Dot Structures Research Articles

Tunability of solar cell with double quantum dot structure

This work introduces a double quantum dot (QD) structure as an intermediate band for high-performance solar cells. The density matrix equations, coupled with the continuity-current equation, are solved numerically to address the interaction between all the states, which is impossible with the rate equation modeling. This modeling allowed us to obtain quantum efficiency (QE). Using the orthogonalized plane wave assumption for WL-QD, the momentum matrix elements of QD-QD, QD-wetting layer (WL), WL-barrier transitions are calculated. Results are simulated in both the excitonic and non-excitonic (electron-hole, eh) cases and exhibit the importance of adding the QD layer.The results confirm the importance of adding the intermediate band (QD layer). Both band-to-band recombination rates and relaxation rates in the DQD structure are modulated according to the energy difference between DQD states. Compared to the eh model, the excitonic model reduces the QD-QD relaxation rates and all band-to-band recombination rates. The quantum efficiency (QE) is attaining at longer recombination and relaxation times that are confirmed depending on the wider energy difference by manipulating the transitions of the DQD structure.

Investigation of electronic structure, electrical and thermal properties of PbS quantum dots for thermoelectric applications

PbS nanoparticles (NPs) and PbS quantum dots (QDs) were prepared by co-precipitation method and simple chemical bath method, respectively and their thermoelectric properties were comparatively investigated. The structural, morphological, optical, functional groups and pore size of PbS NPs and PbS QDs were analysed by XRD, UV-DRS, FTIR and BET analysis. TEM images revealed spherical morphology of PbS QDs with size ranging from 5 to 10 nm. The band gap value of PbS QDs calculated from UV-DRS is 1.20 eV which is relatively higher compared to PbS NPs (0.73 eV). This may due to the effect of quantum confinement. From the BET analysis the specific surface area was obtained as 66.031 m2 g−1. Quantum Espresso package based on Plane Wave-based Pseudopotential (PWSCF) approach was used to calculate the energy spectrum of PbS QDs of various sizes. The calculated energy gap value between the HOMO and LUMO levels is found to vary with PbS cluster size. Thermoelectric measurements showed that pelletized sample of PbS QDs exhibit a large Seebeck coefficient, high power factor and low thermal conductivity resulted in large figure of merit. The figure of merit of pelletized sample of PbS QDs at 403 K is 0.34 which is relatively higher than that of pelletized PbS NPs (0.24) at the same temperature, making it highly suitable for thermoelectric applications.

Preparation of AIE Functional Single-Chain Polymer Nanoparticles and Their Application in H2 O2 Detection through Intermolecular Heavy-Atom Effect.

Single-chain polymer nanoparticles (SCNPs) are soft matter constructed by intrachain crosslinks, with promising prospects in detection and catalysis. Herein, a fluorescent core (SCNPs) with aggregation-induced emission (AIE) is prepared, applying for H2 O2 detection through intermolecular heavy-atom effect. In detail, the SCNPs precursors are synthesized by ring-opening copolymerization. Then the SCNPs are prepared by intramolecularly cross-linking via olefin metathesis. Imitating the structure of AIE dots, SCNPs are encapsulated by H2 O2 -responsive polymers. Probably due to the stable secondary structure of SCNPs, the obtained micelles show stable fluorescence performance. Furthermore, as the heavy-atom, tellurium is introduced into the carriers to construct the heavy-atom effect. In this micelle-based system, the SCNPs act as the fluorescent core, and the stimuli-responsive polymer acts as the carrier and the fluorescent switch. The hydrophilicity of the tellurium-containing segment is affected by the concentration of H2 O2 , resulting in a change in the distance from the SCNPs, which ultimately leads to a change in the fluorescence intensity. Furthermore, tellurium is particularly sensitive to H2 O2 , which can detect low concentrations of H2 O2 . The SCNPs are merged with AIE materials, with the hope of exploring new probe designs.

