Intercalation Studies in Bismuth Selenide

Materials Science and Engineering, B I (1988) 147-154

147

Intercalation Studies in Bismuth Selenide K. PARASKEVOPOULOS, E. HATZIKRANIOTIS, K. CHRISAFIS, M. ZAMANI, J. STOEMENOS and N. A. E C O N O M O U

Physics Department, Universityof Thessaloniki, Thessaloniki (Greece) K. ALEXIADIS

Polytechnic School, UniversityofThessaloniki, Thessaloniki (Greece) M. BALKANSK!

Laboratoire de Physique des Solides, Universit6 Pierre et Marie Curie, Paris (France) (Received June 1, 1988)

Abstract

We investigated the possibility of using Bi2Se~ as an intercalation cathode in solid state ionic devices by the insertion of lithium into this material. The results presented in this paper show that lithium insertion occurs either by chemical or by electrochemical treatment, introducing additional layers perpendicular to the c axis, with a periodicity of about 60 A, as has been revealed by transmission electron studies. The variation of the carrier concentration owing to the transfer of charge from the lithium intercalant was probed by the combined measurement of the conductivity, Hall effect and plasma frequency. Two effects are noticeable and worth commenting on, these being a decrease of the mobility and an increase in the effective mass resulting from the increase of the lithium content. Both of these effects may have a common origin, being the result of the displacement of the Fermi level owing to the charge transfer or possibly the result of more drastic effects on the band structure owing to the filling of van der Waals voids by lithium. Finally, the electrochemical kinetics, at least in the limited range in which they were performed, proved to be completely reversible, whilst side reactions were not observed.

I. Introduction

Lithium intercalation in layered semiconductors has attracted considerable attention in recent years in view of the use of such materials as positive electrodes and electron exchangers in solid state energy storage devices. Investigations 0921-5107/88/$3.50

of the insertion of lithium and silver in In2Se 3 [1] and InSe [2] have recently been reported. The electrical and optical properties of M2V~N3vm layered compounds have been studied earlier [3, 4] in detail. Some more recent investigations of the conduction-band parameters [5], of the point defects of Bi2Se3 [6] and of the incorporation of impurities [7] have been conducted. In particular, in the case of the distribution of germanium in Bi2Se3, Predota et al. [8] have furthered the idea that, at low concentrations, the germanium atoms preferentially enter into the bismuth sublattice forming substitutional defects GeB~ and, for high germanium concentrations, the germanium atoms create new structural planes inside the van der Waals gap of the Bi2Se3 structure, resulting in the formation of a modulated structure. The transport properties and plasma resonance of this crystal doped with cadmium [9] and, more recently, the free-carrier concentration in Bi2_xlnxTe 3 mixed crystals, have been investigated [10] and it has been shown that the freecarrier concentration decreases as x increases. In view of these findings, we investigated the possibility of using Bi2Se3 as an intercalation cathode in solid state ionic devices by the insertion of lithium into this material. The results presented in this paper show that lithium insertion occurs either by chemical or by electrochemical treatment. Transmission electron micrographs have been used to compare the intercalated and pure Bi2Se3. The electrical properties of the intercalated samples were also investigated as well as the charge density trans© Elsevier Sequoia/Printed in The Netherlands

148

ferred during the insertion process. The variation of the free-carrier concentration in BizSe 3 with the level of lithium intercalation is also presented.

2. Crystal structure ofBi2Se 3 Bismuth selenide, Bi2Se3, is a layered narrowgap semiconductor with band gap energy Eg 0.21 eV [11], which crystallizes in the rhombohedral lattice with space group D3d 5 (R3m) [3] and is isostructural with Bi2Te 3 and Sb2Te~. The crystal structure of Bi2Se3 consists of layers perpendicular to the trigonal c axis, formed by alternating planar arrangements of identical atoms. A layer contains five planes of atoms in the sequence ... Se/Bi/Se/Bi/Se Se/Bi/Se/Bi/Se Se/Bi/Se/Bi/Se ... The unit cell is formed by three such layers. The primitive unit cell, containing one formula unit of Bi2Se3 has the dimensions a 0 = 0.414 nm, b 0 = 0.984 nm and c o = 2.864 nm [3]. The stacking sequence in a close-packed structure is given by

Van der Waals gap

=

Se

--

Bi 8e

"

