Peculiarities of the reflection-absorption and transmission spectra of

ISSN 20702051, Protection of Metals and Physical Chemistry of Surfaces, 2013, Vol. 49, No. 2, pp. 189–197. © Pleiades Publishing, Ltd., 2013. Original Russian Text © S.D. Stuchebryukov, S.L. Selektor, D.A. Silantieva, A.V. Shokurov, 2013, published in Fizikokhimiya Poverkhnosti i Zashchita Materialov, 2013, Vol. 49, No. 2, pp. 194–203.

MOLECULAR AND SUPRAMOLECULAR STRUCTURES AT THE INTERFACES

Peculiarities of the Reflection–Absorption and Transmission Spectra of Ultrathin Films under Normal Incidence of Light S. D. Stuchebryukov, S. L. Selektor, D. A. Silantieva, and A. V. Shokurov Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow, 119071 Russia email: [email protected] Received 28 July, 2012

Abstract—This paper analyzes the prospects of studying optically anisotropic monolayers on a liquid sub strate using linearly polarized light. For quantification of the electronic transmission spectra of the studied monolayers on the transparent subphase and Langmuir–Blodgett films transferred onto transparent plates as well, we obtained formulas for optical densities of transmission spectra in a triple system: surrounding medium–ultrathin film–substrate (liquid or solid). The expressions derived in this work are compared with previously obtained formulas for optical densities of spectra measured in the reflection mode for an analogous system. It is shown that, with all other conditions being equal, employment of this measurement technique allows one to obtain spectra the peak intensities of which (measured in absorbance units) are almost an order of magnitude higher than the corresponding values for transmission spectra. The regularities revealed in this work are confirmed by experimental measurements carried out for monolayers of certain conjugated organic compounds of various structures: amphiphilic chromoionofores, photosensitive oligomeric organic conduc tors, and electrochemically multistable rigid macrocyclic compounds. A good quantitative agreement between theory and experiment was obtained. DOI: 10.1134/S2070205113020044

INTRODUCTION A comprehensive theoretical base is needed that will allow proving the reliability and informativity of recorded optical signals in order to fully implement the possibilities of electronic absorbance spectroscopy for studying Langmuir monolayers and Langmuir– Blodgett films (LBFs) on various surfaces. Highly ordered ultrathin films made of various amphiphilic compounds have been attracting more and more attention of researchers in recent years for use in molecular electronic nanodevices and photon ics. It is known that the efficiency of such devices is defined not only by the physicochemical properties of the compounds used, but also by the spatial conditions of intermolecular and energetic interactions and the specifics of the behavior of planar supramolecular sys tems, which is due to their location directly at the air/substrate interface. For example, in many cases, high proneness to aggregation becomes one of the main obstacles in practical application, the type and character of such aggregation may significantly change the whole complex of physicochemical properties of the formed LBF. This is why one of the most promising areas of work in increasing the operation efficiency of information devices is the development of ultrathin planar systems the location of components of which allows for optimal conditions for generation and read ing information.

Solving such problems requires the use of reliable and rapid working methods of in situ control of the state, structure, and properties of the monolayer pre cursor immediately in the process of its formation. Methods of infrared (IR) spectroscopy and electronic absorption spectroscopy are widely used for study of such phenomena in threedimensional systems (for example, in films with a thickness of more than 100 nm). Special modifications of IR spectroscopy for the study of ultrathin films on various surfaces, including air/water interface, have been developed [1, 2]. How ever, vibrational spectra cannot give full information on the physicochemical processes that take place upon formation of monolayers on liquid substrate. It is evident that, in many cases, electronic spectra can be very informative from the standpoint of the study of chemical interactions. The character of changes of electronic spectra upon various structural transformations in bulk objects is also well known [3]. With this, registration of spectra can be carried out in both transmission and reflection modes, depending on the geometrical and optical properties of the studied objects. However, use of such measurements in research of planar systems requires the development of special approaches that will take into account the fact that the film thickness is many times less than the wavelength of incident light, the small intensity of the signal, and the anisotropic properties of twodimen

