Hybrid DNA materials for energy storage R. A. Norwood*a, J. Thomasa, N. Peyghambarian a, J. Wang b, L. Li b, F. Ouchen c, and J. E. Grote c

aCollege of Optical Sciences, University of Arizona, 1630 E. University Blvd., Tucson, AZ 85721; bTIPD, LLC, Tucson, AZ

cAir Force Research Laboratory, Wright-Patterson Air Force Base, Dayton, OH

ABSTRACT We investigate the dielectric and electrical properties of sol-gel/DNA-CTMA blends, with particular interest in capacitor applications in energy storage. Methacryloyloxypropyltrimethoxysilane (MAPTMS) was the sol-gel precursor, and DNA-CTMA was blended in to the resulting sol-gel at various weight percentages. The blends were tested for their dielectric properties and dielectric breakdown strength; the 5% DNA blend was found to be optimal with a dielectric constant in the range of 7.5, while the breakdown strength was greater than 800 V/μm for 1 μm films and about 500 V/μm for 5μm films. Hybrid sol-gel/DNA-CTMA/barium titanate nanoparticle composites were also formulated and their dielectric properties measured. While a high dielectric constant was achieved (38), this came at the expense of a significantly reduced breakdown voltage (160V/μm). We discuss these results as well as other aspects of the dielectric and electrical properties of these blends.

Keywords: sol-gel, DNA, capacitor, dielectric breakdown, nanoparticle composites, energy storage, polymer blends

1. BACKGROUND There has recently been a surge of activity towards improving capacitor technology [1,2], including work

on so-called supercapacitors [3] and nanoparticle composites [4]. Much of this interest is based upon the development of more compact energy storage devices for applications such as memory backup and handheld electronics. At the same time thin film capacitors also find use as building blocks in compact electronic devices, especially if the capacitor technology is easily integrated with other electronics technologies. The field of organic electronics is actively seeking the availability of high power density, long cycle life capacitor technology that is compatible with organic electronic high-volume manufacturing processes, which is expected to ultimately pervade a wide range of consumer applications. This technology is further expected to impact an array of avionics needs ranging from providing energy storage for elements such as solid-state converters, filters, heat sinks and real loads, to providing materials with truly exceptional dielectric properties that can be used for a variety of specialized applications.

Polymer dielectrics have found wide spread use and include materials such as polystyrene, polycarbonate, and biaxially oriented polypropylene, which has a modest dielectric constant of 2.5, but high dielectric strength (~ 200 V μm-1) [5]. Most approaches to increasing the dielectric constant, such as nanoparticle doping, often come at the expense of processability, dielectric loss and dielectric strength. Recent efforts in the nanoparticle composite area have succeeded in combining barium titanate nanoparticles with high dielectric strength polymers [6]. However, these systems still show a reduction in the dielectric strength with nanoparticle loading, and since the energy storage is proportional to the square of the dielectric strength, the result can be zero-sum.

Organically modified sol-gels and DNA-CTMA are both classes of materials that have been considered for a variety of other applications in electronics and photonics. We have shown optimized sol-gels to be practically ideal claddings for EO polymer modulators, benefiting from their high conductivity at typical poling temperatures (~ 150°C), low dielectric constant and dielectric loss at ordinary operating temperatures (0 - 80°C), tunable refractive indices, low optical losses, and low-temperature processing using wet etching

Invited Paper

Nanobiosystems: Processing, Characterization, and Applications III, edited by Norihisa Kobayashi, Fahima Ouchen, Ileana Rau, Proc. of SPIE Vol. 7765, 77650H

© 2010 SPIE · CCC code: 0277-786X/10/$18 · doi: 10.1117/12.862412

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[7]. DNA-CTMA has shown a variety of intriguing properties and has been incorporated in functional devices [8]. With this in mind, we have been investigating blends of DNA-CTMA and sol-gel materials with goal of realizing a material with a moderate dielectric constant (6-8) but exceptional dielectric strength ( > 1 kV/μm), and very low leakage current. This paper discusses our recent progress in this area as well as new efforts to incorporate barium titanate nanoparticles into sol-gel/DNA-CTMA blends.

