Photoresponse and Field-Emission Properties of Bismuth Sulfide Nanoflowers Xuelian Yu and Chuanbao Cao* Research Center of Materials Science, Beijing Institute of Technology, Beijing 100081, P.R. China
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 11 3951–3955
ReceiVed October 13, 2007; ReVised Manuscript ReceiVed June 2, 2008
ABSTRACT: Elegant Bi2S3 nanoflowers were prepared via a simple vapor deposition process. Compared to the Bi2S3 nanorods bundles, the conductivity of the nanoflowers is much more sensitive to simulated sunlight exposure. The light-induced conductivity increase allows us to reversibly switch the nanoflowers between “OFF” and “ON” states, an optical gating phenomenon analogous to the commonly used electrical gating, which show the possibility of creating highly sensitive photodetectors and optical switches using these Bi2S3 nanoflowers. Moreover, the field emission measurements confirm that the present nanoflowers are excellent field emitters with the turn-on field of 7.45 eV. Therefore, these Bi2S3 nanoflowers with large proportion of free open edges may be attractive in novel nanoscale electric and optoelectronic devices.
1. Introduction Over the past several years, the synthesis of nanomaterials with well-defined architecture has attracted great attention because of the potential uses of the nanostructures as building blocks for various nanodevices and nanosystems.1-6 The research on nanostructures has rapidly extended from simple structures to the assembly of nanocrystals into ordered superstructures aiming to achieve increased structural complexity and functionality.7 For example, tetrapods may be important alternatives to fibers and rods as additives for the mechanical reinforcement of polymers,8 and multipod nanocrystals can also have advantages over their counterparts with one-dimensional nanostructures in the generation of hierarchical nanostructural networks.9 However, it is challenging to develop simple and novel synthetic approaches for building hierarchically selfassembled fractal architectures of various systems. Bismuth sulfide (Bi2S3) is a direct band gap layered semiconductor with Eg of 1.3 eV.10 The photoconductivity in Bi2S3 was reported by Case in 1917 based on studies on mineral samples of bismuthinite or bismuth glance.11 This ascribes to Bi2S3 the status of one of the earliest photoconducting materials known until the early 1920s.12 Macrocrystalline thin films of Bi2S3 have been used in electronic devices such as photodiode arrays as its forbidden gap lies between 1.25 and 1.7 eV.13-15 Up to now, Bi2S3 nanoribbons,16 snowflake-like Bi2S3 nanorods,17 nanowires,18 and nanoflowers19-21 have been prepared via microwave-assisted ionic liquid approach,22 solvothermal,23,24 hydrothermal,25 and microemulsion methods.26 As can be seen, most of the reactions take place in the solution, so the obtained Bi2S3 nanomaterials are randomly arranged in a powder form, which limits the investigation the photoconductive property of Bi2S3 nanostructure. Therefore, up to now little has been reported27 on the photoelectric response of the Bi2S3 nanostructures despite their exciting possibilities for use in optoelectronic devices, saying nothing of the hierarchical nanostructure. Here, we report the synthesis of elegant Bi2S3 nanoflowers grown on silicon substrate via a simple vapor deposition method. Furthermore, the shape of the Bi2S3 nanostructure can be modulated from flowers to bundles of nanorods by the control of reactant partial pressure. Compared to the Bi2S3 nanorods * To whom correspondence should be addressed. E-mail: address: cbcao@ bit.edu.cn. Phone: +86 10 6891 3792. Fax: +86 10 6891 2001.
bundles, the conductivity of the nanoflowers is much more sensitive to simulated sunlight exposure. The light-induced conductivity increase allows us to reversibly switch the nanoflowers between “OFF” and “ON” states, an optical gating phenomenon28,29 analogous to the commonly used electrical gating, which shows the possibility of creating highly sensitive photodetectors and optical switches using these Bi2S3 nanoflowers. Moreover, the field emission measurements confirm that the present nanoflowers are excellent field emitters with the turnon field of 7.45 eV. Therefore, these Bi2S3 nanoflowers with a large proportion of free open edges may be attractive in novel nanoscale electric and optoelectronic devices.
