Ring and Disk-Like CdSe Nanoparticles Stabilized

with Copolymers

Amir Wagih Fahmi, Ulrich Oertel,* Volker Steinert, Christian Froeck, Manfred Stamm

Institute of Polymer Research Dresden e.V., Hohe Str. 6, 01069 Dresden, GermanyFax: þ49 (0)351 4658 284; E-mail: oertel@ipfdd.de

Received: April 14, 2003; Revised: June 6, 2003; Accepted: June 10, 2003; DOI: 10.1002/marc.200350005

Keywords: cadmium selenide; diblock copolymers; LB films; nanoparticles; ring structure

Introduction

The fabrication of polymer-containing nanoparticles of

inorganic semiconductors[1–3] is an active research field

because of the interesting physical properties of those

particles (quantum dots with confinements effects), which

can be far from those in bulk. For this reason, they have high

potential for utilization in optical, electronic and electro-

optical devices.[4,5] The control of size, shape and size

distribution of the nanoparticles remains a main focus due

to the correlation with optical and electronic properties. A

very important aim is to control themorphology, orientation

and arrangement of the particles. Thus, the production of

nanoparticles and their manipulation has become one of the

most important topics in nanotechnology.

The assembly of charged nanoparticles on adsorbed

polymer films, such as poly(ethyleneimine) or poly(dial-

lyldimethylammonium chloride), was used to obtain order-

ed structures of metallic,[6] semi-conducting,[7] silicate[8]

and even mineral[9] nanoparticles. Block copolymers have

been used extensively as a surrounding and stabilizing

matrix of the nanoparticles due to their possible bipolar

nature and dedicated phase behavior.[10–12] The use of the

Langmuir-Blodgett (LB) technique to produce monoparti-

cular layers[13] has been demonstrated, too. Ring structures

on the mm scale were found by several authors including

those prepared with the LB technique.[14,15]

In this work, CdSe nanoparticles were prepared and

stabilized with the amphiphilic diblock copolymer poly-

styrene-block-poly(4-vinyl pyridine) (PS-b-P4VP). The

resulting dispersions were stable in both toluene and

chloroform. Since these polymer-stabilized nanoparticles

cannot be dispersed in water, the LB technique was chosen

to prepare ultrathin films containing polymer-stabilized

CdSe nanoparticles. During these studies, some unexpected

regular geometric structures were observed by means of

atomic force (AFM) and scanning electron (SEM) micro-

scopy,whichmayallowapplicationinelectronicsorcatalysis.

Communication: CdSe nanoparticles stabilized with theamphiphilic diblock copolymer polystyrene-block-poly(4-vinylpyridine) were spread from toluene dispersion on thewater surface. Monolayers could be transferred onto solidsubstrates using the Langmuir-Blodgett technique. Bymeansof atomic force and scanning electron microscopy highlysymmetric ring and disk-like structures with diametersranging between 150 nm and 1200 nm were observed.

AFM image of amixedmonolayer of copolymer 12 andCdSenanoparticles stabilized with polystyrene-block-poly(4-vinylpyridine).

Macromol. Rapid Commun. 2003, 24, 625–629 625

Macromol. Rapid Commun. 2003, 24, No. 10 � WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2003 1022-1336/2003/1007–625$17.50þ.50/0

Experimental Part

Materials

PS-b-P4VPwaspurchased fromPolymerSource, Inc. (Canada,polydispersity Mw/Mn¼ 1.07). The molecular weights wereMw¼ 41 300 g/mol and 1900 g/mol for the PS and P4VPblocks, respectively. Cd(CH3COO)2 and Se were purchasedfromAlfa Aesar (Germany).N,N-dimethylformamide (DMF),toluene and chloroform were of analytical grade (Fluka,Germany).Synthesis and application of the amphiphilic copolymer

poly[(maleic acid hexadecylmonoamide)-co-propylene] (12)to prepare LB films has been described previously.[16]

The water used for the Langmuir monolayer experimentswas purified using a Milli-Q Plus system (Millipore, USA,18.2 MO/cm) fed by a Milli-RO-5 instrument (Millipore,USA). Synthetic quartz plates of Spektrosil B (25 mm�75 mm� 0.5 mm, Saint-Gobain Quartz, Germany) were usedas transparent supports for LB film preparation. For AFMexperiments, silicon wafers (SilChem, Freiburg) were used.Both quartz plates and siliconwafers were cleaned in amixtureof H2SO4 and potassium dichromate.

