Exciton Radius Research Articles

(Invited) Microwave-Assisted Synthesis and Characterization of Doped Ge Nanocrystals

Ge is one of the quintessential semiconductors being considered in nanoform for optoelectronic applications. As a group 14 semiconductor, bulk Ge has a small bandgap and a large Bohr exciton radius, making it an attractive semiconductor for various applications, including solar cell technology, photodetectors, and integrated flash memory devices. I will present our work on doped and alloyed Ge nanoparticles via microwave-assisted synthesis and galvanic replacement. I will present new insights from this chemical route that may impact the nanoparticle synthesis of other covalently bonding semiconductors.

Elemental doping tailoring photocatalytic hydrogen evolution of InP/ZnSeS/ZnS quantum dots

Photocatalytic water splitting for hydrogen production has garnered considerable attention as an effective method to alleviate energy shortages. In comparison to cadmium-based quantum dots (QDs) photocatalysts, InP QDs possess a smaller bandgap, larger exciton radius, broader absorption range, and are environmentally friendly. Although InP/ZnS core/shell QDs exhibit immense potential in photocatalysis, they suffer from rapid electron-hole recombination owing to interface defects and lattice mismatch. To address these issues, this study introduces an intermediate ZnSeS shell to reduce defects and tailor QD redox properties through transition metals (manganese and copper) doping. The characterization of QDs was performed from various perspectives, including morphology, element distribution, band structure, and charge transport efficiency. Notably, the photoelectrochemical properties of doped QDs were superior to those of the undoped QDs, while Mn-doped QDs showed inferior catalytic performance compared to the undoped ones. The mechanism of different photoelectrochemical and photocatalytic performances has been studied more intensely. The Mn-doped QDs undergo a process where Mn4+ is converted to Mn2+, consuming electrons in competition with the photocatalytic hydrogen evolution process. Conversely, InP/ZnSeS:Cu/ZnS QDs introduced an additional energy level into the original band structure, capturing some holes and slowing down the electron-hole recombination, thereby providing positive feedback for photocatalytic hydrogen production. Profiting from both the synergies of energy level structure and doping metal state, effective electron-hole separation, and rapid electron transfer to the surface of Cu-doped QDs accomplish the efficient hydrogen generation. The present study offers more possibilities for exploiting the required photocatalytic performance of QD catalysts via elemental doping.

Dicarboxylic Acid‐Assisted Surface Oxide Removal and Passivation of Indium Antimonide Colloidal Quantum Dots for Short‐Wave Infrared Photodetectors

AbstractHeavy‐metal‐free III–V colloidal quantum dots (CQDs) are promising materials for solution‐processed short‐wave infrared (SWIR) photodetectors. Recent progress in the synthesis of indium antimonide (InSb) CQDs with sizes smaller than the Bohr exciton radius enables quantum‐size effect tuning of the band gap. However, it has been challenging to achieve uniform InSb CQDs with band gaps below 0.9 eV, as well as to control the surface chemistry of these large‐diameter CQDs. This has, to date, limited the development of InSb CQD photodetectors that are sensitive to 1400 nm light. Here we adopt solvent engineering to facilitate a diffusion‐limited growth regime, leading to uniform CQDs with a band gap of 0.89 eV. We then develop a CQD surface reconstruction strategy that employs a dicarboxylic acid to selectively remove the native In/Sb oxides, and enables a carboxylate‐halide co‐passivation with the subsequent halide ligand exchange. We find that this strategy reduces trap density by half compared to controls, and enables electronic coupling among CQDs. Photodetectors made using the tailored CQDs achieve an external quantum efficiency of 25 % at 1400 nm, the highest among III–V CQD photodetectors in this spectral region.

Dicarboxylic Acid-Assisted Surface Oxide Removal and Passivation of Indium Antimonide Colloidal Quantum Dots for Short-Wave Infrared Photodetectors.

