Growth and characterization of RuS2 single crystals

Growth and characterization of RuS2 single crystals

ill Mat. Res. B u l l . , Vol. 23, p p . 277-285, 1988. Printed in the USA. 0025-5408/88 $3.00 + .00 Copyright (c) 1988 Pergamon Journals Ltd. GROWT...

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Mat. Res. B u l l . , Vol. 23, p p . 277-285, 1988. Printed in the USA. 0025-5408/88 $3.00 + .00 Copyright (c) 1988 Pergamon Journals Ltd.

GROWTH AND CHARACTERIZATION OF RuS 2 SINGLE CRYSTALS

Ying-Sheng Huang and Shoei-Sheng Lin* Department of Electronic Engineering and Technology National Taiwan Institute of Technology Taipei, Taiwan, 10772 R. O. C.

(Received October 8, 1987; Communicated by A. Wold)

ABSTRACT.

Single crystals of R u ~ have been grown by an oscillating chemical vapor transport method using ICl 3 as transport agent. Optimum conditions are given for growing large high quality crystals. As confirmed by X-ray investigations, the specimens crystallise with the correct pyrite structure. The electrical resistivity and Hall effect measurements have shown n-type semiconducting behavior. Photocurrent versus wavelength measurements show an indirect bandgap of 1.38 eV. MATERIALS INDEX : Ruthenium, Sulfides.

INTRODUCTION RuS 2 belongs to the family of transition metal dichalcogenides crystallising in the pyrite structure (i) The semiconducting behavior of this diamagnetic compound was verified by Hulliger (2) on polycrystalline samples. Recently, it has been the subject of much interest due to its potential application in energy-related technologies (3,4) . It is a promising material for the thermal catalytic processing of organic sulphur and nitrogen compounds in petroleum refining (5). It also has interesting photoelectrochemical behavior (6-8) , and photochemical catalytic properties (9). Single crystals (i0,Ii), poorly crystallized (12), polycrystalline samples (3) have been prepared and evaluated. However, only relatively incomplete experimental information on its solid state properties have been reported. Besides, there is considerable disagreement *Permanent Address

: National Yuen-Lin Institute of Technology, Hu-Wei, Yuen-Lin, Taiwan. 277

278

Y-S.

HUANG, et al.

Vol. 23, No.

among v a r i o u s i n v e s t i g a t o r s c o n c e r n i n g the p r o p e r t i e s (13). M a n y of these d i f f i c u l t i e s have a r i s e n b e c a u s e poor q u a l i t y c r y s t a l s w h i c h w e r e a v a i l a b l e to e a r l i e r tigators.

of RuS 2 of the inves-

An o s c i l l a t i n g chemical vapor t r a n s p o r t m e t h o d has been developed for growing large size single c r y s t a l s of RuS 2. X-ray powder diffraction is u s e d to d e t e r m i n e the lattice constants. The t e m p e r a t u r e d e p e n d e n c e of electrical transport p r o p e r t i e s are s t u d i e d by using the van der Pauw method. W a v e l e n g t h d e p e n d e n c e of p h o t o c u r r e n t measurements are u t i l i z e d to d e t e r m i n e the b a n d g a p of the material.

