Project description:Room temperature femtowatt sensitivity remains a sought-after attribute, even among commercial inorganic infrared (IR) photodetectors (PDs). While organic IR PDs are poised to emerge as a pivotal sensor technology in the forthcoming Fourth-Generation Industrial Era, their performance lags behind that of their inorganic counterparts. This discrepancy primarily stems from poor external quantum efficiencies (EQE), driven by inadequate exciton dissociation (high exciton binding energy) within organic IR materials, exacerbated by pronounced non-radiative recombination at narrow bandgaps. Here, we unveil a high-performance organic Near-IR (NIR) PD via integer charge transfer between Poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (C-14PBTTT) donor (D) and Tetrafluorotetracyanoquinodimethane (TCNQF4) acceptor (A) molecules, showcasing strong low-energy subgap absorptions up to 2.5 µm. We observe that specifically, polaron excitation in these radical and neutral D-A blended molecules enables bound charges to exceed the Coulombic attraction to their counterions, leading to an elevated EQE (polaron absorption region) compared to Frenkel excitons. As a result, our devices achieve a high EQE of ∼107%, femtowatt sensitivity (NEP) of ~0.12 fW Hz-1/2 along a response time of ~81 ms, at room temperature for a wavelength of 1.0 µm. Our innovative utilization of polarons highlights their potential as alternatives to Frenkel excitons in high-performance organic IR PDs.
Project description:The pursuit of optoelectronic devices operating in the mid-infrared regime is driven by both fundamental interests and envisioned applications ranging from imaging, sensing to communications. Despite continued achievements in traditional semiconductors, notorious obstacles such as the complicated growth processes and cryogenic operation preclude the usage of infrared detectors. As an alternative path towards high-performance photodetectors, hybrid semiconductor/graphene structures have been intensively explored. However, the operation bandwidth of such photodetectors has been limited to visible and near-infrared regimes. Here we demonstrate a mid-infrared hybrid photodetector enabled by coupling graphene with a narrow bandgap semiconductor, Ti2O3 (Eg = 0.09 eV), which achieves a high responsivity of 300 A W-1 in a broadband wavelength range up to 10 µm. The obtained responsivity is about two orders of magnitude higher than that of the commercial mid-infrared photodetectors. Our work opens a route towards achieving high-performance optoelectronics operating in the mid-infrared regime.
Project description:Assembling nanomaterials into hybrid structures provides a promising and flexible route to reach ultrahigh responsivity by introducing a trap-assisted gain (G) mechanism. However, the high-gain photodetectors benefitting from long carrier lifetime often possess slow response time (t) due to the inherent G-t tradeoff. Here, a light-driven junction field-effect transistor (LJFET), consisting of an n-type ZnO belt as the channel material and a p-type WSe2 nanosheet as a photoactive gate material, to break the G-t tradeoff through decoupling the gain from carrier lifetime is reported. The photoactive gate material WSe2 under illumination enables a conductive path for externally applied voltage, which modulates the depletion region within the ZnO channel efficiently. The gain and response time are separately determined by the field effect modulation and the switching speed of LJFET. As a result, a high responsivity of 4.83 × 103 A W-1 with a gain of ≈104 and a rapid response time of ≈10 µs are obtained simultaneously. The LJFET architecture offers a new approach to realize high-gain and fast-response photodetectors without the G-t tradeoff.
Project description:Most covalent organic frameworks (COFs) to date are made from relatively small aromatic subunits, which can only absorb the high-energy part of the visible spectrum. We have developed near-infrared-absorbing low bandgap COFs by incorporating donor-acceptor-type isoindigo- and thienoisoindigo-based building blocks. The new materials are intensely colored solids with a high degree of long-range order and a pseudo-quadratic pore geometry. Growing the COF as a vertically oriented thin film allows for the construction of an ordered interdigitated heterojunction through infiltration with a complementary semiconductor. Applying a thienoisoindigo-COF:fullerene heterojunction as the photoactive component, we realized the first COF-based UV- to NIR-responsive photodetector. We found that the spectral response of the device is reversibly switchable between blue- and red-sensitive, and green- and NIR-responsive. To the best of our knowledge, this is the first time that such nearly complete inversion of spectral sensitivity of a photodetector has been achieved. This effect could lead to potential applications in information technology or spectral imaging.
Project description:Extremely thin silicon show good mechanical flexibility because of their 2-D like structure and enhanced performance by the quantum confinement effect. In this paper, we demonstrate a junctionless FET which reveals a room temperature quantum confinement effect (RTQCE) achieved by a valley-engineering of the silicon. The strain-induced band splitting and a quantum confinement effect induced from ultra-thin-body silicon are the two main mechanisms for valley engineering. These were obtained from the extremely well-controlled silicon surface roughness and high tensile strain in silicon, thereupon demonstrating a device mobility increase of ~500% in a 2.5 nm thick silicon channel device.
