Selective formation of monodisperse CdSe nanoparticles on functionalized self-assembled monolayers using chemical bath deposition

Electrochimica Acta 55 (2010) 8126–8134

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Selective formation of monodisperse CdSe nanoparticles on functionalized self-assembled monolayers using chemical bath deposition Peng Lu, Zhiwei Shi, Amy V. Walker ∗ Department of Materials Science and Engineering, University of Texas at Dallas, 800 W. Campbell Rd RL10, Richardson, TX 75080, United States

a r t i c l e

i n f o

Article history: Received 30 November 2009 Received in revised form 15 March 2010 Accepted 16 March 2010 Available online 25 March 2010 Keywords: CdSe Chemical bath deposition Self-assembled monolayers Molecular devices Solar cells

a b s t r a c t Using CdSe chemical bath deposition (CBD) we demonstrate the selective growth and deposition of monodisperse nanoparticles on functionalized self-assembled monolayers (SAMs) using time-of-flight secondary ion mass spectrometry and scanning electron microscopy. We show that the deposition mechanism involves both ion-by-ion growth and cluster-by-cluster deposition. On –COOH terminated SAMs strongly adherent CdSe nanoparticles form via a mixed ion-by-ion and cluster-by-cluster mechanism. Initially, Cd2+ ions form complexes with the terminal carboxylate groups. The Cd2+ -carboxylate complexes then act as the nucleation sites for the ion-by-ion growth of CdSe. After a sufficient concentration of Se2− has formed in solution via the hydrolysis of selenosulfate ions, the deposition mechanism switches to cluster-by-cluster deposition. On –OH and –CH3 terminated SAMs monodisperse CdSe nanoparticles are deposited via cluster-by-cluster deposition and they do not bind strongly to the surface. Finally, under the appropriate experimental conditions we demonstrate the selective deposition of CdSe nanoparticles on patterned –CH3 /–COOH SAMs. © 2010 Elsevier Ltd. All rights reserved.

1. Introduction Cadmium selenide is a II–VI direct bandgap semiconductor (Eg = 1.74 eV) [1]. Nanocrystals of CdSe (diameter < 100 nm) are of particular interest for a wide range of applications because their photoluminescence spans visible wavelengths [2]. CdSe nanoparticles have been employed in light emitting diodes (LEDs) [3–7], sensors [8,9], biomedical imaging [8], lasing [10] and solar cells [11–15]. In many of these applications the CdSe nanocrystals are deposited on, or incorporated into, an organic matrix, such as a polymer. Critical steps in device fabrication using nanocrystals include the precise control of the nanoparticle size and spatial positioning. CdSe nanostructures and thin films have been fabricated with a variety of methods, including thermal evaporation [16,17], electrodeposition [18,19], spray pyrolysis [20], arrested precipitation in solution [2], synthesis in a structured medium (e.g. in a polymer matrix) [2], successive ionic layer adsorption and reaction (SILAR) [21,22], chemical vapor deposition (CVD) [23,24] and chemical bath deposition (CBD) [25–34]. CBD is an attractive technique by which to deposit CdSe on organic thin films because it can be employed at low temperatures (≤50 ◦ C) which are compatible with organic substrates. Further, CBD is suitable for large-area processing and

∗ Corresponding author. Tel.: +1 972 883 5780; fax: +1 972 883 5725. E-mail address: [email protected] (A.V. Walker). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.03.054

is performed under ambient conditions making it an inexpensive technique. Self-assembled monolayers (SAMs) have well-defined, highly organized structures with a uniform density of terminal groups [35,36]. They can be patterned using a variety of techniques including microcontact printing [37–39], scanning probe lithographies [37–39], electron beam lithography [40,41], imprinting [39] and UV photopatterning [42–45] to produce nano- and micro-meter structures. Patterned SAMs have been employed to produce arrays of crystals, either as crystallization directing agents or resists. As resists SAMs have been used to direct the sol–gel deposition of (Pb,La)TiO3 and LiNbO3 [46], the electroless deposition of Ni [47,48] and the CVD of metals [49–52]. The hydrophilic and hydrophobic properties of SAMs have also been employed to direct crystal deposition. For example, Chen et al. [53] employed hydrophilic and hydrophobic interactions to selectively deposit two different sizes of CdSe nanoparticles on microcontact printed SAMs. In this way the photoluminescence of the sample could be spatially controlled with areas of green or red luminescence. However, perhaps the most interesting way to direct crystal growth is to chemical reactions with specific terminal groups. Dressick et al. [54] have selectively electrolessly deposited Co on patterned SAMs using the interaction of Pd2+ ions with pyridyl groups. Using physical and chemical vapor deposition, Walker and co-workers [55,56] have demonstrated the selective deposition of Mg and Al, respectively on patterned SAMs. Semiconductors [57], insulators [55] and polymer brushes [58] can also be deposited using this approach.

