Recent progress on two-dimensional ferroelectrics:Material systems and device applications

Ferroelectrics are a type of material with a polar structure and their polarization direction can be inverted reversibly by applying an electric field.They have attracted tremendous attention for their extensive applications in non-volatile memory,sensors and neuromorphic computing.However,conventional ferroelectric materials face insulating and interfacial issues in the commercialization process.In contrast,two-dimensional(2D)ferroelectric materials usually have excellent semiconductor performance,clean van der Waals interfaces and robust ferroelectric order in atom-thick layers,and hold greater promise for constructing multifunctional ferroelectric optoelectronic devices and nondestructive ultra-high-density memory.Recently,2D ferroelectrics have obtained impressive breakthroughs,showing overwhelming superiority.Herein,firstly,the progress of experimental research on 2D ferroelectric materials is reviewed.Then,the preparation of 2D ferroelectric devices and their applications are discussed.Finally,the future development trend of 2D ferroelectrics is looked at.

Future Applications of GaN Electron Devices

0 Introduction Fifteen years have passed since the first AlGaN/GaN HFET was reported in 1993.The FETs have already commercialized as microwave power devices,but volume production has not yet realized.The main application field is mobile phone base stations.For such applications,GaAs power transistors and silicon LDMOS have already been used.Therefore,advantages compared with these existing devices will be required,such as high efficiency,low distortion,low noise,high reliability and,especially,low cost.However,the latter two items are normally difficult for devices using new materials.

Review of My Research Work on Ferroelectric Films and Si-Based Device Applications: Opportunity and Challenge

Ferroelectric materials have many interesting physical properties such as ferroelectricity, pyroelectricity, piezoelectricity, and opto-electricity, and applying ferroelectric materials in the forms of thin and thick films and integrating them on the silicon substrate as electronic and MEMS devices is a very attractive research area and challenging. In this paper, we report our research works on ferroelectric MEMS and ferroelectric films for electronic device applications. Pyroelectric thin film infrared sensors have been made, characterized, and a 32×32 array with its size of 1cm×1cm has been obtained on Si membrane. Ferroelectric thin films in amorphous phase have been applied to make silicon based hydrogen gas sensors with the metal/amorphous ferroelectric film/metal device structure, and its turn-on voltage of about 4.5V at ～1000 ppm in air is about 7 times of the best value reported in the literature. For the application of electron emission flat panel display, ferroelectric BST thin films with excess Ti concentrations have been coated on Si tips, the threshold voltage of those ferroelectric film coated tips has been reduced about one order from ～70 V/μm to 4～10 V/μm for different Ti concentrations, and however, the electron emission current density has been increased at least 3～4 order for those coated tips compared to that of the bare Si tips. To fulfill in the thickness gap between thin film of typical ～1 μm made by PVD/CVD and polished ceramic wafer of ～50 μm from the bulk, piezoelectric films with thickness in a range of 1～30 μm have been successfully deposited on Si substrate at a low temperature of 650oC by a novel hybridized deposition technique, and piezoelectric MEMS ultrasonic arrays have been very recently obtained with the sound pressure level up to ～120 dB. More detailed results will be presented and mechanisms will be discussed.

Ambient-condition strategy for rapid mass production of crystalline gallium oxide nanoarchitectures toward device application

Currently,the synthesis of nanostructures still encounters two grand challenges:one is the often-required high temperatures and/or high pressures,and the other is the scalable fabrication.Here,to break through such bottlenecks,we demonstrate an ambient-condition strategy for rapid mass production of fourthgeneration semiconductor Ga_(2)O_(3)nanoarchitectures assembled by single-crystalline nanosheets in a controlled manner based on sonochemistry.Their growth is fundamentally determined by the introduced ethanediamine in rationally designed source materials,which could not only protect the metal Ga against reacting with H_(2)O into GaOOH,but facilitate the reaction of Ga with OH·radicals for target crystalline Ga_(2)O_(3)nanostructures.As a proof of concept for applications,the as-fabricated Ga_(2)O_(3)nanoarchitectures exhibit superb performances for electromagnetic wave absorption with a reflection loss value of 52.2 dB at 8.1 GHz,and ammonia sensing with high sensitivity and selectivity at room temperature,representing their bright future to be commercially applied in modern devices.

