766
Langmuir 2003, 19, 766-771
Microstructure Origin of the Conductivity Differences in Aggregated CuS Films of Different Thickness Svetlana Erokhina,* Victor Erokhin, and Claudio Nicolini DISTBIMO, University of Genoa, Corso Europa 30, 16132 Genoa, Italy, Polo Nazionale Bioelettronica, Corso Europa 30, 16132 Genoa, Italy
Francesca Sbrana, Davide Ricci, and Ermanno di Zitti DIBE, University of Genoa, Via Opera Pia 11a, 16145 Genoa, Italy Received August 20, 2002. In Final Form: October 29, 2002 The structure of thin, aggregated layers of CuS nanoparticles, grown in Langmuir-Blodgett film precursors, was investigated with atomic force microscopy along with the study of their electrical conductivity. Very thin layers revealed an essentially insulating behavior. These layers were composed of isolated particle aggregates that had a mean thickness corresponding to the average particle diameter. The increase of the film thickness resulted in the formation of conducting pathways formed by the aggregates in the layer plane. Such samples revealed an increased conductivity. Finally, when the thickness of the initial precursor LB layers was more than 25 bilayers, the resulting aggregated films were uniform and their electrical conductivity was high.
Introduction It is a well-known fact that for very thin metal films it is possible to observe a nonmetallic temperature dependence of the conductivity similar to that found in semiconductors, i.e., an increase of conductivity with temperature.1 This phenomenon originates in the noncontinuous (“island”-like) nature of the film. In fact, the initial stages of the film growth take place at different separated points of the substrate surface.2 If the total thickness of the film is low, the layer will contain isolated metal “islands” separated by insulating gaps. Thermal activation will therefore facilitate the carrier (electrons) transfer from one “island” to the other. In the case of evaporated and epitaxially grown layers, the film formation processes have been well-studied and numerous publications explaining the growth mechanisms and resulting layer properties are available.3-5 In the recent past an alternative technique for the formation of thin inorganic films by the aggregation of nanoparticles grown within organic Langmuir-Blodgett (LB) films has been developed. Usually, these particles are formed by exposing fatty acid salt LB films containing bivalent metals to a hydrogen sulfide atmosphere,6-10 but * Corresponding author. Tel.: +39 010 3538541. Fax: +39 010 3537429. E-mail:
[email protected]. (1) Glocker, D. A.; Shah, S. I. (Eds.) Handbook of Thin Film Process Technology; IOP Publishing: 1995. (2) Markov, I. V. Crystal Growth for Beginners: Fundamentals of Nucleation, Crystal Growth, and Epitaxy; World Scientific: Singapore, 1980. (3) Lewis, B.; Anderson, J. C. Nucleation and Growth of Thin Films; Academic Press: New York, 1978. (4) Moss, T. S. (Ed.) Handbook on Semiconductors; North-Holland, Amsterdam, 1980. (5) Seshan K. Handbook of Thin Film Deposition Processes and Techniques; William Andrew Publishing: New York, 2001. (6) Smotkin, E. S.; Lee, C.; Bard, A. J.; Campion, A.; Fox, M. A.; Mallouk, T. E.; Webber, S. I.; White, J. M. Chem. Phys. Lett. 1988, 152, 265. (7) Zylberajch, C.; Ruaudel-Teixier, A.; Barraud, A. Thin Solid Films 1989, 179, 9. (8) Erokhin, V.; Feigin, L.; Ivakin, G.; Klechkovskaya, V.; Lvov, Y.; Stiopina, N Makromol. Chem. Makromol. Symp. 1991, 46, 359-363.
