J. Phys. Chem. C 2009, 113, 9153–9156
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Ti5Si4 Nanobats with Excellent Field Emission Properties Che-Ming Chang, Yu-Cheng Chang, Chung-Yang Lee, Ping-Hung Yeh, Wei-Fan Lee, and Lih-Juann Chen* Department of Materials Science and Engineering, National Tsing Hua UniVersity, Hsinchu 300, Taiwan, Republic of China ReceiVed: January 27, 2009; ReVised Manuscript ReceiVed: April 4, 2009
Bat-shaped nanorods (nanobats) of Ti5Si4 were grown on titanium foil by a vapor and condensation method without catalysts. The average diameter of the heads and the tails of the nanobats as well as the length of the nanobats are 25, 15, and 350 nm, respectively. The resistivity of the Ti5Si4 nanobat was measured to be 348 Ωµ · cm. Excellent field emission properties were found with the turn-on voltage of the Ti5Si4 nanobats at the current density of 1 µA/cm2 being 1.47 V/µm and field amplification factor being as high as 1350. The possession of remarkable field emission properties indicates that the Ti5Si4 nanobats may be applicable as emitters in flat panel display and vacuum microelectronic devices. Introduction One-dimensional nanomaterials have recently attracted great attention due to their distinct properties compared with bulk materials. The physical properties have been widely investigated. Among various nanowires, silicide nanowires had excellent properties of low resistivity,1-6 high stability, desirable magnetism,7,8 and high compatibility with the processing of silicon-based microelectronics.9-11 As the dimensions of the microelectronic devices continue to shrink, nanostructured silicides will be required in the future. For the sub-100 nm devices, TiSi2, CoSi2, and NiSi have been the competing silicides as contacts and interconnects.12 Consequently, the research on the silicide nanowires is of significant and fundamental importance. Understanding the formation mechanism of silicide nanowires is also a crucial step toward practical application.13-15 Among the titanium silicide nanowires, C54-TiSi2 has been one of the promising field emission materials due to its thermal and chemical stability.16 From the phase diagram, there are five titanium silicide phases. However, scarce related research has been conducted on the Ti-rich phase including Ti3Si, Ti5Si4, and Ti5Si3 and the basic properties of these phases were largely unknown.17-20 In this work, we used titanium foil as the substrate to synthesize the Ti-rich silicide phase bat-shaped nanorods (nanobats). Electrical resistivity, work function, and field emission properties of the nanobats were measured. The growth of the titanium silicide nanostructure on the metal substrate is advantageous in the field emission applications compared with the silicon substrate since the voltage drops between the nanostructures and the substrate are nil. Experimental Section The Ti5Si4 nanobats were grown on titanium through a vapor transport and condensation method in the two-zone alumina tube. Titanium foil (0.25 mm thickness and 99.6% purity) was cut to 1 cm × 1 cm pieces as substrates. The sample pieces were dipped in HNO3 for 30 s to remove the native oxide layer on titanium followed by cleaning sequentially in ethanol and DI water. Two grams of silicon powder (99.8% purity) was used * To whom correspondence should be addressed. E-mail: ljchen@ mx.nthu.edu.tw. Phone: 886-35731166. Fax: 886-35718328.
as the source. The Si powder and substrate were kept at 1100 and 900 °C, respectively. The carrier gas was a mixture of 95 sccm Ar mixed and 5 sccm H2 with a total pressure of 1 Torr. Hydrogen was used to reduce the residual oxygen in the chamber to inhibit the formation of oxide. The growth time ranged from 1 to 3 h. The morphology was investigated with a JEOL 6500F field emission scanning electron microscope (FESEM) operating at 15 kV acceleration voltage. The phases were determined from X-ray diffraction (XRD) spectra with an X-ray diffractometer. A JEOL 2010 transmission electron microscope (TEM) operating at 200 kV was utilized to determine the structure and growth direction. The electrical properties were measured by a multiprobe electronics measurement system (Agilent B1500A). The conducting Pt wires connecting both ends of the nanobat to the gold pads were deposited by a focused ion beam (FIB) system. The work function of the Ti5Si4 nanobat was measured with a photoelectron spectrometer (RIKEN KEIKI Model AC-2) by adjusting the energy of the incident photon on the grown sample. The field emission properties of the Ti5Si4 nanobats were measured by a KEITHLY Model 237 system on the parallel plate with a spacing of 180 µm and contact area of 0.25 cm2 at a pressure of 1 × 10-5 Torr. Results and Discussion A high density of nanobats was grown on the Ti substrate as shown in panels a and b of Figure 1. The diameters of the head and tail are 25 and 15 nm, respectively, and the average length is about 350 nm. The X-ray diffraction spectrum peaks can all be ascribed to the tetragonal structure Ti5Si4 and Ti substrate, as shown in Figure 1c. Figure 2a shows the TEM image of a nanobat. The head was not planar and appeared to be curved. The inset is a selected area electron diffraction (SAED) pattern. The pattern was analyzed to correspond to the [110] zone axis of the tetragonal Ti5Si4 phase with lattice constants a ) 0.671 nm and c ) 1.217 nm (JCPDS card, No-89-2434) with the growth direction along [001]. Analysis of the lattice image, shown in Figure 2b, is consistent with that of the SAED analysis. In addition, from the EDS spectrum shown in Figure 2c, both Ti and Si were detected to be present in the nanobat. In contrast to the Cu
10.1021/jp902082x CCC: $40.75 2009 American Chemical Society Published on Web 05/01/2009
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Figure 1. (a) Top view and (b) side view FESEM images of the Ti5Si4 nanobats grown on titanium foil at 900 °C for 3 h. (c) XRD spectrum of the Ti5Si4 nanobats.
