Photosensing performance of branched CdS/ZnO heterostructures as revealed by in situ TEM and photodetector tests

In Situ Growth Facilitating the Piezo-Photocatalytic Effect of Zn1-xCdxS/ZnO Nanorods for Highly Efficient H2 Production

Photocatalytic H₂ production holds promise for alleviating energy and environmental issues. The separation of photoinduced charge carriers plays vital roles in enhancing the activity of photocatalytic H₂ production. The piezoelectric effect has been proposed to be effective in facilitating the separation of charge carriers. However, the piezoelectric effect is usually restricted by the noncompact contact between the polarized materials and semiconductors. In this study, Zn_1-xCd_xS/ZnO nanorod arrays on stainless steel for piezo-photocatalytic H₂ production are fabricated by an in situ growth method, achieving an electronic-level contact between Zn_1-xCd_xS and ZnO. The separation and migration of photogenerated charge carriers in Zn_1-xCd_xS are significantly improved by the piezoelectric effect induced by ZnO under mechanical vibration. Consequently, under solar and ultrasonic irradiation, the H₂ production rate of Zn_1-xCd_xS/ZnO nanorod arrays achieves 20.96 μmol h^-1 cm^-2, which is 4 times higher than that under solar irradiation. Such a performance can be attributed to the synergies of the piezoelectric field of bent ZnO nanorods and the built-in electric field of the Zn_1-xCd_xS/ZnO heterostructure, which efficiently separate the photoinduced charge carriers. This study provides a new strategy to couple polarized materials and semiconductors for highly efficient piezo-photocatalytic H₂ production.

Light-Induced In Situ Transmission Electron Microscopy─Development, Challenges, and Perspectives

Transmission electron microscopy is a basic technique used for examining matter at the highest magnification scale available. One of its most challenging branches is in situ microscopy, in which dynamic processes are observed in real time. Among the various stimuli, like strain, temperature, and magnetic or electric fields, the light-matter interaction is rarely observed. However, in recent years, a significant increase in the interest in this technique has been observed. Therefore, I present a summary and critical review of all the in situ experiments performed with light, various technical possibilities for bringing radiation inside the transmission electron microscope, and the most important differences between the effects of light and electrons on the studied matter. Finally, I summarize the most promising directions for further research using light excitation.

High-Performance and Broadband Flexible Photodetectors Employing Multicomponent Alloyed 1D CdSxSe1-x Micro-Nanostructures

Low-cost multicomponent alloyed one-dimensional (1D) semiconductors exhibit broadband absorption from the ultraviolet to the near-infrared regime, which has attracted a great deal of interest in high-performance flexible optoelectronic devices. Here, we report the facile one-step fabrication of high-performance broadband rigid and flexible photodevices based on multicomponent alloyed 1D cadmium-sulfur-selenide (CdS_xSe_1-x) micro-nanostructures obtained via a vapor transport route. Photoresponse measurements have demonstrated their superior spectral photoresponsivity (5.8 × 10⁴ A/W), several orders of magnitude higher than the pristine CdSe nanobelt photodevice, high specific detectivity (2 × 10¹⁵ Jones), photogain (1.2 × 10⁵), external quantum efficiency (EQE, 1.4 × 10⁷%), rapid response speed (13 ms), and excellent long-term environmental stability. The multicomponent alloyed CdS_xSe_1-x nanobelt photodevice demonstrated about three times higher photocurrent as well as can operate under multiple color illuminations (200-800 nm) and at a high applied bias of 10 V with the photoresponsivity and EQE being boosted to 4.34 × 10⁵ A/W and 8.96 × 10⁷%, respectively. Furthermore, multicomponent alloyed CdS_xSe_1-x nanobelt flexible photodevices show excellent mechanical and flexural photostabilities with identical photoresponse as rigid nanodevices. The improvement mechanism found in the present research can be exploited to lead to the design of high-performance flexible photodevices comprising other multicomponent nanomaterials.

