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The past decade witnessed the emergence of a new generation of room-temperature infrared detectors based on low-dimensional solids. Among these are topological insulating materials. The present work aims to evaluate this class of photodetectors in the so-called high-temperature infrared (high operating temperature) photodetector family. Their performance, such as current responsivity and detectivity, are compared with available HgCdTe photodiodes, interband quantum cascade photodetectors, colloidal quantum dot detectors, and two-dimensional transition metal dichalcogenides. [ABSTRACT FROM AUTHOR]

PbS colloidal quantum dot (CQD) photodiodes possess high photoresponsivity and broad spectral sensitivity, making them tremendously attractive for low-cost short-wave infrared (SWIR) sensing. A critical challenge in achieving high detectivity in SWIR photodiodes is to minimize their reverse bias dark current density (Jdark) and elucidate the origin of Jdark. We demonstrate in this work how the self-formed AlOx at the ZnO/Al interface dramatically reduced Jdark of the PbS CQD SWIR photodiodes. In such a simple and controlled way, the interfacial states were thoroughly passivated by post-exposure to dry air, which could block the dominant pathways for minority hole injection from Al electrode. The treated photodiode showed low Jdark down to 1.58 × 10−7 A/cm2 at –0.5 V, high photoresponsivity of 0.62 A/W, and high specific detectivity of 2.05 × 1012 Jones at 1413 nm, establishing a readily approach to optimizing detectivity in PbS CQD SWIR photodetectors. [ABSTRACT FROM AUTHOR]

In this work, multiple-beam colloidal quantum dot lasers are achieved in a double waveguide-grating (waveguide-grating-waveguide, W-G-W) microcavity. The grating is fabricated simply using interference lithography, and CdSe/CdS/ZnS colloidal quantum dots (CQDs) are spin-coated as the gain to form an active waveguide-grating structure. The photoresist film, which is not completely etched to the bottom, provides another waveguide-grating structure. In the W-G-W structure, low-threshold multiple-beam laser output is realized under optical pumping with emission peaks at 664.6 and 645.2 nm. The oblique laser is derived from a quasi-propagation mode. The thresholds of the two laser modes are 22.7 and 28.3 μJ/cm2, respectively, and both laser modes are TE0 modes. In addition, the emission wavelengths of the two modes of the designed distributed feedback laser can be flexibly tuned by changing the thickness of the CQDs' waveguide layer. The presence of quasi-propagation modes provides another method for designing compact laser sources, which could help in the design of wireless communication networks, hyperspectral 3D sensing, and color laser displays. [ABSTRACT FROM AUTHOR]

Interface engineering, which efficiently optimizes the interfacial carrier collection and recombination, has been proven to be of importance for the emerging colloidal quantum dot solar cells (CQDSCs). Compared with the attractive modification efforts at the interface between quantum dots and the anode/electron transport layer (ETL), the interface between fluorine-doped tin-oxide (FTO) cathodes and the ETL, for which there exists a band alignment mismatch and high trap density in ZnO, has been investigated less. Herein, two kinds of ethylenimine-based polymers, branched by only ethylenimine groups (b-PEI) and by both ethylenimine/ethoxylated groups (e-PEI), respectively, are introduced as bifunctional interfacial layers (BILs) in lead sulfide (PbS) CQDSCs. PEI-based BILs were utilized to modulate the work function of an FTO cathode for optimizing the band alignment at the FTO/ZnO interface and to control the crystallinity of ZnO for reducing its traps. These BILs suppressed the interfacial carrier recombination and achieved a power conversion efficiency (PCE) of 11.28% in CQDSCs, which was much superior to the PCE of the reference device without BIL (10.29%). Also, the branched side chain of PEI-based BILs plays a crucial role in rationally modulating the Schottky barrier to gain different interface-optimization effects. Our work has laid a foundation for the commercial application of CQDSCs due to the advantage of low-temperature solution processability, low-cost, and scalable manufacturing. [ABSTRACT FROM AUTHOR]

Silicon (Si) is a mature semiconductor detection material because of its appropriate bandgap, high reliability, and low-cost manufacturing process. However, the detection range of Si-based photodetectors is mainly restricted to visible and near-infrared (NIR) ranges. Broadening the detection range of Si-based detectors to ultraviolet (UV) could ignite numerous applications, including flame detection, defense safety, and environmental monitoring. Here, the obtained perovskite colloidal quantum dots (QDs) with high quality are integrated with a Si/graphene heterojunction to broaden and improve response. By the electrical coupling mechanism between graphene and QDs, the built-in potential in detectors is maximized. As a result, the devices exhibit a high UV responsivity of 0.33 A W−1 at 210 nm, 0.73 A W−1 at 400 nm, and a superior detectivity of nearly 1012 Jones under the UV region. Aside from that, the performance of devices under visible and NIR ranges is also dramatically improved compared to that of the one without perovskite QDs. [ABSTRACT FROM AUTHOR]

Mercury telluride (HgTe) nanocrystals (NCs) are very promising for THz technology as they exhibit broad THz absorption resonances and a carrier lifetime of a few picoseconds as well as being easily fabricated using solution synthesis. In this work, we show their light emission properties in the THz spectral range, up until now unexplored, and show how THz pulse generation can be used for microscopic insights into these NCs. In particular, we report on coherent THz emission from large HgTe NCs excited by linearly polarized optical pulses via second-order nonlinear effects. The peak emission frequency is tunable from 0.4 to 0.8 THz by varying incident angles of optical pulses from 0° to 45°. Our results reveal that the THz emission is induced by transient photocurrents arising from both photogalvanic and photon drag effects. By pushing the light emission of colloidal quantum dots down to the THz spectral range, our study expands the application fields of NCs, especially toward the development of easily integrable and tunable THz emitters and quantum THz devices. [ABSTRACT FROM AUTHOR]

