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Size dependent optical properties of quinacridonequinone nanoparticles prepared by liquid laser ablation in water
Chemical Physics Letters 552 (2012) 102–107
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Size dependent optical properties of quinacridonequinone nanoparticles prepared by liquid laser ablation in water Ikuko Akimoto ⇑, Masahiro Ohata, Nobuhiko Ozaki, Ping Gu Faculty of Systems Engineering, Wakayama University, Japan Faculty of Education, Wakayama University, Japan
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Article history: Received 1 April 2012 In final form 20 September 2012 Available online 2 October 2012
a b s t r a c t We investigated optical properties of colloidal solutions of nanoparticles prepared by laser ablation of quinacridonequinone powder in a poor solvent, water. Blue shift of the absorption peak energy was observed corresponding to the increase of laser fluence or to the prolonged irradiation time. For a series of colloidal solutions, we found a linear correlation between the blue shift of the absorption peak energy and the decrease in the diameter of the nanoparticles. The blue shift is considered to be due to the surface state, such as surface excitons or bond modified surface states. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Organic materials have been utilized in the production of optical and electronic devices in recent years [1–7]. However, applications of organic materials have been limited partially because of the high-cost for mass production. The fabrication of devices such as organic light emitting diodes (OLED) and organic field effect transistors (OFET) requires high-cost vacuum sublimation processes for small organic molecules. An alternative way of the process is a wet-process, which has been used for solvable polymer materials [8,9]. Since organic small molecules are difficult to be solved without an addition of hydrophilic moieties on a molecule or any surfactants, the wet-process has not been applied for systems of small organic molecules. Even if they are soluble, environmental load substances and/or toxic ones such as toluene or chloroform are used. If aqueous solutions of simple small organic molecules could be obtained, various new applications of organic molecules would be expected: for example, wet-process device fabrication by application of an ink-print technique, a drag delivery system, and cosmetic application. Modification of the size from micro-crystals to nanoparticles is a promising technique for the preparation of dispersions of small organic materials. Nanoparticles, generally, easily disperse in various solvents including ubiquitous solvents such as water, and the dispersion is maintained for long time by Brownian motion in a solvent. Such a dispersed system of organic nanoparticles is also worth giving a possible stage for the realization of nanoparticle manipulation by resonant radiative
⇑ Corresponding author. Address: Wakayama University Japan, Sakaedani 930, Wakayama 640-8510, Japan. Fax: +81 73 457 8272. E-mail address: [email protected] (I. Akimoto). 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2012.09.048
force with laser beam which was predicted in the reference [10,11]. There are several methods to form nanoparticles, which are classified to build-up or break-down method. As a build-up method, the re-precipitation technique has been known [12]. On the other hand, as a break-down method, pulsed laser ablation in poor solvent has been developed by Masuhara and co-workers [13–24]. Up to now, the validity of the laser ablation has been demonstrated by several groups for various organic systems, such as poly-diacetylene (poly-DCHD) [15], vanadyl phthalocyanine (VOPc) [16–18], quinacridone (QA) [20–22], dendronized perylenediimide (D-PDI) [23,24], pentacene [25], and fullerene [26], which possess enough tolerance for laser power. The laser ablation converts microcrystals of target materials into nanoparticles in poor solvent with laser pulses over the threshold fluence which is settled for each molecule and each absorption band. The size of the formed nanoparticles can be controlled by ablation conditions, such as fluence, wavelength, duration of laser pulse and a type of poor solvent. The colloidal solution is usually stable without any surfactants, except the case of the relatively large molecule, for example, D-PDI [23,24] in which a surfactant was added to stabilize the solution before the laser ablation. Fragmentation mechanism has been considered as follows [16,20,25]: The photo-thermal conversion causes an explosive temperature increase on the surface layers of the target micro-crystals within a penetration depth at the irradiated laser wavelength. When the temperature on the surface layers increases higher than the threshold, nanoparticles eject from the surface and/or micro-crystals fragment into nanoparticles. The cooling process takes place competitively due to thermal diffusion in the solvent. There still remains argument about which mechanism governs the size of ejected particles under higher laser fluences, that is, a higher fluence only gives a large fragmentation rate of
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the same sized nanoparticles assisted by the cooling process as described in the case of VOPc [16] or a higher fluence gives smaller nanoparticles by further fragmentation of hotter particles as described in the case of QA [20] and pentacene [25]. The formation of the nanoparticles has been monitored by UV– VIS absorption spectrum of the colloidal solution. Besides the significant increase of absorbance after laser ablation, the spectral shift depending on nanoparticle size was reported in poly-diacetylene (poly-DCHD) [15], dendronized perylenediimide(D-PDI) [23], and pentacene [25]. The reason of the spectral shift has been discussed individually. The blue shift of absorption peak energy on smaller nanoparticles was attributed to the reduction of effective conjugated length of p-electron due to higher concentration of structural defects in poly-DCHD [15] or to the light scattering effect explained by Mie theory in D-PDI [25]. On the other hand, in QA [20], no blue shift of absorption spectrum was observed, although the particle size became smaller by higher fluences. Instead, significant spectral change was observed due to the change of crystal phase b or c-form by selecting the irradiation wavelength, where QA is known to have polymorphism in a, b, or cforms. Thus, optical properties of colloidal solutions prepared by the liquid laser ablation still need interpretations of case by case, therefore, further collection of successful samples is necessary to understand the optical properties of colloidal solutions. In this Letter, we investigated the liquid laser ablation for yellow pigment quinacridonequinone (QQ) in a poor solvent, water and clarified the size dependent optical properties of the prepared colloidal solutions. The formed colloidal solutions of QQ nanoparticles accompanied with remarkable blue shift of the absorption peak. The analysis of particle diameters by dynamic light scattering (DLS) method in the same colloidal solutions indicates that the blue shift corresponds to the decrease of particle size. The blue shift is considered to be due to the surface state, such as surface excitons or bond modified surface states. The linear correlation between the magnitude of the blue shift and the mode diameter provides us a simple method to estimate the most frequent nanoparticle diameter in colloidal solution from the absorption spectrum in the limited diameter range. 2. Experiment Laser ablation of yellow pigment quinacridonequinone (QQ, Aldrich) was performed; molecular structure is shown in Figure 1. The QQ is a derivative of well-known red pigment quinacridone (QA) and a simple material to observe the absorption spectrum because of its monomorphism. Starting powder was put in the distilled water making dispersed mixture at a concentration of 5 10 5 mol/l. Although we used the water which was degassed by nitrogen bubbling for 30 min, any significant differences were not observed in ablation results between degassed and pristine waters in our preliminary experiments. The mixtures were sonicated for 30 min in capped bottle, so that a little amount of material was solved and water color was changed to pale yellow.
Figure 1. Molecular structure, quinacridonequinone (QQ).
