Recently, black phosphorus (BP) has joined the two-dimensional material family as a promising candidate for photonic applications due to its moderate bandgap, high carrier mobility, and compatibility with a diverse range of substrates. Photodetectors are probably the most explored BP photonic devices, however, their unique potential compared with other layered materials in the mid-infrared wavelength range has not been revealed. Here, we demonstrate BP mid-infrared detectors at 3.39 μm with high internal gain, resulting in an external responsivity of 82 A/W. Noise measurements show that such BP photodetectors are capable of sensing mid-infrared light in the picowatt range. Moreover, the high photoresponse remains effective at kilohertz modulation frequencies, because of the fast carrier dynamics arising from BP's moderate bandgap. The high photoresponse at mid-infrared wavelengths and the large dynamic bandwidth, together with its unique polarization dependent response induced by low crystalline symmetry, can be coalesced to promise photonic applications such as chip-scale mid-infrared sensing and imaging at low light levels.

Lately rediscovered orthorhombic black phosphorus (BP) exhibits promising properties for near- and mid-infrared optoelectronics. Although recent electrical measurements indicate that a vertical electric field can effectively reduce its transport bandgap, the impact of the electric field on light-matter interaction remains unclear. Here we show that a vertical electric field can dynamically extend the photoresponse in a 5 nm-thick BP photodetector from 3.7 to beyond 7.7 μm, leveraging the Stark effect. We further demonstrate that such a widely tunable BP photodetector exhibits a peak extrinsic photo-responsivity of 518, 30, and 2.2 mA W−1 at 3.4, 5, and 7.7 μm, respectively, at 77 K. Furthermore, the extracted photo-carrier lifetime indicates a potential operational speed of 1.3 GHz. Our work not only demonstrates the potential of BP as an alternative mid-infrared material with broad optical tunability but also may enable the compact, integrated on-chip high-speed mid-infrared photodetectors, modulators, and spectrometers.

Recently rediscovered black phosphorus is a layered semiconductor with promising electronic and photonic properties. Dynamic control of its bandgap can allow for the exploration of new physical phenomena. However, theoretical investigations and photoemission spectroscopy experiments indicate that in its few-layer form, an exceedingly large electric field in the order of several volts per nanometre is required to effectively tune its bandgap, making the direct electrical control unfeasible. Here we reveal the unique thickness-dependent bandgap tuning properties in intrinsic black phosphorus, arising from the strong interlayer electronic-state coupling. Furthermore, leveraging a 10 nm-thick black phosphorus, we continuously tune its bandgap from ∼300 to below 50 meV, using a moderate displacement field up to 1.1 V nm−1. Such dynamic tuning of bandgap may not only extend the operational wavelength range of tunable black phosphorus photonic devices, but also pave the way for the investigation of electrically tunable topological insulators and semimetals.

MXenes, a new family of two-dimensional structures, have recently gained significant attention due to their unique physical properties suitable for a wide range of potential applications. Here we introduce TiCT delaminated monolayers as ultrathin transparent conductors with properties exceeding comparable reduced graphene oxide films. Solution processed TiCT films exhibit sheet resistances as low as 437 Ω sq with 77% transmittance at 550 nm. Field effect transistor measurements confirm that these films have a metallic nature, which makes them suitable as electrodes. We show using Kelvin Probe Atomic Force Microscopy that the work function of delaminated TiCT flakes (with OH terminal groups) is 5.28 ± 0.03 eV. These results demonstrate that solution-processed TiCT conducting films could open up a new direction for the next generation of transparent conductive electrodes.