Figure 2. (a) TEM image and diffraction pattern of a Ti5Si4 nanobat. The growth direction is along [001]. (b) HRTEM image of a single-crystal Ti5Si4 nanobat and (c) TEM/EDS spectrum.
nanobats with 5-fold symmetry and multiple twins, the Ti5Si4 nanobats are single crystalline.21 To clarify the growth mechanism, the growth time was varied from 1 to 3 h. The SEM image in Figure 3a shows the formation of protruded bumps after 1 h. Bat-shaped nanorods emerged after 2 h, as shown in Figure 3b. After 3 h of reaction, the nanorods grew longer and the bat-shaped morphology was maintained as shown in Figure 3c. A schematic plot of the growth process is shown in Figure 3d. It is worthwhile to mention that the conditions to grow Ti5Si4 nanobats were rather stringent. For substrate maintained at lower and higher temperatures (800 and 1000 °C), Ti5Si4 nanorod networks and clusters, respectively, were formed. The variation in the diameters of the nanobats is attributed to the fluctuation of the size of protruded bumps, as seen on the substrate after 1 h of reaction at 900 °C (Figure 3a). Vapor-liquid-solid (VLS) and vapor-solid (VS) are the two well-known growth mechanisms of the nanowires. In the present work, since no catalyst was involved in the reaction, the growth of the Ti5Si4 nanobats was not via the conventional VLS mechanism in which a metal particle acted as the catalytic seed site. It was likely that the VS mechanism controlled the growth of the Ti5Si4 nanobats. Initially, the silicon vapor reacted with the titanium substrate and formed the Si-Ti alloy islands.
Continuous feeding of silicon to the quasiliquid Si-Ti islands led to the 1D growth of the Ti5Si4 phase. This growth process can be defined as self-catalytic growth. On the other hand, as the 1D growth continued, the supply of silicon vapor was diminished with the consumption of Si powder. As a result, the dimension of the roots of nanorods became smaller, which led to bat-shaped morphology. The mechanism is similar to that proposed for TiSi nanopins in a previous work.22 The growth of Ti-rich phases on Ti foils was somewhat expected with the ample supply of Ti atoms from the substrate. Formation enthalpy of the different titanium silicide phases was measured with the Knudsen effusion mass spectrometer and thermodynamic calculations were carried out.18 Both experimental measurement and theoretical calculation indicated that the Ti5Si4 possesses the large and negative formation enthalpy. The Ti5Si4 phase was also identified to be present in the initial stage of reaction between Ti thin film and Si by the highresolution TEM technique.19,20 Photoelectron spectrometry was used to measure the work function of the Ti5Si4 nanobats by adjusting the energy of incident photon on the as-grown sample. When the energy of the photon reached about 5.6 eV, the number of exciting electrons was increased greatly. The extrapolation of the
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Figure 5. Electrical transport property of the Ti5Si4 nanobats at 300 K with two-probe measurement. The inset is the SEM image of a nanobat on the silicon oxide connected by the deposited platinum.
Figure 3. FESEM image of the Ti5Si4 nanobats grown on titanium foil at 900 °C for (a) 1, (b) 2, and (c) 3 h. (d) The schematic plots of the formation of the nanobats.
Figure 4. I-V curves of field emission of the Ti5Si4 nanobats grown on titanium foil. The inset is the FN plot of the emission of the nanobats.
