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The piezoelectric effect has been widely observed in biological systems, and its applications in biomedical field are emerging. Recent advances of wearable and implantable biomedical devices bring promise as well as requirements for the piezoelectric materials building blocks. Owing to their biocompatibility, biosafety, and environmental sustainability, natural piezoelectric biomaterials are known as a promising candidate in this emerging field, with a potential to replace conventional piezoelectric ceramics and synthetic polymers. Herein, we provide a thorough review of recent progresses of research on five major types of piezoelectric biomaterials including amino acids, peptides, proteins, viruses, and polysaccharides. Our discussion focuses on their structure- and phase-related piezoelectric properties and fabrication strategies to achieve desired piezoelectric phases. We compare and analyze their piezoelectric performance and further introduce and comment on the approaches to improve their piezoelectric property. Representative biomedical applications of this group of functional biomaterials including energy harvesting, sensing, and tissue engineering are also discussed. We envision that molecular-level understanding of the piezoelectric effect, piezoelectric response improvement, and large-scale manufacturing are three main challenges as well as research and development opportunities in this promising interdisciplinary field.

We present a controllable terahertz (THz) metamaterial switch by manipulating toroidal dipolar mode in simulation. The metamaterial switch consists of periodically patterned metallic split rings with photosensitive silicon stripes. The excitation of toroidal dipolar resonance is closely dependent on the transition between the dielectric and conductive phases of photosensitive silicon. The toroidal dipolar mode is exploited to modulate the ON/OFF-switching transmission of THz wave under wide incident angles. The operation mechanism and frequency tunability are discussed.

Ni-rich layered LiNi0·6Co0·2Mn0·2O2 materials have recently drawn much attention for their high specific capacity. However, they still suffer from the cycling instability arising from Li+/Ni2+ ions mixing in crystal structure and harmful side reactions with electrolyte, especially charged at high cut-off voltage to achieve higher energy density. In this study, the Li(Ni0·6Co0·2Mn0.2)O2 (NCM622) cathode materials were prepared by sol-gel method, and Ce4+ modification was utilized to improve cycling stability and rate performance. Through XRD refinement, XPS, SEM and TEM characterization, it was found that in the case of 0.5 at% level, Ce4+ ions not only entered into the crystal lattice to replace transition metal ions, but also produced CeO2 coating on the surface of NCM622 particles. The results of electrochemical performance showed that after 100 cycles at 1 C rate, the capacity retention of NCM-0.5 sample increased by 11.03% compared to pristine sample. Besides, in the case of 5 C, the capacity of NCM-0.5 sample reached to 114.92 mAh·g−1, much higher than that of NCM-0 (91.92 mAh·g−1). The findings indicate that it is the synergistic effect of Ce4+ that endows NCM622 excellent cycling stability and rate performance at high cut-off voltage.

We propose a birefringent dielectric multi-foci metalens for polarization detection utilizing different transmission phases in two orthogonal directions, which consists of Si nanopillars with different sizes and more than 80% transmission efficiency. Implementing the superposition of the phase profiles in the x- and y-direction, the polarization states of the focal points include linear polarization state in the x-direction, linear polarization state in the y-direction, the incident polarization state, and the polarization state whose y-polarization component more shifted by π/2 compared with the incident polarization state. Based on the intensities of the deflected and converged focal points on the same focal plane, the full polarization information of the incident polarized light can be determined with an analyzer. The multiplexing design method with compact and efficient features renders this technique very attractive for polarization detection and information processing.

We report a ReSe2 passively mode-locked composite YVO4/Nd:YVO4 laser at 1.06 μm, which can stably generate pure continuous-wave mode-locking (CWML) operation at watt-level output. ReSe2, as a new type of 2-D transition metal dichalcogenides (TMDs), has the features of weak-layered dependence, broadband saturable absorption, low modulation depth and saturation intensity, which is beneficial to initiate robust CWML mechanism via nonlinear saturable absorption. By employing few-layered ReSe2 as saturable absorber, the nanosecond mode-locked pulses at fundamental repetition rate of 90.37 MHz was obtained experimentally in a composite YVO4/Nd:YVO4 laser. The maximum output power in pure CWML operation was as high as 1.1 W. The high-power CWML laser source shows potential applications in time-resolved spectroscopy, telecommunication and detection.

