Single Exciton Research Articles

Role of various dielectric environment matrices of InP/ZnS core/shell quantum dot on optical gain coefficient

Theoretical studies on excitonic and optical properties of a single exciton confined in an InP/ZnS heterostructure core/shell quantum dot embedded in various dielectric environments are investigated. The energies are found with and without the inclusion of dielectric mismatch employing single band effective mass approximation using variational formulism. The solution of Poisson-Schrodinger wave equations for the attractive term between the electron and hole is carried out using a self consistent approach in the Hartree approximation. The binding energy due to an exciton and oscillator strength is found with the effect of geometrical confinement. The total absorption coefficients, the injection current density for the optical output and the corresponding threshold optical pump intensity studied in the presence of various dielectric environments are investigated in the InP/ZnS heterostructure core/shell quantum dot. The ratio of core/shell hetero-structured quantum dot radius for various values of dielectric matrices immersed is found. The results show that the obtained properties are considerably enhanced with the incorporation of dielectric environment matrices immersed in the core/shell quantum dot particularly the highest dielectric constant will bring out the better results. It is hoped that to the present study will contribute the understanding of excitonic and optical properties in the group II–VI core/shell heterostructure quantum dots for the potential applications in photovoltaic and light emitting diodes.

Electric field quenching of graphene oxide photoluminescence

With the advent of graphene, there has been an interest in utilizing this material and its derivative, graphene oxide (GO) for novel applications in nanodevices such as bio and gas sensors, solid-state supercapacitors and solar cells. Although GO exhibits lower conductivity and structural stability, it possesses an energy band gap that enables fluorescence emission in the visible/near infrared leading to a plethora of optoelectronic applications. In order to allow fine-tuning of its optical properties in the device geometry, new physical techniques are required that, unlike existing chemical approaches, yield substantial alteration of GO structure. Such a desired new technique is one that is electronically controlled and leads to reversible changes in GO optoelectronic properties. In this work, we for the first time investigate the methods to controllably alter the optical response of GO with the electric field and provide theoretical modeling of the electric field-induced changes. Field-dependent GO emission is studied in bulk GO/polyvinylpyrrolidone films with up to 6% reversible decrease under 1.6 V µm−1 electric fields. On an individual flake level, a more substantial over 50% quenching is achieved for select GO flakes in a polymeric matrix between interdigitated microelectrodes subject to two orders of magnitude higher fields. This effect is modeled on a single exciton level by utilizing Wentzel, Kremer, and Brillouin approximation for electron escape from the exciton potential well. In an aqueous suspension at low fields, GO flakes exhibit electrophoretic migration, indicating a degree of charge separation and a possibility of manipulating GO materials on a single-flake level to assemble electric field-controlled microelectronics. As a result of this work, we suggest the potential of varying the optical and electronic properties of GO via the electric field for the advancement and control over its optoelectronic device applications.

Area and thickness dependence of Auger recombination in nanoplatelets.

The ability to control both the thickness and the lateral dimensions of colloidal nanoplatelets offers a test-bed for area and thickness dependent properties in 2D materials. An important example is Auger recombination, which is typically the dominant process by which multiexcitons decay in nanoplatelets. Herein, we uncover fundamental properties of biexciton decay in nanoplatelets by comparing the Auger recombination lifetimes based on interacting and noninteracting formalisms with measurements based on transient absorption spectroscopy. Specifically, we report that electron-hole correlations in the initial biexcitonic state must be included in order to obtain Auger recombination lifetimes in agreement with experimental measurements and that Auger recombination lifetimes depend nearly linearly on the lateral area and somewhat more strongly on the thickness of the nanoplatelet. We also connect these scalings to those of the area and thickness dependencies of single exciton radiative recombination lifetimes, exciton coherence areas, and exciton Bohr radii in these quasi-2D materials.

How Exciton and Single Carriers Block the Excitonic Transition in Two-Dimensional Cadmium Chalcogenide Nanoplatelets.