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In this paper using CdSe CBD we demonstrate the selective growth and deposition of monodisperse nanoparticles on patterned SAMs. The size of the deposited nanoparticles is controlled using the bath temperature and substrate immersion time. We demonstrate that the deposition mechanism involves both ion-by-ion growth and cluster-by-cluster deposition. Further we show that the dominant deposition mechanism is dependent on the chemical functionality of the SAM terminal group and the concentration of Se2− in the deposition bath. On –COOH terminated SAMs strongly adherent CdSe nanoparticles form via a mixed ion-by-ion and cluster-by-cluster mechanism. Initially, Cd2+ ions form complexes with the terminal carboxylate groups. These Cd2+ -carboxylate complexes act as the nucleation sites for the ion-by-ion growth of CdSe. After a sufficient concentration of Se2− has formed in solution via the hydrolysis of selenosulfate ions (SeSO3 2− ), the deposition mechanism switches to cluster-by-cluster deposition on both the –COOH terminated SAM and in solution. On –OH and –CH3 terminated SAMs neither Cd2+ nor Se2− ions interact with the terminal groups. The dominant reaction mechanism is cluster-by-cluster deposition from solution, leading to the deposition of monodisperse nanoparticles, which do not strongly adhere to the SAM surface. 2. Experimental 2.1. Materials Gold and chromium (99.995%) were obtained from Alfa Aesar Inc. (Ward Hill, MA). Cadmium sulfate (≥99%), selenium powder (99.99%), sodium sulfite (≥98%) and nitrilotriacetic acid trisodium salt (≥98%) (NTA) were purchased from Sigma–Aldrich (St. Louis, MO). Anhydrous ethanol (A.C.S. grade) was obtained from Aaper Alchohol (Shelbyville, KY). Hexadecanethiol (99%) (HDT), 16-mercaptohexadecanoic acid (99%) (MHA), and mercaptohexadecanol (99%) (MHL) were obtained from Asemblon, Inc. (Redmond, WA). All chemicals were used without further purification. Native silicon native oxide wafers (1 1 1 orientation) were purchased from Addison Technologies, Inc. (Pottstown, PA). The wafers were cleaned with piranha etch (H2 SO4 :H2 O2 = 3:1) prior to use. A 200 mM stock solution of sodium selenosulfate (Na2 SeSO3 ) was prepared by dissolving 0.2 M Se into 0.4 M sodium sulfite at 60 ◦ C for 2 h. 2.2. SAM preparation The preparation and characterization of alkanethiolate SAMs adsorbed on gold substrates have been described in detail previously [59–63]. Briefly, first Cr (∼5 nm) and then Au (∼100 nm) were thermally deposited onto clean Si native oxide wafers. Selfassembly of well-ordered monolayers was achieved by immersing the Au substrates into a 1 mM ethanolic solution of the relevant hexadecanethiol molecule (with –COOH, –OH or –CH3 terminal functional groups) for 24 h at ambient temperature (21 ± 2 ◦ C). To ensure that the SAMs were free from contamination, for each batch one SAM sample (∼1 cm2 ) was taken and characterized using single-wavelength ellipsometry (Stokes Ellipsometer, Gaertner Scientific Corp., Skokie, IL) and time-of-flight secondary ion mass spectrometry (TOF SIMS IV, ION TOF Inc., Chestnut Hill, NY) prior to chemical bath deposition. 2.3. CdSe chemical bath deposition The standard plating solution employed in this study was composed of 40 mM cadmium sulfate (CdSO4 , cadmium source), 80 mM sodium selenosulphate (Na2 SeSO3 , selenium source), and 120 mM