Investigation of the growth mechanism and crystallographic structures of GaSb dots nucleation layer and GaSb thin film grown on Si(001) substrate by molecular beam epitaxy

We investigated the growth mechanism and crystallographic structures of GaSb dots as a nucleation layer and GaSb thin films grown on a Si(001) substrate by molecular beam epitaxy using atomic force microscopy and transmission electron microscopy (TEM). The surface morphology of the 100-nm-thick GaSb with GaSb dots drastically changed from that without them. As the GaSb dots gradually grew in size, the coalescence between the adjacent dots was repeated and the space between them was filled, thereby changing the growth mode of GaSb to two-dimensional growth and forming domain structures with terrace surfaces. The high-resolution TEM images and fast Fourier transform patterns revealed that the lattice-mismatched strain in the epitaxial GaSb thin film was almost completely relieved. Because some adatoms were rotated by 60° on the {111} facets and formed a monolayer with a wurtzite structure as a stacking fault in the initial growth stage, twinned GaSb with an inclination of 54.7° from the (001) plane was formed in addition to epitaxial GaSb. The lattice-mismatched strain was nearly relieved in the vicinity of the GaSb/Si interface because of the multiple periodic 90° and 60° misfit dislocation arrays. The formation of GaSb dots, which acted as crystal nuclei and induced periodic misfit dislocation arrays, was useful for the epitaxial growth of GaSb thin films on a Si(001) substrate—a result that will be advantageous for growing high-quality GaSb thin films, with flatter and fewer crystal defects, on a Si(001) substrate in future.

Analisis Miskonsepsi Siswa Kelas X.3 SMAN 1 Belopa pada Materi Pokok Ikatan

This study is a descriptive research which aimed to find out the misconceptions study on chemical bond. The samples in this research is the class X.3 SMA Negeri 1 Belopa as much 36 students. The data was collected by instrument which has 20 items two-tier multiple choice, observation page for teacher, student, and study source and interview to identified cause of misconception. The data was analyzed by descriptive statistic method to obtain percentage of the misconceptions in each item. There is six subconcept that analyzed. The result of this research showed that percentage misconceptions of students average to learning of chemical bond is 11,29 % and low category. Percentage misconceptions of students to element preference to stable is 18,52%; draw of Lewis structure is 9,26%; formation of ion bond is 11,11%; formation of covalent bond is 15,00 %; formation of covalent coordination is 13,89%. The result research show that misconception causes by, 1) teacher not give detailed explanation and 2) student is praconception is wrong

Characterization of electronic charged states of high density self-aligned Si-based quantum dots evaluated with AFM/Kelvin probe technique

We have characterized charged states of a self-aligned Si quantum dot (QD) structure with an areal density as high as ∼1013 cm−2 by surface potential measurements by using atomic force microscopy (AFM)/Kelvin probe force microscopy (KFM). By line scanning with the electrically-biased AFM tip with respect to the substrate in a contact mode, the corresponding area was negatively charged caused by electron injection from the tip into QDs. Cross-sectional profiles of the surface potential were clearly changed, which depends on the tip bias voltages. Also, a stepwise shape in a relationship between the change of the surface potential (ΔV) and the tip bias voltages was observed, reflecting electron charging into the discrete energy levels due to the quantum confinement effect. By assuming a simple equivalent circuit model for KFM measurements of the self-aligned Si-QDs, calculated results for ΔV are in good agreement with the measured results. The agreements precisely clarified the charged states in Si-QDs.

Fully Integrated, 80 GHz Bandwidth, 1.3μ m InAs/InGaAs CW-PW Quantum Dot Passively Colliding-Pulse Mode-Locked (CPM) Lasers for IR Sensing Application

Integrated quantum dot passively mode-locked lasers: IQDMLLs open opportunities for IR sensing. Therefore, here, we investigated structure of quantum dot passively mode-locked lasers (QDMLLs) with colliding-pulse mode-locked (CPM) method in order to reinforce the performance of QDMLLs for ultrahigh-bit-rate compared to self-colliding pulse mode-locking (SCPM) method. The proposed structure is numerically based on finite difference traveling wave (FDTW) method. By considering bias current of 80 mA, which is the bias applied to the gain section, the laser is at its maximum output power. Finally, by comparing the QDMLLs with self-colliding pulse mode-locking (SCPM) method, it is shown that due to the grating caused by the collision of the two counter-propagating pulses in the saturable absorber, the laser bit rate in the structure of CPM is twice the bit rate in structure of SCPM somehow bit rate from 40 GHz in SCPM method increased to 80 GHz in CPM method. Also, grating coupling factor (GCF) changes due to the proposed structure has been shown, so that in the range of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1cm^{ - 1} \le \kappa \le 5cm^{ - 1}$ </tex-math></inline-formula> the laser output power is pulsed wave (PW), while after this range the laser output power changes from a pulsed regime to a continuous wave (CW) regime.

