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Bowl-shaped superstructures of CdSe nanocrystals with the narrow-sized distribution for a high-performance photoswitch
Chemical Physics Letters 633 (2015) 76–81
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Bowl-shaped superstructures of CdSe nanocrystals with the narrow-sized distribution for a high-performance photoswitch Bo Zhang, Yongtao Shen, Yiyu Feng, Chengqun Qin, Zhengcheng Huang, Wei Feng ∗ Tianjin Key Laboratory of Composite and Functional Materials, School of Materials Science and Engineering, Tianjin University, Tianjin 300072, PR China
a r t i c l e
i n f o
Article history: Received 22 March 2015 In final form 7 May 2015 Available online 19 May 2015
a b s t r a c t The bowl-shaped CdSe superstructure with a diameter of 1–2 m and the thickness of hundreds nanometers was synthesized using Cd(SA)2 and Se powder in an organic phase. The CdSe nanocrystals for assembling superstructures had a narrow-sized distribution indicated by a sharp emission peak in the photoluminescence (PL) spectrum. Moreover, an organic–inorganic hybrid photoswitch based on CdSe superstructures were fabricated. The device exhibited an on/off switching ratio of ∼100 with a good cycling stability. The excellent photo-responsible performance illustrates that the superstructures hold a great promise for the application of photoelectric devices. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Photoluminescence colloidal semiconductor nanocrystals, also known as quantum dots, arouse a great interest in fundamental studies and technological applications [1–10]. As one of the most important II–VI nanocrystals, cadmium selenide (CdSe) becomes a ‘model’ nanocrystal system for a hotspot of basic studies on the photoelectrical properties of nanocrystals because of its accessibility of high-quality and stability [11]. Recently, the family of colloidal CdSe with a narrow-sized distribution, which is similar to regular nanocrystals, has attracted much attention of the researchers [12–21]. The CdSe nanocrystals with the narrow-sized distribution hold great promising for the various applications based on a rational design supported by the understanding of nucleation and growth [16]. Much effort has been made in this field. Peng et al. first reported a hot-injection method for the preparation of CdSe with the narrow-sized distribution [17]. The resulting nanocrystals showed a sharp and dominated absorption peak at 349 nm, while these nanocrystals have not be isolated and characterized. Nguyen et al. worked on the systematically characterization of (CdSe)n (n = 1–37) clusters using a combination of structure enumeration, Monte Carlo search and local optimization [18]. The binding energy was calculated using density functional theory to identify the special nanocrystals. Yu et al. prepared multiple families of CdSe with a narrow bandgap via non-injection one-pot syntheses, showing sharp absorption and emission peaks [19]. Results demonstrated
∗ Corresponding author. E-mail address: [email protected] (W. Feng). http://dx.doi.org/10.1016/j.cplett.2015.05.014 0009-2614/© 2015 Elsevier B.V. All rights reserved.
that low acid-to-Cd and high Cd-to-Se feed molar ration are critical for preparing the narrow-sized CdSe [20]. In addition, they proposed a thermodynamic equilibrium driven approach for the formation of CdSe nanocrystals, which featured highly synthetic reproducibility [21]. The assembly of nanocrystals is considered as a vital route for the development of collection performance and tailoring photoelectrical properties [22–24]. Various interparticle forces, such as van der Waals, electrostatic, magnetic, molecular, entropic forces and their balance are used in the nanoscale self-assembly [25,26]. The properties of superstructures are derived from the block units and the interaction between them. Compared with mono-dispersed nanocrystals, superstructures show highly efficient charge transfer, high charge mobility and a broad range of charge collection [27]. Nevertheless, the superstructures of nanocrystals with the narrow-sized distribution are rarely been reported, which might be attributed to the difficulty in modulating the ligands in an organic phase [28]. This problem is potentially overcome by using the nanocrystal supersaturated solution, and the nanocrystals may be deposited from the solution and assemble into superstructures [29,30]. In this letter, we presented a method of assembling CdSe superstructures in an organic phase using cadmium stearate (Cd(SA)2 ) as the Cd source and surface ligands, elemental selenium as Se compounds and 1-Octadecene (1-ODE) as the reaction medium. By decreasing the temperature acutely, we synthesized bowlshaped superstructures of CdSe nanocrystals with the narrow-sized distribution. This simple method was used to produce highquality superstructures. Organic–inorganic hybrid devices based on CdSe superstructures and poly(3-hexylthiophene) (P3HT) were
B. Zhang et al. / Chemical Physics Letters 633 (2015) 76–81
fabricated. The hybrid photoelectric devices, combining the advantages of both organic and inorganic devices, are significantly important for optoelectronic applications including photo detection in visible light region. The hybrid photoelectric devices exhibited an on/off switching ratio of ∼100 with a good cycling stability. These performance enable CdSe superstructures to be used in the application of photocatalysis, energy storage and conversion, and electronic and photoelectric devices.
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optical fiber to avoid the heating effect, and the power intensity delivered on the photoswitch was measured using the light density meter (Beijing Zhongjiaojinyuan Co., Ltd.). The photoelectric properties of the organic–inorganic hybrid device were measured by electrochemical workstation (Chenhua CH1660D) with Pt and Ag/AgCl electrode as the counter and reference electrode. All the photoelectric tests were conducted in 0.1 M KCl at room temperature.
