Photoconductivity and Current-Voltage

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rectifying current-voltage (I-V) characteristics. ... experimental data, and physical processes likely to arise in the devices are discussed. ... Keywords: DNA, Schottky barrier, Schottky barrier height, photoelectric effect, current-voltage ... These studies indicate that electron/hole transport in DNA is mediated primarily.
Photoconductivity and Current-Voltage Characteristics of Thin DNA Films: Experiments and Modeling Ravindra Venkatramani1 De Yu Zang2, Choon Oh2, James Grote3 and David Beratan1 1 Department of Chemistry, Duke University, 5311 French Family Science Center, Durham 27708 2 IPITEK, 2330 Faraday Avenue, Carlsbad, CA 92008 3 US Air Force Research Laboratory, Materials and Manufacturing Directorate, AFRL/RXPS, Wright-Patterson Air Force Base, OH 45333-7707

ABSTRACT We report experimental observations and theoretical modeling of an unusual photoelectric effect in deoxyribonucleic acid (DNA) thin-film devices, under visible and near-infrared illumination. The devices also show diode-type rectifying current-voltage (I-V) characteristics. An equivalent circuit model was constructed that fits the experimental data, and physical processes likely to arise in the devices are discussed. We envisage the formation of a Schottky barrier at the DNA film-metal interface and infer that the photoresponse arises from photoinjection of electrons from the metal into the DNA film. Keywords: DNA, Schottky barrier, Schottky barrier height, photoelectric effect, current-voltage

characteristics, work function, bandgap. 1. INTRODUCTION There is great interest in the development of organic and biological materials for applications in electronic devices. DNA is a promising candidate, as it has a natural propensity to self assemble, and it supports long-range multi-step charge transport. Considerable experimental and theoretical efforts have been devoted to understanding the charge transport properties of DNA [1-9]. These studies indicate that electron/hole transport in DNA is mediated primarily by the stacked nucleobases and that the backbone plays an indirect role. Positive charges (holes) tend to localize on the most easily oxidized guanine (G) bases while negative charges (electrons) localize on the most easily reduced thymine (T) bases. Thus, in hole transport, G:C (C represents cytosine) base pairs play the role of hole traps while stacks of A:T (A represents adenine) base pairs play the role of barriers [3, 10]. Likewise for electron transport, A:T (G:C) base pairs play the role of electron traps (barriers) [11, 12]. Charge transfer in DNA between donor and acceptor nucleobases can take place through several different mechanisms [7, 13, 14], including coherent single-step superexchange, incoherent multi-step hopping, or polaronic transport. The mechanism depends on relative nucleobase energies, interbase couplings and distance between the donor and acceptor. Thermally induced conformational changes cause base energies and couplings to fluctuate, providing access to multiple charge-transfer mechanisms in the systems [15, 16]. Experiments on DNA thin films are relatively unexplored, primarily because of the extensive material processing and treatment required to achieve macroscopic currents. Electrochemical experiments [17] on disordered calf thymus DNA films on carbon electrodes found no currents and concluded that alignment of DNA was necessary for conductance. Orientation can been achieved exploiting the self assembly properties of DNA or its charged backbone [18, 19]. A thiol terminating group is generally used to tether the DNA molecules to a gold electrode. These self-assembled monolayers of nucleic acids provide a promising, systematic and controlled approach to electrochemical devices [20-24] . However, these approaches require extensive chemical processing and their Nanobiosystems: Processing, Characterization, and Applications II, edited by Norihisa Kobayashi, Fahima Ouchen, Ileana Rau, Proc. of SPIE Vol. 7403, 74030B · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.831024

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scalability to build macroscopic electronic devices has yet to be tested. Okahata and coworkers describe methods [25] to create films of oriented DNA molecules The DNA is reacted with cationic amphiphiles that attach to the phosphate backbone, presumably displacing counter-ions. The modified DNA is soluble in organic solvents and forms uniform thin films. DNA alignment is achieved either by uniaxial stretching of a cast film of DNA-lipid complex [26] or by a vertical dipping Langmuir-Blodgett method of anionic DNA with cationic lipid monolayers [27]. Such films were tested for use in various optoelectronic devices and show promising results [28-30]. This study is also a step in this relatively unexplored direction of building thin DNA film (DNAF) based electronic devices. We report the fabrication and characterization of a simple metal-DNAF-ITO (indium tin oxide) sandwich device, and examine the effects of electric poling, on the device. Our device shows a photoresponse and diode like current rectification upon the application of a voltage bias. We model the transient photocurrent using an equivalent circuit model and discuss possible underlying physical processes responsible for the observed photoconductivity.

