Article pubs.acs.org/crystal
Growth of Pentacene-Doped p‑Terphenyl Crystals by Vertical Bridgman Technique and Doping Effect on Their Characterization Qing Ai,†,‡ Peifeng Chen,† Yuxiang Feng,† and Yebin Xu*,† †
School of Optical and Electronic Information, Huazhong University of Science and Technology, Luoyu Road No. 1037, Wuhan, Hubei, 430074, People’s Republic of China ‡ School of Electrical and Electronic Information Engineering, Hubei Polytechnic University, North Guilin Road No. 16, Huangshi, Hubei, 435003, People’s Republic of China ABSTRACT: High quality pentacene-doped p-terphenyl crystals were successfully grown by the modified vertical Bridgman technique with a specially designed double-walled ampule. Upon the dopant addition, the growth crystal exhibits a color change from colorless to purple due to the guest-induced absorption changes. Powder X-ray diffraction, Fourier transform infrared (FTIR), 1H nuclear magnetic resonance (NMR), and fluorescence spectra studies of the doped crystal were carried out and compared with those of a pure p-terphenyl crystal. The decrease in both crystallinity and fluorescent intensity of the doped crystal clearly indicates that the guest molecules appear as defects in the form of irregularly oriented molecules which do not significantly distort the crystal structures. No significant changes on FTIR and 1H NMR spectral features of the doped crystal in relation to the corresponding features observed for the single crystal shows that the crystal doping in such small amounts does not lead to noticeable changes of the intermolecular interaction energy. for reducing crystallographic defects in crystals.15−17 Compared with solution growth and vapor growth, the melt growth technique is a more convenient and fast method for growing bulk crystals.17 The vertical Bridgman technique is the simple and best melt method for the growth of good quality bulk organic crystals with a limited period.18 Selvakumar has grown single crystals of p-terphenyl by using a modified method of selective self-seeding vertical Bridgman technique.19 For the purpose of preparing pentacene-doped p-terphenyl crystals used as the gain medium for the MASER, the present study focuses on the improvement of crystal quality with a specially designed ampule and presents the crystal growth aspects using the modified vertical Bridgman technique. We report results on powder X-ray diffraction (XRD), FTIR, 1H NMR, and fluorescence spectra studies of pentacene-doped p-terphenyl crystal in order to compare them with those of pure p-terphenyl crystal, looking for possible doping effects upon molecular conformational changes in the crystal, which greatly influence the excitonic properties.
1. INTRODUCTION Organic scintillators have vital applications in multiple areas because of their molecular nature of luminescence, including particle counting, nuclear spectroscopy, and radiation detections.1 A p-terphenyl crystal scintillator is commonly used as a particle identifier for measuring the neutron energy spectrum due to its fast timing response and high quantum yield.2 Recently, a bulk mixed organic molecular crystal, p-terphenyl doped with pentacene, has been used as the gain medium for a MASER.3 The MASER, the microwave analogue of the laser, has considerable applied value in the fields of astronomy observation,4 deep-space communication,5 high-precision clocks,6 and electron paramagnetic resonance spectroscopy.7 However, these applications are limited by the demanding operational conditions, such as vacuum pumps,8 cryogenic refrigeration,9 strong magnets,10 and magnetic shielding.11 The recent discovery of the MASER based on a pentacene-doped pterphenyl crystal can operate at room temperature without the need for intense magnetic fields, which constituted an exciting step forward toward reducing the operating requirements.3 As a well-known organic scintillator, p-terphenyl and its doped compounds have been extensively investigated.2,12,13 However, little is known about either the intrinsic nature of the molecular conformational changes or the way it is affected by the doping. The use of pentacene-doped p-terphenyl crystals as a gain medium calls for more details on their nature. Heretofore, the difficulties still remain in the growth of organic crystals with sufficient quality in practice.14 In recent decades, various kinds of growth methods have been developed © 2017 American Chemical Society
2. EXPERIMENTAL SECTION 2.1. Ampule Design for Crystal Growth. A double-walled ampule having the conical end with a narrow opening at the lower end of the inner wall has been extensively used for organic crystal growth in previous studies.18,19 The special design of the bottom of the inner wall can restrain the inferior formation from entering into the inner Received: December 28, 2016 Revised: March 24, 2017 Published: April 10, 2017 2473
DOI: 10.1021/acs.cgd.6b01900 Cryst. Growth Des. 2017, 17, 2473−2477
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tube from the interlayer to ensure that there is only one good quality nucleation in the inner tube at the beginning of the growth period.20 Moreover, material and vacuum in the interlayer act as thermal insulators during and after growth. Hence, the double-walled ampule can prevent thermal fluctuations to ensure superior quality crystal.21 After several experimental tests and modifications, we finalized the design of an improved double-walled ampule for pentacene-doped pterphenyl crystal growth. The improved quartz ampule is shown in Figure 1. One improved design is to increase the diameter of the pipe