Preparation of Cadmium Telluride Quantum Dots Modified by Thioglycolic Acid

A CdTe quantum dot modified with thioglycolic acid as stabilizer was prepared. The structure of CdTe quantum dots was characterized by IR, UV and fluorescence spectra, transmission electron microscopy (TEM) scanning and X-ray diffraction (XRD). The effects of reactants, temperature, time and PH on the luminescence properties of the quantum dots were investigated. It is found that the quantum dots have strong fluorescence intensity. The synthesized QDS have small and uniform particle size and high crystallinity. The optimum conditions were determined as follows: n (Cd2+): n (Te2-): n (TGA) = 2:1:4, heating temperature 140°C, reaction time 60min, pH 11.

Chemistry without the Born-Oppenheimer approximation.

Chemistry without the Born-Oppenheimer approximation.

Chlorine dioxide: An exception that proves the rules of localized chemical bonding.

We employ natural bond orbital and natural resonance theory tools to analyze the enigmatic properties of the C2v-symmetric isomer of chlorine dioxide radical (ClO2), whose many challenges to Pauling-type localized bonding concepts were recognized by Linus Pauling himself. Although spin-contamination is minimal in this species, ClO2 exhibits an unusually strong form of "different Lewis structures for different spins" bonding pattern, intrinsically outside the framework of "maximal pairing" concepts. We show how the novel spin-unpaired donor-acceptor interactions lead to weakened bonding in the supramolecular domain of polyradical (ClO2)n homoclusters and aqueous ClO2(H2O)n heteroclusters. Despite feeble binding energies and large inter-radical separations, the polyradical clusters are found to maintain coherent spin patterns in each cluster component, attesting to the quantal donor-acceptor nature of their interactions and the cooperative and anticooperative couplings that govern intra- and intermolecular spin distributions in such spin-clusters.

Direct Imaging of the Carrier Capture into Individual InP Quantum Dots of a Semiconductor Disk Laser Membrane.

We report on nanoscopic exploration of the luminescence from individual InP quantum dots (QDs) by means of highly spatially resolved cathodoluminescence (CL) spectroscopy directly performed in a scanning transmission electron microscope (STEM). A 7-fold layer stack with high-density InP quantum dots is embedded as an active medium membrane in an external-cavity surface-emitting laser. We characterize the vertical transfer of carriers within the periodic separate confinement heterostructure and determine the capture efficiency of carriers from the cladding layer into the quantum dot layers. Benefiting from the nanoscale resolution of our STEM-CL, we perform single-dot spectroscopy on single isolated QDs in the STEM lamella resolving the details of the excitonic structure of individual quantum dots. Executing highly spatially resolved spectrum line scans within the QD layers, we directly visualize the lateral transport, i.e., the efficient lateral capture of carriers into an individual QD. We observe a characteristic change of the spectral fingerprint during this line scan, while the electron beam is approaching and subsequently receding from the quantum dot position. This directly correlates to the increase and decrease of the numbers of excess carriers reaching the dot, i.e., altering the quantum dot population. The characteristic shift of emission energies visualize the renormalization of the ground-state energy of the single dot, and the intensity ratio of the excitonic recombinations verifies this change of the occupation and the state-filling.