2. EXPERIMENTS: PHTOELECTRIC EFFECTS AND I-V CHARACTRISTICS 2.1.

Device, Materials and Preparation

2.1.1.

Device configuration

The metal-DNAF-ITO devices consists of three layers: top and bottom electrodes (metal Au/Cu and ITO respectively) and a thin DNA film sandwiched between the electrodes. The thickness of the gold and ITO electrodes is ~1000 and 700 Å, respectively, while the DNA film is 13 - 19 μm thick. The DNA film is generally shiny and smooth, with a thickness uniformity of 10% typically in the device working area. The DNA metal electrode area, which defines the device working area is ~ 0.691 cm2. Figure 1 shows the device. Au/Cu ITO

DNA Substrate

(A)

(B)

(C)

Figure 1: Device structure: (A) diagram; (B) unwired device; (C) wired device

2.1.2.

Material and fabrication.

Salmon sperm DNA with a molecular weight of 8 – 10 kbps were obtained from Prof. Naoya Ogata from the department of materials science at Chitose Technology Institute, Hokkaido, Japan. The DNA was dissolved in deionized water with a weight ratio of 1:100. The DNA-water solution was then filtered with a 0.45 μm filter and cast on a ~ 1”×1” ITO-coated glass substrate. The sample was immediately placed in an oven at 50 ° C to dry overnight. The sample was then transported into a sputtering machine to deposit the top metal electrode using a shadow mask. Finally, the top and bottom electrodes were connected to wires. 2.1.3.

Electric Poling.

After wiring, the devices were processed by electrical poling. The devices were placed in a nitrogen chamber, and heated from room temperature to 120°C at ~ 10°C/min. At ~ 100°C, a voltage is turned on and applied to the device. The applied voltage was ~ 20 V and monitored by the current (~ 2 µA) through the devices. The temperature of 120°C is held for 20 minutes, and decreases to the room temperature at a same rate of 10°C/min,, while maintaining the applied potential difference across the electrodes. It should be noted that the devices do not show the photocurrent described below unless they are poled. Before electric poling, the DNA strands are believed to be oriented randomly in the film. Heating the film past the glass transition temperature of DNA melts the double

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helices, and could create mobile single-stranded DNA. While speculative, a possible scenario for the melting, orientation, and annealing is shown in Figure 2. The overall effect of the electric poling process may be to a) attract the negatively charged DNA towards the positively biased metal electrode thus forming contacts with the electrode and b) create oriented fibers of DNA.

Figure 2: A cartoon representation of the possible consequences of electric poling to establish DNA that bridges the Au/Cu (orange) and ITO (white) electrodes. The density of DNA in the film is too high to allow simple rotation of the strands as depicted here. However we expect a fractional DNA orientation between the electrodes.

2.2.

Photoelectric Effect

2.2.1.

Experimental Setup

Figure 3: Device schematic showing Photocurrent measurement setup

2.2.2.

The photoelectric measurement apparatus is shown in Figure 3, which consists of a light source, the device and an ammeter (an autoranging Pico-ammeter, Keithley 485). The device was not electrically biased. The analog output of the ammeter was connected to a computer for sampling and real time recordings. The sampling rate was 100 Hz. The light sources used in the experiments were HeNe lasers and laser diodes with several different wavelengths. When a light beam is launched on ITO-DNA side to pass through the glass substrate, electrons could be excited, which can across over the barrier height into the conduction band in the DNA to generate photocurrent if the light energy is greater than the barrier height.

Results

Figure 2 (A) shows a typical photocurrent response measurement generated by a 0.75 mW laser at 605 nm for the Au-DNAF-ITO device. The photocurrent shows five time regimes: A) prior to laser illumination; B) immediately after laser-illumination; C) steady-state illumination; D) immediately after laser switch off and E) steady state with laser off. Within time domain A, a dark current (~ 0.2 nA) was generated by the open-circuit potential flows through the device. The direction of the current across the ammeter in Figure 3 was negative, flowing from the ITO to gold electrode. In time domain B, a large current spike is observed, presumably caused by the generation of a significant amount of charge in a short time period. The spike is followed by multi-exponential decay. In time domain C, under stable and relatively low power illumination, presumably providing a steady injection of charges from the metal into the DNA film, the photocurrent was nearly constant, with a slow decay that could only be observed when the device was illuminated for very long times (minutes to hours). In time domain D, as the laser is turned off, we find a current transient similar to the case in time domain B, but in the opposite direction: a negative photocurrent spike was generated which subsequently shows a multiexponential decay.

