Advanced Nanofabrication Technologies for Making Photo-Response Devices Based on Emerging Layered Transition Metal Dichalcogenides.

Emerging layered semiconductor materials, such as layered transition metal dichalcogenides (TMDCs), have exhibited attractive optoelectronic properties and hold a significant potential to be implemented for making new photoresponse devices. However, to realize working TMDC-based photoresponse devices for practical photovoltaic and photodetection applications, we need to (i) create new nanofabrication and nanomanufacturing technologies capable of producing TMDC devices with deterministic properties, architectures, and arrangements, (ii) develop new materials processing approaches capable of modulating various TMDCs to enable different device applications, and (iii) advance the device physics for understanding and leveraging the unique photonic properties of layered semiconductors. The research works presented in this thesis sought to advance the scientific/technical knowledge toward addressing the needs mentioned above and specifically focused on three relevant topics: (1) invention and demonstration of plasma-assisted transfer nanoprinting technology capable of producing orderly arranged layered material structures into device sites; (2) development and study of plasma-assisted doping processes, which can result in permanently stable doping effects in layered semiconductors; (3) characterization of a series of photoresponse devices based on emerging layered transition metal dichalcogenides (TMDCs), which were fabricated using various techniques, including plasma-assisted doping, stacking of graphene-TMDC heterostructures, and thin-film-metal-induced surface charge transfer (SCT) doping. The presented works have advanced the scientific knowledge and technical capability toward realizing working TMDC-based photoresponse devices for practical photovoltaic and photodetection applications. Additionally, the nanofabrication approaches developed in these works can be generally used for making other TMDC-based nanoelectronic and photonic devices, and the obtained device physics knowledge is anticipated to greatly leverage the uniquely advantageous optoelectronic properties of semiconducting TMDCs for enabling new device applications.PhDMechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/120740/1/sjwi_1.pd

Transport Characteristics of Multichannel Transistors Made from Densely Aligned Sub-10 nm Half-Pitch Graphene Nanoribbons

Densely aligned sub-10 nm graphene nanoribbons are desirable for scale-up applications in nanoelectronics. We implemented directed self-assembly of block-copolymers in combination with nanoimprint lithography to pattern sub-10 nm half-pitch nanoribbons over large areas. These graphene nanoribbons have the highest density and uniformity to date. Multichannel field-effect transistors were made from such nanoribbons, and the transport characteristics of transistors were studied. Our work indicates that a large ribbon-to-ribbon width variation in a multichannel FET can lead to nonsynchronized switching characters of multiple graphene channels and thus a poor ON/OFF current ratio. Through process optimization, we have created 8 nm half-pitch graphene nanoribbons with the minimal ribbon-to-ribbon width variation of ∼2.4 nm (3σ value). The corresponding transistors exhibit an ON/OFF current ratio >10, which is among the highest values ever reported for transistors consisting of densely arranged graphene nanoribbons. This work provides important insights for optimizing the uniformity and transport properties of lithographically patterned graphene nanostructures. In addition, the presented fabrication route could be further developed for the scalable nanomanufacturing of graphene-based nanoelectronic devices over large areas

Nanoimprint-Assisted Shear Exfoliation + Transfer Printing (NASE+TP) for Producing Emerging Transition Metal Dichalcogenide Heterostructures

Vertically stacked heterostructures of emerging layered materials are needed for modulating the band structures of new nanoelectronic/photonic devices based on layered materials. We present a nanofabrication route for producing arrays of such heterostructures, which uniquely combines nanoimprint-assisted shear exfoliation and transfer printing techniques. Using this route, we have demonstrated the fabrication of uniform arrays of heterostructures consisting of different multilayer transition metal dichalcogenides as well as photoresponse devices based on such structures

MoS₂ Transistors Fabricated <i>via</i> Plasma-Assisted Nanoprinting of Few-Layer MoS₂ Flakes into Large-Area Arrays