Contribution of nicotinamide as an intracyclic N dopant to the structure and properties of carbon dots synthesized using three α-hydroxy acids as C sources

Fixed carbon source and different dopants are mainly used to study the effect of heteroatoms on the structure and properties of carbon dots (CDs). As reactants, some dopants with conjugated structure and high nitrogen content may have important contributions to the structure and properties of doped CDs in addition to providing heteroatoms. Herein, to study the effect of fixed dopant on the structure and properties of CDs, three different CDs were synthesized using nicotinamide (NAA) and three common α-hydroxy acids (4–5 carbon atoms), and the optimal conditions were determined by orthogonal experimentation. Transmission electron microscopic micrographs showed that the average size of CDs based on nicotinamide are relatively large, up to 19.40 nm. X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy demonstrated that these CDs have graphite nitrogen and several functional group structures. Ultraviolet–visible absorption spectra, fluorescence emission spectra, and fluorescence lifetime illustrated that these CDs have similar emission centers (460–470 nm) and fluorescence processes. The influence of carbon source on the surface structure of CDs was determined by systematically analyzing the response of these CDs in different pH ranges. DFT calculations revealed the distribution characteristics of the electrons in the excited state at the HOMO and LUMO energy levels of CDs. All the above characterizations and calculations proved that NAA is a desirable dopant with an important contribution to the structure and properties of CDs.

InGaN Quantum Dot Superlattices as Ratchet Band Solar Cells

In this article, we investigate the construction of ratchet band solar cells using InGaN quantum dots. The strong piezoelectric potentials can spatially separate electron and hole states. This spatial separation reduces both radiative and nonradiative recombination within the quantum dot, providing one of the key characteristics of a ratchet band system. Ratchet band solar cells have proven difficult to realize experimentally, and the piezoelectric potential in InGaN quantum dots could provide a spatial ratchet that can operate at room temperature with broadband absorption. We use a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mathbf {k}{\cdot }\mathbf {p}$</tex-math></inline-formula> model to calculate the electronic structure and absorptance of a superlattice of InGaN/GaN quantum dots. We present an improved method to calculate the absorptance for the bound-to-continuum transition by using bulk <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mathbf {k}{\cdot }\mathbf {p}$</tex-math></inline-formula> states to approximate the host material continuum states. We show an example dot structure that could act as a ratchet band solar cell. Using the absorptances of the quantum dot array, we calculate a detailed balance efficiency of 36% for the system. Optimizing the dot geometry and alloy fraction allows detailed balance efficiencies up to 42% but at the cost of losing the spatial ratchet.

Implementing blended first-year chemistry in a developing country using online resources

Decades of rapid development in information and communication technologies (ICTs) have resulted in tremendous global evolution in computer and online instruction. Many developing countries, however, are still struggling to successfully integrate ICTs in their teaching and learning practices, subsequently leading to slower rates of adapting digital learning pedagogies. To understand how blended instruction might operate in higher education in a developing country, this study explored students’ perspectives on the implementation of blended learning in a first-year chemistry program delivered in the Philippines. Through the resource-based learning framework, multiple types of online learning resources were employed for blended delivery of topics on periodic trends, chemical bonding, Lewis structures, molecular shape and polarity through the learning management system, Moodle. To understand the students’ experiences, a mixed methods approach was employed through a survey, focus groups, and learning analytics. Despite the scarcity of technological resources (such as access to a reliable internet connection) 57.5% of 447 student respondents favoured blended learning because of the flexibility, wider access to different types of interactive learning resources, variety of learning activities, and perceived increase in learning productivity. While the majority of respondents (75.7%) had ICT skills sufficient for education, much fewer had access to computers (19.7%). 40.0% of students self-reported that they preferred a traditional mode of instruction primarily due to the perceived difficulty of chemistry as subject matter and the students’ perceived need for a physically-present teacher to explain concepts face-to-face and to respond to questions that would arise anytime during the learning period.

Preparation of multicolor carbon dots with high fluorescence quantum yield and application in white LED

Using tartaric acid and triammonium citrate as environmentally friendly precursor reactants, multicolor fluorescent carbon dots with adjustable emission wavelength and high quantum yield were successfully synthesized via one-step solvothermal method by only changing the reaction solvent. The morphology, structure and optical properties of the carbon dots were investigated in detail. Characterization results showed that the as-synthesized carbon dots have good dispersibility, rich surface functional groups and high quantum yield (all over 40%). A high-quality white LED with CIE color coordinate (0.34, 0.35) was successfully fabricated using the prepared multicolor carbon dots as the light conversion material. The correlated color temperature was 5336 K, and the color rendering index was up to 83.1. The prepared LED emitted warm white light and was suitable for indoor lighting. It is of great significance to prepare multicolor carbon dots with high quantum yield by single-factor control, and the prepared multicolor carbon dots have potential and broad application prospects in the field of white LED.

