Langmuir 1989, 5, 1015-1020 oxidized (Moo3) material and crystallization of amorphous MoS2. In contrast to oxidized sputtered films, the opposite changes in lattice spacings were observed39for alkali-metal intercalation of NbSez. Niobium, having one less d electron than molybdenum, can be readily intercalated with alkali atoms that ionize by donating an electron to the half-filled Nb d,z orbital. Extended X-ray absorption fine structure (EXAFS) measurements have shown that the intercalated material is expanded in the edge directions and contracted in the basal direction with no change in Nb-Se bond length. Expansion of the trigonal prismatic lattice in the basal direction has been associated with reduced friction for various materials? On the basis of the XRD measurements reported here, and with the aid of the MO interpretation, we predict that substitution within the MoSz lattice, during preparation, of electron acceptor-type dopants (of which oxygen is a special case) will reduce the friction below that of the parent pure material. There is evidence that slight oxidation of sputter-deposited MoS2 lowers its friction coefficient,32and films with slightly elevated S:Mo (greater than 2) ratios have measurably lower friction coefficients than stoichiometric or sulfur-deficient Another system predicted to have improved lubrication properties is MoS2 films that have a small percentage of Nb substituted for the Mo. The ideal lubricant film should be one that adheres well to the substrate material and that has the lowest possible friction. Substitution of some sulfur by another species within the interface region has already been mentioned for improving adhesion. Arsenic might be an appropriate substituent, since it would be an acceptor for some metal electron density, but the precise property that makes it good for adhesion at the interface, its ability to hybridize by using d orbitals, would probably encourage interlayer (39) Bourdillon, A. J.; Pettifer, R. F.; Marseglia, E. A. Physica (Utrecht) 1980,99E,64.
1015
bonding and thus increase friction within the bulk of the film. Instead, an appropriate substituent within the bulk film for reducing friction would be phosphorus, an acceptor that does not have d orbitals available for hybridization and bonding. The preparative task for achieving the best possible lubricant material will be to control interface and bulk compositions independently. The value of studying the electronic structure of solid lubricant materials and using this information to design new or modified lubricants cannot be overstated. Summary a n d Conclusions A qualitative molecular orbital model, based on D3h symmetry and periodic repetition of the Mo(S), unit within the MoSz crystal, agrees well with the results of spectroscopic measurements of valence-level transitions and can be used to interpret crystallographic variations in MoSz thin films. This model involves a total of seven valencelevel molecular oribtals, six of which form the bonds between the central Mo and the six S atoms and one of which is considered to be nonbonding. Such an interpretation contradicts the model based on augmented spherical wave calculations, wherein the uppermost state was assigned as an antibonding (d,2) state with support derived from photochemical studies.13 Our assignment of the highest filled level as a nonbonding level appears to be more physically realistic, because it results in a net bond order around the central Mo of six, a situation that is not realized with the alternative assignment. Our assignment provides explanations of spectroscopic and crystallographic data and can be used to predict substitution chemistry for improving lubrication properties. Acknowledgment. Support for this work was provided by the Defense Advanced Research Projects Agency and the U.S. Air Force Space Division Contract No. F0470185-C-0086. This paper is dedicated to the memory of Dr. C. C. Badcock. Registry No. MoS2, 1317-33-5.
