Organic-inorganic halide perovskite quantum dots (PQDs) constitute an attractive class of materials for many optoelectronic applications. However, their charge transport properties are inferior to materials like graphene. On the other hand, the charge generation efficiency of graphene is too low to be used in many optoelectronic applications. Here, we demonstrate the development of ultrathin phototransistors and photonic synapses using a graphene-PQD (G-PQD) superstructure prepared by growing PQDs directly from a graphene lattice. We show that the G-PQDs superstructure synchronizes efficient charge generation and transport on a single platform. G-PQD phototransistors exhibit excellent responsivity of 1.4 × 108 AW–1 and specific detectivity of 4.72 × 1015 Jones at 430 nm. Moreover, the light-assisted memory effect of these superstructures enables photonic synaptic behavior, where neuromorphic computing is demonstrated by facial recognition with the assistance of machine learning. We anticipate that the G-PQD superstructures will bolster new directions in the development of highly efficient optoelectronic devices.

/pdf/ultrasensitive-and-ultrathin-phototransistors-and-photonic-2xemlff086.pdf

Ultrasensitive and ultrathin phototransistors and photonic synapses using perovskite quantum dots grown from graphene lattice.

Broadband convolutional processing using band-alignment-tunable heterostructures

Two-dimensional crystals have emerged as a class of materials that may impact future electronic technologies. Experimentally identifying and characterizing new functional two-dimensional materials is challenging, but also potentially rewarding. Here, we fabricate field-effect transistors based on few-layer black phosphorus crystals with thickness down to a few nanometres. Reliable transistor performance is achieved at room temperature in samples thinner than 7.5 nm, with drain current modulation on the order of 10(5) and well-developed current saturation in the I-V characteristics. The charge-carrier mobility is found to be thickness-dependent, with the highest values up to ∼ 1,000 cm(2) V(-1) s(-1) obtained for a thickness of ∼ 10 nm. Our results demonstrate the potential of black phosphorus thin crystals as a new two-dimensional material for applications in nanoelectronic devices.

http://absimage.aps.org/image/MAR14/MWS_MAR14-2013-005031.pdf

Black Phosphorus Field-effect Transistors

Photodetectors based on two-dimensional (2D) materials have been the focus of intensive research and development over the past decade. However, a gap has long persisted between fundamental research and mature applications. One of the main reasons behind this gap has been the lack of a practical and unified approach for the characterization of their figures of merit, which should be compatible with the traditional performance evaluation system of photodetectors. This is essential to determine the degree of compatibility of laboratory prototypes with industrial technologies. Here we propose general guidelines for the characterization of the figures of merit of 2D photodetectors and analyze common situations when the specific detectivity, responsivity, dark current, and speed can be misestimated. Our guidelines should help improve the standardization and industrial compatibility of 2D photodetectors. 

/pdf/how-to-characterize-figures-of-merit-of-two-dimensional-1x1aib2p.pdf

How to characterize figures of merit of two-dimensional photodetectors

Ga2O3-based solar blind avalanche photodetectors exhibit low voltage operation, optical filter-free and monolithic integration of photodetector arrays, and therefore they are promising to be an alternative to the bulky and fragile photomultiplier tubes for weak signal detection in deep-ultraviolet region. Here, by deliberate lattice and band engineering, we construct an n-Barrier-n unipolar barrier avalanche photodetector consisting of β-Ga2O3/MgO/Nb:SrTiO3 heterostructure, in which the enlarged conduction band offsets fortify the reverse breakdown and suppress the dark current while the negligible valance band offsets faciliate minority carrier flow across the heterojunction. The developed devices exhibit record-high avalanche gain up to 5.9 × 105 and detectivity of 2.33 × 1016 Jones among the reported wafer-scale grown Ga2O3-based photodetectors, which are even comparable to the commercial photomultiplier tubes. These findings provide insights into precise manipulation of band alignment in avalanche photodetectors, and also offer exciting opportunities for further developing high-performance Ga2O3-based electronics and optoelectronics. 

