Structural, mechanical, thermal, electrical, second- and third-order nonlinear optical characteristics of MCBT NLO crystal for optoelectronics device and laser applications

Structural, mechanical, thermal, electrical, second- and third-order nonlinear optical characteristics of MCBT NLO crystal

for optoelectronics device and laser applications

G J SHANMUGA SUNDAR^1,2, S M RAVI KUMAR^3,* , P SAGAYARAJ⁴, S SELVAKUMAR⁵, C SHANTHI¹, S SIVARAJ⁵and R GUNASEELAN⁶

1PG and Research Department of Physics, Government Arts College, Tiruvannamalai 606 603, India

2Department of Physics, Rajeswari Vedachalam Government Arts College, Chengalpattu 603 001, India

3PG Department of Physics, Sri Subramaniyaswamy Government Arts College, Tiruttani 631 209, India

4Department of Physics, Loyola College, Chennai 600 034, India

5PG and Research Department of Physics, Government Arts College (Autonomous), Chennai 600 035, India

6Department of Physics, Pachaiyappa’s College for Men, Kanchipuram 631 501, India

*Author for correspondence (smravi78@rediffmail.com; ravism23@gmail.com) MS received 13 February 2021; accepted 14 April 2021

Abstract. Single crystal of mercury cadmium bromide thiocyanate (MCBT) has been produced by a slow cooling technique with a mixed solvent of ethanol and deionized water (1:1). The lattice constant of the MCBT crystal evaluated by single-crystal X-ray diffraction analysis illustrates that the titular crystal belongs to orthorhombic system. The second- harmonic generation proficiency of MCBT is 5.64, more prominent than KDP crystal. The transmission spectrum gives UV cut-off wavelength of 328 nm. The thermal stability of 201°C was validated by thermogravimetric and differential thermal analyses. The mechanical limits of MCBT crystal were evaluated by the microhardness test. The dielectric study was carried out with varying frequency at different temperatures. The feature of surface was examined by scanning electron microscope for the titular crystal. Third-order nonlinear optical performance was determined by Z-scan technique.

Keywords. Slow cooling; single-crystal XRD; UV–visible–NIR; thermal studies; microhardness studies; Z-scan technique.

1. Introduction

Current research pointed out that second-order nonlinear optical (SONLO) materials are profoundly fit for changing the frequency of infrared into visible and UV wavelengths;

consequently, those materials are significant in the field of optoelectronics and photonics [1–4]. Presently, organometallic crystals are expected to produce NLO property. Organometallic materials are also having the benefits of both organic and inorganic materials regarding physicochemical properties. Therefore, there are so many organometallic NLO materials that have been reported and succeeded [5–10].

Organometallic compounds have the ability to take charge transfer from metal to ligand and ligand to a metal that builds up the variety of molecular structure. This kind of molecular arrangement modifies the molecule electronic properties and to increase the optical properties (linear and nonlinear) [11].

The literature on organometallic compounds show that the thiocyanate ligand joint with transition metal ions, in

which thiocyanate (SCN) is responsible for combining the versatility ligand with two donor atoms. Being a ligand with potential donors S and N, the thiocyanate (SCN) make a focal point to the researcher, because they are having the structural chemistry of its multifunction co-ordination modes, which is used in development of complexes with NLO activities [12,13].

For the most part, the NLO materials have been uti- lized in numerous fields, including broadcast communi- cations, optical registering, optical data preparing, clinical diagnostics, colour displays, laser-driven fusion and laser remote sensing [14–17]. Hence, there is a requirement of NLO materials with high efficiency, good environmental stability and high resistance to laser damage [12,15].

Most of the bimetallic complexes of thiocyanate crystal in acentric structure exhibit the excellent NLO property.

The bimetallic thiocyanate-based crystals, like CMTC [18], FMTC [19], ZMTC [10], MMTC [20], MMTD [21]

and ZCTC [15], are examined organometallic crystals.

Hence, the attempts have been made with this expansive view and succeeded in the bimetallic thiocyanate complex https://doi.org/10.1007/s12034-021-02491-6

of mercury cadmium bromide thiocyanate (MCBT) crystal.

Among the bimetallic thiocyanate family crystals, MCBT is also a good potential candidate in the view of second- harmonic generation (SHG) efficiency. The point of the structure of MCBT crystal shows that Br, Cd and Hg are directed by the possibility to gain non-centrosymmetric structure by the co-ordination of SCN ligand and to amplify the density of chromospheres in the structure. The function of Br^- anion is to expand the transparency bandwidth of Hg–Br and Cd–Br bonds. The development suggested that the mercury molecule is encased by two sulphur and four bromide atoms designing a deformed octahedron. The cadmium atom joined with six nitrogen atoms gives the distorted prismatic trigonal environment. This is an amazing structural chemistry of cadmium. The metal environment of MCBT is shown in figure1.

