Chem. Mater. 2008, 20, 5845–5850
5845
Axial-Junction Nanowires of Ag2Te-Ag As a Memory Element Sudip K. Batabyal and Jagadese J. Vittal* Department of Chemistry, National UniVersity of Singapore, Singapore 117543 ReceiVed May 23, 2008. ReVised Manuscript ReceiVed July 15, 2008
Silver telluride nanowires incorporating silver were prepared by green chemical method in aqueous solution. First, Ag2TeO3 was prepared from aqueous solutions of sodium tellurite (Na2TeO3) and silver nitrate (AgNO3). At room temperature in basic solution, R-D-glucose reduces Ag2TeO3 to generate silver nanocrystals, whereas solvothermal reaction of Ag2TeO3 with R-D-glucose at 165 °C generates Te, Ag, or Ag2Te-Ag axial hetrojunction nanowires depending upon the pH of the reaction mixture. These heterojunction nanowires, which exhibit electrical bistability at low voltage, can be used as data storage devices.
Introduction Miniaturization of modern devices is the goal of modern scientific research and to achieve this we need rationalized synthesis of new nanostructures with advanced properties. Also the assembly of these nanostructures in a controllable manner is very important to manipulate the properties. Onedimensional (1D) nanostructures such as nanotubes and nanowires are the most studied system for device applications as they afford ideal basis for investigating fundamental phenomena in the mesoscopic system. Now for the practical applications of these 1D systems as the basic building block in a compact circuit, it is necessary to generate junction of two or more components along the 1D axis. Some progress has been made to generate the axial-junction nanowires by catalytic vapor growth, direct solid-solid reaction, and template-based design techniques.1-4 Another approach for junction formation is the variation of the doping concentration along the axis of the 1D structures.5 The formation of junction in nanowires can also be achieved by controlled reduction process. The partial reduction of binary or ternary metal containing semiconducting nanowires to metal may form the metal-semiconductor junction on the nanowires. As the candidate for nanowires, silver telluride, Ag2Te is chosen as it is easy to reduce to silver for making of metalsemiconductor junction along the axis. The low temperature monoclinic phase of Ag2Te is a narrow band gap semiconductor with high electronic mobility and high lattice thermal conductivity.6,7 It exhibits a phase transition around 150 °C * To whom correspondence should be addressed. E-mail:
[email protected]. Fax: 65-6779-1691.
(1) Bjork, M. T.; Ohlsson, B. J.; Sass, T.; Persson, A. I.; Thelander, C.; Magnusson, M. H.; Deppert, K.; Wallenberg, L. R.; Samuelson, L. Nano Lett. 2002, 2, 87. (2) Milliron, D. J.; Hughes, S. M.; Cui, Y.; Manna, L.; Li, J.; Wang, L.; Alivisatos, A. P. Nature 2004, 430, 190. (3) Son, D. H.; Hughes, S. M.; Yin, Y.; Alivisatos, A. P. Science 2004, 306, 1009. (4) Robinson, R. D.; Sadtler, B.; Demchenko, D. O.; Erdonmez, C. K.; Wang, W.; Alivisatos, A. P. Science 2007, 317, 355. (5) Pradhan, B.; Batabyal, S. K.; Pal, A. J. Appl. Phys. Lett. 2006, 89, 233109. (6) Kobayashi, M.; Ishikawa, K.; Tachibana, F.; Okazaki, H. Phys. ReV. B 1998, 38, 3050.
