Bismuth Triiodide Sheet-Assisted Growth and Enhanced Field

Sep 6, 2008 - Haiyan Li, Joshua M. Green, and Jun Jiao* ... We report a bismuth triiodide (BiI3) sheet-assisted solution-phase procedure for the growt...
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J. Phys. Chem. C 2008, 112, 15140–15143

Bismuth Triiodide Sheet-Assisted Growth and Enhanced Field Emission Properties of Cadmium Sulfide Nanowire Array Attached to a Flexible CdS Film Haiyan Li, Joshua M. Green, and Jun Jiao* Department of Physics, Portland State UniVersity, P.O. Box 751, Portland, Oregon 97207 ReceiVed: May 12, 2008; ReVised Manuscript ReceiVed: August 1, 2008

We report a bismuth triiodide (BiI3) sheet-assisted solution-phase procedure for the growth of long CdS nanowire arrays attached to flexible CdS films. The growth was dominated by the coordination between the deposition of CdS on BiI3 sheets and BiI3 decomposition. Electrical properties were measured by using a single CdS nanowire as a transport channel in a field-effect transistor. The arrays of nanowires were also characterized as electron field emitters. The conductive nanowires and the attached CdS film formed excellent connections between the vertical nanowire tips and the Si substrate. This enabled the nanowire array to generate a strong electron field emission with a high emission current density of 320 mA/cm2 at the applied electric field of 14.21 V/µm. Introduction As one of the most important II-VI group semiconductors, CdS nanowires (NWs) have been widely synthesized and demonstrate promising potential in optoelectronics.1-4 Assembling CdS NWs into arrays is an effective approach to constructing complex device architectures in order to maximize their great potential. A common method of assembling CdS NW arrays involves molding the CdS deposition to the orderly aligned holes of a porous aluminum anodic oxidization (AAO) template.5,6 This method has an inevitable drawback: During the template-dissolving process, the NW array collapses due to the lack of support of the template. Using the template-free and Au-catalyzed vapor-liquid-solid (VLS) process, Kar, et al. synthesized CdS NW arrays with a length of 1 µm at 900 °C.7 However, the arrays’ future applications may be complicated by their high reaction temperature and the remaining catalyst particles. Furthermore, Lin, et al. demonstrated a hot-wall metal organic chemical vapor deposition process to synthesize CdS NW arrays at 550 °C.8 For the template- and catalyst-free methods, the separation of NWs from the bundles and the elongation of the NWs must be addressed before they can find applications in device fabrication and integration. Therefore, it is imperative and desirable to develop a low-temperature solution for growing and continuously elongating CdS NW in organized arrays. To accomplish this task, we introduce the bismuth triiodide (BiI3) compound formed with three layers (I-Bi-I) that are connected by the weak van der Waals force. This causes BiI3 to form a sheet with polar Bi-I bonds situated on its surface.9,10 By coordinating the deposition of CdS on the polar surface of BiI3 sheets and the decomposition of BiI3, we have synthesized high-yield CdS square grid planar NW networks using a lowtemperature solution-phase procedure.11 This method involves the acceleration of both the deposition of CdS and the decomposition of BiI3 by increasing both the concentration of reactants and the reaction temperature. The result is the formation of well-dispersed CdS-nucleus-seeding long NWs. The as-grown NW arrays were characterized and discovered to * Corresponding author. E-mail: [email protected].

Figure 1. SEM images of (a) BiI3 sheets; (b and c) carpet-like nanostructures grown for 6 h at 195 and 200 °C, respectively. In contrast to the sample shown in (c), (d) is a sample synthesized with a substrate covered with lower-density BiI3 sheets.

have an organized, carpet-like structure as well as excellent conductivity and field emission properties. Experimental Methods Synthesis. BiI3 sheets were typically deposited on a Si substrate in a mixture of 3 g BiI3 (99.999%, Alfa Aesar) and 10 mL ethanol after being ultrasonicated for 10 min. Figure 1a shows a scanning election microscope (SEM) image of the morphology of the as-deposited BiI3 sheets. The aforementioned Si substrates were then introduced into a mixture of cadmium acetate dihydrate [Cd(CH3COO)2.2H2O, 0.5 g, 99.999%, Alfa Aesar] and Dotriacontane (C32H66, 5 g, 97%, Alfa Aesar), which had been heated at 160 °C for 2 h. After adding 0.5 g S powder, the reaction lasted for 6-48 h at 195-200 °C. Next, the products were washed in isopropanol at 60 °C for 30 min and then dried at 200 °C for 0.5 h under the flow of N2 gas. Structural Characterization. The morphology and internal structure of the CdS NW arrays were analyzed using a FEI Siron

10.1021/jp804209j CCC: $40.75  2008 American Chemical Society Published on Web 09/06/2008

