Chemical Bath Deposition of ZnO on Functionalized Self-Assembled

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Chemical Bath Deposition of ZnO on Functionalized Self-Assembled Monolayers: Selective Deposition and Control of Deposit Morphology Zhiwei Shi, and Amy Victoria Walker Langmuir, Just Accepted Manuscript • DOI: 10.1021/la5040239 • Publication Date (Web): 14 Jan 2015 Downloaded from http://pubs.acs.org on January 19, 2015

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Chemical Bath Deposition of ZnO on Functionalized Self-Assembled Monolayers: Selective Deposition and Control of Deposit Morphology Zhiwei Shi, Amy V. Walker* Department of Materials Science and Engineering, RL 10, University of Texas at Dallas, 800 W. Campbell Rd, Richardson, TX 75080.

KEYWORDS chemical bath deposition, zinc oxide, thin films, ion-by-ion growth, selfassembled monolayer

*

Corresponding Author: Ph:+1 972 883 5780; Fax: +1 972 883 5725; email:[email protected]

ACS Paragon Plus Environment

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ABSTRACT: We have developed a method by which to selectively and reproducibly deposit ZnO films on functionalized self-assembled monolayers (SAMs) using chemical bath deposition (CBD). The deposition bath is composed of zinc acetate and ethylenediamine. The deposition reaction pathways are shown to be similar to those observed for sulfides and selenides, even though ethylenediamine acts as both an oxygen source and a complexing agent. On –COOH terminated SAMs, Zn-carboxylate surface complexes act as nucleation sites for ion-by-ion growth leading to the formation of adherent ZnO nanocrystallites. Cluster-by-cluster growth is also observed which produces weakly adherent micron-sized ZnO crystallites. On –CH3 and –OH terminated SAMs only micron-sized ZnO crystallites are observed because Zn2+ does not complex with the SAM terminal group preventing nucleation of the nanocrystalline phase. The application of either ultrasound (“sonication-assisted CBD”) or stirring promotes ion-by-ion ZnO growth on –COOH terminated SAMs. Stirring produces smoother but less reproducible ZnO films than sonication-assisted CBD.


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1. Introduction Zinc oxide is a widely studied oxide material for technological applications including sensing,1, 2 energy harvesting,3, 4 lasing5, photocatalysis6 and in optoelectronics.7 It is also widely used in paints, catalysis and in cosmetics. It is a direct wide bandgap n-type semiconductor (Eg ~3.3 eV at room temperature)3 with a large exciton binding energy (60 meV)3 which exhibits near UV emission,8 piezoelectricity9 and transparent conductivity.10 Zinc oxide thin films have been deposited by a wide range of methods. These include metalorganic chemical vapor deposition (MOCVD),10 atomic layer deposition (ALD),7,

11, 12

molecular beam epitaxy (MBE),13 pulsed laser deposition (PLD),14 electrodeposition15 and chemical bath deposition (CBD).1, 4, 16-27 Chemical bath deposition is an attractive technique for organic thin film substrates because it can be performed at low temperatures, does not require a conductive substrate and is carried out under ambient conditions. The control of the film morphology is critical for the application of ZnO because the chemical, electrical and optical properties of ZnO are dependent on the detailed film microstructure and its surface area.2,

4, 6, 8, 28

The film structure has been demonstrated to affect the photocatalytic

behavior of ZnO.6 Doutt and co-workers28 showed that surface optical efficiency increases by greater than an order of magnitude as the ZnO film roughness decreases to unit cell dimensions. In contrast for dye-sensitized solar cells rougher films are more applicable because they have a higher energy conversion; ZnO nanoflower films were more efficient than the smoother columnar nanostructured films.4 The sensitivity of ZnO sensors has also been shown to depend on the film morphology which has been attributed to the higher surface area and detailed microstructure of the deposited film.2 The development of selective deposition methods for


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semiconducting materials is also important for their application in electronics, sensing and lithography.29 Although ZnO CBD has been widely studied, the reaction pathways involved in ZnO CBD are not well understood. A number of different ZnO CBD processes have been reported, and the resulting film morphologies (including surface density and grain size), adherence and composition are strongly dependent on the deposition temperature,16, 17 pH,16, 17 reagents,4, 24, 25, 27, 30

post-deposition annealing temperature18-20, 22, 23 and the presence of a seed layer.1, 21