The low-dimensional cesium bismuth halides are intriguing wide-bandgap semiconductors with fruitful photophysics. However, their photodynamics is rather intricate and remains debated. We study the optical properties of the Cs3Bi2Br9 nanoplatelets (NPLs) by using the combined experimental and first-principles calculation methods. The results indicate that the exhibited dominant blue emission band and weak green band arise from two kinds of shallow color centers. The Cs3Bi2Br9 NPLs exhibit Raman active and inactive vibrational modes that are separately ascribed to the localized lattice waves propagating along the edges and interiors of the quantum well-like bromide–bismuth octahedral frameworks in Cs3Bi2Br9. These findings improve our understanding of the unique photodynamics of these multiple quantum well-like semiconductor nanocrystals. [ABSTRACT FROM AUTHOR]

Solution-processed core/shell quantum dot films are of great significance for light-emitting diodes. It is well known that the operation of core/shell quantum dot-based light-emitting diodes largely relies on charge transport. However, the charge transport mechanism in quantum dot films is still under debate and inconclusive. Herein, the temperature-dependent charge transport properties of light-emitting core/shell colloidal quantum dot thin films are characterized and analyzed across a wide temperature range, and the charge transport mechanism is studied. The results reveal that Poole–Frenkel emission conduction is applicable in the high-temperature range. With the decrease in the temperature, the measured current can be described by the Efros–Shklovskii variable-range hopping model. It is worth noting that, in both cases, trap states and disorders in the quantum dot film play a very important role in charge transport. These findings are of great importance for optimizing quantum dot light-emitting diodes and understanding the effects of charge transport on the device performance. [ABSTRACT FROM AUTHOR]

Conventional memory technologies are facing enormous problems with downscaling, and are hence unable to fulfill the requirement of big data storage generated by a huge explosion of digital information. A resistive random access memory device (RRAM) is one of the most emerging technologies for next-generation computing data storage owing to its high-density stacking, ultrafast switching speed, high non-volatility, multilevel data storage, low power consumption, and simple device structure. In this work, colloidal MoS2 quantum dots (QDs) embedded in an insulating matrix of poly-(4vinylpyridine) (PVP) were used as an active layer to fabricate a RRAM device. The MoS2 QDs-PVP based RRAM device reveals an excellent nonvolatile resistive switching (RS) behavior with a maximum current on-off ratio (ION/IOFF) of 105. High endurance, long retention time, and successive "write-read-erase-read" cycles indicate high-performance RRAM characteristics. The ultimate power consumption by this RRAM device is considerably low for energy saving. In addition, the MoS2 QDs-PVP based device shows RS behavior even at 130 °C. High ION/IOFF, low operating power, high endurance, long retention time, and excellent stability with temperatures reveal that the MoS2 QDs-PVP based device can be a promising candidate for high-performance low power RRAM devices that can be operated at relatively higher temperatures. [ABSTRACT FROM AUTHOR]

All-inorganic lead halide perovskite quantum dots (IPQDs) display some intriguing magnetic, electronic, and optical properties by introducing a transition metal Mn element. In this Letter, Mn-doped CsPb(BrCl)3 IPQDs with a cubic structure have been synthesized by the hot injection method. What make us excited is that the photoluminescence properties of a fixed ensemble of Mn-doped CsPb(BrCl)3 IPQDs systematically depend on the varying concentration. This experimental phenomenon can be well explained by the quantitative model based on photon reabsorption. The results combining experiments with theoretical models indicate that the photon reabsorption is of paramount importance in the analysis of the luminescence properties of colloidal quantum dots especially at high concentrations. Further analysis based on time-resolved fluorescence and transient absorption spectroscopy reveals the effect of the solution concentration on the Mn-doped CsPb(BrCl)3 IPQD exciton dynamics. The results suggest that exciton energy transfer can take place both inside and between IPQDs and also can be enhanced by increasing the concentration of a Mn-doped CsPb(BrCl)3 IPQD solution. These results are of great significance to accurately interpret and control the luminescence properties of IPQDs. [ABSTRACT FROM AUTHOR]

Active modulation of quantum dot thin film photoluminescence (PL) has been far-reaching potential applications in biomedical and optoelectronic systems, but challenges remain in achieving large PL modulation depth and fast temporal response. Here, we report an efficient voltage-controlled optical down-converter by optically exciting a colloidal quantum dot thin film within a quantum dot light-emitting diode under reverse bias. Utilizing field-induced luminescence quenching, we show that a large electric field can strongly modify carrier dynamics in this nanostructured device, resulting in stable and reversible photoluminescence quenching. The device exhibits photoluminescence reduction of up to 99.5%, corresponding to a contrast ratio of 200:1 under the applied electric field of 3 MV cm−1 with a 300 ns response time. Using excitation wavelength dependent and transient PL spectroscopy, we further show that the high degree of quenching is achieved by a synergistic interplay of quantum-confined Stark effect and field-induced exciton dissociation. [ABSTRACT FROM AUTHOR]