the process is also successful using other metal-containing compounds.11-19 After formation during the hydrogen sulfide treatment, the particles can be aggregated in thin layers, selectively removing the precursor organic molecules by washing the sample in organic solvents, such as benzene or chloroform.20 The use of the LB technique for this purpose is preferable with respect to other methods, such as colloidal particles precipitation, as it allows a very high level of control on both the total amount and surface density of metal atoms in the precursor layer and, consequently, allows a similar degree of control on the thickness of the resultant aggregated layer. The thickness resolution of this method is, for the aforesaid reasons, very high: the average thickness of a single aggregated layer, obtained from one precursor bilayer, was found to be 0.7 nm.20 Nanoparticles of different materials with a wide variety of physical properties have been fabricated using LB precursors.21-28 The ability to induce aggregation of these (9) Zhao, X. K.; Yang, J.; McCormick, L. D., Fendler, J. H. J. Phys. Chem. 1992, 96, 9933-9939. (10) Furlong, D. N.; Urquhart, R. S.; Mansur, H.; Grieser, F.; Tanaka, K.; Okahata, Y. Langmuir 1994, 10, 899-904. (11) Kimizuka, N.; Mioshi, T.; Ichinose, I.; Kunitake T. Chem. Lett. 1991, 2039-2042. (12) Schmitt, H. J.; Zhu, R.; Wie, Y.; Yuan, C.; Xiao, S.; Lu, Z. Solid State Commun. 1992, 84, 449-451. (13) Wang, J. Y.; Uphaus, R. A.; Ameenuddin, S.; Rintoul, D. A. Thin Solid Films 1994, 242, 127-131. (14) Yang, J. P.; Qadri, S. B.; Ratna, B. R. J. Phys. Chem. 1996, 100, 17255-17259. (15) Nabok, A. V.; Richardson, T.; Davis, F.; Stirling, C. J. M. Langmuir 1997, 13, 3198-3201. (16) Li, L. S.; Jin, J.; Tian, Y. Q., Jiang, S. M.; Zhao, Y. Y.; Li, T. J.; Du, Z. L.; Ma, G. H.; Zheng, N. Supramol. Sci. 1998, 5, 475-478. (17) Guo, S.; Konopny, L.; Popovitz-Biro, R.; Cohen, H.; Porteanu, H.; Lifshitz, E.; Lahav, M J. J. Am. Chem. Soc. 1999, 121, 9589-9598. (18) Nabok, A. V.; Ray, A. K.; Hassan, A. K. J. Appl. Phys. 2000, 88, 1333-1338. (19) Vidya, V.; Kumar, N. P.; Narang, S. N.; Major, S.; Vitta, S.; Talwar, S. S.; Dubcek, P.; Amenitsch, H.; Bernstorff, S. Colloids Surf. A 2002, 198-200, 67-74. (20) Facci, P.; Erokhin, V.; Tronin, A.; Nicolini, C. J. Phs. Chem. 1994, 98, 13323-13327. (21) Peng, X.; Guan, S.; Chai, X.; Jiang, Y.; Li, T. J. Phys. Chem. 1992, 96, 3170-3174.
10.1021/la026433s CCC: $25.00 © 2003 American Chemical Society Published on Web 12/24/2002
Conductivity Differences in Aggregated CuS Films
particles in layers allows one to form thin inorganic films with different conductivity, band-gap, optical properties, etc. The aggregated film formation technique has also been successfully applied for the formation of inorganic superlatticessheterostructures containing alternating ultrathin layers of different materials.29 Recently, a method has been developed for the in-plane patterning of these aggregated layers by the exposure of selected precursor layer areas to an electron beam.30 Precursor molecules in the exposed film areas became cross-linked and insoluble in organic solvents. Thus, during the following aggregation process, these areas remained electrically insulating, while the conductivity in the aggregated areas (not exposed to the electron beam) was determined by the nature of the grown particles. The range of possibilities that have been mentioned above, together with the fact that it does not require complicated equipment and special environmental conditions, allow one to consider the aggregated layers formation technique as a perspective alternative method to ultrathin inorganic film fabrication. To this date, however, the aggregated film formation process and the structure of the resultant layers have not yet been studied in detail, whereas such knowledge seems very important for predicting the electrical properties of these films and for designing functional elements