SSEM) and its characterization using single crystal XRD, NLO test, optical studies, Vickers microhardness, dielectric behaviour, surface analysis and thermal studies. Fundamental parameters like valance electron plasma energy, Penngap, Fermi energy and electronic polarizability of the crystal are calculated and reported for the first time. 2. Experimental 2.1. Synthesis and growth of crystal In the present work, manganese mercury thiocyanate (MMTC) was successfully synthesized using commercially available AR grade ammonium thiocyanate, mercury chloride and manganese chloride substances in the stoichiometric ratio 4:1:1 according to the following reaction: 4NH4SCN+MnCl2+HgCl2→ MnHg(SCN)4+4NH4Cl The synthesized salt was dissolved in deionized water. The obtained product was purified by repeated recrystallization before it was used for crystal growth. The solution was constantly stirred for 24 hours to overcome the concentration gradient in the crystallizer. The pH value played a definite role in the crystallization process [8]. The optimum pH value for MMTC was 2.8.The saturated solutions of MMTC of about 250 ml prepared at 35 °C and are allowed for slow evaporation. Self-nucleated seed crystals are allowed to grow on the base of the beakers. The rate of evaporation was controlled by fixing perforated lids on the beakers. Good quality single crystals of dimensions 14 × 12 × 11 mm3 were obtained in a period of 50–60 days (Fig.1a & b).
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3. Results and discussion
3.1 Single crystal X-ray diffraction analysis Well shaped, transparent single crystal of MMTC was subjected to single crystal X-ray diffraction analysis. Enraf-Nonius CAD4-MV31 single crystal X-ray diffractometer with MoKα (λ=0.71073 Å) radiation was used for single crystal XRD analysis. From single crystal XRD studies, it has been found that the compound crystallizes in the tetragonal crystal system of space group I 4 with the lattice parameters; a=11.274Å, b=11.274Å, c=4.25Å, α=β=γ=90°and V=542(Å)3, which is considered as non-centrosymmetric system and thus
satisfies the requirements for the SHG activity in the crystal. It is observed that lattice parameter values and cell volume of MMTC are in good agreement with the reported values [9]. 3.2 Nonlinear optical test
Quantitative measurement of the conversion efficiency of the crystal was determined using the powder technique developed by Kurtz and Perry [10]. In order to determine SHG efficiency, the crystal was ground into fine powder and then packed in a micro-capillary of uniform bore and exposed to a Q-switched Nd: YAG laser of wavelength 1064 nm. The laser was incident normally on the capillary tube filled with sample and output from the sample was passed through monochromater to collect the intensity of 532 nm component [11]. When a laser input of 2.3 mJ was passed through MMTC, second harmonic signal of 532 nm was produced and the experimental data confirmed the SHG efficiency of the sample nearly 7 times greater than urea.
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3.3 UV–vis–NIR absorbance analysis
Since single crystal is mainly used in optical applications, the optical transmission range and the transparency cut-off wavelength of the material should be known [12]. The optical absorption spectrum for the grown crystal was recorded in the range 200–2500 nm using a VARIAN CARY 5E spectrometer, which covers the entire near-UV, visible, NIR regions. UV-vis-NIR studies also give important structural information because absorption of UV and visible light involves promotion of the electrons in π and n orbital from the ground state to higher energy states. The observed low UV cut-off wavelength of 383 nm (Fig. 2a) indicated that this material is a potential candidate for generating blue-violet light using a diode laser. The optical band gap of the crystal is calculated by using the Tauc’s relation, (1)
α hν = A(hν − E g ) n
where, Eg is the direct transition band gap, n = 1/2 for the direct allowed transition, A is a constant and α is the absorption coefficient. The band gap energy Eg was calculated by the extrapolation of the linear part (Fig. 2b). The band gap is found to be 3.6 eV. As a consequence of the wide band gap, the grown crystal has large transmittance in the entire visible region. 3.4 Vickers microhardness test
It is very important to know the mechanical strength of the specimen before making any device out of the grown crystals [13]. The structure and molecular composition in crystals greatly influence mechanical properties. Hardness of a material is a measure of resistance offered by the lattice for permanent deformation [14]. Microhardness testing is one of the simplest and best methods to understand the strength of the materials.