Low-dimensional semiconductors have been considered excellent materials to construct photodetectors for infrared detection with an easy process and excellent compatibility but suffer from low detectivity mainly owing to the poor light absorption of the ultra-thin body. Here, we demonstrate a thin film transistor (TFT) based short-wave infrared photodetector consisting of a carbon nanotube (CNT) TFT gated by a PbS colloidal quantum dots (CQDs) based heterojunction. The thick PbS CQDs' film efficiently absorbs infrared light and then excites and separates electron–hole pairs to generate a photovoltage at the pn heterojunction of the PbS CQDs/ZnO film. The photovoltage is further amplified and transduced in situ by the CNT TFT under the heterojunction, and then the detector featured a specific detectivity of 5.6 × 1013 Jones under 1300 nm illumination and a fast response of the sub-ms level (0.57 ms). The CQDs based heterojunction gating TFT represents a universal architecture for highly sensitive low-dimensional semiconductor based infrared photodetectors, competitive with state-of-the-art epitaxial semiconductors and enabling monolithic integration technology. [ABSTRACT FROM AUTHOR]

Cheung and Kaake investigate the effect of the exciton binding energy and endothermicity on the potential solar cell efficiency.[17] The binding energy comes from the Coulombic interaction between the electron and hole of the exciton. Solar photovoltaics will play a dominant role in the power generation of the zero-carbon future.[1] Today, the market of large-scale solar power generation is dominated by silicon solar cells, where high-performance lab-scale devices are reaching their detailed-balance efficiency limit.[2] With decreasing module prices,[3] cell efficiency becomes more valuable as it becomes an increasingly important driver for further reductions in the levelized cost of electricity. A theoretically elegant but technically challenging way to overcome the efficiency limit of single-junction solar cells is the concept of I intermediate-gap solar cells. i [[33]] Here, a state within the bandgap of the absorber layer is used to provide a second absorption gap. [Extracted from the article]

Polariton emission from optical cavities integrated with various luminophores has been extensively studied recently due to the wide variety of possible applications in photonics, particularly promising in terms of the fabrication of low-threshold sources of coherent emission. Tunable microcavities allow extensive investigation of the photophysical properties of matter placed inside the cavity by deterministically changing the coupling strength and controllable switching from weak to strong and ultra-strong coupling regimes. Here, we demonstrate room-temperature strong coupling of exciton transitions in CdSe/ZnS/CdS/ZnS colloidal quantum dots with the optical modes of a tunable low-mode-volume microcavity. Strong coupling is evidenced by a large Rabi splitting of the photoluminescence spectra depending on the detuning of the microcavity. A coupling strength of 154 meV has been achieved. High quantum yields, excellent photostability, and scalability of fabrication of quantum dots (QDs) pave the way to practical applications of coupled systems based on colloidal QDs in photonics, optoelectronics, and sensing. [ABSTRACT FROM AUTHOR]

We demonstrate a technique to quickly build and spatially map the frequency response of optoelectronic devices. The transfer function of a linear system is the Fourier transform of its impulse response. Such an impulse response is obtained from transient photocurrent measurements of devices such as photodetectors and solar cells. We introduce and apply Fourier transform impedance spectroscopy (FTIS) to a PbS colloidal quantum dot SiC heterojunction photodiode and validate the results using intensity-modulated photocurrent spectroscopy. Cutoff frequencies in the devices were as high as ∼10 kHz, showing their utility in advanced thin film and flexible electronics. The practical frequencies for FTIS lie in the mHz–kHz range, ideal for composite materials such as quantum dot films that are dominated by interfacial trap states. These can lead to characteristic lengths for charge collection ∼20–500 μm dominated by transmission line effects, rather than intrinsic diffusion and drift length scales, enabling extraction of interfacial capacitances and series/parallel resistances. [ABSTRACT FROM AUTHOR]

Single solid-state quantum dots have significant potential as bright single-photon sources for scalable photonic quantum information technologies. Engineering their radiative relaxation properties is of significant importance for their practical applications. In this study, we demonstrate a cavity-free, broadband approach for modulating and collecting the fluorescence of a single-photon emitter using a fiber taper–silver nanowire plasmonic probe. When the plasmonic probe is located above a single colloidal quantum dot at approximately 20 nm , the photon-emitter interaction increased rapidly and a significant decrease, by an average factor of 3.38, in the lifetime of the quantum dot was observed. The fluorescence signal of the quantum dots was collected by the hybrid probe, with significantly higher efficiency than that of the traditional metal-coated near-field probe. The results of the numerical simulation were in good agreement with the experimental results. The proposed near-field modulation method can be applied to other single-photon sources and proved to be a flexible method for manipulating the luminescence of systems based on single-photon emitters. [ABSTRACT FROM AUTHOR]

When we applied colloidal quantum dots (QDs) for quantum dot light emitting diodes, it was well known that shell thickness played an important role in core protection, confinement of electrons and holes, and charge injection efficiency. However, although the shell thickness dependence of electroluminescence properties was reported, carrier injection efficiency has not been discussed in detail. In this paper, we investigated the effect of shell thickness on the carrier injection efficiency that was evaluated by photoelectrochemical measurements. By comparing the product of internal quantum yield of photoluminescence and the evaluated carrier injection efficiency with external quantum efficiency (EQE) for QDs with various shell thicknesses, we found that the optimal shell thickness for increasing EQE is determined by the balance between protection of QD's surface and carrier injection efficiency. [ABSTRACT FROM AUTHOR]

