Jiuxiang Dai

Nature Physics, Published online: 14 November 2022; doi:10.1038/s41567-022-01812-8

Nature Physics, Published online: 14 November 2022; doi:10.1038/s41567-022-01824-4

A strategy is developed for key generation and device authentication through the utilization of high-degree-of-randomness amorphous (A) state variations concomitant with different operating conditions to address the challenge of realizing efficient roots of trust for resource constrained hardware. This approach enables excellent physical unclonable functions (PUF) performance in uniformity, uniqueness, writing overheads, and machine learning attack resilience.

Abstract

There is an ever-increasing demand for next-generation devices that do not require passwords and are impervious to cloning. For traditional hardware security solutions in edge computing devices, inherent limitations are addressed by physical unclonable functions (PUF). However, realizing efficient roots of trust for resource constrained hardware remains extremely challenging, despite excellent demonstrations with conventional silicon circuits and archetypal oxide memristor-based crossbars. An attractive, down-scalable approach to design efficient cryptographic hardware is to harness memristive materials with a large-degree-of-randomness in materials state variations, but this strategy is still not well understood. Here, the utilization of high-degree-of-randomness amorphous (A) state variations associated with different operating conditions via thermal fluctuation effects is demonstrated, as well as an integrated framework for in memory computing and next generation security primitives, viz., APUF, for achieving secure key generation and device authentication. Near ideal uniformity and uniqueness without additional initial writing overheads in weak memristive A-PUF is achieved. In-memory computing empowers a strong exclusive OR (XOR-) and-repeat A PUF construction to avoid machine learning attacks, while rapid crystallization processes enable large-sized-key reconfigurability. These findings pave the way for achieving a broadly applicable security primitive for enhancing antipiracy of integrated systems and product authentication in supply chains.

Abstract

Exploring advanced electromagnetic wave (EMW) absorbers is one of the most feasible ways to solve the increasing electromagnetic pollution in both military and civil fields. In this work, γ-graphyne (γ-GY) is synthesized by a mechanochemical route using CaC₂ and hexabromobenzene (PhBr₆). Then three-dimensional (3D) reduced graphene oxide/γ-GY (RGO/GY) heterostructures are prepared through facile solvothermal self-assembly and subsequent thermal reduction. The influences of calcination temperature and the content of γ-GY of the composite on EMW absorption performance are fully investigated. The minimum reflection loss (RL) value of the RGO/GY composite foam is −71.73 dB at 10.48 GHz with the matching thickness of 3.54 mm, and the effective absorption bandwidth (EAB) less than −10 dB is 7.36 GHz. Moreover, excellent terahertz (THz) absorption property is also obtained at 0.2–1.6 THz. The RL of 84.08 dB is acquired, and the EAB covers 100% of the entire measured bandwidth. In addition, the composite is also a promising anticorrosive EMW absorber. This work provides encouraging findings, which are also instructive for the potential advantages of graphyne-based materials as highly efficient EMW absorbers in both gigahertz and terahertz band ranges.

An in-depth understanding of the fundamental issues of metal droplet deposition is particularly crucial for developing droplet deposition-based engineering manufacturing technologies. Herein, the fundamental theory of metal droplet deposition, the characterization of deposition behavior, deposition-forming regulation, and three typical engineering manufacturing techniques based on metal droplet deposition, that is, thermal spraying, soldering, and droplet-based 3D printing, is reviewed.

Metal droplet deposition is of fundamental importance in many industrial and manufacturing processes. An in-depth understanding of the fundamental issues of metal droplet deposition is particularly crucial for developing droplet deposition-based engineering manufacturing technologies. This review systematically introduces the evolution of metal droplet deposition stages and characteristic behaviors in terms of metal droplet deposition dynamics. The characteristic morphologies of single-metal droplets after solidification, that is, gas entrapment, ripples, bulging edges, and splashing, are reviewed. The formation mechanisms and influencing factors of different droplet morphologies are summarized. Moreover, the deposition strategy of multiple metal droplets to promote good interfacial bonding is further concluded. Based on these theoretical foundations, three typical engineering manufacturing techniques based on metal droplet deposition, that is, thermal spraying, soldering, and droplet-based 3D printing, are presented to elucidate further the advantages and potential of metal droplet deposition applied to engineering manufacturing. Finally, some existing problems and future research perspectives are discussed.

