Strong Quantum Confinement Effect Research Articles

Concentration controlled QDs ZnS synthesis without capping agent and its optical properties

Facile synthesis of quantum dots (QDs) of ZnS without capping agent is reported. Cubic phase of ZnS is confirmed using both X-ray diffraction and high resolution transmission electron microscopy. Absorption studies using UV–vis spectroscopy reveal strong quantum confinement effect for a typical ZnS of 3nm size with a band gap of 4.46eV in comparison to the bulk value of 3.55eV. Photoluminescence study shows the presence of the surface defects.

EFFECT OF CALCINATION ON THE ELECTRICAL PROPERTIES AND QUANTUM CONFINEMENT OF FE2O3 NANOPARTICLES

Fe2O3 nanoparticles have been prepared by a simple solvothermal method using a domestic microwave oven. X-ray powder diffraction measurement indicates the amorphous nature of the as-prepared sample. Calcined samples were obtained by annealing the as-prepared sample at different temperatures, viz. 400, 500, 600 and 700 o C. Transmission electron microscopic images indicate that all the five samples are spherical in shape. AC electrical measurements were carried out on pelletized samples by the parallel plate capacitor method at various temperatures ranging from 40-130 o C and frequencies ranging from 100 Hz -1 MHz. Results indicate low AC electrical conductivities and consequently show the occurrence of nano confined states. The exciton Bohr radii obtained from the dielectric constant values at 40 o C temperature and 1 kHz frequency are 41.8, 54.8, 55.3, 56.3 and 27.0 respectively for the asprepared sample and samples calcined at 400, 500, 600 and 700 o C which indicate a strong quantum confinement effect. The

Synthesis, characterization, and comparative study of CdSe–TiO2 nanowires and CdSe–TiO2 nanoparticles

Cadmium selenide (CdSe) quantum dots in selective size were prepared by using inverse micelle technique without the presence of trioctylphosphine (TOP) and characterized by UV–visible absorption, Fourier transform infrared (FTIR) and photoluminescence (PL) spectroscopic studies. The UV–visible absorption and PL studies showed strong quantum confinement effect and bandgap energy about 2.1eV in CdSe quantum dots. Particle size and electron diffraction analyses in high resolution transmission electron microscopy (HRTEM) confirmed the formation of CdSe crystalline particles in the range of 3.1–3.9nm. These wide bandgap CdSe quantum dots were used for the preparation of CdSe–TiO2 nanocomposites of TiO2 nanoparticles and TiO2 nanowires. HRTEM images of the CdSe–TiO2 nanowires (CdSe-TNW) and the CdSe–TiO2 nanoparticles (CdSe-TNP) composites revealed the uniform, layered distribution of CdSe quantum dots on TiO2 nanoparticles and on TiO2 nanowires in different dimensions. Further, the differences in the dimensional distributions and optical properties of CdSe-TNW and CdSe-TNP nanocomposites were investigated for their applications to TiO2 sensitized solar cells.

Direct Precipitation and Characterization of ZnO Nanoparticles

ZnO nanoparticles are prepared through hydrolysis and condensation of zinc acetate dihydrate by potassium hydroxide in alcoholic medium at low temperatures. Thermal gravimetric analysis (TGA) of the precursor is made in order to specify the temperature range over which the weight loss and thermal effect are significant. X‐ray diffraction of the as‐prepared specimens shows that the hexagonal (a = 3.2459 Å, c = 5.1999 Å) structure is the predominant crystallographic structure. According to Scherer’s formula, the average size of the nanoparticles is 22.4 ± 0.6 nm. The structural properties of the synthesized ZnO nanoparticles have been confirmed using the TEM micrographs. The optical energy gap of the ZnO nanoparticles, as obtained from applying Tauc’s equation, is equal to 3.52 eV, which is higher than that of the bulk material. Absorption peak of the as‐prepared sample is 298 nm which is highly blue shifted as compared to the bulk (360 nm). Large optical energy gap and highly blue shifted absorption edge confirm that the prepared ZnO nanoparticle exhibits strong quantum confinement effect.