5
The mechanical studies of MMTC single crystals were made by Vickers microhardness test at room temperature. Crystals, free from cracks, with flat and smooth faces, were chosen for the static indentation tests. The crystal was mounted properly on the base of the microscope. Now, the selected faces were indented gently by loads varying from 10 to 100 g for a dwell period of 10 s using Vickers diamond pyramid indenter [15] attached to an incident ray research microscope (Mututoyo MH 112, Japan). The Vickers indented impressions were approximately square in shape. The length of the two diagonals was measured by a calibrated micrometer attached to the eyepiece of the microscope after unloading and the average was found out. For a particular load at least five well defined indentations were considered and the average of all the diagonals (d) was considered. The hardness number (Hv) was calculated using the standard formula [16]: H V = 1.8544 p / d 2
(2)
where, p is the applied load in kg, d is in mm and Hv is in kg mm−2. Fig. 3a shows the variation of Hv as a function of applied load for the MMTC single crystals. It is clear from the figure that Hv increases with increase in load. The mechanical strength of the crystal reveals that it has a good hardness and is useful for any device applications. The Mayer's index number was calculated from the Mayer's law, which relates the load and indentation diagonal length as: P = kd n
(3)
log P = log k + n log d
(4)
where, k is the material constant and ‘n’ is the Meyer's index. In order to find the value of ‘n’, a graph is plotted for log p against log d (Fig.3b) which gives a straight line (after least square fitting). From the slope line, the Meyer's index number ‘n’ was calculated 6
to be 3.476. According to concept of Onitsch, MMTC belongs to soft material category [17]. The above relation indicates that Hv should increase with p if n > 2 and decrease with p when n > 2. 3.5 Dielectric studies
The dielectric properties of the non-conducting optical single crystals are well correlated with electro-optic properties. The different polarization mechanisms in solids such as atomic polarization of lattice, orientation polarization of dipoles, space charge polarization, electric and ionic polarizations can be understood easily by studying the variation of dielectric constant as a function of frequency and temperature. The response of both dielectric loss and dielectric constant with temperature are shown in Fig. 4 (a & b). The dielectric constant is higher at lower frequencies, then decreases sharply with frequency and after that it remains almost constant over the entire higher frequency range. The decrease of dielectric constant in higher frequency region may be due to the fact that the dipoles cannot follow up the fast variation of the applied field. The dielectric constant and loss was evaluated using the relation,
ε ' = Cd / ε 0 A
(5)
ε '' = ε r D
(6)
where, d is the thickness, A is the area of the cross section of the grown crystal and D is dissipation factor. It was found that dielectric constant gradually increased with increase in temperature. This indicates the presence of space charge effect in addition to electronic and atomic conduction in the samples. The characteristic of low dielectric loss at a high frequency range for a given sample suggests that the sample possesses enhanced optical quality with lesser defects and hence this parameter is of vital importance for various NLO materials and 7
their applications. The variation of dielectric constant of MMTC crystal as a function of temperature at different frequencies is shown in Fig. 4c. The valance electron plasma energy is given by, 1/ 2 (7) Zρ ω p = 28 .8 M where, Z is the total number of valance electrons(Z=62), M is the molecular weight
and ρ is the density of MMTC crystal. The dielectric constant at higher frequency is almost constant (ε∞=107.8). The Penn gap (Ep) and Fermi energy (EF) are calculated using the following equations [18],
EP =
ω p
(8)
(ε ∞−1 )1 / 2
(9)
E F = 0 .2948 (ω p ) 4 / 3
The polarizability (α) of the grown material is calculated using the relation,
M (ω p ) 2 S 0 α = × × 0.396 × 10−24 cm −1 (10) 2 2 (ω p ) S 0 + 3E p ρ where, S0 is a constant for a particular material and is given by,
E 1 E S0 = 1 − P + P 4E F 3 4E F
2
(11)
The value of α can also be calculated using Claussius- Mossotti equation [19],
α=
3M ε ∞ −1 4πN A ρ ε ∞ + 2
(12)
8
where, NA is the avagadro number. Table 1 shows the fundamental parameters required for assessing SHG efficiency of MMTC. The SHG efficiency depends upon the polarizability of the medium. Since the polarizability of MMTC is approximately 6.3, the SHG efficiency of MMTC is interpreted as 7 times that of urea. This theoretical prediction of SHG efficiency has been confirmed by our Kurtz and Perry powder SHG test. 3.6 AC conductivity studies
The ac conductivity (σac) was calculated using the relation:
σ ac = ε 0 ε r ω tan δ
(13)
where, ε0 is the permittivity of free space (8.85 × 10 −12 F/m) and (ω = 2πf) is the angular frequency. The temperature dependence of conductivity, σac, for the sample is shown in Fig. 5, as a plot of ln σacT versus 1000/T. It is evident from the graph that the conductivity increases with temperature. The line of best fit for the plot of ln σacT versus 1000/T obeys Arrhenius relationship.
