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Electronic structure of black phosphorus studied by X-ray photoelectron spectroscopy
Solid State Communications, Vol. 44, No. 6, pp. 877-879, 1982. Printed in Great Britain.
0038-1098/82/420877-03503.00/0 Pergamon Press Ltd.
ELECTRONIC STRUCTURE OF BLACK PHOSPHORUS STUDIED BY X-RAY PHOTOELECTRON SPECTROSCOPY Y. Harada Department of Chemistry, College of General Education, The University of Tokyo, Komaba, Meguro, Tokyo 153, Japan K. Murano The National Institute for Environmental Studies, Yatabe, Tsukuba, Ibaraki 305, Japan I. Shirotani Institute for Solid State Physics, The University of Tokyo, Roppongi, Tokyo 106, Japan T. Takahashi Department of Physics, Faculty of Science, Tohoku University, Sendai 980, Japan and Y. Maruyama Department of Chemistry, Ochanomizu University, Otsuka, Bunkyo-ku, Tokyo 112, Japan
(Received 17 June 1982 by H. Kawamura) The X-ray photoelectron spectrum of black phosphorus has been measured for the first time. The features in the valence band spectrum are found to be in good agreement with those of the valence state density recently calculated on the basis of a pseudopotential method. The 2s and 2p core spectrum of black phosphorus is also discussed. LAYERED MATERIALS have recently attracted considerable attention because of their theoretical and practical interests. Black phosphorus, the stable form of phosphorus under normal conditions, exists as puckered layers, with four atoms per unit cell, arranged in an orthorhombic array [1 ]. It is a p-type semiconductor with an intrinsic energy gap of about 0.3 eV [2]. Under high pressure, the orthorhombic form undergoes reversible transformation to successively denser rhombohedral and cubic forms [3]. This structural change is known to correspond to semiconductor-semimetal-metal transition [4]. Although black phosphorus shows these interesting physical properties, the study of its electronic structure has been relatively neglected until recently. In order to study the valence electronic structure of black phosphorus we have measured the X-ray photoelectron spectrum in the present work. Since photoelectron spectroscopy yields direct information on the density of valence states, it serves as an adequate basis to describe the electronic structure. The results of the experiment have been compared with those of the band calculations recently carried out by Morita and his coworkers [5, 6]. The XPS measurements were performed on a VG
Scientific ESCA LAB 5 spectrometer using MgKa or A1Ka radiation. The spectra were recorded with a full width at half maximum of about 1.0 eV. During the measurements a pressure of ca. 5 x 10 -1° torr was maintained. Black phosphorus was prepared from red phosphorus under high pressure using a wedge-type cubic anvil apparatus [7]. Red phosphorus was melted in a carbon furnace at 1100°C under a pressure of 23 kbar and then cooled. The black phosphorus sample thus obtained was nearly a single crystal and could be cleaved along the ac plane. Four pieces of the cleaved sample of about 1 mm thick was fixed on a substrate (10 mm diameter) with silver paste. Immediately before the measurements, under an argon atmosphere, the sample surface was polished with polishing paper (3 ~ mesh) in a dry box attached to the spectrometer chamber. The binding energy of the spectra was calibrated using the energy of the 4 f s / 2 ' 7/2 peaks in a thin gold film evaporated onto the sample surface. Figure 1 shows the MgKal 2 X-ray photoelectron spectrum of black phosphorus In the 2s and 2p region. Both of the main 2p and 2s peaks (0) and @) are accompanied by two satellites (©, ®, ® and ®) in addition to
877
878
ELECTRONIC STRUCTURE OF BLACK PHOSPHORUS
Vol. 44, No. 6
® ®
o
ul
0
J,
®
(.o
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100
1 0
Binding
200
250
energy (eV)
Fig. 1. MgKa, 2 X-ray photoelectron spectrum of black phosphorus in the 2s and 2p region. ® 2p main line; ® 2s main line; @, ®, ®, ® satellites due to plasmon excitations; Q', ®', 2p and 2s main lines due to MgK~, 4 radiation. Table 1. Peak positions in the Mg Ka,, 2 spectrum o f black phosphorus in the 2s and 2p region Binding energy/eV
Energy difference/eV
P 2p (9 @ ®
129.9 150.1 170.2
20.2 20.1
P2s @ ® ®
187.6 207.7 227.7
20.1 20.0