Heavy-metal-free III-V colloidal quantum dots (CQDs) are promising materials for solution-processed short-wave infrared (SWIR) photodetectors. Recent progress in the synthesis of indium antimonide (InSb) CQDs with sizes smaller than the Bohr exciton radius enables quantum-size effect tuning of the band gap. However, it has been challenging to achieve uniform InSb CQDs with band gaps below 0.9 eV, as well as to control the surface chemistry of these large-diameter CQDs. This has, to date, limited the development of InSb CQD photodetectors that are sensitive to 1400 nm light. Here we adopt solvent engineering to facilitate a diffusion-limited growth regime, leading to uniform CQDs with a band gap of 0.89 eV. We then develop a CQD surface reconstruction strategy that employs a dicarboxylic acid to selectively remove the native In/Sb oxides, and enables a carboxylate-halide co-passivation with the subsequent halide ligand exchange. We find that this strategy reduces trap density by half compared to controls, and enables electronic coupling among CQDs. Photodetectors made using the tailored CQDs achieve an external quantum efficiency of 25 % at 1400 nm, the highest among III-V CQD photodetectors in this spectral region.

Giant excitonic magneto-optical Faraday rotation in single semimagnetic CdTe/Cd1-xMnxTe quantum ring

Magnetic tuning of the bound exciton states and corresponding giant Zeeman splitting (GZS) between σ+ and σ− excitonic transitions in CdTe/Cd1-xMnxTe quantum ring has been investigated in the Faraday configuration for various concentrations of Mn2+ ions, using the variational technique in the effective mass approximation. The sp-d exchange interaction between the localized magnetic impurity ions and the delocalized charge carriers has been accounted via mean-field theory with the inclusion of a modified Brillouin function. The enhancement of the GZS, and in turn, the effective g-factor with the application of an external magnetic field, is strikingly manifested in type-I – type-II transition in the band structure, which has been well explained by computing the overlap integral between the electron and hole, and the in-plane exciton radius. This highlights the extraordinary magneto-optical properties, including the giant Faraday rotation and associated Verdet constant, which have been calculated using single oscillator model. The oscillator strength has been estimated, and is found to be larger than in the bulk diluted magnetic semiconductors (DMS) and quantum wells, reflecting stronger confinement inside the quantum ring. More extensive Zeeman splitting and a higher oscillator strength in the DMS-based QR lead to an ultra-high Verdet constant of 2.6×109rad/Teslam, which are a few orders of magnitude larger than in the existing quantum systems and magneto-optical materials.

One-Pot Colloidal Synthesis Enables Highly Tunable InSb Short-Wave Infrared Quantum Dots Exhibiting Carrier Multiplication.

Colloidal quantum dots (CQDs) are emerging materials for short-wave infrared (SWIR, ≈1100-3000 nm) photodetectors, which are technologically important for a broad array of applications. Unfortunately, the most developed SWIR CQD systems are Pb and Hg chalcogenides; their toxicity and regulated compositions limit their applications. InSb CQD system is a potential environmentally friendly alternative, whose bandgap in theory, is tunable via quantum confinement across the SWIR spectrum. However, InSb CQDs are difficult to exploit, due to their complex syntheses and uncommon reactive precursors, which greatly hinder their application and study. Here, a one-pot synthesis strategy is reported using commercially available precursors to synthesize-under standard colloidal synthesis conditions-high-quality, size-tunable InSb CQDs. With this strategy, the large Bohr exciton radius of InSb can be exploited for tuning the bandgap of the CQDs over a wide range of wavelengths (≈1250-1860nm) across the SWIR region. Furthermore, by changing the surface ligands of the CQDs from oleic acid (OA) to 1-dodecanthiol (DDT), a ≈20-fold lengthening in the excited-state lifetime, efficient carrier multiplication, and slower carrier annihilation are observed. The work opens a wide range of SWIR applications to a promising class of Pb- and Hg-free CQDs.

Nonlinear Optical Properties of Semiconductor Microcrystals

The absorption edge of CdSe and CdSSe microcrystals has been observed to undergo a significant short-wavelength shift (of more than 30 meV) during excitation by light pulses at the second harmonic from a Nd:YAG laser. An optical-nonlinearity mechanism is proposed. This mechanism involves the filling of levels in the energy spectrum of a spatially bounded multiexciton system in microcrystals with a size comparable to the exciton radius.

