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Investigation on rapid growth of 4-N,N-dimethylamino-4′-N′-methylstilbazolium p-toluenesulphonate (DAST) crystals by SNM technique
ARTICLE IN PRESS Journal of Crystal Growth 312 (2010) 420–425
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Investigation on rapid growth of 4-N,N-dimethylamino-40 -N0 methylstilbazolium p-toluenesulphonate (DAST) crystals by SNM technique R. Jerald Vijay a, N. Melikechi b, T. Rajesh Kumar a, Joe G.M. Jesudurai a, P. Sagayaraj a,n a b
Department of Physics, Loyola College, Chennai, India Department of Physics and Pre-Engineering, Centre for Research and Education in Optical Sciences and Applications, Delaware State University, Dover, DE 19901, USA
a r t i c l e in f o
a b s t r a c t
Article history: Received 23 July 2009 Received in revised form 14 October 2009 Accepted 15 October 2009 Communicated by M. Fleck Available online 3 December 2009
We have investigated the rapid growth of N,N-dimethylamino-N0 -methylstilbazolium p-toluenesulphonate (DAST) adopting the slope nucleation method and by rapidly evaporating the solvent. Thin plates of DAST are grown within a period of 72 h by carefully optimizing the growth conditions. The structural and optical properties of the crystal are studied by employing powder XRD, FTIR and NMR. The electrical properties of the crystal are investigated by ac, dc and photoconductivity measurements. The surface features and the influence of rapid evaporation of the DAST crystal have been analyzed using scanning electron microscopy. The results suggest that the quality of the crystal grown by this method compares well with those grown by conventional techniques. & 2009 Elsevier B.V. All rights reserved.
Keywords: A1. Surface structure A2. Growth from solutions B2. Nonlinear optical materials B3. Terahertz technology
1. Introduction Recently, THz technology has been an extremely active field of research, and the development of new THz sources and detectors has been filling the THz gap [1]. This new technology has great potential to integrate the electronic and optical devices, which is expected to enable ultrahigh speed computation and communications beyond signal switching rates of 100 Gigabits/s [2]. One of the primary motivations for the development of THz sources and spectroscopy systems is the potential to extract material characteristics that are unavailable using other frequency bands [3]. Organic crystals have been a recent source of interest as THz emitters as they have been reported to generate stronger THz signals than commonly used semiconductor or inorganic electrooptic emitters owing to their large second-order nonlinear electric susceptibility. They offer vast design possibilities to tailor the linear and nonlinear properties, and owing to the almost completely electronic origin of the nonlinearity, they are well suited for future high speed devices [4]. Waveguide structuring for integrated optics with organic crystals by photo bleaching, femtosecond ablation, and ion implantation, as well as electrooptic modulation in thin organic single crystalline films and channel waveguides have been demonstrated [5,6]. Among the various classes of materials investigated worldwide, ionic organic
n
Corresponding author. Tel.: + 9144 28178200; fax: + 9144 28175566. E-mail address: [email protected] (P. Sagayaraj).
0022-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2009.10.067
crystals are of special interest due to their advantageous mechanical, chemical and thermal properties. Compared to the widely investigated poled polymers, organic single crystal are advantageous because of superior long-term thermal and photochemical stability combined with a higher chromophore concentration [5]. However, only a few of organic material could so far be crystallized in reasonable crystal size with high optical quality required for possible application. Organic crystals like N,N-dimethylamino-N0 -methylstilbazolium p-toluenesulphonate (DAST), N-bezyl-2-methyl-4-nitroaniline (BNA) and 2-methyl-4nitroanline (MNA) have very high NLO coefficients and at the same time have a low dielectric constant making them a perfect choice for THz generation [7,8]. Recent research proves that it is possible to synthesize stilbazolium derivatives such as 4-N,Ndimethylamino-40 -N0 -methylstilbazolium 2,4,6-trimethylbenzenesulfonate (DSTMS) and trans-40 -(dimethylamino)-N-phenyl-4stilbozolium hexafluorophospate (DAPSH) with very favorable crystal growth characteristics by carefully modifying the structure with various substitutions on the counter-anion and these materials are projected to be promising alternates for DAST, especially for THz generation [9,10]. Another interesting material developed by adopting the above procedure is 4-N,N-dimethylamino-40 -N0 -methyl-stilbazolium 2-napthalenesulfonate (DSNS), which showed very high nonlinear optical properties even higher than DAST; however this compound do not have the favorable growth characteristics [11]. Zhang et al. first reported THz optical rectification in DAST and confirmed a high electro-optic coefficient ( 4400 pm/V) at 820 nm. The best DAST sample provided a detected THz electric field that was 185 times larger than that
ARTICLE IN PRESS R. Jerald Vijay et al. / Journal of Crystal Growth 312 (2010) 420–425