Metallic thiocyanates and their derivatives are feasibly valuable applicants among the organometallic frameworks, because all of them contain –S=C=N–bridges. In bimetallic complexes, SCN bridges connect the metal atoms with an infinite network that centre the large polarization with the addition of large microscopic nonlinearities. Similar studies have been demonstrated in recent times by many research teams [21–24]. MCBT was examined to different charac- terization studies like X-ray diffraction (XRD), optical, thermal and mechanical properties. The obtained physico- chemical properties of the titular crystal are discussed for the first time in this article.

2. Experimental

2.1 Synthesis

All the substances (purity 99%) of analytical reagent (AR) are utilized as bought. MCBT has been synthesized without

further purification of substances. The synthesis process was of two steps.

Step 1: 6 K SCN½ ð Þ þ2 HgCl½ ₂ þ CdCl₂

!Hg₂Cd SCNð Þ₆þ6KCl

Step 2: Hg₂Cd SCNð Þ₆þ2 HgBrð ₂Þ

!Hg₄CdBr4ðSCNÞ₆

In step 1, mercury cadmium thiocyanate was achieved as a product by using appropriate amounts (6:2:1) of potassium thiocyanate (purity 98%, Merck), mercury chloride (purity 99%, Merck) and cadmium chloride (purity 99%, Merck). This process was completed with solvent double-distilled water.

The product of mercury cadmium thiocyanate is a white pre- cipitate. In step 2, using a mixture solvent of water and ethanol, the mercury cadmium thiocyanate makes as a solution and then mercury bromide (Assay 98%, Nice Chemicals (LR grade)) was added slowly while stirring. The obtained solution was stirred for almost 14 h to acquire a homogenous solution.

2.2 Growth

From the prepared MCBT solution, MCBT was filtered thrice to eliminate any insoluble contaminations and it was kept at a constant temperature bath (accuracy±0.01°C) to control the evaporation rate (slow cooling). Initially, the temperature of the solution was maintained at 40°C for 3 days. To get a saturated solution, the temperature was reduced by 0.2°C per day. Within 10–20 days, moderate transparent and defect-free tiny crystals were formed. In a span of 20–40 days, MCBT crystal is formed with dimension 109792 mm³, and the grown crystal images are shown in figure2.

3. Results and discussion

3.1 Single-crystal XRD analysis

The grown MCBT crystal was investigated by single-crystal XRD. This analysis gives the confirmation that MCBT belongs to an orthorhombic crystal system with a non- centrosymmetric space group Fmm2. The details of the parameters are given in table 1. The obtained result of MCBT is well agreed with already measured values [25].

3.2 Linear optical study

The optical transmittance spectrum of MCBT single crystal was in the scale of 190–1100 nm. Figure3 represents the UV–visible–NIR transmittance spectrum of MCBT crystal.

From the spectrum, the UV cut-off wavelength was deter- mined as 328 nm, which shows that the MCBT crystal is applicable for NLO applications. The small transmittance peaks attained in the visible region due to the occurrence of Figure 1. Metal environment of MCBT crystal.

Br^2? ion in the grown crystal. The evaluated UV cut-off wavelength of MCBT is less than that of other organometallic crystals like TMTM (350 nm) [26], MMTC (373 nm) [27] and CMTG (366 nm) [10]. The linear optical study indicates that MCBT crystal is pertinent for photonics and optoelectronics appliances.

Tauc’s plot was used to measure the optical bandgap (E_g) of the grown crystal MCBT. Figure4shows the value of bandgap (Eg), which was calculated by plotting (aht)²vs.photon inci- dent energy (ht) and it is shown in figure 4. The measured

bandgap (E_g) value was 5.176 eV. This higher bandgap value can cause the crystal to have more optical conductivity without absorption of optical photons within a specific wavelength range. As a result, materials were capable of transmitting light with wavelengths ranging from 328 to 1100 nm.

3.3 SHG efficiency studies

To inspect the NLO property in terms of SHG efficiency, Kurtz and Perry powder technique was utilized [28]. The powder form of MCBT crystal was illuminated by Q-switched high-energy Nd:YAG laser operating at the fundamental wavelength of 1064 nm. The output energy was emitted by the sample as 40.6 mJ with a high-intensity green light. The same procedure was followed for the ref- erence crystal of KDP, which emitted the output energy of 7.8 mJ. The results exposed that MCBT has higher NLO property (5.64 times) than KDP crystal. The SHG efficiency of MCBT crystal is compared with other organometallic crystals and the details are given in table2.

