Tunable light emissions from thermally evaporated In2O3 nanostructures grown at different growth temperatures

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Journal of Crystal Growth 310 (2008) 2264–2267

Tunable light emissions from thermally evaporated In

₂

O

₃

nanostructures grown at different growth temperatures

Tsung-Shine Ko, Chia-Pu Chu, Jun-Rong Chen, Tien-Chang Lu

,

Hao-Chung Kuo

, Shing-Chung Wang

Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, 1001 Ta Hsueh Rd., Hsinchu 30050, Taiwan, ROC Available online 4 December 2007

Abstract

We report the synthesis of the In2O3nanostructures grown at different growth temperatures by using the thermal evaporation method.

The gold nanoparticles were used as the catalyst and were dispersed on the silicon wafer to facilitate the growth of In2O3nanostructures.

The nanostructures of the In2O3were characterized by scanning electron microscopy, transmission electron microscopy, and X-ray

diffraction. The photoluminescence study reveals that In2O3nanostructures could emit different luminescence peaks in the range of

400–600 nm with broad bands by adjusting different growth temperatures. The coverage of the wavelength tuning in the emission peaks of the In2O3nanostructures could be beneﬁcial for possible applications in white light illumination through manipulating the ratio of

each wavelength component.

PACS: 81.10.h; 81.10.BK; 42.70.a

Keywords: A1. Nanostructure; A1. Photoluminescence; B1. Indium oxide; B2. Semiconducting indium compound

1. Introduction

In2O3is an important transparent semiconductor material

with wide band gap energy (3.6 eV) and has been applied in optical and electric devices such as solar cells and liquid crystal devices [1–3]. Previous studies focused on the preparations and characterizations of In2O3 ﬁlms or

nanostructrues [4,5]. However, optical properties, such as photoluminescence (PL) results of In2O3 obtained by

different research groups varied a lot. For instance, Lee et al. obtained the PL emission of 637 nm from In2O3thin ﬁlms

synthesized by thermal oxidation[4]. Liang et al. used InP as the starting material to grow In2O3 nanoﬁbers using a

furnace and their PL spectra exhibited emission of 470 nm

[6]. Zheng et al. observed the PL emission of 429 and 460 nm from their In2O3nanowires using a three-probe DC method

[7]. Li et al. measured PL of In2O3 nanotubes exhibiting

emission of 593 nm[8]. Most of the above PL results come from different densities of oxygen vacancies [9]. However, few groups have developed methods to modulate the emission wavelength of In2O3. In this paper, we report the

synthesis of In2O3 nanoparticles with a special

nanostruc-ture using the two-zone thermal evaporation method. The PL results of In2O3 nanostructures obtained by different

growth conditions exhibited different broad band emission peaks ranging from 400 to 600 nm, which could have potential applications in white light illumination.

2. Experiments

The synthesis procedures of In2O3nanostructures started

with gold nanoparticles produced by the chemical reduction of gold chloride tetrahydrate (HAuCl4) with sodium citrate

spread onto the Si substrates cleaned by the 3-aminopropyl-trimethoxysilan (APTMS) ethanol solution and water. Observed from the SEM (scanning electron microscopy) images, the Au nanoparticles are 10–20 nm in width and the density is around 6 108cm2. Then, the gold nanoparticles

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on Si substrates were sent to a two-zone vacuum furnace to grow the In2O3. The experimental setup for synthesizing

indium oxide nanostructures is schematically depicted in

Fig. 1. The starting material, 5 g indium metal, was placed in a quartz boat located inside a quartz tube reactor, designated as zone 1 with the temperature setting as T1. p-Type silicon (1 0 0) substrates were placed in the downstream of the tube reactor separated from the starting material by 20 cm, designated as zone 2 with the temperature setting as T2. The quartz tube was exhausted by a mechanical pump down to around 0.1 Torr. We grew indium oxide nanostructures under different growth temperatures T2 from 500 to 900 1C for 8 h reaction time. The temperature T1 was set 900 1C for the source zone. We used the oxygen gas of 150 sccm as carrier gas in our furnace system.

For characterization of as-grown samples, we used SEM (JEOL, JSM 6500F) and high-resolution transmission electron microscopy (HRTEM, JEOL, JEM 2010F, operating at 200 kV) to investigate the morphology and microstructure. The compositions were analyzed using energy dispersive spectrometry (EDS) attached to the SEM. Selected area electronic diffraction (SAED) con-ﬁrmed the crystal orientation of In2O3 nanocrystals. The

crystal structure analysis was performed by the XRD measurement with Cu Ka radiation. PL spectra were measured at room temperature with a spectrometer (TRIAX-320) excited by a 25-mW He–Cd laser with the lasing wavelength of 325 nm.

3. Results and discussion

Fig. 2(a)–(c)show SEM images of the In2O3

nanocrys-tals grown at different growth temperatures of 500, 700,

and 900 1C, respectively, with the oxygen ﬂow rate of 150 sccm. The amount of nanostructure was found to be dependent on the growth temperatures. As a result,

Fig. 2(a) shows fewer nanostructures existed and many In2O3 tips are beginning to sprout when the growth

temperature was 500 1C. As the growth temperature was increased to 900 1C, most of In2O3nanocrystals show

well-shaped nanostructure as indicated inFig. 2(c).