Photochemistry and Radiation Chemistry of Colloidal Semiconductors. 33. Chemical Changes and Fluorescence in CdTe and ZnTe Ute Resch, Horst Weller, and Arnim Henglein* Hahn-Meitner-Institut Berlin GmbH, 0-1000 Berlin 39, Federal Republic of Germany Received October 13, 1988. I n Final Form: March 23, 1989 Colloidal solutions of CdTe and ZnTe were prepared via precipitation of the metal ions by NazTe in the presence of polyphosphate. The solutions do not spontaneously fluoresce, but upon illumination or attack by free radicals generated radiolytically the colloids acquire fluorescence, the highest quantum yield being 0.2. ZnTe colloids lose their fluorescence upon standing in the dark but regain it upon exposure to a small dose of light. Co colloids have onsets of absorption and fluorescences that gradually shift with the Cd/Zn ratio. In all cases, the fluorescence band is located at the onset of absorption and the decay of the fluorescence occurs in the 10-ns range. The colloids corrode upon illumination or free radical attack to yield metal, which partially dissolves to produce hydrogen. Tellurium is also formed, colloidal ditelluride formation being an intermediate step. Introduction The chemistry and physics of small semiconductor particles in colloidal solution have attracted increasing interest during the past few years. In the beginning of this research, the photocatalytic properties of these particles were of major interest: upon light absorption electrons and 0743-7463/89/2405-1015$01.50/0
positive holes, which initiate various redox processes, are generated.' More recently, certain physical effects accompanying the chemical reactions were observed. They include fluorescence and fluorescence quenching,2 size quantization e f f e ~ t snonlinear ,~ optical phenomena: and photoelectron emi~sion.~ With respect to preparation and 0 1989 American Chemical Society
1016 Langmuir, Vol. 5, No. 4, 1989
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analysis, new roads were found, surface modification procedures? sandwich formation (combination of different colloidal particles'), and separation by exclusion chromatographye being typical examples. In the present paper, the preparation and various properties of colloidal cadmium and zinc telluride are described. In the macrocrystalline state, these materials are semiconductors with band gaps of 1.5 and 2.3 eV, respe~tively.~ CdTe is an important component in experiments on photoelectrochemica1 cells.lo The colloids are extremely sensitive to oxygen; therefore, all the experiments had to be performed under exclusion of air. Upon illumination or exposure to free radical attack, these colloids underwent chemical changes which were accompanied by unusual fluorescence phenomena.
Experimental Section Preparation of the Colloids. The stoichiometric colloid was prepared by injection of 1 X M NazTe solution into 50 mL of solution containing 1 X lo4 M Cd(C104)zor Zn(C104)zand 1 X lo4 M sodium hexametaphosphate (Riedel de Haen) under argon. The vessel was vigorously shaken during the injection. The pH was adjusted before injection. Both colloids were stable in the pH range from 6 to 12. The stabilizer consisted mainly of polyphosphates of different chain lengths; the molarities given are based on the formula (NaP03)$.In the case of CdTe colloids with an excess of Cd2+,a stoichiometric colloid was first prepared and excess Cd(C104)2added afterwards. ZnTe colloids with excem Zn2+ were prepared by varying the Zn2+concentration before NazTe injection. The NazTe solution was prepared by bubbling an argon stream containing HzTe, generated in the reaction of AlZTe3with 2 N HzS04at 0 O C , through ice-cooled 0.1 M NaOH. Apparatus and Analysis. Stationary illuminations were performed with a 600-W xenon lamp. When illumination with monochromatic light was required, a monochromator (Kratos) was used. In the illumination with polychromatic light, various filters were used to select the desired wavelength range. y-Irradiations were carried out in the field of a 6oCo source. Fluorescence quantum yields were determined by comparing the area of the fluorescence band with that of standard compounds absorbing in the same wavelength range known to fluoresce with (1)(a) Henglein, A. Top. Curr. Chem. 1988,143, 113-180. (b) Kalyanasundaram, K.; Gritzel, M. In Chemistry and Physics of Solid Surfaces V; Vanselow, R., Howe, R., Eds.; Springer Verlag: Berlin, 1984. (c) Thomas, J. K. J. Phys. Chem. 1987,91,267-276. (2) (a) Henglein, A. J. Phys. Chem. 1982,86,2291-2293.