紫外輻射與溶膠凝膠

1、Ultraviolet (UV) IrradiationINTRODUCTIONSolgel methods commonly include a heating process which removes residual organic components and promotes densification and crystallization of amorphous gels. In recent years, lower processing temperatures have been demanded for processing on polymer substrates

2、 and for preparing organicinorganic composite or hybrid materials. Energetic photons in ultraviolet (UV) lights directly activate various kinds of chemical bonds in substances. Thus, it is generally expected that the promotion of chemical reactions in sols or gels and the modification of gel network

3、s are achieved by UV irradiation at low temperatures instead of conventional heating processes. Thermal excitation is relatively important when the local temperature at the surface is increased by irradiation with intense UV lights (phonon mode). However, energetic photons in UV illumination mainly

4、induce electronic excitation, including the activation of organic and inorganic components (photon mode). This is an essentially different point from the effects induced with irradiation techniques using visible and infrared lights. The decomposition of residual organic compounds with UV excitation

5、is effective to lower the processing temperature for the crystallization of solgel films. Another feature of UV irradiation techniques is the achievement of spatial selectivity of excitation. Most studies on UV irradiation in solgel routes have been performed for thin-film processing because the pen

6、etration depth of UV photons is limited in the surface region. UV modification can be achieved even on low-heat-resistance substrates because the excitation is limited in the surface layer of the films. Two-dimensional patterning was also performed by irradiation through a mask. In this chapter, fun

7、damental effects induced by UV irradiation and their application to solgel routes are described.CLASSIFICATION OF UV-INDUCED EFFECTS UPON SOLGEL ROUTESStudies on UV irradiation techniques for solgel processes are summarized in Table 27-1. Densification and crystallization of gel films, and reduction

8、 in gel films were observed through direct activation of the inorganic gel network using coherent and incoherent beams of deep UV ( <300 nm) and vacuum UV ( <160 nm) lights. This type of research started in 1992, where the densification of solgel-derived thin films has been investigated (Ohish

9、i, 1992). Photo-induced crystallization and reduction were reported in 1998 (Imai, 1998). These techniques required no further treatments after UV irradiation or photosensitive additives. In these cases, energetic photons activate the inorganic components through electronic excitation. The charge tr

10、ansfer from O2 to MX+ in the disordered bonds was deduced to be essential for the structural changes. These investigations are scientifically important to clarify photochemical reactions on amorphous states.In another field of research, fine patterning of solgel films using UV irradiation has been r

11、eported since 1993 (Nakao, 1993). UV-induced modification of solgel films containing photosensitive components was utilized for two-dimensional patterning without conventional lithography techniques. The addition of photosensitive components, such as acetylacetone, was needed to change the chemical

12、property of the solgel matrix. Since the solubility of gel films decreased with a photochemical reaction induced by UV irradiation, fine patterning was obtained with subsequent chemical etching.TABLE 27-1. CLASSIFICATION OF UV IRRADIATION TECHNIQUES FOR SOLGEL ROUTESYearDirect activationPatterningPr

13、e-treatment for annealingPhotolysis of metal salts92(d) Ta2O5-LPM (Ohishi)93PZT-KrF laser (Nakao)Ta2O5-KrF laser (Ohishi)94(d) SiO2-SR (lmai)(d) SiO2-LPM (Maekawa)ZrO2-HPM (Shinmou)BST-KrF laser (Soyama)95(d) SiO2, TiO2-LPM (Van de Leest)AI2O3-HPM (Zhao)9697(d) TiO2-LPM (lmai)98(c) In2O3-ArF laser (

14、lmai)Al2O3-SiO2-LPM (Zhao) nitride-KrCI Iamp (Kikuta)99(r) In2O3-LPM (lmai)(c) ZnO-KrF laser (Nagase)TiO2 LPM (Yamada) SnO2-UHPM (Kikuta)TiO2, In2O3, ZnO, SnO2 excimer lasers (Tsuchiya.)00(c) TiO2,Ta2O5, SrTiO-excimer lasers (Asakuma)(d)SiO2-Xe2(Zhang)Ta2O5-Xe2 lamp (Boyd)01(d) SiO2-TiO2-HPM (Ma)SBT

