Langmuir 2001, 17, 1461-1469
1461
Selective Deposition of ZnF(OH) on Self-Assembled Monolayers in Zn-NH4F Aqueous Solutions for Micropatterning of Zinc Oxide Noriko Saito,*,†,‡ Hajime Haneda,‡ Won-Seon Seo,† and Kunihito Koumoto† Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan, and National Institute for Research in Inorganic Materials, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan Received April 25, 2000. In Final Form: October 11, 2000 The deposition manner of ZnF(OH) on the OH- and phenyl-terminated self-assembled monolayers (SAMs) in Zn-NH4F aqueous solutions has been investigated. NH4F plays a role as a control agent of supersaturation by complex formation in the deposition process of ZnF(OH). ZnF(OH) was deposited as spherical particles on a substrate and decomposed into ZnO by annealing at 300 °C in air. A shorter induction time for heterogeneous nucleation and a larger number density of nuclei of ZnF(OH) were found on the OH-SAM surface, which indicates a lower interfacial energy between ZnF(OH) and the OH-SAM surface. Taking advantage of the effects of the surface functional groups on the deposition manner, ZnO was successfully micropatterned by using photopatterned SAMs as templates.
Introduction Future microelectronics technology requires the construction of micro/nanodevices of functional materials,1 and in this sense a micropatterning technique is regarded as inevitable.2 Because any required procedure should be incorporated into the usual semiconductor process, the micropatterning process must be carried out at low temperatures with low cost, and thus biomimetic processing techniques3,4 using organic matrixes are useful. One effective method for micropatterning of ceramic thin films is to use self-assembled monolayers (SAMs). By using SAMs, the surface functional groups of substrates can be modified, and interactions between the materials deposited and the surface of the substrate can be controlled. So far, some ceramic patterning using SAMs as templates for film synthesis have been studied. Payne et al.5,6 reported the patterning of oxide ceramics using microcontact printing of SAMs. Heuer et al. has investigated the effect of SAM on film synthesis7 and obtained patterned TiO2 using a photopatterned SAM.8 Rieke et al.4,9 studied selective deposition of FeOOH on SAMs patterned using electron and ion beam lithographic techniques. Koumoto et al.10 recently have succeeded in * To whom correspondence should be addressed. Tel: 81-29858-5643. Fax: 81-298-52-7449. E-mail:
[email protected]. † Nagoya University. ‡ National Institute for Research in Inorganic Materials. (1) See the papers in: Auciello, O., Ramesh, R., Eds. MRS Bull. 1996, 21 (7). (2) Dressick, W. J.; Calvert, J. M. Jpn. J. Appl. Phys. 1993, 32, 5829. (3) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1998, 37, 550. (4) Bunker, B. C.; Rieke, P. C.; Tarasevich, B. J.; Campbell, A. A.; Fryxell, G. E.; Graff, G. L.; Song, L.; Liu, J.; Virden, J. W.; McVay, G. L. Science 1994, 264, 48. (5) Jeon, N. L.; Clem, P. G.; Nuzzo, R. G.; Payne, D. A. J. Mater. Res. 1995, 10, 2996. (6) Clem, P. G.; Jeon, N. L.; Nuzzo, R. G.; Payne, D. A. J. Am. Ceram. Soc. 1997, 80, 2821. (7) Shin, H.; Agarwal, M.; DeGuire, M. R.; Heuer, A. H. Acta Mater. 1998, 46, 801. (8) Collins, R. J.; Shin, H.; DeGuire, M. R.; Heuer, A. H.; Sukenik, C. N. Appl. Phys. Lett. 1996, 69, 860. (9) Rieke, P. C.; Tarasevich, B. J.; Wood, L. L.; Engelhard, M. H.; Baer, D. R.; Fryxell, G. E.; John, C. M.; Laken, D. A.; Jaehnig, M. C. Langmuir 1994, 10, 619.