2. Experimental Section The samples were prepared via a vapor deposition process carried out in a horizontal tube furnace. Commercial bismuth sulfide (Bi2S3) powder and sulfur (S) powders were used as starting materials. Bi2S3 powders (∼1 g) in a ceramic boat were placed in the middle of the furnace tube, and sulfur powders (∼2 g) were placed between the Bi2S3 and the gas inlet. Several pieces of silicon (Si) wafers were placed in the downstream direction, several centimeters away from the Bi2S3 powders. The tube was then tightly sealed and purged by introducing a high purity argon flow. Then the temperature was raised to 650 °C at a heating rate of 8 °C min-1 and kept at the temperature for 2 h before gradually being reduced to room temperature. During the thermal treatment, the flow rate of Ar (which functioned as both the protecting medium and the carrying gas) was kept at ∼100 sccm (standard cubic centimeter per minute). The products deposited on the Si substrate were collected for the following characterization. The structure and morphology of the products were characterized by X-ray diffraction (XRD, Philips X’pert Pro diffractometer), scanning electron microscopy (SEM, Hitachi S-3500), transmission electron microscopy (TEM, Hitachi H-800), and high-resolution transmission electron microscopy (HRTEM, JEOL-2010). A TEM sample was prepared via the following procedure: some nanoflowers were stripped off and dispersed in the alcohol with the aid of ultrasonic vibration for 15 min, and then a drop of the solution was transferred onto a standard carbon-covered copper grid. Raman spectra (excited with the 488nm Ar+ laser) were recorded by the HORIBA micro-Raman spectrometer (Jobin-Yvon HR800). The dark and photocurrent measurements of the products were measured on the electrochemical workstation system (IMe6, ZAHNER elekrik). Pairs of coplanar silver print electrodes were applied on the film surface. The illumination source was a Xe lamp (CHF-XM35-500W, Beijing Chang Tuo Sci-tech Co. Ltd., 200-2000 nm). Field emission measurements were conducted in a vacuum chamber with the pressure of 1.2 × 10-6 Pa at room temperature. A rodlike stainless steel probe (1 mm diameter) of 0.78 mm2 area was used as the anode. The sample was used as the cathode. The emission
10.1021/cg701001m CCC: $40.75 2008 American Chemical Society Published on Web 09/12/2008
3952 Crystal Growth & Design, Vol. 8, No. 11, 2008
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Figure 3. TEM and HRTEM images of one of the petals of the Bi2S3 nanoflowers. Figure 1. XRD pattern of the as-synthesized Bi2S3 nanostructure.
Figure 2. SEM images of flower-like nanostructures at (a) low magnification view and (b) high magnification view. current was measured by a picoammeter (Keithley 485). A ballast resistor of 10 MV was used to protect the apparatus against circuit shorting.
3. Results and Discussion 3.1. Structural Characterization. Figure 1 presents the XRD pattern of the as-obtained sample. All the diffraction peaks in the pattern are labeled and can be indexed to a pure orthorhombic phase bismuth sulfide with cell constants of a ) 11.14, b ) 11.30, and c ) 3.951, which are in good agreement with the literature value (JCPDS Card No. 170320). No impurity peaks are found in the experimental range. SEM images shown in Figure 2 reveal that numerous welldistributed flower-like nanostructures formed on the silicon substrate about 3 cm away from the reactant Bi2S3 powder. Figure 2a shows an overview of the samples, in which a large number of flower-like structures are observed. The nanoflowers are approximately of 1 µm in size and composed of tens of petals. The nanoflowers reported here obviously differ from the flower-like SiOx nanostructures30 formed via connection of nanofibers. The petals of these Bi2S3 nanoflowers are smooth flake-structures and approximately 250 nm in width and several nanometers in thickness, shown in Figure 2b. Further characterization of these Bi2S3 nanoflowers was performed by TEM and HRTEM. Figure 3a shows a TEM image of one of the petals of the Bi2S3 nanoflowers. Basically, the diameters of the petal under TEM observation are consistent with the SEM results. The SAED pattern confirms the singlecrystal nature of the petal grown along the (100) direction, shown as an inset in Figure 3a. Figure 3b is a HRTEM image taken on the part of an individual petal. The clear lattice fringes with a d-spacing of 0.556 nm are consistent with that of the (200) planes of orthorhombic Bi2S3 and are parallel to the growth direction of the petal. Usually, growth of nanomaterials is sensitive to the experimental parameters.31 Nanostructures with different morpholo-
Figure 4. SEM images of Bi2S3 products obtained under different experimental conditions. (a) 600 °C; (b) 750 °C; (c) vapor flow rate of 30 sccm; (d) vapor flow rate of 150 sccm; (e) higher magnification at the flow rate of 150 sccm; (f) at the heating rate of 5 °C/min.