obtained from a LiTaO3 crystal and 42 times larger than GaAs and InP crystals under the same experimental conditions [12]. Schineider et al. [13] demonstrated that the THz radiation spectrum generated and detected using DAST crystals extended from 0.4 to 6.7 THz depending on the laser excitation wavelength in the 700–1600 nm wavelength range. Though the DAST crystal is the best organic THz emitter ever studied, the growth of high optical quality DAST single crystal is still a challenge, one of the challenges is to reduce the growth time needed to obtain high optical quality DAST, which takes several weeks for crystals with dimensions exceeding 1 cm3 [14]. Faster and easier crystal growth procedure is an important challenge for future applications and therefore optimization of the growth techniques and the development of new molecules for crystal growth are subjects of present research. Different approaches have been adopted to achieve faster and improved growth rate of DAST crystals. The difficulties in growth positioning and nucleation are effectively solved by combining the slope nucleation method (SNM) with the Laser Irradiation Method (LIM) [15]. A cost effective method has been suggested by Brahadeeswaran et al. [16] by using solutions of lower supersaturation coupled with isothermal solvent evaporation and this method facilitated the development of nearly parallel (0 0 1) and (0 0 1¯) faces so as to directly utilize the crystals for EO and THz applications. For many photonic applications, a thin crystal or thin film is more attractive. The experiments conducted by Han et al. [17] proved that DAST crystals with a thickness of a few hundred micrometers are suitable for EO sampling up to few THz. It has been reported that the as-grown and very thin crystals are less sensitive to thermal shock when compared to thick DAST crystals due to large thermal gradients and these thin DAST crystals are preferable to avoid defects [18]. Recently, the as-grown nonpolished DAST crystal was used successfully for THz generation by Taniuchi et al. [19]. The rate of evaporation determines the size of crystals and the rapid evaporation favours the formation of a lot of tiny crystals [16]. Inspired with these facts, we have made an attempt to investigate the rapid growth of DAST crystals by slightly modifying the slope nucleation method and obtained reasonably good quality thin plates of DAST within 72 h. We have characterized the rapidly grown DAST crystals employing the powder XRD, FTIR, NMR, SEM, ac and dc conductivity and photoconductivity studies.
2. Experimental 2.1. Material synthesis DAST was synthesized by the condensation of 4-methyl-Nmethyl pyridinium tosylate, which was prepared from 4-picoline and methyltolunesulfonate and 4-N,N-dimethylamino-benzaldehyde in the presence of piperidine [14]. The synthesized salt appeared reddish, the typical colour of DAST. The product was separated from additives and then kept in an oven at 100 1C for 1 h to prevent absorption of water from the atmosphere. The purity of the product was further improved by successive recrystallization. 2.2. Crystal growth We investigated DAST crystal growth by slope nucleation method (SNM) and the evaporation of the solution was performed at a faster rate. The SNM is a very simple and high yielding process. In this method, a Teflon plate with grooves is inserted
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(in slope shape) into the growth solution. The tiny spontaneous nuclei, which are generated in the supersaturated solution are made to fall down onto the slope. As crystals grow larger, they slip downward along the slope until they arrive at one of the grooves. Finally, the crystals stand and then continue to grow larger on the groove [20]. This arrangement facilitates the simultaneous growth of many high-quality crystals at a single growth run. We have prepared two different DAST-methanol solutions at 45 1C according to their solubility. The first solution with a concentration of 4 g/100 mL and the second solution with a concentration of 2 g/100 mL were prepared and transferred to separate Teflon beakers. Both the solutions were stirred for 1 h at 45 1C to ensure homogeneity. After this, the Teflon plates with grooves were inserted in both the beakers. The solutions were housed in a constant temperature bath and the temperature was maintained at 45 1C for 2 days. The key factor was both the beakers were kept completely open so as to allow fast evaporation. For comparison, one more set of DAST-methanol solutions were also prepared simultaneously with same concentrations as mentioned earlier and then kept in the same constant temperature bath under the same temperature (45 1C), but the difference was that in these beakers the rate of evaporation was controlled by using a cap with a small hole in it. We followed the slope nucleation method with DAST-methanol solutions prepared with two different concentrations and then employed fast evaporation for one set and controlled slow evaporation for another set. Interestingly, we observed the growth of DAST crystals in the form of thin plates of thickness ranging from 0.1 to 1 mm within a period of 72 h, when the experiment was done with faster evaporation. Whereas, the crystals grown under the controlled evaporation process were of larger size but the growth period was nearly 15 times (45 days). Fig.1a and b show the photographs of as-grown DAST crystals that are grown by employing rapid and slow evaporation methods, respectively. It was observed that DAST crystal nucleated within few hours in the unsaturated Teflon beaker and tiny crystals were observed on the Teflon slope plate and also at the bottom of the growth vessel within a day. Since the nucleation depends on the equilibrium concentration, it is found that the metastable zone width increases with decreasing concentration and corresponding temperature [21]. Nathalie Sanz et al. have pointed out that a wide distribution of particle size generally arises from nucleation over a relatively