micropatterning TiO2 thin films using a patterned SAM with OH and phenyl groups as the template. The mechanisms of patterning using SAMs can be classified into two major categories. One is based on the electrostatic interactions between homogeneously nucleated colloid particles and a substrate; 8 negatively charged colloid particles are attracted by a positively charged substrate surface, or vice versa. The other is based on the difference in nucleation energy;9 lower interfacial energy between the nuclei and the substrate surface would lead to lower nucleation energy, resulting in an enhanced nucleation rate. Both mechanisms give rise to selective deposition of particles or films onto specific regions of the substrate. When the deposition takes place by incorporating the two mechanisms on the substrate irrespective of the difference in surface nature, a postdeposition lift-off process would give rise to a micropattern of a deposition film because of the difference in adhesion of the film to the substrate.5,6,10 The objective of the present study is to obtain micropatterned ZnO using SAMs. Zinc oxide is a representative ceramic material because it is widely used for electronic applications,11 such as varistors and SAW filters, and moreover it is expected for other uses such as transparent electrode or phosphor materials. For investigation of the effect of SAMs, film synthesis at low temperatures should be applied. Although many fabrication techniques, such as sputtering,12 chemical vapor deposition (CVD),13,14 a sol-gel method,15,16 spray pyrolysis,17 or electrochemical reaction,18 have so far been developed, one of the useful wet chemical routes is “electroless deposition”.19,20 The (10) Koumoto, K.; Seo, S.; Sugiyama, T.; Seo, W. S.; Dressick, W. J. Chem. Mater. 1999, 11, 2305. (11) Moulson, A. J.; Herbert, J. M. Electroceramics; Chapman and Hall: London, 1990. (12) Minami, T.; Nanto, H.; Tanaka, S. Appl. Phys. Lett. 1982, 41, 958. (13) Shiosaki, T.; Yamamoto, T.; Yagi, M.; Kawabata, A. Appl. Phys. Lett. 1981, 39, 399. (14) Smith, F. T. J. Appl. Phys. Lett. 1983, 43, 1108. (15) Tang, W.; Cameron, D. C. Thin Solid Films 1994, 238, 83. (16) Ohyama, M.; Kozuka, H.; Yoko, T. J. Am. Ceram. Soc. 1998, 81, 1622. (17) Caillaud, F.; Smith, A.; Baumard, J. F. J. Am. Ceram. Soc. 1993, 76, 998.
10.1021/la000607t CCC: $20.00 © 2001 American Chemical Society Published on Web 02/10/2001
1462
Langmuir, Vol. 17, No. 5, 2001
Saito et al.
Table 1. Effect of NH4F and pH on the Precipitation in Solution and the Deposition on the Substrate NH4F; 0.45 M pH ) 7 pH ) 7.5 pH ) 8 pH ) 8.5
NH4F; 0.675 M
NH4F; 0.9 M
solution
substrate
solution
substrate
solution
substrate
precipitation (ZnF(OH)) precipitation (ZnF(OH)) precipitation (ZnF(OH)) precipitation (unknown)
deposition (very slow) no deposition
transparent
deposition (ZnF(OH)) deposition (very slow) deposition (very slow) no deposition
transparent
deposition (very slow) deposition (ZnF(OH)) deposition (ZnF(OH)) no deposition
no deposition no deposition
precipitation (ZnF(OH)) precipitation (ZnF(OH)) precipitation (ZnF(OH))
