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FT-IR surface electromagnetic waves spectroscopy of Langmuir-Blodgett films on metal surfaces
1
Journal of Molecular Liquida, 63 (mW2) 1-14 Elaevier Sciencle Publiahera B.V.. Ameterdam
FT-IR SURFACE ELZETROMAGNETIC BLODGEL'TPILMSONMEPAI,SURl?ACEs*
WAVES
SPECTROSCOPY
G.N. ZHIZRIN, A-A. SIGAREV and V-A-YAKOVIXV Institute of Speutrosaopy. USSR Aoademy Moaoow Region, Troitzk (The USSR) (Received
1 October
of
OF
IlaNGrduIR-
Soienoes.
142092.
1990)
SUMMARY The sensitivities of the SEW and the refleotion-absorption FT methods IR speotrosoopy aompared experimentally and theoret5uallg on the example ozhe 4-n-ootadeoglphenol v Blo ett films. The versatility of the broad-band SEW Bpeotrosoopy in % veetigatlng of thin films on metal surfaoes have been demonstrated, INTROIXJCTION first theoretioal (refa. 1.2) and experimental Already the (refs. 3-5) speotrosoopio works with the use of surface eleotromagnetio waves (SEW) have demonstrated the high sensitivity of SEW to the state of metal surfaues oaused by the near surface oharaoter of the probing field localization. A transition from the laser SEW speotrosuopy (refs. 3-5) to the broad-band speotrosoopy by means of the use of a Fourier transform infrared (FT IR) speutrometer provided with a hm-sensitive reoording system to deteot weak light fluxes has allowed to develop a new universal method of vibrational speotrosoopy for nondestruot%ve study of thin films on metal substrates. Such appmaoh has permitted to obtain broad-band vibrational speotra of th%n dieleotrio films on smooth metal surfaoes by the SEW method with the use of three kinds of ooupling elements for the t,itation of SEW by bulk radiation and for the reverse SEW into bulk oonversion of radiation such as the edges of the metal surface (ref. 6). diffraotson @;ratings on the metal aurfaoe (refs.7.8) and the aperture excitation of SEW (ref. 9). The main advantages of affraotioa gratfngrj used in the present work as ooupling elements are: relatively high trans r033ntdion effioienoy in the middle IR range. the simplioity of optioal adjustment. the plane oonstruotion, the meohanioal stab%lity and thus determined the stability of the transformer efflofenuy. The * Dedicated
to Professor
0167-7322/92/$06.00
8
Paraskeva
1992 -
Sirnova
Elaevier Science Publiehers
B-V. Allrighta reserved
above properties oan beoome espeoially valuable when ooupl~ attaohelements are used in vaouum and t3geotrcmoopio OrgogsniO merits, The neoessity to make diffraotion gratings on the surfaoe of the metal sample beinvestigated is a asadvantage of this version but it oan be overuome by the use of various oonstruotions of oompoaite SEW waveguides (refs. 8.10), Namely a oombination of the SEW method with the adventsgee of F!F speotrometry has allowed the development of a new highsensitive method for the analysis of metal surfaoes: the FL IR SEW speotrosoopyThe SEW method oan be uompared with one of the most highsensitive optioal methods of vibrational speotrosoopy - the refleotion-absorption (RA) speotrosoopy (ref's, 11-13) whtoh is widely used for the investigation of moleoular systems on metal surfaues in the IR range (refs. 14-16). A uommon property of the SEW- and DA-methods is their seleotive sensitivity only to perpendioular to the metal surfaoe components of Uipole moments of v*brational transitions fn a thfn film being on the metal surfaoe (r&s. 7. 2. 12). In the present paper the sensitivity of the IR broad-band SEW speotmsoopg is disoussed compared to that of the DA method and also a possible applioation of a SEW wave&de with two walls to invesfTgate vibrational speotra of thin tieleotrio films on metal surfaoes by the SEW method is oonsidered,
Speotral measurements in the wavenumber iv) range 650-2500 om-' were made on a Digflab P!l!S-20B l?!CIR speotrometer with resolution 4 am-' anU the number of 808118 400. We have used speoial SEW and rerleotion attaobments whose optioal schemes were desoribed in detailsin (ref- 7). and a HgOd!l!e photodeteotor operated at 77 EC. The manufaoturing oonditions and the oonfiguration of oopper substrates with two dirfraotion grate for the SEW input and output used in this work as well as the deposition oonditions for 4-n-outadeaylphenol (ODP) films on substrates using the v Blodgett method (LB films of ODP) are desoribed in (ref- 7.81, The thioknesses of investigated ODP films were equal to the values from 25 2 (1 monomoleoular layer) up to 400 8. IR speotra of the films were obtained in the following wayThe refleotion speotrurn (TotRnl(V)) and the SEW transmission (Y)) for the chosen pure substrate were suooesspedtrum (TotsEIR, sively measur ed with the use of refleotion and SEW attaobments
a respeottvelg
and
put
into
the
memory
of
the
El? speotrometer
computer. Then the dleleotrio film being InvesIgated wan deposited on the aurfaoe of the substrate and the SEW transmission speotrum (U)) and the reflection speutrum CT, tRA, (VI) were measured (T (SEW) __ .~ for the substrate with the film. The absorptflon speotra of the films are given below in the optical density soale (D(V)) whioh was oalaulated for eaoh of the methods by the formula:
N’)=-~gp,(V)/T,.,(v)]
(1 1
REXllXS AND DISCUSSION Conefder the charaotristio features of the El! IR SEW apeotrosaopy tak3ng I&3 films of ODP as an example. The absorption ape&rum of the initial polyoryetallins ODP pressed into a KDr pellet is shown in Fig-la. In the interval 1000-1700 cm-' the strongest abeorption bands correspond to vibrations of the phenyl ring: the band at WA260 cm-' - to deformation vibrations of C-Hthe band at aJ=1518 am-' - tostretiiw groups in the ring plane, vibrations of carbon atoms of the ring (ref. 17)InFig. SEW tranermi ssion apeotra are shown for the substrate with no film and with a I&3 film of ODP about 125 8 thiok (five monolayers). In the speotrum of the sample with an ODJ? film, firl3tJg. over the whole range of SEW exaitation a deoreaee Tn the SEW tmission is observed aompared to the case of a pure substrate and, secondly, tranExnission minima appear in the region of ODP absorption bands. In Fig-Id absorption apeotra of LB film6 of ODP of different thiaknesrses are mown obtained upon transformation of the respective transmi ssion epeotra of Sl3W by the formula (1). !L%e absorption band.8 at 1260 and 1518 cx-' are clearly Been in the speotrum of an ODP monomoleoular film. For oomparison Pig-lb shows the absorption speotrum of an ODP film of five monolayer8 on a aopper sub&rate med by the FU method at the angle of rad.lation tiuidenoe on the sample about 88? It is seen from Figalb,ld that the two methods being give oonsidered similar absorption spectra of the f5_lm - the positions and the half-widthrs of the respeotive absorption bands ooinoide, however, the intensities of the abaorption band6 In the film speutra (AD), equal to the aifferenoes AD=+3 in optiual densities at the maximum of the band (#) and at the level of the background (9) differ appreoiably for the two methods. Experiments and calaulationa have permitted to d&ermine the oharaoter of the nonlinear dependenae of the intensity of absorp-
a
b
T*104 6
3
0
D
0.3 0-2 0-1
0
I
1600
1400
1200
zr, cm-'
Fig, 1. a, The abeorption speotrum of polgory6talline ODP preseed In a KEr pellet. b- The abbe tion speotrum of a LB-film of ODP of five monolayers on oopper Ilp FIA-met&od. the angle of radiation lnoidenoe on the sumple is about 88 ). the speotrum IS shifted the D axia with no ohange of the soale. al0 o, The SEW tranemissYY on Bpeotra for the oo er subBt.rate without a film (I) and with an ODP film about 125 ?? thiqk, (2), An abrupt decrease 5.n the sample transmission at ?~=I316 om is oaused by Bragg refleotion of SEW on the SEW input and output gratws with the period d, The absorption speotra of LB-films of ODP of one (I), II-4 pthree (2) and five (3) monolayers on the surfaoe of oopper (SEWmethod, the distanoe between the SEW input and output gratings 5s equal to 1 om)-
5
tion band in the speotrum of the film on the metal surraoe on the film thiokness ror the SEW and RA speotmsoopy methods (refs.7,38) For organio films under investigation c-ons5derable deviations from proportionality 0r the film thiolazess to the band intensity are already observed at thicknesses of SBVB~ a0zen~ 0r BngstmmThis raot must be taken Into aooount when evaluating quantitat-lvely the flilm parameters by their speutra. In the approximation of a thin dieleotrio film on a metal surfaae. 5-e. when the oondition 12K'~(l-E,)d,4'~(
a 1
(2)
is fulfilled (here 13, is the dieleotrio constant of the film, d, is the CUm thiokness, s is the dieleotruo oonstant of the metal) the intensity of the film absorption band ADsM in the ease of the SRW method me asurements in (rer_ 18): AD
SEW
-
+ 2WPd,
8k%%~'Rd,[Im(-E~1)](lg 1 - &;' + 2&3%;2 ( em
e)[ I + I&.%J;~
31.
