ATOMIC LAYER DEPOSITION OF ULTRATHIN CRYSTALLINE FILMS OF MoS₂ and V₂O₅

Atomic layer deposition (ALD) is a unique thin film deposition method, suitable for depositing uniform and conformal films with atomic accuracy.¹ Single crystalline materials with specific phases are usually preferable as the active and interfacial films for devices but most of the ALD grown materials are limited to polycrystalline and amorphous.² Here we present the growth and characteristics of single crystalline films of pristine MoS₂ and V₂O₅ by ALD. Employing thermal ALD, we have grown crystalline MoS₂ films on c-axis Al₂O₃ substrate. Details of the atomic registry between MoS₂ and sapphire has been revealed by HRTEM and shows (00l) preferred orientation with c-sapphire. The compressive strain of 2% in the first MoS₂ layer and 0.6% in the second layer has been observed at the interface. The crystallographic relationships between the films and the substrate are Al₂O₃ || MoS₂ and Al₂O₃ || MoS₂. Planar faults are observed in the TEM and are due discontinuous Mo and S atomic planes shared between adjacent layers. Interestingly, the films show a high density of nanowalls and exhibit excellent electrochemical characteristics in Na-ion battery.³ Single crystalline epitaxial V₂O₅ films have been deposited on c-Al₂O₃ using plasma enhanced ALD. The HRTEM images show the smooth interface between the Al₂O₃ and V₂O₅ film with epitaxial growth. The crystallographic relations are found be V₂O₅||Al₂O₃ and V₂O₅|| <11̅02>Al₂O₃ and undergo compressive strain at the initial few monolayer growth to adjust epitaxially with the substrate. The observed compressive strain is consistent with Raman signatures. Humidity sensor based on the V₂O₅ film has superior sensitivity relative to other oxide-based sensors.⁴

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2. Wen, L.; Zhou, M.; Wang, C.; Mi, Y.; Lei, Y., Energy Mater. 2016, 6, 1600468.
3. B. Sreedhara, Subhra Gope, Badri Vishal, Ranjan Datta, Aninda J. Bhattacharyya, C. N. R. Rao, J. Mater. Chem. A, 2018, 6, 2302.
4. B. Sreedhara, J. Ghatak, B. Bharath, and C. N. R. Rao, ACS Appl. Mater. Interfaces 2017, 9, 3178.