Influence of highfluence neutron proton irradiation on the optical properties microstructure of r

1、Nuclear Instruments and Methods in Physics Research B 218 (2004) 1111161locate1nimbInfluence of high-fluence neutron and1or proton irradiation on the optical properties and microstructure of rutileTiecheng Lu a,c,*, Libin Lin a, Xiaotao Zu a, Sha Zhu b, Lumin Wang b Department of Physics and Key Lab

2、oratory for Radiation Physics and Technology of Ministry of Education, Sichuan University, Chengdu 610064, PR ChinaDepartment of Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, MI 48109, USAInternational Center for Material Physics, Chinese Academy of Sciences, Shen

3、yang 110015, PR ChinaAbstractRutile (TiO2 ) single crystals of (1 1 0) or (0 0 1) orientation were irradiated by high fluence reactor neutrons with the fluence of 1 x 1023 m2 , by protons with the energy of 0.318 MeV and the fluence from 1 x 1018 to 1.9 x 1022 m2 , respectively. UVVISIR, LRS and HRT

4、EM were used to analyze the optical properties and microstructure of samples. The UVVISIR results showed that high-fluence neutron and proton irradiation induce similar variation of optical properties of samples, especially absorption plateau appear. LRS results showed that the relative intensity of

5、 common vibration modes is dierent for dierent plane, both high fluence neutron irradiation and proton irradiation induce similar variations of the frequency and intensity of Raman vibration modes in the same orientated samples, and proton irradiation also induces several new vibration modes appear.

6、 High fluence irradiation induced large defect clusters, i.e. stacking faults, which is demonstrated by HRTEM observation. In addition, the dierence between neutron-induced irradiation eect and proton-induced irradiation eect has been also discussed.© 2004 Elsevier B.V. All rights reserved.Keyw

7、ords: Rutile; Neutron irradiation; Proton irradiation; Optical absorption; Stacking faults1. IntroductionRutile electrodes have created great interests in the field of semiconductorelectrolyte interfaces since water photoelectrolysis can be achieved without decomposition of the electrode 1. For sola

8、r application, however, TiO2 is not suitableCorresponding author. Address: Department of Physics and Key Laboratory for Radiation Physics and Technology of Ministry of Education, Sichuan University, Chengdu 610064, PR China. Tel.: +86-28-8541-2031; fax: +86-28-8541-7106.E-mail address: lutiecheng (T

9、. Lu).because its gap is 3 eV, too far away in the UV region to absorb the solar energy significantly. But TiO2 remains high in the list of numerous mate- rials since it has been regarded as the most stable material for a long time. So what really counts is to decrease its gap or increase the optica

10、l absorption in the visible region by increasing the number of defects. Therefore the properties of defects in doped TiO2 have been extensively studied 24. Recently, dye-sensitization has become a new modification method for solar application because organic dye has strong optical absorption in UV r

11、egion 5,6. However, organic dye is unstable and subject to ageing. In fact, defects can also be0168-583X1$ - see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016.01.005112T. Lu et al. / Nucl. Instr. and Meth. in Phys. Res. B 218 (2004) 111116introduced deliberately by neutron

12、or ion irradia- tion 714. Unfortunately, a few of papers has been reported about the influence of neutron and1 or proton irradiation on the optical properties of rutile 13,14. In addition, the defect structure in- duced by high fluence irradiation has not been clarified.In this paper, high fluence n

13、eutron and1or pro- ton irradiation modification method is presented and induced defect structure has been studied by XTEM and HRTEM.2. Experimental proceduresThe TiO2 single crystal specimens, 10 x 10 x 0.5 mm in size, were sliced from a stoichiometric rutile single crystal boule. The 10 x 10 mm fac

14、e of sam- ples was (1 1 0) or (0 0 1) plane. The samples were polished, cleaned by acetone and ultrasonic wave, and dried.The neutron irradiation experiment of the samples was carried on, with the fluence of 1 x 1023 m2 , by high-flux-engineering-experiment-reactor of China. Samples show deep bluebl

