Showing papers by "Guozhen Shen published in 2019"

Bioinspired Interlocked Structure-Induced High Deformability for Two-Dimensional Titanium Carbide (MXene)/Natural Microcapsule-Based Flexible Pressure Sensors.

Abstract: Achieving high deformability in response to minimal external stimulation while maximizing human-machine interactions is a considerable challenge for wearable and flexible electronics applications. Various natural materials or living organisms consisting of hierarchical or interlocked structures exhibit combinations of properties (e.g., natural elasticity and flexibility) that do not occur in conventional materials. The interlocked epidermal-dermal microbridges in human skin have excellent elastic moduli, which enhance and amplify received tactile signal transport. Herein, we use the sensing mechanisms inspired by human skin to develop Ti3C2/natural microcapsule biocomposite films that are robust and deformable by mimicking the micro/nanoscale structure of human skin-such as the hierarchy, interlocking, and patterning. The interlocked hierarchical structures can be used to create biocomposite films with excellent elastic moduli (0.73 MPa), capable of high deformability in response to various external stimuli, as verified by employing theoretical studies. The flexible sensor with a hierarchical and interlocked structure (24.63 kPa-1) achieves a 9.4-fold increase in pressure sensitivity compared to that of the planar structured Ti3C2-based flexible sensor (2.61 kPa-1). This device also exhibits a rapid response rate (14 ms) and good cycling reproducibility and stability (5000 times). In addition, the flexible pressure device can be used to detect and discriminate signals ranging from finger motion and human pulses to voice recognition.

Grain-Boundary-Induced Drastic Sensing Performance Enhancement of Polycrystalline-Microwire Printed Gas Sensors

Abstract: The development of materials with high efficiency and stable signal output in a bent state is important for flexible electronics. Grain boundaries provide lasting inspiration and a promising avenue for designing advanced functionalities using nanomaterials. Combining bulk defects in polycrystalline materials is shown to result in rich new electronic structures, catalytic activities, and mechanical properties for many applications. However, direct evidence that grain boundaries can create new physicochemical properties in flexible electronics is lacking. Here, a combination of bulk electrosensitive measurements, density functional theory calculations, and atomic force microscopy technology with quantitative nanomechanical mapping is used to show that grain boundaries in polycrystalline wires are more active and mechanically stable than single-crystalline wires for real-time detection of chemical analytes. The existence of a grain boundary improves the electronic and mechanical properties, which activate and stabilize materials, and allow new opportunities to design highly sensitive, flexible chemical sensors.

Flexible Smart Noncontact Control Systems with Ultrasensitive Humidity Sensors.

Abstract: The development of noncontact humidity sensors with high sensitivity, rapid response, and a facile fabrication process is urgently desired for advanced noncontact human-machine interaction (HMI) applications. Here, a flexible and transparent humidity sensor based on MoO3 nanosheets is developed with a low-cost and easily manufactured process. The designed humidity sensor exhibits ultrahigh sensitivity, fast response, great stability, and high selectivity, exceeding the state-of-the-art humidity sensors. Furthermore, a wearable moisture analysis system is assembled for real-time monitoring of ambient humidity and human breathing states. Benefiting from the sensitive and rapid response to fingertip humidity, the sensors are successfully applied to both a smart noncontact multistage switch and a novel flexible transparent noncontact screen for smart mobile devices, demonstrating the potential of the MoO3 nanosheets-based humidity sensors in future HMI systems.

Bio-Multifunctional Smart Wearable Sensors for Medical Devices

Abstract: Advances in digital health care have driven innovations in high‐performance wearable and smart sensors. One requirement in this field is establishing healthy, secure, and reliable medical devices for precisely monitoring vital signs of the human body or the surrounding environment through flexible sensors with not only high‐sensing performance but also excellent biofunctionality. Smart wearable sensors with excellent biofunctionality furnish medical devices with various smart functions such as biocompatibility, biodegradability, and self‐healing, which have attracted widespread interest from device engineers and materials scientists. Herein, a comprehensive review of the latest progress concerning these smart wearable sensors is presented with a focus on bio‐multifunctional (biocompatible, biodegradable, and self‐healing) device designs. The medical applications of bio‐multifunctional smart wearable sensors are also briefly covered, and to conclude, a discussion of the challenges, opportunities, and future perspectives is provided.