Recent advances in anisotropic two-dimensional materials and device applications

Two-dimensional(2D)materials,such as transition metal dichalcogenides(TMDs),black phosphorus(BP),MXene and borophene,have aroused extensive attention since the discovery of graphene in 2004.They have wide range of applications in many research fields,such as optoelectronic devices,energy storage,catalysis,owing to their striking physical and chemical properties.Among them,anisotropic 2D material is one kind of 2D materials that possess different properties along different directions caused by the intrinsic anisotropic atoms5 arrangement of the 2D materials,mainly including BP,borophene,low-symmetry TMDs(ReSe2 and ReSa)and group IV monochalcogenides(SnS,SnSe,GeS,and GeSe).Recently,a series of new devices has been fabricated based on these anisotropic 2D materials.In this review,we start from a brief introduction of the classifications,crystal structures,preparation techniques,stability,as well as the strategy to discriminate the anisotropic characteristics of 2D materials.Then,the recent advanced applications including electronic devices,optoelectronic devices,thermoelectric devices and nanomechanical devices based on the anisotropic 2D materials both in experiment and theory have been summarized.Finally,the current challenges and prospects in device designs,integration,mechanical analysis,and micro-/nano-fabrication techniques related to anisotropic 2D materials have been discussed.This review is aimed to give a generalized knowledge of anisotropic 2D materials and their current devices applications,and thus inspiring the exploration and development of other kinds of new anisotropic 2D materials and various novel device applications.

Silicon and hybrid silicon photonic devices for intra-datacenter applications: state of the art and perspectives [Invited]

We review the state of the art and our perspectives on silicon and hybrid silicon photonic devices for optical interconnects in datacenters. After a brief discussion of the key requirements for intra-datacenter optical interconnects, we propose a wavelength-division-multiplexing(WDM)-based optical interconnect for intra-datacenter applications. Following our proposed interconnects configuration, the bulk of the review emphasizes recent developments concerning on-chip hybrid silicon microlasers and WDM transmitters, and silicon photonic switch fabrics for intra-datacenters. For hybrid silicon microlasers and WDM transmitters, we outline the remaining challenges and key issues toward realizing low power consumption, direct modulation, and integration of multiwavelength microlaser arrays. For silicon photonic switch fabrics, we review various topologies and configurations of high-port-count N-by-N switch fabrics using Mach–Zehnder interferometers and microring resonators as switch elements, and discuss their prospects toward practical implementations with active reconfiguration.For the microring-based switch fabrics, we review recent developments of active stabilization schemes at the subsystem level. Last, we outline several large challenges and problems for silicon and hybrid silicon photonics to meet for intra-datacenter applications and propose potential solutions.

Special Polymer Optical Fibres and Devices for Photonic Applications

Remarkable progresses have been made in developing special polymer optical fibres and devices for photonic applications in recent years. This presentation will mainly report on the development of electro-optic, photosensitive and photorefractive polymer optical fibres and related devices.

Periodic structural defects in Bragg gratings and their application in multiwavelength devices

In this paper, we present the finding that periodic structural defects(PSDs) along a Bragg grating can shift the Bragg wavelength. This effect is theoretically analyzed and confirmed by numerical calculation. We find that the Bragg wavelength shift is determined by the defect size and the period of the defects. The Bragg wavelength can be well tuned by properly designing the PSDs, and this may provide an alternative method to fabricate grating-based multiwavelength devices, including optical filter arrays and laser arrays. In regards to wavelength precision, the proposed method has an advantage over the traditional methods, where the Bragg wavelengths are changed directly by changing the grating period. In addition, the proposed method can maintain grating strength when tuning the wavelength since only the period of defects is changed. This will be a benefit for devices such as arrays.