Little is known about the role of the substrate chemistry in controlling the deposition efficiency, morphology and selectivity of ZnO deposition. The formation of surface complexes31 and the hydrophobic and hydrophilic properties of SAMs32 have been employed in CBD to control the morphology of ZnS31 and PbS,33 the composition of PbS, and to perform selective growth of ZnS,31, 32 CdSe,34 and PbS.32, 33 In general, the interaction of the surface with the cation in solution has been critical in determining the deposition selectivity. For example, M2+carboxylate surface complexes (M = Zn, Cd, Pb) act as the nucleation sites for the selective deposition of ZnS,31 CdSe34 and PbS.33 The deposition efficiency can be controlled by stirring35 and by applying ultrasonication during growth (sonication-assisted CBD).36-40 Ultrasound causes the formation, growth and implosive collapse of bubbles in liquids (acoustic cavitation). During cavitation, the bubble collapses gives rise to high pressures, hot spots and short lifetimes, which can drive chemical reactions.41 In the case of sonication-assisted CBD the application of ultrasonic vibrations promotes ion-by-ion growth which leads to changes in deposit morphology as well as composition.36-40 There have been few systematic studies of the effects of stirring in CBD. Stirring affects deposit morphology by removing weakly adherent, large clusters, and may alter the phase of the film deposited as well as the deposition rate.35


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In this study we develop a CBD method to selectively deposit crystalline ZnO with controlled morphology on organic thin films, using a bath composed of zinc acetate and ethylenediamine. We investigate for the first time the reaction pathways involved in the chemical bath deposition of ZnO on well-characterized organic surfaces. We employ functionalized alkanethiolate SAMs with –COOH, –OH and –CH3 terminal groups as model organic thin films. We demonstrate that the dominant deposition mechanism and the film morphology are critically dependent on the chemical functionality of the SAM terminal group and the experimental conditions employed. The reaction pathways for ZnO CBD on organic thin films are shown to be similar to those observed for the deposition of sulfides31,

33

and selenides34 even though in ZnO CBD the

complexing agent, ethylenediamine, is also the oxygen source. Using our understanding of the reaction pathways we demonstrate the selective deposition of ZnO thin films on patterned –CH3/–COOH and –OH/–COOH terminated SAMs. Finally, we show that ion-by-ion growth of ZnO on –COOH terminated SAMs can be promoted by both application of stirring and ultrasound to the deposition bath. We find that the stirring speed has little effect on the resulting deposit. Stirring produces a smoother deposit but that the film morphology and composition are not as reproducible as those obtained using sonication-assisted CBD. 2. Experimental 2.1 Materials Zinc acetate hydrate (≥ 98.0%), ethylenediamine (≥ 99.5%), hexadecanethiol (HDT; 99%) and mercaptohexadecanoic acid (MHA; 99%) were obtained from Sigma Aldrich (St. Louis, MO). 16-hydroxy-1-hexadecanethiol (99%) was obtained from Frontier Scientific Inc. (Logan, UT). All reactants were used without further purification. Gold and chromium were obtained from Alfa Aesar Inc. (Ward Hill, MA) and were of 99.995% purity. Silicon wafers (


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orientation) were purchased from Addison Engineering Inc. (San Jose, CA) and etched using piranha etch (H2SO4:H2O2 = 3:1) prior to use. 2.2 SAM Preparation The preparation of SAMs used in this study is described in refs 42, 43. Briefly, first Cr (~ 50 Å) and then Au (~1000 Å) were sequentially thermal-deposited onto freshly etched Si native oxide wafers. Well-organized self-assembled monolayers were formed by placing the prepared Au substrates into a 1 mM ethanolic solution of the appropriate alkanethiol molecule (with –COOH, –OH or –CH3 terminal functional groups) for 24 h at ambient temperature (21 ± 2 °C). The sample size used was ~(1×1) cm2. To ensure that the prepared SAMs were free from significant chemical contamination, for each batch, one sample was taken and characterized using singlewavelength ellipsometry (Gaertner Scientific Corp., Skokie, IL) and time-of-flight secondary ion mass spectrometry (TOF SIMS) (ION TOF Inc., Chestnut Hill, NY) prior to any further experiments. 2.3 Chemical Bath Deposition The ZnO plating solution was 0.018 M zinc acetate and 0.042 M ethylenediamine (complexing agent), and prepared as follows. First, zinc acetate was dissolved in deionized water which lead to the formation of a white precipitate. Ethylenediamine was then added dropwise to the solution until the white precipitate re-dissolved. The pH of the plating solution was 10.4. Depositions were performed at 22 °C (room temperature) and 45 °C for times from 5 min to 120 min. To test the effect of reagent concentration on the deposition, the concentrations of zinc acetate and ethylenediamine were varied from 0.0036 M to 0.042 M and 0.0072 M to 0.063 M, respectively. Films were also deposited while stirring with speeds 0 – 400 rpm and while applying ultrasound to the deposition bath (Aquasonic 75 HT, frequency ~ 40 KHz, power intensity ~2.8 W/cm2

