Excitonic Optical Transitions Research Articles

Diameter-Dependent Excitation Energy Transfer for Enhanced Semiconducting Single-Walled Carbon Nanotube Solar Photoconversion

Tremendous progress and innovation has ushered the incorporation of semiconducting single-walled carbon nanotubes (s-SWCNTs) as active components into organic/inorganic/hybrid solar cells and thermoelectrics. For solar photoconversion, strong and tunable absorption is necessitated, suggesting that s-SWCNTs can play a role as light-harvesting components, however the narrow and distinct excitonic optical transitions found in s-SWCNT systems limit broad-band absorption. As such, we introduce encapsulation of small molecules within the endohedral volume of s-SWCNTs as a route to increase and extend light absorption in regions outside s-SWCNT excitonic transitions. Creation of this self-assembled supra-molecular system can strongly modify the opto-electronic properties of both the s-SWCNT and encapsulated molecule. We investigate excitation energy transfer occurring from encapsulated dye molecules to s-SWCNTs, which results in luminescent SWCNT excitons. We exploit the high-throughput and chiral selectivity of polyfluorene wrapping methods to generate dispersions of s-SWCNTs with dye encapsulation. We demonstrate that the selectivity of the polyfluorene polymer for semiconducting chiral distributions is not affected by the presence of encapsulated molecules. Tracking the excited state transient absorption signatures of excitation energy transfer provides detailed information regarding dynamics of both the encapsulated dye and s-SWCNTs. We observe sub-picosecond excitation energy transfer of excitons generated in dye molecules to the surrounding s-SWCNTs. Photoluminescence excitation maps reveal that the absorption of encapsulated dye molecules depends sensitively on the diameter of the s-SWCNT in which they are encapsulated. We consider a simple molecular exciton model to describe aggregate formation within the s-SWCNT endohedral volume, which reveals that the dye molecules adopt unique aggregate structures that are defined both by a competition between intermolecular interactions between the dye molecules and confinement effects due to the s-SWCNT diameter. Small molecule encapsulation in s-SWCNTs serves as a strong platform to control intermolecular interactions in small molecule systems through confinement, which opens up the possibility of enhancing photo-induced extraction of energy and charge in s-SWCNTs solar photoconversion systems.

Optical properties of lithium gallium oxide

The optical dielectric function tensor of orthorhombic single-crystal LiGaO2 was determined for polarizations along a, b, and c crystal-axis in the photon energy range from 0.04eV to 6.5eV by the generalized spectroscopic ellipsometry. In the far-infrared spectral range from 12.4meV to 40meV, the dielectric function was determined from conventional polarized transmittance and polarized reflectance measurements. Lineshape analysis of the dielectric function tensor major components allowed for a determination of the long-wavelength optical phonon characteristics, refractive indices dispersion, and parameters of interband and excitonic optical transitions.

Excitonic optical transitions characterized by Raman excitation profiles in single-walled carbon nanotubes

We examine the excitonic nature of the ${E}_{33}$ optical transition of the individual free-standing index-identified $(23,7)$ single-walled carbon nanotube by means of the measurements of its radial-breathing-mode and $G$-mode Raman excitation profiles. We confirm that it is impossible to determine unambiguously the nature of its ${E}_{33}$ optical transition (excitonic vs band to band) based only on the excitation profiles. Nevertheless, by combining Raman scattering, Rayleigh scattering, and optical absorption measurements on strictly the same individual $(23,7)$ single-walled carbon nanotube, we show that the absorption, Rayleigh spectra, and Raman excitation profiles of the longitudinal and transverse $G$ modes are best fitted by considering the nature of the ${E}_{33}$ transition as excitonic. The fit of the three sets of data gives close values of the transition energy ${E}_{33}$ and damping parameter ${\mathrm{\ensuremath{\Gamma}}}_{33}$. This comparison shows that the fit of the Raman excitation profiles provides with good accuracy the energy and damping parameter of the excitonic optical transitions in single-walled carbon nanotubes.