the main peaks due to MgKa3 ' 4 radiation (0)', ®'). The peak positions of the spectrum are given in Table 1. Since the satellite peaks appear at nearly equal intervals on the lower kinetic energy side of the main peaks, they are considered to be due to the creation of plasmon excitations. In metals, where one can take a free electron model, the plasmon excitation energy is given by Ep = (47rne2h2/m) 1/2
(1)
in which n is the electron density, e and m are the charge and the mass of an electron. In the semiconductors and insulators, the plasmon frequency of equation (1) must be modified by multiplication by a factor [1 + (Eg/ Ep):] x/2, where Eg is the energy gap of the solid [8]. In black phosphorus (Eg = 0.3 eV), this factor gives little effect to the value o f E p . If we assume that five valence
f
EF
10 15 5 Binding energy (eV)
20
Fig. 2. A1Ka X-ray photoelectron spectrum of black phosphorus in the valence electron region compared with the calculated valence state density [6]. Arrows indicate the positions of the main features in the state density. electrons (3s and 3p electrons) of black phosphorus having the density of 2.69 gcm -3 are involved in the plasmon excitation, the value of n becomes 2.62 x 1023 cm -1 . Substituting this value in equation (1), we have 19.0 eV for the calculated Ep, which is in fair agreement with the observed value, 20.1 eV in Table 1. Figure 2 shows the A1K~ X-ray photoelectron spectrum of black phosphorus in the valence electron region together with the calculated density of the valence states which was recently obtained by Asahina et al. on the basis of a self-consistent pseudopotential method [6]. Since for X-ray photoelectron spectra the momentum k of escaping electron is very large, the k conservation effect usually does not play an important role, and hence good correspondence should exist between the features in the spectrum and the valence state density. This is clearly seen in Fig. 2; there are two well-separated bands mainly due to 3s orbitals in the 8 - 1 8 eV binding energy range and three bands predominantly originating from 3p orbitals up to several eV below the Fermi level EF. In Table 2 the peak positions in the spectrum are compared with the positions of the main features in the calculated valence state density. There is good agreement between the two sets of data. The same group of authors also calculated
Vol. 44, No. 6
ELECTRONIC STRUCTURE OF BLACK PHOSPHORUS
Table 2. Peak positions in the valence electron spectrum of black phosphorus compared with the positions of the main feature in the calculated valence state density Binding energy*/eV Peak in spectrum
Position of main feature in state density t
2.8 4.5 6.1 10.8
2.2 4.4 6.8 10.7
13.9
14.4
* Relative to the Fermi level. t Denoted by arrows in Fig. 2.
879
two s-like bands decreases in the order, P(3.1 eV), As (2.6 eV), Sb (1.7 eV) and Bi (1.2 eV). The latter splitting is mainly due to the splitting caused by the bonding and anti-bonding combinations of s-orbitals belonging to neighbouring atoms and decreases as the nearest neighbour distance increases (2.22, 2.50, 2.90 and 3.10A for P, As, Sb and Bi). In fact, an empirical linear relationship has been proposed between the splitting of the valence s band peaks and the nearest neighbour distance for covalent group IV and V elements [9].
Acknowledgements - The authors wish to thank Prof. I. Ikemoto and Prof. S. Asano, The University of Tokyo, and Prof. A. Morita and Dr H. Asahina, Tohoku University, for their helpful discussions. REFERENCES
the band structure of black phosphorus using a tight binding approach [5], which yielded broader 3s and narrower 3p bands compared with the results of the pseudopotential method. The present experimental results are thus in favour of the pseudopotential calculation. Finally, we wish to compare the valence bands of bulk phosphorus with those of other covalent group V elements, As, Sb and Bi. In contrast to black phosphorus, the latter elements have a rhombohedral (A7) structure at normal conditions, but their over-all valence band structures in the X-ray photoelectron spectra [9, 10] are similar to that of black phosphorus; the spectra show two well-separated s-like bands between 7 and 16 eV and p-like bands near EF. This reflects the similarity of the electronic configuration (s~p a) and the small change in the local arrangement of atoms during the transition from the orthorhombic to the rhombohedral structure. The mean splittings between the s-like and p-like bands are not very much different among the group V elements, being 8.1,8.4, 7.2 and 8.6 eV for P, As, Sb and Bi, respectively, but the splitting of the
1. 2.