189

190

STUCHEBRYUKOV et al. E 兩兩 E⊥

Surrounding medium Ir⊥,兩兩 n1 ⊥,兩兩

Ii

x

θ1 n~2x = n2 + ik2x n~2y = n2 + ik2y n~2z = n2 + ik2z

Monolayer

y z

n3

Subphase

θ3

It⊥,兩兩

Fig. 1. Optical scheme of reflection spectra measurement.

sional films formed upon compression of Langmuir monolayers on liquid substrate. The development of fiber optic electronic spec troscopy in the visible region in the past two decades has opened up wide possibilities for study of both chemical and physical interactions in monolayers of various dyes at air–aqueous subphase interface. In various technical modifications, such methods are used for research of aggregation processes in monolay ers upon compression and structural organization [4– 12] and for obtaining information on the orientation of molecules in monolayer [13–16]. In addition, there are several examples of studies of chemical reactions with the interaction of monolayer components using electronic spectra of monolayers of functionalized amphiphilic compounds on the surface of the water subphase in the literature [17, 18]. Some researchers record the absorbance of the monolayer in a transmission mode using a trough with a transparent sapphire window [6, 7, 18, 19]. As should be expected, the intensity of the measured signal, even for dyes with high extinction, is very small (no more than 0.01 rel. units of absorbance). In other works, a light signal reflected from the surface of the water with deposited monolayer is recorded (in the form of differ ential absorbance spectra) [4, 5, 11–13, 15–17]. In these works, the results of measurements are pre sented as units of absorbance [15–17]. However, the oretical analyses of applied techniques and compari sons of experimental values of peak intensities of obtained spectra were not previously reported by anyone. During the performance of experimental studies of monolayers of amphiphilic hemycyanine dyes, we observed that registration of the light signal reflected from the monolayer surface allows one to obtain spectra that are significantly more contrasting than those obtained upon measurements in the transmission mode. In order to avoid possible misunderstandings, we should note that, in this paper, we will use the term

“reflection spectra” for all types of spectra recorded using specular reflection. The method of processing of true reflection spectra that we applied allows us to call the obtained dependences differential normed reflec tion spectra represented in absorbance units. In our opinion, for brevity, it is quite convenient to use the term “reflection spectra” everywhere in present arti cle. We also should note that we quite thoroughly explain the derivation of formulas (6) and (7), and thus no misunderstanding in terms should arise. The main aim of this article is to prove that the observed advantage of reflection spectra over trans mission spectra has a regular character. For this, in the second section of the article, we show and analyze pre viously obtained [20, 21] formulas of optical densities of reflection spectra recorded in a threelayer optical system: surrounding medium–studied thin film–sub phase. In the third section of the article, we derive for mulas for optical densities of transmission spectra recorded in both an analogous system and in the sys tem surrounding medium–thin film–transparent sub strate as well. In the fourth section of the paper, expressions for correlations of absorbance values in reflection and transmission spectra are shown. A com parison of the theoretical values of these parameters with experimentally obtained ones is also given. Lastly, the seventh and final section of the article is devoted to a discussion of urgently needed methods for improving experimental schemes of spectra recording and possi ble ways to increase the volume of information obtained in these experiments. REFLECTION SPECTRA The studied threelayer optical system is shown in Fig. 1. It is assumed that surrounding medium (1) and subphase (medium 3) are semiinfinite, isotropic, transparent, and characterized by real refractive indi ces n1 and n3 respectively. To the contrary, the studied thin film (medium 2) has finite thickness d, is aniso tropic, and absorbs probing radiation. Three assump tions regarding the optical properties of medium 2 sig nificantly simplify the whole further description. The first assumption is that the x, y, and z axes coincide with the main directions of the optical dielectric con stant tensor in the coordinate system considered below. The second assumption is that the main direc tions of optical dielectric and conductivity tensors coincide with each other. Lastly, the third assumption is that only the conductivity tensor has anisotropy of properties. Taking into account all these assump tions, the complex refractive indices of studied medium can be expressed [22] as n2X = n2 + ik2X, n2Y = n2 + ik2Y, n2Z = n2 + ik2Z. It is understood that strictly quantitative studies of optical systems of such type are possible only with employment of polarized light. Very simple mathe matical expressions that connect the intensities of

PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 49 No. 2 2013

PECULIARITIES OF THE REFLECTION–ABSORPTION AND TRANSMISSION SPECTRA

incident and reflected radiation can be given for lin early polarized light. Below in the article, an upper index ⊥ will be used for designation of all values related to perpendicular polarization and the symbol “||” for parallel polarization. As is shown in Fig. 1, the coordinate system used in this work is chosen so that strength of electric field E⊥ corresponding to perpendicular polarization of prob ing radiation is directed along the y axis. With this, vector E|| corresponding to parallel polarization has projections on both the x axis and z axis as well. Thus, this paper considers the case in which a plane monochromatic, linearly polarized electromagnetic wave with intensity I i⊥, and length λ (in vacuum) falls onto the interface between the surrounding medium and the film at angle θ1. After reflecting (at the same angle) from the optical system film–subphase, the ray, which now has intensity I r⊥,, returns into the sur rounding medium, where it is registered by a detector. The image of the source and receiver (detector) of radiation are displayed on a block scheme of the real experiment shown below (Fig. 3). Figure 1 also shows the wave that passed into the third medium, but we will discuss the mathematical expression that connects its intensity I t⊥, with the value I i⊥,,in the next section of the article. Using Fig. 1, one can easily see that, at θ1 = 0, the zcomponent of E|| equals zero. Thus, experiments with zero θ1 have peculiarities. In fact, using perpen dicularly polarized light, one can measure its absor bance along the y axis and find values of k2Y. By carry ing out experiments with parallel polarized radiation, one can determine the values of k2X. It is clear, how ever, that in these experimental conditions one cannot obtain any information about the values of k2Z. The expressions for the reflectance shown below, R

⊥, ||

⊥, ||

Ir = , ⊥, || Ii

(1)

are obtained in the following way. In their initial form, they were found in [20], in consideration of thin (d λ), totally isotropic films, i.e., complying with the conditions n2X = n2Y = n2Z и k2X = k2Y = k2Z; initially [23, 24], these correlations were applied only in this form. Then, in [21], these expressions were rewritten in such a way that extinction coefficients k2X, k2Y, and k2Z included in them could differ from each other. Furthermore, these modified correlations were used in works [15, 16]. In addition, no justification of such generalization of initial formulas was provided in [21] or [15, 16]. Nevertheless, we assume that, with com pliance to the conditions considering the properties of the film mentioned at the beginning of this section, the correlations presented in [21] are, in fact, true. It should be noted that Hansen provides only the final results of his analytical calculations in [20], but does

191

not explain the details of the mathematical methods that he used in their derivation; thus, in the new con ditions, it is quite difficult to repeat his derivation exactly. However, the relations presented in [21] can be obtained using a scheme of analytical calculations analogous to that which is briefly described in the next section upon derivation of the formulas of transmis sion coefficients. In this paper, the authors limited themselves to a discussion of only normal (θ1 = 0) incidence of prob ing radiation. Thus, the expressions for R presented in [21] will (at θ1 = 0) look like

⊥

and R

||

16πn 1 n 2 k 2Y d ⊥ , R = R 0 1 + 2 2 ( n 3 – n 1 )λ

(2)

16πn 1 n 2 k 2X d || . R = R 0 1 + 2 2 ( n 3 – n 1 )λ

(3)

R0, from formulas (2) and (3), is a coefficient of reflection of radiation from the surrounding medium– subphase interface. At normal incidence, this value is the same for both polarizations and has the following form: 2

( n1 – n3 ) R 0 = 2 . ( n1 + n3 )

(4)

The most convenient method [20] for determina tion of the numerical values of optical constants of the studied thin film implies the use of mathematical expressions of the following form: ⊥, ||

⊥, || R ΔA 1r = – log 10 . R0

(5)

