18717
2008, 112, 18717–18721 Published on Web 11/08/2008
Properties of a Biophotovoltaic Nanodevice Z. G. Chirgwandi* BioNano Systems Laboratory, Department of Microtechnology and Nanoscience, MC2, Chalmers UniVersity of Technology, SE-412 96 Go¨teborg, Sweden
I. Panas and L.-G. Johansson Department of EnViromental Inorganic Chemistry, Chalmers UniVersity of Technology, SE-412 96 Go¨teborg, Sweden
B. Norde´n Department of Physical Chemistry, Chalmers UniVersity of Technology, SE-412 96 Go¨teborg, Sweden
M. Willander Department of Science and Technology, Linko¨ping UniVersity, 601 74 Norrko¨ping, Sweden
D. Winkler Department of Microtechnology and Nanoscience, MC2, Chalmers UniVersity of Technology, SE-412 96 Go¨teborg, Sweden
H. Ågren Department of Theoretical Chemistry, Royal Institute of Technology (KTH), SE-10691 Stockholm, Sweden ReceiVed: September 6, 2008; ReVised Manuscript ReceiVed: October 15, 2008
Properties of an on-chip photovoltaic nanodevice are demonstrated. The dyes comprise green florescent proteins (GFP). Dependence of recently reported zero external potential bias (ZEPB) photocurrent (I) on temperature, power, and wavelength (λ) is shown. Correlation between UV-vis spectrum of the GFP and the ZEPB I(λ) of the device is reported. The temperature dependence suggests the ZEPB photocurrent to reflect a liquid crystal type ordering where the current declines monotonically with increasing temperature. The influence of an external bias on the photocurrent is demonstrated. The resulting light-induced current is analyzed in terms of resistive and quantum mechanical contributions. Introduction Clean energy production is arguably the greatest challenge for achieving a sustainable society. A recent survey compares technologies for CO2 free production of electricity, including hydropower, wind, geothermal, nuclear fission and fusion, biomass, solar, and ocean energy, concluded that solar power is the most promising candidate.1 The harvesting of CO2 free solar energy can be divided into concentration of solar energy, for driving generators at the high end or produce heated water for domestic use at the low end, and photovoltaic (PV) conversion of solar energy to electricity. There are mainly two contemporary strategies in PV, i.e., the production of electricity from sun light. One exploits solid state semiconductor properties of e.g. silicon, gallium arsenide, and copper indium diselenide, which are subject to photoexcitations whereby photo induced charge carriers, i.e., hole-electron pairs, are produced. The second approach is well represented by the Gra¨tzel cell,2 which * To whom correspondence should be addressed.
10.1021/jp807925k CCC: $40.75
combines semiconductor properties of TiO2 nanoparticles, coated with dye molecules which provide the photoelectrons. In this context, the present effort combines the two approaches. Thus, 30 nm anode-cathode spacing is invoked on a silicon chip by means of electron beam lithography, and self-assembled green fluorescent proteins (GFP) are employed as the dye molecules.3 Indeed, it was demonstrated in ref 3 how photoelectric current at zero external potential bias (ZEPB) develops in the submicron PV cell, employing a dielectric medium including enhanced green fluorescent protein EGFP molecules. Photoinduced hole-electron pairs were suggested to form on the GFP molecules, which would serve as electron sinks and sources with respect to the Fermi level of the metallic leads if the Fermi level was bracketed by the molecular HOMO-LUMO gap. The observed net current would result from symmetry breaking aggregation of GFP proteins, which display photon induced conductivity. A motivation of the current work is that the origin of the exceptionally bright fluorescent characteristics of green fluo 2008 American Chemical Society
18718 J. Phys. Chem. C, Vol. 112, No. 48, 2008 rescent proteins, generated by evolution, can be intimately connected to their potential use as effective photoenergy converters, with particular, outstanding, properties. The relatively small size, a 3 nm high cylinder shape, with a diameter of 2 nm, and the completely autocatalytic formation of its light emitting chromophore, make GFP an unprecedented fluorophore, which can be expressed in genetic fusion with other proteins for studying their expression, localization, and function inside the living cell. Relevant for the present strategic undertaking is that GFPs feature a wide range of different spectral properties, with fluorescence colors spanning most of the visible spectrum, from blue-cyan to near-infrared. Such variety is due to different covalent structures of the chromophore and arrangements of its molecular environment. Engineered GFP mutants display sensitivity of their excitation/emission intensity upon pH, or metal and halide ion concentration, hence their use as biosensors for these chemical species in the cell.4 Other nice applications exploit the possibility to optically switch GFPs, in a reversible or irreversible fashion, between molecular states that differ in brightness and color. Indeed, it is remarkable that the quest for novel, simple and sustainable solutions for renewable energy may very well be found in nature itself, as in it we find evolutionary stability