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Optical and photoelectronic properties of melanin
Thin Solid Films 511 – 512 (2006) 362 – 366 www.elsevier.com/locate/tsf
Optical and photoelectronic properties of melanin V. Capozzi a,b,*, G. Perna a,b, P. Carmone a, A. Gallone a, M. Lastella a, E. Mezzenga a, G. Quartucci a, M. Ambrico c, V. Augelli b,d, P.F. Biagi b,d, T. Ligonzo b,d, A. Minafra b,d, L. Schiavulli b,d, M. Pallara e, R. Cicero f a Dipartimento di Scienze Biomediche, Universita` di Foggia, Viale Pinto, I-71100 Foggia, Italy Istituto Nazionale di Fisica della Materia, Sezione di Bari, Via Amendola 173, I-70126 Bari, Italy c Istituto di Metodologie Inorganiche e dei Plasmi del C.N.R., Via Orabona 4, I-70126 Bari, Italy d Dipartimento Interateneo di Fisica, Universita` di Bari, Via Amendola 173, I-70126 Bari, Italy e Dipartimento Geomineralogico, Universita` di Bari, via Amendola 173, Bari, Italy Dipartimento di Biochimica Medica e Biologia Medica, Sezione di Biologia Medica, Facolta` di Medicina e Chirurgia, Universita` di Bari, Policlinico, I-70124 Bari, Italy b
f
Available online 23 January 2006
Abstract A study of the structural, optical and electrical properties of synthetic and natural melanin by means of X-ray diffraction, absorption and photocurrent techniques is reported. The model of the natural melanin film as a network of nano-aggregates of polymeric units based on the indolic structure is proposed to explain the X-ray diffraction results. The shape of the absorption spectra is similar to that of amorphous and disordered semiconductors, with a very strong, broad band UV and visible absorption and an optical gap value of about 0.5 eV. Photosensitivity to sun spectra has been demonstrated by photoconductivity measurements of synthetic melanin pellets under AM1 light source illumination. D 2005 Elsevier B.V. All rights reserved. Keywords: Eumelanin; Optical properties; Photoconductivity
1. Introduction Melanins are an important class of biological pigments largely found in living organisms. They are usually classified in eumelanin, which is a dark pigment containing nitrogen, and pheomelanin, which is yellow-to-reddish-brown pigment also containing sulphur [1]. Eumelanin, the most diffuse form, derives from oxidation of tyrosine [2] although it can be prepared by nonenzymatic way (synthetic melanin). Natural melanins are very complex, because they contain also proteins: purification processes can be used to isolate melanin from the protein component. Although a role as free radicals scavengers and antioxidant has been proposed [3,4], the main function of melanin is to act as skin protector from phototoxic events due to sunlight irradiance [5]. In fact, it largely absorbs UV and visible light by converting the light energy into heat. The broadband spectral absorbance of melanins are very interest* Corresponding author. Dipartimento di Scienze Biomediche, Universita` di Foggia, Viale Pinto, I-71100 Foggia, Italy. 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.12.065
ing for the possibility of use them as active materials for technological applications, as molecular electronic and photovoltaic devices. The technological use of melanin starts in 1974, when McGinnes et al. [6] demonstrated that melanin pellets behave as electrical switch and they postulated that these materials can be considered disordered organic semiconductors. However, several questions remain still open concerning the structural properties of melanins. It is known that eumelanins are macromolecules containing dihydroxyindole (DHI or HQ), dihydroxyde-carboxylic acid (DHICA), 5,6-indolequinone (IQ) and semiquinone (SQ) monomers [7,8]. However, the basic structural unit is still unknown. In particular, it is still a matter of debate whether eumelanin is a highly cross-linked heteropolymer or composed of oligomers condensed into nanoaggregates [9]. This is an important issue for understanding the charge transport in this pigment as well as all the optical properties. Recent results [10] support the nanoaggregate model, according to which the structural units (protomolecule) are based on stacked planar layers composed mainly of indolequinone monomers.