Figure 2. As-grown crystals of MCBT.

Table 1. Crystal data of MCBT single crystal.

Crystal formula Hg4CdBr4(SCN)6

Crystal system Orthorhombic

Space group Fmm2

a(A˚ ) 22.324

b(A˚ ) 18.467

c(A˚ ) 6.378

a=b=c 90°

Volume (A˚³) 2629.377

Z 4

Density (D) 4.006

Figure 3. Optical transmittance spectrum of MCBT crystal.

Figure 4. Tauc’s plot of MCBT crystal.

3.4 Thermal studies

By adopting thermogravimetric and differential thermal analyses (TGA/DTA), thermal studies have been carried out for grown crystal. The achieved thermal spectrum is shown in figure 5. The total weight of the sample 10.31 mg has been taken for this study. MCBT has three stages of weight loss, confirmed from the TGA traces. The temperature between 201.7 and 256°C, crystal suffers first weight loss of about 6.1% (practically 0.63 mg). This confirms the non-appearance of a water molecule in the grown sample. At this point, the sample Hg₄CdBr₄(SCN)₆ may decompose into HgCd(SCN)₆ and further it may get disorganized to CdS/HgS/CS₂/(CN)₂ and N₂. The next stage of weight loss happened between the temperature 256 and 308°C, with a weight loss of about 7% (nearly 0.73 mg). The sample starts the third dissociation at 308°C. It is clearly noted that the grown sample of MCBT

has thermal stability of 201°C. This result indicates MCBT has higher thermal stability when compared with other thiocyanate complex organometallic crystals, like mercury cadmium chloride thiocyanate (MCCTC) [172.78°C] [8], cadmium mercury thiocyanate (CMTC) [198°C] [37], cadmium mercury thiocyanate glycol monomethyl (CMTG) [115°C] [10].

3.5 Microhardness studies of MCBT

The composition of bonds and molecular coordinators in a crystal could cause the mechanical performance of a crystal, which could be examined by microhardness study [38]. The mechanical reliability of a crystal plays an important role in appliance invention. Therefore, the grown MCBT crystal was subjected to microhardness study, in which the hard- ness number was measured for different loads 25, 50, 75 Table 2. Comparison of SHG efficiency of MCBT with other organometallic crystals.

Sample

SHG efficiency with respect

to KDP References

Tetrathiourea cadmium tetrathiocyanato zincate (TCTZ) 2.5 Bhaskaranet al[29]

Nickel mercury thiocyanate crystal (NMTC) 0.66 Ramachandra Rajaet al[30]

Tetrathiourea mercury tetrathiocyanato manganate (TMTM) 0.88 Pabitha and Dhanasekaran [31]

Potassium thiocyanate added KDP 1.31 Dhanarajet al[32]

Manganese mercury thiocyanate-bis(N-methyl formamide) (MMTN) 1.2 Josephine Ushaet al[33]

Calcium bis(thiourea) chloride (CBTC) 1.15 Aniset al[34]

2-Aminopyridine potassium dihydrogen orthophosphate lithium chloride (2APKDPL)

1.14 Sivavishnuet al[35]

Cadmium mercury thiocyanate bis(N-methyl formamide) (CMTN) 5 Subhashiniet al[36]

Mercury cadmium bromide thiocyanate (MCBT) 5.64 This study

Figure 5. TG–DTA curve of MCBT crystal.

and 100 g with indentation time 10 s. The hardness value has been calculated by the following equation,

Hv¼1:8544P=d² ðkg mm²Þ; ð1Þ

where 1.8544 is a constant of the geometrical factor for the diamond pyramid indenter,Pdenotes load applied in g and d stands for the diagonal length of the indentation impres- sion in mm. The measured hardness number (H_v) with applied load (P) is shown in figure6. It is shown that the hardness number (H_v) increases with the increase of applied load (P). From the result, it is observed that MCBT crystal exhibits reverse indentation size effect (RISE) [39]. Mey- er’s law relates applied load (P) and average indentation (d), and this relation is shown in figure7. From the graph, the value of ‘n’ (work-hardening coefficient) was measured as 1.94, which implies that the MCBT crystal belongs to the soft category material.

3.6 Dielectric studies

The sample of MCBT with smooth surface was chosen for analysing the dielectric properties. The MCBT crystal (&3 mm thickness) was coated with graphite on both sides and placed between the electrodes of the parallel plate capacitor.