(b) Spanhel, L.; Haase, M.; Weller, H.; Henglein, A. J. Am. Chem. SOC.1987,109, 5649-5655. (c) Chestnoy, N.; Harris, T. D.; Hull, R.; Brus, L. E. J. Phys. Chem. 1986,90,3393-3399. (3)(a) Fojtik, A.;Weller, H.; Koch, U.; Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1984,88,969-977. (b) Brus, L.E. J.Chem. Phys. 1983,79, 5566-5571. (c) Brus, L. E. J. Phys. Chem. 1986,.O, 2555-2560. (d) Nozik, A. J.; Williams, F.; Nenadovic, M. T.; Rajh, T.; Micic, 0. T. J. Phys. Chem. 1985,89,397-399.(e) Watzke, H. J.; Fendler, J. H. J.Phys. Chem. 1987,91,854-861. (4)(a) Henglein, A.;Kumar, A.; Janata, E.; Weller, H. Chem. Phys. Lett. 1986,132,133-136. (b) Wang, Y.;Mahler, W. Opt. Commun. 1987, 61, 233-236. (5)(a) Haase, M.;Weller, H.; Henglein, A. J.Phys. Chem. 1988,92, 4706-4712. (b) Haase, M.; Weller, H.; Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1988,92,1103-1107. (6)(a) Dannhauser, T.;O'Neil, M.; Johansson, K.; Whitten, D.; McLendon, G. J. Phys. Chem. 1986,90,6074-6076. (b) Spanhel, L.; Haase, M.; Weller, H.; Henglein, A. J. Am. Chem. SOC.1987, 109, 5649-5655. (c) Steigerwald, M. L.; Alivisatos, A. P.; Gibson, J. M.; Harris, D. T.; Kortan, R.; Muller, A. J.; Thayer, A. M.; Duncan, T. M.; Douglass, D. C.; Brus, L. E. J. Am. Chem. SOC.1988,110,3046-50. (d) Fischer, Ch.-H.; Henglein, A. J. Phys. Chem., in press. (7)(a) Spanhel, L.; Weller, H.; Henglein, A. J. Am. Chem. SOC.1987, 109,6632-6637. (b) Spanhel, L.; Henglein, A.; Weller, H. Ber. BunsenGes. Phys. Chem. 1987,91,1359-1363. (8) Fischer, Ch.-H.; Weller, H.; Katsikas, L.; Henglein, A. Langmuir 1989,5,429-432. (9) Landolt-Bbrnstein, Numerical Data and Functional Relationships in Science and Technology; Hellwege, K.-H., Ed.; Springer Verlag: Berlin, 1982; Vol. 17,subvol. b, pp 157,225. (10)Ellis, A. B., Kaiser, S. W.; Bolts, J. M.; Wrighton, M. S. J. Am. Chem. SOC.1977,99,2839-2848.
0
roo
300
600
500
ACnnI
Figure 1. Absorption spectrum of a CdTe solution containing excess telluride. CdTe, 1 X lo4 M; excess Te2-, 5 X 10" M; polyphosphate, 1 X M; pH 9. 1.o
I
lh''\L '.\ -
io0
300
W
u
c
;0.5
I
.
absorption
.
-
0 VI 9
m
400
500 600 700 X Inml Figure 2. Illumination of a CdTe sol by monochromatic light (304 nm). Absorption spectrum before and after 1-h illumination
and fluorescence spectra after various illumination periods. CdTe, 1 X lo4 M; excess Cd2+,2 X lo4 M, polyphosphate, 1 X M; pH 9.
almost 100% yield (Coumarin in the case of ZnTe, Pyridin I1 in the case of red-fluorescingCdTe, and Rhodamine 6G in the case of orange- and yellow-fluorescingCdTe). Hydrogen was determined gas chromatographically. Tellurite, TeO?-, and tellurate, Te042-,were determined polarographically." For the polarographic measurements, 0.1 M tartaric acid was added to the colloidal solution and the pH adjusted to 9.0 by adding ammonia. Zn2+ions were complexed by adding 1 X M EDTA. Particle sizes were determined by transmission electron microscopy.
Results Photolysis of Colloidal CdTe and ZnTe. The absorption spectrum of CdTe samples began in the 650700-nm range, i.e., a t wavelengths shorter than 824 nm, which corresponds to the macrocrystalline band gap of 1.5 eV. In the presence of excess telluride, the spectrum contained a maximum at 420 nm and a shoulder at 510 nm (Figure 1). The stoichiometric colloid and colloids with excess Cd2+ions have an unstructured spectrum (Figure 2). The mean particle size of the stoichiometric colloid was 45 A as revealed by electron microscopy. The blue shift of the absorption threshold is ascribed to the spatial confinement of the charge carriers generated by light absorption in these small particles. Exposure of colloidal CdTe to 304-nm light resulted in a slow decrease in absorption, the quantum yield of phoCdTe molecule consumed/photon tolysis being 4 x absorbed. Figure 2 shows the absorption spectrum before and after illumination. One recognizes a slight decrease in absorption a t shorter wavelengths. A longer wave(11)Baumgarten, s.;Cover, R. E.; Hofsass, H.; Karp, s.;Pinches, P.