V. Capozzi et al. / Thin Solid Films 511 – 512 (2006) 362 – 366
363
In this paper we report a study on the structural, optical and photoelectronic properties of synthetic and natural melanin through X-ray diffraction (XRD), absorption and photoconductivity measurements. The aim of this work is to investigate the melanin basic structural units, which are related to the absorption properties of this biopolymer and to the possibility of obtain photoconductivity effects. 2. Experimentals Powder of melanin prepared by oxidation of tyrosine with H2O2, purchased from Sigma Aldrich, was used as model for synthetic melanin, whereas melanin extracted and isolated from melanosomes of the liver of Rana esculenta L., according to the method of R. Cicero et al. [11], was used as model for natural melanin. Pellets of synthetic melanin were obtained by pressing the powders at a pressure of about 500 MPa for about 3 min. The pellets, having a diameter of 13 mm and a thickness of about 0.5 mm, were used for XRD and photoconductivity measurements. The natural melanin was deposited on a quartz substrate and air dried for several days. This deposit of natural melanin was used for the XRD measurements. The absorption measurements were carried out on melanin solutions. The synthetic and natural melanin solutions were prepared in deionized water. To favour solubility, the pH of the synthetic melanin solution was adjusted to 11.5 by means of 0.01 M NaOH, whereas the natural melanin solution was gently heated and sonicated for 15 min. In both cases a continuous melanin dispersion was produced, because it is well known that melanin is essentially insoluble in any solvent [12]. XRD analysis was performed by using the CuKa radiation ˚ ) of a h – 2h diffractometer. The absorption (k = 1.5406 A spectra of melanin solutions at room temperature were obtained by using the well known Lambert – Beer law T = exp( ad), where T is the sample transmittance and d is the cuvette thickness. The transmittance was measured by means of a double beam spectrophotometer in the spectral range from 200 to 1300 nm. The temperature dependence of the photocurrent was investigated in the 300– 360 K range by exposing the sample to the radiation from a AM1 light source. During the photocurrent measurements the sample was kept in a temperature controlled oven; a Keithley 617 electrometer was used for both supplying 30 V between electrodes and for photocurrent detection. The photocurrent signal was recorded by a personal computer. 3. Results and discussion 3.1. XRD measurements The XRD spectra at room temperature of natural and synthetic melanin are shown in Fig. 1(a) and (b), respectively. Both spectra are characterised by a broad peak (as occurs in amorphous and disordered materials), centred at about 21.5for natural melanin and 25.6- for synthetic melanin. Such peaks are due to X-ray diffraction from parallel planar layers.
Fig. 1. X-ray spectra of (a) a deposit of natural melanin from Rana esculenta L. liver and (b) a pellet of synthetic melanin.
The peak position can give information about the interlayer spacing d, according to the Bragg equation 2dsinh ¼ mk
ð1Þ
where h is the diffraction angle, m is the diffraction order and k is the X-ray wavelength. By considering the first order ˚ for natural melanin diffraction (m = 1) we obtain d = 4.0 A ˚ and d = 3.5 A for synthetic melanin. In particular, the value of ˚ is in good agreement with the literature value of the 3.5 A interlayer spacing in the stacked sheets model of the melanin [10]. In contrast, the increase of the d distance occurring in natural melanin is probably due to the presence of residual molecules (from the purification procedure) intercalated between the layers, which increases the interlayer distance. An estimate of the average melanin grain size can be obtained from the Debye– Schrerrer relationship [13]: D ¼ 0:9k=ðFWHMIcoshÞ
ð2Þ
where FWHM is the full width at half maximum of the ˚ for natural diffraction peak. The obtained D values are 10.1 A ˚ for synthetic melanin. These values melanin and 13.5 A support the nanoaggregate model of melanin: in fact, they may correspond to the lateral or height extension of the melanin stacked sheets protomolecules. In particular, the value ˚ obtained for synthetic melanin corresponds to D = 13.5 A about four stacked sheets of planar structures. The average dimension of the natural melanin protomolecules is lower than
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V. Capozzi et al. / Thin Solid Films 511 – 512 (2006) 362 – 366
that of synthetic melanin ones probably for the presence of proteic residual in the former: it weakens the bonding forces between the layers and, consequently, counteracts the stacking. Therefore, the XRD results support the nanoaggregate model both for synthetic and natural melanin: it may be a general feature present in all types of melanin. 3.2. Absorption measurements The absorption spectra of natural and synthetic melanin solutions at room temperature are reported in Fig. 2(a) and (b), respectively. Both solutions present a decrease of the absorption coefficient vs. k with a very strong and broad UV –visible absorption. Such a behaviour is in agreement with photoprotection function of melanin and potentially useful for solar photon capture for photovoltaic applications. In particular, the absorption of synthetic melanin increases exponentially (the exponential fit is not shown in the figure) towards shorter wavelengths, whereas the absorption spectrum of natural melanin shows weak absorption features at about 350, 270 and 220 nm: these features can be assigned to absorption of light by the residual protein fragments which are not completely removed during the isolation and purification procedure (several amino acids have absorption maxima in this spectral region [14]).
