II–VI semiconductor nanocrystals in thin films and colloidal crystals

Colloids and Surfaces

A: Physicochemical and Engineering Aspects 202 (2002) 135–144

II–VI semiconductor nanocrystals in thin films and colloidalcrystals

Andrey L. Rogach a,b,*, Nicholas A. Kotov c, Dmitry S. Koktysh a,c,Andrei S. Susha a,d, Frank Caruso d

a Physico-Chemical Research Institute, Belarusian State Uni�ersity, 220050 Minsk, Belarusb Institute of Physical Chemistry, Uni�ersity of Hamburg, D-20146 Hamburg, Germany

c Department of Chemistry, Oklahoma State Uni�ersity, Stillwater, OK 74078, USAd Max Planck Institute of Colloids and Interfaces, D-14424 Potsdam, Germany

Received 4 December 2000; accepted 2 February 2001

Abstract

Semiconductor nanocrystals whose optical properties are largely determined by the quantum confinement effect arecurrently being extensively studied in both physics and chemistry. Highly luminescent thiol-capped CdTe and HgTenanocrystals with desirable sizes ranging from less than two to approximately 8 nm have been recently synthesized inaqueous solutions by a wet chemical route. They were used for the preparation of composite multilayer thin films bythe layer-by-layer (LBL) deposition technique. Films containing luminescent nanocrystals were made both on planarsubstrates and on submicron-sized monodisperse polystyrene spheres. Alternatively, nanocrystals have been incorpo-rated as cores into silica spheres of desirable sizes. Composite nanocrystal/silica and core-shell latex/nanocrystalspheres have been used as building blocks for 3-D colloidal photonic crystals. © 2002 Elsevier Science B.V. All rightsreserved.

Keywords: Semiconductor; Nanocrystal; Luminescence; Layer-by-layer deposition; Photonic crystal

www.elsevier.com/locate/colsurfa

1. Introduction

II–VI semiconductor nanocrystals are currentlyof great technological interest as emitting materi-als for thin film electroluminescence devices [1–3]and as optical amplifier media for telecommunica-

tion networks, [4,5] because of their strongbandgap luminescence and size-dependent opticalproperties due to the quantum confinement effect[6]. The incorporation of luminescent semiconduc-tor nanocrystals into photonic crystals [7,8] hasobtained considerable attention recently as apromising pathway to novel light sources withcontrollable spontaneous emission.

Thiol-capped CdTe nanocrystals with size-de-pendent bandgap luminescence in the visible spec-tral region have been synthesized in aqueouscolloidal solutions [9,10] and used for fabrication

* Corresponding author. Tel.: +49-40-428383414; fax: +49-40-42838-3452.

E-mail address: [email protected] (A.L.Rogach).

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A.L. Rogach et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 202 (2002) 135–144136

of light-emitting diodes (LEDs) with light genera-tion in the range 530–650 nm [3], and for impreg-nation of colloidal photonic crystals [8,11,12].Recent experiments have shown that the quantumefficiency of photoluminescence (PL) of water-sol-uble thiol-capped CdTe nanocrystals can be suffi-ciently enhanced through pH control [10] andproper size-selective fractionation [13], up to 25–30% at room temperature. HgTe nanocrystalswith very strong (up to 50% quantum efficiency atroom temperature) bandgap luminescence in thenear-IR spectral range have also been recentlysynthesized in aqueous dispersions [14,15]. Theycan find applications in the telecommunicationindustry [4,5], which is based on fiber-optical net-works operating in the windows of transparencyat 1300 and 1550 nm.

In this paper, we review our recent efforts inprocessing CdTe and HgTe nanocrystal disper-sions into high quality thin films with thicknessescontrollable on the nanometer scale using thelayer-by-layer (LBL) assembly method both onplanar substrates and on colloidal polystyrene(PS) spheres. The LBL assembly is based on thealternating adsorption of oppositely charged spe-cies, such as positively and negatively chargedpolyelectrolyte pairs [16] or polyelectrolytes andnanoparticles [17–19]. It can be equally effectivelyapplied to the coating of both macroscopically flat[16,17] and non-planar (e.g. colloidal particles)[18,19] surfaces. We also briefly introduce amethod of incorporation of semiconductornanocrystals into silica spheres. Finally, wepresent some examples of colloidal photonic crys-tals (also called artificial opals [20]) made frombare latex spheres, composite nanocrystal/silicaand core-shell latex/nanocrystal spheres, and ofthe latex-based photonic crystals with CdTenanocrystals electrophoretically deposited therein[21–25].