B.; Mutes, L. Anal. Chin. Acta 1959,20,397-404.
Langmuir, Vol. 5, No. 4, 1989 1017
Chemical Changes and Fluorescence in CdTe and ZnTe
0.15
1.5
L
0
0
0.10 a
r 1.0 In
U
-
c m
s2
z
0.05 e=
0.5
0
0
200
400
600
X Inml Figure 3. Absorption and fluorescence spectra of a ZnTe solution at various illumination times with 304-nm light. ZnTe, 1 X lo4 M; excess Zn2+, 2 X lo4 M; polyphosphate, 1 X lo4 M; pH 11.
lengths, the absorption increased a little. The colloid did not fluoresce before illumination. However, as can be seen in Figure 2, it started to fluoresce upon illumination. The intensity of fluorescence first increased and finally decreased again, the highest quantum yield being 0.02. Only colloids with excess Cd2+could be activated to fluoresce upon illumination. The best concentration of excess Cd2+ was 2 X 10"' M. The fluorescence was strongest when the pH of the solution was between 8 and 9. Stronger photolysis of colloidal CdTe solutions was observed when they were irradiated with the polychromatic light of the xenon lamp for hours. After the initial decrease in absorption described above, the absorption increased a t all wavelengths in the visible and near UV. Traces of Hzcould be detected. The increased absorption is ascribed to the formation of tellurium. In the case of ZnTe, these effects were more pronounced. They are described in more detail below. The colloidal ZnTe had an absorption threshold close to 500 nm which is only slightly below that of the macrocrystalline material (537 nm). In the presence of excess Zn2+,the spectrum did not show any structure. In the presence of Te2- ions, the specrum has a weak shoulder at 370 nm. Figure 3 shows that illumination at 304 nm causes a strong increase in absorption, the quantum yield of photolysis being 4 X ZnTe molecules consumed per photon absorbed. The figure also shows that the colloid acquired fluorescence upon illumination, the rather narrow fluorescence band lying a t the onset of absorption. The fluorescence color was blue. Only colloids which contained excess Zn2+ions could be activated to fluoresce upon illumination. During photolysis of ZnTe, hydrogen and tellurium are formed. In Figure 4, the concentration of Hzand the increase in the absorption a t 600 nm are shown as functions of the illumination time. The increase in absorption occurs in two steps, the first one being complete after 90 min. As shown below, ZnTe2 is the product a t short illumination times, and at longer times tellurium is formed. Figure 4 also shows the fluorescence quantum yield. In the calculation of this yield, one had to take into account that ZnTe was consumed during the illumination to form ZnTe2. The concentration of ZnTe2 was calculated from the increased 600-nm absorption knowing the absorption coefficient of ZnTe2 at this wavelength (see below). For much longer illumination times than those in Figure 4, much higher conversions of ZnTe were reached. For example, a 10-h illumination yielded an H2 concentration of 7 x lod M corresponding to a 70% conversion according to ZnTe + 2H20 Zn2+ + H2 + 20H- + Te (1)
-
t Iminl Figure 4. Photolysis of ZnTe with 304-nm light:
Hzconcentration, fluorescence yield, and 600-nm absorbance as functions of illumination time. I
U
10
11
12
PH
Figure 5. pH dependence of the fluorescence quantum yield. Various illumination times for activation of the fluorescence. Inset:
fluorescence decay.
The fluorescence of ZnTe strongly depended on the excess Zn2+ concentration and the pH of the solution. Figure 5 shows the pH dependence of a solution containing 2X M Zn2+. At this excess Zn2+concentration and after activation of the solution by a 20-min illumination, the highest fluorescence quantum yield, 0.22, was obtained. The inset of the figure shows the decay of the fluorescence as measured by single photon counting. The decay is multiexponential, the first half-life time being 20 ns. Fluorescing ZnTe solutions showed a most curious effect: upon standing in the dark for a while, their fluorescence gradually disappeared. It could, however, be reactivated by 304-nm illumination, only 10-s illumination being necessary. Radiolysis of Colloidal CdTe and ZnTe. The primary chemical species in the radiolysis of dilute aqueous solutions are hydrated electrons, hydroxyl radicals, and hydrogen peroxide from the decomposition of the solvent. The radiation chemical yields (G values) are G(e,-) = 2.6, G(0H) = 2.7, and G(H202)= 0.8 radical (or molecule) per 100 eV of absorbed radiation energy. If the reactions of OH radicals are to be studied, the irradiation is performed under nitrous oxide as, under these conditions, hydrated electrons are scavenged and additional OH radicals formed:
N20 + eaq-+ H 2 0
-
Nz + OH
+ OH-
(2)
On the other hand, when OH radicals are not desired, an organic scavenger, such as tert-butyl alcohol, is added OH + (CH,)&OH Hz0 + CHz(CH&2COH (3)
-
the organic radical produced being much less reactive than OH. In the presence of both N20 and tert-butyl alcohol, only organic radicals and hydrogen peroxide are generated.
