A TEM image of as-synthesized InP NWs, observed to be not only no evident aggregations but also almost straight shapes, is displayed in Fig. 4.1 (a). Without any catalyst seeds are extra seen at the tip of InP NWs. The distribution of InP nanowire diameters determined in TEM images is estimated and presented in the inset of Fig. 4.1 (a). The size distribution can be fitted with Gaussian distribution, yielding an average diameter of ~ 21.4 nm and a standard deviation of ~ 13.5 nm.
Besides, as-synthesized InP NWs possess 2 ~ 5 μm in length. Due to the large diameter and long length of as-synthesized InP NWs, quantum confinement effect
45
can be excluded in the below discussion. Fig. 4.1 (b) illustrates a typical FE-SEM image of a single InP nanowire embedded in two Ti/Au electrodes. The distance separating the two Ti/Au electrodes contacted to an InP NW is ~ 1 μm. A cartoon schematic for our two-probe InP NW devices is represented in the inset of Fig. 4.1 (a).
Figure 4.1: (a) TEM image of as-synthesized InP NWs with the size distribution and a red curve fitted according to a Gaussian function, shown in the inset. The average diameter and the standard deviation of nanowires are evaluated to be about 21.4 and 13.5 nm, respectively. (b) FE-SEM image of a single InP NW embedded in two Ti/Au electrodes. A cartoon schematic for our two-probe InP NW device is illustrated in the inset.
Fig. 4.2 (a) displays a typical temperature dependence of I-V characteristic of an InP NW device with a RT resistance about 22 MΩ. The black and light-blue curves represent the I-V behavior at 300 and 100 K, respectively. The current is
46
observed to arise with increasing voltage within the applied current range of ± 1 nA at variable temperature from 300 to 100 K. The I-V curve at room temperature reveals a linear and symmetric relationship that is also noticed in all of our InP NW devices, even though they have a striking variance in RT resistances. By analyzing the I-V characteristics of InP NW devices, the resistance at a given temperature can be determined from the regime around the zero-bias voltage. Fig. 4.2 (b) illustrates temperature dependent resistances of InP NW devices (NW-1 ~ NW-6). Although NWs fabricated nanodevices are taken out from the same source sample, their RT resistances, 4662, 931, 650, 530, 321 and 87 MΩ for NW-1, NW-2, NW-3, NW-4, NW-5 and NW-6 nanodevices, alter fairly enormous. It is suggested that the difference of RT resistances, which is up to two orders of magnitude, comes from the unavoidable contribution of contact resistance in two-probe configuration nanodevices [9]. Thermally activated transport (solid lines) can be used well in fitting the data at higher temperature regime, whereas the data at lower temperature regime follow the Mott-VRH transport theory (dashed lines), as depicted in Fig. 4.2 (b). The data at lower temperatures gradually deviate from the thermally activated transport to Mott-VRH conduction, implying that carrier transport is progressively govern by electron hopping through the disorder system, such as a deteriorated interface formed between the Ti/Au electrode and NW. The transition temperature as a function of RT resistance is plotted in the inset of Fig. 4.2 (b). The contact resistance and InP NW resistance have to be comparable and they will donate to the total resistance of the two-probe NW devices. We conclude that the higher RT resistance of InP NW devices is, the more contact resistance dominates. The detail explanation of temperature dependent behaviors has been demonstrated elsewhere [11]. Since the data at higher temperature mainly are described to follow the thermally activated transport, the activated energy in InP NWs could be extracted
47
from the slope of the plot of temperature dependent resistance to be about ~ 50 meV, ascribed possibly to the phosphorus deficiency related defect [12, 13].
Furthermore, the carrier concentration is also evaluated to be ~ 1015 cm-3 in the InP NW-dominated devices with the lowest RT resistance (NW-6). Temperature dependent resistances in InP NW devices facilitate us to distinguish both the contact and NW-dominated devices. Additionally, such a lower concentration in semiconductor materials gives rise to a promising perspective to develop nanoscale optoelectronics.
48
Figure 4.2: (a) I-V curves of a typical InP NW device with a RT resistance of ~ 22 MΩ. (b) Resistance as a function of inverse temperature for two-probe InP NW devices. The solid and dashed lines delineate the best fits to the mathematical equations of thermally activated transport and Mott-VRH, respectively. The inset shows the transition temperature as a function of RT resistance for InP NW devices.