Fig. 3. The rise and decay of photocurrent during switching on and off the radiation from a AM1 light source, at different temperatures for a pellet of synthetic melanin.
These spectra are similar to absorption spectra of typical amorphous semiconductors, in which the distribution of the electronic states does not terminate abruptly at the conduction and valence band edges but tails of localized states are present in the energy gap region. We remark that the energy gap in melanin in solution is associated with HOMO to LUMO transitions between k and k* molecular orbitals. An estimate value of the energy gap in melanin can be obtained by means of the Tauc method [15], which is commonly used to estimate the optical absorption edge in amorphous and disordered semiconductors. According to this method, the energy gap is valued by means of an empirical optical gap E Tauc, which is defined according to the following equation: að E ÞIE ¼ BIð E ETauc Þ2
ð3Þ
where E is the photon energy and B is an edge width parameter. The E Tauc estimate can be obtained by plotting (aE)1 / 2 versus E and extrapolating the linear portion of the plot to the abscissa: the intercept on the abscissa yields the E Tauc value. By means of this method we have estimated an E Tauc value of about 1.1 and 0.6 eV for synthetic and natural melanin in solution, respectively, as shown in the left hand insets of Fig. 2. The dashed line is a linear regression through the linear portion of the (aE)1 / 2 versus E plot. The narrower gap value for natural melanin suggests that it is more disordered with respect to the synthetic one: in fact, the disorder is caused by the presence of protein fragments that were not completely removed in the isolation and purification process. Probably, in agreement with the XRD results, some residual molecules from the protein coat are intercalated between the melanin layers, so increasing the structural disorder of the stacked layers and causing a shrinkage of the optical gap. The larger disorder in the natural melanin solution is also confirmed by considering the absorption profile at low energy, where it follows an exponential behaviour, according to the semiempirical relationship known as Urbach rule [16]: að E Þ ¼ a0 Iexp½ð E E0 Þ=U
Fig. 2. Absorption spectra of (a) a natural melanin solution and (b) a synthetic melanin solution (see text for details). In the inset the estimate of the optical gap (E g) according to the Tauc model [15] and the Urbach energy (U) [16] are reported.
ð4Þ
where a 0 and E 0 are material dependent parameters and the Urbach energy U is proportional to the broadening of the exponential absorption tail and, consequently, to the structural disorder characterizing the melanin solution. A least squares fit of the Eq. (4) to the experimental data (see dashed line in the
V. Capozzi et al. / Thin Solid Films 511 – 512 (2006) 362 – 366 Table 1 Time constants of the process described by Eq. (5)
365
right hand insets of Fig. 2) gives an estimate of the Urbach energy of 391 meV for natural melanin and 224 meV for synthetic melanin.
Table 1 follows quite well the model described by Eq. (6), as shown in Fig. 4, where a semilog plot of the two s r versus reciprocal temperature is reported. The fit of the s r values of Table 1 to Eq. (6) yields the following two values for the activation energy: 211 T 9 and 166 T 43 meV in correspondence of s r1 and s r2, respectively. The nanoaggregate model discussed above suggests a hopping mechanism for the photoconductivity process. Therefore, the trap energies calculated from the photoconductivity measurements represent the energy required to charged carriers to hop between two localized states.
