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Materials Science and Engineering B 138 (2007) 224–227

Synthesis of single crystalline europium-doped ZnO nanowires

Paritosh Mohanty a,∗, Bongsoo Kim a,∗, Jeunghee Park b

a Department of Chemistry, KAIST, Daejeon 305-701, Republic of Koreab Department of Chemistry, Korea University, Jochiwon 339-700, Republic of Korea

Received 22 August 2006; received in revised form 20 November 2006; accepted 7 January 2007

bstract

Single crystalline Eu3+-doped wurtzite zinc oxide (ZnO) nanowires were synthesized by a vapor deposition method under a controlled oxidativetmosphere. The nanowires were deposited on a Si substrate coated with 5 nm gold nanoparticles. The reaction was carried out for 30 min

hich resulted in high yield of vertically grown nanowires of diameter 40–150 nm and length up to several microns. The nanowires were grown

long ±[0 0 0 1] direction. The concentration of dopant Eu3+ in the synthesized nanowires was below 1 atomic %. The crystal structure and theicrostructures of the doped nanostructures were studied and compared with undoped ZnO. 2007 Elsevier B.V. All rights reserved.

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eywords: Single crystalline nanowires; ZnO; Europium doping; XPS

. Introduction

Synthesizing one-dimensional (1D) nanostructures in semi-onducting materials, such as metal sulfides, selenides, oxidesnd nitrides, generated tremendous amount of interest becauseheir optical, electrical, and mechanical properties change dra-

atically with a quantum size effect [1–10]. Recently, oxidesnd nitrides are preferred for electronics and optoelectronicsaterials [6–10]. Zinc oxide (ZnO) is a wide direct band gap

emiconductor with band gap energy of 3.37 eV at room tem-erature [7,8,11,12] Several nanostructures such as nanowires,anotubes, nanobelts, nanocombs, nanorings, and nanobows,tc., were reported for ZnO [6–10]. Furthermore, it is an envi-onmental friendly material, and is expected to become a newttractive material for its potential applicability as blue and ultra-iolet light emitting and detectors [7,11,12].

Rare-earth (RE)-doped semiconductors have been inten-ively studied for their potential use in integrated optoelectronicevices like visible (blue, green, and red) and infrared lumines-ent devices [13–17]. In these materials, the excitation of the

E cations occur by the recombination of photogenerated car-

iers confined in the semiconductor and energy transfer fromhe semiconductor to the RE ions [13–17]. The optical proper-

∗ Corresponding author.E-mail addresses: mparitosh@kaist.edu (P. Mohanty), bongsoo@kaist.ac.kr

B. Kim).

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921-5107/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.mseb.2007.01.007

ies of RE cations mainly depend on the local environment orymmetry of the host materials [18,19]. However, manipulat-ng the concentration of the dopants and controlling the aspectatio of the 1D nanostructures are still remain challenges to theanotechnology community.

Several reports are available on the synthesis and studies ofhe luminescent properties of europium (Eu)-doped semicon-uctors in various morphologies. Cheng and Wang reportedhe synthesis and phosphorescence of europium-doped ZnSanowires [13]. Okuyama and coworkers observed the energyransfer from the ZnO to the Eu3+ ions in europium-dopednO/poly(ethylene glycol) nanocomposites [14]. Europium-oped ZnO thin films are also reported [15]. Recently,anemitsu and coworkers reported the synthesis of Eu-dopednO nanorods by a microemulsion method [17]. To the bestf our knowledge no report is available on the synthesis ofu-doped ZnO nanowires by a vapor phase method. In thisommunication we report the synthesis of single crystallinenO:Eu3+ nanowires by a vapor transport method.

. Experimental details

Eu3+-doped ZnO nanowires were fabricated through a facile

apor-transport and condensation method. Mixtures of highlyure ZnO (50 − x/2) mol%, Zn (50 − x/2) mol% and Eu2O3 (x)ol%, x = 0–7.5, powders were used as the precursor materi-

ls. Si (1 1 1) wafers dispersed with 5 nm gold nanoparticles

and E

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wuoiCatXtodrtptfiemnany of the crystal structure of ZnO or Zn. As it was also observedin the undoped sample so the possibility of observing such peakfrom Eu2O3 is excluded.

