Strong Quantum Confinement Regime Research Articles

Colloidal Aziridinium Lead Bromide Quantum Dots.

The compositional engineering of lead-halide perovskite nanocrystals (NCs) via the A-site cation represents a lever to fine-tune their structural and electronic properties. However, the presently available chemical space remains minimal since, thus far, only three A-site cations have been reported to favor the formation of stable lead-halide perovskite NCs, i.e., Cs+, formamidinium (FA), and methylammonium (MA). Inspired by recent reports on bulk single crystals with aziridinium (AZ) as the A-site cation, we present a facile colloidal synthesis of AZPbBr3 NCs with a narrow size distribution and size tunability down to 4 nm, producing quantum dots (QDs) in the regime of strong quantum confinement. NMR and Raman spectroscopies confirm the stabilization of the AZ cations in the locally distorted cubic structure. AZPbBr3 QDs exhibit bright photoluminescence with quantum efficiencies of up to 80%. Stabilized with cationic and zwitterionic capping ligands, single AZPbBr3 QDs exhibit stable single-photon emission, which is another essential attribute of QDs. In particular, didodecyldimethylammonium bromide and 2-octyldodecyl-phosphoethanolamine ligands afford AZPbBr3 QDs with high spectral stability at both room and cryogenic temperatures, reduced blinking with a characteristic ON fraction larger than 85%, and high single-photon purity (g(2)(0) = 0.1), all comparable to the best-reported values for MAPbBr3 and FAPbBr3 QDs of the same size.

A theoretical study of InAs/InP and InAs/GaAs QDs systems: Formation mechanisms and photoluminescence characterization

In this work, we have studied structural, electronic and optical properties of InAs/InP quantum dots (QDs). Our results were compared with the theoretical and experimental results reported in the literature and with those of QDs InAs/GaAs which is the most studied system. We have shown that, even though InAs/InP and InAs/GaAs QDs have the same dot (InAs), their electronic, optical and structural properties differ considerably.The main shared characteristics and dissimilarities between the InAs/InP and the InAs/GaAs QDs systems have been pointed out. It has been shown that these dissimilarities have a significant impact on their applications. Our photoluminescence (PL) characterization simulation model, presented in a previous work, was used. For InAs/InP QDs system, two cases were considered: the strong quantum confinement regime and the weak one. For each regime, two hypotheses were analysed, one with non-radiative recombination centers and another without. It was highlighted a shift of the PL peaks and a broadening of the spectra with the increase of the QDs size for both systems. But, for InAs/InP QDs system the broadening of the spectra is less strong. This is attributed to the lattice mismatch which is smaller for InAs/InP compared with InAs/GaAs. It has been shown that InAs/InP QDs are less suitable for applications in solar cells, due to the short lifetime of the charge carriers. On the other hand, InAs/GaAs QDs are not able to reach the 1.5 µm wavelength for telecom applications, also called C-band telecom, because they strongly degrade for emission wavelengths larger than 1.3 µm.

Universality of the Förster's model for resonant exciton transfer in ensembles of nanocrystals.

For nanocrystals in a strong quantum confinement regime, it has been confirmed analytically that resonant exciton transfer proceeds in full accordance with the Förster mechanism. This means that the virtual exciton transitions between the nanocrystals of close sizes are governed only by the dipole-dipole interaction of nanocrystals even in very dense ensembles, while the contributions of all other higher-order multipoles are negligibly small. Based on a simple isotropic model of the envelope function approximation and neglecting the electron-hole interaction inside each nanocrystal, we have computed the rate of the resonant exciton transfer between two nanocrystals. Using the obtained result, we have estimated, for some arbitrarily chosen nanocrystal, the total rate of the exciton non-radiative annihilation caused by the possibility of its resonant virtual transitions into all other nanocrystals of the ensemble. The total rate dependence on the nanocrystal size is determined only by the size distribution function of nanocrystals in the ensemble.