EXPERIMENTAL

DETAIL S

Compound Preparatio n The RuS 2 p o w d e r c o m p o u n d w e r e p r e p a r e d from elements having the f o l l o w i n g p u r i t i e s (%) Ru : 99.99, S : 99.9998. Appropriate a m o u n t s (a 2 m o l e % sulphur excess) of the powdered elements were i n t r o d u c e d into a quartz a m p o u l e of internal d i a m e t e r 15 mm, outside d i a m e t e r 19 m m and length about [5 cm. The a m p o u l e was then evacuated to a pressure of I0 -v torr, sealed at the c o n s t r i c t i o n and v i b r a t e d for 30 m i n s to ensure the m i x i n g of the p o w d e r e d elements. The m i x t u r e was d i s t r i b u t e d a l o n g the length of the horizontal a m p o u l e w h i c h was then i n t r o d u c e d into a furnace w h o s e temp e r a t u r e was increased, in steps of 50 °C, from r o o m temperature to a final t e m p e r a t u r e of 1070 °C; the final temperature b e i n g r e a c h e d after i0 hrs. This slow heating was necessary to avoid any p o s s i b i l i t y of explosion due to the strongly e x o t h e r m i c r e a c t i o n b e t w e e n the elements. The ampoule was m a i n t a i n e d at this t e m p e r a t u r e for I0 days and then cooled s l o w l y to r o o m t e m p e r a t u r e and r e m o v e d from the furnace. At this stage the c o m p o u n d was in the form of a f r e e - f l o w i n g fine d a r k grey p o w d e r . X - r a y d i f f r a c t i o n p a t t e r n indicated that single phase p o l y c r y s t a l l i n e RuS 2 was thus produced. The compound was then u t i l i z e d as- starting m a t e r i a l for crystal growth. Crystal G r o w t h The crystals were grown by an o s c i l l a t i n g chemical v a p o r t r a n s p o r t m e t h o d using ICI 3 as the t r a n s p o r t i n g agent. The s t a r t i n g m a t e r i a l s t o g e t h e r w i t h the carrier substance were placed in a quartz a m p o u l e w h i c h was c h i l l e d in liquid nitrogen, e v a c u a t e d to I0 -U torr and sealed. Two ampoules were then inserted into a horizontal g r a d i e n t furnace (Fig. la) (Lindberg Model 54259 three zone tube furnace w i t h Model 59459-A control console), h a v i n g three i n d e p e n d e n t l y c o n t r o l l a b l e zones. Crystal g r o w t h was c a r r i e d out over a variety of temperatures, t e m p e r a t u r e g r a d i e n t s and ICI~ c o n c e n t r a t i o n s . A m o d i f i e d scheme named oscillating method was a d o p t e d in w h i c h the t e m p e r a t u r e s of the charge zone and the g r o w t h end of the ampoule were subjected to a

2

Vol. 23, No.

2

RuS 2 C R Y S T A L S uminum Seal / B Al.... Seal Quartz Ampoule~

279

/Starting Material~

Quartz wool--/ SingleCrystals~ - j

11~

~Q

~ooo

~3 ....

F...... r AluminumTube

Aluminum Seal

............

DISTANCE

Fig.

1

Setup and t e m p e r a t u r e p r o f i l e growth of RuS 2 crystals

for the

periodic o s c i l l a t i o n at the first 10 days of the growth period. The typical record of the oscillating temperatures of the charge zone and g r o w t h end of the ampoule are shown in Fig. 2. It followed a period of normal growth by the fixed t e m p e r a t u r e technique. The typical thermal g r a d i e n t is shown in Fig. lb. The ICI 3 c o n c e n t r a t i o n was about 8mg/cm 3. Crystal g r o w t h was then c o n t i n u e d for a p e r i o d of from 30 to 40 days. At the end of this time, the furnace was allowed to cool to about 200 °C. The ampoule was then removed and wet tissues applied rapidly to the end away from the crystals to condense the ICI 3 vapor. When the ampoule r e a c h e d r o o m temperature, it was opened and the crystals removed. ...... -

IF

. . . . . . . . . .

r

..... F

/

F. . . . .

F

/

.....

The temperature of charge zone The temperature of growth end

-

/

. . . . . . . . . . . . . . . . . . . . . . . . ] - ] ,

i

100C--[ '

I

95O

I

I

I

i

I

I

I

I

I

1

2

3

4

5

6

7

8

9

TIME

Fig.

2

l

I 10

(Days)

Typical record of the o s c i l l a t i n g t e m p e r a t u r e s of the charge zone and growth zone

280

Y-S.

HUANG, et 8/.