2. Experimental 3. Results and discussion 2.1. Materials 1-ODE (tech. 90%) and poly(3-hexylthiophene) (P3HT) were purchased from J&K Chemical Ltd. (Beijing China). Cd(SA)2 (18.8–20% by cadmium oxide) and selenium powder (99.7%) were purchased form Guangfu Chemical Institute (Tianjin, China). All chemical reagents were used without the further purification. 2.2. Synthesis of CdSe nanocrystals Cd(SA)2 (136 mg) and selenium powder (4.0 mg) were added into a 100 mL three necked round-bottom flask, which containing 7 mL 1-ODE. Afterwards, five argon-vacuum cycles and vacuum treatment were executed to remove oxygen and moisture. Then, under a flow of purified argon, the reaction mixture was heated to 230 ◦ C for 30 min at a rate of 5 ◦ C min−1 . The resultant products were cooled down gradually and then washed with isopropyl alcohol by centrifugation and finally dispersed in toluene. 2.3. Synthesis of CdSe superstructures The CdSe nanocrystals with the narrow-sized distribution were synthesized by Cd(SA)2 , Se powder and 1-ODE. First, the mono-dispersed CdSe nanocrystals were synthesized at a high temperature. Then, the solution was cooled down to room temperature. The resultant products were washed with isopropyl alcohol by centrifugation for several times and finally dispersed in toluene. 2.4. Fabrication of the device P3HT (10 mg) was dissolved in 2 mL of toluene. The CdSe superstructures solution (100 L, 10 mg/mL) and P3HT solution (200 L) were well mixed into a homogenous solution, and the device was fabricated by dropping 100 L of the solution onto pre-cleaned Indium Tin Oxide (ITO) glass. The device was dried and stored in vacuum for the measurement. 2.5. Characterization The morphologies of CdSe superstructures were observed by SEM (Hitachi S-4800), TEM (Phlilps Tecnai G2 F20). PL spectra were investigated at room temperature using fluorescence spectrophotometer (Hitachi F-4600). The fluorescence image was also recorded on the microscope with a high pressure mercury lamp as the excitation resource. UV/vis absorption spectroscopy (Persee TU-1901) and photoluminescence spectroscopy (Hitachi F-4600) were equipped to collect the date of optical properties of the samples. The chemical structures and elementary composition were analyzed by X-ray diffraction (XRD, Rigaku D/max-2500) with Cu K␣ and X-ray photo electron spectroscopy (XPS, PERKIN ELMZR PHI 3056 spectrometer) with an Al anode source operated at 15 kV and an applied power of 350 W, respectively. The device was irradiated by a xenon lamp (Beijing Zhongjiaojinyuan Co., Ltd.). This light source delivered a continuous spectrum ranging from 390 to 770 nm. The incident white light was focused and guided by a long
For the as-prepared samples without controlling the decreased temperature acutely, optical properties were investigated by the absorption (red line) and PL (green line) spectra in Figure 1a. The samples exhibit a sharp bandgap PL emission with a narrow full width at half maximum of ca. 30 nm at 550 nm. Three distinguishable absorption peaks were observed in the current system, with the peak positions at 437, 490 and 537 nm, respectively. The discrete and size-dependent optical absorption features are characteristic of quantized electronic transitions of monodispersed nanocrystals. Further investigations indicate that a long reaction time induces the formation of regular nanocrystals (Figure S1). There is no evidence of the evolution of the nanocrystals with the narrow-sized distribution into the regular nanocrystals, but more likely, these nanocrystals dissolve and dissociate into monomers, which feeds the growth of the regular nanocrystals [19]. Typical TEM image of the mono-dispersed CdSe is shown in Figure 1b, indicating that these nanocrystals have the narrow-sized distribution. The samples were further analyzed by the XPS patterns. From the 3d spectrum of Se in Figure 1c, the profile displays a characteristic peak for Se 3d at 54.1 eV, which could be ascribed to Se2− in CdSe. Two peaks in Figure 1d located at 405.1 eV and 412.0 eV are assigned to Cd 3d5/2 and Cd 3d3/2 , respectively. The Cd 3d5/2 at 405.1 eV is ascribed to Cd2+ in CdSe. By decreasing the temperature acutely, bowl-shaped superstructures of CdSe nanocrystals with the narrow-sized distribution were fabricated. Figure 2a shows the morphologies of CdSe superstructures. The bowl-shaped CdSe superstructures are connected to form a three-dimensional network. The diameter and thickness of a single CdSe superstructure are 1–2 m and hundreds of nanometers, respectively. The histogram of superstructural diameter distribution is shown in Figure S2. It reveals that most of superstructures show the size of 1.0–1.4 m. A high-magnification SEM image in Figure 2b shows that the superstructure looks like a bowl with wrinkles on the surface. Figure 2c displays a hollow structure and wrinkles. The high-resolution image (Figure 2d) of the CdSe superstructure presents well-resolved lattice fringes of the (1 1 1) plane with interplanar sapcing of 0.332 nm and 0.342 nm. In the inset of Figure 2d, the selected-area electron diffraction (SAED) pattern of the bowl-shaped structure shows the presence of characteristic diffraction patterns for CdSe (1 1 1) (d = 0.328 nm), (2 2 0) (d = 0.204 nm), and (3 1 1) (d = 0.172 nm). XRD pattern of the CdSe superstructures after removing the excess solvent is shown in Figure S3. The diffraction peaks in this pattern is index to the pure cubic zinc blende phase CdSe (JCPDS No.19-0191) [31], corresponding to the results of SEAD patterns with diffraction rings. The peaks of the CdSe superstructures are broadened compared with bulk CdSe, which is similar to the ordinary nanocrystal [32]. The longrange ordered structure is confirmed by a series of clear small-angle diffraction peaks. Meanwhile, these peaks show a self-assembled stack constructed from the CdSe nanocrystals [33]. The superstructures, the assembly of nanocrystals, reveal different optical properties with mono-dispersed nanocrystals. Optical properties of CdSe superstructures were investigated by absorption
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Figure 1. (a) The absorption (red line) and PL (ex = 350 nm) (green line) spectra of CdSe nanocrystals with narrow-sized distribution. (b) Typical TEM image of the monodispersed nanocrystals. XPS of Se 3d (c) and Cd 3d5/2 and Cd 3d3/2 (d).
