Quantum dot (QD) light-emitting diodes (LEDs) are ideal for large-panel displays because of their excellent efficiency, colour purity, reliability and cost-effective fabrication1-4. Intensive efforts have produced red-, green- and blue-emitting QD-LEDs with efficiencies of 20.5 per cent4, 21.0 per cent5 and 19.8 per cent6, respectively, but it is still desirable to improve the operating stability of the devices and to replace their toxic cadmium composition with a more environmentally benign alternative. The performance of indium phosphide (InP)-based materials and devices has remained far behind those of their Cd-containing counterparts. Here we present a synthetic method of preparing a uniform InP core and a highly symmetrical core/shell QD with a quantum yield of approximately 100 per cent. In particular, we add hydrofluoric acid to etch out the oxidative InP core surface during the growth of the initial ZnSe shell and then we enable high-temperature ZnSe growth at 340 degrees Celsius. The engineered shell thickness suppresses energy transfer and Auger recombination in order to maintain high luminescence efficiency, and the initial surface ligand is replaced with a shorter one for better charge injection. The optimized InP/ZnSe/ZnS QD-LEDs showed a theoretical maximum external quantum efficiency of 21.4 per cent, a maximum brightness of 100,000 candelas per square metre and an extremely long lifetime of a million hours at 100 candelas per square metre, representing a performance comparable to that of state-of-the-art Cd-containing QD-LEDs. These as-prepared InP-based QD-LEDs could soon be usable in commercial displays.

Highly efficient and stable InP/ZnSe/ZnS quantum dot light-emitting diodes

Colloidal quantum dot (QD) solids are emerging semiconductors that have been actively explored in fundamental studies of charge transport1 and for applications in optoelectronics2. Forming high-quality QD solids—necessary for device fabrication—requires substitution of the long organic ligands used for synthesis with short ligands that provide increased QD coupling and improved charge transport3. However, in perovskite QDs, the polar solvents used to carry out the ligand exchange decompose the highly ionic perovskites4. Here we report perovskite QD resurfacing to achieve a bipolar shell consisting of an inner anion shell, and an outer shell comprised of cations and polar solvent molecules. The outer shell is electrostatically adsorbed to the negatively charged inner shell. This approach produces strongly confined perovskite QD solids that feature improved carrier mobility (≥0.01 cm2 V−1 s−1) and reduced trap density relative to previously reported low-dimensional perovskites. Blue-emitting QD films exhibit photoluminescence quantum yields exceeding 90%. By exploiting the improved mobility, we have been able to fabricate CsPbBr3 QD-based efficient blue and green light-emitting diodes. Blue devices with reduced trap density have an external quantum efficiency of 12.3%; the green devices achieve an external quantum efficiency of 22%. A solution-based ligand-exchange strategy enables the realization of close-packed quantum dot solid films with near-unity photoluminescence quantum yield and high charge carrier mobility.

Bipolar-shell resurfacing for blue LEDs based on strongly confined perovskite quantum dots

Near a magic twist angle, bilayer graphene transforms from a weakly correlated Fermi liquid to a strongly correlated two-dimensional electron system with properties that are extraordinarily sensitive to carrier density and to controllable environmental factors such as the proximity of nearby gates and twist-angle variation. Among other phenomena, magic-angle twisted bilayer graphene hosts superconductivity, interaction-induced insulating states, magnetism, electronic nematicity, linear-in-temperature low-temperature resistivity and quantized anomalous Hall states. We highlight some key research results in this field, point to important questions that remain open and comment on the place of magic-angle twisted bilayer graphene in the strongly correlated quantum matter world.

/pdf/graphene-bilayers-with-a-twist-5d0g3ibv3a.pdf

Graphene bilayers with a twist

Modern electronics based on semiconductors is meeting the fundamental speed limit caused by the interconnect delay and large heat generation when the sizes of components reach nanometer scale. Photons as a carrier of the information are superior to electrons in bandwidth, density, speed, and dissipation. More over, photons could carry intensity, polarization, phase, and frequency information which could break through the limitation of binary system as in electronic devices. But due to the diffraction limitation, the photonic components and devices can not be fabricated small enough to be integrated densely. Surface plasmon polariton is quanta of collective oscillations of free electrons excited by photons in metal nanostrucrures, which offers a promising way to manipulate light at the nanoscale and to realize the miniaturization of photonic devices. Hence, plasmonic circuits and devices have been proposed for some time as a potential strategy for advancing semiconductor-based computing beyond the fundamental performance limitations of electronic devices, as epitomized by Moore's law. A variety of individual plasmonic nanodevices have been intensively studied recently, but the crucial and necessary step to enable nanophotonic circuits for future information technology, namely cascade logics integrated on-chip, has not been achieved. Here we demonstrate that a nanophotonic binary logic NOR gate can be realized by cascading plasmonic OR and NOT gates in four-terminal nanowire networks. We explain the operating principle for the device based on quantum dot luminescence imaging, which reveal the interferences for different logic functions between propagating plasmon wave packets in the nanowire network in great detail. Since the NOR gate is logic complete, i.e. any Boolean logic gate can be constructed from it, our results could have a key role in defining a viable path for the development of novel subwavelength optical processor architectures.

