One-step synthesis of PbSe-ZnSe composite thin film
© Abe; licensee Springer. 2011
Received: 20 January 2011
Accepted: 12 April 2011
Published: 12 April 2011
This study investigates the preparation of PbSe-ZnSe composite thin films by simultaneous hot-wall deposition (HWD) from multiple resources. The XRD result reveals that the solubility limit of Pb in ZnSe is quite narrow, less than 1 mol%, with obvious phase-separation in the composite thin films. A nanoscale elemental mapping of the film containing 5 mol% PbSe indicates that isolated PbSe nanocrystals are dispersed in the ZnSe matrix. The optical absorption edge of the composite thin films shifts toward the low-photon-energy region as the PbSe content increases. The use of a phase-separating PbSe-ZnSe system and HWD techniques enables simple production of the composite package.
Quantum-dot solar cells have attracted much attention because of their potential to increase conversion efficiency . Specifically, the optical-absorption edge of a semiconductor nanocrystal is often shifted due to the quantum-size effect. The optical band gap can then be tuned to the effective energy region for absorbing maximum intensity over the solar radiation spectrum. Furthermore, quantum dots produce multiple electron-hole pairs per photon through impact ionization, whereas bulk semiconductors produce one electron-hole pair per photon.
Wide-gap semiconductor sensitized by quantum dot is a candidate material for such use. The quantum dot supports absorbing visible and near-infrared light. Up to now, various nanocrystalline materials (InP , CdSe , CdS [4, 5], PbS , and Ge ) have been investigated as the sensitizer for TiO2. Alternatively, a wide-gap semiconductor ZnO is also investigated, since the band gap and the energetic position of the valence band maximum and conduction band minimum of ZnO are very close to that of TiO2 . Most of these composite materials were synthesized through chemical techniques, however, physical deposition, such as sputtering, is also useful. In the material design for co-sputtering, based on the heat of formation, nanocrystal and matrix are clearly phase-separated in spite of the co-deposition from multiple sources [9, 10]. However, it is generally found that sputtering techniques often damage a film due to contamination of the fed gas and high-energy bombardment of the film surface. Thermal evaporation in a high-vacuum atmosphere seems to be better as a preparation technique from the point of view of film quality. In addition, the present study focuses on the insolubility of the material system, since simultaneous evaporation from multiple sources often provides a solid solution . The PbSe-ZnSe system is a candidate for the composite. In the bulk thermal equilibrium state, the mutual solubility range is quite narrow, less than 1 mol%, at temperatures below 1283 K . In addition, a composite thin film of PbSe nanocrystal embedded in ZnSe matrix is capable of exhibiting the quantum size effect because of the relatively large exciton Bohr radius of 46 nm in PbSe  and the relatively wide band gap of 2.67 eV in ZnSe . Hence, the optical gap of PbSe nanocrystals will probably be tuned to the maximum solar radiation spectrum. The dendritic PbSe nanostructure  and ZnSe nanobelt array , for instance, are hitherto investigated, but there is no report for one-step synthesis of PbSe-ZnSe composite thin film. Furthermore, an evaporation technique should be carefully selected, since the techniques involving a thermal non-equilibrium state, such as molecular beam epitaxy, increase the solubility limit . The use of hot-wall deposition (HWD), which can provide an atmosphere near thermal equilibrium, is therefore indicated here . Based on these considerations, one-step synthesis of a PbSe-ZnSe composite thin film was investigated by simultaneous HWD from multiple sources for the first time.
Results and discussion
The two sources were simultaneously evaporated to prepare a PbSe-ZnSe composite thin film. In the apparatus used, thermal radiation from the wall- and the source-furnace induced an unintentional increase of the substrate temperature up to 515 K without use of the substrate-furnace. The deposition rate of the film was almost the same irrespective of the substrate temperature in the range from 515 to 593 K. A homogeneous color is observed visually in these films. Above a substrate temperature of 593 K, the deposition rate abruptly decreased with increasing temperature, since re-evaporation of PbSe from the film surface became dominant. The films visually exhibit an inhomogeneous yellowish and metallic color, probably caused by a significant reduction in the PbSe while the ZnSe remained, due to the relatively high vapor pressure of PbSe . The wall temperature also induced similar behavior. A substrate temperature of 573 K and a wall temperature of 773 K are therefore adopted throughout the present study.
We investigated the preparation of PbSe-ZnSe composite thin films by a co-evaporating HWD method. The relatively high substrate and wall temperatures induce re-evaporation of PbSe from the substrate surface while the ZnSe remains. The solubility limit of Pb in ZnSe is quite narrow, less than 1 mol% in the film form, indicating that an atmosphere near thermal equilibrium is achieved in the apparatus used. Elemental mapping indicates that isolated PbSe nanocrystals are dispersed in the ZnSe matrix. The optical absorption edge shifts toward the lower-photon-energy region as the PbSe content increases. In particular, onset absorption can be confirmed at approximately 1.0 eV with 16 mol% PbSe, favorably covering the desirable energy region for high conversion efficiency. The insolubility material system and the HWD technique enable a one-step synthesis of PbSe-ZnSe composite thin film. Further investigation is needed to produce a narrower size distribution of the PbSe nanocrystals through the use of a single-crystal substrate, for instance, to control the growth direction, or through using a different phase-separating material system.
