Intravascular ultrasound (IVUS) is a burgeoning imaging technology that provides vital information for the diagnosis of coronary arterial diseases. A significant constituent that enables the IVUS system to attain high-resolution images is the ultrasound transducer, which acts as both a transmitter that sends acoustic waves and a detector that receives the returning signals. Being the most mature form of ultrasound transducer available in the market, piezoelectric transducers have dominated the field of biomedical imaging. However, there are some drawbacks associated with using the traditional piezoelectric ultrasound transducers such as difficulties in the fabrication of high-density arrays, which would aid in the acceleration of the imaging speed and alleviate motion artifact. The advent of microelectromechanical system (MEMS) technology has brought about the development of micromachined ultrasound transducers that would help to address this issue. Apart from the advantage of being able to be fabricated into arrays with lesser complications, the image quality of IVUS can be further enhanced with the easy integration of micromachined ultrasound transducers with complementary metal-oxide-semiconductor (CMOS). This would aid in the mitigation of parasitic capacitance, thereby improving the signal-to-noise. Currently, there are two commonly investigated micromachined ultrasound transducers, piezoelectric micromachined ultrasound transducers (PMUTs) and capacitive micromachined ultrasound transducers (CMUTs). Currently, PMUTs face a significant challenge where the fabricated PMUTs do not function as per their design. Thus, CMUTs with different array configurations have been developed for IVUS. In this paper, the different ultrasound transducers, including conventional-piezoelectric transducers, PMUTs and CMUTs, are reviewed, and a summary of the recent progress of CMUTs for IVUS is presented.

Due to the practical demand in many fields, room-temperature photodetectors in mid/long-wavelength and terahertz ranges have attracted much attention. Photothermoelectric (PTE) detectors based on photothermal conversion and thermoelectric effect can realize ultra-broadband detection of a photon without external bias. In recent years, two-dimensional (2D) materials open up revolutionary opportunities in rapid and sensitive photodetection by virtue of their remarkable electronic and optical properties. Here, we provide a brief review of state-of-the-art photodetectors based on PTE effect and 2D materials. It is worth noting that emerging PTE detectors based on 2D materials, including graphene, transition metal dichalcogenides (TMDCs), black phosphorus (BP) and MXenes, are proposed systematically. Next, we will discuss the existing challenges and prospects in PTE detectors, followed by a conclusion of this review.

The development of photothermoelectric (PTE) detectors has drawn much attention and is a hot research topic because of their superiority of converting broadband terahertz to mid-infrared radiation energy without any bias support in ambient conditions. However, earlier work in PTE detectors is complicated by a sophisticated fabrication process, uncontrollable formation, poor flexibility, and intractable material instability. Herein, we apply doped polyaniline (PANI)/graphene composites on poly(ethylene terephthalate) substrates to propose a new type of PTE detectors. The facile fabrication process, involving tip sonification and magnetic stirring, enhances the dispersion of homogeneously aqueous graphene. Besides the uniformity of the composite, the bias-free photodetector exhibits its highly sensitive responsivity by tuning the graphene concentration, achieving a peak detectivity of 6.8 × 10 7 cm Hz 1/2 W −1 and responsivity of 2.5 V W −1 . In addition, various bending radii (−1.5 to 1.5 cm) and more than 300 multiple bending cycles demonstrate remarkable flexibility of the doped-PANI/graphene composite. We further simulate human interactions by setting fingers 3−5 mm away from detectors and moving fingertips along the perpendicular direction toward the detector in multiple attempts to exhibit a rapid, high-performance photovoltage response of 10 μV. Overall, the striking doped-PANI/graphene composite PTE detectors manifest satisfactory broadband detectivity and provide insights into abundant applications in nondestructive health monitors, future optical detectors, and wearable Internet of Things (IoT) devices.

In the 21st century, photovoltaic (PV) is an emerging renewable energy source. According to its low production cost, organic solar cells (OSCs) exhibit huge potential in the commercialization market. Low-dimensional carbon nanomaterials with superb electronic, optical, mechanical properties have been proposed to serve as different functions in organic solar cells. In this paper, we systematically summarize the progress of carbon nanotube (CNT)- and graphene-based OSCs, including the photoactive, electrode and interfacial layers. It concludes that CNTs and graphene can play a crucial role in OSCs. Also, this review provides a summary and outlook on improving the performance of OSCs. At present, the device is in the direction of the hybrid system, high power conversion efficiency (PCE) and long lifetimes.

Based on the problem that the intensity of excitation source is not easy to regulate by the traditional active control method, this paper presents an accurate temperature control system based on micro-hotplate for the first time. This system realizes the active control of terahertz metamaterial functional devices, and implements various functions by using the proposed accurate temperature control process. The temperature control characteristics of micro-hotplate are introduced into the design of terahertz functional devices by taking a vanadium dioxide (VO₂ ) metamaterial absorber as an example. In this design, a silicon-based micro-hotplate is used to heat the metamaterial absorber. According to the phase transition characteristics of VO₂ , the alteration of temperature leads to conductivity change, so as to realize the active control of the absorber. At the same time, this paper also analyzes the heating and cooling time of the micro-hotplate. The simulation results show that, by using the micro-hotplate to heat the metamaterial functional devices, the temperature adjustment speed is reasonably high and the controllable performance is excellent. The test results shows that the surface temperature can be controlled between 40 ℃ and 80 ℃ , and the temperature difference of the working area can be kept within 1℃ . The temperature control of the micro-hotplate is accurately controlled, resulting in the great performance of the metamaterial functional devices.

Self-powered, flexible, and uncooled mid-wavelength infrared (MWIR) detectors based on the photothermoelectric (PTE) mechanism are promising for the next-generation wearable Internet of Things (IoT).

Photothermoelectric (PTE) detectors functioning on the infrared spectrum show much potential for use in many fields, such as energy harvesting, nondestructive monitoring, and imaging fields. Recent advances in low-dimensional and semiconductor materials research have facilitated new opportunities for PTE detectors to be applied in material and structural design. However, these materials applied in PTE detectors face some challenges, such as unstable properties, high infrared reflection, and miniaturization issues. Herein, we report our fabrication of scalable bias-free PTE detectors based on Ti3C2 and poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) composites and characterization of their composite morphology and broadband photoresponse. We also discuss various PTE engineering strategies, including substrate choices, electrode types, deposition methods, and vacuum conditions. Furthermore, we simulate metamaterials using different materials and hole sizes and fabricated a gold metamaterial with a bottom-up configuration by simultaneously combining MXene and polymer, which achieved an infrared photoresponse enhancement. Finally, we demonstrate a fingertip gesture response using the metamaterial-integrated PTE detector. This research proposes numerous implications of MXene and its related composites for wearable devices and Internet of Things (IoT) applications, such as the continuous biomedical tracking of human health conditions.

In this paper, we propose a packaged PTE detector utilizing a CNTF active layer with MXene electrodes, finally demonstrating the non-destructive imaging capacities.