Layered black phosphorus (BP) has attracted wide attention for mid-infrared photonics and high-speed electronics, due to its moderate band gap and high carrier mobility. However, its intrinsic band gap of around 0.33 electronvolt limits the operational wavelength range of BP photonic devices based on direct interband transitions to around 3.7 μm. In this work, we demonstrate that black arsenic phosphorus alloy (b-As P) formed by introducing arsenic into BP can significantly extend the operational wavelength range of photonic devices. The as-fabricated b-AsP photodetector sandwiched within hexagonal boron nitride (hBN) shows peak extrinsic responsivity of 190, 16, and 1.2 mA/W at 3.4, 5.0, and 7.7 μm at room temperature, respectively. Moreover, the intrinsic photoconductive effect dominates the photocurrent generation mechanism due to the preservation of pristine properties of b-AsP by complete hBN encapsulation, and these b-AsP photodetectors exhibit negligible transport hysteresis. The broad and large photoresponsivity within mid-infrared resulting from the intrinsic photoconduction, together with the excellent long-term air stability, makes b-AsP alloy a promising alternative material for mid-infrared applications, such as free-space communication, infrared imaging, and biomedical sensing.

Optical excitation and subsequent decay of graphene plasmons can produce a significant increase in charge-carrier temperature. An efficient method to convert this temperature elevation into electrical signals can enable important mid-infrared applications. However, the modest thermoelectric coefficient and weak temperature dependence of carrier transport in graphene hinder this goal. Here, we demonstrate mid-infrared graphene detectors consisting of arrays of plasmonic resonators interconnected by quasi-one-dimensional nanoribbons. Localized barriers associated with disorder in the nanoribbons produce a dramatic temperature dependence of carrier transport, thus enabling the electrical detection of plasmon decay in the nearby graphene resonators. Our device has a subwavelength footprint of 5 × 5 μm and operates at 12.2 μm with an external responsivity of 16 mA W and a low noise-equivalent power of 1.3 nW Hz at room temperature. It is fabricated using large-scale graphene and possesses a simple two-terminal geometry, representing an essential step towards the realization of an on-chip graphene mid-infrared detector array.

Recently rediscovered layered black phosphorus (BP) provides rich opportunities for investigations of device physics and applications. The band gap of BP is widely tunable by its layer number and a vertical electric field, covering a wide electromagnetic spectral range from visible to mid-infrared. Despite much progress in BP optoelectronics, the fundamental photoluminescence (PL) properties of thinfilm BP in mid-infrared have rarely been investigated. Here, we report bright PL emission from thin-film BP (with thickness of 4.5 to 46 nm) from 80 to 300 K. The PL measurements indicate a band gap of 0.308 ± 0.003 eV in 46 nm thick BP at 80 K, and it increases monotonically to 0.334 ± 0.003 eV at 300 K. Such an anomalous blueshift agrees with the previous theoretical and photoconductivity spectroscopy results. However, the observed blueshift of 26 meV from 80 to 300 K is about 60% of the previously reported value. Most importantly, we show that the PL emission intensity from thin-film BP is only a few times weaker than that of an indium arsenide (InAs) multiple quantum well (MQW) structure grown by molecular beam epitaxy. Finally, we report the thickness-dependent PL spectra in thin-film BP in mid-infrared regime. Our work reveals the mid-infrared light emission properties of thin-film BP, suggesting its promising future in tunable mid-infrared light emitting and lasing applications.

Nonvolatile charge-trap memory plays an important role in many modern electronics technologies, from portable electronic systems to large-scale data centers. Conventional charge-trap memory devices typically work with fixed channel carrier polarity and device characteristics. However, many emerging applications in reconfigurable electronics and neuromorphic computing require dynamically tunable properties in their electronic device components that can lead to enhanced circuit versatility and system functionalities. Here, we demonstrate an ambipolar black phosphorus (BP) charge-trap memory device with dynamically reconfigurable and polarity-reversible memory behavior. This BP memory device shows versatile memory properties subject to electrostatic bias. Not only the programmed/erased state current ratio can be continuously tuned by the back-gate bias, but also the polarity of the carriers in the BP channel can be reversibly switched between electron- and hole-dominated conductions, resulting in the erased and programmed states exhibiting interchangeable high and low current levels. The BP memory also shows four different memory states and, hence, 2-bit per cell data storage for both n-type and p-type channel conductions, demonstrating the multilevel cell storage capability in a layered material based memory device. The BP memory device with a high mobility and tunable programmed/erased state current ratio and highly reconfigurable device characteristics can offer adaptable memory device properties for many emerging applications in electronics technology, such as neuromorphic computing, data-adaptive energy efficient memory, and dynamically reconfigurable digital circuits.