Colloidal Quantum Wells Enable Phototransistors with Enhanced Visible Light Detection

Colloidal quantum wells, or nanoplatelets, exhibit large, circularly polarized optical Stark effects under sub-band-gap femtosecond illumination. The optical Stark effect is measured for CdSe colloidal quantum wells of several thicknesses and separately as a measure of pump photon energy, pump fluence, and temperature. These measurements show that optical Stark effects in colloidal quantum wells shift the absorption features up to 5 meV, at the intensities up to 2.9 GW·cm-2 and large detuning (>400 meV) of the pump photon energy from the band edge absorption. Optical Stark shifts are underpinned by large transition dipoles of the colloidal quantum wells (μ = 15-23 D), which are larger than those of any reported colloidal quantum dots or epitaxial quantum wells. The rapid (<500 fs), narrow band blue shift of the excitonic features under circular excitation indicates the viability of these materials beyond light emission such as spintronics or all-optical switching.

The dynamics of intersubband relaxation are critical to quantum well technologies such as quantum cascade lasers and quantum well infrared photodetectors. Here, intersubband relaxation in CdSe colloidal quantum wells, or nanoplatelets, is studied via pump-push-probe transient spectroscopy. An initial interband pump pulse is followed by a secondary infrared push excitation, resonant with intersubband absorption, which promotes electrons from the first conduction band of the quantum well to the second conduction band. A probe pulse monitors subsequent electron cooling to the band edge of the quantum well. Using this technique, intersubband relaxation is studied as a function of critical variables such as colloidal quantum well size and thickness, surface ligand chemistry, temperature, and excitation pulse intensity. Larger quantum well sizes, judicious selection of surface ligand chemistry (e.g., thiolates), low temperatures, and elevated push pulse fluences slow intersubband relaxation. However, compared to resonant intraband relaxation in colloidal quantum dots (up to hundreds of picoseconds), intersubband relaxation in colloidal quantum wells is rapid (<1 ps) under all examined conditions. These experiments indicate that rapid relaxation is driven by both LO phonon and surface scattering. The short time scale of relaxation observed in these materials may hinder intersubband technologies such as mid-infrared detectors, although such rapid relaxation may prove valuable in optical switching.

Although achieving optical gain using aqueous solutions of colloidal nanocrystals as a gain medium is exceptionally beneficial for bio-optoelectronic applications, the realization of optical gain in an aqueous medium using solution-processed nanocrystals has been extremely challenging because of the need for surface modification to make nanocrystals water dispersible while still maintaining their gain. Here, we present the achievement of optical gain in an aqueous medium using an advanced architecture of CdSe/CdS@CdxZn1-xS core/crown@gradient-alloyed shell colloidal quantum wells (CQWs) with an ultralow threshold of ∼3.4 μJ cm-2 and an ultralong gain lifetime of ∼2.6 ns. This demonstration of optical gain in an aqueous medium is a result of the carefully heterostructured CQWs having large absorption cross-section and gain cross-section in addition to inherently slow Auger recombination in these CQWs. Furthermore, we show low-threshold in-water amplified spontaneous emission (ASE) from these aqueous CQWs with a threshold of 120 μJ cm-2. In addition, we demonstrate a whispering gallery mode laser with a low threshold of ∼30 μJ cm-2 obtained by incorporating films of CQWs by exploiting layer-by-layer approach on a fiber. The observation of low-threshold optical gain with ultralong gain lifetime presents a significant step toward the realization of advanced optofluidic colloidal lasers and their continuous-wave pumping.

As specialized optical fibers, side-emitting optical fibers (SEOFs) are designed to emit light from their sides rather than their ends for possible applications in curved-surface lighting and sensing. In this study, we propose and demonstrate an in situ decoration of SEOFs with colloidal quantum wells (CQWs) for the first time. The proposed method enables the homogeneous distribution of CQWs in the polymeric matrix of the fiber with high side-emission efficiency, which is based on a simple yet effective method of CQW sheet coating. Two different structures of CQWs, a red-emitting CdSe/CdZnS core/shell and a green-emitting CdSe/CdS core/crown, were synthesized and employed in SEOFs. Accordingly, carefully tuned concentrations of CQWs were incorporated within the hollow-core optical fibers, and the side-scattered light from the fibers was systematically characterized and analyzed. The results confirm excellent side-emitting characteristics that are highly dependent on the optical fiber structure and CQW absorption spectrum. The average quantum yield values of 51 ± 5% and 34.5 ± 5% for different concentrations of red and green CQWs relative to their solution form were measured in our experiment, which is associated with polymeric medium, cluster formation, and fiber tower temperature. The findings pave the way for developing high-performance optical fiber devices based on CQW-doped SEOFs capable of efficient and precise light emission targeting a wide variety of applications ranging from three-dimensional curved-surface lighting to sensing.

