Growth and characterization of RuSe2 single crystals

Growth and characterization of RuSe2 single crystals

Mat. Res. Bull., Vol. 26, pp. 11-17, 1991. Printed in the USA. 0025-5408/91 $3.00 + .00 Copyright (c) 1991 Pergamon Press plc. GROWTH AND CHARACTERIZ...

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Mat. Res. Bull., Vol. 26, pp. 11-17, 1991. Printed in the USA. 0025-5408/91 $3.00 + .00 Copyright (c) 1991 Pergamon Press plc.

GROWTH AND CHARACTERIZATION OF RuSe2 SINGLE CRYSTALS

Jiang-Shing Sheu and Yen-Shiang Shih Department of Chemical Engineering, National Taiwan Institute of Technology Taipei, Taiwan, 10772 R. O. C. $$ Shoei-Sheng Lin and Ying-Sheng Huang Department of Electronic Engineering, National Talwan Institute of Technology Taipei, Taiwan, 10772 R. O. C. (Received November 7, 1990; Communciated by A. Wold)

ABSTRACT.

The successful growth of single crystals of RuSes has been accomplished by an oscillating chemical vapor transport method using ICl3 as transport agent. Optimum conditions are given for growing large single crystals. The stoichiometry of selected single crystals are discussed. The electrical resistivity and Hall effect measurements indicate n-type semiconducting properties. The photoelectrochemical measurements have shown no photoresponse. MATERIALS INDEX : Ruthenium, Selenides. INTRODUCTION RuSe2 belongs to the family of transition metal dichalcogenides crystallizing in the pyrite structure [1-2]. The semiconducting behavior of this diamagnetic compound was verified by Hulliger [3] on polycrystalline sample. In electrochemical investigations, the ruthenium compounds, particularly RuS2, have attracted interest as electrode or photoelectrode * **

Permanent Address : Department of Chemical Engineering, Feng-Chia University, Taichung, Taiwan. Permanent Address : National Yuen-Lin Institute of Technology, Huwei, Yuenlin, Taiwan.

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materials because of their catalytic properties and favorable stability [4-6]. The theoretical and experimental understandings of its solid state properties is still relatively incomplete. Until recently, experimental work on RuSe2 had been done on powdered or polycrystalline samples because of the difficulties in obtaining single crystals. However, Vaterlaus et al. [7] succeeded in growing single crystals and investigated its physical properties. They estimated the energy gap of RuSe2 single crystals at 0.76 eV by optical transmission and reflectivity measurements. The onl:~ electrical transport data for RuSe2 was reported by Vaterlaus et al. [7], it showed a very unusual transport properties. In this article we report the growth of large single crystals on a routine basis. We have studied in detail the conditions under which RuSe2 crystal grow. The stoichiometry of selected single crystals are discussed. The temperature dependence of electrical transport properties are studied by using the four probe technique. The electrochemical and photoelectrochemical property studies of this material are also carried out. EXPERIMENT DETAILS Single crystals of RuSe2 were prepared by an oscillating chemical vapor transport method with IC13 as the transporting agent. First, the RuSe2 powder compound was prepared from the elements (Ru : 99.95 % pure, Se : 99.999 % pure). To improve the stoichiometry, selenium with 2 mole % in excess has been added with respect to the stoichiometric mixture of the constituent elements, 10 g of the powdered elements were introduced into a quartz ampoule (19 mm o.d., 15 mm i.d., 15 cm length) which was then evacuated to a pressure about 10 -8 torr and sealed. The mixture was slowly (about 10 hour) heated to 1070 0C. This slow heating was necessary to avoid any possibility of explosion due to strongly exothermic reaction between the elements. The ampoule was maintained at this temperature for 10 days to bring the reaction to complete. Single phase polycrystaUine RuSe2, as determined by powder X-ray diffraction, was thus produced. For crystal growth, appropriate amount of this material and carrier substance (IC13 : 8 mg/cm~) were placed in a quartz ampoule (36 mm o.d., 32 mm i.d., 21 cm length) which was chilled in liquid nitrogen, evacuated to 10-8 torr and sealed. The ampoule was then inserted into a horizontal tube furnace. At the first day, the growth end of the ampoule was set to higher temperature (1080 0C) than the charge zone (980 0C) to remove nucleating center from the growth zone. During the next 10 days, the temperatures of the charge zone and the growth end of the ampoule were subjected to a periodic oscillation between 1080 0C and 980 0C. The ratio of time periods in the hot and cool zones was kept at 1 : 5, corresponding to one 4-hr period on a reversetransport (" clean ") cycle and one 20-hr period on a forward transport ( deposit ") cycle per day. It then followed a period of 40 days normal growth by the fixed temperature gradient technique (1080 0C -~ 980 0C). At the end of this time, the furnace was allowed to cool down slowly. When the ampoule reached room temperature, it was opened and the crystals removed. In no case did only one crystal grow or all the starting material transport.