3.3. Photoconductivity measurements
4. Conclusion
Fig. 3 shows the photoconductivity signal of a synthetic melanin pellet, measured exposing the sample under AM1 light source, at different temperatures from 320 to 360 K. The photocurrent signal rises when the light source is switched on with a behaviour that can be described as a combination of two exponential functions. The rise of the photocurrent intensity I r has been fitted by means of the following function:
The structural, optical and photoelectronic properties of natural and synthetic melanin have been investigated by means of XRD, absorption spectroscopy and photocurrent measurements. The XRD measurements support a nanoaggregate model for both types of melanin, based on structural units consisting of several stacked layers of indole-based oligomers. In particular, the structural units of the natural melanin are smaller and more disordered that those of synthetic melanin, as confirmed by the absorption measurements. The large UV and visible absorption of both melanins may be potentially useful for photovoltaic applications, because of the solar photons harvesting possibility. Preliminary results about the rise and decay of the photoconductivity intensity of synthetic melanin pellets show the presence of a photocurrent signal under illumination with a AM1 light source. In this disordered structure, the main role during the photoconductivity process is due to hopping mechanism. However, this is only a first step in view of possible application of melanin-based devices. Several efforts should be made before a realistic application. First of all, it is necessary to produce good quality material, mainly thin films of controlled thickness and composition. Next, it is necessary to learn how to improve the electronic properties (as the electrical conductivity) and to explore the possibility of doping. Finally, a coupling of the melanin to a suitable electrode should be studied, in order to obtain an efficient transport of the carriers through the external circuit.
T (K)
s r1 (s)
s r2 (s)
320 340 360
85 T 7 56 T 2 36 T 1
1017 T 211 829 T 15 519 T 11
Ir ¼ IS Ar1 expð t=sr1 Þ Ar2 expð t=sr2 Þ
ð5Þ
where I s is the steady state photocurrent, A r and s r are the amplitude and time constants for rise process. The values of the time constants are reported in Table 1. Both time constants are very long; in addition, they decrease with increasing temperature. These features can be explained by a very large trap density. Further, in the photoconductivity process the time constant is proportional to the trap concentration [17]. So, the photocurrent signal reaches the steady state value more rapidly as the temperature increases. The depth of the traps involved in the photocurrent mechanism can be estimated by considering the temperature dependence of the time constants, according to the following equation [18]: s ¼ s0 expð DE þ kT Þ
ð6Þ
where DE is the energy difference between the trap levels and the band edges and k is the Boltzmann constant. The temperature dependence of the two time constants shown in 100 90 80
(a)
(b)
70 τr1(S)
1000 900 800 τr2(S) 700
60 50 40 30 0.032
600 ΔE=211±9 meV
0.034
0.036
500 400 0.032
ΔE=166±43 meV 0.034
0.036
1/kT (meV) Fig. 4. Semilog plot of photocurrent time constants s r1 (a) and s r2 (b) versus 1 / kT. The continuous line is a least squares fit to Eq. (6), from which the activation energy DE is obtained.
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V. Capozzi et al. / Thin Solid Films 511 – 512 (2006) 362 – 366
References [1] [2] [3] [4] [5] [6] [7] [8]
D.S. Galvao, M.J. Caldas, J. Chem. Phys. 92 (1990) 2630. P.A. Riley, Int. J. Biochem. Cell Biol. 29 (1997) 1235. G. Prota, Melanins and Melanogenesis, Academic Press, San Diego, 1992. M.L. Wolbarsht, A.W. Walsh, G. George, Appl. Opt. 20 (1981) 2184. H. Ou-Yang, G. Stamatas, N. Kollias, J. Invest. Dermatol. 122 (2004) 492. J. McGinnes, P. Corry, P. Proctor, Science 183 (1974) 853. S. Ito, Biochim. Biophys. Acta 883 (1986) 155. L.E. Bolivar-Marinez, D.S. Galvao, M.J. Caldas, J. Phys. Chem., B 103 (1999) 2993. [9] C.M.R. Clancy, J.B. Nofsinger, R.K. Hanks, J.D. Simon, J. Phys. Chem., B 104 (2000) 7871. [10] C.M.R. Clancy, J.D. Simon, Biochemistry 40 (2001) 13353. [11] R. Cicero, A. Mallardi, I. Maida, A. Gallone, G. Pintucci, Pigment Cell Res. 2 (1989) 100.