15、-Xe2 lamp (Hayashi)ZnO-LPM (Sukur)02(r,c) ZnO-LPM (Asakuma)(r) Fe (Van de Leest)(c) TiO2- HPM (Liu)SiO2 hybrid (Soppera)ZrO2, SBT-UHPM (Nishizawa)ZrO2, SBT-Xe2 (Yu)LPM: low-pressure mercury lamp, HPM:high-pressure mercury lamp, UHPM: ultrahigh-pressure mercury lamp, SR: synchrotron radiation.(d) den

16、sification, (c) crystallization, (r) reduction.Irradiation techniques were also utilized for rapid heating and removal of residual organics as a pre-treatment for conventional annealing processes. Assistant effects are generally expected when UV irradiation is used because the absorbed photons readi

17、ly increase the local temperature at the surface and/or decompose organic molecules. The utilization of UV irradiation as a pre-treatment for subsequent annealing started in recent years. Lowering the processing temperature for crystallization, and improving the film morphology were successfully ach

18、ieved by irradiation. At the same period, UV-induced photolysis of films derived from metal salts was investigated although the precursors were not changed into gel films through hydrolysis and condensation reactions.Table 27-2 shows various UV sources that emit incoherent and coherent lights. The p

19、erformance of UV sources, including photon energy and fluence, is an important factor for developing the irradiation techniques utilized to modify solgel processing. Low-, high- and ultrahigh-pressure mercury lamps (LPM, HPM, UHPM) and excimer lamps are widely used as a UV source. Although synchrotr

20、on radiation (SR) is not applicable to mass production processes, it is also utilized for the irradiation of vacuum UV and soft X-rays. Coherent beams from excimer lasers were subjected to solgel-derived films to modify their structures. Since the energy density of laser beams is much higher than th

21、at of UV lamps, irradiation of a laser beam generally accompanies thermal effects.TABLE 27-2. UV SOURCESWavelength (nm) (photon energy eV)IncoherentUltrahigh-pressure mercury lamp270460High-pressure mercury lamp250440Low-pressure mercury lamp254 (4.9), 185 (6.7)XeCI excimer lamp308 (4.0)KrCI excimer

22、 lamp222 (5.6)Xe2 excimer lamp172 (7.2)Synchrotron radiationSoft X-ray visibleCoherentXeCI excimer laser308 (4.0)KrF excimer laser248 (5.0)ArF excimer laser193 (6.4)FUNDAMENTALS OF UV-INDUCED EFFECTSFundamental effects induced by UV irradiation upon solgel materials are classified as illustrated in

23、Figure 27-1. As described in the previous section, energetic photons from UV lamps predominately exhibit electronic rather than thermal excitation. The electronic excitation in organic compounds results in the decomposition of residual alkoxyl groups and chelating agents. The annealing temperature f

24、or crystallization can then be lowered by previous UV treatments. The change in the solubility is also ascribed to photochemical reactions through the electronic excitation of organic bonds. Patterning of solgel films utilized the phenomena induced with UV irradiation. Drastic structural changes, su

25、ch as densification, crystallization, and reduction, are achieved by activation of an inorganic network of gels. In this case, the band-to-band excitation should be achieved by photons with energy above the bandgap of the materials. In general, the excitation of an electron through the band gap of o

26、xides means the charge transfer from O2 to MX+. The excited states are usually deactivated through the recombination of free electrons and positive holes in the crystalline structure. However, the localization of excited electrons and holes at disordered sites in amorphous structures and at the surf

27、ace should cause the cleavage of MOM bonds. The bond cleavage with the excitation is essential for the rearrangement of the inorganic network for densification, crystallization, and reduction (release of oxygen atoms). A schematic model of the excitations of organic and inorganic components in solge

28、l films is illustrated in Figure 27-2. Thermal effects should not be ignored, especially for the exposure to an intense coherent beam from UV lasers. Laser-induced densification and crystallization may be assisted by increasing the temperature under the irradiation. However, photo-reduction is not a

29、ssociated with normal thermal excitation.Figure 27-1. Fundamental effects induced by UV irradiation and their applications.Figure 27-2. Schematic model of excitation of organic and inorganic components.DIRECT ACTIVATION OF THE GEL MATRIX WITH UV IRRADIATIONDensificationTable 27-3 indicates the photo

30、-induced densification of various kinds of solgel films. Amorphous Ta2O5 films were prepared by a solgel route with UV irradiation. In this case, organic components in gel films were removed by ozone-oxidation with 185 nm irradiation (Ohishi, 1993). A significant reduction in the leakage current den