Van der Waals

OaP

... CaBcA BcAbC AbCaB CaBcA ... % ,,

/

one unit cell where the capital letters stand for the chalcogen atoms (i.e. for the selenium atoms) and the lower case letters stand for the positions of the bismuth atoms. Figure 1 presents the crystal structure of Bi2Se3 [12]. The small circles represent selenium atoms and the large circles bismuth atoms. In the fivefold layers, covalent bonding is assumed to dominate the ionic contribution, with the valence states forming highly excited hybrids of the type s p 3 d 2 o r p 3 d 3 around the metal positions. The easy cleavage, perpendicular to the trigonal axis, is attributed to the weak Se-Se interlayer vander-Waals-type bonding. The energetically accessible sites in the van der Waals gap are those usually found between pairs of closely packed atomic layers [13], i.e. one octahedral site and two tetrahedral sites per primitive unit cell. The layered character of VB-VIB compounds leads to two-dimensional conduction [14].

3. Lithium insertion in Bi2Se3 Lithium insertion in Bi2Se3 was carried out by direct exposure to n-butyl lithium. This method has been applied in a variety of hosts such as

Fig. 1. Crystal structure of the rhombohedral, R 3 m ) Bi2Se3. The full straight lines correspond to covalent bonds. The accentuated axis is one of the three within the Wigner-Seitz cell of the crystal.

transition metal dichalcogenides (TX2/ L151, MPX 3 compounds [16] and in some III-VI layered semiconductors [1.2]. The samples used in this work are taken from single-crystal Bi2Se3 ingots grown by the Bridgman method. Samples cut in shapes appropriate for electrical and optical experiments were immersed in a 1.6 M solution of n-butyl lithium in hexane. The experiments were conducted under a controlled argon atmosphere. The stability of the intercalated samples has been tested in vacuum, in an argon atmosphere and also in air. Intercalated samples have undergone chemical analysis. For this purpose, two series of samples, each of about 100 mg, were immersed in an acid mixture composed of 15 ml HCI (99.9999% pure) and 5 ml concentrated H N O 3. The solution was heated on a sand bath (I 20 °C) until the major part of the acid mixture had evaporated. It was then diluted with distilled water and transferred into a 200 ml volumetric flask. The lithium selenium content levels were determined by atomic absorption spectroscopy (AAS) using a

149 TABLE 1

Lithium and selenium content in intercalated

BizSe3 Samph"

D10/2b Dl0/4s

Lithium content

Selenium content

(wt.%)

(mole fraction)

(wt.%)

(mole fraction)

0.086 0.102

0.083 0.099

-38.2

-3.07

Perkin-Elmer 503 spectrometer, whilst the bismuth content was inferred by the difference in weight. The results are summarized in "Fable 1. 4. T r a n s m i s s i o n e l e c t r o n m i c r o s c o p y on l i t h i u m intercalated B i z S e 3

Transmission electron microscopy (TEM) investigation of lithium insertion in BizSe 3 was conducted in plane and in cross-sectional view, i.e. with the electron beam parallel and perpendicular to the c axis respectively. Electron micrographs were taken of samples before intercalation and the same specimen was examined by T E M after intercalation. The intercalated samples were transferred to the microscope under an argon atmosphere. Figure 2(a) shows the plane view of a nonintercalated sample, Fig. 2(b) the corresponding diffraction pattern of the basal plane, and Fig. 2(c) the same area after intercalation for 10 rain. Comparison of Figs. 2(a) and 2(c) shows that the density of defects has increased considerably during the interaction process. Nevertheless, the diffraction pattern did not show any changes after intercalation. Figure 3(a) shows the cross-sectional view of a non-intercalated sample, Fig. 3(b) that of the same sample after 10 rain of insertion, Fig. 3(c) the diffraction pattern in the [001] direction and Fig. 3(d) a thicker area where long straight linear defects appear perpendicular to the c axis. Figure 3(a) shows a relatively low defect density in the material before intercalation whereas after intercalation for 10 rain the same specimen shows a high defect density as shown by the wave and irregular fringes perpendicular to the c axis (Fig. 3(b)). The diffraction pattern of this area (Fig. 3(c)) reveals the existence of satellite spots along the [001] direction. From this set of spots the mean periodicity of the fringes is estimated to be 60 A. The distance between the linear defects shown in Fig. 3(d) varies between 60 and 400 A. This

o

r01]

Fig. 2. Plane-view observation (i.e. the electron beam is almost parallel to the c axis of (a) the pristine specimen (the density of defects is very low), (b) the corresponding diffraction patterns and (c) the same specimen after intercalationfor 10 rain, segments of dislocations and precipitates being evident).