+ (3)
the high-frequency dieleotrio oonstant 0r the ODP rilm. wp is the plasma frequency of the metal, R is the aistanoe between the SEW input and output ooupling elements, One can see from rormula (3) that AD,,grows with the distanoe R between SEW ooupl3ng elements. However, the sensitivity of the speotrometer reoording system restriots the maximum value of R at whioh the SEW speotrum 0811 still be measured with suT+ioTontly Ilre;h signal-to-noise ratio at present values 0r the radiation sonroe power and the effioienoy or bulk racklation and S‘EW ooupling elementsThe opM.mal distanae between the SEW input and output elements oan be ohosen sufrioiently large, R RI ai1 (x-d6), where a0 is the SEW deoay ooeffioient on a smooth metal surraue ror some mean value of the wavenumber rrom the ape&rum region uncler invest5gation. For good oonduotors the value of aiiin the middle IR range uan be of the order of a oentimeter. In the ease of the RA method at the itied angle of radiation 3noidenoe on the sample the intensity of the absorption band in the speotrum of the film on the metal surfaoe Inoreases with the number of refleotions (refs.12, 13); however, the maximum value of the signal-to-noise ratio in the film speotrum will be attained at a quite derinite optimal number of rerleotions wioh depends on the
where
is
parameters
of metal
uonsid_ered
and
the given
radiatfon
tioidenoe
&Lngle (refs. 13.14). oriteria for the evaluation of One of the most important The signal-to-noise ratio in speotrosuopio methods is sensitivity. is the useful signal AT,/6T, where AT, the film speotrum equals of the substrate with a
= Gsm
(VI lexpC-ar(V) RI.
(4)
funotion where a funotion GQEW (V) will be oalled the transmission of the SEW attaubment which uharaoteriees the effioienog of SEW input and output ooupling elements at given parameters of the exoiting radiation beam; a(v) is the SEW deuay ooeffioient for the substrate se&ion between the aoup1in.g gratings; R is the distance between the gratings. In the experiment R=l om. For the RA method in the ease of a s3_ngle refleotion the transmfssion of the sample TM oan be expressed in function of the reflection attaohment terms of the t ransmission (GM) and the ooeffioient of the refleotion Worn the sample (r) by the formula and the optioal density of the T,(~)=G~(v)~r(v). film absorption with the allowance made for (I) 0811 be D&v) represented in the form D,(V)=-lg;C r,W)/ro(v) 1 where r,(V) and r,(Y) are the ooeffioients of reflection from the metal without a film and with a film. respectively, The values of r,(V) and r,(v) have been oaloulated by the exaot formula for the refleotion ooef-
7 flolent in a system "metal - dleleotrlo film of the thlokness d 1 a i r " (z,gf. 20) a t the a n g l e o f znmd1~tion i n e l d e n o e on t h e s a m p l e surface ~=~o. It w a s s u p p o s e d that a t h i n f i l m o n t h e m e t a l s u r f a c e changes negligibly the t r a n s m i s s i o n functions o f the SEW and reflection a t t a c h m e n t s a n d t h a t the c o n t r i b u t i o n o f t h o s e f i l m sections which a r e i n the r e g i o n o f S E W i n p u t a n d o u t p u t g ~ a t l n g s h a s b e e n s u b t r a o t e d f r o m t'_,e e x p e r i m e n t a l v a l u e A D s EW (ref. 18). We asked for oaloulatlons that the dleleotrlo constant of the LB film of OD~ near the single absorption b a n d a t 1 2 6 0 o m -1 is described by a slngle--osoillator model:
E1
E='[~LO ~
.
.
.
l~Ol
.
.
[~TO--~
l%~TO] -1
,
(5)
w h e r e £~--2.25 is t h e h l g h f r e q u e n c y d l e l e o t r i o c o n s t a n t ; ~TO" ~ L O a r e t h e f r e q u e n o l e s a n d 7TO" ~ L O a r e t h e d e c a y f o r t r a n s v e r s e a n d l o n ~ i t u d l n a l o s c i l l a t i o n s of t h e film, z ~ s p e o t i v e l y . T h e values of
t h e p e m s m e t e r s 12i~=1262 om-1 , 12TO=1259.7 em-1 . ~350----7TO=15 om-1 : f o r the band under investigation of the LB film of ODP were determined f r o m t h e s p e c t r a l d a t e o b t a i n e d b y the S E W - a n d RA--methods a n d b y the attenuated total refleotlon method in the case of an ODP film on a go _rz~n4 um substx~te too. The v a l u e of the d~ f f e r e n o e (~O-~TO) was specified to r e a c h t h e b e s t a g r e e m e n t b e t w e e n t h e calculation and the experiment for the SEW method. The 4tteleotrie constant for copper was calculated by the formula =
1
-
(6)
with the following pamameters: pl~usma f r e q u e n c y ~p=65100 o m -1 ; effeotlve frequency of oollisionR of eleotrorus with phonons, impurltles and defects V ¢ = 4 0 0 o m -1 . Theoretloal dependences of t h e r e l a t i v e transmission of the substrate w i t h a n O D P f i l m a t I)=-1260 o m -1 at the trexlsmlssion m~nlmum (~/TO)_ a n d at t ~ e bsokgroua~d l e v e l (~/TO)_ o n the O D P film thickness ar~ shown ~a F~.2 for two methods. For the SEW method a relatively rapid decrease of the backg r o u n d t r a n s m i s s i o n w i t h the i n c r e a s i n g t h l o k n e s s o f t h e o o v e r l n g f i l m is c h a r a c t e r i s t i c (curve 2 i n F i g . 2a), w h e r e a s f o r t h e R A m e t h o d t h e baok4~roumd t ~ i s B i o n somewhat increases (curve 4). I n F i g . 2 b t h e oalouulated d e p e n d e n c e s of t h e z~elativo s i g n a l v a l u e o b t a i n e d f r o m t h e a p p r o p r i a t e c u r v e s 1 - 4 in Fig. 2a are showTl.
