Laser-Assisted Synthesis and Time-Resolved Growth Control of Two-Dimensional Quantum Materials

Abstract

Two-dimensional (2D) layered materials, including transition metal dichalcogenides (TMDCs), have recently been at the heart of quantum materials and information sciences research due to unusual properties associated with their firmly defined dimensionalities. Many efforts have focused on developing new methods for the accelerated growth and discovery of 2D materials, including physical and chemical vapor deposition techniques. However, synthesizing these multi-component crystals in the gas phase has been extremely challenging due to complex and uncontrolled gas-phase reactions and flow dynamics. A novel laser-assisted synthesis technique (LAST) has been demonstrated in response to existing growth complexities to accelerate the growth of 2D materials in this study. This novel bottom-up synthesis approach facilitates the growth of various 2D materials directly from stoichiometric powders through laser vaporization. The directed laser heating allows pressure-independent decoupling of the growth and evaporation kinetics enabling the use of stoichiometric powder as precursors for the growth of high-quality 2D materials, including MoS2, MoSe2, WSe2, and WS2. Controlling and understanding the growth of atomically-thin transition metal dichalcogenides (TMDCs) monolayer two-dimensional (2D) materials is vital for next-generation 2D electronics and optoelectronic devices. However, their growth kinetics is not fully observed or well understood due to the bottlenecks associated with the existing synthesis methods. In pursuit of resolving these issues, this study further explored and demonstrated the time-resolved and ultrafast growth of 2D materials by this novel laser-based synthesis approach that enables rapid initiation and termination of the vaporization process during crystal growth. Stoichiometric powder (e.g., WSe2) minimizes the complex chemistry, while the vaporization and growth process allows rapid initiation/termination control over the generated flux. An extensive set of experiments is performed to understand the growth evolution from both feedstock supply and surface diffusion perspective, achieving sub-second growth as low as 10 ms along with the record-breaking 100 µm/s growth rate on a non-catalytic substrate such as Si/SiO2. The process parameter further predicts that this ultrafast crystal growth rate is highly reproducible and scalable. Tuning the structural and electronic properties of atomically-thin two-dimensional (2D) materials via defect and vacancy engineering is the key to their potential use in various applications, including electronics, energy, and sensing devices. Vacancies are, for instance, becoming highly promising for enhanced interaction of gases and biomolecules with 2D materials in energy and sensing applications. However, the deterministic generation of desirable vacancies with tunable concentration remains a challenge in 2D materials due to the limitations in the current growth methods, such as the complex reaction chemistries and gas flow dynamics. Therefore, engineering defects and vacancies in 2D materials have been mainly limited to destructive top-down processes such as heating, ion bombardments, and laser post-processing. In order to address these challenges, this study introduced a single-step bottom-up synthesis approach of LAST to grow monolayer MoSe2 crystals with tunable vacancy concentrations. This method utilizes the spatiotemporal properties and adjustable power density of the lasers to control the vaporization dynamics of the stoichiometric MoSe2 powders. Such a mechanism in the vaporization allows us to grow tunable stoichiometry monolayer MoSe2-x crystals on the substrates. The localized and time-controlled (250 ms to 2 s) vaporization of the MoSe2 powder by a CO2 laser enables the formation of monolayer crystals with controlled vacancy concentrations ranging from ~1 to 20%. The effects of laser power, laser irradiation time, and background pressure on the vacancy tuning range and subsequent properties of the crystals are investigated and quantified using Raman and photoluminescence spectroscopy, scanning transmission electron microscopy (STEM), and time-correlated single-photon counting (TCSPC). LAST facilitates observation and understanding of the 2D crystal evolution and growth kinetics with time-resolved and sub-second time scales. Furthermore, this bottom-up synthesis is a promising approach that allows deterministic vacancy tuning for future electronics, particularly gas and bio-sensing applications, without the need for further post-processing and potential structural disruption of the crystals. Overall, this research work presents a general yet straightforward approach to accelerating the synthesis and discovery of emerging quantum materials.