Quantum Confinement Of Excitons Research Articles

Boosting exciton dissociation and charge transfer in CsPbBr3 QDs via ferrocene derivative ligation for CO2 photoreduction

Photo-catalytic CO2 reduction with perovskite quantum dots (QDs) shows potential for solar energy storage, but it encounters challenges due to the intricate multi-electron photoreduction processes and thermodynamic and kinetic obstacles associated with them. This study aimed to improve photo-catalytic performance by addressing surface barriers and utilizing multiple-exciton generation in perovskite QDs. A facile surface engineering method was employed, involving the grafting of ferrocene carboxylic acid (FCA) onto CsPbBr3 (CPB) QDs, to overcome limitations arising from restricted multiple-exciton dissociation and inefficient charge transfer dynamics. Kelvin Probe Force Microscopy and XPS spectral confirmed successfully creating an FCA-modulated microelectric field through the Cs active site, thus facilitating electron transfer, disrupting surface barrier energy, and promoting multi-exciton dissociations. Transient absorption spectroscopy showed enhanced charge transfer and reduced energy barriers, resulting in an impressive CO2-to-CO conversion rate of 132.8 μmol g-1 h-1 with 96.5% selectivity. The CPB-FCA catalyst exhibited four-cycle reusability and 72 h of long-term stability, marking a significant nine-fold improvement compared to pristine CPB (14.4 μmol g-1 h-1). These results provide insights into the influential role of FCA in regulating intramolecular charge transfer, enhancing multi-exciton dissociation, and improving CO2 photoreduction on CPB QDs. Furthermore, these findings offer valuable knowledge for controlling quantum-confined exciton dissociation to enhance CO2 photocatalysis.

Entropic Ligand Mixing for Engineering 2D Layered Perovskite from Colloidal Monolayer Building Blocks

AbstractLayered 2D perovskites are solution‐processed quantum‐wells. Their effective band‐gap is determined via the inorganic perovskite layer thickness and exciton quantum confinement effects. Alternatively, by changing the organic moieties, one can tune the dielectric constant and distance between the monolayers modifying the excitonic interactions. In colloidal perovskites, a dynamic equilibrium exists between the free organic moieties in the solution and the surface of the nanocrystal. Colloidal synthesis is used to make single monolayer L2PbBr4 platelets and assemble these into layered 2D stacks. In the experiment, L is an alkylamine surface ligand whose length (4‐18 carbons) determines the interlayer distances between the quantum‐wells. The dynamic equilibrium of ligand mixtures in solution and perovskite surfaces leads to optimal mixing of the molecules. During the self‐assembly of monolayers, the distance between the inorganic layers is thus engineered. The interlayer distance is proportional to the average ligand mixture length. This results in controlled interactions between the 2D‐excitons, enabling red‐shifted absorption and emission and extended lifetimes for longer alkyl chains. Using entropic mixing of ligands for the engineering of 2D excitonic interactions is therefore demonstrated. Formation of layered 2D perovskites from colloidal building blocks allows intermixing of dissimilar materials opening possibilities for new heterostructures and junctions.

Edge-Confined Excitons in Monolayer Black Phosphorus.

Quantum confinement of two-dimensional excitons in van der Waals materials via electrostatic trapping, lithographic patterning, Moiré potentials, and chemical implantation has enabled significant advances in tailoring light emission from nanostructures. While such approaches rely on complex preparation of materials, natural edges are a ubiquitous feature in layered materials and provide a different approach for investigating quantum-confined excitons. Here, we observe that certain edge sites of monolayer black phosphorus (BP) strongly localize the intrinsic quasi-one-dimensional excitons, yielding sharp spectral lines in photoluminescence, with nearly an order of magnitude line width reduction. Through structural characterization of BP edges using transmission electron microscopy and first-principles GW plus Bethe-Salpeter equation (GW-BSE) calculations of exemplary BP nanoribbons, we find that certain atomic reconstructions can strongly quantum-confine excitons resulting in distinct emission features, mediated by local strain and screening. We observe linearly polarized luminescence emission from edge reconstructions that preserve the mirror symmetry of the parent BP lattice, in agreement with calculations. Furthermore, we demonstrate efficient electrical switching of localized edge excitonic luminescence, whose sites act as excitonic transistors for emission. Localized emission from BP edges motivates exploration of nanoribbons and quantum dots as hosts for tunable narrowband light generation, with future potential to create atomic-like structures for quantum information processing applications as well as exploration of exotic phases that may reside in atomic edge structures.

Tunable phononic coupling in excitonic quantum emitters.

