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SERS on photographic silver halide emulsions
Volume 163, number I
CHEMICAL PHYSICS LETTERS
3 November 1989
SERS ON PHOTOGRAPHIC SILVER HALIDE EMULSIONS K. KNEIPP a) W. JAHR b and G. ROEWER ’ a PhysicsDepartment,Humboldt University,DDR-JO40Berlin, GermanDemocraticRepublic b VEB FiImfabrik Wolfen,DDR-4440 Wolfen,GermanDemocraticRepublic c TechnischeHochschuie“CarlSchorlemmer”,DDR-4200Merseburg,GermanDemocraticRepublic Received 14 March 1989; in final form 12 July 1989
Surface-enhanced Raman Scattering (SERS) investigations on a cyanine dye in photographic silver halide emulsions with various photographically relevant additives are reported. In addition to an electromagnetic enhancement due to Giver particles of suitable size and shape a chemical enhancement which is directly connected with the thiosulfate digestion of the emulsion, i.e. the formation of Ag,S, plays an important role. In photographic emulsions sensitivity centres and SERS active sites arc identical. A strong influence of the additives (stabilizers and antifogging agents) on the chemical SERS enhancement was found.
1. Introduction A strong similarity between photographic silver halide materials and effective SERS-active substrates, as well as the strong SERS spectra of sensitizing dyes on silver sols and irradiated silver halide sols, suggest the use of the SERS effect for the elucidation of the photographic process and spectral sensitization [ 1-7 1. Identical SERS spectra of pyridine and several dye molecules on silver sols and silver halide sols, as well as very similar absorption spectra of SERS-active halide sols and typical silver sols used for SERS, provide a strong hint that the origin of the Raman enhancement is similar for both substrates and that a significant contribution to it is electromagnetic in nature, i.e. due to the local field enhancement associated with resonant excitation of surface plasmons [ 4-71. However, there are also experimental results which suggest so-called chemical contributions to the SERS enhancement of silver halides due to a specific interaction between molecule and substrate. Strong support for a chemical contribution comes from the observation of different enhancement factors on electromagnetically identical substrates. The chemical enhancement is connected with the existence of active sites on the SERS surface where the adsorbed molecule undergoes a specific in-
teraction with the adsorbate which results in an enhanced Raman cross section. SERS experiments can be of interest to the silver halide photographic process for the following reasons 171: (i) SERS spectra of sensitizing dyes can provide information about the chemical structure of these molecules, about their adsorption behaviour and their interaction with the substrate, about their aggregation and about the formation of dye radicals. (ii) According to the strong dependence of the electromagnetic SERS enhancement on the size and shape of the silver particles [ 8,9] the SERS intensity can be a probe for monitoring the growth of silver particles in a photographic emulsion [ 71. (iii) Furthermore, relative SERS intensities of the various Raman bands of a molecule can depend on the surface potential of the SERS substrate [lo]. Hence, the surface potential of the particles in a silver halide emulsion - in particular changes of this potential during their growth can be studied by monitoring changes within the relative SERS intensities of calibrated test molecules [ 7 1. (iv) The most interesting aspect of the application of SERS in photographic science seems to lie in the obvious similarities between the mechanisms acting in chemical SERS enhancement and in photographic processes due to a possible charge-transfer pathway
0 009-2614/89/$ 03.50 0 Elsevier Science Publishers B.V. ( North-Holland Physics Publishing Division )
105
Volume 163, number 1
CHEMICAL PHYSICS LETTERS
[ 111. In this way, SERS of sensitizing dyes in silver halide emulsions could become a model process for studying photographic emulsions and additives. In the following paper SERS experiments with a typical sensitizing molecule (pseudoisocyanine) adsorbed on photographic silver halide emulsions with various additives will be reported. In particular, the role of active sites will be studied. The dependence of the SERS spectra on time and the influence of various additives on the SERS enhancement will be investigated.
