Oxygen-assisted charge transfer between ZnO quantum dots and graphene

Ultrahigh-performance photodetectors based on low-dimensional Cs2AgBiBr6/CdS heterojunction

Lead-free double perovskite Cs2AgBiBr6 has garnered increasing attention in photoelectric applications owing to its good stability and excellent photoelectric properties. However, the poor carrier transport in Cs2AgBiBr6 thin films constraints their further application in photodetection. To overcome this issue, we have developed an innovative low-dimensional Cs2AgBiBr6/CdS heterojunction photodetector with substantially improved performance. The device achieved a high responsivity of 6.66 × 103 A/W, an outstanding specific detectivity of 2.10 × 1014 Jones, and an impressive external quantum efficiency of 1.88 × 106 %. Additionally, the on/off current ratio of the heterojunction device reached an impressive 6.18 × 107. These key parameters are significantly better than those of most previously reported Cs2AgBiBr6-based photodetectors. Furthermore, scanning photocurrent mapping and band arrangement analysis were performed to elucidate the mechanism of photocurrent generation and transport in the low-dimensional Cs2AgBiBr6/CdS heterojunction photodetectors. This study highlights the outstanding performance of Cs2AgBiBr6/CdS heterojunction and provides a simple and effective strategy for developing high-performance Cs2AgBiBr6-based photodetectors.

Performance of Low-Dimensional Solid Room-Temperature Photodetectors-Critical View

In the last twenty years, nanofabrication progress has allowed for the emergence of a new photodetector family, generally called low-dimensional solids (LDSs), among which the most important are two-dimensional (2D) materials, perovskites, and nanowires/quantum dots. They operate in a wide wavelength range from ultraviolet to far-infrared. Current research indicates remarkable advances in increasing the performance of this new generation of photodetectors. The published performance at room temperature is even better than reported for typical photodetectors. Several articles demonstrate detectivity outperforming physical boundaries driven by background radiation and signal fluctuations. This study attempts to explain these peculiarities. In order to achieve this goal, we first clarify the fundamental differences in the photoelectric effects of the new generation of photodetectors compared to the standard designs dominating the commercial market. Photodetectors made of 2D transition metal dichalcogenides (TMDs), quantum dots, topological insulators, and perovskites are mainly considered. Their performance is compared with the fundamental limits estimated by the signal fluctuation limit (in the ultraviolet region) and the background radiation limit (in the infrared region). In the latter case, Law 19 dedicated to HgCdTe photodiodes is used as a standard reference benchmark. The causes for the performance overestimate of the different types of LDS detectors are also explained. Finally, an attempt is made to determine their place in the global market in the long term.

Effect of surface ligands on the photoinduced electron transfer rate and efficiency in ZnO quantum dots and graphene oxide assemblies

Apart from biocompatibility, ZnO quantum dots (QDs) are considered to be an efficient luminescence material due to their low cost and high redox potential. Here, we report the synthesis of ZnO QDs by using five different functionalizing ligands like mercaptoacetic acid (MAA), 3-mercaptopropionic acid (MPA), octadecene (ODE), ethylene glycol (EG), and oleyl amine (OLA) and fabricate their assemblies with graphene oxide (GO). We investigate the role of functionalizing ligands as a surface modifier of ZnO QDs for their attachment to GO. The steady-state photoluminescence (SSPL) and time-resolved photoluminescence (TRPL) analyses demonstrate the photoluminescence (PL) quenching of ZnO QDs in ZnO QDs-GO assembly. The highest reduction in PL intensity is observed with ZnO QDs-GO assembly with EG as a surface functionalizing ligand. Cyclic voltammetry (CV) analysis confirms the feasibility of charge transfer from ZnO QDs to the GO. The maximum (79.43%) charge transfer efficiency (ECT) is observed in the case of ZnO-MAA-GO as compared to other assemblies. This means the thiol group-containing ligands facilitate charge transfer as compared to hydroxyl and amine group ligands. This leads to the conclusion that charge transfer in ZnO QDs-GO assemblies depends strongly on the nature of surface ligands.

Isotope Engineered Fluorinated Single and Bilayer Graphene: Insights into Fluorination Selectivity, Stability, and Defect Passivation

Tailoring the physicochemical properties of graphene through functionalization remains a major interest for next-generation technological applications. However, defect formation due to functionalization greatly endangers the intrinsic properties of graphene, which remains a serious concern. Despite numerous attempts to address this issue, a comprehensive analysis has not been conducted. This work reports a two-step fluorination process to stabilize the fluorinated graphene and obtain control over the fluorination-induced defects in graphene layers. The structural, electronic and isotope-mass-sensitive spectroscopic characterization unveils several not-yet-resolved facts, such as fluorination sites and CF bond stability in partially-fluorinated graphene (F-SLG). The stability of fluorine has been correlated to fluorine co-shared between two graphene layers in fluorinated-bilayer-graphene (F-BLG). The desorption energy of co-shared fluorine is an order of magnitude higher than the CF bond energy in F-SLG due to the electrostatic interaction and the inhibition of defluorination in the F-BLG. Additionally, F-BLG exhibits enhanced light-matter interaction, which has been utilized to design a proof-of-concept field-effect phototransistor that produces high photocurrent response at a time <200 µs. Thus, the study paves a new avenue for the in-depth understanding and practical utilization of fluorinated graphenic carbon.

High responsivity of VIS-NIR photodetector based on Ag2S/P3HT heterojunction

Ag₂S quantum dot (QD) photodetectors (PDs) have attracted a lot of attention in the field of imaging system and optical communication. However, the current Ag₂S PDs mainly works in the near-infrared band, and its detection ability in the visible band remains to be strengthened. In this paper, we used poly(3-hexylthiophene) (P3HT) with high carrier mobility and Ag₂S QDs to construct heterojunction PD. Stronger absorption in blends with polymer P3HT compared to single Ag₂S QDs. The optical absorption spectra show that the Ag₂S/P3HT has strong light absorption peak at 394 and 598 nm. The results show that P3HT significantly enhances the absorption of Ag₂S QDs from the visible to near-infrared band. The output characteristics, transfer characteristics and fast switching capability of the device at 405 nm, 532 nm and 808 nm were tested. The device has the responsivity of 6.05 A W^-1, 83.72 A W^-1and 37.31 A W^-1under 405 nm, 532 nm and 808 nm laser irradiation. This work plays an important role in improving the detection performance of Ag₂S QDs and broadening its applications in photoelectric devices for weak light and wide spectrum detection.

