Synthesis of SnO 2-ZnO Core-Shell Nanowires and Their Optoelectronic Properties

Zinc oxides deposited on Tin dioxide nanowires have been successfully synthesized by atomic layer deposition (ALD). The diameter of SnO2-ZnO core-shell nanowires is 100 nm by ALD 200 cycles. The result of electricity measurements shows that the resistance of SnO2-ZnO core-shell nanowires (ALD: 200 cycles) is 925Ω, which is much lower than pure SnO2 nanowires (3.6 × 106 Ω). The result of UV light test shows that the recovery time of SnO2-ZnO core-shell nanowires (ALD: 200 cycles) is 328 seconds, which is lower than pure SnO2 nanowires (938 seconds). These results demonstrated that the SnO2-ZnO core-shell nanowires have potential application as UV photodetectors with high photon-sensing properties.


Introduction
Tin dioxide (SnO 2 ) and zinc oxide (ZnO), world-renown ntype multifunctional semiconductors, have wide direct bandgap energy: 3.6 and 3.37 eV at 300 K, respectively.Furthermore, because of the very large length-to-diameter and surface-to-volume ratios in nanoscale regime, the gas sensing and optoelectronic properties of one-dimensional SnO 2 and ZnO nanostructures are highly sensitive to adsorbed species on their own surface.Owing to special physical and chemical characteristics of one-dimensional SnO 2 and ZnO nanowires, plenty of nanocompounds based on them have been widely applied to many industrial places, such as field emission devices [1], lithium-ion batteries [2], optoelectronic devices [3][4][5][6], and gas sensors [6][7][8][9][10].However, upgrading the optoelectronic sensitivity is still a challenge for SnO 2 nanostructure-based photodetectors.
In recent years, many literatures have demonstrated that nanocompounds of SnO 2 and ZnO which belong to a heterocombination acted with good chemical and physical properties exceeding to their individual materials [11][12][13].Most of all, a synthesis of core-shell nanostructure is a considered method for strongly improving sensing properties of SnO 2 nanowires since surface states affects the sensing properties significantly.
In this study, SnO 2 -ZnO core-shell nanowires were synthesized by a step-by-step manufacture.First, SnO 2 nanowires were fabricated by thermal evaporation.Second, ZnO layers were deposited on SnO 2 nanowires by atomic layer deposition.Atomic layer deposition (ALD), a chemical vapor deposition, is a cutting-edge technique for the good controllable deposition of inorganic layers and has been commonly applied to semiconductor industries.In particular, the self-limiting gas-solid growth features of the ALD growth expedites the growth of thin films and nanomaterials with pretty accurate thickness and good uniformity.A few research of SnO 2 -ZnO core-shell nanowires focuses on enhancement of optoelectronic sensing, so their optoelectronic properties under UV light were investigated.

Experimental
There are two steps in this fabrication of SnO 2 -ZnO coreshell nanowires.In advance, an Au layer (about 30 Å), a catalyst in this fabrication, was coated on aluminum substrates by sputtering.In the first step, Tin oxide nanowires were  In the second step, a ZnO layer was coated on SnO 2 nanowires by atomic layer deposition (ALD) [14][15][16].The precursors were DEZ (Zn(C 2 H 5 ) 2 , Aldrich, purity 98%) and H 2 O (purity 99.999%) in 200 ALD reaction cycles.Every single cycle, a 60 ms DEZ pulse and a 60 ms H 2 O pulse were sequentially delivered into the ALD chamber by 5 s of purging with N 2 (purity 99.9995%).The pressure of chamber was maintained 1 Torr in the ALD reaction.
The morphology, crystal structure, and chemical composition of the SnO 2 -ZnO core-shell nanowires were analyzed by a field emission-scanning electron microscope (FESEM, JEOL JSM-6900F), a MAC glancing incident X-ray spectrometer with an incident angle of 0.5 • , a high-resolution transmission electron microscope (HRTEM, JEOL, JEM-2010), and X-ray photoelectron spectroscopy (XPS Perkin-Elmer Model PHI1600 system).In order to test the optoelectronic properties of SnO 2 -ZnO core-shell nanowires, the measurements of I-V curves via a constant voltage (5 V) and photoresponse curves under UV light (λ = 365 nm and intensity is 17.1 W/m 2 ) were handled.To further investigate the detailed microstructure of SnO 2 -ZnO core-shell nanowires, taking images by TEM was performed.Figures 2(a In order to identify the chemical composition of the SnO 2 -ZnO core-shell nanowires and check the quality of ZnO deposited on SnO 2 by ALD, the core-shell SnO 2 -ZnO nanowires were additionally characterized by XPS. Figure 3(a) shows that the binding energy of Zn (2p 3/2 ) and Zn (2p 1/2 ) are 1021.9eV and 1044.9 eV, respectively.Figure 3  530.9eV. Figure 3(c) shows that binding energy of Sn (3d 3/2 ) and Sn (3d 5/2 ) is 495.1 eV and 487.5 eV.After integration of the area of three main elements "peaks by computer program", the atomic percentages of oxygen, zinc, and tin are 55.27, 41.7, and 3.03, respectively.Judging from XPS analysis, because the atomic percentage of ZnO is large in this compound, ZnO crystalline shell is definitely deposited on SnO 2 nanowires by ALD.By above material analyses, the confirmation of SnO 2 -ZnO core-shell nanowires synthesized sequentially via thermal evaporation, and ALD was done.To determine the potential applicability of SnO 2 -ZnO core-shell nanowires in this study to photodetectors, their I-V curves and UV photon-sensing properties were investigated.Figure 4 shows the current-voltage (I-V) curves of SnO 2 -ZnO core-shell nanowires with ALD 200 cycles reaction (red line) and pure SnO 2 nanowires (black line) at room temperature and atmospheric pressure.Two curves show a good ohmic contact between the nanocompounds and Ag electrodes.The electrical resistivity can be expressed as R = ρ × L/A, and the electrical resistivity of SnO 2 -ZnO nanowires and pure SnO 2 nanowires can be solved as 925 Ω and 3.6 × 10 6 Ω, respectively.The electrical resistivity of core-shell nanostructure is much lower than pure nanowires.Because of that ZnO film of core-shell SnO 2 -ZnO nanowires is more compact than SnO 2 nanowires, this effect is able to increase the film conductivity.