process can be carried out in an aqueous solution, and the procedure is simple. NH4F is used as a freezing agent in order to inhibit homogeneous precipitation of zinc hydroxide in a solution and to make fluoride hydroxide deposit heterogeneously on a substrate instead. The fluoride hydroxide can be oxidized at relatively low temperatures, 6.6. Table 1 shows the effects of NH4F on precipitation and deposition on substrates in a ZnCl2 solution added with NaOH at pH ) 7.0-8.5. When the concentration of NH4F was low and the pH was high, the color of a solution turned to white because of precipitation. Under the conditions of high NH4F concentration and low pH, deposition was observed to occur only slightly after several hours. When the NH4F concentration was high, precipitation did not occur (the solution remained transparent) and deposition only on the substrate and on the wall of the beaker was observed to start after several minutes. In the case of 0.9 M NH4F and pH ) 8.5, no precipitation or deposition was found. XRD patterns of the precipitates in the solution and the deposit on the substrate both agreed with the JCPDS data of zinc fluoride hydroxide, ZnF(OH), except for the precipitate obtained for pH ) 8.5 and 0.45 M NH4F, which was crystalline but could not be identified. Hereinafter, heterogeneous nucleation behavior at 0.9 M NH4F was investigated in detail, which seemed to be suitable for film formation. Figures 1 and 2 show SEM photographs of ZnF(OH) deposited at pH ) 7.5 on the patterned substrate with OH and phenyl groups. The effect of the surface functional groups on the deposition process is remarkable, and a larger number of particles were observed to deposit on the OH surface. ZnF(OH) was deposited as spherical particles consisting of small fibrous crystallites. The thickness of the 100% coverage film, the OH part after 3 h shown in Figure 1c, was estimated to be about 20 µm by SEM observation. Taking advantage of the difference in the deposition rate, selective deposition of ZnF(OH) on the OH region was realized. Figure 3 exemplifies a SEM photograph of micropatterned ZnF(OH) prepared at pH ) 7.7 for the reaction time of 5 min. Figure 4 shows a SEM photograph of ZnO particles obtained by annealing ZnF(OH) at 300 °C in air, which was prepared at pH ) 7.5 for a reaction time of 15 min. The spherical shape of ZnF(OH) remained unchanged, but the crystallite size appears to have become smaller. Figure 5 shows the time dependence of the number density of ZnF(OH) particles deposited on the surfaces with OH and phenyl groups at pH ) 7.5. The difference in the number density on the OH and phenyl surfaces was 1-3 orders of magnitude. The induction period for
Selective Deposition of ZnF(OH)
Figure 1. SEM photographs of the as-deposited ZnF(OH) on the OH surface at pH ) 7.5 for the reaction time of (a) 5, (b) 15, (c) 60, and (d) 180 min.
Langmuir, Vol. 17, No. 5, 2001 1463
Figure 2. SEM photographs of the as-deposited ZnF(OH) on the phenyl surface at pH ) 7.5 for the reaction time of (a) 5, (b) 15, (c) 60, and (d) 180 min. Because deposition on the phenyl surface was subtle, the photographs of a and b were focused on the particles.
1464
Langmuir, Vol. 17, No. 5, 2001
Saito et al.
Figure 3. Micropatterning of ZnF(OH) prepared at pH ) 7.7 for the reaction time of 5 min.
Figure 6. Time dependence of the average particle diameter of ZnF(OH) particles at pH ) 7.5. Error bars with the standard deviations are attached to the data. The triangle indicates the relation of d ∝ t1/2.
Figure 4. SEM photograph of ZnO obtained by annealing ZnF(OH) deposited on the OH surface at pH ) 7.5 for the reaction time of 15 min.
Figure 7. Effect of pH on the number density of ZnF(OH) particles. The reaction time was 15 min.
Figure 5. Time dependence of the number density of ZnF(OH) particles at pH ) 7.5.
