Quantum Confinement Of Excitons Research Articles

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.

Quasicubic model for metal halide perovskite nanocrystals.

We present an analysis of quantum confinement of carriers and excitons, and exciton fine structure, in metal halide perovskite (MHP) nanocrystals (NCs). Starting with coupled-band k · P theory, we derive a nonparabolic effective mass model for the exciton energies in MHP NCs valid for the full size range from the strong to the weak confinement limits. We illustrate the application of the model to CsPbBr3 NCs and compare the theory against published absorption data, finding excellent agreement. We then apply the theory of electron-hole exchange, including both short- and long-range exchange interactions, to develop a model for the exciton fine structure. We develop an analytical quasicubic model for the effect of tetragonal and orthorhombic lattice distortions on the exchange-related exciton fine structure in CsPbBr3, as well as some hybrid organic MHPs of recent interest, including formamidinium lead bromide (FAPbBr3) and methylammonium lead iodide (MAPbI3). Testing the predictions of the quasicubic model using hybrid density functional theory (DFT) calculations, we find qualitative agreement in tetragonal MHPs but significant disagreement in the orthorhombic modifications. Moreover, the quasicubic model fails to correctly describe the exciton oscillator strength and with it the long-range exchange corrections in these systems. Introducing the effect of NC shape anisotropy and possible Rashba terms into the model, we illustrate the calculation of the exciton fine structure in CsPbBr3 NCs based on the results of the DFT calculations and examine the effect of Rashba terms and shape anisotropy on the calculated fine structure.

Cation effect on excitons in perovskite nanocrystals from single-dot photoluminescence of CH3NH3PbI3

The single-dot photoluminescence properties of $\ensuremath{\sim}7\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ perovskite $\mathrm{MAPb}{\mathrm{I}}_{3}(\mathrm{MA}=\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{{\mathrm{H}}_{3}}^{+})$ nanocrystals (NCs) were investigated in the $5\text{--}295\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ temperature range both in spectral and time domains. Repeatable single-dot measurements were facilitated by the use of a protective polymer, which stabilized the NCs. Temperature-induced phase transition and exciton-phonon interactions were revealed as well as the exciton fine structure. A pronounced spectral jump of the emission peak at $140\text{--}160\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, indicating a tetragonal-orthorhombic phase transition, was observed. In addition, the emission linewidth of $\ensuremath{\sim}0.6\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$ was measured, which is the narrowest ever recorded for this perovskite material system. A $\ensuremath{\sim}4.0\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$ phonon mode was identified for the NCs at 5 K, defining the linewidth thermal broadening. In general, the presence of $\mathrm{M}{\mathrm{A}}^{+}$ leads to broader spectra than for $\mathrm{C}{\mathrm{s}}^{+}$ or $\mathrm{F}{\mathrm{A}}^{+}$ containing perovskite NCs. It is attributed to higher polarity of this cation, thus it is more susceptible to spectral diffusion, which is clearly observed here. Photoluminescence decay measurements indicated that the recombination from the lowest energy state of the emission level manifold is partially forbidden. This is opposite to $\mathrm{C}{\mathrm{s}}^{+}$ cation NCs, highlighting the central role of the positive ion in the exchange interaction in perovskites. Finally, delayed luminescence was found to govern the recombination dynamics below room temperature, suggesting an involvement of trap sites for the orthorhombic phase. The reported photophysics of a quantum-confined exciton in this material, which is of interest for various light-converting applications, clarifies the role of the cation in perovskite nanocrystal optical properties.

Linearly Polarized Luminescence of Atomically Thin MoS2 Semiconductor Nanocrystals.

Atomically thin layers of transition-metal dichalcogenides semiconductors, such as MoS2, exhibit strong and circularly polarized light emission due to inherent crystal symmetries, pronounced spin-orbit coupling, and out-of-plane dielectric and spatial confinement. While the layer-by-layer confinement is well-understood, the understanding of the impact of in-plane quantization in their optical spectrum is far behind. Here, we report the optical properties of atomically thin MoS2 colloidal semiconductor nanocrystals. In addition to the spatial-confinement effect leading to their blue wavelength emission, the high quality of our MoS2 nanocrystals is revealed by narrow photoluminescence, which allows us to resolve multiple optically active transitions, originating from quantum-confined excitons (coupled electron-hole pairs). Surprisingly, in stark contrast to monolayer MoS2, the luminescence of the lowest-energy levels is linearly polarized and persists up to room temperature, meaning that it could be exploited in a variety of light-emitting applications.

