Golay-Encoded Ultrasound Monitoring of Simultaneous High-Intensity Focused Ultrasound Treatment: A Phantom Study.

Ultrasound (US) imaging has high potential in monitoring high-intensity focused US (HIFU) treatment due to its superior temporal resolution. However, US monitoring is often hindered by strong HIFU interference, which overwhelms the echoes received by the imaging array. In this study, a method of Golay-encoded US monitoring is proposed to visualize the imaged object for simultaneous HIFU treatment. It effectively removes HIFU interference patterns in real-time B-mode imaging and improves the metrics of image quality, such as peak signal-to-noise ratio (PSNR), structural similarity (SSIM), and contrast ratio (CR). Compared to the pulse-inversion sequence, the N -bit Golay sequence can boost the echo magnitude of US monitoring by another N times and, thus, exhibits higher robustness. Simulations show that a sinusoidal HIFU waveform can be fully eliminated using Golay decoding when the bit duration of the N -bit Golay sequence ( N is the power of 4) coincides with either odd (Case I) or even (Case II) integer multiples of the HIFU quarter period. Experimental results also show that the Golay decoding with Case II can increase the PSNR of US monitoring images by more than 30 dB for both pulse- and continuous-wave HIFU transmissions. The SSIM index also effectively improves to about unity, indicating that the B-mode image with HIFU transmission is visually indistinguishable from that acquired without HIFU transmission. Though Case I is inferior to Case II in the elimination of even-order HIFU harmonic, they together enable a more flexible selection of imaging frequencies to meet the required image resolution and penetration for Golay-encoded US monitoring.

Ultra-Hard (41 GPa) Isotopic Pure 10BP Semiconductor Microwires for Flexible Photodetection and Pressure Sensing.

An urgent demand for electronic and optoelectronic devices able to work in extreme environments promotes a series of research studies on semiconductor materials. Cubic boron phosphide (BP) as a semiconductor material with excellent characteristics shows great application potential. However, since the synthesis conditions required are difficult to achieve and the growth mechanism of BP is still unclear, there are few reports on the basic properties of BP and pure isotope BP, resulting in a narrow understanding of their special physical properties. Here, we successfully obtained highly pure isotopic 10BP crystals by a vapor-liquid-solid (VLS) method unconventionally designed, which successfully overcomes the thermodynamic conflict between the high melting point of the boron element and low sublimation temperature of the phosphorus element. The 10BP achieved owns an aspect ratio as high as 104 and a hardness up to 41 GPa. Besides, as an indirect bandgap semiconductor, it has ultrawide red emission spectra, a p-type conductivity with extremely low resistivity, and excellent photoelectronic and piezoelectric characteristics. Furthermore, compared with other superhard semiconductors like cubic BN and diamond, 10BP has an obvious advantage of lower growth temperature (1200 °C). All these characteristics confirm the prospects owned by 10BP in its applications to the field of high-conductivity, optoelectronic, strain-sensing, and superhard semiconductors.

High-Efficiency Down-Conversion Radiation Fluorescence and Ultrafast Photoluminescence (1.2 ns) at the Interface of Hybrid Cs4PbBr6-CsI Nanocrystals.

The research of fast scintillators in positron emission tomography and other applications based on time-of-flight technology promotes the development of radiation detection. However, because of the current lack of efficient and fast carrier radiation recombination pathways, the research on scintillator radioluminescence (RL) still faces severe challenges. Here, we propose an effective interface carrier transport mechanism: CsI:Na crystal and Cs4PbBr6 nanocrystals (NCs) interface to form a new phase and a continuous heterostructure, providing an effective channel for X-ray excited carrier transfer to Cs4PbBr6. Then, the excited carriers realize efficient recombination luminescence through the self-trapped excitons inside Cs4PbBr6. On the basis of this mechanism, the heterostructure composite scintillator composed of CsI:Na/Cs4PbBr6 exhibits high-efficiency radiant fluorescence and an ultrafast photoluminescence (PL) decay time of 1.22 ns. The effective interface carrier transport shown in this work provides an optimization idea that can be used for reference in the research of fast scintillators.