heterogeneous nucleation on the OH surface was shorter than that on the phenyl surface. The number density increased during a certain period after the reaction started (about 5 and 30 min on the OH and phenyl surfaces, respectively), and only a little nucleation occurred afterward. A slight decrease after a maximum (OH surface)
suggests that some particles became coalesced to form larger particles. Figure 6 shows the time dependence of the average diameter, d, of ZnF(OH) particles deposited on the OH and phenyl surfaces at pH ) 7.5. The diameter of the particles on the phenyl surface was slightly larger than that on the OH surface. The slopes in the log d-log t plots, as evaluated using the least-squares method, were 0.6 on both surfaces. Figures 7 and 8 show the effects of pH on the number density and the average diameter of ZnF(OH) particles at a fixed reaction time of 15 min, respectively. In the pH range of 7.5-8.0, the number density and the average diameter on both surfaces increased and decreased with increasing pH, respectively. At pH ) 7.2, a small amount of deposit was observed only on the OH surface after 30 min. At pH < 7.0 or pH > 8.5, deposition did not occur on either surface. Figure 9 shows the time dependence of the surface coverage of ZnF(OH) deposited at pH ) 7.5 and 7.7. The
Selective Deposition of ZnF(OH)
Langmuir, Vol. 17, No. 5, 2001 1465
the phenyl surface. It should be noted that the induction time for deposition is rather long for the phenyl surface and increases with decreasing pH, while the OH surface shows practically no induction time. Discussion Reactions in a Zn-NH4F Aqueous Solution. Zinc hydroxide usually precipitates in the pH range of ∼6.511 according to the reaction
Zn2+ + 2OH- a Zn(OH)2V
(i)
A small amount of Zn2+ forms complexes with OH-:
Zn2+ + nOH- a Zn(OH)n2-n
(ii)
When fluoride ions, F-, exist, Zn2+ ions form complexes with F-: Figure 8. Effect of pH on the average diameter of ZnF(OH) particles. The reaction time was 15 min. Error bars with the standard deviations are attached to the data.
Zn2+ + F- a ZnF+
(iii)
and zinc fluoride hydroxide precipitates through the reaction between ZnF+ and OH-:
ZnF+ + OH- a ZnF(OH)V
(iv)
Moreover, Zn2+ forms ammine complexes with NH3 under high pH conditions:
Zn2+ + nNH3 a Zn(NH3)n2+
(v)
with n ) 1-4, where NH3 is in equilibrium with NH4+ and OH- as
NH4+ + OH- a NH3
Figure 9. Time dependence of the surface coverage of ZnF(OH) at (a) pH ) 7.5 and (b) pH ) 7.7.
coverage was calculated from the product of the number density and the cross-sectional area of a particle. The coverage rate (the slope of the time dependence) increased with increasing pH for both OH and phenyl surfaces. The coverage rate on the OH surface was larger than that on
(vi)
Because the ionic strength, I, of the present solution is high (for example, in the solution of 0.9 M NH4F and 0.05 M ZnCl2, I ∼ 1), the stability constants of the reactions described above in this system cannot be estimated accurately. However, using the stability constants reported in the literature,22 we could know qualitatively the concentrations of the zinc species in a solution before the precipitation reaction. In Table 2, the stability constants22 used for calculation are shown. The total volume of the solution was assumed to be constant, even though an NaOH solution was added to control the pH. The complexes with OH- at n g 2 were neglected because the pH conditions considered here are not so high ( 6.5. Although chloride ions may affect the state of zinc ions, we ignored the interactions here because it is difficult to deal with the solubility products of several zinc chloride hydroxides and also because we obtained similar experimental results using Zn(NO3)2 instead of ZnCl2. The procedure to calculate the concentrations of zinc species is described in the Appendix subsection i. When the solubility products of Zn(OH)2 and ZnF(OH), Ksp1 and Ksp2, are known, the state of zinc in a solution in equilibrium with the precipitate can also be calculated. Here, we used the values Ksp1 ) [Zn2+][OH-] ) 10-15.5 and Ksp2 ) [ZnF+][OH-] ) 10-8.63. To our knowledge, the data for Ksp2 ) [ZnF+][OH-] have not been reported in the literature and so we assumed that ZnFOH precipitates at pH g 6.8 in the solution with 0.9 M NH4F. The solubility product of Zn(OH)2 was taken to be convenient to (22) Bjerrum, J.; Schwarzenbach, G.; Sillen, L. G. Stability Constants of Metal-ion Complexes; The Chemical Society: London, 1958.
1466
Langmuir, Vol. 17, No. 5, 2001
Saito et al.
