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.

Compared to the most well-known 2D material, graphene, which is a semi-metal, the semiconducting 2H phase of MoS 2 is advantageous in having a band gap suitable for electronic applications. In bulk form, MoS 2 has an indirect band gap of 1.3 eV, which increases as a function of decreasing film thickness. In monolayer MoS 2 (thickness ≈0.6 nm), the band gap becomes direct with a width of 1.8 eV. [1] Importantly, to meet the requirements of different applications, properties of MoS 2 and other TMDCs can be tuned by controlling the thickness, [1] doping and alloying, [5][6][7][8] surface modification and functionalization, [9][10][11] strain, [12,13] and by creating heterostructures with other 2D materials. [6,[14][15][16] The appealing properties of TMDCs have led to a wide range of proposed applications. MoS 2 has been extensively studied as a channel material in conventional field-effect transistors, [17][18][19][20][21] as well as phototransistors and other optoelectronic devices. [16,21,22] The 2D structure of TMDCs plays a crucial role in possible applications relying on more exotic quantum phenomena, such as valleytronics. [23,24] MoS 2 has also shown promise in, for example, catalysis, [25] batteries, [26] photovoltaics, [27] sensors, [28] and medicine. [29] The production of high-quality, large-area MoS 2 films with a thickness controllable down to a monolayer, as required in many of the aforementioned applications, still remains a major challenge. Additionally, in many cases, the processing temperature should be kept as low as possible in order to avoid damaging sensitive substrates, such as polymers or nanostructures. Initially, flakes of monolayer MoS 2 were produced from natural MoS 2 crystals using micromechanical exfoliation, a topdown method capable of producing high-quality monolayers, albeit with poor throughput as well as limited control over flake thickness and dimensions. [4,30,31] Liquid-phase exfoliation of bulk crystals, on the other hand, offers good scalability, but often suffers from limited flake size, poor crystallinity, or contamination. [4,31,32] Bottom-up methods offer a more controllable way to produce MoS 2 films. High-quality MoS 2 thin films are most commonly deposited by chemical vapor deposition (CVD) or sulfurization of metal or metal oxide thin films. The most common Molybdenum disulfide (MoS 2 ) is a semiconducting 2D material, which has evoked wide interest due to its unique properties. However, the lack of controlled and scalable methods for the production of MoS 2 films at low temperatures remains a major hindrance on its way to applications. In this work, atomic layer deposition (ALD) is used to deposit crystalline MoS 2 thin films at a relatively low temperature of 300 °C. A new molybdenum precursor, Mo(thd) 3 (thd = 2,2,6,6-tetramethylheptane-3,5-dionato), is synthesized, characterized, and used for film deposition with H 2 S as the sulfur precursor. Self-limiting growth with a low growth rate of ≈0.025 Å cycle −1 , straightforward thickness control, and large-area uni...

Aluminum fluoride thin films have potential in both optic and lithium-ion battery applications. AlF 3 thin films have mostly been deposited using physical vapor deposition methods. In this study, we present a new atomic layer deposition process for AlF 3 . Our method makes use of a halide−halide exchange reaction with AlCl 3 and TiF 4 as the precursors. With this new chemistry, thin films of AlF 3 can be deposited at a temperature range of 160−340 °C. The films have been studied by UV−vis spectroscopy, field emission scanning electron microscopy, X-ray diffraction, X-ray reflectance, atomic force microscopy, timeof-flight elastic recoil detection analysis (ToF-ERDA), energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. At 220 °C, the growth rate of the films is approximately 1.1 Å per cycle, and the refractive index is 1.36 (at 580 nm). The films show only small amounts of Cl and Ti impurities when deposited at high temperatures, as determined by ToF-ERDA. Surface oxidation of the films due to moisture in ambient air is observed.

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.

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For the first time an element other than a metal was deposited by atomic layer deposition (ALD). Pure and conformal thin films of elemental antimony were prepared by ALD using SbCl3 and (Et3Si)3Sb as precursors. In situ reaction mechanism studies showed that the dehalosilylation reactions involved are very efficient in eliminating the ligands from the growing surface enabling the use of low growth temperatures down to 95 °C. Various antimony compounds, such as GeSb, Sb2Te, GaSb, and AlSb, can also be deposited by reacting (Et3Si)3Sb with other metal halides or mixing Sb growth cycles with other ALD processes. The new antimony ALD process is a major step in the realization of non-volatile phase change random access memories (PCRAM) and ALD of III−V compounds.

2D materials research is advancing rapidly as various new "beyond graphene" materials are fabricated, their properties studied, and materials tested in various applications. Rhenium disulfide is one of the 2D transition metal dichalcogenides that has recently shown to possess extraordinary properties such as that it is not limited by the strict monolayer thickness requirements. The unique inherent decoupling of monolayers in ReS combined with a direct bandgap and highly anisotropic properties makes ReS one of the most interesting 2D materials for a plethora of applications. Here, a highly controllable and precise atomic layer deposition (ALD) technique is applied to deposit ReS thin films. Film growth is demonstrated on large area (5 cm × 5 cm) substrates at moderate deposition temperatures between 120 and 500 °C, and the films are extensively characterized using field emission scanning electron microscopy/energy-dispersive X-ray spectroscopy, X-ray diffractometry using grazing incidence, atomic force microscopy, focused ion beam/transmission electron microscopy, X-ray photoelectron spectroscopy, and time-of-flight elastic recoil detection analysis techniques. The developed ReS ALD process highlights the potential of the material for applications beyond planar structure architectures. The ALD process also offers a route to an upgrade to an industrial scale.

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