Exciton Drift Research Articles

Enhanced Exciton Drift Transport through Suppressed Diffusion in One-Dimensional Guides.

Drift-diffusion dynamics is investigated in a one-dimensional (1D) exciton guide at room temperature. Spatial engineering of the exciton energy in a WSe2 monolayer is achieved using local strain to confine and direct exciton transport. An unexpected and massive deviation from the Einstein relation is observed and correlated to exciton capture by defects. We find that the capture reduces exciton temperature and diffusion so much that drift transport visibility improves to 38% as excitons traverse asymmetrically over regions with occupied defect states. Based on measurements over multiple potential gradients, we estimate the exciton mobility to be 169 ± 39 cm2/(eV s) at room temperature. Experiments at elevated exciton densities reveal that the exciton drift velocity monotonically increases with exciton density, unlike exciton mobility, due to contributions from nonequilibrium many-body effects.

Spatiotemporally controlled room-temperature exciton transport under dynamic strain

Two-dimensional transition metal dichalcogenides (TMDs) provide an attractive platform for studying strain dependent exciton transport at room temperature due to large exciton binding energy and strong bandgap sensitivity to mechanical stimuli. Here, we use Rayleigh type surface acoustic wave (SAW) to demonstrate controlled and directional exciton transport under weak coupling regime at room temperature. We screen the in-plane piezoelectric field using photogenerated carriers to study transport under type-I bandgap modulation and measure a maximum exciton drift velocity of 600 m/s. Furthermore, we demonstrate precise steering of exciton flux by controlling the relative phase between the input RF excitation and exciton photogeneration. The results provide important insight into the weak coupling regime between dynamic strain wave and room temperature excitons in a 2D semiconductor system and pave way to exciting applications of excitonic devices in data communication and processing, sensing and energy conversion.

Imaging Seebeck drift of excitons and trions in MoSe2 monolayers

Hyperspectral imaging at cryogenic temperatures is used to investigate exciton and trion propagation in MoSe2 monolayers (ML) encapsulated with hexagonal boron nitride (hBN). Under a tightly focused, continuous-wave laser excitation, the spatial distribution of neutral excitons and charged trions strongly differ at high excitation densities. Remarkably, in this regime the trion distribution develops a halo shape, similar to that previously observed in WS2 ML at room temperature and under pulsed excitation. In contrast, the exciton distribution only presents a moderate broadening attributed to a much less pronounced halo. Spatially and spectrally resolved luminescence spectra reveal the buildup of a significant temperature gradient at high excitation power, that is attributed to the energy relaxation of photoinduced hot carriers. We show, via a numerical resolution of the transport equations for excitons and trions, that the halo can be interpreted as thermal drift of trions due to a Seebeck term in the particle current. The model shows that the difference between trion and exciton profiles is simply understood in terms of the very different lifetimes of these two quasiparticles.

Nonlinear exciton drift in piezoelectric two-dimensional materials

The noncentrosymmetric nature of single-layer (SL) transition-metal dichalcogenides (TMD) manifests itself in the finite piezoelectricity and valley-Zeeman coupling. We microscopically model nonlinear exciton transport in nanobubbles of SL TMDs. Thanks to the giant piezoelectric effect, we obtain an enormous internal electric field, ${E}_{\mathrm{piezo}}\ensuremath{\sim}{10}^{7}\phantom{\rule{0.16em}{0ex}}\mathrm{V}/\mathrm{m}$, resulting in a built-in dipole moment of excitons. We demonstrate that the piezo-induced dipole-dipole interaction provides a novel channel for the nonlinear exciton transport distinct from the conventional isotropic funneling of excitons and leads to the formation of a hexagon-shaped exciton droplet on the top of circularly symmetric nanobubbles. Moreover, we found that the hexagonal distribution of exciton density is preserved even for strongly elliptic nanobubbles. The effect is tunable via the bubble-size dependence of the piezoelectric field ${E}_{\mathrm{piezo}}\ensuremath{\sim}{h}_{\mathrm{max}}^{2}/{R}^{3}$ with ${h}_{\mathrm{max}}$ and $R$ being the bubble height and radius, respectively.