long period of time, where young nuclei are produced simultaneously along with the growth of older nuclei. Hence, in order to overcome this, they have utilized the advantage of instantaneous nucleation caused by rapid evaporation to obtain crystals of a narrow size distribution. It was found that the rapid evaporation induces germination of high number of nuclei, triggering faster growth [22]. In our case, the excess solvent due to unsaturation was evaporated quickly, which paved the way for saturation leading to nucleation and hence formation of crystal plates. It is well known that DAST is very sensitive to even minute changes in its concentration; here, even though the concentration was reduced, the temperature was maintained high instead of maintaining equilibrium temperature in order to achieve supersaturation and the rate of evaporation causes instantaneous nucleation. The higher supersaturation is normally detrimental for the growth of twin free crystals and hence efficient control over solution supersaturation and growth rate are of vital importance for the successful growth of good quality crystals. In the present case, the solution prepared at an optimum concentration of 2 g/100 mL has resulted in controlled nucleation on the Teflon slope and the higher growth temperature triggered a faster evaporation of methanol solvent leading to the formation of thin plates of DAST. Therefore the lower concentration coupled with faster evaporation created an ideal situation for the growth of
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was found that DAST crystallized into monoclinic system. ˚ b= 11.3007 The lattice parameters are, a =10.273 70.584 A, ˚ c =18.27670.081 A, ˚ and b =91.580 70.1101 and V= 0.626 A, 2120.56070.296 A˚ 3 which are on par with earlier results [7]. The raw data was processed [24] and the recorded XRD pattern is shown in Fig. 2. Since the crystal is grown at a rapid pace, the chances of polycrystalline presence cannot be totally ruled out. 3.2. FTIR and NMR spectral analyses The FTIR spectrum recorded for the wavelength range 400– 4000 cm 1 is shown in Fig. 3. The peak at 3035.96 cm 1 is assigned to the aromatic C–H stretch. The peak at 2914 cm 1 is assigned to the alkyl C–H stretch. The peak at 1645.79 cm 1 is due to the C =C stretch. The peaks at 1584.32 and 1527.63 cm 1 are assigned to the aromatic ring vibrations. The peak at 1369.87 cm 1 is assigned to CH2 bending and C–N stretching mode. The peak at 825.51 cm 1 is assigned to the 1, 4 distributed aromatic ring. The spectrum of the rapidly grown DAST shows that the bands in the range 4000–2500 cm 1 are relatively less intense suggesting that the grown crystal is an ordered single crystal in nature [25]. The sharp peak at 3435 cm 1 is attributed to O–H stretch, which is due to the air bubble as evident from the SEM images. In the proton NMR spectrum (Fig. 4) of DAST, the singlets at 2.37 and 4.215 are assigned to three C–CH3 hydrogens and three N–CH3 hydrogens, respectively. The singlet at 3.08 is due the 6N–(CH3)2 hydrogens and the intensity of the peak justifies the number of contributing nuclei. The doublets at 6.78 and 7.6 are due to the four hydrogens of the N–(CH3)2–C6H4 aromatic ring. The doublets at 7.22 and 7.94 are due to the two aromatic hydrogens ortho to –SO3 and two aromatic hydrogens ortho to –CH3. The doublets at 7.708 and 8.49 are due the four hydrogens ortho to the C5H4N aromatic ring. The doublets at 7.1 and 7.848 are due to the two oliphinic hydrogens (HC= CH). Fig.1. (a) DAST plates grown rapidly after 72 h. (b) DAST crystals grown by slow evaporation.
these crystal plates. During the growth period the solvent was completely evaporated and after 72 h crystal plates of size 4–6 3–4 0.1–1 mm3 (Fig.1a) were harvested. Though there are reports on the growth of similar such thin plates of DAST, the growth processes adopted were complicated and also the growth period was relatively longer [16,23]. In the present work, we have demonstrated a simpler and cost effective method to grow DAST crystal plates of reasonably good quality in a very short period by careful control of concentration of the DAST-methanol solution coupled with faster evaporation. However, with beakers containing the high concentration solution (4 g/100 mL), we could only observe the formation of dendrites and needles of poor quality and this could be attributed to the multi-nucleation caused by higher supersaturation. At the same time, we have obtained large size crystals of DAST with length exceeding 1.7 cm by slope nucleation method, which was prepared under the same conditions but with prolonged period of evaporation. Since our main focus is on the rapid growth of DAST crystals, we limit the characterization to only to those grown by the rapid process.
3.3. Impedance spectral analysis Impedance spectroscopy is an analytical tool whose results like conductivity can be correlated to defects and impurities of solids [26]. The ac conductivity study using complex impedance spectroscopy is performed to characterize the bulk resistance of the crystalline material [27]. In the present case, the complex impedance parameters are measured with HB 4124 LCR meter using silver electrodes by pelletizing DAST crystal into a
3. Results and discussion 3.1. Powder X-ray diffraction The as-grown crystal was characterized by powder X-ray ˚ From the observed 2y values it diffractometry at 1.5406 A.
Fig. 2. Powder XRD pattern of DAST.
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Fig. 3. FTIR spectrum of DAST.
5x105
Z '' (Ω)
4x105
3x105
2x105
1x105
0.0
2.0x105 4.0x105 6.0x105 8.0x105 1.0x106 1.2x106 Z ' ( Ω) Fig. 5. Complex impedance plot of DAST.
