- Email: [email protected]
Semiconductivity in β-alanine
Volume 6, -number 5
CHEMICAL PHYSICSLETTERS
SEMICONDUCTIVITY
1 September 1970
IN /6-ALANINE
T. MOOKHEHJI* Teledyne-Brawn EngSzeevitrg Company, Hunfs&tZe, Alabama, USA
and AHUN P. KUI.SHHESHTHA** NASA,
George C. Marshall @ace Flight -Huntsville. Alabama, USA
Center,
Received 6 July 1970
Temperature dependence of electrical conductivity and optical transmission and reflection have been studied in pure and X-ray irradiated&alanine single crystals and powder. Direct allowed transition with room temperature energy of 5.54 eV has been observed. X-ray damage is found to.produce a broad absorption band overlapping the fundamental band.
The electrical conduction in amino acids in the solid state is a subject with implications for biochemistry. /3-alanine (3 aminopropionic acid) H H-C NH2
H -
C-C !i
//O \
OH
is a representative of this class of biological materi& and very little work has been done on its electronic properties. The dark electrical conductivity of &alanine was first measured by Cardew and Eley [l J, who found an activation energy of 4.07 eV, attributed to a process associated with thermal excitation of electrons into the conduction band. Later, Vasilescu et al. [2] observed an activation energy of 3.8 eV using an improved technique of measuring high resistivities. #?-alanine fluoresces feebly and shows two peaks at wavelengths 459C& (2.70 eV) and 521OA (2.38 cV) when the crystals are excited by light from a high-pressure mercury lamp passing through a Wood glass filter [3]. The discrepancy in the several energy values Physicist, Research Laboratories. Postdoctoral Resident Research Associate of National Academy of Sciences - National Research Counail. USA; assigned to Thermal Environment Physics Branch,. Space Thermophysics Division, Space Sciences Laboratory. :
led us to investigate more thoroughly the electronic conduction in this material. We have used optical transmission and reflection techniques to determine whether the reported activation energies by Cardew and Eley [I] and Vasilescu et al. [Z] could be related to the fundamental band gap. We have also measured the temperature dependence of electrical conductivity to find a suitable correlation with the earlier mentioned activation energies The material, in powder form, was obtained from Eastman Kodak Company; it was recrystallized several times from water solution, and crystals at different stages of purification following the second recrystallization did not show any difference in the neasurements. B-alanlne crystallizes in the orthorhombic class and grows as plates parallel to the (100) face. The crystals have a cleavage plane parallel to the (010) face [4]. We studied the opticaL transmission in a direction perpendicular to the cleavage plane, The electrical conductivity measurements were made both paralleL and perpendicular to this plane, but no appreciable anisotropy could be observed_ As the crystals were very transparent, even for the thicker samples, the reflectivity measurements were made on pressed pellets formed under pressures varying from 5 OOCto 25 000 psi. The reflectivity spectrum‘did not show dependence on the applied pressure; however, the airsoLute values of re.-
423 ._
: Volume 6. nynber 5
CHEMICAL
- --....-.-.29-i 1
PHYSICS
LETTERS
’ “:, 1 September 1970
UNIRRADIATCD --
X-IRRADIATRD
0 Hr)
X-IRRADlAfRD(3Hts,
=. .
’
.’ ;.
RAVELENCTllS
Fig. 1. Transmission
I
\ :
. ‘1 . \ -*. ‘r-r
/
/’
’
,
IN NlClKm
spectra in pure and X-huiiated
single crystals.
UNIRRADIATED X-IRRADIATED
.“.2 ‘.3.~ ‘..4. _
.5
1
.7 .‘.E .-,p-
1.0 :.
1.1
-
1.2 ,13.,:
1.4.
1.5.
