Controlling the stacking and conversion in bilayer crystals and heterostructures by non-equilibrium synthesis and processing is very important to construct 2D moiré quantum materials. Here we will show how to introduce isotopes, laser thinning, and Raman spectroscopy to understand the bilayer growth mechanism in two-step chemical vapor deposition. Then we will describe a feedback approach to reveal and control the transformation pathways in bilayer 2D materials by pulsed laser deposition (PLD). We will focus on the transformation kinetics of bilayer WS2 crystals into Janus WSSe/WS2 and WSe2/WS2 heterostructures by hyperthermal implantation of laser-vaporized Se clusters. In situ ICCD imaging, ion probe, and spectroscopy diagnostics characterize the PLD plasma and are used to precisely control the kinetic energies of the Se species arriving at the substrate. In situ Raman spectroscopy is used to characterize the conversion kinetics and capture the metastable phases during transformation. DFT calculations, XPS, and atomic-resolution HAADF STEM are used to identify the compositions, vibrational modes, and structures for revealing the conversion mechanism of the bilayer crystals.
Automated platforms for synthesis are a necessity to increase the rate of discovery, synthesis, and optimization of materials to match the accelerating pace of theoretical predictions. Here, we introduce a novel pulsed laser deposition (PLD) platform that combines in situ Raman, photoluminescence, and white light reflectance spectroscopies as a material probe with intensified CCD (ICCD) imaging/spectroscopy and ion probe gas-phase PLD plume diagnostics. These diagnostics together with full automation and high-throughput synthesis schemes enable rapid synthesis and characterization with a Python-based dataflow for seamless integration with machine learning algorithms. Examples of in situ spectroscopy of 2D materials during growth and modification will be discussed.
We utilized in situ laser-heating within a TEM to reveal how nanomaterials transform from amorphous precursors, and used electron spectroscopy to characterize the optical properties of these nanostructures in situ and in real time. The recrystallization, grain growth, phase separation, and solid state dewetting of AgNi films were investigated using stepwise laser heating. The experiments reveal a wealth of in situ information, including changes of composition and lattice constants during phase separation. To establish the true and dynamic structure–property relationship of nanostructures, we also characterized the photonic properties of the synthesized materials in situ. For example, the plasmon modes of metallic particles were mapped using electron energy gain induced by photon-plasmon-electron coupling. These in situ TEM studies of laser-induced heating are a valuable discovery tool for the rapid exploration of synthesis pathways and functional properties of nanostructures.
Incorporating dopants in monolayer transition metal dichalcogenides (TMD) can enable manipulations of their electrical and optical properties. Previous attempts in amphoteric doping in monolayer TMDs have proven to be challenging. Here we report the incorporation of molybdenum (Mo) atoms in monolayer WS2 during growth by chemical vapor deposition, and correlate the distribution of Mo atoms with the optical properties including photoluminescence and ultrafast transient absorption dynamics. Dark field scanning transmission electron microscopy imaging quantified the isoelectronic doping of Mo in WS2 and revealed its gradual distribution along a triangular WS2 monolayer crystal, increasing from 0% at the edge to 2% in the center of the triangular WS2 triangular crystals. This agrees well with the Raman spectra data that showed two obvious modes between 360 cm-1 and 400 cm-1 that corresponded to MoS2 in the center. This in-plane gradual distribution of Mo in WS2 was found to account for the spatial variations in photoluminescence intensity and emission energy. Transition absorption spectroscopy further indicated that the incorporation of Mo in WS2 regulate the amplitude ratio of XA and XB of WS2. The effect of Mo incorporation on the electronic structure of WS2 was further elucidated by density functional theory. Finally, we compared the electrical properties of Mo incorporated and pristine WS2 monolayers by fabricating field-effect transistors. The isoelectronic doping of Mo in WS2 provides an alternative approach to engineer the bandgap and also enriches our understanding the influence of the doping on the excitonic dynamics.
Atomically-thin two-dimensional (2D) metal chalcogenides have emerged as an exciting class of materials which have the potential to enable numerous new applications that range from electronics to photonics. Developing new methods for controlled synthesis and manipulation of these layered materials is crucial for emerging applications in functional devices. Here, we demonstrate non-equilibrium laser-based approaches to form and deliver atoms, cluster or stoichiometric nanoparticles with tunable kinetic energies for the synthesis and processing of 2D layered semiconductors. Utilizing the stoichiometric nanoparticles as feedstocks, we demonstrate the formation of either small domain nanosheet networks (~ 20 nm) or large crystalline domains (~100 µm). On the other hand, atomic precursors with tunable kinetic energies are used in doping, alloying and conversion of 2D monolayers. Patterned arrays of lateral heterojunctions between 2D layered semiconductors, MoSe2/MoS2, are formed by e-beam lithography and selective conversion processes. Moreover, we explore the nonequilibrium, bottom-up synthesis of single crystalline monolayers of MoSe2-x with controllable levels of Se vacancies far beyond intrinsic levels (up to 20 %) exhibiting unique optical and electrical properties. These non-equilibrium laser-based approaches provide unique synthesis and processing opportunities that are not easily accessible through conventional methods.
Currently, two-dimensional (2D) layered materials are rapidly emerging as a new platform for many potential applications in nanoscale optoelectronics, optics, flexible electronics, energy, etc. Monolayers of 2D crystals [e.g., transition metals dichalcogenides (TMDs)] are basically surface and therefore, their optoelectronic properties are very sensitive to defects and environment including ambient gases and substrates. However, only limited number of studies is devoted to understanding of the effect of defects on their optical properties. It is not clear if the specific defects have their fingerprints in Raman, absorption, and PL spectra. Here, we report measurements of low temperature (4-150K) Raman and photoluminescence (PL) spectra of TMD monolayers (MoSe2, WS2) with variable and controlled concentrations of specific defects, i.e., chalcogenide atom vacancies, to reveal optical signatures of these defects. The defective TMD monolayers were synthesized using our new laser CVD approach. To identify the type of defects and their concentration the 2D crystals were transferred from a substrate to a TEM grid and atomic resolution STEM and EELS measurements were performed. Low temperature Raman and PL mapping were used to understand spatial distribution of the defects within the 2D crystals. The assignment of the observed spectral features in low temperature Raman and PL spectra was supported by ab initio theoretical modeling.
Synthesis science was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences (BES), Materials Sciences and Engineering Division. Characterization and computational science at CNMS was supported by the Scientific User Facilities Division, BES.
The recent power-packed advent of perovskite solar cells is transforming photovoltaics (PV) with their superior efficiencies, ease of fabrication, and cost. This perovskite solar cell further boasts of many unexplored features that can further enhance its PV properties and lead to it being branded as a successful commercial product. This article provides a detailed insight of the organometal halide based perovskite structure, its unique stoichiometric design, and its underlying principles for PV applications. The compatibility of various PV layers and its fabrication methods is also discussed.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.