Research in the field of colloidal semiconductor nanocrystals (NCs) has progressed tremendously, mostly because of their exceptional optoelectronic properties. Core@shell NCs, in which one or more inorganic layers overcoat individual NCs, recently received significant attention due to their remarkable optical characteristics. Reduced Auger recombination, suppressed blinking, and enhanced carrier multiplication are among the merits of core@shell NCs. Despite their importance in device development, the influence of the shell and the surface modification of the core@shell NC assemblies on the charge carrier transport remains a pertinent research objective. Type-II PbTe@PbS core@shell NCs, in which exclusive electron transport was demonstrated, still exhibit instability of their electron transport. Here, we demonstrate the enhancement of electron transport and stability in PbTe@PbS core@shell NC assemblies using iodide as a surface passivating ligand. The combination of the PbS shelling and the use of the iodide ligand contributes to the addition of one mobile electron for each core@shell NC. Furthermore, both electron mobility and on/off current modulation ratio values of the core@shell NC field-effect transistor are steady with the usage of iodide. Excellent stability in these exclusively electron-transporting core@shell NCs paves the way for their utilization in electronic devices. [ABSTRACT FROM AUTHOR]

Colloidal quantum dots are promising materials for near infrared light emitting diodes (NIR QLEDs) owing to the widely tunable wavelength of emitted light, high quantum efficiency, and full integration with solution processing techniques. However, the imbalance of charge carriers in NIR QLEDs greatly limits their performance, which in turn narrows the scope of their application. Here, we propose an approach for improving the brightness and stability of NIR PbS QLEDs through balancing the device current by an ultra-thin inorganic aluminum oxide (Al2O3) electron suppressing layer. This modification resulted in a 7.42 W/sr/m2 peak radiance at 1.3 μm wavelength. Moreover, the halide-capped PbS-based NIR QLEDs remained stable under constant current drive for over 144 h. [ABSTRACT FROM AUTHOR]

The integration of silicon (Si) and nanomaterials with infrared light harvesting capability is a promising approach to fabricate large area infrared light detecting arrays. However, the construction of a high quality junction between Si and small bandgap colloidal quantum dots (CQDs) remains a challenge, which limited their photodetecting performance in the short wavelength infrared region (1.4 μm–3 μm). Herein, a layer of solution processed ZnO nanoparticles was inserted between silicon and CQDs to passivate the surface dangling bond of silicon. This significantly reduces the carrier recombination between Si and CQDs. Meanwhile, the formation of the Si:CQD heterojunction structure enables effective carrier extraction. As a result, the photodetector shows the detecting range to the short wavelength infrared region (0.8 eV) and achieves a standard detectivity of 4.08 × 1011 Jones at a bias of −0.25 V at room temperature. [ABSTRACT FROM AUTHOR]

Colloidal quantum dots (QDs), which exhibit highly saturated color emission, are expected to be used as emitting materials in wide-color-gamut displays. However, the development of low-toxicity alternatives is necessary because QDs with high color purity and highly efficient emission contain toxic materials such as Cd and Pb. In this study, QD light-emitting diodes (QD-LEDs) with ZnS–AgInS2 solid-solution nanoparticles [(AgIn)xZn2(1−x)S2, ZAIS] as low-toxicity QDs were fabricated and their electroluminescence properties were investigated. The ZAIS QDs exhibited red photoluminescence, with a peak wavelength of ca. 700 nm. Because the charge-injection barrier of ZAIS QDs without a wide-bandgap shell is relatively low, the QD-LEDs with red ZAIS QDs in combination with organic charge-transport materials induce electron injection from an organic electron-transport layer. These QD-LEDs exhibit an external quantum efficiency of 2.2%, a turn-on voltage of 2.4 V, and red emission with chromaticity coordinates (0.66, 0.33). [ABSTRACT FROM AUTHOR]

HgTe colloidal quantum dots are investigated as the active material in photodiodes for extended short-wave infrared up to 2.6 μm. The HgTe colloidal quantum dots photodiodes achieve external quantum efficiencies above 50% and specific detectivities of 1 × 1011 at 2.2 μm at room temperature with a microsecond response time and compete with commercial extended InGaAs photodiodes. [ABSTRACT FROM AUTHOR]

Light-emitting devices (LEDs) based on colloidal semiconductor nanocrystals (NCs) are promising next-generation thin-film display and lighting devices owing to their high luminescence efficiency and size-tunable color. However, the studies of the carrier transport and recombination mechanisms of the NC-LEDs lag far behind. We study the operation mechanisms of the CsPbBr3 NC-LEDs with a working voltage as high as 40 V. They exhibit an anomalous hump-shaped bias dependence of luminescence intensity. A theoretical model is proposed to explain this, and it indicates that there is enhanced Auger recombination in the CsPbBr3 NC-LEDs caused by imbalanced carrier accumulation, which causes efficiency droop similar to that in the nitride-based light-emitting diodes. We also report the intriguing phenomenon of electroluminescence spectrum fluctuation of the CsPbBr3 NC-LEDs, which is ascribed to the inner self-heating effect in the emission layer. [ABSTRACT FROM AUTHOR]

Perovskite semiconductor nanocrystals of lead halides exhibit excellent electronic and optical properties that are suitable for many optoelectronic applications. In this report, we investigate the underlying mechanism of the optical response of the material by probing coherent interaction between the exciton and lattice vibration by two-dimensional electronic spectroscopy. Coherent exciton-phonon coupling has been observed with time-domain oscillations in the dynamics of signals related to either ground or excited electronic states. Furthermore, the spectra of the coherent phonon derived from the dynamics of biexciton formation have larger amplitudes for higher frequency modes, which is attributed to the phonon bottleneck effect. Moreover, the composition-dependent coherent coupling between the exciton and lattice vibration in mixed halide samples reveals a critical role played by the anharmonicity and fluctuation of lattice vibration in the excited-state dynamics of perovskite nanocrystals. [ABSTRACT FROM AUTHOR]