We present a unique scheme to efficiently enhance terahertz third harmonic generation (THG). By exploiting the high-order guided-mode resonance of graphene plasmonic grating, the off-plane transverse localized field can be suppressed, which enables the electromagnetic energy to be tightly concentrated and significantly enhanced in the monolayer graphene. Meanwhile, high density of in-plane hot spots can be excited to form a quasi-continuous enhanced-field profile called as hot surface. This can bring about the giant nonlinearity enhancement of terahertz field-graphene interaction, greatly pushing forward the enhanced THG in monolayer graphene to breakout the limitation of various patterned metamaterials. It is anticipated that the graphene plasmonic grating has promising application for nonlinear terahertz spectroscopy, imaging and communication.

Nanostructured piezoelectric semiconductors offer unprecedented opportunities for high-performance sensing in numerous catalytic processes of biomedical, pharmaceutical, and agricultural interests, leveraging piezocatalysis that enhances the catalytic efficiency with the strain-induced piezoelectric field. Here, a cost-efficient, high-performance piezo-electrocatalytic sensor for detecting l-ascorbic acid (AA), a critical chemical for many organisms, metabolic processes, and medical treatments, is designed and demonstrated. Zinc oxide (ZnO) nanorods and nanosheets are prepared to characterize and compare their efficacy for the piezo-electrocatalysis of AA. The electrocatalytic efficacy of AA is significantly boosted by the piezoelectric polarization induced in the nanostructured semiconducting ZnO catalysts. The charge transfer between the strained ZnO nanostructures and AA is elucidated to reveal the mechanism for the related piezo-electrocatalytic process. The low-temperature synthesis of high-quality ZnO nanostructures allows low-cost, scalable production, and integration directly into wearable electrocatalytic sensors whose performance can be boosted by otherwise wasted mechanical energy from the working environment, for example, human-generated mechanical signals.

The stable operation with passing obstacles is the main challenge for wheel-legged robots in irregular terrains, especially in the gully terrain. An obstacle-surmounting method of wheel-legged hybrid gait based on perception system is proposed in this paper for gully terrain. Firstly, the system composition and communication mode of the wheel-legged robot are described. Secondly, the kinematics and dynamics models are established, and on this basis, the strategy of wheel-legged compound gait crossing gully landform is designed. Finally, the co-simulation based on MATLAB and V-REP is carried out. The simulation results show that the body of robot can maintain horizontal stability in the process of crossing gully terrain, which verifies the effectiveness of the obstacle-surmounting strategy.

We first report on a stable ReSe2 passively Q-switched Nd:Y3Al5O12 bulk laser near room temperature, which can generate near-repetition rate limit of microsecond pulse output at full depth modulation. A new kind of two-dimensional transition metal dichalcogenide, ReSe2, is exploited as an excellent saturable absorber at 1 . 06 μ m with extremely high rate of loss modulation. The microsecond pulse repetition rate of as high as 274 kHz can be achievable under the pump power of 7.3 W, with the pulsewidth of 1 . 08 μ s and the pulse energy of 2.5 μ J. As the pump power varies from 4.4 to 7.3 W, the pulse repetition rate is tunable from 204 to 274 kHz. The good stability in operation is evaluated with power fluctuation less than 1%. The ultrahigh repetition rate of microsecond pulse source can be suitable for differential absorption lidar of monitoring atmospheric environment, which can meet the specific requirements of adequate interaction duration and sufficient sampling rate to improve the signal-to-noise ratio.