Figure 2. Axial temperature profiles of the furnace for pentacenedoped p-terphenyl crystal growth and annealing. The material melted in the high temperature zone and stayed here for 24 h. Accordingly, the material was fully homogenized and the formation of bubbles would be avoided during crystal growth. The translation of the melted substance from the upper zone to the lower zone allows directional freezing of the substance from the bottom to the top of the growth vessel. Due to the low thermal conductivity of organic substances, the growth of high quality crystal can only adopt low growth rates.22 A rate of 1−2 mm/h has been recommended as an upper limit for growing organic crystal.23 The present study concludes that the best descending rate was 1−0.5 mm/ h according to the furnace axial temperature gradient of 2 °C/cm. The initial growth was begun with a translation rate of 1 mm/h, and then a small number of nuclei formed in the interlayer of the ampule. After nearly 5 mm growth, the fast-growing grains attained a larger size and one good quality nucleation entered into the narrow tapered region through the capillary tube. While the crystallization front moves from the narrow part to the wider part of the ampule, the rate was decreased to 0.5 mm/h gradually to maintain a flat interface. It was kept at 0.5 mm/h during the rest of the growth period. After the growth, the temperature curve of the furnace was slowly adjusted with a rate of 2 °C/h to fit the annealing profile, as shown in Figure 2. Thus, the temperature of the bottom and top of the ampule tended to be uniform, and the thermal stress caused by temperature difference decreased. Then, the temperature was slowly reduced to the ambient temperature at a rate of 2 °C/h, in order to avoid the crystal cracking caused by the different thermal expansion coefficients of the glass and crystal. The grown crystal was carefully extracted from the ampule in a time span of 15 days. The as-grown crystal was trimmed and then polished. The light purple transparent crystal boule was 11 mm at the largest diameter and 44 mm long, as shown in Figure 3. The uneven color of the crystal shows that the distribution of pentacene in the crystal is not uniform due to the effects of crystallization and gravity, and the tiny yellow material at the bottom is the collection of heavier particle impurities. As a contrast, a typical colorless transparent p-terphenyl single crystal grown by the vertical Bridgman technique is presented in Figure 4. Upon the dopant addition, the growth crystal exhibits a color change from colorless to light purple due to the guest-induced absorption changes. This color change indicates that the host lattice undergoes a conformational change. Since trace level pentacene is soluble in the crystal, the distortion of the host lattice caused by doping is quite small, as in co-crystallization.24 The extent of the guest-induced color change of the crystal depends on the shape, size, number, and position of the functional group of the guest molecules. 2.3. Characterization Techniques. Powder X-ray diffraction was carried out to demonstrate the crystallinity of the grown crystals. The grown ingots were finely crushed and subjected to powder XRD
Figure 1. Schematic of the improved double-walled ampule. at the top of the ampule. In this case, the material can be loaded into the ampule with the help of a long neck funnel. The material falls into the bottom of the ampule along the funnel and will not adhere to the pipe wall. A vacuum is needed for the Bridgman growth of organic crystals, so as to avoid the oxidation of organic materials at high temperatures. The pipe is sealed with a solid quartz cylinder by using an oxygen-acetylene flame after vacuum pumping. In addition, the cylinder can prevent the very few oxides and decomposition products generated by high temperature flame sealing from falling into the inner tube. Another optimal design is to add a small hole in the upper part of the inner tube. This hole is used for helping to extract air out of the interlayer so that the oxidation of the material can be avoided at high temperature during growth. The final way to optimize the design is to confirm that the appropriate gap width is 1 mm. There is a considerable amount of liquid flowing into the interlayer after material melting. It will drastically reduce the size of the grown crystal in the inner tube. Through multiple experiments, we believe that this width can not only ensure the isolation effect of the interlayer but also make the size of the grown crystal as large as possible. 2.2. Growth of Pentacene-Doped p-Terphenyl Crystals. In the study, pentacene-doped p-terphenyl crystals were grown in the improved ampule using a two-zone vertical Bridgman furnace. The ampule was washed with detergent, acetone, and deionized water by ultrasonic cleaning and then dried at 120 °C for 20 h. The commercially available p-terphenyl and pentacene (0.01 wt %) mixed powder was charged into the ampule. Then, the ampule was evacuated to 10−5 Pa and sealed carefully. The temperature profile for the growth is shown in Figure 2. The temperature in the upper part of the furnace was maintained at 213−220 °C, which was 0−7 °C above the melting point (213 °C) of p-terphenyl. Trace amounts of pentacene were soluble in hot p-terphenyl liquid at this temperature. 2474
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Figure 3. Image of a Bridgman-grown pentacene-doped p-terphenyl crystal.