Nelineynoe pogloshchenie i fotolyuminestsentsiya nanotetrapodov CdTe s nakonechnikami CdSe pri nerezonansnom vozbuzhdenii eksitonov

The nonlinear absorption and photoluminescence of CdTe/CdSe nanotetrapod colloids have been studied by the pump–probe method for the case of single-photon nonresonance excitation of excitons. A competition between the short-wave and long-wave shifts of the peak of photoluminescence associated with an indirect electron–hole transition, which is observed with increasing excitation radiation intensity, has been found and explained. The former shift is associated with an increase in the one-dimensional exciton radius after the exciton state occupation, and the latter shift may be attributed to the charge-induced Stark effect and local heating of nanotetrapods as a result of electron–phonon interaction at nonresonance excitation of the system.

Thermal effect on magnetoexciton energy spectra in monolayer transition metal dichalcogenides

It is widely comprehended that temperature may cause phonon-exciton scattering, enhancing the energy level's linewidth and leading to some spectrum shifts. However, in the present paper, we suggest a different mechanism that allows the thermal motion of the exciton's center of mass (c.m.) to affect the magnetoexciton energies in monolayer dichalcogenides (TMDCs). By the nontrivial but precise separation of the c.m. motion from an exciton in a monolayer TMDC with a magnetic field, we obtain an equation for the relative motion containing a motional Stark term proportional to the c.m. pseudomomentum, related to the temperature of the exciton gas but neglected in the previous studies. Solving the Schr\"odinger equation without omitting the motional Stark potential at room temperature shows approximately a few meV thermal-magnetic shifts in the exciton energies, significant enough for experimental detection. Moreover, this thermal effect causes a change in exciton radius and diamagnetic coefficient and enhances the exciton lifetime as a consequence. Surprisingly, the thermoinduced motional Stark potential breaks the system's SO(2) symmetry, conducting new peaks in the exciton absorption spectra at room temperature besides those of the $s$ states. This mechanism could be extended for other magnetoquasiparticles such as trions and biexcitons.

Hexagonal and tetragonal ScX (X = P, As, Sb) nanosheets for optoelectronics and straintronics

The majority of already industrialized and technically feasible materials fall under the category of non-van der Waals (n-vdW) materials. However, the library of two-dimensional (2D) materials typically consists of nanosheets exfoliated from layered bulk (vdW) compounds. In this work, two phases of non-vdW two-dimensional (2D) ScX (X = P, As, Sb) nanosheets, namely hexagonal and tetragonal, have been designed. One potential solution to the severe energy and environmental problems lies in the direct generation of H2 via photocatalytic water splitting. Herein, using systematic density functional theory (DFT) calculations, we are able to show that the hexagonal nanosheets are strong contenders in this case, as supported by its thermodynamic favourability, suitable exciton binding energy, magnificent optical absorption, water insolubility, high exciton lifetime, and high carrier mobility. Moreover, exciton lifetime ∼100 ns, small exciton radius, and considerable exciton binding energy makes it an embryonic candidate for Bose-Einstein condensation. High sensitivity of the band gap of the tetragonal nanosheets to strain allows bandgap opening under a minor biaxial tensile strain. The tetragonal nanosheets are therefore envisioned to show enormous proficiency in flexible and stretchable nanoelectronics such as strain-based switches, digital data storage devices, movement-controlled sensors, and optoelectronic devices such as 3D printings and displays.

Excitons in metal halide perovskite nanoplatelets: an effective mass description of polaronic, dielectric and quantum confinement effects.

A theoretical model for excitons confined in metal halide perovskite nanoplatelets is presented. The model accounts for quantum confinement, dielectric confinement, short and long range polaron interactions by means of effective mass theory, image charges and Haken potentials. We use it to describe the band edge exciton of MAPbI3 structures surrounded by organic ligands. It is shown that the quasi-2D quantum and dielectric confinement squeezes the exciton radius, and this in turn enhances short-range polaron effects as compared to 3D structures. Dielectric screening is then weaker than expected from the static dielectric constant. This boosts the binding energies and radiative recombination probabilities, which is a requisite to match experimental data in related systems. The thickness dependence of Coulomb polarization and self-energy potentials is in fair agreement with sophisticated atomistic models.

