Elsevier

Applied Surface Science

Volume 252, Issue 22, 15 September 2006, Pages 7941-7947
Applied Surface Science

Pressure induced anisotropy of electrical conductivity in polycrystalline molybdenum disulfide

https://doi.org/10.1016/j.apsusc.2005.10.011Get rights and content

Abstract

Anisotropic specimens of MoS2 are obtained by pressing the microcrystalline powder into special die. This inelastic compression results in a rearrangement of the disulfide micro platelets observed by atomic force microscopy and reflected in the macroscopic anisotropy in electrical conductivity in these samples. The conductivity measured parallel and perpendicular to the direction of applied pressure exhibits an anisotropy factor of 10 at 1 GPa. This behaviour of the conductivity as a function of applied pressure is explained as the result of the simultaneous influence of a rearrangement of the micro platelets in the solid and the change of the inter-grain distances.

Introduction

A very relevant feature of molybdenum disulfide is its hexagonal layered structure, a characteristic that leads to its use as a lubricant [1], as a catalyst [2] and makes it an ideal material to explore intercalation chemistry [3]. Also the transport properties of MoS2 are not an exception [4]. Pure MoS2 is an intrinsic semiconductor with a marked anisotropic behaviour [3]. Single-crystal electrical conductivity along the axis perpendicular and parallel to the layer plane shows a ratio σσof about 103 [5]. However, in spite of the relatively high anisotropy of layered transition metal disulfide, the use of these kinds of materials to develop, for instance, new electronic and electrochemical devices is often discouraged by the difficulties in obtaining single-crystals [6], [7].

The fact that polycrystalline transition metal disulfides are rather more abundant and convenient than single crystal specimens, encourages us to study transport properties of these kinds of samples searching for conditions under which the anisotropy of the materials may be observed. Furthermore, such a procedure will be especially useful for intercalation host-guest derivatives of layered transition metal disulfides, which exhibit intrinsic anisotropic transport properties, typically superior to those of the pristine host, but they may be obtained only as microcrystalline powders [8].

In this paper, the microscopic ordering and the electrical conductivity behaviour of compressed polycrystalline samples of MoS2 as a function of pressure and temperature is described, and a model describing the variations between the applied pressure and the anisotropy of the system is proposed. The dependence of the conductivity on the density of the material shows that after a given pressure, the sample can be considered as a pseudo single-crystal from its conductivity characteristics.

Section snippets

Experimental

Molybdenum disulfide (Aldrich 99%) was used as received without further treatment. Samples for electrical conductivity measurements were prepared by pressing the polycrystalline powder in a specially designed die to obtain parallelepiped pellets of about 5mm×5mm×2 mm. The applied pressure was in the range of 50 MPa–1 GPa. The density was determined from the weight and volume of the samples.

Atomic force microscopy (AFM) investigations were conducted on the faces of the die in directions parallel

Structural characterization of compressed MoS2

Manipulation of polycrystalline powders of layered compounds like MoS2 often induces ordering of the micro crystals in the sample. Indeed, X-ray diffraction patterns obtained at the surface of such samples show reflection intensity ratios that differ from those obtained by transmission through the bulk [9].

In the former case, the {001} plane reflections are notoriously enhanced due to spontaneous ordering taking place on the surface. This feature becomes more pronounced when the surface is

Conclusions

Results discussed above show that for microcrystalline samples of MoS2 lamellar solids become ordered by pressure leading to specimens in which the intrinsic anisotropy of the micro crystals is to some degree macroscopically observed. In the case of polycrystalline MoS2 an anisotropy factor of 10 is obtained at 1 GPa and the electrical conductivity in the direction parallel to the lamellae is also measured to be 10 times that in the perpendicular direction.

It may be assumed that the

Acknowledgements

The support of the FONDECYT (Grants 1050344, 1030102, 7050081, 1050788), Science Foundation Ireland (SFI), the University of Chile, and the Universidad Tecnológica Metropolitana are gratefully acknowledged.

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