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Effects of adsorption and reaction on the second harmonic generation of Langmuir-Blodgett films

Effects of adsorption and reaction on the second harmonic generation of Langmuir-Blodgett films
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  3620 Langmuir 1995,11, 3620-3622 EffectsofAdsorption and Reaction on the SecondHarmonic Generation of Langmuir-Blodgett Filmst Xinsheng Zhao,* Jianhua Xing, Peng Li, Xiaoming Xie, and Xiaohua Xia Department of Chemistry and Znstitute of Physical Chemistry, Peking University,Beijing 100871, China Hui Li and Chunhui Huang Research Center of Rare Earth Chemistry, Peking University, Beijing 100871, China Tiankai Li and Lingge Xu Znstitute of Photographic Chemistry, Academia Sinica, Beijing 10082, ChinaReceived November 29, 1994. In Final Form: August 2, 1995@ The effect of the adsorption and reaction of HC1 and N(CzH& on the second harmonic generation (SHG)of Langmuir-Blodgett (LB) films made of nonlinear optical materials was studied. It was found thatintroduction ofHCl or N(CzH& gases separately greatly reduced the SHG ofthe LB films. This is attributedas due to the modification of the film molecules by the acid or base. However, when the two of gasessubsequently adsorbed, the SHG signal recovered, and in some cases it was several fold of magnitudehigher than that from the srcinal film. Both UV-vis spectroscopy and image of scanning electron microscopysupported the idea that a thin uniform layer of salt was formed on the matrix of the LB film molecules.Second order nonlinear optical susceptibilities of filmsare some of the important properties that have beenstudied extensively n recent years.l Among the methodsof film formation, the Langmuir-Blodgett (LB) method is one ofthe convenient research tools to investigate secondorder nonlinear optical materials. In the literature, thesecond order nonlinear optical properties were all char-acterized in air. However, in applications the materialmay be put in various kinds of atmosphere. Recently, wereported the observation of variations of the secondharmonic generation (SHG) with the chemical environ-ment of the LB films made of hemicyanide dyes2 In thisLetter, we report a new phenomenon that the acid-basereaction on the LB film can enhance the SHG of the filmand that the deposit of the salt product on the film is uniform, which may provide a new way for film formationor manipulation.Three film materials shown in Figure 1 with highmolecular hyperpolarizabilities on the order of esuwere chosen to prepare the LB films.3 The synthesis ofthe materials and the film formation followed the de-scription in ref 3. The apparatus is shown in Figure 2. SHG by 1.064 pm radiation from a Nd:YAG laser atmonolayer LB film with an incident angle of 45" wasmeasured with standard transmission mode4 except thatthe LB film was in a gas cell. The outcome beam after thefundamental frequency was filtered out was focused downonto the slit of a monochromator. The SHG signal wasdetected by a Hamamatsu R955 PMT and recorded andaveraged by an EG&G 4100 boxcar averager or an HP * E-mail: by NSFC and by the Fok Ying Tung Education @ Abstract published in Advance ACS Abstracts, September 1, (1) See, for example,Garito, A.; Shi, R. F.; Wu, M. Phys. Today 1994, (2) Zhao,X.; Xie,X.;Xia,X.; Li, H.; Wang, K.; Huang, C.;Xu, L.;Tian, (3) Li, H.; Huang, H.; Xu, G.; Xu, L.; Li, T.; Zhao, X.; Xie, X. Prog. Foundation. 1995. 47 (51, 51. K. Thin Solid Films, in press. Nut. Sei., in press. Li, T. Lengmuir 1994, 10, 1910. (4) hou, D.; Huang, C.; Wang, K.; Xu, G.; Zhao, X.; Xie, X.; Xu, L.; 0743-74631951241 -3620$09.00/0 A: I MamN=N-@N-Ci~H371CiePMP: ON =?- CH3 'C-C-C-Ci5H3r I1 - II 00 Figure 1. Structure of the film materials labeled A, , nd C in this paper. The arrow indicates the amino nitrogen whichis most likely first protonated in acid environment. 