3. 4. 5. 6.
7. 8. 9. 10.
A. Brown & S. Rundqvist, Acta Cryst. 19,684 (1965). R.W. Keyes, Phys. Rev. 92,580 (1953); D. Warschauer, J. Appl. Phys. 34, 1853 (1963); Y. Maruyama, S. Suzuki, K. Kobayashi & S. Tanuma, Physica 105B, 99 (1981). J.C. Jamieson, Science 139,129 (1963); M.L. Duggin, Z Phys. Chem. Solids 33, 1267 (1972); D. Schiferl, Phys. Rev. BI9,806 (1979). J. Witting & B.T. Matthias, Science 160,994 (1968). Y. Takao, H. Asahina & A. Morita, J. Phys. Soc. Japan 50, 3362 (1981). H. Asahina, A. Morita & K. Shindo, J. Phys. Soc. Japan 49, Suppl. A, 85 (1980); H. Asahina, K. Shindo & A. Morita, J. Phys. Soc. Japan 51, 1193 (1982). I. Shirotani, R. Maniwa, H. Sato, A. Fukizawa, N. Sato, Y. Maruyama, T. Kajiwara, H. Inokuchi & S. Akimoto, J. Chem. Soc. Japan 1604 (1981). R.S. Knox, Theory of Excitons, Solid State Phys. Suppl. 5, p. 98. Academic, New York (1963). L. Ley, R.A. Pollak, S.P. Kowalczyk, R. McFeely & D.A. Shirley, Phys. Rev. B8,641 (1973). T. Takahashi, Y. Harada & H. Hamanaka, Phys. Rev. B24, 3663 (1981).
0038-1098/82/420877-03503.00/0 Pergamon Press Ltd.
ELECTRONIC STRUCTURE OF BLACK PHOSPHORUS STUDIED BY X-RAY PHOTOELECTRON SPECTROSCOPY Y. Harada Department of Chemistry, College of General Education, The University of Tokyo, Komaba, Meguro, Tokyo 153, Japan K. Murano The National Institute for Environmental Studies, Yatabe, Tsukuba, Ibaraki 305, Japan I. Shirotani Institute for Solid State Physics, The University of Tokyo, Roppongi, Tokyo 106, Japan T. Takahashi Department of Physics, Faculty of Science, Tohoku University, Sendai 980, Japan and Y. Maruyama Department of Chemistry, Ochanomizu University, Otsuka, Bunkyo-ku, Tokyo 112, Japan
(Received 17 June 1982 by H. Kawamura) The X-ray photoelectron spectrum of black phosphorus has been measured for the first time. The features in the valence band spectrum are found to be in good agreement with those of the valence state density recently calculated on the basis of a pseudopotential method. The 2s and 2p core spectrum of black phosphorus is also discussed. LAYERED MATERIALS have recently attracted considerable attention because of their theoretical and practical interests. Black phosphorus, the stable form of phosphorus under normal conditions, exists as puckered layers, with four atoms per unit cell, arranged in an orthorhombic array [1 ]. It is a p-type semiconductor with an intrinsic energy gap of about 0.3 eV [2]. Under high pressure, the orthorhombic form undergoes reversible transformation to successively denser rhombohedral and cubic forms [3]. This structural change is known to correspond to semiconductor-semimetal-metal transition [4]. Although black phosphorus shows these interesting physical properties, the study of its electronic structure has been relatively neglected until recently. In order to study the valence electronic structure of black phosphorus we have measured the X-ray photoelectron spectrum in the present work. Since photoelectron spectroscopy yields direct information on the density of valence states, it serves as an adequate basis to describe the electronic structure. The results of the experiment have been compared with those of the band calculations recently carried out by Morita and his coworkers [5, 6]. The XPS measurements were performed on a VG
Scientific ESCA LAB 5 spectrometer using MgKa or A1Ka radiation. The spectra were recorded with a full width at half maximum of about 1.0 eV. During the measurements a pressure of ca. 5 x 10 -1° torr was maintained. Black phosphorus was prepared from red phosphorus under high pressure using a wedge-type cubic anvil apparatus [7]. Red phosphorus was melted in a carbon furnace at 1100°C under a pressure of 23 kbar and then cooled. The black phosphorus sample thus obtained was nearly a single crystal and could be cleaved along the ac plane. Four pieces of the cleaved sample of about 1 mm thick was fixed on a substrate (10 mm diameter) with silver paste. Immediately before the measurements, under an argon atmosphere, the sample surface was polished with polishing paper (3 ~ mesh) in a dry box attached to the spectrometer chamber. The binding energy of the spectra was calibrated using the energy of the 4 f s / 2 ' 7/2 peaks in a thin gold film evaporated onto the sample surface. Figure 1 shows the MgKal 2 X-ray photoelectron spectrum of black phosphorus In the 2s and 2p region. Both of the main 2p and 2s peaks (0) and @) are accompanied by two satellites (©, ®, ® and ®) in addition to