Formulas (5) describe changes in absorbance values of reflection spectra that arise upon the appear ance of the studied film on the substrate surface. As can be seen from (5), coefficient R0 serves here as a baseline. Thus, the spectra measurement procedure consists of preliminary recording of optical densities of reflectance spectra of the surrounding medium–sub phase interface and subsequent registration of the same value in the threelayer system: surrounding medium–film–subphase. The values Δ A1r⊥ and Δ A1r obtained in such a way should be connected to the optical constants of the studied thin film. This can be done by substituting expressions for R⊥ and R|| from (2) and (3) and R0 from (4) into (5). Then, assuming that the second terms (in square brackets) in (2) and (3) are much less than 1, one can easily obtain formulas for Δ A1r⊥ and Δ A1r : ⊥, A1r

16πn 1 n 2 k 2Y d ⊥ , ΔA 1r = – 2 2 ln 10 ( n 3 – n 1 ) λ


(6)

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STUCHEBRYUKOV et al. Surrounding medium

z y

n1

n1

It⊥,||

E ||

Ii⊥,|| E ||,⊥ θ1

n3

x

Transparent plate

Polarizer Film Fig. 2. Optical scheme of transmission spectra measure ment.

16πn 1 n 2 k 2X d || . ΔA 1r = – 2 2 ln 10 ( n 3 – n 1 ) λ

(7)

In analysis of relations (6) and (7), one should note three important facts. First, Δ A1⊥r , and Δ A1r are linear by d/λ. Second, at θ1 = 0 and n1 < n3, both Δ A1⊥r , and Δ A1r are less than zero. This means that the appear ance of a thin film on the surface of the subphase leads to an increase (in comparison to R0) of the numerical values of R⊥ and R||. The situation in which n1 < n3 is most typical, because, as a rule, the surrounding media in which the source and detector of probing radiation are located is air (i.e., n1 = 1) and the sub phase is some kind of diluted aqueous solution (n3 ≈ 1.33) or deionized water (n3 = 1.33). Third, the optical constants that characterize the studied film are included in formulas (6) and (7) in the form of prod ucts of n2 by k2Y and n2 by k2X. Then, taking into consideration the known rela tions [25], ε2Y = 2n2k2Y, (8) ε2X = 2n2k2X,

(9)

these expressions can be rewritten as

8 π n 1 ε 2Y d ⊥ Δ A 1r = – , 2 2 ln 10 ( n 3 – n 1 ) λ

(10)

8 π n 1 ε 2X d || Δ A 1r = – . 2 2 ln 10 ( n 3 – n 1 ) λ

(11)

Functions ε2Y and ε2X in formulas (10) and (11) are imaginary parts of the optical dielectric constants of the studied film along the y and x axes, respectively. Thus, in this case, the dependences of ε2Y and ε2X on λ are actually measured. One should take into con sideration the fact that, with k2Y and k2X not exceeding 0.1, the qualitative differences between transmission (discussed below) and reflection spectra will be small, but an increase of the k2X and k2Y values can (due to the presence of anomalous dispersion) lead to differences

