ALD2020 Session NS1-WeA: 2D Nanomaterials by ALD II
Wednesday, July 1, 2020 1:00 PM in Room Auditorium
NS1-WeA-1 Atomistic Simulation of ALD of 2D Transition-Metal Dichalcogenides
Mahdi Shirazi, Erwin Kessels, Ageeth Bol (Eindhoven University of Technology, Netherlands)
Extensive research has been done during the last decade to unravel the remarkable electronic properties ,  of two dimensional transition-metal dichalcogenides (2D-TMDs) in the monolayer regime. In spite of their astounding electrical properties, these material systems are not ready yet for replacing Si based materials for future nanometer-sized electronic devices. One key challenge is the integration of these materials in bottom-up processes at low temperature (usually < 500 °C) into the semiconductor manufacturing flow. Horizontal growth at wafer scale with a large grain size (typically 1x1 µm2) is required for nano-electronic devices . The cyclic process of atomic layer deposition (ALD)  with tight control over the chemical reactions shows promise as such a bottom-up process. The chemical reactions of ALD are self-limiting and are designed to proceed only at the surface. In this contribution, we have employed density functional theory (DFT) to provide fundamental insight into the reaction mechanisms of the MoS2 growth. We have studied the deposition of MoS2 that is initiated by the exposure of metal precursor Mo(NMe2)2(NtBu)2 (C12H30N4Mo) to the SiO2 surface and then followed by exposure of H2S/H2 as co-reagent in the second pulse . In this so-called hetero-deposition, the involved chemical reactions during ALD lead to the formation of a buffer layer at the surface of SiO2 . After formation of this buffer layer, ALD enters into the steady-growth regime (also called homo-deposition). In the steady growth regime, vertically or horizontally aligned MoS2 structures grow in a layer-by-layer fashion. The calculated reaction energies and activation energies indicate that the reaction kinetics in the hetero-deposition are slower than reaction kinetics in homo-deposition. Artificial intelligence is used to generate an efficient interatomic potential using the calculated energies and forces of configurations obtained by DFT. The generated interatomic potential will be used for larger scale simulations to provide further fundamental insight into the deposition of MoS2 by ALD.
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 S. M. George, Chem. Rev., 2010, DOI:10.1021/cr900056b.
 M. Shirazi, W. M. M. Kessels, A. A. Bol, Phys. Chem. Chem. Phys., 2018, DOI:10.1039/C8CP00210J.
 M. Shirazi, W. M. M. Kessels, A. A. Bol, APL Mater., 2018, DOI:10.1063/1.5056213.
NS1-WeA-4 ALD of MoSe2 using New Precursors
Raul Zazpe (University of Pardubice, Czech Republic); Richard Krumpolec (Brno University of Technology, Czech Republic); Jaroslav Charvot, Ludek Hromadko, Hanna Shopa, Martin Motola, Milos Krbal, Filip Bures, Jan Macak (University of Pardubice, Czech Republic)
2D semiconductor transition metal dichalcogenides (TMDs) have attracted considerable attention due to their layered structure, suitable band gap for visible light absorption, high carrier mobility, electrochemically active unsaturated edges and relatively good stability against photocorrosion . Recently, 2D MoSe2 has been gaining considerable interest due to its higher electrical conductivity as compared to MoS2, its wider inter-layer distance (~0.65 nm), narrow bandgap (1.33–1.72 eV), high resistance to photo-corrosion, high surface area layer, electrochemically active unsaturated Se-edges and close to zero Gibbs free energy edges for hydrogen adsorption.These properties are promising for different applications of MoSe2 including hydrogen evolution , photocatalysis  and Li-ion batteries . However, their low light absorption efficiency, recombination issues of the photogenerated electron–hole pairs and slow charge transfer of the intrinsic semiconducting 2H-phase are a handicap. An efficient strategy to surpass those intrinsic limitations are hybrid nanostructures using conducting supporting materials. In this regard, anodic TiO2 nanotubes (TNTs) are excellent photoactive supporting material providing a high surface area, unique directionality for the charge separation, and highly effective charge collection.  Accordingly, we present anodic TiO2 nanotubes homogenously decorated with MoSe2 nanosheets by atomic layer deposition (ALD). In parallel, we address the current scarcity of convenient ALD Se precursors by the synthesis a set of new selenium precursors - alkysilyl (R3Si)2Se and alkytin (R3Sn)2Se, and cyclic silylselenides compounds. Those Se precursors were extensicely characterized and their reliability as ALD Se precursors explored [6,7]. Several compounds exhibited promising results to be convenient ALD Se precursor as will be presented in the presentation. The synthesis of the MoSe2 nanosheets and their composites with TiO2 NTs, their physical and electrochemical characterization, and encouraging results in electrochemical characterization, hydrogen evolution reaction (HER) and photocatalysis will be presented and discussed.
