Influence of the annealing atmosphere on the photoluminescence of ...

Paper

Journal of the Ceramic Society of Japan 118 [10] 916-920 2010

Influence of the annealing atmosphere on the photoluminescence of ZnSe-core/In 2 O 3 -shell nanowires Chongmu LEE, Hyunsu KIM, Sunghoon PARK, Hyoun Woo KIM and Changhyun JIN³ Department of Materials Science and Engineering, Inha University, Yonghyeondong, Incheon 402–751, Republic of Korea

The influence of thermal annealing on the photoluminescence properties of the ZnSe-core/In2O3-shell nanowires prepared by a two-step process comprising the thermal evaporation of ZnSe powders and the sputter-deposition of In2O3 was investigated. The ZnSe nanowires were a few tens to a few hundreds of nanometers in diameter and up to a few hundreds of micrometers in length. Photoluminescence measurements showed that ZnSe nanowires had an emission band centered at around 630 nm in the orange region. In contrast, ZnSe-core/In2O3-shell nanowires had a stronger emission band centered at around 560 nm in the yellowgreen region. The major emission of the ZnSe nanowires was found to be blue-shifted and enhanced in intensity by coating them with In2O3. The major emission is shifted back to 630 nm and further enhanced by annealing in a reducing atmosphere, whereas it is shifted back to ³620 nm and degraded in intensity by annealing in an oxidative atmosphere. The PL enhancement by annealing in a reducing atmosphere is mainly attributed to the formation of In interstitials in the ZnSe cores during the annealing process. ©2010 The Ceramic Society of Japan. All rights reserved.

Key-words : ZnSe, Nanowires, Annealing, Energy-dispersive X-ray spectroscopy, Photoluminescence spectroscopy [Received July 9, 2010; Accepted August 19, 2010]

1.

Introduction

In recent years, one-dimensional (1D) nanostructures such as nanowires, nanorods, nanobelts, nanoribbons, nanoneedles, nanotubes have attracted significant attention owing to their unique optical and electronic properties. A common technique to control and enhance the properties of the 1D nanostructures is to create core­shell coaxial heterostructures.1)­11) For example, the photoluminescence (PL) emission intensity of the light emitted from core­shell nanostructures can be significantly increased or the wavelength of the emission can be controlled by selecting a proper coating material and a proper coating layer thickness.12)­17) ZnSe is an important compound semiconductor with a wide direct band gap which has attracted significant interest for its potential applications in optoelectronics and electronics. ZnSe is particularly suitable for short-wave optoelectronic device applications including blue-green laser diodes18) and tunable mid-IR laser sources for remote sensing.19) Recently 1D nanostructures of ZnSe have been synthesized using various techniques including thermal evaporation,20) metal-organic chemical vapor deposition (MOCVD),14) molecular beam epitaxy (MBE),21) pulsed lased deposition (PLD),22) atomic layer deposition (ALD),23) and a chemical method through the solution-liquidsolid mechanism.24) On the other hand, In2O3 is an important n-type wide band gap (3.55­3.75 eV, depending on the synthesis method) transparent conducting oxide material. Due to its high electrical conductivity, high transparency to the visible light, and intense interaction with some poisonous gas, In2O3 has been widely used in transparent conductive electrodes,25) optoelectronic devices such as solar cells,26) flat panel display materials27) and gas sensors.28) 1D In2O3 nanostructures are also known to be sensitive to NO2 and NH3 gases29),30) as well as to biomolecules.31) ³

Corresponding author: C. Lee; E-mail: [email protected]

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The photoluminescence (PL) properties of ZnSe 1D nanostructures have been reported recently by many researchers. However, there has been no report on the synthesis and PL properties of ZnSe-core/In2O3-shell 1D nanostructures yet. In this paper, we report on the effects of coating and annealing, in particular, the annealing atmosphere on the PL properties of ZnSe nanowires.

2.