2. HYBRID DNA/SOL-GEL CAPACITORS We have previously demonstrated sol-gel/DNA-CTMA 95/5 (wt/wt) composites with high dielectric

constant (7.5) and breakdown strength (~1000V/μm). To further optimize the composites, we have investigated a new series with the sol-gel/DNA-CTMA ratios changing from the previous 95/5 (wt/wt) to a much wider range, i.e., 80/20, 67/33, 50/50, 33/67, 20/80 and 10/90. These were the volumetric ratios between the MAPTMS/ZPO (95/5) sol-gel and a 10% DNA-CTMA solution in 1:2 1-methoxy-2-propanol/1-butanol mixed solvents. These samples were labeled as SD1(80/20), SD2(67/33), SD3(50/50), SD4(33/67), SD5(20/80), and SD6(10/90). They were all prepared with the same basic procedure and all samples were clear and transparent when initially mixed. However, the SD1 and SD2 samples became opaque on the second day, as was observed with the 95/5 mixture previously. The solutions with higher DNA-CTMA concentrations, i.e., the SD4, SD5, and SD6, remained transparent even after one week. The opaqueness of the solutions was observed to come from a small amount of white precipitate suspended in solution. This precipitate could be larger species resulting from sol-gel oligomers chemically attaching to DNA molecules.

The series of sol-gel/DNA-CTMA thin films were spin coated on indium tin oxide (ITO)/glass substrates, and a thin layer of gold was coated on top of the composite film to define the sample area and serve as the anode. The samples were then baked at 120oC for 5 hours. The capacitance and resistance were measured at room temperature using a Stanford Research LCR Meter SR720. The dielectric constant was found by using the parallel plate capacitor formula

C = εrε0A/d (1)

where C is the capacitance, εr the relative permittivity (dielectric constant), ε0 the permittivity of free space, A the area, and d the separation between the plates. The dielectric constants at 1 kHz are plotted versus the sample area in Figure 1. It is further confirmed that the dielectric constants of the sol-gel/DNA-CTMA blend are in the range of 7-8, falling right between the known dielectric constant of the sol-gel (∼6) and DNA (8-9).

Measurements of the dielectric breakdown strength showed a clear trend, with breakdown strength decreasing with increasing DNA-CTMA concentration, thus we have focused our efforts on the sol-gel(95)/DNA-CTMA(5) composite. One of the major advantages of this composite is its exceptional breakdown strength. Dielectric strengths as high as 800V/μm were measured for sol-gel/DNA-CTMA thin films with thicknesses of 1 μm or below. One 0.82 μm sol-gel/DNA-CTMA film broke down at an applied voltage of 700 V - corresponding to a breakdown strength of 854 V/μm. This sample was made of low molecular weight DNA, and had a dual-layer structure achieved by a double spin coating process. Another sample, made of high molecular weight DNA, and had a single layer thickness of 1.10 μm, and broke down at an applied voltage of 850 V - corresponding to breakdown strength of 772 V/μm. Thinner samples have the advantage of being more internally uniform and containing fewer entrained pinholes, which results in improved dielectric breakdown strength; pinholes are more prone to being formed inside thicker films.

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1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.05.0

5.5

6.0

6.5

7.0

7.5

8.0

8.5

9.0

Die

lect

ric c

onst

ant

Sampling area: μm2

pure DNA; pure sol-gel SD6; SD5; SD3

Figure 1. Dielectric constants (1kHz) of sol-gel/DNA-CTMA films on ITO/glass substrate

However, from a practical point of view, it is preferable that the thin film capacitors be capable of surviving a much higher voltage, e.g., several kilovolts, as is required by potential high voltage applications. In an attempt to improve the dielectric strength performance of thicker films, we have fabricated a series of thicker sol-gel/DNA-CTMA samples that are as thick as 5 μm, which are made with high molecular weight DNA. To get thicker films, the methanol generated in a 95/5 MAPTMS/ZPO sol-gel was removed under vacuum at room temperature. The resulting higher concentration sol-gel was mixed with a 5wt % DNA-CTMA solution in a 1:1 weight ratio.