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However, most of powders remained as precipitations. The isotropic mixture was quickly divided into each capped quartz cuvette of 1 1 5 cm3 with a tiny Teflon coated stirring bar. The microcrystals in water stirred by a magnetic stirrer were irradiated by 5 ns laser pulses from a tunable optical parametric oscillator excited by Q-switched YAG laser operated in 10 Hz repetition rate (Spectra Physics, MOPO). The wavelength of irradiated laser pulse was selected at 430 nm, which corresponds to the lowest absorption band of a QQ molecule. UV–VIS absorption spectrum was measured at room temperature with a conventional system (JASCO, V-560) in a few tens of minutes after the ablation. Colloidal solutions was prepared for different serial of the irradiation time up to 40 min at a fixed laser fluence of 11 mJ/cm2, which is almost on the threshold in the case of the QA [20]. Formation of nano-crystals was confirmed by an AFM image (SII, SPA400) of a deposited film on a Si substrate from a typical colloidal solution. Another colloidal solutions was prepared by laser irradiation for 1 min with different serial fluences from 5.2 to 88 mJ/cm2. We chose the relatively short irradiation time in order to investigate the early stage of the ablation by various laser fluenes. For five solutions from this series, the diameter of the included particles in the ensemble was estimated by DLS measurements (HORIBA, nano-partica), where each supernatant was treated by a centrifuge at 6200 rpm for 30 min before the DLS measurement in order to eliminate dusts or fragments larger than a few-lm. It was confirmed that the treatment by centrifuge did not change the absorption spectrum. We employed the mode diameter instead of the mean diameter, where the mode diameter indicates a most frequent diameter in the ensemble, because our 1 min irradiation time was too short to achieve equilibrium for all particles. Surface electric potential on the nanoparticles was also obtained by f-potential measurements (HORIBA, nano-partica) for three solutions in the series. Typical image of the nanoparticles was observed by the transmission electron microscopy (JEOL, 2000EX). The Debye-Scherrer rings of the nanoparticles were obtained by electron diffraction measurements with a relatively low acceleration voltage: 120 kV. The lattice spacings taken from the electron diffraction pattern was compared to those of starting QQ micro-crystals powder measured by X-ray diffraction (XRD) (Rigaku, MiniFlexII). A vapordeposited film of the QQ with thickness of 46.5 nm was prepared by a vacuumed deposition system of organic thin layers (Eiko, EO-5) to compare the optical absorption spectrum to others and to obtain the absorption coefficient of QQ.
3. Results Figure 2b shows the absorption spectra of the supernatants of QQ before and after laser irradiation for various lengths of times at a fluence of 11 mJ/cm2. For comparison, absorption spectra of relatively well-solved QQ solutions in dichlorobenzene and in ethanol are shown in Figure 2a. For prolonged laser irradiation, characteristic absorption peaks (2.88 eV) increased and scattering tails (2.24 eV) became smaller as shown in Figure 2c. Such changes were accompanied with a visible disappearance of precipitations and appearance of a transparent yellow solution. As any reduction of the absorbance was not observed at 2.88 eV, we think the photodegradation of the QQ molecule didn’t occur in this irradiation condition, which was observed in another fragile molecule [27]. Blue shift and reduction of width of the absorption peak are remarkable for the lowest band around 430 nm. These changes in the absorbance spectrum are the result of converting precipitations to nanoparticles in water. As shown in Figure 3, nanoparticles were actually observed by an AFM image of the prepared colloidal solution deposited on a Si substrate. The image indicates that the par-
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(a)
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Figure 3. A typical AFM image of a deposited film of a colloidal solution of QQ on a Si substrate.
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Figure 2. (a) An absorption spectra of QQ solutions in polar non-protic solvent dichlorobenzene (1) and in polar protic solvent ethanol (2). (b) Absorption spectra before and after various irradiation times at a laser fluence of 11 mJ/cm2. (c) Variation of absorbance at 2.24 eV (open circles) and 2.88 eV (solid circles) by irradiation time. Dotted curves in (c) are guides for eyes.
ticles are in the shape of distorted ellipsoid with a typical shortest axis 78 nm and are dispersed somewhat uniformly. Figure 4a shows the absorption spectra of the supernatants before and after laser irradiation for 1 min at laser fluences of 5.2, 19, 67, and 88 mJ/cm2. As the laser fluence was increased; the absorbance increased, the peak energy was shifted to higher energy side, and each full width at half maximum (FWHM) became narrower (as in the case of the previous series shown in Figure 2). Figure 4b shows absorbance variations by laser fluences for 1 min irradiation at 2.9 and 2.0 eV. When the scattering intensity at 2.0 eV was diminished, the absorbance at 2.9 eV asymptotically reaches the constant value. Therefore, the photo-degradation didn’t occur in this irradiation condition. The threshold intensity was not clear, probably because the QQ is relatively soluble in water. The lowest peak energies and the FWHMs are summarized in Figure 5a against the laser fluences, where the lowest absorption peak is employed since influences of the scattering effect are less than higher energy bands. The peak positions and the FWHM are asymptotically
(b)
Figure 4. (a) Absorption spectra before and after 1 min irradiation at various laser fluences of 5.2, 19, 67, and 88 mJ/cm2. (b) Variation of absorbance at 2.0 eV (open circles) and 2.90 eV (solid circles) by laser fluences for 1 min irradiation. Dotted curves in (b) are guides for eyes.
reaching at constant values, 2.923 and 0.484 eV, respectively. We focus on the blue shift of the absorption peak energy rather than the increase of the absorbance.
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(a)
(b) Figure 7. Typical TEM image (left) and electron diffraction pattern (right) of QQ nano-particles prepared with laser fluence 19.2 mJ/cm2 for 1 min.
Figure 5. (a) Fluence dependence of the lowest absorption peak energy (solid triangles) and its FWHM (solid circles). (b) Fluence dependence of the mode diameter estimated by DLS (open circles). Dotted curves are guides for eyes.
The mode diameters of nanoparticles in the same solutions were estimated by DLS as shown in Figure 5b against the laser fluences. The mode diameter was observed smaller in solutions prepared by higher laser fluences, in the range from 53 to 88 nm with the typical standard deviation 42 nm. The sizes less than a hundred-nm were roughly coincident to the observed particle images by AFM and TEM shown in Figures 3 and 7. We found the linear correlation between the mode diameters and the lowest absorption peak energies as shown in Figure 6. The linear correlation provides us a simple estimation of the most frequent diameter of nanoparticles in an ensemble by observation of absorption peak energy. However, we think the relation might be limited for nanoparticles of diameters, at least, within the range from 55 to 90 nm in the case of QQ. For another colloidal solution prepared by prolonged irradiation conditions, a mode diameter was 37.4 nm and the lowest absorption peak energy was 2.932 eV. This relation deviates from the linear correlation, probably because the lowest absorption peak energy converges at certain value for nanoparticles less than 55 nm. 432
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f-potential of the nanoparticles was also obtained for three solutions in the series. The observed f-potentials were negative and the absolute values tended to be smaller for solutions prepared by higher laser fluences; that is, 68.7, 52.5 and 44.3 mV for the solutions prepared by laser fluences at 5.2, 30, and 88 mJ/cm2, respectively [27]. Apparently, the magnitude of the negative fpotentials was related to the size of the nanoparticles. Charging of the surface causes repulsive interactions between the nanoparticles so as to avoid their aggregation in water. Each of the colloidal solution of QQ was kept at least for 50 days with small reduction of absorbance less than 6%. The repulsive interaction was also demonstrated as well dispersion in the deposited film shown in Figure 3. A TEM image of the QQ nanoparticles prepared at a fluence of 19.2 mJ/cm2 for 1 min is shown in the left of Figure 7. The image of nanoparticles was observed typically in size around 90 nm, which is not far from the mode diameter observed by DLS (77.6 nm). An electron diffraction pattern of the nanoparticle ensemble shows multiple Debye–Scherrer rings as shown in the right of Figure 7, which means that the nanoparticles consist of the crystal structure. The lattice spacings read from the Debye– Scherrer rings were coincident with those obtained by XRD measurements for starting QQ powder within analytical accuracy. Therefore, the crystal structure of the QQ powder maintains after the laser ablation in water. Figure 8 shows absorption spectra of a vapor-deposited film (a) and a colloidal solution prepared by laser ablation at laser fluence of 88 mJ/cm2 for 1 min (b), for comparison that of a solution before irradiation (c) is also shown. The peak position of the colloidal solution in (b) was largely blue shifted by 48 meV (7.0 nm) com600
422 (nm)
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Norm. Absorbance
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(a) vapor deposited film (b) after irrad. nano solution (c) before irrad. solution 1
y=1681-554x 2.88
2.90
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Absorption Peak Energy (eV) Figure 6. Correlation of the mode diameter to the lowest peak energy obtained at various fluences (solid circles). A broken line is a fitting curve obtained by the least squares method.