Theory of intervalley Coulomb interactions in monolayer transition-metal dichalcogenides

Exciton optical transitions in transition-metal dichalcogenides offer unique opportunities to study rich many-body physics. Recent experiments in monolayer WSe$_2$ and WS$_2$ have shown that while the low-temperature photoluminescence from neutral excitons and three-body complexes is suppressed in the presence of elevated electron densities or strong photoexcitation, new dominant peaks emerge in the low-energy side of the spectrum. I present a theory that elucidates the nature of these optical transitions showing the role of the intervalley Coulomb interaction. After deriving a compact dynamical form for the Coulomb potential, I calculate the self-energy of electrons due to their interaction with this potential. For electrons in the upper valleys of the spin-split conduction band, the self energy includes a moderate redshift due to exchange, and most importantly, a correlation-induced virtual state in the band-gap. The latter sheds light on the origin of the luminescence in monolayer WSe$_2$ and WS$_2$ in the presence of pronounced many-body interactions.

(Invited) Photocurrent Spectroscopy of Exciton and Free Particle Optical Transitions in Suspended Carbon Nanotube Pn-Junctions

We study photocurrent generation in individual, suspended carbon nanotube pn-junction diodes formed by electrostatic doping using two gate electrodes. Photocurrent spectra collected under various electrostatic doping concentrations reveal distinctive behaviors for free particle optical transitions and excitonic transitions. In semiconducting nanotubes, the photocurrent generated by excitonic transitions exhibits a strong gate doping dependence, while that of the free particle transitions is gate independent.1 Here, the built-in potential of the pn-junction is required to separate the strongly bound electron-hole pairs of the excitons, while free particle excitations do not require this field assisted charge separation. We observe a sharp, well defined E11 free particle interband transition in contrast with previous photocurrent studies. Several steps are taken to ensure that the active charge separating region of these pn-junctions is suspended off the substrate in a suspended region that is substantially longer than the exciton diffusion length and, therefore, the photocurrent does not originate from a Schottky junction. We present a detailed model of the built-in fields in these pn-junctions, which, together with phonon-assistant exciton dissociation, predicts photocurrents on the same order of those observed experimentally. In addition, we investigate the competing photocurrent mechanisms in quasi-metallic nanotubes over a wide range of bandgaps (30–180 meV).2 Here, we correlate homogenous pn-doping dependence ( Vg1 = – Vg2 ) of the photocurrent with the current–voltage ( I–Vg ) characteristics (Vg1 = Vg2 ) of each device. These I–Vg characteristics, in turn, are used to determine the bandgap of the nanotubes by fitting our data to a Landauer transport model. The mechanism underlying photocurrent generation is then established by correlating the photocurrent transport with the calculated thermoelectric power, the measured conductance, and the extracted bandgap for all devices. 1. Chang, S.W., J. Theiss, J. Hazra, M. Aykol, R. Kapadia and S.B. Cronin, Photocurrent spectroscopy of exciton and free particle optical transitions in suspended carbon nanotube pn-junctions. Applied Physics Letters, 107, 053107 (2015). 2. Amer, M.R., S.W. Chang and S.B. Cronin, Competing Photocurrent Mechanisms in Quasi-Metallic Carbon Nanotube pn Devices. Small, 11, 3119-3123 (2015).

Photocurrent spectroscopy of exciton and free particle optical transitions in suspended carbon nanotube pn-junctions

We study photocurrent generation in individual, suspended carbon nanotube pn-junction diodes formed by electrostatic doping using two gate electrodes. Photocurrent spectra collected under various electrostatic doping concentrations reveal distinctive behaviors for free particle optical transitions and excitonic transitions. In particular, the photocurrent generated by excitonic transitions exhibits a strong gate doping dependence, while that of the free particle transitions is gate independent. Here, the built-in potential of the pn-junction is required to separate the strongly bound electron-hole pairs of the excitons, while free particle excitations do not require this field-assisted charge separation. We observe a sharp, well defined E11 free particle interband transition in contrast with previous photocurrent studies. Several steps are taken to ensure that the active charge separating region of these pn-junctions is suspended off the substrate in a suspended region that is substantially longer than the exciton diffusion length and, therefore, the photocurrent does not originate from a Schottky junction. We present a detailed model of the built-in fields in these pn-junctions, which, together with phonon-assistant exciton dissociation, predicts photocurrents on the same order of those observed experimentally.