3 November 1989 OH
“TAi”, 4-hydroxy-6-methyl-l,3,3a,7_tetraazaindene
2. Experimental Spectra were measured in a 90”~scattering geometry using 5 14 nm Ar+-laser excitation. Laser power was about 200 mW. The SERS samples were mixtures of 1 ml silver halide emulsions and 10 ~1 of a 1O-s mol/Q methanolic solution of I ,l’-diethyl-2,2’-cyanine dye ( pseudoisocyanine ) .
w
“2.mercapto-TAi”, tetraazaindene
We report here measurements on three sample series (I, II, III). The samples of series I did not have any prior chemical sensitization (primitive emulsion). The additives are summarized in table 1. For series II a thiosulfate digestion ( 1.4 X 1Om4 mol Na2S203 per mol AgRr) was performed ( 130 min at 60°C pH~6.5, pAg~8.3 at 40°C). The samples of series I and II are identical as regards their additives and are only distinguished by the thiosulfate digestion of series II. The additives TAi and 2-mercaptoTAi, respectively, were been added after completion of the thiosulfate ripening. In series III for some cases (see table 2) the emulsions were digested with thiosulfate for 60 min at 55°C. The additives TAi, mercapto-TAi or thioether, respectively, were added to the emulsions 15 min before beginning the thiosulfate ripening.
An‘2”5
C2H5
2-mercapto-4-hydroxy-6-methyl-l,3,3a,7-
“PIG”, 1,l ‘-diethyl-2,2’-cyanine
Measurements were performed at different times after the beginning of laser irradiation of the sample. The silver halide emulsions contained cubic AgBr grains (d&s 1.1 pm) precipitated by the double jet method. The following compounds were used as additives: HO-CHZ-CH2-S-CH2-CHrS_CH2-CH2_OH “thioether”, l,&dihydroxy-3,6-dithiaoctan
Table i Sample conditions for series I and II Additives (mol/mol AgBr)
TAix lo-’ 2-mercaptoTAix 1O-3
106
Sample No. 1
2
3
4
5
6
7
8
0
2.2
4.4
8.8
17.2
34.5
-
-
1.8
3.65
9
10
7.3
14.6
Volume 163, number 1
CHEMICAL PHYSICS LETTERS
3 November 1989
Table 2 Sample conditions for series III Additives (moljmol AgBr)
Sample No. 1
TAix lo-’ LmercaptoTAix lo-) thioetherx lo-’ Na,S,O,x 1O-4
-
2
1.04
3
4
5
6
2.0
2.0
2.0
-
-
1.5
3. Results and discussion Fig. 1 shows absorption spectra of samples from the different emulsion types I-III and, for comparision, the absorption spectra of two typical SERS-active silver ~01s.All photographic emulsions show absorption spectra similar to those of the silver sols, i.e. they contain silver particles which can give rise to an electromagnetic SERS enhancement due to their size
3.0
7
8
9
10 2.0
1.65
1.65
1.65
-
1.5
3.0
2.5 -
2.5 -
and shape. (Up to now, we have been unable, to find a SERS signal from silver halides without the formation of silver particles, i.e. without an electromagnetic enhancement. ) The additives influence the formation of the silver particles, in particular, their shape which is reflected by small shifts of the absorption maxima [ 81 of series Ill relative to series I and II, respectively. Samples of series I and II show identical absorption spectra, i.e. a similar electromagnetic SERS enhancement should be expected in both series. However, no SERS signal from series I could be found whereas samples of series II gave rise to strong SERS spectra. SERS spectra of pseudoisocyanine (PIC) in photographic emulsions were similar to those measured in aqueous silver sols (ref. [ 31 and references therein) and on silver electrodes [ 101. Figs. 2b-2e show SERS spectra of PIG on some samples of series II in a characteristic wave number region as a function of time. Fig. 2a shows PIG SERS on a silver electrode with different well-defined surface potentials [ lo] in the same spectral region. The spectra illustrate the dependence of the relative intensities on the surface potential within the line triplet (1390, 1370, 1356 cm-‘). The strong differences in the SERS behaviour of series I and II despite identical electromagnetic enhancement conditions for both series give an unambiguous hint to the important role of chemical enhancement.
Fig. 1. Absorption spectra of photographic silver halide emulsions and silver sols (optical path for all samples 0.2 cm).
