Excitonic Transition Energies Research Articles

Isolation of Single Donors in ZnO.

The shallow donor in zinc oxide (ZnO) is a promising semiconductor spin qubit with optical access. Single indium donors are isolated in a commercial ZnO substrate using plasma focused ion beam (PFIB) milling. Quantum emitters are identified optically by spatial and frequency filtering. The indium donor assignment is based on the optical bound exciton transition energy and magnetic dependence. The emission stability of these single donors in terms of both intensity and frequency, alongside their transition linewidths less than twice the lifetime limit, highlight the promise of single In donors as optically accessible spin qubits. The optical stability of single donors after FIB fabrication is promising for optical device integration required for scalable quantum technologies based on single donors in direct band gap semiconductors.

Linear and nonlinear exciton polariton dispersion in MoS2 nanosheets and its nickel doping dependence

In this paper, we study the influence of Ni-doping on A- and B-exciton polariton dispersion in MoS2 nanosheets. In particular, we have calculated the effect of the Ni-concentration on the energy dispersion and therefore we have deduced the effective masses by adjusting the hopping parameter. The E A , E B -exciton transition energies and the hopping parameter are discerned from the PL of Ni-MoS2 nanosheets. It is shown that the Ni changes considerably the bandgap and as a result the electronic band structure, the effective masses, the optical transition, and the hopping term. We found that the increasing concentration of Ni-dopant can modulate and favors the mixture of A- and B-exciton peaks of MoS2 nanosheets. The diverging Ni-concentration of A- and B-exciton polariton dispersion are also discerned and discussed. Finaly, we can predict that these changes caused by the influence of Ni-doping can be used in most applications involving exciton polariton dispersion for photonics and optoelectronics devices.

Demystifying the modulation effects of the graphene transport layers on the interlayer charge transfer mechanism and broadband optical properties of MoS2/graphene/WSe2 Van der Waals heterojunctions

2D materials and heterojunctions have extraordinary potential in the field of next-generation integrated photodetectors. Recently, inserting graphene (Gra) into Van der Waals heterojunctions as transport layers has been proven to be an effective method for improving the responsivity and response speed of photodetectors. However, how did the physical mechanisms caused by the insertion (interlayer coupling and electron transfer) regulate its optical properties are unclear. Meanwhile, whether the insertion will modulate the broadband optical properties of the heterojunction should also be carefully verified. In this work, the broadband (1.25–6.50 eV) electronic band structures and exciton properties of MoS2/WSe2 and MoS2/Gra/WSe2 were studied using spectroscopic ellipsometry, meanwhile Raman and PL spectroscopy were used to assist in analysis. We found that graphene insertion not only promote the spontaneous charge transfer from WSe2 to MoS2, but also effectively inject electrons into the MoS2 layer, which benefits the interlayer charge separation and net charge accumulation in MoS2. Moreover, the almost stable CP energies near the band edge represent that graphene insertion does not change the electronic band structures of MoS2/WSe2. Furthermore, due to the increasing effective dielectric screening induced by graphene insertion, the exciton binding energies redshift and the exciton transition energy blueshift consequently.

A-Site Cation Influence on the Structural and Optical Evolution of Ultrathin Lead Halide Perovskite Nanoplatelets.

Imposing quantum confinement has the potential to significantly modulate both the structural and optical parameters of interest in many material systems. In this work, we investigate strongly confined ultrathin perovskite nanoplatelets APbBr3. We compare the all-inorganic and hybrid compositions with the A-sites cesium and formamidinium, respectively. Compared to each other and their bulk counterparts, the materials show significant differences in variable-temperature structural and optical evolution. We quantify and correlate structural asymmetry with the excitonic transition energy, spectral purity, and emission rate. Negative thermal expansion, structural and photoluminescence asymmetry, photoluminescence full width at half-maximum, and splitting between bright and dark excitonic levels are found to be reduced in the hybrid composition. This work provides composition- and structure-based mechanisms for engineering of the excitons in these materials.