Vol. 23, No. 2

X-ray Diffraction Lattice p a r a m e t e r s of c r y s t a l s from different experimental conditions were determined. C r y s t a l s w e r e c r u s h e d to p o w d e r and p a t t e r n s w e r e t a k e n by m e a n s of a slowly moving radiation d e t e c t o r and r e c o r d e d on a m o v i n g strip of p a p e r w i t h CuK~ ( I =1.5418A) radiation. The resulting patterns were computer refined to g i v e the lattice p a r a m e t e r s . The p a r a m e t e r s so d e t e r m i n e d are precise to 0.01%. Lau~ b a c k reflection was used to investigate the m o r p h o l o g y of the RuS 2 s i n g l e c r y s t a l s . Electrical Transport Measurements Electrical resistivity was d e t e r m i n e d by the four p r o b e s van der Pauw t e c h n i q u e and the sign, m o b i l i t y and c o n c e n t r a t i o n of the c h a r g e c a r r i e r s w e r e d e t e r m i n e d by Hall measurements. Electrical r e s i s t i v i t y was m e a s u r e d b e t w e e n 10OK to 420°K with a temperature stability of 0.~°K or better. The Hall e f f e c t was s t u d i e d only b e t w e e n 140 K and room t e m p e r a t u r e . Ohmic contacts to the sample were made by s o l d e r i n g the gold w i r e to the c r y s t a l s w i t h p u r e i n d i u m metal. Photocurrent Measurements The RuS 2 e l e c t r o d e s were prepared by first p a s s i n g a copper wire t h r o u g h a glass tube and s o l d e r i n g the free end to one end of the c o p p e r plate. Ohmic back contacts were made using a Ga-In alloy. The crystal was m o u n t e d by c e m e n t i n g it to the copper plate with silver c o n d u c t i n g epoxy. The w i r e and p l a t e w e r e then i n s u l a t e d w i t h epoxyc e m e n t l e a v i n g o n l y a s i n g l e face of crystal e x p o s e d to the electrolyte. Teflon test cells w e r e e q u i p p e d w i t h quartz flat windows. The a c t i v e crystal face was b r o u g h t to about 1 mm of the w i n d o w to m i n i m i z e s o l u t i o n light absorption. The c o u n t e r e l e c t r o d e was a 5 cm 2 Pt plate. The e l e c t r o l y t i c s o l u t i o n was 1 N H 2 S O 4. The light source was a I00 W tungsten h a l o g e n lamp (Oriel C o r p o r a t i o n ) . The b e a m was c h o p p e d at 35 Hz w i t h m e c h a n i c a l c h o p p e r and then focused on the entrance slit of a 0.2 m M c P h e r s o n Model 270 grating monochromator. Both entrance and exit slits w e r e set at 0.i mm. Part of the exit slit was m a s k e d to d e f i n e a spot of 1 m m h e i g h t as f o c u s e d on the e l e c t r o d e face. A Oriel LP 470 f i l t e r w h i c h cuts off wavelengths s h o r t e r then 5000A, was used to p r e v e n t the s e c o n d o r d e r r e s p o n s e at the longer wavelength. Photoresponse spectra u n d e r short circuit c o n d i t i o n s w e r e o b t a i n e d by c o n n e c t i n g the two e l e c t r o d e s to a Standford Research Model 510 l o c k - i n a m p l i f i e r . Light intensities were measured with a c a l i b r a t e d UTD Model 255 Si d e t e c t o r . The s p e c t r a w e r e c o r r e c t e d for the n u m b e r of p h o t o n s r e a c h i n g the cell at the various wavelengths and normalized to unity at their maxima. Corrections for s o l u t i o n light a b s o r p t i o n w e r e not made.

Vol.

23, No.