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sodium nitrilotriacetate (NTA, complexing agent). To investigate the influence of the ratio Cd2+ to SeSO3 2− the sodium selenosulfate concentration was varied. Two other concentrations of sodium selenosulfite were employed, 20 mM and 80 mM. To make the plating solution, cadmium sulfate was first dissolved in deionized water, and then sodium nitrilotriacetate added to the solution. SAM samples (∼1 cm2 ) were usually immersed in this solution for 15 min prior to the addition of Na2 SeSO3 (“seeded” deposition). To investigate the influence of Cd2+ complexation with the –COOH terminal group, sodium selenosulfate was also added to the bath prior to immersion of the MHA SAM (“unseeded” conditions). The deposition was performed in the dark. The pH of the plating solution was 8.0 before the addition of sodium selenosulfite. Upon addition of Na2 SeSO3 , the pH rose to 9.5, and slowly increased. After 2 h (the maximum deposition time) the pH of the solution gradually increased and reached 10.5. Deposition was performed using bath temperatures of 22 ◦ C and 45 ◦ C for times from 1 min to 2 h. After CBD, samples were rinsed thoroughly with copious amounts of deionized water and absolute ethanol, dried using nitrogen gas and immediately transferred to the TOF SIMS IV or SEM for analysis. 2.4. UV photopatterning of SAMs Patterned –CH3 /–COOH terminated SAMs were prepared using UV photopatterning following the procedure described by Zhou and Walker [44]. First a mask (copper TEM grid, Electron Microscopy, Inc., Hatfield, PA) was placed on top of a –COOH terminated SAM (MHA). The sample was then placed approximately 50 mm away from a 500 W Hg arc lamp equipped with a dichroic mirror (to remove IR light) and a narrow-band-pass filter (280–400 nm) (Spectra Physics Inc., Stratford, CT). The sample was then exposed to UV light for 2 h to ensure complete photooxidation of the SAM. The UV photopatterned –COOH terminated SAM was immersed into a freshly made 1 mM solution of a second alkanethiol, HDT (–CH3 terminated alkanethiol) for 24 h. After immersion, HDT was adsorbed in the areas exposed to UV light resulting in a patterned –COOH/–CH3 terminated SAM (HDT/MHA). The patterned surfaces were rinsed with copious ethanol and dried with N2 gas. 2.5. Time-of-flight secondary ion mass spectrometry (TOF SIMS) TOF SIMS analyses were conducted using an ION TOF IV spectrometer (ION TOF Inc., Chestnut Hill, NY) equipped with a Bi liquid metal ion gun. Briefly, this instrument consists of an air lock, and preparation and analysis chambers, each separated by gate valves. The preparation and analysis chambers were maintained at <3.8 × 10−9 mbar. The Bi+ primary ions were accelerated to 25 keV and contained within a ∼100 nm diameter probe beam. The beam was rastered over a 100 ␮m × 100 ␮m area during spectra acquisition and 500 ␮m × 500 ␮m area during imaging. All spectra were acquired within the static regime [64] using a total ion dose of <1011 ions cm−2 . The secondary ions were extracted into a time-of-flight mass spectrometer using 2000 V and were reaccelerated to 10 keV before reaching the detector. Peak intensities were reproducible to within ±10% from scan to scan and from sample to sample. For each CdSe deposition, at least three samples were prepared and three areas on each sample were examined. The ion intensity data presented are an average of these measurements. Three samples of each patterned SAM were prepared, and three spots on each surface examined. The images shown are 500 ␮m × 500 ␮m divided into 128 × 128 pixels. Optical images of the samples were captured using a video camera (ExwaveHAD, Sony) mounted in the TOF SIMS analysis chamber.

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Fig. 1. SEM images of –COOH terminated SAMs after 2 h CdSe chemical bath deposition at (a) 22 ◦ C and (b) 45 ◦ C.

2.6. Scanning electron microscopy (SEM) SEM measurements were conducted on a Field Emission Scanning Electron Microscope (Hitachi s-4500) equipped with a NORAN Instruments energy dispersive X-ray (EDX) microanalysis system, a back scattering detector and a mechanical straining stage. The diameter of the CdSe nanoparticles was obtained from the scanned high resolution SEM images. Between 150 and 200 nanoparticles were measured to obtain the size distribution. 3. Results and discussion 3.1. CdSe deposition on –COOH terminated SAMs After CdSe CBD for 2 h, SEM images indicate that nanoparticles have deposited on MHA (Fig. 1) with more- and largernanoparticles formed at a bath temperature of 45 ◦ C than 22 ◦ C. At 45 ◦ C the diameter of the CdSe nanoparticles was 142 ± 11 nm while at 25 ◦ C the nanoparticle diameter was 56 ± 7 nm. Using EDX the chemical composition of the nanoparticles was confirmed to be CdSe: the deposited nanoparticles contain only Cd and Se in an approximately 1:1 ratio. In the TOF SIMS spectra, no molecular or cluster ion characteristic of MHA, such as AuM2 − (m/z 681; M = –S(CH2 )15 COOH) are observed at 22 ◦ C and 45 ◦ C indicating that the SAM is covered by a CdSe overlayer (Fig. 2). At both 22 ◦ C and 45 ◦ C we observe ions characteristic of the formation of a CdSe overlayer, including Cd+ , Se− and CdSe− . Since at 22 ◦ C only scattered, small CdSe nanoparticles are observed in SEM (Fig. 1), this suggests that a layer of much

Fig. 2. High resolution negative ion TOF SIMS spectra of Au2 M− (m/z 681, M = –S(CH2 )15 COOH), for –COOH terminated SAMs prior to and after CdSe CBD for 2 h at 22 ◦ C and 45 ◦ C.

