Hossein Rokni

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.

A continuum model for the static pull-in behavior of graphene nanoribbon electrostatic actuators with interlayer shear and surface energy effects

Based on multi-beam shear model theory, a continuum mechanics model is developed to investigate the pull-in instability of wedged/curved multilayer graphene nanoribbon (MLGNR) cantilever nanobeams subjected to electrostatic and Casimir forces. The first-order fringing-field correction, the interlayer shear between neighboring graphene nanoribbons (GNRs), surface elasticity, and residual surface tension are incorporated into the analytical model. An explicit closed-form analytical solution to the governing fourth-order nonlinear differential equation of variable coefficients is introduced for the static pull-in behavior of electrostatic nanoactuators using a Fredholm integral equation of the first kind. A comparison study for a [001] silver electrostatic nanoactuator indicates that the proposed analytical closed-form solution yields an improved accuracy over other analytical and numerical methods existing in literature. The results indicate that the interfacial slip between GNRs and the surface material parameters play a significant role in static pull-in behavior of MLGNR electrostatic nanoactuators. From the experimental data and atomistic simulations available in the literature, the value of interlayer shear modulus at the graphene/graphene interface is estimated to be in the order of magnitude of 10−1 GPa. The continuum model proposed in this study will be helpful for characterizing the mechanical properties of GNRs and the design of graphene-based nanoelectromechanical system devices.Based on multi-beam shear model theory, a continuum mechanics model is developed to investigate the pull-in instability of wedged/curved multilayer graphene nanoribbon (MLGNR) cantilever nanobeams subjected to electrostatic and Casimir forces. The first-order fringing-field correction, the interlayer shear between neighboring graphene nanoribbons (GNRs), surface elasticity, and residual surface tension are incorporated into the analytical model. An explicit closed-form analytical solution to the governing fourth-order nonlinear differential equation of variable coefficients is introduced for the static pull-in behavior of electrostatic nanoactuators using a Fredholm integral equation of the first kind. A comparison study for a [001] silver electrostatic nanoactuator indicates that the proposed analytical closed-form solution yields an improved accuracy over other analytical and numerical methods existing in literature. The results indicate that the interfacial slip between GNRs and the surface material pa...

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.

Surface and Thermal Effects on the Pull-In Behavior of Doubly-Clamped Graphene Nanoribbons Under Electrostatic and Casimir Loads

In this study, a comprehensive analytical model is established based on Euler–Bernoulli beam theory with von Karman geometric nonlinearity to investigate the effect of residual surface tension, surface elasticity, and temperature on the static pull-in voltages of multilayer graphene nanoribbon (MLGNR) doubly-clamped beams under electrostatic and Casimir forces and axial residual stress. An explicit closed-form analytical solution to the governing fourth-order nonlinear differential equation of variable coefficients is presented for the static pull-in behavior of electrostatic nanoactuators using a Fredholm integral equation of the first kind. The high accuracy of the present analytical model is validated for some special cases through comparison with other existing numerical, analytical, and experimental models. The effects of the number of graphene nanoribbons (GNRs), temperature, surface tension, and surface elasticity on the pull-in voltage and displacement of MLGNR electrostatic nanoactuaotrs are investigated. Results indicate that the thermal effect on the pull-in voltage is significant especially when a smaller number of GNRs are used. It is found that the surface effects become more dominant as the number of GNRs decreases. It is also demonstrated that the residual surface tension exerts a greater influence on the pull-in voltage in comparison with the surface elasticity.

Effect of Graphene Layers on Static Pull-in Behavior of Bilayer Graphene/Substrate Electrostatic Microactuators

A closed-form solution is obtained for the pull-in instability of curved multilayer graphene/substrate microcantilever electrostatic actuators. The first-order fringing-field correction and the interlayer shear between neighboring graphene layers (GLs) and between the graphene and the substrate are incorporated into the analytical model. In the solution procedure, the governing fourth-order differential equation of variable coefficients is converted into a Fredholm integral equation. The resulting equation is solved for the static pull-in voltages by adopting the first natural mode of the cantilever beam as a deflection shape function. The influence of GLs on the pull-in voltages of the electrostatic microactuators is investigated. It is found that laying 10, 30, and 60 GLs on top of the substrate results in increases of about 95%, 190%, and 295%, respectively, in the pull-in voltage of the straight bilayer graphene/substrate electrostatic microactuators. It is also observed that the classical Euler-Bernoulli beam theory fails to predict the pull-in voltages of the multilayer graphene/substrate electrostatic microactuators, showing that the pull-in voltage is highly affected by the graphene interlayer shear.