in the behavior of the functions ε2Y(λ) and ε2X(λ) in comparison to k2Y(λ) and k2X(λ) becoming significant. We should also emphasize that the abovenoted impossibility of determining the k2Z values from mea surement of spectra at θ1 = 0 is not in any way a nega tive feature. The case is that using incident angle θ1 = 0 we can (using two orthogonal polarizations) find numerical values of k2Y(λ) and k2X(λ) via expressions (6) and (7). Then, carrying out measurements in par allel polarized light, with various values of θ1 (using expressions for Δ A1r (θ1)) we can determine the value k2Z. Such stepbystep registration of spectra can be especially advantageous in studying thin films with one and twoaxis optical anisotropy. We should note that all that was said in the last paragraph on the topic of reflection spectra measurement is equally right for the transmission spectra. TRANSMISSION SPECTRA The optical schemes that we discuss are shown in Figs. 1 and 2. Figure 1 corresponds to the case of trans mission spectra measurement when probing radiation passes through the studied film and aqueous subphase and exits the Langmuir trough through a transparent window. The optical scheme of measurement of the transmission spectra of thin films on substrates is shown in Fig. 2. Here, we are basically talking about the cuvette compartment of a standard spectropho tometer in which a transparent parallelsided plate with a thin film of its surface is located. As in the pre vious section, we assume that the surrounding medium (1) and substrate (medium 3) are isotropic and transparent. Their real refractive indices are n1 and n3. At the same time, medium 1 is semiinfinite from the left and the right side and the studied film (medium 2) and the plate have finite thicknesses d and h, respectively. Regarding the properties of medium 2, we still assume the three previously formulated condi tions, and its complex refractive indices are written down as n2X = n2 + ik2X, n2Y = n2 + ik2Y, n2Z = n2 + ik2Z. Now, we consider the case in which a beam of light with intensity I i⊥, leaves the spectrophotometer radia tion source and falls onto the surrounding medium– thin film interface at angle θ1. Travelling through the film and the substrate and somewhat deflecting (if θ1 ≠ 0) the beam parallel to it, now having intensity I t⊥,, it arrives at the spectrophotometer detector. As previously, it is assumed that this light beam is a plane, monochromatic, and linearly polarized elec tromagnetic wave with length λ (in vacuum). It should be noted that the reflected wave is not shown on this figure, as its intensity is not measured. From comparison of Figs. 2 and 1, we can see that the coordinate system used in consideration of trans mission spectra is analogous to the one that was used previously for reflection spectra. Thus, everything said


PECULIARITIES OF THE REFLECTION–ABSORPTION AND TRANSMISSION SPECTRA (а) Optic fiber of light source

(b) Optic fiber of detector

Moving barriers

193

Ir⊥,||

Optic fiber of light source

Moving barriers

Ii⊥,||

Ii⊥,|| Monolayer

Subphase

Monolayer Subphase It⊥,|| Langmuir trough

Langmuir trough Transparent window

Optic fiber of detector Fig. 3. Experimental scheme of measurement of (a) reflection spectra and (b) transmission spectra.

above in regard to projections of electric fields strengths E⊥ and E || onto the x, y, and z axes remains true here. Some differences in the reflection and transmission spectra are in the fact that, due to the presence of a substrate, thickness of which is many times higher than wavelength of probing radiation, in the second case, it is not enough to limit oneself to considering only coherent light beams. The fact is, the problem of determination of transmission coefficients in practi cally fourlayer (from the optical point of view) sys tems discussed here, is split up into two parts. First, one should consider transmission of radiation through the system surrounding medium–film–substrate and then, considering multiple reflections of light beam inside a transparent parallelside plate, find the total transmission coefficients of this system. In order to solve the first of the stated problems, we can use expressions for transmission coefficients of the threelayer optical system obtained in [26] (in com plex form). In derivation of the formulas for transmis sion coefficients, we generally followed a scheme anal ogous to one proposed in [27]. The required relations are obtained in the following way. In the first step, expressions presented in [26] are expanded into binominal series. Under the condition that d λ, these series is quite simple. Then, using multiplication of expressions obtained in such a way by their complex conjugated values, one can obtain the transmittance. Finally, at the final step of this derivation, it is required to find the imaginary part of the function

ξ2 =

2

2

2

( n 2 + ik 2 ) – n 1 sin θ1 .

(12)

With arbitrary θ1, the expression for Imξ2 has a quite cumbersome form and is not linear by k2. At the same time, it is seen from formula (12) that, if θ1 = 0, then Imξ2 = k2. Thus, the transmittance T

⊥, ||

⊥, ||

It = , ⊥, || Ii

(13)

for θ1 = 0 are written as 4πk 2Y d ⊥ T = T 0 1 – , λ

(14)

4πk 2X d || T = T 0 1 – . λ

(15)