Quantum Ensembles of Silicon Nanoparticles: Discrimination of Static and Dynamic Photoluminescence Quenching Processes.

Porous silicon photoluminescence is characterized by a broad emission band that displays unusually long (tens to hundreds of micro-seconds), wavelength-dependent emissive lifetimes. The photoluminescence is associated with quantum confinement of excitons in silicon nanocrystallites contained within the porous matrix, and the broad emission spectrum derives from the wide distribution of nanocrystallite sizes in the material. The longer emissive lifetimes in the ensemble of quantum-confined emitters correspond to the larger nanocrystallites, with their longer wavelengths of emission. The quenching of this photoluminescence by aromatic, redox-active molecules aminochrome (AMC), dopamine, adrenochrome, sodium anthraquinone-2-sulfonate, benzyl viologen dichloride, methyl viologen dichloride hydrate, and ethyl viologen dibromide is studied, and dynamic and static quenching mechanisms are distinguished by the emission lifetime analysis. Because of the dependence of the emission lifetime on emission wavelength from the silicon nanocrystallite ensemble, a pronounced blue shift is observed in the steady-state emission spectrum upon exposure to dynamic-type quenchers. Conversely, static-type quenching systems show uniform quenching across all emission wavelengths. Thus, the difference between static and dynamic quenching mechanisms is readily distinguished by ratiometric photoluminescence spectroscopy. The application of this concept to imaging of AMC, the oxidized form of the neurotransmitter dopamine that is of interest for its role in neurodegenerative diseases, is demonstrated. It is found that static electron acceptors result in no ratiometric contrast, while AMC shows a strong contrast, allowing ready visualization in a 2-D imaging experiment.

Role of surface passivation on visible and infrared emission of Ge quantum dots formed by dewetting

The dual action of oxide-related defects in the visible and infrared emission of germanium (Ge) self-assembled quantum dots (QDs) is discussed. The Ge particles were fabricated by solid-state dewetting on a thin layer of \({\hbox {SiO}}_{2}\). Subsequent surface passivation by amorphous silicon was carried out for several samples. All samples were encapsulated by \({\hbox {SiO}}_{2}\). Atomic force microscopy analysis indicates a linear relationship between the size of QDs and the initial thickness of the amorphous Ge films. The crystallization of the QDs was evidenced by transmission electron microscopy and Raman spectroscopy. Photoluminescence measurements show that the main visible emission is blue-green centred around 520 nm. The luminescence attributed to the radiative recombination of quantum-confined excitons is only observed when the surface is in-situ passivated prior to the deposition of the oxide matrix. The results of this work are helpful for optimizing the performance of the optoelectronic devices based on the infrared emission of Ge nanocrystals.

Electrical manipulation of the fine-structure splitting of WSe2 quantum emitters

We report on the modulation of the fine-structure splitting of quantum-confined excitons in localized quantum emitters hosted by a monolayer transition metal dichalcogenide (TMDC). The monolayer TMDC, tungsten diselenide (${\mathrm{WSe}}_{2}$), is encapsulated in a van der Waals heterostructure which enables the application of an external electric field on the quantum-dot-like emitters hosted by the monolayer flake. The emitters exhibit quantum-confined Stark effect and a modulation in the fine-structure splitting (FSS) as a function of electric field. A maximum modulation of $1500\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{eV}$ is observed in the FSS from the studied emitters. Finally, we measure the polarization response of the localized exciton emission as a function of electric field exhibiting strong circular polarization with decreasing fine-structure splitting, further confirming the suppression of the anisotropic electron-hole exchange interaction thats causes the FSS.

