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Solar Energy Materials & Solar Cells 93 (2009) 779–782

Contents lists available at ScienceDirect

Solar Energy Materials & Solar Cells

0927-02

doi:10.1

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journal homepage: www.elsevier.com/locate/solmat

Synthesis of type II CdTe/CdSe heterostructure tetrapod nanocrystalsfor PV applications

H. Lee a, S.W. Yoon b, J.P. Ahn b, Y.D. Suh c, J.S. Lee a, H. Lim a, D. Kim a,�

a Department of Materials Science and Engineering, Korea University, Anam-dong, Seongbuk-gu, Seoul 137-713, Republic of Koreab Advanced Analysis Center, Korea Institute of Science and Technology, Hawolkok-dong, Sungbuk-gu, Seoul 130-650, Republic of Koreac Division of Advanced Chemical Materials, Korea Research Institute of Chemical Technology, Daejeon 305-600, Republic of Korea

a r t i c l e i n f o

Article history:

Received 14 January 2008

Accepted 27 September 2008Available online 19 December 2008

keywords:

CdTe/CdSe

Type II heterojunction

Tetrapod nanocrystal

Band engineering

Indirect transition

48/$ - see front matter & 2008 Elsevier B.V. A

016/j.solmat.2008.09.050

esponding author. Tel.: +82 2 928 3584; fax: +

ail address: donghwan@korea.ac.kr (D. Kim).

a b s t r a c t

We report a facile one-pot synthetic method for the formation of CdTe/CdSe tetrapod nanocrystals with

type II band alignment. The crystal growth kinetics can be controlled by changing the injection

temperature, rate and concentration of the chalcogen precursor, allowing the structure of CdTe/CdSe

tetrapod nanocrystals to be synthesized without changing the underlying chemistry. Only the multiple

injection of Se precursor promotes epitaxial growth of a CdSe nanorod on the end of CdTe tetrapod arms.

This synthesis shows that the mechanism of tetrapod nucleated growth may be generally applicable for

creating other non-core/shell heterostructures. The heterostructure nanocrystals are composed of a

CdTe tetrapod core and four CdSe nanorod tips, showing optical properties typical of type II

heterostructures that are well suited for photovoltaic applications.

& 2008 Elsevier B.V. All rights reserved.

1. Introduction

Semiconductor heterostructure nanocrystals made of differentmaterials have attracted a tremendous amount of attention for the‘‘band engineering’’ related to their properties and potentialapplications in optoelectronic and photovoltaic applications [1–3].

As shown in Fig. 1, in type I heterostructure nanocrystals, wherethe conduction band and valence band energy levels of onesemiconductor with lower band gap (Eg) placed in between thoseof another semiconductor with higher Eg, exciton energy transferof the electrons and holes trapping to the lower Eg semiconductoroccurs, so that highly luminescent nanostructures can be obtained.In type II heterostructure nanocrystals, where only one of theconduction and valence energy levels of one semiconductor ishigher than the corresponding energy level of the other semi-conductor, type II heterostructure nanocrystals spatially separatesphotogenerated carriers within the nanostructure such that theelectron wavefunction mainly resides in one semiconductor andthe hole wave function in the other. Recent theoretical [4] andrelaxation dynamics investigations [2,5] of type II CdSe/CdTeheterostructure nanocrystals indicate that they are ideal materialsfor their long-range-photoinduced charge separation, and could beapplied in photovoltaic devices. Meanwhile, CdSe, CdTe and CdTe/CdSe tetrapod nanocrystals perform well in nanocrystal–polymerhybrid solar cells [6–9], and hence they have drawn attention to

ll rights reserved.

82 2 3290 4784.

the synthesis of high-quality tetrapod nanocrystals. Therefore, thedevelopment of simple, inexpensive synthetic methods for type IICdTe/CdSe heterostructure tetrapod nanocrystals is important inachieving high efficiency solar cells.

Previous studies have produced type II structures such as CdSe/CdTe tetrapods and CdTe branches grown off CdSe tetrapods [4],CdTe/CdSe/CdTe quantum rods [10] CdSe tipped CdTe multiple-branched rods [9] and CdTe tipped CdSe nanobarbells [11].However, there is still challenge existing for the synthesis of typeII nanocrystals by addressing issues such as, the high syntheticand environmental cost (expensive alkylphosphonic acids andtoxic trioctylphosphine oxide are used in the synthesis) and thelow yield of tetrapod nanocrystals, have to be addressed.Obviously, the development of simple and inexpensive syntheticmethod may be very important in solving the above problems.

Here, we report a facile one-pot synthetic method for theformation of CdTe/CdSe tetrapod nanocrystals with a type II bandgap offset with an inexpensive, nontoxic and liquid fatty acidligand and 1-octadecene solvent. The crystal growth kinetics canbe controlled by changing the injection temperature, rate andconcentration of the chalcogen precursor, allowing the structureof CdTe/CdSe tetrapod nanocrystals to be tuned without changingthe underlying chemistry.

2. Experimental

We synthesized CdTe/CdSe tetrapod nanocrystals via a mod-ified recipe of Yu et al. [12]. For a typical reaction, a solution of

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Fig. 1. Schematic diagrams of (a) type I and (b) type II heterojunction band

alignment.

