INTRODUCTION
In recent years, thin films of semiconductor nanoparticles have found an increasing number of applications in solar energy conversion, electronics, and light-emitting diodes (LEDs).[1-13] Different approaches, such as Langmuir-Blodgett (LB), self-assembly (SA), and layer-by-layer assembly (LBL), were developed to fabricate thin films.[4,11,14] Among these approaches, layer-by-layer assembly offers a simple scheme to fabricate films consisting of different layers. This may be potentially used to construct films with advanced functions, such as electron and energy transfer between component layers. Layer-by-layer assembly techniques which are based on electrostatic attraction between oppositely charged molecules in adjacent layers were successfully applied to the construction of multilayer films of molecules,[15] nano-particle/polymer mixtures,[16-18] and other inorganic materials.[19] This LBL assembly method is quite simple and versatile.[11] In the preparation process, the substrate, functionalized with a charged layer, is first immersed in a solution containing materials of opposite charge to allow the deposition of the first layer. The resultant film is then rinsed and dried. To deposit the second layer, the same procedure is repeated for materials of opposite charges to the first layer. The deposition cycle can be repeated until the desired number of bilayers is reached. Because of the strong interactions between the oppositely charged layers, the LBL films have shown long-term mechanical stability in air, water, and other polar solvents.[20] Many nanoparticles, such as TiO2, CdSe, and Au, were successfully assembled into films, for which a number of advanced electronic and photonic applications were demonstrated.1-16,17,19-1
As an alternative method to electrostatic interaction-based LBL assembly, multilayer films based on hydrogen bonding have been reported by Stockton and Rubner[21] using polyaniline, and by Wang et al.[22,23] using poly(vinyl pyridine)/poly(acrylic acid) (PVP/PAA). Recently, we extended this hydrogen bonding-based LBL approach to assemble inorganic nanoparticles.[24,25] In this report, we review our group’s work on layer-by-layer assembly of metal and semiconductor nanoparticles. Both films of single- and multiple-particle types (size or chemical nature) have been fabricated. We will describe two approaches of LBL assembly based on hydrogen bonding interaction between adjacent layers. We will discuss the use of UV-visible absorption spectrum to monitor the buildup of the multilayers and Fourier transform infrared (FTIR) spectroscopy to investigate the hydrogen-bonding interaction between the layers.
EXPERIMENTAL SECTION
Materials
Poly(4-vinylpyridine) (PVP) (Mw = 6.0x 104), poly (acrylic acid) (PAA) (Mw=2.4 x 105), polyethylenimine (PEI) (Mw=7.5 x 105), 4-mercaptobenzoic acid (4-MBA), selenium power (99.99%), sulfur powders, tellurium powders, hydrogen tetrachloroaurate(III) trihydrate, cadmium acetate dihydrate (98%), and sodium borohydride were purchased from Aldrich. Methanol and dimethylfor-mide were used as they were found.
Synthesis of Au Nanoparticles with Carboxyl Group Tailored Surfaces (4-MBA-Au)
4-Mercaptobenzoic acid (4-MBA, HS-C6H4-COOH)-capped Au nanoparticles were synthesized in methanol/ acetic acid solution following a published procedure with a slight modification.1-26-1 Briefly, 0.40 mmol of tetrachlo-roauric acid and 1.2 mmol of 4-mercaptobenzoic acid were codissolved in 35 mL of 6:1 methanol/acetic acid, producing a yellow solution. NaBH4 (0.3 g, 8 mmol) in 15 mL of methanol was added with rapid stirring. The solution immediately changed to a black suspension. The suspension was stirred for an additional 30 min. The solvent was then removed under vacuum. The black product was washed several times with diethyl ether and dried under an N2 stream. The sample was soluble in polar solvents such as water and methanol.
