6866
Langmuir 2005, 21, 6866-6871
Growth and Photoluminescence Characterization of Highly Oriented CuI/β-Cyclodextrin Hybrid Composite Film Yang Yang and Qiuming Gao* State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China Received January 3, 2005. In Final Form: May 22, 2005 A highly (111) oriented CuI/β-cyclodextrin inorganic/organic composite film on glass substrates was deposited from a dispersion of CuI and β-cyclodextrin in a mixture of acetonitrile and dimethylformamide (DMF) solvents, in which DMF plays an important role in the orientation of CuI film. The composite film exhibits obviously improved band gap photoluminescence compared with that of the pure CuI film due to the passivation of iodine-related defect sites by β-cyclodextrin matrix. This result indicates that the optical properties of CuI film can be easily adjusted by the formation of composite film with proper ligand agents.
Introduction The low-temperature (below 350 °C) γ-phase CuI of cubic structure is one of a few p-type semiconductors, whose direct fundamental band gap is up to 3.1 eV in the visible region (blue) of the spectrum.1,2 These properties make γ-phase CuI thin films very useful in dye sensitized solidstate solar cells as a hole-collecting agent and potentially applicable in the fabrication of room-temperature blueemitting devices, field emission displays, and vacuum fluorescent displays.3-5 CuI thin films have been reported to fabricate by various techniques including radio frequency/direct current (rf-dc) coupled magnetron sputtering,6 pulse laser deposition,7 hybrid electrochemical/ chemical synthesis,8 vacuum evaporation,9 and others.10,11 However, a continuing need for alternative deposition techniques to grow more reproducible and better quality films independent of complex procedures, sophisticated equipment, or rigid experimental conditions is still necessary. Acetonitrile solvent has been found to have good dissolving ability for CuI samples at room temperature. It is well-known that CuI is a water-insoluble solid. Its good solubility in acetonitrile is ascribed to the formation of weak CuI-acetonitrile adducts whose coordination originates from a significant π back-bonding from cuprous * Corresponding author. Telephone: 86-21-52412513. Fax: 8621-52413122. E-mail address:
[email protected]. (1) Chahid, A.; Mcgreevy, R. L. Physica B 1997, 234, 87. (2) Sekkal, W.; Zaoui, A. Physica B 2002, 315, 201. (3) Tennakone, K.; Kumara, G. R. R. A.; Kumarasinghe, A. R.; Wijayantha, K. U. G.; Sirmanne, P. M. Semicond. Sci. Technol. 1995, 10, 1689. (4) Kumara, G. R. R. A.; Kaneko, S.; Okuya, M.; Tennakone, K. Langmuir 2002, 18, 10493. (5) Tanaka, I.; Nakayama, M. J. Appl. Phys. 2002, 92, 3511. (6) Tanaka, T.; Kawabata, K.; Hirose, M. Thin Solid Films 1996, 281-282, 179. (7) Sirimanne, P. M.; Rusop, M.; Shirata, T.; Soga, T.; Jimbo, T. Chem. Phys. Lett. 2002, 366, 485. (8) Hsiao, G. S.; Anderson, M. G.; Gorer, S.; Harris, D.; Penner, R. M. J. Am. Chem. Soc. 1997, 119, 1439. (9) Kim, D.; Nakayama, M.; Kojima, O. Phys. Rev. B 1999, 60, 13879. (10) Tennakone, K.; Punchihewa, S.; Kiridena, W. C. B.; Ketipearachchi, U. S.; Senadeera, S. Thin Solid Films 1992, 217, 129. (11) Yang, Y.; Li, X. F.; Zhao, B.; Chen, H. L.; Bao, X. M. Chem. Phys. Lett. 2004, 387, 400.
ion to nitrogen, which has been confirmed by infrared and Raman spectra investigations.12,13 The formed CuIacetonitrile adducts are metastable and easy to revert to CuI material with a ligand missing. This interesting property has made solution deposition a fairly convenient technique for coating thin optically transparent CuI film on glass substrates in recent years.14,15 However, CuI film deposited from acetonitrile solution usually consists of large cubic crystallites, so the surface of the obtained CuI film is very rough. Such a structure cannot form stable and firm contact with other functional materials, which distinctly restricts its applications in many optoelectronic devices.16 Besides, when CuI film prepared by solution evaporation is exposed to light, radiation with energy higher than that of the CuI band gap will inevitably photodecompose CuI and liberate iodine, which will strongly affect the stability of its optical and electronic properties. For example, the emission of this film usually originates from iodine-related defects, which is unstable owing to the change of the amount of iodine adsorbed on the CuI surface.15 In the present study, we make use of CuI-acetonitrile solution and demonstrate a novel approach to obtain a new CuI-related inorganic/organic hybrid composite film by introducing β-cyclodextrin to the system. It is found that highly (111) oriented CuI hybrid film with improved quality could be easily prepared on glass substrate, via simple solvent evaporation at room temperature. Furthermore, the composite film exhibits obviously improved band gap photoluminescence, indicating that β-cyclodextrin is a kind of organic species exhibiting a special function for this composite film. To the best of our knowledge, the (12) Zarembowitch, J.; Maleki, R. Spectrochim. Acta, Part A 1983, 39, 43. (13) Bell, A.; Walton, R. A.; Edwards, D. A.; Poulter, M. A. Inorg. Chim. Acta 1985, 104, 171. (14) Tennakone, K.; Kumara, G. R. R. A.; Kottegoda, I. R. M.; Perera, V. P. S.; Aponsu, G. M. L. P.; Wijayantha, K. G. U. Sol. Energy Mater. Sol. Cells 1998, 55, 283. (15) Perera, V. P. S.; Tennakone, K. Sol. Energy Mater. Sol. Cells 2003, 79, 249. (16) Kumara, G. R. A.; Konno, A.; Shiratsuchi, K.; Tsukahara, J.; Tennakone, K. Chem. Mater. 2002, 14, 954.