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SELENIDES

Crystals were ground to powder and patterns were taken by means of a slowly moving radiation detector and recorded on a moving strip of paper with CuKa radiation. The resulting patterns were computer refined to give the lattice parameters. The parameters so determined are precise to 0 . 0 1 % . Laue back reflection was used to investigate the morphology of the RuSes single crystals. Selected crystals were examined by electron probe microanalysis. Electrical resistivity was measured on reasonably well-shaped single crystals using standard four probe technique. The sign, mobility and concentration of the charge carriers were determined by Hall effect measurements. The Hall coefficient was measured in a magnetic field of 10 KG. Electrical resistivity was studied between 20 K to 300 K with a temperature stability of 0.5 K or better. The Hall effect was measured between 75 K and room temperature. Ohmic contacts to the sample were made by soldering the gold wire (0.1 mm in diameter ) to the single crystals with pure indium metal. Electrochemical and photoelectrochemical experiments were carried with a conventional three-compartment cell. The electrical contact to crystal was made with a copper wire attached by a conducting silver paste to the opposite side of the exposed face which was in turn isolated by making the rest of the crystal with a non-contaminating epoxy resin. The electrode was then mounted in a suitable Teflon holder. The counter electrode was a large Pt plate Potentials were measured with respect to a saturated calomel electrode (SCE). The geometric surface of the exposed face was estimated to be 15 mm2. The roughness factor has been assumed to be unity. All solutions were prepared from analytical grade reagents and deionized water. Photo response measurements were using a constant white illumination from a Sylvania tungsten halogen lamp. The light intensity at the electrode surface was about 100 mW/cm2. RESULTS AND DISCUSSION Some representative crystal of RuSe~ grown in our laboratory are shown in Fig. 1. Single crystal of up to 10 x 8 x 6 mm3 with mirror-like surfaces were obtained on a routine basis. The oscillating method allows large crystals to grow at the expense of small one and leads to a small number of larger single crystals. Ennaoui et al. [8] reported successful growth of FeS~ with chemical vapor transport method by addition small amount of Mn with NH4Hal as transport agent. The growth rate increased from 0.005 mg/h to 4 mg/h. We have tried to grow RuSe2 single crystal doped with Mn. The transport rate did not show any increase and the EPR studies [9] indicated that single crystal of RuSe2 doped with Mn can not be grown 1~3; the chemical vapor transport method. Growth with seeds was tried, but no significant improvement was observed. Powder X-ray diffraction patterns showed RuSe2 crystallizing in the cubic, pyrite structure with cell dimension a0 = 5.933 A. The Laue pattern demonstrates a good crystallinity of the crystals. Crystals with various natural faces, such as (100), (110), (111), (121) and (221), were observed. The (111) face appears to be the predominant growth face. The examination by the electron microprobe of selected RuSe2 crystals shows

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Fig. 1 Some representative crystals of RuSes grown in our laboratory

that the crystals are Se deficiency. The nonstoichiometry in the RuSes sample crystals was reported by previous EPR studies [9]. A Se point defect model was established to account for the S = 1/2 paramagnetic species. Fig. 2 shows the typical results of the electrical resistivity measurements for RuSe2 single crystals. The results exhibit a semiconducting-like behavior. The resistivity increase from 0.017 fl-cm at