gies, sizes, and compositions are controllably synthesized through the adjustment of these conditions.32 In our work, this influence on the morphology of Bi2S3 nanoflowers is also researched, as shown in Figure 4. The growth of Bi2S3 flowers is sensitive to several parameters, such as the growth temperature, the flow rate of gas, the heating rate, and the concentration of reagents. Figure 4a,b shows the images of products grown under the temperature of 600 and 750 °C, respectively. As can be seen, nanorods with the diameter of about 300 nm are fabricated, among which several nanorods radiate from the centers of the crystals. Droplets are easily observed at the free end of each nanorod, indicating the catalytic vapor-liquid-solid (VLS) growth mechanism. At higher temperature, a flower-like product is also seen. They are aggregates of nanobelts with the length of tens of micrometers, which is much bigger than the as-synthesized Bi2S3 nanoflowers. The influence of the Ar flow rate is also studied, and Figure 4c,d shows the SEM results obtained at the flow rate of 30 and 150 sccm, respectively. As shown in Figure 4d, flower-shaped products can only be seen when a higher flow rate is adopted. From the higher magnification view in Figure 4e, smaller nanoflowers with 100 nm length
Bismuth Sulfide Nanoflowers
Crystal Growth & Design, Vol. 8, No. 11, 2008 3953 Scheme 1. Schematic Mechanism for Forming the Unique Bi2S3 Nanostructures
Figure 5. SEM images in the different stages of the growth process.
petals appeared accompanied with the usual ones. Figure 4f is the SEM image of product fabricated when the heating rate changed from 8 °C/min to 5 °C/min. Under this condition, sparse nanoflowers are generally seen on the deposited film. Therefore, we think the morphology diversifications in different conditions can be ascribed to the different supersaturation of Bi2S3 vapor, which will be further discussed as follows. 3.2. Possible Growth Mechanism. To clarify the possible growth mechanism of Bi2S3 nanoflowers in our work, the products collected at the downstream of the nanoflowers are also characterized. As can be seen in Figure 5a,b, abundant nanorods with the diameter of 80-100 nm and the length of 1 µm are obtained in the region about 5 cm away from the nanoflowers. While for the product collected 3 cm away from the nanoflowers, aggregations of nanorods are formed instead of dispersed ones in Figure 5c. It is believed to be the earlier stage when the Bi2S3 nanoflowers begin to form. Then, what is the possible growth mechanism for these aggregations forming the final nanoflowers? Usually, vapor-liquid-solid (VLS), vapor-solid (VS), and oxide-assistant (OA) mechanisms are used to explain the growth of various nanostructures. For example, a VS mechanism is used to explain the growth of MgO nanoflowers,33 but it is not suitable for our products because of the existence of catalytic droplets (mainly bismuth element was detected in the EDS measurement) in Figure 4a, a typical VLS process. Thus, the catalytic effect of Bi metal was further researched through increasing the Bi vapor concentration (more Bi2S3 powders were used and a higher concentration of Bi vapor was generated in the reaction). The images of the products collected 3 and 5 cm away from Bi2S3 powder are shown in Figure 5d,e, the appearance of smooth spherical structures indicating the undoubted formation of metal droplets. As seen in Figure 5e, Bi2S3 nanoflowers could only be achieved in the farther region, where it has a lower supersaturation and the Bi droplets have suitable surface energy for sequent nucleation and growth. The appearance of several nanorods radiating from the droplet surface in Figure 5f proved well this VLS subsequent growth on Bi, which has been observed in the formation of silicon oxide nanoflowers similarly.34