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In the time domain E, the photocurrent slowly recovers to its initial state (a small negative initial current in domain A). The recovery time was relatively long (up to a few minutes). A similar photoresponse was observd for the Cu-DNAF-ITO device. However the dark currents were positive (i.e.) in a direction opposite to that of Au-DNAF-ITO device. Typical photocurrent measurements as a function of light power are shown in Figure 2 (B). The photocurrent generated at a specific wavelength was linearly proportional to the light power. 6

200

Device Laser Beam

Current

4

λ = 605 nm P = 0.75 mW 150

Photocurrent (nA)

Photocurrent (nA)

A A

Light-Off

2

0 Light-On

100

50

-2 A

B

E

D

C

0

-4 0

10

20

30

40

50

60

0

Time (s)

50

100

150

200

250

Power (mW)

Figure. 4: Photoelectric Effect for the Au-DNAF-ITO device : (A) Photocurrent Response vs. Time; (B) Photocurrent vs. Light Power

2.3.

Current-Voltage Characteristics

2.3.1.

Experimental Setup for Current-Voltage Measurements

The static current-voltage (I-V) measurement setup is shown in Figure 3. The setup included two external resistors R1 and R2 (R1 = R2 = 0.58 MΩ), a 1×2 switch, a DC power supply and an ammeter A. The voltage from R1 Device R2 A a DC power supply was measured using a voltmeter (HP 3468B Multimeter), and the current 1x2 was measured using an ammeter (Keithley 485, an ON2 Switch auto-ranging Pico-ammeter). Both current and ON1 Off voltage readings were recorded by computer at an acquisition data rate of 100 Hz. The switch has three states: ON 1 (connection of DC power DC Power supply), ON 2 (short circuits) and OFF (not Figure 5: Experimental Setup for I-V Measurements connected). 2.3.2.

Current-Voltage Measurements

The power supply voltage was first adjusted to the desired value when the switch was in the OFF state. Then, it was turned to the ON 1 state to connect the DC power supply to the device. The current -voltage data recording was started a few seconds prior to switching on and continued for ~ 100 seconds. During the measurements, the switch was turned to the OFF state again and quickly turned to ON 2 state at ~ 50 – 60 seconds. When the circuit was short-

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circuit-connected (the ON 2 state), the metal-DNA-ITO device acted as a charged capacitor and it discharged the current in the circuit. For forward bias operation, the Au electrodes and ITO were connected to positive and negative terminals of the voltage source, respectively. For each voltage, the current transient was recorded and the variation of the steady-state current with applied voltage was extracted. 2.3.3.

Steady-State Current Behavior

A typical measurement is shown in Figure 6, which demonstrates that the steady state current is highly nonlinear, even for voltage biases up to ± 1.1 V, with slight rectification. However, for small biases (< ± 0.3 V), the behavior is approximately ohmic. Figure 2 shows a linear fit to this data, from which a resistance of 17.6 MΩ was obtained.

Figure 6: Nonlinear behavior of the steady state current with applied bias. Also shown is the linear best fit to the data at low bias

3. EQUIVALENT CIRCUIT MODEL AND DATA FITTING 3.1.

Equivalent Circuit Model

We model the photoresponse of the metal-DNAF-ITO device with an equivalent circuit. We seek the simplest circuit built from conventional components that reproduces the transient current behavior in Figure 4 A. Such models are often used to analyze the behavior of complex devices in terms of capacitative, resistive and inductive responses under the action of current or voltage sources. The equivalent circuit behavior can be solved using conventional methods to extract parameters that may help determine the underlying physical processes at play. Figure 7: Switched RC circuit and its current response to a square potential.