31、sity was found with the reduction of sub-oxides, impurities, oxygen vacancies, and defects in the films. The formation of active oxygen species is considered to lead to the improvement.UV-induced structural changes in SiO2 gel were reported as a function of the photon energy using synchrotron radiat

32、ion (Imai, 1994a, 1994b). Vacuum UV photons, which have higher energy than the band gap of silica (>8 eV, <160 nm), induced significant densification with dehydration. Figure 27-3 shows a decrease in thickness and an increase in refractive index of an SiO2 gel film induced by the UV irradiatio

33、n. The refractive index was greater than that with thermal treatment at 1000°C (Imai, 1996). The FTIR absorption band due to the stretching mode of the SiO bonds indicated that the SiOSi network was highly distorted in the irradiated films. Therefore, the specific structural change was attribut

34、ed to electronic excitation through the band gap rather than to the normal thermal effects. A slight compaction of solgel-derived silica gel films was also reported to be induced by UV (254 and 184 nm) irradiation at 200°C (Maekawa, 1994). However, this phenomenon may be ascribed to thermal eff

35、ects because silica gels do not exhibit absorption in these wavelengths. Double-layered films of SiO2/SnO2 were densified by UV irradiation (Ohishi, 1997). Formation of porous SiO2 at low temperatures like 25200°C was achieved by photo-induced solgel processing using 172 nm radiation from a Xe2

36、 excimer lamp (Zhang, 2000).TABLE 27-3. DENSIFICATIONComponentsUV sourceRemarksReferencesTa2O5LPMO3 oxidationOhishi, 1992SiO2SR>5 eV with dehydrationlmai, 1994a, 1994b, 1996SiO2LPMMaekawa, 1994SiO2LPMdehydrationVan de Leest, 1995SiO2Xe2Zhang, 2000SiO2-TiO2HPMMa, 2001TiO2LPMVan de Leest, 1995TiO2L

37、PMlmai, 1997Figure 27-3. Changes in thickness and refractive index of SiO2 gel films induced by irradiation with SR radiation (Imai, 1994b).Condensation reactions in solgel-derived TiO2 films were promoted by irradiation with UV light <200 nm (Van de Leest, 1995). Densification of TiO2 gel films

38、was observed by UV irradiation with 254 nm (Imai, 1997). SiO2TiO2 nanocomposite films were densified by irradiation with a high-pressure Hg lamp (Ma, 2001). Electron excitation caused the bond cleavage in gel network. UV irradiation (low-pressure Hg) at an ambient atmosphere improved the alkaline co

39、rrosion resistance of ZrO2 films prepared by solgel methods (Hirai, 1999). The densification of the coating layer due to the elimination of residual organics induced by photo-excitation was responsible for the improvement of the resistance.CrystallizationTable 27-4 shows the crystallization of solge

40、l films with UV irradiation. Transformation of amorphous ZnO into the wurtzite phase was achieved by irradiation of 4.9 eV (254 nm) photons using a low-pressure Hg lamp (Asakuma, 2002a). This change originated from the photo-induced charge transfer from O2 to MX+. Cleavage of the ZnO network with el

41、ectronic excitation and subsequent oxidation with activated oxygen species were essential for the formation of the ordered structure. The photo-induced crystallization was found in relatively dense films annealed at l00°C because photo-reduction predominantly occurred in porous gel films. The p

42、hase transformation of TiO2 nanoparticles from the amorphous phase into anatase was also promoted by irradiation from a high-pressure Hg lamp (Liu, 2002). Coatings of crystalline ITO nanoparticles were cured by UV irradiation, and conducting and anti-glare coatings were prepared on plastic and glass

43、 substrates at a low temperature of 130°C (Al-Dahoudi, 2001). In these cases, nanosized particles with high specific surface area are inferred to be sensitive to UV-induced structural change.TABLE 27-4. CRYSTALLIZATIONComponentsUV sourceRemarksReferencesIn2O3ArF laser1020 mJ/cm2lmai, 1998, 1999

44、KrF laserZnOKrF laser100 mJ/cm2Nagase, 1999TiO2KrF, XeCt lasers Asakuma, 2000Nb2O5KrF, XeCt lasersTa2O5ArF, KrF lasersSrTiO3ArF, KrF, XeCI lasersZnOLPMPre-baked at 100°CAsakuma, 2002aTiO2HPMFine particlesLiu, 2002Crystallization of In2O3, TiO2, Ta2O5, and SrTiO3 gel films was induced by exposur

