Two-dimensional transition metal dichalcogenides (e.g. MoS₂, WS₂) have fascinating optical and electronic properties that make them promising materials for use in next-generation devices. In particular, 2D quantum dots (i.e. nanoparticles of 2D materials) have received great interest in recent years for their effectiveness in catalysis as well as opto-electronic and energy-storage applications [1]. Furthermore, the versatility of 2D materials is greatly enhanced by the fact that they can be assembled with atomically sharp interfaces, resulting in heterostructures that benefit from the combined functionality of multiple 2D materials as well as completely new material properties [2]. Applying heterostructure methods to 2D quantum dots could result in core-shell nanoparticles which combine the useful properties of 2D quantum dots with the versatility of heterostructures. However, little is known about such structures, as most of the literature is aimed at single-material quantum dots.

In this work, we show that 2D monolayer core-shell quantum dots can be grown by atomic layer deposition. Using a supercycle approach, we grow crystalline nano-scale monolayer flakes of MoS2 which are subsequently epitaxially bordered by WS₂. This is achieved by exploiting the reactive edge sites of these materials, by switching from MoS₂ to WS₂ deposition before a closed monolayer is formed. Notably, this method can be extended to the growth of more elaborate structures (e.g. core-shell-shell), whereby the diameter of the core and shell(s) can be straightforwardly and accurately controlled through the number of ALD cycles. In practice, the size control of the grown core-shell structures is limited by the nucleation density of the ALD process, which is linked to the density of reactive sites on the substrate [3,4]. In order to enable the growth of larger and more elaborate structures, we employ strategies to reduce the nucleation density. The growth is investigated in-situ using spectroscopic ellipsometry, and the grown structures are characterized by AFM and TEM, as well as Raman and PL spectroscopy. We expect our technique of growing 2D core-shell quantum dots to extend to other 2D material systems, thereby establishing an important new method of tailoring the properties of 2D quantum dots for applications in catalysis, opto-electronics and energy-storage.

[1] Manikandan, A. et al.Prog. Quantum Electron.68, 100226 (2019)

[2] Novoselov, K. S., Mishchenko, A., Carvalho, A. & Castro Neto, A. H. Science353, aac9439 (2016)

[3] Puurunen, R. L. J. Appl. Phys.97, 121301 (2005)

Tungsten oxide attracts great attention as a functional layer of electrochromic [1] or resistive memory devices [2], but also as a source for the synthesis of two-dimensional ultrathin WS₂ nanosheets, which can serve as a channel material in the post-silicon FET technology [3]. In this regard, ALD utilization for the tungsten oxide growth is highly preferable since it allows the best thickness control and reproducibility over other deposition techniques, which is crucial for mass-production. Previously it has been reported that radical-enhanced ALD also allows precise oxygen deficiency control in WO_x films [4].

In this work, we modify the previously reported WH₂(Cp)₂-based ALD process by the oxygen source exchange to ozone. This process showed a clear saturation behavior on both reactants, good deposition uniformity, and higher oxygen deficiency level, which was monitored by in-situ X-Ray photoelectron spectroscopy to exclude film degradation under ambient conditions. The V_o presence was concluded from the visible electronic states formation in the bandgap, an increase in the concentrations of tungsten in the W5+ state and oxygen in the non-lattice state. An increase in the V_o concentration was accompanied by a significant film resistivity decrease and an abrupt change in the crystalline structure, which was revealed by XRD analysis. The early stages of the suggested WO_x ALD process were studied by angle-resolved XPS and the moment of film continuity was determined. The subsequent sulfurization allowed its’ effective conversion to WS₂ nanosheets with the terrace-terminated structure. Recently it was shown that preliminary seed WO_x film hydrogen treatment allowed to significantly change WS₂ morphology and improve its’ electrical quality [5]. Noteworthy, exactly ultrathin WS₂ layers are of particular interest, therefore, the seed WO_x thickness influence on the resulting WS₂ morphology was also investigated. In particular, such a technique allowed us to obtain the continuous WS₂ nanosheets with a thickness down to 3-4 monolayers. The obtained results expand the opportunities horizon of the 2D materials by the possibility of a fab-oriented production technique.

1. S. Balaji et al., Chem. Mater. 2009, 21, 7, pp. 1381-1389
2. F.-M. Lee et al., DOI: 10.1109/IEDM.2013.6724672
3. T. Schram et al., 2017 Silicon Nanoelectronics Workshop (SNW), 17436971
4. R. I. Romanov et al., J. Phys. Chem. C 2020, 124, pp. 18156-18164
5. M. G. Kozodaev et al., J. Phys. Chem. C 2020, 124, pp. 28169-28177

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are attracting growing interests in recent years due to their unique electronic properties even with thickness at atomic scale. Though tremendous progress has been made in the understanding and implementation of the physical properties of TMDs in advanced micro-/nanoelectronic devices through conventional fabrication methods like mechanical exfoliation or chemical vapor deposition (CVD), the large-scale device integration for system-level applications have been severely bottlenecked by the lack of effective synthesis approaches to achieve wafer-scale, uniform, crystalline and stoichiometric TMD films.

Atomic layer deposition (ALD) is a surface-controlled film fabrication and can provide a promising route towards the synthesis of high-quality TMD thin films since the ALD approach follows the layer-by-layer deposition mechanism. As compared to other synthetic methods like CVD, ALD can enable precise thickness control on atomic scale and excellent film uniformity as well as good stoichiometry and crystallinity with proper annealing steps. Here, we have developed ALD-based synthesis approaches to prepare wafer-scale MoS₂ and WS₂ TMD thin films. Non-toxic MoCl₅, WCl₅ and hexamethyldisilathiane (HMDST) are used as precursors. Film characterizations have confirmed the wafer-level uniformity and good crystallinity by annealing in sulfur atmosphere. The field-effect transistor (FET) device arrays fabricated on the wafer-scale TMD film have shown excellent homogeneity in electrical performance, which provides good platform for further integration. In our work, we have fabricated optoelectronic device arrays as well as inverter, NAND, NOR, AND, and OR logic gates which have shown robust and repeatable logic functions. This has paves solid basis for the system and circuit-level applications.

It should also be noted that we have been designing and optimizing a high-temperature ALD process to grow crystalline TMD thin films eliminating the high-temperature annealing step in the previous flow. Preliminary experimental results have been achieved demonstrating satisfactory film quality, and the FET devices fabricated on it can exhibit repeatable switching performance with on/off ratio over 10³. We believe that this can be a more encouraging synthetic method to prepare wafer-scale TMD thin films for practical electronic applications.