1018 Langmuir, Vol. 5, No. 4, 1989
Resch et al.
...*.. *... e----e..
e..
---4
/
0
AL,TeOi-
0
-
' 0
t [mini Figure 8. Reaction of ZnTe colloid with OH radicals. Concentration of Te02' and Te042-and absorption of Te as functions
of y-irradiation time.
0.3
0
1
[OH] [lO-LMl 2 3 4
I
I
I
0
5
10
15 t Iminl
5
6
1
20
25
Figure. 7. Plot of the 600-nm absorbance (Figure 6) as a function of y-iradiation time or total OH radical concentration generated.
The absorption of a ZnTe solution first increased and later decreased again upon 7-irradiation under NzO, Le., under conditions where OH radicals attacked the colloidal particles (Figure 6). Teh 600-nm absorption of the solution is plotted in Figure 7 as a function of irradiation time, the upper abscissa scale giving the total concentration of OH radicals produced. It can be seen that the absorption builds up in two steps, the first increase being complete after 4 min. At this time, the total OH radical concentration was 1 X lo4 M, an amount equal to that of the initial ZnTe concentration. In the maximum of Figure 7, twice the amount of radicals, 2 x M, had been produced. This two-step oxidation of ZnTe is interpreted as formation of ZnTez in the first step and tellurium formation in the second step:
-
+ 20H ZnTez + 2 0 H
2ZnTe
ZnTez + Zn2+ + 20H-
-
2Te
+ Zn2+ + 20H-
(4)
(5)
Colloidal ZnTez directly prepared from NazTez(5 X M) and Zn(C104)2(3 X lo4 M) shows the same absorption spectrum as ZnTe in Figure 6 after 4 min of irradiation. The Na2Te2solution was obtained by illuminating a Na T solution a t pH 11. The absorption coefficient of Te2% 1X M-' cm-' at 512 nm.lo From the 600-nm absorbance after 4 min, in Figure 7 an absorption coefficient of colloidal ZnTez of 1145 M-' cm-' was calculated. On the basis of these results, we interpret the two-step buildup of the 600-nm absorption in the photolysis of ZnTe (Figure 4) by a similar mechanism. First ZnTez is formed, and later the photolysis of this compound leads to elementary tellurium. The decrease in 600-nm absorption at irradiation times longer than 10 min in Figure 7 is ascribed to the oxidation of tellurium by OH radicals, tellurite and tellurate being
the products. Figure 8 shows the decrease in the 600-nm absorption of Te and the concentrations of the two oxidation products. Tellurite is only an intermediate product and is rapidly oxidized to yield tellurate. Experiments are now described in which the activation of the fluorescence of CdTe and ZnTe was brought about by y-irradiating the solutions. These experiments were performed by irradiating the solutions (a) under argon without additional scavengers (H202, oxidizing OH radicals, and reducing hydrated electrons attacking the colloidal particles), (b) under argon with added tert-butyl alcohol (hydrated electrons, HzOz, and organic radicals attacking), and (c) under nitrous oxide and addition of tert-butyl alcohol (HzO2and organic radicals as reagents). The following observations were made in the three cases. Case a: The y-irradiation (dose rate 1.3 X lo5 rad/h) of CdTe and ZnTe solutions without added scavengers led to fluorescence, the quantum yield being 0.05 for CdTe and 0.005 for ZnTe under the most favorable conditions (pH 8-9, excess [Cd2+]= 2 X M; pH 11, excess [Zn2+]= 2 x lo-* M, respectively). The changes in the absorption spectra of both colloids were similar to the changes observed in photolysis (Figures 2 and 3). Case b: In the presence of tert-butyl alcohol, the y-irradiation (dose rate 1.3 X lo5rad/h) led to much stronger fluorescences, the highest