The role of SnO2quantum dots in improved CH4sensing at low temperature

The role of quantum dots (QDs) of SnO2 in detecting low concentrations of methane (CH4) at a relatively low temperature of ∼150 °C with high response (S ∼ 3.5%) and response time below 1 min is reported. A simple room temperature single step chemical process was adopted for the growth of SnO2 nanoparticles of a size around 2.4 nm. These nanoparticles were subsequently annealed at 800 °C to increase the grain size to 25 nm. The as-prepared SnO2 nanoparticles, being smaller than the corresponding Bohr radius (2.7 nm), showed a strong quantum confinement effect with a blue shift in the band gap energy from 3.6 eV for the bulk SnO2 to 4.37 eV for the QDs. These QDs exhibited a strong sensing response to CH4 in comparison to the annealed sample. A low activation energy of 90 meV, as estimated from the temperature dependent S plot for SnO2 QDs, was found to be the driving force for such unusual high sensitivity at a low operating temperature. X-ray diffraction, transmission electron microscopy, along with Raman spectroscopy measurements are used for the detailed structural studies. The critical role of the chemisorbed oxygen species present at different operating temperatures on the surface of the off-stoichiometric quantum sized SnO2 and bulk-like annealed samples are discussed in light of the adsorption kinetics.

Synthesis of highly-confined CdS nanoparticles by copolymerization of acryloylated starch

A simple method for the synthesis of highly-confined and monodispersed, polyacryloylated starch-capped CdS nanoparticles (PAS-CdS) is reported. The synthesized nanoparticles exhibited a strong quantum confinement effect with respect to the bulk sample. Fourier transform infrared spectra confirmed the presence of acryloylated starch (AS) as a passivating agent. Transmission electron microscopy indicated that the particles were well dispersed and spherical in shape.

Suppression of the quantum-confined Stark effect in AlxGa1−xN/AlyGa1−yN corrugated quantum wells

We report comparative studies of 6-nm-thick AlxGa1−xN/AlyGa1−yN pyroelectric quantum wells (QWs) grown by plasma-assisted molecular beam epitaxy on c-sapphire substrates with a thick AlN buffer deposited under different growth conditions. The Al-rich growth conditions result in a 2D growth mode and formation of a planar QW, whereas the N-rich conditions lead to a 3D growth mode and formation of a QW corrugated on the size scale of 200–300 nm. Time-resolved photoluminescence (PL) measurements reveal a strong quantum-confined Stark effect in the planar QW, manifested by a long PL lifetime and a red shift of the PL line. In the corrugated QW, the emission line emerges 200 meV higher in energy, the low-temperature PL lifetime is 40 times shorter, and the PL intensity is stronger (∼4 times at 4.5 K and ∼60 times at 300 K). The improved emission properties are explained by suppression of the quantum-confined Stark effect due to the reduction of the built-in electric field within the QW planes, which are not normal to the [0001] direction, enhanced carrier localization, and improved efficiency of light extraction.

Controlling the size and optical properties of ZnO nanoparticles by capping with SiO2

The size and shape of the ZnO nanoparticles synthesized through sol–gel method were controlled by capping with SiO2. X-ray diffraction (XRD) and field emission scanning electron microscope (FE-SEM) and High Resolution Transmission Electron Microscope (HR-TEM) results demonstrated that the particle growth of the ZnO nanoparticles has been restricted to 5nm with SiO2 capping. As a result, the absorption spectra of ZnO nanoparticles capped with SiO2 got blue shifted (toward lower wavelength side) due to strong quantum confinement effects. BET (Brunauer–Emmet–Teller) surface area pore size analyzer results showed that surface area of samples increased monotonously with increase of SiO2 concentration. It was observed that the absorption spectra of ZnO capped with SiO2 broadened with increase of SiO2 concentration. Absorption and photoluminescence excitation results (PLE) confirmed that this broadening is due to the absorption of non-bridging oxygen hole centers (NBOHC) of SiO2. These results also indicated that ZnO nanoparticles capped with SiO2 are insensitive to Raman scattering. Maximum UV emission intensity was achieved with 353nm excitation wavelength compared to 320nm in ZnO as well as in SiO2 capped ZnO nanoparticles. Furthermore, there is an enhancement in the intensities of emission peaks related to oxygen vacancies and interstitials with SiO2 capping. The enhancement in the UV intensity is attributed to the surface passivation of ZnO nanoparticles and excitation processes in SiO2.