877
878
ELECTRONIC STRUCTURE OF BLACK PHOSPHORUS
Vol. 44, No. 6
® ®
o
ul
0
J,
®
(.o
"6
100
1 0
Binding
200
250
energy (eV)
Fig. 1. MgKa, 2 X-ray photoelectron spectrum of black phosphorus in the 2s and 2p region. ® 2p main line; ® 2s main line; @, ®, ®, ® satellites due to plasmon excitations; Q', ®', 2p and 2s main lines due to MgK~, 4 radiation. Table 1. Peak positions in the Mg Ka,, 2 spectrum o f black phosphorus in the 2s and 2p region Binding energy/eV
Energy difference/eV
P 2p (9 @ ®
129.9 150.1 170.2
20.2 20.1
P2s @ ® ®
187.6 207.7 227.7
20.1 20.0
the main peaks due to MgKa3 ' 4 radiation (0)', ®'). The peak positions of the spectrum are given in Table 1. Since the satellite peaks appear at nearly equal intervals on the lower kinetic energy side of the main peaks, they are considered to be due to the creation of plasmon excitations. In metals, where one can take a free electron model, the plasmon excitation energy is given by Ep = (47rne2h2/m) 1/2
(1)
in which n is the electron density, e and m are the charge and the mass of an electron. In the semiconductors and insulators, the plasmon frequency of equation (1) must be modified by multiplication by a factor [1 + (Eg/ Ep):] x/2, where Eg is the energy gap of the solid [8]. In black phosphorus (Eg = 0.3 eV), this factor gives little effect to the value o f E p . If we assume that five valence
f
EF
10 15 5 Binding energy (eV)
20
Fig. 2. A1Ka X-ray photoelectron spectrum of black phosphorus in the valence electron region compared with the calculated valence state density [6]. Arrows indicate the positions of the main features in the state density. electrons (3s and 3p electrons) of black phosphorus having the density of 2.69 gcm -3 are involved in the plasmon excitation, the value of n becomes 2.62 x 1023 cm -1 . Substituting this value in equation (1), we have 19.0 eV for the calculated Ep, which is in fair agreement with the observed value, 20.1 eV in Table 1. Figure 2 shows the A1K~ X-ray photoelectron spectrum of black phosphorus in the valence electron region together with the calculated density of the valence states which was recently obtained by Asahina et al. on the basis of a self-consistent pseudopotential method [6]. Since for X-ray photoelectron spectra the momentum k of escaping electron is very large, the k conservation effect usually does not play an important role, and hence good correspondence should exist between the features in the spectrum and the valence state density. This is clearly seen in Fig. 2; there are two well-separated bands mainly due to 3s orbitals in the 8 - 1 8 eV binding energy range and three bands predominantly originating from 3p orbitals up to several eV below the Fermi level EF. In Table 2 the peak positions in the spectrum are compared with the positions of the main features in the calculated valence state density. There is good agreement between the two sets of data. The same group of authors also calculated
Vol. 44, No. 6
ELECTRONIC STRUCTURE OF BLACK PHOSPHORUS
Table 2. Peak positions in the valence electron spectrum of black phosphorus compared with the positions of the main feature in the calculated valence state density Binding energy*/eV Peak in spectrum
Position of main feature in state density t
2.8 4.5 6.1 10.8
2.2 4.4 6.8 10.7
13.9
14.4
* Relative to the Fermi level. t Denoted by arrows in Fig. 2.