X-ray radiation excited ultralong (>20,000 seconds) intrinsic phosphorescence in aluminum nitride single-crystal scintillators.

Phosphorescence is a fascinating photoelectronic phenomenon usually observed in rare-earth-doped inorganic crystals and organic molecular crystals, owning great potential in optical information storage, color display and biological dosimetry. Here, we present an ultralong intrinsic phosphorescence (>20,000 seconds) in AlN single-crystal scintillator through X-ray excitation. We suggest that the long afterglow emission originates from the intra-band transition related to native nitrogen vacancy. Some excited states formed by absorbing X-ray photons cannot satisfy the parity difference between initial and final states required by transition selection rule, so they cannot return to the ground state directly through radiation transitions but through several phonon-assisted intra-band transitions slowly. During this process, a long-term broad-spectra phosphorescence emission is formed. Investigating the X-ray excited phosphorescence emission in the AlN is of great significance to understanding the mechanism of phosphorescence in inorganic materials, and to realizing the practical applications in high-energy ray dosimetry.

Ultra-high Photovoltage (2.45 V) Forming in Graphene Heterojunction via Quasi-Fermi Level Splitting Enhanced Effect.

Owing to the fast response speed and low energy consumption, photovoltaic vacuum-ultraviolet (VUV) photodetectors show prominent advantages in the field of space science, high-energy physics, and electronics industry. For photovoltaic devices, it is imperative to boost their open-circuit voltage, which is the most direct indicator to measure the photoelectric conversion capability. In this report, a quasi-Fermi level splitting enhanced effect under illumination, benefiting from the variable Fermi level of graphene, is proposed to significantly increase the potential difference up to 2.45 V between the two ends of p-Gr/i-AlN/n-SiC heterojunction photovoltaic device. In addition, the highest external quantum efficiency of 56.1% (under the VUV irradiation of 172 nm) at 0 V bias and the ultra-fast photoresponse of 45 ns further demonstrate the superiority of high-open-circuit-voltage devices. The proposed device design strategy and the adopted effect provide a referential way for the construction of various photovoltaic devices.

Ultrawide Band Gap Oxide Nanodots (Eg > 4.8 eV) for a High-Performance Deep Ultraviolet Photovoltaic Detector.

Recently, deep ultraviolet (DUV) detectors based on gallium oxide (Ga2O3) have become promising in industrial and aerospace applications because of their inherently ultrawide band gaps (4.5-4.9 eV). Because most of them are difficult to be prepared, the lattice mismatch with the substrate and the expensive cost have to be taken into consideration. Because of such problems, the solution-processible nanodots (NDs) with ultrasmall size provide a solution. Here, we propose to use γ-Ga2O3 NDs as the DUV-sensitive layer to construct a DUV p-i-n-type detector with photovoltaic properties (p-graphene/γ-Ga2O3 NDs/n-SiC). The device exhibits a high photoresponsivity (5.8 mA/W) and detectivity (7.6 × 1010 jones) with a 250 nm source illumination under 0 V bias. Moreover, the DUV/UV injection ratio (R250/R360) reaches 103. These results demonstrate a new way to manufacture low-cost, high-performance DUV detectors based on γ-Ga2O3.

Near vacuum-ultraviolet aperiodic oscillation emission of AlN films.

An accurate measurement of the refractive index is necessary for the optical design of both deep ultraviolet laser diodes and light-emitting diodes. Generally, the refractive indices along different crystallographic axes of anisotropic thin films are measured using variable angle spectroscopic ellipsometry. However, there are still some limitations concerning this method. Here we proposed a potential method to measure the band edge refractive index of wide bandgap semiconductor. An aperiodic oscillation emission phenomenon due to the Fabry-Perot effect was observed in the fluorescence spectrum of an AlN film with a thickness of 1500 nm. Based on the characteristics of the fluorescence spectrum and the definition of Fabry-Perot effect, we obtained the ordinary refractive index of the AlN thin film near the band edge directly. This refractive index measurement method is a supplement to the variable angle ellipsometry, and it is a more direct and effective method for transferred film and thinner samples to measure the fluorescence spectrum.