based on them. One example can illustrate the influence of the film thickness and, therefore, of the film structure on its properties in such aggregated layers. The specific electrical conductivity of CuS aggregated layers was found to increase by several orders of magnitude when the film thickness was increased.31 Conductivity saturation values (more than 1 S/cm) were obtained when the thickness of the aggregated layer was more than 25 nm. Such behavior cannot be explained only by the increase in thickness: in fact, it indicates more likely a difference in film organization at successive stages of the layer thickness growth. Therefore, it can be stated that the properties of thin aggregated films also depend strongly on the layer structure. The aim of this work is to investigate the structure of aggregated films of different thickness, along with their electrical properties, with the purpose of determining a structure-properties relationship. CuS aggregated layers were chosen as the object of the present study, as they provide the best conductivity values among all investigated aggregated layers of different materials,31 hence allowing one to perform electrical measurements even for very thin layers, maintaining a high signal-to-noise ratio level. Atomic force microscopy (AFM) was chosen as the tool for the structural analysis, as it is a commonly used technique for the investigation of thin films32-34 and particle arrays.35,36 (22) Leloup, J.; Ruaudel-Texier, A.; Barraud, A Thin Solid Films 1992, 210/211, 407-409. (23) Fendler, J. H. Mater. Res. Soc. Symp. Proc. 1992, 255, 355-366. (24) Elliot, D. J.; Furlong, D. N.; Grieser, F. Colloids Surf. A 1998, 141, 9-17. (25) Sun, J.; Hao, E.; Sun, Y.; Zhang, X.; Yang, B.; Zou, S.; Shen, J.; Wang, S. Thin Solid Films 1998, 327-329, 528-531. (26) Elliot, D. J.; Furlong, D. N.; Grieser, F. Colloids Surf. A 1999, 155, 101-110. (27) Dutta, A. K.; Ho, T.; Zhang, L.; Stroeve, P. Chem. Mater. 2000, 12, 3236. (28) Hemakanthi, G.; Dhathathreyan, A. Chem. Phys. Lett. 2001, 334, 245-249. (29) Erokhin, V.; Facci, P.; Gobbi, L.; Dante, S.; Rustichelli, F.; Nicolini, C. Thin Solid Films 1998, 327-329, 503-505. (30) Erokhin, V.; Troitsky, V.; Erokhina, S.; Mascetti, G.; Nicolini, C. Langmuir 2002, 18, 3185-3190. (31) Erokhina, S.; Erokhin, V.; Nicolini, C. Colloids Surf. A 2002, 198-200, 645-650.
Langmuir, Vol. 19, No. 3, 2003 767
Figure 1. Dependence of the specific resistance of aggregated CuS layers on the number of bilayers of copper arachidate LB precursor.
Materials and Methods Arachidic acid and copper sulfate (CuSO4 5H2O) were purchased from Sigma. Milli-Q purified water with 18.2 MΩ·cm resistivity was used in all the experiments. Arachidic acid monolayers were spread on the water subphase containing 10-4 M of CuSO4 5H2O. LB films were deposited on glass slides at the surface pressure of 28.5 mN/m by the vertical lift dipping technique using a MDT Langmuir trough (NT-MDT Co., Moscow, Russia).37 Particle formation was performed by exposing the samples to a hydrogen sulfide atmosphere produced by the reaction of FeS with diluted H2SO4. The exposition time was varied according to the film thickness from 1 h (for one bilayer film) to 12 h (for the 25 bilayer film).20,30 Voltage/current (V-I) characteristics of the films were measured with a Keithley model 6517 electrometer (Keithley Instruments, Inc.) driven by a personal computer. Contacts to the layers were made through indium patches. Nominal specific resistance of the samples was calculated using the nominal layer thickness values obtained from ellipsometry data.20 AFM measurements were performed in intermittent contact mode using a PSI Autoprobe CP microscope (Thermomicroscopes, Sunnivale, CA), equipped with 100 and 5 µm scanners and polysilicon cantilevers (Ultralevers, Thermomicroscopes, Sunnivale, CA). Previous to use, both scanners where calibrated in x,y,z using reference standard gratings (VLSI Standards).
