Langmuir 1991, 7, 520-524
520
Semiconductor Particles Formed at Monolayer Surfaces Xiao Kang Zhao,l Shuqian X U , and ~ Janos H. F e n d l e r * Department of Chemistry, Syracuse University, Syracuse, New York 13244-4100 Received May 14, 1990. I n Final Form: July 16, 1990 Monolayers have been prepared from negatively charged arachidic acid (AA), bovine-brain phosphatidylserine (PS),n-hexadecyl-11-(viny1benzamide)undecylhydrogen phosphate (l),and positively charged dioctadecyldimethylammonium bromide (DODAB) and bis( 2 4n-hexadecanoyloxy)ethyl)methyl(p-vinylbenzy1)ammoniumchloride (2) over aqueous 1.0 x M CdClZ, or ZnClz, or CuS04. Controlled and slow infusion of hydrogen sulfide onto compressed monolayers prepared from AA, PS, 1, and polymerized 1 resulted in the formation of semiconductor particles. In contrast, semiconductor formation could not be observed at the surfaces of monolayers prepared from positively charged DODAB and 2. Reflectivity and refractive index measurements provided information on the time-dependent growth of semiconductor particles. Sulfide nuclei, forming at the monolayer interface, rapidly grew to aligned microclusters that coalesced, predominantly two dimensionally, at the monolayer interface to an interconnected, porous, semiconductor particulate film. The optical thickness of the monolayer-supported particulate semiconductor film increased to a plateau value beyond which no additional semiconductor particle formation could be observed. Subsequent to exposure to the atmosphere, the optical thickness of the monolayer-supported semiconductor particulate film decreased, at a rate of ca. 10 A/h, by spontaneous dissolution of the semiconductor particulates. Introduction of M CuSO4 into the subphase coated by a 225 A thick CdS particulate film on a 1 monolayer resulted in the incorporation of copper ions into the semiconductor layer.
Introduction There is an increasing interest in the preparation, characterization, and utilization of nanosized Size quantization, as the reduction of particles to nanometer size has become known, is accompanied by altered mechanical, electrical, electrooptical, magnetic, and chemical properties that can be potentially exploited in a broad variety of applications. Nanosized particles have been prepared by mechanical reduction of larger structure^,^ by direct chemical synthesis,6 and by colloid chemical techniques using controlled mixing of the precursors under well-adjusted ~onditions.5,'-~~ Maintaining monodispersed particles in (1) Present address: Angstrom Technology, 1815 W 1st Ave., Suite 102, Mesa, AZ 85202. (2) Present address: Chemistry Department, University of Pittsburgh, Pittsburgh, PA 15260. (3) Henglein, A. Top. Curr. Chem. 1988,143,113. Brus, L. A. J. Phys. Chem. 1986,90,2555. Andres, R. P.;Averback, R. S.; Brown, W. L.; Brus, L. E.; Goddard, W. A.; Kaldor, A.; Louie, S. G.; Moskovits, M.; Percy, P. S.;Riley, S. J.; Siegel, R. W.; Spaepen, F.; Wang, Y. J. Mater. Res. 1989, 4, 704. (4) Fendler, J. H. Chem. Reu. 1987,87, 877. (5) Charles, S. W.; Popplewell, J. In Ferromagnetic Material; Wohlfarth, E. P., Ed.; North Holland, Inc.: New York, 1980; Vol. 2. (6) Steigerwald, M. L.; Sprinkle, C. R. J. Am. Chem. SOC. 1987, 109, 7200. Brennan, J. G.; Siegrist, T.; Caroll, P. J.; Stuczynski, S. M.; Brus, L. E.; Steigerwald, M. L. J . A m . Chem. Soc.. 1989, 111, 4141. (7) Meissner, D.; Memming, R.; Kastening, B. Chem.Phys. Lett. 1983, 96, 34. (8) Rosetti, R.; Hull, R.; Gibson, J. M.; Brus, L. E. J. Chem. Phys. 1984, 82, 552. (9) Mau, A. W. H.; Huang, C. B.; Kakuta, N.; Bard, A. J.; Campion, A.; Fox, M. A.; White, M. J.; Webber, S. E. J . Am. Chem. Soc. 1984,106, 6537.
