Perspectives on exfoliated two-dimensional spintronics

Magnetic orderings, i.e., the spontaneous alignment of electron spins below a critical temperature, have been playing key roles in modern science and technologies for both the wide applications of magnetic recording for information storage and the vibrant potential of solid state electronic spin devices (also known as spintronics) for logic operations. In the past decades, thanks to the development of thin film technologies, magnetic thin films via sputtering or epitaxial growth have made the spintronic devices possible at the industrial scale. Yet thinner materials at lower costs with more versatile functionalities are highly desirable for advancing future spintronics. Recently, van der Waals magnetic materials, a family of magnets that can in principle be exfoliated down to the monolayer limit, seem to have brought tremendous opportunities: new generation van der Waals spintronic devices can be seamlessly assembled with possible applications such as optoelectronics, flexible electronics, and etc. Moreover, those exfoliated spintronic devices can potentially be compatible with the famed metal-oxide field effect transistor architectures, allowing the harness of spin performances through the knob of an electrostatic field.

Thermionic electron emission in the 1D edge-to-edge limit

Thermionic emission is a tunneling phenomenon,which depicts that electrons on the surface of a conductor can be pulled out into the vacuum when they are subjected to high electrical tensions while being heated hot enough to overtake their work functions.This principle has led to the great success of the so-called vacuum tubes in the early 20 th century.To date,major challenges still remain in the miniaturization of a vacuum channel transistor for on-chip integration in modern solid-state integrated circuits.Here,by introducing nano-sized vacuum gaps(~200 nm)in a van der Waals heterostructure,we successfully fabricated a one-dimensional(1 D)edge-to-edge thermionic emission vacuum tube using graphene as the filament.With the increasing collector voltage,the emitted current exhibits a typical rectifying behavior,with the maximum emission current reaching 200 p A and an ON-OFF ratio of 10;.In addition,it is found that the maximum emission current is proportional to the number of the layers of graphene.Our results expand the research of nano-sized vacuum tubes to an unexplored physical limit of 1 D edge-to-edge emission,and hold great promise for future nano-electronic systems based on it.

Metal–insulator transition in few-layered GaTe transistors

Two-dimensional(2D)materials have triggered enormous interest thanks to their interesting properties and potential applications,ranging from nanoelectronics to energy catalysis and biomedicals.In addition to other widely investigated 2D materials,GaTe,a layered material with a direct band gap of^1.7 e V,is of importance for applications such as optoelectronics.However,detailed information on the transport properties of GaTe is yet to be explored,especially at low temperatures.Here,we report on electrical transport measurements on few-layered GaTe field effect transistors(FETs)encapsulated by h-BN at different temperatures.We find that by tuning the carrier density,ambipolar transport was realized in GaTe devices,and an electrical-field-induced metal to insulator transition(MIT)was observed when it was hole doped.The mobilities of GaTe devices show a clear dependence on temperature and increase with the decrease of temperature,reaching^1200 cm2 V-1s-1 at 3 K.Our findings may inspire further electronic studies in devices based on GaTe.

Low-viscosity oligoether esters(OEEs)as high-efficiency lubricating oils:Insight on their structure‒lubricity relationship

Development of energy-efficient lubricants is a way to reduce energy consumption for transportation,with the tendency to design molecules that are beneficial in reducing the viscosity of synthetic oils.Oligoether esters(OEEs),as a low-viscosity ester base oil,have characteristics such as simple synthesis and excellent lubrication effect;however,the application of OEEs in tribology field has rarely been investigated.The objective of the present study is to investigate the effect of structure on the lubricating performance of OEEs and to develop a predictive model for OEEs based on quantitative structure‒property relationship(QSPR)through a combination of experiment and statistical modeling.Results showed that glycol chains contribute positively to lubrication with the ether functional groups increasing the sites of adsorption.Compared to branched-chain OEEs,straight-chain OEEs exhibited reduced wear,which was mainly due to the thicker adsorption film formed by the straight-chain structure.Furthermore,carbon films were detected on lightly worn surfaces,indicating that OEEs underwent oxidation during the friction process.Based on the results of principal component analysis(PCA)and partial least squares(PLS),it could be found that the predictive models of viscosity‒temperature performance,thermal stability performance,coefficient of friction(COF),and wear volume(WV)performed well and robustly.Among them,COF and WV can be best predicted with an R2 of about 0.90.