Excitonic feature in CsAg2I3 crystals prepared by Bridgman method

This study investigated the optical spectra of the CsAg2I3 crystals at low temperatures. In the reflection spectrum, remarkable reflection peaks owing to band-edge exciton transitions were observed at approximately 3.8 eV above the fundamental absorption edge at 3.6 eV. Under excitation in the energy region of exciton transitions, an intense luminescence band attributed to a self-trapped exciton (STE) was observed at 3.37 eV. In addition to STE luminescence, a weak luminescence line was observed at 3.77 eV. Because the value of 3.77 eV is practically equal to the lowest exciton transition energy estimated from the reflection spectrum, the luminescence line at 3.77 eV comes from a free exciton (FE). The intensities of the FE and STE luminescence peaks decrease with increasing temperature. The activation energies of the FE and STE were estimated from the quenching of luminescence intensities. The features of the exciton states in the CsAg2I3 crystals are presented.

(Invited) Exciton Transport in Strained 2D Semiconductors

Being atomically thin, flexible, and exhibiting considerable light emission and ultrafast non-equilibrium dynamics, semiconducting transition metal dichalcogenides (TMDs) have been considered as promising candidates for next-generation optoelectronic devices. The optical and electronic properties of TMDs are governed by a rich landscape of tightly bound excitons, including regular bright excitons, as well as optically inaccessible dark exciton states. Recently, strain engineering of monolayer TMDs has been introduced to tune their optical properties, such as the exciton transition energy, exciton-phonon coupling, or the Stokes shift [1-3].Transport of charge carriers is crucial for nanoelectronics. In conventional materials, electronic transport can be conveniently controlled by external electric fields. However, the tightly bound excitons, being neutral particles, are only weakly affected by electrical fields. We demonstrate that mechanical strain can also be used to manipulate the transport of excitons in TMDs. To this end, we apply homogeneous tensile strain to a WS2 monolayer by bending the substrate [1], which causes a redshift of the X0 exciton photoluminescence. By measuring the spatiotemporal photoluminescence after near-resonant excitation with femtosecond laser pulses, we map the spread of excitons and extract the strain-dependent diffusion coefficient [4].Furthermore, we demonstrate the propagation of excitons in an inhomogeneous strain landscape. We create inhomogeneous tensile strain in TMD monolayers by transferring them onto patterned substrates with nanopillars or by a nanoimprint technique [5]. Due to the redshift of the exciton resonances with applied strain, excitons are expected to move towards high-strain regions in an inhomogeneous strain field - the so-called "funneling" effect. We verify this behavior for the "bright" TMD material monolayer MoSe2. In the case of the "dark" monolayer WS2, we observe exactly the opposite effect. Here, the excitons are expelled from the high-strain regions ("anti-funneling") [6]. By comparing our experimental results with a microscopic theory, we explain this observation by the drift of momentum-dark KΛ excitons, which, in contrast to bright excitons, shift to higher energies with strain.Our joint experiment-theory study highlights the dominant role of momentum-dark excitons for the dynamics in monolayer TMDs and provides crucial design guidelines for TMD devices based on exciton transport.

Recombination dynamics and pronounced re-absorption effect in ZnO single crystals under two-photon excitation

Time-resolved photoluminescence measurements were conducted on ZnO single crystals using two-photon (2P) excitation at low temperatures ranging from 25 to 260 K. The decay constants of various emission peaks are determined and compared between one-photon and two-photon excitations using a recently modified localized-state ensemble model. The analysis is focused on the redshift of the phonon-assisted free exciton transition energy with increasing temperature. A significantly longer radiative recombination lifetime of approximately 840 ps is observed under 2P excitation. The extended lifetime of excitons during 2P excitation is achieved through the interplay of multiple mechanisms. Photon-recycling, enabled by high absorption coefficients and total internal reflection, facilitates re-absorption and new carrier generation. Phonon scattering lowers photon energy, allowing their escape from the crystal, while exciton–polariton interactions delay photon travel, collectively contributing to the extension of exciton lifetimes. The re-absorption effect and elongated carrier lifetime observed from our work could be beneficial for applications of ZnO materials in 2P imaging, particularly of biological samples.

Optical properties of AgxCu1–xI alloy thin films

We report on the excitonic transition energy E0 and spin–orbit split-off energy Δ0 of γ-AgxCu1–xI alloy thin films studied by using reflectivity measurements at temperatures between 20 K and 290 K. The observed bowing behavior of the E0 transition as a function of the alloy composition is explained based on first-principles band structure calculations in terms of different physical and chemical contributions within the description of ordered alloys. The spin–orbit coupling is found to increase from a value of 640 meV for CuI to approximately 790 meV for AgI. Furthermore, we show that the temperature-dependent bandgap shift between 20 K and 290 K decreases with increasing Ag-content from 25 meV for CuI to 6 meV for AgI. We attribute this behavior mostly to changes in the contribution of thermal lattice expansion to the bandgap shift.