In-Situ Monitoring of Reciprocal Charge Transfer and Losses in Graphene-Silicon CCD Pixels

Charge-coupled devices (CCD) allow imaging by photodetection, charge integration, and serial transfer of the stored charge packets from multiple pixels to the readout node. The functionality of CCD can be extended to the non-destructive and in-situ readout of the integrated charges by replacing metallic electrodes with graphene in the metal-oxide-semiconductors (MOS) structure of a CCD pixel. The electrostatic capacitive coupling of graphene with the substrate allows the Fermi level tuning that reflects the integrated charge density in the depletion well. This work demonstrates the in-situ monitoring of the serial charge transfer and interpixel transfer losses in a reciprocating manner between two adjacent Gr-Si CCD pixels by benefitting the electrostatic and gate-to-gate couplings. We achieved the maximum charge transfer efficiency (CTE) of 92.4%, which is mainly decided by the inter-pixel distance, phase clock amplitudes, switching slopes, and density of surface defects. The discussion on overcoming transfer losses and improving CTE by realizing a graphene-electron multiplication CCD is also presented. The proof of the concept of the in-situ readout of the out-of-plane avalanche in a single Gr-Si CCD pixel is also demonstrated, which can amplify the photo packet in a pre-transfer manner.

High-Performance Broad-Band Photodetection Based on Graphene-MoS2xSe2(1-x) Alloy Engineered Phototransistors

The concept of alloy engineering has emerged as a viable technique toward tuning the band gap as well as engineering the defect levels in two-dimensional transition-metal dichalcognides (TMDCs). The possibility of synthesizing these ultrathin TMDC materials through a chemical route has opened up realistic possibilities to fabricate hybrid multifunctional devices. By synthesizing nanosheets with different composites of MoS_2xSe_2(1-x) (x = 0 - 1) using simple chemical methods, we systematically investigate the photoresponse properties of three terminal hybrid devices by decorating large-area graphene with these nanosheets (x = 0, 0.5, 1) in 2D-2D configurations. Among them, the graphene-MoSSe hybrid phototransistor exhibits optoelectronic properties superior to those of its binary counterparts. The device exhibits extremely high photoresponsivity (>10⁴ A/W), low noise equivalent power (∼10^-14 W/Hz^0.5), and higher specific detectivity (∼10¹¹ jones) in the wide UV-NIR (365-810 nm) range with excellent gate tunability. The broad-band light absorption of MoSSe, ultrafast charge transport in graphene, and controllable defect engineering in MoSSe makes this device extremely attractive. Our work demonstrates the large-area scalability with the wafer-scale production of MoS_2xSe_2(1-x) alloys, having important implications toward the facile and scalable fabrication of high-performance optoelectronic devices and providing important insights into the fundamental interactions between van der Waals materials.

2D Heterostructures for Ubiquitous Electronics and Optoelectronics: Principles, Opportunities, and Challenges

A grand family of two-dimensional (2D) materials and their heterostructures have been discovered through the extensive experimental and theoretical efforts of chemists, material scientists, physicists, and technologists. These pioneering works contribute to realizing the fundamental platforms to explore and analyze new physical/chemical properties and technological phenomena at the micro-nano-pico scales. Engineering 2D van der Waals (vdW) materials and their heterostructures via chemical and physical methods with a suitable choice of stacking order, thickness, and interlayer interactions enable exotic carrier dynamics, showing potential in high-frequency electronics, broadband optoelectronics, low-power neuromorphic computing, and ubiquitous electronics. This comprehensive review addresses recent advances in terms of representative 2D materials, the general fabrication methods, and characterization techniques and the vital role of the physical parameters affecting the quality of 2D heterostructures. The main emphasis is on 2D heterostructures and 3D-bulk (3D) hybrid systems exhibiting intrinsic quantum mechanical responses in the optical, valley, and topological states. Finally, we discuss the universality of 2D heterostructures with representative applications and trends for future electronics and optoelectronics (FEO) under the challenges and opportunities from physical, nanotechnological, and material synthesis perspectives.

Physical and Chemical Sensors on the Basis of Laser-Induced Graphene: Mechanisms, Applications, and Perspectives

Laser-induced graphene (LIG) is produced rapidly by directly irradiating carbonaceous precursors, and it naturally exhibits as a three-dimensional porous structure. Due to advantages such as simple preparation, time-saving, environmental friendliness, low cost, and expanding categories of raw materials, LIG and its derivatives have achieved broad applications in sensors. This has been witnessed in various fields such as wearable devices, disease diagnosis, intelligent robots, and pollution detection. However, despite LIG sensors having demonstrated an excellent capability to monitor physical and chemical parameters, the systematic review of synthesis, sensing mechanisms, and applications of them combined with comparison against other preparation approaches of graphene is still lacking. Here, graphene-based sensors for physical, biological, and chemical detection are reviewed first, followed by the introduction of general preparation methods for the laser-induced method to yield graphene. The preparation and advantages of LIG, sensing mechanisms, and the properties of different types of emerging LIG-based sensors are comprehensively reviewed. Finally, possible solutions to the problems and challenges of preparing LIG and LIG-based sensors are proposed. This review may serve as a detailed reference to guide the development of LIG-based sensors that possess properties for future smart sensors in health care, environmental protection, and industrial production.

High-performance perovskite photodetectors based on CsPbBr3 microwire arrays

All inorganic perovskite materials have drawn extensive attention, owing to their outstanding performance, facile solution-processed method, and potential applications in optoelectronic devices. However, uncontrollable morphology, high defect density, and instability of perovskites prepared via solution-processed method are the main challenges for their large-scale production and commercialization. Herein, we prepared large-scale CsPbBr₃ microwire arrays with highly ordered morphology and high crystalline quality by a template-assisted method. The photodetectors based on CsPbBr₃ microwire arrays exhibited remarkable on/off photocurrent ratio of 9.02×10³, high detectivity of 1.59×10¹³ Jones, high responsivity of 4.55 A/W, and fast response speed of 4.9/3 ms. More importantly, the photocurrent of the photodetectors hardly changed in air after being stored for two months, indicating remarkable stability. This study demonstrates that CsPbBr₃ microwire arrays provide the possibility for preparing large-scale and high-performance optoelectronic devices.