. K. J . Phys. Bensen-Ges. H. J. Chem.
. Chem. 1985, 89, 1236. (14) Tien, H. T.; Bi, Z. C.; Tripathi, A. K. Photochem.Photobiol. 1986, 44, 779. (15) Lianos, P.; Thomas, J. K. J. Colloid Interface Sci. 1986,117,505. (16) Daunhauser, T.; O'Neil, M.; Johansson, K.; Whitten, D.; McLendon, G. J . Phys. Chem. 1986,90, 6074. (17) Wang, Y.; Mahler, W. O p t . Commun. 1987, 61, 233.
0743-7463/91/2407-0520$02.50/0
controlled sizes is experimentally demanding. Nonaqueous solvents, low temperature, stabilizers, and chemical treatments (capping) have been used for the stabilization of ultrasmall semiconductor particle^.^-^^ Advantage has been taken in our laboratory of organized surfactant aggregates as matrices for the stabilization of colloidal semiconductor particles. In particular, CdS and related semiconductor particles have been in situ generated in reversed micelles,12 in surfactant and polymerized surfactant vesicle^,^^^^^-^^ in bilayer lipid membranes (BLMs) and between the headgroups of LangmuirBlodgett films.31 The general methodology involved the slow exposure of the appropriate metal cation precursor, electrostatically attracted to the oppositely charged surfactant aggregate surface, to HzS. Particularly sensitive control was provided by the BLM since it allowed the spatial separation of the metal-ion precursor and H2S by the membrane.20 Coating only one side of the BLM by metal ions (the cis side) and infusing H2S from the aqueous solution bathing the opposite side (the trans side) of the bilayer led to the gradual appearance of ultrasmall semiconductor particles on the cis side of the membrane. The semiconductor particles rapidly moved around in the matrix of the membrane and grew in size, forming islands that merged with themselves and with a second generation ,20*29930
(18) Wang, Y.; Suna, A.; Mahler, W.; Rasowski, R. J . Chem. Phys. 1987,87, 7315. (19) Wang, Y.; Herron, N. J. Phys. Chem. 1987, 91, 257. (20) Zhao, X. K.; Baral, S.; Rolandi, R.; Fendler, J. H. J. Am. Chem. SOC.1988, 110, 1012. (21) Petit, C.; Pileni, M. P. J. Phys. Chem. 1988, 92, 2282. (22) Steigerwald, M. L.; Alivisatos, A. P.; Gibson, J. M.; Harris, T. D.; Kortan, R.; Muller, A. J.; Tayer, A. M.; Duncan, T. M.; Doughs, D. C.; Brus, L. E. J . Am. Chem. SOC. 1988, 110, 3046. (23) Tricot, Y.-M.; Fendler, J. H. J. Am. Chem. SOC.1984,106, 2475. (24) Tricot, Y.-M.;Fendler, J. H. J . Am. Chem. SOC. 1984,106, 7359. (25) Rafaeloff, R.; Tricot, Y.-M.; Nome, F.; Fendler, J. H. J. Phys. Chem. 1985,89, 533. (26) Rafaeloff, R.; Tricot, Y.-M.; Nome, F.; Tundo, P.; Fendler, J. H. J . Phys. Chem. 1985,89, 1236. (27) Tricot, Y.-M.; Emeren, A; Fendler, J. H. J. Phys. Chem. 1985,89,
-.
A731
I_.
