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Electrical and optical properties of intercalated GaSe compound
Physica 105B (1981) 238--242 North-Holland Publishing Company
E L E C T R I C A L AND O P T I C A L P R O P E R T I E S OF I N T E R C A L A T E D CaSe C O M P O U N D Shoji I C H I M U R A , Chiei T A T S U Y A M A and Osamu U E N O
Department of Electronics, Faculty of Engineering, Toyama University, Takaoka, Toyama, Japan We present the first studies on the formation of an intercalated GaSe compound with iodine by the anode reaction in the electrolyte solution of p-bromoaniline and mercury di-iodine in ethylene glycol. The X-ray diffraction patterns show a continuous decrease in the intensity of the GaSe (0, 0, 10) line at 57.7° and that of the (0, 0, 08) line at 45.5° in the early stage of the anode reaction. The appearance of the new peaks at 45.8 and 58.0° suggested the formation of the intercalated compound. The thickness along the crystal c-axisexpanded by two or three times. The absorption edge shifted about 10 meV to the higher energy side and the electrical resistance in the crystal ab-plane for the intercalated compoundwas found to be the same as that of pure GaSe, but the resistance along the c-axis decreased by about three orders of magnitude.
1. Introduction The intercalated layered compounds for example the intercalated MoS2, WaS2 and graphite, have been studied extensively [1], and the application of these compounds in the field of industry has been studied recently. However, the intercalation of GaSe has not yet been studied. GaSe is generally considered to be chemically inert because of no dangling bond on the interface between layers. Graphite is also considered to be chemically inert, but it is able to interact with a large number of substances under suitable conditions. For example, graphite bisulfate lamellar compound is formed in an anode when sulfuric acid is electrolyzed between a graphite anode and an auxiliary cathode [2]. In anodization process of metals and semiconductors, generally, it has been known that a nonpassive (or active) state and a passive state exist, depending upon the surface potential of the anode materials [3]. A nonpassive film grows with little increase of the anode potential on the surface of the anode materials when anodization is performed under a constant current condition. Thus, when anodization is performed under a proper constant current condition with the use of a proper electrolyte solution, we may expect an intercalated GaSe compound to form in the anode.
In this paper, we present the results of studies on the formation of GaSe intercalation compounds with iodine in the anode when mercury di-iodine is electrolyzed between a GaSe anode and a platinum cathode.
2. Experimental The electrolytic system consists of a platinum cathode, a GaSe anode, and an electrolyte solution. Two kinds of electrolyte solutions were used for the anode reaction. One is a solution of p-bromoaniline (2 g) in ethylene glycol (40 ml) (referred to " A E G " ) and the other is a solution of p-bromoaniline (2 g) and mercury di-iodine (0.5 g) in ethylene glycol (40 ml) (referred to "AMEG"). A p-type GaSe single crystal has been grown from the melt by the Bridgman method. The X-ray diffraction patterns using Cu-K~ 1 show that the lattice constant along the c-axis is 15.96A and the transmission Laue patterns show that the crystal belongs to e-modification with a hexagonal system. The method of anode reaction was as follows. A GaSe single crystal was cleaved into a platelike sample of about 0.5ram thick along the c-axis and cut into dimensions of about 5 x 7 mm 2. An ohmic contact for the sample was obtained by painting indium amalgam on the
0378-4363/81/0000-0000/$2.50 (~ North-Holland Publishing Company and Yamada Science Foundation
$. Ichimura et al./Studies on an intercalated GaSe compound
ab-plane of the sample crystal. By heating the sample on a Cu block at about 250°C for several minutes in N2 atmosphere, Hg was evaporated and In left as forming an ohmic contact [4]. It is strictly ohmic above about 200 K. The contact was covered with an epoxy-paint for insulation. With progress in the anode reaction under a constant current of 150-200#A, the electrode potential changed from +240 to +60 V in several distinct steps. These steps appear to correspond with the stage transformation of an intercalated compound. Therefore, the process in the formation of an intercalated GaSe compound was clarified qualitatively from the correlation between the changes in electrode potential and those in the crystal structure. Chemical quantitative analysis of the compound was not possible, but reproducible formation was made by identical techniques. Iodine in an intercalated compound was detected by the iodo-starch reaction (iodometry) and bromine was detected by the precipitation analysis with silver nitrate. The electrical resistance of the compound was measured by a metal point contact technique. A
239
current passed through the sample was calculated from the voltage at the ends of standard resistance (1 kO). The resistance of the sample was measured as a function of temperature. The reflectance spectra from the crystal of a freshly cleaved ab-plane were measured by unpolarized light at near normal incidence to the surface. The samples were X-rayed before and after the measurement to insure that no change had occurred in composition. 3. Results and discussion
In the early stages of anode reaction, a periodic decreasing of the electrode potential was observed. These periodic structures are formed by repetitions of equal duration, which represent the duration time of nonpassive states. The electrode potentials approach a constant voltage at the end of anode reaction. The duration time was shortened while the intercalation stage of the intercalated compound has been progressed from the high stage to the low stage.