An efficient protocol for excited states of large biochromophores.

Efficient energy transport in photosynthetic antenna is a long-standing source of inspiration for artificial light harvesting materials. However, characterizing the excited states of the constituent chromophores poses a considerable challenge to mainstream quantum chemical and semiempirical excited state methods due to their size and complexity and the accuracy required to describe small but functionally important changes in their properties. In this paper, we explore an alternative approach to calculating the excited states of large biochromophores, exemplified by a specific method for calculating the Qy transition of bacteriochlorophyll a, which we name Chl-xTB. Using a diagonally dominant approximation to the Casida equation and a bespoke parameterization scheme, Chl-xTB can match time-dependent density functional theory's accuracy and semiempirical speed for calculating the potential energy surfaces and absorption spectra of chlorophylls. We demonstrate that Chl-xTB (and other prospective realizations of our protocol) can be integrated into multiscale models, including concurrent excitonic and point-charge embedding frameworks, enabling the analysis of biochromophore networks in a native environment. We exploit this capability to probe the low-frequency spectral densities of excitonic energies and interchromophore interactions in the light harvesting antenna protein LH2 (light harvesting complex 2). The impact of low-frequency protein motion on interchromophore coupling and exciton transport has routinely been ignored due to the prohibitive costs of including it in simulations. Our results provide a more rigorous basis for continued use of this approximation by demonstrating that exciton transition energies are unaffected by low-frequency vibrational coupling to exciton interaction energies.

Tunable lattice dynamics and dielectric functions of two-dimensional Bi2O2Se: striking layer and temperature dependent effects.

Two-dimensional (2D) Bi2O2Se semiconductors with a narrow band gap and ultrahigh mobility have been regarded as an emerging candidate for optoelectronic devices, whereas the ambiguous phonon characteristics and optical properties still limit their future applications. Herein, high-quality centimeter-scale 2D Bi2O2Se films are successfully synthesized to disclose the lattice dynamics and dielectric functions under the control of thickness and temperature. It has been demonstrated that the stronger electrostatic Bi-Se interactions result in a stiffened phonon vibration of thicker Bi2O2Se layers. Three excitons (Ea, Eb, and Ec) exhibit significant red shifts with layer stacking. Interestingly, the dielectric properties in the visible-near infrared region (Ea and Eb) are dominated by the combined effect of the joint density of states and mass density, whereas the dielectric properties in the ultraviolet region (Ec) are dominated by the exciton effect. Furthermore, the temperature-sensitivity of the phonon frequency and exciton transition energies is revealed to be layer-dependent. In particular, the optical response of Eb excitons exhibits a prominent dependence on temperature, which indicates a promising optical modulation by temperature in the visible spectrum. This study enriches the knowledge about phonon dynamics and dielectric properties for 2D Bi2O2Se, which provides an essential reference for high-performance related optoelectronic devices.

Intensive and broad bound exciton emission at cryogenic temperature in suspended monolayer transition metal dichalcogenides

Two-dimensional (2D) transition metal chalcogenides (TMDCs) are expected to play an important role in next-generation optoelectronic devices. However, the properties of 2D TMDCs can be largely affected by defects, and thus, many efforts have been made to characterize and eliminate the defects. Here, we investigate defect-related trap states, particularly in large-area suspended ${\mathrm{WS}}_{2}$ monolayers at cryogenic temperatures, using steady-state and time-resolved photoluminescence (TRPL) spectroscopy. We observed an intensive and broad (full width half maximum $\ensuremath{\sim}170\phantom{\rule{0.28em}{0ex}}\mathrm{meV}$) photoluminescence (PL) emission centered at 1.83 eV, 0.3 eV lower than the free exciton transition energy. From temperature-dependent PL spectra, the thermal activation energy of excitons escaping from trap states is determined to be 30 meV. The TRPL spectrum shows an ultralong lifetime at 13 K. The long-lived emission with the broad PL linewidth is attributed to excitons bound to defects of possibly chalcogenide-site substitution and sulfur vacancies. Compared with the supported monolayer, the defect-related trap states are found to be deeper in the suspended layer and can trap excitons more efficiently due to less dielectric screening. In this letter, we provide further understanding of trap states and decay channels in monolayer TMDCs and show that suspended 2D TMDCs can provide a pure platform for studying defect-related properties.