Native surface oxidation yields SiC-SiO2 core-shell quantum dots with improved quantum efficiency.

Silicon carbide is an important wide-bandgap semiconductor with wide applications in harsh environments and its applications rely on a reliable surface, with dry or wet oxidation to form an insulating layer at temperatures ranging from 850 to 1250 °C. Here, we report that the SiC quantum dots (QDs) with dimensions lying in the strong quantum confinement regime can be naturally oxidized at a much lower temperature of 220 °C to form core/shell and heteroepitaxial SiC/SiO2 QDs with well crystallized silica nanoshells. The surface silica layer enhances the radiative transition rate of the core SiC QD by offering an ideal carrier potential barrier and diminishes the nonradiative transition rate by reducing the surface dangling bonds, and, as a result, the quantum yield is highly improved. The SiC/SiO2 QDs are very stable in air, and they have better biocompatibility for cell-labeling than the bare SiC QDs. These results pave the way for constructing SiC-based nanoscale electronic and photonic devices.

Absorption and emission in the visible range by ultra-small PbS quantum dots in the strong quantum confinement regime with S-terminated surfaces capped with diphenylphosphine

The synthesis and characterization of PbS QDs absorbing and emitting in the visible range is a very challenging task, since it requires the QDs to be within the strong quantum confinement regime (QD size<2.5 nm). It implies not only having “small” QDs, but also stable and monodisperse; characteristics that have been elusive to achieve for many researchers. In the current work, ultra-small PbS QDs (size ~2 nm) were synthesized based on a modification of the Hines method, controlling the reaction time, and adding diphenylphosphine (DPP) which serves as a catalyst and a protective agent in the reaction synthesis. Novel ultra-small PbS QDs with S-terminated surfaces were obtained, which formed at the early stages of the synthesis reaction and are stabilized by the DPP; as it was suggested by the TEM, FTIR and Raman results. The ultra-small PbS QDs display a maximum peak of optical absorption at ~532 nm, with a corresponding optical band gap of 1.82 eV; a maximum peak of emission at ~679 nm, which results in a Stokes shift of 119 nm, smaller than the Stokes shift observed in “larger” PbS QDs. These ultra-small QDs displayed an average size of ~2 nm, with a standard deviation of ~0.3 nm, which was the smallest among the synthesized samples, based on TEM measurements. Finally, the LUMO and HOMO levels were measured by means of cyclic voltammetry and optical absorption spectroscopy. The values of the optical band gap and the energies measured for the LUMO and HOMO levels of these ultra-small PbS QDs were affected by their atomistic surface arrangement and the capping ligand interacting with their surface. Producing variations in their values that doesn’t follow the trends established for quantum confinement effects related to size variation only. Thorough physical and chemical characterization of such ultra-small PbS QDs are crucial in understanding the origin of their optoelectronic properties, which will contribute to better delineate possible future applications.

Stable and Size Tunable CsPbBr3 Nanocrystals Synthesized with Oleylphosphonic Acid.

We employed oleylphosphonic acid (OLPA) for the synthesis of CsPbBr3 nanocrystals (NCs). Compared to phosphonic acids with linear alkyl chains, OLPA features a higher solubility in apolar solvents, allowing us to work at lower synthesis temperatures (100 °C), which in turn offer a good control over the NCs size. This can be reduced down to 5.0 nm, giving access to the strong quantum confinement regime. OLPA-based NCs form stable colloidal solutions at very low concentrations (∼1 nM), even when exposed to air. Such stability stems from the high solubility of OLPA in apolar solvents, which enables these molecules to reversibly bind/unbind to/from the NCs, preventing the NCs aggregation/precipitation. Small NCs feature efficient, blue-shifted emission and an ultraslow emission kinetics at cryogenic temperature, in striking difference to the fast decay of larger particles, suggesting that size-related exciton structure and/or trapping-detrapping dynamics determine the thermal equilibrium between coexisting radiative processes.