Fig. 4. NMR spectrum of DAST.
rectangular specimen of thickness of 0.115 cm and area 0.5024 cm2. The ac conductivity of the sample is determined from the real part of the impedance using the relation
sac ¼ t=Rb A where t is the thickness, A is the area of face in contact with the electrode and Rb is the bulk resistance. The bulk resistance was found to be 1.928 105 O. The calculated ac conductivity was 11.87 10 9 (Om) 1. The plot of Z0 and Z00 for DAST crystal at room temperature is shown in Fig. 5 and the obtained impedance exhibits a good semicircle. The observed value is typical for an insulating material. Interestingly, the observed low ac conductivity suggests that the number of defects or impurities present in the rapidly grown DAST crystal is low. 4. dc Conductivity The dc electrical conductivity measurements were carried out for the DAST crystal using the conventional two-probe
technique in the temperature range 313–363 K. The dc electrical conductivity (sdc) of the crystal was calculated using the relation.
sdc ¼ t=RA where R is the measured resistance, t is the thickness of the sample and A is the area of face in contact with the electrode. The sdc values were fitted into the equation:
sdc ¼ s0 expðEd =kTÞ Electrical conductivity depends on thermal treatment of a crystal. Thus the conductivity at low temperatures depends on the cooling speed from melting point temperature to room temperature. Thus for slow cooling, the remaking of the lattice can occur by the migration of interstitials to vacancies, recombination of Schottky defects or migration of vacancies to the surface or along dislocation channels. On quenching or rapid cooling, a fraction of the vacancies freeze and the preexponential term includes a contribution from those frozen vacancies [28]. The value of conductivity ln sdc is found to
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increase with temperature. The activation energy (Ed) for temperatures from 313 to 363 K is calculated from the slope of the graph between ln sdc versus 1000/T (Fig. 6) and it is found to be 0.0657 eV.
photoconductivity. This phenomenon can be attributed to generation of mobile charge carriers caused by the absorption of photons [29]. 4.2. SEM analysis of DAST crystal
4.1. Photoconductivity studies Fig. 7 shows the variation of both photocurrent and dark current with applied field. Photocurrent is observed due to the absorption of photons, leading to the creation of free charge particles in the conduction band or in valence band. Whereas, dark current is the amount of current that flows through the material when no radiation is incident on the sample. If photocurrent is greater than dark current for a given sample the phenomenon is regarded as positive photoconductivity, and the vice versa represents negative photoconductivity. It is seen from the plots that both photo and dark current of the sample increase linearly with the applied electric field. In the present study, it is observed that the photocurrent is always higher that the dark current, hence it can be concluded that DAST exhibits positive
An ordered morphology of crystal surface is an essential requirement for linear and nonlinear applications. Hence the morphologies of the thin crystal are generally investigated by
-20.90 -20.95 Edc = 0.0657 eV Linear Fit
lnσdc (mho cm-1K)
-21.00 -21.05 -21.10 -21.15 -21.20 -21.25 -21.30 -21.35 -21.40 2.6
2.8
3.0
3.2
103/T (K-1) Fig. 6. The dc conductivity plot of DAST.
Fig. 7. Variation of photo and dark currents with applied electric field.
Fig. 8. (a) SEM photograph of DAST plate with smooth surface. (b) SEM photograph showing exploded air bubbles. (c) SEM photograph showing air bubbles.
ARTICLE IN PRESS R. Jerald Vijay et al. / Journal of Crystal Growth 312 (2010) 420–425
electron and optical microscopic techniques. In the present case, the surface of the as-grown DAST crystal plate was examined by scanning electron microscope. In Fig. 8a, the surface appears to be very smooth and flat even without the help of polishing or flattening. The preparation of DAST single crystals with better surface quality is still a challenge. The smoothness of the surface and the size clearly suggest that this form of crystal can be useful for THz generation [30]. Interestingly, even in the case of DAST crystals grown with great care and optimized conditions, the formation of twins and hillocks are unavoidable [16,23]. However in the present study, we observe only one type of inclusions in the form of air bubbles, which were formed due to rapid evaporation and surprisingly the crystal is free from cracks even after undergoing rapid thermal stress and strain [31]. Fig. 8b reveals small pits created by explosion of air bubble thereby affecting the quality of the crystal. With decreased magnification (Fig. 8c), one can observe many such air bubbles and exploded bubbles on the surface of DAST plates. Efficient THz generation requires crystals with low or without imperfections such as nonflatness, inhomogeneity, misorientation and imperfect surface conditions [17]. By choosing the DAST plate whose surface is almost smooth and flat, the above requirements can be satisfied.
5. Conclusion
facilities for conductivity measurements. The authors are grateful to DST-SERC for the instrumentation facility provided at Loyola College through a project (SR/S2/LOP-03/2007). This work was partially supported by the National Science Foundation Centre for Research Excellence in Science and Technology (CREST), award number 0630388.
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The development of nearly flat DAST crystals requires thorough investigation on the growth procedures. Employing rapid evaporation of the DAST-methanol solution with slope nucleation method, thin plates of DAST crystal of dimension 4–6 3–4 0.1–1 mm3 are harvested in a period of 72 h. Our preliminary study indicates that through proper optimization of growth conditions such as concentration of the solution, temperature and growth rate, it is possible to grow good quality thin plates of DAST by a simple and cost effective method. The grown crystal was characterized by powder XRD, FTIR and NMR techniques. The ac/dc conductivity and photoconductivity studies of the sample have been carried out. The SEM analysis clearly reveals the moderately good surface of the grown crystal. Since DAST is a strategically important material and identified as the best organic THz emitter, its development by faster growth procedure is likely to encourage the direct application of the crystal for variety of applications.