smaller CdSe nanoparticles, or a thin CdSe overlayer, is deposited at this temperature which is too small to be observed in SEM. The color of the sample surface is also orange-red, which suggests that a layer of small nanoparticles (∼3–20 nm diameter) has been deposited [30]. The TOF SIMS data also indicate that Cd2+ complexes with two –COOH terminal groups. In the positive mass spectra we observe the formation of Cd(COO)2 (CH2 )x (CH)y + ions after CdSe CBD for 2 h at 45 ◦ C (Fig. 3). Few such ions are observed at 22 ◦ C indicating that only a small number of these complexes form at this bath temperature. This observation suggests that at 45 ◦ C these Cd2+ -carboxylate complexes are the nucleation sites for subsequent growth of CdSe. There is no evidence in the TOF SIMS spectra that Cd, Se or CdSe has penetrated through the monolayer to the Au/S interface. Fig. 4 displays the growth of CdSe nanoparticles over time at a bath temperature of 45 ◦ C. After 5 min in the deposition bath, we observe the formation of scattered crystallites with a diameter of 34 ± 3 min. After 15 min the nanoparticles have formed a dense layer. Both the size and density (number of CdSe nanoparticles per ␮m2 ) of the deposited CdSe nanoparticles increases with immersion time (Fig. 5). The composition of the plating solution also affects the CdSe nanoparticle growth (Fig. 5). If the ratio of the selenosulfate to Cd2+ ions in solution is decreased to 0.5:1 (20 mM:40 mM), smaller nanoparticles are observed to form (Fig. 5a). However if the ratio is increased to 1:1 the CdSe nanoparticle is the same (within experimental error) as that for a bath with a 2:1 composition (the

Fig. 3. High resolution positive ion TOF SIMS spectra of Cd(COO)2 (CH2 )x (CH)y + ions (x = 14, y = 1) for –COOH terminated SAMs prior to and after CdSe chemical bath deposition for 2 h at 22 ◦ C and 45 ◦ C. The ions were assigned based on accurate mass and were not structurally characterized.

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Fig. 4. SEM images after CdSe chemical bath deposition on –COOH terminated SAMs at 45 ◦ C for immersion times from 1 min to 120 min.

standard bath conditions). We also observe that the number density of CdSe nanoparticles (number of CdSe nanoparticles per ␮m2 ) increases substantially with increasing sodium selenosulfate concentrations. We note in Fig. 5b that at 80 mM Na2 SeSO3 there is a slight decrease in the observed CdSe nanoparticle number density at long deposition times. We do not believe that the number of CdSe nanoparticles is decreasing. Rather, we believe that the CdSe over-

layer has become sufficiently thick that we slightly under-count the number of nanoparticles deposited. Finally we observe that the concentration of sodium selenosulfate also affects the induction time (delay time) for CdSe nanoparticle formation. In Fig. 5b it can be clearly seen that as the Na2 SeSO3 increases the induction time decreases: at 20 mM Na2 SeSO3 the induction time is 60 min, while at 80 mM Na2 SeSO3 it is 5 min. Finally, the pH of the plating solution influences the CdSe deposition on –COOH terminated SAMs. When the bath pH is adjusted to a starting pH of 10.5 using ammonium hydroxide, fewer CdSe nanoparticles are deposited after 2 h at 45 ◦ C (Fig. 6). However, the diameter of the deposited nanoparticles are similar to those deposited using a bath with a starting pH of 8.0 (the “standard” deposition conditions). Similarly to our previous studies of ZnS CBD [57], our standard deposition conditions include a “seeding” step in which the –COOH SAM was immersed in a Cd2+ solution for 15 min prior to the addition of sodium selenosulfate. To investigate whether the complexation of Cd2+ with the –COOH terminal groups affects the CdSe CBD, we compared the nanoparticles formed under “seeded” conditions with those formed under “unseeded” conditions, that is when the SAM is placed in a bath solution containing both Cd2+ and SeSO3 2− . In the first case the Cd2+ ions are able to complex with the terminal groups prior to CdSe formation, while under “unseeded” conditions pre-adsorbed Cd2+ is kept to a minimum. In contrast to ZnS CBD, we observe that “seeding” does not affect the CdSe deposition with nanoparticles of similar size (diameter) and number density forming under both experimental conditions (Fig. 7). After 2 h CdSe CBD at 45 ◦ C the diameter of the deposited nanoparticles was 138 ± 14 nm and 142 ± 11 nm under “unseeded” and “seeded” experimental conditions. Further, the CdSe nanoparticles formed under “seeded” and “unseeded” conditions strongly adhere to the MHA surface and could not be removed using sonication (Fig. 7 b and d). 3.2. CdSe deposition on –OH and –CH3 terminated SAMs

Fig. 5. The variation of the deposited CdSe nanoparticle (a) diameter and (b) number density (number of CdSe nanoparticles per ␮m2 ) for three different Na2 SeSO3 concentrations, 20 mM, 40 mM and 80 mM for immersion times from 1 min to 120 min at 45 ◦ C. The concentration of CdSO4 was 40 mM. The dotted lines are drawn as guides to the eye.