Multi-colored type-I Ag-doped ZnCdS/ZnS core/shell quantum dots with intense emission

Abstract Although doped quantum dots (d-dots) with intense and tunable emission have been studied for long time, there are still great efforts to prepare new ones with promising characteristics. In the present work, we used a mild/effective strategy for preparing high quality and aqueous-soluble Ag:ZnCdS/ZnS core/shell quantum dots (QDs) with N-acetyl- l -cysteine as the capping agent. Through investigating the experimental variables, the impurity-related emission intensity of the as-prepared samples was optimized. The capability of the present work on creating quantum structures with tunable emission across the entire visible spectrum was approved through simple but effective variation in Zn-to-Cd molar ratio. Indeed, as the Zn:Cd molar ratio changed from 2:0 to 0:2, the emission color was changed significantly from blue to red color with a satisfactory photoluminescence quantum yield. The quantum yield value reached ∼41% for the as-prepared core/shell d-dots without any pre/post-treatment, which is a remarkable result for such aqueous-soluble structures. XRD, EDX, ICP, and TEM measurements were applied to determine the structural features of the QDs in a strong quantum confinement regime. The generality of preparation route, its biocompatibility, along with a multi-color emission, can create new opportunities especially for the white light emitting technologies, or multi-color bioimaging for theranostics.

Synthesis of Inorganic-Organic 2D CdSe Slab-Diamine Quantum Nets.

Porous semiconductors attract great interest due to their unique structural characteristics of high surface area as well as their intrinsic optical and electronic properties. In this study, synthesis of inorganic-organic 2D CdSe slabs-diaminooctane (DAO) porous quantum net structures is demonstrated. It is found that the hybrid 2D CdSe-DAO lamellar structures are disintegrated into porous net structures, maintaining an ultrathin thickness of ≈1 nm in CdSe slabs. Furthermore, the CdSe slabs in quantum nets show the highly shifted excitonic transition in the absorption spectrum, demonstrating their strongly confined electronic structures. The possible formation mechanism of this porous structure is investigated with the control experiments of the synthesis using n-alkyldiamines with various hydrocarbon chain lengths and ligand exchange of DAO with oleylamine. It is suggested that a strong van der Waals interaction among long chain DAO may exert strong tensile stress on the CdSe slabs, eventually disintegrating slabs. The thermal decomposition of CdSe-DAO quantum nets is further studied to form well-defined CdSe nanorods. It is believed that the current CdSe-DAO quantum nets will offer a new type of porous semiconductors nanostructures under a strong quantum-confinement regime, which can be applied to various technological areas of catalysts, electronics, and optoelectronics.

Surface Chemistry Control of Colloidal Quantum Dot Band Gap

Surface chemistry modification of as-synthesized colloidal inorganic semiconductor nanocrystals (QDs), commonly referred to as ligand exchange, is mandatory toward effective QD-based optoelectronic and photocatalytic applications. The widespread recourse to ligand exchange procedures on metal chalcogenide QDs often narrows the optical band gap, although little consensus exists on explanation of this experimental evidence. This work attempts at providing a comprehensive description of such a phenomenon by exploiting rationally designed thiol ligands at the surface of colloidal PbS QDs, as archetype of material in the strong quantum confinement regime: the thiol(ate)-induced QD optical band gap reduction almost linearly scales with the inorganic core surface-to-volume ratio and mainly depends on the sulfur binding atom, which is here suggested to contribute occupied 3p orbitals to the valence band edge of the QDs. As opposed to QD models based on the analogy with core/shell heterostructures, the indecomposa...

An insight into the origin of room-temperature ferromagnetism in SnO2 and Mn-doped SnO2 quantum dots: an experimental and DFT approach.