Results and Discussion The dependence of specific resistivity of the CuS aggregated films on the number of initial precursor LB bilayers (and, therefore, aggregated film thickness) is presented in Figure 1. Measurements were performed on 20 different samples of each thickness. For low thickness films, the resistivity values varied significantly from one sample to the other. On the other hand, for thicker samples, the variation in the resistivity value of different samples was negligible. In the following, we show how this behavior can be interpreted by analyzing a series of results obtained on films of increasing thickness. V-I characteristics of the CuS aggregated layer obtained from a single bilayer precursor are presented in (32) Tillmann, R. W.; Hofman, U. G.; Gaub, H. E. Chem. Phys. Lipids 1994, 73, 81-89. (33) Rozlosnik, N.; Antal, G.; Pusztai, T.; Faigel, G. Supramol. Sci. 1997, 4, 215-218. (34) Tachibana, H.; Yamanaka, Y.; Sakai, H.; Abe, M.; Matsumoto, M. Langmuir 2000, 16, 2975-2977. (35) Imae, T.; Ikeo, Y. Supramol. Sci. 1998, 5, 61-66. (36) Dutta, A. K.; Vanoppen, P.; Jeuris, K.; Grim, P. C. M.; Pevenage, D.; Salesse, C.; De Schryver, F. C. Langmuir 1999, 15, 607-612. (37) Nicolini, C.; Erokhin, V.; Antolini, F.; Catasti, P.; Facci, P. Biochim. Biophs. Acta 1993, 1158, 273-278.
768
Langmuir, Vol. 19, No. 3, 2003
Erokhina et al.
Figure 2. V-I characteristics measured on the CuS aggregated layer obtained from one bilayer LB film of copper arachidate.
Figure 4. AFM image of the 10 bilayer LB film after the reaction causing particle formation but without washing and aggregate layer assembly. The image size is 5 µm × 5 µm.
Figure 3. AFM image of the one bilayer LB film of copper arachidate after the particle formation and aggregation processes. The image size is 1 µm × 1 µm.
Figure 2. The very low current and extremely high noise levels allow one to conclude that no uniform conductive aggregated layer was formed in this case. The estimated conductivity (10-9 S/cm) is similar to that of the clean glass surface. AFM investigations confirmed the suggested nonuniform nature of the sample. A variety of film morphologies were found in different regions of the sample. Some areas were not even covered by the particulate layer, while it was possible to observe particle aggregates in other regions. The most representative AFM image (i.e. more often found on the sample) is presented in Figure 3. Analysis of the image together with height distribution histogram and height profiles revealed that the layer contains separated objects with a lateral size in the range of 70-80 nm and a mean height of approximately 2.3 nm. These objects cannot be attributed to individual particles. In fact, from the previously published data on the blue shift of the optical absorption spectrum,6,38 it was found that the effective CuS particle size is in the 2-3 nm range, even after their aggregation.20 On the other hand, ellipsometry has revealed that the average thickness of the aggregated layer obtained from one bilayer precursor is about 0.7 nm. Therefore, the objects in the AFM image can be considered as aggregates of nanoparticles, covering the glass substrate surface in a noncontinuous and nonuniform way. The mean height, taken from the height distribution histogram, is 2.3 nm, and it corresponds well to the effective particle size, measured from optical absorption spectra.6,20 Consequently, we suggest that these particles, formed within one bilayer precursor, tend to aggregate during the organic layer removal, forming 2D arrays at the substrate surface and having a thickness corresponding to the diameter of a single particle. (38) Geddes, N. J.; Urquhart, R. S.; Furlong, D. N.; Lawrence, C. R.; Tanaka, K.; Okahata, Y. J. Phys. Chem. 1993, 97, 13767-13772.