Synthesis of Au Nanoparticles with Pyridine Group Tailored Surfaces (Py-Au)
A total of 55.8 mg of PVP was dissolved in 150 mL of methanol, into which 70.8 mg of HAuCl4 dissolved in 10 mL of methanol was added under rapid stirring.[24] The molar ratio of metal salts to pyridine units was about 1:2. Ten minutes later, 27 mg of NaBH4 in 10 mL of methanol was quickly added. A change of color from yellow to pink was immediately observed, indicating the formation of Au nanoparticles. After 30 min of further stirring, the solution was maintained at 278 K for further use.
Synthesis of CdS (CdSe) Nanoparticles in N,N Dimethylformide (DMF)
Cadmium acetate dihydrate powder was suspended in ethanol and refluxed for 3-6 h, yielding a 0.1 M Cd precursor. NaHS (NaHSe) ethanolic solution (0.56 M) was prepared from the reaction between selenium and sodium borohydride in absolute ethanol.[27] For CdS (CdSe) nanoparticle preparation, 10 mL (1 mmol) of the Cd precursor was evaporated under reduced pressure. The resulting dry residue was dissolved in 100 mL DMF, into which 4-mercaptobenzoic acid (2.5 mmol) was added. The colorless solution was then reacted with 1.0 mL of NaHS (NaHSe) ethanolic solution under argon atmosphere, producing an orange CdS (brown CdSe) colloidal solution. After further stirring for 30 min, the solution was kept at 278 K for future use.
Synthesis of 4-MBA-Capped CdTe Nanoparticles
4-MBA-capped CdTe nanoparticles were synthesized in two steps: first, we prepared TOPO-capped CdTe nanoparticles according to a published procedure;[28,29] then, the TOPO capping groups on CdTe nanoparticles were substituted by 4-MBA in DMF solutions, following literature procedures.[30]
Fabrication of Polymer/Metal (Semiconductor) Nanoparticle Multilayer Films
To fabricate LBL films of polymer/metal (semiconduc-tor)-nanoparticles, it was first necessary to pretreat the substrates. In this step, we functionalized the substrate (e.g., quartz, CaF2 plates, cover glass, and carbon-coated copper grids) with an NH2-tailored surface by immersing into a branched polymer (PEI, 0.5 wt.%) solution. Poly-ethylenimine is an attractive choice because it can adhere to a variety of substrate surfaces. Au nanoparticles with carboxyl group and pyridine group tailored surfaces were assembled according to the following two routes.
LBL assembly of PVP and 4-MBA-Au nanoparticles
The PEI-pretreated substrates were transferred to a PAA (0.252 g/L) methanol solution for 10 min, resulting in a COOH-tailored surface. The substrates were then immersed in a PVP (0.256 g/L) methanol solution for 5 min to add a layer of PVP. After rinsing with methanol, the substrates were then transferred to a methanol/acetic acid (10:1) solution of Au nanoparticles for 10 min to deposit one layer of Au nanoparticles. The procedure was repeated until the desired number of bilayers of PVP/4-MBA-Au was reached as shown in Fig. 1.
LBL assembly of PAA and Py-Au nanoparticles
As schematically shown in Fig. 2, the PEI-pretreated substrates were first immersed in a PAA (0.252 g/L) methanol solution for 10 min, resulting in a COOH-tailored surface. After washing with methanol, the substrates were transferred to a methanol solution of PVP-capped Au nanoparticles for another 10 min, adding one layer of Au nanoparticles. Multilayer films can be obtained by repeating the last two steps.
LBL Assembly of PVP/CdSe and PVP/CdS Multiplayer Films
Briefly, the substrates treated with PEI solutions were transferred to a PAA methanol solution (0.252 g/L) to form a carboxylic acid terminated surface. The resulting substrates were immersed in PVP methanol solution (0.256 g/L) for 10 min to add a PVP layer. After being rinsed with methanol, the substrates were immersed into a dilute CdS (CdSe) colloid (DMF/methanol=1:1) for 10 min to form a CdS (CdSe) layer. Multilayer films can be obtained by repeating the last two steps, as shown schematically in Fig. 1.