10.1021/la050005f CCC: $30.25 © 2005 American Chemical Society Published on Web 06/22/2005
CuI/β-Cyclodextrin Hybrid Composite Film
Langmuir, Vol. 21, No. 15, 2005 6867
use of cyclodextrin inclusion to control the luminescence property of semiconductor CuI film has not been reported so far. Experimental Section Synthesis. A 25.0 mL volume of KI (3.32 g, 20.0 mmol) aqueous solution was added to 25.0 mL of CuCl2‚2H2O (1.70 g, 10.0 mmol) aqueous solution with stirring. Dark brown precipitates could be instantly found in the mixed solution. Subsequently, 50.0 mL of Na2S2O3‚5H2O (2.48 g, 10.0 mmol) aqueous solution was added to the solution, which was stirred for 30 min. The pale brown product was filtered off, and washed with distilled water and ethanol several times to remove excess iodine and other byproducts. The final white product was dried in vacuo at 25 °C and ground into powders for further analysis. In an attempt to fabricate CuI/β-cyclodextrin hybrid composite film, 190 mg of CuI (1.0 mmol) powder was first dissolved in 50.0 mL of anhydrous acetonitrile solvent under ultrasonic condition at 25 °C. A transparent pale yellow solution was obtained. A 110 mg (0.1 mmol) sample of β-cyclodextrin was dissolved in 50.0 mL of dimethylformamide (DMF) solvent, followed by mixing with the above CuI-acetonitrile solution. The clear mixed solution was mechanically stirred for 1 h. The resulting solution was spread on a dry glass plate (1 × 1 cm2), which was rinsed with acetone and distilled water prior to the deposition. After evaporation of the mixed solvents under decompression at 25 °C, a hybrid CuI thin film was formed on the substrate. In addition, the thickness of the film could be increased by repetition of this process. Pure CuI films deposited from a dispersion of CuI in acetonitrile, and in the mixture of acetonitrile and DMF, were also prepared, respectively, in contrast to this hybrid film. Characterization. Surface morphology of the prepared thin films was observed on an atomic force microscope (AFM; Nanoscope IIIa) or a scanning electron microscope (SEM; EPMA8705QH2). Crystalline structure was characterized by X-ray diffraction (XRD; Rigaku Dmax 2200PC, Cu KR). Electrospray ionization mass spectrometric (ESI-MS) spectra of CuI-acetonitrile solution at different concentrations were carried out on an LCQ-Finnigan instrument. UV-vis absorption spectra of the solution samples were recorded on a Shimadzu UV-3101PC spectrophotometer by using cuvettes with a 1 cm optical path length. Emission spectra were studied on a Shimadzu RF-5301PC spectrophotometer. All the measurements were performed at room temperature.
Results and Discussion The surface morphology of a prepared CuI/β-cyclodextrin hybrid composite film was observed in an AFM image, which is presented in Figure 1a. For comparison, an AFM surface image of pure CuI film deposited from the mixture of acetonitrile and DMF is also exhibited in Figure 1b. Both of the obtained films are compact, smooth, and dense in surface structure. The apparent grain size is about 200300 nm, unlike the CuI film directly deposited from CuI/ acetonitrile solution (see below), which is usually composed of large cubic crystallites.16 This result is consistent with the work previous reported, in which the CuI film quality could be noticeably improved, with crystal size reduced, when 1-methyl-3-ethylimidazolium thiocyanate (MEISCN), a kind of crystal growth inhibitor, was introduced in the coating solution.16 Figure 2a shows the XRD pattern of this composite film, in which a very strong peak and two weak ones could be detected. These peaks could be assigned to the (111), (220), and (222) planes of the face-centered-cubic structure (fcc) of CuI. The very high diffraction peak intensity of the (111) plane and the coexistence of the (222) plane indicate that the deposited CuI/β-cyclodextrin film is highly oriented along the [111] crystal axis perpendicular to the substrate surface. No other peaks were observed even by increasing the sensitivity on the expanded scale. In addition, owing to the very strong (111) diffraction of CuI
Figure 1. AFM images of surface of CuI/β-cyclodextrin hybrid composite film (a) and pure CuI film (b) deposited from mixture of acetonitrile and DMF on glass substrates.
film, the broadened XRD pattern originating from the glass substrate could not be observed. Therefore, the prepared hybrid film is of high crystallinity even at room temperature, which can also be confirmed by the ultranarrow (111) full width at half-maximum line width (