Electronic structure and exciton shifts in Sb-doped MoS 2 monolayer

Abstract: The effective manipulation of excitons is important for the realization of exciton-based devices and circuits, and doping is considered a good strategy to achieve this. While studies have shown that 2D semiconductors are ideal for excitonic devices, preparation of homogenous substitutional foreign-atom-doped 2D crystals is still difficult. Here we report the preparation of homogenous monolayer Sb-doped MoS2 single crystals via a facile chemical vapor deposition method. A and B excitons are observed in the Sb-doped MoS2 monolayer by reflection magnetic circular dichroism spectrum measurements. More important, compared with monolayer MoS2, the peak positions of two excitons show obvious shifts. Meanwhile, the degeneration of A exciton is also observed in the monolayer Sb-doped MoS2 crystal using photoluminescence spectroscopy, which is ascribed to the impurity energy levels within the band-gap, confirmed by density function theory. Our study opens a door to developing the doping of 2D layered transition metal dichalcogenides with group-V dopants, which is helpful for the fundamental study of the physical and chemical properties of transition metal dichalcogenides. Excitons in two-dimensional transition metal dichalcogenides can be tuned by incorporation of group-V dopants. A team led by Zhongming Wei at the Institute of Semiconductors, Chinese Academy of Sciences and Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences used chemical vapour deposition to synthesize Sb-doped MoS2 monolayers, and investigated the spectral position of the A and B excitons. Microscopy and spectroscopy results indicated that Sb doping is substitutional and homogeneous, and stably replaces Mo atoms in the MoS2 lattice, forming Mo0.91Sb0.09S2 crystals. A combination of reflection magnetic circular dichroism spectroscopy and photoluminescence spectroscopy revealed that the A and B excitons exhibit a clear shift if compared to pristine MoS2. Density functional theory calculations showed that Sb doping gives rise to formation of impurity-related energy level within the band-gap of stoichiometrically pure MoS2.

MoS2-OH Bilayer-Mediated Growth of Inch-Sized Monolayer MoS2 on Arbitrary Substrates.

Abstract: Due to remarkable electronic property, optical transparency, and mechanical flexibility, monolayer molybdenum disulfide (MoS2) has been demonstrated to be promising for electronic and optoelectroni...

Characterization of atomic defects on the photoluminescence in two???dimensional materials using transmission electron microscope

Abstract: Two‐dimensional material (2D) that possesses atomic thin geometry and remarkable properties is a star material for the fundamental researches and advanced applications. Defects in 2D materials are critical and fundamental to understand the chemical, physical, and optical properties. Photoluminescence arises in 2D materials owing to various physical phenomena including activator/dopant‐induced luminescence and defect‐related emissions, and so forth. With the advanced transmission electron microscopy (TEM) technologies, such as aberration correction and low voltage technologies, the morphology, chemical compositions and electronic structures of defects in 2D material could be directly characterized at the atomic scale. In this review, we introduce the applications of state‐of‐the‐art TEM technologies on the studies of the role of atomic defects in the photoluminescence characteristics in 2D material. The challenges in spatial and time resolution are also discussed. It is proved that TEM is a powerful tool to pinpoint the relationship between the defects and the photoluminescence characteristics.

Skin Adhesives with Controlled Adhesion by Polymer Chain Mobility.

Abstract: Wearable devices have attracted a lot of attention because of their importance in the biomedical and electronic fields. However, as one of the important fixing materials, skin adhesives with controlled adhesion are often ignored. Although remarkable progress has been achieved in revealing the natural adhesion mechanism and biomimetic materials to complex solid surfaces, it remains a great challenge to explore nonirritant, controlled skin adhesives without surface structure. Herein, we present skin-adhesive patches of polydimethylsiloxanes (SAPs) with controlled adhesion by simply modulating polymer chain mobility at the molecular level. The controlled adhesion of SAPs strongly depends on the proportion of polymer chains with different mobility exposed to the solid surface, including free chains, dangling chains, and cross-linking chains. As a proof of concept, we demonstrate that the SAP can act as a skin-friendly fix to monitor the human pulse by integrating with the poly(vinylidene fluoride-trifluorethylene)/reduced graphene oxide (P(VDF-TrFE)@rGO) nanofiber sensor. This study provides a clue to design durable and skin-friendly adhesives with controlled adhesion for wearable devices.