The discrepancy between the height measured with AFM and the ellipsometry data20 can be easily explained by taking into consideration that ellipsometry measurements perform an average of the thickness on a rather large area (corresponding to the laser spot diameter): in our case, the presence of uncoated areas on the sample has the effect of significantly diminishing the measured thickness. Such sample structure, composed of aggregates and containing small isolated nanoparticles that are not in an electrical contact with each other, is responsible for its electrical properties. In fact, it explains the essentially insulating behavior of the sample as shown by the V-I characteristics (Figure 2). The next data refer to samples containing 10 bilayers of Cu arachidate LB film, but examined at two different stages. First we show results after the CuS particle formation through exposure to hydrogen sulfide, but before washing with chloroform and particle aggregation; later we show results on a similar sample that has undergone the full process of particle formation, i.e., organic molecule removal and layer aggregation. An AFM image of the sample only exposed to hydrogen sulfide but unwashed and hence containing nonaggregated particles is shown in Figure 4. The fractal-like morphology shown in this image is similarly found throughout all the sample surface area. The height distribution histogram of the image contains two peaks corresponding to heights of 5.3 and 9.8 nm, respectively. Moreover, cross-section profiles along several lines reveal the presence of contiguous regions in the film having similar height. From these data it is possible to conclude that in this kind of sample there is a tendency to form terrace-like structures. The two peaks correspond to the sample surface having terraces at two altitudes and with a height difference of 4.5 nm. It is known that the thickness of a bilayer in a Cu arachidate LB film is about 5.4 nm.39 The presence of metal ions between amphiphilic molecule headgroups gives rise to a practically vertical orientation of the aliphatic chains in the LB film of fatty acid salts.40 After the reaction with H2S, arachidic acid headgroups became protonated and adjacent monolayers are not attached one (39) Matsuda, A.; Sugi, M.; Fukui, T.; Iizima, S.; Miyahara, M.; Otsubo, Y. J. Appl. Phys. 1977, 48, 771-774. (40) Knoll, W.; Philpott, M. R.; Golden, W. G. J. Chem. Phys. 1982, 77, 219-225.
Conductivity Differences in Aggregated CuS Films
Figure 5. Schematic representation of copper arachidate LB film after the particle formation.
to the other through metal ions anymore.6 Therefore, they can form a new packing structure with a tilted hydrocarbon chain orientation. The bilayer thickness will be decreased in this case. Such tilted orientation of the aliphatic chains in fatty acid LB films with decreased bilayer thickness has been already reported.41 Hence, a value of 4.5 nm for the difference between the peaks in the height histogram can be explained as corresponding to the thickness of an arachidic acid bilayer with tilted hydrocarbon chains, and it is possible to attribute the terraces in the AFM crosssection line profiles to the bilayer steps, whose altitude is modulated by the presence of grown nanoparticles. This result is also in agreement with the large amount of data in the literature that show how thickness inhomogeneities in LB film morphology often correspond to the height of one bilayer, i.e., that the bilayer is the smallest stable structural unit in LB films.42,43 In our case, the film has also undergone the particle formation processes that induce significant displacements in the film structure, hence giving rise to even greater inhomogenieties in the thickness measurements. Nevertheless, the main height steps still correspond to a bilayer thickness. A schematic representation of the structure of a multilayer film containing the unaggregated particles is shown in Figure 5. An AFM image of a similar sample (made from a 10 bilayer Cu arachidate precursor) after the particle formation and the washing and aggregation steps is shown in Figure 6. Judging by sight, the sample morphology seems very similar to that of the unaggregated sample (Figure 4). However, analysis of the height distribution histogram and cross-section line profiles along the lines indicated on the image reveals two significant differences. In the first place, even if also in this case the histogram contains two peaks, their heights are smaller and correspond to 0.5 and 1.5 nm, respectively. In the second place, the crosssection analysis does not reveal contiguous regions of similar height, as found in the case of the unaggregated sample. The similarities in the sample morphologies (Figures 4 and 6) allow one to suggest that the surface structure (41) Erokhin, V.; Lvov, Y.; Mogilevsky, L.; Zozulin, A.; Iljin, E. Thin Solid Films 1989, 178, 153-158. (42) Hui, S. W.; Viswanathan, R.; Zasadzinski, J. A.; Israelachvili, J. N. Biophys. J. 1995, 68, 171-178. (43) Zhavnerko, G. K.; Zhavnerko, K. A.; Agabekov, V. E.; Gallyamov, M. O.; Yaminsky, I. V.; Rogach, A. L. Colloids Surf. A, 2002, 198-200, 231-238.