(28) Tricot, Y.-M.; Fendler, J. H. J . Phys. Chem. 1986, 90, 3369. (29) Baral, S.; Fendler, J. H. J . Am. Chem. SOC.1989, 111, 1604. (30) Zhao, X. K.; Baral, S.; Fendler, J. H. J. Phys. Chem. 1990. 94, 2043. (31) Yi, K. C.; Fendler, J. H. Langmuir 1990, 6, 1519.
0 1991 American Chemical Society
Semiconductor Particles Formed a t Monolayers
Langmuir, Vol. 7,No.3, 1991 521
solution of the chosen surfactant (8 X 10'' molecules/mL) was carefully injected onto the clean, thermostated (25.0 "C), aqueous surface. The surfactants were compressed at a rate of (2-5) X (Az/molecule)/s. Subsequent to 5-15 min of incubation a t the desired surface pressure (25 mN/m for AA and 50 mN/m for PS, DODAB, 1, and 2), 200-250 r L of HzS was slowly injected into the nitrogen-filled Plexiglas hood covering the film balance. Semiconductor particle formation a t the monolayer surface was visually observed and the particulate films formed were transferred onto solid substrates by horizontal lifting for subsequent chara~terization.~~ The schematics of the circular, 4.0 cm deep, Pyrex trough (7.0 cm2 surface area) used for in situ semiconductor particle generation are shown in Figure 1. The trough was placed on a clean (chromic acid, copious amounts of water) flat glass plate and covered by a circular (7.5 cm high, 14.5 cm in diameter) glass jar whose flat frosted bottom provided a gas-tight contact. The Figure 1. Schematics of the experimental arrangements used water surface was cleaned by sweeping it through a water for the generation of semiconductor particles at the negatively aspirator. An appropriate amount of the spreading solution (1.5 charged, surfactant headgroup-aqueous (1.0 X 10-3 M MC12) subX M surfactant in CHC13) was carefully injected onto the phase interface and that used for the in situ monitoring of reclean subphase to give a coverage of 20 Az per molecule of AA flectivities: P, polarizer; D, detector; R, chart recorder. and 40 A2 per molecule of PS, DODAB, 1, and 2. Subsequent to 20 min of incubation, a Hamilton syringe containing 200-500 of particles. Growth ultimately led to a continuous porous FL of HzSwas introduced into the atmosphere covering the monosemiconductor particulate film that grew in thickness in layer via a rubber septum (Figure 1). The barrel of the syringe the direction perpendicular to the BLM. The growth of was kept a t the same position. This ensured the extremely slow the semiconductor particulate film could be controlled by (several hours) diffusion of HzS into the chamber and, hence, the amount and the rate of H2S i n f u ~ i o n .Size~ ~ ~ ~ ~into ~ the ~ ~monolayer-water interface. Formation of semiconductor quantized semiconductor particles have also been incorparticles was monitored by reflectivity measurements (vide infra). The semiconductor particulate films formed were transporated between the hydrophilic layers of LB films.32 ferred onto solid substrates by horizontal lifting for subsequent Generation of semiconductor particles from their metalcharacterization." ion precursors, electrostatically attracted to negatively Monolayers of P S were also formed from vesicles prepared by charged monolayers, is the subject of the present publisonicating 1.0 X 10-3 M aqueous dispersions of the lipid by a cation. The methodology used has been conceptionally Braunson sonicator for 60 min a t 25 "C and 15 W. Injection of similar to that developed for particle formation a t BLMs. vesicles into the cleaned aqueous subphase (to give stoichioH2S was introduced into the atmosphere and was allowed metric 2.0 x M PS) resulted in vesicle-monolayer transto infuse slowly across the compressed monolayer to the formation within a period of 15 h. Slow diffusion of HzS, in a metal counterion. There are several advantages of using manner described above, yielded semiconductor particles analogously to those formed directly from prepared monolayers. monolayer matrices for semiconductor particle generation. In some