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295 K to 0.29 ~2-cm at 25 K. The increase of resistivity is larger than the results of Vaterlaus et al. [7] ( 2.5 ~ 10-a 12-cm at 295 K and 4.1 ~ 10 -3 f~-cm at 25 K ). Activation energy of resistivity defined by the relation P = P0 exp (aE/kT) is 26 meV between 190 K and room temperature. These activation energies are very low and characterize an extrinsic conductivity. Hall effect measurements was studied between 75 K and room temperature which confirmed the n-type semiconducting behavior. The carrier concentration n = 1/qRh and the Hall mobility #h were calculated using a single carrier model. A plot of the temperature dependence of the concentration is given in Fig. 3. The behavior of the increase of concentration with increasing temperature is normal for a semiconductor. The Hall mobility ~h remains nearly constant in the limit of the experimental error as reported by Vaterlaus et al. [7]. At room temperature the carrier concentrations are between 1.2 x 1019 and 1.8 x 1019 cm-3, the Hall mobilities are between 21 and 24 cm2/Vs. The results of Vaterlaus et al. [HT~l were 4.1 x 1019 cm-3 for carrier concentrations and 60 cm2/Vs for the 1 mobility. The measured values varied from sample to sample, and this was attributed to uncontrollable impurity concentrations or nonstoichiometric effects. The large number of extrinsic c h a r g e c a r r i e r s could be due to substitution of halogen for selenium. Iodine a n d / o r chlorine could possibly be i n t r o d u c e d into the sample c r y s t a l s as shallow donors, since IC13 was used as the t r a n s p o r t agent d u r i n g c r y s t a l growth. However, in the p r e v i o u s EPR studies [9], the absence of h y p e r f i n e p a t t e r n rules out halogens as the origin of the spectrum, since iodine has one natural isotope with I=5/2 and chlorine has two natural isotopes with I=3/2. 2O ,,

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hydrogen evolution starts at E = -0.4 V. When the anodic inversion potential exceed 1.4 V, a small peak at E = 0.78 V on the cathodic half-cycles appeared and its height increased after prolonged cycling. It indicates that the oxidation potential of the RuSe2 is comparative to the oxygen evolution reaction potential. Photoelectrochemical measurements show no diode-like behavior, with high dark currents and low photocurrents. The narrow band gap (~ 0.75 eV) and high carrier concentration (~ 1019 cm -3) might lead the difficulty of formation a rectifying junction between RuSe2 and electrolyte. Z8

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Fig. 4 Cyclic voltammetry of the (100) RuSe~ single crystal electrode in 0.5 M H~SO4, 200 mV/sec. CONCLUSIONS Single crystals of RuSe2 up to 10 x 8 x 6 mm3 have been grown by an oscillating chemical vapor transport method with ICla as the transporting agent. The crystals are larger and considerably better formed than those produced by high temperature solution growth technique [10]. Electrical resistivity and Hall effect measurement have indicated the presence of high concentration of extrinsic charge carriers, and that the measured values varied from sample to sample even from samples obtained from the same crystal growth. This has been attributed to uncontrollable impurities or nonstoichiometric effects. Electron microprobe analysis have indicated the difficulty of growing the exact stoichiometry of the material. Photoelectrochemical measurements have shown no photo response. This might related to the lack of a rectifying junction between RuSe~ and electrolyte. ACKNOWLEDGEMENTS The authors would like to thank the supports of the National Science Council of the Repubhc of China.

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SELENIDES REFERENCES

1. O. Sutarno, O. Knop and K. I. G. Reid, Can. J. Chem. 45, 1391 (1967). 2. H. D. Lutz, P. Willich and H. Ha~useler, Z. Naturf. a 31, 847 (1976). 3. F. Hulliger, Nature 200, 1064 (1963). 4. R. Guittard, R. Heindl, R. Parsons, A. M. Redon and H. Tributsch, J. Electroanal. Chem., .!11, 401 (1980). . 5. A. M. Redon, Solar Cells, 15, 27 (1985). 6. T. A. Pecoraro and R. R. Chianelli, J. Catalysis, 67, 430 (1981). 7. H. P. Vaterlaus, R. Bichsel, F. L~vy and H. Berger, J. Phys. C: Solid State Phys. 18, 6063 (1985). 8. A. Ennaoui, S. Fiechter, W. Jaegermann and H. Tributsch, J. Electrochem. Soc. 133, 97 (1986). 9. Jiang-Tsu Yu, Ying-Sheng Huang and Shoei-Sheng Lin, J. Phys. : Condens. Matter, _2, 5587 (1990). 10. S. Fiechter and H. M. Kfihne, Journal of Crystal Growth 83, 517 (1987).

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