Regulating crystallinity in linear conjugated polymer to boost the internal electric field for remarkable visible-light-driven disinfection

Conjugated linear polymers are promising metal-free photocatalysts for visible-light-driven photocatalytic water disinfection,but it was still bottlenecked by the insufficient photogenerated charge separation and transport(CST)process.Herein,we obtained the highly crystalline imine-linked conjugated linear poly-mer(ODA-BPAH)with a greatly enhanced CST process.The highly crystalline ODA-BPAH exhibited excel-lent broad-spectrum water disinfection efficiency up to 99.99999%in 1 h,which is among the reported highest of state-of-the-art photocatalysts.The crystallinity of ODA-BPAH was regulated by simply turn-ing the solvent and the experiment results revealed that the ODA-BPAH with high crystallinity exhibited higher internal electric field strength and photocatalytic performance than that with low crystallinity,which indicates that higher crystallinity in linear conjugated polymers contributes to superior CST ef-ficiency as well as the generation of reactive oxygen species.This work highlights the impact of poly-mer crystallinity on the internal electric field and proves that linear poly-imine could be a new type of promising metal-free photocatalyst for water treatment.

Room temperature ferromagnetism in ultra-thin van der Waals crystals of 1T-CrTe2

Although many emerging new phenomena have been unraveled in two dimensional(2D)materials with long-range spin orderings,the usually low critical temperature in van der Waals(vdW)magnetic material has thus far hindered the related practical applications.Here,we show that ferromagnetism can hold above 300 K in a metallic phase of 1T-CrTe2 down to the ultra-thin limit.It thus makes CrTe2 so far the only known exfoliated ultra-thin vdW magnets with intrinsic long-range magnetic ordering above room temperature.An in-plane room-temperature negative anisotropic magnetoresistance(AMR)was obtained in ultra-thin CrTe2 devices,with a sign change in the AMR at lower temperature,with−0.6%and+5%at 300 and 10 K,respectively.Our findings provide insights into magnetism in ultra-thin CrTe2,expanding the vdW crystals toolbox for future room-temperature spintronic applications.

A gate-tunable artificial synapse based on vertically assembled van der Waals ferroelectric heterojunction

Memtransistor,a multi-terminal device that combines both the characteristics of a memristor and a transistor,has been intensively studied in two-dimensional layered materials(2 DLM),which show potential for applications in such as neuromorphic computation.However,while often based on the migration of ions or atomic defects in the conduction channels,performances of memtransistors suffer from the poor reliability and tunability.Furthermore,those known 2 DLM-based memtransistors are mostly constructed in a lateral manner,which hinders the further increasing of the transistor densities per area.Until now,fabricating non-atomic-diffusion based memtransistors with vertical structure remains challenging.Here,we demonstrate a vertically-integrated ferroelectric memristor by hetero-integrating the 2 D ferroelectric materials CuInP_(2)S_(6)(CIPS)into a graphite/CuInP_(2)S_(6)/MoS_(2)vertical heterostructure.Memristive behaviour and multi-level resistance states were realized.Essential synaptic behaviours including excitatory postsynaptic current,paired-pulse-facilitation,and spike-amplitude-dependent plasticity are successfully mimicked.Moreover,by applying a gate potential,the memristive behaviour and synaptic features can be effectively gate tuned.Our findings pave the way for the realization of novel gate-tunable ferroelectric synaptic devices with the capability to perform complex neural functions.

A monolithically sculpted van der Waals nano-opto-electro-mechanical coupler

The nano-opto-electro-mechanical systems(NOEMS)are a class of hybrid solid devices that hold promises in both classical and quantum manipulations of the interplay between one or more degrees of freedom in optical,electrical and mechanical modes.To date,studies of NOEMS using van der Waals(vdW)heterostructures are very limited,although vdW materials are known for emerging phenomena such as spin,valley,and topological physics.Here,we devise a universal method to easily and robustly fabricate vdW heterostructures into an architecture that hosts opto-electro-mechanical couplings in one single device.We demonstrated several functionalities,including nano-mechanical resonator,vacuum channel diodes,and ultrafast thermo-radiator,using monolithically sculpted graphene NOEMS as a platform.Optical readout of electric and magnetic field tuning of mechanical resonance in a CrOCl/graphene vdW NOEMS is further demonstrated.Our results suggest that the introduction of the vdW heterostructure into the NOEMS family will be of particular potential for the development of novel lab-on-a-chip systems.