Block co‐polymer patterns are attractive candidates for nanoparticle assemblies. Directed self‐assembly of block co‐polymers in particular allows for long range ordering of the patterns, making them interesting scaffolds for the organization of magnetic particles. Here, a method to tune the channel width of polymer‐derived trenches via atomic layer deposition (ALD) of alumina is reported. The alumnia coating provides a much more thermally robust pattern that is stable up to 250 °C. Using these patterns, magnetic coupling in both ferromagnetic and superparamagnetic nanocrystal chains is achieved.
Two-dimensional (2D) layered transition metal dichalcogenides (TMDs) have recently emerged as a new class of atomically thin semiconductors for diverse electronic, optoelectronic, and valleytronic applications. To explore the full potential of these 2D semiconductors requires a precise control of their band gap and electronic properties, which represents a significant challenge in 2D material systems. Here we demonstrate a systematic control of the electronic properties of 2D-TMDs by creating mixed alloys of the intrinsically p-type WSe2 and intrinsically n-type WS2 with variable alloy compositions. We show that a series of WS2xSe2–2x alloy nanosheets can be synthesized with fully tunable chemical compositions and optical properties. Electrical transport studies using back-gated field effect transistors demonstrate that charge carrier types and threshold voltages of the alloy nanosheet transistors can be systematically tuned by adjusting the alloy composition. A highly p-type behavior is observed in selenium-rich alloy, which gradually shifts to lightly p-type, and then switches to lightly n-type characteristics with the increasing sulfur atomic ratio, and eventually evolves into highly n-doped semiconductors in sulfur-rich alloys. The synthesis of WS2xSe2–2x nanosheets with tunable optical and electronic properties represents a critical step toward rational design of 2D electronics with tailored spectral responses and device characteristics.
Author(s): Halim, Udayabagya | Advisor(s): Duan, Xiangfeng | Abstract: The development of graphene and the nanotechnology revolution brought new interest in Layered transition metal dichalcogenides (TMDs). First explored in the 1960s, these TMDs are a subject of interest due to their wide range of material properties form semiconductor, metals, to superconductor. Through intercalation chemistry, there is additional flexibility to tune the materials’ parameters to the desired property. Naturally semiconducting, Group 6 TMDs (i.e. MoS2) have the most potential for future electronic application. Yet these Group 6 TMDs are also the most chemically inert. While over 240 organic-TMD complexes have been made reported for Group 4 (i.e. TiS2) and Group 5 (i.e. TaS2) TMDs, for 50 years, organic complexes of Group 6 TMDs remained unexplored. With the goal of tailoring the properties of Group 6 TMDs, we followed three different experimental approaches. We begin by pursuing liquid phase exfoliation to produce few/mono layer products and use contact angle measurement to infer the thermodynamic of the exfoliated materials in mixed solvents. We expanded a pen-paper vibrational model of 2H-MoS2 to analyze the vibrational spectra of WS2-xSex alloy with a tunable optical gap. Inspired by Lithium based intercalation methods, we propose a new and direct electrochemical intercalation route to produce Organic-MoS2 complexes using quaternary ammonium compounds. We use the same electrochemical concept and show that the theory is sufficiently robust, allowing intercalation onto Layered Black Phosphorus (BP).
The electrochemical molecular intercalation of two-dimensional layered materials (2DLMs) produces stable and highly tunable superlattices between monolayer 2DLMs and self-assembled molecular layers. This process allows unprecedented flexibility in integrating highly distinct materials with atomic/molecular precision to produce a new generation of organic/inorganic superlattices with tunable chemical, electronic, and optical properties. To better understand the intercalation process, we developed an on-chip platform based on MoS2 model devices and used optical, electrochemical, and in situ electronic characterizations to resolve the intermediate stages during the intercalation process and monitor the evolution of the molecular superlattices. With sufficient charge injection, the organic cetyltrimethylammonium bromide (CTAB) intercalation induces the phase transition of MoS2 from semiconducting 2H phase to semimetallic 1T phase, resulting in a dramatic increase of electrical conductivity. Therefore, in situ monitoring the evolution of the device conductance reveals the electrochemical intercalation dynamics with an abrupt conductivity change, signifying the onset of the molecule intercalation. In contrast, the intercalation of tetraheptylammonium bromide (THAB), a branched molecule in a larger size, resulting in a much smaller number of charges injected to avoid the 2H to 1T phase transition. Our study demonstrates a powerful platform for in situ monitoring the molecular intercalation of many 2DLMs (MoS2, WSe2, ReS2, PdSe2, TiS2, and graphene) and systematically probing electronic, optical, and optoelectronic properties at the single-nanosheet level.
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