Layer-by-Layer Insight into Electrostatic Charge Distribution of Few-Layer Graphene

In few-layer graphene (FLG) systems on a dielectric substrate such as SiO2, the addition of each extra layer of graphene can drastically alter their electronic and structural properties. Here, we map the charge distribution among the individual layers of finite-size FLG systems using a novel spatial discrete model that describes both electrostatic interlayer screening and fringe field effects. Our results reveal that the charge density in the region very close to the edges is screened out an order of magnitude more weakly than that across the central region of the layers. Our discrete model suggests that the interlayer charge screening length in 1–8 layer thick graphene systems depends mostly on the overall gate/molecular doping level rather than on temperature, in particular at an induced charge density >5 × 1012 cm−2, and can reliably be determined to be larger than half the interlayer spacing but shorter than the bilayer thickness. Our model can be used for designing FLG-based devices, and offers a simple rule regarding the charge distribution in FLG: approximately 70%, 20%, 6% and 3% (99% overall) of the total induced charge density reside within the four innermost layers, implying that the gate-induced electric field is not definitely felt by >4th layer.

Nanoscale Probing of Interaction in Atomically Thin Layered Materials

We combine conductive atomic force microscopy (CAFM) and molecular dynamics (MD) simulations to reveal the interaction of atomically thin layered materials (ATLMs) down to nanoscale lateral dimension. The setup also allows quantifying, for the first time, the effect of layer number and electric field on the dielectric constant of ATLMs with few-layer down to monolayer thickness. Our CAFM-assisted electrostatic technique shows that high-quality mono- and bilayer graphene is reliably produced at significant yields only by the shear type of bond breaking between layers, whereas the normal type of bond breaking exhibits a very stochastic process mainly due to the coexistence of local delamination and interlayer twist. Our dielectric constant measurements also reveal a very weak dependence on the layer number and the electric field (up to our experimental limit of 0.1 V/Å), which is in contrast with theoretical reports. Owing to unexpectedly large variations in the screening ability of pristine monolayer graphene under ambient conditions, we further demonstrate that the effective dielectric constant of monolayer graphene can be engineered to provide a broad spectrum of dielectric responses (3.5–17) through oxidation and thermal annealing, thus confirming its much higher chemical reactivity than bilayer and few layers.

Scaling behavior of nanoimprint and nanoprinting lithography for producing nanostructures of molybdenum disulfide

Top-down lithography techniques are needed for manufacturing uniform device structures based on emerging 2D-layered materials. Mechanical exfoliation approaches based on nanoimprint and nanoprint principles are capable of producing ordered arrays of multilayer transition metal dichalcogenide microstructures with a high uniformity of feature dimensions. In this study, we present a study on the applicability of nanoimprint-assisted shear exfoliation for generating ultrathin monolayer and few-layer MoS2 structures as well as the critical limits of feature dimensions produced via such nanoimprint and nanoprint-based processes. In particular, this work shows that give a lateral feature size of MoS2 structures that are pre-patterned on a bulk stamp, there exists a critical thickness or aspect ratio value, below which the exfoliated layered structures exhibit major defects. To exfoliate a high-quality, uniform monolayer or few-layer structures, the characteristic lateral feature sizes of such structures need to be in the sub-100 nm regimes. In addition, the exfoliated MoS2 flakes of critical thicknesses exhibit prominent interlayer twisting features on their cleaved surfaces. Field-effect transistors made from these MoS2 flakes exhibit multiple (or quasi-analog-tunable) charge memory states. This work advances the knowledge regarding the limitations and application scope of nanoimprint and nanoprint processes in manufacturing nano/microstructures based on layered materials and provides a method for producing multi-bit charge memory devices.