Insights into the photoluminescence properties of gel-like carbon quantum dots embedded in poly(methyl methacrylate) polymer

Carbon dots (CDs) have become one of the most investigated nanomaterials in the last few years, since they offer excellent prospects to be used in a wide range of technological applications. Nonetheless, the understanding of the photoluminescence (PL) processes remains unclear and solely based on spectroscopic measurements at room temperature, which hampered a full discussion on the nature of recombination mechanisms. Insights into the CDs’ PL and corresponding recombination models can be achieved from temperature, excitation energy and excitation density dependency studies. Thus, in this paper, CDs with a mean diameter of ∼3.0 nm were embedded in poly(methyl methacrylate) (PMMA) and studied by such optical techniques. The recombination of CDs/PMMA composites was found to be due to an overlap of emitting centres, both in the blue and green regions, with intensity and peak position strongly dependent on the excitation density. Moreover, this emission was found to be preferentially populated by two overlapped excitation bands. Temperature-dependent PL measurements corroborate the hypothesis of overlapped emitting optical centres (e.g. small molecules at the CDs’ surface), as no shift on peak position and no changes in spectral shape were observed, which allow to exclude the hypothesis of a conventional free excitonic transition and quantum confinement effects, as observed in typical nanosized semiconductors with dimensions lower than the exciton Bohr radius.

Binding energy and the third-order nonlinear optical susceptibility of an exciton in GaAs/AlGaAs core/shell spherical quantum dot

In this study, we have calculated the energy levels and binding energies for exciton associated with the ground state and some low-lying states in GaAs/AlGaAs core/shell spherical quantum dot (QD) by using the finite difference method. The correctness of the numerical results is verified by comparison with the analytical solution without considering exciton effects. Simultaneously, the third harmonic generation (THG) in GaAs/AlGaAs core/shell spherical QD is theoretically investigated within the framework of the compact-density-matrix approach and iterative method for both cases with and without exciton effects. All paths of the system are considered which generates third harmonics. Our results indicate that the energy levels and binding energies are depended dramatically on the size of the QD. The THG is greatly enhanced because of the quantum confinement of exciton. It is over two times bigger than that obtained by without considering exciton effects. In addition, the resonant peaks and its corresponding to resonant energy are also taken into account.

Layered Inorganic/Organic Hybrid (CdSe) n·Monoamine Nanobelts: Controllable Solvothermal Synthesis, Multiple Stage Amine De-Intercalation Transformation, and Two-Dimensional Exciton Quantum Confinement Effect.

CdSe nanocrystals, including quantum dots and quantum rods, are a research hotspot in nanomaterials. Recently, it has been found that two-dimensional CdSe semiconductor nanobelts show a unique quantum confinement effect owing to their homogeneous nanoscale thickness. In this study, a series of (CdSe) n·monoamine ( n = 1, 2, and 4, monoamine = butylamine, hexylamine, and octylamine) layered inorganic/organic hybrid nanobelts was synthesized by the solvothermal method. These hybrids have a lamellar structure with a few inorganic [CdSe] layers and monoamine layers alternatively stacked along the c axis of their crystallites, which exhibit the typical preferred orientation for layered crystallites and isomorphous [CdSe] layers according to their X-ray diffraction patterns. The [CdSe] layer is a contracted (110) superlattice cell of wurtzite (WZ)-type CdSe with unit cell parameter relations of a ≈ √3 aWZ and b ≈ cWZ, and a lamellar nanobelt morphology with the [010] growth direction. These ordered hybrids exhibit prominent split exciton absorption peaks and sharp band edge luminescence owing to the homogeneous thickness of the [CdSe] layers within the hybrids, all of which are comparable with reported WZ-type CdSe colloidal nanosheets and exhibit a prominent two-dimensional exciton quantum confinement effect. The solvothermal reaction was systematically investigated. The multiple stage amine de-intercalation topotactic process was revealed for the WZ-type CdSe nanobelts with the (CdSe) n·monoamine ( n = 1, 2, and 4) hybrids as the intermediate phases. The solvothermal synthesis method (70-150 °C) is facile and milder than the traditional hot injection method (>200 °C). Therefore, these hybrids can be considered to be intermediate phases or precursors of WZ-type CdSe nanosheets with a topotactic intralayer [CdSe] structure and also a type of multiple quantum well with extremely thin [CdSe] layers orderly stacked among the monoamine insulating blocking layers.

Efficient coupling of disorder states to excitons in an InGaN nanostructure

InxGa1-xN disks in GaN nanowires (DINWs) have emerged as a viable technology for on-chip tunable visible spectrum emission without the use of a phosphor. Here we present a study of the optical emission and absorption dynamics in DINWs that incorporates the important role of background disorder states. We show that the optical emission in the system is dominated by quantum-confined excitons, however the exci-tons are coupled to a large density of background disorder states. Rapid non-radiative decay (compared to other decay rates such as spontaneous emission) from disorder states into excitons is observed after optical excitation of our sample, which can be advantageous for increasing the brightness of the system in future design efforts.

