This content will become publicly available on November 18, 2023
- Award ID(s):
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- NSF-PAR ID:
- Journal Name:
- Physical Chemistry Chemical Physics
- Page Range or eLocation-ID:
- 27241 to 27249
- Sponsoring Org:
- National Science Foundation
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Abstract Monolayer molybdenum disulfide (MoS 2 ) is one of the most studied two-dimensional (2D) transition metal dichalcogenides that is being investigated for various optoelectronic properties, such as catalysis, sensors, photovoltaics, and batteries. One such property that makes this material attractive is the ease in which 2D MoS 2 can be converted between the semiconducting (2H) and metallic/semi-metallic (1T/1T′) phases or heavily n-type doped 2H phase with ion intercalation, strain, or excess negative charge. Using n -butyl lithium (BuLi) immersion treatments, we achieve 2H MoS 2 monolayers that are heavily n-type doped with shorter immersion times (10–120 mins) or conversion to the 1T/1T′ phase with longer immersion times (6–24 h); however, these doped/converted monolayers are not stable and promptly revert back to the initial 2H phase upon exposure to air. To overcome this issue and maintain the modification of the monolayer MoS 2 upon air exposure, we use BuLi treatments plus surface functionalization p-(CH 3 CH 2 ) 2 NPh-MoS 2 (Et 2 N-MoS 2 )—to maintain heavily n-type doped 2H phase or the 1T/1T′ phase, which is preserved for over two weeks when on indium tin oxide or sapphire substrates. We also determine that the low sheet resistance andmore »
Metal halide perovskites (MHPs) are frontrunners among solution-processable materials for lightweight, large-area and flexible optoelectronics. These materials, with the general chemical formula AMX 3 , are structurally complex, undergoing multiple polymorph transitions as a function of temperature and pressure. In this review, we provide a detailed overview of polymorphism in three-dimensional MHPs as a function of composition, with A = Cs + , MA + , or FA + , M = Pb 2+ or Sn 2+ , and X = Cl − , Br − , or I − . In general, perovskites adopt a highly symmetric cubic structure at elevated temperatures. With decreasing temperatures, the corner-sharing MX 6 octahedra tilt with respect to one another, resulting in multiple polymorph transitions to lower-symmetry tetragonal and orthorhombic structures. The temperatures at which these phase transitions occur can be tuned via different strategies, including crystal size reduction, confinement in scaffolds and (de-)pressurization. As discussed in the final section of this review, these solid-state phase transformations can significantly affect optoelectronic properties. Understanding factors governing these transitions is thus critical to the development of high-performance, stable devices.
Two-dimensional semiconductors (2DSCs) are attractive for a variety of optoelectronic and catalytic applications due to their ability to be fabricated as wide-area, monolayer-thick films and their unique optical and electronic properties which emerge at this scale. One important class of 2DSCs are the transition metal dichalcogenides (TMDs), which are of particular interest as absorbing layers in ultrathin optoelectronic devices. While TMDs are known to exhibit excellent photovoltaic properties at the bulk level, it is not yet clear how carriers are transported in these materials at thicknesses approaching the monolayer limit, where distinct changes in band structure and the nature of photogenerated carriers occur. Here, it is demonstrated that electrochemical microscopy techniques can be employed as powerful tools for visualizing these processes in 2DSCs, even within individual monolayers. Carrier generation-tip collection scanning electrochemical cell microscopy (CG-TC SECCM), which utilizes spatially-offset optical and pipet-based electrochemical probes to locally generate and detect photogenerated carriers, was applied to visualize carrier generation and transport within well-defined n-WSe 2 samples prepared via mechanical exfoliation. Data from these experiments directly reveal how carrier transport varies within complex 2DSC structures as layer thicknesses approach the monolayer limit. These results not only provide valuable new insights into carrier transportmore »
Ultrasonic delamination based adhesion testing for high-throughput assembly of van der Waals heterostructuresTwo-dimensional (2D) materials assembled into van der Waals (vdW) heterostructures contain unlimited combinations of mechanical, optical, and electrical properties that can be harnessed for potential device applications. Critically, these structures require control over interfacial adhesion for enabling their construction and have enough integrity to survive industrial fabrication processes upon their integration. Here, we promptly determine the adhesion quality of various exfoliated 2D materials on conventional SiO 2 /Si substrates using ultrasonic delamination threshold testing. This test allows us to quickly infer relative substrate adhesion based on the percent area of 2D flakes that survive a fixed time in an ultrasonic bath, allowing for control over process parameters that yield high or poor adhesion. We leverage this control of adhesion to optimize the vdW heterostructure assembly process, where we show that samples with high or low substrate adhesion relative to each other can be used selectively to construct high-throughput vdW stacks. Instead of tuning the adhesion of polymer stamps to 2D materials with constant 2D-substrate adhesion, we tune the 2D-substrate adhesion with constant stamp adhesion to 2D materials. The polymer stamps may be reused without any polymer melting steps, thus avoiding high temperatures (<120 °C) and allowing for high-throughput production. We showmore »
The interaction of alloying elements with grain boundaries (GBs) influences many phenomena, such as microstructural evolution and transport. While GB solute segregation has been the subject of active research in recent years, most studies focus on ground-state GB structures, i.e., lowest energy GBs. The impact of GB metastability on solute segregation remains poorly understood. Herein, we leverage atomistic simulations to generate metastable structures for a series of  and  symmetric tilt GBs in a model Al–Mg system and quantify Mg segregation to individual sites within these boundaries. Our results show large variations in the atomic Voronoi volume due to GB metastability, which are found to influence the segregation energy. The atomistic data are then used to train a Gaussian Process machine learning model, which provides a probabilistic description of the GB segregation energy in terms of the local atomic environment. In broad terms, our approach extends existing GB segregation models by accounting for variability due to GB metastability, where the segregation energy is treated as a distribution rather than a single-valued quantity.