High-sensitivity integrated devices based on surface plasmon resonance for sensing applications

A metallic nanostructured array that scatters radiation toward a thin metallic layer generates surface plasmon resonances for normally incident light. The location of the minimum of the spectral reflectivity serves to detect changes in the index of refraction of the medium under analysis. The normal incidence operation eases its integration with optical fibers. The geometry of the arrangement and the material selection are changed to optimize some performance parameters as sensitivity, figure of merit, field enhancement, and spectral width. This optimization takes into account the feasibility of the fabrication. The evaluated results of sensitivity(1020 nm/RIU)and figure of merit(614 RIU^(-1)) are competitive with those previously reported.

Recent developments in CVD growth and applications of 2D transition metal dichalcogenides

Two-dimensional(2D)transition metal dichalcogenides(TMDs)with fascinating electronic energy band structures,rich valley physical properties and strong spin–orbit coupling have attracted tremendous interest,and show great potential in electronic,optoelectronic,spintronic and valleytronic fields.Stacking 2D TMDs have provided unprecedented opportunities for constructing artificial functional structures.Due to the low cost,high yield and industrial compatibility,chemical vapor deposition(CVD)is regarded as one of the most promising growth strategies to obtain high-quality and large-area 2D TMDs and heterostructures.Here,state-of-the-art strategies for preparing TMDs details of growth control and related heterostructures construction via CVD method are reviewed and discussed,including wafer-scale synthesis,phase transition,doping,alloy and stacking engineering.Meanwhile,recent progress on the application of multi-functional devices is highlighted based on 2D TMDs.Finally,challenges and prospects are proposed for the practical device applications of 2D TMDs.

Cryogenic minimum quantity lubrication machining: from mechanism to application

Cutting fluid plays a cooling-lubrication role in the cutting of metal materials.However,the substantial usage of cutting fluid in traditional flood machining seriously pollutes the environment and threatens the health of workers.Environmental machining technologies,such as dry cutting,minimum quantity lubrication(MQL),and cryogenic cooling technology,have been used as substitute for flood machining.However,the insufficient cooling capacity of MQL with normal-temperature compressed gas and the lack of lubricating performance of cryogenic cooling technology limit their industrial application.The technical bottleneck of mechanical-thermal damage of difficult-to-cut materials in aerospace and other fields can be solved by combining cryogenic medium and MQL.The latest progress of cryogenic minimum quantity lubrication(CMQL)technology is reviewed in this paper,and the key scientific issues in the research achievements of CMQL are clarified.First,the application forms and process characteristics of CMQL devices in turning,milling,and grinding are systematically summarized from traditional settings to innovative design.Second,the cooling-lubrication mechanism of CMQL and its influence mechanism on material hardness,cutting force,tool wear,and workpiece surface quality in cutting are extensively revealed.The effects of CMQL are systematically analyzed based on its mechanism and application form.Results show that the application effect of CMQL is better than that of cryogenic technology or MQL alone.Finally,the prospect,which provides basis and support for engineering application and development of CMQL technology,is introduced considering the limitations of CMQL.

Flexible electronics based on one-dimensional inorganic semiconductor nanowires and two-dimensional transition metal dichalcogenides

Flexible electronics technology is considered as a revolutionary technology to unlock the bottleneck of traditional rigid electronics that prevalent for decades,thereby fueling the next-generation electronics.In the past few decades,the research on flexible electronic devices based on organic materials has witnessed rapid development and substantial achievements,and inorganic semiconductors are also now beginning to shine in the field of flexible electronics.As validated by the latest research,some of the inorganic semiconductors,particularly those at low dimension,unexpectedly exhibited excellent mechanical flexibility on top of superior electrical properties.Herein,we bring together a comprehensive analysis on the recently burgeoning low-dimension inorganic semiconductor materials in flexible electronics,including one-dimensional(1D)inorganic semiconductor nanowires(NWs)and two-dimensional(2D)transition metal dichalcogenides(TMDs).The fundamental electrical properties,optical properties,mechanical properties and strain engineering of materials,and their performance in flexible device applications are discussed in detail.We also propose current challenges and predict future development directions including material synthesis and device fabrication and integration.