T0 from formulas (14) and (15) is the transmittance of probing radiation through the surrounding media/substrate interface. With normal incidence, this value is the same for both polarizations and is expressed as 4n 1 n 3 T 0 = 2 . (16) ( n1 + n3 ) Considering the transmission of a light beam through a transparent plate with thickness h λ, we should say that expressions for the transmittance of such a plate that consider multiple reflections inside it


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are obtained in [28], but it is pointless to provide them here. The parallelside quartz plate used in our exper iments has refractive index in visible range, which changes from n3 = 1.48 (at λ = 300 nm) to 1.45 (at λ = 700 nm), and assumed n2 value is n2 = 1.5. Upon sub stitution of these values of n2 and n3, and n1 = 1 as well, into formulas from [28], one can see that the obtained total transmission coefficients of the discussed optical system will differ from the values T⊥ and T || from (14) and (15) by less 1%. We should note that the physical meaning of for mulas (14) and (15) becomes absolutely apparent after expressing them in a more usual form. As these formu las were obtained under the condition that 4πk 2Y d 1, λ

(17)

4πk 2X d 1, λ

(18)

they can be written in an equivalent form, 4πk 2Y d ⊥ T = T 0 exp – , λ

(19)

4πk 2X d || T = T 0 exp – . λ

(20)

Moreover, taking into account that αY and αX, i.e., the Lambertian coefficients are 4πk 2Y d α Y = , λ 4πk 2X d α X = , λ

(21)

⊥

||

T = T 0 exp [ – α X d ] .

⊥, ||

⊥, || T ΔA 1r = – log 10 . T0

(25)

Substituting expressions for T⊥ and T|| from (14) and (15) and T0 from (16) into (25), and also consid ering equations (17) and (18), it is easy to obtain rela tions for Δ A1t⊥ and Δ A1t which are linear by d/λ 4π k 2Y d ⊥ Δ A 1t = , ln 10λ

(26)

4π k 2X d || Δ A 1t = . ln 10λ

(27)

As can be seen from expressions (26) and (27), the values Δ A1t⊥ and Δ A1t are directly proportional to the values k2Y and k2X; i.e., with θ1 = 0, in fact, the depen dences of extinction coefficients k2Y and k2X on λ are registered in transmission spectra of thin films, (in contrast to reflection spectra). COMPARISON OF PEAK INTENSITIES OF REFLECTION AND TRANSMISSION SPECTRA Now, in order to compare the intensities of reflec tion and transmission spectra, one should divide expression (10) by formula (26) and expression (11) by formula (27). One, naturally, should also take absolute values for the right parts of expressions (10) and (11). Then, for θ1 = 0, “enhancement coefficients” K⊥ and K || will have the following form: ⊥

(22)

then, formulas (19) and (20) can be written as T = T 0 exp [ – α Y d ] ,

film is to use of algebraic expressions with the follow ing form:

Δ A 1r 4 n1 n2 ⊥ , K = = ⊥ 2 2 Δ A 1t ( n3 – n1 )

(28)

||

Δ A 1r 4 n1 n2 . K = = || 2 2 Δ A 1t ( n3 – n1 ) ||

(23) (24)

It is evident that expressions (23) and (24) are nothing more than the mathematical form of the known Buger–Lambert–Beer law, with T0 here acting as an initial intensity of light beam, and the results of absorbance of this beam by a film with thickness d are described by the second (exponential) coefficients of these expressions. Returning to the calculation of optical constants, we should note that the procedure of measurement of transmission spectra is absolutely analogous to that described above for the reflection spectra. Thus, the most efficient way to determine the numerical values of optical constants of the studied

(29)

Substituting numerical values n1 = 1, n3 = 1.33, and n2Y = n2X = 1.5 into formulas (28) and (29), we obtain K⊥ = K || = 7.8. It is seen that, with all other conditions being equal, the peak intensity of reflection spectra is almost an order of magnitude higher than that of transmission spectra. One can assume (with a high probability) that numerical values K⊥ and K || will remain about the same even upon complication of the model that describes the properties of the studied thin film, for example, in cases when medium 2 will be uniaxial or even biaxial. Such a significant increase of peak intensities is especially important in the study of very thin films— in particular, Langmuir monolayers—due to the fact that, generally, the signal/noise ratio for transmission spectra of such systems is quite low (for example,