and transformed to superionic conducting cubic crystalline phase.8,9 Further nonstoichiometric Ag2Te possesses interesting magnetoresistive properties. Both n-type (Ag-rich) and p-type (Te-rich) doping display large positive magnetoresistance.10,11 Recently, it was reported that binary nanocrystal superlattices of PbTe and Ag2Te enhanced the conductivity of the composite thin films12 thus supporting that the composite nanostructures of Ag2Te-Ag are suitable for device applications. Recent discovery of the fourth basic circuit element, called memrister, adds another dimension to the memory switching phenomenon.13 Memrister is a basic circuit element like resister, capacitor, and inductor that can memorize the flow of charge after removing the bias voltage. In other words, the resistance of the devices at any point in time is a function of history of the device, i.e., how much charge went through it either forward or backward. Introduction of the chargetrapping layer for nonvolatile flash memory devices is currently attracting immense research interest.14-16 Still, it is a challenge to control the charge trap density and distribution. Nowadays, nanoparticles and nanowires-based memory devices have been investigated for this purpose. Controlled assembly of these nanostructures may control the memory phenomena more accurately to the desired level.17-19 The data storage density may be correlated to the nature of (7) Kashida, S.; Watanabe, N.; Hasegawa, T.; Iida, H.; Mori, M. Solid State Ionics 2002, 148, 193. (8) Martin, C. R. Science 1994, 266, 1961. (9) Das, V. D.; Karunakaran, D. J. Appl. Phys. 1989, 66, 1822. (10) Xu, R.; Husmann, A.; Rosenbaum, T. F.; Saboungi, M. L.; Enderby, J. E.; Littlewood, P. B. Nature 1997, 390, 57. (11) Schnyders, H. S.; Saboungi, M. L.; Rosenbaum, T. F. Appl. Phys. Lett. 2000, 76, 1710. (12) Urban, J. J.; Talapin, D. V.; Shevchenko, E. V.; Kagan, C. R.; Murray, C. B. Nat. Mater. 2007, 6, 115. (13) Strukov, D. B.; Snider, G. S.; Stewart, D. R.; Williams, R. S. Nature 2008, 453, 80. (14) Wann, H. C.; Hu, C. IEEE Electron DeVice Lett. 1995, 16, 491. (15) White, M. H.; Adams, D. A.; Bu, J. IEEE Circuits DeV. 200, 16, 22. (16) De Blauwe, J. IEEE Trans. Nanotechnol. 2002, 1, 72. (17) Lee, J.; Cho, J.; Lee, C.; Kim, I.; Park, J.; Kim, Y.; Shin, H.; Lee, J.; Caruso, F. Nat. Nanotechnol. 2007, 2, 790. (18) Das, B. C.; Batabyal, S. K.; Pal, A. J. AdV. Mater. 2007, 19, 4172. (19) Mohanta, K.; Majee, S. K.; Batabyal, S. K.; Pal, A. J. J. Phys. Chem. B 2006, 37, 18231.
10.1021/cm801388w CCC: $40.75 2008 American Chemical Society Published on Web 08/27/2008
5846
Chem. Mater., Vol. 20, No. 18, 2008
Batabyal and Vittal
Table 1. Description of the Reaction Conditions and Morphology of the Products sample pH of the reaction identification mixture S1 S2 S3 S4 S5 S6 a
1 7 10.5 11.5 12.5 14
productsa mixture of Te and Ag7Te4 Te and Ag7Te4 Ag and Ag2Te Ag and Ag2Te composite nanowires of Ag and Ag2Te Ag2Te
ratio of the intensities of Ag and Ag2Te peaks
∼6:1 ∼1:1 ∼1:2
morphology from SEM images agglomerated spherical cluster agglomerated spherical cluster mixture of spherical cluster and elongated particles mixture of spherical cluster and elongated particles nanowires microtubes
Analyzed by XRPD patterns.
Figure 1. X-Ray powder diffraction patterns of the samples prepared at different pH. The peaks marked with X and ∆ are due to Ag and Ag2Te, respectively. The bottom vertical lines represent the standard sample peak according to JCPDF data shhet. The left side label (S1-S6) represents the sample name and the right side label represents the reaction pH.
the patterned nanowires, and the nanowires are expected to store more data depending upon the patterns generated on it. In this paper, nanowires of Ag2Te incorporating some Ag cluster have been explored for the data storage applications. It appears that the Ag segments inside the Ag2Te nanowires act as the charge trapping medium. The Ag2Te-Ag axialjunction nanowires have been synthesized by reducing silver tellurite (Ag2TeO3) using glucose, a soft reducing agent. At 165 °C in alkaline pH, glucose reduces aqueous dispersion of Ag2TeO3 to Ag2Te and Ag. These Ag2Te and Ag formed the axial-junction nanowires. The relative amount of Ag in the Ag2Te nanowires can be controlled by the changing the amount of glucose and the pH of the medium. We investigated how the charge trapping phenomena is influenced by the relative concentration of Ag in the Ag2Te nanowires and how it provided a fruitful pathway for the advancement of data storage technology. Experimental Section Materials. Silver nitrate, sodium tellurite, sodium hydroxide, and R-D-glucose were purchased from Aldrich and all the chemicals were used as received without any further purification; deionized water from the Milli-Q water purification system was used. Synthesis of Ag2TeO3. The aqueous solutions of AgNO3 (1 mM) and Na2TeO3 (0.5 mM) were mixed together in the molar ratio of 2:1 to get instant white precipitate and the Ag2TeO3 was filtered, washed with water and ethanol and dried in vacuum. Synthesis of Ag2Te-Ag Nanowires. In a typical experiment, 0.1 mM Ag2TeO3 and 0.5 mM glucose solution were taken in a
Figure 2. FESEM micrographs of sample S5 prepared at pH 12.5.
