Synthesis of Monolayer MoSe2 with Controlled Nucleation via Reverse-Flow Chemical Vapor Deposition

Abstract

Two-dimensional (2D) layered semiconductor materials, such as transition metal dichalcogenides (TMDCs), have attracted considerable interests because of their intriguing optical and electronic properties. Controlled growth of TMDC crystals with large grain size and atomically smooth surface is indeed desirable but remains challenging due to excessive nucleation. Here, we have synthesized high-quality monolayer, bilayer MoSe2 triangular crystals, and continuous thin films with controlled nucleation density via reverse-flow chemical vapor deposition (CVD). High crystallinity and good saturated absorption performance of MoSe2 have been systematically investigated and carefully demonstrated. Optimized nucleation and uniform morphology could be achieved via fine-tuning reverse-flow switching time, growth time and temperature, with corresponding growth kinetics proposed. Our work opens up a new approach for controllable synthesis of monolayer TMDC crystals with high yield and reliability, which promote surface/interface engineering of 2D semiconductors towards van der Waals heterostructure device applications.

How high is a MoSe$_2$ monolayer?

“…The spectrum at position 1 (yellow) has the A peak at (801 ± 3) nm, in agreement with the peak position of the fold in Figure 2a, and hence of a monolayer as observed in [13,15]. As the spot moves over the island from right to left, the peak position shifts towards 810 nm, as expected for an increasing number of layers [14,15,18]. The B/A ratio increases as the number of layers increases from right to left, and the top layer has a very high ratio of 0.20.…”

“…PL is another widely used technique used to characterize TMDC layer numbers. The position of the monolayer peak in a PL spectrum for exfoliated MoSe 2 samples has been observed at room temperature in the range 788 -816 nm [5,14,15,17,18,[33][34][35]. The PL intensity can be a affected by the presence of defects [19] and adsorbates [15] and is expected to decrease as the number of layers increases to 2 and 3 [5,[13][14][15].…”

“…As with other 2D materials, TMDC layers can be synthesized both via chemical vapor deposition (CVD) [17][18][19] and, most commonly, using mechanical exfoliation [1,2,11,14,16,[20][21][22], in which mono-or few-layer flakes are transferred onto a substrate, often SiO 2 /Si. Studies of factors from the fabrication process influencing the measured step heights and optical properties of monolayers have been emerging mostly on MoS 2 [20,23], whereas the literature for other TMDCs such as MoSe 2 is limited in comparison.…”

Role of processing parameters in CVD grown crystalline monolayer MoSe2

“…32 The above-mentioned features closely match with the earlier published literature of CVDgrown monolayer MoSe 2 . 10,33,34 In order to estimate the atomic ratio, the integrated peak areas of Mo 4+ 3d and Se 2À 3d peaks have been taken into consideration, and peak areas are normalized with respect to the atomic sensitivity factor. The estimated Se/Mo ratio is found to be $1.8, indicating a large amount of Se vacancies are present in as-synthesized MoSe 2 akes.…”

Síntese por CVD do Disseleneto de Molibdênio (MoSe2) e quantificação de defeitos estruturais por espectroscopia Raman

“…O método CVD consiste na decomposição de reagentes a altas temperaturas e sua reorganização num ambiente de atmosfera controlada. As vantagens do método CVD são: revestimento de qualidade em superfícies 3D, boa taxa de deposição 133,116,114,134 , versatilidade da montagem experimental 135,136,137,37,138,139,140,134 , e não requer alto vácuo (para muitos tipos de deposição) 135 . E suas desvantagens são: altas temperaturas 21 , toxicidade a depender dos reagentes 141 e dificuldade em determinar um modelo teórico para o processo de crescimento 142 -devido aos reagentes estarem no estado gasoso e em altas temperaturas -em um regime turbulento.…”