Effects of external perturbations on light absorption by light/heavy hole excitons in a semi-parabolic quantum well

The effects of electric, magnetic, and intense terahertz laser fields on optoelectronic properties are investigated in a GaAs/AlxGa1-xAs semi-parabolic quantum well. The binding energy, exciton light absorption coefficients, interband transition energies, and dipole elements in the matrix are studied within the structure of the parabolic band and effective-mass approximations. The transition energy difference between the ground-state electron energy level and the ground-state heavy hole (light hole) energy level and matrix elements are carried out as functions of external perturbations with a constant well size. The results exhibit that the exciton binding energies of heavy and light holes reduce as long as the laser dressing parameter is larger. The optical absorption of light hole exciton in the system is stronger than the heavy hole exciton. And the excitonic energies, transition energies, and absorption coefficients are much more sensitive and controlled by changing the external perturbations.

First-principles perspective on full-spectrum infrared photodetectors from doping an excitonic insulator

Innovations in imaging technology involve finding strategies and materials suitable for detection applications over the entire infrared range. Herein, we propose a new design concept based on the unique feature of an excitonic insulator, namely, negative exciton transition energy (${E}_{t}$). We demonstrate this concept using first-principles $GW$-Bethe-Salpeter equation calculations on one-dimensional organometallic wire ${(\mathrm{CrBz})}_{\ensuremath{\infty}}$. The pristine ${(\mathrm{CrBz})}_{\ensuremath{\infty}}$ exhibits an excitonic instability due to a negative ${E}_{t}$ for the lowest exciton. Substitutional doping can continuously tune the ${E}_{t}$ from $\ensuremath{\sim}\phantom{\rule{0.16em}{0ex}}0$ to $\ensuremath{\sim}\phantom{\rule{0.16em}{0ex}}0.6$ eV, which shows the ability of photon detection from terahertz to near infrared. This type of detector has advantages of outstanding wavelength selectivity, reduced thermal disturbance, and elevated working temperature. Our paper not only adds another member in the family of rare one-dimensional excitonic insulators, but also opens a new avenue for the development of high-performance infrared photodetectors in the future.

Exciton binding energy and screening length in two-dimensional semiconductors

We approximately solve the Schr\"odinger equation for two-dimensional Wannier-Mott excitons in the effective-mass approximation and derive an expression for the exciton energy levels, hence the exciton binding energy. The electron-hole interaction is described by the Rytova-Keldysh potential in a logarithmic approximation. From the derived expression, we introduce approximate expressions to experimentally determine the exciton binding energy (along with the quasiparticle band gap), the screening length, and the exciton reduced mass from measured exciton transition energies of the ground state ($1s$), and the first ($2s$) and the second ($3s$) excited states. Our results agree well with experimental data and theoretical calculations.

Robust and high-sensitivity thermal probing at the nanoscale based on resonance Raman ratio (R3)

Raman spectroscopy-based temperature sensing usually tracks the change of Raman wavenumber, linewidth and intensity, and has found very broad applications in characterizing the energy and charge transport in nanomaterials over the last decade. The temperature coefficients of these Raman properties are highly material-dependent, and are subjected to local optical scattering influence. As a result, Raman-based temperature sensing usually suffers quite large uncertainties and has low sensitivity. Here, a novel method based on dual resonance Raman phenomenon is developed to precisely measure the absolute temperature rise of nanomaterial (nm WS2 film in this work) from 170 to 470 K. A 532 nm laser (2.33 eV photon energy) is used to conduct the Raman experiment. Its photon energy is very close to the excitonic transition energy of WS2 at temperatures close to room temperature. A parameter, termed resonance Raman ratio (R3) is introduced to combine the temperature effects on resonance Raman scattering for the A1g and E2g modes. Ω has a change of more than two orders of magnitude from 177 to 477 K, and such change is independent of film thickness and local optical scattering. It is shown that when Ω is varied by 1%, the temperature probing sensitivity is 0.42 K and 1.16 K at low and high temperatures, respectively. Based on Ω, the in-plane thermal conductivity (k) of a ∼25 nm-thick suspended WS2 film is measured using our energy transport state-resolved Raman (ET-Raman). k is found decreasing from 50.0 to 20.0 Wm−1 K−1 when temperature increases from 170 to 470 K. This agrees with previous experimental and theoretical results and the measurement data using our FET-Raman. The R3 technique provides a very robust and high-sensitivity method for temperature probing of nanomaterials and will have broad applications in nanoscale thermal transport characterization, non-destructive evaluation, and manufacturing monitoring.