Teflon-lined stainless steel autoclave of capacity 50 mL, and the pH of the mixture was adjusted to 13 by adding 2 mL of 1N NaOH solution. The final volume of the solution mixture was adjusted to 25 mL and the autoclave was heated at 165 °C. After 24 h, it was cooled to room temperature naturally; black cloudy precipitates found inside the autoclave were separated by centrifugation, washed several times with water and then ethanol, and finally dried in a vacuum for further characterization. Characterization. The X-ray powder diffraction (XRPD) experiments were carried out using a D5005 Bruker AXS X-ray diffractometer. The instrument was operated at a 40 kV voltage and a 40 mA current (λ ) 1.5406 Å) and was calibrated with a standard silicon sample. The samples were prepared by making thin films on glass side by drop cast from ethanol dispersion and scanned from 2θ ) 20-70° at the step scan mode (step size 0.01, preset time 1s). The morphology of the nanowires was studied using a transmission electron microscope (TEM JEOL, 2010EX) operated at an accelerated voltage of 200 kV. A drop of dilute solution of the product in ethanol dispersed by sonication on the carbon-coated copper grid was dried finally in vacuum and was directly used for
Axial-Junction Nanowires of Ag2Te-Ag
Chem. Mater., Vol. 20, No. 18, 2008 5847
Figure 3. FESEM micrographs of sample S6 prepared at pH 14. Lower inset represents a higher-magnification image of the tubular morphology.
the TEM experiments. For the field emission scanning electron microscopy (FESEM), the samples were made into paste and mounted on a double-sided carbon tape and were platinum coated by a fine coater, then the samples were observed using JEOL JSM6700F operated at 5kV. The phase transition temperature and enthalpy of phase change were measured by a TA differential scanning calorimeter DSC 2920 under nitrogen atmosphere. The instrument was calibrated with indium before each set of experiment. About 3 mg of sample was taken in crimped-aluminum pans and a heating rate of 10 °C/min was used. The thermal stability of the Ag2TeO3 was measured using a SDT 2960 TGA Instrument under a nitrogen atmosphere at a heating rate 10 °C/min. The Ag2Te-Ag nanowires were dispersed homogeneously in ethanol and drop caste on a microgap electrode (IME 0525.5 FD Au P) from ABTECH scientific for the electrical measurements. These electrodes are interdigitated 5 µm gap Au electrode with electrode length 5 mm, fabricated from magnetron sputter-deposited gold on glass substrate (http://www.abtechsci.com/imes.html). The electrodes were washed before use several times with acetone and confirmed no connectivity between two electrodes by recording I-V plots without any sample maintaining the same bias condition for sample characterization. The current-voltage characteristics were measured inside the glovebox in a clean room laboratory. A Keithley 4200 semiconductor parameter analyzer was used to apply the voltage between the two electrodes and measuring the current. Because it is difficult to deposit the top electrode for making a sandwiched device with nanowire or nanotube thin film and there is always a probability for filament formation, which gives electrical
Figure 4. (a) TEM micrographs of sample S5, darker portion indicated by rectangle; (b) HRTEM micrograph of sample S5 prepared at pH 12.5.
bistability, lateral Au electrodes were used instead of a sandwiched device.
Results and Discussion To synthesize composite nanowires, we prepared the precursor Ag2TeO3 by reacting AgNO3 and Na2TeO3 in aqueous solution. The XRPD pattern matched well with the orthorhombic phase of reported Ag2TeO3 (JCPDF-00046-0036). The precursor was further characterized by its FTIR spectrum from the stretching and bending vibration of Te-O bond as around 617 and 762 cm-1.20 The precursor Ag2TeO3 was then reduced to Ag2Te by glucose under hydrothermal conditions. However, the nature of final products depends on the pH of the reaction medium. It is known that glucose has been used to reduce silver nitrate and aurochloric acid to silver and gold nanoparticles at the basic pH, whereas no reaction was observed at acidic pH.21 Table 1 gives the details of products distribution of the reduction of Ag2TeO3 in the pH range 1-14. The XRPD patterns of the product at pH 1-5 shows that it contains a mixture of products. When the pH is increased to 12.5, Ag (20) Arnaudov, M.; Dimitrov, V.; Dimitriev, Y.; Markova, L. Mater. Res. Bull. 1982, 17, 1121. (21) Raveendran, P.; Fu, J.; Wallen, L. Green. Chem. 2006, 8, 34.
5848
Chem. Mater., Vol. 20, No. 18, 2008
Batabyal and Vittal
Figure 5. DSC curves of (a) sample S6 prepared at pH 14 and (b) sample S5 prepared at pH 12.5.