NS1-WeA-5 Low Temperature Creation of Layered-MoS2 Thin Films on Large Area High Aspect Ratio Substrates
Anil Mane, Devika Choudhury, Steven Letourneau, Jeffrey W. Elam (Argonne National Laboratory)
Thin layers of two dimensional (2D) materials mainly transition metal dichalcogenides (TMDs) and more specifically ultra-thin layered-MoS2 semiconductor possess exceptional properties such as electrical, optical, magnetic, mechanical and chemical properties. This allows the exploration of internal quantum degrees of freedom of electrons and their potential for use in semiconductor microelectronics, optoelectronic, energy, and sensor and detector applications. These exciting results are being achieved mostly by using exfoliation of flecks from bulk MoS2 crystal. However, the biggest challenge in realizing TMDs full potential has been the lack of scalable material synthesis methods for such films with high uniformity, conformality and interfacing with other materials such as oxides, metals and its process compatibility.
Among the various thin film deposition methods, atomic layer deposition (ALD) offers the best combination of precisely controlled layer-by-layer thin film growth at low temperature with very high conformality on complex substrates. Here we will present the growth of layered-MoS2 thin films. To grow high quality layered–MoS2 thin films, we have developed an ALD-based two step processing approach : firstly the growth of well controlled ultra-thin layer of Mo metal using ALD followed by the sulfurization of the ALD Mo layer at various temperatures. This two-steps processing results in high quality layered-MoS2 thin films on large substrates. For the Mo ALD process we used molybdenum hexafluoride (MoF6) and Si2H6 precursors. We used in-situ QCM measurements to study interfacial and nucleation effects in the formation of continuous ultra-thin metal layer of Mo. The composition of both the ALD Mo and the layered MoS2 layers was determined by X-ray photoelectron spectroscopy (XPS). Further, cross-sectional transmission electron microscopy (TEM) was performed to confirm the formation of layered MoS2 on high aspect ratio trenches and Raman analysis to verify the signature of E12g blue shift and A1g red shift in the MoS2 structure. In this presentation we will discuss the details of the two-step thin film growth process for creating layered MoS2 layers via ALD Mo and subsequent sulfurization as well as the properties of the MoS2 films.
 Anil Mane, Devika Choudhury, Steven Letourneau, Jeffrey Elam, (US patent application submitted 2018)
NS1-WeA-6 Gas Sensing Characteristics of MoxW1-xS2 Synthesized by Atomic Layer Deposition
Inkyu Sohn, Youngjun Kim, Minjoo Lee, Ju Sang Park, Hyungjun Kim (Yonsei University, Republic of Korea)
Two-dimensional Transition metal dichalcogenide (2D TMDC) have been attracted great attention as gas sensing materials with high sensitivity in room temperature.  Because of this characteristic, 2D TMDC gas sensor could overcome the oxide-based semiconductor which need heating for gas sensing. Therefore, various 2D TMDC gas sensor studies have been ongoing. Recently, it has been shown that the gas sensor property could be improved through the changing the composition in WS2xSe2-2x alloy. 
Here we report a synthesis method of MoxW1-xS2 alloys for gas sensor by atomic layer deposition. Layer controlled 2D MoS2 and WS2 were synthesized with Mo(CO)6, W(CO) 6 and H2S as precursors and reactant. For the first time, we systematically modulate the composition of MoxW1-xS2 alloys by changing the configuration of low-temperature ALD super-cycles. AFM and Raman spectroscopy results of MoxW1-xS2 alloys demonstrate that the thickness of alloy is accurately controlled by ALD. Also XPS results confirmed that composition of alloy is precisely controlled by ALD super-cycles. Furthermore, gas sensors fabricated by MoxW1-xS2 alloys were evaluated for NO2 gas. It showed response time and recovery characteristics of MoxW1-xS2 alloy gas sensor is dramatically enhanced.
References  ACS Nano 10, 9287-9296 (2016)  ACS Appl. Mater. Interfaces 10, 34163-34171 (2018)