Obviously, this chemical enhancement is directly connected with the sulfur digestion of series II leading to the formation of Ag,S-clusters. For series I where this process did not occur no SERS spectra 107
Volume 163, number 1
CHEMICAL PHYSICS LETTERS
ru
3 November 1989
e
Fig. 2. A characteristic line triplet at about 1370 cm-’ in SERS spectra of PIC on emulsions of series II at various times after the start of laser irradiation (b-e) and on a silver electrode with different potentials versus a saturated calomelelectrode(SCE) (a).
could be found. However, “normal” resonance Raman scattering has been found in samples of series I by increasing the dye concentration. RRS intensities in series I on the level of SERS intensities of series II have been obtained for 10-3-10-4 mol/Q PIC concentration compared to the lo-’ mol/R dye concentration in SERS. This confirms the action of SERS in series II with an enhancement factor of 103-104. In the following we shall discuss the spectra of series II in more detail. Figs. 2b-2e appear in the first minute after laser illumination, go through a maximum in intensity and vanish after about 10 to 15 min. In our experiments 108
we cannot distinguish whether this behaviour is due to the formation and destruction of silver particles of optimum properties for an electromagnetic enhancement or due to the formation and disappearance of special SERS-active sites, i.e. changes in the chemical enhancement during laser illumination. Furthermore, it should be noted that the line triplet in the SERS spectra on photographic emulsions shows characteristic changes in its relative intensities. Starting from an intensity hierarchy I,,,, iZ,370
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CHEMICAL PHYSICS LETTERS
ver electrodes (fig. 2a) this behaviour reflects a change of the surface potential of the SERS substrate with time to more positive values for the samples containing TAi (samples 11/2-11/6). In ref. [ 121 the changes in the surface potential due to TAi were measured by Kelvin’s probe experiment and an analogous influence on the surface potential of AgBr crystals was found. For samples containing mercapto-TAi (11/7-II/ 10) an oscillating behaviour of the relative intensities can be observed, i.e. after some minutes the system recovers and the 1370 cm-’ line and the 1356 cm-’ line show the same intensity again. In fig. 3a and 3b the SERS intensity of the different samples of series II are compared at a constant time after laser illumination to demonstrate the influence of TAi and mercapto-TAi, respectively, on SERS in thiosulfate-ripened emulsions. The concentration dependence of the additives on the SERS intensity (see for comparison table I ) can be explained by competition between their influence on the enhancement factor and their occupation of adsorption sites on PIC. For TAi the SERS intensity goes through a minimum due to competing coadsorption of TAi and pseudoisocyanine on the SERSactive centres. The increase in the SERS intensity at higher TAi concentration can be explained by the different adsorptions of TAi as a function of concentration (flat on or edge-on [ 131). In contrast mercapto-TAi primarily enhances the scattering intensity. At higher concentrations a decrease of the SERS intensity due to competing adsorption with respect to PIC was found. (MercaptoTAi is not able to adsorb edge-on_) Samples of series III contain different sulfur compounds as additives ( see table 2 ) . SERS experiments
Fig. 3. Relative SERS intensities of the samples of series II 3 min after the start of laser irradiation.
3 November 1989
Fig. 4. Relative SERS intensities of PIC on samples of series III for various additives (see table 2).
on these samples have been performed to confirm the importance of the formation of Ag,S for SERS. The samples showed very different SERS behaviour; In some cases no SERS signal was found whereas other samples exhibited enhancement factors of about 104. Fig. 4 gives a comparison of the SERS intensities of the samples of series III after 3 min laser illumination. We could not detect SERS signals from samples III/l, 111/3, 111/4, 111/9, and III/IO. Samples III/l and III/3 do not contain sulfur compounds. Sample III/2 thiosulfate gives rise to a SERS signal. Despite the presence of thiosulfate in sample 111/4, a SERS signal is prevented by TAi. TAi blocks the formation of SERS-active centres, but this can be reversed by a sufficiently high concentration of thiosulfate - increasing the amount of thiosulfate results in a recovery of the SERS spectrum in sample III/ 5. A weak SERS signal could be registered after the addition of mercapto-TAi to the primitive emulsion (sample 111/6). The intensity of the SERS signal is markedly increased in samples containing thiosulfate and mercapto-TAi (III/7 and III/S). The addition of thioether to the emulsions did not result in the appearance of a SERS spectrum for PIC (sample III/9 and III/lo). In contrast to thiosulfate, sulfur atoms in thioether and mercapto-TAi are not exchangeable at temperatures up to 4O”C, i.e. the formation of silver sulfide ( Ag,S) cannot take place. In this way it could be shown that Ag,S and not only elemental sulfur is responsible for the appearance of SERS in silver halide emulsions. (It is possible that mercapto-TAi used in these investigations contained traces of some other sulfur compounds resulting from its preparation. This could explain the weak SERS signal of sample III/&) 109
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CHEMICAL PHYSICS LETTERS
In summary our experiments demonstrate that in addition to an electromagnetic enhancement, a chemical enhancement at SERS-active sites is a prerequisite to achieve sufficiently high SERS enhancement factors for sensitizing dyes in photographic emulsions. Obviously, compounds containing exchangeable sulfur are able to create these SERS-active sites due to the formation of silver sulfide. This suggests that in photographic emulsion SERS-active sites and photographically sensitive centres prove to be identical. In this way, SERS can become a tool to investigate steps in the photographic process, for instance sulfur digestion and antifogging. Our experiments have demonstrated the strong influence of photographically relevant stabilizers and antifogging agents on the SERS enhancement factors of photographic emulsions.