Results and Discussion
When setting up an electric circuit with core-shell nanostructures, there will be a strain in a heterointerface to keep from a deformation happened and decrease the total energy.Moreover, the band gap of core-shell materials will be reduced, and then the conductivity will be raised [17].
To investigate the application of SnO 2 -ZnO core-shell nanostructure and pure SnO 2 nanowires for UV detectors, the optoelectronic response and recovery of the as-fabricated photodetectors were tested under emitted UV lamps (λ = 365 nm).The optoelectronic test unveils that the current very quickly rises to a steady level under UV light (UV light is on) and then increasingly recovers to the initial state in the dark (UV light is off).Figures 5(a) and 5(b) show the period photoresponse curve of pure SnO 2 nanowires and SnO 2 -ZnO core-shell nanowires (200 ALD cycles), respectively.In previous studies [18], the opinion of these photoresponse curves was discussed.When the UV light is off, the species of oxygen molecular structure are chemoadsorbed on the interfaces of nanostructures because oxygen molecules catch free electrons, that is, O 2(g) + e − → O 2(ad) − .Therefore, a low conductivity depletion layer is formed near the surface of nanowires, and the nonmaterial's conductivity will be decreased.When UV light is on, electron-hole pairs follow this reaction, hν → e − + h + , and will be happened in the nanowires.The photoinduced holes will relocate to the surface and recombine with the oxygen ions, that is, h + + O 2(ad) − → O 2(g) .At the same time, the photon-generated electronics are able to play free electrons and upgrade the electron conductance of nanowires.The photocurrent rapidly increases with the reduced depletion layer.
Figure 5(a) shows that since UV light is on, the current is 3.47 × 10 −6 A. After 50 seconds, the current increases to 7.23 × 10 −6 A. The slope is 0.0752.From Figure 5(b), since UV light is on, the current is 3.53 × 10 −5 A. After 50 seconds, the current lifts to 5.35 × 10 −5 A. The slope is 0.0364.Therefore, the device based on pure SnO 2 nanowires has a better sensitivity than based on SnO 2 -ZnO core-shell nanowires.In terms of photosensitivity, the pure SnO 2 nanowires are little larger than the SnO 2 -ZnO core-shell nanowires, and two depletion areas of core-shell nanostructure might be the cause for this phenomenon.However, the recovery time of core-shell SnO 2 -ZnO nanowires is just 328 seconds, which is much shorter than that of pure SnO 2 nanowires (938 seconds).It is a reason that a superposition of the homo-and heterointerfaces formed at the junctions may be the root cause for the decreased recovery time compared with the detector judging from the nanowires without the shell layer [19,20].All in all, the SnO 2 -ZnO core-shell nanowires have positive characteristics and applications in the manufacture of photodetectors.

Conclusion
SnO 2 -ZnO core-shell nanowires have been successfully fabricated by a step-by-step process.First, SnO 2 nanowires were synthesized by thermal evaporation.Sequentially, ZnO shell layers were deposited by ALD.The SnO 2 -ZnO coreshell nanowire-based photodetectors has the low electrical resistance (925 Ω) and the short recovery time (328 seconds), compared with pure SnO 2 nanowires.Thanks to excellent optoelectron abilities, the SnO 2 -ZnO core-shell nanowires are the promising candidates as photodetectors.
fabricated on aluminum substrates by thermal evaporation.High quality 0.4 g Sn powders (Aldrich, purity 99.99%) and several aluminum substrates were situated in an Al boat and then placed the Al boat in the heating area of a quartz tube.In the growth of Sn nanowires, the pressure inside the quartz tube was kept up at 1 Torr by a rotary pump.The working conditions of this process were as follows.(1) Raising the temperature to 950 • C with 20 • C/min was at a constant flow rate of 10 sccm Ar gas.(2) Maintaining the temperature at 950 • C for 1 hour was at a constant flow rate of mixed gas of 10 sccm Ar and 1 sccm O 2 .(3) Cooling to the room temperature was under a constant flow rate of 10 sccm of Ar gas.

Figure 1 (
Figure 1(a) shows the XRD θ − 2θ diffraction pattern of the SnO 2 -ZnO core-shell nanowires obtained after 200 ALD ) and 2(b) show a low-magnification TEM image.The diameter of core-shell nanowires is 100 nm, and the thickness of ZnO shell layer is 25 nm.From Figure2(c), the HRTEM image of core-shell SnO 2 -ZnO nanowires shows that the lattice spacing between (0002) is 0.28 nm, and the structure of ZnO that has hexagonal structure is corresponding.From Figure2(d), the growth direction is [101], the lattice space is 0.26 nm, and the angle between [101] and [−101] is 67.85, which are consistent with the SnO 2 that has tetragonal rutile structure.According to these TEM images, no doubt the core-shell nanostructure is certainly formed via two-step method.
(b) shows that the binding energy of O (1 s) is