Project description:There is broad interest in surface functionalization of 2D materials and its related applications. In this work, we present a novel graphene layer transistor fabricated by introducing fluorinated graphene (fluorographene), one of the thinnest 2D insulator, as the gate dielectric material. For the first time, the dielectric properties of fluorographene, including its dielectric constant, frequency dispersion, breakdown electric field and thermal stability, were comprehensively investigated. We found that fluorographene with extremely thin thickness (5 nm) can sustain high resistance at temperature up to 400 °C. The measured breakdown electric field is higher than 10 MV cm(-1), which is the heightest value for dielectric materials in this thickness. Moreover, a proof-of-concept methodology, one-step fluorination of 10-layered graphene, is readily to obtain the fluorographene/graphene heterostructures, where the top-gated transistor based on this structure exhibits an average carrier mobility above 760 cm(2)/Vs, higher than that obtained when SiO₂ and GO were used as gate dielectric materials. The demonstrated fluorographene shows excellent dielectric properties with fast and scalable processing, providing a universal applications for the integration of versatile nano-electronic devices.
Project description:The structure of a gate-controlled graphene/germanium hybrid photodetector was optimized by splitting the active region to achieve highly sensitive infrared detection capability. The strengthened internal electric field in the split active junctions enabled efficient collection of photocarriers, resulting in a responsivity of 2.02 A W-1 and a specific detectivity of 5.28 × 1010 Jones with reduced dark current and improved external quantum efficiency; these results are more than doubled compared with the responsivity of 0.85 A W-1 and detectivity of 1.69 × 1010 Jones for a single active junction device. The responsivity of the optimized structure is 1.7, 2.7, and 39 times higher than that of previously reported graphene/Ge with Al2O3 interfacial layer, gate-controlled graphene/Ge, and simple graphene/Ge heterostructure photodetectors, respectively.
Project description:Graphene is a very attractive material for broadband photodetection in hyperspectral imaging and sensing systems. However, its potential use has been hindered by tradeoffs between the responsivity, bandwidth, and operation speed of existing graphene photodetectors. Here, we present engineered photoconductive nanostructures based on gold-patched graphene nano-stripes, which enable simultaneous broadband and ultrafast photodetection with high responsivity. These nanostructures merge the advantages of broadband optical absorption, ultrafast photocarrier transport, and carrier multiplication within graphene nano-stripes with the ultrafast transport of photocarriers to gold patches before recombination. Through this approach, high-responsivity operation is realized without the use of bandwidth-limiting and speed-limiting quantum dots, defect states, or tunneling barriers. We demonstrate high-responsivity photodetection from the visible to infrared regime (0.6 A/W at 0.8 μm and 11.5 A/W at 20 μm), with operation speeds exceeding 50 GHz. Our results demonstrate improvement of the response times by more than seven orders of magnitude and an increase in bandwidths of one order of magnitude compared to those of higher-responsivity graphene photodetectors based on quantum dots and tunneling barriers.
Project description:Organic field-effect transistors (OFETs) are of the core units in organic electronic circuits, and the performance of OFETs replies critically on the properties of their dielectric layers. Owing to the intrinsic flexibility and natural compatibility with other organic components, organic polymers, such as poly(vinyl alcohol) (PVA), have emerged as highly interesting dielectric materials for OFETs. However, unsatisfactory issues, such as hysteresis, high subthreshold swing, and low effective carrier mobility, still considerably limit the practical applications of the polymer-dielectric OFETs for high-speed, low-voltage flexible organic circuits. This work develops a new approach of using supercritical CO2 fluid (SCCO2) treatment on PVA dielectrics to achieve remarkably high-performance polymer-dielectric OFETs. The SCCO2 treatment is able to completely eliminate the hysteresis in the transfer characteristics of OFETs, and it can also significantly reduce the device subthreshold slope to 0.25 V/dec and enhance the saturation regime carrier mobility to 30.2 cm2 V-1 s-1, of which both the numbers are remarkable for flexible polymer-dielectric OFETs. It is further demonstrated that, coupling with an organic light-emitting diode (OLED), the SCCO2-treated OFET is able to function very well under fast switching speed, which indicates that an excellent switching behavior of polymer-dielectric OFETs can be enabled by this SCCO2 approach. Considering the broad and essential applications of OFETs, we envision that this SCCO2 technology will have a very broad spectrum of applications for organic electronics, especially for high refresh rate and low-voltage flexible display devices.
Project description:A broad array of imaging and diagnostic technologies employs fluorophore-labeled antibodies for biomarker visualization, an experimental technique known as immunofluorescence. Significant performance advantages, such as higher signal-to-noise ratio, are gained if the appended fluorophore emits near-infrared (NIR) light with a wavelength >700 nm. However, the currently available NIR fluorophore antibody conjugates are known to exhibit significant limitations, including low chemical stability and photostability, weakened target specificity, and low fluorescence brightness. These fluorophore limitations are resolved by employing a NIR heptamethine cyanine dye named s775z whose chemical structure is very stable, charge-balanced, and sterically shielded. Using indirect immunofluorescence for imaging and visualization, a secondary IgG antibody labeled with s775z outperformed IgG analogues labeled with the commercially available NIR fluorophores, IRDye 800CW and DyLight800. Comparison experiments include three common techniques: immunocytochemistry, immunohistochemistry, and western blotting. Specifically, the secondary IgG labeled with s775z was 3-8 times brighter, 3-6 times more photostable, and still retained excellent target specificity when the degree of antibody labeling was high. The results demonstrate that antibodies labeled with s775z can emit total photon counts that are 1-2 orders of magnitude higher than those currently possible, and thus enable unsurpassed performance for NIR fluorescence imaging and diagnostics. They are especially well suited for analytical applications that require sensitive NIR fluorescence detection or use modern photon-intense methods that require high photostability.