Exfoliated single molecular layers of Mn 0.8 PS 3 and Cd 0.8 PS 3
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The layered compounds MnPS3 and CdPS3 were exfoliated to form single molecular layers of Mn0.8PS3 and Cd0.8PS3 in suspension in water by ion exchange. The x-ray diffraction patterns of the two single-layer suspensions showed profound differences in some of the Bragg peaks, and we demonstrated that the differences are not due to the quality or size of the single layers, but are caused by structure factor modulations of the Warren tail for two-dimensional systems. We also demonstrated that the Cd or Mn vacancies generated in the exfoliation process are not ordered at long range, in contrast to an earlier report of vacancy ordering on intercalated MnPS3.
I. INTRODUCTION
Single molecular layers of MoS2, WS2, and MoSe2, dispersed in suspension in water, have been prepared by exfoliation.1,2 These two-dimensional materials are readily identified by their unique powder x-ray diffraction patterns. Recently, we showed that the layer compound CdPS3 can also be exfoliated into single layers.3 Transition metal phosphorous trisulfide layered compounds such as CdPS3 and MnPS3 have attracted the attention of researchers because of the anisotropic properties imposed by their layered structure and their ability to act as host lattices for intercalation compounds.4,5 These compounds have a layered structure (Fig. 1) where a molecular layer is made up of sheets of Cd/Mn and P atoms located between two sheets of S atoms, which is similar to that of transition metal dichalcogenides.6 CdPS3 and MnPS3 are transparent in the visible region and hence have potential applications in many areas of optoelectronics.7,8 The energy band gaps of CdPS3 and MnPS3 were determined to be around 3.5 eV and 3.0 eV respectively.9 Clement and co-workers4 developed an ion-exchange intercalation chemistry for transition metal phosphorous trisulfides which allows the synthesis of many stable intercalation compounds. In this paper, we report that, using the ion-exchange method, both CdPS3 and MnPS3 can be exfoliated and separated into single molecular layers suspended in water with a layer size of about 400 × 400 Å and a thickness of 6.5 Å. We have characterized and compared the two exfoliated systems using x-ray diffraction. We also demonstrate that the Cd or Mn vacancies that are generated in the ion-exchange process a)
Address all correspondence to this author. Present address: Department of Physics, Dalhousie University, Halifax, Nova Scotia, Canada B3M 3J5. e-mail: [email protected] DOI: 10.1557/JMR.2005.0161 J. Mater. Res., Vol. 20, No. 5, May 2005
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are not ordered at long range, in contrast to an earlier report of vacancy ordering on intercalated MnPS3.10 II. EXPERIMENTAL A. Crystals of CdPS3 and MnPS3
Crystals of CdPS3 (or MnPS3) were grown by heating a mixture of stoichiometric amounts of elemental Cd (or Mn), P, and S powder in an evacuated quartz tube at 800 to 900 °C (650 °C for MnPS3) for 10 days. The resulting material contains mostly small crystals, but includes a few large, th
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