[12] Y. Liu, J.D. Simon, Pigment Cell Res. 16 (2003) 72. [13] H.P. Klug, L.E. Alexander, X-ray Diffraction Procedures, Wiley, New York, 1954. [14] J.B. Nofsinger, E.E. Weinert, J.D. Simon, Biopolymers 67 (2002) 302. [15] J. Tauc, R. Grigorovici, A. Vancu, Phys. Status Solidi, A Appl. Res. 15 (1966) 627. [16] F. Urbach, Phys. Rev. 92 (1953) 1324; J. Tauc, Amorphous and Liquid Semiconductors, Plenum, New York, 1974. [17] H. Meier, Organic Semiconductors. Dark- and Photoconductivity of Organic Solids, Verlag Chemie, Weinheim, 1974. [18] K.C. Kao, W. Hwang, Electrical Transport in Solids, Pergamon Press, Oxford, 1981.
Optical and photoelectronic properties of melanin V. Capozzi a,b,*, G. Perna a,b, P. Carmone a, A. Gallone a, M. Lastella a, E. Mezzenga a, G. Quartucci a, M. Ambrico c, V. Augelli b,d, P.F. Biagi b,d, T. Ligonzo b,d, A. Minafra b,d, L. Schiavulli b,d, M. Pallara e, R. Cicero f a Dipartimento di Scienze Biomediche, Universita` di Foggia, Viale Pinto, I-71100 Foggia, Italy Istituto Nazionale di Fisica della Materia, Sezione di Bari, Via Amendola 173, I-70126 Bari, Italy c Istituto di Metodologie Inorganiche e dei Plasmi del C.N.R., Via Orabona 4, I-70126 Bari, Italy d Dipartimento Interateneo di Fisica, Universita` di Bari, Via Amendola 173, I-70126 Bari, Italy e Dipartimento Geomineralogico, Universita` di Bari, via Amendola 173, Bari, Italy Dipartimento di Biochimica Medica e Biologia Medica, Sezione di Biologia Medica, Facolta` di Medicina e Chirurgia, Universita` di Bari, Policlinico, I-70124 Bari, Italy b
f
Available online 23 January 2006
Abstract A study of the structural, optical and electrical properties of synthetic and natural melanin by means of X-ray diffraction, absorption and photocurrent techniques is reported. The model of the natural melanin film as a network of nano-aggregates of polymeric units based on the indolic structure is proposed to explain the X-ray diffraction results. The shape of the absorption spectra is similar to that of amorphous and disordered semiconductors, with a very strong, broad band UV and visible absorption and an optical gap value of about 0.5 eV. Photosensitivity to sun spectra has been demonstrated by photoconductivity measurements of synthetic melanin pellets under AM1 light source illumination. D 2005 Elsevier B.V. All rights reserved. Keywords: Eumelanin; Optical properties; Photoconductivity
1. Introduction Melanins are an important class of biological pigments largely found in living organisms. They are usually classified in eumelanin, which is a dark pigment containing nitrogen, and pheomelanin, which is yellow-to-reddish-brown pigment also containing sulphur [1]. Eumelanin, the most diffuse form, derives from oxidation of tyrosine [2] although it can be prepared by nonenzymatic way (synthetic melanin). Natural melanins are very complex, because they contain also proteins: purification processes can be used to isolate melanin from the protein component. Although a role as free radicals scavengers and antioxidant has been proposed [3,4], the main function of melanin is to act as skin protector from phototoxic events due to sunlight irradiance [5]. In fact, it largely absorbs UV and visible light by converting the light energy into heat. The broadband spectral absorbance of melanins are very interest* Corresponding author. Dipartimento di Scienze Biomediche, Universita` di Foggia, Viale Pinto, I-71100 Foggia, Italy. 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.12.065
ing for the possibility of use them as active materials for technological applications, as molecular electronic and photovoltaic devices. The technological use of melanin starts in 1974, when McGinnes et al. [6] demonstrated that melanin pellets behave as electrical switch and they postulated that these materials can be considered disordered organic semiconductors. However, several questions remain still open concerning the structural properties of melanins. It is known that eumelanins are macromolecules containing dihydroxyindole (DHI or HQ), dihydroxyde-carboxylic acid (DHICA), 5,6-indolequinone (IQ) and semiquinone (SQ) monomers [7,8]. However, the basic structural unit is still unknown. In particular, it is still a matter of debate whether eumelanin is a highly cross-linked heteropolymer or composed of oligomers condensed into nanoaggregates [9]. This is an important issue for understanding the charge transport in this pigment as well as all the optical properties. Recent results [10] support the nanoaggregate model, according to which the structural units (protomolecule) are based on stacked planar layers composed mainly of indolequinone monomers.