Fig. 1. X-raydiffractionpattern of GaSe intercalationcompound.Electrolytesolution: AMEG; anodic current: 200/zA; reaction time: 27 h.
240
S. Ichimura et al./Studies on an intercalated GaSe compound
The duration time also depends on the pH values and types of electrolyte solution. Fig. 1 shows the X-ray (Cu-K~) diffraction pattern of the intercalated compound synthesized by the anode reaction for 27h in the A M E G electrolyte solution. The patterns showed a continuous decrease in the intensity of the GaSe (0, 0, 10) line at 57.7 ° and that of (0, 0, 08) line at 45.5 ° in the early stage of the anode reaction, and the appearance of a new peak. The new peaks at 45.8 and 58.0 ° indicate the formation of the intercalation. In the absence of mercury diiodine in electrolyte solution, the new peak did not appear. In fig. 1 the splitting of each peak due to K~ and K~2 can be seen. Figs. 2-4 show three examples of the transmission Laue pattern of the intercalated compounds. Figs. 2 and 3 are the transmission Laue pattern of the intercalated compounds which
Fig. 3. Transmission Laue pattern of GaSe intercalation compound. Electrolyte solution: AMEG; anodic current: 200/.tA; reaction time: 35 h.
Fig. 2. Transmission Laue pattern of GaSe intercalation compound. Electrolyte solution: AMEG; anodic current: 200/~A; reaction time: 24 h.
were synthesized by the anode reaction for 24 and 35 h, respectively, in the A M E G electrolyte solution under constant current of 200 # A . The figures show that the intercalated compounds were constructed with various order--disorder structure, but there is no evidence that the principal source is the intralayer or interlayer disorder. Fig. 4 is the transmission Laue pattern of the intercalated compound synthesized by the anode reaction for 78 h in the A M E G electrolyte solution under a constant current of 200/~A. In the figure we find a halo pattern which suggests the disordering along the crystal c-axis. It may also be possible that p-bromoaniline polymer has been synthesized on the interlayer of GaSe, but there is no chemical evidence for this polymer. Iodo-starch reaction (iodometry) and precipitation analysis with silver nitrate were used
241
S. lchimura et al./Studies on an intercalated OaSe compound
p
i
PR,T, ~ L
i
- - GaSe --- Gose
\
0 /
J
200
250
301
T(K)
Fig. 5. Temperature dependence of resistance of GaSe before and after anode reaction (along the crystal ab-plane).
_p__
5o
PR.T.
--
GoSe
--- cos,
TION
20
I0
Fig. 4. Transmission Laue pattern of GaSe intercalation compound. Electrolyte solution: AMEG; anodic current: 200/.LA; reaction time: 78 h.
for chemical analysis of iodine and bromine in the intercalated compounds. The results of chemical analysis indicate that iodine has been intercalated into the interlayer of GaSe. No bromine itself was detected, but there is no denying the possibility of intercalation of pbromoaniline. We have made some investigations on the electrical properties of the intercalated compound synthesized in the AMEG electrolyte solution. The compound includes a sequence of different stages. Both the electrical resistances along the ab-plane and the c-axis of the crystal were measured in order to analyze the resistive anomalies in the compound. Figs. 5 and 6 show the temperature dependence of the tr/tYRTof the sample, where O'RTmeans the resistance at room temperature in the ab-plane and along the c-axis of the compound, respectively. According to
O
I
200
I
250
300
T(K)
Fig. 6. Temperature dependence of resistance of GaSe before and after anode reaction (along the crystal c-axis).