Bismuth Triiodide Sheet-Assisted Growth

Figure 2. Schematic diagrams of the growth process demonstrating the fabrication of the CdS NW arrays with the assistance of BiI3. Successive images illustrate the results of the reaction as a function of time.

field emission Scanning Electron Microscope (SEM) equipped with an energy-dispersive X-ray (EDX) spectrometer, as well as a FEI Tecnai F-20 Transmission Electron Microscope (TEM). Photoluminescence (PL) measurements were carried out at room temperature using a nitrogen laser with the wavelength of 337 nm as the excitation source. Electrical Properties Measurement. Transistors incorporating individual CdS NW were fabricated by a dielectrophoretically controlled alignment of NWs.12 As-grown CdS NW arrays were immersed into acetone and then ultrasonicated for 5 min to remove the CdS NWs from the substrate. Three microliters of the top portion of the solution was then dropped onto a substrate patterned with an array of electrodes for dielectrophoresis alignment. An AC external voltage of 10 V and 10 MHz was used for this alignment. After SEM locating the CdS NW attached to two electrodes, I-V characteristics were examined by a probe station with a microchamber and an Agilent 4156C precision semiconductor parameter analyzer. Electron field emission measurements were performed in an ultrahigh vacuum (UHV) system that had a plane-to-plane diode configuration with a variable separation distance between the anode and the cathode. A Keithley 248 high voltage supply provided a voltage with a step size of 10 V, while the field emission current was monitored by a Keithley 2400 ammeter. Results and Discussion Figure 1b and c shows the representative SEM images of two reaction temperature-dependent products. At a reaction temperature of 195 °C, the as-synthesized product shows a carpet-like nanostructure consisting of branched NWs and a flexible film, as shown in Figure 1b. Note that the high yield of branched NWs was grown on top of the film and formed a carpet-like structure. As the reaction temperature was increased to 200 °C, uniform 0.5 µm-long NWs replaced the branched NWs of the carpet-like nanostructure, as shown in Figure 1c. To understand the formation and subsequent transformation of the CdS carpet-like nanostructure, we performed a reaction with a substrate covered with low-density BiI3 sheets. Figure 1d indicates that well-dispersed nanoparticles were formed where the substrate was covered with thin BiI3 sheets. We also performed the reaction without any BiI3 covering the substrate and did not find any NWs on the Si substrate, confirming the importance of BiI3 sheets in the growth and the alignment of CdS NWs. Base on these results, the growth mechanism of carpet-like nanostructures was proposed and depicted in Figure 2. Similar to the chelation-deposition-epitaxy growth of CdS square grid planar NW networks,11 the electrostatic force enabled the Cd2+ cations to align on the surface of the BiI3 sheet. Next, we alternately deposed S and Cd2+ resulting in the formation of a CdS film. In the meantime, the decomposition of BiI3 sheet led the CdS film to separate into tiny pieces. These pieces served as a crystal nucleus which grew vertically and ultimately formed

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Figure 3. (a) SEM image of folded carpet-like nanostructures grown at 200 °C for 24 h. (b) SEM image of carpet-like nanostructures grown at 200 °C for 48 h.

Figure 4. (a) TEM image of a CdS NW grown at 200 °C for 48 h. (b) HRTEM image of a portion of the NW in (a); the inset is its selected area electron diffraction pattern.

the CdS NW array. Some residual BiI3 debris caused the continuous deposition of polycrystalline CdS at the bottom of the CdS NW array, resulting in the formation of a carpet-like nanostructure. The polycrystalline structure enabled the CdS film to exhibit creasing or curling after elastic deformation,13 as shown in Figure 1(b). The reaction temperature of 200 °C was chosen for extended experimentation on the elongation of NWs. Figure 3a shows the large area and uniform carpet-like nanostructure synthesized with a reaction time of 24 h. In contrast to the short NWs shown in Figure 1c, the NWs of this nanostructure reached a height of over 5 µm. Furthermore, Figure 3b demonstrates a SEM image of the carpet-like nanostructure synthesized with a reaction time of 48 h. By tilting the sample substrate during SEM characterization, it suggests that the long NWs are vertically aligned on the substrate and form NW arrays reaching over 25 µm in height. EDX analyses of these two samples indicate both the NW arrays and the flexible films were composed only of Cd and S; no Bi or I was detected from the sample. This was because most of the I was evaporated during the BiI3 decomposition at 200 °C and the residual Bi reacts with S to form Bi2S3 particles in the solution. TEM characterization was employed to assess the crystallinity of the CdS NWs. Figure 4a shows a representative CdS NW, which has a uniform diameter of 60 nm and a smooth surface. A HRTEM image of an edge (Figure 4b) of this CdS NW indicates that the NW is single crystalline. The lattice fringe spacing of 0.67 nm confirms that [0001] is the preferred growth direction for the wurtzite CdS NWs. This is in agreement with the corresponding selected area electron diffraction pattern, shown in the inset of Figure 4b. Photoluminescence (PL) measurements were performed at room temperature to investigate the optical properties of the CdS NW arrays. The PL spectra are shown in Figure 5. A green emission was recorded from the carpet-like nanostructure synthesized at 200 °C with a reaction time of 24 h; this emission corresponds to the near band edge emission of CdS. For the

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Figure 5. Room temperature PL spectra of the samples grown at 200 °C for different reaction times of 24 h (thin line) and 48 h (thick line).