The present work was supported by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (No.18360338). The author gratefully acknowledges the valuable comments and continuous encouragement of President T. Masumoto [Research Institute for Electric and Magnetic Materials (RIEMM), Sendai, Japan]. The author is also grateful to Mr. N. Hoshi and Y. Sato (RIEMM) for assisting in the experiments.
- Nozik AJ: Quantum dot solar cells. Phys E 2002, 14: 115–120. 10.1016/S1386-9477(02)00374-0View Article
- Zaban A, Micic OI, Gregg BA, Nozik AJ: Photosensitization of nanoporus TiO 2 electrodes with InP quantum dots. Langmuir 1998, 14: 3153–3156. 10.1021/la9713863View Article
- Liu D, Kamat PV: Photoelectrochemical behavior of thin CdSe and coupled TiO 2 /CdSe semiconductor films. J Phys Chem 1993, 97: 10769–10763. 10.1021/j100143a041View Article
- Weller H: Quantum sized semiconcuctor particles in solution in modified layers. Ber Bunsen-Ges Phys Chem 1991, 95: 1361–1365.View Article
- Zhu G, Su F, Lv T, Pan L, Sun Z: Au nanoparticles as interfacial layer for CdS quantum dot-sensitized solar cells. Nanoscale Res Lett 2010, 5: 1749–1754. 10.1007/s11671-010-9705-zView Article
- Hoyer P, Könenkamp R: Photoconduction in porus TiO 2 sensitized by PbS quantum dots. Appl Phys Lett 1995, 66: 349–351. 10.1063/1.114209View Article
- Chatterjee S, Goyal A, Shah I: Inorganic nanocomposites for next generation photovoltaics. Mater Lett 2006, 60: 3541–3543. 10.1016/j.matlet.2006.03.048View Article
- Yang W, Wan F, Chen S, Jiang C: Hydrothermal Growth and Application of ZnO Nanowire Films with ZnO and TiO2 Buffer Layers in Dye-Sensitized Solar Cells. Nanoscale Res Lett 2009, 4: 1486–1492. 10.1007/s11671-009-9425-4View Article
- Ohnuma S, Fujimori H, Mitani S, Masumoto T: High-frequency magnetic properties in metal-nonmetal granular films. J Appl Phys 1996, 79: 5130–5135. 10.1063/1.361531View Article
- Abe S, Ohnuma M, Ping DH, Ohnums S: Anatase-Dominant Matrix in Ge/TiO 2 Thin Films Prepared by RF Sputtering Method. Appl Phys Exp 2008, 1: 095001. 10.1143/APEX.1.095001View Article
- Abe S, Masumoto K: Compositional plane and properties of solid solution semiconductor Pb 1-x Ca x S 1-y Se y for mid-infrared lasers. J Cryst Growth 1999, 204: 115–121. 10.1016/S0022-0248(99)00192-XView Article
- Oleinik GS, Mizetskii PA, Nizkova AI: High-frequency magnetic properties in metal-nonmetal granular films. Inorg Mater 1982, 18: 734–735.
- Wise FW: Lead salts quantum dots: the limit of strong confinement. Acc Chem Res 2000, 33: 773–780. 10.1021/ar970220qView Article
- Adachi S, Taguchi T: Optical properties of ZnSe. Phys Rev B 1991, 43: 9569–9577. 10.1103/PhysRevB.43.9569View Article
- Xue D: A template-free solution method based on solid-liquid interface reaction towards dendritic PbSe nanostructures. Mod Phys Lett B 2009, 23: 3817–3823. 10.1142/S0217984909021879View Article
- Liu J, Xue D: Solution-based route to semiconductor film: Well-aligned ZnSe nanobelt arrays. Thin Solid Films 2009, 517: 4814–4817. 10.1016/j.tsf.2009.03.021View Article
- Koguchi N, Kiyosawa T, Takahashi S: Double hetero structure of Pb 1-x Cd x S 1-y Se y lasers grown by molecular beam epitaxy. J Cryst Growth 1987, 81: 400–404. 10.1016/0022-0248(87)90424-6View Article
- Lopez-Otero A: Hot wall epitaxy. Thin Solid Films 1978, 49: 3–57. 10.1016/0040-6090(78)90309-7View Article
- Nelson JB, Riley DP: An experimental investigation of extrapolation methods in the derivation of accurate unit-cell dimensions of crystals. Proc Phys Soc 1945, 57: 160–177. 10.1088/0959-5309/57/3/302View Article
- Mills KC: Thermodynamic Data for Inorganic Sulphide, Selenides and Tellurides. London: Butterworth; 1974.
- Theis D: Wavelength modulated reflectivity spectra of ZnSe and ZnS from 2.5 to 8 eV. Phys Status Solidi B 1977, 79: 125–130. 10.1002/pssb.2220790112View Article
- Zemel JN, Jensen JD, Schoolar RB: Electrical and optical properties of epitaxial films of PbS, PbSe, PbTe, and SnTe. Phys Rev 1965, 140: A330-A342. 10.1103/PhysRev.140.A330View Article
- Loferski JJ: Theoretical considerations covering the choice of the optinum semiconductor for photovoltaic solar energy conversion. J Appl Phys 1956, 27: 777–784. 10.1063/1.1722483View Article
- Guinier A: X-ray Diffraction in Crystals, Imperfect Crystals, and Amorphous Bodies. New York: Dover Publications; 1994.
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.