Experimental

We prepared ZnSe-core/In2O3-shell nanowires on Si(100) substrates. Firstly, ZnSe nanowires were synthesized on a gold (Au)-coated Si(100) substrate by the thermal evaporation of ZnSe powders. The heating furnace used for this thermal evaporation process is schematically shown elsewhere.32) A quartz tube was mounted inside a horizontal tube furnace. The quartz tube consisted of two temperature zones: zone A at 850°C and zone B at 750°C. An alumina boat loaded with pure ZnSe powders and the Au-coated Si substrate were located in zones A and B, respectively. The nitrogen (N2) gas flow rate was 150 standard cubic centimeters per minute (sccm) and the chamber pressure was 0.5 Torr. The synthesis was performed for 1 h. Secondly, coating of the ZnSe nanowires with In2O3 was carried out by sputtering. The sputter-deposition was done at room temperature using a 99.999% In2O3 target in a radiofrequency (rf) magnetron sputtering system. After the vacuum chamber was evacuated to 1 © 10¹6 Torr using a turbomolecular pump backed by a rotary pump. Ar was provided at a flow rate of 30 sccm. Depositions were carried out at room temperature for 15 min. The system pressure and the rf sputtering power were 1.8 © 10¹2 Torr and 100 W, respectively. Some of the as-prepared core­shell nanowire samples were subsequently annealed in an O2 or N2/3-mol % H2 atmosphere at 600°C for 1 h to see the influence of annealing on the PL properties of the wires. Next, the morphologies of the prepared core­shell nanowire samples were checked by using field emission scanning electron microscopy (FESEM, Hitachi S-4200). The microstructures and ©2010 The Ceramic Society of Japan

Journal of the Ceramic Society of Japan 118 [10] 916-920 2010

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Fig. 1. (a) SEM images and (b) EDX spectrum of the ZnSe-core/ In2O3-shell nanowires synthesized on the Si(100) substrate.

compositions of the nanowire samples were further characterized using transmission electron microscopy (TEM, Phillips CM-200) equipped with an energy dispersive X-ray spectrometer (EDXS). X-ray diffraction (XRD) analyses using a X-ray diffractometer (Philips X’pert MRD) with Cu K¡ radiation were employed to identify the morphology and structure of the nanowire samples. Room temperature PL properties of the products were also investigated using a 325 nm He­Cd laser (Kimon, IK, Japan) as the excitation source.

3.

Results and discussion

The SEM image of the as-synthesized ZnSe-core/In2O3-shell nanowires is shown in Fig. 1(a). The core­shell nanowires were a few tens to a few hundreds of nanometers in diameter and up to a few hundreds of micrometers in length. The enlarged SEM image in the inset of Fig. 1(a) clearly shows that a droplet or particle does exist at the tip of the nanwire. The EDX spectrum (Fig. 1(b)) taken from the particle in the enlarged SEM image (inset of Fig. 1(a)) shows that the particle comprises not only Zn, Se, In and O but also Au elements. These two facts suggest that the ZnSe nanowires have grown through a catalyst-assisted vapor­liquid­solid (VLS) mechanism. The growth mechanism of 1D nanostructures could differ for different process parameters. Particularly, it depends on the growth temperature. The VLS mechanism in which 1D nanostructures grow with an assistance of catalysts is dominant at lower temperatures because the thermal energy is not enough for the nucleation and growth of the structure. In contrast, the vapor­solid (VS) mechanism is dominant at higher temperatures where sufficient thermal energy is provided.33) As described in the ‘Experimental’ section, the In2O3 layer was deposited by sputtering which is known as a

Fig. 2. XRD patterns of the ZnSe-core/In2O3-shell nanowires synthesized on the Si(100) substrate: (a) as-synthesized and (b) annealed in a N2/H2 atmosphere at 600°C for 1 h.

typical physical vapor deposition technique. The growth mechanism of the In2O3 shell layer can be described in three different steps as the general nucleation and growth mechanism in sputtering as follows:34) First, incident atoms transfer kinetic energy to the ZnSe nanowire and become loosely bonded ‘adatom’. This energy transfer is efficient, even for energetic sputtered atoms. Second, the adatom diffuse over the surface of each nanowire, exchanging energy with the nanowire and other adsorbed species, until they either are desorbed, by backsputtering or become trapped at low-energy sites. Once the coating becomes continuous, growth proceeds similarly, by adatoms arriving and diffusing over the coating surface until they desorbed or become trapped in low energy sites. Finally, the incorporated atoms readjust their positions within the lattice of the nanowire by bulk diffusion processes. The XRD patterns of ZnSe-core/In2O3-shell nanowires before and after annealing are shown in Figs. 2(a) and (b), respectively. All the reflection peaks in the XRD pattern of the as-synthesized core­shell nanowires belong to crystalline ZnSe with a zinc blend-type face-centered cubic (fcc) structure (JCPDS 15-0105). In contrast, the reflection peaks in the XRD pattern of the annealed core­shell nanowires belong to either crystalline ZnSe 917

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Lee et al.: Influence of the annealing atmosphere on the photoluminescence of ZnSe-core/In2O3-shell nanowires