Fig. 1 Schematic representation of the buildup of multilayer assembly by consecutive adsorption of PVP and metal or semiconductor nanocrystals (NC).
LBL Assembly of CdSe/CdTe Mixed Multilayer Films
Cover glasses were first treated with PEI as described previously. The pretreated substrates were immersed into a CdSe colloid in DMF for 10 min to form a CdSe layer. The resulting substrates were rinsed with methanol and then immersed in PVP methanol solution (0.256 g/L) for 10 min to add a PVP layer. The substrates, after rinsed by methanol, were immersed in a CdTe colloid in DMF for another 10 min to deposit a layer of CdTe. Multilayer films could be fabricated by depositing PVP/CdSe/PVP/ CdTe layers alternately.
LBL Assembly of CdS/Py-Au Multilayer Films
The CdS/Py-Au multilayer films were assembled according to the following procedure: CaF2 plates were sequentially treated in PEI, PAA, and PVP solutions. The substrates were then immersed in a dilute CdS colloid solution (DMF/methanol= 1:1) for 10 min to form a CdS layer. The substrates were then rinsed in methanol and dried. After that, the substrates were immersed into the Py-Au solution for 10 min. Multilayer films could be fabricated by depositing CdS and Py-Au layers alternately.
RESULTS AND DISCUSSION
Layer-by-Layer Assembly of Single-Composite Nanoparticles
LBL assembly of gold nanoparticles
Poly(vinyl pyridine) (PVP) is known to be a strong metal-chelating agent.[31'32] We synthesized stable Au nanopar-ticles in the presence of pure PVP, producing pyridine group tailored surfaces (hydrogen-bonding acceptors). The UV-vis spectrum of Py-Au methanol solution (Fig. 3, upper curve) showed a pronounced surface plasmon (SP) band around 530 nm. Although the UV-vis spectrum of 4-MBA-capped Au nanoparticles in methanol solution (Fig. 3, bottom curve) showed only a weak SP band, transmission electron microscopy (TEM) images indicate that the Au nanoparticles were round in shape and had an average diameter of about 2.6 ±0.9 nm.
Fig. 2 Schematic representation of the buildup of multilayer assembly by consecutive adsorption of PAA and PVP capped metal or semiconductor nanocrystals.
Multilayer film buildup of Au nanoparticles was monitored by using UV-vis spectroscopy. The observed absorption could be attributed to 4-MBA and Au particles because polymer absorption in the visible region was found to be negligible. Fig. 4(a) shows the UV-vis absorption spectra of the multilayer film of PVP/(4-MBA-Au) with different number of bilayers on a quartz slide. The absorption spectra of PVP/(4-MBA-Au) nanoparticle multilayer films were in agreement with that of the Au nanoparticle solution (Fig. 3), indicating the successful assembly of 4-MBA-Au nanoparticles into the film. In addition, a linear increase of the absorbance with the number of bilayers (Fig. 4(a)) was observed, suggesting that approximately the same amount of Au nanoparticles was adsorbed in every deposition cycle.
A similar linear increase of absorbance with the number of bilayers (Fig. 4b) was observed for the PAA/ (Py-Au) LBL film, again indicating a stepwise and uniform assembly process. Such behavior has been observed in many layer-by-layer deposition systems based on electrostatic interaction.[16-18,33,34] Our result demonstrated that layer-by-layer assembly based on hydrogen bonding provided an alternative way to fabricate homogeneous organic/inorganic multilayer films.
The PVP/(4-MBA-Au) LBL film was also studied by small-angle X-ray diffraction (SAXD), which is a convenient and direct method for determining film thickness.[35,36] Fig. 5 shows the SAXD pattern of a 12-bilayer film of PVP/(4-MBA-Au) on a quartz substrate. The X-ray curve revealed well-defined Kiessig fringes, which indicated that the PVP/(4-MBA-Au) LBL film was uniform and flat. The total thickness of the film was estimated to be about 39.6 nm from the oscillation period. Because the UV-vis results demonstrated a stepwise and uniform assembly process, the thickness of one PVP/Au bilayer was calculated to be about 3.3 nm.