Figure 7. (a) Six consecutive loops of current-voltage curves for sample S5 for two sweep directions in log scale. The sign of the current has been reversed to plot in logarithmic scale. (b) Long time response of the ON state (filled symbol) and OFF state (open symbol) as probed by device current at 0.8 V. The ON and OFF states have been induced by -7 and +7 V, respectively.
Figure 6. Current-voltage characteristics of sample S5 prepared at pH 12.5 on microgap electrode for two sweep directions. Upper left corner shows low-magnification FESEM micrograph of the used electrodes with sample, and lower right corner displays the distribution of sample between two electrodes after performing the electrical characterization.
containing Ag2Te nanowires have resulted. Only at pH 14 did the XRPD patterns match well with that of monoclinic Ag2Te (JCPDF data sheet 00-034-0142). The XRPD patterns of the product obtained at different pH are shown in Figure 1. The FESEM studies reveal that uniform particle size and shape were only obtained when the pH >12. For example, at pH 12.5, 1D nanostructures were observed exclusively for sample S5 as shown in Figure 2. The diameter of the nanowires is in the range of 50-200 nm. This micrograph in Figure 2 further infers that the Ag impurity detected by XRPD should be present inside the nanowires and not present as physical mixtures. The FESEM micrographs of the Ag2Te sample S6 prepared at pH 14 shown in Figure 3 exhibits microsized tubular architectures with very small inner
diameters. The typical diameter of this 1D structure varies from 200 to 500 nm and the length is about 10 µm. The TEM micrograph of samples S5 obtained at pH 12.5 reveals interesting morphology and the finer details about the location of Ag in the Ag2Te nanowires in Figure 4. The TEM image clearly shows that these nanowires consist of some dark bands along the length, which infer that these nanowires consist of two different materials and the junction of these two materials are in the cross-section of the nanowires. Further, the HRTEM images reveal that the lattice spacing in the darker and lighter regions are different. The interplaner spacings of the lighter and the darker regions calculated in the HRTEM micrograph can be assigned to (111) and (220) planes of Ag and Ag2Te, respectively. Hence it is concluded that these nanowires are Ag2Te-Ag axial junction nanowires. As the electronic charge density of Ag is more than that of Ag2Te, it should scatter electron beam more than that of Ag2Te and hence produce darken regions. The DSC curves of the Ag2Te microtubes prepared at pH ) 14 (sample S6) and Ag2Te-Ag nanowires (sample S5) shown in Figure 5 reveal crystalline phase transformation from monoclinic to cubic phase as reported in the literature.22,23 From (22) Qin, A.; Fang, Y.; Tao, P.; Zhang, J.; Su, C. Inorg. Chem. 2007, 46, 7403.
Axial-Junction Nanowires of Ag2Te-Ag
Chem. Mater., Vol. 20, No. 18, 2008 5849
Figure 8. (a) FESEM micrographs of Te nanowires (left) and converted Ag2Te nanowires (right); (b) XRPD patterns of Te and Ag2Te nanowires; (c) current-voltage plots of four consecutive loops of Ag2Te on microgap electrode.
DSC curve of Ag2Te microtubes shown in Figure. 5a, it can be seen that the phase transition during the heating starts at 147.3 °C, whereas the reversible phase transition during cooling occurs at 141.5 °C with a hysteresis of 6 °C. Similar reversible phase transition also observed for Ag2Te nanotubes with the inception transition temperatures at 152.5 and 140 °C during heating and cooling cycles respectively with large hysteresis of 12.5 °C.9 The hysteresis of phase transition is lower for Ag2Te-Ag nanowires for heating and cooling as shown in Figure. 5b. The presence of more conducting Ag impurity in the Ag2Te nanowires (sample S5) may be responsible for the lower phase transition temperature and lower hysteresis. The metal segments inside the semiconducting nanowires accelerate the phonon conduction inside the nanowires which may help to lowering the phase transition temperature for Ag2Te-Ag nanowires. Also the required enthalpy of phase transition for the Ag2Te-Ag hybrid nanowires (sample S5), 13 J/g is lower than that of pure Ag2Te microtubes (sample S6), 15.1 J/g. The current-voltage characteristics of the Ag2Te-Ag nanowires, S5 were recoded by scanning applied voltage from +Vmax to -Vmax followed by a reverse scan from -Vmax to +Vmax. All the characteristics curves showed a nonlinear behavior. A typical I-V curve of the Ag2Te-Ag nanowires is shown in Figure 6 for the applied +Vmax ) 7V. The arrows in the figure represent the direction of the applied voltage (23) In a typical Te nanowires synthesis, 10 mL of an aqueous solution of sodium tellurite (50 mM), 10 mL of an aqueous solution of R-D-glucose (50 mM), and 2 mL of HCl (5N) solution were taken in a Teflonlined autoclave of capacity 50 mL; the final volume of the solution was adjusted to 25 mL with water. The autoclave was heated at 165°C for 2 h and cooled to room temperature for 3 h; the black product was filtered, washed with water, and dried under a vacuum.
