Ferroelectric materials, because of their robust spontaneous electrical polarization, are widely used in various applications. Recent advances in modelling, synthesis and characterization techniques are spurring unprecedented advances in the study of these materials. In this Review, we focus on thin-film ferroelectric materials and, in particular, on the possibility of controlling their properties through the application of strain engineering in conventional and unconventional ways. We explore how the study of ferroelectric materials has expanded our understanding of fundamental effects, enabled the discovery of novel phases and physics, and allowed unprecedented control of materials properties. We discuss several exciting possibilities for the development of new devices, including those in electronic, thermal and photovoltaic applications, and transduction sensors and actuators. We conclude with a brief survey of the different directions that the field may expand to over the coming years. Strain engineering can be used to control the properties of thin-film ferroelectric materials, which are promising for electronic, thermal, photovoltaic and transduction applications. This Review addresses fundamental aspects, novel ways to control materials properties and the development of new ferroelectric-based devices.

https://escholarship.org/content/qt51w3s3s1/qt51w3s3s1.pdf?t=p8f5p7

Thin-film ferroelectric materials and their applications

The so-called Boltzmann tyranny defines the fundamental thermionic limit of the subthreshold slope of a metal-oxide-semiconductor field-effect transistor (MOSFET) at 60 mV dec-1 at room temperature and therefore precludes lowering of the supply voltage and overall power consumption 1,2 . Adding a ferroelectric negative capacitor to the gate stack of a MOSFET may offer a promising solution to bypassing this fundamental barrier 3 . Meanwhile, two-dimensional semiconductors such as atomically thin transition-metal dichalcogenides, due to their low dielectric constant and ease of integration into a junctionless transistor topology, offer enhanced electrostatic control of the channel 4-12 . Here, we combine these two advantages and demonstrate a molybdenum disulfide (MoS2) two-dimensional steep-slope transistor with a ferroelectric hafnium zirconium oxide layer in the gate dielectric stack. This device exhibits excellent performance in both on and off states, with a maximum drain current of 510 μA μm-1 and a sub-thermionic subthreshold slope, and is essentially hysteresis-free. Negative differential resistance was observed at room temperature in the MoS2 negative-capacitance FETs as the result of negative capacitance due to the negative drain-induced barrier lowering. A high on-current-induced self-heating effect was also observed and studied.

Steep-slope hysteresis-free negative capacitance MoS 2 transistors

Atomically thin molybdenum disulfide (MoS2) is an ideal semiconductor material for field-effect transistors (FETs) with sub-10 nm channel lengths. The high effective mass and large bandgap of MoS2 minimize direct source–drain tunneling, while its atomically thin body maximizes the gate modulation efficiency in ultrashort-channel transistors. However, no experimental study to date has approached the sub-10 nm scale due to the multiple challenges related to nanofabrication at this length scale and the high contact resistance traditionally observed in MoS2 transistors. Here, using the semiconducting-to-metallic phase transition of MoS2, we demonstrate sub-10 nm channel-length transistor fabrication by directed self-assembly patterning of mono- and trilayer MoS2. This is done in a 7.5 nm half-pitch periodic chain of transistors where semiconducting (2H) MoS2 channel regions are seamlessly connected to metallic-phase (1T′) MoS2 access and contact regions. The resulting 7.5 nm channel-length MoS2 FET has a low ...

MoS2 Field-Effect Transistor with Sub-10 nm Channel Length

The rapid development of information technology has led to urgent requirements for high efficiency and ultralow power consumption. In the past few decades, neuromorphic computing has drawn extensive attention due to its promising capability in processing massive data with extremely low power consumption. Here, we offer a comprehensive review on emerging artificial neuromorphic devices and their applications. In light of the inner physical processes, we classify the devices into nine major categories and discuss their respective strengths and weaknesses. We will show that anion/cation migration-based memristive devices, phase change, and spintronic synapses have been quite mature and possess excellent stability as a memory device, yet they still suffer from challenges in weight updating linearity and symmetry. Meanwhile, the recently developed electrolyte-gated synaptic transistors have demonstrated outstanding energy efficiency, linearity, and symmetry, but their stability and scalability still need to be optimized. Other emerging synaptic structures, such as ferroelectric, metal–insulator transition based, photonic, and purely electronic devices also have limitations in some aspects, therefore leading to the need for further developing high-performance synaptic devices. Additional efforts are also demanded to enhance the functionality of artificial neurons while maintaining a relatively low cost in area and power, and it will be of significance to explore the intrinsic neuronal stochasticity in computing and optimize their driving capability, etc. Finally, by looking into the correlations between the operation mechanisms, material systems, device structures, and performance, we provide clues to future material selections, device designs, and integrations for artificial synapses and neurons.