Figure 3. (a) The absorption (red line) and PL (ex = 350 nm) (green line) spectra of CdSe superstructures. (b) The fluorescence image of bowl-shaped CdSe superstructures under UV excitation.
Fig. 2. Typical SEM images of the bowl-shaped CdSe superstructures (a and b). Typical TEM image of a bowl-shaped CdSe superstructure (c). HRTEM image of a bowl-shaped CdSe superstructure with the inset of corresponding SEAD pattern (d).
(red line) and PL (green line) spectra in Figure 3a. Three distinguishable absorption peaks are observed in current system, with the peak at 452, 503 and 550 nm, respectively. The sample exhibits a sharp PL emission peak with the narrow full width at half maximum of ca. 30 nm at 572 nm. It indicates that the CdSe nanocrystals have the narrow-sized distribution. Compared with CdSe nanocrystals, the emission peaks of the superstructures is red-shifted from 550 to 572 nm. It shows the effects of quantum confinement on the individual nanocrystals as well as the evidence of interparticle interactions. It should be noted that the red-shifted emission peak is
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Figure 4. TEM images of CdSe superstructures via gradual cooling (a–d), natural cooling (e) and rapid cooling (f).
attributed to the interparticle coupling [34]. In addition, the CdSe superstructures show a green PL emission under ultraviolet light irradiation in Figure 3b. The color of the superstructure emission remains the same over its entire superstructures, which is reflective of the constancy of the diameter of CdSe nanocrystals. The sizes of the luminescent superstructures in the image are consistent with SEM observation. It means that the superstructures are optically active. This feature arises from the assembly of CdSe nanocrystals into superstructures. The formation mechanism of bowl-shaped CdSe superstructures is also investigated by different microstructures of CdSe in the solution at different reaction times. First, the organic stabilized CdSe nanocrystals are prepared (Figure 4a). CdSe nanocrystals are deposited from the solution gradually and uniformly to reach the supersaturation with the decreasing temperatures. It is because of that the solubility of CdSe nanocrystals in 1-ODE decreases slowly. The slow deposition enables CdSe nanocrystals to pack into superstructures (Figure 4b) [30]. As shown in Figure 4c, a large number
of CdSe nanocrystals diffuse at the growing surface to form the hexagonal superlattices. The image of CdSe superstructures with surrounding superlattices was shown in Figure 4d. A relatively high rate of cooling may hinder the formation of superlattices (Figure 4e). It is interesting that the part surface of CdSe superstructures is opened and some internal substances spray out from the interior of the superstructure. Similar microstructures were also given in Figure S4. Finally, a bowl-shaped CdSe superstructure is fabricated with a high rate of cooling (Figure 4f). Semiconductor nanocrystals have a great potential for photoelectric devices in the visible of the solar spectra. However, an inhomogeneous distribution of nanocrystals restricts both charge-carrier mobility and charge collection [27]. Herein, the organic–inorganic hybrid photoswitch based on the CdSe superstructures were fabricated. The CdSe superstructures were mixed with P3HT to form a hybrid film (CdSe:P3HT = 1:1). In this system, CdSe superstructures act as an electron acceptor, while P3HT behaves as an electron donor upon photoexcitation (Figure 5a).
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Figure 5. (a) Energy-level diagram of a P3HT/CdSe superstructures hybrid photoelectric device. (b) On/off switching of the P3HT/CdSe superstructures (red line) and P3HT/CdSe nanocrystals (green line) hybrid devices at an incident light intensity of 460 mW/cm2 and a bias voltage of −0.1 V. (c) Characteristic I–V curves of P3HT/CdSe superstructures of dark current and photocurrents at different incident light intensities. (d) Photocurrent measured as a function of incident light intensity at a bias voltage of −0.1 V.
