Applied Surface Science 256 (2009) 499–502

Polymer assisted assembling of the semiconductor particles:Structural and electrical studies

Bhaskar Bhattacharya a,b,*, Jun Young Lee a, Jung-Ki Park a,*a Korea Advanced Institute of Science and Technology, Republic of Koreab Sharda Univeristy, Greater Noida, India

A R T I C L E I N F O

Article history:

Available online 25 July 2009

Keywords:

Semiconductors

Self-assembly

Polymer

Glass transition

Composite

A B S T R A C T

CdS and Si semiconductor nanopraticles were embedded in a polymer matrix and characterized using

various techniques. The surface properties and size distribution of the nanoparticles were monitored by

POM and SEM and found to be uniform but around the crystalline frameworks of the polymer. XRD and

FTIR analysis have been used to ensure the composite nature and particle size of the semiconductor

loaded films. The electrical conductivity of these films were evaluated and found to increase with

semiconductor dispersion and attains a percolation threshold at optimum composition. This

composition and the distribution of the clusters is shown to vary with the type of the semiconductor,

i.e., CdS or Si.

� 2009 Elsevier B.V. All rights reserved.

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1. Introduction

Semiconductor nanoparticles have been widely investigatedbecause they have unique size-dependent optical, magnetic,electronic, electrochemical properties and have the potential tobe used in a wide range of applications [1,2]. For device applications,the assembly of these particles and the nature of the host media is ofprime importance. The interaction between the host and thedispersed particles is also expected to be the least. With the vastprogress in the field of nanotechnology, various nanostructurematerials are prepared through the controlled organization ofparticles into various ordered and disordered structures. The use ofglass, zeolites and various polymers could be seen in literature [3–5].In most of such experiments the particles are initially surfacemodified and then attached with the matrix [6]. These selective andcontrolled modifications of particles result in the feasibility tofabricate the metal–polymer composites. In these studies, theemployment of two different components, especially inorganic–organic materials, has attracted researcher’s interests. A range ofmethods has been used so far such as ultrasonic, electroplating, wet-chemistry and so on to modify or coat substrates with metalnanoparticles [7–9]. But the most promising is wet-chemistrymethod, which has produced Au, Pt, Ag, and CdS nanoparticle-coatedcomposites with excellent surface features.

* Corresponding authors: Department of Physics, 32-34 Knowledge Park III,

Greater Noida 201306, India (B.Bhattacharya); Department of Chemical and

Biomolecular Engineering, Yuseong-gu, Daejeon 305701, Korea (J.-K. Park).

E-mail addresses: bhaskarmiet@gmail.com (B. Bhattacharya),

jungpark@kaist.ac.kr (J.-K. Park).

0169-4332/$ – see front matter � 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2009.07.055

In this paper we report the assembling of semiconductorparticles on a low glass transition polymer. A simple approach hasbeen adopted to prepare well-covered, high-density and stable CdSand Si particles-coated micro-sized polymer beads. The sampleshave been characterized using various techniques like XRD, POM,and SEM for their structural and electrical behavior. The electricalbehavior of the composite films has been explained in terms of thepercolation threshold depending on the concentration of thedispersed particles.

2. Experimental

Poly(ethylene oxide) (PEO, Mw = 5,000,000), cadmium sulphide(CdS), silicon (Si), and lithium iodide (LiI, 99.999%) were purchasedfrom Aldrich. All chemicals were used without further purification.Desired amounts of the polymer, the each semiconductor and thesalt were weighed in a glove box and dissolved in acetonitrile(Merck). The semiconductor contents were varied from 1, 5 and10 wt% of PEO. The solutions were then cast on Petri dishes anddried in N2 atmosphere. For complete elimination of the solvent,the films were further dried in a vacuum oven for 1 day at ambienttemperature.

The composite polymer films were subjected to structural,thermal and electrical characterizations. The X-ray diffraction(XRD, Rigaku D/MAX-RC 12 kW) patterns of the samples wereobtained with 2u values ranging from 10 to 80 degrees at ascanning speed of 18/min. To confirm interaction between thepolymer and the semiconductor, Fourier Transform Infrared (FTIR)spectra were recorded in the attenuated total reflectance (ATR)mode and absorbance mode on a Bruker Tensor 27 spectrometerwith the resolution of 4 cm�1 in the vibrational frequency range of

Fig. 1. XRD pattern of pure PEO film (black); polymer films containing CdS (blue)

and Si (red) particles. (For interpretation of the references to colour in this figure

legend, the reader is referred to the web version of the article.)

B. Bhattacharya et al. / Applied Surface Science 256 (2009) 499–502500

600–4000 cm�1. The structural features of the composite polymerfilm surfaces were studied with an optical microscope (OM, LeicaDM LB) under cross polarizer. Field emission scanning electronmicroscopy (FE-SEM, FEI Sirion) was also used to observe furtherdetails of the surfaces.

Impedance spectroscopic techniques were used to evaluate theionic conductivities of the polymer films. The polymer films weresandwiched between two stainless steel (SS) electrodes, and thesamples were vacuum-packed in an aluminum plastic pouch toavoid contamination. The conductivities of polymer films wereobtained from bulk resistance by ac complex impedance analysisusing a Solartron 1455 frequency response analyzer (FRA) over afrequency range of 100 Hz–1 MHz.