Oscillator strength and quantum-confined Stark effect of excitons in a thin PbS quantum disk

In this paper, we report a study of the effect of a lateral electric field on a quantum-confined exciton in a thin PbS quantum disk. Our approach was performed in the framework of the effective mass theory and adiabatic approximation. The ground state energy and the stark shift were determined by using a variational method with an adequate trial wavefunction, by investigating a 2D oscillator strength under simultaneous consideration of the geometrical confinement and the electric field strength. Our results showed a strong dependence of the exciton binding and the Stark shift on the disk dimensions in both axial and longitudinal directions. On the other hand, our results also showed that the Stark shift’s dependence on the electric field is not purely quadratic but the linear contribution is also important and cannot be neglected, especially when the confinement gets weaker.

Luminescent Silicon Nanocrystal embedded Sodium Silicate Glass

Green luminescent silicon nanoparticles (Si NPs), embedded in sodium silicate (SS) based glass, have been prepared by a simple green chemical synthesis route. (3-aminopropyl) triethoxysilane (APTES) was reduced by sodium ascorbate (SA) in a one-step synthesis method to prepare Si NPs. Extending the same route SS based glass was synthesised by slow evaporation of the aqueous solution of colloidal Si NPs at room temperature. Colloids of Si NPs and the SS-based glass exhibit intense room temperature greenish photoluminescence (PL) that can be detected with the naked eye. Structural investigations reveal that the Si NP sample contains dominant amorphous phase with trace amount of Si nanocrystals (NCs). Analysis of optical properties of the Si NP sample indicates that green PL is dominantly due to the contribution of oxide enriched surface states rather than quantum confinement of excitons and likewise the PL from the SS based glass is due to the presence of trace amount of Si NCs along with agglomerated amorphous oxide embedded in the glass. There is a clear indication of selective reflectance from the prepared SS based glass.

Enhancing the luminescence efficiency of silicon-nanocrystals by interaction with H+ ions.

The emission of silicon nanocrystals (Si-NCs), synthesized by pulsed laser ablation in water, was investigated on varying the pH of the solution. These samples emit μs decaying orange photoluminescence (PL) associated with radiative recombination of quantum-confined excitons. Time-resolved spectra reveal that both the PL intensity and the lifetime increase by a factor of ∼20 when the pH decreases from 10 to 1 thus indicating that the emission quantum efficiency increases by inhibiting nonradiative decay rates. Infrared (IR) absorption and electron paramagnetic resonance (EPR) experiments allow addressing the origin of defects on which the excitons nonradiatively recombine. The linear correlation between the PL and the growth of SiH groups demonstrates that H+ ions passivate the nonradiative defects that are located in the interlayer between the Si-NC core and the amorphous SiO2 shell.

Ligand-Free, Quantum-Confined Cs2SnI6 Perovskite Nanocrystals

Tin-halide perovskite nanocrystals are a viable precursor for lead-free, high-efficiency active layers for photovoltaic cells. We describe a new synthetic procedure for quantum-confined Cs2SnI6 nanocrystals with size-dependent band gaps in the long-visible to near-infrared (1.38–1.47 eV). Hot injection synthesis produces particles with no organic capping ligands, with average diameters that increase from 12 ± 2.8 nm to 38 ± 4.1 nm with increasing reaction temperature. The band gap, energies of the first excitonic peak, ground-state bleach peak (in the transient absorption spectrum), and photoluminescence peak depend linearly on the inverse square of diameter, consistent with quantum-confined excitons with an effective mass of (0.12 ± 0.02)m0, where m0 is the mass of an electron, a factor of 4.6 smaller than that in the bulk material. Transient absorption measurements show that approximately 90% of the bleach amplitude decays within 30 ps, probably because of carrier trapping on unpassivated surface sites....

Large-scale quantum-emitter arrays in atomically thin semiconductors

Quantum light emitters have been observed in atomically thin layers of transition metal dichalcogenides. However, they are found at random locations within the host material and usually in low densities, hindering experiments aiming to investigate this new class of emitters. Here, we create deterministic arrays of hundreds of quantum emitters in tungsten diselenide and tungsten disulphide monolayers, emitting across a range of wavelengths in the visible spectrum (610–680 nm and 740–820 nm), with a greater spectral stability than their randomly occurring counterparts. This is achieved by depositing monolayers onto silica substrates nanopatterned with arrays of 150-nm-diameter pillars ranging from 60 to 190 nm in height. The nanopillars create localized deformations in the material resulting in the quantum confinement of excitons. Our method may enable the placement of emitters in photonic structures such as optical waveguides in a scalable way, where precise and accurate positioning is paramount.