experiments, monolayers prepared in the Lauda film First, monolayers remain stable considerably longer than balance were transferred into the glass jar shown in Figure 1for BLMs. Second, their surface areas and charges are twosemiconductor particle generation. A small (2 cm X 4 cm) Tedimensionally controllable. Third, they, along with the flon trough was placed 2-5 mm under the water surface in the semiconductor particulate films grown in their matrices, well of the Lauda film balance prior to monolayer formation. can be conveniently transferred to solid supports. Subsequent to the formation of the monolayer and its compression to a desired pressure, the small Teflon trough was lifted through Experimental Section the monolayer horizontally and transferred into the system shown in Figure 1. Semiconductor formation was identical with that Bovine brain phosphatidylserine, PS (Avanti Polar Lipids, described above. Inc.), arachidic acid, AA (Sigma), dioctadecyldimethylammoGeneration of semiconductor particles was monitored by an nium bromide, DODAB (Eastman), cadmium chloride, zinc optical microscope. Light (150-W Xe lamp via an optical fiber) chloride, copper sulfate (Fisher), high-purity dry Nz (Union reflected from the monolayer surface was observed through an Carbide), HzS (Matheson), and spectroscopic grade chloroform Olympus PM-10-M microscope coupled to a television monitor (Aldrich) were used as received. Preparation and purification of and video recorder via an NEC NC-8 CCD color camera. polymerizable surfactants n-hexadecyl-11-(viny1benzamide)unThe experimental arrangements for in situ optical reflectivity decyl hydrogen phosphate, 1, and bis(2-(n-hexadecanoyloxy)measurements are illustrated in Figure 1. AHe-Nelaser (Hughes, ethyl)methyl(p-vinylbenzy1)ammonium chloride, 2, have been 13mW) was tightly mounted on a precision rotating optical stage. described.33 Water was purified by a Millipore Milli-Q filter The rotator with the rotating axis in the horizontal direction was system provided with a 0.22-pm Millistack filter at the outlet. supported by an optical post. Care was taken to provide good Semiconductor particles were generated in situ a t the monovibration isolation during measurements. Parallel-polarized laser layer headgroup-aqueous subphase interface. An aqueous metal light (6328A) was directed to themonolayer surface. For incident ion solution (1.0 X lo9 M CdCl2, ZnCl2, or CuS04) constituted angle measurements, the angle of incident light, 0, was changed the subphase. Either a commercial Lauda Model P Langmuir by sliding the post in a horizontal direction parallel to the surface film balance or a simple circular trough was used for monolayer without affecting the optical geometry. , The intensity of the formation and the subsequent generation of semiconductor incident light, corresponding to 100% ' reflectivity, was measured particles. The Lauda film balance was enclosed in a Plexiglas with 28 = 180O. The intensity of the reflected light was measured hood and placed on a Micro-g optical isolation table. The water by means of a Spectra Physics Model 404 silicon photocell power surface was cleaned several times by sweeping with a Teflon meter, which was connected to a chart recorder. barrier prior to monolayer formation. The subphase was deemed to be clean when the surface pressure increase was less than 0.2 Results and Discussion dyn/cm upon compression to l/zoth ofthe original area and when this surface pressure increase remained the same subsequent to Surface Area-Surface Pressure Isotherms and aging for several hours. An appropriate amount of chloroform Semiconductor Particle Generation. Surface pressure~
(32) Xu, S.;Zhao, X. K.; Fendler, J. H. Adu. Mater. 1990, 2, 183. (33) Watzke, H.; Fendler, J. H. J. Phys. Chem. 1987, 91, 854.
~
~~
~
~~
(34) Zhao, X. K.; Yuan, Y.; Fendler, J. H. J. Chem. SOC.,Chem. Commun. 1990, 1248.
522 Langmuir, Vol. 7, No. 3, 1991
R 0
75
r
30
-
I5
-
tc'
0
10
Zhao et al.