Analytical method for determining quantum well exciton properties in a magnetic field

We develop an analytical approximate method for determining the Bohr radii of Wannier-Mott excitons in thin quantum wells under the influence of magnetic field perpendicular to the quantum well plane. Our hybrid variational-perturbative method allows us to obtain simple closed formulas for exciton binding energies and optical transition rates. We confirm the reliability of our method through exciton-polariton experiments realized in a GaAs/AlAs microcavity with an 8 nm In-x Ga1-xAs quantum well and magnetic field strengths as high as 14 T.

Tuning of exciton states in a magnetic quantum ring

The exciton states in a CdTe quantum ring subjected to an external magnetic field containing a single magnetic impurity are investigated. We have used the multiband approximation which includes the heavy hole–light hole coupling effects. The electron–hole spin interactions and the s, p–d interactions between the electron, the hole and the magnetic impurity are also included. The exciton energy levels and optical transitions are evaluated using the exact diagonalization scheme. We show that due to the spin interactions it is possible to change the bright exciton state into the dark state and vice versa with the help of a magnetic field. We propose a new route to experimentally estimate the s, p–d spin interaction constants.

In situ intercalation dynamics in inorganic-organic layered perovskite thin films.

The properties of layered inorganic semiconductors can be manipulated by the insertion of foreign molecular species via a process known as intercalation. In the present study, we investigate the phenomenon of organic moiety (R-NH3I) intercalation in layered metal-halide (PbI2)-based inorganic semiconductors, leading to the formation of inorganic–organic (IO) perovskites [(R-NH3)2PbI4]. During this intercalation strong resonant exciton optical transitions are created, enabling study of the dynamics of this process. Simultaneous in situ photoluminescence (PL) and transmission measurements are used to track the structural and exciton evolution. On the basis of the experimental observations, a model is proposed which explains the process of IO perovskite formation during intercalation of the organic moiety through the inorganic semiconductor layers. The interplay between precursor film thickness and organic solution concentration/solvent highlights the role of van der Waals interactions between the layers, as well as the need for maintaining stoichiometry during intercalation. Nucleation and growth occurring during intercalation matches a Johnson–Mehl–Avrami–Kolmogorov model, with results fitting both ideal and nonideal cases.

Coherent Exciton Delocalization in Strongly Coupled Quantum Dot Arrays

Quantum dots (QDs) coupled into disordered arrays have exhibited the intriguing property of bulk-like transport while maintaining discrete excitonic optical transitions. We have utilized ultrafast cross-polarized transient grating (CPTG) spectroscopy to measure electron-hole wave function overlap in CdSe QD films with chemically modified surfaces for tuning inter-QD electronic coupling. By comparing the CPTG decays with those of isolated QDs, we find that excitons coherently delocalize to form excited states more than 200% larger than the QD diameter.

Exciton optical transitions in quantum wells with spin-orbit coupling

The energy spectrum of light-hole and heavy-hole excitons and optical absorption in a quantum well have been analyzed taking into account Rashba spin-orbit coupling. Interband and intraband exciton transitions have been considered. It has been shown that, in the presence of spin-orbit coupling, the probabilities of the interband and intraband photoelectric effects diverge in the vicinity of the threshold if the electron-hole interaction is neglected. The threshold probabilities of the interband and intraband photoelectric effects become finite when Coulomb interaction is taken into account.

Charge impurity as a localization center for singlet excitons in single-wall nanotubes

A simple model is developed for studying the interaction of bright excitons in semiconducting single-wall nanotubes with charged impurities. The model reveals redshift in the energy of excitonic states in the presence of impurity, thus indicating binding of free excitons in the impurity potential well. Several bound states were found in absorption spectrum below the onset of excitonic optical transitions in the bare nanotube. Dependence of the binding energy on the model parameters, such as impurity charge and position, was determined and analytical fits were derived for a number of tubes of different diameter. The nanotube family splitting is seen in the diameter dependence, gradually decreasing with the diameter. By calculating the partial absorption coefficient for a small segment of nanotube, the local nature of the wave function of the bound states was derived.