In contrast to CdSe CBD on –COOH terminated SAMs (Fig. 1), SEM images indicate that no nanoparticles formed at 22 ◦ C after 2 h (Fig. 8). At 45 ◦ C a layer of CdSe nanoparticles are observed to form in both –CH3 and –OH terminated SAMs. The diameter and number density of CdSe nanoparticles are very similar. The nanoparticle diameter was 76 ± 7 nm, while the number densities were 18 ± 3

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Fig. 6. SEM images after CdSe chemical bath deposition at 45 ◦ C for 2 h on –COOH terminated SAMs using a starting bath pH of 10.5 and 8.0.

and 15 ± 3 nanoparticles per ␮m2 on –OH and –CH3 terminated SAMs, respectively. These similarities suggest that the different surface properties of the SAMs did not affect the CdSe nanoparticle growth process, and that the surfaces did not actively participate in the CBD process. Further, the nanoparticles do not strongly adhere to the surface and can be removed by sonication in deionized water for 5 min (Fig. 8).

The TOF SIMS spectra indicate that CdSe did not form an overlayer on either the –CH3 or –OH terminated SAMs at 22 ◦ C and 45 ◦ C. We observe both molecular and cluster ions in the spectra which are characteristic of the SAM (Fig. 9). The spectra also indicate that Cd2+ does not complex with the –CH3 or –OH terminal groups: no ions of the form Cd(CH)x (CH2 )y ± or Cd(O)x (CH)y (CH2 )z ± , characteristic of a Cd2+ -terminal group interaction, are observed. Finally,

Fig. 7. SEM images after CdSe chemical bath deposition at 45 ◦ C for 2 h on –COOH terminated SAMs under the following experimental conditions: (a) seeded deposition prior to ultrasonication; (b) seeded deposition after 5 min ultrasonication; (c) unseeded deposition prior to ultrasonication; and (d) unseeded deposition after 5 min ultrasonication.

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Fig. 8. SEM images after CdSe chemical bath deposition for 2 h on –OH and –CH3 terminated SAMs at (a) and (d) 22 ◦ C, (b) and (e) 45 ◦ C, and (c) and (f) 45 ◦ C after ultrasonication in deionized water for 5 min.

there is no evidence in the TOF SIMS spectra that Cd, Se or CdSe has penetrated through these SAMs to the Au/S interface. 3.3. Reaction pathways of CdSe nanoparticle deposition on –COOH, –OH and –CH3 terminated SAMs In chemical bath deposition a controlled chemical reaction is employed to deposit a thin film on a substrate by precipitation

[26]. One of the central issues in CBD is whether the deposition proceeds via ion-by-ion growth (deposition via successive cation and anion adsorption on the growing crystal) or via cluster-by-cluster deposition (formation of colloidal particles followed by aggregation). Cluster-by-cluster deposition is often associated with the presence of metal hydroxides in solution, which act as nuclei for subsequent chalcogenide growth [26]. For example, in CdS CBD, Cd(OH)2 is believed to catalyze the hydrolysis of thiourea and thus CdS deposition [65]. The possible reaction pathways for CdSe CBD are given below. We note that this proposed mechanism is simplified and ignores the formation of other complexes, including surface Cd2+ -terminal group complexes and Cd(OH)+ . Cd2+ + NTA3−  Cd(NTA)− + NTA3−  Cd(NTA)2 4−

(1)

SeSO3 2− + OH− → HSe− + SO4 2−

(2)

HSe− + OH−  Se2− +H2 O

(3)

2+

Cd

+ Se

2−

 CdSe(s)

(CdSe)m + Cd2+ + Se2−  (CdSe)m+1 (s)

(5)

(CdSe)m + (CdSe)n  (CdSe)m+n (s)

(6)

Cd2+ + 2OH−  Cd(OH)2 (s)

(7)

Cd(OH)2 + Se2− → CdSe(s) + 2OH−

(8)

Cd(OH)2 + Cd

Fig. 9. High resolution negative ion TOF SIMS spectra of the molecular cluster ion, Au2 M− for (a) –OH terminated SAMs (M = –S(CH2 )15 CH2 OH); (b) –CH3 terminated SAMs (M = –S(CH2 )15 CH3 ) prior to and after CdSe chemical bath deposition for 2 h at 22 ◦ C and 45 ◦ C.

(4)

2+

+ Se

2−

 Cd(OH)2 ·CdSe(s)

(9)

In the above equations (s) denotes a precipitate. Eq. (1) describes the complexation of nitriloacetate with Cd2+ . Reactions (2) and (3) describe the hydrolysis of selenosulfate ions to form Se2− ions which then react with Cd2+ to from CdSe (Reaction (4)). Reaction (5) describes the growth of a CdSe cluster by the additional adsorption of Cd2+ and Se2− ions (ion-by-ion growth). These clusters can grow either by the addition of further Cd2+ and Se2− , or by aggregation with other clusters (Reaction (6), cluster-by-cluster deposition). The above reactions assume that there is no Cd(OH)2 present in the plating solution to act as nuclei for CdSe growth. Reactions (7)–(9) describe a reaction pathway for CdSe formation in the presence of Cd(OH)2 . First Cd2+ ions react with OH− ions present in the bath (Reaction (7)) and then form CdSe in a manner similar to that described if no Cd(OH)2 was present (Reactions (8) and (9)). We believe that this hydroxide cluster mechanism does not play a