3. Results and discussions

3.1. X-ray diffraction and infrared studies

Fig. 1 shows the XRD pattern of the composite polymer filmscontaining CdS and Si particles. For comparison, the diffractionpattern of the host polymer is also included in the figure. Since thehost polymer PEO is partially crystalline at room temperature, thepeaks of either of the semiconductors (CdS or Si) were super-imposed on the broad hallow due to the host polymer PEO [10]. Thepeaks corresponding to the Si and CdS could clearly been identifiedin the spectra and corresponding planes were identified. No otherpeak than the host and the semiconductors could be seen whichindicates multiphase nature of the films. The composite nature isfurther confirmed from the infrared spectra of the films. Fig. 2(aand b) shows the infrared spectra of the CdS and Si loaded polymerfilms. Peaks corresponding to various conformations of thepolymer chain could be identified [11–13]. Absence of any otherpeak re-confirms the composite nature of the films. These resultsconfirm that the polymer only provides a medium for thesemiconductor particles and no chemical interaction takes place.The particles are not even bound inside the back bone of thepolymer.

3.2. POM and SEM studies

The films were observed under the polarized optical microscopefor their possible surface features. Fig. 3(a–d) shows the POM

Fig. 2. The FTIR spectra of the (a) CdS and (b) Si loaded polymer films. Black lines for 1 wt

references to colour in this figure legend, the reader is referred to the web version of

micrographs at different magnifications for the Si and CdS loadedfilms. The general observations are the followings. (i) The filmswith small concentration of the dispersoid, the films appear lightcoloured blackish/orange. As the concentration of the dispersoid isincreased, the films become (deeply) coloured. (ii) The crystallineregions of the polymer, spherulites, could be seen which areseparated by the dark regions [10,14]. The cross polarizer indicatesthese regions as amorphous parts of the polymer. (iii) Theamorphous regions appear more coloured (black in a and b;orange in c and d) than the crystalline parts [15]. This indicates thatthe semiconductor particles prefer to get assembled in theamorphous regions and cannot penetrate into the ordered chainsof the polymer. This statement gets support from our XRD and FTIRresults showing no interaction between the polymer and thesemiconductor. (iv) The distribution of Si is apparently moreuniform than those for CdS. This statement is further clarified bythe SEM studies discussed below.

The films when seen under SEM appear as shown in Fig. 4(a–d).Spherulites with centred strain could be clearly in seen (Fig. 4(aand c)). On further magnification at the edges of the spherulites,regularly arranged particles could be seen for Si (Fig. 4b). However,for CdS, the particles appear to be embedded even on the

%; blue for 5 wt% and red for 10 wt% of the semiconductor. (For interpretation of the

the article.)

Fig. 4. The SEM images of the Si (a, b) and CdS (c, d) loaded polymer films.

Fig. 3. The POM micrographs of Si (a and b) and CdS (c and d) loaded films. For details see the text.

B. Bhattacharya et al. / Applied Surface Science 256 (2009) 499–502 501

Fig. 5. The change in conductivity of the composite films with the concentration of

Si (blue) and CdS (pink) particles. (For interpretation of the references to colour in

this figure legend, the reader is referred to the web version of the article.)

B. Bhattacharya et al. / Applied Surface Science 256 (2009) 499–502502

crystalline parts and the cluster size is also not uniform. Clustersize ca. 4 mm for Si and ca. 100 nm for CdS were observed. It hasbeen reported that, CdS, when grown in situ, show similarclustering. In some of the regions, the particles grow even up to10 mm and in some it remains in �20 nm size [16]. The polymerwhich provides a support matrix is assumed to work as a preventerfor agglomeration often does not work so. In this case, as seen inFig. 4(b and d) the uniformity is better for Si than the CdS. This is ingood agreement with our earlier POM observations. The three-dimensional network of the crystalline regions of the polymerallows the small Si particles to sit on and an almost regularnetwork could be established. Our choice of the polymer host,poly(ethylene oxide) provides an appropriate supportive base tothe particles to sit on. In case of a polymer with more rigid network(higher Tg) the separation between the particles may be more wideand random and any (charge transfer) link may not be established.In contrast, a very low Tg polymer may result to high agglomera-tion and clustering of the particles.

3.3. Electrical studies

The conductivity of the semiconductor loaded films has beencalculated from the impedance spectroscopic data as mentioned inSection 2. Since PEO shows very low conductivity at roomtemperature (�10�9 S cm�1). So, we have added small amountof LiI to improve its conductivity. Details related with these dopingcan be seen elsewhere [10]. Fig. 5 shows the conductivity of thecomposite films containing Si and CdS particles. The conductivityof the films increases with the addition of semiconductor particles.In case of CdS it shows a maximum near 5 wt% (Fig. 5) where as forSi it goes on increasing up to 40 wt% (not shown in this figure). Themaxima in the conductivity are indicative of the percolation

threshold. According to the percolation theory, the surface of theparticles, in order to maintain the neutrality with the host polymermatrix, becomes rich of charges and thus a highly charged channelbecomes available [17,18]. In case of nanoparticles, the surface tovolume ratio is high and thus the increase in conductivity is alsohigh. This effect is more pronounced for smaller particles than thelarger due to difference in their surface area. At this condition, alink between the particles gets established throughout the film andthe conductivity maximizes. Beyond this value, the drop inconductivity is due to the lack of formation of the highly chargedpathway for the conduction. The final (decreased) value is the bulkconductivity of the CdS or Si particles at room temperature. Thus,by changing the concentration of the particles, one can very easilycontrol the conductivity of the film which can further be utilisedfor device applications.

4. Conclusion

Small semiconductor particles can be loaded on a polymer withsemicrystalline nature at room temperature. The particles prefer tobe more around the crystalline parts than into them. CdS is foundto form more bigger and random clusters than Si. Moderatelyuniform distribution could be obtained for Si. For both CdS and Si,the electrical conductivity is found to increase with concentration.The surface percolation model is used to explain the behavior.

Acknowledgement

Financial support from the Brain Korea 21 (BK 21) project underthe Ministry of Education, Science and Technology, Republic ofKorea is gratefully acknowledged.

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