Study of optical properties of electropolymerized melanin films by photopyroelectric spectroscopy

Abstract

Photopyroelectric (PPE) spectroscopy, in the 350-1,075 nm wavelength range, was used to study the optical properties of electropolymerized melanin films on indium tin oxide (ITO) coated glass. The PPE intensity signal as a function of the wavelength lambda, V (n)(lambda) and its phase F (n)(lambda) were independently measured. Using the PPE signal intensity and the thermal and optical properties of the pyroelectric detector, we were able to calculate the optical absorption coefficient beta of melanin in the solid-state. We believe this to be the first such measurement of its kind on this material. Additionally, we found an optical gap in these melanin films at 1.70 eV.

The physical and chemical properties of eumelanin

“…If a fundamental gap does exist then absorption (either in solution or in solid state) is not an appropriate tool to detect and measure it. Recent photopyroelectric spectroscopy (PPS) measurements indicate that there may indeed be some fundamental energy gap in eumelanins at 1.7 eV (the near infrared consistent with eumelanin 'blackness') (de Albuquerque et al, 2005(de Albuquerque et al, , 2006. In PPS, a true absorption coefficient can be measured in even heavily scattering solid samples.…”

Determination of thermal and optical properties of ion implanted polyetheretherketone films by photothermal spectroscopies

“…The measuring system is composed of a lock-in amplifier, locked at the chopper frequency, connected to a microcomputer that stores the data and controls the experiment. The mechanical slotted wheel chopper modulates the incident light whose intensity obeys the expression [14][15][16]19,23 …”

“…The detected signal V͑ , t͒ is proportional to the pyroelectric coefficient p of the detector and to the temperature distribution along the detector thickness: 15,16,18,19,22,23 …”

“…The heat propagation across the whole chamber is governed by heat diffusion equations of each medium coupled via boundary conditions at the interfaces ͑T a = T b and exp͑± R L R ͒Ϸ1, ͉r R ͉ ӷ 1, and R Ϸ 1, where r n = ␤ n / n and n = ͑1+i͒a n , with a n = ͑f / ␣ n ͒ 1/2 ͑n = g , s , p; that is, g = gas, s = sample, and p = pyroelectric͒. Then, the normalized voltage signal results in 16,19,23,24 …”

“…[10][11][12][13] Among the PTS techniques, photoacoustic spectroscopy ͑PAS͒, 14 which is the most traditional one, and the more recent photopyroelectric spectroscopy 15 ͑PPES͒ have been used for studying thermal and optical properties of polymeric films and semiconductors. [16][17][18][19][20][21][22][23][24] In these techniques a pulsed light beam is absorbed in a solid sample and the converted heat diffuses into the bulk structure; the sample expansion, or the temperature gradient, is then detected by an appropriate sensor system. The detected signal depends on the optical and thermal properties of the sample: the optical absorption coefficient ␤͑͒ ͑ being the light wavelength͒, the nonradiative conversion efficiency ͑͒, the thermal conductivity k, and the thermal diffusivity coefficient ␣.…”