σ ac = σ 0 exp(−Eac / kT )
(14)
Therefore, the sample exhibits Arrhenius type conductivity behaviour in the temperature range of investigation. Activation energies were estimated using the slope of the above line plots (E = −(slope)k × 1000). The activation energy of MMTC, for the conduction process, calculated from the plot is found to be 0.2211 eV. Thus, the lower value of activation energy suggests the crystal contains less number of defects.
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3.7 Impedance spectroscopy analysis
Due to the conductivity phenomena, the blocking contact effect may play an important role in the dielectric spectra. To separate the volume polarization and electrode polarization due to ionic and electronic conductivity, the impedance representation of the measured results was used for analysis. The frequency dependence of the complex impedance was obtained from the dielectric spectra according to formula: Z * = 1 / jωε *ε 0
(15)
where, ε* is the complex dielectric permittivity of the sample, ε0 is the dielectric constant, ω = 2πν and ν is measurement frequency [20-25]. The dependence of imaginary part of the impedance versus the real part on sample is presented in Fig. 6 (a, b & c). Electrical conductivity depends on thermal treatment of the crystal. The value of conductivity ln σdc is found to increase with temperature. From the plot of ln σdcT versus 1000/T, activation energy (Edc) is calculated (Fig. 6d) to be 1.85 eV. The low activation energies confirm that the trapping levels originating probably from the defect states are responsible for conduction in the region. 3.8 Thermal studies
The combined thermo gravimetric (TG) and differential thermal analysis (DTA) of MMTC crystal was recorded in the range from room temperature to 700 °C using NETZSCH
STA 449F3 under nitrogen atmosphere, with a heating rate of 10 °C/min. The recorded TG/DTA curves of MMTC crystal are shown in Fig. 7. It is clear from the DTA curve that there is no phase transition before melting. The sharpness of the peak demonstrated good crystallinity and purity of the sample. From the figure, it is observed that there are three stages of decomposition. TG curve of MMTC implies that the sample is stable up to 200 °C 10
and starts to melt around 340 °C. The first step is an exothermic process in the temperature range of 344-357 °C. It implies the loss of 2C (2CO2), 2S (2SO2) and 4N (2N2) and second step involves the loss of one HgS. The SCN group attached with the metal in which sulphur lies in between the metal and C N and not in the terminal position. The last step is the decomposition of MnSO4 and the final product is pure Mn3O4.
3.9 Third-order nonlinear optical properties (Z-scan technique)
To study the third order NLO properties a single beam Z-scan technique was employed, which readily provides the magnitude and sign of nonlinearity. The Z-scan technique is based on the principle of spatial beam distortion and offers simplicity as well as very high sensitivity for distinguishing the contribution of the real part (nonlinear refraction) and the imaginary part (nonlinear absorption) of third-order nonlinear susceptibility χ(3). For measuring the refractive nonlinear property, an aperture was placed in front of the detector and the transmittance was recorded as a function of the sample position on the Z-axis (closed aperture Z-scan). For measuring the nonlinear absorption, the sample transmittance was measured without the aperture as a function of sample position (open aperture Z-scan) [26]. In this experiment, Nd: YAG laser beam of wavelength 532 nm was used. The sample was fixed across the focal region along the axial direction that is the direction of the propagation laser beam. The transmission of the beam through an aperture placed in the far field is measured using photo detector fed to the digital power meter. For an open aperture Z-scan, a lens to collect the entire laser beam transmitted through the sample replaced the aperture. The sample causes an additional focusing or defocusing, depending on whether nonlinear refraction is positive or negative.