Figure 6. Ids-Vds curves measured at various gate voltages. The upperleft inset is a SEM image of the device; bottom-right inset is the Ids-Vgs curve measured at Vds of 2 V.

purposes of comparison, an emission obtained from the NW array synthesized with a reaction time of 48 h was found to be relatively enhanced. This enhancement was due to the improved yield and crystallinity of the NWs with the increasing reaction time. The very low intensity emissions from 600 to 900 nm are of the samples are largely attributed to the surface defects or impurities.14,15 The electrical transport properties of as-synthesized CdS NWs were measured on a transistor device that incorporated a single CdS NW as shown in upper-left inset of Figure 6. The incorporated NW had a diameter of around 65 nm, and the distance between the source and drain electrodes (channel length) is about 2 µm. All electrical measurements on the device were performed at room temperature within a dark microchamber to minimize the influence of light on CdS’s conductivity.3 Current vs bias (I-V) characteristics were recorded by an Agilent 4156C precision semiconductor parameter analyzer. Our calibration measurements indicate the contact resistance between the microprobes and the Pt electrode is only several ohms and

Li et al.

Figure 7. J-E curves of CdS NW arrays grown at 200 °C for 24 and 48 h. The inset is their corresponding F-N plot.

thereby negligible in contrast to the resistances from the single CdS NW and the contacts between the NW and electrodes. Figure 6 exhibits a graph of current vs drain-source bias (I-Vds) under different gate voltages. The current was enhanced by an increase of the gate voltage from -2 to 2 V. Starting at the linear region of the I-Vds curve collected at Vgs ) 0, the roomtemperature resistance remained at about 2.16 MΩ. The conductivity is estimated to be about 0.69 Ω-1 · cm-1, which is close to previously reported values.16 The gate response I-Vgs curve demonstrates a current increase with respect to the increase of applied Vg, as shown in the bottom-right inset of Figure 6. These behaviors suggest that these CdS NWs are n-type semiconductors. In order to further characterize the electronic properties of the CdS NW arrays, we measured the electron field emission of two CdS carpet-like nanostructures grown at 200 °C for 24 h (short NWs) and 48 h (long NWs). Because the difference in length of NWs between these two samples is approximately 20 µm, the tests were performed with fixed separation distances of 100 and 120 µm, respectively, between the anode and the cathode. The emitting area of each sample is 1 mm2, and the vacuum level of the system was kept below 1 × 10-8 Torr during the measurement of each sample. The field emission current density vs electric field (J-E) curves are shown in Figure 7. For the two CdS carpet-like nanostructures, the threshold current density (1 mA/cm2) is achieved at 12.7 V/µm (short NWs) and 10.57 V/µm (long NW), respectively. From the thick carpet-like nanostructures, a high emission current density of 320 mA/cm2 was measured at the applied electric field of 14.21 V/µm. The threshold electric fields are lower than that of a CdS NW array grown on the quartz substrate covered with a 1 µm thick CdS buffer layer.8 This may be due to the good conductivity between the CdS NW array and the Si substrate, which was improved by an attached CdS film. The J-E characteristics were further revealed by the F-N plots, as shown in the inset of Figure 7, in terms of the Fowler-Nordheim (F-N) equation, ln(J/E2) ) ln(Aβ2/Φ) - BΦ3/2/βE, where A ) 1.54 × 10-6A · eV · V-2, B ) 6.83 × 109eV-3/2 · Vm1-,17 β is the field enhancement factor, and Φ is the work function of emitter materials. The quasi-linear behavior indicates that the field emissions from the CdS NW arrays are a barrier-tunneling, quantum mechanical process.18 From the slopes of the F-N plots and by assuming a work function of 4.7 eV for CdS,19 in the low-voltage region, the β of the thin carpet-like nanostructure and the thick carpet-like nanostructure were estimated to be