150

Fig. 3. Cross-sectional observation (i.e. the electron beam is perpendicular to the c axis of (a) the pristine specimen, (b) the same area after 10 min of intercalation, (c) the corresponding diffraction pattern in which satellite spots can be clearly seen and (d) the same specimen at a thicker region showing planar defects perpendicular to the c axis).

observation seems to indicate a non-uniform distribution of the lithium inserted in the host lattice. T h e planar defects p r o d u c e d by the lithium insertion seem to modify the stacking arrangements of the Bi2Se 3 layers which results in the high dislocation density observed in the basal plane view.

5. Electrical properties 5.1. Conductivity and Hall effect measurements One of the fundamental aspects of intercalation is the swelling of the layer separation whilst there is a negligible effect (if any) on the interatomic distances in the layer sandwich. This is in accordance with our results presented in Section 4.

This is the basis for the justification that the quasi-two-dimensional band structure of the host material is preserved in the rigid band approximation (RBA) [ 17]. T h e electrical properties are strongly affected by the intercalated lithium which will also influence the carrier concentration and possibly the mobility of the carriers. T h e exact calculation of the transport properties and of their modification upon intercalation requires knowledge of the band structure and of the flee-carrier scattering mechanism. For the evaluation of the experimental results we have used a model [18] in which the band structure is characterized by a single non-parabolic conduction band located at the F point and in which the

151

scattering of the free carriers at room temperature is determined by isotropic frequencyindependent scattering by acoustical phonons. According to the RBA model, no drastic change in the above band scheme is expected upon intercalation. The variation in the conductivity of the samples with the intercalation time was monitored using a computer-based data accumulation system, with a Van der Pauw arrangement of the electrodes on (0001) cleavage planes and using silver paint as the electrode material. On these samples, with the same arrangement of electrodes, Hall effect measurements were performed after the interruption of the intercalation process at different time intervals with a magnetic field of 5-10 kG. The electron concentation n was estimated using the single-band expression for the Hall constant R [121 R = - l/he

(1)

where e is the electron charge. All samples on which electrical measurements were performed were examined afterwards by electron microscopy to ensure that no structural changes had occurred. Typical results for in-plane measurements of the resistivity p of samples with intermittent intercalation and the free-carrier concentration, estimated from the Hall effect, are presented in Fig. 4 and in Table 2. From the data it can be seen that when lithium is incorporated into the lattice the mobility of the free carriers at room temperature shows a significant change, this being directly related to the level of the insertion dose of lithium, which implies scattering influ-

enced by the lithium impurities. The exact mechanism for this is under investigation•

5. 2. Electrochemical measurements Electrochemical experiments were performed on Li/electrolyte/Li0ABi2Se3 cells using LiC104 dissolved in propylene carbonate as the electrolyte. The cathode material used in these experiments was Bi2 Se3 single crystals intercalated with lithium by the method described in Section 3. The lithium content was determined by AAS. The electrodes were mounted in such a way that only the layers perpendicular to the c axis were exposed to the electrolyte• A separate lithium electrode was used as a reference electrode. The cells were successively charged and discharged using a Phillips PE-1006 source operating in the potentiostatic mode• The initial e.m.f, of the cells was E 0 = 1.95 V, which corresponds to a lithium mole fraction of x=0.1. The cells were then potentiostatically biased to Eo++_AE ( A E = 5 0 mY). As a result, a time-dependent current flows and Li* ions diffuse into (or out of) the bulk of the Li0ABi2Se3 sample, in response to the internal concentration gradient, until a new homogeneous composition (Li0. l+axBi2Se3) is reached• The quantity Ax, i.e. the mole fraction of lithium transferred from or to the cathode, can be calculated from the charge by Faraday's law [19]. Figure 5 shows the transient current and, as can be seen, the experimental points do not follow a simple exponential law. Honders et al. [20] have shown that the transient current density j(t) can be analytically expressed for short times by

AE

.8

j(r) = ~

•O

2

(ZoTl/21erf(~OTl/2 ] kR,,

exp kRo

)

(2)

..O

8 , 'o•

x

TABLE 2 Results of in-plane measurements of intercalated Bi2Se 3

•O"

6

'a"

a. • .a

.4

o" " "...,'.a . ,

x • • .tL . . . .