(100

)

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nsity

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.)

(101

)

(102

) (103

)

(200

)

(201

)

(002

) (11

1)

(110

) (22

0)

(311

)

Zinc blendeJCPDS No.15-0770

20

a

c

b

30 402θ (degree)

WurtziteJCPDS No.19-0193

50

15 s

60 s

20 nm 5 nm

Fig. 2. (a) TEM and (b) HRTEM images of CdTe tetrapod nanocrystals grown at

260 1C for 2 min. (c) XRD patterns for CdTe tetrapod nanocrystals with different

growth times 15 (lower) and 60 s (upper).

H. Lee et al. / Solar Energy Materials & Solar Cells 93 (2009) 779–782780

52 mg of CdO dissolved in 230 mg of oleic acid and 3.72 g of1-octadecene was heated to about 300 1C to obtain a colorless,clear solution of Cd precursor. At this temperature, a solutionof 26 mg of tellurium dissolved in 210 mg of TOP and 1.8 g of1-octadecene was rapidly injected into a reaction solution to formthe CdTe tetrapod nanocrystals. The reaction was carried out atca. 260 1C for 2 min, and then 0.25 mL of a 0.15 M TOP-Se solutionwas rapidly, periodically injected at equal intervals of 2 min in thetemperature range between 200 and 300 1C. After the synthesis,the reaction mixture was cooled to 60 1C. The ODE solution of theas-prepared nanocrystals was mixed with an equal volumemixture of hexane and methanol (1/2, v/v) by vigorous shakingand then centrifuged, and the ODE phase was extracted. Thisprocedure was repeated once and then the nanocrystals in theODE phase were precipitated with excess acetone. The precipitatewas isolated by centrifugation/decantation and dried under argonat room temperature. The final powder product can be redissolvedin a desired solvent (toluene, chloroform or hexane) for certainmeasurements.

Transmission electron microscopy (TEM), high-resolutiontransmission electron microscopy (HRTEM) and energy dispersivespectrometry (EDS) employed a FEI Tecnai G2 F20. Point EDSmeasurement was performed in scanning TEM mode (STEMmode). X-ray diffraction (XRD) patterns were recorded with anAnton Paar TTK 450 low-temperature camera with a monochro-matic radiation (l ¼ 1.54520 A). UV–vis absorption spectra wererecorded on a Scinco S-3100 spectrophotometer. The photolumi-nescence spectra were taken with a Hitachi F-2500 fluorescencespectrophotometer.

Fig. 3. (a) TEM and (b) HRTEM images of CdTe tetrapod nanocrystals with CdSe

nanorod tips.

3. Results and discussion

The prepared CdTe tetrapod nanocrystals exhibit the wurtzitecrystal structure with the arms elongated in [0 0 2] direction asshown in Fig. 2. The as-prepared nanocrystals have arms whichare average 4 nm in diameter and 8 nm in length. All the peaks inthe X-ray diffraction pattern of CdTe tetrapod nanocrystals can beindexed by assigning a hexagonal lattice (JCPDS No. 19-0193). Asshown in Fig. 2c, the peaks due to reflection from (10 0) and (10 1)were detected together with the characteristic (10 3) reflections ofthe wurtzite phase [12].

CdTe/CdSe heterostructure tetrapod nanocrystals were grownby forming CdSe nanorods at the end of CdTe tetrapod nanocrys-tals, from a rapid injection of TOP-Te followed by controlled,sequential injections of TOP-Se. For the growth of CdSe nanorods,the rapid injection of the low concentration TOP-Se solution

(o0.15 M) was needed, while the rapid injection of the highconcentration (40.2 M) TOP-Se solution or high (4280 1C) or low(o220 1C) injection temperature promoted the formation of freeCdSe quantum dots in the reaction solution.

Fig. 3b shows the HRTEM images of the CdTe/CdSe hetero-structure tetrapod nanocrystals. High-magnification TEM obser-vation shows that the heterostructure tetrapod nanocrystal is wellcrystalline. The arms are average 4 nm in diameter and 16 nm inlength. The arms appear to elongate with no change in diameter,and CdSe nanorod tip growth occurred exclusively at the reactive{0 0 2} planes at the tetrapod ends. The interplanar distance in thetip of the arm is 0.37 nm which corresponds to the (10 0) plane of

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a b25

Cd

CdTe

CdTe

Cd

CdTe

Te

Te

Te TeTe

TeCu

Cu

Cu

Cu

Cu

Se SeSe

20

Cou

nts

Cou

nts

1510

O2

O1

0 5 10Energy (KeV)

15 20

0 5 10Energy (KeV)

15 20

50

25201510

50

c

Fig. 4. (a) STEM image of a single particle of CdTe/CdSe heterostructure tetrapod nanocrystals, indicating the points where the spectra were measured. Point EDX spectra

obtained at the end of an arm (point 1) (b) and at the arm near the center of a tetrapod (point 2) (c).

Fig. 5. (a) TEM and (b) HRTEM images of ripened CdTe/CdSe heterostructure

tetrapod nanocrystals synthesized by dropwise addition of TOP-Se.