2. Experimental section

Aqueous colloidal solutions of CdTe nanocrys-tals capped by thioglycolic acid and HgTenanocrystals capped by thioglycerol were pre-pared as described previously [10,14]. Fig. 1 shows

room temperature PL spectra of two series ofthiol-stabilized CdTe (2–5 nm size range) andHgTe (3–8 nm) nanocrystals. The bandgap lu-minescence of CdTe and HgTe nanocrystals iseasily tunable with particle size in the visible andnear-IR spectral regions, respectively, due to thequantum confinement effect. Fig. 1(c) shows aTEM image of a single HgTe nanocrystal with thecorresponding Fast-Fourier-Transformation(FFT), demonstrating the high crystallinity of thenanoparticles.

The average particle size of CdTe and HgTenanocrystals used in this work was 3.5 nm, with asize deviation of 10% for CdTe and 30% for HgTe(as determined by transmission electron mi-croscopy, TEM). Colloidal solutions were used asprepared, with the concentration of CdTe orHgTe nanocrystals of 0.013 M referring to Cd2+

(Hg2+) and pH 10. The surface of nanocrystalswas negatively charged, because of the deproto-nated �OH and �COOH groups of the stabilizers.

Poly(diallyldimethylammonium chloride)(PDDA), poly(allylamine hydrochloride) (PAH)and poly(styrenesulfonate) (PSS) were used forthe LBL film preparation. LBL films on planarsurfaces have been prepared using carefullycleaned [23] ITO-covered glass slides and siliconwafers as substrates. The following standardcyclic procedure was employed, (i) dipping of thesubstrate into a solution of PDDA for 10 min; (ii)rinsing with water for 1 min; (iii) dipping into theaqueous dispersion of nanocrystals (pH 10.0) for20 min; (iv) rinsing with water again for 1 min.On each exposed surface, such a procedure re-sulted in a ‘bilayer’ consisting of a polymer/nanocrystal composite. The cycle can be repeatedas many times as necessary to obtain a multilayerfilm of desirable thickness.

The nanocomposite shells on colloids were as-sembled by depositing negatively charged CdTenanocrystals on sulfate-stabilized PS spheres pre-coated with a three-layer film of the cationicpolymer PAH and the anionic polymer PSS [21].The outermost surface layer (prior to thenanocrystal deposition) was PAH and hence thesurface was positively charged. The procedure canbe easily repeated, providing control over thecomposite multilayer film thickness [19].

A.L. Rogach et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 202 (2002) 135–144 137

Fig. 1. Room temperature PL spectra of thiol-stabilized CdTe (a) and HgTe (b) nanocrystals. Also shown is a TEM image of asingle HgTe nanocrystal with the corresponding FFT (c).


Composite silica spheres containing semicon-ductor nanocrystals were prepared by the follow-ing procedure [24,26]. First, a solution of 2 �l of3-mercaptopropyltrimethoxysilane (MPS) in 80ml of ethanol was added under vigorous stirringto 20 ml of aqueous solutions of CdTe nanocrys-tals with a nanoparticle concentration of 0.5 mM.The mixtures were stirred for �12 h and then 0.4ml of a 0.54 wt.% solution of sodium silicate wasadded to 40 ml of these solutions. The addition ofsilicate resulted in a pH increase from 8.5 to 10.7.The mixtures were allowed to react for about 4 hand then ripened for 5 days. The same procedurewas used to make silica spheres doped with CdSe/CdS nanocrystals [24]. Larger silica spheres dopedwith semiconductor nanocrystals have been pre-pared by modified Stober synthesis [27] usingethanol/water mixtures of either MPS-modifiedsemiconductor nanocrystals or smaller nanocrys-tal/silica composite particles as growth seeds.

Colloidal crystals were made either by slowgravity sedimentation of charge-stabilizedmonodisperse colloidal nanocrystal/silica or core-shell latex/CdTe nanocrystal spheres, followed byevaporation of the solvent, or by electrophoreticdeposition of bare latex spheres on conductiveITO supports and subsequent electrophoretic de-position of negatively charged CdTe nanocrystalsinto opal voids.