--
G~Se
---
GaSe a f t e r ANODIC OXIDATION
>I,-
~_2o
w a:
I0
I 1.9
I 2.0 PHOTON ENERGY (eV)
I 2. I
Fig. 7. Reflectance spectra of GaSe before and after anode reaction•
242
S. Ichimura et al./Studies on an intercalated GaSe compound
these data, the activation energies for the electrical conduction of the intercalated compound are almost the same values in the ab-plane before and after the anode reaction, but, in the c-axis, 0.18eV before and 0.02eV after the anode reaction. Electrical resistance in the abplane has not been affected in the processes of intercalation, but the resistance along the c-axis decreased about three orders of magnitude. Fig. 7 shows the reflectance spectra of the freshly cleaved ab-plane for the intercalated GaSe compound with iodine, at room temperature. We found that the absorption edge of the intercalated GaSe compound shifts about 10 meV to the higher energy side compared with that of pure GaSe. This result may be explained by the charge transfer mechanism [5]. The value of electron affinity of iodine is high, so iodine may accept the electrons from the GaSe. This leads to an increase of free carriers in GaSe, since GaSe is p-type. The increase of free carders makes the absorption edge shift to the higher energy
Acknowledgement We acknowledge T. Tanpo for his assistance in the experiments.
References [1] J.G. Hooley, Chemistry and Physics of Carbon, vol. 5 (Pergamon Press, New York, 1969) p. 321. A.R. Ubbelohde and F.A. Lewis, Graphite and its Crystal Compounds (Oxford University Press, London, 1960). A. Weiss and R. Ruthardt, Z. Naturforsch. B24 (1969) 1066. F.R. Gamble, J.H. Osiecki and F.J. DiSalvo, J. Chem. Phys. 55 (1971) 3525. W.P.F.A.M. Omloo and F. Jellinek, J. Less -Common Metals 20 (1970) 121. [2] H. Thile, Z. Anorg. Allg. Chem. 206 (1937) 407. W. Rudorff, Z. Physik. Chem. B45 (1939) 42. [3] T.P. Hoar, Modern Aspects of Electrochemistry, no. 2 (Butterworths Scientific Publications, London, 1959). [4] C. Tatsuyama and S. Ichimura, Japan. J. Appl. Phys. 15 (1976) 843. [5] C.C. Shieh, R.L. Schmidt and J.E. Fischer, Phys. Rev. 20 (1979) 3351.
E L E C T R I C A L AND O P T I C A L P R O P E R T I E S OF I N T E R C A L A T E D CaSe C O M P O U N D Shoji I C H I M U R A , Chiei T A T S U Y A M A and Osamu U E N O
Department of Electronics, Faculty of Engineering, Toyama University, Takaoka, Toyama, Japan We present the first studies on the formation of an intercalated GaSe compound with iodine by the anode reaction in the electrolyte solution of p-bromoaniline and mercury di-iodine in ethylene glycol. The X-ray diffraction patterns show a continuous decrease in the intensity of the GaSe (0, 0, 10) line at 57.7° and that of the (0, 0, 08) line at 45.5° in the early stage of the anode reaction. The appearance of the new peaks at 45.8 and 58.0° suggested the formation of the intercalated compound. The thickness along the crystal c-axisexpanded by two or three times. The absorption edge shifted about 10 meV to the higher energy side and the electrical resistance in the crystal ab-plane for the intercalated compoundwas found to be the same as that of pure GaSe, but the resistance along the c-axis decreased by about three orders of magnitude.
1. Introduction The intercalated layered compounds for example the intercalated MoS2, WaS2 and graphite, have been studied extensively [1], and the application of these compounds in the field of industry has been studied recently. However, the intercalation of GaSe has not yet been studied. GaSe is generally considered to be chemically inert because of no dangling bond on the interface between layers. Graphite is also considered to be chemically inert, but it is able to interact with a large number of substances under suitable conditions. For example, graphite bisulfate lamellar compound is formed in an anode when sulfuric acid is electrolyzed between a graphite anode and an auxiliary cathode [2]. In anodization process of metals and semiconductors, generally, it has been known that a nonpassive (or active) state and a passive state exist, depending upon the surface potential of the anode materials [3]. A nonpassive film grows with little increase of the anode potential on the surface of the anode materials when anodization is performed under a constant current condition. Thus, when anodization is performed under a proper constant current condition with the use of a proper electrolyte solution, we may expect an intercalated GaSe compound to form in the anode.