Fig. 3. Low-magnification TEM image of a typical ZnSe-core/In2O3shell nanowire.

with a zinc blend-type fcc structure (JCPDS 15-0105) or crystalline In2O3 with a body-centered cubic (bcc) structure (JCPDS 89-4595). The relatively small reflection peaks for In2O3 (Fig. 2(b)) suggest that the In2O3 shells have been crystallized by annealing, but that the crystallization is incomplete presumably because of the insufficiently high annealing temperature or time. The low-magnification TEM image of a typical ZnSe-core/ In2O3-shell nanowire in Fig. 3(a) clearly shows a rod-like core (a dark area inside) and two shell layers (the less dark areas on both edge sides of the dark area) with high thickness uniformity along the length of the nanowire. The thicknesses of the ZnSe core and the In2O3 shell layer are about ³45 and ³12 nm, respectively. The local high resolution TEM (HRTEM) image and the corresponding selected area electron diffraction (SAED) pattern of the core­shell interface region are shown in Figs. 4(a) and (b), respectively. The fringe pattern on the upper left side clearly indicates that the ZnSe core is monocrystalline, but that the In2O3-shells on the lower right side are amorphous. The associated SAED pattern shows only a set of single crystal fcc electron diffraction pattern spots corresponding to the ZnSe core. This result is consistent with the XRD analysis results. The spacing between the two neighboring fringes shown in Fig. 4(a) is 0.325 nm, which is in good agreement with the interplanar spacing of the {002} lattice plane family of fcc ZnSe with a lattice parameter of a = 0.567 nm (JCPDS 15-0105). On the other hand, no fringes are observed in the shell region (the lighter area on the lower right side in Fig. 4(a)). As regards the PL of ZnSe 1D nanostructures, the room temperature PL spectrum is known to be typically dominated by two characteristic emission bands:35)­38) (1) a NBE emission band centered at around 465 nm and (2) a broad deep level emission band in the wavelength range of 500­680 nm. The NBE emission is generally known to be due to bound excitons and donor­ acceptor pairs whereas the deep level emission is mainly due to deep levels such as vacancies, interstitials, and stacking faults. On the other hand, it is well known that bulk In2O3 cannot emit light at room temperature.39) However, Zhou et al. observed PL peaks at 480 and 520 nm from the In O nanoparticles.40) Lee et al. noticed a peak at 637 nm for In2O3 films.41) Liang and coworkers observed a peak at 470 nm from In2O3 nano-fibers42) and recently Wu et al. reported two distinct peaks at 416 and 435 nm for In2O3 nanowires.43) These emissions are commonly referred to as the deep level emissions due to oxygen deficiencies. 918

Fig. 4. (a) HRTEM image and (b) SAED pattern of a typical the ZnSecore/In2O3-shell nanowire.

Influence of the annealing atmosphere on the PL of the ZnSecore/In2O3-shell nanowires. The nanowire samples were annealed at 600°C for 1 h in different atmospheres. Fig. 5.

The PL spectra of the ZnSe-core/In2O3-shell nanowires annealed in different atmospheres are shown in Fig. 5 along with that of the as-synthesized (unannealed) ZnSe and ZnSecore/In2O3-shell nanowires. The PL spectra of the nanowires are dominated by a deep level-related emission and the near-band edge (NBE) emission is negligible. The deep level-related emission of the ZnSe nanowires is at around 630 nm. In contrast,

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Journal of the Ceramic Society of Japan 118 [10] 916-920 2010

have diffused into the core during the annealing process and the In atoms may have resided at the interstitial sites. The increase in the In interstitial concentration in the core, in turn, resulted in the degradation of the deep-level emission. Close examination of the EDXS line-scanning concentration profiles for In (the light blue colored curve) indicates that the In concentration in the core region is higher after annealing in the N2/H2 atmosphere (Fig. 6(c)) than before annealing (Fig. 6(a)). By comparing the In concentration profile (the third light blue-colored one among the four small profiles on the right hand side of each figure) between Figs. 6(a) and (c), we can see that the In peak at the left shell layer region has disappeared and the In concentration in the core region has increased after annealing. On the other hand, during the annealing process in the O2 atmosphere, the ZnSe core spontaneously reacts with O2 as follows:44) 2ZnSeðsÞ þ O2 ðgÞ ¼ 2ZnOðsÞ þ Se2 ðlÞ:
Gf;873 K ¼
245:339 KJ=mol ð3Þ

