Eve D. Hanson

Superior Plasmonic Photodetectors Based on Au@MoS2 Core–Shell Heterostructures

Integrating plasmonic materials into semiconductor media provides a promising approach for applications such as photosensing and solar energy conversion. The resulting structures introduce enhanced light-matter interactions, additional charge trap states, and efficient charge-transfer pathways for light-harvesting devices, especially when an intimate interface is built between the plasmonic nanostructure and semiconductor. Herein, we report the development of plasmonic photodetectors using Au@MoS2 heterostructures-an Au nanoparticle core that is encapsulated by a CVD-grown multilayer MoS2 shell, which perfectly realizes the intimate and direct interfacing of Au and MoS2. We explored their favorable applications in different types of photosensing devices. The first involves the development of a large-area interdigitated field-effect phototransistor, which shows a photoresponsivity ∼10 times higher than that of planar MoS2 transistors. The other type of device geometry is a Si-supported Au@MoS2 heterojunction gateless photodiode. We demonstrated its superior photoresponse and recovery ability, with a photoresponsivity as high as 22.3 A/W, which is beyond the most distinguished values of previously reported similar gateless photodetectors. The improvement of photosensing performance can be a combined result of multiple factors, including enhanced light absorption, creation of more trap states, and, possibly, the formation of interfacial charge-transfer transition, benefiting from the intimate connection of Au and MoS2.

Substrate-induced strain and charge doping in CVD-grown monolayer MoS2

Due to its electronic-grade quality and potential for scalability, two-dimensional (2D) MoS2 synthesized by chemical vapor deposition (CVD) has been widely explored for electronic/optoelectronic applications. As 2D MoS2 can be considered a 100% surface, its unique intrinsic properties are inevitably altered by the substrate upon which it is grown. However, systematic studies of substrate-layer interactions in CVD-grown MoS2 are lacking. In this study, we have analyzed built-in strain and charge doping using Raman and photoluminescence spectroscopy in 2D MoS2 grown by CVD on four unique substrates: SiO2/Si, sapphire, Muscovite mica, and hexagonal boron nitride. We observed decreasing strain and charge doping in grown MoS2 as the substrates become less rough and more chemically inert. The possible origin of strain was investigated through atomic force microscopy roughness measurements of the as-grown layer and substrate. Our results provide direction for device optimization through careful selection of the ...

Controlled synthesis of 2D MX2 (M = Mo, W; X = S, Se) heterostructures and alloys

The advent of two-dimensional materials and van der Waals (vdW) heterostructures has been a boon for the nanoscience community, enabling the fabrication of nanostructures with atomic-scale precision, resulting in high performance opto-electronic devices. Yet, while vdW heterostructures have been widely studied, their fabrication remains rudimentary, relying upon physical stacking and ad hoc collections of recipes, rather than a rational framework. Here, we report our work on the synthesis of vdW heterostructures and monolayer alloys of MoS2-WS2 and MoSe2-WSe2 and the creation of a unifying, diagrammatic approach to heterostructure growth in these materials systems, which we call Time-Temperature-Architecture (TTA) diagrams. We demonstrate the temperature tunable synthesis of in-plane, vertical, and hybrid heterostructures, as well as monolayer alloys within the MoS2-WS2 and MoSe2-WSe2 systems. We use the TTA framework to add previously unexplored entries to this collection: the first ever single-step growth of MoSe2-WSe2 vertical heterostructures and Mo1-xWxSe2 alloys, and a new MoS2-WS2 hybrid architecture that combines the morphologies of both vertical and in-plane heterostructures. The TTA diagrams are a simple framework for vdW heterostructure and alloy growth, which we believe will be crucial, and enable further work on heterostructures and alloys of MoS2-WS2 and MoSe2-WSe2.The advent of two-dimensional materials and van der Waals (vdW) heterostructures has been a boon for the nanoscience community, enabling the fabrication of nanostructures with atomic-scale precision, resulting in high performance opto-electronic devices. Yet, while vdW heterostructures have been widely studied, their fabrication remains rudimentary, relying upon physical stacking and ad hoc collections of recipes, rather than a rational framework. Here, we report our work on the synthesis of vdW heterostructures and monolayer alloys of MoS2-WS2 and MoSe2-WSe2 and the creation of a unifying, diagrammatic approach to heterostructure growth in these materials systems, which we call Time-Temperature-Architecture (TTA) diagrams. We demonstrate the temperature tunable synthesis of in-plane, vertical, and hybrid heterostructures, as well as monolayer alloys within the MoS2-WS2 and MoSe2-WSe2 systems. We use the TTA framework to add previously unexplored entries to this collection: the first ever single-step grow...

Site-Specific Positioning and Patterning of MoS2 Monolayers – The Role of Au Seeding

Monolayers of transition metal dichalcogenides (TMDs) are attractive for various modern semiconductor devices. However, the limited control over the location, yield, and size distribution of the products using current synthesis methods has severely limited their large-scale applicability. Herein, we identify the ability to use metal ( e. g., Au) nanoparticles to seed the growth of MoS2 monolayers and thereby provide a means to achieve programmable and controllable synthesis. In this study, prepatterned Au seeds are used as heterogeneous nucleation sites to induce the formation of desired geometries of MoS2 monolayers via chemical vapor deposition. Our experimental and theoretical results shed light on the growth mechanism driving the formation of MoS2 monolayers at these sites, revealing that the seeding effect originates from the favorable formation energy of MoS2 on the Au surface. A field-effect transistor with a predesigned channel geometry exhibits electronic performance that compares nicely with previously reported MoS2 monolayer devices. We believe this study contributes fundamental insights into controlled synthesis of TMD monolayers, making integration of these materials into emerging electronic devices more attainable.

Correlative and Multiplexed Microscopy for 2-D Chalcogenide Semiconductors

The science and engineering community has been rapidly exploring the landscape of two-dimensional materials, particularly in their singleor few layer forms [1]. Dominating this landscape are the layered chalcogenides [2] which are diverse in chemistry, structure and properties; with over 100 primary members of this materials family already being reported. Driven by quantum confinement single layers (or few, in some cases) of these materials exhibit electronic, optical, and transport properties that diverge dramatically from their bulk counterparts. The field has evolved considerably since the time when single or few layer flakes were “synthesized” by the scotch-tape mechanical cleavage method. New more sophisticated methods for controlled synthesis (or thinning), deposition and chemical exfoliation have been developed that can “dial” the number of layers with large areal coverage on diverse substrates. Further, the 2D chalcogenide layers are being used as “substrates” onto which other dimensionally confined structures are being integrated in the spirit of nanoscale composites. Some composite structures exhibit synergy of multiple functionalities of the individual components while in some cases they represent quantum coupling or unusual behavior that is contrary to nominal synergy or proportional contribution of individual components.