Weiwei Tang

Fabrication of CdS∕Si nanocable heterostructures by one-step thermal evaporation

Coaxial CdS∕Si nanocable heterostructures with a length of hundreds of micrometers and an average diameter of 100nm were fabricated via one-step thermal evaporation of CdS powder under experimentally controlled conditions. The CdS cores have a hexagonal crystal structure. The Si sheaths are amorphous and can be directly grown on the CdS surfaces from the silicon substrate via a vapor-liquid-solid mechanism without an extra Si source. The photoluminescence of the nanocables presents two emission bands, around 510 and 590nm. This simple method may be applied to other Si-sheathed heterostructures, which can be used in nanodevices with various functions.

MoS2 nanosheet photodetectors with ultrafast response

Two-dimensional layered materials, such as molybdenum disulfide, are emerging as an exciting material system for future electronics due to their unique electronic properties and atomically thin geometry. In this work, MoS2-based FETs are fabricated using mechanical cleavage and standard photolithographic and metal evaporation techniques, and the detector exhibits a good ohmic contact. We show that the multilayer molybdenum disulfide photodetector has a fast photoresponse as short as 42 μs. The fast photodetector response is due to the decrease in the trap states in MoS2 flakes compared to monolayer MoS2, making its photoresponse time close to its intrinsic response. The large photocurrent with the responsivity and external quantum efficiency of 59 A/W and 13 800% for the wavelength of 532 nm was also measured. The fast response time, high responsivity, and the ease of fabrication of these devices make them important components for future optoelectronic devices.

Tailoring Active Far-Infrared Resonator with Graphene Metasurface and Its Complementary

Far-infrared part of electromagnetic spectrum and its technological details have been highly sought after due to its myriad applications including imaging, spectroscopy, industry control, and communication. However, lack of efficient components of electronic and photonic sources/detectors working in this particular spectrum has impeded its widespread application. One of the bottlenecks lies in the compact far-infrared polarization-sensitive resonator/modulator in compatible with pixel-detector for far-infrared spectroscopy. In this work, we demonstrate strong electric resonance response in perforated graphene sheet at this particular electromagnetic region. The results demonstrate inherently different natures for the strong electromagnetic response between graphene-based and metallic metamaterials. Unlike the metallic metamaterials relying on the geometrical inductance for magnetic response, the electric resonance caused by localized dipole/multipolar modes is found to be dominated in graphene and thus enabling sub-wavelength confinement of electromagnetic field. The Babinet’s principle is proposed to be applied for broadband far-infrared modulation and resonant filters design of graphene-based metamaterial. The active tunable electric resonance through electrostatic doping on the graphene-based patterns provides efficient route for compact biosensing, far-infrared imaging, and detection.

Towards sensitive terahertz detection via thermoelectric manipulation using graphene transistors

Graphene has been highly sought after as a potential candidate for hot-electron terahertz (THz) detection benefiting from its strong photon absorption, fast carrier relaxation, and weak electron-phonon coupling. Nevertheless, to date, graphene-based thermoelectric THz photodetection is hindered by low responsivity owing to relatively low photoelectric efficiency. In this work, we provide a straightforward strategy for enhanced THz detection based on antenna-coupled CVD graphene transistors with the introduction of symmetric paired fingers. This design enables switchable photodetection modes by controlling the interaction between the THz field and free hot carriers in the graphene-channel through different contacting configurations. Hence a novel “bias-field effect” can be activated, which leads to a drastic enhancement in THz detection ability with maximum responsivity of up to 280 V/W at 0.12 THz relative to the antenna area and a Johnson-noise limited minimum noise-equivalent power (NEP) of 100 pW/Hz0.5 at room temperature. The mechanism responsible for the enhancement in the photoelectric gain is attributed to thermophotovoltaic instead of plasma self-mixing effects. Our results offer a promising alternative route toward scalable, wafer-level production of high-performance graphene detectors.Graphene: Putting a finger on ultra-sensitive light detectionControlling how finger-like electrodes are positioned on graphene transistors makes it simpler to detect terahertz radiation for applications including medical diagnostics. When photons strike graphene sheets, multiple electrons with high kinetic energy are released. Changlong Liu at the Chinese Academy of Sciences, Shanghai, and coworkers have exploited these electrons to improve detection of terahertz light, which is useful for seeing features underneath human skin. The team’s device places single-layer graphene into a channel flanked by two microscale antennas, and then adds a pair of thin metal fingers on top of the 2D carbon film. By manipulating the separation of the finger contacts, the team unlocked an enhanced response to terahertz light at room temperature. Experiments and simulations revealed that temperature gradients between contact points generated a terahertz-sensitive photocurrent through conversion of heat into electricity.Graphene is very promising for THz application, especially in the fields desiring fast THz imaging, like security screening, communication, biomedical and pharmacy control. However, current graphene-based THz detectors are severely hindered by its poor switching behavior, and lack of internal gain to boost up the responsivity in the framework of plasma-wave self-mixing or thermoelectric detections. A proper means to selectively trigger the photoelectric conversion is highly desirable for practical applications. This work offers alternative way for room temperature THz detection via manipulating the hot carriers either electrically and electromagnetically. Internal photoelectric gain is achieved via the bias-field effect, which leads to the giant enhancement of graphene-based detector’s responsivity over 200 V/W. Furthermore, switching behavior between photoconductive and photovoltaic modes can be given rise in a single device, being promising for scalable THz imaging.

Top-gated black phosphorus phototransistor for sensitive broadband detection

The present work reports on a graphene-like material that is promising for photodetection applications due to its high optical absorption and layer-dependent properties. To date, only narrowband photodetectors have been realized; therefore, extending the working wavelength is becoming more imperative for applications such as high-contrast imaging and remote sensing. In this work, we developed a novel detection technique that provides enhanced performance across the infrared and terahertz bands by using an antenna-assisted top-gated black phosphorus phototransistor. By using the proposed sophisticated design, the adverse effect due to the back-gate that is generally employed for a long-wavelength photon coupling can be eliminated. Moreover, the antenna-assisted near-field and dark current can be further tailored electromagnetically and electrostatically by employing a gate finger, thus resulting in improved detection efficiency. Various detection mechanisms such as thermoelectric, bolometric, and electron-hole generation are differentiated on the basis of the device geometry and incident wavelength. The proposed photodetector demonstrated superior performance-excellent sensitivity of more than 10 V W-1, a noise equivalent power value of less than 0.1 nW Hz-0.5, and a fast response time across disparate wavebands. Thus, the photodetector can satisfy diverse application requirements.