Table 2. Stability Constants21 for the Zn/NH4F System Used for the Calculation of the State of Zinc reaction no.a ii (n ) 1) iii v (n ) 1) v (n ) 2) v (n ) 3) v (n ) 4) vi a
stability constant [Zn(OH-)]/[Zn2+OH-]
measurement condition
Kii1 ) ) Kiii ) [ZnF+]/[Zn2+F-] ) 100.77 Kv1 ) [Zn(NH3)2+]/[Zn2+NH3] ) 102.59 Kv2 ) [Zn(NH3)22+]/[Zn2+NH3]2 ) 104.91 Kv3 ) [Zn(NH3)32+]/[Zn2+NH3]3 ) 106.92 Kv4 ) [Zn(NH3)42+]/[Zn2+NH3]4 ) 108.62 Kvi ) [NH3]/[NH4+OH-] ) 104.39 104.40
25 °C, ZnSO4 solution 20 °C, I ) 1 25 °C, 10% NH4Cl solution
22 °C, 2 M NH4NO3 solution
Corresponds to the reaction number in the text.
Figure 10. Calculated NH4F content dependence of the concentrations of zinc species in the Zn-NH4F solution at pH ) 7.5 (a) before and (b) after precipitation.
demonstrate the experimental fact among the reported data.22 The procedure to calculate is described in the Appendix subsections ii and iii. Figure 10a shows the calculated result of the NH4F content dependence of the concentration of zinc species at pH ) 7.5 before precipitation occurs. [Zn2+] decreases and the total concentration of ammine complexes increases monotonically with increasing NH4F content, while [ZnF+] increases at first but decreases above ∼0.4 M NH4F and remains larger than [Zn2+]. [Zn(OH)+] is negligibly small and lower than 4 × 10-4 M. Figure 10b shows the NH4F content dependence of the concentration of zinc species after precipitation has occurred. Zn(OH)2 precipitates below ∼0.4 M NH4F, and ZnF(OH) precipitates at higher NH4F content. The amount
Figure 11. Calculated pH dependence of the concentrations of zinc species in the Zn-NH4F solution at 0.9 M NH4F (a) before and (b) after precipitation.
of precipitate decreased with increasing NH4F content, and no precipitation occurs above ∼1.8 M NH4F. Figure 11a shows the pH dependence of the concentrations of the zinc species before precipitation at the fixed concentration of NH4F (0.9 M). [ZnF+] decreases and the concentrations of ammine complexes increase with increasing pH. The concentration of Zn2+ remains low at this NH4F content. Figure 11b shows the pH dependence of the concentrations of the zinc species after precipitation. At this NH4F content (0.9 M), ZnF(OH) rather than Zn(OH)2 precipitates. The amount of ZnF(OH) precipitate has a maximum at pH ∼ 7.5 and becomes negligible at pH > 7.9. The increase in the amount of the precipitate with increasing pH at pH < 7.5 is due to the increase in [OH-]. The decrease in the amount of the precipitate at higher pH, despite the
Selective Deposition of ZnF(OH)
Langmuir, Vol. 17, No. 5, 2001 1467
geneous nucleation is possible in the intermediate supersaturation region called a “window”. The observation that the pH range for the window condition was wider at 0.9 M NH4F than at 0.45 M NH4F firmly indicated that the degree of supersaturation has a smaller pH dependence at the higher NH4F concentration. Generally, the solubility of metal hydroxide is very low, and it is difficult to control the supersaturation only by pH variation to control the heterogeneous nucleation behavior. However, by using NH4F, through the reactions ii-v, we can control the degree of supersaturation of ZnF(OH), and its deposition on the substrate through heterogeneous nucleation and growth can be realized. Nucleation Stage of ZnF(OH). According to the theory of heterogeneous nucleation with a contact angle, θ, between a substrate and a nucleus,24 the free energy for heterogeneous nucleation on the substrate, ∆Gs, is given by
∆Gs ) (2 + cos θ)(1 - cos θ)2∆G/4 where ∆G is the free energy for homogeneous nucleation and θ is given by the relation
cos θ ) (σ2 - σ3)/σ1 where σ1 is the surface energy of the nucleus, σ2 is the surface energy of the substrate, and σ3 is the interfacial energy between the nucleus and the substrate. On the assumption that the nucleation phenomenon obeys the Boltzmann distribution law, the rate of nucleation, dN/dt, where N is the number density of nuclei, is proportional to the probability to overcome the activation energy by thermal fluctuation and thus obeys the relation
dN/dt ∝ exp(-∆Gs/kT) Figure 12. Calculated supersaturation of Zn(OH)2 or ZnF(OH) in the Zn-NH4F solution for (a) pH ) 7.5 and (b) 0.9 M NH4F.