The HRTEM images of the nanostructure grown with the oxygen ﬂow rate of 150 sccm at 700 1C are shown in

Fig. 3(a) and (b). It is clearly shown that the In2O3

nanostructure was capped with a 20-nm gold nanoparticle. The presence of gold nanoparticles at the top of the In2O3

nanostructures provides a strong evidence of the vapor– liquid–solid (VLS) growth mechanism. However, VLS growth mechanism generally leads to a well-directional growth and further to form nanowire or nanorod structures [10,11], which is different from our results. We proposed that the short In2O3nanowires were nucleated at

the gold nanoparticles by the VLS mechanism at ﬁrst. As the growth time passed by, the growth direction still favored to the top due to the fast reaction provided by the catalyst. However, the over-supply of the indium vapor

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Fig. 1. Schematic diagram of a two-zone furnace for indium oxide nanostructures’ growth.

Fig. 2. In2O3nanostructures grown at different growth temperatures of 500, 700, and 900 1C (a)–(c), respectively.

Fig. 3. (a, b) TEM images of the In2O3 nanostructure’s tip. (c)

High-resolution TEM images of part of the In2O3 nanostructures. (d) The

corresponding selected area electron diffraction pattern. This In2O3

nanostructure was grown at 700 1C.

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interacting with oxygen atoms could further facilitate the lateral growth. The lateral volume could increase and then turned into such a special shape. Fig. 3(c) and (d) show HRTEM image of a partial In2O3 nanostructure and the

corresponding SAED result, which proved that our In2O3

nanostructure was a single crystal structure. The lattice plane of (4 2 2) with an interplanar spacing of 0.207 nm and (6 2 2) with spacing of 0.153 nm can be obtained by the analysis of both TEM images and SAED shown inFig. 3

corresponding to the In2O3crystal lattice planes.

Typical XRD patterns of the In2O3 nanostructures

grown at different growth temperatures are shown in

Fig. 4. All the diffraction peaks could be indexed to a pure cubic phase structure with a lattice constant of a ¼ 1.011 A˚ (JCPDS 71-2195). Both measurement results show that the stronger and sharper XRD phase peaks could be detected as the growth temperatures were increased. Therefore, the percentage of the crystalline phase of In2O3nanostructures

could get higher when the samples were grown at higher growth temperature, which was consistent with the SEM observations.

The normalized PL measurement results of the In2O3

nanostructures prepared by different growth conditions reveal the quality dependent characteristics as shown in

Fig. 5. Since the bandgap energy of In2O3is around 3.6 eV,

we could exclude the origin of the PL as shown in Fig. 5

from the band-to-band transition. Namely, the transition could substantially be ascribed to the carrier recombination between the valence band and the oxygen vacancy-induced donor levels formed in the midst of the In2O3bandgap[12].

However, the different crystallization quality of the In2O3

nanostructures could result in different transition path of carriers due to the different amounts of oxygen vacancies and defects generated during the growth [13]. The reason for electrical n-type property of In2O3is that no

enough oxygen atoms to catch electrons released from indium atom. The more electrons existing in crystal could ﬁll up the oxygen vacancy-induced donor level and the Fermi energy will be raised up higher to be closer to the

conduction band[14]. The Fermi energy levels ﬁlled up by different amount of electrons can be deemed as new energy states for optical transitions. A schematic diagram of relative states is shown in Fig. 6. E1–E3 represents main

energy states between the conduction and the valence band caused by different amounts of electrons ﬁlled up the oxygen vacancy-induced donor levels. In our case, the worse quality sample grown at 500 1C would lead to a shorter wavelength emission and stronger emission inten-sity due to more oxygen vacancies and more electrons formed in the nanostructures which exhibited higher energy state E1. With increasing growth temperature, the fewer

electrons would produce due to better material quality and form lower energy states E2and E3. Therefore, we could

synthesize the In2O3 nanostructures emitting different

wavelength peaks to span the whole blue–orange light region by adjusting these growth conditions. It is worth noting that the growth temperature could inﬂuence the

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20 30 40 50 60 0 600 1200 1800 2400 3000 3600 500°C 700°C

Intensity (arb. units)

2 theta (deg) (222)

(400)

(440)

900_°C

Fig. 4. XRD pattern of the In2O3 nanostructures grown at different

growth temperatures.

Fig. 5. Room temperature PL spectra of the In2O3nanostructures grown

at different growth temperatures.

Fig. 6. Transition and emission mechanisms for In2O3nanostructures.

T.-S. Ko et al. / Journal of Crystal Growth 310 (2008) 2264–2267 2266

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optical transition energy in the nanostrcutures. Consider-ing the oxygen vacancies, the vacancies could be repaired and would diminished in amount accordingly as we increased the growth temperature.

4. Conclusion

We have grown the In2O3 nanostructures at different

growth temperatures by using the thermal evaporation method. The VLS process was dominant in the growth of the In2O3 nanostructures conﬁrmed by the TEM images.

Both SEM and XRD results revealed the clearer phase of In2O3nanostructures as our samples were grown at higher

growth temperature. The PL measurement results showed that In2O3nanostructures could emit different broadband

luminescence peaks ranging from 400 to 600 nm by adjusting growth temperature. Owing to the different amount of oxygen vacancies provided by different growth conditions, the optical transition energy of the In2O3

nanostructures becomes tunable, which could be beneﬁcial for possible applications in white light illumination. Acknowledgments

This work was supported by the MOE ATU program and in part by the National Science Council NSC

95-2120-M-009-008, NSC E009-092-MY3, NSC 96-2221-E009-093-MY3, and NSC 96-2221-E009-094-MY3, Re-public of China.

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