A comprehensive review on emerging artificial neuromorphic devices

The stability of spontaneous electrical polarization in ferroelectrics is fundamental to many of their current applications, which range from the simple electric cigarette lighter to non-volatile random access memories1. Research on nanoscale ferroelectrics reveals that their behaviour is profoundly different from that in bulk ferroelectrics, which could lead to new phenomena with potential for future devices2, 3, 4. As ferroelectrics become thinner, maintaining a stable polarization becomes increasingly challenging. On the other hand, intentionally destabilizing this polarization can cause the effective electric permittivity of a ferroelectric to become negative5, enabling it to behave as a negative capacitance when integrated in a heterostructure. Negative capacitance has been proposed as a way of overcoming fundamental limitations on the power consumption of field-effect transistors6. However, experimental demonstrations of this phenomenon remain contentious7. The prevalent interpretations based on homogeneous polarization models are difficult to reconcile with the expected strong tendency for domain formation8, 9, but the effect of domains on negative capacitance has received little attention5, 10, 11, 12. Here we report negative capacitance in a model system of multidomain ferroelectric–dielectric superlattices across a wide range of temperatures, in both the ferroelectric and paraelectric phases. Using a phenomenological model, we show that domain-wall motion not only gives rise to negative permittivity, but can also enhance, rather than limit, its temperature range. Our first-principles-based atomistic simulations provide detailed microscopic insight into the origin of this phenomenon, identifying the dominant contribution of near-interface layers and paving the way for its future exploitation.

/pdf/negative-capacitance-in-multidomain-ferroelectric-4sjstauriz.pdf

Negative capacitance in multidomain ferroelectric superlattices

The Boltzmann distribution of electrons poses a fundamental barrier to lowering energy dissipation in conventional electronics, often termed as Boltzmann Tyranny. Negative capacitance in ferroelectric materials, which stems from the stored energy of a phase transition, could provide a solution, but a direct measurement of negative capacitance has so far been elusive. Here, we report the observation of negative capacitance in a thin, epitaxial ferroelectric film. When a voltage pulse is applied, the voltage across the ferroelectric capacitor is found to be decreasing with time--in exactly the opposite direction to which voltage for a regular capacitor should change. Analysis of this 'inductance'-like behaviour from a capacitor presents an unprecedented insight into the intrinsic energy profile of the ferroelectric material and could pave the way for completely new applications.

Negative capacitance in a ferroelectric capacitor.

http://absimage.aps.org/image/MAR16/MWS_MAR16-2015-020045.pdf

Negative Capacitance in a Ferroelectric Capacitor

The Boltzmann distribution of electrons poses a fundamental barrier to lowering energy dissipation in conventional electronics, often termed as Boltzmann Tyranny. Negative capacitance in ferroelectric materials, which stems from the stored energy of phase transition, could provide a solution, but a direct measurement of negative capacitance has so far been elusive. Here we report the observation of negative capacitance in a thin, epitaxial ferroelectric film. When a voltage pulse is applied, the voltage across the ferroelectric capacitor is found to be decreasing with time-in exactly the opposite direction to which voltage for a regular capacitor should change. Analysis of this inductance-like behavior from a capacitor presents an unprecedented insight into the intrinsic energy profile of the ferroelectric material and could pave the way for completely new applications.

Transition-metal-mediated cleavage of C-C single bonds can enable entirely new retrosynthetic disconnections in the total synthesis of natural products. Given that C-C bond cleavage inherently alters the carbon framework of a compound, and that, under transition-metal catalysis, the generated organometallic or radical intermediate is primed for further complexity-building reactivity, C-C bond-cleavage events have the potential to drastically and rapidly remodel skeletal frameworks. The recent acceleration of the use of transition-metal-mediated cleavage of C-C single bonds in total synthesis can be ascribed to a communal recognition of this fact. In this Review, we highlight ten selected total syntheses from 2014 to 2019 that illustrate how transition-metal-mediated cleavage of C-C single bonds at either the core or the periphery of synthetic intermediates can streamline synthetic efforts.

Brian Wang

Papers

Negative capacitance in a ferroelectric capacitor.

Negative Capacitance in a Ferroelectric Capacitor

Negative Capacitance in a Ferroelectric Capacitor

Transition Metal-Mediated C-C Single Bond Cleavage: Making the Cut in Total Synthesis.

Übergangsmetallvermittelte Spaltung von C-C-Einfachbindungen