Two-dimensional (2-D) semiconductors have been intensely explored as alternative channel materials for future generation ultra-scaled transistor technology [1] , [2] , [3] , [4] , [5] , [6] , [7] , [8] . However, significant roadblocks (e.g., poor carrier mobilities [9] , [10] , [11] , instability [4,5,10], and vague potential in scaling-up [10,12–15]) exist that prevent the realization of the current state-of-the-art 2-D materials’ potential for energy-efficient electronics. The emergent solution-grown tellurene exhibits attractive attributes, e.g., high room-temperature mobility, large on-state current density, air-stability, and tunable material properties through a low-cost, scalable process, to tackle these challenges [16]. Nevertheless, the fundamental manufacturing science of the hydrothermal processing for tellurene remains elusive. Here, we report on the first systematic, data-driven learning of the process-structure-property relationship in solution-grown tellurene, revealing the process factors’ effects on tellurene’s production yield, dimensions, and transistor-relevant properties, through a holistic approach integrating both the experimental explorations and data analytics. We further demonstrate the application of such fundamental knowledge for developing tellurene transistors with optimized and reliable performance, which can enable the cost-effective realization of high-speed, energy-efficient electronics.

We propose an ultrasensitive tunable terahertz (THz) sensor which consists of single-layer graphene-based gratings integrated with a Fabry–Perot cavity through numerical simulation. The excitation of standing-wave graphene surface plasmon polaritons can make the probing field strongly confined in a gap cavity. This provides an excellent platform for ultrasensitive and high figure-of-merit (FOM) refractive index sensing at the THz band. Our full-wave electromagnetic simulations show a frequency sensitivity as high as 4.2 THz per refractive index unit and an FOM beyond 12.5 at normal incidence in the THz regime. At oblique incidence, the operation angle for effective THz sensing can be maintained as wide as 75° without inducing degeneration of the sensitivity and FOM. The sensing performance can be readily tuned by electrically controlling the chemical potential of graphene. The proposed graphene plasmonic grating scheme is a promising candidate for the development of on-chip integrated THz biochemical sensors.

Smart sensing devices with high stretchability and self-powered characteristics are essential in future generation wearable human-integrated applications. Here we report for the first time scalable synthesis and integration of selenium (Se) nanowires into wearable piezoelectric devices, and explore the feasibility of such devices for self-powered sensing applications, e.g., physiological monitoring. The ultrathin device can be conformably worn onto the human body, effectively converting the imperceptible time-variant mechanical vibration from the human body into distinguishable electrical signals, e.g., gesture, vocal movement, and radial artery pulse, through straining the piezoelectric Se nanowires. Our results suggest the potential of solution-synthesized Se nanowire a new class of piezoelectric nanomaterial for self-powered biomedical devices and opens doors to new technologies in energy, electronics, and sensor applications.

Deformable energy devices capable of efficiently scavenging ubiquitous mechanical signals enable the realization of self-powered wearable electronic systems for emerging human-integrated technologies. Triboelectric nanogenerators (TENGs) utilizing soft polymers with embedded additives and engineered dielectric properties emerge as ideal candidates for such applications. However, the use of solid filler materials in the state-of-the-art TENGs limits the devices' mechanical deformability and long-term durability. The current structural design for TENGs faces the dilemma where the enhanced dielectric constant of the TENG's contact layer leads to an undesirable saturation of the surface charge density. Here, we present a novel scheme to address the above issues, by exploring a liquid-metal-inclusion based TENG (LMI-TENG) where inherently deformable core–shell LMIs are incorporated into wearable high-dielectric-constant polymers. Through a holistic approach integrating theoretical and experimental efforts, we identified the parameter space for designing an LMI-TENG with co-optimized output and mechanical deformability. As a proof of concept, we demonstrated an LMI-TENG based wireless media control system for a self-powered user interface. The device architecture and design scheme presented here provide a promising solution towards the realization of self-powered human-integrated technologies.

We report a full telecomband covered half-wave meta-reflectarray for efficient circular polarization conversion at near-infrared frequencies, which is composed of regular Z-shaped antenna array above a conductive ground layer separated with a dielectric spacer. The calculations of polarization conversion spectra demonstrate the ultra-broadband operation at 690 to 2010 nm continuously covering the whole telecom windows, with more than 90% conversion efficiency in full bandwidth. The relative bandwidth of as high as 98% is far higher than those reported previously never exceeding 50%. Sufficiently high conversion efficiencies of beyond 90% can be steadily performed for either left- or right-handed circular polarization incidence. The underlying mechanism is interpreted through the cross-coupling of multiple intrinsic resonances of meta-atoms. The effects of geometry and oblique incidence on the polarization conversion are discussed, showing the acceptable technical tolerance and wide operating angle. The proposed half-wave meta-reflectarray shows promising application in the field of on-chip integrated spin-based logic devices and optical communications.