Light: Science & Applications, Published online: 11 November 2022; doi:10.1038/s41377-022-00980-9

Compared with traditional silicon-based devices, the unique features of all-2D electronics are atomic flatness and dangling-bond van der Waals interfaces, including all-2D metal/semiconductor, all-2D semiconductor/semiconductor, and all-2D dielectric/semiconductor interfaces. These interfaces provide an excellent device platform for removing lattice matching constraints, suppressing carrier scattering, and exploiting new physical effects.

Abstract

The interface is the device. As the feature size rapidly shrinks, silicon-based electronic devices are facing multiple challenges of material performance decrease and interface quality degradation. Ultrathin 2D materials are considered as potential candidates in future electronics by their atomically flat surfaces and excellent immunity to short-channel effects. Moreover, due to naturally terminated surfaces and weak van der Waals (vdW) interactions between layers, 2D materials can be freely stacked without the lattice matching limit to form high-quality heterostructure interfaces with arbitrary components and twist angles. Controlled interlayer band alignment and optimized interfacial carrier behavior allow all-2D electronics based on 2D vdW interfaces to exhibit more comprehensive functionality and better performance. Especially, achieving the same computing capacity of multiple conventional devices with small footprint all-2D devices is considered to be the key development direction of future electronics. Herein, the unique properties of all-2D vdW interfaces and their construction methods are systematically reviewed and the main performance contributions of different vdW interfaces in 2D electronics are summarized, respectively. Finally, the recent progress and challenges for all-2D vdW electronics are discussed, and how to improve the compatibility of 2D material devices with silicon-based industrial technology is pointed out as a critical challenge.

Chemical tuning including lithiation control and solvent coordination can significantly modify magnetic interaction in molecule-based magnets for high-temperature magnetic order and large coercivity. Through those technical routes and processing, 2D molecule-based magnets with an applicable magnetic performance of ferrites are demonstrated. The considerable energy product evidences its worth for obtaining practical molecule-based magnets.

Abstract

2D magnets provoke a surge of interest in large anisotropy in reduced dimensions and are promising for next-generation information technology where dynamic magnetic tuning is essential. Until recently, the crucial metal-organic magnet Cr(pyz)₂·xLiCl·yTHF with considerable high coercivity and high-temperature magnetic order opens up a new platform to control magnetism in metal-organic materials at room temperature. Here, an in–situ chemical tuning route is reported to realize the controllable transformation of low-temperature magnetic order into room-temperature hard magnetism in Cr(pyz)₂·xLiCl·yTHF. The chemical tuning via electrochemical lithiation and solvation/desolvation exhibits continuously variable magnetic features from cryogenic magnetism to the room-temperature optimum performance of coercivity (H _c) of 8500 Oe and energy product of 0.6 MGOe. Such chemically flexible tunability of room-temperature magnetism is ascribed to the different degrees of lithiation and solvation that modify the stoichiometry and Cr-pyrazine coordination framework. Furthermore, the additively manufactured hybrid magnets show air stability and electromagnetic induction, providing potential applications. The findings here suggest chemical tuning as a universal approach to control the anisotropy and magnetism of 2D hybrid magnets at room temperature, promising for data storage, magnetic refrigeration, and spintronics.

A temporal-efficient noise-resilient ferroelectric encoder, exploiting the time-to-first-spike encoding scheme, is proposed for information preprocessing in spiking neural networks. Our device manifests prominent ferroelectric dynamics, showing an excellent classification accuracy of 95.14%, in addition to resilience against noise attacks. This work manifests the potential to enable spike-driven computations for energy-efficient machine learning and artificial intelligence.

Abstract

Spiking neural network (SNN), where the information is evaluated recurrently through spikes, has manifested significant promises to minimize the energy expenditure in data-intensive machine learning and artificial intelligence. Among these applications, the artificial neural encoders are essential to convert the external stimuli to a spiking format that can be subsequently fed to the neural network. Here, a molybdenum disulfide (MoS₂) hafnium oxide-based ferroelectric encoder is demonstrated for temporal-efficient information processing in SNN. The fast domain switching attribute associated with the polycrystalline nature of hafnium oxide-based ferroelectric material is exploited for spike encoding, rendering it suitable for realizing biomimetic encoders. Accordingly, a high-performance ferroelectric encoder is achieved, featuring a superior switching efficiency, negligible charge trapping effect, and robust ferroelectric response, which successfully enable a broad dynamic range. Furthermore, an SNN is simulated to verify the precision of the encoded information, in which an average inference accuracy of 95.14% can be achieved, using the Modified National Insitute of Standards and Technology (MNIST) dataset for digit classification. Moreover, this ferroelectric encoder manifests prominent resilience against noise injection with an overall prediction accuracy of 94.73% under various Gaussian noise levels, showing practical promises to reduce the computational load for the neural network.