quantum yield being 0.2 for CdTe and 0.02 for ZnTe after 20 min of irradiation. However, the fluorescences became weaker again upon further irradiation. The absorption spectra decreased in intensity during the first 5 min but started to increase at all wavelengths at irradiation times longer than 20 min. We interpret this increase as the formation of cadmium and zinc metal on the colloidal particles. In the case of CdTe, it was possible to trace the metal formed by using the methyl viologen method.12 Addition of methyl viologen, MV2+,to the irradiated solution, under the exclusion of air, produced the blue color of the stable radical cation of methyl viologen, MV+:
-
Cd + 2MV2+ Cd2+ + 2MV+ (6) The method could not be used in the case of ZnTe, as this colloid reacted thermally with MV2+. The irradiated solutions smelt garliclike, which indicates that telluriumorganic compounds had been formed. Case c: The y-irradiation (dose rate 2.4 X lo5 rad/h) of CdTe and ZnTe colloids in the presence of both NzO and tert-butyl alcohol led to a degradation of the colloids, the garliclike odor of the irradiated solutions indicating (12) Gutierrez, M.; Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1983, 87,474-478.
Langmuir, Vol. 5, No. 4, 1989 1019
Chemical Changes and Fluorescence in CdTe and ZnTe
-
absorption
Y
200
400
fluorescence
600
X [nml Figure 9. y-Irradiation of a CdTe colloid (1X lo-' M) containing M). excess Cd2" (2 X lo4 M) and tert-butyl alcohol (2 X Irradiation under N20. Absorption and fluorescence spectra at various irradiation times. The fluorescence spectra were obtained each time by a subsequent 5-min illumination with 304-nm light. 1.0 0 U )
c m n
k 0.5 v)
n m
0
400
600 800 X [nml Figure 10. Absorption and fluorescence spectra of ZnTe-CdTe co-colloids. ZnTe, 1 X lo4 M; excess Zn2+, 2 X lo4 M; polyphosphate, 1 X M; pH 9; Cd2", added in various amounts.
200
the formation of tellurium-organic compounds. Figure 9 shows the changes in absorption for an irradiated CdTe colloid. It is seen that there is a rapid decrease in the intensity of the absorption spectrum, the final products not absorbing above 350 nm. The yield of the CdTe consumption was calculated from the rate of the decrease in 350-nm absorbance during the first 20 min of irradiation. The G value found is 1.0 CdTe molecule/100 eV of absorbed radiation energy. This agrees, within the limits of error, with the radiation chemical yield of hydrogen peroxide. The solution did not fluoresce after the y-irradiation. However, its fluorescence could be activated by subsequent exposure to 304-nm light. This can also be seen in Figure 9, where the activated fluorescence at various times of y-irradiation is shown. Note that the fluorescence band shifted to shorter wavelengths with increasing irradiation time, the color of the fluorescence light changing from red to orange and then yellow. This effect is attributed to size quantization in the colloidal particles, which become smaller during the degradation. Co-Colloids of CdTe and ZnTe. Figure 10 shows the changes in absorption and fluorescence of colloidal ZnTe upon addition of Cd2+ions. As CdTe is much less soluble than ZnTe, it can be assumed that all the Cd2+ions added are built into the lattice of the ZnTe particles, the replaced Zn2+ ions migrating into the solution. The fluorescence was always activated by a 30-min illumination with 304-nm light. With increasing cadmium content, the spectra shift toward longer wavelengths, the fluorescence band always appearing a t the onset of absorption.