44


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VWR Scientific Products, Radnor PA). To ensure that the resulting films were reproducible at least three samples were prepared at each reaction condition. The results shown are representative of the data obtained for each sample. After reaction, the samples were rinsed with deionized water and anhydrous ethanol, and dried using nitrogen gas. Once prepared, the samples were immediately transferred to TOF SIMS, XPS or SEM instruments for analysis. 2.4 UV Photopatterning of SAMs UV photopatterning of SAMs was performed using the procedure described by Zhou and coworkers.45 First, a mask (a copper TEM grid, 150 mesh, Electron Microscopy Inc., Hatfield, PA) was placed on top of the SAM to be patterned (SAM#1). The SAM with mask atop was placed 50 mm from a 500 W Hg arc lamp equipped with a dichroic mirror and a narrow band-pass UV filter (280 to 400 nm) (Thermal Oriel, Spectra Physics Inc., Stratford, CT). The SAM surface was then exposed to the UV light for 2 h to ensure that photooxidation was complete. The UV photopatterned SAM#1 was immersed in a freshly-made 1 mM ethanolic solution of the second alkanethiol (SAM#2) for 24 h. In the areas exposed to UV light, the photooxidized SAM#1 was displaced by SAM#2, and a SAM#2/SAM#1 patterned surface was obtained. The patterned surfaces were rinsed copiously with degassed ethanol and dried with nitrogen gas. 2.5 Time-of-Flight Secondary Ion Mass Spectrometry (TOF SIMS) TOF SIMS spectra were acquired using an ION TOF IV spectrometer (ION TOF Inc., Chestnut Hill, NY) equipped with a Bi liquid metal ion gun. Briefly, the instrument consists of a load lock for sample introduction, preparation and analysis chambers each separated by a gate valve. The pressure of the preparation and analysis chambers were maintained at < 5×10-9 mbar. The primary Bi+ ions had a kinetic energy of 25 keV and were contained in a ~100 nm diameter


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probe beam, which was rastered over a (100 × 100 µm2) area during spectra acquisition and a (500 × 500 µm2) area during image acquisition. All spectra were acquired using a total ion dose less than 1011 ions cm-2, which is within the static SIMS regime.46 The secondary ions were extracted into a time-of-flight mass spectrometer using a potential of 2000 V and were reaccelerated to 10 keV before reaching the detector. The peak intensities were reproducible to within ± 10 % from scan to scan and from sample to sample. 2.6 X-ray Photoelectron Spectroscopy (XPS) X-ray photoelectron spectra were measured with a Versa Probe II (Physical Electronics, Inc., Chanhassen, MN), using an Al Kα source (E = 1486.7 eV). Typically, the vacuum for measurement was lower than 5×10-10 Torr. All XPS spectra were measured with a step energy of 0.2 eV and pass energy of 23.5 eV. The data were collected at 45° to the normal of the sample surface. All spectra were collected using charge compensation using both an electron beam and an ion beam incident on the surface. The binding energies were calibrated using the Au 4f7/2 binding energy (84.0 eV). Spectra were analyzed using CasaXPS 2.3.16 (RBD Instruments, Inc., Bend, OR) and AAnalyzer 1.07. 2.7 Scanning Electron Microscopy (SEM) A dual-beam FIB instrument (Nova 200 Nanolab, FEI Company) was employed for the SEM images. The electron beam energy for measurements was 5 keV. This apparatus is also equipped with an energy dispersive x-ray (EDX) microanalysis system. 3. Results and Discussion 3.1 ZnO Deposition on –COOH Terminated SAMs SEM images indicate that after 1 h ZnO CBD at 45 °C ZnO has deposited on the surface