0
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Photon Energy(eV) Figure 8. Comparison of absorption spectra normalized at the lowest peak of (a) a vapor deposited film (solid line) (b) a colloidal solution (broken line), (c) a starting aqua solution before irradiation (dotted line).
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pared to that of the vapor-deposited film (a). Therefore, the blue shift in the colloidal solutions is a characteristic of the nanoparticles in water.
4. Discussion We investigated size dependence of absorption spectrum of colloidal solutions with QQ nanoparticles formed by the laser ablation at various fluences for 1 min irradiation as shown in Figure 5. We found the size of nanoparticles becomes smaller by higher laser fluences even for 1 min short irradiation. Therefore, the higher fluences cause sequential fragmentations of hotter particles in our QQ system as described in the case of QA [20] and pentacene [25]. From the absorption coefficient a = 6.1 104 cm 1 of the vapordeposited QQ film with 46.5 nm thickness, the penetration depth at laser wavelength 430 nm is estimated to be 164 nm. The value is much larger than the largest mode diameter 88 nm in the investigated series. Therefore, multiple fragmentations of hotter particles take place until the particle size reaches certain critical sizes under the laser ablation conditions in water. However, the asymptotical changes of the peak positions related to the mode diameter indicate that further fragmentations of QQ particles are limited at the higher fluences even for 1 min irradiation. The similarity between the spectral changes shown in Figure 2b and Figure 4a implies that sequential fragmentations by weak pulses for prolonged time irradiation gives the same results as the sequence of the multiple fragmentations by strong pulses. Now, we discuss the reason of the blue shift of the absorption peak energy corresponding to the reduction of particle size, at least, in the range from 55 to 90 nm. Similar size-dependent blue shift of absorption spectrum was reported in poly-diacetylene (poly-DCHD) [15], dendronized perylenediimide(D-PDI) [23] and pentacene [25]. For nanoparticles of spread size from 42 to 160 nm and fiber-like 1–5 lm of poly-DCHD [15], the blue shift of absorption peak energy on shorter nanoparticles was attributed to the reduction of effective conjugated length of p-electron due to higher concentration of structural defects. In nanoparticles of DPDI in which electronic interaction between PDI chromophores is very weak due to the bulky dendron groups in the bay position [23], the size dependence of extinction spectrum was discussed with simulations of both absorption coefficient and light scattering loss based on the Mie theory [29] assuming spherical nanoparticles of diameters from 50 to 500 nm. They concluded that the light scattering effect gives a major contribution for the blue shift of extinction spectrum of colloidal solutions. In pentacene nanoparticle [25], the blue shift of UV–VIS absorption spectrum was described to be due to a decrease in the size of pentacene by higher irradiation power, however, any reason of the blue shift has not been discussed. In the present case, the QQ molecule holds conjugated p-electrons inside a molecule, but no conjugated p-electrons between molecules. There exist the van der Waals interaction and the hydrogen bond interaction between molecules. On the other hand, the Mie scattering effect is usually remarkable for the particles whose diameters are comparable to optical wavelength such as a few hundred-nm for visible light as pointed out in Refs. [23,29]. In our case, we observed the nanoparticles of the mode diameter in the range from 55 to 90 nm, so that we could neglect such a structure-rich scattering effect as simulated in Ref. [24]. In fact, from the simulation of the Mie scattering by the QQ nanoparticles with various diameters in water, we found the absorption peak energy is not influenced by the Mie scattering. We have to seek other origin for the blue shift of the absorption peak energy. Condensed matter effect due to inter-molecular interaction, socalled site shift effect, is usually observed as energy shift of optical spectrum between gas, liquid, and solid phases [30]. The inter-
molecular interaction causes energy stabilization in HOMO and LUMO energy bands differently and makes the lowest absorption peak in crystal phase lower than that in gas phase [31]. At first, we examine the site shift effect in the present case by comparing absorption spectra shown in Figure 8. The absorption spectrum of the starting solution (c) in Figure 8 comes from slightly solved molecules in supernatant after sonication. Thus, the spectrum (c) is of an aqua solution of the QQ, which might be largely red shifted by solvation energy due to surrounding water from that of gas phase. The spectrum (a) is of the thin film which consists of both amorphous and grained solid state phases and is slightly blue shifted by 23 meV (3.5 nm) from the spectrum (c). The absorption photon energy in the spectrum (a) is caused by, at least, two competitive factors; a blue shift due to absence of the solvation energy and a red shift by the bulk site shift due to inter-molecular interaction. The spectrum (b) is of the colloidal solution with the mode diameter 53.8 nm, where the lowest absorption band is blue shifted from that of the film (a) by 48 meV (7.0 nm). The amount is more than twice of that of film. As nanoparticles are wearing negative surface potentials in the colloidal solutions, we have to consider other factors causing the significant blue shift, suppressing red shifts factors by the bulk site shift and by the solvation energy due to surrounding water molecules. We suppose that the size-dependence of energy shift in absorption spectrum is dominated by the surface states. A ratio of a surface area (S) to a particle volume (V), S/V, which is equal to 6/D where D is a diameter, is relatively larger for smaller particle. Therefore, influences from surface states should be involved to explain the energy shift in nanoparticles. The linear correlation shown in Figure 6 also indicates the correlation of inverse of the S/V against the blue shift. Surface exciton of which absorption peak energy are shifted to higher energy side has been discussed [31] and demonstrated in anthracene crystal [32,33] and in an interface of a thin film of BiI3/PbI2 or BiI3/CdI2 [34]. The concept of the surface exciton should be naturally applied to the nano-crystal system. Another surface state could be modified frontier orbital states, HOMO and LUMO, by bond changing to OH- , O_ and so on, at molecules on the surface layer of the nano-crystals. Such modification of the bonds might be a source of the negative f-potential on surface states of the nanoparticles and also be a source of a narrowing of the FWHM of absorption spectrum. Hardening of the molecular vibrational frequency corresponding to the surface potentials or bond