Discussion The illumination of semiconductor colloids leads to the formation of electrons in the conduction band and positive
holes in the valence band. Most of these charge carriers recombine, and only a few initiate the corrosion in CdTe and ZnTe. The corrosion consists of the decomposition of these materials into the elements. The tellurium formed first, is bound to Te2- in the form of ditelluride, and later appears as elemental Te. The metal formed in the photolysis slowly reacts with water to yield hydrogen. These corrosion processes are accompanied by changes in absorption. Metal and ditelluride formation produce a modest increase in absorption at all wavelengths in the visible, while tellurium formation leads to a much stronger increase (Figures 3 and 4). In some cases, illumination did not immediately lead to increased absorption (Figure 2). In fact, a slight decrease could be observed in the beginning of illumination. Possibly, the products, i.e., metal and tellurium, are present on the colloidal particles as isolated atoms or molecules which do not yet possess the absorption of bulk metal or metal ditelluride. As the illumination proceeds, larger islands of products are formed which do have the properties of the bulk materials. The initial decrease in absorption is attributed to the consumption of the colloid. The intact colloids did not fluoresce. Obviously, the charge carriers recombined here without the emission of light, the defect centers for radiationless recombination possibly being located at the surface of the colloidal particles. The fluorescence appeared after illumination or free radical attack of the colloids, i.e., after certain chemical changes had taken place. The defect centers for radiationless recombination must have been destroyed or blocked by the chemical changes. This would be best understood if the chemical changes had occurred preferentially a t the defect sites, ZnTe colloids, the fluorescence of which had been activated by illumination, lost it upon standing in the dark. They regained their fluorescence after exposure to a very small amount of light. The loss of fluorescence in the dark could be ascribed to thermal reformation of defect sites. However, it seems difficult to understand why the fluorescence could be reactivated by such a small dose of light, while much larger doses were necessary to activate it initially. We prefer to invoke an interaction of the charge carriers with the decomposition products as a prerequisite for the occurrence of radiative band gap recombination: illumination or free radical attack leads to spots of metal and ditelluride on the surface of the particles. The metal spots act as an electron acceptor and storage system for electrons generated upon further illumination. This effect of metal deposits on colloidal semiconductor particles is well-known in other cases where the metal was deposited not by decomposition of the colloid itself but from outside.14 When a certain degree of charging up of the metal deposit is reached, further uptake of electrons is slowed down, and many charge carriers can recombine over the band gap and so emit light. When the colloid stands in the dark, the metal deposit is slowly discharged as the stored electrons form hydrogen by reacting with the aqueous solvent. Upon renewed illumination, the metal deposits are recharged, a process that does not require a lot of absorbed light, and the colloid fluoresces again. For both colloids, the fluorescence band was always located at the onset of absorption, which indicates that radiative band gap recombination occurs. On the other hand, the fluorescence decayed in a multiexponential manner, the first half-life time being about 20 ns (Figure (13) Henglein, A.; Gutierrez, M. Ber. Bunsen-Ges. Phys. Chem. 1983,
87, 852-858.
(14) Kraeutler, B.; Bard, A. J. J.Am. Chem. SOC.1978,100,4317-18.
Langmuir 1989,5, 1020-1026
1020
5). This seems too long to be in agreement with direct band gap recombination after excitation. However, delayed band gap recombination might be the explanation, with the electron, very rapidly after its formation in the conduction band, being trapped in shallow traps from which it returns into the conduction band by thermal activation. We turn now to the discussion of fluorescence activation by y-irradiation of the colloidal solutions. This activation was dependent on the nature of the solute that had been added to scavenge certain types of free radicals: In the case of irradiation under argon without additives (case a), oxidizing species, such as H202and OH radicals, and reducing hydrated electrons react with the colloidal particles. The changes produced closely resemble those produced by illumination. This is readily understood, as the hydrated electrons produce similar reductions as the conduction band electrons in photolysis, and OH plus H20z produce oxidations similar to the ones initiated by positive holes. The overall changes have rather small yields in both cases, because the reduction and oxidation processes occur on the particles simultaneously; thus, back reactions can readily occur and lower the overall yield. However, when y-irradiated solutions contain tert-butyl alcohol (case b), the picture changes drastically. The number of oxidizing species attacking the colloidal particles is reduced (eq 3). Instead, tert-butyl alcohol radicals are formed. The hydrated electrons reduce metal ions on the surface of the colloidal particles, and hydrogen peroxide oxidizes telluride anions. The tert-butyl alcohol radicals react with the tellurium formed to yield tellurium-organic compounds. No attempts were made to analyze the latter in detail; the only method af detection was the characteristic smell. The highest fluorescence yields were observed in case b, i.e., under conditions where metal formation was most pronounced and tellurium formation minimized. In irradiated solutions containing both N20 and tertbutyl alcohol, no reducing species were formed that could produce metal on the colloidal particles. Under these conditions, H202oxidized the particles, and the tellurium formed was scavenged by the organic radicals, the result being the rapid disappearance of CdTe or ZnTe (Figure 9). The colloids did not fluoresce after the y-irradiation.