The variation of dielectric constant with respect to log frequency has been illustrated in figure8. The higher value of dielectric constant at lower frequency is owing to four types of polarization. The lesser value of dielectric constant in higher frequency region reveals the diminishing of polarizations [40]. The behaviour of dielectric constant with various temperatures is also observed from figure7, which indicated that the dielectric constant decreases with an increase in temperature. The function of dielectric loss relating to log frequency and a range of temperatures is exposed in figure 9. From the figure, it was observed that the dielectric loss got higher value at lower frequency. The

evaluation of dielectric loss with lower value at higher frequencies is recognized to rotation of dipole. At the stage of greater frequency region, the polarization becomes reduced and the energy is utilized for rotating the dipoles. The attributing of lesser dielectric loss through higher frequency in the MCBT recommend that MCBT has got superior optical characteristic with tiny flaws. This factor is fundamental for the applications of nonlinear optics [41].

3.7 SEM analysis

The observation of defects in the crystal plane is vital, because the crystal quality may depend on the number of defects in the crystal surface. Defect-free crystals could be used in device fabrications. Therefore, the nature of the

20 30 40 50 60 70 80 90 100 110

70 75 80 85 90 95 100 105

Hardness number (Hv) (Kgmm2 )

Load P (g)

Figure 6. Plot between loadvs.hardness number.

1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0

1.40 1.45 1.50 1.55 1.60 1.65

Log P

Log d

Figure 7. The relation between logP vs.logd.

2 3 4 5 6 7

0 100 200 300 400

Dielectric constant

log frequency

308 K 328 K 348 K 368 K 388 K

Figure 8. Dielectric constant vs. log frequency for MCBT crystal.

surface and growth process of MCBT was analysed by the scanning electron microscope (SEM) study. Hence, a crystal with a smooth surface has been selected for this study. The observed SEM photograph of MCBT is shown in figure10.

From the photograph, it is observed that MCBT crystal has a smooth surface with less number of tiny crystals.

3.8 Z-scan studies of MCBT crystal

The third-order NLO parameters like nonlinear refractive index (n₂) and nonlinear absorption coefficient (b), the real and imaginary parts of the third-order susceptibility [Re(v⁽³⁾) and Im(v⁽³⁾)], third-order NLO susceptibility (v⁽³⁾) were measured by Z-scan study. To perform this study, laser power 50 mW was used. The crystal with thickness 1 mm was moved from the positive axis ofZto the negative axis of Z by stepper motor. The laser radiation with different intensities could fall on the crystal. The convex lens (f_L= 20 cm) is used for focusing the Gaussian beam alongZ-axis at

focal pointZ= 0. This process was reiterated with various values of Zpertaining to focus and calculating the signifi- cant transmission. Z-scan transmittances of open and close apertures of MCBT crystal are shown in figures11and12.

Using figures11and12, ‘n₂’ and ‘b’ of MCBT crystal have been calculated. It is observed from figure10, the reduction in transmittance with respect to increase in input laser intensity implies that the MCBT crystal shows signs of two- photon absorption. The nonlinear refractive index, nonlinear absorption and refraction of the MCBT crystal were mea- sured from the closed aperture Z-scan curve (figure12). By taking the value of the valley to peak phase distortion (from figure 12), the refractive index was determined. (DTp–v) gives the difference between the peak and valley transmit- tances and can be written as,

DTpv ¼ 0:406 1ð SÞ^0:25jDUj; ð2Þ

2 3 4 5 6 7

0.0 0.5 1.0 1.5 2.0

Dielectric loss

log frequency

308 K 328 K 348 K 368 K 388 K

Figure 9. Dielectric lossvs.log frequency for MCBT crystal.

Figure 10. SEM image of MCBT showing smooth surface.

-15 -10 -5 0 5 10 15

0.60 0.65 0.70 0.75 0.80 0.85 0.90

Normalized Transmittance

Z Position in mm

Figure 11. Open aperture Z-scan transmittance of MCBT.

-15 -10 -5 0 5 10 15

0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3

Normalized Transmittance

Z position in mm

Figure 12. Closed aperture Z-scan curve of MCBT.

where S is the aperture linear transmittance, calculated using the following relation,

S¼ 1expð2r²_a=x²_aÞ; ð3Þ

wherer_ais the aperture radius andx_athe beam radius at the aperture. The nonlinear refractive index (n2) of the crystal was calculated using the relation,

n2¼ D/

kI0Leff

m²

W ; ð4Þ

wherekis the wavenumber and it can be written ask¼^2p_k (kis the laser wavelength),I₀the intensity of the laser beam at the focus (Z = 0), L_eff the effective thickness of the sample and which can be written as,