The electric field effect on the optical properties of semiconductors is important in terms of both fundamental physics and technological applications. Here, we explored the optical behavior of cesium halide perovskites under a varied electric field (F). We revealed the intrinsically distinct photoluminescence (PL) spectral evolution between the quantum-confined perovskites and the bulk phase, indicating the different carrier recombination behaviors under F. Strong PL quenching along with significant broadening of emission linewidths was observed. Notably, the CsPbBr3 colloidal quantum dots and nanoplatelets exhibit an unusual field-induced bandgap increase, which is attributed to the weakened orbital coupling between the Pb 6s and Br 4p states with the increase in F. These results could advance their application potential in optoelectronics. [ABSTRACT FROM AUTHOR]

PbS colloidal quantum dot solar cells (CQDSCs) have recently achieved remarkable performance enhancement due to the development of the phase-transfer ligand exchange (PTLE) method. However, the lack of compact packing of the PTLE-passivated CQDs impairs the interdot electronic coupling and thereby severely restricts further improvement in performance. To address this electronic coupling issue, we report a simple yet effective process of external pressure (0–2 MPa). We find that the interdot distance is reduced after the application of the pressure. Both optical and electrical measurements clearly demonstrate that the distance reduction can effectively strengthen the interdot electronic coupling, thus promoting the carrier transport of the CQD layer. However, too much pressure (>2 MPa) could accelerate the detrimental carrier recombination processes of CQDSCs. Accordingly, by optimizing the carrier transport and recombination processes, we achieve the maximum power conversion efficiency of 8.2% with a moderate pressure of 1.5 MPa, which is 25.5% higher than the solar cell without the external pressure. This effective strategy of external pressure could also be applied to other CQD-based optoelectronic devices to realize a better device performance. [ABSTRACT FROM AUTHOR]

We report on the noise characterization of photoconductors based on PbS colloidal quantum dots. The devices operate in the near infrared region with peak responsivity exceeding 70 A/W at 1.3 μm at low optical intensity and low voltage bias. The large responsivity, combined with the low dark current of high resistance devices, provides a specific detectivity D* as large as 1011 cm Hz½W-1. The noise characteristics are investigated using noise current spectra measured at different biases both in dark and under optical excitation. The analysis revealed that the noise is clearly dominated by the flicker component up to 100 kHz. The noise performance is investigated at different optical intensities and for different device dimensions and voltage biases. [ABSTRACT FROM AUTHOR]

As an emerging type of optically active material, semiconductor nanocrystals (NCs) stabilized by chiral molecules have attracted much attention. Owing to the wide range of potential applications of chiral perovskite NCs, the development of these materials is of great importance, but there has been a lack of relevant studies. Here, we describe an investigation of the properties of chiral perovskite NCs obtained using post-synthetic ligand exchange (achiral ligand/chiral ligand). These are found to exhibit mirror-image circular dichroism spectra. It is the chirality of the ligand (enantiomeric 1,2-diaminocyclohexane, DACH) that is most likely responsible for the induction of chiroptical activity in these NCs. Furthermore, their chiroptical properties and the corresponding mechanisms are found to depend strongly on the amount of capping ligand. When excess DACH is used to cap the surface of the NCs, their chiroptical properties are induced mainly by aggregation of DACH on the surface in a chiral pattern. In contrast, when small amounts of DACH are used for the capping, it is mainly surface distortion (or defects) and electronic interaction mechanisms that contribute to the chiroptical behavior of the NCs. In both cases, the anisotropy factors of the NCs are of the order of 10-3, which is comparable to or larger than the values reported for other chiral semiconductor and metal NCs. This work opens the door toward further understanding of chiroptical perovskite NCs and their potential applications. [ABSTRACT FROM AUTHOR]

We report a strategy to realize and facilitate the photocarrier transport from mercury selenium colloidal quantum dots (HgSe CQDs) into silicon with the assistance of twisted graphene. A nanocomposite material consisting of HgSe CQDs and twisted graphene has been synthesized. By bringing the nanocomposites into contact with silicon, a HgSe CQD-twisted graphene nanocomposite/ silicon junction was fabricated and demonstrated photoresponses in the long-wave infrared range. In the nanocomposites, the surface of twisted graphene was decorated with HgSe CQDs. Benefiting from the twisted structure in the nanocomposites, the active sensing area and light-matter interaction length are greatly increased. Driven by the interfacial built-in potential, photocarriers directly transfer from HgSe CQDs into the twist graphene, which serves as a fast carrier transport pathway to silicon, leading to high photocarrier collection efficiency. Compared with vertically stacked HgSe CQD film/flat graphene, the application of HgSe CQD-twisted graphene nanocomposites avoids photocarriers transporting via the hopping mechanism and over 2700% enhancement ratio of spectral responsivity was achieved, reaching 31.5mA/W@9 µm. The interfacial energy band diagram was deduced for a better understanding of the photocarrier transfer process occurring at the interface between HgSe colloidal quantum dots, twist graphene, and silicon. [ABSTRACT FROM AUTHOR]