Figure 5. Powder X-ray diffraction patterns of (a) p-terphenyl single crystal and (b) pentacene-doped p-terphenyl crystal.
Table 1. Lattice Parameters of Pure and Pentacene-Doped pTerphenyl Crystals parameters
single crystal
doped crystal
ICDD data
a (Å) b (Å) c (Å) β (deg) V (Å3)
8.112(3) 5.616(2) 13.608(4) 92.05(3) 619.63(21)
8.089(2) 5.606(1) 13.610(3) 91.88(3) 616.92(16)
8.106 5.613 13.613 92.02 618.99
Figure 4. Image of a Bridgman-grown p-terphenyl single crystal.
the dopant. The guest molecules are randomly filled in the defects or vacancies of the crystal lattice,25 and the doping effect is not strong enough to disorder the domain structure. Powder X-ray diffraction can be used to evaluate peak broadening with crystallite size and lattice strain due to dislocation. The crystallite sizes of the crystals have been estimated by the X-ray line broadening method with the Scherrer equation
analysis at room temperature by a PANalytical Empyrean X-ray diffractometer with Cu Kα (λ = 1.54059 Å) radiation in a 2θ range of 10−50° using a tube voltage and current of 40 kV and 40 mA, respectively. The fluorescent properties have been assessed by fluorescence spectrum study. Fluorescence emission was carried out by using a Jasco FP-6500 luminescence spectrometer with the excitation wavelength of 270 nm at room temperature. The FTIR spectrum was recorded by a Bruker VERTEX 70 FTIR spectrum spectrometer using a KBr pelleting technique in the wavenumber range of 400−4000 cm−1 to identify the presence of functional groups in the grown crystals. 1H NMR analysis was carried out to further confirm the grown compound. The samples have been powdered from the grown ingots and were dissolved in chloroform-d (CHCl3). The NMR spectrum was recorded with a Bruker AscendTM 600 MHZ nuclear magnetic resonance spectrometer.
D=
kλ β cos θ
(1)
where D is the average crystallite size, β is the full width at halfmaximum, θ is the Bragg angle, λ is the X-ray wavelength, and k is the shape factor, normally taken as 0.89. The crystallite sizes of two crystals are given in Table 2. The change in crystallite size corresponding to six major planes clearly indicates the decrease in crystallinity of the doped crystal. It is attributed to that the trace amounts of pentacene in the doped crystal act as impurities in the crystal lattice and make crystallographic defects increase. 3.2. Fluorescence Spectrum Studies. Fluorescence spectra of pure and pentacene-doped p-terphenyl crystals are displayed in Figure 6. The single crystal spectrum extends from 290 to 520 nm with two peaks at 373 and 392 nm, while that of the doped crystal shows a prominent peak at 372 nm with a shoulder at 393 nm.
3. RESULTS AND DISCUSSION 3.1. Powder X-ray Diffraction Analyses. The powder Xray diffraction patterns of pure and pentacene-doped pterphenyl crystals are shown in Figure 5. The lattice parameters and the indices of the reflection planes were calculated and compared with ICDD file 39-1727 through MDI Jade software. The calculated lattice parameters and ICDD data are given in Table 1. The result indicates that two crystals both are of the monoclinic system and space group P21/a. No additional peaks are observed in the diffraction patterns of pure and doped crystals, which confirm that there is no change in phase due to 2475
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Table 2. Change in Crystallite Size Corresponding to Different Planes of Pentacene-Doped p-Terphenyl Crystal single crystal
doped crystal
crystal planes
2θ (deg)
β (deg)
D (nm)
2θ (deg)
β (deg)
D (nm)
% change in crystallite
002 110 003 111 201 211
13.065 19.201 19.604 20.384 23.139 28.066
0.250 0.338 0.269 0.326 0.276 0.252
31.630 23.573 29.638 24.485 29.055 32.135
13.097 19.284 19.614 20.499 23.173 28.235
0.352 0.341 0.430 0.411 0.331 0.422
22.465 23.369 18.541 19.425 24.229 19.196
28.97 0.87 37.44 20.67 16.61 40.26
Figure 6. Fluorescence spectra of pure and pentacene-doped pterphenyl crystals.