2

RuS 2 C R Y S T A L S

RESULTS

281

AND DISCUSSION

Typical r e s u l t i n g c r y s t a l s are shown in Fig. 3, samples as large as 3 x 3 x 3 mm3 w i t h mirror-like surfaces were obtained on a r o u t i n e basis. The oscillating method is r e s p o n s i b l e for the success in g r o w i n g large single c r y s t a l s of RuS 2 . W h i l e the c o n v e n t i o n a l chemical vapor transport method, the n u m b e r of the a c t i v e g r o w t h sites in the g r o w t h zone was too large, r e s u l t i n g in m o r e but smaller cyrstals, see Fig. 4. It is s u s p e c t e d that the o s c i l l a t i n g method preferentially evaporates small nuclei and allows large c r y s t a l s to g r o w at the e x p e n s e of small ones.

g g

0

Q O

I

ig. 3 Some r e p r e s e n t a t i v e c r y s t a l s of RuS 2 g r o w n in our l a b o r a t o r y

~

4cM

Fig.

4

Crystals grown with c o n v e n t i o n a l chemical vapor transport method ( m u l t i f i c a t i o n 5.5 times)

Lattice p a r a m e t e r s of c r y s t a l s from d i f f e r e n t experimental c o n d i t i o n s w e r e d e t e r m i n e d by X - r a y d i f f r a c t i o n in an effort to e s t a b l i s h the e f f e c t s of g r o w t h c o n d i t i o n s . The r e s u l t s did not show any consistent variation with growth

282

Y-S. H U A N G ,

et a].

Vol. 23, No. 2

conditions. X-ray diffraction pattern showed RuS 2 crystallizing in the cubic, p y r i t e structure. The average v a l u e s at r o o m t e m p e r a t u r e w e r e found to be 5 . 6 0 9 5 A e 0 . 0 0 0 5 A w h i c h a g r e e d q u i t e well w i t h the p r e v i o u s l y r e p o r t e d r e s u l t s (I). C r y s t a l s a p p e a r e d to f o r m w i t h the (i00) face as the p r e d o m i n e n t face. The L a u e p a t t e r n d e m o n s t r a t e a good cryst a l l i s i n g of the crystals. i

RuS,

t

i

t

i

t

t

g lO-1

10 - 3 0

i

! 20

I 40

i

I 80

i 80

i JOO

I

120

1 0 3 / T ( K -~ )

Fig.

5

T e m p e r a t u r e d e p e n d e n c e of r e s i s t i v i t y in the range of 10 OK to 420 OK for a RuS 2 signle crystal

The typical r e s u l t s of the e l e c t r i c a l r e s i s t i v i t y measurements for RuS z single c r y s t a l s are shown in Fig. 5 The resistivity decreases f r o m 0.056 ~ c m at 420 OK to 0.034 ~ c m at 180 ~ and then i n c r e a s e s to 0.571 ~ c m at i 0 ~ . The b e h a v i o r of the r e s i s t i v i t y a b o v e 180 OK was attributed to the c h a n g e of the m o b i l i t y . The i n c r e a s e of resistivity is r a t h e r small c o m p a r e w i t h the p r e v i o u s report that the r e s i s t i v i t y i n c r e a s e s a b o u t 100 times f r o m room temperature to 25 OK (14). The r e s i s t i v i t y s h o w e d an e x p o n e n t i a l temper a t u r e d e p e n d e n c e P = ~ e x p - ( ~ E / K T ) b e t w e e n ii0 OK and 160 with an activation e n e r g y A E of 34 meV. This value is much smaller then the previous report (14) that the a c t i v a t i o n e n e r g y ~ E is 88 m e V and c h a r a c t e r i s e an e x t r i n s i c conductivity. Hall e f f e c t s t u d y was studied only between 140 OK and r o o m t e m p e r a t u r e w h i c h c o n f i r m e d the n - t y p e semiconducting behavior. The carrier concentration n = I/q~ and the Hall m o b i l i t y #H was c a l c u l a t e d using a single c a r r i e r model. A t r o o m t e m p e r a t u r e the c a r r i e r c o n c e n t r a t i o n s w e r e b e t w e e n 2 x 1017 and 8 x 1017 cm -3, the Hall m o b i l i t i e s b e t w e e n 180 and 2 0 0 c m 2 / V s. The r e s u l t s of E z z a o u i a et al (8 w e r e 1 0 1 7 u p to 5 x 1 0 1 8 c m for c a r r i e r c o n c e n t r a t i o n s and I00 to 400 c m 2 / V s for the Hall m o b i l i t y , w h i l e Bichsel et al (ii r e p o r t e d that the c a r r i e r concentration were b e t w e e n 3.2 and 4.1 x 10 17 cm -3 and the Hall mobilities b e t w e e n 260 and 310 c m ~ V s at r o o m t e m p e r a t u r e . The m e a s u r e d v a l u e s of electrical r e s i s t i v i t y and Hall m o b i l i t y v a r y c o n s i d e r a b l y