L_eff¼ ½1expðaLÞ=a; ð5Þ

wherea is the linear absorption andLthe thickness of the sample. The nonlinear absorption coefficient (b) is calcu- lated from the open aperture Z-scan data andbis calculated from the following formula [42],

b¼2 ffiffiffiffiffiffiffiffiffi p2DT I0Leff

: ð6Þ

In this equation,DTis the peak value at the open aperture Z-scan curve. The value ofbwill be negative for saturated absorption and positive for two-photon absorption. The real

and imaginary parts of third-order NLO susceptibility v⁽³⁾ are defined by

Reð Þv ^ð3ÞðesuÞ ¼10⁴ðe0c²n²₀n₂Þ=p cm²W¹ ð7Þ Imð Þv ^ð3ÞðesuÞ ¼10²ðe0c²n²₀kbÞ=4p² cm W¹

; ð8Þ wheree0is the vacuum permittivity,n₀the linear refractive index andcthe velocity of light in vacuum. The third-order NLO susceptibility was determined by

v^{ð Þ3} ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðRev^{ð Þ3}Þ²þ ðImv^{ð Þ3}Þ² q

: ð9Þ

The estimated third-order NLO parameters are displayed in table 3. The outcome exposed that MCBT crystal has well nonlinear response. Therefore, MCBT crystal has good nonlinear response and hence can be used as an appropriate crystal for device fabrication in the field of nonlinear optics [43–45]. The obtained third-order NLO parameters are compared with reported organometallic NLO crystals, and values are displayed in table4.

4. Conclusion

MCBT (mercury cadmium bromide thiocyanate) was grown by a slow cooling method with a dimension of 109793 mm³ in 20–40 days. The titular crystal belongs to the orthorhombic crystal system, with non-centrosymmetric space group Fmm₂. From the UV–visible–NIR spectrum, the UV cut-off wavelength was found to be 328 nm. The powder SHG efficiency was measured as 5.64 times greater than the KDP crystal. The microhardness study reveals that MCBT crystal belongs to soft category material. The nor- mal dielectric behaviour was observed for the grown MCBT sample. SEM analysis confirm the smooth surface of MCBT crystal. The crystal has thermal stability at 201°C. Rea- sonably higher value of nonlinear refractive index (n₂) and absorption co-efficient (b) are supportive to a larger value of third-order NLO susceptibility (v⁽³⁾) of 4.44819 10^–14 esu. The result implies that MCBT could be helpful for optical limiting applications. Hence, the MCBT crystal is an Table 3. The measured third-order NLO parameters.

Effective thickness (Leff) 0.9159 mm

Nonlinear refractive index (n2) 9.8701910^–16m³W^-1 Nonlinear absorption co-efficient (b) 4.7899910^–9m W^-1 Real part of third-order susceptibility

[Re(v⁽³⁾)]

4.4481910^–14esu Imaginary part of third-order

susceptibility [Im(v⁽³⁾)]

9.1419910^–13esu Third-order nonlinear optical

susceptibility (v⁽³⁾)

4.4481910^–14esu

Table 4. Comparison of third-order NLO values with other organometallic crystals.

Sample n₂(cm²W^–1) b(cm W^–1) v⁽³⁾(esu) References

Bis(picolinic acetate) zinc(II) (BPZ) 2.349910^–8 0.812910^–4 6.74910^–6 Ravichandranet al[46]

Guanidinium pentaborate monohydrate (GPBMH) 7.85910^–9 7.7910^–3 2.2910^–6 Dhatchaiyiniet al[47]

Bis(4-methoxybenzyl ammonium) tetra chloridozincate (4MBZ)

–4.27910^–9 0.3910^–4 1.8910^–6 Karuppasamyet al[48]

Sodium (bis) boro succinate (SBBS) –1.22910^-13 -0.991910^–5 4.219910^–9 Rajasekaret al[49]

Tris (thiourea) potassium barium sulphate (TTPBS) 4.41910^-12 1.10910^–6 3.44910^–6 Azharet al[50]

L-proline cadmium chloride (LPCC) 6.3910^–12 7.29910^–5 2.39910^–4 Aniset al[51]

Manganese mercury thiocyanate (MMTC) –1.88910^–11 8.62910^–6 6.58910^–9 Rajesh Kumaret al[7]

MCBT 9.87910^–10 4.789910^–7 4.44910^–14 This study

applicable candidate for optoelectronic devices and laser applications.

Acknowledgement

Ravi Kumar truthfully thanks the Science and Engineering Research Board (SERB, a statutory body of the Department of Science and Technology, Government of India) for financial support for research project (No. EEQ/2016/

000451).

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