15、ack color. Then two of them were annealed in vacuum fur- nace (less than 3 x 102 Pa) at 1000 oC for 0.5 h. The sample kept the as-irradiated color. Then they were annealed in air at 1000 and 1500 oC for 0.5 h, respectively. The color of the samples became light blue and colorless, respectively.Durin

16、g proton irradiation, two of (1 1 0) ori- ented specimens were implanted, with the energy of 4.9 and 18 MeV respectively and fluence of1 x 1018 m2 , by HI-13 series accelerator in Chinese Academy of Nuclear Energy Science. They were carried on similar annealing experiments as neu- tron-irradiated sa

17、mple. In addition, one of (0 0 1) samples was irradiated, with energy of 0.5 MeV and fluence of 1.9 x 1022 m2 , by 1.7 MV Tande- tron accelerator in Michigan Ion Beam Labora- tory at University of Michigan.Before and after neutron or proton irradiation, as well as after annealing, samples were inves

18、ti- gated by means of ultravioletvisibleinfrared absorption spectroscopy (UVVISIR) and laser Raman spectroscopy (LRS). UVVISIR spectra of samples were measured with PERKIN-ELMERLambda 19 UVVISmiddle IR spectrometer combining NICOLET FT-IR 170SX infrared spectrometer. The LRS of samples were measured

19、 with backscattering mode by SPEX Model 1403 laser Raman spectrometer. Moreover, the (0 0 1) sample was also prepared in cross-section for TEM observation. Radiation-induced microstruc- tures were examined in a JEOL 2010 FEG trans- mission electron microscopy at University of Michigan.3. Results and

20、 discussionsUVVISIR optical absorption spectra of (1 1 0) oriented sample before treatment, after neutron irradiation and after annealing in vacuum or in air, as well as that of (1 1 0) or (0 0 1) sample after proton irradiation with dierent energy and fluence, and after annealing in air are shown i

21、n Figs. 1(a) and (b). Before irradiation, there are two sharp absorption edges at 410 and 7000 nm, respectively. In the wavelength region of 4107000 nm, almost no optical absorption is observed. After neutron irradiation, a very wide absorption band from 450 to 6000 nm, especially the absorption pla

22、teau at 6002500 nm, is observed. After annealing in vacuum at 1000 oC, its intensity decreased slightly, but the shape of plateaus does not vary. After the oxidizing annealed in air at1000 oC, plateau separates into two peaks centered at 760 and 1700 nm, respectively. At 1500 oC, the optical absorpt

23、ion spectrum is similar to that of non-irradiated sample.After proton irradiation with low fluence of1 x 1018 m2 , even if incident energy is so high asto 4.9 MeV (even 18 MeV), two peaks similar toCurve 4 in Fig. 1(a) centered at 760 and 1700 nmrespectively emerge. When the fluence is increasedto 1

24、.9 x 1022 m2 , no matter what orientationsample is, even if incident energy is not so high, i.e.0.5 MeV, the stronger absorption plateau similarto Curve 2 in Fig. 1(a) appear. After all of proton-irradiated samples oxidizing annealed at 1500 oCin air, the absorption spectrum becomes similar tothat o

25、f non-irradiated sample, too.As shown in Fig. 1, oxygen deficiency is a sig-nificant factor that induces optical absorption.T. Lu et al. / Nucl. Instr. and Meth. in Phys. Res. B 218 (2004) 1111161133.02.52.0Absorbance / ( a. u. )1.51.00.51. before neutron irradiation2. after neutron irradiation3. af

26、ter annealing at 1000C in vacuum4. after annealing at 1000C in air5. after annealing at 1500C in air23451(a)CB-0.85 and CB-1.67 eV, respectively. The former corresponds to the 1700 nm optical absorption peak, and the latter corresponds to the 760 nm peak. We may deduce that neutron irradiation induc

27、es F-type centers, e.g. F center, Fþ center, etc. The density of defects, hence, and intensities of optical absorption of Fþ and F center increase with increasing fluence. Meanwhile, since some F centers transform into stable Fþ centers 15,16, the optical absorption of 760 nm peak inc