SnO2 and Mn-doped SnO2 single-phase tetragonal crystal structure quantum dots (QDs) of uniform size with control over dopant composition and microstructure were synthesized using the high pressure microwave synthesis technique. On a broader vision, we systematically investigated the influence of dilute Mn ions in SnO2 under the strong quantum confinement regime through various experimental techniques and density functional theoretical (DFT) calculations to disclose the physical mechanism governing the observed ferromagnetism. DFT calculations revealed that the formation of the stable (001) surface was much more energetically favorable than that of the (100) surface, and the formation energy of the oxygen vacancies in the stable (001) surface was comparatively higher in the undoped SnO2 QDs. X-ray photoelectron spectroscopy (XPS) and first-principles modeling of doped QDs revealed that the lower doping concentration of Mn favored the formation of MnO-like (Mn2+) structures in defect-rich areas and the higher doping concentration of Mn led to the formation of multiple configurations of Mn (Mn2+ and Mn3+) in the stable surfaces of SnO2 QDs. Electronic absorption spectra indicated the characteristic spin allowed ligand field transitions of Mn2+ and Mn3+ and the red shift in the band gap. DFT calculations clearly indicated that only the substitutional dopant antiferromagnetic configurations were more energetically favorable. The gradual increase of magnetization at a low level of Mn-doping could be explained by the prevalence of antiferromagnetic manganese-vacancy pairs. Higher concentrations of Mn led to the appearance of ferromagnetic interactions between manganese and oxygen vacancies. The increase in the concentration of metallic dopants caused not just an increase in the total magnetic moment of the system but also changed the magnetic interactions between the magnetic moments on the metal ions and oxygen. The present study provides new insight into the fundamental understanding of the origin of ferromagnetism in transition metal-doped QDs.

Strong Quantum Confinement and Fast Photoemission Activation in CH3NH3PbI3 Perovskite Nanocrystals Grown within Periodically Mesostructured Films

In this Communication, a synthetic route is demonstrated to obtain stabilized MAPbI3 nanocrystals embedded in thin metal oxide films that display well‐defined and adjustable quantum confinement effects over a wide range of 0.34 eV. Mesostructured TiO2 and SiO2 films displaying an ordered 3D pore network are prepared by evaporation‐induced self‐assembly of a series of organic supramolecular templates in the presence of metal oxide precursors. The pores in the inorganic films obtained after thermal annealing are then used as nanoreactors to synthesize MAPbI3 crystallites with narrow size distribution and average radius comprised between 1 and 4 nm, depending on the template of choice. Both the static and dynamic photoemission properties of the ensemble display features distinctive of the regime of strong quantum confinement. Photoemission maps demonstrate that the spectral and intensity properties of the luminescence extracted from the perovskite quantum dot loaded films are homogeneous over squared centimeters areas. At variance with their bulk counterparts, constant emission intensity is reached in time scales at least four orders of magnitude shorter.

A facile single injection Hydrothermal method for the synthesis of thiol capped CdTe Quantum dots as light harvesters

A facile, Single Injection Hydrothermal (SIH) method has been developed to synthesize high quality 3-Mercaptopropionic Acid (MPA) stabilized aqueous CdTe QDs, entirely in ambient environment. The synthesis protocol eliminates the use of inert atmosphere for reducing elemental Tellurium powder to Te precursor avoiding the oxidation of Te powder. The XRD result revealed that the synthesized QDs are in cubic zincblende type crystalline structure, without signature of Te oxidation. FTIR spectra have confirmed the attachment of short chained organic compound MPA to the surface of QDs by covalent bond. The Quantum confinement effect was clearly evident by shift in Longitudinal Optic (LO) peak of Raman spectra and absorption peak wavelength with respect to bulk CdTe materials. The optical direct band gap energy of CdTe QDs is between 3.63eV to 1.96eV and QDs size below 6nm, confirm the QDs are well under strong Quantum confinement regime. Also, photoluminescence spectra depict a stable and high luminescence emission from green to dark red color. All these results corroborate that the synthesis of CdTe QDs procedure is very advantageous and present a simple, economical and easily up scalable method for large scale production.