changes could cause the narrowing of FWHM. We could not attribute the narrowing of FWHM shown in Figure 5a to reduction of inhomogeneity, because any evidences for systematic reduction of inhomogeneity were not obtained from the standard deviation values by DLS. We tried an ESR measurement of nanoparticles which were dried on a filter paper with a concentration of at least 1 1012 particles, however, no signal was obtained at 20 K. In addition, it is curious how the variation of interactions with surrounding molecules in solvent influences the conversion phenomena by the laser ablation in water and the stability of the formed colloidal solution. The QQ is expected to bear more frequent hydrogen bond interaction in water because of two additional quinone moieties in comparison to the QA [20]. The solubility in a poor solvent water before irradiation actually reflects the difference, that is, the QQ is slightly but more soluble than the QA. The remarkable change of the absorption spectrum by the laser ablation occurs in the same order of the laser fluence as in the QA. The QQ nanoparticles with mode diameter 54 nm was prepared by a laser fluence 88 mJ/cm2 for 1 min at the wavelength 430 nm, while the QA nanoparticles with diameter 1045 nm was prepared by a laser fluence 90 mJ/cm2 for 20 min at wavelength 580 nm. As the penetration depth 75 nm for the QA at the wavelength 580 nm is much shorter than the value 164 nm for the QQ at the wavelength 430 nm, the relatively larger
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size of nano-crystals in the present QQ is due to the differences of the irradiation time and the penetration depth at wavelengths of irradiated laser pulses. Therefore, we could not find any significant influence by the variation of hydrogen bond interaction with surrounding molecules of solvent in the formation of nanoparticles. On the other hand, the stability of the prepared colloidal solutions was affected by such circumstances. The colloidal solution of QQ is more stable than that of QA; the absorbance of the colloidal solutions of the QQ were kept with a slight reduction less than 6% for 50 days, while that of the QA decreased by 30% for 20 days [20]. The magnitude of the negative f-potential ( 52.5 mV) in the QQ nanoparticles in water with a mode diameter 68.5 nm is larger than that ( 34 mV) in the QA nanoparticles in water with a mean diameter 70 nm [21]. As the charging of negative surface potential causes a repulsive interaction between the nanoparticles so as to avoid aggregations, the magnitude of the potential determines the stability of the colloidal solutions of the QQ and the QA. We also carried out another laser ablation on the QQ using pH controlled water as a solvent [28]. As a result, the conversion from micro-crystals to nanoparticles by laser ablation was achieved for wide range of pH 2.510 water solvent, but the stability of prepared solution is getting worse with a departure from pH 7. Therefore, the stability of prepared colloidal solutions was affected by frequent hydrogen bond with surrounding water, while the formation of nanoparticle was not significantly influenced. 5. Summary We carried out the liquid laser ablation method for yellow pigment QQ in a poor solvent, water and investigated size dependent optical properties of the prepared nanoparticles. The blue shift of the absorption peak corresponding to the decrease of particle size is attributed to the surface states, such as surface excitons or bond modified surface states. We propose a simple method to estimate the most frequent diameter of nanoparticles from the absorption spectrum, at least, in the range from 55 to 90 nm, from the linear correlation between the energy of the lowest absorption peak and the mode diameter of nanoparticle ensemble. Acknowledgement This Letter was supported by JST for ‘A Research for Promoting Technological Seeds 2009’, by Wakayama University for Researcher
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Program and partially by the Inter-university Cooperative Research Program of Institute for Materials Research, Tohoku University. Authors greatly acknowledge Research Group of HORIBA Co. for analysis of particle size and f-potential of solutions. References [1] C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 51 (1987) 913. [2] C. Adachi, S. Tokito, T. Tsutsui, S. Saito, Jpn. J. Appl. Phys. 27 (1988) L269. [3] M.A. Baldo, D.F. O’Brien, Y. You, A. Shoustikov, S. Sibley, M.E. Thompson, S.R. Forrest, Nature 395 (1998) 151. [4] J. Kido, K. Hongawa, K. Okuyama, K. Nagai, Appl. Phys. Lett. 64 (1994) 815. [5] H. Klauk, M. Halik, U. Zschieschang, G. Schmid, W. Radlik, W. Weber, J. Appl. Phys. 92 (2002) 5259. [6] T. Manaka1, F. Liu, M. Weis, M. Iwamoto, Phys. Rev. B 78 (2008) 121302R. [7] K. Goushi, K. Yoshida, K. Sato, C. Adachi, Nat. Photonics 6 (2012) 253. [8] J.H. Burroughes et al., Nature 347 (1990) 539. [9] G. Gustafsson, Y. Cao, G.M. Treacy, F. Klavetter, N. Colaneri, A.J. Heeger, Nature 357 (1992) 477. [10] T. Iida, H. Ishihara, Phys. Rev. Lett. 90 (2003) 057403. [11] T. Kudo, H. Ishihara, Phys. Status Solidi C 8 (2011) 66. [12] H. Kasai et al., Jpn. J. Appl. Phys. 31 (1992) L1132. [13] H. Fukuhara, H. Masuhara, Chem. Phys. Lett. 221 (1994) 373. [14] H. Fujiwara, H. Fukuhara, H. Masuhara, J. Phys. Chem. 99 (1995) 11844. [15] V.V. Volkov, T. Asahi, H. Masuhara, A. Masuhara, H. Kasai, H. Oikawa, H. Nakanishi, J. Phys. Chem. B 108 (2004) 7674. [16] Y. Tamaki, T. Asahi, H. Masuhara, Appl. Surf. Sci. 168 (2000) 85. [17] Y. Tamaki, T. Asahi, H. Masuhara, J. Phys. Chem. A 106 (2002) 2135. [18] Y. Tamaki, T. Asahi, H. Masuhara, Jpn. J. Appl. Phys. 42 (2003) 2725. [19] Y. Hosokawa, M. Yashiro, T. Asahi, H. Masuhara, J. Photochem. Photobiol. A: Chem. 142 (2001) 197. [20] T. Sugiyama, T. Asahi, T. Takeuchi, H. Masuhara, Jpn. J. Appl. Phys. 45 (2006) 384. [21] H.-G. Jeon, T. Sugiyama, H. Masuhara, T. Asahi, Jpn. J. Appl. Phys. 46 (2007) L733. [22] T. Asahi, T. Sugiyama, H. Masuhara, Acc. Chem. Res. 41 (2008) 1790. [23] R. Yasukuni et al., Appl. Phys. A 93 (2008) 5. [24] R. Yasukuni, M. Sliwa, J. Hofkens, F.C. De Schryver, A. Herrmann, K. Muellen, T. Asahi, Jpn. J. Appl. Phys 48 (2009) 065002. [25] S. Kita, S. Masuo, S. Machida, A. Itaya, Jpn. J. Appl. Phys. 45 (2006) 6501. [26] H. Tabata, M. Akamatsu, M. Fujii, S. Hayashi, Jpn. J. Appl. Phys. 46 (2007) 4338. [27] I. Akimoto, M. Ohata, N. Ozaki, G. Ping, MRS Proceedings 1455 (2012) mrss121455-ii08-04. [28] I. Akimoto, M. Ohata, N. Ozaki, P. Gu, unpublished. [29] C.F. Bohren, D.R. Huffman, Absorption and Scattering of Light by, Small Particles, Wiley-Interscience, 1983. [30] J.D. Wright, Molecular Crystals, Cembridge University Press, 1987. [31] A.S. Davydov, Theory of Molecular Excitons, Plenum Press, 1971. [32] M.G. Sceats, K. Tomioka, S.A. Rice, J. Chem. Phys. 66 (1977) 4469. [33] V.M. Agranovich, Excitations in Organic Solids, Oxford Science Publications, 2009. [34] K. Watanabe, S. Takeyama, N. Miura, K. Komatsu, Solid State Commun. 74 (1990) 37.
