Mingxuan Cao

Broadband Phototransistor Based on CH3NH3PbI3 Perovskite and PbSe Quantum Dot Heterojunction

Organic lead halide perovskites have received a huge amount of interest since emergence, because of tremendous potential applications in optoelectronic devices. Here field effect phototransistors (FEpTs) based on CH3NH3PbI3 perovskite/PbSe colloidal quantum dot heterostructure are demonstrated. The high light absorption and optoelectric conversion efficiency, due to the combination of perovskite and quantum dots, maintain the responsivities in a high level, especially at 460 nm up to 1.2 A/W. The phototransistor exhibits bipolar behaviors, and the carrier mobilities are determined to be 0.147 cm2V-1s-1 for holes and 0.16 cm2V-1s-1 for electrons. The device has a wide spectral response spectrum ranging from 300 to 1500 nm. A short photoresponse time is less than 3 ms due to the assistance of heterojunction on the transfer of photoexcitons. The excellent performances presented in the device especially emphasize the CH3NH3PbI3 perovskite-PbSe quantum dot as a promising material for future photoelectronic applications.

Bulk- and layer-heterojunction phototransistors based on poly [2-methoxy-5-(2-ethylhexyloxy-p-phenylenevinylene)] and PbS quantum dot hybrids

The responsivity (R) of a thin film photodetector is proportional to the product of its photo-induced carrier density (n) and mobility (μ). However, when choosing between layer heterojunction (LH) and bulk heterojunction (BH) field-effect phototransistors (FEpTs), it is still unclear which of the two device structures is more conducive to photodetection. A comparison study is performed on the two structures based on polymer and PbS quantum dot hybrids. Both devices exhibit ambipolar behavior, with μE ≈ μH = 3.7 cm2 V−1 s−1 for BH-FEpTs and μH = 36 cm2 V−1 s−1 and μE = 52 cm2 V−1 s−1 for LH-FEpTs. Because of the improvements in μ and the channel order degree (α), the responsivity of LH-FEpTs is as high as 101 A/W, which is as much as two orders of magnitude higher than that of BH-FEpTs (10−1A/W) under the same conditions. Although the large area of the BH improves both the exciton separation degree (β) and n in the BH-FEpT, the lack of an effective transport mechanism becomes the main constraint on high de...

Low operating voltage ambipolar graphene oxide-floating-gate memory devices based on quantum dots

Solution processed quantum dots (QDs) were employed as semiconductor layers in low operating voltage nonvolatile memory devices where graphene oxide (GO) was embedded as a charge storage element. Ambipolar PbSe QD layers have the ability to intrinsically transport holes and electrons, which provides an opportunity for the GO layers to trap both holes and electrons efficiently, resulting in bidirectional threshold voltage shifts after the program/erase operations are applied. The memory exhibited a large memory window (ΔVon ∼ 6 V) and a long retention time (>104 s) under low program/erase operating voltages of ±25 V. The magnitude of ΔVon is controlled not only by programming/erasing voltages (VP/E) but also by the bias voltage (VDS), where ΔVon shows a quadratic function to VP/E and linearly depends on VDS. The QD based GO-floating-gate memory devices supply a facile route for fabricating flexible devices with low-cost large-scale integration.

Ambipolar nonvolatile memory based on a quantum-dot transistor with a nanoscale floating gate

Using only solution processing methods, we developed ambipolar quantum-dot (QD) transistor floating-gate memory (FGM) that uses Au nanoparticles as a floating gate. Because of the bipolarity of the active channel of PbSe QDs, the memory could easily trap holes or electrons in the floating gate by programming/erasing (P/E) operations, which could shift the threshold voltage both up and down. As a result, the memory exhibited good programmable memory characteristics: a large memory window (ΔVth ∼ 15 V) and a long retention time (>105 s). The magnitude of ΔVth depended on both P/E voltages and the bias voltage (VDS): ΔVth was a cubic function to VP/E and linearly depended on VDS. Therefore, this FGM based on a QD transistor is a promising alternative to its inorganic counterparts owing to its advantages of bipolarity, high mobility, low cost, and large-area production.

Enhanced amplified spontaneous emission in a quantum dot-doped polymer-dispersed liquid crystal

Quantum dot-doped polymer-dispersed liquid crystals (QD-PDLCs) were prepared by photoinitiated polymerization and sealed in capillary tubes. The concentration of QDs in the PDLC was 1 wt%. Amplified spontaneous emission (ASE) of the quantum dot-doped polymer-dispersed liquid crystals was observed with 532 nm wavelength laser excitation. The threshold for ASE was 6 mJ cm(-2), which is much lower than that for homogeneous quantum dot-doped polymer (25 mJ cm(-2)). The threshold for ASE was dramatically enhanced when the working temperature exceeded the clearing point of the liquid crystal; this result demonstrates that multi-scattering caused by the liquid crystals effectively improved the path length or dwell time of light in the gain region, which played a key role in decreasing the threshold for ASE.