We begin with the observation that the positive and negative current transients in the photoresponse (Figure 4 A) are analogous to the charging and discharging of a capacitor under a potential bias. In Figure 7, for example, the RC circuit (left) connected to a battery exhibits a current response (right) that has positive and negative spikes similar to those in Figure 4 A. When the switch S is at position 1, a large current flows through the circuit, leading to the first positive spike. As the capacitor charges and the voltage across the plates equals that supplied by the battery, the current through the circuit decays to zero. Now, when the switch S is flipped to position 2, the battery is

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disconnected and the capacitor drives a large current in a direction opposite to that due to the battery, thus leading to the negative spike. Finally, as the capacitor discharges, this current decays to zero. The experimental photoresponse in Figure 4 A is more complicated. First, the current decay after the positive and negative spikes is triexponential. This can be modeled by incorporating three distinct RC branches in the model, each with a different time constant. Secondly, the current does not decay to zero but to a steady state value. This can be modeled by including a purely resistive branch in the circuit. Other temporal characteristics of the current, i.e the small steady state dark current when light is switched off, the asymmetry in the positive and negative spikes and the finite rise time of currents during spiking, can be modeled by choosing appropriate temporal profiles for the voltage applied to the circuit.

C1

R1

C2

R2

Vp(t)

V

Vp0

R3

C3

Vpss Vd

R4

t

Vd

V(t)=Vp(t)+Vd

ton

V

toff

Figure 8: Equivalent circuit model (left ) with the applied potential profile (right) used to fit the device photoresponse in Figure 4 A

The simplest voltage profile that reproduce the above features is:

V (t ) = V D + V P Where Vd is constant at all times and

VP = 0

⎡ ⎛ (t − t on ) ⎞⎤ ⎟⎥ V P = VP 0 ⎢1 − exp⎜ − ⎜ ⎟⎥ τ ⎢⎣ inj ⎝ ⎠⎦ ⎡ ⎛ (t − t off ) ⎞⎤ ⎟⎥ V P = VPSS + (VP 0 − VPSS ) ⎢1 − exp⎜ − ⎜ ⎟⎥ τ inj ⎝ ⎠⎦ ⎣⎢

t ≤ ton ton ≤ t ≤ toff

t ≥ toff (1)

Here ton and toff are times when the laser is switched on and off respectively. The resultant equivalent circuit along with the potential profile in equation 1 is shown in Figure 8. Using the form of the applied potential in equation 1, we solve the equivalent circuit in Figure 8 to compute the current flowing through the circuit. The parameters in equation 1 are varied along with the RC time constants to fit the experimental photoresponse in Figure 4 A. The best fits results for the Au-DNAF-ITO device along with best fit parameters values for Au and Cu devices are shown in Figure 9. The physical significance of the various parameters shown in Figure 9, as well as the potential profile shown in Figure 8, are discussed in the next section.

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R2 = 0.986; RMSE=0.014

Parameters

Au-DNA

Cu-DNA

Idss=Vd/R4

-0.027

0.0061

Ipss=Vp0/R4

0.005

0.275

S=(Vp0-Vpss)/Vp0

0.27

0.075

I1=Vp0/R1

2.61

0.048

R1C1= τInj

0.0310

0.0073

I2 = Vp0 τ2 /R2(τ2 -τInj)

0.552

0.322

R2C2

0.554

0.045

I3 = Vp0 τ3 /R3(τ3 -τInj)

0.130

0.741

R3C3

8.348

19.29

Figure 9: Best fit to the photoresponse of the Au-DNAF-ITO device shown in Figure 4 (A) (left) and the best fit parameter values for Au-DNA-ITO and Cu-DNA-ITO devices.

3.3.

Discussion of Likely Physical Processes and the Observed Photoresponse

In this section, based on the potential profile and the parameters obtained from the equivalent circuit model, we make intuitive arguments regarding the physical processes at play in the device and the parameters appearing in equation 1 and Figure 9. These arguments are merely suggestive at this stage; validation of these hypotheses will require more extensive device characterization and theoretical analysis. We first begin with the potential profile of equation 1. The constant current VD , which drives the dark current in the device, is identified as the built-in voltage or the open circuit potential. The origin of Vd can be attributed to several contributions. The first is the difference in the chemical potentials of ITO and the metal (Au/Cu) contacts. A second contribution is the creation of electric dipoles during poling. As discussed above, the effect of poling is to (partially) align DNA molecules. Ions may also move to contribute to a dipole. The fields created by these dipoles may create a potential gradient from the chemical potential difference of the metal and ITO electrodes to give a resulting built in potential that drives the dark current. A third contribution might arise from the charging of the DNAF by the metal electrodes during electric poling. Positive bias of up to 20 V will certainly lower the metal Fermi levels to enhance the injection of holes into the DNAF, thus oxidizing the film. From the directions of the dark currents (Au->DNAF>ITO & ITO->DNAF->Cu), it seems likely that the Fermi level difference might be the dominant contribution to the built-in voltage, and that the latter two contributions have been minimized by discharging the device after electric poling for a few days. The work-functions for Au is ~ 5.1 eV and for Cu is ~ 4.65 eV. Corresponding ITO values range from 4.3 eV to 5.1 eV, so it is plausible that the ITO work-function lies somewhere between those of Cu and Au, thus contributing opposite directions to the dark current as observed in the experiments.