Determination of unscreened exciton states in polar ZnO/(Mg,Zn)O quantum wells with strong quantum-confined Stark effect

We report on recombination dynamics and screening effects due to photogenerated carriers in polar ZnO/(Mg,Zn)O single quantum wells. For small well widths ($\ensuremath{\le}$2.5 nm), the unscreened transition energy and decay time can be determined directly since the influence of the internal electric field is negligible due to strong confinement of the carriers. The energetic dependence of the decay time was reproduced by a relaxation model to describe the localization within potential minima due to well width fluctuations. For larger well widths, screening effects have to be taken into account even for moderate excitation densities in the 1 W/cm${}^{\ensuremath{-}2}$ range. It is very hard to determine the unscreened single-exciton state of these wells with strong quantum-confined Stark effect within reasonable time scales and a significant signal-to-noise ratio. We present an extended relaxation model to determine the decay time of the unscreened single-exciton state. A difference in polarization between ZnO and Mg${}_{x}$Zn${}_{1\ensuremath{-}x}$O of $\ensuremath{\Delta}P/x=0.026\phantom{\rule{0.28em}{0ex}}\text{C}/{\text{m}}^{2}$ is concluded from the dependence of the decay time on the well width.

Pronounced Multiferroicity in Oleic Acid Stabilized BiFeO3 Nanocrystals at Room Temperature

We report on the experimental observation of pronounced multiferroicity in BiFeO3 nanocrystals (size approximately 40 nm) at room temperature. Large scale BiFeO3 nanocrystals are synthesized using a low temperature chemical route and further stabilized with oleic acid. The nanocrystals exhibit a significant distortion in lattice parameter c compared to the bulk. Oleic acid plays an important role in reducing oxygen vacancies and Fe2+ ions at the nanocrystal surface giving rise to a high resistivity (approximately 10(10) omega-cm at 300 K) of the sample. The direct band gap of nanocrystals is measured to be approximately 4.2 eV (about 1.5 times the bulk value) suggesting a strong quantum confinement effect. The nanocrystals show a remarkably high spontaneous saturation magnetization approximately 4.39 emu/g along with a prominent ferroelectric hysteresis loop at room temperature. Particle size effect leading to the appearance of large number of uncompensated spins and suppression of modulated spin structure have resulted a strong spontaneous magnetization in such nanoscale multiferroic materials.

Strong quantum-confined Stark effect from light hole related direct-gap transitions in Ge quantum wells

A strong quantum-confined Stark effect (QCSE) from light hole related transitions at the Γ point (LH1-cΓ1) in Ge/Si0.15Ge0.85 multiple quantum wells is demonstrated from both photocurrent and optical transmission measurements. Our experimental results show a large and sharp optical absorption peak due to LH1-cΓ1 transitions, and its associated strong absorption change based on the QCSE. By exploiting LH1-cΓ1 transitions, optical modulators with improved compactness and competitive extinction ratio and optical loss can be envisioned for low energy chip-scale optical interconnect applications.