H. Lee et al. / Solar Energy Materials & Solar Cells 93 (2009) 779–782 781

wurtzite CdSe, while the interplanar distance at the lower part ofthe arm is 0.37 nm which corresponds to the (0 0 2) of the wurtziteCdTe. So this lattice matching facilitates the growth of CdSenanorod tips on the end of CdTe tetrapod nanocrystals.

Although heterostructure nanocrystals were synthesized byone-pot synthesis, controlled growth temperature (o260 1C)and rapid injection of precursors minimize the contamination ofCdTe tetrapods with Se and the overcoating of the CdTe tetrapodwith CdSe. No discernible shell can be seen in HR-TEM imagesas shown in Fig. 3. STEM measurements confirm this in Fig. 4.EDS spectra were taken at the end of one elongated arm (seeFig. 4b) and at the arm near the core (see Fig. 4c). Although thespatial precision of the instrument is limited [11], the spotmeasured at the end of the arm indicate the concentration of Cd,Te and Se is 26, 28 and 46 at%, respectively. However, theproximity of the Cd and Te peaks in the X-ray spectrumresults in overcounting of tellurium, and no discernible shell canbe seen in HR-TEM images indicating that the tip of the arm iscomposed of CdSe. Fig. 3c confirms the presence of only Cd andTe elements in the arm near the core, and this indicates thatthe tetrapod core and arms near the core were mainly composedof CdTe.

However, the dropwise addition (20 min/ml) of the lowconcentration (0.2 M) TOP-Se solution promoted the growth ofCdSe shell at the ends and the surface of CdTe arms. This can beexplained by long reaction times. CdTe arms ripen into sphericalCdSeTe alloyed nanocrystals owing to long addition time, i.e. theCdSe end caps first ‘‘ball up’’ to form matchstick structures thateventually ripen into CdSeTe spheres (see Fig. 5).

It is noteworthy that the formation of CdTe tetrapod nano-crystals with CdSe nanorod tips simply requires modifying theCdSe quantum dot synthesis only by the presence of tetrapodsthat serve as nucleation sites and rapid injection of precursorsat a temperature high enough for growth, but low enoughto suppress homogeneous nucleation. This synthesis showsthat the mechanism of tetrapod nucleated growth may begenerally applicable for creating other non-core/shell hetero-structures.

Changes in optical properties of the heterostructure tetrapodnanocrystals were also followed during the growth process.Several distinctive features help confirm that heterostructurenanocrystals with type II band offset are evolving. CdTe/CdSeheterostructure tetrapod nanocrystals exhibit absorption spectra(see Fig. 6a) typical of type II heterostructures [1,2,11] withsignificant absorption across the visible spectrum. Absorption

peaks are continually red-shifted and lose their definition aftereach additional injection of precursors. These are expected as thephoton can be absorbed by the CdTe tetrapod, the CdSe nanorodtip, or the intermediate states that exist at the junction. Inaddition, the spatially indirect property of type II transitions isresponsible for the weaker absorption at the band edges. Unlike intype II core/shell heterostructures, there is no observed emission(see Fig. 6b). Since the carriers are spatially well separated, it ispresumed that recombination occurs primarily through nonra-diative pathways such as spatially indirect transition made by atype II band alignment and surface trap states.

4. Conclusions

In conclusion, we report a facile one-pot synthetic method forthe formation of novel type II CdTe/CdSe heterostructure tetrapodnanocrystals by sequential injection of TOP-Te and TOP-Se to acomplex solution of CdO, oleic acid and 1-octadecene mixture,and the nanocrystals were characterized by TEM, HRTEM, EDS,XRD and absorption and PL spectra analysis. Type II CdTe/CdSeheterostructure tetrapod nanocrystals have arms with diameter of4 nm and length of 16 nm. The PL peak of the initial CdTe tetrapodnanocrystals was completely quenched after the subsequentgrowth of the CdSe nanorod tips on the end of CdTe nanocrystals.Therefore, prepared heterostructure tetrapod nanocrystals haveoptical properties typical of type II heterostructures that are wellsuited for photovoltaic applications.

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400 500

Abs

orba

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500 550 600 650 700 750600 700Wavelength (nm) Wavelength (nm)

800

Abs

orba

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Wavelength (nm)400 500 600 700 800

I

IV

IVIIIIII

Fig. 6. The absorption (a) and emission (b) spectra of samples of CdTe/CdSe heterostructure tetrapod nanocrystals: (a) Absorption spectra of samples taken (I) before

addition of TOP-Se, (II) after the first addition of TOP-Se, (III) after the second addition of TOP-Se and (IV) after the forth addition of TOP-Se. The inset shows all 4 absorption

spectra. (b) Emission spectra of a sample of CdTe tetrapod and CdTe/CdSe heterostructure tetrapod nanocrystals.

H. Lee et al. / Solar Energy Materials & Solar Cells 93 (2009) 779–782782

Acknowledgement

This work is the outcome of the fostering project of the BestLab supported financially by the Ministry of Commerce, Industryand Energy (MOCIE).

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