NANO LETTERS
Photoconductive Characteristics of Single-Crystal CdS Nanoribbons
2006 Vol. 6, No. 9 1887-1892
J. S. Jie,† W. J. Zhang,*,† Y. Jiang,† X. M. Meng,‡ Y. Q. Li,† and S. T. Lee*,†,‡ Center Of Super-Diamond and AdVanced Films (COSDAF) and Department of Physics and Materials Science, City UniVersity of Hong Kong, Hong Kong Special AdministratiVe Region, China, and Nano-Organic Photoelectronic Laboratory, Technical Institute of Physics and Chemistry, The Chinese Academy of Science, Beijing 100101, China Received April 18, 2006; Revised Manuscript Received July 12, 2006
ABSTRACT The photoconductive characteristics of CdS single nanoribbons were investigated. The device characteristics, including spectral response, light intensity response, and time response, were studied systematically. It is found that CdS nanoribbon has the response speed substantively faster than those ever reported for conventional film and bulk CdS materials and the size of nanoribbons has a significant influence on the response speed with smaller CdS nanoribbons showing higher response speed. The high photosensitivity and high photoresponse speed are attributable to the large surface-to-volume ratio and high single-crystal quality of CdS nanoribbons and the reduction of recombination barrier in nanostructures. Measurements in a different atmosphere demonstrate that the absorption of ambient gas (mainly oxygen) can significantly change the photosensitivity of CdS nanoribbons through trapping electrons from the nanoribbons.
The construction and integration of functional nanodevices based on the one-dimensional semiconductor materials have advanced rapidly in recent years. Nanodevices such as transistors,1 light emission diodes (LED),2 and gas and chemical sensors3 have been reported, demonstrating exciting progress in the “bottom-up” approach for building newgeneration electronic and photoelectronic systems with reduced size, higher efficiency, and less energy consumption. As an important application of semiconductor materials, photodetectors or optical switches are essential elements in imaging techniques and lightwave communications and possibly in future memory storage and optoelectronic circuits.4 Various semiconductor materials, including group IV elements (Si, Ge), group III-V compounds (GaN, GaAs, InP), and group II-VI compounds (ZnS, CdS, CdS), have been used for the fabrication of photodetectors. And the large variety of band gaps of these materials leads to a wide spectral response ranging from far-infrared to ultraviolet (UV) light.5 While conventional photodetectors are usually in the film or bulk configurations, the unique and significant properties of nanomaterials, particularly one-dimensional ones, have attracted considerable research interest. So far, the photoconductivity of carbon nanotubes,6,7 GaN nanowires,8,9 ZnO nanowires,4,10 SnO2 nanowires and nano* Corresponding authors. E-mail:
[email protected] and apannale@ cityu.edu.hk. † City University of Hong Kong. ‡ Technical Institute of Physics and Chemistry, The Chinese Academy of Science. 10.1021/nl060867g CCC: $33.50 Published on Web 08/03/2006
© 2006 American Chemical Society
ribbons,3,11 and CdS nanoribbons12,13 have been investigated, and some remarkable characteristics such as the strong polarization dependence and better photoresponse of the nanowire devices have been revealed.6,10,11 In addition, quantum size effects in CdSe and InAs nanorods have been investigated by combining tunneling and optical spectroscopy measurements, revealing the strong dependence of band-gap on the diameter of nanorods.14,15 It is expected these nanorods have better properties, such as enhanced optical gain and polarized lasing, than their quantum dots counterparts. Among the compound semiconductors, CdS is the most promising material for detecting visible radiation due to its primary band gap of 2.4 eV (∼516 nm) and high sensitivity. However, the relatively low response speed (>tens of milliseconds)16 of CdS bulk or films seriously limits its applications in high-frequency or high-speed devices, such as for lightwave communications or optoelectronic switches. Herein we report the performance of a single CdS nanoribbon in photoconductive devices. Such devices show a sensitive spectral response and a significant response speed nearly 2 orders of magnitude faster than that ever reported.13,17 Our results imply the CdS nanoribbons are a great candidate for applications in high-sensitivity and high-speed photodetectors and photoelectronic switches in nanoscale. CdS nanoribbons were synthesized in a horizontal tube furnace by a chemical vapor deposition (CVD) method. The CdS powers (Aldrich, purity ∼99.99%) were placed at the center of the alumina tube. Au-coated silicon substrates were