Ultra-Robust Deep-UV Photovoltaic Detector Based on Graphene/(AlGa)2O3/GaN with High-Performance in Temperature Fluctuations.

A strategy of adopting Ga2O3 alloyed with Al element to reduce the oxygen vacancy defect density and enhance the interface barrier height of Ga2O3 heterojunction is proposed to fabricate deep-UV photovoltaic detectors with high thermal stability, high photoresponsivity, and fast response speed. Here, a graphene/(AlGa)2O3/GaN device with a photoresponsivity of ∼20 mA/W, a rise time of ∼2 µs, and a decay time of ∼10 ms is presented at 0 V bias. At the working temperature of 453 K, the device still exhibits a photo-to-dark current ratio (PDCR) of ∼1.8 × 103, which is 1-2 orders of magnitude higher than that of the reported high-temperature deep-UV film detectors. By comparing the formation energy of oxygen vacancy defects and the interface barrier height of the heterojunction at different temperatures in graphene/Ga2O3/GaN and graphene/(AlGa)2O3/GaN systems, the strategy of synthesizing (AlGa)2O3 ternary composite alloy is proved to be reliable for fabricating high-performance deep-UV photovoltaic detectors. The method proposed in this paper can provide reference for the preparation of deep-UV photovoltaic detectors with high photoresponsivity and thermal stability in the future.

All-Inorganic CsCu2 I3 Single Crystal with High-PLQY (≈15.7%) Intrinsic White-Light Emission via Strongly Localized 1D Excitonic Recombination.

Energy-saving white lighting from the efficient intrinsic emission of semiconductors is considered as a next-generation lighting source. Currently, white-light emission can be composited with a blue light-emitting diode and yellow phosphor. However, this solution has an inevitable light loss, which makes the improvement of the energy utilization efficiency more difficult. To deal with this problem, intrinsic white-light emission (IWE) in a single solid material gives a possibility. Here, an all-inorganic lead-free CsCu2 I3 perovskite single crystal (SC) with stable and high photoluminescence quantum yield (≈15.7%) IWE through strongly localized 1D exciton recombination is synthesized. In the CsCu2 I3 , the Cu-I octahedron, which provides most of electron states, is isolated by Cs atoms in two directions to form a 1D electronic structure, resulting a high radiation recombination rate of excitons. With this electronic structure design, the CsCu2 I3 SCs have great potential in energy-saving white lighting.

Ultrafast Photovoltaic-Type Deep Ultraviolet Photodetectors Using Hybrid Zero-/Two-Dimensional Heterojunctions.

Deep ultraviolet (DUV) photodetectors have wide-range applications in satellite communications, air purification, and missile-plume detection. However, the critical barriers for the currently available wide-band gap semiconductor film-based DUV photodetectors are their low efficiency, complicated processes, and lattice mismatch with the substrate. Quantum dot (QD) devices prepared using solution-based methods can solve these problems. However, so far, there are no reports on photovoltaic-type DUV photodetectors using QDs. In this study, we propose a novel methodology to construct a hybrid zero-/two-dimensional DUV photodetector (p-type graphene/ZnS QDs/4H-SiC) with photovoltaic characteristics. The device exhibits excellent selectivity for the DUV light and has an ultrafast response speed (rise time: 28 µs and decay time: 0.75 ms), which are much better than those reported for conventional photoconductive photodetectors.

Graphene Interdigital Electrodes for Improving Sensitivity in a Ga2O3:Zn Deep-Ultraviolet Photoconductive Detector.