PECULIARITIES OF THE REFLECTION–ABSORPTION AND TRANSMISSION SPECTRA

EXPERIMENTAL Monolayer spectra in wavelength range from 240 to 750 nm were recorded using an AvaSpec2048 FTSPU fiber optic spectrophotometer. A UV/VIS reflectomet ric probe with fiber diameter of 400 μm joined with a sixfiber radiation cable was placed perpendicularly to the studied surface at 2–3 mm from the monolayer for measurements by the scheme described above. The sig nal reflected from the subphase surface right before the monolayer deposition was used as a baseline. The electronic absorption spectra of the LBF in the wavelength range from 200 to 900 nm were recorded using a 2450 PC Shimadzu spectrophotometer (Japan). The measurement error of λ was ±0.3 nm. In order to obtain film spectra, monolayers of studied compounds compressed to a surface pressure of 20 mN/m were transferred onto transparent plates with a thickness of 2 mm made of KU grade optical quartz. Chemical schemes of compounds from which monolayers and films were formed are shown as inserts in the figures. RESULTS AND DISCUSSION Proceeding to a comparison of theoretical and experimental results, we should state that the scheme of reflection spectra detection shown in Fig. 3a does not allow, in our view, efficient use of polarizers. Nev ertheless, it is evident from the form of expressions (28) and (29) that they can be used for comparison of peak intensities of reflection and transmission spectra measured without polarizers. This somewhat paradox ical situation can be explained by the fact that the val ues of “enhancement coefficients” are the same for both polarizations in the framework of the assumed model of optical properties of the studied ultrathin film. Thus, the fact that the probing radiation used in the experiments is partially polarized does not affect the numerical values of K⊥ and K ||. The patterns of relationships found in this work are proved by experimental measurements carried out using the example of monolayers of certain conjugated organic compounds of various structures: amphiphilic chromoionophores, photosensitive oligomeric orga nic conductors, and electrochemically multistable rigid macrocyclic compounds (Figs. 2–6). Curve 2 in Fig. 4 is a transmission spectrum of a monolayer of amphiphilic hemycyanine dye on the deionized water recorded on a trough with a sapphire window according to the scheme shown in Fig. 3b at the monolayer pressure of 12 mN/m (before aggrega

C22H45 N+ Br–

0.04 0.03 Absorbance

Fig. 4, curve 2). The later fact significantly compli cates detection of processes that are accompanied by relatively small spectral changes, for example, struc tural transformations in the monolayer upon its com pression.

195

N

1

0.02 0.01

2

0 300

400 500 600 Wavelength, nm

700

Fig. 4. (1) reflection spectrum and (2) transmission spec trum of a monolayer of amphiphilic hemycyanine dye at surface pressure of 12 mN/m.

tion of the dye molecules [6]). Curve 1 is a reflection spectrum (in units of absorbance) for the same mono layer in the same conditions. Not only a higher inten sity of the main band peak of the second spectrum can be see from the figure, but also a very high level of noise in the transmission mode. The abovementioned advantages of reflection spectra allow one to in situ analyze in great detail the processes that lead to changes in the form of studied absorbance bands upon formation of a monolayer. The average value of the “enhancement coefficient” for this system calculated by three experiments was 7.3. Figures 5 and 6 show a comparison of the reflection spectra of monolayers on the surface of the aqueous subphase (curve 1) for the surface pressure at which monolayers were transferred onto quartz plates and the transmission spectra of the films obtained in such a way (curve 2). The transmission spectrum of a two layer LBF is shown for doubledecker cerium crown phthalocyaninate (Fig. 5), and spectra of a sixlayer LBF for crownsubstituted oligothiophene (Fig. 6). As can be seen from the data shown in the figures, the form of monolayer spectra recorded according to the reflection scheme practically coincides with the form of transmission spectra of films in all the considered cases, but the peak intensities of the later in recalcula tion for one monolayer are several times higher. The preservation of the form of the spectra allows us to experimentally prove that there is preservation of the structure and properties of a monolayer upon its trans fer onto solid substrate. Small hypsochromic shifts of all absorbance bands of LBF spectra in comparison to monolayer spectra can be explained by the effect of the polar substrate. The enhancement coefficients calculated by data shown on the figures amounted to 7.9 and 8.2 for monolayers of cerium crownphthalocyaninate and crownsubstituted oligothiophene, respectively.