Stretchable SnO 2 -CdS interlaced-nanowire film ultraviolet photodetectors

Abstract: Stretchable ultraviolet photodetectors with fast response have wide applications in wearable electronics and implantable biomedical devices. However, most of the conventional binary oxide nanowires based photodetectors exhibit slow response due to the presence of a large number of surface defects related to trapping centers. Herein, with interlaced SnO2-CdS nanowire films as the sensing materials, we fabricated stretchable ultraviolet photodetectors with significantly improved response speed via a multiple lithographic filtration method. Systematic investigations reveal that the interlaced-nanowire based photodetectors have lower dark current and much higher response speed (more than 100 times) compared with pure SnO2 nanowire based photodetectors. The relevant carrier generation and transport mechanism were also discussed. In addition, due to the formation of waved wrinkles on the surface of the nanowires/PDMS layer during the prestretching cycles, the SnO2-CdS interlaced nanowire photodetectors display excellent electrical stability and stretching cyclability within 50% strain, without obvious performance degradation even after 150 stretching cycles. As a simple and effective strategy to fabricate stretchable ultraviolet photodetectors with high response speed, the interlaced-nanowire structure can also be applied to other nanowire pairs, like ZnO-CdS interlaced-nanowires. Our method provides a versatile way to fabricate fast speed ultraviolet photodetectors by using interlaced metal oxide nanowires-CdS nanowires structures, which is potential in future stretchable and wearable optoelectronic devices.

Water-proof and thermally inert flexible pressure sensors based on zero temperature coefficient of resistance hybrid films

Abstract: The accurate measurement of pressure sensors realizes the idea of non-interference environmental monitoring, which is very important for the application of electronic skins (e-skins). Here, we designed a thermally inert flexible pressure sensor array based on a zero temperature coefficient of resistance (Z-TCR) hybrid film, consisting of multi-walled carbon nanotubes (MWCNTs) with a negative temperature coefficient (NTC) and graphite powders (GPs) with a positive temperature coefficient (PTC). In addition, to eliminate temperature interference, a water-proof effect can also be achieved by sealing the device in ethylene-vinyl acetate copolymer (EVA). The as-fabricated flexible pressure sensors exhibited excellent performances with ultrahigh sensitivity, fast response time, and good response stability at high frequency. The 5 × 5 pressure sensor array was also used for imaging spatial pressure distributions. The anti-environmental interference properties in combination with the excellent sensitivity, stability and extensibility show that the pressure sensor array has the potential for applications in e-skins.

Highly flexible self-powered photodetectors based on core–shell Sb/CdS nanowires

Abstract: Flexible photodetectors have great applications in flexible image sensors, wearable electronics and smart robots. In this study, we reported the fabrication of highly flexible self-powered photodetectors with core–shell Sb/CdS nanowires as the sensing materials. The fabricated device exhibited a high Ion/Ioff ratio of 3.54 × 103 under zero bias, fast speed of photoresponse and great stability. An open circuit voltage of 0.35 V was generated due to the presence of CdS and CdSb interfaces within the core–shell nanowires. Moreover, the photocurrent of the flexible device was nearly invariable at various bending angles and even after thousands of bending cycles, demonstrating its excellent flexibility and bending stability. The results indicate that the self-powered photodetectors are promising candidates for future passive optoelectronic devices.

Motion recognition by a liquid filled tubular triboelectric nanogenerator

Abstract: The development of flexible electronics has extended the limit of the intelligent devices, which are highly sensitive, soft and capable of sustaining arbitrary deformation. Here, we report a liquid-polymer tubular triboelectric nanogenerator (L-P TENG) that is filled with liquid for a shape-adaptive sensor in various working modes. The L-P TENG is based on liquid–solid contact electrification with the use of displacement current and excited by the shape change of the tubular structure. The high softness of the device makes it possible to be twisted to any curve and bear extreme strain. It can be used to detect a slight difference in pressure from touch, pressing and stretching and is suitable for a wide-range force recognition with high sensitivity. The independent and multifunctional properties of the L-P TENG extend the potential applications through combinations. Such assembled units with crossings can sense the approaching object. This study provides a new direction for flexible electromechanical sensing and has potential applications in self-powered sensors, wearable electronics, smart human–machine interaction and auxiliary motion correction.