and simplicity in both structure and performance of its functional units. Motivated by the outstanding properties of GFPs reviewed above, we address in the present work self-assembled arrays of GFPs where enhanced lifetime of photoinduced hole-electron pairs allows for the competition of electron-hole recombination, whether with electrons from an electrode or by the de-excitation process, the former being a requirement for photoinduced current. The poor conduction properties of the GFP condensate are overcome by the small dimensions of the photovoltaic device. While not of immediate concern here, we recognize the technological potential of FRET5 in condensates of fluorescent molecules as an important aspect which enhances the efficiency of the photoharvesting property. The objective of the present work is to consolidate the results presented in3 by demonstrating the photocurrent dependence on wavelength, intensity of incident light, temperature, source-drain potential bias, and gate potential. Experimental Section An n-type 600 µm thick silicon wafer was thermally oxidized to produce an insulating SiO2 substrate of 1000 Å thickness. Aluminum electrodes were prepared on top of the SiO2 layer by means of electro-lithography, chemical vapor deposition, and lift-off techniques. The backside of the silicon wafer was attached to a gold surface in order to produce an ohmic contact. The device is composed of parallel arrangements of rectangular shaped and 700 Å thick Al2O3 electrodes. The spacings between electrodes are 30, 50, 70, 90, 200, and 400 nm (see ref 3). The schematic cross-sectional view of the fabricated device is shown in Figure 1a, where a droplet of GFP solution is placed on the chip. The present study employs a setup comprising two electrodes separated by 30 nm on the wafer. The current was measured through the electrodes using “16058A fixture” test cage connected to a 4145 HP semiconductor parameter analyzer. The pure recombinant EGFP (0.92 mg/mL EGFP suspended in 20 nM Tris at pH 8.4 with 0.02% NaN3 as antibacterial) was employed as dielectric medium, and subject to photoexcitations. Light emitting diodes LEDs (λ ) 370, 395, 430, 466, 472, 502, and 570 nm) were used to study current-voltage characteristics as functions of wavelength. A Peltier element was mounted in the test cage in order to allow study of temperature dependence.
Letters
Figure 1. (a) Schematic cross section of the device and the measuring system and (b) overall electron transfer process as implied from the experimental results.
Results and Discussion It was observed in ref 3 that ZEPB current flows in an a priori symmetric device as depicted in Figure 1b. Moreover, no current is observed in the absence of the GFP, i.e., when using only the buffer solution. The molecular origin of this spontaneous photocurrent in a GFP electrolyte nanodevice is demonstrated in Figure 2. Thus, I(λ) (Figure 2a) peaks at 472 nm where the UV-vis spectrum of the solvated GFP molecules (Figure 2b) is also found to display its maximum. This correlation is taken to demonstrate the dependence of charge carrier density on photon energy, and it is concluded that the cross-section for photoexcitation controls the conductivity. Having said this, detailed agreement between I-V characteristics and photoabsorption spectrum are not expected because the I-V measurements and the photoabsorption spectrum do not reflect the same physical situation, the former self-assembled GFP on a chip and the latter GFP in solution. Interestingly, applying a gate potential with field gradient orthogonal to the net direction of the current damps the wavelength dependence significantly. This is taken to imply that enhanced lifetime of each dissociated electron-hole pair with applied field, due to the polarization, weakens the resonance requirement in I(λ). Photocurrent at ZEPB requires spontaneous symmetry breaking in the system as whole. If the arrangement of proteins is responsible for this behavior, then the spontaneous current is expected to display temperature dependence. This phenomenon is demonstrated in Figure 3. What is the reason for the symmetry breaking which causes the net photocurrent? A possible explanation to the observed ZEPB photocurrent is that the dielectric medium, i.e., GFP condensate, is symmetry broken. This implies the existence of a “melting” temperature at which the phenomenon is averaged out. In Figure 3a, the spontaneous net current is presented as function of temperature, and indeed a monotonous reduction in current is observed. Interestingly though, the temperature induced structural transition does not cause the net current to vanish completely, implying that symmetry is only partially recovered at 20 °C. Dependence of the ZEPB photocurrent on the power of incident light (λ ) 472 nm) for different temperatures is displayed in Figure 3b, whereas Figure 3c displays the same information as Figure 3b but in terms of the temperature
Letters
J. Phys. Chem. C, Vol. 112, No. 48, 2008 18719
Figure 2. (a) Relative source-drain current versus wavelength for different gate voltages. The absorption maxima was found at λ ) 472 nm of Vg ) 0 V and shifted to λ ) 466 nm by applying Vg ) -5 V; (b) Absorption spectrum of the solvated GFP molecule.