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Fig. 8a depicts the Z-scan data for closed aperture setup which shows the peak followed by a valley-normalized transmittance which is the signature for negative nonlinearity i.e., self-defocusing [27]. The nonlinear absorption coefficient was determined by fitting the experimental open aperture (Fig. 8b) Z-scan data. Similarly, the nonlinear refractive index was obtained by fitting the closed aperture Z-scan experimental data. The n2, βeff, Imχ(3) and Reχ(3) of MMTC crystal were found to be -5.23X10 -11 esu, 2.71X10-6cmW−1, 2.77X10-8m2/V2 and 0.823X10 -9 m2/V2, respectively (Table 2). It was observed that the intercept of βeff with I0 on the vertical axis was non-zero suggesting that a higher order effect, such as excited state absorption (ESA) via two photon absorption contributing to nonlinear absorption (NLA) was present. Thus, the present study confirms the enhancement in the third order nonlinearities due to the presence of SCN ligand. 3.10 Field emission scanning electron microscope analysis
Field emission scanning electron microscopy (FESEM) observations of MMTC sample were carried out using an instrument model SU 6600. The as-grown crystals with well defined planes and of smooth surface were selected for this analysis and no polishing was done. Fig. 9 (a & b) shows the morphology of MMTC crystal. It is clear from the FESEM micrographs that the crystal surface contains voids of irregular size. With increased magnification, the elongated voids are clearly visible and these patterns are possibly caused by the fluctuations of Mn and Hg metal ligands when SCN bridges them to form the three dimensional network. The compositional analysis of grown crystal MMTC was made using energy dispersive X-ray analysis (EDAX). The electron image of the crystal is shown in Fig. 9c. This EDAX spectrum (Fig. 9d) of MMTC shows that the presences of manganese, mercury, sulphur, carbon and nitrogen. 12
3.11 Atomic force microscope analysis
AFM is extensively used for the investigation of the surface properties with nanometre resolution. Freshly cleaved MMTC crystal was taken and the results were obtained by XE-70-Parksystems, Korea. Fig.10 (a & b) shows AFM images recorded on the flat surface of MMTC for different magnifications using micro fabricated cantilever. Spring constant and resonance frequency of the cantilever are estimated to be 0.2 N m−1, 23 KHz respectively. It is observed from the 3D image (Fig. 10c) that crystal surface possesses valleys and hillocks. It is well known that a large number of peaks and valleys in an image significantly affect the average roughness (S a) and root mean square (S q) values [28]. The estimated roughness values, ten point height (S z), surface skewness (Ssk) are given in Table 3.
Conclusion
Single crystals of MMTC were grown by slow evaporation method at room temperature. Single crystal XRD analysis revealed the structure of the crystal as tetragonal. Kurtz and Perry powder SHG test confirms the frequency doubling of the grown crystal and its efficiency is found to be higher than that of urea. The optical absorption studies reveal that the crystal has UV cut-off wavelength around 383 nm. The electronic polarizability of the crystal is found to be 6.3 from the dielectric response. Hardness analysis proved that the MMTC crystal belongs to soft material category. The electrical conductivity studies were carried out and the activation energy is determined. Thermal studies revealed that the crystal is thermally stable up to 200 °C. Closed aperture Z-scan studies revealed the negative nonlinearity in the crystal and the open aperture Z-scan depicted the saturation absorption. The third order nonlinear properties confirm its suitability for nonlinear optical devices.