110
3 November 1989
References [ I] IL Kneipp, G. Hinzmann and D. Fassler, Chem. Phys. Letters 99 ( 1983) 503. [2] X. Gao, Ch. Wan, T. He, J. Li, H. Xin and F. Liu, Chem. Phys. Letters 112 (1984) 465. [ 31 K. Kneipp, H. Kneipp and M. Rentsch, J. Mol. Struct. [4] K. Kneipp and J. Siegel, in preparation. [ 51W. Jian, L. Dawei, X. Houwen, S. Xu, L. Fan-Chen, Spectrochim. Acta 43A (1987) 375. [6] J.Wang,J.Phys.Chem.92 (1988) 1942. [ 71 K. Kneipp, J. Mol. Struct., to be published. [S] M. Kerker, Appl. Opt. 19 (1960) 3373. [ 91 R. Koh, S. Hayashi and K. Yamamoto, Solid State Commun. 64 (1987) 375. [lo] X. Li, B. Gu and D.L. Akins, Chem. Phys. Letters 105 (1984) 263. [ 111B. Simic-Glawaski, J. Phys. Chem. 90 (1986) 3863. (121 W.K. Lam, L. Richter and Y.T. Tan, Phot. Sci. Eng. 28 (1984) 98. [ 131 D.D.F. Shiao, M.T. Nieh and A.H. Herz, J. Phot. Sci. 30 ( 1982) 208.
CHEMICAL PHYSICS LETTERS
3 November 1989
SERS ON PHOTOGRAPHIC SILVER HALIDE EMULSIONS K. KNEIPP a) W. JAHR b and G. ROEWER ’ a PhysicsDepartment,Humboldt University,DDR-JO40Berlin, GermanDemocraticRepublic b VEB FiImfabrik Wolfen,DDR-4440 Wolfen,GermanDemocraticRepublic c TechnischeHochschuie“CarlSchorlemmer”,DDR-4200Merseburg,GermanDemocraticRepublic Received 14 March 1989; in final form 12 July 1989
Surface-enhanced Raman Scattering (SERS) investigations on a cyanine dye in photographic silver halide emulsions with various photographically relevant additives are reported. In addition to an electromagnetic enhancement due to Giver particles of suitable size and shape a chemical enhancement which is directly connected with the thiosulfate digestion of the emulsion, i.e. the formation of Ag,S, plays an important role. In photographic emulsions sensitivity centres and SERS active sites arc identical. A strong influence of the additives (stabilizers and antifogging agents) on the chemical SERS enhancement was found.