a
-(AT/T&= 0
100
200
300
4oo
a 1' B
AT/T
b
0
100
200
300
400
d 1’
2
Fig. 2. a. Theoretical dependences of the values or q/T0 and at v=l260 cm-' Ty/TO on the ttiokness of the LB-film of ODP on the copper surface for the SEW-method at Fgl om (ourvee 1 and 2. respeotively) and for the RA-method at @=E38 (ourves 3 and 4, reseotivsly). b_ Theoretical dependeno_? of the relative signal z T/To of the absorption band 1260 om on the thiokness of the LB-film of ODP on a co per substrate for the SW-method (ourve 1) and for the RA-method P The signs are the ~~%.%.a1 values oi (q/T0)8m -(A), (Ty/To)ssmr - (I ). (AT,/TcJsEW - (*). (q/To)= - (A), (TT/To)~ - ( q ), (AT,/TJRn - (0). For oomparison Fig. 2a shows by pointa the transmission and signal values at the absorption band of the ODP film in the speotra or several speoimens. The error in the determ9nation of the values of did not exoeed the dimenaiona of the aigne in T,‘To and AT/To Pi_& 2, One oan 888 from 2b that ror the two methods be%ng Figdzsoussed the signals in the speatra have tirferent dependenoee on the film thiokness on the metal auriaoe. In the investigated range
9
of thicknesses
of ODP
f11m-
on copper
for the RA-method
the signal
I n c r e a s e s monotonot%sly w i t h i n o r e a s l n g d I , w h e r e a s for the SEWmethod the s i ~ Inox~ases with inomaslng d I at di<275 ~ and deox~ases at ~ d I st d I > 2 7 5 ~. T h e c a l c u l a t e d d e p e n d e n c e s o f A T 1 / T 0 o n d I f o r t h e ~wo m e t h o d s p r o v l d e a oox-±~ot d e s c r i p t i o n o f e X p e r i -~antal ~ t a . The sensitivities of these two methods are compared upon 8n as~n--ptlon t h a t t h e S E W a n d r e f l e o t l o n s l g n a l s r e c o r d e d b y t h e F T spectrometer az~ sufficiently la~ and relative levels o2 noise ~T/T O in the refleotlon _And S E W s p e c t r a a r e t h e s a m e . In the app~oxlm~tion of a tb4n film on a metal ~aoe (2) f r o m t h e e x p r e s s i o n f o r t h e S E W w a v e v e o t o ~ (r~f.18) R n d t h e e x p r e s s l o n f o r the refleotlon coefficient o f p--polsa~Ized l i g h t 2 o r t h e s y s t e m "met~l substl~ate -- f i l m I - a i r " (i~f. 20) m~klng allowance for t h e c h o s e n m o d e l s d e s o r l b i n g dleleotx~lo c o n s t a n t s o f t h e s u b s t r a t a (6) a n d t h e f i l m (5) w e o b t a i n t h e f o l l o w l n g f o r m u l a f o r t h e r a t i o o f s l g n ~ ] s at t h e f i l m absoI~tlon band in the SEW- and RA-methods (s~ngle refleotlon) upon a condition 72 ~ ~p, ~)~ ~ ~p: (ATI/To)s~(AT1/To)RA
~ ~R(oos
~ ) ( ~ p - S l n a ~ ) -1
~o~ an or~anlo film with 6~=2.25 and of the thickness d1=25 on the copper surface with the psrameteI~ Vp=65100 o m -2 a n d ~ = =400ore -1 t h e r a t i o o f s i g n a l s (7) a t R=I ore, ~ - 8 8 ° a n d ~=-1260 om - 1 e q u a l s a p p I ~ X l m ~ t e l ~ 3.5- A s s e e n f r o m F ~ . 2b, t h e 1~atio o f signals (AT1/TO)8~(AT1/To)RA m u s t deoz~.~use w i t h inoreas4ng %AliOkness o f t h e f i l m o n the m e t a l s u r f a c e . Analysis of the formula (7) --nd F i g . 2 b s h o w s t h a t w i t h the appropr~te c h o i c e o f t h e S E W w a v e g u l d e l e n g t h (R ~ ~ I ) the SEWm e t h o d a l l o w s o n e to o b t a i n a 2 - 6 t i m e s l a r ~ e r r e l a t i v e slgnal v a l u e A T I / T O In t h e w a v e n u m b e r ~ 8 0 0 - 2 5 0 0 e m -I ( o o n s e q u e n t l y , a l ~ r s~l-to-nolse ~ a t l o A T I / ~ T a t e q u a l 1,91ativ8 l e v e l s o f n o i s e ~ T / T O i n t h e speotl~a f o r t w o m e t h o d s o o n s i d e x ~ d ) c o m p a r e d to the RA-method even at optimal (~fs. 12, 13) o o n d l t i o n s f o r (RA) method. In the eXperimental epeOtIn~ t h e v a l u e o f ~ T / T O w h e n m e a s u r e d w i t h t h e r e s o l u t i o n 4 e m -I e n d t h e n u m b e r of s c a n s 4 0 0 w a s e q u a l to about 0 . 0 0 3 for the RA-method and about 0.005 for the SEWmethod in the mlddle p~t of the SEW exoitatlon band. Even in the the
10
these uonditions the SEW method provided a higher signal-to-noise ratio in the speutra of ODP films or the thickness d,<3OO 8. Prom evaluations or the rerlection uoerrl0ient by the formula (rer, 20) it rollows that ror an ODP rilm 25 8 thiuk on the copper on the band 1260 um-‘in ease of the optimal aurrace the signal AT number of rerleotions which is equal to four (r-e+_13) exceeds the corresponding value in case of a single reflection by no more (AT,/T,),, than 1.8 times- It means that in this case the value would exceed the value (AT,/T,), too. The above studies show that the SEW method is espeuiallg promising ror the investigation of vibrational spectra of very thin films (including monomoleoular and *'island" films) on comparatively small metal substrates. In the present paper and in the works (refs, 7.8.18) the peculiarities of the use of the SEW method to obtain vibrational apeotra of the Iilms on monolithic metal substrates with two dif+raction gratings on the surface for the input and output of SEW are considered. It is interesting to realize the SEW spectroscopy analysis of dielectriu films being on the surrace of arbitrary plane metal mirrors, As an example the work (ref. 21) uan be mentioned, There in case 0r a laser radiation sour08 a composite two-walls SEW waveguide formed by a tungsten plate and a gold film deposited on a KRr prism was studied. Below we consider bmad-band SEW speutra obtained in case of the use oi a SEW waveguide with two walls. The waveguide scheme is shown inPi& 3, it is analogous to that used in (ref. 21), A metal substrate 5 without a film or with the film 4 being invesigated is brought up to the surface 0r a metal sample 1 between the gratings (see Fig_ 3) of the SEW input and output. The snmple 1 in rixed in a SRW attachment by analogy to the monolithic sample in works (r&s. 7, 18). The arr0ws show the path of the axial ray in the bulk radiation beam and the SEW propagation dire&ion between the gratings 2 and 3. The gap H between the plates 1 and 5 was set equal to 20 or 50 p by means of nylon layingsThe substrate 5 was obtained by evaporation of a copper layer about O-5 w thick on a polished silicium plate of the length R*=5 UXTI.A LB rilm or ODP 375 2 thiuk (15 monolayers) was deposited on the substrate 5. The sumple 1 with diffraction gratings was prepared in the way described in the work (ref. 7)* The distanoes were (see Pig. 3): R% mm, R”=l nnn.