Engineering the coupling between fundamental quantum excitations is at the heart of quantum science and technologies. An outstanding case is the creation of quantum light sources in which coupling between single photons and phonons can be controlled and harnessed to enable quantum information transduction. Here we report the deterministic creation of quantum emitters featuring highly tunable coupling between excitons and phonons. The quantum emitters are formed in strain-induced quantum dots created in homobilayer WSe2. The colocalization of quantum-confined interlayer excitons and terahertz interlayer breathing-mode phonons, which directly modulates the exciton energy, leads to a uniquely strong phonon coupling to single-photon emission, with a Huang-Rhys factor reaching up to 6.3. The single-photon spectrum of interlayer exciton emission features a single-photon purity >83% and multiple phonon replicas, each heralding the creation of a phonon Fock state in the quantum emitter. Due to the vertical dipole moment of the interlayer exciton, the phonon-photon interaction is electrically tunable to be higher than the exciton and phonon decoherence rate, and hence promises to reach the strong-coupling regime. Our result demonstrates a solid-state quantum excitonic-optomechanical system at the atomic interface of the WSe2 bilayer that emits flying photonic qubits coupled with stationary phonons, which could be exploited for quantum transduction and interconnection.

Co-wide-bandgap conformally heteroepitaxial perovskite–monolithic SiC nanowire array with quantum-confined blue luminescence

SiC is a wide-bandgap semiconductor with excellent mechanical and electrical properties and is a crucial template for epitaxially growing other semiconductors. We report the conformally epitaxial growth of the lead halide perovskites on the red-luminescent monolithic 6H-SiC nanowire arrays. The small lattice mismatch (0.8%) between SiC and CsPbBr3 ensures perfect heteroepitaxial growth of the CsPbBr3 quantum dots and nanosheets over the SiC nanowire arrays. The heteroepitaxial perovskites show intense multiband blue luminescence stemming from the strongly quantum-confined excitons with a twice prolonged lifetime compared with free nanocrystals. These blue-luminescent heteroepitaxial semiconductor–semiconductor nanostructures are promising nanophotonic device units.

Near‐Field Nano‐Optical Imaging of van der Waals Materials

Abstract2D van der Waals (vdW) materials are emerging as the next generation platform for optical and electronic devices with their wide coverage of the energy bandgaps. The strong light–matter interactions in 2D vdW layers allow for exploring novel optical and electronic phenomena such as 2D polaritons exhibiting ultrahigh field confinement, defects‐induced new quantum states, and strain‐modulated quantum confinement of 2D excitons. Far‐field optical imaging techniques are extensively used to characterize the 2D vdW materials so far, however, subdiffraction spatial resolution is required for comprehensive investigations of 2D vdW materials of which physical properties are greatly influenced by local defects and strain. This article aims to cover historical advances, fundamental principles, and distinct features of emerging near‐field optical imaging techniques: scattering‐type scanning near‐field optical microscopy, tip‐enhanced Raman spectroscopy, tip‐enhanced photoluminescence techniques, and photo‐induced force microscopy. The recent developments toward spectroscopic analysis of near‐field imaging and applications for unveiling unique properties of 2D polaritons, nanoscale defects, and mechanical strains in 2D vdW materials, are also discussed. This review article provides an understanding of emerging near‐field imaging techniques and suggests prospective applications for exploring 2D vdW materials.

Controlling quantum phases of electrons and excitons in moiré superlattices

Moiré lattices formed in twisted and lattice-mismatched van der Waals heterostructures have emerged as a platform to engineer the novel electronic and excitonic states at the nanoscale. This Perspective reviews the materials science of moiré heterostructures with a focus on the structural properties of the interface and its structural–property relationships. We first review the studies of the atomic relaxation and domain structures in moiré superlattices and how these structural studies provide critical insights into understanding the behaviors of quantum-confined electrons and excitons. We discuss the general frameworks to manipulate moiré structures and how such control can be harnessed for engineering new phases of matter and simulating various quantum phenomena. Finally, we discuss routes toward large-scale moiré heterostructures and give an outlook on their applications in quantum electronics and optoelectronics. Special emphasis will be placed on the challenges and opportunities of the reliable fabrication and dynamical manipulation of moiré heterostructures.

