페이지_ 18권 3호 전체-16-24.pdf1.02MB

• 1. Introduction
• 2. Experiments
• 2.1. Materials
• 2.2. Preparation of DNA surfactant complex
• 2.3. Preparation of DNA and DNA surfactant complex solution and film
• 2.4. The preparation of OTFT memory
• 3. Results and discussion
• 3.1. UV absorption spectra of the DNA complex
• 3.2. CD spectra of the DNA complex
• 3.3. I-V properties of the DNA complex film
• 3.4. AFM of the DNA complex film
• 3.5. Transfer characteristics of the BiOTFTs
• 4. Conclusions
• Acknowledgement

1. Introduction

In recent years, the biopolymers have attracted attention for their possible application for photonic and electronic materials because of the highly ordered structure and unique properties. Furthermore, natural biomaterials are a renewable resource and are inherently biodegradable, which would open up widely application in large area as the environmentally-friendly materials^[1-4]. For instance, organic field effect transistors, conductive, optical amplifiers^[5], semiconductive nanowires^[6], and nonlinear optical electro-optic modulators have been fabricated from DNA-based biopolymers. Such devices have demonstrated high performance that exceeds that of state-of-the-art devices made with currently available organic based materials^[7-11]. In our study, we have already reported the organic light-emitting diodes fabricated with DNA as a template of charge transport materials and light emitting materials^{[12, 13]}. 

The natured and purified DNA was soluble only in water, the resulting film are too water sensitive and have insufficient mechanical strength. Further, DNA is cationic polyelectrolyte which normally contains sodium ions as counter cations. We already reported the BiOTFTmemory device prepared by natural DNA as the gate dielectric shows poor device performance such as the high OFF current because the existence of the sodium ions probably contributed ion conduction. Okahata et al reported preparation of DNA-cetyltrimethylammonium, CTMA complex which was soluble in organic solvent and was effective to reduce the mobility of counter ions^[14]. After this novel finding, in order to exclude the influence of the sodium ions of DNA and to prepare the high quality thin films, researchers utilized the DNA surfactant complex for the application of electronic devices including OTFT. We have also reported the improvement of BiOTFT memory properties by excluding movable ions such as sodium cations with Lau surfactants^[15]. 

In this study, in order to investigate the influence of surfactant structure on BiOTFT device performance, the physicochemical properties of various DNA surfactants complex in solution and film state have been studied. Further, BiOTFT device performance such as transfer property has also been examined and the possible explanation of the device performance has been discussed. 

2. Experiments

2.1. Materials

The sodium salts of DNA (bp = ca. 100 and 10000) were provided by Prof. Ogata and Nippon Chemical Feed Co.,Ltd. Pentacene(98% purity) were bought from Naad Co., Ltd. Lauroylcholine chloride(Lau), OTMA(98% purity), CTMA(98% purity) and butanol were purchased from Tokyo Chemical Industry Co., Ltd. 

2.2. Preparation of DNA surfactant complex

DNA–Lau was prepared by adding 10 mmol/L of DNA (concentration of the phosphate group) aqueous solution into 10 mmol/L of the lauroylcholine chloride aqueous solution, and then the precipitate were filtered and thoroughly washed with ultrapure water and then dried in vacuo at 50 ℃ for at least 24 hours. A write powder was obtained as a yield of 98% and also DNA-OTMA, DNA-CTMA wereprepared through the same preparation process with the same yield. 

2.3. Preparation of DNA and DNA surfactant complex solution and film

The sodium salts of DNA were dissolved in ultrapure deionized water with the concentration equals to 100 mmol/L. The 100 mmol/L DNA-Lau, 100 mmol/L DNA-OTMA or 100 mmol/L DNA-CTMA complexes was dissolved in butanol. The solution thus obtained was spin-coated on ITO glass substrates, and the films were dried for at least 12 h in vacuo. The thicknesses of the DNA film was about 6 μ m and the DNA surfactant complex films were approximately 2 μm respectively.