1. Introduction A strong similarity between photographic silver halide materials and effective SERS-active substrates, as well as the strong SERS spectra of sensitizing dyes on silver sols and irradiated silver halide sols, suggest the use of the SERS effect for the elucidation of the photographic process and spectral sensitization [ 1-7 1. Identical SERS spectra of pyridine and several dye molecules on silver sols and silver halide sols, as well as very similar absorption spectra of SERS-active halide sols and typical silver sols used for SERS, provide a strong hint that the origin of the Raman enhancement is similar for both substrates and that a significant contribution to it is electromagnetic in nature, i.e. due to the local field enhancement associated with resonant excitation of surface plasmons [ 4-71. However, there are also experimental results which suggest so-called chemical contributions to the SERS enhancement of silver halides due to a specific interaction between molecule and substrate. Strong support for a chemical contribution comes from the observation of different enhancement factors on electromagnetically identical substrates. The chemical enhancement is connected with the existence of active sites on the SERS surface where the adsorbed molecule undergoes a specific in-
teraction with the adsorbate which results in an enhanced Raman cross section. SERS experiments can be of interest to the silver halide photographic process for the following reasons 171: (i) SERS spectra of sensitizing dyes can provide information about the chemical structure of these molecules, about their adsorption behaviour and their interaction with the substrate, about their aggregation and about the formation of dye radicals. (ii) According to the strong dependence of the electromagnetic SERS enhancement on the size and shape of the silver particles [ 8,9] the SERS intensity can be a probe for monitoring the growth of silver particles in a photographic emulsion [ 71. (iii) Furthermore, relative SERS intensities of the various Raman bands of a molecule can depend on the surface potential of the SERS substrate [lo]. Hence, the surface potential of the particles in a silver halide emulsion - in particular changes of this potential during their growth can be studied by monitoring changes within the relative SERS intensities of calibrated test molecules [ 7 1. (iv) The most interesting aspect of the application of SERS in photographic science seems to lie in the obvious similarities between the mechanisms acting in chemical SERS enhancement and in photographic processes due to a possible charge-transfer pathway
0 009-2614/89/$ 03.50 0 Elsevier Science Publishers B.V. ( North-Holland Physics Publishing Division )
105
Volume 163, number 1
CHEMICAL PHYSICS LETTERS
[ 111. In this way, SERS of sensitizing dyes in silver halide emulsions could become a model process for studying photographic emulsions and additives. In the following paper SERS experiments with a typical sensitizing molecule (pseudoisocyanine) adsorbed on photographic silver halide emulsions with various additives will be reported. In particular, the role of active sites will be studied. The dependence of the SERS spectra on time and the influence of various additives on the SERS enhancement will be investigated.
3 November 1989 OH
“TAi”, 4-hydroxy-6-methyl-l,3,3a,7_tetraazaindene
2. Experimental Spectra were measured in a 90”~scattering geometry using 5 14 nm Ar+-laser excitation. Laser power was about 200 mW. The SERS samples were mixtures of 1 ml silver halide emulsions and 10 ~1 of a 1O-s mol/Q methanolic solution of I ,l’-diethyl-2,2’-cyanine dye ( pseudoisocyanine ) .
w
“2.mercapto-TAi”, tetraazaindene
We report here measurements on three sample series (I, II, III). The samples of series I did not have any prior chemical sensitization (primitive emulsion). The additives are summarized in table 1. For series II a thiosulfate digestion ( 1.4 X 1Om4 mol Na2S203 per mol AgRr) was performed ( 130 min at 60°C pH~6.5, pAg~8.3 at 40°C). The samples of series I and II are identical as regards their additives and are only distinguished by the thiosulfate digestion of series II. The additives TAi and 2-mercaptoTAi, respectively, were been added after completion of the thiosulfate ripening. In series III for some cases (see table 2) the emulsions were digested with thiosulfate for 60 min at 55°C. The additives TAi, mercapto-TAi or thioether, respectively, were added to the emulsions 15 min before beginning the thiosulfate ripening.
An‘2”5
C2H5
2-mercapto-4-hydroxy-6-methyl-l,3,3a,7-
“PIG”, 1,l ‘-diethyl-2,2’-cyanine
Measurements were performed at different times after the beginning of laser irradiation of the sample. The silver halide emulsions contained cubic AgBr grains (d&s 1.1 pm) precipitated by the double jet method. The following compounds were used as additives: HO-CHZ-CH2-S-CH2-CHrS_CH2-CH2_OH “thioether”, l,&dihydroxy-3,6-dithiaoctan
Table i Sample conditions for series I and II Additives (mol/mol AgBr)
TAix lo-’ 2-mercaptoTAix 1O-3
106
Sample No. 1
2
3
4
5
6
7
8
0
2.2
4.4
8.8
17.2
34.5
-
-
1.8
3.65
9
10
7.3
14.6
Volume 163, number 1
CHEMICAL PHYSICS LETTERS
3 November 1989
Table 2 Sample conditions for series III Additives (moljmol AgBr)
Sample No. 1
TAix lo-’ LmercaptoTAix lo-) thioetherx lo-’ Na,S,O,x 1O-4
-
2
1.04
3
4
5
6
2.0
2.0
2.0
-
-
1.5
