Self-catalytic crystal growth, formation mechanism, and optical properties of indium tin oxide nanostructures
© Liang and Zhong; licensee Springer. 2013
Received: 18 July 2013
Accepted: 6 August 2013
Published: 22 August 2013
In-Sn-O nanostructures with rectangular cross-sectional rod-like, sword-like, and bowling pin-like morphologies were successfully synthesized through self-catalytic growth. Mixed metallic In and Sn powders were used as source materials, and no catalyst layer was pre-coated on the substrates. The distance between the substrate and the source materials affected the size of the Sn-rich alloy particles during crystal growth in a quartz tube. This caused In-Sn-O nanostructures with various morphologies to form. An X-ray photoelectron spectroscope and a transmittance electron microscope with an energy-dispersive X-ray spectrometer were used to investigate the elemental binding states and compositions of the as-synthesized nanostructures. The Sn doping and oxygen vacancies in the In2O3 crystals corresponded to the blue-green and yellow-orange emission bands of the nanostructures, respectively.
Oxide materials are promising constituents for various scientific applications because of their versatile physical properties . Oxide materials in low-dimensional forms are particularly demanded for manufacturing small devices. One-dimensional (1D) metal-oxide nanostructures with a high aspect ratio and good crystallinity are promising as building blocks for functional device architecture. Indium oxide (In2O3) is a wide bandgap semiconductor and has been used in various optoelectronic and electronic devices [2, 3]. For practical applications, In2O3 semiconductors are usually doped with other elements to increase their functionalities [2, 4–6].
Recently, Sn-doped In2O3 has attracted a considerable amount of attention because of its superior transparency in the visible spectral region and low electrical resistivity. Sn-doped In2O3 is the transparent conducting oxide most widely used in scientific and industrial applications. Sn-doped In2O3 can be integrated into solar cells, smart windows, photocurrent generators, displays, and light-emitting diodes [7, 8]. However, most studies on Sn-doped In2O3 have mainly focused on its thin-film structure because of the numerous applications of this material in optoelectronic and electronic devices [9, 10]. By contrast, there are few works on Sn-doped In2O3 regarding its 1D structure. Recently, comprehensive investigations on the 1D nanostructures of In2O3 have been conducted. In2O3 1D nanostructures have been synthesized using several chemical and physical methodologies [11, 12]. Thermal evaporation is the simplest method used to synthesize In2O3 nanostructures with a large density and high crystalline quality . The source materials used to grow 1D In2O3 nanostructures through thermal evaporation include metallic In powder and ceramic In2O3 powders mixed with carbon powders. Generally, a high growth temperature is required to obtain In2O3 nanostructures when using ceramic powders as the source material. In addition to the source materials, the evaporative synthesis of these nanostructures can be further classified depending on whether or not a metallic catalyst is used during crystal growth. For optoelectronic nanodevice applications, In2O3 nanostructures are doped with trace Sn to enhance their optical and electrical characteristics [14, 15]. Sn-doped In2O3 nanostructures have several superior properties including a high metallic conductivity, excellent oxidation resistance, and good thermal stability. However, recent works on Sn-doped In2O3 nanostructures with various morphologies have been limited to synthesis using mixed ceramic powders composed of various elements, such as In2O3 with SnO2 or InN with SnO2, on the substrates, with or without an Au catalyst layer [14, 16]. Some Sn-doped In2O3 nanostructures were synthesized using mixed metallic In and Sn powders on Au catalyst-coated substrates . In this study, Sn-doped In2O3 nanostructures with various morphologies were synthesized using mixed In and Sn powders. No metal catalyst was used to grow the nanostructures. This paper presents the detailed investigation of nanostructures that were produced through self-catalytic growth and reports the related microstructures and self-catalytic growth mechanisms of the In-Sn-O nanostructures.
The synthesis of In-Sn-O nanostructures was performed in a horizontal quartz tube furnace. SiO2/Si (100) and sapphire (0001) are used as substrates. Metallic In and Sn powders were used as the solid precursor. Sn atomic percentage in the source powder is approximately 12%. The mixed powders were placed on an alumina boat and positioned at the center of a horizontal quartz tube furnace. Substrates were loaded on separate alumina boats in the source downstream at different distances (15, 20, and 21 cm apart from the source materials) respectively. The furnace tube was then heated to 800°C for source materials, and the corresponding substrate temperature ranges from 400°C to 500°C. During the growth, the pressure in the reaction tube was kept at about 1 Torr with a constant gas flow rate of 100 sccm Ar. The growth duration of the nanostructures was 1 h. After the system had cooled down to room temperature under a 20 Torr of Ar gas atmosphere, a layer of white product was found deposited on the substrates.
The crystal structure of the samples was investigated by X-ray diffraction (XRD) with Cu Kα radiation. X-ray photoelectron spectroscope (XPS) analysis was performed to determine the chemical binding states of the constituent elements of the In-Sn-O nanostructures. The detailed microstructure of the as-synthesized samples was characterized by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). The composition analysis was performed using energy-dispersive X-ray spectrometer (EDS) attached to the TEM. The room temperature-dependent photoluminescence (PL) spectra are obtained using the 325-nm line of a He-Cd laser.
Results and discussion
In conclusion, crystalline In-Sn-O nanostructures with three morphologies (rod-like, sword-like, and bowling pin-like) were obtained through thermal evaporation using mixed metallic In and Sn powders. The nanostructures were capped with Sn-rich particles of various sizes. Nanostructure formation was achieved through self-catalytic growth. Sn-rich alloy particles promoted the formation of In-Sn-O nanostructures during thermal evaporation. Sn vapor saturation around the substrate played a key role in determining the size of the Sn-rich alloy droplets and thus affected the final morphology of the 1D nanostructures. Detailed composition and elemental binding energy analyses showed that the PL properties of the In-Sn-O nanostructures consisted of blue-green and yellow-orange emission bands and were associated with the Sn content and crystal defects of the nanostructures.
This work is supported by the National Science Council of Taiwan (grant nos. NSC 102-2221-E-019-006-MY3, 100-2628-E-019-003-MY2, and NSC100-2221-E-019-059-MY2) and National Taiwan Ocean University (grant no. NTOU-RD-AA-2012-104012).
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