2.4. The preparation of OTFT memory

OTFT memory devices were fabricated by depositing a pentacene layer (film thickness =50 nm) as an active layer at a pressure of 2×10-3 Pa and an evaporation rate of 0.2–0.4Å s-1 on the ITO/DNA or ITO/DNA complex film. Au as the source and drain electrodes (W/L=5 mm /20 μm) was deposited by vacuum evaporation on this pentacene film. The chemical structure of Lau and OTMA, CTMA serving as surfactant to prepare the DNA based gate dielectric are shown in Figure 1 (a), (b), (c) respectively. BiOTFT structure using a top contact and gate bottom geometry is schematically depicted in Figure 1(d). 

Figure 1. Chemical structure of (a) Lau. (b) OTMA. (c) CTMA. (d) Schematic of the top contact and bottom gate BiOTFT memory

3. Results and discussion

3.1. UV absorption spectra of the DNA complex

The UV absorption of DNA is an important property for determining whether π-πstacking nucleobases occurs or not. The DNA double helix in aqueous solution has a specific absorption band from 220 nm to 300 nm with a λmax at 260 nm^[16]. In order to compare the effect of different molecular weight of DNA with different cationic surfactant on the structural regularity, the UV-visible spectra have been measured.

The lower part in Figure 2 shows the UV-visible spectra of DNA-Lau, DNA-OTMA, DNA-CTMA in butanol solutions. It was clear that both the high (10 kbps) and low (100 bps) molecular length with different DNA complex have the same maximum absorption wavelength at 260 nm, which was assigned as the characteristic absorption of the nucleobases in DNA. And the molecular length had no relationship with the UV spectral. The π-πstacking of nucleobases of the DNA did not change when the molecular weight altered, indicating that both the high and low molecule DNA retained the double helix structure after the ion exchange reaction in the butanol solution. 

Figure 2. UV-vis (lower) and CD spectra (upper) of DNA complex solution with (a) high molecular weight and (b) low molecular weight DNA

3.2. CD spectra of the DNA complex

CD analysis is one of the most useful techniques for probing the conformation of DNA complex in many kinds of solutions, in gels, films as well as fibers [17]. In order to compare the different molecular weight of DNA with different surfactant and to analyze its effect on the double helix structure, both the solution and the film have been investigated. The upper part in Figure 2 and Figure 3 show the CD spectra of the solution and film state. All of the complexes either in butanol solution or in the film state showed positive and negative CD signals. In the case of the solution, a positive Cotton effect at about 280 nm and a negative Cotton effect at about 225 nm and 245 nm have been observed, which is similar in shape to natural DNA. Meanwhile, the A form of the DNA complexes in butanol solution appeared to be transformed to the B form in the film state. In addition, with the increase of the alkyl chains, the CD signal decreased, indicating that the decrease of structure regularity, it would be because the hydrophobic properties of the alkyl chain and steric hindrance between the long alkyl chains. 

Figure 3. UV-vis (lower) and CD spectra (upper) of DNA complex film with low molecular weight DNA

3.3. I-V properties of the DNA complex film

It is known that the resistivity of the DNA complex plays an important role in the property of the BiOTFT memory when using as the insulator layer. Therefore, it is necessary to evaluate the resistivity of the DNA complex. For comparison, the I-V characteristics of DNA alone with different chain length are also shown in Figure 4. 

Figure 4. I-V characteristics of ITO/high and low DNA /Au cells

From Figure 4 and Figure 5, they show that the resistivity of DNA complex increased significantly in comparison with DNA alone with either high or low chain length. It would be because the DNA which is one of the poly-anion could be interacted with cationic surfactant through the ion exchange reaction, leading to the decrease of the movable ions such as sodium ions and to the improvement of the resistivity. 

Figure 5. I-V characteristics of ITO/high (a) and low (b) DNA complex/Au cells

In addition, the Figure 5 also shows that with the increasing of the alkyl chains, the resistivity also increased, it was because the longer alkyl chains provided the higher proportion of the insulative alkyl chains in the DNA complex film, which would improve the insulating property. Besides, compared to the high molecular weight DNA, the low molecular weight DNA showed the better resistivity when the same surfactants were applied. This was due to the decrease of the carrier conductive pathway along the DNA chains when the length of DNA chain was short. In conclusion, the kinds of the surfactant and the chain length of the DNA have great effect on the resistivity of the film. The resistivity of the low molecular DNA-Lau, DNA-CTMA and DNA-OTMA equals to 1.41×10¹³ Ω∙cm, 8.2×10¹³ Ω∙cm and 1.51×10¹⁴ Ω∙cm, respectively. It is means that it is reasonable to use them as the dielectric material in the OTFT memory.