In this paper, we present the results of studies on the formation of GaSe intercalation compounds with iodine in the anode when mercury di-iodine is electrolyzed between a GaSe anode and a platinum cathode.
2. Experimental The electrolytic system consists of a platinum cathode, a GaSe anode, and an electrolyte solution. Two kinds of electrolyte solutions were used for the anode reaction. One is a solution of p-bromoaniline (2 g) in ethylene glycol (40 ml) (referred to " A E G " ) and the other is a solution of p-bromoaniline (2 g) and mercury di-iodine (0.5 g) in ethylene glycol (40 ml) (referred to "AMEG"). A p-type GaSe single crystal has been grown from the melt by the Bridgman method. The X-ray diffraction patterns using Cu-K~ 1 show that the lattice constant along the c-axis is 15.96A and the transmission Laue patterns show that the crystal belongs to e-modification with a hexagonal system. The method of anode reaction was as follows. A GaSe single crystal was cleaved into a platelike sample of about 0.5ram thick along the c-axis and cut into dimensions of about 5 x 7 mm 2. An ohmic contact for the sample was obtained by painting indium amalgam on the
0378-4363/81/0000-0000/$2.50 (~ North-Holland Publishing Company and Yamada Science Foundation
$. Ichimura et al./Studies on an intercalated GaSe compound
ab-plane of the sample crystal. By heating the sample on a Cu block at about 250°C for several minutes in N2 atmosphere, Hg was evaporated and In left as forming an ohmic contact [4]. It is strictly ohmic above about 200 K. The contact was covered with an epoxy-paint for insulation. With progress in the anode reaction under a constant current of 150-200#A, the electrode potential changed from +240 to +60 V in several distinct steps. These steps appear to correspond with the stage transformation of an intercalated compound. Therefore, the process in the formation of an intercalated GaSe compound was clarified qualitatively from the correlation between the changes in electrode potential and those in the crystal structure. Chemical quantitative analysis of the compound was not possible, but reproducible formation was made by identical techniques. Iodine in an intercalated compound was detected by the iodo-starch reaction (iodometry) and bromine was detected by the precipitation analysis with silver nitrate. The electrical resistance of the compound was measured by a metal point contact technique. A
239
current passed through the sample was calculated from the voltage at the ends of standard resistance (1 kO). The resistance of the sample was measured as a function of temperature. The reflectance spectra from the crystal of a freshly cleaved ab-plane were measured by unpolarized light at near normal incidence to the surface. The samples were X-rayed before and after the measurement to insure that no change had occurred in composition. 3. Results and discussion
In the early stages of anode reaction, a periodic decreasing of the electrode potential was observed. These periodic structures are formed by repetitions of equal duration, which represent the duration time of nonpassive states. The electrode potentials approach a constant voltage at the end of anode reaction. The duration time was shortened while the intercalation stage of the intercalated compound has been progressed from the high stage to the low stage.
Fig. 1. X-raydiffractionpattern of GaSe intercalationcompound.Electrolytesolution: AMEG; anodic current: 200/zA; reaction time: 27 h.
240
S. Ichimura et al./Studies on an intercalated GaSe compound
The duration time also depends on the pH values and types of electrolyte solution. Fig. 1 shows the X-ray (Cu-K~) diffraction pattern of the intercalated compound synthesized by the anode reaction for 27h in the A M E G electrolyte solution. The patterns showed a continuous decrease in the intensity of the GaSe (0, 0, 10) line at 57.7 ° and that of (0, 0, 08) line at 45.5 ° in the early stage of the anode reaction, and the appearance of a new peak. The new peaks at 45.8 and 58.0 ° indicate the formation of the intercalation. In the absence of mercury diiodine in electrolyte solution, the new peak did not appear. In fig. 1 the splitting of each peak due to K~ and K~2 can be seen. Figs. 2-4 show three examples of the transmission Laue pattern of the intercalated compounds. Figs. 2 and 3 are the transmission Laue pattern of the intercalated compounds which
Fig. 3. Transmission Laue pattern of GaSe intercalation compound. Electrolyte solution: AMEG; anodic current: 200/.tA; reaction time: 35 h.