Synthesis of CdSe Nanoparticles Stabilized with PS-b-P4VP

Cadmium acetate dihydrate powder was suspended in ethanoland refluxed for 3 h, yielding 0.1 M Cd precursor solution.Ethanolic NaHSe solution (0.506 M) was prepared fromthe reaction between selenium and sodium borohydride inabsolute ethanol according to a procedure given in theliterature.[17] For the preparation of CdSe nanoparticles,10 ml (1 mmol) of the Cd precursor solution was evaporatedunder reduced pressure. The resulting dry residue wasdissolved in 100 ml of DMF containing PS-b-P4VP ([PS-b-P4VP]¼ 0.223 M base units) and stirred for at least 1 h tocompletely dissolve the Cd salt. The colorless solution wasthen stirred with 1.0 ml of the ethanolic NaHSe solution for30 min at room temperature under N2 atmosphere. The yellowdispersion of CdSe/PS-b-P4VP nanoparticles was stored at278 K.

Methods

For the Langmuir monolayer and transfer experiments, theCdSe dispersion (preparation as described above) was dilutedwith toluene by a factor of 25 ([CdSe]¼ 0.0002 M, [PS-b-P4VP]¼ 0.0091 M base units, 4 vol.-% DMF, 96 vol.-%toluene). In the following, this dispersion will be denoted as‘‘CdSe spreading solution’’. The concentration of CdSe wascalculated assuming complete conversion of NaHSe.Langmuir monolayer formation and deposition experiments

were carried out in a clean room (class 10 at theworking place)with the commercial film balances KSV 3000 (KSV Instru-ments, Finland) and FW 2 (Lauda Dr. R. Wobser GmbH,Germany). Compression speed for the isothermswas 29.2 cm2/min. The initial compression speed for the transfer experimentswas 90 cm2/min and lowered to 36 cm2/min and 18 cm2/minafter reaching a film pressure of 5 mN/m and 10 mN/m, res-pectively. Monolayer transfers were performed at a surface

pressure of 17 mN/m. The film areas became constant within90 min after reaching the target pressure. Then monolayerdepositions were started with a dipping speed of 1.3 mm perminute. Brewster angle microscopy was done with a Nima501 LB trough (Nima Technologies, GB) combined with theBrewster angle microscope BWM 1 (Nanofilm Technologies,Germany). For monolayer formation and deposition experi-ments the subphase temperature was 22� 1 8C.

For the preparation of the mixed monolayers appropriateamounts of the CdSe spreading solution and of the amphiphile12 (in chloroform) were mixed before spreading.

UV-vis spectra were recorded on a Lambda 800 spectro-photometer (Perkin-Elmer, Germany). Fluorescence spectrawere measured with a LS 50 fluorescence spectrometer(Perkin-Elmer, UK). The fluorimeter is corrected for thewavelength dependent throughput of the excitation part. Amodified sample cell was used. The incident angle of theexcitation beam was 458.

AFM was carried out with a Dimension 3100 system (DI,Santa Barbara). The tip used for the experiments was anultrasharp noncontact silicon cantilever (NSC16) from mikro-mash (Tallin, Estonia). The area of interest was scanned in thetappingmode and so the damage of the surfacewasminimized.SEM was performed with a Zeiss DSM 982 Gemini.

Results and Discussion

Stable dispersions of polymer-stabilized CdSe nanoparti-

cles were prepared using the procedure described in the

Experimental Part. Figure 1 shows the UV-vis spectrum of

the initially obtained dispersion after dilution with toluene

(curve 1) in comparison to the spectrum of pure PS-b-P4VP

(curve 2) at the same concentration as in the CdSe disper-

sion. The molar extinction coefficient of the CdSe nanopar-

ticles at 370 nm was calculated to be eCdSe¼ 2950 l/mol�cm and was therefore much smaller than reported in the

literature.[18] Possibly, only a minor part of the Cd in the

CdSe spreading solution was converted to CdSe. Absorp-

tion of the CdSe below 400 nm is an indication of the small

Figure 1. UV-vis spectra of the CdSe spreading dispersion(curve1) andofPS-b-P4VP (curve2; [PS-b-P4VP]¼ 9.1�10�3

M

in toluene/DMF (4 vol.-% DMF)), and fluorescence spectrum ofthe CdSe spreading dispersion (curve 3; lexc¼ 325 nm).