However, they acquired fluorescence upon subsequent illumination, i.e., under conditions where a metal deposit could be formed. The existence of a small deposit thus seems to be a prerequisite for fluoresence. The activation of the fluorescence by illumination or free radical attack is only possible when the solutions of CdTe and ZnTe contain an excess of metal ions. This indicates that the formation of the necessary metal deposit is facilitated by Cd2+and Zn2+ions, respectively. To draw a final conclusion about fluorescence activation in these colloids, we may say that the present experiments demonstrate the possibility of such an activation, although we must admit that our knowledge about the mechanism of activation is still fragmentary. The observations made with the co-colloids (Figure 10) can be explained in two ways. First, the question arises whether CdTe and ZnTe are isomorphous, i.e., whether one is dealing with colloidal particles in which there is a statistical distribution of the metal ions in the lattice. Previous studies on co-colloids of CdS and ZnS13 showed that the absorption threshold and peak of the fluorescence band shift gradually with the Cd/Zn ratio. In these experiments, the two metal ions were simultaneously precipitated by H2S. However, in the present experiments, in which Cd2+ ions were added to a ZnTe solution and exchanged with Zn2+ ions in the lattice, the formation of sandwichlike structures may occur, i.e., a ZnTe colloid carrying a CdTe deposit. Such sandwich structures have recently been studied for AgI-Ag,S ~0mbinations.l~In these structures, a shift in the absorption threshold and the fluorescence peak was also observed with changing size of the Ag2Spart of the colloid. The explanation was given in terms of size quantization of the lower band gap component when this component was very small. With increasing size of the Ag,S component, its band gap became larger, and the fluorescence shifted to longer wavelengths. A similar explanation could also hold for the ZnTe-CdTe structures. Registry No. CdTe, 1306-25-8; ZnTe, 1315-11-3; Te, 1349480-9;Hz,1333-74-0;H,O, 7732-18-5;ZnTe,, 12402-39-0;NzO, 10024-97-2; H2Oz,7722-84-1;OH, 3352-57-6;tert-butyl alcohol, 75-65-0. (15) Henglein, A.; Gutierrez, M.; Weller, H.; Fojtik, A.; Jirkovsky, J. Ber. Bunsen-Ges. Phys. Chem. 1989, 93,593-600.
Oxidation of a Sulfide Group in a Self-Assembled Monolayer Nolan Tillman, Abraham Ulman,* and James F. Elman Corporate Research Laboratories, Eastman Kodak Co., Rochester, New York 14650-2109 Received November 1, 1988. I n Final Farm: March 29, 1989 A monolayer film prepared on silicon substrates from ll-[4-(methylthio)phenoxy]-l-(trichlorosilyl)undecane FTIR,XPS, ellipsometry, and contact angle measurements. Treatment of this monolayer with hydrogen peroxide in acetic acid resulted in efficient conversion of the surface-localized sulfide group to a sulfoxide group. The resulting hydrophilic surface could be treated successfully with octadecyltrichlorosilane to produce a bilayer film. (1) was investigated by
Introduction The field of self-assembled monolayer films is currently an area of great interest to several research groups.'-" (1) Sagiv, J. J. Am. Chem. SOC.1980, 102, 92. (2) Nuzzo, R. G.; Allara, D. L. J. A m . Chem. SOC.1983, 105, 4481.
Self-assembly by using alkyltrichlorosilane derivatives has Promise as a method by which ultrathin mon01ayerl-l~and (3) Maoz, R.; Sagiv, J . J . Colloid Interface Sci. 1984, 100, 465. (4) Gun, J.; Iscovici, R.; Sagiv, J. J. Colloid Interface Sci. 1984,101, 201. (5) Allara, D.L.;Nuzzo, R. G. Langmuir 1985, 1, 45.
0743-7463/89/2405-lO20$01.50/0 Q 1989 American Chemical Society
