Youpin Gong

Printable Transfer-Free and Wafer-Size MoS2/Graphene van der Waals Heterostructures for High-Performance Photodetection

Two-dimensional (2D) MoS2/graphene van der Waals heterostructures integrate the superior light-solid interaction in MoS2 and charge mobility in graphene for high-performance optoelectronic devices. Key to the device performance lies in a clean MoS2/graphene interface to facilitate efficient transfer of photogenerated charges. Here, we report a printable and transfer-free process for fabrication of wafer-size MoS2/graphene van der Waals heterostructures obtained using a metal-free-grown graphene, followed by low-temperature growth of MoS2 from the printed thin film of ammonium thiomolybdate on graphene. The photodetectors based on the transfer-free MoS2/graphene heterostructures exhibit extraordinary short photoresponse rise/decay times of 20/30 ms, which are significantly faster than those of the previously reported MoS2/transferred-graphene photodetectors (0.28-1.5 s). In addition, a high photoresponsivity of up to 835 mA/W was observed in the visible spectrum on such transfer-free MoS2/graphene heterostructures, which is much higher than that of the reported photodetectors based on the exfoliated layered MoS2 (0.42 mA/W), the graphene (6.1 mA/W), and transfer-free MoS2/graphene/SiC heterostructures (∼40 mA/W). The enhanced performance is attributed to the clean interface on the transfer-free MoS2/graphene heterostructures. This printable and transfer-free process paves the way for large-scale commercial applications of the emerging 2D heterostructures in optoelectronics and sensors.

Wrapping cytochrome c around single-wall carbon nanotube: engineered nanohybrid building blocks for infrared detection at high quantum efficiency

Biomolecule cytochrome c (Cty c), a small molecule of a chain of amino acids with extraordinary electron transport, was helically wrapped around a semiconductive single-wall carbon nanotube (s-SWCNT) to form a molecular building block for uncooled infrared detection with two uniquely designed functionalities: exciton dissociation to free charge carriers at the heterojunction formed on the s-SWCNT/Cty c interface and charge transport along the electron conducting chain of Cty c (acceptor) and hole conducting channel through s-SWCNT (donor). Such a design aims at addressing the long-standing challenges in exciton dissociation and charge transport in an SWCNT network, which have bottlenecked development of photonic SWCNT-based infrared detectors. Using these building blocks, uncooled s-SWCNT/Cyt c thin film infrared detectors were synthesized and shown to have extraordinary photoresponsivity up to 0.77 A W−1 due to a high external quantum efficiency (EQE) in exceeding 90%, which represents a more than two orders of magnitude enhancement than the best previously reported on CNT-based infrared detectors with EQE of only 1.72%. From a broad perspective, this work on novel s-SWCNT/Cyt c nanohybrid infrared detectors has developed a successful platform of engineered carbon nanotube/biomolecule building blocks with superior properties for optoelectronic applications.

Atomically Thin Al2O3 Films for Tunnel Junctions

Metal-Insulator-Metal tunnel junctions (MIMTJ) are common throughout the microelectronics industry. The industry standard AlOx tunnel barrier, formed through oxygen diffusion into an Al wetting layer, is plagued by internal defects and pinholes which prevent the realization of atomically-thin barriers demanded for enhanced quantum coherence. In this work, we employed in situ scanning tunneling spectroscopy (STS) along with molecular dynamics simulations to understand and control the growth of atomically thin Al2O3 tunnel barriers using atomic layer deposition (ALD). We found that a carefully tuned initial H2O pulse hydroxylated the Al surface and enabled the creation of an atomically-thin Al2O3 tunnel barrier with a high quality M-I interface and a significantly enhanced barrier height compared to thermal AlOx. These properties, corroborated by fabricated Josephson Junctions, show that ALD Al2O3 is a dense, leak-free tunnel barrier with a low defect density which can be a key component for the next-generation of MIMTJs.

Designing the Interface of Carbon Nanotube/Biomaterials for High-Performance Ultra-Broadband Photodetection

Inorganic/biomolecule nanohybrids can combine superior electronic and optical properties of inorganic nanostructures and biomolecules for optoelectronics with performance far surpassing that achievable in conventional materials. The key toward a high-performance inorganic/biomolecule nanohybrid is to design their interface based on the electronic structures of the constituents. A major challenge is the lack of knowledge of most biomolecules due to their complex structures and composition. Here, we first calculated the electronic structure and optical properties of one of the cytochrome c (Cyt c) macromolecules (PDB ID: 1HRC ) using ab initio OLCAO method, which was followed by experimental confirmation using ultraviolet photoemission spectroscopy. For the first time, the highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels of Cyt c, a well-known electron transport chain in biological systems, were obtained. On the basis of the result, pairing the Cyt c with semiconductor single-wall carbon nanotubes (s-SWCNT) was predicted to have a favorable band alignment and built-in electrical field for exciton dissociation and charge transfer across the s-SWCNT/Cyt c heterojunction interface. Excitingly, photodetectors based on the s-SWCNT/Cyt c heterojunction nanohybrids demonstrated extraordinary ultra-broadband (visible light to infrared) responsivity (46-188 A W-1) and figure-of-merit detectivity D* (1-6 × 1010 cm Hz1/2 W-1). Moreover, these devices can be fabricated on transparent flexible substrates by a low-lost nonvacuum method and are stable in air. These results suggest that the s-SWCNT/biomolecule nanohybrids may be promising for the development of CNT-based ultra-broadband photodetectors.