IA
lq
1-S.
l.P..u)‘Zl-
2.2
-22
1
Volume 6, number 5
CHEMICAL PHYSICS LETTERS
flectivity for a particular wavelength increased slightly with higher pressures, which was presumably due to the added smoothing of the pellet surface. Optical transmission and reflection measurements were made using a Beckman DK-2A ratiorecording spectrometer. Temperature dependence of electrical conductivity was measured using a dc electrometer technique, as described elsewhere [5]. Typical transmission and reflection spectra are shown in figs. la and 2a, respectively. The absorption peaks inthevicinity of 2.20 and 1.75 microns (0.56 and 0.71 eV) appear in both the spectra, whereas the peaks near 1.40 and 1.15 microns (0.89 and 1.08 eV) are observed only in the case of powder reflectivity spectrum. The latter energy values could possible be due to the presence of certain impurities in the original powder which were removed by a subsequent recrystallization process. An analysis of the trans-
I September
1970
between 1.3 to 2.2 microns was made to find the possibility of free carrier absorption in this region The optical density in this wavelength region did not depend on A2. However, the region between 1.7 and 2.0 microns showed a k2.7 dependence. The sharp rise in the transmission and the reflection spectra (figs. Ia and Za) beyond 0.2 micron indicates the possibility of existence of a band-gap type absorption. Following the technique of McLean [S], the values of the squares and the square-roots of the optical density, log&/I), were plotted against wavelength, and only the square function could be fitted to a linear: curve, mission spectrum in the,r&on
as shown in fig. 3. This may be taken as evidence in favor of an allowed direct transition with an
energy Eg equal to 5.5 eV.
As mentioned earlier, a fluorescence spectrum in the visible region was observed by Dertrand [3]. In our .case, since the opticaL detection system belongs to the nonmonochromatic class,
Voly&
6, numb82 5:
CHEiMCAL
PHYSICS
LETTERS
1 Septemtre* 1970
-PURE CRYSTALS - - - X-~RRADIATLD CRYSTAL
1x10’;
2.6
I
I
I
2.7
2.8
2.9
I
3.0
I
I
3.1
3.2
I
3.3
1
I
I
3.i
3.5
3.6
Fig. 4. Temperature of electrical resistivfiy in pure and X-irradiated single crystals. the watt position of the fluorescence band cannot be located in the reflectivity mode of meas-
urement. However, beyond the minimum energy required for the excitation of fluorescence, the total integrated intensity from the sample in the reffecfivity spectrum will increase by an amount proportional to the total fluorescence intensity. Fig. 2a possibly depicts this case where the reflectivity spectrum is seen to rise above 100 per cent in the region 0.25 to 0.40 micron. The temperature dependence of electrical resistivity of &alanine is shown in fig. 4. Curves for all the samples yielded only one slope and the calculated activation energy from p - exp(E/2kT) was found to be 0.56 (A 0.05) eV. This value corresponds to one of the absorption bands arising due to impurity in the transmission and reflection spectra of the crystal as already mentioned. The effect of X-ray irradiation on 8-alanine has been found to produce electron spin resonance.and this has been attributed to the formation of free radicals [7]. Since no data are available on other electronic properties of X-irradiated s-alanine, we havti extended our work to study the optical and electrical properties of this material after damaging it with X rays of 20 kV, 20 mA at room temperature for dose times ranging from 1 to 3 hours. No change in the activation energy of ,electrical conduction was observed after this treatment (fig. 4). The change in optical 426
.
,
transmissio? is shown in figs. lb and lc after 1 hour 2nd 3 hours of irradiation dose, respective: Iy. Change in the reflectivity has been shown in fig. 2b for a powder sample that was irradiated for 3 hours. The nature of both these spectra remains essentially the same, except for the creation of a broad absorption band overlapping the fundamental absorption. Further work on the luminescence in pure and X-irrxiiated fi-alanine is in progress to clarify the uncertainties associated with the optical spectra near the band gap. The authors wish to express their appreciation to Mr. G. M. Arnett and Dr. H. J. Watson for their constant encouragement and to Mr. D. R. Wilkes and Mr. S. A. Fields for providing the facilities for this work. REFERENCES [I] M. E. Cardew and D. D. Eley, Discussions Faraday SOC. 27 (l959j 115. [Z] D. Vasilescu,. A. Jolivet, and A. Brau, Rev. Phys. AppL. 2 (1967) 283. [3] D. Bertrand, Bull. Sot. Chim. 12 (1945) 1023. [4].P.Jose and L.M.Pant, Acta Cry&. 18 (1965) 806. [5] A. P.KuIshreshtba and T. Moolfherji, MoI.Cryst.. to bu published. [6] A. F. Gibson et al.,. Progress in semidonductors, Vol. 5 (Heywood, London,, 1950) p. 55. i?] H. Shields and W. Gordy. J. Phys. Chem. Li2 (l958) 789. ” _ -.
.-
.- . .:
_.‘.;.