increase in [OH-], results from the decrease in [ZnF+] due to the increase in the concentration of ammine complexes. The calculated values were different from the experimental results in a quantitative sense because the stability constants and the solubility products used here are not accurate values for all solution conditions. If the stability constants of ammine complexes are larger, the amount of precipitate has a maximum or zero at lower pH. The degree of saturation, δ, before the precipitation reaction can be estimated to be the ratio of the total amount of zinc (0.05 M) to the amount of soluble zinc ions (0.05 M minus the content of the precipitate). Figure 12 shows the calculated degree of supersaturation, δ - 1. The degree of supersaturation decreases with increasing NH4F content, and the pH dependence has a maximum at pH ∼ 7.5 for 0.9 M NH4F. The present experimental facts (in Table 1) that (1) precipitation occurred at low NH4F contents, (2) heterogeneous nucleation on the substrate was observed at higher NH4F contents, and (3) the amount of deposit on the substrate depends on pH all correspond to the decrease in the degree of supersaturation with increasing NH4F content (Figure 12a). According to the usual theory of crystal growth,23 no nucleation occurs in the solution with low supersaturation, homogeneous nucleation occurs at higher supersaturation, and film formation via hetero-
where k is the Boltzmann constant and T is the absolute temperature. When the affinity of the nucleus against the substrate, σ3, is high, θ and ∆Gs become small, and consequently heterogeneous nucleation can easily occur, giving rise to a high density of nuclei. A larger density of nuclei on the OH surface than on the phenyl surface (Figure 5) strongly suggests that ZnF(OH) has a lower interfacial energy with the OH surface than with the phenyl surface. The longer period necessary for the number density to be almost saturated on the phenyl surface than on the OH surface also supports the idea. The saturation of the number density after the initial period implies that the degree of supersaturation in the vicinity of the substrate was lowered to the level where new nucleation could not occur. Particle Growth Stage. Particle growth proceeded after the period of nucleation. Generally, particle growth is classified into two major mechanisms: diffusioncontrolled growth or surface reaction controlled growth.25 By the time dependence of the diameter of the particle, d, the mechanism can be judged; d ∝ t1/2 for the diffusioncontrolled growth, and d ∝ t for the surface reaction controlled growth mechanism. We judged that the data of about 0.6 slopes in the log d-log t plots (Figure 6) approximately obey d ∝ t1/2 and that particle growth is mainly governed by the diffusion-controlled mechanism, (23) Sawada, K. Crystallization Process; Ohtaki, H., Ed.; Wiley: Chichester, England, 1998. (24) Kingery, W. D.; Bowen, H. K.; Uhlmann, D. R. Introduction to Ceramics, 2nd ed.; John Wiley & Sons: New York, 1975. (25) Nielsen, A. E. Kinetics of Precipitation; Pergamon Press: Oxford, U.K., 1964.