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Figure caption
Fig. 1a Seed crystals of MMTC Fig. 1b As grown single crystal of MMTC Fig. 2a UV-vis-NIR absorbance spectrum of MMTC Fig. 2b Tauc’s plot of MMTC Fig. 3a Variation of Vickers microhardness number Hv with applied load Fig. 3b Plot of log of indentation diagonal length versus log of applied load Fig. 4a Variation of dielectric loss with log frequency at different temperatures Fig. 4b Variation of dielectric constant with log frequency at different temperatures Fig. 4c Variation of dielectric constant of MMTC crystal as a function of temperature at different frequencies Fig. 5 Plot of ac conductivity of MMTC versus temperature Fig. 6 (a,b&c) The real and the imaginary part of the impedance spectra of MMTC crystal at different temperatures Fig. 6d Arrhenious plot of MMTC crystal Fig. 7 TG-DTA traces of MMTC Fig. 8 (a&b) Closed and open aperture Z-scan curve of MMTC
Fig. 9 (a&b) FESEM micrographs of MMTC Fig. 9 (c&d) Electron image and EDAX spectrum of MMTC Fig. 10 (a&b) AFM image, (c) 3D image of MMTC
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Table Table 1. Fundamental parameters for assessing SHG efficiency of MMTC crystal Table 2. Third order optical nonlinearity data of MMTC Table 3. Surface parameters of MMTC measured by AFM
18
(a)
(b)
Fig. 1a Seed crystals of MMTC
Fig. 1b As-grown single crystal of MMTC
19
Fig. 3a Variation of Vickers microhardness number Hv with applied load.
22
Fig. 3b Plot of log of indentation diagonal length versus log of applied load
23
Fig. 4aVariation of dielectric loss with log frequency at different temperatures
24
Fig. 4b Variation of dielectric constant with log frequency at different temperatures
25
Fig.4c Variation of dielectric constant of MMTC crystal as a function of temperature at different frequencies
26
Fig. 5 Plot of ac conductivity of MMTC versus temperature
27
(a)
(b)
Fig. 8 (a&b) Closed and open aperture Z-scan curve of MMTC
31
(a)
(b)
Fig. 9 (a&b) FESEM micrographs of MMTC
32
(c)
(d)
Fig. 9 (c&d) Electron image and EDAX spectrum of MMTC
33
(b)
(a)
(c)
Fig. 10 (a&b) AFM image, (c) 3D image of MMTC
34
Fig. 2a UV-vis-NIR absorbance spectrum of MMTC
Fig. 2b Tauc’s plot of MMTC
Fig. 6 (a,b&c) The real and the imaginary part of the impedance spectra of MMTC crystal at different temperatures
Fig. 6d Arrhenious plot of MMTC crystal
Fig. 7 TG-DTA traces of MMTC
Table 1. Fundamental parameters for assessing SHG efficiency of MMTC crystal
S. No 1 2 3 4
5 6
Parameters Density (g/cm3) Plasma energy (eV) Penn gap (eV) Fermi gap (eV) Polarizability using plasma energy value (cm3) Polarizability using Claussius-Mossotti equation
40
Values for MMTC 2.98 18.694 1.809 14.629
6.2567×10-23 6.311×10-23
Table 2. Third order optical nonlinearity data of MMTC S.No 1 2 3 4 5 6
Parameters ∆φ n2 (cm2/W) β (cm/W) Im χ(3) (esu) Re χ(3) (esu) χ(3) (esu)
MMTC 0.11 -63.3 2.71X10 -6 2.77X10 -8 0.823X10-9 3.13X10 -8
41
Table 3. Surface parameters of MMTC measured by AFM S.NO. Parameter MMTC 1 Roughness average (Sa) (nm) 7.0754 2 Root mean square (Sq) (nm) 10.1859 3 Surface skewness (Ssk) 2.10411 4 Surface kurtosis (Sku) 12.3428 5 Ten-point height (Sz) (nm) 193.247 6 Density of summits (Sds) (1/µm²) 40.3
42
Highlights • • • •
MMTC crystallizes in tetragonal crystal system. Plasma energy, Penngap and electronic polarizability have been calculated for the first time. Z-scan study confirms the enhancement in the third order nonlinearities due to the presence of SCN ligand. Surface parameters like average roughness (Sa), root mean square (Sq) and surface skewness (Ssk) were calculated by using AFM analysis.
43
Graphical Abstract
It is observed from the AFM image that crystal surface possesses valleys and hillocks. It is well known that a large number of peaks and valleys in an image significantly affect the average roughness (Sa) and root mean square (Sq) values.
44