Fiber-optic sample illuminator design for the observation of light induced phenomena with transmission electron microscopy in situ: Antimicrobial photodynamic therapy

Antibacterial photodynamic therapy is a promising treatment for problematic infections caused by bacteria and fungi. Despite its undoubted effectiveness, the ultrastructural mechanism of microbial death remains not fully described and distinct organisms respond to the treatment with different efficacy. For this reason, it was decided to try imaging the process using the in situ transmission electron microscopy method. To conduct an observational experiment, the microscope was significantly modified. Liquid cell methods were used, electron doses and their influence on the sample were estimated, and a fiber-optic sample illuminator was designed and built. The modifications allowed for the light-induced characterization of photosensitizer-bacteria interaction. Microscope modification is a promising platform for further studies of light-induced phenomena in both life and material science.

In Situ Analytical Techniques for the Investigation of Material Stability and Interface Dynamics in Electrocatalytic and Photoelectrochemical Applications

Electrocatalysis and photoelectrochemistry are critical to technologies like fuel cells, electrolysis, and solar fuels. Material stability and interfacial phenomena are central to the performance and long-term viability of these technologies. Researchers need tools to uncover the fundamental processes occurring at the electrode/electrolyte interface. Numerous analytical instruments are well-developed for material characterization, but many are ex situ techniques often performed under vacuum and without applied bias. Such measurements miss dynamic phenomena in the electrolyte under operational conditions. However, innovative advancements have allowed modification of these techniques for in situ characterization in liquid environments at electrochemically relevant conditions. This review explains some of the main in situ electrochemical characterization techniques, briefly explaining the principle of operation and highlighting key work in applying the method to investigate material stability and interfacial properties for electrocatalysts and photoelectrodes. Covered methods include spectroscopy (in situ UV-vis, ambient pressure X-ray photoelectron spectroscopy (APXPS), and in situ Raman), mass spectrometry (on-line inductively coupled plasma mass spectrometry (ICP-MS) and differential electrochemical mass spectrometry (DEMS)), and microscopy (in situ transmission electron microscopy (TEM), electrochemical atomic force microscopy (EC-AFM), electrochemical scanning tunneling microscopy (EC-STM), and scanning electrochemical microscopy (SECM)). Each technique's capabilities and advantages/disadvantages are discussed and summarized for comparison.

Planar Growth, Integration, and Applications of Semiconducting Nanowires

Silicon and other inorganic semiconductor nanowires (NWs) have been extensively investigated in the last two decades for constructing high-performance nanoelectronics, sensors, and optoelectronics. For many of these applications, these tiny building blocks have to be integrated into the existing planar electronic platform, where precise location, orientation, and layout controls are indispensable. In the advent of More-than-Moore's era, there are also emerging demands for a programmable growth engineering of the geometry, composition, and line-shape of NWs on planar or out-of-plane 3D sidewall surfaces. Here, the critical technologies established for synthesis, transferring, and assembly of NWs upon planar surface are examined; then, the recent progress of in-plane growth of horizontal NWs directly upon crystalline or patterned substrates, constrained by using nanochannels, an epitaxial interface, or amorphous thin film precursors is discussed. Finally, the unique capabilities of planar growth of NWs in achieving precise guided growth control, programmable geometry, composition, and line-shape engineering are reviewed, followed by their latest device applications in building high-performance field-effect transistors, photodetectors, stretchable electronics, and 3D stacked-channel integration.

Recent Progress of In Situ Transmission Electron Microscopy for Energy Materials

In situ transmission electron microscopy (TEM) is one of the most powerful approaches for revealing physical and chemical process dynamics at atomic resolutions. The most recent developments for in situ TEM techniques are summarized; in particular, how they enable visualization of various events, measure properties, and solve problems in the field of energy by revealing detailed mechanisms at the nanoscale. Related applications include rechargeable batteries such as Li-ion, Na-ion, Li-O₂ , Na-O₂ , Li-S, etc., fuel cells, thermoelectrics, photovoltaics, and photocatalysis. To promote various applications, the methods of introducing the in situ stimuli of heating, cooling, electrical biasing, light illumination, and liquid and gas environments are discussed. The progress of recent in situ TEM in energy applications should inspire future research on new energy materials in diverse energy-related areas.