‘-
_._
_-
,
CHEMICAL PHYSICSLETTERS
SEMICONDUCTIVITY
1 September 1970
IN /6-ALANINE
T. MOOKHEHJI* Teledyne-Brawn EngSzeevitrg Company, Hunfs&tZe, Alabama, USA
and AHUN P. KUI.SHHESHTHA** NASA,
George C. Marshall @ace Flight -Huntsville. Alabama, USA
Center,
Received 6 July 1970
Temperature dependence of electrical conductivity and optical transmission and reflection have been studied in pure and X-ray irradiated&alanine single crystals and powder. Direct allowed transition with room temperature energy of 5.54 eV has been observed. X-ray damage is found to.produce a broad absorption band overlapping the fundamental band.
The electrical conduction in amino acids in the solid state is a subject with implications for biochemistry. /3-alanine (3 aminopropionic acid) H H-C NH2
H -
C-C !i
//O \
OH
is a representative of this class of biological materi& and very little work has been done on its electronic properties. The dark electrical conductivity of &alanine was first measured by Cardew and Eley [l J, who found an activation energy of 4.07 eV, attributed to a process associated with thermal excitation of electrons into the conduction band. Later, Vasilescu et al. [2] observed an activation energy of 3.8 eV using an improved technique of measuring high resistivities. #?-alanine fluoresces feebly and shows two peaks at wavelengths 459C& (2.70 eV) and 521OA (2.38 cV) when the crystals are excited by light from a high-pressure mercury lamp passing through a Wood glass filter [3]. The discrepancy in the several energy values Physicist, Research Laboratories. Postdoctoral Resident Research Associate of National Academy of Sciences - National Research Counail. USA; assigned to Thermal Environment Physics Branch,. Space Thermophysics Division, Space Sciences Laboratory. :
led us to investigate more thoroughly the electronic conduction in this material. We have used optical transmission and reflection techniques to determine whether the reported activation energies by Cardew and Eley [I] and Vasilescu et al. [Z] could be related to the fundamental band gap. We have also measured the temperature dependence of electrical conductivity to find a suitable correlation with the earlier mentioned activation energies The material, in powder form, was obtained from Eastman Kodak Company; it was recrystallized several times from water solution, and crystals at different stages of purification following the second recrystallization did not show any difference in the neasurements. B-alanlne crystallizes in the orthorhombic class and grows as plates parallel to the (100) face. The crystals have a cleavage plane parallel to the (010) face [4]. We studied the opticaL transmission in a direction perpendicular to the cleavage plane, The electrical conductivity measurements were made both paralleL and perpendicular to this plane, but no appreciable anisotropy could be observed_ As the crystals were very transparent, even for the thicker samples, the reflectivity measurements were made on pressed pellets formed under pressures varying from 5 OOCto 25 000 psi. The reflectivity spectrum‘did not show dependence on the applied pressure; however, the airsoLute values of re.-
423 ._
: Volume 6. nynber 5
CHEMICAL
- --....-.-.29-i 1
PHYSICS
LETTERS
’ “:, 1 September 1970
UNIRRADIATCD --
X-IRRADIATRD
0 Hr)
X-IRRADlAfRD(3Hts,
=. .
’
.’ ;.
RAVELENCTllS
Fig. 1. Transmission
I
\ :
. ‘1 . \ -*. ‘r-r
/
/’
’
,
IN NlClKm
spectra in pure and X-huiiated
single crystals.
UNIRRADIATED X-IRRADIATED
.“.2 ‘.3.~ ‘..4. _
.5
1
.7 .‘.E .-,p-
1.0 :.
1.1
-
1.2 ,13.,:
1.4.
1.5.
IA
lq
1-S.