The measured dielectric constants of these thicker films at 1.0 KHz are shown in Figure 2. As expected, the measured dielectric constants fall in the range from 6 to 8.5. The breakdown voltage of these sol-gel/DNA-CTMA films was measured at room temperature with a high-voltage source and a Keithley 2400 Source Meter monitoring the leakage current. The thickness (T), breakdown voltage (Vbreak), and dielectric strength (Ebreak) of this series of samples, JF501, JF502, and JF503, are listed in Table 1 below. In Table 1, it is seen that all of the thick sol-gel/DNA-CTMA thin films have survived applied voltages higher than 2 kV. The maximal applied voltage is 2.5 kV, which is the highest applied voltage that we have ever achieved with sol-gel/DNA-CTMA thin film capacitors. The 2.5 kV breakdown voltage corresponds to a maximal breakdown strength of 450 V/μm, almost twice as large as for previous 5μm samples we have attempted. This encouraging result indicates that with further optimization, the sol-gel/DNA-CTMA thin film capacitors not only can possess high dielectric strength, but also can sustain multi-kV of applied voltage, making them suitable for high voltage applications. To note, these samples are spin coated only once, which results in a single layer structure, thus it is expected that the performance can be further improved with a dual-layer structure by using a double spin-coating process.

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2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.52

4

6

8

10

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lect

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onst

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Sampling area: mm2

Dielectric constants measured at 1.0 KHz JF501; JF502; JF503

Figure 2. Dielectric constants of thicker sol-gel/DNA-CTMA films

Table 1. Breakdown strength of thick sol-gel/DNA-CTMA films

Sample JF501 JF502 JF503

T[μm] 5.105 5.521 5.104

Vbreak[KV] 2.1 2.5 2.2

Ebreak[V/μm] 411 453 431

3. HYBRID DNA/SOL-GEL BARIUM TITANATE NANOPARTICLE CAPACITORS To achieve high dielectric constant composite materials aimed at higher energy density capacitors,

barium titanate (BaTiO3) nanoparticles, with a dielectric constant of 150, have been systematically explored for filling our existing sol-gel/DNA-CTMA blends to result in sol-gel/DNA-CTMA/nanoparticle composites with very high dielectric constants. To enhance the stability of the nanoparticle-dispersed solution, we first pretreated the BaTiO3 nanoparticles with MAPTMS. We then mixed them with DNA-CTMA solution with the assistance of ultrasonic energy. The resulting dispersed solutions were observed to be much more stable than previously obtained solutions that used barium titanate particles without pretreatment.

A series of sol-gel/DNA-CTMA/BaTiO3 nanocomposite thin films (BT1Δ2, BT2, BT3, and SDB1) were all spin coated on indium tin oxide (ITO)/glass substrates, and a thin layer of gold was coated on top of the composite film to define the sampling area and serve as the anode. The fabricated nanocomposite thin films are observed to be less transparent than the sol-gel/DNA-CTMA thin films without nanoparticles added. As shown in Figure 3, the left hand side is the sol-gel/DNA-CTMA/BaTiO3 nanocomposite thin film and the right hand side is a sol-gel/DNA-CTMA thin film sample without nanoparticles. The golden spots and lines represent the deposited electrodes. Note that transparency is not a requirement for the capacitor application, but does give us some idea of the size of the nanoparticle agglomerates that are present.

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Figure 3. Thin film samples with (left) and without (right) nanoparticles added.

The dielectric constants of this series of nanocomposite thin film samples were measured with a Stanford Research Systems (SRS) Model SR720LCR meter at room temperature. A dramatic improvement in the dielectric constant was observed for the sample SDB1, whose value was measured in the range of 36 to 40 and averaged to be 38. This is more than four times larger than that for sol-gel/DNA-CTMA samples without nanoparticles incorporated. The breakdown voltage of the sol-gel/DNA-CTMA/BaTiO3 nanocomposite films were measured at room temperature with a high-voltage source and a Keithley 2400 Source Meter monitoring the leakage current. It was observed that the breakdown strength of the nanoparticles samples, ranging from 166 to 288 V/μm, is lower than the previously achieved value of 700 – 800 V/μm for sol-gel/DNA-CTMA blends. The sample showing the largest dielectric constant, SDB1, has the lowest breakdown strength of 166 V/μm, indicating a potential tradeoff between dielectric constant and breakdown strength.