Evac

laser

kInj

Empty states

EVac

eΦB,e

Au Fermi + Level EAu-F h

kHop

ΦB,h Filled states

ITO Fermi level EITO-F z

Figure 10: Schematic representation of the physical processes that operate within the metal-DNA-ITO device. The vertical scale (E) is energy and the horizontal scale is the device dimension (z).

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The time-dependent voltage VP is the voltage that develops in the device due to laser illumination. When the device is exposed to visible light, it interacts mainly with the metal (Au/Cu) electrode as the DNAF and ITO substrate absorb in the ultraviolet. This may create a distribution of hot photoelectons and holes with energy centered around a specific value above and below the fermi level (say, Ee for electrons and Eh for holes) depending on the laser frequency. Electrons/holes that have a significant momentum perpendicular to the DNA film-metal electrode interface may be injected into the DNA states from the metal either directly over a barrier or by tunneling through a barrier. In principle, both electrons and holes could be injected into the DNAF subject to the condition that the DNA-film “valence” or “conduction” bands have energies matching those of the excited electrons or holes. From the directions of photocurrents observed in both devices (from Au/Cu to ITO through the external circuit), it is clear that electron injection dominates for metal-DNA-ITO devices studied here. Diffusion of the injected charges away from the interface can take place through multi-step hopping through DNA states. If the charge transfer rate (khop) is slower than the rate of charge injection (kinj), an accumulation of charge at the Au-DNAF interface may take place to create a potential difference opposing the built-in potential. As more and more charge builds up, the current may reverse direction and increase with the photo-induced potential. The growth of the photoinduced potential across the device is exponential, with the time constant τinj in equation 1 and VP0 equal to the value at which the potential saturates. Once the potential saturates, the current decreases exponentially as the device charges to a steady-state value. When the light source is switched off, the accumulated charge at the Au-DNAF interface discharges exponentially with time constant τinj, and the potential difference across the device approaches its intrinsic value. However, it is possible that not all the charge at the interface returns to the Au electrode right away, and some of it may be trapped in interface states, lowering the built-in potential from its initial value before the light is switched on (to a value VPSS + VD). Thus, the device may discharge to a new dark current equilibrium value with a direction opposite to the photo-induced current. The processes described here are depicted schematically in Figure 10. The currents IDSS and IPSS flow through the purely resistive branch due to VD and VPSS + VD, respectively The parameter S represents the fraction of injected charge trapped at the DNAF-metal interface after the laser is switched off, and the DNAF discharges into the metal. The current through the RC branches I1, I2, and I3 are currents that arise from different charge-transport mechanisms. Charge can flow via electron or hole hopping mediated mechanisms through the DNA base states (Figure 11). Studies on long DNA fibers and networks [31-35] found an exponential increase of DNA conductivity with humidity, suggesting that the observed conductivity was primarily ionic. Ionic contributions can arise from Na+ ions or OH⎯ and H+ ions associated with the DNA backbone. If the DNA molecules are randomly oriented then the electron/hole or ionic contributions which move in the direction of the DNA helical axis will tend to cancel and cannot produce measurable currents. This could be the case, for instance, for the disordered DNA films in reference [17] and our own thin film samples which were not electrically poled. Without electric poling our samples which do not have any preferred orientation for the DNA, show no photocurrents. After the electric poling treatment a small fraction of the DNA could orient themselves along the direction of the applied field to give the observable photocurrents. However the dark current is always present with or without poling which suggests that a charge transport mechanism different from the ones discussed above might be operational. One possibility is the diffusion of ions through the network of DNA strands under an applied bias (either the built-in potential or the photoinduced potential). While alignment of DNA strands is not necessary for diffusion of ions through a charged polymer matrix, the mobility of ions may be larger when the polymer chains are somewhat organized along the axis between the electrodes. At large voltage biases applied across the electrodes, the fields might start distorting the charged polymer matrix to make the dependence of the ionic diffusion current on the applied bias nonlinear. Further, the junction is inherently asymmetric because the currents do not see the same DNA matrix as they flow from DNA to ITO and ITO to DNA. The large resistance values ~ 17.6 MΩ, the nonlinear I-V characteristics, and the rectifying I-V behavior of the steady state currents (Figure 6) suggest that they might have a significant contribution from the ionic diffusion current flowing through the polymer matrix. At present, however, it is not possible to unambiguously assign currents I1, I2, and I3 to the other candidate charge transport mechanisms discussed above. Irrespective of the charge transport mechanism, it appears that the alignment of DNA strands, between the electrodes, along the field direction might lead to much higher currents. An exciting possibility is to perform photoconductivity and I-V measurements on the thin films of oriented DNA, such as those created by Okahata and coworkers [25]. The electric poling process can also be further refined to control the fraction of DNA aligned and contacting the electrodes and further experiments on small concentrations of DNA in solutions or gels would help understand the process better.