The realization of both high efficiency and strong bending-durability is a critical issue for future applications of colloidal quantum dot solar cells (CQDSCs) in flexible devices. Herein, we fabricated flexible CQDSCs with a three-dimensional electron transport layer (ETL) composed of a ZnO nanowire (NW) array for efficient carrier collection and probed the effect of the NW array on the bendability of flexible cells. The good bendability of cells with ZnO NWs was verified from the consistency of photovoltaic performance during mechanical bending treatment with various bending angles and cycles. The efficient release of stress inside the three-dimensional CQDSCs with the ZnO NW array led to the maintenance of 97% of the initial power conversion efficiency at a bending angle of 160°. In contrast, crack formation on the common planar ETL of CQDSCs formed by ZnO nanoparticles resulted in a degradation of device performance to 77% of the initial one at the same angle. Our research demonstrated that the bending stability of the ZnO NW array may be promising for commercial applications of flexible photovoltaic devices. [ABSTRACT FROM AUTHOR]

Colloidal quantum dot (QD) solar cells have seen remarkable progress in recent past to reach the certified efficiency of 10.6%. Anatase titanium oxide (TiO2) is a widely studied n-type widow layer for the collection of photogenerated electrons in QD solar cells. Requirement of high temperature (~500 °C) processing steps proved to be disadvantageous for its applications in flexible solar cells and roll to roll processing, and it also has adverse commercial implications. Here, we report that solar light exposure to low temperature processed (80 °C-150 °C) TiO2 and niobium doped TiO2 films leads to unprecedented enhancement in their electron densities and electron mobilities, which enables them to be used as efficient n-type layers in quantum dot solar cells. Such photoinduced high conducting states in these films show gradual decay over hours after the light bias is taken off and can be retrieved under solar illumination. On the contrary, TiO2 films processed at 500 °C show marginal photo induced enhancements in their characteristics. In bilayer configuration with PbS QDs, photovoltaic devices based on low temperature processed TiO2 films show improved performance over high temperature processed TiO2 films. The stability of photovoltaic devices also improved in low temperature processed TiO2 films under ambient working conditions. [ABSTRACT FROM AUTHOR]

We report the fabrication of a colloidal quantum dot based photodetector designed for the 3-5 lm mid infrared wavelength range incorporated with optical nano-antenna arrays to enhance the photocurrent. The fabricated arrays exhibit a resonant behavior dependent on the length of the nano-antenna rods, in good agreement with numerical simulation. The device exhibits a three-fold increase in the spectral photoresponse compared to a photodetector device without antennas, and the resonance is polarized parallel to the antenna orientation. We numerically estimate the device quantum efficiency and investigate its bias dependence. [ABSTRACT FROM AUTHOR]

Semiconductors with bandgaps in the near- to mid-infrared can harvest solar light that is otherwise wasted by conventional single-junction solar cell architectures. In particular, colloidal quantum dots (CQDs) are promising materials since they are cost-effective, processed from solution, and have a bandgap that can be tuned into the infrared (IR) via the quantum size effect. These characteristics enable them to harvest the infrared portion of the solar spectrum to which silicon is transparent. To date, IR CQD solar cells have been made using a wasteful and complex sequential layer-by-layer process. Here, we demonstrate ~1 eV bandgap solar-harvesting CQD films deposited in a single step. By engineering a fast-drying solvent mixture for metal iodide-capped CQDs, we deposited active layers greater than 200 nm in thickness having a mean roughness less than 1 nm. We integrated these films into infrared solar cells that are stable in air and exhibit power conversion efficiencies of 3.5% under illumination by the full solar spectrum, and 0.4% through a simulated silicon solar cell filter. [ABSTRACT FROM AUTHOR]

We calculate the effective macroscopic dielectric constant εa of a periodic array of spherical nanocrystals (NCs) with dielectric constant ε immersed in the medium with dielectric constant εm ≪ ε. For an array of NCs with the diameter d and the distance D between their centers, which are separated by the small distance s = D - d ≪ d or touch each other by small facets with radius ρ ≪ d what is equivalent to s < 0, |s| ≪ d we derive two analytical asymptotics of the function εa(s) in the limit ε/εm ≫ 1. Using the scaling hypothesis, we interpolate between them near s = 0 to obtain new approximated function εa(s) for ε/εm ≫ 1. It agrees with existing numerical calculations for ε/εm = 30, while the standard mean-field Maxwell-Garnett and Bruggeman approximations fail to describe percolation-like behavior of εa(s) near s = 0. We also show that in this case the charging energy Ec of a single NC in an array of touching NCs has a non-trivial relationship to εa, namely, Ec = αe²/εad, where α varies from 1.59 to 1.95 depending on the studied three-dimensional lattices. Our approximation for εa(s) can be used instead of mean field Maxwell-Garnett and Bruggeman approximations to describe percolation like transitions near s = 0 for other material characteristics of NC arrays, such as conductivity. [ABSTRACT FROM AUTHOR]

Owing to the Auger recombination effect, multiple excitons (MEs) in semiconductor nanocrystals (NCs) are dissipated nonradiatively at the sub-nanosecond time scale, which sets a stringent limit on the time window within which one can operate with them. Here, we show that this issue can be resolved by utilizing an intrinsic energy transfer system in CdSe NCs, where MEs created in the donor quantized states can be effectively extracted to the acceptor trap states. This was evidenced by the step-like increase in the intensity and the apparent decrease in the rise time of the trap-state photoluminescence with the elevated laser excitation power. With the radiative lifetime being tens of nanoseconds for the trap states, extended storage of MEs has been achieved and marks a crucial step towards flexible manipulations of their optoelectronic properties. [ABSTRACT FROM AUTHOR]

