Article Cite This: Chem. Mater. XXXX, XXX, XXX−XXX
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Initial Growth and Agglomeration during Atomic Layer Deposition of Nickel Sulfide Ran Zhao and Xinwei Wang* School of Advanced Materials, Shenzhen Graduate School, Peking University, Shenzhen 518055, China
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ABSTRACT: Atomic layer deposition (ALD) is a highly useful technique to grow thin film materials, and the initial growth of ALD is of particular importance for achieving highquality materials and interfaces. In this work, we investigate the initial ALD growth of nickel sulfide (NiS) on a SiOx surface, from bis(N,N′-di-tert-butylacetamidinato)nickel(II) (Ni(amd)2) and H2S, by using the in situ techniques of Xray photoelectron spectroscopy (XPS) and low-energy ion scattering (LEIS). The mechanism of the initial ALD growth is found to be rather different from that in the later steady film growth. In the initial ALD cycles, the XPS results show a drastic cyclic variation of the signals for the Ni−O bonds, with prominently observable Ni−O signals after each Ni(amd)2 dose but almost negligible after the subsequent H2S dose. These results suggest that the Ni−O bonds are first formed on the surface in the Ni(amd)2 half-cycles and then mostly converted to NiS in the following H2S half-cycles. To describe this initial ALD growth process, a reaction-agglomeration mechanistic scheme is proposed. In this scheme, the conversion from Ni−O to NiS in the H2S half-cycles is suggested to be accompanied by the spontaneous agglomeration of the ligand-stripped Ni-containing species to afford NiS clusters. This agglomeration can reexpose the surface of SiOx and therefore allow the surface to further react with Ni(amd)2 in the subsequent ALD cycles. With the further use of LEIS, the transition from the initial reaction-agglomeration growth to the continuous steady film growth is found to be between 100 and 300 ALD cycles. Ex situ atomic force microscopy and transmission electron microscopy are further employed to corroborate the presence of the agglomeration during the initial growth. The results reported herein and the implied growth mechanism should be of great representativeness for the ALD of metal sulfides, and therefore, the findings should be highly valuable for future engineering of functional metal sulfide nanomaterials and interfaces by ALD.
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initial surface chemistry during ALD.11−13 Therefore, understanding the initial surface chemistry as well as the growth mechanism is of crucial importance for rationally designing functional materials and interfaces by ALD. Over the past several years, ALD of metal sulfides has aroused great attention, especially for their numerous applications in the emerging energy technologies.4,15−25 Many new ALD processes of metal sulfides have been recently developed, and the metal sulfides of which the ALD processes were only recently available include GaSx,16 GeS,26 MoS2,27 Li2S,17 Co9S8,18,28 NiS,19−21 MnS,22 FeSx,29−31 VS4,32 Bi2S3,33 AlSx,23 and ReS2.34 Among these sulfide compounds, the nickel sulfide (NiS) is of great interest, as NiS has been recently employed in many important applications, such as lithium/ sodium-ion batteries,35−38 supercapacitors,39−42 solar cells,43,44 and water-splitting electrocatalysis.19,20,40,45 The ALD process of NiS was recently developed by using bis(N,N′-di-tertbutylacetamidinato)nickel(II) (Ni(amd)2) and H2S as the
INTRODUCTION Atomic layer deposition (ALD) is a highly useful technique to synthesize thin film materials, such as metal oxides, nitrides, sulfides, and metals themselves.1−4 ALD relies on sequential self-limiting reactions between gas-phase precursor molecules and a solid support to enable thin films to grow one layer at a time.5 This growth fashion allows ALD to produce highly reproducible, large-scale uniform and conformal thin films with sub-nanometer precise control of film thickness on highsurface-area complex or porous solids. In a typical ALD process, the initial nucleation and growth stage is of particular importance, because it not only is crucial for preparing highquality film−substrate interfaces6−8 but also can be engineered to synthesize well-controlled nanoparticles if the initial ALD growth is intentionally designed to fall in the nucleationcontrolled growth regime.9 Particularly, the latter has recently triggered many ALD studies to fabricate functional nanoparticles for varieties of applications, such as noble-metal nanoparticles for catalysis,10−12 PbS or CdS quantum dots for photovoltaics,8,13,14 and so forth. Among these functionalities, many are closely related to the shape, size, and coverage of the nanoparticles, and these attributes depend very much on the © XXXX American Chemical Society
Received: September 17, 2018 Revised: December 19, 2018 Published: December 19, 2018 A
DOI: 10.1021/acs.chemmater.8b03940 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials
The ALD of NiS was performed at 200 °C, using bis(N,N′-di-tertbutylacetamidinato)nickel(II) (Ni(amd)2) and H2S as the nickel precursor and sulfur source, respectively. Ni(amd)2 was kept at 70 °C during deposition to afford a saturated vapor pressure of ∼140 mTorr. Each ALD cycle consisted of one Ni(amd)2 dose and one H2S dose. The Ni(amd)2 dose consisted of 5 pulses of the Ni(amd)2 vapor, which corresponded to a total exposure of 0.1 Torr s for Ni(amd)2. As for H2S, we used diluted H2S gas (3% in Ar), and it was first delivered into an ∼8 mL gas trap and then pulsed into the ALD chamber for the NiS deposition. It should be cautioned that H2S is a toxic, flammable, and corrosive gas, and therefore, it should be handled with great care. The H2S dose consisted of 3 pulses of the diluted H2S gas, which corresponded to a total exposure of 0.3 Torr s for H2S. Note that the above exposure numbers for Ni(amd)2 and H2S were particularly chosen to resemble the conditions used in practical ALD processes.19 Ex situ atomic force microscopy (AFM, Bruker, Multimode 8) was used to examine the surface morphology of the ALD NiS on SiOx/Si substrates, and transmission electron microscopy (TEM, Jeol, JEM3200FS) was used to examine the morphology of the ALD NiS on TEM grids.
precursors,19 and the process was found to follow a typical layer-by-layer ALD growth mode to produce fairly smooth, pure, and conformal NiS films at deposition temperatures ranging from 90 to 200 °C.19 Meanwhile, the growth rate of NiS during the steady ALD growth was found to be fairly slow (