A xenon lamp was used as a white-light source. Upon the illumination with the light with the energy larger than bandgap (Eg ) of CdSe, the electron-hole pairs are generated and holes are trapped on the surface, leaving unpaired electrons. This feature increases the conductivity of the CdSe superstructures under an applied electric field. For the hybrid photoswitch, the interface of P3HT/CdSe superstructures plays an important role in charge dissociation and transportation, enabling efficient charge separation and good mobility. These effects result in a remarkable increase in photocurrent of the hybrid devices [35]. Figure 5b shows the photocurrent of device during the repetitive switching of light irradiation. With the on/off switching of the light, the photocurrent shows two different states, a low-current in dark and a high current under the irradiation. The reversible and fast switching of two states show a high photosensitivity. For the P3HT/CdSe superstructures device (red line), the current is only 0.025 A in dark. However, at an incident light density of 460 mW/cm2 and a bias voltage of −0.1 V, the current increases sharply to 2.5 A, giving an on/off switching ratio of ∼100. The hybrid device also exhibits a good cycling stability with no obvious degradation during scores of cycles. It is interesting to note that the time constant of rising edges is much smaller than that of falling edges. It might be controlled by the traps and other defect states in CdSe nanocrystals. The photogenerated carries may first fill the traps and defect states under the illumination and then the current reach the maximum until all the traps and defect states are saturated. In the decay stage, there is a large time constant because of the release of carries in dark [36]. In contrast, the device based on P3HT/CdSe nanocrystals shows a low photocurrent of 0.5 A at a bias of −0.1 V, an enhancement of
about 50 times compared with the dark current of 0.01 A (green line). The characteristic I–V curves of the device exposed to different intensities of white light were given in Figure 5c. When the intensity of the light is changed, the photocurrent shows a remarkable change, attributed to a change in the photon intensity of the hybrid organic–inorganic device [35]. The white-light illumination modulates the switching behavior by a large number of photogenerated charges. Therefore, the switching effect is controlled by white light with various densities [37,38]. The dark current and photocurrent curves near zero bias demonstrate the existence of photovoltaic effect [39]. As shown in Figure 5d, the photocurrent is relied on the light irradiance. These results indicate a promising potential of the organic–inorganic hybrid device as a photoswitch and a highly photosensitive detector.
4. Conclusion We synthesized the bowl-shaped superstructures of CdSe nanocrystals with a narrow-sized distribution. The diameter and thickness of a single CdSe superstructure are 1–2 m and hundreds of nanometers, respectively. The formation of bowl-shaped CdSe superstructures is attributed to a high rate of cooling with a low solubility of nanocrystals. Additionally, organic–inorganic hybrid devices were fabricated by using CdSe superstructures and P3HT polymers, and the devices show excellent photoelectric performance of a high on/off switching ratio (Ion /Ioff ≈ 100), a good cylcing stability and high photosensitivity, which probably provide a promising application in the field of photoelectric devices.
B. Zhang et al. / Chemical Physics Letters 633 (2015) 76–81
Acknowledgements This work was support by National Natural Science Funds for Distinguished Young Scholars (Grant no. 51425306), National Natural Science Foundation of China (grant nos. 51103094, 51473116 and 51411140036), the Doctor Project for Young Teachers of Ministry of Education (20110032120021) and the Research Fund for the Doctoral Program of Higher Education of China (Grant no. 20110032110067). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.cplett.2015.05.014 References [1] A.P. Alivisatos, Science 271 (1996) 933. [2] I. Robel, V. Subramanian, M. Kuno, P.V. Kamat, J. Am. Chem. Soc. 128 (2006) 2385. [3] G. Konstantatos, Nature 442 (2006) 180. [4] B. Sun, H.W. Li, L.J. Wei, P. Chen, RSC Adv. 4 (2014) 50102. [5] B. Sun, Q.L. Li, W.X. Zhao, H.W. Li, L.J. Wei, P. Chen, J. Nanopart. Res. 16 (2014) 2389. [6] B. Sun, Y.H. Liu, W.X. Zhao, J.G. Wu, P. Chen, Nano-Micro Lett. 7 (2015) 80. [7] B. Sun, C.M. Li, Phys. Chem. Chem. Phys. 17 (2015) 6718. [8] H.F. Meng, Y. Yang, Y.J. Chen, Y.L. Zhou, Y.L. Liu, X.A. Chen, H.W. Ma, Z.Y. Tang, D.S. Liu, L. Jiang, Chem. Commun. 17 (2009) 2293. [9] B. Qin, H.Y. Chen, H. Liang, L. Fu, X.F. Liu, X.H. Qiu, S.Q. Liu, R. Song, Z.Y. Tang, J. Am. Chem. Soc. 132 (2010) 2886. [10] H.S. Kim, S.W. Jung, S.K. Yang, K.S. Ahn, S.H. Kang, Mater. Lett. 111 (2013) 47. [11] Z.A. Peng, X. Peng, J. Am. Chem. Soc. 123 (2001) 183. [12] P. Viviane, R.L. Sthanley, A.A. Acácio, C.A.S. Anielle, O.D. Noelio, Chem. Phys. Lett. 580 (2013) 130. [13] E. Kucur, J. Ziegler, T. Nann, Small 4 (2008) 883.