879
two s-like bands decreases in the order, P(3.1 eV), As (2.6 eV), Sb (1.7 eV) and Bi (1.2 eV). The latter splitting is mainly due to the splitting caused by the bonding and anti-bonding combinations of s-orbitals belonging to neighbouring atoms and decreases as the nearest neighbour distance increases (2.22, 2.50, 2.90 and 3.10A for P, As, Sb and Bi). In fact, an empirical linear relationship has been proposed between the splitting of the valence s band peaks and the nearest neighbour distance for covalent group IV and V elements [9].
Acknowledgements - The authors wish to thank Prof. I. Ikemoto and Prof. S. Asano, The University of Tokyo, and Prof. A. Morita and Dr H. Asahina, Tohoku University, for their helpful discussions. REFERENCES
the band structure of black phosphorus using a tight binding approach [5], which yielded broader 3s and narrower 3p bands compared with the results of the pseudopotential method. The present experimental results are thus in favour of the pseudopotential calculation. Finally, we wish to compare the valence bands of bulk phosphorus with those of other covalent group V elements, As, Sb and Bi. In contrast to black phosphorus, the latter elements have a rhombohedral (A7) structure at normal conditions, but their over-all valence band structures in the X-ray photoelectron spectra [9, 10] are similar to that of black phosphorus; the spectra show two well-separated s-like bands between 7 and 16 eV and p-like bands near EF. This reflects the similarity of the electronic configuration (s~p a) and the small change in the local arrangement of atoms during the transition from the orthorhombic to the rhombohedral structure. The mean splittings between the s-like and p-like bands are not very much different among the group V elements, being 8.1,8.4, 7.2 and 8.6 eV for P, As, Sb and Bi, respectively, but the splitting of the
1. 2.
3. 4. 5. 6.
7. 8. 9. 10.
A. Brown & S. Rundqvist, Acta Cryst. 19,684 (1965). R.W. Keyes, Phys. Rev. 92,580 (1953); D. Warschauer, J. Appl. Phys. 34, 1853 (1963); Y. Maruyama, S. Suzuki, K. Kobayashi & S. Tanuma, Physica 105B, 99 (1981). J.C. Jamieson, Science 139,129 (1963); M.L. Duggin, Z Phys. Chem. Solids 33, 1267 (1972); D. Schiferl, Phys. Rev. BI9,806 (1979). J. Witting & B.T. Matthias, Science 160,994 (1968). Y. Takao, H. Asahina & A. Morita, J. Phys. Soc. Japan 50, 3362 (1981). H. Asahina, A. Morita & K. Shindo, J. Phys. Soc. Japan 49, Suppl. A, 85 (1980); H. Asahina, K. Shindo & A. Morita, J. Phys. Soc. Japan 51, 1193 (1982). I. Shirotani, R. Maniwa, H. Sato, A. Fukizawa, N. Sato, Y. Maruyama, T. Kajiwara, H. Inokuchi & S. Akimoto, J. Chem. Soc. Japan 1604 (1981). R.S. Knox, Theory of Excitons, Solid State Phys. Suppl. 5, p. 98. Academic, New York (1963). L. Ley, R.A. Pollak, S.P. Kowalczyk, R. McFeely & D.A. Shirley, Phys. Rev. B8,641 (1973). T. Takahashi, Y. Harada & H. Hamanaka, Phys. Rev. B24, 3663 (1981).