Multicolor Photodetector of a Single Er3+-Doped CdS Nanoribbon
© Dedong et al. 2015
Received: 21 April 2015
Accepted: 12 June 2015
Published: 8 July 2015
Er3+-doped CdS nanoribbons (Er-CdS NRs) are synthesized by thermal evaporation and then characterized by field emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HRTEM), photoluminescence (PL), and absorption spectra. The Er-CdS NR photodetector is studied systematically, including spectral response, light intensity response, and photoconductance (G) versus temperature (T). It is found that Er-CdS NR has the ability of detecting multicolor light including blue, red, and near-infrared light with higher responsivity (R λ ) and external quantum efficiency (η). The conductance of Er-CdS NR under dark conditions decreases with increasing temperature in the range of 87–237 K, while its conductance increases with increasing temperature in the range of 237–297 K when T is larger than 237 K. These results indicated that ionized impurities and the intrinsic excitation are responsible for the conductance change of Er-CdS NR in the dark. The superior performance of the Er-CdS NR device offers an avenue to develop highly sensitive multicolor photodetector applications.
Semiconductor nanostructures, such as nanowires , nanobelts , colloidal quantum dots , and polymer-inorganic nanocrystal composites , are attractive building blocks for a new generation of high-sensitivity and high-selectivity sensors primarily because of their high surface-to-volume ratios and diverse functions as both device elements and interconnects [5, 6]. As an important application of semiconductor materials, photodetectors, optical switches, or nanoelectromechanical (NEM) switches are essential elements in imaging techniques and light wave communications and possibly in future memory storage as well as optoelectronic circuits [1–7]. Among them, some semiconductors such as ZnS  and ZnO  have been assembled into nanometer “visible-light-blind” or “solar-blind” ultraviolet (UV) light sensors with high sensitivity and selectivity. Some photodetectors have achieved a broad spectral response, such as monolayer MoS2, in which the photocurrent monotonously increased as the wavelength of the incident light decreased from 680 to 400 nm . Generally, every material only detects UV light (GaN) , green light (CdS) , or near-infrared light (CdSe) . Furthermore, slow modulation response speed and narrow response bandwidth (<20 Hz) have been tolerated in these devices, but these limitations severely curtail potential applications. Pan et al. reported that a high-performance photodetector based on the CdS0.49Se0.51/CdS0.91Se0.09 lateral heterostructure has been designed to detect double spectral response bands, with the peaks at around 525 and 602 nm, respectively . In fact, we hope that photodetectors can detect not only UV light but also near-infrared light, even tricolor light. Recently, a far-infrared photodetector in a silicon-doped vanadium material has been investigated, and it was found that as the V concentration increases, an important increase of the photoresponse is observed in the far-infrared region of the spectrum . This idea naturally reminds us of doping.
Doping, the intentional incorporation of impurities into materials, is a primary means of tuning electronic, optical, and magnetic properties of bulk semiconductors. As you know, rare earth (RE) elements are effective luminescent centers for RE-doped semiconductors because the excitation of RE ions can occur by the recombination of the photogenerated carriers confined in semiconductors and subsequent energy transfer to RE ions . Therefore, RE-doped II–VI materials are promising candidates for application in color thin-film electroluminescence devices, nonlinear optics, and multicolor optical switches [16, 17]. Rare earth doping may play an important role in obtaining highly efficient multicolor photodetectors and upconversion signals.
Based on these reasons, we have synthesized Er3+-doped CdS nanoribbons (hereafter referred to as Er-CdS NRs) via thermal evaporation and then investigate photoconductance (PC) of a single Er-CdS NR device. It is found that a single Er-CdS NR device detects not only blue and red light but also the infrared one. Its photoconductance could be tuned over 4, 3, and 2 orders of magnitude illuminated by blue, red, and near-infrared light, respectively. Er-CdS NRs offer a promising platform for multicolor photodetectors with high rate and efficiency of detection due to the fine regulation of their band structure coming from numerous valence states of RE ions. In addition, we study the PC dependence of the single Er-CdS NR on temperature to reveal its photodetection mechanism.