Chemical synthesis and functional properties of magnesium doped ZnSe nanoparticles

Mg2+ doped ZnSe nanoparticles are prepared by facile wet chemical route. N-methylaniline (NMA) is used as a surface passivating agent to obtain the agglomeration free nanoparticles. X-ray diffraction (XRD) pattern exhibits the cubic zinc blende structure of ZnSe. Ultraviolet visible spectroscopy (UV) shows the strong quantum confinement effect and the band gap is 3.56eV. The emission spectrum confirms the incorporation of Mg2+ into the ZnSe. The presence of NMA is confirmed by Fourier transform infrared spectroscopy (FTIR). Transmission electron microscopy (TEM) result shows that the synthesized ZnSe is free from agglomeration and average size of the nanoparticles is about 15nm. EDAX analysis confirms the presence of Mg2+ in ZnSe nanoparticles.

One-pot solvothermal strategy for the synthesis of ultrathin ZnS nanowires

Ultrathin ZnS nanowires with diameter of ca. 1.7nm and length of ca. 150nm were synthetized by using a simple, green, and catalyst-free solvothermal process at 140°C. These as-prepared nanowires were obtained from the reactions between Zn2+ and thiourea by employing oleylamine as a surfactant. The effects of reaction temperature and amount of thiourea on the morphology and size of the ZnS nanowires were studied. The morphology and size of ZnS nanowires can be controlled by varying these experimental conditions. The formation mechanism and optical properties of the nanowires have also been discussed. UV–vis absorption and luminescence emission spectra show that ultrathin ZnS nanowires have strong quantum confinement effect.

Terahertz Bandwidth All-Optical Modulation and Logic Using Multiexcitons in Semiconductor Nanocrystals

Optical pumping of semiconductor nanocrystals with femtosecond pulse sequences was performed in order to modulate multiexciton populations. We show for the first time that control of multiexciton populations produces high speed modulation of stimulated emission. Upon the basis of the speed of multiexcitonic processes in nanocrystals, we show modulation rates approaching 1 THz by virtue of strong quantum confinement effects. Employing femtosecond optical pulse sequences, we demonstrate all-optical logic using these nanocrystals in two forms: an AND gate, and an inverter, a key step toward all optical signal processing.

Preparation and Microstructure Control of One-Dimension Core-Shell Heterostructure of Te/Bi, Te/Bi<sub>2</sub>Te<sub>3</sub> by Microwave Assisted Chemical Synthesis

Microstructure engineering of thermoelectric materials can resolve the conflicts of electrical and thermal transports. Especially, one-dimensional structure can obviously improve the thermoelectric figure of merit because of its crystal anisotropy and strong quantum confinement effect. In this paper, the Te nanowires, one-dimensional core-shell heterostructure of Te/Bi and Te/Bi2Te3 were controlled synthesized by microwave assisted chemical synthesis. The effect of PVP concentration and reductant dropping rate on the microstructure of the Te nanowires were investigated. The experimental results showed that with increasing the amount of PVP, the Te nanowires got less crystallinity and its surface become more rough due to its steric hindrance effect. With decreasing reductant dropping rate, the longer and thiner Te nanowires were obtained. Epitaxial growth can describe the relation of core Te and shell Bi (or Bi2Te3). It has been found that Bi shell uniformly surrounded around Te nanowires core, but Bi2Te3 sheets were perpendicular to the c-axis of Te nanowires. The different core-shell heterostructure structure can be obtained by adjusting reaction conditions and controlling diffusion kinetics of Te and Bi.

Strong quantum confinement effects in SnS nanocrystals produced by ultrasound-assisted method

Nanocrystalline SnS powder has been prepared using tin chloride (SnCl2) as a tin ion source and sodium sulfide (Na2S) as a sulfur ion source with the help of ultrasound irradiation at room temperature. The as-synthesized SnS nanoparticles were quantitatively analyzed and characterized in terms of their morphological, structural, and optical properties. The detailed structural and optical properties confirmed the orthorhombic SnS structure and a strongly blue shifted direct band gap (1.74 eV), for synthesized nanoparticles. The measured band gap energy of SnS nanoparticles is in a fairly good agreement with the results of theoretical calculations of exciton energy based on the potential morphing method in the Hartree–Fock approximation.