tangential line of rapidly increasing current verse voltage curve yields a work function value of 5.6 eV. The field emission properties of the Ti5Si4 nanobats were measured on the parallel plate with a spacing of 180 µm and the contact area was 0.25 cm2 at a pressure of 1× 10-5 Torr. The emission current was recorded with the applying voltage at a step of 5 V. Figure 4 shows the current density as a function of the applied voltage. The turn-on voltage of the Ti5Si4 nanobats was assessed to be 1.47 V/µm with the current density reaching 1 µA/cm2. The ln(J/E2) - 1/E plot showed a straight line, indicating that it was consistent with the Fowler Nordheim (FN) behavior. The calculation was performed by using the simplified FN equation:
J ) (Aβ2E2/Φ) exp(-BΦ3/2/βE) where J is the current density, E is the applied electric field, and Φ is the work function. A and B are constants, corresponding to 1.56 × 10-10 (A V-2 eV) and 6.83 × 103 (V eV-3/2 µm-1), respectively. The field amplification factor β indicates the capability of field emission amplification, which depends on the crystal structure, geometry, and density of the emitting points. Utilizing the FN equation and substituting the work function of the Ti5Si4 nanobats (Φ ) 5.6 eV), the field amplification was calculate to be 1350. The β was considerably higher than or comparable to those of other 1D nanomaterials,
such as Si (β ) 1000),23 TiSi2 (β ) 501),16 NiSi2 (β ) 630),24 TaSi2 (β ) 1200-1800),25,26 and Ti5Si3 (β ) 816).27 The remarkable field emission properties of Ti5Si4 nanobats are attributed to the growth on titanium foil in addition to their intrinsic properties. Ti is a good conductor, compared to silicon, and the electron transport on the side of the emitting surface was assisted by the nil voltage across the nanobat/Ti interfaces to enhance the field emission performance.28,29 The low turnon voltage and high field enhancement factor are advantageous for application as the field emission sources of flat plate display and vacuum nanoelectronics devices. For electron transport property measurement, Pt was selected as the contact metal with a work function of 5.6 eV, which is the same as that of the Ti5Si4 (5.6 eV), and served as good ohmic contacts. The nanobats are relatively short so that the four-probe measurement was not amenable. The diameters of the nanobats were selected to be larger than the average value for the ease of positioning in the electrical measurements. As shown in the inset of Figure 5, the head, tail, and length of the measured nanobat are 90, 70, and 300 nm, respectively, with an average diameter of 80 nm. The I-V plot seen in Figure 5 indicates the presence of an ohmic contact and a resistance of about 276 Ω was obtained for the nanobat at 300 K. The resistivity of the Ti5Si4 was therefore calculated to be 348 Ωµ · cm. Other measurements yielded essentially the same value of resistivity. In thin-film reactions between Ti and silicon, only C49- and C54-TiSi2 are grown to extensive thickness.19 Conventional crystal growth of a pure Ti silicide phase has been difficult and inaccessible. As a result, the physical properties of many Ti silicide phases are not available. In the present investigation, single-crystalline Ti5Si4 nanobats have been grown for the first time. The work function, field emission, as well as electrical transport properties of Ti5Si4 nanobats were measured. The growth of the titanium silicide nanostructures on the metallic substrate is advantageous for field emission applications since the voltage drop across the substrate is nil. The excellent field emission properties for the Ti5Si4 nanobats on Ti foil may lead to important applications as field emitters in a number of optoelectronics devices. Summary and Conclusions In summary, we had successfully synthesized Ti5Si4 nanobats on titanium foil by a vapor and condensation method without catalysts. The growth mechanism of nanobats was clarified to be of self-catalytic growth. The bat-shaped morphology is attributed to the diminishing supply of the consumed Si powder source. The electrical resistivity (348 Ωµ · cm) and work function (5.6 eV) of the Ti5Si4 phase were measured for the first time.
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For field emission properties, Ti5Si4 nanobats possess a low turnon voltage of 1.47 V/µm and a field amplification factor as high as 1350. The growth of the titanium silicide nanostructures on the metallic substrate is advantageous for field emission applications since the voltage drop across the nanobat/substrate interface is nil. The excellent field emission properties of Ti5Si4 nanobats may lead to applications as emitting sources in flatpanel displays and vacuum nanoelectronics devices. Acknowledgment. The research was supported by the Republic of China National Science Council through Grant Nos. NSC 96-2221-E-007-169-MY3 and NSC 97-2120-M-007-003. References and Notes (1) Dong, L.; Bush, J.; Chirayos, V.; Solanki, R.; Jiao, J.; Ono, Y.; Conley, J. F.; Ulrich, B. D. Nano Lett. 2005, 5, 2112–2115. (2) Decker, C. A.; Solanki, R.; Freeouf, L. J.; Carruthers, J. R.; Evans, D. R. Appl. Phys. Lett. 2004, 84, 1389–1391. (3) Okino, H.; Matsuda, I.; Hobara, R.; Hosomura, Y.; Hasegawa, S.; Bennett, P. A. Appl. Phys. Lett. 2005, 86, 233108. (4) Ouyang, L.; Thrall, E. S.; Deshmukh, M. M.; Park, H. AdV. Mater. 2006, 18, 1437–1440. (5) Schmitt, A. L.; Bierman, M. J.; Schmeisser, D.; Himpsel, F. J.; Jin, S. Nano Lett. 2006, 6, 1617–1621. (6) Wang, Q.; Luo, Q.; Gu, C. Z. Nanotechnology 2007, 18, 195304– 195308. (7) Liang, S.; Islam, R.; Smith, D. J.; Bennett, P. A.; O’Brien, J. R.; Taylor, B. Appl. Phys. Lett. 2006, 88, 113111. (8) Magen, C.; Ritter, C.; Morellon, L.; Algarable, P. A.; Ibarra, M. R.; Tsokol, A. O.; Gschneidner, K. A.; Pecharsky, V. K. Phys. ReV. B 2006, 74, 174413–174420. (9) Kim, J.; Anderson, W. A. Nano Lett. 2006, 6, 1356–1359.
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