Graphene (Gr) has been widely used as a transparent electrode material for photodetectors because of its high conductivity and high transmittance in recent years. However, the current low-efficiency manipulation of Gr has hindered the arraying and practical use of such detectors. We invented a multistep method of accurately tailoring graphene into interdigital electrodes for fabricating a sensitive, stable deep-ultraviolet photodetector based on Zn-doped Ga2O3 films. The fabricated photodetector exhibits a series of excellent performance, including extremely low dark current (∼10-11 A), an ultrahigh photo-to-dark ratio (>105), satisfactory responsivity (1.05 A/W), and excellent selectivity for the deep-ultraviolet band, compared to those with ordinary metal electrodes. The raise of photocurrent and responsivity is attributed to the increase of incident photons through Gr and separated carriers caused by the built-in electric field formed at the interface of Gr and Ga2O3:Zn films. The proposed ideas and methods of tailoring Gr can not only improve the performance of devices but more importantly contribute to the practical development of graphene.

Amorphous-MgGaO Film Combined with Graphene for Vacuum-Ultraviolet Photovoltaic Detector.

Vacuum-ultraviolet (VUV) detector equipped on satellites has extensive application in space exploration and cosmic science. For a VUV detector, a semiconductor material with a sufficiently wide band gap is eagerly desired. In this work, a wide-band gap amorphous-MgGaO (a-MGO) film was epitaxially grown on n-type GaN substrate by atomic layer deposition and a p-i-n-type heterojunction device for VUV detection was constructed with a-MGO film as a photosensitive layer and p-type graphene as a transparent conductive layer. The device exhibits a good spectral selectivity of VUV with photovoltaic response, a high responsivity (2 mA W-1) under zero bias, and an ultrafast response speed (rise and decay time of 0.76 µs and 0.56 ms, respectively) under nanosecond VUV pulse irradiation. This newly developed device shows great potential in VUV detection for space exploration.

Vacuum-Ultraviolet Photodetection in Few-Layered h-BN.

Over the past 20 years, astro and solar physicists have been working hard to develop a new-generation semiconductor-based vacuum-ultraviolet (VUV, 100-200 nm) photodetector with small size and low power consumption, to replace the traditional microchannel detection system, which is ponderous and has high energy consumption, and finally to reduce the power load and launch costs of explorer satellites. However, this expectation has hardly been achieved due to the relatively low photoresponsivity and external quantum efficiency (EQE) of the reported VUV photoconductive detectors based on traditional wide-band-gap materials and structures. Here, on the basis of few-layer h-BN, we fabricated a high-performance two-dimensional photodetector with selective response to VUV light. Typically, it has high sensitivity (EQE = 2133%, at 20 V) to the extremely weak 160 nm light (3.25 pW). This excellent photoresponsivity can be attributed to the high carrier collection efficiency and existing surface trap states of few-layer h-BN. In addition, this device can maintain a stable performance in a wide temperature range (80-580 K), which is quite favorable for application in deep space with huge temperature fluctuation.

High-Performance Graphene/ß-Ga2O3 Heterojunction Deep-Ultraviolet Photodetector with Hot-Electron Excited Carrier Multiplication.

Solar-blind ultraviolet (SBUV) detection has important applications in wireless secure communication, early warning, and so forth. However, the desired key device for SBUV detection and high-sensitivity and low-noise "sandwich" photodetector with large detective area is difficult to be fabricated because it is usually hard for traditional wide band gap semiconductors to boast both high conductivity and high SBUV transparency. Here, we proposed to use graphene as the transparent conductive layer to form graphene/ß-Ga2O3 heterojunction. With the help of large-area graphene and hot carrier multiplication, a SBUV photodetector with large detective area, low dark current, and high sensitivity was successfully assembled. Its photoresponsivity is 1-3 orders of magnitude higher than that of the conventional SBUV photodetectors, and its response speed can rival the best device ever reported.

Vacuum Ultraviolet Photodetection in Two-Dimensional Oxides.