Acute Effects of Postactivation Potantiation on Explosive Strength Performance in Fitness Athletes

Aim: The aim of this research is to examine the effects of postactivation potantiation (PAP) on explosive strength performance in terms of the rest period and also to determine the optimal resting time for generating explosive strength after the PAP protocol. Methods: Ten fitness athletes were recruited for this study. The mean age of the participants group of the research was 26.60±1.07, their height was 178.50±3.02, their weight was 80.00±4.49, their maximal squat repetition performance (1-RM) was 130±7.45, their vertical jump performance was 53.80±1.98 and their explosive strength performance was 14.50±.47. In this research, measurements were completed on five seperate days with 72 hours intervals. On the first measurement day, the athletes' height and weight measurements, 1-RM squat measurements, vertical jump performance measurements were taken and explosive strength levels were calculated using the Lewis formula. On the second measurement day, 5 repetitions were performed with 85% of the 1-RM squat and after 15 seconds of rest, the vertical jump performance test was measured. On the third measurement day, 5 repetitions of 1-RM squats with 85% were performed and after 3 minutes of rest, the vertical jump performance test was measured. On the fourth measurement day, 5 repetitions of 1-RM squat with 85% were performed and after a rest period of 6 minutes, the vertical jump performance test was measured. Finally, on the fifth measurement day, 5 repetitions of 1-RM squats with 85% were performed and after 9 minutes of rest, the vertical jump performance test was measured. After all measurements, explosive strength levels were calculated using Lewis formula. Passive rest was performed between the exercise and the measurement. Results: It was determined that explosive strength performance was negatively affected 15 seconds after the loading applied within the scope of the PAP protocol, and the difference between the pre-test and the post-test was statistically significant. Bat it was determined that there was no statistical difference between the pre-test and post-test in explosive strength performance 3 minutes after the loads applied within the scope of the PAP protocol. But also it was observed that the explosive strength performance was positively affected 6 minutes after the loading applied within the scope of the PAP protocol, and the difference between the pre-test and the post-test was statistically significant. Finally, it was determined that there was no statistical difference between the pre-test and post-test in explosive strength performance 9 minutes after the loading applied within the scope of the PAP protocol. Conclusion: As a result of this research, it has been determined that the optimal resting time for explosive strength generation is 6 minutes after the loading applied within the scope of the PAP protocol. In addition, it was determined that explosive strength performance was negatively affected after a short rest period of 15 seconds after the loading applied within the scope of the PAP protocol. In this context, it can be said that optimum performance can occur when fatigue is greatly reduced but the PAP effect is still present. Keywords: Postactivation Potantiation, Explosive Strength Performance, Complex Training

Frustration-controlled quantum phase transition between multiple singular two-stage Kondo behaviors in a tetrahedral quadruple quantum dot structure

Semiconductor quantum dots are considered to be promising candidates for the hardware of quantum information technology and optoelectronic devices. Herein, motivated by a tetrahedrally shaped colloidal quadruple quantum dot structure made from In-based III--V semiconductors, which has been synthesized very recently by Leemans et al. [J. Am. Chem. Soc., 143, 4290 (2021)], we provide timely insight into the electronic transport and the quantum phase transition (QPT) for such an architecture. When the interdot hopping between different side dots (${t}_{2}$) is absent, a singular two-stage Kondo effect is revealed for small central-side coupling ${t}_{1}$. The two screening processes are separated by an energy scale of the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction, and the fitting parameters deviate from the regular ones of the side-coupled double dot system. The RKKY interaction and temperature are well illustrated by functions of ${t}_{1}^{4}/(U{T}_{K1}^{2})$, where $U$ and ${T}_{K1}$ are the on-site electron-electron repulsion and the first Kondo temperature, respectively. When ${t}_{2}$ turns on, the ground state of the side dots transits from a spin quadruplet to a magnetic frustration phase, and then to a singlet, through two first-order QPTs. In the frustration phase, another new two-stage Kondo effect is demonstrated, which includes the process of screening the local spin on the central dot firstly, and then that on the neighborless side dot is screened at a lower temperature. Both the Kondo temperatures are found to be rather sensitive to ${t}_{2}$. When ${t}_{2}$ is large enough, the reappearance of the regular Kondo effect is found. With fixed ${t}_{2}=0$, charging the central dot triggers a transition from an antiferromagnetic correlation among the side dots to a ferromagnetic one, accompanied by a Kondo behavior in the central dot. We adopt the state-of-the-art numerical renormalization group method to implement the above behaviors, combined with analytical arguments.