1 16 Figure 2. Experimental setup: 1, aser; 2, prism; 3, photodiode;4, polarization rotator; 5, Glan-Taylor prism; 6, RG850 filter; 7, eaction cell; 8, LB ilm; 9, aratron; 10, otal reflector of hefundamental wave; 11, BG40 ilter; 12, ocal lens; 13, neutraldensity filter; 14, monochromator; 15, MT; 16, ignal processorand PC computer; 17, valve; 18, eedle valve; 19, ample flask.54510A digitizing oscilloscope. A typical laser energy was4 mJ/pulse with a pulse duration of 6 ns. The cell was 0 1995 American Chemical Society  Letters Langmuir, Vol. 11, No. 10, 1995 3621 9.00 12.00 TIM E( min) O n L c W 0.00 5.00 10.00 15.00 20.00 25.00 TIM E( mi n) b Figure 3. Variation of SHG as vapor pressure of HCl andN(C2Hs)swhen process of operation is (a) ast and (b) slow. TheSHG detection is in the p-p mode, and the film material ismolecule C. vacuum sealed and pumped by a diffusion pump. Thevapor was purified by several cycles of pump and thaw.The vapor pressure was accurately monitored by an MSKbaratron. The digitized SHG signal and vapor pressurein the cell were simultaneously recorded by a PC computer.In the experiment SHG in vacuum was used as ourreference, which could be slightly different from that inair. As reported in ref 2, introduction of methyl, ethyl,and isopropyl alcohols, water, HC1, acetic acid, CHC13,and N(C2H& will greatly effect the SHG of the films. It was found that in all systems but water films, SHGdecreased in both s(fundamenta1 polarization)-p(har-monic polarization) and p-p detection modes as the vaporpressure increased. But, in the water-film systems theSHG signal increased as the water vapor pressureincreased, indicating that the adsorption of water somehowmade the film more ordered than that without water. Inthe experiment, the behavior of the three kinds of LBfilms was similar. On comparison of the effect of gas tothe performance of LB films, it was observed that the LBfilm made of material C is more stable. This is consistentwith the stabilities shown in the n-A isotherm studies3and, again, in agreement with observations from hemi-cyanide type LB films.2What is new in this Letter is our observation that whenthe reaction of HCl or HAC with N( C2H5)3 on the nonlinearoptical LB film was introduced, the SHG signal not onlycould be recovered but in some cases was even higherthan that from the srcinal film. To do so, HC1 or HAC Figure 4. The image by 1910 PE field emission scanningelectron microscope (Amray) of (a, top) the pure LB film and (b, bottom) the one after slowly processed acid-base reactionstopped at the moment that SHG was on its maximum. Thefilm material shown here is molecule B. The two films have a similar picture, indicating that the formation of the salt didnot create large particles. vapor was introduced into the cell where an LB film waslocated and pumped out again. Then, N(C2H& vapor wasled in. Much to our surprise, when inlet of the vapors wasfast, from 2-fold to order of magnitude increase of SHGsignal relative to in vacuum (or air) was observed duringthe course of different runs. The signal went down againwhen more of the second vapor was put in. One suchexample is shown in Figure 3a. The increase of SHG wasdramatic considering that the materials we chose werealready among those known with the highest second ordernonlinear optical susceptibilities. When the vapors wereprocessed slowly, he SHG could consistently recover onlyto about its srcinal level (Figure 3b).To find the reason for the enhancement of SHG, a similarexperiment was done at ambient atmosphere by substi-tuting the LB film by a wet substrate (fused silica), andsubsequently dipped the plate into test tubes saturatedwith the HC1 and N(C2H& vapors, respectively. The sameorder of magnitude of SHG was also observed during thereaction. Under an optical microscope it was observedthat there were crystals much scattered in both shapeand size on the wet substrate. This indicated that theenhancement of SHG signal may well be due to theformation of respective salts, rather than the chemicalmodification of the film molecule. For the sample withfast processing, we did see particles under the opticalmicroscope, but the size and the shape of the particleswere uniform in micrometer dimensions. For the samplewith slow processing, no particles could be observed underthe optical microscope. When studied by scanning electronmicroscopy, the slowly reacted film and pure LB filmlooked similar to those shown in Figure 4.  