Cascaded Logic Gates in Nanophotonic Plasmon Networks

In quantum-confined semiconductor nanostructures, electrons exhibit distinctive behavior compared with that in bulk solids. This enables the design of materials with tunable chemical, physical, electrical, and optical properties. Zero-dimensional semiconductor quantum dots (QDs) offer strong light absorption and bright narrowband emission across the visible and infrared wavelengths and have been engineered to exhibit optical gain and lasing. These properties are of interest for imaging, solar energy harvesting, displays, and communications. Here, we offer an overview of advances in the synthesis and understanding of QD nanomaterials, with a focus on colloidal QDs, and discuss their prospects in technologies such as displays and lighting, lasers, sensing, electronics, solar energy conversion, photocatalysis, and quantum information.

Semiconductor quantum dots: Technological progress and future challenges

Quantum dot light-emitting diodes are promising light sources for applications in displays. However, to date, there have been no reports of devices that simultaneously offer both high brightness and high external quantum efficiency. Here, we report red, green and blue quantum dot light-emitting diodes based on CdSe/ZnSe core/shell structures that have these attributes. We demonstrate devices with maximum external quantum efficiencies of 21.6%, 22.9% and 8.05% for red, green and blue colours with corresponding brightness of 13,300 cd m–2, 52,500 cd m–2 and 10,100 cd m–2. The devices also offer peak luminance of 356,000 cd m–2, 614,000 cd m–2 and 62,600 cd m–2, respectively. We postulate that this high performance is due to the use of Se throughout the core/shell regions and the existence of alloyed bridging layers at the core/shell interfaces. This study suggests that in the future visible quantum dot light-emitting diodes will also be suitable for lighting applications. Red, green and blue CdSe/ZnSe quantum dot light-emitting diodes combine efficient operation with high brightness.

Visible quantum dot light-emitting diodes with simultaneous high brightness and efficiency

Graphene bilayers exhibit zero-energy flatbands at a discrete series of magic twist angles In the absence of intrasublattice interlayer hopping, zero-energy states satisfy a Dirac equation with a non-Abelian SU(2) gauge potential that cannot be diagonalized globally We develop a semiclassical WKB approximation scheme for this Dirac equation by introducing a dimensionless Planck's constant proportional to the twist angle, solving the linearized Dirac equation around AB and BA turning points, and connecting Airy function solutions via bulk WKB wave functions We find zero-energy solutions at a discrete set of values of the dimensionless Planck's constant, which we obtain analytically Our analytic flatband twist angles correspond closely to those determined numerically in previous work

WKB Estimate of Bilayer Graphene's Magic Twist Angles

We study an oblique spacetime crystal realized by a monoatomic crystal in which a sound wave propagates, and analyze its quasienergy band structure starting from a tight-binding Bloch band for the static crystal. We investigate Floquet-Bloch oscillations under an external field, which show different characteristics for different band topologies. We also discover intraband Zener tunneling beyond the adiabatic limit, which effectively converts between different band topologies. Our results indicate the possibility of energy conversion between the sound wave and a dc electric field.

Floquet-Bloch Oscillations and Intraband Zener Tunneling in an Oblique Spacetime Crystal.

Surface plasmon polaritons (SPPs) due to their subwavelength nature could significantly modify electronic transition behaviors in various optoelectronic systems. Here, using a model system with a spherical quantum dot (QD) close to a flat metal surface, we show that the conventional forbidden optical transitions in a QD could be largely enabled by the spontaneous SPP decay. The electronic states of the QD are approximated by a Bloch state combined with wave functions in a spherical potential well, which provides multiple hole states with mixed electronic multipoles. Moreover, the SPP is quantized by using a canonical quantization scheme followed by a Green's function approach to introduce its dissipation. In particular, we find that when the SPP dissipation is included, the spontaneous decay of the corresponding QD exciton is dominant by the transition into the off-resonance mode of SPPs with large momenta. Also, we have studied the dependence of spontaneous decay rates on the size and crystal orientation of a QD, the distance between the QD and metal surface, and the linewidth of SPPs. Some useful scaling relations have been revealed, and the multipole transitions are found to be comparable with the dipole transition under specific system parameters. These findings have important implications for our understanding of the electronic transition at a metal near field and might prove instrumental for the future design of plasmonic and QD devices.

Spontaneous surface plasmon polariton decay of band-edge excitons in quantum dots near a metal surface

We study a periodically driven Bloch system connected to a heat bath by introducing a formalism that combines the Floquet analysis and the quantum Liouville equation. We first obtain the density matrix for such a system without electron-electron interactions, and find that by turning off the thermal contact, the system goes into a diagonal ensemble where the density matrix is diagonal under the Floquet-Bloch eigenbasis. We then calculate the current response that consists of a dissipationless intrinsic part and a dissipative extrinsic correction to the first order in damping. The correction originates from a shift in polarization between the diagonal ensemble and a Bloch thermal equilibrium. We further apply the result to a two-band model with broken time-reversal and inversion symmetries revealing the role of the external driven field strength.

Qiang Gao

Papers

Visible quantum dot light-emitting diodes with simultaneous high brightness and efficiency

WKB Estimate of Bilayer Graphene's Magic Twist Angles

Floquet-Bloch Oscillations and Intraband Zener Tunneling in an Oblique Spacetime Crystal.

Spontaneous surface plasmon polariton decay of band-edge excitons in quantum dots near a metal surface

Diagonal Ensemble in Strongly Driven Floquet-Bloch Systems: Dissipationless Intrinsic Current and Dissipative Extrinsic Correction.