Figure 3. Temperature dependent zero bias current is demonstrated for λ ) 472 nm; (a) 4.8 mW, (b) for a range of powers, and equivalently, (c) power dependence of the photocurrent obtained for different temperatures.
dependence of the ZEPB photocurrent for a set of different values of the power. The robustness of the spontaneous photocurrent may be investigated by applying an external source-drain potential VSD.
In Figure 4a, the influence of a positive potential bias on the photocurrent is demonstrated. Antagonistic mechanisms for charge transport are implied from the dip at low bias potentials. We observe that at bias potentials greater than ∼50 mV, the device displays an almost ohmic behavior. By extrapolation to zero current the spontaneous internal potential bias may be estimated to Vint ∼ 40 mV at VSD ) 0 V, where some 70% of the ZEPB photocurrent is of ohmic nature. The remainder is arguably of quantum mechanical origin, as discussed further below. In Figure 4b, the influence of a source-drain potential, which opposes the intrinsic photocurrent, on the I-V characteristics is displayed. A marked asymmetry is observed, consistent with the physics of a zero-bias photocurrent responding differently to applied external positive or negative biases. The I-V characteristics of the nanodevice are summarized in Figure 4c. The hysteretic behavior arguably reflects the stiffness of the spontaneous symmetry breaking producing the ZEPB net photocurrent. The influence of a gate potential Vg (see Figure 1b) on the ZEPB photocurrent is displayed in Figure 4d for two different temperatures. The gate voltage was changed from -5 to +5 V in steps of 2.5 V. The applied gate field is seen to affect the photocurrent in an asymmetric way. A similar phenomenon was reported by Gruner et al.6,7 for arrays of carbon nanotubes with variable degrees of disorder, where the current was observed to depend sensitively on the network density. Apparently, an analogy between carbon nanotube array density and temperature influence on GFP organization can be made if both are said to affect ordering. The asymmetry in ISD as function of gate voltage represents another device characteristic which the nanotube array shares with that of GFP device. The origin of which may be the up-down asymmetry due to the fact that the nanoparticles are deposited on a substrate. By subtracting the ohmic term from the total I-V curve (see Figure 4a), we are able to isolate the nonohmic potential dependence of possible quantum mechanical origin (see Figure 5b). The simplest phenomenology for describing this contribution to the zero-bias photocurrent is that of particle in a box. The GFP molecules are assumed to be packed densely so that the characteristic time for tunneling is much shorter than the lifetime of the electron in the dielectric
18720 J. Phys. Chem. C, Vol. 112, No. 48, 2008
Letters
Figure 4. (a) Current-voltage characteristics of the device at positive bias. The ohmic contributions to the total photocurrent are indicated by straight lines (in black color) which intersect the x-axis at Vds ) -56.25 mV (T ) 16 °C) and Vds ) -40 mV (T ) 18 °C). The total photocurrents are in red and blue colors); (b) current-voltage characteristics at negative bias for 16 °C (blue) and 18 °C (red); (c) summary of current-voltage characteristics, where the arrows indicate the direction of increased applied voltage; (d) ZEPB source-drain current dependence on gate voltage is depicted.
medium. Consequently, only the external boundary conditions enter in the description of the quantum mechanical (QM) current. An expression for the photocurrent is arrived at by describing the initial states of the charge carriers in terms of a Gaussian wave function γ(σ,x - xn) reflecting the size of a GFP molecule centered at xn. Thus, we consider the particle-in-a-box eigenstates ΨR. Taking R to be the quantum number for the particle-in-a-onedimensional-box
ψR(x) )
2a sin Rπxa
(1)
and the length of the box a ) 30 nm, the linear momentum and thus the current can be extracted, assuming the system to be in the quasi-classical regime
ψ(x) ∝
1 sin √p
m x ∫0x p dξ ∝ √1 sin[ qp ∫0 I dl] ) I
1 m sin qEp √I
[
∫0V I dV] (2) x
where p is Planck’s constant, m is the electron mass, q is the elementary charge, and dl and dV are dummy variables and have length and voltage dimensions, respectively. The potential drop across the cell is aE (E is the electric field) and Vx ) xE.