3. Results and discussion Fig. 1 shows absorption spectra of samples from the different emulsion types I-III and, for comparision, the absorption spectra of two typical SERS-active silver ~01s.All photographic emulsions show absorption spectra similar to those of the silver sols, i.e. they contain silver particles which can give rise to an electromagnetic SERS enhancement due to their size
3.0
7
8
9
10 2.0
1.65
1.65
1.65
-
1.5
3.0
2.5 -
2.5 -
and shape. (Up to now, we have been unable, to find a SERS signal from silver halides without the formation of silver particles, i.e. without an electromagnetic enhancement. ) The additives influence the formation of the silver particles, in particular, their shape which is reflected by small shifts of the absorption maxima [ 81 of series Ill relative to series I and II, respectively. Samples of series I and II show identical absorption spectra, i.e. a similar electromagnetic SERS enhancement should be expected in both series. However, no SERS signal from series I could be found whereas samples of series II gave rise to strong SERS spectra. SERS spectra of pseudoisocyanine (PIC) in photographic emulsions were similar to those measured in aqueous silver sols (ref. [ 31 and references therein) and on silver electrodes [ 101. Figs. 2b-2e show SERS spectra of PIG on some samples of series II in a characteristic wave number region as a function of time. Fig. 2a shows PIG SERS on a silver electrode with different well-defined surface potentials [ lo] in the same spectral region. The spectra illustrate the dependence of the relative intensities on the surface potential within the line triplet (1390, 1370, 1356 cm-‘). The strong differences in the SERS behaviour of series I and II despite identical electromagnetic enhancement conditions for both series give an unambiguous hint to the important role of chemical enhancement.
Fig. 1. Absorption spectra of photographic silver halide emulsions and silver sols (optical path for all samples 0.2 cm).
Obviously, this chemical enhancement is directly connected with the sulfur digestion of series II leading to the formation of Ag,S-clusters. For series I where this process did not occur no SERS spectra 107
Volume 163, number 1
CHEMICAL PHYSICS LETTERS
ru
3 November 1989
e
Fig. 2. A characteristic line triplet at about 1370 cm-’ in SERS spectra of PIC on emulsions of series II at various times after the start of laser irradiation (b-e) and on a silver electrode with different potentials versus a saturated calomelelectrode(SCE) (a).
could be found. However, “normal” resonance Raman scattering has been found in samples of series I by increasing the dye concentration. RRS intensities in series I on the level of SERS intensities of series II have been obtained for 10-3-10-4 mol/Q PIC concentration compared to the lo-’ mol/R dye concentration in SERS. This confirms the action of SERS in series II with an enhancement factor of 103-104. In the following we shall discuss the spectra of series II in more detail. Figs. 2b-2e appear in the first minute after laser illumination, go through a maximum in intensity and vanish after about 10 to 15 min. In our experiments 108
we cannot distinguish whether this behaviour is due to the formation and destruction of silver particles of optimum properties for an electromagnetic enhancement or due to the formation and disappearance of special SERS-active sites, i.e. changes in the chemical enhancement during laser illumination. Furthermore, it should be noted that the line triplet in the SERS spectra on photographic emulsions shows characteristic changes in its relative intensities. Starting from an intensity hierarchy I,,,, iZ,370
Volume 163, number I
CHEMICAL PHYSICS LETTERS
ver electrodes (fig. 2a) this behaviour reflects a change of the surface potential of the SERS substrate with time to more positive values for the samples containing TAi (samples 11/2-11/6). In ref. [ 121 the changes in the surface potential due to TAi were measured by Kelvin’s probe experiment and an analogous influence on the surface potential of AgBr crystals was found. For samples containing mercapto-TAi (11/7-II/ 10) an oscillating behaviour of the relative intensities can be observed, i.e. after some minutes the system recovers and the 1370 cm-’ line and the 1356 cm-’ line show the same intensity again. In fig. 3a and 3b the SERS intensity of the different samples of series II are compared at a constant time after laser illumination to demonstrate the influence of TAi and mercapto-TAi, respectively, on SERS in thiosulfate-ripened emulsions. The concentration dependence of the additives on the SERS intensity (see for comparison table I ) can be explained by competition between their influence on the enhancement factor and their occupation of adsorption sites on PIC. For TAi the SERS intensity goes through a minimum due to competing coadsorption of TAi and pseudoisocyanine on the SERSactive centres. The increase in the SERS intensity at higher TAi concentration can be explained by the different adsorptions of TAi as a function of concentration (flat on or edge-on [ 131). In contrast mercapto-TAi primarily enhances the scattering intensity. At higher concentrations a decrease of the SERS intensity due to competing adsorption with respect to PIC was found. (MercaptoTAi is not able to adsorb edge-on_) Samples of series III contain different sulfur compounds as additives ( see table 2 ) . SERS experiments
Fig. 3. Relative SERS intensities of the samples of series II 3 min after the start of laser irradiation.