11
Fig. 3- The soheme of a oomposite SEW wave&de - metal substrates2 - a S",j.np;tsoz&m; Lating: 4 - a dieleAtri0 film; *
with two walls: 1, 3 - a SBW output
the period of the gratinge 2 and 3 was 11.4 p. the ruling depth of the gratings wae about 0.8 m. The optioal density spectra obtained in aooordanoe with the relation (1) are shown in Fig. 4. where T, is the tranamisbion of the open wave&de (the substrate 1 (see Fig- 3) without the substrate 5 brought up to it). T, is the transmission of the wavegu%de formed by the substrates 1 and 5 as shown on Fig. 3. From the oomparison of Fig- 4a and previous results (ref. 18) one oan oonolude that at the above values of the gap in ease of a SEW waveguide with two walls the intensity of the absorption band at ~=I260 cm-' In the speotrum of the ODP film deposited on the substrate 5 is approximately the same aa that whioh oould be expeotcd when measents are made with the use of an open SEW waveguide (the sample 1 wsthout the plate 5 on Pig-3) for an ODP film of the same thiokness deposited on a seotion (5 nvn long) of the surfaoe of the sample 1 between the &EW input and output for the SEW waveguide with two walls the SEW gratingsHowever. transmission deoreases appreoiably oompared to the ease of a open SEW waveguide of the same length due to an inorease in dieleotrio losses in metal whioh deteriorate the signal-to-noise ratio in apeotra of films, the SBW dfspersion by the We have oaloulated numerioally formula from the work ref. 19) for a system "oopper - air gap H ODP film 375 R thiok - oopper'l us ing the relations (5) and (6) to oonstants of the ODP film and of oopper in the desoribe dieleotrio speotral range under tivest%gation and at the above Parameters in It was obtained that for the formulae (5) and (6). respeotively. the inorease in the deoay ooeffioient for the gaps 20 d H d 70 p
12
0.6 a
i-._+/.==
0.4
---.__-.-.--
--__
-----_rc
0.2
-----
-._
3
‘L ------ah_
‘c-.-
--._
0 ii 1 350
i 1300 _lI_il:m_l
1250
1200
1
6 a4
b
2
0
I
I
200
I
100
1
I
50
40
I
I
30
2o
H I UJII
I?%.
4. a. Absorption spectra of a SEW waveguide with two walls at p 20 and 50 ~&IIfor t e oopper substrate 5 th~_oknesses of the air eee PIg.3) with a LB-f ?alm of ODP about 375 I!?thiok (ourves 1 and reepeotively) and for the pure substrate 5 (ourves 3 and 4. z r&peotively) at R'Pc5 UEU. b. Caloulated dependenoes of the SEW deoay ooeffioIent for the SEW waveguide with two walls (a’) on the thiokness of the air,gap H In the oa8e of the pure oopper substrate 5 at V=1200 om (1) and in the ease or he oopper subs+;? 5 with a LB-fllm_?f ODP or the thiokness 375 & at v=l200 om and at V=1260 om
wave&de mode oorresponding to SEW fs approximately inversely proportional to the value of the gap both for the substrate 5 (see Fig, 3) without an ODP film at V=1200 om and for the substrate 5 with an ODP film outside the band (at V=l200 om-') 88 well as on the absorption band at V=1260 am-' (see Fig. 4b). Consequently, the intensity of the absorpteon band of the film depends weakly on the magnitude of the air gap in the above range of the values of the gap- This oondition agrees qualitatively with the experImenta results presented in Fig. 4-a. Por quantitative oomparieon of the oaloulation with the experi-
18
ment it is neaessary to know the value of the ooeffiaient or SEW **open wave&de - waveguide transmission through the interiaae This aondition es well as a possible existenae with two walls"or several modes with di++erent deaay aoeffiaients in the waveguide hamper numeriaal treatment of the results obtained for the waveguide beaonsidered. in the present paper the h.Tgh sensitivity -LYMX the mus. cleat romagnetia wave6 speatroversatiltty of the Fl! IR surraae This method is intended ror the saopy have been demonstrated. The use of a aomposite onslysfs of the state 0r metal surraaes. SEW waveguide allows to broaden the appliaability of the broadband SEW spectrosaopy in investigating vibrational speatra of thin films on metal substrates in aase of the use of grating and other kinds of aoupling elements.
1
laritons and V-HAgranotiah, Crystal optias of surfaae properties of surfaaes. Sov. Phya- Usp..l8 (1975 r 99-146. 2 R-J. Bell, R.W. Alexander. 0-A. Ward. J.L.Tyler, Introduatory eleatwetia wave speatrosaopy. su33_ theory ror surfaae SOT,) 48 (1975) 253-287. G-N, Zhizhin, Y-A. Woskalova, E.V. Shomins snd V-A. Yakovlev, 3 wsve surfaae eleatromagnetia Seleative absorption of a of a thin dieleatria resenae propagating on a metal in the film .Sov. Phys- JlW!P Lett., 24 P1976) 196-199. 4 R. E3has3~, D- Bryan, R-W, Alexander, R-J. Bell, Absorption in the Tnfrared of surraae eleatromsgnetia waves by absorbed moleaules on a aopper surfaae. J. Chem, Phis,, 64 (1976) 50195C25R-J- Bell, D.A. Bryan, D.L. Be leg, E, Bhesin. R-W, Alexander* 5 eleatro etia R. Cerson. ? ion distanae 0r surfaae Propwaves on two metal - oxMe - air systems, su33_ Soi-, T 1976) 53-62s 6 2. Sahlesinger, A-J. Sievers, Broadbend surraae eleatromagnetia wave speatrosaopy, Surf. SOT.. 102 (1981) I29L34. ev and V-11, Yakovlev, G-N, Zhiehin. M.A. Moskalova. A-A. S 7 ?Tr F!l?-IR fnn?faae eleatromagnetia waves ( EW) absorption speatrosaopy of very thin films on metals, Opt. Comrmn., 43(l) (1982) 31-36. 8 A-A. Sigarev. l?C-IR speatrosaopy of thin films on metals with ThesisMosaow. the use of surfaae eleatromagnetia waves, (1984). (in Russfan)M-A, Chesters, S-l?. Parker and V-11, Yakovlev. Surfaae eleatro9 exaitation. Opt. speatrasaopy us¶ng aperture Xzz!lzia wave (1985) 17-21. 55(q) M.A. Moskalova. E-V. Shomine end V-A. Yakovlev. 10 G-N- &!!hin, Edge erreats due to propagation of surfaae IR eleatromegnet.Ta waves along a metal surfaae. JEI!P Lett. 29 (1979) 486-4-89. Infrared speatra or monolayers on 11 S-A. Fransis. A.H. Ellison, metal mirror. J. Opt, SOS. Amer., 49 (1959) 131-138. R.G. Greenler, Infrared study of adsorbed moleoules on metal 12 surfaaes by refleation teahniques. J. Chem. Phys.. 44(l) (1966) 310-315-
14
13
14 15 16 17 18
19 20
21
R.G. Greenler, Rerleotion method for obtaining the infrared spectrum of a thin layer on a metal surfaoe. 3. Chem. Phys.. 50(5) (1969) 1963-l%= R.G Greenler. Design of a refleution-absorption experiment for adsorbed on a metal studying the IR spectrum of moleoules surface, J_ Vao. Soi-Teohnol-, 12(6) (1975) 1410-1417. J-F. Rabolt, J,Juriuh, J-D. Swalen, Infrared refleotion-absorption studies of thin films at grazing incidence. Appl- Speotr39(2) (1985) 269-272of J-F. Blanke. S-E. Vincent, J- Overend. Infrare d spectrosoopy instrumental uonsiderations. Speotroohfmica surface species; Acta, 3311 (1976) 163-173G.F, Bolshakov, E.A. Glebovskaya. Z-G, Kaplan, Infrared spectra and X-rayograms heteroorganio substances, Leningrad. 1967. (5-n Russian). G.N. Zhishin, V-6, aov, Y-A. Moskalova. A.A. Sigarev, V.A. Yakovlev. The dependence of the intensity of an absorption bznd on the thickness of an orgsnio fll_m on a metal surface for the SEW speotrosoopy and for the refleotion-absorption speotrosoapy. Surfaoe. Phys-. Chem., Mechan..10(1985) S-S.(in Russian) 0-A. Ward, K_ Bhasin, R.J_ Bell, R-W, Alexander, I. Tyler, Multtiedia dispersian relation for surface electromagnetic waves, J. Chem. Phys., 62 (1975) 1674-1676. Q-NJ- Zhizhin. M:,A_ Moskalova. E-1 Firsov. E.V. Shomina and V.I. Absorption of surface eleotromagnetio waves by thin Yakovlev. oxide films on metal surfaoes, Sov, Phys, JETP. 52 (1980) 282289. Y.J. Chabal. A.J. Sievers, High-resolution yered study of zzf;n (1x1) on tangsten (100). Phys- Rev. -m 44 (?980) .