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Size dependent optical properties of quinacridonequinone nanoparticles prepared by liquid laser ablation in water Ikuko Akimoto ⇑, Masahiro Ohata, Nobuhiko Ozaki, Ping Gu Faculty of Systems Engineering, Wakayama University, Japan Faculty of Education, Wakayama University, Japan
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Article history: Received 1 April 2012 In final form 20 September 2012 Available online 2 October 2012
a b s t r a c t We investigated optical properties of colloidal solutions of nanoparticles prepared by laser ablation of quinacridonequinone powder in a poor solvent, water. Blue shift of the absorption peak energy was observed corresponding to the increase of laser fluence or to the prolonged irradiation time. For a series of colloidal solutions, we found a linear correlation between the blue shift of the absorption peak energy and the decrease in the diameter of the nanoparticles. The blue shift is considered to be due to the surface state, such as surface excitons or bond modified surface states. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Organic materials have been utilized in the production of optical and electronic devices in recent years [1–7]. However, applications of organic materials have been limited partially because of the high-cost for mass production. The fabrication of devices such as organic light emitting diodes (OLED) and organic field effect transistors (OFET) requires high-cost vacuum sublimation processes for small organic molecules. An alternative way of the process is a wet-process, which has been used for solvable polymer materials [8,9]. Since organic small molecules are difficult to be solved without an addition of hydrophilic moieties on a molecule or any surfactants, the wet-process has not been applied for systems of small organic molecules. Even if they are soluble, environmental load substances and/or toxic ones such as toluene or chloroform are used. If aqueous solutions of simple small organic molecules could be obtained, various new applications of organic molecules would be expected: for example, wet-process device fabrication by application of an ink-print technique, a drag delivery system, and cosmetic application. Modification of the size from micro-crystals to nanoparticles is a promising technique for the preparation of dispersions of small organic materials. Nanoparticles, generally, easily disperse in various solvents including ubiquitous solvents such as water, and the dispersion is maintained for long time by Brownian motion in a solvent. Such a dispersed system of organic nanoparticles is also worth giving a possible stage for the realization of nanoparticle manipulation by resonant radiative
⇑ Corresponding author. Address: Wakayama University Japan, Sakaedani 930, Wakayama 640-8510, Japan. Fax: +81 73 457 8272. E-mail address: [email protected] (I. Akimoto). 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2012.09.048
force with laser beam which was predicted in the reference [10,11]. There are several methods to form nanoparticles, which are classified to build-up or break-down method. As a build-up method, the re-precipitation technique has been known [12]. On the other hand, as a break-down method, pulsed laser ablation in poor solvent has been developed by Masuhara and co-workers [13–24]. Up to now, the validity of the laser ablation has been demonstrated by several groups for various organic systems, such as poly-diacetylene (poly-DCHD) [15], vanadyl phthalocyanine (VOPc) [16–18], quinacridone (QA) [20–22], dendronized perylenediimide (D-PDI) [23,24], pentacene [25], and fullerene [26], which possess enough tolerance for laser power. The laser ablation converts microcrystals of target materials into nanoparticles in poor solvent with laser pulses over the threshold fluence which is settled for each molecule and each absorption band. The size of the formed nanoparticles can be controlled by ablation conditions, such as fluence, wavelength, duration of laser pulse and a type of poor solvent. The colloidal solution is usually stable without any surfactants, except the case of the relatively large molecule, for example, D-PDI [23,24] in which a surfactant was added to stabilize the solution before the laser ablation. Fragmentation mechanism has been considered as follows [16,20,25]: The photo-thermal conversion causes an explosive temperature increase on the surface layers of the target micro-crystals within a penetration depth at the irradiated laser wavelength. When the temperature on the surface layers increases higher than the threshold, nanoparticles eject from the surface and/or micro-crystals fragment into nanoparticles. The cooling process takes place competitively due to thermal diffusion in the solvent. There still remains argument about which mechanism governs the size of ejected particles under higher laser fluences, that is, a higher fluence only gives a large fragmentation rate of
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the same sized nanoparticles assisted by the cooling process as described in the case of VOPc [16] or a higher fluence gives smaller nanoparticles by further fragmentation of hotter particles as described in the case of QA [20] and pentacene [25]. The formation of the nanoparticles has been monitored by UV– VIS absorption spectrum of the colloidal solution. Besides the significant increase of absorbance after laser ablation, the spectral shift depending on nanoparticle size was reported in poly-diacetylene (poly-DCHD) [15], dendronized perylenediimide(D-PDI) [23], and pentacene [25]. The reason of the spectral shift has been discussed individually. The blue shift of absorption peak energy on smaller nanoparticles was attributed to the reduction of effective conjugated length of p-electron due to higher concentration of structural defects in poly-DCHD [15] or to the light scattering effect explained by Mie theory in D-PDI [25]. On the other hand, in QA [20], no blue shift of absorption spectrum was observed, although the particle size became smaller by higher fluences. Instead, significant spectral change was observed due to the change of crystal phase b or c-form by selecting the irradiation wavelength, where QA is known to have polymorphism in a, b, or cforms. Thus, optical properties of colloidal solutions prepared by the liquid laser ablation still need interpretations of case by case, therefore, further collection of successful samples is necessary to understand the optical properties of colloidal solutions. In this Letter, we investigated the liquid laser ablation for yellow pigment quinacridonequinone (QQ) in a poor solvent, water and clarified the size dependent optical properties of the prepared colloidal solutions. The formed colloidal solutions of QQ nanoparticles accompanied with remarkable blue shift of the absorption peak. The analysis of particle diameters by dynamic light scattering (DLS) method in the same colloidal solutions indicates that the blue shift corresponds to the decrease of particle size. The blue shift is considered to be due to the surface state, such as surface excitons or bond modified surface states. The linear correlation between the magnitude of the blue shift and the mode diameter provides us a simple method to estimate the most frequent nanoparticle diameter in colloidal solution from the absorption spectrum in the limited diameter range. 2. Experiment Laser ablation of yellow pigment quinacridonequinone (QQ, Aldrich) was performed; molecular structure is shown in Figure 1. The QQ is a derivative of well-known red pigment quinacridone (QA) and a simple material to observe the absorption spectrum because of its monomorphism. Starting powder was put in the distilled water making dispersed mixture at a concentration of 5 10 5 mol/l. Although we used the water which was degassed by nitrogen bubbling for 30 min, any significant differences were not observed in ablation results between degassed and pristine waters in our preliminary experiments. The mixtures were sonicated for 30 min in capped bottle, so that a little amount of material was solved and water color was changed to pale yellow.
Figure 1. Molecular structure, quinacridonequinone (QQ).