Graphene-Ta2O5 heterostructure enabled high performance, deep-ultraviolet to mid-infrared photodetection

Ultrafast, high sensitive, low cost photodetectors operating at room temperature sensitive from the deep-ultraviolet to mid-infrared region remain a significant challenge in optoelectronics. Achievements in traditional semiconductors using cryogenic operation and complicated growth processes prevent the cost-effective and practical application of broadband detectors. Alternative methods towards high-performance photodetectors, hybrid graphene-semiconductor colloidal quantum dots have been intensively explored. However, the operation of these photodetectors has been limited by the spectral bandwidth and response time. Here, we have demonstrated hybrid photodetectors operating from the deep-ultraviolet to the mid-infrared region with high sensitivity and ultrafast response by coupling graphene with a p-type semiconductor photosensitizer, nitrogen-doped Ta2O5 thin film. Photons with energy higher than the energy of the defect centers release holes from neutral acceptors. The holes are transferred into graphene, leaving behind ionized acceptors. Due to the advantage of two-dimensional heterostructure including homogeneous thickness, extending in a two-dimensional plane, large contact area between the N-Ta2O5 thin film and graphene, and the high mobility of carriers in graphene, holes are transferred rapidly to graphene and recirculated during the long lifetime of ionized acceptors. The photodetectors achieve a high photo-responsivity (up to 3.0 × 106 A W-1), ultrafast rise time (faster than 20 ns), and a specific detectivity (up to ∼2.2 × 1012 Jones). The work provides a method for achieving high-performance optoelectronics operating in the deep-ultraviolet to mid-infrared region.

Short-wavelength ultraviolet dosimeters based on DNA nanostructure-modified graphene field-effect transistors

Direct and sensitive short-wavelength ultraviolet (UVC) dosimeters could provide a safer disinfection environment against viruses. We developed direct, quantitative, specific and highly sensitive UVC dosimeters based on DNA nanostructure-modified graphene field-effect transistors. Detectable doses of the dosimeters range from 0.005 to 6 kJ m-2 and such dosimeters have at least 5 times better sensitivity than the current direct UV dosimeters.

Colloidal HgTe Quantum Dot/Graphene Phototransistor with a Spectral Sensitivity Beyond 3 µm

Infrared light detection enables diverse technologies ranging from night vision to gas analysis. Emerging technologies such as low-cost cameras for self-driving cars require highly sensitive, low-cost photodetector cameras with spectral sensitivities up to wavelengths of 10 µm. For this purpose, colloidal quantum dot (QD) graphene phototransistors offer a viable alternative to traditional technologies owing to inexpensive synthesis and processing of QDs. However, the spectral range of QD/graphene phototransistors is thus far limited to 1.6 µm. Here, HgTe QD/graphene phototransistors with spectral sensitivity up to 3 µm are presented, with specific detectivities of 6 × 108 Jones at a wavelength of 2.5 µm and a temperature of 80 K. Even at kHz light modulation frequencies, specific detectivities exceed 108 Jones making them suitable for fast video imaging. The simple device architecture and QD film patterning in combination with a broad spectral sensitivity manifest an important step toward low-cost, multi-color infrared cameras.

The Opto-Electronic Functional Devices Based on Three-Dimensional Lead Halide Perovskites

These days, opto-electronic functional devices based on three-dimensional lead halide perovskites (LHPs) are emerging. LHPs could be spin-coated to other materials, making it very convenient to combine LHPs with different categories of materials including metals, semiconductors, and polymers and achieve high-level performances. In this review, we will discuss the development in the LHP-based functional devices in recent years. After a brief presentation of the LHP’s properties, we will focus on the functional devices including lasers, photodetectors, and modulators. Then the fabrication of the LHP-based devices will be presented, which is followed by the summary and outlook.

Temperature-Dependent Charge Carrier Transfer in Colloidal Quantum Dot/Graphene Infrared Photodetectors

Colloidal PbS quantum dot (QD)/graphene hybrid photodetectors are emerging QD technologies for affordable infrared light detectors. By interfacing the QDs with graphene, the photosignal of these detectors is amplified, leading to high responsivity values. While these detectors have been mainly operated at room temperature, low-temperature operation is required for extending their spectral sensitivity beyond a wavelength of 3 μm. Here, we unveil the temperature-dependent response of PbS QD/graphene phototransistors by performing steady-state and time-dependent measurements over a large temperature range of 80-300 K. We find that the temperature dependence of photoinduced charge carrier transfer from the QD layer to graphene is (i) not impeded by freeze-out of the (Schottky-like) potential barrier at low temperatures, (ii) tremendously sensitive to QD surface states (surface oxidation), and (iii) minimally affected by the ligand exposure time and QD layer thickness. Moreover, the specific detectivity of our detectors increases with cooling, with a maximum measured specific detectivity of at least 10¹⁰ Jones at a wavelength of 1280 nm and a temperature of 80 K, which is an order of magnitude larger compared to the corresponding room temperature value. The temperature- and gate voltage-dependent characterization presented here constitutes an important step in expanding our knowledge of charge transfer at interfaces of low-dimensional materials and toward the realization of next-generation optoelectronic devices.

High-Responsivity Photodetector Based on a Suspended Monolayer Graphene/RbAg4I5 Composite Nanostructure

Graphene has excellent electrical, optical, thermal, and mechanical properties that make it an ideal optoelectronic material. However, it still has some problems, such as a very low light absorption rate, which means it cannot meet the application requirements of high-performance optoelectronic devices. Here, we produce a high-responsivity photodetector based on a monolayer graphene/RbAg₄I₅ composite nanostructure. With the aid of poly(methyl methacrylate), we suspend the monolayer graphene on a hollow carving groove with a width of 100 μm. A RbAg₄I₅ film evaporated on the back of the graphene causes the composite nanostructure to generate a large photocurrent under periodic illumination. Experimental results show that the dissociation and recombination of ion-electron bound states (IEBSs) are responsible for the excellent photoresponse. The device has very high (>1 A W^-1) responsivity in wide-band illumination wavelength from 375 nm to 808 nm, especially at 375 nm, where it shows a responsivity of up to ∼5000 A W^-1. We designed the dimensions of the carving groove to allow the light spot to cover the entire groove, and we cut the graphene sheet to match the length of the carving groove. With the structural optimizations, the energy of light can be used more efficiently to dissociate the IEBSs, which greatly improves the photoresponse of optoelectronic devices based on the proposed monolayer graphene/RbAg₄I₅ composite nanostructure.

ZnO Nanoparticle/Graphene Hybrid Photodetectors via Laser Fragmentation in Liquid

By combining the enhanced photosensitive properties of zinc oxide nanoparticles and the excellent transport characteristics of graphene, UV-sensitive, solar-blind hybrid optoelectronic devices have been demonstrated. These hybrid devices offer high responsivity and gain, making them well suited for photodetector applications. Here, we report a hybrid ZnO nanoparticle/graphene phototransistor that exhibits a responsivity up to 4 × 104 AW-1 and gain of up to 1.3 × 105 with high UV wavelength selectivity. ZnO nanoparticles were synthesized by pulsed laser fragmentation in liquid to attain a simple, efficient, ligand-free method for nanoparticle fabrication. By combining simple fabrication processes with a promising device architecture, highly sensitive ZnO nanoparticle/graphene UV photodetectors were successfully demonstrated.