Journal of Molecular Liquida, 63 (mW2) 1-14 Elaevier Sciencle Publiahera B.V.. Ameterdam
FT-IR SURFACE ELZETROMAGNETIC BLODGEL'TPILMSONMEPAI,SURl?ACEs*
WAVES
SPECTROSCOPY
G.N. ZHIZRIN, A-A. SIGAREV and V-A-YAKOVIXV Institute of Speutrosaopy. USSR Aoademy Moaoow Region, Troitzk (The USSR) (Received
1 October
of
OF
IlaNGrduIR-
Soienoes.
142092.
1990)
SUMMARY The sensitivities of the SEW and the refleotion-absorption FT methods IR speotrosoopy aompared experimentally and theoret5uallg on the example ozhe 4-n-ootadeoglphenol v Blo ett films. The versatility of the broad-band SEW Bpeotrosoopy in % veetigatlng of thin films on metal surfaoes have been demonstrated, INTROIXJCTION first theoretioal (refa. 1.2) and experimental Already the (refs. 3-5) speotrosoopio works with the use of surface eleotromagnetio waves (SEW) have demonstrated the high sensitivity of SEW to the state of metal surfaues oaused by the near surface oharaoter of the probing field localization. A transition from the laser SEW speotrosuopy (refs. 3-5) to the broad-band speotrosoopy by means of the use of a Fourier transform infrared (FT IR) speutrometer provided with a hm-sensitive reoording system to deteot weak light fluxes has allowed to develop a new universal method of vibrational speotrosoopy for nondestruot%ve study of thin films on metal substrates. Such appmaoh has permitted to obtain broad-band vibrational speotra of th%n dieleotrio films on smooth metal surfaoes by the SEW method with the use of three kinds of ooupling elements for the t,itation of SEW by bulk radiation and for the reverse SEW into bulk oonversion of radiation such as the edges of the metal surface (ref. 6). diffraotson @;ratings on the metal aurfaoe (refs.7.8) and the aperture excitation of SEW (ref. 9). The main advantages of affraotioa gratfngrj used in the present work as ooupling elements are: relatively high trans r033ntdion effioienoy in the middle IR range. the simplioity of optioal adjustment. the plane oonstruotion, the meohanioal stab%lity and thus determined the stability of the transformer efflofenuy. The * Dedicated
to Professor
0167-7322/92/$06.00
8
Paraskeva
1992 -
Sirnova
Elaevier Science Publiehers
B-V. Allrighta reserved
above properties oan beoome espeoially valuable when ooupl~ attaohelements are used in vaouum and t3geotrcmoopio OrgogsniO merits, The neoessity to make diffraotion gratings on the surfaoe of the metal sample beinvestigated is a asadvantage of this version but it oan be overuome by the use of various oonstruotions of oompoaite SEW waveguides (refs. 8.10), Namely a oombination of the SEW method with the adventsgee of F!F speotrometry has allowed the development of a new highsensitive method for the analysis of metal surfaoes: the FL IR SEW speotrosoopyThe SEW method oan be uompared with one of the most highsensitive optioal methods of vibrational speotrosoopy - the refleotion-absorption (RA) speotrosoopy (ref's, 11-13) whtoh is widely used for the investigation of moleoular systems on metal surfaues in the IR range (refs. 14-16). A uommon property of the SEW- and DA-methods is their seleotive sensitivity only to perpendioular to the metal surfaoe components of Uipole moments of v*brational transitions fn a thfn film being on the metal surfaoe (r&s. 7. 2. 12). In the present paper the sensitivity of the IR broad-band SEW speotmsoopg is disoussed compared to that of the DA method and also a possible applioation of a SEW wave&de with two walls to invesfTgate vibrational speotra of thin tieleotrio films on metal surfaoes by the SEW method is oonsidered,
Speotral measurements in the wavenumber iv) range 650-2500 om-' were made on a Digflab P!l!S-20B l?!CIR speotrometer with resolution 4 am-' anU the number of 808118 400. We have used speoial SEW and rerleotion attaobments whose optioal schemes were desoribed in detailsin (ref- 7). and a HgOd!l!e photodeteotor operated at 77 EC. The manufaoturing oonditions and the oonfiguration of oopper substrates with two dirfraotion grate for the SEW input and output used in this work as well as the deposition oonditions for 4-n-outadeaylphenol (ODP) films on substrates using the v Blodgett method (LB films of ODP) are desoribed in (ref- 7.81, The thioknesses of investigated ODP films were equal to the values from 25 2 (1 monomoleoular layer) up to 400 8. IR speotra of the films were obtained in the following wayThe refleotion speotrurn (TotRnl(V)) and the SEW transmission (Y)) for the chosen pure substrate were suooesspedtrum (TotsEIR, sively measur ed with the use of refleotion and SEW attaobments
a respeottvelg
and
put
into
the
memory
of
the
El? speotrometer
computer. Then the dleleotrio film being InvesIgated wan deposited on the aurfaoe of the substrate and the SEW transmission speotrum (U)) and the reflection speutrum CT, tRA, (VI) were measured (T (SEW) __ .~ for the substrate with the film. The absorptflon speotra of the films are given below in the optical density soale (D(V)) whioh was oalaulated for eaoh of the methods by the formula:
N’)=-~gp,(V)/T,.,(v)]
(1 1
REXllXS AND DISCUSSION Conefder the charaotristio features of the El! IR SEW apeotrosaopy tak3ng I&3 films of ODP as an example. The absorption ape&rum of the initial polyoryetallins ODP pressed into a KDr pellet is shown in Fig-la. In the interval 1000-1700 cm-' the strongest abeorption bands correspond to vibrations of the phenyl ring: the band at WA260 cm-' - to deformation vibrations of C-Hthe band at aJ=1518 am-' - tostretiiw groups in the ring plane, vibrations of carbon atoms of the ring (ref. 17)InFig. SEW tranermi ssion apeotra are shown for the substrate with no film and with a I&3 film of ODP about 125 8 thiok (five monolayers). In the speotrum of the sample with an ODJ? film, firl3tJg. over the whole range of SEW exaitation a deoreaee Tn the SEW tmission is observed aompared to the case of a pure substrate and, secondly, tranExnission minima appear in the region of ODP absorption bands. In Fig-Id absorption apeotra of LB film6 of ODP of different thiaknesrses are mown obtained upon transformation of the respective transmi ssion epeotra of Sl3W by the formula (1). !L%e absorption band.8 at 1260 and 1518 cx-' are clearly Been in the speotrum of an ODP monomoleoular film. For oomparison Pig-lb shows the absorption speotrum of an ODP film of five monolayer8 on a aopper sub&rate med by the FU method at the angle of rad.lation tiuidenoe on the sample about 88? It is seen from Figalb,ld that the two methods being give oonsidered similar absorption spectra of the f5_lm - the positions and the half-widthrs of the respeotive absorption bands ooinoide, however, the intensities of the abaorption band6 In the film speutra (AD), equal to the aifferenoes AD=+3 in optiual densities at the maximum of the band (#) and at the level of the background (9) differ appreoiably for the two methods. Experiments and calaulationa have permitted to d&ermine the oharaoter of the nonlinear dependenae of the intensity of absorp-
a
b
T*104 6
3
0
D
0.3 0-2 0-1
0
I
1600
1400
1200
zr, cm-'
Fig, 1. a, The abeorption speotrum of polgory6talline ODP preseed In a KEr pellet. b- The abbe tion speotrum of a LB-film of ODP of five monolayers on oopper Ilp FIA-met&od. the angle of radiation lnoidenoe on the sumple is about 88 ). the speotrum IS shifted the D axia with no ohange of the soale. al0 o, The SEW tranemissYY on Bpeotra for the oo er subBt.rate without a film (I) and with an ODP film about 125 ?? thiqk, (2), An abrupt decrease 5.n the sample transmission at ?~=I316 om is oaused by Bragg refleotion of SEW on the SEW input and output gratws with the period d, The absorption speotra of LB-films of ODP of one (I), II-4 pthree (2) and five (3) monolayers on the surfaoe of oopper (SEWmethod, the distanoe between the SEW input and output gratings 5s equal to 1 om)-
5
tion band in the speotrum of the film on the metal surraoe on the film thiokness ror the SEW and RA speotmsoopy methods (refs.7,38) For organio films under investigation c-ons5derable deviations from proportionality 0r the film thiolazess to the band intensity are already observed at thicknesses of SBVB~ a0zen~ 0r BngstmmThis raot must be taken Into aooount when evaluating quantitat-lvely the flilm parameters by their speutra. In the approximation of a thin dieleotrio film on a metal surfaae. 5-e. when the oondition 12K'~(l-E,)d,4'~(
a 1
(2)
is fulfilled (here 13, is the dieleotrio constant of the film, d, is the CUm thiokness, s is the dieleotruo oonstant of the metal) the intensity of the film absorption band ADsM in the ease of the SRW method me asurements in (rer_ 18): AD
SEW
-
+ 2WPd,
8k%%~'Rd,[Im(-E~1)](lg 1 - &;' + 2&3%;2 ( em
e)[ I + I&.%J;~
31.