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However, most of powders remained as precipitations. The isotropic mixture was quickly divided into each capped quartz cuvette of 1 1 5 cm3 with a tiny Teflon coated stirring bar. The microcrystals in water stirred by a magnetic stirrer were irradiated by 5 ns laser pulses from a tunable optical parametric oscillator excited by Q-switched YAG laser operated in 10 Hz repetition rate (Spectra Physics, MOPO). The wavelength of irradiated laser pulse was selected at 430 nm, which corresponds to the lowest absorption band of a QQ molecule. UV–VIS absorption spectrum was measured at room temperature with a conventional system (JASCO, V-560) in a few tens of minutes after the ablation. Colloidal solutions was prepared for different serial of the irradiation time up to 40 min at a fixed laser fluence of 11 mJ/cm2, which is almost on the threshold in the case of the QA [20]. Formation of nano-crystals was confirmed by an AFM image (SII, SPA400) of a deposited film on a Si substrate from a typical colloidal solution. Another colloidal solutions was prepared by laser irradiation for 1 min with different serial fluences from 5.2 to 88 mJ/cm2. We chose the relatively short irradiation time in order to investigate the early stage of the ablation by various laser fluenes. For five solutions from this series, the diameter of the included particles in the ensemble was estimated by DLS measurements (HORIBA, nano-partica), where each supernatant was treated by a centrifuge at 6200 rpm for 30 min before the DLS measurement in order to eliminate dusts or fragments larger than a few-lm. It was confirmed that the treatment by centrifuge did not change the absorption spectrum. We employed the mode diameter instead of the mean diameter, where the mode diameter indicates a most frequent diameter in the ensemble, because our 1 min irradiation time was too short to achieve equilibrium for all particles. Surface electric potential on the nanoparticles was also obtained by f-potential measurements (HORIBA, nano-partica) for three solutions in the series. Typical image of the nanoparticles was observed by the transmission electron microscopy (JEOL, 2000EX). The Debye-Scherrer rings of the nanoparticles were obtained by electron diffraction measurements with a relatively low acceleration voltage: 120 kV. The lattice spacings taken from the electron diffraction pattern was compared to those of starting QQ micro-crystals powder measured by X-ray diffraction (XRD) (Rigaku, MiniFlexII). A vapordeposited film of the QQ with thickness of 46.5 nm was prepared by a vacuumed deposition system of organic thin layers (Eiko, EO-5) to compare the optical absorption spectrum to others and to obtain the absorption coefficient of QQ.
3. Results Figure 2b shows the absorption spectra of the supernatants of QQ before and after laser irradiation for various lengths of times at a fluence of 11 mJ/cm2. For comparison, absorption spectra of relatively well-solved QQ solutions in dichlorobenzene and in ethanol are shown in Figure 2a. For prolonged laser irradiation, characteristic absorption peaks (2.88 eV) increased and scattering tails (2.24 eV) became smaller as shown in Figure 2c. Such changes were accompanied with a visible disappearance of precipitations and appearance of a transparent yellow solution. As any reduction of the absorbance was not observed at 2.88 eV, we think the photodegradation of the QQ molecule didn’t occur in this irradiation condition, which was observed in another fragile molecule [27]. Blue shift and reduction of width of the absorption peak are remarkable for the lowest band around 430 nm. These changes in the absorbance spectrum are the result of converting precipitations to nanoparticles in water. As shown in Figure 3, nanoparticles were actually observed by an AFM image of the prepared colloidal solution deposited on a Si substrate. The image indicates that the par-
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(a)
(b)
Figure 3. A typical AFM image of a deposited film of a colloidal solution of QQ on a Si substrate.
(a) (c)
Figure 2. (a) An absorption spectra of QQ solutions in polar non-protic solvent dichlorobenzene (1) and in polar protic solvent ethanol (2). (b) Absorption spectra before and after various irradiation times at a laser fluence of 11 mJ/cm2. (c) Variation of absorbance at 2.24 eV (open circles) and 2.88 eV (solid circles) by irradiation time. Dotted curves in (c) are guides for eyes.
ticles are in the shape of distorted ellipsoid with a typical shortest axis 78 nm and are dispersed somewhat uniformly. Figure 4a shows the absorption spectra of the supernatants before and after laser irradiation for 1 min at laser fluences of 5.2, 19, 67, and 88 mJ/cm2. As the laser fluence was increased; the absorbance increased, the peak energy was shifted to higher energy side, and each full width at half maximum (FWHM) became narrower (as in the case of the previous series shown in Figure 2). Figure 4b shows absorbance variations by laser fluences for 1 min irradiation at 2.9 and 2.0 eV. When the scattering intensity at 2.0 eV was diminished, the absorbance at 2.9 eV asymptotically reaches the constant value. Therefore, the photo-degradation didn’t occur in this irradiation condition. The threshold intensity was not clear, probably because the QQ is relatively soluble in water. The lowest peak energies and the FWHMs are summarized in Figure 5a against the laser fluences, where the lowest absorption peak is employed since influences of the scattering effect are less than higher energy bands. The peak positions and the FWHM are asymptotically
(b)
Figure 4. (a) Absorption spectra before and after 1 min irradiation at various laser fluences of 5.2, 19, 67, and 88 mJ/cm2. (b) Variation of absorbance at 2.0 eV (open circles) and 2.90 eV (solid circles) by laser fluences for 1 min irradiation. Dotted curves in (b) are guides for eyes.
reaching at constant values, 2.923 and 0.484 eV, respectively. We focus on the blue shift of the absorption peak energy rather than the increase of the absorbance.
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(a)
(b) Figure 7. Typical TEM image (left) and electron diffraction pattern (right) of QQ nano-particles prepared with laser fluence 19.2 mJ/cm2 for 1 min.
Figure 5. (a) Fluence dependence of the lowest absorption peak energy (solid triangles) and its FWHM (solid circles). (b) Fluence dependence of the mode diameter estimated by DLS (open circles). Dotted curves are guides for eyes.
The mode diameters of nanoparticles in the same solutions were estimated by DLS as shown in Figure 5b against the laser fluences. The mode diameter was observed smaller in solutions prepared by higher laser fluences, in the range from 53 to 88 nm with the typical standard deviation 42 nm. The sizes less than a hundred-nm were roughly coincident to the observed particle images by AFM and TEM shown in Figures 3 and 7. We found the linear correlation between the mode diameters and the lowest absorption peak energies as shown in Figure 6. The linear correlation provides us a simple estimation of the most frequent diameter of nanoparticles in an ensemble by observation of absorption peak energy. However, we think the relation might be limited for nanoparticles of diameters, at least, within the range from 55 to 90 nm in the case of QQ. For another colloidal solution prepared by prolonged irradiation conditions, a mode diameter was 37.4 nm and the lowest absorption peak energy was 2.932 eV. This relation deviates from the linear correlation, probably because the lowest absorption peak energy converges at certain value for nanoparticles less than 55 nm. 432
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f-potential of the nanoparticles was also obtained for three solutions in the series. The observed f-potentials were negative and the absolute values tended to be smaller for solutions prepared by higher laser fluences; that is, 68.7, 52.5 and 44.3 mV for the solutions prepared by laser fluences at 5.2, 30, and 88 mJ/cm2, respectively [27]. Apparently, the magnitude of the negative fpotentials was related to the size of the nanoparticles. Charging of the surface causes repulsive interactions between the nanoparticles so as to avoid their aggregation in water. Each of the colloidal solution of QQ was kept at least for 50 days with small reduction of absorbance less than 6%. The repulsive interaction was also demonstrated as well dispersion in the deposited film shown in Figure 3. A TEM image of the QQ nanoparticles prepared at a fluence of 19.2 mJ/cm2 for 1 min is shown in the left of Figure 7. The image of nanoparticles was observed typically in size around 90 nm, which is not far from the mode diameter observed by DLS (77.6 nm). An electron diffraction pattern of the nanoparticle ensemble shows multiple Debye–Scherrer rings as shown in the right of Figure 7, which means that the nanoparticles consist of the crystal structure. The lattice spacings read from the Debye– Scherrer rings were coincident with those obtained by XRD measurements for starting QQ powder within analytical accuracy. Therefore, the crystal structure of the QQ powder maintains after the laser ablation in water. Figure 8 shows absorption spectra of a vapor-deposited film (a) and a colloidal solution prepared by laser ablation at laser fluence of 88 mJ/cm2 for 1 min (b), for comparison that of a solution before irradiation (c) is also shown. The peak position of the colloidal solution in (b) was largely blue shifted by 48 meV (7.0 nm) com600
422 (nm)
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(nm)
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Norm. Absorbance
Mode Diameter (nm)
90 80 70 60 50 40 2.86
(a) vapor deposited film (b) after irrad. nano solution (c) before irrad. solution 1
y=1681-554x 2.88
2.90
2.92
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Absorption Peak Energy (eV) Figure 6. Correlation of the mode diameter to the lowest peak energy obtained at various fluences (solid circles). A broken line is a fitting curve obtained by the least squares method.