Capturing of non-hydrogenic Rydberg series of exciton binding energy in two-dimensional mono-layer WS2 using a modified Coulomb potential in fractional space

2D materials exhibit unique electronic states due to quantum confinement. Among the Group-VI chalcogenides, direct mono-layer WS2 is the most prominent where screening is non-localized, having strongly bound excitons with large binding energies and a pronounced deviation of the excitonic states from the hydrogenic series. State-of-the-art experimental and theoretical methods to determine excitonic Rydberg series employ optical spectroscopy and Bethe-Salpeter (BSE) equation, respectively, but incur high costs, paving the way to develop analytical approaches. We present a generalized hydrogenic model by employing a fractional version of the Coulomb-like potential to capture the excitonic Rydberg series of the fundamental optical transition in mono-layer WS2, based on the fractional scaling of the electron-hole pair interactions through the tuning of the fractional-space parameter β, benchmarked with experimental data and that of with numerical computation of the hydrogenic solution involving the Rytova-Keldysh (R-K) potential model. The enhanced electron-hole interactions lead to a strong dielectric contrast between the mono-layer WS2 and its surrounding environment and causes the deviation of the low-lying excitonic states from the hydrogenic series. The fractional Coulomb potential (FCP) model captures the first two non-hydrogenic states at β < 3, to fit a Coulomb-like to logarithmic change with respect to the excitonic radius and the higher hydrogenic states to have Coulombic interactions at β ≈ 3 in mono-layer WS2. A comparison of the proposed model with an existing model based on Wannier theory reveals a reduction in the relative mean square error of up to 30% for the excitonic series, with only the ground state captured as non-hydrogenic by the latter.

Effect of Cerium dopant on third-order nonlinear optical properties of CdS/PEG self-standing nanocomposite films

CdS/PEG, 4 and 7 mol% Ce doped CdS/PEG nanocomposite films were synthesized by in situ method. The XRD pattern confirmed the cubic structure of the CdS and the size of the CdS nanocrystallite is found to be smaller than the Bohr exciton radius for all the samples. SEM analysis shows that CdS and Ce doped CdS nanoparticles are well mono-dispersed in PEG polymer. FTIR spectra confirm the interaction of Ce-doped CdS nanoparticles with PEG polymer. UV–Vis optical absorption studies exhibit a red-shift of CdS nanoparticle with Ce doping. The photoluminescence spectra of all the films show a red emission due to the surface defects. The third-order nonlinear optical properties of all the films have been studied using the Z-scan technique. Closed and open aperture Z-scan plots show self-focusing optical nonlinearity and reverse saturable absorption with positive nonlinear coefficients. The 7 mol% Ce doped CdS/PEG nanocomposite film exhibits the highest value of nonlinear refractive index and nonlinear absorption coefficient which are about 1.649 × 10−9 m2/W and 7.149 × 10−3 m/W. The third-order nonlinear optical susceptibility and figure of merit have also been estimated for all the films and also increase with increase in Ce dopant concentrations. The maximum value of third-order nonlinear optical susceptibility and figure merit are found for 7 mol% Ce doped CdS/PEG nanocomposite film and are about 9.882 × 10−4 esu and 5.902 × 10−8 esu m. The present study strongly recommends that the Ce doped CdS/PEG nanocomposite films are worthy candidates for fabricating the future optical limiting devices.

Evolution of the Electronic and Optical Properties of Meta-Stable Allotropic Forms of 2D Tellurium for Increasing Number of Layers.

In this work, ab initio Density Functional Theory calculations are performed to investigate the evolution of the electronic and optical properties of 2D Tellurium—called Tellurene—for three different allotropic forms (-, - and -phase), as a function of the number of layers. We estimate the exciton binding energies and radii of the studied systems, using a 2D analytical model. Our results point out that these quantities are strongly dependent on the allotropic form, as well as on the number of layers. Remarkably, we show that the adopted method is suitable for reliably predicting, also in the case of Tellurene, the exciton binding energy, without the need of computationally demanding calculations, possibly suggesting interesting insights into the features of the system. Finally, we inspect the nature of the mechanisms ruling the interaction of neighbouring Tellurium atoms helical chains (characteristic of the bulk and -phase crystal structures). We show that the interaction between helical chains is strong and cannot be explained by solely considering the van der Waals interaction.