20
30
A'/molecnlc
Figure 2. Surface pressure-surface area isotherm of arachidic acid on 2.5 X M CdC12 prior (A) and 5 min subsequent (B) to the introduction of H2S over the monolayer. The area loss, Aa, corresponds to the transfer of the monolayer to a substrate at T = 11.5 mN m-l = constant.
surface area isotherms of monolayers prepared from AA, PS, DODAB, 1, and 2, showed the expected behavior (Figure 2). At low surface pressure, molecules occupied large areas lying flat on the subphase. With increasing surface pressure, they began to be squeezed together and orient their hydrophobic tails away from the surface. Transition to a compressed (solid) state manifested in a rapid rise of the surface pressure with relatively little change in the surface area. This solid state prevailed until the collapse of the monolayer. Depending on the subphase (1.0 X M CdC12, ZnClz, or CUSO~), collapsed pressures of monolayers prepared from AA, PS, 1,DODAB, and 2 were found to be 30-40, 60-70, 60-70,48-58, and 44-54 mN/m, respectively. These values were in the expected range.35 Semiconductor particle formation, initiated by the introduction of HzS into the hood covering the trough (see Figure 1 and Experimental Section), was observable in compressed and incubated monolayers prepared from negatively charged AA, PS, and 1 monolayers. In contrast, semiconductor formation could not be observed in monolayers prepared from positively charged DODAB and 2. Infusion of H2S over the aqueous subphase in the absence of monolayers led to the formation of large quantities of yellowish (CdS), whitish (ZnS),and dark brownish (CuS-Cu2S) particles that precipitated in the aqueous solution and settled at the bottom of the trough. A typical pressure-area isotherm is shown in Figure 2. Significantly, no precipitation occurred on using negatively charged monolayers. Nucleation at the surfactant headgroups was followed by two-dimensional particle growth to cover the entire monolayer. Subsequently, the particulate semiconductor film grew in thickness perpendicular to the plane of the monolayer. All of the incipient particles were supported by the monolayer (i.e., there were no colloidal semiconductor particles in the aqueous subphase). Strong electrostatic and particle-particle interactions are, presumably, responsible for maintaining the particulate semiconductor film a t the monolayer interface. Uniform coverage of the monolayer was favored by keeping the surface pressure constant at the middle of the compressed state (25 mN/m for AA and 50 mN/m for PS and I). Keeping the monolayers a t surface pressures where their fluid states predominated (less than 8 mN/m for AA, for example) led to the appearance of disjoined patches of semiconductor particles. At very low monolayer surface pressures (less than 2 mN/m for AA) infusion of HzS resulted in coloring of the subphase and eventual pre(35) Rolandi, R.; Paradiso, R.; Xu, s. Q.; Palmer, C.; Fendler, J. H. J. Am. Chem. SOC.1989, I l l , 5233.
0
I
5
10
TIME
hovil
Figure 3. Typical profiles of reflectivity changes as a function of H2S infusion time over AA monolayers (kept at 20 A/molecule) floating on aqueous 1.0 x 10-3 M ZnCl (curve with a maximum and a minimum) and 1.0 x 10-3 M CdC12. Points A and B indicate the introduction of H2Sand air, respectively, into the chamber covering the monolayer. Angle of incidence, 0 = 10'.