The direct evidence for hydrogen bonding between PVP and 4-MBA on the surface of Au nanoparticles was obtained by using FTIR spectroscopy. Hydrogen bonding between pyridine groups and carboxyl groups resulted in a splitting pattern of the OH stretching bands.[37-40] Fig. 6 shows the IR spectra of a cast film of 4-MBA-Au nanoparticles, a cast film of PVP, and a 10-bilayer PVP/ (4-MBA-Au) film on CaF2 plates. The spectrum of the 4-MBA-Au nanoparticles shows a pronounced C=0 band at 1678 cm"1, which is consistent with that of 4-MBA in KBr glass.[41,42] This band, as well as the broad OH stretch (~ 3450 cm"1), indicates hydrogen-bonding interaction between terminal carboxyl groups of 4-MBA on the surface of Au nanoparticles.[41-43] The absorption peaks at 1595.8, 1556.6, and 1414.2 cm~1 in the spectrum of the cast film of PVP are assigned to ring vibrations of pyridine groups of PVP.[22]
Fig. 3 UV-visible spectra of 4-MBA capped Au nanoparticles and Py-Au nanoparticles in methanol.
Fig. 4 (a) UV-visible absorption spectra of the PVP/(4-MBA-Au) nanoparticle multilayer film with different numbers of bilayers. From the lower to upper curves, the number of PVP/(4-MBA-Au) bilayers is 1, 2, 3, 4, and 5. Shown in the inset is a plot of the absorbance at 400 and 500 nm vs. the number of PVP/(4-MBA-Au) bilayers. (b) UV-visible absorption spectra of PAA/(Py-Au) nanoparticle multilayer thin film with different numbers of bilayers. From the lower to upper curves, the number of PAA/(Py-Au) bilayers is 1, 2, 3, 4, and 5. The inset is a plot of the absorbance at 530 nm vs. the number of PAA/(Py-Au) bilayers.
After Au nanoparticles were assembled into multilayer film, the C=0 stretching vibration was shifted to 1710 cm"1. Another striking feature is the appearance of new bands at 1937 and 2520 cm"1. These bands were assigned to the stretching bands of hydrogen-bonded OH groups based on a similar splitting pattern of OH groups observed in polymer blends containing carboxyl and vinylpyridine groups.[37-40] The presence of these bands in the multilayer thin films indicated strong hydrogen bonding between the carboxyl groups (donors) on the surface of Au nanoparticles and pyridine groups (acceptors) of PVP. The peaks in the spectral region from 1650 to 1400 cm"1 were difficult to assign because of overlapping absorption bands, although they arose mainly from the ring vibration of both 4-MBA and pyridine groups.
Fig. 5 Small-angle X-ray diffraction patterns of a 12-bilayer film of PVP/Au on a quartz substrate.
Fig. 6 FTIR spectra of a cast of film of PVP, a cast film of 4-MBA-Au nanoparticles, and a 10-bilayer PVP/Au film on CaF2 plates.
Fig. 7 FTIR spectra of a cast film of pure PAA and a 10-bilayer PVP/Au film on CaF2 plates.
Fig. 7 shows the FTIR spectra of a cast film of PAA and a 10-bilayer PAA/(Py-Au) film on CaF2 slides. The IR spectrum of pure PAA shows a broad absorption band around 3000 cm"1 and the C=C) stretching vibration at 1709 cm"1, indicating that the polymer was in an associated state.[22] For the PAA/Py-Au films, in addition to a shift of C=0 stretching band to 1723 cm" O—H stretching vibrations at 2530 and 1943 cm"1 were also observed. These spectral features indicated strong hydrogen bonding between the carboxyl groups of PAA and pyridine groups on the surface of Au nanoparticles.[37-40] Ring vibrations of pyridine groups at 1605, 1557, and 1416 cm" 1 were also observed.
