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.

Multiple MoS2 Transistors for Sensing Molecule Interaction Kinetics.

Atomically layered transition metal dichalcogenides (TMDCs) exhibit a significant potential to enable next-generation low-cost transistor biosensors that permit single-molecule-level quantification of biomolecules. To realize such potential biosensing capability, device-oriented research is needed for calibrating the sensor responses to enable the quantification of the affinities/kinetics of biomolecule interactions. In this work, we demonstrated MoS2-based transistor biosensors capable of detecting tumor necrosis factor--alpha (TNF-α) with a detection limit as low as 60 fM. Such a detection limit was achieved in both linear and subthreshold regimes of MoS2 transistors. In both regimes, all sets of transistors exhibited consistent calibrated responses with respect to TNF-α concentration, and they resulted in a standard curve, from which the equilibrium constant of the antibody-(TNF-α) pair was extracted to be KD = 369 ± 48 fM. Based on this calibrated sensor model, the time-dependent binding kinetics was also measured and the association/dissociation rates of the antibody-(TNF-α) pair were extracted to be (5.03 ± 0.16) × 10(8) M(-1) s(-1) and (1.97 ± 0.08) × 10(-4) s(-1), respectively. This work advanced the critical device physics for leveraging the excellent electronic/structural properties of TMDCs in biosensing applications as well as the research capability in analyzing the biomolecule interactions with fM-level sensitivities.

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/cm(2) 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.

Multibit data storage states formed in plasma-treated MoS2 transistors.

New multibit memory devices are desirable for improving data storage density and computing speed. Here, we report that multilayer MoS2 transistors, when treated with plasmas, can dramatically serve as low-cost, nonvolatile, highly durable memories with binary and multibit data storage capability. We have demonstrated binary and 2-bit/transistor (or 4-level) data states suitable for year-scale data storage applications as well as 3-bit/transistor (or 8-level) data states for day-scale data storage. This multibit memory capability is hypothesized to be attributed to plasma-induced doping and ripple of the top MoS2 layers in a transistor, which could form an ambipolar charge-trapping layer interfacing the underlying MoS2 channel. This structure could enable the nonvolatile retention of charged carriers as well as the reversible modulation of polarity and amount of the trapped charge, ultimately resulting in multilevel data states in memory transistors. Our Kelvin force microscopy results strongly support this hypothesis. In addition, our research suggests that the programming speed of such memories can be improved by using nanoscale-area plasma treatment. We anticipate that this work would provide important scientific insights for leveraging the unique structural property of atomically layered two-dimensional materials in nanoelectronic applications.

MoS2 transistors fabricated via 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 cm(2)-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.e., 3.0 ± 1.9 nm) over cm(2)-scale areas. Furthermore, we experimentally show that our plasma-assisted printing process can be generally used for producing other emerging atomically layered nanostructures (e.g., graphene nanoribbons). We also demonstrate working n-type transistors made from printed MoS2 flakes that exhibit excellent properties (e.g., ON/OFF current ratio 10(5)-10(7), field-effect mobility on SiO2 gate dielectrics 6 to 44 cm(2)/(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.

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.