Bismuth Triiodide Sheet-Assisted Growth about 112 for E < 11.20 V/µm and 158 for E < 9.7 V/µm, respectively; in the high-voltage region, the β of the thin carpetlike nanostructure and the thick carpet-like nanostructure were about 276 for E > 11.20 V/µm and 349 for E > 9.7 V/µm, respectively. β is generally related to the geometry, structure, and density of emitting nanostructures,20 and the increase of β from the low-voltage region to the high-voltage region is related to the increasingly vertical alignment of CdS NWs.21 Our SEM and PL characterizations suggest that the enhancement of β in our case was attributed to the improvement of both the aspect ratio and the crystallinity of the CdS NWs as the reaction time increased. We also noticed that the aspect ratio of the CdS NWs from the thick carpet-like nanostructure was around five times higher than that of the CdS NWs from the thin carpet-like nanostructure. The ratio of the two samples’ β values, however, was rather low in comparison to the ratio of the NWs aspect ratios for the two samples, indicating that the screening effect influenced the emission process of the thick carpet-like nanostructure more strongly than it influenced that of the thin carpetlike nanostructure. Conclusions In summary, we have performed the growth of large arrays of long CdS NWs on flexible CdS films by a BiI3 sheet-assisted solution-phase method. The high quality CdS NW arrays show good conductivity and excellent field emission properties. Their stable high field emission current density makes the carpet-like nanostructures a candidate for very bright electron sources. Acknowledgment. This work was supported by the National Science Foundation under grants ECCS-0348277, ECCS0520891, and DMR-0649280 and a grant from ONAMI/DOD’s nanoelectronic program.

J. Phys. Chem. C, Vol. 112, No. 39, 2008 15143 References and Notes (1) Lieber, C. M.; Wang, Z. L. MRS Bull. 2007, 32, 99. (2) Ma, R. M.; Dai, L.; Qin, G. G. Nano Lett. 2007, 7, 868. (3) Jie, J. S.; Zhang, W. J.; Jiang, Y.; Meng, X. M.; Li, Y. Q.; Li, S. T. Nano Lett. 2006, 6, 1887. (4) Dong, L. F.; Jiao, J.; Coulter, M.; Love, L. Chem. Phys. Lett. 2003, 376, 653. (5) Routkevitch, D.; Bigioni, T.; Moskovits, M.; Xu, J. M. J. Phys. Chem. 1996, 100, 14037. (6) Xu, D.; Xu, Y.; Chen, D.; Guo, G.; Gui, L.; Tang, Y. AdV. Mater. 2000, 12, 520. (7) Kar, S.; Chaudhuri, S. J. Phys. Chem. B 2006, 110, 4542. (8) Lin, Y. F.; Hsu, Y. J.; Lu, S. Y.; Kung, S. C. Chem. Commun. 2006, 2391. (9) Nason, D.; Keller, L. J. Cryst. Growth. 1995, 156, 221. (10) Yosim, S. J.; Ransom, L. D.; Sallach, R. A.; Topol, L. E. J. Phys. Chem. 1962, 66, 28. (11) Li, H.; Jiao, J. Chem. Mater. 2008, 20, 3770. (12) Dong, L. F.; Bush, J.; Chirayos, V.; Solanki, R.; Jiao, J.; Ono, Y.; Conley, J. F.; Ulrich, B. D. Nano Lett. 2005, 5, 2112. (13) Wang, G.; Zhao, D. Q.; Bai, H. Y.; Pan, M. X.; Xia, A. L.; Han, B. S.; Xi, X. K.; Wu, Y.; Wang, W H. Phys. ReV. Lett. 2007, 98, 235501. (14) Spanhel, L.; Haase, M.; Weller, H.; Henglein, A. J. Am. Chem. Soc. 1987, 109, 5649. (15) Oneil, M.; Marohn, J.; McLendon, G. J. Phys. Chem. 1990, 94, 4356. (16) Long, Y.; Chen, Z.; Wang, W.; Bai, F.; Jin, A.; Gu, C. Appl. Phys. Lett. 2005, 86, 153102. (17) Fowler, R. H.; Nordheim, L. W. Proc. R. Soc. London.SerA 1928, 119, 173. (18) Dong, L. F.; Jiao, J.; Pan, C. C.; Tuggle, D. W. Appl. Phys. A: Mater. Sci. Process. 2004, 78, 9. (19) Liu, G.; Schulmeyer, T.; Brötz, J.; Klein, A.; Jaegermann, W. Thin Solid Films 2003, 431-432, 477. (20) Lee, C. J.; Lee, T. J.; Jyu, S. C.; Zhang, Y.; Ruh, H.; Lee, H. J. Appl. Phys. Lett. 2002, 81, 3648. (21) Lee, Y. D.; Lee, H. J.; Han, J. H.; Yoo, J. E.; Lee, Y. H.; Kim, J. K.; Nahm, S.; Ju, B. J. Phys. Chem. B 2006, 110, 5310.

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