Tapering-free monocrystalline Ge nanowires synthesized via plasma-assisted VLS using In and Sn catalysts

In and Sn are the type of catalysts which do not introduce deep level electrical defects within the bandgap of germanium (Ge). However, Ge nanowires produced using these catalysts usually have a large diameter, a tapered morphology, and mixed crystalline and amorphous phases. In this study, we show that plasma-assisted vapor–liquid–solid (PA-VLS) method can be used to synthesize Ge nanowires. Moreover, at certain parameter domains, the sidewall deposition issues of this synthesis method can be avoided and long, thin tapering-free monocrystalline Ge nanowires can be obtained with In and Sn catalysts. We find two quite different parameter domains where Ge nanowire growth can occur via PA-VLS using In and Sn catalysts: (i) a low temperature-low pressure domain, below ∼235 °C at a GeH4 partial pressure of ∼6 mTorr, where supersaturation in the catalyst occurs thanks to the low solubility of Ge in the catalysts, and (ii) a high temperature-high pressure domain, at ∼400 °C and a GeH4 partial pressure above ∼20 mTorr, where supersaturation occurs thanks to the high GeH4 concentration. While growth at 235 °C results in tapered short wires, operating at 400 °C enables cylindrical nanowire growth. With the increase of growth temperature, the crystalline structure of the nanowires changes from multi-crystalline to mono-crystalline and their growth rate increases from ∼0.3 nm s−1 to 5 nm s−1. The cylindrical Ge nanowires grown at 400°C usually have a length of few microns and a radius of around 10 nm, which is well below the Bohr exciton radius in bulk Ge (24.3 nm). To explain the growth mechanism, a detailed growth model based on the key chemical reactions is provided.

Unusually large exciton binding energy in multilayered 2H-MoTe2

Although large exciton binding energies of typically 0.6–1.0 eV are observed for monolayer transition metal dichalcogenides (TMDs) owing to strong Coulomb interaction, multilayered TMDs yield relatively low exciton binding energies owing to increased dielectric screening. Recently, the ideal carrier-multiplication threshold energy of twice the bandgap has been realized in multilayered semiconducting 2H-MoTe2 with a conversion efficiency of 99%, which suggests strong Coulomb interaction. However, the origin of strong Coulomb interaction in multilayered 2H-MoTe2, including the exciton binding energy, has not been elucidated to date. In this study, unusually large exciton binding energy is observed through optical spectroscopy conducted on CVD-grown 2H-MoTe2. To extract exciton binding energy, the optical conductivity is fitted using the Lorentz model to describe the exciton peaks and the Tauc–Lorentz model to describe the indirect and direct bandgaps. The exciton binding energy of 4 nm thick multilayered 2H-MoTe2 is approximately 300 meV, which is unusually large by one order of magnitude when compared with other multilayered TMD semiconductors such as 2H-MoS2 or 2H-MoSe2. This finding is interpreted in terms of small exciton radius based on the 2D Rydberg model. The exciton radius of multilayered 2H-MoTe2 resembles that of monolayer 2H-MoTe2, whereas those of multilayered 2H-MoS2 and 2H-MoSe2 are large when compared with monolayer 2H-MoS2 and 2H-MoSe2. From the large exciton binding energy in multilayered 2H-MoTe2, it is expected to realize the future applications such as room-temperature and high-temperature polariton lasing.