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Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Bowl-shaped superstructures of CdSe nanocrystals with the narrow-sized distribution for a high-performance photoswitch Bo Zhang, Yongtao Shen, Yiyu Feng, Chengqun Qin, Zhengcheng Huang, Wei Feng ∗ Tianjin Key Laboratory of Composite and Functional Materials, School of Materials Science and Engineering, Tianjin University, Tianjin 300072, PR China
a r t i c l e
i n f o
Article history: Received 22 March 2015 In final form 7 May 2015 Available online 19 May 2015
a b s t r a c t The bowl-shaped CdSe superstructure with a diameter of 1–2 m and the thickness of hundreds nanometers was synthesized using Cd(SA)2 and Se powder in an organic phase. The CdSe nanocrystals for assembling superstructures had a narrow-sized distribution indicated by a sharp emission peak in the photoluminescence (PL) spectrum. Moreover, an organic–inorganic hybrid photoswitch based on CdSe superstructures were fabricated. The device exhibited an on/off switching ratio of ∼100 with a good cycling stability. The excellent photo-responsible performance illustrates that the superstructures hold a great promise for the application of photoelectric devices. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Photoluminescence colloidal semiconductor nanocrystals, also known as quantum dots, arouse a great interest in fundamental studies and technological applications [1–10]. As one of the most important II–VI nanocrystals, cadmium selenide (CdSe) becomes a ‘model’ nanocrystal system for a hotspot of basic studies on the photoelectrical properties of nanocrystals because of its accessibility of high-quality and stability [11]. Recently, the family of colloidal CdSe with a narrow-sized distribution, which is similar to regular nanocrystals, has attracted much attention of the researchers [12–21]. The CdSe nanocrystals with the narrow-sized distribution hold great promising for the various applications based on a rational design supported by the understanding of nucleation and growth [16]. Much effort has been made in this field. Peng et al. first reported a hot-injection method for the preparation of CdSe with the narrow-sized distribution [17]. The resulting nanocrystals showed a sharp and dominated absorption peak at 349 nm, while these nanocrystals have not be isolated and characterized. Nguyen et al. worked on the systematically characterization of (CdSe)n (n = 1–37) clusters using a combination of structure enumeration, Monte Carlo search and local optimization [18]. The binding energy was calculated using density functional theory to identify the special nanocrystals. Yu et al. prepared multiple families of CdSe with a narrow bandgap via non-injection one-pot syntheses, showing sharp absorption and emission peaks [19]. Results demonstrated
∗ Corresponding author. E-mail address: [email protected] (W. Feng). http://dx.doi.org/10.1016/j.cplett.2015.05.014 0009-2614/© 2015 Elsevier B.V. All rights reserved.
that low acid-to-Cd and high Cd-to-Se feed molar ration are critical for preparing the narrow-sized CdSe [20]. In addition, they proposed a thermodynamic equilibrium driven approach for the formation of CdSe nanocrystals, which featured highly synthetic reproducibility [21]. The assembly of nanocrystals is considered as a vital route for the development of collection performance and tailoring photoelectrical properties [22–24]. Various interparticle forces, such as van der Waals, electrostatic, magnetic, molecular, entropic forces and their balance are used in the nanoscale self-assembly [25,26]. The properties of superstructures are derived from the block units and the interaction between them. Compared with mono-dispersed nanocrystals, superstructures show highly efficient charge transfer, high charge mobility and a broad range of charge collection [27]. Nevertheless, the superstructures of nanocrystals with the narrow-sized distribution are rarely been reported, which might be attributed to the difficulty in modulating the ligands in an organic phase [28]. This problem is potentially overcome by using the nanocrystal supersaturated solution, and the nanocrystals may be deposited from the solution and assemble into superstructures [29,30]. In this letter, we presented a method of assembling CdSe superstructures in an organic phase using cadmium stearate (Cd(SA)2 ) as the Cd source and surface ligands, elemental selenium as Se compounds and 1-Octadecene (1-ODE) as the reaction medium. By decreasing the temperature acutely, we synthesized bowlshaped superstructures of CdSe nanocrystals with the narrow-sized distribution. This simple method was used to produce highquality superstructures. Organic–inorganic hybrid devices based on CdSe superstructures and poly(3-hexylthiophene) (P3HT) were
B. Zhang et al. / Chemical Physics Letters 633 (2015) 76–81
fabricated. The hybrid photoelectric devices, combining the advantages of both organic and inorganic devices, are significantly important for optoelectronic applications including photo detection in visible light region. The hybrid photoelectric devices exhibited an on/off switching ratio of ∼100 with a good cycling stability. These performance enable CdSe superstructures to be used in the application of photocatalysis, energy storage and conversion, and electronic and photoelectric devices.
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optical fiber to avoid the heating effect, and the power intensity delivered on the photoswitch was measured using the light density meter (Beijing Zhongjiaojinyuan Co., Ltd.). The photoelectric properties of the organic–inorganic hybrid device were measured by electrochemical workstation (Chenhua CH1660D) with Pt and Ag/AgCl electrode as the counter and reference electrode. All the photoelectric tests were conducted in 0.1 M KCl at room temperature.
