Available online 22 February 2011.
We experimentally show the detailed electrical transport characteristics of an SnO2 nanomaterial with three-branch junction morphology, termed the nanowire T-junction. By contacting the three branches of a T-junction as the gate, source and drain, some control and tunability has been achieved over the output current, without the need for an external gate, analogous to the operation of a field effect transistor. Our results suggest that SnO2 T-junction nanostructures represent novel current splitters with potential electronic applications. The different resistivities for each branch across the junction can be mainly attributed to differences of the defects and dislocations at the junction. The gating effects are directly related to the resistances of each branch and are electric field dependent. These features mean that SnO2 T-junctions may be useful in nanoelectronic devices.
Being an important n-type semiconductor with a wide band gap, SnO2 possesses many unique electrical properties which are useful for sensing applications such as, a remarkable receptivity variation in gaseous environments, low resistivity, and excellent chemical stability1. There has been an explosion of interest in electronic nanodevices based on SnO2 nanomaterials, including field effect transistors (FET)2, field emissions3, and gas sensors, ,  and . However, most previous reports on electronically addressed nanodevices to date have focused on using traditional materials, such as SnO2 particle films7, nanowire frameworks and , individual SnO2 nanowires, , ,  and , etc. Instead of a single-wire SnO2 nanodevice, we experimentally demonstrate the detailed electrical transport characteristics of a three-branch junction SnO2 nanomaterial, termed the nanowire T-junction, with tungsten (W) tips directly in contact with each probing pad. By contacting the three terminals of a T-junction as the gate, source and drain, some control and tunability has been achieved over the output current, without the need for an external gate, analogous to the operation of a field effect transistor. We believe that such an SnO2 T-junction with three-terminals has great potential to be used in constructing novel nanoelectronic sensing devices.
In the current study the SnO2 T-junctions were synthesized by a thermal evaporation method (see Supplementary information for details). Collected SnO2 nanostructures were ultrasonicated in ethanol solvent, and then dispersed on a silicon wafer for characterization. Fig. 1a provides a closer look at a typical sample using scanning electron microscopy (SEM). It can easily be seen that the branch is perpendicular to the stem, forming a T-junction with three terminals. Using the geometrical shape offered by the T-junction, where two terminals can be regarded as source and drain and the other as gate, the electrical transport characteristics can be tuned, as will be considered in more detail in the following section. Patterns recorded using x-ray powder diffraction (Fig. 1b) suggest that all diffraction peaks of the as-prepared product could be indexed using the tetragonal rutile structure of SnO2 (a = b = 0.4738 nm and c = 0.3187 nm, JCPDS card. 41-1445). No other crystalline forms, such as Sn or other tin oxides, were detected. Fig. 1c is the corresponding transmission electron microscopy (TEM) image. The “T” shaped morphology is further highlighted. Three selective area electron diffraction (SAED) patterns corresponding to the stem, junction, and branch are shown in Figs. 1d, 1e and 1f, respectively. The results indicate that the SnO2 T-junction has the same rutile crystalline lattice and a rectangular cross-section enclosed by ±(010) and ±(101¯) facet planes. These crystallographic configurations of synthesized nanowires are similar to those previously observed and . The growth directions of the stem and branch are parallel to the  and  crystal direction, respectively. High resolution TEM (HRTEM) images provide consistent results with the SAED patterns and confirm the tetragonal rutile structure for T-shaped SnO2 nanowires, as shown in Figs. 1g, 1h and 1i, respectively. The interplanar d-spacing marked in the HRTEM images is about 0.477 nm, which can be indexed to the (010) lattice plane.
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SnO2 T-junction.(a) Representative SEM image of an individual SnO2 T-junction.(b) X-ray diffraction pattern.(c) Representative TEM result.(d-f) Electron diffraction patterns from the T-junction shown in (c).(g-i) The corresponding HRTEM images shown in (c).