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STUCHEBRYUKOV et al. O

O O

O

O

O

O

O

O

N

O

N

N N

0.04

N N

N

O

N

O

Absorbance

O

Ce

O

O

O

O

O

O

O

O N

N N N

N N

O

O

O

O

O

O

O

O

N N

0.02

O

O

O

O

0.03

O

O

O

O

O

O

1 0.01 2 0 300


700

Fig. 5. Reflection spectrum of a monolayer of Ce(R4Pc)2 (Pc = phthalocyaninateion, R = 15crown5) at (1) the air–water interface and (2) transmission spectrum of a twolayer LBF of this compound.

O O O

O

S

O

O S

O

O

1

0.06 Absorbance

O S

O

0.04 2 0.02

0 300


reflection spectra in studies of ultrathin films of vari ous natures. Nevertheless, one should not totally disregard transmission spectra measurement. In our view, a comparison of the peak intensities of reflection and transmission spectra can provide additional informa tion on the optical properties of thin films. As was pre viously noted, discussed spectra differ in physical nature at values of θ1 near zero. Thus, calculation of “enhancement coefficients” analogous to K⊥ and K || can be beneficial. This, in the first place, is important for recording spectra of uniaxial, and even biaxial, films. It is quite possible that, in this case, not only n2, but n2Y, will arise in formula (28), and n2X in expression (29). It is possible that one could determine the numerical values of these parameters in an experiment with collimated beams that have a high degree of polarization. Differences between values of n2Y and n2X can directly indicate that the studied system possesses much more complicated anisotropic properties than was previously assumed in the model that we used. However, all these studies that appear possible now should have a solid basis—correct use of polarized probing radiation. We should note that the most simple and effective method for overcoming the difficulties associated with the use of polarizers is the employment of a measure ment scheme in which the beam incident on the sam ple and the beam reflected from it are spatially sepa rated. In this scheme, it is desirable to separate these beams in such a way that the incidence angle will remain as close as possible to zero. In this case, to determine optical constants, one can, as before, use formulas (6) and (7), naturally (using relations from [21]) determining the systematic errors that arise upon use of these expressions for nonzero θ1. This approach can help preserve the advantages of measurement of spectra at θ1 = 0 and, at the same time, successfully apply any types of polarizers.

700

Fig. 6. Reflection spectrum of a monolayer of crownsub stituted oligothiophene on (1) the air–water interface and (2) transmission spectrum of a sixlayer LBF of this com pound.

The presented results show a good qualitative agree ment of theory and experiment for the transmission spectra of the monolayer on the water surface (Fig. 4) and for films transferred onto quartz plates (Figs. 5, 6). CONCLUSIONS Thus, the theoretically obtained expressions for correlation of peak intensities of absorbance bands of reflection and transmission spectra are experimentally proved and demonstrate a significant advantage of

ACKNOWLEDGMENTS We are grateful to our colleagues from the University of Bordeaux 1 Dr. G. Jonusauskas and Dr. E. Abraham for help in carrying out spectral measurements and to V.M. Sukhov for help in the work on the paper. This work was financially supported by the Russian Foundation for Basic Research (project 110312124 ofim2011) and Program of the Presidium of the Russian Academy of Sciences P8(5). REFERENCES 1. Mendelsohn, R., Brauner, J.W., and Gericke, A., Ann. Rev. Phys. Chem., 1995, vol. 46, p. 305. 2. Mendelsohn, R. and Flach, C.R., Handbook of Vibra tional Spectroscopy, vol. 2., New York: John Wiley & Sons Ltd., 2002.


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