1468
Langmuir, Vol. 17, No. 5, 2001
where the growth rate of d is proportional to 1/d; namely, smaller particles can grow faster. In this manner, the particles had relatively narrow size distribution and the particles formed later on the phenyl surface can grow to a size similar to that of the particles on the OH surface. Because the number density of the particles on the phenyl surface was smaller than that on the OH surface, the ion concentrations around the nuclei on the phenyl surface must be higher than those on the OH surface, which led to a higher diffusion flux supplied to particles to grow larger. Consequently, ZnF(OH) particles on the phenyl surface grew slightly larger than those on the OH surface. The observed fact that the small number density coincides with the large average diameter of particles (Figures 7 and 8) can be explained based on Von Weimarn’s law.26 The law is given by an empirical formula, d∞(δ 1)n ) constant (n being a positive integer or fraction), with an initial degree of supersaturation of the reacting solution of δ - 1 and a particle size after sufficient reaction time of d∞. From this law, it is suggested that the dominant consumption of dissolved ions for precipitation transfers from the particle growth stage to the nucleation stage, with increasing δ. In the preceding section, the increase of the particle number density with pH was interpreted as the result of an increase of the degree of supersaturation. Thus, there is the correlation between the particle size and the number density. Micropatterning. Because of the relation d ∝ t1/2, the surface coverage with ZnF(OH) particles, Γ, satisfies the relation
Γ ) πNd2/4 ∝ Nt After the number density of particles becomes saturated, Γ is proportional to the reaction time and the coverage rate is proportional to the saturated number density. The induction time for the coverage to start increasing is the period necessary for the number density to become dominant, which is near the period necessary for the number density to be saturated. The difference in the coverage rate between the OH and phenyl surfaces corresponds to the difference in the saturated number density of nuclei. The difference in the induction time for deposition corresponds to the difference in the nucleation rate. In the time dependence of the coverage, there are two factors reflecting the nucleation energy: the coverage rate and the induction time for deposition. Thus, for the purpose of selective deposition of ZnF(OH) on one specific surface of a patterned substrate, the differences in the coverage rate (the number density) and the induction time for deposition should be made large. This can be achieved by appropriate combination of surface functional groups for a patterned substrate. Conclusions In the present study, we investigated the manner of deposition of ZnF(OH) in Zn-NH4F aqueous solutions on the OH- and phenyl-terminated SAMs. The results can be summarized as follows: (1) The role of NH4F in the deposition process was clarified as an agent to control the supersaturation of ZnF(OH) through the formation of complexes. (2) Spherical ZnF(OH) particles were deposited on SAM substrates and were decomposed to ZnO by annealing at 300 °C in air. (26) Von Weimarn, P. P. Chem. Rev. 1926, 2, 217.
Saito et al.