3622 Langmuir, Vol. 11, No. 10, 1995 Letters 0 0 W ?I--- b !$ 200 300 400 500 600 0 A (nm) Figure 5. UV-vis spectra of LB film made of molecule A (a)pure LB film with one layer of the film on each side of the fusedsilica plate; (b) same plate after exposed to the HC1 vapor. In (b) one side of the LB film was wiped out due to the requirement of SHG measurement. The spectrometer used was a UV-3100 (Shimadzu).To further elucidate the source ofvariation of SHG withthe chemical environment, we studied the W-vis spectraof the film materials after exposure to HC1 vapor. Figure5a is the W-vis spectrum of the pure film, where twobands characterize the n*-n and n*-n transitions withpeaks at 280 and 570 nm, respectively. After the filmwas exposed to the HC1 vapor, a portion of 570 nmcomponent was blue shifted to 470 nm (Figure 5b). Thiscan be properly explained by the protonation of themolecule at the amino nitrogen (indicated by an arrow inFigure 1). As is seen, the protonation on this nitrogenmade the JC electron distribution more symmetric, and as a result the second order hypopolarizability is red~ced.~The decrease of SHG after the film was exposed to acidwas expected. When the cell was pumped down to vacuum,the combined HCl remained on the position, and thenwhen N(C2H5)s gas was added, surface reaction occurred on a specific site, and at the same time the film moleculewas released to its original form. The recovery of themolecular structure is illustrated by the UV-vis spectra(Figure 6). When the vapor inlet was slow, the reactionwas gentle, so that avery thin layer ofthe salt was formedon the molecular matrix ofthe LB film. When the processwas fast, more acid molecules were left over on the filmand the basic vapor was quickly put in, larger particlesofthe salt could form, but the size and shape still remainedquite uniform, unlike the crystallization in the solution (5) Xiao, X.; Vogel, V.; Shen, Y. R. . Chem. Phys. 1992, 4, 2315. 0 1 t------- I 01 a ). (nm) Figure 6. UV-vis spectra of LB film made of molecule C: (a)pure LB film with one layer of the film on each side of the fusedsilica plate; (b) same plate after slowly processed acid-basereaction and stopped at the moment that SHG was on itsmaximum. In (b) one side of the LB film was wiped out due to the requirement in SHG measurement. The spectrometerused was a UV-3100 (Shimadzu).where the microcrystals were much scattered in both sizeand shape. When the layer of the salt was very thin the SHG was mostly from the nonlinear optical LB film itself.But, when microcrystals of big size were formed, thenonlinear efficiency of the salt became important, andenhancement of the SHG was evident. The size of thecrystal depended on the experimental condition (such ashow fast the vapors were put in and pumped out, howlong the film was in the vapor, and how thorough the cellwas pumped). The results of different magnitudes of SHGsignal among different runs were due to our incapabilityof controlling the reaction and crystallization processesat this stage. The drop-off of the SHG ignal after moreN(C2H& was added may be due to vapor condensationacting as a sovent to'destroy the regular structure of thefilm.There are three indications out of this study worth ofmentioning. First, in application, we should not neglectthe effect of the chemical atmosphere on the stability andthe performance of the films of the second order nonlinearoptical materials. Second, the correlation between ad-sorption of vapors and the SHG signal might haveapplication as chemical sensors. Third, a gas-surfacereaction started from a certain matrix material mightbecome an alternative way to make or grow useful filmsin good quality. LA940941N
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