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role in the CdSe CBD on functionalized SAMs. The solubility product, Ksp , for Reactions (4) (CdSe formation) and (7) (Cd(OH)2 ) are 4 × 10−35 and 2 × 10−14 , respectively [26]. Thus the deposition of CdSe will always be favored except when the concentration of the free Cd2+ concentration is larger than 2 × 10−5 M (pH = 9.5, the pH of the plating solution after addition of sodium selenosulfate and at room temperature). The free concentration of Cd2+ is dependent on the NTA:Cd ratio (Eq. (1)). Under these conditions the critical NTA:Cd ratio to prevent Cd(OH)2 formation is ∼1.7 at room temperature and ∼1.9 at 40 ◦ C [25]. The NTA:Cd ratio employed in these experiments is 3:1 and thus it is unlikely that a hydroxide cluster mechanism is operative. We note that at the longest deposition times, 2 h, the pH of the bath has risen to 10.5. Even at this pH the free Cd2+ concentration is too low for Cd(OH)2 deposition. The experimental data also support this. In the TOF SIMS spectra we find no evidence of Cd(OH)2 formation. Further, we do not observe any sudden changes in the nanoparticle size or number density, suggesting that deposition mechanism has not changed and that Cd(OH)2 does not play a role in CdSe CBD on functionalized SAMs. The observed slow increase in the plating bath pH may also be explained by Reactions (1)–(7). Gorer and Hodes [25] proposed that the first step in hydrolysis of selenosulfate is: 2SeSO3 2− + H2 O  HSe− + SeS2 O6 2− + OH−

(10)

followed by reaction to form sulfate ions and further HSe− ions. This first reaction step leads to an increase in the pH due to the generation of OH− ions. Further, the pH will continue to increase as long as the HSe− ions do not react with Cd2+ ions to form CdSe that is during the earliest stages of deposition. We observe an induction

time in CdSe CBD from 5 min to 60 min depending on the initial sodium selenosulfate concentration. Thus it appears that the above reaction (Reaction (10)) is relatively slow and that an induction time is required to build a sufficient concentration of HSe− and Se2− ions for CdSe CBD to begin. During this time we observe the largest changes in the pH, in agreement with the proposed reaction pathway. Further, if the bath pH is increased the deposition of CdSe is observed to be slower with fewer nanoparticles formed. These observations are consistent with the hypothesis that the observed induction time is due to the hydrolysis of selenosulfate (Reaction (10)): as the pH of the bath is increased, there will be a decrease in the concentration of HSe− ions (by Le Chatelier’s principle) and so the CdSe deposition will be reduced. Finally, we note that there are other possible reactions that lead to an increase in the bath pH including the oxidation of HSe− by dissolved oxygen [25], and these likely cause the observed slow rise in pH during deposition. 3.3.1. CdSe deposition on –COOH terminated SAMs At first glance the CdSe CBD on –COOH terminated SAMs follows a cluster-by-cluster mechanism. The deposited CdSe exhibits non-epitaxial growth – forming nanoparticles on the surface – and the growth rate is strongly temperature dependent. Both these observations indicate that a cluster-by-cluster mechanism is operative. However, there are several features of the CdSe deposition which cannot be explained by cluster-by-cluster growth. First, in the TOF SIMS spectra we observe Cd(COO)2 (CH2 )x (CH)y + ions, indicating that Cd2+ ions complexes with the carboxylate terminal groups and that the surface actively participates in the reaction. Second, the deposited CdSe nanoparticles strongly adhere to the

Fig. 10. (a) Optical, (b) SEM and, (c) and (d) TOF SIMS images after CdSe chemical bath deposition on a patterned –CH3 /–COOH terminated SAM surface for 15 min at 45 ◦ C. The SEM image indicates that the CdSe nanoparticles (dark grey) have deposited only on the –COOH terminated SAM areas. The TOF SIMS images are centered at 78 Se− (m/z 78) (c) and Au2 M− (m/z 651, M = –S(CH2 )15 CH3 ) (d). The intensity of the 78 Se− ion indicates that CdSe has been selectively deposited on the –COOH terminated SAM (MHA) areas. The Au2 M− ion is characteristic of HDT and its high intensity indicates that the SAM is free from chemical contamination and was undamaged by the CdSe CBD. TOF SIMS analysis: primary ion, Bi+ ; kinetic energy = 25 keV; area of analysis = 500 ␮m × 500 ␮m, 128 pixels × 128 pixels; intensity scale: white, highest number of counts; black, zero counts.