Colloidal quantum wells (CQWs) exhibit superior optical characteristics, including giant oscillator strengths and high absorption cross sections, making them attractive for light-sensing applications. In this study, we fabricated light-sensitive nanocrystal skin (LS-NS) devices using a single-layer edge-up oriented CQW film as the active absorber and achieved a significant enhancement in the performance by leveraging the controlled orientation of the self-assembled CQWs. The LS-NS devices with edge-up oriented CQW film show eight times higher sensitivity and three times higher voltage build-up compared to LS-NS devices with spin-coated CQW films. Such enhancements are due to the trapping of the holes on the edges of the CQWs because of defect-ridden edges, which helps hopping of the holes localized within close proximity to the metal-CQW interface. The LS-NS device with edge-up CQW film exhibited a record-high photovoltage buildup of 600 mV, three times greater than the highest previously reported value for similar LS-NS architectures.

The most recent class of semiconductor nanocrystal to be synthesized colloidally is the quantum well, in which carriers are confined quantum mechanically in only one dimension. Electrons and holes in colloidal quantum wells undergo different dynamics than in either colloidal quantum dots or epitaxially grown quantum wells, providing new opportunities for applications. The opportunities presented by cadmium chalcogenide nanoplatelets are particularly exciting, because they can be grown with control over their thickness down to the single atomic layer and with all nanoplatelets in an ensemble having the same thickness. This Feature Article reviews the relaxation and recombination dynamics of electrons and holes, which are tightly bound into excitons, in nanoplatelets. These dynamics are favorable for optical gain and lasing, and this Article reviews the progress that has been made toward practical realization of nanoplatelet lasers, including the demonstration of low thresholds for room-temperature gain and...

Quantum well intersubband transitions are critical for quantum cascade lasers and infrared photodetectors. Control of band offsets allows bound-to-bound intersubband transitions, with confinement of both initial and final states, and bound-to-continuum transitions, in which only the initial state is energetically confined within the potential well. Both types of transitions are also achieved in colloidal CdSe wells by changing the heterostructure shell. Bare wells have narrow intersubband transitions spanning the near-infrared spectrum following effective mass predictions. Atomically precise core/shells enable a readily adjusted potential well for electrons. For CdSe/ZnS, bound-to-bound transitions are narrow and redshift with shell thickness. By contrast, broad bound-to-continuum absorptions are found in CdSe/CdS. Due to small conduction band offsets, higher conduction band states of the well are more delocalized into the CdS shell. These measurements provide unique data to understand the electronic structure of colloidal quantum wells and chart a path to atomically precise optoelectronic materials for the mid-infrared.

Atomically thin monolayer Mn2+‐alloyed 2D Ruddlesden–Popper (RP) perovskite colloidal quantum wells (CQWs) are demonstrated as efficiently tunable emitter with wide color gamut. Ultrathin character evidenced by the well‐controlled thickness of ≈1.8 nm, leads to alloying rather than doping mechanism reported previously, which facilitates uniform and large‐ratio substitution of Pb2+ by Mn2+ within the metal‐halide octahedra layer. Photoluminescence spectra reveal dual color emissions constituted by blue‐greenish intrinsic excitonic emission and reddish Mn2+ emission with a high photoluminescence quantum yield (PLQY) up to 32%. Two individual channels are created for precisely targeting the emission color in a wide gamut by finely controlling the excitonic emission via the chemical compositions of Br/I and the Mn2+ emission via the chemical compositions of Mn/Pb, respectively. Experimentally, high quality single‐component white emitter ultrathin CQWs are achieved with CIE coordinate value of (0.33, 0.32), 21% PLQY, and good film processibility. The results reveal great potential as applicant for next generation lighting and display technologies.

CdTe colloidal quantum wells (CQWs) exhibit very low quantum yields (QYs) (<1%) and suffer significant stability challenges unlike their CdSe counterparts, which feature near unity QY and high stability under ambient conditions. Here, we synthesized highly stable CdSeTe CQWs with photoluminescence quantum efficiency (PLQY) reaching 25%, which far exceeds the previously best-reported PLQYs from CdTe CQWs while preserving the PL spectral characteristics (PL peak position and full-width-at-half-maximum (fwhm) of PL emission) similar to those of CdTe CQWs. In these as-synthesized CdSeTe nanoplatelets, a progressive increase in the Se/Te ratio leads to a notable enhancement in their quantum yield, accompanied by extended photoluminescence lifetimes, strongly indicating that higher Se incorporation effectively mitigates nonradiative recombination channels. In addition, we demonstrate that these CdSeTe CQWs are stable under ambient conditions unlike pristine CdTe CQWs through systematic studies with X-ray photoelectron spectroscopy (XPS). Using density functional theory (DFT) calculations we demonstrate that, under oxygen-rich conditions, O atoms weaken Cd–Te bonds and destabilize CdTe nanoplatelets, whereas in Se-alloyed CdSeTe CQWs, oxygen atoms preferentially adsorb onto the surface without penetrating the structure, effectively suppressing bond destabilization, which is consistent with our XPS observations. The enhanced optical properties, stability, and robustness of these CdSeTe-alloyed nanoplatelets offer a practical solution to overcoming the inherent limitations of CdTe CQWs and position them as highly attractive materials for practical use in advanced optoelectronic applications.