Switchable, Tunable, and Directable Exciton Funneling in Periodically Wrinkled WS2.

The extreme elastic strain of monolayer transition metal dichalcogenides provides an ideal platform to achieve efficient exciton funneling via local strain modulation; however, studies conducted thus far have focused on the use of substrates with fixed strain profiles. We prepared 1L-WS2 on a flexible substrate such that the formation of topographic wrinkles could be switched on or off, and the depth or the direction of the wrinkle could be modified by external strain, thereby providing full control of the periodic undulation of the band gap profile of 1L-WS2 in the range 0-57 meV. Nanoscale photoluminescence (PL) imaging unambiguously evinced that the photoexcited excitons of 1L-WS2 were accumulated at the top regions of the wrinkles with less band gap than the valley region. Our results of broad tunability of the two-dimensional (2D) exciton funneling suggest a promising route to control exciton drift for enhanced optoelectronic performances and future 2D exciton devices.

NURBS-based microstructure design for organic photovoltaics

The microstructure – spatial distribution of electron donor and acceptor domains – plays an important role in determining the photo current in thin film organic solar cells (OSCs). Optimizing the microstructure can lead to higher photo current generation, and is an active area of experimental research. There has been recent progress in framing OSC microstructure design as a computational design problem. However, most current approaches to microstructure optimization are based on volumetric distribution of material, which makes the design space very large. In contrast, we frame the microstructure design optimization problem in terms of designing the interface between the donor and acceptor regions, and thus pose it as a surface representation and optimization problem. This results in substantially reduced number of design variables, thus enabling use of standard optimization tools. In this work, we address the efficient design of OSC microstructure by using surface and curve modeling techniques to model the donor–acceptor interface, and use meta-heuristic, gradient-free optimization techniques to optimize the microstructure for maximum short circuit current generation. Our modeling framework consists of three major components: (1) geometric modeling of OSC microstructure that uses Non-Uniform Rational B-spline (NURBS) curves and surfaces to construct the free-form donor–acceptor interface, (2) photo-current generation modeling that uses a parallel, finite-element based exciton–drift–diffusion (XDD) model, and (3) optimization that utilizes genetic algorithms (GA) to optimize the OSCs microstructure via exploration of the NURBS representation. We apply these methods for the optimization of both 2D and 3D microstructures. Results show substantial improvement in current density compared to the bulk-heterojunction microstructures. These results provide promising microstructures for experimental groups to fabricate. The proposed surface representation approach seems to be a promising approach for interface design in engineered systems.

Derivation and solution of effective medium equations for bulk heterojunction organic solar cells

A drift-diffusion model for charge transport in an organic bulk heterojunction solar cell, formed by conjoined acceptor and donor materials sandwiched between two electrodes, is formulated. The model accounts for (i) bulk photogeneration of excitons, (ii) exciton drift and recombination, (iii) exciton dissociation (into polarons) on the acceptor–donor interface, (iv) polaron recombination, (v) polaron dissociation into a free electron (in the acceptor) and a hole (in the donor), (vi) electron/hole transport and (vii) electron–hole recombination on the acceptor–donor interface. A finite element method is employed to solve the model in a cell with a highly convoluted acceptor/donor interface. The solutions show that, with physically realistic parameters, and in the power generating regime, the solution varies little on the scale of the micro-structure. This motivates us to homogenise over the micro-structure; a process that yields a far simpler one-dimensional effective medium model on the cell scale. The comparison between the solution of the full model and the effective medium (homogenised) model is very favourable for applied voltages less than the built-in voltage (the power generating regime) but breaks down as the applied voltages increases above it. Furthermore, it is noted that the homogenisation technique provides a systematic way to relate effective medium modelling of bulk heterojunctions [19, 25, 36, 37, 42, 59] to a more fundamental approach that explicitly models the full micro-structure [8, 38, 39, 58] and that it allows the parameters in the effective medium model to be derived in terms of the geometry of the micro-structure. Finally, the effective medium model is used to investigate the effects of modifying the micro-structure geometry, of a device with an interdigitated acceptor/donor interface, on its current–voltage curve.

Excitonic Diffusion in InGaN/GaN Core-Shell Nanowires.

