Recent Development of Multifunctional Sensors Based on Low-Dimensional Materials.

With the demand for accurately recognizing human actions and environmental situations, multifunctional sensors are essential elements for smart applications in various emerging technologies, such as smart robots, human-machine interface, and wearable electronics. Low-dimensional materials provide fertile soil for multifunction-integrated devices. This review focuses on the multifunctional sensors for mechanical stimulus and environmental information, such as strain, pressure, light, temperature, and gas, which are fabricated from low-dimensional materials. The material characteristics, device architecture, transmission mechanisms, and sensing functions are comprehensively and systematically introduced. Besides multiple sensing functions, the integrated potential ability of supplying energy and expressing and storing information are also demonstrated. Some new process technologies and emerging research areas are highlighted. It is presented that optimization of device structures, appropriate material selection for synergy effect, and application of piezotronics and piezo-phototronics are effective approaches for constructing and improving the performance of multifunctional sensors. Finally, the current challenges and direction of future development are proposed.

Recent Advances in Flexible Tactile Sensors for Intelligent Systems.

Tactile sensors are an important medium for artificial intelligence systems to perceive their external environment. With the rapid development of smart robots, wearable devices, and human-computer interaction interfaces, flexible tactile sensing has attracted extensive attention. An overview of the recent development in high-performance tactile sensors used for smart systems is introduced. The main transduction mechanisms of flexible tactile sensors including piezoresistive, capacitive, piezoelectric, and triboelectric sensors are discussed in detail. The development status of flexible tactile sensors with high resolution, high sensitive, self-powered, and visual capabilities are focused on. Then, for intelligent systems, the wide application prospects of flexible tactile sensors in the fields of wearable electronics, intelligent robots, human-computer interaction interfaces, and implantable electronics are systematically discussed. Finally, the future prospects of flexible tactile sensors for intelligent systems are proposed.

Single-mode lasing of CsPbBr3 perovskite NWs enabled by the Vernier effect.

Inorganic lead halide perovskite (CsPbX3, X = Cl, Br, I) NWs (NWs) have been employed in lasers due to their intriguing attributes of tunable wavelength, low threshold, superior stability, and easy preparation. However, current CsPbX3 NW lasers usually work in a multi-mode modal, impeding their practical applications in optical communication due to the associated false signaling. In this work, high-performance single-mode lasing has been demonstrated by designing and fabricating coupled cavities in the high-quality single-crystal CsPbBr3 NWs via the focused ion beam (FIB) milling approach. The single-mode laser shows a threshold of 20.1 µJ cm-2 and a high quality factor of ∼2800 profiting from the Vernier effect, as demonstrated by the experiments and finite-different time-domain (FDTD) simulations. These results demonstrate the promising potentials of the CsPbX3 NW lasers in optical communication and integrated optoelectronic devices.

Piezoelectricity in Multilayer Black Phosphorus for Piezotronics and Nanogenerators.

Recently, piezoelectric characteristics have been a research focus for 2D materials because of their broad potential applications. Black phosphorus (BP) is a monoelemental 2D material predicted to be piezoelectric because of its highly directional properties and non-centrosymmetric lattice structure. However, piezoelectricity is hardly reported in monoelemental materials owing to their lack of ionic polarization, but piezoelectric generation is consistent with the non-centrosymmetric structure of BP. Theoretical calculations of phosphorene have explained the origin of piezoelectric polarization among P atoms. However, the disappearance of piezoelectricity in multilayer 2D material generally arises from the opposite orientations of adjacent atomic layers, whereas this effect is limited in BP lattices due to their spring-shaped space structure. Here, the existence of in-plane piezoelectricity is experimentally reported for multilayer BP along the armchair direction. Current-voltage measurements demonstrate a piezotronic effect in this orientation, and cyclic compression and release of BP flakes show an intrinsic current output as large as 4 pA under a compressive strain of -0.72%. The discovery of piezoelectricity in multilayer BP can lead to further understanding of this mechanism in monoelemental materials.

Strain-modulated high-quality ZnO cavity modes on different crystal orientations.

Dynamically regulated coherent light emission offers a significant impact on improving white light generation, optical communication, on-chip photonic integration, and sensing. Here, we have demonstrated two mechanisms of strain-induced dynamic regulation of ZnO lasing modes through an individual ZnO microbelt and microrod prepared by vapor-phase transport method. We systematically explained the dependence on externally applied strain and crystal orientation. Compared with the reduced size of resonant cavity played a major role in the microbelt, the resonant wavelength variation of the microrod under tensile stress is affected by the change in both the cavity size and the refractive index, which tends to antagonize in the direction of movement. It shows that the refractive index can be effectively regulated only when the stress is in the same direction along the c-axis. The results on the linear relationship between the resonance wavelength variation and applied strain imply the capacity of the devices to detect tiny stresses due to the ultra-narrow line width of the cavity mode with a high-quality factor of âˆ¼104. It not only has a positive influence in the field of the modulated coherent light source, but also provides a feasible strategy for implementing color-resolved non-contact strain sensors.

Two Photon-Pumped Whispering-Gallery Mode Lasing and Dynamic Regulation.

Realizing the dynamic regulation of nonlinear optical signals has a great scientific significance for the development of new-type nonlinear optoelectronic devices and expands its application in the field of laser technology, spectroscopy, material structure analysis, etc. Here, two photon absorption-induced whispering-gallery mode lasing from a single ZnO microresonator with a relatively low lasing threshold (15 µW) and high quality factor (Q ≈ 3200) under ambient conditions is demonstrated. Furthermore, success is achieved in obtaining the dynamic regulation of two photon-pumped lasing mode in the UV gain region. The corresponding resonant wavelength can be tuned dynamically from 388.99 and 391.12 to 390.01 and 392.12 nm for TE33 and TE32 modes, respectively. This work provides a new strategy for building high-performance mode-adjustable frequency upconversion lasers.

Crystal-Orientation-Related Dynamic Tuning of the Lasing Spectra of CdS Nanobelts by Piezoelectric Polarization.

Realizing dynamic wavelength tunability could bring about tremendous impacts in laser technology, pressure nanosensing, and lab-on-a-chip devices. Here, we demonstrate an original strategy to operate the lasing mode shift through reversible length changes of a CdS nanobelt, which is determined by the direction of piezoelectric polarization. The relationships between the direction of applied strain, the lasing mode shift, and the tunable effective refractive index are elaborated in detail. The correlation between the piezoelectric polarization-induced lasing mode red shift and the blue shift in the wavelength of the lasing mode output caused by the Poisson effect is discussed in depth, as well. Our study comprehensively considers the influence of both the cavity size variations and refractive index changes on the control of the lasing mode and provides a deeper understanding of the strain-induced lasing mode shift.

Piezoelectric Effect Tuning on ZnO Microwire Whispering-Gallery Mode Lasing.

We report a dynamic tuning on coherent light emission wavelengths of single ZnO microwire by using the piezoelectric effect. Owing to the dominant role occupied by the piezoelectric polarization effect in the wurtzite-structure ZnO microwire, the effective dielectric constant (or refraction index) of the gain media was modulated toward an increasing trend by applying a tensile strain, resulting in a shift of the strain-mediated whispering-gallery mode (WGM) lasing at room temperature. Also, the strain required to resolve the spectra in the two operating types of PL and lasing were systematically analyzed and compared. Because of the narrow line width in the lasing mode, the strain-dependent spectral resolution was improved by an order of magnitude, making it feasible for achieving high-precision, ultrasensitive, and noncontact stress sensing. Our results have an important impact on laser modulation, optical communication, and optical sensing technology.