Cadmium chalcogenide nanoplatelets (NPLs) possess unique properties and have shown great potential in lasing, light-emitting diodes, and photocatalytic applications. However, the exact natures of the band-edge exciton and single carrier (electron and hole) states remain unclear, even though they affect the key properties and applications of these materials. Herein, we study the contribution of a single carrier (electron or hole) state to phase space filling of single exciton states of cadmium chalcogenide NPLs. With pump fluence dependent TA study and selective electron removal, we determine that a single electron and hole states contribute 85% and 12%, respectively, to the blocking of the excitonic transition in CdSe/ZnS core/shell NPLs. These observations can be rationalized by a model of band-edge exciton and single carrier states of 2D NPLs that differs significantly from that of quantum dots.

Microscopic processes of Rubrene-doped devices with various exciplexes as hosts

Recently, exciplex-based organic light-emitting diodes (OLEDs) have become hot topics in organic optoelectronics in terms of improving luminescent efficiency by controlling the energy transfer of excitons. This is because the formation of an exciplex can result in a small energy level difference, Δ E ST between singlet and triplet exciplex states, and with the assistance of external thermal energy, nonradiative triplet exciplex states can transform into radiative singlet ones through the so-called reverse intersystem crossing (RISC) process. Moreover, this RISC process will enhance the luminescence of OLEDs based on host–guest systems through energy transfer from exciplexes to dopants. The energy-transfer capability of OLEDs will affect the number of polaron pairs and exciplexes in host–guest systems. Indeed, it is important to find an effective probing technique for simply studying these microscopic processes. Many literature reports have demonstrated that organic magneto-electroluminescence (MEL) traces could be used for exploring and understanding the formation of and interactions between polaron pairs, excitons, and/or exciplexes. This is because MELs exhibit sensitive fingerprint responses to intersystem crossing (ISC), high-level reverse intersystem crossing (HL-RISC), singlet exciton fission (SF), and triplet exciton fusion (TF). In this study, four different OLEDs with various exciplex hosts have been fabricated, and their MEL curves have been measured at different currents and temperatures. Four exciplexes with different triplet exciton energies were used as host materials, and Rubrene was used as a fluorescent dopant. To study the exciton energy transfer and luminescence mechanism in such doping systems, we fully analyzed the emission spectra of the host materials, absorption spectrum of the guest material, and triplet exciton energy of the host and guest materials. The experimental results show that when the combined energy of the exciplex’s triplet excitons (EX3) is lower than that of the second-order triplet excitons (T2,Rub) of the Rubrene dopant, the MEL curve is dominated by the B -mediated ISC from host polaron pairs (PP1→PP3). Otherwise, the MEL curves are composed of both low-field and high-field components. The low-field components of MEL are governed by the B -mediated HL-RISC (T2,Rub→S1,Rub) process of the Rubrene molecules, while the high-field components of MEL are the result of the TF process between the triplet excitons of Rubrene (T1,Rub+T1,Rub→S1,Rub+S0,Rub) at high current densities or by the SF process of singlet excitons (S1,Rub+S0,Rub→T1,Rub+T1,Rub) at low current densities. In addition, these microprocesses will be markedly affected by the operational temperature and bias-current of the devices, because the Rubrene triplet excitons have long lifetimes (T1,Rub and T2,Rub) at low temperatures, and large quantities of Rubrene excitons are produced at high current densities. Moreover, the energy level difference between the single and triplet excitons of the host and guest will regulate the energy transfer from the former to the latter, i.e., the combined energy from the singlet excitons of the host being higher than that of the guest is the requirement of efficient Forster energy transfer where the smaller is the triplet exciton energy difference between the host and guest, the stronger is the Dexter energy transfer. Consequently, the energy-transfer mechanism will affect the utilization rate of the triplet excitons by adjusting the quantity and longevity of the singlet or triplet excitons on the Rubrene molecules, which ultimately affects the luminous intensity of the device. This work is helpful for further understanding the microscopic mechanisms of Rubrene-based devices and provides a theoretical reference for enhancing their luminescent efficiencies.