Colloidal quantum dots (CQDs) are free-standing nano-structures with chemically tunable electronic properties. This tunability offers intriguing possibilities for nano-electromechanical devices. In this work, we consider a nano-electromechanical nonvolatile memory (NVM) device incorporating a triple quantum dot (TQD) cluster. The device operation is based on a bias induced motion of a floating quantum dot (FQD) located between two bound quantum dots (BQDs). The mechanical motion is used for switching between two stable states, "ON" and "OFF" states, where ligand-mediated effective interdot forces between the BQDs and the FQD serve to hold the FQD in each stable position under zero bias. Considering realistic microscopic parameters, our quantum-classical theoretical treatment of the TQD reveals the characteristics of the NVM. [ABSTRACT FROM AUTHOR]

In an attempt to suppress the dark current, the barrier layer engineer for solution-processed PbSe colloidal quantum-dot (CQD) photodetectors has been investigated in the present study. It was found that the dark current can be significantly suppressed by implementing two types of carrier blocking layers, namely, hole blocking layer and electron blocking layer, sandwiched in between two active PbSe CQD layers. Meanwhile no adverse impact has been observed for the photo current. Our study suggests that this improvement resides on the transport pathway created via carrier recombination at intermediate layer, which provides wide implications for the suppression of dark current for infrared photodetectors. [ABSTRACT FROM AUTHOR]

Using a combination of continuous wave and time-resolved spectroscopy, we study the effects of interfacial conditions on the radiative lifetimes and photoluminescence intensities of sub-monolayer colloidal CdTe/CdS quantum dots (QDs) embedded in a three-dimensional porous silicon (PSi) scaffold. The PSi matrix was thermally oxidized under different conditions to change the interfacial oxide thickness. QDs embedded in a PSi matrix with ~0.4 nm of interfacial oxide exhibited reduced photoluminescence intensity and nearly five times shorter radiative lifetimes (~16 ns) compared to QDs immobilized within completely oxidized, porous silica (PSiO2) frameworks (~78 ns). The exponential dependence of QD lifetime on interfacial oxide thickness in the PSi scaffolds suggests charge transfer plays an important role in the exciton dynamics. [ABSTRACT FROM AUTHOR]

A recently developed enhanced Fourier law is applied to the problem of extracting thermal properties of materials from frequency-domain thermoreflectance (FDTR) experiments. The heat transfer model comprises contributions from two phonon channels: one a high-heat-capacity diffuse channel consisting of phonons of mean free path (MFP) less than a threshold value, and the other a low-heat-capacity channel consisting of phonons with MFP higher than this value that travel quasi-ballistically over length scales of interest. The diffuse channel is treated using the Fourier law, while the quasi-ballistic channel is analyzed using a second-order spherical harmonic expansion of the phonon distribution function. A recent analysis of FDTR experimental data suggested the use of FDTR in deriving large portions of the MFP accumulation function; however, it is shown here that the data can adequately be explained using our minimum-parameter model, thus highlighting an important limitation of FDTR experiments in exploring the accumulation function of bulk matter. [ABSTRACT FROM AUTHOR]

For hybrid light emitting devices (LEDs) consisting of GaN quantum wells and colloidal quantum dots, it is necessary to explore the physical mechanisms causing decreases in the quantum efficiencies and the energy transfer efficiency between a GaN quantum well and CdSe quantum dots. This study investigated the electro-luminescence for a hybrid LED consisting of colloidal quantum dots and a GaN quantum well patterned with photonic crystals. It was found that both the quantum efficiency of colloidal quantum dots on a GaN quantum well and the energy transfer efficiency between the patterned GaN quantum well and the colloidal quantum dots decreased with increases in the driving voltage or the driving time. Under high driving voltages, the decreases in the quantum efficiency of the colloidal quantum dots and the energy transfer efficiency can be attributed to Auger recombination, while those decreases under long driving time are due to photobleaching and Auger recombination. [ABSTRACT FROM AUTHOR]

We report a comparative synchrotron radiation x-ray diffraction study of GaAs1-yNy microstructures obtained by two different patterning methods: spatially selective H incorporation achieved by using H-opaque masks and spatially selective H removal attained by laser writing. These methods are emerging as original routes for fabrication of micro- and nano-structures with in-plane modulation of the bandgap energy. By measuring the out-of-plane and in-plane lattice parameters, we find that for both patterning approaches the largest part of the micro-structure volume remains tensile-strained and pseudomorphic to the substrate, regardless of the compressive-strained hydrogenated barriers. However, a larger lattice disorder is probed in the laser-written micro-structures and attributed to partial removal of H and/or strain changes at the micro-structure boundaries. This larger lattice disorder is confirmed by photoluminescence studies. [ABSTRACT FROM AUTHOR]