Effect of Glass Composition on Luminescence and Structure of CsPbBr3 Quantum Dots in an Amorphous Matrix.

Glass matrix embedding is an efficient way to improve the chemical and thermal stability of the halide perovskite QDs. However, CsPbX3 QDs exhibit distinct optical properties in different glass matrixes, including photoluminescence (PL) peak position, PL peak width, and optical band gap. In this work, the temperature-dependent PL spectra, absorption spectra, high-energy X-ray structure factor S(Q), and pair distribution function (PDF) were integrated to analyze the structural evolution of CsPbBr3 QDs in different glass matrixes. The results show that the lattice parameters and atomic spacing of CsPbBr3 QDs are affected by the glass composition in which they are embedded. The most possibility can be attributed to the thermal expansion mismatch between CsPbBr3 QDs and the glass matrix. The results may provide a new way to understand the effect of the glass composition on the optical properties of CsPbBr3 QDs in a glass matrix.

Rational Design of ZnMn2O4 Quantum Dots in a Carbon Framework for Durable Aqueous Zinc‐Ion Batteries

Manganese oxides are promising cathode materials for aqueous zinc-ion batteries (ZIBs) due to their high energy density and low cost. However, in their discharging processes, the Jahn-Teller effect and Mn3+ disproportionation often lead to irreversible structural transformation and Mn2+ dissolution, deteriorating the cycling stability of ZIBs. Herein, ZnMn2 O4 quantum dots (ZMO QDs) were introduced into a porous carbon framework by in-situ electrochemically inducing Mn-MIL-100-derived Mn3 O4 quantum dots and the carbon composite. In such ZMO QDs and carbon composite, the quantum dot structure endows ZnMn2 O4 with a shorter ion diffusion route and more active sites for Zn2+ . The conductive carbon framework is beneficial to the fast transport of electrons. Furthermore, at the interface between the ZMO QDs and the carbon matrix, the Mn-O-C bonds are formed. They can effectively suppress the Jahn-Teller effect and manganese dissolution of discharge products. Therefore, Zn/ZMO QD@C batteries display remarkably enhanced electrochemical performance.

Rational Design of ZnMn2O4 Quantum Dots in a Carbon Framework for Durable Aqueous Zinc‐Ion Batteries

AbstractManganese oxides are promising cathode materials for aqueous zinc‐ion batteries (ZIBs) due to their high energy density and low cost. However, in their discharging processes, the Jahn–Teller effect and Mn3+ disproportionation often lead to irreversible structural transformation and Mn2+ dissolution, deteriorating the cycling stability of ZIBs. Herein, ZnMn2O4 quantum dots (ZMO QDs) were introduced into a porous carbon framework by in‐situ electrochemically inducing Mn‐MIL‐100‐derived Mn3O4 quantum dots and the carbon composite. In such ZMO QDs and carbon composite, the quantum dot structure endows ZnMn2O4 with a shorter ion diffusion route and more active sites for Zn2+. The conductive carbon framework is beneficial to the fast transport of electrons. Furthermore, at the interface between the ZMO QDs and the carbon matrix, the Mn−O−C bonds are formed. They can effectively suppress the Jahn–Teller effect and manganese dissolution of discharge products. Therefore, Zn/ZMO QD@C batteries display remarkably enhanced electrochemical performance.