Recent advancements and manipulation strategies of colloidal Cs_(2)B^(I)B^(III)X_(6)lead-free halide double perovskite nanocrystals

Double-metallic lead-free halide perovskites,Cs_(2)B^(I)B^(III)X_(6),sharing three-dimensional crystal structure,have been under the spotlight as the promising alternatives for the toxic and instable lead-based counterparts.Interest in Cs_(2)B^(I)B^(III)X_(6)motivates intense research into their colloidal nanocrystals(NCs).Recently,Cs_(2)B^(I)B^(III)X_(6)NCs have made great progress in the optical performance via alloying or doping,but there are still great challenges for optoelectronic applications.In this review,the latest advances of Cs_(2)B^(I)B^(III)X_(6)NCs in synthesis approaches,bandgap engineering,photoluminescence(PL)optimization,and applications are summarized.The focus is put upon the composition-property relationships of Cs_(2)B^(I)B^(III)X_(6)NCs,which is approached by discussing the influences of composition variation on the electronic states,carrier dynamics,and optical properties.The challenges and the corresponding improving strategies in the development of high-effective and stable Cs_(2)B^(I)B^(III)X_(6)NCs for device applications are also highlighted.It is believed that this review can deepen the understanding on this burgeoning material system and shed light on their future research directions.

Raman Spectroscopy and Imaging of Graphene

Graphene has many unique properties that make it an ideal material for fundamental studies as well as for potential applications.Here we review recent results on the Raman spectroscopy and imaging of graphene.We show that Raman spectroscopy and imaging can be used as a quick and unambiguous method to determine the number of graphene layers.The strong Raman signal of single layer graphene compared to graphite is explained by an interference enhancement model.We have also studied the effect of substrates,the top layer deposition,the annealing process,as well as folding(stacking order)on the physical and electronic properties of graphene.Finally,Raman spectroscopy of epitaxial graphene grown on a SiC substrate is presented and strong compressive strain on epitaxial graphene is observed.The results presented here are highly relevant to the application of graphene in nano-electronic devices and help in developing a better understanding of the physical and electronic properties of graphene.

A comprehensive review on Bi_(2)Te_(3)-based thin films: Thermoelectrics and beyond

Bi_(2)Te_(3)-based materials are not only the most important and widely used room temperature thermoelectric(TE)materials but are also canonical examples of topological insulators in which the topological surface states are protected by the time-reversal symmetry.High-performance thin films based on Bi_(2)Te_(3)- have attracted worldwide attention during the past two decades due primarily to their outstanding TE performance as highly efficient TE coolers and as miniature and flexible TE power generators for a variety of electronic devices.Moreover,intriguing topological phenomena,such as the quantum anomalous Hall effect and topological superconductivity discovered in Bi_(2)Te_(3)-based thin films and heterostructures,have shaped research directions in the field of condensed matter physics.In Bi_(2)Te_(3)-based films and heterostructures,delicate control of the carrier transport,film composition,and microstructure are prerequisites for successful device operations as well as for experimental verification of exotic topological phenomena.This review summarizes the recent progress made in atomic defect engineering,carrier tuning,and band engineering down to a nanoscale regime and how it relates to the growth and fabrication of high-quality Bi_(2)Te_(3)-based films.The review also briefly discusses the physical insight into the exciting field of topological phenomena that were so dramatically realized in Bi_(2)Te_(3)-and Bi_(2)Se_(3)‐based structures.It is expected that Bi_(2)Te_(3)-based thin films and heterostructures will play an ever more prominent role as flexible TE devices collecting and converting low-level(body)heat into electricity for numerous electronic applications.It is also likely that such films will continue to be a remarkable platform for the realization of novel topological phenomena.