To extend the result in determination of the contact- or NW-dominated devices and to understand further the impact of nanocontact on optoelectronic devices, the real-time response of InP NW devices to either light (green-light laser) or gas (oxygen) exposures have been studied. Fig. 4.3 (a) exhibits that the contact- or NW-dominated devices unveil distinct sensitive response to a green-light laser exposure at room temperature in an open air environment. The sensitivity is calculated in terms of the ratio of R / R0, where R0 and R denote the NW device
49
resistances before and after exposure, respectively. The R0’s of the contact-dominated device is 4.6 GΩ, whereas it is only 87 MΩ for the NW-dominated device. It is manifestly found that the contact-dominated device always exhibits a much higher ratio in comparison with the NW-dominated device.
The data in Fig. 4.3 (a) delineate that almost recoverable curve in either contact or NW-dominated devices after the light exposure is removed, indicating that photo response is significantly larger than any thermionic or thermoelectric effects that might arise from a light-induced temperature increase. On the other hand, the response ratio, ΔR R/ 0 = R R− 0 /R0 , as a function of RT resistance for our two-probe InP NW devices is given in Fig. 4.3 (b). Interestingly, an unambiguous trend of a rising response ratio, ΔR / R0, with the increase of RT resistances for InP NW devices has been monitored. The response ratio of NW-dominated devices with a RT resistance lower than 102 MΩ is only 10 ~ 20 %, whereas that of contact-dominated devices with a RT resistance close to 104 MΩ almost gets up to 50 % addition in response to the light exposure. Actually, for the photo response, it is well-known that a variation of resistances under light illumination is mainly due to the increase in the number of carriers in semiconductors. For example, in a direct bandgap semiconductor, such as InP, the photo energy absorption is a process that requires no any assistance from lattice vibrations. As the photo energy is absorbed, free electron-hole pairs (EHPs) will be generated in the inner of semiconductor materials. As an electric field applies to semiconductors, these EHPs generated will then be separated and accelerated in opposite directions to contribute carriers and diminish resistances. We conjectured that, in contact-dominated devices, most of carriers are limited not to pass through a barrier existing in metal-semiconductor interface. However, while a light with efficient energy exposes to contact-dominated devices, extra carriers with higher energy will be generated by
50
the incident light and then reduce the barrier height. The distorted barrier permits more tunneling carriers across so that an enhanced photo response ratio can be observed. This observation in contact-dominated devices is consistent with previous reports where a higher photo response has been demonstrated due to a modified Schottky barrier [14-16].
Figure 4.3: (a) The sensitivity of light exposure for contact and NW-dominated two-probe InP NW devices. (b) The response ratio (ΔR / R0) as a function of RT resistance for two-probe InP NW devices.
Fig. 4.4 (a) exhibits that the contact- or NW-dominated devices unveil distinct response to oxygen exposure at room temperature. The R0’s of the contact- and NW-dominated devices are 692 and 28 MΩ, respectively. It is surprising that the contact-dominated device also illustrates a much higher ratio of resistance changes, showing a marked contrast to the NW-dominated device. Unlike the light exposure to InP NW devices, the sensitivity with increasing time of oxygen exposure reflects
51
a gradually elevated behavior and the response time (i.e. the rise and decay time) of oxygen exposure is very blunt to be larger than 12 h. Indeed, the principle of charge transfer between a nanostructure and its surface-adsorbed species has been unfolded to explain the response behaviors in various gas sensors. As InP NW is exposed to oxygen gas, oxygen gas will adsorb on the surface, leading to a formation of surface species, such as In2O3 or P2O5 [17]. The adsorbed species on InP NW surface act as electron acceptors generating an electron-depleted surface layer. As a result, the resistance increases. This discovery is for the first time to observe the oxygen response in InP NW. Meanwhile, the response ratio as a function of RT resistance for two-probe InP NW devices is also given in Fig. 4.4 (b). Since the separation distance between two electrodes was kept a constant and all NWs fabricated devices had a uniform diameter, the area of NW devices to expose oxygen gas is always the same so that the number of adsorbed oxygen on the surface of InP NW devices is the same. With the increase of RT resistances for InP NW devices, the contact resistance gradually governs the carrier transport, indicating an equivalent barrier forms between metal-semiconductor interfaces and the resistances progressively increase. Hence, it is unearthed clearly the contact-dominated devices display a higher response ratio, as shown in Fig. 4.4 (b).