Preparation of Er-CdS Nanoribbons
CdS nanoribbons were synthesized in a horizontal tube furnace with three temperature zones via thermal evaporation. The premixed powders of CdS powers (Aldrich, purity 99.99 %) and erbium(III) acetate hydrate (Aldrich, purity 99.9 %) were placed at the center of an alumina tube. Au-coated silicon substrates were placed at the downstream position of the source material. After that, the tube was evacuated to a base pressure of 5 × 10−6 Torr and then the sources were heated to 840 °C at a rate of 40 °C/min; this temperature was maintained for 2.5 h. A carrier gas of high-purity argon premixed with 5 % hydrogen was fed at a total flow rate of 20 sccm. The pressure inside the alumina tube was maintained at 150 Torr during the whole experimental process. The as-synthesized nanoribbons were bright yellow in color.
These nanoribbons were characterized by scanning electron microscopy (SEM, Quanta 250 FEG) and high-resolution transmission electron microscopy (HRTEM, CM200 FEG operating at 200 kV). Room-temperature photoluminescence (PL) was measured by using the fourth harmonic of a Nd:YAG laser with a wavelength of 244 nm for excitation and a 0.5-m monochromator with an expected spectral resolution of 0.1 nm. The absorption spectra of Er-CdS NRs were measured using a spectrometer (PerkinElmer, Lambda 2S) by dispersing the nanoribbons in alcohol.
Fabrication and Characterization of a Single Er-CdS NR Device
For the fabrication of a single nanoribbon detector, Er-CdS NRs were dispersed into dehydrated ethyl acetate by ultrasonic processes. Subsequently, the suspension solution was dropped on a p-type Si substrate with a SiO2 (500 nm) layer on top, and then a desired NR density was obtained on this substrate. After drying the wafer and locating the position of Er-CdS NRs, patterned Ti (20 nm) and Au (100 nm) electrodes were successively deposited on the two ends of the nanoribbons in high vacuum by e-beam evaporation with the assistance of a mesh-grid mask composed of tungsten wires (10 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. Thus, a single Er-CdS NR device was obtained.
A light system combining a mercury lamp (500 W) and a monochromator (1/4 m, VIS-NIR Cornerstone 260) was used to provide the monochromatic light, which was focused and guided onto the nanoribbons perpendicularly. Current-voltage (I-V) measurements were performed by using a two-probe configuration. The dependence of the conductance on the temperature was measured at a temperature of 87–297 K. To measure the response time of the nanoribbon photodetector to light irradiation, a mechanical chopper (frequency ranging from 1 to 500 Hz) was used to turn on and off the light irradiation.
Results and Discussion
In order to obtain the detailed wavelength-related spectral response, we also measured the photocurrents of the device with the incident light wavelength scanning from 300 to 1350 nm. Figure 3c shows the wavelength-dependent photocurrent response of the device constructed with Er-CdS NR (working voltage 1 V, light density 25 μW/cm2). It can be seen that the Er-CdS NR device exhibits three spectral response bands, with the peaks at around 457.5, 620, and 955 nm. The spectral response peak at 457.5 nm is sharper than those at 620 and 955 nm, with the spectral FWHM of about 22.5, 36.5, and 51.4 nm. At the same time, the photocurrent under illumination of 457.5 nm light is much higher than those under illumination with light of 620 and 955 nm, respectively. As reported in the literatures, the infrared emission spectrum in Cs3Y2Br9 (1 % Er) is at 1548 nm related to the 4I13/2 → 4I15/2 [18, 19] and the strong absorption peak in Y3Sc2Ga3O12 (Er)  is at 1524.9 nm related to the 4I15/2 → 4I13/2, which match with IR detection behavior. Therefore, it is speculated that there is a fourth maximum photocurrent peak at the wavelength of 1540 nm. This result has not been proved due to the limit of the monochromator (the longest wavelength is 1350 nm). Therefore, a multicolor photodetector is obtained.
