Graphene and PbS quantum dot hybrid vertical phototransistor

Electrical Transport Properties of PbS Quantum Dot/Graphene Heterostructures

The integration of PbS quantum dots (QDs) with graphene represents a notable advancement in enhancing the optoelectronic properties of quantum-dot-based devices. This study investigated the electrical transport properties of PbS quantum dot (QD)/graphene heterostructures, leveraging the high carrier mobility of graphene. We fabricated QD/graphene/SiO2/Si heterostructures by synthesizing p-type monolayer graphene via chemical vapor deposition and spin-coating PbS QDs on the surface. Then, we used a low-temperature electrical transport measurement system to study the electrical transport properties of the heterostructure under different temperature, gate voltage, and light conditions and compared them with bare graphene samples. The results indicated that the QD/graphene samples exhibited higher resistance than graphene alone, with both resistances slightly increasing with temperature. The QD/graphene samples exhibited significant hole doping, with conductivity increasing from 0.0002 Ω-1 to 0.0007 Ω-1 under gate voltage modulation. As the temperature increased from 5 K to 300 K, hole mobility decreased from 1200 cm2V-1s-1 to 400 cm2V-1s-1 and electron mobility decreased from 800 cm2V-1s-1 to 200 cm2V-1s-1. Infrared illumination reduced resistance, thereby enhancing conductivity, with a resistance change of about 0.4%/mW at a gate voltage of 125 V, demonstrating the potential of these heterostructures for infrared photodetector applications. These findings offer significant insights into the charge transport mechanisms in low-dimensional materials, paving the way for high-performance optoelectronic devices.

Graphene-PbS Quantum Dot Heterostructure for Broadband Photodetector with Enhanced Sensitivity

Photodetectors converting light into electrical signals are crucial in various applications. The pursuit of high-performance photodetectors with high sensitivity and broad spectral range simultaneously has always been challenging in conventional semiconductor materials. Graphene, with its zero bandgap and high electron mobility, is an attractive candidate, but its low light absorption coefficient restricts its practical application in light detection. Integrating graphene with light-absorbing materials like PbS quantum dots (QDs) can potentially enhance its photodetection capabilities. Here, this work presents a broadband photodetector with enhanced sensitivity based on a graphene-PbS QD heterostructure. The device leverages the high carrier mobility of graphene and the strong light absorption of PbS QDs, achieving a wide detection range from ultraviolet to near-infrared. Employing a simple spinning method, the heterostructure demonstrates ultrahigh responsivity up to the order of 107 A/W and a specific detectivity on the order of 1013 Jones, showcasing significant potential for photoelectric applications.

Measuring the carrier diffusion length in quantum dot films using graphene as photocarrier density probe

The diffusion length of quantum dot (QD) films is a critical parameter to improve the performance of QD-based optoelectronic devices. The dot-to-dot hopping transport mechanism results in shorter diffusion lengths compared to bulk solids. Herein, we present an experimental method to measure the diffusion length in PbS QD films using single layer graphene as a charge collector to monitor the density of photogenerated carriers. By producing devices with different thicknesses, we can construct light absorption and photocarrier density profiles, allowing extracting light penetration depths and carrier diffusion lengths for electrons and holes. We realized devices with small (size: ∼2.5 nm) and large (size: ∼4.8 nm) QDs, and use λ = 532 nm and λ = 635 nm wavelength illumination. For small QDs, we obtain diffusion lengths of 180 nm for holes and 500 nm for electrons. For large QDs, we obtain diffusion lengths of 120 nm for holes and 150 nm for electrons. Our results show that films made of small QD films have longer diffusion lengths for holes and electrons. We also observe that wavelength illumination may have a small effect, with electrons showing a diffusion length of 500 and 420 nm under λ = 532 nm and λ = 635 nm illumination, respectively, which may be due to increased interactions between photocarriers for longer wavelengths with deeper penetration depths. Our results demonstrate an effective technique to calculate diffusion lengths of photogenerated electrons and holes and indicate that not only QD size but also wavelength illumination can play important roles in the diffusion and electrical transport of photocarriers in QD films.

Liquid-Exfoliated 2D Materials for Optoelectronic Applications

Two-dimensional (2D) materials have attracted tremendous research attention in recent days due to their extraordinary and unique properties upon exfoliation from the bulk form, which are useful for many applications such as electronics, optoelectronics, catalysis, etc. Liquid exfoliation method of 2D materials offers a facile and low-cost route to produce large quantities of mono- and few-layer 2D nanosheets in a commercially viable way. Optoelectronic devices such as photodetectors fabricated from percolating networks of liquid-exfoliated 2D materials offer advantages compared to conventional devices, including low cost, less complicated process, and higher flexibility, making them more suitable for the next generation wearable devices. This review summarizes the recent progress on metal-semiconductor-metal (MSM) photodetectors fabricated from percolating network of 2D nanosheets obtained from liquid exfoliation methods. In addition, hybrids and mixtures with other photosensitive materials, such as quantum dots, nanowires, nanorods, etc. are also discussed. First, the various methods of liquid exfoliation of 2D materials, size selection methods, and photodetection mechanisms that are responsible for light detection in networks of 2D nanosheets are briefly reviewed. At the end, some potential strategies to further improve the performance the devices are proposed.