Herein, abundant and uniform nanopores with sub-3-nm sizes are introduced to 1.5 nm-thick nickel-iron layered double hydroxide (NiFe LDH) ultrathin nanosheets via an etching-aging process, realizing remarkable enhancement in oxygen evolution reaction (OER) performance. Detailed analyses revealed that the NiFe LDH phase around the nanopores can be preferentially oxidized into electroactive Fe:NiOOH phase. In addition, the buffering space provided by the nanopores can effectively avoid structural deformation during repeated redox cycling, leading to better electrochemical stability, which synergistically make the catalyst a promising candidate for commercial water splitting. This work highlights the positive role of porous structure in accelerating the formation of active phases beyond just increment of surface area, which can provide insight in designing advanced catalysts.

Three-dimensional (3D) light fields with spatially inhomogeneous polarization and intensity distributions play an increasingly important role in photonics due to their peculiar optical features and extra degrees of freedom for carrying information. However, it is very challenging to simultaneously control the intensity profile and polarization profile in an arbitrary manner. Here we experimentally demonstrate a metalens that can focus light into an arbitrarily shaped focal curve with a predefined polarization distribution. The efficacy of this approach is exemplified through the demonstration of focused curves in 3D space ranging from simple shapes such as a circle to topologically nontrivial objects such as a 3D knot with controlled local polarization states. This powerful control of the light field would be technically challenging with their conventional counterparts. Our demonstration may find applications in beam engineering and integration optics.

Electronics has become an inseparable part of our daily lives, stressing the supply of electrical power at anytime and anywhere. Besides reducing power consumption, increasing the energy density of the power supply component, and developing a sustainable power system that provides power by harvesting energy from the ambient environment are two solutions to address this challenge. To this end, my research goal is to transform advanced manufacturing through innovating designer functional nanomaterials for societally-pervasive areas including health monitoring, energy harvesting, human-machine interaction, and internet-of-things. Piezoelectric and triboelectric effects can convert mechanical energy to electricity, which enables the design of devices to utilize energy generated from the human body. The functional nanomaterials with such unique properties could be rationally synthesized and fabricated as sustainable power sources or self-powered systems. In this dissertation, two kinds of devices have been developed, the nanogenerator to covert mechanical energy from human body to electricity and the self-powered sensor. First, A series of biomaterial and biocompatible materials derived wearable energy harvesting devices were invented by systematically engineering the chemical and surface structures. Second, a versatile platform was developed for the monolithic integration of liquid-solid heterojunction devices through the hybrid manufacturing of bottom-up growth of 2D ZnO piezoelectric nanostructures on additively printed liquid metal electrodes. This new class of wearable devices are conformable to human skins and can sustainably perform non-invasive physiological functions, e.g. detection of pulses and vocal vibration, by harvesting the operation power from the human body. Besides, the controllable manufacturing of functional nanocrystals and their energy-related applications are also included. This dissertation is expected to have a positive impact and immediate relevance to many societally pervasive areas, e.g. energy and environment, biomedical electronics, and human-machine interface.

We present a tunable plasmon-induced transparency (PIT) structure that is composed of dielectric grating and a graphene system to manipulate terahertz (THz) waves. The graphene system consists of a graphene sheet and a graphene ribbon layer, with a spacer between them. By exploiting the diffraction coupling of THz wave with dielectric grating, graphene plasmonic resonance is efficiently excited on both graphene sheet and graphene ribbons. This leads to the surface plasmon mode of the graphene sheet and the localized plasmon mode of the graphene ribbons. The coupling between the two-plasmon modes via near-field destructive interference generates a strong PIT effect with slowing the group velocity of THz waves. A group delay over 0.2 ps and group index beyond 170 can be achievable. The group slowing effect is dynamically tunable with varying the Fermi level of graphene. The work suggests a promising scheme for on-chip graphene slow-wave devices at the THz regime.