scan. It was observed that the sample current decreases with the decrease of the applied voltage from +Vmax following nonohmic path. But whenever the applied voltage changes its polarity, sample current suddenly jumped to very high value and after that it slowly decreases with increase of the applied voltage. Different conducting path were observed for -Vmax to zero in comparison with zero to -Vmax bias voltage scan. This is also observed in the voltage scan of both forward and reverse directions. This means that these nanowires show two distinct conducting states which can be applied for low power data storage applications. The I-V curve shows similar type of behavior for repeated cycles of the same voltage sequence. The current for six consecutive loops are exhibited in the Figure 7a. Whenever the polarity of the applied voltage changes, the current jumps to a higher value shifted the nanowires to a high-conducting state and on increasing the applied voltage, the current decreases and conducting state merges with the low conducting state. Therefore, it is clear that only at low voltage this composite system has two distinct conducting states that can be applied for low power data storage applications. We have also measured the data retention time of the nanowire devices. These nanowires could remember their high and low conducting states for a long time. Once the high state (or low state) was induced, it was probed by applying a small voltage for 20 min; the device current has been found to be consistent on the same state. After a short pulse of -7 V was applied, the current was measured at +0.8 V; it can keep the higher current up to 20 min, i.e., it can remember the high conducting state at least for this time duration. Similarly, when a short pulse of +7 V was applied,
5850
Chem. Mater., Vol. 20, No. 18, 2008
the device current was probed at +0.8 V as the low conducting state currents. The high and low conducting state currents with respect to time are shown in Figure 7b. Sample S4 is found to be highly conducting because of the presence of large amount of silver along with Ag2Te (Table 1) as observed from XRPD and SEM results. The current-voltage curve for sample S4 gives ohomic nature but sample S6 gives nonlinear I-V characteristics in identical condition (see the Supporting Information). To understand the mechanism of switching, some controlled experiment were performed and Ag2Te nanowires were synthesized without silver segment. This has been done as follows: First, Te nanowires were prepared by reducing Na2TeO3 with glucose.23 These Te nanowires were then converted to Ag2Te nanowires by reacting with AgNO3.24 The Ag2Te nanowires do not contain any Ag as observed from the XRPD patterns and the HRTEM images further confirms that the sample composed of only Ag2Te. The SEM images of Te and Ag2Te samples are given in Figure 8a. The XRPD patterns of Te nanowires and after conversion to Ag2Te nanowires are shown in Figure 8b. The I-V characteristics of these Ag2Te have been measured in the same microgap electrode under identical conditions as described for sample S5. As expected the pure Ag2Te nanowires do not exhibit any memory effect at low voltage (Figure 8c). So it could be inferred that the memory phenomenon in the sample S5 is observed only because of the Ag segments inside the Ag2Te nanowires, which behaves (24) Mu, L.; Wan, J.; Ma, D.; Zhang, R.; Yu, W.; Qian, Y. Chem. Lett. 2005, 34, 52.
Batabyal and Vittal
as a charge storage center and enhances the device current whenever the polarity is changed. In both voltage scan direction, i.e., ( 7 V to zero, the Ag segments are stored the electric charges. As soon as the polarity changes, the Ag segments are discharged and producing high current in the device. Thus these metal semiconductor nanowires junction have some promising applications in future nanoscale devices. Conclusions Controlled reduction may be a new synthetic pathway to fabricate composite nanostructures. The pH plays a crucial role in the synthesis of metal telluride nanostructures by glucose reduction. Only at basic pH silver tellurite can be reduced to silver telluride by glucose. Controlled reduction can generate metal containing metal telluride nanowires at pH > 12. These types of metal-metal telluride junctions may find uses as memory elements for data storage applications. Acknowledgment. We are grateful to Prof. Peter K. H. Ho and Dr. Sankaran Sivaramakrishnan for their help in the electrical measurements. The Ministry of Education, Singapore, is gratefully acknowledged for financial support to J.J.V. through NUS (Grant 143-000-283-112). Supporting Information Available: XRPD, FTIR, and TGA analysis of Ag2TeO3, indexed XRPD patterns of S5 and S6, I-V plots (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. CM801388W
