R. Jerald Vijay et al. Journal of Crystal Growth 312 (2010) 420–425

Investigation on rapid growth of 4-N, N-dimethylamino-4’N’-methylstilbazolium

p-toluenesulphonate (DAST) crystals by SNM technique

R. Jerald Vijay a, N. Melikechi

b, T. Rajesh Kumar

a, Joe G. M. Jesudurai and

P. Sagayaraj a*

a Department of Physics, Loyola College, Chennai, India

b Department of Physics and Pre –Engineering,

Centre for Research and Education

in Optical Sciences and Applications, Delaware State University,

Dover DE 19901, US

Abstract

We have investigated the rapid growth of N, N-dimethylamino-N’-

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 hours 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.

Keywords: A1. Surface structure, A2. Growth from solutions, B1. Nonlinear optical

materials, B3. Terahertz technology

PACS: 68.37.Hk, 81.10.Dn, 42.70.Nq, 87.50.U

*Corresponding author

Dr. P. Sagayaraj, Reader in Physics, Loyola College, Chennai – 600 034, India

Email: [email protected], Tel: +9144 28178200; Fax: +9144 28175566

*4. Manuscript

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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/sec [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 electro optic 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 electro-optic 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

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 photo-

chemical 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-N’-methylstilbazolium p-toluenesulphonate (DAST), N-Bezyl-2-

methyl-4-nitroaniline (BNA) and 2-methyl-4-nitroanline (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,N-dimethylamino-4’-N’- methyl-

stilbazolium 2,4,6-trimethylbenzenesulfonate (DSTMS) and trans-4’–

(dimethylamino)-N-Phenyl-4-stilbozolium 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- 4’-N’- 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 (>400 pm/V) at 820nm. The best DAST sample provided a detected THz

electric field that was 185 times larger than that obtained from a LiTaO3 crystal and

42 times larger than GaAs and InP crystals under the same experimental conditions

[12]. Schineider et al demonstrated that the THz radiation spectrum generated and

detected using DAST crystals extended from 0.4 THz to 6.7 THz depending on the

laser excitation wavelength in the 700 nm to 1,600 nm wavelength range [13].

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 by

using solutions of lower super saturation coupled with isothermal solvent evaporation

and this method facilitated the development of nearly parallel (001) and ( ) faces

so as to directly utilize the crystals for EO and THz applications [16].

For many photonic applications, a thin crystal or thin film is more attractive.

The experiments conducted by Han et al proved that DAST crystals with a thickness

of a few hundred micrometers are suitable for EO sampling up to few THz [17]. 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 non-

polished 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 hours. 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-N-methyl 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 0C for 1 hour 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 (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 450C according to their solubility. The first solution with a concentration

of 4 g/100mL and the second solution with a concentration of 2 g/100mL were

prepared and transferred to separate Teflon beakers. Both the solutions were stirred

for 1 h at 450C 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 450C for two 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 (450C), 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 0.1mm to 1mm with in a period of 72 hours, 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 1b show the photographs of as grown DAST crystals which 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 2g/100mL 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 these crystal plates. During the growth period the solvent

was completely evaporated and after 72 hours crystal plates of size

4-6x3-4x0.1-1mm3 (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 (4g/100mL), 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. Results and Discussion

3.1 Powder X-ray Diffraction

The as grown crystal was characterized by powder X-ray diffractometry at

1.5406 Å. From the observed 2θ values it was found that DAST crystallized into

monoclinic system. The lattice parameters are, a=10.273±0.584 Å, b=11.300±0.626

Å, c=18.276±0.081 Å, and β = 91.580±0.110 o and V=2120.560± 0.296 Å3 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 FT-IR and NMR spectral analyses

The FTIR spectrum recorded for the wavelength range 400 to 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 to 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 6 N-(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).

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 in to a 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

σac = t/RbA

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 x 105 Ω. The calculated

ac conductivity was 11.87 x 10-9 (Ωm)-1. The plot of Z′ and Z″ 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.

3.4 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 (σdc) of the crystal was calculated using the

relation.

σdc= 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 σdc values were fitted into the

equation

σdc = σo exp (-Ed/kT)

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 cooing, 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 pre-exponential

term includes a contribution from those frozen vacancies [28]. The value of

conductivity ln σdc is found to increase with temperature. The activation energy (Ed)

for temperatures from 313 to 363 K is calculated from the slope of the graph between

ln σdc versus 1000/T (Fig. 6) and it is found to be 0.0657 eV.

3.5 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 photoconductivity.

This phenomenon can be attributed to generation of mobile charge carriers caused by

the absorption of photons [29].

3.6 SEM analysis of DAST crystal

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 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.

4. Conclusion

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

4-6x3-4x0.1-1mm3 are harvested in a period of 72 hours. 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 photo conductivity 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.

Acknowledgement

The authors acknowledge Dr. S. Austin Suthanthiraraj, Department of Energy,

University of Madras, Chennai for providing 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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Figure captions

Fig.1a. DAST plates grown rapidly after 72 hours

Fig.1b. DAST crystals grown by slow evaporation

Fig.2. Powder XRD pattern of DAST

Fig.3. FTIR spectrum of DAST

Fig.4. NMR spectrum of DAST

Fig.5. Complex impedance plot of DAST

Fig.6. The dc conductivity plot of DAST

Fig.7. Variation of Photo and dark currents with applied electric field

Fig.8a. SEM photograph of DAST plate with smooth surface

Fig.8b. SEM photograph showing exploded air bubbles

Fig.8c. SEM photograph showing air bubbles

Fig.1a. DAST plates grown rapidly after 72 hours

Fig.1b. DAST crystals grown by slow evaporation

Fig.2. Powder XRD pattern of DAST crystal

Fig.3. FTIR spectrum of DAST

Fig.4. NMR spectrum of DAST

0.0 2.0x105

4.0x105

6.0x105

8.0x105

1.0x106

1.2x106

1x105

2x105

3x105

4x105

5x105

Z '' (

W)

Z ' ( W )

Fig.5. Complex impedance plot of DAST

2.6 2.8 3.0 3.2-21.40

-21.35

-21.30

-21.25

-21.20

-21.15

-21.10

-21.05

-21.00

-20.95

-20.90

Edc

= 0.0657 eV

Linear Fit

lns

dc (

mh

o c

m-1

K)

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.8a. SEM photograph of DAST plate with smooth surface

Fig.8b. SEM photograph showing exploded air bubbles

Fig.8c. SEM photograph showing air bubbles