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MHA, and cannot be removed using sonication. Third, the diameter of the deposited CdSe nanoparticles is about twice as large as those formed on –OH and –CH3 terminated SAMs when using the same deposition bath. These observations suggest that an ion-byion mechanism is also operative. We therefore propose the following reaction mechanism for CdSe CBD on –COOH terminated SAMs. During the “seeding” step (under standard deposition conditions) the deposition plating solution has a pH of 8.0 which is lower than the pKa of hexadecanoic acid (∼8.5 to 8.8) [66] indicating that Cd2+ ions cannot complex with the –COOH terminal groups since these groups are not deprotonated at this pH. Upon addition of Na2 SeSO3 to the bath, the pH increases to 9.5 and so ∼90% of the –COOH terminal groups deprotonate. Thus, Cd2+ ions can complex with the SAM terminal groups to form Cd2+ carboxylate complexes [67]. Once formed, these complexes react with Se2− ions to from CdSe nuclei via an ion-by-ion mechanism. These CdSe crystallites are likely to be small and form an underlying CdSe layer, which may not be able to be observed in SEM. This reaction pathway also explains the observation that “seeding” does not affect the CdSe CBD process since the Cd2+ -carboxylate complexes cannot form until the addition of selenosulfate ions to the bath. After a sufficient concentration of HSe− and Se2− has been reached, CdSe colloidal cluster formation begins both in solution and on the MHA surface. At this point cluster-by-cluster growth becomes the dominant reaction mechanism. The clusters formed via heterogeneous nucleation processes on the SAM surface strongly adhere to it and cannot be removed using sonication. 3.3.2. CdSe deposition on –OH and –CH3 terminated SAMs The CdSe nanoparticles deposited on –OH and –CH3 terminated SAMs are of the same diameter, have similar deposition number densities and only weakly adhere to the surface. These observations indicate that these SAM surfaces do not play a role in the deposition mechanism. Further, the CdSe growth rate is strongly dependent on the bath deposition. At 22 ◦ C no CdSe nanoparticle deposition is observed, while at 45 ◦ C a large number of monodisperse nanoparticles are formed after 2 h. Together these data indicate that CdSe CBD on –OH and –CH3 terminated SAMs follows a cluster-by-cluster mechanism. In this case, Cd2+ and Se2− form colloidal CdSe clusters in solution. These clusters then aggregate until they reach a critical size, precipitate, and adsorb to the surface. Since there is no chemical interaction between the CdSe nanoparticles and the SAM the deposited CdSe only weakly adheres to the surface and can be easily removed using sonication. 3.4. Selective deposition of CdSe on patterned SAMs The above date suggest that CdSe can be selectively deposited on patterned –OH/–COOH and –CH3 /–COOH terminated SAMs. Fig. 10 displays a UV photopatterned –CH3 /–COOH terminated SAM after 15 min CdSe CBD at 45 ◦ C and sonication for 5 min in deionized water. The optical and SEM images indicated that CdSe has only deposited in the –COOH terminated SAM areas (“bar” areas). TOF SIMS images confirm these observations: Cd+ (data not shown) and Se2− (Fig. 10c) ions are only observed in the “bar” areas. Further, the data also show that the –CH3 terminated SAM (HDT) is undamaged by the CdSe CBD process. High intensities of molecular and cluster ions characteristic of HDT, such as AuM2 − (m/z 651; M = –S(CH2 )15 CH3 ), are observed in the square areas (Fig. 10d). 4. Conclusions The chemical bath deposition of CdSe on functionalized SAMs proceeds both by ion-by-ion growth and cluster-by-cluster deposition. On –COOH terminated SAMs, Cd2+ ions in solution complex with the terminal group forming Cd2+ -carboxylate complexes.