Colloidal quantum dots (CQDs) are semiconductor nanocrystals synthesized with, processed in, and deposited from the solution phase, potentially enabling low-cost, facile manufacture of solar cells. Unfortunately, CQD solar cell reports, until now, have only explored batch-processing methods--such as spin-coating--that offer limited capacity for scaling. Spray-coating could offer a means of producing uniform colloidal quantum dot films that yield high-quality devices. Here, we explore the versatility of the spray-coating method by producing CQD solar cells in a variety of previously unexplored substrate arrangements. The potential transferability of the spray-coating method to a roll-to-roll manufacturing process was tested by spray-coating the CQD active layer onto six substrates mounted on a rapidly rotating drum, yielding devices with an average power conversion efficiency of 6.7%. We further tested the manufacturability of the process by endeavoring to spray onto flexible substrates, only to find that spraying while the substrate was flexed was crucial to achieving champion performance of 7.2% without compromise to open-circuit voltage. Having deposited onto a substrate with one axis of curvature, we then built our CQD solar cells onto a spherical lens substrate having two axes of curvature resulting in a 5% efficient device. These results show that CQDs deposited using our spraying method can be integrated to large-area manufacturing processes and can be used to make solar cells on unconventional shapes [ABSTRACT FROM AUTHOR]

In this work, we demonstrate a proof-of-concept system for generating highly polarized light from colloidal quantum dots (QDs) coupled with magnetically aligned segmented Au/Ni/Au nanowires (NWs). Optical characterizations reveal that the optimized QD-NW coupled structures emit highly polarized light with an s-to p-polarization (s/p) contrast as high as 15:1 corresponding to a degree of polarization of 0.88. These experimental results are supported by the finite-difference timedomain simulations, which demonstrate the interplay between the inter-NW distance and the degree of polarization. [ABSTRACT FROM AUTHOR]

Size and composition-dependences of band gap energies are important properties for nanocrystal semiconductors, and have attracted extensive attentions for the last two decades. In this letter, a simple method of band gap prediction for nanocrystal alloys is developed. The band gaps of II-VI semiconductor homogeneous alloys with zinc blende and wurtzite structure, such as zb-(ZnS)x(CdS)1-x, zb-(CdS)x(CdSe)1-x, zb-(ZnSe)x(CdSe)1-x, w-(ZnS)x(CdS)1-x, w-(ZnSe)x(CdSe)1-x, and w-(CdSe)x(CdTe)1-x nanocrystal alloys, are calculated. The calculated results are in good agreement with the available experimental data. It provides insights into the effects of structure, size, and composition on the band gap. [ABSTRACT FROM AUTHOR]

In this research, PbS quantum dots (QDs) were assembled directly on ternary metal oxide nanowires by pulsed laser deposition (PLD) for photovoltaic applications, which avoided the difficult ligand exchange processing needed in typical colloidal synthesis processes. Different ligands including oleic acid, oleylamine, and 3-mercaptopropionic acid (MPA) were used to cap the PbS QDs after PLD deposition, and Fourier transform infrared spectroscopy was used to characterize their coverage. The ligand effect on solar cell performance was also investigated, and it was found that the device treated with oleic acid exhibited a significantly improved performance. [ABSTRACT FROM AUTHOR]

An organic floating-gate memory device using CdSe quantum dots (QDs) as a charge-trapping element was fabricated. CdSe QDs were localized beneath a pentacene without any tunneling insulator, and the QD layer played a role as hole-trapping sites. The band bending formed at the junction between pentacene and QD layers inhibited back-injection of holes trapped in CdSe into pentacene, which appeared as a hysteretic capacitance-voltage response during the operation of the device. Nearly, 60% of trapped charge was sustained even after 104 s in programmed state, and this long retention time can be potentially useful in practical applications of non-volatile memory. [ABSTRACT FROM AUTHOR]

Time-resolved Faraday rotation spectroscopy is used to study the electron spin coherence in colloidal CdS quantum dots. Long-lived spin coherence with dephasing time T2*>3ns has been found at room temperature. Spin dynamics unaffected by the faster carrier recombination suggests the spin signal coming from the residual electrons in the dots. A small external transverse magnetic field of 50 mT can extend T2* ∼ 2 times longer compared with that in zero magnetic field. Hyperfine interaction between electron and nuclear spins limits the dephasing time in zero or low magnetic field, while for higher magnetic fields, inhomogeneous dephasing becomes to dominate the spin dynamics. [ABSTRACT FROM AUTHOR]

We report broadband responsivity enhancement in PbS colloidal quantum dot (CQDs) photoconductive photodetectors due to absorption increase offered by a plasmonic scattering layer of Ag metal nanoparticles. Responsivity enhancements are observed in the near infrared with a maximum 2.4-fold increase near the absorption band edge of ∼1 μm for ∼400 nm thick devices. Additionally, we study the effect of the mode structure on the efficiency of light trapping provided by random nanoparticle scattering in CQD films and provide insights for plasmonic scattering enhancement in CQD thin films. [ABSTRACT FROM AUTHOR]

Silicon nanocrystals have shown attractive properties for photonic and photovoltaic applications. We demonstrate all-Si light-emitting diodes based on boron-doped Si nanocrystal/c-Si p-n heterojunction structure, which show electroluminescence in the visible/infrared regions. The electroluminescence spectra of these diodes can be modified by changing the quantum confining barriers from SiO2 to Si3N4. Our results are an important demonstration of electroluminescence from boron-doped Si nanocrystals-a wide band gap absorber material for third generation photovoltaics. [ABSTRACT FROM AUTHOR]

We demonstrate that an array of optical antennas may render a thin layer of randomly oriented semiconductor nanocrystals into an enhanced and highly directional source of polarized light. The array sustains collective plasmonic lattice resonances, which are in spectral overlap with the emission of the nanocrystals over narrow angular regions. Consequently, different photon energies of visible light are enhanced and beamed into definite directions. [ABSTRACT FROM AUTHOR]