Electrospraying preparation of metal germanate nanospheres for high-performance lithium-ion batteries and room-temperature gas sensors.

Abstract: Metal germanate nanospheres including Ca2Ge7O16, Zn2GeO4 and SrGe4O9 were successfully synthesized by a direct and large-scale electrospraying method. As anodes for lithium ion batteries, the prepared metal germanate nanospheres showed high electrochemical lithium storage performance including excellent cycling stability and high rate capacity. Especially, the Ca2Ge7O16 nanosphere anode delivered a high specific capacity of ∼ 670 mA h g−1 at 0.2 A g−1. The capacity retention was maintained around 71% even after 500 cycles. Moreover, when applied as room-temperature ammonia gas sensors, all three prepared germinate nanospheres showed significant sensing response values (6.04 for SrGe4O9, 2.73 for Zn2GeO4 and 1.70 for Ca2Ge7O16), fast response–recovery time, excellent stability and outstanding selectivity.

Magnetic and transport properties of a ferromagnetic layered semiconductor MnIn2Se4

Abstract: Magnetic two-dimensional (2D) materials hold considerable promise for the next generation of spintronic devices. In this study, a ternary 2D layered semiconductor compound, MnIn2Se4, with good photoelectric properties and low-temperature ferromagnetism is introduced. Field-effect transistors based on few-layer MnIn2Se4 exhibit n-type characteristics, with a high on-off ratio of up to 450 times, and exhibit a good photoresponse with an on-off ratio of 25 times. Magnetic measurements show that few-layer MnIn2Se4 exhibits ferromagnetic behavior with a perpendicular anisotropy at 2 K and a Curie temperature of ∼7 K. This study suggests that MnIn2Se4 has potential in applications involving magnetic and optoelectronic devices.

Resonant and Selective Excitation of Photocatalytically Active Defect Sites in TiO2.

Abstract: It has been known for several decades that defects are largely responsible for the catalytically active sites on metal and semiconductor surfaces. However, it is difficult to directly probe these active sites because the defects associated with them are often relatively rare with respect to the stoichiometric crystalline surface. In the work presented here, we demonstrate a method to selectively probe defect-mediated photocatalysis through differential alternating current (ac) photocurrent (PC) measurements. In this approach, electrons are photoexcited from the valence band to a relatively narrow distribution of subband gap states in TiO2 and then subsequently to the ions in solution. Because of their limited number, these defect states fill up quickly, resulting in Pauli blocking, and are thereby undetectable under direct current or continuous wave excitation. In the method demonstrated here, the incident light is modulated with an optical chopper, whereas the PC is measured with a lock-in amplifier. Thin (5 nm) films of TiO2 deposited by atomic layer deposition on various metal films, including Au, Cu, and Al, exhibit the same wavelength-dependent PC spectra, with a broad peak centered around 2.0 eV corresponding to the band-to-defect transition associated with the hydrogen evolution reaction (HER). While the UV-vis absorption spectra of these films show no features at 2.0 eV, photoluminescence (PL) spectra of these photoelectrodes show a similar wavelength dependence with a peak of around 2.0 eV, corresponding to the subband gap emission associated with these defect sites. As a control, alumina (Al2O3) films exhibit no PL or PC over the visible wavelength range. The ac PC plotted as a function of electrode potential shows a peak of around -0.4 to -0.1 V versus normal hydrogen electrode, as the monoenergetic defect states are tuned through a resonance with the HER potential. This approach enables the direct photoexcitation of catalytically active defect sites to be studied selectively without the interference of the continuum interband transitions or the effects of Pauli blocking, which is limited by the slow turnover time of the catalytically active sites, typically on the order of 1 μs. We believe that this general approach provides an important new way to study the role of defects in catalysis in an area where selective spectroscopic studies of these are few.