Hydrostatic pressure dependence of excitonic optical transitions in strained wurtzite GaN/AlN quantum disks

In the framework of the effective mass approximation, the effects of hydrostatic pressure on optical transitions associated with the excitons confined in strained wurtzite (WZ) GaN/AlN quantum disks (QDisks) with the confinement potential of finite depth are investigated by using a variational technique, with considering the influences of the built-in electric field (BEF) and the biaxial strain dependence of material parameters. The Schrödinger equation via the proper choice of the exciton trial wave function is solved. The behaviors of the excitonic optical transition are examined at different pressures for different QDisk sizes. In our calculations, the effective masses of electron and hole, dielectric constants, phonon frequencies, energy gaps, and piezoelectric polarizations are taken into account as functions of biaxial strain and hydrostatic pressure. Numerical results show that the hydrostatic pressure and the QDisk size have a remarkable influence on exciton states. The calculated pressure coefficient of optical transition energy shows a negative value if the QDisk height L>3.2nm, in contrast with the positive pressure coefficient of the GaN band gap. The peculiar pressure behavior is related to the pressure-induced increase of the built-in electric field. For a fixed pressure, the optical transition energy has a red-shift if the QDisk height and radius increase and QDisk height has a more obvious influence on Eph than QDisk radius. Furthermore, the relationship between the radiative decay time and hydrostatic pressure (QDisk height) is also investigated. It is found that the radiative decay time increases with pressure and the increment tendency is more prominent for the large height QDisks. The radiative decay time strongly increases by three orders of magnitude reaching microsecond order if the QDisk height increases from 1nm to 3nm.

Exciton optical transitions in a hexagonal boron nitride single crystal

AbstractNear band gap photoluminescence (PL) of a hexagonal boron nitride single crystal has been studied at cryogenic temperatures with synchrotron radiation excitation. The PL signal is dominated by trapped‐exciton optical transitions, while the photoluminescence excitation (PLE) spectra show features assigned to free excitons. Complementary photoconductivity and PLE measurements set the band gap transition energy to 6.4 eV and the Frenkel exciton binding energy larger than 380 meV. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Exciton states and optical transitions in InGaN/GaN quantum dot nanowire heterostructures: Strong built-in electric field and dielectric mismatch effects

Considering the strong built-in electric field (BEF), dielectric-constant mismatch and 3D confinement of the electron and hole, the exciton states and interband optical transitions in [0 0 0 1]-oriented Ga-rich wurtzite In x Ga 1− x N/GaN strained quantum dot (QD) nanowire heterostructures are investigated theoretically using a variational approach under the effective mass approximation. We find that the strong BEF gives rise to an obvious reduction of the effective band gap of QDs and leads to a remarkable electron–hole spatial separation. The BEF, QD height and radius, and dielectric mismatch effects have a significant influence on exciton binding energy, electron interband optical transitions, and the radiative decay time. Our calculations show that the radiative decay time of the redshifted transitions is large and increases almost exponentially when the QD height increases, which is in good agreement with the previous experimental and theoretical results.

Optical Properties of Single-Walled Carbon Nanotubes Separated in a Density Gradient: Length, Bundling, and Aromatic Stacking Effects