cipitation of large sulfide particles-a behavior quite analogous to that observed in the absence of monolayers. Very little difference was found in the quality of the particulate semiconductor film formed at different negatively charged interfaces. Slow (several hours) infusion of H2S was critical for uniform coverage of the monolayer by semiconductor particles. Reflectivity Measurements. Time-dependent growth of semiconductor particulate films at monolayer interfaces has been monitored by reflectivity measurements. Typical reflectivity profiles for ZnS and CdS, in situ generated at AA monolayers, are illustrated in Figure 3. Prior to the introduction of HzS (at time A in Figure 3), the reflectivity was equal to that observed for a pure water surface. Apparently, reflectivity due to the monolayer could not be detected under the present experimental condition. Twenty to 30 min subsequent to the introduction of H2S, a faint foglike layer became observable a t the monolayer interface. No individual grains of particles could, however, be discerned by the optical microscope using 640-fold magnification. This behavior was in sharp contrast to BLMs, where semiconductor particles could be observed relatively easily at the same magnification. With the continuous slow infusion of HzS, the thickness of the semiconductor particulate film grew, which manifested in observable increase of the intensity of the reflected light to a plateau value for CdS and CuS-Cu2S which corresponded to limiting thicknesses of 225-300 and 300-350 A, respectively. A different behavior was found in the generation of ZnS particles. Sequential color changes of white yellow gold orange pink violet blue green yellow and gold were accompanied by interference maxima and minima. Neglecting the contribution of the monolayer (having an optical thickness of 26.7 A) allowed the treatment of reflectivity as a singlefilm system. The optical thickness of the ZnS particulate semiconductor film at the interference extreme, d,, was approximated by eq 1,where N is the order of the inter-
- -- -
- - -
d, =
( N - '/z)A 212, cos 8,
ference fringe given by integers ( N = 1, 2, 3, ...) for the maxima and by half integers ( N = I / Z , 3/2, 5 / ~ ...) , for the minima, n, is the effective refractive index of the semiconductor particulate film a t wavelength A, and 8, is the
Langmuir, Vol. 7, No. 3, 1991 523
Semiconductor Particles Formed at Monolayers
Table I. Optical Parameters of Semiconductor Particulate Film in Situ Generated at AA Monolayers Brewster bulk effective vol angle refraction refraction fraction semiconductor (OB), deg indexa (nb) index (ns) (F),% ZnS 61.5 2.37 1.84 55.4 CdS 65.0 2.50 2.14 75.5 cus-CUZS 60.0 1.73 Taken from ref 35.
x 10-2
from the measured reflectivities, R36 >
t:
?. 4
5
P i:
0 ‘
10
20
30
40
INCIDENT ANGLE,
50
60
70
80
27rdlnl cos 6, 1
(6)
P2=
27rd,n, cos 6, 1
(7)
0
Figure 4. Incident-angle-dependent reflectivities of a ZnS articulate film in situ grown at an AA monolayer (kept a t 20 XI molecule) floating over 1.0 X 10-3 M ZnClz. The different curves were taken at different times subsequent to the beginning of H2S infusion. The colors and calculated thicknesses are indicated on the curves. The intersection of the curves, at 61.5’, defines the Brewster angle, OB.
angle of refraction of the semiconductor particulate film and is given by
n, cos 6, = (n; - sin2 6)’”
(2)
where 6 is the angle of incidence. Taking 6 = loo, n, = 1.84 (vide infra), optical thicknesses a t the first and second interference maxima and minima were assessed to be 864, 1727, 2591, and 3455 A, respectively. Semiconductor particulate films, formed at the monolayer interface in an optical thickness of d , 1 100 A, are quite fragile. Small vibration of the trough or insertion of a stainless steel needle caused a crack that grew to several centimeters in length. Semiconductor particulate films could not be transferred to a solid support by traditional vertical Langmuir-Blodgett lifting. They could, however, be transferred by horizontal lifting. Furthermore, freshly prepared (Le.,still wet) substrate-supported semiconductor particulate films could be refloated intact by careful horizontal reinsertion into clean water. Effective Refractive Index a n d Optical Thickness