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Langmuir 1997, 13, 3198-3201
Cadmium Sulfide Nanoparticles in Langmuir-Blodgett Films of Calixarenes Alexei V. Nabok and Tim Richardson* Centre for Molecular Materials and Department of Physics, University of Sheffield, Hicks Building, Hounsfield Road, Sheffield S3 7RH, U.K.
Frank Davis and Charles J. M. Stirling Centre for Molecular Materials and Department of Chemistry, University of Sheffield, Dainton Building, Brook Hill, Sheffield S3 7HF, U.K. Received December 19, 1996X CdS nanoparticles have been formed within Langmuir-Blodgett (LB) films of cadmium salts of calix[8]and calix[4]arene by reaction with hydrogen sulfide gas. The presence of CdS nanoparticles in the LB film matrices was checked by measurement of UV-vis absorption spectra, which allows an estimation of the size of the particles. It was shown that smaller particles (1.5 ( 0.3 nm) are formed in calixarene LB films than in Cd stearate films (used for comparison). The particle size does not depend on the type of calixarene or the number of LB layers but increases with increasing pH of the subphase. The formation of CdS was directly confirmed with XPS.
Introduction Observations of the blue-shift of the adsorption edge in chalcogenide glasses were reported by Ekimov and Onushchenko.1,2 This phenomenon, which depends on size-quantization of the energy spectra of group II-VI semiconductor nanoparticles embedded in glass matrices, has been accounted for in the literature.3,4 Subsequently, these ideas have been used for calculation of the size of nanoparticles from their UV-visible absorption edge. Formation of metal sulfide inclusions in LangmuirBlodgett (LB) films of fatty acid salts by chemical reduction with H2S was first reported by Barraud and co-workers.4-6 The initial idea was to create 2D semiconductor layers in LB films. These layers, however, showed unstable behavior due to segregation of metal sulfide particles in the film. 3D clusters, formed in LB film as a result, demonstrated quantum-sized effects in optical absorption.7,8 The technology of ordered organic films, particularly LB films, gives many opportunities to control the size of nanoparticles and form ordered organic-inorganic systems. This direction of molecular engineering consequently has been developed by many researchers9-17 with X
the use of novel experimental methods (STM,12 BAM13,17) and molecular techniques (self-assembly,12 polyelectrolytes,14-16 and colloid particles17,18 ). Increasing interest in such quantum-size II-VI semiconductor systems has been stimulated by their possible applications in electronics. For instance, single-electron tunneling through CdS nanoparticles in fatty acid LB film at room temperature has been recently demonstrated.19 In addition, formation and study of very small particles consisting of a few tens of atoms are of significant interest for fundamental physics. The study of the transformation single molecule (atom) f molecular (atomic) cluster f solid state could provide an answer to the philosophical question: “How many atoms are needed to create the solid state?”. The main goal of the present work was the formation and characterization of CdS nanoparticles in calixarene Cd salt LB films using reaction with H2S. We supposed that calixarene cavitand molecules might be able to fix CdS clusters and restrict the segregation/aggregation process. Smaller CdS nanoparticles (with lower variation of particle size) might be formed by comparison with those reported for fatty acid LB films.8,10,11
Abstract published in Advance ACS Abstracts, May 15, 1997.
(1) Ekimov, A. I.; Onushchenko, A. A. Pis’ma Zh. Eksp. Teor. Fiz. 1981, 34, 363 (in Russian). (2) Brus, L. E. J. Chem. Phys. 1983, 79, 5566. (3) Ekimov, A. I.; Efros, A. L.; Onushchenko, A. A. Solid State Commun. 1985 56, 921. (4) Ruaudel-Teixier, A.; Leloup, J.; Barraud, A. Mol. Cryst. Liq. Cryst. 1986, 134, 347. (5) Zylberajch, G.; Ruaudel-Teixier, A.; Barraud, A. Synth. Met. 1988, 27, B609. (6) Zylberajch, G.; Ruaudel-Teixier, A.; Barraud, A. Thin Solid Films 1989, 178, 535. (7) Weller, H.; Schmidt, H. M.; Koch, U. Chem. Phys. Lett. 1986, 124, 557. (8) Smotkin, E. S.; Lee, C.; Bard, A. J. Chem. Phys. Lett. 1988, 152, 265. (9) Leloup, J.; Ruaudel-Teixier, A.; Barraud, A. Thin Solid Films 1992, 210/211, 407. (10) Peng, X. G.; Lu, R.; Zhao, Y. Y.; Qu, L. H.; Chen, H. Y.; Li, T. J. J. Phys. Chem. 1994, 98, 7052. (11) Dhanabalan,A.; Kudroli, H.; Major, S. S.; Talwar, S. S. Solid State Commun. 1996, 99, 859. (12) Ogava, S.; Fan, F. R. F.; Bard, A. J. J. Phys. Chem 1995, 99, 11182. (13) Kang, Y. S.; Risbud, S. ; Rabolt, J.; Stroeve, P. Langmuir 1996, 12, 4345.