The phase transition, multiple valence bands, and resonant bonding endow GeTe with promising thermoelectric performance. Significant breakthroughs are achieved in GeTe-based materials. This review summarizes the recent progress in developing high-performance GeTe-based materials and devices, including the underlying fundamentals, large-scale production, novel strategies for boosting performance, and techniques of device assembly.

Abstract

Driven by the intensive efforts in the development of high-performance GeTe thermoelectrics for mass-market application in power generation and refrigeration, GeTe-based materials display a high figure of merit of >2.0 and an energy conversion efficiency beyond 10%. However, a comprehensive review on GeTe, from fundamentals to devices, is still needed. In this regard, the latest progress on the state-of-the-art GeTe is timely reviewed. The phase transition, intrinsic high carrier concentration, and multiple band edges of GeTe are fundamentally analyzed from the perspectives of the native atomic orbital, chemical bonding, and lattice defects. Then, the fabrication methods are summarized with a focus on large-scale production. Afterward, the strategies for enhancing electronic transports of GeTe by energy filtering effect, resonance doping, band convergence, and Rashba band splitting, and the methods for strengthening phonon scatterings via nanoprecipitates, planar vacancies, and superlattices, are comprehensively reviewed. Besides, the device assembly and performance are highlighted. In the end, future research directions are concluded and proposed, which enlighten the development of broader thermoelectric materials.

A novel strategy to increase the Seebeck coefficient of 2D PtSe₂ films by stacking the same PtSe₂ layer onto each other as stacked PtSe₂/PtSe₂ homostructures via a wet-transfer method is demonstrated. The Seebeck coefficient increases significantly with increasing number of stacking films in a PtSe₂/PtSe₂ homostructure for a four-stacked homostructure due to the carrier-interface interaction under the longitudinal temperature gradient.

Abstract

When a thermoelectric (TE) material is deposited with a secondary TE material, the total Seebeck coefficient of the stacked layer is generally represented by a parallel conductor model. Accordingly, when TE material layers of the same thickness are stacked vertically, the total Seebeck coefficient in the transverse direction may change in a single layer. Here, an abnormal Seebeck effect in a stacked two-dimensional (2D) PtSe₂/PtSe₂ homostructure film, i.e., an extra in-plane Seebeck voltage is produced by wet-transfer stacking at the interface between the PtSe₂ layers under a transverse temperature gradient is reported. This abnormal Seebeck effect is referred to as the interfacial Seebeck effect in stacked PtSe₂/PtSe₂ homostructures. This effect is attributed to the carrier-interface interaction, and has independent characteristics in relation to carrier concentration. It is confirmed that the in-plane Seebeck coefficient increases as the number of stacked PtSe₂ layers increase and observed a high Seebeck coefficient exceeding ≈188 µV K⁻¹ at 300 K in a four-layer-stacked PtSe₂/PtSe₂ homostructure.

The implementation of new volatile heteroleptic W-complex [W(N^tBu)₂(NⁱPr₂)₂] as precursor in a chemical vapor deposition (CVD) process enabled the growth of highly pure and crystalline WO₃. Advanced surface characterization demonstrates that WO₃ thin film implementation in a custom-built mobile sensor device shows outstanding sensing response at low temperature toward hydrogen.

Abstract

The intrinsic properties of semiconducting oxides having nanostructured morphology are highly appealing for gas sensing. In this study, the fabrication of nanostructured WO₃ thin films with promising surface characteristics for hydrogen (H₂) gas sensing applications is accomplished. This is enabled by developing a chemical vapor deposition (CVD) process employing a new and volatile tungsten precursor bis(diisopropylamido)-bis(tert-butylimido)-tungsten(VI), [W(N^tBu)₂(NⁱPr₂)₂]. The as-grown nanostructured WO₃ layers are thoroughly analyzed. Particular attention is paid to stoichiometry, surface characteristics, and morphology, all of which strongly influence the gas-sensing potential of WO₃. Synchrotron-based ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), X-ray photoelectron emission microscopy (XPEEM), low-energy electron microscopy (LEEM) and 4-point van der Pauw (vdP) technique made it possible to analyze the surface chemistry and structural uniformity with a spatially resolved insight into the chemical, electronic and electrical properties. The WO₃ layer is employed as a hydrogen (H₂) sensor within interdigitated mini-mobile sensor architecture capable of working using a standard computer's 5 V 1-wirebus connection. The sensor shows remarkable sensitivity toward H₂. The high, robust, and repeatable sensor response (S) is attributed to the homogenous distribution of the W⁵⁺ oxidation state and associated oxygen vacancies, as shown by synchrotron-based UPS, XPS, and XPEEM analysis.