Vol. 23, No. 2

RuS 2 CRYSTALS

283

30C

RuS, ~'~ 27C

A

21C

A

<

~: 18(

130

15Q

! 0

2 0

2 0

280

310

T(°K)

Fig.

6

T e m p e r a t u r e d e p e n d e n c e of m o b i l i t y in the range of 140 OK to 300 °K for a RuS 2 single crystal

from sample to sample. This might be e x p l a i n e d by d i f f e r e n t u n c o n t r o l l a b l e impurity c o n c e n t r a t i o n s or nonstoichiometry effects. A plot of the t e m p e r a t u r e d e p e n d e n c e of the Hall mobility is g i v e n in Fig. 6. Over the range b e t w e e n 140 OK and 200 oK, the m o b i l i t y i n c r e a s e s w i t h i n c r e a s i n g temperature, c h a r a c t e r i s t i c of d o m i n a t i o n of i m p u r i t y scattering. A b o v e 200 OK the m o b i l i t y d e c r e a s e s w i t h i n c r e a s i n g t e m p e r a ture. This b e h a v i o r is normal for a semiconductor. However for the d e s c r i p t i o n of the s c a t t e r i n g m e c h a n i s m s , it needs m o r e experimental information+ The w a v e l e n g t h d e p e n d e n c e of the p h o t o c u r r e n t is shown in Fig. 7. The p h o t o c u r r e n t increases from 900nm to 520 nm and then d e c r e a s e s in the short w a v e l e n g t h region. The indirect b a n d g a p of RuS 2 can be d e t e r m i n e d from the 7½ versus hv plot near the b a n d edge r e g i o n (15). Here U is the q u a n t u m efficiency. The q u a n t u m e f f f c i e n c y of u n i t y implies that one e-h pair is g e n e r a t e d for each incident photon and is u s u a l l y d e f i n e d at a p a r t i c u l a r wavelength. Thus,

~(~)

.

.

Iph(A ) / e .

.

.

.

IL(A) / hP

Iph(A ) hv

IL(A) e

w h e r e I -is p h o t o c u r r e n t ; I L is light intensity. As shown in Fig~n8, 7½ vs. hv plot shows a linear r e l a t i o n over an e x t e n d e d spectral range. This indicates an indirect band transition w i t h a band edge at the i n t e r c e p t of this plot (15). The b a n d g a p derived from this figure is about 1.38 eV. The value of the b a n d g a p of RuS 2 has been the subject of m u c h c o n t r o v e r s i a l d i s c u s s i o n in recent years (13). Orginally a figure of 1.8 eV was accepted, based on d i f f u s e optical reflection m e a s u r e m e n t s on powdered samples (2). From optical absorption measurement, Bichsel et al (Ii) estimated the e n e r g y gap of RuS 2 single crystal at 1.3 eV. Evidence for the e n e r g y gap from p h o t o r e s p o n s e spectra at

284

Y-S.

0

,

], 2 5

,

7

Vol. 23, No. 2

,

1.6~

2. 0 5

PHOTON

Fig.

et al.

2. ~5

ENERGY

2. 8 5

3.25

(eV)

R e l a t i v e short c i r c u i t p h o t o c u r r e n t single crystal RuS 2 in I N H2SO ~

spectra

for

!

~

RuS~

0 1 • 25

1.45

! • 65

PHOTON

Fig.

