Structure of layered WSe 2 thin films with ultralow thermal conductivity
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Ngoc T. Nguyen and David C. Johnson Department of Chemistry, University of Oregon, Eugene, Oregon 97403
David G. Cahill Department of Material Science and Engineering and F. Seitz Materials Research Laboratory, University of Illinois at Urbana–Champaign, Urbana, Illinois 61801 (Received 6 September 2007; accepted 4 January 2008)
Crystallographic ordering and defects in WSe2 thin films with ultralow thermal conductivity are characterized by electron imaging and diffraction in cross-sectional geometry. The results show that the film consists of oriented, coherent crystallites that are a few nanometers in diameter. Two films of different thickness with different thermal conductivity are compared. We show that the film with a lower thermal conductivity is characterized by less coherent crystallites with a greater degree of misorientation.
I. INTRODUCTION 1
A recent report by Chiritescu et al. shows that layered WSe2 thin films formed by the elemental-reactants method have a thermal conductivity (k) as small as 0.05 Wm−1 K−1 at room temperature. This low k value is 30 times smaller than the thermal conductivity of singlecrystal WSe2 along the c-axis direction, and a factor of six smaller than the predicted minimum k value of materials with this atomic density and speeds of sound.1 The k value is also much lower than that of inorganic glasses (k ∼ 1 W/m-K) or glassy polymers (k ∼ 0.15 W/m-K). Significant variations in the thermal conductivity, however, were observed in films of different thicknesses. The cause of these variations is not yet known, but Chiritescu et al.1 speculated that the microstructure of the films might depend on film thickness. The purpose of this work was to provide electron microscopy analysis of these thin films to facilitate a better understanding of ultralow thermal conductivity in disordered layered crystals. Single-crystal WSe2 has a hexagonal layered structure with cell constants of a ⳱ b ⳱ 3.282 Å and c ⳱ 12.96 Å.2 The W atoms form a flat hexagonal layer, which is bonded by two layers of Se atoms on each side. The W and Se bonds are strongly covalent with some ionic char-
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0124 1064 J. Mater. Res., Vol. 23, No. 4, Apr 2008 http://journals.cambridge.org Downloaded: 13 Mar 2015
acter. The W layer with its neighboring two Se layers can be considered as a sheet, and the crystal structure consists of the stacking of two-dimensional (2D) WSe2 sheets. The bonding between two adjacent sheets was accomplished by much weaker van der Waals forces.2,3 It has been suggested that the ultralow k of layered WSe2 thin films comes from the random stacking of 2D crystalline WSe2 sheets and the localization of lattice vibrations.1 This suggestion was supported in the initial report by synchrotron x-ray diffraction data. The intensity scan of (10 L) reflections indicated that the crystallographic coherence in the stacking of WSe2 sheets is
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