Simultaneous atomic-level visualization and high precision photocurrent measurements on photoelectric devices by in situ TEM

Herein, a novel in situ transmission electron microscopy (TEM) method that allows high-resolution imaging and spectroscopy of nanomaterials under simultaneous application of different stimuli, such as light excitation, has been reported to directly explore structure–activity relationships targeted towards device optimization. However, the experimental development of a photoelectric system capable of combining atomic-level visualization with simultaneous electrical current measurement with picoampere-precision still remains a great challenge due to light-induced drift while imaging and noise in the electrical components due to background current. Herein, we report a novel photoelectric TEM holder integrating an LED light source covering the whole visible range, a shielding system to avoid current noise, and a picoammeter, which enables stable TEM imaging at the atomic scale while measuring very small photocurrents (pico ampere range). Using this high-precision photoelectric holder, we measured photocurrents of the order of pico amperes for the first time from a prototype quantum dot solar cell assembled inside a TEM and obtained atomic-level imaging of the photo anode under light exposure. This study paves the way towards obtaining mechanistic insights into the operation of photovoltaic devices by providing direct information on the structure–activity relationships that can be used in device optimization.

A photoelectric system is capable of simultaneous atomic-level visualization and pico-ampere-precision.

High-Sensitive Ultraviolet Photodetectors Based on ZnO Nanorods/CdS Heterostructures

The ultraviolet (UV) photodetectors with ZnO nanorods (NRs)/CdS thin film heterostructures on glass substrates have been fabricated and characterized. It can be seen that the UV photoresponsivity of such a device became higher as the ZnO NR length was increased in the investigation. With an incident wavelength of 350 nm and 5 V applied bias, the responsivity of photodetectors based on ZnO NR/CdS heterostructures with the ZnO NR length at 500, 350, and 200 nm and traditional CdS film were at 12.86, 3.83, 0.91, and 0.75 A/W, respectively. The measurement results of the fabricated photodetectors based on ZnO nanorods (NRs)/CdS heterostructures have shown a significant high sensitivity in the range of UV light, which can be useful for the application of UV detection.

Optical and Optoelectronic Property Analysis of Nanomaterials inside Transmission Electron Microscope

In situ transmission electron microscopy (TEM) allows one to investigate nanostructures at high spatial resolution in response to external stimuli, such as heat, electrical current, mechanical force and light. This review exclusively focuses on the optical, optoelectronic and photocatalytic studies inside TEM. With the development of TEMs and specialized TEM holders that include in situ illumination and light collection optics, it is possible to perform optical spectroscopies and diverse optoelectronic experiments inside TEM with simultaneous high resolution imaging of nanostructures. Optical TEM holders combining the capability of a scanning tunneling microscopy probe have enabled nanomaterial bending/stretching and electrical measurements in tandem with illumination. Hence, deep insights into the optoelectronic property versus true structure and its dynamics could be established at the nanometer-range precision thus evaluating the suitability of a nanostructure for advanced light driven technologies. This report highlights systems for in situ illumination of TEM samples and recent research work based on the relevant methods, including nanomaterial cathodoluminescence, photoluminescence, photocatalysis, photodeposition, photoconductivity and piezophototronics.

The light-induced pyro-phototronic effect improving a ZnO/NiO/Si heterojunction photodetector for selectively detecting ultraviolet or visible illumination

In this work, we fabricate a ZnO/NiO/Si heterojunction photodetector, which successfully realizes selective detection of UV or visible light in a single photodetector. In addition, the pyro-phototronic effect has been confirmed as an effective approach for enhancing the performance of photodetectors based on ZnO. The pyro-phototronic effect is carefully studied by comparing with a ZnO/Si heterojunction photodetector under UV and visible illumination at different bias voltages. To enhance the photodetector's wavelength selectivity, the parameters and structure are further optimized for the ZnO/NiO/SiO₂/Si heterojunction photodetector. The newly designed photodetector may be used to selectively detect and distinguish UV and visible illumination in a single photodetector.

Flexible Photodetectors Based on Novel Functional Materials

Flexible photodetectors have attracted a great deal of research interest in recent years due to their great possibilities for application in a variety of emerging areas such as flexible, stretchable, implantable, portable, wearable and printed electronics and optoelectronics. Novel functional materials, including materials with zero-dimensional (0D) and one-dimensional (1D) inorganic nanostructures, two-dimensional (2D) layered materials, organic semiconductors and perovskite materials, exhibit appealing electrical and optoelectrical properties, as well as outstanding mechanical flexibility, and have been widely studied as building blocks in cost-effective flexible photodetection. Here, we comprehensively review the outstanding performance of flexible photodetectors made from these novel functional materials reported in recent years. The photoresponse characteristics and flexibility of the devices will be discussed systematically. Summaries and challenges are provided to guide future directions of this vital research field.