626 A. Wagih Fahmi, U. Oertel, V. Steinert, C. Froeck, M. Stamm

size of the nanoparticles (average particle size< 10 nm) but

the broad absorption peak is due to a considerable size

distribution.[19,20] These CdSe nanoparticles with an aver-

age particle size below 10 nm are stabilized by interaction

with PS-b-P4VP. The assemblies will be further addressed

as polymer-stabilized nanoparticles. The CdSe nanoparti-

cles exhibit a broad but relativeweak fluorescence emission

(curve 3 in Figure 1) with a maximum at 381 nm and a

shoulder at 445 nm.

Attempts to prepare solution cast films of the polymer-

stabilized CdSe nanoparticles in order to determine the size

of individual particles and their distribution failed: They

reveal a tendency to form clusters and large but interesting

structures were observed. Otherwise, the polymer-stabi-

lized CdSe nanoparticles appeared to be stable in the

toluene dispersion. Therefore, monofilms of the polymer-

stabilized nanoparticles were prepared by means of the LB

technique. In a first experiment the dispersion already used

for UV-vis measurements was spread on the water surface.

Compression of the monolayer led to a very inhomoge-

neous film as could be recognized visually. The CdSe

spreading solution was thus mixed with a solution of the

good monofilm-forming polymer poly[(maleic acid hex-

adecylmonoamide)-co-propylene] (12)[16] to prevent

association between the polymer-stabilized nanoparticles

andthestabilizingmacromoleculesofPS-b-P4VP.Figure2a

shows the pressure versus area isotherms as a function of the

total remaining trough area. This scalewas chosen since the

evaluation of a molecular area in such mixedmonolayers is

insignificant and depends on the reference. Brewster angle

microscopy (Figure 2b) gives an impression of the inhomo-

geneous nature of the Langmuir film obtained with the

CdSe spreading solution. Figure 2c indicates that, in the

presence of 12 on a microscopic scale, homogeneous films

(with respect to the point resolution of the Brewster angle

microscope of approximately 4 mm) may be obtained at

surface pressures above 5 mN/m.

To prepare the mixtures of 12/CdSe spreading solutions,appropriate amounts of solutions of 12 and the CdSe spre-

ading solution were mixed to obtain the same volume for

eachmixture. As can be seen from the inset in Figure 2a, the

area at the transfer pressure of 17 mN/m was observed to

follow the additivity rule. This is a hint that, in the mixed

films, the solutions of 12 and polymer-stabilized CdSe

nanoparticles formed separate phases in themonolayerwith

a morphology comparable to those in pure monofilms. The

size of the polymer-stabilized CdSe nanoparticles must be

in the range of the optical resolution (4 mm) of the Brewster

angle microscope or below since they are not resolved.

Mixed monolayers with a high content of polymer-

stabilized CdSe nanoparticles could be successfully trans-

ferred onto hydrophilic quartz supports. The transfer ratios

(TR) turn out to be in the range 1.09–1.14 for downward

dips and 0.56–0.85 for dips upward. The CdSe-related

absorption between 300 and 480 nm was not clearly detec-

table. According to the relatively low extinction coefficient

(see discussion related to Figure 1) this was not expected.

The CdSe emission could not be observed, too.

Further experiments were carried out to determine the

size of the PS-b-P4VP-stabilized nanoparticles. To improve

film transfer (TR should be� 1) and to prevent aggregation

of the polymer-stabilized nanoparticles the excess of the

polyamphiphile 12 in the monolayer was highly increased

by a factor of 10.6. Due to their lower surface roughness in

comparison to conventional quartz plates, silicon wafers

were used as substrates. Under these conditions, the TR of

the monolayers was close to one. Figure 3a shows an AFM

image of such a monolayer on a silicon wafer. A number of

relatively large but nearly exactly circular particles with

dimensions between 150 nm and 1.2 mmwere visible. Their

height was relatively independent of their diameter and

varied between8–10nm.Surprisingly, someof the particles

seemed to have a hole in the middle. With increased

resolution (Figure 3b) the ring structures were observed.