S0743-7463(96)02115-4 CCC: $14.00
Experimental Section tert-Octylcalix[8]arene and tert-octylcalix[4]arene were synthesized from formaldehyde and tert-octylphenol as previously published.20 They were then converted into carboxylic acids using the method of Arduini et al.21 where the phenols were reacted with ethyl bromoacetate to form esters22 (IR carbonyl peak at (14) Gao, M.; Yang, Y.; Yang, B.; Shen, J. J. Chem. Soc., Chem. Commun. 1994, 2229. (15) Gao, M.; Yang,Y.; Yang, B.; Bian, F.; Shen, J. J. Chem. Soc., Chem. Commun. 1994, 2777. (16) Dekany, I.; Nagy, L.; Turi, L.; Kiraly, Z.; Kotov, N. A.; Fendler, J. H. Langmuir 1996, 12, 3709. (17) Kotov, N. A.; Meldrum, F. C.; Wu, C.; Fendler, J. H. J. Phys. Chem. 1994, 98, 2735. (18) Fendler, J. H.; Meldrum, F. C. Adv. Mater. 1995, 7, 607. (19) Facci, P.; Erokhim, V.; Carrara, S.; Nicolini, C. Proc. Natl. Acad. Sci. U.S.A. 1996, 93, 10556. (20) Cornforth, J. W.; D’Arcy Hart, P.; Nicholls, G. A.; Rees, R. J. W.; Stock, J. A. Br. J. Pharmacol. 1955, 10, 73. (21) Arduini, A.; Pchini, A.; Reverberi, S.; Ungaro, R. J. Chem. Soc., Chem. Commun. 1984, 981.
© 1997 American Chemical Society
Cadmium Sulfide Nanoparticles
Langmuir, Vol. 13, No. 12, 1997 3199 of 70° of the X-ray beam corresponds to the mean free path of an electron of 3.38 nm.
Results and Discussion Theoretical Background. CdS particles, having an energy band gap (Eg) of 2.4 eV in the solid state, incorporated into an organic insulating matrix (Eg ) 6-8 eV) can be described as 3D-quantum wells. The limitation of the motion of electrons, holes, or excitons in such a well leads to quantization of their kinetic energy. Two kinds of quantization can take place depending on the size3 of the particle (a). If the size of the particles is larger than the exciton Bohr radius (a . aB), quantization of the exciton as a whole should occur.3 The position of the first exciton absorption band (n ) 1) can be derived as Figure 1. Π-A isotherm of 1a (curve 1) and 1b (curve 2) monolayers on a water subphase containing 5 × 10-4 M CdCl2. 1760 cm-1 and no O-H in the FTIR spectrum) and then hydrolyzed to form the acid (IR carbonyl peak at 1740 cm-1, loss of ethyl group confirmed by 1H NMR). tert-Octylcalix[8]- (and [4]-) arene acids, (1a and 1b) were used with stearic acid for comparison.
hν01 ) Eg - Eex +
(22) Richardson, T.; Greenwood, M. B.; Davis, F.; Stirling, C. J. M. Langmuir 1995, 11, 4623.