Improved Li+ Storage through Homogeneous N-Doping within Highly Branched Tubular Graphitic Foam

A novel carbon structure, highly branched homogeneous-N-doped graphitic (BNG) tubular foam, is designed via a novel N, N-dimethylformamide (DMF)-mediated chemical vapor deposition method. More structural defects are found at the branched portions as compared with the flat tube domains providing abundant active sites and spacious reservoirs for Li⁺ storage. An individual BNG branch nanobattery is constructed and tested using in situ transmission electron microscopy and the lithiation process is directly visualized in real time.

High performance, self-powered ultraviolet photodetector based on a ZnO nanoarrays/GaN structure with a CdS insert layer

A self-powered photodetector based on a ZnO nanoarrays/CdS/GaN structure with a responsivity as high as 176 mA W⁻¹ at 300 nm.

3D-branched hierarchical 3C-SiC/ZnO heterostructures for high-performance photodetectors

The ultra-sensitive photodetection of different wavelengths holds promising applications in high-performance optoelectronic devices and it requires an efficient and suitable semiconductor unit. Herein, we demonstrated the designable synthesis of 3D-branched hierarchical 3C-SiC/ZnO heterostructures by a three-step process and their assembling into an ultrasensitive photodetector. Microstructure analyses using high-resolution transmission electron microscopy reveal that the hierarchical 3C-SiC/ZnO heterostructure is composed of single-crystal 3C-SiC nanowires as a central stem and numerous well-aligned single-crystalline ZnO nanorods as branch shells. Optoelectronic tests on the 3C-SiC/ZnO heterostructure photodetector verify the outstanding photo-detection performance with an ultrahigh EQE (1.69 × 10⁸%), a superior photoresponsivity (4.8 × 10⁵ A W^-1), a very fast response time (a rise time of 40 ms and a decay time of 60 ms), a high photo-dark current ratio of 187.8 and an excellent photocurrent stability and reproducibility, which is significantly advantageous or comparable to those of ZnO and other inorganic semiconductor nanostructure based photodetectors. To understand the excellent photodetection of hierarchical 3C-SiC/ZnO heterostructures, a band-gap energy diagram describing the photogenerated electron transport process is plotted and the corresponding mechanism is discussed. The strategy proposed in the present work will open up more opportunities for the design and boost of ultra-sensitive photodetectors based on semiconductor heterostructures.

Facile fabrication and application of SnO2–ZnO nanocomposites: insight into chain-like frameworks, heterojunctions and quantum dots

We report that a novel SnO₂–ZnO chain-like heterojunction framework embedded with SnO₂ and ZnO quantum-dots shows highly efficient photocatalytic performance.

In situ fabrication of highly crystalline CdS decorated Bi2S3 nanowires (nano-heterostructure) for visible light photocatalyst application

Herein, we have demonstrated the in situ synthesis of the orthorhombic Bi₂S₃ nanowires decorated with hexagonal CdS nanoparticles by facile solvothermal method. The heterostructures have been used as photocatalyst for solar hydrogen production.

Fabrication and characterization of hierarchical multipod silver citrate complex microcrystals with excellent SERS properties

Novel hierarchical multipods silver citrate complexes (SCC) microcrystals have been successfully synthesized in aqueous solution for the first time. The multipods SCC microcrystals showed excellent performance and long-term-stability as SERS substrate.

In situ fabrication and investigation of nanostructures and nanodevices with a microscope

The widespread availability of nanostructures and nanodevices has placed strict requirements on their comprehensive characterization.

Giant UV-sensitivity of ion beam irradiated nanocrystalline CdS thin films

A highly sensitive UV-detector is devised for the first time from ion beam irradiated nanocrystalline CdS thin films. The sensor exhibits improvements in the responsivity, photosensitivity, and efficiency as a function of ion fluence.