One of the figures of merit for energy storage capacitors is the energy density, which is defined as the stored energy (Eg) per unit volume (Vol):

Eg/Vol = (1/2)εrε0V2/d2 = (1/2)εrε0(Ebreak)2 (2) where εr is the relative permittivity (dielectric constant), ε0 is the electric constant, d is the thickness of the dielectric, and V is the applied voltage. (2) shows that the energy density is proportional to the dielectric constant εr, while it has a quadratic dependence on the breakdown strength Ebreak. The benchmark polymer-based polypropylene thin film capacitors have an energy density of 0.45 J/cm3.

Our sol-gel/DNA-CTMA blend thin films have an average dielectric constant of 8 and breakdown strength of 750 V/μm (in the range of 700-800 V/μm as achieved previously), resulting in an energy density of 20 J/cm3. This value is about 45 times as large as that of polystyrene thin film capacitors. With BaTiO3 nanoparticles added, although the nanocomposite thin film samples have an enhanced dielectric constant of 38, a reduced breakdown strength of 200 V/μm reduces the energy density to ~ 7 J/cm3. Thus, the benefit of increasing the dielectric constant of the nanocomposite material is offset by its diminished breakdown strength unless it can be improved to be close to the range of the sol-gel/DNA-CTMA blend thin films. Efforts to improve the nanocomposite film quality are now under investigation.

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4. CONCLUSION In summary we have investigated the dielectric and electrical properties of a series of sol-gel/DNA blends and sol-gel/DNA barium titanate nanoparticle composites. We found that the sol-gel/DNA-CTMA 95/5 (wt/wt) has the best combination of dielectric constant (~ 7.5) and breakdown strength (750V/μm), providing an energy storage capacity of 20 J/cm3 ~ 50 times larger than that of commercial polypropylene capacitors. Optimized sol-gel/DNA barium titanate nanoparticle composites had exceptional dielectric constant (~ 38) but their reduced breakdown strength (165V/μm) leads to reduced energy storage capacity (~ 7 J/cm3). Thick sol-gel/DNA-CTMA 95/5 films (~ 5μm) were developed with only slightly reduced breakdown strength (~ 500V/μm) compared to thinner films. Future work will focus on the development of packaged capacitor prototypes and further investigation of the nanoparticle composite approach.

5. ACKNOWLEDGEMENTS We would like to acknowledge the support of the Air Force Office of Scientific Research and the Air Force Research Laboratory. The University of Arizona acknowledges the support of the National Science Foundation through the STC MDITR, Grant# 0120967.

6. REFERENCES [1] P. Gonon, et. al., “High-density capacitors based on amorphous BaTiO3 layers grown under hydrogen containing atmosphere,” Appl. Phys. Lett. 90, 232902 (2007). [2] C. G. Liu, et. al., “Frequency response characteristic of single-walled carbon nanotubes as supercapacitor electrode material,” Appl. Phys. Lett. 92, 143108 (2008). [3] Y. R. Ahn, et. al., “Enhanced charge-discharge characteristics of RuO2 supercapacitors on heat-treated TiO2 rods,” Appl. Phys. Lett. 90, 122106 (2007). [4] Z.-M. Dang, Y.-H. Lin, and C.-W Nan, “Novel ferroelectric polymer composites with high dielectric constant,” Adv. Mat. 15, 1625 (2003). [5] M. Rabuffi and G. Picci, “Status quo and future prospects for metalized polypropylene energy storage capacitors,” IEEE Trans. Plasma Sci. 30, 1939 (2002). [6] P. Kim, et. al., “Phosphonic acid-modified barium titanate polymer nanocomposites with high permittivity and dielectric strength,” Adv. Mat. 19, 1001 (2007). [7] Y. Enami, et. al., “Hybrid polymer/sol-gel waveguide modulators with exceptionally large electro-optic coefficients,” Nature Photonics 1, 180 (2007). [8] E. Heckman, et. al., “Performance of an electro-optic waveguide modulator fabricated using a deoxyribonucleic-acid-based biopolymer,” Appl. Phys. Lett. 89, 181116 (2006).

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