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Figure 4.4: (a) The sensitivity of oxygen exposure for contact- and NW-dominated two-probe InP NW devices. (b) The response ratio (ΔR / R0) as a function of RT resistance for two-probe InP NW devices.
References
[1] L. Pavesi, F. Piazza, A. Rudra, J. F. Carlin and M. Ilegems, Phys. Rev. B 44, 9052 (1991)
[2] S. M. Sze, K. K. Ng, Physics of Semiconductor Devices; John Wiley & Sons:
Edison, New York (2005)
[3] M. S. Gudiksen, J. Wang and C. M. Lieber, J. Phys. Chem. B 105, 4062 (2001) [4] J. Wang, M. S. Gudiksen, X. Duan, Y. Cui and C. M. Lieber, Science 293, 1455 (2001)
53
[5] X. Duan, Y. Huang, Y. Cui, J. Wang and C. M. Lieber, Nature 409, 66 (2001) [6] C. Thelander, H. A. Nilsson, L. E. Jensen and L. Samuelson, Nano Lett. 5, 635 (2005)
[7] T. Strupeit, C. Klinke, A. Kornowski and H. Weller, ACS Nano 3, 668 (2009) [8] M. D. Moreira, P. Venezuela and T. M. Schmidt, Nanotechnology 19, 065203 (2008)
[9] Y. F. Lin and W. B. Jian, Nano Lett. 8, 3146 (2008)
[10] Z. Liu, K. Sun, W. B. Jian, D. Xu, Y. F. Lin and J. Fang, Chem. Eur. J. 15, 4546 (2009)
[11] Y. F. Lin, T. H. Chen, C. H. Chang, Y. W. Chang, Y. C. Chiu, H. C. Hung, J. J.
Kai, Z. Liu, J. Fang and W. B. Jian, Phys. Chem. Chem. Phys. accepted (2010) [12] A. P. Seitsonen, R. Virkkunen, M. J. Puska, R. M. Nieminen, Phys. Rev. B 49, 5253 (1994)
[13] G. Marrakchi, K. Cherkaoui, A. Karoui, G. Hirt and G. Müller, J. Appl. Phys.
79, 6947 (1996)
[14] R. R. Mehta and B. S. Sharma, J. Appl. Phys. 44, 325 (1973)
[15] N. Matsuo, H. Ohno and H. Hasegawa, Jpn. J. Appl. Phys., Part 2 23, L299 (1984)
[16] O. Katz, V. Garber, B. Meyler, G. Bahir and J. Salzman, Appl. Phys. Lett. 79, 1417 (2001)
54
[17] G. Hollinger, E. Bergignat, J. Joseph and Y. Robach, J. Vac. Sci. Technol. A 3.
2082 (1985)
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Chapter 5
Nano Approach Investigation of
Conduction Mechanism of Polyaniline Nanofibers
5.1 Introduction
Similar to the revolutionary miniaturization in electronic industry began in 1950s, flexible electronics might also change the world in the near future [1, 2]. Organic conducting material is a promising material to offer opportunities in developing flexible electronics. A great amount of theoretical and experimental works in organic conducting material were reported when the first highly doped conjugated polymer, polyacetylene, was discovered to manifest a dramatic decrease in its resistivity via halogen treatment in 1977 [3]. Among all the conducting polymers, Polyaniline (PANI) is one of the potential synthetic metals due to its low-cost synthesis, fast processability, and environmental stability in either the doped conducting or the de-doped insulting form. The applications of PANI, such as chemical sensors [4], field effect transistor [5], and memory devices [6], have been
56
demonstrated as well. Very recently, PANI nanostructures have been synthesized by using the template-free or template synthesis so as to raise the surface/volume ratio and to enhance the sensitivity in comparison with that of their bulk [4, 7]. Despite the abundant achievements in applied aspects, there are still a lot of controversial issues in the fundamental conduction mechanism of the conducting PANI for a long time. In this study, PANI nanofibers were synthesized by using polymerization method and their electrical transport was carried out.