To clarify the origin of the spectral response, the absorption spectrum (red line) of Er-CdS NRs is measured, as depicted in Fig. 3c. It is seen that there is a broad absorption peak at 390–445 nm and a small sharp peak at around 500 nm. The former is related to the transformation of the 4I15/2 → 4F3/2 (4F5/2), and its absorption edge matches with the spectral response band at 457.5 nm. The latter corresponds to the energy band gap. We have carefully observed it and found that there is a steadily increasing absorption in the range of 500–800 nm, which is related to the transition of Er3+ ions with 4I15/2 → 4F9/2 [18, 19], but no obvious absorption peak is observed at 620 nm. However, there is a broad absorption peak at around 900–1100 nm, as shown in the inset of Fig. 3c, which is related to 4I15/2 → 4I11/2 transitions of Er3+ ions [18, 19]. Fascinatingly, the best response wavelengths are nearly coincident with the absorption spectrum at the absorption edge and at the long wavelength position, revealing that the response spectrum is directly related to the energy band structure of Er-CdS NRs, and the transitions of Er3+ ions energy levels, which is reflected by the photocurrent measurement. Thus, it can be concluded that the enhancements of the photoconductive response are due to the electron-hole pairs excited by the incident light with energy larger than the band gap of CdS and the transition energy of Er3+ ion energy levels. Light with a smaller energy has not enough energy to excite electrons from the valence band to the conduction band and thus contributes little to the photocurrent.
For comparisons, the photoluminescence spectrum for Er-CdS NRs is measured, which is displayed in Fig. 3d. It is seen that three emission bands are observed in green, red, and infrared regions at 504.5, 701, 742, 751, 806, and 894 nm. They were associated with 4F9/2 → 4I15/2, 4F7/2 → 4I13/2, 4I9/2 → 4I15/2, and 4I11/2 → 4I15/2 transitions of erbium ions [18–24]. As a consequence, incorporation of the dopant erbium ions into the matrix, rather than simple adhesion to the surface of the CdS nanoribbons, was demonstrated.
A comparison of the critical parameters between this work and various nanostructure photodetectors
Peak response wavelength (nm)
R λ (A/W)
7.15 × 104
2.75 × 105
3.96 × 105
In2Se3 NW: α-phase/κ-phase
3.46 × 104
8.14 × 103
9.19 × 103
Figure 7c depicts the temperature-dependent conductance ratio under irradiation of incandescent light to that under dark conditions. It is seen that the PC ratio increases from 50 to 550 times when the temperature is from 86 to 267 K. But an abrupt decrease in the PC ratio is observed when temperature is raised above 267 K. Therefore, it is appropriate to detect incandescent light for the Er-CdS NR device at 267 K in vacuum.
To elucidate the adsorption effect, we also have measured the dark current and photocurrent of Er-CdS NR in lower vacuum. It is found that the dark current decreases, but the photocurrent ratios of the Er-CdS NR to the dark current are 350, 800, and 1500 under illuminations with torch light, weak table light, and strong table light, respectively (the pressure is 3 × 10−3 Torr), and the dark current (Additional file 1: Figure S1) has increased 2~6 times in vacuum (3 × 10−3 Torr) compared with that in air at room temperature. Combined with Fig. 3b, it is found that the surface absorption and desorption [30–32] (have important influence on) do affect not only the dark current but also the photocurrent ratio of the Er-CdS NR. The PC mechanism of Er-CdS NR is governed by the adsorption of oxygen as well as intrinsic carriers and ionization dopants.
In conclusion, the photoconductance of the Er-CdS NR was investigated. The Er-CdS NR showed higher responses at 457.5, 620, and 955 nm. A multicolor photodetector of the Er-CdS NR was developed, which can simultaneously detect blue, red, and infrared light. The conductance of Er-CdS NRs in the dark decreases with increasing temperature in the range of 87–237 K, which indicates that the impurities are completely ionized and the intrinsic excitation is not primary. When T is larger than 237 K, the conductance of Er-CdS NRs in the dark increases with increasing temperature in the range of 237–297 K, and the intrinsic carriers have a larger contribution to the conductance. In addition, the PC ratio increases from 50 to 550 when the temperature is from 86 to 267 K, while the PC ratio decreases when the temperature is raised above 267 K, which is related to the excitation process of intrinsic carriers and impurities in the semiconductor.
This work was supported by the National Natural Science Foundation (Grant No. 11164034), Yunnan Province Natural Science Foundation (Grant No. 2010DC053), the Key Applied Basic Research Program of Science Technology Commission Foundation of Yunnan Province (Grant No. 2013FA035), and Innovative Talents of Science and Technology Plan Projects of Yunnan Province (Grant No. 2012HA007).
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