P. Mohanty et al. / Materials Science

ere used as substrates. Typically, 0.5 g of the precursor pow-er kept in an alumina boat was placed at the centre of aorizontal quartz tube furnace. The system was pre-evacuatedo 0.3 torr and then purged with 105 standard cubic centime-ers per minute (sccm) of a mixture of argon and oxygen gasith a molar ratio of 20:1 for 30 min, while we maintain theressure of the system at 15–19 torr. The furnace was heatedo 850 ◦C at a rate of 20 ◦C/min and held at this tempera-ure for 30 min. The furnace was then switched off and cooledown to room temperature. The Si substrate was uniformly cov-red with white film. In order to compare the results with thendoped ZnO nanowires, the reaction was carried out under sim-lar experimental conditions without using Eu2O3 powder in therecursors.

X-ray diffraction (XRD) measurements were carried out by aigaku D/Max-RC (12 kW) X-ray diffractometer with a graphiteonochromator and Cu K� radiation (λ = 0.15405 nm) from a

otating anode X-ray generator operated at 40 kV voltage and0 mA current. A step width of 0.01◦ and step time of 1 s waspplied to record the data in the range of 25–70◦ of the 2θ scale.-ray photoelectron spectroscopy (XPS) measurements of the

pecimen were carried out on an ESCA 2000, V.G. MicrotechK, at a pressure of 1 × 10−9 torr using the Mg K� line withphoton-energy of 1253.6 eV. Field emission scanning electronicroscope (FESEM) images were taken on a Phillips XL30S.ransmission electron microscope (TEM), selected area elec-

ron diffraction (SAED) pattern, and energy-dispersive X-raypectrometry (EDS) were taken on Jeol JEM-1230 transmissionlectron microscope operated at 120 kV. A standard procedure

as followed to prepare the sample for the TEM analysisy dispersing the nanostructures in the organic solvents anddrop of the solution was put on the carbon coated copper

rid.

ig. 1. XRD patterns of (a) undoped and (b) europium-doped ZnO nanowires.he precursor composition for the ZnO:Eu3+ nanowires in the diffractogram (b)as ZnO (47.5 mol%), Zn (47.5 mol%) and Eu2O3 (5 mol%). A representative

1 0 0) peak of these diffractograms is shown in the inset demonstrating the shiftf the peak position towards a lower 2θ-value on europium doping.

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ngineering B 138 (2007) 224–227 225

. Results and discussion

The crystal structure of the obtained specimen was studiedith the XRD. Fig. 1a and b show the XRD patterns of thendoped and the Eu-doped ZnO nanowires, respectively. Thebserved peaks in these diffractograms were unambiguouslyndexed to the wurtzite hexagonal crystal structure (space group6mc) of ZnO. All the peaks in the doped sample shifted towardslower 2θ value and also the peaks were broadened compared to

he undoped sample. It can be clearly seen by the high-resolutionRD pattern of a representative (1 0 0) peak shown in the inset

o Fig. 1. This shift in the diffraction peaks towards a lower 2�r in other words the increase of the lattice parameters could beue to the doping of the bigger size Eu3+ cations (effective ionicadii ri = 0.947 A, coordination number CN = VI) compared tohat of the smaller size Zn2+ (ri = 0.74 A, CN = VI) [20]. Thiseak shifting and broadening may also be attributed to the lat-ice mismatching, lattice distortion, strain of the crystal, and thenite size effect. In any of the cases, it indicates the doping ofuropium in the ZnO nanowires. One very small peak (asteriskarked) is observed in both the Eu-doped and undoped ZnO

anowires at ∼38.2◦ in the 2θ scale. It could not be assigned to

ig. 2. FESEM images of (a) undoped and (b) europium-doped ZnO nanowiresemonstrating that the epitaxially grown nanowires were deposited in largeuantity on the Si substrate.

