In this work, an atomic layer deposition approach for the synthesis of MoS2 monolayers is presented. Optical properties of the prepared large-area samples were characterized by Raman and photoluminescence (PL) spectroscopies, yielding homogeneous optical properties in 5 × 5 mm2 areas. High-resolution transmission electron microscopy and atomic force microscopy demonstrate closed films with grain sizes in the micrometer range. Crucial process parameters and their impact on the properties of the resulting layers are discussed, highlighting the resilience of the process with a broad parameter window for obtaining monolayer films with a high PL yield.
stacking nature, [5] strain, [6] and applied voltages. [7] Properties such as natural surface passivation, [8] high carrier mobility, [9] semiconducting band gaps, [1a] valley polarization, [10] strong light-mater interactions, [3,11] and the transitions from an indirect band gap in bulk form to a direct band gap in monolayer form [3,12] make them noteworthy materials to study. TMDCs such as molybdenum disulfide (MoS 2 ) have proven to be photoactive [13] and have been investigated as potential absorber layers in solar cells. [14] Many proof-of-concept devices have been constructed in the past few years, the majority of them based on exfoliated TMDC flakes. [13b,15] Exfoliation approaches are limited to the preparation of micrometer-scale devices and are challenging to scale to larger length scales. Even though there have been a few instances of larger scale TMDC solar cells. [16] In most of these cases, bulk MoS 2 or TMDC materials were used with layer thickness exceeding 100 nm. Therefore, these approaches could not benefit from the intrinsic properties of mono-and few-layer MoS 2 , such as an increased band gap [12] and high absorption coefficient in thin layers, [13b] and do not exploit the potential for transparent and semi-transparent photovoltaic applications. [17] In order to take advantage of the unique mono-and few layer properties of MoS 2 as absorption layers in solar cells, the TMDC films need to be integrated in a device stack that can separate and extract charge carriers from the TMDC light absorbing layer. Charge separation can be realized with carrierselective contact materials, such as titanium oxide (TiO x ) acting as an electron selective contact [18] and molybdenum oxide (MoO x ) acting as a hole selective contact. [19] Where these selective contacts help separate the generated charge carriers due to different work functions of the contact material. [20] TiO x and MoO x have already been reported to form type II heterostructures with MoS 2 allowing for charge separation. [21] One aim of this paper is to investigate the influence of the adjacent TiO x and MoO x contacts on the optoelectronic properties of monoand few layer exfoliated MoS 2 flakes, different from a similar study by Xu et al., [22] that also explores the relationship between MoS 2 and carrier selective contacts. The MoS 2 absorber layers, in our study are transferred onto the contact materials in order to avoid possible damage of the ultrathin absorber during contact deposition, instead of being sandwiched between silicon dioxide (SiO 2 ) and metal oxides. As 2D materials are sensitive Transition metal dichalcogenides are an exciting class of new absorber materials for photovoltaic applications due to their unique optoelectronic properties in the single to few-layer regime. In recent years, these materials have been intensively studied, often utilizing conventional substrates such as sapphire and silicon dioxide on silicon. This study investigates the optical properties of molybdenum disulfide (MoS 2 ) mono-, bi-, and mul...
Micropatterning of transition metal dichalcogenide (TMDC) ultrathin-films and monolayers has been demonstrated by various multi-step approaches. However, directly achieving a patterned growth of TMDC films is still considered to be challenging. Here, we report a solution-based approach for the synthesis of patterned MoS2 layers by dragging a precursor solution droplet with variable velocities across a substrate. Utilizing the pronounced shearing velocity dependence in a Landau-Levich deposition regime, MoS2 films with a spatially modulated thickness with alternating mono/bi- and few-layer regions are obtained after precursor annealing. Generally, the presented facile methodology allows for the direct preparation of micro-structured functional materials, extendable to other TMDC materials and even van der Waals heterostructures.
research and development in this area has demonstrated a host of different material platforms with unique and precisely tunable properties utilizing various quantum confinement effects. [1][2][3][4] Especially the large surface area of 2D semiconducting materials enables prominent interactions with its environment, resulting in a wide range of possible sensing applications, [5] strong interlayer interactions in van der Waals (vdW) heterostructures, [6] and large exciton binding energies. [7] Transition metal di-chalcogenides (TMDCs) are among the most promising 2D semiconductors, owing to their variable electron energy band gaps [8][9][10] and alignment dependent optoelectronic properties in Moiré heterostructures. [11] TMDC materials also provide an opportunity to form heterostructures with atomically sharp interfaces due to the weak vdW interlayer interactions, making them a perfect platform to investigate interlayer interactions at the atomic scale and to develop lowdimension devices with new functionalities. [12][13][14] Most of the device designs showing unique optoelectronic properties are largely based on layers obtained by mechanical exfoliation, that is, by cleaving from bulk TMDCs crystals, [15][16][17] allowing for an understanding of their fundamental physical properties. However, this approach is not suitable for industrial scale device fabrication. [18] Extensive research efforts are being made on developing and fine tuning bottom-up approaches to grow individual layers of TMDCs single-and few-layer films. In particular, large area (millimeter to centimeter range) growth of TMDC layers has been demonstrated using various types of chemical vapor deposition (CVD), [19][20][21][22] atomic layer deposition (ALD), [23][24][25][26] as well as molecular beam epitaxy, [27][28][29] and solution-based deposition approaches. [22,[30][31][32][33][34][35][36][37] A broad range of physical characteristics have been reported, including film morphology, domain size, and charge mobility. [22,26] For TMDC devices, the controlled growth of vdW heterostructures is a prerequisite, which, however, is challenging due to the weak interaction between the growing layers and, up to now, was only described in a few studies. For example, microscale lateral TMDC heterostructures were obtained by the subsequent growth of two TMDC materials, resulting in dispersed flakes laterally connected in some cases. [38][39][40][41][42][43][44] For Despite the plethora of intriguing phenomena observed in heterostructure stacks of 2D transition metal dichalcogenide (2D-TMDC) flakes, their application in functional devices is still hampered due to the lack of reliable growth methodologies for large-area heterostructures. Here, a scalable process for obtaining as-grown transition metal di-chalcogenide heterostructures by a combination of atomic layer deposition of monolayer MoS 2 and solutionbased processing of ultrathin WS 2 is presented. Spatially uniform optical and electrical characteristics of the individual TMDC layers and heterostructure...
Optimization of the sulfurization process of thin MoO3 precursor layers, pushing the reaction towards vapor-phase-assisted routes to obtain large-scale, homogeneous monolayer MoS2.
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