On the basis of these results, the condensed Bi vapor can serve as the catalyst for the growth of Bi2S3 nanostructures. A VLS self-catalyzed growth mechanism is supposed in Scheme 1.35 First, Bi and S vapor were generated through the decomposition of Bi2S3, which were carried to the low-temperature region by Ar gas. There, Bi vapor reacted with the volatilized and decomposed S vapor and Bi2S3 vapor formed (I). Second, Bi/ Bi2S3 droplets were formed and condensed on the substrate, and these droplets can enhance the absorption and diffusion of reagent to form Bi2S3 nanostructures (II).36 Finally, the size of droplets was influenced by the vapor supersaturation, which is the most important factor for the controllable synthesis of nanomaterials. As reported, the supersaturation has an intimate relationship with the temperature, flow rate, concentration, and so forth.37 In the low-temperature region with low supersaturation, smaller catalytic droplets formed and much Bi and S vapor was absorbed on their surface. Bi2S3 nanorods were fabricated through a Bi self-catalyzed growth.38 In the hightemperature region, catalytic droplets were so large that catalyzed growth of single nanorods can not be reached. Instead, Bi2S3 nuclei formed around the surface of the droplets (III). Thus, the petals grew following a similar self-catalyzed process along the preferred growth direction because of the intrinsic anisotropy of Bi2S3, and the flower structure came into being (IV). 3.3. Properties of Bi2S3 Nanoflowers. The Raman spectra of the Bi2S3 nanoflowers and nanorods (shown in Figure 5a) at room temperature were recorded between 100 and 1250 cm-1. The spectrum of the nanoflowers (shown in Figure 6a) exhibits four distinct vibrational peaks at about 145, 305, 431, and 965 cm-1. Compared to the spectrum of nanorods, it has a litter shift and the intensity is stronger. Up to now, there is little work reported on the application of Raman spectroscopy on Bi2S3 nanostructures. Xuzheng39 have investigated the Raman spectra of Bi2S3 nanotubes from 100 to 600 cm-1, revealing three Raman peaks located at around 187, 239, and 261 cm-1. The presence of the 965 cm-1 peak may be related to the surface phonon modes because of the high surface-to-volume ratio. Campbell40 has calculated theoretically that Raman scattering is very sensitive to the microstructure of nanocrystalline materials, and it can lead to a shift, which is in agreement with our results. Furthermore, Raman spectroscopy is a powerful tool for the investigation of the doping concentration, lattice defect identification, and crystal orientation properties of the materials.41 The higher Raman intensity of Bi2S3 nanoflowers may come from the better crystallization and the lower concentration of defects than that of the nanorods. As an important application of semiconductor materials, photodetectors or optical switches are essential elements in imaging techniques, lightwave communications, and possibly in future memory storage and optoelectronic circuits.42 While conventional photodetectors are usually in the film or bulk configurations, the unique and significant properties of nanomaterials are expected to show better properties, such as enhanced optical gain and polarized lasing, and so forth.43 Figure 7 shows the measured photoresponse as a function of time using nanoflowers and nanorods measured as we switch the Xe lamp
3954 Crystal Growth & Design, Vol. 8, No. 11, 2008
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Figure 6. Typical Raman spectra of Bi2S3 nanoflowers(a) and nanorods(b).
Figure 7. Sensitivity of the photoresponse of Bi2S3 nanoflowers to stimulated sunlight exposure.
