Synthesis, Electrical Measurement, and Field Emission Properties of α-Fe2O3Nanowires
© to the authors 2008
Received: 26 June 2008
Accepted: 18 August 2008
Published: 9 September 2008
α-Fe2O3nanowires (NWs) were formed by the thermal oxidation of an iron film in air at 350 °C for 10 h. The rhombohedral structure of the α-Fe2O3NWs was grown vertically on the substrate with diameters of 8–25 nm and lengths of several hundred nm. It was found that the population density of the NWs per unit area (D NWs) can be varied by the film thickness. The thicker the iron film, the more NWs were grown. The growth mechanism of the NWs is suggested to be a combination effect of the thermal oxidation rate, defects on the film, and selective directional growth. The electrical resistivity of a single NW with a length of 800 nm and a diameter of 15 nm was measured to be 4.42 × 103 Ωcm using conductive atomic force microscopy. The field emission characteristics of the NWs were studied using a two-parallel-plate system. A low turn–on field of 3.3 V/μm and a large current density of 10−3 A/cm2(under an applied field of about 7 V/μm) can be obtained using optimal factors ofD NWsin the cathode.
One-dimensional (1D) nanostructures have been extensively studied because of their unique chemical and physical characteristics. Because of their high aspect ratio and sharp tips that enhance the local electrical field, they can be used as emitters in field emission (FE) applications . Factors that affect FE properties include the population density of the emitters (the number of emitters per unit square) , the electronic resistance of a single emitter , and the aspect ratio (radius and length) . The field emission properties of an emitter are affected by nearby emitters, which is called the field screen effect. Many research groups have studied the relationship between FE properties (field screen effect) and the population density of emitters for ZnO nanowires (NWs) , Si NWs , CNTs [2, 3], and CuO NWs . Poor FE properties were obtained when the population density of the emitters was too high.
Investigating the electronic properties of 1D nanomaterials can reveal the relationship between the electronic properties of emitters and the FE properties of the device. In addition, it can provide useful information for the theoretical study of FE characteristics. Electronic measurement methods for NWs are generally categorized into four types: (1) laterally growing NWs between two electrodes and directly measuring them [7, 8], (2) spreading the NWs on a large number of patterned electrodes and measuring a single NW on a pair of electrodes [9, 10], (3) directly measuring vertical NWs using conductive atomic force microscopy (CAFM) [11, 12], and (4) measuring aligned NWs arrays using two electrode films [13, 14]. The CAFM system is a convenient method for the non-destructive characterization of the electronic properties of NWs.
α-Fe2O3 (hematite) is a semiconductor (Eg = 2.1 eV) and the most stable iron oxide under an ambient environment . α-Fe2O3 NWs have recently been synthesized by various research groups [16, 17]. Because they are thermally stable, resistant to oxidation, and have a high aspect ratio, α-Fe2O3 NWs are a candidate emitters for FE applications. It has been reported that an electrical field of 7–8 V/μm is required to obtain a 10−5 A/cm2 emission current using densely packed α-Fe2O3 NWs as emitters . However, there have been no detailed studies on FE properties related to the population density of α-Fe2O3 NWs (D NWs) and their resistance (R NW).
In this study, we report a simple method that can be used to control the population density of α-Fe2O3NWs (D NWs) by varying the film thickness of iron. The FE properties of α-Fe2O3NWs with various population densities and resistances (R NW) were studied. It was found that the best FE properties can be obtained by using the optimalD NWsin the cathode.
α–Fe2O3NWs were grown using an iron film thermally oxidized at 350 °C for 10 h. Iron films with thicknesses of 30 nm, 50 nm, 100 nm, and 150 nm were coated on indium tin oxide (ITO) glass by direct current (DC) sputtering. The iron-coated substrates were heated at 350 °C for 10 h in an oven in the air atmosphere. The morphology and crystalline structure of the as-synthesized NWs were characterized by field emission scanning electron microscopy (FE-SEM, HITACH S-4800) and high-resolution transmission electron microscopy (HR-TEM, JEOL JSM 3010), respectively. The current–voltage (I–V) characteristics of the as-grown NWs were obtained using the CAFM (SEIKO SPA-400) system. A highly conductive polygon shaped tip with a height of 10 μm and a radius of 50 nm (the top of the tip) was used as an electronic probe while ITO was used as an electrode. The applied voltages were −10 V to +10 V. FE properties were measured in a vacuum chamber at 6 × 10−6 torr. The sweep voltage was 0–1100 V and the emission current was measured using Keithley 2410.
Results and Discussion
Summary of the population densities (D NWs), resistances of NWs (R NW), turn–on field (E to), and calculated field enhancement factors (β) for samples A, B, C, and D
Iron thickness (nm)
Population density of the NWs (D NWs) (NWs/cm2)a
Resistance of a NM(R NW)(Ω)
Turn-on field(E to) (V/μm)
Field enhancement factor(β)
R total(Ω) of NEsb
1.6 × 108
1.25 × 1011
7.8 × 108
2 × 1011
1.32 × 109
1.4 × 1011
1.83 × 109
6.25 × 1010
where ρ is the electronic resistivity of an α-Fe2O3NW, andL andr represent the average length (800 nm) and radius (15 nm) of the NW, respectively. We assumed that ρ (4.42 × 103 Ωcm) is a constant for all of the as-produced α-Fe2O3NWs. Using the mean length and radius of the NWs,R NW(orR overall) of samples A, B, C, and D was calculated as 1.25 × 1011, 1.4 × 1011, 1.4 × 1011, and 6.25 × 1010 Ω, respectively. The values are shown in Table 1.
The formation, electronic characterization, and field emission application of α-Fe2O3NWs were studied. α-Fe2O3NWs were grown vertically on the substrate via the thermal oxidation of an iron film in air at 350 °C for 10 h. By increasing the film thickness (to 30, 50, 100, and 150 nm),D NWscould be increased.R NWof the NWs was estimated using the CAFM technique. In the FE study, anE toof 3.3 V/μm and a large current density of 10−3 A/cm2(under an applied field of about 7 V/μm) were obtained with the optimumD NWs. The densely packed NWs caused the screen effect, leading to poor FE performance. The field enhancement factor (β) depends onD NWmore than it does onR NW. This study shows that α-Fe2O3NWs are a candidate for emitters in field emission applications.
The authors would like to thank Mr. Shu-Teng Chou at Advantage Scientific Incorporated for his help in CAFM measurements. Mr. Chien-Wei Huang at National Chung Cheng University is acknowledged for his help in sputtering iron films.
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