Colloidal Synthesis of Quantum Confined Single Crystal CsPbBr3 Nanosheets with Lateral Size Control up to the Micrometer Range.

We report the nontemplated colloidal synthesis of single crystal CsPbBr3 perovskite nanosheets with lateral sizes up to a few micrometers and with thickness of just a few unit cells (i.e., below 5 nm), hence in the strong quantum confinement regime, by introducing short ligands (octanoic acid and octylamine) in the synthesis together with longer ones (oleic acid and oleylamine). The lateral size is tunable by varying the ratio of shorter ligands over longer ligands, while the thickness is mainly unaffected by this parameter and stays practically constant at 3 nm in all the syntheses conducted at short-to-long ligands volumetric ratio below 0.67. Beyond this ratio, control over the thickness is lost and a multimodal thickness distribution is observed.

BCS-BEC Crossover in Quantum Confined Superconductors

Ultranarrow superconductors are in the strong quantum confinement regime with formation of multiple coherent condensates associated with the many subbands of the electronic structure. Here, we analyze the multiband BCS-BEC crossover induced by the chemical potential tuned close to a subband bottom, in correspondence of a superconducting shape resonance. The evolution of the condensate fraction and of the pair correlation length in the ground state as functions of the chemical potential demonstrates the tunability of the BCS-BEC crossover for the condensate component of the selected subband. The extension of the crossover regime increases when the pairing strength and/or the characteristic energy of the interaction get larger. Our results indicate the coexistence of large and small Cooper pairs in the crossover regime, leading to the optimal parameter configuration for high transition temperature superconductivity.

硒化铅量子点聚乙烯醇薄膜的三阶非线性光学特性

PbSe/polyvinyl alcohol composite film with third-order nonlinear optical property was prepared by wet-chemical method.The morphology of the PbSe nanocrystals was characterized by transmission electron microscope,and the surface morphology of the composite film was characterized by scanning electron microscope.The third-order optical nonlinearity of film was investigated by using the laser Zscan technique with 38ps pulses width at 532nm.The experimental results show that the diameter of PbSe nanocrystals is about 10nm,which is in strong quantum confinement regime,and the film has a flat surface.Negative nonlinear refraction and reverse saturable absorption properties were observed,the third-order nonlinear refractive indexχ(3)was found to be 3.6×10-11 esu.

Anisotropic Absorption in PbSe Nanorods

We present absorption anisotropy measurements in PbSe nanostructures. This is accomplished via a new means of measuring absorption anisotropy in randomly oriented solution ensembles of nanostructures via pump-probe spectroscopy, which exploits the polarization memory effect. We observe isotropic absorption in nanocrystals and anisotropic absorption in nanorods, which increases upon elongation from aspect ratio 1 to 4 and is constant for longer nanorods. The measured volume-normalized absorption cross section is 1.8 ± 0.3 times larger for parallel pump and probe polarizations in randomly oriented nanorods as compared to nanocrystals. We show that this enhancement would be larger than an order of magnitude for aligned nanorods. Despite being in the strong quantum confinement regime, the aspect ratio dependence of the absorption anisotropy in PbSe nanorods is described classically by the effects of dielectric contrast on an anisotropic nanostructure. These results imply that the dielectric constant of the surrounding medium can be used to influence the optoelectronic properties of nanorods, including polarized absorption and emission, phonon modes, multiple exciton generation efficiency, and Auger recombination rate.