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-

OH H Intrastrand

Interstrand

C

-P

A

P

Na+ T

P

Ionic transport

G Electron/hole transport

Cross 3’-3’

Cross 5’-5’

G

P

-

T

P

-

A

P

-

C

Figure 11: Possible mechanisms of charge transport in the metal-DNAF-ITO devices. The black and blue solid lines and green and pink dashed lines represent electronic coupling between DNA bases for electron/hole transport. 3.4.

Transient Current as a Function of Applied Voltage

Upon applying a step voltage, the transient current, at first glance, behaves like the observed transient photocurrent. When the positive (negative) bias is switched on (see Figure 3, experimental setup) there is a positive (negative) spike in the current that decays exponentially to a steady-state value. When the voltage is switched off, the current shows a negative (positive) spike and then decays to its open circuit potential value. However, upon closer inspection, some differences are apparent. While the photocurrent transients are described by a tri-exponential decay (plus a steady state constant term), the current transients at low bias show single-exponential decay and, at higher biases, the decays become bi-exponential. This is seen more clearly in semi-log plots of the current transient decay vs. time (Figure 8). While the log of the decay at low bias can be approximated by a straight line, the decays at higher biases cannot. Comparing with the semi-log plot of the photocurrent transient, for the range of biases used, additional mechanisms of charge transport appear to be operational during the photocurrent transients. At low bias, we expect charge injection processes from the metal to the DNAF will be weak, and the applied voltage bias will affect both free and DNA-bound ions in the film. If, as discussed in the preceding section, free ions diffusing through the polymer matrix are responsible for the steady-state current. the ions bound to the oriented DNA strands moving along the DNA helix axis produce the monoexponential decay at low bias (since ions bound to randomly oriented DNA strands produce stray currents that cancel). At higher bias, charge injection over the barrier or via tunneling becomes more significant, and the currents exhibit multiexponential decays as multiple processes begin to contribute. The fastest decay timescale of the photocurrent transient is not present in current transients for voltage biases up to 1.74 V, we conclude that this fastest decay is connected to a charge injection process not energetically accessible in the range of the applied bias.

Vb =0.28 V

Figure 12.: (A) Sample current transients at positive and negative voltage bias VB. (B) Semilog plot of transient current decay vs time (blue lines) showing linear behavior (single exponent) at low bias but which becomes nonlinear (double exponents) at higher bias. Also shown for comparison is the log of the photocurrent transient decay (black line), which is described by a tri-exponential.

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4. CONCLUSIONS We have reported experiments and numerical modeling of an unusual photoelectric (PE) effect under visible and near-infrared illumination as well as a diode-type rectifying current-voltage (I-V) response in DNA-metal contact devices. The photoresponse was modeled using an equivalent circuit model with parameters linked to the underlying physical mechanisms of device operation. We hypothesize that ionic and electron/hole transport mechanisms driven by a combination of intrinsic potentials and a photoinduced potential difference contribute to the observed photoresponse. The intrinsic potential is hypothesized to arise from workfunction differences of Au/Cu and ITO electrodes. The photoinduced potential is hypothesized to arise due accumulated charge injected from the metal at the DNAF-metal interface. The directions of the observed currents supports that the photoinduced potential arises primarilyfrom electron injection. Acknowledgements. This work was sponsored by DARPA DSO seedling project under contract #FA8660-07-C7735 (DSO program manager: Dr. Cynthia Daniell).

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