Graphene Hybrid Structures for Integrated and Flexible Optoelectronics

Graphene (Gr) has many unique properties including gapless band structure, ultrafast carrier dynamics, high carrier mobility, and flexibility, making it appealing for ultrafast, broadband, and flexible optoelectronics. To overcome its intrinsic limit of low absorption, hybrid structures are exploited to improve the device performance. Particularly, van der Waals heterostructures with different photosensitive materials and photonic structures are very effective for improving photodetection and modulation efficiency. With such hybrid structures, Gr hybrid photodetectors can operate from ultraviolet to terahertz, with significantly improved R (up to 10⁹ A W^-1 ) and bandwidth (up to 128 GHz). Furthermore, integration of Gr with silicon (Si) complementary metal-oxide-semiconductor (CMOS) circuits, the human body, and soft tissues is successfully demonstrated, opening promising opportunities for wearable sensors and biomedical electronics. Here, the recent progress in using Gr hybrid structures toward high-performance photodetectors and integrated optoelectronic applications is reviewed.

Microplasma-Enabled Graphene Quantum Dot-Wrapped Gold Nanoparticles with Synergistic Enhancement for Broad Band Photodetection

Plasmonic nanostructure/semiconductor nanohybrids offer many opportunities for emerging electronic and optoelectronic device applications because of their unique geometries in the nanometer scale and material properties. However, the development of a simple and scalable synthesis of plasmonic nanostructure/semiconductor nanohybrids is still lacking. Here, we report a direct synthesis of colloidal gold nanoparticle/graphene quantum dot (Au@GQD) nanohybrids under ambient conditions using microplasmas and their application as photoabsorbers for broad band photodetectors (PDs). Due to the unique AuNP core and graphene shell nanostructures in the synthesized Au@GQD nanohybrids, the plasmonic absorption of the AuNP core extends the usable spectral range of the photodetectors. It is demonstrated that the Au@GQD-based visible light photodetector simultaneously possesses an extraordinary photoresponsivity of ∼10³ A/W, ultrahigh detectivity of 10¹³ Jones, and fast response time in the millisecond scale (65 ms rise time and 53 ms fall time). We suggest that the synergistic effect can be attributed to the strong fluorescence quenching in Au@GQD coupled with the two-dimensional graphene layer in the device. This work provides knowledge of tailoring the optical absorption in GQDs with plasmonic AuNPs and the corresponding photophysics for broad band response in PD-related devices.

Ag2S QDs/Si Heterostructure-Based Ultrasensitive SWIR Range Detector

In the 20th century, microelectronics was revolutionized by silicon-its semiconducting properties finally made it possible to reduce the size of electronic components to a few nanometers. The ability to control the semiconducting properties of Si on the nanometer scale promises a breakthrough in the development of Si-based technologies. In this paper, we present the results of our experimental studies of the photovoltaic effect in Ag2S QD/Si heterostructures in the short-wave infrared range. At room temperature, the Ag2S/Si heterostructures offer a noise-equivalent power of 1.1 × 10-10 W/√Hz. The spectral analysis of the photoresponse of the Ag2S/Si heterostructures has made it possible to identify two main mechanisms behind it: the absorption of IR radiation by defects in the crystalline structure of the Ag2S QDs or by quantum QD-induced surface states in Si. This study has demonstrated an effective and low-cost way to create a sensitive room temperature SWIR photodetector which would be compatible with the Si complementary metal oxide semiconductor technology.

Ultrahigh Responsivity Photodetectors of 2D Covalent Organic Frameworks Integrated on Graphene

2D materials exhibit superior properties in electronic and optoelectronic fields. The wide demand for high-performance optoelectronic devices promotes the exploration of diversified 2D materials. Recently, 2D covalent organic frameworks (COFs) have emerged as next-generation layered materials with predesigned π-electronic skeletons and highly ordered topological structures, which are promising for tailoring their optoelectronic properties. However, COFs are usually produced as solid powders due to anisotropic growth, making them unreliable to integrate into devices. Here, by selecting tetraphenylethylene monomers with photoelectric activity, elaborately designed photosensitive 2D-COFs with highly ordered donor-acceptor topologies are in situ synthesized on graphene, ultimately forming COF-graphene heterostructures. Ultrasensitive photodetectors are successfully fabricated with the COF_ETBC-TAPT -graphene heterostructure and exhibited an excellent overall performance with a photoresponsivity of ≈3.2 × 10⁷ A W^-1 at 473 nm and a time response of ≈1.14 ms. Moreover, due to the high surface area and the polarity selectivity of COFs, the photosensing properties of the photodetectors can be reversibly regulated by specific target molecules. The research provides new strategies for building advanced functional devices with programmable material structures and diversified regulation methods, paving the way for a generation of high-performance applications in optoelectronics and many other fields.

Inkjet-Printed Imbedded Graphene Nanoplatelet/Zinc Oxide Bulk Heterojunctions Nanocomposite Films for Ultraviolet Photodetection

A ZnO sol-gel precursor (ZnOPr) and graphene nanoplatelets (GnPs) are mixed into a composite ink for inkjet printing photodetectors with bulk heterojunctions of ZnO/GnP on a heated SiO2/Si substrate. Heating of the SiO2/Si wafers at ∼50 °C was found optimal to prevent segregated droplets on the hydrophobic surface of the SiO2/Si substrate during printing. After printing the ZnO/GnP channels, thermal annealing at 350 °C for 2 h was performed for crystallization of ZnO and formation of the ZnO/GnP heterojunctions. The GnP concentration was varied from 0, 5, 20, and 30 mM to evaluate optimal formation of the ZnO/GnP bulk heterojunction nanocomposites based on ultraviolet photoresponse performance. The best performance was observed at the 20 mM GnP concentration with the photoresponsivity reaching 2.2 A/W at an incident ultraviolet power of 2.2 μW and a 5 V bias. This photoresponsivity is an order of magnitude better than the previously reported counterparts, including 0.13 mA/W for dropcasted ZnO-graphite composites and much higher than 0.5 A/W for aerosol printed ZnO. The improved performance is attributed to the ZnO/GnP bulk heterojunctions with improved interfaces that enable efficient exciton dissociation and the charge transport. The developed inkjet printing of sol-gel composite inks approach can be scalable and low cost for practical applications.