Measurements. Incident-angle-dependentreflectivity measurements provided information on the optical thicknesses of the semiconductor particulate films, in situ formed a t monolayer surfaces. The data for a typical measurement are shown in Figure 4; the incident-angledependent intensities of the light reflected from differently colored particulate semiconductor films, generated in situ at AA monolayer surfaces subsequent to increasingly longer infusion of H2S, are illustrated. Neglecting the contribution of the monolayer, the intersection of the reflectivities of the ZnS particulate films having different thicknesses defined the Brewster angle, OB (Figure 4), which allowed the assessment of the effective refractive index, n,, from
n, = tan 6~
P1=
(3)
This, in turn, permitted the numerical evaluation of the optical thicknesses of the semiconductor particulate film
Defining ro1, r12, and r23 as the Fresnel reflection coefficients (p-polarized light) at the air-monolayer, monolayerparticulate semiconductor film, and particulate semiconductor film-aqueous subphase interfaces, respectively, nl, n,, and n3 as the refractive indices of the monolayer, the particulate semiconductor film, and the aqueous subphase, dl and d , as the optical thicknesses of the monolayer and the particulate semiconductor film, and 601 as the phase changes at the air-monolayer and using nl = 1.52, n, = 1.84 ( 6 =~ 61.5’), n3 = 1.333, and d l = 26.7 A, the optical thicknesses of the six-colored ZnS particulate films were calculated to be 1163,1305,1427,1555,1664, and 1791A, respectively. Volume fractions of the films, F , were o b t a i n e d b y m e a n s of t h e M a x w e l l - G a r n e t t approximation37 2 2 2 n, -n3 = F n: - n3 n,2 - 2n32 n: + 2n,2
where n b is the refractive index of the bulk semiconductor. By use of n, = 1.84 and n b = 2.37,38the volume fraction of the ZnS semiconductor particulate film, in situ generated at AA monolayer interfaces, was calculated to be 55.4%. Similar measurements were performed on CdS and CuSCu2S particulate films and the obtained values are summarized in Table I. Decomposition of t h e Monolayer-Supported Semiconductor Particulate Films. Subsequent to exposure to the atmosphere (as indicted by time B in Figure 3), the optical thickness of monolayer-supported semiconductor particulate films slowly decreased. The rate of decrease, determined by reflectivity measurements, was found to be on the order of 10 A/h. This was not the consequence of separation of particles, since no semiconductors could be detected (by absorption spectrophotometry; detection limit = lo4 M) in samples withdrawn from the subphase during the decrease of the thickness of the monolayer supported film. Spontaneous dissolution of the ultra(36) Zhao, X. K.; Xu, s.;Fendler, J. H. J.Phys. Chem. 1990,94,2573. (37) Garnett, M. S. C. Philos. Trans. R. SOC.London 1904,203,385. (38)Landolt-Bernstein New Series; Springer: Berlin, 1983;Vol. 111, 17f, p 45. Handbook of Chemistry and Physics, 63rd ed.; CRC Press: Boca Raton, FL, 1982/1983;p B-165.
Zhao et al.
524 Langmuir, Vol. 7, No. 3, 1991
small semiconductor particles (9) (MSIcrysMline M ~aq++ s2-*q is likely to be responsible for the decrease in the amount of semiconductor particulates supported by the monolayer. In the presence of air, semiconductor particle decomposition is also facilitated by +
-
(10) (MS)crysblline + 202 M2++ Semiconductor Particulate Film Formation at the Interface of Polymerized Monolayers. CdS particles have also been formed a t the surfaces of polymerized monolayers, prepared from l.35 Monolayers were kept at a compression of 40 A2 per molecule over 1.0 X M aqueous CdC12. Polymerization was affected by irradiation by a 4.0-W lamp placed 2-3 cm above the monolayer surface for 15-20 min.35 The light intensity was determined to be 10 f 2 kW/cm2 at 250 nm by a Model 2332 UV optical power meter (Mimir Instruments Corp.). Infusion of H2S was initiated subsequent to 10 min of incubation of the polymerized monolayer. Semiconductor particle formation was markedly different from that observed in monolayers prepared from nonpolymerized 1. Monitoring the reflected light during CdS particle formation revealed sequential color changes of light yellow pink blue green yellow pink. The limiting