Multifunctional NdBr3 Modifier Enables Remarkable Enhancement in the Performance of Perovskite Light-Emitting Diodes

Quasi-two-dimensional (quasi-2D) perov- skites possess the preponderances of large exciton binding energy and quantum confinement effects compared with three-dimensional (3D) perovskites. The unique properties of quasi-2D perovskite have attracted extensive attention in the field of optoelectronic devices, especially in the light emitting diode (LED) direction. However, the adverse effects between the perovskite emission layer and the popularly used hole transport layer (HTL) Poly(3,4-ethylenedioxythiophene):poly (styrene sulfonate) (PEDOT:PSS) have degrade the device performance, such as large energy level barrier and exciton quenching. Here we demonstrated high performance of LED with enhanced brightness and efficiency by modifying the PEDOT:PSS with NdBr3, which not only decreases exciton quenching, but also decreases HOMO level of the PEDOT:PSS, thereby improving the charge balance. The final performance of the perovskite light-emitting diode (PeLED) device was obtained with a maximum luminance of 18684 cd m−2, a maximum current efficiency (CE) of 48.4 cd A−1 and an external quantum efficiency (EQE) of up to 15.5% when using NdBr3 modified PEDOT:PSS.

Two-Photon Lasing from Two-Dimensional Homologous Ruddlesden-Popper Perovskite with Giant Nonlinear Absorption and Natural Microcavities.

Two-dimensional Ruddlesden-Popper perovskites (RPPs) with multiple quantum well-like structures, strong excitonic quantum confinement, and high stability are promising optical gain media. However, the lasing from such material with a small number of inorganic well layers is difficult to achieve. Herein, we demonstrate the low-threshold upconversion lasing from the homologous RPP (PEA)2(MA)n-1PbnI3n+1 (n = 2 and 3) microflakes with wavelength varies from 598 to 637 nm under 800 nm laser excitation at low temperature (≤153 K). Using the micro Z-scan technique, we discovered that the RPP flakes have a giant two-photon absorption coefficient β as high as 3.6 × 103 cm GW-1, resulting in the effective upconversion transition under two-photon excitation. Furthermore, the self-formation of Fabry-Pérot microcavities provides the support for lasing emission from the n ≥ 2 RPP flakes. Calculation results and microscopic transient absorption measurements reveal that low-threshold lasing is due to the high differential gain coefficient and the suppressed nonradiative Auger recombination rate inside the quantum confinement structures. These properties enable RPPs as potential gain media for developing upconversion microcavity lasers.

Electrically tunable quantum confinement of neutral excitons.

Confining particles to distances below their de Broglie wavelength discretizes their motional state. This fundamental effect is observed in many physical systems, ranging from electrons confined in atoms or quantum dots1,2 to ultracold atoms trapped in optical tweezers3,4. In solid-state photonics, a long-standing goal has been to achieve fully tunable quantum confinement of optically active electron-hole pairs, known as excitons. To confine excitons, existing approaches mainly rely on material modulation5, which suffers from poor control over the energy and position of trapping potentials. This has severely impeded the engineering of large-scale quantum photonic systems. Here we demonstrate electrically controlled quantum confinement of neutral excitons in 2D semiconductors. By combining gate-defined in-plane electric fields with inherent interactions between excitons and free charges in a lateral p-i-n junction, we achieve exciton confinement below 10 nm. Quantization of excitonic motion manifests in the measured optical response as a ladder of discrete voltage-dependent states below the continuum. Furthermore, we observe that our confining potentials lead to a strong modification of the relative wave function of excitons. Our technique provides an experimental route towards creating scalable arrays of identical single-photon sources and has wide-ranging implications for realizing strongly correlated photonic phases6,7 and on-chip optical quantum information processors8,9.

Two-Photon Excitation Spectroscopy of Silicon Quantum Dots and Ramifications for Bio-Imaging.

Two-photon excitation in the near-infrared (NIR) of colloidal nanocrystalline silicon quantum dots (nc-SiQDs) with photoluminescence also in the NIR has potential opportunities in the field of deep biological imaging. Spectra of the degenerate two-photon absorption (2PA) cross section of colloidal nc-SiQDs are measured using two-photon excitation over a spectral range 1.46 < ℏω < 1.91 eV (wavelength 850 > λ > 650 nm) above the two-photon band gap Eg(QD)/2, and at a representative photon energy ℏω = 0.99 eV (λ = 1250 nm) below this gap. Two-photon excited photoluminescence (2PE-PL) spectra of nc-SiQDs with diameters d = 1.8 ± 0.2 nm and d = 2.3 ± 0.3 nm, each passivated with 1-dodecene and dispersed in toluene, are calibrated in strength against 2PE-PL from a known concentration of Rhodamine B dye in methanol. The 2PA cross section is observed to be smaller for the smaller diameter nanocrystals, and the onset of 2PA is observed to be blue shifted from the two-photon indirect band gap of bulk Si, as expected for quantum confinement of excitons. The efficiencies of nc-SiQDs for bioimaging using 2PE-PL are simulated in various biological tissues and compared to efficiencies of other quantum dots and molecular fluorophores and found to be comparable or superior at greater depths.