Two-Dimensional Covalent Organic Framework-Graphene Photodetectors: Insight into the Relationship between the Microscopic Interfacial Structure and Performance

Graphene is an attractive material for photodetection and optoelectronic applications because it offers a broad spectral bandwidth and ultrafast response speed. However, because of the broad light absorption characteristic, graphene has a lack of selectivity to the wavelength, which limits the performance of graphene-based photodetectors. Here, we demonstrate a novel hybrid photodetector with monolayer graphene covered with an ultrathin film of surface covalent organic frameworks (COFs) with variable structures as the light-harvesting materials. Photodetectors based on surface COF-G show enhanced responsivity in comparison with unmodified graphene and graphene modified with monomers. The submolecular resolution of scanning tunneling microscopy allows us to get a direct insight into the relationship between the microscopic interfacial structure and the performance of the device. We prove that the enhancement in the device performance is directly related with the orderliness of surface COFs, which influences the interfacial charge transfer by tuning π-π stacking between surface COF and graphene.

Dramatic Enhancement of Optoelectronic Properties of Electrophoretically Deposited C60-Graphene Hybrids

Fullerene (C₆₀) and multilayer graphene hybrid devices were fabricated using electrophoretic deposition, where the C₆₀ clusters are electrically charged upon the application of an external bias in a polar solvent, acetonitrile, mixed with toluene, which facilitates their deposition on the graphene membranes. Raman spectroscopy unveiled the unique vibrational fingerprints associated with the A_2g mode of the C₆₀ molecules at ∼1453 cm^-1, while blue shifts of ∼6 and ∼17 cm^-1 were also attributed to the G- and 2D-bands of the hybrids relative to bare graphene, suggestive of p-doped graphene. The intensity ratio of the G- and the 2D-bands I_2D/I_G (hybrid) dropped to ∼0.18 from ∼0.3 (bare graphene), and this reduction in I_2D/I_G is also a signature of hole-doped graphene, consistent with the relatively strong electron accepting nature of C₆₀. The electronic conductance of the two-terminal hybrid devices increased relative to bare graphene at room temperature which was attributed to the increased carrier density, and temperature-dependent electronic transport measurements were also conducted from ambient down to ∼5.8 K. Additionally, a low energy shift in the Fermi level, E_F ≈ 140 meV, was calculated for the hybrids. When the hybrid devices were irradiated with a broadband white light source and a tunable laser source (with a wavelength λ ranging from ∼400-1100 nm), a strong photoresponse was evident, in contrast to the bare graphene devices which appeared unresponsive. The responsivity R of the hybrids was measured to be ∼10⁹ A/W at λ ≈ 400 nm and ∼298 K, while the detectivity and external quantum efficiency were also exceptional, ∼10¹⁵ jones and ∼10⁹%, respectively, at ∼1 V and a light power density of ∼3 mW/cm². The R values are ∼10 times higher compared to other hybrid devices derived from graphene reported previously, such as quantum dot-graphene and few-layer MoS₂-graphene heterostructures. The strong photoresponse of the C₆₀-graphene hybrids reported here is attributed to the doping enhancement arising in graphene upon the adsorption of C₆₀. This work demonstrates the exceptional potential of such hybrid nanocarbon-based structures for optoelectronics.

Photogating in the Graphene-Dye-Graphene Sandwich Heterostructure

In this work, we developed an atomically thin (∼2.5 nm) heterostructure consisting of a monolayer rhodamine 6G (R6G) film as a photoactive layer that was sandwiched between graphene films functioning as channels (graphene-R6G-graphene, G-R-G). Through a comparison of results of both photocurrent measurements and chemically enhanced Raman scattering (CERS) experiments, we found that our G-R-G heterostructure exhibited ∼7 and ∼30 times better performance than R6G-attached single-graphene (R6G-graphene, R-G) and MoS₂ devices, respectively; here, the CERS enhancement factor was highly correlated with the relative photoinduced Dirac voltage change. Furthermore, the photocurrent of the G-R-G device was found to be ∼40 times better than that of the R-G photodetector. The top graphene was highly operative in the monolayer, of which the performance is significantly deteriorated by fluorescence and tailored charge transfer efficiency with the increment of R6G film thickness. Overall, the responsivity of the G-R-G photodetector was ∼40 times higher than that of the R-G photodetector because of the more efficient carrier transfer between the organic dye and graphene induced by weaker π-π interactions between the top and bottom graphene channels in the former device. This atomically thin (∼2.5 nm) and highly photosensitive photodetector can be employed for post-Si-photodiode (PD) image sensors, single-photon detection devices, and optical communications.

Excimer ultraviolet-irradiated exfoliated graphite loaded with carbon-coated SnOx small nanoparticles as advanced anodes for high-rate-capacity lithium-ion batteries

This paper reports a fast and efficient excimer ultraviolet (EUV) radiation method to prepare carbon-coated mixed tin oxide-loaded exfoliated graphite (SnOx@C-G) nanocomposites. The SnOx small nanoparticles (SNPs) are isolated using oxidized sucrose and uniformly deposited onto mildly oxidized exfoliated graphite during the 20-minute EUV radiation process. XPS and ESR analyses suggest the existence of abundant oxygen vacancies in the SnOx SNPs. The electrochemical kinetics of SnOx@C-G, which are determined by in situ electrochemical impedance analysis, demonstrated a high reversible capacity of approximately 740 mA h g-1 after 250 cycles at a current density of 1.6 A g-1, and an impressive reversible rate performance exceeding 450 mA h g-1 can be obtained even at a high current density of 3.2 A g-1 when applied as an anode for lithium storage. This improved cycling stability and rate capability benefit from the carbon coating, which not only buffers the volume change of SnOx SNPs but also provides a path for electron transport on the surface of the SnOx SNPs during the electrochemical process. Furthermore, the oxygen vacancies in SnOx SNPs result in a large capacitive contribution to capacity. The EUV radiation method used to synthesize SnOx@C-graphite nanosheets is universally applicable to prepare a high-performance SNPs/carbon-based anode for lithium-ion batteries.