3 November 1989
Fig. 4. Relative SERS intensities of PIC on samples of series III for various additives (see table 2).
on these samples have been performed to confirm the importance of the formation of Ag,S for SERS. The samples showed very different SERS behaviour; In some cases no SERS signal was found whereas other samples exhibited enhancement factors of about 104. Fig. 4 gives a comparison of the SERS intensities of the samples of series III after 3 min laser illumination. We could not detect SERS signals from samples III/l, 111/3, 111/4, 111/9, and III/IO. Samples III/l and III/3 do not contain sulfur compounds. Sample III/2 thiosulfate gives rise to a SERS signal. Despite the presence of thiosulfate in sample 111/4, a SERS signal is prevented by TAi. TAi blocks the formation of SERS-active centres, but this can be reversed by a sufficiently high concentration of thiosulfate - increasing the amount of thiosulfate results in a recovery of the SERS spectrum in sample III/ 5. A weak SERS signal could be registered after the addition of mercapto-TAi to the primitive emulsion (sample 111/6). The intensity of the SERS signal is markedly increased in samples containing thiosulfate and mercapto-TAi (III/7 and III/S). The addition of thioether to the emulsions did not result in the appearance of a SERS spectrum for PIC (sample III/9 and III/lo). In contrast to thiosulfate, sulfur atoms in thioether and mercapto-TAi are not exchangeable at temperatures up to 4O”C, i.e. the formation of silver sulfide ( Ag,S) cannot take place. In this way it could be shown that Ag,S and not only elemental sulfur is responsible for the appearance of SERS in silver halide emulsions. (It is possible that mercapto-TAi used in these investigations contained traces of some other sulfur compounds resulting from its preparation. This could explain the weak SERS signal of sample III/&) 109
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CHEMICAL PHYSICS LETTERS
In summary our experiments demonstrate that in addition to an electromagnetic enhancement, a chemical enhancement at SERS-active sites is a prerequisite to achieve sufficiently high SERS enhancement factors for sensitizing dyes in photographic emulsions. Obviously, compounds containing exchangeable sulfur are able to create these SERS-active sites due to the formation of silver sulfide. This suggests that in photographic emulsion SERS-active sites and photographically sensitive centres prove to be identical. In this way, SERS can become a tool to investigate steps in the photographic process, for instance sulfur digestion and antifogging. Our experiments have demonstrated the strong influence of photographically relevant stabilizers and antifogging agents on the SERS enhancement factors of photographic emulsions.
110
3 November 1989
References [ I] IL Kneipp, G. Hinzmann and D. Fassler, Chem. Phys. Letters 99 ( 1983) 503. [2] X. Gao, Ch. Wan, T. He, J. Li, H. Xin and F. Liu, Chem. Phys. Letters 112 (1984) 465. [ 31 K. Kneipp, H. Kneipp and M. Rentsch, J. Mol. Struct. [4] K. Kneipp and J. Siegel, in preparation. [ 51W. Jian, L. Dawei, X. Houwen, S. Xu, L. Fan-Chen, Spectrochim. Acta 43A (1987) 375. [6] J.Wang,J.Phys.Chem.92 (1988) 1942. [ 71 K. Kneipp, J. Mol. Struct., to be published. [S] M. Kerker, Appl. Opt. 19 (1960) 3373. [ 91 R. Koh, S. Hayashi and K. Yamamoto, Solid State Commun. 64 (1987) 375. [lo] X. Li, B. Gu and D.L. Akins, Chem. Phys. Letters 105 (1984) 263. [ 111B. Simic-Glawaski, J. Phys. Chem. 90 (1986) 3863. (121 W.K. Lam, L. Richter and Y.T. Tan, Phot. Sci. Eng. 28 (1984) 98. [ 131 D.D.F. Shiao, M.T. Nieh and A.H. Herz, J. Phot. Sci. 30 ( 1982) 208.