+ (3)
the high-frequency dieleotrio oonstant 0r the ODP rilm. wp is the plasma frequency of the metal, R is the aistanoe between the SEW input and output ooupling elements, One can see from rormula (3) that AD,,grows with the distanoe R between SEW ooupl3ng elements. However, the sensitivity of the speotrometer reoording system restriots the maximum value of R at whioh the SEW speotrum 0811 still be measured with suT+ioTontly Ilre;h signal-to-noise ratio at present values 0r the radiation sonroe power and the effioienoy or bulk racklation and S‘EW ooupling elementsThe opM.mal distanae between the SEW input and output elements oan be ohosen sufrioiently large, R RI ai1 (x-d6), where a0 is the SEW deoay ooeffioient on a smooth metal surraue ror some mean value of the wavenumber rrom the ape&rum region uncler invest5gation. For good oonduotors the value of aiiin the middle IR range uan be of the order of a oentimeter. In the ease of the RA method at the itied angle of radiation 3noidenoe on the sample the intensity of the absorption band in the speotrum of the film on the metal surfaoe Inoreases with the number of refleotions (refs.12, 13); however, the maximum value of the signal-to-noise ratio in the film speotrum will be attained at a quite derinite optimal number of rerleotions wioh depends on the
where
is
parameters
of metal
uonsid_ered
and
the given
radiatfon
tioidenoe
&Lngle (refs. 13.14). oriteria for the evaluation of One of the most important The signal-to-noise ratio in speotrosuopio methods is sensitivity. is the useful signal AT,/6T, where AT, the film speotrum equals of the substrate with a
= Gsm
(VI lexpC-ar(V) RI.
(4)
funotion where a funotion GQEW (V) will be oalled the transmission of the SEW attaubment which uharaoteriees the effioienog of SEW input and output ooupling elements at given parameters of the exoiting radiation beam; a(v) is the SEW deuay ooeffioient for the substrate se&ion between the aoup1in.g gratings; R is the distance between the gratings. In the experiment R=l om. For the RA method in the ease of a s3_ngle refleotion the transmfssion of the sample TM oan be expressed in function of the reflection attaohment terms of the t ransmission (GM) and the ooeffioient of the refleotion Worn the sample (r) by the formula and the optioal density of the T,(~)=G~(v)~r(v). film absorption with the allowance made for (I) 0811 be D&v) represented in the form D,(V)=-lg;C r,W)/ro(v) 1 where r,(V) and r,(Y) are the ooeffioients of reflection from the metal without a film and with a film. respectively, The values of r,(V) and r,(v) have been oaloulated by the exaot formula for the refleotion ooef-
7 flolent in a system "metal - dleleotrlo film of the thlokness d 1 a i r " (z,gf. 20) a t the a n g l e o f znmd1~tion i n e l d e n o e on t h e s a m p l e surface ~=~o. It w a s s u p p o s e d that a t h i n f i l m o n t h e m e t a l s u r f a c e changes negligibly the t r a n s m i s s i o n functions o f the SEW and reflection a t t a c h m e n t s a n d t h a t the c o n t r i b u t i o n o f t h o s e f i l m sections which a r e i n the r e g i o n o f S E W i n p u t a n d o u t p u t g ~ a t l n g s h a s b e e n s u b t r a o t e d f r o m t'_,e e x p e r i m e n t a l v a l u e A D s EW (ref. 18). We asked for oaloulatlons that the dleleotrlo constant of the LB film of OD~ near the single absorption b a n d a t 1 2 6 0 o m -1 is described by a slngle--osoillator model:
E1
E='[~LO ~
.
.
.
l~Ol
.
.
[~TO--~
l%~TO] -1
,
(5)
w h e r e £~--2.25 is t h e h l g h f r e q u e n c y d l e l e o t r i o c o n s t a n t ; ~TO" ~ L O a r e t h e f r e q u e n o l e s a n d 7TO" ~ L O a r e t h e d e c a y f o r t r a n s v e r s e a n d l o n ~ i t u d l n a l o s c i l l a t i o n s of t h e film, z ~ s p e o t i v e l y . T h e values of
t h e p e m s m e t e r s 12i~=1262 om-1 , 12TO=1259.7 em-1 . ~350----7TO=15 om-1 : f o r the band under investigation of the LB film of ODP were determined f r o m t h e s p e c t r a l d a t e o b t a i n e d b y the S E W - a n d RA--methods a n d b y the attenuated total refleotlon method in the case of an ODP film on a go _rz~n4 um substx~te too. The v a l u e of the d~ f f e r e n o e (~O-~TO) was specified to r e a c h t h e b e s t a g r e e m e n t b e t w e e n t h e calculation and the experiment for the SEW method. The 4tteleotrie constant for copper was calculated by the formula =
1
-
(6)
with the following pamameters: pl~usma f r e q u e n c y ~p=65100 o m -1 ; effeotlve frequency of oollisionR of eleotrorus with phonons, impurltles and defects V ¢ = 4 0 0 o m -1 . Theoretloal dependences of t h e r e l a t i v e transmission of the substrate w i t h a n O D P f i l m a t I)=-1260 o m -1 at the trexlsmlssion m~nlmum (~/TO)_ a n d at t ~ e bsokgroua~d l e v e l (~/TO)_ o n the O D P film thickness ar~ shown ~a F~.2 for two methods. For the SEW method a relatively rapid decrease of the backg r o u n d t r a n s m i s s i o n w i t h the i n c r e a s i n g t h l o k n e s s o f t h e o o v e r l n g f i l m is c h a r a c t e r i s t i c (curve 2 i n F i g . 2a), w h e r e a s f o r t h e R A m e t h o d t h e baok4~roumd t ~ i s B i o n somewhat increases (curve 4). I n F i g . 2 b t h e oalouulated d e p e n d e n c e s of t h e z~elativo s i g n a l v a l u e o b t a i n e d f r o m t h e a p p r o p r i a t e c u r v e s 1 - 4 in Fig. 2a are showTl.