Large-area few-layer-MoS₂ device arrays are desirable for scale-up applications in nanoelectronics. Here we present a novel approach for producing orderly arranged, pristine few-layer MoS₂ flakes, which holds significant potential to be developed into a nanomanufacturing technology that can be scaled up. We pattern bulk MoS₂ stamps using lithographic techniques and subsequently transfer-print prepatterned MoS₂ features onto pristine and plasma-charged SiO₂ substrates. Our work successfully demonstrates the transfer printing of MoS₂ flakes into ordered arrays over cm²-scale areas. Especially, the MoS₂ patterns printed on plasma-charged substrates feature a regular edge profile and a narrow distribution of MoS₂ flake thicknesses (<i>i</i>.<i>e</i>., 3.0 ± 1.9 nm) over cm²-scale areas. Furthermore, we experimentally show that our plasma-assisted printing process can be generally used for producing other emerging atomically layered nanostructures (<i>e</i>.<i>g</i>., graphene nanoribbons). We also demonstrate working n-type transistors made from printed MoS₂ flakes that exhibit excellent properties (<i>e</i>.<i>g</i>., ON/OFF current ratio 10⁵–10⁷, field-effect mobility on SiO₂ gate dielectrics 6 to 44 cm²/(V s)) as well as good uniformity of such transistor parameters over a large area. Finally, with additional plasma treatment processes, we also show the feasibility of creation of p-type transistors as well as pn junctions in MoS₂ flakes. This work lays an important foundation for future scale-up nanoelectronic applications of few-layer-MoS₂ micro- and nanostructures

Enhancement of Photovoltaic Response in Multilayer MoS₂ Induced by Plasma Doping

Layered transition-metal dichalcogenides hold promise for making ultrathin-film photovoltaic devices with a combination of excellent photovoltaic performance, superior flexibility, long lifetime, and low manufacturing cost. Engineering the proper band structures of such layered materials is essential to realize such potential. Here, we present a plasma-assisted doping approach for significantly improving the photovoltaic response in multilayer MoS₂. In this work, we fabricated and characterized photovoltaic devices with a vertically stacked indium tin oxide electrode/multilayer MoS₂/metal electrode structure. Utilizing a plasma-induced p-doping approach, we are able to form p–n junctions in MoS₂ layers that facilitate the collection of photogenerated carriers, enhance the photovoltages, and decrease reverse dark currents. Using plasma-assisted doping processes, we have demonstrated MoS₂-based photovoltaic devices exhibiting very high short-circuit photocurrent density values up to 20.9 mA/cm² and reasonably good power-conversion efficiencies up to 2.8% under AM1.5G illumination, as well as high external quantum efficiencies. We believe that this work provides important scientific insights for leveraging the optoelectronic properties of emerging atomically layered two-dimensional materials for photovoltaic and other optoelectronic applications

Nanoimprint-Assisted Shear Exfoliation (NASE) for Producing Multilayer MoS₂ Structures as Field-Effect Transistor Channel Arrays

MoS₂ and other semiconducting transition metal dichalcogenides (TMDCs) are of great interest due to their excellent physical properties and versatile chemistry. Although many recent research efforts have been directed to explore attractive properties associated with MoS₂ monolayers, multilayer/few-layer MoS₂ structures are indeed demanded by many practical scale-up device applications, because multilayer structures can provide sizable electronic/photonic state densities for driving upscalable electrical/optical signals. Currently there is a lack of processes capable of producing ordered, pristine multilayer structures of MoS₂ (or other relevant TMDCs) with manufacturing-grade uniformity of thicknesses and electronic/photonic properties. In this article, we present a nanoimprint-based approach toward addressing this challenge. In this approach, termed as nanoimprint-assisted shear exfoliation (NASE), a prepatterned bulk MoS₂ stamp is pressed into a polymeric fixing layer, and the imprinted MoS₂ features are exfoliated along a shear direction. This shear exfoliation can significantly enhance the exfoliation efficiency and thickness uniformity of exfoliated flakes in comparison with previously reported exfoliation processes. Furthermore, we have preliminarily demonstrated the fabrication of multiple transistors and biosensors exhibiting excellent device-to-device performance consistency. Finally, we present a molecular dynamics modeling analysis of the scaling behavior of NASE. This work holds significant potential to leverage the superior properties of MoS₂ and other emerging TMDCs for practical scale-up device applications