Semiconducting two-dimensional (2D) materials are studied intensively because of their promising performance in diverse applications from electronics to energy storage and catalysis. Recently, HfS2 and ZrS2 have emerged as potential rivals for the commonly studied 2D semiconductors such as MoS2 and WSe2, but their use is hindered by the difficulty of producing continuous films. Herein, we report the first atomic layer deposition (ALD) processes for HfS2 and ZrS2 using HfCl4 and ZrCl4 with H2S as the precursors. We demonstrate the deposition of uniform and continuous films on a range of substrates with accurately controlled thicknesses ranging from a few monolayers to tens of nanometers. The use of semiconductor industry-compatible precursors and temperatures (approximately 400 °C) enables facile upscaling of the process. The deposited HfS2 and ZrS2 films are crystalline, smooth, and stoichiometric with oxygen as the main impurity. As an important step toward applications of HfS2 and ZrS2, we show that their sensitivity toward oxidation can be overcome by minimizing the impurities in the reactor and by depositing a protective Al x Si y O z layer on the top without a vacuum break. Finally, we demonstrate HfS2 and ZrS2 photodetectors exhibiting good performance and stable operation in ambient conditions. Photoresponsivity comparable to thin films or even single flakes of HfS2 or ZrS2 deposited at much higher temperatures is achieved, although the response speed seems to be limited by photogating, as is common for 2D photodetectors. We expect the first ALD processes for HfS2 and ZrS2 to enable further exploration of these materials for various semiconductor applications.

Atomic layer deposition (ALD) enables the deposition of numerous materials in thin film form, yet there are no ALD processes for metal iodides. Herein, we demonstrate an ALD process for PbI2, a metal iodide with a two-dimensional (2D) structure that has applications in areas such as photodetection and photovoltaics. This process uses lead silylamide Pb­(btsa)2 and SnI4 as precursors and works at temperatures below 90 °C, on a variety of starting surfaces and substrates such as polymers, metals, metal sulfides, and oxides. The starting surface defines the crystalline texture and morphology of the PbI2 films. Rough substrates yield porous PbI2 films with randomly oriented 2D layers, whereas smooth substrates yield dense films with 2D layers parallel to the substrate surface. Exposure to light increases conductivity of the ALD PbI2 films which enables their use in photodetectors. The films can be converted into a CH3NH3PbI3 halide perovskite, an important solar cell absorber material. For various applications, ALD offers advantages such as ability to uniformly coat large areas and simple means to control film thickness. We anticipate that the chemistry exploited in the PbI2 ALD process is also applicable for ALD of other metal halides.

Hybrid halide perovskite thin films are applicable in a wide range of devices such as light-emitting diodes, solar cells, and photodetectors. The optoelectronic properties of perovskites together with their simple and inexpensive film deposition methods make these materials a viable alternative to established materials in these devices. However, the potential of perovskite materials is compromised by the limitations of the existing deposition methods, which suffer from trade-off among suitability for large-scale industrial production in a batch or roll-to-roll manner, deposition area, film quality, and costs. We addressed these limitations by developing a deposition method that is inexpensive, applicable to large substrate areas, scalable, and yields high-quality perovskite films. In this study, the low-cost electrodeposition (ED) method and sequential exposure to reagent vapors produce CH 3 NH 3 PbI 3 perovskite films with thickness nonuniformity below 9% on a centimeter scale. PbO 2 films are electrodeposited first and then undergo two vapor conversion steps, with HI vapor in the first step and CH 3 NH 3 I vapor in the second step. The second step yields CH 3 NH 3 PbI 3 films that are continuous and consist of micrometer-sized grains. This process allows the preparation of both α- and β-phase CH 3 NH 3 PbI 3 films, offers a simple means to control the film thickness, and works over a wide range of film thicknesses. In this work, films with thicknesses ranging from 100 nm to 10 μm were prepared. ED and vapor conversion are inherently scalable techniques and hence the process described herein could benefit application areas in which large device areas and throughput are required, such as the production of solar cells.

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van der Waals epitaxy holds great promise in producing high-quality films of two-dimensional materials. However, scalable van der Waals epitaxy processes operating at low temperatures and low vacuum conditions are lacking. Herein, atomic layer deposition is used for van der Waals epitaxy of continuous multilayer films of two-dimensional materials HfS2, MoS2, SnS2, and ZrS2 on muscovite mica and PbI2 on sapphire at temperatures between 75 and 400 °C. For the metal sulfides on mica, the main epitaxial relation is (0001)[2110]MS2 || (001)[100]mica. Some domains rotated by 30° are also observed corresponding to the (0001)[1100]MS2 || (001)[100]mica alignment. In both cases, the presence of 60° domains (mirror twins) is also expected. For PbI2 on sapphire, the epitaxial relation is (0001)[2110]PbI2 || (0001)[2110]Al2O3 with no evidence of 30° domains. For all of the studied systems there is relatively large in-plane mosaicity and in the PbI2/Al2O3 system some nonepitaxial domains are also observed. The study presents first steps of an approach towards a scalable and semiconductor industry compatible van der Waals epitaxy method.