Memristors Based on 2D Monolayer Materials

2D materials have been widely used in various applications due to their remarkable and distinct electronic, optical, mechanical and thermal properties. Memristive effect has been found in several 2D systems. This chapter focuses on the memristors based on 2D materials, e. g. monolayer transition metal dichalcogenides (TMDs) and hexagonal boron nitride (h-BN), as the active layer in vertical MIM (metal–insulator–metal) configuration. Resistive switching behavior under normal DC and pulse waveforms, and current-sweep and constant stress testing methods have been investigated. Unlike the filament model in conventional bulk oxide-based memristors, a new switching mechanism has been proposed with the assistance of metal ion diffusion, featuring conductive-point random access memory (CPRAM) characteristics. The use of 2D material devices in applications such as flexible non-volatile memory (NVM) and emerging zero-power radio frequency (RF) switch will be discussed

Electron irradiation-induced defects for reliability improvement in monolayer MoS2-based conductive-point memory devices

Monolayer molybdenum disulfide has been previously discovered to exhibit non-volatile resistive switching behavior in a vertical metal-insulator-metal structure, featuring ultra-thin sub-nanometer active layer thickness. However, the reliability of these nascent 2D-based memory devices was not previously investigated for practical applications. Here, we employ an electron irradiation treatment on monolayer MoS2 film to modify the defect properties. Raman, photoluminescence, and X-ray photoelectron spectroscopy measurements have been performed to confirm the increasing amount of sulfur vacancies introduced by the e-beam irradiation process. The statistical electrical studies reveal the reliability can be improved by up to 1.5× for yield and 11× for average DC cycling endurance in the devices with a moderate radiation dose compared to unirradiated devices. Based on our previously proposed virtual conductive-point model with the metal ion substitution into sulfur vacancy, Monte Carlo simulations have been performed to illustrate the irradiation effect on device reliability, elucidating a clustering failure mechanism. This work provides an approach by electron irradiation to enhance the reliability of 2D memory devices and inspires further research in defect engineering to precisely control the switching properties for a wide range of applications from memory computing to radio- frequency switches.This work was supported in part by the National Science Foundation (NSF) grant #1809017, and an NSF MRSEC under Cooperative Agreement No. DMR-1720595. The authors acknowledge use of Texas Nanofabrication Facilities supported by the NSF NNCI award #1542159. D.A. acknowledges the Presidential Early Career Award for Scientists and Engineers (PECASE) through the Army Research Office (W911NF-16-1- 0277).Center for Dynamics and Control of Material

Memory effect in two dimensional atomically-thin sheets

Two-dimensional (2D) materials have attracted much attention over the last decade in nanoelectronics due to their remarkable electronic, optical, mechanical, and thermal properties. In this dissertation, we report intriguing observation of stable non-volatile resistive switching (NVRS) in single layer 2D atomic sheets sandwiched between metal electrodes. These devices can be collectively labeled atomristor, in essence, memristor effect in atomically-thin nanomaterials or atomic sheets. The systematic investigation for 2D-based-NVRS is conducted in terms of performance, materials, mechanisms, and applications and presented in separate chapters. Chapter 1 is an introduction to 2D materials and non-volatile memories. In Chapter 2, molybdenum disulfide (MoS₂) based atomristor is demonstrated showing large on/off ratio, forming-free characteristic and small switching voltages. In chapter 3, we further expand the collection of 2D materials showing NVRS, including transition metal dichalcogenides (MX₂, M=transition metal atom, e.g. Mo, W, Re, Sn or Pt; X=chalcogen atom, e.g. S, Se or Te), a heterostructure (WS₂/MoS₂) and an insulator (h-BN), indicating the universality of the phenomenon in various 2D atomic sheets. Then a possible mechanism dissociation-diffusion-adsorption (DDA) model is presented in Chapter 4, supported by both first-principle calculation results and scanning tunneling microscope (STM) atomistic imaging. In chapter 5, emerging device concepts in non-volatile flexible memory and zero-static power radio frequency (RF) switches are demonstrated, which could benefit substantially from the wide 2D materials design space. Our findings overturn the contemporary thinking that non-volatile switching is not scalable to sub nanometer and thus motivate further research in defect engineering, interface modification and ionic diffusion, which is discussed in the last Chapter 6Materials Science and Engineerin