Acknowledgements The authors acknowledge Dr. S. Austin Suthanthiraraj, Department of Energy, University of Madras, Chennai for providing
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Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Investigation on rapid growth of 4-N,N-dimethylamino-40 -N0 methylstilbazolium p-toluenesulphonate (DAST) crystals by SNM technique R. Jerald Vijay a, N. Melikechi b, T. Rajesh Kumar a, Joe G.M. Jesudurai a, P. Sagayaraj a,n a b
Department of Physics, Loyola College, Chennai, India Department of Physics and Pre-Engineering, Centre for Research and Education in Optical Sciences and Applications, Delaware State University, Dover, DE 19901, USA
a r t i c l e in f o
a b s t r a c t
Article history: Received 23 July 2009 Received in revised form 14 October 2009 Accepted 15 October 2009 Communicated by M. Fleck Available online 3 December 2009
We have investigated the rapid growth of N,N-dimethylamino-N0 -methylstilbazolium p-toluenesulphonate (DAST) adopting the slope nucleation method and by rapidly evaporating the solvent. Thin plates of DAST are grown within a period of 72 h by carefully optimizing the growth conditions. The structural and optical properties of the crystal are studied by employing powder XRD, FTIR and NMR. The electrical properties of the crystal are investigated by ac, dc and photoconductivity measurements. The surface features and the influence of rapid evaporation of the DAST crystal have been analyzed using scanning electron microscopy. The results suggest that the quality of the crystal grown by this method compares well with those grown by conventional techniques. & 2009 Elsevier B.V. All rights reserved.
Keywords: A1. Surface structure A2. Growth from solutions B2. Nonlinear optical materials B3. Terahertz technology
1. Introduction Recently, THz technology has been an extremely active field of research, and the development of new THz sources and detectors has been filling the THz gap [1]. This new technology has great potential to integrate the electronic and optical devices, which is expected to enable ultrahigh speed computation and communications beyond signal switching rates of 100 Gigabits/s [2]. One of the primary motivations for the development of THz sources and spectroscopy systems is the potential to extract material characteristics that are unavailable using other frequency bands [3]. Organic crystals have been a recent source of interest as THz emitters as they have been reported to generate stronger THz signals than commonly used semiconductor or inorganic electrooptic emitters owing to their large second-order nonlinear electric susceptibility. They offer vast design possibilities to tailor the linear and nonlinear properties, and owing to the almost completely electronic origin of the nonlinearity, they are well suited for future high speed devices [4]. Waveguide structuring for integrated optics with organic crystals by photo bleaching, femtosecond ablation, and ion implantation, as well as electrooptic modulation in thin organic single crystalline films and channel waveguides have been demonstrated [5,6]. Among the various classes of materials investigated worldwide, ionic organic
n
Corresponding author. Tel.: + 9144 28178200; fax: + 9144 28175566. E-mail address: [email protected] (P. Sagayaraj).
0022-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2009.10.067
crystals are of special interest due to their advantageous mechanical, chemical and thermal properties. Compared to the widely investigated poled polymers, organic single crystal are advantageous because of superior long-term thermal and photochemical stability combined with a higher chromophore concentration [5]. However, only a few of organic material could so far be crystallized in reasonable crystal size with high optical quality required for possible application. Organic crystals like N,N-dimethylamino-N0 -methylstilbazolium p-toluenesulphonate (DAST), N-bezyl-2-methyl-4-nitroaniline (BNA) and 2-methyl-4nitroanline (MNA) have very high NLO coefficients and at the same time have a low dielectric constant making them a perfect choice for THz generation [7,8]. Recent research proves that it is possible to synthesize stilbazolium derivatives such as 4-N,Ndimethylamino-40 -N0 -methylstilbazolium 2,4,6-trimethylbenzenesulfonate (DSTMS) and trans-40 -(dimethylamino)-N-phenyl-4stilbozolium hexafluorophospate (DAPSH) with very favorable crystal growth characteristics by carefully modifying the structure with various substitutions on the counter-anion and these materials are projected to be promising alternates for DAST, especially for THz generation [9,10]. Another interesting material developed by adopting the above procedure is 4-N,N-dimethylamino-40 -N0 -methyl-stilbazolium 2-napthalenesulfonate (DSNS), which showed very high nonlinear optical properties even higher than DAST; however this compound do not have the favorable growth characteristics [11]. Zhang et al. first reported THz optical rectification in DAST and confirmed a high electro-optic coefficient ( 4400 pm/V) at 820 nm. The best DAST sample provided a detected THz electric field that was 185 times larger than that
ARTICLE IN PRESS R. Jerald Vijay et al. / Journal of Crystal Growth 312 (2010) 420–425