Langmuir, Vol. 19, No. 3, 2003 769
Figure 6. AFM image of the 10 bilayer LB film of copper arachidate after the particle formation and aggregation processes. Image size is 5 µm × 5 µm
Figure 7. Schematic representation of the aggregated CuS layer.
of the aggregated layers at a mesoscopic level (tens of nanometers) is mainly predetermined during the particle formation process within the precursor layer. The following aggregation process results mainly in the removal of the organic matrix molecules and in only small lateral displacements of the particles, thus leaving a similar film morphology. However, these small displacements of the particles play a very important role in the microstructure formation. Particles of upper layers tend to occupy empty spaces of sublayers during the aggregation process, thus smoothing the whole film surface. The model of the aggregated film organization is schematically shown in Figure 7. Interpreting the first peak in the histogram (0.5 nm) as corresponding to the average corrugation (roughness) of the aggregated film in the same particle layer plane (h), we can attribute the second peak of the histogram (1.5 nm) to the height step due to the particles that were originally formed in the upper bilayer of the precursor film and are now forming the last particle layer plane (H). The thickness of this additional particle layer (measured from the difference in height between the second and the first peak in the histogram) is about 1 nm. This value is significantly less than the particle diameter as measured on a film obtained from one bilayer precursor and is only slightly higher than the average layer thickness, measured by ellipsometry.20 Therefore, it is possible to suppose that the particles, formed in different bilayers of the precursor, tend to form uniform layers during their aggregation. Particles formed in the upper precursor bilayers have high probability to fill in empty spaces in the underlayers, minimizing the total surface energy of the film. The V-I characteristics of this sample are presented in Figure 8. The nominal specific resistance of this sample was calculated to be about 106 Ω·cm. The conductivity of this sample was greater by 3 orders of magnitude than that of the film obtained from one bilayer precursor, while its thickness increased only by 1 order of magnitude. Figure 9 represents an AFM image of the CuS aggregated film obtained from 25 bilayers of precursor. The height distribution histogram of sample 9 contains one
770
Langmuir, Vol. 19, No. 3, 2003
Figure 8. V-I characteristics measured on the CuS aggregated layer obtained from 10 bilayer LB film of copper arachidate.
Figure 9. AFM image of 25 bilayer LB film of copper arachidate after particle formation and aggregation processes. Image sizes are 5 µm × 5 µm.
Figure 10. V-I characteristics measured on the CuS aggregated layer obtained from a 25 bilayer LB film of copper arachidate.
single peak at a mean height value of about 3.2 nm. Moreover, the sample morphology is rather uniform and does not contain any fractal-like structure that was found in the thinner sample. The V-I characteristics of the sample are shown in Figure 10. Specific resistance was calculated to be about 10 Ω·cm. The conductivity of this sample was greater by 5 orders of magnitude with respect to the sample obtained from 10 bilayers of precursor, while its thickness increased only by 2.5 times. Some geometrical considerations on the film organization may be useful for understanding their electrical properties. If we consider, for sake of simplicity, the particles as symmetrical objects (spheres), we can attribute them a mean diameter of 2.3 nm. This value is taken from the height distribution histogram for the particle layer obtained in the sample having only a single bilayer precursor. As in this case, there cannot be any overlapping of particles formed in different bilayers of the precursor during the aggregation process; the corrugation of the image represents very likely the particle height. This value for the particle diameter is also in good agreement with the effective particle size, calculated from the optical absorbance shift.6 Considering the ionic radii44 of Cu2+ and S2-, it is possible to calculate that there are, on the (44) Weast, R. C. (Ed.) Handbook of Chemistry and Physics, 57th ed.; CRC Press: Cleveland, 1977.
Erokhina et al.