The outer diameter of the three visible rings were between

600 and 670 nm while the inner diameter varied between

200 and 330 nm. Even at a resolution of 1 mm no further

Figure 2. (a) Surface pressure vs area isotherms of mixtures ofthe CdSe spreading solutions with copolymer 12 (4.7� 10�3

M inchloroform). Parameter for different curves: volume ratio of CdSespreading to added solution of 12; inset: area at p¼ 17 mN/mdependent on the concentrations of 12 and CdSe in the mixtures.Brewster angle micrographs of the Langmuir monolayers of (b)pure CdSe spreading solution, and (c) the CdSe spreading solutionmixed with 12 (1:1 by volume, [12]¼ 4.7� 10�3 M in chloro-form), both at a surface pressure of 34 mN/m.

Ring and Disk-Like CdSe Nanoparticles Stabilized with Copolymers 627

substructures were detectable. In a number of further

experiments it was proven that observation of the rings was

reproducible. Monolayer transfer experiments at different

pH values demonstrated the reproducibility of ring forma-

tion; an influence of the pH value (the pH value of the

subphase was adjusted by addition of appropriate amounts

of 0.001 M NaOH or HCl, tested: pH 3, 7 and 9) on the ring

yield and their dimensions was not detectable. To further

confirm these observations, SEM measurements were

carried out. Figure 3c clearly shows the presence of ring

structures. The outer diameters of the visible rings were

about 760 and 800 nm, the inner ones were about 380 nm.

These results are in good agreement with those obtained by

means of AFM.

The SEM image in Figure 3c confirms the perfect

geometry of the rings and discs. The darker and lighter areas

can be explained by differences in the local thickness. This

can be seen in Figure 4 in the case of a ring structure. At the

margins of the particles the thickness drops from�10 nm to

zerowithin�45 nm on the horizontal scale. With respect to

the dimensions of theCdSe nanoparticles (< 10 nm) and the

length of an individual polymer chain (< 15 nm) the

maximum diameter of an individual polymer-stabilized

CdSe nanoparticle cannot exceed 40 nm. This value would

be obtained in the case of a micelle-like structure of the

polymer-stabilized nanoparticles and becomes much smal-

ler if the CdSe nanoparticle is stabilized by only one block

copolymer chain. Therefore, the rings and discs observed

must be composed of a large number of such polymer-

stabilized nanoparticles.

On the micrometer scale, ring structures have been

described previously. For instance, Ohta et al.[15] observed

ring structures in Langmuir films of arachidic acid,

methyl arachidate and J-type oxycanine complexes. Similar

Figure 3. (a, b) AFM (6.5� 6.5 mm2) and (c) SEM (4� 3 mm2) images of amixedmonolayer of copolymer 12 and PS-b-P4VP-stabilized CdSe nanoparticles (spreading solution: [CdSe]¼ 4� 10�6

M, [PS-b-P4VP]¼ 1.8� 10�4M,

[12]¼ 10�3M, 0.08 vol.-% DMF, 1.92 vol.-% toluene, 98 vol.-% CHCl3).

628 A. Wagih Fahmi, U. Oertel, V. Steinert, C. Froeck, M. Stamm

self-organized domains with ring patterns (diameter 2–

20 mm) of a samarium complex were obtained by Zhang

et al.[14] Here the driving force was assumed to be

electrostatic interaction caused by highly ordered round

domains of stearic acid. Compared with these results, the

ring structures described in this work exhibit a nearly

perfect geometry with lower dimensions at the nm scale.

Nevertheless, the mechanism for the formation of ring

structures is not clear yet. At the present stage, one can

assume that different interactions may contribute to ring

formation. We believe, that the use of two spreading

solvents with different vapor pressure and, therefore,

different evaporation rates may have a decisive influence

on the phase behavior in the Langmuir monolayer on the

water surface.