To lower the launch cost and prolong the lifetime of a deep space explorer, solar- and astrophysicists and photonics scientists have devoted much time and energy in exploring and developing a compact and low-power-consumption semiconductor-based vacuum ultraviolet (VUV) photodetector. However, the target has not yet been achieved due to the lack of high external quantum efficiency (EQE) VUV photoconductive materials. Here, we found that two-dimensional MgO, obtained via conformal anneal synthesis method, had ultrasensitive photoresponse to VUV light. It can identify an extremely weak VUV signal (0.85 pW), with a high EQE of 1539%. Such ultrasensitive photoresponse is attributed to the high charge-collection efficiency of excited carriers. Our results provide an idea for developing integrated VUV devices with high responsivity and low power consumption, which will prolong the service time and lower the launch cost of a space explorer.

Vacuum-Ultraviolet Photovoltaic Detector.

Over the past two decades, solar- and astrophysicists and material scientists have been researching and developing new-generation semiconductor-based vacuum ultraviolet (VUV) detectors with low power consumption and small size for replacing traditional heavy and high-energy-consuming microchannel-detection systems, to study the formation and evolution of stars. However, the most desirable semiconductor-based VUV photovoltaic detector capable of achieving zero power consumption has not yet been achieved. With high-crystallinity multistep epitaxial grown AlN as a VUV-absorbing layer for photogenerated carriers and p-type graphene (with unexpected VUV transmittance >96%) as a transparent electrode to collect excited holes, we constructed a heterojunction device with photovoltaic detection for VUV light. The device exhibits an encouraging VUV photoresponse, high external quantum efficiency (EQE) and extremely fast tempera response (80 ns, 104-106 times faster than that of the currently reported VUV photoconductive devices). This work has provided an idea for developing zero power consumption and integrated VUV photovoltaic detectors with ultrafast and high-sensitivity VUV detection capability, which not only allows future spacecraft to operate with longer service time and lower launching cost but also ensures an ultrafast evolution of interstellar objects.

High-sensitive and fast response to 255 nm deep-UV light of CH3NH3PbX3 (X = Cl, Br, I) bulk crystals.

Deep-UV light detection has important application in surveillance and homeland security regions. CH3NH3PbX3 (X = Cl, Br, I) materials have outstanding optical absorption and electronic transport properties suitable for obtaining excellent deep-UV photoresponse. In this work, we have grown high-quality CH3NH3PbX3 (X = Cl, Br, I) bulk crystals and used them to fabricate photodetectors. We found that they all have high-sensitive and fast-speed response to 255 nm deep-UV light. Their responsivities are 10-103 times higher than MgZnO and Ga2O3 detectors, and their response speeds are 103 times faster than Ga2O3 and ZnO detectors. These results indicate a new promising route for deep-UV detection.

Balanced Photodetection in One-Step Liquid-Phase-Synthesized CsPbBr3 Micro-/Nanoflake Single Crystals.

Here, we reported a low-cost and high-compatibility one-step liquid-phase synthesis method for synthesizing high-purity CsPbBr3 micro-/nanoflake single crystals. On the basis of the high-purity CsPbBr3, we further prepared a low-dimensional photodetector capable of balanced photodetection, involving both high external quantum efficiency and rapid temporal response, which is barely realized in previously reported low-dimensional photodetectors.

An ultrafast-temporally-responsive flexible photodetector with high sensitivity based on high-crystallinity organic-inorganic perovskite nanoflake.

Flexible cameras are important early warning wearable devices to protect security personnel from dangerous events. However, the desired key component of flexible cameras, a highly-sensitive and high-response-speed flexible photodetector, is difficult to create using conventional inorganic semiconductors. Here, we propose a low-temperature synthesis method to grow perovskite nanoflakes with high flexibility and crystallinity on a polymeric substrate. Furthermore, a high-performance flexible photodetector based on the obtained perovskite nanoflakes was fabricated. Its photoresponsivity rivals the highest values reported and, simultaneously, its response speed is faster than those of most current flexible photodetectors by 1-3 orders of magnitude.