Single-Exciton Gain and Stimulated Emission Across the Infrared Telecom Band from Robust Heavily Doped PbS Colloidal Quantum Dots

Materials with optical gain in the infrared are of paramount importance for optical communications, medical diagnostics and silicon photonics. The current technology is based either on costly III-V semiconductors that are not monolithic to silicon CMOS technology or Er-doped fiber technology that does not make use of the full fiber transparency window. Colloidal quantum dots (CQD) offer a unique opportunity as an optical gain medium in view of their tunable bandgap, solution processability and CMOS compatibility. The 8-fold degeneracy of infrared CQDs based on Pb-chalcogenides has hindered the demonstration of low-threshold optical gain and lasing, at room temperature. We demonstrate room-temperature, infrared, size-tunable, band-edge stimulated emission with linewidth of ~14 meV. Leveraging robust electronic doping and charge-exciton interactions in PbS CQD thin films, we reach gain threshold at the single exciton regime representing a four-fold reduction from the theoretical limit of an eight-fold degenerate system, with a net modal gain in excess of 100 cm-1.

Slow carrier cooling in methylammonium lead bromide quantum dots: Evidence of hot phonon bottleneck

Photoinduced charge carrier dynamics studies in lead halide perovskites helps in explaining key processes like carrier trapping, diffusion and multiexciton dynamics etc. which are of importance to understand the fate of excitons. In the present study, we investigate the dynamics in methylammonium lead bromide perovskite nanocrystals under low and high pump intensity to study hot carrier cooling mechanisms. The single exciton studies conducted under low pump fluences (<N> = 0.3) revealed slow carrier decay with a significant fraction of >1 ns decay times as expected due to long radiative carrier recombination times. This also indicated that fast carrier trapping does not occur as evidenced by absence of any fast decay components. The hot carrier (HC) cooling mechanisms were verified by studying carrier density dependence of carrier temperature decays as extracted from the higher energy part of exciton bleach. These studies revealed a very fast cooling of ∼150 fs under low carrier density 1.5 × 1017/ cm3. However additional experiments under higher carrier densities of ∼1018-1019/cm3 showed a very slow carrier temperature decay with decay time ∼100 ps for the highest intensity investigated. The analysis of hot carrier lifetimes revealed Auger heating and acoustic phonon up-conversion as reason for hot carrier relaxations at higher intensities. Additionally, we carried out experiments under hole quencher, we found lower initial carrier temperatures and faster decay indicating highly efficient hole transfer to the quencher in a time scale faster than cooling of carrier at higher intensities. These results hint at possibility of molecular design for enabling efficient hot carrier extraction from these Quantum Dots (QDs).

Effect of Spectral Overlap and Separation Distance on Exciton and Biexciton Quantum Yields and Radiative and Nonradiative Recombination Rates in Quantum Dots Near Plasmon Nanoparticles

AbstractEfficient biexciton (BX) photoluminescence (PL) from quantum dots (QDs) paves the way to the generation of entangled photons and related applications. However, the quantum yield (QY) of BX PL is much lower than that for single excitons (EX) due to efficient Auger‐like recombination. In the vicinity of plasmon nanoparticles, the recombination rates of EX and BX may be affected by the Purcell effect, fluorescence quenching, and the excitation rate enhancement. Here, the effect of the plasmon resonance spectral position on the EX and BX PL is experimentally studied in two cases: when the plasmon band overlaps with the excitation wavelength and when it coincides with the QDs PL band. In the first case, the EX and BX excitation efficiencies are significantly increased but the EX QY reduced. As a result, the BX‐to‐EX QY ratio is higher than 1 at plasmon–exciton systems separations shorter than 40 nm. In the second case, the radiative recombination rates are enhanced by several orders of magnitude, which led to an increase in BX QY over distances of up to 90 nm. Finally, these two effects are obtained in the same hybrid structure, with the resultant increase in both excitation efficiency and QY of BX PL.