28、reases rapidly with increasing fluence. As a result, up to high-fluence of 1 x 1023 m2 (neutron) or 1.9 x 1022 m2 (proton), the densities of the two centers reach saturation or over-saturation, and two related absorption peaks may combine into a plateau. It may be expected that the absorption platea

29、u is related to color-center-defect-cluster.3.00.30.51234 5 6 7 8 Wavelength / µmThe phenomena on color centers or oxygen deficiency induced by irradiation can be checked with LRS measurements. The space group of rutile1. before proton irradiation2. after proton irradiation ( 4.9 MeV, 1 ×

30、10m)(b)is D14 (P421mnm). There are four kinds of Raman4hvibration modes, i.e. Eg (447 cm1 ), parallel to c2.5Absorbance / ( a. u. )2.01.51.00.53. after proton irradiation ( 18 MeV, 1 × 10m)4. after proton irradiation ( 0.5 MeV, 1.9 × 10m)5. after annealing at 1000C in air43251axis and B1 g

31、 (143 cm1 ), A1 g (612 cm1 ), B2 g (826 cm1 ), perpendicular to c axis. The 232 cm1 peak is related to a multi-phonon process, and the 447 and 612 cm1 peaks are related to Eg and A1 g modes, respectively. As for (1 1 0) sample, the vibration direction of two oxygen ions, which are related to Eg vibr

32、ation mode shown in the insert of Fig. 2(a), sited in C4 axis of oxygen octahedron are parallel to the surface of (1 1 0) plane. As a result, before irradiation, the intensity of Eg is higher than that of A1 g. Based on same reason, as for (0 0 1) sample, the intensity of A1 g is higher than that of

33、 Eg, before proton irradiation, due to vibration direction of A1 g mode paralleling to0.30.51234 5 6 7 8 Wavelength / µmsurface of (0 0 1) orientation. That is to say, therelative intensity of the two peaks are oppositeFig. 1. UVVISIR optical absorption spectra of (1 1 0) ori- ented sample befo

34、re treatment, after neutron irradiation and after annealing in vacuum or in air (a), as well as that of (1 1 0) or (0 0 1) sample after proton irradiation with dierent energy and fluence, and after annealing in air (b).Our research group calculated the electronic structure of F and Fþ center in

35、 reduced rutile with embedded-cluster numerical variation method 15,16. The defect levels of F and Fþ centers arebetween (0 0 1) and (1 1 0) plane. The LRS of (1 1 0) sample before treatment, after neutron irradiation or proton irradiation and after annealing in vac- uum or in air, as well as L

36、RS of (0 0 1) sample before and after proton irradiation are shown in Fig. 2(a)(c). Before neutron irradiation, there are three obvious peaks centered at 232, 447 and612 cm1 , respectively. After neutron irradiation, there are also three peaks around 232, 443 and611 cm1 , respectively, but the inten

37、sity of Eg114T. Lu et al. / Nucl. Instr. and Meth. in Phys. Res. B 218 (2004) 1111161. before neutron+irradiation(a)1. before proton irradiation2. after proton irradiation with energy of(b)2. after neutron- irradiation3. after annealing18MeV and fluence of 1×10Ti3. after annealing in vacuumInte

38、nsity / (a.u.)4. after oxidizationmin vacuumIntensity / (a.u.)4. after oxidizationEgAgOEgAg4433221180.0490.0900.0Raman shift / cm-180.0490.0900.0Raman shift / cm-11. before proton irradiation2. after proton irradiation with energy of0.5 MeV and fluence of 1.9×10m(c)450Eg236356510Ag5966097242451

39、Intensity / (a.u.)Bg144237180.0490.0900.0Raman shift / cm-1Fig. 2. LRS of (1 1 0) oriented sample before treatment, after neutron irradiation (a) or proton irradiation (b) and after annealing in vacuum or in air, as well as LRS of (0 0 1) sample before and after proton irradiation (c). Eg, Ag Raman

40、vibration modes of rutile are shown in the insert (view along c axis).decreases. After annealing in vacuum, there are no changes. However, after oxidizing annealing in air, the LRS almost recover to that before irradiation, i.e. Eg frequency becomes 446 cm1 , and its intensity increases and is stron