obtained from a LiTaO3 crystal and 42 times larger than GaAs and InP crystals under the same experimental conditions [12]. Schineider et al. [13] demonstrated that the THz radiation spectrum generated and detected using DAST crystals extended from 0.4 to 6.7 THz depending on the laser excitation wavelength in the 700–1600 nm wavelength range. Though the DAST crystal is the best organic THz emitter ever studied, the growth of high optical quality DAST single crystal is still a challenge, one of the challenges is to reduce the growth time needed to obtain high optical quality DAST, which takes several weeks for crystals with dimensions exceeding 1 cm3 [14]. Faster and easier crystal growth procedure is an important challenge for future applications and therefore optimization of the growth techniques and the development of new molecules for crystal growth are subjects of present research. Different approaches have been adopted to achieve faster and improved growth rate of DAST crystals. The difficulties in growth positioning and nucleation are effectively solved by combining the slope nucleation method (SNM) with the Laser Irradiation Method (LIM) [15]. A cost effective method has been suggested by Brahadeeswaran et al. [16] by using solutions of lower supersaturation coupled with isothermal solvent evaporation and this method facilitated the development of nearly parallel (0 0 1) and (0 0 1¯) faces so as to directly utilize the crystals for EO and THz applications. For many photonic applications, a thin crystal or thin film is more attractive. The experiments conducted by Han et al. [17] proved that DAST crystals with a thickness of a few hundred micrometers are suitable for EO sampling up to few THz. It has been reported that the as-grown and very thin crystals are less sensitive to thermal shock when compared to thick DAST crystals due to large thermal gradients and these thin DAST crystals are preferable to avoid defects [18]. Recently, the as-grown nonpolished DAST crystal was used successfully for THz generation by Taniuchi et al. [19]. The rate of evaporation determines the size of crystals and the rapid evaporation favours the formation of a lot of tiny crystals [16]. Inspired with these facts, we have made an attempt to investigate the rapid growth of DAST crystals by slightly modifying the slope nucleation method and obtained reasonably good quality thin plates of DAST within 72 h. We have characterized the rapidly grown DAST crystals employing the powder XRD, FTIR, NMR, SEM, ac and dc conductivity and photoconductivity studies.
2. Experimental 2.1. Material synthesis DAST was synthesized by the condensation of 4-methyl-Nmethyl pyridinium tosylate, which was prepared from 4-picoline and methyltolunesulfonate and 4-N,N-dimethylamino-benzaldehyde in the presence of piperidine [14]. The synthesized salt appeared reddish, the typical colour of DAST. The product was separated from additives and then kept in an oven at 100 1C for 1 h to prevent absorption of water from the atmosphere. The purity of the product was further improved by successive recrystallization. 2.2. Crystal growth We investigated DAST crystal growth by slope nucleation method (SNM) and the evaporation of the solution was performed at a faster rate. The SNM is a very simple and high yielding process. In this method, a Teflon plate with grooves is inserted
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(in slope shape) into the growth solution. The tiny spontaneous nuclei, which are generated in the supersaturated solution are made to fall down onto the slope. As crystals grow larger, they slip downward along the slope until they arrive at one of the grooves. Finally, the crystals stand and then continue to grow larger on the groove [20]. This arrangement facilitates the simultaneous growth of many high-quality crystals at a single growth run. We have prepared two different DAST-methanol solutions at 45 1C according to their solubility. The first solution with a concentration of 4 g/100 mL and the second solution with a concentration of 2 g/100 mL were prepared and transferred to separate Teflon beakers. Both the solutions were stirred for 1 h at 45 1C to ensure homogeneity. After this, the Teflon plates with grooves were inserted in both the beakers. The solutions were housed in a constant temperature bath and the temperature was maintained at 45 1C for 2 days. The key factor was both the beakers were kept completely open so as to allow fast evaporation. For comparison, one more set of DAST-methanol solutions were also prepared simultaneously with same concentrations as mentioned earlier and then kept in the same constant temperature bath under the same temperature (45 1C), but the difference was that in these beakers the rate of evaporation was controlled by using a cap with a small hole in it. We followed the slope nucleation method with DAST-methanol solutions prepared with two different concentrations and then employed fast evaporation for one set and controlled slow evaporation for another set. Interestingly, we observed the growth of DAST crystals in the form of thin plates of thickness ranging from 0.1 to 1 mm within a period of 72 h, when the experiment was done with faster evaporation. Whereas, the crystals grown under the controlled evaporation process were of larger size but the growth period was nearly 15 times (45 days). Fig.1a and b show the photographs of as-grown DAST crystals that are grown by employing rapid and slow evaporation methods, respectively. It was observed that DAST crystal nucleated within few hours in the unsaturated Teflon beaker and tiny crystals were observed on the Teflon slope plate and also at the bottom of the growth vessel within a day. Since the nucleation depends on the equilibrium concentration, it is found that the metastable zone width increases with decreasing concentration and corresponding temperature [21]. Nathalie Sanz et al. have pointed out that a wide distribution of particle size generally arises from nucleation over a relatively long period of