on and off. When the lamp is switched on, the light currents increases to 3.5 µA and 0.0008 µA for the nanoflowers and nanorods, respectively. And as soon as the lamp is turned off, the current drops down immediately. It is obvious that the conductivity of the nanoflowers is much more sensitive to the simulated sunlight exposure. The on/off sensing cycles can be repeated many times without any detectable degradation. The rise and decay times of the nanoflowers switches are less than 1 s, which is another key parameter that determines the capability of a photodetector to follow a fast-varying optical signal. All these results imply that the Bi2S3 nanoflowers are a great candidate for applications in high-sensitivity and highspeed photodetectors and photoelectronic switches in the nanoscale, with the dark insulating state as “OFF” and the lightexposed conducting state as “ON”. It is known that oxygen chemisorption plays a central role in regulating the photosensitivity of bulk or thin film Bi2S3. So we believe that a similar mechanism is applicable to our nanostructure, and the photoconductivity mechanism involves a complex process of electron-hole generation, trapping, and recombination. Compared to the nanorods, the deleterious effects of grain-boundary recombination for photoconductors44-47 in the nanoflowers could be avoided thanks to the structurally perfect and single-crystalline nature, which can be proved from the HRTEM and Raman results. The density of traps induced
by defects is drastically reduced; thus, the photocurrent of nanoflowers reaches a higher steady state than that of nanorods. Besides working as photodetectors, we also investigated the field emission property of these Bi2S3 nanoflowers. Field emission measurements were conducted in a vacuum chamber with a pressure of 1.2 × 10-6 Pa at room temperature. A rodlike stainless steel probe (1 mm diameter) of 0.78 mm2 area was used as an anode. The sample was used as the cathode. The curve of the emission current density versus applied field (J-E) is depicted in Figure 8. By definition, the turn-on field and the threshold field are the electronic fields required to generate an emission current density of 0.01 and 1 mA cm-2, respectively. A turn-on field of 7.45V µm-1 and a threshold field of 9.83 V µm-1 are obtained for the Bi2S3 nanoflowers. This turn-on field is lower than the reported single Bi2S3 nanowire (26 V µm-1) while higher than the ultrafine ZnS nanobelt (3.47 V µm-1).25 Besides, the threshold field of these nanoflowers is close to that reported for MoS2 nanoflowers (8 V/µm) and needle-shaped SiC nanorods (8.5V/µm),48 indicating that the prepared Bi2S3 nanoflowers are, in fact, excellent field emitters. As we know, the field-enhancement factor, β, is related to the emitter geometry, its crystal structure, and the spatial distribution of emitting centers.49,50 It can be expressed as β ) h/r, where h is the height and r is the radius of curvature of an emitting center. Thus, materials with elongated geometry and sharp tips or edges
Bismuth Sulfide Nanoflowers
Figure 8. Field emission current density versus electric field curves of the obtained Bi2S3 nanoflowers. The insets show the corresponding Fowler-Nordheim relationships (ln(J/E2)-1/E plots).
can greatly increase an emission current. For example, it has been demonstrated that the field emission performance of ZnO nanowires can be significantly enhanced through increasing the aspect ratio (length-to-thickness ratio).51 So, the low turn-on field and threshold field of Bi2S3 nanoflowers are assumed to result from their ultrathin petals. Besides, the high density of deposited nanostructures also can lead to the excellent field emission performance. An inset in Figure 8 displays the corresponding Fowler-Nordheim (FN) plot of the Bi2S3 nanoflowers. According to the Fowler-Nordheim electron emission theory, emission current can be described as, ln(J/E2) ) ln(Aβ2/ Φ) - BΦ3/2/βE, where A and B are constants, β is the fieldenhancement factor, and Φ is the work function of the emitting materials, which is 5.3 eV for Bi2S3.52 Therefore, the fieldenhancement factor is about 1.23 × 103, which has been calculated from a slope of the fitted straight line in the inset of Figure 8. This value is higher than that for the single Bi2S3 nanowire (about 200)25 but lower than that for the ultrafine ZnS nanobelt with the field-enhancement factor of over 2000.
4. Conclusion In summary, elegant Bi2S3 nanoflowers were prepared via a simple vapor deposition process. Because of the high singlecrystal quality and ultrathin petals of these flower-like nanostructures, they show excellent photoresponse and field emission properties. These photoconducting nanoflowers could serve as highly sensitive photodetectors, chemical and biological sensors, and switching devices for nanoscale optoelectronic applications, where the binary states can be addressed optically. Furthermore, their excellent field emission properties indicate their promising applications in practical flat-panel displays. Acknowledgment. This work was supported by the National Science Foundation of China via Grant 20471007.
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CG701001M