We report on the direct observation of the diffusion of carriers in graded InGaN/GaN quantum wells in a nanowire. By probing the local dynamics at the nanoscale, along the wire for different temperatures between 4 and 250 K, we conclude that this diffusion process is thermally activated. In addition, the analysis of the cathodoluminescence lifetime for different temperatures shows that the carrier motion is isotropic and does not follow the indium gradient. Our observations are interpreted in terms of a hopping process between localized states. We find that the random alloy fluctuations prevent any directional drift of excitons along the In gradient and therefore any carrier accumulation. Our results therefore confirm the potential of core-shell nanowires for lighting devices. Indeed, the short lifetime of m-plane quantum wells together with their large active area and the homogeneous distribution of carrier along the quantum well will decrease influence of any high carrier density effect on the efficiency of these light-emitting diodes.

Exciton localization and drift in tailored-potential quantum nanowires

Exciton recombination dynamics in tailored-potential, site-controlled AlGaAs quantum wires (QWRs) are studied. Time-resolved photoluminescence spectra evidence exciton localization in weakly disordered “uniform” QWRs, whereas deterministic bandgap grading is shown to suppress localization and promote exciton drift along the potential gradient. Measured exciton transit times between two quantum dot probes placed at opposite ends of the potential gradient yield the effective 1D exciton mobility as >1300 cm2/(eVs).

Exciton Drift in Semiconductors under Uniform Strain Gradients: Application to Bent ZnO Microwires

Optimizing the electronic structures and carrier dynamics in semiconductors at atomic scale is an essential issue for innovative device applications. Besides the traditional chemical doping and the use of homo/heterostructures, elastic strain has been proposed as a promising possibility. Here, we report on the direct observation of the dynamics of exciton transport in a ZnO microwire under pure elastic bending deformation, by using cathodoluminescence with high temporal, spatial, and energy resolutions. We demonstrate that excitons can be effectively drifted by the strain gradient in inhomogeneous strain fields. Our observations are well reproduced by a drift-diffusion model taking into account the strain gradient and allow us to deduce an exciton mobility of 1400 ± 100 cm(2)/(eV s) in the ZnO wire. These results propose a way to tune the exciton dynamics in semiconductors and imply the possible role of strain gradient in optoelectronic and sensing nano/microdevices.

Dynamics of long-living excitons in tunable potential landscapes

A novel method to experimentally study the dynamics of long-living excitons in coupled quantum well semiconductor heterostructures is presented. Lithographically defined top gate electrodes imprint in-plane artificial potential landscapes for excitons via the quantum confined Stark effect. Excitons are shuttled laterally in a time-dependent potential landscape defined by an interdigitated gate structure. Long-range drift exceeding a distance of 150 μ m at an exciton drift velocity v d ≳ 10 3 m / s is observed in a gradient potential formed by a resistive gate stripe.

Real-time observation of charged exciton drift in modulation-doped (Cd,Mn)Te quantum well

Time and spatial-resolved circular-polarized photoluminescence (PL) measurements were performed on an $n\ensuremath{-}$type modulation-doped (Cd,Mn)Te quantum well. The spatial extent of PL in the right circular polarization increased with an increase in magnetic fields, although the PL extent showed a tendency to contract with the magnetic fields in the left one. We successfully observed the time development of an electric-field-induced drift of a negatively charged exciton ${X}^{\ensuremath{-}}$. The drift was found to be promoted by the magnetic fields. These peculiar phenomena were resulting from the distinctive properties of the diluted magnetic semiconductor, (CdMn)Te.

Exciton drag and drift induced by a two-dimensional electron gas

We show theoretically that an electric current in a high-mobility quasi-two-dimensional electron layer induces a significant drift of excitons in an adjacent layer through interlayer Coulomb interaction. The exciton gas is shown to drift with a velocity which can be a significant fraction of the electron drift velocity at low temperatures. The estimated drift length is of the order of micrometers or larger during the typical exciton lifetime for $\mathrm{Ga}\mathrm{As}∕{\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ double quantum wells. A possible enhancement of the exciton radiative lifetime due to the drift is discussed.

Observation of charge transport by negatively charged excitons.