placed at the downstream position of the source material. After the tube was evacuated to a base pressure of 10-4 Torr, the source was heated to 860 °C at a rate of 20 °C/min and then was maintained at this temperature for 2-4 h. A carrier gas of high-purity argon premixed with 5% hydrogen was fed at a total flow rate of 20 sccm. The pressure was maintained at 150 Torr during the whole process. The assynthesized nanoribbons are bright yellow in color. The nanoribbons were characterized by scanning electron microscopy (SEM Philips XL 30 FEG) and high-resolution transmission electron microscopy (HRTEM, CM200 FEG operating at 200 kV). The width, length, and corresponding thickness of nanoribbons were also measured by SEM. Room-temperature photoluminescence (PL) measurement was conducted by using a Nd:YAG laser with a wavelength of 266 nm and a pulse width of 6 ns as the excitation source and a 0.5 m spectrometer (Anton Research Corp. Spectra Pro 500i). The absorption spectra of CdS nanoribbons were measured using a spectrometer (Ferkin Elmer, Lambda 2S) by dispersing the nanoribbons in alcohol. For the fabrication of single-nanoribbon detectors, CdS nanoribbons were dispersed on a SiO2 (300 nm thick)/Si wafer with desired density. Patterned Ti(100 nm) and Au (25 nm) electrodes were successively deposited on the nanoribbons in high-vacuum by e-beam evaporation with the assistance of a mesh-grid mask composed of tungsten wires (18 µm in diameter). Since the lengths of the nanoribbons were larger than the diameter of tungsten wires, the electrodes were formed on the uncovered parts of nanoribbons. A fast annealing at 350 °C was carried out in H2(5%)/N2 atmosphere for 3 min to form the ohmic contact between electrodes and nanoribbons. A light system combining a xenon lamp (150 W) and a monochromator (1/8 m, Spectra-physics 74000) was used to provide the monochromatic light, which was focused and guided onto the nanoribbons perpendicularly. I-V measurements were performed by using a two-probe configuration. To measure the time response of nanoribbons to light irradiation, a mechanical chopper (frequency ranging from 0 to 600 Hz) was used to turn on and off the light irradiation, and an oscilloscope (Agilent infiniium) was used to monitor the variation of photocurrent with time. SEM morphology in Figure 1a shows that the surfaces of CdS nanoribbons are clean and smooth. The width and thickness of the nanoribbons are in the range of 2-40 µm and 10-60 nm, respectively, but each ribbon is uniform in width and thickness along its length direction. The typical length of ribbons is about 80-120 µm. HRTEM image and the corresponding selected-area electron diffraction (SAED) pattern (Figure 1b and its inset) indicate that the CdS nanoribbons are hexagonal single crystals growing along [112] orientation. The optical microscopic image of the CdS single nanoribbon device is shown in the left inset in Figure 2a. Ti/Au parallel electrodes 18 µm apart are deposited on the nanoribbon dispersed on a SiO2/Si substrate, and the uncovered part of the ribbon (green color) is exposed to the incidence light. The width and thickness of this nanoribbon are 10.4 µm and 30 nm, respectively. Right inset in Figure 2a is a 1888
Figure 1. (a) SEM morphology of the as-synthesized CdS nanoribbons and (b) HRTEM image of a CdS nanoribbon. The corresponding SAED pattern is shown in the inset. Note that the length of the nanoribbon is along the [112] direction, and the fringe spacing of 0.335 nm corresponds to the (002) lattice spacing of CdS.
schematic diagram of the device configuration for photocurrent measurement, in which a monochromatic light (full width at half-maximum (FWHM) ∼ 3 nm) is illuminated on the nanoribbon in the normal direction, and the I-V measurements are performed by using a two-probe method. Figure 2a shows typical I-V curves obtained when the nanoribbon is exposed to light of different wavelengths (λ) at a constant light intensity of 1.8 mW/cm2. The approximately linear shape of the curves reveals good ohmic contacts of the nanoribbon with the electrodes. Note the photoconductance (Gpho) at zero bias is highly wavelength sensitive: Gpho ) 0.793 nS for λ ) 500 nm, and 128 nS for λ ) 490 nm, vs Gpho ) 0.021 nS measured in dark (refer to the I-V curve in the inset of Figure 3a). However, the conductance of the nanoribbon decreases to 96 and 54 nS for the light of 440 and 400 nm, respectively. The spectral response of the CdS nanoribbon is depicted in Figure 2b. It can be seen that the sensitivity of the CdS device is rather low (