Interface engineering: broadband light and low temperature gas detection abilities using a nano-heterojunction device

Herein, we have designed a nano-heterojunction device using interface defects and band bending effects, which can have broadband light detection (from 365-940 nm) and low operating temperature (50 °C) gas detection abilities. The broadband light detection mechanism occurs because of the defects and band bending between the heterojunction interface. We have demonstrated this mechanism using CoSi2/SnO2, CoSi2/TiO2, Ge/SnO2 and Ge/TiO2 nano-heterojunction devices, and all these devices show broadband light detection ability. Furthermore, the nano-heterojunction of the nano-device has a local Joule-heating effect. For gas detection, the results show that the nano-heterojunction device presents a high detection ability. The reset time and sensitivity of the nano-heterojunction device are an order faster and larger than Schottky-contacted devices (previous works), which is due to the local Joule-heating effect between the interface of the nano-heterojunction. Based on the abovementioned idea, we can design diverse nano-devices for widespread use.

Hierarchical CdS Nanowires Based Rigid and Flexible Photodetectors with Ultrahigh Sensitivity

Hierarchical CdS nanowires were synthesized via a facile vapor transport method, which were used to fabricate both rigid and flexible visible-light photodetectors. Studies found that the rigid photodetectors on SiO2/Si substrate showed ultrahigh photo-dark current ratio up to 1.96 × 10(4), several orders of magnitude higher than previously reported CdS nanostructures, as well as high specific detectivity (4.27 × 10(12) Jones), fast response speed and excellent environmental stability. Highly flexible photodetectors were also fabricated on polyimide substrate, which exhibited comparable photoresponse performance as the rigid one. In addition, the as-prepared flexible devices displayed excellent mechanical flexibility, electrical stability and folding endurance. The results indicate that the hierarchical CdS nanowires may be good candidates for nanoscale optoelectronic devices such as high-efficiency photoswitches and highly photosensitive detectors.

Hierarchical Assembly of SnO2/ZnO Nanostructures for Enhanced Photocatalytic Performance

SnO₂/ZnO hierarchical heterostructures have been successfully synthesized by combining electrospinning technique and hydrothermal method. Various morphologies of the secondary ZnO nanostructures including nanorods (NRs) and nanosheets (NSs) can be tailored by adding surfactants. Photocatalytic performance of the heterostructures was investigated and obvious enhancement was demonstrated in degradation of the organic pollutant, compared to the primary SnO₂-based nanofibers (NFs) and bare ZnO. Furthermore, it was found that the H₂ evolution from water splitting was achieved by photocatalysis of heterostructured nanocomposites after sulfurization treatment. This synthetic methodology described herein promises to be an effective approach for fabricating variety of nanostructures for enhanced catalytic applications. The heterostructured nanomaterials have considerable potential to address the environmental and energy issues via degradation of pollutant and generation of clean H₂ fuel.

In situ fabrication and optoelectronic analysis of axial CdS/p-Si nanowire heterojunctions in a high-resolution transmission electron microscope

A high-precision technique was utilized to construct and characterize axial nanowire heterojunctions inside a high-resolution transmission electron microscope (HRTEM). By an in-tandem technique using an ultra-sharp tungsten probe as the nanomanipulator and an optical fiber as the optical waveguide the nanoscale CdS/p-Si axial nanowire junctions were fabricated, and in situ photocurrents from them were successfully measured. Compared to a single constituting nanowire, the CdS/p-Si axial nanowire junctions possess a photocurrent saturation effect, which protects them from damage under high voltages. Furthermore, a set of experiments reveals the clear relationship between the saturation photocurrent values and the incident light intensities. The applied technique is expected to be valuable for bottom-up nanodevice fabrications, and the regarded photocurrent saturation feature may solve the Joule heating-induced failure problem in nanowire optoelectronic devices caused by a fluctuating bias.

In situ observation of the sodiation process in CuO nanowires

We observed the dynamic evolution of the morphology and phase transformations of CuO nanowires during sodiation using in situ transmission electron microscopy. These results will facilitate our fundamental understanding of the sodiation mechanism of CuO nanostructures used as electrode materials in sodium ion batteries.

Cubic lattice nanosheets: thickness-driven light emission

Silicon has a diamond-like cubic crystal lattice for which two-dimensional (2D) nanometer thickness nanosheet crystallization appears not to be trivial. However, in this issue of ACS Nano, the group led by Heon-Jin Choi demonstrates the gas-phase dendritic growth of Si nanosheets, only 1 to 13 nm thick. Moreover, such nanosheets display strong thickness-dependent photoluminescence in a visible range with red, green, and blue emission each documented.