0
2.0
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Photon Energy(eV) Figure 8. Comparison of absorption spectra normalized at the lowest peak of (a) a vapor deposited film (solid line) (b) a colloidal solution (broken line), (c) a starting aqua solution before irradiation (dotted line).
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pared to that of the vapor-deposited film (a). Therefore, the blue shift in the colloidal solutions is a characteristic of the nanoparticles in water.
4. Discussion We investigated size dependence of absorption spectrum of colloidal solutions with QQ nanoparticles formed by the laser ablation at various fluences for 1 min irradiation as shown in Figure 5. We found the size of nanoparticles becomes smaller by higher laser fluences even for 1 min short irradiation. Therefore, the higher fluences cause sequential fragmentations of hotter particles in our QQ system as described in the case of QA [20] and pentacene [25]. From the absorption coefficient a = 6.1 104 cm 1 of the vapordeposited QQ film with 46.5 nm thickness, the penetration depth at laser wavelength 430 nm is estimated to be 164 nm. The value is much larger than the largest mode diameter 88 nm in the investigated series. Therefore, multiple fragmentations of hotter particles take place until the particle size reaches certain critical sizes under the laser ablation conditions in water. However, the asymptotical changes of the peak positions related to the mode diameter indicate that further fragmentations of QQ particles are limited at the higher fluences even for 1 min irradiation. The similarity between the spectral changes shown in Figure 2b and Figure 4a implies that sequential fragmentations by weak pulses for prolonged time irradiation gives the same results as the sequence of the multiple fragmentations by strong pulses. Now, we discuss the reason of the blue shift of the absorption peak energy corresponding to the reduction of particle size, at least, in the range from 55 to 90 nm. Similar size-dependent blue shift of absorption spectrum was reported in poly-diacetylene (poly-DCHD) [15], dendronized perylenediimide(D-PDI) [23] and pentacene [25]. For nanoparticles of spread size from 42 to 160 nm and fiber-like 1–5 lm of poly-DCHD [15], the blue shift of absorption peak energy on shorter nanoparticles was attributed to the reduction of effective conjugated length of p-electron due to higher concentration of structural defects. In nanoparticles of DPDI in which electronic interaction between PDI chromophores is very weak due to the bulky dendron groups in the bay position [23], the size dependence of extinction spectrum was discussed with simulations of both absorption coefficient and light scattering loss based on the Mie theory [29] assuming spherical nanoparticles of diameters from 50 to 500 nm. They concluded that the light scattering effect gives a major contribution for the blue shift of extinction spectrum of colloidal solutions. In pentacene nanoparticle [25], the blue shift of UV–VIS absorption spectrum was described to be due to a decrease in the size of pentacene by higher irradiation power, however, any reason of the blue shift has not been discussed. In the present case, the QQ molecule holds conjugated p-electrons inside a molecule, but no conjugated p-electrons between molecules. There exist the van der Waals interaction and the hydrogen bond interaction between molecules. On the other hand, the Mie scattering effect is usually remarkable for the particles whose diameters are comparable to optical wavelength such as a few hundred-nm for visible light as pointed out in Refs. [23,29]. In our case, we observed the nanoparticles of the mode diameter in the range from 55 to 90 nm, so that we could neglect such a structure-rich scattering effect as simulated in Ref. [24]. In fact, from the simulation of the Mie scattering by the QQ nanoparticles with various diameters in water, we found the absorption peak energy is not influenced by the Mie scattering. We have to seek other origin for the blue shift of the absorption peak energy. Condensed matter effect due to inter-molecular interaction, socalled site shift effect, is usually observed as energy shift of optical spectrum between gas, liquid, and solid phases [30]. The inter-
molecular interaction causes energy stabilization in HOMO and LUMO energy bands differently and makes the lowest absorption peak in crystal phase lower than that in gas phase [31]. At first, we examine the site shift effect in the present case by comparing absorption spectra shown in Figure 8. The absorption spectrum of the starting solution (c) in Figure 8 comes from slightly solved molecules in supernatant after sonication. Thus, the spectrum (c) is of an aqua solution of the QQ, which might be largely red shifted by solvation energy due to surrounding water from that of gas phase. The spectrum (a) is of the thin film which consists of both amorphous and grained solid state phases and is slightly blue shifted by 23 meV (3.5 nm) from the spectrum (c). The absorption photon energy in the spectrum (a) is caused by, at least, two competitive factors; a blue shift due to absence of the solvation energy and a red shift by the bulk site shift due to inter-molecular interaction. The spectrum (b) is of the colloidal solution with the mode diameter 53.8 nm, where the lowest absorption band is blue shifted from that of the film (a) by 48 meV (7.0 nm). The amount is more than twice of that of film. As nanoparticles are wearing negative surface potentials in the colloidal solutions, we have to consider other factors causing the significant blue shift, suppressing red shifts factors by the bulk site shift and by the solvation energy due to surrounding water molecules. We suppose that the size-dependence of energy shift in absorption spectrum is dominated by the surface states. A ratio of a surface area (S) to a particle volume (V), S/V, which is equal to 6/D where D is a diameter, is relatively larger for smaller particle. Therefore, influences from surface states should be involved to explain the energy shift in nanoparticles. The linear correlation shown in Figure 6 also indicates the correlation of inverse of the S/V against the blue shift. Surface exciton of which absorption peak energy are shifted to higher energy side has been discussed [31] and demonstrated in anthracene crystal [32,33] and in an interface of a thin film of BiI3/PbI2 or BiI3/CdI2 [34]. The concept of the surface exciton should be naturally applied to the nano-crystal system. Another surface state could be modified frontier orbital states, HOMO and LUMO, by bond changing to OH- , O_ and so on, at molecules on the surface layer of the nano-crystals. Such modification of the bonds might be a source of the negative f-potential on surface states of the nanoparticles and also be a source of a narrowing of the FWHM of absorption spectrum. Hardening of the molecular vibrational frequency corresponding to the surface potentials or bond