Time-Resolved and Temperature-Dependent Fractional Amplitude Contributions to the Broadband Emission of CdSe Quantum Dots

The broadband spontaneous emission of excitons in CdSe quantum dots (QDs) is of great interest for the spectral imaging of living organisms or specific substances in the visible spectral region as well as in the biological optical window near the infrared spectral region. Semiconductor QDs that are near the bulk Bohr radius exhibit wide spectral tunability and high color purity due to quantum confinement of excitons within the dot boundary. However, with reducing dot size, the role of the surface-trapped state increases. The temperature-dependent photoluminescence (PL) confirms this with a ~3:1 emission intensity decrease from the surface-trapped state compared to the band edge. Large crystal irregularity, dangling ions, and foreign molecules can introduce new electronic transitions from surface-trapped states that provide broad spontaneous emission in the spectral region from visible to near IR in addition to the band edge emission. The time-resolved PL analyzed the fractional contributions of band edge, surface-trapped states, and possible intermediate trapped states to the broad spectral emission in order to characterize the CdSe QDs.

Promoting Energy Transfer via Manipulation of Crystallization Kinetics of Quasi-2D Perovskites for Efficient Green Light-Emitting Diodes.

Quasi-2D (Q-2D) perovskites are promising materials applied in light-emitting diodes (LEDs) due to their high exciton binding energy and quantum confinement effects. However, Q-2D perovskites feature a multiphase structure with abundant grain boundaries and interfaces, leading to nonradiative loss during the energy-transfer process. Here, a more efficient energy transfer in Q-2D perovskites is achieved by manipulating the crystallization kinetics of different-n phases. A series of alkali-metal bromides is utilized to manipulate the nucleation and growth of Q-2D perovskites, which is likely associated with the Coulomb interaction between alkali-metal ions and the negatively charged PbBr6 4- frames. The incorporation of K+ is found to restrict the nucleation of high-n phases and allows the subsequent growth of low-n phases, contributing to a spatially more homogeneous distribution of different-n phases and promoted energy transfer. As a result, highly efficient green Q-2D perovskites LEDs with a champion EQE of 18.15% and a maximum brightness of 25 800cd m-2 are achieved. The findings affirm a novel method to optimize the performance of Q-2D perovskite LEDs.

Quantum confined Rydberg excitons in Cu2O nanoparticles

The quantum confinement of Rydberg excitons is an important step towards exploiting their large nonlinearities for quantum applications. We observe Rydberg excitons in natural nanoparticles of Cu$_2$O. We resolve up to the principal quantum number $n=12$ in a bulk Cu$_2$O crystal and up to $n=6$ in nanoparticles extracted from the same crystal. The exciton transitions in nanoparticles are broadened and their oscillator strengths decrease as $\propto n^{-4}$ compared to those in the bulk (decreasing as $\propto n^{-3}$). We explain our results by including the effect of quantum confinement of exciton states in the nanoparticles. Our results provide an understanding of the physics of Cu$_2$O Rydberg excitons in confined dimensions.

Size-dependent optical properties of shallow quantum dot excitons close to a dielectric-hyperbolic material interface.

The resonance frequency shift and the radiative decay rate of single quantum dot excitions in close proximity to a dielectric-hyperbolic material interface are theoretically investigated. The previous nonlocal susceptibility model for a quantum-confined exciton in inhomogeneous surroundings has been substantially upgraded in a way to incorporate exciton's envelope functions with a non-zero orbital angular momentum and a dyadic Green function tensor for uniaxially anisotropic multilayer structures. Different eigenstates of spatially localized excitons are considered with a distance to the interface of half-infinite Tetradymites(Bi2Se3), a natural hyperbolic material in a visible-to-near infrared wavelength range. From numerically obtained self-energy corrections (SEC) of the exciton as a function of its spatial confinement, eigenfunction, and distance, where the real and imaginary parts correspond to the resonance frequency shift and the radiative decay rate of the exciton, respectively, both optical properties show a significant dependence on the spatial confinement of the exciton than expected. The SEC of very weakly confined (quasi free) two-dimensional excitons is almost immune to specific choice of the eigenfunction and to anisotropic properties of the hyperbolic material even at a close distance, while such conditions are decisive for the SEC of strongly confined excitons.