Single-walled carbon nanotubes (SWNTs) are promising materials for in vitro and in vivo biological applications due to their high surface area and inherent near infrared photoluminescence and Raman scattering properties. Here, we use density gradient centrifugation to separate SWNTs by length and degree of bundling. Following separation, we observe a peak in photoluminescence quantum yield (PL QY) and Raman scattering intensity where SWNT length is maximized and bundling is minimized. Individualized SWNTs are found to exhibit high PL QY and high resonance-enhanced Raman scattering intensity. Fractions containing long, individual SWNTs exhibit the highest PL QY and Raman scattering intensities, compared to fractions containing single, short SWNTs or SWNT bundles. Intensity gains of approximately ~1.7 and 4-fold, respectively, are obtained compared with the starting material. Spectroscopic analysis reveals that SWNT fractions at higher displacement contain increasing proportions of SWNT bundles, which causes reduced optical transition energies and broadening of absorption features in the UV-Vis-NIR spectra, and reduced PL QY and Raman scattering intensity. Finally, we adsorb small aromatic species on "bright," individualized SWNT sidewalls and compare the resulting absorption, PL and Raman scattering effects to that of SWNT bundles. We observe similar effects in both cases, suggesting aromatic stacking affects the optical properties of SWNTs in an analogous way to SWNT bundles, likely due to electronic structure perturbations, charge transfer, and dielectric screening effects, resulting in reduction of the excitonic optical transition energies and exciton lifetimes.

Dielectric constant model for environmental effects on the exciton energies of single wall carbon nanotubes

The excitonic optical transition energies Eii of single wall carbon nanotubes, that are modified by surrounding materials around the tubes (known as the environmental effect), can be reproduced by defining a dielectric constant κ which depends on the subband index, nanotube diameter, and exciton size. The environmental effects on excitons can be recognized on a plot of the functional form of κ simply by the different linear slopes obtained for different samples. This treatment should be very useful for calculating Eii for any type of nanotube environment, hence providing an accurate assignment of many nanotube (n,m) chiralities.

Spin interactions in a quantum dot containing a magnetic impurity

The electron and hole states in a CdTe quantum dot containing a single magnetic impurityin an external magnetic field are investigated using a multiband approximation whichincludes the heavy hole–light hole coupling effects. The electron–hole spin interactions andsp–d interactions between the electron, the hole and the magnetic impurity are alsoincluded. The exciton energy levels and optical transitions are evaluated using the exactdiagonalization scheme. A novel mechanism is proposed here to manipulate impurity spinin the quantum dot which allows us to drive selectively the spin of the magnetic atom intoeach of its six possible orientations.

Study of temperature and indium concentration-dependent dielectric constant and electron affinity effects on the exciton optical transition and binding energy in spherical [formula omitted] quantum dots

We have study the heavy-hole exciton states in GaSb–GaInAsSb–GaSb type-I spherical Quantum Dots, using temperature-dependent static dielectric constant and electron affinity, with a finite height potential barrier, as a function of the quantum dot radius for several values of Indium concentration. Our calculations have been worked out using interpolating methods to find the temperature and Indium concentration dependence of both the dielectric constant and electron affinity, in order to determine the conduction and valence band-offsets in GaSb–GaInAsSb–GaSb heterostructure by application of the Electron Affinity Rule. We have calculated the exciton binding energy and the corresponding transition energy from the exciton ground state to the heavy-hole level, using a variational procedure within the effective-mass approximation. We have found that the binding energy of the heavy-hole exciton presents changes due to the temperature dependence of the electron affinity and static dielectric constant. However our results for the transition energy from the exciton ground state to the heavy-hole level coincide with those reported in a previous theoretical work, where we had found a very good agreement with photoluminescence and photoreflectance experimental studies at T=12 K in Ga 1 − x In x As y Sb 1 − y films grown over GaSb substrates by liquid phase epitaxy.

Polarization memory in single quantum dots

We measured the polarization memory of excitonic and biexcitonic optical transitions from single quantum dots at either positive, negative or neutral charge states. Positive, negative and no circular or linear polarization memory was observed for various spectral lines, under the same quasi-resonant excitation below the wetting layer bandgap. We developed a model which explains both qualitatively and quantitatively the experimentally measured polarization spectrum for all these optical transitions. We consider quite generally the loss of spin orientation of the photogenerated electron–hole pair during their relaxation towards the many-carrier ground states. Our analysis unambiguously demonstrates that while electrons maintain their initial spin polarization to a large degree, holes completely dephase.