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(Figure 1). However, little, or no, deposition is observed at room temperature, 22 °C (Figure 1). At 45 °C the deposit appears to be composed of two types of crystallite: an underlying layer of ~100 - 500 nm nanoneedles and an overlayer of flower-shaped crystals (~1-2 µm) diameter. The larger flower-shaped crystals do not strongly adhere to the surface and are desorbed by sonication in deionized water for 3 min (Figure 1). In contrast, the smaller nanocrystallites strongly adhere to the surface and are not removed by sonication. Similar ZnO deposit morphology is observed over a wide range of ethylenediamine and zinc acetate concentrations. The zinc acetate and ethylenediamine concentrations can be varied by an order of magnitude; for zinc acetate and ethylenediamine the concentrations range from 0.0036 M to 0.042 M and 0.0072 M to 0.063 M, respectively. The zinc acetate to ethylenediamine ratio can also be changed from 1.75 to 2.33. Since the ethylenediamine concentration also controls the pH, the bath pH varied from 9.4 to 10.5. We note that this is in contrast to previous studies which indicate that at pH < 10, ZnO deposits are powdery and non-uniform on glass substrates.16, 17


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Figure 1: SEM images after ZnO CBD for 1 h on –COOH terminated SAMs at 45 °C, 22 °C, and 45 °C after sonication in deionized water. TOF SIMS spectra confirm these observations. Upon ZnO CBD at 45 °C we observe ions of the form Zn+, ZnOH+ and ZnxOy- indicating that ZnO has been deposited (Figure 2a). Upon sonication of the sample, these ion intensities decrease slightly indicating that some of the deposited layer has been removed. Further no molecular or cluster ions characteristic of the –COOH terminated SAM were observed after ZnO CBD indicating that the SAM is completely covered by the deposited ZnO layer (Figure 2b). After ZnO CBD at 22 °C the SIMS spectra indicate that there is little, or no, deposition of ZnO; the intensities of Zn-containing ions are negligible (Figure 2a) while high intensities of molecular cluster ions characteristic of the –COOH terminated SAM are observed (Figure 2b).


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intensity (a.u.)

a)

-COOH

o

22 C o

45 C after sonication 64 + ZnOH 45oC

C6H9

+

Bare SAM 80.6

80.8

81.0

81.2

m/z, positive ion

b)

-COOH o

intensity (a.u.)

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22 C o

45 C after sonication 45oC

-

Au2MHA

Bare SAM 680.5

681.0

681.5

682.0

682.5

m/z, negative ion

Figure 2: High resolution mass spectra centered at a) m/z 81 and b) m/z 681.5 after ZnO CBD for 1 h on –COOH terminated SAMs at 45 °C, 22 °C, and 45 °C after sonication in deionized water. Au2(MHA)- (MHA = -S(CH2)15COOH) is a molecular cluster ion characteristic of –COOH terminated SAMs. Also shown for reference are the spectra of the bare SAM.


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Figure 3: SEM images after ZnO CBD on –COOH terminated SAM at 45 °C for various deposition times. Figure 3 shows the growth of ZnO layer with deposition time at 45 °C. After 5 min in the plating solution we observe the formation of scattered nanocrystals (~100 – 500 nm). At longer deposition times (15 min) these nanocrystals become more numerous leading to the complete coverage of the –COOH terminated SAM as the deposition continues. There also appears to be nucleation of larger crystallites. At 30-45 min we observe the formation of larger micron-sized flower-shaped crystals which become more numerous as the deposition continues. Using XPS the chemical composition of the ZnO film was confirmed to be zinc oxide. Figure 4 displays the Zn 2p, Zn 3p, Au 4f, C 1s and O 1s core level spectra before and after CBD on the –COOH terminated SAM. The XPS spectra were shifted so that the Au 4f7/2 binding energy was 84.00 eV. The binding energy of Zn 2p3/2 is 1022.2 eV which is within the range reported for ZnO (1021.4 – 1022.5 eV), and the spin-orbit splitting is 23.1eV consistent with Zn in the 2+ oxidation state.12, 47 However, we note that the measured Zn 2p3/2 binding energy also overlaps with Zn(OH)2 (~1021.7 – 1022.7 eV).12, 47 In the O 1s spectra for the bare monolayer, there is a


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single peak at ~532 eV which is assigned to the COO oxygen atoms.43 Upon ZnO CBD we observe 2 peaks with binding energies at 531.0 eV and 532.8 eV, which we assign to the oxygen present in ZnO and –COOH respectively. However it is difficult to distinguish the products of the deposition using O 1s spectra because the peak overlaps with Zn(OH)2 (~532.2 eV),47 ZnO (529.9 – 531.2 eV)47 and the interaction of metals with the carboxylic acid terminal group.