(1)
where Eex is the exciton binding energy, M ) me + mh is the exciton mass and ν01 is the frequency associated with the transition. In the case of a , aB electrons and holes will quantize separately. By use of simple parabolic dispersion of electrons and holes, the following expression for size-quantization energy has been obtained:3 eh ) El,n
Monolayers were formed by spreading 0.5 mg/mL solutions of compounds in chloroform on the surface of pure (Elga UHP) water containing 5 × 10-4 M CdCl2. Cd salt formation was increased by increasing the pH. The nominal pH of the subphase was 5.45 and was increased to 7.5 by adding ammonia to the subphase. A constant perimeter Langmuir trough, equipped with a Wilhelmy balance, was used for LB film production and for measurements of Π-A isotherms. Π-A isotherms of 1a and 1b are shown in Figure 1. Areas per molecule of 3.0 and 1.8 nm2 for 1a and 1b, respectively, are in good agreement with estimated cross-sectional areas (3.0 and 1.5 nm2, respectively) of these molecules confirmed using CPK molecular models. The reflection ∼6-8 mN m-1 observed for the ocatmer may arise as a result of molecular shape transformation during monolayer compression. Calix[8]arenes are known to be more flexible than their tetramer analogues. The collapse pressure, and therefore the stability, of the monolayer of 1b is less than for 1a. Y-type (even number) LB films were transferred to solid hydrophobic substrates at a surface pressure of 25 mN/m (28 mN/m was chosen for stearic acid) and a deposition speed of 5 mm/min for both down- and upstroke. Glass and quartz slides and silicon wafers were cleaned in chromic acid to remove organic contamination and then rinsed many times in pure water with ultrasonic action. The substrates were made hydrophobic by treatment overnight with hexamethyldisilasane ((CH3)3Si-NH-Si(CH3)3) vapor. All transfer ratios were found to be in the range 0.9-1.1. The formation of CdS in the LB films was achieved by exposure to H2S gas overnight in sealed jars. Absorption spectra of LB films transferred to quartz (and glass) slides were measured before and after exposure to H2S using a Unicam UV-vis spectrometer UV4. The spectra were then subtracted and transformed to the form of (k* - k)d/N vs quantum energy [eV], where k and k* are absorption coefficients of the LB films before and after H2S treatment, respectively, d is the film thickness (estimated to be 1.4 nm from CPK models), and N is the number of LB layers. To confirm formation of CdS, X-ray photoelectron spectra of LB films (10 layers) transferred to silicon wafers were measured. Spectra were measured before and after H2S treatment using a VG CLAM 2 photoelectron spectrometer with an Mg KR X-ray source (100 W, pass energy of 100 eV). The angle of incidence
h2 8Ma2
h2Jl,n2 8π2meha2
(2)
where Jl,n is a series of Bessel function roots, l ) 1, 2, 3, ... is the orbital quantum number and the order of the Bessel function. n ) 1, 2, 3, ... is the main quantum number and serial number of the root of the Bessel function. Both light absorption and luminescence correspond to electron transitions between levels of size-quantization for elece h ) and holes (El,n ) with selection rules l, n ) trons (El,n const. The position of the absorption band then can be written:23
hνl,n ) Eg +
m e mh h2 2 J where µ ) (3) l,n me + mh 8π2µa2
The absorption spectra of nanoparticles should consist of a series of separate lines. The dispersion of the size of the particles, however, and electron-phonon interactions lead to broadening of the bands, and a less defined spectrum is obtained as a result. Optical Absorption Spectra. Subtracted UV-vis absorption spectra of 1a Cd salt LB films are shown in Figure 2. All spectra show three well-resolved bands below 380 nm, related to the presence of CdS nanoparticles in the film. It should be noted that the absorption edge of CdS particles formed in fatty acids LB films are usually observed in the range 450-500 nm,8,10,11 and the edge of the bulk CdS material is near 590 nm.24 These spectra presented with an energy scale and normalized (divided) for the number of LB layers are almost coincidental, especially in the energy range 2-4.5 eV, as shown in Figure 3. The noisy spectra of bilayers and some shift of the base line is caused by the very low signal intensity near the limit of the sensitivity of the spectrometer. The same behavior, i.e., increase of absorption of the light with quantum energy above 3.2 eV and independence of the spectra on the number of LB layers, was observed for LB films of 1a and 1b prepared at different pH. It is demonstrated very well by normalized spectra of LB films (23) Kulish, N. R.; Kunetz, V. P.; Lisitsa, M. P. Ukr. Fiz. Zh. (Russ. Ed.) 1990, 35, 1817. (24) Ray, B. II-VI Compounds; Pergamon Press: Oxford, 1969.
3200 Langmuir, Vol. 13, No. 12, 1997
Figure 2. UV-vis absorption spectra of 1a Cd salt LB films (pH 6.5) obtained by subtraction of corresponding spectra after and before exposure with H2S. The number of LB layers is indicated near respective curves.
Nabok et al.