Fluorescence of these organic molecular crystals is mainly attributed to the individual molecules.26 However, defects in the crystal lattice can broaden the different scintillation components and make them indistinguishable.27 The spectra clearly indicate that the fluorescent intensity of the doped crystal is much lower than that of the single crystal. This may be due to that the guest molecules act as entrapped impurities in the crystal lattice. 3.3. FTIR and Transmission Spectral Analyses. Figure 7a displays the optical transmission spectrum of a p-terphenyl single crystal. A peak appearing at 3032.2 cm−1 is assigned to C−H stretch of the aromatic ring. Peaks between 2000 and 1666 cm−1 are all due to overtone and combination bands. The ring skeletal vibrations are observed at 1575.1, 1478.7, 1453.6, and 1402.4 cm−1, respectively. The absorption peaks below 1300 cm−1 are owing to in-plane and out-of-plane wagging vibrations. The C−H bending mode of the middle phenyl ring appears at 837.6 cm−1. The sharp peaks at 745.4 and 685.9 cm−1 predict C−H bending modes of the terminal rings. As shown in Figure 7b, the FTIR spectrum of the doped crystal is similar to that of the single crystal. The unobserved changes in the spectral features from the single crystal to the doped crystal mean that the corresponding functional groups are not sensitive to the low impurity content. These unobserved changes have been interpreted as proving that the very low concentration of impurities does not essentially influence the parameters of the vibrational spectrum.28 3.4. 1H NMR Spectrum Studies. The 1H NMR spectrum of a p-terphenyl single crystal is presented in Figure 8a. The intense singlet at 7.699 ppm represents the four protons in the center ring. The doublet at 7.672 ppm is due to the protons at
Figure 7. FTIR spectra of (a) p-terphenyl single crystal and (b) pentacene-doped p-terphenyl crystal.
the ortho-position of the terminal rings. The triplet at 7.480 ppm is owing to the protons at the meta-position of the terminal rings. The next triplet at 7.382 ppm is assigned to the protons at the para-position of the terminal ring. The four signals with peak integrals nearly in the ratio 2:2:2:1 confirm the four different types of aromatic protons present in pterphenyl. The 1H NMR spectrum of the p-terphenyl single crystal rejects the inclusion of any impurity in the crystal lattice. The 1H NMR spectrum of the doped crystal is almost identical to that of the single crystal, as shown in Figure 8b. The difference is that the chemical shift of each peak has a slight deviation. There is also no other signal in the spectrum except for the p-terphenyl signal due to such small amounts of pentacene in the doped compound.
4. CONCLUSION A light purple transparent pentacene-doped p-terpheny crystal was grown using the vertical Bridgman technique. A novel modified double-walled growth ampule is proposed. Thermal defects and impurities can be effectively reduced in the process of crystal growth by using this type of ampule. With the addition of the dopant, the growth crystal exhibits a color change from colorless to light purple due to the guest-induced absorption changes. The decrease in crystallinity of the doped crystal clearly indicates that the guest molecules appear as 2476
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Figure 8. 1H NMR spectra of (a) p-terphenyl single crystal and (b) pentacene-doped p-terpheny crystal.
defects in the form of irregularly oriented molecules which do not significantly distort the monoclinic crystal structures. The dramatic decrease in fluorescent intensity clearly suggests that the guest molecules appear as impurities in the crystal lattice, resulting in poor fluorescence properties of the doped crystal. The foregoing results on FTIR and 1H NMR spectral features of the doped crystal in relation to the corresponding features observed for the single crystal show that the crystal doping in such small amounts does not lead to noticeable changes of the intermolecular interaction energy.
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AUTHOR INFORMATION
Corresponding Author
*Tel: 86-027-87543855. Fax: 86-027-87544160. E-mail:
[email protected]. ORCID
Qing Ai: 0000-0002-6872-9848 Yebin Xu: 0000-0001-9388-4033 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China under grant numbers 61671214 and 61401152. The authors wish to acknowledge the Analytical and Testing Center in Huazhong University of Science and Technology for XRD, FTIR, NMR, and fluorescence spectra analysis.
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REFERENCES
(1) Vijayan, N.; Bhagavannarayana, G.; Maurya, K. K.; Haranath, D.; Rathi, B.; Balamurugan, N.; Sharma, Y. K.; Ramasamy, P. Mater. Chem. Phys. 2012, 132, 453−457. (2) Selvakumar, S.; Arulchakkaravarthi, A.; Kumar, R.; Muralithar, S.; Sankar, S.; Sivaji, K. Solid State Commun. 2008, 145, 482−486. 2477
DOI: 10.1021/acs.cgd.6b01900 Cryst. Growth Des. 2017, 17, 2473−2477