Ultrafast Photovoltaic-Type Deep Ultraviolet Photodetectors Using Hybrid Zero-/Two-Dimensional Heterojunctions

Deep ultraviolet (DUV) photodetectors have wide-range applications in satellite communications, air purification, and missile-plume detection. However, the critical barriers for the currently available wide-band gap semiconductor film-based DUV photodetectors are their low efficiency, complicated processes, and lattice mismatch with the substrate. Quantum dot (QD) devices prepared using solution-based methods can solve these problems. However, so far, there are no reports on photovoltaic-type DUV photodetectors using QDs. In this study, we propose a novel methodology to construct a hybrid zero-/two-dimensional DUV photodetector (p-type graphene/ZnS QDs/4H-SiC) with photovoltaic characteristics. The device exhibits excellent selectivity for the DUV light and has an ultrafast response speed (rise time: 28 μs and decay time: 0.75 ms), which are much better than those reported for conventional photoconductive photodetectors.

Single-channel UV/vis dual-band detection with ZnCdS:Mn/ZnS core/shell quantum dots

In the ultraviolet detection system, the Si-based photodetector could be sensitised with different kinds of fluorescent material to enhance its response in the short-wavelength range. Thick-shell ZnCdS:Mn/ZnS core/shell quantum dots (QDs) exhibit unique advantages in UV signal sensitisation due to their long PL lifetime, as well as stable emission matched with CCD's response. Herein, a single-channel UV panoramic detection system based on these Mn-doped QDs has been proposed. The QDs@PMMA film was attached on a Si-based CCD camera versus a tapered fibre, and an optical chopper was mounted before the QDs@PMMA film. The long lifetime fluorescence originating from UV signal could be still collected by the CCD camera when the chopper is in the 'off' state, hence the UV/vis signal ratio is significantly enhanced.

Recent Progress and Future Prospects of 2D-Based Photodetectors

Conventional semiconductors such as silicon- and indium gallium arsenide (InGaAs)-based photodetectors have encountered a bottleneck in modern electronics and photonics in terms of spectral coverage, low resolution, nontransparency, nonflexibility, and complementary metal-oxide-semiconductor (CMOS) incompatibility. New emerging two-dimensional (2D) materials such as graphene, transition metal dichalcogenides (TMDs), and their hybrid systems thereof, however, can circumvent all these issues benefitting from mechanically flexibility, extraordinary electronic and optical properties, as well as wafer-scale production and integration. Heterojunction-based photodiodes based on 2D materials offer ultrafast and broadband response from the visible to far-infrared range. Phototransistors based on 2D hybrid systems combined with other material platforms such as quantum dots, perovskites, organic materials, or plasmonic nanostructures yield ultrasensitive and broadband light-detection capabilities. Notably the facile integration of 2D photodetectors on silicon photonics or CMOS platforms paves the way toward high-performance, low-cost, broadband sensing and imaging modalities.

Sensitive and Robust Ultraviolet Photodetector Array Based on Self-Assembled Graphene/C60 Hybrid Films

Graphene has been widely investigated for use in high-performance photodetectors due to its broad absorption band and high carrier mobility. While exhibiting remarkably strong absorption in the ultraviolet range, the fabrication of a large-scale integrable, graphene-based ultraviolet photodetector with long-term stability has proven to be a challenge. Here, using graphene as a template for C₆₀ assembly, we synthesized a large-scale all-carbon hybrid film with inherently strong and tunable UV aborption. Efficient exciton dissociation at the heterointerface and enhanced optical absorption enables extremely high photoconductive gain, resulting in UV photoresponsivity of ∼10⁷ A/W. Interestingly, due to the electron-hole recombination process at the heterointerface, the response time can be modulated by the gate voltage. More importantly, the use of all-carbon hybrid materials ensures robust operation and further allows the demonstration of an exemplary 5 × 5 (2-dimensional) photodetector array. The devices exhibit negligible degradation in figures of merit even after 2 month of operation, indicating excellent environmental robustness. The combination of high responsivity, reliability, and scalable processability makes this new all-carbon film a promising candidate for future integrable optoelectronics.

Assembly of graphene nanoflake-quantum dot hybrids in aqueous solution and their performance in light-harvesting applications

Graphene nanoflakes and CdSe/ZnS quantum dots were covalently linked in environmentally friendly aqueous solution. Raman spectroscopy and photoluminescence studies, both in solution and on surfaces at the single nanohybrid level, showed evidence of charge transfer between the two nanostructures. The nanohybrids were further incorporated into solar cell devices, demonstrating their potential as light harvesting assemblies.

Narrow bandgap oxide nanoparticles coupled with graphene for high performance mid-infrared photodetection

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 (E_g = 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.

One-Dimensional Assembly on Two-Dimensions: AuCN Nanowire Epitaxy on Graphene for Hybrid Phototransistors

The van der Waals epitaxy of functional materials provides an interesting and efficient way to manipulate the electrical properties of various hybrid two-dimensional (2D) systems. Here we show the controlled epitaxial assembly of semiconducting one-dimensional (1D) atomic chains, AuCN, on graphene and investigate the electrical properties of 1D/2D van der Waals heterostructures. AuCN nanowire assembly is tuned by different growth conditions, although the epitaxial alignment between AuCN chains and graphene remains unchanged. The switching of the preferred nanowire growth axis indicates that diffusion kinetics affects the nanowire formation process. Semiconducting AuCN chains endow the 1D/2D hybrid system with a strong responsivity to photons with an energy above 2.7 eV, which is consistent with the bandgap of AuCN. A large UV response (responsivity ∼10⁴ A/W) was observed under illumination using 3.1 eV (400 nm) photons. Our study clearly demonstrates that 1D chain-structured semiconductors can play a crucial role as a component in multifunctional van der Waals heterostructures.

Graphene-Based Light Sensing: Fabrication, Characterisation, Physical Properties and Performance

Graphene and graphene-based materials exhibit exceptional optical and electrical properties with great promise for novel applications in light detection. However, several challenges prevent the full exploitation of these properties in commercial devices. Such challenges include the limited linear dynamic range (LDR) of graphene-based photodetectors, the lack of efficient generation and extraction of photoexcited charges, the smearing of photoactive junctions due to hot-carriers effects, large-scale fabrication and ultimately the environmental stability of the constituent materials. In order to overcome the aforementioned limits, different approaches to tune the properties of graphene have been explored. A new class of graphene-based devices has emerged where chemical functionalisation, hybridisation with light-sensitising materials and the formation of heterostructures with other 2D materials have led to improved performance, stability or versatility. For example, intercalation of graphene with FeCl 3 is highly stable in ambient conditions and can be used to define photo-active junctions characterized by an unprecedented LDR while graphene oxide (GO) is a very scalable and versatile material which supports the photodetection from UV to THz frequencies. Nanoparticles and quantum dots have been used to enhance the absorption of pristine graphene and to enable high gain thanks to the photogating effect. In the same way, hybrid detectors made from stacked sequences of graphene and layered transition-metal dichalcogenides enabled a class of devices with high gain and responsivity. In this work, we will review the performance and advances in functionalised graphene and hybrid photodetectors, with particular focus on the physical mechanisms governing the photoresponse, the performance and possible future paths of investigation.

