Field Emission Properties and Fabrication of CdS Nanotube Arrays
© to the authors 2009
Received: 23 February 2009
Accepted: 14 April 2009
Published: 5 May 2009
A large area arrays (ca. 40 cm2) of CdS nanotube on silicon wafer are successfully fabricated by the method of layer-by-layer deposition cycle. The wall thicknesses of CdS nanotubes are tuned by controlling the times of layer-by-layer deposition cycle. The field emission (FE) properties of CdS nanotube arrays are investigated for the first time. The arrays of CdS nanotube with thin wall exhibit better FE properties, a lower turn-on field, and a higher field enhancement factor than that of the arrays of CdS nanotube with thick wall, for which the ratio of length to the wall thickness of the CdS nanotubes have played an important role. With increasing the wall thickness of CdS nanotube, the enhancement factorβ decreases and the values of turn-on field and threshold field increase.
One-dimensional semiconductor nanostructures have been intensively investigated in recent years due to their interesting optical and electronic properties, and promising applications in nanoscale devices [1–3]. Among the II–VI semiconductors, CdS has been attracted special interest because it exhibits high photosensitivity and its band gap energy (2.41 eV) appears in the visible spectrum leading to many commercial or potential applications in light-emitting diodes, solar cells, field emitter, and other photoelectric devices [4–8]. To date, CdS nanotubes are synthesized via the sol–gel or electrophoretic processing combination of molecular anchor template and various porous membranes including anodic aluminum oxide, polycarbonate, and mesoporous silica. [9–12] In fact, the CdS nanotubes prepared by above-mentioned methods are always free standing, and impossible to be directly used to fabricate nanodevices, because they cannot orderly locate on the solid surface after the removal of the templates. Therefore, the controlling growth of aligned CdS nanotubes and getting ordered nanostructures on conductive substrates by a facile and versatile synthetic method is still a challenge. In this work, a large area of CdS nanotube arrays on silicon wafer is successfully fabricated by the method of layer-by-layer deposition using well-aligned ZnO nanorod arrays as removable templates. The wall thicknesses of CdS nanotubes are precisely tuned by controlling the cycles of layer-by-layer deposition. The field emission (FE) properties of CdS nanotube arrays are investigated. The results indicate that the FE properties of CdS nanotube arrays can be controlled through changing the wall thicknesses of CdS nanotubes. With increasing the wall thicknesses of CdS nanotubes, the enhancement factor β decreases and the values of turn-on field and threshold field increase.
The arrays of CdS nanotubes are characterized and analyzed by field emission scanning electron microscopy (FESEM, Hitachi, S-4300), transmission electron microscopy (TEM, JEOL, JEM-1011), energy-dispersed X-ray microanalysis system (EDXA, Oxford Instrument, UK), Fluorescence spectrophotometer (Hitachi, F-4500), and X-ray diffraction (XRD). The XRD patterns are recorded with a Japan Rigaku D/max-2500 rotation anode X-ray diffractometer equipped with graphite-monochromatized Cu Kα radiation (λ = 1.54178 Å), employing a scanning rate of 0.05°s−1in the 2θ range from 20° to 60°. The FE properties of CdS nanotube arrays are measured using a two-parallel-plate configuration in a homemade vacuum chamber at a base pressure of ~1.0 × 10−6 Pa at room temperature. The sample is attached to one of stainless-steel plates as cathode with the other plate as anode. The distance between the electrodes is 300 μm. A direct current voltage sweeping from 0 to 5000 V was applied to the sample at a step of 50 V. The emission current is monitored using a Keithley 6485 picoammeter.
Results and Discussion
The field emission properties of CdS nanotube arrays
where m n, and c are alterable parameters. The r is the average radius of nanotube, l is the length of nanotube, and t is the thickness of wall. The β is determined by two factors, aspect ratio l/r and the ratio of length to wall thickness l/t. For getting the values of three parameters, we must use formula 3 to fit the values of β which are obtained in experiment. The parameters m n, and c are obtained as 0.9, 3.7, and 6, for the best fit to the experimental datum. Using these parameters, we get three simulating values of βsim for samples A, B, and C, which are 273, 359, and 642, respectively. They are consistent with the experiment values. In our case, l/r of those CdS nanotube arrays are almost same and the value of parameter n is larger than parameter m, which means l/t is the key role to determine the FE properties of CdS nanotube arrays. The FE properties of CdS nanotube arrays increase with the increase of l/t. The length of samples A, B, and C are almost same, so the arrays of CdS nanotube with the thinner wall exhibit the better FE property.
A large area of CdS nanotube arrays is fabricated by a facile way of layer-by-layer deposition. The wall thicknesses of nanotubes are controlled by tuning the deposition cycles. The FE properties of CdS nanotube arrays dependence on wall thicknesses are investigated and show infusive and regular results. With decreasing the wall thicknesses of CdS nanotubes, the value ofEtoandEthdecrease andβ increases. The thinnest walls of CdS nanotubes exhibit the least value ofEtoandEthfor promising candidate materials on field emitters and nanodevices.
This work was supported by the National Nature Science Foundation of China (20531060, 20473102, and 20571078) and the National Basic Research 973 Program of China.
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