Fig. 2. Transmission Laue pattern of GaSe intercalation compound. Electrolyte solution: AMEG; anodic current: 200/~A; reaction time: 24 h.
were synthesized by the anode reaction for 24 and 35 h, respectively, in the A M E G electrolyte solution under constant current of 200 # A . The figures show that the intercalated compounds were constructed with various order--disorder structure, but there is no evidence that the principal source is the intralayer or interlayer disorder. Fig. 4 is the transmission Laue pattern of the intercalated compound synthesized by the anode reaction for 78 h in the A M E G electrolyte solution under a constant current of 200/~A. In the figure we find a halo pattern which suggests the disordering along the crystal c-axis. It may also be possible that p-bromoaniline polymer has been synthesized on the interlayer of GaSe, but there is no chemical evidence for this polymer. Iodo-starch reaction (iodometry) and precipitation analysis with silver nitrate were used
241
S. lchimura et al./Studies on an intercalated OaSe compound
p
i
PR,T, ~ L
i
- - GaSe --- Gose
\
0 /
J
200
250
301
T(K)
Fig. 5. Temperature dependence of resistance of GaSe before and after anode reaction (along the crystal ab-plane).
_p__
5o
PR.T.
--
GoSe
--- cos,
TION
20
I0
Fig. 4. Transmission Laue pattern of GaSe intercalation compound. Electrolyte solution: AMEG; anodic current: 200/.LA; reaction time: 78 h.
for chemical analysis of iodine and bromine in the intercalated compounds. The results of chemical analysis indicate that iodine has been intercalated into the interlayer of GaSe. No bromine itself was detected, but there is no denying the possibility of intercalation of pbromoaniline. We have made some investigations on the electrical properties of the intercalated compound synthesized in the AMEG electrolyte solution. The compound includes a sequence of different stages. Both the electrical resistances along the ab-plane and the c-axis of the crystal were measured in order to analyze the resistive anomalies in the compound. Figs. 5 and 6 show the temperature dependence of the tr/tYRTof the sample, where O'RTmeans the resistance at room temperature in the ab-plane and along the c-axis of the compound, respectively. According to
O
I
200
I
250
300
T(K)
Fig. 6. Temperature dependence of resistance of GaSe before and after anode reaction (along the crystal c-axis).
--
G~Se
---
GaSe a f t e r ANODIC OXIDATION
>I,-
~_2o
w a:
I0
I 1.9
I 2.0 PHOTON ENERGY (eV)
I 2. I
Fig. 7. Reflectance spectra of GaSe before and after anode reaction•
242
S. Ichimura et al./Studies on an intercalated GaSe compound
these data, the activation energies for the electrical conduction of the intercalated compound are almost the same values in the ab-plane before and after the anode reaction, but, in the c-axis, 0.18eV before and 0.02eV after the anode reaction. Electrical resistance in the abplane has not been affected in the processes of intercalation, but the resistance along the c-axis decreased about three orders of magnitude. Fig. 7 shows the reflectance spectra of the freshly cleaved ab-plane for the intercalated GaSe compound with iodine, at room temperature. We found that the absorption edge of the intercalated GaSe compound shifts about 10 meV to the higher energy side compared with that of pure GaSe. This result may be explained by the charge transfer mechanism [5]. The value of electron affinity of iodine is high, so iodine may accept the electrons from the GaSe. This leads to an increase of free carriers in GaSe, since GaSe is p-type. The increase of free carders makes the absorption edge shift to the higher energy
Acknowledgement We acknowledge T. Tanpo for his assistance in the experiments.
References [1] J.G. Hooley, Chemistry and Physics of Carbon, vol. 5 (Pergamon Press, New York, 1969) p. 321. A.R. Ubbelohde and F.A. Lewis, Graphite and its Crystal Compounds (Oxford University Press, London, 1960). A. Weiss and R. Ruthardt, Z. Naturforsch. B24 (1969) 1066. F.R. Gamble, J.H. Osiecki and F.J. DiSalvo, J. Chem. Phys. 55 (1971) 3525. W.P.F.A.M. Omloo and F. Jellinek, J. Less -Common Metals 20 (1970) 121. [2] H. Thile, Z. Anorg. Allg. Chem. 206 (1937) 407. W. Rudorff, Z. Physik. Chem. B45 (1939) 42. [3] T.P. Hoar, Modern Aspects of Electrochemistry, no. 2 (Butterworths Scientific Publications, London, 1959). [4] C. Tatsuyama and S. Ichimura, Japan. J. Appl. Phys. 15 (1976) 843. [5] C.C. Shieh, R.L. Schmidt and J.E. Fischer, Phys. Rev. 20 (1979) 3351.