Fabrication of graphene: CdSe quantum dots/CdS nanorod heterojunction photodetector and role of graphene to enhance the photoresponsive characteristics

Integration of graphene with semiconducting quantum dots (QDs) provides an elegant way to access the intrinsic properties of graphene and optical properties of QDs concurrently to realize the high-performance optoelectronic devices. In the current article, we have demonstrated the high-performance photodetector based on graphene: CdSe QDs/CdS nanorod heterostructures. The resulting heterojunction photodetector with device configuration ITO/graphene: CdSe/CdS nanorods/Ag show excellent operating characteristics including a maximum photoresponsivity of 15.95 AW^-1and specific detectivity of 6.85 × 10¹²Jones under 530 nm light illumination. The device exhibits a photoresponse rise time of 545 ms and a decay time of 539 ms. Furthermore, the study of the effect of graphene nanosheets on the performance enhancement of heterojunction photodetector is carried out. The results indicate that, due to the enhanced energy transfer from photoexcited QDs to graphene layer, light absorption is increased and excitons are generated led to the enhancement of photocurrent density. In addition to that, the graphene: CdSe QDs/CdS nanorod interface can facilitate charge carrier transport effectively. This work provides a promising approach to develop high-performance visible-light photodetectors and utilizing advantageous features of graphene in optoelectronic devices.

High-Performance Vertical Organic Phototransistors Enhanced by Ferroelectrics

Organic phototransistors with high sensitivity and responsivity to light irradiance have great potential applications in national defense, meteorology, industrial manufacturing, and medical security. However, undesired dark current and photoresponsivity limit their practical applications. Here, a novel vertical organic phototransistor combined with ferroelectric materials is developed. The device structure has nanometer channel length, which can effectively separate photogenerated carriers and reduce the probability of carrier recombination and defect scattering, thus improving the device performance of phototransistors. Moreover, by inserting the poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) ferroelectric layer, the Schottky barrier at the interface between the semiconductor and source can be adjusted by the polarization of the external electric field, which can effectively reduce the dark current of the phototransistor to further improve the device performance. Therefore, our phototransistors exhibit a high photoresponsivity of more than 5.7 × 10⁵A/W, an outstanding detectivity of 1.15 × 10¹⁸ Jones, and an excellent photosensitivity of 5 × 10⁷ under 760 nm light illumination, which are better than those of conventional lateral organic phototransistors. This work provides a new approach for the development of high-performance phototransistors, which opens a new pathway for organic phototransistors in practical application.

Efficient FAPbI3–PbS quantum dot graphene-based phototransistors

PbS quantum dots capped with formamidinium ligands were deposited as graphene-based photodetectors. Solid phase exchange improves the infrared photo-detectivity.

Infrared tunable, two colour-band photodetectors on flexible platforms using 0D/2D PbS-MoS2 hybrids

Two-dimensional (2D) MoS₂ nanosheets have been integrated with zero-dimensional (0D) PbS quantum dots to achieve a superior optical response extending to the short-wavelength infrared region along with a broadband visible response for multispectral photodetection. The 0D/2D hybrid nanostructures have been synthesized by a one pot, stabilizer-free solvothermal growth process. Microscopic and spectroscopic studies confirmed the formation of PbS QD decorated semiconducting 2H-MoS₂ layers. The size tunable absorption features with longer photo-generated carrier lifetime of synthesized hybrid nanostructures indicate that the integration of PbS QDs in MoS₂ could be a viable approach for fabricating two-colour band photodetectors, viz. visible broadband and wavelength selective short-wave IR photodetectors. Devices have also been demonstrated on polyethylene terephthalate substrates using a solution-based synthesis technique for flexible and ultrathin optoelectronic device applications. The photodetection performance of fabricated devices suggests that the synergic 0D/2D hybrid nanostructures are significantly superior to solution processed hybrid devices operating in the infrared region. The successful integration of 0D QDs in 2D materials may pave the way for novel, high performance, next-generation CMOS compatible flexible photonic devices.

Improved Charge Extraction Beyond Diffusion Length by Layer-by-Layer Multistacking Intercalation of Graphene Layers inside Quantum Dots Films

Charge collection is critical in any photodetector or photovoltaic device. Novel materials such as quantum dots (QDs) have extraordinary light absorption properties, but their poor mobility and short diffusion length limit efficient charge collection using conventional top/bottom contacts. In this work, a novel architecture based on multiple intercalated chemical vapor deposition graphene monolayers distributed in an orderly manner inside a QD film is studied. The intercalated graphene layers ensure that at any point in the absorbing material, photocarriers will be efficiently collected and transported. The devices with intercalated graphene layers have superior quantum efficiency over single-bottom graphene/QD devices, overcoming the known restriction that the diffusion length imposes on film thickness. QD film with increased thickness shows efficient charge collection over the entire λ ≈ 500-1000 nm spectrum. This architecture could be applied to boost the performance of other low-cost materials with poor mobility, allowing efficient collection for films thicker than their diffusion length.

A highly sensitive and fast graphene nanoribbon/CsPbBr3 quantum dot phototransistor with enhanced vertical metal oxide heterostructures

Although recent breakthroughs in reported graphene-based phototransistors with embedded quantum dots (QDs) have definitely been astonishing, there are still some obstacles in their practical use with regard to their electrical and optical performances. We show that through optimization of the vertical graphene nanoribbon (GNR)/QD/IGZO heterostructure and the ultrahigh efficiency of CsPbBr3 QDs, it is possible to significantly increase the on/off ratio (>103), the subthreshold slope (S.S., 0.9 V dec-1), the device's field effect mobility (μFET, 13 cm-1 V-1 S-1) and other electrical properties. Subsequently, on the basis of the extra optical-electrical characterization, we attribute the enhanced photosensitivity (800), the accelerated detecting speed (141 μs) and the high detectivity (7.5 × 1014 cm Hz1/2 W-1) to the vertical heterostructure associated with the optimized GNR component. To further demonstrate this enhancement phenomenon, the mechanism and theory mode of this vertical heterostructure are analyzed and exploited in this letter. This research indicates that a highly sensitive and fast phototransistor can be realized using the novel GNR/QD/IGZO vertical heterostructure and the long diffusion length of the perovskite QD photosensing component.