2. Experimental 3. Results and discussion 2.1. Materials 1-ODE (tech. 90%) and poly(3-hexylthiophene) (P3HT) were purchased from J&K Chemical Ltd. (Beijing China). Cd(SA)2 (18.8–20% by cadmium oxide) and selenium powder (99.7%) were purchased form Guangfu Chemical Institute (Tianjin, China). All chemical reagents were used without the further purification. 2.2. Synthesis of CdSe nanocrystals Cd(SA)2 (136 mg) and selenium powder (4.0 mg) were added into a 100 mL three necked round-bottom flask, which containing 7 mL 1-ODE. Afterwards, five argon-vacuum cycles and vacuum treatment were executed to remove oxygen and moisture. Then, under a flow of purified argon, the reaction mixture was heated to 230 ◦ C for 30 min at a rate of 5 ◦ C min−1 . The resultant products were cooled down gradually and then washed with isopropyl alcohol by centrifugation and finally dispersed in toluene. 2.3. Synthesis of CdSe superstructures The CdSe nanocrystals with the narrow-sized distribution were synthesized by Cd(SA)2 , Se powder and 1-ODE. First, the mono-dispersed CdSe nanocrystals were synthesized at a high temperature. Then, the solution was cooled down to room temperature. The resultant products were washed with isopropyl alcohol by centrifugation for several times and finally dispersed in toluene. 2.4. Fabrication of the device P3HT (10 mg) was dissolved in 2 mL of toluene. The CdSe superstructures solution (100 L, 10 mg/mL) and P3HT solution (200 L) were well mixed into a homogenous solution, and the device was fabricated by dropping 100 L of the solution onto pre-cleaned Indium Tin Oxide (ITO) glass. The device was dried and stored in vacuum for the measurement. 2.5. Characterization The morphologies of CdSe superstructures were observed by SEM (Hitachi S-4800), TEM (Phlilps Tecnai G2 F20). PL spectra were investigated at room temperature using fluorescence spectrophotometer (Hitachi F-4600). The fluorescence image was also recorded on the microscope with a high pressure mercury lamp as the excitation resource. UV/vis absorption spectroscopy (Persee TU-1901) and photoluminescence spectroscopy (Hitachi F-4600) were equipped to collect the date of optical properties of the samples. The chemical structures and elementary composition were analyzed by X-ray diffraction (XRD, Rigaku D/max-2500) with Cu K␣ and X-ray photo electron spectroscopy (XPS, PERKIN ELMZR PHI 3056 spectrometer) with an Al anode source operated at 15 kV and an applied power of 350 W, respectively. The device was irradiated by a xenon lamp (Beijing Zhongjiaojinyuan Co., Ltd.). This light source delivered a continuous spectrum ranging from 390 to 770 nm. The incident white light was focused and guided by a long
For the as-prepared samples without controlling the decreased temperature acutely, optical properties were investigated by the absorption (red line) and PL (green line) spectra in Figure 1a. The samples exhibit a sharp bandgap PL emission with a narrow full width at half maximum of ca. 30 nm at 550 nm. Three distinguishable absorption peaks were observed in the current system, with the peak positions at 437, 490 and 537 nm, respectively. The discrete and size-dependent optical absorption features are characteristic of quantized electronic transitions of monodispersed nanocrystals. Further investigations indicate that a long reaction time induces the formation of regular nanocrystals (Figure S1). There is no evidence of the evolution of the nanocrystals with the narrow-sized distribution into the regular nanocrystals, but more likely, these nanocrystals dissolve and dissociate into monomers, which feeds the growth of the regular nanocrystals [19]. Typical TEM image of the mono-dispersed CdSe is shown in Figure 1b, indicating that these nanocrystals have the narrow-sized distribution. The samples were further analyzed by the XPS patterns. From the 3d spectrum of Se in Figure 1c, the profile displays a characteristic peak for Se 3d at 54.1 eV, which could be ascribed to Se2− in CdSe. Two peaks in Figure 1d located at 405.1 eV and 412.0 eV are assigned to Cd 3d5/2 and Cd 3d3/2 , respectively. The Cd 3d5/2 at 405.1 eV is ascribed to Cd2+ in CdSe. By decreasing the temperature acutely, bowl-shaped superstructures of CdSe nanocrystals with the narrow-sized distribution were fabricated. Figure 2a shows the morphologies of CdSe superstructures. The bowl-shaped CdSe superstructures are connected to form a three-dimensional network. The diameter and thickness of a single CdSe superstructure are 1–2 m and hundreds of nanometers, respectively. The histogram of superstructural diameter distribution is shown in Figure S2. It reveals that most of superstructures show the size of 1.0–1.4 m. A high-magnification SEM image in Figure 2b shows that the superstructure looks like a bowl with wrinkles on the surface. Figure 2c displays a hollow structure and wrinkles. The high-resolution image (Figure 2d) of the CdSe superstructure presents well-resolved lattice fringes of the (1 1 1) plane with interplanar sapcing of 0.332 nm and 0.342 nm. In the inset of Figure 2d, the selected-area electron diffraction (SAED) pattern of the bowl-shaped structure shows the presence of characteristic diffraction patterns for CdSe (1 1 1) (d = 0.328 nm), (2 2 0) (d = 0.204 nm), and (3 1 1) (d = 0.172 nm). XRD pattern of the CdSe superstructures after removing the excess solvent is shown in Figure S3. The diffraction peaks in this pattern is index to the pure cubic zinc blende phase CdSe (JCPDS No.19-0191) [31], corresponding to the results of SEAD patterns with diffraction rings. The peaks of the CdSe superstructures are broadened compared with bulk CdSe, which is similar to the ordinary nanocrystal [32]. The longrange ordered structure is confirmed by a series of clear small-angle diffraction peaks. Meanwhile, these peaks show a self-assembled stack constructed from the CdSe nanocrystals [33]. The superstructures, the assembly of nanocrystals, reveal different optical properties with mono-dispersed nanocrystals. Optical properties of CdSe superstructures were investigated by absorption
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Figure 1. (a) The absorption (red line) and PL (ex = 350 nm) (green line) spectra of CdSe nanocrystals with narrow-sized distribution. (b) Typical TEM image of the monodispersed nanocrystals. XPS of Se 3d (c) and Cd 3d5/2 and Cd 3d3/2 (d).