The SnO2 T-junction nanodevice was fabricated using the photolithographic technique, four-probe nanomanipulation technique and a focused ion beam system. The T-junction nanodevice was formed on a p-doped silicon-on-insulator (SOI) wafer. A 15 nm layer of titanium was grown on the active silicon area to form the adhesion layer, followed by 100 nm of gold to form the metallic electrodes. An individual SnO2 T-junction was selected and transferred to the central of Au/Ti electrodes (Figs. 2a and 2b). Platinum microleads were deposited using a focused ion beam (FIB) system onto the metallic electrodes and T-junction (Fig. 2c). The nanodevice was electrically tested using the metal pads as contacts (Fig. 2d). (For further details about the fabrication process, see Supplementary information)
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Nanodevice structure of an individual T-junction with three-terminal configuration fabricated using thy four-probe nanomanipulation technique. (a, b) SEM images showing the transfer of an individual SnO2 T-junction to a silicon wafer. (c) SEM image of a T-junction nanodevice with platinum microleads that had been contacted using an FIB system. (d) In situ measuring approach using three tungsten probes. A T-junction (marked by a small square) is connected by larger metal pads.
Prior to measurements of the gating effect of the SnO2 T-junction, I−V characteristics of this T-junction nanodevice across different terminals were first investigated (Fig. 3). It is clear that the device exhibits good linear behavior at the bias voltage ranging from −1 to 1 V, which proves that there is a good ohmic contact between the SnO2 nanowire and platinum microleads. Based on the I−V characteristics, the resistances of each branch of the T-junction have been calculated. For the oa, ob and oc branches, it is about 2.875×107, 17.445×107 and 6.715×107 Ω, respectively. Combined with geometric parameters (length and diameter) of each branch, their resistivity (ρoa, ρoc and ρob) has also been obtained. It is about 3.85 Ω·cm for the oa branch, 3.58 Ω·cm for the oc branch and 32.52 Ω·cm for the ob branch, which is larger than 0.941 Ω·cm for individual SnO2 nanowires15. In addition, it can be found that ρoa of the oa branch is approximately equal to ρoc of the oc branch. However, ρob of the ob branch is about 89 times of ρoa or ρoc. The different resistivities for each branch across the junction could be mainly attributed to the difference of the defects and dislocations at the junction. This contribution was further evidenced by the resistivity of the linear part of the oc branch, without the junction. The result was found to be smaller than that for the oa branch and the oc branch, which is similar to the value previously reported. However, it is also important to consider the effect of the anisotropy of crystalline materials on their electrical behavior. The anisotropic resistivity of various single crystal materials has been reported previously and . Again, for the directions perpendicular and parallel to the tetragonal c-axis of single crystal SnO2, the dielectric functions are also different and anisotropic in theoretical calculations18. Therefore, anisotropic behavior might also be a factor contributing to the different resistivities of the stem of oa and oc along the  direction and the branch of ob along the  direction for a T-shaped SnO2 nanowire.
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I−V characteristics taken on three pairs of branches (bc, ab, ac) of an individual SnO2 T-junction in a vacuum. The insets represent the exact measuring terminals of the nanodevice along with the SEM image.
Analysis of how the source-drain (Id) and gate current (Ig) varies with the gate (Vg) and source-drain voltage (Vd) in the T-junction is shown in Fig. 4. Analogous to the operation of the FET, three terminals of T-junction SnO2 can be used as the gate, source and drain. In this experiment, b is the gate (G) while a and c are the drain (D) and source (S), respectively (Fig. 4a). Fig. 4b presents the corresponding equivalent circuit. In terms of the Kirchhoff's Voltage/Current Law, the current Ig and Id for the equivalent circuit can be expressed by the following equations:
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I−V characteristics taken on three terminals of an individual SnO2 T-junction in a vacuum. (a) Measurement approach of gating effect for an individual SnO2 T-junction using b as the gate, a as the drain and c as the source. (b) Schematic of the equivalent circuit. (c, d) Id−Vd and Ig−Vd curves under different gate voltages, respectively. (e, f) Id−Vg and Ig−Vg curves under different source-drain voltages, respectively.