Figure 5. Part of the spectra showing the edge of absorption: (curve 1) 1a Cd salt LB film (16 layers, pH 5.5); (curve 2) 1b Cd salt LB film (16 layers, pH 5.5); (curve 3) cadmium stearate LB film (16 layers, pH 5.5). Table 1. Values of the Absorption Edge Energy and the Size of CdS Nanoparticles for 1a, 1b, and Cadmium Stearate LB Films Prepared at Different pH compounds 1a 1b cadmium stearate
pH
hνedge (eV)
∆E (eV)
a (nm)
5.5 6.5 7.5 5.5 6.5 5.5 6.5
3.35 3.10 2.53 3.33 2.90 2.70 2.45
0.925 0.675 0.105 0.905 0.475 0.275 0.025
1.80 2.11 5.36 1.82 2.52 3.31 11.0
The size of CdS nanoparticles can be estimated by two methods. The first is based on the dependence of the absorption edge on the size of the particle according to eq 1. As for the bulk CdS, the absorption coefficient near the edge can be derived as Figure 3. Difference spectra presented on an energy scale and normalized by the number of LB layers.
Figure 4. Normalized spectra of LB films (16 layers, pH 5.5) of 1a (curve 1) and 1b (curve 2) on quartz and cadmium stearate (curve 3) on glass.
(16 layers, pH 5.5) of Cd salts of both 1a and 1b, which are shown in Figure 4. The spectrum of 16 LB layers of cadmium stearate is also shown for comparison. The calixarene compounds have nearly the same spectra (i.e., position of absorption edge and all absorption maxima.) By contrast, the absorption spectrum of the LB film is smoother, with the absorption edge shifted to lower energy. It should be noted that such well-resolved peaks, corresponding to electron transitions between levels of sizequantization, are usually observed only at low temperatures.3
k ) A(hν - Eg)1/2 where A is a constant. Data presented in the form of (k* - k)2 vs hν should show therefore linear dependence. Extrapolation of the linear section to the x-axis allows the absorption edge to be defined most precisely. The (k* k)2 spectra of 1a and 1b LB films at pH 5.5, as well as for a cadmium stearate LB film prepared at the same pH, are shown in Figure 5. The absorption edge of both calixarene films is nearly the same (about 3.3 eV), while cadmium stearate gives a lower threshold value (2.7 eV). This last value is very similar to those obtained earlier for group II-VI semiconductor nanoparticles in fatty acid LB films8,10,11 and shows that the size of CdS particles in a calixarene matrix is substantially less than in a stearic acid matrix. Energies of the absorption edge and values of the size of the particles (calculated by eq 1) for all LB films prepared at various pH are collected in Table 1. Values of Eg ) 2.425 eV and m ) 0.125m0 (me ) 0.153m0, mh ) 0.7m0) are used.24 The results show that (i) the size of CdS nanoparticles does not depend on the number of LB layers, and therefore, CdS clusters form in each bilayer of LB films, (ii) particles formed in LB films of 1a and 1b at a nominal pH of 5.5 have similar size (about 1.8 nm), which is much less then those formed in stearic acid LB films (3.3 nm), and (iii) the increase of pH to 6.5 leads to an increase of particle size (2.1 and 2.5 nm for 1a and 1b, respectively). This is caused by the shift in equilibrium toward the formation of Cd salt as a result of the incorporation of more Cd into the floating monolayer. The same pH causes a dramatic increase in the size of the CdS aggregates (up to 11 nm) in cadmium stearate films. Further increase of pH to 7.5 leads to the formation of
Cadmium Sulfide Nanoparticles
Langmuir, Vol. 13, No. 12, 1997 3201 Table 2. XPS Data of LB Films (10 layers) of 1a, 1b, and Cadmium Stearate Prepared at Different pH
C/Cd ratio Cd per molecule S/Cd ratio chemical shift of Cd3D line (eV)
cadmium stearate
1a Cd salt
1b Cd salt
pH 5.5
pH 6.5
pH 5.5
pH 6.5
pH 5.5
pH 6.5
47.74 2.85 1.10 0.34
40.14 3.39 0.76 0.27
66.84 1.02 0.71 0.21
47.43 1.43 0.80 0.49
64.25 0.28 0.78 0.06
33.38 0.54 0.90 2.31
of the Cd3D doublet, are summarized in Table 2. LB films of 1a, prepared at pH 4.5, contain 2.85 Cd atoms per calixarene unit. This number increases to 3.4 at pH 6.5. LB films of 1b contain just one Cd atom per molecule at pH 5.5, this ratio increasing to 1.43 at pH 6.5. This confirms that there is more effective Cd salt formation in calixarene LB films at higher pH. This reaction, however, is not complete because the ratios obtained are less than stoichiometric (4 and 2 for 1a and 1b, respectively). Cd per molecule ratios of 0.54, obtained for the stearic acid (St) films at pH 6.5, correspond very well to the normal stoichiometric formula for cadmium stearate (CdSt2). An average S/Cd ratio of 0.84 ( 0.1 demonstrates almost complete conversion of Cd salt into CdS in the film. The chemical shift obtained corresponds well to the value of 0.25-0.5 eV reported for CdS in comparison with Cd itself.25 The much larger shift observed in the LB films of cadmium stearate at high pH can be explained by hydrolysis of Cd in the floating monolayer following the formation of cadmium hydrosulfide in the LB films. XPS measurements, therefore, directly confirm the formation of a CdS phase in calixarene LB films. Figure 6. Parts of X-ray photoelectron spectra of 1a Cd salt LB film (10 layers, pH 6.5): (a) Cd3D doublet before (curve 1) and after (curve 2) H2S exposure; (b) S2P peak.