Here, the first account of self-resonant fully colloidal μ-lasers made from colloidal quantum well (CQW) solution is reported. A deep patterning technique is developed to fabricate well-defined high aspect-ratio on-chip CQW resonators made of grating waveguides and in-plane reflectors. The fabricated waveguide-coupled laser, enabling tight optical confinement, assures in-plane lasing. CQWs of the patterned layers are closed-packed with sharp edges and residual-free lifted-off surfaces. Additionally, the method is successfully applied to various nanoparticles including colloidal quantum dots and metal nanoparticles. It is observed that the patterning process does not affect the nanocrystals (NCs) immobilized in the attained patterns and the different physical and chemical properties of the NCs remain pristine. Thanks to the deep patterning capability of the proposed method, patterns of NCs with subwavelength lateral feature sizes and micron-scale heights can possibly be fabricated in high aspect ratios.

Sluggish surface reaction is a critical factor that strongly governs the efficiency of photocatalytic solar fuel production, particularly in CO2-to-ethanol photoconversion. Here, inspired by the principles underlying enzyme catalytic proficiency and specificity, we report a biomimetic photocatalyst that affords superior CO2-to-ethanol photoreduction efficiency (5.5 millimoles gram-1 hour-1 in average with 98.2% selectivity) distinctly surpassing the state of the art. The key is to create a class of catalytic pocket, which contains spatially organized NH2…Cu-Se(-Zn) multiple functionalities at close range, over ZnSe colloidal quantum wells. Such structure offers a platform to mimic the concerted cooperation between the active site and surrounding secondary/outer coordination spheres in enzyme catalysis. This is manifested by the chemical adsorption and activation of CO2 via a bent geometry, favorable stabilization toward a variety of important intermediates, promotion of multielectron/proton transfer processes, etc. These results highlight the potential of incorporating enzyme-like features into the design of photocatalysts to overcome the challenges in CO2 reduction.

Single-photon sources (SPSs) are essential for quantum technologies. Colloidal quasi two-dimensional (2D) quantum wells (QWs) possess high emission uniformity, photoluminescence quantum yields (PLQYs), and narrow linewidths at room temperature, all contributing to ideal SPSs. However, their large lateral dimensions make excitons sensitive to environments and enhance multiexciton emission, casting long-standing doubt on their viability as SPSs. Here, we demonstrate bright room-temperature single-photon emission (SPE) with high purity and minimal blinking from in-plane-engineered (IPE) 2D QWs incorporating a doubly gradient architecture. A compositionally graded CdSe/CdSexS1-x core in the in-plane direction tailors electron-hole wavefunctions to control multiexciton Auger dynamics, while a graded CdyZn1-yS shell in the thickness direction suppresses interfacial strain and nonradiative defects. This design unites 0D and 2D structural advantages, achieving near-unity ensemble PLQY and bright SPE (0.6-1.2 × 105 counts/s, NA = 0.65). Controlling the CdSe core size via IPE, we attain 92% single-photon purity (2 nm core), and suppressed blinking with a 96.9% ON-time fraction (8 nm core). This work establishes deterministic design rules for colloidal 2D QWs as high-performance SPSs for scalable quantum technologies.

Giant responsivity of a new InGaZnO ultraviolet thin-film phototransistor based on combined dual gate engineering and surface decorated Ag nanoparticles aspects

Doping of bulk semiconductors has revealed widespread success in optoelectronic applications. In the past few decades, substantial effort has been engaged for doping at the nanoscale. Recently, doped colloidal quantum dots (CQDs) have been demonstrated to be promising materials for luminescent solar concentrators (LSCs) as they can be engineered for providing highly tunable and Stokes-shifted emission in the solar spectrum. However, existing doped CQDs that are aimed for full solar spectrum LSCs suffer from moderately low quantum efficiency, intrinsically small absorption cross-section, and gradually increasing absorption profiles coinciding with the emission spectrum, which together fundamentally limit their effective usage. Here, the authors show the first account of copper doping into atomically flat colloidal quantum wells (CQWs). In addition to Stokes-shifted and tunable dopant-induced photoluminescence emission, the copper doping into CQWs enables near-unity quantum efficiencies (up to ≈97%), accompanied by substantially high absorption cross-section and inherently step-like absorption profile, compared to those of the doped CQDs. Based on these exceptional properties, the authors have demonstrated by both experimental analysis and numerical modeling that these newly synthesized doped CQWs are excellent candidates for LSCs. These findings may open new directions for deployment of doped CQWs in LSCs for advanced solar light harvesting technologies.

Thin film transistors (TFTs) with amorphous InGaZnO (IGZO) channel layer were fabricated by radio frequency magnetron sputtering technique. The IGZO films show optical transparency over 80 % both before annealing and after annealing. It was found that performances of transistors with IGZO thin films annealed in air at 450 °C were significantly improved. Through annealing treatment, Saturation current of TFTs increased from 2.8 to 181 μA at bias of VDS = 20, VGS = 20 V, and saturation mobility is up from 1.49 to 15.8 cm2 V−1 s−1. In addition, X-ray photoelectric spectroscopy (XPS) was performed to provide elemental information on the surface of the IGZO films before and after annealing. O1s XPS spectra of unannealed and annealed IGZO films indicated oxygen vacancy concentration decreased by annealing treatment.