8

1.65

EN~GY

2. 05

2.2S

(eV)

Square root of q u a n t u m efficiency, p l o t t e d as a f u n c t i o n of p h o t o n energy, d e t e r m i n e s indirect b a n d g a p of RuS 2

semiconductor-electrolyte i n t e r f a c e s was also inconclusive. G u i t t a r d et al (3) made the initial o b s e r v a t i o n on this effect using sintered RuS 2 electrodes, and suggested a value of 1.3-1.5 eV. However, in later w o r k from the same laboratory, at single crystal e l e c t r o d e s the value of 1.85eV was r e p o r t e d (8). N o n e t h e l e s s in p u b l i s h e d spectra (i0), a s i g n i f i c a n t r e s p o n s e to photons of energy b e l o w 1.8 eV is remarkably persistent at single crystal surfaces. An even s t r o n g e r p h o t o r e s p o n s e at the red end of the s p e c t r u m was o b s e r v e d using s i n t e r e d e l e c t r o d e s (7) and was i n t e r p r e t e d in terms of donor levels close to the Fermi level and a s s o c i a t e d w i t h structural d e f e c t s in the material. However, the e f f e c t could be i n t e r p r e t e d in terms of a indirect energy gap of a r o u n d 1.4 eV w i t h a low t r a n s i t i o n cross section, f o l l o w e d by a h i g h e r gap of 1.8 eV. A d e f i n i t i v e value of E~ need further experimental evidence, a l t h o u g h our result~ i n d i c a t e that the lower value m i g h t appear.

Vol. 23, No. 2

RuS 2 C R Y S T A L S

285

ACKNOWLEDGEMENTS This w o r k has b e e n s u p p o r t e d of the R e p u b l i c of China.

by the N a t i o n a l

Science

Council

REFERENCES i. 2. 3. 4. 5. 6. 7. 8. 9.

10. Ii. 12. 13. 14. 15.

Sutarno, O. K n o p and K. I. G. Reid, Can. J. Chem., 45, 1391(1967). F. H u l l i g e r , N a t u r e 200, 1064(1963). R. Guittard, R. Heindl, R. Parsons, A. M. R e d o n and H. T r i b u t s c h , J. E l e c t r o a n a l . Chem., iii, 401(1980). A. M. Redon, Solar Cells, 15, 27(1985}. T. A. P e c o r a r o and R. R. C h i a n e l l i , J. Catalysis, 67, 430(1981). H. M. K u h n e a n d H. T r i b u t s c h , J. E l e c t r o c h e m . Soc., !30, 1448(1983). R. Heindl, R. Parsons, A. M. Redon, H. T r i b u t s c h and J. V i g n e r o n , S u r f a c e Science, 115, 91(1981). H. E z z a o u i a , R. Heindl, R. P a r s o n s and H. T r i b u t s c h , J. E l e c t r o n a n l . Chem., 745, 279(1983). D. H. M. W. T h e w i s s e n , E. A. Van der Z o u w e n - A s s i n k , K. Timmer, A. H. A. T i n n e m a n s , a n d A. Mackor, J. Chem. Soc., Chem. Commun., 941(1984). H. E z z a o u i a , J . W. F o i s e and O. G o r o c h o v , M a t . Res. Bull., 20, 1 3 5 3 ( 1 9 8 5 ) . R. Bichsel, F. L e v y and H. Berger, J. Phys. C., 17, L19(1984). J. D. P a s s a r e t t i , K. Dwight, A. Wold, W. J. Croft and R. R. C h i a n e l l i , Inorg. Chem., 20, 2 6 3 1 ( 1 9 8 1 ) . A. J. McEvoy, Mat. Chem. and Phys., 14, 113(1986). H. P. V a t e r l a u s , R. Bichsel, F. L e v y and H. Berger, J. Phys. C, 18, 6 0 6 3 ( 1 9 8 5 ) . J. J. Pankove, "Optical P r o c e s s e s in S e m i c o n d u c t o r s " , D o v e r P u b l i c a t i o n s Inc., New York (1975).