This implies in particular that
pR(xn) ∝
|
d ln ψR(x) √2Rπ Rπ ) cot x dx a a n x)xn
(
)
(3)
We may determine the expansion coefficients of the Gaussia γ(σ,x - xn) in terms of the particle-in-a-box eigenstates
Ri(xn) ) 〈γ(σ;x - xn)|ψi(x)〉
(4)
and employ these to estimate the net electron momentum at a point 0 e xn e a, where a is the box size. Thus, the inversion symmetric momentum as function of position xn of the source is arrived at
∑ i Ri(xn)2pi(xn) ∑ i βi(a - xn)2pi(a - xn) P(xn) ) + ∑ i Ri(xn)2 ∑ i βi(a - xn)2
(5)
which describes both electrodes equivalently (see Figure 5a). It is observed that at zero bias only photon absorption in the vicinity of the electrodes produces current to any appreciable extent and that the net quantum mechanical current is zero due to the symmetry properties of eq 5. For this model to hold, aquantum mechanical zero bias net current implies a “left-right” asymmetry, which was argued already in this section to drive the ohmic contribution to the current. Its origin may be a spontaneous polarization in the GFP arrangement.
Letters
J. Phys. Chem. C, Vol. 112, No. 48, 2008 18721 N
∑
j (xjN) ) 1 P P(xn) N n)1
N
where
jxN )
∑
1 x N n)1 n
(6)
j N decays monotonically as function of It is apparent that P jxN, and that it vanishes when all GFPs become involved in the photocurrent generation. The averages are taken because a constant light intensity is assumed. Contact between model and experiment is made by assuming a linear relationship between the source-drain bias potential VSD, and number of GFP molecules N involved in the photoinduced QM contribution to the current. This model implies homogeneous conditions on the time scale of the electron transport process, i.e., conserved electron momentum p is equivalent to homogeneous and time independent field E. The purpose of this interpretation is 2-fold: (1) to assume the simplest model which connects electric current to quantum mechanical phenomenology, and (2) to reformulate the particle in a box expression into one which employs the parameters of the experiment. Conclusion A comprehensive understanding of the properties of a recently described biophotovoltaic nanodevice emerges based on a condensation of GFP molecules between two electrodes. The condensation invokes spontaneous symmetry breaking with regard to the direction across the cell, as manifested in a zero bias voltage photocurrent. The mechanism for electron transport was found to correlate with the molecular excitation cross section of the GFP fluorophore. Support for a liquid crystal type of ordering was provided by the suppression of spontaneous current with temperature. A nonresistive contribution to the photocurrent of possible quantum mechanical origin was extracted by subtracting the ohmic contribution to the total current. The quantum mechanical contribution was analyzed by exploiting a particle-in-a-box interpretation, assuming the system to be in the quasi-classical regime. Our work capitalizes on that nature has produced evolutionary stable biomolecules with potential use of technical applications with complete environmental compatibility. GFP produced by jellyfish Aequorea Victoria is shown to offer a promising strategy for biophotovoltaics. The full potential, of course, will only be realized if an effective up-scaling of the results of the present work can be obtained, for instance with nanoprint technology, with other boundary conditions such as sustainability and cost, being fulfilled. Such studies will be conducted in the future.
Figure 5. (a) Inversion symmetric momentum, relative to the maximum momentum P(xn)/P(x0) as function of position of the source is shown (compare eq 5). (b) Photocurrent extracted by subtracting the ohmic term from the total I-V curves in Figure 4a. (c) The experimental nonohmic contribution to the current superimposed on the predicted QM current eq 6, where a linear relationship between jxn and VSD is assumed.
Upon applying external voltage, excitations in increasingly larger domains become involved in the actual electron shuttling process. Loss of asymmetry in the sampling of current sources is equivalent to loss of the QM contribution to the current. We write the average momentum and the average photoelectron source position as function of distance from one electrode into the interior of the cell
Acknowledgment. Support from the Swedish Research Council is greatfully acknowledged. We also thank Dr. Daniel González, at the Department of Biology, Robert Renthal Laboratory, Science Building 3.02.27, University of Texas at San Antonio, One UTSA Circle, San Antonio, Texas 78249, U.S.A., for the EGFP. References and Notes (1) Schiermeier, Q.; Tollefson, J.; Scully, T.; Witze, A.; Morton, O. Nature 2008, 454, 816. (2) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737–739. (3) Chiragwandi, Z. G.; Gillespie, K.; Zhao, Q. X.; Willander, M.; Panas, I. Appl. Phys. Lett. 2006, 89, 162909. (4) Richmond, T. A.; Takahashi, T. T.; Shimkhada, R.; Bernsdorf, J. Biochem. Biophys. Commun. 2000, 268, 462–465. (5) Sullivan, K. F.; Kay, S. A. In Methods in cell biology; University of California Press, San Diego, CA, 1999; Vol. 58, Chapter 18. (6) Gruner, G. Bioanal. Chem. 2006, 384, 322–335. (7) Artukovic, E.; Kaempgen, M.; Hecht, D. S.; Roth, S.; Gru¨ner, G. Nano Lett. 2005, 5, 757.
JP807925K