Atomic force microscopy (AFM) images weretaken by using a Nanoscope IIIA (MultiModeScanning Probe Microscope) instrument (DigitalInstruments/Veeco) operating in tapping modewith silicon nitride tips. Scanning tunneling mi-croscopy (STM) images were taken using thesame Nanoscope IIIa instrument with standardPt/Ir tips. Scanning electron microscopy (SEM)pictures were made with a Philips SEM 515 and aJSM 6400 microscope. TEM measurements wereperformed on a Philips CM-300 microscope oper-ating at 300 kV. Absorption and transmissionspectra were taken by using a HP8453 diode arrayHewlett–Packard spectrophotometer. PL spectraof films containing CdTe nanocrystals were mea-sured on a modular Fluorolog 3 SPEX spec-trofluorimeter. PL spectra of films containingHgTe nanocrystals were recorded using a choppedAr-ion laser as the excitation source; the resulting

Fig. 2. Schematic illustration of the LBL assembly of polyelec-trolytes and nanocrystals on planar substrates (a) and colloidalspheres (b).

emission was collected through a Bentham M300monochromator and detected using a liquid nitro-gen cooled InAs photodiode.

3. Results and discussion

Fig. 2 presents a schematic illustration of theLBL assembly of polyelectrolytes and nanocrys-tals on planar substrates and colloidal spheres. Inboth cases, the consecutive electrostatic adsorp-tion of oppositely charged species leads to theformation of multilayer composite structures con-taining nanocrystals.

CdTe- and HgTe-containing films formed byLbL deposition on planar substrates have beeninvestigated by microscopy and spectroscopically.The internal structure of the LBL nanoparticulatelayer can be seen from AFM and STM images(Fig. 3, PDDA/HgTe film on a silicon wafer isshown). The roughness of the film is 6.5 nm,which is slightly larger than the nanocrystal di-ameter. The particles form densely packed films,where each nanocrystal is in contact with adjacentquantum dots. PDDA/CdTe films show the samemorphological features.


The dense packing of nanocrystals andpolyelectrolyte chains in each layer promotes reg-ularity in the deposited layers. The equal amountsof nanocrystals and polyelectrolyte transferred ineach deposition cycle can be seen from the se-quentially recorded absorption spectra of the mul-tilayers (Fig. 4). The optical density at a selectedwavelength increases almost linearly with thenumber of deposited layers. After ten depositioncycles, the coating has a dark glass appearancewithout any apparent defects or color inhomogen-ities. The films are robust and environmentallystable: no sign of peeling or oxidation were ob-served for a number of months.

Fig. 4. Absorption spectra of LBL assembled films of PDDAand CdTe (a) and PDDA/HgTe (b) nanocrystals on ITO as afunction of the number of deposition cycles. The inserts showthe luminescence of CdTe (a) and HgTe (b) nanocrystals in thecorresponding 20-layer films.

Fig. 3. AFM (a) and STM current (b) images of a PDDA/HgTe bilayer on a silicon wafer.

CdTe nanocrystals in composite LBL filmsshow emission in the visible spectral range (insertin Fig. 4(a)). The emission color can be easilycontrolled by using CdTe nanocrystals of appro-priate size, which makes the films potentially use-ful for fabrication of LEDs operating in thevisible spectral range [3,28]. The LBL assembledfilms of HgTe nanocrystals display strong emis-sion in the near-infrared, with a maximum around1600 nm (insert in Fig. 4(b)) and covers the entiretelecommunications spectral region of interest. Byobserving the PL from a PDDA/HgTe film, thedifficulties of solvent re-absorption, which wereobserved previously [14] were eliminated and thefull extent of the spectrum is revealed.


Composite thin films containing luminescentthiol-capped CdTe nanocrystals have also beenprepared by the LBL assembly on spherical PScolloids [21]. Fig. 5(a) shows a TEM image of PSspheres coated with polyelectrolyte/nanocrystalshells. The surface composite layers are relatively

uniform, with a thickness of about 5–8 nm. High-resolution TEM allows one to distinguish singlenanocrystals in a composite layer [21]. Energy-dis-persive X-ray analysis (EDX) shows the presenceof Cd, Te and S in the multilayer shells.