a
-(AT/T&= 0
100
200
300
4oo
a 1' B
AT/T
b
0
100
200
300
400
d 1’
2
Fig. 2. a. Theoretical dependences of the values or q/T0 and at v=l260 cm-' Ty/TO on the ttiokness of the LB-film of ODP on the copper surface for the SEW-method at Fgl om (ourvee 1 and 2. respeotively) and for the RA-method at @=E38 (ourves 3 and 4, reseotivsly). b_ Theoretical dependeno_? of the relative signal z T/To of the absorption band 1260 om on the thiokness of the LB-film of ODP on a co per substrate for the SW-method (ourve 1) and for the RA-method P The signs are the ~~%.%.a1 values oi (q/T0)8m -(A), (Ty/To)ssmr - (I ). (AT,/TcJsEW - (*). (q/To)= - (A), (TT/To)~ - ( q ), (AT,/TJRn - (0). For oomparison Fig. 2a shows by pointa the transmission and signal values at the absorption band of the ODP film in the speotra or several speoimens. The error in the determ9nation of the values of did not exoeed the dimenaiona of the aigne in T,‘To and AT/To Pi_& 2, One oan 888 from 2b that ror the two methods be%ng Figdzsoussed the signals in the speatra have tirferent dependenoee on the film thiokness on the metal auriaoe. In the investigated range
9
of thicknesses
of ODP
f11m-
on copper
for the RA-method
the signal
I n c r e a s e s monotonot%sly w i t h i n o r e a s l n g d I , w h e r e a s for the SEWmethod the s i ~ Inox~ases with inomaslng d I at di<275 ~ and deox~ases at ~ d I st d I > 2 7 5 ~. T h e c a l c u l a t e d d e p e n d e n c e s o f A T 1 / T 0 o n d I f o r t h e ~wo m e t h o d s p r o v l d e a oox-±~ot d e s c r i p t i o n o f e X p e r i -~antal ~ t a . The sensitivities of these two methods are compared upon 8n as~n--ptlon t h a t t h e S E W a n d r e f l e o t l o n s l g n a l s r e c o r d e d b y t h e F T spectrometer az~ sufficiently la~ and relative levels o2 noise ~T/T O in the refleotlon _And S E W s p e c t r a a r e t h e s a m e . In the app~oxlm~tion of a tb4n film on a metal ~aoe (2) f r o m t h e e x p r e s s i o n f o r t h e S E W w a v e v e o t o ~ (r~f.18) R n d t h e e x p r e s s l o n f o r the refleotlon coefficient o f p--polsa~Ized l i g h t 2 o r t h e s y s t e m "met~l substl~ate -- f i l m I - a i r " (i~f. 20) m~klng allowance for t h e c h o s e n m o d e l s d e s o r l b i n g dleleotx~lo c o n s t a n t s o f t h e s u b s t r a t a (6) a n d t h e f i l m (5) w e o b t a i n t h e f o l l o w l n g f o r m u l a f o r t h e r a t i o o f s l g n ~ ] s at t h e f i l m absoI~tlon band in the SEW- and RA-methods (s~ngle refleotlon) upon a condition 72 ~ ~p, ~)~ ~ ~p: (ATI/To)s~(AT1/To)RA
~ ~R(oos
~ ) ( ~ p - S l n a ~ ) -1
~o~ an or~anlo film with 6~=2.25 and of the thickness d1=25 on the copper surface with the psrameteI~ Vp=65100 o m -2 a n d ~ = =400ore -1 t h e r a t i o o f s i g n a l s (7) a t R=I ore, ~ - 8 8 ° a n d ~=-1260 om - 1 e q u a l s a p p I ~ X l m ~ t e l ~ 3.5- A s s e e n f r o m F ~ . 2b, t h e 1~atio o f signals (AT1/TO)8~(AT1/To)RA m u s t deoz~.~use w i t h inoreas4ng %AliOkness o f t h e f i l m o n the m e t a l s u r f a c e . Analysis of the formula (7) --nd F i g . 2 b s h o w s t h a t w i t h the appropr~te c h o i c e o f t h e S E W w a v e g u l d e l e n g t h (R ~ ~ I ) the SEWm e t h o d a l l o w s o n e to o b t a i n a 2 - 6 t i m e s l a r ~ e r r e l a t i v e slgnal v a l u e A T I / T O In t h e w a v e n u m b e r ~ 8 0 0 - 2 5 0 0 e m -I ( o o n s e q u e n t l y , a l ~ r s~l-to-nolse ~ a t l o A T I / ~ T a t e q u a l 1,91ativ8 l e v e l s o f n o i s e ~ T / T O i n t h e speotl~a f o r t w o m e t h o d s o o n s i d e x ~ d ) c o m p a r e d to the RA-method even at optimal (~fs. 12, 13) o o n d l t i o n s f o r (RA) method. In the eXperimental epeOtIn~ t h e v a l u e o f ~ T / T O w h e n m e a s u r e d w i t h t h e r e s o l u t i o n 4 e m -I e n d t h e n u m b e r of s c a n s 4 0 0 w a s e q u a l to about 0 . 0 0 3 for the RA-method and about 0.005 for the SEWmethod in the mlddle p~t of the SEW exoitatlon band. Even in the the
10
these uonditions the SEW method provided a higher signal-to-noise ratio in the speutra of ODP films or the thickness d,<3OO 8. Prom evaluations or the rerlection uoerrl0ient by the formula (rer, 20) it rollows that ror an ODP rilm 25 8 thiuk on the copper on the band 1260 um-‘in ease of the optimal aurrace the signal AT number of rerleotions which is equal to four (r-e+_13) exceeds the corresponding value in case of a single reflection by no more (AT,/T,),, than 1.8 times- It means that in this case the value would exceed the value (AT,/T,), too. The above studies show that the SEW method is espeuiallg promising ror the investigation of vibrational spectra of very thin films (including monomoleoular and *'island" films) on comparatively small metal substrates. In the present paper and in the works (refs, 7.8.18) the peculiarities of the use of the SEW method to obtain vibrational apeotra of the Iilms on monolithic metal substrates with two dif+raction gratings on the surface for the input and output of SEW are considered. It is interesting to realize the SEW spectroscopy analysis of dielectriu films being on the surrace of arbitrary plane metal mirrors, As an example the work (ref. 21) uan be mentioned, There in case 0r a laser radiation sour08 a composite two-walls SEW waveguide formed by a tungsten plate and a gold film deposited on a KRr prism was studied. Below we consider bmad-band SEW speutra obtained in case of the use oi a SEW waveguide with two walls. The waveguide scheme is shown inPi& 3, it is analogous to that used in (ref. 21), A metal substrate 5 without a film or with the film 4 being invesigated is brought up to the surface 0r a metal sample 1 between the gratings (see Fig_ 3) of the SEW input and output. The snmple 1 in rixed in a SRW attachment by analogy to the monolithic sample in works (r&s. 7, 18). The arr0ws show the path of the axial ray in the bulk radiation beam and the SEW propagation dire&ion between the gratings 2 and 3. The gap H between the plates 1 and 5 was set equal to 20 or 50 p by means of nylon layingsThe substrate 5 was obtained by evaporation of a copper layer about O-5 w thick on a polished silicium plate of the length R*=5 UXTI.A LB rilm or ODP 375 2 thiuk (15 monolayers) was deposited on the substrate 5. The sumple 1 with diffraction gratings was prepared in the way described in the work (ref. 7)* The distanoes were (see Pig. 3): R% mm, R”=l nnn.