Universal Non-Volatile Resistance Switching Phenomenon in Atomic Monolayers

We have observed non-volatile resistance switching (NVRS) phenomenon in non-metallic single-layer atomic sheets in a vertical device configuration. Results suggest a rich multi-physics effect persistent in both poly- and single- crystalline atomic sheets below 1nm thickness. NVRS is observed in several TMDs including MoS2, MoSe2, WSe2, and WS2 and also in h-BN. This alludes to a universal effect in non-metallic 2D materials. Our findings overturn the contemporary thinking that non-volatile switching is not scalable below a few nanometers. Emerging concepts in non-volatile flexible memory fabrics, zero static power radio-frequency switches, and brain-inspired (neuromorphic) computing could benefit substantially from the pervasive NVRS effect in atomic sheets. Experimentally results for RF switching have been achieved

Genetic analysis of activin/inhibin β subunits in zebrafish development and reproduction.

Activin and inhibin are both dimeric proteins sharing the same β subunits that belong to the TGF-β superfamily. They are well known for stimulating and inhibiting pituitary FSH secretion, respectively, in mammals. In addition, activin also acts as a mesoderm-inducing factor in frogs. However, their functions in development and reproduction of other species are poorly defined. In this study, we disrupted all three activin/inhibin β subunits (βAa, inhbaa; βAb, inhbab; and βB, inhbb) in zebrafish using CRISPR/Cas9. The loss of βAa/b but not βB led to a high mortality rate in the post-hatching stage. Surprisingly, the expression of fshb but not lhb in the pituitary increased in the female βA mutant together with aromatase (cyp19a1a) in the ovary. The single mutant of βAa/b showed normal folliculogenesis in young females; however, their double mutant (inhbaa-/-;inhbab-/-) showed delayed follicle activation, granulosa cell hypertrophy, stromal cell accumulation and tissue fibrosis. The ovary of inhbaa-/- deteriorated progressively after 180 dpf with reduced fecundity and the folliculogenesis ceased completely around 540 dpf. In addition, tumor- or cyst-like tissues started to appear in the inhbaa-/- ovary after about one year. In contrast to females, activin βAa/b mutant males showed normal spermatogenesis and fertility. As for activin βB subunit, the inhbb-/- mutant exhibited normal folliculogenesis, spermatogenesis and fertility in both sexes; however, the fecundity of mutant females decreased dramatically at 270 dpf with accumulation of early follicles. In summary, the activin-inhibin system plays an indispensable role in fish reproduction, in particular folliculogenesis and ovarian homeostasis

Atomristor: Nonvolatile Resistance Switching in Atomic Sheets of Transition Metal Dichalcogenides

Recently, two-dimensional (2D) atomic sheets have inspired new ideas in nanoscience including topologically protected charge transport,(1'2) spatially separated excitons,(3) and strongly anisotropic heat transport.(4) Here, we report the intriguing observation of stable nonvolatile resistance switching (NVRS) in single-layer atomic sheets sandwiched between metal electrodes. NVRS is observed in the prototypical semiconducting (MX2, M = Mo, W; and X = S, Se) transitional metal dichalcogenides (TMDs),(5) which alludes to the universality of this phenomenon in TMD monolayers and offers forming-free switching. This observation of NVRS phenomenon, widely attributed to ionic diffusion, filament, and interfacial redox in bulk oxides and electrolytes,(6-9) inspires new studies on defects, ion transport, and energetics at the sharp interfaces between atomically thin sheets and conducting electrodes. Our findings overturn the contemporary thinking that nonvolatile switching is not scalable to subnanometre owing to leakage currents.(10) Emerging device concepts in nonvolatile flexible memory fabrics, and brain-inspired (neuromorphic) computing could benefit substantially from the wide 2D materials design space. A new major application, zero-static power radio frequency (RF) switching, is demonstrated with a monolayer switch operating to 50 GHz

Histological analysis of activin βA mutant females during post-pubertal maturation (55–80 dpf).

Follicles from control and βA single mutants (inhbaa-/-, inhbab-/-) showed normal growth and development. In the double mutant (inhbaa-/-;inhbab-/-), abnormal follicles with hypertrophic granulosa cells could often be observed (arrow). The boxed areas are shown at higher magnification below. The numbers shown in the photos indicate the total number of fish examined (lower) and the fish exhibiting similar phenotype to that shown (upper). PG, primary growth; PV, previtellogenic; EV, early vitellogenic; MV, mid-vitellogenic; LV, late vitellogenic; FG, full-grown.</p