Graphene-Based Semiconductor Heterostructures for Photodetectors

Graphene transparent conductive electrodes are highly attractive for photodetector (PD) applications due to their excellent electrical and optical properties. The emergence of graphene/semiconductor hybrid heterostructures provides a platform useful for fabricating high-performance optoelectronic devices, thereby overcoming the inherent limitations of graphene. Here, we review the studies of PDs based on graphene/semiconductor hybrid heterostructures, including device physics/design, performance, and process technologies for the optimization of PDs. In the last section, existing technologies and future challenges for PD applications of graphene/semiconductor hybrid heterostructures are discussed.

Gate-Controlled Graphene-Silicon Schottky Junction Photodetector

Various photodetectors showing extremely high photoresponsivity have been frequently reported, but many of these photodetectors could not avoid the simultaneous amplification of dark current. A gate-controlled graphene-silicon Schottky junction photodetector that exhibits a high on/off photoswitching ratio (≈10⁴ ), a very high photoresponsivity (≈70 A W^-1 ), and a low dark current in the order of µA cm^-2 in a wide wavelength range (395-850 nm) is demonstrated. The photoresponsivity is ≈100 times higher than that of existing commercial photodetectors, and 7000 times higher than that of graphene-field-effect transistor-based photodetectors, while the dark current is similar to or lower than that of commercial photodetectors. This result can be explained by a unique gain mechanism originating from the difference in carrier transport characteristics of silicon and graphene.

Self-Assembled UV Photodetector Made by Direct Epitaxial GaN Growth on Graphene

Hybrid systems based on the combination of crystalline bulk semiconductors with 2D crystals are identified as promising heterogeneous structures for new optoelectronic applications. The direct integration of III-V semiconductors on 2D materials is very attractive to make practical devices but the preservation of the intrinsic properties of the underlying 2D materials remains a challenge. In this work, we study the direct epitaxy of self-organized GaN crystals on graphene. We demonstrate that severe metal-organic chemical vapor deposition growth conditions of GaN (chemically aggressive precursors and high temperatures) are not detrimental to the structural quality and the charge carrier mobility of the graphene base plane. Graphene can therefore be used both as an efficient sensitive material and as a substrate for GaN epitaxy to make a self-assembled UV photodetector. A responsivity as high as 2 A W^-1 is measured in the UV-A range without any further postprocessing compared to simple deposition of contact electrodes. Our study opens the way to build new self-assembled 2D/III-V hybrid optoelectronic devices by direct epitaxy.

Dual role of Ag nanowires in ZnO quantum dot/Ag nanowire hybrid channel photo thin film transistors

High mobility and p-type thin film transistors (TFTs) are in urgent need for high-speed electronic devices. In this work, ZnO quantum dot (QD)/Ag nanowire (NW) channel TFTs were fabricated by a solution processed method. The Ag NWs play the dual role of dopant and providing the charge transfer route, which make the channel p-type and enhance its mobility, respectively. The best sample yields an on/off ratio (I on/I off) of 5.04 × 105, a threshold voltage (V T) of 0.73 V, a high field effect mobility (μ FE) of 8.69 cm2 V-1 s-1, and a subthreshold swing (SS) of 0.41 V dec-1. Owing to the strong ultraviolet (UV) absorption and photo-induced carrier separation ability of ZnO QDs and the fast carrier transport of Ag NWs, the devices acquire a high external quantum efficiency (EQE) and ultra-fast response under 365 nm UV illumination. The UV-modulated ZnO QD/Ag NW hybrid channel photo TFTs have potential for future application in optoelectronic devices, such as photodetectors and photoswitches.

Gold-patched graphene nano-stripes for high-responsivity and ultrafast photodetection from the visible to infrared regime

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.

Stable Graphene-Two-Dimensional Multiphase Perovskite Heterostructure Phototransistors with High Gain

Recently, two-dimensional (2D) organic-inorganic perovskites emerged as an alternative material for their three-dimensional (3D) counterparts in photovoltaic applications with improved moisture resistance. Here, we report a stable, high-gain phototransistor consisting of a monolayer graphene on hexagonal boron nitride (hBN) covered by a 2D multiphase perovskite heterostructure, which was realized using a newly developed two-step ligand exchange method. In this phototransistor, the multiple phases with varying bandgap in 2D perovskite thin films are aligned for the efficient electron-hole pair separation, leading to a high responsivity of ∼10⁵ A W^-1 at 532 nm. Moreover, the designed phase alignment method aggregates more hydrophobic butylammonium cations close to the upper surface of the 2D perovskite thin film, preventing the permeation of moisture and enhancing the device stability dramatically. In addition, faster photoresponse and smaller 1/f noise observed in the 2D perovskite phototransistors indicate a smaller density of deep hole traps in the 2D perovskite thin film compared with their 3D counterparts. These desirable properties not only improve the performance of the phototransistor, but also provide a new direction for the future enhancement of the efficiency of 2D perovskite photovoltaics.

Graphene-Insulator-Semiconductor Junction for Hybrid Photodetection Modalities

A sensitive optical detector is presented based on a deeply depleted graphene-insulator-semiconducting (D²GIS) junction, which offers the possibility of simultaneously leveraging the advantages of both charge integration and localized amplification. Direct read-out and built-in amplification are accomplished via photogating of a graphene field-effect transistor (GFET) by carriers generated within a deeply depleted low-doped silicon substrate. Analogous to a depleted metal-oxide-semiconducting junction, photo-generated charge collects in the potential well that forms at the semiconductor/insulator interface and induces charges of opposite polarity within the graphene film modifying its conductivity. This device enables simultaneous photo-induced charge integration with continuous “on detector” readout through use of graphene. The resulting devices exhibit responsivities as high as 2,500 A/W (25,000 S/W) for visible wavelengths and a dynamic range of 30 dB. As both the graphene and device principles are transferrable to arbitrary semiconductor absorbers, D²GIS devices offer a high-performance paradigm for imaging across the electromagnetic spectrum.