High-performance PbS quantum dot vertical field-effect phototransistor using graphene as a transparent electrode

Solution processed photoactive PbS quantum dots (QDs) were used as channel in high-performance near-infrared vertical field-effect phototransistor (VFEpT) where monolayer graphene embedded as transparent electrode. In this vertical architecture, the PbS QD channel was sandwiched and naturally protected between the drain and source electrodes, which made the device ultrashort channel length (110 nm) simply the thickness of the channel layer. The VFEpT exhibited ambipolar operation with high mobilities of μe = 3.5 cm2/V s in n-channel operation and μh = 3.3 cm2/V s in p-channel operation at low operation voltages. By using the photoactive PbS QDs as channel material, the VFEpT exhibited good photoresponse properties with a responsivity of 4.2 × 102 A/W, an external quantum efficiency of 6.4 × 104% and a photodetectivity of 2.1 × 109 Jones at the light irradiance of 36 mW/cm2. Additionally, the VFEpT showed excellent on/off switching with good stability and reproducibility and fast response speed with a shor...

Random lasing in a colloidal quantum dot-doped disordered polymer.

We report random lasing in colloidal quantum dots (CQDs) doped disordered polymer. The CdSe/ZnS core-shell CQDs are dispersed in hybrid polymer including two types of monomers with different rates of polymerization. After UV curing, spatially localized random resonators are formed owing to long range refractive-index fluctuations in inhomogeneous polymer with gain. Upon the optical excitation, random lasing action is triggered above the threshold of 7mJ/cm2. Through the investigation on the spectral characteristics of random laser, the wavelengths of random lasers strongly depend on pump position, which confirms that random laser modes originate from spatially localized resnonators. According to power Fourier transform of emission spectrum, the average size of equivalent micro resonators is attributed to be 50 μm. The proposed method provides a facile route to develop random lasers based on CQDs, showing potential applications on random fiber laser and laser displays.

Tunable amplified spontaneous emission in graphene quantum dots doped cholesteric liquid crystals

Graphene quantum dots (GQDs) have received much research attention, because of their useful structure and optical absorption/emission. We report the tunable amplified spontaneous emission (ASE) in GQD-doped cholesteric liquid crystal (CLC), which to the best of our knowledge has not been previously observed. The GQDs are uniformly dispersed with a weight ratio of 0.5 wt.% in CLC. Under optical excitation, typical ASE is triggered in the system at pump energies greater than 1.25 mJ cm-2. The emission peak at the long wavelength edge of the photonic bandgap shifts from 662 to 669 nm, as the working temperature is increased from 50 to 90 °C. The preparation of the combined GQDs and CLC is simple and low-cost, and the resulting material is photostable and non-toxic. Combining the GQD gain material with the self-assembled CLC resonator has potential in the fabrication of ASE source and laser devices.

Ambipolar Graphene–Quantum Dot Hybrid Vertical Photodetector with a Graphene Electrode

A strategy to fabricate an ambipolar near-infrared vertical photodetector (VPD) by sandwiching a photoactive material as a channel film between the bottom graphene and top metal electrodes was developed. The channel length in the vertical architecture was determined by the channel layer thickness, which can provide an ultrashort channel length without the need for a high-precision manufacturing process. The performance of VPDs with two types of semiconductor layers, a graphene-PbS quantum dot hybrid (GQDH) and PbS quantum dots (QDs), was measured. The GQDH VPD showed better photoelectric properties than the QD VPD because of the high mobility of graphene doped in the channel. The GQDH VPD exhibited excellent photoresponse properties with a responsivity of 1.6 × 104 A/W in the p-type regime and a fast response speed with a rise time of 8 ms. The simple manufacture and the promising photoresponse of the GQDH VPDs reveal that an easy and effective way to fabricate high-performance ambipolar photodetectors was developed.

Platinum-scatterer-based random lasers from dye-doped polymer-dispersed liquid crystals in capillary tubes

The resonance characteristics of platinum-scatter-based random lasers from dye-doped polymer-dispersed liquid crystals (DDPDLCs) in capillary tubes were researched for the first time, to the best of our knowledge. After adding platinum nanoparticles (Pt NPs) into the liquid crystal mixtures, the emission spectra of DDPDLCs revealed a lower lasing threshold in comparison with those of DDPDLCs without Pt NPs due to light scattering of liquid crystal droplets and the local field enhancement around Pt NPs. Furthermore, the full width at half-maximum (FWHM) and the lasing threshold were determined by the doping density of the Pt NPs. The threshold was decreased by about half from 17.5  μJ/pulse to 8.7  μJ/pulse on the condition that around 1.0 wt. % was the optimum concentration of Pt NPs doped into the DDPDLCs. The FWHM of the peaks sharply decreased to 0.1 nm. Our work provides an extremely simple method to enhance random lasers from DDPDLCs doped with Pt NPs, and it has potential applications in random fiber lasers or laser displays.