/pdf/enhanced-gain-and-detectivity-of-unipolar-barrier-solar-23luthqx.pdf

Enhanced gain and detectivity of unipolar barrier solar blind avalanche photodetector via lattice and band engineering

Unipolar barrier structures are used to suppress dark current in photodetectors by blocking majority carriers. Designing unipolar barriers with conventional materials is challenging due to the strict requirements of lattice and band matching. Two-dimensional materials have self-passivated surfaces and tunable band structures, and can thus be used to design unipolar barriers in which lattice mismatch and interface defects are avoided. Here, we show that band-engineered van der Waals heterostructures can be used to build visible and mid-wavelength infrared unipolar barrier photodetectors. Our nBn unipolar barrier photodetectors, which are based on a tungsten disulfide/hexagonal boron nitride/palladium diselenide heterostructure, exhibit a low dark current of 15 pA, a photocurrent of 20 μA and a detectivity of 2.7 × 1012 cm Hz1/2 W−1. Our pBp unipolar barrier photodetectors, which are based on a black phosphorus/molybdenum disulfide/graphene heterostructure, exhibit a room-temperature detectivity of 2.3 × 1010 cm Hz1/2 W−1 in the mid-wavelength infrared region under blackbody radiation. The pBp devices also show a dichroic ratio of 4.9 under blackbody radiation, and a response time of 23 μs under 2 μm laser illumination. Band-engineered van der Waals heterostructures that block dark current without suppressing photocurrent can be used to build detectors with high room-temperature detectivity for visible light and blackbody infrared light.

Unipolar barrier photodetectors based on van der Waals heterostructures

Two-dimensional (2D) infrared photodetectors always suffer from low quantum efficiency (QE) because of the limited atomically thin absorption. Here, we reported 2D black phosphorus (BP)/Bi2O2Se van der Waals (vdW) photodetectors with momentum-matching and band-alignment heterostructures to achieve high QE. The QE was largely improved by optimizing the generation, suppressing the recombination, and improving the collection of photocarriers. Note that momentum-matching BP/Bi2O2Se heterostructures in k-space lead to the highly efficient generation and transition of photocarriers. The recombination process can be largely suppressed by lattice mismatching-immune vdW interfaces. Furthermore, type II BP/Bi2O2Se vdW heterostructures could also assist fast transport and collection of photocarriers. By constructing momentum-matching and band-alignment heterostructures, a record-high QE of 84% at 1.3 micrometers and 76.5% at 2 micrometers have been achieved in BP/Bi2O2Se vdW photodetectors.

Momentum-matching and band-alignment van der Waals heterostructures for high-efficiency infrared photodetection.

The photodiode is a prevailing architecture for photodetection with the merits of fast response and low dark current. However, an ideal photodiode is also desired for both high responsivity and high external quantum efficiency (EQE), which may facilitate more applications. Here the photoconducting effect in a photodiode is discussed and an Au-PbS colloidal quantum dot (CQD)-indium tin oxide Schottky junction photodiode is fabricated. The long carrier lifetime and improved carrier mobility in tetrabutylammonium iodide-modified PbS CQDs cooperating with the proper band structure and an ultrashort channel in the diode enable the photodiode with high photoconductive gain, realizing an EQE of ≈400% and a responsivity (R) of 5.15 A W-1 while simultaneously achieving a response time of 110 µs and a specific detectivity of 1.96 × 1010 Jones under 1550 nm illumination. In addition, this CQD-based photodiode is stable, low cost, and compatible with complementary metal oxide semiconductor technology. All of these promise this device great potential in applications.

A Colloidal-Quantum-Dot Infrared Photodiode with High Photoconductive Gain.

Unraveling the reaction reversibility and structure stability of nickel sulfide anodes for lithium ion batteries

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Unipolar barrier photodetectors based on van der Waals heterostructures

Momentum-matching and band-alignment van der Waals heterostructures for high-efficiency infrared photodetection.

A Colloidal-Quantum-Dot Infrared Photodiode with High Photoconductive Gain.

Unraveling the reaction reversibility and structure stability of nickel sulfide anodes for lithium ion batteries