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Once formed these complexes react with Se2− ions to form via ionby-ion growth a thin CdSe layer, which serves as the nucleation sites for subsequent CdSe growth. After there is a sufficient concentration of HSe− , and therefore Se2− , ions in solution (generated by the hydrolysis of SeSO3 2− ) the deposition proceeds via a clusterby-cluster mechanism both on the –COOH terminated SAM surface and in solution. On –OH and –CH3 terminated SAMs, neither the Cd2+ nor Se2− ions interact with the SAM surface. In this case, the dominant deposition mechanism is cluster-by-cluster deposition in solution, leading to the deposition of monodisperse nanoparticles which do not strongly adhere to the SAM surface. We also demonstrate that under the appropriate experimental conditions CdSe can be deposited on patterned SAMs. Together with our previous studies of ZnS CBD on functionalized SAMs [57], these results indicate that the morphology, size and coverage of deposited semiconductors can be controlled on organic films using CBD. Critical parameters include the temperature and pH of the bath, as well as the chemical identity of the SAM. These data also show that for stable, adherent overlayers to form on organic thin films, metal-terminal group complexes must first be formed which act as nucleation sites for further film growth. Our data also indicate that –COOH terminated SAMs are well suited for the formation of such complexes particularly for baths with pH ≥ 9. Finally, these studies indicate that CBD is a suitable method for the large-area selective deposition of semiconducting thin films for applications in optoelectronics and sensing. References [1] T. Trindade, P. O’Brien, N.L. Pickett, Chem. Mater. 13 (2001) 3843. ´ [2] N. Tomczak, D. Janczewski, M.Y. Han, G.J. Vancso, Prog. Polym. Sci. 34 (2009) 393. [3] V.L. Colvin, M.C. Schlamp, A.P. Alivisatos, Nature 370 (1994) 354. [4] B.O. Dabbousi, M.G. Bawendi, O. Onitsuka, M.F. Rubner, Appl. Phys. Lett. 66 (1995) 1316. [5] J. Lee, V.C. Sundar, J.R. Heine, M.G. Bawendi, K.F. Jensen, Adv. Mater. 12 (2000) 1102. [6] M.C. Schlamp, X. Peng, A.P. Alivisatos, J. Appl. Phys. 82 (1997) 5837. [7] J. Zhao, J.A. Bardecker, A.M. Munro, M.S. Liu, Y. Niu, I. Ding, J. Luo, B. Chen, A. Jen, D.S. Ginger, Nano Letters 6 (2006) 463. [8] I.L. Medintz, H.T. Uyeda, E.R. Goldman, H. Mattoussi, Nat. Mater. 4 (2005) 435. [9] R.C. Somers, M.G. Bawendi, D.G. Nocera, Chem. Soc. Rev. 36 (2007) 579. [10] V.I. Klimov, S. Tretiak, P.O. Anikeeva, I.V. Bezel, L.P. Balet, M. Achermann, A. Piryatinski, J. Nanda, S.A. Ivanov, J. Phys. Chem. B 108 (2004) 10625. [11] W.U. Huynh, J.J. Dittmer, A.P. Alivisatos, Science 295 (2002) 2425. [12] W.U. Huynh, X.G. Peng, A.P. Alivisatos, Adv. Mater. 11 (1999) 923. [13] I. Robel, S.V. Subramanian, M. Kuno, P.V. Kamat, J. Am. Ceram. Soc. 128 (2006) 2385. [14] N.C. Greenham, X.G. Peng, A.P. Alivisatos, Phys. Rev. B 54 (1996) 17628. [15] B. Sun, E. Marx, N.C. Greenham, Nano Letters 3 (2003) 961. [16] D. Samanta, B. Samanta, A.K. Chaudhuri, S. Ghorai, U. Pal, Semicond. Sci. Technol. 11 (1996) 548. [17] K.N. Shreekanthan, B.V. Rajendra, V.B. Kasturi, G.K. Shivakumar, Cryst. Res. Technol. 38 (2003) 30. [18] Y. Golan, L. Margulis, I. Rubinstein, G. Hodes, Langmuir 8 (1992) 749. [19] H. Wynands, M. Cocivera, J. Electrochem. Soc. 139 (1992) 2052. [20] T. Elango, S. Subramanian, K.R. Murali, Surf. Coat. Technol. 123 (2003) 8. [21] R.B. Kale, S.D. Sartale, B.K. Chougule, C.D. Lokhande, Semicond. Sci. Technol. 19 (2004) 980. [22] H.M. Pathan, B.R. Sankapal, J.D. Desai, C.D. Lokhande, Mater. Chem. Phys. 78 (2003) 11. [23] J.R. Heine, J. Rodriguez-Viejo, M.G. Bawendi, K.F. Jensen, J. Crys. Growth 195 (1998) 564. [24] C.X. Shan, Z. Liu, C.M. Ng, S.K. Hark, Appl. Phys. Lett. 86 (2005) (art. no. 213106). [25] S. Gorer, G. Hodes, J. Phys. Chem. 98 (1994) 5338. [26] G. Hodes, Chemical Solution Deposition of Semiconductor Films, Marcel Dekker, Inc., New York, 2002. [27] H. Cachet, H. Essaaidi, M. Froment, G. Maurin, J. Electroanal. Chem. 396 (1995) 175. [28] P.P. Hankare, V.M. Bhuse, K.M. Garadkar, S.D. Delekar, I.S. Mulla, Semicond. Sci. Technol. 19 (2004) 70. [29] P.P. Hankare, S.D. Delekar, M.R. Asabe, P.A. Chate, V.M. Bhuse, A.S. Khomane, K.M. Garadkar, B.D. Sarwade, J. Phys. Chem. Solids 67 (2006) 2506. [30] G. Hodes, A. Albu-Yaron, F. Decker, P. Motisuke, Phys. Rev. B 36 (1987) 4215. [31] R.C. Kainthla, D.K. Pandya, K.L. Chopra, J. Electrochem. Soc. 127 (1980) 277. [32] C.D. Lockhande, E.-H. Lee, K.-D. Jung, O.-S. Joo, Mater. Chem. Phys. 91 (2005) 200.

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