V. Capozzi et al. / Thin Solid Films 511 – 512 (2006) 362 – 366
363
In this paper we report a study on the structural, optical and photoelectronic properties of synthetic and natural melanin through X-ray diffraction (XRD), absorption and photoconductivity measurements. The aim of this work is to investigate the melanin basic structural units, which are related to the absorption properties of this biopolymer and to the possibility of obtain photoconductivity effects. 2. Experimentals Powder of melanin prepared by oxidation of tyrosine with H2O2, purchased from Sigma Aldrich, was used as model for synthetic melanin, whereas melanin extracted and isolated from melanosomes of the liver of Rana esculenta L., according to the method of R. Cicero et al. [11], was used as model for natural melanin. Pellets of synthetic melanin were obtained by pressing the powders at a pressure of about 500 MPa for about 3 min. The pellets, having a diameter of 13 mm and a thickness of about 0.5 mm, were used for XRD and photoconductivity measurements. The natural melanin was deposited on a quartz substrate and air dried for several days. This deposit of natural melanin was used for the XRD measurements. The absorption measurements were carried out on melanin solutions. The synthetic and natural melanin solutions were prepared in deionized water. To favour solubility, the pH of the synthetic melanin solution was adjusted to 11.5 by means of 0.01 M NaOH, whereas the natural melanin solution was gently heated and sonicated for 15 min. In both cases a continuous melanin dispersion was produced, because it is well known that melanin is essentially insoluble in any solvent [12]. XRD analysis was performed by using the CuKa radiation ˚ ) of a h – 2h diffractometer. The absorption (k = 1.5406 A spectra of melanin solutions at room temperature were obtained by using the well known Lambert – Beer law T = exp( ad), where T is the sample transmittance and d is the cuvette thickness. The transmittance was measured by means of a double beam spectrophotometer in the spectral range from 200 to 1300 nm. The temperature dependence of the photocurrent was investigated in the 300– 360 K range by exposing the sample to the radiation from a AM1 light source. During the photocurrent measurements the sample was kept in a temperature controlled oven; a Keithley 617 electrometer was used for both supplying 30 V between electrodes and for photocurrent detection. The photocurrent signal was recorded by a personal computer. 3. Results and discussion 3.1. XRD measurements The XRD spectra at room temperature of natural and synthetic melanin are shown in Fig. 1(a) and (b), respectively. Both spectra are characterised by a broad peak (as occurs in amorphous and disordered materials), centred at about 21.5for natural melanin and 25.6- for synthetic melanin. Such peaks are due to X-ray diffraction from parallel planar layers.
Fig. 1. X-ray spectra of (a) a deposit of natural melanin from Rana esculenta L. liver and (b) a pellet of synthetic melanin.