l.P..u)‘Zl-
2.2
-22
1
Volume 6, number 5
CHEMICAL PHYSICS LETTERS
flectivity for a particular wavelength increased slightly with higher pressures, which was presumably due to the added smoothing of the pellet surface. Optical transmission and reflection measurements were made using a Beckman DK-2A ratiorecording spectrometer. Temperature dependence of electrical conductivity was measured using a dc electrometer technique, as described elsewhere [5]. Typical transmission and reflection spectra are shown in figs. la and 2a, respectively. The absorption peaks inthevicinity of 2.20 and 1.75 microns (0.56 and 0.71 eV) appear in both the spectra, whereas the peaks near 1.40 and 1.15 microns (0.89 and 1.08 eV) are observed only in the case of powder reflectivity spectrum. The latter energy values could possible be due to the presence of certain impurities in the original powder which were removed by a subsequent recrystallization process. An analysis of the trans-
I September
1970
between 1.3 to 2.2 microns was made to find the possibility of free carrier absorption in this region The optical density in this wavelength region did not depend on A2. However, the region between 1.7 and 2.0 microns showed a k2.7 dependence. The sharp rise in the transmission and the reflection spectra (figs. Ia and Za) beyond 0.2 micron indicates the possibility of existence of a band-gap type absorption. Following the technique of McLean [S], the values of the squares and the square-roots of the optical density, log&/I), were plotted against wavelength, and only the square function could be fitted to a linear: curve, mission spectrum in the,r&on
as shown in fig. 3. This may be taken as evidence in favor of an allowed direct transition with an
energy Eg equal to 5.5 eV.
As mentioned earlier, a fluorescence spectrum in the visible region was observed by Dertrand [3]. In our .case, since the opticaL detection system belongs to the nonmonochromatic class,
Voly&
6, numb82 5:
CHEiMCAL
PHYSICS
LETTERS
1 Septemtre* 1970
-PURE CRYSTALS - - - X-~RRADIATLD CRYSTAL
1x10’;
2.6
I
I
I
2.7
2.8
2.9
I
3.0
I
I
3.1
3.2
I
3.3
1
I
I
3.i
3.5
3.6
Fig. 4. Temperature of electrical resistivfiy in pure and X-irradiated single crystals. the watt position of the fluorescence band cannot be located in the reflectivity mode of meas-
urement. However, beyond the minimum energy required for the excitation of fluorescence, the total integrated intensity from the sample in the reffecfivity spectrum will increase by an amount proportional to the total fluorescence intensity. Fig. 2a possibly depicts this case where the reflectivity spectrum is seen to rise above 100 per cent in the region 0.25 to 0.40 micron. The temperature dependence of electrical resistivity of &alanine is shown in fig. 4. Curves for all the samples yielded only one slope and the calculated activation energy from p - exp(E/2kT) was found to be 0.56 (A 0.05) eV. This value corresponds to one of the absorption bands arising due to impurity in the transmission and reflection spectra of the crystal as already mentioned. The effect of X-ray irradiation on 8-alanine has been found to produce electron spin resonance.and this has been attributed to the formation of free radicals [7]. Since no data are available on other electronic properties of X-irradiated s-alanine, we havti extended our work to study the optical and electrical properties of this material after damaging it with X rays of 20 kV, 20 mA at room temperature for dose times ranging from 1 to 3 hours. No change in the activation energy of ,electrical conduction was observed after this treatment (fig. 4). The change in optical 426
.
,
transmissio? is shown in figs. lb and lc after 1 hour 2nd 3 hours of irradiation dose, respective: Iy. Change in the reflectivity has been shown in fig. 2b for a powder sample that was irradiated for 3 hours. The nature of both these spectra remains essentially the same, except for the creation of a broad absorption band overlapping the fundamental absorption. Further work on the luminescence in pure and X-irrxiiated fi-alanine is in progress to clarify the uncertainties associated with the optical spectra near the band gap. The authors wish to express their appreciation to Mr. G. M. Arnett and Dr. H. J. Watson for their constant encouragement and to Mr. D. R. Wilkes and Mr. S. A. Fields for providing the facilities for this work. REFERENCES [I] M. E. Cardew and D. D. Eley, Discussions Faraday SOC. 27 (l959j 115. [Z] D. Vasilescu,. A. Jolivet, and A. Brau, Rev. Phys. AppL. 2 (1967) 283. [3] D. Bertrand, Bull. Sot. Chim. 12 (1945) 1023. [4].P.Jose and L.M.Pant, Acta Cry&. 18 (1965) 806. [5] A. P.KuIshreshtba and T. Moolfherji, MoI.Cryst.. to bu published. [6] A. F. Gibson et al.,. Progress in semidonductors, Vol. 5 (Heywood, London,, 1950) p. 55. i?] H. Shields and W. Gordy. J. Phys. Chem. Li2 (l958) 789. ” _ -.
.-
.- . .:
_.‘.;.
‘-
_._
_-
,