large particles even in LB films of 1a. This can be explained by greater ionization of acid groups in the floating monolayers, causing the incorporation of a large amount of Cd in the monolayer and further precipitation. The size of CdS nanoparticles in calixarene matrices can be also estimated from the position of each band (3.7, 5.0 and 6.2 eV) in the spectra presented in Figure 4 using eq 3 and the values of Eg and µ used in previous calculations. This method gives particle radius values of 1.52, 1.54, and 1.45 nm, respectively. Taking into account that the effective mass of electrons and holes can deviate from the values mentioned above, especially at higher energy on the edges of respective zones, the conformity between values obtained is noticeable. The difference between two values (1.8 and 1.5 nm), obtained from the absorption edge and peaks position, respectively, may be caused by broadening of absorption bands due to dispersion of the dimensions of the particles. Ignoring other reasons for broadening, e.g., electronphonon interactions, one can estimate the upper limit of dispersion at 0.3 nm. XPS Results. XPS study of LB films of Cd salts of 1a, 1b, and stearic acid demonstrated the following results: (i) the presence of Cd3D doublet in the range of binding energy of 400-420 eV for all samples; (ii) S2P peak at 160-165 eV for all samples treated with H2S; (iii) the chemical shift of Cd3D doublet for samples that were exposed to H2S in comparison to unexposed ones. The C1S peak was used as a reference for calculation of the chemical shift. Typical results for LB films of the Cd salt of 1a are shown in Figure 6. Calculated atomic ratios of C/Cd, S/Cd, and Cd per molecule, as well as values of chemical shift
Conclusions The results obtained demonstrate that CdS nanoparticles sized 1.5 ( 0.3 nm are formed in the matrix of calixarene LB films after exposure to H2S. The particle size does not depend on the type of calixarene or the number of LB layers but increases with increasing pH of the subphase. The particle size is noticeably less than those observed in LB films of cadmium stearate. To explain this, it is useful to calculate the cadmium concentration in the LB films. By use of values of the area per molecule (Figure 1) and XPS data of the Cd/ molecule ratio, the values of 1.14 × 1014, 6 × 1013, and 1.4 × 1014 cm-2 have been found for 1a, 1b, and cadmium stearate LB films, respectively. We do not see any correlation between concentrations obtained and particle size in the LB films. Lateral motion of CdS molecules and small aggregates in the bilayer of calixarene LB films may be limited by their capture in calixarene cavities. Aggregation of CdS in calixarene matrices is therefore less pronounced than in fatty acid films, and as a result, much smaller CdS particles are formed. Small CdS particles containing only several tens of Cd atoms can be successfully described by effective masses typical of bulk material and therefore demonstrate their solid state behavior. Our future work will concentrate on X-ray and ellipsometric characterization of these films in order to probe thickness and structural changes occurring after the formation of the CdS nanoparticles. Additionally, direct observation of nanoparticles using scanning probe microscopies will be attempted. LA962115F (25) Physical Electronics. Handbook of X-ray Photoelectron Spectroscopy; Muilenberg, G. E., Ed.; Perkin-Elmer Corp.: Eden Prairie, MN, 1979.