Fig. 6. EDXS elemental maps of the ZnSe-core/In2O3-shell nanowires: (a) as-synthesized and (b) annealed in an O2 atmosphere, and (c) annealed in a N2/H2 atmosphere at 600°C for 1 h.

the ZnSe-core/In2O3-shell nanowires have a stronger emission band centered at around 560 nm in the yellow-green region. In other words, the major emission of the ZnSe nanowires has been blue-shifted and enhanced in intensity by coating them with In2O3. It is also worthy of noting that the major emission has been degraded in intensity and shifted back to approximately 620 nm by annealing in an oxidative atmosphere. In contrast, the emission has been significantly enhanced in intensity and shifted back to ³630 nm by annealing in a reducing atmosphere. We performed EDXS analysis to investigate the origin of the PL enhancement of the core­shell nanowires in intensity by annealing in a reducing atmosphere. During the annealing process in the H2/N2 atmosphere, the ZnSe core and the In2O3 shell layer, in actuality, may not have been spontaneously dissociated by reacting with H2 as follows:44) ZnSe þ H2 ¼ Zn þ H2 Se:
Gf;873 K ¼ 140:655 KJ=mol

ð1Þ

In2 O3 þ 3H2 ¼ 2In þ 3H2 O:
Gf;873 K ¼ 45:270 KJ=mol ð2Þ Nevertheless, comparison of Fig. 6(a) with 6(b) gives that the O concentration in the In2O3 shell was markedly lower after annealing in the reducing atmosphere than before annealing. This result suggests that some oxygen atoms loosely bound with indium (In) atoms in the shell layer were presumably removed by reacting with hydrogen molecules during the annealing process despite that Reaction 2 is thermodynamically unfavorable at the annealing temperature. The extra In atoms in the shell layer may

It appears that the major phase of the cores changed from ZnSe into ZnO through this reaction. The substantial decrease in the Se concentration in the coaxial nanowire in Fig. 6(b) is an evidence that this reaction occurred during the annealing process. Hence, the deep level-related emission at 630 nm from the core­shell nanowires annealed in an oxidative atmosphere are not from ZnSe but ZnO, i.e., the source for the emission quite differs from that for the emission from the core­shell nanowires as-synthesized or annealed in a reducing atmosphere, although the wavelength of the major emission from the former happens to be similar to that from the latter. The defect-related deep-level emission from ZnO nanostructures strongly depends upon the preparation methods and growth conditions, but the yellow-green emission around 510 nm and the red emission around 650 nm are known to be typical ones.45) The wavelength of the emission from our sample (630 nm) somewhat differs from that of the typical emission from ZnO (650 nm), which may be caused by the high concentration of Se impurities in the ZnO cores in our sample.

4.

Conclusions

ZnSe-core/In2O3-shell nanowires have been prepared by using a two-step process: the thermal evaporation of ZnSe powders on Si(100) substrates coated with Au thin films and the atomic layer deposition of In2O3. The ZnSe nanowires were a few tens to a few hundreds of nanometers in diameter and up to a few hundreds of micrometers in length. The cores and shells of the annealed ZnSe-core/In2O3-shell nanowires are single crystal zinc blend-type fcc ZnSe and single crystal body-centered cubic (bcc) In2O3, respectively. ZnSe nanowires have an emission band centered at around 630 nm in the orange region. In contrast, ZnSe-core/In2O3-shell nanowires have a stronger emission band centered at around 560 nm in the yellow-green region. It appears that the major emission was blue-shifted and enhanced in intensity by coating the ZnSe nanowires with In2O3. The major emission is shifted back to 630 nm and further enhanced by annealing in a reducing atmosphere, whereas it is shifted back to ³620 nm and degraded in intensity by annealing in an oxidative atmosphere. In the case of annealing in a reducing atmosphere, the In interstitial concentrations is increased due to the removal of the loosely bound oxygen atoms and subsequent diffusion of extra indium atoms in the core to the shell, which results in the enhancement in the deep level emission. In contrast, the deep level-related 919

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Lee et al.: Influence of the annealing atmosphere on the photoluminescence of ZnSe-core/In2O3-shell nanowires

emission at 620 nm from the core­shell nanowires annealed in an oxidative atmosphere are not from ZnSe but ZnO, i.e., the source for the emission quite differs from that for the emission from the core­shell nanowires as-synthesized or annealed in a reducing atmosphere, although the wavelength of the major emission from the former happens to be similar to that from the latter. Acknowledgements This work was supported by Korea Science and Engineering Foundation through ‘2007 National Research Lab. Program’.

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