Investigation of Photoluminescence Mechanisms from SiO2/Si:SiO2/SiO2 Structures in Weak Quantum Confined Regime by Deconvolution of Photoluminescence Spectra.

Si nanocrystals embedded in SiO2 matrix were prepared by co-sputtering method followed by a post annealing process in N2 ambient. By fixing sputtering parameters, the effects of annealing time and annealing temperature on the optical properties of Si nanocrystals are investigated. Origin and evolution of the photoluminescence (PL) in weak quantum confinement regime are discussed in the light of X-ray diffraction, Fourier transform infrared, and temperature dependent photoluminescence measurements. For all samples, the PL peaks tend to decompose to four Gaussian peaks in which attributed to the radiative defects in SiO2 matrix, nc-Si/SiO2 interface related localized defects, localized states in the amorphous Si band gap and quantum confinement of excitons in smaller nanocrystals. Considering the observation of luminescence and its decomposition tendency in nanocrystals with average sizes larger than exciton's Bohr radius the necessity to distinguish between the role of smaller and larger nanocrystals in the PL mechanisms is discussed. Furthermore, possible origin of the interface related localized states in particular Si=O double bonds in the nc-Si/SiO2 interface and that of radiative defects in SiO2 matrix are discussed.

Size-Dependent Exciton Formation Dynamics in Colloidal Silicon Quantum Dots.

We report size-dependent exciton formation dynamics within colloidal silicon quantum dots (Si QDs) using time-resolved terahertz (THz) spectroscopy measurements. THz photoconductivity measurements are used to distinguish the initially created hot carriers from excitons that form at later times. At early pump/probe delays, the exciton formation dynamics are revealed by the temporal evolution of the THz transmission. We find an increase in the exciton formation time, from ∼500 to ∼900 fs, as the Si QD diameter is reduced from 7.3 to 3.4 nm and all sizes exhibit slower hot-carrier relaxation times compared to bulk Si. In addition, we determine the THz absorption cross section at early delay times is proportional to the carrier mobility while at later delays is proportional to the exciton polarizability, αX. We extract a size-dependent αX and find an ∼r(4) dependence, consistent with previous reports for quantum-confined excitons in CdSe, InAs, and PbSe QDs. The observed slowing in exciton formation time for smaller Si QDs is attributed to decreased electron-phonon coupling due to increased quantum confinement. These results experimentally verify the modification of hot-carrier relaxation rates by quantum confinement in Si QDs, which likely plays a significant role in the high carrier multiplication efficiency observed in these nanomaterials.

Excitonic quantum confinement modified optical conductivity of monolayer and few-layered MoS2

Theoretical calculations reveal that the excitonic quantum confinement effect significantly modifies the optical conductivities of monolayer and few-layered MoS2.

Effect of Sn Doping on the Properties of Nano-Structured ZnO Thin Films Deposited by Co-Sputtering Technique.

In this study, tin doped zinc oxide (ZnO:Sn) nano-structured thin films were successfully deposited by co-sputtering of ZnO and Sn on top of glass substrate. The effect of Sn doping on the microstructure, phase, morphology, optical and electrical properties of the films were extensively investigated by means of XRD, EDX, SEM, AFM, Hall Effect measurement, and UV-Vis spectrometry. The results showed that the undoped ZnO film exhibited preferred orientation along the c-axis of the hexagonal wurtzite structure. With increase of Sn doping, the peak position of the (002) plane was shifted to the higher 20 values, and ultimately changed to amorphous structure. The absorption edge was shifted to blue region which confirmed the excitonic quantum confinement effect in the films. Consequently, improved surface morphology with optical bandgap, reduced average particle size, reduced resistivity, enhanced Hall mobility and carrier concentration were observed in the doped films after vacuum annealing. Among all of the as-deposited and annealed ZnO:Sn films investigated in this study, annealed film doped with 8 at.% of Sn concentration exhibited the best properties with a bandgap of 3.84 eV, RMS roughness of 2.51 nm, resistivity of 2.36 ohm-cm, and Hall mobility of 83 cm2 V(-1) s(-1).