time, where young nuclei are produced simultaneously along with the growth of older nuclei. Hence, in order to overcome this, they have utilized the advantage of instantaneous nucleation caused by rapid evaporation to obtain crystals of a narrow size distribution. It was found that the rapid evaporation induces germination of high number of nuclei, triggering faster growth [22]. In our case, the excess solvent due to unsaturation was evaporated quickly, which paved the way for saturation leading to nucleation and hence formation of crystal plates. It is well known that DAST is very sensitive to even minute changes in its concentration; here, even though the concentration was reduced, the temperature was maintained high instead of maintaining equilibrium temperature in order to achieve supersaturation and the rate of evaporation causes instantaneous nucleation. The higher supersaturation is normally detrimental for the growth of twin free crystals and hence efficient control over solution supersaturation and growth rate are of vital importance for the successful growth of good quality crystals. In the present case, the solution prepared at an optimum concentration of 2 g/100 mL has resulted in controlled nucleation on the Teflon slope and the higher growth temperature triggered a faster evaporation of methanol solvent leading to the formation of thin plates of DAST. Therefore the lower concentration coupled with faster evaporation created an ideal situation for the growth of
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was found that DAST crystallized into monoclinic system. ˚ b= 11.3007 The lattice parameters are, a =10.273 70.584 A, ˚ c =18.27670.081 A, ˚ and b =91.580 70.1101 and V= 0.626 A, 2120.56070.296 A˚ 3 which are on par with earlier results [7]. The raw data was processed [24] and the recorded XRD pattern is shown in Fig. 2. Since the crystal is grown at a rapid pace, the chances of polycrystalline presence cannot be totally ruled out. 3.2. FTIR and NMR spectral analyses The FTIR spectrum recorded for the wavelength range 400– 4000 cm 1 is shown in Fig. 3. The peak at 3035.96 cm 1 is assigned to the aromatic C–H stretch. The peak at 2914 cm 1 is assigned to the alkyl C–H stretch. The peak at 1645.79 cm 1 is due to the C =C stretch. The peaks at 1584.32 and 1527.63 cm 1 are assigned to the aromatic ring vibrations. The peak at 1369.87 cm 1 is assigned to CH2 bending and C–N stretching mode. The peak at 825.51 cm 1 is assigned to the 1, 4 distributed aromatic ring. The spectrum of the rapidly grown DAST shows that the bands in the range 4000–2500 cm 1 are relatively less intense suggesting that the grown crystal is an ordered single crystal in nature [25]. The sharp peak at 3435 cm 1 is attributed to O–H stretch, which is due to the air bubble as evident from the SEM images. In the proton NMR spectrum (Fig. 4) of DAST, the singlets at 2.37 and 4.215 are assigned to three C–CH3 hydrogens and three N–CH3 hydrogens, respectively. The singlet at 3.08 is due the 6N–(CH3)2 hydrogens and the intensity of the peak justifies the number of contributing nuclei. The doublets at 6.78 and 7.6 are due to the four hydrogens of the N–(CH3)2–C6H4 aromatic ring. The doublets at 7.22 and 7.94 are due to the two aromatic hydrogens ortho to –SO3 and two aromatic hydrogens ortho to –CH3. The doublets at 7.708 and 8.49 are due the four hydrogens ortho to the C5H4N aromatic ring. The doublets at 7.1 and 7.848 are due to the two oliphinic hydrogens (HC= CH). Fig.1. (a) DAST plates grown rapidly after 72 h. (b) DAST crystals grown by slow evaporation.
these crystal plates. During the growth period the solvent was completely evaporated and after 72 h crystal plates of size 4–6 3–4 0.1–1 mm3 (Fig.1a) were harvested. Though there are reports on the growth of similar such thin plates of DAST, the growth processes adopted were complicated and also the growth period was relatively longer [16,23]. In the present work, we have demonstrated a simpler and cost effective method to grow DAST crystal plates of reasonably good quality in a very short period by careful control of concentration of the DAST-methanol solution coupled with faster evaporation. However, with beakers containing the high concentration solution (4 g/100 mL), we could only observe the formation of dendrites and needles of poor quality and this could be attributed to the multi-nucleation caused by higher supersaturation. At the same time, we have obtained large size crystals of DAST with length exceeding 1.7 cm by slope nucleation method, which was prepared under the same conditions but with prolonged period of evaporation. Since our main focus is on the rapid growth of DAST crystals, we limit the characterization to only to those grown by the rapid process.
3.3. Impedance spectral analysis Impedance spectroscopy is an analytical tool whose results like conductivity can be correlated to defects and impurities of solids [26]. The ac conductivity study using complex impedance spectroscopy is performed to characterize the bulk resistance of the crystalline material [27]. In the present case, the complex impedance parameters are measured with HB 4124 LCR meter using silver electrodes by pelletizing DAST crystal into a
3. Results and discussion 3.1. Powder X-ray diffraction The as-grown crystal was characterized by powder X-ray ˚ From the observed 2y values it diffractometry at 1.5406 A.
Fig. 2. Powder XRD pattern of DAST.
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Fig. 3. FTIR spectrum of DAST.
5x105
Z '' (Ω)
4x105
3x105
2x105
1x105
0.0
2.0x105 4.0x105 6.0x105 8.0x105 1.0x106 1.2x106 Z ' ( Ω) Fig. 5. Complex impedance plot of DAST.
