Growth of ZnO Nanowires Using ZnS Substrates with Ga Droplets∗

ZnO nanowires were synthesized by the heat treatment of ZnS substrates with Ga droplets in the air, and their morphological and structural properties were then investigated. The nano-sized ZnO wires were formed on thin ZnO layers accompanied with the formation of ZnGa2O4. It was found that the growth direction of the nanowires is ZnO[0001], and which is parallel to ZnGa2O4[010]. The growth mechanism of the ZnO nanowires is discussed based on the thermal stability or enthalpy of formation of the oxides. [DOI: 10.1380/ejssnt.2009.25]

Recently, the growth of compound materials, in which the reactions are governed by the difference in the enthalpy of formation, has been developed, and Ca 2 Si, Sr 2 Si layers have been grown by a simple heat treatment [6,7].Miyake et al. reported a simple growth technique of epitaxial ZnO layers using ZnS substrates.The oxidation of ZnS causes the formation of ZnO, and excess S atoms are evaporated from the substrates [8].Using ZnS powders with Fe(NO 3 ) 3 , the ZnO whiskers were grown [9], and the ZnO nanowires were grown by thermal evaporation of the ZnS powders [10].
On the other hand, it has been reported that a variety of oxide nanostructures have been synthesized by the heat treatment of silicide alloys with Ga droplets [11].The growth of ZnO nanostructures is expected when the heat treatment with Ga droplets is applied to the ZnO growth using ZnS substrates.
In this paper, we report the successful growth of ZnO nanowires by the heat treatment of ZnS substrates with Ga droplets in the air.In addition, the morphological and structural properties of the nanostructures were investigated.
The nanowires were grown by the heat treatment of ZnS substrates with Ga droplets.Polycrystalline bulk ZnS crystals were used as the source material.Ga was melted around 35-40 • C, then manually applied to the ZnS substrate using a steel or wood stick.As a result, Ga droplets with a mm size were stuck to the ZnS.The ZnS with Ga droplets were loaded into the quartz tube which is open to the air.The nanowire growth was performed by exposure of the ZnS to the air, and the nanowires were grown for 5 h at the temperature of 800 • C.
The resulting nanostructures were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) with selected area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM).The compositional analysis was made using energy dispersive X-ray spectroscopy (EDS).

III. RESULTS AND DISCUSSION
Figure 1(a) shows an SEM image of the nanowires.The EDS result revealed that the observed atomic ratio of Zn/O is near unity, with gallium and sulfur atoms detected at less than 1 at%.The diameters of the nanowires are about 20-200 nm, and the maximum length extends to tens of micrometers.It is interesting to note that some nanosheets, by which the adjacent nanowires are joined to each other, are observed.
Figures 1(b) and (c) show a TEM image of a nanowire and corresponding SAED pattern, respectively.The SAED pattern agrees with that of ZnO with the incident electron beam parallel to the [2 11 0] direction.Based on the compositional analysis mentioned above, the nanowire is confirmed to be ZnO and the growth axis of the nanowire is parallel to the ZnO  Figure 3 shows a TEM image and the corresponding Zn, O, Ga and S EDS mapping.The pictures reveal that the Ga atoms are highly concentrated at the substrate of the nanowire, while the Zn atoms are distributed in both the layer and nanowire.The EDS result reveals that the nanowire mainly contains zinc and oxygen, and the ratio of zinc to oxygen is about 54:45, indicating stoichiometric ZnO.It should be noted that small amount of Ga or S remain in the ZnO layer.The observed stoichiometry of the substrate region does not strictly agree with that of ZnGa 2 O 4 , which might be caused by the co-existence of other Zn or Ga compounds in deeper region of the substrate.
It has been reported that the selective growth of a particular phase is mainly triggered by a specific interface composition, and is governed by the diffusion flux to the interface [12].In this sense, when ZnS is used as the starting material of the growth evolution, O diffusion into ZnS leads to the formation of ZnO near the surface.The enthalpy of formation of ZnO (41.9 kcal/g-atom) [13] is greater than that of ZnS (24.5 kcal/g-atom) [13].The formation of ZnO is thermodynamically favored as compared to that of ZnS.On the other hand, the enthalpy of formation of Ga 2 O 3 (51.8kcal/g-atom) [13] is greater than that of ZnO.If the Ga droplets remained on the ZnS surface, Ga could be oxidized to form Ga 2 O 3 .It is assumed that Ga atoms are consumed to form ZnGa 2 O 4 , and possibly incorporated into ZnO as impurities.No evidence of segregation of the Ga or Ga-related compounds was obtained in the EDS mapping shown in Fig. 3.
On the other hand, no evidence of nanostructure growth was obtained on the ZnO layers, when the ZnS substrates were thermally treated without Ga [14].As reported by Nakanishi et al., Kirkendall voids were observed between a ZnO layer and a ZnS substrate, which means that not only S, but also Zn diffused to the surface region through the ZnO.The Zn atoms on the surface combined http://www.sssj.org/ejssnt(J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/) e-Journal of Surface Science and Nanotechnology with O to form ZnO, and S is evaporated from the surface.When the ZnS is thermally treated with Ga, Zn and Ga atoms are combined with O to form ZnGa 2 O 4 , and a small amount of Ga atoms were incorporated into the ZnO.
The growth mechanism of the crystalline nanowires is mainly explained by a vapor-liquid-solid (VLS) or vaporsolid (VS) mechanism.The former mechanism is generally based on a catalytic reaction.In the VS mechanism, nanowires were directly formed from the vapors of the precursors without a liquid state.For the case of nanowires presented here, no additional catalyst particles are observed on the top or bottom of the nanowires.This result shows that the growth of the ZnO nanowires presented here is not due to the VLS mechanism.Tseng et al. reported that needle-like well-aligned ZnO nanowires with a high density were uniformly grown over the entire Gadoped conductive ZnO films, but only sparse nanowires were obtained on the undoped ZnO film [15], and they proposed that the surface Ga atoms and crystal defects of the ZnO films are nucleation sites.On the other hand, the deposition rate along the ZnO[0001] was faster than that in the lateral directions.This is consistent with the growth model proposed by Li et al. [16].Along with the literature mentioned above, the growth of nanowires by the heat treatment of ZnS with Ga droplets was triggered by Ga the atoms incorporated into the growing ZnO layers.Further investigation will be required to clarify the growth mechanism.Due to Ga distribution on or in the ZnO, the nonhomogeneity of the supersaturation condition in the vapor would be considered [16].However, this simple growth procedure encourages us to synthesize a variety of nanostructures using the compositional conversion of the starting materials.
Figure1(a) shows an SEM image of the nanowires.The EDS result revealed that the observed atomic ratio of Zn/O is near unity, with gallium and sulfur atoms detected at less than 1 at%.The diameters of the nanowires are about 20-200 nm, and the maximum length extends to tens of micrometers.It is interesting to note that some nanosheets, by which the adjacent nanowires are joined to each other, are observed.Figures1(b) and (c) show a TEM image of a nanowire and corresponding SAED pattern, respectively.The SAED pattern agrees with that of ZnO with the incident electron beam parallel to the [2 11 0] direction.Based on the compositional analysis mentioned above, the nanowire is confirmed to be ZnO and the growth axis of the nanowire is parallel to the ZnO[0001].The ZnO nanowires