11
Fig. 3- The soheme of a oomposite SEW wave&de - metal substrates2 - a S",j.np;tsoz&m; Lating: 4 - a dieleAtri0 film; *
with two walls: 1, 3 - a SBW output
the period of the gratinge 2 and 3 was 11.4 p. the ruling depth of the gratings wae about 0.8 m. The optioal density spectra obtained in aooordanoe with the relation (1) are shown in Fig. 4. where T, is the tranamisbion of the open wave&de (the substrate 1 (see Fig- 3) without the substrate 5 brought up to it). T, is the transmission of the wavegu%de formed by the substrates 1 and 5 as shown on Fig. 3. From the oomparison of Fig- 4a and previous results (ref. 18) one oan oonolude that at the above values of the gap in ease of a SEW waveguide with two walls the intensity of the absorption band at ~=I260 cm-' In the speotrum of the ODP film deposited on the substrate 5 is approximately the same aa that whioh oould be expeotcd when measents are made with the use of an open SEW waveguide (the sample 1 wsthout the plate 5 on Pig-3) for an ODP film of the same thiokness deposited on a seotion (5 nvn long) of the surfaoe of the sample 1 between the &EW input and output for the SEW waveguide with two walls the SEW gratingsHowever. transmission deoreases appreoiably oompared to the ease of a open SEW waveguide of the same length due to an inorease in dieleotrio losses in metal whioh deteriorate the signal-to-noise ratio in apeotra of films, the SBW dfspersion by the We have oaloulated numerioally formula from the work ref. 19) for a system "oopper - air gap H ODP film 375 R thiok - oopper'l us ing the relations (5) and (6) to oonstants of the ODP film and of oopper in the desoribe dieleotrio speotral range under tivest%gation and at the above Parameters in It was obtained that for the formulae (5) and (6). respeotively. the inorease in the deoay ooeffioient for the gaps 20 d H d 70 p
12
0.6 a
i-._+/.==
0.4
---.__-.-.--
--__
-----_rc
0.2
-----
-._
3
‘L ------ah_
‘c-.-
--._
0 ii 1 350
i 1300 _lI_il:m_l
1250
1200
1
6 a4
b
2
0
I
I
200
I
100
1
I
50
40
I
I
30
2o
H I UJII
I?%.
4. a. Absorption spectra of a SEW waveguide with two walls at p 20 and 50 ~&IIfor t e oopper substrate 5 th~_oknesses of the air eee PIg.3) with a LB-f ?alm of ODP about 375 I!?thiok (ourves 1 and reepeotively) and for the pure substrate 5 (ourves 3 and 4. z r&peotively) at R'Pc5 UEU. b. Caloulated dependenoes of the SEW deoay ooeffioIent for the SEW waveguide with two walls (a’) on the thiokness of the air,gap H In the oa8e of the pure oopper substrate 5 at V=1200 om (1) and in the ease or he oopper subs+;? 5 with a LB-fllm_?f ODP or the thiokness 375 & at v=l200 om and at V=1260 om
wave&de mode oorresponding to SEW fs approximately inversely proportional to the value of the gap both for the substrate 5 (see Fig, 3) without an ODP film at V=1200 om and for the substrate 5 with an ODP film outside the band (at V=l200 om-') 88 well as on the absorption band at V=1260 am-' (see Fig. 4b). Consequently, the intensity of the absorpteon band of the film depends weakly on the magnitude of the air gap in the above range of the values of the gap- This oondition agrees qualitatively with the experImenta results presented in Fig. 4-a. Por quantitative oomparieon of the oaloulation with the experi-
18
ment it is neaessary to know the value of the ooeffiaient or SEW **open wave&de - waveguide transmission through the interiaae This aondition es well as a possible existenae with two walls"or several modes with di++erent deaay aoeffiaients in the waveguide hamper numeriaal treatment of the results obtained for the waveguide beaonsidered. in the present paper the h.Tgh sensitivity -LYMX the mus. cleat romagnetia wave6 speatroversatiltty of the Fl! IR surraae This method is intended ror the saopy have been demonstrated. The use of a aomposite onslysfs of the state 0r metal surraaes. SEW waveguide allows to broaden the appliaability of the broadband SEW spectrosaopy in investigating vibrational speatra of thin films on metal substrates in aase of the use of grating and other kinds of aoupling elements.
1
laritons and V-HAgranotiah, Crystal optias of surfaae properties of surfaaes. Sov. Phya- Usp..l8 (1975 r 99-146. 2 R-J. Bell, R.W. Alexander. 0-A. Ward. J.L.Tyler, Introduatory eleatwetia wave speatrosaopy. su33_ theory ror surfaae SOT,) 48 (1975) 253-287. G-N, Zhizhin, Y-A. Woskalova, E.V. Shomins snd V-A. Yakovlev, 3 wsve surfaae eleatromagnetia Seleative absorption of a of a thin dieleatria resenae propagating on a metal in the film .Sov. Phys- JlW!P Lett., 24 P1976) 196-199. 4 R. E3has3~, D- Bryan, R-W, Alexander, R-J. Bell, Absorption in the Tnfrared of surraae eleatromsgnetia waves by absorbed moleaules on a aopper surfaae. J. Chem, Phis,, 64 (1976) 50195C25R-J- Bell, D.A. Bryan, D.L. Be leg, E, Bhesin. R-W, Alexander* 5 eleatro etia R. Cerson. ? ion distanae 0r surfaae Propwaves on two metal - oxMe - air systems, su33_ Soi-, T 1976) 53-62s 6 2. Sahlesinger, A-J. Sievers, Broadbend surraae eleatromagnetia wave speatrosaopy, Surf. SOT.. 102 (1981) I29L34. ev and V-11, Yakovlev, G-N, Zhiehin. M.A. Moskalova. A-A. S 7 ?Tr F!l?-IR fnn?faae eleatromagnetia waves ( EW) absorption speatrosaopy of very thin films on metals, Opt. Comrmn., 43(l) (1982) 31-36. 8 A-A. Sigarev. l?C-IR speatrosaopy of thin films on metals with ThesisMosaow. the use of surfaae eleatromagnetia waves, (1984). (in Russfan)M-A, Chesters, S-l?. Parker and V-11, Yakovlev. Surfaae eleatro9 exaitation. Opt. speatrasaopy us¶ng aperture Xzz!lzia wave (1985) 17-21. 55(q) M.A. Moskalova. E-V. Shomine end V-A. Yakovlev. 10 G-N- &!!hin, Edge erreats due to propagation of surfaae IR eleatromegnet.Ta waves along a metal surfaae. JEI!P Lett. 29 (1979) 486-4-89. Infrared speatra or monolayers on 11 S-A. Fransis. A.H. Ellison, metal mirror. J. Opt, SOS. Amer., 49 (1959) 131-138. R.G. Greenler, Infrared study of adsorbed moleoules on metal 12 surfaaes by refleation teahniques. J. Chem. Phys.. 44(l) (1966) 310-315-
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14 15 16 17 18
19 20
21
R.G. Greenler, Rerleotion method for obtaining the infrared spectrum of a thin layer on a metal surfaoe. 3. Chem. Phys.. 50(5) (1969) 1963-l%= R.G Greenler. Design of a refleution-absorption experiment for adsorbed on a metal studying the IR spectrum of moleoules surface, J_ Vao. Soi-Teohnol-, 12(6) (1975) 1410-1417. J-F. Rabolt, J,Juriuh, J-D. Swalen, Infrared refleotion-absorption studies of thin films at grazing incidence. Appl- Speotr39(2) (1985) 269-272of J-F. Blanke. S-E. Vincent, J- Overend. Infrare d spectrosoopy instrumental uonsiderations. Speotroohfmica surface species; Acta, 3311 (1976) 163-173G.F, Bolshakov, E.A. Glebovskaya. Z-G, Kaplan, Infrared spectra and X-rayograms heteroorganio substances, Leningrad. 1967. (5-n Russian). G.N. Zhishin, V-6, aov, Y-A. Moskalova. A.A. Sigarev, V.A. Yakovlev. The dependence of the intensity of an absorption bznd on the thickness of an orgsnio fll_m on a metal surface for the SEW speotrosoopy and for the refleotion-absorption speotrosoapy. Surfaoe. Phys-. Chem., Mechan..10(1985) S-S.(in Russian) 0-A. Ward, K_ Bhasin, R.J_ Bell, R-W, Alexander, I. Tyler, Multtiedia dispersian relation for surface electromagnetic waves, J. Chem. Phys., 62 (1975) 1674-1676. Q-NJ- Zhizhin. M:,A_ Moskalova. E-1 Firsov. E.V. Shomina and V.I. Absorption of surface eleotromagnetio waves by thin Yakovlev. oxide films on metal surfaoes, Sov, Phys, JETP. 52 (1980) 282289. Y.J. Chabal. A.J. Sievers, High-resolution yered study of zzf;n (1x1) on tangsten (100). Phys- Rev. -m 44 (?980) .