Synthesis and Characterization of Europium-Doped Zinc Sulphide (ZnS:Eu) Nano Particles: Nano Red Phosphor

A preliminary work on synthesis of Europium-doped Zinc sulphide nanophosphors is reported. The result shows that pure and well passivated ZnS:Eu3+ nano-particles were synthesized. The average particle sizes show that the nano-particles are in the strong quantum confinement regime which is evident from the stoke-shift in the absorption spectrum and the broadening of the XRD diffraction peaks. Particles sizes determined from the effective mass approximation and hyperbolic band model show higher particle size distribution, as a result of a consequential failure of these models at very small size regime. The intensity of red light emitted from the synthesized ZnS:Eu nanophospors is size and site dependent. There is clear evidence that energy transfer from ZnS host to Eu3+ occurred but this emission has weak intensity.

Sb2S3 quantum dots: diffusion‐controlled growth and characterization

AbstractThe quantum dots of antimony trisulphide, a potential semiconductor for various applications, are grown in glass matrix for the first time and are characterized by various techniques. The dependence of the average dot size on growth parameters like growth temperature and time is systematically studied for the dot size range of 5–80 nm. The linear blue shift of band gap of dots with inverse square of dot size clearly indicates the typical behavior of quantum dots in a strong quantum confinement regime. (© 2013 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Analysis of size quantization and temperature effects on the threshold voltage of thin silicon film double-gate metal-oxide-semiconductor field-effect transistor (MOSFET)

In this paper, we analyze the combined effects of size quantization and device temperature variations (T = 50 K to 400 K) on the intrinsic carrier concentration (ni), electron concentration (n) and thereby on the threshold voltage (Vth) for thin silicon film (tsi = 1 nm to 10 nm) based fully-depleted Double-Gate Silicon-on-Insulator MOSFETs. The threshold voltage (Vth) is defined as the gate voltage (Vg) at which the potential at the center of the channel (Φc) begins to saturate (Φc=Φc(sat)). It is shown that in the strong quantum confinement regime (tsi≤3nm), the effects of size quantization far over-ride the effects of temperature variations on the total change in band-gap (ΔEg(eff)), intrinsic carrier concentration (ni), electron concentration (n), Φc(sat) and the threshold voltage (Vth). On the other hand, for tsi≥4 nm, it is shown that size quantization effects recede with increasing tsi, while the effects of temperature variations become increasingly significant. Through detailed analysis, a physical model for the threshold voltage is presented both for the undoped and doped cases valid over a wide-range of device temperatures, silicon film thicknesses and substrate doping densities. Both in the undoped and doped cases, it is shown that the threshold voltage strongly depends on the channel charge density and that it is independent of incomplete ionization effects, at lower device temperatures. The results are compared with the published work available in literature, and it is shown that the present approach incorporates quantization and temperature effects over the entire temperature range. We also present an analytical model for Vth as a function of device temperature (T).

Loosening Quantum Confinement: Observation of Real Conductivity Caused by Hole Polarons in Semiconductor Nanocrystals Smaller than the Bohr Radius

We report on the gradual evolution of the conductivity of spherical CdTe nanocrystals of increasing size from the regime of strong quantum confinement with truly discrete energy levels to the regime of weak confinement with closely spaced hole states. We use the high-frequency (terahertz) real and imaginary conductivities of optically injected carriers in the nanocrystals to report on the degree of quantum confinement. For the smaller CdTe nanocrystals (3 nm < radius < 5 nm), the complex terahertz conductivity is purely imaginary. For nanocrystals with radii exceeding 5 nm, we observe the onset of real conductivity, which is attributed to the increasingly smaller separation between the hole states. Remarkably, this onset occurs for a nanocrystal radius significantly smaller than the bulk exciton Bohr radius a(B) ∼ 7 nm and cannot be explained by purely electronic transitions between hole states, as evidenced by tight-binding calculations. The real-valued conductivity observed in the larger nanocrystals can be explained by the emergence of mixed carrier-phonon, that is, polaron, states due to hole transitions that become resonant with, and couple strongly to, optical phonon modes for larger QDs. These polaron states possess larger oscillator strengths and broader absorption, and thereby give rise to enhanced real conductivity within the nanocrystals despite the confinement.