The first single-sized (∼1 nm) and periodically ordered array of In2Te3semiconductor quantum dots self-assembled in solution

We report the first example of single-sized quantum dots (QD) of a III–VI semiconductor, In2Te3, self-assembled in solution. The nanometer-sized dots (∼1 nm in diameter) with the formula of In8Te12 are arranged into perfectly ordered arrays via an organic passivating and structure-directing molecule (triethylenetetramine or trien) resulting in a single crystal structure of [In8Te12(trien)4] (monoclinic crystal system, space group C2/c). The In8Te12 dots in the [In8Te12(trien)4] structure show a remarkably strong structure-induced quantum confinement effect which results in a very large blue shift of 2.1 eV in the optical absorption spectrum. The observed increase in the band gap is substantially higher than that of the smallest colloidal quantum dots reported to date. The In8Te12 dots are readily dispersible in suitable solvents to form nanoparticles of an average size of ∼8 to 10 nm. The capability of forming periodic crystal lattices of semiconductor quantum dots offers an attractive way to tune the electronic and optical properties in the same (or larger) extent as those of the smallest nanoparticles while having the advantages of precisely controlling the size and stoichiometry. Such a crystal assembly of QDs may find utility as a new type of molecular-based precursors for the fabrication of energy- and cost-effective solution-processed thin film devices.

Synthesis and Compositional Control of Size Monodisperse SixGe1-x Nanocrystals for Optoelectronic Applications

ABSTRACTAlkyl-terminated SixG1-x nanocrystals are prepared at room temperature by co-reduction of Si and Ge precursors by hydride reducing agents within inverse micelles. Compositional control of the alloy silicon-germanium NCs (ca. 3.6 nm) is achieved by varying the relative amounts of each precursor used in the synthesis. Transmission electron microscopy imaging confirmed that the NCs are highly crystalline with a narrow size distribution; optical spectroscopy shows strong quantum confinement effects, with moderate absorption in the UV spectral range, and a strong blue emission with a marked dependency on excitation wavelength.

Low temperature method for synthesis of starch-capped ZnSe nanoparticles and its characterization studies

A facile method for synthesis of monodispersed, starch-capped ZnSe nanoparticles at room temperature is being reported. The nanoparticles exhibited strong quantum confinement effect with respect to the bulk ZnSe. The transmission electron microscopy image indicated that the particles were well dispersed and spherical in shape. The X-ray diffraction analysis showed that the ZnSe nanoparticles were of the wurtzite structure, with average particle diameter of about 3.50 nm. The Fourier transform infrared spectrum confirmed the presence of starch as passivating agent. At the request of the editor and publisher, this article is being retracted effective 4 May 2022.

Synthesis and structural and optical characterization of Mn2+ doped cadmium sulphide nanoparticles stabilized in DETA matrix

Mn2+ doped cadmium sulphide nanoparticles were prepared with different pH values by chemical precipitation method, at room temperature. Diethylene triamine (DETA) was used as stabilizing agent to control the particle size and prevent agglomeration. The samples were characterized with X-ray diffraction (XRD), high resolution scanning electron microscopy (HRSEM), energy dispersive X-ray analysis (EDAX), diffuse reflectance spectroscopy (DRS), Fourier transform infrared (FTIR) spectroscopy and photoluminescence (PL) studies. The average size of the Mn2+ doped cadmium sulphide nanoparticles, exhibits both cubic and hexagonal structure, calculated from Debye–Scherrer formula was of the order of 2–6nm. The above value of the particle size was confirmed by using Williamson–Hall plot as well as Henglein’s formula. X-ray peak broadening analysis was done using Williamson–Hall plot. The HRSEM images showed the formation of nanoclusters and EDAX spectra confirms the presence of cadmium, sulphide and manganese elements in the sample. The DRS UV–vis spectra of the samples show blue shift, revealing the strong quantum confinement effect of nanoparticles. The formation of DETA capped Mn2+ doped CdS nanoparticles were confirmed by FTIR analysis. The synthesized samples show photoluminescence emission, ranging from 400 to 600nm, in the visible region of the electromagnetic spectrum.