The LBL approach employed to cover colloidsrepresents a general technique to produce com-posite core-shell particles with luminescentnanocrystals being embedded in the multilyerpolyelectrolyte/nanocrystal shells. Another ap-proach to produce composite colloidal spheres,containing semiconductor nanocrystals includedin the core, has been developed recently [24]. Thesurface of thiol-capped CdTe or citrate-cappedCdSe/CdS nanoparticles synthesized in aqueoussolutions was modified by MPS in water–ethanolmixtures. By addition of sodium silicate, ‘raisinbun’-type composite particles were formed, withnanocrystals being homogeneously incorporatedas multiple cores into silica spheres of 40–80 nmsize (Fig. 5(b)). This leads to some alteration ofoptical properties of the nanocrystals and, in par-ticular, to the reduction of the luminescence quan-tum yield. Further, growth of larger silica spheres(100–700 nm) can be performed by the Stobertechnique [27] using either MPS-modified semi-conductor nanocrystals or small nanocrystal-doped composite particles as seeds, which givessemiconductor-doped silica spheres of desirablesizes in the submicrometer range.

Monodisperse latex or silica spheres, eitherbare, doped or covered with a shell, can be usedfor the fabrication of 3-D colloidal crystals. Thesestructures, also called artificial opals, have at-tracted much attention in the last years as proto-types for 3-D photonic crystals working in thevisible and near-IR spectral ranges. Photonic crys-tals are periodic dielectric structures behavingwith respect to electromagnetic waves in the samemanner as common semiconductors do with re-spect to electrons [29,30]. That is, they possessstopbands, or photonic bandgaps, for light propa-gation, because of the redistribution of the densityof photonic states on the lattice with periodicallyvarying refractive index. There are a number ofinteresting phenomena expected from the pho-tonic crystals, and among them is the influence(control, inhibition, or enhancement) of the pho-tonic bandgap on the spontaneous emission of

Fig. 5. TEM images of PS spheres coated with polyelectrolyte/CdTe nanocrystal shells (a) and a silica sphere with embeddedCdTe nanocrystals (b).


Fig. 6. Examples of SEM images of 3-D colloidal crystals made from bare PS spheres by electrophoresis (a) and from compositelatex/CdTe nanocrystal spheres by gravity sedimentation (b).

light-emitting species inside a photonic crystal.One of the most convenient materials to use forthis purpose is luminescent semiconductornanocrystals with size-dependent excitonic emis-sion. Following our previous work on colloidalphotonic crystals and luminescent nanocrystalsembedded within them [8,11,12], we have em-ployed electrophoretic deposition for rapid fabri-cation of high-quality opals from bare latexspheres and for their impregnation with lumines-cent CdTe nanocrystals [25]. We have also pre-pared colloidal photonic crystals from core-shelllatex/CdTe spheres [22] and from nanocrystal-doped silica particles [24] by slow gravity sedi-mentation [31]. Recent experiments have shown

the possibility to employ the electrophoresis forfabrication of opals from core-shell spheres aswell [32].

Fig. 6(a) shows examples of SEM images of3-D colloidal crystals made from bare PS spheresby electrophoresis. The perfect close-packed orderextends over areas of �10 �m, with the hexago-nally packed layers of the (111) face of the face-centered cubic (fcc) lattice favored. The perfect fcccrystalline order extends uniformly in the direc-tion perpendicular to the substrate over the entirefilm thickness when looking at the stacking edges,which is especially important for the optical prop-erties of colloidal crystals. We did not observeterraces at the cleaved edges of the samples which


is a common issue for colloidal crystals grown bythe sedimentation technique. At lower magnifica-tion, vertical cracks perpendicular to the substratewere observed every �10 �m, which usually didnot disturb the fcc order in the films. Interestingly,the crystalline packings on both sides of the gap arealways complementary to each other, indicatingthat the film maintains continuity as it grows,whereas cracking occurs at the SEM sample prepa-ration stage.

Fig. 6(b) shows examples of SEM images of 3-Dcolloidal crystals made from composite latex/CdTenanocrystal spheres by gravity sedimentation [22].The whole set of the observed packing types ischaracteristic again of the (111) plane of a fcclattice. Wide ordered domains are observed in boththe top surfaces and the cleaved edges.

Colloidal photonic crystals are brightly coloredin both transmitted and reflected light, because ofthe optical diffraction on regular multilayers. Opti-cal transmission spectra of the samples elec-trophoretically made from bare latex spheres (theangle between the light propagation vector andsurface normal �=0°) show an existence of apronounced photonic stop band (Fig. 7) due to theBragg reflection on the (111) planes. The positionof this band changes systematically with the size ofthe latex spheres according to the following equa-tion [33]:

�c=2 · neffd, with n eff2 =n latex

2 · f+nair2 (1− f ) (1)

where neff is the effective refractive index of thelatex/air composite, f=0.74 is the filling factor fora close packed structure, d= (2/3)1/2D is the dis-tance between crystalline planes in the direction�=0°, and D is the sphere diameter. Thus, theposition of the dip in the transmission spectra canbe tuned accurately through control of the latexsphere size.