Quantum hydrodynamics of a single particle

Semiconductor devices are strong competitors in the race for the development of quantum computational systems. In this work, we interface two semiconductor building blocks of different dimensionalities with complementary properties: (1) a quantum dot hosting a single exciton and acting as a nearly ideal single-photon emitter and (2) a quantum well in a 2D microcavity sustaining polaritons, which are known for their strong interactions and unique hydrodynamic properties, including ultrafast real-time monitoring of their propagation and phase mapping. In the present experiment, we can thus observe how the injected single particles propagate and evolve inside the microcavity, giving rise to hydrodynamic features typical of macroscopic systems despite their genuine intrinsic quantum nature. In the presence of a structural defect, we observe the celebrated quantum interference of a single particle that produces fringes reminiscent of wave propagation. While this behavior could be theoretically expected, our imaging of such an interference pattern, together with a measurement of antibunching, constitutes the first demonstration of spatial mapping of the self-interference of a single quantum particle impinging on an obstacle.

Discrete interactions between a few interlayer excitons trapped at a MoSe2\u2013WSe2 heterointerface

Inter-layer excitons (IXs) in hetero-bilayers of transition metal dichalcogenides (TMDs) represent an exciting emergent class of long-lived dipolar composite bosons in an atomically thin, near-ideal two-dimensional (2D) system. The long-range interactions that arise from the spatial separation of electrons and holes can give rise to novel quantum, as well as classical multi-particle correlation effects. Indeed, first indications of exciton condensation have been reported recently. In order to acquire a detailed understanding of the possible many-body effects, the fundamental interactions between individual IXs have to be studied. Here, we trap a tunable number of dipolar IXs (NIX ~ 1–5) within a nanoscale confinement potential induced by placing a MoSe2–WSe2 hetero-bilayer (HBL) onto an array of SiO2 nanopillars. We control the mean occupation of the IX trap via the optical excitation level and observe discrete sharp-line emission from different configurations of interacting IXs. The intensities of these features exhibit characteristic near linear, quadratic, cubic, quartic and quintic power dependencies, which allows us to identify them as different multiparticle configurations with NIX ~ 1–5. We directly measure the hierarchy of dipolar and exchange interactions as NIX increases. The interlayer biexciton (NIX = 2) is found to be an emission doublet that is blue-shifted from the single exciton by ΔE = (8.4 ± 0.6) meV and split by 2J = (1.2 ± 0.5) meV. The blueshift is even more pronounced for triexcitons ((12.4 ± 0.4) meV), quadexcitons ((15.5 ± 0.6) meV) and quintexcitons ((18.2 ± 0.8) meV). These values are shown to be mutually consistent with numerical modelling of dipolar excitons confined to a harmonic trapping potential having a confinement lengthscale in the range ell approx 3 nm. Our results contribute to the understanding of interactions between IXs in TMD hetero-bilayers at the discrete limit of only a few excitations and represent a key step towards exploring quantum correlations between IXs in TMD hetero-bilayers.

Excitonic complexes in InAs/InP nanowire quantum dots

InAs quantum dots embedded in InP nanowires form an important platform for basic research studies, as well as for quantum dot applications. Notably, understanding of nanowire quantum dot spectral properties is essential in both cases. Therefore, in this work we use atomistic theory to study spectra of the single exciton $(X)$, the biexciton $(XX)$, the triexciton $(XXX)$, and the positively and negatively charged trions $({X}^{+}$ and ${X}^{\ensuremath{-}})$ confined in these nanostructures. We focus on the role of vertical and lateral confinement, therefore, we systematically study a large family of quantum dots with different heights and diameters, and find the important role of correlations due to presence of higher states. We find that the order of excitonic binding energies is a characteristic feature of InAs/InP nanowire quantum dots being (ordered from negative to positive values): ${X}^{\ensuremath{-}},XX$, and ${X}^{+}$, with strongly bound ${X}^{\ensuremath{-}}$, rather weakly bound $XX$, and typically unbound ${X}^{+}$. Next, we determine the key role of alloy randomness due to intermixing, which turns out to especially important for larger quantum dot heights and phosphorous contents over 40%. In selected cases, the alloying can lead to an unbound biexciton, and can even reverse ordering of excitonic lines.