In this work, a combination of single event effect, displacement damage and total ionizing dose irradiation tests are performed, demonstrating that carbon nanotube transistors feature superior comprehensive radiation effect tolerance compared to their Si counterparts. The ultra-strong radiation tolerance promotes carbon nanotube integrated circuit's (IC) applications in high-energy solar and cosmic radiation environments.

Abstract

Carbon nanotube (CNT) field-effect transistors (FETs) have been considered ideal building blocks for radiation-hard integrated circuits (ICs), the demand for which is exponentially growing, especially in outer space exploration and the nuclear industry. Many studies on the radiation tolerance of CNT-based electronics have focused on the total ionizing dose (TID) effect, while few works have considered the single event effects (SEEs) and displacement damage (DD) effect, which are more difficult to measure but may be more important in practical applications. Measurements of the SEEs and DD effect of CNT FETs and ICs are first executed and then presented a comprehensive radiation effect analysis of CNT electronics. The CNT ICs without special irradiation reinforcement technology exhibit a comprehensive radiation tolerance, including a 1 × 10⁴ MeVcm² mg⁻¹ level of the laser-equivalent threshold linear energy transfer (LET) for SEEs, 2.8 × 10¹³ MeV g⁻¹ for DD and 2 Mrad (Si) for TID, which are at least four times higher than those in conventional radiation-hardened ICs. The ultrahigh intrinsic comprehensive radiation tolerance will promote the applications of CNT ICs in high-energy solar and cosmic radiation environments.

The Au/InSe/Au van der Waals Schottky structures with a pair of the Au electrodes are demonstrated to perform a high-performance self-powered broadband photoresponse due to the asymmetric Schottky barrier heights and contact geometries in Au–InSe junctions.

Abstract

Self-powered photodetectors (SPPDs) are generally carried out in multilayered heterostructures with different semiconductors or in Schottky junctions with different metal electrodes. It is interesting to build an SPPD using metal–semiconductor–metal (MSM) structures with the same type of metal electrodes. Here, an SPPD is fabricated facilely by stacking a piece of irregular InSe nanosheet on a pair of Au electrodes with asymmetric van der Waals contacts. The SPPD performs a high responsivity of 0.103 A W⁻¹, a high on-off current ratio over 10⁴, a high detectivity of 1.83 × 10¹⁰ Jones, a fast response time of 1 ms and a broadband sensing spectrum ranging from 300 to 1000 nm under zero bias. A series of characterization and working mechanism analysis demonstrate the contribution of the asymmetric Schottky barrier heights and contact geometries in Au–InSe junctions to the self-powered performance of the detector. This work offers an effective scheme to construct high-performance SPPDs in simple architecture and processing for potential optoelectronic device applications.

By combining inkjet printing and thermal annealing, large-area patterned MoS₂ are directly synthesized on polymer substrates without photolithography patterning and transferring process. The resultant MoS₂ films exhibit superior mechanical durability (≈2% variation in resistance over 10,000 bending cycles), as well as excellent chemical stability. Also, temperature and biopotential signals are continuously recorded with MoS₂-based sensors even under folding conditions.

Abstract

Synthesis of large-area patterned MoS₂ is considered the principle base for realizing high-performance MoS₂-based flexible electronic devices. Patterning and transferring MoS₂ films to target flexible substrates, however, require conventional multi-step photolithography patterning and transferring process, despite tremendous progress in the facilitation of practical applications. Herein, an approach to directly synthesize large-scale MoS₂ patterns that combines inkjet printing and thermal annealing is reported. An optimal precursor ink is prepared that can deposit arbitrary patterns on polyimide films. By introducing a gas atmosphere of argon/hydrogen (Ar/H₂), thermal treatment at 350 °C enables an in situ decomposition and crystallization in the patterned precursors and, consequently, results in the formation of MoS₂. Without complicated processes, patterned MoS₂ is obtained directly on polymer substrate, exhibiting superior mechanical flexibility and durability (≈2% variation in resistance over 10,000 bending cycles), as well as excellent chemical stability, which is attributed to the generated continuous and thin microstructures, as well as their strong adhesion with the substrate. As a step further, this approach is employed to manufacture various flexible sensing devices that are insensitive to body motions and moisture, including temperature sensors and biopotential sensing systems for real-time, continuously monitoring skin temperature, electrocardiography, and electromyography signals.