Transition from metal to semiconductor by semi-hydrogenation of borophene

Borophene has triggered a surge of interest due to its outstanding properties including mechanical flexibility, polymorphism, and optoelectrical anisotropy. Very recently, a novel semi-hydrogenated borophene, called ${\ensuremath{\alpha}}^{\ensuremath{'}}$-4H, was synthesized in large-scale freestanding samples, exhibiting excellent air-stability and semiconducting nature. Herein, using the density functional theory (DFT) and many-body perturbation theory (MBPT), we investigate the electronic and excitonic optical properties of ${\ensuremath{\alpha}}^{\ensuremath{'}}$-4H borophene. The DFT results reveal that hydrogenation breaks the mirror symmetry and increases the buckling height of pure ${\ensuremath{\alpha}}^{\ensuremath{'}}$ borophene, which results in an orbital hybridization and indirect band gap of 1.49 eV in ${\ensuremath{\alpha}}^{\ensuremath{'}}$-4H borophene. Moreover, the optical spectrum achieved from solving the Bethe-Salpeter equation (i.e., $\mathrm{GW}+\mathrm{BSE}$) shows an optical band gap of 2.40 eV, which corresponds to a strongly bound and stable bright exciton with a binding energy of 1.18 eV. The mean value of absorption within the visible area is 1.68 (1.13)$\ifmmode\times\else\texttimes\fi{}{10}^{7}\phantom{\rule{4pt}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}$ for $E\ensuremath{\parallel}x$ ($E\ensuremath{\parallel}y$) polarization, which shows a linear dichroism for visible light. The effective mass and Bohr radius of the ground-state exciton are 0.78 ${m}_{0}$ and 2.03 \AA{}, respectively, which demonstrates the characteristics of Frenkel exciton. The excitonic states are robust against tension up to 10%, under which the monolayer is dynamically stable. We also study the bilayer ${\ensuremath{\alpha}}^{\ensuremath{'}}$-4H borophene with different stackings. For the weak van der Waals interactions between the layers, the bilayer preserves most of the structural and electronic properties of the monolayer. Our study exposes the underlying physics behind the structural, electronic, and optical properties of ${\ensuremath{\alpha}}^{\ensuremath{'}}$-4H borophene and suggests it as a very promising candidate for flexible optoelectronic applications.

Энергия относительного движения электрона и дырки в экситоне во внешнем электрическом поле в пластине GaAs

The dependence of the energy of the relative electron-hole motion in an exciton on the magnitude of applied electric field for various thicknesses of an ideal flat semiconductor plate is theoretically calculated. It is shown that the variation of the plate thickness significantly affects the dependence of the energy on the electric field. The effect should be observed in plates whose thickness exceeds the Bohr exciton radius by two orders of magnitude.

Energy of electron-hole relative motion in exciton in external electric field in thick GaAs plate

The dependence of the energy of the relative electron-hole motion in an exciton on the magnitude of applied electric field for various thicknesses of an ideal flat semiconductor plate is theoretically calculated. It is shown that the variation of the plate thickness significantly affects the dependence of the energy on the electric field. The effect should be observed in plates whose thickness exceeds the Bohr exciton radius by two orders of magnitude. Keywords: exciton, electrick field, thick plate.

Simulation Engineering in Quantum Dots for Efficient Photovoltaic Solar Cell Using Copper Iodide as Hole Transport Layer

Among the third-generation solar cells, quantum dot solar cell is attracting the researchers and the widespread research has been increasing day by day. It has appropriate electrical and optical properties for a photovoltaic response. Quantum dot has a tunable band gap and its size is less than the Bohr exciton radius. In this work, copper iodide (CuI) is the hole transport layer (HTL), the absorber layer is tetrabutylammonium iodide treated lead sulphide (PbS-TBAI), and titanium dioxide (TiO2) is the electron transport layer (ETL). The device has been rectified to obtain maximum efficiency followed by device optimization. The variation of thickness of the ETL, HTL and absorber layer has been done to optimize the performance of the proposed device. Further, variation of doping density of HTL and interface defect density (IDD) of HTL/PbS-TBAI and PbS-TBAI/ETL has been done to overall optimize the device performance. The interface defect density at both the interfaces (HTL/PbS-TBAI and PbS-TBAI/ETL) has been varied from 1×1010 cm−2 to 1×1017 cm−2 without changing the rest of the device parameters. The result shows notable, diminution in the photo-voltaic performance of the solar cell at higher interface defect density. Quantum efficiency (Q.E) and J-V curve have been investigated. From this study, we have observed that the simulated device structure could be used as an efficient solar cell.