Optical absorption of interlayer excitons in transition-metal dichalcogenide heterostructures.

Interlayer excitons, electron-hole pairs bound across two monolayer van der Waals semiconductors, offer promising electrical tunability and localizability. Because such excitons display weak electron-hole overlap, most studies have examined only the lowest-energy excitons through photoluminescence. We directly measured the dielectric response of interlayer excitons, which we accessed using their static electric dipole moment. We thereby determined an intrinsic radiative lifetime of 0.40 nanoseconds for the lowest direct-gap interlayer exciton in a tungsten diselenide/molybdenum diselenide heterostructure. We found that differences in electric field and twist angle induced trends in exciton transition strengths and energies, which could be related to wave function overlap, moiré confinement, and atomic reconstruction. Through comparison with photoluminescence spectra, this study identifies a momentum-indirect emission mechanism. Characterization of the absorption is key for applications relying on light-matter interactions.

Determination of optical constants and scattering properties of transparent polymers for use in optoelectronics

Knowledge of optical constants, i.e. refractive index n and extinction coefficient k, and light scattering properties of optical polymers are required to optimize micro-optics for light-emitting diodes in terms of efficiency, color properties and light distribution. We present here a model-based diagnostic approach to determine the optical properties of polymers, which should be particularly useful in the development of plastics for optical applications. Optical constants and scattering coefficients were obtained from transmission and reflection measurements in a wavelength range from UV to NIR taking into account scattering effects due to rough surfaces and volume inhomogeneity. Based on the models for the dielectric function, the molecular optical transition energies Eg, critical point energies, Urbach energies and exciton transition energies were determined. Rayleigh and Mie scattering model and van de Hulst's anomalous diffraction theory were applied to characterize scattering due to volume inhomogeneities. Scalar diffraction theory was applied to account for surface roughness scattering. Atomic force microscopy with nanomechanical characterization was used to characterize domains in size and shape and to assign optical scattering to a suitable morphological model. The combined optical and mechanical characterization help to improve the qualification of new polymer materials for optical applications.

Experimental quantification of the robustness of adiabatic rapid passage for quantum state inversion in semiconductor quantum dots

Adiabatic rapid passage (ARP) is demonstrated in a single In(Ga)As quantum dot (QD) over a wide range of laser tuning relative to the exciton transition energy to assess the level of robustness of this quantum state inversion gate for practical QD systems. Our experiments indicate a drop in exciton inversion by only 5% for a detuning of 9.3 meV, indicating accessible detunings that span the typical inhomogeneous broadening of self-assembled QD ensembles. Our findings indicate that ARP is an ideal control protocol for synchronous triggering of quantum light sources for applications in photonic quantum technology.

Temperature Dependent Excitonic Transition Energy and Enhanced Electron-Phonon Coupling in Layered Ternary SnS2-xSex Semiconductors with Fully Tunable Stoichiometry.

In this study, a series of SnS2-xSex (0 ≤ x ≤ 2) layered semiconductors were grown by the chemical–vapor transport method. The crystal structural and material phase of SnS2-xSex layered van der Waals crystals was characterized by X-ray diffraction measurements and Raman spectroscopy. The temperature dependence of the spectral features in the vicinity of the direct band edge excitonic transitions of the layered SnS2-xSex compounds was measured in the temperature range of 20–300 K using the piezoreflectance (PzR) technique. The near band-edge excitonic transition energies of SnS2-xSex were determined from a detailed line-shape fit of the PzR spectra. The PzR characterization has shown that the excitonic transitions were continuously tunable with the ratio of S and Se. The parameters that describe the temperature variation of the energies of the excitonic transitions are evaluated and discussed.

Exciton energy spectra in polyyne chains

Recently, we have experimentally observed signatures of sharp exciton peaks in the photoluminescence spectra of bundles of monoatomic carbon chains stabilized by gold nanoparticles and deposited on a glass substrate. Here, we estimate the characteristic energies of excitonic transitions in this complex quasi-one-dimensional nano-system with use of the variational method. We show that the characteristic energy scale for the experimentally observed excitonic fine structure is governed by the interplay between the hopping energy in a Van der Waals quasicrystal formed by parallel carbon chains, the neutral-charged exciton splitting and the positive-negative trion splitting. These three characteristic energies are an order of magnitude lower than the direct exciton binding energy.