(3) A shorter induction time for the heterogeneous nucleation and a larger density of nuclei of ZnF(OH) were found on the OH surface than on the phenyl surface, which indicated a lower interfacial energy between ZnF(OH) and the OH substrate. (4) Taking advantage of the difference in the deposition manner on two kinds of SAMs with OH and phenyl groups, ZnO was successfully micropatterned on photopatterned SAMs. A novel mechanism of selective deposition of ZnF(OH) from an aqueous solution developed in the present study to fabricate ZnO micropatterns is expected to be applied to various other oxide systems. Appendix Here, the procedure to calculate the concentration of zinc species is described. (i) Before Precipitation. The total amounts of ammonium, fluorine, and zinc species are expressed using stability constants shown in Table 2, as follows: 4
[NH4F] ) [NH3] + [NH4+] +
[Zn(NH3)n2+] ∑ n)1
) [NH3]{1 + 1/Kvi/[OH-]} + 4
[NH3]nKvn} ∑ n)1
[Zn2+]{
(1)
[NH4F] ) [F-] + [ZnF+] ) [F-]{1 + [Zn2+]Kiii}
(2)
4
[ZnCl2] ) [Zn2+] + [ZnF+] +
∑ [Zn(NH3)n2+] + n)1 [ZnOH+] 4
) [Zn2+]{1 + [F-]Kiii +
[NH3]nKvn + ∑ n)1 [OH-]Kii1} (3)
Because the concentrations [NH4F] and [ZnCl2] are known in an experiment, combination of eqs 1 and 2 enables one to express [Zn2+] and [F-] as functions of [NH3]. Putting these functions into eq 3 gives rise to a polynomial equation for [NH3] which can be solved numerically if the pH of the solution is given. Thus, [Zn2+], [ZnF+], [Zn(NH3)n2+], and [ZnOH+] can all be known. (ii) After Precipitation of Zn(OH)2. When Zn(OH)2 coexists in the solution, the concentration of Zn2+ is fixed according to the relation Ksp1 ) [Zn2+][OH-]2 as
[Zn2+] ) Ksp1/[OH-]2
(4)
Then, the concentrations of ZnOH+ and ZnF+ are given by
[ZnOH+] ) [Zn2+][OH-]Kii1 ) Ksp1Kii1/[OH-] (5) [ZnF+] ) [Zn2+][F-]Kiii ) Ksp1Kiii[F-]/[OH-]2 (6) Putting [F-] ) [NH4F] - [ZnF+] (eq 2) in to eq 6, [ZnF+] is given by
Selective Deposition of ZnF(OH)
Langmuir, Vol. 17, No. 5, 2001 1469
[ZnF+] ) [NH4F]Ksp1Kiii/[OH-]2/{1 + Ksp1Kiii/[OH-]2} (7)
[NH4F] ) [NH3]{1 + 1/Kvi/[OH-]} + 4
[NH3]nKvn} ∑ n)1
[Zn2+]{
Equation 1, expressing the total amount of the ammonium species, can be written as a function of [NH3] as follows:
) [NH3]{1 + 1/Kvi/[OH-]} + 4
[NH3]nKvn}Ksp2/[OH-]/[F-]/Kiii ∑ n)1
{ -
[NH4F] ) [NH3]{1 + 1/Kvi/[OH ]} + 4
2+
[Zn ]{
∑ [NH3] Kvn} n
n)1
On the other hand, the relationship for the total amount of fluorine and zinc species is written using the amount of precipitate, [ZnF(OH)], as follows:
[NH4F] ) [F-] + [ZnF+] + [ZnF(OH)]
) [NH3]{1 + 1/Kvi/[OH-]} + 4
∑ [NH3]nKvn}Ksp1/[OH-]2 n)1
{
(8)
4
∑ [Zn(NH3)n2+] + n)1
[ZnOH+] + [ZnF(OH)] (14) Equations 13 and 14 give the following relation:
[NH4F] - [ZnCl2] ) [F-] - [Zn2+] 4
[Zn(NH3)n2+] - [ZnOH+] ∑ n)1
(9)
) [F-] - [Zn2+]{1 + 4
∑ [NH3]nKvn + [OH-]Kii1} n)1
Then, the concentration of Zn2+ is written as
[Zn2+] ) [ZnF+]/[F-]/Kiii ) Ksp2/[OH-]/[F-]/Kiii (10)
4
) [F-] - {1 +
[NH3]nKvn + ∑ n)1
[OH-]Kii1}Ksp2/[OH-]/[F-]/Kiii (15)
Thus,
[ZnOH+] ) [Zn2+][OH-]Kii1 ) Ksp2Kii1/[F-]/Kiii
(13)
and
[ZnCl2] ) [Zn2+] + [ZnF+] +
Thus, [NH3] can be known from eq 8 for a fixed concentration of NH4F and pH, and hence [Zn(NH3)n2+] can be calculated using the stability constants Kv1-Kv4 and eq 4. (iii) After Precipitation of ZnF(OH). When ZnF(OH) coexists in the solution, the concentration of ZnF+ is fixed according to the relation Ksp2 ) [ZnF+][OH-] as
[ZnF+] ) Ksp2/[OH-]
(12)
(11)
The total amount of the ammonium species (eq 1) here is written as
The combination of eqs 12 and 15 gives rise to two polynomial equations for [NH3] and [F-], and numerical solutions would further give us the concentrations of other species, [Zn2+], [Zn(NH3)n2+], and [ZnOH+]. LA000607T