Highly Efficient Rubrene-Graphene Charge-Transfer Interfaces as Phototransistors in the Visible Regime

Atomically thin materials such as graphene are uniquely responsive to charge transfer from adjacent materials, making them ideal charge-transport layers in phototransistor devices. Effective implementation of organic semiconductors as a photoactive layer would open up a multitude of applications in biomimetic circuitry and ultra-broadband imaging but polycrystalline and amorphous thin films have shown inferior performance compared to inorganic semiconductors. Here, the long-range order in rubrene single crystals is utilized to engineer organic-semiconductor-graphene phototransistors surpassing previously reported photogating efficiencies by one order of magnitude. Phototransistors based upon these interfaces are spectrally selective to visible wavelengths and, through photoconductive gain mechanisms, achieve responsivity as large as 10⁷ A W^-1 and a detectivity of 9 × 10¹¹ Jones at room temperature. These findings point toward implementing low-cost, flexible materials for amplified imaging at ultralow light levels.

Plasmonic Silicon Quantum Dots Enabled High-Sensitivity Ultrabroadband Photodetection of Graphene-Based Hybrid Phototransistors

Highly sensitive photodetection even approaching the single-photon level is critical to many important applications. Graphene-based hybrid phototransistors are particularly promising for high-sensitivity photodetection because they have high photoconductive gain due to the high mobility of graphene. Given their remarkable optoelectronic properties and solution-based processing, colloidal quantum dots (QDs) have been preferentially used to fabricate graphene-based hybrid phototransistors. However, the resulting QD/graphene hybrid phototransistors face the challenge of extending the photodetection into the technologically important mid-infrared (MIR) region. Here, we demonstrate the highly sensitive MIR photodetection of QD/graphene hybrid phototransistors by using plasmonic silicon (Si) QDs doped with boron (B). The localized surface plasmon resonance (LSPR) of B-doped Si QDs enhances the MIR absorption of graphene. The electron-transition-based optical absorption of B-doped Si QDs in the ultraviolet (UV) to near-infrared (NIR) region additionally leads to photogating for graphene. The resulting UV-to-MIR ultrabroadband photodetection of our QD/graphene hybrid phototransistors features ultrahigh responsivity (up to ∼10⁹ A/W), gain (up to ∼10¹²), and specific detectivity (up to ∼10¹³ Jones).

An Omnidirectionally Stretchable Photodetector Based on Organic-Inorganic Heterojunctions

Omnidirectionally stretchable photodetectors are limited by difficulties in designing material and fabrication processes that enable stretchability in multiaxial directions. Here, we propose a new approach involving an organic-inorganic p-n heterojunction photodetector comprised of free-standing ZnO nanorods grown on a poly(3,4-ethylenedioxythiophene)-polystyrene sulfonate transport layer coated on a three-dimensional micropatterned stretchable substrate containing bumps and valleys. This structure allows for efficient absorption of stretching strain. This approach allows the device to accommodate large tensile strain in all of the directions. The device behaves as a photogated p-n heterojunction photodetector in which current modulation was obtained by sensing the mechanisms that rely on photovoltage and photogating effects. The device exhibits a high photoresponse to UV light and reliable electrical performance under applied stretching in uniaxial and omniaxial directions. Furthermore, the device can be easily and conformally attached to a human wrist. This allowed us to investigate the response of the device to UV light during human activity.

Improving the Performance of Graphene Phototransistors Using a Heterostructure as the Light-Absorbing Layer

Interfacing light-sensitive semiconductors with graphene can afford high-gain phototransistors by the multiplication effect of carriers in the semiconductor layer. So far, most devices consist of one semiconductor light-absorbing layer, where the lack of internal built-in field can strongly reduce the quantum efficiency and bandwidth. Here, we demonstrate a much improved graphene phototransistor performances using an epitaxial organic heterostructure composed of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) and pentacene as the light-absorbing layer. Compared with single light-absorbing material, the responsivity and response time can be simultaneously improved by 1 and 2 orders of magnitude over a broad band of 400-700 nm, under otherwise the same experimental conditions. As a result, the external quantum efficiency increases by over 800 times. Furthermore, the response time of the heterostructured phototransistor is highly gate-tunable down to sub-30 μs, which is among the fastest in the sensitized graphene phototransistors interfacing with electrically passive light-absorbing semiconductors. We show that the improvement is dominated by the efficient electron-hole pair dissociation due to interfacial built-in field rather than bulk absorption. The structure demonstrated here can be extended to many other organic and inorganic semiconductors, which opens new possibilities for high-performance graphene-based optoelectronics.

Printable Nanocomposite FeS2-PbS Nanocrystals/Graphene Heterojunction Photodetectors for Broadband Photodetection

Colloidal nanocrystals are attractive materials for optoelectronics applications because they offer a compelling combination of low-cost solution processing, printability, and spectral tunability through the quantum dot size effect. Here we explore a novel nanocomposite photosensitizer consisting of colloidal nanocrystals of FeS₂ and PbS with complementary optical and microstructural properties for broadband photodetection. Using a newly developed ligand exchange to achieve high-efficiency charge transfer across the nanocomposite FeS₂-PbS sensitizer and graphene on the FeS₂-PbS/graphene photoconductors, an extraordinary photoresponsivity in exceeding ∼10⁶ A/W was obtained in an ultrabroad spectrum of ultraviolet (UV)-visible-near-infrared (NIR). This is in contrast to the nearly 3 orders of magnitude reduction of the photoresponsivity from ∼10⁶ A/W at UV to 10³ A/W at NIR on their counterpart of FeS₂/graphene detectors. This illustrates the combined advantages of the nanocomposite sensitizers and the high charge mobility in FeS₂-PbS/graphene van der Waals heterostructures for nanohybrid optoelectronics with high performance, low cost, and scalability for commercialization.

Ultra-high sensitivity infra-red detection and temperature effects in a graphene-tellurium nanowire binary hybrid

The optoelectronic performance of hybrid devices from graphene and optically sensitive semiconductors exceeds conventional photodetectors due to a large in-built optical gain. Tellurium nanowire (TeNW), being a narrow direct band gap semiconductor (∼0.65 eV), is as an excellent potential candidate for near infra-red (NIR) detection. Here we demonstrate a new graphene-TeNW binary hybrid that exhibits a maximum photoresponsivity of ∼10⁶ A W^-1 at 175 K in the NIR regime (920 nm-1720 nm), which exceeds the photoresponsivity of the most common NIR photodetectors. The resulting noise-equivalent power (NEP) is as low as 2 × 10^-18 W Hz^-1/2, and the specific detectivity (D*) exceeds 5 × 10¹³ cm Hz^1/2 W^-1 (Jones). The temperature range of optimal operation, which extends up to ≈220 K and ≈260 K for 1720 nm and 920 nm excitation, respectively, is primarily limited by the electrical conductivity of the TeNW layer, and can further be improved by lowering of the defect density as well as inter-wire electronic coupling.