3.4. AFM of the DNA complex film

In general, surface roughness and structure of the thin film play a crucial role for use of any insulating dielectric film for OTFT memory. This stems from the fact that the charge transport takes place at the interface between dielectric and semiconductor film^[18]. 

Figure 6 shows the 2 μm×2 μm surface morphology of different DNA complex with high and low molecular weight. The RMS of the high molecular weight DNA complex were estimated to be 0.8～1.0 nm. In contrast, the RMS of low molecular weight was lower than that which equals about 0.3 nm. In addition, in comparison with the morphology of low molecular weight DNA complex, some bump structure was found in the longer DNA complex. It is possibly because the longer chain length would lead to the entanglement of the DNA complex. Therefore, the smoothness and the film formability of the shorter DNA were higher than that of longer DNA complex extremely. This indicates that low molecular weight DNA looks more favorable in the application of the dielectric layer of the OTFT memory.

Figure 6. AFM topographical images of (a)(d)DNA-Lau film. (b)(e)DNA-CTMA film. (c)(f)DNA-OTMA film

3.5. Transfer characteristics of the BiOTFTs

Figure 7 presents the transfer characteristics which represent the transistor current I_ds poltted as a function of the gate voltage at constant V^d of the BiOTFTs fabricated by using the DNA-OTMA, DNA-CTMA and the DNA–Lau film as well as DNA film alone as a gate dielectric layer respectively.

Figure 7. Transfer characteristics of BiOTFTs with (a) DNA alone and (b) DNA complex

In these all BiOTFTs, the hysteresis behaviors were observed, indicating that both the DNA film and the DNA complex film work as a memory layer. Because of the poor resistivity of thefilm with DNA alone, the OFF current which equals to 3.8×10^-7 Awas too high to make application in the OTFT memory as the insulator. In contrast, when the DNA complex was applied as the dielectric layer, the OFF current decreased remarkably which equals about 10^-9 A because of the improvement of the resistivity. At the same time, the hysteresis of the BiOTFT memory with the DNA complex was significantly increased, and the ON/OFF current ratio at V_G = 0 V was as high as 10⁴. Moreover, compared to DNA-OTMA and DNA-CTMA, when the DNA-Lau was used as the dielectric layer,the higher ON current and the restrain of the OFF current were also observed. This improvement of device performance might be effected by varying the composition and structure of DNA complex, because the roughness of the DNA-Lau film was the lowest and the DNA-Lau has the best structure regularity when we compare it to the DNA-CTMA and DNA-OTMA complex film. The appearance of the hysteresis in DNA based BiOTFT was thought to be due to the accumulation of mobile ions at the interface. However, the larger hysteresis was obtained in DNA complex, indicating that the accumulation of mobile ions is not dominant factor but the ferroelectricity of DNA complex and/or the interface is thought to be a possible explanation. The work is now underway to give the better understanding for this memory performance. 

4. Conclusions

DNA film and various DNA complex films were used as the gate dielectric materials of the BiOTFT memories. In order to exclude the influence of the sodium ions and other impurities, it is necessary to make the DNA complex with cationic surfactant through the ion exchange reaction. At the same time, the molecular weight of DNA also plays an important role on the resistivity property and the morphology of the DNA complex film. And in this study the most favorable surfactant for preparing the Bio-organic thin film transistor is the DNA-Lau through comparing with DNA-OTMA and DNA-CTMA. The ON current and the restrain to the OFF current have been improved extensively by using DNA-Lau as the gate dielectric. 

Acknowledgement

This work was partly supported by a Grant-in-Aid for Scientific Research (No. 20350085) from MEXT, Japan and by the Futaba Electronics Memorial Research Foundation.