Cuong Dang

Si nanowire metal–insulator–semiconductor photodetectors as efficient light harvesters

Novel ITO-Si nanowire (NW) metal-insulator-semiconductor (MIS) photodetectors were fabricated by using n-type Si NWs as detection units and ITO films as top gate electrodes. Measurements on the Si NW based device reveal a significant photoresponse, including photocurrent generation with an external quantum efficiency (EQE) of approximately 35% at a peak wavelength of 600 nm at zero external bias, and with an EQE of 70% at a peak wavelength of 800 nm at - 0.5 V bias. The NW device shows a flat and low reflectance and almost constant EQE up to a 60 degrees incident angle of illumination, demonstrating efficient visible-light harvesting by the Si NW antenna.

Highly Flexible, Electrically Driven, Top-Emitting, Quantum Dot Light-Emitting Stickers

Flexible information displays are key elements in future optoelectronic devices. Quantum dot light-emitting diodes (QLEDs) with advantages in color quality, stability, and cost-effectiveness are emerging as a candidate for single-material, full color light sources. Despite the recent advances in QLED technology, making high-performance flexible QLEDs still remains a big challenge due to limited choices of proper materials and device architectures as well as poor mechanical stability. Here, we show highly efficient, large-area QLED tapes emitting in red, green, and blue (RGB) colors with top-emitting design and polyimide tapes as flexible substrates. The brightness and quantum efficiency are 20,000 cd/m(2) and 4.03%, respectively, the highest values reported for flexible QLEDs. Besides the excellent electroluminescence performance, these QLED films are highly flexible and mechanically robust to use as electrically driven light-emitting stickers by placing on or removing from any curved surface, facilitating versatile LED applications. Our QLED tapes present a step toward practical quantum dot based platforms for high-performance flexible displays and solid-state lighting.

Stable and Low-Threshold Optical Gain in CdSe/CdS Quantum Dots: An All-Colloidal Frequency Up-Converted Laser

An all-solution processed and all-colloidal laser is demonstrated using tailored CdSe/CdS core/shell quantum dots, which exhibit highly stable and low-threshold optical gain owing to substantially suppressed non-radiative Auger recombination.

A Wafer‐Level Integrated White‐Light‐Emitting Diode Incorporating Colloidal Quantum Dots as a Nanocomposite Luminescent Material

High-brightness, color-tunable colloidal quantum dots are incorporated in 3D nanoporous GaN to create a nanocomposite material (CQD/NP-GaN), which is demonstrated to be an effective approach for a wavelength down-conversion nanomaterial in solid-state lighting. The white-light-emitting diode (LED) made from a blue GaN-based LED and the CQD/NP-GaN shows an increase of extraction efficiency by a factor of 2, a controllable white color, and a down-conversion quantum efficiency as high as 82%.

Large ordered arrays of single photon sources based on II–VI semiconductor colloidal quantum dot

In this paper, we developed a novel and efficient method of deterministically organizing colloidal particles on structured surfaces over macroscopic areas. Our approach utilizes integrated solution-based processes of dielectric encapsulation and electrostatic-force-mediated self-assembly, which allow precisely controlled placement of sub-10nm sized particles at single particle resolution. As a specific demonstration, motivated by application to single photon sources, highly ordered 2D arrays of single II-VI semiconductor colloidal quantum dots (QDs) were created by this method. Individually, the QDs display triggered single photon emission at room temperature with characteristic photon antibunching statistics, suggesting a pathway to scalable quantum optical radiative systems.

Highly efficient, spatially coherent distributed feedback lasers from dense colloidal quantum dot films

Colloidal quantum dots (CQD) are now making their entry to full-color displays, endowed by their brightness and single-material base. By contrast, many obstacles have been encountered in their use towards lasers. We demonstrate here optically pumped distributed feedback (DFB) lasers, based on close-packed, solid films self-assembled from type-I CQDs. Notably, the single mode CQD-DFB lasers could reach such a low threshold as to be pumpable with a compact pulsed source in a quasi-continuous wave regime. Our results show the spatially and temporally coherent laser beam outputs with power of 400 μW and a quantum efficiency of 32%.