Another possibility to control the position of thephotonic stopband, as it follows from Eq. (1), is tochange the refractive index of the colloidal spheres,e.g. by doping with semiconductor nanocrystals.Fig. 8(a) shows a shift of the dip minimum tolonger wavelengths for colloidal crystals madefrom silica spheres doped with CdSe/CdSnanocrystals [24] in comparison with undopedones. For the colloidal crystal made of pure silica

with a diameter of 250 nm and nSiO2=1.45, the

peak photonic band gap is expected to be at 550nm (Eq. (1)). For the same opals, incorporatingnanocrystals with a filling factor of 0.05 (calculatedfrom TEM) and nnanoparticle=2.6, the peak positionis estimated to be at 575 nm. In Fig. 8(a), for pureopals the photonic band gap is observed at 561 nm(dotted line), while for nanoparticle-doped opals itis seen at 573 nm (solid line). The position of bothbands coincides quite well with the estimates espe-cially considering some uncertainty in determina-tion of filling factor for the imperfect colloidalcrystal.

To examine the effect of the photonic stopbandin the 3-D colloidal crystals on the spontaneousemission of light-emitting species embeddedtherein, the latex-based crystals have been elec-trophoretically impregnated with luminescentCdTe nanocrystals [25]. A mixture of CdTenanoparticles of different sizes was used whichgave a broad PL spectrum centered around 600 nm

Fig. 7. Normal incidence transmission spectra of colloidalcrystals electrophoretically prepared from bare PS spheres ofdifferent sizes.


Fig. 8. Red-shift of the photonic stopband in transmissionspectra of the colloidal crystal made of silica spheres dopedwith CdSe/CdS nanocrystals in comparison to undoped col-loidal crystals (a); modification of the spontaneous emission ofCdTe nanocrystals electrophoretically deposited into the voidsof colloidal crystal made of PS spheres (b). See text forexplanations.

CdTe nanocrystal spheres was also observed [22],and might represent the same phenomenon. Fur-ther investigations of this effect are currentlyunderway.

4. Conclusions

We have shown that semiconductor nanocrys-tals, with their exciting optical properties, can beused for fabrication of complex and orderedstructures: composite thin films on planar sub-strates and colloids, and composite 3-D colloidalphotonic crystals. The demonstration of lumines-cence in the visible and near-IR spectral rangesfrom CdTe and HgTe nanocrystals included intoa solid film configuration, which could be theprecursor for many different types of devices, issignificant in the context of optoelectronics andoptical telecommunications. Further investiga-tions of photonic crystals with luminescentnanocrystals embedded therein might provide abasis for low threshold lasers and novel nonlinearoptical phenomena.

Acknowledgements

We are grateful to A. Kornowski (University ofHamburg) and J. Ostrander (Oklahoma StateUniversity) for assistance with TEM, and to DrM. Harrison from the Corning Research Center(Ipswich, UK) for the luminescence measurementson HgTe nanocrystals. A.L. Rogach and A.S.Susha have been supported in part by an INTAS-Belarus research grant 97–250. A.L. Rogach ac-knowledges support of the DFG-Schwerpunkt-programm ‘‘Photonic Crystals’’. A.L. Rogach andN.A. Kotov acknowledge the support of the Na-tional Research Council (COBASE grants pro-gram) that made collaborative work on thisproject possible. N.A. Kotav and D.S. Koktyshacknowledge the research scholarship (DGE-9902637) from the NSF-NATO. F. Caruso ac-knowledges support from the BMBF and the MaxPlanck Society.

(Fig. 8(b), dotted line) for impregnation of thecolloidal crystal made from PS spheres of diame-ter 269 nm. The luminescence spectrum of theCdTe nanocrystals changes when it overlaps withthe optical stopband of the colloidal crystal (Fig.8(b)). A dip was observed in the emission spec-trum (solid line), correlating with the spectralposition of the photonic stopband (dashed line),i.e. the inhibition of spontaneous emission ofsemiconductor nanocrystals as a result of amodified photon density of states within this spec-tral region [8].

An asymmetrical broadening of the lumines-cence spectrum of the CdTe nanocrystals in thecolloidal crystal prepared from core-shell latex/


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II–VI semiconductor nanocrystals in thin films and colloidal crystals

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