The peak position can give information about the interlayer spacing d, according to the Bragg equation 2dsinh ¼ mk
ð1Þ
where h is the diffraction angle, m is the diffraction order and k is the X-ray wavelength. By considering the first order ˚ for natural melanin diffraction (m = 1) we obtain d = 4.0 A ˚ and d = 3.5 A for synthetic melanin. In particular, the value of ˚ is in good agreement with the literature value of the 3.5 A interlayer spacing in the stacked sheets model of the melanin [10]. In contrast, the increase of the d distance occurring in natural melanin is probably due to the presence of residual molecules (from the purification procedure) intercalated between the layers, which increases the interlayer distance. An estimate of the average melanin grain size can be obtained from the Debye– Schrerrer relationship [13]: D ¼ 0:9k=ðFWHMIcoshÞ
ð2Þ
where FWHM is the full width at half maximum of the ˚ for natural diffraction peak. The obtained D values are 10.1 A ˚ for synthetic melanin. These values melanin and 13.5 A support the nanoaggregate model of melanin: in fact, they may correspond to the lateral or height extension of the melanin stacked sheets protomolecules. In particular, the value ˚ obtained for synthetic melanin corresponds to D = 13.5 A about four stacked sheets of planar structures. The average dimension of the natural melanin protomolecules is lower than
364
V. Capozzi et al. / Thin Solid Films 511 – 512 (2006) 362 – 366
that of synthetic melanin ones probably for the presence of proteic residual in the former: it weakens the bonding forces between the layers and, consequently, counteracts the stacking. Therefore, the XRD results support the nanoaggregate model both for synthetic and natural melanin: it may be a general feature present in all types of melanin. 3.2. Absorption measurements The absorption spectra of natural and synthetic melanin solutions at room temperature are reported in Fig. 2(a) and (b), respectively. Both solutions present a decrease of the absorption coefficient vs. k with a very strong and broad UV –visible absorption. Such a behaviour is in agreement with photoprotection function of melanin and potentially useful for solar photon capture for photovoltaic applications. In particular, the absorption of synthetic melanin increases exponentially (the exponential fit is not shown in the figure) towards shorter wavelengths, whereas the absorption spectrum of natural melanin shows weak absorption features at about 350, 270 and 220 nm: these features can be assigned to absorption of light by the residual protein fragments which are not completely removed during the isolation and purification procedure (several amino acids have absorption maxima in this spectral region [14]).
Fig. 3. The rise and decay of photocurrent during switching on and off the radiation from a AM1 light source, at different temperatures for a pellet of synthetic melanin.
These spectra are similar to absorption spectra of typical amorphous semiconductors, in which the distribution of the electronic states does not terminate abruptly at the conduction and valence band edges but tails of localized states are present in the energy gap region. We remark that the energy gap in melanin in solution is associated with HOMO to LUMO transitions between k and k* molecular orbitals. An estimate value of the energy gap in melanin can be obtained by means of the Tauc method [15], which is commonly used to estimate the optical absorption edge in amorphous and disordered semiconductors. According to this method, the energy gap is valued by means of an empirical optical gap E Tauc, which is defined according to the following equation: að E ÞIE ¼ BIð E ETauc Þ2
ð3Þ
where E is the photon energy and B is an edge width parameter. The E Tauc estimate can be obtained by plotting (aE)1 / 2 versus E and extrapolating the linear portion of the plot to the abscissa: the intercept on the abscissa yields the E Tauc value. By means of this method we have estimated an E Tauc value of about 1.1 and 0.6 eV for synthetic and natural melanin in solution, respectively, as shown in the left hand insets of Fig. 2. The dashed line is a linear regression through the linear portion of the (aE)1 / 2 versus E plot. The narrower gap value for natural melanin suggests that it is more disordered with respect to the synthetic one: in fact, the disorder is caused by the presence of protein fragments that were not completely removed in the isolation and purification process. Probably, in agreement with the XRD results, some residual molecules from the protein coat are intercalated between the melanin layers, so increasing the structural disorder of the stacked layers and causing a shrinkage of the optical gap. The larger disorder in the natural melanin solution is also confirmed by considering the absorption profile at low energy, where it follows an exponential behaviour, according to the semiempirical relationship known as Urbach rule [16]: að E Þ ¼ a0 Iexp½ð E E0 Þ=U
Fig. 2. Absorption spectra of (a) a natural melanin solution and (b) a synthetic melanin solution (see text for details). In the inset the estimate of the optical gap (E g) according to the Tauc model [15] and the Urbach energy (U) [16] are reported.