Figure 3. (a) The absorption (red line) and PL (ex = 350 nm) (green line) spectra of CdSe superstructures. (b) The fluorescence image of bowl-shaped CdSe superstructures under UV excitation.
Fig. 2. Typical SEM images of the bowl-shaped CdSe superstructures (a and b). Typical TEM image of a bowl-shaped CdSe superstructure (c). HRTEM image of a bowl-shaped CdSe superstructure with the inset of corresponding SEAD pattern (d).
(red line) and PL (green line) spectra in Figure 3a. Three distinguishable absorption peaks are observed in current system, with the peak at 452, 503 and 550 nm, respectively. The sample exhibits a sharp PL emission peak with the narrow full width at half maximum of ca. 30 nm at 572 nm. It indicates that the CdSe nanocrystals have the narrow-sized distribution. Compared with CdSe nanocrystals, the emission peaks of the superstructures is red-shifted from 550 to 572 nm. It shows the effects of quantum confinement on the individual nanocrystals as well as the evidence of interparticle interactions. It should be noted that the red-shifted emission peak is
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Figure 4. TEM images of CdSe superstructures via gradual cooling (a–d), natural cooling (e) and rapid cooling (f).
attributed to the interparticle coupling [34]. In addition, the CdSe superstructures show a green PL emission under ultraviolet light irradiation in Figure 3b. The color of the superstructure emission remains the same over its entire superstructures, which is reflective of the constancy of the diameter of CdSe nanocrystals. The sizes of the luminescent superstructures in the image are consistent with SEM observation. It means that the superstructures are optically active. This feature arises from the assembly of CdSe nanocrystals into superstructures. The formation mechanism of bowl-shaped CdSe superstructures is also investigated by different microstructures of CdSe in the solution at different reaction times. First, the organic stabilized CdSe nanocrystals are prepared (Figure 4a). CdSe nanocrystals are deposited from the solution gradually and uniformly to reach the supersaturation with the decreasing temperatures. It is because of that the solubility of CdSe nanocrystals in 1-ODE decreases slowly. The slow deposition enables CdSe nanocrystals to pack into superstructures (Figure 4b) [30]. As shown in Figure 4c, a large number
of CdSe nanocrystals diffuse at the growing surface to form the hexagonal superlattices. The image of CdSe superstructures with surrounding superlattices was shown in Figure 4d. A relatively high rate of cooling may hinder the formation of superlattices (Figure 4e). It is interesting that the part surface of CdSe superstructures is opened and some internal substances spray out from the interior of the superstructure. Similar microstructures were also given in Figure S4. Finally, a bowl-shaped CdSe superstructure is fabricated with a high rate of cooling (Figure 4f). Semiconductor nanocrystals have a great potential for photoelectric devices in the visible of the solar spectra. However, an inhomogeneous distribution of nanocrystals restricts both charge-carrier mobility and charge collection [27]. Herein, the organic–inorganic hybrid photoswitch based on the CdSe superstructures were fabricated. The CdSe superstructures were mixed with P3HT to form a hybrid film (CdSe:P3HT = 1:1). In this system, CdSe superstructures act as an electron acceptor, while P3HT behaves as an electron donor upon photoexcitation (Figure 5a).
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Figure 5. (a) Energy-level diagram of a P3HT/CdSe superstructures hybrid photoelectric device. (b) On/off switching of the P3HT/CdSe superstructures (red line) and P3HT/CdSe nanocrystals (green line) hybrid devices at an incident light intensity of 460 mW/cm2 and a bias voltage of −0.1 V. (c) Characteristic I–V curves of P3HT/CdSe superstructures of dark current and photocurrents at different incident light intensities. (d) Photocurrent measured as a function of incident light intensity at a bias voltage of −0.1 V.