226 P. Mohanty et al. / Materials Science a

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sof tetrahedrally coordinated O2− and Zn2+ ions were stakedalong the c-axis [7–9]. It has three types of growth directionsof [2110], [0110], and ±[0 0 0 1]. The facets [2110] and [0110]have lower energy than ±[0 0 0 1] facet [7–9]. Under thermody-

ig. 3. TEM micrograph of Eu3+-doped ZnO nanowire. The SAED pattern isiven in the inset which shows the nanowire is single crystalline with ±[0 0 0 1]rowth direction.

The microstructures of the doped and the undoped ZnOanowires were studied with FESEM images as shown in Fig. 2and b. In both cases, the nanowires were epitaxially grown on aubstrate and their diameters vary between 40 and 150 nm andengths up to several micrometers. Details about the microstruc-ure and the growth direction of the nanowires were studied withhe TEM and the SAED pattern. TEM image of a representativeuropium-doped ZnO nanowire is shown in Fig. 3 and the cor-esponding SAED pattern is given in the inset of this figure. The

anowire is straight with a diameter ∼75 nm. From the SAEDattern it can be clearly seen that the nanowire is single crys-alline with ±[0 0 0 1] growth direction and the side surfaces areefined by [2110] and [0110]. Wurtzite ZnO has a hexagonal

ig. 4. XPS wide scan spectrum of (a) undoped and (b) europium-doped ZnOanowires.

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nd Engineering B 138 (2007) 224–227

tructure with space group C6mc in which the alternating planes

ig. 5. High-resolution XPS spectra of ZnO:Eu3+ nanowires showing the indi-idual (a) Eu 4d, (b) Zn 2p, and (c) O 1s peaks.

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P. Mohanty et al. / Materials Science

amic equilibrium conditions, the structure tends to minimizehe area of ±[0 0 0 1] facet. Thus, after an initial period of nucle-tion, a crystallite will prefer to grow along ±[0 0 0 1] directiony maximizing the surface areas of [2110] and [0110] facetsside surfaces) resulting in the growth into nanowire morphol-gy. Several other morphologies such as nanobelts, nanocombsnd hierarchical nanostructures were also reported by manipu-ating the growth directions and side surfaces in ZnO [7–9].

In order to measure the concentration of europium we tookDS patterns of different nanowire samples. In these cases, theoncentration of europium was maximum up to 0.15 atomic %.

hen we repeated the reaction by taking various mole percent ofhe Eu2O3 in the precursors ranging from 0.5 mol% to 7.5 mol%,he concentration of europium is not raised above 0.15 atomic

as studied with the EDS. It is in fact below the detectionimit of the EDS analysis. In order to confirm the europiumoping we studied XPS of the specimen. The wide scan XPSpectra of the undoped and the Eu-doped ZnO sample is shownn Fig. 4a and b, respectively. The peak at 136 eV in Fig. 4b dueo Eu 4d binding energy confirms the europium doping in thenO nanowires. This peak is not observed in the undoped ZnOample. The high-resolution XPS spectrum reveals the Eu 4d5/2nd Eu 4d3/2 peaks at 138.4 eV and 140.3 eV, respectively, ashown in Fig. 5a. The high-resolution XPS spectrum of Zn 2p3/2nd Zn 2p1/2 peaks at 1021.1 eV and 1044.0 eV are displayedn Fig. 5b. The energy difference of these two peaks is 22.9 eV,hich agrees well with the standard value of 22.97 eV [21]. Theigh-resolution XPS spectrum due to the O 1s peak at 530.6 eVs shown in Fig. 5c.

. Conclusion

Epitaxial Eu3+-doped ZnO nanowires were grown on gold-oated Si wafers by a vapor deposition method. Compared withhe undoped counterpart, the lattice parameters were increasedy europium doping because of the bigger size of Eu3+ cations.

he nanowires are single crystalline with ±[0 0 0 1] growthirection. The concentration of Eu3+ in the nanowires was belowatomic %, and such a small amount of the dopant (Eu3+) con-

entration is useful for studying their optical absorption and

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ngineering B 138 (2007) 224–227 227

mission properties. The study of the optical properties of Eu3+

n the ZnO host and also the energy transfer from the host to theopant europium are under progress.

cknowledgment

This work was financially supported by the Center for Nanos-ructured Materials Technology (grant No. 05K1501-01210).

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