average, about six Cu crystal lattice periods in one particle. In the precursor layer, Cu2+ ions occupy the same area per molecule than arachidic acid, a value that can be obtained from the π-A isotherm of the monolayer at the air/water interface. Thus, it is possible to calculate the filling ratio (sum of all particle volumes divided by the total film volume), considering the resulting aggregated film to have a thickness of 2.3 nm and supposing that all the available Cu2+ ions were involved in the particle formation reaction. To perform such estimations, we must also take into account that not all the arachidic acid molecules were in the form of a salt in the LB precursor layer, but some of them were in the acid form:45 we assume that about 70% of arachidic acid molecules are in the form of salt, while the remaining 30% are in the acid form. We can then estimate that the film of 2.3 nm height (medium height of the histogram of the image shown in Figure 3), obtained by the aggregation of particles formed in one bilayer of precursor, must be 8.5% filled with CuS particles and the remaining 91.5% of the layer space must be empty. These estimated values clarify the electrical behavior we have observed for this sample. In fact, it contains mainly uncoated glass surface with an addition of some separated particle aggregates, which are not even in electric contact with each other. Therefore, the sample reveals a practically insulating behavior. In the case of the sample formed from 10 bilayers of precursor, the amount of the aggregated matter was increased by 10 times. From the previous considerations regarding the single bilayer film, there would be enough room for the total collapsing of the resulting aggregated film down to the same thickness of 2.3 nm, resulting from a filling of the layer volume by particles up to about 85%. However, we can state that the film thickness was increased for, at least, 1 nm or even more. Such an increase of the film thickness corresponds to the height peaks difference in the histogram of this sample (image in Figure 6) and it was attributed to the medium height of the upper particle layer, formed over the sublayer (Figure 7). The decreased thickness of this upper layer with respect to the particle diameter indicates the preferential aggregation of the particles of the upper layers in the empty spaces of the sublayers. Assuming the total aggregated layer thickness to be not less than 3.3 nm [the medium particle diameter (2.3 nm) plus the additional thickness variation due to the upper layer (1 nm)], the volume filling in this case would be not more than 59%. Such filling of the film can provide electrical contacts between different particle aggregates. Thus, the observed fractal-like features in the sample morphology can be conducting CuS “wires”, formed by aggregates. The presence of these conducting pathways results in the significant decrease of the specific resistance value with respect to the film obtained from one bilayer of precursor. In the case of the film obtained from 25 bilayers of precursors, the sample morphology allows us to consider it as rather homogeneous with the medium height variation of about 3.2 nm. The increased nonuniformity of the layer roughness with respect to thinner films can be due to the fact that the increased number of the precursor layers results in a more random formation of the particles. As the final layer structure is mainly predetermined at this stage, the aggregation process results in the formation of the film with the increased height nonuniformity. The filling ratio in this case seems to be rather high, as the specific resistance value for this film is very low. (45) Kurnaz, M. L.; Schwartz, D. K. J. Phys. Chem. 1996, 100, 1111311119.
Conductivity Differences in Aggregated CuS Films
Supposing that the average thickness of the aggregated layers corresponds to that obtained from ellipsometry data,20 it is possible to estimate the relative variation of this film thickness by taking the medium height of the AFM height distribution histogram of the image in Figure 9. Assuming the total thickness to be about 20 nm, we obtain approximately 15% thickness variation over the sample surface. For a sample of such low thickness, this roughness value is comparable or even better than the values obtained on films fabricated using other methods. Conclusions CuS nanoparticles, formed in LB precursors, can be aggregated into thin inorganic layers. The structure and electrical properties of these layers are determined by the thickness of the precursor films. When this thickness is low (one bilayer of precursor), the particles form aggregates with the lateral size of about 70-80 nm and with a thickness of one individual particle diameter (2.3 nm). The layer is not uniform and homogeneoussthe film volume is only 8.5% filled by particle aggregates. Such “porosity” of the film determines its practically insulating behavior. Increased thickness of the precursor film results in significant changes of the aggregated layer structure and
Langmuir, Vol. 19, No. 3, 2003 771
properties. Particles of the upper precursor layers tend to occupy empty spaces of the sublayers during the aggregation process, resulting in the decrease of the height nonuniformity in the final film. Increased filling of the film volume by particle aggregates provides the formation of conducting pathways and a significant decrease of the film specific resistance. Further increase of the precursor film thickness results in the increase of the height nonuniformity of the aggregated film, providing a further increase of the filling of the film volume by particle aggregates, resulting in a pronounced decrease of the film specific resistance. The results obtained have demonstrated that effectively conducting thin inorganic layers can be prepared by the aggregation of CuS nanoparticles, produced in LB films, when the initial precursor thickness is more than 25 bilayers. In this case, the layers are rather homogeneous. The variation in the film roughness is about 3 nm, corresponding to a variation of 15% of the average thickness, and electrical properties of these layers are stable and reproducible. In the case of thinner films, the structure is not uniform and the instability degree of the electrical properties increases with the film thickness decrease. LA026433S