Electric Field Quenching of Graphene Oxide Photoluminescence

With the advent of graphene, there has been an interest in utilizing this material and its derivative, graphene oxide (GO) for novel applications in biotechnology, electronics, photonics, materials and environmental science. Although GO exhibits lower conductivity and structural stability, it possesses an energy band gap that enables fluorescence emission in the visible/near infrared leading to a plethora of optoelectronic applications. In order to enable fine-tuning of its optical properties in the device geometry, new physical techniques are required that unlike existing chemical approaches yielding substantial alteration of GO structure, enable electronically-controlled and, ideally, reversible changes in GO optoelectronic properties. In this work, we for the first time investigate the methods to controllably alter the optical response of GO with the electric field and provide theoretical modelling of the electric field-induced changes. Field-dependent GO emission is studied in bulk GO/PVP films with up to 6% reversible decrease under 1.6 V/µm electric fields. On an individual flake level a more substantial 50% quenching is achieved for select GO flakes in polymeric matrix between interdigitated microelectrodes subject to two orders of magnitude higher fields. This effect is modelled on a single exciton level by utilizing WKB approximation for electron escape form the exciton potential well. In an aqueous suspension at low fields GO flakes exhibit electrophoretic migration indicating a degree of charge separation and a possibility of manipulating GO materials on a single-flake level to assemble electric field-controlled microelectronics. As a result of this work, we suggest the potential of varying the optical and electronic properties of GO via the electric field for the advancement and control over its optoelectronic device applications.

Observation of double indirect interlayer exciton in WSe2/WS2 heterostructure.

Interlayer excitons (IX) are produced by the spatially separated electron-hole pairs due to the robust Coulomb interactions in van der Waals transition metal dichalcogenide (TMDC) heterostructures (HSS). IX is characterized by a larger binding energy, and its lifetime is orders of magnitude longer than that of the direct excitons, providing a significant platform for the manufacture of long-lived exciton devices and the exploration of exciton quantum gas. However, the studies are restricted to the single interlayer exciton, and the simultaneous capture and study of double IX remain challenging in the WSe2/WS2 HS. Here, we demonstrate the existence of double indirect IX in the WSe2/WS2 HS with the emission centers at 1.4585eV (∼25.9meV wide) and 1.4885 eV (∼14.4 meV wide) at cryogenic temperature. Interestingly, the intensities of the double IX emission peaks are almost equal, and the energy difference between them is in a good agreement with the cleavage value of the WS2 conduction band (CB). Additionally, diverse types of excitons in the individual materials were successfully observed in the PL spectra at 8 K. Such unique double IX features, in combination with excellent exciton identification, open up new opportunities for further investigations for new physical properties of TMDCs and explorations for the technological innovation of exciton devices.

Calculation of the biexciton shift in nanocrystals of inorganic perovskites

We calculate the shift in emission frequency of the trion and biexciton (relative to that of the single exciton) for nanocrystals (NCs) of inorganic perovskites ${\mathrm{CsPbBr}}_{3}$ and ${\mathrm{CsPbI}}_{3}$. The calculations use an envelope-function $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ model combined with self-consistent Hartree-Fock and a treatment of the intercarrier correlation energy in the lowest (second) order of many-body perturbation theory. The carriers in the trion and biexciton are assumed to have relaxed nonradiatively to the ground state at the band edge before emission occurs. The theoretical trion shifts for both ${\mathrm{CsPbBr}}_{3}$ and ${\mathrm{CsPbI}}_{3}$ are found to be in fair agreement with available experimental data, which include low-temperature single-dot measurements, though are perhaps systematically small by a factor of order 1.5, which can plausibly be explained by a combination of a slightly overestimated dielectric constant and omitted third- and higher-order terms in the correlation energy. Taking this level of agreement into account, we estimate that the ground-state biexciton shift for ${\mathrm{CsPbBr}}_{3}$ is a redshift of order 10--20 meV for NCs with an edge length of 12 nm. This value is intermediate among the numerous high-temperature measurements on NCs of ${\mathrm{CsPbBr}}_{3}$, which vary from large redshifts of order 100 meV to blueshifts of several meV.