48, 49

Since in previous studies the deposition of mixed ZnO/Zn(OH)2 films was observed,18-20 we used the Zn LMM Auger peak and the modified Auger parameter which are more sensitive to differences in chemical environment. Figure 5 shows the Zn LMM Auger peak of the deposited ZnO. The kinetic energy of the Zn LMM peak is 987.7 eV which is assigned to Zn2+ in ZnO (987.7 – 988.9 eV)12,

47

rather than Zn2+ in Zn(OH)2 (986.5 - 987.3 eV).12,

47

Further, the

modified Auger parameter for the deposited film is 2009.9 eV indicating that ZnO (2009.5 – 2011.0 eV)47 has deposited. The core level of the bare –COOH terminated SAM indicates that two types of carbon are present, at binding energies 284.6 eV and ~290 eV, which are assigned to –CH2– and –COOH respectively.43 Upon ZnO CBD, the methylene C peak has shifted to a higher binding energy (285.5 eV) and decreased in intensity. The high binding energy peak also increases in energy but in contrast to the methylene peak does not decrease in intensity. Given the energy shift from the methylene peak (~4 eV) and the approximately constant peak intensity, we assign this peak to the formation of zinc carbonate,50 which is due to the reaction of ZnO with atmospheric carbon dioxide. There is also a third, new peak observed with binding energy ~287 eV indicative of the interaction of metals with the SAM terminal group (Figure 4).43, 48


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-COOH, O 1s

-COOH, Zn 2p

Relative Intensity

Relative Intensity

Zn 2p3/2 Zn 2p1/2

2h ZnO CBD

2h ZnO CBD Bare

Bare 1055 1050 1045 1040 1035 1030 1025 1020 1015

538

536

534

Binding Energy (eV)

528

526

Au 4f7/2

Au 4f5/2

Relative Intensity

Relative Intensity

530

-COOH, Zn 3p, 4f -CH2interaction

2h ZnO CBD -COOH Bare

292

532

Binding Energy (eV)

-COOH, C 1s

Zn 3p3/2 2h ZnO CBD

Bare 290

288

286

284

282

Binding Energy (eV)

94

92

90

88

86

84

82

80

Binding Energy (eV)

Figure 4. Zn 2p, O 1s, C 1s and Au 4f XPS spectra after ZnO CBD for 2h on –COOH terminated SAMs at 45 °C. The fits to the data are shown as dotted lines.

60000

Zn LMM 55000

CPS

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50000 45000 40000 35000 980

985

990

995

1000

kinetic energy (eV)

Figure 5. Zn LMM spectrum after ZnO CBD for 2h on –COOH terminated SAMs at 45 °C.


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The SIMS data also indicate that Zn2+ interacts with the –COOH terminal groups. After 1 h CBD in the positive ion spectra Zn(COO)2(CH2)x(CH)y+ ions are observed indicating that Zn2+ complexes with two –COOH terminal groups (Figure 6). Further there is no evidence that Zn or ZnO has penetrated through the –COOH terminated SAM to the Au/S interface; no AuxZnySz± or AuxZnySzOa± ions are observed in the spectra (data not shown).

64

-COOH

+

Zn(COO)2(CH2)11(CH)3

+

64

Zn(COO)2(CH2)12(CH)2

+

66

Zn(COO)2(CH2)11(CH)3 68 + Zn(COO)2(CH2)11(CH)3 + 64 Zn(COO)2(CH2)12(CH)2

Intensity (a.u.)

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After deposition Bare SAMs 345

346

347

348

349

350

m/z, positive ions

Figure 6. High resolution positive ion mass spectra of Zn(COO)2(CH2)x(CH)y+ after ZnO CBD for 1 h on –COOH terminated SAMs at 45 °C. Also shown for reference is the spectrum of the bare SAM. 3.2 ZnO Deposition on –OH and –CH3 Terminated SAMs Similar to –COOH terminated SAMs, SEM and TOF SIMS data indicate that ZnO deposition is strongly temperature dependent. At 22 °C no ZnO crystallites were observed to form on –OH and –CH3 terminated SAMs (data not shown). At 45 °C only micron-sized ZnO crystallites deposit (Figure 7). These crystallites do not strongly adhere to the surface and can be removed by sonication in deionized water for 3 min.