The capability of sensor systems to efficiently scavenge their operational power from stray, weak environmental energies through sustainable pathways could enable viable schemes for self-powered health diagnostics and therapeutics. Triboelectric nanogenerators (TENG) can effectively transform the otherwise wasted environmental, mechanical energy into electrical power. Recent advances in TENGs have resulted in a significant boost in output performance. However, obstacles hindering the development of efficient triboelectric devices based on biocompatible materials continue to prevail. Being one of the most widely used polymers for biomedical applications, polyvinyl alcohol (PVA) presents exciting opportunities for biocompatible, wearable TENGs. Here, the holistic engineering and systematic characterization of the impact of molecular and ionic fillers on PVA blends' triboelectric performance is presented for the first time. Triboelectric devices built with optimized PVA-gelatin composite films exhibit stable and robust triboelectricity outputs. Such wearable devices can detect the imperceptible skin deformation induced by the human pulse and capture the cardiovascular information encoded in the pulse signals with high fidelity. The gained fundamental understanding and demonstrated capabilities enable the rational design and holistic engineering of novel materials for more capable biocompatible triboelectric devices that can continuously monitor vital physiological signals for self-powered health diagnostics and therapeutics.

Flexible optoelectronic devices have attracted considerable attention due to their low weight, portability, and ease of integration with other devices. However, major issues still exist: they are subject to repeated stresses, which often leads to damage; and the current fabrication methods such as photolithography and nano-imprint lithography can be very time-consuming or costly. This work aims to develop a novel cost-effective and time-efficient laser metasurface fabrication (LMF) technique for production of flexible optoelectronic devices. The experimental results have shown that the laser patterned flexible surfaces exhibit high visible transmittance, low sheet resistance, and extraordinary mechanical durability under repeated bending cycles. The laser patterned flexible surfaces have also demonstrated the potential to be utilized as heaters, which renders them new de-icing or de-fogging applications. This innovative laser patterning method will provide a new avenue for fabrication of multifunctional optoelectronic devices.

Tellurium (Te) has a trigonal crystal lattice with inherent structural anisotropy. Te is multifunctional, e.g., semiconducting, photoconductive, thermoelectric, piezoelectric, etc., for applications in electronics, sensors, optoelectronics, and energy devices. Due to the inherent structural anisotropy, previously reported synthetic methods predominantly yield one-dimensional (1D) Te nanostructures. Much less is known about 2D Te nanostructures, their processing schemes, and their material properties. This review focuses on the synthesis and morphology control of emerging 2D tellurene and summarizes the latest developments in understanding the fundamental properties of monolayer and few-layer tellurene, as well as the recent advances in demonstrating prototypical tellurene devices. Finally, the prospects for future research and application opportunities as well as the accompanying challenges of 2D tellurene are summarized and highlighted.

Piezoelectricity provides an ideal electromechanical mechanism with emerging applications in wearable devices due to its simplicity and self-powered nature. However, the 3D printing of piezoelectric devices still faces many challenges, including material printability, high energy poling process, and low dimensional accuracy. This study demonstrates, for the first time, a tellurium nanowire-based piezoelectric device fabricated by a hybrid printing method integrating highly complementary aerosol jet printing and extrusion printing in a single printing platform. The aerosol-jet-printed tellurium nanowire demonstrates piezoelectric properties without the need for any poling processing due to the unique properties of the tellurium nanowires. The silver nanowire electrodes printed by aerosol jet printing demonstrate excellent conductivity and stretchability without the need for sintering. An extrusion method is employed to print the silicone films, which serve as the stretchable substrate and the electrical insulation layers between the printed tellurium and silver. The printed wearable piezoelectric devices were attached to a human wrist to detect different hand gestures and to a human neck to detect heartbeat without using an external power source. The fully printed, sintering-free and poling-free, and stretchable piezoelectric device opens enormous opportunities for facile integration with a broad range of printed electronics and wearable devices.