intensity of the reflected light corresponded to an optical thickness of 3000 A for the CdS particulate film, in situ grown a t the polymerized monolayer interface. This value is substantially greater than that observed at the interfaces of nonpolymerized monolayers (225-300 A). Polymerization of 1monolayers results in pulling the surfactant tails together and, thereby, causing pinholes a t the water surface.35 It is tempting to speculate that the accelerated passage of H2S through these pinholes is primarily responsible for the formation of larger semiconductor particles in greater thicknesses. In Situ Defect Formation in the MonolayerSupported Semiconductor Particulate Films. Exchange of the aqueous subphase allowed the introduction of different species into the monolayer-supported particulate semiconductor. Thus, for example, introduction of CuSO4 M) into a subphase coated by a 225 A thick CdS particulate film on a 1 monolayer resulted in a gradual change of the color from light yellow to greenblue over an 8-h period. Copper ions apparently incorporated into the particulate film and partially exchanged with cadmium ions. The process was accompanied only by a small increase in the optical thickness (to 340 A). Proposed Model for the Generation of Semiconductor Particulate Films at Monolayer Surfaces. The presence of well-packed, negatively charged monolayers is an essential requirement for the in situ formation of semiconductor particles. Negative charges, available in high density a t the surfactant headgroup-aqueous subphase interface, effectively concentrate cations from the aqueous subphase. Thus, while the stoichiometric metal M, it can be as high as 5.0 ion concentration is 1.0 X M at the monolayer interfa~e.~~,*O Slow infusion of H2S has resulted in the formation of covalent metal-sulfide bonds at a large number of sites at the monolayer interface. Not more than a few molecules have constituted the nascent metal sulfide nuclei. They have rapidly grown to aligned microclusters. With continued infusion of H2S, the microclusters have coalesced, predominantly two
-
-
- - -
(39)Beth, J. J.; Pethica, B. A. Trans. Faraday SOC.1956, 52, 1581. (40)Yazdanian, M.; Yu, H.; Zografi, G. Langmuir 1990, 6, 1093.
+++++
+++
+++++
a
++++++
+++++ ++++ b
+ + +++++ ++++ + + + C
Figure 5. Proposed schematics for the initial (a) and subsequent (b and c) growth of a monolayer-supported,porous, semiconductor
particulate film.
dimensionally, at the monolayer interface to an interconnected semiconductor particulate film whose thickness initially ranged between 50 and 100 A. It was only a t this point that particle formation at the monolayer interface had become optically detectable. Sulfide and/or bisulfide ions, produced in the aqueous subphase from H2S, have adsorbed onto the surface of the initially formed semiconductor particulate film and attracted metal cations in high local concentrations which, in turn, seeded the formation of a new set of clusters and semiconductor particles. These particles have loosely interconnected and formed additional layers of porous semiconductor particulate films (see Figure 5). The thickness of monolayersupported semiconductor particulate films has increased to a plateau value (Figure 3) beyond which no additional semiconductor particle formation could be observed. The obtainable optical thicknesses have varied from system to system and were found to depend on the volume fraction of the given semiconductor particulate film (see Table I). The smaller the volume fraction of the monolayersupported semiconductor particulate film, the thicker it could grow. This behavior is rationalized by considering the rate and the extent to which the infusing H2S can penetrate the growing semiconductor particles. The more porous the particulate film (the smaller its volume fraction), the greater the depth to which H2S is able to penetrate. Very dense semiconductor particulate films (high volume fraction) promptly block the penetration of H2S.
Acknowledgment. Support of this work by a grant from the National Science Foundation is gratefully acknowledged. Registry No. 1,116437-66-2; 1 (homopolymer), 116437-67-3; 2,96478-22-7; DODAB,107-64-2; AA, 506-30-9; CdS, 1306-23-6; ZnS,1314-98-3; Cul& 121857-74-7; Cu, 7440-50-8.