Figure 2 (
a) shows an enlarged TEM image of the nanowire.It was found that the surface of the nanostructure is covered by a thin layer.Figures 2(b), (c) and (d) show HRTEM images with SAED patterns in the insets.The plane spacing perpendicular to the growth direction is about 0.26 nm, which is consistent with that of the (0002) planes of the ZnO crystal.Stacking faults lying on the (0001) planes are observed perpendicular to the growth direction, and the streaking SAED pattern in the inset of Fig. 2 (c) is caused by the existence of the stacking faults.As shown in Fig. 2(d), the average distance of the lattice planes is about 0.43 nm and 0.26 nm.The angle between them is about 75 • .The spacing lattice conforms to that of the cubic ZnGa 2 O 4 crystal (a = 0.8335 nm; JCPDS Card No. 38-1240).The SAED pattern in the inset of Fig. 2(d) shows that the ZnGa 2 O 4 grew along the [010] direction.It was found that the growth direction of the nanowires is ZnO[0001], and which is parallel to ZnGa 2 O 4 [010].In addition, ZnO[2 11 0] is nearly parallel to ZnGa 2 O 4 [301].This relationship is roughly equivalent with the relationship as ZnO( 1100)//ZnGa 2 O 4 (101) and ZnO(11 20)//ZnGa 2 O 4 ( 101).The mismatch of the plane spacings between d ZnO( 1100) (0.2815 nm) and d ZnGa2O4(202) (0.2947 nm) is about 4.6%, and that of the plane spacings between d ZnO(11 20) (0.1625 nm) and d ZnGa2O4( 404) (0.1474 nm) is about 9.3%.This crystallographic relationship shows relatively lower lattice mismatch configuration for the ZnO and ZnGa 2 O 4 couple.It is considered that the observed relationship is provided by the result of ZnO( 1100)//ZnGa 2 O 4 (101) and ZnO(11 20)//ZnGa 2 O 4 ( 101) crystallographic orientation relationships with lower lattice mismatch.

FIG. 3 :
FIG. 3: (a) TEM image of the nanowire shown in Fig. 2(a), and the corresponding EDS mapping images of (b) Zn, (c) O, (d) Ga and (e) S. The observed EDS results are also shown in (f).

The
ZnO nanowires were successfully synthesized by the heat treatment of ZnS with Ga droplets in the air.ZnO nanowires were formed on the ZnO thin layers.The formation of ZnGa 2 O 4 also occurred and the growth direction of the nanowires is ZnO[0001], and which is parallel to ZnGa 2 O 4 [010].When the ZnS is thermally treated with Ga, the Zn and Ga atoms are combined with O to form ZnGa 2 O 4 , and a small amount of Ga atoms were incorporated into the ZnO as impurities.It is considered that the nanowire growth of ZnO is triggered by Ga atoms incorporated into the growing ZnO layers, and the growth rate anisotropy of the ZnO crystal.