Evidence for the Dominance of Carrier-Induced Band Gap Renormalization over Biexciton Formation in Cryogenic Ultrafast Experiments on MoS2 Monolayers.

Transition-metal dichalcogenides (TMDs) such as MoS2 display promising electrical and optical properties in the monolayer limit. Due to strong quantum confinement, TMDs provide an ideal environment for exploring excitonic physics using ultrafast spectroscopy. However, the interplay between collective excitation effects on single excitons such as band gap renormalization/exciton binding energy (BGR/EBE) change and multiexciton effects such biexciton formation remains poorly understood. Using two-dimensional electronic spectroscopy, we observe the dominance of single-exciton BGR/EBE signals over optically induced biexciton formation. We make this determination based on a lack of strong PIA features at T = 0 fs in the cryogenic spectra. By means of nodal line slope analysis, we determine that spectral diffusion occurs faster than BGR/EBE change, indicative of distinct processes. These results indicate that at higher sub-Mott limit fluences, collective effects on single excitons dominate biexciton formation.

Coherent and incoherent coupling dynamics in a two-dimensional atomic crystal embedded in a plasmon-induced magnetic resonator

Strong light-matter interaction realized by an individual nanoparticle with large coupling energy has attracted much interest due to its great importance in both fundamental quantum science research and potential applications in quantum information devices. We theoretically investigate the strong coupling between a plasmon-induced magnetic resonance supported by a single nanorod and excitons in a two-dimensional atomic crystal. We demonstrate a record of Rabi splitting over $220\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$ at ambient condition with the involvement of only a few excitons. Importantly, a full quantum model is established to quantitatively describe the coherent and incoherent coupling dynamics of the hybrid modes. We reveal that the extra repopulation/depopulation of high-energy/low-energy polariton states induced by the incoherent coupling process is the microscopic origin of the subradiance/superradiance of the hybrid modes. In particular, large incoherent coupling strength can lead to not only the drastic population accumulation, but also the great enhancement of the scattering intensity in a high-energy polariton state, which is expected to hold promise for realizing polariton lasing without population inversion, as well as for other quantum optics applications.

The g-factor anisotropy of trapped excitons in CH&lt;sub&gt;3&lt;/sub&gt;NH&lt;sub&gt;3&lt;/sub&gt;PbBr&lt;sub&gt;3&lt;/sub&gt; perovskite