We report transport of electron-hole complexes in semiconductor quantum wells under applied electric fields. Negatively charged excitons (X-), created by laser excitation of a high electron mobility transistor, are observed to drift upon applying a voltage between the source and drain. In contrast, neutral excitons do not drift under similar conditions. The X- mobility is found to be as high as 6.5 x 10(4) cm2 V-1 s-1. The results demonstrate that X- exists as a free particle in the best-quality samples and suggest that light emission from opto-electronic devices can be manipulated through exciton drift under applied electric fields.

Linear and Nonlinear Excitonic Absorption and Photoluminescence Spectra in Cu2O: Line Shape Analysis and Exciton Drift

A complete analysis of the line shape of the excitonic absorption and photoluminescence lines of the yellow series of Cu2O is given. A detailed fit to the absorption lines up to the n = 5 exciton according to Toyozawa's theory gives precise values for the excitonic resonance energies, allowing to calculate the excitonic Rydberg. The fit to the photoluminescence lines shows very good agreement with experimental data at different temperatures. At high excitation, the fit to the excitonic absorption lines reveals the fundamental mechanism for bleaching of exciton absorption lines in Cu2O. Based on these findings, a new all-optical method for the observation of exciton transport in Cu2O is proposed.

Schottky barrier breakdown in Si stimulated by exciton drift in a nonuniform electric field at 4.2 K

Schottky barrier breakdown in Si stimulated by exciton drift in a nonuniform electric field at 4.2 K

Drift of excitons in an inhomogeneous electric field in silicon at 4.2K

The results are presented of the experimental detection and investigation of the phenomenon of the drift of excitons in an inhomogeneous electric field. The investigations were performed on silicon structures with Shottky barriers at 4.2 K. A study was made of the influence of the photoexcitation intensity and electric field strength on the breakdown dynamics of the structure. The experimental results were used to show that breakdown of a reverse-biased Shottky barrier involves a narrowing of the width of the space-charge region due to a drift of excitons in the inhomogeneous electric field and to recombination of electrons on ionized impurity centers of the barrier.

Inter-Island Energy Transfer in AlGaAs/GaAs Quantum Wells Grown by Molecular Beam Epitaxy

The results of photoluminescence, time-resolved photoluminescence, photoluminescence excitation and photoluminescence kinetics studies are presented for a Al0.3 Ga0.7Αs/GaΑs quantum well system grown without growth interruptions at the interfaces. The time-resolved photoluminescence measurements show drift of excitons towards lower energy states induced in a quantum well by potential fluctuations. We present also a first direct evidence for migration of free excitons from the 24 to 25 ML regions of the quantum well and interpret these results within a linear rate model, deriving the transition rate of 290 ps-1. Such inter-island migration processes have been observed till now only in growth interrupted structures. PACS numbers: 78.66.Fd, 78.47.+ρ, 73.20.Jc

Drift of excitons induced by static electromagnetic field

Under experimentally achievable conditions, an observable drift of excitons is predicted in the direction E × B and their average velocity is estimated.

Ionized impurity induced photocarrier generation in organic energy conversion systems

Due to the high density of impurities, Schottky barriers in organic semiconductors usually have quite thin depletion regions, of the order of a few hundred angstroms. For typical built in potentials of few tenths of a volt, the average field in the barrier is of the order of 10 V/μm. In this paper, the influence of the localized nature of the ionized impurities on field dependent carrier generation will be investigated. Assuming the existence of exciton diffusion, it will be shown that carrier generation due to a strong local field of ionized impurities can far exceed the generation efficiency predicted on the basis of the average electric field in the barrier. The concept of exciton drift currents will be introduced, which will lead to a unique light collection mechanism by exciton funnelling to the ionized impurities, thus increasing further the probability of photocarrier generation. Experimental evidence will be presented which indicates that the ionized impurity induced carrier generation is the dominant mechanism of carrier production in x-metal-free phthalocyanine photovoltaic cells. The analogy between the proposed mechanism and photosynthesis will be discussed. It will be suggested that the assumption of a charged ’’special pair’’ could naturally explain the low probability for the back reaction in the chain of primary photosynthetic events.