The outstanding ability of optical metasurfaces to tailor light's amplitude, phase, and polarization with a subwavelength resolution has attracted big interest. A multifunctional metasurface device is reported and experimentally demonstrated that integrates an arbitrary polarization profile for image concealment and a computer‐generated hybrid hologram. Herein, a high‐resolution image is hidden in the polarization profile of the light beam, while the holographic images are encoded in its phase profile for the reconstruction of two different types of holographic images. The designed polarization profile is realized based on the superposition of two different phase profiles for circular polarization states with opposite handedness. This method can further extend the design degree of freedom in optical metasurfaces, which may open an alternative avenue for anticounterfeiting, optical image encoding, and optical information security.

Piezoelectric nanogenerators have attracted intensive interest in harvesting the stray mechanical energy in the environment to power miniaturized electronics and sensors. However, their efficient integration into systems and compatibility with existing technologies for practical applications remains challenging. Here, we report for the first time the systematic, data-driven learning of the process-property-performance relation in ZnO nanowires piezoelectric nanogenerators that are synthesized and integrated through a laser-induced chemical process. An experiment-derived behavioral model was established to reveal the apparent connections between the production parameters and the output performance of the ZnO piezoelectric nanogenerator. We further demonstrated the application of such knowledge for integrating the optimized ZnO nanowires piezoelectric nanogenerator with a photosensor into a self-powered sensor system, exhibiting the potential for future system-level improvements.

We present a dynamically tunable anomalous electromagnetic induced transparency (EIT) of a cavity-integrated metallic grating by the coupling of guided-mode resonance (GMR) with cavity-mode resonance (CMR) in the terahertz regime. The strong group slowing effect of terahertz waves results from the EIT mechanism under simultaneous excitation of GMR and CMR at a degenerate state. With the introduction of graphene as a functional layer overlying the grating structure, the enhanced group delay can be achievable beyond 6.1 ps with stable operation frequency and signal efficiency by tuning the graphene Fermi level. The work could provide an efficient scheme to manipulate the group velocity of terahertz signals.

Trigonal tellurium (Te) is a chiral semiconductor that lacks both mirror and inversion symmetries, resulting in complex band structures with Weyl crossings and unique spin textures. Detailed time-resolved polarized reflectance spectroscopy is used to investigate its band structure and carrier dynamics. The polarized transient spectra reveal optical transitions between the uppermost spin-split H4 and H5 and the degenerate H6 valence bands (VB) and the lowest degenerate H6 conduction band (CB) as well as a higher energy transition at the L-point. Surprisingly, the degeneracy of the H6 CB (a proposed Weyl node) is lifted and the spin-split VB gap is reduced upon photoexcitation before relaxing to equilibrium as the carriers decay. Using ab initio density functional theory (DFT) calculations, we conclude that the dynamic band structure is caused by a photoinduced shear strain in the Te film that breaks the screw symmetry of the crystal. The band-edge anisotropy is also reflected in the hot carrier decay rate, which is a factor of two slower along the c-axis than perpendicular to it. The majority of photoexcited carriers near the band-edge are seen to recombine within 30 ps while higher lying transitions observed near 1.2 eV appear to have substantially longer lifetimes, potentially due to contributions of intervalley processes in the recombination rate. These new findings shed light on the strong correlation between photoinduced carriers and electronic structure in anisotropic crystals, which opens a potential pathway for designing novel Te-based devices that take advantage of the topological structures as well as strong spin-related properties., The complex band structure and carrier decay response upon photoexcitation of the chiral semiconductor tellurium remain to be unveiled. Here, the authors report unusual dynamic band modifications in Te near band-edge structure due to photoinduced symmetry breaking and strong anisotropy in carrier decay dynamics.

We propose a reflective terahertz (THz) metalens with four focal points for polarization detection of THz beams. The metalens is composed of Z -shaped resonators with spatially variant orientations, a reflective gold layer, and a dielectric spacer between them. The polarization states of the focal points include left circular polarization, right circular polarization, an incident polarization state, and a polarization state whose major axis is rotated π / 4 in comparison with that of the incident polarization. The handedness, ellipticity, and major axis of the polarization state can be determined based on the light intensities of the focal points. The uniqueness of the designed device renders this technique very attractive for applications in compact THz polarization detection and information processing.