Surface-emitting red, green, and blue colloidal quantum dot distributed feedback lasers

We demonstrate surface emitting distributed feedback (DFB) lasers across the red, green, and blue from densely packed colloidal quantum dot (CQD) films. The solid CQD films were deposited on periodic grating patterns to enable 2nd-order DFB lasing action at mere 120, 280, and 330 μJ/cm2 of optical pumping energy densities for red, green, and blue DFB lasers, respectively. The lasers operated in single mode operation with less than 1 nm of full-width-half-maximum. We measured far-field patterns showing high degree of spatial beam coherence. Specifically, by taking advantage of single exciton optical gain regime from our engineered CQDs, we can significantly suppress the Auger recombination to reduce lasing threshold and achieve quasi-steady state, optically pumped operation.

Temperature-dependent Optoelectronic Properties of Quasi-2D Colloidal Cadmium Selenide Nanoplatelets

Colloidal cadmium selenide (CdSe) nanoplatelets (NPLs) are a recently developed class of efficient luminescent nanomaterials suitable for optoelectronic device applications. A change in temperature greatly affects their electronic bandstructure and luminescence properties. It is important to understand how and why the characteristics of NPLs are influenced, particularly at elevated temperatures, where both reversible and irreversible quenching processes come into the picture. Here we present a study of the effect of elevated temperatures on the characteristics of colloidal CdSe NPLs. We used an effective-mass envelope function theory based 8-band k·p model and density-matrix theory considering exciton-phonon interaction. We observed the photoluminescence (PL) spectra at various temperatures for their photon emission energy, PL linewidth and intensity by considering the exciton-phonon interaction with both acoustic and optical phonons using Bose-Einstein statistical factors. With a rise in temperature we observed a fall in the transition energy (emission redshift), matrix element, Fermi factor and quasi Fermi separation, with a reduction in intraband state gaps and increased interband coupling. Also, there was a fall in the PL intensity, along with spectral broadening due to an intraband scattering effect. The predicted transition energy values and simulated PL spectra at varying temperatures exhibit appreciable consistency with the experimental results. Our findings have important implications for the application of NPLs in optoelectronic devices, such as NPL lasers and LEDs, operating much above room temperature.

Single-shot multispectral imaging with a monochromatic camera

Multispectral imaging plays an important role in many applications, from astronomical imaging and earth observation to biomedical imaging. However, current technologies are complex with multiple alignment-sensitive components and spatial and spectral parameters predetermined by manufacturers. Here, we demonstrate a single-shot multispectral imaging technique that gives flexibility to end users with a very simple optical setup, thanks to spatial correlation and spectral decorrelation of speckle patterns. These seemingly random speckle patterns are point spread functions (PSFs) generated by light from point sources propagating through a strongly scattering medium. The spatial correlation of PSFs allows image recovery with deconvolution techniques, while the spectral decorrelation allows them to play the role of tunable spectral filters in the deconvolution process. Our demonstrations utilizing optical physics of strongly scattering media and computational imaging present a cost-effective approach for multispectral imaging with many advantages.

Spectroscopy of optical gain in low threshold colloidal quantum dot laser media: dominance of single-exciton states at room temperature

Experimental studies of amplified spontaneous emission (ASE) and lasing from various colloidal II-VI semiconductor nanocrystals have been used as inputs to several microscopic models for underlying optical gain, usually involving permutations of quantum confined multiple excitonic states. Here we focus on particular types of CdSe/ZnCdS and CdSe/ZnS/ZnCdS colloidal quantum dot (CQD) films and elucidate on the discovery of single-exciton states at the fundamental edge as a dominant mechanism for optical gain at room temperature. Pump-probe spectroscopic techniques enable us to measure the onset of gain at ensemble-average exciton occupancy per CQD, = 0.6 and 0.7 for the two types of CQD films at room temperature. Time-resolved measurements, in turn, show how optical gain persists well into the time regime associated with spontaneous emission (nanoseconds), thus providing direct evidence for how the non-radiative Auger recombination processes (~100 ps) can be thwarted. In addition to benefits of the material assets of densely packed CQD films with high luminescence efficiency (quantum yield ~90%) and nanoparticle monodispersity therein, we propose that access to the single-exciton gain regime at room temperature requires a careful spectral balance between the lowest exciton absorption resonance and its corresponding red-shifted spontaneous emission maximum (“Stokes shift”).