Conclusions

It could be shown that, bymeans of theLB technique, highly

symmetric disc and ring structures (height: approximately

10 nm, diameter: 150–1200 nm) formed reproducibly on

solid substrates by interaction among polymer-stabilized

CdSe nanoparticles. The driving force for structure forma-

tion is not clear yet. Better knowledge about the underlying

processes and principles will open a way for an optimized

selective procedure to prepare ring and disc structures

and to vary their dimensions. So far it has not been clarified

whether CdSe is required, and whether amphiphilic

polymer 12 takes part in structure formation. Additionally,

the role of solvents used to spread the materials may be

important. At this stage, the working hypothesis and

starting point for future work is based on the assumption

that the use of two spreading solvents with different vapor

pressures (toluene and CHCl3) to prepare the mixed mono-

films is essential. Further work will focus on the validation

of this hypothesis.

Acknowledgement: We gratefully acknowledge assistance byBettina Pilch and Hans-Georg Braun during film preparation andscanning electron microscopy. This work was supported within aBMBF project (No. 01RC0176) and by a DFG hot topic project(No. 03N8627B).

[1] Y. N. C. Chan, R. R. Schrock, R. E. J. Cohen, J. Am. Chem.Soc. 1992, 114, 7295.

[2] J. M. J. Frechet, J. H. Golden, H. Deng, F. J. Disalvo, P. M.Thompson, Science 1995, 268, 1463.

[3] D. E. Fogg, L. H. Radzilowski, R. Blanski, R. R. Schrock, E.L. Thomas, Macromolecules 1997, 30, 417.

[4] M. Sastry, A. Gole, S. R. Sainkar, Langmuir 2000, 16, 3553.[5] C. Damie, S. Cole, M. J. Sastry, Mater. Chem. 2000, 10,

1389.[6] J. Schmitt, G. Decher, W. J. Dressick, S. L. Brandow, R. E.

Geer, R. Dhashidhar, J. M. Calvert, Adv. Mater. 1997, 9, 61.[7] M. A. Correa-Duarte, M. Giersig, N. A. Kotov, L. M. Liz-

Marzan, Langmuir 1998, 14, 6430.[8] K. Ariga, Y. Lvov, M. Onda, I. Chinose, T. Kunitake, Chem.

Lett. 1997, 125.[9] K. Hotta, M. Inukai, A. Taniguchi, Chem. Lett. 1997, 83.[10] W. A. Lopes, H. M. Jaeger, Nature 2001, 414, 735.[11] M. J. Fasolka, A.M.Mayes,Annu. Rev.Mater. Res. 2001, 31,

323.[12] T. L. Morkved, H. M. Jaeger, Europhys. Lett. 1997, 40, 643.[13] E.Wolert, S.M. Setz, R. S.Underhill, R. S. Duran, Langmuir

2001, 17, 5671.[14] R. J. Zhang, S. P. Zheng, M. Q. Wang, K. Y. Yang, J. B. Li,

J. F. Hu, Thin Solid Films 2001, 396, 229.[15] N. Ohta, M. Nakamura, I. Yamazaki, Langmuir 1998, 14,

6226.[16] U. Oertel, D. Appelhans, P. Friedel, D. Jehnichen, H.

Komber, B. Pilch, B.Hanel, B.Voit,Langmuir 2002,18, 105.[17] D. L. Klayman, T. S. J. Griffin, J. Am. Chem. Soc. 1973, 95,

197.[18] V. N. Soloviev, A. Eichhofer, D. Fenske, U. Banin, J. Am.

Chem. Soc. 2000, 122, 2673.[19] Z. A. Peng, X. Peng, J. Am. Chem. Soc. 2001, 123, 183.[20] X. Peng, J. Wickham, A. P. Alivisatos, J. Am. Chem. Soc.

1998, 120, 5343.

Figure 4. AFM section analysis of a ring structure displayed in Figure 3a (The line from the lower leftto the upper right corner in the left image indicates the direction of the section analysis; the markers inboth images correspond to identical points).

Ring and Disk-Like CdSe Nanoparticles Stabilized with Copolymers 629