A xenon lamp was used as a white-light source. Upon the illumination with the light with the energy larger than bandgap (Eg ) of CdSe, the electron-hole pairs are generated and holes are trapped on the surface, leaving unpaired electrons. This feature increases the conductivity of the CdSe superstructures under an applied electric field. For the hybrid photoswitch, the interface of P3HT/CdSe superstructures plays an important role in charge dissociation and transportation, enabling efficient charge separation and good mobility. These effects result in a remarkable increase in photocurrent of the hybrid devices [35]. Figure 5b shows the photocurrent of device during the repetitive switching of light irradiation. With the on/off switching of the light, the photocurrent shows two different states, a low-current in dark and a high current under the irradiation. The reversible and fast switching of two states show a high photosensitivity. For the P3HT/CdSe superstructures device (red line), the current is only 0.025 A in dark. However, at an incident light density of 460 mW/cm2 and a bias voltage of −0.1 V, the current increases sharply to 2.5 A, giving an on/off switching ratio of ∼100. The hybrid device also exhibits a good cycling stability with no obvious degradation during scores of cycles. It is interesting to note that the time constant of rising edges is much smaller than that of falling edges. It might be controlled by the traps and other defect states in CdSe nanocrystals. The photogenerated carries may first fill the traps and defect states under the illumination and then the current reach the maximum until all the traps and defect states are saturated. In the decay stage, there is a large time constant because of the release of carries in dark [36]. In contrast, the device based on P3HT/CdSe nanocrystals shows a low photocurrent of 0.5 A at a bias of −0.1 V, an enhancement of
about 50 times compared with the dark current of 0.01 A (green line). The characteristic I–V curves of the device exposed to different intensities of white light were given in Figure 5c. When the intensity of the light is changed, the photocurrent shows a remarkable change, attributed to a change in the photon intensity of the hybrid organic–inorganic device [35]. The white-light illumination modulates the switching behavior by a large number of photogenerated charges. Therefore, the switching effect is controlled by white light with various densities [37,38]. The dark current and photocurrent curves near zero bias demonstrate the existence of photovoltaic effect [39]. As shown in Figure 5d, the photocurrent is relied on the light irradiance. These results indicate a promising potential of the organic–inorganic hybrid device as a photoswitch and a highly photosensitive detector.
4. Conclusion We synthesized the bowl-shaped superstructures of CdSe nanocrystals with a narrow-sized distribution. The diameter and thickness of a single CdSe superstructure are 1–2 m and hundreds of nanometers, respectively. The formation of bowl-shaped CdSe superstructures is attributed to a high rate of cooling with a low solubility of nanocrystals. Additionally, organic–inorganic hybrid devices were fabricated by using CdSe superstructures and P3HT polymers, and the devices show excellent photoelectric performance of a high on/off switching ratio (Ion /Ioff ≈ 100), a good cylcing stability and high photosensitivity, which probably provide a promising application in the field of photoelectric devices.
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Acknowledgements This work was support by National Natural Science Funds for Distinguished Young Scholars (Grant no. 51425306), National Natural Science Foundation of China (grant nos. 51103094, 51473116 and 51411140036), the Doctor Project for Young Teachers of Ministry of Education (20110032120021) and the Research Fund for the Doctoral Program of Higher Education of China (Grant no. 20110032110067). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.cplett.2015.05.014 References [1] A.P. Alivisatos, Science 271 (1996) 933. [2] I. Robel, V. Subramanian, M. Kuno, P.V. Kamat, J. Am. Chem. Soc. 128 (2006) 2385. [3] G. Konstantatos, Nature 442 (2006) 180. [4] B. Sun, H.W. Li, L.J. Wei, P. Chen, RSC Adv. 4 (2014) 50102. [5] B. Sun, Q.L. Li, W.X. Zhao, H.W. Li, L.J. Wei, P. Chen, J. Nanopart. Res. 16 (2014) 2389. [6] B. Sun, Y.H. Liu, W.X. Zhao, J.G. Wu, P. Chen, Nano-Micro Lett. 7 (2015) 80. [7] B. Sun, C.M. Li, Phys. Chem. Chem. Phys. 17 (2015) 6718. [8] H.F. Meng, Y. Yang, Y.J. Chen, Y.L. Zhou, Y.L. Liu, X.A. Chen, H.W. Ma, Z.Y. Tang, D.S. Liu, L. Jiang, Chem. Commun. 17 (2009) 2293. [9] B. Qin, H.Y. Chen, H. Liang, L. Fu, X.F. Liu, X.H. Qiu, S.Q. Liu, R. Song, Z.Y. Tang, J. Am. Chem. Soc. 132 (2010) 2886. [10] H.S. Kim, S.W. Jung, S.K. Yang, K.S. Ahn, S.H. Kang, Mater. Lett. 111 (2013) 47. [11] Z.A. Peng, X. Peng, J. Am. Chem. Soc. 123 (2001) 183. [12] P. Viviane, R.L. Sthanley, A.A. Acácio, C.A.S. Anielle, O.D. Noelio, Chem. Phys. Lett. 580 (2013) 130. [13] E. Kucur, J. Ziegler, T. Nann, Small 4 (2008) 883.
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