5.2 Experimental Method
Polyaniline nanofibers were synthesized by using a rapidly mixed reaction [8]. A description of the synthesis method is given as follows. A solution of ammonium persulfate (0.183g, 0.8 mmol, >99%, Sigma-Aldrich Ltd.) dissolved in 10 mL of 1M HCl was carefully poured in a solution of aniline (0.298 g, 3.2 mmol, >99%, Sigma-Aldrich Ltd.) dissolved in 10 mL of 1M HCl. The mixed solution was stirred immediately at room temperature. The polymerization began and the mixed solution had turned to deep green color. This solution was stirred continuously for 24 hours. To store polymer solution, 0.5 mL of this solution was fetched and diluted with distilled water of 10 mL. Polyaniline nanofibers were stocked in the water solution. The schematic diagram for the synthesis of PANI nanofibers was illustrated in Fig. 5.1. Dimensions and morphology of polyaniline nanofibers were all recorded by using high-emission scanning electron microscopy (FE-SEM, JELO JSM-7000F).
Micrometer-scale Ti/Au (~10/60 nm thickness) electrodes were made by means of conventional photolithography on Si substrates, which were capped with a 400-nm thick SiO2 layer, in order to prevent from any possible contribution of current leakage through the substrate. The as-synthesized polyaniline solution was
57
protonated (deprotonated) to doped (dedoped) form by washing it with 0.25M HCl (0.2M NH3‧H2O) solution. Several drops of the doped, as-synthesized, and dedoped polyaniline solutions were deposited onto the pre-patterned Si substrate and then were dried at room temperature in air to get thin-film devices. On the other hand, the standard electron-beam lithography technique was adopted to define two submicrometer-scale Ti/Au (~20/100 nm thickness) current leads with a gap of 150-300 nm in width between them, connecting to the pre-patterned micrometer-scale electrodes of Si substrates. Before polyaniline nanofibers were positioned into the gap, the resistance of the gap at room temperature was inspected to be an order of magnitude much higher than 1 TΩ. The high resistance guaranteed that a good insulation existed between two current leads so as to detect the intrinsic electrical properties of the polyaniline nanofibers after positioned into the gap. Several drops of the as-synthesized polyaniline solutions were introduced to put on the gap substrates and a dielectrophoresis technique with a sinusoidal wave at a frequency of 1 MHz was applied across the two current leads for 3 min to move and position the nanofibers into the gap. In the dielectrophoresis procedure, the alternating voltage was selected in the range of 3 to 6 V, depending on the separation distance of the gaps. Besides, a series capacitor of 10 μF was placed in the equivalent circuit to filter out any direct current component and to avoid electrochemical reactions. Subsequently, the drop of solution was gently blown off the substrate with a stream of dry nitrogen gas. The as-fabricated nanofiber devices were exposed to electron beam with a dose of 3 × 104 C/m2 s for 1 h, especially at the contact area between the current lead and the nanofiber, to reduce the device resistance. The two respective procedures to fabricate PANI nanofiber thin-film and nanoscale devices are revealed briefly in Fig. 5.2.
All of the thin-film and nanofiber devices were located in a cryostat (Variable
58
Temperature Insert Cryostat, CRYO Industries of America Inc.) with helium gas (>99.99 %) at a pressure of 760 Torr for acquiring temperature dependence of two-probe electrical properties. The current-voltage characteristics were performed by using an electrometer with a current resolution of 10 pA and a voltage resolution of 1 mV. The standard deviation of the estimated resistance around the zero bias voltage at a given temperature was less than 0.1 %. The transport properties were determined at temperatures ranging from 300 down to 80 K.
Figure 5.1: A schematic illustration of polyaniline nanofibers synthesis in a rapidly-mixed reaction.
59
Figure 5.2: Schematic diagrams outlining the fabrication procedure of (a) polyaniline nanofiber thin-film device and (b) nanoscale nanodevices.