ð4Þ
where a 0 and E 0 are material dependent parameters and the Urbach energy U is proportional to the broadening of the exponential absorption tail and, consequently, to the structural disorder characterizing the melanin solution. A least squares fit of the Eq. (4) to the experimental data (see dashed line in the
V. Capozzi et al. / Thin Solid Films 511 – 512 (2006) 362 – 366 Table 1 Time constants of the process described by Eq. (5)
365
right hand insets of Fig. 2) gives an estimate of the Urbach energy of 391 meV for natural melanin and 224 meV for synthetic melanin.
Table 1 follows quite well the model described by Eq. (6), as shown in Fig. 4, where a semilog plot of the two s r versus reciprocal temperature is reported. The fit of the s r values of Table 1 to Eq. (6) yields the following two values for the activation energy: 211 T 9 and 166 T 43 meV in correspondence of s r1 and s r2, respectively. The nanoaggregate model discussed above suggests a hopping mechanism for the photoconductivity process. Therefore, the trap energies calculated from the photoconductivity measurements represent the energy required to charged carriers to hop between two localized states.
3.3. Photoconductivity measurements
4. Conclusion
Fig. 3 shows the photoconductivity signal of a synthetic melanin pellet, measured exposing the sample under AM1 light source, at different temperatures from 320 to 360 K. The photocurrent signal rises when the light source is switched on with a behaviour that can be described as a combination of two exponential functions. The rise of the photocurrent intensity I r has been fitted by means of the following function:
The structural, optical and photoelectronic properties of natural and synthetic melanin have been investigated by means of XRD, absorption spectroscopy and photocurrent measurements. The XRD measurements support a nanoaggregate model for both types of melanin, based on structural units consisting of several stacked layers of indole-based oligomers. In particular, the structural units of the natural melanin are smaller and more disordered that those of synthetic melanin, as confirmed by the absorption measurements. The large UV and visible absorption of both melanins may be potentially useful for photovoltaic applications, because of the solar photons harvesting possibility. Preliminary results about the rise and decay of the photoconductivity intensity of synthetic melanin pellets show the presence of a photocurrent signal under illumination with a AM1 light source. In this disordered structure, the main role during the photoconductivity process is due to hopping mechanism. However, this is only a first step in view of possible application of melanin-based devices. Several efforts should be made before a realistic application. First of all, it is necessary to produce good quality material, mainly thin films of controlled thickness and composition. Next, it is necessary to learn how to improve the electronic properties (as the electrical conductivity) and to explore the possibility of doping. Finally, a coupling of the melanin to a suitable electrode should be studied, in order to obtain an efficient transport of the carriers through the external circuit.
T (K)
s r1 (s)
s r2 (s)
320 340 360
85 T 7 56 T 2 36 T 1
1017 T 211 829 T 15 519 T 11
Ir ¼ IS Ar1 expð t=sr1 Þ Ar2 expð t=sr2 Þ
ð5Þ
where I s is the steady state photocurrent, A r and s r are the amplitude and time constants for rise process. The values of the time constants are reported in Table 1. Both time constants are very long; in addition, they decrease with increasing temperature. These features can be explained by a very large trap density. Further, in the photoconductivity process the time constant is proportional to the trap concentration [17]. So, the photocurrent signal reaches the steady state value more rapidly as the temperature increases. The depth of the traps involved in the photocurrent mechanism can be estimated by considering the temperature dependence of the time constants, according to the following equation [18]: s ¼ s0 expð DE þ kT Þ
ð6Þ
where DE is the energy difference between the trap levels and the band edges and k is the Boltzmann constant. The temperature dependence of the two time constants shown in 100 90 80
(a)
(b)
70 τr1(S)
1000 900 800 τr2(S) 700
60 50 40 30 0.032
600 ΔE=211±9 meV
0.034
0.036
500 400 0.032
ΔE=166±43 meV 0.034
0.036
1/kT (meV) Fig. 4. Semilog plot of photocurrent time constants s r1 (a) and s r2 (b) versus 1 / kT. The continuous line is a least squares fit to Eq. (6), from which the activation energy DE is obtained.
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