.o"

D

6 ....

Time Resistivity (h)

(xl0

3~cm)

Free-carrier concentration

Mobiliff (×103cm2V

( x lOl~cm 311

o" .o •

~,o

~oo intercalation

~5o 200 time Chours)

220

Fig. 4. Time evolution of the resistivity p and the freecarrier concentration n upon lithium intercalation with nbutyl lithium.

0 23 44 64 95 139 164 190 214

0.429 0.401 0.381 (t.366 0.341 0.320 0.361 0.295 0.284

2.449 3.548 4.180 4.815 5.830 7.060 7.566 8.280 9.110

0.595 0.439 0.392 0.355 0.314 0.277 0.229 0.256 0.242

is

')

152 1.5 . . . . ''O

I

(Amp)

O E

• +0. O O o o

~1.0! + F

m

00"....

Io-G

\

Lio,Bi2S~3

o o+

Lio~Si=Se+

o oo~(~,

"-.

~

i ~/

:o DISCHARGE o S/fP

O

O O

o

CHARGE ~TEP

IJ.5 to

0

!o

0

o

10-7

,

100

, ,,~+,,I

n

I01

,,

,,,,,I

_

,

10Z

.......

l

103

,

1 O

0

}

O~

lime~sec,

Fig. 5. Transient response of the charge step of ki/IJCIO4/ Li01Bi2Se3 cell.

0

2

3

time

5

4 (103 SSC

i +

...... [ _ _

6

Fig. 6. Successive potentiostatic charge-discharge cycles ol the Li/LiClO4/Li0jBi2Se3 cell. The initial e.m.f. E, = 1.95 V and AE = 50 inV.

and for long times by AE [ 1 rZ0 J(~) = R0 + zu/3 exp / ~ - R cm 3. These results are presented in Fig. 8. Comparison of the concentrations calculated from the plasma frequency shift, observed as the time of intercalation increases and using a constant value for the effective mass, with the corresponding values from Hall measurements reveals that the former are systematically less than expected. Gobrecht et a l [11] and Tichy and Horak [27] found that the effective mass in Bi2Se3 depends on the position of the Fermi level. In our case more drastic effects caused by the insertion of lithium should not be excluded, a phenomenon that is under investigation.

E~o O)p

~0~ +i~07

7. Conclusions

Analysis of the experimental reflectivity spectra in the whole region using eqns. (6) and (7) yields the plasma frequencies wp = 752 c m - 1 for pure BizSe 3 and we'= 1296 cm -~ for lithiumintercalated Li0jBi2Se > with e ~ = 3 0 and

The results presented in this paper indicate that insertion of lithium into Bi2S % occurs by either chemical or electrochemical reaction, introducing additional layers perpendicular to the

1300

.•..," ...,0•'

o o o [io. , Bi2se3 00

...... ""

Bi2se3

.75 >, 40

O"

1100

o

g

_a

.... .O "'''O ....

>

• •

~.,,'C)

.5 -

g

q-

900

..O"

0""

•"

.0""

O'"

4

i

7OO NOO

i000

[500

wavenumber

2000

cm - 1

Fig. 7 Reflectivity spectra in the region of plasma frequency in Bi2Se ~ and Li0~ BizSe 3 samples. The full line represents the calculated spectra•

I

I

I

I

50

100

150

Z00

inlercalaljon tim8

~50

(hours)

Fig. 8. T h e time evolution of the plasma frequency me and the resulting carrier concentration with constant effective mass upon lithium intercalation.

154

c axis with a periodicity of about 60 A, as has been revealed by transmission electron studies. The variation of the carrier concentration owing to the charge transfer from the lithium intercalant was probed by combined conductivity, Hall effect and plasma frequency measurements. Two effects are noticeable and worth commenting on, these being the decrease of the mobility and the increase in the effective mass resulting from the increase of the lithium content. Both of these effects may have a common origin, being the result of the displacement of the Fermi level owing to the charge transfer or may be as a result of more drastic effects on the band structure caused by the filling of the van der Waals voids by lithium. Finally, the electrochemical kinetics, at least in the limited range in which they were performed, proved to be completely reversible, whilst side reactions were not observed.

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