Fig. 4. NMR spectrum of DAST.
rectangular specimen of thickness of 0.115 cm and area 0.5024 cm2. The ac conductivity of the sample is determined from the real part of the impedance using the relation
sac ¼ t=Rb A where t is the thickness, A is the area of face in contact with the electrode and Rb is the bulk resistance. The bulk resistance was found to be 1.928 105 O. The calculated ac conductivity was 11.87 10 9 (Om) 1. The plot of Z0 and Z00 for DAST crystal at room temperature is shown in Fig. 5 and the obtained impedance exhibits a good semicircle. The observed value is typical for an insulating material. Interestingly, the observed low ac conductivity suggests that the number of defects or impurities present in the rapidly grown DAST crystal is low. 4. dc Conductivity The dc electrical conductivity measurements were carried out for the DAST crystal using the conventional two-probe
technique in the temperature range 313–363 K. The dc electrical conductivity (sdc) of the crystal was calculated using the relation.
sdc ¼ t=RA where R is the measured resistance, t is the thickness of the sample and A is the area of face in contact with the electrode. The sdc values were fitted into the equation:
sdc ¼ s0 expðEd =kTÞ Electrical conductivity depends on thermal treatment of a crystal. Thus the conductivity at low temperatures depends on the cooling speed from melting point temperature to room temperature. Thus for slow cooling, the remaking of the lattice can occur by the migration of interstitials to vacancies, recombination of Schottky defects or migration of vacancies to the surface or along dislocation channels. On quenching or rapid cooling, a fraction of the vacancies freeze and the preexponential term includes a contribution from those frozen vacancies [28]. The value of conductivity ln sdc is found to
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increase with temperature. The activation energy (Ed) for temperatures from 313 to 363 K is calculated from the slope of the graph between ln sdc versus 1000/T (Fig. 6) and it is found to be 0.0657 eV.
photoconductivity. This phenomenon can be attributed to generation of mobile charge carriers caused by the absorption of photons [29]. 4.2. SEM analysis of DAST crystal
4.1. Photoconductivity studies Fig. 7 shows the variation of both photocurrent and dark current with applied field. Photocurrent is observed due to the absorption of photons, leading to the creation of free charge particles in the conduction band or in valence band. Whereas, dark current is the amount of current that flows through the material when no radiation is incident on the sample. If photocurrent is greater than dark current for a given sample the phenomenon is regarded as positive photoconductivity, and the vice versa represents negative photoconductivity. It is seen from the plots that both photo and dark current of the sample increase linearly with the applied electric field. In the present study, it is observed that the photocurrent is always higher that the dark current, hence it can be concluded that DAST exhibits positive
An ordered morphology of crystal surface is an essential requirement for linear and nonlinear applications. Hence the morphologies of the thin crystal are generally investigated by
-20.90 -20.95 Edc = 0.0657 eV Linear Fit
lnσdc (mho cm-1K)
-21.00 -21.05 -21.10 -21.15 -21.20 -21.25 -21.30 -21.35 -21.40 2.6
2.8
3.0
3.2
103/T (K-1) Fig. 6. The dc conductivity plot of DAST.
Fig. 7. Variation of photo and dark currents with applied electric field.
Fig. 8. (a) SEM photograph of DAST plate with smooth surface. (b) SEM photograph showing exploded air bubbles. (c) SEM photograph showing air bubbles.
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electron and optical microscopic techniques. In the present case, the surface of the as-grown DAST crystal plate was examined by scanning electron microscope. In Fig. 8a, the surface appears to be very smooth and flat even without the help of polishing or flattening. The preparation of DAST single crystals with better surface quality is still a challenge. The smoothness of the surface and the size clearly suggest that this form of crystal can be useful for THz generation [30]. Interestingly, even in the case of DAST crystals grown with great care and optimized conditions, the formation of twins and hillocks are unavoidable [16,23]. However in the present study, we observe only one type of inclusions in the form of air bubbles, which were formed due to rapid evaporation and surprisingly the crystal is free from cracks even after undergoing rapid thermal stress and strain [31]. Fig. 8b reveals small pits created by explosion of air bubble thereby affecting the quality of the crystal. With decreased magnification (Fig. 8c), one can observe many such air bubbles and exploded bubbles on the surface of DAST plates. Efficient THz generation requires crystals with low or without imperfections such as nonflatness, inhomogeneity, misorientation and imperfect surface conditions [17]. By choosing the DAST plate whose surface is almost smooth and flat, the above requirements can be satisfied.
5. Conclusion
facilities for conductivity measurements. The authors are grateful to DST-SERC for the instrumentation facility provided at Loyola College through a project (SR/S2/LOP-03/2007). This work was partially supported by the National Science Foundation Centre for Research Excellence in Science and Technology (CREST), award number 0630388.
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The development of nearly flat DAST crystals requires thorough investigation on the growth procedures. Employing rapid evaporation of the DAST-methanol solution with slope nucleation method, thin plates of DAST crystal of dimension 4–6 3–4 0.1–1 mm3 are harvested in a period of 72 h. Our preliminary study indicates that through proper optimization of growth conditions such as concentration of the solution, temperature and growth rate, it is possible to grow good quality thin plates of DAST by a simple and cost effective method. The grown crystal was characterized by powder XRD, FTIR and NMR techniques. The ac/dc conductivity and photoconductivity studies of the sample have been carried out. The SEM analysis clearly reveals the moderately good surface of the grown crystal. Since DAST is a strategically important material and identified as the best organic THz emitter, its development by faster growth procedure is likely to encourage the direct application of the crystal for variety of applications.
Acknowledgements The authors acknowledge Dr. S. Austin Suthanthiraraj, Department of Energy, University of Madras, Chennai for providing
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