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Figure 7. SEM images after ZnO CBD for 1 h on –OH and –CH3 terminated SAMs at 45 °C before and after sonication in deionized water. The TOF SIMS spectra indicate that Zn2+ ions do not complex with –OH and –CH3 terminated SAMs, and in agreement with the SEM data few Zn-containing ions are observed even after deposition at 45 °C (see Supporting Information). Upon sonication, no Zn-containing ions are observed indicating that the ZnO layer does not adhere to –OH and –CH3 terminated SAMs. The data also show that ZnO does not form an overlayer on either –OH or –CH3 terminated SAMs; in the negative ion spectra molecular and cluster ions characteristic of the SAMs are observed (see Supporting Information). These ions have a similar intensity to that observed for the bare monolayers. 3.3 Reaction Pathways Involved in ZnO CBD on –COOH, –OH and –CH3 Terminated SAMs In CBD a controlled ion exchange reaction is employed to deposit a thin film on a substrate by


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precipitation. The reaction rate is generally controlled by the concentration of the “free” metal ion, which in this reaction is Zn2+. However, we note that a critical concentration of OH- ions must also be reached for ZnO precipitation to occur. In the plating solution ethylenediamine (en) hydrolysis leads to an increase of pH and thus OH- concentration: ܰ‫ܪ‬ଶ (‫ܪܥ‬ଶ )ଶ ܰ‫ܪ‬ଶ + 2‫ܪ‬ଶ ܱ ⇋ ሾܰ‫ܪ‬ଷ (‫ܪܥ‬ଶ )ଶ ܰ‫ܪ‬ଷ ሿଶା + ܱ‫ି ܪ‬

(1)

Under our reaction conditions the pH of the solution upon addition of ethylenediamine is 10.4. The ethylenediamine also acts as a complexing agent for Zn2+ ions. At pH 10.3, previous studies indicate that the predominant species present in solution is Zn(en)32+: 51 ܼ݊ଶା + 3ܰ‫ܪ‬ଶ (‫ܪܥ‬ଶ )ଶ ܰ‫ܪ‬ଶ ⇋ ሾܼ݊(ܰ‫ܪ‬ଶ (‫ܪܥ‬ଶ )ଶ ܰ‫ܪ‬ଶ )ଷ ሿଶା

(2)

Other species likely present in solution include Zn(en)22+ and Zn[(en)2OH]+.51 Upon heating, the equilibrium in reaction 2 moves to the left leading to an increase in the “free” Zn2+ concentration. Concomitantly the hydrolysis rate of ethylenediamine increases (reaction 1) leading to an increase in OH- concentration. Once the solubility product (Ksp) of Zn(OH)2/ZnO is exceeded a precipitation reaction occurs and ZnO is deposited: ܼ݊ଶା + ܱ‫)ܪܱ(ܼ݊ ⇋ ି ܪ‬ଶ ܼ݊(ܱ‫)ܪ‬ଶ ⇋ ܼܱ݊ + ‫ܪ‬ଶ O Our experiments indicate that the reaction pathways involved in ZnO CBD are more complicated than the above discussion suggests. Any proposed reaction mechanism must be able to explain the following experimental observations. On –COOH terminated SAMs two different types of crystallites are observed: an underlayer of ~100-500 nm nanocrystallites and large micron-sized flower-like crystals (Figure 1). In contrast on –OH and –CH3 terminated SAMs only micron-sized crystallites are observed which do not strongly adhere to the substrate (Figure 7). One might argue that the differences in film growth observed are due to differences in the


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interfacial charge density or a hydrophobic/hydrophilic effect. However, –CH3, –OH and –COOH

terminated

SAMs

all

exhibit

similar

negative

ζ

potentials.52,

53

The

hydrophilicity/hydrophobicity of the SAM surfaces also cannot account for the observed deposity because –OH and –COOH terminated SAMs are both hydrophilic (and have similar water contact angles).52,

54

We propose that the deposition occurs in the following way. The

growth of the nanocrystallites can be explained via a kinetically controlled reaction in which there is a low concentration Zn2+-carboxylate complexes while there is a large concentration of OH- present in the surface region. The pKa of palmitic (hexadecanoic) acid is ~8.5-8.8.55 Thus under our reaction conditions it is likely that the –COOH terminal groups are deprotonated leading to the formation of COO- groups on the surface. These carboxylate groups are able to form complexes with Zn2+ (Figure 6), and these Zn-carboxylate complexes provide the nucleation sites for the subsequent ion-by-ion growth of ZnO via the dehydration of Zn(OH)2. The equilibrium constant for the complexation of Zn2+ and carboxylic acids is very low (

Chemical Bath Deposition of ZnO on Functionalized Self-Assembled

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