Hybrid organic-inorganic perovskites show large potential applications in solar cells, light emitting diodes and low threshold lasers because of the high tolerance of defects compared with other semiconductor materials. Normally they have been synthesized by dilution method, generating a device with high performance, but they also introduce lots of defects. So far, investigations have been done intensively on ensemble defects both in theory and experiment, but single-defect related trapped excitons are yet to be explored. In this work, we prepared high-quality CH&lt;sub&gt;3&lt;/sub&gt;NH&lt;sub&gt;3&lt;/sub&gt;PbBr&lt;sub&gt;3&lt;/sub&gt; perovskite nanowires with the length of about 1 μm and the width of several hundred nanometers by “reverse” ligand assisted reprecipitation method, and performed the magneto-photoluminescence measurement of different trapped excitons in single perovskite nanowires at a low temperature with a standard confocal microscopic system. The photoluminescence (PL) peak with narrow linewidth has been observed from trapped excitons with high luminescence intensity and the trapped excitons can be coupled with phonons in different ways. Both Zeeman splittings and diamagnetic effects have been observed in single trapped excitons under the magnetic field, and we found that the different trapped excitons have different Zeeman splittings and diamagnetic effects which is caused by the different defects near the trapped excitons. At the same time, we have extracted the g-factor of the trapped excitons under different magnetic field angles. The extracted exciton g-factors show anisotropic, which can be ascribed to the limitation of the lattice structure of the perovskite and the trapped exciton wave-function anisotropy under a vector magnetic field. Our results demonstrate that trapped excitons with narrow linewidth have very good luminescence properties and studying the magneto-optical properties from single trapped excitons can provide a deep understanding of trapped excitons in perovskites for applications in quantum light sources and spintronics. Furthermore, our results can also provide a possibility to control the electron spin in single-trapped-excitons-based hybrid organic-inorganic perovskites by manipulating the g-factor through an applied vector magnetic field, which promotes the application of the perovskite-based spintronics.

Strain-induced control of a pillar cavity-GaAs single quantum dot photon source

Herein, we present the calculated strain-induced control of single GaAs/AlGaAs quantum dots (QDs) integrated into semiconductor micropillar cavities. We show precise energy control of individual single GaAs QD excitons under multi-modal stress fields of tailored micropillar optomechanical resonators. Further, using a three-dimensional envelope-function model, we evaluated the quantum mechanical correction in the QD band structures depending on their geometrical shape asymmetries and, more interestingly, on the practical degree of Al interdiffusion. Our theoretical calculations provide the practical quantum error margins, obtained by evaluating Al-interdiffused QDs that were engineered through a front-edge droplet epitaxy technique, for tuning engineered QD single-photon sources, facilitating a scalable on-chip integration of QD entangled photons.

Nonlinear Photocurrent Spectroscopy of Layered 2D Perovskite Quantum Wells.

Two-dimensional coherent photocurrent spectroscopies directly probe the electronic states and processes that are relevant to the performance of a photovoltaic device. In this Letter, we apply two-pulse nonlinear photocurrent spectroscopy to a photovoltaic device based on layered perovskite quantum wells. The method effectively decomposes the photovoltaic response into contributions from separate quantum wells and excited-state species (i.e., either single excitons or biexcitons). Our experiments show that the efficiency of photocurrent generation increases with the size of the quantum well. Overall, the results suggest that energy funneling processes in layered perovskites, which are most prominent in transient absorption spectroscopies, are largely irrelevant to the function of a photovoltaic cell.

Square disks‐based crossed architectures of SnO2 for ethanol gas sensing applications—An experimental and theoretical investigation

A facile synthesis and characterization of square disks‐based crossed architectures of SnO2 are reported in this paper. The prepared crossed architectures of SnO2 were analyzed by different techniques to study their structural, morphological, crystalline, optical, vibrational and scattering properties. From FESEM analysis an average length of the side and width of the square disks shaped structures were ∼ 4–5 μm and 1.0 ± 0.2 μm, respectively. Pure tetragonal rutile phase and crystallite size of 38.26 nm were confirmed by XRD studies. A single well-defined exciton peak at 373 nm corresponding to the energy band gap of 3.33 eV was observed from UV–vis spectrum. SnO2 square disks‐based crossed architecture based gas sensor showed gas responses of 70.5, 107.5 and 110.9 at operating temperatures of 350, 400 and 450 °C, respectively. The response time of 2 s and recovery time of 2018 s, 1081 s, and 708 s were observed for ethanol gas at 350, 400 and 450 °C, respectively. Herein, the interaction between ethanol and SnO2 is simulated by Density functional theory (DFT) calculations. B3LYP/LanL2dz level of theory was employed in this study. It was found that the sensitivity of SnO2 sensor to ethanol molecule may be attributed to the depositing of an oxygen atom from the media to the SnO2 surface.