It is clear from these electrical characteristics that Id and Ig can be modulated by the source-drain or gate voltage, despite this the operation mode of the device is quite different from that of a conventional FET that is controlled by the carrier density and the thickness of the channel19. The nature of the T-junction means that it possible for the junction to act as a current splitter that can be controlled by the gate or source-drain voltage. Again, the gate is not a perfect insulating gate. A leakage current would be diverted from one path to another when a gate voltage is applied, resulting in a shift in current. This is similar to that seen in the three-way electrical gating effect of Y-shaped carbon nanotubes,  and , Co-doped Y-shaped ZnO23 and the ZnO tetrapod and .
Fig. 5 shows the I−V characteristics of the SnO2 T-junction using different terminals as the gate, source and drain, under different source-drain voltages. The schematic measurement approaches are presented in the inset of Figs. 5a, c and e. It is noted that the source-drain current is also observed to linearly increase (Figs. 5a, c and e) and the gate current is also observed to decrease (Figs. 5b, d, and f) when the source-drain voltage is increased. The main difference is the slopes of the Id−Vd and Ig−Vd curves. Based on the current proposed equations ((1) and (2)), it is believed that the resulting slope differences are due to the resistances of each branch, which could be either “(Rs + Rg)/R” (i.e., Id−Vd curve) or “−Rs/R” (i.e., Ig−Vd curve). This suggests that the gating effect behavior of T-junction SnO2 nanostructures is directly related to the resistance of each branch, which is determined by the resistivity and geometry of each branch. Further, the differences of the proportional displacement of the Id−Vd and Ig−Vd curves are also seen with increasing Vg. This is because the proportional displacement of the Id−Vd and Ig−Vd curves depends on −Rs/R and (Rs + Rd)/R, at a given Vg, respectively.
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I−V characteristics taken on three terminals of an individual SnO2 T-junction in a vacuum under different gate voltages. (a, b) Id−Vd and Ig−Vd curves using b as the gate (a: source, c: drain). (c, d) Id−Vd and Ig−Vd curves using c as the gate (b: source, a: drain). (e, f) Id−Vd and Ig−Vd curves using c as the gate (a: source, b: drain).
To determine whether the Id−Vg and Ig−Vg curves behavior is similar to that observed in the above experiments, I−V characteristics were measured under different source-drain voltages and the results are presented in Fig. 6. As expected, Id decreases and Ig increases linearly with the increase of Vg, when the drain voltage Vd is fixed, respectively. The similar behavior was observed using the a terminal as the gate, and the I−V curves are shown in [Fig. S1] and [Fig. S2] (see Supplementary information). So far, it is necessary to point out that such a T-junction nanodevice might be more advanced than those double-terminal sensors if employed as a sensor. By making use of the current splitter feature, it could be designed as a multiterminal sensor, i.e., a sensor which can give multiple responses to a single signal at the same time. Further research on the sensing applications of SnO2 T-junction nanodevices would be of value and is in hand.
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I−V characteristics taken on three terminals of an individual SnO2 T-junction in a vacuum under different source-drain voltages. The arranged terminals are presented in panels (a), (c), and (e). (a, b) Id−Vg and Ig−Vg curves using b as the gate (a: source, c: drain). (c, d) Id−Vg and Ig−Vg curves using c as the gate (b: source, a: drain). (e, f) Id−Vg and Ig−Vg curves using c as the gate (a: source, c: drain).
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I−V characteristics taken on three terminals of an individual SnO2 T-junction in a vacuum. (a) Approach for measurement of the gating effect for an individual SnO2 T-junction, using a as the gate, b as the drain and c as the source. (b) Schematic of the equivalent circuit. (c) and (d) Id−Vd and Ig−Vd curves under different gate voltages, respectively. (e) and (f) Id−Vg and Ig−Vg curves under different source-drain voltages, respectively.
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