Observation of quantum-confined exciton states in monolayer WS2 quantum dots by ultrafast spectroscopy.

Monolayer transition metal dichalcogenide quantum dots (TMDC QDs) could exhibit unique photophysical properties, because of both lateral quantum confinement effect and edge effect. However, there is little fundamental study on the quantum-confined exciton dynamics in monolayer TMDC QDs, to date. Here, by selective excitations of monolayer WS2 QDs in broadband transient absorption (TA) spectroscopy experiments, the excitation-wavelength-dependent ground state bleaching signals corresponding to the quantum-confined exciton states are directly observed. Compared to the time-resolved photophysical properties of WS2 nanosheets, the selected monolayer WS2 QDs only show one ground state bleaching peak with larger initial values for the linear polarization anisotropy of band-edge excitons, probably due to the expired spin-orbit coupling. This suggests a complete change of the band structure for monolayer WS2 QDs. In the femtosecond time-resolved circular polarization anisotropy experiments, a valley depolarization time of ∼100 fs is observed for WS2 nanosheets at room temperature, which is not observed for monolayer WS2 QDs. Our findings suggest a strong state-mixing of band-edge valley excitons responsible for the large linear polarization in monolayer WS2 QDs, which could be helpful for understanding the exciton relaxation mechanisms in colloidal monolayer TMDC QDs.

Photocurrents in GaAs quantum wires including excitonic effects

Photocurrent induced by ultrashort laser pulses in [100]-oriented GaAs quantum wires with square cross section is investigated using the multiband semiconductor Bloch equations with the inclusion of electron-hole interaction. Influences of excitonic effects and quantum confinement effects on the photocurrent are evaluated and discussed.

Exciton energy oscillations induced by quantum beats

In this paper, we experimentally demonstrate an oscillating energy shift of quantum-confined exciton levels in a semiconductor quantum well after excitation into a superposition of two quantum confined exciton states of different parity. Oscillations are observed at frequencies corresponding to the quantum beats between these states. We show that the observed effect is a manifestation of the exciton density oscillations in the real space similar to the dynamics of a Hertzian dipole. The effect is caused by the exciton-exciton exchange interaction and appears only if the excitons are in a superposition quantum state. Thus, we have found clear evidence for the incoherent exchange interaction in the coherent process of quantum beats. This effect may be harnessed for quantum technologies requiring the quantum coherence of states.

Strongly Quantum-Confined Blue-Emitting Excitons in Chemically Configurable Multiquantum Wells.

Light matter interactions are greatly enhanced in two-dimensional (2D) semiconductors because of strong excitonic effects. Many optoelectronic applications would benefit from creating stacks of atomically thin 2D semiconductors separated by insulating barrier layers, forming multiquantum-well structures. However, most 2D transition metal chalcogenide systems require serial stacking to create van der Waals multilayers. Hybrid metal organic chalcogenolates (MOChas) are self-assembling hybrid materials that combine multiquantum-well properties with scalable chemical synthesis and air stability. In this work, we use spatially resolved linear and nonlinear optical spectroscopies over a range of temperatures to study the strongly excitonic optical properties of mithrene, that is, silver benzeneselenolate, and its synthetic isostructures. We experimentally probe s-type bright excitons and p-type excitonic dark states formed in the quantum confined 2D inorganic monolayers of silver selenide with exciton binding energy up to ∼0.4 eV, matching recent theoretical predictions of the material class. We further show that mithrene's highly efficient blue photoluminescence, ultrafast exciton radiative dynamics, as well as flexible tunability of molecular structure and optical properties demonstrate great potential of MOChas for constructing optoelectronic and quantum excitonic devices.

Size, porosity, and surface-termination dependence of the radiative and nonradiative relaxation processes of porous silicon

The role of porosity and surface-termination on the radiative and the nonradiative relaxation processes of luminescent porous silicon is investigated using temperature-dependent, time-resolved photoluminescence spectroscopy. We show that, for porous silicon having low- to mid-porosity, radiative relaxation times should be associated with the quantum confinement of excitons (the confined photo-excited electron–hole pairs), while nonradiative relaxation processes are related to the state of the surface (e.g., the surface chemistry), in agreement with previous reports. However, for high-porosity films of porous silicon, we have found much faster low-temperature relaxation times, which are associated with radiative transitions from the triplet excitonic state. This state becomes partially allowed due to a strong coupling and mixing with the singlet state in high-porosity films of porous silicon containing fairly small silicon nanocrystallites.