41、ger than that of A1 g.After proton irradiation with the energy of 18MeV and fluence of 1 x 1018 m2 , as for (1 1 0)sample, there are two new very weak peaks cen- tered at 354 and 508 cm1 , respectively, besides three peaks centered at 233, 445 and 610 cm1 , respectively. The relative intensities of

42、Eg and A1 g are dierent between before and after irradiation, too. After annealing in vacuum, there are no changes. However, after oxidizing annealing, theT. Lu et al. / Nucl. Instr. and Meth. in Phys. Res. B 218 (2004) 111116115LRS almost recover to that before irradiation, i.e. intensity of Eg inc

43、reases and is stronger than that of A1 g and two weak peaks disappear. As for (0 0 1) sample, after high fluence proton irradia- tion, the relative strength of Eg and A1 g is changed romantically, too. Moreover, three new peaks centered at 356, 510 and 724, respectively, appear. Of course, before pr

44、oton irradiation, the weak B1 g mode can be shown due to parallel to (0 0 1) plane based on above analysis.After neutron or proton irradiation, oxygen ions are easy to be lost under irradiation condition due to low displacement threshold energy. As a result, the vibration frequency and intensity of

45、most relative Raman mode for certain orientation sample, e.g. Eg mode for (1 1 0) sample, may be seriously aected. Thus it can be understood that the intensity of this mode decreases obviously after irradiation due to disturbance or destruction of related crystal symmetry. After annealing in oxi- di

46、zation, the LRS almost recovers to that before irradiation because oxygen ions are replenished. On the contrary, the LRS keeps invariable after annealing in vacuum. Therefore, no matter kinds of irradiation particles and surface orientation of samples, the influence of irradiation on LRS of rutile i

47、s similar to UVVISIR of rutile. As for proton-irradiated new peaks, the reason is that oxygen insuciency destroy crystal symmetry so that the non-Raman vibration mode is shown. In addition, it is shown that proton irradiation in- duces stronger eective, which aects the crystal symmetry and induces s

48、everal new LRS peaks, on the microstructure of rutile than neutron irradia- tion due to the charge of implanted protons or Hþ ions.Figs. 3(a) and (b) show a bright field cross-sectional TEM and a HRTEM image (g ¼ 111)of (1 1 0) plane of proton-irradiated (0 0 1) sam-ple with the energy of

49、0.5 MeV, respectively. Theypresent direct witness for above defect analy-sis. The two figures show high density of defectclusters (stacking faults or dislocation loops) inthe depth of 36004200 nm below the surface. Thedamage peak was about 600 nm in width. Thedepth of damage peak is consistent with

50、calcula-tion results by SRIM 2000. The selected electrondiraction pattern shown in the insert of Fig. 3(b)Fig. 3. A bright field cross-sectional TEM image (a) and a HRTEM image (b) of (1 1 0) plane of proton-irradiated (0 0 1) sample with the energy of 0.5 MeV and fluence of 1.9 x 10m. A selected el

51、ectron diraction pattern taken along 1 1 0 of TiOis shown in the insert.indicated that there is no phase transition or amorphization after proton irradiation. The for- mation procedure of stacking faults can be de- scribed as follows. Oxygen vacancies usually exist in rutile due to be grown from red

52、uction condition of flame fusion method. Bonding energy of oxygen ions neighbor to these vacancies is lower than that in stoichiomatric area. They are easier to lost than others during irradiation. The number of lost oxygen ions increase, hence, and the damage area116T. Lu et al. / Nucl. Instr. and

53、Meth. in Phys. Res. B 218 (2004) 111116enlarges with increasing fluence. As a result, the fluence is increased to higher, high density of oxygen vacancies or color centers is induced and aggregate to defect clusters, i.e. stacking faults, which induce strong absorption plateau shown in the UVVISIR spectra. But most of oxygen octahedrons keep stable, so the electron diraction patterns keep invariable, too.4. ConclusionsHigh fluence neutron and/or proton irradiation induces strong optical absorption at UVVISIR region, especially absorption plateau at 600 2500 nm.