changes could cause the narrowing of FWHM. We could not attribute the narrowing of FWHM shown in Figure 5a to reduction of inhomogeneity, because any evidences for systematic reduction of inhomogeneity were not obtained from the standard deviation values by DLS. We tried an ESR measurement of nanoparticles which were dried on a filter paper with a concentration of at least 1 1012 particles, however, no signal was obtained at 20 K. In addition, it is curious how the variation of interactions with surrounding molecules in solvent influences the conversion phenomena by the laser ablation in water and the stability of the formed colloidal solution. The QQ is expected to bear more frequent hydrogen bond interaction in water because of two additional quinone moieties in comparison to the QA [20]. The solubility in a poor solvent water before irradiation actually reflects the difference, that is, the QQ is slightly but more soluble than the QA. The remarkable change of the absorption spectrum by the laser ablation occurs in the same order of the laser fluence as in the QA. The QQ nanoparticles with mode diameter 54 nm was prepared by a laser fluence 88 mJ/cm2 for 1 min at the wavelength 430 nm, while the QA nanoparticles with diameter 1045 nm was prepared by a laser fluence 90 mJ/cm2 for 20 min at wavelength 580 nm. As the penetration depth 75 nm for the QA at the wavelength 580 nm is much shorter than the value 164 nm for the QQ at the wavelength 430 nm, the relatively larger
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size of nano-crystals in the present QQ is due to the differences of the irradiation time and the penetration depth at wavelengths of irradiated laser pulses. Therefore, we could not find any significant influence by the variation of hydrogen bond interaction with surrounding molecules of solvent in the formation of nanoparticles. On the other hand, the stability of the prepared colloidal solutions was affected by such circumstances. The colloidal solution of QQ is more stable than that of QA; the absorbance of the colloidal solutions of the QQ were kept with a slight reduction less than 6% for 50 days, while that of the QA decreased by 30% for 20 days [20]. The magnitude of the negative f-potential ( 52.5 mV) in the QQ nanoparticles in water with a mode diameter 68.5 nm is larger than that ( 34 mV) in the QA nanoparticles in water with a mean diameter 70 nm [21]. As the charging of negative surface potential causes a repulsive interaction between the nanoparticles so as to avoid aggregations, the magnitude of the potential determines the stability of the colloidal solutions of the QQ and the QA. We also carried out another laser ablation on the QQ using pH controlled water as a solvent [28]. As a result, the conversion from micro-crystals to nanoparticles by laser ablation was achieved for wide range of pH 2.510 water solvent, but the stability of prepared solution is getting worse with a departure from pH 7. Therefore, the stability of prepared colloidal solutions was affected by frequent hydrogen bond with surrounding water, while the formation of nanoparticle was not significantly influenced. 5. Summary We carried out the liquid laser ablation method for yellow pigment QQ in a poor solvent, water and investigated size dependent optical properties of the prepared nanoparticles. The blue shift of the absorption peak corresponding to the decrease of particle size is attributed to the surface states, such as surface excitons or bond modified surface states. We propose a simple method to estimate the most frequent diameter of nanoparticles from the absorption spectrum, at least, in the range from 55 to 90 nm, from the linear correlation between the energy of the lowest absorption peak and the mode diameter of nanoparticle ensemble. Acknowledgement This Letter was supported by JST for ‘A Research for Promoting Technological Seeds 2009’, by Wakayama University for Researcher
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Program and partially by the Inter-university Cooperative Research Program of Institute for Materials Research, Tohoku University. Authors greatly acknowledge Research Group of HORIBA Co. for analysis of particle size and f-potential of solutions. References [1] C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 51 (1987) 913. [2] C. Adachi, S. Tokito, T. Tsutsui, S. Saito, Jpn. J. Appl. Phys. 27 (1988) L269. [3] M.A. Baldo, D.F. O’Brien, Y. You, A. Shoustikov, S. Sibley, M.E. Thompson, S.R. Forrest, Nature 395 (1998) 151. [4] J. Kido, K. Hongawa, K. Okuyama, K. Nagai, Appl. Phys. Lett. 64 (1994) 815. [5] H. Klauk, M. Halik, U. Zschieschang, G. Schmid, W. Radlik, W. Weber, J. Appl. Phys. 92 (2002) 5259. [6] T. Manaka1, F. Liu, M. Weis, M. Iwamoto, Phys. Rev. B 78 (2008) 121302R. [7] K. Goushi, K. Yoshida, K. Sato, C. Adachi, Nat. Photonics 6 (2012) 253. [8] J.H. Burroughes et al., Nature 347 (1990) 539. [9] G. Gustafsson, Y. Cao, G.M. Treacy, F. Klavetter, N. Colaneri, A.J. Heeger, Nature 357 (1992) 477. [10] T. Iida, H. Ishihara, Phys. Rev. Lett. 90 (2003) 057403. [11] T. Kudo, H. Ishihara, Phys. Status Solidi C 8 (2011) 66. [12] H. Kasai et al., Jpn. J. Appl. Phys. 31 (1992) L1132. [13] H. Fukuhara, H. Masuhara, Chem. Phys. Lett. 221 (1994) 373. [14] H. Fujiwara, H. Fukuhara, H. Masuhara, J. Phys. Chem. 99 (1995) 11844. [15] V.V. Volkov, T. Asahi, H. Masuhara, A. Masuhara, H. Kasai, H. Oikawa, H. Nakanishi, J. Phys. Chem. B 108 (2004) 7674. [16] Y. Tamaki, T. Asahi, H. Masuhara, Appl. Surf. Sci. 168 (2000) 85. [17] Y. Tamaki, T. Asahi, H. Masuhara, J. Phys. Chem. A 106 (2002) 2135. [18] Y. Tamaki, T. Asahi, H. Masuhara, Jpn. J. Appl. Phys. 42 (2003) 2725. [19] Y. Hosokawa, M. Yashiro, T. Asahi, H. Masuhara, J. Photochem. Photobiol. A: Chem. 142 (2001) 197. [20] T. Sugiyama, T. Asahi, T. Takeuchi, H. Masuhara, Jpn. J. Appl. Phys. 45 (2006) 384. [21] H.-G. Jeon, T. Sugiyama, H. Masuhara, T. Asahi, Jpn. J. Appl. Phys. 46 (2007) L733. [22] T. Asahi, T. Sugiyama, H. Masuhara, Acc. Chem. Res. 41 (2008) 1790. [23] R. Yasukuni et al., Appl. Phys. A 93 (2008) 5. [24] R. Yasukuni, M. Sliwa, J. Hofkens, F.C. De Schryver, A. Herrmann, K. Muellen, T. Asahi, Jpn. J. Appl. Phys 48 (2009) 065002. [25] S. Kita, S. Masuo, S. Machida, A. Itaya, Jpn. J. Appl. Phys. 45 (2006) 6501. [26] H. Tabata, M. Akamatsu, M. Fujii, S. Hayashi, Jpn. J. Appl. Phys. 46 (2007) 4338. [27] I. Akimoto, M. Ohata, N. Ozaki, G. Ping, MRS Proceedings 1455 (2012) mrss121455-ii08-04. [28] I. Akimoto, M. Ohata, N. Ozaki, P. Gu, unpublished. [29] C.F. Bohren, D.R. Huffman, Absorption and Scattering of Light by, Small Particles, Wiley-Interscience, 1983. [30] J.D. Wright, Molecular Crystals, Cembridge University Press, 1987. [31] A.S. Davydov, Theory of Molecular Excitons, Plenum Press, 1971. [32] M.G. Sceats, K. Tomioka, S.A. Rice, J. Chem. Phys. 66 (1977) 4469. [33] V.M. Agranovich, Excitations in Organic Solids, Oxford Science Publications, 2009. [34] K. Watanabe, S. Takeyama, N. Miura, K. Komatsu, Solid State Commun. 74 (1990) 37.