5.3 Results and Discussion
The as-synthesized PANI nanofibers, as displayed in Fig. 5.3 (a), are unfolded entangled appearances. The length of PANI nanofibers is observed to be at most in the range of several hundreds of nanometers. The distribution of PANI nanofiber diameters determined in FE-SEM image is estimated and presented in the inset of Fig. 5.3 (a). The size distribution can be fitted with Gaussian distribution, yielding an average diameter of ~ 45.0 nm and a standard deviation of ~ 19.3 nm. The somewhat large standard deviation in our nanofiber diameters may arise from the secondary growth, which causes the over-polymerized reaction of nanofibers. So as to comprehend and investigate the intrinsic conduction mechanism in PANI nanofibers, the dedoped, as-synthesized and doped nanofiber thin-film devices are first fabricated for comparisons. The I-V curves and temperature dependent
60
resistance of dedoped, as-synthesized and doped nanofiber thin films are illustrated in Fig. 5.3 (b) and (c), respectively. The I-V curves at room temperature for all thin-film devices reveal linear in the measurement range. As seen in Fig. 5.3 (b), PANI nanofiber thin film, doped with HCl acid, responds with a higher resistance decrease of up to ~ 2 orders of magnitude. It has been well-known that the conducting difference in PAIN thin films comes from the degree of protonation [9].
As for the corresponding temperature behaviors, as delineated in Fig. 5.3 (c), in both as-synthesized and dedoped (doped) thin films the resistance as a function of the square root of the inverse temperature appears straight manifestly. The slope of temperature-dependent resistance increases with the increasing of room-temperature resistance of PANI nanofiber thin films. Unlike the inorganic nanostructure such as ZnO nanowires [10] which can be detached from the source sample to disperse onto the substrate, PANI nanofibers cannot be placed arbitrarily on an insulating substrate for elucidating their electrical properties. By utilizing DEP technique, the nanofibers are positioned onto the top of two pre-patterned Ti/Au electrode by electron-beam lithography, as shown in Fig. 5.3 (d). As a result of the shrinking contact area in nanofiber devices, the contact resistance may contribute extremely to the total resistance of two-probe system [11]. Fig. 5.3 (e) depicts the I-V curves of a PANI nanofiber device under electron-beam exposure at room temperature. After several times of the electron-beam exposure to the contact area of the PANI nanofiber device, the current increases by at least three orders of magnitude and the nonlinear I-V curve gradually is converted to a linear line, indicating an ohmic contact forms and contact resistance can be neglected. The room-temperature resistance of the PANI nanofiber device as a function of electron-beam exposing time is shown in Fig. 5.3 (f). It is emphasized that all of PANI nanofiber devices, discussed in the below paragraphs, are exposed for 1 h.
61
Figure 5.3: (a) FE-SEM image of as-synthesized polyaniline nanofibers with the corresponding size distribution and a red curve fitted according to a Gaussian function, shown in the inset. The average diameter and the standard deviation of nanofibers are evaluated to be about 45.0 and 19.3 nm, respectively. (b) I-V behaviors and (c) R-T of dedoped, as-synthesized, and doped nanofiber thin films.
(d) SEM image of a polyaniline nanofiber device. (e) The change of I-V curves of a polyaniline nanofiber device at room temperature after electron-beam exposure. (f) The room-temperature resistance of the nanoscale devices as a function of the electron-beam exposing time.
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Resistances as a function of inverse temperature for a series of two-probe PANI nanofiber thin-film devices (L01 and L02) and nanofiber devices (S01-S06), as revealed in Fig. 5.4 (a), have been well-described to follow a hopping transport form for carriers
R R= 0exp
( (
T T0/)
1/p)
(5.1) , where R and T are resistance and temperature, T0 is a characteristic temperature, R0 is a weak temperature-dependent constant, and p is the exponent parameter. The solid lines give the best fitting to data in Fig. 5.4 (a) to derive the exponent parameter, p, shown in the inset. The average value and standard deviation of the exponent parameter p’s are 2.08 and 0.275, respectively. The average of T0obtained for present nanofiber devices is drawn out to be about 41874 K as well. In addition, it should be noticed that with the variation of the geometric structure from PANI nanofiber thin-film to nanofiber devices, room-temperature resistance increases by 4 orders of magnitude. To our best knowledge, we surmise assertively
obtained for present nanofiber devices is drawn out to be about 41874 K as well. In addition, it should be noticed that with the variation of the geometric structure from PANI nanofiber thin-film to nanofiber devices, room-temperature resistance increases by 4 orders of magnitude. To our best knowledge, we surmise assertively