45、e to coherent beams from UV lasers with the fluence lower than 50 mJ/shot (Imai, 1998, 1999; Asakuma, 2000). On the other hand, BaTiO3, LiTaO3, and LiNbO3 gel films were not crystallized by the irradiation although the crystallization temperature is almost the same as that of the former gels. An inc

46、rease in temperature at the surface under irradiation was insufficient for thermal crystallization. Thus, the structural change was attributed to the UV-induced electronic excitation rather than to thermal effects.Transparent conducting ITO films were successfully prepared on a plastic substrate usi

47、ng the UV laser irradiation technique (Asakuma, 2003a). The resistivity of ITO films prepared on a PET substrate was about 101 cm. Exposure of ZnO gel films to KrF excimer laser beams produced crystalline films with high crystallinity and strong orientation (Nagase, 1999). In this case, however, the

48、 crystallization of the gels was attributable to thermal effects with the laser irradiation because a high fluence above 100150 mJ/shot was required for crystallization. Photo-induced structural changes are influenced by the components and structures of gel films, and crystallization may be suppress

49、ed by the presence of residual organics in gel films.ReductionPhoto-induced reduction was reported as shown in Table 27-5. This phenomenon is characteristic of UV irradiation because reduction is not observed by regular annealing processes in air. UV light promoted the reduction of In2O3 and ZnO gel

50、 films (Imai, 1999; Asakuma, 2002b), which was often accompanied by darkening of the films. Figure 27-4 shows the photo-darkening of a ZnO gel film irradiated with a low-pressure mercury lamp. Deficiency of oxygen and formation of metallic particles were found in the irradiated films, indicating tha

51、t these changes basically originated from the photo-induced charge transfer from O2 to MX+. The activation was localized in the amorphous structure and then induced bond cleavage in the network structure. The formation of mobile oxygen atoms causes the release of oxygens, and the metallic components

52、 remain.TABLE 27-5. REDUCTIONComponentsUV sourceReferencesIn2O3LPMlmai, 1999ZnOLPMAsakuma, 2002bFe in ferriteLPMVan de Leest, 2002Figure 27-4. Change in absorption spectra of ZnO gel films induced by irradiation with a low-pressure mercury lamp (Asakuma, 2002b).The photo-reduction was observed in ge

53、l films containing In3+, Zn2+, Sn4+, and Pb 2+ (Asakuma, unpublished). These cations are known to be easily reduced in reductive conditions. The porous body of the gel films is associated with the release of oxygen atoms.Ferrite and nickelzinc ferrite films were prepared from solgel precursors by UV

54、 irradiation and subsequent rapid thermal processing (Van de Leest, 2002). The irradiation was reported to be effective in reducing the iron ions (Fe(III)Fe(II). This phenomenon is also classified into the photo-reduction in gel films. HydroxylationHydroxylation of the surface of solgel-derived ZnO

55、films was observed on irradiation with UV photons with energy above the bandgap (Asakuma, 2003b). The formation of hydroxyl groups was attributed to a reaction of atmospheric water molecules with defective sites induced via photo-reduction. Two types of hydroxyl groups were produced from two kinds o

56、f defective sites associated with oxygen vacancy and bond cleavage. It is known that highly hydrophilic ZnO and TiO2 surfaces are induced by UV irradiation (Wang, 1997; Sun, 2001). The photo-induced hydrophilicity is ascribed to the formation of hydroxyl groups with reduction at the surface of the s

57、emiconductor oxides.Figure 27-5. Patterning process based on a UV-induced reaction of modified gel films.PATTERNING OF GEL FILMS CONTAINING PHOTOSENSITIVE COMPONENTSThe solubility of gel films fabricated from photosensitive solgel solutions is drastically changed by UV irradiation. This change is ap

58、plicable to the fine patterning of oxide films. Gel films obtained from alkoxides chemically modified with -diketones show absorption bands in the UV range (250350 nm), which are characteristic of the * transition in the chelate rings of, -diketonate complexes. The exposure of UV light and lasers corresponding to the transition dissociates the chelate bonds and changes the solubility of the gel films in water, acidic solutions, a